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Scheme 5. Benzoylpyridinium decomposition mechanism. Reproduced with permission from reference (21). Copyright 2020 Wiley. Aza-Aromatic Compounds Flavin mononucleotide (FMN), alloxazines, and phenazines are three biomimetic aza-aromatic compounds. Meng et al. first explored FMN (Entry 1 in Table 5) in alkaline aqueous RFBs, initiating a novel research direction of biochemical organic RFBs (80). Gaining inspiration from naturally occurring flavin cofactors, Aziz et al. developed an alloxazine 7/8-carboxylic acid (ACA, Entry 2 in Table 5) in aqueous RFBs (81). The introduction of the carboxylic acid group on ACA improves its solubility in alkaline electrolytes. ACA presented a redox potential of –0.62 V vs. SHE, giving the ACA/K4Fe(CN)6 battery a potential of 1.13 V (Figure 6a). The ACA/K4Fe(CN)6 battery with 0.5 M ACA displayed a peak power density of 350 mW/cm2 at a current density of 0.58 A/cm2 (Figure 6b). A CE of 99.7% and a capacity retention of ~90% over 400 cycles was achieved (Figure 6c), indicating that the aza-aromatic redox-active species are promising materials for aqueous organic RFBs. Phenazine is not soluble in water; however, molecular modification on phenazine with -OH, -NH2, -SO3H and -NR3+ enhances the solubility in water (82, 83). Wang et al. explored 7,8- dihydroxyphenazine-2-sulfonic acid (DHPS, Entry 3 in Table 5) as the anolyte material (82). The two hydroxy groups increase the nearly insoluble phenazine to a solubility of 1.8 M, corresponding to a capacity of 96.5 Ah/L. In addition, the introduction of -OH groups lowered the redox potential of phenazine derivatives, increasing the power density of aqueous RFB. Jin et al. investigated the impact of ortho-substituents of the hydroxyl groups on the performance of phenazine derivatives (83). In this work, compound 2-hydroxyphenazine (Entry 4 in Table 5) displayed a higher solubility; however, it showed instability in its reduced state. Compound 2-amino-3-hydroxyphenazine (AHP, Entry 5 in Table 5) presented an improved stability; nonetheless, the insufficient solubility (0.43 M) limited the energy density of the AHP-based battery. Compound benzo[a]hydroxyphenazine-7/8- carboxylic acid (BHPC, Entry 6 in Table 5) displayed a satisfying solubility (1.55 M, corresponding to a theoretical capacity of 83.1 Ah/L) and high cyclability. The BHPC/Na4Fe(CN)6 battery based on 0.5 M BHPC anolyte delivered a maximum capacity of 25.9 Ah/L and a capacity retention of 98% over 1305 cycles (Figure 7), indicating the tremendous potential of phenazine derivatives. 21 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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