Synergistic Effect of Sn and Fe in Fe-N Site Formation and Activity in Fe-N-C Catalyst for ORR

Iron-nitrogen-carbon (Fe-N-C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O₂ in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe-N-C materials were co-doped with Sn as a secondary metal center. Sn-N-C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H₂SO₄ electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn-N-C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe-N₄ site density, whereas the possible synergistic interaction of Fe-N₄ and Sn-N found no confirmation. The presence of Fe-N₄ and Sn-N was thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe-N-C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe-N-C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (j_k = 2.11 vs 1.83 A g^-1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests.