Solid oxide cells (SOCs) show great potential for green hydrogen production and energy conversion, yet their limited durability remains a bottleneck to their widespread commercialisation. This study investigates the impact of humidity on oxygen electrode degradation, using a state-of-the-art cell architecture of LSCF|LSCF-GDC|GDC|YSZ|Ni-YSZ. Long-term durability tests were conducted at 800°C under varying humidity content (dry, 3%, and 8%) in both electrolysis (SOEC) and fuel cell (SOFC) operating modes. Electrochemical impedance spectroscopy (EIS) measurements were performed to assess cell performance and identify degradation mechanisms before and after extended testing.

EIS analyses revealed a notable increase in the medium-frequency semicircle of the oxygen electrode during humid SOEC operation compared to dry condition, indicating electrode degradation (Figure 1-a). All the tested cells showed an increase in high frequency arc, which was attributed to hydrogen electrode degradation [1]. The increase in ohmic resistance classically observed under dry SOEC mode was found to be aggravated under humid condition. Post-test characterizations identified a clear elemental inter-diffusion across various cell layers, which was observed in all samples already after cell manufacturing [1]. The degradation of the hydrogen electrode was mainly caused by Ni depletion and coarsening in the SOEC mode under dry air conditions. The degradation of the oxygen electrode was only noticeable during ageing in humid air conditions, resulting in the formation of a dense Sr oxide layer on the top surface of the current collector layer (Figure 1-b) [2]. To complement the experimental findings, The Density Functional Theory (DFT) method was employed to calculate the energy of LSCF decomposition according to different scenarios under dry and humid conditions. The results provided insights into the Sr oxide growth in humid vs dry air conditions.

The study highlights the detrimental effect of humidity on oxygen electrode degradation in SOCs, particularly in SOEC mode. The identification of Sr oxide formation as a key factor in oxygen electrode degradation under humid conditions paves the way for targeted strategies to mitigate this issue and enhance SOC durability for practical applications.

[1] G. Sassone et al., Effect of the Operating Temperature on the Degradation of Solid Oxide Electrolysis Cells. submitted.

[2] G. Sassone et al., Impact of Air Moisture and Operating Mode on the Degradation of Solid Oxide Cells. In preparation.