Synthesis Of N-doped Carbon Composites And Their Applications In Oxygen Reduction,Supercapacitors And Li-S Batteries

Recently,the widely applications of carbon nanomaterials significantly facilitate the advancement of materials science.Carbon nanomaterials endowed with excellent physical and chemical properties including good electronic and ion conductivity,chemical stability,thermal stability and biocompatibility are originated from rigid carbon skeleton and high strength covalent bonds.These properties make them a special and unique choice for various applications in divergent fields such as energy storage and conversion,biomedical,heterogeneous catalysts,chemical sensing,gas adsorption/separation and drug delivery.To further explore the advantages of carbon materials,functionalizing it is an excellent choice.Anisotropic and synergistic effects arising from the heteroatom doping,heterostructure and composite structure show high potential especially in electrochemical energy storage and conversion including supercapacitors,oxygen reduction reaction and batteries.Here,various kinds of functionalized carbon nanomaterials including nitrogen doped porous carbon nanosheets,defects site engineering carbon nanofibers and Ni nanoparticles embedded in carbon nanofibers were designed and prepared and we studied their electrochemical properties.The detailed research content was as follows:(1)We report the synthesis of oxygen-containing porous nitrogen-doped carbon(ONC)nanosheets through the carbonization of a mixture that contained glucose and dicyandiamide(DCDA).In situ formed graphitic carbon nitride(g-C3N4)derived from DCDA provided a nitrogen-rich template,thereby facilitating the formation of ONC nanosheets.The resultant ONC materials with high nitrogen content,high specific surface areas,and highly mesoporous total volume displayed excellent electrochemical performance,including a similar ORR onset potential,half-potential,a higher diffusion-limited current,and excellent tolerance to methanol than that of the commercial Pt/C catalyst,respectively.(2)we reported a facile strategy for the large-scale synthesis of nitrogen doped porous carbon via carbonization of core-shell structured protonated g-C3N4@polypyrrole(P-g-C3N4@PPy)nanocomposites.Polypyrrole provided both the carbon and heteroatom sources,while P-g-C3N4 acted not only as a sacrificial template but also as nitrogen sources.Porous carbon derived from P-g-C3N4@PPy(denoted as PCN@PPy-C)exhibited a specific capacitance of 350 F g-1 at a current density of 5 A g-1 and a high-rate capability(320 F g-1)aqueous solution,as well as good long-term stability after 1000 cycles.In addition,PCN@PPy-C also displayed excellent electrocatalytic performances in ORR in aqueous solution.(3)the preparation of NiO nanoparticle-dispersed electrospun N-doped porous CNF(NiO/PCNF)and as free-standing flm electrode for high-performance electrochemical supercapacitors is reported.Polyacrylonitrile and nickel acetylacetone are selected as precursors of CNF and Ni sources,respectively.Dicyandiamide not only improves the specifc surface area and pore volume,but also increases the N-doping level of PCNF.Benefting from the synergistic effect between NiO nanoparticles(NPs)and PCNF,the prepared free-standing NiO/PCNF electrodes show a high specifc capacitance of 850 F g-1 at a current density of 1 A g-1 in 6 m KOH aqueous solution,good rate capability,as well as excellent long-term cycling stability.(4)carbonyl group functionalized porous carbon nanofibers assembled with nickel(Ni/PCNFO)are proposed to serve as an efficient sulfur host in Li-S batteries.Such a Ni/PCNFO-S composite cathode exhibits outstanding electrochemical performances,which are attributed to three factors:1)the large inner space of the PCNF can afford a high S content and accommodate the volume expansion;2)high electrical conductivity is provided by the carbon nanofiber skeleton and the electrocatalytically active Ni species embedded in the PCNF significantly facilitate the redox kinetics of the S species;and 3)the carbonyl group anchored on the Ni/PCNF can effectively suppress the polysulfide effect via strong chemical affinity/adsorption with polysulfides.With these advantageous features,the Li-S batteries based on Ni/PCNFO-S cathodes exhibit a high specific capacity(1320 mA h g-1).excellent rate capability(780 mAh g-1),and long cycling stability(1070 mA h g-1 after 100 cycles at 0.2C).