3.1 Structure and Morphology Analysis of NiS electrode
Owing to understand the structure of NiS cathode material better, some chara-cterization equipment was used to characterize its composition, morphology and structure. In order to determine the phase composition of the products, the products obtained from the reaction were analyzed by X-ray diffraction analysis (XRD). As shown in Fig. 3-1a, diffraction peaks of NiS samples were observed at 18.43°, 30.31°, 32.21°, 35.71°, 40.48°, 48.85°, 50.14°, and 52.64°. Corresponding to the (110), (101), (300), (021), (211), (131), (410) and (401) crystal faces of the standard card (PDF#98-000-0308), respectively. It shows that the NiS generated by the reaction exists in the form of hexagonal crystal system. With the increase of soluble starch concentration in the precursor, diffraction peaks with increasing intensity were observed at 30.15°, 34.67°, 45.91° and 53.55°, corresponding to (100), (101), (102) and (110) crystal faces of standard card (PDF#97-002-9313), respectively. The results showed that the content of NiS hexagonal phase with higher crystal density increased with the increase of soluble starch concentration. Through hydrothermal vulcanization, all the precursors were converted to NiS, and no other impurity peaks were found in the XRD pattern, indicating that NiS pure phase was formed.
Additionly, in order to analyze the effects of different starch concentrations on the microstructure of NiS, scanning electron microscopy (SEM) tests were performed on NiO, NIS-1, NIS-3, NIS-5, NIS-7, as well as NiS prepared without starch assisted crystallization. The results are shown in Fig. 3-1b-g. As can be seen from Fig. 3-1b, the annealed NiO was in the shape of nanoflake accumulation and NiS was irregular and accompanied by particle agglomeration. Figure 3-1c shows the SEM image of NIS-1. It can be seen that the size of NiS particles is small, most of them are spheroidal morphology, and the agglomeration of NiS particles is obvious. As the soluble starch concentration increased to 3g, the particle size increased slightly, and irregular rod-like morphology appeared, as shown in Fig. 3-1d. Later, with the increase of soluble starch concentration, the rod-like morphology gradually decreased and the irregular flaky structure increased, as shown in Fig. 3-1e.When the concentration of soluble starch increased to 7g, most of the morphology changed into an irregular sheet structure, as shown in Fig. 3-1f. It can be seen from the SEM test diagram that NiS particles prepared without starch assisted crystallization are more serious than those obtained with starch assisted crystallization, and the particle size is larger, so the specific surface area is relatively small, as shown in Fig. 3-1g. The specific morphology transformation process is shown in Fig. 3 − 1. The Ni(OH)2 precursor is an irregular, flaky-like structure, which is transformed into NiO after annealing, and its morphology is a nano-flower structure stacked with nanosheets. In the subsequent vulcanization process, due to the high temperature and high pressure environment, the nanosheets are decomposed and self-assembled into NiS nanomaterials with small particle size and uniform particles.
The chemical states of elements in NiS were further studied by X-ray photoelectron spectroscopy (XPS) analysis. Figure 3-2a shows the full XPS spectrum of NiS. The characteristic peaks in the spectrum further confirm the presence of Ni and S elements. The spectrum of C located at 285.6eV is used as the standard element for binding energy correction. The high-resolution XPS spectrum of Ni 2p is shown in Fig. 2-2b, with two spin-orbital splitting double peaks corresponding to Ni 2p3/2 and Ni 2p1/2 orbits, accompanied by two satellite peaks at 861.8 eV and 880.1 eV. The wide peaks of Ni 2p3/2 and Ni 2p1/2 are further deconvolved into four peaks at 853.2eV, 855.8eV, 870.8eV and 874.7eV by fitting. The peaks at 853.2eV and 870.8eV belong to Ni2+. The peaks at 855.8 eV and 874.7 eV belong to Ni3+. The high resolution XPS spectra of Ni 2p demonstrate the existence of multiple valence states of Ni during charge and discharge, and the presence of Ni3+ indicates the progress of redox reaction.The high-resolution XPS spectrum of S 2p is shown in Fig. 3-3c. The XPS peaks of S 2p are fitted as two peaks of binding energy at 161.6eV and 162.8eV, corresponding to the S 2p3/2 and S 2p1/2 orbits, respectively, which belong to S2- in Ni = S.
