Deciphering the Thermal and Electrochemical Behaviors of Dual Redox-Active Iron Croconate Violet Coordination Complexes

Abstract

Interest in coordination compounds based on non-innocent ligands (NILs) for electrochemical energy storage have raised in the last years. We have focused our attention on an overlooked redox active linker, croconate violet, which has not yet been addressed in this field although closely related to standard NILs such as catecholate and tetracyanoquinodimethane (TCNQ). Two anionic complexes consisting of Fe(II) and croconate violet (-2) with balancing potassium cations were isolated and structurally characterized. By a combination on in and ex situ techniques (powder and single crystal X-ray diffraction, infrared and 57 Fe Mössbauer spectroscopies), we have shown that their dehydration occurs through complex patterns, whose reversibility depends on the initial crystal structure, but that the structural rearrangements around the iron cations occur without any oxidation. While electrochemical studies performed in solution clearly shows that both the organic and inorganic parts can be reversibly addressed, in the solid state poor charge storage capacities were initially measured, mainly due to the solubilization of the solids in the electrolyte. By optimizing the formulation of the electrode and the composition of the electrolyte, a capacity of > 100 mAh g -1 after 10 cycles could be achieved. This suggests that this family of redox active linkers deserve to be investigated for solid state electrochemical energy storage, although it requires to solve the issues related to the solubilization of the derived coordination compounds.