supercapacitor

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2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 43) 1300816wileyonlinelibrary.comAdv. Energy Mater. 2014, 4, 1300816

Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities

Jun Yan , Qian Wang , Tong Wei , and Zhuangjun Fan *

1 . Introduction

Due to the rapid development of the global economy, the growing human population worldwide, a fast-growing market for portable electronic devices, and the development of hybrid electric vehicles, global energy consumption has been acceler-ating at an alarming rate. [ 13 ] The exhaustion of global energy will soon become unavoidable in the near future at current con-sumption rate. It is reported that our global energy needs will roughly double by mid-century and triple by 2100. [ 4 ] Thus, there has been an ever-increasing and urgent demand for vigorous development of not only clean, renewable, and sustainable alternative energies (solar, wind, and tide), but also advanced, low-cost, and environmentally friendly energy conversion and storage devices to satisfy the needs of modern society and

emerging ecological concerns. [ 2,5 ] Among various energy conversion and storage devices, lithium-ion batteries (LIBs) [ 6 ] and supercapacitors [ 7,8 ] are at the forefront as illustrated in the Ragone plot ( Figure 1 ). Although their high energy densities can be achieved as high as 180 Wh kg 1 , LIBs usually suffer from a somewhat slow power delivery or uptake. [ 5 ] Upon that, the wide-spread application of LIBs is thus greatly inhibited, especially in energy-storage sys-tems where fast and higher-power storage devices are highly required. [ 5,9 ] As a conse-quence, this heavy burden has been given to the supercapacitors. [ 10 ]

Supercapacitors, also known as elec-trochemical capacitors or ultracapacitors, have attracted a great deal of attention from both industry and academia due to their high power density, superior rate capability, rapid charging/discharging rate, long cycle life (>100 000 cycles), simple principles, fast dynamics of charge

propagation and low maintenance cost. [ 17,18 ] Since General Electric for the fi rst time demonstrated and patented in 1957, [ 19 ] supercapacitors have continued to attract considerable attention from both scientists and engineers as indicated by the number of published articles in this area ( Figure 2 ). In addition, they

DOI: 10.1002/aenm.201300816

Dr. J. Yan, Q. Wang, Prof. T. Wei, Prof. Z. J. FanKey Laboratory of Superlight Materials and Surface Technology Ministry of Education, College of Material Science and Chemical Engineering Harbin Engineering University Harbin , 150001 , P. R. China E-mail: [email protected]

In recent years, tremendous research effort has been aimed at increasing the energy density of supercapacitors without sacrifi cing high power capability so that they reach the levels achieved in batteries and at lowering fabrication costs. For this purpose, two important problems have to be solved: fi rst, it is critical to develop ways to design high performance electrode materials for supercapacitors; second, it is necessary to achieve controllably assembled supercapacitor types (such as symmetric capacitors including double-layer and pseudo-capacitors, asymmetric capacitors, and Li-ion capacitors). The explosive growth of research in this fi eld makes this review timely. Recent progress in the research and development of high performance electrode materials and high-energy supercapacitors is summarized. Several key issues for improving the energy densities of supercapacitors and some mutual relationships among various effecting parameters are reviewed, and chal-lenges and perspectives in this exciting fi eld are also discussed. This provides fundamental insight into supercapacitors and offers an important guideline for future design of advanced next-generation supercapacitors for industrial and consumer applications.

Figure 1. Specifi c power against specifi c energy, also called a Ragone plot, for various electrical energy storage devices. Data obtained from Ref. [ 11 ] ( ), [ 12 ] ( ), [ 13 ] ( ), [ 14 ] ( ), [ 15 ] ( ) and [ 16 ] ( ).

http://doi.wiley.com/10.1002/aenm.201300816

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2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1300816 (2 of 43) wileyonlinelibrary.com Adv. Energy Mater. 2014, 4, 1300816

have triggered an explosion of interest for a wide and growing range of applications where require high power density such as energy back-up systems, consumer portable devices, electrical/hybrid electric vehicles and other devices. [ 5,10,20 ] Supercapaci-tors currently bridge the power gap between batteries and tra-ditional solid state and electrolytic capacitors, delivering higher power bursts than batteries and storing more energy than capacitors. Although the energy density of most of the commer-cially available supercapacitors (less than 10 Wh kg 1 ) is much higher than conventional dielectric capacitors, it is still sig-nifi cantly lower than batteries and fuel cells. [ 18 ] With the rapid development of the global economy, there is an urgent need for supercapacitors with high stored energy. Thus, tremendous research effort has been performed aiming at increasing the energy density of supercapacitors without sacrifi cing their high power capability to be close to or even beyond that of batteries as well as lowing fabrication costs all over the world in recent years. [ 11,14,2124 ]

The fi eld of research and development of high energy density supercapacitors is currently undergoing an exciting develop-ment with increasing achievements. In spite of many reviews exploring numerous materials applied in supercapacitors, such as carbon based materials, metal oxides or conducting poly-mers, metal oxide-carbon hybrid materials and the design of nanostructured materials, [ 17,2432 ] there are just several reviews focusing specifi cally on supercapacitors with high energy den-sities as a whole. [ 33,34 ] As we all know, an increasing number of signifi cant breakthroughs have been made on advanced elec-trode materials with high specifi c capacitance (SC) in recent years. Therefore, it is feasible to achieve high energy density for supercapacitors originated from high capacitance of the electrodes.

Apart from advanced electrode materials, there are other major parameters that are important in determining high energy density of supercapacitors, such as electrolytes, assemble types of supercapacitor and reasonable matching of negative/positive electrodes. With the new emergence of gra-phene, redox-active electrolytes, and carbon/carbon superca-pacitors with high working voltage in recent years, the research in supercapacitors with high energy density is springing up

Jun Yan received his PhD degree in material science from Harbin Engineering University in 2010 and carried out his post-doctoral research at Harbin Engineering University (2010-2012). Now, he is a Lecturer in the College of Material Science and Chemical Engineering, Harbin Engineering University. His research interests mainly

focus on design, synthesis, and functionalization of carbon nanomaterials as well as their applications in electrochem-ical energy conversion and storage devices.

Zhuangjun Fan received his PhD in 2003 from the Institute of Coal Chemistry, Chinese Academy of Sciences. He became a full professor in the College of Material Science and Chemical Engineering in 2006, and he is now the director of the Institue of Advanced Carbon Based Materials at Harbin Engineering University. His

research interests focus on the design and controlled syn-thesis of carbon nanomaterials, such as carbon nanotubes and graphene, and their application in energy-related areas such as supercapacitors, Li-ion batteries, and full cells.

around the world. Therefore, it is imperative and important to provide timely updates on progress in this promising fi eld, and systematically present these key issues for improving the energy densities of supercapacitors as well as deeply reveal the mutual relationships among various effecting parameters along with some discussions on challenges and perspectives. This review fi lls this potential gap focusing solely on the research progress in high energy density supercapacitors with compre-hensive tables provided. We fi rst provide a brief introduction to the basic principles and performances of supercapacitors. A variety of promising strategies to improve the energy densities of supercapacitors are also briefl y discussed, followed by an in-depth summary of signifi cant research progress in the devel-opment of high energy density supercapacitor technologies demonstrated in recent years. Particular emphasis is focused on recent research breakthroughs achieved by rational design and development of various recently emerging positive/nega-tive electrode materials with high SC to maximize the electro-chemical performance of the devices. Considerable aqueous and non-aqueous electrolytes springing up in supercapacitors over the past several years as well as recently designed and fabricated asymmetric supercapacitors (ASCs) appeared in

Figure 2. Trends in the number of publications on supercapacitors (data obtained from Web of Science on Aug 27, 2013).

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is of critical signifi cance for it to simultaneously possess high SC, wide operating cell voltage, and minimum ESR. Nowadays, from a practical point of view, the most important challenge facing material scientists and engineers worldwide is to increase the energy density higher than 10 Wh kg 1 with low fabrication cost while using environmentally friendly materials.

Based on Equation ( 2 ), the important parameters for high energy density supercapacitors are shown in Figure 3 . This is an effective approach to improve energy density through increasing the capacitance, which can be realized by improving the SC of both positive and negative electrodes. Therefore, there has been an explosion of interest in designing and devel-oping advanced nanostructured electrode materials with high SC in supercapacitor research and development in recent years. To be specifi c, with regard to the carbon materials, the increase of SC can be realized through increasing the SSA and optimizing the pore sizes and PSD by developing hierarchically porous structure without sacrifi cing the good electrical conduc-tivity. As for the pseudocapacitive materials, it can be achieved by synthesizing nanosized electroactive materials with large SSA to provide suffi cient electroactive sites for faradaic reac-tions and creating hierarchical porosity of electroactive mate-rials with excellent conductivity to ensure suffi cient electrolyte ions and electrons participating in the faradaic reactions at high rates simultaneously. Moreover, the enhancement of SC can be achieved by introducing redox species to contribute additional faradic pseudocapacitance to the overall capacitance, such as doping function groups/heteroatoms (including N, O, S, B, and P) to carbon materials and adoption of redox-active electrolytes.

On the other hand, the energy density of a supercapacitor is also proportional to the square of the operating voltage. Thus, increasing the operation voltage is regarded as a promising strategy to improve the energy density of a supercapacitor and can be achieved by selecting a proper electrolyte with a large oper-ating voltage, for instance, organic electrolytes (up to 2.53.0 V) and ionic liquids (ILs, up to 4 V). Moreover, an attractive alter-native approach to maximize the operating voltage window is to develop ASCs because organic electrolytes are usually more expensive and fl ammable as well as less conductive than aqueous electrolytes; [ 11,15 ] this has created considerable interest from sci-entifi c community over the past years. For an ASC, a battery-like Faradaic electrode (as energy source) and a capacitive electrode (as power source) are combined in a cell system in which both electrical double-layer capacitance and faradaic pseudo-capaci-tance mechanisms occur simultaneously. Therefore, ASC pos-sesses the advantages of both supercapacitors (rate, cycle life)

literature are highlighted. However, we do not claim that this review covers all of the published work about supercapacitors owing to the explosion of publications in this exciting fi eld. We apologize to those authors whose work we have left out. Finally, the prospects and future developments in this exciting fi eld of high-power and high-energy supercapacitors are also suggested.

