The importance of ion size and electrode curvature on electrical double layers in ionic liquids
- 15 November 2010
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Physical Chemistry Chemical Physics
- Vol. 13 (3), 1152-1161
- https://doi.org/10.1039/c0cp02077j
Abstract
Room-temperature ionic liquids (ILs) are an emerging class of electrolytes for supercapacitors. We investigate the effects of ion size and electrode curvature on the electrical double layers (EDLs) in two ILs 1-butyl-3-methylimidazolium chloride [BMIM][Cl] and 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6], using a combination of molecular dynamics (MD) and quantum density functional theory (DFT) simulations. The sizes of the counter-ion and co-ion affect the ion distribution and orientational structure of EDLs. The EDL capacitances near both planar and cylindrical electrodes were found to follow the order: [BMIM][Cl] (near the positive electrode) > [BMIM][PF6] (near the positive electrode) ≈ [BMIM][Cl] (near the negative electrode) ≈ [BMIM][PF6] (near the negative electrode). The EDL capacitance was also found to increase as the electrode curvature increases. These capacitance data can be fit to the Helmholtz model and the recently proposed exohedral electrical double-cylinder capacitor (xEDCC) model when the EDL thickness is properly parameterized, even though key features of the EDLs in ILs are not accounted for in these models. To remedy the shortcomings of existing models, we propose a “Multiple Ion Layers with Overscreening” (MILO) model for the EDLs in ILs that takes into account two critical features of such EDLs, i.e., alternating layering of counter-ions and co-ions and charge overscreening. The capacitance computed from the MILO model agrees well with the MD prediction. Although some input parameters of the MILO model must be obtained from MD simulations, the MILO model may provide a new framework for understanding many important aspects of EDLs in ILs (e.g., the variation of EDL capacitance with the electrode potential) that are difficult to interpret using classical EDL models and experiments.Keywords
This publication has 51 references indexed in Scilit:
- Microstructure and Capacitance of the Electrical Double Layers at the Interface of Ionic Liquids and Planar ElectrodesThe Journal of Physical Chemistry C, 2009
- Ionic Liquid Near a Charged Wall: Structure and Capacitance of Electrical Double LayerThe Journal of Physical Chemistry B, 2008
- Differential Capacitance of the Electrical Double Layer in Imidazolium-Based Ionic Liquids: Influence of Potential, Cation Size, and TemperatureThe Journal of Physical Chemistry C, 2008
- Surface Structure at the Ionic Liquid−Electrified Metal InterfaceAccounts of Chemical Research, 2008
- Ionic Liquid Structure Dependent Electrical Double Layer at the Mercury InterfaceThe Journal of Physical Chemistry C, 2008
- Polarization Relaxation in an Ionic Liquid Confined between Electrified WallsThe Journal of Physical Chemistry B, 2007
- Electrochemical Aspects of Ionic LiquidsPublished by Wiley ,2005
- Ionic Liquids with Low Melting Points and Their Application to Double-Layer Capacitor ElectrolytesElectrochemical and Solid-State Letters, 2002
- Room-Temperature Ionic Liquids. Solvents for Synthesis and CatalysisChemical Reviews, 1999
- Differential Capacitance Measurements in Solvent‐Free Ionic Liquids at Hg and C InterfacesJournal of the Electrochemical Society, 1997