Structure–Stability Relationship of Amorphous IrO₂–Ta₂O₅ Electrocatalysts on Ti Felt for Oxygen Evolution in Sulfuric Acid

Abstract

A mixed layer of IrO2 and Ta2O5 deposited on a Ti substrate is an effective electrode for the catalysis of oxygen evolution reactions (OERs) in acidic solutions. In our previous work, IrO2–Ta2O5 catalysts supported on Ti fibers exhibited an amorphous structure, a large electrochemically active surface area (ECSA), and a high stability in acidic media. However, the local structure of amorphous OER electrocatalysts has not been studied extensively. In this study, the properties of an amorphous IrO2–Ta2O5 layer were analyzed through X-ray absorption spectroscopy (XAS) to understand the factors that contribute to its high OER stability. Herein, the addition of polyethylene glycol (PEG) to a precursor composed of H2IrCl6 and TaCl5 resulted in the formation of IrO2 nanoparticles with low chlorine residues even after thermal decomposition at a low temperature of 350 °C. X-ray diffraction and Raman spectroscopy results showed that the synthesized IrO2 nanoparticles were amorphous. However, from the XAS results, the local [IrO6] octahedron structure, which is similar to that of rutile IrO2 crystals, was revealed. During the preparation process, PEG assisted a complete ligand exchange from the [IrCl6] octahedron to the [IrO6] octahedron. This local structure of the IrO2 nanoparticles makes the amorphous IrO2–Ta2O5 layer on the Ti fibers stable during OER in acidic media. Furthermore, sintering is unlikely to occur during the synthesis because of the low thermal decomposition temperature of the precursor. As such, the synthesized amorphous IrO2 nanoparticles exhibited a large ECSA. In addition, the high intrinsic activity of the [IrO6] local structure resulted in a small Tafel slope. Thus, the amorphous IrO2 nanoparticles with an [IrO6] local structure are essential not only to achieve high activity but also high stability during the OER.

Tantalum Pentoxide: From Crystal Structures to Applications in Water Splitting

“…It has been reported that tantalum oxide can assist in solar-driven processes for water splitting. However, tantalum oxide alone does not exhibit sufficient enhancement in OER performance under acidic conditions and needs to be combined with other metals like IrO 2 . − Figure A and B summarizes recent Ta-based electrocatalysts for OER in both acidic and alkaline environments, demonstrating their potential for electrocatalytic water-splitting applications.…”

Amorphous versus nanocrystalline RuO2 electrocatalysts: activity and stability for oxygen evolution reaction in sulfuric acid

“…We have reported the coating of an IrO 2 -Ta 2 O 5 electrocatalyst on a Ti-fibre felt, which serves as a conductive substrate material. [27][28][29] The felt has a three-dimensional porous structure and high surface area. IrO 2 -Ta 2 O 5 /Ti-felt electrodes exhibit greater OER activity in aqueous 0.1 M H 2 SO 4 than electrodes supported on a conventional Ti plate due to their larger ECSAs.…”