Proton-conducting polymer electrolyte membranes based on sulfonated PEEK and blends of protic ionic liquids with different acidities

Protic ionic liquids (PILs) immobilised in an ionogene host polymer represent an intriguing alternative to the state-of-the-art commercial fluoropolymer-based membranes as proton conducting electrolytes for future intermediate-temperature fuel cells (T > 100 °C, IT-PEMFCs). In this study, two PILs, N, N-diethyl-3-sulfopropane-1-ammonium trifluoromethanesulfonate [DESPA][TfO] and diethyl-methylammonium trifluoromethanesulfonate [DEMA][TfO], were investigated for their suitability as proton conducting species in a sulfonated poly(ether-ether-ketone) membrane (sPEEK). Both PIL membrane casts exhibit good thermal stability until 200 °C and a high ionic conductivity of up to 5.28 mS.cm^-1(120 °C, 40%RH). The polarisation curves showed a maximum power density of 3.5 mW.cm^-2 for [DESPA][TfO] membrane cast and 6.5 mW.cm^-2 for [DEMA][TfO] membrane cast, proving the suitability of the PIL membrane cast as a polymer electrolyte membrane (PEM) for the IT-PEMFCs.