3.1 Crystal structure characterization (XRD)
Fig. 1(a) shows the XRD patterns of all samples prepared in a mixed solvent of glycerol and ethylene glycol with a ratio of 1:3. All samples are within the range of 15~80°, and have the same diffraction peaks at 16.34°, 26.83°, 31.59°, 38.32°, 47.41°, 50.46°, 55.33° and 58.07°, which are the same as the card (PDF-20-0782), corresponding to (111), (220), (311), (400), (422), (511), (440) and (620) crystal planes of NiCo2S4 (PDF-20-0782), respectively.16, 17 Obviously, the longer the reaction time, the better the crystallinity of the corresponding sample. Therefore, the crystallinity of GENCS-1,3,6 samples is poor, while that of GENCS-9, 12, 15, 18 samples is obviously better than that of the previous. Certainly, the XRD also reflects that there is a fluctuation about crystallinity around 12 hours, which may be due to environmental factors. Figure 1(b) shows that the distribution of Ni, Co, and S in the detection tone is represented by an element mapping image, which further shows the coexistence of NiCo2S4.
3.2 Morphology characterization and Electrochemical characterization
Fig.2(a) and (b) are the SEM pictures of GENCS-1,3 and show that there are some opened cavities in the samples, which looks like a blooming flower. In fact, the TEM pictures, as shown in Fig.2(c), (d), present that the products are some irregular nano-particles with cavity. Our previous research suggests that when the cavity walls are thin, and at the same time the water pressure in the cavity is large enough, the cavity walls may be broke.18, 19 Therefore, it can be inferred that the product is synthesized within 3 hour, and its cavity wall is thin,, and in this case the cavity wall is easily opened by a steam breakthrough. Fig.2(e), (f) show the lattice stripes and the spacing of the lattice stripes, which was measured to be 0.234nm, 0.186nm, 0.301nm and 0.297nm, respectively, corresponding to (400), (511), (220), (311) of NiCo2S4. Some electrochemical measurements were also carried out here. It was tested their charge-discharge curve (GCD) and charge-discharge curve at different current densities. Fig.2(g), (h) show the GCD curves and rate curves, respectively. Using the GCD discharge curve of 1A/g current density, the discharge capacitance of the samples can be calculated to be 1874F/g and 1816F/g, respectively. But, at 10A/g, the calculated value is 1260F/g and 1620F/g, respectively. Obviously, GENCS-1 performance declined significantly with current rising, which may be related to its poor structural stability caused by its too thin cavity wall. Certainly, these two samples must benefit from their flower-like structure to achieve high capacitance performance. The open holes will expose more active sites, thereby increasing the ion exchange rate.
The SEM images of the remaining samples are shown in Fig.3(a) - (e). Their reaction time is 6h, 9h, 12h, 15h and 18h, respectively. It is observed that almost all these samples show platelet shapes, with a size of about 100 nm, but there is no any flower-like structure. This may indicate that when the reaction time is more than 6 hours, not only the cavity wall becomes thicker, but the atoms on the wall are arranged more orderly, which leads to stronger walls and all the cavities not to be broken again. The discharge test (GCD) and capacitance performance test under different current density conditions were performed on these samples, as shown in Fig. 3(f) and 3(g). These data indicate that at 1 A/g current density, the capacitance of these samples is between 1000~1600 F/g, and when the current density rises to 10 A/g, the capacitance attenuates between 960 and 1350 F/g, showing a significant slip compared to the previous two samples, especially compared with GENCS-3. It seems that the better the crystallinity, the worse the corresponding capacitance property. On the other hand, it can be seen from Fig. 2(h) and Fig.3 g that the rate performance of the samples with good crystallinity will be better. Therefore, GENCS-18 has a capacitance of 1020F/g at 1A/g and 960F/g at 10A/g, showing an excellent capacitance retention rate of 94.12%.
