Operando Studies of Electrochemical Interfaces By High-Energy Surface x-Ray Scattering
Authors: Magnussen Olaf M. 
Published: February 2024

Knowledge on the atomic-scale structure of the electrode-electrolyte interface is of key importance for understanding electrochemical reactivity. This includes the structure of the atomic layers at the electrode surface, the arrangement of chemisorbed species on the surface, and the near-surface structure of the adjacent electrolyte, i.e., the electrochemical double layer. Surface X-ray scattering (SXRD) methods can provide such information and have been used for decades to obtain important insights into various structural aspects of electrochemical systems. However, conventional SXRD is limited by the considerable time required for a full characterization of the three-dimensional interface as well as in terms of the structural detail that can be reliably extracted from the diffraction data by structural models. This is related to the slow sequential data acquisition necessary for such measurements.
We here describe in situ and operando studies of electrochemical interface structure by High Energy Surface X-ray Diffraction (HESXRD), where the interface structure is probed by hard X-rays with high photon energy (in this work 70 keV). This method has been originally developed for studies of catalyst surfaces in the gas phase [1], but is starting to be applied increasingly to electrochemical systems. In combination with the highly brilliant beams provided by emerging hard X-ray 4th generation synchrotron, HESXRD allows to obtain very large datasets in short time. From these the interface structure can be determined with unprecedented detail by a quantitative analysis of the measured crystal truncation rods (CTRs). Furthermore, measurement of restricted datasets is possible with even high time resolution down to the second or sub-second regime, which is ideal for monitoring fast kinetic changes in operando.
We illustrate the capabilities of HESXRD by studies of platinum surface oxidation and magnetite single crystal electrodes under oxygen evolution conditions. For the case of platinum we present data on Pt(111), Pt(100), and Pt(110) in 0.1 M HClO4 that reveal distinct differences in the structure and formation mechanisms of the Pt surface oxide [2]. Because of these differences, the irreversible surface restructuring and Pt dissolution during oxidation/reduction cycles depends strongly on the crystallographic orientation. In addition, we demonstrate for Pt(111) that the extraction of Pt atoms out of the electrode surface in the initial stages of oxidation is not directly coupled to the charge transfer associated with the formation of adsorbed oxygen species [3]. Studies on magnetite focus on Fe3O4(100) in 0.1 M NaOH, where previous SXRD studies showed that the (√2x√2)R45° reconstructed surface formed under UHV conditions can be maintained [4]. HESXRD allowed to obtain extended CTR datasets, which provide deeper insights into the structure of this oxide model electrocatalyst.
[1] J. Gustafson et al., Science 343, 758 (2014)
[2] T. Fuchs et al., Nature Catalysis 3, 754 (2020)
[3] T. Fuchs et al., J. Phys. Chem. Lett. 14, 3589 (2023), https://doi.org/10.1021/acs.jpclett.3c00520
[4] D. Grumelli et al., Ang. Chem. Int. Ed. 59, 49 (2020)