Advancing Bromine Flow Batteries

Researchers Develop New System for High-energy-density, Long-life Batteries

New research from the Dalian Institute of Chemical Physics suggests a innovative approach to mitigate corrosion in zinc-bromine flow batteries, potentially enhancing their energy density and cycle life while lowering costs. By introducing a bromine scavenger into the electrolyte, scientists have developed a system that reduces harmful bromine concentrations, enabling stable operation for grid-scale energy storage.

Traditional Bromine-Based Flow Batteries

Flow batteries hold promise for storing renewable energy due to their scalability and safety. Zinc-bromine variants, in particular, stand out for their high energy densities and use of inexpensive electrolytes. However, the generation of corrosive and volatile elemental bromine (Br₂) during charging poses significant hurdles. This bromine can degrade battery components like electrodes, membranes, and even auxiliary equipment such as pipes and tanks, leading to shortened service life and environmental concerns.

Traditional solutions involve complexing agents, such as quaternary ammonium salts, which capture bromine but often result in phase separation, uneven electrolyte distribution, and persistently high bromine levels—up to hundreds of millimoles per liter. These issues compromise kinetics, increase costs, and fail to fully eliminate corrosion risks. Results from prior studies indicate that without addressing these limitations, zinc-bromine batteries struggle to achieve long-term stability for widespread adoption in renewable energy grids.

A Novel Multi-Electron Reaction

In a study published in Nature Energy, a team led by Xianfeng Li from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, introduced sodium sulfamate (SANa) as a bromine scavenger in the catholyte. This compound reacts rapidly with bromine produced during charging, forming a milder product called N-bromo sodium sulfamate (Br-SANa). The reaction reduces free bromine concentration to an ultra-low level of approximately 7 mM, far below typical values in conventional systems.

Unlike the single-electron transfer in traditional setups—from bromide ions (Br⁻) to Br₂—this approach enables a two-electron transfer from Br⁻ to Br-SANa. During discharge, Br-SANa dissociates back into bromine via chemical equilibrium, maintaining the battery's functionality. The process, akin to a disproportionation reaction, relies on the formation of a covalent Br–N bond, which eliminates free bromine species and minimizes corrosivity.

Researchers screened various amino compounds with electron-withdrawing groups to optimize the scavenger. SANa emerged as the most effective, showing the highest oxidation peak current in cyclic voltammetry tests, indicating a rapid bromination rate. The electrolyte's pH also plays a crucial role; in near-neutral conditions, the reaction proceeds spontaneously, avoiding bromine accumulation seen in acidic environments.

Key Findings from Battery Testing

The team assembled a zinc-bromine flow battery using this electrolyte paired with a zinc anode and a low-cost sulfonated polyetheretherketone (SPEEK) membrane, which is typically not corrosion-resistant. Results indicated superior performance compared to traditional designs. The energy density reached 152 Wh L⁻¹—versus 90 Wh L⁻¹ in conventional systems—at a 2 M bromide concentration, with potential to hit 200 Wh L⁻¹ at higher concentrations.

Cycling tests demonstrated encouraging durability. The single-cell battery maintained an energy efficiency of 82% at a current density of 40 mA cm⁻², operating stably for over 600 cycles (more than 2,000 hours). In contrast, traditional zinc-bromine batteries with SPEEK membranes failed after about 30 cycles due to corrosion. No degradation was observed in key components like current collectors, electrodes, or membranes, even after extended cycling.

Scaling up, the researchers built a 5-kW stack delivering 6.6 kWh output. It achieved an energy efficiency of 78% and ran for over 700 cycles (approximately 1,400 hours) without corrosion issues. These outcomes highlight the system's potential for practical, large-scale applications, where reduced material requirements could lower overall costs.

Implications for Renewable Energy Storage

This innovation offers a pathway to more reliable bromine-based flow batteries, addressing barriers to their commercialization. By enabling the use of affordable, non-fluorinated membranes and eliminating the need for corrosion-resistant auxiliaries, the approach could make these batteries more economically viable for grid-scale storage—essential for integrating intermittent renewables like solar and wind.

"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." - Xianfeng Li