Development of high temperature polymer electrolyte membranes is essential for advanced energy conversion and storage technologies. Herein, an imidazolium type ionic liquid (BPMIm) with a long side-chain of 5-bromopentyl is synthesized and employed as the quaternization reagent for ether-free poly(p-terphenyl-co-N-methyl-piperidine) (PTP). Grafting the flexible long side-chain imidazolium group into the ether-free polymer backbone not only improves the polymer solubility in organic solvents but also provides acid-base interaction sites for the following phosphoric acid doping that supports proton conductivity at above 100 °C. Compared with pure and iodomethane quaternized PTP membranes (i.e. PTP and PTP-Me), the prepared imidazolium ionic liquid grafted PTP membrane (i.e. PTP-PMIm) exhibits an enhanced phosphoric acid uptake and hence a superior anhydrous proton conductivity of 138 mS cm−1 at 180 °C. The technical feasibility of the PTP-PMIm membrane is demonstrated in H2-air fuel cells at temperatures from 160 °C–200 °C, which reaches a high peak power density of around 456 mW cm−2 at 200 °C at ambient pressure.