Colourful Chemistry – from Hybrid Flow Batteries to a Powerful Redox Flow Battery with Impressive Colour Changes for a Phenomenological Approach

The energy transition towards a larger share of renewables requires energy storage devices with redox flow batteries playing a central role for stationary large-scale storage. The most promising and developed one is the vanadium redox flow battery. However, vanadium compounds are toxic, harmful to the environment and hardly suitable for use in schools – in addition, the worldwide occurrence of vanadium minerals is limited. In Freiburg, the aim of a research project was to make the topic of storage systems accessible to students both at university and in chemistry classes, because they will be the generation that will or must help shape this development. Therefore, hybrid flow batteries with harmless electrolytes, a common metal electrode and a simplified construction needed to be built for school application. Utensils from medical technology such as plastic syringes or extension lines are well suited for building cost-effective hybrid flow batteries for chemistry lessons. With such materials, electrolytes can be circulated with a pump that generates an efficient electrolyte flow. Freiburg’s design makes it possible to charge and discharge electrolytes in just a few minutes and the electrochemical reactions get visible through colour changes. In the field of redox flow batteries where the energy is stored in two electrolytes, a high-performance cell based on iron and cerium salts is also presented. With this battery, the anolyte and catholyte can be oxidised or reduced in a short time and electrolytes which change their colours during the electrochemical reaction are used as well. Thus, the operating principle of flow batteries can impressively be demonstrated by changing the properties of the electrolytes at the phenomenological level