Carbon materials for high volumetric performance

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Energy & Environmental Science View Article Online

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Cite this: Energy Environ. Sci., 2016, 9, 729

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Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities Qian Wang, Jun Yan* and Zhuangjun Fan* Volumetric performance, a much more reliable and precise parameter for evaluating the charge-storage capacity of supercapacitors compared with gravimetric performance, has aroused more and more interest in recent years owing to the rapid development of miniaturized, portable and wearable electronic devices as well as electric vehicles. Various carbon materials are widely used as electrode materials in supercapacitors. However, their intrinsically low specific capacitance and relatively low bulk density lead to a relatively low volumetric performance, significantly limiting their future application. This critical review points out the crucial importance of volumetric performance and reviews recent achievements of high volumetric performances obtained through the rational design and development

Received 11th October 2015, Accepted 3rd December 2015

of novel carbon-based materials. Particular emphasis is focused on discussing the factors influencing

DOI: 10.1039/c5ee03109e

an in-depth summary of various promising approaches used to make significant research breakthroughs

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in recent years. Current challenges, future research directions and opportunities in this fascinating field of supercapacitors with high gravimetric and volumetric performances are also discussed.

the volumetric performance of carbon materials from a structural design point of view. We then make

Broader context Volumetric performance has aroused more and more interest in recent years owing to the rapid development of miniaturized, portable electronic devices and electric vehicles. Among various energy storage devices, supercapacitors have attracted tremendous attention due to their outstanding features. Various carbon materials are widely used as electrode materials in supercapacitors. However, their intrinsically low specific capacitance and relatively low bulk density lead to a relatively poor volumetric performance, significantly limiting their future application. This critical review clarifies the crucial importance of volumetric performance and comprehensively summarizes recent achievements in volumetric performances obtained through the rational design and development of novel carbon-based materials. Various factors influencing the volumetric performance of carbon materials are discussed from a structural design point of view. An in-depth summary of various promising approaches used to make significant research breakthroughs in recent years is made. Current challenges, future research directions and opportunities in this fascinating field of supercapacitors with high gravimetric and volumetric performances are also discussed. This critical review will not only provide fundamental insights into supercapacitors but will also provide important guidelines for the future design of advanced next-generation electrochemical energy storage devices for both industrial and consumer applications.

1. Introduction With the rapid development of the global economy, rapid depletion of fossil fuels and increasingly worsening environmental pollution, there is an ever-increasing demand for sustainable and renewable power sources, which has stimulated intensive research on efficient, clean energy conversion and storage devices to meet urgent future energy requirements

Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China. E-mail: [email protected], [email protected]; Fax: +86 451 82569890; Tel: +86 451 82569890

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worldwide.1–6 Among various energy storage devices, supercapacitors have attracted tremendous attention due to their outstanding features, such as high power density, ultra-fast charge and discharge rate, low maintenance, long cycle life, excellent stability and safe operation, making them the nextgeneration energy-storage systems for potential applications in the now ubiquitous portal electronics, power back-ups and hybrid vehicles.7–9 Recently, supercapacitors were successfully used in the emergency doors of the Airbus 380.2 Unfortunately, the energy density of commercially available supercapacitors (normally less than 10 W h kg 1) is remarkably lower than those of batteries and fuel cells, which prevents them from fulfilling the ever-growing energy demands, especially for

Energy Environ. Sci., 2016, 9, 729--762 | 729

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modern/next-generation portable devices and electric vehicles.10 With the rising progress of the global economy and industry, it is imperative to increase the energy density of supercapacitors to solve these obstacles. In recent years, tremendous research efforts around the world have been actively dedicated to increasing the gravimetric energy density and lowering the fabrication costs of supercapacitors without sacrificing their high power delivery and cycle life. One promising and effective strategy is the synthesis of nanostructured materials with multiple components,11–15 involving the design and optimization of advanced material/electrode architecture (hierarchical, core–shell, sandwich-like and array nanostructures).16–25 More specifically, the electrochemical properties of carbon materials can be effectively improved through various design strategies as follows: the incorporation of pseudocapacitive materials (metal oxides, electrically conducting polymers) into various dimensional conductive carbon materials to synthesize multi-component composites;11,15,26–32 increase of the specific surface area (SSA) and optimization of the pore sizes and pore size distribution of carbon materials by developing hierarchically porous structures without sacrificing the electrical conductivity;17,33–39 synthesis of nanosized

Qian Wang received her PhD degree in materials science from the Harbin Engineering University in 2014. She is now a lecturer at the Harbin Normal University. Her current research includes the controllable synthesis of carbon-based materials for energy-storage devices.

Qian Wang

Jun Yan

Jun Yan received his PhD degree in materials science from Harbin Engineering University in 2010 and carried out his postdoctoral research in the Harbin Engineering University (2010– 2012). He is now a professor at the College of Material Science and Chemical Engineering, Harbin Engineering University. His research interests mainly focus on the design, synthesis, and functionalization of carbon nanomaterials as well as their applications in electrochemical energy conversion and storage devices.

730 | Energy Environ. Sci., 2016, 9, 729--762

pseudocapacitive materials with large SSAs to provide sufficient electroactive sites for faradaic reactions and to create hierarchical porosity in electroactive materials with excellent conductivity to ensure a sufficient number of electrolyte ions and electrons participate in faradaic reactions simultaneously at high rates;13,15,40–43 doping functional groups/heteroatoms (including N, O, S, B, and P) into carbon materials17,35,36,44–46 and adoption of redox-active electrolytes.47–51 Alternatively, increasing the operating voltage of supercapacitors turns out to be a more effective approach to improving their gravimetric energy density. This method can be achieved through selecting a suitable electrolyte with a large operating voltage (organic electrolytes: up to 2.5–3.0 V; ionic liquids: up to 4 V) and the development of asymmetric supercapacitors.13,29,30,33,40,42,52–54 Despite these huge achievements in gravimetric performances, supercapacitors remain limited in their application since only gravimetric capacitive performance has been traditionally employed as the figure-of-merit to evaluate electrodes/devices in most of these cases and the majority of publications do not report the volumetric capacitance of the materials studied. With regard to practical application, other device components (current collector, electrolyte, separator, binder and packing) also need to be considered. In this respect, the energy and power densities per weight of a material on a Ragone plot (Fig. 1a) may not give a realistic picture of the performance that the assembled device could reach. Volumetric performance reflects how much and how fast energy can be stored in a unit volume of material/packed device. More recently, Gogotsi and Simon proposed that the volumetric performance should be a much more reliable and precise parameter for evaluating the charge-storage capacity of supercapacitors compared with the gravimetric performance,55 which may be a milestone towards the future design and development of supercapacitor devices. In particular, the volumetric performance would be even more important from an application standpoint if supercapacitors are employed in a limited space such as mobile electronics, electric vehicles and other compact electronic devices. For a low-density graphene electrode (0.3 g cm 3) with an extremely

Zhuangjun Fan received his PhD in 2003 at the Institute of Coal Chemistry, Chinese Academy of Sciences. He became full professor at the College of Material Science and Chemical Engineering in 2006 and now he is the director of the Institute of Advanced Carbon Based Materials at Harbin Engineering University. His research interests focus on the design and controlled synthesis of carbon Zhuangjun Fan nanomaterials, such as carbon nanotubes and graphene, and their application in energy-related areas such as supercapacitors, Li-ion batteries and fuel cells.


