Highly stable aqueous all‑iron alkaline flow battery based on a 6-coordinate Fe(THEED) as active material for anolyte

A stable 6-coordinate Fe III compound K[Fe III (THEED)] (H 4 THEED = N,N,N′,N′ -tetrakis(2-hydroxyethyl)ethylenediamine) was used as the anodic active material for all‑iron alkaline redox flow battery (ARFB), which shows a redox potential of E 1/2 = −0.93 V ( vs. Ag/AgCl). When it was paired with K 4 [Fe(CN) 6 ] as the cathodic counterpart, it gives a cell voltage of ∼1.3 V. Fe(THEED) exhibits the enhanced resistance to metallic iron deposition as compared with the 5-coordinate Fe-triethanolamine complex Fe(TEOA), mainly attributed to its saturated 6-coordinate structure with a large stabilization constant that effectively inhibits various side reactions. The cell performance of ARFB can be significantly improved by using Fe(THEED) as the negative active material. The ARFB containing 0.5 M Fe(THEED) and Fe(CN) 6 electrolytes demonstrated a discharge capacity of 14.59 Ah L −1 at a current density of 80 mA cm −2 , with a coulombic efficiency (CE) of 99.8 %, and an energy efficiency (EE) of 79.9 %. The peak power density can reach 194 mW cm −2 at a current density of 240 mA cm −2 . The system can exhibit exceptional cycling stability with 99.9 % capacity retention over 100 cycles (∼36 h). The low cost of Fe-THEED and Fe(CN) 6 components increases the economic viability, as compared to conventional V-based and previously reported Fe-based systems. • The solubility of Fe(THEED) can reach 1.12 M, which is 10 times higher than the original estimation. • 6-coordinate Fe(THEED) exhibit the excellent stability in long-term cycling. • The ARFB of 0.5 M Fe(CN) 6 /Fe(THEED) demonstrate the exceptional performance with an EE of 79.9% and a CE of 99.8%. • The initial capacity retained 99.9% after 100 cycles with a decay rate of 0.0001% per cycle.