Preparation of MoFs-Derived Cobalt Oxide/Carbon Nanotubes Composites for High-Performance Asymmetric Supercapacitor

Metal–organic frameworks (MOFs)-derived metallic oxide compounds exhibit a tunable structure and intriguing activity and have received intensive investigation in recent years. Herein, this work reports metal–organic frameworks (MOFs)-derived cobalt oxide/carbon nanotubes (MWCNTx@Co3O4) composites by calcining the MWCNTx@ZIF-67 precursor in one step. The morphology and structure of the composite were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) characterization. The compositions and valence states of the compounds were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Benefiting from the structurally stable MOFs-derived porous cobalt oxide frameworks and the homogeneous conductive carbon nanotubes, the synthesized MWCNTx@Co3O4 composites display a maximum specific capacitance of 206.89 F·g−1 at 1.0 A·g−1. In addition, the specific capacitance of the MWCNT3@Co3O4//activated carbon (AC) asymmetric capacitor reaches 50 F·g−1, and has an excellent electrochemical performance. These results suggest that the MWCNTx@Co3O4 composites can be a potential candidate for electrochemical energy storage devices.


Introduction
The desire to reduce society's dependence on fossil fuels has made the exploration of new energy and high energy utilization efficiency one of the most important issues faced by national governments in the 21st century. Energy storage, which is an intermediate step toward the efficient utilization of energy, has attracted large-scale concern and increasing research interest [1]. Among various emerging energy storage technologies, supercapacitors (SCs), also known as electrochemical capacitors (ECs), are a new concept of energy storage devices that was developed at the forefront of the available environmentally friendly electrochemical energy storage systems. According to the energy storage mechanisms, there are two types of supercapacitors: electrical double-layer capacitors (EDLCs) and pseudocapacitors. The core of supercapacitors is the electrode material, which directly dominates the performance of energy storage [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18].
In recent years, pseudocapacitors based on transition metal oxides/hydroxides with variable valence exhibit a higher specific capacitance. Moreover, transition metal oxides and hydroxides have attracted extensive interest due to their high theoretical capacity, low cost, environmental friendliness and great flexibility in morphology and structures, such as Co 3 O 4 , NiO, Fe 3 O 4 and MnO 2 for pseudocapacitor electrodes, which achieve ultrahigh values of electrode capacitance [19][20][21][22][23][24][25][26]. Among the transition metal oxides, due to its

Results and Discussion
The SEM of the MWCNTx@Co 3 O 4 sample is shown in Figure 1. As can be seen from Figure 1a-d, Co 3 O 4 in MWCNTx@Co 3 O 4 samples all show a rhomboidal dodecahedron morphology, indicating that Co 3 O 4 retains the morphology of ZIF-67 and is not damaged by high temperature. The dispersion of MWCNT in MWCNT 1 @Co 3 O 4 and MWCNT 2 @Co 3 O 4 samples was also observed because the content of carbon nanotubes was less than that of Co 3 O 4 . On the other hand, it is possible that the surface oxidation of MWCNT has oxygencontaining functional groups that repel each other under electrostatic action so that the agglomeration of MWCNT is greatly reduced [37]. As can be seen from Figure 1d, because the MWCNT content is the highest, the whole MWCNT 4 @Co 3 O 4 sample is wrapped by MWCNT, resulting in the Co 3 O 4 polyhedron structure not being obvious. It even hindered the growth of the polyhedral structure, and the morphology distribution was uneven, which may be determined by the growth mechanism of ZIF-67 on the MWCNT surface. However, the distribution of MWCNTS in MWCNT 3 @Co 3 O 4 is more reasonable, as shown in Figure 1c. The length and diameter of MWCNTs is 500 nm and 20 nm, respectively. The Co 3 O 4 rhomboidal dodecahedron is clear and uniform in size, and the MWCNT is evenly dispersed. This phenomenon may be caused by the growth mechanism of MWCNTx@ZIF-67: under the action of a PVP dispersant, MWCNT is uniformly dispersed and can only adsorb a certain amount of Co 2+ on the surface, while the content of ZIF-67 crystal is fixed and, after calcination, uniformly dispersed MWCNT 3 @Co 3 O 4 [38] is obtained. According to the above characterization analysis, the morphology of the MWCNT 3 @Co 3 O 4 composite is the most uniform.
