Supercapacitor performance of porous nickel cobaltite nanosheets

In this work, nickel cobaltite (NiCo2O4) nanosheets with a porous structure were fabricated on nickel foam as a working electrode for supercapacitor applications. The nanosheets were fabricated by electrochemical deposition of nickel–cobalt hydroxide on the nickel foam substrate at ambient temperature in a three-electrode cell followed by annealing at 300 °C to transform the coating into a porous NiCo2O4 nanosheet. Field emission scanning electron microscopy and transmission electron microscopy revealed a three-dimensional mesoporous structure, which facilitates ion transport and electronic conduction for fast redox reactions. For one cycle, the NiCo2O4 electrodeposited nickel foam has a high specific capacitance (1734.9 F g−1) at a current density (CD) of 2 A g−1. The electrode capacitance decreased by only approximately 12.7% after 3500 cycles at a CD of 30 A g−1. Moreover, a solid-state asymmetric supercapacitor (ASC) was built utilising the NiCo2O4 nanosheets, carbon nanotubes, and a polyvinyl alcohol-potassium hydroxide gel as the anode, cathode, and solid-state electrolyte, respectively. The ASC displayed great electrochemical properties with a 42.25 W h kg−1 energy density at a power density of 298.79 W kg−1.

solutions (Ag/AgCl) [16][17][18][19] . It has been proved that the introduction of graphene can effectively enhance the total capacitance and stability, primarily because graphene can withstand the basic structures of polyaniline and evade mechanical deformation in the redox process 20 . Therefore, ternary composites with carbon nanomaterials, using transition metal oxides and polymers, have been studied and showed an enhancement in the electrochemical performance that open a new fabrication pathway for next generation high-performance electrochemical electrodes 21 . Ternary CuCo 2 S 4 is well known as an electrode material for electrochemical capacitors because of its synergistic effects, high conductivity, and low cost. The supercapacitive performance of CuCo 2 S 4 electrodes for electrochemical capacitors was investigated by Xu et al. In their study, thin layers of CuCo 2 S 4 were deposited on conductive substrates, and the results showed better properties than single crystal CuCo 2 S 4 22 . Electrodeposition has been utilised for different applications, such as microelectronics and energy conversion. Because of the need for enhanced performance and device miniaturization, nuanced control of the growth process is needed; electrochemical deposition is a technique that can fulfil those requirements 23 .
Chen et al. successfully deposited Ni-Co-S nanosheet arrays on carbon fibres through one-step electrodeposition of the ternary sulfides to provide an effective and facile approach for large scale applications, which was a significant advantage compared to other multistep synthesis techniques. They found that the Ni-Co-S-4 interconnected nanosheet exhibits the best electrochemical performance as a supercapacitor electrode 24 .
One effective method of enlarging the potential window of the NiCo 2 O 4 electrode to attain a high energy density is the utilisation of asymmetric devices 16,[25][26][27] . For example, Yedluri et al. fabricated chain-like NiCo 2 O 4 / NiCo 2 O 4 nanofile arrays using a facile hydrothermal and thermal decomposition approach and reported a specific capacitance of 2312 F g −1 at a current density (CD) of 2 mA cm −228 . They also studied the electrochemical performance of NiCo 2 O 4 @NiCo 2 O 4 composite nanoplates and NiCo 2 O 4 nanoplates decorated with NiMoO 4 honeycombs for high performance supercapacitor applications 29,30 . Herein, we developed a solid asymmetric supercapacitor with a pre-synthesised NiCo 2 O 4 mesoporous electrode without binders, where a nickel foam (with high conductivity) was selected as a current collector, and the NiCo 2 O 4 provided a large surface area along with a unique mesoporous nanostructure. Our sample delivered a specific capacitance of 1734.9 F g −1 at a 2 A g −1 CD with a retention rate of 87.3% at a 30 A g −1 CD for 3500 cycles. Furthermore, the application of a NiCo 2 O 4 -based binder-free electrode to a solid asymmetric supercapacitor resulted in a 42.25 W h kg −1 energy density at a power density of 298.79 W kg −1 (Fig. 1). The images show that a smooth, uniform array of NiCo 2 O 4 nanosheets is grown on the nickel foam surface, and the nanosheets are interlaced to form a mesoporous structure. Figure 2 (a) shows that the NiCo-LDH possesses an interconnected nanosheet microstructure. Figure 2(b, c) shows SEM images of NiCo 2 O 4 . These nanosheets, which are several hundred nanometers in size, have a porous structure that is intercrossed, which contains electroactive surface sites and plentiful vacancies [31][32][33] .

