The Electric Double Layer in Carbon Nanotubes and -slits

WOLFGANG SCHMICKLER; FERNANDA JUAREZ; DOMINGUEZ-FLORES, FABIOLA; ELISABETH SANTOS; PAOLA QUAINO

Belgrade

Congreso; 71st Annual Meeting of the International Society of Electrochemistry; 2020

International Society of Electrochemistry

The diameters of nanotubes and nanopores can be much smaller than the Debey lengths of electrolytesolutions. In this case, conventional double-layer theory cannot be applied. In order to understand thebehavior of electrolytes under such conditions, we explore the interactions between ions, molecules, andcarbon nanostructures such as graphene and nanotubes. Because of the low density of electronic states theadsorption or insertion of ions modifies the electronic properties of the carbon structures. In particular,they make semiconducting nanotubes locally conducting, converting them into degenerate n- or p-typesemiconductors.In contrast, the interaction of water with carbon structures is weak. In the case of narrownanotubes, the image charge on the walls of the tube weakens the solvation of ions with water. Also, thepresence of water hardly affects the interaction of ions across the walls of a nanotube.On the basis of the elementary interaction of ions with carbon materials we have performedMonte Carlo simulations for an ensemble of ions inside a carbon nanoslit in the grand-canonicalensemble. Two regimes can be distinguished: A gas-like filling of the slit with a low density of ions, anda disordered, glass-like structure at high ionic concentrations. The two regimes are separated by atransition region reminiscent of a phase transition, in which transient, string-like structures are formed –see figure. Both the charge and the electrochemical potential are well defined in these ensembles. Thisallows us to calculate capacity curves without introducing adjustable parameters. The structure of thesecurves, in particular the number of capacity humps, depends on the details such as the hydrophilicity ofthe slit, and the charge number of the ions. A few typical examples will be presented.