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The effect of pressure on solid oxide fuel cell performance
Current work in solid oxide fuel cells (SOFCs) is on cathode-supported, anode-supported, or electrolyte-supported cells. In electrode-supported cells, a thin film (5 to 30 microns) of an electrolyte (YSZ) is deposited on a relatively thick, porous electrode. In electrolyte-supported cells, the electrolyte thickness is typically greater than or equal 150 microns upon which thin electrodes are screen printed. Both types of SOFCs are being explored for hybrid applications, that is, in combination with a gas turbine, for which the exit gases from an SOFC generator must be at a high pressure (3 to 15 atm) for input into a gas turbine. It is necessary to examine the expected performance of an SOFC under a high pressure. Work at Westinghouse Ontario Hydro has shown that the performance improvement at high pressures is greater than that can be expected based on an increased Nernst potential alone. This increased performance can in part be attributed to a lower concentration polarization. The objective of this work was to conduct a preliminary analysis of the effect of pressure on the performance of both cathode-supported and electrolyte-supported cells. Flux equations for transport through porous electrodes are formulated and are solved in combination with those for electrochemical operation of an SOFC for cathode-supported and electrolyte-supported cells. The analysis shows that the overall cell performance increases significantly with increasing pressure in the case of cathode-supported cells due to a lowering of concentration polarization at high pressures. Similar effects (not presented here) are also observed on anode-supported cells. By contrast, only a modest improvement is observed in the case of electrolyte-supported cells, commensurate with the fact that in the latter, the ohmic contribution of the electrolyte is the most dominant one, which is not altered by pressure