2 . Important Parameters for High Energy Density Supercapacitors

The performances of supercapacitors are mainly evaluated based on the following aspects: [ 17 ] 1) a high SC; 2) a substan-tially high power density; 3) a relatively high energy density; 4) an excellent cyclability; 5) fast charge/discharge rates within seconds; 6) low self-discharging; 7) safe operation, and 8) low cost. It is well-known that several factors signifi cantly affect the performances of a supercapacitor as summarized by Pandolfo and Hollenkamp in their review, [ 19 ] such as pore structure of electroactive materials (specifi c surface area (SSA) and pore size distribution (PSD)), intrinsic properties of the electrolyte, micro-structure/morphology and electrical conductivity of electroactive materials, and the interface between electrode and electrolyte. In addition to the above factors, there are some other infl uencing parameters, such as operating voltage, reasonable matching of negative/positive electrode, and asymmetric design, that strongly affect the energy density for supercapacitors.

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:

1/CT = 1/Cp + 1/Cn (1) in which C p and C n are the capacitance of the positive and neg-ative electrodes based on a three-electrode setup, respectively. If the two electrodes are identical ( C p = C n ), the corresponding supercapacitor is called a symmetric supercapacitor and the total capacitance C T will be one half of either ones capacitance. In other case, i.e., the positive and negative electrodes use dif-ferent materials corresponding to ASCs, and C T will depend on the relative smaller value between C p and C n .

Energy and power densities are two crucial parameters for evaluating the electrochemical performance of supercapacitors. The maximum energy ( E in Wh kg 1 ) and power densities ( P in W kg 1 ) of a supercapacitor can be obtained using Equations ( 2 ) and ( 3 ), respectively: [ 3,17 ]

E = CV 2/2 (2) P = V 2/4R (3) where C T is the total capacitance of the cell (in F); V (in V) is the cells operating voltage, which is determined by the thermody-namic stability (stability window) of an electrolyte and dependent upon electroactive electrode materials; and R is the equivalent series resistance (ESR, in ), composed of the intrinsic resist-ance of the electroactive materials, contact resistance between the electroactive materials and the current collector, diffusion resistance of ions in the electrode materials and through the separator, and ionic resistance of electrolytes. Therefore, in order to achieve excellent performance for a supercapacitor, it

Figure 3. Schematic illustration of different approaches to improve energy density of a supercapacitor.

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store charge electrostatically through reversible adsorption of electrolyte ions onto the electroactive materials ( Figure 5 ). The surface electrode charge generation involves surface dissocia-tion and ion adsorption from both the electrolyte and crystal lattice defects, [ 5 ] thus there is no charge transfer across the elec-trode/electrolyte interface and energy storage is a true capaci-tance effect. [ 44 ]

The capacitance ( C ) of an electrical double-layer at each elec-trode/electrolyte interface is generally assumed to follow that of a parallel-plated capacitor (Equation ( 5 ): [ 5,17 ]

C =grg0

dA

(5)

where r (a dimensionless constant) is the relative dielectric constant of electrolyte; 0 (in F m 1 ) is the dielectric constant of the vacuum; d (in m) is the effective thickness of the electrical double-layer; and A (in m 2 g 1 ) is the SSA of the electrode acces-sible to the electrolyte ions. It has been demonstrated the thick-ness of the electrical double-layer, which is dependent upon the electrolyte concentration and the size of electrolyte ions, is on the order of 0.51 nm for concentrated electrolytes. The elec-trical double-layer capacitance is estimated to be 1020 F cm 2 for a smooth electrode depending on the used electrolyte, as demonstrated by Ktz and Carlen. [ 45 ] SC obtained in aqueous alkaline or acid electrolytes is generally higher than in organic electrolytes and ionic liquids, but organic electrolytes are more widely used in practical applications because they can provide a higher operation voltage.

Commonly, porous carbon materials in different forms such as activated carbons (ACs), [ 4655 ] carbon xerogels, [ 8,5661 ] carbon nanotubes (CNTs), [ 23,6269 ] mesoporous carbons, [ 48 ] templated carbons, [ 7073 ] carbide-derived carbons (CDCs), [ 7477 ] graphene, [ 35,7886 ] porous carbon spheres, [ 8794 ] and carbon

and advanced batteries (energy density). In the following sec-tions, an in-depth summary of the signifi cant research progress in the development of high energy density supercapacitor tech-nologies demonstrated in recent years will be discussed.

3 . Electrode Materials with High SC

Nanostructured materials have signifi cantly accelerated the development of supercapacitors because of their several advan-tages over bulk counterparts as described below: 1) Nanostruc-tured materials can be designed to have a high SSA, which provides more ion adsorption or active sites for the formation of electrical double-layer and charge-transfer reactions, resulting in the enhanced SC. 2) Nanoscale active materials have short diffusion and transport pathways of electrolyte ions within the particles, facilitating the transport of electrolyte ions and accordingly improving the effective electrochemical utilization of active materials and high rate charge/discharge capability. In classical electrochemistry theory, the ion diffusion time con-stant ( ) can be expressed by the Equation ( 4 ):

J = L 2 / 2D (4) where L is ion diffusion length and D is ion transport coeffi -cient. [ 31 ] It is obvious that the ion diffusion time decreases with decreasing particle sizes. 3) The small size of particles could effectively buffer the stress from the expansion and shrinkage of the electrodes during the charge/discharge process, pre-venting the pulverization of electrode and improving the cycle stability. 4) The large surface area of nanostructured materials increases the contact area between electrode and electrolyte, resulting in higher ion fl ux compared to the bulk one.

In general, the electrode materials can be categorized into three principal types, namely porous carbon materials, [ 17,3539 ] transition metal oxides and hydroxides, [ 4042 ] and electrically conducting polymers, [ 4042 ] as shown in Figure 4 .

3.1 . Porous Carbon Materials

Porous carbon materials are commonly used as electrode mate-rials for electrical double-layer supercapacitors (EDLCs) that

Figure 5. a) Schematic illustration of an EDLC in its charged state. b) Schematic of a commercial spirally wound double layer capacitor. c) Assembled device weighing 500 g and rated for 2600 F. d) A small button cell, which is just 1.6 mm in height and stores 5 F. Panels (bd) reproduced with permission. [ 5 ] Copyright 2008, Macmillan Publishers Ltd.

Figure 4. Capacitive performance of various electrode materials reported in the literature. Reproduced with permission. [ 43 ] Copyright 2008, The Electrochemical Society.

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cost, excellent chemical and thermal stability as well as rela-tively good electrical conductivity. ACs are derived from various types of carbon-rich organic precursors (coconut shells, wood, pitch, coal, polymers, etc.) by carbonization in inert atmos-phere with subsequent physical and/or chemical activation to increase the SSA and pore volume. After the activation process, the SSA and porosity of the resulting materials are signifi cantly enhanced compared with the carbonized samples. Depending on the activation process and the used carbon precursors, a variety of ACs with different physicochemical properties and well developed SSA as large as 3000 m 2 g 1 have been prepared and used as electrodes for supercapacitors over the past years ( Table 1 ).

Due to the electrical double-layer storage mechanism of carbon materials, high surface areas, more electrolyte ions

onions, [ 20,95101 ] have been investigated extensively as the elec-trode materials of supercapacitors. It has been demonstrated both theoretically and experimentally that several factors sig-nifi cantly affect the electrochemical performances of carbon materials including SSA, electrical conductivity, pore size and distribution, and pore volume. [ 5,102105 ]

It is usually anticipated that the larger SSA, the higher SC. Initial research on carbon materials was directed towards sig-nifi cantly increasing the pore volume by developing materials with large SSA. However, the increase in capacitance is relatively limited because not all micropores in the carbon materials are electrochemically accessible to from the electrical double-layer when they are immersed in electrolytes. It has been demon-strated that there is no linear relationship between the SSA and the capacitance, i.e., the SC of various carbon materials does not increase linearly with SSA. Despite ACs possessing SSA as high as 25003000 m 2 g 1 , only a relatively small SC

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capacitance and stored energy density due to the increase in surface area of regular pore distributions vs. broad distribu-tions of pores, [ 9 ] which means that monodispersed pores and elaborately optimized PSD would be an ideal candidate for the energy storage of supercapacitors. [ 150 ] More recently, Kondrat et al. demonstrated that the optimal pore size providing the maximal energy density increases with increasing operating

can be accumulated at the electrode/electrolyte interface. Therefore, many approaches have been used to increase the SSA of carbon materials, including heat treatment, alkaline treatment, physical or chemical activation, and plasma sur-face treatment. [ 51,108,144149 ] In addition to SSA, PSD is a sig-nifi cant factor affecting the electrochemical performances of ACs. Generally, narrowing PSD leads to an increase in

Table 1. AC materials for EDLC electrodes reported recently in the literature.

Carbon source Activation method

S BET [m 2 g 1 ]

C a) [F g 1 ]

Potential [V]

Scan rate E max [Wh kg 1 ]

Cycles CR b) [%]

Electrolyte [mol L 1 ]

Ref.

