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

• Electrochemical behavior of electrolytes with different cu-cl complexes distributions. • Hysteresis in voltabsorbometries linked to the cu-cl complexes stability. • Quantum-chemical calculations get insights into the structure of the complexes. • Highly coordinated cu-cl complexes recommended in concentrated electrolytes. Redox Flow Batteries (RFB) are an ideal choice for large stationary applications. Among the different chemistries that can be exploited, all-copper aqueous RFB (CuRFB) use low-cost, earth-abundant raw materials with a well-defined European supply chain. The CuRFB takes advantage of the three oxidation states of copper. As Cu(I) is not stable in aqueous media, the system is based on the chlorocomplexation of the copper cations. We demonstrated that it is possible to evaluate the complexation characteristic of the concentrated solutions used in CuRFB by investigating the speciation of copper (II) in electrolytes with increasing Cu(II) concentration. Spectro-electrochemical tests in diluted solution give information on the electrochemical behavior of electrolytes with a different chloro‑complexes distribution. Quantum chemical calculations elucidate the molecular structure and electronic transitions of water solvated copper chloro‑complexes, thus complementing the experimental picture. [ABSTRACT FROM AUTHOR]