Probing the Electrode-Liquid Interface Using Operando total-Reflection X-Ray Absorption Spectroscopy
Authors: Grespi Andrea , Larsson Alfred , Gajdek Dorotea , Eidhagen Josefin , Pan Jinshan , Merte Lindsay , Lundgren Edvin 
Published: February 2024

Traditional operando methods to study electrochemical (EC) processes, although successful, are based on EC measurements, lacking in direct sensitivity of chemical and structural changes. A fundamental understanding of the underlying mechanism occurring during an EC reaction could allow to improve performances and properties of applied material systems. EC processes are localized at the electrode-electrolyte interfacial regions, but several surface sensitive experimental techniques are often limited to ex situ conditions, owing to the need of a Ultra High Vacuum (UHV) environment, e.g., Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS), Low Energy Electron Diffraction (LEED), etc.1 On the contrary, synchrotron-based hard x-rays provide an intense and highly energetic source that can penetrate the electrolyte without undergoing significant attenuation. Furthermore, the x-ray beam can be focused on a flat metal surface adopting a grazing incidence geometry, increasing surface sensitivity up to a few nanometers in subcritical condition. The latter case is achieved when the incident angle is set below the critical angle for total external reflection (TER), characterized by the occurrence of an evanescent wave confined at the surface. As a result, x-ray photon-in photon-out techniques can be used to probe the electrode-liquid interface in working conditions.2,3

In this contribution, we performed operando Grazing Incidence X-ray Absorption Spectroscopy (GI-XAS) in total external reflection (TER) mode4 (Fig. 1) at P64 (DESY, Hamburg), using our EC setup designed for multimodal studies with hard x-ray scattering and spectroscopy techniques3. XAS can provide unique information about the local chemical and structural environment of absorbing atoms during electrochemical polarization. The electro-oxidation of polycrystalline gold, relevant for the understanding of electrolysis, and the surface passive film development and breakdown of Ni-Cr-Mo corrosion resistant alloys, have been investigated. Despite the presence of the electrolyte and the PEEK walls of the EC cell, 1-3 nm thick surface oxides film can be detected in TER mode using energies down to 8 keV. Advantages and challenges of this method will be discussed showing experimental results.

Figure 1 Left: Schematic of our EC cell showing sample geometry for GI-XAS in TER mode, the incidence angle q is always lower than the critical angle. Right: GI-XAS in TER mode from a Ni 59 alloy sample at the Ni k-edge at increasing EC polarization in 1M NaCl and pH 12.

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Figure 1