Next Article in Journal
In Situ and Operando Characterization Techniques in Stability Study of Perovskite-Based Devices
Next Article in Special Issue
Printed Electrodes Based on Vanadium Dioxide and Gold Nanoparticles for Asymmetric Supercapacitors
Previous Article in Journal
Nanostructure Design of Catalysts: Latest Advances and Prospects
Previous Article in Special Issue
Asymmetric Supercapacitors Using Porous Carbons and Iron Oxide Electrodes Derived from a Single Fe Metal-Organic Framework (MIL-100 (Fe))
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Nanomaterials
Volume 13
Issue 13
10.3390/nano13131981
nanomaterials-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

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

by
Hammad Mueen Arbi
1,†,
L. Vijayalakshmi
2,†,
Yedluri Anil Kumar
3,4,
Salem Alzahmi
3,4,*,
Chandu V. V. Muralee Gopi
5,
Andrivo Rusydi
6 and
Ihab M. Obaidat
1,4,*
1
Department of Physics, United Arab Emirates University, Al Ain P.O. Box 15551, United Arab Emirates
2
Department of Automotive Engineering, Yeungnam University, Gyeongsan-si 38541, Republic of Korea
3
Department of Chemical & Petroleum Engineering, United Arab Emirates University, Al Ain P.O. Box 15551, United Arab Emirates
4
National Water and Energy Center, United Arab Emirates University, Al Ain P.O. Box 15551, United Arab Emirates
5
Department of Electrical Engineering, University of Sharjah, Sharjah P.O. Box 27272, United Arab Emirates
6
Advanced Research Initiative for Correlated-Electron Systems (ARiCES), Department of Physics, National University of Singapore, Singapore 117551, Singapore
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Nanomaterials 2023, 13(13), 1981; https://doi.org/10.3390/nano13131981
Submission received: 28 April 2023 / Revised: 21 June 2023 / Accepted: 23 June 2023 / Published: 30 June 2023
(This article belongs to the Special Issue Nanomaterials for Supercapacitors)
Browse Figures
Versions Notes

Abstract

:
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.

1. 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 and release it on demand, albeit with a limited cycling span [9,10]. The attempts toward improving the cycling span of the batteries have led to the pioneering of SCs [11]. SCs are mainly classified as electrochemical double-layer capacitors (EDLCs) and pseudocapacitors (PCs) [12]. The sample electrodes utilized for EDLCs are carbon-type elements that include graphene, carbon nanotubes (CNTs), graphite, etc. These electrodes possess an enlarged specific surface [13,14,15].
Transition metal oxides (TMOs), a representative class of PC nanomaterials, have captured a lot of attention due to their abundant complexity valence state and their rapidly reversible procedures. Among them, RuO2 material possesses a standard TMO status and achieves excellent energy storage properties, but it has serious limitations due to its notably high prices and large toxicities. [16]. Surprisingly, spinel NiCo2O4 is a promising candidate for conventional TMOs, offering superior benefits in electrode samples due to its cost-effective and efficient electrochemical process [17,18]. One out of three Co atoms in Co3O4 is replaced with a Ni atom [19,20,21]. The NiCo2O4 structure is ordered as there are two metal sites in this structure, one being tetrahedral (the 8a site) and the other being octahedral (the 16d site) [22,23,24]. Moreover, the Ni metallic ions would produce plentiful redox reactions to achieve energy storage activities in contrast to Co3O4 [25,26].
NiCo2O4 was rarely noticed as the energy material for the electrode composites of SCs due to its restricted capacities (407 F·g−1) [27]. On the other hand, Co(OH)2 was one of the classic PC electrodes, displaying superior theoretical capacities (3460 F·g−1), and well-redox reactions, in addition to being a hydrotalcite-type compound [28]. The expanded interlayer spaces could supply superior specific surfaces to transport the ions/electrons rapidly, which, in turn, enhances the electrochemical activities [29]. However, Co(OH)2 displays lower specific capacity and low-level cycling stabilities behavior [30,31]. These limits were ascribed to the relatively very low conductivities and correspondingly fewer availabilities of active sites [32,33,34]. To achieve better performances, Co(OH)2 has been designed into a microscale composite with a conductive foam. In this regard, the as-developed electrode materials consist of expanded specific surface areas, shorter electron transport paths, and plenty of energy-storing active sites. By combining NiCo2O4 and Co(OH)2, the capacitance of NiCo2O4 can be improved. Evidently, a combination of Co(OH)2@NiCo2O4 with a conductive matrix-type nickel foam can enhance the cycling performances and rate capabilities of materials. To the best of our understanding, the capabilities of a Co(OH)2@NiCo2O4 were not adequate, and it was the main endeavor to further boost the electrochemical activities of SCs. Compared to the recently published composites, it would be anticipated that the Co(OH)2@NiCo2O4 might produce stronger cooperatives effects, encouraging enhanced electrochemical performances for SCs.
By combining the excellent electrochemical performances of Co(OH)2 and the high conductivity of NiCo2O4, a unique nano-structure of Co(OH)2@NiCo2O4 was in situ assembled on Ni foam as a binder-free sample. This composite material not only forms an open inter-network for easier ions diffusion but also enhances the specific area to offer abundant electro-active sites for the Faradic procedures. NiCo2O4 nanoparticles as core nanomaterial were developed using a hydrothermal procedure where the Co(OH)2 shell could be simply coated onto the interface of NiCo2O4 nanoparticles via a hydrothermal route with high efficacy. Co(OH)2 nanomaterial, when made as a nanocomposite with NiCo2O4, offers improved electrochemical reactions due to its expanded surface region, hierarchical morphologies, well mechanical properties, good chemical stabilities, and excellent electrical kinetics. The as-designed Co(OH)2@NiCo2O4 nanosheet achieved high specific capacitances of 1308 F·g−1 at 0.5 A·g−1 with 92.83% capacity retention over 6000 cycles. Thus, we deduce that combining Co(OH)2 with NiCo2O4 effectively improved the kinetics of the composite material, the specific capacitances, the cycling stabilities, and the rate capabilities of SCs.

