Advances and Challenges of Intermediate Temperature Solid Advances and Challenges of Intermediate Temperature Solid Oxide Fuel Cells: A Concise Review Oxide Fuel Cells: A Concise Review

: Fuel cell is an electrochemical energy conversion device to directly convert the chemical energy of fuels to electricity. Among all types of fuel cells, solid oxide fuel cells (SOFCs) operating at intermediate temperatures of 600 ~ 800 益 offer an attractive option that is much more fuel flexible than low temperature fuel cells such as proton exchange membrane fuel cells, and is suitable for a wide range of applications. However, two main challenges remain towards the commercial viability and acceptance of the SOFC technologies: the cost and durability. Both are critically dependent on the process, fabrication, performance, chemical and microstructural stability of various cell components, including anode, cathode, electrolyte, interconnect, and seal. Manifold and balance of plant materials also need to be carefully selected to ensure the structural stability and integrity with minimum volatile species. This article aims at providing a concise review and outlook of materials and components that have studied for SOFCs. The opportunities and challenges for the new generation of SOFCs technologies are briefly discussed.


Introduction
The demand for clean, secure and sustainable energy sources has simulated great interests in electrochemical energy storage and conversion technologies such as advanced batteries, fuel cells and supercapacitors.Among them, fuel cell is particularly attractive as fuel cell is considered to be the most efficient, and less polluting power-generating technology.Fuel cell is an electrochemical device that directly converts the chemical energy of fuels such as hydrogen, natural gas, methanol, ethanol and hydrocarbons to electricity and is a potential and viable candidate to moderate the fast increase in power requirements and to minimize the impact of the increased power consumption on the environment.Fuel cells are also versatile devices ranging from low temperature (<100 o C) fuel cells such as alka-line fuel cells (AFC) and proton exchange membrane fuel cells (PEMFCs) to high temperature (500 ~1000 o C) molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs).Among all types of fuel cells, SOFCs offer great promise for the most efficient utilization of various readily available carbon-containing fuels such as natural gas, hydrocarbons, gasified coal and solid carbon.For stand-alone applications, chemical to electrical efficiency of a SOFC is 45 to 65%, based on the lower heating value (LHV) of the fuel, which is twice that of an internal combustion engine.
A SOFC consists of a porous anode, a fully dense solid electrolyte, and a porous cathode.Driven by the differences in oxygen chemical potentials, oxygen ions migrate through the electrolyte to the anode where they are consumed by oxidation of fu-els such as hydrogen, methane and hydrocarbons (C n H 2n+2 ).Fig. 1  wider materials selections for interconnect and compressive nonglass/ceramic seals, as well as reduced balance of plant (BOP) costs.However, reduction in operation temperature results in a significant increase in the electrolyte and electrode resistivity and the polarization losses.To compensate for the performance losses, the thickness of electrolyte layer has to be reduced in order to lower the ohmic resistance of the cell.Using a thin electrolyte layer, the electrolyte can no longer mechanically support the cell.Thus, electrode-supported cells, typically anode-supported cells have been developed.The oxygen reduction reaction on the cathode is the most sluggish reaction when the temperature is reduced to < 800 o C [1] .Thus, the development of highly active and stable cathodes for IT-SOFCs is also one of the main tasks at the intermediate temperature region.
There are significant activities in the research and development in the materials, fabrication technologies and stack designs for intermediate temperature SOFCs, or IT-SOFCs [1][2] .This article aims to provide a concise review of the current status, advances and challenges of IT-SOFCs technologies.the most commonly used anodes [3] .The function of the oxide component is primarily to reduce the sintering of the Ni metal phase, to decrease the thermal expansion coefficient (TEC) and to improve the electrochemical performance of the anode.Thus, the composition and phase distribution are important for the thermal, electrical and ionic conductivity properties, microstructure and performance of the anodes.
For example, with addition of 30% (by volume) YSZ, the TEC of the Ni/YSZ cermet is ~12.5伊10 - K -1 , which is close to that for the YSZ electrolyte [4] .
