Synthesis,Property Tuning And Electrochemical Characterization Of Novel Heteroatom-doped Porous Carbon

With the huge economic and environmental problems caused by the consumption of non-renewable energy resource,the development of green materials and novel energy devices with high efficiency is one of the most important ways to solve this problem.Porous carbon materials with the advantages of environmental friendliness and low cost have been widely used in adsorption,energy storage and catalysis.Depending on their characteristics of high specific surface area,high stability,high electron transfer capacity,easy modification and controllable structure,heteroatom-doped porous carbon material has been proved to be a promising electrochemical material and successfully applied in supercapacitors and electrocatalysis.Till now,the improvement of porous carbon material focuses on the control of the pore structure and heteroatom doping,which are also the fundamental factors affecting the application properties.Researchers have developed various synthesis methods and doping forms of porous carbon.However,there are still some problems as follows in the reported methods that need to be improved or solved:1.The preparation process is usually tedious and high-cost,meanwhile,the pollution problem is profound.2.It is difficult to control the pore structure quantitatively through simple synthesis.3.Most of established methods of structure control are hard to extend to other synthesis systems.Therefore,it is of great significance to develop simple methods of preparing porous carbon materials with quantitatively controlled pore size.This thesis focuses on the efficient synthesis and flexible control of novel heteroatom-doped porous carbon materials with cheap raw materials.The effects of pore morphology and heteroatom doping on the application performance for capacitance and oxygen reduction catalysis were evaluated by systematic electrochemical analysis.This thesis contains six chapters.Chapter 1:The concept,classification,characteristics and applications of porous carbon materials were introduced briefly,in the senario of electrochemical analysis technologies and electrode materials.The research progress of porous carbon materials in synthesis,morphology control and doping was introduced emphatically.Chapter 2:A new method for quantitative control of the micropore size of porous carbon was developed by using extended?CN₂?_x structures as nano-modulators,and the capacitive property of the materials were greatly improved.This chapter consists of two sections:in the first section,the microporous carbon material?named as NC-T?with high specific surface area was prepared simply using melamine as the nitrogen source,furfural as the liquid environment as well as the supporting polymer,and potassium carbonate as the activator.By exploring the influence of carbonization temperature on the pore size of NC-T,it is concluded that temperature control had a great influence on the porosity but almost no effect on the pore size distribution.More importantly,we found that melamine sacrificed to form micropores,which inspired us to further study on micropore control.In the second section,urea,melamine,melem,melon and carbon nitride,including extended CN₂structures,were selected as the nitrogen sources and micropore nano-modulators.At the same carbonization temperature,the micropore sizes of product and nano-modulator were correspondingly increased.In addition,different nano-modulators mixing in different proportions,could flexibly control the distribution of micropores in the product.This method has the advantages of template method and activation method,and realizes the simple and quantitative control of micropore size and distribution.By predesigning the size of the nano-modulators,the pore diameter can be flexibly changed between 0.2 nm-2.3nm.The hydrophilicity and electrical conductivity of PC_?CN2?xCN2)x materials increased successively,especially the specific capacitance increased from 260 F g^-11 to 523 F g^-1(the current density was 0.2 A g^-1).Chapter 3:A template-free method was proposed to control the pore sizes at different dimensions by cascade self-assembling of multiple heteroatom dopants,and the capacitance of the materials increased with the change of pore hierarchies.Three heteroatom dopants including sodium sulfanilate,diphenylcarbazide and melamine were mixed and dispersed in furfural/potassium carbonate environment in different ratios.The mixture was directly carbonized by an one-step method to obtain HIPC-x materials.Due to differences in the number and variety of functional groups,three heteroatom dopants were assembled to form unstable assemblies with extended sizes in furfural induced by intermolecular force.Therefore,the pore morphologies were changed from micropore to micro-/mesopore,then to micro-/meso-/macropore,and the pore size could be controlled flexibly between 2 nm to 500 nm.Furthermore,by selecting different heteroatom dopants,doping form of N and S heteroatoms in the products can be optimized to improve the application performance.The specific surface area of HIPC-3 is up to 2420 m² g^-1,with the energy density of 16.1 Wh kg^-1at 100 W kg^-1.In addition,the HIPC-3 material exhibites excellent adsorption/oxidative removal properties of a variety of organic contaminations.Chapter 4:Based on the method of control the pore structure by localized assemblies of heteroatom dopants,a template-free method for constructing Fe-N co-doped mesoporous carbon sphere was developed and used to catalyze the oxygen reduction reaction.At begining,diphenylcarbazide and ferric chloride dissolved in ethanol and cooperate to form the reaction center,then added furfural to form polymeric microspheres.During carbonization,the unstable localized assemblies in the sphere decomposed to form mesopores,at the same time,the Fe-N complexes tended into FeN₄ activity sites with high catalytic activity.Finally,Fe-N co-doped mesoporous carbon sphere material named Fe-N/C-T was obtained.We studied the influencing factors on the pore structure,such as the addition ratio of nitrogen-compounds,carbonation temperature and heating rate.The obtained material has excellent catalytic performance in oxygen reduction reaction.The onset potential of the catalytic reaction was 0.93 V?vs.RHE?and the electron transfer number is close to 4.At the same time,Fe-N/C-T materials show much higher stability and outstanding tolerance toward methanol than commercial Pt/C catalyst.Chapter 5:Taking the aforementioned concept of simple synthesis and pore-control to the preparation of porous carbon using biomass,a method was developed to maintain the macroporous skeleton of carbon using cotton as the carbon precursor,meanwhile,the transformation of three-dimensional hierarchical pore structures to the carbon nano-onions encapsulated metallic cobalt core/shell structures was flexibly controlled.This chapter consists of two sections:in the first section,we found that the cotton aerogel could be transformed into porous carbon with macroporous skeleton,but collapsed at high carbonization temperature.In order to protect the macroporous structure,glutaraldehyde or aniline,self-polymerizable liquid reactant,was selected to polymerize on the inner and outer surface of the macropores of the aerogel to form a homogenous macroporous shell,which remained stable during carbonization.At the same time,micropores were introduced through activation to obtain hierarchical porous carbon materials.After the control of the pore morphology,the electrical conductivity,mass transfer performance and capacitance performance are improved obviously.In the second section,in order to increase the utilization of biomass raw materials,we added cobalt chloride in the reaction system.Finally,the three-dimensional hierarchical pore structure changed to the carbon nano-onions encapsulated metallic cobalt core/shell structure.The cobalt nanoparticles shown the dual functions of the catalytic graphitization of carbon and introduction of catalytic active sites.Using the obtained materials as the electrocatalyst of oxygen reduction reaction,the material shows the same catalytic activity as the commercial Pt/C catalyst,as well as higher toxicity resistance and stability.Chapter 6:This chapter summarized the work described in this thesis indicated the issues that need to be improved and proposed the future research direction.