Equivalent circuits and efficiencies of fuel cells

Abstract

By introducing an equivalent thermal potential fixed in terms of the heat of reaction, a thermodynamic equivalent circuit is determined for a fuel cell which yields the correct current-potential curve of the cell for low current by formal application of Ohm's law. The thermodynamic power balance and the thermodynamic efficiency also are given correctly. A linear electrical equivalent circuit is constructed which represents closely the electrical performance of the cell over a wide range of current (not approaching zero). The effect of polarization processes on the electrical power and efficiency is taken into account through a single constant parameter. Introduction of the equivalent thermal potential into the circuit yields a thermodynamic equivalent circuit whose output computed by formal use of Ohm's law coincides exactly with that given by the electrical equivalent circuit. The correct electrical properties, thermodynamic power balance, and thermodynamic efficiency follow directly from the circuit. A general theorem is formulated, independently of the validity of an equivalent circuit, connecting the thermodynamic and electrical efficiencies of a fuel cell. Confirmatory experimental results based on hydrogen—oxygen cells are presented, which underline the large reduction in available power brought about by polarization processes and entropy changes in a fuel cell.