Browsing by Author "Penkova Y."

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    Anodic Oxidation of Tungsten under Illumination-Multi-Method Characterization and Modeling at the Molecular Level
    (2023-11-01) Bojinov M.; Penkova Y.; Betova I.; Karastoyanov V.
    Tungsten oxide has received considerable attention as photo-anode in photo-assisted water splitting due to its considerable advantages such as significant light absorption in the visible region, good catalytic properties, and stability in acidic and oxidative conditions. The present paper is a first step in a detailed study of the mechanism of porous WO3 growth via anodic oxidation. In-situ electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS) during oxidation of W illuminated with UV and visible light are employed to study the ionic and electronic processes in slightly acidic sulfate-fluoride electrolytes and a range of potentials 4–10 V. The respective responses are discussed in terms of the influence of fluoride addition on ionic and electronic process rates. A kinetic model is proposed and parameterized via regression of experimental data to the EIS and IMPS transfer functions.
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    Mechanism of Anodic Dissolution of Tungsten in Sulfate–Fluoride Solutions
    (2024-09-01) Bojinov M.; Penkova Y.; Betova I.; Karastoyanov V.
    Thin passive films on tungsten play an important role during the surface levelling of the metal for various applications and during the initial stages of electrochemical synthesis of thick, nanoporous layers that perform well as photo-absorbers and photo-catalysts for light-assisted water splitting. In the present work, the passivation of tungsten featuring metal dissolution and thin oxide film formation is studied by a combination of in situ electrochemical (voltammetry and impedance spectroscopy) and spectro-electrochemical methods coupled with ex situ surface oxide characterization by XPS. Voltametric and impedance data are successfully reproduced by a kinetic model featuring oxide growth and dissolution coupled with the recombination of point defects, as well as a multistep tungsten dissolution reaction at the oxide/electrolyte interface. The model is in good agreement with the spectro-electrochemical data on soluble oxidation products and the surface chemical composition of the passive oxide.