A Long-Lifetime All-Organic Aqueous Flow Battery Utilizing TMAP-TEMPO Radical

The massive-scale integration of renewable electricity into the power grid is impeded by its intrinsic intermittency. The aqueous organic redox flow battery (AORFB) rises as a potential storage solution; however, the choice of positive electrolytes is limited, and the aqueous-soluble organic positive redox-active species reported to date have short lifetimes. Here we report a stable organic molecule for the positive terminal, 4-[3-(trimethylammonio)propoxy]-2,2,6,6-tetramethylpiperidine-1-oxyl (TMAP-TEMPO) chloride, exhibiting high (4.62 M) aqueous solubility. When operated in a practical AORFB against a negative electrolyte comprising BTMAP-viologen at neutral pH, the flow cell displayed an open-circuit voltage of 1.1 volts and a Coulombic efficiency of >99.73%. The capacity retention rate is among the highest of all-organic AORFBs reported to date, at 99.993% per cycle over 1,000 consecutive cycles; the temporal capacity fade rate of 0.026% per h is independent of concentration. [Display omitted] •Highly soluble and long lifetime TMAP-TEMPO for pH 7 aqueous flow batteries•The posolyte features a concentration-independent capacity retention rate•Temporal capacity retention rate of >99.974% per h for over 1,000 cycles•Rational molecular design leads to improved stability With the rapidly falling cost of photovoltaic and wind energy generation, grid-scale integration of renewable energy becomes inevitable. This, however, is impeded by the intrinsic intermittency of these renewable energy resources. Safe, cost-effective storage could solve this problem. Aqueous organic redox flow batteries (AORFBs), which store energy externally in low-cost electroactive water-soluble organic molecules, have emerged as promising for this application. Current AORFBs are limited by short lifetimes and the limited choices for redox-active material for the positive electrolyte. We report a long-lifetime TMAP-TEMPO/BTMAP-Vi all-organic aqueous flow battery, the capacity retention rate of which is among the highest of all-organic AORFBs. We discuss the potential cause of the stabilization of the free radical posolyte molecule. The results we report here constitute an important step toward massive-scale intermittent renewable energy penetration into the future electric grid. 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 conc