NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γphases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure MM centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.

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Electrochemical CO2 reduction has attracted much attention, because of its advantageous ability to convert CO2 gas to useful chemicals and fuels. Herein, we have developed prism-shaped Cu catalysts for efficient and stable CO2 electroreduction by using an electrodeposition method. These Cu prism electrodes were characterized by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Electrochemical CO2 reduction measurements show improved activities for C2H4 production with a high partial current density of −11.8 mA/cm2, which is over four times higher than that of the planar Cu sample (−2.8 mA/cm2). We have demonstrated that the enhanced C2H4 production is partially attributed to the higher density of defect sites available on the roughened Cu prism surface. Furthermore, stability tests show a drastic improvement in maintaining C2H4 production over 12 h. The enhanced performance and durability of prism Cu catalysts hold promise for future industrial applications.

Synthetic pentlandite (FeNiS) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction.