Fabrication of Bimetallic Oxides (MCo 2 O 4 : M=Cu, Mn) on Ordered Microchannel Electro-Conductive Plate for High-Performance Hybrid Supercapacitors

: AB 2 O 4 -type binary-transition metal oxides (BTMOs) of CuCo 2 O 4 and MnCo 2 O 4 were successfully prepared on ordered macroporous electrode plates (OMEP) for supercapacitors. Under the current density of 5 mA cm − 2 , the CuCo 2 O 4 /OMEP electrode achieved a speciﬁc capacitance of 1199 F g − 1 . The asymmetric supercapacitor device prepared using CuCo 2 O 4 /OMEP as the positive electrode and carbon-based materials as the negative electrode (CuCo 2 O 4 /OMEP//AC) achieved the power density of 14.58 kW kg − 1 under the energy density of 11.7 Wh kg − 1 . After 10,000 GCD cycles, the loss capacitance of CuCo 2 O 4 /OMEP//AC is only 7.5% (the retention is 92.5%). The MnCo 2 O 4 /OMEP electrode shows the speciﬁc and area capacitance of 843 F g − 1 and 5.39 F cm − 2 at 5 mA cm − 2 . The MnCo 2 O 4 /OMEP-based supercapacitor device (MnCo 2 O 4 /OMEP//AC) has a power density of 8.33 kW kg − 1 under the energy density of 11.6 Wh kg − 1 and the cycle stability was 90.2% after 10,000 cycles. The excellent power density and cycle stability prove that the prepared hybrid supercapacitor fabricated under silicon process has a good prospect as the power buffer device for solar cells.


Introduction
Currently, with the continuous expansion of energy demand, as an important device to stabilize energy input and output, the research on energy storage components has become more and more the focus of future sustainable development [1]. After the solar cell generates electric energy, storing this energy through the energy storage system integrated with microelectronic technology to fulfill the stable and safe utilization of energy remains a challenge for the efficient application of solar energy [2,3]. The complex environment of solar cell power generation and storage is pursuing the characteristics of high-power density, long lifecycle and low cost, which are consistent with the research concept of electrode materials for supercapacitors [4]. Binary transition metal oxides (BTMOS) are important multifunctional materials composed of two metal elements. Due to their unique structure and properties, they have attracted extensive attention in catalysis, heavy metal ion adsorption, electrocatalyst, sensor, lithium-ion batteries and supercapacitors [5][6][7][8]. Among those bimetallic oxides, cobalt-based AB2O4 BTMOS such as CuCo 2 O 4 , ZnCo 2 O 4 , NiCo 2 O 4 , MnCo 2 O 4 and FeCo 2 O 4 have attracted extensive attention due to their high specific capacitance, better rate performance and stability electrochemical properties, and are considered to have the most potential electrode material for energy storage, especially supercapacitors [9][10][11][12][13].
Cu, Mn, Fe and other metal atoms replace Co atoms in Co 3 O 4 to form a MCo 2 O 4 type bimetallic oxide, which increases the Faraday redox reaction site with a higher capacity and reduces the production cost of the active material without changing the spinel structure of Co 3 O 4 [11,14,15]. However, the untreated bimetallic oxides have low conductivity and are easy to agglomerate during the reaction, which limits their further application in supercapacitors [16,17]. Selecting appropriate bimetallic oxides, and then advanced electrode structure design and nanomaterial preparation process, is an effective means to enhance the ion transport and structural stability of bimetallic oxides in the process of electrochemical reaction. Among those MCo 2 O 4 type materials, CuCo 2 O 4 and MnCo 2 O 4 are two kinds of active materials with different electrical active elements, which have received extensive attention in recent years [16,[18][19][20]. Zhang et al. [21] reported the specific capacitance of CuCo 2 O 4 @CuCo 2 O 4 /Ni foam electrode up to 888.9 F g −1 at 2 mA cm −2 ; Pang et al. worked on δ-MnO 2 -added CuCo 2 O 4 which shows a maximum specific capacitance of 1180 F g −1 at 1 A g −1 and remains 93.2% after 5000 GCD cycles [22]. Lee's MnCo 2 O 4 -NiCo 2 O 4 composite electrodes exhibit a high specific capacitance of 1152 F g −1 at 1 A g −1 , and excellent rate capability and superior cycling stability (95.38% retention after 3000 cycles) [23]. These studies showed that the cobalt-based bimetallic oxide has a good application prospect for energy storage equipment.
