Remarkably high activity of electrodeposited IrO2 film for electrocatalytic water oxidation

A homogenous and transparent IrO2 film was prepared on an ITO electrode by anodic electrodeposition under galvanostatic conditions from an aqueous solution containing 2 mM K2IrCl6 and 40 mM oxalic acid that is aged at 37 °C and pH 10 for ca. 10 days. The absorption spectral change of the solution suggested that an IrO2 colloid is formed in the solution during ca. 10 day-aging. The scanning electron microscopic (SEM) measurement displayed homogeneous deposition of IrO2 particles with 100-250 nm of a diameter on the surface of the film. The X-ray diffraction (XRD) measurement indicated that IrO2 in the film is amorphous. The cyclic voltammogram (CV) of the IrO2-coated ITO electrode dipped in a 0.1 M KNO3 aqueous solution exhibited a steep rise of an anodic current at 1.0 V vs SCE for catalytic water oxidation, as well as an anodic wave at 0.3 V and a corresponding cathodic wave at −0.1 V that are assigned as an IrIV/IrV redox. The anodic current at 1.3 V on the CV was 660 times higher than that for a blank bare ITO electrode. Ir electrodeposited on the ITO electrode was also shown to be electrocatalytically active for water oxidation. However, the anodic current at 1.3 V on the CV for the Ir-coated ITO electrode was 14 times lower than that for an IrO2-coated electrode in spite of the 34 times higher coverage of Ir. The potential static electrochemical water oxidation using the IrO2-coated ITO electrode produced a significant amount of O2 above 1.1 V vs Ag/AgCl, in contrast to no O2 detected even at 1.3 V using a bare ITO electrode. The maximum turnover frequency (TOF) of the IrO2 catalyst was provided as 16,400 ± 450 h−1 at 1.3 V vs Ag/AgCl from the slope of the linear plots of the amount of O2 vs coverage of IrO2 in the range of ∼1.5 × 10−9 mol. The TOF was 450 times higher than that (36.4 ± 1.4 h−1 at 1.3 V) for electrodeposited Ir showing the very high catalytic activity of the IrO2 film.