Cooperative Atomically Dispersed Fe–N4 and Sn–Nx Moieties for Durable and More Active Oxygen Electroreduction in Fuel Cells

Abstract

One grand challenge for deploying porous carbons with embedded metal–nitrogen–carbon (M–N–C) moieties as platinum group metal (PGM)-free electrocatalysts in proton-exchange membrane fuel cells is their fast degradation and inferior activity. Here, we report the modulation of the local environment at Fe–N₄ sites via the application of atomic Sn–N_x sites for simultaneously improved durability and activity. We discovered that Sn–N_x sites not only promote the formation of the more stable D2 FeN₄C₁₀ sites but also invoke a unique D3 SnN_x–Fe^IIN₄ site that is characterized by having atomically dispersed bridged Sn–N_x and Fe–N₄. This new D3 site exhibits significantly improved stability against demetalation and several times higher turnover frequency for the oxygen reduction reaction (ORR) due to the shift of the reaction pathway from a single-site associative mechanism to a dual-site dissociative mechanism with the adjacent Sn site facilitating a lower overpotential cleavage of the O–O bond. This mechanism bypasses the formation of the otherwise inevitable intermediate that is responsible for demetalation, where two hydroxyl intermediates bind to one Fe site. As a result, a mesoporous Fe/Sn-PNC catalyst exhibits a positively shifted ORR half-wave potential and more than 50% lower peroxide formation. This, in combination with the stable D3 site and enriched D2 Fe sites, significantly enhanced the catalyst’s durability as demonstrated in membrane electrode assemblies using complementary accelerated durability testing protocols.