Here, MCo 2 O 4 (M=Cu, Mn) nanosheets were prepared on a three-dimensional ordered macroporous electrode plate (OMEP) through the hydrothermal method as highperformance multi-dimensional Faraday electrodes [24]. Although there are many studies on porous materials in the field of energy storage in recent years, there is little similar research on ordered porous materials such as OMEP, and the research materials combined with AB 2 O 4 bimetallic oxides have just emerged [21]. OMEP, composed of millions of neatly arranged porous structures fabricated by the silicon micromachining process, acts as a conductive substrate in the multidimensional composite electrode to effectively prevent the agglomeration of nanomaterials [25,26]. A stable nickel coating on the OMEP surface not only can improve the electrical conductivity of the electrode, but it also acts as the nucleation center of the active material, allowing the active material to adhere tightly to the surface of the nickel layer, without the need for an additional conductive agent or polymer binder. Therefore, the electrodes prepared with OMEP can objectively reflect the performance characteristics of specific active substances.
In this paper, CuCo 2 O 4 /OMEP electrodes and MnCo 2 O 4 /OMEP electrodes were successfully prepared by using nickel-plated ordered macroporous electrode-conductive plates (OMEP) as the substrate. These composite electrodes have a unique three-dimensional porous structure, which facilitates the transport of electrons and ions, increases the liquidsolid interface area, improves the utilization efficiency of the active material and reduces the weight and size of the electrode, which makes the active material obtain a large specific capacitance for miniature supercapacitors [27]. The MnCo 2 O 4 /OMEP electrode shows the specific and area capacitance of 843 F g −1 and 2.7 F cm −2 at 5 mA cm −2 . The MnCo 2 O 4 /OMEP-based supercapacitor device (MnCo 2 O 4 /OMEP//AC) has a power density of 8.33 kW kg −1 under the energy density of 11.6 Wh kg −1 and the cycle stability was 90.2% after 10,000 cycles. The CuCo 2 O 4 /OMEP electrode has high cycle stability and maintains a good ratio at a higher current density and CV scanning speed. The specific capacitance of the electrode is 1199 F g −1 at the current density of 5 mA cm −2 . The asymmetric supercapacitors prepared using the CuCo 2 O 4 /OMEP electrode and activated carbon electrode (CuCo 2 O 4 /OMEP//AC) has obtained the power density of 14.58 kW kg −1 under the energy density of 11.7 Wh kg −1 . After 10,000 GCD cycles, the loss capacitance of CuCo 2 O 4 /OMEP//AC is only 7.5%.

Preparation of OMEP
The silicon macroporous microchannel plate (Si-MCP, Figure 1a,b) is the skeleton structure of OMEP. Its fabrication process can be divided into three steps: pretreatment, deep etching and stripping. Pretreatment stage: select (100) crystal p-type silicon wafer, grow silica mask layer through thermal oxidation, define the hole pattern by lithography, use buffered oxide etchant (BOE) etching solution to expose the holes that need to be etched from the silica mask layer, remove the photoresist to obtain the desired pattern. The wafer is anisotropically etched in 25% tetramethylammonium hydroxide solution and stops etching when a distinct cross pattern is seen under the microscope. The pretreated silicon wafer was subjected to deep photochemical etching [28] and the peel steps are followed to obtain the required Si-MCP. A layer of nickel was uniformly deposited on the outer and inner surfaces of Si-MCP by liquid flow method [29][30][31] to fabricate the OMEP (Figure 1c-f). To further improve the electrical conductivity and specific surface area of the OMEP, the electroplating nickel method was carried out to deposition a nickel particle layer on OMEP after chemical deposition. At room temperature of (23 ± 1) • C, the electrolyte consisted of 2 M ammonium chloride and 0.1 M nickel chloride, and the pH was adjusted to 3.5. After nickel deposition, the morphologies of the surface and sidewalls at different magnification are shown in Figure 1c-f. It can be seen from the figure that a layer of nickel evenly covers the surface and sidewalls of the Si-MCP.

