solid oxide fuel cell (sofc) technical challenges and solutions from nano aspects

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. (2009)Published online in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/er.1600

Solid oxide fuel cell (SOFC) technical challenges and solutionsfrom nano-aspects

Bin Zhu1,2,�,y

1Department of Energy Technology, Royal Institute of Technology (KTH), S-100 44, Sweden2GETT Fuel Cell AB, S-10314, Stockholm, Sweden

SUMMARY

The classical (over 100 years) oxygen ion conductor and theory for solid oxide fuel cells (SOFCs) have met criticalchallenges, which are caused by the electrolyte material, the heart of the SOFC. Ionic conductivity of 0.1 S cm�1 as a basicrequirement limits conventional SOFC electrolyte material, yttrium stabilized zirconia (YSZ) functioning at ca. 10001C.Such high temperature prevents SOFC technology from commercialization. Design and development of materialsfunctioning at low temperatures are therefore a critical challenge. State of the art of the nanotechnology remarks a greatpotential for SOFCs. Through a review of typical SOFC electrolyte materials and analysis of the ionic conduction theoryas well as constrains and disadvantages in single-phase materials, the need for design, development and theory of newmaterials are obvious. Our approach is to design and develop two-phase materials and functionalities at interfacesbetween the constituent phases in nanotech-based composites, that is nanocomposites. The nano- and compositetechnologies can realize superionic conduction by constructing the interfaces as ‘ion highways’. Manipulation of theinterphases of the nanocomposites can overcome SOFC challenges and thus enhance and improve material conductivityand FC performance at significantly lower temperatures (300–6001C). Copyright r 2009 John Wiley & Sons, Ltd.

KEY WORDS: low temperature solid oxide fuel cell; nanocomposites; single-phase material; two-phase material;interfaces; superionic conduction; bulk mechanism; interfacial mechanism

1. INTRODUCTION

Solid oxide fuel cells (SOFCs) are one of the mostpromising fuel cell (FC) technologies. But thechallenges are critical and problems are caused bythe electrolyte materials that function only at high

temperatures (HT) and prevent the SOFC com-mercialization. The currently used SOFC material,yttrium stabilized zirconia (YSZ), was discovered100 years ago by Nernst [1] and used for SOFCsin the 1930s [2]. For over half a century, extensiveSOFC R&D based on the YSZ have been

*Correspondence to: Bin Zhu, Department of Energy Technology, Royal Institute of Technology (KTH), S-100 44, Sweden.yE-mail: [email protected]

Contract/grant sponsor: Swedish Agency for Innovation Systems (VINNOVA)Contract/grant sponsor: Swedish Agency for Energy (STEM)Contract/grant sponsor: EC FP6 NANOCOFC; contract/grant number: 32308

Received 29 April 2009

Revised 28 June 2009

Accepted 28 June 2009Copyright r 2009 John Wiley & Sons, Ltd.

undertaken. An electrolyte conductivity of0.1 S cm�1 is a basic requirement for FCs. TheYSZ reaches 0.1 S cm�1 at ca. 10001C, thuscausing the HT SOFC technology. Designingand development of functional materials at lowertemperatures (LT) are a critical challenge [3] andtherefore there is a world tendency to developSOFCs for low temperatures. Many efforts havebeen done by using thin-film technologies on YSZto reduce the operational temperature [4–7]. But athin film electrolyte can not guarantee a longSOFC life because FC operation involves masstransport processes which can affect the electrolyteproperty in one or another aspect, thus causingserious degradation. Even if using the thin filmYSZ the operational temperature requires stillabove 7001C or so.

