Molecular dynamics studies of the Nafion®, Dow® and Aciplex® fuel-cell polymer membrane systems

The Nafion, Dow and Aciplex systems - where the prime differences lies in the side- chain length - have been studied by molecular dynamics ( MD) simulation under standard pressure and temperature conditions for two different levels of hydration: 5 and 15 water molecules per ( H) SO3 end- group. Structural features such as water clustering, water- channel dimensions and topology, and the dynamics of the hydronium ions and water molecules have all been analysed in relation to the dynamical properties of the polymer backbone and side- chains. It is generally found that mobility is promoted by a high water content, with the side-chains participating actively in the H3O+/ H2O transport mechanism. Nafion, whose side- chain length is intermediate of the three polymers studied, is found to have the most mobile polymer side- chains at the higher level of hydration, suggesting that there could be an optimal side- chain length in these systems. There are also some indications that the water- channel network connectivity is optimal for high water- content Nafion system, and that this could explain why Nafion appears to exhibit the most favourable overall hydronium/ water mobility.