Chapter 03 - Sessions A09, A12 - EFCF · Chapter 03 - Sessions A09, A12 - 2/122 Stack design and characterisation ... Institute of Engineering Thermodynamics Pfaffenwaldring 38-40

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Proceedings of

12th

European SOFC & SOE Forum 2016

Chapter 03 - Sessions A09, A12

A09: Cell design and characterisation A12: Stack design and characterisation

Edited by

Prof. Nigel Brandon (Chair) Dr. Antonio Bertei Dr. Paul Boldrin

Dr. Richard Dawson Dr. Kristina Kareh Dr. Jung-Sik Kim

Dr. Zeynep Kurban Dr. Mardit Matian Dr. Paul Shearing

Dr. Farid Tariq Dr. Enrique Ruiz Trejo Dr. Vladimir Yufit

Co-Edited by

Olivier Bucheli Gabriela Geisser Fiona Moore Dr. Michael Spirig

Copyright © European Fuel Cell Forum AG

These proceedings must not be made available for sharing through any open electronic means.

ISBN 978-3-905592-21-4

12th European SOFC & SOE Forum www.EFCF.com/Lib ISBN 978-3-905592-21-4 5 - 8 July 2016, Lucerne/Switzerland

Cell design and characterisation ............................. Chapter 03 - Sessions A09, A12 - 2/122 Stack design and characterisation

Chapter 03 - Sessions A09, A12 ..................................................................................... A09: Cell design and characterisation ....................................................................................... A12: Stack design and characterisation Content Page A09, A12 - ..

A0901 ..................................................................................................................................... 6

Mechanics of SOFC Contacting

Zhangwei Chen (1), Xin Wang (2), Nigel Brandon (3), Alan Atkinson (2) (1) Earth Science and Engineering (2) Department of Materials (3) Sustainable Gas Institute Imperial College. London SW7 2AZ UK

A0902 (Published in EFCF Special Issue Series, www.EFCF.com/LIB) ....................... 13

Relation between shape of Ni-particles and Ni migration in Ni-YSZ electrodes – a hypothesis

Mogens B. Mogensen, Anne Hauch, Xiufu Sun, Ming Chen, Youkun Tao, Sune D. Ebbesen, Peter V. Hendriksen

Department of Energy Conversion and Storage Technical University of Denmark (DTU) Frederiksborgvej 399, DK-4000 Roskilde

A0903 (Abstract only, published elsewhere) .................................................................. 14 Cation diffusion at the CGO barrier layer region of solid oxide fuel cells

M. Morales (1)*, V. Miguel-Pérez (1), A. Tarancón (1), M. Torrell (1), B. Ballesteros (2), J. M. Bassat (3), J. P. Ouweltjes (4), D. Montinaro (5), A. Morata (1)

(1) IREC, Catalonia Institute for Energy Research, Dept of Advanced Materials for Energy Applications, Jardins de les Dones de Negre 1, Planta 2, 08930, Sant Adriá del Besós, Barcelona, Spain. (2) ICN2, Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, Bellaterra, (Barcelona), Spain. (4) CNRS, ICMCB, 87 avenue du Dr. A. Schweitzer, F-33608 Pessac, France. (4) HTceramix SA, Avenue des Sports 26, CH-1400 Yverdon-les-Bains, Switzerland (5) SOLIDPower SpA, Viale Trento 117, 38017 Mezzolombardo, Italy.

A0904 (Abstract only, published elsewhere) .................................................................. 15

Direct-Methane Solid Oxide Fuel Cells with Ceria-Coated Ni Layer at Reduced Temperatures

Jin Goo Lee (1), Ok Sung Jeon (1), Ho Jung Hwang (2), Jeong Seok Jang (1), Yeyeon Lee (2), Sang-Hoon Hyun (3), Yong Gun Shul (1,2)

(1) Department of Chemical and Bio-molecular Engineering, Yonsei University, Seoul/Republic of Korea (2) Department of Graduate Program in New Energy and Battery Engineering, Yonsei University, Seoul/Republic of Korea (3) Department of Materials Science and Engineering, Yonsei University, Seoul/Republic of Korea

A0905 (Abstract only, published elsewhere) .................................................................. 16 Investigation of high performance low temperature ceria-carbonate composite fuel cells

Muhammad Imran Asghar (1), Ieeba Khan (2), Suddhasatwa Basu (2), Peter D. Lund (1)

(1) Department of Applied Physics, Aalto University, P. O. Box 15100, 00076, Finland. (2) Department of Chemical Engineering, Indian Institute of Technology, New Delhi-110016, India.



A0906 (Abstract only, published elsewhere) .................................................................. 17 1D numerical modeling of direct ammonia solid oxide fuel cells

Masashi Kishimoto, Yuki Matsui, Hiroshi Iwai, Motohiro Saito, Hideo Yoshida Department of Aeronautics and Astronautics, Kyoto University Nishikyo-ku, Kyoto 615-8540 Japan

A0907 ................................................................................................................................... 18 Electrochemical and microstructural characterization of Micro-Tubular SOFC: The effect of the operation mode

M. Torrell (1), A. Hornés (1), A. Morata (1), K. Kendall (2), A. Slodczyk (1), A. Tarancón (1)

(1) Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre, 1, 08930 Sant Adrià de Besòs, Barcelona, Spain (2) Adelan, 112 Park Hill Road, Birmingham B17 9HD, UK

A0908 ................................................................................................................................... 28

CFY-Stacks: Progress in Development S. Megel (1), M. Kusnezoff (1), W. Beckert (1), N. Trofimenko (1), C. Dosch1, A. Weder (1), M. Jahn (1), A. Michaelis (1), C. Bienert (2), M. Brandner (2), S. Skrabs (2), W. V. Schulmeyer (2), L. S. Sigl (2)

(1) Fraunhofer Institute for Ceramic Technologies and Systems, Winterbergstrasse 28, 01277 Dresden, Germany (2) Plansee SE, Metallwerk-Plansee Strasse 71, 6600 Reutte, Austria Contact: Dr. Stefan Megel

A0909 ................................................................................................................................... 38 New all-European high-performance stack (NELLHI): Experimental evaluation of an 1 kW SOFC stack

Christoph Immisch (1), Andreas Lindermeir (1), Matti Noponen (2), Jukka Göös (2) (1) Clausthaler Umwelttechnik-Institut GmbH Leibnizstrasse 21+23, D-38678 Clausthal-Zellerfeld, Germany (2) Elcogen Oy Niittyvillankuja 4, FIN-01510 Vantaa, Finland

A0910 (Published in EFCF Special Issue Series, www.EFCF.com/LIB) ....................... 50 Triode Solid Oxide Fuel Cell operation under Sulphur poisoning conditions

Priscilla Caliandro, Stefan Diethelm, Jan Van herle FUELMAT, École Polytechnique fédérale de Lausanne 1951 Sion, Switzerland

A0911 (Abstract only, published elsewhere) .................................................................. 51 Pressurized Operation of a 10 Layer Solid Oxide Electrolysis Stack

Marc Riedel, Marc P. Heddrich, K. Andreas Friedrich German Aerospace Center (DLR) Institute of Engineering Thermodynamics Pfaffenwaldring 38-40 70569 Stuttgart Germany

A0912 ................................................................................................................................... 52

Evaluation of Zr doped BaCe0.85Y0.15O3-δ as PCFC electrolyte

Ha-Ni Im, Dae-Kwang Lim, Jae-Woon Hong, In-Ho Kim and Sun-Ju Song Ionics Laboratory, School of Materials Science and Engineering Chonnam National University, Gwang-Ju 61186, Republic of Korea



A0913 (Abstract only, published elsewhere) .................................................................. 61 Homogenization of the thermo-elastic properties of pristine and aged Ni-YSZ samples

Toni Vešović (1, 2), Arata Nakajo (2), Fabio Greco (2), Pierre Burdet (2, 3), Jan Van herle (2), Frano Barbir (1)

(1) Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia (2) Fuelmat Group, Faculty of Engineering Sciences and Technology STI, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland (3) Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

A0914 ................................................................................................................................... 62 Evaluation of H2O/CO2 co-electrolysis of LSCF6428-GDC Electrode SOFC on microstructural parameters

Sang-Yun Jeon(1)*, Young-Sung Yoo(1), Mihwa Choi(1), Ha-Ni Im(2), Jae-Woon Hong(2), Sun-Ju Song (2)*

(1) Renewables & ESS Group, Energy New Business Lab., Korea Electric Power Research Institute (KEPRI), Korea Electric Power Corporation (KEPCO) 105, Munji-Ro, Yuseong-Gu, Daejeon, 43056, Republic of Korea (2) Ionics Lab., School of Materials Science and Engineering, Chonnam National University, 77 Yongbong-Ro, Buk-gu, Gwang-Ju, 61186, Republic of Korea

A0915 ................................................................................................................................... 69 Temperature effect on elastic properties of SOFC layers

Alessia Masini, Zdeněk Chlup and Ivo Dlouhý Institute of Physics of Materials, Academy of Science of the Czech Republic 22 Zizkova, 616 62 Brno/Czech Republic

A0917 ................................................................................................................................... 75 Characterization of the performance and long-term degradation of fuel electrode supported multilayered tape cast Solid Oxide Cells

M. Torrell (1)*, D. Rodríguez (2), B. Colldeforns (1), M. Blanes (2), A. Morata (1), F. Ramos (2), A. Tarancón (1)

(1) IREC, Catalonia Institute for Energy Research, Dept of Advanced Materials for Energy Applications, Jardí de les Dones de Negre 1, Planta 2, 08930, Sant Adriá del Besós, Barcelona (2) FAE, Francisco Albero SAU, L'Hospitalet de Llobregat, Spain

A0918 (Abstract only, published elsewhere) .................................................................. 85 Hydrogen membrane fuel cell using Ni-Zr alloy membrane

SungBum Park (1), Sung Gwan Hong (1), Yong-il Park (1) (1) Kumoh National Institute of Technology 61 Daehak-ro, Gumi, Gyeongbuk, Korea

A1201 (Abstract only, published elsewhere) .................................................................. 86 Stability of SOFC cassette stacks during redox-thermal-cycling

Ute Packbier (1), Tim Bause (2), Qingping Fang (1), Ludger Blum (1), Detlef Stolten (1)

(1) Forschungszentrum Jülich GmbH Institute of Energy and Climate Research Electrochemical Process Engineering (IEK-3) Wilhelm-Johnen-Straße, 52428 Jülich, Germany (2) ElringKlinger AG, Max-Eyth-Straße 2 72581 Dettingen, Germany

A1202 (Published in EFCF Special Issue Series, www.EFCF.com/LIB) ....................... 87 Evaluation of a SOEC stack for hydrogen and syngas production: a performance and durability analysis

Mikko Kotisaari (1), Olivier Thomann (1), Dario Montinaro (2), Jari Kiviaho (1) (1) VTT Technical Research Centre of Finland Ltd., Biologinkuja 5, 02150 Espoo, Finland (2) SOLIDpower SpA, Viale Trento 115/117, 38017 Mezzolombardo, Trento, Italy



A1203 ................................................................................................................................... 88 Investigation of a 500W SOFC stack fed with dodecane reformate

Massimiliano Lo Faro, Stefano Trocino, Sabrina C. Zignani, Giuseppe Monforte, Antonino S. Aricò

CNR-ITAE, Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy A1204 .................................................................................................................................. 97

Performance Characteristics of Elcogen Solid Oxide Fuel Cell Stacks

Matti Noponen, Jukka Göös, Pauli Torri, Daniel Chade, Heikki Vähä-Piikkiö, Paul Hallanoro

Elcogen Oy 01510 Vantaa, Finland

A1205 (Abstract only, published elsewhere) ................................................................ 106 Performance and degradation of an SOEC stack with different air electrodes

Y. Yan (1), Q. Fang (1), L. Blum (1), W. Lehnert (1, 2) (1) Forschungszentrum Jülich GmbH Institute of Energy and Climate Research Wilhelm-Johnen-Straße 52425 Jülich/ Germany (2) RWTH Aachen University, Modeling in Electrochemical Process Engineering 52072 Aachen/ Germany

A1206 ................................................................................................................................. 107 Fuel Distributions in Anode-Supported Honeycomb Solid Oxide Fuel Cells

Hironori Nakajima(1), Tatsumi Kitahara (1), Sou Ikeda (2) (1) Department of Mechanical Engineering, Faculty of Engineering, Kyushu University (2) Department of Hydrogen Energy Systems, Graduate School of Engineering, Kyushu University 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

A1208 (Abstract only, published elsewhere) ................................................................ 114

Potential for critically-high electrical efficiency of multi-stage SOFCs with proton-conducting solid electrolyte

Yoshio Matsuzaki (1,2), Yuya Tachikawa (3), Takaaki Somekawa (1,4), Kouki Sato (2), Hiroshige Matsumoto (5), Shunsuke Taniguchi (2,3,6), Kazunari Sasaki (2,3,4,5,6)

(1) Fundamental Technology Department, Tokyo Gas Co., Ltd., 1-7-7 Suehiro-cho, Tsurumi-ku, Yokohama City, Kanagawa 230-0045, Japan (2) Next-generation Fuel Cell Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan (3) Center for Co-Evolutional Social Systems (CESS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan (4) Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan (5) International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan (6) International Research Center for Hydrogen Energy, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan

A1209 ................................................................................................................................. 115 Performance testing for a SOFC stack with bio-syngas

Ruey-Yi Lee (1)*, How-Ming Lee (1), Ching-Tsung Yu (1), Yung-Neng Cheng (1), Szu-Han Wu (1), Chien-Kuo Liu (1), Chun-Hsiu Wang (2), Chun-Da Chen (2)

(1) Institute of Nuclear Energy Research No. 1000 Wenhua Road, Longtan District, Taoyuan City / Taiwan (R.O.C.) (2) China Steel Corporation No. 1 Chung-Kang Road, Hsiao Kang District, Kaohsiung / Taiwan (R.O.C.)



A0901

Mechanics of SOFC Contacting

Zhangwei Chen (1), Xin Wang (2), Nigel Brandon (3), Alan Atkinson (2) (1) Earth Science and Engineering

(2) Department of Materials (3) Sustainable Gas Institute

Imperial College. London SW7 2AZ UK Tel.: +44 2075946780

[email protected]

Abstract

Assembly of a planar SOFC or SOE stack involves the lamination of cells and interconnect plates under an applied force. In most designs a pattern of ribs on the interconnector makes contact with a layer of porous ceramic current collector on the air side of the cells. These localised contacts are regions of increased stress on the cells and can cause damage if the stresses become too large. In this contribution we studied experimentally the response of an anode supported cell to a localised load applied using a spherical indenter. FIB/SEM cross sections were used to characterise the deformation of the cell and it was found that the main damage mode was through-cracking of the electrolyte due to bending of the electrolyte layer. Similar experiments and finite element simulation were carried out to determine the mechanical response of each individual layer in the cell structure. A key feature of the FE simulations was inclusion of a sub-model to describe the collapse and densification of the porous anode support and cathode materials under the compressive loading. The FE simulations were used to analyse the indentation experiments and thus determine the critical stress for fracture of the 8YSZ electrolyte to be approximately 2 GPa, which is consistent with the defects seen in the electrolyte layer. Finite element simulations were then carried out for a typical interconnector/cell geometry to study the stress distribution at an interconnector rib contacting the cathode side of the cell. The stiffness of the anode support was found to be a key parameter determining the likelihood of cell damage.

mailto:[email protected]



Introduction All SOFC and SOE devices require electronic current collection to the electrodes. In stacks of planar cells this involves making a contact between the electrode and the interconnector or bipolar plate. On the fuel side this is usually achieved using a nickel mesh or nickel felt. On the air side, the contact is usually made between ribs on the interconnector and the porous air electrode itself, or a similar porous ceramic current collector layer. When the stack is assembled forces are applied to compress the stack, for example by tie-bars between relatively stiff end plates. This applied load serves to accommodate deviations from cell flatness and control or compress seals. It also pushes the interconnector ribs into the air electrode to make the current collection contact. In this respect, the externally applied force becomes a local load where the interconnector rib contacts the air electrode. Penetration of the rib into the electrode structure is beneficial as it improves contact and also accommodates variations in manufacturing tolerances. However, if the local load becomes too high it can cause damage to the cell. This is a particular risk for cell structures having a thin electrolyte layer such as anode-supported SOFCs. Fracture of the electrolyte can allow direct combustion of fuel and air during operation, leading to a local hot-spot and eventual destruction of the cell. As the interconnector rib makes contact with the cathode (in a SOFC) it compresses the cathode leading to collapse of the porous ceramic structure and its local densification. The load is transferred downwards deforming the electrolyte layer and eventually the anode support, which is also porous and capable of densification.

1. Scientific Approach The aim of the current work is to study the mechanics of this process in order to be able to model the cell contacting and to identify a suitable criterion for preventing damage to the electrolyte in an anode-supported SOFC (ASC). The approach adopted involves studying the local loading of the SOFC component materials and layered structures using indentation experiments and finite element (FE) simulations. Particular attention is given to a suitable description of the collapse and densification of the porous electrodes under compressive loading and the criterion for failure of the electrolyte. The results are then used in a FE simulation of contacting between an ASC cathode and interconnector rib.

2. Experiments and Simulations 2.1 Materials The cathode material was La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and was studied in both porous bulk and thin film forms. Bulk specimens were made by isostatic pressing LSCF powder (Fuel Cell Materials, USA) at 200 MPa and films by tape casting onto dense CGO disc substrates. The specimens were sintered in air at peak temperatures of 900, 1000, 1100 and 1200 °C for 4 h. Further details are given in [1]. Typical SOFC porous NiO-8YSZ anode substrates (oxidised condition) approximately 500

µm thickness with a 10 m 8YSZ electrolyte on one side were supplied by JÜLICH Germany. In order to study the specimens with the anode in the reduced condition, some of the as-received specimens were reduced in flowing 92N2/8H2 gas by heating to 900 °C



at a rate 3 °C/min, holding for 3 h, and then cooling to room temperature at a rate of 5 °C/min. As a result, the anode substrates were fully reduced to Ni-8YSZ. 2.2 Indentation experiments Indentation experiments were carried out at room temperature using a spherical diamond indenter tip of 25 µm radius using a NanoTest platform (Micromaterials, UK). Each loading and unloading curve was analysed using the conventional Oliver and Pharr method [2] to give apparent indentation hardness and apparent elastic modulus. The surface and cross-sectional microstructures as well as indentation-induced damage of the specimens were investigated post-indentation using the FIB-SEM slice and view technique. 2.3 Finite element simulation 2D axisymmetric FE models for indentation simulations were performed in Abaqus CAE

6.12 environment (Dassault Systemes, USA) as described in [3]. In brief, a thin (10 m)

dense layer was built on a porous substrate having much larger thickness (500 m) The interface between the layers was perfectly bonded (i.e. no delamination or slippage was allowed). The presence of the anode functional layer is neglected in the model as it is expected to have very similar properties to the substrate. The spherical indenter and the rib on the interconnector were modelled as rigid bodies. The materials were assumed to be homogeneous and isotropic and having perfect elastic-plastic behaviour characterised

by Young’s modulus, E, Poisson’s ratio and yield stress y. Adaptive meshing was performed in the regions close to the indenter contact point to improve the resolution of the stress distribution. The loading and unloading parts of the indentation were simulated by stepwise vertical displacement of the indenter into and off the structures. A key feature of the simulation is the way it deals with collapse and densification of the porous materials. This was done using the Gurson model [4] in which the yield condition is a function of the porosity (f) and is given by the following expression, Eq. (1),

2

232 cosh 1 0

2d dy y

q pf f

(1)

where dy is the yield stress of the dense matrix material, q is the effective von Mises

macroscopic stress and p is the macroscopic hydrostatic stress. Since we apply this

relationship outside the limits of its strict validity we treat dy as an adjustable parameter

whose value is obtained by a best fit to the indentation response of the porous material (this is not necessarily the same as the yield stress of the dense matrix). Substituting the

measured porosity and the fitted value of dy into Equation 1 and considering a uniaxial

stress state gives a value for the uniaxial yield stress of the porous material. The resulting mechanical properties for the materials in the simulation are summarised in Table 1.



Table 1 Mechanical property parameters for materials in simulation

Material Porosity (%) E (GPa) dy (GPa) y (GPa)

8YSZ 5 200 3.80 3.58

NiO/YSZ 14.6 152 3.00 2.38

Ni/YSZ 30.1 73 0.96 0.67

LSCF 900 44.9 34.2 0.83 0.43

LSCF 1000 36.3 44.5 0.9 0.55

LSCF 1100 28.7 80.2 2.19 1.34

LSCF 1200 5.2 174.3 4.56 4.29

In addition the electrolyte layers on these substrates are known to have an in-plane equi-biaxial residual stress of -600 MPa in the oxidised state and -400 MPa in the reduced state [5,6]. These stresses were incorporated in the simulations as initial conditions. The balancing stress in the substrate is much smaller (because it is much thicker than the electrolyte) and was ignored.

3. Results From the SEM images after indentation (e.g. Fig. 1), detectable ring cracks in the indented contact area could be seen after a threshold load was exceeded. For the oxidised specimen this was at 1900 ± 100 mN and 1300 ± 100 mN for the reduced specimen. For loads significantly above the threshold, the cracking of the electrolyte was more severe for the reduced specimen than for the oxidised one as shown in Fig.1. a) b)

a Fig. 1 Top surface and cross-sectional features of the oxidised and reduced specimens after indentation onto the YSZ electrolyte. Oxidised specimen after indentation at 7000 mN peak load and (b) reduced specimen after indentation at 2000 mN peak load. The damage is seen to have three components: concentric ring cracks on the surface inside the indent circle; cone-like cracks in the YSZ electrolyte layer and delamination at the interface between the electrolyte and the substrate. The ring cracks have negligible penetration and probably form during loading. The cone-like cracks are more severe.

Delamination

Cone Crack

Ring Crack

10 µm 10 µm


Cell design and characterisation ........................... Chapter 03 - Sessions A09, A12 - 10/122 Stack design and characterisation

They probably form on loading and then open up on unloading as the elastic stress field relaxes. The delamination probably forms during unloading as the elastic relaxation of the electrolyte is greater than that of the substrate, which has suffered greater irreversible plastic deformation by densification in the region of the interface. Fig. 1 clearly shows the large influence that the substrate has on the susceptibility to damage in the electrolyte. The greater porosity of the substrate in the reduced state gives much less mechanical support to the electrolyte for a given indentation load. For a circular penny-shaped crack-like defect in a homogeneous far-field stress in Mode I (opening mode) loading the critical stress required to extend the defect is given by,

1

2

4c IcK

c

(2) where KIc is the Mode I critical stress intensity and c is the radius of the defect. For 8YSZ KIc = 1.61 MPa m1/2 [7] and typical defects seen in Fig. 1 have a maximum size of

approximately 1 m. Thus Eq. (2) predicts a strength for the YSZ layer of 2.02 GPa. Cracking in the complicated multi-axial stress field of the indentation is difficult to predict. Nevertheless, a commonly used criterion is that cracking initiates in regions where the maximum principal stress is tensile and propagates initially in a direction normal to this stress. The general form of the deformation shown in Fig. 1 is biaxial bending of the YSZ layer due to the less stiff substrate. When the sphere is loaded there will be a compressive region in the upper part of the YSZ as in indentation of a bulk material, and a tensile region near the lower surface of the YSZ due to the bending of the YSZ layer. Fig 2 shows the distribution of the maximum principal stress at the threshold loads for the oxidised and reduced specimens computed in the FE simulations in the absence of any cracking. Note that “maximum principal stress” respects the normal sign convention of tensile stress being positive and compressive stress being negative. From the simulation result in Fig. 2(b) it is seen that the maximum principal tensile stress for the reduced specimen when loaded at the threshold load of 1300 mN is predicted to be between 1.9 and 2.7 GPa in the region at the bottom of the YSZ under the indenter. This is similar to the critical stress of 2 GPa estimated above and suggests that cone-like cracks would be initiated in this specimen at the threshold load of 1300 mN. This is also consistent with the more extensive cracking seen after indentation at the higher load of 2000 mN in Fig. 1(b). The simulated stress field in Fig. 2(a) for the oxidised specimen at its threshold load shows a peak tensile maximum principal stress of less than 0.5 GPa at the lower surface of the YSZ. This is significantly lower than 2 GPa and therefore cone cracks are not expected to form at this load in the oxidised specimen. This is consistent with the much higher load of 7000 mN required to produce the cone cracks in the oxidised specimen seen in Fig. 1(a) Note that the residual compressive stress in the electrolyte is not apparent in the plots in Fig. 2, as mathematically the maximum principal residual stress is zero and is normal to the plane of the electrolyte. Nevertheless, the compressive residual stress is important as it deflects the cone cracks towards the plane of the electrolyte and prevents open cracks forming across the electrolyte. This is important in the context of a fuel cell since open cracks crossing the electrolyte will lead to direct combustion of fuel.



Fig 2. Maximum principal stress distributions computed in the FE simulations. (a) At the threshold load of 1900 mN for the oxidised specimen. (b) At the threshold load of 1300 mN for the reduced specimen. The contour values are in GPa.

Fig. 3 Maximum principal stress distribution from FE simulation of interconnect rib contacting the cathode of an anode supported SOFC in the oxidized state at an applied load corresponding to a mean pressure of 1.5 GPa.

(b) (a)

Electrolyte

Anode

Interconnect rib

Cathode 30 m

Electrolyte 10 m

Support 500 m

200 m



The same FE model was then used to simulate the contact between a cell and an interconnector rib and the maximum principal stress distribution is shown in Fig. 3. The simulation is in plane strain and is invariant in the third dimension. In the simulation the interconnector rib was assumed to be perfectly rigid, the substrate was in the oxidized condition and the cathode was sintered at 1000°C. The radius of curvature at the corner of

the contacting rib was 25 m and the applied load was equivalent to a mean pressure of 1.5 GPa on the top surface of the rib. The results show similar features to those in Fig.2 in that the YSZ layer is bent around the edge of the rib. However, the peak maximum principal stress in the electrolyte is now in the upper region of the YSZ layer and in this example reaches 2.4 GPa, which is sufficient to damage the electrolyte. In addition, the cathode layer is greatly densified under the rib, but is “squeezed out” at the edge.

References [1] Z. Chen, X. Wang, V. Bhakhri, F. Giuliani, and A. Atkinson, Nanoindentation of

porous bulk and thin films of La0.6Sr0.4Co0.2Fe0.8O3−δ, Acta Materialia 61 (15), 5720-5734 (2013).

[2] W.C. Oliver and G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, Journal of Materials Research 7(6),1564-1583 (1992).

[3] Z. Chen, X. Wang, A. Atkinson, N. Brandon, Spherical indentation of porous ceramics: Elasticity and hardness, Journal of the European Ceramic Society, 36, 1435-1445 (2016).

[4] A.L. Gurson, Continuum theory of ductile rupture by void nucleation and growth: Part I—Yield criteria and flow rules for porous ductile media, Journal of Engineering Materials and Technology 99(1), 2-15 (1977).

[5] B. Sun, R. A. Rudkin and A. Atkinson, Effect of thermal cycling on residual stress and curvature of anode-supported SOFCs, Fuel Cells, 9, 805-813 (2009).

[6] W. Fischer, J. Malzbender, G. Blass and R.W. Steinbrech, Residual stresses in planar solid oxide fuel cells, Journal of Power Sources, 150, 73-77 (2005).

[7] A. Selçuk and A. Atkinson, Strength and toughness of tape-cast yttria-stabilized zirconia, Journal of the American Ceramic Society, 83, 2029-2035 (2000).

Acknowledgements This research was carried out as part of the Hydrogen and Fuel Cell SUPERGEN Hub supported by EPSRC grant EP/J016454/1, part of the RCUK energy programme. The Energy Programme is an RCUK cross-council initiative led by EPSRC and contributed to by ESRC, NERC, BBSRC and STFC. The authors are grateful to Dr N. Menzler for provision of specimens.



