Holding it together: noncovalent cross-linking strategies for ionogels and eutectogels

Abstract

When a hydrogel simply won’t cut it – either because it dries out too quickly, or it does not tolerate more than roughly one volt when applied in an electrochemical device – where is the savvy materials researcher to turn? This is where two important classes of nonaqueous gel counterparts, known as ionogels and eutectogels, can truly shine. Replacing the aqueous liquid phase of a hydrogel with either an ionic liquid (IL) or a deep eutectic solvent (DES) allows one to realize an array of versatile gel electrolyte materials that offer outstanding nonvolatility, wider windows of electrochemical stability, reasonably high ionic conductivity, and nearly unlimited chemical design possibilities. In addition to choosing a specific IL or DES, there are a myriad of options when it comes to constructing a solid, three-dimensional, volume-spanning network (or scaffold) that will support the nonaqueous liquid phase of an ionogel or eutectogel. In this focused review, several recent approaches to forming these gels using noncovalent scaffold assembly and cross-linking are examined, and the primary noncovalent interactions responsible (e.g. hydrogen bonding, solvophobicity, coulombic interactions) are identified. Noncovalent scaffold assembly in nonaqueous, ion-dense electrolytes often leads to supramolecular gel materials that can exhibit extreme stretchability, good toughness, and an ability to self-heal in many cases. After reviewing several strategies that have been recently employed for creating ionogels and eutectogels, a brief inspection of some motivating noncovalently cross-linked scaffolds reported for hydrogels is presented with the hopes that these may provide inspiration for the future design of novel ionogels and eutectogels by the materials research community.