Effect of coating MoS 2 nanoflakes onto CuCo 2 O 4 on the electrochemical performance as electrode materials of supercapacitors

Abstract

CuCo₂O₄@MoS₂ microflowers have been synthesized on Ni foam using a hydrothermal method, and their electrochemical performances as supercapacitor electrodes are investigated. The influence of MoS₂ loading on the specific capacity and cycle stability of the nanocomposites is systematically analyzed. It is found that the addition of MoS₂ significantly enhances the specific capacity of the CuCo₂O₄ microflowers, while the cycle stability varies with MoS₂ content. Competitive mechanistic insights into the interplay between MoS₂ content, electrochemical behavior, and structural stability during cycling are provided. Notably, the CuCo₂O₄@MoS₂-60 sample exhibits an impressive specific capacity of 11.1 F cm⁻² at a discharge current density of 0.003 A cm⁻², with a remarkable capacity retention rate of 94.6% after 1800 charge–discharge cycles. Moreover, the practical application prospects are demonstrated through the fabrication of symmetric supercapacitors. This study offers valuable guidance for the rational design and optimization of CuCo₂O₄@MoS₂ nanocomposites for high-performance supercapacitor applications.

Introduction

Supercapacitors have emerged as a promising energy storage technology due to their high power density, rapid charge–discharge rates, and long cycle life,^1–5 making them ideal for a wide range of applications, including portable electronics, ⁶ electric vehicles, ⁷ and renewable energy systems. ⁸ The performance of supercapacitors is largely determined by the electrode materials, which directly impact the specific capacity, rate capability, and cycling stability. ⁹ Transition metal oxides have garnered significant attention as electrode materials for supercapacitors due to their high specific capacitance and abundant redox-active sites.^10,11 Among these materials, bimetallic oxides have shown great potential for supercapacitor applications owing to their unique advantages. Bimetallic oxides offer the ability to combine the properties of multiple transition metals, allowing for synergistic effects that can enhance the electrochemical performance of the resulting materials.^12–15

CuCo₂O₄, a spinel-type copper cobalt oxide, has demonstrated high theoretical specific capacity and excellent electrochemical stability, making it an attractive candidate for supercapacitor applications.^16–20 However, the practical utilization of CuCo₂O₄ is hindered by its low electrical conductivity and poor rate capability. To address these limitations, the integration of CuCo₂O₄ with highly conductive and electrochemically active materials has been proposed to improve the performance of the electrode material.^21–24 For example, Wang et al. ²¹ have synthesized urchin-like core–shell CuCo₂O₄@CuCo₂S₄ heterostructure on carbon cloth substrate through a hydrothermal-assisted electrodeposition method, which not only spatially separates the redox reactions but also enhances the electrochemical stability. Liu et al. ²² prepared porous CuCo₂O₄/CuO microspheres and nanosheets, demonstrating excellent electrochemical performance and significant electrochemical energy storage potential. Xiao et al. ²³ coated CoSe shell on an urchin-like CuCo₂O₄ electrode for high-performance supercapacitors and demonstrated that the core–shell structure can greatly enhance the stability and electrochemical performance. By combining CuCo₂O₄ with other materials, the resulting composites can leverage the unique properties of both components, leading to enhanced supercapacitor performance. One such material, molybdenum disulfide (MoS₂), has garnered attention for its high theoretical specific capacitance, excellent electrochemical activity, and good mechanical flexibility.^25–27 MoS₂ has been widely investigated for applications in energy storage and conversion devices, including lithium-ion batteries,^28–30 hydrogen evolution reactions,^31–33 and supercapacitors.^34–36 For instance, Wang et al. ³⁷ synthesized nanoflower-like MoS₂@CuCo₂O₄ heterostructures and exhibited a high sensitivity for real-time glucose detection. Koohdareh et al. ³⁸ investigated the synthesis of MoS₂ nanoflowers decorated with CuCo₂O₄ and their catalytic activities toward the electrochemical hydrogen evolution reaction.

