Solid Oxide Fuel Cells: Technology Status

Int. J. Appl. Ceram. Technol, 1 [1] 5-15 (2004)

Ceramic Product Development and Commercialization

Solid Oxide Fuel Cells: Technology Status Prabhakar Singh Pacific Northwest National Laboratory, Richland, WA 99352

Nguyen Q. Minh GE Power Systems, Torrance, CA 90502

In its most common configuration, a solid oxide fuel cell (SOFC) uses an oxygen-ion conducting ceramic electrolyte membrane, perovskite cathode, and nickel cermet anode electrode. Cells operate in the 600-1000ºC temperature range and utilize metallic or ceramic current collectors for cell-to-cell interconnection. Recent developments in engineered electrode architectures, component materials chemistry, cell and stack designs, and fabrication processes have led to significant improvements in the electrical performance and performance stability as well as reduction in the operating temperature of such cells. Large kW-size power-generation systems have been designed and field demonstrated. This paper reviews the status of SOFC power-generation systems with emphasis on cell and stack component materials, electrode reactions, materials reactions, and corrosion processes.

Introduction

Science and technology of the solid oxide fuel cell (SOFC) has been extensively reviewed in the literature. Some of the noteworthy reviews are by Minh1, Steele2, and Yamamoto3. Although SOFCs containing an oxygen-ion conducting yttria-stabilized zirconia solid electrolyte were first constructed and electrically tested by Baur and Pries4 in 1937, after the discovery of solid oxide electrolyte by Nernst5 in 1899, much of the engineering and materials development and technology scale-up has been relatively recent (1980 and onward). During this period, significant technical advances were made in the field of high-temperature tubular SOFC materials, fabrication processes, and field demonstration of kW-class generators6. Long-term stable operation of such fuel cells was also demonstrated on pipeline natural gas7. As a result, several generator prototypes were subsequently assembled and successfully operated at 800-1000ºC for thousands of hours on reformed pipeline natural gas.

In recent years, SOFC technology development effort has focused on electrical performance improvement along with the lowering of the cell operation temperature to 600-800°C. High-performance cathode, carbon- and sulfur-tolerant ceramic anode, low-cost metallic current collectors, and composite seals are being developed and tested under the Solid state Energy Conversion Alliance (SECA) initiative of the US DOE that targets the development of low-cost ($400/kW) modular 3–10 kW mass customized SOFC power-generation systems for stationary, mobile, and military markets8. This paper summarizes recent development in SOFC materials. Materials challenges and development trends are also presented.

Fuel cells directly convert the chemical energy of gaseous or liquid fuels into electrical energy by a highly efficient and clean electrochemical oxidation process. Not limited by the Carnot cycle, fuel cells offer high chemical-to-electrical conversion efficiency. Absence

6 International Journal of Applied Ceramic Technology Vol. 1, No. 1, 2004 of high-temperature combustion processes and an open flame front in fuel cells also eliminates the formation of harmful pollutants such as SOx, NOx, volatile organic carbon (VOC), and particulate matters. Compared to conventional combustion-based energy conversion systems, fuel cells offer additional advantages of higher electrical efficiency at part load, ability to hybridize with gas turbines, combined heat and power production, modular construction, fuel flexibility, and noiseless operation.

A fuel cell device comprises an anode electrode (exposed to fuel), electrolyte, and a cathode electrode (exposed to oxidant). The electrolyte separates anode and cathode electrodes and facilitates the ionic transport required for the oxidation of fuel. Electron flow in the external circuit produces usable electrical power. Several individual cells are connected in series to form a “stack” to obtain usable cell voltage and power. In a conventional SOFC, a dense yttria-stabilized zirconia oxygen-ion conducting electrolyte membrane separates a porous nickel-zirconia cermet anode and a doped lanthanum-manganite-perovskite cathode. Among the fuel cell technologies currently under development (phosphoric acid fuel cells, polymer electrolyte membrane fuel cells, molten carbonate fuel cells, and alkaline fuel cells, for example, as shown in Table 1), solid oxide fuel cell technology offers the advantage of all solid-state construction, multi-fuel operational capability, high electrical conversion efficiency, and a simpler balance of plant. SOFC technology also offers some of the unique advantages in terms of:

• Use of non-strategic and non-noble materials for cell component fabrication.

• Fuel flexibility and use of existing fuel infrastructure to bridge the hydrogen infrastructure gap.

• Modular solid state construction for W, kW, and MW products.

