Applied Energy 106 (2013) 163–175

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Breakthrough fuel cell technology using ceria-based multi-functionalnanocomposites

0306-2619/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.01.014

⇑ Corresponding author. Tel.: +46 (0)87907403; fax: +46 (0)8204161.E-mail address: binzhu@kth.se (B. Zhu).

Bin Zhu a,⇑, Liangdong Fan a,b, Peter Lund c

a Department of Energy Technology, Royal Institute of Technology (KTH), Stockholm S-10044, Swedenb Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR Chinac Department of Applied Physics, Aalto University, FI-00076 Aalto, Espoo, Finland

h i g h l i g h t s

" NANOCOFC methodology for LTACFCs is introduced." Development of ceria-salt composites and nanocomposites." Breakthrough research – EFFC, a joining FC and solar cell device, is highlighted." Semi-ion conductive nanocomposites for advanced energy technologies.

a r t i c l e i n f o

Article history:Received 6 September 2012Received in revised form 1 January 2013Accepted 3 January 2013Available online 21 February 2013

Keywords:Ceramic fuel cellsNANOCOFCCeria-based compositeElectrolyte-free fuel cellSingle componentNanocomposite

a b s t r a c t

Recent scientific and technological advancements have provided a wealth of new information about solidoxide-molten salt composite materials and multifunctional ceria-based nano-composites for advancedfuel cells (NANOCOFC). NANOCOFC is a new approach for designing and developing of multi-functional-ities for nanocomposite materials, especially at 300–600 �C. NANOCOFC and low temperature advancedceramic fuel cells (LTACFCs) are growing as a new promising area of research which can be explored invarious ways. The ceria-based composite materials have been developed as competitive electrolyte can-didates for low temperature ceramic fuel cells (LTCFCs). In the latest developments, multifunctionalmaterials have been developed by integrating semi- and ion conductors, which have resulted in anemerging insight knowledge concerned with their R&D on single-component electrolyte-free fuel cells(EFFCs) – a breakthrough fuel cell technology. A homogenous component/layer of the semi- and ion con-ducting materials can realize fuel cell all functions to avoid using three components: anode, electrolyteand cathode, i.e. ‘‘three in one’’ highlighted by Nature Nanotechnology (2011). This report gives a shortreview and advance knowledge on worldwide activities on the ceria-based composites, emphasizingon the latest semi-ion conductive nanocomposites and applications for new applied energy technologies.It gives an overview to help the audience to get a comprehensive understanding on this new field.

� 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1642. NANOCOFC approach and materials development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

2.1. NANOCOFC approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1642.2. Materials development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

2.2.1. The ceria-salt composite and nanocomposite for LTCFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1652.2.2. Development of multi-functionalities for nanocomposite materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

2.3. The electrolyte-free fuel cell (EFFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

164 B. Zhu et al. / Applied Energy 106 (2013) 163–175

3. Summarizations and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

1. Introduction

Low temperature (LT) ceramic fuel cells (CFCs) or solid oxidefuel cells (SOFCs) demand high conducting electrolytes. Lowtemperatures, say 300–600 �C, CFCs can offer higher Nernst volt-ages and allow the use of cheap stainless steel for the bipolarplates, gaskets and the balance of the plant. Thus they avoidhigh temperature (HT, 1000 �C) material and technical problemsresulted in the high cost for conventional SOFCs, and the inter-diffusion of the ingoing elements between the electrolyte andelectrode structure. By lowering the operating temperature itcan also put simpler and easier technical requests that, in turn,will give LTCFCs a cost advantage over conventional HTSOFCtechnology.

The electrolyte plays a central role in SOFC R&D, which becomesactually a key limiting factor for realizing the SOFC commercializa-tion. Facing this challenge, there are world activities to reduce theelectrolyte thickness using various advanced thin film technologies[1,2]. The thin film SOFCs are able to reduce the operational tem-peratures from conventional 800–1000 �C to around 700 �C. How-ever, the advanced fabrication technologies will in turn increasethe cost and complexity of fuel cell system. In addition, the thin-film electrolyte used for SOFCs easily caused failures at the extre-mely operational atmospheres and certain high temperatures.Our LTCFC R&D follows a new route to develop high ion conductingelectrolytes for low temperature operation (300–600 �C), based onour network and international joint efforts on multifunctionalnano-composites for advanced fuel cells (NANOCOFC) – an EC-Chi-na Multifunctional Nanocomposites for Advanced Fuel Cells net-work, www.nanocofc.com. Manipulation of interphases ofnanotech-based composites, so-called nanocomposites providesuperionic conductivity and dual H+/O2� conductivity. The nano-composites will tremendously reduce working temperature of con-ventional SOFCs from 1000 �C to 300 �C, which opens newopportunities and successes by employing composite and nano-technology. The NANOCOFC approach has created a new pathand some breakthrough to develop advanced fuel cell (AFC) mate-rials and technologies for LTCFCs since conventional structural de-sign of SOFCs could not bring the new functional materials for 300–600 �C.

Based on the NANOCOFC our recent developments on electro-lyte -free fuel cells (EFFCs) have physically removed the electrolytecomponent/layer by using one component/layer of the ceria- andtransition metical oxide nanocomposite [3–6]. It is the break-through point over the conventional anode/electrolyte/cathodethree-layer fuel cells (FCs), in which the electrolyte is indispens-able for charge separation and energy extraction from the chemicalenergy of fuels. The experimental results had demonstrated similarcell performance to that of the conventional electrolyte-based fuelcells (EFFCs), but at much lower materials/system cost and muchgreater engineering feasibility. The development of this novel de-vice will extend and integrate the working principles of FCs and so-lar cells [6], which are strongly deserved for extensive activitiesboth from technology and fundamental aspects.

