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Supplementary informationToward highly efficient in-situ dry reforming of H2S contaminated
methane in solid oxide fuel cells via incorporating coke/sulfur resistant bimetallic catalyst layer
Bin Hua a, Ning Yan b, Meng Li c, Yi-Fei Sun a, Jian Chen d, Ya-Qian Zhang a, Jian Li
c, Thomas Etsell a, Partha Sarkar e, Jing-Li Luo a, *
* Corresponding author
a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada, Tel.: +1 780 492 2232; fax: +1 780 492 2881. E-mail address: [email protected]
b Van’t Hoff Institute for Molecular Sciences (HIMS), University of Amsterdam, Amsterdam, 1098XH, The Netherlands, Tel.: +31 020 525 6468; E-mail address: [email protected]
c Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China
d National Institute for Nanotechnology, Edmonton, Alberta T6G 2M9, Canada
e Environment & Carbon Management Division, Alberta Innovates-Technology Futures, Edmonton, Alberta, T6N 1E4, Canada
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2016
Methods
Preparation of the catalysts
NiCu-Ce0.8Zr0.2O2 (ZDC) catalyst was prepared by a glycine-nitrate process
(GNP). Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Zr(NO3)4·5H2O and Ce(NO3)3·6H2O were
dissolved into distilled water at a molar ratio of 6:1:2:8 with glycine added. This
solution was then combusted at 200 °C and calcined at 800 °C for 2 h to form NiCuO-
Ce0.8Zr0.2O2 powder, which was in-situ reduced in H2 to form NiCu-Ce0.8Zr0.2O2
catalyst. The volume percentage of metal phase (Ni-Cu alloy) in NiCu-ZDC cermet is
around 23.3%, ensuring adequate electronic conductivity for SOFC operation. In a
control group, we also prepared the Ni-ZDC (molar ratio of Ni:Zr:Ce is 7:2:8), NiCo-
ZDC (molar ratio of Ni:Co:Zr:Ce is 6:1:2:8) and NiFe-ZDC (molar ratio of
Ni:Fe:Zr:Ce is 6:1:2:8) using the same method.
Preparation of the cathode
To prepare La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode material,
La(NO3)3·6H2O, Sr(NO3)2, Co(NO3)2·6H2O and Fe(NO3)3·9H2O were dissolved in
distilled water with EDTA/citric acid added as the chelating agent. The aqueous
solution was heated at 80 °C under agitation to a viscous gel, and then dried at 180 °C
to form a black foamy intermediate product, which was ground into fine powder and
calcined at 800 °C for 2 h in air to obtain perovskite structure LSCF.
Cell fabrication
Tape-casting/screen-printing/sintering processes were used for fabrication of the
anode supported cells. NiO (Type A Standard, Inco) and YSZ (TZ8YS, Tosoh) in
57:43 weight ratio were ball milled for 24 h in toluene/ethanol solvent with fish oil as
the dispersant and corn starch as the pore former. It was further milled for another 24
h after adding polyvinyl butyral (Richard E. Mistler Inc., USA) as the binder and
polyethylene glycol (Richard E. Mistler Inc., USA) as the plasticizer. Such prepared
slurry was cast into sheet (anode support layer) by using a tape casting machine, then
dried in air to obtain the anode support (ϕ16×1.4 mm), to which functional NiO-YSZ
(60:40 wt. %, Type F Standard, Inco-TZ8Y, Tosoh) and electrolyte (TZ8Y, Tosoh)
were then screen printed in sequence, prior to sintering at 1390 °C for 3 h. To prepare
the baffle and cathode, Ce0.9Gd0.1O1.9 (GDC, NIMTE, CAS) and LSCF-GDC (70:30)
pastes were then screen-printed successively on the surface of the sintered YSZ
electrolyte, followed by sintering separately in air at 1300 °C and 950 °C for 2 h to
complete the fabrication of the anode-supported cells. The size of the obtained SOFC
button cell was about ϕ13×1 mm with an active area of 0.5 cm2 (cathode). The
NiCuO-Ce0.8Zr0.2O2 slurry was painted on the outer surface of anode and sintered at
900 ºC for 2 h in air.
Preparation of the LSCF-GDC cathode and NiCuO-ZDC catalyst slurry
The cathode and the catalyst pastes were prepared by ball milling the powder and
a self-made binder with a weight ratio of 50:50 for 1 h. The self-made binder was
prepared by adding 4 wt. % cellulose into 96 wt. % terpilenol, followed by stirring
and heating at 80 °C to completely dissolve the cellulose.