A0902 (Published in EFCF Special Issue Series, www.EFCF.com/LIB)

Relation between shape of Ni-particles and Ni migration

in Ni-YSZ electrodes – a hypothesis

Mogens B. Mogensen, Anne Hauch, Xiufu Sun, Ming Chen, Youkun Tao, Sune D. Ebbesen, Peter V. Hendriksen

Department of Energy Conversion and Storage Technical University of Denmark (DTU)

Frederiksborgvej 399, DK-4000 Roskilde Tel.: +45-46775726

[email protected]

Abstract This is an attempt to explain a phenomenon of total depletion of Ni next to the electrolyte in Ni-YSZ cermet electrodes in solid oxide electrolysis cells during electrolysis at high current density/overpotential. Intuitively, we would think that Ni would always migrate down the steam partial pressure (pH2O) gradient as previously observed [1], but in the present cases Ni seems to migrate up the pH2O gradient. However, it is also observed that there is a preceding phase in this Ni-YSZ electrode degradation, namely that the Ni-particles closest to the YSZ electrolyte loose contact to each other. This means that the active three phase boundary (TPB) moves away from the electrolyte and causes a significant increase in the ohmic resistance as is also observed in electrochemical impedance spectra. We hypothesize that the cause of this loss of contact is due to a change in Ni-particle shape at very negative potential due to change in surface energy with polarization and to contraction of YSZ upon reduction. Based on micrographs of the Ni-YSZ electrode structures we postulate that the original irregular, elongated shaped Ni-particles get more ball shaped with increasing negative potential, i.e. the surface energy of the Ni increases with decreasing potential. Before the loss of contact of the Ni- and YSZ-particles, the Ni will migrate towards the YSZ electrolyte during negative polarization. Depending on the exact operation conditions, the Ni-particles may lose contact before much migration has taken place. If this happens, there will be no pH2O gradient in the volume between the active TPB (now moved away from the electrolyte) and the electrolyte. Furthermore, as the potential of the non-contacted Ni-particles will be determined simply by the steam/hydrogen ratio, while the Ni at the TPB is significantly negatively polarized, i.e. there is a clear electrochemical potential difference between them. We know that the migration of Ni takes place in form of Ni-OH complexes in the family of Ni(OH)x, but maybe with Ni in a lower positive oxidation state than +2. Anyway, the activity of Ni in a positive oxidation state will be lowest at the most reducing condition, i.e. at the most active TPB some distance (max. few microns) away from the electrolyte. Consequently the Ni should diffuse, probably in the gas phase, to the active TPB and be precipitated there. This will cause the Ni-particles at the TPB (which is now a little away from the electrolyte) to grow, and this is actually observed. At some stage a significant increase in Ni-particle size at the active TPB has taken place and no loss of contact between them will then happen, but thereafter a too dense Ni-layer may form. Remark: The full paper is published in EFCF Special Issue Series (www.EFCF.com/LIB,

SI EFCF 2016) in Journal "FUEL CELLS - From Fundamentals to Systems".

http://www.efcf.com/LIB





A0903 (Abstract only, published elsewhere)

Cation diffusion at the CGO barrier layer region of solid oxide fuel cells

M. Morales (1)*, V. Miguel-Pérez (1), A. Tarancón (1), M. Torrell (1), B. Ballesteros (2), J. M. Bassat (3), J. P. Ouweltjes (4), D. Montinaro (5), A. Morata (1)

(1) IREC, Catalonia Institute for Energy Research, Dept of Advanced Materials for Energy Applications, Jardins de les Dones de Negre 1, Planta 2, 08930, Sant Adriá del Besós,

Barcelona, Spain. (2) ICN2, Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, Bellaterra,

(Barcelona), Spain. (4) CNRS, ICMCB, 87 avenue du Dr. A. Schweitzer, F-33608 Pessac, France.

(4) HTceramix SA, Avenue des Sports 26, CH-1400 Yverdon-les-Bains, Switzerland (5) SOLIDPower SpA, Viale Trento 117, 38017 Mezzolombardo, Italy.

[email protected]

Abstract

The difficulty of achieving a long term stable operation represents one of the main hindrances for the commercialization of solid oxide fuel cells. The important progress achieved in the last years has reduced many of the major classical problems to a minimum. The remaining degradation phenomena produce subtle decreases of performance that reveal their importance at long operating times. Understanding the involved processes and assessing their relative importance in the whole degradation is a major concern.

A well-known phenomenon of degradation of SOFCs is the reaction of La1-xSrxCo1-yFeyO3

(LSCF) cathode with the conventional 8YSZ electrolyte, forming the insulating phases SrZrO3, La2Zr2O7 and (Co, Fe)3O4 [1-3]. The solution adopted to avoid the appearance of these phases is to introduce a dense gadolinium-doped ceria (CGO) barrier layer between the cathode and the YSZ electrolyte [4]. In this work, the solid state reaction and inter-diffusion phenomena between the YSZ electrolyte, the CGO interlayer and the LSCF cathode are analysed. A non-operated reference cell is compared with one subjected to 3000 h working under real conditions in a stack. Exhaustive observations have been carried out using XRD, Raman spectroscopy, SEM-WDX and STEM-EDX-EELS. The results show that insulating phases and solid solutions are formed at both interfaces in pristine and the tested cells and throw light on the inter-diffusion mechanisms taking place.

Remark: Only the abstract is avaiable, because the authors chose to publish elsewhere.

Please see Presentations on www.EFCF.com/LIB or contact the authors directly.






Direct-Methane Solid Oxide Fuel Cells with Ceria-Coated Ni Layer at Reduced Temperatures

Jin Goo Lee (1), Ok Sung Jeon (1), Ho Jung Hwang (2), Jeong Seok Jang (1), Yeyeon Lee (2), Sang-Hoon Hyun (3), Yong Gun Shul (1,2)

(1) Department of Chemical and Bio-molecular Engineering, Yonsei University, Seoul/Republic of Korea

(2) Department of Graduate Program in New Energy and Battery Engineering, Yonsei University, Seoul/Republic of Korea

(3) Department of Materials Science and Engineering, Yonsei University, Seoul/Republic of Korea

Tel.: +82-02-2123-2758 Fax: Not Available

[email protected]

Abstract

Natural gas constitutes a promising energy source in the intermediate future because of the existing supply infrastructure and ease of storage and transportation. Although a solid oxide fuel cell can directly convert chemical energy stored in the hydrocarbon fuel into electrical energy at high temperatures, carbon formations on the nickel-based anode surfaces cause serious degradation of the long-term performance. Here, we report highly coke-tolerant ceria-coated Ni catalysts for low-temperature direct-methane fuel cells. The catalyst shows the high activity for CO oxidations, which is beneficial to avoid carbon formations induced by CO disproportionation at low temperatures. When the ceria-coated Ni catalysts were applied to the solid oxide fuel cells as a catalyst layer, the cell generates a power output of 1.42 W cm-2 at 610 C in dry methane and operates over 1000 h at a current density of 1.2 A cm-2. Remark: Only the abstract is available, because the authors chose to publish elsewhere.







Investigation of high performance low temperature ceria-carbonate composite fuel cells

Muhammad Imran Asghar (1), Ieeba Khan (2), Suddhasatwa Basu (2), Peter D. Lund (1)

(1) Department of Applied Physics, Aalto University, P. O. Box 15100, 00076, Finland. (2) Department of Chemical Engineering, Indian Institute of Technology, New Delhi-

110016, India. [email protected]

Abstract

This work will be submitted to Nano Energy journal 2016, more details of this study

can be found here [1]. In this work, high performance ceria-carbonate composite fuel

cells (CCCFC) are fabricated and characterized using electrochemical impedance

spectroscopy (EIS) and current-voltage measurements under fuel cell conditions at 550oC.

The nanocomposite electrolyte of the cell consists of Ce0.85Sm0.15O2 (65%), referred as

SDC, and eutectic mixture of ternary carbonates Na2CO3, Li2CO3 and K2CO3 (35%)

referred as NLK. The ionic conductivity of the electrolyte was obtained through EIS, which

resulted in 0.44 S/cm and 0.55 S/cm at 550oC and 600oC, respectively. CCCFCs are

fabricated with this electrolyte and composite electrodes (anode = NiO 50wt% and

Electrolyte 50wt%) (cathode = LSCF 50wt% and Electrolyte 50wt%) through cold pressing

method. The cells produced 1.04 W/cm2 at 550oC. The EIS reveals low resistances to

oxidation-reduction and hydrogen-oxidation reactions.

The CCCFC materials were further characterized by X-ray diffraction (XRD) for a wide

range of temperatures (25oC – 600oC) and differential scanning calorimetry (DSC) to see

the structural stability and phase changes. It was found that the carbonates were

transformed into molten phase at around 393oC. The solid phases of NiO, LSCF and SDC

remained stable at least up to 600oC. The CCCFCs were further characterized using

Brunauer-Emmett-Teller (BET) analysis, scanning electron microscopy (SEM), and

transmission electron microscopy (TEM) coupled with X-ray energy dispersive

spectroscopy. Furthermore, effect of supplying CO2 to the cathode in addition to supplying

air was studied, and it was found that the open circuit voltage (OCV) of the CCCFCs

improved from 1.1 V to 1.2 V. This study will provide a deeper insight into the transport

mechanisms and electrode reactions in the fuel cells. Another CCCFC was manufactured

using a composite electrolyte (30wt% SDC, 70wt% NKL) prepared through freeze drying

method. The anodes and cathodes were prepared in a similar fashion as for the previous

CCCFCs. This CCCFC produced even higher power output power 1.1 W/cm2 at 550oC.

Remark: Only the abstract is available, because the authors chose to publish elsewhere.







1D numerical modeling of direct ammonia solid oxide fuel cells

Masashi Kishimoto, Yuki Matsui, Hiroshi Iwai, Motohiro Saito, Hideo Yoshida Department of Aeronautics and Astronautics, Kyoto University

Nishikyo-ku, Kyoto 615-8540 Japan Tel.: +81-75-383-3652 Fax: +81-75-383-3652

[email protected]

Abstract

Ammonia is receiving attention as a hydrogen carrier because of a number of advantages over other hydrogen careers, such as larger hydrogen content, easy liquefaction and no carbon emission. Mass production process of ammonia has also been fully established and well known as the Haber-Bosch process. Solid oxide fuel cells (SOFCs) are one of the candidates that can be operated with ammonia-based fuels because the excess heat generation from the cells can be effectively utilized for ammonia decomposition to produce hydrogen. In this study we have developed a one-dimensional numerical model that predicts performance of direct ammonia SOFC cells. Catalytic decomposition of ammonia and the electrochemical reaction of hydrogen are simultaneously considered within the electrode. Microstructural parameters of the porous electrode were obtained using focused ion beam scanning electron microscopy (FIB-SEM). Empirical formula for the ammonia decomposition in the Ni-YSZ anode was developed in our group and applied to the model. The results were compared with experimental data for validation. From a button cell experiment, the performance of an anode-supported cell with ammonia fuel at 700 °C was found to be comparable to that with fully decomposed gas (H2:N2=3:1). The concentration overpotential was slightly larger when the ammonia fuel was supplied. The numerical results revealed the distribution of the ammonia decomposition and the electrochemical reaction within the anodes. In anode supported cells, most of the ammonia was decomposed before it reached the anode-electrolyte interface, with the

decomposition area being ca. 200 m from the anode surface. The electrochemical reaction occurred in the vicinity of the anode-electrolyte interface and the active thickness

was 20-35 m, which is similar to that observed when hydrogen-based fuel is supplied. Remark: Only the abstract is available, because the authors chose to publish elsewhere.






A0907

Electrochemical and microstructural characterization of Micro-Tubular SOFC: The effect of the operation mode

M. Torrell (1), A. Hornés (1), A. Morata (1), K. Kendall (2), A. Slodczyk (1), A. Tarancón (1)

(1) Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre, 1, 08930 Sant Adrià de Besòs, Barcelona, Spain

(2) Adelan, 112 Park Hill Road, Birmingham B17 9HD, UK Tel.: +34-93-356-2615 Fax: +34-93-356-3802 [email protected]

Abstract Due to the excellent thermal shock and mechanical resistance of Micro-Tubular Solid Oxide Fuel Cells (mSOFC), many orders of magnitude larger than the planar SOFC, mSOFC have been applied in the transport sector market. The main objective of the SAFARI project is the development of a SOFC based system as an Auxiliary Power Unit (APU) fed by LNG for road trucks. In previous publication the authors detailed the long-term degradation of single micro-tubular SOFC operating at 700 ºC supplying 7 W. In the present work the influence of the fuel utilization (Fu) in the degradation of the cells has been studied by means of Fu cycling experiments. Obtained results are discussed and related with the information extracted from the post-mortem microstructural characterization. The results also evidence mass transport issues at low carrier gas flows which are ascribed to the evacuation of produced water at the anode active sites. When high Fu is employed, which means lower H2-to-carrier ratio, this situation is prevented. An improvement of the cell performance and long term resistance is observed when the carrier gas flow is increased, even at higher Fu, identifying the carrier gas flow as a key factor for enhancing fuel efficiency of the cells.




Introduction Micro-Tubular Solid Oxide Fuel Cells (mSOFCs) exhibit an excellent thermal shock and mechanical resistance, many orders of magnitude higher than that characteristic of planar SOFCs [1-3]. Moreover, bearing in mind their very promising power density and almost instantaneous start-up, it is not surprising that they have been applied in the transport sector market [1-5]. The main objective of the SAFARI project is the development of Auxiliary Power Units (APUs) for road trucks fed by liquid natural gas (LNG) [6,7]. Designed APU will provide 100W of power at operation temperatures around 700ºC, with a maximum starting up time of 20min, showing a durability of at least 1000h in continuous operation of 100 start-stop cycles. These requirements are accomplished by using 16 single mili-tubular SOFC cells with high thermal and mechanical stability [6-8]. Figure 1 presents the scheme of a single mSOFC used in this project: the nickel-modified yttria stabilized zirconia (Ni-YSZ) cermet is applied as an anode, the lanthanum strontium cobalt ferrite (LSCF) perovskite as a mixed ion-electron conducting (MIEC) cathode whereas the YSZ plays a role of electrolyte. In order to avoid undesirable reaction between the YSZ and LSCF giving rise to the presence of secondary insulating phases [9], the thin layer of samaria doped ceria (SDC) is used as a barrier layer. Silver paste has been used as a current collector covering the 20cm2 of cathode active area, helping the collection of the electrons through the 15mm length cathode.

1. Scientific Approach The performance optimization and comprehension of the ageing mechanism of single mSOFC are the main targets to achieve the SAFARI project goals. The study has been carried out through careful microstructural and electrochemistry characterization of the pristine and operated cells under different conditions. In a previous publication, the authors claim a remarkable performance at 700ºC, supplying up to 7W as well as high thermal shock and operation stability of the single micro-tubular SOFC during long-term degradation tests [8]. The aim of the presented work was to improve and understand the key parameters that govern the efficiency and durability of the cell under operation conditions. Consequently, the long term stability of the single cell undergoing the 100 simultaneous current and thermal cycles was carefully analyzed. In order to distinguish the effect of current and temperature in the observed degradation, a thermal cycling test was performed separately. Moreover, the influence of the fuel utilization (Fu) in the cell degradation has been carefully studied by means of the Fu dual cycling experiments. The role of the total gas carrier flow in the fuel efficiency is also discussed.

2. Experimental methods Micro-tubular solid oxide fuel cells with 5.5 mm of internal diameter and 20cm length were prepared by high shear extrusion method. Figure 1 summarizes the composition and thickness of each characteristic layer. The single cell was mounted into a metallic hardware with a gas controller and monitoring system, and an electrical connections panel. Fuel cell performance (V-I polarization curves, current-temperature cycling and dual Fu cycling) and Electrochemistry Impedance Spectroscopy (EIS) measurements were then performed in a home-made station equipped with a furnace, El-Flow Bronkhorst mass flow controllers enabling H2 and Ar mixture as well as electrical characterization equipment (Kepco KLP power source, TrueData electronic load, Solartron 1260 impedance analyzer and Keithley temperature controller). This allows the V-I curves and EI spectra recording



from OCV to 15A at operating temperature, i.e. 700ºC and different fuel utilization. Microstructural analysis has been performed using a Zeiss Auriga Scanning Electron Microscope.

Fig. 1 Scheme and SEM cross section showing the thickness and composition of characteristic layers constituting the single mSOFC employed in SAFARI project. Note the

presence of silver current collector on the top of the LSCF cathode.

3. Results Initial performance of micro-tubular SOFC Presented here mSOFCs have been firstly characterized by V-I polarization curves (Fig. 2). As it can be observed, the OCV value of 1.1V together with the fact that currents higher than 9A can be easily applied obtaining a total maximum power above 6W confirm a high cell performance [1-5]. Note, the degradation tests have been carried out at milder conditions applying 6A giving rise to the power around 4W per cell.

Fig2. V-I and power curves of a single mSOFC cell at 725oC under pure wet H2.

LSCF (25 µm)

SDC (2 µm)

YSZ (15 µm)

Ni-YSZ (550 µm)

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Thermal and current cycling Performed cycling aging current and thermal tests allow simulating the real operation of the single cells when operate on the device [10]. It is known that one of the most critical points of a system is the on/off process and the heating-up [3-5]. The behavior of the cell and the obtained performance has been studied and analyzed. Figure 3a shows an example of three last cycles from the studied series of 100 recorded between 300 and 700ºC. During each cycle, once 700ºC achieved, the current was changed from OCV to 6A. The slight increase of the in-cell temperature due to the Joule effect was observed. The presented cycles reveal high stability of the system in very good agreement with the time dependence of the cell power extracted from each cycle (Fig. 3b). As it can be see, the extracted cell power value slightly decreased with the operation time and accumulative number of cycles, pointing to a degradation rate of 4% after 100 temperature - current cycles of operation.

Fig. 3 a) Current and thermal cycles applied to the single SAFARI mSOFC – here example of the last 3 cycles from 100 recorded. b) Time evolution of the cell power extracted from

each cycle.

V-I polarization curves and electrochemistry impedance spectra (EIS) were recorded before and after the 100 cycles of operation (Fig. 4). The comparison of initial and final voltage-current density/power density curves (Fig. 4a) confirms a very small loss of cell performance due to the cell degradation. The analysis of EI spectra presented in Fig. 4b reveals that observed differences in the cell resistance can be mostly assigned to the ohmic contribution. According to the SEM analysis (Fig. 5), anode microstructure modifications after operating are of minor importance showing a very small and localized oxidation and coarsening of nickel near the electrolyte-anode interface. Taking into account a high stability of YSZ electrolyte, especially at this intermediate operation temperature, the detected ohmic resistance increase can be then ascribed to the degradation of silver current collectors as a function of time and temperature. Note that the tendency of Silver to agglomerate has already been described above 700ºC [11,12]. This suggests that the used silver current collectors here appear as the weakest issue of the actual cell design, as previously mentioned in literature [13].

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Fig. 4 Comparison of a) V-I and power curves and b) EIS recorded before and after 100 cycles of current and thermal operation.

Fig. 5 SEM micrographs showing a) cross sections and b) electrolyte – anode interfaces with the focus on the anode microstructure of the single cell before (left) and after (right)

100 current thermal cycles.

In order to clarify whatever thermal or current cycling plays a dominant role on the cell degradation, the 100 thermal cycles were performed in the same temperature conditions (insert in Fig. 6a). The V-I curves and EI spectra recorded before and after this thermal cycling, presented in Figure 6, show very similar behavior to that observed during simultaneous current-thermal cycling tests. This reveals that the temperature cycling can be considered as the main cause of the cell degradation. It seems also clear that the performance drop of the cell is related mostly to the ohmic resistance.

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Fig. 6 Comparison of a) V-I and power curves and b) EI spectra recorded before and after 100 thermal cycles. Fig. 6a insert shows thermal cycles as a function of time.

Fuel utilization In the understanding of cell degradation process one cannot forget the important impact of the used fuel utilization [14, 15]. In previous study, the authors showed that the best long-term degradation behavior was obtained with the 40% of Fu [8]. In order to achieve better results from economic and industrial points of view the tests have been performed using higher Fu. It should be noticed that a high Fu gives rise to high production of water and in consequence to the presence of locally more oxidizing atmosphere in the anode sites [15, 17]. This enhances the undesirable oxidation and agglomeration of nickel and can even lead to the electrolyte-anode interlayer decohesion. In order to deal with this issue, the fuel utilization cycling in the so called dual fuel utilization mode, i.e. 60 and 25% of Fu, have been performed. Note two different values of total gas flow (H2 and Ar) were compared, pure H2 being responsible of the Fu value. Time evolution of voltage recorded in such dual mode is presented in Figure 7a. The results

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point to two important features. First of all a higher degradation rate is detected when low Fu conditions are used. This reveals the mass transport issues, starting already in the beginning of operation, which can be related to the evacuation of produced water. When high Fu is employed, which means lower H2-to-carrier ratio, this situation is prevented. The important role of carrier gas flow on the degradation process constitutes then the second feature. As it can be seen (Fig. 7a), the higher the carrier gas flow, the lower the cell degradation rate. This is well confirmed by the data presented in Fig. 7b, obtained for severe Fu of 80%. The measurements performed under 300ml/min of total gas flow point to the small degradation rate of 0.39 mV/h whereas in the case of the cell operating under reduced flow loss of cell efficiency is detected already after 80 hours of working.

Fig. 7 Time evolution of voltage recorded during a) cyclic dual operation at Fu of 60 and 25%, and b) galvanostatic measurements at Fu of 80% under 200 ml/min or 300 ml/min of

total gas flow. As presented in Fig.8, once the high total gas flow is used, the cell can operate 1000 hours even at the highest Fu of 80% satisfying perfectly the industrial lifetime and Fu requirements. Moreover, very small degradation rates were detected (Fig 8) after 1000h of operation. It is important to highlight the presence of three different regions of degradation with slightly different rates: 0.39 mV/h (< 200h), 0.15 mV/h (200h-800h) and 0.19 mV/h (> 800h), which is very characteristic for SOFCs in agreement with their highly dynamic behavior in terms of microstructure [18]. The above results show clearly that an improvement of the cell performance and long term resistance can be achieved when the carrier gas flow is increased, even at higher Fu, identifying the carrier gas flow as a key factor for enhancing fuel efficiency of the cells.

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Fig.8 Long time evolution of voltage recorded during galvanostatic measurements at Fu of

80% and under 300 ml/min of total gas flow.

4. Conclusion

The applied long term: current – thermal and thermal cycles as well as Fu dual mode tests allow us to study the performance of a single mSOFC under operation mode. The main conclusions obtained from the cycling experiments can be summarized as follows: i) low degradation rate, i.e. 4%, shows that the cell performance is rather well preserved during the long term thermal and current cycling, ii) the thermal cycling plays the dominant role in the cell degradation, iii) the loss of cell performance is mostly related to the degradation of silver current collectors detected in the evolution of the ohmic contribution while the electrolyte and anode microstructures as well as the interphase are preserved. The dual Fu tests using different total gas flow shows clearly that Fu has as important influence on the cell degradation. However, the negative effect of high Fu operation conditions can be significantly improved by the use of higher flow of carrier gas. These results are in very good agreement with the previously published ones [8] and confirm well a longtime stability and high potential of mSOFC.

Acknowledgement

The SAFARI project is funded by Europe’s Fuel Cell and Hydrogen Joint Undertaking (FCH-JU) under Grant Agreement No. 325323. Information contained in the paper reflects only view of the authors. The FCH-JU and the Union are not liable for any use that may be made of the information contained therein.

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References [1] V. Lawlor, S. Griesser, G. Buchinger, A.G. Olabi, S. Cordiner, D. Meissner, Review of the micro-tubular solid oxide fuel cell. Part I: Stack design issues and research activities, Journal of Power Sources 193 (2009) 387–399. [2] V. Lawlor, Review of the micro-tubular solid oxide fuel cell. Part II: Cell design

issues and research activities, Journal of Power Sources 240 (2013) 421-441.

[3] K.S. Howe, G.J. Thompson, K. Kendall, Micro-tubular solid oxide fuel cells and stacks, Journal of Power Sources 196 (2011) 1677-1686. [4] Y. Alyousef, K. Kendall, Characterization of the electrochemical performance of micro‐ tubular solid oxide fuel cell (SOFC), Journal of Taibah University for Science 2 (2009) 14-21. [5] M.H. Chen, T.L. Jiang, The analyses of the start-up process of a planar, anode-supported solid oxide fuel cell using three different start-up procedures, Journal of Power Sources 220 (2012) 331-341. [6] http://safari-project.eu/ [7] K. Kendall, J. Newton, M. Kendall, Microtubular SOFC (mSOFC) System in Truck APU Application. ECS Transactions 68 (2015) 187-197. [8] M. Torrell, A. Morata, P. Kayser, M. Kendall, K. Kendall, A. Tarancon, Performance and long term degradation of 7 W micro-tubular solid oxide fuel cells for portable applications, Journal of Power Sources 285 (2015) 439-448. [9] G. Kostogloudis, G. Tsiniarakis, C. Ftikoset, Chemical reactivity of perovskite oxide SOFC cathodes and yttria stabilized zirconia, Solid State Ionics 135 (2000) 529-535. [10] M.F. Serincan, U. Pasaogullari, N.M. Sammes, Effects of operating conditions on the performance of a micro-tubular solid oxide fuel cell (SOFC), Journal of Power Sources 19 (2009) 414-422. [11] P. Singh, Z. Yang, V. Viswanathan, J.W. Stevenson, Observations on the structural degradation of silver during simultaneous exposure to oxidizing and reducing environments, Journal of Material Engineering Performance 13 (2004) 287-294. [12] Y. Liu, M. Mori, Y. Funahashi, Y. Fujishiro, A. Hirano, Development of micro-tubular SOFCs with an improved performance via nano-Ag impregnation for intermediate temperature operation, Electrochemistry Communications 9 (2007) 1918–1923. [13] H. Zhu, R.J. Kee, The influence of current collection on the performance of tubular anode-supported SOFC cells. Journal of Power Sources 169 (2007) 315-326. [14] K. Huang, Fuel utilization and fuel sensitivity of solid oxide fuel cells, Journal of Power Sources 196 (2011) 2763–2767.

http://safari-project.eu/



[15] P. Nehter, A high fuel utilizing solid oxide fuel cell cycle with regard to the formation of nickel oxide and power density, Journal of Power Sources 164 (2007) 252–259. [16] Y. Liu, Effects of impurities on microstructure in Ni/YSZ–YSZ half-cells for SOFC. Solid State Ionics 161 (2003) 1–10. [17] T. Suzuki, Z. Hasan, Y. Funahashi, T. Yamaguchi, Y. Fujishiro, M. Awano, Impact of anode microstructure on solid oxide fuel cells, Science 325 (2009) 852-855. [18] J.T.S. Irvine, D. Neagu, M.C. Verbraeken, C. Chatzichristodoulou, C. Graves, M.B. Mogensen, Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers, Nature energy DOI: 10.1038/NENERGY.2015.14



A0908

CFY-Stacks: Progress in Development

S. Megel (1), M. Kusnezoff (1), W. Beckert (1), N. Trofimenko (1), C. Dosch1, A. Weder (1), M. Jahn (1), A. Michaelis (1),

C. Bienert (2), M. Brandner (2), S. Skrabs (2), W. V. Schulmeyer (2), L. S. Sigl (2) (1) Fraunhofer Institute for Ceramic Technologies and Systems, Winterbergstrasse 28,

01277 Dresden, Germany (2) Plansee SE, Metallwerk-Plansee Strasse 71, 6600 Reutte, Austria

Contact: Dr. Stefan Megel Tel.: +49(0)351/25537-505, Fax: +49(0)351/2554-187

[email protected]

Abstract

The development of electrolyte supported cells and the components for high efficient and robust stacks are in the focus of the R&D activities of Fraunhofer IKTS for a long time. Since 2010, the CFY-stack design MK351 is produced for a broad range of prototype applications. The change to the new design MK352 has advantages in operation, integration, quality and shall lead to a commercial production. In close collaboration with Plansee SE, a symmetrical design of the interconnect was developed, which allows the compensation of tolerances resulting from near-net shape pressing technology and simpler stack integration in modules. By revision of tolerance chains for all stack components, better robustness in manufacturing and performance has been achieved. The program of validation tests for cells, glass sealings, interconnects, protection and contact layers for the stack will be shown on the example of the new stack design MK352. CFY stacks are the heart for several SOFC/SOEC systems and shows equal characteristics for a wide operating window. The background for that will be explained in this article by testing different gas compositions with local temperature measurements inside the stack.

Figure 1: Different designs of interconnect plates made by Plansee SE, designed bei Fraunhofer IKTS

(top/back MK351, bottom/front MK352)



1. Introduction For more than 15 years, Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) has

been developing SOFC stacks based on Chromium-Iron-Yttria (CFY) alloy interconnects from

Plansee SE [1, 2]. The stack concept with electrolyte supported cells (ESC) has the highest potential

for realization of reliable SOFC stacks. Fully Scandia stabilized Zirconia has a high ionic

conductivity and a thermal expansion coefficient (TEC) perfectly matched by the CFY material.

This enables stacks with high power density and efficiency comparable to anode supported cell

(ASC) stacks [3, 6, 7].

Now a new design MK352 follows the successful MK351 stack design [4, 7] and has passed all

validation tests. The main advantages are higher geometric accuracy, symmetric manifolds and

lower pressure drop at the air side. Performance maps, thermal cycling without reducing gas and

long-term tests will be shown. Whereas the performance and stability have influence to system

power and operation time, low degradation in the cycling allows an efficient cycling as well as

surviving uncontrolled shut downs at the systems. The validation tests should deliver the needed

information for system development and calculations. Because of the wide variety of SOFC

systems, several gas mixtures have to be tested at the stack to recommend an operation strategy. It

was shown previously [4] that stack testing for large units can be easily performed in hotboxes with

preheated gases. For scientific comparison of different gas composition a surrounding was

developed to simulate the hotbox condition with comparable temperature behavior. The power

output regarding different gas composition up to high CH4 contents will be discussed. The aim of

the present work is to show the advantages of CFY-stacks for system operation and demonstrate the

capabilities for stationary applications in the power range of 0.5-200 kWel.



