Confinement, Desolvation, And Electrosorption Effects on the Diffusion of Ions in Nanoporous Carbon Electrodes

Abstract

Supercapacitors are electrochemical devices which store energy by ion adsorption on the surface of a porous carbon. They are characterized by high power delivery. The use of nanoporous carbon to increase their energy density should not hinder their fast charging. However, the mechanisms for ion transport inside electrified nanopores remain largely unknown. Here we show that the diffusion is characterized by a hierarchy of time scales arising from ion confinement, solvation, and electrosorption effects. By combining electrochemistry experiments with molecular dynamics simulations, we determine the in-pore conductivities and diffusion coefficients and their variations with the applied potential. We show that the diffusion of the ions is slower by 1 order of magnitude compared to the bulk electrolyte. The desolvation of the ions occurs on much faster time scales than electrosorption.

Multi-scale modelling of supercapacitors: From molecular simulations to a transmission line model

“…More recently, He et al have also oberved such effects in molecular dynamics simulations when the size of the slitpores match the ionic dimensions 10 . In the case of the adsorption of electrolytes in more complex nanoporous carbons with several types of pores, the enhancement of the diffusivity is not observed 11,12 . However the diffusion coefficient values remain reasonable since they are on average lower than the bulk liquid ones by one order of magnitude only, which explains the fast charging of these supercapacitors.…”

Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy

“…Theoretical studies based on molecular dynamics (MD) and meanfield theories have shown that ion-ion interactions, ion-carbon interactions and the electrode pore size all influence the rates of ionic diffusion in supercapacitor electrodes, 16,18,23,24 though there is not yet a clear consensus on which factors are most important, nor the order of magnitude by which diffusion is influenced by confinement and by charging. Experimental methods to directly probe inpore motion in an ion-selective and electrode-selective way have until now been lacking.…”

“…23 PFG NMR experiments provide additional dynamic information about the exchange (or swapping) of ions between in-and ex-pore environments (Figure 2d). A in-pore gives the mole fraction of in-pore ions that have remained in the nanopores during the observation time, Δ.…”

“…This is unsurprising since diffusion measurements on neat electrolyte showed that the solvent diffuses 2.0 times faster than the anions, and 2.5 times faster than the cations. MD simulations have also shown that (de)solvation of in-pore ions with acetonitrile takes place as quickly as ~1 ps, 23 and a recent study of dimethyl carbonate showed a relatively modest reduction of effective diffusion (by a factor of ~ 0.2) upon confinement in a nanoporous carbon. 35 Overall, the slower diffusing ions move through a highly dynamic solvent medium in the nanopores.…”

“…This increase of in-pore ion population correlates with the reduction of D in-pore for both ions: i.e., it appears that increasing ion-ion interactions lead to a reduction of D in-pore with 7 voltage for negative polarisations. In principle ion-carbon interactions may also influence D in-pore , 23 though these appear to be a minor factor, since the anions and cations show such similar variations of D in-pore with voltage.…”