The electrical conductivity of typical Ni/YSZ cermet (with a volume ratio of YSZ to Ni of 40 颐 60 and porosity of 30 ~40 % ) is in the range of 102 to 103 S窑cm -1 .For a simple two-phase system the theory predicts the percolation threshold at ~30 % (by volume) of the phase with higher conductivity for the transition from dominant ionic conductivity to dominant electronic conductivity [5] .Adding electrochemically active and mixed conducting interlayer, such as yttria-d oped ceria (YDC) between anode and electrolyte was shown to significantly reduce the electrode polarization resistance for methane oxidation on Ni/YSZ anode [6] .
An important issue regarding the Ni-based cermet anodes is the microstructure degradation under SOFC operating cond itions.The predominant microstructure change is agglomeration and coarsening of the Ni phase, primarily due to the poor wettability between the metallic Ni and YSZ oxide phase [7] .At SOFC operation temperatures, coarsening kinetics is fast and agglomeration of Ni occurs relatively rapidly [8] .Focus ion beam-scanning electron microscopy (FIB-SEM) technique is particularly useful in obtaining three dimensional phase distribution in SOFC electrodes to study the microstructure change of the electrodes [9][10] .Fig. 2 shows the simulated microstructural evolution of Ni-YSZ anode functional layers using phase-field approach [10] .The simulations assumed that the YSZ phase did not change whereas  Ni, Deep color represents pore, and the YSZ phase is transparent) [10] .
Carbon deposition and sulfur poisoning are two of the major issues facing the long-term stability and practical applications of SOFC technologies based on direct utilization of hydrocarbon fuels ranging from natural gas to diesel in which the most abundant impurity is sulfur.Sulfur is a major impurity in coal, so sulfur tolerance is also a major issue for SOFC power plants designed to utilize gasified coal.Ni/YSZ based cermet anodes have a very limited tolerance to H 2 S. As shown by Zha et al., the sulfur poisoning is generally characterized by a two-stage behavior: a rapid drop in the cell performance upon exposure to H 2 S, followed by a gradual but persistent deterioration in performance [11] .
Nickel and alloys based metals are highly active for carbon cracking and deposition of carbon can result in the blocking of the active sites and disintegration of bulk metals and alloys into metal particles at high temperatures (300 ~850 o C).The carbon cracking can be avoided by providing a high enough steam to carbon (S/C) ratio in the fuel gas, e.g., by mixing the fuel with H 2 O [12] .However, high S/C ratio is not attractive for fuel cells as it lowers the electrical efficiency of the fuel cells by diluting the fuel.The endothermic nature of the steam reforming reaction can also cause local cooling and steep thermal gradients potentially capable of mechanically damaging the cell stack.Thus, development of anodes with high tolerance towards carbon deposition and sulfur poisoning is critical for SOFCs utilizing hydrocarbon fuels.
Replacing Ni with carbon-inert Cu to form Cu/YSZ cermet anodes [13] and using Ni-Cu/YSZ cermet anodes prepared by impregnation of porous YSZ layer with copper and nickel nitrate solution [14] were found to improve the resistance towards carbon deposition.One main issue with Cu-based cermet anodes is the microstructure instability of copper phase.A number of groups have shown that perovskite oxides, e.g., (La,Sr)(Cr,Mn)O 3 (LSCM) and (La,Sr)VO 3 (LSV), exhibit better sulfur tolerance [15][16][17][18] ; LSV makes it possible to work with >10% H 2 S in the fuel [17] .Unfortunately, LSV appears Co-Fe alloy, showing good performance with high sulfur tolerance and coking resistance [19] .One of the main concerns of the ceramic oxide based anodes is their generally low electronic conductivity as compared to metallic Ni [20][21] .In the case of LSCM, the electrical conductivity is 38 S窑cm -1 in air at 900 o C and drops significantly to 5 S窑cm -1 in 5% H 2 [22] .The poor electrical conductivity can lead to the significant increase in the electrode ohmic and polarization resistance and contact resistance between the electrode coatings and current collector sulfur poisoning [26][27][28][29] .Infiltration of GDC nanoparticles in Ni/YSZ cermet anodes showed the significantly improved electrode performance and stability in methane without causing carbon deposition [30] .Most recently Yang et al showed that introducing barium oxide nanoparticles drastically increase the carbon tolerance in dry C 3 H 8 by promoting water-mediated carbon removal (see Fig. 3 [ 31] ).Surface modification of existing Ni-based cermet anodes remains an attractive option for the significant enhancement of sulfur and carbon tolerance without comprising performance [32] .(water was formed on the anode by electrochemical oxidationof dry C 3 H 8 ) [31] .
