Researchers at the University of Wisconsin-Madison have invented a water-soluble chemical additive — a soft-hard zwitterionic trapper that improves the performance of bromide aqueous flow batteries.

The solution is an additive called a complexing agent. The researchers used molecular design to engineer over 500 candidate organic molecules they call “soft-hard zwitterionic trappers.” They synthesized and tested 13 of these representative molecules as potential additives for the bromide batteries.

The resulting multi-functional additives solved the flow battery’s main problems. It encapsulated the bromide ions while allowing them to remain water-soluble, and since the resulting complex is now larger, they cannot pass through the membrane. The ions are also “phase-stable,” preventing them from separating from the water-electrolyte or creating toxic bromine gas.

Significantly, the additives dramatically improve the flow battery’s performance, increasing the efficiency and longevity of the chemical system.

The researchers said devices with the additive functioned without decay for almost two months compared to ones without it, which typically fail within a day. This was vital for green energy storage, where the storage period extends from 10 to 20 years.

The study was published in the journal Nature.

Currently, giant tractor-trailer-sized lithium-ion battery packs store energy for the grid, but technical limitations exist. Lithium batteries have safety concerns due to the potential for fires and explosions and a complicated international supply chain.

Aqueous flow batteries could make grid-scale storage safer and cheaper. In these batteries, positive and negative liquid electrolytes circulate over electrodes that are separated by a membrane. Since the batteries use ions dissolved in a liquid (water), they can be scalable, sustainable, and safe. Despite being a promising solution, bromide-based products have many electrochemical problems.

The most commercially mature flow batteries are based on vanadium ions, which, like lithium, are expensive and hard to source. However, another version of these flow batteries relies on bromide, a cheap, widely available ion that performs similarly to vanadium on paper.

However, tiny bromide ions cause many problems in flow batteries. They can pass through the membrane that separates the electrodes and reduce the battery’s efficiency. Sometimes, the ions precipitate out of the electrolyte and form a messy oil that “sinks” to the bottom of the solution. Occasionally, the ions also form toxic bromine gas.

Researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory have repurposed nitrogenous triphosphate or nitrilotrimethyl phosphonic acid , which is commonly used in water treatment facilities, for large-scale battery energy storage.

Material scientists at the Department of Applied Physics and Applied Mathematics, Columbia University, combined potassium and sodium with sulfur to create a high-energy, low-cost solution for storing energy for longer durations.

Researchers at Stanford University have made progress on an emerging technology that uses liquid organic hydrogen carriers to create a ‘liquid battery’ for storing renewable energy from wind and solar power.