This tiny chemistry change makes flow batteries last far longer
Bromine-based flow batteries store energy using a chemical reaction between bromide ions and elemental bromine. This chemistry is attractive
Bromine-based flow batteries store energy using a chemical reaction between bromide ions and elemental bromine. This chemistry is attractive because bromine is widely available, has a high electrochemical potential, and dissolves well in liquid electrolytes. The downside appears during charging, when large amounts of bromine are produced. This reactive material can attack battery components, reduce how many charge cycles the battery can handle, and raise overall system costs. Additives known as bromine complexing agents can help limit corrosion, but they often cause the electrolyte to separate into different phases, which disrupts uniformity and makes the system harder to manage.
In a study published in Nature Energy, researchers led by Prof. Xianfeng Li from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) reported a new approach to bromine-based battery chemistry. The team designed a bromine-related reaction that transfers two electrons instead of one and successfully applied it to a zinc-bromine flow battery. Their results show both a working proof of concept and successful scale-up toward a long-life battery system.
Capturing Bromine to Boost Performance
The researchers achieved this by adding amine compounds to the electrolyte, where they act as bromine scavengers. During battery operation, the bromine (Br2) formed through electrochemical reactions is converted into brominated amine compounds. This process lowers the amount of free Br2 in the electrolyte to an ultra-low level of about 7 mM. Traditional bromine chemistry relies on a single-electron transfer from bromide ions to Br2. In contrast, the new process enables a two-electron transfer from bromide ions to the brominated amine compounds, which increases energy density. At the same time, keeping Br2 levels extremely low greatly reduces corrosive effects, helping extend battery lifespan.
Long-Term Stability and Lower Costs at Scale
The team then tested this chemistry in zinc-bromine flow batteries under practical conditions. Because the electrolyte contains very little free Br22, the battery can operate reliably using a standard non-fluorinated ion exchange membrane (SPEEK), which helps bring down costs. In a 5 kW scale-up test, the battery ran stably for more than 700 cycles at a current density of 40 mA cm-2 and reached an energy efficiency above 78%. With the Br2 concentration kept so low, no corrosion was detected in critical components — including current collectors, electrodes, and membranes — either before or after cycling.
Implications for Future Energy Storage
“Our study provides a novel approach to the design of long-life bromine-based flow batteries and lays the foundation for the further application and promotion of zinc-bromine flow batteries,” said Prof. Li.
