Bromine-based flow batteries operate through the redox reaction between bromide ions and elemental bromine, having advantages such as abundant resources, high redox potential and good solubility. However, the substantial bromine generated during the charging process can corrode battery components, shorten cycle life, and increase system costs.

Traditional bromine complexing agents can alleviate corrosion to some extent, but they often induce phase separation, compromising electrolyte homogeneity and adding complexity to the system.

In a study published in Nature Energy, a research team led by Prof. LI Xianfeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences developed a novel bromine-based two-electron transfer reaction system, and demonstrated both the proof of concept and the system-scale up of a long-life zinc-bromine flow battery by using it.

Researchers proposed an innovative bromine two-electron transfer reaction by introducing amine compounds as bromine scavengers into the electrolyte. They found that the bromine generated during the electrochemical reaction could be converted into brominated amine compounds, reducing the concentration of bromine in the solution to an ultra-low level of approximately 7 mM. 

Unlike traditional single-electron transfer reaction from bromide ions to bromine, this new reaction enabled a two-electron transfer from bromide ions to brominated amine compounds, increasing the battery's energy density. Meanwhile, the ultra-low bromine concentration substantially reduced electrolyte corrosivity, extending the battery life.

Researchers applied this reaction to zinc-bromine flow batteries. Due to the extremely low bromine concentration in the electrolyte, the long-term stable operation was achieved by assembling a single battery using a conventional non-fluorinated ion exchange membrane (SPEEK), thereby reducing battery costs. 

In a 5 kW system scale-up test, the battery operated stably for over 700 cycles under a current density of 40 mA cm^-2, achieving an energy efficiency of over 78%. With the drastically reduced bromine concentration, no corrosion was observed in key materials including current collectors, electrodes, and membranes either before or after cycling.

"Our study provides a novel approach to design long-life bromine-based flow batteries, and lays a foundation for further application and promotion of zinc-bromine flow batteries," said Prof. LI, one corresponding author of the study.