Theoretical Structure Design And Performance Research Of Two-Dimensional Molybdenum-Based Electrocatalysts For Hydrogen Evolution Reaction

The development of modern society needs more and more energy.On the one hand,the excessive consumption of traditional fossil energy causes environmental pollution.On the other hand,the reserves of traditional fossil energy are limited,so it is urgent to develop green new energy.Among the various alternative energy resources,hydrogen（H₂）has aroused great attention due to its clean,renewable,high energy density,and zero-carbon emission.Although electrocatalytic hydrogen evolution reaction（HER）of water splitting becomes a feasible method for the production of high-purity and large amounts of H₂,such reaction is quite demanding on catalyst performance.Currently,Pt-based materials are the optimal catalysts,but the serious scarcity and high cost severely limit their large-scale industrial applications.Therefore,the development of high-efficiency and low-cost electrocatalysts for the HER has become a key problem to be solved urgently in the development and utilization of hydrogen energy.Through extensive effort and exploration,two-dimensional（2D）materials have become one of the most promising electrocatalysts.On the other hand,theoretical calculations can reveal the origin of catalyst activity and design catalysts with excellent performance,which play an important role in boosting the development of new materials.In this thesis,based on first-principles structure search calculations,focusing on 2D non-precious metal Mo-based materials,on the basis of reliable research on the HER activity of the existing 2D Mo₂C,a series of novel Mo-based 2D materials are designed by introducing elements and changing chemical components,of which the electrocatalytic activity and catalytic mechanism are investigated.The results provide a valuable theoretical basis for the experimental synthesis of high-performance catalysts.The main research contents and conclusions are as follows:The potential effective strategies to improve the HER performance of 1T and 2H Mo₂C phases are explored theoretically.Based on first-principles calculations,the formation conditions of point defects,the stable sites for oxygen functionalization,their electronic properties with/without strain,and the HER activity are determined.It is found that the intrinsic 1T phase is catalytically active relative to the HER,while the intrinsic 2H phase is catalytically inert,which is consistent with existing research reports.Further research found that Mo defect can improve the catalytic performance of the HER,while C defect is inactive.The combination of oxygen functionalization and Mo vacancy is an effective way to achieve the best HER activity of 1T-Mo₂C,mainly due to the charge transfer from Mo to O atom which reduces the bound degree of O to H atom.In general,strain engineering has little effect on the HER activity of the system.This work not only reveals the intrinsic mechanism of Mo₂C monolayer on the HER,but also has guiding significance for further improving the activity.Phosphorus-rich Janus Mo₂P₃monolayer is theoretically designed,with intrinsic metallicity and high HER activity.Through first-principles swarm-intelligence structural search calculations,structural stability of a series of Mo_xP_y（x=1-3,y=1-6）chemical compositions is determined,and two novel Mo₂P₃and Mo₃P₂monolayers are found with high thermal,dynamical,and mechanical stability,showing intrinsic metallicity.The P-rich Mo₂P₃has a Janus structure（in-plane asymmetry）and contains a dumbbell P₂unit.The P atoms in the dumbbell P₂units and bridge bond of two Mo atoms are identified as active sites toward hydrogen adsorption.Moreover,the Mo₂P₃monolayer has a high d_{active–site}of 2.65×10¹⁵site/cm²,which can be attributed to the dumbbell P₂units and Janus structure.The HER activity of some active sites is comparable to that of Pt.For the HER mechanism,2D Mo₂P₃favors the V-H pathway,and can produce hydrogen with high efficiency.For Mo-rich Mo₃P₂,only Mo atoms are active sites.The ternary 2D Mo-C-N structures as electrocatalytic catalysts are studied theoretically for the HER.Based on the variable composition structure search calculations,four monolayers structures are found,i.e.,Mo CN,Mo C₂N,Mo CN₂,and Mo C₂N₂,all of which have excellent thermodynamic,dynamical,thermal,mechanical stabilities,as well as good electrical conductivity,mainly due to the contribution of Mo 4d orbital.The calculations of the HER activity indicate that these monolayers show good HER activities,especially the activity of Mo CN monolayer is comparable to that of noble metal Pt,such as the free energy value of hydrogen adsorption is close to zero,which can still be maintained at high hydrogen coverage.This may be attributed to the obvious electron transfer process between the strong electron-withdrawing chemical groups cyano（CN）and Mo,which could effectively adjust the electron density on the relevant atoms and make Mo CN exhibit excellent HER catalytic activity.The Mo PC monolayer with Janus structure is theoretically designed,which is expected to be a high-performance electrocatalytic catalyst for the HER.The high HER activity of the binary Mo-C and Mo-P monolayers,and the synergistic charge transfer in the ternary system in favor of enhancing the catalytic activity inspire the author to investige of the Mo-P-C system.Theoretical calculations found the Mo PC monolayer with a Janus structure,of which the spatial configuration is similar to that of 2H-Mo S₂,with the distribution of P and C atoms on both sides of the Mo atom,showing an antisymmetric sandwich structure.The distinct feature of this structure is that all the constituent atoms exhibit near-zero hydrogen adsorption free energy,which can maintain high catalytic activity under high hydrogen coverage,indicating excellent catalytic activity for the HER.In addition,this kind of novel monolayer can possess very high active sites density up to 1.46×10¹⁵site/cm²,which is even comparable to that of Pt.Based on this structure and the idea of high-throughput computing,the Cr PC and Mn PC monolayers are also found to possess the high HER activity by substituting Mo atoms with 28 transition metals.