The microstructure and composite structure of MWCNT 3 @Co 3 O 4 were further studied by TEM characterization, as shown in Figure 2. Figure 2a,b show low and highmagnification TEM images of MWCNT 3 @Co 3 O 4 . It can be seen that the Co 3 O 4 surface is connected to MWCNT. This composite structure can improve the conductivity of Co 3 O 4 , thereby enhancing its electrochemical performance. The size of cobalt oxide in MWCNT 3 @Co 3 O 4 is between 120-350 nm, consistent with the size captured in the SEM image. Figure 2c is the high-resolution transmission electron microscope image of the MWCNT 3 @Co 3 O 4 sample. Lattice fringes of different widths were obtained by analyzing the lattice spacing, and were 0.239 nm (the (311) crystal plane of Co 3 O 4 nanoparticles) and 0.29 nm (the (220) crystal plane of Co 3 O 4 nanoparticles), respectively. The polycrystalline properties of MWCNT 3 @Co 3 O 4 composites are described. Figure 2d is the selected electron diffraction diagram of the MWCNT 3 @Co 3 O 4 composite material, which indicates four bright concentric diffraction rings. The radius of the diffraction ring corresponds to the crystal surfaces of the cubic Co 3 O 4 nanoparticles (511), (400), (200) and (311), which are consistent with the XRD results, and also indicate the polycrystalline property of the material. In summary, the successful preparation of MWCNT 3 @Co 3 O 4 composite materials is indicated by the above experimental results. The microstructure and composite structure of MWCNT3@Co3O4 were further studied by TEM characterization, as shown in Figure 2. Figure 2a and b show low and highmagnification TEM images of MWCNT3@Co3O4. It can be seen that the Co3O4 surface is connected to MWCNT. This composite structure can improve the conductivity of Co3O4, thereby enhancing its electrochemical performance. The size of cobalt oxide in MWCNT3@Co3O4 is between 120-350 nm, consistent with the size captured in the SEM image. Figure 2c is the high-resolution transmission electron microscope image of the MWCNT3@Co3O4 sample. Lattice fringes of different widths were obtained by analyzing the lattice spacing, and were 0.239 nm (the (311) crystal plane of Co3O4 nanoparticles) and 0.29 nm (the (220) crystal plane of Co3O4 nanoparticles), respectively. The polycrystalline properties of MWCNT3@Co3O4 composites are described. Figure 2d is the selected electron diffraction diagram of the MWCNT3@Co3O4 composite material, which indicates four bright concentric diffraction rings. The radius of the diffraction ring corresponds to the crystal surfaces of the cubic Co3O4 nanoparticles (511), (400), (200) and (311), which are consistent with the XRD results, and also indicate the polycrystalline property of the material. In summary, the successful preparation of MWCNT3@Co3O4 composite materials is indicated by the above experimental results. The EDS analysis method was used to obtain the element composition and distribution map of the MWCNT3@Co3O4 composite. It can be observed from Figure 3 that the content of the C element is the highest, which is mainly attributed to the conversion of the organic skeleton into a C source after calcination in ZIF and the addition of an appropriate amount of MWCNT. It can also be observed that Co and O elements are evenly distributed on the surface of the polyhedron structure, which confirms the existence of Co3O4, which  The EDS analysis method was used to obtain the element composition and distribution map of the MWCNT 3 @Co 3 O 4 composite. It can be observed from Figure 3 that the content of the C element is the highest, which is mainly attributed to the conversion of the organic skeleton into a C source after calcination in ZIF and the addition of an appropriate amount of MWCNT. It can also be observed that Co and O elements are evenly distributed on the surface of the polyhedron structure, which confirms the existence of Co 3 O 4 , which is the basic condition for obtaining a good pseudocapacitance performance. The EDS analysis method was used to obtain the element composition and distr tion map of the MWCNT3@Co3O4 composite. It can be observed from Figure 3 tha content of the C element is the highest, which is mainly attributed to the conversion o organic skeleton into a C source after calcination in ZIF and the addition of an approp amount of MWCNT. It can also be observed that Co and O elements are evenly distrib on the surface of the polyhedron structure, which confirms the existence of Co3O4, w is the basic condition for obtaining a good pseudocapacitance performance.   Table 1 shows the composite ratio of MWCNT@Co 3 O 4 . As can be seen from the table, with an increase in the amount of MWCNT added, the content of the C element in the composites gradually increased from 21.23% to 28.50%, whereas the content of the Co element in the composites gradually decreased from 50.65% to 44.32%. This confirms the addition of MWCNT. Through the above analysis, the successful synthesis of the MWCNT x @Co 3 O 4 composite materials is confirmed.  successfully obtained after calcination. In addition, it can be seen from the XRD pattern of the MWCNTx@Co 3 O 4 composite that the carbon-derived peak (002) is not obvious, which may be caused by the relatively large the diffraction peak intensity of Co 3 O 4 on the one hand and the low crystallinity of carbon after calcining MWCNTx@ZIF-67 on the other hand.
