Recent advances in porous graphene materials for supercapacitor applications

Abstract

Driven by the environmental problems and energy crisis, the development of clean and renewable energy materials as well as their devices is urgently demanded. Supercapacitors, also called electrochemical capacitors, which store energy using either ion adsorption or fast surface redox reactions, are supposed to be a promising candidate for alternative electrical energy storage devices due to their high power density, exceptional cycle life, and low maintenance cost. The performance of supercapacitors is highly dependent on the properties of electrode materials. Graphene based materials exhibit great potential application for supercapacitors because of their unique structure and excellent intrinsic physical properties. Introducing porous structures to graphene is an effective strategy to obtain high surface area and high specific capacitance. In this review, we provide a brief summary of the recent developments about the synthesis and application of porous graphene materials for supercapacitors.

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1. Introduction

Recently, there has been increasing worldwide demand for the development of alternative energy techniques with higher efficiency and sustainability due to the rapid depletion of fossil fuels and increasingly worsened environmental pollution and global warming. However, power production from sustainable energy resources, including solar, wind, and geothermal energies, is not always coincident with energy demand. For example, implementation of solar or wind energy causes big challenges to power grid management and grid stability because of the large fluctuations in electricity generation that are completely decoupled from the actual energy demand.¹ Therefore, the development of large-scale energy-storage systems is very important to resolve this problem. Pumped hydro, flywheels, compressed air energy storage (CAES), superconducting magnetic energy storage (SMES), hydrogen storage, and electrochemical energy storage are usually-proposed alternative and/or competitive solutions for storing energy.² Among them, electrochemical energy storage devices, such as batteries and electrochemical capacitors (ECs), are becoming leading electrical energy storage (EES) technologies today. Fig. 1 shows the Ragone plot for various electrical energy storage devices.

Fig. 1 Ragone plots for various electrical energy storage devices. Times shown are the time constants of the devices, obtained by dividing the energy density by the power. Reproduced with permission from ref. 3, Copyright 2008 NPG.

Supercapacitors, also called electrochemical capacitors, have attracted intense attentions because of their higher power densities than batteries and higher energy densities than conventional dielectric capacitors. With high power capability at relatively high energy densities, exceptional cycle life and reliability, supercapacitors have been used in a variety of applications ranging from power electronics, and computer memory backup systems, to large scale transport systems including subway trains and buses, and to energy storage at intermittent generators including windmills, and smart grid applications.⁴ The fast charge/discharge characteristics and long cycle life of supercapacitors are particularly well suited for recycling energy from repetitive motion (e.g. automotive braking, elevator operation, seaport rubber-tired gantry crane) that would otherwise be wasted, leading to the improved energy efficiency.⁵ In a hybrid power system, supercapacitors can be combined with batteries or fuel cells in order to enhance the system lifetime and increase the overall efficiencies.^6,7 Supercapacitors can also be used as uninterruptible power supplies (UPS) in the industry and load-leveling to provide high quality power, which can greatly reduce economic losses resulted by power disruptions.⁸ Many excellent reviews about the fundamental considerations of supercapacitors can be found in the literatures.^3,9–12

General electric first patented a low voltage electrolytic capacitor in 1957. Later, the Standard Oil of Ohio (SOHIO) Company invented a device that stored energy by the electric double layer (EDL) mechanism in 1966. Following the commercial introduction by NEC in 1978 under license from SOHIO, supercapacitors have evolved through several generations of designs.^13,14 Today, there are a number of manufacturers producing supercapacitors, such as Maxwell, NESSCAP, Nippon ESMA, Ioxus and Cap-XX, and so on. However, the energy density of most commercially available supercapacitors (less than 10 W h kg⁻¹) is still significantly lower than batteries. The electrode materials are usually considered to play the most important role in the performance of supercapacitors.

Graphene, a single layer of sp² hybridized carbon atoms arranged in a hexagonal lattice, has emerged as one of the most attractive carbon allotropes for energy storage applications in recent years due to its unique structure and properties. It has a large theoretical specific surface area (2630 m² g⁻¹), high intrinsic carrier mobility (2 × 10⁵ cm² V⁻¹ s⁻¹), excellent thermal conductivity (∼5000 W m⁻¹ K⁻¹), high Young's modulus (∼1.0 TPa), good optical transmittance (∼97.7%), and ultrahigh electrical conductivity at room temperature (10⁶ S cm⁻¹).^15–19 Graphene can be prepared by various approaches, including mechanical cleavage of graphite with a Scotch tape,²⁰ epitaxial growth on metal surfaces or single-crystal SiC,^21,22 chemical vapor deposition (CVD),^23–25 liquid phase exfoliation,^26,27 chemical reduction,^28–33 thermal reduction,^34,35 electrochemical synthesis,^36–38 and high energy ball milling.^39–42 The application of graphene material as the supercapacitor electrode material was first explored by Ruoff and co-workers in 2008, in which the reduction of graphene oxide (GO) by using hydrazine hydrate exhibited specific capacitances of 135 and 99 F g⁻¹ in aqueous and organic electrolytes, respectively.⁴³ However, graphene sheets tend to form irreversible agglomerates or even restack to form graphite due to the interlayer strong π–π stacking and van der Waals interactions among the graphite single layers, resulting in a dramatic decrease of the surface area and a portion of graphene surfaces practically inaccessible for electrolyte ions.

Making graphene into porous structures is an effective strategy to prevent the agglomerates of graphene nanosheets and to obtain graphene materials with high surface area and high specific capacitance. In this review, we mainly focus on the synthetic approaches of porous graphene materials, based on the strategies of chemical activation, template approach, and self-assembly of graphene nanosheets. Furthermore, the application of porous graphene materials for supercapacitor is also summarized.

2. Supercapacitors: principle and characteristics

2.1. Theoretical background

The main energy storage of supercapacitors originates from the reversible reaction on the surface of electrode materials, including charge separation and faradic redox reaction at the electrode/electrolyte interface.^44,45 Because “surface charge storage” does not require bulk ionic diffusion within the inner crystalline framework of electrode materials, supercapacitors can offer ultrafast charge/discharge rates (i.e., high power density), but relatively poor specific capacitance (i.e., low energy density) compared with batteries.^46,47 Therefore, many efforts have been devoted to improve the capacitance and energy density.

The electrode/electrolyte interface can be regarded as a conventional dielectric capacitor, and the capacitance C is defined by eqn (1):

where ε₀ is the vacuum permittivity (ε₀ = 8.854 × 10⁻¹² F m⁻¹), ε_r is the relative permittivity of the dielectric electrolyte, and d is the effective thickness of the double layer (Debye length). A two-electrode supercapacitor cell can be considered as two capacitors in series, and the total capacitance (C_T) of the cell can be calculated as follows:in which C_p and C_N represent positive capacitance and negative capacitance, respectively.

The energy density (E) of a capacitor is expressed by eqn (3), in which V is the charging potential:

When the stored energy is released through discharge of the capacitance, the maximum power (P_max) is expressed by eqn (4)

where ESR and M represent the equivalent series resistance and the total mass of the two electrodes, respectively. The energy improvement of a supercapacitor can be achieved by maximizing the specific capacitance or cell voltage.

Supercapacitors can be classified into two categories based on the energy storage mechanism.^48,49 One is the electrochemical double layer capacitor (EDLC), in which no electrochemical reaction is involved in the electrode material during the charging and discharging processes, while only pure physical charge accumulation occurs at the electrode/electrolyte interface. The other type is the faradaic capacitor (FC), in which the electrode material store and release charges by electrochemical redox reactions during the charging and discharging processes.

