The electric double layer is generally viewed as simply the boundary that interpolates between an electrolyte solution and a metal surface. Contrary to that view, recent studies have shown that the interface between ionic liquids and metallic electrodes can exhibit structures and fluctuations that are not simple reflections of surrounding bulk materials [1]. The charge of the electrode is screened by the interfacial fluid and induces subtle changes in its structure, which cannot be captured by the conventional Gouy-Chapman theory.

 In recent years, this topic has been more intensively addressed in order to develop more efficient supercapacitors [2]. The latter are electrochemical devices that store the charge at the electrode/electrolyte interface through reversible ion adsorption. In order to understand the molecular mechanisms at play, we have performed molecular dynamics simulations on a variety of systems made of ionic liquids and electrodes of different geometries ranging from planar to nanoporous. A key aspect of our simulations is to use a realistic model for the electrodes, by allowing the local charges on the atoms to vary dynamically in response to the electrical potential caused by the ions and molecules in the electrolyte [3].

These simulations have allowed us to gain strong insight on the structure and dynamics of ionic liquids at electrified interfaces. From the comparison between graphite and nanoporous carbide-derived carbon electrodes, we have elucidated the microscopic mechanism at the origin of the increase of the capacitance enhancement in nanoporous carbons [4]. The simulations also provide us the diffusion coefficients of the ions and the charging times for the full supercapacitor device [5]. More recently, we have focused on the study of graphene-based supercapacitors [6], as well as of highly concentrated electrolytes, namely the family of « water-in-salts » that has recently been introduced [7].

References:

1.       Fedorov, M.V., Kornyshev, A.A., Chem. Rev., 114 (2014), 2978-3036

2.       Salanne, M., Rotenberg, B., Naoi, K., Kaneko, K., Taberna, P.L., Grey, C.P., Dunn, B., Simon, P., Nature Energy, 1 (2016), 16070

3.       Siepmann, J.I., Sprik, M., J. Chem. Phys., 102 (1995), 511-524         

4.       Merlet, C., Rotenberg, B., Madden, P.A., Taberna, P.L., Simon, P., Gogotsi, Y., Salanne, M., Nature Materials, 11 (2012), 306-310

5.       Pean, C., Daffos, B., Rotenberg, B., Levitz, P., Haefele, M., Taberna, P.L., Simon, P., Salanne, M., J. Am. Chem. Soc., 137 (2015), 12627-12632

6.       Mendez-Morales, T. et al., Energy Storage Materials, 17 (2019), 88-92

7.       Dubouis, N. et al., ACS Cent. Sci., 5 (2019), 640-643