• Use of low-cost conventional ceramic fabrication processes for large-scale manufacturing.

• Systems for “zero” or “near zero” emissions.

Table 1. Charge carriers and fuel cell operation temperature

Stationary Power, APUFuel flexibility

O=600-1000oCSOLID METAL OXIDE

SOFC

TransportationPure H2, CO intolerant

H+~50oCION EXCHANGE MEMBRANE

PEMFC

Stationary Power, Transportation

Relatively pure H2

H+~200oCPHOSPHORIC ACID

PAFC

Stationary PowerFuel flexibility

CO3=~650oCMOLTEN

CARBONATESMCFC

SpacePure H2,

CO, CO2 intolerant

OH –~ 80oCPOTASSIUM HYDROXIDE

AFC

APPLICATION & FUEL COMMENTS

CHARGE CARRIER

OPERATING TEMPERATURE

ELECTROLYTEFUEL CELL TYPE

Operation of large kW-class SOFC and SOFC–

micro-turbine hybrid power plants have been recently demonstrated9. Development and demonstration plans for SOFC power plants with CO2 sequestration capability have been announced. Operation of several planar and radial SOFC power-generation systems have also been demonstrated at kW levels by Honeywell, Sulzer-Hexis, Global Thermoelectric, Juelich, and Delphi Automotive10-12. One of the biggest hurdles of the technology remains to be the high cost (>$5000/kW).

The schematic of a solid oxide fuel cell and electrode reactions is shown in Fig. 1. The fuel (H2, CO, or hydrocarbons) is oxidized at the anode, while the oxidant (air or oxygen) is reduced at the cathode. Oxygen ions are transported across a predominantly oxygen-ion conducting electrolyte (tion ~1) from the cathode to the anode, where they react with the fuel to form H2O or CO2. Theoretical cell voltage, also called open circuit voltage (OCV) or Nernst voltage, is governed by the chemical potential gradient in oxygen across the electrolyte and is given by: VNernst = RT/4F ln (PO2oxidant/ PO2fuel)

The actual operating cell voltage (VCell) during a given current flow is dictated by ohmic losses due to cell internal resistance and electrode losses, also called polarization losses due to activation, and mass transport:

VCell = VNernst – ohmic losses – polarization losses

Solid Oxide Fuel Cells: Technology Status 7

Fig. 1. Schematic of a solid oxide fuel cell. Oxidation of fuel at the anode electrode and reduction of oxygen is shown at the cathode electrodes. Useful power is generated in the external circuit.

Unit reaction processes and related mathematical derivations of cell polarization losses have been well documented in the literature13-15. The electrical output of an electrochemical cell is represented by a “Current–Voltage” relationship as shown in Fig. 2. At lower current densities, electrical performance loss in the cell is attributed to activation or electron exchange process limitations (Region 1) whereas at higher current densities, resistive (Region 2) and mass transport (Region 3) processes limit the overall performance. Cell cathode and electrolyte interaction and formation of an insulating pyrochlore (A2B2O7), and formation of corrosion products at the current collector surfaces, are two major contributors to increases in cell resistance. The presence of gas phase contaminants such as sulfur, hydrated Cr and Si vapors16 in the fuel and oxidant gases lead to extensive carbon formation, reduction in reformation kinetics, and surface deposition of insulating phases.

Among various cell configurations studied and reported in the literature (tubular, planar, monolithic, radial in electrode and electrolyte supported configurations), tubular and planar cells have attracted the most attention in recent years. While the tubular cell design developed by Westinghouse Electric Corporation offers the advantage of a seal-less generator design and cell-to-cell connection over the axial ceramic interconnection strip exposed to a fuel environment (Fig. 3), planar cells offer the potential

for higher power density, low-temperature operation, and use of a metallic current collector for cell-to-cell connection (Fig. 4). Planar cell configurations with central manifold are also amenable to minimal seal requirements. Other design parameters such as the fuel and oxidant flow pattern (co-, cross-, or counter-flow), gas manifolding (internal or external), fuel processing schemes (steam methane reforming, partial or catalyzed partial oxidation, auto thermal reformation or direct oxidation of fuel), and fuel-oxidant separation (seal or seal-less, compressive or rigid seals) also dictate the cell, stack, and system configurations.