While the integration of MoS₂ with different electrode materials has shown promising results, the potential of CuCo₂O₄–MoS₂ nanostructures for supercapacitor applications remains relatively unexplored. This study aims to address this gap by investigating the synthesis and electrochemical performance of CuCo₂O₄–MoS₂ composite materials for supercapacitor electrodes. The synergistic effects arising from the combination of CuCo₂O₄ and MoS₂ are expected to provide increased active sites for charge storage, improve electrical conductivity, and facilitate charge transfer kinetics, leading to enhanced supercapacitor performance. The findings of this research could provide valuable insights into the development of advanced electrode materials for high-performance supercapacitors, contributing to the advancement of energy storage technologies.

Results and discussion

The X-ray diffractometer (XRD) patterns of CuCo₂O₄ and CuCo₂O₄/MoS₂-60 are shown in Figure 1(a). CuCo₂O₄ sample exhibits characteristic diffraction peaks at 2θ values of 18.8°, 31.2°, 36.7°, 44.6°, 55.4°, 59.2°, and 65.0° corresponding to (111), (220), (311), (400), (422), (511), and (440) crystallographic planes in the standard spectrum of spinel CuCo₂O₄ (JCPDS card no. 37-0878), respectively. For CuCo₂O₄/MoS₂-60 sample, in addition to the characteristic peaks of CuCo₂O₄, new diffraction peaks are observed at 2θ values of 32.8°, 33.7°, and 50.5°, which are indexed to the (100), (101), and (105) crystallographic planes of MoS₂ (JCPDS card no. 87-2416), confirming the successful incorporation of MoS₂ into the CuCo₂O₄ nanostructures without vulcanization.

Figure 1.

(a) XRD pattern of CuCo₂O₄ and CuCo₂O₄/MoS₂-60, (b) SEM image of CuCo₂O₄ on Ni form, (c) magnified SEM image of CuCo₂O₄ cluster, (d) SEM image of CuCo₂O₄/MoS₂-60 microflower, (e) TEM image of CuCo₂O₄/MoS₂-60 microflower, and (f) high-resolution TEM image.

Figure 1(b) is the scanning electron microscopy (SEM) image of CuCo₂O₄, revealing that the cluster-like CuCo₂O₄ structures are well formed on the Ni framework and uniform in size with a hierarchical morphology. Figure 1(c) is a magnified SEM image of a CuCo₂O₄ cluster, displaying the 2-µm diameter cluster consists of interconnected ultrathin nanosheets. The surface of CuCo₂O₄ cluster exhibits a porous and rough texture, providing a high surface area for electrochemical reactions and charge storage. The SEM image of the CuCo₂O₄/MoS₂-60 composite shows the uniform distribution of MoS₂ nanoflakes on the surface of the CuCo₂O₄ nanostructures with the diameter expanding from 2 to 10 µm (Figure 1(d)). The MoS₂ nanoflakes appear to coat the CuCo₂O₄ clusters, forming a conformal and continuous layer with microflower morphology. Figure 1(e) is a transmission electron microscopy (TEM) image of a CuCo₂O₄/MoS₂-60 microflower, showing that CuCo₂O₄ cluster is totally covered with MoS₂ nanoflakes. Distinct lattice fringes can be observed in the high-resolution TEM image (Figure 1(f)). The irregular and lattice fringes with distances of 0.25 and 0.62 nm are in good agreement with the (311) plane of CuCo₂O₄ and the (002) plane of MoS₂, respectively.

Figure 2(a) is the energy-dispersive X-ray spectrometer (EDS) spectra of CuCo₂O₄ and CuCo₂O₄/MoS₂-60. Co, Cu, and O are originated from CuCo₂O₄, while Ni comes from the Ni foam substrate. In CuCo₂O₄/MoS₂-60 spectrum, the peak belonging to the Mo and S can be observed, implying the formation of MoS₂. The elemental mapping profiles of CuCo₂O₄@MoS₂-60 microflower are shown in Figure 2(b), exhibiting a distinct spatial arrangement. Cu and Co are primarily concentrated in the core region, further indicating the dominant presence of CuCo₂O₄. Mo and S are found in the outer layer, indicating the existence of MoS₂. This elemental distribution suggests the successful formation of a well-defined core–shell structure, where CuCo₂O₄ acts as the core material and MoS₂ forms the shell.