2.1.1 Electrochemical double layer capacitor. The electrical double-layer capacitance comes from the potential-dependent reversible accumulation of electrostatic charge at the interface of electrode and electrolyte, where the excess or deficit charges on the electrode surfaces trigger the electrolyte ions with counterbalancing charge to build up across the electrode/electrolyte interface, in order to remain electroneutrality.^3,9 The charge generation includes surface dissociation as well as ions adsorption from both the electrolyte and crystal lattice defects, which solely involve the electrostatic accumulation of surface charge. During the process of charging, cations travel towards the negative electrode while anions accumulate near the positive electrode surface. When the charges are released, the reverse process takes place within the electrolyte. The mechanism of EDL determines that no charge transfer or ions exchange happens across the electrode/electrolyte interface. This implies that the electrolyte concentration remains constant during the charging and discharging processes. As a result, energy is stored in the electric double-layer interface.⁵⁰ Fig. 2 gives a schematic of the charge storage mechanism of EDLC.

Fig. 2 Charge storage mechanism of EDLC. Reproduced with permission from ref. 51, Copyright 2013 Elsevier.

It should be noted that only the surface accessible to electrolyte ions can contribute to charge storage; therefore, optimization of pore size, pore structure, surface properties and conductivity of the electrode materials are desired. This storage mechanism allows very fast energy uptake and delivery and high stability of EDLCs during millions of charge/discharge cycles.

2.1.2 Faradaic capacitors. In contrast to EDL capacitance, pseudo-capacitance originates from thermodynamics reasons and it is dependent on charge acceptance (dq) and potential change (dV). The derivative capacitance corresponds to the pseudo-capacitance, which is defined as
where C is the capacitance of the pseudo-capacitor, q is the quantity of charge, and V is the potential.

The main difference between pseudo-capacitance and EDL capacitance lies in the fact that pseudo-capacitance is faradaic in origin, involving fast and reversible redox reactions between the electrolyte ions and electro-active species on the electrode surface. The typical electrode materials include conducting polymers and transition metal oxide and hydroxide.

When a potential is applied to a FC, fast and reversible faradaic reactions (redox reactions) take place on the electrode materials and involve the transport of charge across the double layer, similar to the charging and discharging processes occurred in batteries, resulting in faradaic current passing through the supercapacitor cell.⁴⁴ Three types of faradaic processes occur at FC electrodes:

(1) reversible adsorption of active species on the surface of electrode,

(2) redox reactions of transition metal oxides or hydroxides,

(3) reversible electrochemical doping/dedoping of conductive polymer based electrodes.

It has been demonstrated that the electrochemical processes occur both on the surface and in the bulk near the surface of the solid electrode, which can increase the specific capacitance of supercapacitors.⁵² Therefore, FCs can provide much higher energy density compared with EDLCs. However, FCs usually suffer from relatively lower power density than EDLCs because the redox electrodes usually have poor electrical conductivity and faradaic processes are normally slower than non-faradaic processes. In addition, they often endure poor stability during cycling owe to redox reactions.

Besides, hybrid supercapacitors with an asymmetrical electrode configuration (e.g. one electrode utilizing EDL mechanism while the other consists of faradaic capacitance material) have been extensively studied recently to utilize the advantages of both electrode materials to improve overall cell voltage, energy, and power densities. With respective to this hybrid supercapacitor, both electrical double-layer capacitance and faradaic capacitance mechanisms work simultaneously, but usually one of them is dominant.

2.2. Electrode materials

2.2.1 Carbon materials. A large variety of carbons can be considered as active materials in EDLC electrodes, including high surface area activated carbon,^53–55 carbon aerogel,⁵⁶ carbon nanotube (CNT),⁵⁷ template porous carbon,⁵⁸ activated carbon nanofiber (CNF),⁵⁹ and graphene.^43,60–67 The capacitance largely depends on the electrode material, especially on its specific surface area, pore size distribution, pore structure, electrical conductivity and surface wettability. Up to now, activated carbons produced by different activation processes from various precursors (e.g., wood, coal, nutshell) are the most common commercial electrode materials.⁵³

Among various carbon materials, graphene is a unique and attractive electrode material because of its peculiar structure and excellent properties. The theoretical specific surface area of graphene is as high as 2623 m² g⁻¹, which is twice larger than that of single-walled carbon nanotubes (CNTs) and much higher than that of most carbon black and activated carbons.⁶⁸ Therefore, graphene is a promising candidate to deliver high electrochemical double layer capacitance. The interfacial double-layer capacitance on one side of a single graphene sheet is about 21 μF cm⁻²; thus, the theoretical maximum gravimetric specific capacitance of graphene is approximately 550 F g⁻¹.⁶⁹ Graphene materials can be obtained at relatively low costs in large scale from graphite as the precursor. The superior electron mobility of graphene facilitates charge transfer during charge/discharge processes, which is beneficial for improving the performance of supercapacitors. The high mechanical, thermal and chemical stabilities of graphene also enhance its cycle stability. Thus graphene is considered as a very promising candidate to replace activated carbons as the electrode materials in high-performance supercapacitors. Fig. 3 shows that 2D graphene layer can be regarded as the basic building block for carbon materials of all other dimensionalities, i.e. 0D fullerene, 1D nanotubes, and 3D graphite.

Fig. 3 2D Graphene building block for carbon materials with different dimensionalities. Reproduced with permission from ref. 15, Copyright 2007 NPG.

2.2.2 Conducting polymers. Conducting polymers (CPs) are attractive to researchers due to their special properties, such as low cost, high conductivity in a doped state, high storage capacity/porosity/reversibility, and adjustable redox activity through chemical modification.^52,70,71 The capacitive behavior of CPs originates from the fast reversible faradaic processes. In oxidation reaction, ions are inserted into the polymer backbone, while the ions during the reduction process are deserted from polymer backbone into the electrolyte. Different from carbon materials that only utilize the accessible surface to electrolyte for energy storage, the redox reaction of conducting polymers takes place throughout the bulk. The charging/discharging reactions do not involve any structural changes such as phase transformation, which make the redox processes highly reversible.⁷² CPs can be positively or negatively charged with ion insertion in the polymer matrix to balance the injected charge. Commonly used conducting polymers include polypyrrole (PPy),^73–75 polyaniline (PANI),^76–80 polythiophene (PTh),^81,82 and poly(3,4-ethylenedioxythiophene) (PEDOT).^83–85

It is well-known that carbon materials have excellent cyclic stability as supercapacitor electrodes, whereas degradation of conducting polymers often occurs less than a thousand of cycles due to stress destroy caused by the doping/de-doping (intercalation/deintercalation) of ions.⁸⁶ Although higher specific capacitance may be achieved by increasing the doping level of polymers, the accompanying volume change ascribed to more counter ion insertion/de-insertion is supposed to be harmful to the cycle performance. An effective strategy is to form conducting polymers composites with carbon materials to improve the capacitive performance of conducting polymers.

2.2.3 Transitional metal oxide/hydroxide. Metal oxides/hydroxides store charges not only like electrostatic carbon materials but also exhibit electrochemical faradaic reactions between electrode materials and ions within appropriate potential windows. Therefore, they provide higher energy density than conventional carbon materials, and better electrochemical stability than polymer materials. Various metal oxides have been studied extensively, including RuO₂,^87–89 IrO₂,⁹⁰ MnO₂,^91–101 Mn₃O₄,^102–106 NiO,^107,108 Co₃O₄,¹⁰⁹ NiCo₂O₄,^110–114 V₂O₅,^115,116 SnO₂,^117–119 TiO₂,^120–122 MoO₃,^123–125 Fe₂O₃,^126–128 etc., while metal hydroxides generally include Co(OH)₂,^129,130 Ni(OH)₂,¹³¹ or layered double hydroxides (LDHs).^132–135

However, metal oxides/hydroxides may not be adopted as commercial supercapacitor electrodes due to the following issues.

