Theoretical Study On Electrocatalytic Hydrogen Evolution And Nitrogen Fixation Performance Of Two-dimensional Materials

Getting rid of the dilemma faced by the current energy crisis and realizing high-efficiency electrolytic water hydrogen production and electrocatalytic ammonia production has always been a hot research content in the field of new energy.The main difficulty of current research is how to exploit high-performance and cheap electrocatalysts to decrease the excessively high reaction energy barrier.Two-dimensional（2D）materials hold high research prospects in electrocatalytic reactions because of their large specific surface area,abundant reactive sites and unique physical and chemical properties.In this thesis,we take the novel 2D materials as the research object,with the help of the first-principles calculation,to investigate the effects of introducing impurity atoms and constructing defect on the hydrogen evolution performance of MoSSe and MoSi₂N₄ and the nitrogen fixation performance of Fe₃GeTe₂.At the same time,their catalytic activity and reaction mechanism were also revealed,which provides a theoretical reference for the research of other new 2D materials in the field of electrocatalysts.The main contents are as follows:（1）Similar to other transition metal dichalcogenides,it is urgent to activate the relatively large inert basal plane of MoSSe monolayer.Based on the idea of single-atom catalysts,the effects of a series of transition metals（from Ti to Zn and from Zr to Cd）anchored at different sites of MoSSe on the HER activity were systematically studied.The results show that the single Co,Zn and Cd atom doping model possess high HER activity and the Gibbs free energy change in the hydrogen process（ΔG_H）is close to zero.The origin of high HER performance is mainly attributed the doping of Co,Zn and Cd,which improves the conductivity of the original MoSSe,and provides unoccupied electronic states near the Fermi level reducing the energy barrier of the electron transfer between free radical H and the doping model.Meanwhile,the free radical H and the doped structures form a stable bonding-antibonding hydrogen adsorption state above and below the Fermi level,which also improves the adsorption strength between the MoSSe basal plane and free radical H.（2）By the means of the first-principles method,the high-efficiency MoSi₂N₄-based HER catalysts were screened out of a series of single non-precious transition metal（TM）and non-metal（NM）atom catalysts.The results show that the N defective MoSi₂N₄ monolayer doped with O atom（O@MoSi₂N₄^v）and the Si defective MoSi₂N₄ monolayer doped with P/Fe/Nb（P@MS^vN,Fe@MS^vN and Nb@MS^vN）exhibit excellent HER performance with theΔG_H of-0.120 e V,0.075 e V,-0.020 e V and 0.013 e V,respectively.The origin of high HER activity is ascribed to the increased conductivity and the newly formed impurity states near the Fermi level.Moreover,the novel HER activities of someTM doped structures were explained by the“states filling”model.The energy level of the first available unoccupied states for accommodating hydrogen drops after the introduction of TM atom,which further modulates the hydrogen binding strength.（3）Spin polarization plays a vital role in the electrocatalytic N₂ reduction reaction（NRR）.In regard this,the potential of new 2D vd W ferromagnetic（Fe₃GeTe₂）as a NRR catalyst was systematically studied with the aid of the first-principles calculations.The results show that theTe defective Fe₃GeTe₂ acts as a NRR catalyst with outstanding electrocatalytic activity and high selectivity.Notably,all six hydrogenation steps are exothermic and spontaneous.The highly centralized spin-polarization on the exposed Fe4 center in defective Fe₃GeTe₂ leads to strongπ_2p^*backdonation and substantial activation of the N≡N triple bond.Moreover,the metallic character of the defective Fe₃GeTe₂ ensures high-speed transport of carriers.