(103d) Increasing the Charge Storage Capacity of Phenothiazine-Based Electrolytes for Nonaqueous Redox Flow Batteries

Stationary energy storage systems are anticipated to play an important role in facilitating the integration of intermittent, renewable energy sources and in improving the efficiency, reliability, and resiliency of the existing fossil fuel based infrastructure. Redox flow batteries (RFBs) are rechargeable electrochemical devices that are well suited for grid storage due to decoupled power and energy scaling to meet specific installation needs, long operating lifetimes (~10 years), and simplified manufacturing¹. Despite this promise, further cost reductions are needed for widespread adoption motivating research and development into new redox chemistries, electrolyte formulations, and reactor designs.^2,3 The combination of organic redox active materials and nonaqueous electrolytes is an emerging concept that aims to lower system costs through increased energy density.⁴ A key challenge is the design and implementation of stable, soluble, and scalable redox couples that enable high capacity redox electrolytes.

Here, using N-ethylphenothiazine and its derivatives as a learning platform, we systematically investigate the impact of substituent group addition on the molecular properties to develop structure-function relations that may enable deterministic multi-property optimization. Specifically, we couple molecular engineering and electrochemical analysis to increase the equivalent charge concentration of phenothiazine-containing electrolytes by enhancing the solubility⁵ and intrinsic storage capacity⁶. We find that through careful selection and positioning of substituent groups, significant performance improvements may be realized. The lessons learned here are portable and can be used to curate design campaigns for other redox couples, hence accelerating the development of new RFB materials.

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