Laser induced oxidation Raman spectroscopy as an analysis tool for iridium-based oxygen evolution catalysts†

Abstract

The study of degradation behavior of electrocatalysts in an industrial context calls for rapid and efficient analysis methods. Optical methods like Raman spectroscopy fulfil these requirements and are thus predestined for this purpose. However, the iridium utilized in proton exchange membrane electrolysis (PEMEL) is Raman inactive in its metallic state. This work demonstrates the high oxidation sensitivity of iridium and its utilization in analysis of catalyst materials. Laser induced oxidation Raman spectroscopy (LIORS) is established as a novel method for qualitative, chemical and structural analysis of iridium catalysts. Differences in particle sizes of iridium powders drastically change oxidation sensitivity. Oxidation of the iridium powders to IrO₂ occurred at a laser power density of 0.47 ± 0.06 mW μm⁻² for the 850 μm powder and at 0.12 ± 0.06 mW μm⁻² and 0.019 ± 0.015 mW μm⁻² for the 50 μm and 0.7–0.9 μm powders respectively. LIORS was utilized to assess possible deterioration of an iridium electrocatalyst due to operation under electrolysis. The operating electrocatalyst exhibited higher oxidation sensitivity, suggesting smaller iridium particle size due to catalyst dissolution. Peak shifts of the IrO₂ signal were utilized to assess differences in transformation temperatures. The operated electrocatalyst transformed to IrO₂ at lower temperature (8 cm⁻¹ redshift) relative to the pristine catalyst (10 cm⁻¹ redshift), demonstrating that pre-oxidation of the iridium to amorphous IrO_x during electrolysis diminishes the energy barrier needed for IrO₂ formation. Thus, LIORS can be utilized as a straightforward screening method for the analysis of iridium electrocatalysts in the industrial application of PEMEL.