Effects of Water Structure on Proton Transport in Nafion Thin Films with Molecular Dynamics Simulations
In this study, we analyze proton transport properties in the cathode CL using molecular dynamics simulations, focusing on the dependences of the diffusion coefficient on water content. We consider the proton transport mechanism as a combination of the Vehicular mechanism and the Grotthuss mechanism. The Grotthuss mechanism is represented using empirical valence bond methods. It has been shown by previous researches in PEM that the Grotthuss mechanism is a dominant factor for proton diffusion in relatively high water contents. Thus, it is essential to consider the Grotthuss mechanism for analysis of proton transport in CL as well. The system consists of graphite surface and Nafion thin films. Pt particles are neglected because of a restriction of calculation cost. In PEM analysis, the diffusion coefficient of protons increases with increasing water contents. First, we report whether the same trend as observed in PEM can be seen in CL and the correlation between diffusibility and structures. Next, unique characteristics of proton transport properties in CL compared to the PEM analysis are reported. They are evaluated in terms of mass distribution of the thin films and solvent molecules. From this analysis, it is found that the diffusibility is not much affected by surface because of a lack of solvent molecules on surface, which is attributed to the hydrophobic properties of graphite.