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Fig. 1 Two Ragone plots for the same supercapacitors based on (a) a gravimetric (per weight) basis and (b) a volumetric basis, showing that excellent properties of carbon materials will not translate to medium- and large-scale devices if thin-film and/or low-density electrodes are used. Reproduced with permission.55 (c) Research target for advanced supercapacitors with both high gravimetric and volumetric performances in the future.

high gravimetric energy density of 85 W h kg 1, its volumetric density will be 25.5 W h L 1 for the electrode and B5 W h L 1 for the device; a thinner device will have even lower values (Fig. 1b) considering other cell components, including the current collector, electrolyte, separator, binder and packing.55 It should be noted that areal performance is also becoming an increasingly important criterion to consider for practical applications with microdevices and thin-film devices, because the weight of the active materials on a chip or a smart fabric and the volume of the devices are usually negligible.13,28,56–62 Currently, the volumetric energy density of most commercially available supercapacitors is about 5–8 W h L 1, which is considerably lower than those of lead-acid (50–90 W h L 1) and Li-ion batteries (250–850 W h L 1). To this end, it remains a significant challenge for materials scientists to design and develop advanced supercapacitors with high gravimetric and volumetric performances as well as a long cycle life to satisfy the urgent requirement for clean energy for future practical applications. Until now, even though some research efforts have been directed towards improving the volumetric performance of supercapacitors,28,63–71 to the best of our knowledge, there is a distinct lack of reviews focusing specifically on supercapacitors with a high volumetric performance as a whole. Carbon-based porous materials, such as activated carbons (ACs), mesoporous carbon, carbon nanofibers (CNFs), carbide-derived carbons (CDCs), carbon nanotubes (CNTs) and recently-emerging graphene, have been intensively investigated as electrodes for supercapacitors due to their large SSA, high electrical conductivity and excellent chemical and thermal stability, as well as their good compatibility with polymers and/or metal oxides.1 In this critical review, we point out the importance of volumetric performance and review recent achievements obtained through rational design and development of novel carbon-based materials with high volumetric performance. Particular emphasis is focused on discussing the factors influencing the volumetric performance of carbon materials from a structural design point of view. We then make an in-depth summary of the significant research breakthroughs achieved through various approaches in recent years. Finally, current challenges, future directions and opportunities in this fascinating field of supercapacitors with high gravimetric and volumetric performances are briefly discussed.


2. Research progress of high volumetric-performance carbon materials In general, the capacitance and charge storage of supercapacitors are strongly dependent upon the electrode materials used. Various porous carbon materials are the most commonly used electrode materials for EDLCs, which store charge electrostatically through reversible adsorption of electrolyte ions at the electrode/electrolyte interface. Surface electrode charge generation involves surface dissociation and ion adsorption from both the electrolyte and crystal lattice defects, thus no charge is transferred across the electrode/electrolyte interface and energy storage is a true capacitance effect. Carbon-based porous materials, such as ACs, mesoporous carbon, CDCs, CNTs and graphene, have been intensively investigated as electrodes for supercapacitors due to their large SSA, high electrical conductivity and excellent chemical and thermal stability. Therefore, in this section, a brief summary of significant research in the development of carbon materials with a high volumetric performance will be presented. We would like to emphasize here that the extensive list of references provided in this review is not meant to be comprehensive, due to the explosion of publications in this exciting field. We apologize to those authors whose work we have left out. ACs have been the most commonly used electrode materials for supercapacitors during recent years due to their large SSA, moderate cost and excellent chemical and thermal stability, as well as good electrical conductivity. ACs are generally produced from various types of natural or synthetic carbon-rich organic precursors by carbonization in an inert atmosphere, with subsequent physical and/or chemical activation to increase the SSA and pore volume.68 Depending on the activation process and the carbon-enriched precursors used, the SSA of ACs was able to exceed 3000 m2 g 1 in recent years.1 During this time, tremendous achievements have been obtained through the joint efforts of scientists worldwide. A variety of ACs with SSAs in the range of 900–3500 m2 g 1 have been obtained, leading to gravimetric capacitances of 200–550 F g 1 in aqueous electrolytes and 130–230 F g 1 in non-aqueous electrolytes.1


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Despite this progress, the volumetric capacitance of ACs is about 19–611 F cm 3 in aqueous electrolytes and 12–160 F cm 3 in non-aqueous electrolytes due to their relatively low density (0.5 g cm 3) (Table 1). ACs can be produced through carbonization and/or activation of various synthetic carbon-rich organic precursors,22,72–74 natural biomass (e.g. seaweeds, chicken egg whites and sucrose)36,75–77 and agricultural byproducts (e.g. apricot shell and pollens).78–81 For instance, Li’s group demonstrated a facile template-free synthesis route to create macroscopically monolithic ACs through carbonization and activation of common chicken egg whites.76 The resulting ACs are both highly nitrogen rich and highly microporous with SSAs up to 1405 m2 g 1 but have a high density of 1.1 g cm 3, consequently showing both high gravimetric and volumetric capacitances of 550 F g 1 and 611 F cm 3 in an aqueous electrolyte, which are among the highest values reported in the literature for carbon-based electrodes. These promising results, together with the advantages of cheap carbon sources, green method and facile process hold great promise for high performance supercapacitors in practical applications. CDCs are generally produced through selective extraction of non-carbon atoms from carbides through treatments in supercritical water or halogens as well as vacuum thermal decomposition. CDCs have attracted much interest in the past due to their high SSA with pore sizes that can be fine-tuned by precisely controlling the chlorination conditions and by the choice of starting precursors.1,82 The as-synthesized CDCs possess an average pore size of 0.6–2 nm and a SSA up to 3100 m2 g 1, allowing them to exhibit impressive electrochemical performances. Gogotsi and coworkers devoted much effort towards the synthesis of CDCs for high-performance supercapacitors.82–84 For instance, they developed CDC films for microsupercapacitor electrodes, which exhibited excellent volumetric capacitances up to 180 and 160 F cm 3 in organic and aqueous electrolytes, respectively.82,83 Through etching supercapacitor electrodes into conductive TiC substrates (Fig. 2), the monolithic CDC films showed a volumetric capacitance exceeding those of micro- and macroscale supercapacitors reported thus far by a factor of 2.82 Since the synthesis method made use of processes compatible with electronic device fabrication, it could be implemented in manufacturing supercapacitors integrated with silicon chips. CNTs are 1D carbon allotropes with cylindrical structures, composed of either one rolled-up graphitic sheet (single-walled CNTs (SWNTs)) or several coaxial ones (multi-walled CNTs (MWNTs)). During the past few decades, CNTs have attracted increasing interest with respect to supercapacitor applications, due to their excellent electrical conductivity, unique pore structure, and exceptional mechanical, chemical and thermal stability.1,2,85 Nevertheless, their low SSA generally leads to low specific capacitance and energy density. Moreover, the relatively low density of CNTs (B0.3 g cm 3) is unfavorable for the volumetric performance, which is undesirable for their practical application in the near future. During the past decade, considerable efforts have been devoted towards improving the volumetric capacitance of CNTs through various approaches to



