Phase Transformation of YSZ Electrolyte in Anode-Supported SOFCs

Abstract

Cubic-stabilized zirconia doped with 8 mol% Y2O3 (8YSZ) is a highly popular electrolyte material for solid oxide fuel cells (SOFCs) due to its exceptional oxide ionic conductivity, which remains consistent across a broad range of oxygen partial pressures. However, previous studies [1,2] have shown that the addition of nickel oxide (NiO) to 8YSZ can expedite the phase transformation process (from cubic to tetragonal) when exposed to a reducing atmosphere at relatively high temperature as 900℃. This phase transformation is closely related to the degradation of 8YSZ's conductivity. Therefore, it is crucial to investigate whether NiO, which should dissolve into the YSZ electrolyte during the co-sintering process with the NiO-YSZ anode at high temperatures, can lead to a similar phenomenon in anode-supported cells (ASCs). Hence, investigating the NiO solid solution phenomenon and its effect on the phase transformation in the YSZ electrolyte of ASCs can offer insights into its degradation behavior.In this study, NiO-YSZ anode support and YSZ electrolyte were co-sintered at 1370℃, 1450℃, and 1550℃, respectively, for 2 hours to fabricate the half-cell samples. Subsequently, these samples were reduced at 900℃ and 700℃, respectively, with a flow of 50ccm dry H2. High-resolution secondary ion mass spectrometry (SIMS) was used to detect the presence of NiO in YSZ, and Raman spectroscopy was used to detect the phase transformation (cubic to tetragonal) occurring in the YSZ.The SIMS results showed that the amount of NiO in the YSZ electrolyte increased with higher sintering temperatures. Moreover, after reducing the cells at 900℃, phase transformation was detected in all samples. However, there was no significant difference among those cells in terms of the degree of phase transformation, which indicated that phase transformation was not accelerated by the increase of dissolved NiO in YSZ. On the other hand, no phase transformation was observed for the cells reduced at 700℃. The mechanism of phase transformation of YSZ in anode-supported SOFCs and its related conductivity degradation will be discussed in the presentation.