A Molecular Dynamics Study of the Effects of V²⁺ and V³⁺ on the Local Structure of Hydrated Nafion

Because Nafion is a commonly employed membrane in vanadium redox flow batteries, we have undertaken classical molecular dynamics simulations to understand the effects of absorbed V 2+ and V 3+ on the local structure of the ionomer. The simulations reveal that at low hydration the vanadium ions directly interact with the sulfonate groups forming contact ion pairs. As the water content is increased to nearly fully hydrated conditions (18 H 2 O/SO 3 – ), the probability for direct interaction is significantly reduced because of solvation of the cations by the water molecules. Surprisingly, V 3+ showed less affinity to the SO 3 – groups than did V 2+, as demonstrated by fewer V 3+ around the SO 3 – groups. This is attributed to the stronger attraction of the first hydration shell water molecules to V 3+ . Despite the inhomogeneity of the system, the population in the first hydration shell is similar to that observed in bulk water. The pair correlation function between the SO 3 – groups and the H 3 O + cations is strongly affected by the presence of the vanadium ions at all hydration levels. We attribute this to the availability of more unoccupied sulfonate sites and the disruption and perturbation of the structure of the water.