NiMoO₄ Nanosheets Embedded in Microflake-Assembled CuCo₂O₄ Island-like Structure on Ni Foam for High-Performance Asymmetrical Solid-State Supercapacitors

Abstract

Micro/nano-heterostructure with subtle structural design is an effective strategy to reduce the self-aggregation of 2D structure and maintain a large specific surface area to achieve high-performance supercapacitors. Herein, we report a rationally designed micro/nano-heterostructure of complex ternary transition metal oxides (TMOs) by a two-step hydrothermal method. Microflake-assembled island-like CuCo₂O₄ frameworks and secondary inserted units of NiMoO₄ nanosheets endow CuCo₂O₄/NiMoO₄ composites with desired micro/nanostructure features. Three-dimensional architectures constructed from CuCo₂O₄ microflakes offer a robust skeleton to endure structural change during cycling and provide efficient and rapid pathways for ion and electron transport. Two-dimensional NiMoO₄ nanosheets possess numerous active sites and multi-access ion paths. Benefiting from above-mentioned advantages, the CuCo₂O₄/NiMoO₄ heterostructures exhibit superior pseudocapacitive performance with a high specific capacitance of 2350 F/g at 1 A/g as well as an excellent cycling stability of 91.5% over 5000 cycles. A solid-state asymmetric supercapacitor based on the CuCo₂O₄/NiMoO₄ electrode as a positive electrode and activated carbon as a negative electrode achieves a high energy density of 51.7 Wh/kg at a power density of 853.7 W/kg. These results indicate that the hybrid micro/nanostructured TMOs will be promising for high-performance supercapacitors.

1. Introduction

Supercapacitors, as an emerging energy storage device, have been attracting considerable attention due to their high power density, fast charge/discharge rates and long cycling life [1,2,3,4]. For large-scale practical applications, the energy densities of supercapacitors are still far behind rechargeable batteries [5,6]. In light of the critical parameters that are related to the energy density ($E=1/2\mathrm{C}{V}^2$) [7], elevating the specific capacitance is a direct and effective method to increase energy storage. Since the pseudocapacitors rely on rapid faradaic reactions of electrodes to store energy, pseudocapacitive materials usually possess higher capacitance than electric double-layer capacitors [8]. In recent years, ternary transition metal oxides such as NiCo₂O₄ [9,10], ZnCo₂O₄ [11,12], FeCo₂O₄ [13,14] and MnCo₂O₄ [15,16] have been extensively studied for the pseudocapacitor applications because they can provide multiple oxidation states for efficient redox reactions. Among various ternary transition metal oxides, CuCo₂O₄ is considered a potential material for supercapacitors due to electrochemical activity [17]. Nevertheless, owing to relatively low specific capacitance and electrical conductivity, single-component CuCo₂O₄ as an electrode material is not adequate to meet the requirements of a supercapacitor’s application completely. Nowadays, much research effort has been focused on multicomponent composite, such as CuCo₂O₄@MnMoO₄ [18], CuCo₂O₄@MoNi-LDH [19] and CuCo₂O₄@Co(OH)₂ [20]. Indeed, these composites can significantly improve electrochemical performance according to data of the above-mentioned research articles. Thus, designing composite materials based on CuCo₂O₄ is an effective strategy to achieve high specific capacitance of supercapacitors. NiMoO₄, as a member of pseudocapacitive materials, possesses a high specific capacitance because of the high electrochemical activity of Ni atoms with different oxidation states and good electrical conductivity of Mo atoms [21]. Therefore, constructing CuCo₂O₄@NiMoO₄ nanostructure composite is expected to achieve high capacitance by combining the advantages of two-component materials. However, poor cycle stability and inferior structural stability of NiMoO₄-based electrodes resulting from the excessive volume change and aggregation after long-term cycles limit its applications [22,23].

