Tailoring Two-Electron-Donating Phenothiazines to Enable High-Concentration Redox Electrolytes for Use in Nonaqueous Redox Flow Batteries

This study aims to advance our understanding of the physical and electrochemical behavior of nonaqueous redox electrolytes at elevated concentrations and to develop experimentally informed structure-property relationships that may ultimately enable deterministic design of soluble multielectron-transfer organic redox couples for use in redox flow batteries. To this end, we functionalized a phenothiazine core to simultaneously impart two desired properties: high solubility and multiple electron transfer. Specifically, we report the synthesis, solubility, and electrochemical analysis of two new phenothiazine derivatives, 3,7-dimethoxy-N-(2-(2-methoxyethoxy)ethyl)phenothiazine and N-ethyl-3,7-bis(2-(2-methoxyethoxy)ethoxy)phenothiazine, both of which are two-electron donors that are miscible with nonaqueous electrolytes. This dual-property improvement compared to previous phenothiazine derivatives allows for extended symmetric flow cell experiments for 460 h of cycling of a multielectron transfer system at high concentrations (0.3 M active material, 0.6 M faradaic concentration), better representing practical devices.