Ionic liquids at charged surfaces: Insight from molecular simulations

Abstract

Understanding of molecular level structure and mechanisms of the formation of electric double layers in realistic ionic liquid-based electrolytes on charged electrode surfaces is one of scientifically and technologically key areas that has attracted a lot of attention over the last decade. Extensive experimental, theoretical, and modeling studies have been dedicated to this challenging topic in order to establish fundamental correlations between the details of molecular structure of electrolyte and the properties of the electric double layers (EDL) forming on various electrodes. While great progress has been made in advancing our understanding of EDL properties and their influence on the performance of supercapacitors, batteries, and other energy storage devices, there are still a number of challenges and controversies that have not been resolved. In this manuscript, we demonstrate how atomistic molecular dynamics simulations provide a powerful tool for dealing with these challenges and can facilitate the design of novel materials for advancing energy storage technologies.data. For example, Figure 1 shows a comparison of several reported in the literature experimental data for differential capacitance (DC) obtained on gold, platinum, and glassy carbon electrodes for the same (or very similar) RTILs [iii-vii]. The DC is one of the key properties used to characterize structural changes in the EDL as a function of electrode potential as well as a direct measure of the energy stored by a given electrode/electrolyte combination. As we can see from Figure 1, the shape and the magnitude of DC reported by different studies can significantly deviate from each other. The magnitude of DC from two different measurements can be different up to a factor of five, particularly at low potentials. Note that the inconsistencies shown in Figure 1 were obtained for the simplest electrode/electrolyte set up geometry, i.e., flat electrode surfaces in contact with bulk electrolyte. Needless to say that interpretation of experimental results become even more complicated for the technologically more practical nanoporous electrodes, where electrolyte EDL layers are forming in nanopores with very heterogeneous pore size, shape, and surface structure distributions [viii,ix]. In these electrode materials, electrolyte is subjected to nanoconfinement, local curvature, and atomic scale roughness of exposed electrode surface, all of which can significantly influence the magnitude and dependence of DC [x].Several theoretical models have focused on the prediction and explanation of correlations between the EDL structure and the dependence of DC as a function of electrode potential [xi-xxvii]. These models found that on a flat surface, in general, the DC can be either a camel-shaped with a minimum near the potential of zero charge (PZC) or a bell-shaped with a maximum at low potentials. The camel-shape DC typically has a low-voltage minimum (the U-shape region) flanked by two maxima at higher electrode potentials. These dependencies were explained b...

Tracking the structural arrangement of ions in carbon supercapacitor nanopores using in situ small-angle X-ray scattering

“…Even more important is the unique usefulness and the plethora of complementary information when we consider the advanced stage of modeling that now even taps into charge/discharge kinetics and adopts more realistic pore networks. 4,[56][57][58] Yet, most experimental techniques fall short to provide a molecular picture of the ion electrosorption process and we believe that our approach adds an advanced technique to the limited number of electrochemical in situ methods. More generally, this novel approach might be of interest for a variety of fields where ion adsorption within confined geometry plays an important role.…”

Computational Insights into Materials and Interfaces for Capacitive Energy Storage

“…However, in both studies, the structural difference becomes significant when the potential difference is high (over 5 V) . The constant‐potential method is about one order of magnitude slower than the constant‐charge method, but it allows MD simulations to explore more complex and inhomogeneous electrode systems, such as rough surfaces, activated carbon and amorphous carbide‐derived carbons (CDCs), where the constant‐charge method is not applicable.…”

“…The simplified electrode models in the CMD simulations usually lead to lower capacitances . Vatamanu and coworkers showed that the surface roughness (Figure e) could increase the energy storage . Palmer and coworkers conducted quench molecular dynamics (QMD) simulations to generate porous carbon models by mimicking the thermal quenching of systems of carbon atoms .…”

Substrate Effects in Graphene‐Based Electric Double‐Layer Capacitors: The Pivotal Interplays between Ions and Solvents

“…The effect of substrates was further extended to the capacitive properties of graphene‐based systems. Based on the as‐obtained electrolyte structuring, the electric potential of single electrode U electrode was computed, defined as the potential drop within electrode/electrolyte interfacial district relative to the potential of zero charge ( PZC ), [Eq. ]: …”

“…[34,35] Nevertheless, the density oscillations became vanished about~12 Å away from the electrode surface, consistent with previous theoretical studies. [24,36] Noteworthy, according to prior theoretical studies [37,38] , the capacitive behaviors of EDLCs were predominantly determined by the electrolyte distributions in the first layer or interfacial districts at the nanoscale, indicating that the contributions of ionic density profiles from second peak to bulk region were rather marginal. For example, based on the ion number within the first layer, Jo and colleagues have proposed a novel and useful measurement (i. e., differential ion capacity) to interpret the as-obtained capacitive results well.…”