Preparation And Electrochemical Performance Of Biomass Derived Three-dimentional Porous Carbon

With the rapid development of science and technology and the continuous improvement of people’s living standards,the role of electrical energy in the modern life has become increasingly prominent.As a new type of green and efficient electrochemical energy storage device,supercapacitors are very important for achieving sustainable energy supply of a new generation of clean and renewable energy.The superiority of their comprehensive performance mainly depends on the electrode materials.Carbon materials have been one of the top candidates for supercapacitor electrode materials due to their excellent electrochemical performance,the diversity in structure and function regulation,and the relatively low price.Compared with fossil resource precursors,carbon materials derived from cheap,readily available and renewable biomass display obvious advantages in terms of raw material supply,environmental pressure,preparation cost and adjustability of structure and performance,and have aroused widespread interest in the development and application of energy storage materials in recent years.In this thesis,wheat flour and the cultivating residues from tremella were used as raw materials for prapration of three-dimensional（3D）hierarchical porous carbon materials by exploring easy and controllable synthesis strategies.The microstructure and porosity characteristics,chemical composition and electrochemical behavior of the as-prepared carbons were characterized to probe the structure-function relationship.The results obtianed in this work provide basic experimental data and theoretical reference for facile,low-cost and controllable design and preparation of high performance carbon-based electrode materials from biomass.The main research contents of this paper are as follows:（1）The B/N co-doped three-dimensional hierarchical porous carbons were prepared by using wheat flour as carbon precursor,boric acid as a sacrificial template,activting agent as well as boron source,and urea as N dopant.According to the single factor experiment,it was found that when the mass ratio of boric acid to wheat flour was 10:2～10:3,the as-prepared doped porous carbons showed good double-layer capacitance.Further,urea was added as the nitrogen source and through the analysis of BET and XPS,it can be seen that when the mass ratio of boric acid/wheat flour/urea was 10:2:2,the sample BCN-2-2 has a good pore structure and high pore volume,a relatively high level of heteroatom doping and exhibited good electrochemical performance of 214 F g^-1 at 0.5 A g^-1,when the power density was 3000 W kg^-1,the energy density of the assembled supercapacitor device was 4.2 Wh kg^-1,and obtained great cycle stability.（2）Sponge-like carbon foam derived from wheat flour（WFCF）was prepared by steaming,foaming and high temperature carbonization with baking soda as chemical foaming agent and activator.A series of WFCF@Mn O₂ composite materials were further synthesized via in-situ carbothermal reduction method with varied mass concentration of KMn O₄ solution.The results of BET、XRD、SEM showed that the carbon foam displayed a honeycomb-like 3D hierarchical porous structure with a specific surface area of 670 m²g^-1 and mesoporosity of 65%.The manganese dioxide grown and deposited on the surface of WFCF carbon matrix is of birnessite type（δ-Mn O₂）,which showed desirable micro-nano structure with furry lawn-like micro-morphology.The electrochemical tests showed that the highest specific capacitance of WFCF@Mn O₂-2.0 was 208 F g^-1(57.8 m Ah g^-1),and the normalized specific capacitance based on Mn O₂ mass loading was 636 F g^-1.The kinetic parameter analysis indicates that the WFCF@Mn O₂-2.0composite electrode material exhibits a hybrid charge storage mechanism,which is contributed by a combination of capacitive effect and diffusion-dominated ion intercalation reactions.Impressively,an energy density of 20.3 Wh kg^-1 at 2282 W kg^-1 can be achieved for the assembled symmetric supercapacitor based on WFCF@Mn O₂-2.0electrode materials,and exhibited great cycle stability..（3）3D hierarchical porous carbon with high nitrogen-doping（7.78%）was prepared from spent culture substrate of white fungus by pretreatment with a mixture of Na OH/urea（with a mass ratio of 7:12）aqueous solution followed by high-temperature carbonization.The results of BET、XRD、XPS showed that the sample of BC-5-800 has a specific surface area of 1568 m²g^-1,total pore volume of 1.53 cm³g^-1 with mesoporosity up to 83%.When tested in a three electeode system,the BC-5-800 displayed a high specific capacitance of 278 F g^-1 at 0.5 A g^-1and it could still maintained at 230 F g^-1 under 10 A g^-1.In the two electrode test system,the energy density of BC-5-800 can reach 7.77 Wh kg^-1 at 1400 W kg^-1 and great cycle stability.（4）Inspired by the ancient yeast fermentation-blowing technique,3D porous carbon foam for self-supporting electrode materials was prepared by using wheat flour as raw material.Further,the electrochemical properties of the prepared materials can be largely improved by adding trace graphene（0.1%）as a conductive additive together with post-activation by different concentrations of KOH.The results of BET、XRD、SEM showed that the as-prepared carbon foam exhibits a good 3D porous structure and compressive strength yet light weight.After activation by KOH,the CFG-2.0 sample indicated a well-developed hierarchical porous structure with a specific surface area of 974 m²g^-1 and pore volume of 0.54 cm³g^-1.In addition,the obtained CFG-2.0 had effective self-doping of N and O（N:1.16%,O:8.89%）.In the electrochemical tests,the overall conductivity and capacitance of CFG-X samples modified by trace graphene have been greatly improved.As for the CFG-2.0,the charge transfer resistance(R_ct)was decreased to0.4Ωand the specific capacitance can reach 197 F g^-1 at 0.5 A g^-1after deducting the voltage drop.