Graphene aerogels: a review

GO as a hydrophilic compound forms a stable solution in water [94], but this breaks down in strong acidic aqueous media because of the insufficient mutual repulsion [95]. However, if the pH of the GO solution is decreased, the electrostatic repulsion weakens and the hydrogen bonding increases due to the protonation of carboxyls. This leads to the strengthening of the hydrogen bonding and, the formation of a stable GO gelation. Generally speaking, cross-linking agents promote gelation of GO sheets by strengthening the bonding force between the sheets. Common cross-linkers are hydroxyl [70], oxygen-containing [94] or nitrogen functional groups [96].

It has been found that the self-assembly and gelation of GO sheets is promoted by divalent and trivalent ions like Ca²⁺, Mg²⁺,Pb²⁺,Cr³⁺ [94, 97] and other. These metal ions interact with single GO by a bonding force. The concentration of GO affects the pore density and size.

The sol–gel method is another strategy to develop GAs. In this method, the bonding between GO sheets is stronger than the cross-linking method, because polymerization creates covalent bonds between the sheets [72]. Meng et al [44] have reported that alkali-treated GO (AGO) acts as solid base catalyst to initiate the polymerization of resorcinol with formaldehyde (figure 1), which by pyrolization turns into graphene. The graphene/carbon aerogels that were synthesized can be used in supercapacitors because of their specific capacitance (122 F g⁻¹ at a constant current density 50 mA g⁻¹). The final composite graphene/carbon aerogels show large surface area (361–763 m² g⁻¹), low density (0.11–0.19 g cm⁻³), high pore volume, narrow pore size distribution (10–50 nm) and high conductivity (528 S m⁻¹).

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Lin et al [100] presented a freeze gelation (RTFG) method at RT to prepare pristine graphene (PG) aerogels starting from exfoliated material (PG, and, both rGO and rGO solutions). This method is very close to aqueous freeze gelation but here the water is substituted by an organic solvent with a boiling point higher than RT and high vapour pressure. In RTFG, the starting material is mixed with the solvent above its boiling point, usually for temperatures between 50 and 120 °C, and cooled to form a solid state material at RT. The solvent should have a higher vapour pressure than the solid at RT and ambient pressure conditions in order to obtain rapid sublimation which yields a solid material with high porosity [101–103]. The advantages of this method over aqueous freeze gelation is, firstly, that the solid is formed through conventional processing methods prior to sublimation, while with aqueous methods freezing takes place strictly in a container, and secondly, the plurality of solvents that can be used which allow materials to be dispersed without functionalization agents or surfactants.

In figure 2(a), there is an illustration of RTFG application for GAs. At first, PG flakes are soluted to the desired concentration in a solvent at high temperature, with sonication. Then, the solution is treated with the appropriate forming process such as moulding, extrusion or printing and cooled to be solidified. The solvent is evaporated at RT leaving behind a nanostructured porous GA.

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3D printing has also been exploited to prepare GO aerogels with complex shapes [104–106]. In these cases, an aqueous solution of GO is formed in a polymer solution to promote gelation after deposition. Gelation is achieved through water removal either by freeze drying or supercritical drying. Subsequently, an rGO aerogel is formed by reduction at elevated temperatures. The RTFG method can also be used with starting materials mixtures of rGO and multiwalled carbon nanotubes (MWCNTs), mixtures of graphene flakes and MWCNTs, while the polymers addition in these mixtures enhances the strength of the final aerogel. The microstructure of the final product depends on the graphene concentration, the solvents employed, the concentration of the polymer in the solution and the cooling rate. In figures 2(b) and (c), the microstructure of PG aerogels is shown. Solute rejection during solidification leads to solvent rich regions, that create a microporosity of relatively large voids after evaporation, and graphene rich regions that lead to a structure containing random packing of the PG flakes leading to nanoporosity within the microporous structure walls. As shown in figure 2(b), the aerogel with phenol has a characteristic lamellar structure which is very close to the corresponding microstructures formed with conventional aqueous freeze casting [107]. In contrast, the aerogel formed using camphene shows an equiaxed microstructure with no directionality.