State of the art of nanotechnology remarks agreat potential for SOFC or ceramic FC areas,now moving down from HTs (above 7001C) tolow temperature SOFCs. Nanostructured solid-state ionic electrolyte, coined as ‘nanoionics’ [8,9],has recently become one of the hot areas ofresearch related to nanomaterials, since they canbe used in advanced energy conversion and storageapplications [10], for example SOFCs [11–17].A more effective way is to develop new materialswith improved performance, for example super-ionic conduction at LT, say 0.1 S cm�1 at300–6001C [18–20]. The 300–6001C FC technologyis a gap in the worldwide FC R&D. The300–6001C FCs open many opportunities ofapplying new architectures of nanocompositethat yield high performance and conductivitiesresulted from nanomaterials blocks and two-phase interfaces. The ability to manipulate andengineer material synthesis and processes atatomic and molecular level can create radicalnew architectures, materials and structureswith unique functionalities and characteristics.Manipulating interphases of nanotech-basedcomposites, so-called nanocomposites provide su-perionic conductivity at significantly LTs (from10001C to 3001C). Development of 300–6001CSOFC technology also opens up new marketopportunities since applications of nanotechno-logy for FCs give a great potential to reach ultralow cost.

In the 300–6001C FC the high performance isattributed to superionic conduction at interfaces innanocomposite electrolytes. This interfacial mecha-nism opens a new scientific disciplinary todesign and develop advanced functionalities andmaterials with new opportunities to overcomeSOFC challenges. Our ongoing NANOCOFC (EC(Turkey)-China Nanocomposites for advanced FCtechnology) network, www.nanocofc.org, hascarried out extensively innovative FC R&D basedon multi-functional nanocomposites. This report,which covers a brief review and analysis of theSOFC electrolyte materials, the ionic conductiontheory, as well as constrains and disadvantages insingle-phase materials, presents new design anddevelopment of two-phase materials and func-tionalities, especially in nanotech-based two-phasematerials, that is nanocomposites, and solutionsfrom nano-aspects in comparison to the conven-tional materials commonly described in literature.

2. BASIC OF SOFC MATERIALS

It is very important to recognize the current SOFCelectrolytes’ constrains and disadvantages, whichhave caused challenges for SOFCs. We may findsolutions based on the understanding of the SOFCmaterial science and physical properties behind.Recent SOFC material developments made byusing nanotechnology will also help identifyingproblems and solutions.

2.1. Oxygen ion conductivity: structure effects

Currently used SOFC electrolytes are based onoxides, for example YSZ and ceria with a fluoritestructure (see Figure 1). Zirconium or ceriumatoms in a face-centered pattern contain a cube ofoxygen atoms (purple). Zirconia and ceria oxidesform the same crystal structure. Fluorite has asimple structure–space group Fm3m. A structuralunit cell contains fully four cerium and eightoxygen ions, the 1:2 stoichiometry is maintained.

The pure zirconia or ceria are very poor oxygenion conductors. The oxygen ion conductivity takesplace by aliovalent doping to create oxygenvacancies inside their structures. From a phasestructure perspective, stabilized zirconia, for

B. ZHU

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Energy Res. (2009)

DOI: 10.1002/er

example YSZ or ion doped ceria is a single-phasematerial since the doping or guest phase, usually8mol% Y2O3 in ZrO2 or 10–20mol% Sm2O3 inCeO2 to form a solid solution that the Y2O3 orSm2O3 is fully ‘dissolved’ in the zirconia or cerialattice to maintain the host fluorite structure.

The effect of dopant oxides on the ionic con-ductivity in ZrO2-based or CeO2-based binarysystems has been extensively investigated. Themain dopants used for stabilizing ZrO2 includetrivalent rare-earth or metal oxide: Y2O3, Yb2O3,Sc2O3, etc.; divalent metal oxide: CaO, MgO, etc.;while Gd2O3, Sm2O3, La2O3 CaO, MgO, SrO, etc.used for CeO2. The fluorite is a very open structurewhere large contents of dopant oxides can bedissolved into the lattice, in the same time to createlarge amounts of the oxygen vacancies.

For example, when a divalent or trivalent metaloxide MxOy is doped in the zirconia or ceriamatrix to form a solid solution, the followingdefect reactions would occur:

MO�!ZrO2

M00Zr=CeþV��OþO

�O ð1aÞ

M2O3 �!ZrO2

2M0Zr=CeþV��OþO

�O ð1bÞ

The oxygen ion migration or diffusion canoccur only when the neighboring oxygen site isa vacancy. The existence of a large number ofoctahedral vacancies created by doping in struc-ture makes it easier for an O2� to migrate fromone site to the neighboring one, in case that thelatter is a vacancy.

