Hierarchically Developed Ni(OH)2@MgCo2O4 Nanosheet Composites for Boosting Supercapacitor Performance

MgCo2O4 nanomaterial is thought to be a promising candidate for renewable energy storage and conversions. Nevertheless, the poor stability performances and small specific areas of transition-metal oxides remain a challenge for supercapacitor (SC) device applications. In this study, sheet-like Ni(OH)2@MgCo2O4 composites were hierarchically developed on nickel foam (NF) using the facile hydrothermal process with calcination technology, under carbonization reactions. The combination of the carbon–amorphous layer and porous Ni(OH)2 nanoparticles was anticipated to enhance the stability performances and energy kinetics. The Ni(OH)2@MgCo2O4 nanosheet composite achieved a superior specific capacitance of 1287 F g−1 at a current value of 1 A g−1, which is higher than that of pure Ni(OH)2 nanoparticles and MgCo2O4 nanoflake samples. At a current density of 5 A g−1, the Ni(OH)2@MgCo2O4 nanosheet composite delivered an outstanding cycling stability of 85.6%, which it retained over 3500 long cycles with an excellent rate of capacity of 74.5% at 20 A g−1. These outcomes indicate that such a Ni(OH)2@MgCo2O4 nanosheet composite is a good contender as a novel battery-type electrode material for high-performance SCs.


Introduction
With the increasing demand for future-generation renewable energy storage cells, supercapacitors (SCs) are holding on as one of the most favored electrochemical devices, due to their ability to provide greater charge/discharge rates, prolonged cycling life, and superior power densities [1][2][3]. Nevertheless, most SCs are limited by lower energy densities that significantly hamper their further application. As established by the equation (E = 1/2CV 2 ), work voltage and electrode samples have pivotal roles in determining the energy density [4]. In order to attain greater energy densities, a hybrid SC setup could be designed to increased work voltages [5,6]. SCs possess a battery-type faradaic sample as the energy key and a capacitor nanomaterial as the power initiator [7]. Moreover, constructing unique electrodes to achieve greater specific capacities has been contemplated as a way to Nanomaterials 2023, 13, 1414 3 of 12 pieces of MgCo 2 O 4 nanoflakes loaded on Ni foam was then moved to a 40 mL autoclave and maintained at 110 • C for 3 h. After being cooled down to a normal temperature, the samples were rinsed with ethanol and DI water several times and dried at 70 • C for 5 h in a vacuum. The mass loading of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites was 3.7 mg cm −2 . For comparison, Ni(OH) 2 was fabricated on Ni foams using a similar procedure. The mass loading of the MgCo 2 O 4 nanoflake electrode and Ni(OH) 2 electrode was 3.7 mg cm −2 and 1.9 mg cm −2 , respectively.

Measurements and Characterizations
X-ray powder diffraction (XRD, Bruker D8 Advance with Cu Kα radiation) was used to characterize the crystalline phases of the samples. Field emission scanning electron microscopy (FE-SEM, JSM-7800F) was used to study the morphologies. High-resolution transmission electron microscopy (HRTEM, JEM-2100F at 200 kV) was used to study the microstructures and the elemental compositions. Photoelectron X-ray spectroscopy (XPS; ESCCALAB 250Xi, Busan, Republic of Korea) was used to study the chemical valence states and compositions of samples.

Measurements and Characterizations
All electrochemical measurements were conducted using a 3-electrode configuration in a 3 M KOH aqueous electrolyte. The fabricated samples, a Ag/AgCl electrode, and a platinum wire were utilized as working electrodes, reference electrodes, and counter electrodes, respectively. Galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV) were acquired on a Bio-Logic-SP-150C electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was employed in the frequency width from 0.01 to 100 kHz. The specific capacitance (C s , F g −1 ) was calculated from the GCD plots using the following equation [30]: where I (A), ∆t (s), and m (g) have their conventional meanings.

Results and Discussion
The synthesis procedure of Ni(OH) 2 @MgCo 2 O 4 is demonstrated in Figure 1. Initially, MgCo 2 O 4 nanoflakes were developed on Ni foam under simple hydrothermal and annealing technologies. After that, MgCo 2 O 4 nanoflakes were directly engaged as a skeleton to construct Ni(OH) 2 @MgCo 2 O 4 through secondary hydrothermal procedures. As an outcome, MgCo 2 O 4 nanoflakes gradually dissolved and generated fresh Ni(OH) 2 , manifesting a uniform sheet-like Ni(OH) 2 @MgCo 2 O 4 composite.

