The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core-shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

Hydrogen produced from water and renewable energy could fuel a large fleet of proton-exchange-fuel-cell vehicles in the future. However, the dependence on expensive Pt-based electrocatalysts in such fuel cells remains a major obstacle for a widespread deployment of this technology. One solution to overcome this predicament is to reduce the Pt content by a factor of ten by replacing the Pt-based catalysts with non-precious metal catalysts at the oxygen-reducing cathode. Fe-and Co-based electrocatalysts for this reaction have been studied for over 50 years, but they were insufficiently active for the high efficiency and power density needed for transportation fuel cells. Recently, several breakthroughs occurred that have increased the activity and durability of non-precious metal catalysts (NPMCs), which can now be regarded as potential competitors to Pt-based catalysts. This review focuses on the new synthesis methods that have led to these breakthroughs. A modeling analysis is also conducted to analyze the improvements required from NPMC-based cathodes to match the performance of Pt-based cathodes, even at high current density. While no further breakthrough in volume-specific activity of NPMCs is required, incremental improvements of the volume-specific activity and effective protonic conductivity within the fuel-cell cathode are necessary. Regarding durability, NPMCs with the best combination of durability and activity result in ca. 3 times lower fuel cell performance than the most active NPMCs at 0.80 V. Thus, major tasks will be to combine durability with higher activity, and also improve durability at cell voltages greater than 0.60 V.

Micropores are largely responsible for Fe/N/C catalytic activity, but are also intrinsically responsible for the rapid initial performance loss in PEMFC.

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Hydrogen-air polymer-electrolyte-membrane fuel cells (PEMFCs) show promise for the replacement of gasoline internal-combustion engines for vehicle propulsion and other applications. However, the high cost of components, which is largely due to the use of platinum-based catalysts for the O 2reduction reaction (ORR), remains an impediment. [1] For a production of 500 000 PEMFC stacks a year, electrocatalysts alone were estimated to account for nearly half the cost of a stack. [2] Recent studies on pyrolyzed Fe/nitrogen/carbon and Co/ nitrogen/carbon catalysts for the ORR have increased their initial performance close to the level reached by platinumbased catalysts, and other studies have demonstrated promising durability. [3] We reported the use of a Zn II zeolitic imidazolate framework (ZIF) as a microporous support for ferrous acetate (Fe II Ac 2 ) and 1,10-phenanthroline to prepare a catalyst precursor which, after pyrolysis in Ar and then in NH 3 , resulted in unprecedented activity and power performance. [4] The investigated ZIF, referred to as ZIF-8, was a commercial product (Basolite Z1200 from BASF). ZIFs are a subclass of metal-organic frameworks (MOFs), which were first used for the preparation of platinum-free catalysts by Liu and co-workers. [5] MOFs are now actively investigated for electrochemical applications. [6] Herein, we describe our investigations on the replacement of 1,10-phenanthroline (phen) with 2,4,6-tris(2-pyridyl)-striazine (TPTZ) in our synthesis with ZIF-8. The use of TPTZ was investigated previously by Zhang and co-workers, who used a high-surface-area carbon material as a host. [7] However, the current density in the resulting PEMFC was only approximately 0.1 A cm À2 at 0.6 V, [7c] as compared to the value of 1.2 A cm À2 observed with Fe/phen/ZIF-8 precursors. [4] We show herein that a high performance can also be reached with TPTZ by the use of an appropriate synthesis procedure based on an improved understanding of the coordination chemistry of the Fe II /ligand/ZIF-8 catalyst precursor.We first prepared an Fe/TPTZ/ZIF-8 catalyst precursor of weight composition 1:10:90 (see the Supporting Information) by wet impregnation followed by drying and planetary ball milling. This composition results in a TPTZ/Fe molar ratio of about 2:1. The blue color characteristic of [Fe II (TPTZ) 2 ] was immediately observed when Fe II Ac 2 and TPTZ were dissolved. ZIF-8 was then dispersed in the solution, whose color slowly changed to gray-blue and then ochre. The absorption peak at 596 nm characteristic of [Fe II (TPTZ) 2 ] [8] was no longer observed in the UV/Vis spectrum after 2 h (Figure 1 a). Thus, [Fe(TPTZ) 2 ] had reacted with ZIF-8. We expected 2methyl-imidazole (2-MeIm), the structuring ligand of ZIF-8, to compete with TPTZ for ferrous cations. Indeed, the absorption peak of [Fe(TPTZ) 2 ] also vanished after the addition of 2-MeIm (see Figure S1 in the Supporting Information). In contrast, this competition for Fe II cations between 2-MeIm of ZIF-8 and the phen ligand was not observed for th...

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Nanostructures constituted of Pt nanoparticles (NPs) supported on carbon materials are considered to be among the most active oxygen reduction reaction (ORR) catalysts for fuel cells. However, in practice, the usage of such ORR catalysts is limited by their insufficient durability caused by the low physical and chemical stability of Pt NPs during the reaction. We herein present a strategy to synthesize highly durable and active electrocatalysts composed of Pt NPs supported on carbon nanotubes (CNTs) and covered with an ultrathin layer of graphitic carbon. Such hybrid ORR catalysts were obtained by an interfacial in situ polymer encapsulation− graphitization method, where a glucose-containing polymer was grown directly on the surface of Pt/CNTs. The thickness of the carbon-coating layer can be precisely tuned between 0.5 nm and several nanometers by simply programming the polymer growth on Pt/CNTs. The resulting Pt/CNTs@C with a carbon layer thickness of ∼0.8 nm (corresponding to ∼2−3 graphene layers) showed high activity, and excellent durability, with no noticeable activity loss, even after 20 000 cycles of accelerated durability tests. These ultrathin carbon coatings not only act as a protective layer to prevent aggregation of Pt NPs but they also lead to better sample dispersion in solvent which are devoid of aggregates, resulting in a better utilization of Pt. We envision that this polymeric nanoencapsulation strategy is a promising technique for the production of highly active and stable ORR catalysts for fuel cells and metal−air batteries.

F2-Fluorination of Fe/N/C catalysts poisons FeN4, but not CNx sites. Main causes of instability in PEMFCs are either FeN4 demetalation for Fe/N/C or H2O2 when FeN4 sites are poisoned by fluorination or absent as in MOF_CNx_Ar + NH3.