(1) Except for RuO₂, most metal oxides/hydroxides possess very poor electric conductivity, which increases both the sheet resistance and the charge transfer resistance of the electrode, and especially causes a large IR drop at a large current density.¹³⁶ Thus the poor power density and the rate capability of metal oxides/hydroxides hinders their practical application.

(2) The internal strain in metal oxides/hydroxides during the charge-discharge processes gives rise to large volume change, leading to poor cycle stability.

(3) It is difficult to design and tailor the surface area, pore distribution and porosity of metal oxides/hydroxides.

As described above, either metal oxides/hydroxides or conducting polymers, if used alone, can't meet the requirements for practical purposes. Thus, it is very necessary to develop high-performance electrode materials for next-generation supercapacitors. Graphene based materials exhibit great potential for application in supercapacitors thanks to their superior intrinsic physical properties. The fabrication of graphene with porous structures is an effective strategy to obtain graphene materials with large surface areas and high electrochemical performance. The following sections will mainly focus on the recent progress of porous graphene and their applications in supercapacitors.

3. Preparation of porous graphene as supercapacitor electrodes

According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), micropores mean the diameter of pores are less than 2 nm, while macropores are pores with a diameter larger than 50 nm, and mesopores are in between.¹³⁷ It has been recognized that macropores facilitate the penetration and wetting of electrolyte ions to the interior surface and hence improve the rate capability and cycle stability of the electrodes, while mesopores provide a large accessible surface area for ion transport/charge storage, and micropores contribute to increasing the specific surface area (SSA) and specific capacitance.^138–141 Several methods have been developed to prepare porous graphene materials, mainly including chemical activation, template approach, and self-assembly of graphene nanosheets.

3.1. Chemical activation

The chemical activation process consists of the heat-treatment of a mixture of graphene-based precursor and activating agent at a temperature around 450–900 °C.^142–145 KOH, H₃PO₄ and ZnCl₂ are most commonly used for chemical activation. KOH acts as an oxidant, whereas H₃PO₄ and ZnCl₂ act as dehydrating agents.^146,147 Chemical activation is an effective method to produce porous graphene materials without the assistance of templates. Experimental data for supercapacitors prepared with porous activated graphene-based materials (mostly chemical activation with KOH) are listed in Table 1.

Table 1 Supercapacitor performance of porous activated graphene-based materials present in the literature (two-electrode cell system)

Material	Surface area (m^2 g^−1)	Electrolyte	Capacitance^a (F g^−1)	E^a (W h/kg)	P^a (kW kg^−1)	Ref.
a The maximum value.b Values of capacitance for the three-electrode cell system.
Activated MEGO	2400	BMIM BF4/AN	166	∼70	∼250	142
	3100	EMIM TFSI	200	—	—
Activated MEGO	3100	BMIM BF4/AN	172	—	—	143
Activated reduced graphene oxide films	2400	1 M KOH	∼300	∼18	—	144
		1 M TEA BF4/AN	120	40	500
N-Doped activated MEGO	1929	6 M KOH	420	—	—	148
Activated sMEG-O	3290	EMIM TFSI/AN	174	74	338	149
Activated porous graphene nanoribbons	1249	EMIM BF4	130	55.2	—	150
Activated nitrogen-doped graphene	555	1 M KOH	132.4^b	—	—	151
Activated graphene	1315	1 M H2SO4	∼240	—	—	152
Activated graphene-incorporated nitrogen-rich carbon composite	1646	6 M KOH	300	8.4	—	153
Porous graphene/activated carbon composite	2106	6 M KOH	210	—	—	154
		1 M TEA BF4/PC	103	22.3	—
Activated carbon/graphene composites	2566	6 M KOH	334	∼10	—	155
Graphene–activated carbon composite	798	6 M KOH	122	6.1	—	156
		EMIM BF4	179 (80 °C)	99.2	20
Nitrogen-doped porous graphene/carbon composite	1882.3	1 M H2SO4	405^b	—	—	157
Activated 3D porous graphene	3523	TEA BF4/AN	202	51	109	158
		EMIM BF4	231	98	137
Activated microporous carbon nanoplates	2557.3	H2SO4	264	—	—	159
		BMIM BF4/AN	168	133	217
Activated graphene-like carbon nanosheets	2287	BMPY TFSI	∼160	∼40	77	160
Porous graphene-like nanosheets	1874	6 M KOH	276	9.58	7.1	161
		1 M TEA BF4/PC	196	54.7	∼30
CO2-activated macroscopic graphene	829	1 M H2SO4	278.5^b	—	—	162

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Ruoff and co-workers reported a facial activation process of microwave-exfoliated GO (MEGO) and thermally exfoliated GO with KOH (Fig. 4a), to achieve a specific surface area value up to 3100 m² g⁻¹, a high electrical conductivity of ∼500 S m⁻¹, and a low oxygen and hydrogen content (the C/O atomic ratio of up to ∼35).¹⁴² These electronic microscopy images clearly indicate that MEGO was etched in the activation process and a three dimensional (3D) distribution of micro- and meso pores was generated with a size distribution between ∼1 and ∼10 nm in the resulting materials (Fig. 4b–e). The two-electrode symmetrical supercapacitor cells on the basis of a-MEGO (SSA ∼2400 m² g⁻¹) and 1-butyl-3-methyl-imidazolium tetrafluoroborate/acetonitrile (BMIM BF₄)/AN electrolyte was constructed and measured. A specific capacitance of 166 F g⁻¹ was obtained at a current density of up to 5.7 A g⁻¹ (Fig. 4f and g). Under the working voltage of 3.5 V, the energy density was calculated to be ∼70 W h kg⁻¹, and the power density was also very high at ∼250 kW kg⁻¹. Moreover, the a-MEGO showed outstanding cycling stability. After 10000 constant current charge/discharge cycles at a current density of 2.5 A g⁻¹ in BMIM BF₄ electrolyte, 97% of its capacitance was retained. By using this activation process that has already been commercially demonstrated for activated carbons (ACs), the scale up of producing activated-graphene-based materials for supercapacitors may be realized in a short period.

Fig. 4 Porous graphene-based electrode materials prepared by activation of microwave-exfoliated GO. (A) Schematic showing the microwave exfoliation/reduction of GO and the following chemical activation of MEGO with KOH. (B) Low-magnification SEM image of a 3D a-MEGO fragment. (C) High-resolution SEM image of a different sample region. (D) Annular dark field scanning transmission electron microscopy (ADF-STEM) image of the same area as in (C), acquired simultaneously. (E) High-resolution phase contrast electron micrograph of the thin edge of an a-MEGO fragment. (F) Cyclic voltammetry (CV) curves of a-MEGO (SSA ∼2400 m² g⁻¹) for different scan rates. (G) Galvanostatic charge/discharge curves of a-MEGO under different constant currents. Reproduced with permission from ref. 142, Copyright 2011 AAAS.

Ruoff and co-workers further investigated the influence of activation parameters such as the temperature and amount of KOH during the synthesis of a-MEGO on the specific surface area (SSA) of a-MEGO and electrochemical capacitance of a-MEGO electrodes.¹⁴³ At 800 °C and with KOH/MEGO mass ratio of 6.5, a maximum specific surface area of 3100 m² g⁻¹ was obtained and a high specific capacitance of 172 F g⁻¹ was measured in a two-electrode cell with a-MEGO electrodes in BMIM BF₄/AN electrolyte. Increasing the activation temperature from 600 °C, both the BET SSA and the specific capacitance increased, while they decreased when the activation temperature was up to (or higher than) 800 °C at a constant KOH/MEGO ratio of 6.5 (Fig. 5a). The lower BET SSA and the specific capacitance values at 900 and 1000 °C were due to excess carbonization and collapse of the porous structure. Under the activation temperature of 800 °C, the BET SSA and specific capacitance peaked at a KOH/MEGO loading ratio of 6.5; while loading beyond a ratio of 9, the nonconductive samples with very small carbon content were obtained (Fig. 5b).

