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 E1/2 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 cm2 s−1 and 5.8 10−6 cm2 s−1 respectively. Bulk electrolysis experiments at pH 9 demonstrated the chemical stability of both the FeII‐ and the FeIII‐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 FeII/III‐EDDHA complexes make them interesting candidates for future use as negative redox electrolytes in redox flow batteries (RFB).

2‐Methoxyhydroquinone from Vanillin for Aqueous Redox‐Flow Batteries

“…Currently, non-sustainable large scale storage technologies involve redox flow batteries (RFB). [5] Their core is the electrolyte, which nowadays is mainly based on mined sources (such as vanadium, bromine), [6][7][8] with significant environmental impact. Storage capacity of RFB is directly linked to the size of the tanks filled with electrolytes.…”

2‐Methoxyhydroquinone from Vanillin for Aqueous Redox‐Flow Batteries

“…Currently, non‐sustainable large scale storage technologies involve redox flow batteries (RFB) [5] . Their core is the electrolyte, which nowadays is mainly based on mined sources (such as vanadium, bromine), [6–8] with significant environmental impact. Storage capacity of RFB is directly linked to the size of the tanks filled with electrolytes.…”

Activation of carbon tow electrodes for use in iron aqueous redox systems for electrochemical applications

“…33 In this work we present a study of desizing and activation methods applied to CF rovings for use in electrochemical applications, such as redox flow cells. The recently described nontoxic, aqueous, all-iron redox flow system 34,35 The isolation of the Na[Fe III -racEDDHA] complex was performed according to the procedure published in the literature. 35 In brief, approx.…”

“…The recently described nontoxic, aqueous, all-iron redox flow system 34,35 The isolation of the Na[Fe III -racEDDHA] complex was performed according to the procedure published in the literature. 35 In brief, approx. 500 ml hot ethanol extract of crude fertilizer was evaporated to dryness.…”

“…The purity of 95.6% wt. was determined via photometry from the absorbance of an aqueous solution and the molar absorptivity 35 at 480 nm (ε = 5026 L mol -1 cm -1 ) using Beer´s law.…”