| Summary: | 博士 === 國立交通大學 === 材料科學與工程系所 === 94 === Fuel cells are regarded as the most potential power source to replace fossil oil due to their low pollution, high efficiency and low noise. Among all, direct methanol fuel cell (DMFC) which can be minified catches everyone eyes. However, high catalysts loading, low utility of catalysts and low reaction rates are advantages of DMFC. In this thesis, high surface area and high conductivity carbon nanotubes (CNTs) were used as the electrodes to improve the utility of catalysts of the direct methanol fuel cell. This research was divided into two parts. The first part includes directly synthesizes CNTs on carbon cloth as the catalyst supports and disperses metallic catalysts on CNTs by sputtering and wet impregnation. After that, we fabricate and characterize the membrane electrode assembly of a DMFC. We found that although dispersing catalysts by sputtering method can form Pt catalysts with small size (about 3nm), the mass efficiency will decrease with long sputtering time due to the shield effect of CNTs. This will result in the degradation of cell performance. On the other hand, the shield effect can be solved by wet impregnation method, but the agglomeration of catalysts occurred and strongly decreases the performance of DMFC.
The second part of this thesis focuses on the surface modification/ functionalization of CNTs for solving the agglomeration of catalysts and improving the cell performance. The surface modifications of CNTs were carried out by refluxing pristine CNTs in HNO3 for attaching the functional groups. From the results of cyclic voltammograms (CVs) and single cell test of Pt/MWCNTs electrodes, we can found the significant improvement of cell performance of DMFC after modifying MWCNTs. The power density of Pt dispersed on HNO3 treated MWCNTs is 15.8, 12.6 and 8.6 mW/cm2 at 70oC, 50oC and 30oC with 0.4 mg/cm2 Pt loading ,respectively. This is much higher than the DMFCs with Pt/pristine-MWCNTs and sputter deposited Pt/MWCNTs based cells. Furthermore, we introduce a fast and effective method for modifying CNTs by microwave digestion method. The functionalization of CNTs for increasing more anchoring sites can be achieved in a short time by this approach. The CVs results show that the microwave digestion modified Pt/MWCNTs electrode exhibits the larger electrochemical Pt surface area and higher current density of methanol oxidation than pristine and HNO3 treated Pt/MWCNTs electrodes. This technique can be widely used for effective modifying CNTs and shorting the process time. Finally, the effects of functionalization of CNTs with different functional groups, such H2SO4, KOH and 2-mercaptoethanol were discussed in this part. The Pt/sulfonated-MWCNTs has the maximum methanol oxidation current density about 152.1 mA/cm2 and mass efficiency about 304mA/g. The power density of Pt dispersed on thiolated MWCNTs is 20.6 mW/cm2 at 70oC, with 0.5 mg/cm2 Pt loading, which is much higher than the Pt/HNO3 treated-MWCNTs.
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