18.1° (222), 22.1° (114), 24.5° (233), 26.5° (134), 29.6° (044), 31.3° (244), and characteristic peaks of 32.5° (235) and 43.1° (100) that are consistent with those reported in the literature [39], which proves the successful synthesis of the MWCNTx@ZIF-67 precursor. The XRD results of MWCNTx@Co3O4 are shown in Figure 4b. As can be seen from the picture, MWCNT1@Co3O4, MWCNT2@Co3O4, MWCNT3@Co3O4 and MWCNT4@Co3O4 have (511), (400), (200), (111), (311) and (422) crystal plane diffraction peaks consistent with Co3O4 (JCPDS card number 74-2120). It is proved that the MWCNTx@Co3O4 sample was successfully obtained after calcination. In addition, it can be seen from the XRD pattern of the MWCNTx@Co3O4 composite that the carbon-derived peak (002) is not obvious, which may be caused by the relatively large the diffraction peak intensity of Co3O4 on the one hand and the low crystallinity of carbon after calcining MWCNTx@ZIF-67 on the other hand. The chemical composition and corresponding valence states of the MWCNT3@Co3O4 composite were studied by means of XPS characterization. All XPS maps were standardized with reference to the C 1s peak (284.6 eV). Figure 5a shows the full XPS spectrum of the MWCNT3@Co3O4 composite, which mainly contains C, O and Co elements. As can be seen from the Co 2p map in Figure 5b, 779.9 eV and 796.6 eV correspond to Co 2p3/2 and Co 2p1/2, respectively, which are two peaks of Co 2+ and Co 3+ , and the experimental results are consistent with the literature [40]. As can be seen from the C 1s map in Figure 5c, there The chemical composition and corresponding valence states of the MWCNT 3 @Co 3 O 4 composite were studied by means of XPS characterization. All XPS maps were standardized with reference to the C 1s peak (284.6 eV). Figure 5a shows the full XPS spectrum of the MWCNT 3 @Co 3 O 4 composite, which mainly contains C, O and Co elements. As can be seen from the Co 2p map in Figure 5b, 779.9 eV and 796.6 eV correspond to Co 2p3/2 and Co 2p1/2, respectively, which are two peaks of Co 2+ and Co 3+ , and the experimental results are consistent with the literature [40]. As can be seen from the C 1s map in Figure 5c, there are three obvious characteristic peaks at 284.3, 285.8 and 286.4 eV. The peaks at 284.3 eV and 285.8 eV belong to C-C and C=C, respectively, whereas the peaks at 284.6 eV may belong to Co-O-C [41]. Figure 5d shows the O1s map, which mainly contains three peaks of 533.5 eV, 531.5 eV and 530.0 eV, among which the peaks of 530.0 eV and 531.5 eV belong to lattice oxygen species and surface-adsorbed oxygen [42]. These results further confirm the successful synthesis of composite materials, and are further consistent with XRD and EDS results. Figure 6 shows the pore structure and specific surface area of the MWCNT x @Co 3 O 4 composites. As can be seen from Figure 6a, the adsorption/desorption isotherm of MWCNT x @Co 3 O 4 slowly rises at first, and, when it reaches a certain pressure, it rises sharply, resulting in a vertical tail ring and a hysteresis ring. This feature indicates that MWCNT x @Co 3 Figure 6b. The pore volumes of MWCNT 1 @Co 3 O 4 , MWCNT 2 @Co 3 O 4 , MWCNT 3 @Co 3 O 4 and MWCNT 4 @Co 3 O 4 were 3.39, 3.79, 3.06 and 3.30 nm, respectively. It can be well seen in the figure that the pore size distribution of MWCNT 3 @Co 3 O 4 is mainly mesoporous, mainly due to the insertion of MWCNTs into Co 3 O 4 , resulting in a larger pore size of the overall material. Furthermore, the mesoporous structure is conducive to ion diffusion, thereby improving the electrochemical performance of the material. Therefore, the obtained electrode material should have a good electrochemical performance. are three obvious characteristic peaks at 284.3, 285.8 and 286.4 eV. The peaks at 284.3 eV and 285.8 eV belong to C-C and C=C, respectively, whereas the peaks at 284.6 eV may belong to Co-O-C [41]. Figure 5d shows the O1s map, which mainly contains three peaks of 533.5 eV, 531.5 eV and 530.0 eV, among which the peaks of 530.0 eV and 531.5 eV belong to lattice oxygen species and surface-adsorbed oxygen [42]. These results further confirm the successful synthesis of composite materials, and are further consistent with XRD and EDS results.  