Redox coupled ion exchange in cobalt oxide films

Ion exchange coupled to oxidation–reduction cycling of electrochromic cobalt oxide films was studied using Probe Beam Deflection (PBD) techniques. The films were deposited by potentiodynamic cycling of glassy carbon electrodes in aqueous Co⁺² solution. The cyclic voltammograms of cobalt oxide films present three peaks in basic (pH=13) solution. The PBD signal is positive during oxidation indicating insertion of hydroxide ions. The results are confirmed by monitoring the PBD signal during pulse potential experiments at varying distances of the beam from the electrode. Using the theory of PBD behaviour for discontinuous reactions, a diffusion coefficient for the mobile ion of 1.68×10⁻⁵ cm²/s was measured, which agreed with that observed for NaOH diffusion in similar systems. The results are explained on the basis of a scheme of solid state oxidations from Co^II to Co^IV in three redox steps, coupled to hydroxide ion insertion.