Ambient-Pressure X-Ray Photoelectron and Absorption Spectroscopy to Study Electrode Materials Under Working Conditions

The combination of ambient-pressure X-ray photoelectron spectroscopy (APXPS) and X-ray absorption spectroscopy (XAS) has revolutionized our understanding of working functional interfaces by providing access to both the composition and chemical state of electrode materials under reaction conditions. This line of research has been enabled by technological advances on the analytical side and the development of tailored electrochemical cells compatible with spectroscopic requirements.¹ In the present contribution, we will discuss the development of XPS from a purely ultra-high vacuum (UHV)-based technique towards being able to collect spectra while catalytic reactions proceed at surfaces under (close-to) atmospheric conditions. The focus will be on electrochemical studies of electrode materials, especially those materials that are catalyzing the oxygen evolution (OER) and CO₂ reduction (CO2RR) reactions.^2–5 We will highlight the capabilities, advantages, and challenges of the developed electrochemical cell types and discuss the insights that have been generated using these approaches. Importantly, we will describe how a detailed understanding is commonly obtained by tightly integrating operando experimental observations with computational spectroscopy, i.e. by comparing measured spectroscopic fingerprints with first-principles simulated spectra.⁶ While these experiments have so far mainly been performed at synchrotron end stations, multi-colored APXPS systems are starting to become available as lab systems with the advantage of providing flexible daily access for rapid feedback during material development processes. Within the German excellence cluster <i>e</i>-conversion, we have recently ordered a powerful and versatile lab-based APXPS system, available for user operation from summer 2023. Our APXPS features a vertically mounted electron analyzer, operable in gas phase or vapor environments of up to 100 mbar, while also being perfect for probing samples in contact with ‘open’ liquids. Three monochromatized X-ray sources (1.5 - 5.4 keV) allow us to probe surfaces as well as buried solid|solid, solid|liquid, and hybrid interfaces. A dedicated gas-phase manipulator enables <i>operando </i>experiments of heterogeneous (photo)catalysts in different gas atmospheres at a wide range of temperatures. A second, electrochemical manipulator features an ‘open-liquid’ three-electrode electrochemical cell, continuous electrolyte flow capabilities, and versatile connections for custom-made cells. In addition, surfaces and interfaces can be illuminated to investigate photo(electro)catalytic processes and modified photoelectrode properties or degradation processes. In this respect, we will discuss the possibilities and challenges of such lab-based systems. We will conclude with an outlook on future developments within the field of APXPS to study energy materials.<br/><br/>1. Velasco Vélez, J. J. <i>et al.</i> <i>Faraday Discuss.</i> (2022) doi:10.1039/d1fd00114k.<br/>2. Pfeifer, V. <i>et al.</i> <i>Chem. Sci.</i> <b>8</b>, 2143–2149 (2017).<br/>3. Velasco Vélez, J.-J. <i>et al.</i> <i>J. Am. Chem. Soc.</i> <b>143</b>, 12524–12534 (2021).<br/>4. Nong, H. N. <i>et al.</i> <i>Nature</i> <b>587</b>, 408–413 (2020).<br/>5. Velasco Vélez, J. J. <i>et al.</i> <i>ACS Energy Lett.</i> <b>5</b>, 2106–2111 (2020).<br/>6. Streibel, V. <i>et al.</i> <i>Curr. Opin. Electrochem.</i> <b>35</b>, 101039 (2022).