Study On The Construction Of Nitrogen-rich Biomass-based Carbon-carbon Symmetrical Electrode Based On Green Alcoholization Method

Although fossil resources play a very important role in solving energy problems,the energy crisis and environmental problems caused by their excessive consumption cannot be ignored.It is an important means to solve the current crisis by gradually reducing the dependence in a planned way on non-renewable energy and developing renewable clean new energy and new energy storage system,which is also a common problem faced by the whole world.Supercapacitor is a new type of energy storage device,which has attracted the attention of researchers because of its good cycle stability,fast charge and discharge and high power density.Among many electrode materials for supercapacitors,carbon materials are difficult to meet the needs of cleaner production due to its complex technology and high cost,while biomass has the advantages of high abundance,renewable and low cost,and has great potential for development.Therefore,it is of great significance to use biomass waste as electrode material to prepare porous carbon with high specific surface area,appropriate pore size distribution and excellent conductivity,and assemble it into supercapacitors with high energy density.In this thesis,nitrogen-rich biomass soybean straw was used as the precursor to prepare soybean straw-derived porous carbon by pre-carbonization and mild activation,and assembled into a supercapacitor with high energy density,which realized the conversion from three-electrode system to two-electrode system.In addition,on the basis of green（Polyethylene glycol 400）liquefaction,the electrochemical performance is improved by introducing heteroatom doping and other strategies.The specific conclusions are as follows:（1）Using soybean straw from agricultural and forestry wastes as raw material,the layered porous carbon material was successfully prepared and controlled through multi-step carbonization and mild activation route.The pre-carbonization treatment could retain the vascular bundle structure in the soybean straw as a 3D template and create pores on the tube wall to facilitate the dispersion of the subsequent activator and improve the activation efficiency.The specific surface area of the obtained sample is controlled by adjusting the pyrolysis temperature,and reaches 2266.19 m²/g in the presence of a small amount of alkaline activator（mass ratio of soybean straw carbon:KOH=1:2）.Moreover,the abundant porosity and specific chemical structure of nitrogen and oxygen in soybean straw-based carbon materials are favorable for an better electrochemical behavior of carbon-based supercapacitor devices.In the three-electrode system,the optimal sample（SSC-700）exhibits the capacitance as high as 380.5 F/gat a current density of 0.5 A/g,and capacitance retention 73.97%after 10000 charge and discharge cycles,and produces a high energy density of 13.2 W h/kg at a power density of 52.03 W/kg.In the two-electrode system,the SSC-700-based symmetric supercapacitor exhibits a high energy density of 8.95 Wh/kg at a power density of 25 W/kg and are able to maintain 5 Wh/kg at 2500 W/kg Furthermore,carbon-based symmetrical supercapacitors also show good cycle capacity,with a capacity loss rate of 0.5%in 5000 cycles.（2）The interlayer nanoflower porous carbon was prepared in this thesis by coupling green and sustainable liquefied with in-situ doping using soybean straw as a biomass precursor.The pore structures of the obtained samples were reasonably controlled with non-toxic potassium citrate.The specific surface area of SSL-N/S-K-800 reaches 1756.74m²/g,and the microporosity is 75.38%.In the three-electrode system test,SSL-N/S-K-700 exhibits a specific capacitance of 220 F/g at a current density of 0.5 A/g.Moreover,in the two-electrode system test,the SSL-N/S-K-700 assembled symmetric supercapacitor exhibits an energy density of 11.24 Wh/kg at a power density of 400 W/kg.Notably,excellent cycling stability（capacitance retention>99.2%）is presented after10000 cycles.The superior electrochemical properties are attributed to the ion diffusion channels provided by the macropores and mesopores,and the abundant micropores can provide ion adsorption and desorption sites.Additionally,the synchronizing N S doping effectively enhances the pseudo-capacitance of the material.（3）It is important to maintain supercapacitor performance at high current density by regulating the micropore/mesopore balance and surface functional groups of biomass carbons.Here,the biomass porous carbon was successfully prepared from agroforestry waste,soybean straw by coupling liquefication in situ doping with green activation.Effective liquefaction and doping environment for non-metal N/S doping was achieved via green liquefaction using polyethylene glycol 400（PEG400）.The micropore/mesopore ratio,hydrophilic properties,and active sites of the carbon materials were regulated and enhanced by combing N in-situ doping and green activation with potassium citrate.The obtained soybean straw-based carbon with N doping（SSL-N₁）showed higher surface area（1698.9 m²/g）and mesopore ratio（78.15%）.The mesopore/micropore ratio shortened the ion adsorption distance and increased the ion surface accessibility.The specific capacitance of materials was 276.8 F/g at a current density of 0.5 A/g.Especially,the specific capacitance reached 216 F/g when the current density increased 20 times and the capacitance retention rate was 78.0%.Notably,the SSL-N₁//SSL-N₁ symmetrical supercapacitor generated an energy density of 10.14 Wh/kg when the power density reached 250.03 W/kg.This work explores a green,efficient and sustainable way for the high value-added utilization of soybean straw based on the relationship between the porous structure and surface structure of biomass-derived carbon materials and the electrochemical performance of supercapacitors.