This review is based on our more than 20 years of experiencewith innovative CFCs, and especially our recent 10 years of workon multifunctional nanocomposite materials for AFCs, based onthe NANOCOFC. It will emphasize on the latest breakthrough onthe single component EFFCs. It also gives a brief view on world

activities related to the ceria-composites LTCFCs and AFCs. This re-view consists of the following parts:

(i) NANOCOFC methodology.(ii) The ceria-salts composites in the last decade focus on exten-

sive LTCFC performance testing also material and technologydevelopments, especially, recent R&D on multifunctionalceria-based nanocomposite materials for AFCs – NANOCOFC.

(iii) The breakthrough of the EFFC in the FC R&D, which is real-ized by the NANOCOFC approach for further developingmultifunctional nanocomposites using semiconductor-ceria-composite materials. The EFFCs had been highlightedby Nature Nanotechnology [7] with a fantastic name. Threein one: ‘‘Bin Zhu and colleagues at the Royal Institute ofTechnology (KTH), Stockholm, have now been able toremove the electrolyte altogether, creating a fuel cell froma single homogenous layer. . .’’. The research is also high-lighted by Materials Views (2011) [8]: ‘‘This efficient designwill pave the way towards more affordable and efficient fuelcells (FCs) and perhaps even help to hasten the arrival of thehydrogen economy.’’

Under the framework of the European R&D program, an EC-China NANOCOFC network was established to develop AFC basedon nanocomposites. Within this platform, the NANOCOFC meth-odology and functional ceria two-phase composite materialsmade by the nano and composite technologies have been success-fully demonstrated for LTCFCs or LTSOFCs [9–19], and resulted inan emerging R&D field with worldwide activities [20–59]. Thetwo-phase nanocomposite method employs nano-particles toconstruct new functional materials possessing the desired interfa-cial super-ionic conduction for electrolyte and other multi-func-tion. The hybrid semi-ion conducting nanocomposites havecreated the EFFCs, a great breakthrough in FC R&D. Thus, thereis an emerging need to review this field as a new scenario of FCR&D and also for new energy technologies, e.g. joining FC and so-lar cell [6].

2. NANOCOFC approach and materials development

2.1. NANOCOFC approach

NANOCOFC is a new scientific methodology for developing thenanocomposite functionalities. It uses the two-phase nanocompos-ite method that employs nano-particles to construct new func-tional materials possessing the desired interfacial properties, e.g.,superionic conduction for electrolyte and other functions. TheNANOCOFC has a number of scientific characteristics and newknowledge in fuel cell R&D summarized as below:

1. Advanced material designs. The NANOCOFC emphasizes ondesign and development of the material properties on inter-faces between two or multi-constituent phases. The interfacehas low activation energy demand and high mobile ion concen-tration as well as long moving path for ions. Thus it may cause.

2. Interfacial superion conduction. It is a strong conductivityenhancement phenomenon attributed by the interfacial proper-ties and interfaces without involving any structural changes

B. Zhu et al. / Applied Energy 106 (2013) 163–175 165

within the constituent phases. Conversely the conventionalsuperion conduction can only occur accompanying a structurechange [13].

3. Dual H+ and O2� conduction and contribution to high powerdensity of the fuel cell devices. The interfaces can integrate dif-ferent mobile ions that are charge carriers, e.g. H+ and O2�, forthe fuel cell current. This can enhance the device power outputcompared to that with only single ion source, e.g., H+ or O2�,transport [11,12,20,60–62].

4. Based on non-structure designs, i.e. on the interfaces, there areno critical limits on the criteria to select the materials. That is tosay, any structure and solid materials could be used in designfunctional nanocomposites as long as functions can be builtbetween the constituent phases by nano- and composite tech-nologies [63].

5. Besides, the interfacial redox is another radical new and usefulmechanism. In our ceria-composite electrolyte developmentswe have used the redox element, e.g. Ce4+–Ce3+ to successfullyextract the electron conduction through the interfacial or sur-face redox reaction process. This interfacial redox mechanismand process have resulted in a revolutionary fuel cell technol-ogy and breakthrough – EFFC based on only a single layer witha homogenous layer of the hybrid semi-ion conductivematerials.

It should be noted that in the long FC R&D history since its firstinvention in 1839, the electrolyte has always been a main streamfor research and actually becomes a bottleneck to limit the FC per-formance and cost as well as FC commercialization. A removal ofthe electrolyte in FC, i.e. EFFC would signify a true breakthroughand revolution in FC research and development. Based on the EFFC,a new emerging cross link discipline and new generation energyconversion technologies, e.g. joining FC and solar cell are exploredas a lead. It may have strong impacts on development of the nextgeneration of advanced solar cell and fuel cell as well as new ap-plied energy technologies.

2.2. Materials development

2.2.1. The ceria-salt composite and nanocomposite for LTCFCsComposite solid electrolytes have got more attention as com-

pared to pure electrolytes due to their higher conductivity and en-hanced mechanical strength [64–71]. The control of the electrolyteproperties through the variability of the conductance type is veryflexible. The composite electrolytes are mixtures of two or morematerials/phases. These are also called heterogeneously dopedmaterials or dispersed electrolytes [64–66]. These composite elec-trolytes are considered a new type of electrolyte with high conduc-tivity which occurs through interfaces but at significantly lowertemperatures.

The ceria-based composite research activity over the last dec-ade has focused on extensive fuel cell performance testing of var-ious ceria-based composite ceramics, such as samarium-dopedceria (SDC) incorporated with different salts and hydrates, suchas chlorides, fluorites, carbonates, sulphates and hydrates. In thecase of composites, or heterogeneous doping, there are 2 possibil-ities: (a) The insertion of second phase particles into a ceria matrixand (b) The production of a matrix of a second phase in which youdisperse fine ceria particles.

The properties of such composites (or heterogeneous dopingsystems) depend on the compositions of the 2nd and host phase,either in possibility 1 or 2. An example of the second case is thework of the ceria-salt composites [11], where essentially a matrixof a ‘salt’ is formed, often by melting, in which fine powder of agood ion conductor is dispersed. As an illustration, a salt mixtureof Na2CO3/Li2CO3 is often used as a second phase.

