Investigations on solid state boosters and polyoxometalates for redox flow batteries

Abstract

The current fossil fuel based electricity production is unsustainable and a transition towards a decarbonised energy economy urgently needed. However, unlike electricity produced from fossil fuels, many renewable power sources provide power only intermittently, creating the necessity for expanding large scale energy storage. Among a multitude of technologies in this field, Redox-Flow batteries (RFBs) are of special interest for large scale applications, particularly due to their flexibility in capacity and power rating as well as their inherent safety advantages over other types of batteries. However, current RFB technologies often suffer from low energy densities and high capital costs, which prevent widespread use. In this work, ways are explored to increase the capacity and energy density of RFBs by employing solid-state booster materials based on organic redox active materials. These capacity boosters are designed to be insoluble in the electrolytes while being able to exchange electrons with the redox active species therein, combining the flexible scalability and safety of a RFB with the higher concentration of active material in solid matter. Various attempts of demonstrating a capacity increase attributed to a booster are described and a successful proof of concept is shown, where the capacity of a half-cell could be increased by ca. 200% using a booster (from 22.3 mAh for the electrolyte alone to 67.6 mAh with booster). However, one of the major challenges found during this work was slow electron exchange between booster and electrolyte. Potential reasons and implications are discussed alongside possible solutions. The current development status of the concept of redox boosters is critically evaluated and teachings from the various investigations presented here are taken into account to gauge what future research is necessary to improve this concept. Furthermore, a method of calculating the electrochemical potential in solution for a species with more than two possible oxidation states is presented. The theoretical basis is derived from thermodynamic principles and experimental verification of the predictions is provided.