Understanding the stabilization effect of the hydrous IrOx layer formed on the iridium oxide surface during the oxygen evolution reaction in acid

Abstract

Various iridium-based catalysts have been intensively developed for the oxygen evolution reaction (OER) in acid due to their promising catalytic activity and stability. However, there have been several reports about the surface amorphization resulting in quick formation of a hydrous IrO_x layer during the OER, whereas the formation mechanism and catalytic function of such a hydrous IrO_x layer are not clear so far. In this work, BaIrO₃ and SrIrO₃ are employed as model catalysts for the investigation of surface degradation behaviors in acid. BaIrO₃ displays quick fading of the OER activity due to the formation of BaSO₄ deposits on its surface, which not only block the reactive Ir sites but also prevent the formation of the hydrous IrO_x layer. In contrast, the surface of SrIrO₃ quickly undergoes amorphization, resulting in stable catalytic activity and ∼8 times suppressed Ir dissolution compared to that of BaIrO₃. We therefore hypothesize that the amorphous IrO_x layer may serve as a protective layer, which is beneficial for achieving a dynamic balance at the catalyst–electrolyte interface effectively by preventing the formation of high-valence Ir^V species and their dissolution. Results of this work provide an in-depth understanding of the surface degradation processes involved during the OER in acid.