A Doping-Induced SrCo0.4Fe0.6O3/CoFe2O4 Nanocomposite for Efficient Oxygen Evolution in Alkaline Media.

Perovskite and spinel oxides are promising alternatives to noble metal-based electrocatalysts for oxygen evolution reaction (OER). Herein, a novel perovskite/spinel nanocomposite comprised of SrCo0.4Fe0.6O3 and CoFe2O4 (SCF/CF) is prepared through a simple one-step method that incorporates iron doping into a SrCoO3- δ matrix, circumventing complex fabrication processes typical of these materials. At a Fe dopant content of 60%, the CoFe2O4 spinel phase is directly precipitated from the parent SrCo0.4Fe0.6O3 perovskite phase and the number of active B-site metals (Co/Fe) in the parent SCF can be maximized. This nanocomposite exhibits a remarkable OER activity in alkaline media with a small overpotentional of 294 mV at 10 mA cm-2. According to surface states analysis, the parent SCF perovskite remains in its pristine form under alkaline OER conditions, serving as a stable substrate, while the second spinel CF is covered by 5/8 monolayer (ML) O*, exhibiting considerable affinity toward the oxygen species involved in the OER. Analysis based on advanced OER microkinetic volcano model indicates that a 5/8 ML O* covered-CF is the origin for the remarkable activity of this nanocomposite. The results reported here significantly advance knowledge in OER and can boost application, scale-up and commercialisation of electrocatalytic technologies toward clean energy devices.

First-principles calculations and electron density topological analysis of covellite (CuS).

Covellite (CuS) is an important mineral sulfide that can be used in many technological applications. It has a simple formula but a complex structure consisting of alternating layers of planar CuS3 triangles and CuS4 tetrahedrons with S-S bonds. Accurate first-principles calculations are performed for covellite structure (CuS), aiming to provide insights about its structural, mechanical and electronic properties and to unveil the nature of its chemical bonding. DFT and DFT+U methods have been used and showed to be sensitive to the correlation treatment (U value). Although it is not possible to extract a universal value of the U, this study indicates that U = 5 eV is an adequate value. The electronic structure analysis shows a significant metallic character due to p(S)-d(Cu) orbital interactions up to Fermi level. The projected density of states indicates that most of the contribution comes from the atomic orbitals in the [001] plane of the covellite, explaining the conductivity anisotropy observed experimentally. Topological analysis of the electron density was performed by means of quantum theory of atoms in molecules (QTAIM). Two different topological charges in Cu and S were calculated, confirming an ionic model with mix-charges. This mineral presents ionic degree of ∼ 32%. On the basis of the QTAIM analysis, the covalent character of S-S bond is confirmed, and the favored cleavage of CuS at the [001] surface might be at the Cu-S bond. The S atoms occupy most of the cell volume, and their contributions dominate the crystal compressibility: κ(S) ≈ κ(CuS).