Preparation of a hybrid Cu2O/CuMoO4 nanosheet electrode for high-performance asymmetric supercapacitors

Supercapacitors have attracted considerable attention due to their fast charge–discharge ability, excellent rate capability and long-term stability. In this study, a novel electrode composed of Cu2O/CuMoO4 nanosheets directly grown on Ni foam was synthesized via a facile hydrothermal method. The as-prepared electrode exhibits remarkably improved specific and areal capacitance (up to 4264 F g 1 and 9.38 F cm 2 at a current density of 1 A g ). To the best of our knowledge, the specific capacitance of 4264 F g 1 is the highest in reported studies although the areal capacitance of 9.38 F g 1 is slightly lower than the highest value of 16 F cm 2 when Ni(OH)2/carbon nanotubes on Ni foam was used as the electrode. An asymmetric supercapacitor is fabricated using the Cu2O/CuMoO4 nanosheet electrode as the positive electrode and activated carbon (AC) as the negative electrode. The operation voltage could expand to 1.7 V, at which the energy density can reach 75.1 W h kg 1 with a power density of 420 W kg . Additionally, the asymmetric supercapacitor exhibits very high rate capability and good cycling stability. The capacitance remains around 86.6% after 3000 cycles at a current density of 5 A g . This indicates that the Cu2O/CuMoO4 nanosheet electrode is promising for applications in renewable energy storage in the future.


Introduction
Energy and power demands keep increasing year by year due to the fast development of the world. Traditional energy supply is mainly based on the combustion of fossil fuels which causes many environmental issues. To avoid and address these problems, many researchers have started to develop alternative energy storage and conversion systems. The electrochemical capacitor (EC), also called supercapacitor, as one of the promising new classes of energy storage devices, has attracted signicant interest due to its rapid charge-discharge rate, high power density and long life span. [1][2][3][4][5][6] Presently, most commercial supercapacitors are symmetric and carbon material-based, which are known as electric double layer capacitors (EDLCs). [7][8][9] The carbonbased material usually has low energy density which restricts its further application compared to batteries. 10,11 Therefore, it is necessary to develop high quality supercapacitors with improved energy density without sacricing the power density. Asymmetric capacitors are considered as one of the best alternatives because the redox reactions during charge-discharge processes can lead to larger specic capacitance. 12,13 In a typical asymmetric supercapacitor, one electrode is derived from carbon-based materials while the other one is made of pseudocapacitive materials. It can combine the advantages of the two types of materials to exhibit high operating potential, and achieve high energy density. The performances of asymmetric supercapacitors are determined by the properties of the specic electrode materials. Therefore, it is important to develop novel pseudocapacitive materials.
Many efforts have been made to explore the pseudocapacitive materials for supercapacitors, and transition-metal oxides/hydroxides are the commonly used materials. [14][15][16][17] In particular, mixed metal oxides/hydroxides usually offer better electrical performances because of their improved conductivity as well as their synergic effects. [18][19][20][21][22] For example, Liu et al. found that CoMoO 4 -NiMoO 4 exhibited higher specic capacitance than CoMoO 4 and better rate capability than NiMoO 4 . 23 Similarly, the investigation on Co-Ni double hydroxides has proved that the hybrid hydroxide electrodes signicantly outperform either Co(OH) 2 or Ni(OH) 2 alone. [24][25][26] Nevertheless, the performance of pseudocapacitive materials still suffers from intrinsic poor electric conductivity and insufficient diffusion of the electrolyte into the active materials, namely, a large portion of the active materials are not accessible to the electrolyte to undergo faradaic reactions during the charge-discharge process thus no contribution to the specic capacitance. 27 Developing nanostructured materials with a large surface area is regarded as an effective strategy to overcome these issues. In addition, directly growing the active materials on the conductive substrate can reduce internal resistance and make full use of the active materials, therefore improving the capacitance and stability. [28][29][30] Among different oxide materials for supercapacitors, copper oxides have drawn much attention in supercapacitors due to their low cost, environmental benignity and abundance. [31][32][33] Shinde et al. prepared 3D-ower-like CuO on copper foil which exhibited 498 F g À1 at 5 mV s À1 . 34 Wang et al. measured the properties of CuO nanosheets, and a capacity of 569 F g À1 was obtained at a current density of 5 mA cm À2 in 6.0 M KOH electrolyte. 35 However, it is still a big challenge for commercial utilisation of Cu oxide supercapacitors, and more studies should be conducted to achieve the commercial requirements. By combining the aforementioned strategies, it can be deduced that better performance would be obtained if nanostructured copper oxide hybrids can be directly grown on conductive substrates. On the other hand, to the best of our knowledge, the application of CuMoO 4 for supercapacitors has not been reported. Herein, Cu 2 O/CuMoO 4 nanosheets directly grown on Ni foam have been synthesized. The as-prepared electrode exhibits high specic and areal capacitance and also displays good rate capability. In addition, excellent electrical performances are obtained in the asymmetric supercapacitor with AC as the negative electrode. An energy density of 75.1 W h kg À1 and a power density of 420 W kg À1 can be achieved with the voltage window at 1.7 V. Taking the fascinating performances and easy fabrication procedures into account, the hybrid Cu 2 O/CuMoO 4 nanosheet electrode is promising for commercial application in high-energy storage in the future.

