The evolution of O2 on Ir-based catalysts requires the complete oxidation of their surface to Ir+5

Polymer electrolyte water electrolyzers (PEWEs) are very well suited for the production of hydrogen using renewable electricity. However, PEWE-commercialization could be severely limited due to the reliance of this technology on scarce and expensive Ir-based materials to catalyze the evolution of O₂ in PEWE-anodes. This translates in the need for a significant decrease of the amount of Ir used in these electrolyzers, which will in turn be possible by improving our understanding of how these materials catalyze the evolution of O₂ (i.e., of the so-called reaction mechanism). In this context, there is a vivid debate regarding the changes in oxidation state undergone by the surface of the Ir-based catalysts during the O₂-evolution reaction (OER).

To get a better understanding of this question, in this work we selected four Ir-based catalysts with very different surface areas, initial oxidation states and O₂-evolution activities, and studied them under operative conditions using X-ray absorption spectroscopy (XAS). We performed these measurements using a setup available at the SuperXAS beamline of the Swiss Light Source that allowed us to continuously record X-ray absorption spectra with an acquisition frequency of one spectrum per second. The analysis of these time-resolved data showed that the surface of all catalysts becomes linearly more and more oxidized as the applied potential increases. Most importantly, this oxidation process systematically reaches a plateau at a potential specific to each catalyst, but that systematically coincides with the onset of O₂-evolution on its surface. Finally, we compared these spectra acquired under operative conditions with others recorded on reference Ir-compounds with well-defined oxidation states; this allowed us to conclude that for all catalysts the surface oxidation state triggering O₂-evolution corresponds to Ir⁺⁵. As such, our results discard previous reaction mechanisms that postulated that in order to catalyze the OER, Ir-based surfaces only reach oxidation states < +5.