Cathode: LSM is still the materials of choice for cathodes of SOFCs with YSZ electrolytes [33] .
LSM shows a high thermal stability and compatibility with YSZ; more important, if compared to other cathode materials, LSM has an excellent microstructural stability and its long-term performance stability has been well established.
However, LSM is predominant electronic conductor with negligible oxygen ion conductivity.As the incorporation and bulk diffusion of oxygen inside LSM cannot be expected to occur to a significant degree [34] , the O 2 reduction reaction sites primarily occur at TPB areas, which becomes a limiting factor for the application of LSM cathode for SOFC operation at intermediate temperatures of 600 ~800 o C. Various strategies have been developed to improve the electrocatalytic activity of the LSM-based cathodes.For example, addition of YSZ phase to LSM significantly reduced the electrode polarization resistance [35] .The electrochemical performance of a LSM cathode can also be enhanced substantially by introducing catalytically active nanoparticles, such as doped CeO 2 , into the LSM porous structure [36] .LSCF-based perovskites with compositions La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 or LSCF6428 may have desirable properties for IT-SOFC cathode applications and has been extensively studied [37] .Acceptor-doped cobaltites are characterized by enhanced lattice oxygen vacancy formation and, hence high ionic conductivity.The oxygen self-diffusion coefficient of cobaltite based materials is several orders of magnitude higher than that of the manganites [34] .Fig. 4 shows a comparison of polarization performance of LSM and LSCF cathodes measured in air under identical conditions [38] .LSCF shows a much higher activity towards oxygen reduction reaction as compared to that on LSM.The main hurdles with the Co-rich perovskite oxides is the high TECs (~16-18伊10 -6 K -1 ) and high chemically activity with YSZ electrolytes.Replacing lanthanum with barium at the A-site of LSCF substantially enhances its electrochemical activity for the O 2 reduction reaction at intermediate temperatures.Shao and Haile [39] applied Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 (BSCF) as cathode to a doped ceria electrolyte cell and achieved the power densities of 1.01 W窑cm -2 and 402 mW窑cm -2 at 600 o C and 500 o C, respectively, when operated with hydrogen as the fuel and air as the cathode gas.However, its high TEC (~20伊10 -6 K -1 [40] ), low electrical conductivity (~25 S窑cm -1 at 800 o C [40] ) and instability in air [41] are some of the challenges for the practical application of BSCF cathodes in SOFCs.Coating a CO 2 -protective shell by infiltration of conductive and stable La 2 NiO 4 to BSCF porous scaffold was shown to be effective to substantially enhance the resistance of the BSCF cathode towards CO 2 attack [ 42] .
Cathode can be gradually degraded and deactivated by contaminants such as chromium, sulfur, boron which can be either in the air stream or from the volatile species of cell components, such as metallic interconnect, sealant and manifold [43][44][45] .
Chromium deposition and poisoning on SOFC cathodes have been extensively investigated and are due to the fact that the gaseous chromium species from the chromia-forming metallic interconnect migrate through the cathode and deposit in the form of low valence chromium (III) species either at the electrode/electrolyte interface, inside the bulk of the electrode and/or on the electrode surface.The accumulation of Cr deposits poisons and leads to irreversible polarization losses of the cathode and performance degradation of the cell [44,[46][47][48][49][50][51] .Compared to chromium, deposition and poisondue to the source of sulfur and boron contaminants is not as well recognized as that of chromium from chromia-forming air [38] .
alloys.The presence of impurities such as volatile species (Na, B, etc.) from ceramic-glass seals and sulphur or other minor elements such as Si and Al can have serious degradation effect on the cathode of SOFC [52][53][54][55] .The impurities in the air stream may not be avoidable due to the multi-components and multi-processes involved in the synthesis and fabrication of stacks; therefore it is critical to develop cathodes with not only the high activity but also the high tolerance towards impurities and contaminants in air stream.

