There is an increasing demand for stationary energy storage systems to facilitate the integration of intermittent, renewable energy sources (e.g. wind, solar) and to improve the efficiency, reliability, and resiliency of the existing fossil fuel infrastructure. Redox flow batteries (RFBs) are electrochemical energy storage devices that are well suited for grid storage due to decoupled power and energy scaling, long operating lifetimes, and simplified manufacturing¹. While state-of-the-art RFBs utilize transition metal salts to store energy, they are currently too expensive to meet the stringent targets set by the U.S. Department of Energy ($100/kWh)², motivating research in novel organic charge storage materials which may enable a pathway to low cost through tunable molecular structure and inexpensive synthesis routes³.

This work focuses on the use of phenothiazine and its derivatives as a robust learning platform to investigate the impact of substituent group addition on the molecular properties with an overarching goal of developing structure-function relations that enable deterministic multi-property optimization. Specifically, we seek to improve the equivalent charge concentration of redox electrolytes containing phenothiazines, by enhancing the solubility and intrinsic storage capacity through a combination of molecular engineering and electrochemical analysis. Building from N-ethylphenothiazine, we find judicious application of substituent groups can lead to significant performance enhancements, but care must be taken to avoid improving one property at the expense of others.

References

1. A. Z. Weber et al., J. Appl. Electrochem., 41, 1137–1164 (2011).
2. A. A. Akhil et al., Ed Albuq. NM Sandia Natl. Lab. (2013) http://www.emnrd.state.nm.us/ECMD/RenewableEnergy/documents/SNL-ElectricityStorageHandbook2013.pdf.
3. J. A. Kowalski, L. Su, J. D. Milshtein, and F. R. Brushett, Curr. Opin. Chem. Eng., 13, 45–52 (2016).