Fig. 5 (a) Effect of activation temperature on BET SSA and specific capacitance of a-MEGO, at a constant KOH/MEGO ratio of 6.5; (b) effect of activation KOH/MEGO ratio on BET SSA and specific capacitance of a-MEGO, at a temperature of 800 °C.¹⁴³ Reproduced with permission from ref. 143, Copyright 2012 Elsevier.

Subsequently, Ma and co-workers developed a route of chemical activation from noncovalent functionalized graphene with KOH to prepare a graphene–activated carbon composite (GAC) with a high specific surface area.¹⁵⁶ It is clearly observed that the KOH activated method can create micropores or mesopores in the activated carbon covered on graphene. Hence part of the surface of graphene sheets with a few layers is exposed because of etched activated carbon. Simultaneously the pores in the activated carbon also increase its SSA. In addition, the generation of gas in the activated process also prevents severe agglomeration of graphene sheets to some extent. As a result, the SSA of the final product greatly increases to 798 m² g⁻¹. The two-electrode symmetrical coin-size capacitor cells were assembled by using 6 M KOH aqueous solution and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF₄) as the electrolytes, respectively. The resulting graphene–activated carbon composite shows a good capacitance of 122 F g⁻¹ and energy density of 6.1 W h kg⁻¹ in 6 M KOH electrolyte. Moreover, the supercapacitor exhibits maximum energy densities of 52.2 and 99.2 W h kg⁻¹ in EMIM BF₄ electrolyte at room temperature and 80 °C, respectively (Fig. 6). It is worth noted that the energy density in ionic liquid electrolyte at 80 °C exceeds that at room temperature, indicative of extraordinarily fast ion transport even for a large molecule such as EMIM BF₄ at a high temperature.

Fig. 6 Properties of GAC supercapacitors. (a) Charge/discharge curves at 1 A g⁻¹, (b) rate capability, (c) electrochemical impedance spectroscopy (EIS) plots and its magnification in high frequency in the inset, (d) Ragone plots of GAC in KOH at RT, EMIM BF₄ at RT and EMIM BF₄ 80 °C.¹⁵⁶ Reproduced with permission from ref. 156, Copyright 2013 RSC.

Recently, Chen and co-workers reported that porous 3D graphene-based bulk materials were synthesized by hydrothermal polymerization/carbonization of the mixture of GO and industry carbon sources such as biomass, phenol–formaldehyde (PF), and polyvinyl alcohol (PVA), and subsequently followed by chemical KOH activation (Fig. 7a).¹⁵⁸ Various structural and morphology analyses demonstrated that these materials consist of almost entirely defected/wrinkled single layer graphene nanosheets (Fig. 7b–e). The as-prepared porous graphene-based materials simultaneously showed excellent bulk conductivity (up to 303 S m⁻¹) and ultrahigh SSA (up to 3523 m² g⁻¹), with the pore size in the mesopore size range. Meanwhile, those materials exhibited a high specific capacitance of 202 F g⁻¹ in 1 M tetraethylammonium tetrafluoroborate in AN (TEA BF₄/AN) and 231 F g⁻¹ in EMIM BF₄ electrolyte systems (Fig. 7f and g), corresponding to the gravimetric energy densities as high as 51 and 98 W h kg⁻¹ based on the total weight of active material, respectively. The excellent performance for supercapacitor was attributed to the high accessible SSA and rational distribution of pore sizes of 3D graphene-based bulk materials.

Fig. 7 (a) A schematic show of the simple and green process of synthesizing porous 3D graphene-based materials. (b) Low magnification and (c) high-resolution SEM images of products from the mixtures of PF and GO with optimized ratios, which exhibited sponge-like morphology and porous structure. (d) Low magnification and (e) high-resolution TEM images of products from the mixtures of PF and GO with optimized ratios, which also showed a dense 3D pore structure with highly curved or wrinkled surface. Galvanostatic charge/discharge test results of supercapacitors based on the optimized porous 3D graphene-based materials in (f) 1 M TEA BF₄/AN and (g) neat EMIM BF₄ electrolytes under different current densities. Reproduced with permission from ref. 158, Copyright 2013 NPG.

Physical activation is also a conventional technology to achieve microporosity in porous carbons by enlarging the surface area.^146,163 Park and co-workers prepared CO₂-activated macroscopic graphene architectures with trimodal pore systems that consist of 3D inter-networked macroporosity arising from self-assembly, mesoporosity which is derived from the intervoids of nanosheets, and microporosity through CO₂ activation at 900 °C (Fig. 8a–d).¹⁶² The existence of micropores in hierarchical structures of trimodal porous graphene frameworks (tGFs) contributes to greatly increase the surface area up to 829 m² g⁻¹ and pore volume to 2.829 cm³ g⁻¹. A specific capacitance of 278.5 F g⁻¹ was obtained for tGFs in an aqueous 1 M H₂SO₄ electrolyte with a three-electrode configuration (Fig. 8e–f). The high capacitance and good rate capability of the tGFs were attributed to hierarchical trimodal porosity and surface chemistry.

Fig. 8 (a) Schematic illustration of preparation of the tGFs through self-assembly and CO₂ activation. (b) Optical image of the resultant tGFs. (c) Concept of fast mass and ion transport through a continuous macroporous pathway. (d) Micro- and meso-porosity of the tGFs and their origin. (e) CV curves of tGF at the scan rates of 1 to 100 mV s⁻¹. (f) Galvanostatic charge/discharge curve of tGF at 1 A g⁻¹. (g) Rate capability of tGF at a scan rate of 5–100 mV s⁻¹. Reproduced with permission from ref. 162, Copyright 2014 RSC.

3.2. Template approach

Template assisted synthesis, which typically involves various inorganic or organic nanostructures as templates, is one of the most effective methods to prepare porous graphene materials. We broadly divide these approaches into three categories: (1) hard templating synthesis, (2) soft templating synthesis, and (3) self-generating templating synthesis.

3.2.1 Hard templates. Preparation of porous graphene by hard templates generally involves four major steps: (1) preparation of hard templates; (2) functionalization/modification of template surface to achieve favorable surface properties; (3) coating the templates with graphene or its precursor like graphite oxide (GO) by various approaches; and (4) selective removal of the templates. The silica particles, MgO platelet, nickel foam and powder, ice, and block copolymer films have been used as templates for the fabrication of porous graphene.^164–169 GO suspension is most commonly used as the precursor because of its various kinds of oxygen-functionalized groups which enable the stability of the suspension and the interaction with the template.

