| Summary: | 博士 === 國立交通大學 === 機械工程系所 === 95 === Recently, the increase of global energy demand will propel a more rapid depletion of world’s fossil fuel reserves and the burning of the fossil fuels for generating electricity will release greenhouse gases into the atmosphere. The requirements for developing the renewable energy and clean energy technology become the most important issue for the human being in this century. Much attention has been devoted to the developments of the fuel cells because they are clean, high efficient and capable of module. There are two major topics of the R&D programs for the PEM fuel cell systems. First, the improvement of the performance and the decrease of the cost, secondly, enhance the reliability and durability of the fuel cells.
Fuel cell performance and life time are strongly influenced by impurities in the fuel gas (cation and CO). Reforming from methanol or gasoline fuels is the most widely used method to generate hydrogen fuel. Even trace amount CO would reduce the hydrogen electro-oxidations effectively by occupying the Pt reacting surface which results in a decrease in the cell performance and life time. To keep a long time and stable operation, how to reduce the CO concentration effectively from the reformer and enhance the tolerance for CO of the fuel cell will become a significant topic. In this work, a comprehensive theoretical model of the poisoning effect of PEM fuel cells by CO is investigated to promote the tolerance for CO and thus elucidate the transport phenomena inside the cell.
In the first part of this study, a one-dimensional transient mathematical model is applied to simulate the carbon monoxide poisoning effect on the performance of the PEM fuel cell. The transient behavior of CO poisoning process across the anode catalyst layer is investigated. The results show that the hydrogen coverage, �嘕, decreases with the time due to the CO adsorption on the catalyst site. A higher CO concentration results in a less available catalyst site for hydrogen electro-oxidation and a more significant decrease in the response time to reach steady state tss. Increasing anode overpotential and gas porosity would result in an increase in the current density, especially at low level of CO concentration.
Second, a one-dimensional, two-phase mathematical model was developed to analyze the CO poisoning effect on the performance of a PEM fuel cell. Both vapor and liquid water transport are examined inside the cell. The theoretical results indicate that a higher CO concentration results in large CO coverage across the anode catalyst layer. The slowing of the chemical reactions at both the anode and the cathode reduce the liquid water saturation level in the catalytic layers. At high CO concentration and dilute hydrogen feed, the effect of the electro-osmotic drag is small and less liquid water is generated at the cathode catalyst layer, causing the liquid water distribution to have a small slop across the membrane. The distribution of liquid water depends more strongly on the CO concentration than on dilution of hydrogen in the MEA of the fuel cell. A large dropping rate of the current density is observed in the range between 10-50 ppm CO. Increasing the amount of pure hydrogen drastically increases the current density for a wide range of CO contents, promoting the tolerance for CO of the fuel cell.
Third, a one-dimensional, two-phase, transient mathematical model was extended to analyze how carbon monoxide poisoning affects the performance of a PEM fuel cell. The theoretical results indicate that a higher CO concentration results in less hydrogen coverage and a large drop in the time to reach steady state tss. Increasing the amount of pure hydrogen drastically increases tss for a wide range of CO contents. At 10 ppm CO, a long tss can be achieve using pure hydrogen, especially at high cell voltage, promoting the tolerance for CO and providing the desired performance of the fuel cell.
Finally, high temperature proton exchange membrane fuel cells have drawn great attentions due to high CO tolerance and overcoming water managements. In this wrok, theoretical and experimental studies were made to analyze the transient CO poisoning process. Experimental results were measured at different temperature and suffered from various CO contents. Higher performance was obtained at elevated temperature due to faster chemical kinetics. Only 26% of performance loss is obtained under 3% CO and 40%H2. The effects of temperature, CO contents and H2 dilutions on the fuel cell performance and the time to reach steady tss are all investigated. The predictions of the degradation of fuel cell performance show good agreements with experimental results under various fuel compositions. Thus, the present results can provide comprehensive information for designing fuel cell system and methanol reformer.
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