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Replacing Pt-based catalysts with cost-effective, highly efficient, and durable platinum group metal-free catalysts for the oxygen reduction reaction (ORR) is crucial for commercializing hydrogen fuel cells. Herein, we present a highly active Fe–N–C electrocatalyst that contains both Fe nanoparticles and FeNx active sites derived from an Fe-doped carbonized zeolitic imidazolate framework (ZIF-8). It is found that adjusting the doping amount of Fe in the Fe-doped ZIF-8 precursor alters the morphology of the catalyst after heat treatment. The Fe–N–C-300 composite catalyst with the optimized Fe doping amount exhibits excellent activity, good stability, and remarkable methanol tolerance in the challenging acid environment. This study reveals that a suitable amount of Fe nanoparticles in the catalyst can alter the structure of the FeNx active moieties and increase three-phase boundaries to boost the mass transport, thus leading to improved fuel cell performance. This will have implications for using Fe–N–C catalysts in real applications, as the formation of Fe NPs during the synthesis and reaction is almost inevitable. [ABSTRACT FROM AUTHOR]

Low-cost, high-performances and durable hydrogen fuel cells are crucial for the success of the hydrogen economy. While Fe–N–C structures containing Fe-N x active sites are amongst the most promising platinum (Pt)-group metal (PGM)-free catalysts for the oxygen reduction reaction, their highest performances-to-date are still inferior to commercial Pt in real proton exchange membrane fuel cells. Herein, we shed light on this performance gap by using the distribution of relaxation times to quantify the proton transport and oxygen reduction reaction kinetics of a high-performance Fe–N–C catalyst (1.08 W cm−2) and a commercial Pt catalyst (1.7 W cm−2) in hydrogen fuel cell. This study unveils that the Fe–N–C catalyst has slower proton transport and oxygen reduction reaction kinetics than Pt as the Fe–N–C nanoporous carbon matrix limits active site accessibility. Furthermore, while increasing the Fe–N–C catalytic mass loading (from 1 to 3 mg Fe-N-C cm−2) enhances the power density in hydrogen fuel cells, it also slows down proton transport and oxygen reduction reaction kinetics by lengthening the gas, electron, and proton pathways to the active sites. This finding will drive the development of PGM-free catalysts for hydrogen fuel cells and of single-atom catalysts for electrochemical applications. [Display omitted] • The distribution of relaxation times captures ORR kinetics of PGM-free catalysts. • PGM-free catalysts nanoporous structure reduces ORR and proton transport kinetics. • Using high loadings of PGM-free catalysts increases the pathway to active sites. [ABSTRACT FROM AUTHOR]

Fe-N-C catalysts are an attractive low-cost alternative to Pt for the oxygen reduction reaction (ORR) in high-temperature proton exchange membrane fuel cells (HT-PEMFCs) as they are less sensitive to phosphate poisoning, but they suffer from poor stability and unclear degradation mechanisms. In this study, the activation and degradation mechanisms of HT-PEMFCs with several classes of Fe-N-Cs operating for 90 h are identified using a combination of operando electrochemical and post-mortem physical analysis. While simultaneous activation in terms of accelerated ORR processes takes place by acid redistribution, the degradation of the catalyst leads to drastic performance loss (– 27%) within the first 24 h. Carbon corrosion has a lower impact and the strongest degradation is attributed to active site deactivation e.g. by transformation to inactive nanoparticles or by site loss. This study helps to understand the degradation of Fe-N-Cs in HT-PEMFC and points to the necessary and inevitable modification of single-atom catalysts. [Display omitted] • Voltage decay within first 24 h of HT-PEMFC operation independent of Fe-N-C type. • Stability of Fe-N-C in HT-PEMFC independent of electrochemical characterization in single-cell. • Voltage loss attributed to reactive oxygen species formation and Fe-N x deactivation. • No positive effect from acid distribution due to degradation of catalyst in beginning of operation. [ABSTRACT FROM AUTHOR]