Theoretical Design And Mechanism Study Of Two-dimensional MOF As Electrocatalysts For Oxygen Reduction And Carbon Dioxide Reduction Reaction

Renewable energy conversion technologies are effective methods to solve the global energy crisis and environmental problems.Fuel cells and carbon dioxide electrolyzers have attracted the attention of many scientific researchers as two new,efficient,and green renewable energy devices.In these two energy conversion devices,oxygen reduction reaction（ORR）is essential for fuel cells,and CO₂ reduction reaction（CO₂RR）is an effective strategy for reducing CO₂ into fuels and value-added chemicals using intermittent renewable electricity.Both of these key electrochemical reactions require highly efficient electrocatalysts to reduce their overpotential to increase reaction rate and efficiency.Although noble metal-based catalysts（such as Pt and Cu）are highly efficient ORR and CO₂RR electrocatalysts,their commercial applications are severely limited by their high cost,low reserves,and poor stability.Therefore,exploring low-cost,abundant,efficient and stable ORR and CO₂RR catalysts is key to renewable energy technology.Metal organic framework（MOF）has shown great application in the field of electrocatalysis due to its super high specific surface area and structural diversity.Based on the first principles,this paper systematically studied the catalytic performance of a series of MOF systems as ORR and CO₂RR electrocatalysts,and discussed the internal reaction mechanism in depth.The main content and research results of the paper are as follows:1.A series of bis（iminothiolato）metals（TMIT,TM=Mn,Fe,Co,Ni,Cu,Ru,Rh,Pd,Ir and Pt）were theoretically investigated as ORR electrocatalysts.The calculations show that ORR is a 4e^-reaction process on TMIT.The scaling relations are obtained between the Gibbs adsorption free energies of the oxygenated intermediates(ΔG_*OOH,ΔG_*O,ΔG_*OH).The volcano plot of the ORR activity betweenΔG_*OHand overpotential（η）is established.For the pathway*OOH→*O+H₂O,Fe IT has the best ORR catalytic activity with the working potential of 0.71 V,close to 0.78 V for pure Pt.The migration of O atom from the S site to the Fe top site is the rate-determining step with an energy barrier of 0.59 e V,smaller than0.80 e V for pure Pt.2.A series of TMIT（TM=Mn,Fe,Co,Ni,Cu,Ru,Rh and Pd）were theoretically investigated as CO₂RR electrocatalysts.It is found that the CO₂RR activity of TMIT depends on the choice of the central metal atom.Our results demonstrated that HCOOH is the main product for TMIT（TM=Mn,Fe,Co,Ni,Cu,Pd）.Among them,Mn IT and Fe IT are promising CO₂RR catalysts for the HCOOH production due to the good selectivity and catalytic activity with the low overpotential of 0.34 V and 0.35 V,respectively.For Ru IT and Rh IT,the main product is CO,and the strong adsorption of CO on the catalyst surfaces would poison the catalysts.This makes Rh IT and Ru IT are not good CO₂RR catalysts.3.A series of metal hexahydroxybenzene frameworks（M₃（HHB）₂,M=Cr-Cu,Mo,Ru-Ag and W-Au）were theoretically investigated as CO₂RR electrocatalysts.The results demonstrated that the catalytic activity of M₃（HHB）₂ depends on the intensity of interaction between CO₂RR intermediates and metal atoms,and can be adjusted by changing the metal atoms.Among the studied candidates,M₃（HHB）₂（M=Cr,Mo,Ru,and Rh）are predicted to be potential electrocatalysts toward the CO₂RR due to low limiting potential of-0.49,-0.67,-0.63,and-0.68 V,respectively,which are comparable to that of other reported CO₂RR catalysts.In particular,CH₄ is the favorable product on M₃（HHB）₂（M=Cr and Mo）via*HCOO pathway,while the main product of M₃（HHB）₂（M=Ru and Rh）is CH₃OH via*COOH channel.For W3（HHB）2,it cannot effectively catalyze CO₂ due to ultrahigh limiting potential of -1.87V.