Size-Dependent Core–Shell Fine Structures and Oxygen Evolution Activity of Electrochemical IrO_<i>x</i> Nanoparticles Revealed by Cryogenic Electron Microscopy

Electrochemically oxidized amorphous iridium oxides (IrO_<i>x</i>) offer significantly improved electrocatalytic activities on the oxygen evolution reaction (OER) compared to crystalline IrO₂, yet the origin of their decent activity and their size-dependent properties have not been fully understood. An important argument is the formation of deprotonated oxygen species not only at the topmost surface but also at the near surface, which creates an electrophilic character that activates the OER electrocatalysis. However, high spatial resolution identification of the electrophilic oxygen species remains unachieved. We address this hitherto-unresolved problem on size-selected electrochemical IrO_<i>x</i> nanoparticles (NPs) by using cryogenic scanning transmission electron microscopy combined with electron energy loss spectroscopy, which enables simultaneous atomic detection of the near surface compositional and electronic structures with minimal damage that are further correlated with their size-dependent OER activities. Depending on the particle size, the electrochemical IrO_<i>x</i> NPs showed distinctly different core-shell fine structures ranging from amorphous and hydrous IrO_<i>x</i>H_<i>y</i> NPs to a "metallic Ir core/sub-stoichiometric IrO_<i>x</i> interlayer/amorphous IrO_<i>x</i>H_<i>y</i> shell" NP structure. Moreover, the formation of deprotonated, electrophilic oxygen is directly identified at the substoichiometric IrO_<i>x</i> interface layer. These features account for a previously unestablished particle size effect of the electrochemical IrO_<i>x</i> NPs, showing increasing water oxidation reactivity with an increasing nanoparticle size. Our results provide important insights into how subsurface oxygen chemistry controls the surface reactivity in the nanoscale Ir-based OER electrocatalysts.