Atomistic investigation of polymer electrolyte membrane nanostructure and dynamics of molecular transport in fuel cells

Abstract

In Polymer Electrolyte Membrane (PEM) fuel cells, the PEM is sandwiched between the electrodes to facilitate the proton transport from anode to cathode. A molecular study of structural and dynamical properties of various PEMs from simulations can facilitate the development of aqueous/non-aqueous PEMs for fuel cell applications. In this thesis, polymer membranes like the aliphatic Perfluorosulfonic acid (PFSA) membranes [for example: Nafion (Dupont), Aciplex (Asahi Chemicals Co.) and Dow (Dow Chemicals)] and hetero aromatic benzimidazole based membranes (ABPBI) are chosen for investigation using Molecular Dynamics (MD) simulations. A characterization of ammonium based ionic liquids (ILs) using MD simulations and experimental techniques is also presented in this thesis. MD simulations on trifluoromethanesulfonic (triflic) acid and triflate ion-water mixtures are performed to mimic the functional group of hydrated PFSA polymer electrolyte membranes where the sulfonate group is responsible for proton conduction. Subsequently, a comparative study of pendant-water mixtures of Dow, Aciplex and Nafion membranes is performed to investigate the role of the ether oxygen in side chain and the side chain length. The nanostructure of the full Aciplex ionomer membrane and dynamics of hydronium ions and water molecules is also investigated. A molecular investigation of the hydrated ABPBI polymer membrane doped with phosphoric acid (PA) and triflate ion (TFA) is performed for high temperature PEM fuel cells. Classical MD simulations are employed to compare the structure and dynamics of ABPBI+PA, ABPBI+TFA and ABPBI+PA+TFA blends at varying hydration. The effect of different parameters such as thermostat, coupling time and system size for the accurate determination of shear viscosity is examined using an Extended Simple Point Charge (SPC/E) water model. The relation of electrostatic, cation-phenyl and C-H/phenyl interactions with the structure and dynamics of benzyl-NX3(X=methyl, ethyl) trifluoromethanesulfonate ILs is characterized using MD simulations and Electro-chemical Impedance Spectroscopy (EIS). The characterization of the role of asymmetry of cation and N-H/pi interactions in proton transfer using ab initio MD can be the focus of future activities. Such a study can provide further insights to design novel protic ILs for various technological applications.