A Facile Two-Step Hydrothermal Synthesis of Co(OH)2@NiCo2O4 Nanosheet Nanocomposites for Supercapacitor Electrodes

The composites of NiCo2O4 with unique structures were substantially investigated as promising electrodes. In this study, the unique structured nanosheets anchored on nickel foam (Ni foam) were prepared under the hydrothermal technique of NiCo2O4 and subsequent preparation of Co(OH)2. The Co(OH)2@NiCo2O4 nanosheet composite has demonstrated higher specific capacitances owing to its excellent specific surface region, enhanced rate properties, and outstanding electrical conductivities. Moreover, the electrochemical properties were analyzed in a three-electrode configuration to study the sample material. The as-designed Co(OH)2@NiCo2O4 nanosheet achieves higher specific capacitances of 1308 F·g−1 at 0.5 A·g−1 and notable long cycles with 92.83% capacity retention over 6000 cycles. The Co(OH)2@NiCo2O4 nanosheet electrode exhibits a long life span and high capacitances compared with the NiCo2O4 and Co(OH)2 electrodes, respectively. These outstanding electrochemical properties are mainly because of their porous construction and the synergistic effects between NiCo2O4 and Co(OH)2. Such unique Co(OH)2@NiCo2O4 nanosheets not only display promising applications in renewable storage but also reiterate to scientists of the unlimited potential of high-performance materials.


Introduction
Unprecedented fossil fuel consumption became one of the most significant global challenges due to the ongoing increase in the world's population [1][2][3]. Facing this critical issue of fossil fuel depletion as well as the climate emergency crises, the petition for synthesizing eco-friendly and renewable energies became more and more important [4][5][6]. Hence, new energy storage and harvesting technologies became the focus of ongoing global research. Supercapacitors (SCs) and batteries were among the energy-storing devices that significantly improved for better energy-harvesting devices [7,8]. Batteries are typically energy-storing assets that use faradaic electrochemical procedures to store electrical energy 2. Experimental 2.1. Synthesis of Co(OH) 2

@NiCo 2 O 4 Nanosheet Composite
The chemicals used in this study were purchased from Sigma-Aldrich, Busan, Republic of Korea, and utilized without additional purification; these are potassium hydroxide (KOH), cobalt nitrate hexahydrate (Co(NO 3 ) 2 ·6H 2 O), ammonium hydroxide (NH 4 OH), nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O), hydrochloric acid (HCl), and urea (CH 4 N 2 O). A clean Ni foam (with a size of 2 × 4 cm 2 ) was immersed into a mixed solution of 10 mmol Co(NO 3 ) 2 , 20 mL of 28 wt% ammonia solution, 15 mmol NH 4 F, and 25 mL of H 2 O. The upside of the Ni foam was covered from solution contaminations by uniformly coating it with polytetrafluorethylene tapes. The autoclave was collected and put at 130 • C for 12 h for nanoparticle growth. Then, the product was collected and rinsed multiple times with DI water to detach the residual particle debris.
The Co(OH) 2 @NiCo 2 O 4 electrodes were fabricated via an easy one-step hydrothermal route. All the reagents were analytically graded and organized without further purifications. In the particular fabrication experimental part, 0.94 g of Co(NO 3 ) 2 ·6H 2 O and 0.503 g of Ni (NO 3 ) 2 .6H 2 O were dissolved in 30 mL DI water using a stirrer. The relevant quantity of polyethylene glycol was dissolved in 15 mL DI water using a stirrer. The precursors of Ni 2+ and the Co 2+ solutions were included in the solution of polyethylene glycol concurrently under a continuous magnetic stirrer. Then, 25 mL ammonium hydroxide was included in the above mixture solutions. After continuous stirring for 45 min to form a final mixture solution, the final suspension precursor was shifted into a 70 mL autoclave and optimized at 160 • C for 10 h. Then, after the autoclave came to normal temperature, the final products were gathered, washed, and dried at 50 • C for 18 h. Scheme 1 described the information on the synthesis of Co(OH) 2 @NiCo 2 O 4 nanosheet composited electrode.

