Aqueous organic redox flow batteries (RFBs) could enable widespread integration of renewable energy, but only if costs are sufficiently low. Because the levelized cost of storage for an RFB is a function of electrolyte lifetime, understanding and improving the chemical stability of active reactants in RFBs is a critical research challenge. We review known or hypothesized molecular decomposition mechanisms for all five classes of aqueous redox-active organics and organometallics for which cycling lifetime results have been reported: quinones, viologens, aza-aromatics, iron coordination complexes, and nitroxide radicals. We collect, analyze, and compare capacity fade rates from all aqueous organic electrolytes that have been utilized in the capacity-limiting side of flow or hybrid flow/nonflow cells, noting also their redox potentials and demonstrated concentrations of transferrable electrons. We categorize capacity fade rates as being “high” (>1%/day), “moderate” (0.1–1%/day), “low” (0.02–0.1%/day), and “extremely low” (≤0.02%/day) and discuss the degree to which the fade rates have been linked to decomposition mechanisms. Capacity fade is observed to be time-denominated rather than cycle-denominated, with a temporal rate that can depend on molecular concentrations and electrolyte state of charge through, e.g., bimolecular decomposition mechanisms. We then review measurement methods for capacity fade rate and find that simple galvanostatic charge–discharge cycling is inadequate for assessing capacity fade when fade rates are low or extremely low and recommend refining methods to include potential holds for accurately assessing molecular lifetimes under such circumstances. We consider separately symmetric cell cycling results, the interpretation of which is simplified by the absence of a different counter-electrolyte. We point out the chemistries with low or extremely low established fade rates that also exhibit open circuit potentials of 1.0 V or higher and transferrable electron concentrations of 1.0 M or higher, which are promising performance characteristics for RFB commercialization. We point out important directions for future research.

This work demonstrates a new, organic redox-flow battery (RFB) that outlives its predecessors, offering the longest-lived high-performance organic flow battery to date. It appears to be the first aqueous-soluble organic RFB chemistry to meet all the technical criteria for commercialization. The potential low reactant and membrane costs of this chemistry offer the potential for RFBs of this type to be used cost effectively at the gigawatt scale in order to enable massive penetration of intermittent renewable electricity.

TMAP-TEMPO represents an extremely stable redox-active radical organic for an AORFB posolyte. An all-organic AORFB based on TMAP-TEMPO and BTMAP-Vi exhibits an OCV of 1.1 V and a long lifetime, featuring a concentrationindependent temporal capacity retention rate of >99.974% per h, or a capacity retention rate of 99.993% per cycle over 1,000 consecutive cycles.

high cost limits their usage in grid-scale systems. Among the most promising solutions are redox-flow batteries (RFBs), which store the electro-active chemical species separately from the power-generating electrode stack, through which the reactants are pumped during operation. This design allows the energy capacity of the entire system to be scaled independently of its maximum power output so that cost-effective long-duration discharge can be achieved. [2] All-vanadium systems now have the largest market-share among RFBs, but their penetration is limited by the relatively low Earth-abundance and high cost of vanadium. [3] In contrast, the low cost of some organic molecules and the Earth-abundance of carbon offer promising advantages of redox-active organics for massive penetration of grid-scale energy storage. [4] Moreover, the chemical tunability of organic molecules permits improvements in solubility, redox potential, and stability, which can enhance the energy density, power density, and lifetime of a battery.There have been numerous reports regarding RFB chemistries based on quinone, [4a,c,5] viologen, [4b,6] ferrocene, [6a,c] alloxazine, [4d] nitroxide radical motifs, [4b,7] and phenazine [7c,8] in the past four years which, while demonstrating promising performance, fall short of meeting all of the technical requirements for practical deployment. Due to the generally low chemical stability of these reactants, most existing systems experience high temporal capacity fade rates on the order of 0.1-10% per day, which limits their long-term use and renders most of these chemistries unsuitable for commercialization. Voltage trades off against stability in many cases. This is most readily apparent in cells utilizing substituted viologens against substituted ferrocenes, where adequate stability has been accompanied by large compromises in open-circuit voltage (OCV). [6a,c,d] Recently, we reported a negative electrolyte (negolyte) comprising 4,4-((9,10-anthraquinone-2,6-diyl)dioxy) dibutyrate (2,6-DBEAQ), [9] that combines high chemical stability with an OCV of ≥1.0 V against a potassium ferri/ferrocyanide positive electrolyte (posolyte). This flow battery exhibited a capacity fade rate of 0.04% per day, which was the lowest of any quinone species at the time. In the current work, we report an aqueous RFB employing a phosphonate-functionalized A highly stable phosphonate-functionalized anthraquinone is introduced as the redox-active material in a negative potential electrolyte (negolyte) for aqueous redox flow batteries operating at nearly neutral pH. The design and synthesis of 2,6-DPPEAQ, (((9,10-dioxo-9,10-dihydroanthracene-2,6-diyl) bis(oxy))bis(propane-3,1-diyl))bis(phosphonic acid), which has a high solubility at pH 9 and above, is described. Chemical stability studies demonstrate high stability at both pH 9 and 12. By pairing 2,6-DPPEAQ with a potassium ferri/ferrocyanide positive electrolyte across an inexpensive, nonfluorinated permselective polymer membrane, this near-neutral quinone flow bat...

A water-miscible anthraquinone with polyethylene glycol (PEG)-based solubilizing groups is introduced as the redox-active molecule in a negative electrolyte (negolyte) for aqueous redox flow batteries, exhibiting the highest volumetric capacity among aqueous organic negolytes. We synthesized and screened a series of PEG-substituted anthraquinones (PEGAQs) and carefully studied one of its isomers, namely, 1,8-bis­(2-(2-(2-hydroxyethoxy)­ethoxy)­ethoxy)­anthracene-9,10-dione (AQ-1,8-3E-OH), which has high electrochemical reversibility and is completely miscible in water of any pH. A negolyte containing 1.5 M AQ-1,8-3E-OH, when paired with a ferrocyanide-based positive electrolyte across an inexpensive, nonfluorinated permselective polymer membrane at pH 7, exhibits an open-circuit potential of 1.0 V, a volumetric capacity of 80.4 Ah/L, and an energy density of 25.2 Wh/L.