Increasing the surface area of iridium oxide as an oxygen evolution reaction catalyst

Abstract

Hydrogen is a promising clean energy carrier for transportation and chemical industries, helping reduce fossil fuel emissions and combat climate change. Proton exchange membrane water electrolysis (PEMWE) efficiently produces high-purity hydrogen with minimal CO₂ emissions when powered by renewable energy sources like solar and wind. However, the oxygen evolution reaction (OER) at the PEMWE anode is kinetically challenging and requires significant amounts of noble metal oxide electrocatalysts. Iridium oxide (IrO₂) is the preferred OER catalyst due to its high activity and corrosion resistance, but its high cost and scarcity hinder widespread use. To overcome this, researchers are exploring ways to reduce iridium content, such as using support materials to improve efficiency and extend the catalyst duration. This study focused on exploring different pathways to increase the surface area of iridium oxide-based and mixed oxide catalysts from metallic iridium supplied by local mines. Modifications were applied using various oxidising agents in the Adams’ fusion method to adjust the catalyst’s oxidation state, porosity, surface area, and morphology. Post-treatment processes involving different cooling rates were applied to modify the phase and crystallinity of metal oxides. Dispersing IrOx on high-surface-area support material and mixed oxide to increase the surface area and OER performance. The physicochemical properties of the in-house nanomaterials were confirmed using various analytical techniques. The X-ray spectroscopy (XRD) and high-resolution-scanning transmission electron microscopy (HR-STEM) analyses revealed that the synthesised catalysts were amorphous/low crystalline nanomaterials with an average particle size range ± 2 - 5 nm. X-ray photoelectron spectroscopy (XPS) showed that the in-house are in the Ir3+/ Ir4+ oxidation state. According to the scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), the nanomaterials exhibit different iridium loading. The stability and activity of the iridium oxide-based nanomaterials for oxygen evolution reaction (OER) were evaluated using the rotating disk electrode (RDE) technique in an acidic electrolyte. The highest OER performance was obtained from the catalyst produced by using sodium peroxide oxidising agent compared to barium peroxide. Different cooling rates did not result in a significant increase in catalyst surface area. A notable increase in surface area was achieved due to the usage of barium peroxide from 11.0 m2 /g to 31.7 m2 /g, while for mixed-oxide catalyst 18.4 m2 /g was obtained. Furthermore, the electrochemical stability of the catalyst was achieved for IrOx-(Ba) and TaIrOx catalysts, however at the expense of OER activity. The IrOx-(Na) catalyst outperformed both IrOx-(Ba) and TaIrOx mixed oxide with greater mass-specific activity at 1.525 V. Even though the IrOx-(Na)-(R) catalyst had the highest OER compared to all catalysts it was the least stable. For TaIrOx, XPS revealed that Ta is present as Ta5+ (Ta2O5) at the catalyst surface, this likely affected OER activity since it is a non-conductive material.