Kinetic and Mechanistic Investigation of Cobalt Oxide Hydroxide Thin Films by Square-Wave Voltammetry and Multi-Frequency Electrochemical Faradaic Spectroscopy

Abstract

In this study, a detailed investigation of the redox behavior of cobalt(II/III) oxide-hydroxide is presented. With the enhanced sensitivity and unique pulse nature of square-wave voltammetry (SWV), two distinct electrode processes could be observed, proposing a two-step oxidation of Co(OH)₂ according to: Co(OH)₂→CoO(OH)→CoO₂. In the initial scan of cyclic voltammetry, Co(OH)₂ is irreversibly oxidized to CoO(OH) with the quasireversible Co₃O₄/CoO(OH) redox system prevailing in subsequent scans. The system was further kinetically characterized. Theoretical studies and the unique peak splitting in SWV revealed that the electrode reaction is associated with an anodic charge transfer coefficient of $$ =0.59 and an apparent standard rate constant of k_s,app=(2.9±0.1) ⋅ 10⁻⁵ cm s⁻¹. To cope with the complexity of the electrode process pertinent to the redox couple Co₃O₄/CoO(OH), the recently introduced and advanced multi-frequency electrochemical Faradaic spectroscopy (MEFS) was applied. With rapid measurement times of only 4 s, compared to multiple, hour-long experiments for square-wave and cyclic voltammetry, an apparent standard rate constant of k_s,app=(2.2±0.2) ⋅ 10⁻⁵ cm s⁻¹ was obtained with MEFS, which is quite close to the established methods, highlighting the advantages of this novel square-wave derived technique.