Investigation of Fe(II) Complexes with 1,10-Phenanthroline and 2,2′;6′,2“-Terpyridine for Aqueous Flow Battery Applications

Iron(II) complexes with 1,10-phenanthroline (phen) and 2,2′;6′,2“-terpyridine (terpy) ligands bearing different functional groups (methyl, 4-pyridyl, chloro, carboxylic acid) were evaluated for aqueous flow battery applications, detecting oxidation processes followed by coupled chemical reactions. Redox potentials of these compounds were sufficiently high for suitable positive electrolytes (0.88–1.29 V vs. SHE). Randles-Ševčík equation and finite element modelling with COMSOL Multiphysics were utilized in evaluating the diffusion coefficient and the apparent rates of the electron transfer and coupled chemical reactions for the compounds studied by cyclic voltammetry. The systems experience weak adsorption of reactants at glassy carbon, leading to difficulties in determining the latter kinetic parameters. Flow battery tests indicate sufficient flow battery performance with dimethyl functionalized phenanthroline complex [Fe(II)(DMe-phen)₃]²⁺ with 0.06 % per cycle (2.78 % per day) capacity decay. However, [Fe(II)(DMe-phen)₃]²⁺, as well as [Fe(II)(phen)₃]²⁺, experience the discharge at two different thermodynamic conditions, suggesting dimer discharge as the source of the lower voltage plateau. The energy efficiency of [Fe(II)(DMe-phen)₃]²⁺ battery was improved by cycling at higher cut-off voltage for 10 cycles, after which the lost capacity was recovered with lower cut-off voltage in one cycle. [Fe(II)(terpy)₂]²⁺ had too many side reactions at lower potentials to be suitable for flow battery applications.