纳米线阵列电极的制备及其对甲醇电催化氧化特性的研究

【作者】 徐明丽；

【导师】 杨显万；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2007， 博士

【摘要】 近年来，随着各类便携式电子产品的不断涌现，对小型燃料电池的需求随之增加，直接甲醇燃料电池（DMFC）因具有结构简单、污染小、易于小型化、使用方便等优点而受到越来越广泛的关注与重点研究。然而，目前用于催化氧化甲醇的电极材料其催化活性和催化稳定性均不是很高。因此，本论文的主要目的是研制对甲醇具有高催化活性、高稳定性且成本较低的阳极催化剂。目前，多数催化剂均采用负载型纳米催化剂，本研究中则是创新性地将催化剂制成金属纳米线阵列电极的形式，从而大大提高了对甲醇的电催化氧化性能。本论文借助阳极氧化铝（AAO）模板，通过交流电沉积的方式制备了金属Pt、Pd、Ni和Pd-Ni纳米线，并通过电化学测试方法对各纳米线阵列电极电催化氧化甲醇的性能进行了对比性研究，从而得出一种高催化性能、稳定性较好的纳米线阵列电极催化剂。论文的主要内容和研究结论如下：1．对纳米线阵列的应用和DMFC进行了概述，重点阐述了甲醇阳极氧化的电催化机理，系统介绍了DMFC的各种阳极催化剂，总结了目前研究阳极催化剂所面临的问题，并指出催化剂纳米化是当前研究催化剂的主要思路。2．确定了制备AAO模板的具体工艺流程。采用草酸溶液作为电解质，通过二次阳极氧化的方法来制备AAO模板，并通过逐级降电压的方法来减薄阻挡层。本研究工艺具有“利用较短时间能够制备出较大孔径、较薄阻挡层、较厚模板、排列高度有序的多孔氧化铝模板”的优点，与其它获得同样模板的工艺相比，大大缩短了制备时间。3．氧化铝膜的生长过程中存在着膜的电化学生成和化学溶解两个过程。多孔氧化铝膜的形成过程分为阻挡层的形成、微孔萌生和多孔层稳定生长三个阶段。从动力学角度来讲，氧化过程是一个溶液中反应粒子多相传质的过程。在前人对膜层结构形成机理的理论模型基础上提出本论文的理论模型一一“电场支持下的体膨胀应力溶解模型”。4．确定了制备金属纳米线及其阵列电极的具体工艺流程。本研究中通过交流电沉积的方式向AAO模板纳米孔洞中沉积制备了金属Pt、Pd、Ni和Pd-Ni纳米线，与文献中制备金属纳米线的工艺相比，本研究中的制备方法具有操作简单、方便易行、稳定性好的优点，这样既缩短了实验时间，又节约了成本，同时提高了制备样品的成功率。5．通过TEM、SEM、AFM、EDS、XPS和XRD等测试手段对不同处理条件下的AAO模板和金属Pt、Pd、Ni和Pd-Ni纳米线的形貌、结构与成分等进行了表征与分析。主要的研究结论如下：（1） AAO模板具有孔径约70～80nm的排列规则有序的纳米孔结构，孔密度为10⁹～10¹⁰cm^-2，其膜厚可达47～48μm；模板中的Al:O原子基本符合化学计量比2:3，且在未经磷酸处理时AAO模板均为介于无定形Al₂O₃和晶态γ-Al₂O₃之间的过渡态多晶Al₂O₃，而经磷酸处理后则呈多晶态的AlPO₄。（2）纳米线呈实心线，中间无间断，其表面不够光滑呈毛刺状。线径约为65～75nm左右，与模板孔径基本接近；当沉积时间小于30min时，长径比为55～150左右，但若沉积时间延长至60min时，纳米线会超出模板表面横向生长以致彼此连成一片而覆盖模板。（3） XPS检测结果表明，Pt、Pd纳米线均为单质态，而Ni纳米线由于自然氧化以及在后处理过程中与碱液接触其表面生成了NiO与Ni（OH）₂，Pd-Ni纳米线中Pd、Ni处于简单的混合状态，而非固溶体；XRD检测结果表明这四种金属纳米线均为多晶态，而选区电子衍射（SAED）测试结果则表明Pd纳米线呈单晶态，这可能是由于Pd纳米线的特殊生长方式以及采用不同测试手段时电子束照射样品方式不同的缘故。6．研究了KOH碱性介质中甲醇分别在Pt、Pd、Ni和Pd-Ni平板电极与相应纳米线阵列电极上的循环伏安行为，分析了其在不同电极上的电催化氧化性能，得到如下结论：（1）在KOH溶液碱性介质中，Pt、Pd、Ni和Pd-Ni平板电极均表现出一定的催化氧化甲醇的能力，其催化活性顺序为：Ni＞Pd-Ni＞Pd＞Pt；纳米线阵列电极对甲醇的电催化氧化活性很高，比相应的平板电极均提高了约10倍左右，其催化活性顺序仍为：Ni＞Pd-Ni＞Pd＞Pt。（2）本研究中制备的金属纳米线阵列电极催化剂具有很高的催化活性，其催化氧化甲醇的氧化峰电流密度是文献中负载型纳米催化剂的5～70倍左右。这主要是由于纳米线阵列电极中金属纳米线拥有更高的电化学活性面积和更好的分散度，能够大大提高甲醇催化氧化活性点数量的缘故。7．采用循环伏安法和交流阻抗法对KOH碱性介质中Pt、Pd、Ni和Pd-Ni纳米线阵列电极上不同测试条件对甲醇电催化氧化性能影响的规律以及电极过程动力学进行了分析研究，主要结论如下：（1）随着扫描次数的增加，伏安曲线中甲醇的氧化峰电流密度均呈现不同程度的衰减。结果表明：Pt纳米线电极衰减最多，而Pd-Ni纳米线电极衰减最少，分别为30％和0.44％，从而确定了Pd-Ni纳米线阵列电极是一种稳定性很好且成本相对较Pt低的纳米线催化剂。（2）甲醇浓度为0.1mol/L时，Pt、Pd、Ni、Pd-Ni这四种纳米线阵列电极催化氧化甲醇的CV曲线中正扫氧化峰电流密度均与v^1/2成正比关系，这表明甲醇浓度为0.1mol/L时这四种电极上的催化氧化反应均受扩散步骤控制。若溶液中甲醇浓度增大，则电极上催化氧化甲醇的氧化峰电流密度均随之增大，表明电极表面甲醇被吸附氧化的量相应增加。（3）交流阻抗谱测试结果表明，电极电位为0.2V下，当甲醇浓度为0.1mol/L时，阻抗谱为一条直线，Pt、Pd、Ni、Pd-Ni纳米线阵列电极上甲醇的催化氧化反应均受扩散步骤控制；当甲醇浓度逐渐增大或电极电位逐渐增大时，阻抗谱变为半径逐渐减小的半圆弧，电极过程逐渐由扩散控制转变为电化学控制，且甲醇电催化氧化的速率均随之增加。由此可以得出，在KOH碱性介质中，Pt、Pd、Ni、Pd-Ni这四种纳米线阵列电极中，催化活性最高的是Ni纳米线电极，而催化稳定性最好的是Pd-Ni纳米线电极，催化活性与稳定性最差的是Pt纳米线电极。目前，虽然小型DMFC已产业化，但其催化剂并非纳米线阵列催化剂。经查新，本文研制的Pt、Pd、Ni、Pd-Ni四种金属纳米线阵列电极对甲醇的电催化氧化在国内外文献中均未见报道。综上所述，本论文已完成了Pt、Pd、Ni、Pd-Ni这四种金属纳米线阵列电极在KOH碱性介质中催化氧化甲醇的性能研究工作，对今后研究小型DMFC阳极催化剂开辟了新的思路，为制备纳米线阳极催化剂的研究工作提供了一定的技术措施与理论指导，对研究有机小分子的电催化氧化在理论上具有一定的借鉴与指导意义。更多还原

