Sulfur Poisoning of Electrochemical Reformate Conversion on Nickel/Gadolinium-Doped Ceria Electrodes

The aim of the present study is the measurement and understanding of sulfur poisoning phenomena in nickel/gadolinium-doped ceria (CGO) based solid oxide fuel cells (SOFC) operating on reformate fuels. The sulfur poisoning behavior of commercial, high-performance electrolyte-supported cells (ESC) with Ni/Ce0.9Gd0.1O2-delta (CGO10) anodes operated with different fuels was thoroughly investigated by means of current-voltage characteristics and electrochemical impedance spectroscopy and compared with Ni/Yttria-stabilized zirconia (YSZ) anodes. Various methane and carbon monoxide containing fuels were used in order to elucidate the underlying reaction mechanism. The analysis of the cell resistance increase in H-2/H2O/CO/CO2 fuel gas mixtures revealed that the poisoning behavior is mainly governed by an inhibited hydrogen oxidation reaction at low current densities. At higher current densities, the resistance increase becomes increasingly large, indicating a particularly severe poisoning effect on the carbon monoxide conversion reactions. However, the ability of Ni/CGO anodes to convert carbon monoxide even at H2S concentrations up to 20 ppm was demonstrated, while this was not possible for Ni/YSZ. The sulfur poisoning behavior of Ni/CGO in reformate fuels was fully reversible for short exposure times. From methane steam re-forming experiments, it is deduced that the Ni surface is blocked and, thus, the water-gas shift reaction is fully deactivated as well. However, electrochemical CO oxidation on the CGO surface was shown to be still active. The present results clearly demonstrate that the high sulfur tolerance of Ni/CGO not only is limited to H-2/H2O fuel systems but also extends to CO-containing gases.