Structural Design Of Nano-carbon-based Iron Single-atom Catalysts And Electrocatalytic Reduction Performance

Energy and the environment are the two primary issues facing humanity in the 21st century.Due to the extreme consumption of fossil fuels,the dramatic increase in the energy demand,leading to catastrophic pollution and high levels of greenhouse gas pollution,electrochemical reduction technology,the core of new fuel cells,can fully utilize the available clean and renewable energy sources and reduce the environmental impact.The overpotential and kinetic rate during the reduction reaction depend on the catalyst structure and the design of the active site.Precious metal catalysts are expensive and have poor stability,which limits their large-scale application.Transition metal Fe-based catalysts are considered one of the best materials to replace noble metal catalysts because of their abundance,high activity,and stability.However,iron oxide and iron-nitrogen-carbon(Fe-N-C)catalysts’ active sites,stability,and catalytic mechanism have been controversial.In this thesis,four iron-agent catalysts with high catalytic performance were developed by focusing on iron-based catalysts and achieving the precise design of the structure and active sites of the materials through strategies such as macroscopic morphology,microstructure,modulation of chemical components,electronic structure modulation,and defect construction.The main research ideas are as follows:1.To address the problems of poor catalytic stability and unclear catalytic mechanism,a heterogeneous catalyst with atomic iron sites immobilized on a porous carbon(FesA-NO-C)substrate with N,O-doped in anti-opal structure was designed and prepared,which has a high yield of NH3 3 1.9 μgNH3 h-1 mg-1 and an electrocatalytic Faraday efficiency of 11.8%(at-0.4 V),which exceeds almost all previously reported atomically dispersed metal-nitrogen-carbon catalysts.Theoretical calculations show that the high NRR catalytic activity of FesA-NO-C catalysts is mainly due to the charge transfer optimization between adjacent O and Fe atoms uniformly distributed on the porous carbon carrier,which not only significantly facilitates the transport of carbon N2 and ions but also effectively reduces the binding energy between the separated Fe atoms and the*N2 intermediate as well as the thermodynamic of the ratedetermining step(*N2→*NNH)Gibbs free energy.2.In response to the competition with hydrogen precipitation reaction during NRR reaction,iron single atoms dispersed in nitrogen-sulfur co-doped carbon material(FesA-NSC)were designed and developed.The catalyst exhibited excellent NRR activity in 0.1 M HCl solution with an NH3 yield of 30.4 μgNH3 h-1 mg-1 and Faraday efficiency of 21.9%.Both experimental and theoretical calculations show that the introduction of S in Fe-N molecules effectively activates the ortho-monovalent Fe sites and enables the ideal eg electron(t2g6eg1)filling of ortho-monovalent Fe in FeN3Si,which makes the catalyst moderately active with nitrogen with proper adsorption energy and facilitates the kinetic process of N2 reduction.15N isotope labeling experiments demonstrate that nitrogen in NH3 is from nitrogen reduction,in situ infrared spectroscopy experiments confirm that the catalyst promotes the process of the nitrogen hydrogenation reaction,and density flooding theory(DFT)calculations indicate that FesA-NSC acts with nitrogen with moderate force and has appropriate bond length and adsorption energy,which is favorable to promote the kinetic process of NRR(*N2→*NNH).In addition,the Zn-N2 cell with FesA-NSC catalyst as the cathode was able to achieve the ability to produce electricity and convert N2 to NH3 while providing a power density of up to 4.0 mW cm-2 and an energy density of 793 Wh kg-1.3.A reasonable morphology and porous structure design can promote the accessibility of active sites and the migration of reactants/products and accelerate the reaction kinetics.To address the problems of low catalytic activity,poor stability,and complicated preparation methods,one-dimensional porous iron/nitrogen-sulfur co-doped carbon nanorods(FeSA-NS/C)catalysts were designed and synthesized.Among them,different ligand structures were obtained by regulating different pyrolysis temperatures during the pyrolysis process to obtain the optimal catalytic performance.One of the ligand structures is the performance of the FeN3S1 catalyst for the production of hydrogen peroxide(H2O2)by oxygen reduction(ORR).The selectivity of H2O2 was 91.5%in 0.1 M KOH solution with a production rate of 1181.3 μmol h-1 g-1,which was applied to a gas diffusion electrode with a practical useful cumulative concentration of up to 1.5 mol/L.In situ synchrotron radiation and DFT theoretical calculations showed that electron transfer from Fe to O2,an increase in the density of states near the Fermi energy level,and an increase in adsorption energy,processes that contribute to the reduction of the electrocatalytic ORR H2O2 production energy potential.