| Fuel cell is an energy converting device which could directly convert the chemical energy stored in a fuel into the electricity by electrochemical reactions. The polymer electrolyte membrane fuel cell (PEMFC), which has a high energy density, good energy-conversion efficiency, a low operation temperature, and is environmentally friendly and free of electrolyte erodent, has been considered as the most promising alternative which has wide potential applications in transportation, power station, and portable electronic devices. So far, the most important limitation of the commercialization of fuel cells is that the catalyst Pt is very expensive, limited in its reserves. There are two main approaches that can be used to eliminate the bottleneck of the development of PEMFCs. First, novel non-Pt-based catalysts used in acid fuel cells can be invented. Second, anion exchange membrane used in alkaline fuel cells can be developed, because in the alkaline environment, metals such as Ag or Ni have a very good catalytic activity in the oxygen reduction reaction (ORR).In this thesis, new techniques of developing PEMFCs are studied from these two perspectives: novel metal porphyrin macrocyclic compound non-Pt-based catalysts and anion exchange membrane in alkaline fuel cells. The thesis investigates the solutions to the difficulties caused by the use of noble metal catalysts in PEMFCs, and provides a brand new idea of developing the low temperature fuel cells.A. The non-noble metal catalysts in proton-exchange membrane fuel cells1. Metal based compouded with metal porphyrin catalysts (MeOx--MeP/C, Me=Co, Fe and Ni)A two-step procedure is adopted to synthesize the metal group adsorbed metal porphyrin macrocyclic compound (denoted as MeOx--MeP/C, Me=Co, Fe and Ni). The morphology, structure and property of the metal group in the MeOx--MeP/C are first investigated. Then the effects on the electrochemical activity of the oxygen reduction reaction by different metal groups, different center atoms of the metal porphyrin macrocyclic compound are discussed, and the mechanism of the oxygen reduction reaction happened on the surface of the catalysts are systematically studied. After that, the effects on the electrochemical stability, the mechanism of catalysis, and the electrochemical activity of the oxygen reduction reaction of the macrocyclic compound by the combined action of metal group and metal porphyrin macrocyclic compound are thoroughly investigated. It can be concluded that(1) CoOx-CoP/C has the best oxygen reduction reaction performance compared with the porphyrin without metal center (CoOx-P/C) and the porphyrin with the metal center of Ni and Fe (CoOx-NiP/C; CoOx-FeP/C).(2) Structures of the catalysts are analyzed by TEM and XPS analysis, which reveal that the transition metals are under the oxide form. TEM images of FeOx/C show well-defined spherical nanoparticles with an average diameter of about 20 nm, and are uniformly distributed over the carbon support. For most of them, the lattice is clearly visible at high magnification, revealing their crystalline form. In the case of NiOx and CoOx, a lot of clusters, with the size of 50 nm and larger, are observed all over the carbon. These clusters show the smaller structures like the agglomerated nanoparticles. Only weak crystallinity is detected for NiOx/C. In addition to clusters, TEM images of CoOx/C reveal the crystalline needle-shaped structures.(3) The XPS results reveal that the metal based nanoparticals are oxide rather than elementary substance.(4) Rotational disk electrode measurements demonstrate a better catalytic activity of all these materials in acidic medium compared with the simple cobalt porphyrin supported on carbon, and their activities follow the sequence CoOx-CoP/C>NiOx-CoP/C>FeOx-CoP/C.(5) The mechanism of the CoOx-CoP/C catalysts oxygen reduction reaction is investigated using the Koutecky-Levich formulation. By adding the metal-based particles, the oxygen reduction pathway of the double-metal porphyrin macrocycle compound is transformed to a single 4e pathway instead of the original 2e-4e mixed pathway of the single-metal porphyrin macrocycle compound CoP/C, which significantly improves the utilization ratio of the oxygen.