Study On Controlled Synthesis And Electrocatalytic Activity Of Platinum And Platinum-copper Nanoparticles

Up to present,with the consumption of fossil fuels,researchers are increasingly focusing on the development of alternative energy sources.Direct fuel cell(DFC)has become a research hotspot for researchers due to its high energy efficiency and low pollution.It is well known that Pt-based nanomaterials are the most commonly used and highly active anode catalysts in DFC.However,the high cost and low stability of Pt-based catalysts limit their widespread applications.Therefore,it is necessary to develop platinum based electrocatalysts with high durability,which can promote the development of direct fuel cell technology.In this dissertation,the effect of morphology,size,phase-structure,composition of Pt and Pt-Cu nanoparticles and the optimization of their components on catalytic activity and stability have been studied.The morphology,size,composition and phase-structure of platinum-based nanoparticles could be controlled by suitable synthesis methods.The effect of these factors on the electrocatalytic activities of catalysts was analyzed.The research findings possess important theoretical and practical significance for improving the working efficiency of direct fuel c ells.In this dissertation,a simple chemical reduction method was used to control the morphology,size and phase-structure of Pt nanoparticles at low temperature by adjusting the p H value of the initial solution,the DEG volume fraction and the addition rate of reducing agent.TEM,EDS,XPS,BET and XRD were used to characterize the morphology,size,conposition and phase-structure of the nanoparticles.The effects of initial p H value and DEG volume f raction on the morphology,size and phase-structure of Pt nanoparticles are studied.Meanwhile,the electrocatalytic properties of Pt nanoparticles synthesized under different conditions were determined by a electrochemical workstation.The influence of the morphology,size and phase-structure on the electrochemical catalytic properties of the nanoparticles are deeply studied.The following innovative results were obtained:(1)The effect of initial solution p H on the average size and the phase-structure of Pt nanoparticles was studied:In the synthesis by low-temperature chemical reduction,the average size of Pt nanoparticles can be changed to a certain extent by adjusting the initial solution p H value.The size gradually increases with the solution p H from 3 to 7;however,the average size of Pt nanoparticles can decrease with the solution p H from 7 to 11 because the hydrogen ion concentration in the solution affects the number of Pt nucleation,so as to control the average particle size.In addition,the average size of Pt nanoparticles can be also controlled by slowing down the metal atomic reduction rate and diffusion rate.The p H value of the initial solution is changed from 3 to 7,the dominant-facet index on crystal surface is gradually transformed from(111)to(200).However,when the p H values of the solution changed from 7 to 11,the dominant-facet index on the crystal surface of of Pt nanoparticles is reversely transformed from(200)to(111).This phenomenon appears due to the fact that hydrogen ions play the role of end sealer in the synthesis of Pt nanoparticles under acidic reaction condition,but,sodium ions surve as end sealer under alkaline reaction condition.(2)The change of DEG volume fraction in initial solution severely affects the morphology and phase-structure of Pt nanoparticles.With the increase in DEG volume fraction,the morphology of Pt nanoparticles synthesized gradually appears from isolated microparticles to aggregation of reticular nanowires(meshes),and from meshes to thick particles.Such a variation rule is determined by the concentration of hydroxyl groups in the DEG solution.Meanwhile,the DEG concentration in initial solution also plays an important role in controlling the dominant facet index exposed on the the crystal surface of Pt nanoparticles.(3)The electrocatalytic activities of Pt nanoparticles for the ethylene glycol,methanol and ethanol oxidation reaction:The axtivities closely depends on the average size and the phase-structure of Pt nanoparticles.On the whole,the smaller Pt nanoparticles with more(111)facet exposure possess the higher electrocatalytic activity for the ethylene glycol,methanol and ethanol electrocatalytic oxidation.Their mass activity and specific activity are much higher than those of commercial Pt/C.The excellent electrocatalytic stability of Pt nanoparticles can be also attributed to the fact that the Pt nanoparticles with dominant(111)facet and the smaller size can provide a large number of catalytic active sites.(4)Pt nanoparticles synthesized at 50%DEG solution include morphology of nanowires mixed with nano-nets so that(111)-dominant facet exposure and a large number of grain boundaries exist.Therefore,these Pt nanoparticles have much higher catalytic activity and stability for ethanol oxidation reaction and ethylene glycol oxidation reaction than the commercial Pt/C.(5)Pt/Cu ratio in Pt-Cu alloy nanoparticles is basically as agreeable with that of platinum and copper in the initial precursor.there are closer dimensions among of Pt-Cu nanoparticles with different co mpositions.the results of XRD and HRTEM show that Pt-Cu nanoparticles of different compositions all have the phase structure of(111)-dominant facet exposure.Therefore,we found that Pt-Cu alloy nanoparticles with controllable alloy composition can be sy nthesized by low temperature chemical reduction.Among the alloy nanoparticles,Pt₇₁Cu₂₉nanoparticles have the best electrocatalytic activity and stability for the electrooxidation of ethylene glycol and ethanol.