Figure 6 shows the pore structure and specific surface area of the MWCNTx@Co3O4 composites. As can be seen from Figure 6a, the adsorption/desorption isotherm of MWCNTx@Co3O4 slowly rises at first, and, when it reaches a certain pressure, it rises sharply, resulting in a vertical tail ring and a hysteresis ring. This feature indicates that MWCNTx@Co3O4 composite materials have microporous, mesoporous and macroporous structures. The specific surface areas of MWCNT1@Co3O4, MWCNT2@Co3O4, MWCNT3@Co3O4 and MWCNT4@Co3O4 are 104.20, 66.44, 99.56 and 136.97 m 2 g −1 , respectively. In addition, the pore diameter distribution curve of MWCNTx@Co3O4 is shown in Figure 6b. The pore volumes of MWCNT1@Co3O4, MWCNT2@Co3O4, MWCNT3@Co3O4 and MWCNT4@Co3O4 were 3.39, 3.79, 3.06 and 3.30 nm, respectively. It can be well seen in the figure that the pore size distribution of MWCNT3@Co3O4 is mainly mesoporous, mainly due to the insertion of MWCNTs into Co3O4, resulting in a larger pore size of the overall material. Furthermore, the mesoporous structure is conducive to ion diffusion, thereby improving the electrochemical performance of the material. Therefore, the obtained electrode material should have a good electrochemical performance.  Figure 7a shows the cyclic voltammetry curves of composites with different carbon nanotube contents at 20 mV s −1 . As can be seen, when the scanning speed is 20 mV s −1 , the CV curve area of the MWCNT3@Co3O4 composite material is the largest, indicating that the addition of appropriate carbon nanotubes can improve the specific capacitance of the electrode. This result can be attributed to the synergistic effect of cobalt tetroxide and carbon nanotubes. MWCNT1@Co3O4, MWCNT2@Co3O4 and MWCNT4@Co3O4 decreased, which may be caused by a too high or too low MWCNT content and the overall electrical conductivity of the material. In addition, obvious redox peaks were observed, which were mainly due to different cobalt redox states, such as Co 4+ /Co 3+ and Co 3+ /Co 2+ . The main reaction mechanism is shown in (1) and (2) [43]:  Figure 7a shows the cyclic voltammetry curves of composites with different carbon nanotube contents at 20 mV·s −1 . As can be seen, when the scanning speed is 20 mV·s −1 , the CV curve area of the MWCNT 3 @Co 3 O 4 composite material is the largest, indicating that the addition of appropriate carbon nanotubes can improve the specific capacitance of the electrode. This result can be attributed to the synergistic effect of cobalt tetroxide and carbon nanotubes. MWCNT 1 @Co 3 O 4 , MWCNT 2 @Co 3 O 4 and MWCNT 4 @Co 3 O 4 decreased, which may be caused by a too high or too low MWCNT content and the overall electrical conductivity of the material. In addition, obvious redox peaks were observed, which were mainly due to different cobalt redox states, such as Co 4+ /Co 3+ and Co 3+ /Co 2+ . The main reaction mechanism is shown in (1) and (2) [43]:  Figure 7b shows the GCD curve of the MWCNTx@Co3O4 composite material, which was tested at a current density of 1 A g −1 . It is observed that the curve is almost triangular in symmetry, and there is no particularly rapid drop in voltage, indicating that the synthesized MWCNT1@Co3O4, MWCNT2@Co3O4, MWCNT3@Co3O4 and MWCNT4@Co3O4 materials have particularly good electrochemical reversibility. While the GCD curves of the MWCNT4@Co3O4 composite is different from others, its charge time is longer