Figure 2.

(a) EDS spectra of CuCo₂O₄ and CuCo₂O₄/MoS₂-60 and (b) elemental mapping profiles of CuCo₂O₄@MoS₂-60.

X-ray photoelectron spectroscopy (XPS) is conducted to further elucidate the chemical bonding and oxidation states of the elements presented in CuCo₂O₄/MoS₂-60 sample, as shown in Figure 3. The survey spectra of CuCo₂O₄ reveal the presence of Cu, Co, and O elements, consistent with the expected composition of the spinel-structured CuCo₂O₄ (Figure 3(a)). For CuCo₂O₄/MoS₂, new peaks corresponding to Mo and S are observed, indicating the successful incorporation of MoS₂ into the composites. The high-resolution XPS spectra of CuCo₂O₄/MoS₂ are shown in Figure 3(b)–(f). In Cu 2p spectrum (Figure 3(b)), the peaks at 962.4, 943.8, and 942.1 eV correspond to the shake-up satellite peaks of Cu²⁺, while 932.6 and 952.7 eV correspond to Cu 2p_3/2 and Cu 2p_1/2, respectively. Six peaks appear in the spectrum of Co 2p, as shown in Figure 3(c). Generally, the peaks located at 793–803 and 780–785 eV belong to the Co 2p_1/2 and Co 2p_3/2, respectively. In this case, the two main pairs of peaks belong to Co³⁺ and Co²⁺, while the peaks at 786.6 and 803.1 eV are raised from satellite peaks, proving the hybrid chemical states and spinel structure of CuCo₂O₄. ³⁹ Figure 3(d) shows the Mo 3d spectrum, showing that Mo 3d_5/2 and Mo 3d_3/2 are located at approximately 228.2 and 231.4 eV, respectively. In S 2p spectrum, the peak can be well bimodal fitted with the peaks at 163.3 and 161.9 eV (Figure 3(e)). ⁴⁰ The fine spectrum of O 1s in Figure 3(f) can be devolved into three peaks, which are classified as metal–oxygen–metal bond (530.6 eV), metal–oxygen–hydrogen bond (531.4 eV), and oxygen–hydrogen bond (532.5 eV) in surface-adsorbed water, respectively. ⁴¹ These analyses provide comprehensive insights into the elemental composition, crystal structure, and morphology of CuCo₂O₄ and the CuCo₂O₄/MoS₂ microflowers, highlighting the potential of the CuCo₂O₄/MoS₂ composite for advanced energy storage applications.

Figure 3.

XPS spectra of CuCo₂O₄/MoS₂: (a) survey, (b) Cu 2p, (c) Co 2p, (d) Mo 3d, (e) S 2p, and (f) O 1s.

The cyclic voltammetry (CV) curves of the CuCo₂O₄ and CuCo₂O₄/MoS₂ samples are shown in Figure 4. Figure 4(a) exhibits one pair of reduction/oxidation peaks in a CV cycle of CuCo₂O₄, which remains almost unchanged and exhibits good symmetry, revealing the pseudo-capacitance nature of CuCo₂O₄ with good reversibility at different scan rates. Under the scan rate of 0.005 C s⁻¹, the reduction/oxidation potential of CuCo₂O₄ is 0.19/0.40 V. With increasing scan rate, the peak current increases gradually. The anode peak shifts positively, while the cathode peak shifts negatively, which is caused by the polarization of the electrode and the uncompensated resistance. As shown in Figure 4(b)–(d), the CV curves of CuCo₂O₄/MoS₂ remain one pair of reduction/oxidation peaks but the peaks shift. The oxide peaks of CuCo₂O₄/MoS₂-40 are less positive than those of CuCo₂O₄ under the same scan rate, suggesting the coating of MoS₂ can induce fast ion–electron transfer. Once CuCo₂O₄ is coated with MoS₂, the closed area significantly increases and varies with MoS₂ content, suggesting the improved capacity of CuCo₂O₄/MoS₂.

