Highly Efficient Electrocatalytic Water Oxidation by a Transparent Ordered Mesoporous Film of Intermediate IrO_x(OH)_y

Artificial photosynthesis technologies require transparent thin film catalysts working efficiently for water oxidation in neutral pH media for penetration of light to the inner light-harvesting layer. However, only a few catalysts meet the requirement so far, and developing such catalysts is a still great challenge. A transparent 2D hexagonally ordered mesoporous film (ca. 70–80 nm thickness) of intermediate IrOx(OH)y on fluorine-doped tin oxide (FTO) electrodes was developed to work efficiently for electrocatalytic water oxidation at pH 7.0. A homogeneous Ir-precursor solution containing a polymeric surfactant, Pluronic F127, was spin-coated over a FTO electrode, and then the as-made film was calcined at 300 °C to give an adherent intermediate IrOx(OH)y film but containing charred organic/carbonaceous residues due to the incomplete combustion of Pluronic F127. A simple UV-ozone treatment strategy has been established to remove the organic/carbonaceous residues without hampering the intermediate IrOx(OH)y film. This strategy resolved the synthesis dilemma of previous approaches with calcination at ≥400 °C that resulted in a mesoporous structure but converted the intermediate IrOx(OH)y to a lesser active amorphous/crystalline IrOx in the film. A 11.2-fold higher specific surface area was attained by the 2D hexagonal mesoporous IrOx(OH)y film (125 m2 g–1) compared to the nanoporous IrOx(OH)y film (4 m2 g–1). The observed catalytic current (Icat) of the mesoporous IrOx(OH)y electrode was 2.6-fold higher than that of the mesoporous IrOx electrode (calcined at 400 °C) due to the 2.8-fold higher intrinsic catalytic activity. Icat of the mesoporous IrOx(OH)y electrode was 1.5-fold higher than that of the nanoparticle IrOx(OH)y electrode due to the 1.2-fold increased amount of the electroactive Ir sites and 1.2-fold higher intrinsic catalytic activity. Electrochemical impedance data revealed that charge transfer resistances (29.6–61.9 Ω) for bulk charge transport through the mesoporous IrOx(OH)y film were lower than those (42.3–169 Ω) through the mesoporous IrOx electrode, which are explained by the more efficient electron transport in the former film. The mesoporous IrOx(OH)y film provided a low overpotential (η) of 0.24 V and Tafel slope of 39 mV dec–1 for water oxidation at pH 7.0, which are comparable with those (η = 0.20–0.25 V and Tafel slope of 40–50 mV dec–1) for hitherto-reported highly efficient IrOx electrocatalysts.