Design And Mechanism Study Of Electrochemical Catalysts For CO_<sub>2 Reduction And Hydrogen Evolution Reaction

The energy crisis caused by excessive dependence on fossil fuels and the environmental problems caused by excessive emission of carbon dioxide（CO₂）seriously threaten the sustainable development of human society.It is of great value and practical significance to resolve the energy crisis and realize the low-carbon economy to reduce small molecules of CO₂,H₂O,etc.into useful chemicals and fuels by electrochemical methods.However,the high cost of catalyst,high overpotential,poor durability and selectivity are still the key factors restricting further industrialization.Therefore,the design and development of cost-effective electrocatalysts is still a major challenge.Based on the first-principles method,a series of single metal atom dispersion catalysts（SAC）and alloy catalysts were designed by doping and anchoring according to the two-dimensional porous carbon materials and alloys synthesized in the experiment,and the effects of coordination environment,single metal atom loading,supports and metal doping on the performance of catalysts were systematically investigated.Through detailed research on the reaction mechanism and activity origin,the catalytic performance of these catalysts for electrochemical CO₂reduction（CO₂RR）and hydrogen evolution reaction（HER）was predicted.The specific contents and results are as follows:1.In addition to the widely studied coordination atoms such as C and N,heteroatom B is also expected to be used as the coordination atom of SACs to achieve efficient CO₂electroreduction.We used the single or double vacancy graphene supported Fe C_x（x=3 or 4）SACs as the model prototype to explore the effect of the introduction of different content B atoms on the activity and product selectivity of catalysts.Through density functional theory calculations,eight catalysts are identified as effective electrocatalysts for CO₂reduction by effectively activate CO₂molecules and significant inhibit competitive HER.Among them,Fe C₃gives priority to HCOOH product with a limiting potential of-0.62 V,while the final product of other catalysts is CH₄.In particular,Fe B₂C and Fe B₂C₂^h（^hrefers to a cis structure）show the higher CO₂RR activity with the less negative limiting potentials of-0.24 and-0.40 V,indicating the optimal content for doping B atoms.Remarkably,the adsorption energy of reaction intermediate O has a high linear correlation with the limiting potential.The activity mechanism shows that the CO₂RR activity and product selectivity of catalysts are significantly improved after the introduction of B atom,The more negative d band center and the optimal magnetic moment of Fe atom contribute to the high activity of the catalyst.2.Employing density functional theory computations,the graphdiyne（GDY）and holey graphyne（HGY）supported single-atom catalysts（SACs,M/GDY and M/HGY）have been demonstrated to be promising candidates for CO₂RR.Among the 60 catalysts constructed,25catalysts were found to activate CO₂effectively and inhibit competitive hydrogen evolution,and 8 of them（namely V/GDY,Cr/GDY,Mn/GDY,Fe/GDY,Ru/GDY,Os/GDY,Fe/HGY and Co/HGY）showed higher activity for CH₄production than Cu（211）due to lower limiting potentials.Remarkably,changing supports was found to greatly affect limiting potentials and reaction pathways,even lead to a“inert-active”transition for some central metal.The resulting Mn/GDY,Co/HGY,Ru/GDY and Os/GDY can achieve high activity at low limiting potentials of-0.22～-0.58 V.Further analysis found that the binding strength of OH intermediate can be used to govern the activity of catalysts by adjusting the adsorption energy of other reaction intermediates.Machine learning further revealed that the polarized charge and magnetic moment of the central metal atom as well as the binding strength between metal atoms and supports play the key role in affecting the activity.Among them,the central metal with smaller polarized charge is more likely to induce the moderate activation of CO₂molecules to achieve high CO₂RR catalytic activity.The built machine learning model has been proved to be also applicable to other types of SACs.3.Based on the experimental Ru Ir surface as the model prototype,the active sites of hydrogen evolution reaction on the transition metal atom-doped Ru Ir surface（TM-Ru Ir,TM=Fe,Co,Ni,Cu,Zn）were systematically investigated.Through stability analysis,we first determined the（111）surface terminated by Ru and carried out subsequent hydrogen evolution test.The results show that the best hydrogen adsorption site is near Ir site before doping.After doping transition metal,the best hydrogen adsorption site is transferred from Ir site to Ru site near the doping site.Particularly,Zn-Ru Ir（111）has the best HER performance,which is superior to Pt（111）,and the corresponding Gibbs free energy change and exchange current are-0.03 e V and 1.68 A·cm^-2.The electronic property analysis shows that the positive charge carried by the Ru atom near the doping site is the key to activate the Ru site to achieve high activity.In addition,Ir atoms near the doping sites also participate in the activation process.