2. MK352 stack design

The cross flow design with internal gas- and open air manifolds is a possibility of designing

compact stacks [9]. It allows an easy gas sealing at the end plates and an uniform air distribution

over all cells by external parts. The explosion view of the stack in Figure 2 shows the low amount

of single parts and the easy assembly to a stack. The interconnects of the new stack design (MK352)

has the same outer dimensions like the MK351 design. They are with 130x150x3.2 mm³ the biggest

CFY interconnects and has a high ratio of active/passive area (65 %). The high thermal conductivity

of the CFY material (35-45 W/m/K 20-900°C) and thick plates promote homogeneous temperature

distribution over the active area and more homogeneous current density distribution.

Layout changes in the air channels of the MK352 interconnect flow field led to a pressure drop

that was more than 50 % lower than that of the actual MK351 design [7]. Therefore, the total

system efficiency can be enhanced due to the lower energy consumption of the air blower to supply

air to the stacks.

Figure 2: MK352 stack design

The sealing by semi crystallizing glass leads to a tight stack even without compression at room

temperature. Two layer of glass sealing elements, directly stamped out from the tape casted foil,

quits the step of lamination and allows better quality at the abutting edges. The electrolyte

supported cell has an active area of 110x115 mm² (127 cm²). The CFY material and the other

components in the stack are exactly matched to the thermal expansion of 10Sc1CeSZ electrolyte

and minimize the mechanical stress caused by thermal- and load cycling.



3. Results

To validate the robustness of stacks and proof the suitability of new components in the stack,

IKTS considered three main tests caused by the customer needs over the last years:

Performance map: 30-cell stack in (simulated) hotbox

Thermal cycling without flushing gas: 30-cell stack in hotbox

Long term test: 10-cell stack in furnace

The performance map shows the power output of the stack over a wide operating window and is

tested with 30-cell stacks integrated in the hotbox. The thermal cycling is also carried out in this

configuration, because it clones the operation of a system. For long-term tests it is sufficient to test

with 10-cell stacks in a furnace to reach a better temperature uniformity, monitor all cell voltages

and operate the stack more stable with preheated gases and furnace heating. All these tests were

done with new generation of stacks including new components or designs.

Operating a hotbox with preheated gases leads to a small window of operation and is strongly

depending on the flow and thermal parameter of the test rig. To avoid this problem a 30-cell stack

was tested in a furnace with semi insulated integration, so it is possible to control thermal loss over

the surface of the stack by furnace temperature. Figure 3 shows the temperature distribution in the

30-cell stack by hotbox operation and the comparable results by adjusting parameters (furnace

temperature, air- and gas preheaters). Considering the accuracy of the measurements, the

differences in the values are negligible.

Figure 3: @35 A, fuel: 40% H2 +5 %H2O in N2, FU=75%, air: 100 sl/min, black values are

simulated hotbox measurement, blue values are real hotbox tests

Additional thermocouples in the middle of the stack (cell 14 and 15) enable the sensing of local

temperature distribution (Figure 4) and are in good accordance with simulated values [6]. The

temperature at the air outlet is easy to measure and is set as reference temperature of the stack. The

very low temperature difference of <50 K over the whole active area is the result of the high

thermal conductivity of the interconnect and is one of the main advantages of this stack technology.

Because of the low temperature difference, the active cell area can be operated at optimum

temperature.



Figure 4: Temperature distribution of a middle cell in a 30 cell stack operated in a simulated hotbox

(temperature values as numbers) compared with a 2D simulation @35 A, fuel: 40% H2 +5% H2O in

N2, FU=75%, air: 100 sl/min, orange dots are positions of thermocouples

In this configuration a performance map was measured (Figure 5). It shows a linear behavior with

low dependence of power output to fuel utilization and a wide operation window depending on

current density. There is no influence by +/-25% air flow to the power output (not shown in the

diagram). At the reference point of 35 A and FU=75% a variation from 775°C to 865°C leads to a

power output of 780 W respectively 866 W and results in an average temperature coefficient of

about 1.0 W/K. The wide operation window gives the possibility to applicate the stack in different

systems and operate up to 40 W/cell.

Figure 5: Performance map 30 cell stack in a simulated hotbox, fuel: 40% H2 +5% H2O in N2,

FU=75%, air: 100 sl/min, at a constant reference temperature T_Cat_o=835°C

20

25

30

35

40

600

700

800

900

1000

1100

1200

0,5 0,55 0,6 0,65 0,7 0,75 0,8 0,85

Po

we

r in

W/c

ell

Po

we

r in

W/3

0 c

ell

Fuel utilization

Performance Map MK 352Gaszusammensetzung: N2 : H2 : H2O (55% : 40% : 5%)Betriebspunkte: stationär @ ΔPel < 1 %/1000h

isotherm @ TKat_A= (830°C - 840 °C)VLuft = 100 Nl/min; konst.

50 A

40 A

35 A



System operation

After validating the temperature profile with a real hotbox test, a wide variety of operations were

tested. The selection of gas compositions were derived from real operation of different systems

(Table 1) and is arranged by amount of CH4.

Table 1: Gas composition and power output for 30-cell stack tests @35A, FU=75%, air: 100 sl/min,

at constant reference temperature T_Cat_o=835°C

System operation H2 N2 CO CO2 CH4 H2O Vanode Pel

in vol% in mol/min in W

1. CPOx reformate 40 55 5 1,099 822

2. Woodchip reformate 29 21 11 13 25 1,083 775

3. Diesel reformate 19 7 41 5 29 1,013 767

4. Biogas reformate 14 3 3 40 5 35 1,390 756

5. Anode gas rec 75% 6 3 27 10 54 0,888 766

6. Anode gas rec 35% 10 5 16 38 31 0,266 834

Whereas case 1-4 are real gas compositions from existing systems, case 5 and 6 are theoretical

approaches for a small, compact CH4 driven system with anode off gas recirculation. Instead of a

reformer an anode off gas blower will be used. The advantage of this system approach is an higher

efficiency by lower need for air cooling and a compact system assembly because of smaller size of

heat exchangers, start burner and after burner. The low pressure drop of the stack at the anode side

is important for systems operating with high anode gas flow caused by gas recycling. The low

pressure drop at the air side of MK352 [7] is essential to save blower power in systems operating

with dry synthesis gases (CPOx or woodchip reformate).

Figure 6 schematic flow diagram of system with green marked parts which are not necessary for a

compact CH4 system compared with “state of the art” systems



Figure 7 shows the temperature difference and the power output (values in the legend) caused by

the different gas composition. The temperatures of the cover- and the ground plate are values for

interacting with the surrounding and can be neglected. The temperature of anode inlet (An_in pipe)

and cathode outlet (Cat_o) are controlled values of the preheater and are at a constant level. The

cathode inlet temperature (Cat_in) is the controlling parameter for the cathode outlet temperature

and a measure for heat balance. Focusing on the thermocouples inside the stack, it is obvious, that

the water content has a high influence to the power output (all cases), but no influence in

temperature (case 1 vs. 2). It is known, that methane will lower the temperature in the stack [6] but

the temperature differences up to high CH4 contents compared to the CPOx case 1 are small and can

be compensated by the cathode inlet temperature. The maximum temperature difference is <100 K

(case 2 to 6) and is proportional to the cooling by internal reforming.

The highest temperature difference is measured in case 6 because of the very high amount of CH4

(38 %). It is important to note, that it is possible to operate CFY-stacks with these high amounts of

CH4. Especially at the cold places of the stack (gas inlet, air inlet –An_in /Cat_in) the temperature is

high enough that no Carbon deposition occurs.

Figure 7: Temperature distribution of a middle cell in a 30-cell stack operated in a simulated hotbox

(positions indicated in Figure 4) @35 A, different fuels, FU=75%, air: 100 sl/min, at constant

reference temperature T_Cat_o=835°C

-300

-260

-220

-180

-140

-100

-60

-20

20

60

100

500

550

600

650

700

750

800

850

900

950

1000

1050

1100

Gro

un

d p

late

E15

A

n_in

/Ca

t_in

E15

A

n_in

Ce

nte

r

E15

sta

ck

ce

nte

r

E15

A

n_o

Ce

nte

r

E14

A

n_o

/Cat_

o

E14

A

n_in

/Ca

t_o

An

_in

pip

e

Cat_

in

Cat_

o

Co

ve

r p

late

Te

mp

era

ture

dif

fere

nc

e in

K

Te

mp

era

ture

in

°C

1380b Auswertung Blatt Temp PM+CH4 StackeingangTemperturvergleich verschiedener Gase

1. CPOx reformate 822 W 2. Woodchip reformate 775 W

3. Diesel reformate 767 W 4. Biogas reformate 756 W

5. Anode gas rec 75% 766 W 6. Anode gas rec 35% 834 W

inside stack



Long-term test

The current generation of MK352 stacks is running at a constant long term test since August 2015

at T=830°C with one maintenance and one unplanned stop (Figure 6). The starting power was 285

W in the reference point @35 A, FU=75%, 40%H2 in N2 and 40 sl/min air. After 6000 h a

degradation rate of P/P0=0.6 %/1000 h was calculated. This validates the results from the MK351

stack (P/P0=0.7 %/1000 h over 20.000 h). Understanding the reasons for degradation is the first

step to solve the problem. Sample tests show that the main part of the degradation comes from the

resistance caused by oxidation of the interconnect and the interaction with the protection layer [8].

In the future a degradation rate of P/P0<0.5 %/1000 h seems to be possible by fine tune the

protection layer and interconnect surface.

Figure 6: Long-term test of a 10-cell stack in a furnace: @35 A, fuel: 40% H2, 5%H2O in N2,

FU=75% air: 40 sl/min T_cat_o=830°C

0

200

400

600

800

1000

0

50

100

150

200

250

300

350

400

450

500

0 1000 2000 3000 4000 5000 6000 7000

Te

mp

era

ture

in

°C

, C

urr

en

t d

en

sit

y i

n m

A/c

m²

Po

we

r in

W/1

0-c

ell

Time in h

Power Current density T_Cat_O



Thermal cycling

In addition to the evaluation of performance and degradation behavior of the stack, cyclic testing of

a 30-cell stack in a hotbox was carried out. Thermal cycling between >800°C and <100°C were

performed using no purge/reducing anode flow gas during temperature change, which can only be

withstand by electrolyte supported cells. The heating and cooling rates were set to 2-4 K/min.

During the heating cycle above 750°C, the anode gas of 40% H2 in N2 was flushed for 15 minutes

before adjusting the current at 1 A/min to 35 A. After 5 complete cycles 50 h constant operation

was used to adjust the temperature to 835°C by varying the air inlet temperature and monitor the

power degradation at 35 A (Figure 7). After 120 cycles a degradation of 5.9 % occurs and gives a

specific power loss of P/P0=0.5%/10 cycles. The degradation in the first 35 cycles is a little bit

lower than the following, but it stays linear. The reason for degradation was dedicated to a change

in the anode structure as well as an amount of power loss attributed to long-term operation over

4500 h @35 A. Adjusting the air outlet temperature in the reference point @35A (T_Cat_o) to

exactly 835°C is the key to precise power measurement. While cycle 35 and 75 the temperature was

a little bit shifted to higher values and leads to an increased power. This low values of degradation

by cycling without flushing gas enables simple efficient operation of small CHP systems with high

numbers of start/stop procedures.

Figure 7: Degradation by cycling without flushing gas of 30-cell MK352 stack in a hotbox module

@35 A, fuel: 40% H2 in N2, FU=75%, air: 80 sl/min

For systems of higher power special procedures for saving reducing atmosphere at the anode are

developed. Rarely but with high probability and mostly at inappropriate time an uncontrolled shut

down quits this procedures and destroys the stacks. Therefore it is better to have robust stacks.

700

720

740

760

780

800

820

840

860

880

900

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

5 15 25 35 45 55 65 75 85 95 105 115

Te

mp

era

ture

in

°C

ΔP

/P0

in %

Number of system cycles

VCat=80 sl/min, VAn=24,3 sl/min, 40% H2, 60% N2

ΔP/Po T_Cat_o



4. Conclusion

The development of CFY-stacks is a long-standing focus of R&D activities at Fraunhofer IKTS.

With the stack design MK351, a good platform for enabling proliferation of SOFCs in a wide range

of applications was created. CFY-stacks with high efficiencies and low degradation rates

(P/P0=0.7 %/1000 h over > 20.000 h) perform as reliable components in a variety of SOFC/SOEC

systems. In close collaboration with Plansee SE, Fraunhofer IKTS was able to improve the MK351

stack design. The new MK352 stacks have a high power output, can be easily integrated into SOFC

systems, and feature a lower pressure drop along the air path. Moreover, this stack design enables a

more stable production, which is very important for commercialization of the CFY-stack

technology. The symmetrical interconnect design enables compensation of tolerances resulting from

net-shape pressing technology and simpler stack integration into larger modules. By modifying the

tolerance chain for all of the stack components, it was possible to improve manufacturing system

and performance robustness. In hotbox tests with a 30-cell stack, a new benchmark for start/stop

cyclability was set. The stack showed a power degradation of 0.5 %/10 cycles over more than 120

cycles. Low pressure drop and a wide, efficient operating window with several gas composition

lead to a high total system efficiency.

5. Acknowledgments

The authors acknowledge FFG for the funding in the projects “Hydrocell and SOFCool” as well as

Plansee SE for financial and technical support. Furthermore, the authors thanks all colleagues of

AVL, Convion, as well as IKTS for using CFY stacks, valuable discussions and fruitful

collaboration.

6. References

[1] M. Kuznecov, P. Otschik, K. Eichler, N. Trofimenko and S. Megel, in Proc. Int. Symp.

SOFC IX, S. C. Singhal and J. Mizusaki, Editors, PV 2005-07, p. 200, The Electrochemical

Society Proceedings Series, Pennington, NJ (2005).

[2] A. Venskutonis, G. Kunschert, E. Mueller and H.-M. Höhle, ESC Trans., 7(1), 2109 (2007).

[3] V. V. Kharton, F.M.B. Marques and A. Atkinson, Solid State Ionics 174, 135 (2004).

[4] S. Megel, M. Kusnezoff and N. Trofimenko, et.al, ESC Transactions, 35 (1), 269 (2011).

[5] N.Trofimenko, M.Kusnezoff and A.Michaelis, in 10th European Fuel Cell Forum, Lucerne,

B0703 (2012).

[6] S. Megel, M. Kusnezoff, N. Trofimenko, V. Sauchuk, J. Schilm, J. Schoene, W. Beckert, A.

Michaelis, C. Bienert, M. Brandner, A. Venskutonis, S. Skrabs and L.S. Sigl, CFY-Stack:

From Electrolyte supported Cells to High Efficiency SOFC Stacks, 10th European Fuel Cell

Forum, A1203, Lucerne (2012)

[7] C. Bienert, M. Brandner, S. Skrabs, A. Venskutonis, L.S. Sig, S. Megel, W. Becker, N.

Trofimenko, M. Kusnezoff, A. Michaelis, CFY-Stack Technology: The Next Design, ECS

SOFC-XIV, A0272, Glasgow (2015)

[8] M. Brandner, C. Bienert, S. Megel, M. Kusnezoff, N. Trofimenko, V. Sauchuk, A.

Venskutonis, W. Kraussler A. Michaelis and L.S. Sigl, Long Term Performance of Stacks

with Chromium-based Interconnects (CFY), 11th European Fuel Cell Forum, Lucerne

(2014)

[9] S. Megel, Kathodische Kontaktierung in planaren Hochtemperatur-Brennstoffzellen,

Schriftenreihe Kompetenzen in Keramik, Band 6, ISBN 978-3-8396-0066-5 Fraunhofer

IKTS, Dresden; (2009)



A0909 New all-European high-performance stack (NELLHI):

Experimental evaluation of an 1 kW SOFC stack

Christoph Immisch (1), Andreas Lindermeir (1), Matti Noponen (2), Jukka Göös (2) (1) Clausthaler Umwelttechnik-Institut GmbH

Leibnizstrasse 21+23, D-38678 Clausthal-Zellerfeld, Germany Tel.: +49-5323-933-209 Fax: +49-5323-933-100

[email protected]

(2) Elcogen Oy Niittyvillankuja 4, FIN-01510 Vantaa, Finland

Tel.: +358-40-732-9696 [email protected]

Abstract

The NELLHI project, supported by the Fuel Cells and Hydrogen Joint Undertaking (FCH JU), combines European know-how in single cells, coatings, sealing, and stack design to produce a novel 1 kW SOFC stack with improved electrical efficiency, robustness and considerable cost reductions by establishing mass production pathways. Elcogen stacks are optimized for reduced operating temperatures of 600 to 700 °C and based on products of industrial partners. Elcogen AS supplies the cells, AB Sandvik Materials Technology produces interconnector plate material and coating, Borit manufactures the interconnector plates and Flexitallic Ltd addresses the sealing issue. All components merge together at Elcogen Oy for the design and assembly of the stack. By this, a complete high-quality industrial supply chain is set-up in Europe. Within the NELLHI project, three stack generations shall be developed and evaluated at CUTEC and VTT to proof their performance and long-term stability. Results of tests done at CUTEC with a 15 cell stack of the 1st generation are very promising and a detailed performance map of the stack at different operating parameters like temperature, anode feed gas composition and flow rate was recorded. Additional test gave deeper insight in the capabilities of the stack under aggravated conditions. Current cycle tests were executed over a total time under load of more than 140 h. The presentation shows the status of stack performance within the NELLHI project and gives an outlook on the ongoing developments.

mailto:christoph.immisch@cutec.




Introduction CUTECs task in the NELLHI project is the experimental evaluation of the stacks produced by the industrial partners. Two stacks (1st and 2nd generation) were already tested. Both are mostly identically designed and constructed, except of some minor changes in the design of the seals. The already existing stack test bench at CUTEC needed to be adapted to the specific footprint, gas supply design and compression requirements of the Elcogen stack. This includes CFD-assisted design and construction of an adapter plate for the connection of the stack to the gas supply and the electrical part of the test bench. In addition, a new compression system was designed and built up to supply the stack compression force for the tests.

1. Scientific Approach Prior to the stack tests, the CUTEC stack test bench was modified to fit the Elcogen stack dimensions and requirements. Particularly the gas supply of anode and cathode has to be adapted and a compression system was developed to ensure sufficient compression force to the stacks. CFD simulation where carried out, to ensure that the design of the gas inlet system enables a homogeneous gas distribution throughout all stack layers. For the performance mapping, a test matrix was developed in close collaboration with Elcogen. Tests at different temperatures, anode flow rates and anode gas compositions were intended. The chosen composition of the fuel gas derived from a questionnaire answered by several potential customer companies and from a calculation for an anode recycle. For every test case of this matrix a full polarization curve was measured. After characterizing the stacks within their designated specifications aggravated test were carried out. For the stack of the first generation long-term test with daily current cycles were done and for the stack of the 2nd generation periodic load throw-off cycles were tested. Al stacks were repeatedly tested with a reference anode gas mixture of hydrogen and nitrogen to determine degradation effects during the test series and for the purpose of comparison between the different stacks.

2. Experiments/Calculations/Simulations Stack compression system The compressive seals used in the stack need a compression force of at least 3000 N to ensure gas tightness, corresponding to a weight of about 300 kg. In previous tests with stacks from other suppliers steel weights applied on the top plate of the stack were used to maintain the force during testing. Because of the thereby increase in the thermal mass of the set-up inside the high temperature zone and thus long heating and cooling times this approach is not feasible. An compression system with external springs seems to be more promising. The compression system was designed for a maximum working temperature of 800 °C. The compression system is shown in Figure 1.



Figure 1: stack compression system

CFD simulations on flow distribution The flow towards the anode inlet gas as well as the anode offgas junction is not symmetrical due to the gas channel design of the adaptor plate. This may result in unbalanced gas supply inside the stack with potential problems concerning gas utilization and temperature distribution. To ensure a homogenous gas distribution CFD flow field studies were carried out. The following boundary conditions were chosen for the CFD simulations:

- temperature: 650 °C (isothermal) - cathode flow rate: 20 slm (N2) - anode flow rate: 2 slm (N2, for simplification) - model specifications: turbulent flow allowed

The first outcome of the simulation was that for a single layer of one cathode or one anode compartment the gas distribution is homogeneous as shown in Figure 2.

Figure 2: Cathode flow field (left) and anode flow field (right)

The next question was, if there is a difference in the gas distribution of two next neighbor anode and cathode compartments. Figure 3 shows the overall gas distribution visualized



as particle traces. The velocity in the cathode gas compartment is much higher because of the higher cathode volume flow (20 slm) compared to the anode volume flow (2 slm). The flow simulation tool (AutoCAD CFD) allows calculating the volume flow through predefined areas. Comparing the gas volume flow rates of the two anode inlet channels results in a deviation of only 0.5 %. This affirms a sufficiently balanced gas distribution to both anode gas inlet channels.

Figure 3: Flow distribution (particle traces) inside the anode flow field, cross-section view

(top) and isometric view (bottom) Figure 4 shows the relative volumetric flow rates for the anode and cathode side of each layer obtained from calculations. The deviation from the mean value was less than 1 percent point, confirming both, homogeneity of the gas distribution through all the layers and the reliability of the CFD-simulations.

Figure 4: Relative distribution (100 % corresponds to the mean value) of gas flow rates in

the single anode and cathode layers Stack Tests The tests were done according to IEC 62282-7-2 TS Ed.1 „Fuel cell technologies – Single cell/stack-performance test methods for solid oxide fuel cells (SOFC)“ [1]. For the test program five different anode gas compositions were agreed upon among the project partners and the industrial advisory board:

- two different mixtures of hydrogen and nitrogen - one reformate gas typical for methane reforming with anode of gas recycle (AOGR) - two reformate gases as obtained from on a single-pass steam reformer (without

AOGR). For the definition of the reformate gas compositions thermodynamic calculations of a natural gas were conducted, assuming that the reformer is capable of reaching



thermodynamic equilibrium. For operation in AOGR mode, this should be possible at least at beginning of its lifetime (“bol”). As temperature for the single-pass steam reformer 600 and 700 °C were considered. Table 1 summarizes the test gas compositions. A volumetric air flow rate of 33 slm was chosen for the cathode.

Table 1: Test Gas compositions

The stack was tested with the latter four gas compositions at three different operating temperatures and three different anode gas flow rates (80 %, 100 %, 120 %, see Table 2), resulting in nine test cases for every gas composition and a total of 36 tests. The 50/50 H2/N2 mixture was repeatedly used as reference case at a temperature of 650 °C to monitor possible degradation.

Table 2: Conditions for standard tests

For each test case a full I/V-characteristic was measured starting at open current voltage (OCV), increasing the current with a rate of 1 A/min to a maximum of 35 A. The maximum current was maintained for 15 minutes before decreasing the current load back to OCV. The following data were recorded online during each test:

- cathode and anode feed flow rates, - temperatures inside furnace, at stack top plate and stack adapter plate, - inlet and outlet gas temperatures for cathode and anode, - inlet and outlet gas pressures for cathode and anode, - stack current, - overall stack voltage - stack voltage at electronic load, - single cell voltages for the three bottom and two top cells, pairwise for the remaining

ten cells, - concentration of CO, CO2, H2 and CH4 in the anode offgas, - ambient pressure.

The influence of the anode feed flow rate was determined in depth by tests with the bol reformate at an operating temperature of 650 °C. Here the relative anode gas flow rate was varied in a wider range from 65 to 120 % and in steps of 5 percentage points. These tests were done at a constant current of 30 A.

anodefeed cathodeair

H2 N2 CO CO2 CH4 H2O

[vol.%][vol.%][vol.%][vol.%][vol.%][vol.%] [slm] [slm]

H2/N250/50reference 50.0 50.0 - - - - 15.6 33.0

H2/N260/40 60.0 40.0 - - - - 8.75 33.0

reformate,bol 25.7 - 3.5 23.9 15.9 31.0 5.65 33.0

reformate,TRef=600°C 51.2 - 6.2 8.5 6.2 27.9 6.45 33.0

reformate,TRef=700°C 59.6 - 11.3 6.4 0.7 22.0 7.05 33.0



After finishing this program with many changes of operation point long term were carried out tests with parameters kept constant during one day . As anode gas reformate (TRef, simulated @ 600 °C) with the standard flow rate (100 %) was used and the furnace operation temperature was set to 650 °C. In each test cycle stack current was increased at the beginning of the test in the morning up to 30 A at a rate of 1 A/min and lowered back to zero at the same rate in the evening. Over night and over the weekends the stack was held in a “hot standby” mode without current load at 650 °C furnace temperature under reducing atmosphere on the anode side (1 slm H2 and 10 slm N2). These tests were done for 11 days (resulting in 11 current cycles) with a total accumulated time under load of 144 hours.

3. Results 1st Generation Stack Tests: Variation of Operating Conditions Figures 5 to 9 show examples of the recorded IV-curves, of the resulting power density curves and fuel utilization characteristics. The fuel utilization was calculated according to [1] annex B “calculation of effective fuel utilization”. The term “efficiency” (“eta” in the diagrams) in this paper means the stack efficiency and is defined as the ratio of electrical power output of the stack to direct chemical power input (anode feed) to the stack. The gross efficiency of a fuel cell system of course may be higher, for example by using reforming, high degree of internal heat recovery and possibly anode off gas recycling. In all cases the cell voltages are depicted as mean value of all measured cells.

Figure 5: Influence of anode gas composition: two H2/N2-mixtures and bol reformate



Figure 6: Influence of anode gas gas composition: bol reformate and reformates

corresponding to reformer temperatures of 600 °C and 700 °C

Figure 7: Influence of temperature for a reformate gas feed with low CH4 content

Figure 8: Influence of temperature for a reformate gas feed with high CH4 content



Figure 9: Influence of anode feed gas flow rate

The following itemization summarizes the main results of the depicted tests:

The influence of anode feed gas flow rate on the I/V characteristic is rather small. Of course the fuel utilization and the electrical stack efficiency is affected.

With bol reformate lower cell voltages occur but higher stack efficiency was reached compared to the hydrogen/nitrogen mixtures.

For the 700 °C-equivalent reformate the temperature of the stack has a moderate impact on its performance. The maximum current of 35 A (0.29 A/cm2) was only reached for temperatures of 625 °C and 650 °C. At an operating temperature of 600 °C the maximum stack current was 29 A for the lower cell voltage limit of 700 mV.

The influence of operating temperature is higher for the bol reformate with a higher CH4 concentration. Here the performance at an operating temperature of 700 °C is approximately as good as the performance with the previously discussed gas composition at an operating temperature of 650 °C. For lower temperatures the performance drops significantly.

1st Generation Stack: Additional Tests To get a better understanding of the effect of CH4 in the anode feed gas on stack performance an additional test with bol reformate was done at an operation temperature of 650 °C. The anode gas flow rate was increased in steps of 5 percentage points from 65% to 120 % while drawing a constant current of 30 A. The results are shown in Figures 10 and 11. Results are plotted against the chemical input power. The maximum electrical stack power is about 376 W at 1120 W chemical power input. Further increase of chemical power input led to a decrease in stack power.



Figure 10: Influence of feed gas flow rate on FU, el. power output and stack efficiency

Figure 11: Influence of feed gas flow rate on cell voltage and anode and, respectively,

cathode outlet temperatures

In general, CH4 concentrations in the anode offgas were quite low (0.12 – 0.48 vol.%) for all tested anode feed gases, indicating an almost complete internal reforming of CH4 inside the stack. Higher CH4-amounts in the anode feed lead to decreasing temperatures beyond the point of maximum electrical power output because of the increased internal endothermic reforming. This drop of the anode outlet temperatures can be seen in Figure 12 resulting in a voltage drop and thereby, finally, in a power drop (Figure 11). 1st Generation Stack: Current Cycles After finishing the test program above it was decided together with Elcogen to conduct tests with daily current cycles. As described above these were done with the 600 °C-reformate gas, a relative anode feed gas flow rate of 100 % and an operating temperature of 650 °C. Results are shown in Figure 12.



Figure 12: Daily current cycle tests

During these tests, different incidents with the test equipment occurred. The green arrow indicates a change of the CH4 gas bottle. Therefore the stack current was lowered to run without CH4 for a short time. The red arrow indicates a total power blackout in Clausthal-Zellerfeld that also affected the CUTEC building. Several elements of the control system were damaged and had to be replaced. Nevertheless a controlled manual shutdown of the stack with a moderate temperature ramp could be achieved. After repairing the control system the test was continued. The blue arrow indicates a malfunction of the water supply. After the latter two incidents a negative effect on the stack performance was detected, noticeable from the lower cell voltage. To determine stack degradation repeated measurements at the hydrogen/nitrogen reference point were made. The open current voltage (OCV) shows no degradation over the total testing time. The cell voltage at the maximum current of 35 A (= 0.29 A/cm2) dropped only by 0.354 % along the regular testing period. The stack than withstand two unintended harsh black outs, one from electricity blackout and the other from water supply failure. These blackouts had no influence on OCV and the voltage under full load only dropped by 3 %. 2nd Generation Stack Tests: Current Cycles The 2nd generation stack was tested with a shorter current cycle compared to the 1st generation testing and an additional abrupt load interruption. The cycle starts with a current ramp up to 30 A with a rate of 1 A/min. Afterwards the current is hold at 30 A for 30 min followed by an instant shutdown of the electrical load. After 30 min at open current voltage the cycle starts again. The measured data for a single cycle are shown in Figure 14.



