Co-doped Ni hydroxide and oxide nanosheet networks: laser-assisted synthesis, effective doping, and ultrahigh pseudocapacitor performance

Abstract

Morphology control and impurity doping are two widely applied strategies to improve the electrochemical performance of nanomaterials. Herein, we report an environmentally friendly approach to obtain Co-doped Ni(OH)₂ nanosheet networks using a laser-induced cobalt colloid as a doping precursor followed by an aging treatment in a hybrid medium of nickel ions. The shape and specific surface area of the doped Ni(OH)₂ can be successfully adjusted by changing the concentration of sodium thiosulfate. Furthermore, a Co-doped Ni(OH)₂ nanosheet network was further converted into Co-doped NiO with its pristine morphology retained via facile thermal decomposition in air. The structure and electrochemical performance of the as-prepared samples are investigated with scanning and transmission electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, Fourier transform infrared spectroscopy, the nitrogen adsorption–desorption isotherm technique, and electrochemical measurements. The Co-doped Ni(OH)₂ electrode shows an ultrahigh specific capacitance of 1421 F g⁻¹ at a current density of 6 A g⁻¹, and a good retention level of 76% after 1000 cycles, in sharp contrast with only a 47% retention level of the pure Ni(OH)₂ electrode at the same current density. In addition, the Co-doped NiO electrode exhibits a capacitance of 720 F g⁻¹ at 6 A g⁻¹ and 92% retention after 1000 cycles, which is also superior to the corresponding values of relevant pure NiO electrodes. The Co²⁺ partially substitutes for Ni²⁺ in the metal hydroxide and oxide, resulting in an increase of free holes in the valence band, and, therefore, enhancement of the p-type conductivity of Ni(OH)₂ and NiO. Moreover, such novel mesoporous nanosheet network structures are also able to enlarge the electrode–electrolyte contact area and shorten the path length for ion transport. The synergetic effect of these two results is responsible for the observed ultrahigh pseudocapacitor performance.