Active Sites Engineering And Structural Control Of Iron-based Catalysts For Electrocatalysis

Electrochemical energy conversion and storage technologies provide an effective approach to address the global warming and energy crisis.Rationally designing and engineering of high-performance catalysts are extremely crucial for enhancing the electrocatalytic efficiency and accelerating commercialization of the renewable energy devices.Transition metal iron-based materials are recognized as one of the most promising candidates for partially or totally substituting precious metal electrocatalysts.Unfortunately,current iron-based electrocatalysts e.g.,iron-nitrogen-carbon(Fe-N-C)and iron oxide,have insufficient catalytic activity and stability for viable applications.In this thesis,aiming to obtain Fe-based electrocatalysts with high efficiency and durability,we design and synthesize some unique structures through favorable morphology control,composition control,defect engineering,and confinement effects.1.A simple and gentle protection strategy was reported to realize effective nitrogen reduction reaction(NRR)performance using defect-rich and hollow shell structured Fe₃C/Fe₃O₄ heterojunction coated with carbon frameworks(Fe₃C/Fe₃O₄@C).The Fe₃O₄ part contains significant defects which play a role as active sites,and the Fe₃C serves as a trigger to induce the occurance of in-situ oxygen vacancy.The catalyst achieves NH₃ yield of 25.7?g h^-1 mg^-1_cat.and Faradic efficiency(FE)of 22.5%at-0.20 V vs.RHE in 0.1 M HCl solution along with remarkable stability through the synergistic effect between oxygen vacancy and heterojunction.Illustrated by density functional theory(DFT)calculations,the surface oxygen vacancies improve the N₂ adsorption and activation in the electrocatalytic process,and the rate-determining step is formation of NH₃*via the enzymatic mechanism.2.A molecular iron catalyst i.e.,tetraphenylporphyrin iron chloride(Fe TPPCl),with well-defined Fe N₄ configuration structures was chosen as a model to elucidate the complex multiple proton and electron transfer NRR processes competing with the undesirable hydrogen evolution reaction(HER).It exhibits promising NRR activity with the highest NH₃ yield(18.28±1.6?g?h^-1?mg^-1_cat.)and FE(16.76±0.9%)at-0.3 V vs.RHE in neutral electrolytes.Importantly,¹⁵N isotope labeling experiments confirm that the synthesized NH₃originated from the direct reduction of N₂in which ¹H NMR spectroscopy and colorimetric methods were performed to quantify NH₃ production.Also,in situ electrochemical Raman spectroscopy studies reveal that the breakage of the Fe-Cl bond in the Fe TPPCl catalyst is a prerequisite for initiating the NRR.DFT calculations further verify that the active species is Fe porphyrin complex[Fe(TPP)]^2-and the rate-determining step is the first hydrogenation of N₂ via the alternating mechanism on the[Fe⁰]^2-sites.3.An N-enriched hierarchically porous carbon electrocatalyst containing trace Fe was designed and synthesized.The formation of complex active sites caused highly improved electrocatalytic activity compared with singly N-or Fe-doped active sites because of a synergistic effect.The structural advantages prompted the NCF-900 catalyst to demonstrate outstanding performance in multifunctional electrocatalysis with excellent overall oxygen electrode activity(?E=E_OER,10-E_ORR,1/2 of 0.770 V),impressive durability in 0.1 M KOH and self-assembled rechargeable Zn-air batteries,and remarkable NRR activity and selectivity(no by-product N₂H₄).4.An effective step-by-step self-assembly strategy was developed to allocate single Fe and Ni(or Co)sites on the outer and inner walls,respectively,realizing separate-sided different single-atom functionalization.Such a core-shell nanostructure engineering could maximize the potential of M-N-C for multistep catalytic reactions that required the presence of active sites with different functionalities simultaneously.Also,the unique method was versatile in regulating different catalytic activities by controlling the active sites of different building units.The p-FeNC@Co NC catalyst exhibited the remarkable oxygen reduction reaction(ORR)activity,excellent stability,and encouraging durability in H₂–air fuel cells,while the p-FeNC@Ni NC catalyst demonstrated outstanding CO₂ reduction reaction(CO₂RR)activity.