| Summary: | 碩士 === 國立中央大學 === 能源工程研究所 === 98 === The effect of operating parameters of fuel cells such as performance temperature, cathode humidification temperature and cathode stoichiometric feeding on the efficiency of energetic conversion were concerned in this study. The exploration was conducted by evaluation the I-V resulted from DC electrochemical measurement and Electrochemical Impedance Spectroscopy (EIS) measurement by means of numeric estimation of the equivalent circuit. This investigation was focused on a proton exchange membrane fuel cell by using gold-coated nickel foam instead of the traditional graphite flow field plate. .
Simulation of the EIS data by the commercial software Z-view, we obtained the equivalent circuit consisting of ohmic resistance (Rohm) , charge transfer resistance (Rct), constant phase element of the double electric layer (CPE1), mass transfer resistance (Rmt) and the constant phase element of the mass transport (CPE2). The magnitude of the elements was determined individually as follows: Rohm determined by the nature the membrane, Rct and CPE1 determined by the charge transfer on oxygen reduction reaction (ORR) process, Rmt and CPE2 determined by the mass transfer effect.
The resistance for both charge transfer (Rct) and mass transfer (Rohm) decreases with increasing the operating temperature of the fuel cells. This means that the reactivity increases with increasing the operation temperature. It is also found that the resistance for both charge transfer (Rct) and mass transfer (Rmt) increases with increasing the cathode humidification temperature. This implies that higher content of water was carried into membrane and even led to flooding of the membrane electrode assembly (MEA) at higher cathodic humidification temperatures thus causing blocking of oxygen transport. With increasing the cathode stoichiometry, the resistance of charge transfer decreases but that of mass transfer (Rohm) is almost unchanged. This result is ascribed to acceleration of oxygen reduction resulted from sufficient supply of oxygen on the cathode.
According to the results, the optimal operation condition of this system could be summarized as follows: Performing of the fuel cells at 60℃, with cathode humidification temperature at 50℃ and the cathode stoichiometry at 5.
Compared the EIS data between those coming from the fuel cell with gold-coated nickel foam and from that with traditional graphite flow field plate in the performance of proton exchange membrane fuel cell under the same condition, we found that the Rct is much less (0.008Ω)for the gold-coated nickel foam than for the graphite (0.024Ω). It may be ascribed to much more sufficient supply of oxygen through nickel foam than the graphite..
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