Graphene and boron nitride foams for smart functional applications

Graphene and boron nitride (BN) foams, as two types of three‐dimensional (3D) nanomaterials consisting of two‐dimensional (2D) nanosheets, can inherit a series of excellent properties of the 2D individuals. The internal 3D network can prevent aggregation or restacking between isolated graphene or BN nanosheets, and provide a highway for phonon/electron transports. Moreover, the interconnected porous structure creates a continual channel for the mass exchange of exotic species. The light‐element graphene and BN foams can thus possess the characteristics of low density, high porosity, high surface area, and excellent mechanical, thermal, and electrical properties. Benefiting from these advantages, they show a wide range of applications. The usual synthesis methods and the recent functional applications of graphene and BN foams are reviewed herein, including their applications as supporting materials, elastic materials, acoustic shielding materials, thermal interface materials, electromagnetic shielding materials, adsorption materials, electrocatalysis and thermal catalyses materials, electrochemical energy storage, and thermal energy storage materials. Current challenges and outlooks are additionally discussed.

graphene has an ultrathin structure and ultrahighspecific surface area (2630 m 2 /g). (2) Graphene possesses extremely high carrier mobility at room temperature (15,000 cm 2 /(V s)) and the resistivity is 10 −6 Ω cm, lower than copper or silver. 2,4 (3) With a tensile modulus of 1 TPa (150,000,000 psi), graphene has high mechanical properties, which are 200 times stronger than steel. 5 (4) Graphene has a high thermal conductivity of up to 5300 W/(m K), which is higher than carbon nanotubes and diamonds. 6 (5) The active functional groups on graphene material can greatly improve the reactivity with organisms, so it has biological compatibility.
As a sister of graphene, boron nitride (BN) is a compound with an equal number of nitrogen and boron atoms arranged alternately. Its crystal types mainly include three kinds: sp 2 -hybridized hexagonal BN (h-BN), sp 3 -hybridized wurtzite BN, and cubic BN. Among them, h-BN is also known as "white graphene" because it possesses the same layered structure and similar lattice parameters to graphene. In addition, because it is isoelectronic with graphene, h-BN shares similar characteristics with graphenes, such as high thermal conductivity, good thermodynamics, and low expansion coefficient. [7][8][9][10] However, it is worth noting that the B-N bond is partially ionic, thus h-BN and graphene have diametrically opposite optical and electrical properties. For example, since the N atom is more polar than the B atom, the electrons of π bonds tend to be fixed around the N atom and cannot move freely, so the band-gap width of h-BN can be up to 5-6 eV. Combined with these similarities and differences in properties with respect to graphene, h-BN has also been widely developed in energy, catalysis, sensing, and other fields.
Herein, we first summarize the various preparation methods for the two materials, including mainly solution-based assembly, 3D printing, chemical vapor deposition (CVD), and template-free pyrolysis. Then the applications of the two materials in physics and chemistry disciplines are discussed, including their applications as supporting materials, elastic materials, acoustic shielding materials, thermal interface materials, electromagnetic shielding materials, adsorption materials, electrocatalysis and thermal catalyses materials, electrochemical energy storage, and thermal energy storage materials ( Figure 1). Lastly, current challenges and perspectives are summarized, hoping to provide directions for the study of GFs and BN foams in broader fields.

| Solution-based assembly for graphene and BN foams
As the main method to prepare GFs and BN foams, the solution-based assembly can be mainly divided into the cross-linking method, hydrothermal reduction method, and template-assisted method. For the solution-based assembly method, the starting materials are usually dispersions of graphene oxide (GO), pristine graphene, or h-BN nanosheets (BNNS). Owing to the anisotropic structure and high aspect ratios, gelation can occur when the concentration of GO sheet solution is higher than a certain critical value. The crosslinking agent can also promote the generation of gelation, so as to obtain a 3D structure. At present, commonly used crosslinking agents have polymers, metal ions, and so on. [42][43][44][45] Shi et al. synthesized a porous GF that uses the DNA ( Figure 2A) as a crosslinking agent and the pore diameters range from submicrometer to several micrometers ( Figure 2B). Meanwhile, the pore walls consist of very thin layers of stacked GO sheets ( Figure 2C). 46 In addition, Yu et al. also reported a GF through the use of ferrous ions. 47 Although cross-linking agent possesses great application potential in the synthesis of GFs, it is hard to get pure GFs. In contrast, a hydrothermal reduction method is an effective approach. For the GO sheet solution, the hydrophilic functional groups of the GO sheet can cause the random distribution of GO in the solution. When these hydrophilic oxygen function groups are reduced, the van der Waals forces between the graphene sheets increase, promoting the formation of GFs. In 2010, Shi et al. first applied the hydrothermal reduction method to synthesize GFs ( Figure 2D). 47 The GF was obtained by  heating the GO aqueous dispersion in a Teflon-lined autoclave at 180°C for 12 h. The resulting GF demonstrates a 3D porous structure that is interconnected on the submicron to several microns scale ( Figure 2E,F). Meanwhile, the pore walls consist of thin layers of stacked graphene sheets. Later, various GFs were synthesized by the hydrothermal reduction method inspired by this work. For example, Wu et al. synthesized an N,B codoped GF for the supercapacitors. 49 Figure 2G demonstrates the hydrothermal process of the GF, and its scanning electron microscope (SEM) images are shown in Figure 2H. Apart from the GO sheets, the GF can also be fabricated from the pristine graphene sheets selecting polyvinyl alcohol (PVA) as a binder. 50,51 For the template-assisted method, templates can be combined into the gelation process of GO sheets to tailor the porous structure of GF. Usually, these templates can be classified as ice templates, hard templates (polystyrene, polymethylmethacrylate, silica nanospheres, etc.), and soft templates (emulsion, gases, etc.). Using the ice template, Zhang et al. prepared a vertically aligned GF by bottomup directional freezing of a mold containing a mixture of GO and ethanol. 52 Adopting a hard template, Huang et al. fabricated a lamellar-like structure GF. They used the methyl-modified silica with hydrophobic properties as a template to induce GO assembly. 53 For the soft template, Li et al. adopted hexane emulsion to induce GO assembly into a 3D network in a hydrothermal environment and finally obtained the GF. 54 As compared with GO sheets, the BNNS typically have much smaller lateral sizes. Therefore, BNNS are hard to form liquid crystals. 55 Moreover, there are no functional groups on BNNS is another factor that affects the stability of BNNS dispersion in water. 56 So, additives are often added to stabilize the BNNS in the suspension. 57,58 For instance, Ajayan et al. use water-soluble PVA to produce a strong interaction between BNNS, thus forming a BN foam. 59 The foam possesses a better mechanical property than the 3D structure composed of pure BNNS and a high surface area that can absorb 320 mg/g CO 2 molecules at 54 bar.

