Computational Investigation of Structure, Dynamics and Proton Transport in Polymer Electrolyte Membrane Fuel Cells

Abstract

Polymer electrolyte membrane fuel cells are environmentally friendly energy conversion devices where electrical energy is derived from chemical energy. The key role of the membrane is to prevent the mixing of the reactant gases, to conduct protons from anode to cathode and to provide insulation to the transfer of electrons. Experimental techniques and computer simulations have been employed extensively to study structure, surface morphology, membrane deformation, dynamics, hydrophilicity, etc. of various polymer electrolyte membranes. In this thesis, molecular dynamics (MD) simulations (using all-atom force field) is employed to examine structure/dynamics of molecular transport in hydrated perfluorosulfonic acid and N,N-diethylethylammonium triflate([dema][TfO]) ionic liquid (IL) doped poly-benzimidazole (PBI) fuel cell environments. In the first part, MD simulations are employed to examine the effect of atomic charge delocalization on the pendant side chain of hydrated Nafion on the structural and dynamical properties. The sulfur-sulfur radial distribution functions suggest that the sulfonate groups of the pendant side chain have closer geometric proximity with an increase in charge delocalization. A complex interplay between sulfur-sulfur, sulfur-water/hydronium interactions, and water cluster distribution plays a key role in the magnitude of the diffusion coefficient of water molecules and hydronium ions. In the second study, the simulations predict ionic conductivity increases with wt% of [dema][TfO] IL (in IL-doped PBI) and temperature and is found to be in qualitative agreement with experimental measurements. Also, the simulations predict that anions of IL preferably interact with the interaction site on the PBI. In the final investigation, quantum chemistry calculations are employed to examine proton transport pathways in base rich imidazolium methanesulfonate (IMMSA) IL. When IMMSA interacts with two imidazole molecules, one of the pathways shows barrierless rotation of imidazole molecules. This could be the reason for high proton conductivity in base rich imidazolium ILs.