Simulation of carbon black aggregate and evaluation of ionomer structure on carbon in catalyst layer of polymer electrolyte fuel cell

In polymer electrolyte fuel cells (PEFCs), an ionomer is needed to maintain the proton conductivity in the catalyst layer; however, it causes oxygen diffusion resistance because of its thickness on the platinum surface and the blockage of the void spaces. Therefore, understanding the ionomer distribution on a platinum/carbon black (CB) support catalyst is extremely important, because this knowledge can contribute to a reduction in the resistance for the cathode oxygen reduction reaction. In this study, a three-dimensional CB aggregate structure is simulated using numerical analysis with various experimental information (surface volume, aggregate size, and anisotropy); from the viewpoint of the roughness and morphology of CB, the ionomer distribution on its surface is simulated. The relationship between the ionomer content and the coverage of the ionomer on carbon is determined. Moreover, the effect of the surface structural properties on the ionomer distribution in the catalyst layer is studied using simulations. Based on the results, the relationship between the surface roughness and the ionomer connectivity is determined. The coverage and thickness of the ionomer do not change linearly upon changing its content. This condition strongly depends on the surface roughness. Insights gathered from this study can assist in designing optimum catalyst layer.