| 3D printing for graphene and BN foams
Due to the advantages of rapid prototyping, at present, the 3D printing method used in the fabrication of smart functional material has gradually received more and more attention. Benefiting from precise computer control, 3D printing technology has great potential in fabricating porous 3D structures. At present, the preparation of GFs using 3D printing technology can be divided into two styles. One is the GO ink which is used directly for printing. Another one is to print out the template and then prepare the final product based on the template. For the first way, it is important to control the viscosity of the GO ink because the GF is prepared by continuously extruding the ink wire through a micronozzle. 60,61 Adopting the first style, Lu et al. fabricated a GF with an electrical conductivity of 41.1 S/m. 61 For the second style, Ding et al. fabricated a few-layered bicontinuous porous GF through the combination of CVD and 3D printing. 62 First, they use 3D printing technology to fabricate a 3D porous silica template, and then the CVD method is used to deposit graphene sheets on the silica template to obtain GF. Figure 3A shows the process of synthesis. Figure 3B,C shows the SEM and transmission electron microscope (TEM) images of the GF, respectively. Finally, the obtained GF exhibits a wide range of good properties, including excellent electrical conductivity, strong mechanical properties, and so forth. Meanwhile, it shows huge potential applications in solar steam generators, catalyst supports, and sensors. Besides, through combing 3D printing and thermal annealing, Lin et al. also fabricated a GF that shows a truly 3D geometric configuration consisting of overhang beam and column elements with hollow-carved topological morphologies. 63 Figure 3D exhibits the synthesis process and Figure 3E demonstrates the SEM image of the GF. First, they fabricate a hollow polymer architecture as a template, then inject the GO-based ink into the template. Finally, after the process of thermal annealing and freeze drying, the GF with interconnected porous structure is fabricated. In general, the 3D printing method shows huge application potential in preparing GFs with ordered porous structures. At present, the challenges are mainly in two aspects. One is the possibility to print the foam structure by inheriting the unique characteristic of the original graphene sheets. The other one is designing suitable inks that possess appropriate thixotropic and viscous properties.

| CVD for graphene and BN foams
CVD is a technology in which inorganic or organic polymers are used as templates, and the reactive substances react under gaseous conditions to produce solid substances. Adopting CVD, high-quality graphene can be produced through pyrolyzing of precursors (such as CH 4 , C 2 H 6 , and glucose) on templates. 64 Notably, GFs can be obtained when the template possesses the foam structure. For example, Cheng et al. fabricated a GF with monolayer and few-layer graphene sheets using the Ni foam as the template and CH 4 as the precursor ( Figure 4A). 20 When the temperature reaches 1000°C, the CH 4 starts to decompose and then the graphene films start to precipitate on Ni foam. Later, the graphene-Ni composite is coated with a thin layer of poly (methyl methacrylate) to protect the GF during the subsequent operation of etching the Ni using HCl or FeCl 3 solution.
Besides GFs, CVD is also suitable for the preparation of BN foam. Guo et al. fabricated a h-BN foam using the Ni foam as a template and solid borazane as the precursor in low pressure. 29 At 100°C, boronazane will produce boronazane vapor, and then the h-BN membrane skeleton forms on the nickel surface at 1000°C. Later, the surface of the h-BN foam is coated with a thin layer of poly methyl methacrylate to protect h-BN foam during the process of etching the nickel skeleton with HCl solution. Apart from the Ni template, other templates have also been adopted to fabricate BN foams. Beitollahi et al. fabricated a 3D interconnected porous h-BN foam using a polymer as the template. 66 The h-BN foam not only replicates the structure of polymer foam but also exhibits high water purification capacity. In addition, the porous SiO 2 foam is also a suitable template for CVD growth. Golberg et al. fabricated a tubular h-BN cellular-network foam, which used the SiO 2 foam as the template ( Figure 4B-D). 65 At 1100°C, N-doped graphene was in situ deposited on the SiO 2 foam by CVD method, and then the internal SiO 2 was removed by hydrofluoric acid etching to form N-doped GF that inherited the structure of SiO 2 foam. Then, the N-doped GF was used as a template to prepare h-BN foam by carbothermal and substitution reaction at 1700°C.

