Atomistic simulations of nanostructure and dynamics of phosphoric acid-benzimidazole systems: A fuel cell initiative

Abstract

Phosphoric acid doped benzimidazole membranes like poly[2,2-(m-phenylene)-5,5-bibenzimidazole] (PBI) and poly(2,5-benzimidazole) (ABPBI) have been investigated as fuel cell electrolytes to operate at elevated temperatures. Several experimental studies have synthesized and characterized various physical, chemical and electrochemical properties of these phosphoric acid doped benzimidazole systems. In this thesis, computer simulation methods such as Molecular Dynamics is employed to examine structural and dynamical properties of phosphoric acid-benzimidazole systems. The insights from computation can spur further experimental investigations on fuel cell membranes for anhydrous proton conduction. Since, benzimidazole moiety is an important constituent of these membranes, the interactions in phosphoric acid-benzimidazole mixtures is first examined. The structural properties (Radial Distribution Function), dynamical properties (diffusion) and hydrogen bond lifetime calculations allude to the possibility that benzimidazole and phosphoric acid molecules exhibit dual proton-acceptor/donor functionality. A subsequent examination of interactions between phosphoric acid and ABPBI shows that the inter-chain and intra-chain interactions in ABPBI membrane remain unaffected with chain length and temperature. However, these interactions are significantly changed with phosphoric acid doping. The radius of gyration is found to increase linearly with increasing ABPBI chain length but remains invariant to phosphoric acid doping and temperature. The end-to-end distance deviates from linearity with chain length of ABPBI which suggests increased coiling of membrane (independent of phosphoric acid doping and temperature). The diffusion coefficient of phosphoric acid increases with phosphoric acid doping and temperature, but remains constant with polymer chain length. The activation energy of diffusion of phosphoric acid decreases significantly with an increase in polymer chain length at low phosphoric acid doping, but remains unaffected at higher phosphoric acid doping.