(699b) Molecular Modeling of Proton Transport in Polymer Electrolyte Membrane
A polymer electrolyte membrane (PEM) is a semipermeable ionomer, designed to conduct protons, while being impermeable to reactants. The ?ideal' PEM characteristics include: (i) high proton conductivity at elevated temperatures, (ii) impermeability to fuel and other reactants, (iii) mechanical and chemical robustness, and (iv) reasonable cost. Among these, the most critical characteristic is controlling conductivity, which is a strong function of the PEM hydration level. This entails a detailed understanding of various molecular interactions and transport processes within the PEM. In this study, we examined the proton transport mechanism in PEM using a molecular dynamics (MD) simulation to establish a relationship between the nano-scopic PEM structure and the macroscopic transport properties by examining proton conduction and water diffusion mechanisms. Nafion® molecules were modeled via a united-atom model with Lennard Jones, and electrostatic potential energy models via Newton's second law. To examine the water uptake at molecular level, long-range Coulombic interactions between water molecules and the Nafion® beads were explicitly accounted in our model. MD simulations were performed from this total potential energy input. We have examined how water and proton transport processes are influenced by water distribution and Nafion® structure. In our preliminary studies, we examined the PEM structural changes for various water uptakes; the PEM density decreased with an increase in water uptake, which is consistent with experimental data. To examine transport coefficients from the nano-structural inputs, we examined the radial distribution functions (g) for water, polar (sulfur), and nonpolar groups . For preliminary testing, we calculated g for proton-water and proton-polar groups, and found that more counterions and water molecules exist in a hydrophilic clustered region. We further examined nano-scale transport processes for protons and water molecules. Specifically, we extracted self-diffusion coefficients of protons for various water contents up to percolation threshold via the Einstein relation. We found drastic differences in diffusional mechanisms for isolated water-islands versus connected islands, which are currently under investigation. We intend to investigate various coarse-grained potential energies accurately from quantum mechanical calculations and link current MD simulations to continuum dynamics to fully explore molecular design of PEMs.
1. C. H. Cheng, P. Y. Chen, and C.W. Hong, J. Electrochem. Soc., 155, B435 (2008).