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

Abstract

Redox flow batteries (RFBs) are a promising stationary energy storage technology for leveling power supply from intermittent renewable energy sources with demand. A central objective for the development of practical, scalable RFBs is to identify affordable and high-performance redox-active molecules as storage materials. Herein, we report the design, synthesis, and evaluation of a new organic scaffold, indolo[2,3-b]quinoxaline, for highly stable, low-reduction potential, and high-solubility anolytes for nonaqueous redox flow batteries (NARFBs). The mixture of 2- and 3-(tert-butyl)-6-(2-methoxyethyl)-6H-indolo[2,3-b]quinoxaline exhibits a low reduction potential (-2.01 V vs Fc/Fc⁺), high solubility (>2.7 M in acetonitrile), and remarkable stability (99.86% capacity retention over 49.5 h (202 cycles) of H-cell cycling). This anolyte was paired with N-(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT) to achieve a 2.3 V all-organic NARFB exhibiting 95.8% capacity retention over 75.1 h (120 cycles) of cycling.

Figure 1

Representative organic anolyte scaffolds with reduction potentials more negative than −1.9 V (vs Fc/Fc⁺) for the NARFBs.

Figure 2

(a) Reported quinoxaline-based anolytes for aqueous and nonaqueous RFBs. (b) Design of indolo[2,3-b]quinoxaline as a new anolyte scaffold.

Figure 3

(a) Synthetic scheme of 5a and chemical structures of 8 and 9. (b) Calculated and experimental reduction potential of 5a, 8, and 9 in acetonitrile. (c) Comparison of calculated HOMO of 5a and the SOMO of radical anion 5a^•–. (d) Charge and spin density changes upon reduction from 5a to 5a^•–.

Figure 4

(a) CV of 5a (5 mM) in 0.5 M TBAPF₆/MeCN solution with a glassy carbon working electrode at a scan rate of 100 mV/s. (b) CVs of 5a (blue, top), 8 (red, middle), and 9 (black, bottom). (c) Scan-rate-dependent CVs of 5a. Inset: Linear relationship between the peak current density versus the square root of scan rates. (d) LSV of different rotation rates of the RDE scans of 2 mM 5a in 0.1 M TBAPF₆/MeCN. Inset: Linearly fitted Levich plots of 5a in 0.1 M TBAPF₆/MeCN. (e) Koutecky–Levich plots of 5a in 0.1 M TBAPF₆/MeCN. Inset: Linearly fitted Tafel plots of 5a based on the Butler–Volmer equation as a function of overpotentials. (f) Discharge capacity versus cycle number of 5a (black), 8 (blue), and 9 (red) and Coulombic efficiency for cycling of 5a (green) (5 mM in 0.5 M TBAPF₆/MeCN) in a static H-cell.

Figure 5

Evaluation of reduction potential, H-cell cycling stability, and solubility of indolo[2,3-b]quinoxaline derivatives 5b–5i.

Figure 6

(a) Discharge capacity and Coulombic efficiency versus cycle number of mixed 5 mM 5h and 5 mM 10 solution (in 0.5 M TBAPF₆/MeCN) in a static H-cell cycling. (b) Potential versus capacity for the 2nd, 3rd, 9th, and 202nd H-cell cycling (91% SOC). CV of the anolyte (c) and catholyte (d) before and after H-cell cycling between 5h and 5h^•–of mixed 5 mM 5h and 5 mM 10 solution (5 mM in 0.5 M TBAPF₆/MeCN) with a glassy carbon working electrode at a scan rate of 100 mV/s.

Figure 7

(a) Anolyte and catholyte electrochemical reactions in the flow battery of 5h and 10. (b) Charge capacity, discharge capacity, Coulombic efficiency, and energy efficiency versus cycle number for flow cell cycling of 50 mM 5h and 50 mM 10 (c) and 100 mM 5h and 100 mM 10 (d) in 0.5 M TBAPF₆/MeCN solution. CVs of the 5 mM diluted anolyte (e) and catholyte (f) before and after cycling the flow battery (100 mM 5h and 100 mM 10 in 0.5 M TBAPF₆/MeCN solution) with a glassy carbon working electrode at a scan rate of 100 mV/s.