The Theoretical Energy Conversion Efficiency of a High Temperature Fuel Cell Based on a Mixed Conductor

Abstract

We have calculated the voltage vs. current characteristic and the energyconversion efficiency of a fuel cell based on a mixed ionic-electronic conductor.The transport equations for the electrolyte are solved exactly and the materialparameters which determine the shape of the characteristic are determined.The equations are then applied to the special case of doped ceria, whichbecomes a mixed conductor only at low oxygen pressures. We then showthat for the purpose of efficiency calculations, the finite but small ratio of ionicto electronic mobility valid for ceria can be safely taken as zero, and we comparethe resulting simple equations with the equivalent circuit approach to afuel cell. The characteristic is found to be curved instead of being a straightline and for typical operating conditions the best energy conversion efficiencyis, for the transport theory approach, 50% higher than for the equivalentcircuit one. We conclude from the calculated new values of conversionefficiency that the equivalent circuit underestimates the conversion efficiencyof a mixed conductor seriously and that doped ceria as a solid electrolyte is amore serious contender to doped zirconia than believed up to now.