Mass transport in gas-diffusion electrodes : A diagnostic tool for fuel-cell cathodes

Two mathematical models of gas-diffusion electrodes, one for liquid electrolytes and one for ion-exchange polymer electrolytes, are presented to investigate the effects of mass-transport limitations on the polarization characteristics of a reaction obeying Tafel kinetics. The focus is on low-temperature fuel-cell cathodes, and in particular, contrasting two limiting cases that may be encountered at high current densities: control by kinetics and dissolved oxygen mass transport vs. control by kinetics and ionic mass transport. It is shown that two distinct double Tafel slopes may arise from these two limiting cases. The former is first order, and the latter is half-order with respect to oxygen concentration. How the modeling results may be applied to diagnose the performance of fuel-cell cathodes is also presented. Since the ionic-mass-transport-limited case has generally been neglected in previous gas-diffusion electrode models, specific examples of fuel-cell cathode data from the literature which display the behavior predicted by the models in this case are given and briefly discussed.