Atomic Scale Insight Into High Voltage Degradation Mechanism in <scp>YSZ</scp> Electrolyte for Solid Oxide Electrolyzer Applications

With the rapid development of affordable energy, hydrogen can serve as an ideal long‐term energy storage medium and a commodity chemical/fuel precursor for energy conversion. Solid oxide electrolysis cells produce hydrogen from water with a high electrical efficiency in an environmentally friendly manner. However, the long‐term durability of solid oxide electrolysis cells is yet to be fully understood, especially in terms of decoding the fundamentals behind the degradation mechanisms. In this work, testing of solid oxide electrolysis cells was conducted at voltages well above the thermoneutral voltage of ~1.3 V to expose potential degradation mechanisms that could occur when solid oxide electrolysis cell is tested in a galvanostatic mode with voltage increasing over time. While standard electrochemical techniques did not reveal obvious degradation beyond the initial break‐in period, high‐resolution analyses uncovered intergranular and transgranular voids and cracks in the yttria‐stabilized zirconia electrolyte that were not observed during normal operation at 1.3 V. A comprehensive study of representative state‐of‐the‐art and thin‐film electrolyte and barrier layers was performed to establish a correlation between high voltages and structural changes in yttria‐stabilized zirconia. Void formation in yttria‐stabilized zirconia was attributed to strain‐vacancy interactions at high voltages, whereas the barrier/electrolyte interdiffusion layer and Ni precipitation in yttria‐stabilized zirconia were ruled out as triggers of void formation. This study provides insight into the operational boundaries of real‐time solid oxide electrolysis cells service, indicating that operating at voltages well above the thermoneutral voltage should be avoided.