Development of organic redox‐active materials in aqueous flow batteries: Current strategies and future perspectives

Abstract

Aqueous redox flow batteries, by using redox‐active molecules dissolved in nonflammable water solutions as electrolytes, are a promising technology for grid‐scale energy storage. Organic redox‐active materials offer a new opportunity for the construction of advanced flow batteries due to their advantages of potentially low cost, extensive structural diversity, tunable electrochemical properties, and high natural abundance. In this review, we present the emergence and development of organic redox‐active materials for aqueous organic redox flow batteries (AORFBs), in particular, molecular engineering concepts and strategies of organic redox‐active molecules. The typical design strategies based on organic redox species for high‐capacity, high‐stability, and high‐voltage AORFBs are outlined and discussed. Molecular engineering of organic redox‐active molecules for high aqueous solubility, high chemical/electrochemical stability, and multiple electron numbers as well as satisfactory redox potential gap between the redox pair is essential to realizing high‐performance AORFBs. Beyond molecular engineering, the redox‐targeting strategy is an effective way to obtain high‐capacity AORFBs. We further discuss and analyze the redox reaction mechanisms of organic redox species based on a series of electrochemical and spectroscopic approaches, and succinctly summarize the capacity degradation mechanisms of AORFBs. Furthermore, the current challenges, opportunities, and future directions of organic redox‐active materials for AORFBs are presented in detail.

Progresses and Perspectives of All‐Iron Aqueous Redox Flow Batteries

“…Different from metal‐ion‐based RFBs, the recently developed aqueous organic redox flow battery is regarded as an environmental‐friendly and tunable choice for large‐scale energy storage because of its potential for low cost, high natural abundance, adjustable electrochemical properties, and wide‐ranging structural diversity. [ 41–47 ] Molecules such as quinone, [ 48,49 ] viologen, [ 50 ] fluorenone, [ 43 ] and phenazine [ 42,51 ] derivatives are easily synthesized and have been researched and determined as viable options. [ 44 ] However, these new‐type RFBs also face challenges in the stability, efficiency, crossover issue, and the cost for the inexpensive preparation of active materials.…”

Quinones for Aqueous Organic Redox Flow Battery: A Prospective on Redox Potential, Solubility, and Stability

“…These species exhibit different redox potentials relative to the standard hydrogen electrode (SHE), normal hydrogen electrode (NHE), and Ag/AgCl in acidic, neutral, and alkaline environments, respectively. [ 86,94,148–151 ]…”

“…Assessing the solubility of organic redox‐active molecules is challenging as they are often organic salts that dissolve after ion solvation and ionic bond breakage, making their solutions unique from other dissolved species. [ 44,45 ] To prevent clogging of the felt electrodes and ensure maximum utilization of the electrolyte, it is crucial to avoid the precipitation of organic redox‐active molecules during battery charging. [ 46 ] Various methods have been developed to address this issue, including the proper selection of the counterion, functionalizing the redox moiety with solubilizing groups, and so on.…”

Enabling Long-Life Aqueous Organic Redox Flow Batteries with a Highly Stable, Low Redox Potential Phenazine Anolyte

“…Aqueous organic redox flow batteries (AORFBs) utilize organics dissolving in aqueous solutions as active species in ARFBs and are being explored as a new direction for energy storage systems . These organics consist of earth-abundant elements such as C, H, O, N, P, and S, making AORFBs more promising in terms of cost reduction compared to vanadium-based ARFBs. , Since the first report of AORFBs based on 9,10-anthraquinone-2,7-disulfonic acid (AQDS) by Aziz, there has been substantial progress in the development of high-performance AORFBs, marked by the exploration of novel organic compounds and advancements in electrode designs. − The success of AQDS-based AORFBs has encouraged more researchers to focus on highly soluble and electro-active organic compounds for AORFBs. − Despite progress in the development of AORFBs, many previous reports on this technology are yet to be translated into practical applications due to challenges such as short cycling time (less than 1 month), ,− ,− serious capacity fade (greater than 0.1% per day), ,,,− and low open-circuit voltage (e.g., less than 1 V in al...…”

“…14 These organics consist of earth-abundant elements such as C, H, O, N, P, and S, making AORFBs more promising in terms of cost reduction compared to vanadiumbased ARFBs. 15,16 Since the first report of AORFBs based on 9,10-anthraquinone-2,7-disulfonic acid (AQDS) by Aziz, 17 there has been substantial progress in the development of high-performance AORFBs, marked by the exploration of novel organic compounds and advancements in electrode designs. 18−26 The success of AQDS-based AORFBs has encouraged more researchers to focus on highly soluble and electro-active organic compounds for AORFBs.…”