First Principles Study Of MXenes Based Bifunctional Single Atom Electrocatalysts

With the global energy consumption and environmental pollution becoming more and more serious,people's demand for environmentally friendly and sustainable green energy is increasing.Among many green energy systems,electrochemical energy technology has become one of the most potential new energy technologies due to its advantages of clean,environmental protection and secondary utilization.Among them,fuel cell and metal air cell are energy devices which use oxygen electrode reaction as driving force to store and release electric energy.Due to the "zero emission" of pollutants and the high efficiency of clean energy conversion,the development and use of fuel cells and metal air cells have been widely concerned.However,it is a pity that the energy technology of these two kinds of batteries has been restricted by the slow kinetics of oxygen reduction reaction(ORR,discharge process)and oxygen evolution reaction(OER,charging process)in practical application.Therefore,the development of bifunctional ORR/OER electrocatalysts with high activity,high stability and low cost is of great significance for improving the performance of fuel cells and metal air cells,alleviating the energy crisis and environmental pollution.First principles calculations based on density functional theory(DFT)and quantum mechanics play an important role in the selection and design of highly active catalyst materials.It can not only reasonably simulate the chemical reaction conditions,predict the stability and activity of the catalyst,but also understand the catalytic reaction process and basic principle from the micro perspective.This paper makes full use of the relevant advantages of first principles calculation,through the study of ORR and OER reaction mechanism,puts forward a systematic and reasonable design scheme for high efficiency,low cost and sustainable development of dual effect catalyst.As a new type of two-dimensional transition metal based carbon(nitrogen)compound materials,MXenes show great advantages in the application of catalyst substrate materials due to its excellent conductivity,good electrochemical stability,high specific surface area,adjustable surface electrochemical properties and other characteristics.In addition,the single atom catalyst is the smallest atomic catalyst,which can greatly optimize the activity of the catalyst while improving the utilization of metal,and has a very broad application prospect in the field of electrochemical research.Therefore,in this paper,mxenes was used as the base material and metal atoms were introduced to design efficient ORR/OER bifunctional catalysts.The main research contents are as follows:(1)The first principles methods were used to design efficient bifunctional SACs based on MXenes.The Pd/Pt single atom were introducing to the surface of Nb₂CT₂(T= O;F;OH)to regulate its electronic structure.When Pt single doped on the surface of Nb₂CO₂ and Nb₂CT₂(expressed as Nb₂CO₂-Vo-Pt and Nb₂CF₂-VF-Pt),the SACs exhibit both excellent ORR and OER catalytic activities.Especially for Nb₂CF₂-VF-Pt,its activity even exceeds the traditional Pt(111)and IrO₂(110)catalysts,and the theoretical over potential of ORR and OER is lower than 0.40 V and 0.37 V.The high efficiency of the Nb₂CF₂-VF-Pt was revealed by the electronic structure,volcano diagram and charge transfer mechanism in the catalytic process.The high activity depended on the electron delivery ability of the SACs and the synergistic effect of functional groups with Pt atoms.In addition,the utilization of Nb₂CF₂-VF-Pt was reached 100%.(2)Based on the first principles calculation method and the catalyst model of the previous part,the properties of Pt SACs with 26 kinds of typical MXenes were systematically studied.The stability of the single atom catalyst was evaluated by the calculated formation energy and the diffusion energy barrier of Pt atom.By calculating the charge transfer mechanism,it revealed that Pt is not only the active center of the SACs,but also the charge transfer medium between MXenes and reactant.The main factors affecting the catalytic performance were also investigated,including the electronic states of Pt-5d,the work function of SACs,the electronegativity of transition metal layers in MXenes,the ability of C and N layers obtaining electrons,and the number of electron empty orbitals in O and F functional groups.It was found that when the MXenes composed with V,Ti,Nb and Cr metals,the SACs showed effective ORR activities.Particularly,Nb₂CF₂-VF-Pt,Nb4C3F2-VF-Pt and Cr2 Ti C2F2-VF-Pt,which also had excellent OER activity.This study not only screened out efficient bifunctional SACs,but also revealed the deep reasons for their high activities,which highlighted the great potential of MXenes in designing bifunctional ORR/OER SACs.(3)Combined with previous literature reports and the previous two parts of research work,based on the first principles calculation,this study explored all possible MXenes materials,and aimed at screening O-termianted MXenes based Pt SACs with higher stability and activity.Firstly,by calculating the catalytic properties of series Pt SACs based on the 21-MXenes.Results showed that the low activity of the O functional terminated SACs were due to the low electron density around Pt and C atoms.Then a property method was proposed to adjust the electronic structure of MXenes by introducing a new metal layer to the middle layer of the 21-type MXenes,where the introduced middle layer should have lower work functions.Based on this research idea,18 kinds of potential O-functionalized Pt SACs were designed,and their catalytic activities were evaluated by calculating the over potential,solvation effect,reaction energy barriers.Four kinds of O-functionalized catalysts(Cr2Nb2C3O2-VO-Pt,Cr2Ta2C3O2-VO-Pt,Cr2 Nb C₂O₂-VO-Pt and Cr2 Ta C₂O₂-VO-Pt)with excellent ORR/OER bifunctional activities were selected.It was found that the target SACs were all supported by the Cr-based MXenes,which demonstrated the importance of Cr based MXenes in designing of bifunctional Pt SACs.(4)In this study,first principles calculations were used to design and screen a series of highly efficient MXenes based non noble metal SACs.Firstly,the electronegativity and work function values of the later transition metals and M(M represents the transition metals in MXenes)were summarized.Then,representative materials were selected from each group to design single atom catalyst.By calculating the charge transfer mechanism between the metal atoms and the MXenes,and the adsorption energy of O2 molecules on the surface of these SACs,feasible MXenes screened out for designing non noble SACs,that was M = Mo or W.Then,using Mo2CT2 and W2CT2(T = O;F)as base materials and Mn,Fe,Co,Ni,Cu,Zn as active sites,24 possible nonnoble SACs were constructed.By calculating the formation energy of catalysts and the theoretical overpotential of ORR and OER,four kinds of non noble SACs with bifunctional catalytic activity were selected.Especially for Mo2CO2-VO-Fe,both the ORR and OER activities were better than the traditional catalysts,and even better than those Pt SACs based on MXenes.In addition,we compared the effects of Mo2CT2 and W2CT2 on the activities,and the essential reason determine the high activity was because of the higher electronegativity of Mo2CT2 than W2CT2.This work provided a reliable theoretical basis for the study of bifunctional nonnoble SACs based on MXenes.