Since 1998, LTCFC material innovations were devoted to devel-oping composites based on the ceria-salts, and ceria-oxide com-posite electrolytes. The ceria was used more commonly as an iondopant, e.g., Gd-CeO2 (GDC), Sm-CeO2 (SDC) and Y-CeO2 (YDC).The ion doped ceria is well known to be a good oxygen ion conduc-tor (10�3 to 10�2 S cm�1 at 600 �C). In addition, these salts haveexhibited proton conduction in FC in our previous LTCFC R&D[72–77], which is compatible with the oxygen ion conduction. Ifwe could combine them together to function in one bulk, couldthey become more functional electrolytes for the new LTCFCs? An-other consideration is based on a super-ion conductor theory. Thefast ion conduction takes place in one mobile ion sub-lattice whichcan be considered to be a melt, and another type of the ions is insolid sub-lattice which carries the melt mobile ions. A typicalexample of this is a-AgI. Extremely high Ag+ ion conductivity issuggested due to highly disordered Ag ions, or the melt Ag-ion lat-tice. A similarity in comparison is a composite containing one mol-ten phase carried by another rigid phase, it would make anartificial super-ion conductor structure. In this case, the fast ionconduction may be facilitated by the molten phase. Compositesconsisting of ceria (GDC, YDC, SDC) and MxCO3 (M = Li, Na, K, Ca,Sr, Ba, x = 1, 2) or hydroxide (such as LiOH, KOH, NaOH etc.) aretypical examples. These innovative ceria-based composites havecreated new advanced electrolytes and excellent CFC performanceof 1 W cm�2 at 600 �C. Using these materials the CFC could func-tion at lower temperatures, e.g., close 0.3 W cm�2 obtained at400 �C and even at a temperature as low as 200 �C [45–48]. Scho-ber and Ringel [64,65] have also made extensive examples of aBCY-carbonate composite system; they reported various BCY-car-bonate composites that showed super-ionic conduction. For exam-ple, Y-doped Ba cerate with a Li2CO3–Na2CO3 (2:1) Mixture, aBCY20 + Li2CO3 composite, a BCY20 + NaOH composite, a BCY20-LiCl composite, and GDC + 20wt.% Li2CO3 + Na2CO3.

Several ceria-salt composite electrolytes, e.g. ceria-chloride[77,78], ceria-sulphate [63], ceria-hydrates [11,17] and most com-monly ceria-carbonates [9–16,18,20–51,53–58] have been studied.These are crystalline in ceria constituent phases, and the salts areamorphous. Typical examples of crystalline ceria-salt compositesare SDC-NaCl and SDC-CaCO3. In our recent research and develop-ment on LTCFCs, which is a combination of SOFC and molten car-bon fuel cell (MCFC) technologies, we have developed a newapproach of nanocomposite materials for advanced fuel cell tech-nology. Functional ceria-based two-phase composite electrolytematerials developed by combining nanotechnology and compositetechnology have been successfully demonstrated for LTCFCs.

Nanocomposites for AFC technology, i.e. NANOCOFC, is anemerging LTCFC or LTSOFC R&D effort that was established byDr. Zhu and his colleagues over the last 10 years, and it has beensignificantly expanded and extended by the efforts of other groupsaround the world. The distinctly different efforts that have contrib-uted to these R&D activities in the last 10 years have emphasizednanotechnology to create new functionalities in the composites,especially by using nanotechnology to develop composites whichminimize significantly the amount needed for the 2nd salt phasein a composite but maintain the same material functionality. Atthe same time, by enhancing the salt stability in typical core–shellor 3D-homogenous nano-particle dispersions, we can create a newstructure stable phase. These new nanocomposites have explored anew promising AFC or NANOCOFC area. To realize these, two ap-proaches have been developed and used:

2.2.1.1. Nanocomposites. The LTSOFC environment puts critical de-mands on the nano-structured materials. Normal nano-structuredmaterials cannot function in the LTSOFCs because the strongly re-duced and oxidized LTSOFC environments at hundreds of degree Ccan easily destroy the nano-structures. To solve this problem a un-

Fig. 1. SEM images for the normal SDC-LiNaCO3 micro-composite (a) and SDC-Na2CO3 nanocomposite (b).

Fig. 2. HRTEM images of (a) a pure SDC particle and (b) core–shell SDC-Na2CO3 nanocomposite particle [79].

166 B. Zhu et al. / Applied Energy 106 (2013) 163–175

ique approach has been developed using a second phase materialto create a new nano-composite: a nano-core particle with anano-cover-layer formed by the 2nd phase, both are functionalmaterial components. This invention can prevent the nano-func-tional particles from the energetic growth and strong activity in re-duced or oxidant atmospheres at high temperatures. According topercolation and effective-medium theories, the ion conductivityfor such a composite system can also be strongly enhanced,becoming a super-functional LTSOFC material.

As an example, we can see in conventional SDC-LiNaCO3 com-posites prepared by the two-step process, i.e. first preparation ofSDC using co-precipitation, then mixing by LiNaCO3 in dryingway, much larger particle sizes in the range of several lm as shownin Fig. 1a. Clear interfaces between ceria and carbonate phases areseen. The composite displays the size in a wide range from tens ofnm to couple lm with very uneven particle sizes and morpholo-gies, Fig. 1a. They show two-phase particle separation regions withclearly different morphology for salt and ceria enrich areas. Whilein a ceria-carbonate nanocomposite using one-step preparationthrough wet chemical co-precipitation [79], the homogenous dis-tribution of carbonate and ceria particles are shown in Fig. 1b, ingenerally, the carbonate and ceria form a core–shell structure.

Fig. 2a shows high resolution transmission electron microscopy(HRTEM) images of ceria particles both as pure SDC (Samariadoped ceria) and SDC in a composite (2b). In Fig. 2b the ‘‘d’’ spacingof the (111) plane is identified to be around 0.317 nm which isslightly smaller than that of pure CeO2 (0.357 nm) due to partialsubstitution of Sm atoms for Ce atoms in CeO2.

