Molecular Tailoring of Phenothiazine Derivatives for High Performance Non-Aqueous Organic Redox Flow Batteries
Authors: Liang Zhiming 
Published: November 2023

Non-aqueous organic redox flow batteries (NAORFBs) have gained significant attention as a promising electrochemical energy storage technology, offering numerous advantages such as grid-scale electricity production with variable intermittent delivery, decoupled energy and power density, and simplified manufacturing processes. However, despite their promising attributes, this system faces certain challenges including limited solubility, poor electrochemical stability of radicals, low redox potential, and inadequate ionic conductivity, hampering their widespread commercial utilization. In recent years, molecular engineering has emerged as a transformative tool for designing and synthesizing of high-performance redox-active molecules for NAORFBs. These molecules, such as quinone, nitroxyl radicals, dialkoxybenzenes, phenothiazine, phenazine, pyridiniums, and viologen derivatives, offer precise control over solubility, stability, and redox potential through the strategic introduction or removal of functional groups. Our team has focused extensively on molecular engineering, with a specific focus on the phenothiazine core. Through a concise synthesis process, we have successfully synthesized a series of finely tuned phenothiazine derivatives with adjustable redox potentials, solubility, and stability. These tailored derivatives have demonstrated remarkable longevity when applied in NAORFBs. The selection of appropriate supporting electrolytes and membranes plays a significant role in achieving high performance RFBs. In this account, we commence by summarizing the results of our comprehensive examination of various supporting electrolytes and commercially available membranes, assessing their effects to stability, electrochemical reversibility, and crossover rates of redox-active molecules such as N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT). This comprehensive analysis provides a fundamental framework for evaluating the supporting materials for novel catholytes or anolytes. In efforts to fine-tune the solubility and redox potential of phenothiazines, we have employed molecular tailoring techniques. For instance, the introduction of alkyl or alkoxy groups on the nitrogen atom of phenothiazine has significantly improved solubility and electrochemical stability. These modifications have provided neutral molecules miscible with the commonly used organic solvents and allowed their radicals to reach concentrations of up to 0.5 M. However, monosubstituted phenothiazines still exhibit a relatively low usable oxidation potential, at 0.3 V vs. ferrocene/ferrocenium (Fc/Fc+). To address this limitation, we further modified the phenothiazine core by introducing substituents at positions 3 and 7. This alteration resulted in a notable increase in the stable oxidation potential. Notably, N-ethyl-3,7-bis(2-(2-methoxyethoxy)ethoxy)phenothiazine (B(MEEO)EPT) exhibited an oxidation potential of 0.65 V vs. Fc/Fc+. In pursuit of cost-effective NAORFBs, we developed a novel ionic compound named ethylpromethazine bis(trifluoromethanesulfonyl)imide (EPRT-TFSI). Through a concise three-step synthesis, EPRT-TFSI compound displayed a high redox potential of 1.12 V vs Fc/Fc+, a solubility of up to 1.3 M, and an ionic conductivity as high as 25 mS cm-1. The utilization of such ionic compounds eliminates the necessity for additional supporting salts in the electrolytes. Furthermore, we conducted investigations into the stability of radicals at various concentrations, employing different counter anions while maintaining controlled moisture levels in ambient environments. These stability tests provide valuable insights and guidelines for synthesizing other stable redox-active molecules. Our dedicated research efforts have been instrumental in advancing the development of high-performance NAORFBs, bringing us closer to realizing their potential as a prominent energy storage technology.