0 Introduction Supercapacitors possess power densities more than 10 times higher than batteries and energy densities tens of times higher than electrostatic capacitors. They also offer advantages such as fast charging and discharging speeds, no environmental pollution, and long cycle life, making them a promising new type of green energy for the 21st century. It is well known that amorphous hydrated ruthenium oxide is the most promising electrode material for high-power, high-energy-density supercapacitors, but its high price and scarcity, coupled with the environmental pollution caused by the electrolyte used, significantly limit its commercial development. NiO and other oxide electrode materials have similar functions to RuOxH₂O and are inexpensive, attracting considerable attention from researchers. In recent years, various methods for synthesizing nano-NiO have been explored [1]. In the liquid phase, precursor precipitates prepared at room temperature often exhibit poor crystallinity and are prone to aggregation. Adding surfactants during synthesis can reduce the aggregation degree of the precursor, but it still cannot eliminate the aggregation phenomenon caused by salt bridging and rapid crystal transformation during high-temperature calcination. If the precursor synthesized at room temperature is hydrothermally modified before calcination, the strong reactivity and high dispersibility of the high-temperature aqueous solution can be used for "wet sintering" to improve the crystallinity and dispersion properties of the precursor [s], thereby preparing oxide nanoparticles with good dispersibility. This paper uses NiSO₄·6H₂O and NaOH as raw materials and adopts a process of room temperature synthesis-hydrothermal modification-medium temperature calcination to prepare nano-nickel oxide and studies its capacitance properties. 1 Experiment 1.1 Preparation of nano-nickel oxide Take 20 mL of 0.1 mol/L nickel sulfate aqueous solution into a 100 mL beaker. Under magnetic stirring (keeping the stirring speed at 500 r/min), add about 20 mL of 0.2 mol/L sodium hydroxide aqueous solution. The reaction is complete and a light green precipitate is obtained. Adjust the pH of the mother liquor to 13.4. The reaction mixture was injected into a 50 mL high-pressure reactor lined with polytetrafluoroethylene (PTFE), with a filling density of 80%. The reactor was sealed and placed in a DHG-9070A type electric thermostatic drying oven at 180°C for 24 hours via hydrothermal reaction. After natural cooling to room temperature, the mixture was filtered, washed, and dried in a vacuum drying oven at 80°C for 4 hours to obtain hexagonal flake-like Ni(OH) particles with regular edges. The obtained Ni-(OH) was placed in a crucible and calcined in a high-temperature box furnace at a programmed temperature of 300°C for 2–3 hours. The calcined product was naturally cooled to room temperature and then ground to obtain nano-nickel oxide. The precursor Ni(OH) of NiO was subjected to thermogravimetric analysis using a DT-40 thermal analyzer. The morphology and particle size of the product were observed using an S-430O scanning electron microscope, and the crystal structure of the NiO particles was determined using a Bruker D-8 X-ray diffractometer. 1.2 Preparation of Nickel Oxide Electrode The prepared nano-nickel oxide powder was mixed with conductive agent (acetylene black) and PTFE (binder) in a ratio of 7:2:1, and uniformly coated onto accurately weighed nickel foam to form the working electrode. A saturated calomel electrode (SCE) was used as the reference electrode, a 2cm x 2cm nickel mesh as the auxiliary electrode, and KOH solution as the electrolyte to form a three-electrode system. 2 Results and Discussion 2.1 Thermogravimetric Analysis of Nickel Hydroxide Figure 1 shows the TDTA curve of the nano-nickel hydroxide precursor prepared by the hydrothermal method. As can be seen from the figure, the TG curve has two weight loss steps. The first step has a temperature range below 200℃ and a small slope, which is the process of the nano-nickel hydroxide crystals slowly losing their water of crystallization. The DTA curve corresponds to a broad endothermic peak, indicating that the obtained sample still has water of crystallization after drying. The second step occurs in the temperature range of 200–292℃. The weight loss at this higher temperature is caused by the thermal decomposition of nickel hydroxide to generate nickel oxide, corresponding to a sharper endothermic peak on the DTA curve. When the temperature exceeds 292°C, the TG curve becomes essentially horizontal, indicating that the precursor decomposition is basically complete, and the remaining crystals are mainly nickel oxide, which provides us with a basis for the pyrolysis temperature. [align=left]2.2 X-ray diffraction analysis of nickel oxide The X-ray diffraction pattern of nickel oxide powder is shown in Figure 2. The position and