meet the future requirements.86,87 For instance, a volumetric capacitance of 135 F cm 3 with an energy density of 75 W h L 1 was obtained for densified A-CNTs, which was much higher than that of AC electrodes (20 W h L 1) in an ionic liquid. Hata et al. presented a rational and general method to fabricate a new macroscopic bulk form of a CNT material (high-density packed and aligned SWNTs) using the zipping effect of liquids to draw tubes together.87 Such a bulk material retained the intrinsic properties of individual SWNTs such as high SSA, flexibility and electrical conductivity. As a result, the supercapacitor device yielded a volumetric capacitance of 11.4 F cm 3 with an energy density of B40 W h L 1 in an organic electrolyte. Importantly, through controlling the fabrication process, it was possible to fabricate a wide range of solids in numerous shapes and structures, which is advantageous for numerous applications. Layer-by-layer (LBL) techniques can also be used to prepare densely packed and functionalized CNT films to improve the volumetric performances of CNTs.88,89 The assembled CNT thin-film electrodes yielded high volumetric capacitances of 132 and 180 F cm 3 in aqueous and organic electrolytes, respectively.88 More recently, Gogotsi and coworkers fabricated a flexible and sandwich-like MXene/CNT composite paper electrode with high volumetric capacitance through alternating filtration (Fig. 3).70 The sandwich-like electrode yielded a volumetric capacitance of 390 F cm 3, much higher than those of pure MXene and randomly mixed MXene/CNT papers. Graphene has been widely regarded as a promising electrode material because of its high intrinsic conductivity, excellent mechanical strength and chemical stability, exceptionally large theoretical SSA (2630 m2 g 1) and theoretical gravimetric capacitance of B550 F g 1.33 Graphene can be assembled into various promising structures, such as 1D fibers, 2D films and 3D foams, to enhance its gravimetric capacitance.5 However, like other carbon nanomaterials, graphene usually possesses a low packing density with abundant spacious voids and random stacking of graphene sheets, thus leading to a small volumetric capacitance (Table 1), even though the volumetric capacitance is critical for real capacitor applications. In general, there is usually a trade-off relationship between the packing density and the porosity. On the one hand, a highly porous electrode may provide a large accessible SSA and facilitate the rapid diffusion and transport of electrolyte ions, enhancing the gravimetric capacitance but usually causes a low volumetric capacitance owing to its relatively low packing density. On the other hand, a highly compact electrode can enhance the volumetric capacitance but decrease the ion-accessible SSA and diffusion rate of electrolyte ions, leading to a low gravimetric capacitance and poor rate capability. To this end, achieving graphene materials with a highly compact structure while retaining a high porosity is still a huge challenge for materials scientists wishing to obtain high volumetric capacitance carbon electrodes. In order to restrict face-to-face restacking of graphene and improve the volumetric capacitance of graphene to further meet the urgent requirements for practical application, a considerable number of interesting approaches have sprung up during the past few years such as mechanical compression,112,126


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Energy & Environmental Science Table 1

Carbon materials for EDLCs reported recently in the literature

Materials


Review

Carbon sphere Carbon sphere AC AC AC AC film HFAC-2 N-doped AC N-doped porous carbon N-doped carbon fiber N-doped porous carbon N-doped SGC B-/N-porous carbon N,P co-doped carbon S-carbon CNFs 3D porous carbon Cu modified AC PF15G-HA MHCN Porous carbon films Activated MCMB Activated carbon xerogel ACM Porous carbon Porous carbon Porous carbon Porous CNs Edge-enriched porous carbon CDC CDC CDC CDC CDC A-CNTs MWNTs CNT/MXene EM-CCG Compressed-25 NS-rGO aMEGO aMEGO aligned aMEGO Graphene HLrGOP Graphene film VArGO HPGM Crumpled RGO 3D HPG 3D graphene 3D graphene FGN-300 RGO RGO Holey graphene film Holey graphene Activated graphene S-porous carbon/graphene

Electrolyte 30% KOH 6 M NaOH 6 M KOH EMIMBF4 2 M H2SO4 1 M H2SO4 1 M H2SO4 1 M KOH 1 M TEABF4/PC 6 M KOH 6 M KOH 1 M H2SO4 6 M KOH 1 M Na2SO4 6 M KOH 1 M H2SO4 TEATFB/PC/DMC 6 M KOH 6 M KOH TEATFB 1 M TEABF4/AN EMIMBF4 1 M TEABF4/AN 0.5 M H2SO4 6 M KOH 1 M H2SO4 6 M KOH 6 M KOH 1.5 M Et4NBF4/AN 1 M H2SO4 6 M KOH 1 M TEABF4 1 M Et4NBF4/PC 1 M H2SO4 1 M TEMABF4/AN 1 M TEABF4 1 M H2SO4 1.5 M TEMABF4/AN 3 M EMIBF4/PC 1 M H2SO4 1 M MgSO4 EMIMBF4/AN 1 M H2SO4 EMIMBF4/AN 6 M KOH BMIM BF4/AN [EMIM][TFSI]/AN 2 M EMIM TFSI/AN H2SO4/PVA 1 M KOH 6 M KOH 6 M KOH 6 M KOH 1 M TEABF4/AN 6 M KOH TEABF4/PC 6 M KOH 1 M H2SO4 6 M KOH 6 M KOH 1 M Na2SO4 6 M KOH 1 M H2SO4 EMI:TFSI 6 M KOH EMIMBF4/AN 2 M BMIBF4/PC 6 M KOH


