Assessment of Active Areas for the Oxygen Evolution Reaction on an Amorphous Iridium Oxide Surface

Abstract Electrocatalytic “green” production of hydrogen from water for sustainable energy provision schemes is currently inefficient due to the sluggish kinetics of the oxygen evolution reaction (OER) at the anodes of the electrolysers. In the case of acidic polymer electrolyte membrane electrolysers, iridium (Ir) oxide catalysts pose a promising compromise between good OER activity and stability. However, the structure-activity relations for these materials remain largely unknown because the surface of a “real” oxide catalyst under reaction conditions becomes amorphous. In order to contribute to the understanding of these systems, we use electrochemical scanning tunnelling microscopy under reaction conditions (‘noise’ or n-EC-STM). With this technique, active areas can be detected by an increased noise level of the STM signal compared to inactive sites. The n-EC-STM measurements are applied to an amorphous iridium oxide surface, which is formed during electrochemical cycling of Ir(111). By doing so, we can monitor OER activity in-situ while simultaneously assessing the surface morphology. In order to elucidate the active areas, step and terrace sites were quantitatively compared to each other. The measurements reveal that terraces, step sites and concavities lead to a similar noise level increase in the STM signal. We, thus, conclude that the OER on the amorphous extended iridium oxide surface shows little structure-sensitivity. Subsequently, we suggest that in contrast to, e.g., metallic Pt for the oxygen electro-reduction, the shape of amorphous IrOx nanoparticles in an acidic medium should not significantly influence the OER turnover frequency.