Effect of Tin Precursor Ligand in SnFe–N–C Single-Site Catalysts for Oxygen Reduction Reaction

Iron–nitrogen–carbon materials are still the most promising alternative to platinum group materials for the electrochemical reduction of O2. Doping with a secondary metal center is a possible way to further enhance the activity, and Sn seems to be a valuable choice. Here, SnFe–N–C materials featuring both Fe and Sn single-site atoms were prepared from Sn precursors to study the effect on physical-chemical and electrochemical properties. Sn could act as a structural promoter for iron, regulate charge distribution, and favor the formation of more chemically stable sites. Here, we show that oxygen-containing ligands favor the fixation of Sn and Sn/Fe alloy, while others do not. Interestingly, the presence of nitrogen is not fundamental in the Sn precursor. Indeed, SnCl2 is a valuable precursor. At the same time, (NH4)2SnCl6 is a valid and cheaper alternative to Sn(phen)Cl2 to fix a higher amount of Fe-Nx single-site. Even if the amount of Sn remains low in all samples (as a single site), all the bimetallic catalysts outperform the sole iron one (the best catalysts show a 3-fold increment). In addition, in situ electrochemical XAS confirms the redox behavior of Fe, while Sn does not show any oxidation or coordination changes under operating conditions.