Abstract: This paper introduces the fabrication process of an activated carbon-based supercapacitor using an organic electrolyte system. Six different organic electrolytes were compared and assembled into supercapacitors, and their electrochemical performance was tested. The results show that the EhNBF4/PC system is suitable as an electrolyte for supercapacitors; the LiPF6/PC and LiPF6/EC+PC systems are unsuitable for supercapacitors due to decomposition reactions. Keywords: Supercapacitor, Electric Double Layer Capacitor, Organic Electrolyte, Activated Carbon Supercapacitors (Supercapacitors) are widely used in the electronics industry due to their high power, long lifespan, environmental friendliness, and high efficiency. Activated carbon with high specific surface area has excellent adsorption performance and flexible electrode structure, and is widely used in the industrialization of supercapacitors. Organic electrolytes have a significant impact on the capacitance, internal resistance, and temperature characteristics of supercapacitors. This paper introduces the fabrication process of supercapacitors and investigates the performance of six organic electrolytes used in supercapacitors. 1. Experiment 1.1 Physical Property Testing of Activated Carbon The physical property parameters of the activated carbon used as the electrode material were tested. Specific surface area and pore size distribution were tested using an ASAP2010 instrument with an adsorbate concentration of 77 KN2. Particle size was tested using a Malvern laser particle size analyzer. Tap density was tested using a Quanta Chrome instrument, according to GB/T 5162-1985 standard. 1.2 Electrolyte Physical Properties Testing Six electrolytes (all with a concentration of 1 tool/L) were selected for comparative testing, labeled as E1-E6 electrolytes, and their specific compositions are shown in Table 1. The conductivity of the electrolytes at different temperatures was tested using a DDS-11C digital conductivity meter, with a temperature range of -20 to 60℃. The thermal stability of the electrolytes was tested using a Netzaeh-Tase-414/4 thermal analyzer, with a temperature range of 25 to 350℃, a heating rate of 5℃/min, and N2 atmosphere protection. 1.3 Assembly of the Supercapacitor: Activated carbon, acetylene black, and PTFE (polytetrafluoroethylene) binder were weighed at a mass ratio of 80:10:10, dry-mixed, and then an appropriate amount of water was added. The mixture was stirred for 3 hours to adjust the viscosity to 6.5–7.0 kPa·s. The slurry was uniformly coated onto a 20 μm thick aluminum foil current collector using an electrode coating machine, with the double-sided electrode thickness controlled at 240 μm. The electrodes were cut to 35 mm × 62 mm specifications, stacked, and assembled into a supercapacitor. The outer packaging was an aluminum foil bag used for lithium-ion batteries, and the separator was a grafted polypropylene film. 1.4 Electrochemical Performance Testing: Constant current charge-discharge performance tests were conducted at different temperatures using an American MC.4 supercapacitor tester. The test current was 1 A, and the voltage range was 0–2.8 V. An AC impedance spectroscopy test was conducted using a Zahner IM6 electrochemical workstation to determine the DC internal resistance of the supercapacitor, with a frequency range of 5 kHz–0.1 Hz. 1.5 Gas Chromatography Analysis An Agilent 7093 gas chromatograph was used to analyze the decomposition gas during the constant current test. The test method was as follows: 1 mL of decomposition gas was extracted and injected into a capillary column for split testing. The split ratio was 12.6:1, the column inlet temperature was 240℃, and the furnace temperature was 300℃. 2. Results and Discussion 2.1 Physical Properties of Activated Carbon The particle size of SUP-AC activated carbon was 4.8 μm, the specific surface area was 1660 m²/g, and in the total pore volume (0.85 cm³/g), micropores accounted for 62% and mesopores accounted for 24%. 2.2 Physical Properties of Electrolyte Figure 1 shows the conductivity curves of the electrolyte tested at different temperatures. The conductivity of the electrolyte directly affects the internal resistance of the supercapacitor. The change in internal resistance at different temperatures has a significant impact on the temperature characteristics of the capacitor. As shown in Figure 1, the conductivity of the electrolyte increases with increasing temperature. Electrolyte E3 exhibits the best conductivity, with a room temperature conductivity of 1.15 S/m, demonstrating excellent performance at both high and low temperatures. Electrolyte E5 exhibits the worst conductivity, with a room temperature conductivity of only 0.57 S/m and a conductivity of 1.09 S/m at 60℃. As an electrolyte for supercapacitors, it is crucial to maintain thermal stability within a specific temperature range. Figure 2 shows the thermogravimetric (TG) and differential thermal (DSC) analyses of the six electrolytes. As shown in Figure 2, the E1 electrolyte exhibits three distinct exothermic peaks: peaks 1-3 represent the volatilization peaks of DMC (boiling point 90 °C), EMC (boiling point 110 °C), and EC (boiling point 248 °C), respectively. Peak 1 has a relatively low initial temperature (60 °C). During prolonged high-current charging and discharging