Protic ionic liquids as next-generation proton exchange membrane materials: Current status & future perspectives

The development of protic ionic liquids (PILs) in proton exchange membrane has drawn significant interest. The intrinsic properties of high thermal stability and proton conductivity of PILs at high temperature (> 100 °C) enables fuel cell operation at anhydrous condition. This review discusses the roles of PILs in different types of polymeric membranes, including Nafion®, sulfonated poly(ether ether ketone) (SPEEK), poly(vinyl alcohol) (PVA), polybenzimidazole (PBI) and sulfonated polyimide (SPI). The plasticisation effect attributed to the presence of PILs increases the segmental mobility of the polymer chains and hence enable fast protons transport. The selection of anions and cations of PILs towards the improved proton conductivity of membrane was discussed. Novel synthesis strategies of next generation PILs-based solid polymer membrane such as polymerisation and chemical modification are highlighted to overcome the existing PIL leaching issue from the membrane. This PILs-based PEM mimics the structural of Nafion® by having ions attached to the polymeric chains in serving as the backbone of PEM and maintaining the mechanical integrity. Finally, the challenges beneath the development of PEM using PILs also been addressed along with future perspective to take the research into next phase.