Though high doping concentration results inhigh defect (oxygen vacancy) concentration tofacilitate the oxygen ion mobility or conductivity,after reaching a limit a stronger defect associationtakes place, for example

M0Zr=CeþV��O ! ½M

0Zr=CeV

��O�� ð1cÞ

2M0Zr=þV��O½M

0Zr V

��OM

0Zr�� ð1dÞ

This equals to occupation of the oxygen va-cancy to make the oxygen sublattice ordering, thusreducing the O2� conductivity. This prevents fur-ther increasing the material conductivities.

The ionic conduction is described by Arrheniusequation:

sT ¼ s0 expð�Ea=kTÞ ð1eÞ

A simple theoretical analysis [21,22] can deduceEquation (1f ) below, and help us to understandthe Arrhenius relation (1e) and ionic conductionphenomenon.

s0 ¼nz2e2gd2o0

6kð1fÞ

The advanced ion conductor design is thus tomaximize s0 and minimize Ea. Two parameters(d, ion mean jump distance and n, mobile ionconcentration) are important to maximize the s0.From this analysis we may identify the following

O2-

M4+ (host cation)

Oxygen vacancy

M2+ or M3+(aliovalent cation)

Figure 1. Fluorite structure for SOFC electrolytes.

SOLID OXIDE FUEL CELL


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disadvantages and limitations for the single-phasematerials:

(i) Within the structure unit, a strong interactionbetween oxygen ions and cations contributesmajor part of the high Ea values in the order of1.0 eV (while o0.5 eV is required for super-ionic conduction);

(ii) s0 is limited by the crystal unit cell at least intwo aspects, (a) n is limited by the degree ofdoping resulting in a low concentration, forexample 8mol% of Y2O3 in YSZ; though thehigher doping can increase the oxygen vacancyconcentration, in the same time the higherdefect (oxygen vacancy) concentration willreduce the oxygen ion conductivity due tothe defect association; (b) d is limited in a unitcell, that is of the order of some Angstrom.

These structural effects put strong limitationson the conventional SOFC materials, resulting inhigh activation energy, low oxygen ion con-centration, and mobility; thus HT is required toactivate sufficient ionic mobility, for example10001C for YSZ. Added to these the nonstructuraleffects are even more crucial.

2.2. Oxygen ion conductivity: microstructural effects

The basic considerations are as follows:

(i) Ceramic electrolytes are polycrystalline andconsist of grains, grain boundaries, pores, etc.as shown in Figure 2;

(ii) Grain boundaries (gb) often acting a predo-minating role have a significant influence onoverall properties;

(iii) Studies of grain boundary behavior areessential to design ceramic materials and tocontrol and optimize their properties.

In the majority of oxygen ion conductors, thesmaller the grain size, the higher the resistivitybecause of:

(i) inter-grain neck and contact;(ii) space charge effects that tend to lower oxygen

vacancy concentration near the grain boundaries;

(iii) insufficient oxygen vacancies in the grainboundaries [23].

The conductivity of the polycrystalline electro-lyte is contributed from both grains and grainboundaries. The grain boundary resistivity instabilized zirconia or doped ceria is often in mag-nitude higher than the grain resistivity, despite theion transport activation energy typically rangingbetween 1.08 and 1.16 eV, higher than that of thegrain [24]. Figure 3 shows schematically thesituation in the polycrystalline electrolytes, wherecontinuous grain boundaries’ network with highresistivity surrounds grains. This indicates thatthough the structure of the stabilized zirconia ordoped ceria is optimum for the high ionic con-ductivity, the polycrystalline nature from the non-structural effects, the microstructural morphology,etc., is more important to determine the finalconductivity. The emphasis should be, therefore,placed on engineering of the grain boundaries byenhancing the local oxygen vacancy concentration

grain

Grainboundary

Figure 2. Polycristalline electrolyte from a SEM view.

