**2. Synthesis of porous graphene**

According to the standard specified by the International Union of Pure and Applied Chemistry (IUPAC), microporous materials have pore diameters of less than 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters of greater than 50 nm. There are basically two main methods, which can be used to fabricate porous graphene materials. These are the template and template-free methods, which will be described in greater details below.

#### **2.1. Template approach**

Template synthesis is an effective method for the transformation of graphene into porous graphene. It uses various inorganic and organic structures as templates for the transformation. Depending on the required size and morphology of pores, the appropriate template could be selected. This method can be divided into two categories: (1) soft-template method and (2) hard-template method.

#### *2.1.1. Soft-template methods*

Different kinds of amphiphilic molecules, such as surfactants and copolymers are used as structure directing agents under mild operating conditions in the soft-template methods. A bottom-up approach has been used for the preparation of mesoporous materials with twodimensional (2D) sandwich structure consisting of graphene layers and mesoporous silica with the use of cationic surfactant, cetyltrimethyl ammonium bromide (CTAB) [30]. The presence of oxygen functionalized groups makes graphene oxide (GO) sheets negatively charged. CTAB has the capability of electrostatically adsorbing and self-assembling onto the surface of negatively charged GO in alkaline solution. The GO-based silica hybrid is formed after the hydrolysis of silicon precursor, tetraethylorthosilicate (TEOS), and removal of CTAB. Thermal annealing at high temperature gives graphene-based silica sheets. The adsorption data have indicated a high specific area of 980 m2 /g. In another method, mesostructured graphene-based SnO2 composite is prepared by hydrothermally treating a suspension of GO, CTAB, and SnCl4 [31].

Two-dimensional ordered mesoporous carbon nanosheets have been prepared by low molecular weight phenolic resols on graphene sheets using a triblock copolymer called Pluronic F-127 as the structure-directing agent [32]. After mixing an aqueous GO dispersion with the above prepolymer, hydrothermal treatment and further thermal annealing were carried out to prepare mesoporous carbon/graphene composite. It was reported that, Bruna‐ uer-Emmett-Teller (BET) surface area decreases with increase in the GO ratio in the composite. In another case, the same hydrothermally driven low-concentration micelle assembly ap‐ proach was used with the help of anodic aluminum oxide (AAO) membranes to provide a large surface area [33]. After the hydrothermal treatment, AAO membrane was carbonized at 400–500°C for 2 h in argon atmosphere, followed by further carbonization at 700°C for 2 h in the same environment. Finally, mesoporous graphene sheets were obtained by dissolving the AAO substrate (**Figure 1**). The TEM images suggest that these nanosheets displayed ordered mesostructures, having an average pore size of 9 nm and wall thickness of 4 nm. Wen et al. [34] used a dual template method with Pluronic F-127 as the soft template and SiO2 as the hard template to fabricate three-dimensional graphene-based hierarchically porous carbon (3DGHPC). Carbonization was carried out to convert the layer of coated polymers on SiO2 spheres to carbon phase and simultaneously reduce GO. Finally, the 3DGHPC was obtained by treating as-prepared composite with 10% HCl to remove the SiO2 template followed by plenty of washing with Deionized (DI) water and drying at 50°C for 24 h. The as-prepared 3DGHPC displayed a specific area of 384.4 m2 /g with a pore volume of 0.73 cm3 /g.

**Figure 1.** Schematic representation of the formation of ordered mesoporous graphene nanosheets [33]. Reprinted with the permission of the American Chemical Society.

#### *2.1.2. Hard-template methods*

two-dimensional hexagonal lattice of sp2

1962 Recent Advances in Graphene Research

**2. Synthesis of porous graphene**

described in greater details below.

**2.1. Template approach**

hard-template method.

*2.1.1. Soft-template methods*

and fuel cells.

hybridized carbon atoms and since its discovery in

2004, significant efforts have been put in exploring its potential applications. Various synthe‐ sis methods have been developed to produce graphene including epitaxial growth of gra‐ phene on metal or SiC substrates [14, 15], chemical vapor deposition (CVD) [16–18], chemical reduction[19,20],thermalreduction[21,22],electrochemical synthesis [23,24],andliquidphase exfoliation [25, 26]. However, because of the strong π-π stacking and van der Waals interac‐ tions between graphene sheets, the experimentally obtainable surface area is far below the theoretical value. To overcome this problem, increasing effort has been put to transforming graphene into porous structures to achieve higher surface area [27–29]. Along with the inherent properties of graphene, porous graphene has a clear edge over other porous carbon materials. For example, the excellent electrical conductivity can be used as a perfect current collector for the rapid diffusion of electrons/ions while its high mechanical strength provides mechanical stability to the porous framework. These unique properties make porous graphene a highly promising material for energy storage and conversion applications like lithium-ion batteries (LIBs), lithium-sulfur (Li-S) batteries, supercapacitors, the dye-sensitized solar cells (DSSCs),