Synthesis of the CuCo 2 O 4 /OMEP and MnCo 2 O 4 /OMEP Electrode
To fabricate the CuCo 2 O 4 /OMEP electrode, a mixture of 1 mmol copper nitrate and 2 mmol cobalt nitrate was added with 15 mmol urea as precipitators. The solution was fully stirred and mixed and put into a reactor with OMEP in front of it. The hydrothermal reaction lasted for 8 h at 120 • C and then cooled to room temperature naturally. The CuCo 2 O 4 /OMEP electrode was prepared after the samples were rinsed and dried at 60 • C for 24 h. The preparation method and steps of the MnCo 2 O 4 /OMEP electrode are the same as the CuCo 2 O 4 /OMEP electrode, but the difference is that the copper nitrate used in CuCo 2 O 4 /OMEP fabrication is replaced by manganese sulfate in hydrothermal reaction. The procedure for preparing samples on the surface of nickel foam is consistent with that of OMEP-based samples, except that the substrate is changed from OMEP to nickel foam. At the same time, the CuCo 2 O 4 and MnCo 2 O 4 fabricated on nickel foam were named CuCo 2 O 4 /NF and MnCo 2 O 4 /NF, respectively.

Characterization
Scanning electron microscopy (SEM, JEOLJSM-7001F, Tokyo, Japan) was used to study the morphology and microstructure of various electrodes. X-ray diffraction (XRD, Rigaku, RINT2000, Tokyo, Japan) was used to study the crystal structure and X-ray photoelectron spectroscopy (XPS, Kratoms Axis Ultra DLD) was used to determine the elemental composition of the fabricated electrodes.
In this paper, CHI660E electrochemical workstation was used to perform performance tests in 2 M KOH electrolyte. OMEP, Co 3 O 4 /OMEP, CuCo 2 O 4 /NF, MnCo 2 O 4 /NF, CuCo 2 O 4 /OMEP and MnCo 2 O 4 /OMEP electrodes were used as the working electrodes, saturated calomel electrode as the reference electrode and platinum wire electrode as the counter electrode. The electrochemical properties of the composite electrodes were characterized by cyclic voltammetry (CV), charge-discharge analysis (GCD) and electrochemical impedance spectroscopy (EIS).

Electrodes SEM Morphology Analysis
As shown in Figure 1a,b, the Si-MCP composed of millions of neatly arranged porous structures acts as the foundation for making OMEP that compatible with the microelectronics process. After the two-step nickel deposition methods on Si-MCPs, the OMEP (  Figure S1a, it is found that the atomic percentage of Ni is 59%. The ratio of Cu, Co and O atoms was close to 1:2:4, which were 6%, 13% and 22%, respectively. Figure 3 shows the SEM image of MnCo 2 O 4 /OMEP composite electrode. According to the SEM image in Figure 2a- Figure S1b shows the SEM mapping of the CuCo 2 O 4 /OMEP electrode, from which the atomic percentage of Ni is 39%. Cu, Co and O atoms accounted for 9%, 18% and 34%, respectively. To make a comprehensive analysis of the performance of OMEP as the supercapacitor substrates, the electrodes based on nickel foam were also prepared and characterized under the same conditions. Figures S2 and S3 show the SEM images of CuCo 2 O 4 and MnCo 2 O 4 deposited on the surface of nickel foam, respectively. From these diagrams, it can be seen that the nanomaterials formed on nickel foam are nanowires, which are completely different from those deposited on the OMEP surface, although their preparation conditions are identical. The above observation and analysis show that the unique substrate structure and surface characteristics can control the morphology of the prepared nanomaterials, and further affect the electrochemical performance of the electrodes.

