Sun and wind are not oriented to our electricity needs – regenerative energy must therefore be stored until it is consumed, for example in redox flow batteries. These are both cycle-stable – their capacity does not decrease noticeably even after thousands of cycles – and non-flammable, and their performance and capacity can be tailored to requirements. In addition, they do not require critical materials and their electrolytes can be fully recovered. But: Until now, they were simply too expensive for the mass market.
Cost effective, lightweight and compact
Researchers at Fraunhofer UMSICHT have now been able to find a sustainable solution to this problem: They have reinvented the manufacturing method of the central electrically conductive plastic so that it remains flexible and can be welded. This new process has significant implications for redox flow batteries. “The developed stack, the heart of every redox flow battery, is 40 percent cheaper in terms of material costs, and production costs have also been significantly reduced. The stack weighs 80 percent less than a conventional stack and is only about half the size,” summarizes Prof. Christian Doetsch. The stack is marketed by the spin-off Volterion. For this development, Christian Doetsch and Lukas Kopietz from Fraunhofer UMSICHT as well as Dr. Thorsten Seipp from Volterion GmbH & Co. KG receive the Joseph von Fraunhofer Prize. The jury justified its decision among other things with “the spin-off and the successful exit of Fraunhofer, which prototypically show the way of marketing new manufacturing technologies”.
Typically, the stacks consist of 160 stacked components held together with a variety of screws and solid metal plates and sealed with numerous gaskets. Some of these components are injection molded and are as brittle as pencil lead due to the high pressures and temperatures required for injection molding. To get around this, the research team used similar starting materials, i.e. graphites and carbon blacks, but approached the process in a different way: pellet-shaped plastic is cooled to as low as minus 80 degrees, ground into powder and mixed with 80 percent graphite by weight. The research team sends the resulting powder through a system of several rollers at different temperatures and speeds. Between the rollers, the powder is briefly melted at moderate temperatures and low pressures, kneaded, rolled into an “endless sheet” and finally rolled up. “The new material acquires thermoplastic properties in the process, so it is pliable and weldable, even though the plastic only accounts for 20 percent of the material,” explains Lukas Kopietz. This means that the stack does not require a single seal, and screws are also superfluous – the cells are simply welded together. Another advantage is that this method not only allows bipolar plates to be produced much faster and thus more cost-effectively, but there are also no longer any size limits. Bipolar plates with up to several square meters are possible without any problems.
Implemented in Volterion GmbH & Co KG up to the battery
The second decisive step, because it reduces costs, was the development of a continuous production process: the powder-to-roll process, in which the bipolar plates can be produced as a continuous roll. Very thin sheets can be produced in this way. Whereas the sheet thickness in injection moulding is limited to several millimetres for production reasons, it can be between 0.1 and 0.4 millimetres thin in the powder-to-roll process. This means that significantly less material is required, which in turn lowers the price and enables lighter, more compact stacks. “All of this provides completely new possibilities in design, which we have implemented at Volterion GmbH & Co. KG, right up to the entire battery,” says Thorsten Seipp. In the meantime, Volterion has already built and sold over a thousand stacks.