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

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.


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 = 1/2CV 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 NiCo 2 O 4 [9,10], ZnCo 2 O 4 [11,12], FeCo 2 O 4 [13,14] and MnCo 2 O 4 [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, CuCo 2 O 4 is considered a potential material for supercapacitors due to electrochemical activity [17].Nevertheless, owing to relatively low specific capacitance and electrical conductivity, single-component CuCo 2 O 4 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 CuCo 2 O 4 @MnMoO 4 [18], CuCo 2 O 4 @MoNi-LDH [19] and CuCo 2 O 4 @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 CuCo 2 O 4 is an effective strategy to achieve high specific capacitance of supercapacitors.NiMoO 4 , 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 CuCo 2 O 4 @NiMoO 4 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 NiMoO 4 -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 NiMoO 4 nanosheetscoated NiCo 2 O 4 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 MnCO 3 showed superior capacitance retention of 98.79% after 10,000 cycles [26].To further improve the specific capacitance of CuCo 2 O 4 @NiMoO 4 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 CuCo 2 O 4 /NiMoO 4 heterostructures on Ni foam through a facile and stepwise hydrothermal approach.The combination of 2D microflake CuCo 2 O 4 assembled 3D architecture and secondary inserted units of NiMoO 4 ultrathin nanosheets make full use of the merits of individual components and micro/nanostructure.The microflakes assembled island-like CuCo 2 O 4 serve as the backbone material, and subsequent embedded NiMoO 4 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.

Characterization
Figure 1 displays the morphologies of pristine CuCo 2 O 4 and CuCo 2 O 4 /NiMoO 4 heterostructures.As can be seen in Figure 1a, the island-like CuCo 2 O 4 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 NiMoO 4 nanosheet growth.From Figure 1b, it can be clearly seen that the interstices between CuCo 2 O 4 microflakes are filled with ultrathin NiMoO 4 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 CuCo 2 O 4 /NiMoO 4 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 NiMoO 4 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 NiMoO 4 nanosheets are nearly uniformly aligned on the Ni foam.
Molecules 2023, 28, x FOR PEER REVIEW 3 of 13 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/Ni-MoO4 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).To further identify the structure and composition of the CuCo 2 O 4 and CuCo 2 O 4 /NiMoO 4 heterostructures, the elemental mapping is carried out under X-ray spectroscopy (EDS) mapping analysis.The elemental distribution of the CuCo 2 O 4 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 CuCo 2 O 4 .After loading the NiMoO 4 nanosheets, all the elements (Cu, Co, Ni, Mo and O) are detected and homogenously distributed on CuCo 2 O 4 /NiMoO 4 hybrid heterostructures (Figure 2).
Molecules 2023, 28, x FOR PEER REVIEW 3 of 13 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/Ni-MoO4 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.

Electrochemical Measurements
To evaluate CuCo 2 O 4 @NiMoO 4 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 NiMoO  S7a) or NiMoO 4 nanosheets (Figure S7c).Apparently, individual components (CuCo 2 O 4 or NiMoO 4 ) 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 CuCo 2 O 4 /NiMoO 4 electrode corresponds to that of CuCo 2 O 4 or NiMoO 4 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 CuCo 2 O 4 and NiMoO 4 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, CuCo 2 O 4 /NiMoO 4 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 CuCo 2 O 4 /NiMoO 4 electrodes, CuCo 2 O 4 and NiMoO 4 electrodes show inferior performance in terms of specific capacitance and rate capability (Table 1).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 Furthermore, cycle stability is evaluated by repeating a charge/discharge process of the electrode materials.As the skeleton, island-like CuCo 2 O 4 structure manifests exceptional cycling stability and is able to reach 91.0% of its initial value even after 5000 cycles (Figure S8a).After inserting NiMoO 4 nanosheets into the skeleton, the CuCo 2 O 4 /NiMoO 4 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: NiMoO 4 nanosheets are inserted into layered CuCo 2 O 4 microflakes, which inhibit the microflake collapse and self-agglomeration of nanosheets, while CuCo 2 O 4 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 CuCo 2 O 4 and CuCo 2 O 4 /NiMoO 4 are almost preserved, whereas NiMoO 4 nanosheets suffer from severe aggregation and deformation after longterm cycles (Figure S8d).Thus, it can be further inferred that enhanced cycling stability of CuCo 2 O 4 /NiMoO 4 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 CuCo 2 O 4 @NiMoO 4 heterostructures not only exceeds that of either CuCo 2 O 4 island-like structure or NiMoO 4 nanosheets but also is superior to that of many previously reported mixed-metal oxides in terms of specific capacitance and cycling performance (Table S2).