from its  Figure 7b shows the GCD curve of the MWCNTx@Co 3 O 4 composite material, which was tested at a current density of 1 A·g −1 . It is observed that the curve is almost triangular in symmetry, and there is no particularly rapid drop in voltage, indicating that the synthesized MWCNT 1 @Co 3 O 4 , MWCNT 2 @Co 3 O 4 , MWCNT 3 @Co 3 O 4 and MWCNT 4 @Co 3 O 4 materials have particularly good electrochemical reversibility. While the GCD curves of the MWCNT 4 @Co 3 O 4 composite is different from others, its charge time is longer from its discharge time. This is mainly due to the higher resistance, which is consistent with the result of Figure 7f. When applying the same voltage in the MWCNT 4 @Co 3 O 4 composite, the current is unable to charge quickly, which leads to the GCD curve of the sample MWCNT 4 @Co 3 O 4 composite being asymmetrical. The discharge time of MWCNT 3 @Co 3 O 4 is longer than that of MWCNT 1 @Co 3 O 4 , MWCNT 2 @Co 3 O 4 and MWCNT 4 @Co 3 O 4 . Therefore, among the four materials prepared, the MWCNT 3 @Co 3 O 4 composite has the largest specific capacitance, whereas the MWCNT 1 @Co 3 O 4 composite material has the smallest specific capacitance. It is also comparable to other Co 3 O 4 -based materials as listed in Table 2.  Figure 7c shows the cyclic voltammetry curves of MWCNT 3 @Co 3 O 4 at sweep speeds of 5, 10, 20, 50 and 100 mV·s −1 . It is observed that the CV curve of the electrode material is similar with an increase in the sweep speed, indicating that the MWCNT 3 @Co 3 O 4 electrode material has good performance. Figure 7d shows the GCD curves of MWCNTx@Co 3 O 4 composites at 1, 2, 3, 4 and 5 A·g −1 current densities. When the applied current density is 1 A·g −1 , the MWCNT 3 @Co 3 O 4 composite has the longest discharge time and its specific capacitance is 206.89 F·g −1 , which is higher than MWCNT 1 @Co 3 O 4 , MWCNT 2 @Co 3 O 4 and MWCNT 4 @Co 3 O 4 (132.73 F·g −1 , 194.64 F·g −1 , 89.36 F·g −1 , respectively). It is shown that the composite has an excellent electrochemical performance. Figure 7e shows the specific capacitance as a function of current density. As the specific current density increases, the specific capacitance decreases due to the limitation of diffusion on the electrode surface [50]. The MWCNT 3 @Co 3 O 4 electrode shows a specific capacitance of 191.67 F·g −1 at 5 A·g −1 , and the specific capacity of MWCNT 3 @Co 3 O 4 is 3.2 times higher than that of MWCNT 4 @Co 3 O 4 and 2 times higher than that of MWCNT 1 @Co 3 O 4 at the same current density. By adding an optimal proportion of MWCNT, the capacity of MWCNT@Co 3 O 4 is increased, thus promoting the charge-transfer ions on the surface. Figure 7f shows the electrochemical impedance spectroscopy (EIS) of the MWCNTx@Co 3 O 4 composite. It can be observed that the linear slope of the composite is the highest in the low-frequency region MWCNT 3 @Co 3 O 4 , which indicates that the ion diffusion resistance of MWCNT 3 @Co 3 O 4 is smaller than that of MWCNT 1 @Co 3 O 4 , MWCNT 2 @Co 3 O 4 and MWCNT 4 @Co 3 O 4 electrodes. In the high-frequency region, the MWCNT 3 @Co 3 O 4 electrode has the smallest semicircle diameter, indicating that its charge transfer resistance is smaller than that of MWCNT 1 @Co 3 O 4 , MWCNT 2 @Co 3 O 4 and MWCNT 4 @Co 3 O 4 , indicating that the charge transfer rate of MWCNT 3 @Co 3 O 4 is faster. It is also further explained that the MWCNT 3 @Co 3 O 4 electrode material has better electrical conductivity. Figure 8 shows the cycle performance experiment conducted on MWCNT 3 @Co 3 O 4 with a current density of 1 A·g −1 . As can be seen, the maintenance rate of the specific capacitance of MWCNT 3 @Co 3 O 4 is 78.9% after 1000 cycles of charge and discharge, which indicates that the MWCNT 3 @Co 3 O 4 composite electrode has a good cycle life.
