【作者】 李伟伟；

【导师】 卢世刚；

【作者基本信息】 北京有色金属研究总院 ， 有色金属冶金， 2011， 博士

【摘要】 直接甲醇燃料电池由于原料来源广泛、噪音低、对环境污染小、可靠性高等特点,成为研究的热点和重点。影响直接甲醇燃料电池商业化的主要技术难点来自于电催化剂材料,甲醇氧化的电化学极化和Pt催化剂受“CO”毒化现象是导致电池性能下降的主要原因。根据甲醇电化学氧化机理,在甲醇氧化过程中活性“-OH”的存在能改善Pt催化剂的“CO”毒化现象,因此在催化剂制备中引入能提供表面活性含氧物种的组分是有效的措施。铌酸和氧化铌具有独特的固体酸性质,特别是在有水分子参与或生成的反应中表现出优异的催化性能。本文以氧化铌作为Pt/C和PtRu/C的助催化剂,研究了水合氧化铌的制备工艺及其性能,重点分析了以水和氧化铌为助剂的Pt/C和PtRu/C催化剂的结构和性能,并对其在甲醇氧化中的电化学活性进行了研究。以草酸铌为前驱体,分别在水溶剂、乙二醇-水混合溶剂的体系中,采用氨水沉淀法制备水合氧化铌。利用XRD、TG、IR、SEM等分析了产物的结构、组成和形貌,结果表明：采用不同的反应体系,制备的水合氧化铌具有不同的组成和形貌,但是都在600-700℃之间转化为氧化铌晶体。根据对反应过程的分析,以水为溶剂的体系中,Nb2O5·nH2O的生成以成核-凝并生长为主,而以乙二醇-水为混合溶剂的体系中,Nb2O5·nH2O的生成以外延与表面反应生长方式为主。循环伏安(CV)测试表明,采用乙二醇-水混合溶剂制备的Nb2O5·nH2O,700℃热处理后对氧气还原为过氧化氢的电化学反应表现出一定的活性。。通过优化工艺参数,制备了20%Nb2O5·nH2O/Co 20%Nb2O5·nH2O/C在Ar气氛中700℃对热处理后,TEM分析表明,Nb205的晶粒大小约10nm,比较均匀的分布在C黑表面。CV测试表明,热处理后的20%Nb2O5·nH2O/C对氧气还原为过氧化氢的电化学反应表现出一定的活性。采用乙二醇-水混合溶剂为反应体系,制备了不同含量(5%、10%、20%)水合氧化铌的20%Pt/C催化剂,采用CV法比较了电化学比表面积,结果表明10%Nb2O5·nH2O-20%Pt/C具有最高的比表面积。进一步采用XRD和TEM分析10%Nb2O5·nH2O-20%Pt/C的结构和形貌特性,结果表明：随着热处理温度的升高,催化剂材料中水合氧化铌由无定形态转化为结晶态,但是氧化铌没有改变Pt的晶体结构和在C载体上的分布状态。比较不同温度热处理(Ar气氛中)的10%Nb2O5·nH2O-20%Pt/C催化剂,相对于20%Pt/C催化剂,10%Nb2O5·nH2O-20%Pt/C中Pt晶粒大小变化较小,说明添加水合氧化铌能阻碍Pt晶粒在热处理中的长大。利用]HRTEM分析700℃热处理的10%Nb2O5·nH2O-20%Pt/C催化剂,可以观察到Pt和Nb205的晶格相分布在载体C上Pt和Nb205相互接触。采用XPS对10%Nb2O5·nH2O-20%Pt/C进行分析,结果表明：随着热处理温度的升高,催化剂中零价Pt含量降低,而且氧化铌没有改变Pt的化学态,但是相对于20%Pt/C,10%Nb2O5·nH2O-20%Pt/C催化剂中“-OH”含量明显增加。CV测试表明,无热处理的20%Pt/C和10%Nb2O5·nH2O-20%Pt/C对甲醇的催化氧化活性基本一样,10%Nb2O5·nH2O-20%Pt/C催化剂对甲醇的催化氧化活性随着热处理温度的升高而增强；700℃热处理的10%Nb2O5·nH2O-20%Pt/C对甲醇的氧化峰电流,是无热处理10%Nb2O5·nH2O-20%Pt/C的349倍,是700℃热处理20%Pt/C的2.59倍。计时电流测试表明,10%Nb2O5·nH2O-20%Pt/C的抗“CO”中毒能力随着热处理温度的升高而增强。采用乙二醇-水混合溶剂为反应体系,制备了10%Nb2O5·nH2O-20%Pt10%Ru/C催化剂。利用XRD和TEM分析10%Nb2O5·nH2O-20%Pt10%Ru/C的结构和形貌,结果表明：随着热处理温度的升高,20%Pt10%Ru/C和10%Nb2O5·nH2O-20%?Pt10%Ru/C催化剂中Pt、Ru合金化的程度都增强；比较同温度热处理的两种催化剂,发现10%Nb2O5·nH2O-20%Pt10%Ru/C中Pt、Ru合金化程度较差,而且活性组分颗粒较小说明Nb2O5·nH2O能阻碍热处理过程中活性颗粒的长大和Pt、Ru的合金化。对10%Nb2O5·nH2O-20%Pt10%Ru/C进行XPS分析,结果表明：氧化铌基本没有改变催化剂中Pt的化学态,但是随着热处理温度的升高,催化剂中氧化钉含量增加。CV测试表明,700℃热处理的10%Nb2O5·nH2O-20%Pt10%Ru/C对甲醇的催化氧化活性最好,300℃热处理的20%Pt10%Ru/C对甲醇的催化氧化活性最好,而且前者的氧化峰电流是后者的1.45倍。计时电流测试表明,同温度热处理的10%Nb2O5·nH2O-20% Pt10%Ru/C和20%Pt10%Ru/C催化剂,前者具有较好的抗“CO”中毒能力。总之,对于甲醇氧化电催化剂Nb2O5·nH2O-Pt/C和Nb2O5·nH2O-PtRu/C, Nb2O5·nH2O是一种良好的助催化剂。更多还原

【Abstract】 Direct methanol fuel cells(DMFCs) have been attracted much attention due to easy storage and supply of fuel, low noise pollution, little environment pollution and high security. The widely used Pt-based electro-catalysts for DMFC anodes were hampered the development of commercial DMFCs, electrochemistry polarization of methanol oxidation and Pt easily poisoned by the intermediates of methanol oxidation were the main reason what