Scan rate 1

2 mV s 5 mV s 1 1 mV s 1 1Ag 1 — 1 mV s 1 0.25 A g 1 0.25 A g 1 1 mA cm 2 2 mV s 1 1Ag 1 0.25 A g 1 0.20 A g 1 0.50 A g 1 2 mV s 1 0.50 A g 1 0.50 A g 1 0.50 A g 1 0.50 A g 1 0.50 A g 1 0.20 A g 1 1Ag 1 1Ag 1 0.50 A g 1 10 mV s 1 0.20 A g 1 0.125 A g 1 0.05 A g 1 2 mV s 1 2 mV s 1 2 mV s 1 0.05 A g 1 1 mV s 1 1 mA cm 2 — 1 mV s 1 — — 20 mV s 1 0.10 A g 1 50 mV s 1 2 mV s 1 0.10 A g 1 0.10 A g 1 100 mV s 1 1.00 A g 1 2.80 A g 1 2.10 A g 1 1.00 A g 1 1Vs 1 10 mV s 1 0.50 A g 1 0.10 A g 1 0.05 A g 1 0.10 A g 1 0.10 A g 1 0.40 A cm 3 1.00 A g 1 0.50 A g 1 1.00 A g 1 1 mV s 1 0.50 A g 1 2 mV s 1 1.00 A g 1 0.30 A g 1 3.00 A g 1 1.00 A g 1 1.00 A g 1 0.50 A g 1 0.05 A g 1

Cga (F g 1)

Cv (F cm 3)

Eg (W h kg 1)

Ev (W h L 1)

Ref.

164 (2) 328 (3) 339 (2) 207 (2) 400 (2) 510 (2) 556 (3) 525 (3) 52 (2) 305 (3) 202 (3) — (2) 298 (3) 481 (3) — (2) 247 (3) 206 (3) 28 (2) 280 (3) 318 (3) 79 (2) 202 (2) 231 (2) 103 (2) 180 (3) 306 (2) 251 (3) 348 (2) 271 (3) 156 (2) 198 (2) 262 (2) 150 (2) 180 (2) 190 (2) 116 (2) — (2) — (2) — (2) 260 (2) 159 (3) 134 (3) 196 (2) 203 (2) 147 (2) 237 (2) 166 (2) 174 (2) 154 (2) — (2) — (2) 255 (3) 226 (2) 145 (2) 238 (2) 108 (2) — (3) 178 (2) 305 (2) 250 (2) 341 (3) 456 (3) — (2) 182 (2) 57 (2) 45 (2) 310 (2) 298 (2) 186 (2) 109 (2)

170 383 171 104 200 390 611 578 25 287 200 — 161 212 — 101 261 12 88 118 62 80 92 80 220 160 166 162 252 145 180 214 60 160 140 75 180 160 180 130 132 390 261 256 110 51.4 62.9 100 177 71.6 575.4 196 174 171 376 171 330 103 177 30 436 470 — 255 87 54 221 212 214 65

— — — 88 — — — — 41.7 — — 7.11 — — 25.7 — — — — — 43.9 51 98 22.4 — — — — 9.4 31.2 7.4 — 30 — — 28 — — — 150 — — 47.9 — 63 — 70 74 66 — — — — 6.3 8.3 23.5 — 38 — 7 16.2 — 26.4 4.4 — 10 — 127 — 4.9

— — — 44 — — — — 19.6 — — 7.04 — — 11.3 — — — — — 34.7 20.2 38.8 17.3 — — — — 8.8 29.0 6.7 — 12 — — 18 — — — 75 — — 59.9 — 48 — 26.5 44 75.3 2.5 — — — 7.43 13.1 37.1 — 22 — 1 20.7 — 27.2 6.2 — 12 — 90 — 2.9

90 91 78 76 72 92 36 36 93 94 95 95 96 21 21 97 98 99 100 101 102 79 79 22 103 104 105 106 107 107 108 109 38 110 84 111 82 82 83 86 88 70 67 67 112 113 33 114 66 115 116 117 117 63 64 64 118 119 119 120 121 122 122 123 124 125 126 126 127 128


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Energy & Environmental Science (continued)


Materials RGO/AC Graphene/CNT film Graphene/CNT film Graphene/CNT film Graphene/CNT film RGO/CNT N,B co-doped graphene N-graphene a

Electrolyte 1 M TEABF4 1 M LiPF6 0.5 M H2SO4 EMIMBF4 1 M TEABF4/PC H3PO4/PVA H2SO4/PVA 1 M TEABF4/PC

Scan rate 1

1.00 A g 1 mV s 1 50 mV s 1 0.50 A g 1 1.00 A g 1 0.1 mA cm 10 mV s 1 2 mV s 1

2

Cga (F g 1)

Cv (F cm 3)

Eg (W h kg 1)

Ev (W h L 1)

Ref.

117 (2) 155 (2) 175 (3) 199 (2) 110 (2) — (2) — (2) — (2)

20 307 160 211 165 158 488 57.4

37 230 — 110.6 — — — —

6.4 450 — 117.2 — 3.5 16.9 8.0

129 130 131 69 65 132 45 133

The numbers 2 and 3 refer to two- and three-electrode devices, respectively.

Fig. 2 (a) Schematic of the fabrication of a micro-supercapacitor integrated onto a silicon chip based on the bulk CDC film process.82 (b–e) SEM images of CDC films on highly ordered pyrolytic graphite (b), Al2O3 (c), glassy carbon (d), and oxidized Si wafer (e).83 Reproduced with permission.82,83

liquid-mediated dense integration,67 evaporation-induced drying64 and construction of 3D porous structures.114,118,121,134 For instance, by taking full advantage of the intrinsic microcorrugated 2D configuration and self-assembly behavior of chemically reduced graphene oxide (RGO), Li and coworkers demonstrated liquid-mediated dense integration of graphene materials through capillary compression of adaptive graphene gel films in the presence of a nonvolatile liquid electrolyte.67


The packing density of these films could be readily controlled from 0.13 to 1.33 g cm 3 by changing the ratio of volatile and nonvolatile liquids. As a result, the EM-RGO films yielded a volumetric capacitance of 255.3 F cm 3 in an aqueous electrolyte and 261.3 F cm 3 with an energy density of B110 W h L 1 in an organic electrolyte. Graphene-based films have been recently explored as advanced electrodes for supercapacitors due to their polymer binder-free process, low macropore volume