Structural engineering is another critical factor in reaching optimal supercapacitor performance, especially for cycling and rate performances. Among various structures, 2D structures have demonstrated their merits in various applications owing to their anisotropic structure and high surface-to-bulk ratio, which grant a short diffusion path for electrons and ions and rich active sites [24]. For instance, Zhao et al. reported that NiMoO₄ nanosheets-coated NiCo₂O₄ exhibited an ultrahigh specific capacitance of 2806 F/g at 5 A/g and good rate capability with 1408 F/g at 30 A/g [25]. Nevertheless, 2D structures face the problems of self-aggregation and structure collapses during cycling, which lead to irreversible capacitance loss and lifetime decay. Micro/nanostructures with subtle morphologies have been demonstrated to stabilize the skeleton structure during the charge/discharge process. Ravi et al. reported that asymmetric supercapacitors assembled with micro-nano MnCO₃ showed superior capacitance retention of 98.79% after 10,000 cycles [26]. To further improve the specific capacitance of CuCo₂O₄@NiMoO₄ composites without compromising structural stability, novel micro/nanostructured materials with 2D structure need to be explored.

Herein, we have proposed and validated the rational design and fabrication of the 3D hierarchical CuCo₂O₄/NiMoO₄ heterostructures on Ni foam through a facile and stepwise hydrothermal approach. The combination of 2D microflake CuCo₂O₄ assembled 3D architecture and secondary inserted units of NiMoO₄ ultrathin nanosheets make full use of the merits of individual components and micro/nanostructure. The microflakes assembled island-like CuCo₂O₄ serve as the backbone material, and subsequent embedded NiMoO₄ nanosheets can provide more active sites and electron/ion transport channels to facilitate the reaction kinetics. In addition, building a binder-free integrated structure with current collectors creates a highly efficient electron conducting pathway and avoids a “dead surface”. Therefore, the micro/nanostructure with multicomponent composite electrode exhibits excellent electrochemical performance.

2. Results and Discussion

2.1. Characterization

Figure 1 displays the morphologies of pristine CuCo₂O₄ and CuCo₂O₄/NiMoO₄ heterostructures. As can be seen in Figure 1a, the island-like CuCo₂O₄ microstructures are nearly uniformly anchored on Ni foam with an average diameter of about 10 µm. With a closer view (Figure 1c), these islands are assembled by microflakes with a thickness of about 40 nm. Moreover, the microflakes are shown to have plenty of open spaces between these adequately separated microflakes, which is beneficial for electrolyte transportation. The 3D structure can also serve as the ideal conductive skeleton for the subsequent NiMoO₄ nanosheet growth. From Figure 1b, it can be clearly seen that the interstices between CuCo₂O₄ microflakes are filled with ultrathin NiMoO₄ nanosheet. Under a lower magnification (Figure S1b), the surface of the Ni foam substrate is uniformly covered by the island-like structure while the surface of pristine Ni foam is smooth (Figure S1a), suggesting that CuCo₂O₄/NiMoO₄ anchored on the Ni Foam can be used directly as binder-free integrated electrodes for supercapacitors. The magnified image in Figure 1d shows that the NiMoO₄ nanosheets are typically interconnected with each other with a thickness of less than 5 nm. Such configuration is of great importance to facilitate electron and ion transfer, increase active sites and maintain structural stability. As a control sample (Figure S2), well-defined NiMoO₄ nanosheets are nearly uniformly aligned on the Ni foam.

To further identify the structure and composition of the CuCo₂O₄ and CuCo₂O₄/NiMoO₄ heterostructures, the elemental mapping is carried out under X-ray spectroscopy (EDS) mapping analysis. The elemental distribution of the CuCo₂O₄ microflake structure displays that the elements of Cu, Co and O were distributed uniformly on the whole area (Figure S3). In addition, the Cu/Co atomic ratio is about 0.5 (Figure S4), which matches the formula of CuCo₂O₄. After loading the NiMoO₄ nanosheets, all the elements (Cu, Co, Ni, Mo and O) are detected and homogenously distributed on CuCo₂O₄/NiMoO₄ hybrid heterostructures (Figure 2).

