Simple modification of N-ethylphenothiazine (left) with electron-donating substituents (right) increases the molecular charge-storage capacity of this donor.
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-(2methoxyethoxy)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.
The lack of suitable membranes for nonaqueous electrolytes limits cell capacity and cycle lifetime in organic redox flow cells. Using soluble, stable materials, we sought to compare the best performance that could be achieved with commercially available microporous separators and ion-selective membranes. We use organic species with proven stability to avoid deconvoluting capacity fade due to crossover and/or cell imbalance from materials degradation. We found a trade-off between lifetime and coulombic efficiency: non-selective separators achieve more stable performance but suffer from low coulombic efficiencies, while ion-selective membranes achieve high coulombic efficiencies but experience capacity loss over time. When electrolytes are pre-mixed prior to cycling, coulombic efficiency remains high, but capacity is lost due to cell imbalance, which can be recovered by electrolyte rebalancing. The results of this study highlight the potential for gains in nonaqueous cell performance that may be enabled by suitable membranes. File list (2) download file view on ChemRxiv UK-MIT04 MS for submission.pdf (1.45 MiB) download file view on ChemRxiv UK-MIT04 SI for submission.pdf (3.59 MiB)
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