Molecular Dynamics Study of the Thickness Dependence of Structure and Mass Transport in Ionomer Thin Film

In order to improve cell performance of polymer electrolyte fuel cells, it is important to understand mass transport phenomena at the nanoscale. In this study, we have investigated the effects of thickness of ionomer thin films on proton diffusivity using molecular dynamics simulations. We have obtained self-diffusion coefficient of protons, maximum cluster lengths corresponding to connectivity of water clusters, and water density distributions at different ionomer thickness. As a result, we found that at high water contents, proton diffusion coefficients have a little relationship with ionomer thickness, whereas at low water contents, proton diffusion coefficients show a peak with the ionomer thickness of around seven nm. The maximum cluster lengths show the similar trends as the proton diffusion coefficient.