Thin Shell Model for Proton Conduction in PEM Fuel Cell Cathodes

Abstract

A mathematical model is presented for proton conduction in the thin ionomer layer that covers the carbon supported catalyst in proton exchange membrane (PEM) fuel cells. The cathode proton resistance has been previously measured in a cell using ac impedance as a function of ionomer-to-carbon mass ratio , ionomer equivalent weight, and relative humidities (RH). After considering the effect of roughness and ionomer absorption in the carbon support, the model favorably agrees with the experimental results. It shows that the cathode proton resistivity is indeed an intrinsic property, which for a given type of carbon support depends on the volumetric ionomer-to-carbon ratio, which accounts for the RH-induced ionomer swelling. Further analysis indicates that the roughness and porosity of the carbon particles play a significant role for the proton conduction. The amount of ionomer absorbed into the pores and the roughness factor of the carbon particles can be extracted from a comparison of the model and data.