Design Of Carbon-nitrogen Materials As A Novel Support For Their Application In Electrocatalysis And Energy Storage Device

In recent years,the development of doping technologies provides the potential to the application of more and more high efficient catalysts and good performance electrode materials.It is possible to effectively regulate and improve the electronic structure,chemical inertness and pore distribution of the carbon nanomaterials by achieving the incorporation of the heteroatoms to their intrinsic structure,more importantly,this phenonmenon plays a crucial role in enhancing their interactions with the metal particles or the metal oxide particles.In this paper,a series of novel carbon-nitrogen nanomaterials have been designed and fabricated,and the formation mechanism,chemical properties and electronic structure of these carbon-nitrogen nanomaterials were studied clearly.By adjusting the ratio of raw materials,pretreatment conditions and calcination temperature,an optimal carbon-nitrogen nanomaterials can be obtained,which possesses considerable active sits and significant stable structures.Furthermore,they are used to load metal or metal oxide nanoparticles to obtain a series of nanocomposites towards direct liquid fuel cell anode catalyst,lithium ion battery anode material,photoelectrochemical cell photoelectric anode material.Meanwhile,the influence of the the interactions among the active components,mircostructures,and morphology of the resulting nanocomposites on the electrochemical properties was also carefully investigated.The main contributions of this dissertation can be described as follows:1.Controlled synthesis and property study of covalently bonded graphitic carbon nitride and graphene hybrid supported Pd catalysts.Graphite oxide(GO)is a widely used graphene derivatives with rich oxygenated functional groups like epoxy,hydroxyl and carboxyl,etc.Among them,the epoxy group possess superior reactivity,which could undergo a nucleophilic substitution with melamine to form a melamine chemically modified GO sheet via covalent C-N bonds.After calcination at 550 ?,both the thermal polymerization of melamine and the elimination of oxygenated functional groups of GO can be achieved,producing a covalently coupled hybrid of graphitic carbon nitride with graphene(g-C3N4-rGO).We have employed various characterization methods to confirm their formation mechanism.The g-C3N4-rGO was further used to load Pd nanoparticles.Due to the strong electron affinity and the coordination ability of g-C3N4,Pd nanoparticles supported on g-C3N4-rGO shows a uniform dispersion with a small size,far surpassing than supported on traditional carbon materials such as graphene,carbon nanotubes and carbon black.On the other hand,the ternary Pd-g-C3N4-rGO nanocomposite exhibits excellent electrocatalytic properties toward both formic acid and methanol electrooxidation,such as extremely large electrochemically active surface area(ECSA)values,significantly high forward peak current densities and reliable long-term stability,which may be attributed to the specific characteristics of their unique nanostructure and the concerted effects of the individual components.DFT computations suggest that the absorption of Pd on g-C3N4 reveals a complicated“exchange-transfer mechanism”,thus reducing Pd cluster migration,agglomeration,and dissolution during the electrocatalysis process.2.Controlled synthesis and property study of 3D hierarchically porous graphitic carbon nitride modified graphene hybrid supported Pt catalysts.The design concept of covalently bonded graphitic carbon nitride and graphene hybrid is further extended to a three dimensional system.Taking advantage of the self-assembly properties of GO,a 3D hierarchically porous graphitic carbon nitride modified graphene hydrogel was successfully constructed and then used to load Pt nanoparticles by hydrolysis of platinum nitrate.Pt nanoparticles with a narrow particle size distribution are homogeneously anchored onto the 3D g-C3N4-rGO nanosheets.Most importantly,compared with graphene hydrogel supported Pt catalyst,the in situ formation of g-C3N4 brings a lot crumples to graphene sheets,which not only plays an important role in increasing the surface area of 3D architectures and preventing the stacking of graphene layers,but also leads to a hierarchically multimodal pore-size distribution.This unique feature significantly facilitates the fast diffusion of electrolytes,the sufficient transfer of reactants and the prompt emission of products,resulting