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 fuelcell electrolytes to operate at elevated temperatures Several experimental studies havesynthesized and characterized various physical chemical and electrochemical propertiesof these phosphoric acid doped benzimidazole systems In this thesis computer simulationmethods such as Molecular Dynamics is employed to examine structural and dynamicalproperties of phosphoric acid benzimidazole systems The insights from computationcan spur further experimental investigations on fuel cell membranes for anhydrous protonconduction Since benzimidazole moiety is an important constituent of these membranes the interactions in phosphoric acid benzimidazole mixtures is first examined Thestructural properties Radial Distribution Function dynamical properties diffusion andhydrogen bond lifetime calculations allude to the possibility that benzimidazole and phosphoricacid molecules exhibit dual proton acceptor donor functionality A subsequent examination of interactions between phosphoric acid and ABPBI showsthat the inter chain and intra chain interactions in ABPBI membrane remain unaffectedwith chain length and temperature However these interactions are significantly changedwith phosphoric acid doping The radius of gyration is found to increase linearly withincreasing ABPBI chain length but remains invariant to phosphoric acid doping and temperature The end to end distance deviates from linearity with chain length of ABPBIwhich suggests increased coiling of membrane independent of phosphoric acid dopingand temperature The diffusion coefficient of phosphoric acid increases with phosphoricacid doping and temperature but remains constant with polymer chain length Theactivation energy of diffusion of phosphoric acid decreases sig newline newline