Three-dimensional Co₃O₄@C@Ni₃S₂ sandwich-structured nanoneedle arrays: towards high-performance flexible all-solid-state asymmetric supercapacitors

Abstract

In this paper, we report the design and fabrication of a novel hierarchical Co 3 O 4 @C@Ni 3 S 2 sandwich-structured nanoneedle arrays (NNAs) electrode for supercapacitor application. The supercapacitor performance based on the Co 3 O 4 @C@Ni 3 S 2 NNAs electrodes are investigated in detail. A lightweight and flexible asymmetric supercapacitor (ASCs) is successfully fabricated using the Co 3 O 4 @C@Ni 3 S 2 NNAs as the positive electrode and activated carbon (AC) as the negative electrode, which delivers an output voltage of 1.8 V, and high energy/power density (1.52 mWh cm -3 at 6 W cm -3 and 0.920 mWh cm -3 at 60 W cm -3 ), as well as remarkable cycling stability (~91.43% capacitance retention after 10000 cycles), owing to the unique 3D porous sandwich-structured nanoneedle array architecture and a rational combination of the three electrochemically active materials. As a result, the ternary hybrid architectural design demonstrated in this study provides a new approach to fabricate high-performance metal oxides/sulfides composites nanostructure arrays for next-generation energy storage devices.Co 3 O 4 @Ni 3 S 2 NNAs electrodes are also shown in Figure S7a-d, respectively. These charging and discharging curves of the ternary electrode are highly symmetric and show fairly linear slopes between -0.05 and 0.45 V, arising from the ideal capacitance behavior and fast Faraday reaction. The current density dependence of the areal capacitance for the Ni 3 S 2 nanoflakes, Co 3 O 4 NNAs, Co 3 O 4 @C NNAs and Co 3 O 4 @Ni 3 S 2 NNAs electrodes are compared in Figure 6e. It can be found that the discharge areal capacitance of the Co 3 O 4 @C@Ni 3 S 2 NNAs electrode is 3.564 F cm -2 at 1 mA cm -2 , which is nearly three times as that of the Co 3 O 4 NNAs (1.202 F cm -2 ) and about seven times as that of Ni 3 S 2 nanoflakes (0.546 F cm -2 ), respectively.The intrinsic electrochemical and kinetic mechanism of the hybrid electrode was further analyzed by the electrochemical impedance spectroscopic (EIS) studies. Figure 6f illustrates the Nyquist plots of EIS spectra of all electrodes in the frequency range from 10 kHz to 0.01Hz. An equivalent circuit (inset in Figure S8b), including the ohmic resistance (R e ), the charge transfer resistance (R ct ), the Warburg impedance (Z w ), the double-layer capacitance (CPE), has been adopted to simulate the experimental data. In the low-frequency area, the inclined line represents the Warburg (W) impedance corresponding to the electrolyte diffusion in porous electrode and proton diffusion in host materials. [46,47] The Co 3 O 4 @ C@Ni 3 S 2 NNAs electrode has the largest slope in all electrodes, indicating its best capacitive performance with lower diffusion resistance. This can be attributed to the decoration of Ni 3 S 2 nanoflakes with large surface area and thus facilitates the supply of OHto the entrance of Co 3 O 4 nanopores and nickel foam microspores. In high frequency area, the intercept to x axis represents the bulk resistance of the electrochemical system (R e ), and the sem...

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