in impressively high electrocatalytic properties toward methanol electrochemical oxidation.Moreover,DFT computations forecast that g-C3N4 can significantly reduce the interfacial binding of CO and Pt,thus more active Pt sites can be exposed for methanol electrooxidation.3.Controlled synthesis and property study of graphitic carbon nitride modified nitrogen-doped graphene hybrid supported Pd catalysts.Nitrogen-doping has been demonstrated to be an effective protocol to adjust the spin density and charge distribution of carbon atoms on graphene.The g-C3N4-rGO obtained by the above-mentioned step is treated via higher temperature.Due to the protection of graphene,g-C3N4 nanpsheets split into many smaller nanoflakelets and couple with the substrate more intimately instead of decomposition,and part of nitrogen transferred from g-C3N4 to rGO via some intermediates(e.g.,,C2N2+,C3N2+)generated in pyrolysis of g-C3N4,obtaining the desired graphitic carbon nitride modified nitrogen-doped graphene hybrid(CN-NG).The morphology,microstructure and the formation mechanism of CN-NG was carefully confirmed.Benefiting from the inherently considerable microporous and mesoporous,active nitrogen species and edge sites,small-sized Pd nanoparticles could be effectively stabilized with a highly dispersion.The as-obtained Pd-CN-NG nanocatalyst exhibits an unprecedented high and stable electrocatalytic activity for both formic acid electrooxidation in acid media and methanol electrooxidation in alkaline media.This may attributed to the synergetic effects,where g-C3N4 acts as a co-catalyst accelerating the formation of-OH by water dissociation and oxidative removing the absorbed intermediate poisoning species on the Pd sites during catalytic reactions;nitrogen-doped graphene not only maintains acceptable conductivity and reduces the charge transfer resistance,but also providing a high rate of diffusion of the electrolyte via pores inside;the covalent interactions between g-C3N4 and NG via the C-N bond ensure the structure integrity,long-term electrochemical stability and durability.4.Controlled synthesis and property study of graphitic carbon nitride modified nitrogen-doped graphene hybrid supported Co3O4 nanocrystals.Transition metal oxides,especially Co3O4,are considered as a high performance anode material in lithium ion batteries,but limited by the large volume expansion during the cycling of Li+ insertion and extraction.In this paper,we fabricate a ternary anode material by growing well-distributed Co3O4 nanocrystals on graphitic carbon nitride modified nitrogen-doped graphene hybrid.Owing to the unique distribution of ? electrons,the Co3O4 nanocrystals display a special morphology of blunt edge nanocubes and a well demarcated dispersion.Most importantly,there are abundant low index crystallographic facets exposed.This plane not only possesses low surface energy,but also is directly involved in the electrochemical reactions for lithium ions shuttling in/out of active hosts.Combining the textural features of CN-NG including high electric/ionic conductivities,considerable Li+ accessible active sites,strong interactions with Co3O4 and robust structural integrity,as well as enough flexibility to accommodate the volume expansion/contraction,the Co3O4/CN-NG exhibits a large reversible capacity,high rate capability,and prominent cyclic stability.5.Controlled synthesis and property study of single site nickel embedded into carbon nitride.The single site nickel embedded into carbon nitride has been bottom-grown on the surface of FTO by coupling supramolecular and coordination effects.By adjusting the ratio of nickel salt,three dimensional porous films are able to be obtained with different nickel valence states and microstructures.As a photoanode in photoelectrochemical cells,3%Ni-CNx exhibits an optimal photoelectrocatalytic properties and Incident photon-to-electron conversion efficiency(IPCE).The structure has been discussed in detail,and it is found that the electric conductivity and conduction band possess a gradient distribution from the surface to the bottom.The latter directs the electrons flow toward the conductive substrate and improve the carrier separation efficiency across the layer.Meanwhile the nickel species(predominantly Ni3+)not only facilitated the charge transfer across the semiconductor/electrolyte interface and thus suppressed the electron-hole recombination,but promoted the formation of hydroperoxy(OOH)species(key intermediates in the OER)and subsequent conversion to oxygen.