Specific surface area test (BET) is a method to determine the specific surface area and pore size distribution of powdery materials. We conducted BET tests on powders of different electrode materials, and the test data are shown in Table 1.The obtained N2 adsorption-desorption isotherm is shown in Fig. 3-3a, which expresses the functional relationship between gas adsorption and relative pressure. It can be seen from the figure that the analyzed sample has a type IV isotherm with a type H1 hysteresis ring[26]. Such adsorption and desorption isotherms often exist in mesoporous materials formed by particle agglomeration, and the mesoporous pores in NiS materials are the result of particle agglomeration. Among different electrode materials, NIS-3 material has the largest specific surface area, 21.5357m2·g− 1. The large specific surface area provides more active sites for faradaic reaction, thus improving the charge storage capacity of the electrode.The pore size distribution of different electrode materials is shown in Fig. 3-3b. It can be seen that the peak value is concentrated in the middle of 2-3nm, indicating that most of the prepared materials are composed of mesoporous structures. This mesoporous structure proves that the materials have a large specific surface area and a large number of active sites promote the faradaic reaction. In the figure, there is a low peak around 100nm, indicating that the material also has the characteristics of large pores. The formation of large pores is caused by the gap formed between the large particles after agglomeration. This large pore structure speeds up the process of electrolyte infiltration into the electrode, thus promoting the faradaic reaction.
Table 1
Specific Surface Area and Pore Size of NiS Electrode Materials with Different Starch Concentrations
| NiS-1 | NiS-3 | NiS-5 | NiS-7 | No soluble starch |
BET(m2·g− 1) | 17.6787 | 21.5357 | 18.7160 | 21.3849 | 16.9913 |
Aperture(nm) | 28.4636 | 29.3095 | 29.0874 | 28.2816 | 23.4407 |
3.2 NiS Electrochemical Properties of the electrode material
Owing to further explore the electrochemical properties of the prepared material, the prepared electrode material was used as the working electrode, the platinum electrode as the counter electrode, and the mercury oxide electrode as the reference electrode. Electrochemical tests were carried out in a three-electrode system in a 6mol·L− 1 potassium hydroxide electrolyte.
Table 2
Impedance of NiS Electrode Materials with Different Starch Concentrations
| NiS-1 | NiS-3 | NiS-5 | NiS-7 | No soluble starch |
R1(Ω) | 0.96 | 0.72 | 0.89 | 0.91 | 0.97 |
R2(Ω) | 15.35 | 3.60 | 9.04 | 14.00 | 23.03 |
Figure 3-4a shows the cyclic voltammetry (CV) curve at a sweep speed of 10mV·s− 1. Electrode materials synthesized with different starch concentrations have similar CV curves, and the obvious faradaic peak indicates the presence of reversible faradaic reaction. The results show that the NiS-3 electrode has excellent pseudocapacitance performance, superior charge storage performance and higher specific capacity. This is the uniform distribution of mesoporous pores formed by the aggregation of NiS-3 electrode materials, which promotes charge transfer and thus improves the electrochemical performance of the electrode.
Figure 3-4b shows the constant current charge and discharge (GCD) curve of the synthesized electrode with different starch concentrations when the current density is 1A·g− 1. Obvious charge and discharge platforms can be seen in the figure, indicating that the prepared NiS is a typical pseudo-capacitor storage mechanism. The discharge times of NiS-1, NiS-3, NiS-5 and NiS-7 electrodes are 705s, 950s, 881s and 757s, respectively. It can be seen that the NiS-3 electrode has the highest specific capacitance and the best capacitance performance, indicating that the uniform distribution of mesopoles accelerates the electron transport and the small rod-like structure increases the ionic conductivity.