Nanoscale and Nanostructured Elec鄄 trodes
The nanoscale and nano-structured electrode approach by impregnation or infiltration method attracts increasing attention as the viable alternative for the development of new electrodes for IT-SOFCs [56][57][58][59] .
Infiltration method is a two-step sintering process, Fig. 5 shows the scheme of nano-structured electrodes by two-step infiltration route [56] .The infiltrated catalytic nanoparticles can form discrete distribution or a thin and continuous network on the surface of the porous scaffold.The porous scaffold can be electronic conducting electrode materials such as LSM or ionic conducting electrolyte materials such as YSZ and doped ceria.The latter requires deposition of a continuous nanoparticle layer with high electronic conductivity as well as high electrocatalytic activity, and multiple infiltration steps are necessary to achieve sufficient electron conduction [60] .Such repeated infiltration process is time-consuming and hinders the practical application of the infiltration approach.Using a concentrated LSM nitrate precursor solution with surfactant (e.g.,Triton-X 100), Sholklapper et al showed that it may be possible to form a continuous LSM nanoparticles on YSZ scaffold with a single-step infiltration [59,61] .However, from practical point of view, it would be very difficult for concentrated precursor solutions to penetrate and infiltrate uniformly into micro-and nanopores of the scaffold by capillary force, and multiple infiltration-calcination steps are generally required, particularly in the case of thick anode-substrate supports.On the other hand, the infiltration process can be accelerate by electrodeposition and electroless deposition methods [62][63] .
The enhancement in the electrochemical performance of nanostructured electrodes is truly remarkable.For instance, Jiang et al showed that in the case of LSM cathodes, the electrode polarization resistance ( E ) is 11.7 赘窑cm 2 at 700 o C. With the infiltration of 5.8 mg窑cm -2 GDC nanoparticles, E is reduced dramatically to 0.21 赘窑cm 2 , which is 56 times smaller than that of the pure LSM cathode at the same temperature [64] .The R E for the O 2 reduction reaction on the nano-structured Pd+YSZ is 0.11 赘窑cm 2 at 750 o C and 0.22 赘窑cm 2 at 700 o C, and this is significantly lower than that of the LSM (9 to 54 Fig. 5 Scheme of the infiltrated nano-structured electrodes onpre-sintered porous electrode or electrolyte scaffold/skeleton [56] .赘窑cm 2 at 700 o C [65][66] ), LSM/YSZ (2.5 赘窑cm 2 at 700 o C [67] ) and LSM/GDC (1.1 赘窑cm 2 at 700 o C [65] ) composite cathodes.The most distinctive advantage of the nanostructured approach is the flexibility in the selection and combination of highly active catalytic materials with structurally stable and highly electronic or ionic conducting scaffold to meet stringent requirements of anodes and cathodes of SOFCs.For example, the high performance SOFC cathodes can be developed by the combination of highly active MIEC but thermally incompatible materials such as BSCF with the structurally stable and highly conductive LSM scaffold.In such nanostructured BSCF-infiltrated LSM composite cathode, the uniformly distributed BSCF nanoparticles significantly enhance the electrochemical activity, while the LSM scaffold provides an effective electron transfer path, TEC matching and thermal stability with the YSZ electrolyte.The interfacial reaction between the infiltrated BSCF and YSZ is minimized due to the low phase formation temperature of BSCF.Fig. 6 shows the performance of anode-supported YSZ film cells with pure LSM and 1.1 mg窑cm -2 BSCF-infiltrated LSM cathodes at different temperatures under H 2 /air [68] .The cell with thin YSZ electrolyte film and the nanostructured BSCF-LSM cathode exhibits maximum power densities of 1.21 and 0.32 W窑cm -2 at 800 o C and 650 o C, respectively, substantially higher than 0.51 and 0.08 W窑cm -2 for the cells with the pure LSM cathode under identical conditions.
The remarkable promotion effect of the nanostructured electrodes on the performance of anodes and cathodes of SOFCs is a direct result of the formation and uniform deposition of nano-sized and catalytically active phases on the surface of porous electrode or electrolyte scaffolds.Consequently, the most significant challenge in the application and development of nanostructured ele ctrodes is the microstructure stability of the infiltrated nanoparticles.
As the particle size of the infiltrated phase is very fine (20 ~100 nm), the tendency for sinte ring and grain growth at operation temperature of SOFCs (500 ~800 o C) would be high due to the large surface energy associated with the nano-sized oxide or metallic phase [69] .In the case of infiltrated Pd nanoparticles on porous YSZ scaffold, the agglomeration and grain growth resulted in the formation of continuous and dense Pd films on the YSZ scaffold surface, leading to the increase in the polarization losses due to the blocking of the oxygen diffusion path [70] .
Al loying with cobalt, manganese and silver has been found to increase the thermal stability and enhance the performance stability of Pd-infiltrated electrodes [70][71] .
Reduction in operation temperature of SOFCs is expected to significantly benefit the microstructure stability of nanostructured electrodes.

