北九州市大　天野先生との共同研究がJ. Phys. Chem. Cに受理されました．

A mixed layer of iridium dioxide (IrO₂) and tantalum pentoxide (Ta₂O₅) coated on titanium substrate is an effective electrocatalyst for oxygen evolution reaction (OER) in acidic solutions. The IrO₂-Ta₂O₅ layer supported on titanium fibers was amorphous and had a large electrochemically active surface area (ECSA), and also had high stability in OER. However, the local structure of the amorphous OER electrocatalyst has not been clarified so far. In this study, we analyzed the amorphous IrO₂-Ta₂O₅ layer by X-ray absorption spectroscopy (XAS) to clarify the factors of high OER stability. The addition of polyethylene glycol (PEG) to the precursor composed of H₂IrCl₆ and TaCl₅ resulted in the formation of IrO₂ nanoparticles with low chlorine residue after thermal decomposition even at a low temperature of 350 °C. We confirmed that the IrO₂ nanoparticles were amorphous by X-ray diffraction and Raman spectroscopy but exhibited a local [IrO₆] octahedron structure similar to that of rutile IrO₂ crystals by XAS. The reason why the amorphous IrO₂-Ta₂O₅ layer formed on the titanium fiber was stable during OER is that the [IrO₆] octahedron has high stability like rutile IrO₂ crystals. The addition of PEG assists the complete ligand exchange from the [IrCl₆] octahedron to the [IrO₆] octahedron in the preparation process. Since the thermal decomposition of the precursor proceeds at a low temperature, sintering is unlikely to occur, and the amorphous IrO₂ species has a large ECSA. In addition, the high intrinsic activity of the [IrO₆] local structure resulted in the small Tafel slope for OER. We found that the amorphous IrO₂ nanoparticles with [IrO₆] local structure are essential not only for high activity but also for high stability during OER.