Using molecular probes that have unique spectral signatures and have strong selective binding to the potential active sites allows elucidating the mechanism of different reactions. The mechanism of oxygen reduction reaction in metal−nitrogen−carbon (MNC) catalysts has been studied by using a bisphosphonate complexing agent, which improves the selectivity of ORR by blocking the protonated and hydrogenated nitrogen that are catalyzing the partial reduction of oxygen to hydrogen peroxide. A combination of theoretical, electrochemical and spectroscopic with focus on near-ambient pressure X-ray photoelectron spectroscopy is used to directly probe the competition between binding of oxygen and molecular probe to the surface of MNC catalyst and to identify the role of different types of nitrogen in the mechanism of ORR.

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.