| Template-free pyrolysis for graphene and BN foams
In addition to the above methods, template-free pyrolysis methods also possess great application potential to fabricate GFs and BN foams. For example, getting inspiration from an ancient food art of "blown sugar," Wang et al. fabricated a chemical-blowing technique to fabricate GFs with mono-and few-layered graphitic membranes ( Figure 5A). 67 In this method, sugar is used as the carbon source and NH 4 Cl is used as the foaming agent. During the heating process, NH 4 Cl decomposed into NH 3 and HCl to make the black-like polymer foaming. As the bubble expands, the bubble wall of the foam gradually becomes thinner, and finally, the thickness of the polymer bubble wall can be as thin as about 20 nm. After annealing at 1350°C, the GF is prepared. The obtained GF possesses an excellent mechanical elasticity and it can still recover after 80% compressive strain. At the same time, the graphene stacking problem is eliminated, and the specific surface area of the GF can be up to 1005 m 2 /g. In addition, Wang et al. recently adopted a zinc-assisted pyrolysis method to fabricate a Zn-guided 3D GF with mono/fewatomic layers and a mean thickness as small as 2.2 nm. Figure 5B-I demonstrates its morphological evolution in different steps. 68 Through this method, the powder of glucose and zinc is pressed into a composite ingot, which is then calcined to fabricate GF with the interconnected porous structure ( Figure 5J,K). The zinc powder can impregnate and delaminate the solid char into multiple membranes. Meanwhile, it also can promote the process of graphitization and carbonization as catalyze. Moreover, Wang et al. also fabricated a reactive template-free pyrolysis method to fabricate GFs. 69 First, they adopted the O 2 -NH 3 reactive pyrolysis method achieving the transfer of cellulose fiber to graphenic sheets. Then, they proceeded to assemble these graphenic sheets into 3D structures.
The chemical blowing method can also be adopted to produce BN foams. Wang et al. prepared a h-BN foam by heating aminoborane to make it bubble spontaneously ( Figure 5L). 70 During the heating process, aminoborane polymerizes into polyaminoborane, polyimylene borane, and it releases H 2 to make polyaminoborane and polyimylene borane foaming. Finally, the h-BN foam is obtained by sintering at 1200°C. In addition, Miele et al. prepared a h-BN foam by spark plasma sintering method using B-tri(methylamino)-borazine as the precursor of h-BN. 71 Finally, the surface area of the obtained BN foam can be up to 123-171 m 2 /g and the thermal conductivity can reach 0.674 W/(K m).

| Other preparations for graphene and BN foams
In addition to the above preparation method, several other methods have also been attempted to fabricate graphene and BN foams such as ion beam irradiationassisted crosslinking, 72 SiC epitaxial growth, 73 electrospinning. 74  The irradiation of the ion beam can strengthen the weaker physical connections between graphene sheets to promote the synthesis of GF. Figure 6A shows the irradiation process on the graphene layers stack. It is noteworthy that the beam energy, beam diameter, and the distance between graphene layers are essential for the synthesis of GF. In addition, the SiC epitaxial growth method is also an effective method to prepare homogeneosus GF. In this method, SiC crystals are heated in a vacuum (120-1600°C). Since the sublimation temperature of silicon is higher than that of carbon, the former is removed and the latter is reassembled into GF under heating conditions. Adopting this method, Berger et al. prepared single and multilayer GF and investigated their properties. 73 However, this method requires vacuum (or inert atmosphere), ultrahigh temperature heating, and other harsh conditions, resulting in a great impact on the conductivity of GF, and the product is not easy to separate from the substrate, so it is difficult to produce GF in large quantities. Adopting the electrospinning method Chen et al. fabricated a BN foam, which uses polyvinylidene fluoride as the matrix. 74 The BNNS can orient along the in-plane direction of the polymer film and interlink to form BN foam ( Figure 6B). The scheme of the preparation of the BN foam is demonstrated in Figure 6C. In addition, Xu et al. mixed BN with NH 4 HCO 3, molded it at 300 MPa, and then heated it to decompose NH 4 HCO 3 , thus obtaining BN foam ( Figure 6D). 75 The process is shown in Figure 6E. It is worth noting that no polymer binder or dispersant is added to the network, which makes the formed BN 3D network thermal conduction path more smooth.

| Application of graphene and BN foams as supporting and elastic materials
Benefiting from the high porosity and low density, the foam material is a light-but-strong structural supporter.
Liu et al. designed a GF with a lightweight of 12 mg/cm 3 and excellent mechanical strength to support 14,000 times its own weight. 23 Golberg et al. fabricated a h-BN foam with 98.5% porosity that can support about 25,000 times its own weight. 65 Benefiting from the excellent mechanical support property, GFs and BN foam also have a relatively high energy absorption capacity. By adopting freeze-casting technology and GO as a precursor, Li et al. designed a GF with the hierarchical structure of natural cork ( Figure 7A) and it shows an energy loss coefficient of 87% ( Figure 7B). Notably, this performance is superior to the conventional carbon, metallic, and polymer foams. 76 In addition, GFs are also excellent elastomers. Inspired by the hierarchical structure of natural cork, Li et al. fabricated a GF with a cork-like hierarchical structure using GO as a precursor by combining graphene chemistry with ice physics. 76 As an excellent elastomer, at 50,000 times its own weight, the GF still can maintain its structural integrity and quickly recover 80% after the external force is removed. Furthermore, thanks to the excellent thermal stability, Chen et al. also prepared a GF elastomer, which shows reversible superelastic properties at −196°C and 900°C, and Figure 7C shows the superelastic performance of the GF at different temperature. 77 For the further application of GFs elastomer, Li et al. experimentally studied its dynamic mechanical and electromechanical properties and revealed its piezoresistive behavior. 80 The GF elastomer can provide a rapid electrical response to dynamic pressures ranging from 0 to 2000 Hz, while also being able to detect pressures below 0.082 Pa. Thanks to the excellent piezoresistive behavior, GFs have extensive application potential in pressure sensors. [81][82][83][84][85][86] The current application of GFs sensors is mainly focused on three points. The first aspect is changing the content of conductive fillers or using composite conductive fillers to adjust the conductivity of GF. 78,79,[87][88][89][90][91] By introducing polyimide into the GF, Yang et al. fabricated a GF that can increase the sensitivity of the sensor to 0.36 kPa −1 . 78 Figure 7D,E shows its SEM image and compressive stress-strain curves, respectively. The second aspect is redesigning the porous structure of GFs to achieve different pore sizes and porosity. 62,79 For example, Ding et al. fabricated a layered continuous porous GF, which use the CVD method and 3D-printed silica as templates to achieve a better electrical and mechanical properties ( Figure 7F). 62 Using a two-step freezing method, Tong et al. fabricated a GF that present core-shell structure. The GF exhibited an excellent compressive strength that can reach 60 kPa at 70% strain, and Figure 7G-I demonstrate the different compressive strength test of the GF. 79 The third aspect is the structural design of on GFs to promote the sensitivity of GFs sensor. [92][93][94] For example, Tang et al. designed a unique sandwich-like structure (GF/polyurethane/GF) sensor that possesses the sandwich-like structure to fabricate a GF-based pressure sensor, which shows an excellent sensitivity of approximately 1.5 kPa −1 . 94 3.2 | Application of graphene and BN foams as acoustic shielding materials At present, noise pollution is threatening our physical and mental health. [95][96][97] Therefore, a variety of acoustic absorbing materials have received extensive attention. 98,99 Most of them have a similar porous honeycomb structure consisting of open, semiopen, and closed cells. Due to the interconnected porous structure that can enhance air-viscous resistance damping, they have a satisfactory acoustic absorption performance. Therefore, GFs with interconnected porous structures also have great potential as acoustic absorbing materials. In addition, its nature such as lightweight, high stability, low thermal expansion, and high corrosion resistance of graphene can also give GFs-based acoustic absorbers more application scenarios.