Preparation of Ni(OH)2@MgCo2O4 Nanosheet Composite
Following a typical procedure, 2 mmol NiCl2·6H2O and 4 mmol urea were put into 20 mL of DI water and stirred for 30 min to obtain a clear solution. The mixed precursor with pieces of MgCo2O4 nanoflakes loaded on Ni foam was then moved to a 40 mL autoclave and maintained at 110 °C for 3 h. After being cooled down to a normal temperature, the samples were rinsed with ethanol and DI water several times and dried at 70 °C for 5 h in a vacuum. The mass loading of the Ni(OH)2@MgCo2O4 nanosheet composites was 3.7 mg cm −2 . For comparison, Ni(OH)2 was fabricated on Ni foams using a similar procedure. The mass loading of the MgCo2O4 nanoflake electrode and Ni(OH)2 electrode was 3.7 mg cm −2 and 1.9 mg cm −2 , respectively.

Measurements and Characterizations
X-ray powder diffraction (XRD, Bruker D8 Advance with Cu Kα radiation) was used to characterize the crystalline phases of the samples. Field emission scanning electron microscopy (FE-SEM, JSM-7800F) was used to study the morphologies. High-resolution transmission electron microscopy (HRTEM, JEM-2100F at 200 kV) was used to study the microstructures and the elemental compositions. Photoelectron X-ray spectroscopy (XPS; ESCCALAB 250Xi, Busan, Republic of Korea) was used to study the chemical valence states and compositions of samples.

Measurements and Characterizations
All electrochemical measurements were conducted using a 3-electrode configuration in a 3 M KOH aqueous electrolyte. The fabricated samples, a Ag/AgCl electrode, and a platinum wire were utilized as working electrodes, reference electrodes, and counter electrodes, respectively. Galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV) were acquired on a Bio-Logic-SP-150C electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was employed in the frequency width from 0.01 to 100 kHz. The specific capacitance (Cs, F g −1 ) was calculated from the GCD plots using the following equation [30]: ∆t ∆V (1) where I (A), Δt (s), and m (g) have their conventional meanings.