2. Experimental

2.1. Synthesis of Co(OH)2@NiCo2O4 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(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 cm2) 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 Ni2+ and the Co2+ 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.

2.2. Physical Characterization

The crystallinity and structure of Co(OH)2@NiCo2O4 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 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, NiCo2O4 and Co(OH)2@NiCo2O4 nanosheet. The valence states of the developed nano-constructions 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.

2.3. Electrochemical Characterization

The electrochemical properties were achieved using a 3-electrode configuration with Co(OH)2@NiCo2O4 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]:
C s c = I × Δ t m × Δ v
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).

3. Results and Discussions

Figure 1 demonstrates the XRD spectra of the as-grown samples. For all the electrodes, the main diffraction points at 2θ values of 44.5°, 51.9°, and 76.5° are due to the Ni skeleton. Diffraction peaks at 2θ = 18.79°, 35.39°, 36.87°, 62.35°, and 64.83° correspond to (220), (311), (222), (440), and (531) facets, respectively. They were effectively indexed with the spinel spacing of NiCo2O4 (JCPDS card no. 73–1702) in good correlation with recent reports [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 NiCo2O4 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@NiCo2O4 interface. Thus, the as-prepared sample was confirmed to be a composite of NiCo2O4 and Co(OH)2. All angle diffraction facets provided evidence for the admirable crystallinity of Co(OH)2@NiCo2O4 nanosheet composite on Ni foam.
FTIR spectra of the as-obtained NiCo2O4 electrode and Co(OH)2@NiCo2O4 nanosheet are recorded and shown in Figure S2. It can be seen that the characteristic peaks of NiCo2O4 appear at about 553 and 643 cm−1 (Figure S2a), belonging to metal–oxygen M−O vibrations of the NiCo2O4. The peaks at around 3413 and 1628 cm−1 are attributed to the vibrational mode of absorbed H2O, and the peak at 1383 cm−1 may be associated with the presence of physically adsorbed CO2. In contrast, in the spectrum of Co(OH)2@NiCo2O4 nanosheet (Figure S2b), the characteristic peaks of NiCo2O4 appear at about 560 and 648 cm−1, showing slightly red-shift compared with the pure NiCo2O4. Our observation of both the difference in peak position and intensity between the as-prepared Co(OH)2@NiCo2O4 nanosheet and the pure NiCo2O4 reported in the literature indicates a mutual interaction 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.
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 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 Co3+, and the fitting of Co 2p peaks at 782.5 eV and 797.7 eV correspond to Co2+. 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 Ni2+, while the other two peaks at 875.7 eV and 856.8 eV correlate with Ni3+ [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 as-prepared 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 spectra (Figure S1b) reveal only the existence of Ni, Co, and O elements in the NiCo2O4 electrode material.
Figure 4e shows that the isotherms of NiCo2O4 display a type-IV mesopore feature with a hysteresis loop in the middle-pressure area. Their pore size distribution values are consistent with the isotherms. On the other hand, Figure 4f shows the isotherm of Co(OH)2@NiCo2O4 nanosheet composite, which was a typical type-I isotherm owing to the sharp increase at a lower relative pressure (P/P0 < 0.1), which reveals the existence of micropores. The isotherm adsorption line illustrates a vital hysteresis loop in the middle-pressure part (P/P0 = 0.4–0.9), which was due to the condensation of N2 molecules under lower pressure sections, and then the pores are occupied. The pore size distribution of Co(OH)2@NiCo2O4 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 m2·g−1, and 41.3 m2·g−1 for Co(OH)2@NiCo2O4, and NiCo2O4, respectively. The Co(OH)2@NiCo2O4 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, NiCo2O4, and Co(OH)2@NiCo2O4 nanosheet electrode samples were analyzed using a three-electrode cell in an electrolyte of 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]:
Co(OH)2 + OH− ⇔ CoOOH + H2O + e−
NiCo2O4 + OH− + H2O ⇔ NiOOH + 2CoOOH + e−
CoOOH + OH− ⇔ CoO2 + H2O + e−
A pairing of redox peaks appeared in the plot of the NiCo2O4 sample, which is due to the faradic reactions of Co4+/Co3+ and Ni3+/Ni2+. In the Co(OH)2 sample, two couples of faradic peaks Co4+/Co3+ and Co3+/Co2+ were revealed in Equations (2)–(4). Thus, the Co(OH)2@NiCo2O4 nanosheet composite electrode discloses coupling pairs of redox peaks. Figure 5b demonstrates the CV plots of the Co(OH)2@NiCo2O4 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@NiCo2O4 nanosheet electrode consists of prolonged discharge times (573 s) compared with the remaining electrodes of NiCo2O4 (303 s) and Co(OH)2 (196 s). At 0.5 A·g−1, according to Equation (1), the composite Co(OH)2@NiCo2O4 nanosheets electrode possesses the higher specific capacitances of 1308 F·g−1, while for NiCo2O4, 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@NiCo2O4 nanosheet electrode possesses greater specific capacitances than the other Co(OH)2 and NiCo2O4 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 NiCo2O4 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 NiCo2O4 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@NiCo2O4 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 NiCo2O4 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 NiCo2O4 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@NiCo2O4 nanosheet sample exhibits a 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 three-electrode 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

