Theoretical Studies On The Design And Relevant Catalytic Mechanisms Of Hydrogen Evolution And Oxygen Evolution Catalysts Based On Two-dimensional Nanomaterials Containing Group IVA Elements

Energy shortage and environmental problems caused by the overuse of fossil fuels have become two major challenges facing mankind.In order to solve these problems,people begin to pay much attention to how to develop and utilize clean and eco-friendly new energy to replace traditional fossil energy.Hydrogen has been expected to become the best alternative to fossil fuels,due to its high energy density,pollution-free combustion,easy storage and other advantages.Among many hydrogen production technologies,electrochemical water splitting is considered to be the most promising hydrogen production method in view of the simple equipment,pollution-free products and high purity of hydrogen products.Water electrolysis includes two important half reactions of hydrogen evolution（HER）and oxygen evolution reaction（OER）,both of which have sluggish kinetic processes and need to be carried out with the participation of efficient catalysts.At present,noble metal Pt and Ru O₂/Ir O₂are recognized as the most advanced HER and OER catalysts,respectively.However,these catalysts are not cheap and their reserves are very low,which limits their large-scale use.Two-dimensional（2D）materials with unusual planar structure show great application prospects in catalyst design.As one of the most important members of the 2D material family,graphene has been widely used in the field of catalyst design.However,due to the strongπconjugation between the C atoms,it is difficult for pristine graphene surface to absorb the relevant species involved in the HER/OER,resulting in its poor catalytic activity.Considering that other Group IVA elements（such as Si,Ge and Sn）have weak inter-atomicπconjugation properties,this paper proposes that the 2D nanosystems composed of these elements can be used to design a series of electrocatalysts with high HER/OER catalytic performance.In addition,this paper also proposes that the electrocatalysts with high HER/OER catalytic activities can be designed by constructing a 2D carbon-based structure completely composed of anti-aromatic structural units.The main research work and results are as follows:（1）Compared with carbon atom,silicon has weak ability to formπ-bonding.In this paper,the structural stabilities,conductivity and HER catalytic performance of a series of 2D BSi_n（n=1-4）are systematically investigated.In these BSi_n（n=1-4）systems,Si atom could prefer to move out of the BSi_nplane and change from sp²to sp³hybridization when H attacks,which is conducive to the adsorption of the reaction species on the surface,thus exhibiting high HER catalytic performance.The Si–Si bridge sites or Si-sites can uniformly serve as the most active sites.With increasing the Si/B atomic ratio,higher HER activity can be achieved,due to the change from weak aromaticity to strong anti-aromaticity for the correlative B_xSi_ysix-membered rings.In addition,the HER activity of the studied systems can be further improved by doping P,S,Ge and C atom with the different electronegativity.These systems also have good conductivity and high structural stability,which are expected to become excellent candidates for cheap and efficient HER catalysts.This study is the first time to report the application of 2D metal-free BSi_n（n=1-4）in HER catalysis and can provide valuable theoretical information for the experimental design of new efficient and cheap HER electrocatalysts based on silicon-based materials.（2）In this paper,we systematically investigate the electrocatalytic activities of2D systems composed of Group IVA elements（such as Si,Ge and Sn）in the high period.Unlike graphene,these 2D systems（Ge Si,Sn Si and Sn Ge）can reduceπbonding and form mixed sp²-sp³orbitals,which will be favorable for the attack of reactive species.The charge transfer can also take place between two adjacent atoms with different electronegativity,which can adjust the electron density on the relevant atoms and is advantageous for the HER or OER catalytic activity of the material systems.The computed results reveal that the Sn Si monolayer can show good HER catalytic activity,while the Sn Ge monolayer can display remarkable OER catalytic activity.In particular,2D Ge Si monolayer can even exhibit excellent bifunctional HER/OER electrocatalytic activity.In addition,it can be found that applying biaxial strain and doping heteroatoms（particularly for P）can effectively improve HER catalytic performance of these monolayers.The doped Ge Si and Sn Si systems can usually exhibit higher HER activity than the doped Sn Ge systems.This study can provide new ideas for the design of non-noble metal electrocatalysts for overall water splitting.（3）In this paper,the structure and catalytic performance of the R-graphyne system,which is completely composed of anti-aromatic rings,are systematically investigated.It was found that compared with inert graphene,R-graphyne composed of anti-aromatic structural units,can exhibit certain HER catalytic activity.Doping transition metal atom in Group VIIIB can effectively enhance the HER or OER catalytic activity of this system.Among them,the system doped with Fe,Os,Rh and Ir can exhibit higher HER catalytic activities due to the formation of more active sites.Usually,the shorter the distance between the TM and C atoms is,the better the HER activity of C-site is.In addition,doping Co,Ni and Rh atoms can improve the OER catalytic performance of R-graphyne.In particular,embedding Ni and Rh atom can induce considerably high OER catalytic activity,which can be comparable to that of Ir O₂.Obviously,these 2D carbon nanostructures based on completely anti-aromatic rings are expected to become excellent candidates for cheap and efficient HER/OER catalysts.In summary,this thesis has designed a series of efficient and cheap HER and OER catalysts using the 2D nanosystems composed of Group IVA elements in high period and using the 2D carbon nanosystem completely composed of anti-aromatic structural units.It is expected to design and screen a series of cheap and efficient HER/OER catalysts based on the Group IVA elements,which can provide valuable theoretical guidance for the experimental synthesis of relevant catalysts.