Although ceria particles in the composites have the same struc-tural images as pure ceria in the edging areas, i.e. interfaces sur-rounding the ceria particle, additional lattice defects ordistortions are observed (Fig. 2b). The evidence observed for thelattice distortion and defect accumulation in the interface areasupport interfacial functionalities such as ionic transportation

[80]. Moreover, the SDC-carbonate nanocomposites form a core(SDC) – shell (carbonate) structure. The shell layer is amorphousand makes the progress of the ionic transportation smooth, whichwill be discussed later in the paper. From Fig. 2b it can be seen thatthe interface layer/shell thickness of the ceria-carbonate nanocom-posite is 5–10 nm.

2.2.1.2. Hybrid H+/O2� conductors and composite technology. Anotherunique approach is to use the composite technology to develop hy-brid ion conductors combining protons and oxygen ions to makethem function together thus to enhance significantly the FC perfor-mance. To realize this concept, the composite technology andresulting composite materials are the key. The composite materialshave the following characteristics: (a) it is not a single phase, but atwo- or multi-phase-composite; (b) the two or multi-phases arenot a simple mixture. There are some physical interactions be-tween the constituent phases to enhance the conductivity, but noother compound is formed in the composites; (c) the conductivityis not limited by the structure (e.g. doping ion kinds, sites, and con-centrations), but mainly determined by the ‘‘composite effect’’, i.e.the conductivity is strongly enhanced by the interfaces betweenthe constituent phases; (d) it can merge or integrate multi-kindsof ion conduction into one composite material system, formingthe hybrid-ion conductors and creating the new functions.

Wang et al. [48] proposed the Swing model, as shown in Fig. 3,for explaining proton conduction in the ceria-composite electro-lyte. They assumed that O2� conduction was due to a bulk vacancymechanism. When protons approach the composite electrolytefrom the anode, they can form metal-stable hydrogen bonds withoxygen ions from both the SDC surface and CO2�

3 groups. Whenthe operating temperature is above the transition temperature ofthe amorphous carbonate phase, the bending and stretching vibra-tion of C–O bonds are enhanced, as well as the mobility and rota-tion of the CO2�

3 group itself. These enhanced movements facilitate

Fig. 3. A schematic illustration of dual H+/O2� conduction pathways: the protonsare transported by the interface in the core–shell SDC/Na2CO3 nanocomposite,while oxygen ion conduction is through the grains of SDC [48].

B. Zhu et al. / Applied Energy 106 (2013) 163–175 167

rapid breaking and forming of hydrogen bonds in the interface re-gion, leading to effective long-range proton transportation drivenby a proton concentration gradient or the fuel cell electrical field.In this process, carbonate serves as a ‘‘bridge’’ for protons to move(transfer) from one hydrogen bond to another. In this case, protonconductivity was found to be one order of magnitude higher thanoxygen ion conductivity. The proton conductivity was close tothe 0.1 S cm�1 level at 600 �C. This model gives another simpleexplanation to the dual H+ and O2� transport but is limited as itdoes not cover other ceria-based composites, where both H+ andO2� make comparable contributions to conductivities and in somecases, O2� can play even a dominant role [81]. As stated abovesince multi-parameters determine the final ceria-composite mate-rial electrical properties and H+/O2� conductivities, it has to bestudied from case to case.

On the other hand, highly mobile CO2�3 anions from the carbon-

ate phase can also promote oxygen ion mobility [82,83]. Undersuch circumstances, the oxygen ions (vacancies) formed on ceriananoparticle surfaces can contribute to the high mobility of O2�

along the interfacial region of the ceria phase in contact with thecarbonate. Two paths and conduction processes can thus exist inparallel.

In some cases, H+ and O2� could collide in the electrolyte andform water, i.e. 2H+ + O2� = H2O. As a matter of fact, this case doesnot mean any ’’electrochemical’’ leakage since the half-cell reac-tions H2 = 2H+ + 2e� and 1/2O2 + 2e� = O2� that occurred in bothelectrode processes have already generated electricity to the exter-

nal circuit. This is clearly different from the mechanical leakagethat would yield H2 + 1/2O2 = H2O (combustion) and produce heatonly. Therefore, the enhancement of total ionic conductivity in ananocomposite electrolyte by H+/O2� dual ion conduction is useful.

The nanocomposite architectures for electrolyte materials mayshow high dual H+/O2� conductivity at low temperatures. Ceria-based nanocomposite (nano-particles, nano-rods, nanotubes) canbe built with other H+/O2� conductors using different system con-figurations (core–shell, capping, bonding, linking). By using thetwo-phase material (TPM) nanocomposite approach, the two-phase architectures can be directly made by integrating two protonand oxygen conductive oxide materials. Thus H+ and O2� conduc-tion paths/channels can be constructed with dual H+/O2� conduc-tivity. This has been discovered in the composite of protonconductor BaCe0.9Y0.1O3�d and the oxygen ion conductor SDC(samaria doped ceria) [84].

Proton conduction is even more important for LTSOFCs becauseit can be activated easier than oxygen ions in the low temperatureregion (300–600 �C). As shown earlier, the proton and oxygen ionconductivities are comparable at higher temperatures, e.g. 550 �C,but the proton conductivity is several times to one order magni-tude higher than that of the oxygen ion at 450 �C. Huang et al.[85] observed that in some cases such a composite material couldbe a pure proton conductor. The nanocomposite/TPM approach canintegrate multi-ion functions, typically the hybrid or dual H+/O2�

conduction that enhances the material conductivity and increasesfuel cell power output.

