**3. Applications of porous graphene materials**

Unique porous structure of graphene along with its superior properties makes graphene a potential candidate for energy storage and conversion applications. The following sections review several key applications of porous graphene in LIBs, Li-S batteries, supercapacitors, and the dye-sensitized solar cells.

#### **3.1. Lithium-ion batteries**

Lithium-ion battery has a widespread increasing demand because of its high energy density, flexibility, low maintenance, and longer lifespan compared with other battery technologies [62]. To further increase the energy density, charging efficiency, and cycle life of lithium-ion batteries, it is essential to look at new electrode materials that have good lithium storage capability. Porous graphene with exceptional properties holds a great potential as an electrode material for the lithium-ion battery. The high surface area of graphene can significantly increase the diffusion of lithium ions and electrons. Furthermore, the superior electrical conductivity provides a good conductive network within the electrodes. Graphene can construct a 3D framework with a strong tolerance to the volume change of electrochemically active materials during charge-discharge cycles [63].

An anode material for Li-ion battery was made by hierarchical mesoporous and macroporous carbon using the spinodal decomposition of a mesophase pitch (MP) carbon precursor and polystyrene as a soft template [64]. Scanning Electron Microscope (SEM) images of this structure revealed a 3D bicontinuous network of macropores and according to Hg porosimetry the average macropore size was recorded as 100 µm. The first reversible capacity of 470 mAh/ g was recorded at a discharge-charge rate of C/5. When discharge-charge rates were increased to 1 and 5 C, reversible capacities of 320 and 200 mAh/g were obtained. Yang et al. [30] managed to synthesize a graphene-based mesoporous carbon anode which performed better than previous graphitic anode. Two-dimensional sandwich like graphene structure increases the surface area while each nanosheet acts as a mini-current collector. They facilitate the rapid transportation of electrons during charge-discharge cycles. At the rate of C/5, its reversible capacity stabilized at 770 mAh/g. When the discharge-charge rates were increased to 1 and 5 C, the reversible capacities recorded 540 and 370 mAh/g, respectively.

Graphene materials loaded with macroporous structures have shown positive results as anode materials for the Li-ion batteries. Mn3O4-graphene [65], Co3O4-graphene [66], and Fe3O4 graphene [67–69] have been studied extensively as potential anode materials for Li-ion materials. Chen et al. [69] reported a 3D graphene-Fe3O4 hybrid prepared by chemical reduction of the GO in the presence of Fe3O4 nanoparticles. The as-prepared hybrid was tested as an anode material for LiBs and exhibited capacities of 990 and 730 mAh/g at current densities of 800 and 1600 mA/g, respectively.

#### **3.2. Li-sulfur batteries**

For more than 20 years, the Li-ion battery has dominated the rechargeable battery market for portable devices and it is still the best choice for electric vehicles. But, when it comes to the electrical performance, a significant improvement is less likely as the performance of the Liion battery has almost reached its theoretical limits [70, 71]. Therefore, Li-S battery is consid‐ ered as one of the potential candidates to replace the Li-ion battery as the next generation rechargeable battery. Sulfur is considered as the 10th most abundant element in the Earth. When employed as a cathode, it has a high specific capacity of 1675 mAh/g and it can deliver a specific energy of 2600 Wh/kg. However, several key issues have prevented the practical applications of Li-S batteries so far. The issues which need to be addressed are (i) poor electrical conductivity of sulfur and its final discharge products (Li2S/Li2S2); (ii) large volume change of sulfur electrode during electrochemical cycling; and (iii) dissolution of polysulfides, inter‐ mediate reactant products in the organic electrolyte leading to deposition of Li2S2/Li2S at the electrode interface. To overcome these drawbacks, extensive researches have been carried out to use graphene materials as scaffolds for cathodes in Li-S batteries [72–76].

**3.1. Lithium-ion batteries**

2006 Recent Advances in Graphene Research

active materials during charge-discharge cycles [63].

of 800 and 1600 mA/g, respectively.

**3.2. Li-sulfur batteries**

C, the reversible capacities recorded 540 and 370 mAh/g, respectively.

