| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 102 === This thesis applies a recently-designed pressurized button cell setup in an already established high-pressure double-chamber solid oxide fuel cell (SOFC) testing platform to measure electrochemical impedance spectra (EIS) of a symmetric cathode button cell with a symmetric bi-layer structure made of LSM/LSM-GDC materials. In order to understand effects of system pressure (p) and temperature (T) as well as the influence of oxygen concentration ([O2]) on the oxygen reduction reaction mechanism of the cathode, we measure EIS data over a range of p from 1atm to 5atm, a range of T from 700℃ to 850℃, and a wide range of [O2] varying from 4.2% to 100% by changing only one parameter at a time while keeping the other two parameters fixed. All experiments studied here use a fixed flow rate of 0.5slpm by diluting with nitrogen except for the case of [O2] = 100%. Results show that the cathode’s oxygen reduction reaction resistance decreases with increasing p at any fixed T and [O2]. Such resistance decrease due to the effect of pressurization is more significant from p = 1atm to 3atm and then becomes more gradually from p = 3atm to 5atm. This is because pressurization starting from normal pressure can be more effectively to increase the cathode oxygen concentration and thus increase the contact area between oxygen molecules and the cathodic catalyst, while the constant areas tends to be saturated at higher p. Besides, the pressure effect is more profound at lower T than at higher T. It is found that the ohmic resistance is almost unchanged with pressurization, but it decreases with increasing T, so as to the polarization resistance. This leads to an increase of the exchange current density and thus the oxygen reduction reaction rate can be effectively increased. It is concluded that the effect of T is more significant than the effect of p on the cathode oxygen reduction reaction. As to the effect of [O2], increasing [O2] can increase the oxygen partial pressure, allowing more oxygen molecules to react with the cathodic catalyst at any fixed p and T and thus, the oxygen reduction reaction resistance can be reduced and the cathode cell performance can be increased. Further, the pressure effect is found to be more effective at lower [O2].
By analyzing the dependence of the cathode polarization resistance (RP) with oxygen partial pressure (p_(O_2 )) as a form of R_P^(-1)∝p_(O_2)^n where n is the power law exponent, the rate-determining steps in the cathode can be obtained. At p = 1atm, values of n = 0.370~ 0.379 at T = 700℃ and T = 750℃, the rate-determining steps should be the atomic oxygen diffusion process along the interface between the electrode and the electrolyte; at higher T = 850℃, n = 0.278 and the oxygen ion diffusion process from the surface of LSM to the triple-phase boundary should be the rate-determining step based on the comparison with available literature data. In the cases of pressurization, the corresponding rate-determining step is the oxygen ion diffusion process of the LSM surface at T = 700℃where n = 0.614 and the dissociated oxygen adsorption process when T = 750℃~T = 850℃where values of n = 0.455~0.548. These electrochemical results are important to our understanding of the mechanism of oxygen reduction reaction in the cathode under pressurization conditions. Moreover, the current pressurized button cell test rig is a useful tool to test and to develop the new cathode materials.
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