Reversible Solid Oxide Fuel Cell Technology for Green Fuel

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Reversible Solid Oxide Fuel Cell Technology for Green Fuel and Power Production by Nguyen Q. Minh and Mogens B. Mogensen Areversible solid oxide fuel cell (RSOFC) is a device that can operate efficiently in both fuel cell and electrolysis operating modes. Thus, in the fuel cell mode, an RSOFC functions as an SOFC, generating electricity by electrochemical combination of a fuel (hydrogen, hydrocarbons, alcohols, etc.) with air (oxygen in the air). In the electrolysis mode, an RSOFC functions as an electrolyzer (in this case, referred to as a solid oxide electrolysis cell or SOEC), producing hydrogen (from water) or chemicals such as syngas (from mixtures of water and carbon dioxide) when coupled with an energy source (fossil, nuclear, renewable). Figure 1 illustrates the operating principles of the RSOFC. The RSOFC has the following attractive features (demonstrated or potential): compatibility (environmentally compatible with reduced CO2 emissions in power generation mode), flexibility (fuel flexible and suitable for integration with any type of energy sources), capability (useful for different functions), adaptability (suitable for a variety of applications and adaptable to local energy needs), and affordability (competitive in costs).1 The RSOFC thus possesses all the desired characteristics to serve as a technology base for green, flexible, and efficient energy systems in the future (Fig. 2). Sustainable energy systems based on the RSOFC for the future is feasible. An example of such a system is shown schematically in Fig. 3. In this system, the RSOFC, operating in the electrolysis mode, uses a renewable energy supply (e.g., solar, wind, hydro) to produce hydrogen (from H2O) or syngas (H2+CO) (from mixtures of H2O and CO2). The chemicals produced can be used to generate power by the same RSOFC operating in the fuel cell mode or can be stored or converted to other chemicals/fuels for subsequent uses. Similarly, the RSOFC can generate power from biomass-derived fuels and the electricity generated can then be used for a variety of power generation applications. The RSOFC is both the SOFC and SOEC incorporated in a single unit. Since the SOEC is the SOFC operated in reverse mode and traditionally derived from the SOFC, the RSOFC being developed is typically based from the more technologically advanced SOFC. Thus, materials for the RSOFC are those commonly used in the SOFC, i.e., yttria stabilized zirconia (YSZ) for the electrolyte, perovskites (such as lanthanum strontium manganese oxide or LSM, lanthanum strontium cobalt iron oxide or LSCF) for the oxygen electrode, nickel/YSZ cermet for the hydrogen electrode and for stacking, conductive oxides (such as lanthanum strontium chromium perovskite or LSC) or stainless steels for the interconnect (depending on the operating temperature). Like the SOFC, the RSOFC operates in the temperature range of 600o-1000oC. Specific operating temperature depends on cell/stack designs and selected materials.2,3 Solid Oxide Fuel Cell Technology The RSOFC is fundamentally and technologically based on SOFC technology. In the past 20 years, the SOFC has received significant attention as a clean and efficient energy conversion technology for a variety of practical fuels and has been under development for a broad spectrum of power generation applications. The key features of the SOFC are its all solid state construction (ceramic and metal) and high operating temperature (600o-1000oC). The combination of these features leads to a number of distinctive and attractive attributes for the SOFC including cell and stack design flexibility, multiple fabrication options, multi-fuel capability, and operating temperature choices. SOFC cells can be configured to be self supporting (electrolyte-supported, anode- supported, cathode-supported) or external supporting (interconnect-supported, sub- strate-supported). Stack designs being developed for the SOFC include the tubular design, the segmented-cells-in-series design, the monolithic design, and the planar design, with the planar design currently being the most common. These design options permit flexibility to shape the SOFC into a structure having the desired electrical and electrochemical performance along with required thermal management, mechanical integrity, and dimensional constraint (if any) to meet operating requirements of specified power generation applications.4 A wide range of fabrication processes have been investigated for making SOFC cells. Fabrication processes developed for the SOFC include conventional ceramic processing methods (e.g., tape casting, tape calendering, screen printing, and extrusion) and deposition techniques (e.g., plasma spraying, spin coating, dip coating, electrochemical vapor deposition, electrophoretic deposition).4 The key step in any selected process is the fabrication of (continued on next page) Fig. 1. Operating Principles of an RSOFC (written for hydrogen fuel in SOFC mode and steam electrolysis in SOEC mode). The Electrochemical Society Interface • Winter 2013 55

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