Assessment of Sulfur Poisoning of Ni/CGO-Based SOFC Anodes

Abstract

The presence of fuel impurities, such as hydrogen sulfide, siloxane and phosphine, in biogas, diesel and natural gas can cause Solid Oxide Fuel Cell (SOFC) degradation due to surface poisoning of Ni-containing anodes. In this regard, Ni/CGO anodes have shown higher sulfur tolerance than Ni/YSZ anodes and a comparable high performance. In order to allow for a more profound understanding of the processes underlying sulfur poisoning, this study presents an extensive experimental investigation of commercial Ni/CGO-based SOFC operating on H2/H2O fuel gases and reformate fuel mixtures with trace amounts of hydrogen sulfide (H2S). The short-term poisoning behavior of high-performance electrolyte-supported Ni/CGO10-based cells was systematically investigated by means of transient voltage stability experiments and electrochemical impedance measurements for a wide range of operating conditions. The effects of temperature (800 – 950 °C) and current density (OCV – 0.75 A·cm‒2) on the extent of sulfur poisoning (1 – 20 ppm H2S) was evaluated. The poisoning behavior was shown to be completely reversible for short exposure times in all cases. The chemical capacitance of Ni/CGO10 anodes was demonstrated to be strongly dependent upon temperature and gas phase composition reflecting a changing Ce3+/Ce4+ ratio in the CGO phase. Using a model reformate as fuel gas, it was shown that CO can still be electrochemically converted under sulfur exposure. Furthermore, long-term experiments of 1500 h were conducted at 900 °C and 0.5 A·cm‒2 with and without sulfur exposure and the degradation progress was monitored by impedance spectroscopy.