In Situ Study of the Corrosion Behavior of Pd-/Pt-Based Noble Metal Nanocrystals with Liquid Cell TEM

Author:Shan Zuo

Supervisor:deng tao zuo jian bo


Degree Year:2018





The fuel cell is a device that mixes the fuel(such as hydrogen,methanol,etc.)with oxygen and converts the chemical energy to the electric energy in the presence of electrocatalysts without direct combustion.The developments of green energy make important contributions to optimize the current structure of energy used in our country,in order to achieve sustainable energy development and to effectively alleviate the existing environmental pollution problems.In the proton exchange membrane fuel cells,the widely used catalysts are noble platinum-(Pt-)based nanoparticles no matter in cathode and anodic reactions.However,the expensive price and the limited reserves of Pt restrict its large-scale commercial applications.In addition,the change and evolution of the morphologies,structures and components of Pt-based electrocatalysts induced by corrosion during the electrochemistry reactions,may decay their electrocatalytic activities.It is important to develop new Pt-based electrocatalysts with high activities,good stability,and low Pt content,which will undoubtedly promote the improvements of fuel cells.In situ liquid transmission electron microscopy(TEM)is an emerging technology in recent years.It uses two-layer electron beam-permeable thin film(Si Nx or graphene)to encapsulate a small amount of liquid between two chips to build a micro reactor with liquid environment.Then the behaviors of nanomaterials and their chemical reactions are able to be observed and recorded in liquid phase under static or flowing liquid environment in TEM.By means of the high time and high tempo resolution of TEM,this technique can be used to study the dynamic evolution of nanomaterials in liquid phase and further analyze the reactions mechanisms.In order to figure out the above scientific questions and techniques,we synthesized different morphologies Pd@Pt core-shell nanoparticles(cube,octahedron,and icosahedron)with low Pt content and study their corrosion behaviors by in situ liquid TEM,revealing the mechanisms of their corrosion and degradation,and relating influence factors.The main research and results are as follows:1.We successfully obtained Pd@Pt core-shell nanoparticles with the morphologies of cube,octahedron,and icosahedron with ultrathin Pt atomic layer(3-5 layers)via seed growth method and epitaxial deposition technology in liquid.The nanoparticles were uniform and the average size was 14 nm.2.We studied and compared the corrosion behaviors of regular and defected(corner defected and terrace defected)Pd@Pt cubes by in situ liquid TEM.Results showed that the inner Pd atoms were etched out and formed Pt cubic frames.Further study indicated that the corrosion mode in regular cube was galvanic etching,while there was another halogen etching induced by Br-ions in defected cubes.Galvanic etching occurred at corners,decided by the coordination number and surface free energies;while halogen etching occurred at defects sites,where inner Pd atoms contacted and reacted with Br-ions in the liquid directly and formed [Pd Br4]2-.This reaction was faster due to the lower redox potential comparing with the galvanic etching between Pd and Pt,which made halogen etching dominant in defected cubes.The entire etching processes of regular and corner defected cubes could be divided into three stages of initialized(I),retarded(II)and rapid(III)with a relationship of Stage III > Stage I > Stage II in average etching rate.However,in the case of the terrace defected cubes,only two stages were found and showed and order of corrosion: Stage II > Stage I.Halogen corrosion and galvanic corrosion competed with each other within one defected cube.Halogen corrosion inhibited galvanic corrosion,the faster the halogen corrosion,the more inhibited galvanic corrosion would be.3.We studied and compared the corrosion behaviors of 14 nm and 37 nm Pd@Pt octahedron by in situ liquid TEM.Results showed that inner Pd atoms of the big octahedron were etched out within 5 min,while the small one kept unchanged.There were two different etching modes: corner etching and terrace etching,which may appear on one nanoparticle separately or together,causing the etching of Pd atoms.Atomic HAADF-STEM characterization showed that the deposition of Pt atomic layer on the surface of Pd octahedral seed determined the corrosion of octahedron nanoparticles.Somewhere was not covered by Pt,showing some defects on the big octahedron,at which sites inner Pd atoms contacted with Br-ions directly.However,the surface of the small octahedron were fully covered by Pt,isolating the inner Pd atoms from Br-ions,avoiding the corrosion behavior.4.We studied the corrosion behavior of Pd@Pt icosahedron by in situ liquid TEM.Results showed that the corrosion initially started at twin defects and preferentially etched at {111} surface.Comparing the different corrosion behaviors between monocrystalline cubes and twin structural icosahedron,there were several differences.Firstly,corrosion in icosahedron was faster than that in cubes.Secondly,cubes had better stability than icosahedron.Thirdly,the corrosion in cubes were etched from corners to center gradually,while the corrosion in icosahedron initiated at twin defects and preferentially etched at {111} surface.Electron beam dose rate could also affect the corrosion behavior.No corrosion occurred when no electron beam irradiation.The corrosion time was 16 s when the dose rate was small(563 e-/?2·s).When the electron beam dose was large(1600 e-/?2·s),the etching time was 6 s.5.We obtained ultrathin Pd nanosheets with an average diameter by wet-chemical method.The thickness of a nanosheet was 2.2 nm.We observed the corrosion behavior of Pd nanosheets and their structure and performance changes in liquid.By utilizing cationic/anionic surfactant(CTAB/SDS),we could realizing the controllable of assembly and disassembly of Pd nanosheets in different size(10 nm,15 nm,and 25 nm).Not only could the oxidation corrosion resistance of Pd nanosheets be improved by this means,but also their performances could be long-term stored.Our results also showed that free-standing Pd nanosheets were extremely unstable and decomposed into porous or nanoparticles within 9 days,whereas the assembled Pd nanosheets stacks maintained their twodimensional structure and preserved the methanol oxidation reaction(MOR)properties after disassembling.After 9 days,the mass activity of the sample after disassembled from the assembled Pd nanosheets decreased only by 13%.While the mass activity decreased by 74% for the free-standing Pd nanosheets after 9 days.Moreover,Pd nanosheets were not stable under electron beam irradiation.The surface of nanosheets were prone to etched into porous.This study will focus on the investigation of corrosion phenomena of fuel cell catalysts in applications,revealing the corrosion kinetics of nanoparticles,and further guide and design the porous and nanoframesstructured electrocatalysts with more active sites.This will be of great significance to the development of fuel cells.