All‐copper Flow Batteries

In this chapter, an overview of current insights of the all-copper Flow Battery (CuFB) is presented and discussed. Although early investigations in all-copper battery systems were already addressed in the mid-1970s, the first practical approach of CuFBs appeared in 2014. The system consists of a hybrid FB based on chloride-rich aqueous electrolytes, where Cu(I)/Cu(II) and Cu 0 /Cu(I) are the redox couples involved in the reactions at the positive and negative half-cells respectively, providing an open circuit voltage of 0.65 V. The composition of the supporting electrolyte strongly affects the electrochemical and physical properties of the copper species, which needs to be carefully selected to ensure the stability of cuprous species, high conductivity and low density and viscosity, enhancing reversibility, kinetics, and stability in solution. HCl and CaCl 2 , as well as mixtures thereof, have been widely investigated as supporting electrolytes providing excellent results. Also, the use of a variety of electrode materials has been proposed, attending to performance and cost criteria. Finally, low-cost polymeric separators and microporous membranes have shown great potential for their application in CuFBs. The use of non-expensive materials coupled with the excellent electrochemical performance and the high availability and recyclability of copper, place this technology well ahead in terms of cost-effectiveness and sustainability compared to other FBs. Different prototypes of the CuFB at laboratory scale have been developed so far, showing promising results. Ongoing research is focused on the optimization of the stack design and in the scale-up of these prototypes up to kW level.