The optical images of the Ni foam and the annealed NiO nanosheets are shown in Figure 2: from left to right: Ni foam, as-grown nickel hydroxide, and NiO nanosheets (Figure 2a). Figure 2b,c shows the optical images for the flexible electrode material, which can withstand strain relaxation and mechanical deformation. Figure 3a shows a low-magnification field emission scanning electron microscopy (FESEM) image of the nickel foam substrate. Figure 3b is the FESEM image of the NiO nanosheets supported on the nickel foam substrate. As can be seen, the nanosheets are uniformly grown on Ni foam. To further reveal its microstructure, Figure 3c shows a high-magnification FESEM image of the NiO nanosheet with a lateral size of several hundred nanometers and a thickness of several nanometers. This characteristic will benefit electron transmission. Figure 3d shows the FESEM image of the NiO nanosheets after charging/discharging for 3,000 cycles at a current density of 11.8 mA cm-2. As can be seen, the morphology of the nanosheets is retained well.
Figure 2Optical images. (a) From left to right: optical photos of the Ni foam, as-grown nickel hydroxide, and NiO nanosheets. (b, c) Optical images for the flexible electrode material.
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Figure 3FESEM images of Ni foam and NiO nanosheets. (a) Low-magnification FESEM image of the nickel foam. (b, c) Low- and high-magnification FESEM images of the NiO nanosheets. (d) FESEM images of the NiO nanosheets after charging/discharging for 3,000 cycles.
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The NiO nanosheets are further characterized by using X-ray diffraction and Raman spectroscopy. Typical XRD patterns of the annealed NiO nanosheets are shown in Figure 4. All of the reflections in the XRD pattern can be indexed to face-centered cubic phase NiO (JCPDS card no. #47-1049). The three characteristic peaks at 37.2°, 43.2°, and 62.8° correspond to the (111), (200), and (220) diffraction planes, respectively. The high peak intensity indicates that the NiO nanosheets are of high crystallinity. No peaks from other phases are detected, indicating that the product is of high purity. Moreover, no peaks from the Ni substrate are detected, suggesting that the NiO nanosheets are uniformly grown upon the Ni foam surface. The average crystallite size calculated using the Scherrer equation based on the half-width of the (200) peak is about 14.3 nm. Additional file 1: Figure S1 shows the Raman spectrum of NiO. Three Raman peaks located at about 436, 519, and 1,154 cm-1 are observed in the spectra, corresponding to the shaking peaks of NiO. The former two peaks could be attributed to the first-order transverse optical (1TO) vibration mode and longitudinal optical (1LO) phonon modes of NiO, respectively. The peaks at 1,154 cm-1 could be assigned to two-phonon (2P) 2LO modes of NiO. Such a result further confirms that the crystalline structure of the NiO nanostructure has been obtained.
Figure 4XRD pattern of NiO nanosheets.
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The TGA curve of the Ni(OH)2 nanosheets is shown in Figure 5. The initial weight loss of about 4% between 20°C and 200°C is due to the removal of physically adsorbed water molecules. The rapid weight loss of about 34% between 200°C and 450°C is due to the removal of the crystalline water molecules and the decomposition of Ni(OH)2. Beyond 500°C, all the intercalated water molecules and NiO are formed as the final product. Moreover, there is no obvious weight loss when the temperature is higher than 500°C, and hence, it can be concluded that all Ni(OH)2 was converted into NiO.
Figure 5TGA curve of Ni(OH) 2 precursor from 20°C to 600°C.
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To highlight the merits of the unique architecture, we directly apply the hybrid structure of ultrathin NiO nanosheets as an electrode for supercapacitors. Figure 6a shows the cyclic voltammetry (CV) curves of the NiO nanosheet electrode with various sweep rates ranging from 2 to 50 mV s-1. A distinct pair of current peaks can be identified during the cathodic and anodic sweeps, whose intensity increases with the scan rate. It can be attributed to the following reversible redox reaction:
Figure 6Electrochemical characterizations of the NiO nanosheets on Ni foam as electrodes for supercapacitors. (a) CV curves at various scan rates ranging from 2 to 50 mV s-1. (b) Average specific capacitance at various scan rates. (c) Charging/discharging voltage curves at various current densities ranging from 5 to 25 A g-1. (d) Specific capacitance of NiO nanosheets at various discharge current densities.
