International audienceSupercapacitors are electrochemical energy storage devices that operate on the simple mechanism of adsorption of ions from an electrolyte on a high-surface-area electrode. Over the past decade, the performance of supercapacitors has greatly improved, as electrode materials have been tuned at the nanoscale and electrolytes have gained an active role, enabling more efficient storage mechanisms. In porous carbon materials with subnanometre pores, the desolvation of the ions leads to surprisingly high capacitances. Oxide materials store charge by surface redox reactions, leading to the pseudocapacitive effect. Understanding the physical mechanisms underlying charge storage in these materials is important for further development of supercapacitors. Here we review recent progress, from both in situ experiments and advanced simulation techniques, in understanding the charge storage mechanism in carbon- and oxide-based supercapacitors. We also discuss the challenges that still need to be addressed for building better supercapacitors

Lightweight, low-cost supercapacitors with the capability of rapidly storing a large amount of electrical energy can contribute to meeting continuous energy demands and effectively levelling the cyclic nature of renewable energy sources. The excellent electrochemical performance of supercapacitors is due to a reversible ion adsorption in porous carbon electrodes. Recently, it was demonstrated that ions from the electrolyte could enter sub nanometre pores, greatly increasing the capacitance. However, the molecular mechanism of this enhancement remains poorly understood. Here we provide the first quantitative picture of the structure of an ionic liquid adsorbed inside realistically modelled microporous carbon electrodes. We show how the separation of the positive and negative ions occurs inside the porous disordered carbons, yielding much higher capacitance values (125 F g(-1)) than with simpler electrode geometries. The proposed mechanism opens the door for the design of materials with improved energy storage capabilities. It also sheds new light on situations where ion adsorption in porous structures or membranes plays a role.

Liquids exhibit specific properties when they are adsorbed in nanoporous structures. This is particularly true in the context of supercapacitors, for which an anomalous increase in performance has been observed for nanoporous electrodes. This enhancement has been traditionally attributed in experimental studies to the effect of confinement of the ions from the electrolyte inside sub-nanometre pores, which is accompanied by their partial desolvation. Here we perform molecular dynamics simulations of realistic supercapacitors and show that this picture is correct at the microscopic scale. We provide a detailed analysis of the various environments experienced by the ions. We pick out four different adsorption types, and we, respectively, label them as edge, planar, hollow and pocket sites upon increase of the coordination of the molecular species by carbon atoms from the electrode. We show that both the desolvation and the local charge stored on the electrode increase with the degree of confinement.

We report a Monte Carlo and molecular dynamics simulations study of carbon dioxide in hydrated sodium montmorillonite, including thermodynamical, structural and dynamical properties. In order to simulate the behaviour of a clay caprock in contact with a CO 2 reservoir, we consider clays in equilibrium with H 2 O−CO 2 mixtures under conditions close to relevant ones for geological storage, namely a temperature T =348 K, and pressures P=25 and 125 bar, and under which two bulk phases coexist: H 2 O-rich liquid on the one hand and CO 2-rich gas (P=25 bar) or supercritical fluid (P=125 bar) on the other hand. We first use grand-canonical MC simulations to determine the number of stable states in clay, their composition and the corresponding equilibrium interlayer distances. The vertical, horizontal and radial distribution functions of the confined mixture, subsequently obtained using molecular dynamics, reveal some structural feature induced by the presence of CO 2. Finally, the simulations indicate that carbon dioxide considerably influences the diffusion of mobile species in clays. We discuss these results by comparing them with those obtained for the bulk mixtures, as well as for Namontmorillonite in equilibrium with a pure water reservoir water at the same temperature and pressure.

Supercapacitors based on an ionic liquid electrolyte and graphite or nanoporous carbon electrodes are simulated using molecular dynamics. We compare a simplified electrode model in which a constant, uniform charge is assigned to each carbon atom with a realistic model in which a constant potential is applied between the electrodes (the carbon charges are allowed to fluctuate). We show that the simulations performed with the simplified model do not provide a correct description of the properties of the system. First, the structure of the adsorbed electrolyte is partly modified. Second, dramatic differences are observed for the dynamics of the system during transient regimes. In particular, upon application of a constant applied potential difference, the increase in the temperature, due to the Joule effect, associated with the creation of an electric current across the cell follows Ohm's law, while unphysically high temperatures are rapidly observed when constant charges are assigned to each carbon atom.

Using molecular dynamics simulations with recently developed importance sampling methods, we show that the differential capacitance of a model ionic liquid based double-layer capacitor exhibits an anomalous dependence on the applied electrical potential. Such behavior is qualitatively incompatible with standard mean-field theories of the electrical double layer, but is consistent with observations made in experiment. The anomalous response results from structural changes induced in the interfacial region of the ionic liquid as it develops a charge density to screen the charge induced on the electrode surface. These structural changes are strongly influenced by the out-of-plane layering of the electrolyte and are multifaceted, including an abrupt local ordering of the ions adsorbed in the plane of the electrode surface, reorientation of molecular ions, and the spontaneous exchange of ions between different layers of the electrolyte close to the electrode surface. The local ordering exhibits signatures of a first-order phase transition, which would indicate a singular charge-density transition in a macroscopic limit.

We report a molecular dynamics study of the structure and dynamics of water at a clay surface. The negative charge of the surface and the presence of surface oxygen atoms perturbs water over two to three molecular layers, while the nature of the counterions (Na(+)or Cs(+)) has only a small effect. In the first molecular layer, approximately half of the water molecules are H-bonded to the surface. We also analyze the H-bond network between surface water molecules. The diffusion of water molecules along the surface is slowed down compared to the bulk case. As far as the orientational order and dynamics of the water dipole are concerned, only the component normal to the clay surface is perturbed. We investigate the surface H-bond formation and dissociation dynamics and their coupling to the release of molecules from the first molecular layer. We introduce a simple kinetic model in the spirit of Luzar and Chandler [Nature, 1996, 379, 55] to allow for a comparison with bulk water dynamics. This model semi-quantitatively reproduces the molecular simulation results and suggests that H-bond formation is faster with the surface than in the bulk, while H-bond dissociation is slower.

International audienceA recently developed coarse-grained model (J. Phys. Chem. B, 2010, 114, 12629-12631), previously validated against experimental data for a number of bulk properties, is used in molecular dynamics simulations of two different interfaces involving the ionic liquid [BMI][PF6]. First, simulations of the liquid-vapor interface demonstrate that the model is able to predict the surface tension of the fluid (for which we obtain a value of 39.4 mN*m-1 at 400 K). Second, simulations were performed at constant potential differences applied between two graphite electrodes. From simulations with different applied potentials, the differential capacitances of the positive and negative electrodes can be calculated. It appears that both capacitances (C+ = 3.9 μF*cm-2 for the positive electrode and C- = 4.8 μF*cm-2 for the negative electrode) agree very well with simulations results obtained with an all-atom model. The coarse-grained model also accurately reproduces the two-dimensional structure observed at the graphite-ionic liquid interface, namely, a defective hexagonal lattice with a lattice spacing of approximately 10 Å