Figure 14: Single current cycle

The results obtained up to now from the ongoing tests are shown in Figure 15. Currently, after almost 68 h and 43 cycles of testing there is no loss in the open current voltage. The averaged single cell voltage of the first five cycles at beginning of the tests was at 849.8 mV and drops only by 3.0 mv to 846.8 mV at the last five cycles.

Figure 15: Tests with load interrupted current cycles

Conclusion The adaption of the SOFC test bench at CUTEC to the Elcogen design enabled the successful installation of the 1st and 2nd generation stacks. Initial simulations showed that the gas distribution to anode and cathode is homogeneous in spite of the asymmetrical gas inlet. A detailed performance map of the stack at different operating parameters as e.g. temperature, anode feed gas composition and flow rate was recorded. Additional test under aggravated conditions gave deeper insight in the capabilities of the stack. Current cycle tests were executed over a total time under load of more than 140 h with a 1st generation stack. Problems occurring occasionally on the test bed periphery like small leakage, power blackout or interruption in water supply did no significant harm or damage to the stack. The 1st generation stack, however, showed some degradation after the total



interrupt in water supply resulting in a 3 % of its performance (voltage drop). As a result this kind of stress was included in the subsequent test conditions. The tests with the 2nd generation stack are still ongoing. After 68 hours of testing with repeated load throw-offs OCV was still constant. The degradation of average single cell voltage was 0.354 % in 68 hours.

References [1] International Electrotechnical Commission, draft technical specification : “IEC 62282-

7-2 TS Ed.1 Fuel cell technologies – Single cell/stack-performance test methods for solid oxide fuel cells (SOFC)“, Germany, 2013




Triode Solid Oxide Fuel Cell operation under Sulphur

poisoning conditions

Priscilla Caliandro, Stefan Diethelm, Jan Van herle FUELMAT, École Polytechnique fédérale de Lausanne

1951 Sion, Switzerland Tel.: +41-21-693-3549

[email protected]

Abstract

The Triode SOFC is a three electrodes configuration. The third electrode, so called auxiliary, is connected in a way to run in electrolysis mode, while cathode and anode operate in normal fuel cell mode. This mixed operation allows to reach anode–cathode potential differences which are not accessible in normal operation. In this work, the benefits of triode operation under S-poisoning conditions are shown. In particular, the difference between conventional and triode operation mode under 2ppm of H2S in H2 are analyzed. The partial reversibility of sulphur poisoning is investigated and the observed regeneration processes are discussed for both the conventional and triode operation. After each sequence of exposure and regeneration, IV and EIS characteristics are taken. The electrochemical impedance spectra are further processed by computing the distribution of relaxation times (DRT). During triode operation, less degradation during exposure and faster stabilization after exposure and regeneration with respect to conventional operation mode are observed. Keywords Triode SOFC; Degradation; Sulphur poisoning; DRT; EIS. Remark: The full paper is published in EFCF Special Issue Series (www.EFCF.com/LIB,







Pressurized Operation of a 10 Layer Solid Oxide Electrolysis Stack

Marc Riedel, Marc P. Heddrich, K. Andreas Friedrich German Aerospace Center (DLR)

Institute of Engineering Thermodynamics Pfaffenwaldring 38-40

70569 Stuttgart Germany

Tel.: +49-711-68628205 [email protected]

Abstract

Solar and wind energy are becoming the fundament of the power supply system. However, the increasingly significant amount of renewable electrical power associated with natural intermittency due to varying weather conditions requires flexible storage options. One promising path is the solid oxide electrolysis cells (SOECs) technology which can provide hydrogen or derived hydrocarbons as fuel in transport, as chemical in industry or for repowering or heating. SOECs offer a great potential for a highly efficient energy conversion due to their high operating temperature that may lead to reduced electrochemical losses. Previous studies have shown that the efficiency of solid oxide fuel cells (SOFCs) can be significantly improved by operating at elevated pressure. Similar effects on the electrochemistry can also influence the cell when operated in electrolysis mode and may also cause improved performance. Another reason for pressurization is the use of pressurized hydrogen in downstream processes like storage or fuel synthesis, e.g. methanation or Fischer-Tropsch synthesis in co-electrolysis. Preliminary experimental results of water electrolysis in a pressurized SOEC stack are presented in this paper. More results are presented at the poster presentation. The stack consists of ten electrolyte supported cells. The pressure ranges from 1 to 8 bar. Reactant gas composition (0.80 - 0.98), steam utilization (0.60 - 0.85) and temperature (750 - 850 °C) are the experimental parameters that are varied. Pressure influence on open circuit voltage (OCV) and power density is examined. Furthermore current voltage characteristics and impedance spectroscopy are performed to investigate the influence of pressure on the stack performance. Remark: Only the abstract is available, because the authors chose to publish elsewhere.






A0912

Evaluation of Zr doped BaCe0.85Y0.15O3-δ as PCFC

electrolyte

Ha-Ni Im, Dae-Kwang Lim, Jae-Woon Hong, In-Ho Kim and Sun-Ju Song Ionics Laboratory, School of Materials Science and Engineering

Chonnam National University, Gwang-Ju 61186, Republic of Korea *Tel: +82-62-530-1706, Fax: +82-62-530-1699,

[email protected]

Abstract

Fuel cells as an energy conversion device have generated paramount importance in ensuring the efficient way of utilizing the limited resource of hydrocarbon-based fuels. Among various fuel cell configurations, oxygen ion-conducting electrolyte based solid oxide fuel cells (SOFCs) operating at temperatures >800◦C have been widely studied and utilized for power generation because of advantage of fuel flexibility. However, the mechanical and economic constraints arising from the requirement of high temperature operations so far have been limiting factors in widespread commercialization of these SOFCs. Consequently, over the years great efforts have been made to develop new electrolyte and electrode materials which can bring down the operating temperature of high temperature fuel cells to the intermediate temperature range. Recently, proton-conducting oxide materials have been expected to be potential electrolytes for the new fuel cell configurations operating in intermediate temperature range. A number of cathode material for BCFC, the hydration/dehydration kinetics of Ba0.5Sr0.5Co0.8Fe0.2O3−δ

(BSCF5582) have reported protonic conductivity on humidify the BSCF5582 bulk phase. This observation has broadened the scope of BSCF5582 being used as cathode in intermediate temperature proton-conducting ceramic-electrolyte fuel cells (IT-PCFCs) as well, as the protonic conduction in BSCF5582 would be helpful in PCFCs because of the possibility of extending three-phase boundary deep into cathode during the PCFC operation. Otherwise, the use of LSM as cathode in PCFC would limit the cathode reaction at the three phase boundary (TPB) and keep the TPB close to the electrolyte, thus compared both cathode material we can identified electrochemical active area. In this work, we have fabricated two type of PCFC 1)BaCe0.85Y0.15O3−δ (BCY15) electrolyte with LSM cathode to confirm of sub-process, 2) BaCe0.45Zr0.4Y0.15O3−δ electrolyte with BSCF5582 cathode which enhanced chemical stability of electrolyte.




1. Introduction Lowering of the operating temperature of solid oxide fuel cells (SOFCs) to <750◦C, in order to reduce many operational and maintenance difficulties associated with the high temperature SOFC operations, has been one of the important objectives of the fuel cell research. However, the operation at <750◦C presents some challenges in terms of lower electrocatalytic activity of cathode materials for oxygen reduction reaction at the reduced temperatures. Therefore, the development of new materials with good electrocatalytic activity for oxygen reduction at the lower temperatures has been a key issue for the lowering of operating temperature of SOFCs. In early 1980s, Iwahara et al. found numerous perovskite oxides exhibiting high proton Conductivity in the temperature range 450-600 ◦C [1-2]. These proton conducting perovskite oxides are gaining enormous interests to be used as electrolyte for SOFCs which are called PCFCs operating in intermediate temperature range. Not only the operating temperature but also one additional advantage of PCFCs is that PCFCs form water at the cathode side keeping pure fuel at anode side hence there is no requirement of recirculate ion which is in contrast with the oxygen ion conducting SOFCs. The investigations on oxides like SrCeO3, BaZrO3 and BaCeO3 show high proton conductivities and they are successfully tested in various electrochemical devices. [3] Barium Cerate based proton conducting electrolytes are still the most widely used in Intermediate Temperature PCFCs (IT-PCFCs). One of the best examples for this is Yttrium doped Barium Cerate(BaCeO3, BCY). It has many advantages like high ionic and low electronic conductivity and durability. In BCY the substitutions of Ce4+ by Y3+ cause the formation of oxygen vacancies and this yttrium doped barium cerate absorbs water as it is hygroscopic and dissociation of water results into the formation of hydroxyl groups. Above 400-500 °C, hydrogen atoms in the structure become mobile leading to proton conductivity. [4-5] It is reported that BaCeO3 has poor chemical and thermal stabilities during redox cycles, as they are basic in nature, they may react with acidic byproducts such as CO2 or SO2 or SO3. [6-7] In contrast Barium Zirconate (BaZrO3) has an excellent chemical stability in humid and CO2 containing atmosphere which makes it to be used as an electrolyte in PCFCs. But its high sintering temperature (1600-1700°C) and low grain boundary conductivity impede its application in solid state devices. [8-9] To achieve good sinterability, high proton conductivity and good thermochemical stability, we have focused on the combination of both Y-doped BaCeO3 (BCY) and Y-doped BaZrO3 (BZY ) and on analysis of its material properties. The material properties of perovskite-type BZY-BCY were not systemized in the previous study even few researches were overlapped at similar time. Further it is mainly reported that these two perovskite oxides measured for the conductivity and phase stability under hydrogen atmosphere in hydration condition and CO2 atmosphere. [10-15] A number of materials, such as BSCF5582, La0.6Sr0.4Co0.2Fe0.8O3-δ, Ba0.5Sr0.5FeO3-δ, Sm0.5Sr0.5CoO3-δ, La0.7Sr0.3FeO3-δ- BaZr0.1Ce0.7Y0.2O3-δ, La0.8Sr0.2MnO3-BaCe0.85Y0.15O3-δ, have been employed as cathode in PCFCs[16-25]. Specially, the hydration/dehydration kinetics of BSCF5582 have shown that it is possible to humidify the BSCF5582 bulk and create the protonic conductivity in it.[26-27] This observation has broadened the scope of BSCF5582 being used as cathode in intermediate temperature PCFC (IT-PCFCs) as well, as the protonic conduction in BSCF5582 would be helpful in PCFCs because of the possibility of extending three-phase boundary deep into cathode during the PCFC operation. In this work, we have fabricated two type of PCFC 1) BaCe0.85Y0.15O3−δ (BZCY0) electrolyte with LSM cathode which is shown non-hydration kinetics to confirm of sub-process, 2) BaCe0.45Zr0.4Y0.15O3−δ(BZCY40) electrolyte with BSCF5582 cathode which is shown hydration kinetic.



The PCFC performance is measured as a function of oxygen partial pressure and hydrogen partial pressure to improve cell performance at 700oC.

2. Experiments/Calculations/Simulations

The porous NiO-BZCY0 and NiO-BZCY40 anode support (AS) was prepared by tape-casting. A slurry of the commercial powder BaCe0.85Y0.15O3−δ (BZCY0) (Kceracell; BET = 8.5 m2g−The 25 mm diameter circles of anode tapes were cut by punching from the tape-casted sheet (thickness ~350 µm) and the circular anode tapes were sintered stepwise at 1000 ºC for 2 h.

The anode functional layer (AFL), BZCY0 and BZCY40 electrolyte layer was coated onto the as-prepared each AS by dip-coating the electrolyte coating slurry prepared with ethanol based solvent and BZCY0 (or BZCY40). Finally, the AS/AFL/electrolyte assembly was sintered at 1400-1500 ºC for 5 h in air atmosphere. The linear shrinkage rate along the diameter of sintered sample was ~ 20 %.

Polycrystalline BSCF5582 (Ba0.5Sr0.5Co0.8Fe0.2O3-δ) powders were prepared by solid state reaction method and BaCO3 (purity 99.99%), SrCO3 (purity, 99.9%), Co3O4 (purity, 99.9%) and Fe2O3 (purity, 99.9%) were used as starting materials, purchased from Alfa Aesar. The starting materials were mixed in each stoichiometric amounts and ball-milled with stabilized zirconia balls. The BSCF5582 powder was calcined at 950 °C for 10 h in air. The prepared BSCF5582 and commercial La0.8Sr0.2MnO3 (LSM, Kceracell, BET=5~10 m2g-1, d50=0.3~0.6 µm) was used for the fabrication of cathode in the each electrolyte cell.

The slurry was coated onto the electrolyte side by screen printing method to complete the assembly of button cell.



3. Results (1) Fuel cell performance with different cathodic pO2

Figure 1. (a) Power performance of LSM at different anodic pO2, (b) impedance Nyquist plot, (c) DRT analysis of the EIS data and (d) variations of ASRs with anodic pO2

During the operation of the fuel cells the oxygen reduction reaction occurs at the

cathode, and any change in the pO2 in the cathodic compartment may have profound effect on the overall fuel cell performance of the PCFC. In the present work, the effect of variation of cathodic pO2 on the fuel cell performance was studied at 700 °C and the data are reported in Fig. 1 and Fig 2. In Fig. 1 and Fig 2 (a) showing the I-V-P characteristics of the fuel cell with variations in cathodic pO2 the power density and cell voltage at the higher current densities are improved with increasing cathodic pO2. Similarly, in Fig. 1 and Fig 2 (b) showing the corresponding EIS response of the fuel cell under OCV conditions, significant decrease in the size of impedance arc with the increasing pO2 can be observed, indicating that the lower frequency EIS response, which corresponds to the electrode sub-processes, is clearly affected by the changes in pO2.

0.0 0.2 0.4 0.60.0

0.2

0.4

0.6

0.8

1.0

Air 200 + N2 0 (log pO

2/atm = -0.67)

Air 150 + N2 50 (log pO

2/atm = -0.80)

Vo

lta

ge

(V

)

Current Density (A/cm2)

0.00

0.05

0.10

0.15

0.20

0.25

(a)

Po

we

r D

en

sity (

W/c

m2)

700oC

LSM/BZCY0/Ni-BZCY0

0.7 1.4 2.10.0

0.2

0.4

0.6

0.8

1.0

1.2LSM/BZCY0/Ni-BZCY0

(b)


2/atm = -0.67)

Air 150 + N2 50 (log pO

2/atm = -0.80)

-Z"(c

m2)

Z'(cm2)

700oC

104 Hz

103 Hz

102 Hz

-1.4 -1.2 -1.0 -0.8 -0.6

-0.8

-0.4

0.0

0.4LSM/BZCY0/Ni-BZCY0

(d)

700oC

Peak 2

Peak 3

Electrolyte

Peak 1

log

(A

RS

/

cm

2)

log(pO2 / atm)

R(pO2)

-1/4

Total

1E-7 1E-5 1E-3 0.1

0.0

0.5

1.0

(c)

Peak 3

Peak 2

g(

)

/ sec


2/atm = -0.67)

Air 150 + N2 50 (log pO

2/atm = -0.80)

Peak 1

LSM/BZCY0/Ni-BZCY0

700oC



Figure 2. (a) Power performance of BSCF5582 at different anodic pO2, (b) impedance Nyquist plot, (c) DRT analysis of the EIS data and (d) variations of ASRs with anodic pO2

In order to clearly ascertain the identity of the electrode sub-processes affecting the fuel

cell performance with the changes in cathodic pO2, the Distribution function of relaxation time (DRT) of the EIS data was performed and is given in Fig. 1 and Fig 2 (c). As can be seen, the high frequency sub-process P1 is nearly independent of the cathodic pO2 variation, but the sub-process P2 and P3 shift to lower values with increasing cathodic pO2. The ASR values related with various sub-processes were calculated from EIS data using the equivalent circuit, as earlier mentioned in Fig. 6.2 (d), and are plotted in Fig. 6.3 (d). As can be seen, the ohmic ASR and the ASRs corresponding to the P1 and P3 are nearly independent of cathodic pO2 but the ASR corresponding to the sub-process P2 shows a clear

type-dependence on the cathodic pO2. In literature, [217] such a dependence on pO2 has been attributed to the charge-transfer from the cathode However, in case of BSCF5582 cathode cell, DRT analysis revealed that peak P1 and P2 are nearly independent of change in pO2 but the intensity of peaks P3 decreases with increase in pO2. This is further supported by ASR calculations which are shown in Fig. 6.13 (d). The ASRs corresponding to the peaks P1 and P2 are almost constant with variation of cathodic pO2 but ASRs corresponding to P3 shows decreasing trend with change in pO2 confirming its dependence on oxygen reduction reaction. The cathodic polarization due to both the charge transfer reaction and oxygen dissociation are the major contributor towards the overall electrode polarization resistance. (2) Fuel cell performance with different anodic pH2O

0.24 0.27 0.30

0.00

0.02

0.04

0.06

0.08

log(pO2) = -0.67

log(pO2) = -0.80

-Z"

(c

m2)

Z' (cm2)

104Hz

102Hz

101Hz

100Hz

700oC

BSCF5582/BZCY40/Ni-BZCY40

0.0 -0.6 -1.2 -1.8 -2.4

0.4

0.6

0.8

1.0

Vo

lta

ge

(V

)

Current Density (Acm-2)

0.0

0.3

0.6

0.9

log(pO2) = -0.67

log(pO2) = -0.80

Po

we

r D

en

sity (

Wc

m-2)

700oC


-1.4 -1.2 -1.0 -0.8 -0.6-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Slope = -3/8

P1

P2

P3

Ohmic

log

(AS

R/c

m2)

log(pO2/atm)

Total


700oC

1E-5 1E-4 1E-3 0.01 0.10.00

0.02

0.04

log(pO2) = -0.67

log(pO2) = -0.80

Peak 3

Peak 2

g ()

(s)

Peak 1


700oC



Figure 3 (a) Power performance of LSM at different cathodic pH2O, (b) impedance Nyquist plot, (c) DRT analysis of the EIS data and (d) variations of ASRs with cathodic

During the PCFC operation, the water is formed at the cathode side and therefore

solves the problem of fuel dilution, which is otherwise observed in oxygen ion-conducting electrolyte based SOFCs. However, the anode used in the present work is a composite of electron conducting NiO (after reduction) and proton-conducting oxide, and the presence/absence of the moisture in the anode compartment can have significant effect on the protonic conductivity of the BZCY0 component of the composite anode. The effect of variation of anodic pH2O on the fuel cell performance was studied at 700 °C and the data are reported in Fig. 3 and Fig 4. In Fig. Fig. 3 and Fig 4 showing the I-V-P characteristics of the fuel cell, a drop in power density at the higher current densities is observed with increasing pH2O of the anode compartment. Similarly, there is an overall decrease in cell voltage with increasing anodic pH2O.

1E-7 1E-5 1E-3 0.1

0.0

0.4

0.8

1.2

(c)

g(

)

/ sec

Dry condition

20oC wet

25oC wet

Cathode : Dry gas

Anode : Wet gas

Peak 2

Peak 3

Peak 1

700oC

LSM/BZCY0/Ni-BZCY0 700oC

0.0 0.2 0.4 0.60.0

0.2

0.4

0.6

0.8

1.0

1.2700

oC

Dry condition

20oC wet

25oC wet

30oC wet

35oC wet

40oC wet

Vo

lta

ge

(V

)

Current Density (A/cm2)

0.00

0.05

0.10

0.15

0.20

0.25

(a)

Po

we

r D

en

sity (

W/c

m2)

LSM/BZCY0/Ni-BZCY0

-4 -3 -2 -10.0

0.5

1.0

1.5

2.0

2.5

(c)

Total

Electrolyte

Peak 1

Peak 2

Peak 3

AR

S /

cm

2

log(pH2O / atm)

Anode : wet gas

700oC

Peak 2

Peak 3

Electrolyte

Peak 1

Total

LSM/BZCY0/Ni-BZCY0

0.5 1.0 1.5 2.00.0

0.5

1.0

(b)

Dry condition

20oC wet

25oC wet

-Z"(c

m2)

Z'(cm2)

104 Hz 10

3 Hz

102 Hz

LSM/BZCY0/Ni-BZCY0

700oC



Figure 4 (a) I-V-P performance of the PCFC at different cathodic pO2 at 700 °C, (b) The EIS data for different cathodic pO2 at 700 °C, (c) DRT analysis of the EIS data and (d) Variations of ASRs with cathodic pO2.

In Fig. 3 and Fig 4 (b) showing the corresponding EIS response of the fuel cell under

OCV conditions, an increase in the size of impedance arc with the increasing pH2O can be observed, indicating that the anodic pH2O variations are clearly affecting the electrode processes. The decrease in fuel cell performance at the higher current densities and the variations in EIS can be understood in terms of the hindrances caused by the pH2O molecules at the TPB in the anode compartment. Though the humidification can increase the protonic-conductivity of the composite anode, water molecules can block the supply of H2 or hinder the anodic reaction at the TPB, thereby causing an increase in anodic overpotential. As can be seen from the DRT analysis of the EIS data, in Fig. Fig. 3 and Fig 4 (c), the intensity of high frequency sub-process P1 significantly decreases after initial humidification from dry condition, indicating possible increase in protonic conductivity of anode and consequently an increased proton exchange at the electrode/electrolyte interface. On the other hand, the sub-process P2, which was earlier been attributed mainly to the cathodic charge transfer reaction, is nearly independent of the anodic pH2O variation, but the sub-process P3 shift to lower values with increasing anodic pH2O.

However in case of BSCF5582 cathode, DRT analysis of the EIS data is presented in Fig. 4 (c). The sub-process corresponding to the peak P1 associated with the reactions occurring at the anode remains unchanged although for a change in cathodic pH2O. But the sub-processes corresponding to the P2 and P3 are significantly affected by the humidification. The EIS data was fitted in the equivalent circuit from Fig 6.10 (e) and ASR calculations were done from the same as shown in Fig. 4 (d). It is observed that Ohmic

0.24 0.27 0.30 0.33 0.36

-0.02

0.00

0.02

0.04

0.06Cathode

10-1Hz

Dry condition

pH2O = 0.023 atm

pH2O = 0.031 atm

-Z"

(c

m2)

Z' (cm2)

104Hz

102Hz

103Hz

101Hz

100Hz

700oC


0.0 -0.6 -1.2 -1.8 -2.4

0.4

0.6

0.8

1.0

Vo

lta

ge

(V

)

Current Density (Acm-2)

0.0

0.3

0.6

0.9

Dry Condition

pH2O = 0.023 atm

pH2O = 0.031 atm

Po

we

r D

en

sity (

Wc

m-2)

700oC

Cathode


1E-5 1E-4 1E-3 0.01 0.10.00

0.02

0.04

0.06

Peak 3

Peak 2

Dry condition

pH2O = 0.023 atm

pH2O = 0.031 atm

g ()

(s)

Cathode (200 sccm)


700oC

Peak 1

-4 -3 -2 -1-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Slope = -1/2

log

(AS

R/c

m2)

log(pH2O/atm)

P1

P2

P3

Ohmic

Total


700oC



ASR and ASR for peak P1 are independent of vapour pressure while for P2 and P3, ASRs increased initially as vapour pressure increased from dry condition. But, after that, the P2 is reduced with a gradient -1/2 and P3 remain almost constant for further increase in vapour partial pressure. Concerning the BSCF5582, the peak P2 can be attributed to the proton transfer at electrode/electrolyte interface while peak P3 to the elementary reactions involving the oxygen species at cathode compartment. Detail results of experiment is described on ref 28. In ref. 28, single PCFC cells were fabricated using BCY15 electrolyte, LSM cathode and NiO-BCY anode and their fuel cell performances were measured in different thermodynamic conditions in 600-750 °C temperature range. Various electrode sub-processes were identified by the DRT analysis of the EIS data obtained by varying the gas flow rate, pH2O and pO2 at different temperatures.

References

[1] H. Iwahara, H. Uchida, K. Ono and K. Ogaki, Proton Conduction in Sintered Oxides Based on BaCeO3 Journal of The Electrochemical Society 135(2) (1988) 529-533.

[2] H. Iwahara, H. Uchida, K. Morimoto, High Temperature Solid Electrolyte Fuel Cells Using Perovskite-Type Oxide Based on BaCeO3, Journal of The Electrochemical Society 137(2) (1990) 462–465.

[3] Klaus-Dieter Kreuer, Proton-Conducting Oxides, Annual Review of Materials Research 33 (2003) 333-359.

[4] N. Malikova, J. M. Zanotti and C. K. Loong, H2. Proton Conduction in Yttrium Doped Barium Cerate, Scientific Report (2005–2006) 70–71.

[5] K. S. Knight, M. Soar and N. Bonanos, Crystal Structures of Gadolinium and Yttrium-Doped Barium Cerate, Journal of Materials Chemistry 2(7) (1992) 709-712.

[6] Sewook Lee, Inyu Park, Hunhyeong Lee, Dongwook Shin, Continuously Gradient Anode Functional Layer for BCZY Based Proton-Conducting Fuel Cells, International Journal of Hydrogen Energy 39 (2014) 14342–14348.

[7] Cameron W. Tanner and Anil V. Virkar, Instability of BaCeO3 in H2O-Containing Atmospheres, Journal of The Electrochemical Society 143(4) (1996) 1386–1389.

[8] M. Shirpour, R. Merkle, J. Maier, Space Charge Depletion in Grain Boundaries of BaZrO3 Proton Conductors, Solid State Ionics 225 (2012) 304–307.

[9] Yangzhong Wang, Jin Huang, Tingting Su, Wei Liu, Huijun Qi, Jinlong Yang, Synthesis, Microstructure and Electrical Properties of BaZr0.9Y0.1O3-

δ:BaCe0.86Y0.1Zn0.04O3-δ Proton Conductors, Materials Science and Engineering: B 196 (2015) 35–39.

[10] Nadja Zakowsky, Sylvia Williamson, John T.S. Irvine, Elaboration of CO2 Tolerance Limits of BaCe0.9Y0.1O3-δ Electrolytes for Fuel Cells and Other Applications, Solid State Ionics 176 (2005) 3019-3026.

[11] Emiliana Fabbri, Daniele Pergolesi, Alessandra D’Epifanio, Elisabetta Di Bartolomeo, Giuseppe Balestrino, Silvia Licoccia and Enrico Traversa, Design and Fabrication of a Chemically-Stable Proton Conductor Bilayer Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs), Energy & Environmental Science 1 (2008) 355-359.

[12] Maria A. Azimova, Steven McIntosh, Transport Properties and Stability of Cobalt Doped Proton Conducting Oxides, Solid State Ionics 180(2-3) (2009) 160-167.

[13] Koji Katahira, Yoshirou Kohchi, Tetsuo Shimura, Hiroyasu Iwahara, Protonic Conduction in Zr-Substituted BaCeO3, Solid State Ionics 138 (2000) 91-98.



[14] Emiliana Fabbir, Alessandra D’Epifanio, Elisabetta Di Bartolomeo, Silvia Licoccia, Enrico Traversa, Tailoring the Chemical Stability of Ba(Ce0.8-xZrx)Y0.2O3-δ Protonic Conductors for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs), Solid State Ionics 179 (2008) 558-564

[15] Alessio Bassano, Vincenzo Buscaglia, Massimo Viviani, Marta Bassoli, Maria Teresa Buscaglia, Mohamed Sennour, Alain Thorel, Paolo Nanni, Synthesis of Y-Doped BaCeO3 Nanopowders by a Modified Solid-State Process and Conductivity of Dense Fine-Grained Ceramics, Solid State Ionics 180 (2009) 168-174.

[16] Ye Lin, Ran Ran, Yao Zheng, Zongping Shao, Wanqin Jin, Naqing Xu and Jeongmin Ahn, Evaluation of Ba0.5Sr0.5Co0.8Fe0.2O3−δ as a potential cathode for an anode-supported proton-conducting solid-oxide fuel cell, J. Power Sources, 180, (2008), 15-22.

[17] Dengjie Chen, Zongping Shao, Surface exchange and bulk diffusion properties of Ba0.5Sr0.5Co0.8Fe0.2O3−δ mixed conductor, Inter. J. Hydrogen Energy, 36, (2011), 6948-6956.

[18] Bin Lin, Hanqing Ding, Yingchao Dong, Songlin Wang, Xiaozhen Zhang, Daru Fang, Guangyao Meng, Intermediate-to-low temperature protonic ceramic membrane fuel cells with Ba0.5Sr0.5Co0.8Fe0.2O3-δ–BaZr0.1Ce0.7Y0.2O3-δ composite cathode, J. Power Sources, 186, (2009), 58-61.

