Self-assembled polyoxometalate enables efficient and stable proton conduction in high-temperature polymer electrolyte membranes.

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are attractive for their high fuel-impurity tolerance and simplified water/thermal management, but their performance is often limited by phosphoric acid (PA) loss from the membrane, especially below 100 °C, where condensed water accelerates PA leaching. Here we report a self-assembled polyoxometalate (POM) strategy to immobilize PA and promote stable PA-mediated proton conduction. A polyethylene glycol-grafted POM (GSiW11) was designed and co-incorporated with PA into a poly(terphenyl piperidinium) (PTP) matrix to prepare hybrid high-temperature proton exchange membranes (HT-PEMs). GSiW11 plays two important roles. At the molecular level, it forms supramolecular complexes with PA, lowering the proton dissociation energy of PA and facilitating proton transport. At the nanoscale, it induces microphase separation of PTP to generate continuous ionic domains that confine PA and construct stable proton-conducting networks. The resulting hybrid membrane exhibits a high proton conductivity of 91 mS cm-1 at 180 °C with a relatively low PA uptake of 125%. The corresponding H2/O2 fuel cell delivers a peak power density of 1130 mW cm-2 at 160 °C and operates stably over a broad temperature range of 80-160 °C. This work demonstrates the potential of self-assembled POMs as functional additives for designing HT-PEMs with efficient and stable proton-conduction performance.