Further insights into the crystal structure of CuCo₂O₄ and CuCo₂O₄/NiMoO₄ heterostructures are elucidated by XRD patterns. As shown in Figure 3, the obtained patterns of CuCo₂O₄ microflakes match well with the standard patterns for the cubic spinel phase of CuCo₂O₄ (JCPDS No. 01-1155). For the heterostructures, several weak diffraction peaks attributed to NiMoO₄ (marked as triangle) are observed except for the peaks of the CuCo₂O₄ skeleton (marked as diamond), indicating relatively low crystallinity compared with the CuCo₂O₄ phase. Moreover, Figure S5 shows the identified peaks of the control sample, which can be mainly assigned to monoclinic NiMoO₄ (JCPDS No. 45-0142) with a small amount of α-NiMoO₄ phase (JCPDS No. 33-0948).

The detailed structural characterization of CuCo₂O₄ and CuCo₂O₄/NiMoO₄ are further investigated by TEM and HRTEM. Under a low-magnification TEM image (Figure S6a), these CuCo₂O₄ microflakes are shown to have a smooth surface. An enlarged TEM view (Figure 4a) reveals that the microflakes are porous and composed of many interconnected crystalline nanoparticles (~10 nm). Furthermore, the selected-area electron diffraction (SAED) patterns in the inset of Figure 4a demonstrate the polycrystalline nature of the CuCo₂O₄ skeleton, which is consistent with the XRD result. HRTEM measurements (Figure 4b) also clearly display two sets of visible lattice fringes with an interplanar spacing of 0.46 nm and 0.24 nm, corresponding to the (111) and (311) planes of CuCo₂O₄. Compared with the smooth CuCo₂O₄ microflakes, noticeable wrinkle-like structures are found on the surface of heterostructures (Figure 4c, Figure S6b). Meanwhile, the well-defined SAED pattern is also observed in the inset of Figure 4c, and the three diffraction rings correspond to the (422), (330) and (202) crystal planes of NiMoO₄. Figure 4d shows the HRTEM image of hybrid heterostructures. The measured lattice spacing of 0.21 nm and 0.24 nm are in good agreement with the (330) plane of NiMoO₄ and (311) plane of CuCo₂O₄, respectively. The above results demonstrate the formation of CuCo₂O₄/NiMoO₄ heterostructures.

