Microstructure, electrochemical surface and electrocatalytic properties of IrO2+Ta2O5 oxide electrodes

X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used to characterize physical structure of IrO₂+Ta₂O₅ films over the whole composition range by thermodecomposition of chloride solutions heated at 450°C. Solid solubilization between Ta component and IrO₂ rutile in the mixed films was measured, and three typical surface morphologies of the oxide coatings were observed. The surface electrochemical properties of Ti/IrO₂-Ta₂O₅ electrodes were studied by cyclic voltammetry at varying potential scan rate, and a double-layer electrochemical structure containing the inner and outer layers has been distinguished. The voltammetric charge appears to decline with the decrease of grain size of oxide coatings as a result of the effect of surface tension. However, the coatings of 70% IrO₂+30% Ta₂O₅ with the finest grains still exhibit the highest apparent activity for oxygen evolution evaluated by the anodic current at a constant potential. This result is interpreted by the measurements of open-circuit potential (E _oc) and double-layer capacitance (C _dl) using electrochemical impedance spectroscopy (EIS). Thereby, the reliability of voltammetric charge obtained in double-layer potential region in determining the real electrocatalytic activity for O₂ evolution has been discussed.