Highly active nano-sized iridium catalysts: synthesis and operando spectroscopy in a proton exchange membrane...

A stable and cost effective oxygen evolution reaction (OER) catalyst is crucial for the large-scale market penetration of proton exchange membrane (PEM) water electrolyzers. We show that the synthesis of iridium nanoparticles in either low purity ethanol or water, or in the absence of a surfactant, is detrimental to the electrocatalytic properties of the materials. Adding NaBH₄ in excess improves the purity of the catalyst enhancing the OER activity up to 100 A g_Ir^-1 at 1.51 V vs. RHE, the highest value reported so far for high purity Ir nanoparticles. The measured OER activity correlates with the capacitive current rather than with the charge corresponding to the Ir^III/Ir^IV oxidation peak. Operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on membrane electrode assemblies (MEAs) with the synthesized catalysts reveals a metallic core surrounded by a thin layer of Ir^III/IV oxides/hydroxides. Oxidation of Ir^III leaves behind a porous ultrathin layer of Ir^IV oxides/hydroxides, which dominate the surface during the OER, while Ir^V was not detected.