Azobenzene‐Based Low‐Potential Anolyte for Nonaqueous Organic Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) using organic materials in organic solvents are particularly promising because of the diversity of organic materials and the wide redox‐innocent window of organic solvents, yet the further development of this type of RFB has been hindered by the lack of suitable anolyte materials with high potentials, solubility, and stability. Herein, we examine the physical and electrochemical properties of azobenzene (AzoPh) compounds and investigate their degradation mechanism in nonaqueous organic RFB. The azobenzene displayed two‐electron activity at −1.69 and −2.20 V vs. Ag/Ag+. Paired with a well‐established PEGylated phenothiazine (PEG3‐PTZ ) catholyte, the battery presents a high theoretical cell voltage of 2.08 V. The azobenzene‐based RFB displays a capacity retention of 93.3 % over 50 cycles with an average fade rate of 0.13 % per cycle. The capacity decay mechanism was probed by proton nuclear magnetic resonance spectroscopy, cyclic voltammetry, and high‐resolution mass spectrometry, and revealed proton‐assisted degradation mechanisms.

Azo Compounds as Active Materials of Energy Storage Systems

“…Wang et al revealed that this behavior was due to the fact that one-electron reduction accompanied by protonation in the organic solvent assisted degradation into aniline or dimerization. [55] As mentioned above, azo molecules act as highperformance anolytes because of their high solubility, fast redox reaction, and the coupling between the redox reaction, which high capacity and voltage increase (Table 5).…”

Benzotriazoles as Low-Potential Anolytes for Non-aqueous Redox Flow Batteries

“…We next changed the solvent to DMF on the basis of its comparable dielectric constant (ε = 38 compared to 39 for MeCN) and literature reports showing the improved redox stability of low-potential anolytes in this medium. ,, As shown in Figure (red), the redox potentials of 1 and 2 in DMF are lower than those in MeCN (−2.91 and −2.71 V vs Fc/Fc + , respectively). Furthermore, the peak height ratios ( i pc / i pa values) are closer to 1, at 0.87 and 0.91, respectively.…”

Azo Compounds as Active Materials of Energy Storage Systems

“…Wang et al. revealed that this behavior was due to the fact that one‐electron reduction accompanied by protonation in the organic solvent assisted degradation into aniline or dimerization [55] …”

“…Wang et al revealed that this behavior was due to the fact that one-electron reduction accompanied by protonation in the organic solvent assisted degradation into aniline or dimerization. [55] As mentioned above, azo molecules act as highperformance anolytes because their high solubility, fast redox reaction, and the coupling between the redox reaction, which provides high capacity and voltage increase (Table 5).…”