To better explore the practical application of the electrode materia MWCNT3@Co3O4//AC asymmetric supercapacitor was assembled for the electroch test. Figure 9a shows the cyclic voltammetry curves of the MWCNT3@Co3O4//AC ca at 5, 10, 20, 50 and 100 mV s −1 . It can be seen from the figure that the CV curve p the characteristics of double electric layers and pseudocapacitors, which are the con for the capacitor to have a good electrochemical performance. Figure 9b shows th curve under different current densities. The mass specific capacitance of the capaci der 1, 2, 3, 4 and 5 A g −1 current densities is calculated as 50.0, 46.25, 34.31, 30.0 and g −1 , respectively. To better explore the practical application of the electrode material, the MWCNT 3 @Co 3 O 4 //AC asymmetric supercapacitor was assembled for the electrochemical test. Figure 9a shows the cyclic voltammetry curves of the MWCNT 3 @Co 3 O 4 //AC capacitor at 5, 10, 20, 50 and 100 mV·s −1 . It can be seen from the figure that the CV curve presents the characteristics of double electric layers and pseudocapacitors, which are the conditions for the capacitor to have a good electrochemical performance. Figure 9b shows the GCD curve under different current densities. The mass specific capacitance of the capacitor under 1, 2, 3, 4 and 5 A·g −1 current densities is calculated as 50.0, 46.25, 34.31, 30.0 and 25.0 F·g −1 , respectively. Figure 9c shows the relationship between specific current and specific capacitance. It can be seen that the capacitor still has a 57.1% specific capacitance maintenance rate at a current density of 5 A·g −1 , indicating that the capacitor has an excellent multiplier performance. Figure 9d shows the performance of the device after 1000 cycles. At a 1 A·g −1 current density, the initial capacitance remains at 87.2% after 1000 times of charging and discharging, indicating that the capacitor has a good cycle life. This is mainly attributed to the polyhedral structure and high porosity of MWCNT 3 @Co 3 O 4 composites, which reduces the damage of electrode materials during charging and discharging. Figure 10 shows the relationship between energy density and power density. Under current densities of 1 A·g −1 , 2 A·g test. Figure 9a shows the cyclic voltammetry curves of the MWCNT3@Co3O4//AC capacitor at 5, 10, 20, 50 and 100 mV s −1 . It can be seen from the figure that the CV curve presents the characteristics of double electric layers and pseudocapacitors, which are the conditions for the capacitor to have a good electrochemical performance. Figure 9b shows the GCD curve under different current densities. The mass specific capacitance of the capacitor under 1, 2, 3, 4 and 5 A g −1 current densities is calculated as 50.0, 46.25, 34.31, 30.0 and 25.0 F g −1 , respectively.  Figure 9c shows the relationship between specific current and specific capacitance. It can be seen that the capacitor still has a 57.1% specific capacitance maintenance rate at a current density of 5 A g −1 , indicating that the capacitor has an excellent multiplier performance. Figure 9d shows the performance of the device after 1000 cycles. At a 1 A g −1 current density, the initial capacitance remains at 87.2% after 1000 times of charging and discharging, indicating that the capacitor has a good cycle life. This is mainly attributed to the polyhedral structure and high porosity of MWCNT3@Co3O4 composites, which reduces the damage of electrode materials during charging and discharging. Figure 10 shows the relationship between energy density and power density. Under current densities of 1 A g −1 , 2 A g −1 , 3 A g −1 , 4 A g −1 and 5 A g −1 , the power densities of the device are 800, 1600, 2400, 3200 and 4000 W kg −1 , respectively. The corresponding energy densities are 17.78, 16.44, 12.20, 10.67 and 8.89 Wh kg −1 , respectively.

Experimental Section
This section may be divided by subheadings. It should provide a concise and precise description of the experimental results and their interpretation, as well as the experimental conclusions that can be drawn.

Materials
All of the chemical reagents in this experiment were of analytical purity and directly used without any further purification.

