Water Transport in Polymer Electrolyte Membranes Investigated by Dissipative Particle Dynamics Simulation

Abstract

In order to deepen the insight into water transport in polymer electrolyte membranes (PEMs), we calculated the self-diffusion coefficient of water, DW, by dissipative particle dynamics (DPD) simulation. The simulation target materials are a Nafion membrane and radiation-grafted PEMs in the fully-hydrated states. DW was obtained by the following procedures: (i) molecular modeling with the coarse-grained particles representing groups of several atoms; (ii) calculation of the diffusivity of water particles, DWParticle, in the PEM systems in the equilibrium states; (iii) determination of the unit of time in the DPD simulation by comparing DWParticle with DW in bulk water; (iv) conversion of DWParticle of the PEM systems in the DPD unit into DW in the standard SI unit. Interestingly, DW was found to decrease with the diffusion time period, Δt, probably owing to the geometrical confinement effect by water-transport hydrophilic regions. Quantitative analysis of this relationship provided us with the size of hydrophilic regions.