Heteroanionic materials exhibit great structural diversity with adjustable electronic, magnetic, and optical properties that provide immense opportunities for materials design. Within this material family, perovskite oxynitrides incorporate earth-abundant nitrogen with differing size, electronegativity, and charge into oxide, enabling a unique approach to tuning metal-anion covalency and energy of metal cation electronic states, thereby achieving functionality that may be inaccessible from their perovskite oxide counterparts, which have been widely studied as electrocatalysts. However, it is very challenging to directly obtain such materials due to the poor thermal stability of late transition metals coordinated with N and/or at high valence states. Herein, we introduce an effective strategy to prepare a perovskite oxynitride with a small fraction of sites substituted with Ir and adopt it as the first electrocatalyst in this material family, thereby enabling high activity and efficient utilization of precious metal content. From a series of characterization techniques, including X-ray absorption spectroscopy, atomic resolution electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction, we prove the successful incorporation of Ir into a strontium tungsten oxynitride perovskite structure and discover the formation of a unique Ir–N/O coordination structure. Benefitting from this, the material exhibits a high activity toward the hydrogen evolution reaction, which exhibits an ultralow overpotential of only 8 mV to reach 10 mA/cm2 geo in 0.5 M H2SO4 and 4.5-fold enhanced mass activity compared to commercial Pt/C. This work opens a new avenue for oxynitride material synthesis as well as pursuit of a new class of high-performance electrocatalysts.

Molybdenum nitrides and oxynitrides have been increasingly realized as (electro)catalysts for a variety of reactions. In this context, the cubic "γ-Mo 2 N", also known to contain oxygen in the bulk, is of particular interest. The γ phase is typically derived from ammonolysis of MoO 3 , and a high temperature is needed to fully react the stable MoO 2 intermediate that often forms along the reaction pathway. In this study, ammonolysis of atypical bronze (H x MoO 3 ) and peroxo (H 2 MoO 5 ) precursors was undertaken to avoid the formation of this undesired intermediate with the aim of synthesizing "γ-Mo 2 N" at reduced temperatures and thus with a high surface area. It was found, using in situ powder diffraction, that, when the phase I bronze (x ≈ 0.3) served as the precursor, MoO 2 formed as an intermediate and was retained in the reaction product until 700 °C. In contrast, ammonolysis of the phase III bronze (x ≈ 1.7) and of H 2 MoO 5 circumvented the MoO 2 intermediate. From these latter two precursors, "γ-Mo 2 N" was formed at the lowest maximum reaction temperatures reported in the literature, namely, 480 °C in the case of H x MoO 3 -III and 380 °C for H 2 MoO 5 . The resulting products displayed extremely high surface areas of 206 and 152 m 2 /g, respectively, presumably as a consequence of the low synthesis temperatures. While the H x MoO 3 -III precursor showed evidence of a topotactic transformation pathway, with morphological similarity between precursor and product phases, H 2 MoO 5 transformed via amorphization. Electrochemical characterization showed moderate activity for the hydrogen evolution reaction (HER), which increased after exposure to reducing potentials and loosely scaled with the catalystspecific surface area. This work points toward new low-temperature synthesis pathways for accessing molybdenum (oxy)nitrides with high surface areas.

Carbon-supported Pt nanoparticles are the leading catalysts for the cathode oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. However, these ORR catalysts suffer from poor electrochemical durability, particularly the loss of electrochemical surface area (ECSA) due to Pt nanoparticle dissolution and agglomeration. Here, Pt loss is mitigated through a Pickering emulsion-processing strategy that employs graphene nanoplatelet dispersions stabilized by the polymer ethyl cellulose. The resulting graphene-Pt/Vulcan carbon (Pt/C) catalysts exhibit superior durability and ECSA retention throughout an accelerated stress test compared with a commercial Pt/C standard catalyst, both in a diagnostic-rotating disc electrode setup and in a membrane electrode assembly full cell. These graphene-Pt/C catalysts also improve durability at high-voltage conditions, providing further evidence of their exceptional electrochemical stability. Consistent with density functional theory calculations, postelectrochemical characterization reveals that Pt nanoparticles localize at graphene defects both on the basal plane and especially at the edges of the graphene nanoplatelets. Since this Pt nano particle localization suppresses Pt nanoparticle dissolution and agglomeration without hindering accessibility of the reactant species to the catalyst surface, the ORR performance under both idealized and practical experimental conditions shows significantly improved durability while maintaining high electrochemical activity.