Synthesis of Co(OH)2@NiCo2O4 Nanosheet Composite
The chemicals used in this study were purchased from Sigma-Aldrich, Busan, South Korea, and utilized without additional purification; these are potassium hydroxide (KOH), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), ammonium hydroxide (NH4OH), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), hydrochloric acid (HCl), and urea (CH4N2O). A clean Ni foam (with a size of 2 × 4 cm 2 ) was immersed into a mixed solution of 10 mmol Co(NO3)2, 20 mL of 28 wt% ammonia solution, 15 mmol NH4F, and 25 mL of H2O. The upside of the Ni foam was covered from solution contaminations by uniformly coating it with polytetrafluorethylene tapes. The autoclave was collected and put at 130 °C for 12 h for nanoparticle growth. Then, the product was collected and rinsed multiple times with DI water to detach the residual particle debris.
The Co(OH)2@NiCo2O4 electrodes were fabricated via an easy one-step hydrothermal route. All the reagents were analytically graded and organized without further purifications. In the particular fabrication experimental part, 0.94 g of Co(NO3)2·6H2O and 0.503 g of Ni (NO3)2.6H2O were dissolved in 30 mL DI water using a stirrer. The relevant quantity of polyethylene glycol was dissolved in 15 mL DI water using a stirrer. The precursors of Ni 2+ and the Co 2+ solutions were included in the solution of polyethylene glycol concurrently under a continuous magnetic stirrer. Then, 25 mL ammonium hydroxide was included in the above mixture solutions. After continuous stirring for 45 min to form a final mixture solution, the final suspension precursor was shifted into a 70 mL autoclave and optimized at 160 °C for 10 h. Then, after the autoclave came to normal temperature, the final products were gathered, washed, and dried at 50 °C for 18 h. Scheme 1 described the information on the synthesis of Co(OH)2@NiCo2O4 nanosheet composited electrode. Scheme 1. Schematic illustration of the synthesis of Co(OH)2@NiCo2O4 nanosheet composite electrode. Scheme 1. Schematic illustration of the synthesis of Co(OH) 2 @NiCo 2 O 4 nanosheet composite electrode.

Physical Characterization
The crystallinity and structure of Co(OH) 2 @NiCo 2 O 4 nanosheet were studied via an XRD analysis (Rigaku, XRD, Bruker D8 Advance, a wavelength of 1.540 Å, Bruker AXS LTD., Busan, Republic of Korea). The microstructures of the developed nanomaterials were Nanomaterials 2023, 13,1981 4 of 14 studied using FE-SEM (FE-SEM, JSM-7800F equipping, JEOL LTD., Busan, Republic of Korea) instrument operated at an accelerating voltage of 5 kV. The composite elements were studied using energy-dispersive X-ray (EDAX, BRUKER, Thermo Scientific LTD., Busan, Republic of Korea) equipment. A high-resolution transmission electron microscope (TEM, JEM-2100F, JEOL LTD., Busan, Republic of Korea) instrument operated at an accelerating voltage of 200 kV was employed to find the HR-TEM images of the grown Co(OH) 2 , NiCo 2 O 4 and Co(OH) 2 @NiCo 2 O 4 nanosheet. The valence states of the developed nanoconstructions were registered using an X-ray photoelectron spectrometer (XPS, ESCCALAB 250Xi, Thermo Scientific LTD., Busan, Republic of Korea) with a maintaining power of 60 W.

