Reversible solid bromine complexation into Ti3C2Tx MXene carriers: a highly active electrode for bromine-based flow batteries with ultralow self-discharge

Abstract

Bromine-based flow batteries (Br-FBs) are appealing for stationary energy storage because of their high energy density and low cost. However, the wider application of Br-FBs is hindered by the sluggish reaction kinetics of the Br₂/Br⁻ redox couple and serious bromine crossover. Adding bromine complexing agents (BCAs) into electrolytes can inhibit bromine crossover effectively, but it generally deteriorates the reaction kinetics. Here, we use hexadecyl trimethyl ammonium bromide (CTAB) to intercalate Ti₃C₂T_x MXene as electrodes for Br-FBs. In this design, CTAB acts as the BCA to form robust solid bromine complexes, which are accompanied by highly active Ti₃C₂T_x MXene carriers. Soluble bromine species diffusion is perfectly suppressed based on such a strong and reversible solid complexation effect, while the adverse effect of solid complexes on reaction kinetics is well overcome. Thus, the assembled zinc-bromine flow battery delivered a remarkable improvement in suppressing self-discharge, achieving an unprecedently high capacity retention rate of 82.93% after standing for 24 h at 80 mA cm⁻². At a high current density of 180 mA cm⁻², this battery exhibited the highest voltage efficiency of 66.76% and energy efficiency of 66.06% ever reported, and also showed an outstanding long-term durability for 580 cycles with a high coulombic efficiency of 99.30%. This work provides a new strategy for designing electrodes with ultralow self-discharge, high power density and long cycle life for Br-FBs.