PNNL

Abstract

One grant challenge for deploying porous carbons with embedded metal-nitrogen-carbon (M-N-C) moieties as platinum group metal (PGM) free electrocatalysis in proton-exchange membrane fuel cells is their fast degradation and inferior activity. Here we report the modulate of local chemical environment of Fe-N4 sites via the application of atomic Sn-Nx sites toward simultaneously improved durability and activity. We discovered that Sn-Nx sites not only promotes formation of the more stable D2 FeN4C10 sites but also invoked a unique D3 SnNx-Fe?N4 site characterized with bridging atomic Sn and Fe. This new D3 site exhibited significantly improved stability against demetallation and several times higher turnover frequency for the oxygen reduction reaction (ORR) due to the shift of reaction pathway from single site associative mechanism to a dual-site dissociative mechanism with adjacent Sn site assist lower overpotential cleavage of O-O bond. This mechanism bypasses the otherwise inevitable demetallation-responsible intermediate with two hydroxyl intermediates bind with one Fe site. As a result, mesoporous Fe/Sn-PNC catalyst exhibited positively shifted ORR half wave potential and reduced peroxide formation by more than 50%. This, in combination with the stable D3 site and enriched D2 Fe sites, significantly enhanced the catalyst durability and reduced demetallation by more than 40%. The enhanced stability is further demonstrated in membrane electrode assemblies using complimentary accelerated durability testing protocols.