Crystalline Strontium Iridate Particle Catalysts for Enhanced Oxygen Evolution in Acid

Iridium-based mixed metal oxide phases have been shown as promising electrocatalysts for the oxygen evolution reaction (OER) because of their ability to stabilize unique Ir-based surface sites with modulated properties and improved activity. Herein, the effect of crystal structure on the OER activity of Sr iridate particles is explored. Phase-pure Sr₄IrO₆, Sr₂IrO₄, and SrIrO₃ micrometer-scale particles show high activity toward the OER with the electrode area-based geometric activity increasing in the order of IrO_x < SrIrO₃ < Sr₄IrO₆ < Sr₂IrO₄ at a constant Ir mass electrode loading. Particularly, Sr₂IrO₄ displays superior activity and stability compared to commercial Ir/C (Premetek) nanoparticles, including more than an order of magnitude improvement in the catalyst surface area normalized specific activity. This translated to a similar Ir-based mass activity for Sr₂IrO₄ despite significantly larger average particles sizes (0.1–3 μm for Sr₂IrO₄ versus 2–3 nm for Ir/C) and a 40-fold improvement in Ir-based mass activity in comparison to IrO_x particles synthesized by a similar thermochemical procedure. During electrochemical testing of the Sr iridate materials, initial Sr leaching results in the formation of a stable Ir-rich catalyst surface with a modified electronic environment compared to Ir-only materials, potentially leading to enhanced OER activity. The superior intrinsic activity of Sr iridates illustrates the ability of surface-leached crystalline materials to stabilize high activity surface sites capable of significantly improving catalyst performance toward economical OER-based technologies.