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

Abstract

Non-aqueous redox flow batteries hold promise as a technology for electrochemical energy storage based on the large potential window of organic solvents compared to that of their aqueous counterparts. However, to date, the realization of this promise has been limited by a dearth of redox active molecules that leverage the full potential window of non-aqueous solvents. This report addresses this challenge through the development of an organic storage material based on the benzotriazole scaffold. Using a combination of iterative molecular design, organic synthesis, electrochemical evaluation, and density functional theory, a 2-aryl benzotriazole derivative is developed that exhibits a redox potential below −2 V (−2.3 V vs ferrocene/ferrocenium), a ≥0.4 M solubility in the electrolyte solution in all oxidation states, and stable electrochemical cycling in a prototype flow battery (≤90% capacity retention over 100 cycles, which represents approximately 505 h of cycling).

Indolo[2,3-b]quinoxaline as a Low Reduction Potential and High Stability Anolyte Scaffold for Nonaqueous Redox Flow Batteries

“…The change in NBO charges upon reduction of 5a to 5a •− and the condensed Mulliken spin densities of 5a •− (Figure 3d) show that the added electron, while fully delocalized in the πsystem, has relatively more density on N5, N11, and C13, which may hint at possible decomposition pathways. 35 Mass Transport and Electrokinetics of 5a. The electrochemical kinetics of ROMs, including the diffusion coefficient (D) and the heterogeneous electron-transfer kinetics (k 0 ), are critical for the performance of RFBs.…”

“…The HOMO of 5a confirms that the p z atomic orbital of the nitrogen has the correct orbital symmetry to participate in electron delocalization, which helps expand the conjugated π-system and stabilize the charged intermediate. The change in NBO charges upon reduction of 5a to 5a •– and the condensed Mulliken spin densities of 5a •– (Figure d) show that the added electron, while fully delocalized in the π-system, has relatively more density on N5, N11, and C13, which may hint at possible decomposition pathways …”

“…The 2,2′-bipyrimidines display two-electron reductions where only the second reaches such a low reduction potential; furthermore, they exhibit moderate stability and solubility . On the other hand, benzotriazoles display an extremely low reduction potential (−2.3 V vs Fc/Fc + ) and high solubility but only moderate electrochemical cycling stability . Benzophenone-containing scaffolds also exhibit low reduction potentials (−2.26 V vs Ag/Ag + ) and good solubility but maintain only moderate cycling stability .…”

“…34 On the other hand, benzotriazoles display an extremely low reduction potential (−2.3 V vs Fc/Fc + ) and high solubility but only moderate electrochemical cycling stability. 35 Benzophenonecontaining scaffolds also exhibit low reduction potentials (−2.26 V vs Ag/Ag + ) and good solubility but maintain only moderate cycling stability. 32 Therefore, developing organic electrolytes with reduction potentials at the extreme ends of the electrochemical window of acetonitrile (<−1.9 V Fc/Fc + ) along with high stability remains a significant unmet challenge, especially when considering the solubility requirements necessary to achieve high energy density.…”

“…In recent years, there has been significant progress in the development of new ROM scaffolds for catholytes, − anolytes, ,− and bipolar molecules. − These include several elegant examples of novel anolytes; ,− however, only a few of them leverage the wide potential window of nonaqueous solvents by reaching reduction potentials more extreme than −1.9 V (vs Fc/Fc + ) (Figure ). The 2,2′-bipyrimidines display two-electron reductions where only the second reaches such a low reduction potential; furthermore, they exhibit moderate stability and solubility .…”