Study On The Structure Of The Transition Metal-based Material And Its Effects On Electrochemical Performance

With energy shortages and serious environmental pollution problems,the development of new environmentally friendly energy sources has become an urgent problem to be solved.Among them,the use of electrocatalytic technology to convert electric energy into a new type of high-energy-density energy source is considered a very effective strategy.Transition metal basic catalysts have become one of the most widely used catalyst materials due to their wide sources,abundant reserves,and low price.However,transition metal-based catalysts still face problems such as poor stability,low activity,and high reaction energy barriers.In response to these problems,reasonable regulation of the active site structure is considered to be one of the most effective strategies.In this process,accurate analysis of the atomic and electronic structure of the catalyst and a full understanding of the mechanism that these structures affect performance are important foundations for rational design of catalyst performance.However,current research in this area is still lacking.Therefore,in this paper,we will adjust the structure of the active centers of transition metal-based materials to obtain catalyst materials with excellent performance,and combine advanced material structure characterization,first-principles calculations,electrochemical performance characterization and other means to combine the system.The research structure determines the relationship to its performance.details as follows:(1)In the first work,we designed a single Cu atom as an active site for CO electro-reduction of CO molecules.By loading Cu single atoms on the transition metal carbide Ti₃C₂T_x(also a kind of MXene material)nanosheets.The electrocatalytic test results show that Cu-SA/Ti₃C₂T_xhas an ultra-high selectivity of98%for the formation of C₂products(C₂H₄and Et OH).It has a selectivity of up to71%for C₂H₄products at a voltage of-0.7v relative to the reversible hydrogen electrode.In addition,its electrochemical activity can be stable for 68 hours without any degradation.The spherical aberration correction electron microscope and synchrotron radiation XAFS method were used to successfully analyze the coordination structure of Cu single atoms,and further combined with first-principles calculations to accurately explain the structure determining the stability and reduction activity of the catalyst.The interaction with the support Ti₃C₂T_xmakes the Cu single atom firmly stabilized on the Ti₃C₂T_xsurface,forming a single-atom catalyst with stable structure.The coordination environment of Cu single atom modulates the electronic structure of Cu,which makes it have more excellent CO reduction activity.It can be seen that Ti₃C₂T_x(an MXene material)is a very good substrate material that stabilizes and adjusts the single-atom structure of the metal.(2)In the second work,we further extended the MXene substrate to the Pt single-atom catalyst,and used the interaction with the substrate to adjust the electronic structure of the active site of Pt,resulting in a unique electronic structure of local electric field polarization.Through spherical aberration correction electron microscope and synchrotron radiation XAFS characterization methods,it is analyzed that Pt single atom atoms form an asymmetric coordination environment on the MXene substrate by the bonding mode of Pt-O and Pt-Ti.Density functional theory(DFT)calculation results show that this asymmetric coordination environment leads to polarization of the local electric field around the single Pt atom.The HER electrochemical performance test in an alkaline electrolyte environment shows that the Pt-SA/MXene catalyst only needs a low overpotential of33 m V to reach a current density of 10 m A cm^-2,and its quality under an overpotential of 100 m V The activity(23.5 A mg Pt^-1)is even 29.4 times higher than commercial Pt/C.Unlike the comparative samples MXene and Pt(111)when there is no polarizing electric field,the H*configuration adsorbed on the active site of Pt-SA is no longer the exact apex adsorption configuration,but occurs along the direction of the polarizing electric field.Deviate and the adsorption energy barrier is closer to zero.Therefore,combining theoretical calculations and literature research results,we speculate that the locally polarized electric field formed around a single Pt atom is the main reason for the excellent HER activity of the Pt-SA/MXene catalyst.In addition,the analysis of the projected density of states(PDOS)further proved the positive effect of the local polarization electric field on the improvement of the catalytic activity of HER from the perspective of the electronic structure.(3)Through the above two works,the important role of structural modification of active sites in improving catalyst performance is fully demonstrated,and it is proved that the combination of structural characterization methods and first-principles calculations can clearly analyze the relationship between catalyst structure and performance.Therefore,in the third work,we will further promote this work to analyze the three-dimensional atomic structure of point defects in transition metal oxide catalysts,and combine the first-principles calculation method to predict this structure The effect that performance will have.The three-dimensional configuration of Fe atom vacancies and Frenkel defects in NiFe₂O₄film is analyzed by using the atomic-scale HAADF image simulation technology.Firstly,several possible defect structure models were inferred from the results of the HAADF experimental atomic image analysis of NiFe₂O₄.Then,using density functional theory(DFT)and HAADF image simulation technology,a defect atomic structure model that matches the HAADF experimental image was confirmed.Afterwards,the electronic structure and OER catalytic performance under the NiFe₂O₄defect structure were theoretically calculated.The results show that the electronic structure of NiFe₂O₄with Fe vacancies and Frenkel defects produces favorable modulation,which improves its OER performance.In the above work,excellent catalyst materials were successfully obtained through the modification of the active site structure of transition metal catalysts.The combined application of structural characterization methods and theoretical calculations helped us fully understand the effect of structure on performance,and even helped us predict the effect of the structure on performance.The electrocatalytic performance of the designed structure provides important guidance for the rational design of the catalyst.