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Title: 核殼鈣鈦礦陰/陽極與鈰基電解質共燒之中溫固態氧化物燃料電池研究
Study on Core-shell Perovskite Anode and Cathode Co-fired with Ceria-based Electrolyte for Intermediate Temperature Solid Oxide Fuel Cell (ITSOFC)
Authors: Wang, Yao-Ming
王耀明
Contributors: 國立臺灣海洋大學:輪機工程學系
Keywords: 鈰基電解質;BSF-Ce核殼陰極;LST-Ce核殼陽極;中溫固態氧化物燃料電池
ceria-based electrolyte;BSF-Cecore-shell cathode;LST-Cecore-shell anode;intermediate-temperature solid oxide fuel cell (ITSOFC)
Date: 2017
Issue Date: 2018-08-22T03:40:14Z
Abstract: 固態氧化物燃料電池(SOFC)將電化學反應轉變為電能,與傳統內燃機遵循卡諾循環的工作機制不同。主要結構由陰極、陽極與電解質等陶瓷組成。其輸出效率至少有55%,大於目前內燃機效率,是大動力廠的良好選擇。目前商業化主要限制在於操作溫度過高,約1000 oC,因此在實際應用有所阻礙。本研究以中溫固態氧化物燃料電池(ITSOFC)材料為主軸,探討陰極、陽極與電解質材料及其共燒電池組合,達到工作溫度500-800 oC的高功率ITSOFC。 本研究選用傳導氧離子的多元摻雜氧化鈰基(La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891 (LSBC)電解質材料為核心,利用簡易固態球磨,高溫燒結獲得緻密的電解質結構體。在中溫650 oC運作下,電解質的傳導率達商業化的0.01 S/cm。搭配AC交流阻抗分析,解析電解質晶粒與晶界及電極三者之阻抗,結果顯示晶界在氧離子傳導上扮演著重要的角色。LSBC超過1400 oC燒結溫度可達95%相對密度,在操作溫度550 oC以後,晶界阻值貢獻幾乎消失,使LSBC足以成為中溫電解質。在不同的燒結條件下,晶界活化能大於晶粒活化能。當燒結溫度1400 oC,晶界所貢獻的活化能就不再改變,約0.90 eV,晶粒的活化能約0.84 eV。在晶粒中氧空缺形成所需要的焓值(Ha)約0.90 eV,在晶粒中氧空缺移動所需要的焓值(Hm)約0.62 eV,足夠低的(Ha + Hm)值,讓氧離子傳導效率提高。 Ba0.5Sr0.5FeO3 (BSF)陰極材料在1150 oC持溫6小時可獲得相對純相結構,BSF鐵系陰極材料對於CO2以及潮濕環境有靈敏反應,半有機法製備的核殼BSF可以減緩環境的影響,增益電性與電催化特性。1150 oC燒結的核殼陰極,隨Ce的包覆,對BSF有穩定Fe-O結構效果,FTIR證明Ce殼層可避免水合氧化鐵的產生。當Ce的包覆計量達到10 mol%以上,將析出含鈰二次相。隨Ce包覆量增加,在空氣中量測的電子傳導與離子傳導轉換溫度(Tc) 510C將移至較高溫,BSF-15 mol%Ce可獲得最高直流導電率;15 mol%Ce以上披覆量接近LSBC電解質的熱膨脹係數。AC阻抗分析證實15 mol%Ce以上披覆量能有效降低BSF/LSBC界面阻抗與陰極之擴散阻抗。1150 oC共燒的BSF-20 mol%Ce/LSBC/Pt半電池在操作溫度750 oC,核殼陰極半電池的開路電位約0.8 V,功率密度可以達到250 mW/cm2。 La0.3Sr0.7TiO3 (LST)核殼陽極不論氧化或還原燒結LST-x mol%Ce (x=0.75, 1.5, 3, 6, 12),皆會使Ce擴散進入LST核之晶格中,愈高燒結溫度有愈高的Ce擴散量,還原燒結比氧化燒結可得到更高的Ce擴散量。Raman及XPS分析1300 C以及1500 C燒結之核殼陽極,擴散進入LST核晶格中的Ce,空氣氧化燒結為Ce4+,活性碳還原燒結為Ce3+。氧化燒結的LST-x mol%Ce核殼結構呈現結晶CeO2殼層與結晶LST核的清晰核殼界面;而還原燒結的LST-x mol%Ce核殼結構呈現結晶CeO2殼層、非晶擴散層與結晶LST核的三區核殼界面。核殼陽極不管是氧化或還原燒結,導電率隨披覆量的增加而增加。兩者的差異在於氧化燒結塊材在披覆超過3 mol%計量後,導電率就會反轉下降,還原燒結則是隨披覆量一直提升。核殼陽極以LST-3 mol%Ce在還原環境下,擁有最佳導電性與最低極化阻抗。750C量測的LST-x mol%Ce/LSBC/Pt陽極半電池(1300 oC共燒)可以有效提升發電功率與開路電位;與純陽極結構半電池相比較,核殼陽極半電池達到3.5倍峰值功率的增進。 本研究BSF-20 mol%Ce/LSBC/LST-3 mol%Ce全陶瓷電池擁有最佳的發電效率。AC交流阻抗分析得知此全電池的陰極極化阻值貢獻大於陽極。500m厚度電解質支撐的全電池,具有核殼陰陽極改善了與電解質的界面燒結阻抗問題以及延伸電極與電解質三相區間,增進擴散反應。在800 oC操作溫度下,全陶瓷電池LST-3 mol%Ce/LSBC/BSF-20 mol%Ce得到開路電壓0.8 V,發電峰值功率約355 mW/cm2。 關鍵字:鈰基電解質、BSF-Ce核殼陰極、LST-Ce核殼陽極、中溫固態氧化物燃料電池
Solid oxide fuel cell (SOFC) converts chemical energy to electrical energy directly, it is different from Carnot cyclic internal combustion engine needing multiple mechanical processes. The SOFC components compose ceramic structures including electrolyte, cathode and anode. The efficiency of SOFC is about 55% larger than trandtional internal combustion engine. This dissertation is focused on investigating materials of intermediate-temperature solid oxide fuel cell (ITSOFC) which is operated among 500~800 oC. The oxygen ions conducting multiple elements doped ceria (LSBC) is utilized as electrolyte for cell support. The solid state oxides prepared electrolyte was densified by conventional high temprature sintering. The conductivity arrives to 0.01 S/cm at 650 oC in this study. The AC impedances of grains and grain boundaries (GB) in electrolyte and of metallic electrode indicate the most important impedance of grain boundaries. LSBC electrolyte has 95% relative density after 1400 oC sintering. The GB impedance almost disappears when the operation temperature higher than 550 oC. This approves the LSBC to be an electrolyte of intermediate temperature operation. The GB activation energy is higher than grain activation energy in various sintering conditions. The GB activation energy is hardly changed as 0.90 eV when the sintering temperature over 1400 oC. The