Self-Supported Co3O4@Mo-Co3O4 Needle-like Nanosheet Heterostructured Architectures of Battery-Type Electrodes for High-Performance Asymmetric Supercapacitors

Herein, this report uses Co3O4 nanoneedles to decorate Mo-Co3O4 nanosheets over Ni foam, which were fabricated by the hydrothermal route, in order to create a supercapacitor material which is compared with its counterparts. The surface morphology of the developed material was investigated through scanning electron microscopy and the structural properties were evaluated using XRD. The charging storage activities of the electrode materials were evaluated mainly by cyclic voltammetry and galvanostatic charge-discharge investigations. In comparison to binary metal oxides, the specific capacities for the composite Co3O4@Mo-Co3O4 nanosheets and Co3O4 nano-needles were calculated to be 814, and 615 C g−1 at a current density of 1 A g−1, respectively. The electrode of the composite Co3O4@Mo-Co3O4 nanosheets displayed superior stability during 4000 cycles, with a capacity of around 90%. The asymmetric Co3O4@Mo-Co3O4//AC device achieved a maximum specific energy of 51.35 Wh Kg−1 and power density of 790 W kg−1. The Co3O4@Mo-Co3O4//AC device capacity decreased by only 12.1% after 4000 long GCD cycles, which is considerably higher than that of similar electrodes. All these results reveal that the Co3O4@Mo-Co3O4 nanocomposite is a very promising electrode material and a stabled supercapacitor.


Introduction
With broad applicability in the electronics, power systems, communication systems, and automobile sectors, supercapacitors have been recognized as being highly reliable among the numerous forms of energy storage devices. Supercapacitors exhibit high power densities and high charge storage abilities with long-term stability compared to batteries [1]. The cost-effective and less hazardous properties of supercapacitor components have motivated researchers to explore different electrode materials. Still, the practical usability of supercapacitors is limited due to their low energy densities for which battery-type electrodes are currently preferred. A wide range of transition mono-/multi-metallic oxides or hydroxides has been investigated as electrode materials for supercapacitors [2,3]. Among these, cobalt oxides have been found to possess high theoretical specific capacities [4].

Synthesis of Co 3 O 4 and Co 3 O 4 @Mo-Co 3 O 4
The hetero-structure nanomaterial was synthesized via a facile hydrothermal method followed by low-temperature calcination. Pure precursors, i.e., cobalt nitrate, sodium molybdate, and urea, were used as-purchased. First, 2 mmol of Co(NO 3 ) 2 .6H 2 O (0.582 g), 1 mMol NaMoO 4 (0.206 g), and 10 mM urea (0.6 g) were completely dissolved in distilled water (25 mL) and ethanol (10 mL). Then, the 35-mL solution was transferred to a Teflonlined autoclave for hydrothermal synthesis. The autoclave was subjected to heating to 120 • C and maintained for 10 h. Then, the autoclave was cooled to room temperature. The hydrothermal sample was washed with distilled water and ethanol to remove unreacted precursors or impurities, followed by drying at 60 • C. Finally, the obtained powder was calcinated at 350 • C for 3 h at a heating rate of 5 • C/min. Figure 1 presents a schematic view of the synthesis of the heterostructured Co 3 O 4 nanoneedles and Mo-Co 3 O 4 nano-petals. The low temperature used in the hydrothermal treatment allowed nucleation and the growth of nanoneedles and nanosheets to occur, while the post-annealing improved the crystallinity of the synthesized oxides. 814 C g −1 (1 A g −1 ) and retention of 90% after 4000 long-cycles. In addition, an asymmetric supercapacitor (ASC) with continuous cycling stability was assembled utilizing Co3O4@Mo-Co3O4 and activated carbon (AC). The fabricated Co3O4@Mo-Co3O4//AC ASC displayed great electrochemical capacity, stability, and superior conductivity.

