The Anomalous Behavior of Cesium in the Electrical Double Layer: an In-Situ X-ray Spectroscopy Study on Graphene

Understanding the behavior of ions at electrified interfaces is crucial for the vast majority of electrochemical processes, including energy storage, corrosion, and catalysis. The electric double layer (EDL), formed at the interface between an electrode and an electrolyte, plays a pivotal role in governing these processes. In particular, large ions with a weak charge, such as cesium, exhibit extraordinary behaviors in the EDL, rendering these ions a means to manipulate EDL properties such as solvation, potential drop and local hydrophilicity. In this study, we utilize X-ray and electron transmissive graphene electrodes to investigate the influence of cesium-ions within the EDL using advanced X-ray spectroscopy techniques. By employing synchrotron-based operando X-ray photoelectron spectroscopy (XPS) and electron yield X-ray absorption spectroscopy (XAS), we determine the ion concentrations and elucidate the electronic structure and chemical environment of cesium-ions near the electrode surface. Our results reveal intricate ion-specific interactions within the EDL, shedding light on ion adsorption, desorption, and redistribution phenomena, in dependence of the interfacial cesium concentration. Furthermore, we explore the impact of electrolyte bulk processes, such as ion pairing and surface charge density on the EDL structure and dynamics. Understanding this ion behavior in the EDL is vital for designing electrochemical processes from the electrolyte side. Ease in applicability, high degree of control and dynamic manipulation of this approach, renders it vital for the electrochemistry driven energy revolution.