Electrolytes
Any SOFC electrolyte must meet the requirements of fast ionic transport, negligible electronic conduction, and thermodynamic stability over a 200 mL窑min -1 ; oxidant: stationary air) [68] .

电 化 学
2012 年 wide range of temperature and oxygen partial pressure.In addition, they must have thermal expansion compatible with the electrodes and other cell components, good mechanical properties and negligible interaction with electrode materials under operation and processing conditions.The most common SOFC electrolyte is zirconia-based electrolyte materials, typically yttria-stabilized zirconia, because of its superior stability [72] .Although it is a good oxygen conductor, its conductivity is significantly lower than doped ceria and LaGaO 3 perovskites-based electrolytes (see Fig. 7 [73] ).
In the Y 2 O 3 -ZrO 2 system, a maximum of the conductivity of ZrO 2 electrolyte with respect to the dopant content occurs around 8 ~9% (by mole) Y 2 O 3 [74]   .This maximum corresponds to the minimum dopant level required to fully stabilize the hightemperature cubic phase.Scandia-doped zirconia (ScSZ) provides a higher conductivity, attributed to the smaller mismatch in size between Zr 4+ and Sc 3+ ions [75] .ScSZ with 9.0% (by mole) Sc 2 O 3 (9ScSZ) has the conductivity of 0.34 S窑cm -1 at 1000 o C [ 76 ] .The segregation of impurities such as silica [77] is detrimental, but can be improved by minor oxide additives [78] .
Pure ceria is not a good oxygen ion conductor, but its conductivity can be increased significantly by substituting Ce 4+ with divalent alkaline earth or trivalent rare earth ions [79] .Highest conductivities have been reported for either Gd-or Sm-doped ceria (GDC and SDC) with the maximum conductivity obtained around 10 ~20 % (by mole) Gd 2 O 3 or Sm 2 O 3 [80-83]   .Ceria-based electrolytes suffer from the partial reduction of Ce 4+ to Ce 3+ .However, the leakage current decreases substantially with the reduction in operation temperature [84] .
Tab. 1 lists the typical values of the oxygen ion conductivity and its activation energies in the lowand high-temperature ranges for the main electrolyte materials employed in SOFCs.Note that higher conductivity values are sometimes observed in thin films and other nanostructured systems.Kosacki et al. [87] investigated the ionic conductivity of highly textured YSZ films deposited on MgO substrate and observed exceptionally high ionic conductivity for film thickness < 60 nm.Recently Garcia-Barriocanal et al. [88] reported a huge ionic conductivity enhancement at interfaces of epitaxial YSZ/SrTiO 3 heterostructures.This indicates that nanoscale effects at the interface could be manipulated to enhance the ionic conductivity of zirconia-based electrolytes for low and intermediate temperature SOFC applications.

窑 窑
蒋三平院 中温固体氧化物燃料电池优势和挑战的简要评述 第 6 期 Tab.1 Conductivity and activation energy for some electrolyte materials employed in SOFCs 0.037 赘窑cm 2 , 2 orders of magnitude lower than 1.259 赘窑cm 2 for YSZ at the same temperature [93] .The high oxygen mobility is a result of weak metal-oxygen bonds and thus Bi 2 O 3 -based materials have lower stability under reduced partial pressure of oxygen at the anode side, resulting in the decomposition to metallic Bi.However, the cell stability can be substantially improved by using functionally graded ceria/bismuth oxide bilayered structure [94] .A high power density of ~2 W窑cm -2 at 650 o C was reported on an anode-supported cell with ceria/bismuth oxide bilayered structured electrolyte [95] .

