Molecular Dynamics Analysis of Proton Diffusivity in Hydrated Nafion Membranes Contaminated with Ferrous Ions

A molecular dynamics simulation is performed to understand the effects of ferrous ion contaminations on the proton transport property and nanostructures of solvent molecules in hydrated Nafion membranes while considering the Grotthuss mechanism. At low hydration conditions, the proton diffusivity has a local maximum at a certain concentration of ferrous ions. In the case of low ferrous ion concentration (≈25% of total cation charge), proton diffusivity is similar to that in the pure membrane. Also, in the case of middle ferrous ion concentration (50%), proton diffusivity is enhanced because the water clusters are more connected compared with those in the pure membrane. In the case of high ferrous ion concentration (larger than 70%), proton diffusivity is inhibited because the water clusters are disintegrated by the ferrous ions. Moreover, the correlation between proton diffusivity and nanostructure of the Nafion membrane is different among low, middle, and high water content. The results imply that the proton diffusivity is affected by ferrous ion contaminations by different mechanisms according to water content.