Finally, renewable energies seem suitable for everyday use. But critics still fear a “Dunkelflaute,” – dark days without either sunshine or wind, when too little green electricity, or none at all, can be produced. If coal-burning and nuclear power plants are to be shut down once and for all, energy security must be ensured for days like these as well. This means the true breakthrough in green power depends to a crucial extent on the possibility of keeping sufficient electricity available every single day of the year – and thus on the performance capacity of energy storage technologies.

Christina Roth, a professor of physical chemistry at Freie Universität Berlin, is studying one of the most promising solutions: the redox flow battery. This is a special form of liquid battery that should be in use at large wind and solar farms soon. “Batteries are one of the hottest topics in chemistry and materials science right now,” Roth says. “But most people are thinking mainly about lithium-ion batteries.”

These commonplace batteries, which are installed in mobile phones and computers and are also expected to help bring about a breakthrough in electric mobility, are small. “Redox flow batteries, by contrast, are designed for stationary storage of large volumes of electricity,” Roth explains. “They aren’t suitable for electric cars – but they definitely are for large-scale industrial applications.”

Lithium-ion batteries, which are known primarily from mobile phones, laptops, and tablets, ensure that devices can be operated for long periods without recharging. These kinds of batteries are small, lightweight, and solid on the inside, making them advantageous for portable applications. Redox flow batteries, on the other hand, are large (it is not uncommon to find them in the size of a shipping container), and they store energy by chemical means, in liquid electrolyte solutions.

A vanadium redox flow battery (VRFB) like that used by Roth and her team uses two electrolyte solutions that are physically separated by a membrane. The only active element these solutions contain is vanadium. They use its four stages of oxidation for storage purposes.

The solution on one side of the membrane is oxidized during charging when it flows through the porous electrode, while a reduction reaction takes place on the other side at the same time (hence the name “redox flow,” for reduction and oxidation). The direction of the processes is reversed during discharge. “The advantage of this battery is that it has a longer shelf life and almost never discharges itself, unlike lithium-ion batteries. The electrolytes do not use themselves up,” Roth says. “You can charge and discharge them practically any number of times, with almost no loss.” The biggest plus, though, is that energy and power density can be scaled independently as needed based on the volume of electrolytes (the tank size) and the size of the electrodes.

Plans call for these batteries to be used in private homes at some point as well. In this way, those who generate their own power with solar cells on the roof can build up a private store of energy in the basement. A German start-up is already offering solutions for this. Roth estimates that the battery will be competitive on a large scale in five to ten years.

“We already understand a lot about the redox flow battery,” Roth explains. “The fundamental research is largely done by now. The goal now is to make the battery easier to use and improve performance.” Roth’s team consists of three doctoral candidates working primarily on the porous electrode in the battery through which the electrolyte solutions flow. “The electrode is woven of carbon fiber,” Roth explains. “In principle, it looks like a piece of black felt.” For their experiments, Roth’s working group uses a device that allows them to make their own electrodes through a method known as electrospinning.

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The most striking factor when visiting Roth’s lab is how international the group is. People from Italy, Syria, and India sit at the equipment, shifting back and forth between German and English as a matter of course. “We are a diverse group, and all of us show consideration for each other,” Roth says.

One of her doctoral candidates is Abdulmonem Fetyan. For his dissertation, he is studying how changing the structure of the electrode can increase its capacity. He shows several electrodes he has produced using electrospinning: small black rectangles that do indeed look like common pieces of felt.

An industrially manufactured electrode has a close, grid-like weave. Fetyan’s electrode is fuzzier. A microscopic image shows a wild tangle of fine carbon threads. “This gives the electrode up to 100 times more surface,” Fetyan says. And that, in turn, should increase the battery’s capacity while keeping material costs the same.

Alongside the doctoral candidates, students also work on Roth’s project, either as assistants or writing their master’s theses on the subject. Last year, Bengü Sahin, a secondary school student from Berlin who worked with Fetyan on a redox flow topic, won a special award in the “Jugend forscht” youth research competition. The redox flow battery is also firmly established in teaching activities. “Our study regulations offer wide latitude, so we can get students involved in our research early on,” Roth says. “Everyone involved really enjoys working on a subject with such practical real-world implications.”