Bunching and Immobilization of Ionic Liquids in Nanoporous Metal–Organic Framework

Abstract

Room-temperature ionic liquids (ILs) are a unique, novel class of designer solvents and materials with exclusive properties, attracting substantial attention in fields like energy storage and supercapacitors as well as in ion-based signal processing and electronics. For most applications, ILs need to be incorporated or embedded in solid materials like porous hosts. We investigate the dynamic structure of ILs embedded in well-defined pores of metal–organic frameworks (MOFs). The experimental data combined with molecular dynamics simulations unveil astonishing dynamic properties of the IL in the MOF nanoconfinement. At low IL loadings, the ions drift in the pores along the electric field, whereas at high IL loadings, collective field-induced interactions of the cations and anions lead to blocking the transport, thus suppressing the ionic mobility and tremendously decreasing the conductivity. The mutual pore blockage causes immobilized ions in the pores, resulting in a highly inhomogeneous IL density and bunched-up ions at the clogged pores. These results provide novel molecular-level insights into the dynamics of ILs in nanoconfinement, significantly enhancing the tunability of IL material properties.

Insights in the Ionic Conduction inside Nanoporous Metal-Organic Frameworks by Using an Appropriate Equivalent Circuit

“…Thus, we believe the presented method will help to explore the MOFsupport interface and will contribute to enhanced MOF-device performances. pore filling was previously determined [27]. For the preparation of the IL@MOF-pellets, we followed standard directions [45].…”

“…The experimental data can be well described by the suggested equivalent circuit. [27] with copyright permission from 2019 American Chemical Society.…”

“…This indicates that a (purely phenomenological) description of the data by a Randles-cell-like circuit results in a larger deviation than by the (R-CPEdl)‖CPEgeo circuit. Indeed, in comparison to the fits with the Randles-cell-like circuit [26,27], the (R-CPEdl)‖CPEgeo circuit allows a significantly better reflection of the experimental data (see Appendix A, Figures A2-A5). As a reference, the Nyquist plots of pure IL and of protic liquids, triazole, deionized water, and butanediol, on the substrates with the deposited electrodes are shown in the appendix, Figure A1.…”

“…Often, in addition to the high-frequency semi-circle, a low-frequency ray or spike is also observed. In many studies [ 20 , 24 , 25 , 26 , 27 , 28 , 29 ], the experimental impedance data are then described by an equivalent circuit similar to the Randles circuit, where the Nyquist plot shows a semi-circle at high frequencies with a diameter of an ohmic resistance connected to a low-frequency ray with an angle of 45°. The Randles equivalent circuit is composed of an ohmic resistance in serial with a Warburg (diffusion) element in parallel combination with a capacitance, i.e., it is an RC -element with a Warburg resistance in serial to R [ 14 ].…”

Introducing electrical conductivity to metal–organic framework thin films by templated polymerization of methyl propiolate

“…This is about 6 orders of magnitude larger than the conductivity of the empty SURMOF system 3, were we find a value of 8 pS/m, which is in line with other published values for SURMOFs and for HKUST-1 loaded with 7,7,8,8-tetracyanoquinododimethane. [54][55][56]…”