The Electrical Property of Y0.20Zr0.04Bi0.76O1.5+δ (YZDB204) Coated on Gd-doped Ceria (GDC) Electrolyte for Intermediate Temperature Solid Oxide Fuel Cell
vi, 64 p.
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Solid oxide fuel cell (SOFC) is a promising device to convert chemical energy into electricity without toxic byproducts. The conventional SOFCs are operating at 800-1000oC. As a result, the commercialization is limited due to problems related to high operating temperature such as high cost fabrication and long-term stability. To reduce the operating temperature, alternative electrolyte materials showing higher ionic conductivity than conventional Y-stabilized zirconia (YSZ) have been studied. Ga-doped ceria (GDC) and Er-doped bismuth oxide (EDB) exhibit one or two order higher ionic conductivity compared to YSZ. However, those two materials are thermodynamically unstable in reducing atmosphere; in other words, GDC would be mixed ionic and electronic conductor (MIEC) and EDB would be decomposed into metallic Bi. However, anode/GDC-EDB/cathode bi-layer cell structure is a promising approach to avoid thermodynamical problems. By applying GDC layer, EDB would be stable and also by using EDB layer adjacent to GDC, electron conduction hinders. The objective of this thesis is to evaluate Y0.20Zr0.04Bi0.76O1.5+δ (YZDB204) as an electronic blocking layer and also GDC stability as enforcing layer. First of all, electrical properties of YZDB204 have been studied by measuring the impedance spectra of button pellet. Then, GDC-YZDB204 bi-layer effects have been evaluated with various GDC thickness (4.5-21 um). YZDB204 exhibits higher electrical conductivity than Y0.25Bi0.75O1.5+δ (YDB25) and similar to Er0.20Bi0.80O1.5+δ (EDB20). That could be related to the lattice parameter which YZDB204 shows similar lattice parameter to EDB20. When dealing with GDC thickness below 10um, GDC-YZDB204 bi-layer exhibits higher OCV than GDC single cell. On the other hand, the effect of GDC-YZDB204 bi-layer is negligible in over 10 um thicknesses of GDC. It could be related to the grain size of GDC. Below 10um, GDC shows relatively smaller grain size than over 10um, so the fracture force initiated by volume change of Ce reduction would be minor. In addition, YZDB204 layer has enhanced GDC stability. During OCV development in single GDC, OCV fluctuation is observed frequently compared to that of YZDB204 bi-layer, According to the microstructure, YZDB204 dense layer could hinder GDC crack propagation. Regarding to the effectiveness of YZDB204, the performance of GDC cell could be promisingly enhanced.