The dual-function from proton and oxygen ion conduction wasconfirmed under a constant discharge where water was formed,both on the anode and the cathode side, indicative of proton andoxygen ion transport. Initial experimental results have demon-strated that the composite technology to integrate two ion conduc-tors into one bulk to form a new functional ion conductor or hybridproton and oxygen ion conductor is realistic. It may become a use-ful methodology to develop new advanced CFCs materials. Theinteresting examples were hybrid conductors based on all oxides,e.g. proton conducting oxide, e.g. BaCe0.8Y0.2O2.9 (BCY20) andBaTi0.9Y0.1O2.95 and oxygen ion conductor, SDC and GDC. TheSDC-BCY20 electrolyte CFCs reached a performance of0.25 W cm�2 at 550 �C [84]. In this similar composite system, Scho-ber and Ringel [64] observed a super-ionic conduction at ca. 560 �C,which is agrees with the above fuel cell results. At 550 �C the fuelcell showed a significant enhancement in performance. By formingcomposite or heterogeneous doping, Lin et al. [86] used high tem-perature sintering to achieve a fuel cell power density of 0.4 and0.6 W cm�2 at 700 �C by sintering BCY-GDC at 1450 and 1550 �C,respectively. Most activities on LTCFCs or LTSOFCs reported in lit-erature have focused on the carbonate used as one constituentfunctional component in the composite together with ceria. Tosome extent this ceria-carbonate composite system may be consid-ered as an integrated new advanced technology for fuel cells be-tween the MCFC (molten carbon fuel cell) and SOFC. By using thecarbonate as one important functional component with naturalrelation to MCFC though the ceria-carbonate and LTCFC most func-tion not based on the carbonate anions. By adjustment of the com-posite composition (i.e. the portion of the carbonate, usually above40 wt.%) and adding MCFC operational gases, we can operate theLTCFC in MCFC mode; or we can operate in CFC/SOFC mode if thecarbonate is less than 40 wt.% and the system is at moderately hightemperatures, 500–600 �C (in most cases the system is under anH2/air environment, where the dual-ion of H+ and O2� are typicalto make the fuel cell function). Moreover, combined MCFC andSOFC operation model is also possible. Li et al. [39,82] proposedthe ceria-carbonate (40 wt.%) composites as a hybrid-ionic trans-port electrolyte, which was operated by adding CO2 at cathode sideand achieved 1.7 W cm�2 at 650 �C. It should be pointed out that

168 B. Zhu et al. / Applied Energy 106 (2013) 163–175

the dual ions of H+ and O2� or hybrid ions of H+, O2� and CO2�3 func-

tion is one of the important and unique characteristics for LTCFCsthat is different from existing fuel cells.

Fig. 6. (a) A high-resolution TEM image showing a core–shell structure of a SDCnanoparticle which is covered by a thin layer of LiZn-oxide; (b) simulated HRTEMimage of ZnO coating at [210] projection [92].

2.2.2. Development of multi-functionalities for nanocompositematerials

The new functional nanocomposite materials and their designand development have greatly attracted the attention of scientists’who are interested in lowering the operational temperatures of theSOFCs with an eye towards commercialization [87]. A conductivityof 0.1 S cm�1 level is a basic requirement for high performanceSOFCs. The conventional SOFC using yttrium stabilized Zirconia(YSZ) reaches this conductivity at �1000 �C. Designing and devel-oping super oxygen ion conductors for advanced applications atlow enough temperatures to be technically useful, especially forLT (<600 �C) SOFCs, is a challenge for Material Science and Physics[87,88]. Oxygen ionic conduction through the structural vacancymechanism has been known and dominated SOFC science andtechnology since the first SOFC was created [89]. Conventionalhigh O2� conductivity is realized by ion-doping to create oxygenvacancies in a single-phase, YSZ structure [90]. However, thismechanism limits the high temperatures needed to activate ions,thus O2� conductors are infeasible at LT. The conventional electro-lyte, YSZ, is polycrystalline, and the continuous grain boundarynetwork (Fig. 4) significantly limits ion conduction [88]. Thus, even

Fig. 4. A schematic of continuous grain boundaries network that are highlyresistant surround grains in the polycrystalline electrolyte [88].

Fig. 5. Temperature dependence of the conductivity for core–shelled ceria-carbonate nanocomposites [14].

a thin film YSZ electrolyte SOFC still needs to operate above 700 �C,or ca. 1000 �C for bulk YSZ.

We have developed NANOCOFC focusing on two-phase materialinterfaces: interfacial phases/structures and properties, e.g., super-ionic conduction. In the two-phase composite material, ion dopedceria and carbonate form a core–shelled structure (Fig. 2b) [80,91].The clear interface between the ceria and carbonate two-phasenanocomposites leads to a super-ionic phase transition around300 �C and brings a super-ionic conduction, 0.2 S cm�1, as shownin Fig. 5.

A sharp conductivity leap takes place below 300 �C, giving thesame conductivity level as YSZ at 1000 �C. Against traditional SOFCmaterial ion conduction via oxygen vacancy and bulk diffusionmechanism in a single-phase, the interfacial superionic conductionis a new approach employing two-phase nanomaterial compositesto create new functionality beyond that of each phase in isolation.This interfacial mechanism has characteristics of high ion (defect)concentration and mobility and extremely low activation energy(at the interface there is no need to activate from the structure).This is a powerful tool to design and construct the interface struc-ture and property through nano-engineering and fabricating formultifunctional nanocomposites, especially for LTSOFCs. The dualO2� and H+ conduction and bi-functionality of the LT SOFC pro-cess/science and correspondingly new technology have also beendeveloped [88]. All these developments within the NANOCOFChave opened up new horizons and opportunities for SOFC science,technology and applications.

The latter developments were devoted to develop Ceria-LiZn-Oxide core–shell structured nanocomposite electrolytes [92]. Aceria, e.g. SDC as a core and an amorphous LiZn-oxide layer in 1–2 nm thickness covered on the SDC core particles as shown inFig. 6. These new functional nanocomposite materials have beensuccessfully used for LTCFCs resulting in above 0.6 W cm�2 at520 �C. This may be a new generation of the AFC technology.

More significant developments of the NANOCOFC materialfunctionalities expanded to integrate semi- and ion conductingmaterials in a nanocomposite and its applications have resultedin a revolutionary FC technology: EFFCs from our very recent workfollowing also by others [3–6,93–97].

2.3. The electrolyte-free fuel cell (EFFC)

The breakthrough was made to use single component/layer de-vice (SC/LD) technology for EFFCs [3–6,93–97]: to remove the elec-trolyte – a bottleneck of the FC technology and commercialization

Fig. 7. Schematics of how a conventional three component FC configuration (a) replaced by the configurations of the single-component FC device (b) and the dual-componentFC device (c) invented by Zhu [4,93].

