Ab Initio Study of Proton Transfer and Interfacial Properties in Phosphoric Acid-Doped Polybenzimidazole

Quantum mechanics calculations and hybrid ab initio/empirical molecular dynamics simulations using the ONIOM scheme have been used to investigate proton transfer and interfacial properties in phosphoric acid (PA)-doped polybenzimidazole. Quantum mechanics calculations indicate that hydrogen bonding between PA and benzimidazole (with PA as the hydrogen donor) is the major interaction between PA and polybenzimidazole at low PA-doping levels. Comparing energy barriers for different proton transfer pathways indicates that proton transfer is prone to occur between the same molecules or between a molecule and its corresponding ion. The atom-centered density matrix propagation approach coupled with ONIOM using density functional theory and universal force field calculations indicates that protonation of the =N- atom on the imidazole ring occurs when there are two or more PA molecules surrounding each imidazole ring. These conclusions agree with the experimental results in the literature.