Material advances will continue to play a large role in the enhanced performance and commercialization of solid oxide fuel cells



There has been a widespread and growing interest in fuel cells over the past several years. All major automotive manufacturers are involved in the development of fuel cell vehicles, and numerous other companies are working on various aspects of fuel cell technology-from hydrogen storage, raw materials development, and cell and stack production, to integrating power units into the grid. Governments around the world are allocating an increasing amount of resources for the development and implementation of fuel cells, and major fuel cell organizations are creating partnerships and cooperative agreements.

The dependency on foreign fossil fuels and their limited availability, the need to protect the environment, and the high efficiency of fuel cells have all contributed to this growing interest. Fuel cell applications vary widely from fueling laptops and cell phones to transportation and large stationary power units, and fuel cells have been shown to work in more than 100 NASA space missions.¹ However, much work has to be done to reach wide-scale commercialization.

One of the most important areas of advances is in the materials used to produce the fuel cell components-particularly the ceramic powders used in solid oxide fuel cells (SOFCs). Suppliers continue to develop powders with more tightly controlled properties, and SOFC researchers and manufacturers are constantly learning more about how the interactions between these materials can affect fuel cell performance. These advances are helping to drive fuel cell technologies forward.

Fuel Cell Basics

The fuel cell reaction is one of reverse electrolysis, in which oxygen and hydrogen combine to create electricity, heat and water. Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electric energy. Similar to batteries, fuel cells consist of an electrolyte sandwiched between an anode and a cathode; however, unlike batteries, which function as storage devices and therefore stop providing energy as soon as the reactants are used up, fuel cells operate continuously as long as fuel (hydrogen) is supplied. Fuel cells provide energy without combustion and without emitting any pollutants.

A fuel cell produces energy in the form of heat and DC electricity according to the following reaction (for a non-carbon-containing fuel):

Anode: H₂ + O^2- H₂O + 2e^-

Cathode: 1/2 O₂ + 2e^- O^2-

The Nernst Equation determines the voltage of an individual fuel cell. To generate a significant amount of energy, it is necessary to combine several cells into a stack. The cells are separated by metallic or ceramic interconnects that direct the reaction gases via channels and also collect the generated electrical current. Several stacks are integrated into a system that includes the control and monitoring units, also known as the balance of plant (BOP).

Fuel cells (FCs) are categorized by their electrolyte material and include polymer electrolyte (PEFC), alkaline (AFC), phosphoric acid (PAFC), molten carbonate (MCFC) and solid oxide (SOFC). A summary of the various types of fuel cells, including their operating temperature, can be found in Fuel Cell Handbook, Sixth Edition.²

Fuel cells offer several advantages over other forms of energy generation. Since they operate without combustion and moving parts, fuel cells demonstrate a high efficiency grade. They produce no pollutant emissions, operate quietly, offer flexibility regarding fuel types, and have the potential to reduce our dependency on foreign fossil fuels. Fuel cells are modular and can be built in various sizes to power anything from cell phones to large power plants. Current drawbacks include the lack of long-term experience in operating fuel cells, the lack of a hydrogen supply infrastructure, the lack of customer acceptance and high material costs.

SOFCs have generated significant interest in recent years. They are the second most developed fuel cell technology after PEFCs, and the value of the business generated from SOFCs is expected to be $200 million by 2008.³ SOFCs are high-temperature fuel cells (800-1000°C) and provide a potential fuel efficiency of up to 70%. They are flexible in regard to the types of fuels that can be used, and in certain cases they eliminate the need for external reforming. SOFCs offer a broad spectrum of potential applications, including decentralized power distribution, cogeneration of heat and power (CHP), and auxiliary power units (APUs) for transportation technology (trucks, cars, boats, etc.).

Two major types of cell designs have evolved for SOFCs, namely planar and tubular. In the planar design, fuel cells are stacked on top of each other and are connected via interconnects that allow the flow of air and fuel to the cathode and anode, respectively. In a tubular SOFC, the cell components are designed around a tubular hollow cathode.

SOFC Materials

The major components of a SOFC are the anode, cathode, electrolyte and interconnect. In addition, a seal is applied to a fuel cell stack to separate the fuel and oxidant gas flows. Each of these components carries out a specific electrochemical function; the components are in close contact with each other, and the entire stack is exposed to high temperatures, thermal cycling, and to an oxidizing/reducing atmosphere. These conditions impose some strict material requirements on fuel cell components with regard to conductivity (electronic and ionic), thermal expansion and chemical reactivity. On top of these physical/chemical requirements, fuel cell materials and components must be cost-effective and compatible with mass-production processes.

The Electrolyte

Oxygen ionized at the cathode migrates through the electrolyte toward the anode, where it reacts with the fuel to generate a voltage. Accordingly, the electrolyte must posses a high ionic conductivity and have no (or minimal) electronic conductivity. In addition, it must be nonporous to prevent short-circuiting of the reactive gases, and it must also be as thin as possible to minimize ohmic loss. Cubic zirconia stabilized with yttria (Y₂O₃), or YSZ, has been the most commonly used material of choice as a SOFC electrolyte. It is stable in the SOFC operating temperature range (800-1000°C), its ionic conductivity is comparable with liquid electrolytes, it is stable in both reducing and oxidizing atmospheres, and it has a matching coefficient of thermal expansion (CTE) with the contacting layers. YSZ can be applied through electrochemical vapor deposition, plasma spraying, spray coating or dip coating, or it can be produced as a substrate by slip casting. Other electrolyte materials include scandium-doped zirconia and gadolinium-doped ceria.

