Correlations between Synthesis and Performance of Fe-Based PGM-Free Catalysts in Acidic and Alkaline Media: Evolution of Surface Chemistry and Morphology

The effects of the synthesis steps of a platinum group metal-free (PGM-free) catalyst on the surface chemistry, morphology, and electrochemical activity in acidic and alkaline media toward the oxygen reduction reaction (ORR) were studied. Each step exhibits a positive impact on catalyst activity. In acid media, etching of the silica template is the major contributor to the enhancement of the half-wave potential, as the ORR active sites formed during the first pyrolysis become more accessible. Further processing steps result even in higher accessibility and utilization of the 4e^- transfer sites. In alkaline media, the second pyrolysis is a critical step that favors the complete reduction of oxygen to water, as the peroxide production is significantly diminished. The large heterogeneity in the porosity at each synthesis step indicates that this parameter needs to be further studied to attain better control of the morphology of the PGM-free catalyst, as it is an important factor that contributes to the active site utilization. The acid etching and second pyrolysis increase the meso- and macroporosity. Understanding the effects of each of the synthesis steps on the chemical composition, morphology and ORR activity of the PGM-free catalyst provides the necessary feedback for the design of synthetic schemes that increase the catalysts' performance.