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Fig. 3 (a) Schematic of the preparation of sandwich-like MXene/CNT paper. (b) Digital image and (c) SEM image of MXene/CNT paper. Reproduced with permission.70

and good adhesion between current collector and active materials,45,59,66,124 which are of significant importance in achieving high volumetric capacitance. Within this context, aligned activated microwave-exfoliated graphite oxide (aMEGO) with a density of B1.15 g cm 3 obtained through vacuumassisted self-assembly exhibited a very high volumetric capacitance of 177 F cm 3 with an energy density of 75.3 W h L 1 in an organic electrolyte due to its highly dense and well-ordered structure.66 Such an outstanding performance may introduce a realistic energy storage device configuration to satisfy various technological demands. 3D porous graphene structures have been demonstrated to be effective in restricting face-toface restacking of graphene and in facilitating the transport of electrolyte ions.114,121,134 Highly porous graphene-derived carbons with hierarchical pore structures obtained through chemical activation of graphene-based hollow carbon spheres achieved both high gravimetric (174 F g 1) and volumetric (100 F cm 3) specific capacitances in an ionic liquid electrolyte.114 In addition, the assembled cell displayed high energy densities of 74 W h kg 1 and 44 W h L 1 owing to the unique pore structure of the graphene-based carbon materials. Mesoporous graphene with a 3D structure and high packing density (1.28 g cm 3) showed high volumetric capacitances of 436 and 212 F cm 3 with energy densities of 20.7 and 67.2 W h L 1 in aqueous and organic electrolytes, respectively.121 In order to prevent self-restacking or self-aggregation caused by the strong p–p interaction between interlayers and increase the penetration of electrolyte ions, it seems to be effective to introduce other carbon materials with different dimensions as spacers between graphene sheets to increase the interlayer spacing.5,8,21,65,69,130,132 For instance, hierarchically structured carbon microfibers made of an interconnected network of aligned SWNTs with interposed N-doped RGO sheets exhibited a high packing density and a large ion-accessible surface area, leading to volumetric capacitances of 305 F cm 3 in H2SO4 and 300 F cm 3 in PVA/H3PO4.5 The fabricated all-solid-state micro-supercapacitors achieved an ultrahigh volumetric energy density of B6.3 W h L 1, about tenfold higher than those of state-of-the-art commercial supercapacitors and even comparable to the 4 V/500 mA h thin-film


Review

lithium battery. A self-standing, binder-free functionalized graphene/MWNT electrode with a hierarchical structure and relatively high density (41 g cm 3) displayed high energy densities of 230 W h kg 1 and 450 W h L 1.130 Another interesting measure to enhance ion accessibility is to construct in-plane or vertical structures,63 which can effectively increase accessibility and penetration of electrolyte ions between two vertically aligned electrodes. A highly dense and vertically aligned RGO (VArGO) electrode prepared by simple handrolling and cutting showed a high volumetric capacitance of 171 F cm 3 and an energy density of 7.43 W h L 1 in an aqueous electrolyte due to its vertically aligned and open-edged graphene structure with a high packing density (1.18 g cm 3).63 Such a simple and facile process may provide an exciting opportunity for the large-scale application of graphene in compact energy storage devices. To improve the specific capacitance and energy density of carbon materials while maintaining high power performance and good cycling stability, the introduction of heteroatoms, including O, N, P, B and S, on the surface or in the bulk of carbon materials has been considered to be a feasible approach to combine the advantages of carbon EDLCs with pseudocapacitance induced by the heteroatoms.21,35,45,94–97,108,122,135,136 Heteroatoms providing a pair of electrons can significantly modulate the electron donor–acceptor characteristics of carbon materials and create additional functional groups on the carbon surface, leading to an increased specific capacitance. For instance, N-doped porous CNFs synthesized by carbonization of macroscopic-scale CNFs coated with PPy exhibited reversible specific capacitances of 202 F g 1 and 200 F cm 3 with a high rate-capability and a volumetric energy density of B7 W h L 1 in an aqueous electrolyte.95 A N-doped, sandwich-like graphene/ porous carbon composite prepared through one-step pyrolysis of a mixture of GO/PANI and KOH exhibited a high specific surface area (2927 m2 g 1), hierarchical interconnected pores, moderate pore volume, short ion diffusion paths and a high nitrogen level (6 at%), resulting in an unparalleled gravimetric (481 F g 1) and an outstanding volumetric capacitance (212 F cm 3) in an aqueous electrolyte (Fig. 4).21 P and N-enriched porous carbon obtained by direct heat treatment of a honeycomb-patterned H3PO4/polyacrylonitrile composite showed high gravimetric and volumetric capacitances of 205.7 F g 1 and 261 F cm 3 in an aqueous electrolyte.98 The high contents of N and P containing groups were found to be critical in obtaining the impressive electrochemical performances. A supercapacitor based on nitrogen and boron co-doped graphene films yielded a remarkable pseudocapacitance with an ultrahigh volumetric capacitance of B488 F cm 3 and an outstanding rate capability of up to 2000 V s 1.45 Such a synthesis approach could hold great potential for other thin-film energy storage and conversion devices. More recently, Yan and coworkers developed a novel strategy to prepare rationally functionalized graphene sheets via low temperature (300 1C) thermal reduction of GO with a slow heating rate, using a removable Mg(OH)2 template.122 Owing to its dented sheets with high SSA and considerable stable oxygen-containing groups, the as-obtained graphene


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Fig. 4 (a and b) TEM images of GO/PANI composite (a) and SGC sample (b). (c) Gravimetric and volumetric specific capacitances versus current densities for SGC sample. (d) Comparison of the volumetric and gravimetric capacitances of SGC electrode with other reported carbon electrodes. Reproduced with permission.21

delivered ultrahigh specific gravimetric and volumetric capacitances of 456 F g 1 and 470 F cm 3, almost 3.7 and 3.3 times higher than those of RGO, respectively. This novel route is simple but effective for large-scale production of a graphene material and is expected to greatly promote the development of high volumetric performance supercapacitors where space is limited. Incorporation of pseudocapacitive materials, such as transition metal oxides and electrically conducting polymers, is one of the most efficient strategies for improving the capacitance of carbon materials by providing additional pseudocapacitance.28,124,137–148 For example, an ordered mesoporous tungsten oxide/carbon composite displayed a high average volumetric capacitance (340 F cm 3) and gravimetric capacitance (103 F g 1) owing to its interconnected mesoporous structure, high surface area, low internal resistance and high intrinsic density of tungsten oxide.138 The asymmetrical supercapacitor, based on a 3D carbon nanosheet array/MnO hybrid, delivered a high gravimetric energy density of 184 W h kg 1 with a super-high volumetric energy density of 138 W h L 1, which was 4 times higher than that of the symmetric device.137 Vanadium nitride is considered as a promising pseudocapacitive candidate for next-generation high performance supercapacitors due to its excellent electrical conductivity and high pseudocapacitance. All-solid-state supercapacitors based on a thin, lightweight and flexible free-standing mesoporous VN nanowire/CNT hybrid electrode displayed a high volumetric capacitance of 7.9 F cm 3 and an energy density of 0.54 W h L 1.140 Freestanding MoO3 x/CNT composite films showed a specific capacitance of 337 F g 1 (based on the mass of MoO3 x) and a high volumetric capacitance of 291 F cm 3