4. Conclusions

In summary, the composite Co(OH)2@NiCo2O4 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@NiCo2O4 nanosheet was carried out using SEM and TEM experiments. The NiCo2O4 nanoparticles components cohabited with the aggregations of Co(OH)2 particles in unique constructions. Owing to the porous constructions and the synergistic effects of NiCo2O4 and Co(OH)2, the as-designed composite Co(OH)2@NiCo2O4 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@NiCo2O4 nanosheet electrode is a promising candidate for wider applications prospects in high-performance SCs.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano13131981/s1, Figure S1. XPS full survey spectrum of Co(OH)2 and NiCo2O4 electrode materials. Figure S2 FTIR spectra of the as-prepared NiCo2O4 electrode (a), and Co(OH)2@NiCo2O4 nanosheets electrode (b). Figure S3. (a) CV curves and (b) CD plots of pure NiCo2O4 electrode at various scan rates and different current densities grown on Ni foam, and Figure S4. (a) Nyquist plots, and (b) Cycling stability of Co(OH)2 electrode material in three-electrode electrochemical test.

Author Contributions

Conceptualization, I.M.O. and S.A.; Supervision and validation, I.M.O. and S.A.; Writing, Y.A.K., H.M.A. and I.M.O.; Investigation and visualization, Y.A.K., H.M.A., L.V. and C.V.V.M.G.; Validation, A.R., Y.A.K. and I.M.O.; All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