The mechanical properties of the PG aerogels are shown in figure 3. The PG aerogels with graphene concentration of 20 and 40 mg cm⁻³ had Young's modulus values equal to 7.7  ±  1.0 and 82.5  ±  4.0 kPa, respectively, and yield strength of 0.4  ±  0.01 and 2.1  ±  0.2 kPa (figure 3(c)). With the addition of poly(vinyl alcohol) (PVA) in the solution with the phenol, the Young's modulus of the aerogels increased to 553  ±  85 and 893  ±  82 kPa for the same concentrations of 20 and 40 mg cm⁻³, respectively, and yield strengths of 25  ±  2 and 51  ±  3 kPa (figure 3(d)).

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The Raman spectra of the PG flakes and the aerogel produced using phenol are shown in figure 4(a). The 2D peak shifts from 2666 cm⁻¹ for PG to 2656 cm⁻¹ for the aerogel, while the 2D:G peak intensity ratio increases from 0.4 to 0.63. The observed shift and the increase of the 2D peak intensity indicate that the graphene in the aerogel is of higher quality than the PG, noting that the PG may be further exfoliated during mixing. This is indicative of the presence of high quality few layer graphene sheets in the aerogel, without flake restacking and aggregation during processing. Gas adsorption was used to find the mean specific surface area of the aerogels (figures 4(b) and (c)). A small amount of MWCNTs that was added to the PG solution (20 wt.% MWCNTs) affects significantly the aerogel microstructure and the physical properties, like density (to 6.5 mg cm⁻³ from 2.5 mg cm⁻³). The PG and PG/MWCNTs aerogels show specific surface areas of approximately 400 and 700 m² g⁻¹, respectively (figure 4(c)). These values can be compared to those from GAs produced by other methods [104, 108]. The pore-size distribution lies of the 10–200 nm range has a peak pore diameter at 73 nm for the PG aerogel and 83 nm for the PG/MWCNTs aerogel (figure 4(c)). The electrical conductivity of the PG aerogel as a function of density (figure 4(d)) shows a maximum value of 9 S cm⁻¹ at a density value of 20 mg cm⁻³. Although this value is slightly lower than those produced by CVD onto sacrificial templates [56], it is higher than those of rGO-based aerogels [109] and CNT foams [72].

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The synthesized aerogels show high specific capacitance, very good electrochemical stability and a high reversibility. These aerogels can be used as electrodes in supercapacitors. Attention should be paid to the choice of solvent as it affects the microporous structure.

Zhu et al [106] reported a method of direct ink writing for fabricating GAs. This is done via a 3D printing technique known as direct ink writing. This technique uses a three-axis motion to create 3D structures by mechanically extorting a continuous ink filament through a micronozzle at RT and depositing it layer-by-layer [110]. The 3D printing technique is a very convenient method for fabricating several aerogel architectures which can be applied to different fields. The so-made GAs are lightweight, exhibit high conductivity and remarkable supercompressibility (up to 90% compressive strain). Also, the Young's moduli of the aerogels (≈25 MPa) appear to be higher than those of other bulk graphene aerogels (≈2.5 MPa).

The stability of the cyclic resilience of printed GAs and the compression cycling of the GA at 50% strain are shown in figures 5(a) and (b). As seen, the energy loss coefficient of printed aerogels decreases from 60 to 30% in the first three cycles and then remains constant. The highest stress for each cycle in figure 5(a) also shows a similar behaviour (figure 5(b)). The electrical resistance of the printed GAs is also shown versus cyclic compression (figure 5(c)). We can notice a small decrease after multiple compressions, confirming the high structural resilience of the GA microlattices. The Young's modulus, E, as a function of density for the printed GAs and standard GAs (bulk) vis-à-vis values obtained from other carbon, carbon nanotube and graphene assemblies reported in the literature [46, 86, 109, 111–115] are shown in figure 5(d). In fact, the bulk aerogel data are close to literature data, while the printed aerogel data are clearly off the known curve. For both cases, Young's modulus, E, was found to scale linearly with density, ρ^2.5. The similar values of the exponent (≈2.5) suggest that both printed and bulk aerogels have the same bending behaviour under compression. But, for the graphene microlattices Ε is about one order of magnitude larger than that of most bulk GAs for the same densities. So, the printed GAs can exhibit higher stiffness with much lower density compared to higher density bulk aerogels.

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The real challenge for this fabrication is the development of printable GO inks with appropriate composition and rheology. Nevertheless, this proposed method is very promising and the GA structures of desired shapes can lead to new energy storage devices, graphene-based electronics, separation devices and catalytic scaffolds.

