Protic phosphonium-based ionic liquids for intermediate temperature polymer electrolyte membrane fuel cells

Intermediate-temperature polymer electrolyte membrane fuel cells (IT-PEMFCs) with operating temperatures around 100 °C represent a future technology to circumvent problems arising in commercially available PEMFCs by simplifying water and heat management. Elevated temperatures necessitate electrolytes enabling proton conduction in anhydrous states. As such, ionic liquids, particularly protic sulfoalkylphosphonium-based ionic liquids (PPILs), emerge as promising candidates, due to their protonic conductivity at elevated temperatures and anhydrous conditions. Sulfoalkylphosphonium-based cations manifest high proton-donor activity, conferring Oxygen Reduction Reaction (ORR), as well as apolarity, allowing sufficient oxygen solubility, ensuring high transport-limited current density. Sulfoalkylphosphonium-based cations exhibit lower charge density and higher apolarity. In this study, six protic sulfoalkylphosphonium-based ionic liquids (PPILs) were investigated, combining two anions, trifluoromethanesulfonate [TfO] and bis(trifluoromethane)sulfonimide [TFSI], with three cations, tributyl(3-sulfopropyl) phosphonium [tBP], tributyl(4-sulfobutyl) phosphonium [tBB], and trioctyl(3-sulfopropyl) phosphonium [tOP]. The conducted measurements revealed higher oxygen solubility than for the N-analogue (10^-5 mol cm^-3 vs. 10^-6 mol cm^-3 at 120 °C). [TfO]-based PPILs oxygen diffusivities are lower compared to the [TfO]-based N-analogue (10^-7 cm² s^-1 vs. 10^-5 cm² s^-1 at 120 °C), but [TFSI]-based PPILs exhibited similar values. Altogether, higher limiting current densities for ORR can be measured (10^-1 mA cm^-2 vs. 10^-2 mA cm^-2 at 80 °C). The thermal stability of [TfO]-based PPILs is sufficient for future IT-PEMFCs.