Water Oxidation at Hematite Photoelectrodes with an Iridium-Based Catalyst

The iridium complex [Cp*Ir(H2O)3](SO4) was used as an organometallic source for the electrodeposition of iridium oxide onto Fe2O3. The new iridium-containing electrode allowed us to study the coupling between the photocatalytic properties of hematite and the electrocatalytic properties of the iridium-based material. A cathodic shift of the photocurrent for water oxidation upon electrodeposition of the iridium complex was observed, which increased with increasing surface concentration of IrOx on Fe2O3. The shift for the highest surface concentration of iridium tested amounts to 300 mV at 200 μA·cm−2 current density. The catalytic mechanism of the IrOx layer was unveiled by impedance spectroscopy measurements fitted to a physical model and can be explained on the basis of a highly capacitive layer, which enhances charge separation and stores photogenerated holes at Fe2O3, subsequently oxidizing water. These findings improve our understanding of the mechanism of water oxidation by heterogeneous Ir-based catalysts coupled to semiconductor electrodes.