The development of efficient and durable oxygen evolution reaction (OER) electrocatalysts is critical for advancing proton exchange membrane water electrolyzers (PEMWEs). This study presents a nanostructured iridium oxide anchored on high-surface-area carbon (IrOx/C) OER electrocatalyst, synthesized through ultrasonication-assisted deposition of hydrolysis-derived colloidal IrOx nanoparticles onto the carbon support. The as-prepared IrOx/C exhibits a mass activity exceeding five times that of unsupported benchmark commercial IrO2, attributed to its uniform nanoparticle distribution and enhanced surface chemistry. Post-synthesis heat-treatments at two different temperatures (200 or 300 °C) were employed to stabilize the catalyst structure. Notably, the sample treated at 200 °C retained 84% of its initial activity after accelerated stress testing, outperforming the commercial counterpart, which retained 67 % under identical conditions. Comprehensive structural and morphological analyses revealed that the heat-treatment increased the IrOx particle size and decreased the Ir³⁺/Ir⁴⁺ ratio while leaving the amorphous nature of IrOx unaffected, which combinedly led to the improved durability. These findings offer valuable insights into designing stable low-iridium OER electrocatalysts for acidic PEMWE applications.