Coal tar pitch KOH 1003 224 (2) 1.0 0.1 A g 1 7.84 1000 98.5 KOH (6) [ 48 ]

Mesophase pitch KOH 2258 145 (2) 2.5 20 A g 1 31 Et 4 NBF 4 (1) [ 112 ]

Coke KOH 1397 350.9 (2) 1.0 2 mA cm 2 6000 40 H 2 SO 4 (1) [ 113 ]

Lignite KOH 2580 377 (2) 1.0 0.05 A g 1 500 92.8 KOH (3) [ 114 ]

Petroleum coke KOH 1590 330 (2) 1.0 0.02 A g 1 21.2 200 93.5 KOH (6) [ 115 ]

Deoiled asphalt NaOH 1778 235 (2) 1.0 0.05 A g 1 KOH (7) [ 116 ]

Green needle coke KOH 3347 348 (2) 1.0 0.05 A g 1 1000 91.4 KOH (6) [ 117 ]

Camellia oleifera shell ZnCl 2 1935 374 (3) 1.0 0.2 A g 1 5000 91.3 H 2 SO 4 (1) [ 118 ]

Rice hull NaOH 3969 368 (3) 1.0 2 mV s 1 KOH (6) [ 119 ]

Coconut shell Steam 1532 192 (2) 0.8 1.0 A g 1 38.5 3000 61.3 KOH (6) [ 47 ]

Walnut-shell KOH 2390 202.8 (2) 0.9 1 mA cm 2 1000 79.4 H 2 SO 4 (3) [ 120 ]

Argan seed shells KOH 2100 355 (3) 0.75 0.125 A g 1 H 2 SO 4 (1) [ 121 ]

Beer lees KOH 3557 188 (2) 0.9 1 mA cm 2 H 2 SO 4 (0.1) [ 122 ]

Sugar cane bagasse ZnCl 2 1788 300 (2) 1.0 0.25 A g 1 10.0 5000 83 H 2 SO 4 (1) [ 123 ]

Coffee grounds ZnCl 2 1019 368 (2) 1.2 0.05 A g 1 20.0 10000 95 H 2 SO 4 (1) [ 124 ]

Corn grains KOH 3199 257 (2) 0.9 1 mA cm 2 KOH (6) [ 125 ]

Coffee endocarp CO 2 709 176 (3) 0.9 10 mA H 2 SO 4 (1) [ 126 ]

Sunfl ower seed shell KOH 2509 311 (2) 0.9 0.25 A g 1 9.0 30%KOH [ 127 ]

Pig bones KOH 2157 185 (2) 1.0 0.05 A g 1 KOH (7) [ 128 ]

Cellulose KOH 2457 187 (2) 2.3 0.1 A g 1 TEABF 4 (1) [ 129 ]

Potato starch KOH 2342 335 (2) 1.0 0.05 A g 1 900 90 KOH (6) [ 130 ]

Bamboo KOH 1293 55 (2) 1 mA cm 2 30%H 2 SO 4 [ 131 ]

Eucalyptus wood KOH 2967 232 (2) 2.3 0.1 A g 1 TEABF 4 (1) [ 132 ]

Apricot shell NaOH 2074 339 (2) 1.0 0.05 A g 1 KOH (6) [ 132 ]

Pistachio shells Steam 1096 120 (3) 1.0 10 mV s 1 H 2 SO 4 (0.5) [ 133 ]

Fish scale KOH 2273 168 (2) 1.0 0.05 A g 1 KOH (7) [ 134 ]

Wheat straw KOH 2316 251.1 (2) 2.0 2 mV s 1 MeEt 3 NBF 4

(1.2)

[ 135 ]

Rubber wood saw CO 2 683.6 33.7 (3) 1.0 1 mV s 1 H 2 SO 4 (1) [ 136 ]

Firwoods KOH 1064 180 (3) 1.0 10 mV s 1 H 2 SO 4 (0.5) [ 133 ]

Tea leaves KOH 2841 330 (3) 1.0 1.0 A g 1 2000 92 KOH (2) [ 137 ]

D -glucosamine KOH 571 300 (3) 0.8 0.1 A g 1 50.0 2000 93 H 2 SO 4 (1) [ 138 ]

Sucrose CO 2 2102 163 (2) 0.6 1 mV s 1 10000 100 H 2 SO 4 (1) [ 139 ]

Phenolic resin CO 2 1025 56.0 (2) 3.5 2 mA cm 2 70.0 EMImBF 4 [ 140 ]

BDD copolymer H 3 PO 4 633 220 (2) 1.0 5.0 A g 1 16.3 H 2 SO 4 (1) [ 141 ]

Polyfurfuryl alcohol KOH 2600 150 (2) 2.5 0.15 A g 1 32.0 5000 90 Et 4 NBF 4 (1) [ 142 ]

PANI KOH 1976 455 (3) 1.0 1 mV s 1 2000 88.7 KOH (6) [ 143 ]

PPy KOH 3432 290 (2) 2.3 0.1 A g 1 10000 92 EMImBF 4 [ 52 ]

a)The numbers 2 and 3 refer to two- and three-electrode tests, respectively. ; b)CR = capacitance retention.

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capability due to its high SSA (1976 m 2 g 1 ), narrow PSD (

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seaweeds for symmetric supercapacitors exhibited a SC of 125 F g 1 and a high energy density of 10.7 Wh kg 1 with a cell voltage of 1.6 V. [ 154 ] However, it has been demonstrated that the presence of surface functionalities or moisture usually leads to the poor stability of electrodes during cycling, an increased series resistance, an increase of leakage current, decomposition of the organic electrolyte, and deterioration of capacitance. [ 19 ] Thus, surface chemistry of ACs should be elaborately opti-mized to enhance the long-term cycling stability.

Nowadays, a variety of ACs produced through physical or chemical activation from various precursors are the most exten-sively used electrode materials for commercial supercapaci-tors. Nevertheless, their practical applications are still limited to some extent as the energy density is still relative low and the control of PSD and pore structure is still a great challenge. Therefore, it is of great signifi cance to design and produce ACs with elaborately tailored PSD, minimized pore tortuosity, inter-connected pore structure, short pore length, and controlled surface chemistry, facilitating the ion transport and enhancing energy storage without sacrifi cing power capability and cycle life.

3.1.2 . Templated Carbons

Most porous carbon materials have a very wide PSD with pores randomly connected, much more disordered and complicated structures, resulting in poor conductivity, ionic transport, and very limited rate capability for supercapacitor applica-tions. Additionally, it is extremely diffi cult to precisely control the structure of conventional carbons at the nanometer level. Therefore, templating method is considered to be an effective, unique, and versatile way to provide well-designed and precisely controlled carbon materials. [ 168 ] The resulting carbons exhibit a medium SSA, high porosity, well controlled narrow PSD, and an interconnected pore network, making them intriguing can-didates for energy storage applications. Until now, there have been signifi cant advances in the synthesis of nanostructured carbons through templating methods. [ 17 ] Generally, the syn-thetic procedure for templated carbons involves impregnation of carbon precursors (e.g., sucrose, propylene, pitch, furfuryl alcohol, phenolic resin, or polymer solution) into the porous structure of the template, carbonization treatment, followed by the removal of the template to liberate the resulting porous carbon. According to the used templates, templating technology can be classifi ed into hard-template and soft-template methods. The former refers to replication synthesis with pre-synthesized hard templates through infi ltration, carbonization, and removal of templates. Various inorganic materials, such as silica nanoparticles, [ 169 ] zeolites, [ 73 ] anodic aluminum oxide (AAO) fi lms, [ 170 ] mesoporous silica, [ 171 ] CaCO 3 , [ 172 ] and MgO, [ 48 ] have been used as hard templates. Contrarily, the latter is defi ned as self-assembly with soft templates through the cocondensa-tion and carbonization process without the need for removal of template. Various commercial available triblock copolymers PEO-PPO-PEO (PO: propylene oxide; EO: ethylene oxide), such as F127 (EO 106 PO 70 EO 106 ), [ 173 ] P123 (EO 20 PO 70 EO 20 ), [ 174 ] F108 (EO 132 PO 50 EO 132 ), [ 175 ] have been widely used as the soft templates in recent years. Depending upon the used tem-plates and carbon precursors, microporous, mesoporous, and

macroporous carbons with different structures can be prepared using templating methods ( Figure 9 ).

Various porous carbons with controllable micropores, mesopores, and/or macropores prepared with different tem-plates and precursors have been investigated intensively for supercapacitors. [ 70,73,76,171,176181 ] Due to their walls with uni-form thickness of

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co-workers demonstrated the facile synthesis of highly ordered mesoporous carbon nanofi ber arrays (MCNAs) with triblock copolymer Pluronic P123 and natural crab shell as the soft- and hard-templates, respectively ( Figure 10 ). [ 189 ] The obtained mate-rials were composed of a mesoporous carbon nanofi ber (70 nm in mean diameter and 11 nm in mesopore), an interspacing void (70 nm) between nanofi bers and 1 micrometer of pores between nanofi ber arrays. The MCNAs exhibited a SSA of 1266 m 2 g 1 with a large pore volume of 4.3 cm 3 . A maximum SC of 152 F g 1 can be obtained in organic electrolyte, which is much higher than that of CMK-3 (90 F g 1 ). Additionally, 95% of the initial SC could be maintained after 1000 cycles. The remarkable electrochemical performance could be attributed to the unique structure providing a more favorable path for pen-etration and transportation of electrolyte ions and good elec-tronic conductivity. The soft-template method is an economical, fast, and environmentally friendly route to synthesize porous carbon materials with large mesopores and high SSA.

