Tracking Microstructural Evolution of the Ni-Cermet Fuel Electrode in Solid Oxide Electrolyzers Using X-Ray Nano Computed Tomography
Authors: Dzara Michael , Slomski Heather S , Zhu Zhikuan , Gorman Brian , Ginley David , Tucker Mike C , Shulda Sarah 
Published: December 2024

Decarbonization of hydrogen production is critically important to the renewable energy economy, indeed, the US Department of Energy recently released the Hydrogen Shot, setting the target at reducing the cost of clean hydrogen to $1 per kilogram in 1 decade. Of the electrolysis technologies that might meet the Hydrogen Shot goal, high temperature electrolysis based on solid oxide electrolysis cells (SOECs) is particularly promising due to its high efficiency and the ability to avoid the use of expensive and scarce metal catalysts.1 However, the harsh environment (high temperature, reducing/oxidizing environments, interfacial compatibility, and high current density) that SOECs are operated in presents a major challenge with respect to material stability. To reach relevant SOEC lifetimes on the order of sixty to eighty thousand hours of operation, drastic improvements in cell durability must be made. However, ageing cells on the order of tens of thousands of hours to characterize physicochemical changes at the material level to inform mitigation strategies is prohibitively slow. Accelerated stress test (AST) protocols must be developed to activate degradation mechanisms more quickly in the SOEC, enabling comparison of different materials and fabrication methods in a timely fashion. This approach will enable mitigation strategies to directly address degradation pathways identified as a function of testing conditions, SOEC physicochemical properties, and cell processing history.
Several possible degradation pathways for SOECs have been hypothesized and identified, including cation migration into/across the electrolyte, particle coarsening in the electrodes and subsequent change in porosity, undesired phase formation, and others.2 These can lead to increased electrical resistance, decreased mass transport rate of gaseous reactants or products, and deactivation of active sites through formation of inactive species. Particular attention has been paid to microstructural evolution of the fuel electrode comprised of Ni and yttria-stabilized zirconia (YSZ). The hydrogen evolution reaction (HER) occurs electrocatalytically at the triple-phase boundary (TPB) of Ni (catalysis and electron conduction) YSZ (oxygen ion conduction) and pore space (gas phase transport), and any change in distribution of these three phases can result in both increased resistances and loss of active HER sites. Therefore, extensive effort has been made to understand Ni redistribution and optimize the Ni-YSZ microstructure to decrease the rate of change in Ni properties. However, there is still debate within the literature on the exact mechanism for Ni redistribution as both a surface hydroxide mediated transport and a particle curvature-based hypothesis have been suggested.3 Furthermore, clear correlation of degradation symptoms, including Ni particle size change, migration towards/away from the electrolyte, Ni/YSZ de-wetting, and isolation of pore space/TPB with variations in initial microstructure or different operating conditions is still lacking.
This work focuses on illuminating these relationships between performance losses in Ni-YSZ fuel electrodes with dynamic, AST operating conditions using X-ray nanoscale computed tomography (nano-CT). Nano-CT yields 3D microstructural information through image segmentation, which can then be used to track changes in TPB density, phase fraction depth profile, phase size distribution, and phase connectivity/isolated fractions.4 This extensive set of properties can aid in identifying whether loss of fuel electrode performance is due to Ni coarsening, de-wetting, or migration. Understanding how Ni migration may be influenced by operating conditions will inform efforts to effectively suppress and slow degradation of the fuel electrode. These efforts will drive SOEC technology advances towards longer operating lifetimes, enabling cheaper lifetime hydrogen production.

References

(1) Hauch, A.; Küngas, R.; Blennow, P.; Hansen, A. B.; Hansen, J. B.; Mathiesen, B. V.; Mogensen, M. B. Recent Advances in Solid Oxide Cell Technology for Electrolysis. Science
2020, 370 (6513).
(2) Moçoteguy, P.; Brisse, A. A Review and Comprehensive Analysis of Degradation Mechanisms of Solid Oxide Electrolysis Cells. Int. J. Hydrog. Energy
2013, 38 (36), 15887–15902.
(3) Mogensen, M. B.; Chen, M.; Frandsen, H. L.; Graves, C.; Hauch, A.; Hendriksen, P. V.; Jacobsen, T.; Jensen, S. H.; Skafte, T. L.; Sun, X. Ni Migration in Solid Oxide Cell Electrodes: Review and Revised Hypothesis. Fuel Cells
2021, fuce.202100072.
(4) Heenan, T. M. M.; Bailey, J. J.; Lu, X.; Robinson, J. B.; Iacoviello, F.; Finegan, D. P.; Brett, D. J. L.; Shearing, P. R. Three-Phase Segmentation of Solid Oxide Fuel Cell Anode Materials Using Lab Based X-Ray Nano-Computed Tomography. Fuel Cells
2017, 17 (1), 75–82.

Figure 1