Modelling ionic liquids at electrochemical interfaces
- Benjamin Rotenberg (CNRS and University Pierre and Marie Curie, Paris, France)
- Mathieu Salanne (University Pierre et Marie Curie, France)
- Paul A. Madden (Queen's College, Oxford, United Kingdom)
- Alexei Kornyshev (Imperial College London, United Kingdom)
Ionic liquids are room-temperature molten salts, composed mostly of organic ions that may undergo almost unlimited structural variations. The scientific and technological importance of ionic liquids now spans a wide-range of applications . Here we will focus on the electrochemical applications of ionic liquids only. These can be separated in two main families: electrodeposition on the one hand and energy storage and conversion on the other hand. For the former, ionic liquids allow for the development of processes that are otherwise impossible in water; for example the electroplating of aluminium in order to protect steel from corrosion . As for the very-active field of electrochemical storage of energy, many synthesis routes involve the use of ionic liquids . They are also used in replacement of the conventional organic solvents as electrolytes in battery , fuel cell or supercapacitor  devices, allowing for their operation under a large electric potential window. In particular, it is worth underlining the case of electrical double layer capacitors (EDLC), which have attracted much attention in recent years . The discovery of nanoporous electrode materials with enhanced performances when using an ionic liquid with ion sizes matching the pore size opens the way for a widespread use of supercapacitors in many contexts where high power electrical output is required . As an extension, the behavior of the ionic liquids at charged interfaces even allows to forecast the development of electroactuators  which could be used as artificial muscles, sensors, and even energy generators in turbulent flows or sea-tide.
Despite the wide range of applications and the fundamental questions raised by the modelling of ionic liquids at electrochemical interfaces, detailed below, no previous CECAM workshop has been devoted to this important, challenging and timely topic. Only a few events have partly considered related aspects in a broader context, e.g. "Liquid/Solid interfaces [...]" in 2011 and 2013, "New Challenges in Electrostatics of Soft and Disordered Materials" in 2012, or the earlier one devoted to "Computational models of RTIL" in 2009. The present workshop aims at gathering the experts in the theory and simulation on different scales of these challenging systems, as well as a few leading experimentalists in this field, which has known spectacular developments in the last few years. We propose to organize this event in the CFCAM-IdF node (note that a smaller scale discussion meeting on this topic is organized in september 2013, in order to set up which are the most important questions to address in a full workshop).
The considerable amount of experimental results, some of them somewhat unexpected, has spurred the interest of the theoreticians community. Despite the underseen simulation work of Heyes and Clarke , the interface between electrodes and ionic liquids had barely remained explored until the last decade. Then several studies have focused on the structure of ionic liquids on charged surfaces [10, 11], showing that the ions exhibit a pronounced oscillatory structure close to the interface. In parallel, the development of a mean-field theory based on the Poisson-Boltzmann lattice-gas model showed that it is compulsory to account for the finite volume occupied by the ions, resulting in a dramatic departure from the Gouy-Chapman law for the capacitance-potential curves . It is worth noting that the latter work has actually preceded experiments, and that numerous studies have reported differential capacitance results in qualitative agreement with the mean-field theory since then [13, 14].
In a second step, simulation studies have focused on a better understanding of the existence of particular capacitance-potential curve shapes, such as e.g. the « bell-shaped » or « camel-shaped » ones. In particular, the roles of the anisotropy of size between the ions or of the charge distribution inside a given ion have been adressed [15, 16, 17, 18]. It was also shown that for the particular case of the interface between the molten salt LiCl and an aluminium electrode, a potential-induced ordering transition of the adsorbed layer was at the origin of a peak of the capacitance [19, 20]. Such a situation has also been reported for imidazolium ionic liquids systems in experiments based on in situ STM [21, 22]. Lately, the adsorption on atomically corrugated electrodes was shown to induce a systematic increase in the differential capacitance compared to the case of electrodes [23, 24].
The technologically-important case of porous electrode has also recently been investigated. Simulations involving slit-like pores  or carbon nanotubes [25, 26] provided a first insight of the structure of ionic liquids in confined environment. But it is the correct introduction of polarization effects by the metallic walls which could allow for a complete understanding of the formation of a superionic state [27, 28]. The mechanism at the origin of the enhanced capacitance in nanoporous electrodes was then fully understood from molecular dynamics simulations involving realistic carbon materials and taking into account the polarization in an appropriate way . These ingredients were then used in order to predict the optimal pore size distribution for enhancing the energy density of supercapacitors . In order to underline the current vigor of the simulation community in this field, we may mention the following example: the existence of a capacitance varying with an oscillatory behavior depending on the pore size was simultaneously reported by three different groups in the same week [31, 32, 33]! Among the questions which are now being addressed, the dynamic properties (i.e. charge/discharge time) is probably the most important one. Up to now it has only been studied by using a mean-field model , but we expect molecular simulations to provide additional informations in the future.
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