Current Density (A /cm2)

Volta

ge (V

)

VNernst

Activation Limitation

Ohmic Limitation

Transport Limitation

1. Reduce activation polarization : Region 1 Move V-I Curve upward-

2. Reduce ohmic polarization : Region 2 Reduce V-I Slope-

3. Reduce diffusional polarization : Region 3Extend V-I Range before bend-

V Cell = VNernst - iR- Φ (Activation + Diffusional)

Region 1 Region 2 Region 3

Increase in cell resistance increases the slope

Fig. 2. Current–voltage characteristic of an electrochemical cell. Electrical performance losses are attributed to activation (Region 1), resistance (Region 2), and mass transport (Region 3). Cell electrical performance degradation is attributed to electrode deactivation, poisoning, and increase in cell resistance.

Fig. 3. Schematic of an air electrode supported tubular SOFC showing axial interconnection for cell-to-cell attachment.

www.ceramics.org/ACT

8 International Journal of Applied Ceramic Technology Vol. 1, No. 1, 2004

Fig. 4. Planar cell design with through-plane current collection. Bi-polar interconnects separate fuel and air and provide cell-to-cell connection in a stack.

Cell and Stack Component Materials

The cell and stack component materials (repeat and non-repeat components) of a SOFC stack are exposed to relatively higher temperatures (600-1000°C) and a complex gas environment. The desire to maintain stable electrical performance (<0.1%/1000 hr) and longer life (40,000 hr) requires that the cell components must remain chemically and structurally stable. Components must be low cost and be resistive to micro-structural changes, interfacial reactions, electro-catalytic poisoning, and corrosion.

Electrolyte, gas separation seals, and cell-to-cell interconnections must remain dense during the entire cell life to prevent mixing and burning of fuel and oxidant. Cell electrodes, on the other hand, are expected to maintain their porous structure to allow for the diffusion of gas phase reactants to the respective electro-catalytic sites. Component stresses must also be minimized to prevent cracking and delamination during cell operation and fabrication processes. Advanced computational tools, available today, have been extensively used in predicting stress levels in cells and stacks under steady-state and transient operations17. These tools have also helped in optimizing cell and stack designs for thermal, flow, and current generation profiles.

Interactions between cell components and exposure environments must be controlled and minimized during cell fabrication and operation in order to reduce the formation of resistive interfaces at the electrode-electrolyte and interconnection–electrode interfaces. US Patents18-19 document such interactions and provide methodologies for solving such problems. The metallic current collector, exposed

to both fuel and oxidant gas atmospheres (bi-polar condition) must form conducting oxide scales to lower resistive losses in the cell. Scales formed at the current collector and electrode interfaces should also remain non-reactive and not contaminate the electrode. Stack containment materials should provide resistance to oxidation and ease of fabrication during welding and heat treatment20. In addition to chemical and structural compatibility, cell component materials must also have low cost.

Recent advances in the electrolyte, anode and cathode electrodes, interconnection, and gas seal materials are presented below.

Electrolytes

Although oxygen ion (O=) and proton (H+) conduction in a solid electrolyte of a SOFC device can electrochemically oxidize the fuel, the majority of SOFC development activities have focused on the use of oxygen-ion conducting membranes as the electrolyte of choice. The status of SOFC electrolyte materials has been reviewed and presented by Minh21. Three classes of electrolyte material based on zirconia, ceria, and lanthanum gallate have been widely researched and have found applications in high- and intermediate-temperature SOFCs.

For high-temperature cells operating above 800°C, doped zirconia, and especially yttria-stabilized zirconia (YSZ), remains the material of choice for the electrolyte. The long-term chemical stability of YSZ and compatibility with electrode materials are well proven. The synthesis of reactive zirconia powder and thin dense electrolyte film fabrication by tape casting, screen printing, calendaring, and chemical–electrochemical vapor deposition (EVD) processes are also well understood for implementation in large-scale manufacturing. Electrolyte film deposition by plasma spray technique has attracted attention in recent years for low-cost manufacturing22. For electrical, chemical, and mechanical properties of various zirconia-based electrolyte compositions, readers are referred to Science and Technology of Zirconia, edited by Heuer, et al.23.

Doped ceria- and lanthanum gallate-based electrolyte materials have been studied extensively for use in low- and intermediate-temperature SOFCs operating in the 500-800°C temperature range. Several review articles summarizing defect chemistry, chemical stability and compatibility with electrode systems, and electrical and physical properties have appeared in recent literature24-26. The significantly smaller


electrolytic domain of ceria-based electrolyte remains a concern as it reduces the cell open-circuit voltage due to an internal short and compromises the electrical output at 700-800°C. The use of composite ceria–zirconia electrolytes have been proposed to minimize such losses27 with little success. In addition, interactions between ceria and zirconia have been reported during sintering at higher temperatures28. Large oxygen losses from the ceria lattice in a reducing environment have been related to lattice expansion that contributes to poor structural stability under thermal or oxygen pressure cycling conditions (lattice expansion and contraction), leading to fracturing and separation of the electrolyte29.