Figure 4.

CV curves of (a) CuCo₂O₄, (b) CuCo₂O₄/MoS₂-40, (c) CuCo₂O₄/MoS₂-60, and (d) CuCo₂O₄/MoS₂-80.

To determine the specific capacity of the samples, galvanostatic charge–discharge (GCD) curves are obtained by applying a constant current density during the charge and discharge processes, as shown in Figure 5. Each curve exhibits typical plateaus, which is the characteristic of the redox reactions of transition metal oxide materials. At current density of 0.003 A cm⁻², CuCo₂O₄ exhibits a specific capacitance of 4.0 F cm⁻² (Figure 5(a)), while CuCo₂O₄@MoS₂-40, CuCo₂O₄@MoS₂-60, and CuCo₂O₄@MoS₂-80 demonstrate excellent specific capacity of 7.7, 11.1, and 21.1 F cm⁻², respectively (Figure 5(b)–(d)). The high capacitance of CuCo₂O₄@MoS₂-80 is mainly attributed to its outstanding morphological structure and good conductivity. ⁴² The rate capabilities are shown in Figure 6(a). As the current density increases to 0.01 A cm⁻², the specific capacities of the four samples drop to 12.4, 6.3, 3.9, and 2.4 F cm⁻², respectively. The specific capacity of the electrodes rapidly decreases with the increase in the current density, which may be attributed to concentration polarization and electrochemical polarization at high current density. Electrochemical impedance spectroscopy (EIS) analysis is conducted at the open-circuit potential and an amplitude of 5 mV with frequency ranging from 0.01 Hz to 100 kHz, and the obtained Nyquist plots are depicted in Figure 6(b). All the Nyquist plots consist of a semicircle in the high-frequency region and a straight line in the low-frequency region, representing the charge transfer resistance and diffusion resistance of the electrode, respectively. It can be observed that both the charge transfer resistance and diffusion resistance of CuCo₂O₄ are the highest. However, as the MoS₂ content increases, both resistances decrease simultaneously. This suggests that MoS₂ effectively enhances the conductivity of the electrode, leading to improved electrochemical performance and facilitating Faradaic redox reactions.

Figure 5.

CV curves of (a) CuCo₂O₄, (b) CuCo₂O₄/MoS₂-40, (c) CuCo₂O₄/MoS₂-60, and (d) CuCo₂O₄/MoS₂-80.

Figure 6.

(a) Rate capabilities and (b) EIS curves.

To further study the sulfuration reaction, repeated charge–discharge experiments are conducted for 1800 cycles at a current density of 300 mA cm⁻² to evaluate the cycle stability performance under rapid charge–discharge condition, as shown in Figure 7. With the increasing cycle number, the specific capacity of CuCo₂O₄ increases to a final retention of 142.5% (Figure 7(a)), indicating the slow self-activation process of pure CuCo₂O₄ clusters. The Coulombic efficiency remains ~100%, implying no irreversible chemical reaction occurs in charge–discharge cycle. For CuCo₂O₄@MoS₂-40, CuCo₂O₄@MoS₂-60, and CuCo₂O₄@MoS₂-80, the specific capacities rise and fall alternately several times, but the whole shows downward trend in varying degrees, resulting in capacity retentions of 74.3%, 94.6%, and 70.8%, respectively (Figure 7(b)–(d)). It is also shown that the capacity retention is affected by the MoS₂ content. Figure 8 shows the SEM images of the CuCo₂O₄@MoS₂-80 sample before and after the 1800 charge–discharge cycles. It is clearly observed that plenty of microflowers detach from the Ni foam framework after the recycle test, leading to the rapid decline of specific capacity. It is possible that the excessive MoS₂ shell layer leads to a decrease in the binding force of electrode materials, making them more prone to detachment from the Ni foam. In the future works, we should pay more attention to optimizing the synthesis and processing conditions to enhance the mechanical adhesion between microflower and framework, and balance the competition of capacity and stability.

