Flow batteries offer direct scalability, discharge over long periods of time and flexibility in operating mode. They are a technological alternative to alleviate the expected bottlenecks in the future supply of Li-ion batteries for stationary applications. State-of-the-art vanadium flow batteries have excellent performance, but are currently still too expensive for most regions of the world and depend on critical materials.

As a result, flow batteries based on organic molecules with redox activity have increasingly come into focus. It is expected that these compounds, produced from readily available chemical feedstocks, could eventually prove competitive for stationary energy storage. Low cost, high capacity, ease of synthesis and chemical stability are fundamental issues here. The selection of compatible, stable chemicals and membranes, the improvement of energy density and power output as well as pilot scale performance under realistic operating conditions are important ongoing tasks.

This project pursued the improvement of flow batteries based on sulfonated anthraquinone, which is available as an industrial redox mediator for gas desulfurization. This compound is currently one of the few that are commercially available and thus represents a benchmark for the development of its synthetic derivatives. Ways were sought to combine this compound with low-cost, semi-organic or inorganic redox species. The activities also included the joint introduction of new viologen derivatives with the Institute of Organic Chemistry at Clausthal University of Technology. Here, synthetic strategies were sought to improve energy density and capacity retention. The large-scale production of these types of compounds by the agrochemical industry was utilized. The experimental work included the electrochemical characterization of redox-active molecules and the evaluation of energy storage performance in laboratory flow batteries.

Dr. Luis Fernando Arenas was funded by the Alexander von Humboldt Foundation.