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at the grain boundary entails then significantlylowering the grain-boundary resistance in order toobtain material high conductivity.

2.3. Remarks

State-of-the-art of the current SOFC materials andoxygen ion conductor/tion in the single-phasematerials may tell us:

(i) Ion doping is a way to generate structuraldefects (oxygen vacancies) in structure forcreating the oxygen ion mobility and con-ductivity; however, the microstructure plays acrucial role to determine the material finalconductivity;

(ii) Though the fluorite structure has large capa-city to accept high doping contents, above adoping level limit, the defect (vacancies)association occurs to decrease conductivity;

(iii) Because of structural limits and high activa-tion energy, the sufficient O2� conductivity/mobility occurs only at HTs, for example0.1 S cm�1 at 10001C for YSZ or 8001C fordoped ceria;

(iv) Grain boundaries or interior contacts betweenparticles are barriers to limit the ionicconductivity since the grain boundary con-ductivity is much lower than (order) that of

the grain thus blocks the ionic transport.Engineering of the grain boundaries is a topoption to improve and develop material ionicconductivity.

2.4. Nano-effects and new principles

However, the situation changes at the nano-levelwith new materials, functionalities, and principles.Recent developments are directed by extensivelyusing nanotechnology and nano-size materialsfor the conventional SOFC electrolytes and sup-port new scientific principles. These new studiesfrom nano-aspects have shown the common factsthat the nanostructured ceria-based materials[12–15] and YSZ [16,17] exhibited strong conduc-tivity enhancement, due to significantly largerarea of grain-boundary or interface in nanostruc-tured systems, which increases the concentrationof mobile defects in the space-charge zone [9,10].The grain-boundary in ionic conductionsuddenly turns from negative/side effect in thenormal-sized (say micrometer level) to positiveeffect as promoter in the nano-sized materials.These new developments obviously disagreewith the mainstream of the current SOFC Sciencebuilt on the conventional materials as describedabove.

Figure 3. A schematic of continuous grain boundaries network that are highly resistant surround grains in thepolycrystalline electrolytes.



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The different particle size levels may causecompletely different ion transport phenomena. Inthe normal particle size the grain boundary blocksthe ion transport ways, thus causing extremelyhigh resistivity gap between the grains. The ionsmigrate from one grain into another forming adiscontinuous path thus causing low ion con-ductivity (see Figure 3); while in nano-scale, thesituation changes to opposite where the grainboundaries become ion conduction highways.Compared to this the grain contribution becomesless important. In the state-of-the-art of the con-ventional materials, the approach is to eliminatethe grain boundaries to a minimum level in orderto obtain high material conductivity. Conversely,in nano-scale grain boundaries in the single phase,or interfaces and interfacial contacts in the two-phase materials should be optimized to a max-imum. An ionic conduction principle presented inFigure 4 [25] may help to understand the differentphenomena and principles. In the micrometerlevel, the O2� ions can migrate through the grains/structure; while in the nano-scale, the difference ofthe sizes between the mobile ions and the grain issignificantly decreased (hundreds of times less thanthat in the micrometer level), which causes astrong expulsion force to resist the O2� migrationwhile the ion approaching the grain, thus causing a‘by-path’ for ion pass around the grain, that isgrain boundary or interface path.

The design and development of functionalitiesand advanced materials in the mm and nm levelsrequire thus very different approach, principleand theory. We need to fully recognize their dif-ferent scientific natures/characteristics to guidenew material designs, developments and theories.Nano-structured materials can enhance the con-ductivity due to strong grain-boundaries’ effects.However, the nano-sized oxide conductors in the

single-phases are not stable, especially for SOFCenvironment at HTs. The nano-structured single-phase materials, typically YSZ and doped ceriamay not function for SOFCs, where hundreds ofdegrees Celsius can easily destroy the nano-structures in addition to strongly reducing andoxidizing environments. Moreover, in the nano-ceria case following side effects can occur [26]:

(a) transition from ionic to e� conduction whenthe particle turn to the nano-scale,

(b) electronic conductivity (se�) increases fornano-sized particle,

(c) ionic activation energy, Ea is also increasedwith decreasing grain sizes.

