Redox-targeted catalysis for vanadium redox-flow batteries

Vanadium redox-flow battery (VRB) as a promising electrochemical power source for large-scale energy storage, suffers from various polarization losses despite that it has been extensively studied in the past decades. Among these losses, the sluggish interfacial charge transfer of the vanadium species on the respective electrode renders large overpotentials giving rise to inevitable hydrogen and oxygen evolutions during the charging process. In this study, we report an unprecedented method based on the redox targeting concept to tackle the above issues. Prussian blue (PB) and a Prussian blue analogues (PBA) with identical redox potentials to VO2+/VO2+ and V2+/V3+ are grafted on cathode and anode, respectively. Upon operation, the reversible proton-coupled redox targeting reactions between PB and VO2+/VO2+ on cathode, PBA and V2+/V3+ on anode facilitate the interfacial charge transfer of the vanadium species and concomitantly inhibit the hydrogen and oxygen evolutions, which improves the selectivity of the redox reactions and considerably enhances the round-trip energy efficiency and cycling performance of VRB in a wide range of current densities. The above redox-assisted catalytic reactions were scrutinized and the mechanisms are unequivocally manifested with various electrochemical and spectroscopic measurements. We anticipate the surface immobilized redox catalysis approach demonstrated here would generically provide a paradigm for improving the sluggish kinetic processes in a variety of electrochemical devices.