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improved solubility in non-aqueous electrolytes. This work opened a new avenue to utilizing these molecules for high-energy RFB applications. Wang et al. applied DBMMB in a full NAORFB (106). The stability of radical cation (DBMMB•+) was dependent on the type of supporting electrolyte: the tetraethylammonium bis(trifluoromethylsulfonyl)imide (TEATFSI)/DME electrolyte afforded the highest chemical stability to both radicals. Zhang et al. synthesized two novel bicyclical substituted dimethoxybenzene derivatives as alternatives to the bulky tert-butyl groups: 9,10-bis(2- methoxyethoxy)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanenoanthracene (BODMA, Entry 6 in Table 8) and 9,10-bis(2-methoxyethoxy)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-diethano-anthracene (BODEA, Entry 7 in Table 8) (107). The substitution in the arene ring addressed the instability issue originated from intermolecular annihilation of radical cations. Coupled with lithium metal, the Li/BODEA battery delivered a discharge plateau at 3.6 V and a discharge capacity of 69 mAh/g, corresponding to 92% of the theoretical capacity. A capacity retention of 99.97% per cycle over 150 cycles was achieved, indicating the advantage of bicyclical substituted dimethoxybenzene. As the dialkoxybenzenes exhibit redox potentials within the electrochemical window of aqueous electrolytes, it is good research direction to develop water-soluble dialkoxybenzene molecules. It is essential to develop high-solubility dialkoxybenzenes in non-aqueous electrolytes. Phenothiazines Phenothiazines (PTZs) belong to the thiazine-class of heterocyclic compounds. The Odom group studied the reactivity of PTZ derivatives in lithium ion batteries and demonstrated the overcharge protection failure mechanisms. In view of the stable redox property and high solubility of PTZ derivatives, they have attracted great attention as catholytes in RFBs due to their double- electron redox property (Scheme 10) (52, 53, 71, 108–110). Scheme 10. Structure and redox reaction of PTZ derivatives. The Odom group systematically studied the property-structure relationship of PTZ derivatives. In non-aqueous electrolytes, PTZ derivatives demonstrated two one-electron redox processes (52, 53). The utilization of the second-electron activity of PTZ derivatives not only doubles the capacity of corresponding RFBs but also improves the power density by increasing the battery potential. However, not all of the PTZs at the dication state were electrochemically stable. Some of them were subject to rapid decomposition, such as N-(2-methoxyethyl)phenothiazine (MEPT, Entry 8 in Table 9) and N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT, Entry 9 in Table 9). To address this issue, Odom et al. demonstrated that substituents of PEG units at the 3 and 7 positions lead to extended charge delocalization (Entries 9 and 10 in Table 9), which stabilized the dication state of PTZs (52, 53). While PTZ is highly soluble in organic solvents, the solubility in water is very limited. A PTZ derivative, methylene blue (MB, Entry 13 in Table 9), also known as methylthioninium chloride, has been widely used as a redox indicator in analytical chemistry. At low pH (<5.6), MB undergoes 31 Qin and Fan; Clean Energy Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2020.PDF Image | Electroactive Materials Next-Generation Redox Flow Batteries
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