Interconnect and Sealant Materials
Alkaline-earth (AE)-doped MCrO 3 (M= La, Y and Pr) are the mostly studied ceramic interconnect materials for the high temperature SOFCs [96][97] .The increase in AE content results in a higher TEC, which causes thermal stresses and thus decreases long-term stability [98] .The praseodymium in PrCrO 3 -based oxides exists in two valence states, Pr 3+ andPr 4+ [99] , which decreases chemical and dimensional stability.La 0.7 Ca 0.3 CrO 3 -doped CeO 2 composite [100] and Nd 0.75 Ca 0.25 Cr 0.98 O 3-啄 [101] were also considered for interconnects.However, the decrease in their electrical and thermal conductivities with decreasing temperature is a major challenge in the developments of ceramic interconnect for IT-SOFCs.
Metallic materials based on transition met-al-based oxidation resistant alloys have been considered to be the primary candidates as the interconnect materials of IT-SOFCs, due to the economic and easy processing benefits in addition to the high electrical and thermal conductivities.These include Ni(-Fe)-Cr based heat resistant alloys, Cr alloys, and chromia-forming ferric stainless steels [102][103] .The alloys with the formation of a protective and semi-conductive chromia scale to minimize further environmental attack during the high temperature operation and with TECs of 11.0 to 12.5伊10 -6 K -1 are the preferred candidates.The conductivity of chromia oxides is ~10 -2 S窑cm -1 at 800 o C in air [104] .A good example in this category is Plansee Ducralloy with a composition of 94% Cr, 5% Fe, and 1% Y 2 O 3 (as Cr 5 FeY 2 O 3 ) [105] .To further increase the electrical conductivity of the scale and to reduce the chromium vaporization, a new alloy which contains 0.5% Mn (Crofer 22 APU) was developed [106] .The oxide scale consisted of a (Mn,Cr) 3 O 4 spinel top layer shows a higher electrical conductivity [107] .
However, without effective protective coatings, the vaporization of chromium species from chromia scale poisons the cathodes and seriously degrades the cell performance [44,[46][47][48][49][50][51]108] . To reuce the growth rate of the oxide scale and the vaporization of chromium species, a thin and dense oxide coating with high electrical conductivity such as LSM and LSCo is often deposited on the metallic interconnect.The chromium volatility can also be suppressed by modification of the metallic interconnect materials.Hua et al. [109] reported a novel Ni-Mo-Cr alloy with a TEC value of 13.92伊10 -6 K -1 between 35 and 800 o C.After oxidation treatment at 750 o C for 1000 h, the area specific resistance (ASR) of this alloy is 4.48 m赘窑cm 2 .Poisoning study using LSM cathode indicates that the Cr deposition and poisoning of the Ni-Mo-Cr alloy is remarkably reduced as compared to the conventional Fe-Cr alloy [110] .
The sealing material has been regarded as one of the most significant technical challenges in the development of planar SOFCs while sealing is much less of a problem for tubular SOFCs [111] .The sealants can be broadly classified into rigid bonded seals, compressive seals, and compliant bonded seals.Each offers advantages and limitations.In rigid bonded sealing, the sealant forms a joint that is non-deformable at room temperature.Because the final joint is brittle, it is critical for the sealant to match the TEC of the adjacent substrates.High temperature glass and ceramic-glass such as alkali silica glasses and BaO-CaO-SiO 2 [112] are among the most important rigid bonded sealants employed in joining SOFC stacks.
These materials have acceptable stability in the reducing and oxidizing atmospheres, are generally inexpensive, and can be readily applied to the sealing surfaces as a powder paste or a tape cast sheet.They are electrically insulating and their TEC can be adjusted to those of electrolyte and metallic interconnect.However, the brittle nature of glasses and ceramic-glasses makes these seals vulnerable to cracking, and they tend to transform in phases and react with the cell components and interconnect materials under SOFC operation conditions in a long run, due to their intrinsic thermodynamic instability [52,113] .Recent results also show that volatile boron species from borosilicate glass has significant detrimental effect on the microstructure stability and electrochemical activity of nano-structured GDC-LSM cathodes of SOFCs [114] .
Compressive seals have been developed to avoid the disadvantages of the rigid bonded seals, with the merit of flexibility and compressibility, allowing the cells and interconnects to expand and contract freely during thermal cycles and operation.This type of sealing relies on the compressive load of the stack.So far two kinds of compressive seals have been considered, i.e., the deformable metallic seals and the mica-based seals.The deformable metallic seals include ductile silver [115] and corrugated or C-shaped superalloy gaskets [116] , but their application is limited due to their high electronic conductivity.The most common compressive sealing material is based on mica [117] .By incorporating a compliant interlayer such as a deformable metal or glass at the interface to form the hybrid mica-based seals, the sealing properties of mica are significantly improved [118][119] .Compliant bonded seals are based on metallic braze.Metallic materials have lower stiffness as compared to ceramics and can undergo plastic deformation, which allow for accommodation of thermal and mechanical stresses.Silver and gold are stable in air and are commonly used as metal braze materials [120] .A major challenge in obtaining a good metal-ceramic joint is adequate wetting of the ceramic by the braze metal.Comprehensive review articles have recently been published on sealant materials used for SOFCs [111,121] .

