Controllable hydrothermal synthesis of Cu-doped δ-MnO2 films with different morphologies for energy storage and conversion using supercapacitors

The δ-MnO2 and Cu-doped δ-MnO2 grown on Ni foams are synthesized by a simple hydrothermal method for supercapacitor electrode application. The samples are characterized by XRD, ICP-AES, SEM, and BET for its composition, structure/morphology, and BET surface area. The electrochemical properties of the electrodes are studied by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) in 1 M Na2SO4 solution. The results show that copper addition can vary the self-assembly of δ-MnO2 nanosheets, inducing the formation of flower-like structure. The morphology and electrochemical performance of the formed δ-MnO2 electrodes can be controlled by simply tuning the copper doping concentration. The 2 at.% Cu-doped δ-MnO2 film obtains the maximum specific capacitance as high as 296 F g−1 at 1 A g−1, which is 80% higher than that of the pure δ-MnO2 film. Furthermore, it also shows a better cyclic stability than undoped δ-MnO2 film. The enhanced electrochemical properties may be attributed to its optimized hybrid nanosheet network and flow-like structure. A possible explanation for such enhancement in electrochemical properties of Cu-doped δ-MnO2 films has been discussed in this paper