Abstract
Electrostatic capacitors have high power density but low energy density. In contrast, batteries and fuel cells have high energy density but low power density. However, supercapacitors can simultaneously achieve both high power density and energy density. Herein, we propose a supercapacitor, in which etched nickel wire was used as a current collector due to its high conductivity. Two redox reactive materials, MnO2 nanoflakes and NiCo2O4 nanoneedles, were used in a hierarchical structure to cover the roughened surface of the Ni wire to maximize the effective surface area. Thus, a specific capacitance, energy density, and power density of 14.4 F/cm3, 2 mWh/cm3, and 0.1 W/cm3, respectively, was obtained via single-electrode experiments. A fiber-shaped supercapacitor was prepared by twisting two electrodes with solid electrolytes made of KOH and polyvinyl alcohol. Although the solid electrolyte had a low ionic conductivity, the energy density and power density were determined to be 0.97 mWh/cm3 and 49.8 mW/cm3, respectively.
Highlights
Supercapacitors comprise two current collectors, electrodes, and electrolytes
electrical double layer capacitors (EDLCs) are similar to electrostatic capacitors, except that the charges are stored on the electrical double layer, composed of an inner Helmholtz plane (IHP) and the diffuse layer in the bulk electrolyte
It appears that two of the strongest peaks at 2θ = 44.62◦ and 52◦ for both un-etched and etched Ni substrate were the characteristic (111) and (200) peaks of nickel, and they appeared in the X-ray powder diffraction (XRD) pattern of the NiCo2 O4 /MnO2 hierarchical nanostructure
Summary
Supercapacitors comprise two current collectors, electrodes, and electrolytes. Supercapacitors can overcome the limitations of energy density in electrostatic capacitors and of power density in Li-ion batteries [1,2,3]. Due to poor electric conductivity (10−5 –10−6 S cm−1 ), MnO2 exhibits much lower specific capacitance than the theoretical value [14] Metal foams such as nickel foam have been widely used in the formation of current collectors to increase the effective surface area. These electrodes were synthesized using materials with high redox activity and exhibited improved energy density and power density These fiber-shaped supercapacitors provided machinability, breathability, and manufacturing flexibility to woven fabrics for application in textiles [22,23,24]. The stability and flexibility of this device were shown to be superior, but the device exhibited a moderate specific capacitance of 1.55 F cm−2 , as the energy density and power density were limited by the small surface area of the electrode and the poor conductivity of the carbon fiber current collector. The performance of the NiCo2 O4 /MnO2 hierarchical nanostructured electrode (HNE) and its super-capacitive performance was thoroughly investigated, and its performance was attributed to a large increase in surface area and the excellent conductivity of the etched Ni current collector
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