SOFC Structures and Configura鄄 tions
There are number of cell support structures and each is classified according to the layer that mechanically supports the cell.These include electrolyte-supported, anode-or cathode-supported as well as porous substrate-or metal-supported structures, see Fig. 8 [2] .
Due to the thick electrolyte (typically in the range of ~100 滋m) needed to mechanically support other cell components, the electrolyte-supported SOFCs are primarily developed for operation at high temperatures (~900 o C or above) [122] .The rapidprogress in the thin-film technology [123] , as well as the identification of alternative electrolyte materials with higher ionic conductivity such as GDC has significantly reduced the ohmic losses associated with solid elect rolyte.The use of thin electrolyte layers requires the electrolyte to be supported on an 488 窑 窑 appropriate substrate.The substrates can be functional anode or cathode, or porous supports providing gas diffusion and transportation for fuel cell reactions.As the substrate is the principal structural component in these cells, it is necessary to optimize the conflicting requirements of mechanical strength and high gas permeability.
Up to now, the most popular supported cell structure is the anode-supported thin film SOFCs.
The state-of-the-art anode-supported SOFC is based on porous Ni/YSZ cermets as a support.The anode support usually consists of a relatively thick porous supporting substrate (200 ~500 滋m) and a thin and fine structured anodic function layer (AFL).To reduce the electrolyte ohmic resistance and to enhance the cell efficiency, the electrolyte layer deposited should be as thin as possible.As a general role, the film thickness is inversely proportional to the pore size and/or propagated roughness of the surface, which means that the larger the pore size, the more difficult it is to get a electrolyte.
Thus the AFL is also required to have proper pore structure with low surface roughness.Tape-casting and tape-calendering processes are the common techniques in the fabrication of anode-supported structure for thin film SOFCs [124][125] .Power density as high as 1.8 W窑cm -2 at 800 o C was reported for such anode-supported cells [126] .However, the porous composite anode support is relatively weak mechanically and can have difficulty withstanding the thermal and mechanical stresses generated by rapid temperature fluctuation.Moreover Ni/NiO redox cycling induced by air diffusion into the anode compartment during the loss of fuel supply and other operational excursion can disrupt the anode microstructure, leading to performance degradation [127] .
Metal-supported structures are often used to overcome the problems associated with anodesupported cells [128][129] 袁 in which the Ni/YSZ anode support is replaced by a metal (usually stainless steel).
Metal support improves thermal shock resistance, reduces temperature gradients due to the high thermal conductivity of metals and thus enhances the robustness of SOFCs.

Conclusions
SOFCs have tremendous potential for numerous applications, from stationary to mobile power, with high fuel flexibility and high system efficiencies.However, common to all new and emerging technologies, cost and durability are two of the most technical barriers for the commercial viability and acceptance of the SOFC technologies.The high cost and low durability makes the fuel cells-based power systems not commercially competitive to existing conventional power generation technologies such as internal combustion engine and gas fired power plant.Though material costs would remain essentially constant, it is anticipated that the production costs will come down with volume.Fabrication process takes a lion's share of the cost of SOFCs.Beside cost reduction, durability is the most important issue to be solved before any fuel cell technologies can be commercially successful.The durability of a SOFC or stack is affected by many internal and external factors, such as cell design, materials degradation, operational conditions, fuel choice, thermal cycling Fig. 8 Illustration of different types of cell support architec tures for SOFCs [2] .and impurities or contaminants.The performance degradation may not avoidable, but can be minimized through a comprehensive understanding of degradation and failure mechanisms and development of appropriate solutions.It is evident that current understanding of these mechanisms on fuel cell components and particularly on the stack level is far from sufficient.Development of new characterization techniques, especially those which allow in situ and quantitative measurements in the atomic level [130][131] , and of advanced computational modeling is critical for the quantum leap in understanding of the fundamental issues.
The challenge of meeting the world爷s gigantic energy demands in a sustainable manner with low environmental impact is undoubtedly one of the most important challenges of this century.IT-SOFC as the most efficient energy conversion technologies should be a technology of choice and can play a critical role to our current and future energy solutions.
illustrates the operation principle of a SOFC.The state-of-the-art SOFCs are based on yttria-stabilized zirconia (YSZ) electrolyte, lanthanum strontium manganite (La 1-x Sr x MnO 3-啄 , LSM) or lanthanum strontium cobalt-iron (La 1-x Sr x Co 1-y Fe x O 3-啄 , LSCF) perovskite oxide cathodes and nickel-YSZ cermet anode.Traditional SOFCs operate at 1000 o C, because of the relatively low oxygen ion conductivity and high activation energy of oxide electrolytes such as YSZ.However, the high operating temperature is a key technical issue that has limited the development and wide deployment of this technology due to high system cost, high performance degradation rate, slow start-up and limited shutdown-startup cycles.Lowering of the operating temperature of SOFCs to intermediate range of 600 ~800 o C brings substantial technical and economic benefits due to