Rate capability is an important factor for the practical appli-cations of supercapacitors, especially in the case of high cur-rent densities. A good electrochemical energy storage device is expected to provide high energy density (or SC) at a high charge-discharge rate. [ 9 ] Commonly, porous carbons suffer from serious electrolyte kinetic problems owing to the inner-pore ion-transport resistance and long ion diffusion distance. Hierarchically porous carbon, combining different pore size systems in nanocarbons, can provide highly effi cient mass transport through macro/mesopores and a large SSA from the micro/mesopores to achieve excellent performances and thus have exhibited great potential for high performance superca-pacitor applications in recent years. [ 176,179,180,191193 ] The hier-archical carbons are usually produced through impregnation of pre-synthesized macroporous templates with precursor sols, carbonization of precursors, and liberation of macropore voids. Based on this strategy, 3D ordered/aperiodic hierarchical porous graphitic carbon (HPGC) was successfully prepared using Ni(OH) 2 nanoparticles as template and phenolic resin as the carbon precursor and exhibited super-high energy and power densities as a electrode materials for high-rate super-capacitor in both aqueous and organic electrolytes. [ 180 ] Such

citric acid. Carbon nanocage with high SSA (2053 m 2 g 1 ) and high purity prepared by an in situ MgO template method with benzene precursor was adopted as supercapacitor electrodes and showed remarkable performance with SC of 260 F g 1 and quite good electrochemical stability (10% deterioration after 10 000 cycles). [ 76 ] In addition, mesoporous carbon can also be produced using CNT-based composites as templates, such as MWNT@mesoporous silica [ 185 ] and MWNT/MnO 2 . [ 87 ] Recently, Zhao and co-workers demonstrated the synthesis of MWNT@mesoporous carbon composite with core-shell confi guration using MWNT@mesoporous silica and furfural alcohol as tem-plate and carbon resource, respectively. [ 185 ] Such composite exhibited greatly enhanced SC from 9.0 to 48.4 F g 1 and 6.8 to 60.2 F g 1 in 1.0 M (C 2 H 5 ) 4 NBF 4 and 6.0 M KOH, good rate performance with 60% retention of the initial capacitance at 20 A g 1 and high cyclability (94% after 1000 cycles).

As a novel class of porous material, metal-organic frame-works (MOFs) have permanent nanoscaled cavities and open channels providing congential conditions for small molecules to access, thus have recently been demonstrated as poten-tial templates to synthesize porous carbons. [ 72,186188 ] In this synthetic method, furfuryl alcohol is impregnated and subse-quently polymerized inside the micropores of MOFs. During the carbonization process, the formation of porous carbon networks and the decomposition of MOFs happen simultane-ously. MOFs act as both the sacrifi cial template and a supple-mentary carbon source. As a result, the SSA of the resulting porous carbons can achieve 5003400 m 2 g 1 with pore volume ranging from 0.13 to 3.14 cm 3 , contributing to a SC as high as 110310 F g 1 in aqueous electrolytes. [ 72,186188 ]

In the hard template synthesis, the use of silica makes the preparation complicated, high-cost and time-consuming, and therefore unsuitable for large-scale production and indus-trial applications. Additionally, the removal of silica requires extremely corrosive and toxic hydrofl uoric acid. Thus, a great deal of effort has been made to develop a simple route for synthesizing porous carbons with homogeneous pore sizes. Recently, a soft-template method using various commercially available triblock copolymers has attracted particular atten-tion to produce carbons with micropores and unique size due to the decomposition of soft templates above 400 C. [ 189,190 ] In this synthetic method, the templating species are mixed with organic precursors in solution and occluded in the growing carbon frame, generating pores in carbons after their removal. Such templating can be regarded as endotemplating, contrary to exotemplating in which the templates are materials with structural pores where the carbon frame is formed. Commonly, the structures of the obtained carbons are fl exible and their generation intimately depends on temperature, type of solvent, and ionic strength. Recently, ordered graphitic mesoporous carbon nanocomposites with tunable mesopore sizes was pre-pared by a brick-and-mortar soft-templating approach with phenolic resins as carbon sources and triblock copolymers (F127) as templates. [ 190 ] In the carbon nanocomposite, phe-nolic resin-based mesoporous carbons act as the mortar to highly conductive carbon blacks and carbon onions (bricks). After carbonization, carbons with larger mesopore volumes, widths, and excellent electrical conductivity were obtained, resulting in a SC of 50 F g 1 in organic electrolyte. Xia and

Figure 10. Schematic representation of the highly ordered MCNAs. Reproduced with permission. [ 189 ] Copyright 2010, The Royal Society of Chemistry.

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3.1.3 . CDCs

CDCs are generally produced through selective extraction of non-carbon atoms from carbides (TiC, B 4 C, ZrC, Ti 3 SiC 2 , Ti 2 AlC, WC, and SiC) at high-temperature (8001200 C). [ 75,77,194197 ] High-temperature chlorination and vacuum decomposition are the most widely used approaches for CDC synthesis:

MC + xCl2 MCl2x + C (6)

MC(s)vacuum M(g) + C(s) (7)

The as-synthesized CDCs possess a narrow and tunable PSD with a sub- precision, an average pore size of 0.62 nm, and SSA up to 3100 m 2 g 1 , [ 198 ] allowing them to exhibit out-standing electrochemical performances in aqueous, [ 77,194,195 ] organic, [ 75,196,198,199 ] and IL electrolytes. [ 105,200,201 ] The mean pore size, PSD, pore volume, and SSA of the CDCs can be controlled and tailored through selecting the precursors and the chlorina-tion conditions.

Since the spatial distribution of carbon atoms in different carbides may differ substantially even when densities of cor-responding CDCs are similar, the microstructure of the pre-cursors may have a crucial impact on the microstructure (pore size, shape, and volume) of the produced CDCs. Theo-retical calculations suggested that a wide range of precursors could achieve a wide range of theoretical pore volumes from 55% to 85% without further post-treatment. [ 202 ] The PSD and

impressive performances are believed to be attributed to its hierarchical structure, namely, macroporous cores as ion-buff-ering reservoirs, mesoporous walls with smaller ion-transport resistance, micropores for charge accommodation, and a local-ized graphitic structure for enhanced electric conductivity ( Figure 11 ). 3D hierarchical ordered porous carbon (HOPC) with partially graphitic nanostructures was prepared using triblock copolymer F127 and monodisperse polystyrene latex spheres as templates and phenol-formaldehyde resin as carbon precursor, possessing good graphitization domains, inter-connected ordered macropores, mesopores, and micropores ( Figure 12 ). Despite its low SSA (296 m 2 g 1 ) and low gravi-metric SC (73.4 F g 1 ), the resulting carbon showed good rate performance, high area SC (24.8 F cm 2 ), and excellent cycling performance (10 000). [ 179 ]

Templated carbons possess uniform pore sizes, ordered structure, large pore volumes, and high SSA, making them promising candidates for supercapacitor with high energy and power densities. However, templated carbons have some disad-vantages such as relatively high production costs, low produc-ibility, and safety considerations. During the synthesis process, the amount of the used template materials is usually several times that of the produced carbons and they are not available by kilogram or ton order, which is not suitable for large-scale production. On the other hand, some template materials are expensive and their eventual removal usually requires the use of highly corrosive and toxic reagents such as HF. Thus, the potential commercial applications of template carbons have been seriously suppressed and it is urgent to develop a simple, economical, and environmentally benign template route to sat-isfy their broad applications.

Figure 12. a) Illustration of the synthetic routes to 3D HOPC through one-pot and evaporation-induced self-assembly. b) SEM and c) TEM images of the as-prepared 3D HOPC. Reproduced with permission. [ 179 ] Copyright 2012, Wiley VCH.

Figure 11. a) Schematic representation of the 3D hierarchical porous texture. b) Ragone plot comparing the performance of HPGC material. Reproduced with permission. [ 180 ] Copyright 2008, Wiley VCH.

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decrease the particle sizes of CDCs to sub-micrometer dimen-sions, however, the overall improvement of the power charac-teristic was demonstrated to be rather moderate. [ 205 ] Templated, ordered, mesoporous silicon CDC was applied in high-rate supercapacitors for the fi rst time and exhibited a SSA up to 2430 m 2 g 1 , high SC of 170 F g 1 , outstanding capacitance retention of 85% at current densities up to 20 A g 1 and spec-tacular energy density (140 Wh kg 1 ) in 1 mol L 1 tetraethyl-ammonium tetrafl uoroborate in acetonitrile solution. [ 197 ]

Another promising strategy to overcome the limitation of slow ion transport in small pores is to develop/design hierar-chical porous CDCs with straight ordered mesopores. [ 75,200 ] The ordered mesoporous channels are favorable for retention and immersion of the electrolyte and can serve as ion-highways and allow for very fast ionic transport into the bulk CDCs. Mean-while, the micropores on the mesopore walls can increase the SSA to provide more sites for charge storage. Recently, the same research group produced hierarchical micro- and mesoporous silicon CDCs with the surface area in the range of 2364 to 2729 m 2 g 1 , [ 200 ] which is among the highest values ever reported for CDCs. Due to the presence of straight mesoporous channels combined with a high micropore content, the pro-duced carbon not only demonstrated very high SC of 202 F g 1 in the aqueous electrolytes, but also showed exceptionally high power performance with up to 90% of the capacitance retained when the current density was increased from 0.1 to over 20 A g 1 . The combination of superb energy and power char-acteristics of the samples could not be matched by the state-of-the-art activated carbons or microporous CDCs. [ 200 ]

CDCs still have very limited commercial potential due to their high cost, serious safety and environmental concerns associated with production, and requirement of high temper-ature. However, from a research point of view, CDCs are of importance to study since they can provide valuable informa-tion about the effect of pore size, channel structures and other parameters on the ion diffusion and charge storage in carbon nanmaterials.

