成果報告書詳細
管理番号20120000000698
タイトル*平成23年度中間年報 固体酸化物形燃料電池システム要素技術開発 基礎的・共通的課題のための研究開発 三相界面についての劣化現象と微細構造変化の相関付け
公開日2014/5/9
報告書年度2011 - 2011
委託先名国立大学法人京都大学大学院工学研究科
プロジェクト番号P08004
部署名新エネルギー部
和文要約
英文要約itle: Development of System and Elemental Technology on Solid Oxide Fuel Cells / Basic research for improving durability and reliability / Degradation behavior related to microstructural change at the triple phase boundary (FY2008-FY2011) FY2011 Annual Report

The triple phase boundary (TPB) is the active site for the electrochemical reaction and significantly affects the performance. Thus, the changes in TPB, such as length and surface composition, during long-term operation lead to the performance deterioration. In this research, we aim to clarify the deterioration factors at the TPB and to simulate degradation behavior through the analysis of electrode reaction, chemical state in the vicinity of TPB, and electrode microstructure. Final target is to recommend the promising strategies for the prediction of deterioration rate, fabrication method and microstructure of electrodes resistive to deterioration, and operating conditions, etc. The experimental results obtained in this fiscal year are as follows:

1) The performance stability of the cell with Ni-YSZ anode was examined at 1000ºC by feeding fuels with a fixed P(O2) of 1.2×10-15 atm. In H2O-H2 fuel, a sudden drop in the terminal voltage was observed within 10 h. Although the similar behavior was confirmed in CO-H2O, the stable power generation was possible in H2-CO2 and CO-CO2. Thus, P(H2O) was the crucial factor for the degradation rather than P(O2).

2) The changes in performance and microstructure of Ni–YSZ were studied during redox cycles by feeding H2 and O2 alternatively to the anode. The surface area of Ni increased after redox cycles, indicating that Ni particles became finer and more complicated in shape. Despite this nickel deformation, TPB length decreased from 2.49 (initial) to 2.39 and 2.11 μmμm3 after first and fourth redox cycles, respectively. The decrease in TPB length was clearly correlated with an increase in polarization resistance of anode during the early stage of redox cycles. Microstructures and compositional variations in the vicinity of TPB were investigated using FE-TEM to clarify the detailed mechanisms of deterioration. In the cell with degraded cathode, the surface of YSZ adjacent to the TPB on the cathode side was confirmed to be covered by nano-scale surface layer containing La and Mn, which is partly responsible for the degradation of the cathode by reducing the active TPB length.

3) Microstructural parameters of porous electrode are quantified from two different structural data: real 3D structure observed by FIB-SEM and 2D cross-sectional image. By comparing the results obtained by each method, the validity and applicability limit of 2D porous model were clarified. The tortuosity factors evaluated from 2D image through “concept of contiguity”, for example, fairly match to the values evaluated by random walk calculation using 3D data when the volume fraction is sufficiently large. However when the volume fraction is small, their differences become prominent as the 2D model cannot properly consider connectivity of the structure.

4) Microstructural parameters are quantitatively evaluated. A random-walk-based diffusion simulation is effectively used for quantification. As an application of the quantified parameters, 1D numerical simulation of an anode is conducted. The predicted anode overpotential agrees well with the experimental counterparts in the condition of 3.0%H2O–97% H2, 1273K, while it is overestimated at high humidified and low temperature conditions.
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