lead to descend of DMFCs performance. According to the mechanism of methanol electro-oxidation, the methanol oxidation reaction rate and anti-poisoning ability of electro-catalysts could be improve if "-OH" existed in methanol electro-oxidation process. A effective method is provide activity "-OH" in catalysts prepared process. Niobium oxide was a typical solid acid, and was very stability when H2O as reactant or product in chemical reaction. In this paper, we used Nb2O5·nH2O as cocatalyst for Pt/C and PtRu/C, the preparation and properties of Nb2O5·nH2O was study. Further, emphaticaly analyzes the structure and properties of 10%Nb2O5·nH2O-20%Pt/C and 10%Nb2O5·nH2O-20%Pt10%Ru/C, and their performance for methanol oxidatio were study, too.Nb2O5·nH2O was prepared by ammonia precipitation in the aqueous solvent system, and the aqueous and glycol mixed solvent system using niobium oxalate as precursor. The structure, composition and morphology of product were researched by XRD,TG, IR and SEM. The results showed:Nb2O5·nH2O have different composition and morphology which prepared in different reaction systems, and they would translate into crystal when heat treatment at 600~700℃. According to analysis of reaction process, in aqueous solvent system, Nb2O5·nH2O grain was formed by nucleation-coagulation mode. In aqueous and glycol mixed system, Nb2O5·nH2O grain was formed by addition-surface reaction mode. Cyclic voltammetry(CV) tseted show that the Nb2O5·nH2O which heat treatment at 700℃has reaction activity for reduced H2O2 by O2.Under the optimum conditions, we prepared 20%Nb2O5·nH2O/C.TEM results showed:when 20%Nb2O5·nH2O/C heat treatment at 700℃in inert atmosphere, the particle size of Nb2O5·nH2O is about 10nm which on C surface uniformly. CV tseted show that the 20%Nb2O5·nH2O/C which heat treatment has reaction activity for reduced H2O2 by O2.In aqueous and glycol mixed systems,5%Nb2O5·nH2O-20%Pt/C,10%Nb2O5·nH2O-20%Pt/C and 20%Nb2O5·nH2O-20%Pt/C were prepared. Their electrochemical surface area were study by CV method, the results showed:10%Nb2O5·nH2O-20%Pt/C have the biggest electrochemical surface area. Further, the structure and morphology of 10%Nb2O5·nH2O-20%Pt/C were study by XRD and TEM. The results showed:with the higher of heat treatment temperatur, Nb2O5 would translate in crystal from amorphous state, the crystal structure and distribution state on C surface of Pt were not change by Nb2O5. When 10%Nb205-nH20-20%Pt/C was heat treatment at different temperature, the Pt particle size was study. Compared with 20%Pt/C, the Pt size of 10%Nb2O5·nH2O-20% Pt/C was few change. When 10%Nb2O5·nH2O-20%Pt/C was heat treatment at 700℃,it morphology was study by