Doped lanthanum gallate (LGO) electrolytes have shown excellent electrical and chemical stability at intermediate temperatures. Unlike ceria-based electrolytes, LGO shows negligible electronic conductivity over a wide oxygen partial pressure range. The perovskite ABO3 type structure of LGO also remains stable. A large number of oxygen-ion vacancies can be introduced into the lattice by partial substitution of cation A or B with lower valance cations contributing to higher ionic conductivity30. LSGM8282 has the highest ionic conductivity among the LGO perovskite oxides24. The sintering of LSGM requires high temperature, resulting in large grain size and presence of secondary phases such as SrLaGa3O7 and La4Ga2O9 along the grain boundaries. High-temperature sintering and densification of the material in contact with nickel anode has shown formation of lanthanum-nickel oxidean insulating phase that reduces the electrical performance31. During sintering, it was also found that components of LSGM9182 migrated to both cathode and anode. Ga reacted with alumina tubes and silica sealants, and vaporized32. Densification of LSGM using activated microwave sintering has been recently studied33 and dense electrolyte structures (>95% dense) have been obtained at 1350°C (after 20-min sintering). A schematic of the microwave sintering apparatus used for densification of palletized electrolyte samples is shown in Fig. 5. Fine-grained microstructures showed no undesirable second phase (Figs. 6 and 7). Microwave sintering can reduce processing time while achieving desirable mechanical and electrical properties.

Fig. 5. Experimental microwave sintering set-up used for studying the sintering behavior of pelletized LSGM samples.

Fig. 6. Surface morphology of sintered LSGM sample. (Sintering condition: 1350oC, 20 min).



Fig. 7. XRD patterns of samples microwave sintered at two power levels (1.5 and 1.75 kW) and two sintering times (20 and 30 min).

Anodes

Recent advances in the SOFC anode technology include the development of a tailored nickel-zirconia cermet anode electrode34, a copper-based electrode for direct oxidation of hydrocarbons35-36, and red-ox- and sulfur-tolerant ceramic electrodes37. Conventional nickel-zirconia cermet electrodes have demonstrated long-term stability, low over-voltages for fuel (H2, CO, NH3) oxidation, and high electrical and thermal conductivity over wide temperature ranges (600-1000°C). Tape calendaring and tape casting, slurry dip coating, and EVD have been used extensively for the fabrication of a functional anode electrode or an anode electrode support. Conventional cermet microstructure consists of uniform zirconia dispersion in the metal with metallic inter-particle contact for electronic conduction. In the anode produced by the EVD process, a thin film of zirconia covers and encapsulates the metallic grains and retards sintering and pore closure. Issues with nickel anodes are red-ox tolerance, sulfur poisoning, and carbon deposition in hydrocarbon fuels. Nickel is easily oxidized above 500°C. Presence of H2S or sulfur odorants (thiophenes, mercaptans, etc.) in the fuel leads to reversible electrode poisoning (<1-2 ppm S) and electrical performance reduction. At higher sulfur levels, Ni3S2 sulfide formation and molten Ni/Ni3S2 eutectic formation can be expected. Nickel anode also remains prone to carbon formation in the presence of hydrocarbons due to hydrocarbon cracking (Fig. 8).

Copper-ceria cermet anode has been studied for the direct oxidation of hydrocarbon fuels38. Copper suppresses carbon formation during direct oxidation

while ceria improves the reaction kinetics. Unlike nickel, copper does not catalyze the carbon formation. If proven adequate for moderate to high power density operation, the direct oxidation scheme for fuel utilization in SOFC can potentially eliminate external or integral reforming of fuels and simplify the balance of plant (BOP) requirements. Copper-based anodes are susceptible to sulfur poisoning, high-temperature sintering, and remain prone to oxidation above 500°C. Ceramic anodes such as gadolina-doped ceria and titanates have been investigated recently to minimize carbon deposition and increase tolerance to sulfur. This has made the “all ceramic cell” a reality.

Fig. 8. Pyrolytic carbon formation on the nickel fuel electrode7.

