Thermochemical Stability and Eletrode Activity for Oxygen Molecule Dissociation and Oxygen Reduction of the Lanthanum-Nickel-Iron Oxide Cathode(ランタンニッケル鉄酸化物カソードの熱化学的安定性および酸素分子解離と酸素還元に対する電極活性)

氏名 Potejanee Sornthummalee
学位の種類 博士(工学)
学位記番号 博甲第440号
学位授与の日付 平成19年9月30日
学位論文題目 Thermochemical Stability and Eletrode Activity for Oxygen Molecule Dissociation and Oxygen Reduction of the Lanthanum-Nickel-Iron Oxide Cathode (ランタンニッケル鉄酸化物カソードの熱化学的安定性および酸素分子解離と酸素還元に対する電極活性)
 主査 教授 佐藤 一則
 副査 教授 松下 和正
 副査 教授 梅田 実
 副査 准教授 松原 浩
 副査 特任教授 井上 泰宣

平成19(2007)年度博士論文題名一覧] [博士論文題名一覧]に戻る.


Chapter 1. Introduction
 1.1 The fuel cell p.1
 1.2 The history of fuel cell p.2
 1.3 Type of fuel cells p.3
 1.4 Basic principles of solid oxide fuel cell p.8
 1.5 Materials for SOFC p.18
 1.6 Background p.20
 1.7 Objectives of research p.24
 1.8 Scopes of this work p.25
 References p.27
Chapter 2. Electrochemical Characterization
 2.1 SOFC preparatino and confifuration p.31
 2.2 Electrode polarization characterization p.36
 2.2.1 AC Impedance analysis p.36
 2.2.2 Current interruption DC method p.39
 2.3 Electorde polarization p.41
 2.4 Electrical conductivity measurement p.42
 References p.44
Chapter 3. Thermocheamical Stability and Ploarizatin Resistance of The La(Ni0.6Fe0.4)O3 Cathode
 3.1 Introduction p.45
 3.2 Experimental p.47
 3.2.1 Cell fabrication p.47
 3.2.2 Electrochemical measurement p.48
 3.2.3 Microstructure characterization p.52
 3.2.4 Phase stability identification p.52
 3.3 Results and Discussion p.53
 3.3.1 Comparison of the performance between LNF and LSM p.53
 3.3.2 Effect of the current-loading time on the electrochemical properties of LNF p.59
 3.3.3 Thermochemical stability of LNF p.63
 3.4 Conclusion p.65
 References p.66
Chapter 4. Electrochemical Performance and Microstructure of the LaNi0.6Fe0.4O3-Sm0.2Ce0.8O01.9 Composite Cathode for SOFCs
 4.1 Introduction p.67
 4.2 Experimental p.70
 4.2.1 Sample perparation and characterization p.70
 4.2.2 AC impedance analysis p.71
 4.2.3 Cell performance and cathodic overvoltage measurement p.72
 4.3 Results and Discussion p.74
 4.3.1 Phase stability and microsturcture characterization p.74
 4.3.2 Impedance analysis p.78
 4.3.3 Cell perfomance and cathodic overvoltage p.81
 4.3.4 Interface microstructure between LNF/SDC cathode and YSZ electrolyte p.86
 4.4 Conclusion p.88
 References p.89
Chapter 5. Thermochemical Stability, Nonstoicheometry, ElectricalConductivity, and Electrochemical Performance of LaNi0.6Fe0.4O3-δ Cathode
 5.1 Introduction p.90
 5.2 Experimental p.91
 5.2.1 X-ray photoelectron microscopy p.91
 5.2.2 Iodometric titration p.92
 5.2.3 Thermogravimetric analysis p.94
 5.2.4 Phase stability characterization p.94
 5.2.5 Electrical conductivity p.95
 5.2.6 Impedance analysis p.95
 5.3 Results and Discussion
 5.3.1 X-ray photoelectron microscopy and iodometric titration p.96
 5.3.2 Thermogravimetric analysis p.100
 5.3.3 Phase identification p.102
 5.3.4 Electrical Conductivity p.102
 5.3.5 Electrochemical performance p.105
 5.4 Conclusion p.109
 References p.110
Chapter 6. Summary p.113
Acknowledgements p.115
List of Journal Papers and Proceedings p.117
List of International Conferences p.119