3.1.4 . Carbon Nanotubes

CNTs have attracted increasing interest for supercapacitor applications due to their excellent electrical conductivity, unique pore structure, exceptional mechanical, chemical and thermal stability. [ 17,206,207 ] However, the SC of CNTs is still low due to the limitation of their surface area (less than 600 m 2 g 1 ). [ 208,209 ] Notably, although the surface area of CNTs is relatively mod-erate, their area SC can reach up to 50.4 F cm 2 , which is higher than those of ACs (2050 F cm 2 ) due to the perfect electrolyte accessibility by the tube entanglement. [ 209 ]

Over the past few years, there has been increasing research interest in fl exible and lightweight energy-storage systems to meet the demands for portable electronic devices, including roll-up displays, stretchable integrated circuits, and wearable systems for personal multimedia or medical devices. [ 210,211 ] Recently, CNTs have been printed on plastics, papers, or coated on textiles to fabricate thin and fl exible supercapaci-tors. [ 212215 ] In these electrodes, CNTs not only act as highly conductive and fl exible active materials, but also increase the effective surface area in the fi lms maximizing the effi ciency of

SSA are infl uenced signifi cantly by the chlorination tempera-ture. Generally, low-temperature chlorination (

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Large contact resistance between active materials and cur-rent collectors would greatly limit the power performance. Therefore, various methods have been employed to reduce the internal resistance of the CNT electrodes to achieve high power capabilities. [ 217224 ] An effective approach to overcome this issue is to assemble binder-free CNT fi lms, which can be realized through layer-by-layer (LBL) assembly [ 217,218 ] and elec-trophoretic deposition (EPD). [ 219,220 ] LBL assembly usually con-sists of repeated and sequential immersion of a substrate into stable dispersions of negatively and positively charged CNTs. This allows the precise control of fi lm thickness and mor-phology through adjusting the immersion times and pH condi-tion of the solution. [ 217 ] LBL CNT electrodes were demonstrated ultrahigh capacitance of 160 F g 1 in 1 mol L 1 H 2 SO 4 , which is considerably higher than those of ACNTs and conventional CNTs. [ 218 ] Compared to LBL assembly, EPD is relatively quick to fabricate fi lms, which generally adopts an oppositely charged metal current collector to attract CNTs in CNT suspension due to the electrostatic attraction. [ 219,220 ] This approach has several advantages involving short formation time, simple equipments, suitability for mass production. The fabricated supercapacitors possess a small resistance, high power density, and superior frequency response. Despite the low SC of 21 F g 1 , MWNT thin fi lms fabricated by this approach exhibited high power density over 20 kW kg 1 and superior frequency response with a knee at 7560 Hz in a two-electrode system. [ 219 ]

Another promising alternative to reduce the internal resist-ance of CNT electrodes is to grow CNTs directly on conductive substrates, such as graphite-foil, [ 221 ] aluminum, [ 225,226 ] , Au, [ 227 ] and Inconel alloy. [ 228,229 ] This approach minimizes the contact resistance between the active material and the current-collector and greatly simplifi es electrode fabrication. Talapatra et al. reported the growth of ACNTs on an Inconel 600 substance through vapor-phase catalyst delivery, which showed a power density of 7 kW kg 1 at 1 V s 1 in 6 mol L 1 KOH solution. [ 228 ] Arrays of multi-segmented hybrid nanostructures of CNT and gold nanowires (AuNW) have been synthesized using a combi-nation of CVD and electrodeposition methods ( Figure 15 ). [ 227 ] Such hybrid structures exhibited excellent electrochemcial performance with a maximum power density of 48 kW kg 1 , much higher than the reported values for CNT-based superca-pacitors due to well adhered interface between CNT and AuNW segments.

Several attempts have been made to improve the SC and energy density of CNTs by increasing their SSA through chemical or plasma activation. [ 216,230232 ] The activation treat-ment can not only substantially increase the SSA of CNTs through opening their end tips and introducing defects while keeping the nanotubular morphology, but also induce oxygen-ated functional groups contributing some pseudocapacitance to the overall SC. Dai and co-workers demonstrated that ACNT arrays synthesized by vacuum CVD were subjected to oxygen plasma activation, leading to the opening of CNT end-tips and increasing of SSA to 400 m 2 g 1 . [ 216 ] With the combined contri-bution from double-layer capacitance and redox pseudocapaci-tance, the activated ACNTs showed a remarkable capacitance (440 F g 1 ), high energy density (148 Wh kg 1 ) and high power density (315 kW kg 1 ) with a high cell voltage (4 V) in IL electro-lyte, potentially exceeding those of the current supercapacitor

thin fi lm CNT supercapacitors. Thin-fi lm supercapacitors fab-ricated with SWNT-coated plastic serving as both electrodes and current collectors exhibited very high energy (6 Wh kg 1 ) and power (70 kW kg 1 ) densities in organic electrolyte. [ 213 ] A compact-designed supercapacitor was fabricated using large-scale, free-standing, and fl exible SWNT fi lms as both the anode and cathode; this exhibited high energy density (43.7 Wh kg 1 ) and power density (193.7 kW kg 1 ) due to the small internal resistance ( Figure 14 a). [ 212 ] Recently, a stretchable and wear-able supercapacitor using SWNTs coated textiles as electrodes (Figure 14 b) [ 214 ] has exhibited a high SC of 140 F g 1 and spectacular energy density of 20 Wh kg 1 at a specifi c power of 10 kW kg 1 in 1 mol L 1 LiPF 6 electrolyte. In addition, it showed extremely good cycling stability with only 2% variation and change in capacitance over 130 000 cycles. [ 214 ]

Moreover, aligned CNTs (ACNTs) have aroused particular interest for supercapacitor applications due to their intriguing advantages over randomly entangled CNTs. [ 23,69,216 ] ACNTs pos-sess relatively regular pore structures and conductive channels, leading to higher effective SSA, facilitating fast ion and electron transportation and providing improved charge storage/deliver properties, which is highly desirable for high-rate applications. Recently, vertically ACNT forests with high SSA synthesized by water-assisted chemical vapor deposition (CVD) as durable electrodes for symmetric supercapacitors, could be operated at a higher voltage (4 V) while maintaining durable full charge/discharge cyclability with an energy density (94 Wh kg 1 , 47 Wh L 1 ) and power density (210 kW kg 1 , 105 kW L 1 ), far exceeding those of AC both gravimetrically and volumetri-cally. [ 23 ] The ordered pore structure of SWNT electrodes results in a lower tortuosity, enabling fast ion transport and thus higher power capability.

Figure 14. a) Rolled design of the separator with SWNT fi lms and b) the resulting compact-designed supercapacitor. Reproduced with permis-sion. [ 212 ] Copyright 2011, The Royal Society of Chemistry. c) Conductive textiles are fabricated by dipping textile into an aqueous SWNT ink fol-lowed by drying in oven at 120 C for 10 min. d) A thin, 10 cm 10 cm textile conductor based on a fabric sheet with 100% cotton. Reproduced with permission. [ 214 ] Copyright 2010, American Chemical Society.

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technology. In addition, the plasma-activated ACNTs signifi -cantly overcome the disadvantage of high viscosity of the ILs by providing a highly accessible pathway to electrolyte ions. How-ever, oxygenated functional groups induced by activation deco-rated on CNTs may potentially deteriorate the cycling stability and cause high leakage current, which is quite undesirable in practical commercial applications.

In spite of these tremendous achievements, a number of issues still need to be addressed to further promote the industrial application of CNTs; these issues include limited SSA, highly variable purity, high production cost, and limited availability.

3.1.5 . Graphene

Graphene is a 2D single-atom-thick carbon allotrope tightly arranged in honeycomb lattices and has stimulated enormous research since its discovery by isolation from bulk graphite using adhesive tape. [ 233235 ] Because of its unique structure, gra-phene possesses ultrahigh theoretical SSA (2630 m 2 g 1 ) and extraordinary electronic, mechanical, thermal, and optical prop-erties. Thus, the emergence of graphene holds great promise for potential applications in high-performance supercapacitors. However, there are still several obstacles signifi cantly inhibiting its commercial application in supercapacitors.

1) Large-scale synthesis of high-quality graphene. The most important challenge facing current researchers in this area is the large-scale synthesis of high-quality graphene. Until now, tremendous efforts have been made to develop synthesis methods to achieve large-scale production of graphene with

high quality. Mechanical cleavage from bulk graphite can pro-duce high quality graphene, but the process is rather tedious, hard to control and the yield is extremely low. [ 234 ] Abundant alternative synthesis routes have been developed to produce graphene, including epitaxial growth on SiC, CVD growth on metal substrates and substrate-free gas phase synthesis; how-ever, the uniform growth of single-layer graphene is still a chal-lenge and these methods generally need high temperature, spe-cial equipment, and precise control over cooling rates, which limit their practical application in large-scale production. [ 236 ] Although different methods continue to be explored, chemical exfoliation of graphite to graphene oxide (GO) followed by con-trollable reduction might be one of the most promising routes among all the strategies pursued. [ 237,238 ] This method is facile and scalable, providing the possibility of large-scale production for a broad range of applications. However, the disadvantage of this approach is the presence of large amount of defects and oxygen-containing functional groups during the oxidization process, resulting in low mobility and electrical conductivity. Over the past years, graphene synthesized through various approaches has been investigated intensively as electrode mate-rials for supercapacitors ( Table 2 ).