Cathodes

The doped ABO3 perovskite family of compounds (A = Lanthanide group and B = transition metal group) has been extensively investigated for use as SOFC cathode39. Oxygen reduction mechanisms have been investigated40-42 and effectively utilized for the optimization of bulk compositions, interface modifications, and microstructure. Sr- and Ca-doped lanthanum manganites (LSM and LCM) have been conventionally used at 800-1000°C with adequate performance. Further performance improvement in such electrodes has been obtained through microstructural modifications along with the development of layered engineered structures. Dimensional stability of these electrodes has been improved through A and B site modifications. Fig. 9 compares the thermal cyclic shrinkage of a


(a)

(b)

Fig. 9. Structural stability of doped lanthanum perovskite electrodes during thermal cycling in air43. (a) Cyclic shrinkage observed on a Ca-doped lanthanum manganite during thermal cycles. (b) Structurally stable lanthanum manganite obtained after A and B site doping.

conventional and modified manganite electrode material. Cost reduction of such an electrode is also expected due to the use of lower-cost lanthanide raw materials43. At lower temperatures (<800oC), manganite-based electrodes show higher polarization as the ionic conductivity decreases.

For intermediate-temperature SOFC, cathode research efforts in recent years have mostly focused on the use of mixed ionically-electronically conducting (MIEC) cathode materials such as doped lanthanum ferrites and composite CeO2-lanthanum ferrites44-45. Copper doping (up to 20%) in the lanthanum ferrite has been studied46 and found to reduce the ASR (~0.12 Ω·cm2). Bi-layered engineered electrode architecture, developed through computational design analysis, has indicated the electrode performance approaching 3.1 A/cm2 at 0.078 V47. Fig. 10 compares the current generation profile and the electrode overvoltages at 700°C. Authors suggest that the chemical and structural compatibility of some of these newly developed high performance cathode materials be further studied and long term stability be evaluated.

Bi-layered electrode

-0.2

0

0.2

0.4

0.6

0.8

0.2093

0.2094

0.2095

0.2096

0.2097

0.2098

0.2099

0.21

-0.2 0 0.2 0.4 0.6 0.8 1 1.2

Current(A/cm2)Volts(V)

PO2(x)

Cur

rent

(A/c

m2)

, Vol

tage

(V)

PO2(x)

x (fraction from gas surface)

-0.2

0

0.2

0.4

0.6

0.8

0.2075

0.208

0.2085

0.209

0.2095

0.21

-0.2 0 0.2 0.4 0.6 0.8 1 1.2

Current(A/cm2)Volts(V)

PO2(x)

Cur

rent

(A/c

m2)

, Vol

ts (V

)

PO2(x)

x (fraction from gas surface)

Single Layer0.78 A/cm2 at 0.11 V

Double Layer0.78 A/cm2 at 0.066 V

Fig. 10. Bi-layered electrode structure comprising an electrochemically active inner layer at the electrolyte-electrode interface followed by an outer

current-carrying layer47.


12 International Journal of Applied Ceramic Technology Vol. 1, No. 1, 2004 Interconnects

Cell interconnects provide cell-to-cell electrical connection and separate fuel and oxidant gas atmospheres in a cell stack. Interconnect requirements have been reviewed by Zhu48. Two classes of SOFC interconnect materials have been extensively used in high-temperature and intermediate-temperature SOFCs, electronically conducting perovskite chromite ceramics, and metallic alloys.

For cells operating above 800oC, doped lanthanum chromites have demonstrated acceptable long-term chemical and structural stability in oxidant and fuel environments as well as compatibility with the cathode and anode electrodes49. The interconnect shows adequate electronic conductivity in both air and fuel. The perovskite ABO3 structure remains relatively stable; however, some structural stability concerns have surfaced in highly doped compounds exposed to reducing environments50. Mn diffusion from the cathode electrode in the interconnection and the development of a barrier layer to prevent such diffusion have been reported18. Chromite interconnects suffer from processing difficulties and high cost (high-temperature sintering, reactive powder synthesis, liquid phase additives, etc.).

