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Preparation And Characterization Of New Carbon Composites Supported Precious Metal Nanoparticles Catalysts And Their Electrocatalytic Properties

Posted on:2014-10-18Degree:DoctorType:Dissertation
Country:ChinaCandidate:F X JiangFull Text:PDF
GTID:1261330398496882Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Low-temperature fuel cell has attracted a great attention due to its wide applications.The performance of fuel cell system is essentially governed by the electrocatalyst includingmetal nanoparticles and support materials, since they strongly influence the electrocatalystperformance, durability and efficiency. It is well known that carbon black is the mostconventional support material with high specific surface area but contributes mostly withmicropores of less than1nm, which will lead to the limited accessibility and low activityof catalysts. Therefore, the development of alternative low cost, large surface area,high-performing and stable support materials are urgently required. In this paper, weprepared a series of new carbon support materials such as conducting polymers, conducingpolymers/graphene and carbon black/graphene composites in order to improve thedispersion, stability and electrocatalytic activity of metal nanoparticles. Theelectrocatalytic performance of metal or bimetallic alloy nanoparticles on the new supportmaterials toward methanol or ethanol oxidation in acid or alkaline media were investigatedby electrochemical measurements and compared with commercial Pt/C (JM20wt.%)catalyst. To further evaluate the electrocatalytic performance of as-prepared catalysts, theeffects of support materials on metal nanoparticles were discussed. These studies of newelectrocatalysts may provide an idea and a basic theoretical reference for futureprospecting of catalyst support materials.(1) Pt-based catalysts with free-standing poly[poly(N-vinyl carbazole)](PPVK) assupport on a glassy carbon electrode (GCE) have been successfully prepared by anelectrochemical method (Pt-M/PPVK, M=Pd, Au, and Ru). Cyclic voltammetric (CV)and chronoamperometric (CA) methods are used to investigate the electrocatalytic activityand stability of as-prepared catalysts. It is found that PPVK film as support effectively enhances the catalytic acitivity of Pt nanoparticles for methanol electrooxidation inalkaline medium. The bimetallic Pt-M nanoparticles obtained potentiostatically on thePPVK film show a higher electrocatalytic activity than the bare Pt/GCE and JM Pt/Cattributed to the porous structure and large surface area of PPVK. Poly(5-cyanoindole)(P5CN), an excellent electron and proton conducting polymer with porous structure, wasincorporated with highly dispersed and small sized Pt-Cu nanoclusters on GCE byelectrochemical CV co-electrodeposition. The Pt-Cu nanoclusters-decorated P5CN(Pt-Cu/P5CN) shows remarkable electrocatalytic activity toward methanol oxidation inacid medium. CV and CA methods were used to investigate the catalytic activity andstability of as-prepared catalysts. The experimental results indicate that the P5CN assupport and Cu introduced into Pt enhance the catalytic activity due to the good dispersionof Pt-Cu nanoclusters with the large electrochemical active surface area and the synergisticeffect.(2) Layered electrochemically reduced graphene oxide (ER-GO) sheets incorporatedwith poly(3,4-ethylenedioxythiophene)(PEDOT) have been fabricated as an efficientsupport for Pt nanoparticles on GCE. The as-prepared Pt-loaded PEDOT/ERGO compositeelectrode exhibits not only the high mass peak current density (390A g-1) but also the goodlong-term catalytic stability toward the ethanol electrooxidation. The Pt/PEDOT/ER-GOalso shows stronger tolerance to poisoning species compared with the JM20%Pt/Celectrode. The high electrocatalytic activity of Pt/PEDOT/ER-GO is mainly described tothe good electrochemical activity of PEDOT/ER-GO composites and the well-dispersed Ptnanoparticles resulting in the large electrochemical active surface area of Pt (47.1m2g-1).