Thermolytic formation and microstructure of IrO 2 + Ta 2O 5 mixed oxide anodes from chloride precursors

Abstract

The thermolytic formation of IrO 2+Ta 2O 5 mixed oxides from chloride precursors is studied by thermogravimetry (TGA) and differential thermal analysis (DTA). The structure and morphologies of the corresponding oxide films coated on titanium bases are determined by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), respectively. The experimental results showed that, as a result of the interaction between Ir and Ta components, especially, the formation of solid solution phases during the thermolysis processes, the oxidative dissociation of the H 2IrCl 6+TaCl 5 mixture is facilitated. The catalytic effect reached the maximum at a nominal IrO 2 content of 70 mol% in the expected product, i.e. IrO 2+Ta 2O 5 mixed oxides, accompanied by the highest solid solubility between the two oxides and the finest rutile-structured crystalline grains in the oxides. For the mixed precursors with a low iridium content (e.g. 10 mol% nominal IrO 2 in IrO 2+Ta 2O 5) or a low tantalum content (e.g. 80 mol% nominal IrO 2), however, the decomposition of the major component is inhibited by the minor one at high temperatures (610–800 °C). The results show that the solid solution at low Ir contents (<30 mol% IrO 2) is unstable since it decomposes at high temperatures (≥750 °C). Two or more IrO 2 based rutile-constructed solid solution phases are thermolytically formed from the mixed precursors with nominal IrO 2 contents ≥30 mol%. The rutile-structured phases stably exist only in the case of IrO 2 contents ≥60 mol%.