Kinetics and Thermodynamics of Sr Permeation in CeO₂-Based Barrier Layers for Solid-Oxide Electrolyzer Cells

Solid oxide electrolyzer cells (SOECs) are promising devices for producing hydrogen from the electrolysis of water, as they can be operated using excess heat as may be produced at a nuclear reactor or power plant. However, due to their high operating temperatures, they can be subject to materials degradation, including from the segregation of cation species across the cells. Notably, in SOECs based on yttria-stabilized zirconia (YSZ) as the oxide-conducting electrolyte, strontium segregation from the air electrode through the Gd-doped CeO 2 (GDC) barrier layer toward the electrolyte can lead to the formation of certain unwanted secondary phases, including SrO and SrZrO 3 . Here, we use density-functional theory (DFT) calculations in conjunction with energy dispersive x-ray spectroscopy (EDS) and mesoscale modeling to characterize Sr diffusion through GDC. We find that Sr diffusion in bulk crystalline GDC is prohibitively slow, but surface diffusion proceeds rapidly. By applying these results to mesoscale models, we show that Sr permeation will be dictated almost exclusively by the porosity of GDC: past a critical porosity, Sr movement will be facile and lead to secondary phase formation. However, we do not expect Sr to form crystalline inclusions of SrO. Instead, Sr accumulation will result in highly distorted Sr- and O-containing surface species within the GDC layer, or SrZrO 3 near YSZ. This finding is significant due to the higher ionic conductivity of SrZrO 3 compared with SrO; as a consequence, it is less likely to degrade overall cell performance. Our results are supported both by thermodynamic calculations and by experimental analysis of post operando SOECs, in which Sr-containing inclusions within GDC pores lack significant crystallinity. Our work provides a comprehensive analysis into the mechanisms of Sr segregation in YSZ-based SOECs, along with insights that can be used to engineer better performing and more resilient devices. Part of this work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Part of this work was performed by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308.