Kim and co-workers reported a straightforward self-assembly method to fabricate mechanically flexible macroporous RGO with tunable open porous morphologies (Fig. 9). Polymer-grafted GO platelets were dispersed in an organic solvent to form stable dispersion, and then cast onto a SiO₂ substrate and exposed to a stream of humid air. Endothermic evaporation of the volatile organic solvent resulted in the spontaneous condensation and close packing of aqueous droplets at the organic solution surface. The pore size and the number of porous layers were effectively controlled by the concentration of the organic precursor dispersion and the length of polymer chains grafted to the GO surface. Built by this method, nitrogen doping (N-doping) RGO was also assembled and showed improved electrical properties. The specific capacitances of the as-pyrolyzed and N-doped macroporous RGO assemblies were measured to be 86.7 and 103.2 F g⁻¹ in 1 M aqueous H₂SO₄ aqueous electrolyte, respectively. In contrast, the planar RGO electrode only showed 62.9 F g⁻¹ in capacitance.¹⁷⁰

Fig. 9 (A) Procedure for the self-assembly of RGO into macroporous carbon films. (B) A photograph of a mechanically flexible, semi-transparent macroporous RGO film on PET. (C) A water contact angle of 152° was measured for the superhydrophobic macroporous RGO film. (D) Plane-view and (E) 60°-tilted SEM images of an RGO film. (F and G) Plane-view SEM images of porous RGO film upon (F) and after (G) deformation. Reproduced with permission from ref. 170, Copyright 2010 Wiley.

After that, methyl group grafted silica spheres with hydrophobic surface and uniform size are chosen as the hard template to fabricate graphene foams. By mixing the silica spheres and GOs in a neutral aqueous solution, the hydrophobic nature of both the silica templates and GOs central planes induces self-assembled lamellar-like structures. The as-made composites are transformed into graphene after calcination under inert atmosphere. This nanoporous graphene foam has a high surface area of 851 m² g⁻¹ and an ultra-large pore volume of ∼4.28 cm³ g⁻¹.¹⁶⁴ A three-dimensional bubble graphene film possessed controllable and uniform macropores and adjustable microstructure was achieved by using polymethyl methacrylate latex spheres as a hard template. This macroporous graphene film had BET specific surface area of 128 m² g⁻¹ and exhibited good electrochemical capacitance at a high scan rate of 1.0 V s⁻¹. The capacitance of macroporous bubble graphene film and compact graphene film electrodes were determined to be 92.7 and 30.6 F g⁻¹, respectively.¹⁷¹

The inorganic nanoparticles without surface decoration are also used as hard templates. A RGO sample possessing a hierarchical pore structure was prepared using commercially available low-cost hydrophobic CaCO₃ spheres with different particle sizes. This porous graphene with an average diameter of about 50 nm had a specific surface area of 540 m² g⁻¹ and exhibited a gravimetric charge–discharge capacitance of 201 F g⁻¹ at a current density of 0.1 A g⁻¹. After 1000 cycles, the electrode retained about 98.4% of its initial capacitance.¹⁷² Since the above-mentioned hard templates have large particle size, the obtained graphene composed of abundant numbers of large pore and macropores. By incorporating MnO₂ nanoparticles into graphene and then selectively removing the nanoparticles, porous graphene with a high surface area of 1374 m² g⁻¹ and a mesoporous structure (∼2.4 nm) was obtained. These mesoporous graphene could effectively promote contact with the electrolytes and electrode material, and shorten the diffusion pathway of ions through the porous sheets, resulting in an improved specific capacitance of 154 F g⁻¹ at 500 m V s⁻¹ and a low capacitance loss of 12% after 5000 cycles.¹⁷³

Freeze drying, a wet shaping technique for porous materials, has been explored to produce graphene composite aerogels.¹⁶⁸ Directly put aqueous homogeneous mixtures containing polystyrene sulfonate-stabilized RGO sheets and PVA (or GO sheets and PVA) into liquid nitrogen, followed by freeze-drying of the frozen samples produced graphene composite aerogels with well-ordered microstructures. In a series of follow-up studies, a variety of modified freeze drying methods for graphene aerogels have been reported.^174–177 Some graphene and graphene composites like graphene–CNT aerogel are also exploited as supercapacitor electrode material.

3D N-doped graphene–CNT networks can be obtained by hydrothermal treatment, freeze-drying and subsequent carbonization of graphene oxide-dispersed pristine CNTs in the presence of pyrrole. The resulting samples used as supercapacitor electrodes show specific capacitance of 180 F g⁻¹ at a current density of 0.5 A g⁻¹, and retain 96% of the initial capacitance after 3000 cycles.¹⁷⁸ When 3-D macroporous nickel foam substrate was used, graphene aerogel–nickel foam hybrid material prepared through freeze-drying and the subsequent thermal annealing of the precursors showed improved electrochemical performance. The resulted graphene aerogels possessed hierarchical porosity and high conductivity. When it was used as binder-free electrode in supercapacitors, a high rate capability, good electrochemical cyclic stability, and a high specific capacitance of 366 F g⁻¹ at a current density of 2 A g⁻¹ were achieved.¹⁷⁹

Macroporous graphene films can be prepared by simply electrophoretic depositing (EPD) GO or RGO onto 3-D macropore nickel foam. EPD offers remarkable advantages over other methods for the synthesis of porous graphene films. It does not require expensive instrumentation, high temperatures, or low-vacuum pressures. This method is also not time-consuming, since materials grown in this fashion have a high growth rate.^180,181

Graphene nanosheets are deposited on nickel foams with 3D porous structure by an EPD method using the colloids of graphene monolayers in ethanol. A high specific capacitance of 164 F g⁻¹ is obtained from cyclic voltammetry measurement at a scan rate of 10 mV s⁻¹. When the current densities are set as 3 and 6 A g⁻¹, the specific capacitance values still reach to 139 and 100 F g⁻¹, respectively. The high capacitance is attributed to nitrogen atoms in oxidized product of p-phenylene diamine adsorbed on the surface of the graphene nanosheets.¹⁸² EPD approach had also been developed for the fabrication of RGO composites based on GO suspension and nickel ions decorated GO colloidal suspension. The zeta-potential of GO aqueous solution is negative, which makes anode deposition feasible, and have positive value when GO is positively charged by adsorption of nickel ion, accordingly cathode deposition. Direct assembly by EPD in water medium facilitated the transformation from GO to RGO and resulted in 3D porous or flowerlike RGO hybrid films on different substrates. By adjusting the concentration of the precursor, the EPD voltage and time, and the pH value of the solution, the morphologies, the film thickness and the pore structure could be tailored. Moreover, the electrochemical capacitive behavior was improved for both RGO and its composites after EPD process.¹⁸³

Although GO and RGO are good candidates to synthesize the meso/macro-porous graphene based on solution process, the resulted samples usually showed low electrical conductivity because long-order sp² carbon was only partially reestablished. Hence, some templating methods based on directly grown graphene were reported. CVD, an excellent method to grow graphene film with controlled layers and high electrical conductivity, was reported to synthesize graphene foams with a graphene 3D network monolith. These graphene foams showed extraordinary electrical and mechanical properties.¹⁶⁶ The growth mechanism of graphene film on nickel foam is analogous to that on nickel substrate.¹⁸⁴

Similar to other dielectrics like ZnS, SiO₂, Al₂O₃, Si₃N₄, Rh-YSZ (yttria-stabilized-zirconia)–Si(111) surface, which had already been used as substrate to grow graphene,^185–189 MgO particle was found to be a good template to form graphene using CVD method based on catalytic decomposition of the carbon source precursor and adsorbing carbon atom. A template CVD approach was explored to produce nanomesh graphene with a well-controlled structure on the gram scale. MgO layers with meshes were firstly prepared by a boiling treatment to construct Mg(OH)₂ layers and followed by calcination at high temperature to remove water (Fig. 10). After the introduction of methane to CVD growth of graphene, one or two graphene layers were formed on the MgO surface along (200) MgO lattice planes. Graphene had one or two graphene layers and specific surface areas of up to 1654 m² g⁻¹. Its unique porous structure brought excellent electrochemical capacitance up to 255 F g⁻¹.¹⁶⁵ When methane was changed into ferrocene as the carbon precursor and porous MgO as sacrificial template, the obtained graphene layers showed a specific surface area up to 1754 m² g⁻¹ and a highly porous structure with small mesopores of 4–8 nm, large mesopores of 10–20 nm and additional macropores. As a result, a specific capacitance of 303 F g⁻¹ (areal capacitance up to 17.3 mF cm⁻²) and a nearly tenfold shorter time constant were achieved when compared with those of nonporous and stacked graphene electrodes.¹⁹⁰

Fig. 10 (a) Illustration of the formation of the polygonal nanomesh graphene; (b–e) electrochemical properties of the nanomesh graphene electrode. (b) CV curves from 10 to 500 mV s⁻¹; (c) specific capacitance vs. scan rate; (d) galvanostatic charge/discharge curves at different constant currents. Specific capacitance values were calculated from the discharge curve for each current; (e) cycling performance (200 mV s⁻¹).¹⁶⁵ Reproduced with permission from ref. 165, Copyright 2011 RSC.

