【作者】 孙俊哲；

【导师】 张忠华；

【作者基本信息】 山东大学 ， 材料加工工程， 2014， 博士

【摘要】 本文中,为了获得高效的直接甲醇燃料电池催化剂,我们对通过机械合金化或快速凝固技术得到的Al基含Pt前驱体合金,进行两步去合金化处理(分别采用NaOH和HN03溶液),制备了高效的纳米多孔二元和三元Pt基催化剂。另一方面,利用阳极氧化和循环伏安还原相结合的方法对金属Pd片进行处理,获得了具有高催化性能的纳米结构Pd催化剂。在机械合金化制备Al2(Cu95Pt5)和Al74Ni25Pt1三元前驱体合金中,对前驱体合金进行两步去合金化处理,通过X射线衍射、扫描/透射电镜等分析发现获得了具有双连续结构的三维纳米多孔二元Pt3Cu和PtNi合金。电化学测试表明,纳米多孔Pt3Cu具有比商用PtC催化剂更好的甲醇催化和氧还原性能。DFT计算纳米多孔Pt3Cu合金d带中心及CO/O2吸附能的结果表明,与Cu合金化可以优化Pt的电子结构,提高催化性能。同时,三维的多孔结构及相互联通的韧带结构为物质和电子传输提供了更好的通道,加速了反应的动力学。类似的,通过机械合金化和两步去合金化法制备的纳米多孔PtNi合金同样具有优异的甲醇催化性能。通过快速凝固技术与两步去合金化相结合的方法,制备了具有三维双连续超细多孔结构的二元PtM(M=Co, Cu, Ni)催化剂。电化学测试表明PtM二元催化剂的甲醇催化性能和氧还原性能均优于商用PtC催化剂,同时不同过渡元素Co,Cu和Ni的添加对催化性能的影响不同。我们认为,不同合金元素的添加对于合金电子结构改变不同,合金中电子结构的改变对于催化性能有着重要的影响作用,因此提高了PtM催化剂的电催化性能。通过机械合金化法制备AlCuPtPd四元前驱体合金,然后对前驱体合金采用不同的去合金化处理工艺(采用不同的腐蚀溶液),获得了具有不同成分配比的三元PtPdCu系列合金(T1-T6合金)。通过X射线衍射、扫描/透射电镜等分析发现均获得了具有双连续结构的纳米多孔PtPdCu合金。电化学分析表明,不同合金(T1-T6合金)的甲醇催化性能和氧还原性能存在很大的差别,这与不同去合金化工艺形成合金的不同成分有关。最终我们确定在现有条件下具有最优的甲醇催化性能和氧还原性能的工艺参数,参数为在2M NaOH和1M HNO3溶液中依次去合金化(T3合金)。通过机械合金化法制备AlCuPtRu/AlNiPtRu四元前驱体合金,然后对前驱体合金采用两步去合金化处理,获得了纳米多孔结构和单质结构共存的三元PtRuCu和PtRuNi合金。电化学分析结果表明,三元纳米多孔PtRuCu合金具有优于商用PtC和PtRuC催化剂的甲醇催化性能,虽然其抗CO中毒能力要稍逊于商用PtRuC催化剂。三元纳米多孔PtRuNi催化剂对于甲醇没有催化效果,但是对甲酸存在一定的催化效果。通过在硫酸溶液中对金属Pd进行阳极氧化和循环伏安还原相结合的方法处理,可以自发形成Pd纳米颗粒。与平板Pd电极相比,阳极氧化Pd电极在硫酸溶液中表现出了不同的循环伏安曲线特征,包括分离的氢吸附／脱附峰以及相对较大的还原峰面积,较大的还原峰面积对应于更大的电化学活性面积(ECS A),因此阳极氧化Pd电极表现出更好的甲醇、乙醇和甲酸催化性能。同时,阳极氧化采用的硫酸浓度、施加电位和氧化时间对ECSA均有很大的影响,我们通过试验得到了最佳ECSA的条件为在1.0M H2SO4溶液中施加2.0V(vs. MSE)处理90min,最好的ECSA可以达到其几何面积的890倍。这个方法也提供了一种有前景的制备具有超高ECSA纳米结构Pd催化剂的途径。更多还原