In 2008, Ruoff and co-workers pioneered supercapacitor based on hydrazine reduced GO that exhibited a SC of 135 and 99 F g 1 in aqueous and organic electrolytes, respectively ( Figure 16 ). [ 86 ] These encouraging results illustrate the exciting potential of graphene-based materials for high performance, electrical energy storage devices. Compared to conventional porous materials, the effective surface area of graphene highly depends on the number of graphene layers rather than the distribution of pores at solid state. [ 86,239 ] Thus, graphene mate-rials with single or few layers with less agglomeration are expected to exhibit high effective SSA and excellent electro-chemical performance. The gas-solid hydrazine reduced gra-phene materials displayed a lower degree of agglomeration with a SSA of 320 m 2 g 1 , which achieved the maximum SC of 205 F g 1 at 1.0 V in aqueous electrolyte with an energy density of 28.5 Wh kg 1 at a power density of 10 kW kg 1 and excellent cycle life with 90% capacitance retention after 1200 cycles. [ 239 ] The impressive performance could be attributed to the high accessibility by electrolyte ions and effective use of SSA and high electrical conductivity.

Due to the highly toxic and potentially explosive chemical (hydrazine), many environmentally friendly candidates have been developed, such as hydrohalic acids, [ 242 ] NaBH 4 , [ 240,254 ] alcohols, [ 243 ] vitamin C, [ 245 ] urea, [ 241 ] glutathione, [ 246 ] and metals. [ 244,255,256 ] Yu and co-workers reported the preparation of ultrathin, transparent fi lms fabricated with NaBH 4 reduced GO for use in supercapacitor applications. [ 240 ] These fi lms dem-onstrated excellent optical transparency, homogeneous mor-phology, and an ideal electrical double layer behavior with SC of 135 F g 1 and a high energy density of 15.4 Wh kg 1 in 2 M KCl electrolyte. [ 240 ] The partially reduced GO using HBr showed the SC of 348 and 158 F g 1 in aqueous and IL, respectively, due to some oxygen functional groups that facilitated the penetra-tion of aqueous electrolyte and introduced pseudocapacitive effects. [ 242 ]

Apart from chemical reduction, graphene can be pre-pared through thermal exfoliation of GO. [ 247,249,257 ] Graphene

Figure 15. a) Schematic showing the fabrication of CNT/AuNW hybrid structures inside the AAO template. b) SEM image showing the CNT/AuNW segments and the Cu back layer. c) TEM image clearly showing the CNT/AuNW interface. Reproduced with permission. [ 227 ] Copyright 2008, The Royal Society of Chemistry.

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prepared by exfoliation of GO at 1050 C exhibited a SSA of 925 m 2 g 1 and a SC of 117 F g 1 in aqueous H 2 SO 4 electro-lyte. [ 257 ] In addition, low-temperature exfoliation of GO (200 C) in a vacuum environment was developed to avoid high energy

consumption. [ 247 ] The as-obtained graphene materials showed SC of 264 and 122 F g 1 in aqueous and organic electrolytes, respectively. Unfortunately, it is found that the C/O ratio was as low as 10, indicating the presence of abundant residual oxygen-containing groups. [ 247 ] Recently, another simple and versatile method to simultaneously achieve the exfoliation and reduction of GO has been realized using convenient and rapid microwave irradiation. [ 248 ] Ruoff et al. treated GO precursor in a commer-cial microwave oven for less than 1 min to obtain crumpled graphene with a SSA of 463 m 2 g 1 , which exhibited a SC of 191 F g 1 in KOH electrolyte. This simple preparation process could provide a promising route for the scalable and cost-effec-tive production of graphene materials.

A mild solvothermal method was also adopted to reduce GO for supercapacitors. [ 251,258,259 ] In this process, a relatively low temperature was used without the addition of reducing agent and the density of functionalities can be controlled through changing the reduction time. [ 258 ] The obtained materials showed SC up to 276 F g 1 in H 2 SO 4 electrolyte with good rate performance and cycling stability, which is due to the surface oxygen-containing groups contributing to large pseudocapaci-tacne, less aggregation, and good wetting properties.

Electrochemical reduction of GO has been recently put for-ward as another environmentally friendly strategy toward gra-phene. [ 252 ] In addition, photocatalytically reduced GO with UV, [ 260 ] focused solar, [ 261 ] or mercury-lamp [ 253 ] is a rapid, chem-ical-free, cost -ffective route for high throughput production of graphene without the use of high temperature. Interestingly, the reduction degree and therefore the electrical conductivity of reduced GO (RGO) can be controlled by varying the irradiation time. The mercury-lamp irradiation reduced GO delivered an

Figure 16. a) SEM image of chemically reduced graphene oxide (CRG) particle surface and b) TEM image showing individual graphene sheets extending from CRG particle surface. c) Schematic of test cell assembly. Reproduced with permission. [ 86 ] Copyright 2008, American Chemical Society.

Table 2. Summary of performances of graphene materials prepared through different methods for EDLCs.

Preparation methods SSA [m 2 g 1 ]

SC a) [F g 1 ]

Scan rate E [Wh kg 1 ]

Cycle life b) Electrolyte [mol L 1 ]

Ref.

Hydrazine reduction 705 135 (2) 10 mA KOH (5.5) [ 86 ]

99 (2) 10 mA TEABF 4 (1) [ 86 ]

Gas-based hydrazine reduction 320 205 (2) 0.1 A g 1 28.5 1200 ( 90%) KOH (30%) [ 239 ]

NaBH 4 reduction 135 (2) 0.75A g 1 15.4 KCl (2) [ 240 ]

Urea reduction 590 255 (3) 0.5 A g 1 1200 (93%) KOH (6) [ 241 ]

HBr reduction 348 (3) 0.2 A g 1 3000 (100%) H 2 SO 4 (1) [ 242 ]

158 (3) 0.2 A g 1 BMIPF 6 [ 242 ]

Benzyl alcohol reduction 9.6 35 (2) 25mV s 1 KOH (6) [ 243 ]

Zn reduction 220 116 (3) 0.05A g 1 5000 (100%) KOH (6) [ 244 ]

Vitamin C reduction 512 128 (2) 0.05A g 1 KOH (6) [ 245 ]

Glutathione reduction 317 238 (3) 0.1 A g 1 1000 (97%) H 2 SO 4 (1) [ 246 ]

Low-temperature exfoliation 400 264 (2) 0.1 A g 1 100 (97.0%) KOH (5.5) [ 247 ]

Microwave assisted exfoliation 463 191 (2) 0.15 A g 1 KOH (5.0) [ 248 ]

Thermal exfoliation 524 150 (3) 0.1 A g 1 500 (100%) KOH (30%) [ 249 ]

Hydrothermal reduction 175 (2) 10 mV s 1 KOH (5.0) [ 250 ]

Solvothermal reduction 346 218 (3) 0.4 A g 1 700 (100%) H 2 SO 4 (0.5) [ 251 ]

Electrochemical reduction 165 (3) 20 mV s 1 8000 (100%) Na 2 SO 4 (0.1) [ 252 ]

Photocatalytical reduction 220 (2) 1 mV s 1 5 20000 (92%) H 2 SO 4 (2) [ 253 ]

a)The numbers 2 and 3 refer to two- and three-electrode tests, respectively ; b)The percentage in brackets represents the capacitance retention in the given conditions.

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energy density of 5 Wh kg 1 at a high power of 1 kW kg 1 and showed superior stability with 92% capacitance retention after 20000 cycles in an H2SO4 aqueous solution. [ 253 ]

2) Serious aggregation and restacking of graphene. Another big challenge for graphene application in supercapacitors lies in its compact and dense structure that is adverse for the rapid transport of electrolyte ions during rapid charge/discharge pro-cess. Thus, a great deal of efforts have been made to develop 3D self-assemble, macrostructured and binder-free graphene foams, [ 83 ] hydrogels, [ 250,262 ] aerogels, [ 245 ] and organogels [ 263 ] for high-rate supercapacitors ( Figure 17 ). For instance, lightweight and electrically conductive RGO foams with open porous and continuously crosslinked structures can be obtained by an auto-claved leavening and steaming strategy ( Figure 18 ). [ 83 ] Thermal steaming of GO layered fi lms with hydrazine is believed to be the key to the formation of RGO foams. Compared to regular RGO layered fi lms (17 F g 1 ), the resulting foams showed signif-icantly improved performance with SC of 110 F g 1 for fl exible supercapacitors. Recently, the intrinsic capacitance of graphene has been experimentally determined to be 21 F cm 2 , [ 264 ] thus the theoretical SC of graphene would be up to 552 F g 1 if the theoretical SSA (2630 m 2 g 1 ) is fully used. That is to say, the

previously reported SC is signifi cantly compromised by the irreversible agglomeration of graphene sheets. Therefore, the inhibition of aggregation is of particular importance for gra-phene sheets as electrode materials in energy storage fi elds. In order to minimize the aggregation, a number of efforts have been devoted, including incorporating spacers (e.g., metal nanoparticles, [ 265,266 ] oxide/hydroxide nanoparticles, [ 4042,267270 ] water molecule, [ 271 ] conducting polymers, [ 272274 ] carbon nano-materials [ 94,275281 ] , preparation of porous or crumpled graphene sheets, [ 35,282287 ] and activation of graphene [ 79,81 ] ). As a conse-quence, these attempts can not only facilitate the transport of electrolyte ions but also greatly enhance the electrochemical utilization of graphene. [ 79 ]

More interestingly, anchoring carbon nanostructures or metal nanoparticles on 2D graphene sheets may effectively inhibit the aggregation of sheets and result in a mechani-cally jammed, exfoliated graphene agglomerate with very high surface area ( Figure 19 ). [ 91,94,265,275280,288 ] Functionalized 2D graphene sheets and acid-treated 1D CNTs hybrid fi lms via electrostatic interactions exhibited a nearly rectangular CV curve and an average SC of 120 F g 1 even at a high scan rate of 1 V s 1 . [ 279 ] A novel strategy was reported to prepare 3D sandwich-like graphene/CNT composite with CNT pillars in situ grown in between graphene layers through CVD process (Figure 19 b), [ 275 ] which exhibited a maximum SC of 385 F g 1 at a 10 mV s 1 in 6 M KOH, and a capacitance increase of ca. 20% of the initial capacitance was observed after 2000 cycles, indicating an excellent electrochemical stability. In another case, CoMgAl layered double hydroxides were used as both cat-alysts and the template for the in situ synchronous growth of graphene and SWNTs from methane by CVD. [ 281 ] The obtained graphene/SWNT composite exhibited a high SSA of 807 m 2 g 1 and a SC of 98.5 F g 1 in an aqueous electrolyte. Such a fabrica-tion method is believed to be easy to scale up for their further

Figure 18. a) Schematic drawings illustrating the leavening process to prepare RGO foams. b) Schematic diagram and c) optical image of the fl exible RGO foam supercapacitor. Reproduced with permission. [ 83 ] Copy-right 2012, Wiley VCH.

