INFLUENCE OF GAS-PHASE MASS-TRANSFER LIMITATIONS ON MOLTEN-CARBONATE FUEL-CELL CATHODES

The purpose of this paper is to elucidate to what extent mass transfer limitations in the gas phase affect the performance of porous molten carbonate fuel cell cathodes. Experimental data from porous nickel oxi de cathodes and calculated data are presented. One and two-dimensional models for the current collector and electrode region have been used. Shielding effects of the current collector are taken into account. Th e mass balance in the gas phase is taken into account by using the Ste fan-Maxwell equation. For standard gas composition and normal operatin g current density, the effect of gas-phase diffusion is small. The dif fusion in the gaseous phase must be considered at operation at higher current densities. For low oxygen partial pressures, the influence of mass transfer limitations is large, even at low current densities. To eliminate the influence of conversion on polarization curves recorded on laboratory cell units, measurements should always be performed with a five to tenfold stoichiometric excess of oxygen. Two-dimensional ca lculations show rather large concentration gradients in directions par allel to the current collector. However, the influence on electrode pe rformance is still small, which is explained by the fact that most of the current is produced close to the electrolyte matrix.