Unveiling Ionic/Electronic Contributions to the Potential Development of Electrical Double Layer Using XPS

Time-dependent X-ray photoelectron spectroscopy (XPS) data are recorded for capturing dynamics of the electrical double layer (EDL) formation on two multilayered graphene electrodes, configured as a coplanar-capacitor, with an ionic liquid (IL) as the electrolyte. The device is subjected to 2 V biasing cycles, reversing its polarity every hour, while iteratively recording the O 1s peak representing the IL with 0.5 s steps, which turns XPS into a recording chemical voltmeter, since variations in binding energy position of the O 1s peak on electrodes report directly the local electrical potential of the IL medium. If the device is physically shorted during measurements, the potential suddenly jumps to the opposite polarization since this process nulls the electronic component of the potential, retaining the ionic one. Complementing action is captured on the drain electrode, which exhibits voltage decay(s) toward the system's ground. On the other hand, data recorded when the circuit is temporarily opened reveal only decays without polarity reversal. Generally, these potentials capture decay with relatively long and multiple time constants, associated with ionic motion. The polarity reversal is explained by a simple model, considering the huge time constant difference between electronic and ionic motions.