The potential pathways to increase the energy storage in electric double-layer (EDL) supercapacitors using room-temperature ionic liquid electrolytes and carbon-based nanostructured electrodes are explored by molecular dynamics simulations. A systematic comparison of capacitances obtained on nanoparticles of various shape and dimensions showed that when the electrode curvature and the length scale of the surface roughness are comparable to ion dimensions, a noticeable improvement in the capacitive storage is observed. The nanoconfinement of the electrolyte in conductive electrode pores further enhances the capacitance due to mismatch in ion−electrode surface interactions and strong electrostatic screening. We show that nanoporous structures made of arrays of conductive carbon chains represent a synergy of all three favorable factors (that is, high curvature, atomic scale roughness, and nanoconfinement) and can generate non-Faradic capacitance ranging from 260 to 350 F/g, which significantly exceeds the performance of the current generation of nanostructured electrodes. SECTION: Energy Conversion and Storage; Energy and Charge Transport

A molecular dynamics simulation study of electric double layer (EDL) structure and differential capacitance (DC) of two 1-butyl-3-methylimidazolium (BMIM)-based room temperature ionic liquids, i.e. [BMIM][BF4] and [BMIM][PF6], has been conducted on basal and prismatic graphite as well as (001) and (011) gold electrode surfaces. The influence of the electrode surface and electrolyte structure on electrode capacitance and EDL structure are discussed. For a given electrode surface both the [BMIM][BF4] and [BMIM][PF6] electrolytes generate very similar DC and EDL structures. The DC for these ionic liquids in contact with atomically flat surfaces (i.e. basal graphite and (001)Au) shows very small variations within the electrolyte chemical stability potential window and fluctuates around an average value of ∼5 μF cm(-2). On atomically more corrugated surfaces (i.e., Au(011) and prismatic graphite) the DC shows more variation with electrode potential and depends on the correspondence between dimensions of the surface roughness and electrolyte ion sizes. The trends and dependencies obtained for DC are used to discriminate between mutually contradictory experimental data reported in the literature for related systems.

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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...

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