Controllable Preparation Of Mesoporous Carbon Spheres And Their Applications In Supercapacitors

Since the 21st century,the global energy crisis has become increasingly serious.How to realize the conversion and storage of energy and promote the sustainable development of human society has become an important issue.Supercapacitor is a new type of energy storage device.Due to the advantages of fast charge and discharge speed,high power density,long cycle life,low cost and environmental protection,it has received more and more attention in recent years.At present,the selection and optimization of electrode materials are the core issues in the research and development of supercapacitors.Carbon materials,as one of the earliest electrode materials for supercapacitors,have high electrical conductivity,excellent cycle stability,and a wide range of raw material sources.Among the carbon electrode materials with various morphologies,porous carbon spheres stand out due to their regular morphology,good fluidity,large specific surface area,and controllable internal structure.They have become research hotspots in the field of supercapacitors.The carbon-based electrode mainly relies on the contact interface between the carbon material and the electrolyte to perform the adsorption/desorption of electrolyte ions,thereby storing and releasing the charge,and finally generating an electric double layer capacitor.Therefore,one of the main methods for increasing the capacitance of the carbon material is to increase its surface area,thereby increasing the interface contact between the material and the electrolyte ions.The introduction of 2-50 nm mesopores in carbon spheres is an important method to increase the contact area between the interface and electrolyte ions.Secondly,the cavity is made inside the carbon spheres,that is,the carbon spheres made of synthetic hollow structure or yolk-shell structure can also increase its surface area.However,the optimization of electrode performance only by increasing the surface area of the material has certain limitations.First,an increase in the surface area of the carbon material introduces additional resistance.In addition,as the porosity of the carbon material is increased,its stability is also greatly reduced,and it may even cause a collapse phenomenon inside the material.Recent studies have shown that the introduction of heteroatoms（such as N）into the carbon skeleton can enhance the conductivity and wettability of carbon materials,and the doping of N atoms can also increase the redox plutonium capacitance,thereby enhancing the capacitance of supercapacitor carbon electrodes.There is no doubt that the dual synergistic effect of increasing the specific surface area and introducing N atoms can also improve the electrochemical performance of carbon materials.The main research contents of this thesis are as follows:1)Resorcinol/formaldehyde polymer resin spheres（RF）with different sizes were obtained using the improved St?ber method.After carbonization under N₂,solid carbon spheres（CS）with different particle sizes were obtained.Among them,520 nm CS showed uniform morphology and maximum specific surface area.When used as an electrode material for supercapacitors,it shows excellent performance.2)During the polymerization of RF resin,the catalyst was simply adjusted to ethylenediamine,and the in-situ doping of N atoms was achieved by a one-pot method to obtain NRF.After carbonization,N-doped mesoporous carbon spheres（NCS）was finally obtained.The prepared NCS has high specific surface area,certain amount of N doping,and uniform pore size distribution.Its excellent capacitance performance and rate performance are highlighted in the electrochemical test.3)On the basis of NRF,a novel“confined-pyrolysis”strategy was proposed,and N-doped hollow carbon spheres（NHCS）were directly and simply obtained.The obtained NHCS has clear hollow structure,micro/mesoporous structure and high N content.It also exhibits excellent capacitance performance and cyclic stability in supercapacitors.4)Using n-hexane-assisted silica“confined-pyrolysis”strategy,RF underwent expansion-contraction-pyrolysis-deposition process to finally obtain yolk-shell structure mesoporous carbon spheres（YCS）with active core.In the BET test,YCS showed a uniform mesoporous structure and a large specific surface area.In subsequent electrochemical tests,it also showed its excellent capacity,rate performance,and cycle life.This article provides a simple,green,template-free method to adjust the internal structure of mesoporous carbon spheres from solid to hollow and core-shell structures.