Degradation of yttria-stabilized zirconia electrolytes: impact of NiO dissolution with and without phase transformation

Abstract

Solid oxide cells (SOCs) utilized for power generation and hydrogen production via electrolysis coupled with renewable energy sources are promising technologies for enabling the transition into a zero-carbon society. To ensure the long-term performance of SOCs under fuel cell and electrolysis operation, elucidating the factors governing degradation of the cell's core components such as the electrodes and electrolytes is crucial to accelerating their practical implementation. In this study, the degradation behavior driven by nickel oxide (NiO) dissolution during co-sintering process used in typical cell preparation is comprehensively investigated for state-of-the-art 8 mol% yttria-stabilized zirconia (8YSZ) electrolytes with Ni-YSZ cermet support. Employing advanced characterization techniques including ¹⁸O oxygen isotope exchange technique with high-resolution secondary ion mass spectrometry (SIMS) imaging, micro-Raman spectroscopy, and scanning/transmission electron microscopy (S/TEM), it is revealed that the degradation of oxide ion diffusivity of 8YSZ proceeds via two distinct pathways. The first pathway is activated by the instantaneous reduction of NiO to metallic Ni in the 8YSZ bulk, resulting in a drastic drop in oxide ion diffusivity; the second one proceeds with a decay time constant which is dependent on the reduction temperature. For the high-temperature regime of 900 °C, an accelerated and extensive phase transformation of 8YSZ from cubic to tetragonal phase occurs. This is accompanied by a preferential precipitation of metallic Ni both within the bulk and grain boundaries of 8YSZ and a relatively fast degradation of oxide ion diffusivity. On the other hand, for the intermediate-temperature regime of 700 °C, even if there is no obvious phase transformation nor Ni precipitation, a gradual progression of degradation of the oxide ion diffusivity of 8YSZ is nevertheless observed. Understanding these underlying degradation mechanisms provides useful insights into the long-term stability of 8YSZ electrolytes and their impact on the long-term performance of SOCs.