Synergistic Effect of Sn and Fe in Fe–N_<i>x</i> 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–Nx found no confirmation. The presence of Fe–N4 and Sn–Nx 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 (jk = 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