Preparation And Electrochemical Properties Of Chitosan-oligosaccharide-based In Situ Nitrogen-doped Mesoporous Carbon Materials

Nitrogen-doped ordered mesoporous carbon has the advantages of high specific surface area,high pore volume,uniform and adjustable pore size,good electrical conductivity and stability and easy modified mesoscopic surface,making it have huge application prospects in catalysis,energy storage,biomedicine,electrochemistry and other fields.Chitooligosaccharides(COS,molecular weight?3200Da)can be used as carbon and nitrogen source precursor for preparing in-situ nitrogen-doped mesoporous carbon due to its rich sources and high carbon and nitrogen content.However,the molecular weight of chitosan oligosaccharides is relatively large,when the hard template is used to prepare mesoporous carbon,it is difficult to effectively enter the template pores due to its large molecular weight,resulting in poor pore size distribution of the prepared carbon material;When using soft membrane plates to prepare mesoporous carbon,the interaction force between COS and the template is weak due to the large molecular weight,so it is difficult to precisely control the nano-scale structure and morphology.It can be seen that the high degree of polymerization is the limitation that chitooligosaccharides are used as carbon and nitrogen sources and it is difficult to obtain mesoporous carbon with uniform and adjustable pore size distribution.Reducing the degree of polymerization of chitooligosaccharides is an effective method to prepare mesoporous carbon with uniform and adjustable pore size.See from the structure of chitosan oligosaccharides,it is found that the glycosidic bonds connected at positions 1 and 4 on the long-chain structure of chitosan oligosaccharides can be broken by auxiliary means.Among many methods,ultrasound cavitation and sulfuric acid catalysis are the easiest methods to reduce the degree of polymerization.Use the high energy generated in ultrasonic cavitation to degrade the COS polymer,break the glycosidic bond and generate free radicals,and use the strong catalytic hydrolysis of H₂SO₄ to break the glycosidic bond to reduce the molecular weight and polymerization degree of chitosan oligosaccharides,further achieve the purpose of preparing in-situ nitrogen-doped mesoporous carbon material with uniform pore diameter.Based on the above factors,the paper proposes a simple preparation method of in-situ nitrogen-doped ordered mesoporous carbon.Hard template is used to prepare in-situ nitrogen-doped ordered mesoporous carbon NOMC(us-x@sa-y)with uniform and adjustable pore size under the synergistic effect of ultrasound and H₂SO₄;Soft membrane plate and sol-gel method are used to prepare in-situ nitrogen-doped mesoporous carbon with uniform and adjustable pore diameter under ultrasound condition.N₂ adsorption/desorption,XRD,Raman,XPS,TEM and other characterization methods are used to study the structure,surface properties,micro morphology,and element composition of in-situ nitrogen-doped mesoporous carbon materials,and its application in supercapacitors was investigated.The specific research content is as follows:1.The preparation of chitosan-oligosaccharide-based in-situ N-doped ordered mesoporous carbon with uniform and adjustable pore size and its electrochemical performance as supercapacitors was investigated.Using biomass macromolecule COS as the nitrogen and carbon source and SBA-15 as hard template,the in-situ nitrogen-doped ordered mesoporous carbon material NOMC(us-x@sa-y)was prepared through the synergistic effect of ultrasound and sulfuric acid.The effects of ultrasonic time,H₂SO₄ content and their synergistic effect on the pore structure,apparent morphology,degree of graphitization,and element content of the in-situ nitrogen-doped ordered mesoporous carbon were studied.The research results show that by controlling the sulfuric acid content and the ultrasonic time of the COS solution,the pore size and specific surface area of the in-situ N-doped ordered mesoporous carbon material can be uniformly adjusted.When the ultrasonic time of COS solution is increased from 0 min to 60 min,the pore size of NOMC(us-x@sa-0.5)is uniformly controllable between 2.5 nm?4.5 nm,and the specific surface area is also within the range of 511.69 m²g^-1?1085.98 m²g^-1 The ratio of mesoporous pore volume has also increased from 65.48%to 99.07%.Research on the application performance of