XAS analysis of electrolytes for copper-based redox flow batteries: unraveling the Cu(I) speciation and the role of supporting electrolytes

Redox flow batteries (RFB) have gained significance due to their capacity to store MWh of energy independently from the power. In this scenario, Cu-based RFB (CuRFB) takes advantage of the copper European supply chain. A high Cl- concentration stabilizes copper-chloride complexes, that is fundamental due to Cu(I) inherent instability in an aqueous media. On the spectroscopic side, this study aims to clarify univocally the coordination structure in acidic condition at different potentials of the Cu(I), never reported to our knowledge; on the battery science side, it aims to clarify the role of the copper speciation on the kinetics of the electrochemical reactions and the effect of a supporting electrolyte. The speciation and kinetic parameters are expected to depend to the State of Charge (SoC) of the CuRFB. These results can be achieved studying the copper coordination number and distances by ex situ and operando investigation of the Cu K-edge with XANES and EXAFS.