Aqueous redox batteries including novel copper-ligand complex determined through optimization of ligand

Copper (Cu) is attractive active material for aqueous redox batteries (ARBs) due to its abundance and cheap price, although this is rarely used for this purpose previously. In this study, new water-soluble Cu-ligand complexes are synthesized based on ligand M (N,N-dimethylethylenediamine) and ligand J (N¹,N¹-dimethyl-N²-(pyridin-2-ylmethyl)ethane-1,2-diamine), and they are used as active material for ARBs. They are then identified by using spectroscopic analyses, such as ¹H NMR, ¹³C NMR, UV–Vis, and ESI-Mass and are stabilized in Cu ion to ligand ratio of 1 to 2 (Cu-M₂) and 1 to 1 (Cu-J). More specifically, difference in potential for two Cu ion redox reactions (Cu²⁺ + e⁻ ↔ Cu⁺ (RR1) and Cu⁺ + e⁻ ↔ Cu⁰ (RR2)) is larger in Cu-J complex (0.33 V) than in Cu-M₂ complex (0.1 V), meaning that the two reactions in Cu-J complex are easily controlled to avoid undesirable metallization of Cu ions occurring by RR2. This corroborates that Cu-J complex is more stable than Cu-M₂ complex for ARBs in pH 9.25. When performance of ARBs using Cu-J and ferrocyanide is measured, they show outstanding columbic efficiency of 99 %, preserving excellent stability over 200 cycles with low capacity decay rate of 0.001 Ah L⁻¹ cyc⁻¹.