Study of solid oxide electrolysis cells operated in potentiostatic mode: Effect of operating temperature on durability

Potentiostatic operation of solid oxide electrolysis cells (SOECs) at the thermoneutral voltage enables operation of the SOECs close to 100% electrical efficiency and simplifies the heat management at the stack and system level. In this work, we investigate the long-term durability of Ni/yttria-stabilized zirconia (YSZ) fuel electrode supported SOECs operated for electrolysis of steam at the thermoneutral voltage of 1290 mV, by testing cells at 800, 750, and 700 degrees C for periods exceeding 1000 h each. The cells show different initial performance due to different operating temperatures, but all experience a significant degree of initial degradation within the first 300 h, with a degradation rate of 2.80, 1.93, and 1.77 A cm(-2) kh(-1) (similar to 1.80, 1.51 and 1.90% kh(-1)) for the cells tested at 800, 750, and 700 degrees C, respectively. Hereafter the cells show stable performance or even slight activation. After 300 h of operation, the cell tested at 750 degrees C exhibits performance comparable to the cell tested at 800 degrees C and more than 74% higher than the cell tested at 700 degrees C, and this is maintained until the end of the test, suggesting that among three operating temperatures 750 degrees C is the optimum one. The different degradation behavior of the cells is further investigated by electrochemical impedance spectroscopy (EIS) analysis and post-test microstructural characterization. The cause of degradation is discussed in terms of electrode overpotential. Our findings highlight the significance of operating temperature optimization on the long-term durability of SOECs when operated in potentiostatic mode.