Structure and electrochemical performances of porous biochar from rice straw at different temperatures

Abstract: A large amount of crop straw waste has been generated every year in China at present. The crop straw needs to be treated in an environmentally friendly way for the sustainable development of agricultural production. Fortunately, the porous carbon materials can be fabricated from the biomass crop straw as a carbon source in recent years. Among them, the porous biochar has been one of the most important electrode materials in the field of the electrochemical energy systems, due to the developed pore structure with a large specific surface area, abundant surface functional groups, and excellent electrochemical performance. But, there are only a few surface-active locations, non-developed pore structures, and low electrochemical performance of crop straw porous biochar prepared by one-step pyrolysis activation technology. In this study, a new preparation test of porous biochar was conducted at different activation temperatures ranging from 500-900 ℃. A microwave hydrothermal pretreatment was also used to prepare the precursor, i.e., the phosphoric acid hydrothermal carbon of rice straw. Meanwhile, a systematic investigation was made on the chemical composition, surface groups and defects, as well as the pore structures and electrochemical properties of porous biochar at different activation temperatures. The results showed that the yield of porous biochar from the rice straw hydrothermal carbon was reduced from 50.31% to 33.47%, with the increase of activation temperature, where the yield of gas increased, and no tar was formed after 800°C. The highest carbon content of porous biochar (74.09%) was achieved at 800 ℃ during treatment. The H/C and (N+O)/C values decreased in the porous biochar, as the activation temperature increased, indicating that the increasing temperature strengthened the aromatization structure of the porous biochar. Hydrothermal carbonization and KHCO3 activation introduced a large amount of O element and some active oxygen-containing functional groups into the porous biochar. Specifically, the surface of porous biochar presented some oxygen-containing groups, such as -OH and C-O-C coupling, and the nitrogen-containing groups, such as pyridine N-6, pyrrolin N-5, graphite nitrogen N-Q, and nitrogen oxide N-O. These functional groups were beneficial to the wettability of the porous biochar, further reducing the ion transfer resistance in the electrolyte solution. The disorder degree and the carbon defects first increased and then decreased in the pore biochar, as the activation temperature increased. The largest surface defect of pore structure in the porous biochar was obtained after the full reaction of KHCO3 with the carbon atoms at 800 ℃. An optimal condition was achieved for the adsorption, transport, and storage of electrolyte ions, with a specific surface area of 1 002.20 m2/g, a maximum total pore volume of 0.79 cm3/g, and a mesopore volume ratio of 45.57%. Correspondingly, the best porous biochar was obtained under the three-electrode and KOH electrolyte system at 800 ℃, indicating the largest specific capacitance, the best rate performance, and the lowest resistance. The specific capacitance at 1 A/g current density of porous biochar was 312.81 F/g. The energy density of the symmetrical capacitor was up to 10.73 W·h/kg at the power density of 228 W/kg. Meanwhile, the specific capacitance retention of the symmetrical capacitor was 95.82% at the current density of 10 A/g after the 5 000 cycles of charge-discharge, showing excellent circulating stability in the prepared porous biochar.