Electrodes Structure and Composition Analysis
The structures of OMEP and nanocomposite electrodes were determined by X-ray diffraction, as shown in Figure 4a.    Figure 4b, it is found that the peak position of C 1s appears at 285 eV, which is consistent with the XPS standard energy spectrum. In Figure S4, O 1s peaks are subdivided into metal-oxygen bonds (M-O, M: Co, Cu and Mn) and oxygen-containing groups (H-O) [32]. XPS images show that the adsorption of oxygen and hydroxyl groups in CuCo 2 O 4 and MnCo 2 O 4 crystal on the surface of the nanostructure are the two main factors affecting the nanostructure [33]. Compared the Figure S4a,b, it can be found that the O 1s peak of the MnCo 2 O 4 /OMEP electrode is shifted to the right, which may be caused by the more abundant valence states of Mn. Figure 4c shows the interval XPS map of Cu 2p. The peak of Cu 2p 3/2 appears at 936 eV, and the peak of Cu 2p 1/2 at 956 eV. Cu 2+ and Cu + constitute the main valence states of Cu 2p [19,34]. The corresponding Figure 4e shows the Mn 2p pattern of the MnCo 2 O 4 /OMEP electrode. The peak of Mn 2P 3/2 appears at 643 eV, and the peak of Co 2p 1/2 at 655 eV. Mn 2+ and Mn 3+ are the main valence states of Mn 2p [35,36]. Figure 4d,f show that Cu and Mn do not affect the spectral arrangement of Co 2p. The peak of Co 2p 3/2 appears at 782 eV, the peak of Co 2p 1/2 at 798 eV and the satellite peaks (Sat.) of Co 2p 3/2 and Co 2p 1/2 appear at 787 eV and 803 eV, respectively. Among them, Co 3+ and Co 2+ are the main valence states of Co 2p [37].

Electrodes Electrochemical Performance Analysis
The redox reaction principle of CuCo 2 O 4 and MnCo 2 O 4 nanomaterials in alkaline solution as follows [38,39]:  In Figure 5e, the redox peak of the MnCo 2 O 4 /OMEP electrode is obvious. When the scanning speed exceeds 15 mV s −1 , obvious deformation occurs in the closed region of the CV curve of the MnCo 2 O 4 /OMEP electrode. Figure 5f shows the GCD curve of the MnCo 2 O 4 /OMEP electrode, indicating that the specific capacitance of this electrode is mainly contributed by the double layer capacitance [40]. Figures S5 and S6 reflect electrochemical chemistry performance of CuCo 2 O 4 /NF and MnCo 2 O 4 /NF at different CV scanning rates and different current densities after activation. It can be seen from these two pictures that the CV curve and GCD curve of electrodes prepared by the same process on nickel foam are quite different from those based on OMEP. The peaks of the CV curve based on foamed nickel are more obvious, while the OMEP-based electrode curve is more similar to a rectangle, which reflects that OMEP-based samples have more pseudo capacitance components.