【Abstract】 In this paper, in order to fabricate high-performance catalysts for direct methanol fuel cells (DMFCs), the Al-based precursor alloys containing Pt were fabricated by mechanical alloying or rapid solidification process, and then the precursor alloys were dealloyed in NaOH and HNO3solutions. Finally, the nanoporous Pt-based binary or ternary alloys can be obtained with high performance for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) in DMFCs. On the other hand, Pd nanoparticles with superior electro-catalytic properties were spontaneously formed through the combination of anodization and cyclic voltammogram (CV) reduction processes.Through mechanical alloying and subsequent two-step dealloying method, ultrafine nanoporous Pt3Cu (np-Pt3Cu) and PtNi (np-PtNi) alloys have been successfully fabricated from Al2(Cu95Pts) and Al74Ni25Pt1precursor alloys, respectively, which were characterized by the X-ray diffraction (XRD) and electron microscopic characterization analysis. Electrochemical tests indicate that the obtained np-Pt3Cu alloy exhibits superior electro-catalytic activity and stability towards MOR, as well as enhanced ORR activities in acidic circumstance compared to the commercial PtC catalyst. Density functional theory (DFT) calculations reveal that the electronic structure of Pt has been modified with the shift of Pt d-band center due to alloying with Cu, which can decrease CO poisoning and enhance the MOR and ORR activities. Moreover, the nanoporous structure and connecting ligaments of the np-Pt3Cu alloy are much favorable for the mass and electron transport in three-dimension, which greatly accelerates the reaction kinetics on the nanoporous alloy modified electrode surfaces. Similarly, the np-PtNi alloy fabricated by mechanical alloying and two-step dealloying method also exhibits enhanced MOR activity compared to the commercial PtC catalyst.Through rapid solidification and subsequent two-step dealloying method, nanoporous PtM (M=Co, Cu, Ni)(np-PtM) alloys have been successfully fabricated. Electrochemical tests indicate that the obtained np-PtM alloys exhibit superior MOR and ORR activities and stability compared to the commercial PtC catalyst, and different elements additions have an obvious effect on the improvement degree of the electro-catalytic activities. It is thought that the electronic structure varies from that of pure Pt among these alloys, which is responsible for the improved electro-catalytic activities of np-PtM alloys.By mechanical alloying and subsequent different dealloying processes, nanoporous PtPdCu (np-PtPdCu) ternary alloys with different compositions (T1-T6alloys) have been obtained from the AlCuPtPd precursor, which was characterized by the XRD and electron microscopic characterization analysis. Electrochemical tests suggest that there are obvious difference of electro-catalytic MOR and ORR activities among these alloys (T1-T6alloys), which is mainly caused by the different compositions due to the different dealloying conditions. Finally, the as-dealloyed T3alloy (dealloying in2M NaOH and1M HNO3solutions) exhibits the best electro-catalytic activities for MOR and ORR.By mechanical alloying and subsequent dealloying process, nanoporous PtRuCu and PtRuNi ternary alloys have been fabricated from the AlCuPtRu and AlNiPtRu precursors, respectively. Electrochemical tests demonstrate that nanoporous PtRuCu exhibits better electro-catalytic activities for MOR than the commercial PtC and PtRuC catalysts, while its CO poisoning resistance ability is a little poor than the commercial PtRuC catalyst. Meanwhile, the nanoporous PtRuNi alloy shows no electro-catalytic activity for MOR and certain electro-catalytic activity for electro-oxidation towards formic acid.The anodization and subsequent CV reduction processes result in the formation of Pd nanostructures on the electrode surface. Compared to the bulk Pd, the anodization of Pd in H2SO4solutions leads to different CV behaviors including well separated adsorption/desorption peaks in the hydrogen region and relatively larger reduction peak areas. The improvement of electrochemically active surface areas (ECSAs) corresponding to the larger reduction peak areas of the anodized Pd samples results in better electro-catalytic activities towards methanol, ethanol and formic acid compared to the bulk Pd. Meanwhile, the electrolyte concentration, the applied potential and polarization time have a significant influence on the anodization process of Pd. The ECSAs of the anodized Pd obtained under the optimum polarization conditions (1.0M H2SO4,2.0V vs. MSE,90min) can reach as large as890times compared to its geometric area. The present findings provide a promising route to fabricate nanostructured Pd electrocatalysts with ultrahigh ECSAs.更多还原