Results and Discussion
The synthesis procedure of Ni(OH)2@MgCo2O4 is demonstrated in Figure 1. Initially, MgCo2O4 nanoflakes were developed on Ni foam under simple hydrothermal and annealing technologies. After that, MgCo2O4 nanoflakes were directly engaged as a skeleton to construct Ni(OH)2@MgCo2O4 through secondary hydrothermal procedures. As an outcome, MgCo2O4 nanoflakes gradually dissolved and generated fresh Ni(OH)2, manifesting a uniform sheet-like Ni(OH)2@MgCo2O4 composite.   [12,16,29]. Additionally, three longer peaks were identified at 2θ = 44.5 • , 51.85 • , and 76.37 • that came from the Ni foam skeleton (JCPDS No. 04-0850). Meanwhile, the line angles at 11.4 • , 23.7 • , and 33.4 • were matched to Ni(OH) 2 (JCPDS No. 38-0715) [31,32]. In general, the diffraction angles of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite were in good agreement with Ni(OH) 2 and MgCo 2 O 4 [28]. The XRD patterns of the Ni(OH)2, MgCo2O4, and Ni(OH)2@MgCo2O4 nanosheet electrodes are displayed in Figure 2. In the XRD tests of MgCo2O4, except for the diffraction angles from the nickel foam matrixes that remained, long peaks could be clearly identified in the MgCo2O4 phases [14]. The diffraction peaks positioned at 31°, 36.8°, 44.6°, 58.89°, and 64.98° were attributed to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), and (4 4 0) lattice planes of MgCo2O4 (JCPDS NO. 02-1073) [12,16,29]. Additionally, three longer peaks were identified at 2θ = 44.5°, 51.85°, and 76.37° that came from the Ni foam skeleton (JCPDS NO. 04-0850). Meanwhile, the line angles at 11.4°, 23.7°, and 33.4° were matched to Ni(OH)2 (JCPDS No. 38-0715) [31,32]. In general, the diffraction angles of the Ni(OH)2@MgCo2O4 nanosheet composite were in good agreement with Ni(OH)2 and MgCo2O4 [28]. XPS was utilized to detect the chemical valence states and compositions of each element in the developed Ni(OH)2@MgCo2O4 nanosheet composites. Figure 3a depicts the survey spectra, which illustrate the coexistence of Ni, Mg, Co, and O in the composite sample. The Ni 2p XPS spectra are shown in Figure 3b. The angles situated at 873.5 eV in Ni 2p1/2 and 855.6 eV in Ni 2p3/2 were ascribed to the Ni 2+ states, and those located at 876.6 eV in Ni 2p1/2 and 858.76 eV in Ni 2p3/2 were attributed to the Ni 3+ states [33][34][35]. The straight spectra of Mg 2p seen in the XPS results are shown in Figure 3c, where the peaks in the binding source at 51.2 eV confirm the existence of magnesium oxides. The Co 2p XPS spectra (Figure 3d) possess two spin orbits and two shake-up satellites (denoted as "Sat."). The phases at 794.2 eV and 779.4 eV were attributed to Co 3+ , whereas the remaining peaks at 798.4 eV and 782.5 eV were ascribed to Co 2+ [34]. The percentage of oxidation of cobalt ions was calculated based on the fitted peak areas of all the individual peaks in the Co 2p XPS spectra for the Ni(OH)2@MgCo2O4 nanosheet composites, and was determined to be 41:59 (Co 2+ :Co 3+ ). As depicted in Figure 3e, the O 1 s XPS spectra are composed of three different peaks labeled as O1, O2, and O3 [36,37]. The O1 peak at 531.2 eV was assigned to metal-oxidized bonds. The O2 peak at 532.8 eV was attributed to the OH − groups procured from Ni(OH)2. The O3 peak at 535.2 eV was ascribed to the chemisorbed oxidized atoms on the interface. In addition, Figure S1 of XPS full spectra of MgCo2O4 nanoflakes grown on Ni foam. XPS was utilized to detect the chemical valence states and compositions of each element in the developed Ni(OH) 2 @MgCo 2 O 4 nanosheet composites. Figure 3a depicts the survey spectra, which illustrate the coexistence of Ni, Mg, Co, and O in the composite sample. The Ni 2p XPS spectra are shown in Figure 3b. The angles situated at 873.5 eV in Ni 2p 1/2 and 855.6 eV in Ni 2p 3/2 were ascribed to the Ni 2+ states, and those located at 876.6 eV in Ni 2p 1/2 and 858.76 eV in Ni 2p 3/2 were attributed to the Ni 3+ states [33][34][35]. The straight spectra of Mg 2p seen in the XPS results are shown in Figure 3c, where the peaks in the binding source at 51.2 eV confirm the existence of magnesium oxides. The Co 2p XPS spectra (Figure 3d) possess two spin orbits and two shake-up satellites (denoted as "Sat."). The phases at 794.2 eV and 779.4 eV were attributed to Co 3+ , whereas the remaining peaks at 798.4 eV and 782.5 eV were ascribed to Co 2+ [34]. The percentage of oxidation of cobalt ions was calculated based on the fitted peak areas of all the individual peaks in the Co 2p XPS spectra for the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites, and was determined to be 41:59 (Co 2+ :Co 3+ ). As depicted in Figure 3e, the O 1 s XPS spectra are composed of three different peaks labeled as O1, O2, and O3 [36,37]. The O1 peak at 531.2 eV was assigned to metal-oxidized bonds. The O2 peak