2.2. Electrochemical Measurements

To evaluate CuCo₂O₄@NiMoO₄ heterostructures as a potential electrode material for the supercapacitor, CV and GCD tests are performed to measure electrochemical performance in a 6 M KOH solution in a three-electrode system. Figure 5a shows the CV curves of the pure NiMoO₄, CuCo₂O₄ and CuCo₂O₄/NiMoO₄ heterostructures at the scan rate of 5 mV/s. It should be noted that the enclosed CV curve area of the CuCo₂O₄/NiMoO₄ heterostructures is apparently larger than those of the pure NiMoO₄ or CuCo₂O₄, indicating that the CuCo₂O₄/NiMoO₄ heterostructures have a larger specific capacitance than the other two materials. This fact can be further confirmed by GCD curves. As shown in Figure 5b, CuCo₂O₄/NiMoO₄ heterostructures display a much longer discharge time in comparison with the other two materials at a current density of 2 A/g. The results prove that the NiMoO₄ nanosheets embedded in island-like CuCo₂O₄ skeleton can significantly improve the capacitive performance. Figure 5c shows CV curves of CuCo₂O₄/NiMoO₄ at different scan rates. All CV curves have similar shapes within a pair of redox peaks. The same phenomenon is also observed in a single component of either CuCo₂O₄ skeleton (Figure S7a) or NiMoO₄ nanosheets (Figure S7c). Apparently, individual components (CuCo₂O₄ or NiMoO₄) and composite both show pseudocapacitive behavior due to faradaic redox reactions [27]. Moreover, with the increasing of scan rates from 5 mV/s to 50 mV/s, the oxidation peak shifts towards a more positive position and the reduction peak towards a more negative position, which is supposed to be related to the internal resistance of the electrode and limitation of charge transfer kinetics [28]. It is worth noting that smaller shifts in the peak position of the CuCo₂O₄/NiMoO₄ electrode corresponds to that of CuCo₂O₄ or NiMoO₄ electrode in a tenfold increase in the scan rate, implying relatively low resistance and fast redox reactions. GCD curves further intuitively explain the phenomenon. As can be seen in Figure 5d, good symmetry of the profiles reveals the excellent reversibility of the charge/discharge behavior. A pair of potential plateaus in the charge and discharge processes is observed, which is consistent with the above CV results. The GCD curves of CuCo₂O₄ and NiMoO₄ electrodes at various current densities are shown in Figure S7b,d, respectively. The calculated specific capacitance as a function of the discharge current density is plotted in Figure 5e. Specifically, CuCo₂O₄/NiMoO₄ electrode delivers an ultrahigh capacitance of 2350 F/g at a current density of 1 A/g and an impressive capacitance as high as 1235 F/g can be achieved even at a current density of 10 A/g. In total, 52.5% of the initial capacitance is retained when the current density increases from 1.0 to 10 A/g, indicating excellent high-rate capability. Compared with CuCo₂O₄/NiMoO₄ electrodes, CuCo₂O₄ and NiMoO₄ electrodes show inferior performance in terms of specific capacitance and rate capability (

Table 1. Comparison of the three-electrode performance based on similar composition and morphology.

Electrode Materials	Max Capacitance	Min Capacitance	Rate Capability	Retention (Cycles)
NiMoO4 nanosheet arrays	1220 (1 A/g)	490 (10 A/g)	40.2%	69.7% (3000)
CuCo2O4 nanosheet-assembled island-like	797 (1 A/g)	366 (10 A/g)	45.9%	91.0% (5000)
CuCo2O4/NiMoO4 hierarchically structure	2350 (1 A/g)	1235 (10 A/g)	52.5%	91.5% (5000)

). EIS techniques are used to investigate the insights into the advantages of these electrodes. In Figure 5f, all Nyquist plots consist of three regions according to the different frequency ranges corresponding to the different interfacial processes. Briefly, the high-frequency intercept on the real Z′ axis represents the series resistance (Rs). In the middle-frequency region, the semicircle can be attributed to the charge transfer resistance (Rct) on the electrode surface [29]. As shown in the inset of Figure 5f, the CuCo₂O₄/NiMoO₄ electrode is found to have a much smaller intercept and diameter of the semicircle than the CuCo₂O₄ or NiMoO₄ electrode, indicating a lower Rs and Rct of the CuCo₂O₄/NiMoO₄ electrode. The straight line in the low frequency is associated with the ion diffusion of electrolytes. The almost vertical straight lines at a low frequency of both the CuCo₂O₄/NiMoO₄ electrode and the NiMoO₄ electrode demonstrated their superior electrolyte ionic diffusion behavior. The fitted values can provide a more intuitive interpretation, as shown in Table S1. The R_s value is rather small and similar for three electrodes, showing the advantage for in situ growth. The CuCo₂O₄/NiMoO₄ and NiMoO₄ electrode have similar values of R_ct and W, which are lower than that of the CuCo₂O₄ electrode. The rapid charge transfer rate and ion diffusion of CuCo₂O₄/NiMoO₄ is attributed to thin nanosheets and the in situ growth of CuCo₂O₄ on Ni Foam. Therefore, the improved conductivity and mass transport in CuCo₂O₄/NiMoO₄ micro/nanostructure enhances its electrochemical performance.