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

Acknowledgments

This work was financially supported by the UAEU-AUA research program under grant no. 12R131.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Yoon, J.H.; Kumar, Y.A.; Sambasivam, S.; Hira, S.A.; Krishna, T.N.V.; Zeb, K.; Uddin, W.; Kumar, K.D.; Obaidat, I.M.; Kim, S.; et al. Highly efficient copper-cobalt sulfide nano-reeds array with simplistic fabrication strategy for battery-type supercapacitors. Energy Storage 2020, 32, 101988. [Google Scholar] [CrossRef]
  2. Mun, C.H.; Gopi, C.V.V.M.; Vinodh, R.; Sambasivam, S.; Obaidat, I.M.; Kim, H.J. Microflower-like nickel sulfide-lead sulfide hierarchical composites as binder-free electrodes for high-performance supercapacitors. J. Energy Storage 2019, 26, 100925. [Google Scholar] [CrossRef]
  3. Gopi, C.V.V.M.; Vinodh, R.; Sambasivam, S.; Obaidat, I.M.; Kalla, R.M.N.; Kim, H.J. One-pot synthesis of copper oxide–cobalt oxide core–shell nanocactus-like heterostructures as binder-free electrode materials for high-rate hybrid supercapacitors. Mater. Today Energy 2019, 14, 100358. [Google Scholar] [CrossRef]
  4. Kumar, Y.A.; Sambasivam, S.; Hira, S.A.; Zeb, K.; Uddin, W.; Krishna, T.N.V.; Kumar, K.D.; Obaidat, I.M.; Kim, H.J. Boosting the energy density of highly efficient flexible hybrid supercapacitors via selective integration of hierarchical nanostructured energy materials. Electrochim. Acta 2020, 364, 137318. [Google Scholar] [CrossRef]
  5. Yang, J.; Jo, M.R.; Kang, M.; Huh, Y.S.; Jung, H.; Kang, Y.M. Rapid and controlled synthesis of nitrogen doped reduced graphene oxide using microwave-assisted hydrothermal reaction for high power-density supercapacitors. Carbon 2014, 73, 106–113. [Google Scholar] [CrossRef]
  6. Yang, J.; Park, S.; Lee, S.; Kim, J.; Huang, D.; Gim, J.; Lee, E.; Kim, G.; Park, K.; Kang, Y.-M.; et al. High-voltage deprotonation of layered-type materials as a newly identified cause of electrode degradation. J. Mater. Chem. A 2023, 11, 3018–3027. [Google Scholar] [CrossRef]
  7. El-Kady, M.F.; Shao, Y.; Kaner, R.B. Graphene for batteries, supercapacitors and beyond. Nat. Rev. Mater. 2016, 1, 16033. [Google Scholar] [CrossRef]
  8. Dang, A.; Sun, Y.; Fang, C.; Li, T.; Liu, X.; Xia, Y.; Ye, F.; Zada, A.; Khan, M. Rational design of Ti3C2/carbon nanotubes/MnCo2S4 electrodes for symmetric supercapacitors with high energy storage. Appl. Surf. Sci. 2022, 581, 152432. [Google Scholar] [CrossRef]
  9. Kumar, K.S.; Choudhary, N.; Jung, Y.; Thomas, J. Recent Advances in Two-Dimensional Nanomaterials for Supercapacitor Electrode Applications. ACS Energy Lett. 2018, 3, 482–495. [Google Scholar] [CrossRef]
  10. Sharma, P.; Sundaram, M.M.; Watcharatharapong, T.; Jungthawan, S.; Ahuja, R. Tuning the Nanoparticle Interfacial Properties and Stability of the Core–Shell Structure in Zn-Doped NiMoO4@AWO4. ACS Appl. Mater. Interfaces 2021, 13, 56116–56130. [Google Scholar] [CrossRef]
  11. Wickramaarachchi, K.; Sundaram, M.M.; Henry, D.J.; Gao, X. Alginate Biopolymer Effect on the Electrodeposition of Manganese Dioxide on Electrodes for Supercapacitors. ACS Appl. Energy Mater. 2021, 4, 7040–7051. [Google Scholar] [CrossRef]
  12. Wang, C.; Wang, J.; Wu, W.; Qian, J.; Song, S.; Yue, Z.J. Feasibility of activated carbon derived from anaerobic digester residues for supercapacitors. J. Power Sources 2019, 412, 683–688. [Google Scholar] [CrossRef]
  13. Brousse, T.; Bélanger, D.; Long, J.W. To be or not to be pseudocapacitive? J. Electrochem. Soc. 2015, 162, A5185. [Google Scholar] [CrossRef] [Green Version]
  14. Gu, C.; Ge, X.; Wang, X.; Tu, J.J. Cation–anion double hydrolysis derived layered single metal hydroxide superstructures for boosted supercapacitive energy storage. Mater. Chem. A 2015, 3, 14228–14238. [Google Scholar] [CrossRef]