As an alternative to ceramic interconnects, corrosion-tolerant conductive oxide scale forming commercial and experimental metallic alloys are also currently being investigated for use as current collectors in intermediate-temperature SOFCs. Metals and alloys offer the potential for lower cost, ease of fabrication and joining, excellent thermal conductivity, and commercial availability. Existing metallurgical fabrication processes (rolling, stamping, brazing, and welding, for example) can also be readily used for large-scale manufacturing of flat or intricately shaped interconnects. Corrosion behavior of Fe- and Ni-base alloys have been extensively studied under simulated fuel cell interconnect exposure conditions49-51. Metal loss and scale morphology results show that Fe-based austenitic or ferritic alloys experience accelerated localized corrosion in the oxidant gas under a dual atmosphere or bi-polar (simultaneous exposure to fuel and oxidant gases) conditions compared to exposure to air-only environment. It should be noted that the above exposure condition represents and simulates bi-polar interconnects in a SOFC stack as it separates both fuel and oxidant, and hence remains

simultaneously exposed to both gases. Fig. 11 shows such localized attack and nodular oxide growth formation on the air side of an austenitic AISI 304 tubular sample after 100 hr exposure. Accelerated corrosion is postulated to be due to hydrogen transport through the metal and red-ox (H2-H2O) formation in the oxide scale52. Hydrogen transported through the metal could also modify the oxide scale defect structure and accelerate the oxide growth. Another important issue with the metallic interconnect is related to the formation of Cr-containing vapor and related contamination of the cathode53.

(a)

(b)

Fig. 11. Surface oxide morphology developed on AISI 304 during oxidation at 800oC. (a) Localized iron rich oxide nodule formation during simultaneous exposure to dual atmosphere exposure conditions at 800oC (H2-3%H2O//Air, 100 hr). (b) Uniform oxide growth in air only (Air//Air).

Advanced metallic and ceramic coatings, alloy formulations and surface treatments are currently being investigated under the SECA Core Technology


Program in the authors’ laboratories to address these issues.

Gas Separation Seals

Gas seals separate and prevent intermixing of fuel and oxidant, fuel and ambient, and oxidant and ambient atmospheres, as well as electrically isolate cells in fuel cell stacks. Between rigidly bonded and compliant compressive seal technologies currently being developed for SOFC application, the rigid glass-based seal technology appears more mature but needs well-tailored glass for chemical compatibility and coefficient of thermal expansion (CTE) match. The compressive seal appears promising with respect to CTE but requires external load frame structure. Status of SOFC seals including challenges, requirements, and failure modes have been a topic of interest and discussion in technical papers and review articles54-56. Seal failure can result in localized burning of the fuel, hot spot and corrosion or oxidation of metallic or ceramic cell components, leading to premature electrical performance degradation and failure. Issues related to glass chemistry, glass-metal interactions, glass evaporation, and structural degradations due to porosity formation and crystallization have been studied57. Stress development in seals has been modeled58 and failure modes have been examined. Next-generation technologies involving self-healing seals, graded seals, composite compliant and wet seals warrant further investigation.

Other Stack Components

Irrespective of the cell and stack designs, SOFC stacks require a host of peripheral components such as: thermal and electrical insulation; passages and manifolds for fuel and oxidant distribution, delivery and exhaust; electrical contacts and buss bars for DC current; and containment for isolation from the ambient. These peripheral components play a significant role in determining the robustness of the stack design and lowering of the stack cost. Topics related to low-cost insulation materials and fabrication processes, corrosion-tolerant containment and gas manifolding materials, high-conductivity oxidation-tolerant buss bars are suggested for further study. For internal and/or direct reforming SOFC systems, it is proposed that both sulfur- and carbon-tolerant catalysts, gas passages and cell components be examined in detail.

Engineering studies are recommended for further optimization of cell designs that allow for uniformity of temperature, stresses, current density, and gas chemistry in a cell or a stack.

Summary

SOFC power-generation systems offer the potential for clean and efficient power generation from a wide variety of fuel sources. The trend toward the development of intermediate-temperature SOFC allows for significant cost reduction and utilization of commercial large-scale ceramic manufacturing technologies. Near-term technical challenges include development of corrosion-tolerant interconnects, chemically stable and structurally reliable gas seals, and poison-tolerant cathode materials and electrode architectures. Long-term R&D needs include the development of sulfur-tolerant anode and related catalysts, chemically stable electrode-electrolyte interfaces, as well as corrosion-resistant high-conductivity coatings and current collection systems.

The Solid state Energy Conversion Alliance (SECA) initiative of the US DOE is addressing the above R&D issues and plans to demonstrate a cost-effective technology by 2010.

Acknowledgments

The authors acknowledge the contributions of their many colleagues whose work is described in this paper. Drs. Vish Viswanathan, Jeff Stevenson, and Larry Pederson are acknowledged for their valuable technical suggestions.

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Solid Oxide Fuel Cells: Technology Status