First-principles Study Of Single-Stom Electrocatalysts For Carbon Dioxide

With the rapid development of new energy industry,electrocatalytic energy conversion and storage technology play important roles in reducing carbon dioxide（CO₂）emissions,obtaining high value-added chemicals and reducing dependence on fossil energy.The design and development of high-performance electrocatalysts are the current imperative for realisation of energy conversion.Single-atom catalysts（SACs）have been widely used in various chemical reactions as they not only achieve 100%utilization of metal atoms,but also have high activity and selectivity of reactions.In this work,we have investigated the CO₂ reduction reaction（CO₂RR）performance on two-dimentional materials,including Mo Se₂ supporting transition metal（TM@Mo Se₂）and metalloporphyrin（MPP）SACs using density functional theory（DFT）,and further explore the effects of ions(Cl^-,Na⁺and S^2-)in the reaction environment on the catalytic performance.We have explained in detail the intrinsic mechanism of the CO₂RR at the atomic scale,and provided corresponding theoretical guidance for future research on the design of high-performance CO₂ catalytic materials.The main contents of this thesis are concluded as follows:1.The CO₂ reduction performance of TM@Mo Se₂ SACs was investigated.The low electrical conductivity and few catalytic active centers of Mo Se₂ material lead to its low reaction efficiency,which is not conducive to the electrocatalytic reduction reaction.The results of DFT calculations show that Fe,Co,Ni and Cu doping can successfully activate the inert surface of Mo Se₂ and regulate its electronic structure to improve electrical conductivity.Detailed calculations of the CO₂RR selectivity and free energy change on TM@Mo Se₂ reveal that the reduction products of these materials are all CH₄,with Cu SACs supported on the hollow position and the top of the Mo atom of Mo Se₂ are able to carry out the reduction reaction at lower overpotentials（η=0.486 V and 0.487 V）.Our work shows that TM@Mo Se₂ is very promising as an alternative to noble metal catalysts and provides a guiding role for the experimental synthesis of SACs with high catalytic efficiency.2.The CO₂ reduction performance of MPP SACs was investigated.MPPs are widely used in catalytic reactions because they have similar structural and functional characteristics to enzymes.Mn,Fe,Ni,Cu,Mo,Pd,Ir,and Pt transition metal atoms are capable of binding to porphyrin rings to form stable complexes.The results indicate that these porphyrin-based SACs with different metal centers exhibit different catalytic performance during CO₂RR due to their different electronic structures.Ni,Cu,Pd and Pt-based porphyrin catalysts have reduction overpotentials greater than 1 V and are the less desirable materials,while Mn,Fe,Mo and Ir-based catalysts can produce HCOOH at overpotentials of 0.028 V,0.271 V,0.529 V and 0.675 V,respectively.The results provide insight into the effect of changes in electronic structure on the reaction and faciliate the development and application of catalysts for CO₂RR reduction reactions.3.The effects of the reaction environment near the catalytic site on the CO₂catalytic performance of MPPs were investigated.The ions(Cl^-,Na⁺and S^2-)were introduced in the vicinity of the metal active site by doping,and the calculated results show that the doping of Cl^-changes the product selectivity of Mn,Fe and Ir-based catalysts and the most favorable reduction pathway for Mo-based catalyst,as well as reduces the reaction overpotentials of Ni,Cu,Pd and Pt-based catalysts.Further studies of Mn,Fe and Ir-based catalysts with HCHO as products reveal that the doping of Na⁺and S^2-also changes their CO₂ reduction selectivity.The calculations confirm that the doping of these ions affects the reduction reaction by changing the electronic structure and the chemical valence of the metal centers.Our work will provide theoretical guidance for the experimental design of electrolyte compositions that promote CO₂reduction reactions.