Investigation of sulfur poisoning of CN x oxygen reduction catalysts for PEM fuel cells

Graphical abstract Examining the role of Fe in CN x materials for oxygen reduction reaction by deliberate poisoning with H2 S. Display Omitted Highlights The role of Fe used to catalyze N-containing carbon nanostructures was examined. H2 S poisoning was used as a probe for active sites in CN x and Pt ORR catalysts. Pt catalyst showed significant activity loss following H2 S treatment. CN x prepared with Fe as a growth catalyst showed no activity loss toward ORR. If Fe were the active site, there should be a significant activity loss with H2 S. The role of the transition metal used during the growth of non-noble metal electrochemical oxygen reduction CN x catalysts was investigated through sulfur treatment, a well-known poison for transition metal-based catalysts. The intent of sulfur poisoning was to show the existence of an electrocatalytic active site in CN x that did not depend on iron. The sulfur treatment was shown to be effective on a platinum catalyst, as seen by the decreasing onset potential. The same treatment, however, not only showed no negative effect on the CN x catalyst, but enhanced its performance, as seen by the increase in the onset potential. This suggests that, if there are iron-based active sites in these catalysts, they are either sulfur tolerant or they do not participate in the electrocatalytic oxygen reduction. The deposition of sulfur onto CN x catalyst was verified by temperature-programmed oxidation and X-ray photoelectron spectroscopy. Iron K-edge X-ray absorption near edge structural analysis of the CN x catalyst suggested that the iron phase, which was primarily composed of nanometer-sized metallic particles, was unchanged by sulfur poisoning, suggesting that the residual iron left in these materials is not catalytically accessible. [PUBLICATION ABSTRACT]