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 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 NiMoO 4 nanosheets as secondary inserted units embedded inside CuCo 2 O 4 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 CuCo 2 O 4 microflakes and NiMoO 4 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.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 Rs 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 Rct 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, Ni-MoO4 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 selfagglomeration of nanosheets, while CuCo2O4 microflakes possess robust stability first.As indicated by the FESEM images of electrode materials after the cycling test (Figures 6b  and 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 uous 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 Ni-MoO4 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: , 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  In order to evaluate the practical application, an electrochemical asymmetric supercapacitor (ASC) is assembled by using the as-prepared CuCo 2 O 4 /NiMoO 4 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 − × ∆V − )/(C + × ∆V + ), the optimal mass ratio between AC and CuCo 2 O 4 @NiMoO 4 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 8558.2W /kg, which is superior to the recently reported value (Figure 8f) [30][31][32][33].
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 8558.2W /kg, which is superior to the recently reported value (Figure 8f) [30][31][32][33].

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

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.CuCo 2 O 4 , NiMoO 4 and CuCo 2 O 4 /NiMoO 4 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).

Synthesis of CuCo 2 O 4 Island-like Structure
The materials were synthesized similarly according to previously reported procedures with minor modifications [34].In brief, Co(NO 3 ) 2 •3H 2 O (2.0 mmol), Cu(NO 3 ) 2 •3H 2 O (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 CuCo 2 O 4 microflake's island-like structure was obtained.dures 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.

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, 0.249g Ni(CH3COO)2•4H2O, 0.2g (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.

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 Xray 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.

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.

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 CuCo 2 O 4 , CuCo 2 O 4 /NiMoO 4 and NiMoO 4 were approximately 1.6, 2.7 and 2.1 mg cm −2 , respectively.

Further
insights into the crystal structure of CuCo 2 O 4 and CuCo 2 O 4 /NiMoO 4 heterostructures are elucidated by XRD patterns.As shown in Figure 3, the obtained patterns of CuCo 2 O 4 microflakes match well with the standard patterns for the cubic spinel phase of CuCo 2 O 4 (JCPDS No. 01-1155).For the heterostructures, several weak diffraction peaks attributed to NiMoO 4 (marked as triangle) are observed except for the peaks of the CuCo 2 O 4 skeleton (marked as diamond), indicating relatively low crystallinity compared with the CuCo 2 O 4 phase.Moreover, Figure S5 shows the identified peaks of the control sample, which can be mainly assigned to monoclinic NiMoO 4 (JCPDS No. 45-0142) with a small amount of α-NiMoO 4 phase (JCPDS No. 33-0948).Molecules 2023, 28, x FOR PEER REVIEW 4 of 13

Figure 3 .
Figure 3. XRD patterns of the CuCo 2 O 4 microflakes and CuCo 2 O 4 /NiMoO 4 micro/nanoheterostructures.The detailed structural characterization of CuCo 2 O 4 and CuCo 2 O 4 /NiMoO 4 are further investigated by TEM and HRTEM.Under a low-magnification TEM image (Figure S6a), these CuCo 2 O 4 microflakes are shown to have a smooth surface.An enlarged TEM view (Figure4a) 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 Figure4ademonstrate the polycrystalline nature of the CuCo 2 O 4 skeleton, which is consistent with the XRD result.HRTEM measurements (Figure4b) 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 CuCo 2 O 4 .Compared with the smooth CuCo 2 O 4 microflakes, noticeable wrinkle-like structures are found on the surface of heterostructures (Figure4c, FigureS6b).Meanwhile, the well-defined SAED pattern is also observed in the inset of Figure4c, and the three diffraction rings correspond to the (422), (330) and (202) crystal planes of NiMoO 4 .Figure4dshows 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 NiMoO 4 and (311) plane of CuCo 2 O 4 , respectively.The above results demonstrate the formation of CuCo 2 O 4 /NiMoO 4 heterostructures.
, the CuCo 2 O 4 /NiMoO 4 electrode is found to have a much smaller intercept and diameter of the semicircle than the CuCo 2 O 4 or NiMoO 4 electrode, indicating a lower Rs and Rct of the CuCo 2 O 4 /NiMoO 4 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 CuCo 2 O 4 /NiMoO 4 electrode and the NiMoO 4 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 CuCo 2 O 4 /NiMoO 4 and NiMoO 4 electrode have similar values of R ct and W, which are lower than that of the CuCo 2 O 4 electrode.The rapid charge transfer rate and ion diffusion of CuCo 2 O 4 /NiMoO 4 is attributed to thin nanosheets and the in situ growth of CuCo 2 O 4 on Ni Foam.Therefore, the improved conductivity and mass transport in CuCo 2 O 4 /NiMoO 4 micro/nanostructure enhances its electrochemical performance.