【Abstract】 Recently, with the appearance of various portable electronic products,small fuel cells are getting more and more needed. Direct Methanol Fuel Cell（DMFC） is gaining extensive attention and research because of its manyadvantages, such as simple structure, little pollution, easy to be miniaturizedand convenient to be used, and so on. However, at present, electrode materialsused in DMFC have lower electro-catalytic oxidation activity for methanol andlower stability. Therefore, the main purpose of this thesis is preparing newelectro-catalysts with higher electro-catalytic oxidation activity, favorablestability and lower cost. Nowadays, most of catalysts are load nano-catalysts.The catalysts in this thesis are innovatively prepared into metal nanowires arrayelectrode so as to effectively enhance electro-catalytic oxidation activity formethanol.In the thesis, anodic aluminum oxide （AAO） film is used as template toprepare platinum, palladium, nickel and palladium-nickel nanowires byalternating current （AC） electro-deposition. Electro-catalytic oxidationactivities for methanol on the different nanowires array electrodes are comparedby electro-chemical testing. So a new nanowires array electrode catalyst isobtained which has higher electro-catalytic activity and favorable stability. Themain contents and conclusions in the thesis are as follows:1. Application of nanowires array and DMFC are reviewed briefly. Theelectro-catalytic mechanism of anodic oxidation for methanol is elaboratedemphatically. Various anodic catalysts in DMFC are introduced systematically.Many problems on researching anodic catalysts are summarized. The thesispoints that nano-catalyst is one of main thoughts in the research of catalysts.2. The technological process of preparation for AAO template is obtained.In this thesis, the oxalic acid is used as the electrolyte to prepare highly orderedAAO template by two anodic oxidation steps. The thickness of barrier layerslowly becomes thinner with the decrease of the anodic oxidation voltage stepby step. Compared with other technologies, the technology in this research is so advantageous that AAO template can be obtained in the short time. Moreover,the template has bigger pore diameter, thinner barrier, bigger thickness andhighly ordered pores.3. During the growing of alumina film, there are two courses, electro-chemical growth and chemical dissolution. The formation process of porousalumina film can be distinguished as formation of barrier layer, germination ofmicro-pores and stable growth of porous layer. From dynamics, anodicoxidation is a process of multiphase mass transfer of reaction particles in thesolution. On the base of the formation mechanism of AAO film from the formers,the thesis presents new mechanism that is named the model of stress anddissolution with the volume expand on the electronic action.4. The technological process of preparing metal nanowires and nanowirearray electrodes is obtained. In the thesis, metal platinum, palladium, nickel andpalladium-nickel are respectively deposited into nano-pores of AAO template byAC electro-deposition. So these four nanowires are successfully prepared.Compared with other technologies, the technique in this research has manyadvantages, such as simple step, convenient action and excellent stability, andso on. It can not only shorten experiment time, but also save cost and increasethe efficiency of preparing sample.5. Transmission electron microscopy （TEM）, scanning electron microscopy（SEM）, atomic force