IrO2–SiO2 binary oxide films: Geometric or kinetic interpretation of the improved electrocatalytic activity for the oxygen evolution reaction

Our previous paper (Electrochim Acta 2010; 55: 4587–4593) reported that doping silica, the largest reserved oxide in the world, significantly improved the apparent electrochemical activity of Ti/IrO2 electrodes for oxygen evolution reaction (OER). In the present work, the electrochemical surface structure of Ti/IrO2–SiO2 composite 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 “total” surface area, of the binary oxide films exhibits much higher values than pure IrO2 film. Although the apparent electrocatalytic activity for the OER at composite electrodes is obviously higher than that at IrO2 electrode, 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 IrO2 electrodes might be merely a result of geometric effect.