The electrochemical CO 2 reduction reaction (ECO 2 RR) is a promising approach to generate renewable fuels and commodities with the integration of renewable energy. Cu-based catalysts produce an array of products resulting from the transfer of 2e − to 18e − during ECO 2 RR. Valueadded C 2+ products are of great interest yet difficult to selectively produce. Oxide-derived Cu (OD-Cu)-type catalysts have shown improved selectivity and activity over metallic Cu catalysts that have not been preoxidized. Undercoordinated Cu sites on OD-Cu-type catalysts are suggested to be the active sites for enhanced C 2+ production. However, the stability of undercoordinated Cu sites remains largely unexplored in alkaline ECO 2 RR conditions. In this work, we prepare strontium copper oxide catalysts of varying Sr−Cu ratios and crystalline phases. We identify a SrCuO 2 tetragonal phase catalyst to be highly selective toward C 2+ products, exhibiting a 53% C 2+ Faradaic efficiency at −0.83 V vs reversible hydrogen electrode (RHE). Ex situ X-ray absorption spectroscopy (XAS) indicates that SrCuO 2 catalysts are able to retain or recover oxidized Cu species after exposure to reductive ECO 2 RR conditions for almost 1 hour, whereas OD-Cu remains in a metallic state. Furthermore, operando XAS of SrCuO 2 catalysts under alkaline ECO 2 RR conditions in a gas diffusion electrode-type flow cell reveals the formation and enhanced stabilization of Cu metallic moieties with a low coordination number of 6.3. This work suggests that tuning copper oxides via incorporation of secondary cations in the crystal lattice can further improve the stability of undercoordinated and higher-valence-state Cu sites for improved ECO 2 RR performance.
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.
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