STABLE HIGH CONDUCTIVITY BILAYERED ELECTROLYTES FOR LOW TEMPERATURE SOLID OXIDE FUEL CELLS (2002)

Solid oxide fuel cells (SOFCs) are the future of energy production in America. They offer great promise as a clean and efficient process for directly converting chemical energy to electricity while providing significant environmental benefits (they produce negligible hydrocarbons, CO, or NO{sub x} and, as a result of their high efficiency, produce about one-third less CO{sub 2} per kilowatt hour than internal combustion engines). Unfortunately, the current SOFC technology, based on a stabilized zirconia electrolyte, must operate in the region of 1000 C to avoid unacceptably high ohmic losses. These high temperatures demand (a) specialized (expensive) materials for the fuel cell interconnects and insulation, (b) time to heat up to the operating temperature and (c) energy input to arrive at the operating temperature. Therefore, if fuel cells could be designed to give a reasonable power output at low to intermediate temperatures tremendous benefits may be accrued. At low temperatures, in particular, it becomes feasible to use ferritic steel for interconnects instead of expensive and brittle ceramic materials such as those based on LaCrO{sub 3}. In addition, sealing the fuel cell becomes easier and more reliable; rapid startup is facilitated; thermal stresses (e.g., those caused by thermal expansion mismatches) are reduced; radiative losses ({approx}T{sup 4}) become minimal; electrode sintering becomes negligible and (due to a smaller thermodynamic penalty) the SOFC operating cycle (heating from ambient) would be more efficient. Combined, all these improvements further result in reduced initial and operating costs. The problem is, at lower temperatures the conductivity of the conventional stabilized zirconia electrolyte decreases to the point where it cannot supply electrical current efficiently to an external load. The primary objectives of the proposed research is to develop a stable high conductivity (> 0.05 S cm{sup -1} at {le} 550 C) electrolyte for lower temperature SOFCs. This objective is specifically directed toward meeting the lowest (and most difficult) temperature criteria for the 21st Century Fuel Cell Program. Meeting this objective provides a potential for future transportation applications of SOFCs, where their ability to directly use hydrocarbon fuels could permit refueling within the existing transportation infrastructure. In order to meet this objective we are developing a functionally gradient bilayer electrolyte comprised of a layer of erbia-stabilized bismuth oxide (ESB) on the oxidizing side and a layer of SDC or GDC on the reducing side, see Fig. 1. Bismuth oxide and doped ceria are among the highest ionic conducting electrolytes and in fact bismuth oxide based electrolytes are the only known solid oxide electrolytes to have an ionic conductivity that meets the program conductivity goal. In this arrangement, the ceria layer protects the bismuth oxide layer from decomposing by shielding it from very low P{sub O{sub 2}}'s and the ESB layer serves to block electronic flux through the electrolyte. This arrangement has two significant advantages over the YSZ/SDC bilayers investigated by others [1, 2]. The first advantage is that SDC is conductive enough to serve as an intermediate temperature SOFC electrolyte. Moreover, ESB is conductive enough to serve as a low temperature electrolyte. Consequently, at worst an SDC/ESB bilayered SOFC should have the conductivity of SDC but with improved efficiency due to the electronic flux barrier provided by ESB. The second advantage is that small (dopant) concentrations of SDC in ESB or ESB in SDC, have been found to have conductivities comparable to the host lattice [3, 4]. Therefore, if solid solutioning occurs at the SDC-ESB interface, it should not be detrimental to the performance of the bilayer. In contrast, solid solutions of SDC and YSZ have been found to be significantly less conductive than SDC or YSZ. Thus, it bears emphasizing that, at this time, only SDC/ESB electrolytes have potential in low temperature SOFC applications.