Electrocatalytic Oxidation of Water by OH − ‐ and H 2 O‐Capped IrO x Nanoparticles Electrophoretically Deposited on Graphite and Basal Plane HOPG: Effect of the Substrate Electrode **

Iridium oxide (IrO x ) is one of the most efficient electrocatalysts for water oxidation reaction (WOR). Here, WOR electrocatalysis by 1.6 nm IrO x nanoparticles (NPs) electrophoretically deposited onto spectroscopic graphite (Gr) and basal plane highly ordered pyrolytic graphite (HOPG) was studied as a function of NPs’ capping ligands and electrodeposition substrate. On Gr, OH − ‐ and H 2 O‐capped NPs exhibited close sub‐monolayer surface coverages and specific electrocatalytic activity of 18.9‐23.5 mA nmol −1 of Ir IV/V sites, at 1 V and pH 7. On HOPG, OH − ‐capped NPs produced films with a diminished WOR activity of 5.17±2.40 mA nmol −1 . Electro‐wettability‐induced changes impeded electrophoretic deposition of H 2 O‐capped NPs on HOPG, WOR currents being 25‐fold lower than observed for OH − ‐capped ones. The electrocatalysis efficiency correlated with hydrophilic properties of the substrate electrodes, affecting morphological and as a result catalytic properties of the formed IrO x films. These results, important both for studied and related carbon nanomaterials systems, allow fine‐tuning of electrocatalysis by a proper choice of the substrate electrode.