Phase- and Surface Composition-Dependent Electrochemical Stability of Ir-Ru Nanoparticles during Oxygen Evolution Reaction

Abstract

The increasing scarcity of iridium (Ir) and its rutile-type oxide (IrO2), the current state-of-the-art oxygen evolution reaction (OER) catalysts, is driving the transition toward the use of mixed Ir oxides with a highly active yet inexpensive metal (IrxM1–xO2). Ruthenium (Ru) has been commonly employed due to its high OER activity although its electrochemical stability in Ir-Ru mixed oxide nanoparticles (IrxRu1–xO2 NPs), especially at high relative contents, is rarely evaluated for long-term application as water electrolyzers. In this work, we bridge the knowledge gap by performing a thorough study on the composition- and phase-dependent stability of well-defined IrxRu1–xO2 NPs prepared by flame spray pyrolysis under dynamic operating conditions. As-prepared NPs (IrxRu1–xOy) present an amorphous coral-like structure with a hydrous Ir-Ru oxide phase, which upon post-synthetic thermal treatment fully converts to a rutile-type structure followed by a selective Ir enrichment at the NP topmost surface. It was demonstrated that Ir incorporation into a RuO2 matrix drastically reduced Ru dissolution by ca. 10-fold at the expense of worsening Ir inherent stability, regardless of the oxide phase present. Hydrous IrxRu1–xOy NPs, however, were shown to be 1000-fold less stable than rutile-type IrxRu1–xO2, where the severe Ru leaching yielded a fast convergence toward the activity of monometallic hydrous IrOy. For rutile-type IrxRu1–xO2, the sequential start-up/shut-down OER protocol employed revealed a steady-state dissolution for both Ir and Ru, as well as the key role of surface Ru species in OER activity: minimal Ru surface losses (<1 at. %) yielded OER activities for tested Ir0.2Ru0.8O2 equivalent to those of untested Ir0.8Ru0.2O2. Ir enrichment at the NP topmost surface, which mitigates selective subsurface Ru dissolution, is identified as the origin of the NP stabilization. These results suggest Ru-rich IrxRu1–xO2 NPs to be viable electrocatalysts for long-term water electrolysis, with significant repercussions in cost reduction.