Accelerated Stress Testing of Solid Oxide Electrolysis Cells in a Symmetric Steam-Rich Atmosphere

Abstract

Temperature, current density, steam utilization variability, and local steam starvation or excess may cause diverse degradation processes to occur in solid oxide electrolysis cells. Typical degradation mechanisms for SOEC with nickel-based fuel electrode materials include nickel migration/evaporation and agglomeration, which may lead to reduced active sites at the triple-phase boundary. However, the phenomenological cause of such degradation mechanisms is still actively debated, making the selection of a durable fuel electrode material challenging. It is desirable to understand the process by which these phenomena occur not only to minimize it, but to intentionally induce it to develop accelerated stress test protocols for cell lifetime prognostics. The degradation of solid oxide electrolyzers has been reported to be accelerated by certain stressful operating conditions. Brisse and Mocotuguy conducted a review of the degradation mechanisms of solid oxide button cells; they reported that high steam partial pressures can lead to the formation of nickel hydroxide at the cathode, which then diffuse to the surface of YSZ1. Sun et al. hypothesize that this is the main mechanism for nickel migration, due to the positive oxidation state of nickel and the direction of the electric potential2. Changes in the surface morphology of the fuel electrode due to redox cycle-induced degradation has been reported, showing the dynamics of nickel migration and agglomeration under these conditions3 . Redox cycling was found to be a destructive test protocol; efforts are currently being made by the group of Daria Vladikova to prevent fracturing of the electrodes and electrolyte during such stressful testing4. Königshofer et al. have reported through multiple investigations that protocols inducing operation at high steam conversion rates (>90%) and large current densities (0.8 A/cm2) have been effective at reproducing long-term SOEC voltage degradation5,6.The objective of this research is to establish a protocol that will help determine the causes that trigger Ni migration or evaporation, and that will mimic the electrode degradation mechanism to standardize the procedure for comparing the long-term performance of solid oxide electrolysis cells. Symmetric button cells of various fuel electrode chemistries with a diameter of 20 mm (active area 1.23 cm2) are tested under symmetric atmospheres with steam/H2 blends. The following two accelerated stress tests (AST) protocols are employed: cycling of steam partial pressure and cycling of current density. In the former, the steam molar fraction is varied from 75% to 95% in a 20-minute cycle for at least 400 cycles (i.e., 1,176 h). In the latter protocol, current density is controlled using a galvanodynamic 10-minute long cyclic ramp profile between 0.8 A/cm2 and 1.5 A/cm2 at a constant steam molar fraction between 75-95% for at least 800 cycles (i.e., 1,176 h).For all tests, the cells are operated at a constant furnace temperature between 700°C and 850°C. The performance characteristics of the various cells are expressed in terms of electrochemical impedance spectroscopy (EIS) and polarization curves every 12 h during the AST. X-ray diffraction (XRD) and fluorescence (XRF) are used to analyze the cells at beginning of life and post-mortem to identify phase changes which occur during operation. Scanning Electron Microscopy - Energy Dispersion Spectroscopy (SEM-EDS) and Glow-discharge spectroscopy (GD) are used to identify and map the nickel concentration on the cathode surface and cross-section. Preliminary results suggest that the high frequency resistance may increase by nearly 500% under a constant high-steam condition (> 90%) in a Ni-YSZ|YSZ|Ni-YSZ cell during a 200 h operation. P. Moçoteguy and A. Brisse, Int J Hydrogen Energy, 38, 15887–15902 (2013).X. Sun, T. L. Skafte, and S. H. Jensen, Fuel Cells (2021).S. Yang et al., J Alloys Compd, 921, 166085 (2022).D. Vladikova et al., Energies (Basel), 15 (2022).B. Königshofer et al., J Power Sources, 523 (2022).B. Königshofer et al., J Power Sources, 497 (2021).