Preparation Of Pd (Pt) Based Catalysts And Their Electrochemical Performance In Fuel Cells

In recent years, direct alcohol fuel cells (DAFCs) based on liquid fuels have attracted enormous attention as power sources for portable electronic devices and vehicles due to their high power density, low pollution and low operating temperature. However, the electrooxidation of alcohol is not very easy so catalysts with excellent catalytic activity and stability are usually desired. As the most important catalysts of DAFCs, noble metal catalysts such as Pt and Pd have received wide recognition for years. Nevertheless, the high cost and limited reserves of the noble metal materials block the development of DAFCs. Therefore, the study on electrocatalysts focuses on the improvement of the catalytic activity and reduction of the cost.To imporove the catalytic activity and lower the cost of catalyst, graphene supporting PdAu nanoparticles and one-dimensional Pd or Pt based materials were parepared in this dissertation. Their structure, composition, morphologies and electrochemical performace were detailedly studied. The main points are summarized as follows:(1) Chemically reduced graphene oxide sheets-supported PdAu (PdAu/CRG) nanocomposites were prepared facilely by co-reduction of graphene oxide sheets, PdCl2and HAuCl4. Then the PdAu/CRG nanocomposites were characterized by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results reveal that PdAu bimetallic nanoparticles with an average diameter of6nm are dispersed uniformly on the chemically reduced graphene oxide sheets (CRG). The electrocatalytic performance of the PdAu/CRG catalyst was studied by cyclic voltammetric and chronoamperometric measurements. Electrochemical experiments show that the PdAu/CRG catalyst has better catalytic activity and stability than carbon nanotubes and carbon podwer supported PdAu nanocomposites for ethanol oxidation, indicating that the readily available CRG is a good catalyst carrier for ethanol oxidation in alkaline media.(2)Te nanowires with the diameter of10nm and length of several micrometers were prepared by a hydrothermal process, and the nanowires were used as templates and reductants to systhsize Pd nanowires and nanotubes. Scanning electron microscopy and transmission electron microscopy were employed to characterize the structure, composition and morphologies of the Pd nanowires and nanotubes, and the results show that the Pd products have similar morphologies to Te nanowires. Electrochemical results indicate that the catalytic stabilities of Pd nanowires and nanotube for ethanol oxidation are obviously better than that of Pd nanoparticles supported by carbon.(3)PdPt bimetallic nanotubes were prepared by the self-assembly of Pt and Pd on Te nanowires at room temperature. The morphologies of the as-prepared PdPt nanotubes were investigated by scanning electron microscopy and transmission electron microscopy, and the results display a large amount of PdPt bimetallic nanotubes with a diameter of10-20nm and a length of several micrometers. The composition and structure of the nanotubes were characterized by X-ray diffraction, high-resolution transmission electron microscopy, scanning transmission electron microscopy, and energy spectrum analysis, and the results display uniform compositional distributions of both elements (Pd and Pt). The mechanism of the formation of the nanotube structure was supposed. The electrocatalytic performance of PdPt nanotubes were studied by cyclic voltammetry and chronoamperometry. Electrochemical results show that the as-prepared PdPt nanotube catalysts have not only high activity but also good stability for ethanol oxidation in alkaline medium. The catalytic stability of PdPt nanotubes is obviously better than that of carbon supported PdPt nanoparticles.(4)Pd nanowires with diameter of10-20nm and length of several micrometers were prepared and monolayer of Pt was deposited on the Pd nanowires by using copper underpotential deposition and subsequent replacement of Cu by Pt. The products were characterized by using scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and energy spectrum analysis. The electrocatalytic performance of PdPt nanowires was studied by cyclic voltammetry and chronoamperometry. Electrochemical results show that the monolayer of Pt can improve not only the activity of Pd nanowires but also the stability for ethanol oxidation in alkaline medium. The Pt nanoparticles obtained by means of the replacement process between H2PtCl6and the Cu (deposited at normal potential) could improve the catalytic activity of Pd nanowires, while the stability descend.(5) Porous Pt layer was deposited on the surface of Pd nanotubes using a chemical process. The scanning electron microscopy and transmission electron microscopy images show that the diameter of Pd nanotubes become larger and the surface become rough after deposition of Pt. The Pt layer is composed of many small Pt nanoparticle with diameter about2-3nm. The gaps of Pt naoparticles structure the porous Pt layer. Electrochemical results show that catalytic activity of PdPt nanocomposites exceeds carbon supproted Pd nanoprticles after the deposition of Pt layer. (6) Long and porous Pt botryoidal nanowires were facilely synthesized simply using Te nanowires as hard template accompanying chemical reduction ascorbic acid and inducement of polyvinylpyrrolidone. The scanning electron microscopy and transmission electron microscopy images show the as-prepared product is botryoidal nanowires with the diameter of20-30nm and length of several micrometers. High resolution transmission electron microscopy shows the Pt botryoidal nanowires are composed of many small Pt nanoparticles (about3nm in diameter), which is just like that many grapes grow on the branch. These small nanoparticles make Pt nanowires have a botryoidal and porous structure. Moreover, the diameter of Pt BNWs can be adjusted by changing the dosage of Pt precursor, polyvinylpyrrolidone and L-ascorbic acid. The electrocatalytic performance of Pt botryoidal nanowires is studied, which shows that the as-prepared Pt botryoidal nanowires catalysts have not only better activity but also better stability for oxygen reduction reaction than carbon supported Pt nanoparticles.