Identification of electrochemically formed metal oxides by coupling high-temperature cyclic voltammetry with Raman spectroscopy

Abstract

High-temperature cyclic voltammetry (HT-CV) is a versatile technique for accurately characterizing metal oxidation processes. This method uses YSZ (Yttria-stabilized zirconia), a solid-state oxygen ion conductor, as an electrolyte. By applying defined potentials, different oxide species are produced. Hence, thermodynamic processes such as the stability of oxides and kinetic properties, like growth rates, can be investigated. However, in electrochemical reactions the current cannot be assigned to specific reactions unequivocally. To address this issue, in situ analytical methods are necessary. This is crucial when investigating diffusion processes and reaction products, as they require additional analytical information. This work presents a novel method that combines high-temperature electrochemistry with Raman spectroscopy to investigate metal oxidation processes and diffusion phenomena directly at the solid-electrolyte/metal interface. The HT-CV method is used to achieve controlled oxidation states of metals. To this end, a commercial Raman heating stage was adapted to enable electrochemical measurements in the cell. The use of a transparent YSZ single crystal allows Raman measurements, as the laser penetrates through the material to the metal (oxide)/electrolyte interface. With Raman spectroscopy, oxides produced under controlled conditions can be analyzed. Furthermore, optical microscope images of the oxides can be taken during their formation. Our combined method also enables the investigation of metallic interdiffusion. The electrochemical control of the potential allows for the selective oxidation of diffusing species whilst the layer through which the diffusion takes place remains in the reduced state. The results obtained with Raman spectroscopy confirm the existence of the species predicted from electrochemical experiments.