Influence of H2S Poisoning on Anode Layer of SOFC

Abstract

The influence of sulfur compounds on Ni-YSZ anode of SOFC cell was investigated by measuring cell voltage under a constant current density for Ni-YSZ anode / YSZ electrolyte / (La,Sr)MnO 3 cathode cell. Two-step degradation of cell voltage was observed by supplying H 2 S containing fuel. The first-step degradation of cell voltage was recovered completely by switching to sulfur-free fuel, while the cell voltage wasn't recovered completely once the cell voltage went down under 0 V. After the cell with irreversible degradation was cooled down under pure N 2 atmosphere, it was analyzed by FE-SEM, EDX and Raman Spectroscopy. The results of these analyses revealed nickel sulfides formation in H 2 S containing fuel. It is supposed that melted nickel sulfides diffused in the anode and electrolyte layer. The irreversible cell poisoning might be caused by these nickel sulfides.

From Ni-YSZ to sulfur-tolerant anode materials for SOFCs: electrochemical behavior, in situ characterization, modeling, and future perspectives

“…Taking the Ni-YSZ cermet anode as an example, the initial quick poisoning by low concentration (ppm-level) H 2 S leads to large increase in anode interfacial resistance but not bulk resistance. 27,54,55,58,100,150 This is in good agreement with the mechanism of sulfur adsorption on the nickel metal surface as the bulk resistance is determined by the electrolyte and the electrode ohmic resistance, which are not expected to change by a surface adsorption process. However, as stated, some researchers have also observed 2 nd stage slow degradation after the rapid initial poisoning.…”

“…However, some other studies show that, in addition to the initial quick poisoning effect, the cell power output continues to degrade with a much slower but still significant rate (usually in an almost linear way) for a long period of time. Zha et al, 54 Sprenkle et al, 57 and Ishikura et al 58 named this continued degradation 2 nd stage slower poisoning. Within this category, there are also two different scenarios.…”

“…4 are different sulfur poisoning behaviors observed for SOFCs with Ni-YSZ anodes upon exposure to hydrogen contaminated with low concentration (ppm level) H 2 S. In all cases, there is a rapid initial drop in power output upon exposure to ppm-level H 2 S, which is associated with a large and rapid increase in anode polarization resistance and has been the focus of most studies in the literature. 27,35,[52][53][54][55][56][57][58][59][60][61] In this section, we will provide a review of the sulfur poisoning behavior of conventional Ni-YSZ anodes with a focus on clarifying the effects of various factors on the observed initial rapid sulfur poisoning, including operating conditions (e.g., temperature and gas concentration), cell structures, and materials used. The mechanism for the initial rapid degradation due to sulfur poisoning will also be discussed based on observed electrochemical behavior and theoretical modeling.…”

“…Bulk sulfidation of either Ni or YSZ also seems unlikely as they are thermodynamically unfavorable, especially for those studies using only low ppm of H 2 S. 52,57,58,61,83 It is unlikely due to the degradation of other cell components since the drop in performance usually appears to be much more severe than those observed in fuels without sulfur. 54,57,58 On the other hand, the study by Hagen and co-workers suggests that the loss of the Ni-percolation network near the anode-electrolyte interface was observed for the Ni-YSZ anode in H 2 S-containing fuels under large polarization conditions, which also display 2 nd stage degradation for hundreds of hours. 84 Recently, the study by Yang et al shined some light upon the possible origins for the 2 nd stage slower poisoning.…”

“…Reversibility of sulfur poisoning. Upon removal of H 2 S from the fuel, both complete recovery 27,[52][53][54]58,73,79,85,86 and partial recovery of power outputs 11,54,58,61,81,87,88 have been observed. Cell failure is rare, but still observed sometimes.…”