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

Abstract

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 IrO x nanoparticles acting as efficient cocatalyst for water oxidation can be photoelectrochemically deposited from hexahydroxoiridate solutions into the porous structure of TiO 2 -PH (polyheptazine, "graphitic carbon nitride") hybrid photoanodes for water photooxidation. As compared to photoanodes loaded with IrO x by the conventional colloidal deposition method, hybrid photoanodes with photodeposited IrO x exhibit significantly enhanced dioxygen evolution under long-term irradiation with visible light (λ > 420 nm). Photocurrent transient measurements show that the undesired accumulation of holes in the TiO 2 -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 ∼6.

Enabling visible-light water photooxidation by coordinative incorporation of Co(II/III) cocatalytic sites into organic-inorganic hybrids: quantum chemical modeling and photoelectrochemical performance

“…Figure 6 shows the photocurrent response and oxygen evolution of TiO 2 -PH-Co(II) under visible light (λ > 420 nm) irradiation at 1.06 V versus RHE. We note that at relatively low bias potentials (~0.2 V vs. RHE), photocurrents were detectable but very small due to the fast back electron transfer (primary recombination) [29]. A higher bias potential was therefore applied in order to obtain larger photocurrents and detectable amounts of oxygen.…”

“…We have shown that porous TiO 2 -PH photoanodes loaded with IrO x or CoO x acting as oxygen-evolving cocatalysts are capable of visible 2 O.V. Khavryuchenko et al light-driven oxygen evolution in aqueous electrolytes [22][23][24][25][26]29]. The presence of a cocatalyst was absolutely necessary since no dioxygen evolution is observed at TiO 2 -PH photoanodes without cocatalyst.…”

“…The presence of a cocatalyst was absolutely necessary since no dioxygen evolution is observed at TiO 2 -PH photoanodes without cocatalyst. In the absence of an efficient water oxidation cocatalyst, the photogenerated holes cannot oxidize water, accumulate in the surface PH layer, and eventually induce photodegradation of the hybrids by cleaving the bonds between PH and TiO 2 [29]. The deposition of the cocatalyst and the quality of its coupling to the TiO 2 -PH light absorber is therefore of crucial importance.…”

“…The deposition of the cocatalyst and the quality of its coupling to the TiO 2 -PH light absorber is therefore of crucial importance. The cocatalyst deposition has been so far typically carried out by colloidal deposition or photoelectrochemically induced deposition and precipitation of IrO x or CoO x nanoparticles into the porous structure of the TiO 2 -PH photoanode [22][23][24][25][26]29]. One disadvantage of using metal oxide nanoparticles as cocatalysts is that the nanoparticles themselves can absorb light and thus block light absorption by the absorber [30].…”

The effect of crystallinity on photocatalytic performance of Co3O4 water-splitting cocatalysts

“…11 In the case of OER, a clear dominant cocatalyst has yet to be established. Several inorganic cocatalysts, such as IrOx, 12 CoOx, 10 Co3O4, 13 NiOx, 9,13 and RuO2, 14 were found to enhance the photocatalytic reaction.…”