Supercapacitor has higher power density compared with better battery, and has higher specific capacity and energy density compared with better conventional capacitor   , and mainly used in aerospace, computers, electric toys and other fields   . Carbon materials, metal oxides and their composites, conductive polymers and other materials were used in ultracapacitors     . Meng prepared graphite oxide by Hummers, then synthesis grapheme-zinc oxide by solvothermal method; the specific capacity was only 9 F・g−1 at a current density of 1・g−1  . Using RuO2 in ultracapacitors, specific capacitance could reach 551 F/g. But the very expensive costs of RuO2 would limit its application and development. So, science researcher must seek low cost metal oxides. Now, MnO2, NiO and CuO Metal oxide composites were used in making ultracapacitors  -  .
In this paper, copper-nickel oxide was successfully prepared by hydrothermal method. In 6 mol/L KOH electrolyte, the specific capacitance was 225.67 F∙g−1 at the charge-discharge current of 1 A∙g−1 and induced charge transfer resistance (Rct) and diffusion impedance (Warburg impedance)   were relatively low.
2.1. Preparation of Electrode
The amount of 0.5 mol/L CuCl2 solution 5 mL and 0.5 mol/L NiCl2 solution 15 mL mixed in a beaker, then, weigh 1 mol/L NaOH solution 20 mL, at 60˚C water bath magnetic stirring the pot, titration 40 minutes, then transferred to the reaction vessel, the degree of filling is 80%. Under the condition of 150˚C, respectively reflect 10 h, 20 h, 30 h. Under the condition of 135˚C, 150˚C, reflect 30 h. The hydrothermal sample with distilled water and absolute ethanol washed six times, 90˚C in blast oven dried 6 hours, then the dried sample was placed in a muffle furnace 300˚C calcined 3 h, finally obtained the active material. The preparation of the active substance, acetylene black (AB), polyvinylidene fluoride (PVDF), the mass ratio of 80:15:5 placed in a beaker, grinding evenly, then added N-methyl-2-pyrrolidone network (NWP), stir into a paste- like. Then the mixture was uniformly coated on a foamed nickel, a coating area of 1 × 1 cm2, under the condition of 90˚C, dried for 12 hours, weighed its quality. Subtracting the mass of the uncoated nickel foam, the quality of the active substance can be calculated. Finally, under the 10 Mpa pressure, the sample was tablet and placed in 6 mol/L KOH solution and soaked about 24 hours.
2.2. Performance Testing
The test device is a Shimadzu SHIMADZU’s XRD-6000, test conditions: CuKα as a radiation source target, the tube voltage 30 kV, tube current 50 mA, scanning speed 4˚/min, scanning range (2θ) 10˚ - 80˚. And used Shimadzu SHIMADZU’s SSX-550 scanning electron microscope (SEM) to observe the sample surface morphology, observed speed is 5000 times. In this study, the electrochemical analysis test was used Shanghai Chen Hua companies producing CHI604B/630B electrochemical workstation by three-electrode system, the working electrode was an electrode active materials which had prepared, the counter electrode was a platinum electrode, the reference electrode was a saturated calomel electrode.
3. The Analysis of Experimental Data
3.1. The SEM Characterization
Figure 1. SEM image of NiO/CuO composites under 120˚C 30 h.
Figure 2. SEM image of NiO/CuO composites under 135˚C 30 h.
Figure 3. SEM image of NiO/CuO composites under 135˚C 20 h.
Figure 4. SEM image of NiO/CuO composites under 150˚C 10 h.
Figure 5. SEM image of NiO/CuO composites at 150˚C 30 h.
hydrothermal conditions (5000×). As can be seen from the figures, the samples have different degrees of agglomeration. In Figure 3, the sample has poor electrochemical performance, because it with smooth surface and without pores and gully, that may affect the embedding and extraction of the electrolyte ion. The agglomeration of Figure 3 is the most obvious among all figures. Compared to Figure 3, Figure 1 and Figure 4 have formed voids. The sample of Figure 5 has a deeper porosity, rougher surface, a relatively low degree of crystallinity. So, it would conclude that the sample in Figure 5 possess best capacitance performance among all samples.
3.2. The Analysis of XRD Phase
Figure 6. XRD patterns at different hydrothermal times under 150˚C.
Figure 7. XRD patterns at different hydrothermal temperatures in 30 h.
Figure 7 appears Na6Cu2O6, probably because the precipitation did not clean up. At diffraction angle 2θ of 37.248˚, 43.286˚, 62.852˚, emerge diffraction peaks of NiO and Ni0.75Cu0.25O and at 35.571˚ and 38.726˚, as do CuO. At 150˚C, with the increase of hydrothermal time the diffraction peaks of NiO and Ni0.75Cu0.25O become more and more obvious, and the occurrence of the diffraction peak also width broadening, content of NiO and Ni0.75Cu0.25O increase too and particles become smaller, the electrochemical activity would improve  . From Figure 7, under the condition of same hydrothermal time (30 h), with the increase of hydrothermal temperature, the diffraction peaks of NiO and Ni0.75Cu0.25O become more and more obvious, and the width of the peak broaden.
3.3. Cyclic Voltammetry
Cu:Ni = 1:3 30 h at different hydrothermal temperatures and different scanning rates from Figure 8 to Figure 10, at the scanning rate of 50 mv∙s−1, the edox peaks is not obvious, just sweep the reduction peak, the current is the largest. With the reduce of the scanning rates, the redox peak of the three samples become more obvious. The CV curves of Cu:Ni = 1:3 150˚C at different hydrothermal times and different scanning rates. From Figure 11 to Figure 13, with the decrease of scanning rate, redox peaks of three samples become more obvious, and at the hydrothermal condition of 150˚C 30 h, have good electrochemical properties Equations.
3.4. AC Impedance
AC impedance curve of the sample can be divided into two parts, the high-frequency and low-frequency region area. Intersection of curve and the real axis represent inner electrode impedance (equivalent to Rb), it concludes impedance of electrolyte ions, the intrinsic impedance of the electrode material and contact resistance of Electrically active material and the current collector. From Figure 9 and Figure 10, intersection value of five samples and the Z’ axis is very small, indicating that the internal impedance is small and almost the same. The semicircle of high frequency region represents induced
Figure 8. CV curves of 50 mv∙s−1 scanning rates.
Figure 9. CV curves of 10 mv∙s−1 scanning rates.
Figure 10. CV curves of 5 mv∙s−1 scanning rates.
charge transfer resistance, frequency image in the embedded thumbnail in Figure 13 and Figure 14. Under 150˚C 30 h hydrothermal conditions, induced charge transfer resistance is the minimum. In low frequency region, slightly inclined straight line repre- sents diffusion impedance (Warburg impedance). Impedance spectroscopy to achieve the desired behavior at low frequency capacitance line for line parallel to Z" linear axis. So sample at 150˚C 30 h hydrothermal conditions with minimal resistance.
Figure 11. CV curves of 50 mv∙s−1 scanning rates.
Figure 12. CV curves of 10 mv∙s−1 scanning rates.
Figure 13. CV curves of 5 mv∙s−1 scanning rates.
Figure 14. AC impedance of Cu:Ni = 1:3 150˚C at different hydrothermal times.
Figure 15. AC impedance of Cu:Ni = 1:3, 30 h at different hydrothermal temperatures.
3.5. Constant Current Charge and Discharge Characteristics
Figure 16 to Figure 19 are the charge-discharge curves of Cu:Ni = 1:3 at different hydrothermal times and different hydrothermal temperatures. From Figure 16 and Figure 17, at the 135˚C has longest charging and discharging time. In 1 A∙g−1 current density, three sets of samples of the discharge duration will remain at 100 s, while at 135˚C has maximum charging time. Compared to the condition of 1 A∙g−1 current density, discharge time of 0.5 A∙g−1 current density is the twice. And under the condition of 135˚C 30 h, charge and discharge time is still the longest.
Figure 16. Charge-discharge curves of Cu:Ni = 1:3 30 h at different hydrothermal temperatures in 1 A∙g−1.
Figure 17. Charge-discharge curves of Cu:Ni = 1:3 30 h at different hydrothermal temperatures in 0.5 A∙g−1.
From Figure 18, at 1 A∙g−1 current density, under the hydrothermal condition of 150˚C 30 h, the charge and discharge time is the longest. At 0.5 A∙g−1 current density, under the hydrothermal condition of 150˚C 10 h, the charge and discharge time is the longest, and under the hydrothermal condition of 150˚C 20 h, has the maximum charge-discharge voltage, and it can reach 0.55 V.
Figure 18. Charge-discharge curves of Cu:Ni = 1:3 30 h at different hydrothermal times in 1 A∙g−1.
Figure 19. Charge-discharge curves of Cu:Ni = 1:3 30 h at different hydrothermal times in 0.5 A∙g−1.
In the experiment, we successfully made nickel oxide and copper oxide composites by hydrothermal synthesis. By comparing 150˚C under different hydrothermal time and 30 h under different hydrothermal temperature, to find different electrochemical properties, it was found that after 150˚C 30 h, hydrothermal reaction sample has the best electrochemical performance. Under the condition of 150˚C 30 h, CV curve redox peaks are obvious; charge and discharge time is the longest. The specific capacitance was 225.67 F∙g−1 at the charge-discharge current of 1 A∙g−1. Different hydrothermal time, hydrothermal temperature and heating rate are very important for the growth of sample particle morphology
The project was supported by the National Natural Science Foundation of China (51365001).