Structural and electrochemical studies of Co oxide films formed by the sol-gel technique

Abstract

Two different types of Co oxide films, each having a distinct electrochemical signature correlated with the film drying temperature, were formed using the sol-gel (SG) technique. Two different states of gelation of the film precursor were also explored. Cyclic voltammograms, collected in alkaline solutions for the low temperature films, displayed two pairs of peaks corresponding to the Co(II) to Co(III) and Co(III) to Co(IV) transitions, centered at 1.2 and 1.4 V, respectively, while the high temperature films underwent only the Co(III) to Co(IV) redox process at 1.4 V. The charge densities obtained for the lower temperature films (particle sizes 2–10 nm in diameter) ranged between 40 and 70 mC/cm2; charge densities for the higher temperature films (particle sizes of 5–40 nm), otherwise formed identically, were between 10 and 20 mC/cm2. The more viscous Co oxide gels led to significantly higher charge densities than less viscous gels, as well as greater film stability during electrochemical cycling. Using a wide range of film characterization techniques, it was shown that Co oxide films formed at > 180∘C are composed mainly of Co3O4 spinel, while films formed at < 180∘C consist predominantly of CoO.