To date, the use of GFs alone as an acoustic absorber has been sparsely reported. Yang et al. report a GF templated by bubbles as acoustic absorber material. 100 By adding a foaming agent to GO dispersion and then stirring to speed up the production of bubbles, in this case, the GO sheets will accumulate on the bubble walls and then the 3D foam structure was obtained by freeze-drying. The SEM image of the GF is demonstrated in Figure 8A. As for the GFs-based acoustic absorber, it exhibits an excellent wide-frequency acoustic absorption performance, in which the normalized absorption coefficient can reach 0.9 from 60 to 6300 Hz ( Figure 8B,C). In addition, it also shows good moisture resistance and nonflammable characteristics.
In particular, the more common GF-based acoustic absorbers are graphene sheets covered with polymer frames to form composite foams as acoustic absorbing materials. [101][102][103][104][105][106] Losic et al. fabricated an acoustic absorber that uses interconnected self-assembled GF supported by a melamine skeleton ( Figure 8D). 101 Thanks to the characteristic of porous structure and the increased surface area caused by the graphene sheet, the sound absorber shows approximately 60.3% enhancement at a wide range of 128-4000 Hz ( Figure 8E,F). Later, Li et al. fabricated a GF-based acoustic absorber consisting of GO, functionalized carbon nanotubes, and melamine. 105 Compared with pure melamine foams and graphene/melamine composite foams, the graphene/carbon nanotubes/ melamine foam shows approximately 100% and 20% enhancement in broadband absorption at 250-1600 Hz, respectively. Aside from the melamine, the polyurethane foam can also be used as a skeleton to fabricate GF for acoustic absorbing materials. Jung et al. fabricated an acoustic absorber based on GF, which impregnates the GO sheets into polyurethane foam by a step-by-step vacuum-assisted process. 103 At a broadband absorption of 800-6300 Hz, the average acoustic absorption coefficient is increased by more than four times. Moreover, Gao et al. recently designed a GF-based acoustic absorber, which constructs the ultrathin graphene membranes into polymer foams. 102 The mechanism of this acoustic absorption is illustrated in Figure 8G. Interestingly, the GF consisting of ultrathin graphene sheets can dissipate sound transmission by producing large out-of-plane resonances when it absorbs sound. Finally, under the combined action of porous structure and resonance effect, the acoustic absorber shows a remarkable enhancement of approximately 320% from 200 to 6000 Hz ( Figure 8H).

| Application of graphene and BN foams as thermal interface materials
At present, polymers occupy an important position in electronic packaging. Nevertheless, the low thermal conductivity (0.2 W/(m K)) of polymers usually cannot meet the actual demand. 107,108 At present, it is an effective method to add high thermal conductivity filler to the polymer. [109][110][111] Graphene, with a high thermal conductivity (5300 W/(m K)), is an ideal filler for thermal management. 112 However, the restacking between individual graphene sheets is an important obstacle limiting the use of graphene as a thermal conductivity filler. 113 This restacking phenomenon would increase the number of interfaces between the graphene sheets, causing a lot of phonon scattering. 114,115 In addition, to improve the mechanical properties of the graphene-based composite, the random arrangement of graphene sheets also needs to be solved. For this situation, the structural design is an effective method. [116][117][118] With a high porosity that is usually more than 95% and interconnected porous structures, GFs are an ideal structural design for composite fillers. First, the interconnected 3D network structure of GF can boost the transport of phonons. Second, the high mechanical properties of GFs can provide stronger support for the composite. Chen et al. prepared a GF and put it in silicone rubber to prepare thermal interface materials. 119 At the 0.5 wt% GF content, the thermal conductivity of the thermal interface materials can be up to 1.26 W/(m K). Notably, GFs fabricated through the hydrothermal method usually possess an isotropic structure, which will reduce the transport of phonons in the intrinsic in-plane. For this situation, Yu et al. fabricated an anisotropic GF that possesses a vertically oriented structure. 120 First, ascorbic acid was added to GO dispersion as a reducing agent and then the graphene hydrogel was obtained by heating and stirring. Finally, the GF was obtained by directional freeze-drying from the bottom surface to the top surface. Figure 9A,B exhibits the mechanism of heat conduction of the GF/epoxy composite and the top-view SEM images of the GF, respectively. Benefitting from the anisotropic structure, the obtained composite exhibits an excellent thermal conductivity that can be up to 6.57 W/(m K) at a low GF content (0.75 vol%). The detailed thermal properties of the composite are demonstrated in Figure 9C,D. Although GFs have shown great promise in thermal interface materials, their high electrical conductivity has limited their use against the insulated application scenes. In view of this, BN foams with good insulation and thermal conductivity are suitable thermal interface materials in insulated application scenes. For instance, Wong et al. constructed a BN foam network and then infiltrated it into the epoxy matrix. 123 The thermal interface materials show a high thermal conductivity that can reach 2.85 W/(m K) at 9.29 vol% BN. Moreover, Jiang et al. prepared h-BN foam and filled it in epoxy, achieving a thermal conductivity of 1.97 W/(m K) at 28.7 wt% BN ( Figure 9E). Figure 9F exhibits the conductivity of BN nanocomposites following the temperature change. 121 By using sodium dodecyl sulfate as a foaming agent and surfactant, and by adding gelatin to ensure the integrity of the foam, Wong et al. prepared an anisotropic h-BN foam ( Figure 9G) by direct foaming method. Then the h-BN foams were filled in epoxy and used as thermal interface materials. 122 At 24.4 wt% BN, the thermal conductivity of the obtained thermal interface materials can reach 5.19 W/(m K) (in plane) and 3.48 W/(m K) (out-of-plane) ( Figure 9H). Meanwhile, its thermal conductivity is also better than other reported BN/ polymer composites ( Figure 9I).