The Anode

Fuel oxidation takes place at the anode. Anode requirements include good electronic conductivity, porosity, stability in a reducing atmosphere and a matching CTE to the electrolyte layer. Metals have an excellent electronic conductivity and can operate in reducing atmospheres, and nickel in particular is a favorable candidate due to its catalytic properties and affordability. However, nickel sinters at the high temperatures of SOFCs, resulting in the unwanted densification of the structure. In addition, its CTE does not match that of the YSZ layer. These problems can be solved by combining nickel with YSZ into a Ni-cermet, and by firing the composite together with pore-forming resins. Remaining problems with nickel include the potential reaction with carbon from the fuel, and its sensitivity to sulfur and an oxidizing atmosphere-all of which contribute to the degradation of the cell's performance. Alternative materials to Ni-YSZ-cermets include Cu-ceria, Ti-doped YSZ, and some perovskite materials.

The Interconnect

The cell components in a SOFC stack are connected to each other via interconnects (ICs) (also referred to as bipolar plates). The interconnects fulfill several functions, including the distribution of fuel and air to the anode and cathode, respectively. They also serve as current collectors, and they provide a barrier between the anode and the cathode. Accordingly, an interconnect should have excellent electronic conductivity, oxidation-reduction resistance, matching CTEs to the contacting layers, chemical stability at the operating temperature, and no porosity to prevent the oxygen and hydrogen from mixing. Depending on the operating temperature, doped La-chromites, ferritic stainless steels and certain ceramics are candidate materials for ICs. The interconnects reflect the most expensive component of the fuel cell stack because of the materials and fabrication processes used. Both the metallic ICs, such as chromium alloys made by powder metallurgy, and ceramic ICs, such as zirconia multilayer tapes, that are used in pre-commercial units are rather costly. In an effort to reduce the material costs, various new steels are being researched that could provide corrosion resistance and temperature stability in the 800-850°C range. Cheaper steel grades will require lower operating temperatures.

The Seals

The seals must also have matching CTEs with the contacting materials. Additionally, they cannot react in any way with the stack components or with the gases, and they must be electronic insulators. Several materials, including glass, are being studied for use as sealing materials. Glass offers the advantage that its CTE can be tailored by adjusting the glass composition; however, it is brittle and tends to react with the cell components. Mica, which is a naturally occurring sheet silicate, is a candidate material in connection with compressive, non-bonding seals.

The Cathode

At the cathode, oxygen is reduced to ions at a temperature range of 700-1000°C, depending on the type of fuel cell. The cathode requirements are similar to those of the anode-i.e., the cathode should have a high electronic conductivity and sufficient porosity to allow oxygen diffusion to the anode/electrolyte interface, it must be chemically stable with the contacting layers, it must have a matching CTE with the electrolyte, and it must possess a catalytic activity for oxygen reduction. In the tubular SOFC design, the cathode also provides mechanical support.

The state-of-the-art cathode material used in conjunction with YSZ electrolytes is lanthanum manganite (LaMnO₃). This perovskite-structured material is used for both tubular and planar SOFC designs. Lanthanum manganite has a high electrochemical activity for oxygen reduction, and it is thermally compatible (i.e., matching CTE) and chemically stable with the YSZ electrolyte.⁴ The electronic conductivity of lanthanum manganite is enhanced by doping it with Ca (LSC) or Sr (LSM). LSM can be manufactured through several methods, each of which offers various advantages and disadvantages in terms of performance and commercialization. These methods include solid-state reaction, sol-gel techniques, spray pyrolysis and co-precipitation.⁵ Another cathode material for YSZ electrolytes includes strontium doped lanthanum ferrite (LSF).

A strong relationship exists between the LSM powder characteristics and the resulting cathode performance. The LSM powder should be a single-phase material with homogeneous particle morphology, narrow particle size distribution, defined stoichiometry and high crystallinity. These characteristics affect performance criteria such as polarization losses, oxygen-reduction activity and shrinkage behavior. At the heart of the matter are the so-called triple phase boundaries (TPBs), which are the contact surfaces between the ion conducting phase (YSZ), the electronic conducting phase (LSM) and the pores. A balanced ratio between these phases and the existence of a percolating system to the current collector on one hand, and to the electrolyte on the other, while minimizing possible reactions with the contacting interconnect, are crucial factors for the performance and efficiency of SOFCs. Therefore, the ability to control and adjust certain powder properties is critical. It is beyond the scope of this article to discuss in detail the various relationships between powder properties and cathode performance; however, the example given by Jiang, S.P.⁶ demonstrates that an anistropic structure, such as a plate- or needle-like structure, can result in fluctuating shrinkage behavior between 20-34%, which can cause polarization losses and low electronic conductivity.