(based on the whole electrode) with an excellent rate capability.139 Zhao et al. demonstrated the synthesis of an aMEGO/MnO2 composite through a self-controlled redox process.144 A gravimetric capacitance of 256 F g 1 was obtained in an aqueous electrolyte, corresponding to a volumetric capacitance of 640 F cm 3 with a retention ratio of 87.7% after 1000 cycles. The excellent electrochemical performances could be attributed to the following aspects: (i) aMEGO with high electrical conductivity and nanoscale pore size distribution provided an excellent scaffold for the deposition of MnO2 nanoparticles. (ii) The resultant 3D composite architecture facilitated the transport of both electrolyte ions and electrons to the electrode surface. (iii) The uniform size and spatial distribution of MnO2 nanoparticles enhanced the utilization of the pseudocapacitive materials. Freestanding and flexible graphene/PANI composite paper prepared by an in situ anodic electropolymerization displayed a strong tensile strength of 12.6 MPa and large specific capacitances of 233 F g 1 and 135 F cm 3 for gravimetric and volumetric capacitances, outperforming many other currently available carbon-based flexible electrodes.145 Such low-cost, high-performance composites using earth-abundant and environmentally friendly materials created by a scalable solution-based process could hold great promise in grid-scale energy storage device applications. Although significant research advances have been achieved during the past few years, significant challenges remain to be overcome. First, due to the trade-off relationship between the packing density and porosity, carbon materials usually possess either high gravimetric but low volumetric performances or low


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gravimetric but high volumetric performances. Synthesis of ideal carbon materials with both high gravimetric and volumetric performances is expected to meet the requirements of future applications. Second, various factors influencing the volumetric performances of carbon materials and their intrinsic interrelationship are still not fully understood. Finally, volumetric performance could be seriously affected by the fabrication procedure of electrodes and precise estimation of the thickness of electrodes is much more difficult than precise estimation of electrode weight, so volumetric capacitances may be prone to calculation errors. Therefore, scientists and engineers around the world should make a concerted effort to develop a more advanced equipment/technology for estimating electrode thickness more precisely to reduce errors.

3. Factors Influencing the volumetric performance of carbon materials The volumetric performance of an energy storage device is strongly dependent upon electrode materials, binder, additives, current collector, separator and electrolyte as well as packaging. Electrode materials are the key components in supercapacitors and play an important role in determining their ultimate performances. The binder is a key additive and can ensure excellent contact between particles of electroactive materials as well as increasing the contact area between electroactive materials and current collector. The conducting agent can enhance the electrical conductivity of electroactive materials and greatly decrease the internal resistance. The current collector collects the current generated by electroactive materials to form a large output current and provides paths for electron transfer. The separator, a porous or ion-conducting barrier, is used to separate the anode and cathode in electrochemical systems to prevent short circuits and permit the electrolyte ions to pass through to arrive at the electrodes. The cell voltage of a supercapacitor is commonly limited by electrolyte decomposition at high potentials. From a materials science point of view, the volumetric performance of carbon materials is heavily influenced by their density, morphology, pore structure, volumetric SSA, heteroatom doping and electrical conductivity (Fig. 5). In this section, various factors influencing the volumetric performance of carbon materials are briefly discussed, which will provide important guidelines for designing high volumetric performance supercapacitors without sacrificing the gravimetric performance for future industrial and consumer applications. 3.1.

Density

With regard to electrode materials, their volumetric energy density can be calculated through the following equations:149 Ev = 0.5CU2/V

(1)

C = mCg

(2)

m = rV

(3)


Fig. 5

Factors influencing the volumetric performances of carbon materials.

where Ev is the volumetric energy density, C is the total capacitance of the electrode, U is the charge/discharge potential, V is the volume of the electrode and m, Cg and r are the weight, gravimetric specific capacitance and density of the electrode materials, respectively. For a specific electrode material, the charge/discharge potential and gravimetric specific capacitance are decided by the intrinsic nature of the selected electroactive materials. Therefore, they could be regarded as constants. The electrode volume is usually dependent on the fabrication process and conditions, so it can be considered to be a constant for a specific fabrication process. Therefore, the packing density of electrode materials plays an important role in determining their volumetric performance. Volumetric performances of carbon materials are straightforwardly dependent upon the electrode density or packing density of carbon materials and gravimetric performances (Fig. 6). In general, there is a trade-off relationship between gravimetric and volumetric capacitance for a selected carbon material. On the one hand, a carbon material with high density may exhibit high volumetric capacitance but have a decreased ion-accessible active surface area and diffusion rate of electrolyte ions, giving rise to a relatively poor gravimetric capacitance, rate capability and power output (the light green region in Fig. 6). On the other hand, if the carbon material is highly porous with a very small packing density, it commonly possesses a large SSA and well-developed ion diffusion channels, leading to high gravimetric capacitance and an excellent rate performance at the expense of an inferior volumetric capacitance (the light purple region in Fig. 6). Moreover, electrodes or devices fabricated with low-packing-density active materials usually have plenty of empty spaces, which are flooded by electrolyte, consequently increasing the total weight of the device without contributing to the capacitance and lowering the total energy density normalized by the total weight of the entire device.55,126 To this end, effectively increasing the packing density of carbon


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Fig. 6 Volumetric and gravimetric specific capacitance of previously reported carbon materials in aqueous electrolytes such as ACs,78,150 flat and crumpled graphene,151 RGO,152 liquid electrolyte-mediated RGO (EM-RGO),67 graphene/CNTs,65 HPGM,64 FGN-300,122 dense carbon monoliths,153 nitrogen and fluorine co-doped carbon microspheres (CM-NF)154 and porous carbon/graphene.21

materials while maintaining a high gravimetric capacitance is still a serious challenge for materials scientists and engineers attempting to prepare carbon materials with high volumetric capacitance. It seems to be advisable to realize both high gravimetric and volumetric capacitance through designing and synthesizing carbon materials with packing densities approaching 1. 3.2.