  15. Chhetri, K.; Tiwari, A.P.; Dahal, B.; Ojha, G.P.; Mukhiya, T.; Lee, M.; Kim, T.; Chae, S.; Muthurasu, A.; Kim, H.Y. A ZIF-8-derived nanoporous carbon nanocomposite wrapped with Co3O4-polyaniline as an efficient electrode material for an asymmetric supercapacitor. J. Electroanal. Chem. 2020, 856, 113670. [Google Scholar] [CrossRef]
  16. Chhetri, K.; Dahal, B.; Mukhiya, T.; Tiwari, A.P.; Muthurasu, A.; Kim, H.Y. Integrated hybrid of graphitic carbon-encapsulated CuxO on multilayered mesoporous carbon from copper MOFs and polyaniline for asymmetric supercapacitor and oxygen reduction reactions. Carbon 2021, 179, 89–99. [Google Scholar] [CrossRef]
  17. Dang, A.; Li, T.; Xiong, C.; Zhao, T.; Shang, Y.; Liu, H.; Chen, X.; Li, H.; Zhuang, Q.; Zhang, S. Long-life electrochemical supercapacitor based on a novel hierarchically carbon foam templated carbon nanotube electrode. Compos. Part B Eng. 2018, 141, 250–257. [Google Scholar] [CrossRef]
  18. Zheng, J.; Yan, B.; Feng, L.; Zhang, Q.; Zhang, C.; Yang, W.; Han, J.; Jiang, S.; He, S. Potassium citrate assisted synthesis of hierarchical porous carbon materials for high performance supercapacitors. Diam. Relat. Mater. 2022, 128, 109247. [Google Scholar] [CrossRef]
  19. Wang, M.; Sui, X.; Wang, Y.; Juan, Y.; Lyu, Y.; Peng, H.; Huang, T.; Shen, S.; Guo, C.; Zhang, J.; et al. Manipulate the electronic and magnetic states in NiCo2O4 films through electric-field-induced protonation at elevated temperature. Adv. Mater. 2019, 31, 1900458. [Google Scholar] [CrossRef]
  20. Liu, Y.; Wang, Z.; Zhong, Y.; Tade, M.; Zhou, W.; Shao, Z. Molecular design of mesoporous NiCo2O4 and NiCo2S4 with sub-micrometer polyhedron architectures for efficient pseudocapacitive energy storage. Adv. Funct. Mater. 2017, 27, 1701229. [Google Scholar] [CrossRef]
  21. Kumar, Y.A.; Kim, H.-J. Preparation and electrochemical performance of NiCo2O4@NiCo2O4 composite nanoplates for high performance supercapacitor applications. New J. Chem. 2018, 42, 19971–19978. [Google Scholar] [CrossRef]
  22. Wang, X.; Fang, Y.; Shi, B.; Huang, F.; Rong, F.; Que, R. Three-dimensional NiCo2O4@NiCo2O4 core–shell nanocones arrays for high-performance supercapacitors. Chem. Eng. J. 2018, 344, 311–319. [Google Scholar] [CrossRef]
  23. Shao, Y.; Zhao, Y.; Li, H.; Xu, C. Three-dimensional hierarchical NixCo1−xO/NiyCo2−yP@C hybrids on nickel foam for excellent supercapacitors. ACS Appl. Mater. Inter. 2016, 8, 35368–35376. [Google Scholar] [CrossRef]
  24. Liu, Y.; Fu, N.; Zhang, G.; Xu, M.; Lu, W.; Zhou, L.; Huang, H. Design of Hierarchical NiCo@NiCo Layered Double Hydroxide Core-Shell Structured Nanotube Array for High-Performance Flexible All-Solid-State Battery-Type Supercapacitors. Adv. Func. Mater. 2017, 27, 1605307. [Google Scholar] [CrossRef]
  25. Liu, B.; Kong, D.; Huang, Z.X.; Mo, R.; Wang, Y.; Han, Z.; Cheng, C.; Yang, H.Y. Three-dimensional hierarchical NiCo2O4 nanowire@Ni3S2 nanosheet core/shell arrays for flexible asymmetric supercapacitors. Nanoscale 2016, 8, 10686–10694. [Google Scholar] [CrossRef]
  26. Ko, T.H.; Lei, D.; Balasubramaniam, S.; Seo, M.-K.; Chung, Y.-S.; Kim, H.-Y.; Kim, B.-S. Polypyrrole-Decorated Hierarchical NiCo2O4 Nanoneedles/Carbon Fiber Papers for Flexible High-Performance Supercapacitor Applications. Electrochim. Acta 2017, 247, 524–534. [Google Scholar] [CrossRef]
  27. Mohamed, S.G.; Attia, S.Y.; Hassan, H.H. Spinel-structured FeCo2O4 mesoporous nanosheets as efficient electrode for supercapacitor applications. Microporous Mesoporous Mater. 2017, 251, 26–33. [Google Scholar] [CrossRef]
  28. Suksomboon, M.; Khuntilo, J.; Kalasina, S.; Suktha, P.; Limtrakul, J.; Sawangphruk, M. High-performance energy storage of Ag-doped Co(OH)2-coated graphene paper: In situ electrochemical X-ray absorption spectroscopy. Electrochim. Acta 2017, 252, 91–100. [Google Scholar] [CrossRef]
  29. Zhao, T.; Jiang, H.; Ma, J. Surfactant-assisted electrochemical deposition of α-cobalt hydroxide for supercapacitors. J. Power Sources 2011, 196, 860–864. [Google Scholar] [CrossRef]
