Iridium Oxidation as Observed by Surface Interrogation Scanning Electrochemical Microscopy

The formation of surface oxides on most metal, including noble metal, electrodes occurs before the onset of the oxygen evolution reaction (OER). An understanding of changes in surface structure and composition caused by the oxidation process is important to the field of electrocatalysis of the OER. In this work, the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of −OH(ads) and −H(ads) species generated at the surface of polycrystalline iridium ultramicroelectrodes (UMEs) in 2 M NaOH. This system was selected because the iridium oxides are among the most effective and stable electrocatalysts for the OER. We introduce the redox pair Fe(III/II)−TEA as a mediator for stable surface interrogation at pH ≥ 12. This is the first time that SI-SECM experiments have been carried out at such an extreme pH. Monolayer coverage of −OH(ads) and −H(ads) was Qθ=1,OH = 456 ± 2.0 μC cm−2 and Qθ=1,H = 224.2 ± 0.2 μC cm−2, respectively. At potentials more positive than 0.20 V, a clear change in the kinetics of the chemical reaction between Fe(II)−TEA and the hydrous oxides of Ir was observed. The kinetic results are interpreted with the aid of a simulation model based on finite element analysis (FEA). We present evidence that Ir(III), Ir(IV), and Ir(V) coexist on the surface of Ir during the OER under these conditions.