- 學位論文
不銹鋼連接材對SOFC陰極材料毒化現象之研究
Poisoning effect of stainless steel interconnects on cathode materials for SOFC
摘要
SOFC使用之連接材(Interconnect material)，早期都使用陶瓷材料之連接材，可是因陶瓷材料之機械加工性差，造成製造成本居高不下，因此近年來改採用金屬做為連接材，最廣泛使用就屬不鏽鋼，歸因於不銹鋼具有良好加工性且抗腐蝕性佳，但過去之研究文獻指出，使用含鉻的金屬連接板會出現陰極毒化現象，高價態的Cr化合物會從合金中揮發，高揮發性的CrO3(g)和 Cr(OH)2O2產生的蒸氣會在電極表面產生電化學或化學還原反應，部份會在三相界面沉積，向多孔陰極擴散，阻塞活性電極表面，使得電池性能降低。 由於不鏽鋼之Cr元素在高溫操作下會造成SOFC陰極毒化，本實驗選用Crofer22APU及SUS430兩種不銹鋼連接材，分別進行第一部分：在600~800°C之高溫操作環境，與LSCF/GDC半電池進行110 h電化學阻抗頻譜(EIS)檢測。第二部分：對LSCF陰極材料進行100 h毒化熱處理兩組實驗。兩組實驗經ESCA分析，毒化後LSCF表面皆出現三價Cr及六價Cr的訊號，以XRD相分析發現表面二次相為SrCrO4。以SEM進行表面微結構觀察，SrCrO4隨著溫度與時間的增長有晶粒變多且成長的趨勢，以800°C/100 h熱處理後表面影響最明顯，二次相厚度約1 μm。EIS結果顯示，持溫110 h時效實驗，發現歐姆阻抗隨時間增加，低導電性的二次相生成於不銹鋼與陰極界面，阻抗值隨之增加；極化阻抗也隨熱處理時間增長有明顯增加的趨勢，此乃毒化物附著於三相界面位置而不利於氧還原。兩種不銹鋼毒化對電性影響以SUS430對LSCF較為嚴重。
並列摘要
In early stage of development, the material used for connecting SOFC (interconnect materials) was mostly made of ceramic. However, the machining of ceramic materials is difficult, and high fabrication cost of ceramic.Therefore, intermetallic metals have replaced ceramics as an alternative interconnect recently. The most widely used metal which is be used is stainless steel, because of the benefit of better fabrication and corrosion resistance. According to some research literatures, interconnect containing chromium would cause poisoning effect on cathode. High state of chromium vapor, like high volatile CrO3(g) and Cr(OH)2O2(g), might generate from the alloy. Chromium vapor deposited on the cathode surface, then hinder electrochemical or chemical reduction reaction the three-phase boundary. As a result, the efficiency of the SOFC was dramatically decreased. Since chromium in stainless steel caused poisoning effect on cathode under high temperature. Crofer22APU and SUS430 stainless steels were chosen as test metals in this study. Two experiments were conducted as following. In part one, at 600 ~ 800°C high operating temperature, test was conducted in a LSCF / GDC half-cell for 110 hr by using electrochemical impedance spectroscopy (EIS) measurement. In part two, poisoning was treated with LSCF cathode for 100 hr for two metals. After treatment, surface analysis and electrochemical measurement were conducted. After poisoning treatment, Cr signals were observed on LSCF surface by using ESCA analysis. XRD analysis showed that the minor phase on the surface is SrCrO4. The SEM observation indicated that SrCrO4 particles grew and got more with heating time and temperature. And the thickness of the minor phase is about 1 μm. The most rapid growth occurred on the surface after treating at 800°C for 100 hr. EIS results showed that, after 110 hr aging, ohm resistance increased obviously with aging time. The increasing of ohm resistance was attributed to the low electronic conductivity minor phase formed on cathode. Polarization resistance also significantly increased with the heat time, which was also attributed to formation of the minor phase on the three-phase boundary, in turn, oxygen reduction become more difficult. On electrical properties for LSCF, SUS430 revealed relatively serious poisoning effect between two tested steels.