Abstract
Since fuel cells have the high energy density and low pollution in the reaction process, they are recognized as one of the most promising green energy devices. Even with many excellent properties, commercialization of fuel cells still faces many challenges. The two most significant issues are how to reduce the cost and increase the durability of the fuel cell. The cost of the catalyst accounts for about fifty percent of the fuel cell, so how to reduce the amount of catalyst used and maintain the efficiency of fuel cell at the same time is a major issue that we are facing. It is also the main purpose of this research. Atomic layer deposition (ALD) technique was adopted to prepare platinum catalyst nanoparticles with uniform particle size and well dispersion. Furthermore, innovative nanostructures were used to replace the traditional carbon support. It was attempted to reduce the amount of catalyst and to improve the efficiency of proton exchange membrane fuel cells (PEMFCs). The dissertation is divided into two parts, the first part focuses on the preparations of various innovative nanostructured catalyst and supports, and the second part is to study the fuel cell durability. In the first section, carbon nanotube (CNT) was chosen to be as the support because of its high electrical conductivity, high specific surface area, and high chemical stability. Because the surface reactivity of CNT is poor, pre-treatment is needed to create defects and functional groups on the surface of CNT for depositing the catalyst. In this study, two different pre-treatment processes are chosen to modify the surface of CNT. The first one is oxygen plasma treatment and the other one is acid treatment. After the pre-treatment, Pt nanoparticles with good dispersion and uniformity are deposited by ALD. The membrane electrode assembly (MEA) performances of PEMFCs made with acid treated CNT are better than that made with oxygen plasma treated and close to that of commercial E-Tek electrodes. The most remarkable finding is that the ultra-low Pt loading of electrode, 0.019 mg/cm2. This is much lower than commercial one (0.5 mg/cm2), has the specific power density 11 times higher than that made with commercial E-Tek electrodes. In addition to CNT, Ni nanohoneycomb and TiN inverse opal structures as the catalyst supports are also fabricated. The Ni nanohoneycomb structure is a three-dimensional porous structure. Apart from high surface area and high conductivity, the electrical property of Pt deposited on Ni substrate is similar to that of Pt-Ni alloys. Inverse opal structure is also a three-dimensional porous structure, and the multilayer structure with a regular arrangement would enhance the specific surface area of the support. In order to apply to fuel cells, TiN is chosen as the support material. In addition to the characteristics that are suitable for the fuel cell, the conductivity of TiN is better than that of carbon black. Therefore the TiN inverse opal structure could enhance the specific surface area and the support conductivity at the same time. The second part of the dissertation is to study the fuel cell durability. MEA made by acid treated CNTs as the catalyst support is used in the experiment. In general, durability test often takes thousands of hours. In order to reduce the test time, a dynamic load method is used to accelerate the aging process (accelerated degradation test, ADT), which could achieve the degradation target in a shorter time. It could achieve 60,000 circulating current cycles in 100 hours of ADT test. The electrochemical and surface analysis methods are adopted to analyze the catalyst degradation after ADT of the fuel cells.