Very recently, Scaffaro et al [116] reported the synthesis of an ultralight graphene oxide-polyethylene glycol aerogel (GPA) with some attractive characteristics such as high porosity, high surface area, biocompatibility, cytocompatibility and outstanding mechanical behaviour. As shown in figure 6, firstly, GO is coupled with an amino-terminated polyethylene glycol (PEG-NH₂) sample using carbodiimide (EDC). The coupling approves the creation of covalent bonds between –NH₂ functional group of the polymer and the epoxy and carboxyl groups of GO. Afterwards, the hydrogel is converted into aerogel by freeze-drying.

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Wang et al [74] have reported the fabrication of GAs with a highly aligned porous structure, ultralow densities, large surface areas and excellent electrical conductivities. This fabrication was accomplished through a novel unidirectional freeze casting method. The highly orientated graphene sheets resulted from the expulsion of GO sheets by the rapidly advancing ice fronts that forced them to be placed between the aligned ice crystals, as shown in figure 8(a). The final unidirectional GA (UGA, figure 8(b)) was converted by vacuum-assisted infiltration into of liquid epoxy UGA/epoxy composites. These composite materials show a very low percolation threshold of 0.007 vol.% because of the highly aligned, 3D interconnected conductive networks of UGA. An analytical model was developed based on the average interparticle distance (IPD) theory [117] to simulate the percolation behaviour and define the percolation threshold of these UGA/epoxy composites. According to the model, V_c, which is the required volume fraction for single type fillers with several shapes to form conductive networks, is given by:

$$\begin{eqnarray}&&{{V}_{\text{c}}}=\frac{ \pi }{4}\left(\frac{1}{\text{R}}\right)\end{eqnarray} \tag{ 1 }$$

where R is the percolation factor which depends on the size, shape and the orientation state of the fillers in the composite materials. For 2D disc-shaped fillers like graphene, the percolation factor, R_S, is given by:

$$\begin{eqnarray}&&{{R}_{\text{S}}}\approx \left(\frac{D}{t}\right){{\langle {{\cos}^{2}}\theta \rangle}^{3}}\end{eqnarray} \tag{ 2 }$$

where D and t are the diameter and thickness of graphene, respectively, and ${{\cos}^{2}}\theta$ is the average orientation angle between the fillers and the direction of preferred orientation. In the IPD model, it is assumed that the composite is divided into cubic elements, each containing one filler in the centre, and the percolation occurs when the unit volume is filled with single type of cubic elements. Similarly, for composites containing two different groups of fillers, we may assume that the percolation occurs when the unit volume is filled with two groups of cubic cells, as shown in figure 7.

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The unidirectional freeze casting method is illustrated in figure 8(a). GO dispersions with different concentrations are poured into a propylene (PP) container with a conductive aluminum bottom for thermal exchange with the underlying liquid nitrogen. The PP container is insulated with styrofoam and mounted on the top of a metal scaffold. The container along with the scaffold is rested in a pool of liquid nitrogen with the same depth. Then, a large temperature gradient is applied between the bottom (−196 °C) and the top (RT) of the GO dispersions and so, the ice crystals grow along a vertical direction. After freeze drying of the freeze-cast dispersion, a GO aerogel is formed. This aerogel is reduced to UGA by controlled heat treatment. The UGA/epoxy composition is illustrated in figure 8(c). Epoxy resin, curing agent and acetone are mixed with specific weight ratio. This mixture is mounted in a vacuum oven to sublime acetone at RT, and then is heated to 60 °C to lower the viscosity of resin. Then, the mixture is poured into the UGA, followed by infiltration under vacuum and curing. Nevertheless, the UGAs that were produced with low concentration of GO (e.g. 0.5 mg ml⁻¹) did not exhibit a similar behaviour. Loosely interconnected graphene sheets with little alignment were observed and also the GO sheets were not close to each other in the dispersion.

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Wei et al [118] have prepared Ni-doped graphene/carbon cryogels (NGCCs) by adding R and F to a GO suspension, using Ni²⁺ as catalysts for the gelation. The Ni²⁺ enrich the crosslinking between GO and RF skeletons, strengthening the cryogel. The starting hydrogel is converted to aerogel through freeze-drying and subsequent carbonization under an inert atmosphere. After this phase, 3D interconnected structures are observed that enhance the mechanical properties. Ni²⁺ are converted into Ni nanoparticles during carbonization, and consequently, they are incorporated in the interconnected structures. The NGCCs can be applied in water purification since their porosity within the interconnected structures enables them to absorb oils and some organic solvents, while it can be recycled as a bulk material.