  30. Jagadale, A.D.; Guan, G.; Li, X.; Du, X.; Ma, X.; Hao, X.; Abudula, A. Ultrathin nanoflakes of cobalt-manganese layered double hydroxide with high reversibility for asymmetric supercapacitor. J. Power Sources 2016, 306, 526–534. [Google Scholar] [CrossRef]
  31. Gupta, V.; Gupta, S.; Miura, N. Al-substituted α-cobalt hydroxide synthesized by potentiostatic deposition method as an electrode material for redox-supercapacitors. J. Power Sources 2008, 177, 685–689. [Google Scholar] [CrossRef]
  32. Casella, I.; Gatta, M. Study of the electrochemical deposition and properties of cobalt oxide species in citrate alkaline solutions. J. Electroanal. Chem. 2002, 534, 31–38. [Google Scholar] [CrossRef]
  33. Kulurumotlakatla, D.K.; Yedluri, A.K.; Kim, H.J. Hierarchical NiCo2S4 nanostructure as highly efficient electrode material for high-performance supercapacitor applications. J. Energy Storage 2020, 31, 31–38. [Google Scholar] [CrossRef]
  34. Arbi, H.M.; Yadav, A.A.; Anil Kumar, Y.; Moniruzzaman, M.; Alzahmi, S.; Obaidat, I.M. Polypyrrole-Assisted Ag Doping Strategy to Boost Co(OH)2 Nanosheets on Ni Foam as a Novel Electrode for High-Performance Hybrid Supercapacitors. Nanomaterials 2022, 12, 3982. [Google Scholar] [CrossRef]
  35. Moniruzzaman, M.; Anil Kumar, Y.; Pallavolu, M.R.; Arbi, H.M.; Alzahmi, S.; Obaidat, I.M. Two-Dimensional Core-Shell Structure of Cobalt-Doped@MnO2 Nanosheets Grown on Nickel Foam as a Binder-Free Battery-Type Electrode for Supercapacitor Application. Nanomaterials 2022, 12, 3187. [Google Scholar] [CrossRef]
  36. Wang, L.; Dong, Z.H.; Wang, Z.G.; Zhang, F.X.; Jin, J. Layered α-Co(OH)2 Nanocones as Electrode Materials for Pseudocapacitors: Understanding the Effect of Interlayer Space on Electrochemical Activity. Adv. Funct. Mater. 2013, 23, 2758–2764. [Google Scholar] [CrossRef]
  37. Zhou, X.; Yue, X.; Dong, Y.; Zheng, Q.; Lin, D.; Du, X.; Qu, G. Enhancing electrochemical performance of electrode material via combining defect and heterojunction engineering for supercapacitors. J. Colloid Interface Sci. 2021, 599, 68–78. [Google Scholar] [CrossRef]
  38. Pang, H.; Li, X.; Zhao, Q.; Xue, H.; Lai, W.-Y.; Hu, Z.; Huang, W. One-Pot Synthesis of Heterogeneous Co3O4-Nanocube/Co(OH)2-Nanosheet Hybrids for High-Performance Flexible Asymmetric All-Solid-State Supercapacitors. Nano Energy 2017, 35, 138–145. [Google Scholar] [CrossRef]
  39. Zhang, X.; Wang, Y.; Yu, X.; Tu, J.; Ruan, D.; Qiao, Z.J. High-performance discarded separator-based activated carbon for the application of supercapacitors. J. Energy Storage 2021, 44, 103378. [Google Scholar] [CrossRef]
  40. Kong, D.; Ren, W.; Cheng, C.; Wang, Y.; Huang, Z.; Yang, H. Three-dimensional NiCo2O4@polypyrrole coaxial nanowire arrays on carbon textiles for high-performance flexible asymmetric solid-state supercapacitor. ACS Appl. Mater. Interface 2015, 7, 21334–21346. [Google Scholar] [CrossRef]
  41. Yang, Y.; Wang, X.; Huang, F.; Zhao, J.; Wang, X. Ni(OH)2 nanodot-decorated Co-Co LDH/C hollow nanocages for a high peformance supercapacitor. Dalton Trans. 2020, 49, 17310–17320. [Google Scholar] [CrossRef]
  42. Xu, L.; Zhang, L.; Cheng, B.; Yu, J. Rationally designed hierarchical NiCo2O4–C@Ni(OH)2 core-shell nanofibers for high performance supercapacitors. Carbon 2019, 152, 652–660. [Google Scholar] [CrossRef]
  43. Tholkappiyan, R.; Vishista, K. Tuning the composition and magnetostructure of dysprosium iron garnets by Co-substitution: An XRD, FT-IR, XPS and VSM study. Appl. Surf. Sci. 2015, 351, 1016–1024. [Google Scholar] [CrossRef]
  44. Tholkappiyan, R.; Vishista, K. Combustion synthesis of Mg–Er ferrite nanoparticles: Cation distribution and structural, optical, and magnetic properties. Mater. Sci. Semicond. Process. 2015, 40, 631–642. [Google Scholar] [CrossRef]
  45. Zhang, J.; Wang, Z.; Deng, T.; Zhang, W. Ni(OH)2 derived Ni-MOF supported on carbon nanowalls for supercapacitors. Nanotechnology 2021, 32, 195404. [Google Scholar] [CrossRef]