intensity of each diffraction peak of the sample are basically consistent with the cubic NiO crystal system, indicating that the product is a cubic NiO crystal. The three typical diffraction peaks are (111), (200) and (220) crystal planes, which are basically consistent with JCPD1049. No impurity phases were observed from the X-ray diffraction peaks, indicating that the purity of the product is very high and the precursor has been completely decomposed. [/align][align=left]2.3 Morphological characterization of nickel oxide The product was observed and characterized using an S-4300 scanning electron microscope. As shown in Figure 3, the obtained nano-nickel oxide has good dispersibility and a morphology of regular hexagonal plates with an average particle size of about 40 nm. The crystal belongs to the cubic crystal system, which is consistent with the XRD results. 2.4 Electrochemical Performance of Nano-Nickel Oxide 2.4.1 Effect of Electrolyte Concentration on the Capacitive Performance of Nickel Oxide Using a NiO electrode as the research electrode, cyclic voltammetry tests were conducted with different concentrations of KOH electrolyte (1 mol/L, 2 mol/L, 3 mol/L, 4 mol/L, 5 mol/L, 6 mol/L, and 7 mol/L) in ascending order of concentration within a potential range of -400 to 400 mV and scan rates of 20 mV/s and 50 mV/s. The results are shown in Figure 4. Figure 4 shows that within a certain concentration range, at a scan rate of 50 mV/s, the specific capacity of the nickel oxide electrode increases with increasing KOH electrolyte concentration; however, when the concentration increases to a certain level (Stool/L), the trend of specific capacity change with concentration becomes less obvious. At a lower scan rate of 20 mV/s, the effect of concentration on the electrode specific capacity is relatively small. Therefore, a concentration of 5 mol/L KOH should be used as the electrolyte when operating at a smaller current. 2.4.2 Changes in the specific capacity of the nickel oxide electrode with the number of cycles Another advantage of supercapacitors is their long service life; the capacitance remains essentially constant with increasing cycle count. Figure 5 shows the relationship between the specific capacity of the nickel oxide electrode and the number of cycles in a 5 mol/L KOH electrolyte, under the conditions of a potential range of -400 to 400 mV and a scan rate of 20 mV/s. As shown in Figure 5, the capacitance change went through three stages: first increasing, then decreasing, and finally remaining constant. In the first few dozen charge and discharge cycles, the active material of the nickel oxide electrode was activated, and its activity became increasingly higher, increasing the specific capacity with the increase in the number of cycles. As the number of charge and discharge cycles continued to increase, the nickel oxide content in the electrode material gradually decreased, leading to a gradual decrease in specific capacity until it remained at around 25 oF/g. 2.4.3 Influence of working potential on the performance of nickel oxide capacitors The specific capacity of the nickel oxide electrode at different working potentials was investigated. Figure 6 shows the cyclic voltammetry of the electrode in 5 mol/L KOH electrolyte at a scan rate of 20 mV/s and different potential ranges. As can be seen from Figure 6, the specific capacity of the NiO electrode varies significantly with the working potential; increasing the working potential significantly improves the specific capacity. 2.4.5 Constant Current Charge-Discharge Performance The electrode was charged and discharged in 5 mol/L KOH at a current of 20 mA within a potential range of 0–500 mV. The charge-discharge curve is shown in Figure 7, and the obtained specific capacity is 252 F/g. 3 Conclusion A nickel oxide electrode material with a regular hexagonal plate morphology and a particle size of approximately 40 nm was prepared using a room-temperature synthesis-hydrothermal modification-medium-temperature calcination process. The material exhibited Faraday pseudocapacitive properties in KOH solution. Its specific capacitance increased with increasing KOH concentration until the KOH concentration reached 5 mol/L, at which point the specific capacitance of NiO no longer changed significantly. The specific capacitance went through three stages with increasing cyclic voltammetry scan cycles: first increasing, then decreasing, and finally remaining constant. The specific capacitance reached 252 F/g during 20 mA constant current charge-discharge. References 1 Xiang L, Deng XY, Jin Y. Experimental study on synthesis of NiOnano-partieles. Scr Mater, 2002, 47(4): 219 2 Zou Bingsuo, Wang Li, Lin Jingu. The electronic structures of NiOnanopartieles coated with stearates. Solid State Commun, 1995, 94(10): 847 Click to download: Preparation of nano-nickel oxide and research on supercapacitor performance Editor: Chen Dong