Visualizing Potential‐Induced Pitting Corrosion of Ultrathin Single‐Crystalline IrO₂(110) Films on RuO₂(110)/Ru(0001) under Electrochemical Water Splitting Conditions

Sophisticated IrO 2 (110)‐RuO 2 (110)/Ru(0001) model electrodes are employed in the oxygen evolution reaction (OER) under acidic conditions. The potential‐induced pitting corrosion of such electrodes is confirmed by a variety of experimental techniques, including scanning electron microscopy (SEM), time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), and operando scanning flow cell‐inductively coupled plasma mass spectrometry (SFC‐ICP‐MS). The structure of the pits is reminiscent of a cylinder (evidenced by focused ion beam scanning electron microscopy: FIB‐SEM), where the inner surface of the pits is covered by hydrous RuO 2 (cyclic voltammetry, ToF‐SIMS) that is formed by electrochemical oxidation of the metallic Ru(0001) substrate. The time evolution of the corrosion process at a fixed electrode potential (1.48 V vs. SHE) is followed via cyclic voltammetry and SEM. The passivating IrO 2 (110) layer results in an “induction period” for the pit growth that is followed by rapid corrosion of the RuO 2 (110)/Ru(0001) substrate. The observed narrow and time‐independent size distribution relative to the mean size of the pits is attributed to a sluggish removal of the corrosion products by diffusion across the cracks of the pits covering IrO 2 layer, leading to steady state corrosion during a total polarization time of 20 to 60 minutes.