An organic-based aqueous hybrid flow battery with high power and long cycle life: a tetrapyridophenazine/ferrocyanide system

Aqueous hybrid flow batteries (AHFBs) have emerged as promising systems for large-scale electrical energy storage. We report a stable conjugated organic compound, tetrapyridophenazine (TPPHZ), as an active anode material for AHFBs, which exhibits highly negative redox potential, fast electrochemical reaction kinetics and extreme insolubility in aqueous alkaline electrolytes. When operated in a practical hybrid flow cell against a positive electrolyte comprising ferrocyanide salt, the cell displayed an open-circuit voltage of 1.37 V, specific power of 28.4 W g(-1) at 100% state of charge (SOC) and energy efficiency of 87.2% at a current density of 10 A g(-1) (10.2 mA cm(-2)). The capacity retention rate is among the highest of AHFBs reported to date, at 99.9980% per cycle over 1200 consecutive cycles.