Characterization
FE-SEM images of flower-like CuO and CuO/NCNO are shown in Fig. 1. As the FE-SEM images of copper oxide show, a flower-like morphology can be observed for this nanostructure, when NCNO is present in CuO synthesis. The same flower-like structure can be observed in the presence of NCNO spherical nanoparticles.
The XRD pattern of flower-like CuO and CuO/NCNO is shown in Fig. 2. The peaks observed at 2θ = 30.7, 36.9, 42.8, 43.7, 50.6, 61.6, 74.3, and 77 degrees are related to the (110), (002), (111), (-200), (202), (-113), (022) and (310) planes, respectively (according to JCPDS 0661-05) [95]. The same peaks can also be seen in the CuO/NCNO nanocomposite; however, the peak related to NCNO is not observed in the XRD pattern of the composite due to the low amounts of NCNO compared to CuO.
TEM images of CuO, NCNO, and their nanocomposite are shown in Fig. 3. Tiny spherical nanoparticles are related to NCNO. Also, the images related to the flower-like CuO show the synthesis of this nanoparticle; the same images can be seen for CuO in the presence of NCNO.
Supercapacitor Properties Investigation For The Prepared Electrodes
The supercapacitor behavior of CuO and CuO/NCNO nanocomposite in potassium hydroxide solution was investigated by cyclic voltammetry in a potential range of -0.2\(-\)0.65 V at a scan rate of 50 mV/s. The experiments were performed in a three-electrode system, including the reference electrode (silver/ silver chloride saturated), the counter electrode (platinum wire), and the working electrode (CuO and CuO/NCNO). The cyclic voltammograms of CuO and CuO/NCNO are shown in Fig. 4. The cyclic voltammogram of the CuO electrode shows a pair of oxidation and reduction peaks that correspond to the conversion of copper (I) to copper (II) and vice versa, however, its current is not very high, so it is a quasi-reversible reaction. A better reversibility reaction with a higher oxidation and reduction current for CuO is observed when the flower-like copper oxide is formed in the presence of NCNO. Also, the surface area under a curve of CuO/NCNO electrode is greater than that of the CuO electrode due to the high surface area of NCNO, which results from a synergistic effect of components CNO and CuO. The oxidation and reduction current and surface area under the curve are more for this electrode. The shape of CV in the CuO/NCNO electrode is more significant than that of the copper oxide electrode, indicating the CuO/NCNO electrode’s higher capacitance than the single copper oxide electrode.
Cyclic voltammograms of CuO and CuO/NCNO electrodes at different scan rates are shown in Fig. 3. According to the figure, the oxidation and reduction current increases slowly with increasing scan rate, indicating this electrode’s ideal capacitive behavior. Also, moving the anodic peaks and cathodic peaks toward the positive and the negative potentials, respectively, verify a quasi-reversible reaction for all the curves. The diagram of the current versus the square root of the scan rate is also displayed in Fig. 2. There is a linear relationship between the oxidation peak currents and the square root of the scan rate for both CuO and CuO/NCNO electrodes, which shows that a quasi-reverse reaction has occurred on the surface of the electrode under the diffusion-controlled process.
The specific capacitances of CuO/NCNO and CuO electrodes were investigated in a three-electrode system with potassium hydroxide solution as an electrolyte and chronopotentiometry method. The charge/discharge curve was obtained in the potential window of -0.2 to 0.6 V and the current density of 1, 2, 3, and 4 A/g. As the current density decreases, the charge/discharge time becomes longer. Figure 5 shows the charge and discharge curves of CuO and CuO/NCNO electrodes. The obtained capacitances for the CuO at current densities of 1, 2, 3, and 4 A/g are 441.25, 367.5, 221.25, and 155 F/g; however, the capacitances values for CuO/NCNO electrodes at the mentioned current densities are 673.75, 572.5, 382.5 and 315 F/g, respectively.
The electrochemical impedance spectrometer of the mentioned electrodes was tested in potassium hydroxide to check the electrodes’ resistance and conductivity. The results are shown in Fig. 6. As the results also indicate, the charge transfer resistance of CuO/NCNO is lower than that of CuO electrode, which suggests that the electron transfer rate on the surface of this electrode is higher than that of the CuO electrode; therefore, the conductivity and capacitance of the electrode are higher. A comparison of the performance of the capacitive electrodes based on CuO and CuO/NCNO is reported in Table 1. The capacitance in this work is comparable and even better than other electrodes due to the synergistic effect of CuO and NCNO. The CuO/NCNO capacitive electrode has a higher surface area and more active sites for the diffusion of the ions and Faraday reactions; therefore, better electrical conductivity is obtained at a more suitable charging time.
Table 1
A comparison of the performance of the capacitive electrodes based on CuO
Electrode | Current density (A.g) | specific capacitance (F.g) | Ref |
RGO/CuO/Cu2O | 1 | 173 | [39] |
PPD/CuO/RGO | 1 | 512 | [40] |
CuO/CuCo2O4 | 1 | 443 | [41] |
RGO/CuO | 0.2 | 188 | [42] |
Cu-CuO/Cu2O | 1 | 85 | [43] |
CuO/g-C3N4 | 2 | 384 | [44] |
CuO/NCNO | 1 | 673.75 | This Work |
The supercapacitor behavior of a symmetrical CuO/NCNO electrode system in potassium hydroxide solution was investigated by the cyclic voltammetry method. Cyclic voltammograms of the CuO/NCNO electrode in Fig. 7a show a pair of oxidation/reduction peaks that appears for this electrode, which is shifted by increasing the scan rate. In fact, a quasi-reversible behavior in the potential region of -0.2\(-\)0.6 V is related to the oxidation/reduction reaction of copper with a favorable oxidation/reduction current in the presence of NCNO. Electrode capacitance, power, and energy density of CuO/NCNO electrode were investigated through chronopotentiometry. The obtained capacitances for NCNO electrode in the current density of 1, 2, 3, and 4 A/g are 1072, 1015, 577, and 450 F/g, respectively (Fig. 7b). Also, the maximum power and energy density for this electrode are 98 W/kg and 3200 Wh/kg, at the current density of 1 and 4 A/g, respectively. The Ragone plot, Fig. 7c shows that good quasi-capacitive behavior can be considered for this electrode due to the appropriate power and energy density. To study the stability of the CuO/NCNO electrode, three thousand consecutive charge/discharge cycles were recorded at a current density of 4 A/g. The results showed that the capacitance change of the CuO/NCNO electrode in the final cycle compared to the first cycle is 4%, so it can be said that the CuO/NCNO capacitive electrode has acceptable stability (Fig. 7d). Also, the charge transfer resistance in potassium hydroxide solution based on the Nyquist diagram is 3 Ω indicating an excellent conductivity of this electrode (Fig. 8)