Preparation And Electrochemical Performance Of Carbon-based Composites Based On Holocellulose

With the growing shortage of fossil energy and deteriorating environmental conditions, it is of great significance to exploit renewable biomass resources and transform them into functional materials with high performance. As an important part of biomass, the development and utilization of forestry residues have been widely concerned. Therefore, it is no doubt that transforming forestry residues into carbonaceous materials and then developing them into electrode material in supercapacitor is an efficient path to improve the use efficiency and additional value of forestry residues. It can not only expand the application area of forest resources, but also contribute to the cross-disciplines fusion. Besides, it is of great significance in ensuring the ecological safety and energy safety, reducing environment pollution, promoting energy saving and emission reduction, and establishing resource-conserving society.In this research, the author took holocellulose which is derived from forestry residues as the raw materials, adopted hydrothermal carbonization to produce carbon sphere with good dispersibility and uniform sizes. Thermal treatments and composite treaments were carried out on hydrochar. As a result, the carbonaceous materials with rich oxygen-containing functional group C=O, carbonaceous materials with graphite structure and pore structure as well as the carbon sphere/iron oxide composites with core-shell structure were obtained, and were applied to the electrode material in supercapacitor. The structures and properties of the materials were analyzed and summarized by using various characterization and testing methods. The following conclusions can be drawn from this research:(1) Taking the wood fiber and holocellulose as the precursors, the wood fiber-derived hydrochar and holocellulose-derived hydrochar were obtained by the method of hydrothermal carbonization. The influence of removal of lignin on the structure properties, chemical properties and thermal properties of the hydrochar was investigated. The results show that the initial carbonization temperature of holocellulose is 210℃, which is lower than 220℃ of wood fiber, and the chemical constituents of holocellulose-derived hydrochar are more simple, and the thermal stability are higher.(2) The author took holocellulose as precursor, and adopted hydrothermal carbonization to produce carbon sphere with good dispersibility and uniform sizes. The influence of carbonization temperature and reaction time on crystalline structure, microstructure, chemical constituents, elementary composition and thermal stability of products were investigated. The results show that with the increasing of temperature or prolong of reaction time, the crystalline region of holocellulose disappeared and was transformed into amorphous carbon sphere with uniform size and good dispersibility gradually, the degree of carbonization was higher gradually, and the thermal stability was rising gradually. When t=8h, the initial carbonization temperature of holocellulose is 210 ℃. When T=220℃,the initial carbonization time of holocellulose starts from 6h. When T=230℃ (t=6h) or t=14h (T=230℃), the chemical constituents were tending towards stability, and there were rich oxygen-containing functional groups such as hydroxyl and carbonyl on the surface.(3) Taking holocellulose-derived hydrochar as the treatment object, the author carried out low-temperature treatment on them in air atmosphere, as a result, carbonaceous materials with rich oxygen-containing functional groups such as C=O were acquired. The results show that the contents of the oxygen-containing functional groups such as C=O increased along with the temperature increasing. The author also found that the specific capacitance of carbonaceous materials acquired in air atmosphere was first increased and then decreased along with the increasing of temperature, wherein the electrochemical performance of carbonaceous material acquired in 250℃ was the best, and its specific capacitance was 83.15 F/g when the current density was 0.2 A/g.(4) Taking holocellulose-derived hydrochar as the treatment object, the author carried out high-temperature treatment on them in nitrogen atmosphere, as a result, the carbonaceous materials with graphite crystal structure and pore structure was acquired. The results show that the graphite crystallization and degree of ordering of the carbonaceous materials increased with the increasing of the temperature. The specific surface area and micropore volume of the products were first increased and then decreased along with the increasing of temperature, wherein the values of pecific surface area and micropore volume of H-N-800 reached maximum value,429.4 m2/g and 43.59%, respectively. The results of analysis of the electrochemical performance showed that the specific capacitance of carbonaceous materials acquired in nitrogen atmosphere was first increased and then decreased along with the increasing of temperature as well, wherein the carbonaceous materials acquired in 800℃ had the best specific capacitance values and rate capability, and cycle performance was stable. Its specific capacitance was 198.12 F/g when the electric current density was 0.5 A/g; The specific capacitance reduced by 50.3% when the electric current density was increased from 0.5 A/g to 5 A/g, which showed an excellent rate capability. The carbonaceous materials obtained at 800℃ shows its great application in supercapacitor electrode material.(5) The author took the carbon sphere acquired in air atmosphere under 250℃ condition as the matrix, adopted hydrothermal method and produced carbon sphere/Fe2O3 composite materials with core-shell structure. The specific capacitance of composite materials was higher than that of Fe2O3 particles, and it was rising along with the increasing of addition of carbon sphere. The electrochemical performance of C3/Fe2O3 composite material was the best when the addition of carbon sphere was 0.05g. The specific capacitance of C3/Fe2O3 composite material was 104.38 F/g when the discharge current density was 0.4A/g, which was 2.64 times that of pure Fe2O3 particles, but the cycling stability of composite materials was lower than that of pure Fe2O3 particles, which needs to be further improved.(6) The author took the carbon sphere acquired in air atmosphere under 250 ℃ condition as the matrix, adopted chemical coprecipitation method and compounded carbon sphere/Fe3O4 composite materials with core-shell structure. The specific capacitance of composite materials was higher than that of Fe3O4 particles, and it was rising along with the increasing of addition of carbon sphere. The electrochemical performance of C3/Fe3O4 composite material was the best when the addition of carbon sphere was 0.26g. The specific capacitance of C3/Fe3O4 composite material was 174.1 F/g when the scan rate was 5 mV/s, which was 1.37 times that of Fe3O4 particles, but the cycling stability of carbon sphere/Fe3O4 composite materials needs to be further improved.