Double-layer properties of several modifications of carbon - a theoretical study

WOLFGANG SCHMICKLER; NOELIA LUQUE; PAOLA QUAINO; FERNANDA JUAREZ; LEILA MOHAMMADZADEH

Niagara Falls

Congreso; 15th Topical Meeting of the International Society of Electrochemistry; 2014

the International Society of Electrochemistry

Carbon is a very versatile electrode material and exists in various forms, from graphite to graphene, nanotubes, and diamond. We have investigated the double-layer properties of several modifications of interest to electrochemistry. Graphite, which is the most common electrode carbon electrode, has an indirect band gap and a low density of states at the Fermi level. As a consequence, an external electric field can penetrate over a distance of several Angstroms into the surface. Consequently, the double-layer capacity of a graphite electrode in contact with an aqueous solution is small, and it has a pronounced minimum at the potential of zero charge. Our model calculations, performed for high electrolyte concentrations, agree quantitatively with experimental data. Doping carbon with nitrogen significantly changes its electronic properties. In particular, it enhances the density of states near the Fermi-level, which entails a better screening of external fields and an increase of the interfacialcapacity. However, it still does not quite reach the capacity of a metal in contact with the same solution.Carbon nanotubes are of great interest for supercapacitors. Experimental and theoretical results indicate that the capacity of very small nanotubes filled with an electrolyte solution is greatly enhanced by the image interaction of the ions with the surroundings. In order to investigate this effect, we have performed DFT calculations for a cesium atom inside a thin carbon tube. The cesium atom loses an electron and forms a cation; the negative charge is distributed on the surrounding carbon rings. From the charge distribution, we can calculate the image potential experienced by  the cation. This potential is a prerequisite for understanding the capacity of nanopores.