Conductivity-enhanced swelling-induced triphenylphosphine-functionalized adamantane-containing poly(aryl ether ketone) membranes for vanadium redox flow batteries

To develop highly conductive membranes for VRFB, triphenyl quaternary phosphonium cationic groups were introduced into unique adamantane-containing poly(aryl ether ketone) to prepare TAPEK membranes. The selective swelling-induced microphase separation strategy was also adopted to strengthen membrane conductivity. The quaternary phosphonium groups give the membrane the Donnan repulsion effect, so the TAPEK membrane exhibits low area resistance and a good barrier to cross-mixing vanadium ions. The TAPEK-100 membrane exhibited a low area resistance of 0.12 Ω.cm-2, lower than the current state-of-the-art Nafion212 membrane (0.14 Ω.cm-2). TAPEK membranes were used for VRFB for the first time, and the TAPEK-100 exhibited excellent energy efficiency in VRFB (EE=91.0% at 80 mAcm-2, EE=85.1% at 200 mAcm-2, EE=80.36% at 280 mAcm-2, and EE=79.32% at 300 mAcm-2). TAPEK membrane showed insufficient stability in in-situ and ex-situ stability tests. The battery efficiency of VRFB with TAPEK-90 membrane dropped significantly after more than 823 cycles in the cycling test. Given the high conductivity of TAPEK membranes, more work related to enhancing the stability of quaternary phosphonium groups-containing membranes needs further study.