Catalytic activity evaluation
The catalytic activity and sulfur tolerance of Ni-YSZ (57:43, sintered at 1390 ºC)
and NiM (M= none, Co, Cu, Fe)-ZDC were compared. Catalytic activity
measurements for dry reforming of methane (DRM) reaction were performed at
atmospheric pressure using a compound of 0.2 g catalysts and 0.4 g
catalytically inactive quartz powder, which was sieved into the particle size ranging
from 30 to 60 mesh and packed on a bed of quartz tube. Prior to the catalytic
evaluation, the samples were heated up to 850 ºC and reduced in H2 for 5 h. As regard
the assessments of the sulfur tolerance, the reduced catalysts were exposed to H2-500
ppm H2S for 5 h prior to the test. The gas mixtures of sweet CH4-CO2 (mole ratio=1:1)
or sour CH4-CO2 (mole ratio=1:1, and balanced with 50 ppm H2S) were fed into the
reactor at the flow rate of 20 ml min-1. Compositional analysis of the effluent gases
was performed with a gas chromatography (GC, Hewlett Packard Series two). The
catalytic reactions were performed at the temperatures ranging from 550 to 800 ºC up
to 48 h. The percentages of CH4 conversion and CO selectivity were calculated
according to Eqs.1 and 2, separately.
Carbon deposition resistance evaluation
The carbon deposition resistance of the catalysts was evaluated by analyzing the
nature of the carbon deposited on the catalyst through Raman spectroscopy. To
accelerate the rate of carbon formation on the catalyst, we exposed the as-
reduced/treated catalysts to pure CH4 at 800 °C for 30 min and cooled them down to
room temperature in H2, followed by carrying out the Raman tests.
Other Characterizations
The X-ray diffraction (XRD) of as-synthesized and reduced powder was
identified by using Cu Kα radiation at a tube voltage of 40 kV and a tube current of
44 mA, within a 2θ range between 20° and 80° at 1 deg. min-1. The microstructures of
the samples were examined by using scanning electron microscopy (SEM, JEOL
6301F). Raman spectrometry (Thermo Nicolet Almega XR Raman Microscope) was
employed to detect the graphitization degree of deposited carbon on the catalyst. The
NiCu-ZDC was also analyzed by transmission electron microscopy (TEM, JEOL
2200 FS). A SDT-Q600 (TA instrument, USA) machine was used to carry out the
thermogravimetry analysis (TGA) experiments of the catalyst reduction process. The
carbon depositions on the catalysts were quantitatively investigated by using
temperature programmed oxidation [TPO via coupled TGA-Mass Spectrometer (MS,
Pfeiffer Vacuum GmbH)].
Results and discussion
Table S1 Crystallite sizes of NiM bimetallic alloys calculated by Scherrer
equation.
Ni NiCo NiCu NiFeCrystallite sizes (nm) 17.8 19.2 20.3 15.9
Calculated by using the (111) metallic Ni plane. The Scherrer equation size is similar to the TEM results.
Table S2. Comparison of the fuel cell performance
Anode Electrolyte/Cathod
e
Temperature (°C) Fuel composition PPD (W cm-2) Ref. No.
Ni-YSZ YSZ/LSCF-GDC 800 CH4-CO2 (1:1, 50 ppm H2S) 0.96 Present study
SDC-Ni-YSZ YSZ/LSM-SDC 800 Dry H2 0.525 1
(Ni0.75Fe0.25-MgO)-YSZ YSZ/LSM-YSZ 800 Wet CH4 (3% H2O) 0.648 2
3 wt % Ru-Al2O3-Ni-YSZ YSZ/LSM-YSZ 800 CH4-CO2 (2:1) 0.705 3
LiLaNi-Al2O3-Ni-ScSZ ScSZ/LSM-ScSZ 850 CH4-H2O (2:1) 0.532 4
GdNi-Al2O3-Ni-YSZ YSZ/LSM-YSZ 800 CH4-CO2 (2:1) ~0.8 5
4 wt % Au-Ni-GDC YSZ/LSM-YSZ 850 CH4-H2O (2.07:1) 0.41 6
Figure S1
Figure S1. XRD patterns of as-synthesized La0.6Sr0.4Co0.2Fe0.8O3-δ perovskite.
Compared to the JCPDS file <01-082-1961>, it is confirmed as-prepared powder
formed a simple perovskite.
Figure S2
Figure S2. XRD patterns of as-synthesized and reduced NiMO-Ce0.8Zr0.2O2.
The XRD results of the as-prepared and reduced catalysts show that 20 mol. % ZrO2
doped CeO2 (ZDC) was identical in both reducing and oxidizing atmospheres while
NiM(M=Cu, Co, Fe)O solid solution was fully reduced to Ni or Ni alloys in H2.
Figure S3
Figure S3. Temperature dependent weight changes of NiM-Ce0.8Zr0.2O2 catalysts
in 10 % H2.
