Aqueous redox flow batteries are a good solution for grid-scale energy storage. This is because their ions dissolve in water, and their store of energy is hence scalable, sustainable and safe. The design that relies on cheap, widely available bromide would fit the bill, if we could resolve several teething problems. Scientists at University of Wisconsin-Madison have developed an adaption to bromide-based aqueous electrolyte, that resolves these concerns.
Why Is This Adaption to Bromide-Based Aqueous Flow Necessary?
Most commercially-available flow batteries use vanadium ions. These are difficult to obtain and hence expensive, which makes bromide ions such an attractive alternative. In practice, though, the latter option has been impractical until now for several reasons, including:
- Bromide ions can pass through the flow battery’s membrane separating the electrodes. This reduces the system’s efficiency.
- Moreover, the bromide ions may on occasion precipitate out of the electrolyte, and sink to the bottom as an ‘oily mess’.
These disadvantages – which also include occasionally forming toxic bromine gas – have effectively hindered commercial adoption.
The team at University of Wisconsin-Madison theorized that a ‘complexing agent’ might help them resolve these issues. These agents are able to form stable complexes with metal ions, increase their solubility, and prevent them precipitating out of a solution.
The Complexing Agent That Solved These Long-Standing Problems
The team of young scientists, and their assistant professor, may have sensed they were close to discovering their adaption to bromide-based aqueous electrolyte. But what would their optimum solution be, among the 500 candidates on their possibility list? They synthesized and tested the 13 most likely candidate molecules.
The solution they chose among these encapsulated the bromide ions, while allowing them to remain water-soluble. The resulting complex was now too large to pass through the membrane. The ions were also “phase-stable,” which meant they did not separate out of the water electrolyte, or create toxic bromine gas.
Their adapted bromide flow battery displayed ‘dramatically improved performance’, and greater efficiency and longevity too. We find it remarkable how one simple change could spin out into so many improvements. Our image shows the separated-out bromide ions on the left, and the improvement on the right.
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