Ni-YSZ Electrode Stability in Solid Oxide Electrolysis Cells Operated in 90-98% Steam

Abstract

Ni-YSZ electrode-supported SOECs were tested at 750 and 800oC using extremely high steam concentrations, 90, 95, and 98%. The performance was compared to SOECs tested in 50% steam with 50% hydrogen. Multiple cells were tested simultaneously with several repeats, all cells configured into a single furnace for each test to ensure a reasonable side-by-side comparison and results reproducibility. The Ni/YSZ electrodes was reduced by hydrogen in situ at each operating temperature and then 50-98% steam was added. Cells were tested at a fixed current or voltage for 1000-3000 hours. The electrochemical impedance data was periodically collected followed by the DRT analyses to identify any changes in polarization losses in time. Minimal to no changes in cell performance in time was observed when compared cell operation in 90+% steam vs 50% steam. Slightly higher degradation was seen for cells tested at 800oC vs those tested at 750oC during the initial 500 hours. After the initial break-in period, the degradation rates for all cells in the following >1000 hours remained the same.A separate set of tests was performed to characterize break-in period and to understand how the initial Ni thermal aging at different temperatures affects the time to a pseudo steady-state, i.e., if the Ni aging/conditioning could be used to accelerate the initial degradation and shorten break-in. Symmetrical cells with two Ni-YSZ electrodes on each side of 10 micron YSZ electrolyte were also tested.The extensive SEM/EDS analyses were performed on all cells to quantify the Ni particle size and describe any microstructural changes. The 2-dimensional SEM images were reconstructed into the 3D microstructures to calculate the triple phase boundary density, species transport paths tortuosity, and particle/pore size distribution.