The Chemistry of Interfacial Ions: In Situ XPS and XAS

Abstract

The choice of electrolyte ions has significant impact on the performance of electrochemical systems, affecting properties such as the electrocatalytic activity and capacitance. To understand how ions influence electrochemical processes, a key step is to determine the chemical behavior of ions at the electrode-electrolyte interface. Here, I will show how X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) allow one to obtain such information. Traditionally, these techniques are vacuum-based, only providing a detailed chemical analysis before or after electrochemistry. However, advanced spectro-electrochemical cell design now makes it possible to interface the vacuum needed for XPS and soft XAS with wet electrochemical environments, enabling us to study the electrode-electrolyte interface during electrocatalytic reactions. Using recent case studies from my group, I will show how operando XPS and soft XAS are able to detect the interfacial ion concentration, the hydration shell structure of the ions, and the electrostatic potential in the double layer. These insights allowed us, for example, to follow the intercalation of Na+ ions into NiFeOx and IrOx in alkaline electrolytes, and to obtain a measure of the double layer layer thickness at a RuOx electrode in 0.1 M H2SO4. Finally, I will give a perspective on how to bring operando XPS from the synchrotron to the laboratory, which will be an important step towards widespread implementation of the technology. Figure 1