Fabrication And Performance Study On Pt-Based Nanomaterials For Electroreduction Of Oxygen

Energy is the driving force of social development.For a long time,traditional energy sources（coal,oil,natural gas,etc.）have been exhausting,followed by environmental problems have become more and more serious.These factors result in that the sustainable development of human society is facing a severe challenge.The development and utilization of renewable clean energy has become the focus of global attention.As a new type of clean energy,fuel cells have received great attention in recent years due to their high energy conversion efficiency,high reliability,and low prices.Therefore,the development of fuel cells with excellent performance and environmental friendliness not only has important significance for energy,but also has shown infinitely bright prospects in various applications.However,since the fuel cell’s energy conversion efficiency is greatly limited by the sluggish kinetics of the cathode oxygen reduction reaction（ORR）,inexpensive and efficient electrocatalysts are needed to ensure the performance of the fuel cell.This thesis aims to develop high-performance electrocatalysts for fuel cells,and focuses on the design and synthesis of core-shell structures（Pt-based core-shell nanoparticles）,hollow structures（Pt-based nanocages）,and highly open dimensional（3D）structures（Pt-based alloys）at the nanoscale dimension.A new strategy is explored that wet chemical synthesis is used to prepare target materials as ORR electrocatalysts for fuel cells.The correlation of structure and catalytic performance is established by a series of characterization techniques to obtain electrocatalysts with high catalytic performances.In addition,this paper takes a systematic study on how structures,components,and compositions affect ORR performances,which provide reliable experimental data and valuable technical solutions for the development and application of fuel cells.The main content and conclusions are as follows:1.Pd@Pt core-shell nanoparticles with precisely adjustable shell thickness were designed.The influence of metal precursors,ligands and surfactants on the structure of the product was investigated to propose the formation mechanism of core-shell structures.The effect of shell thickness on surface strain and ORR performances was investigated.First,Pd@Pt core-shell nanoparticles were prepared by one-pot solvothermal method,and the reaction temperature was used to control the thickness of the shell layer.When the temperature increased from 140℃to 180℃,the thickness of the shell layer increased from 3 to 14 atomic layers,particularly for every 10℃increase in temperature,the thickness of the corresponding shell layer increased by 3 Pt atomic layers successively.When the Pt shell thickness is adjustable from 3 to 14 atomic layers,corresponding surface strain can be adjusted from-1.85%to-0.18%.Compared to the Pt/C catalyst,Pd@Pt_{n L}/C（n=3.4,5.3,8.2,11.0 and 13.9）showed enhanced ORR performances.Among them,the Pd@Pt_{n L}/C（n=3.4）catalyst has the highest catalytic activity(0.95 A mg _Pt^-1),which is 5.3 times that of the Pt/C catalyst.Quantum mechanics/molecular dynamics coupled simulation method calculations showed that the surface compressive strain decreases with the increase of the shell thickness.Pd@Pt_3.4L with the largest strain and showed the best Pt-O binding ability,and thus indicating the optimum activity.After8000 cycles,compared with Pt/C catalyst,Pd@Pt_{n L}/C catalyst showed significantly improved ORR stability.2.Pd@Pt-based core-shell nanoparticles with adjustable shell compositions were designed.Shell components in Pd@Pt-based core-shell structures could be tuned by changing the types of transition meatal precursors add in reaction solution,such as Pd@Pt-M1（M1=Ni,Co,Fe or Cu）and Pd@Pt Ni-M2（M2=Fe or Cu）core-shell nanoparticles.Taking Pd@Pt-Ni core-shell nanocrystals as an example,the effect of metal precursors and protective agents on the structure of the product were investigated to propose the formation mechanism of core-shell structures.Further,we studied the effect of the shell components on the ORR performance.Furthermore,the optimizable ORR activity can be achieved by doping Fe or Cu into the Pt Ni alloy shell.Compared with Pt/C catalyst,Pd@Pt Ni/C showed superior ORR activity（1.29 A mg-1Pt）.When the alloy Pt Ni shell was doped with Fe or Cu,the ORR mass activity of the obtained catalyst is improved to 1.45 A mg _Pt^-1 and 1.79 A mg _Pt^-1,respectively.The stability tests showed that compared with Pt/C,Pd@Pt Ni/C,Pd@Pt Ni Fe/C and Pd@PtNiCu/C have good ORR stability.3.Pd Pt alloy nanocages with precisely controllable wall thickness were designed.The effect of the etching environments on the product structure was investigated to propose the formation mechanism of nanocages,followed by study on the effect of wall thickness on ORR activity.Firstly,Pd@Pt core-shell nanoparticles were prepared by the one-pot method,and the reaction temperature was used to control the thickness of the shell.Then,the acid etching treatment was used to prepare alloy nanocages,and the wall thickness was controllable in the range of 5.3 to 13.9 monolayers.The reaction temperature increased by 10°C from 150°C to 180°C,and the corresponding wall thickness increased by 3 monolayers（5.3,8.2,11.0 and 13.9）.The electrocatalytic test showed that the nanocage catalyst has superior catalytic ORR activity and stability compared with the Pt/C catalyst.Amongst these,the Pd Pt_8.2L nanocage catalyst has the best activity,reaching 1.17 A mg _Pt^-1.After 8000 cycles,the Pd Pt_5.3L nanocage catalyst showed the best ORR stability.4.Pt Cu alloy catalysts were designed,followed by study on the effect of the controllable three-dimensional（3D）structures and surface composition on the ORR performances.First of all,we proposed a new wet chemical method to adjust the three-dimensional structure of Pt-based alloys by adding Ni（acac）₂,switching 3D structures between Pt Cu nested skeleton cubes（NSCs）and solid Pt Cu octahedral stars（OSs）.In the presence of Ni（acac）₂,achieving Pt Cu NSCs were assemble by one-dimension ultra-fine nanostructures.The product become Pt Cu OSs when Ni（acac）₂ is not added.Studies on the growth mechanism showed that Ni²⁺plays a positive dual role in the anisotropic growth of Pt Cu NSCs,which not only gives a faster reduction kinetics for Cu precursor,but also acts as a structure-directing agent.The surface of the Pt Cu NSCs/C catalyst treated with acetic acid（represented as Pt Cu A-NSCs/C）exposed a Pt-rich skin structure,and its ORR mass activity is as high as 5.13 A mg _Pt^-1,which is an increase factor of～26compared with commercial Pt/C catalyst.After 10,000 cycles,the Pt Cu A-NSCs/C catalyst showed excellent stability in ORR and 3D structures.