The current interest in ionic liquids (ILs) is motivated by some unique properties, such as negligible vapour pressure, thermal stability and non-flammability, combined with high ionic conductivity and wide electrochemical stability window. However, for material applications, there is a challenging need for immobilizing ILs in solid devices, while keeping their specific properties. In this critical review, ionogels are presented as a new class of hybrid materials, in which the properties of the IL are hybridized with those of another component, which may be organic (low molecular weight gelator, (bio)polymer), inorganic (e.g.carbon nanotubes, silicaetc.) or hybrid organic–inorganic (e.g.polymer and inorganic fillers). Actually, ILs act as structuring media during the formation of inorganic ionogels, their intrinsic organization and physicochemical properties influencing the building of the solid host network. Conversely, some effects of confinement can modify some properties of the guest IL, even though liquid-like dynamics and ion mobility are preserved. Ionogels, which keep the main properties of ILs except outflow, while allowing easy shaping, considerably enlarge the array of applications of ILs. Thus, they form a promising family of solid electrolyte membranes, which gives access to all-solid devices, a topical industrial challenge in domains such as lithium batteries, fuel cells and dye-sensitized solar cells. Replacing conventional media, organic solvents in lithium batteries or water in proton-exchange-membrane fuel cells (PEMFC), by low-vapour-pressure and non flammable ILs presents major advantages such as improved safety and a higher operating temperature range. Implementation of ILs in separation techniques, where they benefit from huge advantages as well, relies again on the development of supported IL membranes such as ionogels. Moreover, functionalization of ionogels can be achieved both by incorporation of organic functions in the solid matrix, and by encapsulation of molecular species (from metal complexes to enzymes) in the immobilized IL phase, which opens new routes for designing advanced materials, especially (bio)catalytic membranes, sensors and drug release systems (194 references).

The confinement of ionic liquids within a porous silica matrix was performed by a one-step non-hydrolytic sol-gel route, leading to hybrid materials (called “ionogels”) featuring both the mechanical and transparency properties of silica gels and the high ionic conductivity and thermal stability of ionic liquids.

The surfaces of alumina particles were modified by grafting with phenylphosphonic acid and its organic-soluble ester derivatives (diethyl phenylphosphonate and bis(trimethylsilyl) phenylphosphonate). Solid-state ³¹P NMR spectroscopy indicated that in aqueous media the formation of bulk aluminium phosphonate phases could be avoided by using phenylphosphonic acid at pH 6. The formation of such phases was detected in organic media in the presence of phenylphosphonic acid or its silyl ester. On the other hand the use of the dialkyl ester derivative in organic media allowed controlled grafting, excluding the formation of phosphonate phases even under prolonged heating. Alternatively a two-step sol–gel process was carried out, which involved first the non-hydrolytic condensation between aluminium alkoxide and phenylphosphonic acid (or the parent bis(trimethylsilyl) ester), then the hydrolysis–condensation of the remaining Al–OR groups. ³¹P and ²⁷Al NMR spectroscopy proved the homogeneity of the solids obtained, even for P/Al ratios as high as 1.

This highlight article covers recent reports (2008–2013) in materials chemistry involving non-hydrolytic polycondensation reactions. Initially applied to non-hydrolytic sol–gel processing of silicas and metal oxides, nowadays these non-classical reactions have been extended to other processes (solvothermal synthesis, chemical solution deposition, atomic layer deposition), solvents (ionic liquids), O-donors (including cellulose), activation modes (microwaves), and materials (metal sulfides, carbon nanocomposites). A specific emphasis is put here on materials for electrochemical devices and environmental sustainability.

SnO₂ translucent monolith ionogels were obtained by a sol–gel processing using bis(2-methylbutan-2-oxy)di(pentan-2,4-dionato)tin as a precursor in the presence of various ionic liquids: [BMI][Br], [BMI][TFSI], [BMI][BF₄]. The confinement of ionic liquids within the gels was evidenced by Differential Scanning Calorimetry, FTIR and FT-Raman spectroscopy. The ionic liquids could be efficiently washed off, which resulted in supermicroporous solids. Calcination in air at 550 °C of the dried monoliths resulted in nanoporous nanocrystalline cassiterite tin dioxide particles with crystallite sizes of about 8–12 nm and mean pore sizes around 5 nm.