Homogeneous Complexation Strategy to Manage Bromine for High-Capacity Zinc-Bromine Flow Battery

Zinc-bromine flow batteries (ZBFBs) have received widespread attention as a transformative energy storage technology with a high theoretical energy density (430 Wh kg-1). However, its efficiency and stability have been long threatened as the positive active species of polybromide anions (Br2n+1-) are subject to severe crossover across the membrane at a high concentration. Herein, a novel highly hydrophilic complexing agent, N-methyl-N, N-bis(2-hydroxyethyl)-1-propanaminium bromide (PMDA), is developed to effectively manage bromine in a homogeneous posolyte, which realizes a low bromine crossover at a high operating concentration in ZBFBs. Both theoretical and experimental results suggest that the PMDA interacts with Br2n+1- and forms a larger-size complex PMDABr2n+1. When adding 0.40 M PMDA, the bromine stays homogeneous at a high concentration up to 1.20 M, and its permeability is remarkably decreased by 74%. For demonstration, the ZBFB achieves a operating capacity record of 57.2 Ah L-1 in a homogeneous bromine posolyte with a high Coulombic efficiency of ~90.0% and superior cycling stability (capacity retention rate of 100.0% per cycle). This work provides one innovative bromine management strategy to realize a high capacity and superior stability in ZBFBs.