Electrochemical Behaviors Of IrO₂Electrodes Modified By SiO₂

IrO2is the best catalytic material for oxygen evolution in the most common environment. Although having low oxygen evolution over-potential, the other electrochemical performance of pure IrO2coating is not satisfactory, for example, the IrO2coating is easy to peel off, therefore having very short service life. Doping with foreign less noble oxides is one of the most economical approaches for IrO2coating preparation, exhibiting both high activity and high stability. Many kinds of this type of electrodes were investigated, for instance Ir-Co, Ir-Ta, and Lr-Sn electrodes, among which the combination of IrO2-Ta2O5was approved as the best electrode for oxygen evolution reactions (OER) in acid media because of its high electrocatalytic activity and long service life. Silica has high electrical insulation, high thermal stability and homogeneous dispersion when mixed with other components in a mixture system. Moreover, silica possesses amorphous structure in a wide temperature range. In our previous work, we prepared IrO2-SiO2composite films on Ti substrate by the thermal decomposition. The obtained binary oxide electrodes presented a significant improvement in both the electrocatalytic activity and long-term stability for the OERs. In the present work, we investigated the electrochemical surface structure of Ti/IrO2-SiO2composite electrodes and the reaction kinetics of the OER thereon; we also used electrodeposited SiO2film as a template to prepare IrO2electrodes through electrodeposition and investigate the electrochemical behavior of this kind of electrodes.The electrochemical surface structure of Ti/IrO2-SiO2composite electrodes and the reaction kinetics of the OER thereon are investigated in details, to deeper understand the positive role of silica incorporation. Both the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements indicate either "inner" or "outer" active surface area of the mixed oxide electrodes, which is quantified by the voltammetric charge (q*) or by the double-layer capacitance (Cdl), constantly increases when increasing the silica content therein under the investigated range of Ir/Si molar ratio. The "porosity", defined as the ratio of the "inner" to the "total" surface areas, of the binary oxide films exhibits much higher values than pure IrO2film. Although the apparent electrocatalytic activity for the OER at composite electrodes is obviously higher than that at IrO2electrode, the real surface area-normalized activity declines after silica incorporation. The kinetic rate constants of the OER, approximated from the normalized polarization curves, also show dramatically decreased values at silica-doped electrodes. The above-mentioned results suggest that the enhanced apparent electrocatalytical activity of silica-doped IrO2electrodes might be merely a result of geometric effect.Using electrodeposited SiO2film as a template, the amount of IrO2electrodeposited onto ITO substrate is found being lower than that of non-templated film. However, the electrocatalytical activity of templated ITO/IrO2electrode is improved. The scanning electron microscopy (SEM) images show that the improvement of electrocatalytical activity of ITO electrode modified by SiO2is a result of the more porous surface structure.