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

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 (IrₗM₁-ₗO₂). Ruthenium (Ru) has been commonly employed due to its high OER activity although its electrochemical stability in Ir-Ru mixed oxide nanoparticles (IrₗRu₁-ₗO₂ 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 IrₗRu₁-ₗO₂ NPs prepared by flame spray pyrolysis under dynamic operating conditions. As-prepared NPs (IrₗRu₁-ₗOₘ) 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 IrₗRu₁-ₗOₘ NPs, however, were shown to be 1000-fold less stable than rutile-type IrₗRu₁-ₗO₂, where the severe Ru leaching yielded a fast convergence toward the activity of monometallic hydrous IrOₘ. For rutile-type IrₗRu₁-ₗO₂, 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 IrₗRu₁-ₗO₂ NPs to be viable electrocatalysts for long-term water electrolysis, with significant repercussions in cost reduction.