Electrochemical Characterization
The electrochemical properties were achieved using a 3-electrode configuration with Co(OH) 2 @NiCo 2 O 4 nanosheet, Ag/AgCl, and platinum as the working, reference, and counter electrodes, respectively, in an electrolyte of 2 M KOH solution using the electrochemical (Bio-Logic Model: SP-150, Busan, Republic of Korea) equipment. The galvanostatic charge/discharges (GCD) analyzer was conveyed in the window of 0.0 to 0.5 V at a current density in the range of 10-50 A·g −1 . The cyclic voltammetry (CV) examinations were carried out in the window ranges from 0.0 to 0.5 V at scan rates in the range of 0.5-10 mV·s −1 . The electrochemical impedance spectroscopy (EIS) measurements were performed at several frequencies from 1 mHz to 100 kHz at a magnitude of 15 mV. By using GCD plots, the specific capacitances were determined by the following formula [35,36]: where I is the current (A), ∆t is the discharging time (secs), m is the mass of sample (g), and ∆V is the potential (V).  [37]. Diffraction peaks located at 2θ = 11.2 • , 33.1 • , and 59.73 • correspond to (003), (012), and (110) facets, which were well-marked with the Co(OH) 2 phases (JCPDS card no. 46-0605) [38,39]. It is noticed that there are very slight peaks shifts of NiCo 2 O 4 and Co(OH) 2 phases. We believe that these peak shifts could be due to the existence of some stress at the Co(OH) 2 @NiCo 2 O 4 interface. Thus, the as-prepared sample was confirmed to be a composite of NiCo 2 O 4 and Co(OH) 2 . All angle diffraction facets provided evidence for the admirable crystallinity of Co(OH) 2 @NiCo 2 O 4 nanosheet composite on Ni foam.

Results and Discussions
FTIR spectra of the as-obtained NiCo 2 O 4 electrode and Co(OH) 2 @NiCo 2 O 4 nanosheet are recorded and shown in Figure S2. It can be seen that the characteristic peaks of NiCo 2 O 4 appear at about 553 and 643 cm −1 ( Figure S2a between Co(OH)2 and NiCo2O4, further confirming that the Co(OH)2 has been coated closely on the NiCo2O4 electrode surface. The SEM mappings ( Figure 2) of the electrode material demonstrate the morphologies of the samples. Figure 2a discloses the nanostructure of pure NiCo2O4 nanoparticles that consist of major tips and a length of about 10 μm. The nanoparticles were collected from many smaller particles to make porous constructions. The mesoporous in NiCo2O4 nanoparticles would enable the ion diffusions, which further enhances the electrochemical activities. Figure 2b displays the low-magnification SEM image of the Co(OH)2@NiCo2O4 nanosheet composite. When compared with binary NiCo2O4 nanoparticles, the surface shifts from smooth to rough with the coating of Co(OH)2 nanoparticles (Figure 2c). The coatings were mesoporous structures that were networked with each other to form an excellent facilitator surface area. In particular, the Co(OH)2 particles were significantly anchored onto NiCo2O4 nanoparticles as backbones to construct a unique core-shell morphology ( Figure 2d). The porous NiCo2O4 nanoparticles and Co(OH)2 particles make the interior of the composite nanosheets feasible for the electrolytes. From Figure 2, the elemental mapping images of Co, Ni, and O elements once again confirm the formation of Co(OH)2@NiCo2O4. Atomic percentages of Co, Ni, and O are obtained to be 19.41%, 43.56%, and 29.12%, respectively. Their atomic molar ratio is thus about 2:5:2. The detailed explanation is given in the revised manuscript. These are percentages normalized to 100% as the sum of percentages for elements detected and quantified. Not all elements, such as very light ones, are detected and therefore are not included in the 100%. Also, elements present in a small quantity, even, e.g., carbon, etc., below about 0.3 wt% cannot be quantified or are quantified with big uncertainty. The SEM mappings (Figure 2) of the electrode material demonstrate the morphologies of the samples. Figure 2a discloses the nanostructure of pure NiCo 2 O 4 nanoparticles that consist of major tips and a length of about 10 µm. The nanoparticles were collected from many smaller particles to make porous constructions. The mesoporous in NiCo 2 O 4 nanoparticles would enable the ion diffusions, which further enhances the electrochemical activities. Figure 2b displays the low-magnification SEM image of the Co(OH) 2 @NiCo 2 O 4 nanosheet composite. When compared with binary NiCo 2 O 4 nanoparticles, the surface shifts from smooth to rough with the coating of Co(OH) 2 nanoparticles (Figure 2c). The coatings were mesoporous structures that were networked with each other to form an excellent facilitator surface area. In particular, the Co(OH) 2 particles were significantly anchored onto NiCo 2 O 4 nanoparticles as backbones to construct a unique core-shell morphology ( Figure 2d). The porous NiCo 2 O 4 nanoparticles and Co(OH) 2 particles make the interior of the composite nanosheets feasible for the electrolytes. From Figure 2, the elemental mapping images of Co, Ni, and O elements once again confirm the formation of Co(OH) 2 @NiCo 2 O 4 . Atomic percentages of Co, Ni, and O are obtained to be 19.41%, 43.56%, and 29.12%, respectively. Their atomic molar ratio is thus about 2:5:2. The detailed explanation is given in the revised manuscript. These are percentages normalized to 100% as the sum of percentages for elements detected and quantified. Not all elements, such as very light ones, are detected and therefore are not included in the 100%. Also, elements present in a small quantity, even, e.g., carbon, etc., below about 0.3 wt% cannot be quantified or are quantified with big uncertainty.
