Heteroatom Doped Hollow Carbon Spheres With Hierarchical Porous Structure:Fabrication And Electrochemical Energy Storage Performance

Porous carbon materials are widely used as electrode materials in supercapacitors due to the abundant porous structure,good thermal stability and controllable morphology.The physical and chemical properties of carbon materials have an important influence on their performance.The capacitive properties of materials can be enhanced by doping heteroatoms which can improve hydrophilicity and pseudocapacitance.As known to all,the pore structure of the material also paly a crucial role in the performance of capacitance and the single channel structure cannot meet the demand of development.In this paper,heteroatoms doped hollow carbon spheres with hierarchical macro-meso-microporous structure has been prepared through co-assembly of melamine-formaldehyde resin and silica nanospheres.silica nanospheres not only acted as a pore forming agent for macropores,but also played an important role in the formation of hollow structures.The effect of doped heteroatoms on the capacitance of the material was also investigated.The co-doped atoms are beneficial to enhance the capacitive performance of the material due to the synergetic effect.Owing to the hierarchical porous structure,the material possessed an excellment rate performance even at high current density.The details are as follow:(1)Nitrogen-doped hollow carbon microspheres(NHCMs)have been fabricated rapidly in 15 minutes through co-assembly of melamine formaldehyde resin and silica nanospheres.The use of silica nanospheres affords the formation of spherical carbon architectures covered in macropores.The obtained NHCMs possess large hollow cores and hierarchical porous shell with interconnected macropores of 250 nm and a broad micro/meso pore size ranging from 1.5 to 10 nm.The NHCMs carbonized at different temperatures exhibits high surface area of 216-659 m2g-1 with a large pore volume of 0.19-0.46 cm3g-1 and high nitrogen content(2.35-17.53%),which manifesting a good capacitors capability(106F g-1 at 0.5 A g-1)and stability(95%retention after 1000 cycles)due to its unique architectures.(2)Nitrogen-phosphorus co-doped hierarchical porous carbon microsphceres(NPHCMs)have been prepared through combining polymerization of melamine-formaldehyde resin with co-assembly of colloidal silica nanosphercs by using melamine and formaldehyde as carbon and nitrogen source.1-hydroxyethylidene-1.1-diphosphonic acid as catalyst and phosphorus source.After carbonization and silica removal,the obtained NPHCMs possess high surface areas of 563-720 m2 g-1 with micropores,bi-model mesopores(2.6 and 3.7 nm)and macropores(250 nm)distributed in the shell.Owing to the incorporation of abundant N(7.0-15.35%)and P(0.09-0.16%)in the carbon matrix and the interconnected micro-,meso-,and macroporous structure,the NPHCMs exhibit good capacitive performances.The 800?-carbonized sample shows a high specific capacitance and good rate capability(200 F g-1 at 0.5 A g-1,132 F g-1 at 20 A g-1)in 6 M KOH electrolyte.The all-solid-state electrode fabricated by the NPHCMs present high specific capacitance of 180 F g-1 at 0.5 A g-1 and long cycle life with 91%capacitance retention after 5000 cycles at 0.5 A g-1 in a polyvinyl alcohol/KOH gel electrolyte,suggesting its great potentials in the development of high-performance energy storage devices.(3)Nitrogen-rich sulfur-containing hollow carbon spheres with open macroporous structure and wrinkled surface were fabricated through co-assemble of melamine resin and silica nanospheres by using methionine as catalyst and sulfur source.The obtained materials possessed a high nitrogen content(14.16-16.59%)anda controllable sulfur doping level(0.06-0.23%),as well as high specific surface area(600-733 m2 g-1).Benefited from the enriched doped heteroatoms and hierarchical micro-,meso-,macroporous structure,the as-prepared materials show good capacitive performance with a high specific capacitance of 230 F g-1 at 0.5 A g-1 and a good rate capability(80%,from 1 to 10 A g-1)in 6 M KOH electrolyte.In addition,the carbons investigated by two-electrode system with polyvinyl alcohol/KOH gel electrolyte exhibited a high capacitance of 208 F g-1 at 0.5 A g-1 and long-term stability with 94%capacitance retention after 5000 cycles,showing their promising application in energy storage field.