Redox-flow battery based on organic polymers and water

Researchers from JenaBatteries GmbH and the Friedrich Schiller University Jena have developed a new redox-flow battery based on organic polymers and water.

"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," said Dr Martin Hager.

In contrast to conventional batteries, the electrodes of a redox-flow battery are not made of solid materials (eg, 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 oxidised, 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 Hager.

Traditional redox-flow systems mostly use the heavy metal vanadium, dissolved in sulfuric 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," noted Hager. In the redox-flow battery of the Jena scientists, on the other hand, novel synthetic materials are used: in their 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. "Thus we are able to use a simple and low-cost cellulose membrane and avoid poisonous and expensive materials," Tobias Janoschka, first author of the new study said.

"This polymer-based redox-flow battery is ideally suited as energy storage for large wind farms and photovoltaic power stations," said Professor Dr Ulrich Schubert. He is chair for Organic and Macromolecular Chemistry at the FSU Jena and director of the CEEC Jena, an energy research centre run in collaboration with the Fraunhofer Institute for Ceramic Technologies and Systems Hermsdorf/Dresden (IKTS).

In first tests the redox-flow battery could withstand up to 10,000 charging cycles without losing a crucial amount of capacity. The energy density of the system presented in the study is 10 watt-hours per litre. Yet, the scientists are already working on larger, more efficient systems. In addition to the fundamental research at the university, the chemists develop their system, within the framework of the start-up company JenaBatteries GmbH, towards marketable products.