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

Abstract

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.

Dual function organic active materials for nonaqueous redox flow batteries

“…Based on our previous CV results of an N-substituted phenothiazine in different electrolytes, we suspect that a higher concentration of TFSIstabilizes the dication form of EPRT. 14,36,48,49 For MEEV-(TFSI) 2 , in addition to a reversible first reduction, the second reduction is also chemically reversible in both the presence and absence of the supporting salt (peak current ratios of 1.08 and 1.03). Although the second reduction of MEEV-(TFSI) 2 is electrochemically reversible in 0.5 M TEATFSI / ACN (peak separation of 57 mV), it shows noticeably less electrochemical reversibility in the absence of the supporting salt (109 mV).…”

“…While recent molecular engineering efforts have focused on multi-property optimization to increase active species solubility and stability for high energy density RFBs, 14,16 to date, only a few molecular cores have shown promise as potential active materials, including derivatives of benzoquinone, 20,21 anthraquinone, 18,[22][23][24] nitroxyl radicals, 19,25 dialkoxybenzenes, 26,27 phenazine, 13,17,28 cyclopropeniums, 12,29 pyridiniums, 16,30 viologen,31-35 and phenothiazine. 6,14,35,36 Although a large variety of organic redox couples demonstrate stable cycling in nonaqueous RFBs, in many cases, results were obtained from flow cells operated at low concentrations, 4,26,30,33,36 some of which are a result of limited active species solubility -especially in their charged forms. It is important to evaluate active materials at concentrations used in commercial cells so that performance metrics and limitations can be identified.…”

“…[9][10][11] Transitioning from aqueous chemistries to nonaqueous counterparts offers an opportunity to achieve higher energy densities by raising cell voltages via accessing redox events with greater differences in potential, enabled by the wider electrochemical windows of nonaqueous electrolytes compared to their aqueous counterparts. [12][13][14][15] Additionally, the design space of redox couples can be expanded with nonaqueous electrolytes by allowing the inclusion of species that are otherwise incompatible with aqueous systems due to limited solubility and/or stability in aqueous solutions, or that possess redox potentials that lie outside the electrochemical stability window of aqueous electrolytes. 13,14,16 Redox-active organic molecules are of particular interest for low-cost RFBs because they consist of earth abundant elements (e.g.…”

“…[12][13][14][15] Additionally, the design space of redox couples can be expanded with nonaqueous electrolytes by allowing the inclusion of species that are otherwise incompatible with aqueous systems due to limited solubility and/or stability in aqueous solutions, or that possess redox potentials that lie outside the electrochemical stability window of aqueous electrolytes. 13,14,16 Redox-active organic molecules are of particular interest for low-cost RFBs because they consist of earth abundant elements (e.g. carbon, oxygen, and hydrogen).…”

Symmetric, Robust, and High-voltage Organic Redox Flow Battery Model Based on a Helical Carbenium Ion Electrolyte

“…In addition to preventing cross contamination, symmetric ORFBs can access three different oxidation states of the electrolyte within the same compartment, resulting in a charged to one polarity or to the opposite polarity via one-electron cycles in opposite directions. To date, only three examples of symmetric ORFBs have been reported, (Figure 1, green box), [31][32][33] and these either possess a high OCV with low cyclability (Figure 1, example iv), or a better reliability (max 100 cycles) but a small OCV (Figure 1 (Cyclability = capacity (Q) retention >90% after n cycles) with their electrolyte couples: (i), 28 (ii), 29 (iii), 30 (iv), 31 (v) 32 and (vi) 33 .…”