Thus the single nano-ceria materials for SOFCscannot be approved. To develop stable andfunctional nano-materials qualified for SOFCapplications is, therefore, a critical challenge and atough job.

3. NANOTECHNOLOGY ANDAPPROACHES AND ADVANCED

MATERIAL DESIGNS

3.1. Basic of advanced ionic conductors

The basic material design on advanced ionicconductors can be considered from fast ionicconduction or superionic conduction theory. Theprinciple is to design and realize the structure thatcan offer long-distance channels or networks forionic conduction/transport. From this aspect,though YSZ or ion-doped ceria are viewed as theopen structure with large feasibility to createstructure defects by ion-doping for ionic conductiv-ity, it is very much under demands of the superionicconduction, since the oxygen ion does not migrate inthe long-distance or networking in these materials.

In the two- or multiphase nanocompositematerials the two-phase co-existence creates theinterfaces and particle surfaces between the con-stituent phases, where the superionic conductiontakes place. Also, the energy to form defects onthe surfaces or interfaces is significantly lowerthan that inside the bulk structure. Thus, lowtemperature can activate sufficient ion mobility.

nm scaleion grain

µm scale

Figure 4. A principle presentation of the ionic migrationat micrometer and nanometer levels, where grain

boundaries act as side paths for ion pass.

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The interface has, in principle, no bulk structurallimit to create high concentration of mobile ions,and can thus be very disordered. This implies thatthe interfaces have the capacity to contain highermobile ion concentration and also longer rangetransport channels/networks (from particle toparticle at nanometer level) in the interfaces thanthose of the bulk effect occurring within a crystal(e.g. YSZ) unite cell (at Angstrom level) of thebulk, thus strongly enhance the conductivity.

3.2. Two-phase architectures and functionalitiesof the nanocomposites

Multifunctional nanocomposites for advancedLTSOFCs are based on developing and modifyingnano-particles’ surface physical and chemicalproperties to obtain new desired properties thatthe bulk materials do not possess. The surfaceeffect becomes more significant with the particlesizes decreasing, especially o10 nm. At this levelatoms mostly concentrated/shifted on the nano-particle surfaces from the bulk (e.g. at 1 nm levelalmost all atoms 490% will stay on the surfaces).These atoms lack coordination or configurationwith very high surface energy and are thus easilyready to interact with other material’s atoms to bestabilized. Such activated or unstable surfaceatoms (ions) become highly mobile, resulting insuperionic conduction at interfaces. Clear under-standing of physics behind the ionic conductivitywould be very helpful for the development of

complex architectures of nanocomposites (shapeand configuration). The superionic conductivity40.1 S cm�1 in the 300–6001C range can berealized by two-phase nanocomposites with newarchitecture by constructing interfaces as ionhighways to result in superionic conduction innetwork with continuous paths. This has beendealt with by a theoretical approach reportedbefore on the ceria-carbonate (as the 2nd phase)composites, see Figure 5 [27]. The ion transportmechanism in interfaces is under study and needsmore investigations both experimentally andtheoretically since there is a lack of knowledgeand theory on this subject. Based on the nano-science principle, several concepts of the advancedmaterial architectures may be presented below.

3.2.1. Architecture 1. The concept of ‘nanocom-posite’ [28–30] by adding a secondary phase asinclusion can effectively hinder the grain growth ofnanostructures. Nanocomposite approach cancreate a nano-core particle with a nano-layer-shellformed by the 2nd phase, or by interaction (layer/shell) between the constituent phases, both coreand shell/layer are material components to createfunctionalities. This approach can prevent thenano-functional particles from energetic growth atHTs and high activity in extreme atmospheres. Inthe same time it can create new functions/proper-ties at interfaces between the two constitutephases. It is common that nano-sized particle insingle-phase materials can, on one hand, create

Interfacial

paths

Figure 5. A schematic of continuous interfaces between the two constituent phases acting as high ionic conductingways or network where the interfacial conduction between the two constituent phases are indicated by arrows (after

Reference [27]).



