Reversible water induced phase changes of cobalt oxide nanoparticles

Cobalt oxides have been identified as highly active catalysts for the electrochemical water splitting and oxygen evolution reaction. Using near ambient pressure resonant photoelectron spectroscopy, we studied changes in the metal amp; 8722;oxygen coordination of size selected core amp; 8722;shell CoOx nanoparticles induced by liquid water. Indry conditions, the nanoparticles exhibit an octahedrally coordinated Co2 core and a tetrahedrally coordinated Co2 shell. In the presence of liquid water, we observe a reversible phase change of the nanoparticle shell into octahedrally coordinated Co2 as well as partially oxidized octahedrally coordinated Co3 . This is in contrast to previous findings, suggesting an irreversible phase change of tetrahedrally coordinated Co2 after the oxygen evolution reaction conditioning. Our results demonstrate the appearance of water induced structural changes different from voltage induced changes and help us to understand the atomic scale interaction of CoOx nanoparticles with water in electrochemical processe

Rotating Ring–Disk Electrode Study of Oxygen Evolution at a Perovskite Surface: Correlating Activity to Manganese Concentration

Transition-metal oxides with the perovskite structure are promising catalysts to promote the kinetics of the oxygen evolution reaction (OER). To improve the activity and stability of these catalysts, a deeper understanding about the active site, the underlying reaction mechanism, and possible side reactions is necessary. We chose smooth epitaxial (100)-oriented La_0.6Sr_0.4MnO₃ (LSMO) films grown on Nb:SrTiO₃ (STNO) as a model electrode to investigate OER activity and stability using the rotating ring−disk electrode (RRDE) method. Careful electrochemical characterization of various films in the thickness range between 10 and 200 nm yields an OER activity of the epitaxial LSMO surface of 100 μA/cm²_ox at 1.65 V vs RHE, which is among the highest reported for LSMO and close to (110)-oriented IrO₂. Detailed post-mortem analysis using XPS, XRD, and AFM revealed the high structural and morphological stability of LSMO after OER. The observed correlation between activity and Mn vacancies on the surface suggested Mn as the active site for the OER in (100)-oriented LSMO, in contrast to similar perovskite manganites, such as Pr_1–<i>x</i>Ca_<i>x</i>MnO₃. The observed Tafel slope of about 60 mV/dec matches the theoretical prediction for a chemical rate-limiting step that follows an electrochemical pre-equilibrium, probably O–O bond formation. Our study established LSMO as an atomically flat oxide with high intrinsic activity and high stability

Resonant X-ray photoelectron spectroscopy: identification of atomic contributions to valence states.

Valence electronic structure is crucial for understanding and predicting reactivity. Valence non-resonant X-ray photoelectron spectroscopy (NRXPS) provides a direct method for probing the overall valence electronic structure. However, it is often difficult to separate the varying contributions to NRXPS; for example, contributions of solutes in solvents or functional groups in complex molecules. In this work we show that valence resonant X-ray photoelectron spectroscopy (RXPS) is a vital tool for obtaining atomic contributions to valence states. We combine RXPS with NRXPS and density functional theory calculations to demonstrate the validity of using RXPS to identify atomic contributions for a range of solutes (both neutral and ionic) and solvents (both molecular solvents and ionic liquids). Furthermore, the one-electron picture of RXPS holds for all of the closed shell molecules/ions studied, although the situation for an open-shell metal complex is more complicated. The factors needed to obtain a strong RXPS signal are investigated in order to predict the types of systems RXPS will work best for; a balance of element electronegativity and bonding type is found to be important. Additionally, the dependence of RXPS spectra on both varying solvation environment and varying local-covalent bonding is probed. We find that RXPS is a promising fingerprint method for identifying species in solution, due to the spectral shape having a strong dependence on local-covalency but a weak dependence on the solvation environment