Preparation Of The Nanowire Array Electrode For Methanol Electro-catalytic Oxidation Characteristics

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.