TEM measurements ( Figure 3) were carried out to disclose the comprehensive structure of the as-designed samples. Figure 3a illustrates the TEM images, (b) the SAED pattern, and (c) the HRTEM image of the Co(OH) 2 @NiCo 2 O 4 nanosheet composite. Figure 3a shows the NiCo 2 O 4 particle formations, indicating that the single crystalline nature of the material is porous and constructed with many irregular particles, which correlated with SEM data. Meanwhile, the pattern of SAED reveals the polycrystalline (co-centric rings) nature of the samples of NiCo 2 O 4 [34]. The good-indexed lattice fringe with interplanar spaces of 0.28 nm was ascribed to the (220) facets of the NiCo 2 O 4 spinel angle. This demonstrates further evidence of the formation of crystallized nickel cobaltite. Nanomaterials 2023, 13, x FOR PEER REVIEW 6 of 15 TEM measurements ( Figure 3) were carried out to disclose the comprehensive structure of the as-designed samples. Figure 3a illustrates the TEM images, (b) the SAED pattern, and (c) the HRTEM image of the Co(OH)2@NiCo2O4 nanosheet composite. Figure 3a shows the NiCo2O4 particle formations, indicating that the single crystalline nature of the material is porous and constructed with many irregular particles, which correlated with SEM data. Meanwhile, the pattern of SAED reveals the polycrystalline (co-centric rings) nature of the samples of NiCo2O4 [34]. The good-indexed lattice fringe with interplanar spaces of 0.28 nm was ascribed to the (220) facets of the NiCo2O4 spinel angle. This demonstrates further evidence of the formation of crystallized nickel cobaltite. XPS analysis was performed to investigate the oxidational elements and valence states of the as-designed samples, which are displayed in Figure 4. The full spectra (Figure 4a) reveal only the existences of Ni, Co, C, and O elements, which correlates with the XRD results. The major peaks from Co and the weaker peaks from Ni are due to NiCo 2 O 4 being entirely covered by Co(OH) 2 particles. Figure 4b illustrates two strong peaks originating at 797.4 eV and 780.7 eV, which are attributed to Co 2p 1/2 and Co 2p 3/2 , respectively. The other satellite points are noticed at 803.1 eV and 786.4 eV. The fitting of Co 2p peaks at 780.6 eV and 796.4 eV are indexed to Co 3+ , and the fitting of Co 2p peaks at 782.5 eV and 797.7 eV correspond to Co 2+ . The data also were correlated to the Co spectrum of Co(OH) 2 in recent notices [40]. In Figure 4c for Ni 2p spectra, excluding two satellite points, the peaks pointed at 873.6 eV and 855.7 eV for Ni 2p 1/2 and Ni 2p 3/2 , respectively, were assigned to Ni 2+ , while the other two peaks at 875.7 eV and 856.8 eV correlate with Ni 3+ [41,42]. The O 1s spectrum was deconvoluted (Figure 4d) into three peaks, namely O1, O2, and O3. The O1 at 530.1 eV binding energy supports the oxygen-metals bonding, the O2 at 531.4 eV is ascribed to oxygen in the OH − group and defect sites, whereas O3 at 531.4 eV represents the -H-O-H content absorbed at the sample [43,44]. As displayed in Figure 4d, the XPS spectrum was separated into three peaks around 527.7 eV, 529 eV, and 530 eV, which were assigned to Al 2 O 3 (36%), hydroxides (47%), and carbonates (17%) from the side of lower binding