By changing MgO platelets with MgCO₃·3H₂O fibers, one-dimensional highly electroconductive mesoporous graphene nanofibers (GNFs) were obtained by a CVD method. GNFs exhibited good structural stability, high surface area, mesopores in a large amount, and high electrical conductivity (three times that of carbon nanotube aggregates). It was used as an electrode in a 4 V supercapacitor, and exhibited high energy density in a wide range of high power density and excellent cycling stability. The short diffusion distance for ions of ionic liquids electrolyte to the surface of GNFs yielded high surface utilization efficiency and a capacitance up to 15 μF cm⁻², higher than that of single-walled carbon nanotubes.^191,192 Besides CVD methods, direct carbonization of carbon source precursor was also realizable. For example, 3D pillared-porous carbon nanosheets with supporting carbon pillars in the adjacent carbon layers had been prepared using MgO hard template. The unique structure endows the high-rate transportation of electrolyte ions and electrons throughout the electrode matrix, resulting in an excellent electrochemical performance.¹⁹²

The layered structural materials whose interlayer spacing is comparable to one-to-two graphene thickness are also used as hard templates to synthesize porous graphene. Scalable single graphene sheets could be synthesized when zeolite Ni-MCM-22 was used as both catalyst and template. Ni-MCM-22 was produced from MCM-22 (IZA code: MWW) which has typical layer spacing similar with the thickness of one or two layered graphene. As a confined space, the interlayers of MCM-22 can be filled with sucrose and nickel ions by dipping. The obtained graphene has a specific surface area of 784 m² g⁻¹, relatively high conductivity of 73.6 S m⁻¹, and controllable two-dimensional sizes. Its reveals excellent electrochemical double layer capacitance and galvanostatic charge/discharge properties with specific capacitances of 233 F g⁻¹ in aqueous KOH.¹⁹³ Three-dimensional sponge-like graphene nanoarchitectures was prepared using a cobalt phthalocyanine molecules as a hard template. The morphology of graphene was shaped by acid-functionalized multiwalled carbon nanotubes. The sequential “bottom-up” molecular synthesis and subsequent carbonization process were fast to complete. The 3D nanoarchitectures are able to deliver an energy density of 7.1 W h kg⁻¹ even at an extra high power density of 48 000 W kg⁻¹. Moreover, the electrode exhibited high cyclic stability (>90% after 10 000 cycles) in ionic liquids and 1 M H₂SO₄, respectively.¹⁹⁴

3.2.2 Soft templates. Hard (solid) templates method is inarguably the most effective and common employed approach for synthesizing porous graphene. However, hard templates have several intrinsic disadvantages, such as the inherent difficulty of achieving high product yields, the multistep synthetic process and the lack of structural robustness of the shells upon template removal. The issues have prompted interests in developing simpler synthetic approaches to prepare porous graphene that permit easy interaction and release of guest species. Among these approaches, soft (liquid or gaseous) templates have attracted more attentions and achieved some positive progress.

A solution deposition method via a controlled low-concentration monomicelle close-packing assembly approach was reported to synthesize two-dimensional ordered mesoporous carbon nanosheets (Fig. 11). These obtained carbon nanosheets possess only one layer of ordered mesopores on the surface of a substrate, typically the inner walls of anodic aluminum oxide pore channels, and can be further converted into mesoporous graphene nanosheets by carbonization. The atomically flat graphene layers with mesopores provide high surface area while the ordered mesopores perpendicular to the graphene layer facilitates ion transport as well as volume expansion flexibility.¹⁹⁵

Fig. 11 (a) Schematic of the formation process for the ordered mesoporous graphene nanosheets; (b) TEM image of the mesoporous graphene nanosheets synthesized from the low-concentration close-packing assembly of monomicelles; (c) SEM images of the mesoporous graphene nanosheets. Reproduced with permission from ref. 195, Copyright 2013 ACS.

A three-dimensional graphene-based hierarchically porous carbon (3DGHPC) was prepared by a dual-template strategy following a coating–solvent evaporation–thermal polymerization–carbonation–etching template step. The coated polymers on SiO₂ spheres were converted to carbon phase, and the GO was efficiently reduced to graphene simultaneously. The 3DGHPC exhibits a higher specific surface area of 384.4 m² g⁻¹ and an improved pore volume of 0.73 cm³ g⁻¹.¹⁹⁶ Interestingly, using an anti-solvent hexane to stabilize the GO suspension, Lee and co-workers reported a non-stacked reduced graphene oxide (NS-rGO) with a high surface area (1435.4 m² g⁻¹) and a ultrahigh pore volume (4.11 cm³ g⁻¹). Hexane is a non-polar aprotic molecule and does not have any interaction with various oxygen groups of GO, but it can provide a hydrogen bonding interaction of oxygen functional groups within GO, which hinders the re-stacking of the GO sheets. It is crucial that electrode materials have both a high surface area and high pore volume at the same time to develop supercapacitors with a high capacitance and rate capability. A specific capacitance of 236.8 F g⁻¹ was obtained for NS-rGO electrode at a current density of 1 A g⁻¹ in 6 M KOH electrolyte.¹⁹⁷

A novel soft template method is developed to synthesize a mesoporous carbon/graphene (MCG) composite. The specific capacitance of MCG composite is up to 242 F g⁻¹ at the current density of 0.5 A g⁻¹, which is much higher than mesoporous carbon, graphene and a sample made by mechanical mixing of mesoporous carbon with graphene. When the current density is increased to 1, 2 and then to 4 A g⁻¹, the specific capacitances of the sample decrease to 203, 168 and 154 F g⁻¹, respectively. Such a decrease is attributed to the insufficient time available for ion diffusion and adsorption inside the smallest pores within the large particles.¹⁹⁸ Without addition of surfactant, hollow graphene oxide spheres (HGOSs) were fabricated from graphene oxide nanosheets (GONs) utilizing a water-in-oil (W/O) emulsion technique. The oxidation time for preparing GONs is a crucial factor for the formation and morphology of HGOSs. With increasing oxidation time, the morphology and surface topography of HGOSs vary from irregular and rough to uniform and smooth shape with decreasing diameter. The hollow structure, thin and porous shells consisting of graphene also exhibited improved electrochemical performance.¹⁹⁹

3.2.3 Self-generating templates. Soft template-based synthetic approaches for synthesis of porous graphene are based on solution synthesis, in which the concentration of precursors is usually very low. Scaling-up graphene synthesis to produce commercial-scale quantities for applications is expected to introduce significant challenges for morphology, good electrical conductivity, and platelets thickness control.