According to the standard specified by the International Union of Pure and Applied Chemistry (IUPAC), microporous materials have pore diameters of less than 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters of greater than 50 nm. There are basically two main methods, which can be used to fabricate porous graphene materials. These are the template and template-free methods, which will be

Template synthesis is an effective method for the transformation of graphene into porous graphene. It uses various inorganic and organic structures as templates for the transformation. Depending on the required size and morphology of pores, the appropriate template could be selected. This method can be divided into two categories: (1) soft-template method and (2)

Different kinds of amphiphilic molecules, such as surfactants and copolymers are used as structure directing agents under mild operating conditions in the soft-template methods. A bottom-up approach has been used for the preparation of mesoporous materials with twodimensional (2D) sandwich structure consisting of graphene layers and mesoporous silica with the use of cationic surfactant, cetyltrimethyl ammonium bromide (CTAB) [30]. The presence of oxygen functionalized groups makes graphene oxide (GO) sheets negatively charged. CTAB has the capability of electrostatically adsorbing and self-assembling onto the surface of negatively charged GO in alkaline solution. The GO-based silica hybrid is formed after the

When preparing porous graphene by hard template method, the template should initially be prepared. This includes the preparation of hard template itself and functionalization of its surface to get the required properties. Then, depending on the requirement, the template should be coated with graphene or GO. The final step is the selective removal of the template without destroying its structure.

Huang et al. [35] used methyl group grafted silica spheres as a hard template to prepare nanoporous graphene foams. These graphene foams had pore sizes of 30–120 nm and ultrahigh pore volumes of 4.3 cm3 /g. The surface area was reported to be 851 m2 /g. Hydrophobic surface of methyl group grafted silica spheres interacts with the hydrophobic basal planes of GO to induce self-assembled lamellar like structures. Choi et al. [36] were able to use polystyrene (PS) colloidal particles as sacrificial templates to synthesize macroporous embossed chemically modified graphene (CMG) sheets with an average pore size of 2 µm. Initially, free-standing PS/CMG film was made by vacuum filtration of a mixed suspension of CMG and PS. PS particles were then removed to generate 3D macropores. Three-dimensional macroscopic graphene foams (GFs) were made by the chemical vapor deposition (CVD) method using nickel (Ni) foam as the 3D scaffold template followed by the removal of the template by hot HCl [37–43]. In 2011, Cheng et al. [44] reported a flexible 3D GF using template directed CVD. The as-prepared GF had a specific surface area, up to 850 m2 /g, corresponding to an average number of layers of ~3. Poly methyl methacrylate (PMMA) can be used as a hard template to prepare macroporous graphene materials. Chen et al. [45] fabricated macroporous bubble graphene film by PMMA directed ordered assembly method. GO was mixed with the PMMA suspension and vacuum filtration was conducted to make a sandwich type assembly of the PMMA spheres and GO. Composite film was then peeled off from the filter, air dried and calcinated at 800°C to remove the template and reduce GO. As-prepared macroporous graphene film has a specific surface area of 128.2 m2 /g with an average pore diameter of 107.3 nm.

#### **2.2. Template-free approach**

In the template-free approach, defects are introduced in the graphene basal planes by different methods. Chemical etching or chemical activation is one such method which had been used extensively to prepare porous carbon materials. It is an effective and relatively easy method to fabricate porous graphene sheets without using any template.

Zhu et al. [46] produced porous carbon by a simple activation with KOH of microwave exfoliated GO (MEGO) and thermally exfoliated GO (TEGO). A mixture of the MEGO and KOH was thermally treated for 1 h at 800°C in a tube furnace in argon atmosphere at a pressure of 400 torr. Pores ranging from ~1 to ~10 nm were generated in the carbon matrix by the activation with KOH. The activation of carbon with KOH proceeds as, 6KOH + C ↔ 2K + 3H2 +2K2CO3, followed by the decomposition of K2CO3 and reaction of K/K2CO3/CO2 with carbon [46, 47].

Porous graphene hybrids can also be produced by thermally treating a mixture of graphene and porous components [48–55]. Rui et al. [48] produced a V2O5/rGO composite by thermal pyrolysis of a hybrid of vanadium oxide (VO) and rGO at the temperature of 350°C for 30 min under a heating rate of 10°C/min in air. In the thermal pyrolysis process, reduced VO (rVO) is converted into polycrystalline V2O5 porous spheres ranging from 200 to 800 nm.