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NiO+ OH - ↔NiOOH+ e -
(1)
suggesting the pseudocapacitive characteristic of the NiO nanosheets. A pair of redox peaks is located at around 0.125 and 0.281 V with the scan rate of 2 mV s-1. With the 25-fold increase in the sweep rate from 2 to 50 mV s-1, the position of the cathodic peak shifts slightly from 0.125 to 0.047 V. This observation suggests a relatively low resistance of the electrode because of the good contact between the electroactive NiO nanosheets and the conductive Ni foam substrate. The specific capacitance of the electrode can be calculated from the CV curves according to the following equation [21]:
C=∫ I dV/ υmV
(2)
where C is the specific capacitance (F g-1) based on the mass of the electroactive materials, I is the response current density (A), V is the potential (V), v is the potential scan rate (mV s-1), and m is the mass of the electroactive materials in the electrodes (g). Based on these CV curves, the specific capacitance of the sample can be calculated to be 817.6, 602.3, 438.4, 367.6, and 254.3 F g-1 at the scan rates of 2, 5, 10, 20, and 50 mV s-1, respectively (Figure 6b). The charging/discharging measurements are carried out in a 3 M KOH electrolyte at various current densities ranging from 5 to 25 A g-1, as shown in Figure 6c. The specific capacitance of the electrode can be calculated from the CV curves according to the following equation [22]:
C m =IΔt/ Δ Vm
(3)
where C m (F g-1) is the specific capacitance, I (A) is the discharge current, Δt (s) is the charging/discharging time, ΔV (V) is the voltage window for discharge, and m (g) is the mass of the active NiO material in the electrode. Thus, the specific capacitance can be calculated to be 943.5, 791.2, 613.5, 480, and 457.5 F g-1 at the scan rates of 5, 10, 15, 20, and 25 A g-1, respectively (Figure 6d). The specific capacitance of NiO nanosheets is much higher than that of NiO nanobelts, nanorods, and nanosheets reported previously [22–26]. To evaluate the important role of NiO nanosheets for high-performance electrodes, the specific capacitances of Co3O4 nanoneedles and NiO powders are also tested at the scan rates of 5, 10, 15, 20, and 25 A g-1, respectively. The specific capacitances of NiO nanosheets win out over those of Co3O4 nanoneedles and NiO powders (Additional file 1: Figure S2).
Different rates of charging/discharging are used to investigate the high rate capability of the NiO nanosheet electrode as shown in Figure 7a. The Ni foam-supported NiO nanosheet electrode is first cycled with a current density of 5 A g-1, and then the current density is increased to 10, 15, 20, and 25 A g-1, successively. Along with the increment of current density, the corresponding specific capacitances within 100 cycles are measured at 942.6, 791.4, 613.2, 479.6, and 457.1 F g-1, respectively. As can be seen in Figure 7a, when the current density is decreased to 5 A g-1 again, a specific capacitance is recovered at 904.2 F g-1, about 95.9% of the specific capacitance of the initial 100 cycles at 5 A g-1, illustrating an excellent rate capability. In addition, the cycling stability of the NiO nanosheet electrode is also evaluated by the repeated charging/discharging measurement at constant current densities of 11.8 and 23.5 mA cm-2, as shown in Figure 7b. When a discharge current density of 11.8 mA cm-2 is applied, the areal capacitance reaches a value of 1.98 F cm-2 in the 50th cycle. After 3,000 cycles, the supercapacitor displays an excellent long cycle life with only 6.8% deterioration of its initial specific capacitance, demonstrating superior long-term electrochemical stability [27]. Even at a high charging/discharging current density of 23.5 mA cm-2, the areal capacitance can still reach 1.68 F cm-2 in the first cycle and gradually decreases to 1.46 F cm-2 over 3,000 cycles, with a capacitance loss of 13.1%. Figure 7c (curve 1) displays the specific capacitance vs. charging/discharging cycle number of bent electrode at a current density of 5 A g-1. At a discharge current density of 5 A g-1, the areal capacitance can still reach 961.4 F g-1 in the first cycle and gradually decreases to 875.6 F g-1 over 1,200 cycles, equivalent to 91.1% of the capacitance delivered in the first few cycles. Figure 7c (curve 2) displays the specific capacitance of flat electrode at a current density of 5 A g-1. Remarkably, the NiO nanosheets exhibit an excellent retention capacitance of 932.6 F g-1 at the end of 1,200 charging/discharging cycles [27–29], equivalent to 95.3% of the capacitance delivered in the first few cycles. Figure 7d shows the charging/discharging voltage curves of the NiO nanosheets at a current density of 5 A g-1 for the last 10 cycles, and a coulombic efficiency of ≈ 100% can be reached for each cycle.
Figure 7Cycling performance, capacitance, specific capacitance vs. charging/discharing cycle number, and galvanostatic charge and discharge voltage curves. (a) Cycling performance of different samples at progressively varying current densities. (b) The capacitance as a function of cycle number at constant current densities of 11.8 and 23.5 mA cm-2. (c) The specific capacitance vs. charging/discharging cycle number of bent and flat electrodes at a current density of 5 A g-1. (d) Galvanostatic charge and discharge voltage curves of the flat electrode at a current density of 5 A g-1 for the last 10 cycles.
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The improved electrochemical performance could be related to the following structural features. Firstly, the aligned NiO nanosheets with a high surface area facilitate ion diffusion from the electrolyte to each nanosheets, making full use of the active materials [30]. Secondly, the vertical NiO nanosheets could ensure good mechanical adhesion, and more importantly, vertical nanosheets can build up a shortcut and high-speed bridge between the current collector and active materials (Additional file 1: Figure S3). Thirdly, Ni foam as the platform for sustaining nanosheets can withstand strain relaxation and mechanical deformation, preventing the electrode materials from seriously swelling and shrinking during the insertion-deinsertion process.