Synthesis of Co3O4 and Co3O4@Mo-Co3O4
The hetero-structure nanomaterial was synthesized via a facile hydrothermal method followed by low-temperature calcination. Pure precursors, i.e., cobalt nitrate, sodium molybdate, and urea, were used as-purchased. First, 2 mmol of Co(NO3)2.6H2O (0.582 g), 1 mMol NaMoO4 (0.206 g), and 10 mM urea (0.6 g) were completely dissolved in distilled water (25 mL) and ethanol (10 mL). Then, the 35-mL solution was transferred to a Teflonlined autoclave for hydrothermal synthesis. The autoclave was subjected to heating to 120 °C and maintained for 10 h. Then, the autoclave was cooled to room temperature. The hydrothermal sample was washed with distilled water and ethanol to remove unreacted precursors or impurities, followed by drying at 60 °C. Finally, the obtained powder was calcinated at 350 °C for 3 h at a heating rate of 5 °C/min. Figure 1 presents a schematic view of the synthesis of the heterostructured Co3O4 nanoneedles and Mo-Co3O4 nanopetals. The low temperature used in the hydrothermal treatment allowed nucleation and the growth of nanoneedles and nanosheets to occur, while the post-annealing improved the crystallinity of the synthesized oxides.

Characterizations
Detailed characterizations and an electrochemical analysis are available in the supporting file. In addition, the device fabrication procedure and calculation method of the specific capacity, energy, and power densities are provided and applied to the methods and calculations reported in previous literature [15,17,[19][20][21].

Asymmetric Supercapacitor Preparation
For a three-electrode configuration, Co 3 O 4 and Mo-Co 3 O 4 coated on Ni foam were utilized as a working electrode, platinum wire as a counter electrode, and Ag/AgCl as a Nanomaterials 2022, 12, 2330 4 of 14 reference electrode, with 2.0 M KOH as the active electrolyte. For a two-electrode setup, Mo-Co 3 O 4 coated on Ni foam was utilized as a positive electrode and activated carbon (AC) as a negative electrode, while Whatman filter paper-dipped in 2 M KOH solution and then placed between the positive and negative electrodes-was used as a separator. The mass-loading in the three-electrode setup was around 3 mg; in the two-electrode setup, an active mass of 8.7 mg was loaded onto the positive electrode (M) and 1 mg was loaded onto the negative electrode (AC). These values were chosen to balance the charge on both electrodes. The CV measurements for the ASC device were carried out in the potential range of −0.1 to 0.6 V at various scanning rates from 10 to 30 mV s −1 , whereas GCD characterizations were performed in potential ranges from 0 to 1.6 V at various current densities, i.e., ranging from 1 to 10 A g −1 . Moreover, EIS studies for both the two and three-electrode setups were carried out with an open circuit potential (OCP) ranging from 1 Hz to 100 kHz, with an AC perturbation of 5 mV.
In a three-electrode setup, the specific capacities were calculated from the GCD curves using the below equation: where C is the specific capacity (C/g), I indicates the discharge current (A), ∆t is the discharge time (s), and m represents the mass of the active material (g). In the two-electrode setup, the specific capacitance (measured from GCD plots) of the active electrodes was calculated from the following equation: To assemble the supercapacitor device, the charge stored on the positive and negative electrodes was described by q + = q − . The mass balance of the positive and negative electrodes was estimated by: Also, in the two-electrode setup, the energy density (Wh/Kg) and power density (W/Kg) were measured as follows: where C s is the specific capacitances (F/g), ∆t is the discharge time (s), ∆V is the operating voltage window (V), m is the active mass of the working material (g), E is the energy density (Wh/kg), and P is the power density (W/kg).