Apart from using organic and inorganic species to carry out the template-free approaches to produce porous graphene, the amphiphilic nature of GO itself can also be used to fabricate foam-like structures of macroscopic graphene. The pore sizes of these 3D macroscopic structures are in the range of submicrometer to several micrometers. Because of macroscopic nature, they possess high mechanical strength, compressibility, excellent conductivity, and adsorption characteristics [56–59]. Xu et al. [60] prepared a self-assembled graphene hydrogel (SGH) by heating the GO dispersion sealed in a Teflon-lined autoclave at 180°C for 12 h. The hydrothermally reduced GO had a well-defined 3D interconnected porous network (**Figure 2**). The framework of SGH was assembled on partial overlapping of flexible graphene sheets because of π-π stacking interactions. The as-prepared SGH showed excellent mechanical strength and a good electrical conductivity of 5 × 103 S/cm. Later, the same research group reported a highly conductive graphene hydrogel which was reduced by hydrazine hydrate or hydrogen iodide to improve the conductivity by further removing its residual oxygenated groups [61].

surface to get the required properties. Then, depending on the requirement, the template should be coated with graphene or GO. The final step is the selective removal of the template

Huang et al. [35] used methyl group grafted silica spheres as a hard template to prepare nanoporous graphene foams. These graphene foams had pore sizes of 30–120 nm and ultrahigh

/g. The surface area was reported to be 851 m2

of methyl group grafted silica spheres interacts with the hydrophobic basal planes of GO to induce self-assembled lamellar like structures. Choi et al. [36] were able to use polystyrene (PS) colloidal particles as sacrificial templates to synthesize macroporous embossed chemically modified graphene (CMG) sheets with an average pore size of 2 µm. Initially, free-standing PS/CMG film was made by vacuum filtration of a mixed suspension of CMG and PS. PS particles were then removed to generate 3D macropores. Three-dimensional macroscopic graphene foams (GFs) were made by the chemical vapor deposition (CVD) method using nickel (Ni) foam as the 3D scaffold template followed by the removal of the template by hot HCl [37–43]. In 2011, Cheng et al. [44] reported a flexible 3D GF using template directed CVD.

number of layers of ~3. Poly methyl methacrylate (PMMA) can be used as a hard template to prepare macroporous graphene materials. Chen et al. [45] fabricated macroporous bubble graphene film by PMMA directed ordered assembly method. GO was mixed with the PMMA suspension and vacuum filtration was conducted to make a sandwich type assembly of the PMMA spheres and GO. Composite film was then peeled off from the filter, air dried and calcinated at 800°C to remove the template and reduce GO. As-prepared macroporous

In the template-free approach, defects are introduced in the graphene basal planes by different methods. Chemical etching or chemical activation is one such method which had been used extensively to prepare porous carbon materials. It is an effective and relatively easy method

Zhu et al. [46] produced porous carbon by a simple activation with KOH of microwave exfoliated GO (MEGO) and thermally exfoliated GO (TEGO). A mixture of the MEGO and KOH was thermally treated for 1 h at 800°C in a tube furnace in argon atmosphere at a pressure of 400 torr. Pores ranging from ~1 to ~10 nm were generated in the carbon matrix by the activation with KOH. The activation of carbon with KOH proceeds as, 6KOH + C ↔ 2K + 3H2 +2K2CO3, followed by the decomposition of K2CO3 and reaction of K/K2CO3/CO2 with

Porous graphene hybrids can also be produced by thermally treating a mixture of graphene and porous components [48–55]. Rui et al. [48] produced a V2O5/rGO composite by thermal pyrolysis of a hybrid of vanadium oxide (VO) and rGO at the temperature of 350°C for 30 min under a heating rate of 10°C/min in air. In the thermal pyrolysis process, reduced VO (rVO) is

converted into polycrystalline V2O5 porous spheres ranging from 200 to 800 nm.

The as-prepared GF had a specific surface area, up to 850 m2

to fabricate porous graphene sheets without using any template.

graphene film has a specific surface area of 128.2 m2

**2.2. Template-free approach**

carbon [46, 47].

/g. Hydrophobic surface

/g, corresponding to an average

/g with an average pore diameter of 107.3

without destroying its structure.

pore volumes of 4.3 cm3

1984 Recent Advances in Graphene Research

nm.

**Figure 2.** (a) Photographs of a 2 mg/ml homogeneous GO aqueous dispersion before and after hydrothermal reduction at 180°C for 12 h; (b) photographs of a strong SGH allowing easy handling and supporting weight; (c-e) SEM images with different magnifications of the SGH interior microstructures; (f) room temperature *I*-*V* curve of the SGH exhibit‐ ing Ohmic characteristic, inset shows the two-probe method for the conductivity measurements [60]. Reprinted with the permission of the American Chemical Society.
