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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]

Platinum metal (PtM, M=Ni, Fe, Co) alloys catalysts show high oxygen reduction reaction (ORR) activity due to their well‐known strain and ligand effects. However, these PtM alloys usually suffer from a deficient ORR durability in acidic environment as the alloyed metal is prone to be dissolved due to its high electronegativity. Herein, we report a new class of PtMn alloy nanodendrite catalyst with low‐electronegativity Mn‐contraction for boosting the oxygen reduction durability of fuel cells. The moderate strain in PtMn, induced by Mn contraction, yields optimal oxygen reduction activity at 0.53 A mg−1 at 0.9 V versus reversible hydrogen electrode (RHE). Most importantly, we show that relative to well‐known high‐electronegativity Ni‐based Pt alloy counterpart, the PtMn nanodendrite catalyst experiences less transition metals' dissolution in acidic solution and achieves an outstanding mass activity retention of 96 % after 10,000 degradation cycles. Density functional theory calculation reveals that PtMn alloys are thermodynamically more stable than PtNi alloys in terms of formation enthalpy and cohesive energy. The PtMn nanodendrite‐based membrane electrode assembly delivers an outstanding peak power density of 1.36 W cm−2 at a low Pt loading and high‐performance retention over 50 h operations at 0.6 V in H2‐O2 hydrogen fuel cells. [ABSTRACT FROM AUTHOR]

Platinum metal (PtM, M=Ni, Fe, Co) alloys catalysts show high oxygen reduction reaction (ORR) activity due to their well‐known strain and ligand effects. However, these PtM alloys usually suffer from a deficient ORR durability in acidic environment as the alloyed metal is prone to be dissolved due to its high electronegativity. Herein, we report a new class of PtMn alloy nanodendrite catalyst with low‐electronegativity Mn‐contraction for boosting the oxygen reduction durability of fuel cells. The moderate strain in PtMn, induced by Mn contraction, yields optimal oxygen reduction activity at 0.53 A mg−1 at 0.9 V versus reversible hydrogen electrode (RHE). Most importantly, we show that relative to well‐known high‐electronegativity Ni‐based Pt alloy counterpart, the PtMn nanodendrite catalyst experiences less transition metals' dissolution in acidic solution and achieves an outstanding mass activity retention of 96 % after 10,000 degradation cycles. Density functional theory calculation reveals that PtMn alloys are thermodynamically more stable than PtNi alloys in terms of formation enthalpy and cohesive energy. The PtMn nanodendrite‐based membrane electrode assembly delivers an outstanding peak power density of 1.36 W cm−2 at a low Pt loading and high‐performance retention over 50 h operations at 0.6 V in H2‐O2 hydrogen fuel cells. [ABSTRACT FROM AUTHOR]

Catalytic synergy is an unresolved activity descriptor in energy conversion reactions. Here, this study reports the synergistically enhanced intrinsic oxygen evolution reaction activity of a FeNi model catalyst modified with molybdate oxo‐anions via concertedly boosted surface/interface interactions such as interfacial charge transfer, surface intermediate adsorption, in situ phase transformation, and gas bubble evolution. Capturing such cosynergistic impact reveals the accelerated transition of oxygen deprotonation to oxyhydroxide state, ready‐to‐function Fe and Ni active sites, the instant transformation of Fe and Ni local microenvironments to γ ‐FeOOH and β ‐NiOOH phases, and ultrafast gas bubble growth and release. The accelerated oxygen bubble evolution dynamic is monitored in operando with a decreased dynamic variation of the interfacial Faradaic resistance. Such cosynergistic molybdate modification results in a tenfold increase in oxygen evolution turnover frequency as well as a 55 mV decrease in the overpotentials to deliver 10 and 1000 mA cm−2 with respect to the FeNi‐model catalyst. [ABSTRACT FROM AUTHOR]

Fe–N–Cs are an alternative to Pt/C catalysts as they show promising activity towards the oxygen reduction reaction. While several studies are based on thin-film analysis in diluted electrolytes, only few reports show the application of Fe–N–Cs in gas diffusion electrodes (GDE) and their tests under high temperature proton exchange membrane fuel cell (HT-PEMFC) conditions. Here, a series of Fe–N–Cs with different physical properties are analyzed. GDEs are fabricated using ultrasonic spray and doctor blade coating to investigate the effect of coating method on the morphology and performance. Morphological analysis identified thinner catalyst layers for spray coating and inhomogeneous PTFE distribution for doctor blade coated GDEs. Electrochemical characterization in the HT-PEM half-cell displays significantly lower performance for ultrasonic spray-coated GDEs, possibly due to more pronounced phosphoric acid penetration. On the contrary, implementation of GDEs as cathodes in the HT-PEMFC yields comparable performances for both coating methods but differences in ORR and proton transport resistances determined by distribution of relaxation times analysis. Moreover, it was shown that inhomogeneous PTFE distribution caused by high amounts of hydrophilic O-/N-functionalities negatively affects HT-PEMFC performance. This study helps to understand the impact of GDE fabrication and Fe–N–C catalyst properties in HT-PEM half- and single-cell application. • Comparison of two coating processes for Fe–N–C-based GDEs. • Morphological analysis of GDEs using μ-CT and confocal microscopy. • Implementation of Fe–N–Cs in HT-PEM half-cell setup and HT-PEMFC. • Advanced characterization of MEAs using CV and DRT analysis. [ABSTRACT FROM AUTHOR]