Results and Discussion
The X-ray diffraction patterns of Co 3 O 4 and Mo-Co 3 O 4 were studied in the 2θ range of 10-80 • , as presented in Figure 2a. The diffraction patterns demonstrated that the prepared heterostructure was polycrystalline, presenting several peaks which were consistent with Co 3 O 4 planes. The diffraction patterns of the Co 3 O 4 and Mo-Co 3 O 4 heterostructure showed peaks at the (220), (311), (400), (511), and (440) planes, which corresponded to the cubic crystal structure of the Co 3 O 4 phase. All of the planes matched with the JCPDS card no: 042-1467 [19]. The diffraction pattern showed no peaks corresponding to cobalt or molybdenum, indicating that no extra phases existed in the heterostructure. The XRD pattern of the Mo-doped Co 3 O 4 heterostructure exhibited an additional peak at the (002) plane. Irrespective of the (002) plane, all the remaining planes showed pure Co 3 O 4 , indicating that no structural deformation had occurred in the host Co 3 O 4 lattice upon Mo-doping. corresponded to the cubic crystal structure of the Co3O4 phase. All of the planes matched with the JCPDS card no: 042-1467 [19]. The diffraction pattern showed no peaks corresponding to cobalt or molybdenum, indicating that no extra phases existed in the heterostructure. The XRD pattern of the Mo-doped Co3O4 heterostructure exhibited an additional peak at the (002) plane. Irrespective of the (002) plane, all the remaining planes showed pure Co3O4, indicating that no structural deformation had occurred in the host Co3O4 lattice upon Mo-doping. This demonstrates that Mo ions were successfully substituted in the Co lattice positions in the Co3O4 matrix. Rietveld refinement was also used to determine the presence of Mo in the Co3O4 lattice for the Co3O4@Mo-Co3O4 heterostructure. In this analysis, R weighted profile (Rwp), R profile (Rp), R structure factor, R Bragg factor (RBragg), and goodness of fit (GOF) structural parameters were evaluated (as shown in Figure S1) using the EXPO software. The unit cell parameters were also calculated using the Rietveld refinement results. Table  1 shows the estimated Rietveld refinement parameters, including Rp, Rwp, GOF, R structure factor, RBragg, and the unit cell parameters. The quality of Mo substitution in the Co3O4 matrix can be determined from the GOF value, which is 0.805.  Rietveld refinement was also used to determine the presence of Mo in the Co 3 O 4 lattice for the Co 3 O 4 @Mo-Co 3 O 4 heterostructure. In this analysis, R weighted profile (R wp ), R profile (R p ), R structure factor, R Bragg factor (R Bragg ), and goodness of fit (GOF) structural parameters were evaluated (as shown in Figure S1) using the EXPO software. The unit cell parameters were also calculated using the Rietveld refinement results. Table 1 shows the estimated Rietveld refinement parameters, including R p , R wp , GOF, R structure factor, R Bragg , and the unit cell parameters. The quality of Mo substitution in the Co 3 O 4 matrix can be determined from the GOF value, which is 0.805.   Table 2 lists the structural parameters, while Figure 2 depicts the crystal structure.   [5]. Several small peaks appeared due to the scattering of the F 2g or E g phonons [7]. These bands can be integrated into Mo-O-Mo, Mo-O, and MoO 4 vibrations [8]. The results suggest that the synthesis of the Co 3 O 4 @Mo-Co 3 O 4 heterostructure composite was successful.

