A long-life hybrid zinc flow battery achieved by dual redox couples at cathode

Highlights: • Hybrid zinc flow battery was demonstrated using dual cathode redox couples. • Zinc accumulation, the cycle life limited causes under practical operation, was completely solved. • Hybrid zinc flow battery has an ultra-long cycle life over 1100 h with ignored degradation. -- Abstract: Zinc nickel flow battery is one of the most promising energy storage technologies for intermittently renewable solar and wind power. However, unpaired coulombic efficiency of nickel hydroxide cathode and zinc anode causes zinc accumulation in practical operation, which shortens the cycle life and impedes the commercialization of the battery. Here, we induce an additional O₂/OH⁻ redox couple integrated with NiOOH/Ni(OH)₂ as cathode energy storage materials. Benefit from the new reaction mechanism of the hybrid cathode, deposited zinc upon charge can be fully consumed during discharge, which completely avoids zinc accumulation issue. The new designed battery vigorously operates for more than 1100 h with negligible performance degradation, while the energy efficiency of pristine zinc-nickel flow battery dramatically reduces after 440 h. More importantly, the specific capacity of the cathode synchronously increases by 2.5 times and thereby the energy density. This system dexterously integrates dual redox couples into one hybrid unit at cathode complementing advantages of one another, demonstrating a broader method to solve the problem of unpaired coulombic efficiency in zinc based flow batteries.