| Direct methanol PEM fuel cell (DMFC) is recognized as a new alternative to the present power sources because of its characteristics of high efficiency, high power density, low emission and easy fuel carriage. DMFC is expected to find wide application as a portable or mobile power, exemplified by battery for cellular phones or engine for electric vehicles. Liquid feed DMFC is receiving more attention because it is more convenient to use in practice. However, the lack of high performance membrane electrode assembly (MEA), the heart of DMFC, has become one of the key obstacles to its application. The goal of this study is to optimize MEAs through investigation of factors affecting their performance, with an emphasis on elucidating the effects of methanol crossover. The performance of liquid DMFC is evaluated as a function of the composition and structure of MEAs, hot-pressing condition and activation process by means of V-I polarization and AC impedance spectrometry. It is discovered that a process of MEA activation may greatly shorten the time needed for the DMFC to attain to the best discharge state, which is though not changed by it. The investigation reveals that the performance of a MEA is mainly controlled by the conductivity of membrane, load of catalyst and CO2 holding in the anode, ion conductivity and specific of active area in both anode and cathode. Matching these factors is the key to improve the DMFC performance. The experimental results demonstrate that the suitable MEA hot-pressing temperature and pressure are 120℃ and 15.5MPa, respectively. The optimum catalyst loading in anode is 4 mg Pt/cm2, and the best contents of Nafion ionomer in anode and cathode layer are 45wt% and 35.3wt%, respectively. The decrease of active surface area in the anode caused by CO2 accumulation is reduced through treating the diffusion layer and catalyst layer with PTFE, resulting in increased DMFC performance. Improvement of power output has also been achieved by increasing the porosity of the anode catalyst layer, so that the CO2 produced there can be easily released. In addition, treating the cathode diffusion layer with PTFE can further enhance the cell performance.In order to better understand the processes and phenomena that determine the performance of a liquid feed DMFC, a more comprehensive steady-state mathematical model of DMFC is established. Special attention is paid to the consequences of methanol crossover in cathode reaction and cathode over-potential. Good agreement is found between simulations and experiments in regard to the V-I character of DMFCs. Based on the theory of parallel electrode reaction, it becomes possible to obtain quantitatively the value of over-potential caused by methanol crossover, which is either implicit or not included in the overall cathode over-potential in previous models. Simulation results show that cathode over-potential under low current density is considerably increased because of methanol crossover, but its effect becomes much lower under high current density. It also shows that between the two parameters characterizing a polymer electrolyte membrane-proton conductivity and methanol permeability, the former has more impact on the performance of a DMFC.
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