Reporting on flow battery environment using the intrinsic fluorescence of an organic redoxmer

Lily A. Robertson

Chemical Sciences & Engineering Division, Argonne National Laboratory

Abstract: To meet increasing discrepancies between the supply and demand of the electric grid, large-scale energy storage devices are needed. Redox flow batteries are frequently proposed to fill this gap as their liquid-filled tanks of redox-active material are highly scalable. Liquid electrolytes using nonaqueous solvents are of particular interest due to their wide electrochemical stability windows with inexpensive organic compounds acting as the redox-active species. In general, methods to track the state of health of the batteries lag far behind the development and study of new materials. The questions of how and why materials fail and whether batteries may be cured like a patient are pertinent. Most monitoring techniques examine state of charge changes and/or require aliquots of the flow battery solution. Not as many focus on in situ methods or other health issues such as species crossover. Therefore, we pursued a detailed study of new organic redox-active species (“redoxmers”) that are both strong flow battery performers and have an orthogonal “self-reporting” property that does not interfere with battery operation.[i] Here, we show that an organic anolyte redoxmer, 2,1,3-benzothiadiazole, can be synthetically modified using π-extending acetamide groups to have strong emissive properties useful for self-reporting. The redoxmer design had substantial impact on electrochemical performance with simple N-alkylation of the acetamide group giving optimal performance. Strong ion-molecule interactions were observed depending on supporting electrolyte choice in both states of charge. Finally, we used the strong emission of the redoxmer to track crossover of several practical flow solution conditions in real time. To our knowledge, our study is the first time fluorescence has been used to monitor a state of health property in a flow cell design.