In recent years, significant research has been aimed at lowering the operating temperature of SOFCs to the low range of 500-700°C and the intermediate range of 650-800°C. The performance of LSM-cathodes is limited to a temperature range of 800-1000°C due to large over-potentials at lower temperatures; however, in combination with other materials, LSM-based cathodes are suitable for a wide range of operating temperatures. One approach to improving the performance of LSM-cathodes at lower temperatures is to prepare composite cathodes, i.e., mixing LSM with another material. By applying vacuum plasma spraying, Barthel and Rambert⁷ prepared and later tested the performance of such composite cathodes between 700-900°C. They found that the electrochemical behavior of the LSM-YSZ composites was enhanced compared to the pure LSM cathodes. Lee et.al.,⁸ who also demonstrated the improvement of YSZ-LSM composite cathodes, suggested that the improvement may be linked to the increased length of the triple-phase boundaries. The use of LSM-containing composite cathodes has also been reported by Armstrong and Virkar,⁹ who demonstrated an improvement of a composite LSGM (LaSrGaMg)-LSM cathode at a weight ratio of 50/50%.

One final, interesting aspect of LSM-cathodes is the possibility that they could function as the supporting element in SOFCs. Most of the planar SOFC-related research describes either anode-supported or electrolyte-supported structures. Yamahara, et.al.,¹⁰ suggest that LSM-cathode-supported structures may offer advantages over Ni-YSZ cermets. Accordingly, the material costs are lower due to the significantly lower price of LSM vs. YSZ, and carbon deposition can be suppressed when using certain fuels.

Continuing Advances

The idea of a clean and efficient form of energy that uses hydrogen as its fuel is fascinating and has been the focus of numerous technological developments in recent years. Although fuel cells have been shown to work in space missions and elsewhere, much work has yet to be completed before large-scale commercialization takes place. Production costs must be lowered, technical barriers must be resolved, and uniform testing guidelines and standards must be determined. Advances in the materials used to produce SOFCs and other types of fuel cells will undoubtedly continue, and will help to propel the technology to the next level. The motivation is high, and so are the stakes.

For more information about LSM and other high-performance materials, contact H.C. Starck Inc., 45 Industrial Place, Newton, MA 02461-1951; (617) 630-4879; fax (617) 559-3906; e-mail jonathan.margalit@hcstarck.com ; or visit http://www.hcstarck.com . (In Europe, contact H.C. Starck GmbH, P.O. Box 1610, 95090 Selb, Germany; (49) 9287-807-153; fax (49) 9287-807-450; e-mail helmut.feuer@hcstarck.com .)

References

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2. Fuel Cell Handbook, Sixth Edition, by EG&G; Technical Services, Inc. Science Applications International Corp., Under Contract No. DE-AM26-99FT40575, U.S. Dept. of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Morgantown, W. Virginia, November 2002.

3. Colson-Inam, S., "Solid Oxide Fuel Cells-Ready to Market?" January 2004 (http://www.fuelcelltoday.com ).

4. Chen, X.J.; Khor, K.A.; and Chan, S.H., "Identification of O2 Reduction Processes at Yttria-Stabilized Zirconia Doped Lanthanum Manganite Interface," Journal of Power Sources, Vol. 123, No. 1, September 15, 2003, pp. 17-25.

5. Pross, E.; Laube, J.; Weber, A.; Mueller, A.; Ivers-Tiffeee, E., "Low Cost (La, Sr)MnO3 Cathode Material with Excellent Electrochemical Properties," Proceedings of the 8th Int. Symp. on SOFC, Eds. S. C. Shinghal and M. Dokiya, The Electrochemical Society, 2003, pp. 391-399.

6. Jiang, S.P., "Issues on Development of (La,Sr)MnO3 Cathode for Solid Oxide Fuel Cells," Journal of Power Sources, Vol. 124, No. 2, November 24, 2003, pp. 390-402.

7. Barthel, K. and Rambert, S., "Processing and Electrochemical Behavior of LSM-YSZ Composite Cathodes by Vacuum Plasma Spraying," published in 3rd European Solid Oxide Fuel Cell Forum, Nantes, F, 1998, ISBN/ISSN: 3-905592-00-2, pp. 11-18.

8. Lee, Y.K.; Kim, J.Y.; Kim, I.; Moon, H.S.; Park, J.W.; Jacobson, C.P.; and Visco, S.J., "Conditioning Effects on La1-xSrxMnO3-Yttria Stabilized Zirconia Electrodes for Thin-Film Solid Oxide Fuel Cells," Journal of Power Sources, Vol. 115, 2003, pp. 219-228.

9. Armstrong, T. and Virkar, A., "Performance of Solid Oxide Fuel Cells with LSGM-LSM Composite Cathodes," Journal of the Electrochemical Society, Vol. 149, No. 12, , 2002, pp. A1565-A1571

10. Yamahara, K.; Jacobson, C.P.; Visco, S.J.; and De Jonghe, L.C., presentation at the 28th International Cocoa Beach Conference & Expo, Cocoa Beach, Fla., January 26-20, 2004.