Study On Two-Dimensional Structure Search And Graphene Modification For Functionalizational Application And Intrinsic Mechanism

Catalytic reaction,energy conversion and storage with electrochemical technology as the core provide many feasible means to solve the energy crisis and environmental pollution.Investigating the structural information and electronic properties of catalysts and storage materials,clarifying the intrinsic electronic processes,revealling reaction mechanisms,they are important issues to deeply understand functional materials.The bonding structures of functional materials are closely related to their electronic properties and electrochemical performances.Taking chemical simulations to design and identify special two-dimensional structures with targeted bondings and functions,or to modify and decorate existing two-dimensional materials to realize the functional applications is significant.In this thesis,structure search was employed to identify a TMC（TM=Co,Ni,Cu）monolayer containing entirely planar pentacoordinate carbons（pp C）,four novel B atoms-riched two-dimensional materials,and dual-metals electrocatalysts were constructed based on N-modified grapheme.Moreover,their stability,electronic properties,and electrochemical performance were investigated in detail.The specific works are as follows:（1）According to the crystal symmetry and bonding characters,C₂@TM₆is proposed to be a feasible solution to obtain pp C,and the directive structure search identifies the TMC（TM=Co,Ni and Cu）monolayers containing entirely pp C.Their stability,electronic properties,and magnetic characteristics are further analyzed in detail.Moreover,the C-Ctransition from Co C without the classical C-Csingle bond to Cu C with in-planeσbonds and out-of-planeπbonds.（2）The directional structure search identifies the B₇P₂monolayer,which possesses the huge advantage as anode materials for Lithium/Sodium-Ion Batteries（LIBs/SIBs）.B₇P₂monolayer possesses the essential properties as a promising ultrahigh capacity LIB/SIB anode material:its superior thermodynamic,kinetic,thermal and mechanical stabilities,preserves its intrinsic metallicity at upon Li/Na adsorption and desorption,and a high Li/Na mobility and appropriate open-circuit voltages.Encouragingly,its capacity for Li and Na reaches up to3117 m A h g^-1.This investigation indicates that the monolayers with proper pore size and distribution are three key factors,which provides a reliable theoretical basis for the further development of other 2D high-capacity materials.（3）The Ni₂B₅monolayer with high stability and multi-adsorbed sites was obtained by means of the structure search,where the Ni and B are both active sites to effectively adsorb the CO molecule.The maximum kinetically energy barrier for CO migration among different sites is only 0.64 e V,and the computed ultralow barrier of direct*CO dimerization is only0.17 e V.Moreover,the limiting potentials for CH₂CH₂（-0.13 V）and CH₃CH₂OH（-0.17 V）reach the optimal value.Meanwhile,the hydrogen reduction side reaction is uncompetitive with the CO electrochemical reduction on all possible adsorption sites.This work demonstrates that efficient migration of*CO among different sites is crucial for C-C coupling and subsequent protonation to synthesize multi-carbon products.（4）Motivated by the excellent stability and larger size of B₇-unit,12 global-minimum MM’B₇monolayers with the elongated heterometal double-sites as promising nitrogen reduction reaction（NRR）electrocatalysts has been identified from the 10179 candidates using high-throughput screening.These data adequately verify that long-distance heterometal double-sites is an excellent strategy to active and polarize the N≡N triple bond due to elongated bonding length,asymmetric interaction and favorite orbital matching for side-on adsorbed N₂molecule on double-sites.Moreover,9 global-minimum systems are proposed to possess the excellent catalytic activity and selectivity for NRR.Most importantly,this work opens a new avenue to design and develop efficient NRR electrocatalysts with the long-distance heterometal double-sites.（5）The global-minimum Cu B₁₂monolayer with superior stability has been identified based on first-principles computations,and the most significant is that the Cu B₁₂monolayer possesses the best catalytic activity among the reported urea catalysts thermodynamically and kinetically.All possible reaction pathways to form urea（NH₂CONH₂）starting from the CO₂molecule and N₂molecule,as well as the kinetic energy barriers of six possible C-N coupling reactions,are systematically investigated.The lowest kinetic energy barrier of C-N bond formation is 0.54 e V through the reaction*CO+*NHNH→*NHCONH,and the limiting potential of urea production is 0.23 V through the CO₂pathway and OCOH pathway.This work enriches the competitive relationship between the synthesis mechanism of urea and the elementary reactions.（6）72 stable systems were screened from 378 initial configurations by ab initio molecular dynamics simulations,and the activated*N₂intermediate with side-on configuration and the free CO molecule were pre-selected reactants for urea production.Their reaction mechanism to form the*NCON precursor were the key pre-selected C-N coupling pathway.Based on the computed Gibbs free energy of these elementary reactions and other important factors,a database was constructed.Moreover,the principal descriptor（ΔE（*NCONH））was established,together with six significant linear correlations were found.Meanwhile,the effective range（-1.0 e V<ΔE（*NCONH）<0.5 e V）is identified via eight optimal systems,and 72%low-performance systems can be filtered out.This study not only suggests that dispersed dual-metals via MN₃moiety can serve as promising active sites for urea production,but also identifies the principal descriptor in high-throughput methods.