Boosting the kinetics of bromine cathode in Zn-Br flow battery by enhancing the electrode adsorption of the droplet of bromine sequestration agent/polybromides complex

Zinc-bromine (Zn-Br) flow battery is a promising option for large scale energy storage due to its scalability and cost-effectiveness. However, the sluggish reaction kinetics of Br-2/Br- have hindered further advances. In this study, we report that a nitrogen-doped carbon felt electrode derived from a metal-organic framework can facilitate the adsorption of N-methyl N-ethyl pyrrolidinium polybromide (MEP-pBr) complex droplet dispersed in aqueous electrolyte. Density functional theory (DFT) calculation and Zeta potential measurement suggest that the adsorption is primarily due to the positively charged surface induced by graphitic nitrogen defects. Electrochemical analysis indicates that the enhanced adsorption of MEP-pBr droplet significantly boosts the redox kinetics and decreases the crossover of bromine-bearing species. Consequently, the cell performance improves, achieving an energy efficiency of over 79 % during 900 cycles at a current density of 100 mA cm(-2). Our work underscores the importance of providing access of MEP-pBr droplet to the electrode surface in augmenting the energy efficiency of Zn-Br flow battery.