Results and discussion
The products were further examined by X-ray diffraction (XRD) analysis. Figure 3 34,35 .
Transmission electron microscopy (TEM) measurements were performed to further investigate the structure of the synthesised NiCo 2 O 4 nanosheets, as shown in Fig. 4. Figure 4(a, b) shows a NiCo 2 O 4 nanosheet with a folding, silk-like morphology and transparent features, indicating its interconnected nature. Due to the significant difference between the lateral size and thickness, bending and crumpling are clearly observed. The spacing between adjacent fringes is ~ 0.29 nm, which is close to the theoretical interplane spacing of spinel NiCo 2 O 4 (311) planes. Thus, the interconnected nanosheets are composed of 1-3 layers of NiCo 2 O 4 atomic sheets. The selected area electron diffraction (SAED) pattern ( Fig. 4 (c)) presents distinct diffraction rings, indicating polycrystalline characteristics. Furthermore, several interparticle mesopores, with sizes ranging from 1 to 3 nm, in these interconnected nanosheets can be evidently seen ( Fig. 4 (a,b)). It is believed that the mesoporous structures in nanosheets are imperative in facilitating the electrolytes' mass transport within the electrodes for double-layer www.nature.com/scientificreports/ charging/discharging and quick redox reactions 8 . The porous structure also significantly increases the contact area of electrolyte/electrode, and consequently enhances the electrochemical performance [9][10][11][12][13][14][15] . To show the advantages of this architecture, the interconnected mesoporous NiCo 2 O 4 nanosheets with hybrid structure fabricated on Ni foam was directly applied as an electrode for a supercapacitor. Figure 4(d) show the energy dispersive X-ray spectroscopy (EDS) analysis of the NiCo 2 O 4 sample; Co, Ni, and O are detected. This result confirms the chemical composition of the NiCo 2 O 4 structure. X-ray photoelectron spectroscopy (XPS) tests, along with the corresponding fitting results are shown in Fig. 5. Gaussian fitting method was used to best fit the Ni 2p and Co 2p (with two spin-orbit doublets for each), characteristic of Ni 2+ and Ni 3+ and Co 2+ and Co 3+ , respectively and two shake-up satellite (indicated as "Sat. ") for both Ni 2p and Co 2. These data show that the surface of the as-prepared NiCo 2 O 4 contains Co 2+ , Co 3 + , Ni 2 + , and Ni 3+ . where the atomic ratio of Co to Ni elements is ca. 2.2:1, which is close to that in the precursor electrolyte.  www.nature.com/scientificreports/ Nitrogen adsorption/desorption measurements were utilised to examine the porosity and BET surface area and of samples. All the N 2 adsorption/desorption isotherms in Fig. 6 exhibit a typical IV isotherm with a hysteresis loop in the P/P 0 range of 0.25-1.0, suggesting the materials have a mesoporous structure. These curves were based on the IUPAC classification of type IV isotherms with loop hysteresis. The specific surface area calculated for the nanocomposite created in the current density of -6.0 mA/cm 2 were 40.2 m 2 /g. The resulting structure had several advantages in electrochemical supercapacitors. Interconnected NiCo 2 O 4 sheets grown directly on  www.nature.com/scientificreports/ the nickel foam using electrochemical deposition method provided an integrated and orderly electrode that facilitated the transport of ion and electrons, thereby reducing the electrode resistance. Figure 7(a) shows the cyclic voltammetry (CV) curves of NiCo 2 O 4 with different numbers of electrodeposition cycles at 5 mV s −1 . The NiCo 2 O 4 _1 electrode demonstrates higher peak currents and larger integrated areas compared with the NiCo 2 O 4 _2, NiCo 2 O 4 _4, and NiCo 2 O 4 _6 electrodes. Further, the properties of NiCo 2 O 4_ 1 shown in Fig. 7(b) indicate better electrochemical capacity than those of Co 3 O 4 and NiO. Figure 7(c) shows the CV curves of the NiCo 2 O 4 electrode at scan rates from 5 to 100 mV s −1 with a 0-0.6 V potential window (vs. HgCl reference electrode). Even at a relatively high scan rate, a couple of redox peaks are evident, a battery characteristic signifying quick kinetics 16 . The current response enhanced with increasing sweep rate without noticeable alteration in the trend of the CV curves. Furthermore, these peaks are attributed primarily to the faradaic redox reactions involving M-O-OH/M-O (where M signifies Co or Ni) 36 . Figure 7(d, e) presents the galvanostatic charge-discharge (GCD) measurements of Co 3 O 4 , NiO, and NiCo 2 O 4_ 1 with different numbers of electrodeposition cycles at 2 A g −1 from 0 to 0.55 V. Among the samples, NiCo 2 O 4_ 1 had the best charge-discharge properties. The GCD curves at various CDs are demonstrated in Fig. 7 (f). These CD curves are nonlinear, indicating typical battery-type capacitive behaviour 37,38 . These results confirm that the NiCo 2 O 4 _1 electrode had a much higher