Proton transport through interfaces in nanophase-separation of hydrated aquivion membrane: Molecular dynamics simulation approach

Abstract

We investigate the effect of temperature and hydration on the nanophase-separated structure and proton conductivity in Aquivion and Nafion membranes using molecular dynamics simulation method. By considering two different temperatures (298 and 353 K) and water contents (10 and 20 wt%), our study reveals that Nafion exhibits better nanophase separation and a more mature internal structure of the water phase compared to Aquivion, leading to facilitated proton dissociation and improved proton conduction through vehicular and hopping mechanisms. This study also identifies the formation of a hydronium-mediated bridge configuration that restricts vehicular mobility at low hydration, resulting in a decrease of the vehicular contribution to proton conduction. From quantitative evaluation of the extent of nanophase-separation using structure factor analysis, we find that Nafion has a shorter correlation length with greater concentration contrast in comparison to Aquivion. At high hydration conditions, the hopping mechanism dominates at low temperature while the vehicular mechanism is dominant at high temperature. At low hydration, the vehicular mechanism is significantly decreased due to hydronium-mediated bridge configurations. It is interesting to note that the proton conductivity in the Aquivion membrane is higher despite the Nafion membrane having a higher proton diffusion. We discover that the greater proton conductivity of Aquivion membrane is caused by its higher proton concentration. In general, our findings offer a fundamental knowledge of the connection between nanophase-separated structure and proton transport characteristics in Nafion and Aquivion systems under a range of temperature and hydration circumstances.

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