Wood-Derived Doped Activated Carbon Based On Alkali/Urea Solvent Pretreatment And Its Electrochemical Performance

With the development of the economy and society,energy contradictions are becoming more and more serious,and ecological and environmental problems are prominent.It is especially important to build clean,efficient,safe and sustainable energy.Supercapacitors have broad prospects in the field of energy storage due to their high power density,fast charge-discharge rate,and ultra-long cycle life.Biomass-based carbon electrode materials have attracted much attention due to their environmental friendliness,abundant natural resources,and special porous structures.In this paper,based on the dissolution of cellulose by the alkali/urea solvent,wood raw materials and cellulose as carbon sources,urea and thiourea as doping sources,melamine and melamine phosphate as auxiliary（dopant）materials,and alkali as an activator.Doped activated carbon products with developed specific surface area,and introduced conductive polymers,metal sulfides,etc.for compounding.The elemental composition,chemical properties and microcrystalline structure of cellulose during the dissolution process,as well as the pore structure of doped activated carbon,the content of heteroatom elements and the change of binding state,and the cross-linking mechanism of doped organic compounds and cellulose are clarified.It aims to reveal the dissolution mechanism of cellulose in wood materials,the cross-linking reaction mechanism of cellulose and doped organic compounds,and the mechanism of in-situ doping.Novel carbon materials with high electrochemical performance and multifunctionality provide the scientific basis.The details are as follows:（1）Nitrogen-doped activated carbon（NDAC）was prepared by a melamine-assisted Na OH/KOH/urea water solvent using Chinese fir sawdust as raw material,and the effects of activation temperature and melamine dosage on the performance of NDAC were studied.The results showed that the specific surface area(S_BET)and nitrogen content of NDAC were greatly improved compared with those without the addition of melamine,from 1040 m²g^-1 and 3.52%to 1382 m²g^-1 and 5.16%,respectively.Under the conditions of activation temperature of 850°C and melamine of 10 g,the NDAC electrode material has a specific capacitance as high as 196 F g^-1 at 1 A g^-1,capacitance maintained after5000 cycles at 5 A g^-1 and the symmetric supercapacitor exhibits an energy density of 9.7 Wh kg^-1 at a power density of 261.44 W kg^-1.（2）Based on green and low-carbon development,using Chinese fir sawdust as raw material,melamine solid waste（oxhydryl and amino triazine,OAT）as the nitrogen-rich source,dissolving cellulose in wood raw materials based on alkali/urea solvent,and preparing nitrogen-doped activated carbon by one-step pyrolysis.The effects of activation temperature and OAT dosage on the adsorption performance and electrochemical performance of activated carbon were investigated.The results show that with the increase of OAT mass fraction,the yield and adsorption performance of activated carbon first increase and then decrease.The addition of OAT is beneficial to improving the yield,nitrogen content,adsorption performance and electrochemical performance of nitrogen-doped activated carbon.When the activation temperature was 900°C,the mass fraction of OAT was 15%,the yield of nitrogen-doped activated carbon was 34.2%,the adsorption value of iodine was 1116 mg g^-1,the adsorption value of methylene blue was 165mg g^-1,the specific surface area was 1324 m² g^-1,nitrogen content is 3.5%,specific capacitance up to 193 F g^-1.（3）Using Chinese fir sawdust as raw material,and dissolving cellulose based on Na OH/thiourea solvent,N and S co-doped porous carbon（NSAC）with high specific surface area was prepared.N,S co-doped porous carbon/polyaniline composite（NSAC-PANI）was prepared by solution in-situ polymerization method using N,S co-doped porous carbon（NSAC）as the substrate.The results show that the polyaniline can be uniformly attached to the surface of the N,S co-doped porous carbon in a fluffy state.The porous carbon has a high specific surface area and N,S active sites,which is beneficial to the aniline monomer through electrostatic attraction andπ-πstacking acting an load on NSAC.PANI-2sample has high specific capacitance(304 F g^-1),assembled symmetric supercapacitor with the power density of 395 W kg^-1 and energy density up to 23.6 Wh kg^-1,cycling stability of 90.2%capacitance retention after5000 cycles.（4）The N and S co-doped porous carbon-Ni Co₂S₄ composite（NSAC-Ni Co₂S₄）was prepared by hydrothermal treatment of NSAC with nickel and cobalt nitrates,and the influence of nickel-cobalt ratio on its morphology and electrochemical performance was investigated.The results show that when the ratio of nickel to cobalt is 1,NSAC-Ni Co₂S₄-2exhibits a micro/nanoparticle spherical shape and uniformly adheres to the surface of the carbon matrix,and has a specific capacitance up to 762F g^-1.Using the NSAC-Ni Co₂S₄-2 sample as the positive electrode and NSAC as the negative electrode,an asymmetric supercapacitor was assembled,and the energy density was as high as 36.8 Wh kg^-1 at the power density of 401 W kg^-1,cycling stability of 88.9%capacitance retention after 5000 cycles.（5）Using cellulose as raw material,Na OH/thiourea aqueous solution as cellulose solvent,activator and N,S source,and melamine phosphate as N,P source,high mesoporous multi-component co-doped activated carbon was prepared by one-step pyrolysis method and used in the field of supercapacitors.The results show that the synthesized activated carbon has an extremely high mesoporosity（98.26%）and co-doping properties of N（3.78 at%）,P（0.32 at%）and S（3.36 at%）.The high mesoporosity carbon material performs an excellent specific capacitance of 314 F g^-1 at the current density of 1 A g^-1,the energy density is 16.4 W h kg^-1 at the power density of 492 W kg^-1 and cycling stability of 91%capacitance retention after 5000 cycles.（6）During the dissolution process,the doping organic compounds infiltration the interior of the cellulose in the wood material,and produce a cross-linking reaction with the cellulose,which is conducive to the formation of a stable structure on the surface of the activated carbon material in situ and increases the doping amount of the activated carbon product.Compared with the samples without adjuvant,adding melamine and melamine phosphate helps to improve the specific surface area and electrochemical performance of the product.The Na OH/thiourea solvent has a stronger dissolving ability than the Na OH/urea solvent,resulting in a more obvious etching effect and more developed pores of the carbon material.