Chelation approach to long-lived and reversible chromium anolytes for aqueous flow batteries

Iron chromium flow battery (ICFB) has the advances of low cost, safety, and independent design of power and capacity, but is restricted by the deactivation of chromium anolytes. Here, a complex of diethylenetriaminepentaacetic acid with chromium ion (CrDTPA) is designed with minimum capacity loss rate and best cycling stability. DTPA is an octadentate ligand and chelates with chromium ions in a seven-coordinated manner. The coordination structure was studied by Nuclear Magnetic Resonance, Fourier transform infrared and UV-vis absorption spectra, and identified as a robust structure. CrDTPA has good electrochemical activity and reversibility with a redox potential of -1.145 V versus saturated calomel electrode. A novel iron chromium flow battery (NICFB) is designed by coupling CrDTPA anolytes and Fe(CN)6 catholytes. NICFB displays high energy conversion efficiency with coulombic efficiency of 99.0 % and energy efficiency of 82.2 % at 40 mA cm(-2). Importantly, NICFB shows superior cycling stability without performance decay for 160 cycles, ranking the best among recently reported ICFBs. This chelation approach provides a simple and practical method to solve chromium anolytes deactivation and improve cycling stability.