- 學位論文
直接甲烷固態氧化物燃料電池之特性研究
Study on Characteristics of Direct Methane Solid Oxide Fuel Cell
摘要
本研究係以甲烷為固態氧化物燃料電池(solid oxide fuel cell，簡稱SOFC)的燃料，進行SOFC陽極之積碳與去積碳之研究。SOFC係解決本世紀能源問題最有潛力的技術之一。近年來，使用甲烷為SOFC燃料的研究已有初步發展，然而陽極的積碳問題相當嚴重，且燃料使用率相當低。本研究以不同操作電壓、溫度及改變SOFC構成材料等變因，探討SOFC在操作時之陽極電化學反應；並於陽極側添加燃料處理觸媒層，對燃料及脫離陽極的生成物進行處理，以提高甲烷燃料的使用效率。 本研究以直接甲烷固態氧化物燃料電池(direct methane SOFC，簡稱DM-SOFC)進行發電研究，於陽極電化學反應過程中發現”晶格氧抽取之電化學促進(electrochemical promotion of lattice oxygen extraction )”的現象。此現象可抽取SOFC陽極側構成中的晶格氧來與陽極表面的碳反應，有助於減緩DM-SOFC陽極的積碳問題。此現象又引發另一現象，即SOFC可於無燃料時，經由陰極側氧離子填充至SOFC陽極側構成中之氧空缺而放出電流，此電流由本研究定名為”無燃料電流(fuel-free current)”。 在不同操作電壓的測試中，氧物種(Oδ-)在晶格中傳導所需之電荷δ隨操作電壓的增加而減少。因此，隨著操作電壓升高，電化學促進之效應越明顯。而於不同溫度發電時，氧物種在晶格中傳導所需之電荷則隨操作溫度的增加而減少。由溫度的變化可計算出SOFC在發電過程中晶格氧抽取的活化能為124 kJ/mol，遠將小於無發電狀態時之活化能(262 kJ/mol)。 於SOFC之陽極側添加金屬催化層的實驗結果顯示，該層的添加可令燃料進入SOFC前，先行進行燃料重組反應，減少DM-SOFC操作時陽極表面的積碳現象。出口組成的分析顯示，無水汽的生成且一氧化碳選擇率由0.702降至0.547，確認有催化層的存在時，可將SOFC反應後的產物進一步處理。本研究結果顯示，在DM-SOFC的操作中添加金屬催化層可以有效地提高甲烷燃料的使用效率，並可長時間地維持SOFC的操作活性。
並列摘要
In this study, we devoted to research the carbon deposition (coking) and de-coking on the anode of solid oxide fuel cell (SOFC), which fed with the methane flow. This work is considered as the most potential technology to solve the energy problems in this century. In recent years, there are some fuel cell researches using the methane as the fuel, but the fuel efficiency is quite low due to the carbon deposit on anode easily. Herein, we investigated the electrochemical reaction of the SOFC anode with different operating parameters, such as tuning the voltage, temperature or changing the SOFC material. Moreover, in order to improve fuel efficiency, the catalyst layer was added into anode side to deal with the fuel and product. In the study of direct methane solid oxide fuel cell (DM-SOFC) , "Electrochemical promotion of lattice oxygen extraction" was observed in the process of electrochemical reaction. This phenomenon was oxygen extracted from lattices on anode side of SOFC reacted with the carbon on the surface of the anode. It can retard the carbon deposition. Furthermore, this phenomenon also brought a kind of current called "the fuel-free current". It’s be resulted the oxygen from the cathode-side three phase boundary (TPB) refilled the vacancies of the bulk lattice-oxygen on the anode side, especially in the absence of fuel. As the experimental results, the charge δ of oxygen species (Oδ-) decreased with the operating voltage increased. Thereby, the electrochemical effect was more significant. It also found that the oxygen species for transmission in the lattice decreased with the operating temperature increased. Moreover, the activation energy of lattice oxygen extracted under close-circuit was 124 kJ/mol, smaller than the activation energy under open-circuit (262 kJ/mol). When the catalyst layer was introduced to the anode side, the catalyst layer can promote the methane reforming reaction, reduced the coking on the surface of the anode in DM-SOFC. As the results, the carbon monoxide selectivity was about 0.702~0.547, and no water be detected. It showed that the further reaction of the products can be proceeded in presence of the catalyst layer. In this study, the addition of the catalyst layer in the DM-SOFC operation effectively improved the fuel efficiency and maintained the SOFC activity in a long-term operation.