Oxide incorporation into Ni-based solid oxide fuel cell anodes for enhanced sulfur tolerance during operation on hydrogen or biogas fuels: A comprehensive thermodynamic study

The effect of the incorporation of oxides (CeO2, CaO, MgO, SrO, BaO) into SOFC anodes on sulfur poisoning of Ni catalysts is investigated by means of a comprehensive thermodynamic study. The results demonstrate that sulfur chemical potential controls the value of bulk nickel sulfide activity, which, in turn, is a function of sulfur coverage on Ni surface. It is found that oxide incorporation into anodes can reduce the sulfur chemisorption on Ni by lowering the sulfur chemical potential. BaO incorporation may be the best option for IT-SOFCs. The strong affinity of BaO towards sulfur significantly reduces the sulfur coverage on Ni surface, from values in the range of 0.91-0.96 to 0.54-0.58, for wet hydrogen atmosphere, in such a way that catalyst poisoning can be completely prevented. In situ regeneration of BaO could occur by means of a local reaction between BaS and OH species that are generated by dissociative chemisorption of H2O. The highest H2S concentration allowed in a BaO-modified anode depends on fuel composition: 100 ppm for wet hydrogen (3%H2O), and 30-45 ppm for biogas, varying according to CH4/CO2 molar ratio. In the case of biogas, enhanced sulfur tolerance can be achieved provided that the BaCO3 phase cannot be formed. For modified and unmodified anodes, the degree of sulfur poisoning was found to decrease with increasing CH4/CO2 molar ratio in biogas. Although ceria can effectively suppress carbon deposition due to oxygen storage capacity, it cannot alleviate sulfur poisoning of Ni under carbon-free conditions. At carbon deposition boundary, ceria increases the tolerance toward H2S in the biogas only modestly.