The development of better-performing electrocatalysts for the reactions at play in (co‑)electrolyzers (i.e., CO₂-reduction, H₂- and O₂-evolution) is of paramount importance for the ongoing decarbonization of the energy sector. The design of such improved materials requires a better understanding of the parameters that determine their activity, stability and selectivity under operation-relevant conditions. As a result, the last years have witnessed a surge of interest in the in situ / operando characterization of electrocatalysts for these reactions, and X-ray absorption spectroscopy (XAS) is often the technique of choice for such studies. However, the completion of XAS-measurements in the most common transmission configuration (i.e., with X-ray intensity detectors placed before and after the sample / absorbent) often requires the use of highly loaded electrodes with a correspondingly large thickness. This can in turn entail limitations in the transport of reactants, products and/or charges along the catalyst layer (CL-) thickness and, as we will show in this contribution, these may affect the electrochemical and spectroscopic results derived from these measurements.

To study these effects in detail, we have focused our attention on two types of electrocatalysts and corresponding reaction environments. Specifically, we will first present our results in the operando XAS study of an iridium oxide catalyst for the evolution of O₂ in acidic electrolyte. We will show that transmission XAS measurements with a large catalyst loading and a high-intensity X-ray beam lead to the accumulation of O₂-bubbles within the CL-pores and a localized loss of potential control that can be wrongly ascribed to a decrease of iridium’s oxidation state at high potentials. These effects can be circumvented by decreasing the beam flux and CL-thickness, whereby the latter also entails a remarkable improvement of the electrochemical data quality. Secondly, we will present our operando XAS results monitoring the formation of Pd-hydride in a palladium aerogel vs. a carbon-supported Pd-nanoparticle (Pd/C) catalyst in a neutral pH electrolyte representative of CO₂-electroreduction conditions. We will show that the seemingly lower rates of Pd-hydride formation on Pd/C vs. Pd-aerogel inferred from the operando XAS results are reversed when this effect is assessed in a rotating disk electrode configuration entailing the use of thin CLs. Again, this discrepancy can be rationalized based on thickness-induced effects in the Pd/C CLs used for the transmission XAS measurements, including mass-transport limitations and the concomitant buildup of pH-gradients along the catalyst layer.

In summary, this contribution will showcase possible pitfalls in the completion of operando XAS studies, and provide guidelines to prevent such effects and/or aid in the interpretation of the derived results.