The Effect of Porous Composite Electrode Structure on Solid Oxide Fuel Cell Performance: I. Theoretical Analysis

The effect of porous composite electrodes on the overall charge‐transfer process in solid‐state devices, such as solid oxide fuel cells, is theoretically examined by taking into account various parameters such as electrolyte thickness, intrinsic charge‐transfer resistance, electrode thickness, and porosity. A model is presented that accounts for ionic transport within the electrolyte, electronic conduction through electrocatalyst, and charge‐transfer at the electrolyte‐electrocatalyst interface. Diffusion of gaseous species in porous electrodes is assumed to be rapid so as not to be rate limiting. The conduction of electrons in the electrocatalyst is assumed to introduce negligible resistance. The activation overpotential as a function of current density is assumed to be ohmic, and an effective charge‐transfer resistance is defined. The transport equations are solved numerically in two dimensions using a finite difference technique and analytically in one dimension. The analysis predicts that the use of composite electrodes in devices employing solid electrolytes can significantly increase performance under conditions where the intrinsic charge‐transfer resistance is high in comparison to the area‐specific resistance of the electrolyte. The results indicate a low effective charge‐transfer resistance is obtained for relatively thick electrodes with a fine microstructure as long as the porosity is sufficient to ensure negligible concentration polarization.