Controlled-Fabrication of Sandwich-Structured NiO/Ni/NiO Nanotube Arrays for Energy Storage

An experimental study is presented on designing and electrochemical fabrication of sandwich-structured NiO/Ni/NiO nanotube arrays for suppercapacitor applications. The electrochemical performance of different NiO based electrodes with different values of specific capacitance has been reported, including NiO film (∼309 F g⁻¹)[1], NiO nanosheets (∼600 F g⁻¹)[2], NiO nanotubes (∼266 F g⁻¹)[3] and NiO-TiO₂ based nanotubes (∼300 F g⁻¹)[4]. These reported pseudocapacitors are still far from the theoretical value of NiO based capacitance, which is largely attributed to the poor electronic conduction. To overcome this difficulty, we designed and fabricated novel sandwich-structured NiO/Ni/NiO nanotube arrays as pseudocapacitor electrodes on conductive substrates. Fabrication of these nanotube arrays starts with the growth of ZnO nanorods, which then act as sacrificial template. The Ni nanotube arrays subsequently are synthesized by electrodeposition of a Ni layer onto the surface of ZnO nanorods, followed by dissolving the sacrificial template in sodium hydroxide aqueous solution. The final sandwich-structured NiO/Ni/NiO nanotube arrays are then formed by annealing of the Ni nanotubes at temperature of 450°C in air. The aspect ratio of the Ni nanotubes is conformable with the morphology of the ZnO nanorods template and their wall-thickness is determined by the electrodeposition. The thickness of the NiO layer can be further controlled by adjusting the processing parameters of the annealing process (i.e., time and temperature). The results of electrochemical tests, that is, the cyclic voltammetry and galvanostatic charge-discharge cycling measurements, show that the NiO/Ni/NiO sandwich-like nanotubes electrode clearly displays pseudo-capacitive behavior with improved capacitive performance. Experimental data suggest that the metal Ni core in the NiO/Ni/NiO sandwich-like nanotube structures uniquely serves as a channel for high electron transfer rate from current collector to support the rapid redox reaction activated in the bilateral NiO shells with a higher electrolytic accessible surface area, which is responsible for the high performance of energy storage.