Premature degradation study of a cathode-supported solid oxide electrolysis cell

Degradation-related issues are among the main limitations to make solid oxide electrolysis cells (SOEC) meet performance targets economically viable for long-term operation. In this study, the considered cell presents a premature degradation during electrolysis operation observed after sealing the cell holder and the support pieces providing and releasing H₂ electrode gas. This premature degradation is characterized by unusual polarization curve slopes, and appearance of a new impedance contribution at the lowest frequencies of the impedance diagrams recorded. To the best of our knowledge, this new contribution has never been reported for SOECs. Post-mortem analysis of the cell by scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) shows the presence of Si, Al, Na, K, and Ca at the H₂ electrode interface (surface and first dozens microns in the volume) and at the Ni-yttria-stabilized zirconia (YSZ)/YSZ interface, contrary to similar cells tested before sealing the pieces. This degradation is related to Si deposition, notably at the Ni/YSZ/H₂O triple phase boundaries. Concomitantly, the new contribution observed, leading to a beneficial effect on the cell functioning, is assimilated to a “reactivation” contribution. This reactivation contribution is associated with an H₂O adsorption phenomenon and characterized by a relaxation frequency of [1–10 mHz] and a capacitance of ~ 100 F cm⁻². The evolution with time of the resistive and capacitive contributions is consistent with the interpretation of the premature cell degradation. A mechanism explaining the cell behavior after this premature degradation is proposed.