Aqueous redox flow battery using iron 2,2-bis(hydroxymethyl)-2,2′,2′-nitrilotriethanol complex and ferrocyanide as newly developed redox couple

An all-iron aqueous redox flow battery using iron (Fe) 2,2-bis(hydroxymethyl)-2,2′,2′-nitrilotriethanol (BIS-TRIS) complex (Fe(BIS-TRIS)) and Ferrocyanide (Fe[CN]₆) as redox couple is newly suggested. The redox potential of Fe(BIS-TRIS) is −1.11 V (vs Ag/AgCl) and this makes Fe(BIS-TRIS) appropriate as active material for anolyte, while Fe(CN)₆ is proper for catholyte due to its excellent redox reactivity, redox potential, and cheap cost. According to quantitative evaluations, Fe(BIS-TRIS) does not produce any side reactions and is more stable than Fe triethanolamine (TEA) (Fe(TEA)) complex that is conventionally considered for the purpose. This fact is confirmed by computational analysis using density functional theory. In the calculation, energy barrier of Fe(BIS-TRIS) suppressing the occurrence of undesirable side reactions is higher than that of other Fe-ligand complexes, indicating that desirable redox reaction of Fe(BIS-TRIS) occurs more stably. In redox flow battery (RFB) tests, RFBs using Fe(BIS-TRIS) do not show any side reactions even after 250 cycles with excellent performances, such as capacity of 11.7 Ah L⁻¹ and coulombic efficiency and capacity retention rate of 99.8 and 99.9%, respectively. This corroborates that RFBs using Fe(BIS-TRIS) have excellency in both performance and stability, while the cheap cost of BIS-TRIS and Fe(CN)₆ enhances the economic benefit of RFBs.