of the supercapacitor, the internal temperature increases, leading to electrolyte volatilization and increased internal resistance. Peak 2 reaches a maximum temperature of 180 °C, significantly higher than the boiling point of EMC (I10 °C). This is due to the reversible reaction HJ, where EMC transforms into DEC and DMC. The four volatilization peaks of the E2 electrolyte, in temperature order, are DMC, EMC, GBL (boiling point 202 °C), and EC. Due to the presence of DMC, a small amount of volatilization still begins at 60 °C. The endothermic peak 1 of the E3 electrolyte is the volatilization peak of the solvent PC (boiling point 241 ℃), and the exothermic peak 2 is the decomposition peak of the electrolyte Et4NBF4, occurring at a temperature of 312 ℃. The E3 electrolyte shows almost no thermal sensitivity below 100 ℃, indicating stable performance. The E4 electrolyte exhibits poor thermal stability, showing thermal weight loss starting from 50 ℃. Both the E5 and E6 electrolytes show a small step peak around 100 ℃, possibly due to the decomposition of the electrolyte LiPF6 at high moisture content. The endothermic peaks around 250 ℃ are the volatilization peaks of the solvents PC and PC/EC, respectively. 2.3 Electrochemical Performance Testing: Supercapacitors were assembled using different electrolytes. The finished product dimensions were 3.8 mm × 62.0 mm × 35.0 mm, with a total mass of 12.6 g. In a controlled temperature chamber, constant current charge-discharge tests were conducted on the assembled supercapacitor at different temperatures, and the results are shown in Figure 3a. The AC impedance spectrum of the supercapacitor was measured to obtain the equivalent DC internal resistance, and the results are shown in Figure 3b. Figure 3a shows that in the low-temperature region of -20 to 25°C, the electrolyte capacity increases with increasing temperature; in the high-temperature region of 25 to 60°C, the electrolyte capacity decreases with increasing temperature; the E3 electrolyte system has the highest capacity, 57 F at 25°C, and also exhibits good low-temperature performance. Figure 3b shows that the equivalent DC internal resistance of the electrolyte decreases with increasing temperature; the E3 electrolyte system has the lowest internal resistance, 0.20 at 25°C. The ion adsorption energy storage process in the double-layer principle of supercapacitors is heat-sensitive, and the adsorption reaction is an endothermic process. When the temperature rises, ionic activity increases, which is unfavorable for the occurrence of stable adsorption reactions. The amount of charge adsorbed on the same area decreases, resulting in a lower capacity. Conversely, when the temperature decreases, the adsorption process is favorable, the amount of charge adsorbed on the same area increases, and the capacity increases. On the other hand, when the temperature rises, the ionic conductivity of the electrolyte increases, and the internal resistance decreases. Because the voltage consumed by the internal resistance decreases, the voltage range available for energy storage widens, increasing the capacity. When the temperature decreases, the ionic conductivity of the electrolyte decreases, and the internal resistance increases, narrowing the voltage range available for energy storage and decreasing the capacity. Temperature changes have a dual impact on the capacity and internal resistance of supercapacitors. Based on the test data from this experiment, temperature has a dominant effect on the adsorption process and is the main factor affecting the capacity of supercapacitors; the change in internal resistance caused by temperature changes has a relatively small impact on the increase or decrease in capacity. 2.4 Gas Chromatography Analysis During the constant current charge-discharge test, gas bubbling was observed in the E5 and E6 electrolyte systems, with the amount of internally decomposed gas gradually increasing over time. To understand the cause of gas decomposition, a sample of the E5 electrolyte system was selected for gas chromatography testing, and the results are shown in Figure 4. Figure 4 shows a downward negative peak at 45.72 s, a typical characteristic peak of H2, accounting for 0.122% of the total gas. The two most abundant gases are CO2 and CO, accounting for 58% and 39% respectively. This is because, with the participation of Li, PC solvent molecules and EC solvent molecules undergo irreversible redox reactions at the edges of the activated carbon microcrystals, not only generating gas but also causing capacity decay due to the continuous reduction of adsorption area. 3. Conclusions a. Conductivity and TG-DSC analysis of the electrolyte show that the E3 electrolyte has high conductivity and thermal stability, a result verified in electrochemical performance testing. b. Electrochemical performance testing of the six electrolytes shows that, with other components remaining constant, temperature dominates the adsorption process and has a significant impact on the capacitance change of the supercapacitor. c. In comparison, E3 electrolyte is more suitable as an organic electrolyte for supercapacitors, while E1, E2, and E4 electrolyte systems have relatively low adsorption capacity on activated carbon surfaces. E5 and E6 electrolyte systems exhibit significant solvent decomposition reactions.