Effects of Water Structure on Proton Transport in Nafion Thin Films with Molecular Dynamics Simulations

Polymer electrolyte fuel cell (PEFC) is expected to play important roles for next generation power systems in terms of high efficiency and low environmental burden and capability for emergency use, but there are still many hurdles to be largely commercialized. One of them results from problems in catalyst layers (CL). What required for CL is, for example, reducing the amount of Pt particles with keeping high current density. To accomplish this, knowledge about optimum structures of CL based on material transport properties is needed because there is a strong correlation between current density and transport properties. Although there are many reports about material transport properties in the polymer electrolyte membrane (PEM), it is predicted that material transport properties in CL show some unique characteristics because of effects of interfaces in CL. It means that the knowledge obtained from PEM should not be applied to CL. In addition, the researches about CL have been tried by means of experiments and simulations, but especially the transport properties have not been clarified yet. There are nanoscopic structures in Nafion thin films and it is hard to capture the transport phenomena in such a small area by experiments and macroscopic 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.