The influence of sulfur impurities in industrial COx gases on solid oxide electrolysis cell (SOEC) degradation

High-temperature solid oxide cells (SOC) offer unrivaled Faradaic and energetic efficiencies during carbon dioxide electrolysis. However, it is still unclear which gas quality and impurity level is required for low degradation rates in their commercial application. Here, it is shown that the polarization resistance of electrolyte -supported cells (ESC) with Ni/Gadolinia-doped ceria (Ni/CGO) degrades strongly over time periods of up to 72 h when operated in CO/CO2 gas mixtures. X-ray photoemission spectroscopy (XPS) and high-resolution secondary ion mass spectrometry (SIMS) imaging techniques were used to show that this degradation effect is related to sulfur impurities in the feed gas and their poisoning of the Ni surface. Variation of the inlet gas composition showed that impurities are present in both CO and CO2. The degradation was significantly less pronounced for Ni/CGO fuel electrodes compared to Ni/Yttria-stabilized zirconia (YSZ) and also when hydrogen was introduced into the feed gas. Generic natural gas cleaning desulfurization units were not able to remove the impurities from the feed gases. After initial electrode deactivation, stable cell performance was obtained for >900 h. The results clearly underline the importance of feed gas purity and the necessity for advanced cleaning units for the commercialization of high-temperature CO2 electrolysis.