Epitaxial Stabilization and Oxygen Evolution Reaction Activity of Metastable Columbite Iridium Oxide

Non-rutile polymorphs of binary iridium oxide such as columbite IrO2 (α-IrO2) are promising candidates for highly active acid-stable oxygen evolution reaction (OER) catalysts, yet their synthesis has been challenging due to the dominant thermodynamic stability of rutile IrO2 (R-IrO2). Here, we report the growth of α-IrO2 via epitaxial thin films using pulsed laser deposition. We observe that, in competition with R-IrO2 (100), the films can be optimized to be predominantly (100)-oriented α-IrO2. Surprisingly, the activity of α-IrO2 shows a large discrepancy of ∼0.2 V in the overpotential compared to its predicted activity, which is resolved via theoretical calculations to be a crystal orientation effect. This demonstrates that the electrocatalytic activity can be significantly varied upon crystal orientation, a parameter that is difficult to control in conventional polycrystalline systems but accessible in epitaxial thin films. In total, this study demonstrates epitaxial thin film growth as a powerful technique, which can overcome large energetic instabilities on the order of ∼300 meV to stabilize metastable material structures inaccessible by bulk synthesis. This provides unique opportunities to effectively identify the atomic structure of active catalysts by combining investigations of metastable materials with theoretical predictions. Structures and energies available at https://www.catalysis-hub.org/publications/LeeEpitaxial2020