To further analyze the positive influence of the fabrication nanomaterials on OMEP and preparation of bimetallic oxides on the performance of supercapacitors, systematic electrochemical tests were also carried out on OMEP and Co 3 O 4 /OMEP. Unlike the CV from CuCo 2 O 4 /OMEP and MnCo 2 O 4 /OMEP, as shown in Figure 6a, the CV from Co 3 O 4 /OMEP has a clear redox peak, while CuCo 2 O 4 /OMEP and MnCo 2 O 4 /OMEP are more likely to be a surface electrochemical pseudocapacitive reaction at high scanning rates. Figure 6b indicates that Co 3 O 4 /OMEP has a low specific capacity and surface capacitance because it has fewer active sites and lower conductivity. The specific capacitance of the electrode C s and the surface capacitance of the CV curve C CV can be calculated according to Equations (6) and (7), and the energy (E) and the power (P) density were determined according to the equations as follows [41]: where C s (F/g) is the specific capacitance, I(A) is the current density of charge and discharge, ∆t(s) is the discharge time, m(g) is the mass of the active substance in the electrode and ∆V(V) is the pressure drop in the test. v (V/s) is the scanning speed of the CV curve. idV is the closed area of the CV curve. According to Figure 5d, Figure 6d and Equation (6), the specific capacitances of the CuCo 2 O 4 /OMEP electrode are 1199 F g −1 (5 mA cm −2 ), 1111 F g −1 (10 mA cm −2 ), 970 F g −1 (15 mA cm −2 ), 928 F g −1 (20 mA cm −2 ) and 844 F g −1 (25 mA cm −2 ). According to Equation (7), the surface capacitances of the corresponding GCD curve can be further calculated as follows: 4.76 F cm −2 (5 mA cm −2 ), 4.40 F cm −2 (10 mA cm −2 ), 3.85 F cm −2 (15 mA cm −2 ), 3.82 F cm −2 (20 mA cm −2 ) and 3.49 F cm −2 (25 mA cm −2 ). It can be found that the surface capacitance of the CuCo 2 O 4 /OMEP electrode maintains good magnification characteristics, and the surface capacitance is stable at about 2 F cm −2 at each scanning speed. According to Figure 6d and Equations (6) and (7), the specific capacitances of the MnCo 2 O 4 /OMEP electrode are 843 F g −1 (5 mA cm −2 ), 728 F g −1 (10 mA cm −2 ), 648 F g −1 (15 mA cm −2 ), 564 F g −1 (20 mA cm −2 ) and 520 F g −1 (25 mA cm −2 ). The surface capacitances corresponding to GCD curves are 5.39 F cm −2 (5 mA cm −2 ), 4.60 F cm −2 (10 mA cm −2 ), 4.27 F cm −2 (15 mA cm −2 ), 3.71 F cm −2 (20 mA cm −2 ) and 3.25 F cm −2 (25 mA cm −2 ). The results show that with the increase in scanning rate and current density, the surface capacity of the MnCo 2 O 4 /OMEP electrode decreases sharply. The specific capacity and surface capacitance of the electrodes mentioned in this paper are shown in Table 1.   Figure 6e shows the EIS pattern of the composite electrode before and after the cycle. It can be observed that the Nyquist curve of the composite electrode consists of an arc in the high-frequency region and an oblique line in the low-frequency region. Among them, the CuCo 2 O 4 /OMEP electrode (red line) before the cycle has the shortest arc radius and the highest slope of the inclined line in the low-frequency region, which means that the composite electrode has the lowest impedance and the higher conductivity of the CuCo 2 O 4 /OMEP electrode. In addition, the arc radius (red, green line) in the highfrequency region of the composite electrode before the cycle is smaller than the arc radius (orange, blue line) after the cycle, and the slope of the slope in the low-frequency region is higher than the slope of the slope after the cycle. It shows that after 10,000 cycles, the electrical conductivity of the electrode is lost to varying degrees. It can be seen from the intersection of the Nyquist curve