Heteropoly acid negolytes for high-power-density aqueous redox flow batteries at low temperatures

Operating aqueous redox flow batteries (ARFBs) at low temperatures is prohibited by limited solubility of redox-active materials, freezing electrolytes and sluggish reaction kinetics. Here we report a multi-electron heteropoly acid (H6P2W18O62, HPOM) negolyte that enables high-performance ARFBs at low temperatures. The proton (H+) in HPOM warrants a much higher solubility of polyoxometalates (POMs) (0.74 M at 25 °C and 0.5 M at −20 °C) compared with other cations (Li+/Na+/K+) owing to the strong solvation shell of H+ preventing precipitation. The HPOM also exhibits an exceptionally low freezing point and high conductivity owing to its high solubility and Grotthuss proton-conduction mechanism. These merits warrant HPOM as an ideal POM candidate for high-power-density low-temperature ARFB applications. Using a 0.5 M HPOM electrolyte, the ARFBs demonstrate power density (282.4 mW cm−2) and stability (79.6 Ah l−1negolyte at 160 mA cm−2 over 1,200 h without decay) at −20 °C, showing promising application potential under cold weather conditions.