Electrochemical Evaluation of Phenylenediamine/Quinonediimine Redox Systems As Organic Electroactive Materials

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Quinones are one of the most promising and widely investigated classes of redox active materials for organic aqueous redox flow batteries, because they have advantages such as low cost, excellent reaction rate, and wide variety. However, they have a serious problem of the limited solubility in water. In order to solve the problem, there have been many reports on the introduction of polar functional groups into quinones. On the other hand, quinonediimines are quinone equivalents, which allow the introduction of polar functional groups on the nitrogen atoms. Therefore, quinonediimines are easier to introduce polar functional groups than quinones. With these facts in mind, we evaluated the electrochemical properties of phenylenediamine/quinonediimine redox systems in this work.First, cyclic voltammetry of 1,2-phenylenediamine (1) and 1,4-phenylenediamine (2) was performed to evaluate their electrochemical reversibility in aqueous (0.1 M KCl/water) and non-aqueous (0.1 M Bu4NBF4/MeCN) solutions. As a result, 2 was reversible, but 1 was irreversible in both solutions. However, the reversibility of 2 was lost on the second scan in the aqueous solution, suggesting that water makes a nucleophilic attack on the oxidized form of 2.In order to suppress the nucleophilic attack of water, we used and measured the cyclic voltammograms of 2, 2,5-dimethyl-1,4-phenylenediamine (3), and 2,3,5,6-tetramethyl-1,4-phenylenediamine (4) in aqueous and non-aqueous solutions. Their reversibility improved with increasing the number of methyl groups in both solutions. This seems to be due to the steric hindrance of the methyl group, which suppressed the nucleophilic attack of water. In addition, as the number of electron-donating methyl groups increased, the redox potential shifted to the lower potential side.Finally, in order to verify the electrochemical cycling stability, cyclic voltammetry of 4 was performed with 1000 consecutive scans in aqueous and non-aqueous solutions. In aqueous solution, the redox wave of 4 changed from two waves to one wave. In contrast, the redox wave was maintained after 1000 scans in non-aqueous solution. These suggest that the oxidized form of 4 is stable in non-aqueous solution even under multiple scans, whereas it is unstable in aqueous solution.We will present the electrochemical properties of various phenylenediamine/quinonediimine redox systems and also introduce the introduction of polar functional groups into them in this presentation. Figure 1