Synthesis Of Heavily Nitrogen-doped Carbon-based Materials Via Space-confined Pyrolysis Of Transition Metal Cyanamide Compounds And Their Applications

Nitrogen-rich carbon and its composites have important applications in the fields of energy storage and catalysis.However,it is extremely challenging to prepare these materials efficiently.The traditional preparation methods mainly include the pyrolysis of nitrogen-rich organic polymer,chemical vapor deposition in ammonia atmosphere,post-treatment with amine,and so on.In these processes,problems arise including low utilization rate of raw materials,high consumption of ammonia gas,and poor consistency of chemical composition and microstructure of the final products.Therefore,it is urgent to develop a new method for the simple and efficient preparation of nitrogen-rich carbon materials and their composites.In this thesis,transition metal cyanamide compounds are used as precursors for pyrolysis preparation of nitrogen-rich carbon and its composites.A novel method of space-confined pyrolysis is developed using the cyanogen group which can serve as both carbon and nitrogen sources.The transition metals can catalyze or regulate the morphology and structure of the nitrogen-rich carbon and its composites.Through in-depth analysis of the different interaction mechanism between transition metals and carbon or nitrogen,a general law of space-confined pyrolysis of transition metal cyanamide compounds is summarized.Potential applications of the novel nitrogen-rich carbon and its composites in catalysis and energy storage are explored.The main innovative work is as follows:1.M1(NCN)n(n=1,M1=Cu,Zn,Cd;n=1/2,M1=Ag)is pyrolyzed via space-confined pyrolysis,and ZnNCN is selected as the precursor to synthesize heavily nitrogen-doped hollow carbon spheres by zinc-template effect,and their electrocatalytic oxygen reduction performance is investigated.Late transition metals such as Cu,Ag,Zn and Cd have a weak affinity with carbon,and the pyrolysis of their cyanamide compounds can generate metals and nitrogen-doped carbon.Based on the gas-liquid-solid transformation of zinc,a two-temperature-zone space-confined pyrolysis method is proposed to prepare nitrogen-rich hollow carbon spheres.By reasonably setting the temperature of the two zone,the liquidation of zinc vapor can be controlled and the morphology of the carbon material can be regulated,so the nitrogen-rich hollow carbon spheres are successfully synthesized.The two-temperature-zone space-confined pyrolysis method makes full use of the pyrolysis products of the precursor After pyrolysis,nitrogen-doped hollow carbon spheres are directly obtained without any post-processing,and the nitrogen content is as high as 12 wt%.Compared with the traditional pyrolysis method,the space-confined pyrolysis has significant advantages.When used as an electrocatalytic oxygen reduction catalyst,the heavily nitrogen-doped hollow carbon spheres exhibit a higher initial potential(0.84 V v.s.RHE)and better cycling stability(1000 CV cycles).2.M2(HNCN)2(M2=Fe,Co,Ni)is pyrolyzed via space-confined pyrolysis,and heavily nitrogen-doped carbon tubes with M2 nanoparticles coated are prepared through M2 metals catalyzing growth of sp2 carbon,and their electrocatalytic oxygen reduction performance is studied.Group VIII transition metals(Fe,Co,Ni)possess strong carburization and properties of catalyzing growth of sp2 carbon.Pyrolysis of their corresponding cyanamide compounds can generate nitrogen-doped carbon tubes with metal nanoparticles coated.M2(HNCN)2 can produce ammonia gas in the decomposition process.Combining with the space-confined pyrolysis method,a chemical vapor deposition process is simulated to prepare carbon nanotubes with nitrogen content of as high as 7.58 at%,which are higher than that prepared by traditional pyrolysis method(6.18 at%).When applied to electrocatalytic oxygen reduction,the nickel-coated nitrogen-doped carbon nanotubes exhibit excellent electrochemical stability and resistance to methanol poisoning.3.Amorphous complex M3-(H2NCN)x(M3=Mo,W)is prepared,and Mo-(H2NCN)x is selected as a novel precursor to prepare nitrogen-doped carbon-coated molybdenum carbide nanodots,whose electrochemical lithium-storage performance is explored.Early transition metals such as Mo and W are easily combined with carbon to form carbides,and space-confined pyrolysis of their cyanamide compounds can generate nitrogen-doped carbon-coated carbide nanodots.Due to the atomic level dispersion of molybdenum in the precursor and effective carbon coating and anchoring,the average particle size of the prepared molybdenum carbide nanodots can be as small as 1.2 nm.The synergistic effect of nitrogen-doped carbon and molybdenum carbide nanodots can not only provide more electrochemically active sites,but also can relieve the stress caused by the volume expansion of the nanodots during battery cycling and prevent the nanodots from agglomerating.Therefore,when used as anodes for lithium-ion batteries,nitrogen-doped carbon-coated MoCi-x nanodots show superior specific capacity(1040.0 mA h g-1),good rate capability(369.1 mA h g-1 at 5 A g-1)and remarkable cycling stability(500 cycles at 1 A g-1 without decline).4.Amorphous complex M4-(H2NCN)x(M4=Ti,V,Nb,Ta)is prepared,and Ta-(H2NCN)x is selected as a novel precursor to prepare nitrogen-doped carbon-coated tantalum nitride,whose visible-light photocatalytic performance is explored.M4 transition metals have a strong affinity with nitrogen and are easily combined with nitrogen to form nitrides,so their corresponding cyanamide compounds can be pyrolyzed to form nitrogen-doped carbon-coated nitrides.Carbon-coated Ta3Ns is successfully prepared using Ta-(H2NCN)x as the precursor via space-confined pyrolysis.Thermogravimetry coupled with Fourier transform infrared analysis reveals the conversion mechanism of Ta-(H2NCN)x to Ta3N5 that NH3 generated during the decomposition of the precursor contributes to the direct generation of the nitrogen-richer phase Ta3N5.As a narrow bandgap semiconductor(2.1 eV),Ta3N5 can degrade methylene blue rapidly in visible light.The combination of the space-confined method and the novel precursor Ta-(H2NCN)x not only breaks the limit of requiring small-sized tantalum precursors by traditional preparation methods but also considerably reduces ammonia usage,simplifying the preparation process.