Can Tin Enhance the Electrocatalytic Activity of Fe-N _x -C Materials toward the Oxygen Reduction Reaction?

Hydrogen fuel cells are essential for addressing the energy transition process. However, the use of expensive platinum-based electrocatalysts poses a significant challenge for large-scale commercial deployment. Although Pt appears necessary to enhance the sluggish oxygen reduction reaction (ORR) kinetics, extensive research has been conducted aiming to replace it. Particularly, transition metals coordinated with nitrogen atoms and embedded in a conductive carbon framework (TM-N _x -C) demonstrated promising results. In particular, Fe-N _x -C electrocatalysts have shown superior electrocatalytic activity. Other monometallic or bimetallic systems have been less studied or have shown lower ORR metrics. Recently, a few reports have claimed the employment of tin (Sn-N _x -C) as a beneficial secondary active site for fabricating electrocatalysts with good activity in both acidic and alkaline environments. In this scenario, the present work aimed to synthesize monometallic Sn-N _x -C and Fe-N _x -C, and bimetallic Fe-Sn-N _x -C electrocatalysts following a simple, direct, and straightforward preparation method. Particularly, the corresponding tin- and iron-phthalocyanine precursors were blended with a conductive carbon substrate and subjected to a pyrolysis treatment at 600 or 800 °C. The obtained materials were thoroughly characterized and tested for ORR in both acidic and alkaline environments using a rotating ring disk electrode (RRDE). Sn-based electrocatalysts showed less electrocatalytic activity compared to Fe-based ones. Furthermore, a negligible or negative effect of tin co-functionalization was observed in each bimetallic sample. Although the proper blend of Sn and Fe precursors in the sample Sn/Fe(3:1)-Pc_600 led to an increased limiting current value compared to the iron counterpart, the other kinetic metrics were slightly negatively affected, especially in an acid electrolyte. Thus, the obtained results suggest that Sn co-functionalization seems to offer no noticeable enhancement in electrocatalytic activity.