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trimethylammonio)propyl viologen tetrachloride (BTMAP-Vi, Entry 2 in Table 3) (25, 69), and presented excellent cycling performance with a capacity loss of 11.3% per year. Chemically modified viologens can also be applied in NAORFBs. When bis(trifluoromethanesulfonyl)imide (TFSI–) was used as the counterion, the methyl viologen TFSI– (MVTFSI, Entry 9 in Table 3) displayed a solubility of 0.98 M in MeCN (28). However, the lack of ion-selective membrane has been one of the main limitations of NAORFBs. To suppress the crossover of viologen through the separator, Jiang et al. developed PEG-modified viologen as the anolyte materials (70, 71). In addition, they developed a composite nanoporous aramid nanofiber (CANF) membrane as the ion-selective separator. The PEG functionalized viologen (PEG12-V, Entry 10 in Table 3) displayed low permeability for the CANF separator via physically blocking and Coulomb repulsion mechanisms. Moreover, Rodríguez-López group proposed viologen-based polymers as the active materials (Entries 12–14 in Table 3) (23, 72–75), which displayed a decreased crossover for porous membranes because of the large size of polymer. More reported viologen materials are summarized in Table 3. For viologens-based AORFBs, the utilization of double-electron redox activity has interested researchers in recent years. The strategy proposed by prof. Liu opened a new avenue to designing energy-dense redox species (26). The battery based on two-electron storage viologens deliverer double energy density than the corresponding one-electron storage viologens. However, the concentration of double-electron viologens in aqueous anolyte should be further improved for practice application. Pyridinium Pyridiniums are relatively new anolyte materials in aqueous and non-aqueous RFBs. Compared with dipyridiniums (viologens), pyridiniums exhibited a more negative redox potential. The pyridinium cation forms pyridine radical via one-electron reduction and forms enolate when receiving two electrons (Scheme 4). Similar to viologens, pyridiniums can also serve as anolytes in both aqueous and non-aqueous electrolytes. Scheme 4. Structure and redox steps of pyridiniums. To address the potential limitation, Sanford et al. developed a benzoylpyridinium salt (Entry 1 in Table 4) as the anolyte material for aqueous RFB (77). The cyclic voltammetry (CV) of benzoylpyridinium exhibited a 190-mV cathodic shift (–0.85 V vs. Ag/AgCl) compared to the first redox peak of dipyridinium (viologen, –0.66 V vs. Ag/AgCl). Benzoylpyridinium underwent two one-electron reduction processes, however, the second-electron step was irreversible due to the side reaction between doubly reduced benzoylpyridinium and water. By contrast, pyridiniums showed great promising in non-aqueous RFBs as the anolyte materials (54, 78, 79). The Sanford group synthesized and studied a series of pyridiniums for applications in non-aqueous RFBs (54, 78, 79). A combination tool of CV studies and independent synthesis of reduced pyridiniums was used to screen the most promising pyridinium (54). 16 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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