Promoted efficiency of zinc bromine flow batteries with catalytic Co-N-C composite cathode

Zinc-bromine flow batteries (ZBFBs) are regarded as one of the most appealing technologies for stationary energy storage due to their excellent safety, high energy density, and low cost. Nevertheless, their power efficiency and cycling life are still limited by the sluggish reaction kinetics of the Br2/Br− redox couple and the shuttle effect of bromine species. Herein, we report a catalytic cathode for ZBFBs enabled by graphite felt electrode functionalized by cobalt and nitrogen-containing carbon nanocomposites (Co-N/C@GF). The nitrogen-rich groups and metallic cobalt nanocrystal inculsions in the Co-N/C@GF promote the absorption of Br2 and provide abundant catalytic active sites for the Br2/Br− redox reaction. Consequently, the electrochemical performance of ZBFB has been significantly enhanced. The ZBFB using Co-N/C@GF exhibits a high voltage efficiency of 86.06% and excellent energy efficiency of 84.7% at a current density of 80 mA cm−2. It also demonstrates suppressed self-discharge characteristic (keep high open circuit voltage within 84 h) and long cycle life of more than 400 cycles. Furthermore, the ZBFB can also operate stably at ultra-high current density of 180 mA cm−2, and the voltage efficiency reaches 62.8%. Therefore, this work provides an effective approach to developing high-performance ZBFBs. Cobalt and nitrogen-containing carbon nanostrutures are successfully grown on graphite fiber electrode (Co-N/C@GF), which catalyse the Br2/Br− redox reaction, suppresses the shuttling effect of polybromine ions and enhance the electrochemical performance of Zinc-bromine flow battery. [Display omitted] •An ultra-hydrophilic composite cathode with catalytic Co-N/C doping is designed.•Co-N/C@GF greatly improves the redox kinetics and reversibility of Br2/Br−.•The battery achieves excellent energy efficiency of 84.7% at 80 mA cm−2.