specific capacitance than the NiCo 2 O 4 electrodes with more deposition cycles. The remarkable electrochemical performance of the NiCo 2 O 4 _1 electrode could be attributed to the excellent adhesion to the nickel foam substrate, with a large surface area and electrical connection of the active material to the current collector to ensure effective accessibility of the electrolyte ions and electrons. Equation 1 defines a numerical calculation of specific capacitance during current density characterization 39 : where m, I, ΔV, Δt, and C are the active materials mass, discharge current, drop in potential, total discharge time, and specific capacitance, respectively. respectively. This shows that when the charge-discharge rate increases from 2 to 50 A g −1 , around 87.3% of the capacitance is retained.
Electrochemical impedance spectroscopy (EIS) tests were carried out to examine the ion transport properties of the synthesised materials in the 0.01 Hz-100 kHz frequency range. Figure 9 shows the EIS results for the NiCo 2 O 4 _n (n = 1, 2, 4, and 6), NiO, and Co 3 O 4 materials. The Nyquist plots of the electrodes contain straight and semicircular curves in the low and high frequency regions, respectively [40][41][42][43] . The intercept of Z 0 (the real axis) with the semicircle, in the high-frequency region, is identical to R s (the internal resistance), which comprises of the ohmic resistance of the active materials, the resistance of the electrolyte, and the contact resistance at the interface of the active material/nickel foam. The internal resistance values of the NiCo 2 O 4_ 1, NiO, and Co 3 O 4 electrodes were 0.35, 0.22, and 1.58 Ω, respectively. The semicircle represents C dl (the double-layer capacitance), which is related to the surface properties of the electrode, and the semicircle diameter indicates R ct (the charge transfer resistance), which is related to the corresponding faradaic reactions at the interface of the electrode-electrolyte. On the contrary, the slope of the curves in the low frequency region signifies the Warburg resistance, which is related to the diffusion of the electrolyte in the electrodes. It is known that the electrochemical performance of supercapacitors can be effectively enhanced by reducing this resistance. Based on the results of GCD, an extraordinary specific capacitance of 1734.5 F g −1 was obtained at a current density of 2 A g −1 . The CV curves of NiCo 2 O 4 with different numbers of electrodeposition cycles at 5 mV s −1 are given in Fig. 9 The cycle life performance of NiCo 2 O 4 at a CD of 30 A g −1 for 3500 cycles is presented in Fig. 10. The NiCo 2 O 4 deposited on the nickel foam electrode shows steady cycling stability. From the calculation for the discharge curves, an approximate decrease of only 12.7% in the specific capacitance value after 3500 cycles is obtained. At low current densities, some side reactions occurred during the electrochemical redox reaction, leading to incomplete discharge. As the current density increased, the charge and discharge time decreased. The electrochemical process was mainly affected by the electric double layer, so the Coulomb efficiency increased as well 9 . Given the superior electrochemical behaviour of the NiCo 2 O 4 electrode, it is the best choice for use in electrochemical supercapacitors characterised by both excellent rate capability and long cycle life.
Furthermore, to study the practical performance of the synthesised electrode in this work, asymmetric supercapacitors (ASCs) were assembled, in which carbon nanotubes (CNTs) and NiCo 2 O 4 were the cathode and anode, respectively, with a polyvinyl alcohol-KOH gel polymer electrolyte. Figure 11(a) displays the CV curves of the anode and cathode at 5-100 mV s −1 scan rates. Figure 12(b) indicates that the NiCo 2 O 4 and CNT electrodes operate in voltage ranges of 0.0-0.6 V and − 1.0 to 0.0 V, respectively. Consequently, the NiCo 2 O 4 /CNT ASC can operate in a 1.6 V voltage range 12 . As the scan rate increases from 5 to 100 mV s −1 , the shape of the CV curve does not change, indicating that the device has good and rapid charge-discharge properties. The charge-discharge behaviour of the device is depicted in Fig. 11(b). These charge-discharge curves are nonlinear, indicating battery-type capacitive behaviour. Equations 2 and 3 were used to further investigate the excellent rate capability and high capacitance to evaluate the performance indicators of specific power density (P) and specific energy density (E) from the discharge curves 13,44-47 . www.nature.com/scientificreports/ where ΔV, C, and Δt are the potential drop during discharge, specific capacitance derived from the charge-discharge calculations, and full discharge duration, respectively. Figure 12 depicts the power and energy density plot (Ragone plot) of the devices at different charge-discharge rates.

