| 摘要: | 直接甲醇燃料電池是利用氧化還原反應產生電能，分別在陽極端與陰極端直接供給甲醇與氧氣。在近年來，直接甲醇燃料電池的焦點在於更高的效率與更低的成本，在本論文中引進奈米尺寸的概念，同時將觸媒與碳載體尺寸縮小至奈米等級，在碳載體的部份本論文利用MPECVD系統直接成長於碳布上的奈米碳管，有別於傳統塗佈製程，本論文之奈米碳管作為載體，降低了碳管與碳布之界面阻抗，故擁有高表面積與高電子傳遞的特性，適合當作承載電觸媒的載體。
在觸媒的部份，本論文使用化學法製備鉑觸媒奈米顆粒，化學法具有成本低、實驗設備簡便及觸媒分佈均勻的優點，本論文在奈米碳管上鉑觸媒負載量約0.1 mg/cm2，由TEM得知其平均粒徑約2 nm，其相對應之觸媒表面活性面積為800 cm2/mg，而在zeta電位測試結果得知在pH值11的膠體溶液中具有最大電斥力，其zeta電位約為60 mV，對照TEM結果可知當zeta電位越大可得到越小粒徑之鉑觸媒奈米顆粒。
本論文成功的使用醇類還原法得到奈米級的鉑觸媒，在電觸媒活性有非常大的提升。另一方面觸媒分散於直接成長奈米碳管以提高載體之導電性，在低觸媒用量即可得到極佳之全電池表現。
Direct methanol fuel cell (DMFC) is a kind of power source which generates electrical power by a redox reaction involving methanol fuel and oxygen fed directly to the anode and cathode, respectively. To date, there are still some important issues, such as electrocatalysts, electrode membrane material that need to be improved for better DMFC performance. Carbon nanotubes (CNTs), which have high surface area and good electronic conductivity, are considered better suited for supporting the electrocatalysts. On the other hand, electrocatalysts play an important role for the redox reaction in DMFC system. Platinum is catalytically active in room temperature electro-oxidation reactions of interest to the anode and cathode applications. The activity of platinum-based electrocatalysts is strongly dependent on particle size, size distribution, particle dispersion and so on. In this thesis, CNTs are employed to support electrocatalysts. The nano-sized platinum particles are prepared by alcohol reduction process.
To improve the DMFC performance, CNTs were directly grown on carbon cloth (CNTs-carbon cloth electrode) by microwave plasma-enhanced chemical vapor deposition and well-dispersed Pt nanoparticles (Pt NPs) were subsequently grown on CNTs-carbon cloth electrode by the alcohol reduction process. From scanning electron microscopy (SEM) images, the well-distributed Pt NPs on highly-packed CNTs-carbon cloth electrode were observed. From transmission electron microscopy (TEM) images, the as-grown CNTs were found to be bamboo-like structure with diameter 20-30 nm and the nano-sized Pt NPs are highly dispersed on CNTs. The grain size of Pt NPs can be controlled by the pH value of the solution during alcohol reduction process. It is found that the average diameter of Pt NPs is around 2 nm at pH 11, which aids in the relatively narrow-sized distribution suitable for the redox reaction. The Pt electrochemical surface activity area is about 800 cm2-mg-1 at pH 11. The membrane electrode assembly (MEA) are composed of the as-prepared 0.1 mg-cm-2 of Pt NPs on CNTs-carbon cloth electrode for the cathode, 4.0 mg-cm-2 of Pt-Ru supported by activated carbon for the anode and the sandwiched membrane of Nafion® 117. The DMFC performance is about 33mW-cm-2 at 80 ℃ by feeding 1 M methanol and oxygen to anode and cathode, respectively. |