Figure 7.

Specific capacities and Coulombic efficiencies of (a) CuCo₂O₄, (b) CuCo₂O₄/MoS₂-40, (c) CuCo₂O₄/MoS₂-60, and (d) CuCo₂O₄/MoS₂-80 in 1800 cycles.

Figure 8.

(a) SEM image of CuCo₂O₄@MoS₂-80 before cycling test and (b) SEM image of CuCo₂O₄@MoS₂-80 after cycling test.

MoS₂ is a two-dimensional material with a layered structure, allowing for high surface area and abundant active sites for charge storage. To quantitatively measure the active surface area, Brunauer–Emmett–Teller (BET) and electrochemical surface area (ECSA) tests are conducted, as shown in Figure 9. Figure 9(a) shows that the CuCo₂O₄@MoS₂-80 exhibits the highest surface area of 93.65 m² g⁻¹ compared to the surface area of 60.64 for CuCo₂O₄ and 84.71 m² g⁻¹ for CuCo₂O₄@MoS₂-60, indicating that the MoS₂ coating contributes to the increment in surface area. Similar results are also observed from the ECSA test. The calculated double-layer capacitance (C_DL) values of CuCo₂O₄, CuCo₂O₄@MoS₂-40, CuCo₂O₄@MoS₂-60, and CuCo₂O₄@MoS₂-80 are 29.8, 51.7, 54.5, and 57.9 mF cm⁻², respectively, as shown in Figure 9(b). The higher C_DL values of CuCo₂O₄@MoS₂ samples provide abundant active surface area for charge storage. The incorporation of MoS₂ nanosheets into the CuCo₂O₄ enhances the charge storage capability of the composite material. As the MoS₂ content increases, more active sites are available for charge storage, leading to an increase in the specific capacity of the composite. The efficient charge transfer enabled by MoS₂ leads to a higher utilization of the active material, allowing for more charge to be stored and delivered during the charge–discharge cycles. In addition, the high conductivity of MoS₂ helps to reduce the resistance at the electrode–electrolyte interface, ensuring better electrochemical kinetics.

Figure 9.

(a) BET-specific surface areas and (b) C_DL-related ECSA.

To verify the feasibility of CuCo₂O₄@MoS₂ material as a supercapacitor electrode, the two-electrode CV and GCD performances are conducted using two CuCo₂O₄@MoS₂-60 samples as the cathode and anode, respectively. In Figure 10(a), the CV curve shows an extended voltage window up to 1 V, maintaining excellent reversibility. In Figure 10(b), the GCD curves at different currents indicate a significant decrease in specific capacitance compared to the three-electrode method, but still exhibit a relatively high capacity. At a current of 0.003 A, the capacitance can reach 1.21 F cm⁻². These results demonstrate the feasibility of CuCo₂O₄@MoS₂ material as a supercapacitor electrode.

Figure 10.

(a) CV curves of two CuCo₂O₄@MoS₂-60 electrodes and (b) GCD curves of two CuCo₂O₄@MoS₂-60 electrodes.

Conclusion

In conclusion, CuCo₂O₄@MoS₂ microflowers are successfully synthesized on Ni foam using a hydrothermal method and their electrochemical performances as supercapacitor electrodes are investigated. The incorporation of MoS₂ has been found to significantly enhance the specific capacity of the CuCo₂O₄ microflowers. In addition, the cycle stability of the nanocomposites is influenced by the content of MoS₂, where the specific capacity and stability are competitive. Through competitive mechanistic insights, we have gained a deeper understanding of the interplay between MoS₂ content, electrochemical behavior, and structural stability during cycling. Particularly, the CuCo₂O₄@MoS₂-60 sample has exhibited impressive specific capacity and remarkable capacity retention rate after 1800 charge–discharge cycles. Furthermore, the practical application prospects of the CuCo₂O₄@MoS₂ nanocomposites are demonstrated through the fabrication of symmetric supercapacitors. This study provides valuable guidance for the rational design and optimization of CuCo₂O₄@MoS₂ nanocomposites for high-performance supercapacitor applications.