Furthermore, cycle stability is evaluated by repeating a charge/discharge process of the electrode materials. As the skeleton, island-like CuCo₂O₄ structure manifests exceptional cycling stability and is able to reach 91.0% of its initial value even after 5000 cycles (Figure S8a). After inserting NiMoO₄ nanosheets into the skeleton, the CuCo₂O₄/NiMoO₄ still maintains similar or even better cycling stability (Figure 6a). On the other hand, NiMoO4 nanosheets show a capacitance loss of 30.3% after only 3000 cycles (Figure S8c). Therefore, the unique construction enhances structural stability: NiMoO₄ nanosheets are inserted into layered CuCo₂O₄ microflakes, which inhibit the microflake collapse and self-agglomeration of nanosheets, while CuCo₂O₄ microflakes possess robust stability first. As indicated by the FESEM images of electrode materials after the cycling test (Figure 6b and Figure S8b), the structure of both island-like CuCo₂O₄ and CuCo₂O₄/NiMoO₄ are almost preserved, whereas NiMoO₄ nanosheets suffer from severe aggregation and deformation after long-term cycles (Figure S8d). Thus, it can be further inferred that enhanced cycling stability of CuCo₂O₄/NiMoO₄ heterostructures results from the unique structural features owing to the intrinsic rigid island-like skeleton and mutual support between nanosheets and microflakes. The comparison of the electrochemical performance of the three electrode materials is listed in Table 1. The electrochemical performance of CuCo₂O₄@NiMoO₄ heterostructures not only exceeds that of either CuCo₂O₄ island-like structure or NiMoO₄ nanosheets but also is superior to that of many previously reported mixed-metal oxides in terms of specific capacitance and cycling performance (Table S2).

Such desirable pseudocapacitive performance of the CuCo₂O₄/NiMoO₄ heterostructures can be explained as electron and ion transfer of unique structures. As illustrated in Figure 7, the electron/ion transfer path is as follows: (1) Two-dimensional NiMoO₄ nanosheets connect with each other to form the horizontal transport channel of electron and ion. (2) Two-dimensional CuCo₂O₄ microflakes are vertically grown on Ni foam, which are favorable for fast ion diffusion and electron transfer reaction. (3) Porous structures on microflakes and between nanosheets can increase specific surface area and facilitate electrolyte penetration. As shown in Figure S9, the BET-specific surface areas of heterostructures, CuCo₂O₄ microflakes and NiMoO₄ nanosheets are 98.5, 38.3 and 31.1 m²/g, respectively. From pore size distribution analysis, heterostructures composite has richer pore structure than the other two materials. Similarly, the micro/nanostructure is used to provide insights into superior cycling lifetime: (1) Microflakes assembled a 3D island-like structure provide good mechanical stability, and ensure structural integrity under continuous charge/discharge cycles. (2) Ultrathin NiMoO₄ nanosheets as secondary inserted units embedded inside CuCo₂O₄ framework, which have mutually reinforcing effects and provide more active sites. According to the above analysis, a hierarchical micro/nanostructure achieves the synergistic effect between CuCo₂O₄ microflakes and NiMoO₄ nanosheets by combining the advantages of both composition and unique structure. electrochemical performances of composite materials with micro/nanostructure surpass over that of single-component oxides or a 2D single structure.

