Design And Structure Regulation Of 2D Carbon Nanocomposite Catalysts For Electrochemical Oxygen Reduction

Oxygen reduction reaction（ORR）is an important electrochemical reaction for energy storage and conversion devices.And it is also regarded as an effective way of the synthesis of hydrogen peroxide.Electrochemical ORR can be divided into two electron process and four electron process.The ORR process is very complex,thus,the design of new and efficient catalyst to improve the selectivity and efficiency for ORR is an important research direction.Nanocomposite material is an important part for the design of ORR catalysts because of the special properties.In this thesis,a variety of 2D carbon based nanocomposite catalysts have been designed and prepared.Applications in electrochemical ORR of them have been investigated.1)Graphene has unique electronic properties,but its intrinsic ORR activity is low.Therefor,it can be composied with another material to inhance the ORR activity.In this part,Ag-TCNQ/r GO was synthesis by the composition of Ag-7,7,8,8-tetrachyanylbenzoquinone dimethane（Ag-TCNQ）and r GO hydrogel.It has been used as ORR catalysts for 2e^-process to prepare hydrogen peroxide and show good performance.Ag-TCNQ/r GO hydrogel possess remarkable hydrophilicity,unique metal-organic framework structure,abundant porous network.These are the main factors to make it have good catalytic performance.Hydrophilicity of the hydrogel（water content up to 97%）in favor of the contact of the material and the reactants（O₂）.And high porosity provided by graphene’s rich network structure not only increasing the specific surface area of the material to make it easier to contact with oxygen,but also conducive to the delivery of ion and product.Diameter of Ag-TCNQ nanodots is about 1.8nm.Ag-TCNQ nanodots,which provides abundant active sites for electrocatalytic reaction,evenly distributed on the surface of the graphene.The electro transfer number is about 2.0,and the highest hydrogen peroxide yield is 0.78 mg/h cm² while the potension is 0.26 V vs.RHE.The performance of Ag-TCNQ/r GO hydrogel is higher than r GO、Ag-TCNQ、Ag/r GO,it shows that the synergistic effect of each component improves the performance of the material.2)In the first part,0D material is composited with 2D material,the synergistic effect between them result in the changement of structure.Inspired by this,Fe-N-C and MXene is composited in this part.Heterostructure has good electrochemical performance because of the special electronic structure.But the application of heterostructure has been limited because the synthetic process is rigorous.In this part,Positive C₃N₄ and negative MXene are recombined by the force of charge in a liquid to synthesize Fe-N-C/MXene（2D/2D）.Fe nanocluster in Fe-N-C can regulate the surface change and also provide more active sites.Fe-N-C/MXene has good 4e^-ORR performance in alkaline electrolyte.Electron transfer number is 3.50～3.96,half-wave potension is 0.84 V vs.RHE,which is close to commercial catalysts.And it also process excellent electrochemical stability.The preparation process of Fe-N-C/MXene is mild and simple,with large-scale industrialization prospect.Metal clusters and nanosheet used in this work can also be replaced by others.A series of heterostructure with ideal performance can be synthesis by charge force of the surface chanrge.And they can be widely used in electrocatalysis,fuel cell,energy storage and conversion.3)The second part work comfirm the heterostructure has excellent electrochemical performance,but for the most of nanomaterials cincluding 2D heterostructure,reunion limits its performance.Take example by the 0D/2D composite materials in the fierst part,this part will composite heterostructure with 2D materials,and get an excellent electrochemical performance.In this part,In₂S₃-In₂O₃ hetrostructure/2D carbon nanosheet has been synthesized（In₂S₃-In₂O₃/C）.The combination of heterstructure and 2D nanomaterial is realized.In₂S₃-In₂O₃/C has an excellent ORR catalyst to synthesize H₂O₂.Take In salt,Cd salt,2,5-dicarboxylic thiophene as the precursor,In-MOF nanosheet was been synthesized by Ion Exchange,and final product is In₂S₃-In₂O₃/C after heat treatment.The catalysis has the best 2e^-ORR performance when the treatment temperature is 500℃and get the highest hydrogen peroxide yield 32.57mg/L h while the potential was 0.47 V vs.RHE.The material has good stability and keep good electrocatalytic performance after 50 h continuous working.4)The process of ORR is very complicated.The materials synthesized in the previous chapters promote different ORR reaction processes.On this basis,Ag-TCNQ/high defect graphene and Ag-TCNQ/low defect graphene were synthesized and the ORR catalytic performance of them were compared.The results show that the high defect of materials is more favorable to the 2e^-ORR process,eletron transfer number of Ag-TCNQ/high defect graphene is 2.35～2.71,hydrogen peroxide yied is 0.62 mg/h,Faraday efficiency is 64.45%.And Ag-TCNQ/low defect graphene is more利于conducive to 4e^-ORR process.Electro transfer number of Ag-TCNQ/low defect graphene is 3.19～4.71,hydrogen peroxide yield is significantly lower than that of Ag-TCNQ/high defect graphene.The synergistic effect of composite improves the catalytic activity.The existence of defects has an important influence to the structure and electron distribution of materials,which directly affect the electrochemical activity.Therefore,a fundamental understanding of the role and mechanism of defects in catalysis will be an important progress of electrocatalytic material.5)The research and optimization of ORR should be finally implemented in practical applications.In this part,Ag-TCNQ/graphene hydrogel and Fe-N-C/MXene materials were selected as electrode modified material.And the liquid flow electrolytic cell and zinc-air battry were respectively assembled.When Ag-TCNQ/graphene hydrogel was used as the cathode electrode modification material of the liquid flow cell,the highest hydrogen peroxide yield was also appear at 0.26 V vs.RHE,but the hydrogen peroxide yield was nearly 16 times higher than that of H-cell.A simple zinc-air battery was assembled by Fe-C-N material and Ag-TCNQ/low-defect graphene modified electrode,which had good charge-discharge performance and kept the output voltage stable after several charge-discharge cycles.These shows that the synthesized material can be applied to liquid flow cell,metal air battery and even other devices.In this paper,a variety of two-dimensional nanocomposite materials have been designed and applied in ORR.Different process of ORR have been studied.The synergistic effect makes the material exhibit good catalytic performance,which shows the application value of 2D nanocomposites in ORR.