| Application of graphene and BN foams as electromagnetic shielding materials
At present, electronic devices have been integrated into every aspect of our life, such as medical treatment, transportation, communication, and other fields. However, the large number of applications of electronic equipment also leads to the increasing electromagnetic pollution. [124][125][126] Therefore, electromagnetic absorbing materials have become the research hotspot. The common absorbing materials are ferrite, magnetic metal particles, ceramic materials, and so forth. However, they possess some shortcomings such as narrow absorption band, high density and single loss mechanism, which can no longer meet the diversified needs of military and civilian applications.
Benefiting from excellent characteristics such as low density, good conductivity, and large specific surface area, graphene is considered to be a suitable wave absorption material. So far, it has been found that assembling 2D graphene into 3D structures not only makes the material lighter and more flexible but also enhances the absorbing effect. However, considering the poor impedance matching of graphene, it is not suitable to be used as a wave absorber directly. Typically, they are combined with other conductive polymers or magnetic materials. Polyaniline (PANI) is a kind of electrically loss-absorbing material, which possess great development potential. Therefore, GF/PANI composites have received wide attention in the field of electromagnetic shielding. For example, through the hydrothermal in situ polymerization method, Luo et al. fabricated a GF/PANI, and Figure 10A demonstrates the SEM image of the GF. 127 At 11.2 GHz, the strongest reflection loss (R L ) of the obtained GF/PANI can reach −42.3 dB, and Figure 10B,C exhibits the 3D characterization of the GF and GF/PANI from 2 to 18 GHz, respectively. In addition, Zhang et al. fabricated a GF by self-assembly method and then polymerized PANI nanorods with the GF to obtain GF/PANI. 128 Benefiting from the excellent absorption mechanism of the GF/PANI composite ( Figure 10D), the R L of the obtained composite can reach −52.5 dB at 13.8 GHz ( Figure 10E) and −11.5 dB at 14 GHz ( Figure 10F).
Apart from the PANI, polypyrrole (PPy) with the advantages of nontoxic, little environmental harm, good air stability, and high electrical conductivity, also possesses an important application prospect in electromagnetic shielding. It can significantly improve the absorption rate of electromagnetic waves and widen the absorption frequency band of the composite. By uniformly dispersing GO on a PPy foam and then using a hydrothermal method to reduce GO to r-GO, Wang et al. fabricated a GF/PPy. 130 Under the low load of the filler (10 wt%), the electromagnetic absorption bandwidth of the GF/PPy could reach 6.76 GHz. In addition, adopting the self-assembly method, Xu et al. also fabricated a GF/ PPy composite. 129 The addition of PPy nanorods can form the interconnection network in the composite. Meanwhile, it also can effectively modify the electromagnetic parameters and promote microwave absorption performance. Figure 10G shows the microwave dissipated mechanism of the GF/PPy composite. Finally, the R L of the obtained GF/PPy composite can be up to −51.12 dB at 6.4 GHz ( Figure 10H) and −46.8 dB at 6.6 GHz ( Figure 10I).

| Application of graphene and BN foams as adsorption materials
With large specific surface area and excellent pore structures, GFs and BN foam have been demonstrated as effective absorbers for attaching organic pollutants, and oil/water separation. For example, Ruoff et al. fabricated a GF and studied its performance in absorbing oils and organic solvents, in which the surface area can reach 430 m 2 /g. 131 The results show that the GF can absorb more than 20 times its weight in oil or organic pollutants. What's more, the GF can be recycled after adsorbing pollutants. For the absorption performance of BN foam, Tang et al. reported a h-BN foam with a complete open porous structure, which is formed from one-dimensional porous h-BN microfibers by unique selfassembly. 34 With a high porosity (∼99.3%), the h-BN foam also has an excellent performance in absorbing oil and organic pollutants.
