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

Molecular dynamics simulations were carried out to 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-Jones wall model was used as the support model for the ionomer to generate two different hydrophobic walls: 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 confirmed that it well reproduces the experimentally measured proton self-diffusion coefficient in a Nafion membrane.) The obtained proton self-diffusion coefficient (DH+) indicated that the DH+ for the H-wall case is larger than for the L-wall case. This is related to the morphology of the films. For the H-wall case, the sulfonic groups that form 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 opposite from the wall in the lower side of the film, which can lead to the alignment of Nafion molecules and also create lamellar water structures in the film. It was also confirmed that such water structures have better cluster connectivity and larger cluster size, meaning that they serve as better proton transport pathways.