(3) The support materials play a critical role for the electrocatalytic oxidation ofethanol on precious metal catalysts in fuel cells. The poly(3,4-ethylenedioxythiophene)combined with reduced graphene oxide (PEDOT-RGO) as the support of Pd nanoparticles(NPs) for ethanol electrooxidation in alkaline medium was investigated. The as-preparedPd/PEDOT-RGO composite catalysts are characterized by Raman spectrometer, X-raydiffraction, transmission electron microcopy, and scanning electron microcopy.PEDOT-RGO composite with the porous structure facilitates the dispersion of Pd NPs witha smaller size leading to the increase of electrochemical active surface area. The electrochemical properties and electrocatalytic activities of Pd/PEDOT-RGO hybrid areevaluated by cyclic voltammetry, chronoamperometry, CO stripping voltammetry,electrochemical impedance spectroscopy (EIS) and Tafel analysis. The results suggest thatPd/PEDOT-RGO hybrid shows a higher electrocatalytic activity, a better long-termstability, and the poisoning tolerance for the ethanol electrooxidation than Pd on carbonblack. EIS and Tafel analysis indicate that PEDOT-RGO improves the kinetics of ethanolelectrooxidation on the Pd NPs and is an efficient support in fuel cells.(4) Chemical reduced graphene oxide (RGO), inexpensive and available in largequantity, has been recognized as the potential support materials in fuel cells. Unfortunately,graphene sheets suffer from the agglomeration in solution during the reduction reaction.One effective route for applications would be to incorporate graphene sheets in compositematerial to prevent the agglomeration of graphene. Carbon black (C) as the commonsupport may be a good candidate for carbon composite due to the π-π interaction betweenthem. In this study, the PtRu/C-RGO composite catalysts were prepared by incorporatingthe carbon black and graphene with PtRu alloy nanoparticles. The electrocatalytic activitiesof PtRu/C with different Pt/Ru atom ratio and PtRu/C-RGO with different RGO contentwere investigated by cyclic voltammetry and chronoamperometry towards the carbonmonoxide and ethanol oxidation in acid media. PtRu/C with the Pt/Ru atom ratio of59:41shows the highest electrocatalytic activity and stability. For the PtRu/C-RGO catalyst, thehighest mass peak current density and the best catalytic stability for ethanolelectrooxidation was observed on the PtRu/C-RGO3with the50wt.%RGO. It is also beenfound that the presence of carbon black is key to prevent the agglomeration of RGO sheetsand obtaining a large electrochemical active surface area for support material. Also, theRGO plays an important role for the dispersion of PtRu nanoparticles and the enhancementof CO tolerance.(5) The PtPd catalyst has received wide attention in fuel cells for alcoholelectrooxidation. The Pt1-xPdx/C (x=0,0.27,0.53,0.77,1) alloy catalysts were preparedusing formic acid as a reductant and their electrocatalytic performance toward ethanoloxidation were tested and compared by a series of electrochemical measurements such aslinear sweep voltammetry (LSV), CV and CA in acid and alkaline media. Pt1-xPdx/C catalyst (x <0.77) shows an enhanced electrocatalytic activity, CO tolerance and stabilitytoward ethanol and CO oxidation in acid media. By comparison, Pt1-xPdx/C catalystpresents a much larger electrocatalytic activity in alkaline media. The electrocatalyticperformance of Pt1-xPdx/C depends on the Pt/Pd atoms ratio and the highest electrocatalyticperformance can be found to be Pt73Pd27/C in acid media and Pt23Pd77/C in alkaline media,respectively. The enhanced electrocatalytic activity may be due to the electron structure ofPtPd alloy in alkaline media toward ethanol electrooxidation. In addition, to pin down theexternal influencing factors, systematic studies have been conducted on the ethanolelectrooxidation at Pt1-xPdx/C catalysts as a function of ethanol concentration, H2SO4andKOH concentrations, and scan rates.
Keywords/Search Tags:Conducting polymer, Reduced graphene oxide, Precious metals nanoparticle, Composite, Electrocatalytic oxidation
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