As for adsorbing heavy metal ions, GF with high surface areas also possesses a high potential. In addition, the functional groups on GO sheets can also act as binding sites for metal ions, thus promoting the adsorption performance. Zhang et al. designed GF/ Fe 3 O 4 nanocomposites and investigated their ability to adsorb Cr(IV). 132 The nanocomposites exhibit an excellent absorption capacity that can up to 258.6 mg/g benefiting from their high surface area (574.2 m 2 /g). In addition, by direct oxidation of GF, Zhang et al. designed an oxide GF ( Figure 11A) and explored the performance of adsorption of heavy metal ions. 133 Benefiting from the high face area (578.4 m 2 /g) ( Figure 11B) and a large number of active adsorption sites, the oxide GF exhibits superior adsorption ability for many metal ions. In particular, the adsorption capacity for Fe 2+ can reach 578.4 m 2 /g ( Figure 11C). Meanwhile, the BN foam also possesses huge application potential in adsorbing heavy metal ions. Adopting the template-free pyrolysis method, Golberg et al. fabricated a h-BN foam with interconnected porous structures ( Figure 11D) and its specific surface area can be up to 1406 m 2 /g. 41 Finally, the BN foam exhibits an excellent adsorption ability for Cd 2+ (561 m 2 /g, Figure 11E) and oil absorption performance ( Figure 11F).
Thanks to the large range of structural and chemical advantages of GFs, they are also applied in gas-sensing devices. Moreover, the thin graphene sheet of GF can also provide high sensitivity for the sensor. For example, Koratkar et al. designed a gas sensor based on GF, which only show high sensitivity to NH 3 and NO 2 in the gas mixture ( Figure 11G-I). 134 In the aspect of gas absorption, BN foam also has research potential. Through chemical cross-link and in-situ freeze-drying, Ajayan et al. fabricated a h-BN foam with low density, lightweight, and high surface area (124.4 m 2 /g). 59 Notably, the obtained BN foam can realize a high CO 2 capture performance that can reach more than 320 m 2 /g at 54 bar.

| Application of graphene and BN foams in electrocatalysis and thermal catalysis
At present, the increasing energy and environmental crisis has greatly promoted the progress of electrochemical energy. [135][136][137][138][139] Notably, the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO 2 RR) are very important parts of electrochemical energy storage and conversion technologies. Therefore, searching for a suitable electrocatalyst is crucial. Pt-based catalysts have always played an important role in the field of electrocatalysis, but their high price and easy deactivation are also important obstacles to their further application. [140][141][142] GFs as electrocatalysts have the following advantages. First, the high mechanical strength of the GFs gives the electrode high structural stability. Second, the GFs can be fabricated to the binderfree electrode, thus improving the overall conductivity. Third, the interconnected porous structure can boost the transport of electrons and electrolyte ions. At last, the GFs can accommodate more active sites, thus enhancing the electrocatalytic performance.
GFs can be used as electrocatalysts after doping heteroatom. [143][144][145][146] For instance, Wang et al. fabricated an N-doped GF through a supramolecular assembly method. 143 Benefiting from doping of the N atom, the N-doped GF shows an excellent ORR performance. Ajayan et al. also fabricated an N-doped GF ( Figure 12A) as the catalyst for CO 2 RR. 147 Using the methane as a precursor and the Ni foam as a template, the GF was first fabricated by CVD. Then, use a solid precursor of g-C 3 N 4 at the temperature range of 700-1000°C to vary the N atomic concentration. For the N-doped GF catalyst, CO can be easily produced at a low onset overpotential of −0.19 V. Meanwhile, its maximum Faradaic efficiency can reach ∼85% ( Figure 12B) at −0.47 V. Moreover, Wang et al. fabricated an S-doped GF through the CVD method, which uses the thianthrene sulfur dopant. 148 Meanwhile, they introduced many nanoholes and defective sites into the S-doped GF. Finally, benefiting from the synergistic effect of S-doping and defective effect, the S-doped GF shows a low onset overpotential (237 mV) and a low Tafel slope (64 mV/dec) ( Figure 12C).
Moreover, codoping can be also used in GFs for electrocatalysis, and their electrocatalytic performance can be further improved benefiting from the synergistic effect of various heteroatoms. For example, Shan et al. fabricate a P,N-codoped GF used in ORR. 146 Noticeably, the foam exhibits an excellent ORR performance owing to the abundant coupled P-N sites endowed by the doping of N and P atoms. In 0.1 mol/L HClO 4 , the half-wave and onset potential of the GF can reach 0.78 and 0.98, respectively. Moreover, Qiao et al. designed a N,O-dual-doped CNT@GF and shows a high OER activity thanks to the numerous catalytic centers provided by the C-N and C-O-C. 152 In addition to being applied as a catalyst alone, GFs-based composites also can be used as catalysts in electrocatalysis. 149,153,154 Unal et al. fabricated a nickel/ nickel oxide@GF ( Figure 12D) by hydrothermal reaction used in OER. 149 Under the condition of 40% nickel load, it exhibits a low overpotential of 320 mV ( Figure 12E). Moreover, Xie et al. fabricated a M x P(M = Co/Ni)@C-NPs/GF composite as the HER electrocatalyst. 155 Benefiting from the composite structure, it exhibits high catalytic activities that can reach 57 mV/dec in an acidic environment and 66 mV/dec in an alkaline environment. In addition, Choi et al. fabricated a 3D iron porphyrinbased GF as an electrocatalyst for the CO 2 RR to form CO. 150 The obtained GF exhibits high catalytic activity, which the faradaic efficiency can up to 96.2% at 280 mV ( Figure 12F). Moreover, when selecting suitable electrocatalysts in GFs-based composites, other reduced products can also be produced. [156][157][158] For example, Bian et al. fabricated a Pd-In@GF as the electrocatalysts for CO 2 RR to form formate. 156 For this obtained Pd-In@GF, its faradaic efficiency can be up to 85.3% at −1.6 V for the electrochemical reduction of CO 2 . Moreover, Zhang et al. fabricated an Ag-anchored N-doped GF and use it as CO 2 RR electrocatalysts to convert CO 2 to C 2 H 5 OH. 157 Finally, the obtained GF can efficiently transfer CO 2 to C 2 H 5 OH and the Faradaic efficiencies can be up to 82.1%-85.2%.
