Herein we show that protic ionic liquids (PILs) are promising electrolytes for fuel cells operating in the temperature range 100–120 °C. N,N-Diethyl-N-methyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DEMSPA]­[HSA]), N,N-diethyl-N-methyl-3-sulfopropan-1-ammonium triflate ([DEMSPA]­[TfO]), N,N-diethyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DESPA]­[HSA]), and N,N-diethyl-3-sulfopropan-1-ammonium triflate ([DESPA]­[TfO]) are investigated in this study with regard to their specific conductivity, thermal stability, viscosity, and electrochemical properties. The [DEMSPA]­[TfO] and [DESPA]­[TfO] electrolytes offer high limiting current densities for the oxygen reduction reaction (ORR) on platinum electrodes, that is, about 1 order of magnitude larger than 98% H3PO4. This is explained by the minor poisoning of the Pt catalyst and the significantly larger product of the oxygen self-diffusion coefficient and concentration in these two PILs.

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In this study, physicochemical and electrochemical characterization of mixtures of proton-conducting ionic liquids (PILs) is reported for potential future use as a novel electrolyte in polymer electrolyte membrane fuel cells operable at 100−120 °C. By blending two PILs, 2-sulfoethyl-methyl-ammonium triflate [2-Sema][TfO] and diethyl-methyl-ammonium triflate [Dema][TfO], exhibiting different cation acidities and different oxygen diffusivities/ solubilities, it was apparent that the superior physicochemical and electrochemical properties of both PILs can be favorably combined. Improved thermal stability of the blend, compared to neat [2-Sema][TfO], was observed. The viscosity of the mixtures decreased when increasing the fraction of [Dema][TfO], which led to increased proton conductivity and oxygen transmission coefficients, that is, D × c. In combination with the highly acidic [2-Sema] + cations, which served as strong proton donors for the oxygen reduction reaction (ORR), it led to an increase in the ORR limiting current density at cell potentials relevant for fuel cell operation.

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 abilitywith 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.