Local process investigations on composite electrodes: on the way to understanding design criteria for spray coated anodes in Zn electrowinning

摘要

Several possible physico-chemical properties of composite electrodes for oxygen evolution are presented to describe experimental data for which a mathematical model had been developed. On this basis, local electrical and electrochemical properties of a composite electrode were investigated with conductive atomic force microscopy (CAFM) and scanning electrochemical microscopy (SECM). It could be shown by CAFM measurement that the boundary between matrix and catalyst particles seemed to have special advantageous electrical properties. The SECM measurements showed the presence of mass transport phenomena with increased surface concentrations, whilst the thickness of the Nernst diffusion layer was very small. An intermediate was detected and assigned to be hydrogen peroxide. From all species involved in the oxygen evolution reaction (H_2O_2, H+ and O_2), it was concluded that local active spots exist on the electrode on which hydrogen peroxide reacts to oxygen and protons. A two-step two-material process was suggested to explain the whole oxygen evolution mechanism