As an important component of the petroleum industry, propylene is an intermediate in various chemicals, such as polymers and oxygenates. Therefore, the Oxidative dehydrogenation of propane (ODHP) possesses a broad application prospect. At present, the common catalysts for ODHP have metal oxides and alkaline-earth metal oxychlorides, but they possess an excessive oxidation phenomenon that the formed propylene will be further oxidized to CO 2 (10%-60%). 159 Notably, the ODHP is a highly exothermic reaction, which will form hot spots on the catalyst surface and result in secondary oxidation. In this case, h-BN with excellent chemical and thermal stability is a potential catalyst for ODHP. Moreover, the high thermal conductivity of h-BN can also effectively decrease the production of hot spots and suppress the emergence of overoxidation. Currently, h-BN has been used as a catalyst in ODHP reactions and has shown impressive reactivity, which is superior to the traditional catalyst. 160,161 Notably, assembling h-BN into 3D structures can bring better mass transfer, lower pressure, and short residence time distribution and thus receive more and more attention. For example, Lu et al. fabricated a h-BN monolithic catalyst ( Figure 12G) using boron acid and urea as precursors by the CVD method. 151 The BN monolithic catalyst exhibits a high selectivity for propylene (82.1%) ( Figure 12H) and it is still stable with a high selectivity even at high temperatures ( Figure 12I). Moreover, no CO 2 is detected in the final product, further suggesting that the use of BN as a catalyst for ODHP could indeed greatly inhibit the generation of excessive oxidation.

| Application of graphene and BN foams in electrochemical and thermal energy storage
As active electrode material, GFs can be directly applied to electrochemical energy storage. For example, Li et al. fabricated a GF with a high specific surface of (835 m 2 /g) and conductivity (400 S/m) in a mild and friendly environment and used it as a binder-free anode for lithium-ion batteries (LIBs). 162 At 100 mA/g, its specific capacity can be up to 932 mAh/g. Meanwhile, the Coulombic efficiency can reach 61.3%. In sodium-ion batteries (SIBs), Li et al. synthesized an N-doped GF used as an anode for SIBs by combining the hydrothermal and freeze-drying methods. 163 The effect of nitrogen doping can introduce defects into graphene sheets, which further facilitates the transport of large sodium ions. Thanks to the nitrogen doping and interconnected porous structure, the N-doped GF exhibits a good sodium ion intercalation/deintercalation performance. At 100 mA/g, the specific capacity can be stable at 287.9 mAh/g.
As active electrode material, GFs have been also applied on supercapacitors. About the GFs as binder-free electrodes in supercapacitors, Liu et al. prepared a GF with a high specific surface area (512 m 2 /g) and interconnected porous structure. 23 Notably, the specific capacitance of the obtained GF can be up to 128 F/g. Duan et al. designed a GF as the electrode for supercapacitors, which shows a hierarchical porous structure ( Figure 13A). 164 By adding a certain amount of H 2 O 2 solution to a well-dispersed GO aqueous dispersion, H 2 O 2 molecules can create pores by oxidizing and etching carbon atoms around the active defective sites of GO while the GO sheets self-assemble. Thanks to the porous structure, the gravimetric capacitance and volumetric capacitance can reach 298 F/g and 212 F/cm 3 ( Figure 13B,C), respectively. Benefiting from the increase of electrical conductivity after doping heteroatom, the heteroatoms dopped-GFs also exhibit excellent performance in supercapacitors. For instance, Han et al. fabricated an N-dopped GF with a high specific surface area of 814 m 2 /g through a hydrothermal method. 165 At 0.2 A/g, the N-dopped GF electrode exhibits a high capacitance of 223 F/g. Meanwhile, it also shows an excellent long-cycle performance that can maintain 98% of its initial capacity after 2000 cycles at 1 A/g. Moreover, GF can be also used as a monolithic matrix to coat various active substances for electrochemical energy storage devices. At present, there has been a large range of research on the application of GFs-based material in LIBs. [168][169][170][171][172][173][174] Compared with the bare active materials, the electrochemical performance of the GFsbased composite electrode is both improved. For SIBs, encapsulating the active material into GFs is an effective strategy to solve the volume change of active substances during the reaction. For example, Guo et al. dispersed the red P nanoparticles into the GF-based architecture and use it as an anode for SIBs. 175 Benefiting from the advanced structure, the GF@P composite electrode exhibits excellent electrochemical performance that can reach 720 mAh/g at the current density of 1 C. Moreover, the electrochemical properties of SnO 2 , MoS 2 , WS 2 , Sb 2 S 5 , and Bi 2 S 3 are further improved after they are combined with GFs and used in SIBs. [176][177][178][179][180] As host material, GFs also have huge application potential in Li-S batteries. Through the freeze-dried method, Zhou et al. prepared a GF ( Figure 13D) and pressed it in slices to serve as a host material for lithium polysulfides. 166 At the high active materials loading of 4.6 mg/cm 2 , the capacity of the cathode can reach 1200 mAh/g at 0.2 C ( Figure 13E). Meanwhile, it also exhibits a low resistance ( Figure 13F). In addition, combining GFs with polar polymers can also improve the ability of GFs to inhibit the sulfur shuttle effect. [181][182][183] Meanwhile, to further improve the ability to inhibit sulfur shuttling, it is also an effective strategy to dop heteroatoms, such as N, S, B, F, and so forth, in GFs. [184][185][186] As a monolithic matrix in supercapacitors, Lu et al. designed a MnO 2 /GF via electrochemically deposited MnO 2 nanoparticles on graphene. 187 With a high MnO 2 loading of 61 wt%, the capacitance of the MnO 2 /GF can reach 410 F/g. Apart from MnO 2 , Zhuang et al. fabricated a hierarchical PANI/GF ( Figure 13G) as the electrode of supercapacitors. 