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 V2+ and V3+ 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 H2O/SO3–), the probability for direct interaction is significantly reduced because of solvation of the cations by the water molecules. Surprisingly, V3+ showed less affinity to the SO3– groups than did V2+, as demonstrated by fewer V3+ around the SO3– groups. This is attributed to the stronger attraction of the first hydration shell water molecules to V3+. 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 SO3– groups and the H3O+ cations is strongly affected...