Molecular dynamics study of the effect of wettability of the carbon support on proton transport in nafion ionomer thin films

Abstract

Molecular dynamics simulations were carried outto elucidate the effect of the wettability of the carbon support used for Nafion ionomer thin films on proton transport in the ionomer, which is related to the power density of polymer electrolyte fuel cells. The Lennard-Joneswall model was used as the supportmodel for the ionomer to generate two different hydrophobicwalls: the high hydrophobic wall (H-wall) and the low hydrophobic wall (L-wall). The proton transport model,including the Grotthuss mechanism,was used to express real proton transport phenomena (In early work, we confirmedthat it well reproduces the experimentally measuredproton self-diffusion coefficient in a Nafion membrane.) The obtained proton self-diffusion coefficient(DH+) indicated that the DH+ for the H-wall case islarger than for the L-wall case. Thisisrelated to the morphology of the films. For the H-wall case, the sulfonicgroups thatform part of the hydrophilic Nafion side chains were confirmed to be oriented in the direction to the wall in the upper side of the film and oppositefrom the wall in thelower side of the film, which can lead tothe alignment of Nafion moleculesand also createlamellar water structuresin the film. It was also confirmed that suchwater structures havebetter cluster connectivity and largercluster size, meaning that they serve as better proton transportpathways.