Andrey Shavorskiy

Surface oxidation of noble metals under electrochemical conditions has a decisive impact on the performance of electrocatalysts for electrolyzers and fuel cells. However, the experimental detection of such oxides on well‐defined model electrodes under operando conditions is a major challenge, limiting our ability to establish clear structure–performance relationships. Using surface‐sensitive total‐reflection X‐ray absorption spectroscopy in real time during cyclic voltammetry, combined with inductively coupled plasma mass spectrometry dissolution measurements, we demonstrate that an ultrathin (~5 Å) hydroxide film forms on the gold (111) surface prior to the onset of the oxygen evolution reaction. This film, composed of Au ³⁺ in similar coordination as in the bulk oxyhydroxide, thus governs its catalytic properties, dissolution, and restructuring. The results highlight the importance of surface oxidation in water splitting catalysis even for Au(111), the most inert surface of the most inert noble metal.

The increasing demand for advances in efficient electrochemical (EC) energy conversion requires a fundamental understanding of the EC interface, which is challenging to study in situ. High-resolution ambient-pressure X-ray photoelectron spectroscopy, in combination with an EC cell, has been used in a dip-and-pull approach to study the EC oxidation of a Au(111) model electrode during oxidation and water electrolysis conditions. The behavior of Au(111) in 0.05 M H2SO4 was studied in a controlled gaseous environment of ∼13 mbar of water vapor after fully withdrawing the sample from the electrolyte at increasing anodic potential steps. Above 1.5 VRHE, a thin 2D Au(OH)3 film of ≈ 5 Å is observed and exhibits a stable thickness until the onset of the oxygen evolution reaction (OER, ≈ 1.8 VRHE). From the onset of OER, a thicker bulk-like 3D Au(OH)3/Au2O3 mixture rapidly grows to a thickness of ≈ 21 Å at 2.02 VRHE. The initial 2D film exhibits self-limited kinetics, typically described by the Cabrera–Mott model and the point-defect model. The subsequent rapid growth of thicker bulk-like 3D islands suggests a disruption of the passive state coupled with the onset of OER. These results highlight surface passivation, which inhibits further oxidation due to surface oxidation, and provide quantitative and detailed insights into changes in the surface chemical composition of Au(111) under anodic polarization.

Gaining insight into structural and compositional transformations occurring at the electrode/electrolyte interface during the operation of electrochemical systems is fundamental to understanding and, thus, optimizing their performance. Such an analysis must be performed in operando conditions, owing to the potential, electrolyte and time dependence of these transformations. Here, the use of X-ray photoelectron spectroscopy (XPS) is particularly attractive due to its surface sensitivity and ability to provide quantitative information on the oxidation state and chemical environment of an element. In specific instrumental configurations [ e.g. in `dip-and-pull' (D&P) or `meniscus' setup], it can be used to analyse not only the electrode but also the electrolyte side of the interface, under in situ / operando conditions. In this article, we discuss how D&P XPS can provide unique information on both sides of the electrode/electrolyte interface, briefly review publications demonstrating its capabilities, highlight the challenges the method faces, and share our views on its future developments. This article aims to provide a practical guide to new D&P synchrotron users and help them to understand the technique, and physical phenomena that may impede the acquisition of reliable data.