The CV curves of the GENCS-3 sample are also tested under different scanning rates of 5 mV/s, 10 mV/s, 20 mV/s, 30 mV/s, 40 mV/s, 50 mV/s and 100 mV/s, as shown in the Fig. 5(a). It can be seen in the figure that with the increase of scanning rate, the area of the curve also increases, and the position of the redox peak also moves to both ends. Due to the change of sweep speed, the polarization of the electrode occurs, and the redox peak current is also increasing. In a certain voltage range, the peak current does not reach the peak point, so the peak is moving. Fig. 5(b) shows the GCD curves of the GENCS-3 at 1 A/g, 3 A/g, 5 A/g, 8 A/g and 10 A/g in a potential window of 0-0.5. It can be seen that all GCD curves have symmetry under different current density, indicating that the sample has good charge-discharge reversibility. Based on Fig. 5(b), the specific capacitance of GENCS-3 is calculated under different current density, showing a value of 1816 F/g, 1752F/g, 1710F/g, 1648F/g, 1620F/g, respectively. Fig. 5(c) provides the CV curves of all samples at 5 mV/s sweep speed, and there are obvious redox peaks of pseudocapacitance, which are mainly attributed to the Faraday redox reaction between Co2+/Co3+(Ni2+/Ni3+) cations and OH- anions. Fig. 5(d), (e) shows the impedance plots of the low-frequency and high-frequency regions of all the samples, respectively, where the intercept of the curve with the X axis represents the base resistance of the electrode solution. It can be seen from the figure that the slope of GENCS-3 in the low frequency region is greater than that of other samples. This shows that its ion transmission efficiency to electrolyte is better than others. The EIS of GENCS-1, 3 has a larger slope in the low frequency region, indicating their better diffusion behavior.20 However, the Rs value (4.4 Ω) of the sample GENCS-1 is much greater than that of the sample GENCS-3 (0.5 Ω).
Fig. 5(f) shows the Ip ~v1/2 fitting curve of the oxidation peak of the cyclic voltammetry curve of all samples at different scan rates. Here, Ip is the peak electricity (A) of the oxidation peak or reduction peak and v is the cycle Scanning rate of ring voltammetry (mV/s). The oxidation peak current (Ip) is proportional to the square root of the scan rate, indicating that the process of OH- embedding in the NiCo2S4 electrode is a diffusion process. The relationship between CV peak current (Ip) and scan rate is:
Where is the OH- diffusion coefficient. The fitting curve slope of sample GENCS-3 is 1.5, which is much larger than that of other sample fitting curves. Therefore, the diffusion coefficient () of GENCS-3 sample is larger than that of other samples, thus showing excellent electrochemical performance. Fig. 5(g) is retention rate chart of all samples. It can be seen that the retention rate of GENCS-3 is 89.5%, which is a little lower than that of the GENES-18, not to exceed 5%. The degradation is due to the long-term cycling that causes certain damage to the electrode.
Here are two samples selected for BET testing, one with maximum capacitance and the other with minimum capacitance. Fig. 6(a), (b) show that the BET results of the samples of GENCS-3 and GENCS-18, respectively. As showing in Fig. 6(a), (b), both samples have many micropores, few mesopores, and few macrospores. However, the pore size of GENCS-3 is mostly in the micropore area, while the pore size distribution of GENCS-18 is not very uniform in the pore area, and the pore size fluctuates greatly. The average pore size of GENCS-3 is 7nm, and that of GENCS-18 is 10nm. And GENCS-3 has the surface area of 40.3953 m²/g, but the GENCS-18 only has a surface area of 10.2466 m²/g. As thought, GENCS-3 has a larger specific surface area, which is mainly due to its poor crystallinity and more defects. Of course, larger specific surface area can enlarge the effective release area between the electrode and the electrolyte ions, which is beneficial to enhance the diffusion of electrolyte ions in the electrode, thereby improving electrochemical performance.21