gain activation energy is 0.84 eV. The activation energy of grain affects oxygen vacancies conducting behavior. The formation enthalpy (Ha) and migration enthalpy (Hm) of oxygen vacancy were calculated by Ln(T)-(1/T) data. The Ha in grains is about 0.90 V. The Hm in grains is about 0.62 V. Such enough low value of (Ha + Hm) promotes high oxygen ions conducting efficiency. Barium strontium ferrate (Ba0.5Sr0.5FeO3, BSF) cathode material is almost obtained pure pseudo-cubic phase while sintered at 1150 oC. The BSF material is sensivitive to moisture and carbon dioxide environment. It results in structure instability. The core-shell cathode of BSF-x mol%Ce prepared by semi-organic method can solve the above mentioned problems. Ce-coated BSF obtained stable Fe-O bonding. The FTIR analyses prove Ce-coating to avoid generation of hydrous iron oxides. When Ce-coating amount is over 10 mol%, the Ce-contained second phase will be segregated. The transition temperature (Tc ~ 510 C) measured in air atmosphere that represents the transition of electronic to ionic conduction shifts to high temperature with high Ce-coatings. The best DC conductivity is obtained by BSF-15 mol%Ce. The thermal expansion coefficient (TEC) of core-shell BSF over 15 mol% Ce-coatings matches with LSBC. When the Ce-coating is higher than 15 mol%, the interface impedance of BSF/LSBC and diffusion impedance in cathode can be reduced according to the AC-impedance analyses. For 1150 oC co-fired half-cell of BSF-20 mol%Ce/LSBC/Pt operated at 750 oC, the open circuit voltage of half-cell is about 0.8 V and the peak power densty is about 250 mW/cm2. The semi-organic method prepared core-shell structure of La0.3Sr0.7TiO3 (LST)-x mol%Ce (x=0.75, 1.5, 3, 6, 12) either sintered in oxidation or reduction atmosphere, the Ce component diffuses into the core lattice of LST. The more sintering temperature is higher, the more Ce diffuses. Also, high content of Ce in LST is obtained in reduction sintering. The core-shell anode sintered at 1300 C and 1500 C analyzed by Raman and XPS shows that the Ce valences in LST lattice are Ce4+ by air oxidation sintering and Ce3+ by activated carbon reduction sintering. The oxidation sintered LST-x mol%Ce core-shell structure exhibits crystallized CeO2 shell and crystallized LST core with two intimately core-shell interface. However, there are three regions in redcuction sintered core-shell LST-x mol%Ce structure including crystallized CeO2 layer, amorphous zone and crystallized LST core. The conductivity of core-shell anode increases whether oxidation sintering or reduction sintering. The lowest impedance is obtained for LST-3 mol%Ce after reduction treatment. The 1300 oC co-fired half-cell of LST-x mol%Ce/LSBC/Pt has 3.5 times higher peak power density than half-cell without core-shell anode. The best peak power density is achieved by BSF-20 mol%Ce/LSBC/LST-3 mol%Ce full ceramic cell. The cathode polarization impedance is more imprtant than anode contribution investigated by AC impedance analyses. The 500 m thick LSBC electrolyte suppoted cell with core-shell anode and cathode improves interface sintering mismatching, three phase boundary extension and gas diffusion reaction. LST-3 mol%Ce/LSBC/BSF-20 mol%Ce single cell achieves open circuit voltage of 0.8 V and peak power density of 355 mW/cm2 under operation temperature of 800 oC. Keywords: ceria-based electrolyte, BSF-Ce core-shell cathode, LST-Ce core-shell anode, intermediate-temperature solid oxide fuel cell (ITSOFC)
URI: http://ethesys.lib.ntou.edu.tw/cgi-bin/gs32/gsweb.cgi?o=dstdcdr&s=G0D98660002.id
http://ntour.ntou.edu.tw:8080/ir/handle/987654321/48358
Appears in Collections:[輪機工程學系] 博碩士論文

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