Fig. 8. (a) Morphology and (b) electrical conductivity in hydrogen and in air,respectively, of the LiNiZn oxide samples.

B. Zhu et al. / Applied Energy 106 (2013) 163–175 169

is, therefore, overcome. Besides, the SC/LD technology makes moreefficient energy conversion and ultra-low cost products. Conven-tional FCs have historically consisted of the three components ofanode, electrolyte and cathode since their invention by Grove[98] in 1839 (see Fig. 7a), where the electrolyte is the key compo-nent and often a limiting factor. The PI invented SC/LD EFFCs inJuly, 2010 by integrating all anode, electrolyte and cathode func-tions (Fig. 7b) into one. This novel device design uses only a singlehomogeneous multifunctional nanocomposite layer without ‘‘mac-roscopic’’ layered electrolyte, anode, and cathode constructions.This SC/LD can provide the same function and performance as con-ventional FCs but with simple fabrication and design. It clearly dif-ferentiates (and should not be confused with) the scienceassociated with the new devices herein from conventional FCR&D. The two types of devices/technologies have very differentarchitectures, and the balance of processes, reactions and mecha-nisms are very different. In addition, the electrolyte-free fuel cellcould be also constructed by two-layer configuration, see Fig. 7c,which will be further discussed later. The EFFC represents a newtype of energy conversion technology and crosslinking science.This is maximizing the research and knowledge gained from theprevious NANOCOFC project to further develop a multifunctionalcomponent by integrating ionic and semi-conductors/conductionand p–n bulk heterojunction and band gap properties as well ascatalysts for H2 production using solar light (photocatalysts). Inthis way one single-component can realize multi-functions and en-ergy conversion on site more efficiently than conventional ways.

The EFFC is realized using the NANOCOFC approach by develop-ing multifunctional nanocomposites of ceria-carbonate and semi-conductors. Compared to the conventional three-component FCdesign, this novel structure consists of only a homogenous com-posite layer of a mixture of transition-metal oxides and an ionicconductor. This layer works as a bi-catalyst for the hydrogen oxida-tion reaction (HOR) and the oxygen reduction reaction (ORR).When H2 is supplied to one side of the device, it is ionized to H+,releasing electrons as an anode function. The side exposed to O2

then acts as a cathode, receiving the electrons and producingO2�. During this process, a cell potential is generated and energycan be extracted. The results show a FC reaction, i.e., conversionof H2 and O2 into H2O and electricity by an electrochemical routeas in a traditional FC.

Obtained results show performance as good as existing three-component FCs and prospects for further improvements are excel-lent. This will lead to very simple constructions and significantreduction of cost, opening up opportunities for earlier commercial-ization. It can also greatly reduce the device expenditure and thecomplexity, helping pave the way towards more cost efficientFCs and improved marketing competitiveness.

Fig. 8a shows a TEM micrograph for the EFFC used single com-ponent: LiNiZn-oxide (LNZ) sample. They exhibit nano-scale parti-cles with sizes in a range of 50–60 nm. The homogenous one layercomponent contains both LNZ and SDC and, thus, contains both io-nic (O2�) and electronic (n-type and p-type) conductivities. Westudied electrical properties and electrochemical impedance spec-tra (EIS) of the LNZ-SDC layer. As shown in Fig. 8b, the material

Fig. 9. (a) schematics of a conventional three-component FC and EFFC withhydrogen and air supplies and (b) with electron release and acceptance at particlesurfaces in the material, respectively. H2O is generated via 2H+ and O2� combina-tion and the electricity is generated in the processes.

170 B. Zhu et al. / Applied Energy 106 (2013) 163–175

exhibits high conductivity (both electronic and ionic conductivity),typically higher than 0.25 S cm�1 over 600 �C. In a H2 atmosphere aslight enhancement is observed compared to that in air.

Conventional FCs with the anode/electrolyte/cathode threecomponents or MEA (membrane electrode assembly) realizes theelectricity generation via ion transportation through the electro-lyte. The electrolyte acts as a critical separation barrier betweenthe electronic and ionic conduction phases (see Fig. 9a), besidestwo interfaces between electrolyte/anode and electrolyte/cathodecreate big polarization losses to limit serious FC performance[99–103]. Compared to the MEA FCs, we have shown that our sin-gle homogenous layer device (Fig. 9b) can realize the FC reactions/functions directly with H+ and O2� ions by ionization, movementand reaction joint with electrons in the processes.

In the first case, the device can function as a FC in an electro-chemical way, i.e.

At H2 contacting side : H2 ! 2Hþ þ 2e� ð1Þ

and at air ðO2Þ contacting side :12

O2 þ 2e� ! O2� ð2Þ

overall reactions : H2 þ12

O2 ! 2Hþ þ O2� ð3aÞ

2Hþ þ O2� ! H2O ð3bÞ

Combining 3a and 3b, we get

H2 þ12

O2 ! H2O ð4Þ

Thus the SC/LD has realized the same function, Eq. (4), as con-ventional anode-electrolyte-cathode 3-component FCs. However,Eqs. (1)–(3) (3a and 3b) indicate the difference from a conventionalFC due to the avoidance of an electrolyte separator in the new de-vice. The electron transfer/reaction and electricity generation canbe realized directly between the H+ and O2� ions not involvingion transportation through a bulk electrolyte separator. While inconventional FCs, ion transported through the electrolyte is criticalin order to complete the FC reactions. All reactions take place inthis one homogenous layer. Therefore, our device may be namedan ‘‘electrolyte-free fuel cell (EFFC)’’ which means an ‘‘electrolyteseparator-free fuel cell,’’ as regards its ability to realize the samefunctions without using an electrolyte separator. This concepthas been further developed as two-layer configuration EFFC de-vices, Fig. 7c, i.e. using the n-conducting and p-conducting materialrespectively mixed with the electrolyte in appropriate ratios toconstruct the EFFC devices without the electrolyte separator [85].The two-layer EFFC device could also achieve comparable resultswith the single-layer EFFC and three-layer conventional FCs.

