Electrochemical Characterization of Stable Cu(II)/Cu(I) Electrolytes for Redox Flow Battery
Authors: Morais William Gomes , Groiss Emanuele Maria , La Valle Valentina , Macchi Edoardo Gino 
Published: February 2024

The path to reach carbon neutrality cannot be paved without further development of long life and low-cost batteries, considering also cyclability, calendar life, and round-trip efficiency. In this context, Redox Flow Batteries (RFB) have gathered researchers’ attention as possible candidates to play a major role in the incoming of the next generation batteries. Their unique capability to decouple power and energy makes possible of have flexible modular design and operation, along with outstanding scalability, plus moderate maintenance costs and long-life cycling. [1]
Although vanadium based redox flow batteries (VRFB) have been the most studied and commercialized RFB system, in the past years concern has been raised due to vanadium toxicity, low availability and high costs. On the other hand, copper redox flow batteries (CuRFB) have the possibility of using different membranes than expensive perfluorinated ones, furthermore, copper has more competitive costs and a well-consolidated value chain in Europe when compared to vanadium. [2]
In this study, the effect of the copper-chloride complexes used to stabilize electrolytes for CuRFB was investigated, as well as the influence of secondary cations from the supporting electrolyte, e.g., Ca2+, K+, H+. Different solutions with Cu:Cl ratio varying from 1:4 to 1:9 were characterized by physical-chemical and electrochemical methods. It was observed that the chloride complexation efficiently stabilized cuprous cations, at room temperature (Figure 1), avoiding metal Cu to be formed. Obtained diffusion coefficients (D0
) for Cu2+ and Cu+ showed that mobility of the species is influenced by the concentration of the copper itself, and also by Cl- and the secondary cation concentration. Solutions with 2M of Cu2+ shown higher values of D0
compared to 1M, though with reduced reversibility.
Highly concentrated CaCl2 solutions, 1 to 4 M, were used as Cl- is required to form stable [CuClx]2-x complexes, however high Ca2+ concentration might compete with copper species to form [CaClx]2-x, which limits the availability of complexing ligand for Cu+. [3] HCl was then used as additive to enhance the conductivity of the solution while it allows to reduce the CaCl2 concentration by keeping the same molar proportion. Plus, it promoted higher mobility of copper species, with and values about ten times higher than without acid.
Finally, further discussions will be conducted with the aid of electrochemical parameters such as kinetic and exchange currents, along with the heterogenous rate constant and the Nicholson parameter, besides conductivity and viscosity of the solutions, to evaluate ligand composition effect on the Cu2+/Cu+ reaction kinetics and overall battery performance.
Acknowledgements
This work is funded by the Italian Ministry of Enterprises and Made in Italy in the framework of the Important Project of Common European Interest (IPCEI) European Battery Innovation (project IPCEI Batterie 2 - CUP: B62C22000010001). The IPCEI European Battery Innovation is also funded by public authorities from Austria, Belgium, Croatia, Finland, France, Germany, Greece, Poland, Slovakia, Spain and Sweden.
References
[1] E. Sánchez-Díez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, Redox flow batteries: Status and perspective towards sustainable stationary energy storage, J Power Sources. 481 (2021). https://doi.org/10.1016/j.jpowsour.2020.228804.
[2] G. Lacarbonara, L. Faggiano, S. Porcu, P.C. Ricci, S. Rapino, D.P. Casey, J.F. Rohan, C. Arbizzani, Copper chloro-complexes concentrated solutions: An electrochemical study, Batteries. 7 (2021). https://doi.org/10.3390/batteries7040083.
[3] L. Sanz, D. Lloyd, E. Magdalena, J. Palma, M. Anderson, K. Kontturi, Study and characterization of positive electrolytes for application in the aqueous all-copper redox flow battery, J Power Sources. 278 (2015) 175–182. https://doi.org/10.1016/j.jpowsour.2014.12.034.

Figure 1