The Introduction Of Transition Metal Atoms Into Two-dimensional Conductive Materials As Electrocatalysts:Two Case Studies

The rapid economic development has caused serious problems such as energy shortage and environmental pollution.Novel electrocatalysts are developed for oxygen reduction reaction（ORR）,hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,carbon dioxide reduction reaction（CO₂RR）,nitrogen reduction reaction（NRR）and other small-molecule reactions in order to realize energy conversion and change the energy structure,which lay the foundation for achieving sustainable development and implementing environmental protection concepts.Due to its special two-dimensional（2D）single-layer structure,graphene possesses the advantages of large specific surface area,outstanding electrical conductivity and thermal conductivity.It has become the research hotspot in the field of catalysis.As the research further develops,the shortcomings of graphene are gradually exposed.Therefore,lots of researchers began to try to seek functionalized carbon-based or novel two-dimensional materials to realize these electrocatalytic reactions.In this work,a series of transition metal atoms on the proposed C₈N₈ andβ₁₂-borophene monolayers are investigated.By density functional theory calculations（DFT）,the catalytic effect of the TM@C₈N₈ monolayer for the water splitting reaction and catalytic activity of the TM₂@β₁₂-borophene monolayers for carbon dioxide reduction reaction（CO₂RR）are verified,which provides a theoretical basis for the extended application of two-dimensional materials in the field of small-molecule catalysis.The main contents of this thesis are as follows:1.Development of efficient single-atom-catalysts（SACs）is a promising strategy for electrochemical water splitting.High over-potential and poor stability of catalysts remain to be challenges for overall water splitting.In this work,a novel series of TM@C₈N₈monolayers constructed by embedding transition metal atoms in the proposed C₈N₈based on poly（pyrazine-2,3-diamine）units are screened using the kinetic stability and the projected density of states.Their catalytic performance of intrinsic TM-N₄ moiety is investigated for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）using the Gibbs free energy change（ΔG）of each elementary step.Especially,Rh@C₈N₈ exhibits superior catalytic activity toward HER and OER with hydrogen adsorption free energy(ΔG_H*)/over-potential（η）of0.08 e V/0.49 V at p H=7.In addition,it is unbroken at high temperatures（1000 K）.Therefore,Rh@C₈N₈ monolayer can perform as an efficient bifunctional catalyst for overall water splitting.It is also expected that the results can serve as the theoretical basis to open the door for future experimental research on novel CN layered-materials containing Rh-N₄ moiety.2.In order to mitigate the energy crisis and environmental pollution,converting carbon dioxide（CO₂）into value-added chemical products or practicable fuels through electrocatalytic method shows great development prospects.Seeking the stable,efficient and highly selective CO₂RR electrocatalysts are the key to solve current problems.In this study,TM₂@β₁₂-borophene diatomic catalysts（DACs）are constructed by anchoring a series of transition metal dimers onβ₁₂-borophene monolayer.The density of state and stability of TM₂@β₁₂-borophene monolayers are calculated.The comparative study is made between the TM₂@β₁₂-borophene monolayers and TM@β₁₂-borophene single-atom catalysts（SACs）on the adsorption configuration,relative energy,Gibbs free energy change,overpotential and the main product of CO₂RR.The result indicates that TM₂@β₁₂-borophene（TM=Ni,Pd,Cu,Au,Cr,Mn）monolayers exhibit excellent catalystic performance and stability.The final reduction product of Ni₂@β₁₂-borophene is HCOOH with the overpotential of0.05 V.In addition,due to the more active sites and synergistic effect between the two metal atoms,Cu₂@β₁₂-borophene and Cr₂@β₁₂-borophene have lower potentials and exhibit more significant catalystic activity than the corresponding SACs.The work can inspire more experimental and theoretical research for evolution of novel electrocatalyst based on borophene monolayers.