【作者】 崔春华；

【导师】 俞书宏；

【作者基本信息】 中国科学技术大学 ， 纳米化学， 2011， 博士

【摘要】 近年来,随着环境意识的增强和对有限自然资源认识的加深,为了减少对化石能源等不可再生资源的依赖,需要寻求一种更清洁,更廉价,更小型化和更有效的能源供给方式。因此,针对燃料电池成本高,反应活性低和稳定性差等制约其商业化的主要因素出发,展示材料的合理设计,通过提高材料的物理和化学性质,寻找可行的新型催化剂材料路线显得尤为重要。本论文具体内容主要包括以下几个方面:1、发展了一种在非水溶剂(极性非质子溶剂,二甲亚砜,DMSO)中采用模板电化学合成一维纳米颗粒管燃料电池催化剂的普适新方法。从溶剂的物理化学性质着手分析和解释了合成出高质量、高长径比,管壁厚度可控且非常均匀的Pt、Pd、Au和Ag一维纳米管催化剂的机理。其中合成的Pd管和Pt管具有多重等级结构,即纳米管是由40纳米左右的半球体刺球组成,而刺球是由3-6纳米的颗粒构成。该研究结果在《化学通讯》(C.H.Cui,et al Chem. Comm. 2010, 46, 940)上发表。2、氧化物支持的金属催化剂由于其氧化物-金属界面电子传递,在燃料电池领域有重要的应用。采用非水溶剂电化学方法,合成出Cu组分一定,Au/Pd组分比例逐渐增大的三元PdAuCu异质阴极催化剂材料并用于氧气还原反应。研究结果表明,相对于PdCu二元催化剂,Au的加入降低了Cu的氧化电位,使得Cu在电解液中更容易氧化成为CuO,有利于吸附氧和载氧,从而提高了催化活性。不仅如此,Au的存在提高了催化剂的稳定性,对双氧水也有极强的电催化还原能力。因此,这种材料使得发展双氧水/氧气混合燃料催化剂成为可能。相关研究成果在《应用化学》上发表(C.H.Cui,et al Angew. Chem. Int. Ed. 2010, 49, 9149.).在成功制备一维纳米管材料的基础上,通过调节电解质浓度、外加电压大小等条件,进一步拓展了非水溶剂电化学方法,制备出组分可调,管壁厚度可控,长度可控的PdAu异质纳米颗粒管并研究其对乙醇的电催化氧化性质。通过对PdAu组分比例的调节实现对Pd和Au界面的调控。研究结果表明,随着Au/Pd组分比例的提高,金属表面电子结构随之发生改变,而少量Au的加入电子结构变化最大,对乙醇电氧化催化的活性最高,稳定性也最好。相关研究成果在ACS Nano上发表(C.H.Cui,et al ACS Nano, 2011, 5, nn-2011-010602)。3、金属纳米催化剂表面重构是一种很普遍的表面现象。通过对非水溶剂电化学方法合成的PtCu纳米颗粒管在还原性气氛条件下(氢气+氩气)进行退火处理可制备出表面富Pt的多孔管状合金催化剂材料并用于氧气还原电催化研究。这种催化剂由于Cu相对较小原子半径而具有很强的晶格压缩应力,在循环伏安电化学扫描条件下,由于表面羟基和过氧基的吸附和脱附,其表面会发生大尺度的表面重构现象。研究结果表明,随着退火温度的升高晶体的有序程度提高,而晶格有序程度与表面重构密切相关,有序度越高,表面重构越强烈。通过对催化剂氧气还原电催化能力的研究表明电催化活性和稳定性随着重构能力的增强而增强。而当在氧气条件下对催化剂进行一万次循环稳定测试后再进行电化学处理,发现重构能力越高,经过电化学处理去除表面污染物后,电化学活性表面积降低的越小。在此基础上,我们进一步扩展到PtNi合金体系,研究发现PtNi合金经过表面重构以后,表面活性位点增多,活性增强,而且相对于商业的Pt/C催化剂有更强的恢复能力。相关研究成果已被《化学科学》接收(C.H.Cui,et al Chem Sci, 2011)。4、发展了一种制备Pt单层和Pt壳层低Pt催化剂电化学表面处理新方法。通过设计一种低Pt含量的三元PtPdCu体系同时展现了表面重构和表面分离现象。研究发现前体材料通过退火处理以后,Pd和Pt迁移到表面形成PtPd壳层,大量的Cu在体相内与Pt和Pd形成有序结构并产生较大的晶格应力,成为表面重构和表面分离的驱动力之一。表面重构极大的提高了表面粗糙度进而提高了电化学活性表面积;由于Pt和Pd原子在主体Cu原子中的表面分离能差异促使Pt趋向于迁移到壳层而Pd迁移到体相区域,同时Cu趋向于保持在体相内部而不会分离到表面而溶解到溶液中。所以PtPdCu体系能很好的防止非贵金属刻蚀溶解。另外,通过光电子能谱测试表明随着电化学循环次数的增加,近表面Pt/Pd组分比例从百分之十一增加到百分之五十,进一步选择甲酸作为探针分子检测表面Pt/Pd组分比例变化,从而定量和定性的展现了表面分离现象。在证明Cu在酸性电化学环境下不会刻蚀滤去的情况下,可认为表面应力影响相当。随着电化学循环次数增加,表面分离能差异导致表面PtPd原子比例连续增加,连续地改变了金属表面电子结构,导致羟基还原脱附电位连续地正移70毫伏。从而建立起了氧气还原活性与吸附键能的“火山”关系,完整的展现了“萨巴蒂尔”效应。更多还原