Figure 17. a,c,e) Digital photographs of graphene hydrogel (a), aerogel (c), and foam (e). b,d,f) SEM image of the interior microstructures of graphene hydrogel (b,d) and foam (f). Reproduced with permission from ref. [ 250 ] (a,b); ref. [ 245 ] (c); ref. [ 262 ] (d); and ref. [ 83 ] (e,f). Copyright 2010, American Chemical Society (a,b); copyright 2011, The Royal Society of Chemistry (c); copyright 2011, The Royal Society of Chemistry (d); and copyright 2012, Wiley VCH (e,f).

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applications in the fi elds of composites, energy storage, catal-ysis, and devices.

Moreover, carbon black nanoparticles can also be used as spacers to separate graphene sheets through ultrasonication approach (Figure 19 c). [ 276 ] Carbon black particles mainly depos-ited on the edge planes of graphene and could enhance the electrochemical utilization of graphene layers and facilitate the electrolyte ions diffusion and migration. As a consequence, the resulting composite showed SC of 175 F g 1 and excel-lent cycling stability with capacitance retention of 91% after 6000 cycles, superior to those of pure graphene. In addition to carbon black, carbon spheres were also employed as spacers to intercalate between graphene sheets to prepare 3D hierarchical structures (Figure 19 d). [ 91,94 ] Zhao et al. reported 3D architec-tures composed of mesoporous carbon spheres intercalated between graphene sheets by template assisted CVD process. [ 94 ] During the CVD process, the formation of mesoporous carbon spheres and the reduction of GO occurred simultaneously. Carbon spheres uniformly intercalated into the slightly crum-pled graphene sheets and formed randomly distributed open channels with hundred nanometer dimensions, providing easy access of electrolyte to the surface of graphene sheets to form electric double layers. The 3D carbon electrode exhibited a max-imum energy density of 5.5 Wh kg 1 and an excellent electro-chemical cyclability with 94% capacitance retention after 1000 cycles. The method demonstrated in this work opens up a new route for the preparation of 3D graphene-based architectures for supercapacitor applications.

Inspired by the fact that all biological tissues are more or less hydrated and the hydration can provide strong repulsive forces to prevent cells and tissues from collapse, Yang and co-workers recently reported an interesting and novel strategy to prevent the restacking of CRG using water molecules as an effective spacer to allow the multilayered graphene structure to behave as monolayered graphene. [ 271 ] The solvated graphene

exhibited unprecedented electrochemical performance. High SC of 215 F g 1 at 0.1 A g 1 and 156.5 F g 1 at 1080 A g 1 as well as a maximum power density of 414.0 kW kg 1 were obtained in an aqueous electrolyte. Notably, the IL-exchanged solvated graphene fi lm based supercapacitor could offer SC of up to 273.1 F g 1 and an energy density and maximum power density of up to 150.9 Wh kg 1 and 776.8 kW kg 1 , respectively. This simple, bioinspired strategy will open up numerous opportuni-ties for applications of graphene in a bulk form.

Incorporating pseudocapacacitive materials such as various metal oxides/hydroxides, conducting polymers into graphene as spacers to form composites is demonstrated to be an effective strategy to prevent graphene agglomeration. [ 4042,267270,272274 ] On the one hand, these spacers can effectively suppress the irreversible restacking to maintain the intrinsic high SSA and provide more active sites to form EDLs. On the other hand, these added guest materials may contribute pseduocapacitance to the overall SC. Unfortunately, this will usually seriously compromise the cyclability of the electrode due to the inherent instability of these guest materials under electrochemical con-ditions. [ 94 ] This will be discussed in a later section.

In order to increase the effective utilization of graphene nanosheets, facilitate electrolyte ion transport, and improve the rate-performance of graphene electrodes, porous or crumpled graphene sheets have also been extensively studied for superca-pacitors recently. [ 35,282287,289 ] For instance, Ning et al. developed a template CVD approach to synthesize graphene nanomesh on the gram scale. [ 283 ] The porous structure of the graphene nano-mesh helped prevent agglomeration of the graphene sheets. Due to the unique porous structure and high SSA (1654 m 2 g 1 ), the electrode showed high SC of up to 255 F g 1 , excellent cycla-bility, and rate performance. Fan and co-workers reported a facile synthesis of porous graphene through the etching of gra-phene using KMnO 4 . [ 282 ] Due to its open layered and mesopore structures that facilitate the effi cient access of electrolytes to the electrode material and shorten the ion diffusion pathway through the porous sheets, the porous graphene provided SC of 154 F g 1 and excellent cycle stability with 88% SC retained after 5000 cycles. In the later work from the same group, highly cor-rugated graphene sheets (HCGS) were prepared by a rapid, low cost and scalable approach through rapid cooling the thermal reduced GO with liquid nitrogen. [ 286 ] The wrinkling of the gra-phene sheets can signifi cantly prevented them from agglom-erating and restacking with one another face to face and thus increased the electrolyte-accessible surface area. The maximum SC of 349 F g 1 at 2mV s 1 was obtained for the HCGS elec-trode in aqueous solution. Additionally, the electrode showed no capacitance deterioration after 5000 cycles. More recently, Fengs group developed an effi cient and facile strategy to fabri-cate highly crumpled N-doped graphene sheets (C-NGNSs) by thermal treatment with cyanamide as the nitrogen source. [ 35 ] The C-NGNSs exhibited signifi cant improvement SC as high as 248.4 F g 1 , good rate capability, and excellent electrochemical stability (96.1% retention after 5000 cycles) in organic electro-lyte due to the abundant wrinkled structures, high pore volume (3.42 cm 3 g 1 ), high nitrogen doping (10%) and improved electrical conductivity. Noteworthy, the doped-nitrogen con-centration in fi nal C-NGNSs could be tailored by adjusting the amount of C 3 N 4 or the annealing temperatures.

Figure 19. SEM images of graphene separated by other carbon nano-structures as spacers. a,b) CNTs, c) carbon black, and d) carbon spheres. Reproduced with permission from ref. [275] (a); ref. [288] (b); ref. [276] (c); and ref. [91] (d). Copyright 2011, American Chemical Society (a); copy right 2010, Wiley VCH (b); copyright 2010, Elsevier (c); and copyright 2011, The Royal Society of Chemistry (d).

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Importantly, Ruoff and co-workers presented a novel method to prepared porous graphene through chemical activation of RGO powders or papers. [ 79,81 ] The activated microwave exfo-liated GO (a-MEGO) achieved a high SSA up to 3100 m 2 g 1 , which could be readily controlled by the ratio of KOH vs. MEGO ( Figure 20 ). The symmetric supercapacitor based on a-MEGO (SSA: 2400 m 2 g 1 ) offered the SC of 166 F g 1 with a meas-ured power density of 250 kW kg 1 at an energy density of 70 Wh kg 1 . Thus a practical energy density of above 20 Wh kg 1 for a packaged device is obtained, which is four times higher than that reported for hybrid electrochemical devices and nearly equal to those of lead acid batteries. [ 81 ] However, the area-normalized capacitance is relatively low (6 mF cm 2 ), which can be addressed through doping heteroatoms.

In another interesting work, Yoo et al. employed an in-plane fabrication approach for ultrathin supercapacitors based on electrodes comprised of pristine graphene and multilayer RGO. [ 290 ] The in-plane design is straightforward to implement and exploits effi ciently the surface of each graphene layer for energy storage. Additionally, the 2D design allows for exploring the unique electrochemical properties of graphene edges along with the basal planes of graphene. As a consequence, this novel supercapacitor device exhibited maximum SC of 247 F g 1 and normalized area SC of 394 F cm 2 , i.e., 3 times higher than that of the stacked device (140 F cm 2 ). From a practical point of view, this device geometry could be easily extended to other thin-fi lm-based supercapacitors and adapted to various struc-tural and hybrid designs for energy storage devices. [ 290 ]

Due to its extremely high theoretical SSA, excellent electrical conductivity, and outstanding mechanical performance, graphene has attracted particularly extensive interest from scientifi c com-munity in applications of supercapacitors in recent years. Despite tremendous achievements, the researches in this fi eld are just in the infancy and a number of serious challenges still remain in practical applications. First, how to prepare high-quality graphene

with controllable layer thickness on a large scale in a cost-effective and environmentally friendly way is the major technical obstacle preventing the further application of graphene as an alternative to supercapacitor electrodes. Therefore, various low-cost, scal-able, effective, and environmentally benign approaches to control the number of layers, the content of defects and surface function-alities are highly urgent to be developed. Second, graphene mate-rials tend to agglomerate and restack with each other during the synthesis or electrode preparation process, which is usually una-voidable and result in a low SSA and compromised electrochem-ical performance. Hence, it is of great signifi cance to take effec-tive measures to inhibit the irreversible restacking to ensure the utilization of high SSA. Last but not least, a better understanding of the correlation between the electrochemical performance and graphene structures, physical properties, and interactions within hybrids is essential. It is believed that a revolution of clean and renewable energy materials will be realized after fully exploiting the potential of graphene.

