Surface Sites Density and Utilization of Precious Group Metal (PGM)-free Fe-NC and FeNi-NC Electrocatalysts for the Oxygen Reduction Reaction
Pyrolyzed Iron-based precious group metal (PGM)-free nitrogen-doped single site carbon catalysts (Fe-NC) are possible alternatives to Platinum-based carbon catalysts for the oxygen reduction reaction (ORR). Bimetallic PGM-free M1M2-NC catalysts and their surface sites, however, have been poorly studied to date. The present study explores the active accessible sites of mono- and bimetallic Fe-NC and FeNi-NC catalysts. Combining CO cryo chemisorption, X-ray absorption and 57Fe Mössbauer spectroscopy, we evaluate the number and chemical state of metal sites at the surface of the catalysts along with an estimate of their dispersion and utilization. Fe L3,2-edge X-ray adsorption spectra, Mössbauer spectra and CO desorption all suggested an essentially identical nature of Fe sites in both monometallic Fe-NC and bimetallic FeNi-NC; however, Ni blocks the formation of active sites during the pyrolysis and thus caused a sharp reduction in the metal accessible site density, while with only a minor direct participation as catalytic site in the final catalyst. We also use the site density utilization factor, ,ΦSD surface/bulk as a measure of the metal site dispersion in a PGM-free ORR catalysts. ΦSD surface/bulk enables a quantitative evaluation and comparison of distinct catalyst synthesis routes in terms of their ratio of metal accessible site. It gives guidance for further optimization of the accessible site density of M-NC catalysts.