Modelling the performance of the cathode catalyst layer of polymer electrolyte fuel cells

A known macrohomogeneous model for the cathode catalyst layer of a low temperature fuel cell: which includes the kinetics of oxygen reduction at the catalyst \ electrolyte interface, proton transport through the polymer electrolyte network, and oxygen diffusion through hydrophobized voids, is considered. Analytical expressions in the relevant ranges of parameter values are obtained. These are the limits of (i) small currents, (ii) fast oxygen diffusion, (iii) fast proton transport, (iv) high current densities (when the layer is depleted in oxygen). They help to trace the main system properties and operation modes, and the routes for the structure-function optimization. Namely, they rationalize the explicit effect of the kinetic parameters of the oxygen reduction on the performance, the effect of the catalyst layer thickness, the width and the position of the active fraction of the layer, and the dependence of the performance on the oxygen partial pressure. In current-voltage plots, three regions are distinguished, each dominated by one of the three main physico-chemical processes in the cathode. Ranges of the optimum cathode thickness are revealed depending on the current density regimes. A bridge between the structural composition of the cathode and its performance is built with the help of an effective medium theory; the model reproduces the experimentally observed trends. (C) 1998 Elsevier Science S.A. All rights reserved.