Lithium-ion battery has a widespread increasing demand because of its high energy density, flexibility, low maintenance, and longer lifespan compared with other battery technologies [62]. To further increase the energy density, charging efficiency, and cycle life of lithium-ion batteries, it is essential to look at new electrode materials that have good lithium storage capability. Porous graphene with exceptional properties holds a great potential as an electrode material for the lithium-ion battery. The high surface area of graphene can significantly increase the diffusion of lithium ions and electrons. Furthermore, the superior electrical conductivity provides a good conductive network within the electrodes. Graphene can construct a 3D framework with a strong tolerance to the volume change of electrochemically

An anode material for Li-ion battery was made by hierarchical mesoporous and macroporous carbon using the spinodal decomposition of a mesophase pitch (MP) carbon precursor and polystyrene as a soft template [64]. Scanning Electron Microscope (SEM) images of this structure revealed a 3D bicontinuous network of macropores and according to Hg porosimetry the average macropore size was recorded as 100 µm. The first reversible capacity of 470 mAh/ g was recorded at a discharge-charge rate of C/5. When discharge-charge rates were increased to 1 and 5 C, reversible capacities of 320 and 200 mAh/g were obtained. Yang et al. [30] managed to synthesize a graphene-based mesoporous carbon anode which performed better than previous graphitic anode. Two-dimensional sandwich like graphene structure increases the surface area while each nanosheet acts as a mini-current collector. They facilitate the rapid transportation of electrons during charge-discharge cycles. At the rate of C/5, its reversible capacity stabilized at 770 mAh/g. When the discharge-charge rates were increased to 1 and 5

Graphene materials loaded with macroporous structures have shown positive results as anode materials for the Li-ion batteries. Mn3O4-graphene [65], Co3O4-graphene [66], and Fe3O4 graphene [67–69] have been studied extensively as potential anode materials for Li-ion materials. Chen et al. [69] reported a 3D graphene-Fe3O4 hybrid prepared by chemical reduction of the GO in the presence of Fe3O4 nanoparticles. The as-prepared hybrid was tested as an anode material for LiBs and exhibited capacities of 990 and 730 mAh/g at current densities

For more than 20 years, the Li-ion battery has dominated the rechargeable battery market for portable devices and it is still the best choice for electric vehicles. But, when it comes to the electrical performance, a significant improvement is less likely as the performance of the Liion battery has almost reached its theoretical limits [70, 71]. Therefore, Li-S battery is consid‐ ered as one of the potential candidates to replace the Li-ion battery as the next generation rechargeable battery. Sulfur is considered as the 10th most abundant element in the Earth. When employed as a cathode, it has a high specific capacity of 1675 mAh/g and it can deliver a specific energy of 2600 Wh/kg. However, several key issues have prevented the practical applications of Li-S batteries so far. The issues which need to be addressed are (i) poor electrical

For the first time, Wang et al. [77] synthesized a sulfur-graphene (S-GNS) composite by heating a mixture of graphene nanosheets and elemental sulfur. The electrochemical performance of the battery was unsatisfactory as S-GNS electrode contained only 17.6 wt% sulfur. Wang et al. [78] improved the performance of this cathode by increasing the sulfur content up to 44.5 wt % using the same synthesis method. The reversible capacities of the electrode were recorded as 662 mAh/g at 1 C and 391 mAh/g at 2 C after 100 cycles.

Kim et al. [79] produced mesoporous graphene-silica composite (m-GS) as a cathode structure to host sulfur for Li-S batteries. With the help of the ternary cooperative assembly of triblock copolymer (P123), silica precursor and graphene, porous silica structure was made parallel to graphene sheets. Sulfur was infiltrated into the mesoporus structure by melt diffusion at 155°C for 12 h. S intercalated graphite oxide cathode was made by in situ sulfur reduction and intercalation of graphite oxide [80]. By heating a mixture of S8 and graphite oxide at 600°C under vacuum, would break large molecules of S8 into S2 and in the meantime reduce graphite oxide to graphene. Interplanar distance of the carbon matrix allows S2 to intercalate into GO. To minimize the capacity decay, surface S8 could be removed by CS2. This specified cathode was able to maintain a reversible capacity of 880 mAh/g after 200 cycles.

