Rational Design And Research Of Two-dimensional Materials For Electrocatalytic Carbon Dioxide Reduction And Nitrogen Reduction

With the depletion of fossil fuels,a series of issues such as climate change,pollution and resource scarcity have aroused intense concern.Therefore,finding clean and renewable energy has become one of the challenges of social sustainable development.Electrochemical energy conversion,such as electrocatalytic reduction of CO2(CO2RR)and electrochemical nitrogen fixation(NRR),can effectively achieve the regeneration of energy and chemicals,thereby reducing the burden on the environment and the dependence on fossil fuels.At present,seeking and developing efficient electrocatalysts is the key to achieve this goal.Two-dimensional(2D)materials offer great possibilities for designing efficient CO2RR and NRR electrocatalysts due to their outstanding mechanical properties and electronic structures.Through first-principles calculations based on density functional theory,this thesis mainly studies the electrochemical CO2 reduction ability on 2D SnSe2 monolayer,transition metal(TM)and non-metallic B atoms supported on vacant C3N surface(V-C3N).;and the possibility of nitrogen fixation on novel MXenes materials.In addition,the catalytic system was further modified by defect engineering and strain engineering with the aim to explore its catalytic performance in electrocatalytic reduction.The specific research contents of the thesis are as follows:(1)First,we chose a surface defect-size engineering strategy to optimize the catalytic activity on 2D SnSe2 nanosheets for CO2 reduction.Our results show that the basal plane of SnSe2 can be activated through the formation of a Se vacancy,where the CO2 can be efficiently captured and reduced into CH4 with a low limiting potential of-0.58 V.By applying strain and constructing the Janus V-SnSeS structure,the structure-activity relationship is established,which provides an idea for further regulating the catalytic activity of CO2RR.It is found that the theoretical limiting potential of SnSe2 monolayer can be as low as-0.12 V at 6%strain.(2)Next,the CO2RR performance of the V-C3N supported transition metal(TM)and boron atom catalysts was systematically studied.The results show that the existence of the"dual-active pair" site can effectively activate up to three CO2 molecules,providing conditions for the formation of multi-carbon products.Charge layout analysis and free energy calculations show that a pair of TM-B can significantly activate at least one CO2 molecule,with TM anchoring C atoms and B trapping O atoms.Through the screening of 3d transition metals,Mn2-,Fe2-,Co2-,Ni2-B2@C3N have good stability,high activity and exclusive CO2RR selectivity.Subsequently,we separately explored the selectivity of the preliminary hydrogenation reaction with different numbers of adsorbed CO2 molecules.The results show that in all the catalysts,the proton-electron pair preferentially reacts with the O atom to break one of the C-O bonds of CO2 to form*OH and*CO,which are adsorbed on the B atom and TM,respectively.Among them,the coupling reaction on Mn2-,Co2B2@C3N to generate*CO.CH intermediates are thermodynamically unfavorable.After successive hydrogenation reduction to*CO.CH2,the two parts will be reduced independently,and finally C1 products,such as CH4 are formed.For Fe2-,Ni2-B2@C3N,the reaction of dehydration produces two*CO and continuously hydrogenates to form*CO.CH intermediates and then smoothly couples to form C2 products,such as CH3CH3 and CH3CH2OH.In addition,Fe2-B2@C3N successfully formed C3 products,such as C3H7OH and C3H8 through two-step coupling on the basis of activating three CO2 molecules with an onset potential of only-0.71 V.It is found that the maximum C-C coupling energy barrier for the formation of C2+products is only 0.16 eV,which is much lower than the competitive reaction of protonation.(3)Finally,the centimeter-scale MoSi2N4 monolayer synthesized by chemical vapor deposition is a novel 2D MXenes material(Science,2020,369,670).The MoSi2N4 structure with N-Si-N-Mo-N-Si-N spacer layer can be viewed as a MoN2 layer sandwiched between two Si-N bilayers.The Si atoms in the surface layer are considered to be the active phase for fixing N2.We systematically evaluate the catalytic performance of a series of MSi2N4(M=Ti,Zr,Hf,V,Nb,Ta,Cr,Mo,and W)monolayers for NRR after exposure of N vacancies.It is found that the pre-hydrogenated surface was more likely to generate N vacancies to expose active silicon atoms.Then,through a three-step screening criterion:activity,surface reconstruction and stability,it was found that TiSi2N4 and TaSi2N4 monolayers are the most potential and promising electrocatalysts because they can(ⅰ)successfully active N2,(ⅱ)undergo one round of catalytic cycling without strong surface reconstruction,and(ⅲ)be stabilized at high temperatures.In addition,a comprehensive NRR mechanism study revealed that the NRR process proceeds through the Mars-van Krevelen(MvK)mechanism,where the calculated limiting potentials of TiSi2N4 and TaSi2N4 are only-0.41 and-0.46 V,respectively,indicating that both are the most potential electrocatalysts.