Fig. 10 displays comparative performances for these three typesof fuel cells. The two-layer EFFC has further proven the electro-lyte-free concept and shown vast possibilities for new develop-ments. The OCV of SCD and two-layer EFFC are around 1.0 V andboth devices give a maximum power density around600 mW cm�2 at 550 �C, similar to the conventional three-compo-nent FC as shown in Fig. 10a and b. Especially, the SCD device hasdelivered a power density of 300 mW cm�2 at 430 �C. These resultsshow a FC reaction, i.e. converting H2 and O2 to H2O and electricityvia an electrochemical route in the SCD and two-layer EFFC withsuper advantages operating at the low temperature [4–6].

Fig. 11 shows calculations on the EFFC device efficiency by com-parison with the conventional SOFC. The calculations are based onthe following considerations: (i) anode and cathode activations andconcentrations are assumed to be the same for the EFFC and con-ventional SOFC, just removing the contribution of the electrolyteto the voltage/efficiency loss; (ii) the same current efficiency (orfuel and oxidant utilization) and theoretical efficiency (or deltaG/delta H). The overall efficiency equals to voltage efficiency/cur-rent efficiency/theoretical efficiency. In this case, up to 18% higheroverall efficiency can be gained in the EFFC compared to that of theconventional SOFC [104].

It is surprising how a one-component layer can combine theproperties of a complete three-layer/component FC and its func-tions for the anode, cathode and electrolyte? The electrolyte isthe core constituent of all traditional FCs, and it acts as a criticalcomponent for transporting the ions, as well as, at the same timeas a separator to the ion and electron phases blocking electronspassing through to prevent the device from short-circuiting. Con-versely, in the new device a single homogenous layer mixing bothelectron/hole and ion conductors can reach the same voltage andperformance as normal FCs under H2/air operations. Our resultsshow that a single layer can perform in the same way as a complexthree-layer FC, including ion transport and ion–electron junction/function between the anode/electrolyte and electrolyte/cathode.We examined the semiconducting properties of an LNZ-SDChomogenous layer. Fig. 12a displays the I-V measurement obtainedby applying an external voltage on the LNZSDC sample. Afterapplying a certain bias voltage (ca. 1.1 V), the current increases sig-nificantly, indicating a semiconducting property of the material [4].At this moment there is still lack of the detailed scientific knowl-edge of all the processes involved, but we have thus far reachedan understanding that our new device functions joining electro-chemical fuel cell and physical p–n junction, same as in a solar cellprinciples, as shown in Fig. 12b [93].

On the other hand, the device physical principle involves chargeseparation and p–n junction. The single component consists of aseries of metal (Ni, Zn etc.) oxides. Some of them are n-type semi-conductor, like ZnO. The others are p-type semiconductor, like NiO.When a fuel (such as H2) and air are supplied, H2 is adsorbed anddissociated into H+ at surfaces of the n-type semiconducting parti-cles. At the same time, O2 is adsorbed and dissociated into O2� atthe surface of the p-type particles. Negative and positive chargesare generated on surfaces of the n-type and p-type semiconductors,respectively. The junction between them becomes depleted ofcharge carriers, i.e. non-conduction, like a p–n junction. Conse-quently, a cell potential is generated and electric energy can be ta-ken out of the device. The situation is similar to already knownresources of photovoltages that can arise from the gradients ofboth the photoelectric field, and the equilibrium densities andmobilities of electrons and holes [105]. In FC process, a proton acti-vation process is similar to photon activation for a photovoltaicprocess as in a solar cell based on a p–n junction. This is a key toenable the device to avoid the short circuit as a power device. Asa fact, two-layer device using n (electrolyte mixture) and p (elec-trolyte) materials can also prove this.

Fig. 10. (a) and (b) Electrochemical performance comparisons of single-layer and dual-layer fuel cells with conventional three-layer fuel cells at 550 �C. (c) V–I and P–Icharacteristics of one homogenous layer device at various temperatures [4,93].

B. Zhu et al. / Applied Energy 106 (2013) 163–175 171

Based on these principles and SC/LD, a joining FC and solar celldevice is under development for a new applied energy technologyas shown in Fig. 13. This new crosslink research has multi-impacts

on the research and development: (i) introducing semiconductingand physical principles to study fuel cell, to deepen understandingof the FC science and developing new technology, the EFFC is such

Fig. 11. EFFC device efficiency by comparison with the conventional SOFC [93].

172 B. Zhu et al. / Applied Energy 106 (2013) 163–175

an example. Besides, the synergic ion and semi-conducting effectscan enhance greatly both FC and solar cell performances; (ii) thusnext generation high power density and large photovoltaic (PV)power station is possible; (iii) joining FC and PV device and tech-nology can be also possible based on the SC/LD technology andthe joining function principles. The continued research displaysgreat promising future and vast possibilities to the energy field,which is currently carried out concerning both fundamental prin-ciples and technical developments for applications.

Fig. 12. (a) I–V curve at 500 �C and (b) scheme of a joined

3. Summarizations and conclusions

R&D of LTCFC and AFC has been carried out from overall aspects,e.g., materials, fundamentals, technologies, fabrication/construc-tion, electrochemistry, theoretical studies and system engineering.The FC technology, one of the greatest high technologies for the21st century today, still faces many challenges. The material iseventually critical issue. Table 1 gives an overview on major chal-lenges and features for Ceria-based nanocomposite LTCFCs andAFCs with the existing SOFC technologies.

The material R&D for LTCFCs and AFC can be classified as fallinginto several areas, (i) proton conduction dominating conductors;(ii) oxygen ion conduction dominating conductors; (iii) H+/O2� hy-brid ion conductors, and (iv) oxygen ion and electronic mixed con-ductors. The last area has only been employed for CFC electrodeapplications, while the former three areas are functional electro-lytes. Among them the ceria-based composites are the most func-tional materials for LTCFCs. The nano-composite based on thesemi- and ion conducting materials are new developments toMaterials Science and Solid State Ionics. It can be believed thatthe material innovations developed for LTCFCs and AFCs and theircontinuous development will accelerate the FC commercialization.