3.1.6 . Heteroatom-Doped Carbon Materials

Although most of porous carbon materials possess high SC, their electrical conductivity usually suffers from deteriora-tion with increasing porosity and SSA due to the destruction of conductive pathways, which consequently greatly limits the power capability. To further enhance the energy density and power output, the strategy of introducing pseudocapacitance through doping function groups/heteroatoms (such as N, O, S, B, and P) to carbon materials has attracted increasing attention recently. [ 291296 ] The heteroatoms providing a pair of electrons can signifi cantly change the electron donor-acceptor character-istics of carbon materials, which accordingly in turn give pseu-docapacitive reaction. [ 138,147 ]

The oxygen-containing surface groups formed in most of the carbon materials are usually acidic in nature, thus introducing electron-acceptor properties into the carbon surface. [ 138 ] The oxygen-containing functional groups are commonly formed by carbonization and activation, [ 297 ] electrochemical oxidation, [ 298 ] oxidation in O 2 [ 299 ] or HNO 3 , [ 298 ] and oxygen-plasma treat-ment. [ 300 ] In aqueous solution, they can greatly enhance the SC of carbon materials by improving the surface wettability and inducing redox reaction that contribute pseudocapacitance to the overall capacitance as follows: [ 173 ]

>C = O + H+ + e >C OH (8) COO + H+ + e COOH (9) >C = O + e >C O (10)

However, they would be detrimental in organic electrolytes due to irreversible reactions between oxygen and the electrolyte ions, which can cause decomposition of electrolyte, high self-discharge rates, increase of internal resistance of the electrode and leakage current, and thereby inferior cycle life. [ 19,299 ] In organic electrolytes, it is favorable to employ hydrophobic func-tional groups to improve the wettability of the electrode. [ 301 ] Sur-face modifi cation by a sodium oleate surfactant has been proven to greatly improve the wettability of carbon materials in organic electrolytes, leading to a high usable SSA, small internal resist-ance, and consequently increased SC and energy density. [ 301 ]

Figure 20. a) Schematic showing the preparation of a-MEGO. b) Low-magnifi cation and c) high-resolution SEM images of a 3D MEGO sheet. d) ADF-STEM image of the same area as (c). e) High resolution phase-contrast electron microscopy image of the thin edge of a MEGO sheet. f) Exit wave reconstructed TEM image from the edge of a-MEGO. Repro-duced with permission. [ 81 ] Copyright 2011, American Association for the Advancement of Science.

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Among various heteroatoms, N-doping has been investi-gated most extensively in supercapacitor applications due to its effect on capacitance in both aqueous and organic electro-lytes. [ 88,138,147,294,302304 ] Commonly, N-containing functionali-ties such as pyridinic N, porrolic N, quaternary N, and N-oxide ( Figure 21 ) [ 138 ] have electron-donor properties that can enhance the wettability of electrodes and improve the electrical conduc-tivity and capacitance performance. [ 80,138,167,303,305 ]

The doping of nitrogen can be achieved in a number of dif-ferent ways, such as treating carbon materials with N-containing reagents (NH 3 and amines) [ 305 ] or carbonization/activation of N-rich carbon precursors, such as melamine, [ 88,303 ] cyana-mide, [ 35 ] polyacrylonitrile, [ 306 ] PANI, [ 50 ] PPy, [ 294 ] and chicken eggshell membranes. [ 307 ] To date, NH 3 has been the most fre-quently employed reagent to treat carbon materials to introduce nitrogen. When treated with ammonia at high temperatures, ammonia will decompose into numerous free radicals that can attack carbon and etch carbon fragments to generate N-enriched functional groups and increase the porosity. [ 308 ] More recently, Ruoff and co-workers prepared N-doped activated graphene through introducing ammonia gas during the activation pro-cess. [ 305 ] The content of nitrogen doping was easily controlled by varying the fl ow rate of ammonia gas. It is observed that the area-normalized capacitance of lightly N-doped activated gra-phene (2.3 at%) with similar porous structure increased from 6 to 22 F cm 2 compared to pristine graphene. The quantum capacitance is closely related to the N-doping concentration, and N-doping can provide an effective way to increase the density of the states of monolayer graphene. The amount of nitrogen doped into carbons depends upon both the ammoxidation tem-perature and the fl ow rate as well as the pressure of ammonia gas. Chen et al. reported the hierarchically aminated graphene honeycombs obtained through vacuum assisted thermal expan-sion of GO followed by amination. [ 309 ] With the increase of ami-nation temperature from 200 to 600 C, the introduced nitrogen increased gradually from 2.79 at% to 3.91 at%, and N-doped graphene electrode possessed a maximum SC of 207 F g 1 and an energy density of 7.2 Wh kg 1 .

Commonly, ammoxidation of carbons can result in the decrease of SSA and pore volume although a further heat treat-ment could recover the pore parameters. N-enriched functional groups generated in this manner are usually unstable and the

content of nitrogen is usually rather low. To overcome this drawback, various N-enriched precursors can be employed to synthesize N-doped porous carbons, including PANI, [ 50,302 ] cyanamide, [ 35 ] PPy, [ 294 ] melamine, [ 88,291 ] polyacrylonitrile, [ 310 ] and quinoline. [ 311 ] For example, crumpled N-doped graphene nanosheets using cyanamide as the nitrogen source exhibited a SC of 248.4 F g 1 at 5 mV s 1 , which is much higher than that of RGO (106.3 F g 1 ). [ 35 ] A supercapacitor based on N-doped porous carbon nanofi bers synthesized by carbonization of macroscopic-scale carbonaceous nanofi bers coated with PPy showed a reversible SC of 202 F g 1 at 1 A g 1 and a maximum power density of 89.57 kW kg 1 in aqueous electrolyte. [ 294 ] In addition, a maximum volumetric energy density of 19.6 Wh L 1 was obtained for N-doped silk-derived carbon in organic elec-trolyte. [ 304 ] Nevertheless, too much nitrogen content will give rise to a negative effect on the electrochemical performance because of the decomposition of the organic electrolytes. [ 312 ]

Recently, other approaches such as plasma treatment, CVD with N-rich carbon precursors as the N source, and hydro-thermal carbonization were also used to dope nitrogen into carbons. [ 80,138,313 ] It was reported that N-doped graphene pro-duced by a simple plasma process exhibited a high SC of up to 280 F g 1 (four times larger than that of pristine graphene), excellent cycle life (>200 000), and high power capability. The outstanding performances presumably resulted from the N-doped sites at basal planes. [ 80 ] However, this process is very complicated to operate and needs special equipment. N-doped CNTs were also obtained through thermal CVD with C 2 H 2 and NH 3 gases as carbon and nitrogen sources, respectively. [ 313 ] It was indicated that the SC did not increase with the increase in nitrogen content entirely. As an alternative, hydrothermal car-bonization has attracted tremendous attention in recent years as it can maintain high content of nitrogen into fi nal carbons, does not require high temperature (180200 C) and employ only water as a medium. The maximum SC of 300 F g 1 with an energy density of 50 Wh kg 1 was reported for the hydro-thermal carbonization N-doped porous carbon. [ 138 ] However, the increase of nitrogen content does not always lead to the increase of SC, and the relationship between nitrogen content, SSA, and SC has not been fully understood.

Additionally, other elements such as B, P, and S incorpo-rated into carbons could enhance the electric double-layer capacitances and further introduce pseudocapacitances to the electrodes. [ 141,292,295 ] Boron is electrodefi cient with three valence electrons and can substitute carbon at the trigonal sites, resulting in the promotion of oxidation resistance, a shift in Fermi level to con-ducting band, and corresponding modifi cation of the electronic structure of carbon. [ 314 ] Similar to N-doping, B-doping could increase the electrical conductivity and electrochemical activity and produce additional functional groups on the carbon surface, thereby greatly enhancing the electrochemical performances of carbon materials. In addition, it was demonstrated to improve the thermal stability and increase the disorder degree of carbons. B-doping could also increase the hydrophobicity and wettability of carbons in organic electrolyte. [ 315 ] Recently, innovative B-doped RGO prepared by a one pot reduction of GO with borane-tetrahy-drofuran displayed excellent supercapacitor performance with a SC of 200 F g 1 in aqueous electrolyte. [ 292 ] B-doped mesoporous carbon prepared by co-impregnation and carbonization of sucrose

Figure 21. Schematic types of N-doping: a) porrolic N, b) pyridinic N, c) quaternary N, and d) N-oxide.

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and boric acid exhibited substantial improvement of interfacial capacitance by 1.51.6 times higher than boron-free carbon in aqueous electrolytes, [ 314 ] which could be attributed to the low-level boron doping showing catalytic effect on oxygen chemisorptions at edge planes and altering electronic structure of space charge layer of carbon. P-doping enhances the charge delocalization of carbon atoms and gives rise to the morphogy of carbons with many open edge sites. [ 316 ] P-enriched microprous carbon with SSA of 633 m 2 g 1 prepared by simple H 3 PO 4 activation could be operated stably at voltages larger than 1.3 V in H 2 SO 4 and yielded the maximum SC of 220 F g 1 . [ 141 ] Such carbon mate-rial also showed a large energy density of 16.3 Wh kg 1 and excellent long cycle life. S-doped mesoporous carbon displayed excellent supercapacitor performance with 38% increase of SC compared to S-free mesoporous carbon owing to a larger electro-lyte dielectric constant and the charge transfer process facilitated by further polarization of the surface. [ 295 ] Interestingly, the SC was maintained at a steady level with slight improvement with the continuous increase of S content, indicating that the presence of aromatic sulfi de crucially modifying the environment of the carbon surface rather than its concentration.

Nowadays, binary co-doping of heteroato