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Reducing the cost of redox flow batteries

Redox flow batteries (RFBs) are a promising alternative to lithium-ion batteries for storing large quantities of renewable energy (see Chem. Eng., September 2016, pp. 14–20), but they have always been too expensive for the mass market. Now, researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology (Umsicht; Oberhausen, Germany; www.umsicht.fraunhofer.de) have completely redesigned the heart of a RFB — the stack — resulting in a “massive” reduction in material usage and costs.
“The stack that has been developed is 40% more cost-effective in terms of material costs,” says professor Christian Doetsch. “Production costs have also been significantly reduced. The stack weighs 80% less than a conventional stack and is only about half the size,” he adds.
Stacks usually comprise 160 stacked components that are held together by a large number of screws and solid metal plates and sealed with numerous gaskets. Some of these components are injection molded, meaning they are brittle. To avoid this problem, the team of researchers used similar base materials — graphite and carbon black — but approached the process differently: plastic pellets are cooled to temperatures as low as –80°C, then ground into powder and mixed with 80 wt.% graphite. The powder is sent through a system of several rollers moving at different speeds and heated to different temperatures. As a result, the powder is briefly melted between the rollers at moderate temperatures and low pressures, and then kneaded and rolled up. The process makes it possible to manufacture bipolar plates of up to several square meters in size. This powder-to-roll process is the key to reducing production costs, because very thin (0.1–0.4 mm) plates can be produced. Because less material is used, the costs, weight and footprint of the device are reduced.
The stack is being marketed by the spin-off Volterion GmbH (Dortmund, Germany; www.volterion.de). The company has already built and sold more than a thousand stacks. Christian Doetsch and Lukas Kopietz from Fraunhofer Umsicht and Thorsten Seipp from Volterion have been awarded the Joseph von Fraunhofer Prize for this development.

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