Molecular-level Modeling of the structure and wetting of Electrode/Electrolyte interfaces in hydrogen fuel cells

Molecular dynamics (MD) simulations were performed to investigate the structural and dynamical behavior of water and hydronium ions at the electrode/electrolyte interface of hydrogen polymer electrolyte membrane (PEM) fuel cells. Specifically, we have studied the hydrated Nafion membrane, humidified for four different water contents, 5, 10, 15, and 20%, at 300 K. We analyzed the three-phase interface where the hydrated PEM is in contact with the vapor phase and with either the catalyst surface (platinum in this paper) or the catalyst(-)support surface (graphite in this paper). These molecular simulations represent portions of interfaces that exist within the PEM fuel cells. We observed significant wetting of the catalyst surface by a mixture of polymer, water, and hydronium ions but not beyond a monolayer. We observed virtually no wetting of the graphite surface. On the catalyst surface, the degree of wetting of the catalyst surface depends strongly on the level of membrane humidity. The pair correlation functions indicate that the water molecules adsorbed in a monolayer on the catalyst surface form small domains of ordered structures, which are bound by fragments of Nafion on the surface. The diffusion of protons from the catalyst surface into the membrane must proceed across this highly inhomogeneous surface.