【作者】 黄中原；

【导师】 旷亚非；

【作者基本信息】 湖南大学 ， 化学工程与技术， 2014， 博士

【摘要】 近年来,以液态醇类为燃料的直接醇类燃料电池由于其高效、清洁等优点受到了人们广泛关注。然而,将醇类直接电化学氧化比较困难,因此需要高催化活性及稳定性的催化剂。其中,贵金属催化剂,如Pd和Pt,其优异的催化性能已被人们所熟知。但是,贵金属的储量有限,价格相对昂贵,大大阻碍了直接醇类燃料电池的发展与应用。因此,目前有关燃料电池方面的研究大都集中在如何提高催化剂的活性及降低催化剂的成本等方面。本论文从降低催化剂成本、提高催化活性及稳定性等问题入手,制备了以石墨烯为载体的Pd基金属催化剂以及无载体的一维Pd、Pt纳米催化剂,并对其形貌、结构及组成进行了研究,同时详细考察了其电化学性能。研究内容主要包括以下几个方面：(1)以硼氢化钠为还原剂共还原氧化石墨、氯化钯及氯金酸成功制备了石墨烯的负载PdAu双金属纳米颗粒(PdAu/CRG)。采用X射线衍射、扫描电子显微镜及透射电子显微镜等对其结构、形貌及组成进行了分析,结果表明所制备的PdAu纳米颗粒的平均直径约为6nm,且均匀分布在石墨烯的片层中。采用循环伏安法及计时电流法对PdAu/CRG的电化学催化乙醇性能进行了研究,结果表明PdAu/CRG在碱性条件下有着比碳纳米管和碳粉负载的PdAu纳米颗粒更优异的乙醇催化活性及稳定性,是一种较为理想的碱性乙醇燃料电池催化剂。(2)采用水热法制备了直径约为10nm、长度为微米级的Te纳米线,并以Te纳米线为模板和还原剂,制备了Pd纳米线和Pd纳米管。采用扫描电子显微镜及透射电子显微镜等对Pd纳米线及Pd纳米管结构、形貌及组成进行了分析,结果表明所制备的Pd纳米材料与Te纳米线有着类似的形貌。电化学结果表明,Pd纳米线和纳米管对乙醇的催化催化稳定性明显优于碳负载的Pd纳米颗粒。(3)以Te纳米线为模板及还原剂合成了PdPt双金属纳米管,其扫描电子显微镜及透射电子显微镜结果显示所制备的PdPt纳米管的直径在10nm-20nm左右,长度可以达到微米级。采用X射线衍射、能谱、及扫描透射电子显微镜对其组成进行了分析,结果显示Pd和Pt原子较均匀地分布在PdPt纳米管中。同时对纳米管的形成机理进行了假设。电化学结果表明所制备的PdPt纳米管在碱性条件下有着较好的催化活性,且其催化稳定性比碳负载的PdPt纳米颗粒更好。(4)采用欠电位法在直径为10-20nm、长度为微米级的Pd纳米线表面沉积了单原子层的Cu,并通过化学置换法得到了单原子层的Pt,得到了单原子层Pt修饰的Pd纳米线,并采用扫描电子显微镜、透射电子显微镜、X光电子能谱及能谱仪对其结构、形貌、组成进行了研究。通过循环伏安法及计时电流法考察了单原子层Pt对Pd纳米线在碱性条件下对乙醇氧化性能的影响,结果表明单原子层的Pt不仅能提高其催化活性,更能提高其催化稳定性。而常规电化学沉积Cu并通过置换法得到的Pt虽然能提高催化活性,但其催化稳定性却有所降低。(5)采用化学法在Pd纳米管表面沉积了多孔的Pt,扫描电镜及透射电镜图表明沉积Pt后纳米管的直径变大而且表面变得较为粗糙,其表面是由许多直径约2-3nm的Pt纳米颗粒组成,颗粒之间形成了许多缝隙,从而构成了多孔的Pt。电化学结果表明沉积Pt之后,纳米管的乙醇催化活性有较大提高,甚至比碳载体负载的Pd纳米颗粒更好,但其催化稳定性却有所降低。(6)以Te纳米线为模板及还原剂,同时采用抗坏血酸及聚乙烯吡咯烷酮控制Pt的形貌,得到具有多孔状的Pt纳米线。其扫描电镜及透射电镜结果表明,所制备的Pt纳米线的直径在20-30nm左右,其由很多平均尺寸在3nm左右Pt纳米颗粒组成,类似于许多“葡萄”长在树枝上,并且纳米颗粒之间形成了许多小孔。通过改变H2PtCl6、PVP和AA的剂量,可以成功地制备出不同直径的葡萄状Pt纳米线。电化学结果表明该Pt纳米线具有比碳粉负载的Pt更好的氧还原催化活性及稳定性。更多还原

【Abstract】 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.更多还原