Enhanced hydronium ion diffusion in proton exchange membranes reinforced with multilayer graphene oxide: new insights into water retention and ion mobility using molecular dynamics simulation

Abstract

Graphene oxide (GO) reinforced perfluorosulfonic acid (PFSA) based proton exchange membranes (PEMs) show enhanced ion diffusion resulting in elevated polymer electrolyte fuel cell (PEFC) performance. However, the mechanisms by which GO influences water dynamics and ion (hydronium) transport are relatively less explored in the literature. In addition, it is expected that the interlayer spacing of multilayer GO plays a crucial role in promoting ion mobility. To this end, this research article explores the possibility of providing new insights into the water/ion dynamics as well as identifying the impact of interlayer spacing of GO on the ion diffusion. Molecular dynamics (MD) simulation is implemented to elucidate the behaviour of multilayer-GO with PFSA structure and to examine the interactions between functional groups (epoxy and hydroxyl) on the GO surface with water molecules and hydronium ions. The retention of water molecules adjacent to the multilayer-GO plays a crucial role in forming transport channels that significantly enhance ion mobility within the membrane structure. The optimal interlayer spacing of 9.5 Å is identified as the critical threshold value where ion diffusion is observed at its peak. In comparison with pristine Nafion®, the ion (hydronium) diffusion coefficient in the multilayer-GO with PFSA polymer shows an improvement of ∼17% and ∼30% at 300 K and ∼9% and ∼12% at 350 K for hydration levels (λ) of 13 and 20, respectively.