Ir(hkl) Surface Electrochemistry in a Nonadsorbing Acidic Medium

The fundamental properties of electrochemical materials depend on the multiple and often complex interactions between electrode surface sites and electrolyte species at the electrochemical interface. Despite the use of iridium in electrolyzer systems, much of its surface electrochemistry remains underexplored. This study investigates the surface electrochemistry of the Ir(111), Ir(100), and Ir(110) surfaces in acidic media. Using cyclic voltammetry and CO charge displacement experiments, we establish the charge states and adsorbate coverages as a function of the electrode potential, revealing the presence of hydrogen and hydroxyl coadsorption at low potentials on (111), and almost no coverage of H_ad on the (110) facet. In situ Shell Isolated Nanoparticle Enhanced Raman Spectroscopy experiments provide direct evidence of the formation of key adsorbate species, such as hydrogen, hydroxyl, and oxygen, but most importantly, their interactions with interfacial water, confirmed by Density Functional Theory calculations. Our findings highlight the role of coadsorption with microkinetic adsorption voltammetry simulations, corroborating the influence of lateral interactions on adsorption dynamics. These insights help us understand the trends in hydrogen oxidation and evolution and oxygen reduction reactions, where both adsorbate coverage and the interfacial water network determine Ir(hkl) reactivity. Our results underscore the importance of interfacial water and hydrogen bonding networks in shaping the electrochemical behavior on Ir surfaces, refining our baseline understanding of the Ir surface electrochemistry necessary for the targeted use of advanced Ir-based electrochemical materials.