Structural and Reactivity Effects of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction

Whileimproved activity was recently reported for bimetallic iron-metal-nitrogen-carbon(FeMNC) catalysts for the oxygen reduction reaction (ORR) in acidmedium, the nature of active sites and interactions between the twometals are poorly understood. Here, FeSnNC and FeCoNC catalysts werestructurally and catalytically compared to their parent FeNC and SnNCcatalysts. While CO cryo-chemisorption revealed a twice lower sitedensity of M-N-x sites for FeSnNC and FeCoNC relative toFeNC and SnNC, the mass activity of both bimetallic catalysts is 50-100%higher than that of FeNC due to a larger turnover frequency in thebimetallic catalysts. Electron microscopy and X-ray absorption spectroscopyidentified the coexistence of Fe-N-x and Sn-N-x or Co-N-x sites, while no evidence was found for binuclearFe-M-N-x sites. Fe-57 Mo''ssbauer spectroscopyrevealed that the bimetallic catalysts feature a higher D1/D2 ratioof the spectral signatures assigned to two distinct Fe-N-x sites, relative to the FeNC parent catalyst. Thus, the additionof the secondary metal favored the formation of D1 sites, associatedwith the higher turnover frequency.

Automated summary

Although the site density of FeSnNC and FeCoNC catalysts is lower than that of FeNC and SnNC catalysts, the bimetallic catalysts show higher mass activity in the oxygen reduction reaction due to a larger turnover frequency. The coexistence of Fe-N-x and Sn-N-x or Co-N-x sites was confirmed by electron microscopy and X-ray absorption spectroscopy, while no evidence was found for binuclear Fe-M-N-x sites. Fe-57 Mössbauer spectroscopy revealed that the addition of the secondary metal favored the formation of D1 sites, which are associated with a higher turnover frequency.