Vladislav IVANISTSEV1, Iuliia VOROSHYLOVA3, Kaija POHAKO-ESKO2, Xueran TAO2, Heigo ERS2
1University of Copenhagen, Copenhagen, Denmark
2University of Tartu, Tartu, Estonia
3Universidade do Porto, Porto, Portugal
Electrochemical applications of ionic liquids (IL) stimulate studies of their electrical double-layer (EDL) structure and properties. One of the open challenges regarding the EDL in ILs is locating interfacial structures and related phenomena on the potential scale.
In this work, we developed a “potentials of saturation charge” (PSC) concept. It relates well-defined interfacial structures to “milepost” potentials. We identify two promising PSCs – P?SC and P?SC, at which the 1st and 2nd interfacial layers are saturated with counter- or co-ions, respectively. By simulating ion packing at a single-electrode model with molecular dynamics, we evaluated PSC values for 40 ions. By comparing the predictions with the empirical data, we conclude that exact surface charge screening by a monolayer of counter-ions at P?SC is improbable within electrochemical stability windows of common ions. In other words, within ±2 V, counter-ions form overscreening adlayers instead of crowding. In support, Density Function Theory calculations indicate that ions decompose closer to P?SC rather than P?SC.
This presentation addresses a possible counter-argument that reported inverse square-root capacitance-potential dependence indicates crowding (above P?SC). We provide a theoretical justification for power law capacitance–potential dependence between P?SC and P?SC. Thus, we reason that ionic adlayers (visualized with microscopy) are most probably formed in the vicinity of P?SC.
Our findings extend the modern paradigm of EDL in ILs. We present a classification of ions intending to facilitate meta-analysis of previous results and new studies. Furthermore, based on the classification, we suggest hypothetical ions can, within ±2 V, form monolayers (exactly screening the surface charge) as well as crowd the interface.