Sulfur Poisoning of Ni/CGO Anodes: A Long-Term Degradation Study

Abstract

Although continuous progress is achieved with respect to lifetime and performance improvements, solid oxide fuel cells (SOFC) continue to struggle with commercialization. One important factor to possibly reduce overall costs of SOFC systems would be the omission of desulfurization units during the fuel processing. However, sulfur-containing impurities in the most common fuels such as natural gas and biogas lead to significant performance drops upon exposure to Ni-based anodes. In this regard, it has been shown that Ni/gadolinium-doped ceria (CGO) anodes display lower performance drops upon short-time sulfur exposure than Ni/YSZ. However, their long-term resistivity towards sulfur exposure has not been investigated so far, although this is likely to be the most vital factor determining SOFC lifetime. This work presents the first in-depth analysis of long-term degradation due to sulfur poisoning of Ni/CGO10-based SOFC. A parameter study of the sulfur-induced long-term degradation of commercial, high-performance single cells was conducted with an accumulated cell testing time of more than one year. The long-term degradation behavior is investigated at 900 °C for different H2S concentrations and varying H2/H2O/N2 fuel gas atmospheres. The sulfur poisoning periods of the different cells varied from 100 up to 1500 h. The progress of the degradation during the experiments was monitored by means of electrochemical impedance spectroscopy. It is shown that Ni/CGO anodes can be exposed to 10 ppm H2S and operated without significant irreversible degradation at a current density of 0.5 A·cm–2 for 1500 h for certain fuel gas mixtures, demonstrating their promising long-term stability. However, for reduced hydrogen partial pressures in the fuel gas, the same H2S concentration leads to a considerable voltage decrease that is reflected by an increase in anode charge transfer and ohmic resistance. Furthermore, the microstructural evolution of the Ni/CGO is examined ex-situ by means of SEM and correlated to the anode performance degradation. The existence of a stable operating regime for Ni/CGO anodes under sulfur exposure is clearly shown and the critical operating parameters are outlined. The presented results give important insights into the degradation processes occurring during long-term operation of Ni/CGO anodes as well as strategies for stable SOFC operation. Moreover, the results encourage to further optimize the performance and sulfur tolerance of Ni/CGO anodes.