Controllable Praparation And Electrochemical Performance Study Of Biomass-Based Carbon Materials

As the vision and strategy target of peak carbon dioxide emissions before 2030 and carbon neutrality before 2060 are proposed,our country constantly increases the demand and support dynamics of clean and renewable energy,such as wind energy,water energy,tidal energy,and solar energy.However,these renewable energies are intermittent,ascribing to the influences of climate and environment.In order to achieve the high value utilization of clean energy,the energy storage equipment is needed to collect and stably output these energies.In this regard,the supercapacitor has a great potential to be applied in the fields of renewable energy storage and release due to its high power density,fast charging-discharging rates,as well as long cycle life.At present,the lower specific capacitance and lower energy density restrict the further development of supercapacitor.The electrochemical properties of supercapacitor is mainly determined by the basic performance of electrode materials,such as the content of heteroatoms,effective specific surface area,wettability,and the number of redox sites,etc.Therefore,how to prepare electrode materials with specific chemical composition and excellent structural characteristics is an urgent problem.Here,the elemental composition,microstructure and electrical conductivity of biomass based electrode materials were adjusted by means of physical doping,chemical modification,and photocatalytic reduction.Accordingly,the electrochemical properties of biomass-based carbon material are improved.As follows:（1）The presence of heteroatoms can not only provide pseudocapacitance,but also improve the porosity utilization rate of the electrode material by enhancing the hydrophilicity,thus enhancing the electrochemical properties.However,the loss of non-carbon elements in the carbonization process will cause the reduction of specific capacitance.In response to this problem,a method which using the immobilization of nitrogen and sulfur elements by grapheme,was proposed to increase the content of heteroatoms in carbon materials.The biomass material,i.e.lignin,is taken as the raw material to prepare the carbon nanofiber with high content of heteroatom by the process of graphene doping,electrospinning and carbonization.The heteroatom contents（N:11.98 wt%,S:0.70 wt%）in the carbon material could effectively increase as the adsorption and barrier effect of graphene sheets for gases such as HCN,NH₃ and H₂S from carbonization process.As a result,the hydrophilicity of the carbon material significantly increases,and the contact angle of water decreases from 128.9° to 97.5°.Moreover,the twisted graphene sheet could generate the defect structure in the carbonization process due to the stress concentration,leading to the increase in specific surface area(from 1008 m²g^-1 to 2439 m 2 g^-1,)of carbon nanofiber.The specific capacitance of of a single electrode assembled with this material is 267.32 F g^-1,and the energy density is 9.28 Wh kg^-1.（2）The contact resistance at the interface between fibers is larger,resulting in more loss of charges during the transfer and storage process,as well as the poor electrochemical performance.The macrostructure of carbon nanofiber is regulated by a molecular modification to improve the electrochemical properties of electrode materials.Firstly,the butyric anhydride is used for the esterification modification of lignin.The degree of freedom of lignin molecules increases by decreasing the intramolecular hydrogen bond formation,and the glass-transition temperature of precursor fiber decreases.And then,the esterified carbon nanofibers form the inter-fibers bonding at the thermal stability stage in the process of heat treatment.The structure builds more conductive networks and increases the conductivity of the electrode material.The testing results show that the Rs decreases by 64%,compared with that of the unmodified material,and the Rct decreases by 60%.Moreover,the specific capacitance significantly increases to 320.07 F g^-1.As for the supercapacitors assembled by this active material electrode,the energy density reaches 17.92 Wh kg^-1.Furthermore,the specific capacitance retention rate is still around 94.51%after 5000 cycles.The results suggest that the macroscopic properties can be effectively controlled by the microstructural regulation at the molecular scale.（3）The cellulose-based super absorbent resin with a larger molecular weight and higher crystallinity swells up in the methylene blue solution.As such,the charges attraction,hydrogen bonding force and π-π interaction between molecular chains of cellulose could be indirectly controlled,thus synchronously regulating the microstructure and heteroatom content of biomass-based carbon material.With the gradual increase of methylene blue concentration,the specific surface area of porous activated carbon material firstly increases and then decreases,the pore size gradually reduces,and the nitrogen/sulfur content steadily becomes higher,feasibly regulated in the ranges of 400～800 m²g^-1,1.69～2.22 nm,0～2.63/0～0.37 at.%,respectively.These characteristics expose more active reaction sites for electrode materials and accelerate the electrolyte ion transfer rate.Notably,as this active material is used for the electrode material,the specific capacitance is reaches 360.37 F g^-1,and the coulomb efficiency approaches 100%.In terms of the supercapacitors assembled by the active material electrode,the energy density reaches 24.58 Wh kg^-1.After 10000 ycles,the specific capacitance retention rate is 94.59%.（4）In order to further improve the energy storage performance of the supercapacitors,the flexible asymmetric supercapacitors is obtained with the pseudocapacitance composites and all-solid-state assembly way.Firstly,the hollow carbon micro-skeleton（HCMS）is prepared by sol-gel in combination with hard template method to increase the permeability of electrolyte ion.Combined with hydrothermal method,there generate the NiCo₂O₄ nanoflowers on the surface of carbon skeleton with the redox reactions,so as to provide a large amount of pseudocapacitance.In order to improve the charge transfer rate between HCMS and NiCo₂O₄,silver nanoparticles are in situ generated on the interface of HCMS and NiCo₂O₄ by photocatalytic reduction method.This method not only avoids the agglomeration of silver nanoparticles,but also reduces the problem of high interphase contact resistance of multicomponent composites.And hence,this three component composite electrode has typical pseudocapacitance,with a specific capacitance of 527.40 F g^-1.By asymmetric assembly,the energy density of the all-solid-state supercapacitors is 42.67 Wh kg^-1.Furthermore,the specific capacitance retention rate and the coulomb efficiency of this supercapacitors stays reaches 97.47%and 100%sfter undergo 5000 cycles,respectively.Besides,the all-solid-state supercapacitors also presents excellent mechanical flexibility and significant durability,especially,after any bending angle and multiple to-and-fro folding,its energy storage still remains stable.（5）In view of the complex process and the extra dopant is necessarry in the process of preparing high-performance electrode materials from traditional biomass,this chapter focuses on the precursors of generalized biomass and explores its application in the field of energy storage.By using the advantages of self-template and self-doping,the low specific capacitance of traditional biomass-based carbon materials due to simple pore structure and single heteroatomic species is solved.Here,The locust is used as the precursor.In dependence on the advantages of self-template and self-doping,the one-step activation carbonization method is adopted to obtain the activated carbon material with three-dimensional porous structure,doped with heteroatoms.Accordingly,this kind of electrode material has a larger specific surface area,and a higher doping amount of heteroatoms（O:11.4 at.%,N:6.9 at.%,S:0.5 at.%）.Its specific capacitance is up to 433.73 F g^-1,and the specific capacitance retention rate reaches 79%with a high current density of 10 A g^-1.The specific capacitance of the electrode is further increased to 633.51 F g^-1 by bimetallic oxide loading.After symmetric assembly,the energy density of supercapacitors is 23.04 Wh kg^-1.Besides,the specific capacitance retention rate remains at 93.17%.After asymmetric assembly,the energy density of A-LPCs@NiCo₂O₄-Ag//A-LPCs increases to 49.52 Wh kg^-1 at the power density of 400 W kg^-1.These are far higher than those of plant biomass-based electrode material.