Impact of ionomer film on effective transport properties and performance of cathode catalyst layer of proton-exchange membrane fuel cells

Abstract

The ionomer films coated over the agglomerate surfaces in the cathode catalyst layer (CL) are an essential component of proton-exchange membrane fuel cells as they provide pathway for protons to move from the membrane to the catalyst sites. Excessive ionomer coating, however, could reduce the space for gaseous oxygen to move and increase the resistances for dissolved oxygen to diffuse. We investigate these using pore-scale modelling and tomography in this paper. Pore-scale models were developed to simulate proton transport and oxygen diffusion in the structure assuming that the proton transport is through the ionomer and gaseous oxygen diffusion is through the secondary pores. The results indicate that the increase of the effective proton conductivity with ionomer content can be fitted to the Bruggeman’s formula. With the ionomer content increasing, the effective gas diffusion coefficient decreases much faster than predicted by the Bruggeman’s formula, which reveals that the interaction between the bulk and Knudsen diffusions at pore scale creates an extra resistance after the volumetric average. We also elucidated the impact of the ionomer films on cell performance and found that the optimal ionomer content is approximately 20% in the model.