Unveiling the role of carbon oxidation in irreversible degradation of atomically-dispersed FeN4 moieties for proton exchange membrane fuel cells

Abstract

Nonprecious Fe–N–C catalysts containing atomically-dispersed FeN₄ moieties are today the best candidates to replace platinum in proton exchange membrane fuel cell (PEMFC) cathodes. However, limited understanding of problematic operando degradation mechanisms in these catalysts largely impedes widespread commercialization. Recent experiments have shown that there exist durable and non-durable FeN₄ sites in Fe–N–C catalysts for PEMFCs [J. Li et al., Nat. Catal., 2021, 4, 10–19]. Yet, the identification of which FeN₄ sites are durable and which are not – and why – remains unclear. Using first-principles density functional theory (DFT) computations, we investigated the irreversible degradation of FeN₄ catalysts at the atomic level, caused by Fe de-metalation and chemical oxidation of carbon via a proposed new carbon oxidation pathway. Our computational results show that oxidation of surface carbon next to FeN₄ moieties at interior sites is essentially reversible under operando electrochemical conditions; whereas oxidation of carbon next to FeN₄ moieties at the edge sites leads to accelerated Fe de-metalation, inducing irreversible degradation of FeN₄ catalysts. From amongst six FeN₄ moieties established experimentally, we identify three durable and three non-durable configurations. This work resolves the controversy as to which FeN₄ moieties are durable under PEMFC operando conditions and provides a deeper understanding of the irreversible degradation mechanism of FeN₄ catalysts in acidic media, furnishing a practical guide for rational design of FeN₄ catalysts with long-term durability.