Theoretical simulations of proton conductivity: Basic principles for improving the proton conductor

Classical molecular dynamics (MD) simulations of proton conduction have been performed, to get insight into basic principles of potential improvements of proton conductivity in polyelectrolyte membranes. For the simulations the reactive force field for water (RWFF) was used, which allows bond dissociation of water, acids and hydronium ions. The effects are shown to be fundamental relevance for the diffusion of protons in membranes. One and two-dimensional conductors, and a Nafion™™ membrane have been modeled in our simulations. The two-dimensional model imitates a metal phosphate; the one-dimensional model imitates an idealized pore of hydrated Nafion membranes. The MD simulations of proton conductivity of the metal phosphate show the dissociation of the acid POH groups and their participation in the proton transport. Several simulations are performed with acids of different strength and the effect of the acid’s strength on the diffusion and on the conductivity is analyzed. The importance of the ion coupling on the conductivity is firstly proved in tubes, which imitate an ideal pore inside a membrane. Afterwards, the coupling is investigated in a real hydrated Nafion membrane by a non-equilibrium MD simulation. The results suggest a soliton-like behavior for proton conductivity in membranes.