Understanding the Effect of Monomeric Iridium(III/IV) Aquo Complexes on the Photoelectrochemistry of IrOx·nH2O-Catalyzed Water-Splitting Systems

Soluble, monomeric Ir(III/IV) complexes strongly affect the photoelectrochemical performance of IrOx·nH2O-catalyzed photoanodes for the oxygen evolution reaction (OER). The synthesis of IrOx·nH2O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through monomeric intermediates that were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions, the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)5(H2O)]2– and [Ir(OH)6]2–, respectively. These monomeric anions strongly adsorb onto IrOx·nH2O colloids but can be removed by precipitation of the colloids with isopropanol. The monomeric anions strongly adsorb onto TiO2, and they promote the adsorption of ligand-free IrOx·nH2O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode redox cycle and thus dr...