(6) By comparing the electrochemical stability of the CoOx-CoP/C and CoP/C using the half wave potential obtained by the electrochemical reduction scanning, it is found out that the electrochemical stability of the CoOx-CoP/C is much better than that of the CoP/C, which reveals that the combination of metal and metal porphyrin macrocycle compound can effectively increase the electrochemical stability of the double-metal porphyrin macrocycle compound.2. Transition metal compounded with Ru/Se non-Pt catalysts (Se/Ru-M/C, M=Fe, Mn, Co and Ni)A kind of non-Pt catalysts for oxygen reduction reaction, Ru-M/C (M=Mn, Fe, Co and Ni), is synthesized by using Chemical Co-precipitating method in aqueous solution which includes four steps:Ru-M/C (M=Mn, Fe, Co and Ni) synthesis, annealing, electrochemical cleaning, and adding Se. The morphology, structure, component and the activity of catalysts (Se/Ru-M/C, M=Fe, Mn, Co and Ni) have been studied by TEM, ED AX, RDE. The following conclusions can be made.(1) The Se/Ru-Fe/C catalyst nanoparticle has the smallest diameter which is 7.89 nm in average. The Se/Ru-Ni/C catalyst nanoparticle has a larger average diameter of 8.82nm. The Se/Ru-Mn/C catalyst nanoparticle has an even larger average diameter of 15.16nm. The Se/Ru-Co/C catalyst nanoparticle has the largest average diameter of 19.41nm, and is not uniformly distributed.(2) After the step of electrochemical cleaning, the EDAX analysis reveals that in the remainer of the Ru-M/C, most of the Fe remains,50% of the Co left in the remainer,20% of the Ni left in the remainer, and almost all the Mn disappears.(3) According to the content of the transient metal in the remainer after the step of electrochemical cleaning, the possible structures of the Se/Ru-M/C can be deduced. In the Se/Ru-Fe/C, most of the Fe is surrounded by the Ru. The effective combination of Fe and Ru can reduce the consumption of Ru. In the Se/Ru-Co/C and Se/Ru-Ni/C, a part of the Co and Ni are surrounded by the particles, and some others may be exsited in the form of elementary substance. The Mn, however, is either completely departed from Ru, or has a weak combined action with Ru. As a result, almost all the Mn disappears after the step of electrochemical cleaning. (4) In the steps of annealing and electrochemical cleaning, the oxygen reduction activity of the Se/Ru-Mn/C catalysts almost remains the same, while that of the Se/Ru-Fe/C decreses slightly after electrochemical cleaning. Generally speaking, they both have an activity of 0.58V vs. RHE. After the Se is increased, the Se/Ru-Ni/C has the highest activity, which is 0.68V vs. RHE, and those of the other Se/Ru-M/C catalysts are about 0.66V vs. RHE. It can be concluded that, the Se/Ru-Fe/C and the Se/Ru-Ni/C catalyst nanoparticles are the best products among different Se/Ru-M/C catalysts. Therefore, they can be used as the non-Pt catalysts in the application of the PEMFCs.B. Anion exchange membrane using in alkaline fuel cellsA series of alkaline polyvinyl alcohol/1-ethyl-3-methylimidazolium hydroxide (PVA/[Bmim]OH) electrolyte membranes are developed via a direct blending and solution casting method. The morphology structure, thermal, mechanical and conductive properties of PVA/[Bmim]OH membranes with various concentrations of [Bmim]OH have been systematically studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and AC impedance spectroscopy. The results revealed that:(1) The strength and modulus of PVA/[Bmim]OH membranes increase while the tensile strength of the membranes noticeably decrease with the increase of [Bmim]OH concentration;(2) The morphology and structure of PVA/[Bmim]OH is determined by the [Bmim]OH concentration. The amorphous region of PVA increases with the increase of [Bmim]OH concentration and the carbon and nitrogen elements are smoothly distributed throughout the membrane, suggesting the homogeneous blending of [Bmim]OH and PVA weight ratio of [Bmim]OH to PVA;(3) There is an improvement in the thermal stability of PVA/[Bmim]OH membranes when PVA blended by [Bmim]OH. However, this improvement decrease with the increase of [Bmim]OH concentration;(4) When blended, the PVA/[Bmim]OH membrane exhibits superior ionic conductive and the maximum ionic conductivity is found around 0.0196 Scm-1 when the weight ratio of [Bmim]OH to PVA is 2.0 which also have the best property in all aspects;(5) A model is presented to illustrate the structure of PVA/[Bmim]OH membranes and the effect of [Bmim]OH on the ionic conductivity of the PVA matrix. The results and the model indicate that the addition of [Bmim]OH could significantly improve the electrochemical properties of the membranes, which is a promising candidate for anion exchange membrane fuel cells applications. |
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