State-of-the-art proton-exchange membrane water electrolyzers (PEMWEs) use nanoparticulate iridium-based anode electrocatalysts owing to their high activity and durability in catalyzing the oxygen evolution reaction (OER). To attain high mass activity and reduce the required Ir loading, the synthesis of agglomeration-free Ir-based ultrasmall nanoparticles (USNPs) is essential. In this work, we report the base media precipitation of suspended Ir nanoparticles using a Ni(OH)2 template, followed by pH-selective removal of the Ni component to achieve Ir USNPs. The pH-controlled washing step, where selective removal of Ni(OH)2 occurs through the formation of [Ni(NH3)6]2+ as a water-soluble complex, was found to be crucial for controlling the Ni content and morphology of the produced Ir USNPs. Detailed structural and morphological analyses of the Ir USNPs carried out using X-ray and microscopic techniques confirmed their ultrasmall structure (size <2 nm) and metallic nature. The synthesized Ir USNPs exhibited higher electrocatalytic OER activity and stability than the corresponding values for a commercial IrO2 benchmark electrocatalyst. This concept may also be adopted as a general strategy for the synthesis of different platinum-group-metal-based USNPs.