【Abstract】 Increasing awareness of the environment and limited energy rescources to reduce the reliance on fossil fuels make us search for a cheaper, cleaner, smaller and more efficient energy supply mode. Therefore, it is important to explore a feasible and novel fuel cell catalyst route by improving the chemical and physical properties of these materials against the drawbacks of commercialization due to high cost, sluggish kinetics and long-term stability of fuel cell catalysts. The main results can be summarized as follows:1. A general tenplated-directed electrochemical method has been developed to synthesize one dimensional nanoparticle tubes in nonaqueous solution (polar aprotic solvent, DMSO). The synthesis mechanism of Pt, Pd, Au and Ag tubular materials with uniform tube wall, high aspect ratio, and high quality has been analyzed and explained base on the physical and chemical properties of solvent medium. Especially, the Pd and Pt tubes have hierarchical structure, in which the tube is built by semi-aristate sphere of 40 nm that consist of nanoparticles with 3-6 nm.2. Oxide-supported noble metal catalysts have potential application in fuel cells. The developed electrochemical method has also been used to synthesize the ternary PdAuCu heterostructure catalysts for oxygen reduction reaction, in which the Cu component almost keep the same, while the Au/Pd component ratio increase continuously. The results indicate that the oxygen reduction activity is improved because the Cu component can be easier to be oxidized to CuO which demonstrates both adsorption sites and mediator function for O2 when Au component is added in PdCu system. Moreover, this catalyst demonstrates highly stability for H2O2 reduction due to the introduction of Au. Therefore, this material provides possibility for development of mixed gas/liquid oxidant fuel cells. The nonaqueous solution electrochemical method has been further developed to synthesize Pd/Au heterostructure nanoparticle tubes with controlled component and length by tuning the concentration ratio of Pd/Au and applied potential. The electrocatalytic activity for ethanol oxidation has also been studied and improved by controlling the component ratio and thus adjusting the interface area. The results indicate that the optimal activity and ability for ethanol oxidation was obtained when little amount of Au in Pd/Au system, where the electronic structure was highly modified. 3. The surface restructuring on metal surfaces is a very common phenomenon. The Pt-rich porous PtCu alloy catalysts have been obtained by annealing the AAO template-supported PtCu nanoparticle tube in reductive conditions. This material demonstrates high lattice strain owing to the smaller atomic radius in the bulk, which can be one of the driving forces for lage scale restructuring under potential cycling assisted by the adsorption/desorption of oxygenated species. The results indicate that increasing the thermal annealing temperature can increase the lattice ordering which will highly affect the restructuring. The electrocatalytic activity and stability increase with the increase of the restructuring ability. After 10,000 cycles of stability test and subsequently cleaning of the surface contaminants, the catalyst with higher restructuring ability has higher restorable ability. Based on these results, the rearranged surfaces of PtNi system show high surface active sites, high activity and high restorable ability than those of commercial Pt/C catalyst.4. A novel electrochemical surface treatment method has been developed to prepare Pt monolayer or Pt shell low Pt content electrocatalyst. A low Pt content PtPdCu is used to investigate the surface segregation and restructuring phenomena. After thermal annealing treatment, the Pt/Pd migrates to the surface and a mass of Cu stay in the bulk results in large lattice strain. The results indicate that the surface restructuring highly enhanced the surface roughness and thus improved electrochemical active surface area. Moreover, the solute Pt and Pd atoms respectively tend to migrate to the surface and core region due to surface segregation differences, whereas Cu atoms with almost no leaching remain in the core after potential cycling. Thus, the PtPdCu materials can inhibite the leaching of Cu. The surface Pt/Pd component ratio increases from 11 to 50 atom % with increasing the potential cycling. The continuous atomic ratio change between Pt and Pd on the surface is further monitored in situ by selecting formic acid as probe molecule. Therefore, the surface segregation was confirmed by both quantitative and qualitative methods. The Pt/Pd atomic ratio continuously increase with increasing the potential cycling number, which modified the surface electronic structure and positively shift 70 mV of the desorption of oxygenated species. Therefore, the adsorbate bond energy and activity relationship was established and perfectly demonstrate the“Sabatier principle”.更多还原