energy, respectively. Through the XPS and XRD outcomes, we can confirm that the as-prepared sample was the Co(OH) 2 @NiCo 2 O 4 composite. Figure S1 displays the XPS full survey spectrums of Co(OH)2 and NiCo2O4 electrode materials. The full spectra ( Figure S1a) reveal only the existence of Co and O elements in the Co(OH) 2 material. The full spectra ( Figure S1b)   XPS analysis was performed to investigate the oxidational elements and valence states of the as-designed samples, which are displayed in Figure 4. The full spectra (Figure 4a) reveal only the existences of Ni, Co, C, and O elements, which correlates with the XRD results. The major peaks from Co and the weaker peaks from Ni are due to NiCo2O4 being entirely covered by Co(OH)2 particles. Figure 4b illustrates two strong peaks originating at 797.4 eV and 780.7 eV, which are attributed to Co 2p1/2 and Co 2p3/2, respectively. The other satellite points are noticed at 803.1 eV and 786.4 eV. The fitting of Co 2p peaks at 780.6 eV and 796.4 eV are indexed to Co 3+ , and the fitting of Co 2p peaks at 782.5 eV and 797.7 eV correspond to Co 2+ . The data also were correlated to the Co spectrum of Co(OH)2 in recent notices [40]. In Figure 4c for Ni 2p spectra, excluding two satellite points, the peaks pointed at 873.6 eV and 855.7 eV for Ni 2p1/2 and Ni 2p3/2, respectively, were assigned to Ni 2+ , while the other two peaks at 875.7 eV and 856.8 eV correlate with Ni 3+ [41,42]. The O 1s spectrum was deconvoluted (Figure 4d) into three peaks, namely O1, O2, and O3. The O1 at 530.1 eV binding energy supports the oxygen-metals bonding, the O2 at 531.4 eV is ascribed to oxygen in the OH − group and defect sites, whereas O3 at 531.4 eV represents the -H-O-H content absorbed at the sample [43,44]. As displayed in Figure 4d, the XPS spectrum was separated into three peaks around 527.7 eV, 529 eV, and 530 eV, which were assigned to Al2O3 (36%), hydroxides (47%), and carbonates (17%) from the side of lower binding energy, respectively. Through the XPS and XRD outcomes, we can confirm that the asprepared sample was the Co(OH)2@NiCo2O4 composite. Figure S1 displays the XPS full survey spectrums of Co(OH)2 and NiCo2O4 electrode materials. The full spectra ( Figure  S1a) reveal only the existence of Co and O elements in the Co(OH)2 material. The full   Figure 4f shows the isotherm of Co(OH) 2 @NiCo 2 O 4 nanosheet composite, which was a typical type-I isotherm owing to the sharp increase at a lower relative pressure (P/P 0 < 0.1), which reveals the existence of micropores. The isotherm adsorption line illustrates a vital hysteresis loop in the middlepressure part (P/P 0 = 0.4-0.9), which was due to the condensation of N 2 molecules under lower pressure sections, and then the pores are occupied. The pore size distribution of Co(OH) 2 @NiCo 2 O 4 nanosheet composite describes two peaks at 1.4 nm and 4.2 nm, which further confirms the presence of micropores and mesopores. The BET surface areas are measured to be 54.6 m 2 ·g −1 , and 41.3 m 2 ·g −1 for Co(OH) 2 @NiCo 2 O 4 , and NiCo 2 O 4 , respectively. The Co(OH) 2 @NiCo 2 O 4 nanosheet composite had the highest surface area of the three samples and is thus expected to have excellent electrochemical performance.

