Rational Design Of Highly Efficient Alloy Catalysts And The Investigation Of Their Catalytic Mechanism:From Low To High-entropy

Fuel cells and water electrolysis with hydrogen energy as the carrier are important component of sustainable energy systems.And electrocatalytic reactions（such as oxygen reduction reaction（ORR）,hydrogen evolution reaction（HER）,alcohol oxidation reaction,etc.）play a key role in these systems.Platinum（Pt）catalyst exhibits efficient electrochemical activity due to the unique d electronic structure.Therefore,rational design Pt-based catalysts and improve the utilization of Pt active sites are important for the development of sustainable energy systems.Introducing other metals to adjust the electronic structure of Pt,combined with optimizing the catalyst morphology to expose more active sites,is an efficient strategy to improve the catalytic activity and stability.In the thesis,a series of Pt-based low-entropy and high-entropy alloys（HEAs）with unique morphology were proposed to regulate the electronic structure of active sites and increase the mixed entropy of the catalysts in combination with alloying,for further enhanced the catalytic activity.The electrochemical activities in ORR and alkaline HER of low-entropy and high-entropy Pt-based alloys are higher than that of commercial Pt/C,as well as stability.Firstly,open Pt Co nanoflowers（Pt Co NFs）composed of ultra-thin nanosheets were synthesized through a facile one-pot solvothermal method.Meanwhile,developing acid etching strategy to remove unstable Co species and obtain unsaturated coordinated Pt-rich Pt Co NFs assembled by radial ultra-thin nanosheets（～1.5 nm）.The Pt-rich Pt Co NFs exhibit excellent ORR performance in aspect of mass and specific activity(2.63 A mg _Pt^-1,11.23 m A cm^-2),which are 17.5 and 38.7 times higher than that of commercial Pt/C,respectively.And the current density achieves 1,854 m A cm^-2 at0.6 V in fuel cell.The superior ORR performance is mainly attributed to the unsaturated coordination of Pt,which is induced by the optimized catalytic interface including defect sites,high-index facets and twin boundary.Also,theoretical calculation reveals that the ORR performance of low-coordination site is much higher than that of high-coordination site,and the tendency of which is in contrary to d-band center.These kinds of opening Pt-based catalysts not only effectively expose more active sites,but also accelerate the transport of protons and exchange of electrons.Secondly,based on the structure of highly efficient NFs with ultra-thin nanosheets,we develop one step wet chemical strategy to synthesize Pt（Co/Ni）Mo Pd Rh NFs with a thickness of 1.7 nm.The formation mechanism of Pt Co Mo Pd Rh NFs was established by the surfactant content and the quaternary alloys in Pt Co Mo Pd Rh system.And the effect of constituent metals on the electronic structure of HEA and strain effect induced by alloying and morphology were also investigated.The obtained Pt Co Mo Pd Rh NFs exhibit enhanced HER activity in 1.0 M KOH electrolyte with a mass activity 6.38times higher than that of commercial Pt/C.The multiple active sites of HEA and the alloying effect favor the reduction of water dissociation energy barrier and H*adsorption/desorption,as well as the relationship between the strain induced by the morphological structure and H*adsorption/desorption is established.This work not only provides a general method for synthesizing unique structure of HEAs,but also provides a fundamental understanding for the study of HEAs in catalysis.Thirdly,On the basis of the one-step wet chemical method for the synthesis of HEA NFs,Pt,Ni,Mo,Pd and Rh with different d-orbital states were introduced in purpose of regulating 3d,4d and 5d electronic orbitals,and a series of Pt Mo Pd Rh Ni nanocrystals（NCs）composed of nanosheets were synthesized for the first time by tuning the atomic ratio of constitute metals.The obtained Pt₂₈Mo₆Pd₂₈Rh₂₇Ni₁₅ NCs with the strongest d-d electron interaction display the highest alkaline HER activity,with the overpotential as low as 9.7 m V at a current density of-10 m A cm^-2.Combining in situ surface enhanced Raman spectroscopy with theoretical calculation to identify the active sites of Pt Mo Pd Rh Ni NCs in alkaline HER.The superior HER performance is mainly attributed to the multiple active sites with different local chemical environments and the interaction of d-d electron in Pt,Mo,Pd,Rh and Ni.This work not only elucidates the complex structural information and catalytic mechanism of multielement HEA system in depth for further rational design of efficient catalyst,but also highlights HEAs as sufficiently advanced catalysts and accelerate the research in energy-related applications.In this work,not only the ultrathin nanoflowers structure from low-entropy to high-entropy alloys were synthesized,but also the one-step wet chemical method was further developed from the preparation of ultrathin nanoflowers to nanocrystals of HEAs,which is a guideline for the design in morphological of HEAs in the future.Secondly,the relationship between coordination number and ORR activity is established by combining experiment and theory,and the enhancement of the overall HER activity by multiple active sites in HEAs,the strain effect on the H*adsorption energy of HEAs and the electronic regulation of constituent elements in the alloys are explored.