In order to evaluate the practical application, an electrochemical asymmetric supercapacitor (ASC) is assembled by using the as-prepared CuCo₂O₄/NiMoO₄ heterostructures as a positive and activated carbon as a negative with PVA/KOH gel as the electrolyte. By matching the charges stored in the two electrodes according to formula: $m^{+}/m^{-}=(C^{-}\times \Delta V^{-})/(C^{+}\times \Delta V^{+})$, the optimal mass ratio between AC and CuCo₂O₄@NiMoO₄ is calculated to be around 7.74:1 based on the obtained CV results and GCD (Figure S10a–c). Figure 8a shows CV curves of the device collected at 20 mV/s with different potential windows ranging from 1.0 to 1.8 V. The result indicates that the maximum potential of the prepared device is 1.7 V because an obvious oxygen evolution occurs when the operating potential window exceeds 1.7 V. After obtaining this critical parameter, CV tests are researched at various scans with a fixed potential of 1.7 V (Figure 8b). All the CV shapes remained with barely any deformations with the increasing sweeping rate, demonstrating desirable charge/discharge behavior with superior reversibility. The GCD curves with different current densities from 1 to 10 A/g are shown in Figure 8c. The specific capacitance of the device as a function of current density is presented in Figure 8d. A remarkably high specific capacitance of 128.8 F/g is obtained at 1 A/g. Even at a high current density of 10 A/g, the device still has a specific capacitance of 63.2 F/g, which retained about 49.1% of the initial capacitance with a tenfold increase in the current density. Figure 8e shows the cycling stability of the device at a current of 5 A/g. After 3000 charge/discharge cycles, only about 9.2% capacitance loss was observed, revealing its excellent cycling stability. To find out the reason for capacity decay, nyquist plots are investigated (inset of Figure 8e). It can be clearly seen that intercept and semicircle at high-frequency region become slightly larger after cycles while slopes at a low frequency remain almost unchanged. The result confirms increased charge transfer resistance and internal resistance, which is associated with lifetime decay. The device can power a red commercial LED light (2.0 V) using two electrodes in series, as shown in Figure 8e (inset). Furthermore, the device delivers a high energy density of 51.7 Wh /kg at a power density of 853.7 W /kg and an energy density of 25.2 Wh /kg even at a high power density of 8558.2 W /kg, which is superior to the recently reported value (Figure 8f) [30,31,32,33].

3. Experimental

All of the reagents used in the experiments were of analytical grade and were used without further purification. Prior to the synthesis, the Ni foam was cleaned by acetone, ethanol and deionized water for 20 min in sequence with sonication. CuCo₂O₄, NiMoO₄ and CuCo₂O₄/NiMoO₄ were prepared by the hydrothermal method. In this work, activated carbon (YP50) was purchased from Kuraray Co., Ltd. (Tokyo, Japan). The other reagents were purchased from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China).

3.1. Synthesis of CuCo₂O₄ Island-like Structure

The materials were synthesized similarly according to previously reported procedures with minor modifications [34]. In brief, Co(NO₃)₂·3H₂O (2.0 mmol), Cu(NO₃)₂·3H₂O (1.0 mmol) and urea (5 mmol) were dissolved in 40 mL deionized water and ethanol (volume ratio = 5:3). The as-obtained solution and nickel foam were transferred to a 50 mL Teflon-lined stainless-steel autoclave. Subsequently, the reaction mixture was maintained at 120 °C for 6 h in an electric oven. After the reaction, the solution was cooled down to room temperature naturally. The as-obtained sample was rinsed with distilled water and ethanol thoroughly and dried in an oven at 60 °C for 12 h. Finally, the sample was annealed at 350 °C for 3 h, and the CuCo₂O₄ microflake’s island-like structure was obtained.

3.2. Synthesis of CuCo₂O₄/NiMoO₄ Heterostructures on Ni Foam

The synthesized CuCo₂O₄ was used as the skeleton for the growth of NiMoO₄ active materials. In total, 0.249 g Ni(CH₃COO)₂·4H₂O, 0.2 g (NH₄)₆Mo₇O₂₄·4H₂O, and 0.24 g urea were dissolved into 40 mL DI water. The mixed solution and CuCo₂O₄@Ni foam were transferred to a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and maintained at 140 °C for 2 h and then cooled to room temperature naturally. The sample was taken out and rinsed with distilled water and alcohol several times. Finally, the CuCo₂O₄/NiMoO4 was obtained by annealing at 400 °C for 2 h in the argon atmosphere. As shown in Figure 9, NiMoO₄ nanosheets (light blue) were tightly anchored on the surface of CuCo₂O₄ microflakes (purple) to form micro/nano-heterostructure. As a control experiment, NiMoO₄ nanosheets were synthesized according to a reported hydrothermal method [35]. Ni(NO₃)₂ 6H₂O (1 mmol) and Na₂MoO₄·2H₂O (1 mmol) were dissolved in a mixed solvent of 15 mL of H₂O and 15 mL of ethanol. The mixed solution and cleaned Ni Foam were transferred to the autoclave. Afterwards, the autoclave was sealed and maintained at 160 °C for 6 h to synthesize Ni-Mo precursor nanosheet arrays. The final product was obtained by annealing at 450 °C for 2 h in the argon atmosphere.

