Old workhorse of oxide investigations : new features of Co3O4

The electrochemical behaviour of Co3O4 from exhaustive cobalt nitrate decomposition (T = 260-850 °C) is investigated on graphite-supported electrodes, mainly relying on quasi-reversible results. Two well defined 1-e- redox systems are observed related to Co2+/Co3+ and Co3+/Co4+ reactions of non-equivalent oxide surface sites. Oxide stoichiometry is directly estimated by voltammetry in `wet' conditions and depends on preparation temperature as in ex situ solid state data. Adsorption of surface intermediates involved in charge transfer is examined. Langmuir and Temkin-type adsorption isotherms are obeyed for the Co2+/Co3+ and Co3+/Co4+ redox systems, respectively. Unusually negative (attractive) lateral interaction parameters are calculated for the latter system. Separation of oxide and graphite currents is achieved to permit real electrode surface area determination.