New flow battery that stores energy in a simple organic compound

The intermittent supply of green electricity requires large-scale storage to maintain the stability of our electricity networks.

Since normal batteries don’t scale very well, the idea of ​​using flow batteries, which store electricity in a fluid, is appealing. However, these batteries contain rare metals and are expensive.

Scientists from the University of Groningen (Netherlands) have designed a electrolyte for flow batteries that could solve both problems.

Flow batteries.

Flow batteries are not much different from the ones we usually use. The big difference is that the energy is stored in two separate fluids with dissolved chemicals to store the charge.

Electricity is stored (then released) by pumping these fluids through an electrochemical cell which contains a membrane through which ions can be exchanged.

The energy content of such a battery is scalable simply by using larger storage tanks for fluids.

Very high price.

China recently installed flow batteries to reduce variability in green electricity generation.

Large-scale storage capacity is needed when intermittent sources, such as solar and wind, become more important in the electricity mix, as the grid could be destabilized. The type of battery used by the Chinese was designed in the 1980s and is based on a solution containing vanadium.

Edwin Otten, Associate Professor of Molecular Inorganic Chemistry at the University of Groningen.

This metal is only mined in a few places on Earth.

This means that supply cannot always be guaranteed and is quite expensive.

Edwin Oten

In addition, a special membrane is required to separate the two fluids, which also increases costs. Otten’s research group, together with colleagues from the University of Eindhoven in the Netherlands and the Technical University of Denmark, therefore set out to design a new type of material for flow batteries.

Blatter Radicals

We wanted a symmetrical battery in which both reservoirs held the same fluid. Also, we wanted it to be based on an organic molecule and not on a metal.

Edwin Oten

The two sides of the circulation battery generally contain fluids of different composition. Symmetrical batteries were designed by joining molecules used on both sides and filling both reservoirs with the resulting hybrid molecule.

The disadvantage of this approach is that only part of the molecule is used on each side. And, during use, reactive radicals appear which degrade over time. This makes stability an issue.

Edwin Oten

New focus.

Otten and his team used a different approach. They looked for a single stable molecule capable of accepting or donating electrons and therefore of being used on both sides of the battery.

The most promising compound was a Blatter radical, a dipolar organic compound that can accept or donate an electron in a redox reaction.

The molecule we selected was also intrinsically stable.

Edwin Oten

They tested the compound in a small electrochemical cell. It worked well and was stable for 275 charge and discharge cycles.

We have to take it to thousands of cycles; however, our experiences are proof of concept. It is possible to produce a battery with symmetrical flow which has good stability.

Edwin Oten

Blatter’s organic radical is relatively easy to manufacture, and although it is not currently produced in the industry, it should be possible to scale it.

Imbalance.

Another benefit of our symmetrical design is that it’s not a big deal if some of our compound gets through the membrane during use. This could result in a slightly higher volume in one of the reservoirs, but any imbalance is easily reset by simply reversing the polarity.

Edwin Oten

During their testing, they have shown that it works well as expected. Other experimental symmetrical battery designs were not stable enough to achieve the number of cycles needed to prove this.

The next step is to create a water-soluble version of the Blatter radicals. Most flow cells are designed for water-based fluids because water is cheap and non-flammable.

The doctoral students in my group are already working on it.

Edwin Oten

An additional step is to increase the stability and solubility of the Blatter radical and test it on a larger scale.

The crucial test is whether our compounds will be stable enough for commercial applications.

Edwin Oten

More information: acs.org (English text).

Via www.rug.nl

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