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53 63 54 OO Figure 2.2: Structural formulas of the basic benzoquinone (left) and anthraquinone (right) structures in their oxidised form. The possible sites for substitution of functional groups are numbered according to conventional nomenclature. In aqueous solution, quinones generally undergo reversible two-electron redox reactions, which are proton-coupled depending on the pH [40]. The possible electron and proton transfer reactions are illustrated in Figure 2.3 for the benzoquinone (BQ)/hydroquinone (HQ) couple. Due to the proton-coupling, the redox potential is strongly pH-dependent. At pH values below pKa1 (the upper row in Figure 2.3), the redox potential decreases by 59 mV per pH unit. When pKa1 < pH < pKa2 (the middle row in Figure 2.3), the redox potentil decreases by 30 mV per pH unit. Finally, at pH values above pKa2 (the bottom row in Figure 2.3), the redox potential becomes pH-independent [27, 30]. 2.2. Quinone-Based Electrolytes benzoquinone (BQ) and anthraquinone (AQ) subgroups are depicted in Figure 2.2. OO 81 6272 BQ Electron transfer OOO e– e– (E1) (E2) OOO H+ (pKa1) H+ (pKa1') H+ (pKa1'') OH OH OH e– e– (E1') (E2') OOO H+ (pKa2) H+ (pKa2') H+ (pKa2'') OH OH OH e– e– (E1'') (E2'') HO HO HO HQ Figure 2.3: Schematic of the electron (horizontal) and proton (vertical) transfer pro- cesses that lead to the conversion of BQ to HQ, and vice versa, in aqueous media. The blue arrows represent proton-coupled electron transfer. 9 Proton transferPDF Image | Organic Redox Flow Batteries 2023
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