Molecular scale structure and dynamics at an ionic liquid/electrode interface

Abstract

After a century of research, the potential-dependent ion distribution at electrode/electrolyte interfaces is still under debate. In particular for solvent-free electrolytes such as room-temperature ionic liquids, classical theories for the electrical double layer are not applicable. Using a combination of in situ high-energy X-ray reflectivity and impedance spectroscopy measurements, we determined this distribution with sub-molecular resolution. We find oscillatory charge density profiles consisting of alternating anion- and cation-enriched layers at both cathodic and anodic potentials. This structure is shown to arise from the same ion-ion correlations dominating the liquid bulk structure. The relaxation dynamics of the interfacial structure upon charging/discharging were studied by impedance spectroscopy and time resolved X-ray reflectivity experiments with sub-millisecond resolution. The analysis revealed three relaxation processes of vastly different characteristic time scales: a 2 ms scale interface-normal ion transport, a 100 ms scale molecular reorientation, and a minute scale lateral ordering within the first layer.

Surface structure evolution in a homologous series of ionic liquids

“…Thus, a quantitative determination of the structure's characteristics (layering periodicity and decay length) for a broad n range in a single RTIL series, as done here, should also provide an important yardstick for testing theoretical and simulation studies. Finally, the interface structure of RTILs is of great importance not only to basic science but also to the many applications involving interfaces: supercapacitors, solar cells, batteries, metal extraction from wastes, chemical reactions and catalysis, and more (28,(32)(33)(34). Elucidation of the RTIL-air interface's influence and its variation with n should provide insights also for other interfaces, including the RTIL-solid one, involved in the above applications.…”

Static Capacitance at the Electrochemical Liquid-liquid Interface Between Ionic Liquids and Eutectic Ga-In Alloy Measured Using the Pendant Drop Method

“…In contrast to the progress of the understanding of the EDL of ILs from the theoretical side, experimental confirmation of the predicted IL-peculiar behavior is not straightforward. It has been found, from studies using electrochemical impedance spectroscopy (EIS), that C for the EDL of ILs has strong frequency dependence [11][12][13][14][15][16][17][18] and shows potential hysteresis, 11,[19][20][21] both of which seem to result from the structural ordering of IL interface [22][23][24][25][26][27][28][29] and the ultraslow dynamics [29][30][31][32][33][34][35][36][37][38] of such ordered structure. These tendencies have hampered rigorous evaluation of C experimentally.…”

Electrode polarization effects on interfacial kinetics of ionic liquid at graphite surface: An extended lagrangian‐based constant potential molecular dynamics simulation study

“…The constant potential simulation showed a slower kinetics of the EDL relaxation (subnanosecond timescale) compared to the constant charge simulations, but experiments show much slower relaxation timescales of millisecond or longer . A reasonable origin for the much faster timescale in the MD simulation is expected to be the small‐sized electrochemical cell, in particular the short separation between the two cell electrodes (Fig.…”