at 532.8 eV was attributed to the OH − groups procured from Ni(OH) 2 . The O3 peak at 535.2 eV was ascribed to the chemisorbed oxidized atoms on the interface. In addition, Figure S1 of XPS full spectra of MgCo 2 O 4 nanoflakes grown on Ni foam. Nanomaterials 2023, 13, x FOR PEER REVIEW 5 of 12 The intrinsic nanostructures and morphologies of the Ni(OH)2@MgCo2O4 nanosheet composite were analyzed via FE-SEM. The morphological structure of the MgCo2O4 nanoflake electrode is shown in Figure 4a-c from lower to higher magnifications. Uniform nanoflakes were grown at the edge of the surface, as shown in Figure 4a. Figure 4b,c indicate that the nanoflakes nearly uniformly covered the initial micro-morphology, and their formation was followed by an annealing process. Figure 4d demonstrates that the nanosheets were grown perpendicular to the skeleton. As illustrated in Figure 4e,f, it is obvious that the interface of the MgCo2O4 nanoflakes was covered with Ni(OH)2 nanoparticles. The buildup of Ni(OH)2 nanoparticles did not demolish the intrinsically ordered nanostructure. The nanoflakes and nanosheets would supply efficient transfer channels for the electrolytes during the charge storage procedures. The MgCo2O4 nanoflakes were fully positioned on Ni(OH)2 nanoflakes, which favored the effective conductive, preservative furnishing of MgCo2O4 nanoflakes with ion transportations in the electrolyte, and also provided safe structural stabilities. In addition, the Ni(OH)2@MgCo2O4 nanosheet composite supplied abundant active sites for the faradic redox process.  Figure 4a-c from lower to higher magnifications. Uniform nanoflakes were grown at the edge of the surface, as shown in Figure 4a. Figure 4b,c indicate that the nanoflakes nearly uniformly covered the initial micro-morphology, and their formation was followed by an annealing process. Figure 4d demonstrates that the nanosheets were grown perpendicular to the skeleton. As illustrated in Figure 4e,f, it is obvious that the interface of the MgCo 2 O 4 nanoflakes was covered with Ni(OH) 2 nanoparticles. The buildup of Ni(OH) 2 nanoparticles did not demolish the intrinsically ordered nanostructure. The nanoflakes and nanosheets would supply efficient transfer channels for the electrolytes during the charge storage procedures. The MgCo 2 O 4 nanoflakes were fully positioned on Ni(OH) 2 nanoflakes, which favored the effective conductive, preservative furnishing of MgCo 2 O 4 nanoflakes with ion transportations in the electrolyte, and also provided safe structural stabilities. In addition, the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite supplied abundant active sites for the faradic redox process.
The specific microstructures and morphology of the MgCo 2 O 4 nanoflakes ( Figure 5a) and Ni(OH) 2 @MgCo 2 O 4 nanosheet composite (Figure 5b,c) were obtained using TEM and HRTEM. Figure 5a shows that the MgCo 2 O 4 nanoflakes existed as the array type, and the nanoflake could be clearly seen. Figure 5b shows the TEM images of a Ni(OH) 2 @MgCo 2 O 4 nanosheet composite, demonstrating MgCo 2 O 4 has a sheet-type construction with a diameter range of 50 nm [34]. The HRTEM image of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite illustrates well-defined lattice spacings with interplanar fringes of 0.288 nm and 0.244 nm, which were assigned to the (2 2 0) and (3 1 1) planes of MgCo 2 O 4 [18,29]. The SAED image (inset in Figure 5c) obtained from the nanosheets reveals their polycrystalline characteristic, which greatly enhances the active sites needed for the Faraday capacity process. As shown in Figure 5d-g, the elements of Ni, Mg, and Co were homogeneously coated in the whole nanosheet, indicating the uniform coexistence of a Ni(OH) 2 @MgCo 2 O 4 nanosheet composite.  The specific microstructures and morphology of the MgCo2O4 nanoflakes ( Figure 5a) and Ni(OH)2@MgCo2O4 nanosheet composite (Figure 5b,c) were obtained using TEM and HRTEM. Figure 5a shows that the MgCo2O4 nanoflakes existed as the array type, and the nanoflake could be clearly seen. Figure 5b shows the TEM images of a Ni(OH)2@MgCo2O4 nanosheet composite, demonstrating MgCo2O4 has a sheet-type construction with a diameter range of 50 nm [34]. The HRTEM image of the Ni(OH)2@MgCo2O4 nanosheet composite illustrates well-defined lattice spacings with interplanar fringes of 0.288 nm and 0.244 nm, which were assigned to the (2 2 0) and (3 1 1) planes of MgCo2O4 [18,29]. The SAED image (inset in Figure 5c) obtained from the nanosheets reveals their polycrystalline characteristic, which greatly enhances the active sites needed for the Faraday capacity process. As shown in Figure 5d-g, the elements of Ni, Mg, and Co were homogeneously coated in the whole nanosheet, indicating the uniform coexistence of a Ni(OH)2@MgCo2O4 nanosheet composite.

