Transient studies of oxygen species adsorbed on platinum microelectrodes

Abstract

Perry and Denuault reported the presence of an oxygen species that adsorbed on Pt microelectrodes after the electrode was exposed to dissolved oxygen [1, 2, 3]. This species could only be detected at the millisecond scale and when using a neutral and unbuffered media. Although they suggested that the species was oxygenated in nature, its true identity and relation with O₂ remained unknown. In the present work, the species were studied with the use of Pt microelectrodes of different sizes, in different electrolytes and with sampled current voltammetry and scanning electrochemical microscopy. The results show strong evidence that the adsorbed species are oxides formed under a higher local pH, where OH' is being produced by the oxygen reduction reaction proceeding through the alkaline path. This was observed by exploiting hindered diffusion in the presence of O₂, where the oxide formation region appeared as a continuous wave, similar to results in bulk alkaline media. Furthermore, a diffusion controlled oxidation wave appeared that was limited by the local concentration of OH' . Throughout this work, sampled current voltammetry was exploited to study the oxygen reduction reaction and the reduction of adsorbed oxygen species, and as such, methodologies to obtain kinetic information were proposed. An equivalent Koutecky-Levich analysis was applied to sampled current voltammograms of the oxygen reduction reaction, obtaining Tafel slopes similar to those reported in the literature, and an analytical solution for sampled current voltammograms of adsorbed species in the absence of diffusion was also derived; comparison with established methodologies is also presented.

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ORCID for Guy Denuault: orcid.org/0000-0002-8630-9492

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