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large particle surfaces, but on the other hand theparticles are not stable because of high surfaceenergy and the active and chemically unboundedsituation. In two-phase materials large use of theappropriate 2nd phase material/particles cancreate interfaces and surfaces with effective func-tions in two-phase nano-particles while, in thesame time, it can stabilize and modify the surfaceproperties. The key is to use the appropriate 2ndphase, which can construct particle–particle andphase–phase interactions and interfacial electricalfield by fixing the surface configuration andstabilizing the surface properties. Our recentachievements on the core-shell ceria-carbonateelectrolytes [31] have demonstrated such advances,where the core-shelled structure have exhibited agreat thermal stability compared to single-phaseceria, see Figure 6 [32]. In the same time createdsuperionic conduction, see Section 3.3, Nanocom-posite approach makes it possible to applynanomaterials to a new field of SOFC R&D, since

the surface energy and diffusion rate of thenanostructured materials are greatly modified byaccession of the second phase materials, resultingin suppression of grain growth under HT.

3.2.2. Architecture 2. Nano-wires and long ionicconduction paths/channels

As one of the attractive nanomaterials with uniquechemical and physical properties, one-dimensional(1D) nanowires have attracted a lot of interest forboth scientific research and technological applica-tions [33–36]. Based on the interface conductiontheory, the 1D nanowires structure is supposed tohave a longer continuous interface when formingthe two phase nanocomposite electrolyte, whichindicates it may achieve higher ionic conductivitycompared to nanoparticles, the 3D nanomaterials.Therefore, application of doped ceria nanowires inSOFCs field is a challenge and will open up a newarea in nanocomposite electrolyte R&D.

Figure 6. SEM image of: (a) uncoated SDC (samarium doped ceria) annealed at 7001C for 2 h; (b) uncoated SDCannealed at 7001C for 24 h; (c) SDC/Na2CO3 nanocomposites annealed at 7001C for 2 h; and (d) SDC/Na2CO3

nanocomposites annealed at 7001C for 24 h. Scale bar: 200 nm (after Reference [32]).

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The SDC nanowires structure in the compositecan be optimized and better aligned to achievesuperionic conductivity. It is certainly a verypotential electrolyte material, since one dimen-sional nanowires structure is supposed to havelonger continuous conductive path at interfaces innanocomposite electrolyte, which indicates it mayachieve higher ionic conductivity compared to the3D nanoparticles. It opens a new interestingsubject for research.

3.2.3. Architecture 3. Redox properties in two-phase materials: The redox property of ceria hasbeen applied in catalyst field for decades withsuccessful applications. This property is based onreversible Ce41-Ce31 process accompanyingoxygen depletion and deposition. The single-phaseceria alone causes both electrical and mechanicalproblems for SOFC applications. The new ideamay be that we could take the advantage of theCe41-Ce31, then use two-oxide with redox proper-ties in the two-phase material architecture. Typicalexamples are CeMO2 (M5 Sm, Gd, Y, Tb, Pr,Mn, Ca, etc.)-MxOy (M5Ti, Bi, V, Co, Ni, Cu,Fe, etc., x5 1–3, y5 1–4) through the redox cyclebetween two-phase components to enhance andadjust the material properties and create newfunctions. This architecture has a significant rolein creating functional interfaces. For example, forelectrolyte materials it may create superionicconduction and mechanical stability; also ceria-based two-phase material nanocomposites(nano-particles, nano-rods, and nanotubes) maybe constructed with other H1/O2� conductors indifferent configurations/architectures (core–shell,capping, bonding, and linking). It has beendiscovered that a number of ceria-salt compositespossessing dual H1/O2� conduction eitherforming intermediate bonding or cationicvacancies with the salts [37,38]; these are moreconsidered as an interfacial mechanism [39];for electrodes, the ceria- two-phase materialsincluding electronic conductor and catalysts, forexample NiO, CuO, WO3, MoO3, and carbonnanotubes, act the electrode functions. We haverecently been using the nanocomposite orNANOCOFC approach successfully developednanocomposite electrode materials, as shown in

Figure 7, where the nano-catalyst particles(5–10 nm range) are highly homogenously distrib-uted in the composite electrode, resulting in highcatalyst and electrode functions and LTSOFCperformances.