Experimental section

Synthesis of CuCo₂O₄ microflowers on Ni foam

The 2 × 4 cm² Ni foam was cleaned by ultrasonicating in 3M HCl, deionized water, and ethanol for 30 min to remove the oxide layer on the surface, and then placed in dilute HCl for later use. CuCo₂O₄ microflowers were synthesized via a simple hydrothermal method. In a typical synthesis process, 4 mmol Co(CH₃COO)₂·4H₂O and 2 mmol Cu(CH₃COO)₂·H₂O were dissolved in 60 mL of deionized water to form a homogeneous solution. Subsequently, 18 mmol CH₄N₂O were added under continuous stirring for 30 min. The resulting mixture and as-cleaned Ni foam were transferred to a 100-mL Teflon-lined stainless steel autoclave and maintained at 120°C for 6 h. After the hydrothermal reaction, the Ni foam was thoroughly rinsed with deionized water and ethanol, followed by drying at 60°C in a vacuum oven.

Synthesis of CuCo₂O₄/MoS₂ microflowers on Ni foam

A certain mass of Na₂MoO₄·2H₂O and CH₄N₂S with mass ratio of 1:1.5 was dissolved in 30 mL of deionized water with continuous stirring to form a clear solution. Subsequently, the mixture and as-grown Ni foam were transferred to a 50-mL Teflon-lined stainless steel autoclave and heated at 200°C for 24 h. After the hydrothermal reaction, the Ni foams were rinsed with alcohol and deionized water, and dried in a vacuum oven at 60°C. The obtained composite materials with the Na₂MoO₄·2H₂O usage of 40, 60, and 80 mg were named CuCo₂O₄@MoS₂-40, CuCo₂O₄@MoS₂-60, and CuCo₂O₄@MoS₂-80, respectively.

Characterization

XRD (Bruker D8 Advance) was used to analyze the crystal structure and phase composition at a scanning rate of 0.02° s⁻¹. TEM (JEM 2100F, JEOL) and SEM (Regulus 8100, Hitachi) were conducted to characterize the crystal structure and morphology. EDS (7021-H, Horiba) and XPS (K-Alpha, Thermo) were employed to examine the surface chemical composition and oxidation state. The specific surface area was measured by N₂ adsorption–desorption analysis with a Micromeritics ASAP 2000 apparatus and calculated using the BET method.

Electrochemical measurements

The electrochemical performances of the CuCo₂O₄/MoS₂ microspheres were evaluated in a three-electrode electrochemical workstation (CHI660E, CH Instruments) using GCD and CV methods. Ag/AgCl electrode and Pt foil were used as the reference and counter electrode, respectively. The Ni foam was cut into 1 × 3 cm² in area used as working electrode with 1 cm² immersed in electrolyte (3M KOH). CV measurements were cycled within a potential window ranging from 0 to 0.5 V at different scan rates of 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05 V s⁻¹. GCD was carried out with a working voltage between 0 and 0.42 V during the charge and discharge processes by applying a constant current density (0.003, 0.005, 0.01, 0.015, 0.03, 0.05, and 0.1 A cm⁻²). The area-specific capacity (C, F cm⁻²) was calculated following equation (1)

$$C=\frac{\sigma t}{\mathrm{\Delta }V}$$

(1)

where $\sigma ,t,and\mathrm{\Delta }V$ are the discharge current density, discharge time, and window potential, respectively. The cycle stability of CuCo₂O₄/MoS₂ was evaluated by continuously repeating the charge–discharge process for 1800 cycles at current density of 300 mA cm⁻². The capacity retention was calculated based on the ratio of C/C₀, where C₀ represents the specific capacity in the first cycle. The ECSA was determined through the measurement of the C_DL in a non-Faradaic potential window of 0.16–0.26 V at different scan rates by CV. The C_DL was obtained by plotting the difference of anodic and cathodic currents at 0.21 V against the scan rate in the range of 0.005–0.05 V s⁻¹.