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

Abstract

Nonaqueous redox flowb atteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems,b ut their current performance is limited by lowr edox material concentration, cell efficiency,cycling stability,and current density.W er eport an ew nonaqueous all-organic flow battery based on high concentrations of redox materials,w hich 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 flowc ells.T his finding not only increases our fundamental understanding of performance degradation in flow batteries using radical-based redox species,b ut also offers insights towardrational electrolyte optimization for improving the cycling stability of these flowb atteries.The redox flow battery (RFB) has recently attracted agreat deal of interest as as afe and cost-effective energy storage technology for power grid applications.[1] By storing dissolved electroactive materials in external tanks,t he RFB decouples the energy and the power thus gaining tremendous design flexibility to meet different energy-to-power ratio (E/P) applications.C onventional aqueous RFBs are generally lowenergy systems limited mainly by the narrow electrochemical stability window of water.[2] Although the energy density is generally not considered as ac ritical performance criterion for stationary applications,the low energy density of aqueous RFBs has nevertheless disqualified them from other energy storage markets.T herefore,r ecent efforts have shifted to developing nonaqueous RFBs (NRFBs) because of their potential high energy density due to the wider electrochemical window of nonaqueous electrolytes.[3] Current NRFB research is primarily focused on seeking viable redox materials for various flow chemistries,f or example,m etal coordination compounds [4] and organic redox molecules.

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.…”

A bipolar nitronyl nitroxide small molecule for an all-organic symmetric redox-flow battery

“…The combined organic phases were dried over MgSO 4 and then the solvent was removed under reduced pressure to obtain 1 as a colorless solid (5.24 g, 0.045 mol) in a yield of 45%. 1 H NMR (300 MHz, CDCl 3 , δ): 1.24 (brs, 4H, NH 2 ), 1.08 (s, 12H, CH 3 ); 13 Compound 1 (5.2 g, 0.045 mol) was dissolved in tetrahydrofuran (100 ml) and a solution of 2 (8.6 g, 0.021 mol) in tetrahydrofuran (50 ml) was added dropwise. The reaction mixture was stirred at room temperature until the condensation was completed (monitored via thin layer chromatography).…”

“…[6][7][8][9][10] The development of an all-organic RFB with inexpensive and sustainable redox-active materials and low-cost membranes may overcome these drawbacks. 7,[10][11][12][13][14] Among others, Darling et al 15 conducted cost analyses, which revealed that the price of the active material itself and the membrane represent the main costs of these systems. Cost-efficient organic charge-storage materials can feature a price advantage compared with metal-based RFBs; in particular, if the raw materials are affordable, no synthesis or fewer synthesis procedures are required and the avoidance of elaborate purification steps can be achieved.…”

“…For example, a brief calculation for the anthraquinone disulphonate/bromine system yields a price of $27 per kW h for the redox-active organic charge-storage materials used, which is significantly lower than the price of $81 per kW h for vanadium systems. 16 In recent years, several semi-organic, [17][18][19][20][21][22][23][24][25][26][27][28][29] metal-free organic/ inorganic 16,30 and all-organic RFBs 7,[11][12][13][31][32][33][34][35] have been reported. Commonly, two different redox-active materials are used as chargestorage materials, one within the catholyte and the other within the anolyte.…”

Nuclear magnetic resonance studies of the solvation structures of a high-performance nonaqueous redox flow electrolyte

“…[1][2][3][4] Benefiting from wider electrochemically stable voltage windows, the emerging nonaqueous RFB (NRFB) holds great potential for overcoming the low energy density challenge present in current state-of-the-art aqueous RFB technologies. [3][4][5][6] Recent research on NRFBs focuses on the development of redox active materials including organometallic compounds, [7][8][9] redox organic materials, [10][11][12] redox active polymers, [13] and the synergy between flow battery and Li-ion or Li-metal battery chemistries. [14][15][16][17][18][19][20] However, the current performance of NRFBs has lagged far behind their inherent capability.…”