Electrochemical Properties
The electrochemical properties of the Co(OH) 2 , NiCo 2 O 4 , and Co(OH) 2 @NiCo 2 O 4 nanosheet electrode samples were analyzed using a three-electrode cell in an electrolyte of Nanomaterials 2023, 13, 1981 9 of 14 2 M KOH solution at the ambient temperature. The CV plots were maintained within the potential window of 0.0-0.5 V. Figure 5a demonstrates the CV measurements of various samples at fixed scan rates, e.g., at 5 mV·s −1 . The noticeable faradic peaks reveal redox reactions. The faradic procedures are given by the following formulas [45]: A pairing of redox peaks appeared in the plot of the NiCo 2 O 4 sample, which is due to the faradic reactions of Co 4+ /Co 3+ and Ni 3+ /Ni 2+ . In the Co(OH) 2 sample, two couples of faradic peaks Co 4+ /Co 3+ and Co 3+ /Co 2+ were revealed in Equations (2)-(4). Thus, the Co(OH) 2 @NiCo 2 O 4 nanosheet composite electrode discloses coupling pairs of redox peaks. Figure 5b demonstrates the CV plots of the Co(OH) 2 @NiCo 2 O 4 nanosheet composite at various scan rates. The reduction and oxidation points switch significantly toward positive and negative windows with increased scans rate because the diffusion outlay of the electrode ions decreases at high scan rates, which prevents ions adsorptions/desorption. Figure 5c illustrates the GCD plots of the same samples under a current density rate in the range of 0.5-10 A·g −1 , while the voltage window ranges from 0.0 to 0.5 V to acquire direct comparisons. As anticipated, the composite Co(OH) 2 @NiCo 2 O 4 nanosheet electrode consists of prolonged discharge times (573 s) compared with the remaining electrodes of NiCo 2 O 4 (303 s) and Co(OH) 2 (196 s). At 0.5 A·g −1 , according to Equation (1), the composite Co(OH) 2 @NiCo 2 O 4 nanosheets electrode possesses the higher specific capacitances of 1308 F·g −1 , while for NiCo 2 O 4 , it was 926 F·g −1 , and for Co(OH) 2 , it was 413 F·g −1 .
These outcomes reveal that the composite Co(OH) 2 @NiCo 2 O 4 nanosheet electrode possesses greater specific capacitances than the other Co(OH) 2 and NiCo 2 O 4 electrodes. Moreover, the growth material of Co(OH) 2 not only supplies plentiful electroactive sites but also possesses open networks for electrolytes to facile transportations. In addition, the high conductivity of NiCo 2 O 4 resulted in a reduced resistance between the Co(OH) 2 electrode and Ni foam, improving the integral conduciveness of the composite sample. The uniform growth Co(OH) 2 on NiCo 2 O 4 nanoparticles efficiently resulted in the aggregations of Co(OH) 2 particles.
The GCD plots of the as-developed composite electrode at different current densities from 0.5 to 10 A·g −1 are depicted in Figure 5d. Figure 5e outlined the dependences of capacitances on current densities, indicating the rate activities of the sample materials. The specific capacitances of composite Co(OH) 2 @NiCo 2 O 4 nanosheets electrode were recorded to be 1308, 1101, 1013, 842, and 639 F·g −1 at 0.5, 1, 3, 5, and 10 A·g −1 , respectively, which are much greater than those of Co(OH) 2 and NiCo 2 O 4 electrodes. The capacitances of the electrode materials significantly decrease as the current increases because some electrons will not be able to completely access the interior of the samples. Figure S3a,b displays CV and GCD plots of pure NiCo 2 O 4 electrodes at various scan rates and different current densities grown on Ni foam.
