Molecular Study of Nonequilibrium Transport Mechanism for Proton and Water in Porous Proton Exchange Membranes

The transport process of proton and water through a porous proton exchange membrane (PEM) under pressure difference is significant to fuel cell performance. Nonequilibrium molecular dynamics simulations were conducted to investigate the diffusion mechanisms of hydronium ions and water molecules under pressure differences. Here, different driving forces (0.15 kcal/(mol center dot angstrom)-0.45 kcal/(mol center dot angstrom)) were applied to hydronium ions and water molecules as they transported through Nafion 117 membrane at temperatures between 300 K and 350 K. Results indicated that the transport diffusion coefficient of water molecules was larger than that of hydronium ions under different pressure drops due to the lower molecular weight and diffusion activation energy of water molecules (20.25 kJ/mol). Hydronium ions formed stronger hydrogen bonds with sulfonic acid groups than water molecules, which resulted in a higher diffusion activation energy for hydronium ions (21.15 kJ/mol). The proton conductivity rose with the increase in pressure difference. Moreover, the transport diffusion coefficient of water molecules and hydronium ions were positively correlated with temperature. This is because the kinetic energy of molecules increased and the pore size of the Nafion membrane enlarged at high temperatures. In addition, the dynamics of the hydrogen bond between hydronium ions and sulfonic acid groups/water molecules were accelerated at elevated temperatures, which further promoted proton transfer.