To obtain better electrochemical performance, Zhang et al. [81] created dense nanopores on the surface of graphene nanosheets by chemically activating hydrothermally reduced gra‐ phene oxide (rGO). Sulfur was infiltrated into the KOH-activated graphene hydrogels by the melt diffusion method (**Figure 3**). The rGO hydrogel served as a trap for soluble polysulfides.

**Figure 3.** (a) Schematic representation of the preparation route of activated graphene nanosheets through self-assem‐ bly of GO, ion diffusion and chemical activation strategy. (b) Proposed scheme for the constrained electrochemical re‐ action process of the graphene/sulfur composite. (c) Cycling performances of graphene/sulfur composite electrode at 0.5 C and 1 C [81]. Reprinted with the permission of the Royal Society of Chemistry.

According to results from nitrogen sorption measurements, the surface area of the mesoporous system was 2313 m2 /g and the mean value of nanopores was 3.8 nm. At 0.5 C and 1 C, the graphene/sulfur composite electrode delivered high reversible capacities of 1143 and 927 mAh/ g, respectively.

Evers and Nazar [82] prepared a graphene-sulfur cathode material with a sulfur loading of 87 wt% by a simple one pot method. A mixture of GO and soluble polysulfide was oxidized *in situ* as a one pot reaction. Because of the formation of insulating Li2S layer, the initial discharge capacity of 705 mAh/g at 0.2 C decreased drastically after 50 cycles. S/rGO composite material for Li-S battery cathode was made by concurrently oxidizing sulfide and reducing GO [83]. In this method, Na2S and Na2SO3 were mixed with the GO solution. The composite material is obtained by the reduction of GO by Na2S. The composite with a sulfur loading of 63.6 wt% delivered a reversible capacity of 804 mAh/g after 80 cycles at 0.186 C and 440 mAh/g after 500 cycles at 0.75 C. Gao et al. [84] prepared a sulfur cathode composed of sulfur nanoparticles wrapped in graphene by using Na2S2O3 as a precursor of sulfur. In acidic medium, Na2S2O3 can also serve as a reducing agent of GO. Polyvinylpyrrolidone (PVP) was used to prevent the S particles from aggregation and to keep the sulfur particles at submicrometer range. By using (NH4)2S2O3 as a sulfur precursor, Xu et al. made a graphene-encapsulated sulfur composite. In this synthesis method, a mild reducing agent, urea, was used to reduce GO.

#### **3.3. Supercapacitors**

Supercapacitor is another major alternative solution for the energy storage applications. Supercapacitors have higher power densities than batteries and also higher energy densities than dielectric capacitors [46, 85–88]. The first attempt to use graphene as a supercapacitor was done by Rouff et al. [89] in 2008. In that method, GO was reduced by using hydrazine hydrate and the surface area as measured by BET method was 705 m2 /g. Specific capacitances of 135 and 99 F/g were obtained in aqueous and organic electrolytes, respectively. However, strong π-π stacking and van der Waals attractions among inter layers cause irreversible agglomera‐ tion to form graphite, resulting a decrease in surface area which may hinder the diffusion of the electrolyte. Therefore, making graphene in to a highly open porous structure is an effective way to increase the accessible surface area and the specific capacitance.

Zhu et al. [46] were able to make a carbon based supercapacitor by chemically activating the microwave exfoliated GO (MEGO) and thermally exfoliated GO (TEGO) using the KOH to obtain surface area values up to 3100 m2 /g and a high electrical conductivity of 500 S/m with a C/O atomic ratio of 35 . The specific capacitance values calculated from the charge-discharge curves were 165, 166, and 166 F/g at current densities of 1.4, 2.8, and 5.7 A/g, respectively.

Chen et al. [90] discovered a route to convert noncovalent functionalized graphene to a graphene-activated carbon composite by chemically activating with the KOH, which consisted of a specific surface area of 798 m2 /g. Stable graphene colloids absorbed by oligomers of pphenylene diamne (PPD) were converted to a graphene-activated carbon composite by the KOH activation annealing method. The KOH activation created micro/mesopores in the activated carbon covered on graphene whereas pores in activated carbon also contributed the high surface area of the composite. The as-prepared graphene composite exhibited a specific capacitance of 122 F/g and energy density of 6.1 Wh/kg in aqueous electrolyte. Maximum energy density values of 52.2 and 99.2 Wh/kg were obtained in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) electrolyte at room temperature and 80°C, respectively.