  46. Xin, Y.; Dai, X.; Lv, G.; Wei, X.; Li, S.; Li, Z.; Xue, T.; Shi, M.; Zou, K.; Chen, Y.; et al. Stability-Enhanced α-Ni(OH)2 Pillared by Metaborate Anions for Pseudocapacitors. ACS Appl. Mater. Interf. 2021, 13, 28118–28128. [Google Scholar] [CrossRef] [PubMed]
  47. Zhao, Y.F.; Ran, W.; He, J.; Huang, Y.Z.; Liu, Z.F.; Liu, W.; Tang, Y.F.; Zhang, L.; Gao, D.W.; Gao, F.M. High-performance asymmetric supercapacitors based on multilayer MnO2/graphene oxide nanoflakes and hierarchical porous carbon with enhanced cycling stability. Small 2015, 11, 1310–1319. [Google Scholar] [CrossRef]
  48. Tholkappiyan Ramachandran, Fathalla Hamed, Electrochemical performance of plate-like zinc cobaltite electrode material for supercapacitor applications. J. Phys. Chem. Solids 2018, 121, 93–101. [CrossRef]
  49. Xu, K.; Zou, R.; Li, W.; Liu, Q.; Liu, X.; An, L.; Hu, J. Design and synthesis of 3D interconnected mesoporous NiCo2O4@CoxNi1−x (OH)2 core-shell nanosheet arrays with large areal capacitance and high rate performance for supercapacitors. J. Mater. Chem. A 2014, 2, 10090–10097. [Google Scholar] [CrossRef]
  50. Liu, W.W.; Lu, C.; Liang, K.; Tay, B.K. A three dimensional vertically aligned multiwall carbon nanotube/NiCo2O4 core/shell structure for novel high-performance supercapacitors. J. Mater. Chem. A 2014, 2, 5100–5107. [Google Scholar] [CrossRef] [Green Version]
  51. Huang, L.; Chen, D.; Ding, Y.; Wang, Z.L.; Zeng, Z.; Liu, M. Hybrid composite Ni(OH)2@NiCo2O4 grown on carbon fiber paper for high-performance supercapacitors. ACS Appl. Mater. Interface 2013, 5, 11159–11162. [Google Scholar] [CrossRef]
  52. Nguyen, V.H.; Shim, J.J. Three-dimensional nickel foam/graphene/NiCo2O4 as high-performance electrodes for supercapacitors. J. Power Sources 2015, 273, 110–117. [Google Scholar] [CrossRef]
  53. Wei, Y.; Chen, S.; Su, D.; Sun, B.; Zhu, J.; Wang, G. 3D mesoporous hybrid NiCo2O4@graphene nanoarchitectures as electrode materials for supercapacitors with enhanced performances. J. Mater. Chem. A 2014, 2, 8103–8109. [Google Scholar] [CrossRef]
  54. Wang, H.W.; Hu, Z.A.; Chang, Y.Q.; Chen, Y.L.; Wu, H.Y.; Zhang, Z.Y.; Yang, Y.Y. Design and synthesis of NiCo2O4 -reduced graphene oxide composites for high performance supercapacitors. J. Mater. Chem. 2011, 21, 10504–10511. [Google Scholar] [CrossRef]
  55. Wang, Z.; Hong, P.; Peng, S.; Zou, T.; Yang, Y.; Xing, X.; Wang, Z.; Zhao, R.; Yan, Z.; Wang, Y. Co(OH)2@FeCo2O4 as Electrode Material for High Performance Faradaic Supercapacitor Application. Electrochim. Acta 2019, 299, 312–319. [Google Scholar] [CrossRef]
  56. Li, G.; Li, W.; Xu, K.; Zou, R.; Chen, Z.; Hu, J. Sponge-like NiCo2O4/MnO2 ultrathin nanoflakes for supercapacitor with high-rate performance and ultra-long cycle life. J. Mater. Chem. A 2014, 2, 7738–7741. [Google Scholar] [CrossRef]
Nanomaterials 13 01981 sch001 550
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@NiCo2O4 nanosheet composite electrode.
Nanomaterials 13 01981 sch001
Nanomaterials 13 01981 g001 550
Figure 1. XRD pattern of as-synthesized Co(OH)2@NiCo2O4 nanosheet grown on Ni foam.
Figure 1. XRD pattern of as-synthesized Co(OH)2@NiCo2O4 nanosheet grown on Ni foam.
Nanomaterials 13 01981 g001
Nanomaterials 13 01981 g002 550
Figure 2. SEM images of (a) NiCo2O4 nanoparticles, (b) low-magnification of Co(OH)2@NiCo2O4 nanosheet, and (c,d) high-magnification images of Co(OH)2@NiCo2O4 nanosheet composite grown on Ni foam.
Figure 2. SEM images of (a) NiCo2O4 nanoparticles, (b) low-magnification of Co(OH)2@NiCo2O4 nanosheet, and (c,d) high-magnification images of Co(OH)2@NiCo2O4 nanosheet composite grown on Ni foam.
Nanomaterials 13 01981 g002
Nanomaterials 13 01981 g003 550
Figure 3. (a) TEM, (b) SAED pattern, and (c) HRTEM images of the composite Co(OH)2@NiCo2O4 nanosheet electrode grown on Ni foam.
Figure 3. (a) TEM, (b) SAED pattern, and (c) HRTEM images of the composite Co(OH)2@NiCo2O4 nanosheet electrode grown on Ni foam.
Nanomaterials 13 01981 g003
Nanomaterials 13 01981 g004 550
