Highly-Coordinated Iron Complex with Multidentate Ligands for Aqueous Redox Flow Battery Electrolytes

Redox flow batteries (RFBs) possess a longer cycle life than other secondary batteries owing to their limited electrode deterioration upon charge/discharge cycling. Commercial RFBs use vanadium ions as active materials, resulting in a high cost. Therefore, more abundant and less expensive iron-based complexes are desirable as catholyte active materials for RFBs. However, to develop stable iron-based aqueous electrolytes, precipitation of ferric hydroxides under a mild alkaline condition should be suppressed. One potential strategy is to use polydentate ligands that stabilizes the iron coordination structure owing to a chelating effect. For example, a heptadentated [Fe(DTPA)] 2 − (DTPA: diethylenetriaminepentaacetate) shows an excellent cycle stability of 0.029% per cycle as an RFB posolyte. [2] In this study, we further improve the cycle stability of iron-based complexes using an octadentate tetraazacyclododecatetraacetate (DOTA) ligand. The target complex, K[Fe(DOTA)], was prepared by adding FeCl 3 •6H 2 O to an aqueous solution of deprotonated DOTA. Single-crystal X-ray structural analysis reveals an eight-coordinated structure of the iron complex. Cyclic voltammetry shows a redox peak at 0.37 V (vs. SHE) for K[Fe(DOTA)], which is higher by 0.088 V than that for K 2 [Fe(DTPA)] and by 0.12 V than that for K[Fe(EDTA)] (Figure 1). Levich plots obtained by rotating desk electrode measurements reveal the diffusion coefficient of 9.9×10 −7 cm 2 s −1 and the electron transfer rate constant of 5.0×10 −3 cm s −1 for K[Fe(DOTA)] , which are comparable to those of K 2 [Fe(DTPA)]. UV-Vis spectroscopy shows the aqueous solubility of 0.57 mol L −1 for K[Fe(DOTA)], which is lower than that of K 2 [Fe(DTPA)] (1.3 mol L −1 ). The low solubility of K[Fe(DOTA)] should arise from large lattice energy owing to its symmetric structure. Charge/discharge measurements using an H-type cell with 0.10 mol L −1 KCl aqueous solution reveal that K[Fe(DOTA)] exhibits a greater capacity retention rate of 99.973% per cycle than K 2 [Fe(DTPA)] (99.333%/cycle) (Figure 2). Reference: [1] H. J. Schugar, C. Walling, R. B. Jones, H. B. Gray, J. Am. Chem. Soc. 1967 , 89 , 3712. [2] S. E. Waters, B. H. Robb, M. P. Marshak, ACS Energy Lett. 2020 , 5 , 1758. Figure 1