Experimental evaluation of copper redox couples in aqueous and aprotic electrolytes for their application in a flow battery

The present manuscript deals with research and study of electrolytes and electrode materials to be contemplated in an all-copper-based redox flow battery (RFB). In the first instance, process reversibility and half-wave potentials of copper redox pairs were evaluated using NO3- ligands in addition to commonly used Cl- ligands provided by CuCl2 salts in these kinds of batteries, proposing its use in aqueous and aprotic (MeCN) electrolytes. Cyclic voltammetry techniques were used to evaluate the electrochemical response and the half-wave potential in different electrolytes employed, while the electrochemical reductions were studied by in-situ spectrophotometric techniques, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM), in electrolytes with low copper concentrations (0.02 M) to avoid interferences in such spectrometric techniques. For the aqueous electrolyte, electrochemical techniques studies demonstrate that the use of massive copper electrodes in this kind of system should be avoided at the first instance since electrochemical responses evidence quasi-reversible behavior. Potential-current curves show good reversibility of redox couples when a graphite electrode was used, and deposits with well-defined and uniform geometries are visualized in SEM over the entire surface of the working electrode. Nevertheless, UV-vis tests exhibit a low stabilization of Cu+ species and calculated half-wave potentials suggested that the cell potential reached in the RFB system will be low employing both Cl- and NO3- ligands. For the aprotic electrolytes, the calculation of half-wave potentials indicates that those systems are adequate to increase the expected cell potential for charge-discharge reactions, moreover, reversible behavior is improved when the NO3- ligand is employed. This fact was confirmed by the performing of EIS, in situ UV-vis spectrophotometric, and SEM characterization, where the reversibility of Cu2+/Cu+ and Cu+/Cu-0 redox couples was evidenced.