Electrochemical Properties of Electrode Materials
To better evaluate the electrochemical behaviors of Ni(OH)2 nanoparticle electrodes, MgCo2O4 nanoflake electrodes, and Ni(OH)2@MgCo2O4 nanosheet composites, CV, GCD, and EIS measurements were conducted using a three-electrode configuration in a 2 M KOH aqueous electrolyte solution ( Figure 6). Figure 6a shows the results of the CV tests

Electrochemical Properties of Electrode Materials
To better evaluate the electrochemical behaviors of Ni(OH) 2 nanoparticle electrodes, MgCo 2 O 4 nanoflake electrodes, and Ni(OH) 2 @MgCo 2 O 4 nanosheet composites, CV, GCD, and EIS measurements were conducted using a three-electrode configuration in a 2 M KOH aqueous electrolyte solution ( Figure 6). Figure 6a shows the results of the CV tests of as-developed electrodes of the Ni(OH) 2 nanoparticle, MgCo 2 O 4 nanoflake electrodes, and Ni(OH) 2 @MgCo 2 O 4 nanosheet composites at a scanning rate of 5 mV s −1 . All electrode samples possessed an obvious couple of reversible redox peaks, demonstrating that active electrodes have typical battery-type characteristics. Particularly, as depicted in the CV test results shown in Figure 6a, the specific area of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite was much bigger than that of the Ni(OH) 2 nanoparticle electrode and MgCo 2 O 4 nanoflake electrode at similar scan rates, revealing the notable capacitance of Ni(OH) 2 @MgCo 2 O 4 nanosheet composites. Figure 6b depicts the GCD plots of the Ni(OH) 2 nanoparticle electrode, MgCo 2 O 4 nanoflake electrode, and Ni(OH) 2 @MgCo 2 O 4 nanosheet composites at an identical current density of 1 A g −1 . Figure 6d,f display the GCD curves of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite and MgCo 2 O 4 nanoflake electrodes at different current densities. Based on the GCD tests in Figure 6d,f, the C s of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites was measured as 1287 F g −1 at 1 A g −1 . At higher current density values of 2, 5, 10, and 20 A g −1 , the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites also reached good capacitances of 1071, 926, 661, and 459 F g −1 , respectively (Figure 6d), which were attributed to the large specific area and excellent ion interlayer exchanges of the composites. Figure 6c,e display the CV test results for the MgCo 2 O 4 nanoflake electrode and Ni(OH) 2 @MgCo 2 O 4 nanosheet composites at scanning rates ranging from 5 to 25 mV s −1 . The CV tests show that the specific area of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites was larger, indicating the C s of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites was higher.
The specific capacitances of the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites, MgCo 2 O 4 nanoflake electrodes, and Ni(OH) 2 nanoparticle electrodes based on the GCD tests were determined to be 1287 F g −1 , 1084 F g −1 , and 531 F g −1 , respectively, at a current density of 1 A g −1 (Figure 7a). Importantly, the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite electrode was found to exhibit notable rate capabilities with 74.5% retention of the starting capacitances at 20 A g −1 .
EIS measurements were conducted on the samples, and the corresponding Nyquist plots are presented in Figure 7b. The outline of the impedance spectra possesses three parts. In the high-frequency area, the real axis with intersections reveals the electrolyte's resistances (Rs), including the internal resistance, the interface resistance, and ionic resistance of the electrolytes between the working electrodes and current collector [38]. The semicircle diameters give the charge transfer impedances (Rct) [39]. The line slope in the lower-frequency region is attributed to the Warburg resistance (Rw), which is close to OH − in the aqueous solution [40,41]. We can see that the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite sample has abundant vertical lines and semicircles with a smaller diameter than those of the MgCo 2 O 4 nanoflake electrodes and Ni(OH) 2 nanoparticle electrodes, indicating rapid electron transportation kinetics and high diffusion ion rates. The EIS outcomes reveal that the Ni(OH) 2 @MgCo 2 O 4 nanosheet composites had lower Rs (0.59 Ω) and Rct (0.07 Ω) compared to the MgCo 2 O 4 nanoflake electrode (Rs was 0.81 Ω and Rct was 0.36 Ω) and Ni(OH) 2 nanoparticle electrode (Rs was 0.93 Ω and Rct was 0.51 Ω), indicating superior electrical conductivities and fast electron transportation kinetics.
Cycling stability is also a crucial factor in studying the characteristics of SCs. Thus, continuous charge/discharge procedures were maintained for Ni(OH) 2 @MgCo 2 O 4 nanosheet composites, MgCo 2 O 4 nanoflake electrodes, and Ni(OH) 2 nanoparticle electrodes at a current density of 2 A g −1 over 3500 long cycles. As illustrated in Figure 7c, the Ni(OH) 2 nanoparticles and MgCo 2 O 4 nanoflake electrodes retained 78.3% and 81.2% of their starting capacitances, respectively, over 3500 cycles. However, the hybrid Ni(OH) 2 @MgCo 2 O 4 nanosheet composite electrode (with an initial specific capacitance of 1071 F g −1 ) retained 85.6% of its initial capacitance after 3500 long cycles. The unique hybrid constructions achieved outstanding cycling stability results. The specific capacitance values of the Ni(OH) 2 @ MgCo 2 O 4 electrode were compared with those found in previous studies, as shown in Table 1.

