Exploring the electrochemical properties of mixed ligand Fe(II) complexes as redox couples

Abstract

Redox flow batteries are a promising technology for large scale energy storage systems. Although some types of the battery have been commercialized, the narrow working voltage ranges in the aqueous electrolytes limit their further applications. Organic solvents have shown to allow the use of a wider range of redox couples and the idea of non-aqueous redox flow batteries has been proposed. However, the low solubilities and poor electrochemical properties including stability and reversibility of the redox couples in the organic electrolytes remain a significant issue. In this work, a series of heteroleptic (mixed ligand) iron(II) complexes based on a combination of 2,2′-bipyridine, 1,10-phenanthroline and 4,4′-dimethyl-2,2′-bipyridine ligands were synthesized and their chemical and electrochemical properties investigated. For Fe (II) complexes, it was confirmed that ligand equilibration occurs in solution. The electrochemical properties of the complexes were studied by cyclic voltammogram and chronoamperometric methods. The mechanism of the redox processes of these complexes are proposed and how the mixed ligands will affect the redox potentials of the Fe (II) complexes is discussed. Furthermore, we show that the mixed ligand system can improve the total solubility of redox active species. The mixed ligand approach will also be beneficial for designing other new high solubility redox materials for non- aqueous flow batteries.