Ionic liquids as solvents for rare-earth metals: a combined XAS and Molecular Dynamics study

In this work a detailed investigation of the structural organization of ILs, both monocationic and dicationic, and their water mixtures has been carried out by combining XAS spectroscopy and Classical MD simulations. The same joint XAS-MD approach has been also applied to the study of the solvation properties of Ln(III) salts dissolved in ILs. The original application of EXAFS and MD simulations paves the route for the systematic use of an integrated approach, with increased reliability, in the structural investigation of ILs. All together these issues are expected to be of great help in the systematic design of IL systems to meet the requirements of key applications

Ions Adsorbed at Amorphous Solid/Solution Interfaces Form Wigner Crystal-like Structures.

When a surface is immersed in a solution, it usually acquires a charge, which attracts counterions and repels co-ions to form an electrical double layer. The ions directly adsorbed to the surface are referred to as the Stern layer. The structure of the Stern layer normal to the interface was described decades ago, but the lateral organization within the Stern layer has received scant attention. This is because instrumental limitations have prevented visualization of the ion arrangements except for atypical, model, crystalline surfaces. Here, we use high-resolution amplitude modulated atomic force microscopy (AFM) to visualize the lateral structure of Stern layer ions adsorbed to polycrystalline gold, and amorphous silica and gallium nitride (GaN). For all three substrates, when the density of ions in the layer exceeds a system-dependent threshold, correlation effects induce the formation of close packed structures akin to Wigner crystals. Depending on the surface and the ions, the Wigner crystal-like structure can be hexagonally close packed, cubic, or worm-like. The influence of the electrolyte concentration, species, and valence, as well as the surface type and charge, on the Stern layer structures is described. When the system parameters are changed to reduce the Stern layer ion surface excess below the threshold value, Wigner crystal-like structures do not form and the Stern layer is unstructured. For gold surfaces, molecular dynamics (MD) simulations reveal that when sufficient potential is applied to the surface, ion clusters form with dimensions similar to the Wigner crystal-like structures in the AFM images. The lateral Stern layer structures presented, and in particular the Wigner crystal-like structures, will influence diverse applications in chemistry, energy storage, environmental science, nanotechnology, biology, and medicine

Oligomerization of lithium ions in water-in-salt electrolytes

Water-in-salts (WiS) have recently emerged as promising electrolytes for energy storage applications, ranging from aqueous batteries to supercapacitors. Here, ab initio molecular dynamics is used to study the structure of a 21 m LiTFSI WiS. The simulation reveals a new feature, in which the lithium ions form oligomer-like nanochains that involve up to 10 ions. Despite the strong Coulombic interaction between them, the ions in the chains are found at a distance of 2.5 Angstroms. They display a drastically different solvation shell compared to the isolated ions, in which they share on average two water molecules. The nanochains have a highly transient character due to the low free energy barrier for forming/breaking them. Providing new insights into the nanostructure of WiS electrolytes, our work calls for re-evaluating our current knowledge of highly concentrated electrolytes and the impact of the modification of solvation of active species on their electrochemical performances

The role of hydrophobic hydration in the free energy of chemical reactions at the gold/water interface: size and position effects

Metal/water interfaces catalyze a large variety of chemical reactions, which often involve small hydrophobic molecules. In the present theoretical study we show that hydrophobic hydration at the Au(100)/water interface actively contributes to the reaction free energy by up to several hundreds of meV. This occurs either in adsorption/desorption reaction steps, where the vertical distance from the surface changes in going from reactants to products, or in addition and elimination reaction steps, where two small reactants merge into a larger product and viceversa. We find that size and position effects cannot be captured by treating them as independent variables. Instead, their simultaneous evaluation allows to map the important contributions, and we provide examples of their combinations for which interfacial reactions can be either favoured or disfavoured. By taking a N2 and a CO2 reduction pathway as test cases, we show that explicitly considering hydrophobic effects is important for the selectivity and rate of these relevant interfacial processes

Speciation of the proton in water-in-salt electrolytes

Water-in-salt (WiS) electrolytes are promising systems for a variety of energy storage devices. Indeed, they represent a great alternative to conventional organic electrolytes thanks to their environmental friendliness, non-flammability, and good electrochemical stability. Understanding the behaviour of such systems and their local organisation is a key direction for their rational design and successful implementation at the industrial scale. In the present paper, we focus our investigation on the 21 m bis(trifluoromethanesulfonyl)imide (LiTFSI) WiS electrolyte, recently reported to have acidic pH values. We explore the speciation of an excess proton in this system and its dependence on the initial local environment using ab initio molecular dynamics simulations. In particular, we observe the formation of HTFSI acid in WiS system, known to act as a superacid in water. This acid is stabilised in the WiS solution for several picoseconds thanks to the formation of a complex with water molecules and a neighboring TFSI– anion. We further investigate how the excess proton affects the microstructure of WiS, in particular, the oligomerisation of lithium cations, and report possible notable perturbations of lithium nanochain organisation in some cases. These two phenomena are particularly important when considering WiS as electrolytes in batteries and supercapacitors, and our results contribute to the comprehension of these systems on the molecular level

2D-HB-Network at the air-water interface: A structural and dynamical characterization by means of ab initio and classical molecular dynamics simulations

International audienceFollowing our previous work where the existence of a special 2-Dimensional H-Bond (2D-HB)-Network was revealed at the air-water interface [S. Pezzotti et al., J. Phys. Chem. Lett. 8, 3133 (2017)], we provide here a full structural and dynamical characterization of this specific arrangement by means of both Density Functional Theory based and Force Field based molecular dynamics simulations. We show in particular that water at the interface with air reconstructs to maximize H-Bonds formed between interfacial molecules, which leads to the formation of an extended and non-interrupted 2-Dimensional H-Bond structure involving on average ∼90% of water molecules at the interface. We also show that the existence of such an extended structure, composed of H-Bonds all oriented parallel to the surface, constrains the reorientional dynamics of water that is hence slower at the interface than in the bulk. The structure and dynamics of the 2D-HB-Network provide new elements to possibly rationalize several specific properties of the air-water interface, such as water surface tension, anisotropic reorientation of interfacial water under an external field, and proton hopping

Nanostructural Organization in a Biredox Ionic Liquid

International audienceIonic liquids generally display peculiar structural features that impact their physical properties, such as the formation of polar and apolar domains. Recently, ionic liquids functionalized with anthraquinone and TEMPO redox groups were shown to increase the energy storage performance of supercapacitors, but their structure has not yet been characterized. In this work, we use polarizable molecular dynamics to study the nanostructuration of such biredox ionic liquids. We show that TEMPO nitroxyl functions tend to aggregate, while the anthraquinone groups favor stacked arrangements. The latter eventually percolate through the whole liquid, which sheds some light on the mechanisms at play within biredox ionic liquid-based supercapacitors