Synthesis Of Highly Nitrogen-doped 3D Carbon Materials Via High Temperature And Their Supercapacitor Applications

Military and civilian high-tech equipment requires high-power electrochemical power supplies.Supercapacitors have extremely high power density(?15 k W kg^-1)and can realize rapid charge and discharge,but are limited by their low energy density(?10Wh kg^-1),which is difficult to apply widely.The capacity of the high nitrogen content and conductivity three-dimensional carbon materials is much higher than that of traditional activated carbon(?150-200 F g^-1),and these materials are the key materials for future supercapacitors.However,the preparation of highly nitrogen-doped and high conductivity three-dimensional carbon materials is extremely challenging.The traditional method of preparing high conductivity carbon materials requires the high temperature environment(?800°C),but the doped nitrogen atoms are easily decomposed under this condition.Consequently,it is difficult to achieve high nitrogen content doping.Therefore,the high nitrogen content doping and conductivity in carbon materials are mutually restricted,and there is an urgent need to develop new strategies and methods for the preparation of highly nitrogen-doped and high conductivity three-dimensional carbon materials.Sol-gel method combined with high-temperature chemical vapor deposition is used in this thesis and nitrogen-philic elements are introduced to build high-temperature nitrogen-fixing chemical environment,regulating the type and content of nitrogen doping of three-dimensional carbon materials.In this thesis,the mutual constraints of high conductivity and nitrogen content are synergistically optimized,and a series of three-dimensional carbon materials with both high conductivity and nitrogen content are successfully developed.Through analyzing the mechanism of nitrogen fixation in different chemical environments,the general rule for preparing high nitrogen and conductivity three-dimensional carbon materials is summarized.These materials as electrode materials for supercapacitors has achieved excellent performance.The main innovative results are as follows:1.High nitrogen content and conductivity three-dimensional carbon materials are prepared through high-temperature catalysis of Group ? transition metals(Fe,Co,Ni)cooperating with Si-O-Si network.Fe is preferred as the catalytic center.The high-efficiency catalysis of sp² carbon growth and the high-temperature nitrogen fixation of Si-O-Si network are used to change the microstructure of carbon materials to prepare high nitrogen content and conductivity three-dimensional carbon materials and their supercapacitor performances are investigated.Group ? transition metals,such as Fe,Co and Ni,have the characteristics of catalyzing the growth of sp² carbon and for catalytic ability,Fe>Ni/Co.Based on this,it is preferable to introduce Fe in the high-temperature reaction to impart high conductivity to the carbon material(12 S cm^-1).On the other hand,Fe can modify the microstructure of carbon materials,transforming disordered single-layer graphene sheets into few-layer graphene-like and onion-like structures.In addition,the introduction of Si-O-Si network in the high temperature reaction can fix nitrogen atoms while constructing a three-dimensional network,increasing the specific surface area of carbon materials and introducing a hierarchical pore structure.The three-dimensional carbon material with ultra-high specific surface area(2076 m² g^-1,close to graphene(2630 m² g^-1))and high nitrogen content(3.06at%)at the high temperature of 1000°C.As supercapacitor electrode materials,the material has high capacity(315 F g^-1@0.50 A g^-1),excellent rate performance(174 F g^-1@40 A g^-1)and cycle stability(2%loss after 20000 cycles).2.High nitrogen content and conductivity three-dimensional carbon materials are prepared through coordination between Group ? transition metal(Fe,Co,Ni)ions and dicyandiamide assisting Si-O-Si network.Preferably,Ni²⁺is used as the coordination ion.Nitrogen-rich precursor is formed through the coordination of Ni²⁺and dicyandiamide ions to prepare high nitrogen content and conductivity three-dimensional carbon materials and their supercapacitor performances are investigated.Group ? transition metal(Fe,Co,Ni)ions can form coordination bonds with nitrogen-containing compounds with lone electron pairs due to the existence of empty orbitals in the electronic structure,so that N atoms can be more stably doped into carbon materials.Considering that the Ni-N coordination bond has better effect than Fe-N and Co-N,Ni²⁺is preferred as the coordination ion.During the sol process,Ni²⁺will form a nitrogen-rich complex precursor with dicyandiamide.After high-temperature(1000°C)treatment,high nitrogen content doping(4.17 at%)is achieved in the three-dimensional carbon material.In addition,Ni can catalyze the growth of sp² carbon in high-temperature reactions and improve the conductivity of the material(20 S cm^-1).As supercapacitor electrode materials,the three-dimensional carbon material has high capacity(301 F g^-1@0.50 A g^-1),excellent rate performance(180 F g^-1@20 A g^-1)and cycle stability(5%loss after 10000 cycles).3.High nitrogen content and conductivity three-dimensional carbon materials are prepared through Group IIIA metal oxides(Al₂O₃,Ga₂O₃,In₂O₃)at high temperature cooperating with Si-O-Si network.Al₂O₃ is preferable.High nitrogen content and conductivity three-dimensional carbon materials are prepared by the synergistic effect of Al₂O₃ and Si-O-Si network to selectively fix pyrrole nitrogen(N-5)and pyridine nitrogen(N-6)and their supercapacitor performances are investigated.Group IIIA metal oxides can form nitrides at high temperature and for electronegativity Al>Ga>In,so that Al₂O₃ is preferable.The Al in Al₂O₃ can form Al-N bond with N doped into the C six-membered ring at high-temperature environment,and at the same time,it has a synergistic effect with the Si-O-Si network to enhance the effect of nitrogen fixation,thereby making N atoms dope into carbon materials more stable.The three-dimensional carbon material with high nitrogen content(5.29 at%@1000°C),high conductivity(27S cm^-1)and wide pore size distribution(0.5-60 nm)is prepared.The three-dimensional carbon material as supercapacitor electrode materials has high capacity(302 F g^-1@1A g^-1),excellent rate performance(177 F g^-1@120 A g^-1)and cycle stability(8%loss after 20000 cycles).4.Pyrrole nitrogen-rich and high conductivity three-dimensional carbon materials are prepared by in-situ doping of heterogeneous atoms(B,P)selectively regulating N-5 atoms.Preferably,B is used as the doping atom.Electron-deficient B coupling with electron-rich N is cooperated with Si-O-Si network to construct the three-dimensional network structure,achieving the selective anchoring of high-concentration N-5 atoms.Pyrrole nitrogen-rich and high conductivity three-dimensional carbon materials are prepared and their supercapacitor performances are investigated.Heteroatoms such as B and P can be doped into the C six-membered ring to change the electronic structure of adjacent carbon atoms,and due to the electronegativity of B<P<C<N,it can neutralize the lone pair of electrons of pyrrole nitrogen in carbon materials.First-principles calculations show that the doping of B atoms will reduce the energy of the system near the N atoms in the C six-membered ring(especially N-5),thus making the N-5 structure more stable.In the presence of the Si-O-Si network,the in-situ doping of B atoms can selectively tune the N-5 content in the three-dimensional carbon material,achieving high nitrogen content doping(7.80 at%@1000°C).At the same time,chemical vapor deposition at high temperature promotes the material to have high conductivity(30 S cm^-1).The three-dimensional carbon material as supercapacitor electrode materials has high specific capacity(412 F g^-1@1 A g^-1),excellent rate performance(284 F g^-1@20 A g^-1)and cycle stability(3%loss after 10000 cycles).