Morphology

Nowadays, enormous research efforts have been devoted to designing and exploiting advanced carbon nanomaterials with a high SSA, controlled porosity, high electrical conductivity and appropriate pore size for next-generation supercapacitors with high gravimetric energy density, high power capability and excellent cycling stability. To date, various carbon materials with different structures, such as zero-dimensional (0D), 1D, 2D and 3D structures, have been extensively investigated for supercapacitor applications. In the past few years, various ACs, templated carbons, CDCs, CNFs, CNTs, onion-like carbons (OLCs) and newly-emerging graphene have been widely studied as electrode materials for supercapacitors (Fig. 7).1 Each type of carbon structure has its own advantages and disadvantages. OLCs provide a moderate SSA of 500–600 m2 g 1, which is fully accessible to ion adsorption, and have a relatively high conductivity, resulting in high power capability but a limited capacitance of B40 F g 1,155 which could be improved through chemical activation or oxidation in a mixture of nitric and sulfuric acids. Nevertheless, their production cost is high and their volumetric specific capacitance remains inferior (o10 F cm 3). Untreated CNTs yield a specific capacitance of up to 15–80 F g 1 in both organic and aqueous electrolytes with SSAs ranging from 100 to 500 m2 g 1, depending on their morphology, purity and treatment process. Similar to OLCs, the high electrical conductivity and highly accessible outer surface area of CNTs make them a promising material for high-power supercapacitors.156 However, their specific capacitance and energy density are seriously restricted due to their moderate SSA, which could be improved by post



Fig. 7 Volumetric and gravimetric specific capacitance for different carbon materials for supercapacitors.

treatments, such as chemical activation and plasma treatment, during which defects and oxygen-containing functional groups can be incorporated. In addition, the low density of CNTs (B0.3 g cm 3) hinders the volumetric performance, which is unfavorable for their practical application. Moreover, the present difficulty in purification and low productivity still limit their practical applications. Despite its extremely high theoretical SSA (2630 m2 g 1), graphene typically exhibits a SSA of less than 800 m2 g 1 because it tends to irreversibly agglomerate and restack due to strong van der Waals interactions. The specific capacitance of graphene in the pioneering work is only 135 and 99 F g 1 in aqueous and organic electrolytes, respectively.157 In order to reduce self-agglomeration, increase the SSA and further enhance the specific capacitance of graphene materials, a variety of approaches have been pursued such as developing promising synthesis methods, using environmentally friendly and effective reducing agents, incorporating various spacers, preparation of porous or crumpled graphene sheets, and activation of graphene.1 As a consequence, the specific capacitance of graphene is significantly improved to 385 and 250 F g 1 in aqueous and organic electrolytes, respectively.158,159 However, a number of serious challenges still remain with respect to practical applications of graphene, including the presence of numerous defects and functionalities on the surface related to poor electrical conductivity, low packing density (o0.5 g cm 3) leading to low volumetric performance, and high leakage current resulting in poor charge storage ability. More importantly, preparation of 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. ACs prepared through physical or chemical activation usually possess SSAs as high as 3000 m2 g 1, thus their specific capacitances are typically in the range of 100–400 F g 1 in aqueous electrolytes and 50–250 F g 1 in organic electrolytes, relatively higher than those of other carbon materials. Nevertheless, ACs usually have a moderately broad pore size distribution composed of micropores, mesopores and macropores, resulting in low porosity utilization and poor rate capability. Thus, their applications are still limited to a great extent since precise control of the pore size distribution and pore structure remains a challenge.


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In addition, the presence of functionalities and heteroatoms usually worsens the electrical conductivity. Furthermore, with a low density (B0.5 g cm 3), ACs also exhibit a very low volumetric capacitance (50–80 F cm 3).85 CDCs display a high SSA of 1000– 3100 m2 g 1 and a narrow and tunable pore size distribution. Depending upon the initial carbide precursors and the chlorination conditions, the specific capacitance of CDCs typically ranges from 60 to 220 F g 1. Due to their moderate density, the volumetric capacitance of CDCs is moderate (o180 F cm 3). In addition, CDCs still have very limited commercial application due to their moderate production costs and requirement for high temperatures. Templated carbons possess uniform pore sizes, ordered structure, large pore volumes and high SSA, making them promising candidates for supercapacitors with high energy and power densities. However, templated carbons still have some disadvantages such as relatively high production costs, low producibility and safety considerations related to the removal of templates. When carbon materials are used as ideal electrode materials for electrochemical energy storage, they should fulfill the following intrinsic characteristics: (1) high SSA to provide a large and accessible surface area for charge accommodation; (2) controlled hierarchical interconnected pores with narrow pore size distribution as channels for high-rate transport and diffusion of electrolyte ions; (3) outstanding electrical conductivity for efficient transfer of electrons, ensuring high rate capability and power density; (4) high thermal and chemical stability as well as robust mechanical performance for a long cycle life; (5) relatively high packing density for high volumetric performance; (6) low costs of raw materials and production. As mentioned above, none of the respective carbon structures can meet all the requirements. Therefore, it is advisable to design and construct novel 3D carbon architectures by combining the advantages of different carbon building blocks, which possess both interconnected hierarchical pores with high SSA and high electrical conductivity as well as excellent mechanical stability. Then, the properties of each of the individual building blocks will combine to give a synergistically enhanced performance. 3.3.

Volumetric SSA

The volumetric performance of a material was recently recommended as a more reliable parameter than gravimetric performance. Supercapacitors can be described by their volumetric specific capacitance and energy density, which can be expressed as follows: Cv = Cgr

(4) 2

Ev = Egr = 0.5CgU r

(5)

Sv = Sgr

(6)

where Cv, Cg, Ev, Eg, U, Sv, Sg and r are the volumetric specific capacitance, gravimetric specific capacitance, volumetric energy density, gravimetric energy density, charge/discharge potential range, volumetric SSA, gravimetric SSA and particle density of carbon materials, respectively. From the abovementioned


equations, it can be found that a high gravimetric specific capacitance and a large particle density can contribute to a high volumetric specific capacitance and energy density. It is well-known that the gravimetric SSA of carbon materials is usually directly related to the gravimetric specific capacitance. As a consequence, the volumetric performances (specific capacitance and energy density) of carbon materials strongly depend upon their gravimetric SSA and particle density. In other words, a high gravimetric SSA and/or particle density may give rise to a high volumetric performance. To this end, according to eqn (6), a high volumetric SSA may lead to a high volumetric performance. Therefore, it is advisable to synthesize carbon materials with a high volumetric SSA to obtain a high volumetric performance. 3.4.

