Low-cost material doubles energy density of redox flow battery without vanadium

Researchers at the Energy Storage Research Department at the Korea Institute of Energy Research have found an alternative for vanadium, an important component of redox flow batteries, an active area of research globally.

A study found that vanadium that is mined from the ground can be replaced by easily available molecules made from carbon and oxygen.

Renewable energy technology such as wind and solar can only be adopted on a large scale if a suitable energy storage solution is available. This storage solution can help overcome the intermittent nature of energy generation of this technology and supply power when needed, even if the sun is not shining or the wind isn’t blowing.

A reliable energy storage solution should be able to hold a charge for over eight hours and readily supply it when demand increases. Redox flow batteries are a much-preferred solution in this domain.

Why are redox flow batteries so good?

Redox flow batteries are electrical cells storing energy in the electrolyte instead of an electrode, as in lithium-ion batteries.

Redox flow batteries are better than lithium-ion batteries because they can have a flexible layout, drastically decreasing the risk of fire. The battery’s power output can be scaled up easily by increasing the size of the stack.

The lack of solid-solid phase changes ensures that the battery has a longer shelf life and delivers superior energy performance even after two decades of operation.

The only drawback of the redox flow battery is that it uses vanadium , which is not a rare Earth mineral but is already used in commercial processes and has limited reserves. To avoid a similar fate as lithium, researchers are looking for more abundant replacements for vanadium and found them in viologens.

How are viologens helpful?

Viologens are organic compounds made from abundantly available elements, such as carbon and oxygen that do not need to be mined. Previous research on using viologens as a replacement for vanadium faced the hurdle of low solubility in the electrolyte.

The lower solubility of viologens leads to lower battery energy density and instabilities in the charging and discharging processes. To overcome these hurdles, the researchers introduced functional groups in the viologens, which fit into these organic molecules like assembly blocks and improved their stability.

The research team used the functional groups sulfonate and ester, which are water-friendly, making their dispersion easier in the electrolyte.

The addition of functional groups also solved another issue with using viologens. Their molecular shape is similar to a sandwich, and at times, two layers of viologens combine, making them unsuitable for holding charge.

The researchers added alpha-methyl functional groups that prevent the combination of two viologens, ensuring that they are always available for storing energy. The end result of adding these functional groups was that the researchers achieved an energy density twice that of a flow battery made with vanadium.

After 200 charge-discharge cycles, the researchers found that their new battery demonstrated 99.4 percent coulombic efficiency and 92.4 percent capacity retention, both indicators of better performance and stability.

The research findings were published in the journal ACS Applied Materials and Interfaces .