Application of aerogel and its composite materials in energy related technologies

Aerogel was first synthesized by Samuel Kistler in 1932, who defined aerogel as a material that keeps its pores and network structure intact after pore fluid is exchanged with gas. Supercritical drying is the key technology to prevent network collapse during wet gel exchange. In general, aerogel is a special form of very low-density porous material that consists of individual nanoparticles connected to each other to form a three-dimensional network. The unique properties of these highly porous materials with open pores and high specific surface areas are attributed to their irregular solid structure, which can be adjusted by appropriate preparation conditions.

Kistler synthesized a variety of inorganic aerogels, including silicon dioxide, alumina, tungsten oxide, iron oxide, tin oxide and nickel tartrate. In addition to inorganic aerogels, Keast also prepared organic aerogels based on cellulose, nitrocellulose and AGAR. After pacala developed resorcinol formaldehyde aerogel in 1989, the area of organic aerogel expanded rapidly. Subsequently, phenol formaldehyde, melamine formaldehyde, polyacrylamide, polyacrylonitrile, polyacrylate, cyanurate, polyimide, polystyrene, polyurethane and epoxy aerogel were developed. These studies subsequently led to the development of a new class of porous carbons, carbon aerogels obtained through the pyrolysis of organic aerogels. Due to the loss of oxygen and hydrogen functions during pyrolysis, they are high purity carbon structures. Today, scientists are still developing new aerogels that perform better than current aerogels for a variety of uses. Graphene or metal chalcogenide aerogels can be examples of these new aerogels.

Synthesis and energy-related applications

Gel technique is a common method for preparing any type of aerogel. Metal alcohols are common chemical precursors for inorganic aerogel synthesis, while organic monomers are the precursors for organic aerogel synthesis. Hydrolysis and condensation reactions form independent colloidal particles dispersed in solution, known as sol. The subsequent condensation reaction that occurs in this solution results in the formation of a gel, which is a continuous rigid network consisting of submicron sized holes filled with liquid and polymer chains with average lengths greater than microns. Because of the importance of these hydrolysis and condensation reactions to control network structure, they have been extensively studied. These gels, also known as wet gels (or alcohol gels), are first aged in an appropriate solution to complete the unfinished hydrolysis and condensation reactions and strengthen the solid network. Then through the solvent exchange step, the liquid in the hole is exchanged with the solvent soluble in supercritical carbon dioxide. Supercritical drying is the last step in the preparation of aerogel. Its purpose is to remove the solvent from the aerogel hole without causing the collapse of the aerogel hole.

Aerogel is a promising new energy related material with excellent properties such as low thermal conductivity, low sound velocity, high optical transparency, high electrical conductivity and high specific surface area. Aerogel can be used for thermal insulation, catalysis, photocatalysis, adsorption and energy storage. There are already a variety of aerogel insulation products on the market. Aerogel is being studied as a carrier for fuel cell (FCs) electrocatalysts, as an adsorbent for CO2 removal or H2 storage, and as an energy storage element for double electric layer (EDL) capacitors.

Silica aerogel is the most widely studied type of inorganic aerogel, mainly due to its superior thermal insulation properties. They belong to the traditional sol-gel synthesis of mesoporous materials, with low density, narrow aperture distribution, low thermal conductivity, large specific surface area, good transparency and other characteristics. The physicochemical properties of these aerogels depend on the chemical properties of the precursor, the concentration of reactants, reaction conditions and drying process. Various forms of silica aerogel, such as whole, granular or powder, have been used for insulation purposes. Silica aerogel also has good properties, making it suitable for use as a sound insulation material, adsorbent, catalyst or storage medium. Because nonporous organic materials have lower thermal conductivity than inorganic materials under comparable conditions, organic aerogels are also promising for thermal insulation applications. The preparation of these organic aerogels is based on the same sol-gel technique in which multifunctional organic monomers are polymerized in dilute solution prior to supercritical drying. The most common organic aerogel is resorcinol formaldehyde aerogel, which is prepared from the alkali-catalyzed condensation of resorcinol and formaldehyde in aqueous solution and then through a series of steps such as curing, solvent exchange and supercritical drying. The porous network structure of these organic aerogels is similar to that of inorganic aerogels. They are also used as absorbents, shock absorbers, nuclear waste storage materials, batteries, sensors and catalysis.

Carbon aerogel is carbonized by pyrolytic pyrolysis of resorcinol - formaldehyde aerogel in inert atmosphere. As with other aerogels, the aperture and particle size of the mesoporous and microporous solids of these open structures can be adjusted by changing the synthesis conditions. They have high porosity, high specific surface area, low density and sharp pore size distribution. Carbon aerogels are attractive as examples of energy-related technologies such as high-temperature thermal insulators, electrodes in supercapacitors, and gas diffusion layers in FCs. Graphene aerogels developed in recent years have high thermal conductivity and have broad application prospects in energy storage, catalysis and sensors.