Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

Abstract

excellent stability can provide a good choice to meet this demand. [2][3][4][5] However, the extensively studied all-vanadium RFBs suffer some long-standing problems, such as the high costs of cation exchange membranes and vanadium-based active materials and the electrolyte crossover and corrosion issues in strong acidic conditions. [6] Aqueous organic redox flow batteries (AORFBs), after being reported, [7,8] received particular attention among the existing flow battery technologies, primarily because organic redox-active materials possess numerous advantages including great natural abundance, facile structural tailorability, tunable electrochemical properties, and potentially low costs. The solutions of a number of organic active molecules, such as viologen, anthraquinone, phenazine, and azobenzene derivatives, have been investigated as the anolytes for AORFBs. And a few organic molecules are available for the catholytes, typically, 2,2,6,6-tetramethyl piperidin-1-yl)oxyl (TEMPO) and metallocene derivatives. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Considering the high redox potential of TEMPO and its derivatives (ranged from 0.8 to 1.0 V versus standard hydrogen electrode, SHE), the introduction of highly hydrophilic groups, such as the quaternary ammonium group, is a promising approach to further improve the energy density. [20,23] However, up to date, the capacity degradation mechanism on the TEMPO/viologen systems remains unclear, and a lack of appropriate spectroscopic methods was established to study the electrochemical reversibility of redox-active organic species, especially in radical involved systems.Given that the chemically stable pyrrolidinium group has an ultrawide electrochemical window and strong polarity, it has been frequently utilized as a cationic moiety of ionic liquids. [33,34] When coupled with Clion, the pyrrolidinium functionalized organic species also appear to be highly watersoluble. Herein, we report the successful synthesis of pyrrolidinium cation functionalized TEMPO and extended viologen (Pyr-TEMPO and [PyrPV]Cl 4 ), which can serve as a highlysoluble organic redox pair with high cell voltage and excellent redox reversibility in AORFBs (Figure 1). The introduction Aqueous organic redox flow batteries (AORFBs) are regarded as a promising candidate for grid-scale, low-cost and sustainable energy storage. However, their performance is restricted by low aqueous solubility and the narrow potential gap of the organic redox-active species. Herein, a highly-soluble organic redox pair based on pyrrolidinium cation functionalized TEMPO and extended viologen, namely Pyr-TEMPO and [PyrPV]Cl 4 , which exhibits high cell voltage (1.57 V) and long cycling life (over 1000 cycles) in AORFBs is reported. The intrinsic hydrophilic nature of the pyrrolidinium group enables high aqueous solubilities (over 3.35 m for Pyr-TEMPO and 1.13 m for [PyrPV]Cl 4 ). Furthermore, the interaction of nitroxyl radicals with water is observed, which may be helpful to prevent collision-induced...

Novel Ethylene Glycol Substituted Benzoxadiazole and Benzothiadiazole as Anolytes for Nonaqueous Organic Redox Flow Batteries

“…[11] Application of organic solvents such as acetonitrile, propylene carbonate, or dimethylformamide allows expanding of the operational potential of the batteries up to 5 V owing to their wider electrochemical stability window. [12] Currently, the widely used catholytes for organic redox flow batteries are nitroxyl radicals [13][14][15][16] (TEMPO derivatives), alkoxybenzenes, [17,18] and phenothiazines. [19][20][21] Among the anolytes benzoquinones, [22][23][24] anthraquinones, [25][26][27][28][29][30] phenazines, [31][32][33] and alkyl viologens [14,[34][35][36] should be mentioned.…”

“…Currently, the widely used catholytes for organic redox flow batteries are nitroxyl radicals [13–16] (TEMPO derivatives), alkoxybenzenes, [17,18] and phenothiazines [19–21] . Among the anolytes benzoquinones, [22–24] anthraquinones, [25–30] phenazines, [31–33] and alkyl viologens [14,34–36] should be mentioned.…”

Modular TEMPO Dimerization for Water-in-Catholyte Flow Batteries with Extreme Energy Density, Power, and Stability

“…1), especially in low resistance membranes. 32,[41][42][43][44][45][46][47][48] Recent efforts have significantly improved the cycling stability performance of TEMPO derivatives upon the earliest AORFB demonstration of OH-TEMPO 32 through appending charged groups and extended tails to the 4-position, but there are still significant extrapolated capacity decay rates and/or dramatically sacrificed volumetric capacity that prevents commercial application: TMAP-TEMPO demonstrated a decay rate of 227.8% per year at 1.5 M, 47 N2-TEMPO has an estimated decay rate of over 200% per year at 1 M, 46 and Pyr-TEMPO showcased a decay rate of 1,267% per year at 0.5 M. 45 Hence, building on the knowledge gleaned in previous works, new design strategies must be explored to prevent crossover of TEMPO derivatives while retaining high volumetric capacity.…”

“…This contrasts extended TEMPO monomer strategies that inflate the mass with redox inert functionalization. [45][46][47] Saturated dimer solutions have an average density of 1.25 g•mL -1 , resulting in an estimated range of 1 to 20 water molecules per TEMPO dimer (Table S5). Moreover, under WiC dynamics, the stable voltage window of water was extended because there are the limited water molecules will be within the inner solvation sphere with strong intramolecular force between the redox molecules.…”

“…Reported RFB cycling demonstrations of TEMPO monomers have usually sacrificed power for stability with low-power, highly selective AMV/AMVN anion-exchange membrane. [45][46][47] To showcase this important but often overlooked metric for evaluating TEMPO performance, we conducted systematic comparison of the RFB power and cycling performances with AMVN and DSVN membranes for the TEMPO monomers and dimers. The rDC of the DSVN flow cell (1.26 Ω cm 2 ) was 43.9% of that of the AMVN flow cell (2.87 Ω cm 2 ) at 50% SOC (Fig.…”