Oxygen Evolution Electrocatalysis in Acids: Atomic Tuning of the Stability Number for Submonolayer IrO_x on Conductive Oxides from Molecular Precursors

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Proton-exchange-membrane water electrolyzers (PEMWEs) produce high-purity H₂, withstand load fluctuations, and operate with a pure-water feed but require platinum-group-metal catalysts for durability, such as IrO₂ and Pt, due to the acidic environment. At the anode, the slow oxygen evolution reaction (OER) requires a high overpotential to achieve relevant current densities (>2 A·cm^–2) even with a high loading of IrO₂. Using a simple commercial 1,5-cyclooctadiene iridium chloride dimer precursor, we synthesized submonolayer-thick IrO_x on the surfaces of conductive metal oxides to make every Ir atom available for catalysis and reach the ultimate lower limit for Ir loading. We show that the reaction on Sb/SnO₂ and F/SnO₂ conductive oxides is surface-limited and that a continuous Ir–O–Ir network provides improved stability and activity. We cover IrO_x with a thin layer of acid-stable TiO_x by atomic-layer deposition. The effects of TiO_x on the catalyst’s performance were assessed by inductively coupled plasma mass spectrometry (ICP-MS) coupled in situ with an electrochemical flow cell and ex situ by X-ray photoelectron spectroscopy. Tuning the binding environment of IrO_x by TiO_x addition enhances the intrinsic activity of the active sites, simultaneously accelerating the dissolution of the catalyst and the metal-oxide support. We illustrate the interplay between the support, catalyst, and protection-layer dissolution with OER activity, and highlight the effects of annealing to densify the TiO_x protection layer on stability/activity. These ultrathin supported Ir-based catalysts do not eliminate the long-standing issue of the catalyst and support instability during OER in acids, but do provide new insight into the catalyst–support interactions and may also be of utility for advanced spectroscopic investigations of the OER mechanism.