To investigate the kinetic reaction of the electrode samples, EIS analysis was performed at a frequency from 100 kHz to 0.01 Hz. Figure 5f displays the Nyquist plots of all samples. Each curve consists of an inclining line in the lower frequencies area and a mini semicircle in the higher-frequencies zone [46][47][48]. As per the equivalent circuit diagram, which consists of equivalent series resistance (Rs), charge-transfer resistance (Rct), and Warburg resistances (W), the composite Co(OH) 2  The GCD plots of the as-developed composite electrode at different current densities from 0.5 to 10 A·g −1 are depicted in Figure 5d. Figure 5e outlined the dependences of capacitances on current densities, indicating the rate activities of the sample materials. The The long cycling stability is one of the significant parameters of SCs. To evaluate the stability of the composite Co(OH) 2 @NiCo 2 O 4 nanosheet electrode and pure NiCo 2 O 4 electrode, the samples were analyzed using GCD at 3 A·g −1 . As illustrated in Figure 6, after 6000 cycles, the capacity retention remained at 92.83%, indicating superior reversible kinetics and cycling stabilities. The surface between the electrolyte and electrode would infiltrate through the ionic solution with restraining diffusions, and the material samples would improve the active sites via 'ions' diffusions with electrolytes and electrodes [48]. The specific capacitance of the as-prepared Co(OH) 2 @NiCo 2 O 4 nanosheet electrode value is larger than that of the previously reported SCs, as shown in Table 1 small outcome (0.913 Ω) compared with the NiCo2O4 (1.763 Ω) and Co(OH)2 (2.137 Ω) electrodes. For the composite Co(OH)2@NiCo2O4 nanosheets materials, Rct obtained from the partial semicircle is 0.096 Ω, while the Rct values for NiCo2O4 and Co(OH)2 samples are 0.172 Ω and 0.181 Ω, respectively. These values signify that composite Co(OH)2@NiCo2O4 nanosheet electrodes possess greater conductivities. The inclination of the plots in lower frequencies indicates the diffusion resistivity (W) of electrolyte ions. The observed values reveal that a good thickness of Co(OH)2 covered on NiCo2O4 nanoparticles could enhance the electrochemical activities of the electrode. Figure S4a displays Nyquist plots and Figure S4b Cycling stability of Co(OH)2 electrode at 3 A·g −1 in the threeelectrode electrochemical test.
The long cycling stability is one of the significant parameters of SCs. To evaluate the stability of the composite Co(OH)2@NiCo2O4 nanosheet electrode and pure NiCo2O4 electrode, the samples were analyzed using GCD at 3 A·g −1 . As illustrated in Figure 6, after 6000 cycles, the capacity retention remained at 92.83%, indicating superior reversible kinetics and cycling stabilities. The surface between the electrolyte and electrode would infiltrate through the ionic solution with restraining diffusions, and the material samples would improve the active sites via 'ions' diffusions with electrolytes and electrodes [48]. The specific capacitance of the as-prepared Co(OH)2@NiCo2O4 nanosheet electrode value is larger than that of the previously reported SCs, as shown in Table 1

Conclusions
In summary, the composite Co(OH) 2 @NiCo 2 O 4 nanosheet samples on Ni foam were successfully prepared via easy hydrothermal preparation. The elemental compositions and higher crystallinities were validated using XPS and XRD analyzers. The morphological structure analysis of the Co(OH) 2 @NiCo 2 O 4 nanosheet was carried out using SEM and TEM experiments. The NiCo 2 O 4 nanoparticles components cohabited with the aggregations of Co(OH) 2 particles in unique constructions. Owing to the porous constructions and the synergistic effects of NiCo 2 O 4 and Co(OH) 2 , the as-designed composite Co(OH) 2 @NiCo 2 O 4 nanosheet electrode achieved higher specific capacitance of 1308 F·g −1 at 0.5 A·g −1 and a notable cycle stability with 92.83% capacity retention over 6000 long-cycles. All the outcomes substantiate that composite Co(OH) 2 @NiCo 2 O 4 nanosheet electrode is a promising candidate for wider applications prospects in high-performance SCs.

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