Ionic (Proton) transport and molecular interaction of ionic Liquid–PBI blends for the use as electrolyte membranes

Abstract

Protic ionic liquids (PILs) are discussed as new candidates for the use as non-aqueous electrolytes for fuel cells operating at temperatures above 80 °C. The molecular interactions in Diethylmethylammonium triflate ([Dema][TfO]) doped polybenzimidazole (PBI) blend membranes and the proton transport mechanism were investigated by means of TGA, IR and NMR. The mobility of the PIL ions is restricted to the PBI host polymer. The [Dema]+ cations and [TfO]− anions interact strongly via H bonds with the polar groups of the PBI chains. This will significantly confine the proton conductivity of the membrane to vehicular transport. The proton transport was investigated by comparing to an analogous liquid state model using the monomer benzimidazole (BIm) instead of the PBI polymer. During fuel cell operation, it is unavoidable that residual water is present in significant quantities. Resulting from 1H NMR and PFG self-diffusion measurements, proton transport in the liquid state model takes place via a cooperative mechanism involving all of the species NH[Dema]+/NHBIm/H2O depending on the water fraction. Thus, it is suggested that conductivity in the PIL–PBI membrane be mainly provided by the cooperative transport of the protons. This study is intended to broaden understanding of the structure and proton transport mechanism, as well as to give possible ways to optimize PIL electrolyte doped polymer blend membranes for intermediate operating temperatures.