Construction And Electrochemical Mechanism Study Of Multi-component Synergistic Transition Metal-Nitrogen-Carbon Oxygen Reduction Catalysts

Fuel cells and metal-air batteries have garnered attention as highly effective and environmentally friendly energy conversion devices.However,due to the expensive nature of the platinum（Pt）-based catalysts used in the cathode,their widespread application has been severely hampered.Therefore,it is essential to develop non-precious metal catalysts to replace Pt-based catalysts and profoundly address the issue of high prices.Transition metal-nitrogen-carbon（MNC）catalysts have been received extensive research due to their low cost,controllable structure,and oxygen reduction reaction（ORR）activity that can be comparable to expensive Pt/C catalysts under alkaline conditions,and even superior to Pt/C catalysts in terms of stability.However,MNC catalysts and Pt/C catalysts still show distinct levels of activity under acidic conditions.In order to narrow the gap to meet commercial applications,it is particularly important to further improve the activity of MNC catalysts.Multicomponent regulation of MNC sites is an effective way to optimize the adsorption and desorption process of active sites and reaction intermediates,and further accelerate the kinetics of the reaction.However,the issues such as unclear synergistic effect mechanisms and complex preparation methods are still exist.In this thesis,a series of efficient and stable multicomponent regulation of MNC catalysts were constructed through precise control of single-site MN₄ structures through long-range interactions of sulfur functionalities,short-range interactions of phosphorus atoms,synergistic effects of M ₁N₄ and M₂N₄,and electronic regulation of atomic clusters on MN₄.Additionally,the mechanism of introducing components and the formation of MN₄ structures and the structure-performance relationship were systematically investigated.The main contents are as follows:（1）Through the self-sacrificial template strategy modified by small molecules,atomically dispersed Fe-N₄ sites were successfully constructed and embedded into nitrogen,sulfur doped carbon materials.The study discovered that the introduction of small-molecule sulfur sources can alter the density of single-atom Fe active sites in addition to acting as a pore-forming agent to promote the formation of a microporous structure.Further,the different sulfur sources will affect the proportion and c ontent of sulfur functionalities（C-S-C and C-SO_x-C）,as well as the relative content of active site Fe-N₄.X-ray absorption spectroscopy and theoretical calculations showed that C-S-C and C-SO_x-C have electron-donating properties,which can transfer electrons to Fe-N₄ through the conjugatedπbond of the graphitic carbon layer,thereby weakening the strong binding ability of Fe sites to reaction intermediates of*O,accelerating the kinetic process of ORR.Based on this synergistic effect,microporous structure and abundant active sites density,the optimized Fe SNC-TA catalyst exhibited half-wave potentials of 0.76 V and 0.91 V vs.RHE in 0.5 M H ₂SO₄ and 0.1 M KOH,and excellent stability.（2）A soft-template method was adopted to prepare Fe₂P atomic clusters decorated with asymmetric N,P dual-coordinated Fe sites.Scanning transmission electron microscopy and X-ray absorption spectroscopy analyses confirmed that the atomically dispersed iron atoms were coordinated with three nitrogen atoms an d one phosphorus atom,forming a"Fe N₃P₁"configuration,accompanied by the embedding of Fe ₂P atomic clusters in the carbon layer structure.Thanks to this unique coordination mode and hierarchical porous structure,the half-wave potential of the optimized Fe PNC catalyst showed 0.76 V and 0.90 V vs.RHE in 0.5 M H₂SO₄ and 0.1 M KOH.Theoretical calculations showed that the electronegativity and atomic radius differences of N and P atoms allowed for P coordination and Fe ₂P atomic clusters to break the electronic symmetric structure of Fe centers,thus optimizing the adsorption free energy of Fe center sites with*O.The presence of Fe₂P atomic clusters can weaken the strong adsorption capacity of the Fe N₃P₁ central site and*OH.And the two synergistically promote the reaction kinetics.（3）Based on the fact that metal atoms will escape from the surface of metal blocks under high temperature and high pressure conditions,the coexistence of single-atomic Fe and Cu sites decorated nitrogen-doped porous carbon has been successfully fabricated by hydrothermal synthesis of Fe,Cu co-doped ZIF-8 in the presence of Fe and Cu foam with subsequent NH₃ pyrolysis.According to experimental findings,introduction of Cu foam can greatly increase the density of Fe active sites.Scanning transmission electron microscopy and X-ray absorption spectroscopy confirmed that Fe and Cu were atomically dispersed and coordinated with four nitrogen atoms to form Fe N₄ and Cu N₄ coordination structures.Theoretical calculations showed that the introduction of Cu N₄ sites optimized the electronic structure of Fe N ₄ sites,accelerated the desorption of the*OH intermediate,and promoted the ORR kinetics.Benefiting from this,the half-wave potentials of Fe Cu SACs/NC catalysts with bimetallic sites showed 0.75 V and 0.89 V vs.RHE in 0.5 M H₂SO₄ and 0.1 M KOH,respectively.（4）A strategy for the preparation of active sites composited with bimetallic atom clusters and single-atom sites was developed.By using a Mn-doped Cd metal-organic complex as a precursor,the pre-coordinated Mn sites and amorphous carbon rich in defect structures were produced by low-temperature treatment,which was used as the substrate for anchoring Fe³⁺ions.Then,after a high-temperature atomic rearrangement process,atomically dispersed Mn sites with Fe Mn atom clusters were successfully prepared.Scanning transmission electron microscopy and X-ray absorption spectroscopy confirmed that Mn was atomically dispersed and coordinated with four nitrogen atoms to form a Mn N₄ configuration,while Mn in the Fe Mn atom cluster was coordinated with two N bonds on the plane.Finally,a composite structure of Fe Mn atom clusters and Mn-N₄ was obtained.According to electrochemical analysis,the catalyst for the composite structure showed half-wave potentials of 0.79V and 0.90 V vs.RHE in 0.5 M H₂SO₄ and 0.1 M KOH,respectively.Theoretical calculations showed that the active sites of this composite structure were conducive to the cleavage of O-O bonds and promoted the kinetic process of ORR.