X-ray Absorption Spectroscopy in Electrochemistry: From Fundamentals to Fixed Energy X-ray Absorption Voltammetry.

X-ray absorption spectroscopy (XAS) represents a powerful and versatile tool for studying the electronic and atomic structure of matter on a local scale: in principle, the sample can be in any aggregation state and it applies to any element. The use of XAS for the investigation of electrode materials dates back to the mid-1980s, mainly as one of the possible spectroelectrochemistry techniques for studying electrochemical reactions, particularly on the electrolyte side. Since then, thanks to the improved efficiency and number of available synchrotron radiation sources needed for collecting XAS spectra in a reasonable time interval, examples of the use of x-ray absorption techniques in electrochemistry have been constantly growing in number. This is due to the impressive amount and importance of information that can be derived from the evaluation of x-ray absorption spectra: the interaction of x-ray and matter strongly depends on the chemical properties of the absorbing atom and its surroundings, that is, the oxidation state and the nature, distance, and coordination number of neighboring atoms. For the particular case of the use of XAS in electrochemistry, it is worth immediately noting two main features: