Cooperative Atomically\nDispersed Fe–N<sub>4</sub> and Sn–N<sub><i>x</i></sub> Moieties for Durable\nand More Active Oxygen Electroreduction in Fuel Cells

One grand challenge for deploying porous carbons with\nembedded\nmetal–nitrogen–carbon (M–N–C) moieties\nas platinum group metal (PGM)-free electrocatalysts in proton-exchange\nmembrane fuel cells is their fast degradation and inferior activity.\nHere, we report the modulation of the local environment at Fe–N<sub>4</sub> sites via the application of atomic Sn–N<sub><i>x</i></sub> sites for simultaneously improved durability and\nactivity. We discovered that Sn–N<sub><i>x</i></sub> sites not only promote the formation of the more stable D2 FeN<sub>4</sub>C<sub>10</sub> sites but also invoke a unique D3 SnN<sub><i>x</i></sub>–Fe<sup>II</sup>N<sub>4</sub> site that is\ncharacterized by having atomically dispersed bridged Sn–N<sub><i>x</i></sub> and Fe–N<sub>4</sub>. This new D3\nsite exhibits significantly improved stability against demetalation\nand several times higher turnover frequency for the oxygen reduction\nreaction (ORR) due to the shift of the reaction pathway from a single-site\nassociative mechanism to a dual-site dissociative mechanism with the\nadjacent Sn site facilitating a lower overpotential cleavage of the\nO–O bond. This mechanism bypasses the formation of the otherwise\ninevitable intermediate that is responsible for demetalation, where\ntwo hydroxyl intermediates bind to one Fe site. As a result, a mesoporous\nFe/Sn-PNC catalyst exhibits a positively shifted ORR half-wave potential\nand more than 50% lower peroxide formation. This, in combination with\nthe stable D3 site and enriched D2 Fe sites, significantly enhanced\nthe catalyst’s durability as demonstrated in membrane electrode\nassemblies using complementary accelerated durability testing protocols.