Sui et al [119] have fabricated hybrid aerogels of carbon nanotubes (CNTs)-graphene via supercritical CO₂ drying after heating aqueous suspensions of GO and CNTs with Vitamin C. Figure 9 shows a schematic diagram for the synthesis of the hybrid aerogels (on the left) and a photo of a capacitive deionization electrode made by pressed hybrid aerogel into nickel foam (on the right). These hybrid aerogels can be applied in water purification since they can deionize light metal salts, remove organic dyes and enrich heavy metal ions like Pb²⁺, Ag⁺, etc. They exhibit high conductivity of 7.5 S m⁻¹, large surface area of 435 m² g⁻¹, pore volume of 2.58 cm³ g⁻¹ and maximum desalination capacity of 633.3 mg g⁻¹. Nevertheless, supercritical CO₂ drying has a negative environmental impact, so the claimed by the authors 'green' synthesis is not fully convincing.

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Hou et al [120] have reported the preparation of a 3D hybrid of layered MoS₂/nitrogen-doped graphene nanosheet aerogels (3D MoS₂/N-GAs) by a hydrothermal process. Their intention was to examine the possible application of the aerogel as a novel catalyst for hydrogen production from organic wastes and particularly from microbial electrolysis cells (MECs). A high output current density of 0.36 mA cm⁻² with a hydrogen production rate equal to 0.19 m³ H₂/m³ · d was observed for the material at 0.9 V bias. Figure 10 describes the preparation process for the 3D MoS₂/N-GAs. At the beginning, MoS₂/NS was produced by liquid exfoliation method using MoS₂ as the source material. Then, the MoS₂/NS was mixed with GO and ammonia solutions and afterwards this mixture was assembled hydrothermally to form a 3D MoS₂/N-GA. Following this process, the MoS₂/NS was deposited on graphene nanosheets with the simultaneous reduction of GO and the enhancement of nitrogen particles into the graphene network.

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Zhang et al [77] have presented a synthesis of 3D porous hybrids consisted of ultrathin defect-rich MoS₂ nanosheets (dr-MoS₂ NSs) and conductive graphene nanosheets (GNS) through hydrothermal co-assembly procedure. Firstly, the ultrathin dr-MoS₂ NSs are dispersed in aqueous solution, and then, are mixed with GO to co-assemble into 3D porous dr-MoS₂ NSs structures. The mixed dispersion, after sealing it tightly into a Teflon-lined stainless steel autoclave, is heated at 180 °C for 12 h. Finally, the thermal reduction of GO to GNS, takes place at 800 °C for 2 h in N₂ atmosphere. These synthesized hybrids show a large specific surface area that facilitates rapid diffusion of Li⁺ to access active materials. The dr-MoS₂ NSs, due to their additional active edge sites, are responsible for the intercalation of Li⁺ that results in higher specific capacity. The interconnected graphene network contributes to higher conductivity (better charge transfer and Li⁺ transport), while it secures the material's stability during lithiation/delithiation. So, these materials exhibit an excellent electrochemical performance (reversible capacity of 1130.9 mA · h g⁻¹ at a current density of 0.1 A g⁻¹), something that is ascribed to their 3D porous structure and the synergetic effect between ultrathin dr-MoS₂ NSs and conductive graphene network. Consequently, some new promising anode materials can be based on these structures. In table 1, GAs produced by different approaches and methods are listed.

Table 1. Concentrating table for GAs synthesized with different techniques.

GA name	Synthesis method	Properties	Photo		Reference
Graphene/carbon aerogels	Sol–gel	Specific capacitance of 122 F g−1 at a constant current density 50 mA g−1, surface area of 361–763 m2 g−1, density of 0.11–0.19 g cm−3, high pore volume, narrow pore size distribution (10–50 nm) and high conductivity.		Reprinted with permission from Meng F, Zhang X, Xu B, Yue S, Guo H and Luo Y 2011 Alkali-treated graphene oxide as a solid base catalyst: synthesis and electrochemical capacitance of graphene/carbon composite aerogels J. Materi. Chem. 21 18537. Copyright (2011) by The Royal Society of Chemistry.	[44]
PG aerogels	Room temperature freeze gelation (RTFG)-freeze or supercritical drying	High specific capacitance, good electrochemical stability and high reversibility		Reprinted with permission from Lin Y, Liu F, Casano G, Bhavsar R, Kinlock I A and Derby B 2016 Pristine graphene aerogels by room-temperature freeze gelation Adv. Mater. 28 7993. Copyright (2016) by Wiley Online Library.	[100]
GPA	Polymerization-freeze drying	High porosity, specific surface area, biocompatibility, cytocompatibility, good mechanical properties		Reprinted with permission from Scaffaro R, Maio A, Lopresti F, Giallombardo D, Botta L, Bondi M L and Agnello S 2016 Synthesis and self-assembly of a PEGylated-graphene aerogel Compos. Sci. Technol. 128 193. Copyright (2016) by Elsevier Ltd.	[116]
UGA	Freeze casting-freeze drying	Highly aligned porous structure, ultralow densities, large surface areas and excellent electrical conductivities	—		[123]
GA microlattices	Direct ink writing	Light weight, high conductivity and remarkable supercompressibility: up to 90% compressive strain		Reprinted with permission from Zhu C, Han T Y-J, Duoss E B, Golobic A M, Kuntz J D, Spadaccini C M and Worsley M A 2015 Highly compressible 3D periodic graphene aerogel microlattices Nat. Commun. 6 6962. Copyright (2015) by Nature Research.	[106]
Hybrid aerogels	Chemical reduction-supercritical drying	High conductivity of 7.5 S m−1, large surface area of 435 m2 g−1, pore volume of 2.58 cm3 g−1 and maximum desalination capacity of 633.3 mg g−1.		Reprinted with permission from Sui Z, Meng Q, Zhang X, Ma R and Cao B 2012 Green synthesis of carbon nanotube-graphene hybrid aerogels and their use as versatile agents for water purification J. Mater. Chem. 22 8767. Copyright (2012) by The Royal Society of Chemistry.	[119]
3D MoS2/N-GAs	hydrothermal	High output current density of 0.36 mA cm−2 with a hydrogen production rate equal to 0.19 m3 H2/m3 · d at 0.9 V bias		Reprinted with permission from Hou Y, Zhang B, Wen Z, Cui S, Guo X, He Z and Chen J 2014 A 3D hybrid of layered MoS2/nitrogen-doped graphene nanosheet aerogels: an effective catalyst for hydrogen evolution in microbial electrolysis cells J. Mater. Chem. A 2 13795. Copyright (2014) by the Royal Society of Chemistry.	[120]

A novel procedure to fabricate graphene/carbon aerogels with multimodal pores (from micropores to mesopores and macropores) via carbonization of graphene crosslinked polyimide (PI) aerogels, was reported by Zhang et al [121]. At first, GO-poly (amic acid) (PAA, polyimide precursor) aerogels were synthesized by mixing PAA and a GO suspension. Then, the as-synthesized aerogels were amidated in nitrogen atmosphere under 100, 200 and 300 °C for 1 h in order to crosslink PAA with GO and GO to be reduced to graphene. At the last stage, graphene/carbon aerogels are observed by carbonization of graphene crosslinked PI aerogels. Graphene seems to have a powerful crosslinking action which accelerates the gelation process, improves the porous structures inside carbon aerogels (reducing the pore size while increasing the amount of micro- and meso-pores), and increases the specific surface area and conductivity of the aerogels (forming a 3D conductive network). With graphene incorporation, these aerogels show a specific surface area of 998.7 m² g⁻¹ and specific capacitance of 178.1 F g⁻¹ at a current density of 1 A g⁻¹, instead of the respective values for pure carbon aerogels (193.6 m² g⁻¹ and 104.2 F g⁻¹). The dual interest of this process is that it produces high performance of carbon aerogels with hierarchical structures while broadens the applications of polyimide.

Also, some PI composite aerogels with enhanced flame-retardant property have been produced after the addition of friendly flame-retardant additives, like graphene and montmorillonite (MMT) through freeze-drying and followed by thermal imidization method [122]. The GO/MMT hybrid, is well dispersed in PI matrix due to the strong interaction between the two materials and that improves the mechanical, thermal and flame-retardant properties of the aerogels. For the composite aerogels, compression modulus has been found to be equal to 14 MPa, specific modulus equal to 155.5 MPa · cm³ g⁻¹, while 574.3 °C were needed to decompose 10% of the weight of these PI/graphene/MMT aerogels, a value that is 20 °C higher than that of pure PI aerogels. Last but not least, the composite aerogels increased the limiting oxygen index (LOI) up to 55. A high LOI is equivalent to a better flame-retardant performance.