167 Benefiting from the special structure, the hierarchical PANI/GF electrode exhibits a high specific capacitance ( Figure 13H) and energy density ( Figure 13I). Meanwhile, the capacity of the capacitor attenuates only 10% after 3000 cycles at 100 mV/s. In thermal energy storage, the GFs and BN foams also possess huge application potential as phase change materials (PCMs). First, the light and strong characteristics can provide strong shape support for PCMs. Second, the introduction of GF and BN foams can also enhance the thermal conductivity of the PCMs. Until now, many efforts have been made to obtain GFs with high shape stability for use in PCMs. For example, by freeze-drying, Yang et al. designed a GF and formed a PCM with polyethylene glycol, which possesses a high shape stability benefiting from additional bonding interaction between GFs and PCMs matrix provided by the oxygen functional groups on GO. 188 In addition, using the capillary forces and hydrogen bonds between GO and graphene nanosheets, Yang et al. fabricated a PCM with excellent shape stability by introducing graphene nanosheets into GF and combining them with polyethylene glycol. 189 Regarding obtaining GFs with high thermal conductivity used in PCMs, ultrahigh temperature graphitization is an effective strategy because it can repair the surface defects of GO and enhance phonon transport. At 2800°C, Chen et al. fabricated a GF/paraffin PCM with a high thermal conductivity and excellent shape stability through the ultrahigh temperature graphitization method. 190 At the same graphitization temperature, Yu et al. synthesized a GF/1-octadecanol PCM by combining freeze-drying selfassembly. 191 At 5 wt% GF, the thermal conductivity of the obtained PCM can reach 4.28 W/(m K). Moreover, Han et al. also fabricated a GF by the ultrahigh temperature graphitization method at 2800°C. 192 Subsequently, they impregnated the GF into octadecanol to obtain a GF/octadecanol PCM with high thermal conductivity. But, considering the excellent electrical conductivity of GFs, the research of BN foams in PCMs began to rise in recent years due to its superior thermal conductivity and remarkable dielectric properties. For example, Zhang et al. constructed a h-BN foam with high porosity (∼97.6%) and low density (∼1.7 mg/cm 3 ) and used it in PCMs. 38 Moreover, by the GO-assisted dispersion method, Wang et al. fabricated a hybrid rGO/BN foam. 193 Subsequently, they produced a PCM by encapsulating polyethylene glycol in the rGO/BN foam. Finally, the obtained composite PCM exhibits excellent structural stability and its thermal conductivity can reach 0.79 W/(m K).

| SUMMARY AND OUTLOOK
This review discussed the fabrications and applications of GFs and BN foams. The fabrication methods that are commonly used to fabricate GFs and BN foams are described, including solution-based assembly methods, 3D printing, CVD, and template-free pyrolysis.
The functional applications of GFs and BN foams are reviewed. (1) As supporting materials, GFs, and BN foams have high porosity and low density, so they are light structural supports. Moreover, thanks to their high conductivity and elasticity, GFs possess piezoresistive behavior for pressure sensors. (2) As acoustic shielding materials, GFs have an interconnected porous honeycomb structure that can enhance the air-viscous resistance damping, used for the acoustic absorber. (3) As thermal interface materials, GFs, and BN foams with high thermal conductivity are excellent thermal interface materials. As compared with GFs, the insulating BN foams can endow them for insulation and thermal management. (4) As electromagnetic shielding materials, GFs are an excellent absorbing material owing to the high specific surface area and the porous structure. They can also be used to fabricate the wave absorber. (5) As adsorption materials, with a large specific surface, GFs and BN foam have been demonstrated to be effective adsorbers for organic pollutants, oil/water separation, CO 2 capture, H 2 and CH 4 uptake, heavy metal ion, and pollutant disposal. Moreover, given the excellent electrical conductivities of GFs, it can be also applied in gassensing devices. (6) In electrocatalysis, GFs and GFs-based materials are electrocatalysts for energy conversion, benefiting from the high electrical conductivity and the high specific surface. Meanwhile, BN foams are emerging as a thermal catalyst for ODHP. (7) In energy storage, GFs can be used as the binder-free electrode owing to their structural merits for batteries and supercapacitors. In addition, the GFs and BN foams have applications in PCMs owing to their high thermal conductivity.
Although GFs and BN foams have a variety of applications, there are still challenges toward further development. For material preparations, although template-guided CVD can produce high-quality GFs and BN foams, it is still difficult to realize large-scale production because of the use of templates. For solutionbased assembly, although GFs and BN foams can realize mass production, the prepared GFs and BN foams usually have many defects. In the future, the following directions call for more attention. (1) It is valuable to search for suitable templates and/or catalysts. The transition metals and their compounds can be systematically studied to find suitable ones for preparing GFs and BN foams. (2) It is beneficial to develop defect repair techniques. High-temperature annealing is one of the posttreatments for improving crystalline degree, where it is also of great value to reduce the annealing temperature by applying pressure or adding a metal catalyst.
For material applications, the functional modification and decoration of GFs and BN foams is a new highlight. The doped heteroatom or the atomically dispersed metals in/on GFs or BN foams are the popular research to realize functional modification. However, the atomically dispersed metals have high surface free energy, thus causing agglomeration of these single atoms. Realizing controllable preparation with special doping contents is still a challenge.
With a series of advantages, GFs, and BN foams have shown strong application value in diverse disciplines. The rational design, the synthesis exploration, and the practical applications of GFs and BN foams are running for the bright future.