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During the oxygen evolution reaction, manganese rapidly dissolves, however, it has a disordered oxide layer with a steady state thickness.

High-entropy oxides (HEOs), which are a new class of single-phase solid solution materials, have recently attracted significant attention as an anode material for lithium-ion batteries (LIBs). In this study, (MgCoNiZn)1–x Li x O (x = 0.05, 0.15, 0.25, and 0.35) HEOs were synthesized and their electrochemical performances as the anode material were observed in LIBs. X-ray photoelectron spectroscopy (XPS) analysis showed that the increase in the lithium cation concentration causes generation of more oxygen vacancies, which greatly affected the electrochemical performance of (MgCoNiZn)1–x Li x O HEO anodes, in the structure. The more the oxygen vacancy concentration in the anode, the higher the discharge capacity in the LIB. The (MgCoNiZn)0.65Li0.35O anode had 1930 mA h g–1 initial and 610 mA h g–1 stable (after 130 cycles) discharge capacities at a current density of 1000 mA g–1. This work clearly indicated that designing a HEO with abundant oxygen vacancies in the structure was a very efficient strategy to improve the electrochemical performance of the HEO electrode for LIBs.

The widely used engineering material copper is a prototype of an electrochemically passive metal. In this work, the passive films on evaporated copper in 0.1 M NaOH are investigated in situ and operando by spectroscopic ellipsometry and Raman spectroscopy, both conducted during oxidation in potentiostatic step experiments. Oxide growth is initiated by jumping from a potential at which the surface is oxide-free to -0.1 V vs. Ag|AgCl|3 M KCl (+0.11 V vs. standard hydrogen electrode, SHE). At subsequent electrode potential jumps, no corresponding jumps in the thickness are observed; instead, oxide growth proceeds steadily. Above +0.3 V vs. Ag|AgCl|3 M KCl (+0.51 V vs. SHE), the oxide layer thickness remains constant at ≈7 nm. Raman spectra show a peak at 530 cm(-1), which agrees with the dominant peak in spectra of copper mixed oxide, Cu4O3 (Cu2(I)Cu2(II)O3). Crystalline Cu4O3 nucleates from a precursor state showing strong photoluminescence (PL), which hints at the involvement of Cu2O. Overall, the PL spectra of the growing oxide and absorption spectra indicate the presence of Cu2O in the thin films. Absorption spectra cannot be understood as a superposition of the spectra from different well-described copper oxides, which points to defect-rich oxides that show rather different spectra. Raman spectra also point to an involvement of both crystalline and amorphous oxides that coexist. The results show that the passive layers on copper are more complex than the duplex layers described in the literature; they do contain an oxide with a mixed valency of copper.

The mechanism of the hydrogen evolution reaction, although intensively studied for more than a century, remains a fundamental scientific challenge.

Formation and dissolution of oxide on copper under transpassive conditions, i.e. during OER and transpassive dissolution, in alkaline electrolyte was investigated by a combination of electrochemical techniques and in situ and operando Raman and photoluminescence (PL) spectroscopy, as well as spectropscopic ellipsometry. Experiments were conducted under potentiodynamic and potentiostatic polarisation in 0.1M NaOH.In chronoamperometry experiments with steps between potentials, oxide thickness continued increasing beyond the onset of OER. The thickness dropped significantly from >10 nm to <5 nm Ȃ400 mV above the OER onset. The presence of CuO, Cu 2 O and Cu 4 O 3 was observed by Raman spectroscopy after the onset of OER. Correlating with the thickness drop, strong PL was observed at 1.55 eV, indicating the formation of singly charged oxygen vacancies V + O , following the classical PL spectrum interpretation from the literature. PL observation speaks against vacancy pair coalescence as mechanism of dissolution. After electrochemical experiments, the films were n-type semiconductors, not p-type conductors as expected for copper oxides. Results indicate that transpassive dissolution may be triggered by the instability of the oxide with respect to defect formation.

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Corrosion inhibitors are added in low concentrations to corrosive solutions for reducing the corrosion rate of a metallic material. Their mechanism of action is typically the blocking of free metal surface by adsorption, thus slowing down dissolution. This work uses electrochemical impedance spectroscopy to show the cyclic oligosaccharide β-cyclodextrin (β-CD) to inhibit corrosion of zinc in 0.1M chloride with an inhibition efficiency of up to 85%. Only a monomolecular adsorption layer of β-CD is present on the surface of the oxide covered metal, with Raman spectra of the interface proving the adsorption of the intact β-CD. Angular dependent X-ray photoelectron spectroscopy (ADXPS) and ultraviolet photoelectron spectroscopy (UPS) were used to extract a band-like diagram of the β-CD/ZnO interface, showing a large energy level shift at the interface, closely resembling the energy level alignment in an n–p junction. The energy level shift is too large to permit further electron transfer through the layer, inhibiting corrosion. Adsorption hence changes the defect density in the protecting ZnO layer. This mechanism of corrosion inhibition shows that affecting the defect chemistry of passivating films by molecular inhibitors maybe a viable strategy to control corrosion of metals.