 The development of a novel cathode for solid oxide fuel cells (SOFCs) have been focused on their thermochemical stability, catalytic activation for oxygenmolecule dissociation and oxygen reduction, and electronic conductivity, LaNi0.6Fe0.4O3δ(LNF). Cell performance, cathodic overpotential, and impedance were examined by compared to that of La0.8Sr0.2MnO3(LSM), which has been conventional used as the cathode for SOFC. We therefore investigated the singlecell that consists of a NiO-SASZ cermets anode, SASZ electrolyte and LNF cathode and performed the electrochemical measurement in the range of intermediate temperature 700-850℃. The performance of cell used LNF showed significantly high compared with used LSM as the cathode. Microstructure of LNF and LSM on SASZ electrolyte have been discussed and found that a smaller LNF grain size could be increased the three phase boundary (TPB) where the reductionof oxygen participated. The thermochemical stability of LNF in the prolonged current-loading results in a change of LNF surface morphology and activation energy which suggests that an occurrence of the reaction mechanism change at the cathode. As expected in a long time current loading for a cell, the oxygen partial pressure at the interface of cathode and electrolyte could be reduced due to the concentration gradient of oxygen decrease. The decrease the oxygen partial pressure therefore seems to induce the positional change for Ni and Fe ions in the perovskite lattice and affect to the oxygen deficiency in LNF. The investigation of the mixed the ionic and electronic conductivity materials, LNF-SDC as a composite cathode have been performed in order to improve the ability of oxygen reduction at the interface between the cathode showed higher maximum power density and lower cathodic overvoltage than that using LNF cathode.
Enhancement of the electrochemical performance in the LNF-SDC composite cathodeis most likely to be cased by the SDC particles connecting LNF particles with the electrolyte surface. The electrical conductivity of LNF has been studied as a function of the oxygen partial pressure (pO2) using four-probe DC method. The accurate oxygen content of the as-prepared LNF was determined by iodometric titration. The nonstoichiometry in LNF was examined by thermogravimetric measurements in the atmosphere of Ar/O2 at 500℃-900℃. The effect of the pO2 onthe electrical conductivity of LNF indicates the formation of an oxygen vacancyin the LNF lattice since Fe3+ can partially reduced to Fe2+ to produce the oxygen vacancies that agree with the X-photoelectron spectroscopy (XPS). The dependence of oxygen partial pressure on the electrode polarization resistance indicates that the rate determining step of oxygen reduction in LNF are the surface diffusion of adsorbed oxygen atoms and the charge transfer process at TPB between the electrode and electrolyte.

 本論文は「Thermochemical Stability and Electrode Activity for Oxygen Molecule Dissociation and Oxygen Reduction of the Lanthanum-Nickel-Iron Oxide Cathode(ランタンニッケル鉄酸化物カソードの熱化学的安定性および酸素分子解離と酸素還元に対する電極活性)」と題し、6章より構成されている。
 第3章では、 LaNi0.6Fe0.4O3- (LNF)空気極、ニッケルサーメット(Ni-YSZ)燃料極、およびスカンジア安定化ジルコニアを用いて作製した測定セルに対する燃料電池性能評価を、従来型の空気極材料であるランタンストロンチウムマンガン酸化物(LSM)と比較した。特に、長期過負荷発電状態における過電圧の経時変化の温度依存性を詳細に検討し、LNF空気極における熱化学的安定性と電極活性に影響を与える基本因子を明らかにしている。
 第4章では、 LNFにサマリア添加セリア(SDC: 10 mol%Sm2O3-CeO2)を組み合わせた複合型空気極がLNF空気極の熱化学的安定性と電極性能を高める効果を検討した。この空気極においてLNF粒子表面に接合状態で存在するSDC粒子が、LNF粒子同士の接触性向上と
 以上のように本論文では、LNF空気極 が従来の空気極を用いたSOFCに比べて800℃以下の低い温度においても発電効率を高めることが可能であることを、固体化学的な基礎的観点のみならず実用的な観点からも低温動作型SOFC開発に対して有用な知見を与えている。