3.3. Materials Characterization

The structure and phase were characterized by X-ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan) using Cu/Kα radiation (λ = 0.154 nm). Morphologies were observed by a field-emission scanning electron microscope (FESEM, Inspect F50, FEI, Hillsboro, OR, USA). The surface elemental analysis was confirmed by the elemental mapping using X-ray energy dispersive spectroscopy (EDS). More structural information was detected by transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM, G2 20, Tecnai, Hillsboro, OR, USA). Selected area electron diffraction (SAED) patterns were recorded by a Gatan CCD camera in a digital format.

3.4. Electrochemical Measurements

The electrochemical tests including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) measurement and electrochemical impedance spectra (EIS) were performed on CHI 760E electrochemical workstation using three-electrode systems in 3.0M KOH solution with Pt foil as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. EIS was carried out in the frequency range of 0.01 Hz~1 MHz at the open circuit potential with an AC potential amplitude of 5 mV. The EIS test data was fitted with Zview software (Scribner Associates, version 2.9c). Cycling performance was conducted using a LAND battery program-control test system (Wuhan LAND Electronics, Wuhan, China) in the potential range of 0~0.45 V. The corresponding mass loadings of CuCo₂O₄, CuCo₂O₄/NiMoO₄ and NiMoO₄ were approximately 1.6, 2.7 and 2.1 mg cm⁻², respectively.

3.5. Assemble of Asymmetric Supercapacitor

A solid-state asymmetric supercapacitor assembled by CuCo₂O₄/NiMoO₄ as a positive electrode, activated carbon (AC) as a negative electrode, and polyvinyl alcohol (PVA)/KOH gel both as the electrolyte and separator were carried out. The AC electrode was prepared by mixing 95 wt% of activated carbon and 5 wt% of polytetrafluoroethylene and then spread on a 10 mm × 20 mm Ni foam. (PVA)/KOH gel was synthetized by the following procedure [36]: 6 g of KOH was dissolved in 60 mL of deionized water, followed by the addition of 6 g of PVA power. The mixture was heated to 90 °C under stirring until the solution became clear. The PVA/KOH polymer electrolyte was obtained at room temperature. The as-prepared CuCo₂O₄/NiMoO₄/Ni and AC/Ni were soaked in PVA/KOH gel for 10 min under ultrasonic treatment to allow the electrolyte to diffuse into nanoporous active materials. The two electrodes were fabricated into a sandwich-structure solid-state supercapacitor when the PVA-KOH gel electrolyte was solidified under hot air.

4. Conclusions

In summary, we have successfully developed a hierarchical CuCo₂O₄/NiMoO₄ heterostructures by using microflakes assembled island-like CuCo₂O₄ as a skeleton and ultrathin nanosheets NiMoO₄ as a secondary structure. Owing to the combination of the chemical composition and the spatial scale/dimension, such a novel electrode material can effectively provide a large amount of reactive active sites, multi-access channels of ions and the favorable structural stability. The resulting CuCo₂O₄/NiMoO₄ heterostructures exhibited greatly enhanced electrochemical performance. The asymmetric supercapacitor achieves a high energy density of 51.7 Wh/kg at a power density of 853.7 W/kg and a superior cyclability with a capacitance retention of 90.8% after 3000 cycles.