Water Oxidation at Hematite Photoelectrodes with an Iridium-Based Catalyst

Abstract

The iridium complex [Cp*Ir(H 2 O) 3 ](SO 4 ) was used as an organometallic source for the electrodeposition of iridium oxide onto Fe 2 O 3 . 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 IrO x on Fe 2 O 3 . 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 IrO x 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 Fe 2 O 3, subsequently oxidizing water. These findings improve our understanding of the mechanism of water oxidation by heterogeneous Ir-based catalysts coupled to semiconductor electrodes.