Tapping into Charge Storage with Operando-XPS Using a Multi-Layer Graphene Coplanar Capacitor and an Ionic Liquid Mixture

X-ray Photoelectron Spectroscopy under bias tracks surface population and electrical potentials on graphene electrodes with an ionic liquid solution, composed of one anion (TFSI–) and two cations (DEME+ and Rb+). As bias increases, ions are enriched at the analyzed vacuum/graphene interface due to electrosorption, and the measured binding energies shift due to both bias and ion screening. The relative population of Rb+ increases, but that of DEME+ decreases to maintain electroneutrality. After prolonged biasing, induced currents increase by more than 1 order of magnitude, attributed to a significant contribution from electrosorbed, smaller, and mobile Rb+ cations, causing a sizable asymmetry with respect to the polarity of the bias. The evolving relative concentrations of Rb+ and DEME+ allow us to elaborate on the dynamics of the ionic structure inside the electrolyte. These findings bring a new understanding of the surface composition of electrified interfaces in terms of anion and cation balance.