Preparation of N, P, S co-doped biomass porous carbon and its electrochemical properties

With the development of the economy and society, energy contradictions are becoming more and more serious, and ecological and environmental problems are prominent. It is especially important to build clean, efficient, safe, and sustainable energy. Supercapacitors have broad prospects in the field of energy storage due to their high power density, fast charge-discharge rate, and ultra-long cycle life. Biomass-based carbon electrode materials have attracted much attention due to their environmental friendliness, abundant natural resources, and special porous structures. Chinese fir sawdust is the waste generated during the Chinese fir processing and production process, which is usually buried or burned, easily causing environmental pollution. In order to enhance the high value-added utilization of agricultural and forestry wastes in the field of energy storage, this study used Chinese fir sawdust as raw material, melamine phosphate as phosphorus and nitrogen source, based on frozen NaOH/thiourea system to dissolve cellulose in wood-based raw material, which can effectively separate cellulose, hemicellulose and lignin, and make alkali enter into the interior of the wood more adequately, and cross-linking and restructuring of the active groups of melamine and lignocellulose raw material, which can regulate pore structure. At the same time, thiourea and melamine phosphate enter into the wood pore more evenly to achieve uniform and effective doping. Then, a biomass-based doped porous carbon material with developed pore structure was prepared by one-step pyrolysis, and the effects of activation temperature, NaOH/Chinese fir sawdust mass ratio, and freezing conditions on the structure and electrochemical properties of the porous carbon were investigated. The surface structure and pore structure of the material were studied through X-ray photoelectron spectroscopy (XPS) and specific surface area (BET). The electrochemical properties of the material were characterized using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). The results show that the surface area, total pore volume and specific capacitance exhibit a trend of first increasing and then decreasing with the increase of activation temperature and NaOH/Chinese fir sawdust mass ratio. Freezing conditions and the addition of melamine phosphate can increase the specific surface area and full pore volume of porous carbon and enhance the electrochemical performance. When the activation temperature is 900 °C, the NaOH/Chinese fir sawdust mass ratio is 1.2, the prepared nitrogen, phosphorus, and sulfur co-doped porous carbon has a specific surface area of 2 048 m²/g, and a total pore volume of 1.655 cm³/g, the mesoporosity is 99.7%, and the nitrogen, phosphorus, and sulfur contents are 3.41%, 0.29%, and 1.40%. In the three-electrode system, the specific capacitance of the NPS-900-1.2 electrode is 261 F/g at a current density of 0. 5 A/g and 235 F/g when the current density is increased to 10 A/g. The Coulombic efficiency and the capacitance retention rate of the symmetrical supercapacitor assembled with NPS-900-1.2 was close to 100% and 92% at the current density of 5 A/g after the 5,000 cycles, respectively. The energy density of the symmetrical supercapacitor could reach 17.2 Wh/kg at the power density was 248 W/kg, and the energy density can still maintain 12.5 Wh/kg when the power density reaches 4 509 W/kg.