In Situ XPS Reveals Voltage Driven Asymmetric Ion Movement of an Ionic Liquid through the Pores of a Multilayer Graphene Electrode

Under application of a voltage bias, asymmetric ion movement of an ionic liquid (IL) through a multilayered graphene (MLG) electrode has been detected by X-ray photoelectron spectroscopy, via recording the intensity of the two nitrogen peaks. Accordingly, we observe that upon increasing the bias, the two peaks representing the cationic and anionic fragments of the IL start appearing with increasing intensity, together with an asymmetry in their ratio, differing from unity by about 10%. Bias-dependent binding energy shifts followed through atomic features of the IL (F 1s, N 1s, and C 1s) and the graphene electrode (C 1s) indicate that a distinct solid-liquid interface develops throughout the entire intercalation process with an additional and pertinent evidence for finite potential drops across the two electrical double layers. This evidence is bolstered by the fact that the measured binding energy difference between the F 1s of the liquid and C 1s peak of the semisolid MLG electrode is only about half of the applied bias, where the rest of the applied voltage is screened by the two electrical double layers at the solid-liquid interfaces between (i) the MLG-IL and (ii) IL-bottom metal electrode. A simple electrostatic estimation indicates that even this small 10% ion imbalance would lead to 4 orders of magnitude larger voltage development between the IL and the MLG phases and suggests the need for amendment(s) to the current understanding of the dielectric description of ILs.