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.
