Improvements in aqueous redox flow batteries employing low cost and sustainable TEMPO-OH catholyte

Solar and wind energy generation and their grid-scale integration are growing rapidly, however their efficient utilization is impeded by the intrinsic intermittency of these renewable energy sources. Aqueous organic redox flow batteries (AORFBs) employing synthetically tailorable organic electroactive compounds composed of earth-abundant elements have received significant attention for energy storage technologies for their low cost, safety, and tunable properties. A redox flow battery (RFB) stores its energy in redox-active materials dissolved in liquid electrolytes circulating between external reservoirs and the battery stack, gives rise to their main advantages of decoupled energy and power, and excellent scalability. Regardless of this, they are facing daunting challenges such as low utilization ratio, high system complexity, low solubility, insufficient lifetimes, and scale-up difficulties. Therefore, to achieve high-energy-density flow batteries, electrolytes with electroactive materials that have high concentrations, high redox potentials, and ultra-low-capacity fade need to be developed. The nitroxyl radical 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-HO-TEMPO) and its derivatives has been evaluated as a cost-effective catholyte for AORFBs in pH-neutral supporting electrolytes. TEMPO compounds can operate at high current density (up to 100 mA cm−2) and deliver impressive cycling performance due to its considerable positive redox potential. However, imperfect chemical stability and capacity decay, which are exacerbated at high concentrations, are still unacceptably high for TEMPO species. The presented research highlights the possibility to enhance the longevity of (4-HO-TEMPO) at high concentrations when paired with 1,10-bis(3-sulfonatopropyl)-4,4 -bipyridinium (SPr)2V anolyte. Moreover, this research is focused specifically how pH impacts redox reversibility, diffusion coefficient and kinetic rate constant by inducing or suppressing disproportionation of TEMPO and other side reactions. Herein we build upon recent progress with this promising catholyte species and improve performance by using buffers to maintain the pH of electrolytes to avoid degradation and thus high-capacity fade rates of TEMPO.