microscope（AFM）, energy disperse spectroscopy （EDS）,X-Ray photoelectron spectrometer（XPS） and X-Ray diffractomer （XRD） areemployed to investigate the morphology, crystal structure and elementcomposition of AAO template and metal platinum, palladium, nickel andpalladium-nickel nanowires obtained under different conditions. So the mainresults are as follows:（1） AAO template has highly ordered porous structure with 70～80nmpore diameter. Its thickness reaches about 47～48μm. Its pores density is about10⁹～10¹⁰ cm^-2. The atom ratio of aluminum and oxygen basically accords withtheory value 2:3. Crystal structure of the AAO template is intergradationmulti-crystal alumina between amorphism alumina and crystalloidγ-aluminawithout being etched by phosphorus acid. However, it is multi-crystal aluminum phosphate with being etched by phosphorus acid.（2） Nanowire is solid lines with no gap. It’s not smooth but coarse on thesurface. The diameter of nanowire is about 65～75nm, which is close to poresdiameter of AAO template. When the deposition time is less than 30mins, theratio of length and diameter of nanowire is around 55～150. However, when thedeposition time is prolonged to 60mins, nanowires can surpass the templatesurface so that they connect with each other and cover the template.（3） XPS analysis indicates that platinum and palladium nanowires are allmono-atomic. Meanwhile, it shows that there are nickelous oxide and nickeloushydroxide on the surface of nickel nanowire because nickel may be autooxidatedin the atmosphere or react with alkaline solution. It also indicates thatpalladium-nickel is not solid solution but simple mixture with palladium andnickel in Pd-Ni nanowire. XRD analysis indicates that these four nanowires areall multi-crystal structure. However selected area electron diffraction （SAED）analysis indicates that palladium nanowire is monocrystal. It may be the reasonof the peculiar growth of palladium nanowire and different electron beamirradiation in different detection ways.6. The electro-catalytic oxidation of methanol on platinum, palladium,nickel and palladium-nickel plane electrodes and those on the nanowires arrayelectrodes are all investigated in the potassium hydroxide alkaline solution.Electro-catalytic oxidation activities of these four electrodes are respectivelyanalyzed. The results are as follows:（1） These four plane electrodes all exhibit some electro-catalytic activityfor methanol. The sequence is nickel＞palladium-nickel＞palladium＞platinum.Electro-catalytic activities for methanol on the nanowires array electrodes areall 10 timeshigher than those on the corresponding plane electrodes. But thesequence of electro-catalytic activity is still nickel＞palladium-nickel＞palladium＞platinum.