Polymer chain length, phosphoric acid doping and temperature dependence on structure and dynamics of an ABPBI [poly(2,5-benzimidazole)] polymer electrolyte membrane

Abstract

An ABPBI [poly(2,5-benzimidazole)] membrane doped with phosphoric acid (PA) are promising electrolytes in high temperature fuel cells. In the present work, we employ Molecular Dynamics (MD) simulations to characterize the effect of polymer chain length using a dimer to hectamer. Results from our MD simulations (dimer to decamer) show the following trends: the inter-chain and intra-chain interactions in membrane are unaffected with polymer chain length and temperature, though a significant increase with PA doping is observed. The radius of gyration linearly increases with polymer chain length and remains unchanged with PA doping and temperature. However, the end-to-end distance deviates from linearity with polymer chain length which suggests increasing coiling of the membrane. The diffusion coefficient of PA increases with PA doping and temperature, but remains constant with polymer chain length. The activation energy of diffusion of PA decreases significantly with an increase in polymer chain length at low PA doping, but remains unaffected at higher PA doping. A comparison of the structural and dynamical properties of a decamer and a hectamer shows that the decamer represents the optimum polymer chain length beyond which no significant change in properties is observed.