Fabrication Of Graphene/Hexagonal Boron Nitride Nanocomposites For Electric Catalytic Applications

With the continuous development of economy and society,problems such as limited reserves of traditional fossil fuels and environmental pollution caused by combustion are becoming increasingly prominent.Therefore,pollution-free and highly efficient hydrogen energy has been widely concerned.At present,hydrogen is mainly obtained by the electrolysis of water,which mainly includes hydrogen evolution reaction（HER）at cathode and oxygen evolution reaction（OER）at anode.OER is composed of four electron transfer process.Due to high overpotential and poor cyclic stability of materials in the reaction,the efficiency of electrocatalytic oxygen evolution is limited.Therefore,it is urgent to develop efficient and stable OER electrocatalyst.At present,the application of precious metal OER catalysts represented by iridium oxide and ruthenium oxide is limited due to their scarce resources and high cost.The discovery of graphene,hexagonal boron nitride（h-BN）,molybdenum disulfide and other two-dimensional materials,makes them ideal catalysts for hydrogen production from electrolysis of water due to the advantages including large specific surface area,multiple active sites,simple synthesis process and low cost.Therefore,in this thesis,based on the previous work on chemical exfoliations of graphene and h-BN nanosheets（BNNSs）,we exploited exfoliated graphene and BNNSs for fabrication of graphene/BNNSs nanocomposites with amounts of defects,and systematically studied their performance as OER catalysts experimentally and theoretically.Firstly,BNNSs and graphene oxide（GO）were directly exfoliated from h-BN bulk powder and graphite powder by mixed acid solution.The nanocomposites（r GO/BNNSs）were prepared by one-step hydrothermal synthesis.The morphology,structure and chemical compositions as well as other properties of the nanocomposites were characterized by FTIR,Raman,XRD,XPS,SEM,TEM and EDS Mapping.The prepared BNNSs exhibited a transverse size of 50 nm-2μm distributed within the r GO surfaces with a transverse size of 30-50μm.The nanocomposites maintain the integrity of the macroscopic crystal structure,albeit numerous point defects on the surfaces.Secondly,electrochemical tests were carried out on r GO/BNNSs composites with variable mass ratio of 1:1-1:7.The performance measurements showed that as compared with pristine r GO,BNNSs,and other groups of samples,the r GO/BNNSs composites with the mass ratio of 1:3 showed enhanced OER catalytic activity.Linear sweep voltammetry（LSV）showed a low overpotential of 446 m V@10 m A·cm^-2 and a Tafel slope of 279 m V·decade^-1.Electrochemical impedance spectroscopy（EIS）showed lower interfacial resistance and higher electron transfer efficiency than r GO/BNNSs composites with other mass ratios.Finally,density functional theory（DFT）simulation was used to systematically study the configuration,charge transfer and electrocatalytic oxygen evolution mechanism of graphene/h-BN van der Waals heterostructure with point defects.The calculation results show that r GO/BNNSs composite has better electrocatalytic oxygen evolution performance than pure graphene with point defects,and the reaction decision step（RDS）can be effectively reduced after composite.By analyzing the electron states,it is found that the h-BN sheet containing B vacancy defects and graphene sheet containing C vacancy defects form van der Waals heterostructures can enhance the electron and spin state transport,which is conducive to the formation of reaction intermediates and the desorption of O₂.At the same time,the valence band edge XPS（XPS-VB）analysis shows that r GO and BNNSs form an interface potential barrier,which is conducive to the accumulation and transfer of electrons,providing a favorable condition for the electrocatalytic oxygen evolution process,showing a broad prospect in the field of metal-free electrochemical oxygen evolution catalyst.