Redox-flow batteries: Higher performance at lower costs revealed

October 26, 2015 // By Paul Buckley
A team of researchers at the Friedrich Schiller University Jena, in the Center for Energy and Environmental Chemistry and the JenaBatteries GmbH claim to have taken a step towards developing a redox-flow battery which is simple to handle, safe and economical.

“What's new and innovative about our battery is that it can be produced at much less cost, while nearly reaching the capacity of traditional metal and acid containing systems," explained Dr. Martin Hager. The scientists have presented their battery technology in the current edition of the renowned scientific journal 'Nature'.

In contrast to conventional batteries, the electrodes of a redox-flow battery are not made of solid materials (e.g., metals or metal salts) but they come in a dissolved form: The electrolyte solutions are stored in two tanks, which form the positive and negative terminal of the battery. With the help of pumps the polymer solutions are transferred to an electrochemical cell, in which the polymers are electrochemically reduced or oxidized, thereby charging or discharging the battery. To prevent the electrolytes from intermixing, the cell is divided into two compartments by a membrane.

“In these systems the amount of energy stored as well as the power rating can be individually adjusted. Moreover, hardly any self-discharge occurs," said Martin Hager.

Traditional redox-flow systems mostly use the heavy metal vanadium, dissolved in sulphuric acid as electrolyte. “This is not only extremely expensive, but the solution is highly corrosive, so that a specific membrane has to be used and the life-span of the battery is limited,” pointed out Hager. In the redox-flow battery of the Jena scientists, on the other hand, novel synthetic materials are used. In the core structure they resemble Plexiglas and Styrofoam (polystyrene), but functional groups have been added enabling the material to accept or donate electrons. No aggressive acids are necessary anymore; the polymers rather 'swim' in an aqueous solution.

“We are able to use a simple and low-cost cellulose membrane and avoid poisonous and expensive materials,” explained Tobias Janoschka, first author of the new study. "This polymer-based redox-flow battery is ideally suited as energy storage for large wind farms and photovoltaic power stations,” said Prof. Dr. Ulrich S. Schubert. He is chair for Organic and Macromolecular Chemistry at the FSU Jena and director of the CEEC Jena, an energy research center run in collaboration with the Fraunhofer Institute for Ceramic Technologies and Systems Hermsdorf/Dresden (IKTS).