Improving the Performance of Hybrid Photoanodes for Water Splitting by Photodeposition of Iridium Oxide Nanoparticles

The efficient coupling between light-harvesting absorbers and cocatalysts allowing for chemical transformation along multielectron pathways is of fundamental importance for the development of solar-fuel-producing photochemical systems. Herein we demonstrate that IrOx nanoparticles acting as efficient cocatalyst for water oxidation can be photoelectrochemically deposited from hexahydroxoiridate solutions into the porous structure of TiO2-PH (polyheptazine, graphitic carbon nitride) hybrid photoanodes for water photooxidation. As compared to photoanodes loaded with IrOx by the conventional colloidal deposition method, hybrid photoanodes with photodeposited IrOx exhibit significantly enhanced dioxygen evolution under long-term irradiation with visible light (lambda > 420 nm). Photocurrent transient measurements show that the undesired accumulation of holes in the TiO2-PH absorber is significantly reduced due to improved coupling between the absorber and the photodeposited cocatalyst. This decreases significantly the recombination rate, leads to more efficient dioxygen evolution, and improves the stability against photocorrosion. Photocurrent measurements under potentiodynamic conditions revealed that at low bias potentials (<0.6 V vs RHE) the photoconversion efficiency of hybrid photoanodes is limited by fast primary back electron transfer and by reduction of Ir(IV) to Ir(III). The performance and stability of hybrid photoanodes are also found to be drastically influenced by the solution chemistry (electrolyte composition and pH). The highest photoconversion efficiency was observed in sulfate-based electrolytes at pH similar to 6.