Abstract
Direct methanol fuel cell (DMFC) is a good portable power source due to its low operating temperature (room – 60 oC) and high energy density (~ 5000 Wh L-1). At present, low power density and high catalyst loading of the membrane electrode assembly (MEA) are the major barriers that inhibit its commercialization. However, the behaviors inside the MEA are extremely complicated and are heavily dependent on the operation condition, the constituting materials, MEA structure, and the manufacture procedure etc. In this dissertation, we focus on some key issues, methanol crossover, the properties of catalyst and water management for DMFC. First, a semi-empirical equation was proposed to simulate the discharging behavior of direct methanol fuel cell (DMFC). The individual voltage losses in DMFC due to methanol crossover and overpotentials of both cathode and anode can be distinguished as well. Three sets of experiments were designed and carried out to account for the three voltage losses. Values of each parameter in the model were then calculated and the computation showed the fitted result and the experimental data were well matched. The relation between each significant phenomenon and each parameter was also discussed. The model quantitatively identified the major voltage losses to be both the sluggish reaction of methanol oxidation on the anode and the slow oxygen reduction on the cathode. Impact on cell performance by manipulating individual parameter was also issued. Then, a new process to synthesize unsupported Pt-Ru colloid was introduced. The characteristics of synthesized nanoparticles were identified by XRD, TEM/EDX and SEM and it shows that Ru atoms are incorporated into Pt fcc structure and the well-dispersed particles (diameter approx. 4 nm) possess Pt-rich feature. This catalyst shows hydrophobic characteristic which can adsorb very well on the hydrophobic-treated carbon paper or carbon cloth without the need of Nafion. Accordingly, this method can avoid particle agglomeration and the synthesized catalyst demonstrates strong adsorption with carbon paper. In addition, this colloid-type Nafion-free catalyst was measured via linear sweep voltammetry (LSV) and exhibited electrochemical activity for methanol oxidation comparable to the commercial one with Nafion binding. It shows high potential to be applied in the DMFC anodic catalyst layer. Finally, a dynamic model for PEMFC was established on macroscopic scale. This model combines a pseudo steady state model with a time-dependent calculation procedure. To describe the relation between water vapor and liquid water, the evaporation was taken into account and a useful equation for the evaporation rate coefficient was derived. The simulation results successfully demonstrate the dynamic response of cell voltage when a current-step change occurs. The dynamic changes only happen at the initial stage of current-step change and the results also indicate that the dynamic response of cell voltage deeply depends on the liquid saturation and oxygen concentration. Moreover, the sudden variation of cell voltage under steady-state operation was simulated by the present model with a pulse change of inlet liquid saturation.