High-Performance Cobalt–Tungsten All-Heteropolyacid Redox Flow Battery with a TiZrO₄-Decorated Advanced Nafion Composite Membrane

Designing an ion-selective membrane that can deliver high capacity, high cycling efficiency, and low self-discharge rate is imperative for developing a cobalt–tungsten all-heteropolyacid redox-flow battery (H6[CoW12O40] RFB). Herein, for the first-time, a potential composite membrane of Nafion decorated with single-phase TiZrO4 nanotubes (Nafion/TiZrO4NTs) is proposed for the H6[CoW12O40] RFB. An outstanding proton conductivity of 207.9 mS cm–1 and 6-times higher ion selectivity (14.55 × 106 S min cm–3) are achieved using the Nafion/TiZrO4NT composite membrane, as compared to the commercial Nafion-212 membrane (111.4 mS cm–1 and 2.39 × 106 S min cm–3, respectively). Subsequently, the H6[CoW12O40] RFB shows a higher discharge capacity (44.8 mA h), an impressive voltage efficiency (88.9%), and an energy efficiency (87.5%) with the use of a Nafion/TiZrO4NT composite membrane than those of the Nafion-212 membrane (30.2 mA h, 82.9, and 81.4%, respectively). Moreover, excellent battery cycling efficiency with different current densities and a remarkable improvement of open-circuit voltage of 190 mV are obtained using the Nafion/TiZrO4NT composite membrane, ensuring that the incorporation of a TiZrO4NT filler into the Nafion matrix reduces the battery cell resistance by improving the proton conductivity of the Nafion/TiZrO4NT composite membrane. The designed composite membrane will be a promising candidate for high-performance H6[CoW12O40] RFBs and other electrochemical energy-storage devices.