Copper Chloro-Complexes Concentrated Solutions: An Electrochemical Study

Abstract

Basic studies on concentrated solutions are becoming more and more important due to the practical industrial and geological applications. The use in redox flow batteries is one of the most important applications of these solutions. Specifically, in this paper we investigated high-concentrated copper chloro-complexes solutions with different additives. The concentration of ligands and additives affects the physicochemical and electrochemical properties of 2 M solutions of Cu(I) and Cu(II). Solutions with calcium chloride and HCl as Cl− source were investigated with Cu:Cl ratios of 1:5 and 1:7, the 1:5 Cu:Cl ratio being the best performing. The substitution of calcium chloride with ammonium chloride increased the conductivity. However, while the effect on the positive electrode process was not very evident, the reversibility of the copper deposition–stripping process was greatly improved. Orthophosphoric acid could be a viable additive to decrease the complexation of calcium with chloride anions and to improve the stability of Cu(II) chloro-complexes. Absorption spectroscopy demonstrated that phosphate ions do not coordinate copper(II) but lead to a shift in the distribution of copper chloro-complexes toward more coordinated species. Electrochemically, the increased availability of chloride anions in solution stabilized the Cu(II)-rich solution and led to increased reversibility of the Cu(II)/Cu(I) redox process.

A spectroelectrochemical study of copper chloro-complexes for high performance all-copper redox flow batteries

“…However, the increase of the copper concentration, needed to RFB for delivering high energy, brings a tremendous increase of the ionic strength of the solution to maintain a suitable copper-to-halide ratio. In these conditions, the interactions between ions are stronger, and the decrease of water activity highly affects the physicochemical parameters like viscosity and density, as well as the conductivity and the diffusion coefficient [41,42].…”