Using electron microscopy (EM), the phasestructures of nanoparticles and their interfaces canbe examined. It also allows physical parameters(such as shape and size) of these nanoparticles tobe characterized. With advanced EM techniquessuch as high-resolution TEM (HRTEM), electronenergy loss spectroscopy and the selected areaelectron diffraction it is expected that the ionictransport within the particles and their boundariesas well as two-phase interfaces can be clarified. Allthese microstructural information will be veryvaluable to help improve the design of materials

Figure 7. (a) The SEM picture of a nanocompositeelectrode; (b) the TEM image of a nanocomposite

electrode.



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and manufacture of FCs. We are conductingextensive studies in this field.

3.3. Nanocomposites and interfacial functionalities

Development of synthesis methods (e.g. sol–geltechnique, co-precipitation, combustion, microwaveheating, multi-layer growing, nano-colloid stabiliza-tion, combinatorial and high-throughput synthesisand testing, thermal spray, nano-coatings, etc.) areimportant to develop the new materials withcontrolled microstructure, porosity and bulk andsurface/interface composition with new physical andchemical properties. New architectures with desiredfunctionalities may be developed. For example,nanocomposites by intercalation of polymers andbiopolymers into layered inorganic compounds;Preparation of stabilized multi-component nano-structures based on nanosized supported metals andmixed oxides; well-defined model interface/layeredsystems; Self-assembling properties of materials playimportant role in fabrication of new compositionswith new physical and chemical properties; Dis-ordering structures by interactions between surfac-tant and the 2nd guest phase particles introduced bynanotechnology. Furthermore, conventional dopingmethodology in developing functional nanocompo-sites is not only used to create structural defectssame as the conventional way, but more importantlyas the particle surface modifier of the nano-environment to build functionalized interfaces uponusing controlled surface reactions in both hetero-geneous and homogeneous. Many new nanocom-posites and functionalized interfaces or interfacialfunctionalities will be developed for successful LT,300–6001C SOFCs, next generation FC R&D [39].

Among all interfacial functionalities, the ionicconductivity or low temperature, 300–6001Csuperionic conduction is most important forLTSOFC R&D. Unlike the conventional super-ionic conduction that appears in a single-phasematerial accompanying a structural change, thesuperionic conduction in the composites isdetermined by the interfaces, that is a processbreaks symmetry of matrix by introducing astructural discontinuity [9]. In contrast with thebulk, where electroneutrality must be obeyed, atthe interface a narrow charged zone, the so-called

space-charge zone is tolerable and thermo-dynamically necessary [40]. The defect concentra-tions in the space-charge zone are much higherthan that in the bulk, which accounts for higherionic diffusivity and mobility than bulk. One effi-cient method to enhance the space-charge effect isto decorate the grain boundaries with surface-active second-phase materials, which is also calledheterogeneous doping process [41]. The ceria-based composite electrolyte with remarkable ionicconductivity is a victorious case of superionictransitions excited by heterogeneous doping pro-cess, where interface supplies high conductivitypathways for ionic transportation and conduction.

Though in single-phase materials the nano-effects can effectively increase the materialconductivity, the Debye length is significantlydecreased to reduce the concentration (number) ofthe mobile species in the nano-scale, resulting in noeffect on enhancement of the ionic conductivity.The nanocomposite approach can overcome thesingle-phase material bulk nano-side-effect be-cause of creating tremendous surfaces/interfaceswhere the ionic conduction takes place. In nano-sized materials, calculations show that the meanfree volume is significantly larger at boundaries,which enhances the mobility of ions at theboundary core. The enhanced boundary diffusionleads to significant enhancement of effective dif-fusion coefficients of materials with a highboundary density. In the case of ionic materials,this boundary mechanism or interfacial mechan-ism in the two-phase nanocomposites may makethe superionic conduction in the nano-regime.

