The influence of reconstructive transformation from micro-/nanostructured copper(II) hydroxide to copper oxides as electrodes for high-performance supercapacitors

Copper(II) hydroxides, particularly Cu(OH)2 micro/nanostructures, are metastable under normal conditions. Notably, Cu(OH)2 micro-/nanostructures undergo structural restructuring and phase transitions during electrochemical reactions. In this study, the effects of electrolyte concentration, current, and duration of the electrochemical reaction on the crystal morphology, structure, and chemical composition of Cu(OH)2 micro-/ nanostructured composite electrodes were investigated by anodic oxidation. The results showed that a Cu(OH)2 nanorod-like structure was formed on the electrode surface in the KOH electrolyte at a concentration of 3 M, while Cu2O crystals were synthesized at electrolyte concentrations of 1 M and 6 M. The growth current of the Cu (OH)2 nanorod-like structures ranged from 30 mA to 70 mA. At a reaction time of 10-80 min, the micro-/ nanostructure of the electrode surface changed from an unstable Cu2O to a stable CuO phase through metastable Cu(OH)2. The synthesized Cu(OH)2 composite electrode exhibited an area-specific capacitance of 2777 mF cm-2 at a scanning rate of 10 mV s-1. In particular, the relationships between the surface morphology, structure, and chemical composition of the Cu(OH)2 composite electrode and the electrolyte concentration during cyclic voltammetry were studied. The results showed that in the KOH electrolyte at 1 M concentration, the Cu(OH)2 micro-/nanostructure transformed into Cu2O under electrochemical action, Cu(OH)2 transformed into both the Cu2O and CuO phases at 3 M electrolyte concentration, and Cu(OH)2 transformed into the CuO phase when the electrolyte concentration was increased to 6 M, indicating that the concentration of the alkaline electrolyte had a significant effect on the reconstruction and phase transformation of the Cu(OH)2 micro-/nanostructures. These results are expected to address the pressing issues in the application of metastable Cu(OH)2 micro/nanostructured composite electrodes in high-performance supercapacitors, such as insufficient energy, mass transfer, and structural instability.