Figure 4. XPS spectra Co(OH)2@NiCo2O4 nanosheet composite of (a) full survey spectrum, (b) Ni 2p, (c) Co 2p, and (d) O 1s, respectively, and (e,f) N2 adsorption/desorption isotherms and pore diameter distributions of NiCo2O4, Co(OH)2@NiCo2O4.
Figure 4. XPS spectra Co(OH)2@NiCo2O4 nanosheet composite of (a) full survey spectrum, (b) Ni 2p, (c) Co 2p, and (d) O 1s, respectively, and (e,f) N2 adsorption/desorption isotherms and pore diameter distributions of NiCo2O4, Co(OH)2@NiCo2O4.
Nanomaterials 13 01981 g004
Nanomaterials 13 01981 g005 550
Figure 5. (a) CV curves of electrodes at 10 mV·s−1, (b) CV curves of composite Co(OH)2@NiCo2O4 nanosheets electrode at various scan rates, (c) GCD curves of electrodes at 0.5 A·g−1, (d) GCD curves of composite Co(OH)2@NiCo2O4 nanosheets electrode at different current densities, (e) specific capacitance of electrodes at various current densities, and (f) Nyquist plots for the electrodes.
Figure 5. (a) CV curves of electrodes at 10 mV·s−1, (b) CV curves of composite Co(OH)2@NiCo2O4 nanosheets electrode at various scan rates, (c) GCD curves of electrodes at 0.5 A·g−1, (d) GCD curves of composite Co(OH)2@NiCo2O4 nanosheets electrode at different current densities, (e) specific capacitance of electrodes at various current densities, and (f) Nyquist plots for the electrodes.
Nanomaterials 13 01981 g005
Nanomaterials 13 01981 g006 550
Figure 6. Electrochemical performances of cycling performance at the current density of 3 A·g−1 for composite Co(OH)2@NiCo2O4 nanosheet and pure NiCo2O4 electrode.
Figure 6. Electrochemical performances of cycling performance at the current density of 3 A·g−1 for composite Co(OH)2@NiCo2O4 nanosheet and pure NiCo2O4 electrode.
Nanomaterials 13 01981 g006
Table
Table 1. Comparison of the performances of recently reported NiCo2S4-based nanomaterials for supercapacitor applications in a three-electrode configuration.
Table 1. Comparison of the performances of recently reported NiCo2S4-based nanomaterials for supercapacitor applications in a three-electrode configuration.
ElectrodeFabrication MethodElectrolyteCapacitance (Current Density)Cycling Stability (No. of Cycles)Ref.
NiCo2O4@CoxNi1-x (OH)2 electrochemical deposition1 M KOH 5.71 F·cm−1 (5.5 mA·cm−2)80% (3000 cycles) [49]
carbon nanotube/NiCo2O4electrochemical deposition6 M KOH694 F·g−1 (1 A·g−1)91% (1500 cycles)[50]
Ni(OH)2@NiCo2O4electrochemical deposition1 M KOH5.71 F·cm−1 (2 mA·cm−2) 36% (1000)[51]
graphene/NiCo2O4electrochemical deposition3 M KOH1950 F·g−1 (7.5 A·g−1)92.8% (10,000 cycles)[52]
NiCo2O4@graphene nanoarchitecturesHydrothermal2 M KOH778 F·g−1 (1 A·g−1)90% (10,000 cycles)[53]
NiCo2O4–RGO compositeSelf-assembly6 M KOH835 F·g−1 (1 A·g−1)80% (4000 cycles)[54]
Co(OH)2@FeCo2O4Hydrothermal6 M KOH1173.43 F·g−1 (1 A·g−1)95.4% (5000)[55]
sponge-like NiCo2O4/MnO2 ultrathin nanoflakeselectrochemical deposition1 M KOH935 F·g−1 (1 A·g−1)103.1% (25,000)[56]
Co(OH)2@NiCo2O4 nanosheetHydrothermal2 M KOH1308 F·g−1 (0.5 A·g−1)92.83% (6000)This work
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Arbi, H.M.; Vijayalakshmi, L.; Anil Kumar, Y.; Alzahmi, S.; Gopi, C.V.V.M.; Rusydi, A.; Obaidat, I.M. A Facile Two-Step Hydrothermal Synthesis of Co(OH)2@NiCo2O4 Nanosheet Nanocomposites for Supercapacitor Electrodes. Nanomaterials 2023, 13, 1981. https://doi.org/10.3390/nano13131981

AMA Style

Arbi HM, Vijayalakshmi L, Anil Kumar Y, Alzahmi S, Gopi CVVM, Rusydi A, Obaidat IM. A Facile Two-Step Hydrothermal Synthesis of Co(OH)2@NiCo2O4 Nanosheet Nanocomposites for Supercapacitor Electrodes. Nanomaterials. 2023; 13(13):1981. https://doi.org/10.3390/nano13131981

Chicago/Turabian Style

Arbi, Hammad Mueen, L. Vijayalakshmi, Yedluri Anil Kumar, Salem Alzahmi, Chandu V. V. Muralee Gopi, Andrivo Rusydi, and Ihab M. Obaidat. 2023. "A Facile Two-Step Hydrothermal Synthesis of Co(OH)2@NiCo2O4 Nanosheet Nanocomposites for Supercapacitor Electrodes" Nanomaterials 13, no. 13: 1981. https://doi.org/10.3390/nano13131981

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop