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Article

Novel Design and Synthesis of Ni-Mo-Co Ternary Hydroxides Nanoflakes for Advanced Energy Storage Device Applications

by
Zhao Wang
,
Peifeng Li
,
Zhuolun Tang
and
Ka Yuen Simon Ng
*
Department of Chemical Engineering and Material Science, Wayne State University, Detroit, MI 48201, USA
*
Author to whom correspondence should be addressed.
Energies 2024, 17(16), 3881; https://doi.org/10.3390/en17163881
Submission received: 1 June 2024 / Revised: 11 July 2024 / Accepted: 28 July 2024 / Published: 6 August 2024
(This article belongs to the Section F3: Power Electronics)
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Abstract

:
Three-dimensional interconnected mesoporous nanoflakes of amorphous Ni-Mo-Co trimetallic hydroxides were successfully deposited on a Ni foam (NF) using a facile, environmentally friendly, and scalable electrochemical deposition method. The elemental composition of the nanoflakes, including Ni2+, Mo6+, and Co2+, was characterized by X-ray photoelectron spectroscopy (XPS), while the morphology and particle size of the synthesized nanomaterials were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Ni-Mo-Co trimetallic hydroxides on NF were employed as binder-free electrodes for supercapacitors. The Ni-Mo-Co trimetallic hydroxides with a Ni/Mo/Co ratio of 1/1/0.4 exhibited outstanding long-term cyclability over 5000 cycles, with a high reversible specific capacitance of 2700 F g−1 and a high capacitance retention of 96.63% at 10 A g−1. Furthermore, they demonstrated excellent rate performance, maintaining a capacitance of 2429 F g−1 at a current of 50 A g−1, which corresponds to approximately 80% capacitance retention compared to the capacitance at 2 A g−1. The superior performance of these Ni-Mo-Co trimetallic hydroxides can be attributed to their mesoporous hierarchical architecture, which provides large open spaces between the interconnected nanoflakes, numerous electroactive surface sites, facile electron transmission paths, and the synergistic effects of the trimetallic components. These findings demonstrate that Ni-Mo-Co trimetallic hydroxides are promising electrode materials, offering both high capacitance and long-term cyclability for supercapacitors.

1. Introduction

Currently, the development of electric vehicles and consumer electronics is hindered by the inadequate energy and power density, limited cycle life, and safety concerns of current energy storage systems. Batteries have a high energy density but require a long time to recharge. To achieve a high energy density with rapid power delivery and recharging (i.e., high power density), electrochemical capacitors (ECs), also known as supercapacitors (SCs), are promising advanced energy storage systems that can fill the gap between batteries and capacitors [1,2,3].
Supercapacitors, especially pseudocapacitors, have been attracting tremendous attention in recent years as a promising approach to address the relatively low power density of batteries and low energy density of the electric double layer capacitors (EDLCs), the other type of SCs. Different from EDLCs, which store energy via the separation of charge in a Helmholtz double layer, pseudocapacitors store energy via reversible surface Faradaic redox reactions involving charge transfer. As a result, there is a substantial difference in energy density between pseudocapacitors and EDLCs due to their distinct energy storage mechanisms [4]. EDLCs fall short of meeting the stringent requirements of energy-intensive applications due to the limited electric energy stored at the interface between the electrodes, typically composed of porous carbon, and the electrolyte. For instance, carbon-based materials in EDLCs exhibit a capacitance of no more than 150 F g−1 [5]. In pseudocapacitors, transition metal oxides/hydroxides, such as Ni (OH)2, Co (OH)2, NiO, CoO, Co3O4, RuO2, MnO2, and Fe3O4, have garnered increasing interest as pseudocapacitive materials owing to their high theoretical specific capacitance [6,7,8]. For example, by synthesizing a Co (OH)2 nanowire using a dual-template method, Xue et al. demonstrated a specific capacitance of 993 F g−1 at a current density of 1 A g−1 [9]. Hu et al. reported that Ni (OH)2 pseudocapacitive material shows a high specific capacitance of 2217 F g−1 [10]. Although monometallic hydroxides in pseudocapacitors exhibit a higher capacitance compared to EDLCs, several issues remain to be addressed, such as low-rate performance and challenges in achieving a high theoretical capacitance. These issues can be attributed to the intrinsic limitations of the aforementioned monometallic oxides and hydroxides, including poor electrical conductivity, poor electrochemical reversibility, cycling instability, and low-rate performance [11,12,13]. Additionally, the high cost of certain rare metals poses another challenge. For instance, RuO2, despite having a high theoretical specific capacitance of approximately 1000 F g−1, is prohibitively expensive, with costs exceeding USD 1 million for a vehicle-sized supercapacitor [13].
Recently, bimetallic oxides and hydroxides, such as NiCo2O4, NiMoO4, CoMoO4, MnMoO4, MnCo2O4, CoMn(OH)4, NiMn(OH)4, and CoFe(OH)5, have emerged as novel and promising electrode materials for high-performance supercapacitors, aiming to overcome the intrinsic limitations associated with monometallic oxides and hydroxides [14,15,16,17,18,19,20,21,22,23,24,25]. These bimetallic compounds demonstrate synergistic effects that mitigate the defects inherent in their monometallic counterparts. Moreover, the presence of multiple oxidation states in these multi-metallic compounds enables the generation of higher specific capacitances compared to single-metallic oxides [26,27]. For example, Ni (OH)2 suffers from low cyclic stability, poor electric conductivity, and poor rate performance, whereas NiCo(OH)4 possesses a high capacitance and excellent cyclic stability [28,29].
Similarly, as compared to mono-/double-metallic hydroxides/oxides, triple hydroxides/oxides may provide further electrochemical improvement by introducing one more metallic component [27]. For instance, in electrocatalyst applications, NiCoFe layered triple hydroxide nanosheets can be used as an electrocatalyst for water splitting and showed an improved performance compared to their mono-/double-metallic components [30]. NiMnCo or LiNiMnFeO materials have attracted a lot of attention due to their ability to address the intrinsic limitation of LiCoO2 or LiNiO2 for battery applications [31,32,33]. Although tri-metallic compounds exhibit improved electrocatalytic and electrochemical performance compared to mono-/bi-metallic compounds, no tri-metallic hydroxides are currently used in supercapacitors, and the effects of the composition and morphology of tri-metallic hydroxides on supercapacitor capacitance and durability have not been fully elucidated [3].
In the realm of supercapacitors, electrodes commonly rely on amorphous materials due to their potential to achieve higher capacity and superior electrochemical activity compared to crystalline counterparts [34,35]. However, the utilization of amorphous materials may incur challenges, such as pulverization and capacity degradation, owing to their inferior mechanical stabilization properties [36,37]. It is noteworthy that these drawbacks can be mitigated through the optimization of material morphology, for instance, by employing nanoscale three-dimensional structures [15,37,38,39], such as nanotubes [15], nanosheets [40,41], as well as nanobristles [42].
Herein, 3D interconnected mesoporous nanoflakes of amorphous Ni-Mo-Co triple metallic hydroxides (THs), deposited on Ni foam (NF) using a facile, environmentally friendly and scalable electrochemical deposition method, were investigated as supercapacitor electrode materials. The 3D interconnected mesoporous Ni-Mo-Co hydroxides nanoflakes grew vertically onto the surface of the conductive Ni foam, which can be directly used as binder-free electrodes for SCs. A summary of the SC values obtained using Ni, Co, or Mo hydroxide/oxide nanostructured electrodes during the last decade is shown in Table 1. Compared with the reported materials, Ni/Mo/Co of 1/1/0.4 on NF in this study was found to exhibit a better performance, with a reversible specific capacitance of 3074 F g−1 at a current of 2 A g−1 after the aging process. A series of tri-/bi-/mono-metallic hydroxide compounds were prepared for SCs, and correlations between Ni, Mo, and Co composition, as well as morphology, and their electrochemical capacitive performance were investigated in this research.

2. Experimental Section

2.1. Preparation of Ni-Mo-Co Triple Hydroxide (THs) Nanoflakes

All chemicals were of reagent-grade quality and used without further purification. The Ni foam (10 × 10 mm) was ultrasonically cleaned with 2 M HCl, ethanol, and deionized (DI) water and dried in a vacuum oven for 24 h before electrodeposition. The electrodeposition of Ni-Mo-Co triple hydroxides (THs) was performed in a standard three-electrode electrochemical cell at room temperature—graphite foil (20 × 10 mm) as the counter electrode, Ni foam as the working electrode, and Ag/AgCl as the reference electrode—using a CHI 660E model Electrochemical Workstation (Champaign, IL, USA). Electrodeposition was carried out in an aqueous solution containing various Ni/Mo/Co molar ratios of Ni (NO3)2, Na2MoO2, and Co (NO3)2. The total metal cation concentration was 1.2 mM. The THs were electrodeposited onto the dry Ni foam at a constant current of 0.5 mA/cm2 for 300 s at room temperature, right after soaking into the as-prepared solution for 900 s. The as-obtained coated Ni foam was dried in a vacuum oven for another 24 h. The mass of the active materials deposited on the Ni foam was calculated based on the weight of the sample before and after electrodeposition. Typical material loading was around 0.1 mg.

2.2. Material Characterization

X-ray photoelectron spectroscopy (XPS) spectra were taken using a Perkin-Elmer PHI 570 ESCA/SAM spectrometer. The morphology, surface structure, and metal composition of the as-prepared samples were analyzed by field emission scanning electron microscopy (FE-SEM, JEOL JEM-7000F) and energy-dispersive X-ray spectroscopy (EDS, BRUKER QUANTAX EDS for SEM). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were obtained using a JEOL JEM-2010 microscope with a LaB6 Filament gun.

2.3. Electrochemical Measurement

Single-electrode supercapacitor measurements were carried out at room temperature using a three-electrode system with an as-prepared Ni foam substrate deposited with active materials, Pt foil, and Hg/HgO as the working electrode, counter electrode, and reference electrode, respectively. Then, 30 c.c. of 1 M aqueous KOH was used as the electrolyte. The cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and galvanostatic cycling performance (GCP) were tested using an electrochemical workstation (CHI 660E).

3. Results and Discussion

3.1. Electrochemical Performance of the Supercapacitors (SCs)

The electrochemical performances of Ni-Mo-Co hydroxides with varying Ni/Mo/Co ratios were systematically investigated to elucidate the impact of composition on the properties of Ni-Mo-Co triple hydroxides (THs). Figure 1a depicts the cyclic voltammetry (CV) curves for the Ni-Mo-Co hydroxides within a potential window of −0.05 to 0.55 V (vs. Hg/HgO) at a scan rate of 20 mV s−1. The redox peaks observed for the Ni foam are significantly smaller than those of the as-deposited Ni-Mo-Co hydroxide electrodes, indicating that the capacitance contribution from pure Ni foam is negligible (as demonstrated in Figure 1a). The Ni/Mo/Co (1/1/0) hydroxides exhibit a pair of narrow redox peaks within the potential range from 0.33 to 0.47 V. Notably, the anodic peak intensifies and broadens, indicating a higher capacitance, and shifts towards more negative potentials with increasing Co ratios.
The specific capacitance (Cs) as a function of current density of the Ni-Mo-Co TH nanoflakes were plotted in Figure 1b, calculated based on the discharge process in the chronopotentiometry (CP) measurement, using the following equation [34,56]:
C s = I × ∆ t / m × ∆ V
where Cs represents the specific capacitance (F g−1), I is the constant discharge current (A), ∆t (s) is the duration of the discharge, m (g) is the mass of active material, and ∆V (V) is the voltage windows in the CP measurements. As expected, the specific capacitance of all samples decreases with the increase in current density. This common pseudocapacitive behavior is due to lower rate of redox reactions at a higher current density as a result of a shorter time for ion diffusion [10,57,58]. The Cs of Ni/Mo/Co (1/0/1), Ni/Mo/Co (1/1/1), Ni/Mo/Co (0/0/1), and Ni/Mo/Co (0/1/1) are 2026.4 F g−1, 2557.2 F g−1, 1232.4 F g−1, and 1124.4 F g−1, respectively, at the current density of 2 A g−1. This suggests the incorporation of Ni lead into a high electrochemical energy storage capacity. Similarly, the incorporation of Mo resulting in a higher specific capacitance, as the Cs Mo containing samples of Ni/Mo/Co (0/1/1), Ni/Mo/Co (1/1/0), and Ni/Mo/Co (1/1/1) are 1124.4 F g−1, 2026.4 F g−1, and 2557.2 F g−1 (Figure 1b), respectively, at the current density of 2 A g−1. On the other hand, the Cs of Ni/Mo/Co (0/0/1), Ni/Mo/Co (1/0/0), and Ni/Mo/Co (1/0/1) are 1133.4 F g−1, 1885.64 F g−1, and 2026.4 F g−1, respectively, at the same current density, which are lower than the Mo-containing metal hydroxides. Among all samples, the Ni/Mo/Co (1/1/0.4) sample shows the highest specific capacitances of 3074 F g−1 at 2 A g−1, and 2429 F g−1 at a highest current of 50 A g−1 (ca. 80% capacitance retention), indicating an excellent rate-performance and capacitance retention. On the other hand, the capacitance retention values of Ni/Mo/Co (1/1/0) and Ni/Mo/Co (1/1/1) are just ca. 72% and ca. 74%, respectively. This excellent electrochemical performance may be attributed to the high electrochemical reversibility of the optimized amount of Co-based hydroxides [59,60]. Moreover, the potential difference of anodic and cathodic peaks for Ni/Mo/Co (1/1/0.4) of ca. 100 mV is smaller than those for Ni/Mo/Co (1/1/0) (ca. 140 mV) and Ni/Mo/Co (1/1/1) (ca. 110 mV) (Figure 1a). This suggests a fairly high electrochemical reversibility of Co from charge to discharge [60]. Ni/Mo/Co (1/1/0.4) shows higher Cs (3074 F g−1 at 2 A g−1) than Ni/Mo/Co (1/1/1) and all other samples, which may be the result of the optimal composition of triple metallic compositions. These results indicate that the tri-metallic hydroxide can further improve the specific capacitance as compared to bi-metallic hydroxide, which may be attributed to the high electroactivity, stability, and excellent reversibility with the addition of Co [10,28,59,60]; fast electron transport, rapid ion diffusion, high electrical conductivity with Mo addition [51,61,62]; and remarkable electrochemical energy storage capacity resulting from Ni [63].
Energy density (E) and power density (P) are both critical factors in evaluating the electrochemical performances of energy storage devices including supercapacitors. Figure 2 presents the calculated gravimetric energy density and power density for the as-prepared hydroxide materials (for a detailed calculation, see ESI†). It illustrates that the energy density of Ni/Mo/Co (1/1/0.4) is higher than that of others at the same current density. The Ni/Mo/Co (1/1/0.4) shows the highest energy density of 464.96 W h kg−1 among all materials at a low current density, with a power density of 0.6 kW kg−1 and energy density of 367.39 W h kg−1, which remains the highest among all materials at a high current density with a high power density of 15.13 kW kg−1, indicating a remarkable electrochemical performance.

3.2. Morphology and Structural Analysis

Ni-Mo-Co triple hydroxide (TH) ultrathin mesoporous nanoflakes (NFs) were electrodeposited on Ni foam in the aqueous solution containing appropriate concentrations of Ni (NO3)2, Na2MoO2, and Co (NO3)2. The possible formation mechanism can be ascribed to the co-deposition of Ni2+, Co2+, and Mo6+ cations under alkaline condition. First, hydroxide ions (OH−) are produced on the surface of the cathode via a reduction reaction of NO3− and H2O, as shown in Equations (2) and (3) [64]:
NO3− + 7H2O + 8e− → NH4+ + 10OH−
2H2O + 2e− → H2 + 2OH−
Subsequently, the Ni-Mo-Co hydroxide was co-electrodeposited according to Equation (4):
xNi2+ + yCo2+ + zMo6+ + 2(x + y + 3z) OH− → NixCoyMoz(OH)2(x+y+3z)
The valence states and surface compositions of the as-prepared samples were characterized using X-ray photoelectron spectroscopy (XPS), as illustrated in Figure 3. The XPS survey spectra (Figure 3a) reveal the presence of Ni, Mo, Co, O, and a minor amount of C. High-resolution XPS spectra for Ni 2p, Mo 3d, Co 2p, and O 1s are presented in Figure 3b–e, respectively. Specifically, the Ni 2p spectrum (Figure 3b) exhibits two prominent peaks with binding energies at 873.5 eV for 2p1/2 and 855.9 eV for 2p3/2, corresponding to Ni2+ in nickel hydroxides [65]. The Mo 3d spectrum (Figure 3c) can be deconvoluted into two principal peaks with binding energies at 232.4 eV for 3d5/2 and 235.5 eV for 3d3/2, which are typically attributed to Mo6+ [66]. Similarly, the high-resolution Co 2p spectrum (Figure 3d) displays two main peaks at 781.6 eV and 797.7 eV, along with shake-up satellite peaks at 787.5 eV and 804.0 eV, indicative of Co2+ in cobalt hydroxides [67,68]. The O 1s XPS spectrum (Figure 3e) features a peak at 531.1 eV, which is attributed to metal hydroxide bonds [69]. The XPS analysis confirms that the as-prepared sample contains Ni2+, Mo6+, and Co2+, indicating the successful synthesis of Ni-Mo-Co ternary hydroxides.
The SEM images (Figure 4a,b) show that Ni-Mo-Co THs, with a Ni/Mo/Co solution concentration ratio of 1/1/0.4, are deposited uniformly onto the macroscopic 3D skeleton of the Ni foam. Figure 4b displays the SEM images recorded at a higher magnification of the Ni foam surface, as marked in Figure 4a. The image shows as-deposited ultrathin films consisting of interconnected Ni-Mo-Co hydroxide platelets, forming a mesoporous 3D nanostructure around 50 nm in length and 5 nm in thickness. These nanostructures tend to be vertically oriented on the surface of Ni foam, forming a nanoporous structure, with pore sizes of about 50–100 nm. Different from pure Ni foam substrate, the 3D structure of the highly rippled ultrathin nanoflakes have abundant open space and a large surface-to-volume ratio, which can provide more electroactive surface sites. These 3D structures allow the effective migration of the electrolyte, and thus enhance mass/charge transfer at the electrode/electrolyte interface [70]. The morphology and structure of the as-deposited composites were further investigated by TEM. Figure 4e shows a panoramic view of the as-deposited Ni-Mo-Co TH nanoflakes. The interconnected mesoporous characteristic with a distinct light/dark contrast suggested the existence of nanoflakes and mesoporous structures with open space. Moreover, the nanoflakes show a rippled flake structure with dimensions around 50–100 nm, in accordance with the SEM results. These nanoflakes are quite transparent, indicating their ultrathin features. Figure 4g–k present the EDS mapping images of the Ni-Mo-Co TH nanoflakes supported on the Ni foam, and Figure 4g is the scanning background of the test area. It clearly reveals that Mo, Ni, and Co are uniformly distributed, suggesting the homogeneous deposition of the Ni-Mo-Co TH nanoflakes onto the skeleton of Ni foam.
The high-resolution TEM image (Figure 4f) shows there is no observable lattice fringes for Ni, Mo, and Co of Ni-Mo-Co hydroxide composites, suggesting that the as-deposited Ni-Mo-Co TH nanoflakes are amorphous. This can be further supported by the electron diffraction (SAED) pattern of the as-prepared nanoflakes (the inset of Figure 4e), which show a broad and diffused halo ring. As compared to crystalline metallic hydroxides, amorphous hydroxides exhibited enhanced electrochemical performance, which may be attributed to the following aspects: 1. The long-range disorder and short-range ordered structure can improve the electronic conductivity of the electrode materials. 2. The easier access of the amorphous structure for intercalation and deintercalation of charges; 3. Adequate defects on the amorphous materials favor the charge-transfer rate [34,71]. Moreover, the hierarchically structured electrode with mesoporous Ni-Mo-Co hydroxides formed on a microporous Ni foam could benefit the ion/mass transfer while charging/discharging [72].

3.3. Electrochemical Test of Ni-Mo-Co (1/1/0.4) THs

The electrochemical properties were measured by CV and CP techniques. Figure 5a presents the CV curves of Ni-Mo-Co (1/1/0.4) TH nanoflake electrodes at various scan rates ranging from 1 mV s−1 to 100 mV s−1. A pair of relatively symmetric, board redox peaks can be clearly observed in all CV curves, which is characteristic of a pseudocapacitive behavior. The redox peaks were attributed to the Faradaic reactions of Equations (5)–(7), respectively.
Ni (OH)2 + OH− ↔ NiOOH + H2O + e−
Co (OH)2 + OH− ↔ CoOOH + H2O + e−
CoOOH + OH− ↔ CoO2 + H2O + e−
At the scan rate of 20 mV s−1, the redox peak potentials are found to be 0.37 V and 0.27 V for anodic and cathodic peaks, respectively. Moreover, all of these CV curves exhibit similar shapes, except for the slight shift in the redox peak position with the increasing scan rates resulting from concentration polarization. These CV curves suggest the good electrochemical reversibility, high-rate performance and good electron conductivity of amorphous Ni-Mo-Co TH nanoflakes. The GCD tests were performed within the potential window of −0.05 to 0.55 V (vs. Hg/HgO) at current densities varying from 2 to 50 A g−1 to evaluate the electrochemical capacitive performance (Figure 5b) using the CP technique. The nonlinear, highly symmetric shape, as well as the clear charge/discharge plateaus of the CP curves, are characteristic of the typical pseudocapacitance behavior [10]. Additionally, even at a high current density of 50 A g−1, the charge/discharge curve still shows an almost symmetrical nature without a clear IR decrease (inset of Figure 5b). This suggests that the amorphous Ni-Mo-Co TH nanoflakes exhibit rapid I-V response characteristics and reversible redox reactions, resulting in good electrochemical reversibility.
Moreover, the linear relationship between the current of the redox peaks in CV curves and the corresponding square root of the scan rate (Figure 5c) strongly suggest that the electrochemical process is a diffusion-controlled process of hydroxyl ions [38].
Figure 5d shows the long-term performance of the Ni/Mo/Co (1/1/0.4) hydroxide nanoflake electrode at a current density of 10 A g−1. Interestingly, the specific capacitance of the Ni-Mo-Co hydroxide gradually increased during the first 1300 cycles, which may be attributed to activation of the electrode [73]. After 5000 charge/discharge cycles, a loss of 3.37% of the specific capacitance was observed as compared to initial capacitance (for a detailed calculation, see ESI†), and the morphology of Ni/Mo/Co (1/1/0.4) remains relatively stable (Figure 4c,d). Thus, the remarkable cycling stability of this electrode can be attributed to the stable electrodeposited active materials on the Ni foam, as well as the resulting good structural stability. Moreover, the Ni-Mo-Co ternary hydroxide electrode shows a high specific capacitance, long cyclability, as well as excellent rate performance, which can also be attributed to the following: (1) the mesoporous hierarchical architectures of Ni-Mo-Co hydroxides nanoflakes [74] and the 3D structure affording a continuous network as well as extra active sites for the redox reaction with charge/discharge curves [26]; (2) the amorphous nature of the as-obtained triple metallic hydroxide with more defects, long-range disorder, as well as short-range order, which help to obtain an excellent electrochemical performance [34]; and (3) the binder-free electrode system that can promote electrolyte penetration into the active material, resulting in the electrons being able to transport at a sufficiently fast rate for high rate capability [70,75]. The mesoporous nanostructure of ternary hydroxides (THs) formed on microporous nickel foam substantially increases the specific surface area, thereby offering a greater number of reactive sites for electrochemical reactions in supercapacitors. Furthermore, nanoflake architecture enhances the mechanical stability of the amorphous Ni-Mo-Co THs. Consequently, the as-prepared samples exhibit significantly improved electrochemical reversibility, high-rate performance, and cycling stability in supercapacitor applications.

4. Conclusions

In summary, amorphous Ni-Mo-Co triple-hydroxide 3D hierarchical structures with ultrathin nanoflakes as building units were deposited on a Ni foam via a facile template-free, and scalable electrodeposition method. This Ni-Mo-Co (1/1/0.4) TH on a Ni foam as a binder-free electrode for SCs exhibited a superior performance, with a high specific capacitance and capacity (3074 F g−1 at 2 A g−1, and 2429 F g−1 at highest current of 50 A g−1), outstanding rate performance (80% capacitance retention from 2 A g−1 to 50 A g−1), and remarkable cyclability (96.63% of capacitance retention after 5000 cycles at current density of 10 A g−1), which could be attributed to the large open spaces between the interconnected mesoporous nanoflakes, abundant electroactive surface sites and facile electron transmission paths. This performance is better than most reported mono-/bi-/tri-metallic hydroxides as electrodes for SCs. The Ni-Mo-Co THs were demonstrated to be promising high-capacitance electrode material with long-term cyclability for SCs.

Author Contributions

Conceptualization, K.Y.S.N. and P.L.; methodology, P.L.; validation, P.L. and Z.W.; formal analysis, Z.W. and P.L.; investigation, Z.T.; resources, P.L.; data curation, P.L.; writing—original draft preparation, P.L.; writing—review and editing, Z.W.; visualization, Z.T.; supervision, K.Y.S.N.; project administration, K.Y.S.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Electrochemical properties of Ni-Mo-Co hydroxide supercapacitors formed in solution with different Ni/Mo/Co ratios: (a) CV curves at scanning rate of 20 mV/s; (b) evolution of specific capacitance with different current densities.
Figure 1. Electrochemical properties of Ni-Mo-Co hydroxide supercapacitors formed in solution with different Ni/Mo/Co ratios: (a) CV curves at scanning rate of 20 mV/s; (b) evolution of specific capacitance with different current densities.
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Figure 2. Ragone chart of Ni-Mo-Co hydroxide formed in different solutions, calculated from GCD data.
Figure 2. Ragone chart of Ni-Mo-Co hydroxide formed in different solutions, calculated from GCD data.
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Figure 3. XPS spectra of the Ni-Mo-Co hydroxide formed in Ni/Mo/Co (1/1/0.4) solution: (a) Survey scan, (b) Ni 2p spectrum, (c) Mo 3d spectrum, (d) Co 2p spectrum, (e) O 1s spectrum.
Figure 3. XPS spectra of the Ni-Mo-Co hydroxide formed in Ni/Mo/Co (1/1/0.4) solution: (a) Survey scan, (b) Ni 2p spectrum, (c) Mo 3d spectrum, (d) Co 2p spectrum, (e) O 1s spectrum.
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Figure 4. Morphology of Ni-Mo-Co hydroxide formed in Ni/Mo/Co (1/1/0.4) solution: (a) low- and (b) high-magnification images of Ni-Mo-Co TH nanoflakes supported on Ni foam; the morphology and structure of Ni-Mo-Co TH nanoflakes (c) before cycling and (d) after 5000 cycles. (e) Low- and (f) high-resolution TEM images of Ni-Mo-Co TH nanoflakes; the low-left inset in (e) represents the corresponding selected area diffraction pattern. (g–k) EDS elemental mapping.
Figure 4. Morphology of Ni-Mo-Co hydroxide formed in Ni/Mo/Co (1/1/0.4) solution: (a) low- and (b) high-magnification images of Ni-Mo-Co TH nanoflakes supported on Ni foam; the morphology and structure of Ni-Mo-Co TH nanoflakes (c) before cycling and (d) after 5000 cycles. (e) Low- and (f) high-resolution TEM images of Ni-Mo-Co TH nanoflakes; the low-left inset in (e) represents the corresponding selected area diffraction pattern. (g–k) EDS elemental mapping.
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Figure 5. (a) CV curves of the Ni-Mo-Co TH supercapacitor (formed in Ni/Mo/Co (1/1/0.4) solution) at different scan rates from 1 to 100 mV s−1; (b) galvanostatic charge/discharge (GCD) curves of the Ni-Mo-Co THs at different current densities from 2 to 50 A g−1. (c) Correlation between Ip and V1/2 of the Ni-Mo-Co TH nanoflake electrode; (d) galvanostatic cycling performance at a current density of 10 A g−1 for 5000 cycles.
Figure 5. (a) CV curves of the Ni-Mo-Co TH supercapacitor (formed in Ni/Mo/Co (1/1/0.4) solution) at different scan rates from 1 to 100 mV s−1; (b) galvanostatic charge/discharge (GCD) curves of the Ni-Mo-Co THs at different current densities from 2 to 50 A g−1. (c) Correlation between Ip and V1/2 of the Ni-Mo-Co TH nanoflake electrode; (d) galvanostatic cycling performance at a current density of 10 A g−1 for 5000 cycles.
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Table 1. Summary of supercapacitance values obtained using nickel, cobalt, or molybdenum hydroxide/oxide nanostructured electrodes synthesized by different methods during the last decade. Note: the most cited paper(s), in each year, was (were) only considered for comparison.
Table 1. Summary of supercapacitance values obtained using nickel, cobalt, or molybdenum hydroxide/oxide nanostructured electrodes synthesized by different methods during the last decade. Note: the most cited paper(s), in each year, was (were) only considered for comparison.
YearChemical
Composition		MorphologySynthesis MethodCapacitance (F g−1) and Current DensityDecay% (after No. of Cycles)Ref.
2019NiCo2O4 aerogelsAerogelEpoxide addition procedure1400Excellent stability (2000)[43]
2010Gold/MnO2NanoporousConventional chemical method~1145N/A[44]
20123D graphene/Co3O4NanowireChemical vapor deposition~1100 at 10 A g−1N/A[45]
2012Co3O4/NiONanowiresTwo-step solution-based method853 at 2 A g−1Excellent stability (6000)[46]
2013Ni-Co hydroxideNanosheetsHydrothermal1734 at 6 A g−114% (1000)[47]
2014Ni-Co hydroxideNanosheetsOne-step process2682 at 3 A g−118% (5000)[48]
2014NiMoO4 NanowiresFacile hydrothermal method1517 at 1.2 A g−14.7% (1000)[49]
2015Ni-Mo metal oxideUltrathin mesoporousConventional chemical method954 at 2 A g−122.3% (5000)[50]
2015NiMoO4Ultrathin nanosheetsElectrodeposition1694 at 1 A g−17.2% (9000)[51]
2017Ni-Co@Ni-Co layered double hydroxideCore–shell structured nanotubeConventional chemical method319 at 2 A g−13.1% (3000)[52]
2017Mn-Co layered double hydroxideHierarchical hollow cagesConventional chemical method511 at 2 A g−110% (2000)[53]
2017Ni-Al nanosheetsNanosheetsConventional chemical method1919 at 2 A g−1Recover to 433 after 3000[54]
2017NiCoMn2 metal oxideVarious morphologiesHydrothermal1434.2 at 2 mA cm−25.7% (3000)[55]
NowNi-Mo-Co ternary hydroxideNanosheets One-step electrodeposition 3074 at 2 A g−13.37% (5000)
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Wang, Z.; Li, P.; Tang, Z.; Ng, K.Y.S. Novel Design and Synthesis of Ni-Mo-Co Ternary Hydroxides Nanoflakes for Advanced Energy Storage Device Applications. Energies 2024, 17, 3881. https://doi.org/10.3390/en17163881

AMA Style

Wang Z, Li P, Tang Z, Ng KYS. Novel Design and Synthesis of Ni-Mo-Co Ternary Hydroxides Nanoflakes for Advanced Energy Storage Device Applications. Energies. 2024; 17(16):3881. https://doi.org/10.3390/en17163881

Chicago/Turabian Style

Wang, Zhao, Peifeng Li, Zhuolun Tang, and Ka Yuen Simon Ng. 2024. "Novel Design and Synthesis of Ni-Mo-Co Ternary Hydroxides Nanoflakes for Advanced Energy Storage Device Applications" Energies 17, no. 16: 3881. https://doi.org/10.3390/en17163881

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