Leonard DICK1, Barbara KIRCHNER1
1Clausius Institute of Physical and Theoretical Chemistry, University of Bonn, Bonn, Germany
Electric double-layer capacitors (EDLCs) which are built with porous conductive carbon electrodes show an exceptional electrochemical performance in combination with ionic liquids. The storage device offers an outstandingly large power density, which is achieved through reversible ion adsorption at and inside the porous structures. Although the energy density is low compared to conventional batteries, EDLCs are important components for short term energy storage and in applications requiring fast charge/discharge cycles.
It is shown in literature, that a significant increase in capacitance can be achieved by adjusting the pore size of the porous carbon material to the ion size of the ionic liquid. The mechanism of the effect, however, is not completely understood as of yet.
By using classical molecular dynamics simulations and an all atom force field, we investigated the behaviour of an ionic liquid ([BMIM][OAc]) in contact with CNTs of different sizes to model the porous character of a carbon electrode in contact with the electrolyte in an EDLC. The behaviour of the ionic species in confined space is studied in detail, with various new tools being developed to extract the wanted properties from the produced trajectory file. By comparing the ions differing conformations and intermolecular ordering in the bulk and inside the CNT throughout the simulation, we identified conformational changes as well as structural changes of the liquid in confinement. These structural changes of the IL inside a CNT, depending on the CNTs pore size and also referred to as 'frustrated packing', leads to a decrease of the ILs density and therefore to the increase of voids between the ions.