Imagine being able to cover large surfaces with layers of oxides with controlled porosity, or to prepare glasses at room temperature: this is exactly what is possible with the sol-gel process, which emerged in the mid-19thcentury with the discovery by Frenchman Jacques-Joseph Ebelmen that silicic ether was gradually transformed into translucent silica by the action of moisture. The sol-gel process makes it possible to prepare materials with structures optimized for specific applications, using simple molecular precursors and easily controlled chemical reactions. The fundamental aspects of the process are described, including the choice of precursors and acid or basic catalysts, and the degree of progress and kinetics of hydrolysis and condensation/polymerization reactions. Building on these foundations, we present the sol-gel synthesis of organic/inorganic hybrid materials, distinguishing between class 1 and class 2 hybrid gels, with the latter involving the formation of covalent bonds between organic and inorganic parts.
Just as important as the formation of gels is their drying, which is governed by problems of capillarity, surface tension and liquid-vapor interface, the origins of which are explained. Drying with supercritical fluid (CO2) and freeze-drying, which short-circuits the liquid-vapor interphase, produce aerogels, while drying with a polar/non-polar solvent combination and evaporation produce materials with lower porosity, referred to as ambigel and xerogel respectively, each with their own specific applications. These are mentioned here, but only energy storage applications are detailed. The benefits of V2O5 aerogels for lithium-ion batteries are highlighted. Such applications require adjustments to the sol-gel process in order to prepare binary oxides whose Mn+4 is not stable in aqueous media, or even iron oxides and others. Described here are sol-gel processes based on the use of propylene oxide as a precursor for hydroxyl groups, which can then be condensed to produce aerogels. Aerogels of 3D metals (FeOX, CoOX) and rare earths, and even spinel compounds, have been obtained and their performance in batteries and fuel cells reported. Finally, we look at organic resorcinol-formaldehyde gels, which form the basis of carbon aerogels with a wide range of architectures and structures. To extend this lecture, we detail the extension of the polyol process to the synthesis of chalcogenide gels, composites and even sol-gel approaches in aqueous media, where oxygen is not supplied by water but by the reacting alcohol.
We conclude with the contribution of these open architectures to electrochemical storage, which offer considerable performance in terms of power, thanks to the presence of interconnected networks for ion and even oxygen transport, as well as attractive gravimetric performance. The fact remains, however, that the volumetric performance of these open structures is disastrous, which explains why they have difficulty getting past the laboratory door for applications in this field.