The results show the effect of the alloying elements on the reduction temperatures and
rates of the catalysts. The NiCuO solid solution exhibits the lowest reduction
temperature and highest reduction rate.
Figure S4
Figure S4. (a) TEM bright field (BF) image; (b) EDX elemental mappings of Ni;
(c) and (d) High resolution TEM (HRTEM) images of NiCu bimetallic
nanoparticles at the corresponding sites labeled in BF image.
Figure S5
Figure S5. TEM BF image and EDX mappings of Ni, Co, Ce, Zr and O.
This EDX mapping confirms that the elements well disperse in the catalyst from a
relatively macro perspective comparing to Figs. S4 and 2c.
Figure S6
Figure S6. Raman spectra of fresh and H2S treated catalysts after exposing to
dry CH4 at 800 °C for 30 min.
The nature of the deposited carbon in the samples was analyzed by Raman
spectroscopy and the results are shown in Fig. S8. Two intense bands related to the
deposited carbon appeared in the test range, i.e., the D (defect) band associated with
disorder structure of carbon and the G (graphite) band featuring the graphitic layers
and the tangential vibration of carbon atoms. The intensity ratio of these two bands R
(= ID/IG) has been widely used to characterize the graphitization degree of carbon. A
higher value of the R corresponds to a lower degree of graphitization and an active
carbonaceous species that is readily removable. The results showed that NiCu alloy
had the best carbon deposition resistance, and H2S treatment improved the carbon
deposition resistance of all the samples.
Figure S7
Figure 7. Cross-sectional microstructures of the as-prepared cell and cell
components: (a) reduced cell; (c) LSCF-GDC cathode/GDC baffle/YSZ
electrolyte; (b) and (d) NiCuO-Ce0.8Zr0.2O2/Ni-YSZ supported layer.
As the microstructure of the reduced cell shown, porous anode is well adhered to
the dense YSZ electrolyte, which is approximately ~12 μm in thickness. The
reduction of NiO in the anode support and functional anode generates a significant
amount of pores to guarantee fuel gas permeability and three phase boundaries (TPBs)
for electrochemical reaction. The LSCF-GDC cathode, forming a well-sintered porous
structure, is also showed to establish adequate coherence to the anode/electrolyte
substrate. After sintering at 900 °C for 2 h, porous NiCuO-Ce0.8Zr0.2O2 catalyst layer
is integrated to NiO-YSZ supported layer, ensuring efficient electronic transportation.
Figure S8
Figure S8. I-V and I-P curves of C-SOFC and TA-SOFC in various fuels at 800
°C before H2S treatment.
Figure S9
Figure S9. I-V and I-P curves of C-SOFC and TA-SOFC in various fuels at 800
°C after H2S treatment.
Figure S10
Figure S10. Electrochemical impedance spectra of H2S treated TA-SOFC in H2
and CO under -0.5 V over potential.
The EIS are featured by flatten arcs, intersected by the real axis at high and low
frequencies. The intercept of high-frequency is estimated as the ohmic resistance (RΩ),
including electronic resistance of electrode, ionic resistance of the electrolyte and
interfacial contact resistance. The low frequency intercept corresponds to the overall
resistance (RO), while the difference between the RΩ and RO represents the electrode
interfacial polarization resistance (RP).
Figure S11
Figure S11. The current dependent exhaust gas composition of C-SOFC after
H2S treatment fed with sweet CH4-CO2.
Figure S12
Figure S12. I-V and I-P curves of C-SOFC and TA-SOFC in CH4-CO2 at
temperatures between 650 °C and 800 °C before H2S treatment.
Figure S13
Figure S13. I-V and I-P curves of H2S treated C-SOFC and TA-SOFC in CH4-
CO2 at temperatures between 650 °C and 800 °C.
Figure S14
Figure S14. The mass spectroscopic data from the effluent that obtained during
the temperature-programmed oxidation (TPO) of the catalyst after DRM test in
sour CH4-CO2.
A typical mass spectrogram, obtained from the TPO test, demonstrated that the
removal of carbon deposition is accompanied by the consumption of O2 and
generation of CO2. It is also known from the spectrogram that the process of carbon
oxidation started at the temperature around 400 °C.
References of the electronic supplementary information
1. Zhang, L. et al., Journal of Alloys and Compound. 2009, (482), 168.
2. Liu, Y. et al., Journal of Power Sources. 2011, (196), 9965.
3. Wang, W. et al., International Journal of Hydrogen Energy. 2011, (36), 755.
4. Wang, W. et al., Journal of Power Sources. 2011, (196), 90.
5. Wang, W. et al., Journal of Power Sources. 2011, (196), 3855.
6. Niakolas, D. K. et al., International Journal of Hydrogen Energy. 2010, (35), 7898.