Monitoring the Structural\nChanges in Iridium Nanoparticles\nduring Oxygen Evolution Electrocatalysis with <i>Operando</i> X‑ray Total Scattering

Understanding the structure of nanoparticles under (electro)catalytic\noperating conditions is crucial for uncovering structure–property\nrelationships. By combining <i>operando</i> X-ray total\nscattering and pair distribution function analysis with <i>operando</i> small-angle X-ray scattering (SAXS), we obtained comprehensive structural\ninformation on ultrasmall (<3 nm) iridium nanoparticles and tracked\ntheir changes during oxygen evolution reaction (OER) in acid. When\nsubjected to electrochemical conditions at reducing potentials, the\nmetallic Ir nanoparticles are found to be decahedral. The iridium\noxide formed in the electrochemical oxidation contains small rutile-like\nclusters composed of edge- and corner-connected [IrO<sub>6</sub>]\noctahedra of a very confined range. These rutile domains are smaller\nthan 1 nm. Combined with complementary SAXS data analysis to extract\nthe particle size, we find that the OER-active iridium oxide phase\nlacks crystalline order. Additionally, we observe an iridium oxide\ncontraction under OER conditions, which is confirmed by <i>operando</i> X-ray absorption spectroscopy. Our results highlight the need for\nmultitechnique <i>operando</i> studies for a complete understanding\nof the electrochemically formed Ir oxide active in OER.