Studies of the initial oxidation of cobalt in alkaline solutions using scanning electrochemical microscope

Abstract

Scanning electrochemical microscopy and atomic force microscopy were used together with cyclic voltammetry and rotating ring disc experiments to study the initial oxidation of cobalt in alkaline solution. The results show that the mechanism of cobalt oxidation involves a solid state path as well as dissolution combined with precipitation. In the concentration region studied, 0.1-1 M NaOH, the solid state path seems to predominate.The AFM images clearly show a three-dimensional growth mechanism and that the initial oxidation starts at active sites at the surface.Metal oxide and hydroxide films are of practical interest because of their electrocatalytic properties, photochemical activity and charge storage capabilities. Electrochemically generated films are generally more active and it is therefore of great interest to study the initial oxidation mechanism. Two main questions are stressed in many film-forming systems: first, whether the reaction proceeds through a solid state route or by a dissolution/precipitation path, and second, whether the film growth is two-or three-dimensional. In the dissolution/precipitation route, soluble species are formed and can be detected, for example, by conventional rotating ring disc electrode (RRDE) measurements, provided that the precipitation reaction is not too fast. The film growth is generally inhomogeneous and three-dimensional. In this work, the initial oxidation of cobalt in alkaline solution was investigated with these questions in mind. Scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) were used together with cyclic voltammetry and RRDE. Most spectroscopic and diffraction techniques only provide information averaged over macroscopic regions of the electrode surface. This is in contrast with scanning probe methods (SPMs) such as atomic force microscopy (AFM), where the electrode surface can be studied with nanometer to atomic resolution, ex situ or in situ. The high spatial resolution of the surface is of interest because structural and chemical inhomogeneities can have a significant influence on the local mechanism of electrochemical reactions on the electrode. Thus, by performing AFM measurements one gets topographical information about the electrode surface. Furthermore, AFM reveals whether small oxide particles have formed on the electrode surface where other techniques, such as X-ray grazing angle measurements, fail to detect them. AFM can be used to image both conducting and nonconducting samples. SECM was used to detect soluble species formed during the oxidation process. By keeping the tip at constant potential during the polarization of the cobalt sample, these experiments resemble the RRDE measurements. However, the tip electrode can be located close to the surface and more shortlived species can be detected.The results show that soluble species can be detected by RRDE and SECM when the oxidation takes place in 1 M NaOH. Preliminary results show that a larger fraction of soluble species can be detected by SECM. In 0.1 M NaOH the detec...

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