and ordinate zero axis that the solution resistance (Rs) of the CuCo 2 O 4 /OMEP electrode and the MnCo 2 O 4 /OMEP electrode before circulation are 2.01 and 2.42 ohms, respectively. After 10,000 cycles, the solution resistance of the CuCo 2 O 4 /OMEP electrode was reduced to 1.88 ohms, while the solution resistance of the MnCo 2 O 4 /OMEP electrode was increased to 2.68 ohms. Figure 6f shows 10,000 cycle stability tests for CuCo 2 O 4 /OMEP electrodes and MnCo 2 O 4 /OMEP electrodes. The measured specific capacitances of the CuCo 2 O 4 /OMEP electrode and the MnCo 2 O 4 /OMEP electrode at the current density of 5 mA cm −2 before the cycle is 1199 F g −1 and 843 F g −1 , respectively. After 10,000 cycles, the specific capacitance of the CuCo 2 O 4 /OMEP electrode at the current density of 5 mA cm −2 is 1081 F g −1 , resulting in a 9.8% capacitance loss, while the specific capacitance retention of the MnCo 2 O 4 /OMEP electrode in the same condition is 83.5%. The above data indicate that the CuCo 2 O 4 /OMEP electrode is more stable than the MnCo 2 O 4 /OMEP electrode in terms of cycle stability. This may be due to the introduction of Cu 2+ and the special structure of CuCo 2 O 4 /OMEP, which make it difficult to dissolve and agglomerate in the electrolyte.  Figure 7. Figure 7a,c shows that the CV curve closing area of MnCo 2 O 4 /OMEP//AC is larger than that of CuCo 2 O 4 /OMEP//AC. This is because the active material mass in CuCo 2 O 4 /OMEP//AC is 2.4 mg, far less than that in MnCo 2 O 4 /OMEP//AC (4.2 mg). More active material provides higher CV surface capacitance of the supercapacitor. According to Figure 7b and Equations (8) and (9) (11.7 Wh kg −1 ). Figure 7d shows the charge-discharge curve of MnCo 2 O 4 supercapacitor, and the power density (energy density) can be obtained as follows: 1.67 KW kg −1 (25.9 Wh kg −1 ), 3.33 KW kg −1 (20.2 Wh kg −1 ), 5.00 KW kg −1 (16.8 Wh kg −1 ), 6.67 KW kg −1 (14.6 Wh kg −1 ) and 8.33 KW kg −1 (11.6 Wh kg −1 ). The comparison of the power and energy density of the two supercapacitors is shown in Figure 7e. In addition, the cyclic stability test results of the supercapacitor are shown in the lower-left corner of Figure 7e. The test showed that after 10,000 cycles, the CuCo 2 O 4 supercapacitor maintained a specific capacitance of 92.5%, while the MnCo 2 O 4 supercapacitor maintained 90.2% capacitance.

Conclusions
AB 2 O 4 -type binary-transition metal oxides (BTMOs) of CuCo 2 O 4 and MnCo 2 O 4 were successfully prepared on ordered macroporous electrode plates (OMEP) that are compatible with silicon micro-nano processes for supercapacitors. These composite porous electrodes have a unique three-dimensional structure, which facilitates the transport of electrons and ions increases the liquid-solid interface area, improves the active material utilization efficiency and reduces the weight and size of the electrode, which allows the active material to obtain a large specific capacitance. The CuCo 2 O 4 /OMEP has a capacitance of 1199 F g −1 at the current density of 5 mA cm −2 and 844 F g −1 at the current density of 5 mA cm −2 . After 10,000 cycles, the capacitance stability achieved 92.5%. The asymmetric supercapacitor device of CuCo 2 O 4 /OMEP//AC achieves a power density of 14.58 kW kg −1 at the energy density of 11.7 Wh kg −1 . The MnCo 2 O 4 /OMEP electrode shows a better area capacitance at a small current density (5.76 F cm −2 at 5mA cm −2 ) because its compact structure can load more active substances per unit area. The MnCo 2 O 4 supercapacitor's power density is 8.33 kW kg −1 with an energy density of 11.6 Wh kg −1 and its cycle stability was 90.2%, maintained after 10,000 cycles.