Preparation,Structure Optimization,and Electrochemical Performance Of Hollow Carbon Spheres With Few-layer Microporous Carbon/Graphene Framwork

Highly porous carbon materials have great potential applications in supercapacitors,lithium ion batteries and fuel cells due to their excellent properties such as high specific surface area?SSA?,high electronic conductivity,chemical and electrochemicalstability,andso on.Growing technologyrequiresthat supercapacitors have high energy density and power density.This demands that the carbon electrode material can store a large amount of charge?such as electrolyte ions?during charge/discharge,and the carbon framework can rapidly transfer sufficient charge?ions and electrons?.The design and preparation of nanostructured framework carbon materials with high density of effective micropores?adsorption sites?,high conductivity for electron transport and excellent pore connectivity becomes a key for improvement of elechemical energy storage.In addition,the porous carbon with above characters is used as non-noble ORR?oxygen reduction reaction?electrode material for fuel cells,and would improve synergistic catalytic activity,electrochemical corrosion resistance and cycle stability by loading of angstrom-sized catalysts?Fe cluster?and introduction of heteroatoms.Among carbon materials,porous hollow carbon spheres?HCSs?have advantages in regulation of pore and phase structure,doping modification,and so on.Therefore,the purpose of this thesis is to prepare hierarchically porousHCSs with few-layer graphene?FLG?framework,regulate poreand phase structure,modify by doping and study their electrochemical properties.The content of thesis mainly contains the following aspects:?1?Firstly,micropores were constructed in hypercrosslinked PS-DVB microspheres?HHPSs?bythecrosslinkingreactionusingsolid polystyrene-divinylbenzene?PS-DVB?microspheres as the precursor,an hydrous FeCl₃ as the catalyst.Secondly,the FeCl₃ was loaded in the micropores of HHPSs by an anti-solvent effect.Finally,hierarchically porous HCSs were obtained and nanometer-thick microporous carbon on FLG was constructed in the wall by carbonizing HHPSs with Fe-based catalysts,followed by removal of Fe-based salts.The formation mechanism of hollow structure of carbon spheres and FLG,and the influence of FeCl₃ loading mass,carbonization temperature,activation on FLG structure of HCSs,thickness of microporous carbon and pore connectivity were studied by TG-DSC,SEM,TEM,XRD,Raman,element mapping,EDS,and nitrogen adsorption/desorption.The results show that formation of hollow structure of carbon spheres is attributed to etching of Cl₂ from decomposition of FeCl₃ in the interior of HHPSs.Microporous carbon/FLG hereostructure derives from concerted catalysis of Fe clusters and the thickness of microporous carbon decreases with increase of loading mass of catalyst.The HCSs obtained at 700 ? have largest SSA,hierarchically porous structure,showing a specific capacitance of 256 F·g^-1at 5.0mV·s^-1.The HCSs activated by KOH indicate a high specific capacitance of 373 F·g^-1at 5.0 mV·s^-1,increased by about 50%due to good pore connectivity,and high micropore utilization.?2?In order to further reduce the wall thickness of HCSs,shorten the length of micropores,and increase the effective utilization of micropores,the solid PS-DVB microspheres were replaced by hollow PS-DVB capsules?HPCs?for synthesis of hierarchically porous thin-walled HCSs with FLG framework by concerted catalysis of Fe clusters.The effect of the crosslinking density of HPCs on FLG structure,microporous carbon thickness,and pore connectivity of HCSs was investigated by TG-DSC,SEM,TEM,XRD,Raman,element mapping,EDS,and nitrogen adsorption/desorption.The results show that the crystallinity and size of FLG in HCS shells decreases with the increase of crosslinking density of precursors.HCS20-D has specific capacitance as high as 561 F·g^-1 at 0.5 A·g^-1 with good cycling performance.It is important that the HCS20-D electrode with loading mass of 1.16 mg still remains high specific capacitance and excellent cycling stability due to large SSA?805 m²/g?,high micropore utilization,good electrical conductivity and pore connectivity.?3?HCSs with heterojunction had great potential as non-noble metal electrode materials in ORR due to high SSA,good conductivity and pores connectivity.Because Fe clusters with high catalytic activity were embedded in the carbon lattice of HCSs,demonstrating strong corrosion resistance in acidic environments.Therefore,catalytic activity,electrochemical corrosion resistance,and cycle stability could be improved by introducing heteroatoms which cooperate with Fe clusters.Here,Fe-N or Fe-N-S co-doped HCSs with heterojunction were prepared by treating Fe cluster-loading HCSs with N,S-containing substance mixture at high temperatures.The influences of catalyst content,carbonization and doping temperature on the content and chemical environment of Fe,Nand S in the HCSs were studied by SEM,XRD,Raman,EDS and XPS.And the ORR properties were also tested.The results show that Fe-N or Fe-N-S were successfully co-doped in HCSs with heterojunction at 700°C using urea/nitrogen as dopant,and the N or N-S content in the product increased with the increase of the amount of urea/thiourea.The iron-free hollow carbon spheres have excellent ORR performance with an initial potential of 1.04 V,a limiting current density of 7.82 mA·cm^-2 at 1600 rpm.