FOR PEER REVIEW 8 of 12
Ni(OH)2@MgCo2O4 nanosheet composites was measured as 1287 F g −1 at 1 A g −1 . At higher current density values of 2, 5, 10, and 20 A g −1 , the Ni(OH)2@MgCo2O4 nanosheet composites also reached good capacitances of 1071, 926, 661, and 459 F g −1 , respectively ( Figure  6d), which were attributed to the large specific area and excellent ion interlayer exchanges of the composites.  was found to exhibit notable rate capabilities with 74.5% retention of the starting capacitances at 20 A g −1 . EIS measurements were conducted on the samples, and the corresponding Nyquist plots are presented in Figure 7b. The outline of the impedance spectra possesses three parts. In the high-frequency area, the real axis with intersections reveals the electrolyte's resistances (Rs), including the internal resistance, the interface resistance, and ionic resistance of the electrolytes between the working electrodes and current collector [38]. The semicircle diameters give the charge transfer impedances (Rct) [39]. The line slope in the lower-frequency region is attributed to the Warburg resistance (Rw), which is close to OH − in the aqueous solution [40,41]. We can see that the Ni(OH)2@MgCo2O4 nanosheet composite sample has abundant vertical lines and semicircles with a smaller diameter than those of the MgCo2O4 nanoflake electrodes and Ni(OH)2 nanoparticle electrodes, indicating rapid electron transportation kinetics and high diffusion ion rates. The EIS outcomes reveal that the Ni(OH)2@MgCo2O4 nanosheet composites had lower Rs (0.59 Ω) and Rct (0.07 Ω) compared to the MgCo2O4 nanoflake electrode (Rs was 0.81 Ω and Rct was 0.36 Ω) and Ni(OH)2 nanoparticle electrode (Rs was 0.93 Ω and Rct was 0.51 Ω), indicating superior electrical conductivities and fast electron transportation kinetics.
Cycling stability is also a crucial factor in studying the characteristics of SCs. Thus, continuous charge/discharge procedures were maintained for Ni(OH)2@MgCo2O4

Conclusions
In summary, we successfully fabricated a Ni(OH) 2 @MgCo 2 O 4 nanosheet composite electrode on Ni foam using a facile two-step hydrothermal route followed by annealing techniques. The as-synthesized sample electrodes achieved notable electrochemical activities due to their synergistic effects. Combining the synergistic effects of the outstandingly aligned MgCo 2 O 4 nanoflakes and unique specific area of Ni(OH) 2 nanoparticles, the as-developed Ni(OH) 2 @MgCo 2 O 4 nanosheet composite demonstrated a superior C s of 1287 F g −1 at a current density of 1 A g −1 and rate capabilities superior to those of MgCo 2 O 4 nanoflakes and Ni(OH) 2 nanoparticle electrodes alone. Moreover, the Ni(OH) 2 @MgCo 2 O 4 nanosheet composite showed a notable cyclic stability of 85.6% over 3500 long cycles. These features of battery-type electrode materials with outstanding electrochemical performances demonstrate their promise as high-performance SCs.

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