[19] Peng Ranran, Wu Yan, Yang Lizhai, Mao Zongqiang, Electrochemical properties of intermediate-temperature SOFCs based on proton conducting Sm-doped BaCeO3 electrolyte thin film, Solid State Ionics, 177, (2006), 389-393.

[20] Lei Yang, Ze Liu, Shizhong Wang, YongMan Choi, Chendong Zuo, Meilin Liu, A mixed proton, oxygen ion, and electron conducting cathode for SOFCs based on oxide proton conductors, J. Power Sources, 195, (2010), 471-474.

[21] Wengping Sun, Zhen Shi, Shumin Fang, Litao Yan, Zhiwen Zhu, Wei Liu, A high performance BaZr0.1Ce0.7Y0.2O3-δ-based solid oxide fuel cell with a cobalt-free Ba0.5Sr0.5FeO3-δ–Ce0.8Sm0.2O2-δ composite cathode, Inter. J. Hydrogen Energy, 35, (2010), 7925-7929.

[22] Lei Yang, Chendong Zuo, Shizhong Wang, Zhe Cheng, Meilin Liu, A Novel Composite Cathode for Low-Temperature SOFCs Based on Oxide Proton Conductors, Adv. Mater., 20, (2008), 3280-3283.

[23] Wenping Sun, Litao Yan, Bin Lin, Shangquan Zhang, Wei Liu, High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3−δ–Ce0.8Sm0.2O2−δ composite cathode, J. Power Sources, 195, (2010), 3155-3158.

[24] Tianzhi Wu, Ranran Peng and Changrong Xia, Sm0.5Sr0.5CoO3 − δ–BaCe0.8Sm0.2O3-δ composite cathodes for proton-conducting solid oxide fuel cells, Solid State Ionics, 179, (2008), 1505-1508.

[25] Wenping Sun, Shunmin Fang, Litao Yan, Wei Liu, Proton-Blocking Composite Cathode for Proton-Conducting Solid Oxide Fuel Cell, J. Electrochem. Soc., 158, (2011), B1432-B1438.

[26] H. Schichlein, A. C. M¨uller, M. Voigts, A. Kr¨ugel, and E. Ivers-Tiffee, Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells, J. Appl.Electrochem., 32, (2002), 875-882.

[27] A. Leonid, B. Rüger, A. Weber, W. A. Meulenberg and E. Ivers-Tiffée, Impedance Study of Alternative ( La , Sr ) FeO3 − δ and ( La , Sr ) ( Co , Fe ) O3 − δ MIEC Cathode Compositions, J. Electrochem. Soc., 157, (2010), B234-B239.

[28] Dae-Kwang Lim, Ph. D thesis, Chonnam National University, 2016

http://jes.ecsdl.org/search?author1=B.+R%C3%BCger&sortspec=date&submit=Submit

http://jes.ecsdl.org/search?author1=E.+Ivers-Tiff%C3%A9e&sortspec=date&submit=Submit




Homogenization of the thermo-elastic properties of pristine and aged Ni-YSZ samples

Toni Vešović (1, 2), Arata Nakajo (2), Fabio Greco (2), Pierre Burdet (2, 3), Jan Van herle (2), Frano Barbir (1)

(1) Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia

(2) Fuelmat Group, Faculty of Engineering Sciences and Technology STI, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

(3) Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

Tel.:+385-91-174-6059 [email protected]

[email protected]

Abstract

Solid oxide fuel cell (SOFC) materials are exposed to varying types of mechanical loads and degradation processes during operation and the malfunction of a single cell can cause the end of service of the whole stack. The knowledge of the mechanical properties of the materials is essential to predict the lifetime of the SOFC stack. Several of the thin SOFC layers are heterogeneous materials. The measurement of their properties by mechanical testing is challenging. Methods based on 3-D imaging and computational homogenization are of interest to overcome these difficulties.

This study is focused on the evolution of the thermo-elastic properties of nickel-yttria stabilized zirconia (Ni-YSZ) electrode upon SOFC operation. Focused ion beam-scanning electron microscopy (FIB-SEM) serial sectioning has been performed to obtain 3-D reconstructions of the anode material in the pristine state and after short stack operation for 4700 h. The coefficient of thermal expansion (CTE) and elastic constants have been computed using homogenization. The analysis started with the validation of the developed image processing and numerical procedure. A grid and volume independence study have been first performed to estimate the spatial resolution and minimum volume required for characterizing the investigated Ni-YSZ material. The computed thermo-elastic properties have been then compared to measurements of the pristine anode from dilatometry and four-point bending tests. After these validation steps, the changes in the properties caused by operation have been characterized and the relationship with the evolution of the metric and topological properties discussed. Remark: Only the abstract is available, because the authors chose to publish elsewhere.


mailto:[email protected]@fesb.hr

mailto:[email protected]@fesb.hr




A0914

Evaluation of H2O/CO2 co-electrolysis of LSCF6428-GDC Electrode SOFC on microstructural parameters

Sang-Yun Jeon(1)*, Young-Sung Yoo(1), Mihwa Choi(1), Ha-Ni Im(2), Jae-Woon Hong(2), Sun-Ju Song (2)*

(1) Renewables & ESS Group, Energy New Business Lab., Korea Electric Power Research Institute (KEPRI), Korea Electric Power Corporation (KEPCO)

105, Munji-Ro, Yuseong-Gu, Daejeon, 43056, Republic of Korea (2) Ionics Lab., School of Materials Science and Engineering, Chonnam National

University, 77 Yongbong-Ro, Buk-gu, Gwang-Ju, 61186, Republic of Korea Tel.: +82-62-530-1713 Fax: +82-62-530-1699 *[email protected]

Abstract

High temperature co-electrolysis of steam and CO2 based on solid oxide electrolysis cell (SOEC) to produce syngas as a feedstock for the well-known Fischer-Tropsch process is the main aim of the present research. Here Ni-8YSZ/8YSZ/LSCF6428-GDC button cells were fabricated and the effect of the different microstructural parameters like fuel electrode porosities and thermodynamic parameters like gas composition, temperature, on the performance of SOEC has been investigated thoroughly. The SOEC with air electrode and fuel electrode having 20 vol% PMMA contents, addition of YSZ-GDC adhesion layer gives the better performance in overall results and the voltage obtained for this SOEC at current density of 0.8 A.cm-1 is ~ 1.3 V. To identify the electrochemical processes occurring at the electrodes of SOEC, distribution function of relaxation time (DRT) analysis of the electrochemical impedance (EIS) data is carried out. The optimized microstructural composition of the SOEC is conceded forward to study effect of thermodynamic parameters. By controlling the upstream gas composition, I-V and EIS performance of SOEC is evaluated.



Introduction Co-electrolysis of CO2 and water are gaining much interest because it can directly generate syngas using reversible direction of SOFC, a ratio of CO and H2 that can be further converted into a high purity liquid hydrocarbon fuel such as methane with the help of Fischer-Tropsch process [1]. The process of co-electrolysis is more complicated than the water electrolysis because there are three mains competing processes occurring simultaneously and those are the electrolysis of CO2 to CO, electrolysis of steam to H2 and reverse water-gas shift (RWGS) reaction [2-3]:

H2+CO2 ↔ H2O + CO (1) Recently syngas has been generated successfully by using SOEC based on electrodes made from Ni-YSZ cermet. Mahmood et al. [4] has reported the performance of Ni-YSZ supported SOEC with YSZ electrolyte and LSM as an air electrode, for different operating conditions such as temperature, feed gas composition and operating voltage. They observed high concentration polarization at the fuel electrode above high current densities ~ 0.8 A.cm-2 and due to this concentration polarization, performance of SOEC is decreased. In order to minimize concentration polarization, the need of optimization of fuel electrode architecture such as thickness, porosity etc. have been specified clearly. In case of LSM, it undergoes chemical expansion due to the excess oxygen when it is used in the SOEC and it blocks further oxygen ions hence there is loss of oxygen diffusivity as well as the delamination of LSM from the surface of electrolyte takes place. To solve this problem, the use of LSM-YSZ composite electrode is highly proposed. Graves et al. [5] has reported performance and durability of SOEC with Ni-YSZ fuel electrode, YSZ electrolyte and LSM-YSZ air electrode. They have studied distribution function of relaxation times (DRT) analysis of the electrochemical impedance spectroscopy (EIS) data to identify different processes occurring at the electrodes that contribute to the cell resistance. While Ni-YSZ is generally used as fuel electrodes in SOEC, the choice of an air electrode is remained as a main challenge [6]. The series of LSCF is proved to be the better choice of air electrode but its main problem is that it is reacting with YSZ at high temperatures. Therefore, the formation of composite electrodes as well as addition of a barrier layer of another electrolyte between electrolyte and air electrodes which is not reacting with YSZ may be one of the possible solution [7,8]. In the present work, we are going to report the systematic study of the fabrication and performance of SOEC by observing the effect of different microstructural parameters such as fuel electrode porosity on SOEC performance. Hence we attempted to fabricate the Ni-YSZ fuel electrode supported SOEC with different porosities of Ni-YSZ cermet to reduce the concentration polarization, YSZ electrolyte, GDC barrier layer and LSCF6428-GDC composite as an air electrode with addition of adhesion layer of YSZ-GDC composite between barrier layer and electrolyte in order to improve interface. The performance of SOEC is mainly analyzed by I-V characteristics and EIS as well as DRT analysis of EIS data is carried out to identify the different sub-processes contributing towards the total cell resistance.

1. Scientific Approach The operation of an SOEC involves complex chemicals, electrochemical and mass transport processes, and electrochemical impedance spectroscopy has been proved to be an important tool for the analysis of such systems [9]. EIS not only helps in analyzing the performance of SOCs but also the transformation of the impedance data onto the distribution of relaxation times enables to identify different sub-processes contributing towards total cell resistance. The DRT method has recently been applied to analyze the impedance spectra obtained during fuel cell operation, because it directly gives information



about the impedance related reactions to any prior assumption of the individual reactions, and thereby eliminates the need to arbitrarily construct the equivalent circuit for the analysis of unknown complicated systems by rule-of-thumb estimations of impedance spectra [10,11]. In the simplest case, a polarization process of an electrochemical system can be described by an equivalent circuit made of a parallel connection with an ohmic resistance R and a capacitance C. This RC circuit is characterized by its time constant τ = RC=1/(2πf). More complex polarization mechanisms require a distribution of time constants. In order to describe this mathematically, the DRT g(τ) is introduced. In a serial connection of RC-element polarization resistance, Rpol,i=γn·Rpol. Rpol represents the total polarization resistance of the real part and γn weighs the contribution to the nth polarization process of the total polarization loss [12]. The impedance is given by

N

1n n

npol

N

1n n

n,pol

polωτj1

γR

ωτj1

R)ω(Z with 1γ

N

1n

n

(2)

where ω is angular frequency, N is the number of RC elements and j is the imaginary unit. We can extend Eq. (1) and use an infinite number of RC circuits with time constants reaching from 0 to ∞ by introducing the continuous distribution function of relaxation times g(τ)

0

polpol τdωτj1

)τ(γR)ω(Z with 1τd)τ(γ

0

(3)

In Eq. (3), τdωτj1

)τ(γ

represents the fraction of the overall polarization resistance with

relaxation times between τ and τ+dτ. This implies that the area comprised by a peak equals the total polarization resistance of the respective dynamic process. The real and imaginary parts of the impedance data of a linear, time invariant system are connected by the Kramers-Kronig transformations. Therefore, it is sufficient to consider the imaginary part of the impedance only, as given below

0

2polpol τd)τ(γ)ωτ(1

ωτR)ω("Z)ω(Zlm (4)

From linear system theory, it is well known that the impedance of each entirely capacitive electrical system can be Fourier space transformed into the form of Eq. (4). But this approach may not be valid [13]. This problem is known to be ill-posed and requires special methods to be solved in order to avoid false peaks and oscillations. Eq. (4) can be regarded as a Fredholm integral equation of the first kind which can be described by the following equation

b

a

)x(zdy)y(g)y,x(K (5)

2. Experiments/Calculations/Simulations The Solid Oxide Electrolysis Cell (SOEC) with 300 m Ni-8YSZ fuel electrodes, 6 m Ni-8YSZ functional layer (FL), 9 m 8YSZ electrolyte, 3 m GDC barrier layer and La0.6Sr0.4Co0.2Fe0.8O3-GDC air electrode has been fabricated. The fuel electrode substrate was fabricated by tape casting method. Ni-YSZ functional layer and YSZ electrolyte layer was coated on Ni-YSZ substrate by dip coating method. The electrolyte coated samples was sintered at 1450°C for 5h in the air atmosphere to get as-prepared NiO-YSZ/YSZ assembly. The GDC barrier layer was coated into the YSZ electrolyte surface of NiO-YSZ/YSZ assembly by screen printing method. After that LSCF6428- GDC composite air electrodes was screen printed and the whole assembly were again heated at 1100oC for



2h. In the present report, Here the porosity of fuel electrode is varied from different PMMA50 contents 11 vol% to 15 vol% in NiO-YSZ to evaluated porosity effect. Finally, the optimized cell was measured under various gas composition and temperature condition. The geometric area of the LSCF6428-GDC oxygen electrode was 0.5 cm2. The SOFC and SOEC performance of button cells were evaluated at 800 °C using a laboratory-made test system. The fuel electrode side was sealed with a high temperature ceramic adhesive. As for current collector, platinum paste (Heraeus 6926) and Pt mesh (Alfa Aeasar 100 mesh) was used on both the fuel and air electrodes. For SOFC operations, current-voltage (I-V) curves and EIS were measured at various temperatures using electrochemical interface (Wonatech, zive MP5). The impedance was measured under OCV condition with Vrms of 50 mV in the frequency range of 105-10−2 Hz with 20 points per frequency decade. For co-electrolysis test, air (200 sccm) was supplied to the air electrode side and a gas mixture of hydrogen, CO2; steam with N2 valance gas was supplied to the fuel electrode side. The flow rate of gases was controlled by mass flowmeter while the flow of steam gas was controlled by a syringe pump.

3. Results The concentration polarization occurring in the SOEC can be reduced by controlling the porosity of the fuel electrode. Therefore, to study the effect of porosity of Ni-YSZ on the gas concentration polarization at high current density, it is decided to control the porosity of Ni-YSZ fuel electrode by varying the PMMA content. Fig. 1 shows SEM images of the fractured Ni-YSZ with 11 vol% and 15 vol% PMMA content respectively. The porosity of Ni-YSZ increases with PMMA content of 15, 20 and 25 vol%. Also graph of apparent porosity versus PMMA content is plotted and it is shown in Fig.2. Similar to SEM images it is observed that porosity increases with PMMA content.

Figure 1. fracture image of anode support with (a) 5vol% (b) 10vol% (c) 15vol%

(d) 20 vol% and (e) 25vol% PMMA50 contents

Vol % Porosity / %

11 64.98

15 67.83

Table 1. porosity of PMMA50 contents



The performance of SOEC with different porosity of Ni-YSZ was measured in 49% N2, 5% H2, 23% CO2 and 23% H2O gas mixture at fuel electrode side and 200sccm air at air electrode side at 800 °C. Fig. 3(a) shows the I-V characteristics of SOEC with 11 vol%, 15 vol%, 20 vol% and 25 vol% PMMA content. Across open circuit voltage (OCV), the I-V curves show good continuity for all the SOECs with different porosities. Above current density of 0.4 A.cm-2, deviation from ohmic region starts to appear for 11 vol%, 15 vol% and 25 vol% PMMA content while for 20 vol%, it starts above 0.8 A.cm-2. At the current density of 0.8 A.cm-2 the voltage of 1.30 V is obtained for the SOEC having fuel electrode with 20 vol % PMMA content and it is shown in Fig.3 (a) with dotted lines. Fig. 3(b) shows the Nyquist plot for EIS measurements in similar gas condition as I-V measurements at 800°C. It can be seen that the high frequency intercept on x-axis indicates ohmic resistance associated with electrolyte, electrodes, contact resistance etc. and it increases with PMMA content indicating that ohmic losses are increasing with increase in the porosity. The DRT analysis of impedance data was carried out and plotted as shown in Fig. 3(c). Three peaks observed in DRT analysis. The intensity of peaks decreases with increase in the porosity of Ni-YSZ. The intensity of peak 1 is more for 11 vol % PMMA than others, it may be due to the sub-process associated with the peak 1 becomes dominant with 11 vol % PMMA in fuel electrode. Also peak 1 and peak 2 shifts to the lower time region indicating that time required for the sub-processes occurring in the SOEC becomes shorter with increasing porosity of Ni-YSZ. The impedance data was fitted into the series of 3RC circuit elements and the area specific resistances were calculated and plotted as shown in Table 2. It can be observed that the high frequency (HF), middle frequency (MF) and low frequency (LF) ASRs decrease with increase in the porosity of Ni-YSZ and the total ASR for 20 vol% PMMA content is smaller.

Figure 2. (a) DC polarization (i-V) curves for subsequent SOFC and Co-EC

operations (b) Nyquist plot of impedance spectra (c) Distribution of relaxation time



Vol% Ohmic / Ωcm2 LF/ Ω cm2 MF/ Ω cm2 HF/ Ω cm2 total/ Ω cm2

11 0.130 0.188 0.114 0.289 0.695

15 0.127 0.190 0.093 0.246 0.691

Table 2. Variations of ASRs with PMMA content

Detail results of experiment is described on ref 14. In ref. 14, they are going to report the systematic study of the fabrication and performance of SOEC by observing the effect of different microstructural parameters such as air electrode thickness, fuel electrode porosity, adhesion layer and thermodynamic parameters like gas composition, operating temperature, on SOEC performance.[14]

References [1] Wenying Li, Hongjian Wang, Yixiang Shi, Ningsheng Cai, Performance and methane

production characteristics of H2O–CO2 co-electrolysis in solid oxide electrolysis cells, Int. J. Hydrogen Energy 38 (2013) 11104–11109.

[2] Wenying Li, Yixiang Shi, Yu Luo, Ningsheng Cai, Elementary reaction modeling of CO2/H2O co-electrolysis cell considering effects of cathode thickness, J. Power Sources 243 (2013) 118–130.

[3] Shisong Li, Yuanxin Li, Yun Gan, Kui Xie, Guangyao Meng, Electrolysis of H2O and CO2 in an oxygen-ion conducting solid oxide electrolyzer with a La0.2Sr0.8TiO3+δ composite cathode, J. Power Sources 218 (2012) 244–249

[4] Asif Mahmood, Saira Bano, Ji Haeng Yu, Kew-Ho Lee, Effect of operating conditions on the performance of solid electrolyte membrane reactor for steam and CO2 electrolysis, J. Membrane Sci. 473 (2015) 8–15.

[5] Christopher Graves, Sune D. Ebbesen, Mogens Mogensen, Co-electrolysis of CO2 and H2O in solid oxide cells: Performance and durability, Solid State Ionics 192 (2011) 398–403.

[6] Wensheng Wang, Yingyi Huang, Sukwon Jung, John M. Vohs, and Raymond J. Gorte, A Comparison of LSM, LSF, and LSC for Solid Oxide Electrolyzer Anodes, J. Electrochem. Soc. 153 (11) (2006) A2066–A2070.

[7] Wenqiang Zhang, Bo Yu, Jingming Xu, Investigation of single SOEC with BSCF anode and SDC barrier layer, Int. J. Hydrogen Energy 37 (2012) 837–842.

[8] Moon-Bong Choi, Bhupendra Singh, Eric D. Wachsman, Sun-Ju Song, Performance of La0.1Sr0.9Co0.8Fe0.2O3−δ and La0.1Sr0.9Co0.8Fe0.2O3−δ–Ce0.9Gd0.1O2 oxygen electrodes with Ce0.9Gd0.1O2 barrier layer in reversible solid oxide fuel cells, J. Power Sources 239 (2013) 361–373.

[9] A. Nechache, M. Cassir, A. Ringuedé, Solid oxide electrolysis cell analysis by means of electrochemical impedance spectroscopy: A review, J. Power Sources, 258 (2014) 164-181.

[10] H. Schichlein, A. C. Müller, M. Voigts, A. Krügel, E. Ivers-Tiffée, Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells, J. Appl. Electrochem., 32 (2002) 875-882.



[11] A. L. Smirnova, K. R. Ellwood, G. M. Crosbie, Application of Fourier-based transforms to impedance spectra of small-diameter tubular solid oxide fuel cells J. Electrochem. Soc., 148 (2001) A610-A615.

[12] A. Leonide, SOFC modelling and parameter identification by means of impedance spectroscopy, KIT Scientific Publishing, Karlsruhe, Germany, 2010, 22-24.

[13] J. R. Macdonald, Impedance Spectroscopy, John Wiley & Sons, Hoboken, NJ, 1987. [14] Tae-Ryong Lee, Ha-Ni Im, Sang-Yun Jeon, Young-Sung Yoo, Archana U. Chavan,

, Sun-Ju Song, Dependence of H2O/CO2 Co-Electrolysis Performance of SOEC on Microstructural and Thermodynamic Parameters, Journal of The Electrochemical Society, 163 (7) (2016) F728-F736



A0915

Temperature effect on elastic properties of SOFC layers

Alessia Masini, Zdeněk Chlup and Ivo Dlouhý Institute of Physics of Materials, Academy of Science of the Czech Republic

22 Zizkova, 616 62 Brno/Czech Republic Tel.: +420-774-98-2727

[email protected]

Abstract

The goals EU set by year 2020 include the saving of fossil fuels and the decrease of carbon dioxide emissions; hence, more environmentally friendly and efficient means of energy conversion are needed. Solid oxide fuel cells (SOFCs) and reversible solid oxide electrolyser cells (SOECs) are two of them; thanks to their ability to directly convert chemical energy of fuels into electricity, they are attracting considerable attention nowadays. In order to make these devices competitive in the energy market, it is necessary to improve their durability and reliability. Further development of SOCs requires the simulation of their operational behaviour by thermo-mechanical models, which in turn require reliable values for the thermal and mechanical properties of the materials involved. It is known that mechanical damage caused by thermal loading is the most serious problem that may cause degradation or even destruction of the cell and consequently lower the lifetime and efficiency of the whole system. Thus, it is of high importance to understand mechanical properties of SOFC and SOEC components, especially under long term operating conditions. This study is targeted to the behaviour of the individual cell, as it is the main component and its failure compromises the operation of the whole stack. Although exist several literature sources dealing with mechanical properties of the most common electrolytes and electrodes, the knowledge is usually limited to the behaviour of single layers or the same material in the bulk form. The effects of interfaces and layers co-sintering effects have been up to now not well understood. In this contribution we have investigated the overall behavior of the cell, focusing on the role that interface between layers plays in the changing of resulting elastic properties. For this purpose, the effects of added layers were analysed using high temperature impulse excitation technique. The relationship of Young’s modulus and presence of various combinations of layers was measured from room temperature up to a temperature lying above the service one. Obtained trends can be potentially used to extract the elastic behaviour of individual layers suitable as an input for simulations.




Introduction In the last decades, environmental issues have become of worldwide importance; global warming, air pollution, acid precipitation and ozone depletion are just some of the threats that our planet is facing and an effective action is needed [1]. The generation of energy by clean, efficient and environmental-friendly means is one of the biggest challenges nowadays. Fuel cells, with their high efficiency, reliability, modularity, fuel adaptability and very low emission of pollutants, seem to be a promising power source. They convert the chemical energy of a fuel gas directly into electrical and thermal energy; since no combustion is needed, fuel cells have much higher conversion efficiency and also significantly lower output of pollutants compared to conventional thermomechanical methods. Their operating principles are similar to those of batteries, but they don’t need to be recharged since they operate as long as both fuel and oxidant are supplied to the electrodes [2]. There are many types of fuel cells. Solid Oxide Fuel Cells, despite being the most demanding from a material point of view, have a great potential market competitiveness thanks to several aspects: their efficiency in generating electricity; their adaptability in the choice of the fuels; their modular and solid state design; their high temperature operation, that produces high quality heat byproduct which can be used for co-generation and allows the reformation of fuel gas within the stack, eliminating the need for an external reformer; the absence of noble metals in the structure, that could be problematic in resource availability and price issue; their extremely low emissions by eliminating the danger of carbon monoxide in exhaust gases, as any CO produced is converted to carbon dioxide at the high operation temperature; their quiet and vibration-free operation; their potential long life expectancy [1,3]. SOFCs opened a way for a – necessary – revolution in the power generation industry; hence, the importance of the development and improvement of these devices.

1. Scientific Approach In this work, the focus is on the cell layered structure of SOC, as the understanding of its behaviour is crucial for both the design and operation of the whole device. The main goal was to investigate the elastic behaviour of the cell, focusing on the influence that interface between layers has in the changing of resulting elastic properties. The cell is made up of co-sintered individual layers and its overall properties are affected by the constraints arising between them. Rectangular-shaped samples cut from the cell were analysed to investigate the mentioned effects. The starting point was the verification of the methodology used for determining the properties of the electrolyte: several samples of 3YSZ – Yttria Stabilised Zirconia – were tested non-destructively using an impulse excitation technique in the temperature range from room temperature up to 900°C. After that, the effects of added layers were analysed: the same tests were repeated on samples composed by two up to four layers, adding one layer at a time and comparing the behaviour between consecutive samples. All tests were carried out with the impulse excitation technique on samples with optimised geometry. Obtained data can be potentially used to extract the elastic behaviour of individual layers suitable as an input for numerical simulations. In literature, the usual approach is the measurement of elastic properties on individual bulk materials [4,5]. The main innovation of this work is the direct measurement of the elastic properties of the layers when joined together, taking into account co-sintering effects which cannot be analysed when individual bulk materials are investigated.



2. Experiments The main goal of this contribution was to investigate the overall behaviour of the cell and the influence of the subsequently added layers in the changing of elastic properties; thus, different types of samples were analysed. The aim was to characterize co-sintering effects acting in the cell. Table 1 shows a schematic representation of the samples examined. In order to better recognise them, they were named as shown below. The nominal thickness of each type of sample is also indicated.

Table 1. Scheme of the layered structures used for experiments with brief description and nominal thicknesses

Specimens used for the tests were cut from the cell using a precise diamond saw Isomet 5000 (Buehler, USA). Elastic properties were investigated by means of an impulse excitation technique device RFDA MF and RFDA HT 1600, (IMCE N.V., Belgium). In this technique, an automatized tapping device makes the sample vibrate with a preset fixed interval – we have chosen a 10s interval; the vibration signal is caught by a microphone placed on the edge of a waveguide tube and sent to a software able to transform it into a spectrum, from which it is possible to extrapolate the natural frequency of the sample, using the fast Fourier transformation. The tests were carried out from room temperature up to 900˚C in air atmosphere, with the constant heating and cooling rate set to 3˚C/min. The samples were 13mm long and 5mm wide; their dimensions were kept the same for all layers combinations to avoid undesired geometric effects. Elastic modulus was calculated from the resulting main flexural resonance frequency and the sample dimensions using the following equation:

where E, m, b, L, t, ff, T1 are the Young’s modulus [Pa]; the mass of the specimen [g]; the width [mm], the length [mm] and the thickness of the specimen [mm]; the fundamental flexural resonant frequency of the specimen [Hz] and the correction factor for fundamental flexural mode to account for finite thickness of bar according to ASTM standard [6]. In order to calculate Young’s modulus, the variations of Poisson’s ratio with the temperature were neglected and its value at room temperature was used. Young’s modulus was first determined at room temperature for each sample, and then measurements in the furnace were carried out. The room temperature data were used for the calibration of the high temperature set up because of their higher accuracy. In fact, once the sample was placed into the furnace, some unexpected external frequencies – most probably coming from the heating elements



– were disturbing the signal of the sample for the high temperature measurement. Other difficulties were connected to the very small sample thickness which provided only a weak signal, besides damped in the waveguide. To overcome these inconveniences, a proper digital band stop filter was set and the relevant data could be extracted over all the range of temperatures. Both measurements, at room and high temperature, were performed on relatively small and thin samples compared to the geometry recommended by standards. As a consequence, the analysis was more complicated and the data scatter was relatively high. In order to have some reliability in the results, four samples for each layered structure were tested under the same conditions. The congruence of the repeated results, confirmed their reliability.

3. Results The values of Young’s modulus (E) obtained at room temperature for the 3YSZ samples and for the following layered samples are summarised in Fig.1a). In the graph, an average value of four samples selected for each material type (see Table 1) is plotted with the scatter bars. The statistical data for each sample were determined from at least 20 independent measurements of the natural flexural frequency. Since the adding of the very first layer, a continuous drop in the value of the elastic modulus is observed. For the SOC0 specimen, which corresponds to the simple 3YSZ electrolyte, E has an average value of 204 GPa; this result is in good agreement with literature data for similar materials [4,5]. With the addition of the GDC – Gadolinium Doped Ceria - layer (SOC1), the measured elastic modulus drops significantly to 164 GPa. When also the anode is joined (SOC2) E reaches a value of about 109 GPa and it finally has a significant drop to a value of 40 GPa when also the cathode LSCF – Lanthanum Strontium Cobalt Iron oxide - layer is present (SOC3).

(a) (b)

Fig.1. Elastic modulus at room temperature of different type of layered samples. Scatter bars (a) and average bars (b).

