A fundamental understanding of the origin of oxygen evolution reaction (OER) activity of transition-metal-based electrocatalysts, especially for single precious metal atoms supported on layered double hydroxides (LDHs), is highly required for the design of efficient electrocatalysts toward further energy conversion technologies. Here, we aim toward single-atom Au supported on NiFe LDH (Au/NiFe LDH) to clarify the activity origin of LDHs system and a 6-fold OER activity enhancement by 0.4 wt % Au decoration. Combining with theoretical calculations, the active behavior of NiFe LDH results from the in situ generated NiFe oxyhydroxide from LDH during the OER process. With the presence ofAu, Au/NiFe LDH possesses an overpotential of 0.21 V in contrast to the calculated result (0.18 V). We ascribe the excellent OER activity ofAu/NiFe LDH to the charge redistribution of active Fe as well as its surrounding atoms causing by the neighboring Au on NiFe oxyhydroxide stabilized by interfacial CO and HO interfacing with LDH.

In-situ X-ray absorption spectroscopy (XAS) at the oxygen K-edge was used to investigate the role of oxygen during the oxygen evolution reaction (OER) in an electrodeposited Ni-Fe(OxHy) electrocatalyst in alkaline pH. We show the rise of a pre-peak feature at 529 eV in the O K-edge spectra, correlated to the appearance of a shoulder at the Ni L3-edge and formation of oxidized Ni3+/4+-O. Then, for the first time, we track the spectral changes in a dynamic fashion in both the soft and hard X-ray regimes during cyclic voltammetry (in situ CV-XAS) to obtain a fine-tuned resolution of the potential-related changes. The pre-peak feature at the O K-edge likely signifies formation of an electron deficient oxygen site. The electrophilic oxygen species appears and disappears reversibly in correlation with the Ni2+ ↔ Ni3+/4+ process, and persists during OER catalysis as long the metal is oxidized. Our study provides new insight into OER electrocatalysis: Before onset of the O-O bond formation step, the catalytic oxyhydroxide has accumulated electron deficiencies by both, oxidation of transition metal ions and formation of partially oxidized oxygen sites.

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Resonant inelastic X-ray scattering (RIXS) and X-ray absorption (XA) experiments at the iron L- and nitrogen K-edge are combined with high-level first-principles restricted active space self-consistent field (RASSCF) calculations for a systematic investigation of the nature of the chemical bond in potassium ferrocyanide in aqueous solution. The atom- and site-specific RIXS excitations allow for direct observation of ligand-to-metal (Fe L-edge) and metal-to-ligand (N K-edge) charge-transfer bands and thereby evidence for strong σ-donation and π-backdonation. The effects are identified by comparing experimental and simulated spectra related to both the unoccupied and occupied molecular orbitals in solution.

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Aqueous ions are central to catalysis and biological function and play an important role in radiation biology as sources of damage-inducing electrons. Detailed knowledge of solute-solvent interactions is therefore crucial. For transition-metal ions, soft X-ray L-edge spectroscopy allows access to d orbitals, which are involved in chemical bonding. Using this technique, we show that the fluorescence-yield spectra of aqueous ionic species exhibit additional features compared with those of non-aqueous solvents. Some features dip below the fluorescence background of the solvent and this is rationalized by the competition between the fluorescence yields of the solute and solvent species, and between the solute radiative (fluorescence) and non-radiative channels; in particular, electron transfer to the water molecules. This method allows us to determine the nature, directionality and timescale of the electron transfer. Remarkably, we observe such features even for fully ligated metal atoms, which indicates a direct interaction with the water molecules.

A new emerging water oxidation catalyst (WOC) generated via electrodeposition from aqueous solutions containing methylphosphate (MeP i ) and Mn 2+ has been studied by in situ X-ray absorption spectroscopy (XAS) in transmission mode and in situ extended X-ray absorption fine-structure (EXAFS) in fluorescence mode. XA spectra were obtained for freshly prepared and activated MnP i films under different potentials and after different potential durations. Via linear combination fitting the contribution of different Mn species was revealed from the L-edge spectra. The XAS results show that the freshly prepared film at OCP contains a dominant contribution of MnO 2 (∼75%) and a contribution from a birnessite-like material (∼25%). No or only a neglectable percentage of MnO, Mn 3 O 4 , or Mn 2 O 3 -like Mn species was found in the freshly prepared sample. After 51 min of in situ activation at 1.2 V at pH 7.0, the birnessite contribution increased to 75% in the spectrum. We correlate these changes to the material conversion into an efficient WOC. After activation, the film behavior is reversible in potentials above and below water oxidation regime. EXAFS results are found to be consistent with our L-edge spectra fitting results.