Reactive Molecular Dynamics Study of Proton Transport in Polymer Electrolyte Membranes

Dynamical properties of water and protons in Nafion with an equivalent weight of 1144 are studied using the recently developed reactive molecular dynamics (RMD) algorithm at various water contents. The structural diffusion of a proton along the aqueous domains is modeled via a mechanism similar to that observed in bulk aqueous systems. The algorithm implements reactivity in classical MD simulations by three steps: (i) satisfaction of the trigger, (ii) instantaneous reaction, and (iii) local equilibration. Two different schemes (Method 1 and Method 2) of execution of the algorithm are investigated, which differ in terms of the range of the local environment involved in the reaction. Both methods are parametrized to the experimental rate constant of proton transport in bulk water. The mean lifetime of the protons increased with the water content in Nafion. Water diffusivities of the reactive systems using the two schemes increased with the hydration level and were higher than the diffusion coefficients in the nonreactive system. The more detailed scheme in Method 2 which included the relaxation of the extended hydrogen bonding network around the proton-hopping reaction lowered the water diffusivity compared to that of Method 1 and also affected both the structural and vehicular components of diffusion. The total charge diffusion, vehicular component, and structural component increased with water content, and the two components of the charge diffusion were found to be negatively correlated. The negative correlation is due to preferential structural diffusion of the proton toward the sulfonate group, whereas vehicular diffusion tends to move the H 3 O + ion away from the sulfonate group.