Molecular Dynamic Simulations of the Effect on the Hydration of Nafion in the Presence of a Platinum Nanoparticle

Platinum catalysts play a critical role in fuel cell technology. Current optimization efforts focus on reducing the amount of Pt in the system and optimizing the utilization of that which remains. The effect of the presence of Pt nanoparticles on the local structure and morphology of the polymer electrolyte membrane, water, and hydronium ions has been studied at molecular level in this work. Classical molecular dynamics simulation has been used to examine a system containing a 4 nm fcc cubic ({100} face) platinum nanoparticle at the center surrounded by Nafion polymer, water molecules, and hydronium ions at λ = 3, 6, 9, 15, and 22. The changes in density and orientation distribution of sulfonic acid groups in the side-chains, water, and hydronium as a function of distance from platinum surface are analyzed in this study. Sulfonic acid groups and hydronium ions show a very high increase in density near the platinum surface, and they approach the bulk value as they move away from the platinum surface. At lower water contents (λ = 3 and 6), water is strongly attracted to platinum surface, increasing the density near the platinum surface. However, at the highest humidity level studied, the density of water farthest from the platinum surface (>20 Å) is higher than the bulk value and the density of water nearest to the platinum surface. The above observed phenomenon is explained using the probability distribution of orientation computed at various distances from platinum surface. As the water content increases, sulfonic acid groups show a preferential orientation for the side-chains to align themselves horizontally on the platinum than vertically, thereby covering a larger area of platinum and pushing the water away from the platinum surface. This causes the water density to drop near the platinum surface. Water molecules and hydronium ions show preference to certain orientations near the platinum surface, but usually to none as they move away from the platinum surface.