Thesis

Effects of Water Transport and Freezing on the Performance Characteristics of Polymer Electrolyte Fuel Cells
Year of publication 2009
Title of paper Effects of Water Transport and Freezing on the Performance Characteristics of Polymer Electrolyte Fuel Cells
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Abstract The objectives of this study are to observe the behavior of water and to investigate its effect on the performance and the degradation characteristics of the polymer electrolyte fuel cells (PEFC). The effects of external humidification and dynamic behavior of the PEFC owing to the self-humidification were studied about the variation of membrane characteristics by varying water content. Water transport was visualized, extraordinarily, from the anode side. Durability studies were carried out at freezing conditions to observe the variation of mass transport characteristics. The effects of external humidification to the membrane were investigated by varying the humidification side. The best performance of the cell was achieved by both-side humidification. However, as the humidity condition approached saturation state, anode humidification yielded comparable performance to both-side humidification. The performance decreased with the increase of temperature for anode humidification because of membrane dry-out. For the cathode humidification and both side humidification, the performance showed the highest performance at 70 oC when the temperature was varied from 50 oC to 80 oC. The dynamic behavior of polymer electrolyte fuel cells was investigated experimentally at sudden load change conditions. Steady-state results for various temperatures and humidities were used as the basic data for the analysis of dynamic behavior. Electrochemical impedance spectroscopy (EIS) showed that the ohmic resistance was reduced with the increase of humidity and current while the total polarization resistance including the mass transfer effect showed various trends according to cell temperature. The dynamic behavior of the cells was measured with time. The current increment just after an abrupt voltage reduction jumped to a certain level and then increased gradually, showing a logarithmic-shape curve. The stabilization time showed various trends according to cell temperature, humidity, and voltage range. Water transport through the membrane of a PEFC was investigated by not only measuring the voltage variation but also visualizing the water accumulation at the anode according to the variations of the values of operating parameters such as humidity, current density, stoichiometry, humidification side, and membrane thickness. The PEFC was operated in the dead-end mode to prevent the discharge of water from the anode. The water transport in the PEFC was characterized by the elapsed time for the voltage to reach its limit value. Anode visualization showed the water transport under various conditions. In addition, the mass balance of water at the anode of the PEFC was considered, and the variations of water diffusion and electro-osmotic drag were analyzed based on the experimental results. Variation of gas diffusion layer (GDL) characteristics by repetitive freezing was investigated by using porosimetry, gas permeability measurement, and surface morphology. Five types of GDLs were employed to the experiment. As a whole the porosity was decrease by freezing cycles, but the gas permeability increased for most of GDLs. By surface morphology, a lot of cracks on the adhesive material were found after freezing cycles. Especially, GDL 35BC showed a lot of expanded crack on MPL. Degradation of performance of the PEFC under repetitive freezing conditions was investigated for two types of GDL. The slope of performance degradation was higher for GDL 35BC. The fluctuation of performance was appeared owing to the Pt area loss and recovery. The total polarization resistance increased continuously until 27th cycle for GDL 35BC while it stopped increasing after 9th cycle for GDL 10BA.
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박사학위논문