According to results from nitrogen sorption measurements, the surface area of the mesoporous

graphene/sulfur composite electrode delivered high reversible capacities of 1143 and 927 mAh/

Evers and Nazar [82] prepared a graphene-sulfur cathode material with a sulfur loading of 87 wt% by a simple one pot method. A mixture of GO and soluble polysulfide was oxidized *in situ* as a one pot reaction. Because of the formation of insulating Li2S layer, the initial discharge capacity of 705 mAh/g at 0.2 C decreased drastically after 50 cycles. S/rGO composite material for Li-S battery cathode was made by concurrently oxidizing sulfide and reducing GO [83]. In this method, Na2S and Na2SO3 were mixed with the GO solution. The composite material is obtained by the reduction of GO by Na2S. The composite with a sulfur loading of 63.6 wt% delivered a reversible capacity of 804 mAh/g after 80 cycles at 0.186 C and 440 mAh/g after 500 cycles at 0.75 C. Gao et al. [84] prepared a sulfur cathode composed of sulfur nanoparticles wrapped in graphene by using Na2S2O3 as a precursor of sulfur. In acidic medium, Na2S2O3 can also serve as a reducing agent of GO. Polyvinylpyrrolidone (PVP) was used to prevent the S particles from aggregation and to keep the sulfur particles at submicrometer range. By using (NH4)2S2O3 as a sulfur precursor, Xu et al. made a graphene-encapsulated sulfur composite. In

Supercapacitor is another major alternative solution for the energy storage applications. Supercapacitors have higher power densities than batteries and also higher energy densities than dielectric capacitors [46, 85–88]. The first attempt to use graphene as a supercapacitor was done by Rouff et al. [89] in 2008. In that method, GO was reduced by using hydrazine hydrate

and 99 F/g were obtained in aqueous and organic electrolytes, respectively. However, strong π-π stacking and van der Waals attractions among inter layers cause irreversible agglomera‐ tion to form graphite, resulting a decrease in surface area which may hinder the diffusion of the electrolyte. Therefore, making graphene in to a highly open porous structure is an effective

Zhu et al. [46] were able to make a carbon based supercapacitor by chemically activating the microwave exfoliated GO (MEGO) and thermally exfoliated GO (TEGO) using the KOH to

a C/O atomic ratio of 35 . The specific capacitance values calculated from the charge-discharge curves were 165, 166, and 166 F/g at current densities of 1.4, 2.8, and 5.7 A/g, respectively.

Chen et al. [90] discovered a route to convert noncovalent functionalized graphene to a graphene-activated carbon composite by chemically activating with the KOH, which consisted

phenylene diamne (PPD) were converted to a graphene-activated carbon composite by the KOH activation annealing method. The KOH activation created micro/mesopores in the

/g. Specific capacitances of 135

/g and a high electrical conductivity of 500 S/m with

/g. Stable graphene colloids absorbed by oligomers of p-

this synthesis method, a mild reducing agent, urea, was used to reduce GO.

and the surface area as measured by BET method was 705 m2

obtain surface area values up to 3100 m2

of a specific surface area of 798 m2

way to increase the accessible surface area and the specific capacitance.

/g and the mean value of nanopores was 3.8 nm. At 0.5 C and 1 C, the

system was 2313 m2

2028 Recent Advances in Graphene Research

**3.3. Supercapacitors**

g, respectively.

Zhang et al. [91] introduced a method to produce porous 3D graphene-based bulk materials with ultrahigh specific area of 3523 m2 /g and excellent bulk conductivity (up to 303 S/m) by *in-situ* hydrothermal polymerization/carbonization of a mixture of industry carbon sources and the GO followed by KOH activation (**Figure 4**). The carbon sources used in this method were biomass, phenol-formaldehyde (PF), polyvinyl alcohol (PVA), sucrose, cellulose, and lignin. Graphene-PF composite material gave the highest specific capacitance values of 202 F/ g in 1 M TEABF4/AN and 231 F/g in neat EMIMBF4 electrolyte systems, respectively.

