X-Ray Absorption Spectroscopy Studies of Ir-Oxide Based Oxygen Evolution Catalysts Revisited

Abstract

Hydrogen plays a pivotal role in the future decarbonization of the energy sector [1], but its production in a sustainable way is crucial for it to have a positive environmental impact. Due to its high power density, quick start-up and response time as well as pressurized hydrogen delivery, polymer electrolyte membrane water electrolysis (PEMWE) is perfectly suited to facilitate H2 production from renewable electricity [2]. However, at large scale application, the use of scarce and expensive, Ir-based catalysts in PEM water electrolyser anodes limits the proliferation of this technology [3, 4]. For this reason, significant research activities are being conducted to better understand the oxygen evolution reaction (OER) mechanism on benchmark IrO2 based catalysts, as to further improve catalyst design and decrease the efficiency losses related to this reaction.When trying to elucidate a reaction mechanism, the use of operando and/or in situ techniques is often beneficial, and thus in last decades the OER mechanism on Ir-based materials has been extensively investigated through operando / in-situ X-ray absorption as well as X-ray photoelectron spectroscopy (XAS, XPS) [5-10]. Despite enormous efforts in this research area, there is still a lack of consensus regarding the Ir oxidation state under OER conditions and the nature of the OER-active sites. In line with this, in this study we employed operando XAS to investigate the Ir oxidation state in commercial IrO2 (Umicore®) catalyst under OER polarization curve conditions. These XAS measurements were performed both in transmission and fluorescence modes; for the former, electrodes with loadings ≈ 40 fold higher than those used in rotating disk electrode (RDE) measurements were required in order to achieve appropriate XAS data quality. In fluorescence mode, however, thinner electrodes with only four times higher loadings than in RDE could be investigated. Prior to the operando XAS measurement, the utilization of these IrO2 catalyst layers in the operando flow cell (FC) was assessed to ensure that the acquired spectroscopic data was representative of the processes occurring under real conditions. Achieving a good catalyst layer utilization was especially important in the case of the thick electrodes used in transmission XAS measurement, since their resistance is much higher compared to that in thin film RDE measurements. The so obtained results were additionally affected by beam damage of the Nafion ionomer used as the catalyst layer binder. All these factors can further lead to data misinterpretation. Regarding the beam damage, reducing the beam flux helped to prevent this effect and allowed us to obtain reliable results. Additionally, the use of thin catalyst layers in the operando FC and its combination of time-resolved XAS (so called “quickXAS”) in fluorescence mode led to novel insight on the effects of evolved O2 bubbles on the spectroscopic results and subsequent interpretation thereof.In summary, this contribution will provide an overview of common pitfalls and good practices encountered when conducting operando XAS studies of OER-electrocatalysts.