Anodic deposition of colloidal iridium oxide thin films from hexahydroxyiridate(IV) solutions

A facile, in-situ deposition route to stable iridium oxide (IrO _x·nH₂O) nanoparticle thin films from [Ir(OH) ₆]^2- solutions is reported. The [Ir(OH)₆] ^2- solution, made by alkaline hydrolysis of [IrCl₆] ^2-, is colorless and stable near neutral pH, and forms blue IrO _x·nH₂O nanoparticle suspensions once it is adjusted to acidic or basic conditions. IrO_x·nH₂O nanoparticle thin films are grown anodically on glassy carbon, fluorine-doped tin oxide, and gold electrodes by electrolyzing [Ir(OH)₆] ^2- solutions at +1.0-1.3 V versus Ag/AgCl. The thickness of the IrO_x·nH₂O films can be controlled by varying the concentration of [Ir(OH)₆]^2-, the deposition potential, and/or the deposition time. These thin films are stable between pH 1 and 13 and have the lowest overpotential (η) for the oxygen evolution reaction (OER) of any yet reported. Near neutral pH, the Tafel slope for the OER at a IrO_x·nH₂O film/Au rotating disk electrode was 37-39 mV per decade. The exchange current density for the OER was 4-8 × 10^-10 A cm^-2 at a 4 mC cm^-2 coverage of electroactive Ir.