Unit Cell Parameters
In addition, FTIR analysis was performed on the Co 3 O 4 @Mo-Co 3 O 4 needle-like nanosheet heterostructure for additional phase evolution, as presented in Figure S2. This is the first reported FTIR analysis of Mo-doped Co 3 O 4 in a supercapacitor-related publication. The sharp peaks at 560 and 665 cm −1 correspond to pure Co 3 O 4 , comprising Co-O stretching vibrations [22]. These two bands confirm the presence of Co 3 O 4 , which is related to the OB3 vibration (B = Co 3+ in an octahedral hole); the other band is related to the ABO 3 vibration (A = Co 2+ in a tetrahedra hole) [23]. The small peaks at around 3425 and 1645 cm −1 resulted from the O-H groups [24]. The shift could be observed for the Mo-doped Co 3 O 4 , and was attributed to a change in the surface area as well as a surface defect due to the doping [25]. The peak at 1058 cm −1 belonged to the carbonate groups which resulted from air contamination during the reaction of oxide with Co 2 [26].
The chemical composition and valance state of the Co 3 O 4 @Mo-Co 3 O 4 needle-like nanosheet heterostructure were analyzed using X-ray photoelectron spectroscopy (XPS), revealing the oxidization state of the transition metal ion. Figure 2c displays the spectra of Mo-Co 3 O 4 , showing Mo, Co, and O peaks in addition to C. The core-level photoelectron spectra of Mo3d, Co2p, and O1s are presented in Figure 2d-f. The Mo3d core transition was considered by a doublet due to the spin-orbit coupling resulting from the 3d 5/2 and 3d 3/2 components. The binding energies of Mo3d 5/2 and Mo3d 3/2 were 231.92 ± 0.1 eV and 235.06 ± 0.1 eV, respectively. The local structure of the Mo atoms in the Co 3 O 4 lattice offered information about its chemical state and was primarily responsible for these peak placements. The detected binding energy values of Mo differed from those of Mo-O-related compounds, indicating that the Mo cations had been perfectly substituted in the Co sites of the cubic structure. Figure 2c shows two distinct peaks at 780.15 ± 0.1 eV and 795.64 ± 0.1 eV, which belong to 2p 3/2 and 2p 1/2 , respectively. The energy difference between the Co 2p 3/2 and 2p 1/2 splitting was 15.49 eV, which indicated the existence of Co 2+ and corresponded to the existence of Co 3 O 4 . It can be noted that the binding energy difference ∆E between the Co2p 1/2 and Co2p 3/2 (15 eV) observed in the complex was equal to that found for Co(III), which was in agreement with the reported value [27]. The peak detected at 530.26 ± 0.1 eV was attributable to O1s linked to Co and Mo atoms in the lattice oxygen (see Figure 2d). Because the activation energy for oxygen diffusion is substantially higher than that for interstitial Co atoms and Mo 6+ ions, enough oxygen atoms from the atmosphere can diffuse into the Co 3 O 4 lattice to fill up the new oxygen vacancies created by the increase of Co(III) and Mo 6+ ions during Mo:Co 3 O 4 development. The existence of Co and O in XPS spectra is consistent with the XRD patterns.
The morphology of Co 3 O 4 @Mo-Co 3 O 4 was characterized by FESEM, as shown in Figure 3. The one-pot hydrothermal synthesized Co 3 O 4 @Mo-Co 3 O 4 was found to be heterostructured. It is worth mentioning that the heterostructures composed of the monolayer Co 3 O 4 and some Mo-Co 3 O 4 nanosheets demonstrated eminent structural formation. The images in Figure 3a-f show the Co 3 O 4 @Mo-Co 3 O 4 at different magnifications. A bundle of nanoneedles of Co 3 O 4 was arranged over a bud to give a flower-like nanostructure, while the Mo-Co 3 O 4 nanosheets were stuck on/in the nanoneedles. The fact that the self-supported heterostructure formed without the addition of any surfactant or template is quite interesting. The Co 3 O 4 and Mo-Co 3 O 4 heterostructures exhibited excellent metallic characteristics and great structural stability, delivering better electronic conductivity and structural integrity than pristine composites. Moreover, self-assembled particles reduce the surface energy, enhancing the stability of the structure. Within the hierarchical heterostructures, both core and shell are active materials, and the core-shell heterostructures enable easy access to electrolytes. Therefore, both of them can effectively contribute to the capacity. Figure 3g shows the EDS spectra obtained for the elemental studies, whereas Figure 3h-j shows the elemental mapping for the Mo, Co, and O present in the Co 3 O 4 @Mo-Co 3 O 4 needle-like nanosheet heterostructure. It is expected that the architecture can be beneficial for boosting electrochemical performance.
HRTEM images of the Co 3 O 4 @Mo-Co 3 O 4 heterostructure are depicted in Figure 4, where the resolutions in a, b, and c are 50, 10, and 5 nm, respectively. Figure 4a,b clearly convey that there are different shapes of nanoneedles, i.e., a flower and small sheets attached to it. It was observed that the wide exposed nanoneedles in flower form provided space for the small sheets inside them, as well as sharp tips to hold nanosheets. Thus, the hydrothermal growth of the heterostructured Co 3 O 4 nanoneedles and Mo-Co 3 O 4 nanoplates resulted in the creation of interesting nanostructured electrode materials. The highest resolution image (Figure 4c) shows fringes, where the SAED pattern (inset in Figure 4c) confirms the highly crystalline nature of the hetero-structured nanomaterials. The precise spacings of 0.24, 0.29, and 0.46 nm were well-matched with the (311), (220), and (111) planes of Mo-Co 3 O 4 . The well-organized rings and dots seen in the SAED pattern prove the crystallinity nature of the electrode materials that could be beneficial in structure retention for long-term stability. There is no doubt that such an architecture would support efficient electrochemical performance in supercapacitor applications. Nanomaterials 2022, 12, x FOR PEER REVIEW 8 of 15 HRTEM images of the Co3O4@Mo-Co3O4 heterostructure are depicted in Figure 4, where the resolutions in a, b, and c are 50, 10, and 5 nm, respectively. Figure 4a,b clearly convey that there are different shapes of nanoneedles, i.e., a flower and small sheets attached to it. It was observed that the wide exposed nanoneedles in flower form provided space for the small sheets inside them, as well as sharp tips to hold nanosheets. Thus, the hydrothermal growth of the heterostructured Co3O4 nanoneedles and Mo-Co3O4 nanoplates resulted in the creation of interesting nanostructured electrode materials. The highest resolution image (Figure 4c) shows fringes, where the SAED pattern (inset in Figure 4c) confirms the highly crystalline nature of the hetero-structured nanomaterials. The precise spacings of 0.24, 0.29, and 0.46 nm were well-matched with the (311), (220), and (111) planes of Mo-Co3O4. The well-organized rings and dots seen in the SAED pattern prove the crystallinity nature of the electrode materials that could be beneficial in structure retention for long-term stability. There is no doubt that such an architecture would support efficient electrochemical performance in supercapacitor applications.

