Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All‐Organic Redox Flow Battery

Abstract

Nonaqueous redox flow batteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems, but their current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. We report a new nonaqueous all-organic flow battery based on high concentrations of redox materials, which shows significant, comprehensive improvement in flow battery performance. A mechanistic electron spin resonance study reveals that the choice of supporting electrolytes greatly affects the chemical stability of the charged radical species especially the negative side radical anion, which dominates the cycling stability of these flow cells. This finding not only increases our fundamental understanding of performance degradation in flow batteries using radical-based redox species, but also offers insights toward rational electrolyte optimization for improving the cycling stability of these flow batteries.

A Membrane‐Free Redox Flow Battery with Two Immiscible Redox Electrolytes

“…[9] Another encouraging research line is the substitution of aqueous electrolytes by non-aqueous electrolytes [10] or even ionic liquids, [11] which are more electrochemically stable and would allow achieving higher battery voltages and energy densities. [12,13] In the last few years, RFBs based on organic redox molecules, such as quinones, phenothiazine, nitroxides, viologens, and pyridines [14][15][16][17] have experienced a great deal of interest, becoming one of the hottest topics in electrochemical energy storage (see Table S1 in the Supporting Information). [18,19] Regardless of the chemical nature of the electroactive species and the type of electrolytes, most RFBs rely on ion-selective membranes to separate the two redox electrolytes and to prevent the crossover of active compounds while allowing the migration of charge carriers.…”

Towards Low Resistance Nonaqueous Redox Flow Batteries

“…The implementation of the MeCN-based electrolyte and Celgard 2500 separator is critical in achieving low ASR, but the Celgard 2500 separator is impractical for a NAqRFB device since it offers no selectivity for small redox active molecules. Implementing Celgard 2500 in full flow cell would require mixed active species electrolytes, 21,23,41,83 which would be cost prohibitive, 7,12,23,41 or emerging large polymeric active species, [55][56][57] which may yield high viscosity electrolytes with poor mass transfer characteristics. 84 Additionally, the highly soluble Fc1N112 +/2+ model active species and low viscosity MeCN-based electrolyte facilitates small mass transfer resistances.…”

“Wine-Dark Sea” in an Organic Flow Battery: Storing Negative Charge in 2,1,3-Benzothiadiazole Radicals Leads to Improved Cyclability

“…[30][31] A major cause is the decomposition of charged ROM molecules (in their reactive radical ion forms) that may chemically react with each other, neutral ROM molecules, supporting salts, and solvents. 32 Therefore, development of highly soluble ROMs with suitable redox potentials and high chemical stability is the key to achieving promising cell performance in nonaqueous organic flow batteries.…”

“…As shown in Figure 3b 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 addition, even more stable cycling with almost no capacity fading for 40 cycles was obtained in a flow cell containing 0.3 M ROMs in 1,2-dimethoxyethane (DME) ( Figure S9 in the Supporting Information), as DME is generally more stable towards organic radical anions 32,39 . So far, the high-concentration cell efficiency and stability are among the best flow cell performance achieved in nonaqueous flow batteries, although further improvement is still needed to make this system practically attractive.…”