**Figure 4.** (a) Schematic representation of the synthesis procedure of the porous three-dimensional graphene-based ma‐ terials. Galvanostatic charge/discharge test results of supercapacitors based on the optimized porous 3D graphenebased materials in (b) 1 M TEABF4/AN and (c) neat EMIMBF4 electrolytes under different current densities. CV curves of PF16G-HA based supercapacitor under different scan rates in (d) 1 MTEABF4/AN and (e) neat EMIMBF4 electrolyte. Reproduced with permission [91], Copyright 2013 NPG.

The electrochemical performance of carbon based materials can be enhanced by doping carbon network with nitrogen and boron [92–95]. Nitrogen and boron co-doped 3D graphene aerogel (BN-GA) was fabricated by using the GO and ammonia boron trifluoride (NH3BF3) [96]. The interconnected framework of graphene nanosheets had a surface area of 249 m2 /g with a macroporous structure. BN-GAs were directly processed into thin electrodes without destroy‐ ing the 3D continuous frameworks and used in all-solid-state supercapacitors (ASSSs). Because of the unique structure and strong synergetic effects of nitrogen and boron co-doping, a specific capacitance of 62 F/g and energy density of 8.65 Wh/kg were obtained. 3D graphene aerogels with mesoporous silica frameworks (GA-SiO2) were fabricated by the hydrolysis of TEOS with graphene aerogel and CTAB as the soft template [97]. Graphene aerogel-mesoporous carbon (GA-MC) with a surface area of 295 m2 /g was generated by infiltrating a sucrose solution into the GA-SiO2 followed by carbonization at 700°C for 3 h in argon. The as-prepared GA-MC exhibited a specific capacitance of 226 F/g when it was used as a supercapacitor.

#### **3.4. Dye-sensitized solar cells**

The dye-sensitized solar cells are among third generation photovoltaic devices that are costeffective and highly efficient. It consists of a mesoporous TiO2 photoanode with a dye to increase light absorption, a counter electrode (CE), and electrolyte. The CE should reduce redox species, which are used to regenerate the sensitizer after electron ejection. To increase the efficiency of DSSC, it is essential to select a CE material with low sheet resistance, high catalytic activity for the reduction of redox species, excellent chemical stability, and low cost. Recently, graphene-based CEs have been studied extensively as a potential cost-effective replacement for platinum based CEs.

Compared with other graphene-based materials, functionalized or doped graphene exhibits exceptional electocatalytic activity. In 2012, Xu et al. prepared Hemin, an iron-containing porphyrin functionalized rGO by microwave irradiation [98]. The Hemin-rGO hybrid exhibited a power conversion efficiency (PCE) value of 2.45 %. Yen et al. [99] reported a nitrogen-doped graphene prepared using a hydrothermal method. The nitrogen-doped domains on the graphene surface act as electroactive sites, which have selectivity for redox species in the reduction reaction. The as-prepared nitrogen-doped graphene CE exhibited the PCE value of 4.75%. Xue et al. [100] managed to prepare 3D nitrogen-doped graphene foams with a nitrogen content of 7.6% freeze drying the GO foams followed by annealing at 800°C in ammonia/argon mixture for 1 h. Because of the high content of nitrogen, the PCE value of 7.07% was obtained.

#### **4. Conclusion**

In conclusion, owing to its high surface area, unique pore structure, and remarkable electro‐ chemical performances, porous graphene has attracted great attention in the fields of energy storage and conversion. However, there are several key issues, which need to be addressed. The precise control of pore size, pore morphology, and wall thickness is necessary for the assembly of hierarchically structured porous graphene materials. Introduction of different sizes of pores into graphene matrix is essential to produce porous graphene materials to obtain synergic effects of different pores. With increasing research efforts in the field, we believe that there would be significant advances in the synthesis and application of porous graphene in the near future, benefiting development of high performance energy conversion and storage devices. This research was partially supported under the Australian Research Council Discovery Project (DP150101717). Kimal Chandula Wasalathilake acknowledges the QUTPRA scholarship from the Queensland University of Technology.
