Electrochemistry of Iron(II/III)‐N,N'‐ethylene‐bis‐(o‐hydroxyphenylglycine) Complexes in Aqueous Solution Indicates Potential for Use in Redox Flow Batteries

Abstract The iron complexes of the two diastereomers of N,N′‐ethylene‐bis‐(o‐hydroxyphenylglycine) (EDDHA), racemic (R,R−S,S) and meso (R,S−S,R) were isolated from commercial Fe‐EDDHA fertilizer. Cyclic voltammetry in aqueous solution in the pH range between 4.0 and 11.5 indicated reversible behaviour at both mercury and glassy carbon electrodes and quasi reversible characteristics at a copper electrode. An E 1/2 value of −642 mV (racemic diastereomer) and −615 mV (meso form) vs. Ag/AgCl 1 M NaCl reference electrode was observed at pH 9. In unbuffered solutions the complex of the racemic diastereomer shows almost pH independent redox behaviour over the whole pH range studied. The iron complex of the meso isomer hydrolyses at pH values above 9. Diffusion coefficients of the complex species were determined from the voltammograms for the meso isomer and the racemic form with 5.4 10 −6 cm 2 s −1 and 5.8 10 −6 cm 2 s −1 respectively. Bulk electrolysis experiments at pH 9 demonstrated the chemical stability of both the Fe II ‐ and the Fe III ‐form of the complex with racemic EDDHA. Good cycling stability and current efficiency were obtained in subsequent charge/discharge experiments using a carbon felt electrode. The redox properties and the stability of Fe II/III ‐EDDHA complexes make them interesting candidates for future use as negative redox electrolytes in redox flow batteries (RFB).