Preparation of hybrid Cu 2 O/CuMoO 4 nanosheets on Ni foam
All the reagents were used without further purication. Cu 2 O/ CuMoO 4 nanosheets were synthesized by a facile hydrothermal method. Prior to the synthesis, the Ni foam (1 Â 1 cm 2 ) was cleaned by ultra-sonication in 2 M HCl solution, ethanol and deionized water for several minutes, respectively. The solution containing 0.05 M Cu(NO 3 ) 2 and 0.05 M Na 2 MoO 4 was obtained by dissolving copper(II) nitrate hemi(pentahydrate) and sodium molybdate dihydrate in deionized water under constant stirring. The prepared solution was transferred into a Teon lined stainless steel autoclave. Ni foam was immersed in the homogeneous solution, and then the autoclave was heated at 160 C for 15 h and naturally cooled down to room temperature. The Ni foam was taken out and washed with water and ethanol to remove surface ions and molecules, and then followed by drying at 60 C overnight in a vacuum oven. Finally, the Ni foam with the as-grown hydrate precursors was calcined at 450 C for 2 h in an argon atmosphere with a heating rate of 1 C min À1 . The mass loading of the active materials was obtained by comparing the weight of the Ni foam before the hydrothermal reaction and aer the calcination, which was determined to be around 2.2 mg cm À2 . For the sake of comparison, the precursor solution without Na 2 MoO 4 was applied to fabricate electrodes by the same process.

Assembly of the Cu 2 O/CuMoO 4 //AC asymmetric supercapacitor
To fabricate the asymmetric supercapacitor, the as-prepared Cu 2 O/CuMoO 4 electrode was used as the positive electrode and an activated carbon (AC) electrode acted as the negative electrode. The AC electrode was obtained by mixing the activated carbon (Black Pearl 2000, Cabot) and polytetrauoroethylene at a mass ratio of 95 : 5 in water under constant magnetic stirring. The prepared slurry was spread onto a Ni foam (1 Â 1 cm 2 ) and then pressed and dried at 60 C overnight under vacuum. The mass ratio of Cu 2 O/CuMoO 4 nanosheets and AC was optimized to be around 2.2 and 5.6 mg cm À2 . To assemble the full cell, the negative electrode and positive electrode were placed face-toface into a container in which 2 M KOH was added as the electrolyte.

Physical characterization
X-ray Diffraction (XRD) was carried out to identify the crystal structures on a Panalytical X'Pert Pro Multi-Purpose Diffractometer (MPD) with Cu Kalpha1 radiation working at 45 kV and 40 mA. Scanning electron microscopy (SEM) (ZEISS SUPRA 55-VP) and transmission electron microscopy (TEM) (JEOL 2100) were applied to observe the morphologies. Energy-dispersive Xray spectroscopy (EDX) attached to the SEM was employed to analyze the elemental compositions.

Electrochemical measurements
The electrochemical properties of the as-prepared Cu 2 O/ CuMoO 4 and AC electrodes were studied in a conventional three-electrode cell conguration. In particular, the as-obtained electrode (Cu 2 O/CuMoO 4 or AC) was used as the working electrode. The working electrode was soaked in 2 M KOH for 2 days and activated for 1000 cyclic voltammetry (CV) loops before the test. A piece of Pt mesh (1 Â 1 cm 2 ) and an Ag/AgCl electrode (sat. KCl) were used as counter and reference electrodes, respectively. The electrochemical performances of the asymmetric supercapacitor were explored in a two-electrode mode in which Cu 2 O/CuMoO 4 acted as the positive electrode while the AC electrode was used as the negative electrode. All the measurements were performed in 2 M KOH solution at room temperature and recorded on a Solartron 1470E multichannel cell test system. The a.c. impedance was recorded at open circuit voltage by using an integrated Solartron 1455 frequency response analyzer at 5 mV bias with a frequency range of 100 kHz to 0.01 Hz.