Fig. 1b) summarises the average values for each category of specimen and indicates the percentage drop when the next layer is added, relatively the electrolyte. From the plotted data it can be stated that the significant drop in Young’s modulus when adding a thin layer, can be explained by the influence of co-sintering effects, of layers porosity and by the role of interfaces. Fig. 2 shows the variation of the elastic modulus of 3YSZ as a function of temperature.



As already mentioned, the scatter of the measured data is significantly high compared to room temperature measurements; the main reasons are the small size of the sample and the high noise level accompanied by signal losses. However, the trend is well identifiable and was confirmed by repeated measurements. To emphasize the trend, a polynomial regression curve was calculated and superposed to the individual measurements. The observed behaviour is in good agreement with the one reported in literature [4,5,7] and it can be basically divided into four regions. From room temperature up to around 300˚C, Young’s modulus decreases first quite strongly, and then more slowly until reaching about 600˚C. It stays approximately constant between 600˚C and 800˚C, and finally slightly increases above 800˚C.

Fig.2. Elastic modulus of 3YSZ as a function of temperature.

Since the dimensions of the sample are very important parameters, their variations should be taken into account. In literature, a discontinuity in the thermal expansion coefficient for 3YSZ is detected around 650˚C by Gibson and Irvine [8]; this discontinuity indicates a second-order transition that could explain the increase of Young’s modulus, due to a higher stress level in the sample. However, in this work the coefficient of thermal expansion was neglected over the whole range of temperature because it was not known for all layers combinations. It has to be noted that the error caused by changes of dimensions with temperature is relatively small compared to the natural data scatter. Fig. 3 summarises the behaviour of E versus temperature for all the samples analysed. It is evident how the biggest drop in the value of E (more than 50%) occurs when adding the cathode layer. From the present graph, it can be observed that the drop of elastic modulus with increasing temperature is suppressed with rising number of deposited layers; in fact, the behaviour for SOC3 becomes rather constant over the whole range of temperature under investigation.



Fig.3. Comparison of the Elastic modulus behaviour of SOC0, SOC1, SOC2, SOC3 as a function of temperature.

4. Conclusions The room temperature data obtained for each category of specimen show a continuous drop when adding following layers. The influence of co-sintering effects, layers porosity and the role of interfaces are effects that can explain the observed drop. The change in Young’s modulus with increasing temperature becomes smaller with the rising number of deposited layers.

References [1] A. Boudghene Stambouli, E. Traversa, Solid oxide fuel cells (SOFCs): a review of an

environmentally clean and efficient source of energy. Renewable and Sustainable Energy Reviews 6 (2002) 433-455

[2] S.P.S. Badwal, K. Foger, Solid Oxide Electrolyte Fuel Cell Review. Ceramic International 22 (1996) 257-265

[3] S.C. Singhal, Advances in solid oxide fuel cell technology. Solid State Ionics 135 (2000) 305-313

[4] T. Kushi. K. Sato, A. Unemoto, S. Hashimoto, K. Amezawa, T. Kawada, Elastic modulus and internal friction of SOFC electrolytes at high temperatures under controlled atmospheres. Journal of Power Sources 196 (2011) 7989-7993

[5] S. Giraud, J. Canel, Young’s modulus of some SOFCs materials as a function of temperature. Journal of the European Ceramic Society 28 (2008) 77-83

[6] ASTM E1876-15 Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio by Impulse Excitation of Vibration

[7] T. Kushi. K. Sato, A. Unemoto, K. Amezawa, T. Kawada, Investigation of High Temperature Elastic Modulus and Internal Friction of SOFC Electrolytes Using Resonance Method. 216th ECS Meeting, Abstract #1237, Vienna, Austria, October 2009

[8] I.R. Gibson, J.T.S. Irvine, Study of the order-disorder transition in yttria-stabilised zirconia by neutron diffraction. Journal of materials Science, 1996, 6(5), 895-898



A0917

Characterization of the performance and long-term degradation of fuel electrode supported multilayered

tape cast Solid Oxide Cells

M. Torrell (1)*, D. Rodríguez (2), B. Colldeforns (1), M. Blanes (2), A. Morata (1), F. Ramos (2), A. Tarancón (1)

(1) IREC, Catalonia Institute for Energy Research, Dept of Advanced Materials for Energy Applications, Jardí de les Dones de Negre 1, Planta 2, 08930, Sant Adriá del Besós,

Barcelona (2) FAE, Francisco Albero SAU, L'Hospitalet de Llobregat, Spain

(*) [email protected]

Abstract

After the efforts of the scientific community focused on the development and optimization of the Solid Oxide Cells (SOC) carried out during past years, nowadays their final implementation depends mainly on the long term stability of the systems. Improving the durability and characterize the aging mechanisms is shown as key factor for the final introduction of SOC systems as real alternative energy devices. The market penetration of SOC will probably be promoted not only as power generator systems, working as Solid Oxide Fuel Cells (SOFC) but also as Solid Oxide Electrolyzer Cells (SOEC) for chemical energy storage for power to gas and power to liquid routes [1-3]. In the present work, anode supported cells (ASC) have been fabricated by an innovative multilayer tape casting process at industrial scale at FAE S.A.U facilities. NiO-YSZ and YSZ tapes have been cast and jointly sintered to produce the cell supports (fuel electrode and electrolyte). La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) oxygen electrode has been deposited by screen printing with a Gd0.2Ce0.8O2 (CGO) diffusion barrier to avoid the generation of insulator secondary phases such as SrZrO3 [4]. A complete microstructural characterization of the cells has been carried out. The cells have been electrochemically characterized under SOFC and SOEC mode in terms of performance and long term stability. Results are discussed analyzing EIS spectra obtained under different operation modes and conditions. Power densities above 700mW/cm2 have been achieved under wet hydrogen at 750ºC in SOFC mode showing more than 2000h of stability. In addition, current densities of 1A/cm2 have been injected to the same cell operating as electrolyzer at 850ºC. SOEC aging test for 150h has been carried out at voltages above the thermoneutral.



Introduction Experimented increase in power demands detected during the last decades jointly with the continuous shortage of fossil fuels and the aim to diminish the emissions, has leaded the observed rise of alternative green energy technologies. New proposed systems have to take advantage from the renewable sources of energy while presenting low or zero emissions with high efficiency to accomplish the demand [2, 5, 6]. In this scenario, hydrogen acts as the most promising candidate to be used as energy vector for sustainable energy cycles. Hydrogen can be used, and produced, for different types of fuel cells, and electrolysers, respectively [2, 7]. Proton Exchange Membrane cells (PEM) have focused the major research efforts and is already technological mature. The same situation is for alkaline cells (AC), which are already commercially available. Solid Oxide Cells (SOC) technology is less mature, since are the most demanding from materials standpoint due to their high operation temperatures. However, SOC awake high interest due to their potential market competitiveness mainly lead by the higher efficiencies and fuel flexibility [7]. Solid Oxide Fuel Cells (SOFCs) operate at high temperatures (700-950ºC) and are robust, modular and quiet devices producing electricity, heat and water as final by-product when hydrogen is used as fuel [8]. When SOFC operate in a reverse mode are able to electrolyse water, chemically storing the energy as Hydrogen. SOC electrolysers are also capable to co-electrolyze CO2 and water mixtures generating syngas, which can be catalytically converted in synthetic liquid fuels [9-14]. SOC are then an efficient solution for the power-to-gas and gas-to-power (P2G and G2P) routes being able to couple the two major energy infrastructures of our modern society, i. e. gas and electricity networks, while solving the intrinsic renewables sources mismatch between energy productions and demand [15]. Nowadays, an important part of the SOC research is focused into understanding the mechanisms occurring during operation, which governs the cells degradation and their relation with the initial microstructure and operation conditions [16, 17]. Materials selection, microstructure, quality and attachment of the interfaces, as well as stability of used materials become very important for the good operation and durability of the SOC systems. Particularly the SOC critical features are electrode contact overpotentials, reactivity and changes in materials due to the cations interdifusion and the grain coarsening of the Ni that can lead catalytic deactivation of the fuel electrode [18-21]. For the cells operation it is also very important to control the temperature and fuel composition gradients over the active area [20]. The present work presents a new manufacturing method of fuel electrode supported SOC, which presents remarkable results in terms of performance and durability under SOFC and SOEC modes.

1. Experiments

Half-cells composed of YSZ over YSZ-Ni anode functional layer (AFL) and anode support layer (ASL) have been prepared by tape casting. YSZ (8% Y2O3-ZrO2, KCeracell, South Korea), NiO (Kceracell, South Korea) and starch from rice (Sigma-Aldrich, Germany) have been used to prepare both fuel electrode layers and electrolyte. PEG 400 (Panreac, Spain), Duramax B1000 (Rohm and Hass, Germany) and Dolapix PC 75 (Zschimmer and Schwarz, Germany) have been used as plastizicer, binder and dispersant respectively, and water as solvent. The ratio of the YSZ slurry was 23:5:4:1:1 powder:solvent:binder:plastizicer:dispersant.



AFL slurry has been prepared following the same approach, mixing NiO:YSZ in a 3:2 ratio by ball milling and slurries prepared as YSZ slurries. ASL slurry has been prepared mixing NiO:YSZ:Starch in a ratio 7:4:0-1 by ball milling and slurries prepared as YSZ and AFL ones. The pore former has been removed at 500ºC for 1h and samples sintered at 1350ºC for 3h. Obtained half-cells presented an area of 1.5 cm2 and a total thickness around 280 μm (ASL: 200μm AFL: 20-30 μm Electrolyte: 12-20 μm). La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) oxygen electrode has been deposited by screen printing (SONY Si P-850) and sintered at 1000ºC for 5h, after a new screen printing layer of Gd0.2Ce0.8O2 (CGO) has been deposited as diffusion barrier of cations migration in order to avoid their reactivity and the generation of insulator secondary phases such as SrZrO3 [4] For the electrochemical characterization, current collectors based on Pt meshes and nickel paste were used in the fuel electrode side while a single Pt mesh and commercial La0.6Sr0.4Co0.2 Fe0.8O3 (LSCF) paste (Kceracell) was employed in the oxygen side. Ceramabond™ ring covering the sample edge has been used to ensure gas tightness between the fuel and oxygen chambers. The electrochemical fuel cell tests have been carried out in a commercial ProboStatTM (NorECs AS) sample holder placed inside a high temperature tubular furnace. The cells were electrically characterized at a constant temperature of 750 ºC for SOFC and 850ºC for SOEC by employing a potentiostat/galvanostat PARSTAT 2273. Electrochemical Impedance Spectroscopy (EIS) have been performed in a range of frequencies from 1 MHz to 0.1 Hz and applying an AC signal of 50 mV-100mV, under OCV conditions. In order to obtain a more complete electrochemical characterization of the cells EIS measurements have been recorder applying a DC bias voltage of 0.7 V (SOFC) and 1.4V (SOEC). Synthetic air was used as oxidant while pure humidified hydrogen was the inlet fuel. The morphology of the samples was characterized by a Zeiss Auriga scanning electron microscope (SEM). The cells show a remarkable performance at 750ºC under SOFC mode fed by pure Hydrogen and more than 2250h of life time under potentiostatic mode. An operation point of 0.72V has been selected for the aging test to demonstrate the possible use of the presented cells in commercial systems. Moreover, the behavior of the cell under SOEC operation has been also characterized to assess the potential of the fabricated cells as reversible systems, which could operate as power generator and chemical energy storage device.

2. Results The architecture of the fuel electrode supported studied cells is shown in SEM micrographies of the figure 1a and 1b where 170µm 65%wt NiO-YSZ support is observed with a functional layer of 20µm with 40% of NiO. The 20 µm thick YSZ electrolyte is also clearly observed with close porosity to ensure the gas tightness. A 2 µm CGO barrier layer has been used to avoid the reactivity between oxygen electrode and electrolyte, especially while sintering the 35 µm of LSCF porous oxygen electrode. It has to be highlighted the presence of the thin and homogeneous barrier layer of CGO especially clear on the back scattered electrons (BSE) SEM image of the figure 1b. On the figure 1c is can be observed the high density of the YSZ electrolyte. Figure 1c and 1d are included in order to emphasize the good attachment of the interlayers since they are described as one of the key points of the cell performance and stability.



Figure 1. SEM micrographies of the fuel electrode supported fabricated cells a) general view of the cross section by SE-SEM b) General closer

view of the cross section by EBS-SEM c) detail of both electrode/electrolyte interfaces and d) detail of the oxygen electrode/barrier

layer/electrolyte interfaces

V-I polarization curve presented in the figure 2 shows the SOFC performance of the studied cells. More than 700mW/cm2 of power density has been achieved under pure wet H2 at voltages above 0.7V. This is a remarkable performance result for the fabricated fuel electrode supported cells operating at 750ºC. These results confirm the idoneity of the manufacturing method, selected materials and obtained microstructures as it can be observed in figure 1. Fuel diffusion or mass transport contributions are not detected in the polarization curves even at high current densities above 1A/cm2. Cell performance obtained under DC test is in good agreement with the one observed by AC Electrochemical impedance spectroscopy measurements. Figure 3 shows the EIS results that have been studied through the Nyquist arc plots obtained at open circuit voltage (OCV) and at SOFC operation point (0.7V). Resistance contributions have been split by fitting the Nyquist arc plots for an equivalent circuit presented as inset of figure 3. Change in the polarization resistance, Rp= 0.547 ohm.cm2 at OCV and Rp=0.235 ohm.cm2 at 0.7V, is ascribed to an important contribution of the activation resistance of the oxygen electrode. The slight difference in ohmic resistance has not been considered for the presented analysis.

Fig.2 V-I polarization and power density curve obtained at 750ºC under wet pure H2.



Fig. 3. Nyquist plots of the EIS spectra obtained at 750ºC under OCV and SOFC operation conditions (0.7V). Used equivalent circuit for fitting the arc obtaining the different

resistance contributions is presented as an inset.

After the electrochemical characterization, the same cell has been operated under potentiostatic SOFC mode for more than 2250 hours at a fixed voltage of 0.72V and 60ml/min of pure humidified hydrogen as fuel. Figure 4 presents the long term evolution of the power density and total extracted current. Two zones can be differentiate, the first one, called zone I, where a cell stabilization is taking place, it is typically from long-term studies and in this case has been observed during the first 300 hours of test. This zone shows a stepped degradation rate, while the microstructure adapts the ionic and electronic percolation paths and the porosity distribution to the required morphology for the imposed operation conditions [22]. Extrapolating the degradation rate of this first zone a high value of 10%/1000h is obtained. After the first 300h the cell degradation is stabilized in the Zone II, reaching a virtually none degradation step for 2000h bringing more than 400mW/cm2 of constant power density at 750ºC.



Figure 4. Total extracted current and power density evolution of an ASC at a constant

voltage of 0.72V at 750ºC. The presented cell is maintained operating under the same conditions showing the same degradation rates of Zone II during the preparation of the present extended abstract. A detailed microstructure study of post operation characterization by SEM, XRD and Raman will clarify in the future the main mechanisms which lead the accumulated degradation. In order to fully characterize the cell behavior under different operation conditions, a complete polarization V-I curve, from SOFC to SOEC modes has been measured for a cell with the same architecture as the one previously characterized as SOFC. The feeding gas composition has been changed from the pure wet H2 to 25% of steam and 75% of Hydrogen, thus promoting a decrease of the OCV and of the general performance. Nevertheless, more than 350mW/cm2 of power density has been achieved under SOFC operation, and 750mA/cm2 has been injected for H2 generation (5.2mlH2/cm2.min considering 100% of faraday efficiency) at voltages above the thermoneutral for steam electrolysis (1.29V at 850ºC) [21]. The maximum injected current has been 1.15mA/cm2 (8mlH2/cm2.min at faraday efficiency of 100%). V-I curve of the figure 5 confirms the remarkable performance of the cells on both operation modes, showing a smooth transition between both modes. However, the SOEC mode presents a slightly more stepped V-I slope, which reveals a higher area specific resistance (ASR) of the cell when operating as electrolyzer. At the light of the V-I curve and EIS tests (Figure 6) the higher ASR value of the electrolyser comes from the polarization resistance. Obtained ohmic resistance is similar for the different operation points (OCV, 0.7V and 1.4V) while the polarization resistance drastically changes at 1.4V. This increase of the resistance under electrolysis is typically ascribed to diffusion issues due to the water vapor transport on the fuel electrode. However, there are no clear evidences in the EIS results of this correlation with the mass transport phenomena, so the concentration resistance and the reduction process of the water molecules can also play a role in the increase of the final resistance. At 1.4V an ASR=0.872ohm.cm2 is obtained operating at 750ºC. The fitting of the EIS spectra to the equivalent circuit is in agreement with the observed behavior on the V-I



curve for high injected current densities. Obtained value of the polarization resistance at 0.7V (SOFC) has been Rp= 0.183ohm.cm2.

Figure 5. V-I polarization curve of the ASL cell at 850ºC and 25% of steam. A complete

range of current densities, from 1.2A/cm2 (SOFC) to -1.2A/cm2 (SOEC) has been scanned.

Figure 6. Nyquist plots of the EIS spectra obtained at 850ºC under OCV, SOFC operation conditions (0.7V) and SOEC operation conditions (1.4V). Used equivalent circuit to fit the



arc to extract the different resistance contributions as inset; and a zoom of the ohmic resistance zone. The EIS at 1.4V after 150h of operation is also shown on the same axis.

Aging test under SOEC conditions has been performed, as it has been done in the SOFC mode. In this case, the cell has been studied during 150h under galvanostatic operation at 650mA/cm2 at 850ºC (Figure 7). The obtained initial voltage was 1.34V, which evolves to 1.42V after the first 19 hours of test. Then the degradation rate was stabilized in the zone II with a value of 2%/150h, which is not a very promising result but probably can be improved for longer term aging tests. In addition, a poor homogeneity on the steam flow has affected the homogeneity of the obtained voltage values. The authors consider that, after 150h of test, the cell is still in the first accommodation zone where the microstructure is changing. The same phenomenon has been previously observed in other works [22, 23] as well as in the long term SOFC test of the present work.

Figure 7 Long term galvanostatic aging test under SOEC mode at 850ºC.

Figure 6, previously commented, also shows the evolution of the cell after the first 150h of operation. Main degradation contribution is clearly ascribed to the ohmic resistance, which is doubled from the initial point (Rsinitial=0.422), while the polarization resistance diminishes from the initial 0.458 ohm.cm2 to 0.247 ohm.cm2. This increase in the ohmic contribution is ascribed to changes in contact resistance at the oxygen electrode/electrolyte interface [24, 25]. Usually the oxygen electrode/electrolyte interface is the most affected for the degradation under SOEC operation, due to the incorporation of oxygen and/or cation interdiffusion between the oxygen electrode and electrolyte as it is well described for LSM oxygen electrodes [26]. However, this behavior is not clear for LSCF, as the degradation rates seems to be smaller and the microstructure re-crystallises minimizing the surface defects under an electrochemical gradient [24, 27]. Microstructural characterization of the final cell under longer SOEC operation times will be necessary to elucidate the degradation mechanisms and the stability of the tested cells.



Conclusions Innovative manufacturing method of multilayer tape casting of the NiO-YSZ electrode and YSZ electrolyte as a single step sintered support has been developed and industrially optimized with remarkable results. Fabricated cell has been electrochemically and microstructurally characterized showing excellent morphology of the different layers with a good attachment, thus minimizing the contact resistance contributions. More than 700W/cm2 of power density has been achieved at 750ºC when the cell operates as a SOFC. Moreover, the cell shows a virtually none degradation for more than 2000h, after a first stabilization step. Current densities around 750mA/cm2 have been injected (>5ml of H2/cm2.min) at voltages above the thermoneutral voltage for steam electrolysis (1.29V). The degradation rates under SOEC mode are higher and especially focused on the ohmic resistance contribution, as it has been concluded by EIS analysis.

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[14] Irvine, J. T. S. et al. Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers. Nat. Energy 1, 15014 (2016)

[15] A. Tarancón, C. Fábrega, A. Morata, M. Torrell and T. Andreu, Materials for Energy, Chapter: power to fuel and artifi cial photosynthesis for chemical energy storage, ed. D. Muñoz and X. Moya, Pan Stanford Publishing, 2015

[16] Gemmen, R. S., Williams, M. C. & Gerdes, K. Degradation measurement and analysis for cells and stacks. J. Power Sources 184, 251–259 (2008).

[17] Tietz, F, Sebold, D., Brisse, A. & Schefold, J. Degradation phenomena in a solid oxide electrolysis cell after 9000 h of operation. J. Power Sources 223, 129–135 (2013).

[18] Moçoteguy, P. & Brisse, A. A review and comprehensive analysis of degradation mechanisms of solid oxide electrolysis cells. Int. J. Hydrogen Energy doi:10.1016/j.ijhydene.2013.09.045

[19] Gemmen, R. S., Williams, M. C. & Gerdes, K. Degradation measurement and analysis for cells and stacks. J. Power Sources 184, 251–259 (2008).

[20] Yokokawa, H., Horita, T., Yamaji, K., Kishimoto, H. & Brito, M. E. Degradation of SOFC Cell/Stack Performance in Relation to Materials Deterioration. J. Korean Ceram. Soc. 49, 11–18 (2012).

[21] Laguna-Bercero, M. A. Recent advances in high temperature electrolysis using solid oxide fuel cells: A review. J. Power Sources 203, 4–16 (2012).

[22] L. Almar, A. Morata, M. Torrell, M. Gong, T. Andreu, M. Liu and A. Tarancon, J. Power Sources. Submitted 2016

[23] Almar, L. et al. Synthesis and characterization of robust, mesoporous electrodes for solid oxide fuel cells. J. Mater. Chem. A (2016). doi:10.1039/C6TA00321D

[24] Choi, M.-B., Singh, B., Wachsman, E. D. & Song, S.-J. Performance of La0.1Sr0.9Co0.8Fe0.2O3−δ and La0.1Sr0.9Co0.8Fe0.2O3−δ–Ce0.9Gd0.1O2 oxygen electrodes with Ce0.9Gd0.1O2 barrier layer in reversible solid oxide fuel cells. J. Power Sources 239, 361–373 (2013).

[25] Tietz, F., Sebold, D., Brisse, A. & Schefold, J. Degradation phenomena in a solid oxide electrolysis cell after 9000 h of operation. J. Power Sources 223, 129–135 (2013).

[26] Chen, K., Liu, S.-S., Ai, N., Koyama, M. & Jiang, S. P. Why solid oxide cells can be reversibly operated in solid oxide electrolysis cell and fuel cell modes? Phys. Chem. Chem. Phys. 17, 31308–15 (2015).

[27] Nguyen, V. N., Fang, Q., Packbier, U. & Blum, L. Long-term tests of a Jülich planar short stack with reversible solid oxide cells in both fuel cell and electrolysis modes. Int. J. Hydrogen Energy 38, 4281–4290 (2013).

Acknowledges

The authors thank to Centro para el Desarrollo Tecnológico Indrustrial (CDTI) for financial support (project Eureka-SCAPE-SOFC) as well as all the involved researchers of the project partners: AMES, FAE, IREC and Kceracell.




Hydrogen membrane fuel cell using Ni-Zr alloy membrane

SungBum Park (1), Sung Gwan Hong (1), Yong-il Park (1) (1) Kumoh National Institute of Technology 61 Daehak-ro, Gumi, Gyeongbuk, Korea

Tel.: +82-54-478-7753 [email protected]

Abstract

The purpose of this study is to develop HMFC (hydrogen membrane fuel cell), a new concept fuel cell different from the existing thin membrane type SOFC, by utilizing the high-degree of mechanical stability of metal complex electrolyte including metal hydrogen separation membrane. In the case of metal separation membrane mainly used for hydrogen separation membrane, it is excessively dependent upon Pd with effective hydrogen storage and transmission performances and an alternative material has not been found. Accordingly, there is a need for studies on non-Pd system hydrogen permeable membrane that can be used in high temperatures with hydrogen permeable rate similar to existing Pd. This study fabricated and analyzed the characteristics of Ni-Zr system hydrogen separation membrane with high selectivity and permeability of hydrogen while substituting existing Pd that has been used as hydrogen separation membrane. In addition, HMFC was composed by using the Ni-Zr system metal hydrogen permeable membrane fabricated to evaluate its performance. Remark: Only the abstract is available, because the authors chose to publish elsewhere.







Stability of SOFC cassette stacks during redox-thermal-cycling

Ute Packbier (1), Tim Bause (2), Qingping Fang (1), Ludger Blum (1), Detlef Stolten (1)

(1) Forschungszentrum Jülich GmbH Institute of Energy and Climate Research

Electrochemical Process Engineering (IEK-3) Wilhelm-Johnen-Straße, 52428 Jülich, Germany

(2) ElringKlinger AG, Max-Eyth-Straße 2 72581 Dettingen, Germany Tel.: +49-2461-615170 Fax: +49-2461-616695

[email protected]

Abstract

At Forschungszentrum Jülich measurements regarding redox-stability of anode supported cells integrated in 5-layer cassette design stacks were performed. In potential SOFC system applications like off-grid power generators the anode side of these stacks may be exposed to air during system start and/or stop. During the stack tests combined thermal and redox cycles were performed in order to determine the temperature at which the cells are irreversibly damaged. Two 5-layer cassette design stacks provided by ElringKlinger AG were thermal-cycled in a temperature range between 300 to 750 °C. While cooling down, the anode side was flushed with 100 mlmin-1 air as soon as the stack reached a certain temperature. At a temperature of 300 °C air was replaced by 4% H2 in Ar and the stack was heated back to operation temperature. During the stack test the temperature at which flushing with air was started was increased stepwise from 500 to 700 °C for the first stack, and up to 630 °C for the other one. Within each cycle cell voltages at 0.3 A/cm² were recorded at defined conditions for comparison. OCVs under dry atmosphere were measured for detecting possible leakage in each cell. After testing the stacks were examined regarding damages related to redox-cycling by post-mortem analysis. Up to a redox temperature of 600 °C no decrease in cell performance and OCV was observed. At higher redox temperatures starting from 620 °C a noticeable decrease in performance and OCV was measured. At a redox temperature of 700 °C the decrease in OCV indicated a severe damage of the cells. Remark: Only the abstract is available, because the authors chose to publish elsewhere.






Evaluation of a SOEC stack for hydrogen and syngas

production: a performance and durability analysis

Mikko Kotisaari (1), Olivier Thomann (1), Dario Montinaro (2), Jari Kiviaho (1) (1) VTT Technical Research Centre of Finland Ltd., Biologinkuja 5, 02150 Espoo, Finland

(2) SOLIDpower SpA, Viale Trento 115/117, 38017 Mezzolombardo, Trento, Italy Tel.: +358-40-483-7715 Fax: +358-20-722-7048 [email protected]

Abstract

Solid oxide electrolysers (SOE) are gaining growing interest in research because they can convert electricity into a chemical fuel with high efficiency. The present work investigates the performance of a 6-cell SOE stack (80 cm2 active area) in electrolysis and co-electrolysis modes for the purpose of producing synthetic fuel. Initially, the stack was operated and characterized in fuel cell mode at 750 °C. Operation was then changed to electrolysis mode and the stack performance was characterized with a test matrix consisting of four different inlet gas compositions of various ratios of inlet steam and carbon dioxide at temperatures of 700, 750 and 800 °C. It was found that the stack performance depends primarily on the operation temperature and only to a small extent on the inlet gas composition. Finally, a steam electrolysis durability test of 1500 hours was performed at a current density of -0.775 A/cm2 (50 % of reactant utilization) and at a temperature of 750 °C. The voltage trend showed that no degradation could be measured, which is a very promising result. In conclusion, the investigated stack appears suitable for syngas production. In the future, co-electrolysis durability tests will be conducted to evaluate the effect of addition of carbon dioxide on the stack durability. Remark: The full paper is published in EFCF Special Issue Series (www.EFCF.com/LIB,







A1203

Investigation of a 500W SOFC stack fed with dodecane

reformate

Massimiliano Lo Faro, Stefano Trocino, Sabrina C. Zignani, Giuseppe Monforte, Antonino S. Aricò

CNR-ITAE, Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy Tel.: +39-090-624231 Fax: +39-090-624247

[email protected]

Abstract

A proof-of-concept Solid Oxide Fuel Cell (SOFC) system of 500 Wel fed with n-dodecane reformate was realized in order to prove the reliability of SOFC technology for naval uses. The cells used for the prototype consisted of Ni-YSZ/YSZ/YDC/LSFC whereas the catalyst for the reformer of n-dodecane was Rh-CeO2-ZrO2. At the preliminary stage and as to a propaedeutic approach, a microplant consisting of a reformer for the treatment of 7 Wh of dodecane and a single button cell were coupled in order to determinate the proper conditions of operation and the degradation effects occurring during 300 h of stressed tests. Then, a single large area cell and a stack were fed with n-dodecane reformate to determinate the performance achievable under practical conditions. Electrochemical ac-impedance spectra (EIS) and polarizations curves were carried out to study the systems above mentioned. As well, post-operation scanning electron microscopy analysis (SEM) on the cell and thermal analysis on the catalyst were conducted in order to demonstrate the ageing effect observed during the operation of the coupled system.




