Abstract
In this work, we report surface-modified nickel (Ni) wire/NiCo2O4/reduced graphene oxide (Ni/NCO/RGO) electrodes fabricated by a combination of facile solvothermal and hydrothermal deposition methods for wire-shaped supercapacitor application. The effect of Ni wire etching on the microstructural, surface morphological and electrochemical properties of Ni/NCO/RGO electrodes was investigated in detail. On account of the improved hybrid nanostructure and the synergistic effect between spinel-NiCo2O4 hollow microspheres and RGO nanoflakes, the electrode obtained from Ni wire etched for 10 min, i.e., Ni10/NCO/RGO exhibits the lowest initial equivalent resistance (1.68 Ω), and displays a good rate capability with a volumetric capacitance (2.64 F/cm3) and areal capacitance (25.3 mF/cm2). Additionally, the volumetric specific capacitance calculated by considering only active material volume was found to be as high as 253 F/cm3. It is revealed that the diffusion-controlled process related to faradaic volume processes (battery type) contributed significantly to the surface-controlled process of the Ni10/NCO/RGO electrode compared to other electrodes that led to the optimum electrochemical performance. Furthermore, the wire-shaped supercapacitor (WSC) was fabricated by assembling two optimum electrodes in-twisted structure with gel electrolyte and the device exhibited 10 μWh/cm3 (54 mWh/kg) energy density and 4.95 mW/cm3 (27 W/kg) power density at 200 μA. Finally, the repeatability, flexibility, and scalability of WSCs were successfully demonstrated at various device lengths and bending angles.
Highlights
As one kind of significant energy storage device, supercapacitors (SCs) can function at a high charge/discharge rate over a large number of cycles, covering the gap amongst high energy batteries and high power conventional electrostatic capacitors [1,2]
We report the successful fabrication of high-performance flexible twisted wire-shaped supercapacitor (WSC) based on Ni wires coated with NiCo2 O4 (NCO) microspheres and reduced graphene oxide (RGO) nanoflakes using simple and inexpensive direct solvothermal and hydrothermal growth methods, respectively
Results revealed that the microstructure, surface morphology, and electrochemical performance of the Ni/NiCo2 O4 /RGO
Summary
As one kind of significant energy storage device, supercapacitors (SCs) can function at a high charge/discharge rate over a large number of cycles, covering the gap amongst high energy batteries and high power conventional electrostatic capacitors [1,2]. The reduced graphene oxide (RGO) has been widely applied as a support for metal oxides in electrochemical electrode materials because of its large surface area, high conductivity, high chemical stability, and excellent mechanical flexibility [17,18,19,20] Combining both these materials as hybrid nanostructures for the fabrication of the electrodes for SCs becomes indispensable given the electrochemical performance improvement due to their synergistic effects. Considerable research has been conducted into flexible SCs, to the best of our knowledge, hitherto there are no reports on the enhancement of electrochemical performance of symmetric WSCs with electrodes fabricated by facile direct solvothermal and hydrothermal growth of electroactive NiCo2 O4 /RGO hybrid nanostructures onto surface modified Ni wires as a current collector. The device shows highly stable electrochemical characteristics under different deformations such as bending and winding
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