Materials and methods
Preparation of nickel cobaltite. The electrochemical deposition was carried out at ambient temperature to synthesize NiCo 2 O 4 nanosheets onto nickel foam in a 3-electrode cell in which nickel foam, saturated calomel, and Pt foil utilized as working, reference, and counter electrodes, respectively. Nickel-Cobalt layered double hydroxide precursor was deposited over nickel foam in an aqueous mixed electrolyte of 2 mM cobalt nitrate and 1 mM nickel nitrate using a ZIVE SP2 electrochemical workstation. Versatile numbers of cycles like 1, 2, 4, and 6 cycles were selected for electrodeposition of working electrodes with the potential of − 1.2-0.5 V (vs. SCE). The nickel foams were ultrasonically cleaned and rinsed three times with distilled water and ethanol after electrodeposition and then dried at ambient temperature. Then, the electrodeposited working electrodes was placed in a muffle furnace and annealed for two hours at 300 °C, to transform the coating into interconnected mesoporous NiCo 2 O 4 nanosheets; it was carefully weighed after annealing (Fig. 1). The obtained NiCo 2 O 4 specimens are called as NiCo 2 O 4 _n (where n is the number of cycles, n = 1, 2, 4, and 6), and henceforth "NiCo 2 O 4 " denotes the n = 1 sample when NiCo 2 O 4 is compared with other materials.
Preparation of flexible solid electrolyte. The solution-casting technique was used to prepare the potassium hydroxide and PVA polymer electrolyte. 2 g of polyvinyl alcohol was added into 30 ml of double-distilled www.nature.com/scientificreports/ water and stirred for three hours at 80 °C. Then 6 M potassium hydroxide was mixed the prepared solution with agitation at room temperature for 3 h. After dissolution was complete, the final mixture was constantly agitated until a homogeneous viscous product was obtained. Finally, to attain a jelly electrolyte, the mixture was transferred to an oven (vacuum) and kept at 70 °C overnight.
Characterization of NiCo 2 O 4 . The morphology and microstructure of specimens were examined by FESEM (FESEM, LEO-1550) equipped with EDS at a 5 kV applied voltage. XRD tests were carried out (Bruker D8 Advance X-ray diffractometer) with Cu K-α radiation (λ = 0.154056 nm) at 30 mA and 40 kV. The speed of scanning was 5° min −1 with 0.02° steps. Elemental mapping (FEI Talos microscope operating at a 200 kV accelerating voltage), high-angle annular dark-field scanning, and TEM (HAADF-STEM-EDS) were used to characterize the specimens. Thermo VG Escalab 250 photoelectron spectrometer was used for XPS analysis. The pore structures were evaluated by N 2 adsorption at 77 Kusing volumetric equipment (Quantachrome AS-1-MP) after pre-evacuation for 2 h at 423 K while maintaining a base pressure of 10 -4 Pa.
Electrochemical characterizations. The electrochemical characterizations containing electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge (GCD), and cyclic voltammetry (CV) were carried out in a conventional three-electrode configuration at ambient temperature in which the NiCo 2 O 4 on Ni foam electrode was utilized as the working electrode, and a Hg/HgO and Pt foil were the reference and counter electrodes. The electrochemical characterizations were done using a 6 M potassium hydroxide solution with the aid of ZIVE SP2 workstation (10 μHz-4 MHz). The voltage range was from 0 to 0.6 V vs. Hg/HgO for the NiCo 2 O 4 electrode. To examine the energy storage performance of the working electrode for practical applica- www.nature.com/scientificreports/ tion, the anode electrode material was nickel cobalt oxide, and the cathode electrode material was carbon nanotubes (CNTs). The voltage range was from 0 to 1.5 V vs. Hg/HgO for the CNT electrode.