We have taken as a case study on the ceria-carbonate two-phase material to discuss interfacialmechanism. The calculations showed that theoxygen ion activation energy in such interface iso0.2 eV (to well meet the superionic conductionrequest) [27], while it is around 1.0 eV via the bulkeffect in the single-phase material YSZ or ceria.The results obtained from the HRTEM analysishas proven a clear interface in the ceria-carbonatetwo-phase nanocomposites between the SDC(Samarium-doped ceria) and carbonate forming acore-shelled structure, see Figure 8(a), and suchinterface (being usually amorphous) has resultedin superionic conduction, 0.2 S cm�1, a sharp

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conductivity leap takes place below 3001C, seeFigure 8(b). As a fact melting point of Na2CO3 isat ca. 8501C, while individual SDC and Na2CO3

are almost nonconductive at 3001C. Evidencesof the interfaces and significantly improvedperformance for the ceria-carbonate two-phasematerial system proves that interfaces play adominant role in this effect. The 3001C superionicconduction with the same conductivity level asYSZ at 10001C and advanced two-phase nano-composites show indeed a breakthrough in theSOFC materials.

This research area can strongly support newgeneration SOFC R&D and eventually presentsolutions to realize SOFC commercialization.Based on the new functional materials several

advanced technologies, for example industrialmaterial technology for cost-effective LTSOFCs[42] and direct alcohol (biomass-base produced)fuelled 300–6001C FCs [43] are under develop-ments within the Swedish company GETT FCAB, KTH and other joint industrial efforts. InKTH, the existing EXPLORE: future energyplatform established in the Department of EnergyTechnology has the FC polygeneration as one ofthe most active components. These present anemerging need on innovative FC R&D and solu-tions to realize the LTSOFC commercialization.

4. CONCLUSIONS AND SUMMARY

Classical oxygen conduction and theory in single-phase materials encounter critical challenges. State-of-the-art of the conventional SOFC materialscontains a number of limitations and disadvantagesthat do not meet demands of applications. Beyondthe structural limit, nonstructural or microstructureeffects play even more crucial role with negativeeffects on the ionic conductivity. Nanotechnologyand nanoeffects discovered in the conventionalmaterials, for example YSZ and ceria exhibit agreat potential and open up new scientific areas.However, nano-sized single-phase alone cannotovercome the challenges that SOFCs face. Thereis, therefore, an emerging need to develop newmaterials and new Science, where NANOCOFCand two-phase nanocomposites Science and techno-logy are greatly highlighted.

Material designs and advanced nanocompositearchitectures integrated by nanotechnology haveunique advantages over conventional single-phasematerials. The two-phase nanocomposite approachcan create a wide range of functional materials, notlimited by structures, as long as the selected twoappropriate phases can create desired functions atinterfaces. However, extensive studies on inter-facial ionic conduction mechanisms are funda-mental requirements to circumvent the presentstate of the art where the experimental and tech-nological exploitation of these materials is appar-ently moving faster than the understanding oftheir performance. Fundamental studies are thusstrongly needed.

(a)

1.0

-7.5-7.0-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.50.0

550

(°C)

lgs

(S/c

m)

1000/T (1/K)

500 450 400 350 300 250 200 150

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6(b)

Figure 8. (a) The TEM of core-shelled ceria-carbonatenanocomposites; (b) temperature dependence of theconductivities for core-shelled ceria-carbonate nano-

composites (after Reference [31]).



DOI: 10.1002/er

ACKNOWLEDGEMENTS

This work was supported by the Swedish Agency forInnovation Systems (VINNOVA) and the SwedishAgency for Energy (STEM) through GETT Fuelcell AB industrialization projects and the financialsupport form EC FP6 NANOCOFC project (contractNo. 32308). The author would also like to thankcontributions of knowledge to this area from theNANOCOFC consortium partners and associatedpartners.

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B. ZHU


DOI: 10.1002/er

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