Synthesis And Structure-Activity Relationship Of Single-Atom/Low-Nuclear Cluster Electrocatalysts

In recent years,single-atom catalysts have been widely studied in the field of renewable energy conversion due to their 100%theoretical atomic utilization,unique coordination structure and electronic properties,and the relatively uniform active sites.Scientific problems in the study of single-atom catalysis include:(1)How to reduce the surface energy of metal atoms in the synthesis process to avoid particle agglomeration;(2)How to regulate the structure of the active site and optimize its binding energy with the reaction intermediates to improve the catalytic performance.However,studies at home and abroad have shown that it is difficult to combine precise preparation with scalable synthesis,the binding energy between active site and reaction intermediate is not ideal,and the structure-activity relationship needs to be further optimized and improved.Aiming at the above problems,this dissertation starts with the application of single-atom/low-nuclear cluster electrocatalysts in the field of renewable energy conversion.Then several novel and mild defect adsorption-photoreduction and low temperature gas migration methods are proposed to achieve precise and scalable synthesis.At the same time,several novel active site structures of metal-oxygen coordination,bimetallic single-atom and low-nuclear cluster are designed to optimize their binding energy with the reaction intermediates and explore their structure-activity relationships.The main research contents and results are as follows:(1)A cationic defect adsorption-photoreduction method is developed to anchor platinum clusters in anatase nanocrystals/graphene.Spherical aberration electron microscopy and synchrotron radiation results showed that Pt atoms coordinated with oxygen atoms in anatase are in the form of dimer to form Pt2-O4-Tix microstructure.The theoretical calculation shows that under the excitation of electric or optical field,part of the charge will transfer from the bulk phase of the catalyst to the active center(Pt2-O4-Tix),which reduces the energy barrier of the rate-limiting steps of the hydrogen evolution reaction(HER).Hence,the synthetic catalyst shows excellent activity and stability in both electro-and photocatalytic HER.(2)An oxygen-containing functional groups adsorption-photoreduction method is developed to anchor platinum single atoms on the surface of multi-walled carbon nanotubes.Functionalized multi-walled carbon nanotubes simultaneously play three important roles:(i)photoelectronic generator to reduce platinum ions under irradiation;(ii)carrier to anchor Pt ions;(iii)conductive network connecting all the active sites.Spherical aberration electron microscopy and synchrotron radiation results showed that platinum is mainly in the form of single atom and coordinated with oxygen atoms on f-MWCNTs to form the microstructure of Pt-O4-C.It is proved that the coordination structure of Pt-O is very important for the mass activity and stability of Pt single-atom catalyst.(3)A facile low temperature gas transfer method is developed to synthesize transition metal low nuclear cluster catalysts with oxygen coordination configuration.Spherical aberration electron microscopy showed that Ni,Co and Fe atoms were dispersed on the surface of MWCNT in the form of atomic clusters.Synchrotron radiation results showed that Ni atoms were coordinated with four oxygen atoms in the form of pentamers to form Ni5-O4-C microstructure.The experimental results show that the synthesized Ni-MWCNTs electrocatalyst exhibited the best intrinsic activity and stability of oxygen evolution under alkaline conditions.(4)A co-pyrolysis and impregnation reduction method is developed to synthesize Fe/Co bimetallic single-atom catalyst.Spherical aberration electron microscopy,synchrotron radiation and theoretical calculation results show that Fe and Co are atomically anchored on graphene nanosheets,which are coordinated with three nitrogen atoms respectively and bridged by one oxygen atom to form a stable N3Fe-O-CoN3 dual-site.The introduction of Co atom leads to a more uniform distribution of Fe 3d orbitals,which reduces the binding energy of the overall oxygen-containing intermediates.Hydrogen bonding caused by bridge oxygen stabilizes*OH and*OOH intermediates,and ultimately reduces the energy barrier of oxygen reduction and evolution reaction.Therefore,the synthetic catalyst has outstanding performance of bifunctional oxygen electrocatalysis and rechargeable zinc-air batteries.(5)An ion exchange-molten salt template method is developed for the synthesis of nitrogen-doped porous graphene-based low-nuclear nickel cluster catalysts.Spherical aberration electron microscopy and synchrotron radiation results showed that nickel was dispersed in the N-doped graphene in the form of hexamer clusters,and was coordinated with N and C atoms to form the Ni6-N/C microstructure.The synthetic catalyst has many excellent functions as the cathode host in lithium-sulfur batteries:(?)selectively adsorbing soluble lithium polysulfides(LiPSs),(?)accelerating liquid-solid phase transfer at the active site,and(?)accelerating the conversion kinetics of LiPSs and inhibiting the shuttling effects.