Effect of metal substitution on the intrinsic activity of iridium-based oxides for the oxygen evolution reaction

Iridium-based oxides are among the most promising catalysts for the acidic oxygen evolution reaction (OER) owing to their high catalytic activity and stability. Substituting iridium with earth-abundant elements could lower costs and potentially boost its intrinsic activity even further; however, no unambiguous structure-activity relationships describing the physical origins of the effect of the substituent for this class of electrocatalysts have been established. In this work, we utilized a series of IrO _<i>x</i> (:M) nanoparticle catalysts to correlate their <i>in situ</i> structural changes with intrinsic OER activity. We observe that IrO _<i>x</i> (:M) with M = W and In feature a significantly higher Ir-mass-normalized OER activity than IrO _<i>x</i> , however the activity enhancements have a different origin. While the increased activity of IrO _<i>x</i> : In stems from a higher number of electrochemically active iridium centers (due to the leaching of indium), IrO _<i>x</i> : W features a higher intrinsic OER activity compared to IrO _<i>x</i> , due to electronic effects of W on neighboring Ir/O sites. Furthermore, <i>operando</i> electrochemical mass spectrometry experiments and density functional theory (DFT) calculations revealed that the enhanced OER activity of IrO _<i>x</i> (:M) does not originate from a promotion of the lattice oxygen coupling mechanism, but is instead associated with a facilitated conventional adsorbate evolution mechanism.