Electrochemical Double Layers in Confined Geometries

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In this project we want to lay the fundamentals for a description of electrolytes in nanoconfined structures at an atomic level. During the first part we have investigated single ions and ion pairs in gold and carbon nanotubes by density functional theory (DFT). Alkali and halide ions placed in narrow tubes ionized spontaneously, the transferred charge forming an image charge on the walls of the tube. We characterized the electrostatic interactions in the tube by the concept of an effective image radius, so that we could transfer our results to ensembles of ions in nanotubes, which we examined by statistical mechanics and by grand-canonical Monte Carlo simulations. In the first period we published six articles, one of them an invited chapter. Perhaps the most spectacular result is an apparent attraction between two Li-ions inside and outside a semiconducting carbon nanotube, which may explain a similar attraction observed in Li-batteries. The elucidation of this effect, for which we have a working hypothesis, will be one of the centers of the second period. In additions we shall follow our original work plan and progress to larger and more complicated systems. Thus we will pass from one-dimensional to two-dimensional systems and then to slits with a finite width, in order to study the effect of the geometry on the confined electrolytes. In addition we shall study the influence of solvation on ions in nanotubes and slits, including transport in these structures. Another topic is the effect of doping of carbon tubes on their ability to screen an external field, which influences the capacity.Our general aim is to obtain the basic properties of the nanosystems and the interaction between the various particles from DFT, and use them to describe ensembles of particles in a grand-canonical formalism, in which both the interfacial charge and changes in the electrode potential are well defined.We have agreed to cooperate with two experimental groups: Prof. Kaiser, Ulm, who investigates carbon structures with high-resolution transmission electron microscopy, and Prof. Unwin, Warwick, who can investigate electrochemical processes with ultra-high resolution.We are principally interested in the fundamental properties of these confined systems: the distribution of particles and of the electrostatic potential; their capacity to store charge; their equilibrium with a bulk solution. The systems we investigate can be used in supercapacitors and in lithium ion batteries.