Elastic Analysis of Fuel Cell Polyelectrolyte Membrane from Coarse-Grained Molecular Dynamics Simulations

Fuel cells have been developed as an automotive power source. The improvement of their durability would be needed for widespread commercial use. One of the important problems regarding the fuel cell durability is hydrogen crossover, which makes fuel efficiency low. The amount of crossover through the polyelectrolyte membrane increases during hydration/dehydration cycles, because the mechanical strength of the membrane becomes weakened. To investigate the cause of weakening membrane, we analyzed elastic modulus values from the viewpoint of molecular structure in the membrane at various hydration levels, by using coarse-grained molecular dynamics simulations. Potential parameters derived from atomistic simulations enabled quantitative agreement with experimental densities and cluster spacing of hydrated membranes. The water content dependence of elastic modulus is also reproduced well. Molecular structures in hydrated membrane were analyzed. We could clarify that each water particle bridges two sulfonic acids lobe a quasi cross-link. The number of quasi cross-links, which have a similar effect of cross-links between polymer chains, clearly relates to the elastic modulus.