Calculations
The specic capacitance of the electrode in three-electrode mode was calculated from the galvanostatic charge-discharge curves according to the following equation: 19,36 where C s is the specic capacitance (F g À1 or F cm À2 ), I is the discharge current (A), Dt is the discharge time (s), m is the mass of the active materials on the Ni foam (g), DV is the potential change excluding the IR drop in the discharge step (V), and A is the geometric area of the electrode (cm 2 ). In terms of the specic capacitance of the asymmetric supercapacitor, it was also calculated from the galvanostatic charge-discharge curves with the same formula: 37,38 where m 2 is the total mass of the active materials in positive and negative electrodes (g). The energy densities and power densities of the asymmetric supercapacitor were obtained by the following equations: 37,38 where E is the energy density (W h kg À1 ), C s is the specic capacitance (F g À1 ), V is the capacitor potential window excluding the IR drop (V), P is the power density (W kg À1 ) and Dt is the discharge time (s).

Results and discussion
3.1. Physical characterization of the electrode magnications. The Ni foam shows a nice 3D porous structure (Fig. S1, ESI †) and its surface is at and clean aer acid treatment ( Fig. S1b and c, ESI †). However, the images at low magnication ( Fig. 1a and b) indicate that the Ni foam surface becomes rough aer the synthesis procedure as plenty of materials are deposited on it, which proves that the applied hydrothermal process can facilitate growing materials on the substrate. From the enlarged pictures ( Fig. S1c and d †), it can be seen that nanosheet clusters are uniformly distributed on the surface forming a grass-like morphology. Moreover, the nanosheets are interconnected with each other ultimately producing porous texture. Thus, a large surface area is generated and most of the nanosheet surface could be easily accessed by the electrolyte during the electrochemical test. It should be noted that the electrode was washed and calcined aer the hydrothermal process; the overall nanostructure is still well dened, which indicates excellent stability. Fig. S2  To identify the elemental compositions of the materials grown on Ni foam, energy-dispersive X-ray spectroscopy (EDX) analysis has been conducted on the electrode and the results are shown in Fig. S3 and S4 (ESI †). The EDX spectrum indicates that the nanosheets are composed of Ni, Cu, Mo, and O elements (Fig. S3a, ESI †), which is also conrmed by the EDX mapping ( Fig. S3c-f, ESI †). Besides the Ni signal derived from Ni foam, the results are consistent with those of the raw materials used in the hydrothermal reaction.
The compositions of the grown materials are also identied by X-ray diffraction (XRD). No impurities are detected on the Ni foam substrate except the Ni peaks (Fig. S5a, ESI †). As shown in Fig. 2, the background of the Ni foam is very strong but the diffraction patterns still reveal that the composition of the grown materials is the hybrid of Cu 2 O, CuMoO 4 and Cu as the remaining peaks can be assigned to the standard Cu 2 O (ICDD 04-013-0188), CuMoO 4 (ICD 00-047-0511) and Cu (ICDD: 00-004-0850). In terms of the sample prepared with only Cu(NO 3 ) 2 as precursor solution (Fig. S5b, ESI †), Cu 2 O and Cu were detected. A peak at 2q z 27.1 was not identied, which needs further investigation. The CuMoO 4 phase was not well crystallised, and its existence is further conrmed by transmission electron microscopy (TEM) (Fig. 3). The high-resolution TEM (HRTEM) images with lattice fringes are presented in Fig. 3a and b, and the corresponding Fourier transform patterns (inset of Fig. 3b Fig. 4a. The appearance of distinct redox peaks between 0 and 0.6 V vs. Ag/AgCl conrms the pseudocapacitive behaviour of the materials and implies good electrochemical reversibility. Obviously, the current density increases with increasing scan rate, while the anodic and cathodic peaks slightly shi towards the more positive and negative potentials respectively, which is supposed to be related to the internal resistance of the electrode 38 and limitation of charge transfer kinetics. 27 Fig. 4b shows the galvanostatic charge-discharge curves at various current densities from 1 to 50 A g À1 . The nonlinear charge-discharge proles further support the pseudocapacitive characteristics of the electrode, which is in good agreement with the CV curves as shown in Fig. 4a. Based on the discharge curves, the corresponding specic and areal capacitances are calculated (Fig. 4c). The specic capacitance is 4264 F g À1 (9.38 F cm À2 ) at a current density of 1 A g À1 , and it decreases to 3121 F g À1 (6.87 F cm À2 ) when the current density increases to 10 A g À1 . To the best of our knowledge, the specic capacitance of 4264 F g À1 is the highest in reported studies although the areal capacitance of 9.38 F g À1 is slightly lower than the highest value of 16 F cm À2 when Ni(OH) 2 /carbon nanotubes on Ni foam was used as the electrode. 39 It is believed that the degradation of capacitance  with the increase of the current densities is mainly due to the incremental IR drop and insufficient active material involved in redox reactions at higher current densities. 27 However, as the current density keeps increasing, the electrode exhibits remarkable rate capability, namely, the capacitance still remains at 2563 F g À1 (5.64 F cm À2 ) at a current density as high as 50 A g À1 . The capacitance values presented here are much higher than those reported in previous studies using similar materials: Cu 2 O/Cu nanoneedle arrays (510.2 F g À1 , 0.88 F cm À2 at 2.9 A g À1 ); 38 mesoporous NiO nanosheets (2504.3 F g À1 , 0.376 F cm À2 at 13.4 A g À1 ); 37 NiMoO 4 nanowires (3298 F g À1 , 4.94 F cm À2 at 5.33 A g À1 ); 27 MnCo 2 O 4 /Ni(OH) 2 core-shell nanoowers (2124 F g À1 , 3.19 F cm À2 at 5 A g À1 ); 19 NiCoO 2 (508 F g À1 at 0.5 A g À1 ). 40 To further investigate the electrochemical behavior of the Cu 2 O/CuMoO 4 electrode, electrochemical impedance spectroscopy (EIS) measurement is carried out in the frequency range from 100 kHz to 0.01 Hz at open circuit potential with the amplitude at 5 mV. As shown in Fig. 4d, the Nyquist plots of the Cu 2 O/CuMoO 4 electrode are composed of a semicircle in the high-frequency region and a linear component in the low-frequency region. The intercept of the plots at the real axis represents the equivalent series resistance (R s ), including the ionic resistance of the electrolyte, intrinsic resistance of active materials and the contact resistance between the active materials and current collector, [41][42][43] which is determined to be 0.334 U (inset of Fig. 4d). In terms of the semicircle, its diameter indicates the charge transfer resistance (R ct ) of the system. [41][42][43] The corresponding R ct of the Cu 2 O/CuMoO 4 electrode is only 0.02 U, implying the easy and rapid charge transfer on the electrode. In the low-frequency region, the straight line presents the Warburg impedance (Z w ), 41-43 associated with the diffusion of the electrolyte ions along the Cu 2 O/CuMoO 4 nanosheets. The large slope of the straight line shown in Fig. 4d demonstrates a very small Z w of the electrolyte ion diffusion.
The small values of R s , R ct and Z w suggest that there is a large electro-active surface area and higher electrical conductivity with the Cu 2 O/CuMoO 4 nanosheet, which could extend the reaction zone of the electrode, making more electrode materials available for the charging/discharging process, leading to high capacitance. The dramatic high capacitance and outstanding rate capability of the Cu 2 O/CuMoO 4 hybrid electrode can be attributed to its synthesis technique, nanostructure, and the possible synergistic effect from Cu 2 O and CuMoO 4 . The active materials are directly grown on the Ni foam current collector without any binders and conductive additives, which greatly reduces the contact resistance and improves the electrical conductivity of each nanosheet, leading to excellent conductivity of the electrode. Additionally, the specic surface area is signicantly expanded with the nanosheets, which not only increases the electrolyte/electrode contact areas but also creates more active sites for rapid redox reactions with the anions and water molecules, enhancing the utilization of the Cu 2 O/CuMoO 4 . Moreover, the nanostructure could effectively facilitate the transport of electrolyte ions and electrons, and simultaneously, it is compatible with larger volume changes during the fast surface-dependent faradaic processes.  electrode under practical conditions. The electrochemical performance of AC presents a typical feature of carbon-based materials (see Fig. S7, ESI †). In order to nd the optimal working potential, a series of CV measurements in different voltage windows were conducted at 5 mV s À1 . As seen from Fig. 5a, the operating windows are stable from 1.0 to 1.7 V. When it reaches 1.8 V, the polarization trend becomes very obvious, which means that severe water electrolysis happens. Therefore the best operating voltage window is determined to be 1.7 V. Fig. 5b presents the CV performances of the asymmetric supercapacitor at different scan rates ranging from 1 to 20 mV s À1 . Unlike the single Cu 2 O/CuMoO 4 or AC electrode, the asymmetric supercapacitor shows quasi-rectangular CV curves in all scans, which means that the capacitance is derived from both electric double-layer capacitance and pseudocapacitance. The operation voltage of 1.7 V here should be dependent on this remarkable synergy effect. The galvanostatic charge-discharge curves of the asymmetric supercapacitor were recorded at the  current densities from 0.5 to 20 A g À1 (Fig. 5c). It can be seen that the discharge curves are nonlinear especially at low current densities, indicating the double contribution of electric doublelayer capacitance and pseudocapacitance, which is consistent with the CV results. Based on the total mass of the positive and negative electrodes, the specic capacitance at various current densities is plotted in Fig. 5d. The calculated capacitances are 191, 156, 114, 92, 83.7, 79.8 and 75.1 F g À1 at current densities of 0.5, 1, 2, 5, 10, 15 and 20 A g À1 , respectively. It can be found that the as-fabricated asymmetric supercapacitor reveals excellent rate capability from 5 to 20 A g À1 . To further investigate the durability of the asymmetric supercapacitor, its cycling performance was recorded at 5 A g À1 (Fig. 5e). The specic capacitance still retains 86.6% of the initial value even aer 3000 cycles, which demonstrates excellent electrochemical stability. It is believed the decline of the capacitance is caused by the destruction of the electrode lm under numerous rapid redox reactions. 19 This is also evidenced by rst and last ten chargedischarge curves (Fig. S8, ESI †).
The Ragone plots of the asymmetric supercapacitor derived from the discharge curve based on eqn (3) and (4) are displayed in Fig. 5f. The maximum energy density can be determined to be 75.1 W h kg À1 at the average power density of 420 W kg À1 . It can still maintain 21.5 W h kg À1 even at a high power density of 10 435 W kg À1 . It is found that the performance of the Cu 2 O/CuMoO 4 //AC asymmetric supercapacitor is better than that of many previously reported systems, such as Cu 2 O@Cu// AC, 38 MnCo 2 O 4 @Ni(OH) 2 //AC, 19 Ag 6 Mo 10 O 30 //AC, 44 CoMoO 4 -NiMoO 4 //AC, 23 NiO/Ni//CNCs 45 and MnO 2 //CNTs 46 but a higher energy density at 41.1 W h kg À1 was achieved with the power density of 17 002 W kg À1 by a NiMoO 4 //AC system. 20 To further probe the stability of the hybrid electrode, the sample aer the stability test was observed by SEM. Fig. 6 presents the SEM images aer 3000 charge-discharge cycles at 5 A g À1 ; it can be clearly seen that the physical microstructures of the electrode maintain very well, which are still in nanosheets. As no notable changes can be found on the electrode before and aer the cycling measurements, it proves the good stability of the hybrid electrode.

Conclusions
In summary, we synthesized a hybrid Cu 2 O/CuMoO 4 nanosheet electrode via a facile hydrothermal method and it exhibits remarkably improved specic and areal capacitance (up to 4264 F g À1 and 9.38 F cm À2 at a current density of 1 A g À1 ). To the best of our knowledge, the specic capacitance of 4264 F g À1 is the highest in reported studies although the areal capacitance of 9.38 F g À1 is slightly lower than the highest value of 16 F cm À2 when Ni(OH) 2 /carbon nanotubes on Ni foam was used as the electrode. In addition, the electrode presents good rate capability; specically, the capacitance still retains 2563 F g À1 (5.64 F cm À2 ) at a current density as high as 50 A g À1 . Moreover, an asymmetric supercapacitor is fabricated using the Cu 2 O/ CuMoO 4 nanosheet electrode as the positive electrode and AC as the negative electrode. The operation voltage of the asymmetric supercapacitor could achieve 1.7 V, at which the energy density is 75.1 W h kg À1 with a power density of 420 W kg À1 . The asymmetric supercapacitor also displays excellent rate capability and cycling stability. The capacitance remains around 86.6% aer 3000 cycles at a current density of 5 A g À1 . Considering the fascinating performances of Cu 2 O/CuMoO 4 nanosheet electrodes and easy fabrication procedures, the electrodes are promising for applications in renewable energy storage.