Reaction Kinetics and Mass Transfer Synergistically Enhanced Electrodes for High-Performance Zinc-Bromine Flow Batteries

Zinc-bromine flow batteries (ZBFBs) hold great promise for grid-scale energy storage owing to their high theoretical energy density and cost-effectiveness. However, conventional ZBFBs suffer from inhomogeneous zinc deposition and sluggish Br₂/Br^- redox kinetics, resulting in a short cycle life and low power density. Herein, a multiscale porous electrode with abundant nitrogen-containing functional groups is developed by growing zeolitic imidazolate framework-8 in situ on graphite felts, followed by a facile carbonization process to simultaneously tackle both the challenges. Theoretical and experimental results reveal that nitrogen-containing functional groups exhibit a high adsorption energy toward zinc atoms, while the microstructures promote pore-level mass transport, thereby resulting in compact and uniform zinc deposition. In the meantime, the electrode boosts the Br₂/Br^- reaction kinetics due to its high catalytic activity and large surface area. As a result, the ZBFBs equipped with optimized electrodes at both negative and positive sides can operate at an ultrahigh current density of 250 mA cm^-2 while maintaining an energy efficiency of 68.0%, far surpassing that with pristine graphite felts (50.7%). Remarkably, the battery exhibits excellent cycling stability over 2000 cycles without obvious decay. This study provides a simple yet effective method for developing high-performance electrodes to tackle the critical challenges in ZBFBs, thereby promoting the commercialization of this promising energy storage technology.