Multiscale Simulation of a Polyelectrolyte Membrane for Fuel Cells

A multiscale simulation is presented to study the proton-conducting polyelectrolyte membrane Nafion for a fuel cell. Firstly, the mesoscopic structure of the hydrated Nafion membrane was predicted by dissipative particle dynamics simulation. In this method, a molecular structure is represented using a coarse-grained model, and the interaction parameters are estimated by calculating the energy of mixing for each pair of components. A sponge-like structure was spontaneously formed. Secondly, an atomistic structure of the water channel is generated based on the obtained mesoscopic structure by mapping atoms to the concentration profile of each component using a Monte Carlo technique. Then, a molecular dynamics (MD) simulation is performed. The calculated self-diffusion coefficients of water molecule are consistent with the experimental study with respect to both the magnitude and dependence on the water content. The number of water molecules in the first coordination shell around the sulfonic acid group decreases with a reduction in the water content. Thirdly, the electronic state is calculated for a hydronium ion explicitly accounting for the ambient electrostatic energy by the mesoscopic structure. The distribution of the dielectric constant is defined on the basis of the mesoscopic structure and a hydronium ion is then placed in the water region. By iteratively solving Poisson’s equation for the electrostatic potential and Schrödinger’s equation for the electronic state, the energy of the hydronium ion is obtained. We compared the stability of a hydronium ion in the mesoscopic structure.