Self-Reporting Redoxmers: State of Health Metrics for Redox Flow Batteries
Authors: Robertson Lily A. , Shkrob Ilya A. , Agarwal Garvit , Yu Zhou , Zhao Yuyue , Assary Rajeev S. , Cheng Lei , Moore Jeffrey S. , Zhang Lu 
Published: July 2021

Redox-active organic molecules (redoxmers) are popular for nonaqueous redox flow batteries (NRFBs). However, many parameters need to be achieved for their success. These include high solubility, high conductivity, little to no crossover, wide voltage windows, and high stability in all states of charge. Metrics are needed to track these properties and the overall state of health (SOH) of the battery. Here, we describe the concept of self-reporting redoxmers, where an intrinsic property of the molecule can be used to trace a SOH property. In this case, 2,1,3-benzothiadiazole, an anolyte redoxmer, was functionalized with a pi-extending acetamide groups, which initiated fluorescence, a highly sensitive property. In turn, the fluorescence of the molecules was used to detect species crossover in a simplified H-cell in different electrolyte conditions. Using these methods, we were able to correlate permeability with electrolyte environment. Further, changes in fluorescence emission with different electrolyte salts were direct indicators of solvation environment. Importantly, the synthetic design of the molecule substantially affected electrochemical performance, with a simple methylation of the pi-extending acetamide group giving high stability. The fluorescence also served as a handle to probe and report on solvation environment. In acetonitrile, electrolyte salts containing small lithium cations quenched the redoxmer fluorescence due to a strong chelation effects that were not observed for larger cations like tetraalkylammonium cations. However, this chelation ability was removed when solvents like N,N-dimethylformamide that also have strong chelation effects were used as supported by computational binding calculations. These chelation effects may also correspond with changes in half-wave potential in cyclic voltammetry experiments. Finally, we extend these studies to larger, more complex molecular architectures, which we hypothesize will lead to improved battery performance. Our studies represent the fundamental design of new organic redoxmers with an applied basis to electrolyte fluids while providing molecular-level understanding and SOH diagnosis, which together will provide successful implementation in a flow battery environment.
The research was financially supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.