New material innovations generate a new FC system based onthe hybrid proton and oxygen ion conduction and dual electrode/FC processes. Theoretical studies are carried out on the fundamen-

fuel cell and solar cell principle for the EFFC [4,93].

Fig. 13. Integrated technologies containing (a) SCD electrolysis, (b) SCD EFFC and (c) SCD solar cell, respectively. The hybrid system of the SCD FC and solar cell is maintainedat elevated temperatures by integration of solar heating technology.

Table 1Overview of major challenges and features for ceria-based nanocomposite LTCFCs and AFCs with existing SOFC technology.

Types Challenges Features

Ceria-basednanocomposite AFCs,LTCFCs

Compatible electrodesa Low temperatures, 300–600 �CWell established science Scaling up andfabricationb Long-life test need

Two-or multi-phase composites, typical example, Ceria-carbonate, ceria-YBC, ceria-LiZnO2

Dual or hybrid ion transportInterfacial conduction dominatedAnode/electrolyte/cathodeThick electrolyte, 200–1000 lm

Single component EFFCLTAFCs

Urgent new science need long-life test need Low temperatures, 300–600 �CTwo-or multi-phase composites of the ion and semiconductors, typically, ceria-LiNiCuZn oxideMulti-charge, H+, O2�, e�, h. transport Interfacial or surface conduction and reactionSynergic ion and semiconducting propertiesSingle-component deviceThickness 600–1000 lmEasy engineering and scaling up

Solid oxide fuel cells(SOFCs)

Sufficient low temperatures low costs High temperature, 800–1000 �C and technologySingle phase and single ion, O2� typically, Yttrium stabilized zirconia (YSZ) electrolyteStructural bulk conduction anode/electrolyte/cathodeThick electrolyte, 200 lm

Thin film SOFCs Expensive technologies scaling up andfabrication long-life test needs low costs

Low temperatures, 500–700 �CCeria single phase material single ion, O2�

Thin film, typically, a few to 10 lm

a In conventional anode/electrolyte/cathode configuration, it needs to develop compatible electrodes, anode and cathode, with the functional nanocomposite electrolytes.However, in the latest breakthrough technology, single component fuel cell device, such challenge has been avoided.

b Scaling up processes and fabrication technologies need to be developed since nanocomposites request special shaping processes.

B. Zhu et al. / Applied Energy 106 (2013) 163–175 173

tal electrochemistry and the nature of the new systems. Technicaldevelopments are the key to realize the challenges and make theLTCFCs and AFCs for practical applications. In addition to conven-tional effective routes, innovative and cost effective approachesare needed to scale up and engineer the LTCFCs and AFCs.

The LTCFC and AFC are a new FC generation. It requires strongsystem engineering and development through integration ofadvantages from other existing FC systems to create a new ad-vanced system. Correspondingly, many new fundamental and ap-plied subjects, NANOCOFC, are open for study. To meet the R&Dstrategy and new system demands, extensive and intensive na-tional and international cooperation as well as other joint effortshave been established and developed based on our EC-ChinaNANOCOFC network and various joint efforts. We will continu-ously strengthen and develop the network and global efforts torealize the LTCFCs and AFCs, a new FC generation, from both fun-damental and applied fields.

Given our EFFC breakthrough results to date, it can be expectedto catch a glimpse of possible new directions for FC research, liber-ated from the constraints of electrolytes and complex multi-lay-ered structures. In case of success, a new reality would appearconcerning FC science, material and technology and commerciali-zation. A new crosslink scientific disciplinary: SEMIONICS (semi-conductor-ionics) to deal with such new nanocompositematerials combining both semi- and ion conductors and theirapplications, e.g. EFFCs, has been proposed under developing. Fur-ther calculations show that, except demonstrating promisingadvantages as concern cost, simplicity and stability, the one-layerdevice (or the EFFC) is also able to improve energy conversion effi-ciency compared to conventional three-component FCs. Since ourdevice functions can be realized through surface reactions/pro-cesses; while in conventional FC devices, interfaces between theanode/electrolyte and electrolyte/cathode cause ion and electronconduction barriers that put strong constrains for achieving high

174 B. Zhu et al. / Applied Energy 106 (2013) 163–175

FC performances. The EFFC presents a new solution/way to realizethe FC challenges to speed up the FC commercialization.

Acknowledgments

This work was supported by the Swedish Research Council (VR,No. 621-2011-4983), the Swedish governmental agency for Inno-vation Systems (VINNOVA), and Finnish Funding Agency for Tech-nology and Innovation (TEKES). The lead author would like tothank Aalto University (Helsinki) for a visiting professor grant.We would also like to thank our group’s colleagues from KTHand Aalto University, especially for Dr. Rizwan Raza (KTH) andMr. Janne Patakangas (Aalto) supports.

Appendix A

SOFCs Solid oxide fuel cells based on solid oxide electrolyte(LT)SOFCs Low temperature, 300–600 �C, solid oxide fuel cells(HT) SOFCs High temperature, 800–1000 �C, solid oxide fuel

cellsCFCs Ceramic fuel cells, fuel cells using the ceramic material

electrolytes which could be able to transport any fuel cellions, e.g. H+, O2�, OH�, CO32�etc.

(LT)CFCs Low temperature, 300–600 �C, ceramic fuel cellsusing composite electrolytes

AFCs Low temperature, 300–600 �C, advanced fuel cells usingmultifunctional nano-composite electrolytes

NANOCOFC (Multi-functional nanocomposites for advancedfuel cell technology)

MCFCs Molten carbonate fuel cellsEFFC Electrolyte-free fuel cellsSC/LD Single-component/layer deviceFC Fuel cellsSDC Sm0.2Ce0.8O2–ä

YSZ Y0.08Zr0.92O2�ä

MEA Membrane electrode assemblyLNZ-SDC LiZnO-SDC nanocompositeBCY20 BaCe0.8Y0.2O2.9

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