A Two‐Electron Storage Nonaqueous Organic Redox Flow Battery

Abstract

impact the performances of RFBs. Traditionally, aqueous RFBs based on inorganic materials with an emphasis on metal species have been extensively developed, but suffer substantially from limitations, such as low cell voltage, inferior energy density, and high cost. [1] To push the boundaries of grid storage, nonaqueous RFBs are being investigated to pursue high energy density, enabled by the wide voltage windows of nonaqueous electrolytes that lead to higher cell voltages and more choices of materials candidates. [2] A variety of redox-active materials and cell designs have been developed for nonaqueous RFBs including metal coordination complexes, [3] redox-active polymers, [4] semisolid suspensions, [5] Li metal RFBs, [6] and redox targeting RFBs. [7] Notably, redox-active organic materials (ROMs) have gained significant momentum for nonaqueous RFBs. [8] This is mainly ascribed to the great potential of ROMs in expanding the library of material candidates and tailoring the energy density and cyclability of RFBs via molecular engineering. Currently, considerable progress has been achieved in the discovery and prototyping of promising ROMs for nonaqueous RFBs. [9] However, one of the major challenges for ROM-based nonaqueous RFBs is low energy density. Adopting ROMs that allow multiple electron transfers for RFBs is an effective strategy to boost the energy density. A few ROMs, including quinones, N-alkylated pyridiniums, and alloxazines, have been reported to show 2e − transfers in nonaqueous or aqueous RFBs. [10] In this communication, we demonstrate a new two-electron nonaqueous organic RFB based on phenothiazine (PT) catholyte and anthraquinone (AQ) anolyte materials. PTs have been demonstrated to have stable 1e − reactions for nonaqueous RFBs and lithium ion battery overcharge protection applications. [9c,11] Our studies indicate that PT also features 2e − transfers in nonaqueous electrolytes. When coupled with AQ derivatives, a proof-of-concept 2e − PT/AQ RFB was demonstrated with decent cyclability. PT is oxidized by stepwise losing 2e − to form the PT 2+ dication during charging while AQ is reduced by stepwise acquiring 2e − to form the AQ 2− dianion. Reverse processes occur during discharging.A series of structurally tailored AQ and PT derivatives were prepared using the synthetic approaches illustrated in Scheme 1 and the Supporting Information. AQs were synthesized in one step from the corresponding hydroxyanthraquinones and 2-bromoethyl methyl ether with high yields under the treatment of Cs 2 CO 3 in N,N-dimethylformamide (DMF).

Stability of molecular radicals in organic non-aqueous redox flow batteries: A mini review

“…Several carbonyl-containing compounds, which produce ketyl radicals, have been explored for NA RFB energy storage [19,24,25,28,[34][35][36]. Quinones are of particular interest as they are reduced to dianions, via radical anion semiquinone intermediates, allowing two electrons to be stored per molecule [34,37,38]. Stability of the radical intermediate arises from high delocalisation, a property further demonstrated by diaminoanthraquinones (DB-134) which are capable of a reversible five-membered redox series; dianion to dication, allowing a symmetric RFB to be constructed whereby two electrons are stored per DB-134 molecule in both the catholyte and anolyte [37].…”

“…with electron-donating or withdrawing groups have also been examined with BCF3EPT giving an increase in redox potential and stability with 2 M solubility [32,38,41,50,52,54]. The intrinsic capacity of phenothiazine has been further increased by stabilising the irreversible second oxidation of phenothiazine to a dication, via addition of para-N methyl [38] or methoxy groups [54], allowing two electrons to be stored per PT molecule. [57].…”

Azobenzene‐Based Low‐Potential Anolyte for Nonaqueous Organic Redox Flow Batteries

“…Several catholyte materials with high positive potential (up to 1.3 V vs. Fc/Fc + ), such as cyclopropenium, [12] 2,2,6,6‐tetramethylpiperidine‐1‐oxyl, [13] phenothiazine ( PTZ ), [11a,12b,14] have been investigated and applied in nonaqueous electrolyte systems. The anolyte materials, such as viologens, [4b,15] pyridinium, [16] fluorenone, [17] and anthraquinone, [18] possess redox potentials ranging from −0.8 to −2 V vs. Fc/Fc + . Despite several promising reports on novel anolyte systems, [19] the further advancement of anolyte materials is limited by insufficient stability, tedious synthesis, or unidentified degradation mechanism, necessitating the fundamental investigation on possible surrogates with superior properties.…”

Toward High‐Voltage, Energy‐Dense, and Durable Aqueous Organic Redox Flow Batteries: Role of the Supporting Electrolytes

“…Analytical methods to examine the reaction mechanisms and to characterize the reversibility and durability of organic molecules should be developed for optimizing performance and broadening applicability for industrial use. So far, time‐resolved UV‐Vis, Fourier transform infrared spectroscopy (FTIR), NMR and ESR spectroscopy have been successfully used to identify the chemical structure of organic compounds before and after battery cycling and to study their decay mechanisms.…”