（2） The nanowires array electrode catalyst in the research has quite highelectro-catalytic activity for methanol, which is 5～70 times than that of manyformer research. It is mainly because metal nanowires have biggerelectro-chemical active area and better disperse state so that they can greatly increase the quantity of electro-catalytic active points for methanol.7. By cyclic voltammetry and alternating current impedance,electro-catalytic oxidation of methanol in potassium hydroxide solution onplatinum, palladium, nickel and palladium-nickel nanowires array electrodes arecompared. The influences of catalytic oxidation activity for methanol in thedifferent testing conditions and the dynamics process on the electrodes arefurther studied. The results are as follows:（1） With the increase of scanning rounds, peak current densities ofoxidation on these four nanowires array electrodes all decrease in some extendin cyclic voltammetric curve. The results show that the decrease on platinumnanowires electrode is the most, while the decrease on palladium-nickelnanowires electrode is the least, respectively 30% and 0.44%. Therefore,palladium-nickel nanowires electrode is a new catalyst of favorable stability andlower cost compared with platinum nanowires electrode.（2） When the concentration of methanol is 0.1mol/L, peak current densitiesof oxidation during the positive scanning on these four nanowires electrodes areall proportional to the square of the scanning rate. It shows that oxidationreaction of methanol is controlled by the diffusing step. Moreover, peak currentdensities are all increasing with the concentration of methanol, which shows thatthe quantities of methanol absorbed on the electrodes are also correspondinglyincreasing.（3） When the concentration of methanol is 0.1mol/L, alternating currentimpedance appears a straight line under 0.2V. This demonstrates that theelectrode reaction is controlled by the diffusing step. With the increasingconcentration of methanol or the higher potential of electrode gradually,alternating current impedance appears an arc with the decreasing radius. Thisshows that the controlling step of electrode reaction changes from the diffusingstep to the electrochemical step. Simultaneously, the rate of electro-catalyticoxidation of methanol is also correspondingly increasing.So, in the potassium hydroxide solution, it can be seen that theelectro-catalytic activity for methanol on nickel nanowires array electrode is thehighest and the stability of palladium-nickel nanowires array electrode is the best. It also concludes that the electro-catalytic activity and the stability ofplatinum nanowires array electrode are the worst in the above four nanowiresarray electrodes.At present, although small DMFC has been already industrialized,catalysts in it are not metal nanowires array electrode catalysts. It has been notreported that electro-catalytic oxidation of methanol on platinum, palladium,nickel and palladium-nickel nanowires array electrodes in potassium hydroxidealkaline solution after studying the present lots of Chinese and foreignmaterials.Altogether, the thesis has already studied electro-catalytic oxidation ofmethanol on platinum, palladium, nickel and palladium-nickel nanowires arrayelectrodes in potassium hydroxide solution. It creates a new thinking ofresearching anodic catalyst used in small DMFC in the future. It can providesome techniques and theoretical direction for research in the future. It has usefulreference value and guide significance in the theory for researchers to studyelectro-catalytic oxidation of other organic small molecules.更多还原