Assessing the H$_2$S-induced resistance increase of a Ni/GDC cermet electrode under OCV and cathodic polarization

It is widely accepted that sulfur-induced resistance increase is mitigated by the anodic current density in the fuel cell mode. On the other hand, the role of cathodic current in the electrolyzer mode is yet only poorly explored. This study therefore aims to identify the role of cathodic current on the poisoning behavior in solid oxide electrolyzers. We have performed an electrochemical impedance spectroscopy-based study on a commercial Ni/GDC electrode in a H$_2$S concentration range from 0.1 ppm to 1 ppm and current densities from 0 Acm−2 up to −1 Acm−2 in steam-electrolysis and up to −0.8 Acm−2 in CO$_2$-electrolysis. Results at OCV show a critical influence of H$_2$ and CO concentration on the poisoning response. Under cathodic polarization, the impact of sulfur on the resistance is lower when load is applied, compared to the OCV case, and this effect is more pronounced during CO$_2$-electrolysis than in H$_2$O-electrolysis. In poisoned H$_2$O-electrolysis, the electrode overpotentials match expectations from the inherent Tafel behavior of the electrode. In contrast, overpotentials during poisoned CO$_2$-electrolysis appear to be lower than one would expect from the inherent Tafel behavior of the electrode, demonstrating a mitigation effect of cathodic current on the H$_2$S poisoning in CO$_2$-electrolysis.