Tuning Polybenzimidazole Membrane by Immobilizing a Novel Ionic Liquid with Superior Oxygen Reduction Reaction Kinetics

Protic ionic liquid (PIL) is a promising nonaqueous electrolyte alternative to replacing phosphoric acid for fuel cells operating at temperatures above 100 °C. In this study, the physical and electrochemical properties of stoichiometric and nonstoichiometric PILs are investigated focusing on their acid/base ratio. The study involves a series of PILs, generically indicated as N,N-diethyl-3-sulfopropane-1-ammonium trifluoromethanesulfonate ([DESPA+][TfO–]), varying from an excess of the proton acceptor (N,N-diethyl-3-aminopropane-1-sulfonic acid) to an excess of the proton donor (trifluoromethanesulfonic acid, TfOH). Compared to a state-of-the-art electrolyte, i.e., concentrated phosphoric acid, the nonstoichiometric [DESPA+][TfO–] shows superior oxygen reduction reaction kinetics on the investigated Pt catalysts and oxygen permeation ability (DO2·cO2). [DESPA+][TfO–] with a base-to-acid molar ratio of 1:2 achieves a current density ∼10 times larger than that of concentrated phosphoric acid at 110 °C and 0.8 V. Membranes including polybenzimidazole as a host polymer and stoichiometric and nonstoichiometric [DESPA+][TfO–] as the conductive electrolyte exhibit promising properties in terms of thermal stability and conductivity. At 120 °C and 40% relative humidity, conductivities of 2 and 16 mS cm–1 are achieved by the membranes employing stoichiometric and excess acid [DESPA+][TfO–], respectively