Electrochemical Properties of Electrode Materials
The hydrothermally obtained heterostructures were tested for three-electrode system electrochemical performance in 2M KOH electrolytes. Figure 5 illustrates a comparison of the Co3O4 and Co3O4@Mo-Co3O4 needle-like nanosheet heterostructure electrodes. It was found that the doping of Mo into Co3O4 enhanced the electrochemical properties, such as

Electrochemical Properties of Electrode Materials
The hydrothermally obtained heterostructures were tested for three-electrode system electrochemical performance in 2M KOH electrolytes. Figure 5 illustrates a comparison of the Co 3 O 4 and Co 3 O 4 @Mo-Co 3 O 4 needle-like nanosheet heterostructure electrodes. It was found that the doping of Mo into Co 3 O 4 enhanced the electrochemical properties, such as peak current and discharge time (Figure 5a,b). Notably, the cyclic voltammetry (CV) curves and the galvanic charge-discharge (GCD) curves demonstrated that doping did not shift the redox peaks of the Co 3 O 4 . This indicated that Co 3 O 4 undergoes redox reactions while Mo facilitates the electrochemical performance of Co 3 O 4 . The CV curves in the potential window of −0.1 to 0.6 V at different scan rates, i.e., 10-30 mV/s, obtained for Co 3 O 4 and the Co 3 O 4 @Mo-Co 3 O 4 heterostructures, are shown in Figure 6c,d. At each scan rate, clear oxidation and reduction peaks (0.35 V and 0.2 V) were found in the same place, with an increase in the current density with increasing scan rate. Figure 5e,f presents the GCD profiles in a potential window of 0 to 0.45 V at different current densities, i.e., 1-10 A/g, obtained for Co 3 O 4 and the Co 3 O 4 @Mo-Co 3 O 4 heterostructure. The battery-type behavior is clearly visible in the plateau-like charge/discharge pattern. The Co 3 O 4 and Co 3 O 4 @Mo-Co 3 O 4 heterostructure revealed similar humps, indicating the occurrence of redox reactions due to cobalt, where Mo-Co 3 O 4 has a higher discharge time than Co 3 O 4 . The Mo atoms did not show redox activity during charge/discharge reactions but promoted electrochemical performance. The calculated specific capacity is given in Figure 5g. The faradaic reactions for Co 3 O 4 are given below [28]: Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 15  The advantage of Mo-doping is to facilitate the electrochemical ability. The heterostructure formed by the addition of Mo improved the charge storage capacity by 30%. The pristine Co3O4 electrode exhibited a specific capacity of 615 C/g (1464 F/g), whereas the Co3O4@Mo-Co3O4 needle-like nanosheet heterostructure had a specific capacity of 814 C/g (1850 F/g). Besides Mo-doping, the enhanced electrochemical performance could be attributed to the heterostructure architecture of the obtained nanomaterials. The Mo-Co3O4 flower-like nanosheets provided a large area, leading to better electrolyte diffusion into the electrodes. In addition, the Mo-Co3O4 flower-like nanosheets provided an active electrochemical surface for redox reactions. The self- The advantage of Mo-doping is to facilitate the electrochemical ability. The heterostructure formed by the addition of Mo improved the charge storage capacity by 30%. The pristine Co 3 O 4 electrode exhibited a specific capacity of 615 C/g (1464 F/g), whereas the Co 3 O 4 @Mo-Co 3 O 4 needle-like nanosheet heterostructure had a specific capacity of 814 C/g (1850 F/g). Besides Mo-doping, the enhanced electrochemical performance could be attributed to the heterostructure architecture of the obtained nanomaterials. The Mo-Co 3 O 4 flower-like nanosheets provided a large area, leading to better electrolyte diffusion into the electrodes. In addition, the Mo-Co 3 O 4 flower-like nanosheets provided an active electrochemical surface for redox reactions. The self-supported structure comprised a large surface area for charge storage and fast electron transfer reactions. The advantages of this morphology are reflected in the Nyquist plot given in Figure 5h. The solution resistance was found to be 1 Ω and 0.65 Ω for Co 3 O 4 and for the Co 3 O 4 @Mo-Co 3 O 4 heterostructure, respectively (inset in Figure 5h). However, the reduced resistance would have increased the charge kinetics during the electrochemical process. Further, the stability test for 4000 cycles of continuous charge/discharge process in the KOH electrolyte was done with the Co 3 O 4 @Mo-Co 3 O 4 heterostructure electrode. Even at the 4000th cycle, around 90% capacity retention was observed for the Co 3 O 4 @Mo-Co 3 O 4 electrode. The inset of the last 10 similar GCD cycles corroborates the structural stability of the Co 3 O 4 @Mo-Co 3 O 4 heterostructure electrode during long-term cycling. Thus, the excellent morphology and crystallinity of the Co 3 O 4 @Mo-Co 3 O 4 needle-like nanosheet heterostructure electrode indicate that it is a very promising electrode material for supercapacitor applications.

