To store energy from solar and wind power over a longer period of time, flow batteries (redox flow batteries) are promising from both an economic and an ecological point of view, according to current findings. Indeed, being able to store this energy is a basic requirement if any proposed energy transition is to succeed. This is why scientists led by Prof. Birgit Weber, Professor of Inorganic Chemistry at the University of Bayreuth, Germany, have set themselves the goal of optimizing this type of battery. They want to significantly increase the efficiency and storage capacity of environmentally-friendly iron-based flow batteries.
Compared to lithium-ion batteries, flow batteries have several advantages. They work neither with solid electrolytes nor do they release environmentally harmful substances. They also have a relatively long service life and their modular design means that energy storage can be spatially decoupled from the charging and discharging processes. And that’s not all.
Lower energy density – higher storage capacity
Despite the low energy density in its liquid electrolytes, flow batteries can be used to build up very high storage capacities. In recent years, it has become apparent that especially flow batteries whose electrolyte containers contain dissolved vanadium are very promising as storage media. But vanadium is a rare and expensive metal that often has impurities, says Prof. Dr. Birgit Weber. “Today, iron is considered by far the most promising candidate for flow batteries. It is an abundant, inexpensive metal characterized by low toxicity. It can also be used in various molecular environments.”
Iron occurs in two different forms: the divalent form Fe²⁺ and the trivalent form Fe³⁺ (iron (II) and iron (III)). “Both forms of iron form molecular complexes that exist in different quantum mechanical states – so-called spin states,” the researchers explain. External stimuli, such as a change in ambient temperature, could cause the spin state to change.
Significantly increasing the share of sustainable energy sources in the energy supply
The researchers now want to take advantage of this property and further develop flow batteries by exploiting the possibility of “being able to specifically induce a certain spin state of the iron complexes by changing the temperature.” It is possible, she said, that the efficiency of flow batteries in which the vanadium has been replaced by iron (II) and iron (III) could be significantly increased if the redox potential of the iron complexes can be controlled by selectively changing their spin state. “We already know that the spin state of iron complexes affects their redox potential.”
The goal of the project is to develop environmentally-friendly, highly efficient flow batteries based on iron, Weber says. “Because of their high storage capacity, these will be able to help significantly increase the share of sustainable energy sources in the energy supply.”
The project is being funded by the Volkswagen Foundation from the “Experiment!” program for 18 months with around €120,000. With “Experiment!”, the Volkswagen Foundation supports the start-up phase of projects in which new and unusual research ideas are tested. In the best-case scenario, these ideas provide a valuable basis for innovations in important areas of the economy and society – not least of all in the energy sector.
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Cover photo: Prof. Birgit Weber, head of the new research project funded by the Volkswagen Foundation. Photo: Christian Wissler