Electrochemical AC impedance test (EIS) was used to compare the transmission characteristics of the four electrodes, as shown in Fig. 3-4c, and the impedance data are shown in Table 2. The Nyquist curve of the three electrodes is shown as a semicircle and an oblique straight line. The diameter of the semicircle in the low frequency region reflects the charge transfer resistance (Rct) at the interface between the electrolyte and the electrode, and the slope of the oblique line is related to the diffusion resistance of ions in the electrolyte to the electrode surface. The intercept between the curve and the real axis at the high frequency region represents the equivalent series resistance (Rs) composed of the ionic resistance of the electrolyte and the resistance of the electrode itself. It can be clearly seen from the figure that the NiS-3 electrode has the smallest semicircular diameter, indicating that its charge transfer resistance is the smallest, which is more conducive to the diffusion of the electrolyte. This is because the large specific surface area and uniform distribution of mesoporous holes improve the conductivity and charge transfer rate, resulting in better electrochemical performance of the NiS-3 electrode.
3.3 Electrochemical Characteristics of capacitors
In this investigation, graphene oxide material was employed as the negative electrode, NiS material as the positive electrode, and a 0.5mol·L-1 Na2SO4 solution as the electrolyte to construct a water-based hybrid supercapacitor, with the aim of examining the feasibility of electrode preparation. The CV curve of rGOH electrode and NiS-3 electrode in a three-electrode system with a sweep speed of 10mV·s-1 is illustrated in Fig. 3-5a. It can be seen from the figure that the voltage window of rGOH electrode is -1V-0V, and that of NiS-3 electrode is in the range of 0V-0.63V. CV curves of supercapacitors under different voltage Windows are shown in Fig. 3-5b. It can be seen from the figure that polarization occurs when the voltage window is 0V-1.6V, which is caused by polarization reaction of the electrode. Therefore, the range of 0-1.5V is selected as the voltage window of the device. Figure 3-5c shows the CV curve of the supercapacitor under different sweep speeds.
When the sweep rate increases from 5mV·s-1 to 100mV·s-1, the CV graph does not change significantly, indicating that the device has good reversibility and stability, which is due to the uniform mesopore structure of the electrode material that promotes the transfer of electrolyte ions. Moreover, the structure is not easy to be destroyed in the process of redox, which enhances the stability of the capacitor. Figure 3-5d is the GCD curve of the device under different current densities. It can be seen that when the current density increases from 1A·g-1 to 10A·g-1, the curves all show the shape of isosceles triangle, and the charging and discharging time of the device is basically the same, indicating that the device has a good coulomb efficiency, and the shape of the curve does not change, which again confirms the CV results.
When the current density is 1A·g− 1, 2A·g− 1, 3A·g− 1, 4A·g− 1, 5A·g− 1, 6A·g− 1, 8A·g− 1 and 10A·g− 1, the specific capacitance is 50.6F·g− 1, 45F·g− 1 and 43.2F·g− 1,38.6 F·g− 1, 37.2F·g− 1, 35.7F·g− 1, 33.3F·g− 1, respectively. when the current density increases to 10A·g− 1, the capacity retention rate is 65.8%, as shown in Fig. 3-6a. The energy density and power density of capacitors are shown in Fig. 3-6b. When the power density of the device is 750 W·kg− 1, the energy density of 15.8 Wh·kg− 1 can be provided. when the power density reaches 7500 W·kg− 1, the energy density of 10.4 Wh·kg− 1 can still be obtained. It shows that the assembled device has certain practical application value. Figure 3-6c shows the Coulomb efficiency and capacity of the assembled device after 10,000 times of constant current charge and discharge at the current density of 3A·g− 1. It can be seen from the figure that after 10,000 cycles, the capacity retention rate is 52.8%, and the Coulomb efficiency remains at 100%, indicating that the device has good cycle stability.