Carbon Nanotubes Paper/Ferroferric Oxide As Cathode Catalysts For Oxygen Reduction Reaction In Fuel Cell

Being one of the major category of conversion devices, fuel cells have attracted much attention due to their high energy conversion efficiency and low negative environmental impact. For most of the fuel cells, the kinetics of oxygen reduction reaction at the fuel cells are often slow, which determines the overall performance of a fuel cell. Thus, so there is the constant search for highly efficient cathode catalysts to promote the cathodic reaction and to increase the electron transfer number (n). Carbon/platinum (Pt/C), as the most commonly used catalyst, has high catalytic activity with oxygen reduction and has a number approaching the theretical maximum value of 4 electrons. However, Pt/C catalyst is expensive and has significant issues including decomposition, condensation, poisoning etc during operation. Recent studies showed that the carbon nanotubes paper (CNP) with a porous three dimensional network structure become a potential catalysts for oxygen reduction in fuel cells. For such applications, it can also play roles of fixation, dispersion and protection for other catalysts.Traditionally, to determine the electron transfer number at an electrode surface, we need to conduct cyclic voltammograms and linear sweep measurements on rotating disk eletrodes. To test powder samples, particles are often dispersed in solvent as an ink and then coated on a rotating disk electrode. Obviously, this method is not suitable for the porous structures studied here. Thus a major objective of this study is to establish an oxygen diffusion model for porous structures such as the carbon nanotubes paper (CNP). In the meanwhile, CNP samples with different thicknesses were experimented in order to determine the diffusion coefficient. Measurements show that the average value of O2 diffusion coefficient for different CNP thicknesses remains as a constant. Such a constant diffusion coefficient reflects the feasibility of building a model that can lead to the accurate determination of electron transfer number. In this thesis, we also developed a new approach to calculate the electron transfer number for the porous materials.In addition, we report on the investigation of Fe3O4 nanoparticles growth on CNP using electrochemical synthesis and evaluated the catalytic behavior of the composite structures. For the potentiostatic experiments, both the deposition voltage and time effects are studied. The CNP/Fe3O4 structure are used as the fuel cell cathodes and their performances are compared to the model we developed. Experimental results demonstrated that the fabricated CNP/Fe3O4 has high catalytic activity for oxygen reduction reaction with two and four electron transfer modes proceeding simultaneously.We also chemical synthesis CNFs/Fe3O4 composite for the application of the microwave absorptions. The reflection loss of the CNFs/Fe3O4 composite can reach 11 dB at the frequency of 7 GHz.