First Principles Study On The Catalytic Performances Of Surface Terminal Of Two Dimensional Metal Carbonitride

The development of advanced battery and fuel technology are essential to the improvement of the aerospace power technology.Chemical energy,incuding hydrogen,oxygen,hydrocarbons and so on are the important components of the spacecraft power system.The main focuses of chemical energy research is to develop catalysts for the efficient preparation of these gases and compounds.The two dimensional transition metal carbonitrides?MXenes?with high stability,large specific surface area and high conductivity have potential application prospects in the field of energy conversion.In this thesis,to realize design the efficient electrocatalysts,we take MXenes as the research object to systematic study the catalytic performances of MXenes by using first principles calculations,and hope this study can provides a theoretical guidence for synthesizing of high efficiency catalysts.The possibility of synthesis MXenes is the premise for the application of MXenes.In the experiment,the two-dimensional MXenes structure is obtained by etching and exfoliation ceramic phase MAX.In this thesis,we firstly systematic explore the exfoliation possibility of more than 50 MAX phases.The results show that for most of MAX phase,the higher the binding energy between A and MX,the higher exfoliation energy of MAX exfoliated into MXenes.The exfoliation energy is lower than 0.253 eV/?2 will favor the exfoliation of MAX and higher 0.253 eV/?2 will nor favor exfoliation.By etching Cr2 Al C into HF solution,the final product is the Cr₂CO₂,and the Cr₂CF2 and Cr₂C?OH?2 will translate into Cr₂CO₂ even if they were generated firstly during etching reactions,indicating that the highly stability of Cr₂CO₂.In addition,the delamination process requires Li+?Li F?cation,and carbon vacancy is easily to be generated during etching and exfoliation reaction.Hydrogen energy is a new kind of fuel,has the characteristics of high combustion value and cleanness,which is of great significance to replace the traditional fossil fuel and realize the enhancement of space power.The HER activity of MXenes was systematically investigated.Furthermore,with Cr2 C as the case,the effect of transition metal and carbon vacancy defects on the HER performance of Cr₂CO₂ was studied.Results show that the surface terminated O-p orbital center??p?of MXenes can describe the HER activity of MXenes.The MXenes can obtained optimal HER performances at ?p from-4.1 to-3.3 eV,and the HER obey theVolmer-Heyrovsky mechanism.After modulating by Ni and Co of Cr₂CO₂,the structure of Ni/Cr₂CO₂ and Co/Cr₂CO₂ can deliver optimal HER at suitable Ni or Co coverage,?GH* approximate to 0 eV.At same time,the lower carbon concentration can modulate the HER performances of Cr₂CO₂.In addition,the M?2M?C2O₂ MXene also can obtain optimal HER activity at suitable h ydrogen coverage.Electrolysis water evolution of hydrogen is controlled by the other half of the reaction,the oxygen evolution?OER?.For the OER involving multiple electrons and protons,this thesis takes Cr₂CO₂ as the case,and further discusses the HER and OER mechanism of Ni₂ P interface composite and single metal atom anchored on the surface of Cr₂CO₂.Results show that the overpotential of HER and OER on Ni₂P/Cr₂CO₂ are 0.09 and 0.80 V,respectively,which indeed improved the catalytic performances of Cr₂CO₂ compare to the pre-composite systems.The HER on Ni₂P/Cr₂CO₂ interface obey Volmer-Heyrovsky mechanism,and the activation energy lower as 0.68 eV.Further study show that Ni anchored on the surface of Cr₂CO₂ possesses highest HER and OER activity among the study system,with the corresponding overpotential of HER and OER are 0.16 and 0.46 V,respectively.At the same time,the structure of the Ni anchored system is stable and it is not easy to form Ni nanoclusters.Carbon/nitrogen/hydrogen compounds have important applications in the space field.Therefore,this thesis investigates the possibility of MXenes as ORR activity for fuel cells and CO₂ reduction into CO?CO₂RR?.Results show that Sc₂CO₂,W2NO₂ have the highest ORR activity,with the corresponding overpotential of ORR are 0.68 and 0.54 V,respectively.Nb₂CO₂ and Ta₂CO₂ have highest CO₂ redution into CO activity,with corresponding limiting potential are 0.23 and 0.37 V,respectively.The appearing F terminal will weaken the ORR and CO₂ RR activity of MXenes,thus the F terminal should be avoided appearred in experiment sysnthesis of MXenes.In addition,the activity of different transition metal supports as N2 reduction catalyst on the surface of Ti₂NO₂ was studied.Results indicate that the Mo/Ti₂NO₂ system show metallic characteristics.Due to the hybridization of Mo and N2,Mo could effectively promote N2 dissociation.In the system studied,the Mo anchored on the surface of Ti₂NO₂ has the lowest NRR overpotential,which is 0.16 V.In this thesis,the catalytic activity and mechanism of O* terminal MXenes were theoretically predicted,and the interface composite and the single-metal supported MXenes catalyst was designed to provide theoretical guidance for the experimental synthesis of excellent HER,OER and oxygen/nitrogen/carbon dioxide reduction catalysts.