Electrochemical Properties of a Co 3 O 4 @Mo-Co 3 O 4 //AC Device
For practical applications, the Co 3 O 4 @Mo-Co 3 O 4 heterostructure electrode was assembled with an AC electrode in a KOH electrolyte as an asymmetric supercapacitor, as represented in Figure 6a. The comparative CV curves of the negative AC and positive Co 3 O 4 @Mo-Co 3 O 4 electrodes are presented in Figure S1. The asymmetric (Co 3 O 4 @Mo-Co 3 O 4 //AC) device potential window was optimized, as shown in Figure 6b,c. The CV curves in Figure 6b at 80 mV/s and the GCD profile at a current density of 3 A/g (Figure 6c) were made at different potential windows. The optimum potential window for the constructed device was found to be 0-1.6 V. The CV curves for the device were obtained at different scan rates, i.e., 50-100 mV/s, and GCD curves at various current densities, i.e., 1-10 A/g, as demonstrated in Figure 6d,e.
The calculated specific capacity for the Co 3 O 4 @Mo-Co 3 O 4 //AC device is shown in Figure 6f. It showed a maximum capacitance of 154.6 F g −1 at a current density of 1 A g −1 .
The superior supercapacitor performance might be accredited to the strong self-supported structure. The Ragone plot (Figure 6g Table 3, affirms improved capacitive performance of fabricated Co 3 O 4 @Mo-Co 3 O 4 //AC device compared to previously reported works. The EIS study summarized in Figure 6h reveals minimal increase in solution resistance, even after 4000 cycles of charge/discharge. As shown in Figure 6i, the high retention (89%) also proved the long-term durability of the asymmetric device. Thus, the Co 3 O 4 @Mo-Co 3 O 4 //AC device is promising for use in advanced electronics. Such significant electrochemical outcomes could be attributed to the crystallinity and morphology of the Co 3 O 4 @Mo-Co 3 O 4 needle-like nanosheet heterostructure electrode, the self-supported structure with good mechanical and chemical stability, and the numerous nanoneedles, which offer enough surface area and active sites for electrochemical reactions and elec-trolyte intake. Additionally, the one-pot synthesis of the aforementioned heterostructure is a facile, rapid and cheap method.

Conclusions
A novel Co 3 O 4 @Mo-Co 3 O 4 nanosheet composite for supercapacitor applications was synthesized and its physical/electrochemical characteristics were investigated. The Co 3 O 4 @Mo-Co 3 O 4 nanosheet composite achieved a superior specific capacity of 814 C g −1 at 1 A g −1 and capacity retention of 90% with a good rate capability. The asymmetric SC fabricated using this composite material achieved a capacitance of 154.6 F g −1 at 1 A g −1 , a specific energy of 51.35 Wh Kg −1 , and a specific power of 790 W kg −1 . Moreover, the Co 3 O 4 @Mo-Co 3 O 4 //AC device possesses superior rate capabilities and long cycles, with 89.7% of the starting capacitance remaining after 4000 continuous cycles at 2 A g −1 . From this work, it may be concluded that the nanocomposite Co 3 O 4 @Mo-Co 3 O 4 nanosheets coated over a Ni foam skeleton displayed superior capacities and could be considered for use in ultra-capacitor devices in the future.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.