Nanomaterials in Advanced, High-Performance Aerogel Composites: A Review

Abstract

Aerogels are one of the most interesting materials of the 21st century owing to their high porosity, low density, and large available surface area. Historically, aerogels have been used for highly efficient insulation and niche applications, such as interstellar particle capture. Recently, aerogels have made their way into the composite universe. By coupling nanomaterial with a variety of matrix materials, lightweight, high-performance composite aerogels have been developed for applications ranging from lithium-ion batteries to tissue engineering materials. In this paper, the current status of aerogel composites based on nanomaterials is reviewed and their application in environmental remediation, energy storage, controlled drug delivery, tissue engineering, and biosensing are discussed.

Keywords: aerogel; aerogel composites; biomedical engineering; cellulose; cellulose nanofibers; energy storage; graphene; nanocomposites; sensors; tissue engineering.

Figure 1

Comparison of aerogel fabrication strategies showing typical transitions into an aerogel. (a) shows the supercritical drying process where precursor materials undergo gelation prior to supercritical drying. Often, these processes include a solvent exchange step after gelation to provide better fluids for supercritical drying. (b) shows a standard freeze-drying technique where an aqueous solution is frozen and the ice crystal formation dictates the alignment of the precursor materials and thus, the resulting pore structure of the dried aerogel.

Figure 2

A typical phase diagram for pure compounds. Two methods are shown for the gel to aerogel transition, indicated by I → II. The solid-gas transition depicts the transition from a frozen gel (I) to the dried porous gel (II) during freeze-drying (Section 2.2). The transition from a liquid to gas during supercritical drying requires a rise in temperature and pressure (curved arrow from I → II) to avoid crossing the liquid-gas phase boundary (Section 2.1). This pass into the supercritical region eliminates surface tension and capillary forces.

Figure 3

Schematic depiction of the three main 3D-printing techniques employed for the fabrication of aerogels. (a) Stereolithography, where a laser is used to transform the sol to a gel during the printing process; (b) ink-jet printing, where a solution is printed into its desired structure prior to observing gelation; and (c) direct ink writing, where the gel is formed prior to printing and the gel is extruded in order to achieve the desired structure.

Figure 4

(a) Fabrication of CNT aerogels using (P3HT-b-PTMSPMA), (b) scanning electron microscopy (SEM) image of the macroporous honeycomb structure of the aerogel with ~100-nm-thick walls, and (c) SEM image of the aerogel walls with entangled CNTs. Reprinted from [56] with permission, copyright American Chemical Society, 2010.

Figure 5

The DIW of aerogels in two distinct macrostructures, (a) a cube and (b) an ear. (c) The structural porosity, accounting for pores in the range of 600 μm (green box) and the aerogel porosity, accounting for pores in the range of 20 to 800 μm. Reprinted from [90] under open access license.

Figure 6

Depiction of the DIW of CNF aerogels (a) and four different methods of drying (b). From left to right, the CNF hydrogel drying procedures are (i) air drying), (ii) air drying in the presence of surfactants, (iii) solvent exchange before drying, and (iv) freeze-drying. (c) SEM micrographs of the resulting microstructures of such dried CNF aerogels. Adapted from [92] with permission, copyright John Wiley & Sons, 2016.

Figure 7

The processing schematic of aerogel fabrication via the electrospinning of nanofiber mats. Reprinted from [102] with permission, copyright American Chemical Society, 2018.

Figure 8

(a) HTC synthesis of nitrogen-doped carbon nanofiber aerogels; (b) image of large-scale quantities of such a product, and (c) and (d) SEM micrographs of images at different magnifications. Reproduced from [107], copyright Elsevier, 2015.

Figure 9

Schematics and images from the radially grown GO aerogels. (a) Illustrates the ice-templating process, (b) shows scanning electron microscopy (SEM) images of the radially aligned aerogel, (c) illustrates the decreasing width of the radial channels, which decrease from edge to center, and (d–f) show SEM micrographs of the sections outlined in (c). Adapted from [22] with permission, copyright American Chemical Society, 2018.

Figure 10

Reversible compressibility of various graphene aerogels created via DIW. Image (a) shows the behavior of a bulk graphene aerogel (31 mg cm⁻³), (b) 3D-printed graphene aerogel (16 mg cm⁻³), (c) bulk graphene aerogel (123 mg cm⁻³), and (d) 3D-printed graphene aerogel (53 mg cm⁻³) using resorcinol-formaldehyde. Reprinted from [130] under open access license.

Figure 11

Schematic of 3D graphene lattice fabrication with photocurable hollow polymer architecture. Adapted from [31] with permission, copyright American Chemical Society, 2018.

Figure 12

Structure design and fabrication of the ceramic aerogel metamaterial. (a) Illustration of the metastructure design of ceramic aerogels. The units of the colored scale bars are as follows: kilopascals for NPR and percentage (with strain zoomed by 30 times) for NTEC. (b) Illustration of the CVD synthesis process of the double-paned hyperbolic ceramicaerogels. (c) An optical image showing an h-BNAG sample resting on the stamen of a flower. (d) SEM image of h-BNAG. (e) SEM images of the double-pane wall structure of h-BNAGs. Scale bars, 20 nm. Adapted from [138] with permission, copyright the American Association for the Advancement of Science, 2019.

Figure 13

(a) A schematic illustration of fabricating hydrophobic cellulose aerogels. (b) A picture showing water droplets on the aerogels. (c) Pictures showing recovering of an aerogel from compression. Adapted from [152] with permission, copyright the American Chemical Society, 2019.

Figure 14

Optical interrogation of aerogel fabrics’ gross and fine structure. Image of a dyed-green water droplet and Iraq oil on the surface of (a) TW (6 mm thick) and (d) SL (10 mm thick). Insets are images from the WCA measurements. Please note that the TW used is thinner than SL. SEM images of the surfaces of (b and c) TW and (e and f) SL, where aerogel particles are visible on the fibers in high-resolution. Reprinted from [153] with permission, copyright American Chemical Society, 2016.

Figure 15

(a) Schematic illustration showing the fabrication process of P-GA. (b,c) SEM images of the GA (b) and P-GA (c). (d) HRTEM images of P-GA. (e) Schematic showing ASC device construction using MnO₂ and P-GA electrodes. (f) Ragone plot reflecting the superiority of P-GA nanostructures over other electrode materials. The inset shows a red LED powered by using two devices connected in series. Reprinted from [167] with permission, copyright American Chemical Society, 2015.

Figure 16

Schematic of the processing of 3D-printed graphene-based composites for supercapacitors. Reproduced from [174] with permission, copyright John Wiley & Sons, 2018.

Figure 17

Scanning electron microscopy (SEM) images of microgel particles printed via IJP of aqueous alginate solutions into baths of CaCl₂ with (a,c–e) direct or (b) sequential solvent exchange. Reproduced from [200] with permission, copyright Elsevier B.V., 2018.

Figure 18

Schematic illustration of the fabrication of 3D hybrid aerogels from polymer nanofibers and bioglass nanofibers. Reprinted from [192] with permission, copyright John Wiley & Sons, 2018.

Figure 19

(A) Wound healing study where a–f (top row) show the collagen control aerogel at day 0, 3, 9, 12, 15, and 18 and g−l (bottom row) show the 12% collagen-wheat grass aerogel at day 0, 3, 9, 12, 15, and 18. Image (B) shows the wound healing as a percent of wound reduction. Adapted from [205] with permission, copyright American Chemical Society, 2017.