Fig. 1
Fig. 1 Schematic representation of operating principle of a solid oxide fuel cell. 2 Development of Key Materials 2.1 Anode and Cathode Anode: For SOFCs based on YSZ electrolyte, metal-ceramic composite cermet, especially Ni/YSZ or Ni/gadolinia-doped ceria (Ni/GDC) cermets, are

the
Ni phase was allowed to coarsen via Ni surface diffusion.An experimentally obtained three-dimensional reconstruction of a functional layer from an anode-supported SOFC is used as the initial microstructure.The evolution of the microstructure is characterized by examining the three phase boundary (TPB) density, interfacial area per unit volume, and tortuosity versus time.Considerable coarsening, as well as smoothening of surfaces, is evident comparing the results in Fig. 2.

Fig. 2
Fig. 2 3D image renderings of a Ni/YSZ anode functional layer in the as-prepared state (A) and after coarsening using a phase-field model (B) (Light color represents been investigated to modify Ni-based materials to enhance the sulfur-tolerance and carbon cracking resistance for direct utilization of hydrocarbon fuels.Replacing YSZ with doped ceria in the Ni-based cermet also enhances the electrocatalytic activity towards oxidation of hydrocarbon fuels and sulfur-tolerance due to the redox transition between Ce 4+ /Ce 3+ mixed valent cation [24-25] .Impregnation or infiltration of nano-sized electrocatalysts such as doped ceria, Pd, and a proton-conductor SrZr 0.95 Y 0.05 O 3 has shown some success, since the introduced nano-sized electrocatalysts can significantly promote the electrochemical activity of conventional Ni-based anodes towards the oxidation of hydrogen and hydrocarbon fuels and suppress carbon cracking and

Fig. 3
Fig. 3 Current-voltage characteristics and the corresponding power densities measured at 750 o C for cells with a configuration of BaO/Ni-YSZ|YSZ|SDC/LSCF when dry C 3 H 8 was used as the fuel and ambient air as the oxidant (A), and Terminal voltages measured at 750 o C as a function of time for the cells with and without BaO/Ni interfaces operated at a constant current density of 500 mA窑cm -2 with dry C 3 H 8 as the fuel(B)

Fig. 4
Fig. 4 Steady state polarization curves of LSM (A) and LSCF (B) electrodes measured at different temperatures in which effectively separate the catalytic active phase formation temperature from the high sintering temperature as required to establish the intimate electrode/electrolyte interface bonding in the standard SOFC electrodes.The relatively low formation temperatures (< 700 ~800 o C) for the catalytic active phases minimizes the grain growth and agglomeration, resulting in the deposition of nano-sized particles onto structurally stable and compatible scaffolds such as LSM, Ni/YSZ, and YSZ or doped ceria.The infiltration method opens a new horizon in the electrode development as the method expands the selection of variable electrode materials combinations with the minimized TEC mismatch and the suppression of possible detrimental reactions between electrode and electrolyte materials.

Fig. 6
Fig. 6 Performance of the Ni/YSZ anode-supported YSZ elec trolyte film cells with pure LSM cathode and (A) 1.1 mg窑cm -2 BSCF-impregnated LSM composite cathode (B) atdifferent temperatures in H 2 /air (H 2 flow rate: to be insufficiently stable for use in SOFCs.Recently Yang et al reported the development of a composite anode consisting of Pr 0.8 Sr 1.2 (Co,Fe) 0.8 Nb 0.2 O 4+啄 scaffold with homogeneously dispersed nano-sized