Figure 4 .
Figure 4. (a) TEM image of CuCo 2 O 4 nanosheets and corresponding SAED pattern, (b) HRTEM image of the CuCo 2 O 4 nanosheets, (c) TEM image of CuCo 2 O 4 /NiMoO 4 heterostructures and corresponding SAED pattern and (d) HRTEM image of the CuCo 2 O 4 /NiMoO 4 heterostructures.Such desirable pseudocapacitive performance of the CuCo 2 O 4 /NiMoO 4 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 NiMoO 4 nanosheets connect with each other to form the horizontal transport channel of electron and ion.(2) Two-dimensional CuCo 2 O 4 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, CuCo 2 O 4 microflakes and NiMoO 4 nanosheets are 98.5, 38.3 and 31.1 m 2 /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 NiMoO 4 nanosheets as secondary inserted units embedded inside CuCo 2 O 4 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 CuCo 2 O 4 microflakes and NiMoO 4 nanosheets

Figure 5 .
Figure 5. (a) CV curves of the CuCo2O4, NiMoO4 and CuCo2O4/NiMoO4 electrodes at 5 mV/s, (b) GCD curves of these electrodes at 2 A/g, (c) CV curves of the CuCo2O4/NiMoO4 electrode at various scan rates, (d) GCD curves of the CuCo2O4/NiMoO4 electrode at different current densities, (e) calculated

Figure 5 .
Figure 5. (a) CV curves of the CuCo 2 O 4 , NiMoO 4 and CuCo 2 O 4 /NiMoO 4 electrodes at 5 mV/s, (b) GCD curves of these electrodes at 2 A/g, (c) CV curves of the CuCo 2 O 4 /NiMoO 4 electrode at various scan rates, (d) GCD curves of the CuCo 2 O 4 /NiMoO 4 electrode at different current densities, (e) calculated specific capacitance of the three electrodes as a function of current density and (f) EIS Nyquist plots, inset showing the enlarged picture of high-frequency region and electrical equivalent circuit.

Figure 7 .
Figure 7. Schematic illustration of the electron transport pathway and ion diffusion of the CuCo2O4/NiMoO4 on Ni foam.

Figure 7 .
Figure 7. Schematic illustration of the electron transport pathway and ion diffusion of the CuCo 2 O 4 /NiMoO 4 on Ni foam.

Figure 8 .
Figure 8.(a) CV curves of the device with different potential windows varying from 1.0 to 1.8 V at 20 mV/s.(b) CV curves of the device at various scan rates.(c) GCD curves of the device at different current densities.(d) Calculated specific capacitance of the device as function of current density.(e) Cyclic stability of the device at 5 A/g.(Inset: the red LED powered by the device and Nyquist plots of solid-state supercapacitor before and after 3000 cycles).(f) Ragone plots of the device compared with previously reported data[19,[33][34][35][36].

Figure 8 .
Figure 8.(a) CV curves of the device with different potential windows varying from 1.0 to 1.8 V at 20 mV/s.(b) CV curves of the device at various scan rates.(c) GCD curves of the device at different current densities.(d) Calculated specific capacitance of the device as function of current density.(e)Cyclic stability of the device at 5 A/g.(Inset: the red LED powered by the device and Nyquist plots of solid-state supercapacitor before and after 3000 cycles).(f) Ragone plots of the device compared with previously reported data[19,[33][34][35][36].
CuCo 2 O 4 /NiMoO 4 Heterostructures on Ni Foam The synthesized CuCo 2 O 4 was used as the skeleton for the growth of NiMoO 4 active materials.In total, 0.249 g Ni(CH 3 COO) 2 •4H 2 O, 0.2 g (NH 4 ) 6 Mo 7 O 24 •4H 2 O, and 0.24 g urea were dissolved into 40 mL DI water.The mixed solution and CuCo 2 O 4 @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 CuCo 2 O 4 /NiMoO4 was obtained by annealing at 400 • C for 2 h in the argon atmosphere.As shown in Figure 9, NiMoO 4 nanosheets (light blue) were tightly anchored on the surface of CuCo 2 O 4 microflakes (purple) to form micro/nano-heterostructure.As a control experiment, NiMoO 4 nanosheets were synthesized according to a reported hydrothermal method [35].Ni(NO 3 ) 2 6H 2 O (1 mmol) and Na 2 MoO 4 •2H 2 O (1 mmol) were dissolved in a mixed solvent of 15 mL of H 2 O 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.
4 , CuCo 2 O 4 and CuCo 2 O 4 /NiMoO 4 heterostructures at the scan rate of 5 mV/s.It should be noted that the enclosed CV curve area of the CuCo 2 O 4 /NiMoO 4 heterostructures is apparently larger than those of the pure NiMoO 4 or CuCo 2 O 4 , indicating that the CuCo 2 O 4 /NiMoO 4 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, CuCo 2 O 4 /NiMoO 4 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 NiMoO 4 nanosheets embedded in island-like CuCo 2 O 4 skeleton can significantly improve the capacitive performance.Figure 5c shows CV curves of CuCo 2 O 4 /NiMoO 4 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 CuCo 2 O 4 skeleton (Figure

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

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