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LaGaO3계 고체산화물 연료전지의 La0.2Sr0.8TiO3 대체 연료극 특성

LaGaO3계 고체산화물 연료전지의 La0.2Sr0.8TiO3 대체 연료극 특성
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Solid Oxide Fuel Cell (SOFC) is an electrochemical device that converts the chemical energy of combustible fuels into electrical energy. Since a SOFC employing YSZ (Yttria Stabilized Zirconia) requires high operating temperature (750
1000oC), much efforts have been concentrated on finding electrolyte materials having high oxygen-ion conductivity to lower the operating temperature of SOFC. LaGaO3, doped with Sr for A-site and Mg for B-site, typically La0.9Sr0.1Ga0.8Mg0.2O3-δ (hereafter referred to as LSGM) is a promising material that serves such a need. The typical anode materials are Ni-based anodes where Ni is mixed with an electrolyte material, such as YSZ or Gd-doped ceria (GDC). Although this Ni-based anode shows good performance in H2, it has many operational problems that impedes its utilization. A critical problem is the deactivation of Ni-based anode by carbon deposits when exposed to hydrocarbon fuels. The Ni-based anode activate hydrocarbon cracking and facilitate the formation of carbon-carbon bonds, leading to the development of solid carbon deposits. In addition, Ni-based anode has poor tolerance to small amounts of H2S in the fuel. Ni-based anode also has the sintering problem of Ni particles and the Ni-NiO volume-change problem during the redox cycling at SOFC operating temperature. Most of all, the interfacial reaction between Ni and LSGM results in a significant performance degradation. For example, the formation of an insulative-phase LaSrGa(Ni)O4-δ and LaNiO3 was reported under high temperature sintering (>1350oC). A La-doped SrTiO3 is a candidate as an alternative anode material to solve the problems of Ni-based anode. La-doped SrTiO3 has many advantages as an anode material such as good chemical stability, high electronic conductivity under reducing conditions, strong carbon deposition resistance, and high tolerance of sulfur in the fuel. In this study, the composition of La0.2Sr0.8TiO3-δ(LST) was selected due to its high electronic conductivity under low PO2, redox-cycle stability and TEC compatibility with electrolyte materials. Thus, LST was applied on the LSGM electrolyte in this study, and its possible usage as an alternative anode material for intermediate temperature SOFCs has been studied. In addition, the performance of LST-based anode was optimized by the controlling parameters (ex. composite ratio, weight of impregnated-Ni, heat schedule of impregnation, doping ration of acceptor, and etc.). A single-phase LST and an LST-GDC composite were tested as the possible anodes on LSGM electrolyte. In order to further improve the anodic performance, Ni was impregnated into the LST-GDC composite anode. The performance was examined from 600oC to 800oC by measuring impedance of the electrolyte-supported, symmetric (anode/electrolyte/anode) cells. The polarization resistance (Rp) of LST-GDC anode was much reduced from that of LST anode. When Ni was impregnated into LST-GDC composite, the Rp value was further reduced to ~10% of the single-phase LST anode. Furthermore, LST–
GDC anodes of various compositions (LST/GDC ratios) were evaluated in order to optimize the performance of composite anode. A LST30-GDC70 (LST:GDC = 30:70 wt%) composite anode showed the lowest polarization-resistance value at 800oC (Chapter 3, 4). In order to improve the anodic performance of LST, Co (1, 2, 4, 8 mol%) was doped. The Rp of anode was minimum for 2mol% Co-doped LST (LSTC2) at 600oC in 5% H2+95% Ar. While the Rp of LST anode rapidly increased with time, e.g., ~1.6 times after 8 h, the Rp of 1 or 2 mol% Co-doped LST was relatively stable with time (Chapter 5). Ni was impregnated into LST anode with various Ni-weight ratios since impregnated-Ni weight was not optimized to show the best anode performance. When the impregnated Ni-contents in anode was over 19 wt%, it was shown that the good performance of Ni-impregnated anode was stabilized. This result was comparable with the Ni-percolation study shown in porous GDC bulk. The maximum power-density (MPD) of cell with Ni-impregnated LST anode was 460 mW/cm2 at 800oC (Ni-contents: 37 wt%, thickness of electrolyte: ~500㎛) (Chapter 6). The long-term stability of SOFC performance using the LST-based anode in H2 and CH4 fuel has been investigated. The stability were tested on two single cells with different anodes
La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF, cathode) / LSGM / Ni-GDC (anode), and LSCF (cathode) / LSGM / Ni-impregnated LSTC-GDC (anode). The performances of two single cells were compared for 30 hours in H2 and CH4. In H2 fuel, the performance of cell with Ni-GDC anode was almost stable. However, the MPD of cell with LST-based anode increased from 638 to 924 mW/cm2 at 800oC during 30 hours (Ni-contents in anode : 19 wt%, thickness of electrolyte : ~ 250㎛). From the impedance analysis, the increase was due to the decrease the Rp of anode. In CH4 fuel, while the cell performance with Ni-GDC decreased ~25% during 30 hours, that with LST-based anode was relatively stable during 30 hours. In this study, LST-based anode exhibited good tolerance to hydrocarbon fuels compared to conventional Ni-based anode (Chapter 7).
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