Preparation Of High Mass Loading Of Biomass Derived Carbon Materials And Their Electrochemical Properties

Biomass-derived porous carbons（BDPCs）are one of the most studied electrode materials for supercapacitors（SCs）,which can be obtained from cheap,diverse and abundant renewable green resources.Existing activated carbon synthesis methods can obtain carbon with a specific surface area（SSA）as high as 3000～4000 m²/g and a high pore volume exceeding 2 cm³/g.However,high SSA alone does not guarantee the achievement of high electrochemical performance.The parameters such as pore size,shape and heteroatom content affect the surface accessibility of electrolyte ions,which affects the performance of carbon materials as SCs electrode materials.In this thesis,using abundant and renewable biomass as carbon precursor,three kinds of high mass loading BDPCs with different morphology（honeycomb,small cube and orange network）are prepared by water bath,pre-carbonization,hydrothermal and activation.The high mass loading BDPCs exhibit high capacitance,which meets the needs of SCs in practical application.The research contents of this thesis are as follows:1.Using gelatin as the carbon source and glutaraldehyde as the cross linking agent,a novel honeycomb like porous carbon has been successfully prepared through simple cross linking reaction and KOH activation at high temperature.The effects of glutaraldehyde on the morphology,graphitization degree and specific surface area of carbon materials are studied.The study found that the carbon material prepared by adding glutaraldehyde forms a dense and uniform honeycomb morphology,high SSA（1608.89m²/g）,which increased the abundant active sites;the contents of N and O in carbon skeleton increased from 3.70 at%and 10.69 at%to 8.41 at%and 16.69 at%,respectively,which provided abundant pseudocapacitance;the higher degree of graphitization（I_G/I_D=0.93）has improved the conductivity of the carbon material,making this honeycomb porous carbon show excellent electrical properties under high mass loading.When the mass loading of honeycomb like porous carbon is about 10 mg/cm²,the specific capacitance(C_sp)of Gel/G-1.0 is 392 F/g at current density of 1 A/g in 2 mol/L H₂SO₄,with high retention of 64%at current density of 10 A/g.More importantly,the carbon material exhibits a high energy density of 6.39 W h/kg with the power density of 50.03W/kg,even at the power density of 502.79 W/kg,the energy density remains 5.00 W h/kg.2.Using oat as precursor,through pre-carbonization and adding surfactants（CTAB,SDBS and SDS）for hydrothermal reaction,combined with KOH high-temperature activation,it can be transformed into high-performance electrode materials.Hydrothermal method can retain the oxygen element in the carbon source,and the prepared hydrothermal coke contains rich oxygen-containing functional groups;surfactants not only play an important role in changing the surface properties of products,such as morphology and structure,but also affect the hydrophobicity of the electrode surface,so as to improve the stability of the electrode.The carbon material prepared by the synergistic action between surfactant and hydrothermal method has rich heteroatoms,which improves the wettability of the surface of carbon material,makes it easier for electrolyte ions to infiltrate into the electrode,and greatly promotes the improvement of its electrical properties.The prepared carbon materials exhibited small cube morphologies of different sizes.By exploring the reasons for the formation of small cubes and the role of surfactants,it is speculated that the small cube is aγ-cyclodextrin-based metal-organic framework（γ-CD-MOF）,which is formed by the functional groups such as carboxyl and hydroxyl groups contained in biomass materials and K⁺in KOH to form H···OK coordination,and the presence of surfactants adjusts the particle size of small cubes.The obtained electrode materials exhibit ideal electrical properties under high mass loading due to their small cube morphologies of different sizes,high SSA,well-developed pore structure,and abundant heteroatoms.When the mass loading of the electrode material is 10 mg/cm²,and the current density is 1.0 A/g,compared with the carbon material without surfactant(C_sp is 333 F/g),the C_sp of the carbon material with CTAB is381 F/g,the C_sp of SDBS is 533 F/g,and the C_sp of SDS is 447 F/g.the addition of surfactant increases the content of O atom in carbon skeleton from 8.8 to 12.17 at%,O doping can change the interaction between carbon material and electrolyte cation,and redox reaction occurs in the process of charge and discharge to produce pseudo-capacitance,so as to improve the electrochemical performance of carbon material.The addition of surfactants makes the carbon materials show excellent cycling stability.At10.0 A/g,after 10000 cycles,the C_sp of OCHC is 80.06%of the initial capacity,OSHC is60.32%,and ODHC is 93.05%.More importantly,the prepared carbon materials exhibit excellent power density.The energy density of the sample ODHC is 30.28 W h/kg when the power density is 140.01 W/kg,and when the power density is increased to 2800.26W/kg,the energy density is 11.87 W h/kg.3.Using SDBS-modified oat as the precursor,KOH and K₂Fe O₄as the activators,respectively,the porous carbon materials with different morphologies are prepared by hydrothermal method and high temperature activated carbonization method.Among them,KOH has certain damage to the equipment during use,and K₂Fe O₄ is often used as a green disinfectant for drinking water,it is a green,environmentally friendly and non-polluting reagent,which can be decomposed into KOH and Fe substances,is a relatively mild activator and catalyst.The chapter discusses the effects of KOH and K₂Fe O₄ as activators respectively on the morphology,graphitization degree,SSA and pore structure of the prepared carbon materials.The study shows that,compared with KOH,the morphology of carbon materials prepared by K₂Fe O₄ as activator is orange network;the degree of graphitization（I_G/I_D increased from 0.74 to 1.25）aggrandized,SSA（1620.04m²/g to 1820.87 m²/g）and pore volume（0.76 cm³/g to 1.29 cm³/g）increased.In conclusion,compared with KOH,K₂Fe O₄ is able to convert biomass pyroltic carbon into graphitic porous carbon with high SSA and good electrical conductivity without destroying its porous structure.The mass loading of the active material is 10 mg/cm²,compared with KOH,the mass loading carbon material prepared with K₂Fe O₄ as the activator has excellent rate performance(the C_sp at 10.0 A/g retains 1.0 A/g 53.21%),and outstanding long-cycle stability（98.03%capacitance retention after 10000 cycles at a current density of 10.0 A/g）.To sum up,different activators are used,which makes the morphology,SSA and electrochemical properties of the prepared carbon materials different.Therefore,the choice of activator has a great influence on the preparation of carbon materials.