Effects of pyrolysis pretreatment and addition of KMnO₄ on the electrochemical properties of peanut shell modified carbon

This study aims to enhance the high-value utilization of agricultural wastes in the field of energy storage materials. A comparison was also made on the effects of the pyrolysis pretreatment and KMnO₄ addition on the electrochemical properties of the peanut shell-modified carbon. The raw material was taken as the peanut shell powder. The specific procedure was set as the solid mixing and milling, nitrogen environment, pyrolysis temperature of 800 °C, and deionized water admittance rate of 0.2 mL/min. The KMnO₄ powder was used as the modifying agent. The electrochemical properties of the peanut shell modified carbon were characterized by scanning electron microscope (SEM), elemental analyzer (EA), fully automated specific surface and porosity analyzer (BET). X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to identify the elemental content, surface morphology, and pore structure of the modified carbon. Galvanostatic charge/discharge and AC impedance (EIS, electrochemical impedance spectroscopy) were used to test the electrochemical properties. The test results showed that the modified carbon without pyrolytic pretreatment shared the relatively high hydrophilicity, the larger specific surface area, the a rich pore structure, and the electrochemical properties; . The specific surface area of the modified carbon increased gradually, with the increase of KMnO₄ addition. The hydrophilicity was enhanced to elevate the specific capacitance. Furthermore, the better performance was achieved after pyrolysis pretreatment, when the mass ratio of the peanut shell charcoal to KMnO₄ powder was 1.6∶1. The modified charcoal shared the best hydrophilicity, the largest specific surface area of 401.16 cm3/g, the mesoporosity of 5.88%, and the micropore rate of 64.71%, and the largest specific capacitance, which was 86.90 F/g at the current density of 0.5 A/g. When the mass ratio of peanut shell powder to KMnO₄ powder was 1.6:1 without pyrolysis pretreatment, the modified carbon shared the best hydrophilicity, the largest specific surface area of 1 422.79 cm³/g, and the highest pore richness, with a mesoporosity of 36.11% and a microporosity of 63.89%. Meanwhile, the largest specific capacitance was observed with a specific capacitance of 92.02 F/g at a current density of 0.5 A/g, which was higher than that of pyrolytic pretreatments. There was 1.2 times of the best coupled modified carbon. The optimal coupling-modified chars without and with pyrolytic pretreatment shared the specific capacitances of 94.00% and 80.26% of those at 0.5 A/g at a current density of 2 A/g, respectively. There was the specific capacitance retention after 10,000 charges and discharges at a current density of 5 A/g of 93.17% and 86.06%, respectively, with the electron transfer resistances of 0.02 and 0.04 Ω, respectively. The specific capacitance of the coupling-modified carbon was somewhat lower than that of the modified carbon prepared by the KMnO4 solution impregnation. The coupling-modified carbon shared the a much lower electron transfer resistance, higher specific capacitance retention, and stable cycling performance at high current density. The production cost was reduced significantly. This finding can provide an important reference to explore the electrode materials from the peanut shells.