Study Of Synergistic Regulation On Electron And Proton For Highly Selective Carbon Dioxide Electroreduction Over Single-atom Iron Catalytic Sites

Direct usage of intermittent renewable energy in electrochemical CO2 reduction reaction(CO2RR)is one of the greenest routes to large-scale production of value-added chemicals for sustainable carbon cycle.Despite attractive prospects,it still faces fiendish puzzles thrown by the extremely inert C=O bond(806 kJ mol-1),doomed to undergo complex elementary steps one by one,in which two critical factors determine the overall process and efficiency of the reaction:electron,as "fuel",carries the electricity driving force;proton,as "key",attacks the oxygen atom off and unlocks the C=O bond.Carbon-based single-atom catalysts(SACs),composed of homogeneous,well-defined transition metal-nitrogen moieties with high turnover frequency,have demonstrated striking activity for CO2RR to yield CO.Nevertheless,their biggest drawback exactly lies in the structural simplicity that leaves little leeway to deploy efficiently the electrons and protons,what the parasitic hydrogen evolution reaction(HER)heavily competes for.Hence,appropriate function should be installed into SACs,serving as a "dispatcher" for delicate plan of fast electron tansfer and streamlined proton supply precisely targeted at CO2RR,in order to realize a high-rate,highselectivity CO2 conversion over single-atom sites.In this dissertation,a suitable Fe-N-C platform containing atomically dispersed Fe sites is engineered to implement the concept of synergistic regulation on electrons and protons by incorporation of condensed Fe3C nanoparticles.Major interests are highlighted below:In theoretical prediction section,density functional theory(DFT)calculations are performed to elucidate electronic structure of the atomic-Fe and nanostructured-Fe3C(A/N-Fe)active pairs anchored on graphitic layer.Charge density difference mappings suggest that there might be a combination of interactions of zero-valence Fe3C with,respectively,graphitic layer and atomic Fe,which establishes a favorable pattern of free energy on the catalytic system for electron transfer from Fe center to CO2RR intermediates.Density of states(DOS)curves verify that ferromagnetic Fe3C can spin polarize the d-shell of atomic Fe,which help weaken the adsorption of site-blocking*CO and undesirable*H on the Fe site,and downshift the bonded*COOH’s π*molecular orbitals for more antibonding electron occupation and thus easier C-O breaking.In practical verification section,a series of catalysts comprising N-doped carbon nanosheets loaded with different A/N-Fe configurations is prepared by altering paramaters of the pyrolysis using hemin and melamine precursor.Novel methodologies for kinetic analysis are proposed to reveal fine details about how A/N-Fe manipulates protons’ behavior.Electrochemical tests and in situ IR spectroscopy verify that A/N-Fe can rapidly activate CO2 to form*CO2·-and greatly relax C-O bond of the ensuing*COOH，both of which create favorable conditions for proton utilization.Kinetic isotope effect(KIE)analysis indicate that A/N-Fe can constrain and suppress the excessive protons to avert their involvement in HER.All these merits,collectively,manage the overwhelming priority of CO2RR over HER.The catalyst equipped with optimal A/N-Fe achieves a broad window of 300 mV for CO Faradic efficiency>90%(98%maximum),even surpasses numerous cutting-edge SACs.