Recently, Ma and co-workers developed a facile, rapid and environmentally friendly process, called “burn-quench method”, which allows for controllable synthesis of mesoporous nanographene with large quantity, lower than 5 layers, high surface area and electric conductivity (Fig. 12).²⁰⁰ This alternative way is ignition of magnesium ribbons in carbon dioxide gas, followed by in situ quenching technique. Apart from being eco-friendly, the process has several advantages such as high-yield, low-cost, and time-saving. 1 g of nanographene can be obtained from approximately 25 g of Mg ribbons by the burn-quench method shown in Fig. 12a. The resulting Mg ions in the solution after dissolving Mg and MgO can be re-converted into Mg materials for recycled usage of further nanographene preparation. So this burn-quench method has potential for the scale-up synthesis of graphene materials. Electrochemical electrodes composed of nanographene mesoporous architecture both for supercapacitors and lithium ion batteries yield high specific capacitance, rate capability, energy density and cyclic stability.

Fig. 12 (a) Scheme of the burn-quench method and assembly of the electrochemical cells; (b–j) TEM and AFM images of nanographene sheets. (b and c) low-magnification TEM images, (c) enlargement of (b). Insets in (b) and (c) are the corresponding SAED patterns; (d–g) HRTEM images of the controlled layers of graphene with monolayers, double layers, four layers, and five layers, respectively; (h) AFM image of the nanographene sheets on a mica substrate; (i) enlargement of the AFM image in (h); (j) height of the nanographene sheets marked in (i). Reproduced with permission from ref. 200, Copyright 2013 Wiley.

With further controlling the reaction of CO₂ with Mg metal at desired temperature and atmosphere, shape-controlled synthesis of several different types of nanocarbons including mesoporous graphene, carbon nanotubes, and hollow carbon nanoboxes was achieved (Fig. 13). The hollow carbon nanoboxes were firstly reported by this method. The method described here allows effective control of the shape and dimension of nanocarbons through manipulation of reaction temperature. The formation mechanisms of nanocarbons were discussed by analysis of their morphology and preferred orientations in combination with different phase interface reaction and self-generated template-guided growth mechanism. The as-synthesized nanocarbons were used as electrodes for symmetrical supercapacitor, which exhibits high capacitance, good cycling stability, and high energy density value of 80 W h kg⁻¹.²⁰¹ The studies of supercapacitors based on porous graphene obtained by template methods are summarized in Table 2, where the synthetic method and the capacitance values are given.

Fig. 13 TEM images of nanocarbons synthesized at different temperatures, (a) sheet-like morphology of MPG in which scrolls and corrugations present; (b) high-resolution image of 5–10 nm diameter mesopore in the MPG; (c–d) low- and medium-magnification images of carbon tubular nanostructures; (e) low magnification image of HCB shows uniform particle size, hollow core microstructure and cubic shape; (f) medium-magnification image of HCB shows that the shell thickness and the side length of HCB are approximately as large as 80–100 and 230 ± 30 nm; (g–h) the specific capacitance of nanocarbon electrodes in 6 M KOH aqueous and EMIMBF₄ electrolytes, respectively. Reproduced with permission from ref. 201, Copyright 2013 NPG.

Table 2 Supercapacitor performance of templating graphene-based materials reported by the literatures

Materials	Synthetic method	SSA (m^2 g^−1)	Conductivity (S m^−1)	Electrolyte	CS^a (F g^−1)	Ref.
a The maximum value.b Values of capacitance for the three-electrode cell system.
N-Doped RGO films	Self-assembly of GO platelets	—	64 900	1 M H2SO4	103.2^b	170
Nanomesh graphene	CVD using MgO nanomesh as template	1654	—	6 M KOH	255^b	165
Macroporous bubble graphene film	PMMA latex spheres as hard template	128	—	1 M H2SO4	93^b	171
Hierarchical porous RGO	CaCO3 nanospheres as hard template	540	5	1 M H2SO4	201	172
N-Doped graphene/CNT networks	Hydrothermal treatment, freeze drying and carbonization of pyrrole	—	—	6 M KOH	180^b	178
Graphene aerogels	Freeze drying to form GO aerogels followed by thermal reduction	463	71	6 M KOH	366^b	179
Porous graphene	Etching graphene of graphene sheets by MnO2 nanoparticles	1374	—	25% KOH	241^b	173
Mesoporous carbon/graphene composite	Triblock copolymer Pluronic F12 as soft template	546	—	6 M KOH	242^b	198
Functionalized graphene sheets	Thermal annealing of GO attached on Mg(OH)2 nanosheet	285	3	5 M KOH	456^b	202
Anti-solvent derived non-stacked RGO	Thermal reduction of hexane stabilized GO	1435	3000	6 M KOH	236.8	197
Graphene sheets with nanopores	CVD using MgO as template and ferrocene as carbon source	1754	188	6 M KOH	303	190
Sponge-like graphene	Microwave synthesis and carbonation of CoPc using AF-MWCNT as supports followed by exfoliation of graphite	418	—	1 M H2SO4	68^b	194
Graphene films on nickel foam substrate	EPD of RGO	—	—	6 M KOH	164^b	182
Mesoporous graphene nanofibers	CVD using MgCO3·3H2O as template	1280	—	EMIM BF4	193	191
Porous graphene	Zeolite Ni-MCM-22 as a catalyst and template	794	—	6 M KOH	233	193
3-D graphene-based frameworks	Hard templating method and freeze drying	295	—	1 M H2SO4	226^b	140
Ordered mesoporous nanographene	Burn-quench method	756	150	EMIM BF4	95	200
Mesoporous graphene	Controlled reaction of Mg with CO2	762	—	EMIM BF4	145	201

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3.3. Self-assembly of graphene nanosheets

Self-assembly of two-dimensional graphene nanosheets is an important strategy to produce 3D macroscopic graphene architectures.^203–206 The pore sizes of such structures are in the range from sub-micrometer to several micrometers, which bring about ultralight, high mechanical strength, compressibility, and excellent conductivity properties. The macroporous structure not only prevents individual graphene nanosheets from aggregating and restacking during the process of assembling, but also ensures a sufficient contact area between the electrolyte and electrode.²⁰⁷ Moreover, 3D porous frameworks provide fast ion diffusion channels while the framework itself acts as a continuous conductive network ensuring fast electron transfer, resulting in excellent rate capability.

Shi and co-workers reported a self-assembled graphene hydrogel (SGH) via a convenient one-step hydrothermal method at 180 °C for 12 h.²⁰⁸ The SGH has a well-defined and interconnected 3D porous network, and the framework of SGH is formed by the partial overlapping or coalescing of flexible graphene nanosheets with physical cross-linking sites (Fig. 14a–e). It is believed that the formation of the SGH was driven by the residual hydrophilic oxygenated groups of reduced GO sheets and the π–π stacking interactions of graphene sheets. In addition to the excellent mechanical strength, the SGH is electrically conductive with a conductivity of about 5 × 10⁻³ S cm⁻¹ (Fig. 14f). The SGH can be cut into slices with a knife, and they were used as the electrodes to assemble symmetric supercapacitors without using a binding agent and conducting additive because of its excellent mechanical strength, high porosity and conductivity (Fig. 14g). The specific capacitances of 175 and 152 F g⁻¹ for the SGH were obtained at potential scan rates of 10 and 20 mV s⁻¹ in 5 M KOH aqueous electrolyte, respectively (Fig. 14h).

Fig. 14 (a) Photographs of a 2 mg mL⁻¹ 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; (f) room temperature I–V curve of the SGH exhibiting Ohmic characteristic, inset shows the two-probe method for the conductivity measurements; (g) schematic of SGH-based supercapacitor device; (h) CVs of the SGH-based supercapacitor at two different scan rates; (c) galvanostatic charge/discharge curves of the SGH-based supercapacitor at a constant current of 1 A g⁻¹. Reproduced with permission from ref. 208, Copyright 2010 ACS.