Experimental Section
This section may be divided by subheadings. It should provide a concise and precise description of the experimental results and their interpretation, as well as the experimental conclusions that can be drawn.

Materials
All of the chemical reagents in this experiment were of analytical purity and directly used without any further purification.

Materials Characterization
XRD analysis was performed by a powder X-ray diffraction system (XRD-6100, Rigaku, Tokyo, Japan) equipped with Cu Kα radiation (λ = 0.15406 nm) to determine crystalline structures of the obtained samples. The XPS measurements were performed by a Thermo ES-CALAB 250Xi spectrometer (USA) with monochromated Al Kα radiation (hγ = 1486.6 eV). All XPS spectra were calibrated with respect to the C 1s peak at 284.6 eV. The morphology and microstructure of the samples were characterized by field emission scanning electron microscopy (FE-SEM) (JSM-6480A, JEOL, Tokyo, Japan) and a TEM (JEM-2000FX, Electronics Corporation, Tokyo, Japan). The BET (Brunauer-Emmett-Teller) and pore size distrubtion of the samples were characterized by N 2 adsorption Nitrogen and desorption test (TriStar II3flex, Mike, Scottsdale, AZ, USA).

Fabrication of Supercapacitor Electrode
(1) Preparation of Single Electrode All single electrodes in this paper were prepared in the following way: firstly, certain amounts of active substance and acetylene black (Superconducting K90, Ron reagent, Harbin, China) were weighed, dissolved in 5 mL of absolute ethanol and ultrasonicated for 30 min. Then, a certain amount of PTFE (60.0 wt%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) lotion (the mass ratio of active substance, acetylene black and PTFE lotion (5wt%) was 8:1:1) was dropped in proportion, ultrasonicated for 10 min and then dried in a constant temperature blast-drying oven at 60 • C for 12 h. Finally, the obtained black sample was scraped onto one side of foam nickel of 1 × 1 cm, pressed with a tablet press and compacted evenly. By weighing the total mass of foam nickel before and after scraping, the total mass of active substance can be calculated.
(2) Assembly of Asymmetric Supercapacitors In order to evaluate the practical application of the prepared electrode material, the electrode material and AC prepared in this article were used to prepare a single electrode in the same manner as in (1). Subsequently, when assembling the supercapacitor, they were assembled on the positive electrode and the negative electrode of the supercapacitor, respectively. At the same time, the positive and negative electrodes were separated by a membrane, and an asymmetric supercapacitor was assembled. The electrolyte was prepared as a 3.0 M KOH (≥85.0%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) solution.

Electrochemical Characterization
Electrochemical workstation (CHI 760E) was used to observe the electrochemical performance of the MWCNTx@Co 3 O 4 composite electrode in a three-electrode installation. In this test, platinum was used as a counter electrode, Ag/AgCl as a reference electrode, MWCNTx@Co 3 O 4 composites as a working electrode and a solution of 3 M KOH as electrolyte. Cyclic voltammetry (CV) was performed between 0 V and 0.6 V at various scan rates (from 5 to 100 mV·s −1 ). Galvanostatic discharge texts were measured in a voltage window from 0 V to 0.55 V at various current densities. EIS measurements were carried out with a 5 mV sinusoidal voltage in a frequency from 100 kHz to 0.01 Hz.

Conclusions
In summary, a diamond-dodecahedron-structured MWCNT x @Co 3 O 4 composite was successfully fabricated using a smart approach. With an increase in the MWCNT content, the electrochemical properties of the MWCNT X @Co 3 O 4 composite first increased and then decreased. Benefiting from the unique structure, high specific surface area and reasonable pore size distribution, the as-obtained MWCNT 3 @Co 3 O 4 composite exhibits satisfactory capacitive behavior: 206.89 F·g −1 at a current density of 1 A·g −1 ; an excellent cycling stability of 87.2% capacitance retention over 1000 continuous cycles. An asymmetric supercapacitor cell was fabricated through MWCNTx@Co 3 O 4 and AC as a positive and negative electrode, respectively. The cell can deliver a high energy density of 17.78 Wh·kg −1 at a power density of 800 W·kg −1 . Our results suggest that the MWCNTx@Co 3 O 4 composite can be used in actual high-power devices. The preparation strategy offers a facile and variable route to rationally design and prepare cobalt oxide electrode materials for a variety of applications in energy storage and conversion, catalysis and environmental treatment.