(241c) A Reactive Molecular Dynamics Study of Proton Transport In Polymer Electrolyte Membranes

Proton transport in an aqueous media occurs through a combination of conventional diffusion (vehicular diffusion) and hopping mechanism (structural diffusion). Proton transport in the aqueous domains of a hydrated proton exchange membrane (PEM) is highly influenced by the acidic environment due to the sulfonic acid groups and confined environment due to the nano-channels. We have examined the dynamical properties of water and protons in Nafion with an equivalent weight of 1144 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. The triggers are parameterized to the experimental rate constant of proton transport in bulk water. The mean lifetime of the protons increases with the water content in Nafion. Water diffusivities increase with the hydration level. The total charge diffusion, vehicular component and structural component increase with water content and the two components of the charge diffusion are found to be negatively correlated. The negative correlation is due to preferential structural diffusion of the proton towards the sulfonate group, whereas vehicular diffusion tends to move the H₃O⁺ ion away from the sulfonate group. Different schemes of execution of RMD algorithm, which varies in terms of the range of local environment involved, are investigated. Both the structural and vehicular components of diffusion are influenced by extending the relaxation of hydrogen bonding network around the proton-hopping reaction.