Stability Studies of Single-Crystalline IrO2(110)-Based Model Electrodes under Electrochemical Conditions

Abstract

Acidic water electrolysis utilizing polymer electrolyte membrane (PEM) electrolyzers is a promising way to store excess electric energy from intermittent renewable sources (solar, wind) in the chemical bond of molecular hydrogen H2. The performance of PEM electrolyzers is limited by the oxygen evolution reaction (OER) at the anode due to sluggish kinetics and (in)stability issues. Iridium oxide IrO2 is the state-of-the-art electrocatalyst for the OER under acidic conditions owing to its adequate activity and stability. However, IrO2 corrodes as well under the strongly oxidizing conditions at the anode, albeit slowly. There is a variety of studies investigating IrO2-based electrocatalyst materials, providing valuable insight into their stability. Yet, clear-cut experiments allowing for an atomic-scale understanding of the corrosion and dissolution processes under OER conditions are missing. Hence, it is the main objective of the present thesis to gain insight on these processes on a microscopic scale. For this reason, well-defined single-crystalline IrO2(110) films are employed as model electrodes for stability studies under OER conditions. Furthermore, their stability under cathodic conditions in the hydrogen evolution reaction (HER) potential region is studied, as IrO2-based electrodes may be subject to cathodic conditions due to the intermittent operation of PEM electrolyzers. In order to obtain an overall picture of the stability of IrO2(110), an experimental approach comprising in situ or operando (surface X-ray diffraction (SXRD), X-ray reflectivity (XRR), inductively coupled plasma mass spectrometry (ICP-MS)) and ex situ techniques (scanning electron microscopy (SEM), scanning tunneling microscopy (STM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS)) is introduced and explained. In a proof-of-principle study of the electrochemical reduction of RuO2(110)/Ru(0001) under cathodic conditions the suitability of this experimental approach is positively demonstrated. Next, under OER conditions, IrO2(110)-RuO2(110)/Ru(0001) model electrodes are shown to degrade via potential-induced pitting corrosion starting at surface grain boundaries rather than dissolution of IrO2(110). A subsequent study utilizing IrO2(110) films supported on an inert TiO2(110) substrate reveals the former to be very stable under OER conditions: the film thickness is preserved within 0.1 monolayer equivalents upon ≈26 h at a current density of 50 mA·cm-2. Complementary ICP-MS experiments indicate an influence of the operation conditions (steady state vs. dynamic) on the stability. Under cathodic conditions in the HER potential region the IrO2(110)-TiO2(110) model electrodes are shown to be very stable as well: down to a cathodic potential of -1.20 V vs. the reversible hydrogen electrode the IrO2(110) film does not lose its crystallinity, in contrast to RuO2(110). However, fitting of the SXRD data indicates the incorporation of protons in the bulk of the film. An electrochemical reduction of IrO2(110) to hydrous IrO2 or metallic Ir, though, can be excluded. In conclusion, single-crystalline IrO2(110) is shown to be very stable both under anodic and cathodic conditions.