supercapacitors shows that the performance of NOMC(us-x@sa-0.5)increases first and then decreases with the increase of ultrasonic time.When the COS solution is ultrasonicated for 30 minutes,the capacitance of NOMC(us-30@sa-0.5)is high,when the current density is 0.5 Ag^-1,the capacitance reaches 430 Fg^-1.After 5000cycles of the current density of 10 Ag^-1,the specific capacitance of this material drops from the initial 168 Fg^-1 to 160 Fg^-1,with only 5%capacitance loss and 95%capacitance retention.It has excellent cycling ability and stability.2.The preparation of chitosan-based N-doped mesoporous carbon materials by s oft membrane method and its application on supercapacitors.Using COS as the carbon source and F127 as the soft template under the condition of ultrasound,the mass ratio of F127/COS,synthesis temperature and ultrasound time were adjusted by self-assembly hydrothermal method to prepare in-situ nitrogen-doped mesoporous carbon with uniform and adjustable pore size material NMC-n.The research results show that by controlling synthesis temperature and ultrasonic time,the pore size of the in-situ N-doped ordered mesoporous carbon material can be uniformly adjusted.When the synthesis temperature rises from 50?to 70?,the pore size of NMC-y is uniformly controllable between 3.37 nm?6.47nm,and the pore volume is 0.16 cm³g^-1?0.37 cm³g^-1;when the ultrasound time is increased from 0 min to 30 min,the pore size of NMC-z is uniformly controllable between 3.37 nm?7.88 nm,and the pore volume is 0.12 cm³g^-1?0.37 cm³g^-1.When the F127/COS mass ratio is 1:1,the synthesis temperature is 60?,and the ultrasonic time is 15 min,the pore structure parameters of the carbon material NMC are the best,the pore size,pore volume and specific surface area are 3.37 nm,0.37 cm³g^-1 and370.19 m²g^-1 respectively.This is because under the action of ultrasonic cavitation,the COS glycosidic bond is broken to form a low degree of polymerization and uniformly dispersed into the solution,so that the interaction between COS and F127 is enhanced.In the process of hydrothermal reaction and aggregation,the collision probability of COS and F127 was increased due to the polymer chain becomes shorter,which makes the Gibbs free energy in the whole polymerization process tend to the lowest value,and the reaction is more stable.The application performance of the carbon material in supercapacitors shows that when the current density increases from 0.5 Ag^-1 to 20 Ag^-1,the capacitance drops from 179 Fg^-1 to 82 Fg^-1.After 5000cycles at a current density of 10 Ag^-1,the specific capacitance increased from the initial 101 Fg^-1 to 104 Fg^-1,which increased the capacitance by almost 3%and reached a Coulomb efficiency of 103%.3.Study on the electrochemical performance of in-situ N-doped mesoporous carbon materials with rigid framework prepared by sol-gel method and their application as supercapacitors.Using silicate oligomer(TEOS)as the inorganic precursor,F127 as the soft template,COS as the nitrogen and carbon source,under the action of ultrasound,the in-situ nitrogen doped mesoporous carbon material with uniform pore size(COS-NMC-x)of the rigid skeleton structure is prepared by three-component co-assembly by the sol-gel method.The effect of the ultrasonic time to COS precursor solution on the pore structure,apparent morphology,degree of graphitization,and element content of mesoporous carbon materials was studied.The research results show that when the ultrasonic time of COS solution is increased from 0 min to 45 min,the pore diameter of COS-NMC-x changes from 3.0 nm to 3.8 nm,and the specific surface area changes from 75.5 m²g^-1 to 114.9 m²g^-1.When the COS solution was sonicated for 15 min,the specific surface area and pore volume of the prepared COS-NMC-x were the largest,reaching 145 m²g^-1 and 0.2 cm³g^-1,respectively,and the nitrogen doping content also reached 7.59%.Research on the application performance of supercapacitors shows that the performance of COS-NMC-x increases first and then decreases with the increase of ultrasonic time.When the COS solution is ultrasonicated for 15 min,the capacitance of the prepared COS-NMC-15 is highest,and when the current density is 0.5 Ag^-1,The capacitance is 189 Fg^-1.After 5000cycles of the current density of 10 Ag^-1,the specific capacitance increased from the initial 105 Fg^-1 to 120 Fg^-1,which increased the capacitance by almost 14%and reached the Coulomb efficiency of 114%.