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Stationary electric energy storage is anticipated to play an increasingly important role in the efficient, reliable, and sustainable delivery of electricity, especially with increasing deployment of low cost intermittent renewable energy sources. Redox flow batteries (RFBs) are a promising electrochemical technology whose decoupling of power and energy scaling, long operational lifetimes, and safety are particularly appealing for energy storage with long discharge duration. The recent emergence of aqueous organic redox flow batteries (AORFBs) offers intriguing new pathways to inexpensive energy storage through the use of charge storage materials that are composed of earth-abundant elements and have the potential for cost-effective mass production, and whose electrochemical and physicochemical properties can be tuned through molecular functionalization. Further, redoxactive organics possess several secondary benefits, including compatibility across a wide pH range, rapid redox kinetics with multielectron transfer, and low permeability through polymeric membranes. Over the past decade, many organic and organometallic molecular families have been proposed and pursued, including but not limited to quinones, viologens, nitroxide radicals, aza-aromatics, and iron coordination complexes. In addition, several startup companies have emerged seeking to translate these scientific advances into cost-competitive energy solutions, such as Kemiwatt (France), Green Energy Storage (Italy), XL Batteries (United States), and JenaBatteries and CMBlu (Germany). Despite this progress, molecular lifetime remains a significant challenge for practical deployment for AORFBs. Initial durability experiments tend to involve repetitive galvanostatic charge−discharge cycles, typically in a bulk electrolysis cell, at low concentrations and/or small electrolyte volumes to reduce the materialsand time-intensity of experiments. Thus, early claims of long cycle life are often based on a large number of cycles without significant decay performed over a short period of time. With recent advances in the field, it has become evident that all reasonably long-lived molecular redox couples (here, defined as having a half-life greater than a day) of which we are aware, when sufficiently thorough experiments to distinguish between cycleand calendar-denominated fade have been performed, have shown decay to be time-dependent and a function of temperature, state of charge (SOC), molecular concentration, pH, and electrostatic potential. Generally, albeit with some exceptions, the molecule is more suspectible to decay in its “energized” state (i.e., the oxidized state for the posolyte or the reduced state for the negolyte). The continued development of AORFBs requires the adoption of more rigorous testing protocols to accurately assess molecular lifetimes, to elucidate the subtle failure modes of increasingly robust molecules, and to maintain the iterative design−test−improve cycle. Moreover, as the cost-effectiveness of AORFBs is dependent on the chemical costs and fade rates of finite-lifetime organics, as opposed to the current stateof-the-art vanadium, we aim to help guide and accelerate research efforts by articulating a feasible design space for the community to target. In this Viewpoint, we first outline experimental protocols that we urge those active in the field to adopt to better quantify capacity fade. Subsequently, we extend an existing bottom-up capital cost model to estimate AORFB system cost as a function of molecular lifetime, chemical costs, and the interest rate for discounting. While we limit our focus to aqueous-soluble organics, we anticipate the following approaches are applicable to all molecular redox-active materials for which lifetime is uncertain. Capacity Fade Rate Protocol. The procedures summarized below are recommended because they improve the accuracy and simplify the interpretation of capacity fade rates due to molecular decomposition. It is encumbent upon the researcher to determine how closely to follow these procedures, based on prior evidence, the value of time in a study, or whether certain analyses should be expanded upon to address chemistrydependent phenomena. We note that these procedures are representative and intended as guidance to the greater community. There is room and need for continued innovation and advancement in this discipline. Cell Conf iguration. A volumetrically unbalanced, compositionally symmetric flow cell configuration, in which the same compound fills both reservoirs at an intermediate state of charge (SOC), is recommended for characterizing capacity fade rates (Figure 1a). The symmetric flow cell technique offers a controlled electrolyte environment by removing the need for a counter electrode of dissimilar material, suppressing

On Lifetime and Cost of Redox-Active Organics for Aqueous Flow Batteries