Engineering encapsulated ionic liquids for next-generation applications

Abstract

Ionic liquids (ILs) have attracted considerable attention in several sectors (from energy storage to catalysis, from drug delivery to separation media) owing to their attractive properties, such as high thermal stability, wide electrochemical window, and high ionic conductivity. However, their high viscosity and surface tension compared to conventional organic solvents can lead to unfavorable transport properties. To circumvent undesired kinetics effects limiting mass transfer, the discretization of ILs into small droplets has been proposed as a method to increase the effective surface area and the rates of mass transfer. In the present review paper, we summarize the different methods developed so far for encapsulating ILs in organic or inorganic shells and highlight characteristic features of each approach, while outlining potential applications. The remarkable tunability of ILs, which derives from the high number of anions and cations currently available as well as their permutations, combines with the possibility of tailoring the composition, size, dispersity, and properties (e.g., mechanical, transport) of the shell to provide a toolbox for rationally designing encapsulated ILs for next-generation applications, including carbon capture, energy storage devices, waste handling, and microreactors. We conclude this review with an outlook on potential applications that could benefit from the possibility of encapsulating ILs in organic and inorganic shells.

Fig. 1. Number of publications per year that include “ionic liquid” (black bars) or “encapsulated ionic liquid” (red bars) in the title, abstract, or keywords (data obtained from Web of Science in February 2021).

Fig. 2. Schematic representations of the different methods used for the preparation of encapsulated ILs.

Fig. 3. Optical micrograph (a), SEM (b), and TEM micrograph of ultrathin cross-section (c) of poly(triethylene glycol dimethacrylate/n-butyl methacrylate)/poly(n-butyl methacrylate)/[HMIM][TFSI] (P(TEGDM-BMA)/P(BMA)/[HMIM][TFSI]) composite particles prepared by microemulsion polymerization. [Reprinted with permission from ref. . Copyright (2013) Springer].

Fig. 4. SEM micrographs of [BMIM][PF₆] in polyurea capsules prepared via emulsion polymerization in water (top) and in oil (bottom). [Reprinted from React. Funct. Polym., Weiss, E.; Gertopski, D.; Gupta, M. K.; Abu-Reziq, R. Encapsulation of Ionic Liquid BMIm[PF₆] within Polyurea Microspheres. 2015, 96, 32–38, with permission from Elsevier].

Fig. 5. (a) Schematic representation for the preparation of encapsulated IL using either IL-in-water or IL-in-oil Pickering emulsions stabilized by GO nanosheets or alkylated GO nanosheets followed by interfacial polymerization; (b) photograph (i) and SEM micrographs ((ii) and (iii)) of ILs capsules. [Reprinted with permission from ref. . Copyright (2019) American Chemical Society].

Fig. 6. SEM (a) and TEM (c) micrographs of empty hollow carbonaceous submicrocapsules. SEM (b) and TEM (d) micrographs of hollow carbonaceous submicrocapsules containing 1-butyl-methylimidazolium acetate ([BMIM][OAc]). [Reprinted with permission from ref. . Copyright (2019) American Chemical Society].

Fig. 7. TEM micrographs of Pd/[BMIM][PF₆] in SiO₂, with black circles highlighting Pd nanoparticle formation within the capsules. [Reprinted with permission from ref. . Copyright (2014) American Chemical Society].

Fig. 8. (A) Schematic of a column containing IL capsules that is employed for extracting phenol from hexane solutions. Photographs of the IL capsules and packed column are also displayed. (B) Phenol removal for the case of [BMIM][BF₄] capsules (blue trace) and the capsule shell (red trace, after IL extraction). [Reprinted with permission from ref. . Copyright (2019) American Chemical Society].

Fig. 9. Specific capacitances for coin-cell supercapacitors made of anode/cathode material containing rGO-IL capsules, rGO-IL capsule shells, and YP-50 as a function of the cyclic voltammetry scan rate (red trace, after IL extraction). [Reprinted with permission from ref. . Copyright (2019) American Chemical Society].