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(-PO3Na2), ammonium (-NR3+), and poly(ethylene glycol) (PEG) to the molecules can increase the solubility in acidic, alkaline, or pH-neural aqueous medium (25, 26, 30, 47, 48). The attachment of non-ionic PEG chains to redox materials can enhance the solubility in both aqueous and non- aqueous media. In addition, the asymmetrical structure is beneficial to enhance the solubility by increasing the transition dipole moments (49, 50). (2) Redox potential. The redox potentials of electroactive materials dictate the working potential of the battery. The potential of electroactive materials can be tuned by the electron-withdrawing and donating groups. Electron-withdrawing groups, such as trifluoromethyl (-CF3), nitro (-NO2), -NR3+, -SO3H, -COOH, and cyano (-CN), increase the electron affinity of molecules, leading to a decreased highest occupied molecular orbital, resulting in increased redox potential (51, 52); while electron-donating groups, such as -NH2, -OH and -OR, lead to decreased redox potential (47, 53). (3) Chemical stability. The chemical stability of organic electroactive materials is dependent on the substituent groups (54). The unstable substituent groups decrease the stability of electroactive materials. For example, the C-O bond in PEG chains undergoes side reactions at high voltage. And (4) Crossover. The crossover of electrochemical materials pertains to the properties of the membrane separator. For anion exchange membranes, functionalized molecules with -NR3+ display lower permeability. Similarly, for cation exchange membranes, the introduction of -SO3Na units to molecules leads to a lower crossover. Alternatively, enlarging the molecules, such as the formation of oligomers and polymers, can also alleviate the crossover issue. Anolyte Materials The cell voltage of an RFB is determined by the potential difference of catholyte and anolyte (Vcell = Vcatholyte – Vanolyte). Anolyte materials with more negative redox potentials typically result in higher cell voltage. Several kinds of redox-active compounds have been explored as the anolyte materials, such as quinones, viologens, pyridiniums, aza-aromatic compounds, phthalimides, and polycyclic aromatic hydrocarbons. Quinones Quinones are one of the most extensively studied type of organic electroactive materials. The elemental redox steps of different quinones are almost identical. Taking anthraquinone (AQ) as the example, the reaction of AQ at different pH can be presented by a 9-membered square scheme (Figure 3a) (55). Under strong acidic condition (pH < pKa1), quinones undergo rapid two-electron- two-proton reduction to yield dihydroquinone; under strong alkaline condition (pH > pKa2), however, quinones undergo rapid two-electron reduction to yield quinone dianion. Within a mild pH range (pKa1 < pH < pKa2), quinones undergo rapid two-electron-one-proton reduction to yield hydroquinone. The redox potential of AQ in aqueous electrolytes is calculated from Nernst equation: 7 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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