| 摘要: | 本論文利用本實驗室已建立之高壓雙腔體固態氧化物燃料電池(solid oxide fuel cell, SOFC)性能量測平台，並搭配一最近設計之加壓鈕扣型SOFC實驗載體，分別針對不同系統壓力範圍p = 1~5atm、不同操作溫度範圍T = 700℃~850℃和不同陰極氧濃度範圍[O2] = 4.20%~100%，進行一系列鈕扣型陰極對稱型電池之電化學阻抗頻譜量測與分析，目的為探討壓力、溫度和氧濃度效應對陰極氧還原反應機制之影響。本實驗所採用之陰極電池材料為LSM/LSM-GDC，具雙層對稱結構，我們以固定體積流率(0.5slpm)來進行所有實驗。實驗結果顯示，在任一固定溫度和氧濃度條件下，提高系統壓力有助於降低陰極氧還原反應阻抗，其中又以p = 1~3atm最為顯著，而p = 3~5atm 漸趨平緩，這是因為在常壓時，開始加壓會使陰極氧濃度較有效地增加，而在系統壓力已達3atm時，氧分子與陰極電極觸媒接觸面積漸趨飽和，故壓力效應會漸趨平緩。此外，亦發現加壓效應在較低溫度時比在高溫時來得顯著，即在T = 700℃時壓力效應較在T = 850℃時來得顯著。我們發現歐姆阻抗幾乎不隨壓力變化而改變，但提高操作溫度可使歐姆阻抗與極化阻抗同時下降，進而使交換電流密度增加，促使氧還原反應速率有效地提升。是故，溫度效應對陰極氧還原反應比壓力效應來得顯著。有關氧濃度效應方面，在任一固定壓力和溫度條件下，提升氧濃度可使氧分壓增加，使更多氧分子與陰極觸媒接觸反應，因此可有效降低氧還原反應阻抗，提升電池性能。另外，加壓效應於低氧濃度條件下比在高氧濃度條件下會更加顯著。
經由陰極極化阻抗(RP)與氧分壓(p_(O_2 ))之相依性分析，即R_P^(-1)?p_(O_2)^n，其中n為冪次法則之指數，我們找到在常壓條件下，T = 700℃和750℃之n = 0.370~0.379，經由文獻資料比對歸納分析後，可推斷其反應速率決定步驟應為氧原子沿電極與電解質界面擴散之過程；在T = 850℃，n = 0.278，其反應速率決定步驟則為氧離子於LSM表面擴散至三相邊界層的過程。至於在加壓條件下，當T = 700℃，n = 0.614，其相對應之反應速率決定步驟應為氧離子在LSM表面擴散的過程；而當T = 750℃~850℃，n = 0.455~0.548，此指數值範圍則為吸附氧分解的過程。本研究成果，對了解加壓SOFC陰極半電池之還原反應過程機制，有重要之助益。並且，本團隊所建立之加壓鈕扣型電池測試平台，亦可針對不同陰極材料作性能量測分析，可作為開發新型SOFC陰極材料之有用量測平台。
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. |