Introduction The Solid Oxide Fuel Cells (SOFC) are electrochemical devices that may find application in stationary as well in oversize vehicles such as aircraft or ships [1, 2]. Accordingly, the SOFC systems must demonstrate reliability towards the utilization of conventional heavy hydrocarbon fuels such as diesel especially for on-board applications, as auxiliary power unit (APU). The SOFC has already demonstrated a level of maturity in terms to manufacturing of cells having Ni-based anode, stacks, and systems [3]. Nowadays, this type of technology is quite far from its use in real environmental conditions where H2 is not yet available. The main manufacture companies have forecasted that the global SOFC market will grow by 10.3 % over the period 2015-2019, as reported by TechNavio [4]. Therefore, the main possibility for a rapid penetration of such technology into the market of generators for distributed energy is the utilization of a chemical processor coupled to the SOFC [5]. The utilization of SOFC technology may maximize the efficiency for the conversion of fuels to electricity [6]. Furthermore, considering also the recovery of thermal waste, the heat integration between the stack and the fuel processor that generally operate at about 700-800°C, may increase the overall efficiency of the system [7]. Besides, these coupled devices reduce the risk of carbon deposition over the SOFC’s anode and consequently improve the durability [8]. Diesel is a complex mixture having thousand hydrocarbon-species, most of which have carbon number from 10 to 22 and boiling points ranging from 185 to 290 °C [9]. Nowadays, the diesel is utilized exclusively in traditional internal combustion engines especially in small and large size vehicles, because of its intrinsic high volumetric and gravimetric energy densities particularly desirable for on-board and/or powertrain applications [10]. Therefore, based on the above issues, the integration of a fuel processor with a SOFC is nowadays the only solution that may compete with the traditional internal combustion engines [11]. Regarding the research for new materials devoted to the conversion of fuels to electricity, a good comprehension of the processes occurring during the transformation of a pure and single hydrocarbon molecule is necessary. Therefore, the utilization of n-dodecane as reference for tests concerning the next use of diesel in SOFCs may give significant information. Generally, the conversion of hydrocarbon fuels to hydrogen can be carried out by several different reforming approaches, including Steam Reforming (SR), Partial Oxidation (POX), and Autothermal Reforming (ATR) [12]. The conduction of the steam reforming reaction is the preferred one from industrial sector, mainly because it is easy in the management and theoretically this reaction offers the highest H2 concentration; for SOFCs, it may take a further advantage also because waste heat from SOFC operation can be recovered to heat-up the liquid vaporizer and reforming units [13]. SOFC systems fed with diesel fuel have been developed in the framework of EU and US programs dealing with the application of such devices in auxiliary power units for trucks e.g. the EU-FP5 DIRECT project [14]. Moreover, n-dodecane reforming over Ni catalyst and related issues have been studied by Gould et al. showing promising characteristics [15]. The present work is addressing the flexibility of operation of a combined n-dodecane reformer with a SOFC stack. The study was carried out on a micro-scale to get insights useful to scale up the integrated system. Moreover, a study of ageing effects for a conventional SOFC cell fed with n-dodecane reformate is carried out using ac-impedance spectroscopy. This approach is different than those previously reported in the literature since it is dealing with the investigation of coupling a conventional SOFC cell with an efficient n-dodecane reformer and not using alternative strategies for the SOFC anode. This study can be useful for benchmarking SOFC devices utilising diesel fuel.



1. Scientific Approach Preliminary experiments have been carried out in a microscale plant consisting of a microscale reforming unit coupled with a button SOFC of 2.5 cm2. The reforming unit consisted of a quartz tube placed into a furnace containing 0.42 g of Rh-CeO2-ZrO2 (commercial catalyst) in pellets form (200–600 μm) diluted with quartz particles with same size and amount. Three K-type chromel-alumel thermocouples were placed in correspondence of the initial, median, and final section of the catalyst in order to evaluate the gradient of temperature across the entire catalytic zone. The catalyst was activate by using hydrogen (50 vol.% H2 in N2) at 200 °C and then the reactor was purged in N2 flow and heated to the reaction temperature (800 °C). The steam reforming reactions were carried out at a total gas hour spatial velocity (GHSV) of 16000h-1, whereas the reactants were balanced by adding N2 as well maintaining a total flow of 140 cc min-1. S/C ratios ranging from 3 to 1 (at regular intervals of 0.5) were used in order to investigate the feasibility of the entire microscale system. Management of liquids (dodecane and water) consisted of two chromatography pumps. The pipelines were pre-heated at 450 °C whereas the gasified reagents were injected into the reactor by using N2 carrier. A condenser operating at room temperature was used to remove the excess of water from the outlet gas. Then wet gas (3 wt. %) was fed to the button SOFC. An Agilent 6890 Plus gas chromatograph equipped with thermal conductivity (TCD), flame ionization detectors (FID), and a mass spectrometer intercepted the pipeline and analysed the gas composition every 1 h. A button solid oxide fuel cell with a composition Ni-YSZ/YSZ/YDC/LSFC was used for this test. A CO2 laser was used to obtain the button cell (2.5 cm2) from a planar large area commercial-type cell (81 cm2). The cell was mounted in an alumina tube by using ceramic glass (AREMCO 516) as sealant. The cell was treated in air up to 800 °C and then the anode was reduced under a progressive reducing environment (from 10 vol. % H2 in He to pure H2 in 10 h). Instead, the cathode was maintained in static air during the entire test. The temperature of the cell was monitored by a thermocouple closeness to the anode shielded by an closed alumina tube in order to avoid any catalytic effect on the fuel) and by an infrared camera focused on the exposed cathode, which served also to check the integrity of the cell. Additional tests were carried out using a large area (81 cm2) solid oxide fuel cell of the same type. This was tested in a test rig for large area SOFCs. The SOFC data were collected by feeding simulated syngas with a composition very close to the reformate (but without N2 carrier). The flow rate was approximately 2.3 times the stoichiometry value at the maximum current. Based on the previous experiments carried out at short size level, an integrated system consisting of a reformer for the treatment of 800 Wh of dodecane and a stack for 500 Wel were assembled together and investigated in order to asses a proof-of-concept for this technology.

2. Results The button cell coupled with the reformer was investigated by a potentiostatic procedure in presence of different fuel streams. During the fuel transitions, diagnostics consisting in I-V polarization curves and EIS analysis at open circuit voltage (OCV), and 0.8 V were carried out. Fig. 1a shows the current density achieved at 0.8 V for more than 300 h. It consisted of the tests made day-by-day and carried out in presence of H2 during the initial period of analysis, diluted H2 during the week-end (for safety reasons), and feed coming from the reformer unit which operated under steam reforming of n-dodecane by changing S/C ratios in the range between 3 and 1.



Figure 1. a) Time-study of the current density for a solid oxide button cell operating at 0.8

V, at 800 °C, and fed at the anode with a variable composition of the fuel. b) Products composition during the n-dodecane SR performed over Rh-CeO2-ZrO2 commercial

catalyst. As expected, this experiment demonstrated that the cell may achieve performances directly related to the composition of the fuel up to S/C=1.5 used for the reforming of n-dodecane. At S/C lower than 1.5 a fast deactivation of the reforming catalyst caused the formation of high volume percentage of secondary products including methane, ethane, and ethane. In addition, about 0.5 vol % of n-dodecane unconverted was directly fed to the SOFC. As consequence, the cell suffered of a dramatic loss of performance. This decay was irreversible since after the introduction of diluted H2, the cell did not recovery its previous performance. In a subsequent experiment, an endurance test has been carried



out for more than 300 h at 800 mV by feeding the reformate produced at S/C=2.5. A specific protocol for the start-up, shut-down and gas transition was adopted. During the start-up and conditioning, the cell was fed with inert gas, diluted or pure H2 was fed in order to promote the complete reduction of the supporting anode. Following this protocol, the complete reduction of the anode required about 6 h and during this period the cell potential was recorded. Immediately after the switching of the cell under operation condition (800 mV), the current decreased and reached a steady-state performance approximately after 16 h achieving a current density of about 480 mA cm-2. The cell was thus fed with the syngas mixture. The total flow for the button cell was 150 cc min-1 including 30 vol.% of N2 as carrier. The gas composition determined for the outlet stream of the SR reactor is reported on a dry basis. The reforming catalyst has shown constant activity over about 300 h of reaction at the selected operating condition, with good total n-dodecane conversion and high H2 concentration (≈50%). This means that the reformate composition fed to the SOFC equipment can be considered almost constant. Moreover, byproducts as CH4, C2H4, C2H6 have not been revealed, although their presence in traces below the detection limit, can not be excluded. Under reformate gas feed to the button cell, it was achieved a stable current of about 320 mA cm-2. A large flow rate significantly higher than the reaction stoichiometry was used for the button cell. This does not represent a real condition. However, an excess of fuel is used to test the anode resistance to carbon deposition. Another term of stress for the cell is represented by the polarization curves carried out up to short circuit. Test was completed after about three weeks showing a moderate decay of current ( 0.05 mA cm-2 h-1) that at the end of the test was approximately 300 mA cm-2. In order to get insights on the possible causes of degradation occurred in the SOFC, the series resistances values obtained from impedance spectra vs. time have been reported in the fig 2. A similar behaviour was also observed by reporting the polarization resistances (Rp, is difference between the low and high frequency resistances). The spent cell discharged under reducing conditions, was analysed by SEM and EDX carried out along the cross section. The high magnification of the outermost layers showed different morphologies for the anode, dual-layer electrolyte and cathode. The cathode was compact and may be responsible for diffusive constraints at high current density. A magnification on the internal part of anode, allowed to reveal Ni aggregates, in opposition to that observed for the fresh anode showing finer Ni particles. In general, the particle growth is responsible for a loss of electrocatalytic sites in agreement with the reduction of the triple-phase boundary extension. This particle growth is essentially related to the high temperature operation. An increase of resistance as consequence of the loss of electronic percolation is also possible. Therefore, this observation may cause the observed increase of resistance during the endurance test.



Figure 2. Time-study of the series resistance (Rs) for the solid oxide button cell

operating at 800 °C. In a different test, a large area cell (81 cm2 of active area), of the same type of the button cell has been investigated electrochemically in presence of a simulated reformate stream (CO2=9.99%; CO=18.73%; CH4=0.12%; H2=71.16%) according to the average composition of the gas fed to the button cell. The galvanostatic test (30 A) carried out at 800 °C is shown in Fig. 3. In this case, the test has been carried out without the use of N2 as carrier, without humidification of the fuel and with a total gas flow of 500cc min-1. The performance achieved was slightly higher (337 mW cm-2) than that achieved for the button cell. Such behaviour is mainly due to the absence of N2 dilution. This test was carried out with a fuel utilization of approximately 41.8 %. This test wa useful to validate the process on a practical scale and propaedeutic for stack development for this application.



Figure 3. Galvanostatic test carried out at 30 A on a large area SOFC.

On the basis of these previous experiments a prototype of coupled system based on SOFC fed with reformate n-dodecane was realized and tested. The stack consisted of 60 cells of 55 cm2 stacked. The pilot plant also include pre-heaters, gas and liquid flow meters, an electrical backup system and a diagnostic equipment. These hardware were designed to support the operation of stack up to 2 kWel. Therefore, such pilot plant may be used as a test bench for testing of stacks. The technical data of the pilot plant are summarized as following: - Size of hotbox stack: 814.5*532*490 mm - Size of entire pilot plant including system for electrical backup, mass flow meters, pre-heaters and diagnostic tools: 600*600*2200 mm - Nominal maximum power of stack fed with H2:700 Wel -Temperature of operation: 750 – 800 °C - Software for control and acquisition: LabVIEW - Supported on wheels and easily transportable even on boats. The electrochemical tests were conducted at 700 °C in hydrogen reaching performance of 1 kW and then by feeding n-dodecane reformate produced at S/C=2.5 reaching a maximum performance of about 600 W (Fig. 4).



Figure 4. I-V curves of SOFC stach fed with H2 and n-dodecane reformate.

Altogether, the maximum performance achieved in n-dodecane reformate is referred to a voltage efficiency of 66%. Therefore, considering that the fuel converted in this experiment was approximately 80% of the fuel fed, this means that the overall efficiency recorded was approximately 50 %.

Acknowledgements

The present work was carried out within an Agreement between the Italian Ministry of Economic Development (MSE) and National Research Council (CNR) in the framework of a Research Program for the Electric System (sub-activity: Development of materials and components, design, demonstration and optimization of FC systems for co-generative applications). The authors also acknowledge the Italian Ministry of Education, Universities and Research (MIUR) for the grant agreement PON2_00153_2939517 for the project entitled TESEO

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[5] R.M. Ormerod, Solid oxide fuel cells, Chemical Society Reviews, 32 (2003) 17-28. [6] S.H. Chan, C.F. Low, O.L. Ding, Energy and exergy analysis of simple solid-oxide

fuel-cell power systems, Journal of Power Sources, 103 (2002) 188-200. [7] K.W. Bedringås, I.S. Ertesvåg, S. Byggstøyl, B.F. Magnussen, Exergy analysis of

solid-oxide fuel-cell (SOFC) systems, Energy, 22 (1997) 403-412. [8] M. Lo Faro, P. Frontera, P. Antonucci, A.S. Aricò, Ni–Cu based catalysts prepared by

two different methods and their catalytic activity toward the ATR of methane, Chemical Engineering Research and Design, 93 (2015) 269-277.

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A1204

Performance Characteristics of Elcogen Solid Oxide

Fuel Cell Stacks

Matti Noponen, Jukka Göös, Pauli Torri, Daniel Chade, Heikki Vähä-Piikkiö, Paul Hallanoro Elcogen Oy

01510 Vantaa, Finland Tel.: +358-40-732-9696

[email protected]

Abstract

Elcogen E1000 and E3000 stacks were characterized according to IEC 62282-7-2 for their electrochemical performance including rated power tests, current-voltage characteristics tests, effective fuel utilization dependency tests, long term durability tests, and internal reforming performance tests. Stacks show similar performance characteristics at equivalent testing conditions indicating repeatable quality of the stack design, components, assembly and conditioning process. Stack durability has been tested with reformate gas over 7300 h. Elcogen stacks enable high system efficiencies as stack gross efficiency was measured to reach 72 %-LHV with below 5 mbar pressure drops both for fuel and air side at 650 °C.



Introduction Elcogen is a private company focusing on commercialization of solid oxide fuel cell (SOFC) and electrolysis (SOE) technology. Elcogen manufactures both unit cells and stacks. Elcogen solid oxide fuel cell unit cells and stacks provide excellent performance characteristics at reduced operation temperatures between 600 – 700 °C. Elcogen stacks combine unique features of Elcogen’s well-proven unit cell technology together with innovative sealing, contact and flow distribution solutions combined with cost optimized design for mass manufacturing. Through modular stack design, Elcogen provides stack solutions from micro-CHP to commercial stationary applications. Elcogen E1000 with closed air manifold structure is optimized for 0.7 – 1.5 kWe electricity output, and E3000 with open air manifold structure from 1.5 – 3 kWe as a single stack setup up-to hundreds of kWe as multiple stack assemblies.

1. Elcogen stack specifications

Figure 3. (left) Elcogen E1000. (right) Elcogen E3000

Table 1. General specifications for Elcogen stacks

Stack specifications E1000 E3000

Rated power (DC, beginning of life) 1000 W 3000 W

Number of unit cells 39 pcs 119 pcs

Nominal current 30 A 30 A

Maximum temperature 700 °C 700 °C

Inlet temperature for air 600 ± 20 °C 600 ± 20 °C

Maximum working pressure 50 mbar(g) 50 mbar(g)

Air manifold system closed open



2. Experiments This paper summarizes performance characterization of Elcogen stacks. The stacks are characterized according to IEC 62282-7-2 for their electrochemical performance including rated power tests, current-voltage characteristics tests, effective fuel utilization dependency tests, long term durability tests, and internal reforming performance tests. All characterization tests conducted with hydrogen-nitrogen mixtures are made by Elcogen Oy, characterization tests conducted for E1000 under reformate conditions by VTT Technical Research Centre of Finland Ltd, and reformate tests for short stack with 15 unit

layers (E350) by Clausthal Institute of Environmental Technology (CUTEC). All test systems are in general equal; they are composed of the anode gas supply, cathode gas supply, anode gas exhaust, and cathode gas exhaust submodules, the temperature measuring points, current lead points, voltage measuring points, and mechanical load application points. The gas supply subsystems are equipped with mass flow controllers and pressure gauge measurements, the gas exhaust subsystems are equipped with pressure gauge measurements, and temperatures are measured K-type thermocouples located in center line of the piping systems. Stack inlet and outlet temperatures are measured from approx. 5 mm from the stack bottom in which the gas inlet and outlet ports are located in the case of closed manifold stack designs, E350 and E1000. Air outlet temperature is measured with K-type thermocouple from the center of Elcogen E3000 from 10 mm from the stack outlet surface. Stack current is measured with the load unit. Stack voltage is measured with differential analog input measurement terminals and summed over all unit layers. Stack compression is implemented with pneumatic cylinders. All measurements are conducted in furnace environment. Measurement accuracy of different measurement equipment used in Elcogen’s test system are given in Table 2.

Table 2. Instrument accuracies of Elcogen’s test system

Measured Measurement region Accuracy

Flow, H2 and N2 2 to 100 Nl/min ±0.8% R, ±0.3% FS*

Flow, air 10 to 500 Nl/min ±1.5% R, ±0.3% FS*

Pressure, all fluids -5 to 25 mbar ±0.5% FS*

Temperature, all fluids -200 to 1250 °C max(±2.2°C; ±0.75% R*)

Voltage -10 to 10 V ±1 mV

Current 0 to 50 A ±0.1 A (*R – sensor reading, FS – sensor full scale value)

Electrochemical performance and rated power are recorded after the stack has reached the stable state, voltage, current, and other control and output parameters are measured repeatedly. Current-voltage characteristics tests and internal reforming performance tests are conducted galvanostatically at 1 A/min sweep rate from nominal current to open circuit conditions. Effective fuel utilization dependency tests, effective oxygen utilization dependency tests and long term durability tests are conducted galvanostatically at 30 A. All tests are carried out with constant flow rates and constant temperatures for furnace and inlet gases. In addition to the standard tests, pressure drop was recorded both for the fuel and air sides simultaneously with fuel utilization and oxygen utilization tests.


Cell design and characterisation ......................... Chapter 03 - Sessions A09, A12 - 100/122 Stack design and characterisation

3. Results Table 3 depicts the electrochemical performance and rated power test results for Elcogen E1000 and Elcogen E3000 stacks. I is current (A), U is stack voltage (V), P is stack power (W), Vfuel is volumetric flow rate of fuel delivered to stack given at NTP conditions (lN/min), Vair is volumetric flow rate of air delivered to stack given at NTP conditions (lN/min), xfuel is molar fraction of hydrogen in fuel rest being nitrogen (-), Tfurnace is furnace temperature (°C), Tfuel is fuel temperature at stack inlet (°C), and Tair is air temperature at stack inlet (°C).

Table 3. Electrochemical performance and rated power of Elcogen E1000 and E3000

Stack type

I (A)

U (V)

P (W)

Vfuel (lN/min)

xfuel (-)

Vair (lN/min)

Tfurnace (°C)

Tfuel (°C)

Tair (°C)

E1000 30.0 35.0 1050 40.1 0.50 85.2 606 589 586

E3000 30.0 107 3200 124 0.50 324 616* 593 592

*Furnace temperature set point 600°C, no power fed to furnace resistors as the heat from stack reactions increasing the internal furnace temperature. Current-voltage characteristics test results are depicted in Figure 2 for Elcogen E1000 (left) and Elcogen E3000 (right). Black line is stack voltage, blue line is stack power, red dotted line is measured air inlet temperature, and red dashed line is measured air outlet temperature as a function of current drawn from the stack. The open circuit voltages were 46.5 V and 142 V respectively for Elcogen E1000 and Elcogen E3000. Fuel flow, air flow, fuel composition, and fuel flow temperature were the same as given in Table 3. Figure 3 (left) depicts Elcogen E1000 (white dashed line) and Elcogen E3000 stack (black squares) current-average unit layer voltage characteristics. The relative difference in unit layer voltages is below ±10 mV at comparable conditions between the stack designs as shown in Figure 3 (right).

Figure 4. Current-voltage characteristics for Elcogen E1000 (left) and Elcogen E3000 (right).

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Figure 5. (Left) current-average unit layer voltage characteristics for Elcogen E1000 and Elcogen E3000. (Right) Difference of average unit layer voltages as function of current

between Elcogen E1000 and Elcogen E3000.

Effective fuel utilization dependency test result (left) and effective oxygen utilization dependency test result (right) for Elcogen E1000 is given in Figure 4 and for Elcogen E3000 in Figure 5. Tests were conducted at conditions given in Table 3 expect changing either fuel flow rate (effective fuel utilization dependency test) or air flow rate (effective oxygen utilization dependency test). Black line in the figure is stack voltage, red dotted line is measured air inlet temperature, and red dashed line is measured air outlet temperature as a function of current drawn from the stack. Elcogen E1000 was characterized up to 85% fuel utilization level and Elcogen E3000 up to 80 % fuel utilization level. The average unit layer voltage for Elcogen E1000 at 80 % fuel utilization was 0.859 V and for Elcogen E3000 at same conditions 0.851 V. Both stack types were verified to have constant voltage response as function of oxygen utilization at between 20 % to above 35 % oxygen utilization levels.

Figure 6. Effective fuel utilization dependency (left) and effective oxygen utilization dependency (right) for Elcogen E1000.

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Figure 7. Effective fuel utilization dependency (left) and effective oxygen utilization dependency (right) for Elcogen E3000.

Pressure drop dependency test results for fuel side (left) and air side (right) are depicted for Elcogen E1000 in Figure 6 and for Elcogen E3000 in Figure 7. Test was conducted simultaneously with effective fuel utilization dependency tests and effective oxygen utilization dependency tests by measuring gas pressures at the stack inlet and stack outlet. Black squares in the figure are measured pressure differences, black dash line is the least square fit to the pressure drop data, red dotted line is measured air inlet temperature, and red dashed line is measured air outlet temperature as a function of current drawn from the stack.

Figure 8. Pressure drop dependency of fuel side (left) and air side (right) for Elcogen E1000.

Figure 9. Pressure drop dependency of fuel side (left) and air side (right) for Elcogen E3000.

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Table 4 and Figure 8 (right) depicts the internal reforming test results for Elcogen E1000 at rated power. Test system includes pre-reformer that is used to control gas composition at fuel inlet. Reformer outlet temperature was set to 600 °C resulting in stack inlet composition of x[CH4] = 7 %, x[CO] = 7 %, x[H2] = 53 %, x[CO2] = 8 %, x[H2O] = 25 %. Reformer validation is shown by Tallgren et al. [1]. Stack power increase rate in the test is depicted in Figure 8 (left). Rated stack power (1000 W) was obtained 30-31 A. In Table 4, I is current (A), U is stack voltage (V), P is stack power (W), VNG is volumetric flow rate of natural gas at test system inlet given at NTP conditions (lN/min), Vsteam is volumetric flow rate of steam at test system inlet given at NTP conditions (lN/min), Vair is volumetric flow rate of air delivered to stack given at NTP conditions (lN/min), Treformer is adiabatic reformer outlet temperature (°C), Tfurnace is furnace temperature (°C), Tfuel is fuel temperature at stack inlet (°C), and Tair is air temperature at stack inlet (°C). Black line in Figure 8 is stack voltage, blue line is stack power, red dotted line is measured air inlet temperature, and red dashed line is measured air outlet temperature as a function of measurement time (left) and current drawn from the stack (right). Table 4. Electrochemical performance and rated power of Elcogen E1000 with reformate

I (A)

U (V)

P (W)

VNG (lN/min)

Vsteam

(lN/min) Vair (lN/min)

Treformer (°C)

Tfurnace (°C)

Tfuel (°C)

Tair (°C)

30.1 32.90 990 3.41 7.68 86.0 600 595 577 594

31.1 32.87 1024 3.41 7.68 86.1 600 595 577 594

Figure 10. Time-voltage characteristics with reformate for Elcogen E1000 (left) and current-voltage characteristics for Elcogen E1000 (right).

Long term durability test result is depicted in Figure 9 over 7300 hours for Elcogen E1000 at conditions given in Table 4. Test period included two full thermocycles at 1100 hours and 5550 hours due to test system scheduled maintenance and one emergency shutdown at 3300 hours during which current was shutdown immediately and 5% H2 in N2 was fed to the stack anode and air to the cathode. Temperature was decreased down to 500 °C during the emergency shutdown. Exception from conditions given in Table 4 is the air flow rate that was increased after 5317 hours from 86.0 lN/min to 90 lN/min in order to ensure that stack outlet temperature was not increasing above 700 °C. The average unit layer voltage decay over the test period has been 0.01 V in 1000 hours. The percentage voltage decay against initial voltage given in Table 4 was 1.5 % in 1000 hours. The long term durability is still ongoing (May, 2016).

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Elcogen E1000 stack with reduced unit layers (15, E350) was further tested by CUTEC with variety of simulated natural gas fuel feeds. Full description of this hardware-in-a-loop tests is given by Immisch et al in [2]. The gas composition matrix was predefined for three different system layouts and these gas compositions were mixed with mass flow controllers. Here data is shown for a system utilizing anode exhaust gas recirculation in which the reformer outlet temperature is 700 °C and gas composition is calculated at 30 A current and recycle ratio of 70 %, i.e. stack inlet flow rate for fuel side was V(H2) = 3.45 lN/min, V(CO) = 0.62 lN/min, V(CH4) = 0.05 lN/min, V(H2O) = 1.29 lN/min, V(CO2) = 0.4 lN/min. The system inlet gas composition equals to 0.218 lN/min natural gas flow. Air side stack inlet flow rate was 32.87 lN/min. Stack was located in a furnace with set point temperature of 650 °C. Figure 10 (left) depicts current-voltage characteristics and stack fuel utilization-voltage characteristics (black line) with measured inlet (red dotted line) and outlet (red dashed line) air temperatures, and stack power (blue line). Figure 10 (right) depicts the current-gross efficiency (blue line), determined as stack power divided by the natural gas input flow and enthalpy to system. Figure 10 (right) also depicts average unit layer voltage and computational recycle ratio in shown in red line. System fuel utilization is shown in the other x-axis. Stack was measured to reach at 30 A and at 650 °C gross efficiency of 72 %-LHV.

Figure 12. Internal reforming test with gross efficiency test of Elcogen E350 (E1000 design with 15 unit layers)

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Conclusions Elcogen E1000 and E3000 stacks show similar performance characteristics at equivalent testing conditions indicating robust design with repeatable quality of the components, assembly and conditioning process. Stack durability has been tested with reformate gas over 7300 h including two thermocycles and one emergency shutdown indicating good robustness of the stack design and material combinations. Elcogen stacks enable high system efficiencies at reduced temperature level as the stack gross efficiency was verified to reach 72 %-LHV at below 5 mbar pressure drops both for the anode and cathode sides.

Acknowledgements Financial support from FCH JU (FP7) co-funded project “New all-European high-performance stack: design for mass production” (NELLHI), Grant agreement no: 621227, FCH JU (H2020-EU) funded project “Development of innovative 50 kW SOFC system and related value chain” (INNO-SOFC), Grant agreement no: 671403, EU (FP7-people) co-funded project “High efficiency low temperature SOFC stack” (HELTSTACK), Grant agreement no: 612431, and Tekes – the Finnish Funding Agency for Innovation are gratefully acknowledged. Project partners are acknowledged and especially Olli Himanen and Johan Tallgren from VTT Technical Research Centre of Finland Ltd and Andreas Lindermeir and Christoph Immisch from Clausthal Institute of Environmental Technology for conducting the reformate testing for stacks.

References [1] J. Tallgren, O. Thomann, M. Halinen, O. Himanen, J. Kiviaho, “Development of a fuel feeder for a solid oxide fuel cell test station”, Int. J. Energy Research, 39 (15), 2015 pp. 2031–2041. [2] C. Immisch, A. Lindermeir, M. Noponen, J. Göös, “New all-European high-performance stack (NELLHI): Experimental evaluation of an 1 kW SOFC stack”, 12th European SOFC & SOE Forum 2016, Switzerland 2016, A0909.