Subsequently, in order to obtain higher conductivity of graphene hydrogel (GH), Shi and co-workers further used hydrazine (Hz) or hydroiodic acid (HI) for reduction of the GH.²⁰⁹ The chemical reduction of GH was carried out by immersing it into an aqueous solution of HI (55%) or hydrazine monohydrate (50%) in a sealed cuvette. The reduction was conducted for 3 or 8 h at 100 °C for HI and 95 °C for Hz, respectively. The chemically reduced GHs possess high conductivities of 1.3–3.2 S m⁻¹ and SSAs in the range from 780 to 950 m² g⁻¹. Moreover, the further treatment of GH with reducing agent has less impact on the microscopic pore structure of GH (Fig. 15a–d). The supercapacitor based on the Hz-reduced GH exhibited a high specific capacitance of 220 F g⁻¹ at 1 A g⁻¹ in 5 M KOH aqueous electrolyte, and this capacitance can be maintained for 74% as the discharging current density was increased up to 100 A g⁻¹ (Fig. 15g and h). Furthermore, the capacitor exhibited a high power density of 30 kW kg⁻¹ and energy density of 5.7 W h kg⁻¹ at 100 A g⁻¹. This is possibly attributed to the improved conductivity of reduced hydrogels accelerating its charge-transfer during the discharge processes at high current densities.

Fig. 15 (a) Photographs of GH, GH-HI3, GH-HI8, GH-Hz3, and GH-Hz8 (from left to right); (b–d) SEM images of the interior microstructures of freeze-dried GH (b), GH-HI8 (c), and GH-Hz8 (d), respectively; (e) XRD patterns of graphite, GO, and GHs; (f) N₂ adsorption–desorption isotherms of freeze-dried GH, GH-HI8, and GH-Hz8; plots of specific capacitance versus discharging current density for GH-HI (g) and GH-Hz (h) compared with GH. Reproduced with permission from ref. 209, Copyright 2011 ACS.

Since their pioneering work, hydrothermal or solvothermal reaction had been widely applied to fabricate 3D macroscopic graphene architecture via self-assembly of graphene nanosheets.^207,210 Yu and co-workers reported the hydrothermal synthesis of macroscopic nitrogen-doped graphene hydrogels (GN-GH) by using organic amine and graphene oxide as precursors.²¹¹ The organic amine is not only acting as nitrogen sources to prepare the nitrogen-doped graphene, but also as an important structural modifier to control the assembly of graphene sheets in the 3D structures (Fig. 16a–d). The obtained nitrogen-doped graphene hydrogel exhibited remarkably enhanced ultrafast supercapacitor performance. Even at an ultrafast charge/discharge current density of 185 A g⁻¹, a specific capacitance of 113.8 F g⁻¹ and a high power density of 205.0 kW kg⁻¹ could be obtained in 5 M KOH electrolyte (Fig. 16e and f). The studies of 3D macroscopic graphene architecture based supercapacitors are summarized in Table 3, where the synthetic method and the capacitance values are given.

Fig. 16 (a) Photographs of a typical GN-GH using GO and ethylenediamine (100 μL) as precursors after hydrothermal process at 180 °C for 12 h; (b) SEM image of the typical GN-GH microstructures; (c) magnified SEM image of the typical GN-GH microstructures; (d) the high-resolution XPS spectra of the N 1s region; (e) galvanostatic charge/discharge curves of the GN-GH supercapacitor under different constant current densities; (f) plots of specific capacitance versus discharging current density. Reproduced with permission from ref. 211, Copyright 2013 Elsevier.

Table 3 Supercapacitor performance of 3D macroscopic graphene architectures present in the literature (two-electrode cell system)

Synthetic method	SSA (m^2 g^−1)	Conductivity (S m^−1)	Electrolyte	CS^a (F g^−1)	P^a (kW kg^−1)	Ref.
a The maximum value.b Values of capacitance for the three-electrode cell system.
Hydrothermal treatment of GO aqueous dispersion	—	0.5	5 M KOH	175	—	208
Graphene hydrogels further reduced with hydrazine	951	3.2	5 M KOH	222	30	209
Hydrothermal treatment of organic amine and GO	—	—	5 M KOH	190.1	245.0	211
Crosslinking reaction with ethylene diamine followed by hydrazine reduction	745	1351	2 M KOH	232	—	212
Hydrothermal treatment of urea and GO	1521	600	6 M KOH	326^b	—	213
Solvothermal treatment of hydroxylamine and GO	—	—	25% KOH	205	20.5	214
Hydrothermal treatment of ascorbic acid and GO	—	—	H2SO4–PVA gel	196	5	215
Chemical reaction of L-glutathione and GO	315.2	2	0.5 M Na2SO4	157.7	5	216
Chemical reaction of sodium ascorbate and GO	—	1	1 M H2SO4	240^b	—	217
Chemical reaction of pyrrole monomers and GO	—	—	1 M H2SO4	498^b	—	218
Hydrothermal treatment of pyrrole monomers and GO	463	—	3 M NaClO4	350^b	—	219
Hydrothermal treatment of HCl and GO	322	—	6 M KOH	220	—	220
Hydrothermal treatment of GO and glucose	—	—	5 M KOH	140	—	221
Solvothermal treatment of GO and activated carbon	1266	—	1 M TEA BF4/PC	116.5	7	222
Graphene hydrogels deposited in nickel foams reduced by vitamin C	1725	—	5 M KOH	45.6 mF cm^−2	—	223
Electrochemically reduced GO	—	—	1 M Na2SO4	131.6^a	—	224

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4. Conclusions and perspectives

Owing to their high surface area, remarkable thermal conductivity, excellent electronic conductivity and mechanical properties, porous graphene materials have aroused great academic interests. Porous structures are favorable for fast ion/electron transport, and facilitate sufficient contact between electrolyte and graphene materials. In broad terms, low-cost, and time-saving methods for future production of porous graphene with controllable nanostructures are still desired. Besides, the pore structures and morphologies of porous graphene materials highly depend on the processing conditions, which make the precise tailoring for surface area, interconnected porosity, and pore size distribution an imminent technological challenge. Therefore, further application of porous graphene materials still requires the proper realization of their distinct structure.

Although high specific surface area enhances mass transfer during electrochemical reactions, the conductivity of electrode materials may be negatively affected due to loosing contact among graphene sheets. This greatly destroys the prospect for porous graphene in practical applications. To solve the paradox between high specific surface area and excellent conductivity, the promising strategies include hetero-atom doping, chemical-bonding, perfect lattices and removal of oxygen atoms. Besides, theoretical research should also be promoted systematically to reveal the relationship between porosity and electrical properties.

Graphene-based composites are promising candidates as future high-performance supercapacitor electrode materials because the synergetic effects can provide full scope to the potentiality of each component. Combination of conducting polymers or metal oxides/hydroxides with graphene offers a pathway for material selection and optimization. Faradaic materials are well-known for their high capacitance and highly reversible redox reactions, while graphene is an ideal matrix for composites due to the chemical stability and sophisticated porosity.

In conclusion, porous graphene, as an attractive electrode material, has inspired both academic and industrial interests. The graphene paradigm determines the novel pathways for the fabrication of high-performance electrode materials for supercapacitors. There is no doubt that the extensive investigation of porous graphene should surely open a colourful vista for next-generation energy storage devices in future.

Acknowledgements

This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (no. KJCX2-YW-W26), Beijing Municipal Science and Technology Commission (no. Z111100056011007), and the National Natural Science Foundation of China (no. 51403211, 51472238, and 51025726).

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