Surface Ir⁺⁵ Formation as a Universal Prerequisite for O₂ Evolution on Ir Oxides

Abstract

The large-scale deployment of polymer electrolyte water electrolysis (PEWE) is largely limited by the use of O 2 evolution reaction (OER) catalysts based on scarce and expensive iridium in PEWE anodes. The ensuing need for betterperforming, Ir-based OER catalysts requires an improved understanding of the relation between these materials' activity and their physicochemical operando properties. To shed light on this matter, here, we employed operando modulation excitation X-ray absorption spectroscopy to determine the oxidation state of surface Ir in a range of Ir oxides with different surface compositions, crystal structures, and OER activities. Our results reveal that, irrespectively of these diverging properties, the surface Ir in all catalysts systematically undergoes a linear, potential-driven oxidation that stabilizes once a +5 state is reached. The completion of this surface oxidation process is then showed to correlate with the onset of O 2 evolution, thus strongly hinting at the involvement of Ir in oxidation states ≥+5 in the OER and indirectly discarding those mechanisms that do not consider such states as a part of the reaction sequence.

Role of Electrolyte pH on Water Oxidation for Iridium Oxides

“…At potentials >1.4 V RHE , the Ir oxidation state begins to plateau, with the oxidation state being higher in alkaline electrolytes compared to acid. Prior work has suggested the presence of higher oxidation states of iridium at this potential region; , however, recent reports also suggest that the redox transitions at a high potential correspond, in part, to the formation of holes on surface oxo groups. ,, The trends observed in the XANES spectra are also consistent with the shortening of the Ir–O bond with increasing potential, as observed in the X-ray absorption fine structure (EXAFS). The Fourier transforms of EXAFS spectra and the simulations of the spectra (Figures S14–S15 and Table S2) revealed the presence of characteristic Ir–O bond distances and very weak Ir–Ir interaction peaks, showing a short-range disorder structure of amorphous IrO x .…”

“…Combining the operando XANES with our previous DFT calculations, 42 oxidation state being higher in alkaline electrolytes compared to acid. Prior work has suggested the presence of higher oxidation states of iridium at this potential region; 46,47 however, recent reports also suggest that the redox transitions at a high potential correspond, in part, to the formation of holes on surface oxo groups. 35,48,49 The trends observed in the XANES spectra are also consistent with the shortening of the Ir−O bond with increasing potential, as observed in the X-ray absorption fine structure (EXAFS).…”

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

“…63,409−412 Also, operando Ir L-edge experiments indicate much lower redox activity of crystalline oxides, 413 basically mimicking their reduced pseudocapacitance. 414 Similarly, effective charge accumulation with amorphous IrO x was also evidenced by in situ evanescent wave spectroscopy. 415 Some controversy surrounds Ir L 3 -edge experiments that suggest iridium reduction under OER potentials.…”

Iridium Oxide Coordinatively Unsaturated Active Sites Govern the Electrocatalytic Oxidation of Water

“…To date, operando studies of the electronic structure of Ir‐based OER catalysts have focused on X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption spectroscopy (XAS) respectively. [ 10,19,20–22,24–32 ] Such studies have shown Ir oxidation states as high as Ir V may be reached [ 10,26,28,30 ] during the OER. Moreover, XAS at the O K‐edge has shown that at these high oxidation states, a variety of electron‐deficient oxygen species form, including bridging µ 2 ‐O bound to two iridium atoms and the terminal µ 1 ‐O species bound to a single iridium atom.…”