Performance and degradation of an SOEC stack with different air electrodes

Y. Yan (1), Q. Fang (1), L. Blum (1), W. Lehnert (1, 2) (1) Forschungszentrum Jülich GmbH

Institute of Energy and Climate Research Wilhelm-Johnen-Straße 52425 Jülich/ Germany

(2) RWTH Aachen University, Modeling in Electrochemical Process Engineering 52072 Aachen/ Germany

Tel.: +49 2461 61-5487 Fax: +49-2461-616695

[email protected]

Abstract High temperature water electrolysis with Solid Oxide Electrolysis Cell (SOEC) is a promising method for hydrogen production. In order to study the performance and degradation behavior of different air electrodes under electrolysis mode, a 4-cell stack was assembled in JÜLICH’s F10-design with two types of air electrodes based on La0.6Sr0.4CoO3–δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). The performance of the stack was characterized in both SOFC and SOEC modes in the temperature range of 700~800 °C. The durability of the stack was first investigated by conducting a long-term stationary operation with a constant current density of -0.5 Acm-2 and steam conversion rate of 50% at 800 °C. Electrochemical Impedance Spectroscopy (EIS) was utilized in the study of the electrochemical performance of the stack, as well as the degradation behavior during the long-term electrolysis operation. To improve the quality and reliability of the equivalent circuit fitting, the Distribution of Relaxation Times (DRT) analysis was applied. Remark: Only the abstract is available, because the authors chose to publish elsewhere.






A1206

Fuel Distributions in Anode-Supported

Honeycomb Solid Oxide Fuel Cells

Hironori Nakajima(1), Tatsumi Kitahara (1), Sou Ikeda (2) (1) Department of Mechanical Engineering, Faculty of Engineering, Kyushu University

(2) Department of Hydrogen Energy Systems, Graduate School of Engineering, Kyushu University

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Tel.: +81-92-802-3161 Fax: +81-92-802-3161

[email protected]

Abstract

An anode-supported honeycomb solid oxide fuel cell can achieve high volumetric power density and improve thermo-mechanical durability at high temperatures. We have so far fabricated the honeycomb cell with conventional materials for a cathode layer (LSM) and an electrolyte layer (8YSZ) on a porous anode honeycomb substrate of Ni/8YSZ. The anode-supported honeycomb cell exhibited promising volumetric power densities [1]. Effect of flow channel configurations on the cell performance was investigated in terms of the hydrogen partial pressure distributions in the cell under operation as well [1]. In this study, we compare the differences of measured current-voltage and current-power density curves among the honeycomb cells having different porous substrate thicknesses shown in and different flow channel configurations under different inlet hydrogen flow rates. Hydrogen partial pressure distributions associated with the anode-substrate thickness and the flow channel configuration on the cell performance possibly give the performance differences.




Introduction Solid Oxide Fuel Cells (SOFC) have advantages such as power generation with high efficiency and fuel flexibility by its high operating temperature. Moreover, high operating temperatures enable SOFCs to work with base metal electrocatalysts. To date, planar and tubular type cells have been developed for SOFC systems. However, both types still have problems to be resolved. For example, planar type SOFCs need to be developed the thermal mechanical stress and the flow channel design, while tubular types are required to improve volumetric power density. Thus, improvement of durability at high temperature and volumetric power density of SOFC systems are necessary. Honeycomb SOFCs exhibit respectable durability and volumetric power density at high temperatures, and short time start-up [1-7]. However there have been few researches on anode-supported honeycomb SOFC. The anode-supported cell concept can decrease the ohmic loss by thin electrolyte layer and anode overpotential by increase in the active anode surface area [8]. The aim of the present study is testing the performance of the anode-supported honeycomb cells fabricated in-house having different anode and cathode flow channel configurations under different hydrogen flow rates to evaluate hydrogen mole fraction distributions in the honeycomb cell.

1. Experiments An NiO/8YSZ (8 mol% yittria stabilized zirconia, 65/35 wt%, 37% porosity after reduction) honeycomb porous substrate with 3×3 flow channels (Repton Co. Ltd.) was used as the anode. The substrate had 6 mm square channels with a wall thickness of 0.5 mm or 1.0 mm. The cell height is 18 mm. This anode substrate was coated with 8YSZ electrolyte slurry by dip-coating, being followed by co-firing at 1420 °C for 2 hours. Figure 1 shows those honeycomb cells after firing. The cathode slurry was a mixture of La0.7Sr0.3MnO3 (LSM) and 8YSZ with a weight ratio of 10/3 [9]. The LSM-YSZ composite cathode slurry was brush-coated on the electrolyte layer and fired at 1150 °C for 2 hours. Different flow channel configurations were achieved with special masking methods. Ag paste was employed between the cathode surface and the edge face as current collector. Both edge faces are electrically connected with Ag lead wires [1, 7].

Figure 1. Anode-supported honeycomb SOFCs with different thickness of the porous anode substrates (a) 0.5 mm and (b) 1.0 mm. We prepared four various honeycomb cells having two types of flow channel configuration as illustrated in Figure 2 with different porous anode substrate of 0.5 mm and 1.0 mm shown in Figure 1. In the Type-B cell, five channels are anodes, while the other four are



cathodes. Since H2 diffuses inside the porous anode substrate, we regard the whole cathode surface as electrochemical active area. Figure 2. Illustrations of the (a) Type-A and (b) Type-B honeycomb SOFCs. A and C represent anode and cathode channels, respectively. Temperature of the honeycomb cell was maintained at 850°C by an electric furnace at open circuit voltage (OCV). Mass flow controllers (SEC-E40MK3, Horiba STEC) were controlled by LabView 8.5 (National Instruments Inc.) on a personal computer through an I/O device (NI USB-6008, National Instruments Inc.). Anode and cathode were supplied with H2/N2 mixture gas and air, respectively. The NiO anode was reduced to Ni by feeding H2/N2 mixture gas for two hours prior to measurements. During measurements, anode and cathode were fed in co-flow configuration with mixtures of H2/N2 and dried air at constant flow rates, respectively. In the experiment of Type-A cell, gas flow rates for the anode were H2/N2: 40/40, 60/60, 80/80 cm3/min at 25°C, while that for the cathode was air: 800 cm3/min in total, respectively. In the experiment of Type-B cell, gas flow rates for the anode were H2/N2: 40/40, 60/60, 80/80, 100/100, and 200/200 cm3/min in total, while that for the cathode was air: 400 cm3/min in total, respectively. Current voltage (I-V) characteristics of the cells were measured with an electric load (PLZ164WA, Kikusui Electronics Corp.). Ohmic loss was evaluated by electrochemical impedance spectroscopy and current interrupt method.

3. Results Figures 3 and 4 depict the I-V characteristics of the four types of the honeycomb cells. Dependence of the I-V characteristics on the gas flow rate is small for the Type-A05 cell as presented in Figure 3. The thicker porous anode substrate gives the lower power densities possibly due to more hydrogen diffusing in the porous anode substrate in the upstream, and thus hydrogen mole fraction is smaller in the downstream. Gas flow rates, gas utilization at the maximum power, and maximum volumetric power density derived from the I-V characteristics are listed in Tables 1 and 2. The maximum power densities for the Type-A05 and A10 cells under different gas flow rates were 0.17, 0.19, 0.19 W/cm3 at the hydrogen utilizations of 49, 32, 26% and 0.09, 0.12, 0.13 W/cm3 at 26, 16, 15%, respectively.

(a) (b)



Figure 3. I-V characteristics of the (a) Type-A05 and (b) Type-A10 honeycomb cells at 850 °C, H2/N2: 40/40, 60/60, and 80/80 cm3/min, Air: 800 cm3/min in total.

Table 1. Gas Flow Rates and Total Fuel Utilizations for the Type-A05 Cell at the Maximum Power.

Flow rate (cm3/min) [Utilization at the maximum power]

Maximum volumetric power density (W/cm3)

H240 [49%], N240, Air800[6%] 0.17 H260 [32%], N260, Air800[6%] 0.19 H280 [26%], N280, Air800[6%] 0.19

Table 2. Gas Flow Rates and Total Fuel Utilizations for the Type-A10 Cell at the Maximum Power.






On the other hand, in the case of Type-B, the cell voltage in Figure 4 drastically drops as the fuel utilization increases owing to higher concentration overpotential. The maximum power densities for the Type-B05 and B10 cell were 0.03, 0.12, 0.18, 0.21, 0.29 W/cm3 at 2, 7, 13, 13, 17% and 0.04, 0.11, 0.11, 0.11, 0.25 W/cm3 at 2, 7, 9, 9, 15%, respectively, from the I-V characteristics in Figure 4 (Tables 3 and 4).

These power densities are promising compared with the electrolyte-supported honeycomb SOFCs owing to the hydrogen utilizations [3, 4]. This is possibly ascribed to lower ohmic loss achieved by the thin electrolyte layer and 3-dimensional hydrogen transport in the porous anode-supported honeycomb cell.

Figure 4. I-V characteristics of the (a) Type-B05 and (b) Type-B10 honeycomb cells at 850°C, H2/N2: 40/40. 60/60, 80/80, 100/100, and 200/200 cm3/min, Air: 400 cm3/min in total.

Table 3. Gas Flow Rates and Total Fuel Utilizations for the Type-B05 Cell at the Maximum Power.



H240 [9%], N240, Air400[2%] 0.03 H260 [19%], N260, Air400[7%] 0.12




Table 4. Gas Flow Rates and Total Fuel Utilizations for the Type-B10 Cell at the Maximum Power.



H240 [9%], N240, Air400[2%] 0.04

H260 [19%], N260, Air400[7%] 0.11


In general, performance of the Type-A cell is higher than that of Type-B considering hydrogen utilization and gas flow rates. This is attributed to smaller hydrogen mole fraction in each flow channel in the downstream by the larger number of the anode flow channels in Type-B cell. Hence, the concentration overpotential becomes larger [1].

5. Conclusion Mass transport of fuel and performance change ascribed to the difference of the porous anode substrate thickness are investigated by using the cells having different porous anode substrate thickness. Moderate thinning the porous anode substrate shows that fuel transport in the downstream is maintained, thus the volumetric power density becomes higher, giving promising peak power compared with electrolyte-supported honeycomb cells. The anode and cathode flow channel configurations and the porous anode substrate thickness require optimization to minimize the hydrogen partial pressure distribution in the anode when we take into account the relation between the fuel utilization and power density.

Acknowledgments This research is partially supported by the Center of Innovation Program from Japan Science and Technology Agency, JST. We thank a graduate student, Shota Kotake (presently Kyushu Electric Power), for help in the experiments.

References [1] S. Kotake, H. Nakajima and T. Kitahara, Flow channel configurations of an anode-

supported honeycomb solid oxide fuel cell, ECS Transactions, 57 (1), (2013) 815-822.

[2] J. P. Ackerman and J. E. Young, U.S. Pat. No. 4476198, October 9 (1984). [3] H. Zhong, H. Matsumoto, A. Toriyama and, T. Ishihara, Honeycomb-type solid oxide

fuel cell using La0.9Sr0.1Ga0.8Mg0.2O3 electrolyte for high volumetric power density, Journal of the Electrochemical Society 156 (1), (2009) B74-B79.

[4] Z. Wang, S. Shimizu and Y. Yamazaki, Evaluation of extruded cathode honeycomb monolith-supported SOFC under rapid start-up operation, ASME Journal of Fuel Cell Science and Technology, 5, (2008) 031211.



[5] T. Yamaguchi, S. Shimizu, T. Suzuki, Y. Fujishiro and, M. Awano, Evaluation of extruded cathode honeycomb monolith-supported SOFC under rapid start-up operation, Electrochimica Acta, 54, (2009) 1478-1482.

[6] J. C. Ruiz-Morales, D. Marrero-López, J. Peña-Martínez, J. Canales-Vázquez, J. Josep Roa, M. Segarra, S. N. Savvin and P. Núñez, Performance of a novel type of electrolyte-supported solid oxide fuel cell with honeycomb structure, Journal of Power sources, 195, (2010) 516-521.

[7] A. Fukushima, H. Nakajima and T. Kitahara, Performance evaluation of an anode-supported honeycomb solid oxide fuel cell, ECS Transactions, 50 (48), (2012) 71-75.

[8] H. Nakajima, T. Kitahara and T. Konomi, Electrochemical impedance spectroscopy analysis of an anode-supported microtubular solid oxide fuel cell, Journal of the Electrochemical Society, 157 (11), (2010) B1686-B1692.

[9] J-D. Kim, G-D. Kim, J-W. Moon, Y-i. Park, W-H. Lee, K. Kobayashi, M. Nagai and C-E. Kim, Characterization of LSM–YSZ composite electrode by AC impedance spectroscopy, Solid State Ionics, 143, (2001) 379-389.




Potential for critically-high electrical efficiency of multi-stage SOFCs with proton-conducting solid electrolyte

Yoshio Matsuzaki (1,2), Yuya Tachikawa (3), Takaaki Somekawa (1,4), Kouki Sato (2), Hiroshige Matsumoto (5), Shunsuke Taniguchi (2,3,6),

Kazunari Sasaki (2,3,4,5,6) (1) Fundamental Technology Department, Tokyo Gas Co., Ltd.,

1-7-7 Suehiro-cho, Tsurumi-ku, Yokohama City, Kanagawa 230-0045, Japan (2) Next-generation Fuel Cell Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan

(3) Center for Co-Evolutional Social Systems (CESS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan

(4) Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan

(5) International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan

(6) International Research Center for Hydrogen Energy, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka City, Fukuoka 819-0395, Japan

Tel.: +81-806-5418-9431 Fax: +81-3-5604-8285

[email protected]

Abstract

Recently we developed and reported a conceptual design that has a potential to realize a critically-high fuel-to-electricity conversion efficiency of up to as high as 85% (LHV, gross DC), in which a high-temperature multi-stage electrochemical oxidation is combined with a proton-conducting solid oxide electrolyte. In the conceptual design a protonic transport number of a proton-conducting electrolyte was assumed to be unity. However, the protonic transport number of the proton-conducting solid oxide electrolyte depends on the material and operating conditions such as temperature, partial pressures of oxygen and steam, and so on, and would affect the electrical efficiency. In this study, the influence of the conductivities of oxide-ion as well as electron and hole in the proton-conducting solid electrolyte with multi-stage configuration on the electrical efficiency has been investigated. The existence of measurable conductions of electron and/or hole was found to cause leakage current resulting in obvious deterioration of the electrical efficiency. Remark: Only the abstract is available, because the authors chose to publish elsewhere.






A1209

Performance testing for a SOFC stack with bio-syngas

Ruey-Yi Lee (1)*, How-Ming Lee (1), Ching-Tsung Yu (1), Yung-Neng Cheng (1), Szu-Han Wu (1), Chien-Kuo Liu (1), Chun-Hsiu Wang (2), Chun-Da Chen (2)

(1) Institute of Nuclear Energy Research No. 1000 Wenhua Road,

Longtan District, Taoyuan City / Taiwan (R.O.C.) (2) China Steel Corporation No. 1 Chung-Kang Road,

Hsiao Kang District, Kaohsiung / Taiwan (R.O.C.) *Tel.: +886-3-471-1400 Ext. 6761

Fax: +886-3-471-3980 [email protected]

Abstract

The purpose of this study is to assess the adaptability of the bio-syngas as a fuel for a SOFC power system. The eucalyptus wood chips, provided by China Steel Corporation, were fed into a Plasma-Assisted Gasification System at INER to obtain the bio-syngas. Subsequently, it was passed through a set of cleanup processes to remove gaseous impurities and particulates, and then compressed and preserved in the storage tanks. Performance testing was conducted for 3-cell stacks fuelled with the cleaned bio-syngas. In the first run, the stack experienced fluctuations of open circuit voltage (OCV), a relatively high degradation rate as well as severe carbon deposition onto the catalysts of reformer. The situation was significantly improved for the 2nd run, while a series of deep cleanup processes were employed to reduce impurities of the bio-syngas to below ppm levels. The results indicate: (1) the bio-syngas is successfully produced through the plasma-assisted gasification system, the total concentration of hydrogen and carbon monoxide is higher than 50%, the lower heating value of the syngas is around 7~8 MJ/Nm3. (2) the concentration of hydrogen sulfide is below 1.0 ppm after the deep cleanup processes, while the total concentration of sulfur, phosphorus, and chlorine is below 0.01 ppm, 0.01 ppm, and 0.30 ppm, respectively. (3) OCV of the 3-cell stack is 2.89 V, power output 77 W (power density 317 mW/cm2, @ 32 A, 750 oC), and an overall degradation is around 0.6 % for a test period of 103 hours. (4) it is experimentally proved that SOFC can be fuelled with well purified bio-syngas.




1. Introduction A SOFC is a power generation device that converts the chemical energy of a fuel into electricity through electrochemical reactions. Versatile types of hydro-carbon rich materials, such as hydrogen, ammonia, carbon monoxide, methane, syngas as well as biogas can be utilized as fuels for a SOFC. The biomass is contemporarily regarded as a sustainable energy source as its production and consumption can be appropriately coordinated [1]. The integration of a SOFC system with biomass would be well suited as the fuel flexibility, high electric efficiency and operating regimes are taken into account [2]. The employment of biomass into a SOFC system, however, the solid biomass is needed to be converted into gaseous form. As the biomass sources are diverse, different energy conversion pathways, such as: thermal-chemical conversion, bio-chemical conversion and mechanical extraction, can be utilized to comply with different applications [3]. Of which, biomass gasification is considered as a viable technology of thermal-chemical conversion process to produce bio-syngas [4]. The compositions of the bio-syngas are mainly composed of hydrogen, carbon monoxide, carbon dioxide, water, methane, nitrogen and trace species such as particulates, tars, alkali, sulfur, phosphorus, and chlorine, hydrogen sulfide (H2S), and carbonyl sulfide (COS), etc. While those trace species are usually harmful to the downstream equipments and detrimental to the SOFC stack, cleanup processes are of a necessity to reduce the amount of those undesirable containments to some acceptable levels [5]. Investigations indicated that those harmful trace containments and impurities in the bio-syngas poison the catalysts and degrade the performance of SOFC stacks [6~8]. A stringent fuel quality is required for a SOFC fuelled with bio-syngas. The institute of Nuclear Energy Research (INER) has committed to developing the SOFC technology and established the technology from powder to power for a SOFC power system [9,10]. Meanwhile, INER has also developed the plasma torch technologies for two decades and set up a 500-kWth pilot-scale plasma-assisted gasification plant [11,12]. Additionally, the syngas cleanup technology is being demonstrated, which includes sour contaminants, mercury, and CO2 capture in warm gas cleanup system [13-15]. China Steel Corporation (CSC) is one of Taiwan’s leading enterprises, which pursues growth, environmental protection, energy saving and value innovation. As the biomass gasification integrated with a high efficiency SOFC system is granted as a promising candidate of clean energy production, a joint project between INER and CSC, in the period of 1st February to 31st August 2015, was conducted to evaluate the adaptability of the bio-syngas as a fuel for a SOFC power system. In the study, the eucalyptus wood chips, provided by China Steel Corporation, were fed into the Plasma-Assisted Gasification System at INER to obtain the bio-syngas. After a sequence of cleanup processes to remove gaseous impurities and particulates, performance testing was carried out for 3-cell stacks with primary and cleaned bio-syngas.

2. Experimental

In this study, two test runs for SOFC stacks fuelled with the bio-syngas were carried out. Three batches of fragmented eucalyptus wood chips (< 5mm), with a feeding rate of 20~100 kg/h, were fed into the fixed-bed plasma-assisted gasifier to produce the primary bio-syngas. The main elements of eucalyptus wood chips, provided by the CSC, were carbon, hydrogen, oxygen, and nitrogen with its weight percentage of 50.77%, 5.13%, 44.07%, and 0.13%, respectively. During the gasification process, the power of plasma was around 35~45 kW. The oxygen generator (AirSep Model: ASL-1200) provided the oxygen at a flow rate of 100 slpm and concentration of 85~93% as oxidant gas. The temperature of bio-syngas at the outlet of the gasifier was around 300 oC. The bio-syngas



went through a series of cleanup processes to remove impurities, and then served as the fuel for a SOFC stack. A brief flowchart is illustrated in Figure 1. Some sampling points for primary, before and after deep-cleanup of bio-syngas are illustrated in the figure.

The on-line direct reading instrument (ADC MGA3000) and gas chromatography (GC 2010 plus) were employed to analyze the main compositions (CO, CO2, H2, O2, CH4) and concentrations of primary bio-syngas. The remainder of the gas was supposed to be nitrogen and some other impurities. The major impurities included tar, particulates, sulfur-, chloride-, and nitrogen-containing compounds, as well as metals, etc. The main compositions and concentrations of gasified bio-syngas in the three batches of gasification of eucalyptus wood chips are illustrated in Table 1. As the operational parameters were not optimized, variations of concentrations for different species in the gasification were noticed. The heating values of bio-syngas were calculated on the basis of the compositions and fractions of gasified bio-syngas. As mentioned in the previous section the fuel quality is essential to the SOFC system, a series of cleanup processes were set up to purify the bio-syngas. The venturi and packed bed scrubbers were utilized to remove particulates, tars, alkali metals, and water soluble elements. Refine filters (< 0.01 µm) were employed to remove the fine particulates. The bio-syngas was then compressed into a storage tank with a pressure of 7~60 bar. Condensation process (< 5 oC) below the dew-points of tars was used to reduce the content of tars. Activated carbons were utilized to adsorb tars, heavy metals, sulfur compounds, nitrides and other impurities. Zeolites, copper-base, or zinc oxide adsorbents were used to reduce the content of sulfur in the bio-syngas. In this investigation, the concentration limits of H2S, HCl, halogens, and heavy metals were set to below 1 ppm for a SOFC test with bio-syngas.

Table 1 Main compositions and concentrations of gasified bio-syngas.

Test

Run

(%)

H2

(%)

CO

(%)

CO2

(%)

CH4

(%)

O2

(%)

N2

(ppm)

H2S*

(MJ/Nm3)

LHV HHV

TR1 17.67 28.95 24.97 4.47 0.64 23.30 7.2 7.8

TR2A 16.24 29.35 14.77 3.51 0.55 35.59 6.7 7.2

TR2B 17.84 31.72 18.02 4.23 3.22 24.97 7.4 8.0

*Concentration of H2S was below the detection limits (below 1.0 ppm) in the measurement.

Two test runs for two 3-cell stacks fuelled with primary bio-syngas and deep cleanup bio-syngas were carried out. The 3-cell stacks were assembled and tested with hydrogen as fuels to check the consistence of stack performance. After the validation tests with hydrogen, the natural gas and/or bio-syngas as fuels for the SOFC stack performance testing were executed. The home-made catalysts were installed on the plate-type reformer. The open circuit potential (OCV), stack voltage, current, and power were measured under specific test conditions. As the main purpose of this study is to assess the

Figure 1 Flowchart of gasification, cleanup of bio-syngas for a SOFC test.



adaptability of the bio-syngas as a fuel for a SOFC power system, the period for a performance test was set around 100 hours. In addition, the test could be terminated as the degradation of stack performance was over 10%. In the first test run (TR1), the stack was fuelled with primary bio-syngas. However, the test experienced substantial degradation and severe carbon deposition were found in the adjacent pipelines as well as surfaces of reformer catalysts. Thus, in the second test run (TR2), another 3-cell stack was assembled and tested and the pipelines were cleaned and renewed. Furthermore, deep bio-syngas cleanup processes were employed. As the amount of bio-syngas in the second batch (TR2A) of gasification was insufficient to fulfill the performance test for over 100 hours, another batch (TR2B) of gasification was performed.

3. Results and Discussion

Figure 2 depicts the current-voltage-power (I-V-P) curves for a 3-cell stack fuelled with hydrogen, methane and bio-syngas, respectively, in the test run TR1. As fuelled with hydrogen, the OCV was 3.6 V and power output reached to 90 watts at a current of 32 A. While the fuel was methane, the OCV was around 3.0V and its power output was 86 watts at a current of 32 A. As a result of lower heating value of bio-syngas, the power output was the lowest one among three types of fuels. A small amount of water was added as fuelled with methane or bio-syngas to assist the water-gas-shift reformation for hydro-carbon fuels. Subsequently, performance test for durability was carried out. Figure 3 shows the performance test of a 3-cell stack fuelled with primary bio-syngas in test run TR1. A constant current of 32 A was imposed to perform the test. The current was sequentially reduced to 20 A, as the stack was unable to keep steady at higher current density. The power output gradually reduced to 44.6 watts as shown in the test period D of Figure 3. In the test period E, the fuel was shifted from bio-syngas to hydrogen, and the power output recovered to 82.1 watts, which is a little lower than the origin power output of 90 watts. The post-examination on the dismantled reformer and stack indicated that stack remained its integrity. However, half of the catalyst pellets in the reformer were deposited by black matters, as shown in Figure 4. Figure 5 shows the microstructures of nanotube-like materials on the catalyst pellets. The EDS analyses at 4 different locations shown in Figure 5 indicated that the deposited matters were carbons. In practice, the typical tar concentration in the plasma-assisted gasification was in the order of 0.1% or less. Presumably, the nanotube-like carbons might come from synergistic effects of both tars and metals/HCs/CO/CO2 interactions (i.e., carbon nanotube syntheses), especially by Ni on the reforming catalysts and Fe/Ni on the stainless piping walls.

Figure 2 Current-voltage-power curves for different fuels in the TR1 test.



Figure 3 Performance test of a 3-cell stack fuelled with bio-syngas in test run

TR1.

Figure 4 (a) original and (b) black matters deposited on catalyst pellets.

Figure 5 SEM Microstructures of the deposited matters on the catalyst pellets.

Results of TR1 indicated deep cleanup of bio-syngas is required before it fed to a SOFC stack. Figure 6 shows the sulphide contents of bio-syngas at different sampling points. Table 2 indicated the concentrations of S, Cl and P at different sampling points. Concentration of tars was reduced by the condensation and deep cleanup processes. It indicated that the impurities in the bio-syngas were effectively reduced after the deep-cleanup processes.



Figure 6 Sulphide contents of bio-syngas at different sampling points.

Table 2 Concentrations of S, Cl and P at different sampling points (unit: ppm)

Test Run Syngas cleaning [S] [P] [Cl]

TR1 No deep cleaning 2.90 0.03 0.65

TR2 Before deep cleaning 0.50 <0.01 1.30

TR2 After deep cleaning <0.01 <0.01 0.30

In the second test run (TR2), I-V-P curves were measured to check the consistent stack performance while fuelled with hydrogen or bio-syngas, as shown in Figure 7. During the bio-syngas test, a small amount of water (0.8 c.c./minute) was pumped into the pipelines to minimize the likelihood of carbon formation onto the reformer catalysts. While the 3-cell stack fuelled with bio-syngas, the OCV was 2.89 V. At a current of 40 A, the power output was 94.24 watts, and its fuel utilization and electric efficiency reached to 65.3% and 41.1%, respectively. The durability test of the stack fuelled with bio-syngas was subsequently carried out for over 100 hours. Variations of voltages and currents versus time were plotted as Figure 8. As the fuel shifted from hydrogen to bio-syngas, the tafel measurements were carried out with hydrogen as fuel to check the consistence of stack performance. The nominal power output was around 77 W (power density 317 mW/cm2) under a constant current of 32 amperes and operating temperature of 750 oC. The fuel utilization and electric efficiency were 52.1% and 31.2, respectively. In the test, the stack was fuelled with bio-syngas in the test period of A, B, D, E and F. As the amount of bio-syngas was insufficient to complete a test period of 100 hours, the stack was fuelled with hydrogen in the intermittent period C. The system was shut down and restarted in the time period between D and E. Meanwhile, another batch (TR2B) of gasification was processed to provide the renewed bio-syngas. Then the test for stack fuelled with bio-syngas continued in the test period of E and F. The test was terminated because of a break of pipeline at the end of time period F. The post-examination on the dismantled reformer indicated only very minor deposited matters were observed on the catalyst pellets. The degradation of stack performance was about 0.52% in the test period A (~18 hours), while

point 1point 2

point 3

point 3(24 hr)

point 3(50 hr)

0

20

40

60

80

100

Su

lph

ide

co

nte

nt

(%)

Syngas

Sulphide content



the degradation from test period B to F (~103 hours) was 0.17%. The results indicate that the SOFC stack can be fuelled with the bio-syngas as it is well purified.

Figure 7 Current-voltage-power curves for different fuels in the test run TR2.

Figure 8 Performance for a 3-cell stack fuelled with bio-syngas in the test run

TR2.

4. Conclusions The eucalyptus wood chips were successfully converted to bio-syngas by the Plasma-Assisted Gasification System at INER. Performance testing was then conducted for 3-cell stacks with the primary and deep cleanup bio-syngas. Some conclusion remarks are listed as follows:

The SOFC stack can be fuelled with the bio-syngas as it is well purified.

Tars in the bio-syngas are deleterious to the performance of reformer and stack. It is prerequisite to reduce the concentration of tars to some acceptable level. Additionally, providing some extra water/vapour to the system is beneficial to enhance the water-gas-shift reactions and reduce the carbon deposition on the reformer and stack.



Long-term durability tests as well as optimizations and integrations on system level are highly recommended to ensure its economic and practical feasibility prior to the applications of biomass-powered SOFC systems.

References

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IASS, Potsdam¸ Sustainable biomass in the context of climate change and rising Demand, Brief for GSDR 2015, 2015.

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12. H. M. Lee, S.H. Chen, and, C.C. Tzeng, Plasma-assisted biomass gasification, Proceedings of the 7th Asia-Pacific International Symposium on the Basics and Applications of Plasma Technology (APSPT7), Taipei, Taiwan, April 14–16, 2012.

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