HRTEM, we can see that the lattice phases of Nb2O5 and Pt on C surface, and they were contected. The surface chemical state of 10%Nb205·nH2O-20% Pt/C was study by XPS, the results showed:with the higher of heat treatment temperatur, the percentage content of Pt0 was decreased, the electro structure of Pt was not change by Nb2O5·nH2O. Compared with 20%Pt/C, the percentage content of "-OH" in 10%Nb2O5·nH2O-20%Pt/C was increased significantly. CV test results showed:the catalytic activity of 20%Pt/C and 10%Nb2O5·nH2O-20%Pt/C which no heat treatment were little different. With the higher of heat treatment temperatur, the catalytic activity of 10%Nb2O5-nH2O-20%Pt/C was increased. Compared with 10%Nb2O5-nH2O-20%Pt/C that no heat treatment, the oxidation peak current of 10%Nb2O5-nH2O-20%Pt/C that heat treatment at 700℃was increased to 3.49 times. Compared with 20%Pt/C that heat treatment at 700℃, the oxidation peak current of 10%Nb2O5-nH2O-20%Pt/C that heat treatment at 700℃was increased to 2.59 times.Chronoamperogram test results showed: with the higher of heat treatment temperatur, the performance of"CO" tolerance of 10%Nb2O5-nH2O-20%Pt/C was increased.10%Nb2O5-riH2O-20%Ptl0%Ru/C was prepared in aqueous and glycol mixed systems. The structure and morphology of catalysts were study by XRD and TEM. The results showed:with the higher of heat treatment temperatur, the alloying extent of Pt-Ru in 10%Nb2O5-nH2O-20%Pt10%Ru/C and 20%Pt10%Ru/C were increased. Compared 10%Nb205-nH20-20%Pt10%Ru/C with 20%Pt10%Ru/C when they were heat treatment at same temperature, the the alloying extent of Pt-Ru in 10%Nb2O5·nH2O-20% Ptz0%Ru/C was worse, and the size of activated particles in 10%Nb2O5·nH2O-20% Pt10%Ru/C was smaller. These showed that Nb2O5·nH2O could hinders the aggregation of activated particles and alloying extent of Pt-Ru in heat treatment process. The surface chemical state of 10%Nb2O5·nH2O-20%Pt10%Ru/C and 20%Pt10%Ru/C were study by XPS, the results showed:the binding energr of Pt was less effect by Nb2O5·nH2O. With the higher of heat treatment temperatur, the percentage content of RuOx was increased. CV test showed:20%Pt10%Ru/C has the best preformance when it was heat treatment at 300℃,10%Nb2O5·nH2O-20%Pt10%Ru/C has the best preformance when it was heat treatment at 700℃. Compared their oxidation peak current, the latter was 1.45 times than that of the former. Chronoamperogram test results showed:wten they heat treatment at same temperatur, the performance of "CO" tolerance of 10%Nb2O5·nH2O-20%Pt10% Ru/C was better than that of 20%Pt 10%Ru/C.Anyhow, Nb2O5·nH2O was a good cocatalyst for Nb2O5·nH2O-Pt/C and Nb2O5·nH2O-PtRu/C which were used to methanol oxidation.更多还原