Pore structure

Although SSA is a vital parameter with respect to the electrochemical performances of carbon materials, other factors such as pore shape and structure, pore size distribution and regularity of pores can also affect the electrochemical performances of carbon materials. The pore structure of carbon materials is closely related to the processes of ion transport and electron transfer and is therefore a crucial aspect influencing their specific capacitance and rate capability. ACs with very large SSAs up to 3000 m2 g 1 have been prepared and used as electrode materials for supercapacitors. However, they typically have a broad pore size distribution from micropores (0.3 nm) to macropores. The micropores (o0.5 nm) are believed to be inaccessible to electrolyte ions, seriously deteriorating the energy and power densities of carbon materials. In addition, high surface areas may increase the risk of decomposition of the electrolyte at the dangling bond positions.108 It is generally believed that the specific capacitance of carbon materials always increases with their SSA. However, it has been demonstrated that there is no linear relationship between SSA and specific capacitance,2 meaning that the increase in specific capacitance with an increase in SSA is very limited. This is because not all the micropores are electrochemically accessible to electrolyte ions moving to and from the electrical doublelayer. It has been demonstrated that a plot of gravimetric specific capacitance versus SSA exhibits a plateau for carbon materials for a SSA above 1200 m2 g 1, which is probably ascribed to a space constriction for charge accommodation inside the pore walls.160 The pore size of carbon materials plays an important role in their ultimate performance. An adequate pore size is more important with respect to obtaining a high specific capacitance than is a high surface area. The accessibility of micropores to electrolyte ions is heavily dependent upon both the solvated ion size and the pore diameter. When the pores are smaller than the solvated ions, the ions cannot gain access through the pores. It has been observed that the overall porosity of carbon materials is remarkably underused due to a large fraction of the pores being smaller than the effective size of electrolyte ions. Only when the diameters of pores match the ion size, will the micropores contribute to the capacitance. The optimal pore


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sizes are found to be 0.7 and 0.8 nm in aqueous media and organic electrolytes, respectively (Fig. 8a),161 indicating that for such sizes, micropores are electrochemically accessible by electrolyte ions to form an electrical double-layer and are of great importance in maximizing the capacitance. Furthermore, Largeot and co-workers experimentally determined the relationship between ion size and pore size for an EDLC and observed that the pore size leading to the maximum double-layer capacitance was very close to the ion size.164 Both larger and smaller pores lead to a significant drop in capacitance due to the ion sieving effect. When the pore sizes are larger than the ion sizes, the shape of cyclic voltammetry (CV) curves is typically rectangular. On the contrary, smaller pores give rise to a triangular-shaped CV curve.163 In another study, Gogotsi et al. found an anomalous increase in capacitance at pore sizes below 1 nm for CDCs from 55 to 80 F cm 3 due to the loss of the salvation shell around the ions.165 More recently, it has been found that the surface-related capacitance of microporous carbons is practically constant for pore widths between 0.7 and 1.8 nm and the volumetric capacitance with small micropores is higher.166,167 In addition, the volumetric capacitance of microporous carbons in the organic electrolyte is found to be an inverse function of the average micropore width, meaning that for a given micropore volume, the gravimetric capacitance is larger for carbons with smaller pores due to the higher volumetric SSA.167 A smaller micropore size is effective in increasing the particle density of carbon, resulting in a high volumetric SSA and accordingly a high volumetric capacitance. Zeolite-templated carbon with three-dimensionally arrayed and mutually connected 1.2 nm micropores exhibited a much higher volumetric SSA (1693 m2 cm 3) and capacitance (83 F cm 3) as well as a high rate performance.168 Moreover, Itoi and co-workers suggested that a pore size of 1.2 nm could be the best nanopore size, which successfully balances high volumetric capacitance and high rate performance in a 1 M Et4NBF4/PC solution.168


Finally, the energy density is greater for a smaller pore at low voltages even when it is in the saturation regime. However, the saturation energy increases as the pore widens. For a fixed voltage, the energy density first increases with increasing pore width and then it decreases for wider pores due to the presence of an inactive electro-neutral zone in the center of the large pore (Fig. 8b).162 Although too many small micropores can lead to a large SSA and a high volumetric capacitance, they will give rise to poor rate capability and inferior power density. On the other hand, the mesopores could act as ion highways and facilitate fast transport of electrolyte ions into the bulk of the electrode, consequently ensuring high power densities and rate capability. The macropores can serve as ion-buffering reservoirs, minimizing the diffusion distance of ions from the external electrolyte to the interior surfaces. The pore aspect ratio is a geometrical criterion defined as the ratio between the pore length and the pore size of a porous material.169 In classic electrochemistry theory, the ion transport time (t) can be expressed as follows: t = L2/D

(7)

where L is the ion transport length and D is the ion diffusion coefficient. Obviously, when the mesopores are shorter and larger, the ion transport time will be shorter, indicating better ion transport behavior. The pore aspect ratio combines both pore length and pore diameter and can therefore be used to compare different porous carbon materials and provide important guidelines for the design and development of highperformance carbon materials. Thus, suitable mesopores with a large pore size and short ion diffusion length can reduce the ion transport barrier. On the other hand, large mesopores and macropores usually give rise to a relatively low SSA and a large pore volume, leading to low energy density and poor volumetric performance. By forming large interconnected mesopores, as

Fig. 8 (a) Relationship between the volumetric capacitance (theoretical and experimental) and the pore size in aqueous or organic electrolytes for ACs.161 (b) The storage energy density as a function of pore width.162 (c) The storage energy density as a function of voltage for monodispersed and polydispersed porous electrodes.162 Reproduced with permission.161–163



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well as minimizing and even eliminating the micropores within carbon materials, the average coefficient of ion diffusion would be greatly enhanced but at the cost of a significantly deteriorated volumetric capacitance and a notable increase in the relative weight and volume of inactive components, resulting in inferior gravimetric and volumetric energy densities in the final supercapacitor devices. Recently, Yushin and co-workers experimentally observed that the volumetric capacitance of mesoporous carbons is significantly inferior to that of small micropores, which is largely attributed to the large pore sizes.170 In addition, they also demonstrated that micropores could facilitate rapid ion transport and that mesopores are not needed for high-power operation, even at ultralow temperatures. Pore shape also has a crucial influence on the electrochemical performance of carbon materials.84 Pore size and shape play an important role in determining the ion adsorption behavior. At low temperatures and high charge/discharge rates, the micropores could lead to high ohmic resistance due to ‘ion traffic jams’, resulting in a decrease in capacitance.171 It has been demonstrated that highly curved and tortuous inner pore surfaces will cause more diffusion losses and less effective ion adsorption as compared to more planar ones. In addition, the surface area normalized capacitance increases with increasing pore size in the range of 2–5 nm. In addition to pore shape, structure and pore size distribution, pore regularity is another critical influencing factor that needs to be considered, as it reflects the content of pore defects. The presence of more defects will decrease the regularity of the pores. A narrow pore size distribution is favored since it can reduce ion scattering and consequently improve the electrode kinetics.169 The performance of mesoporous electrodes is significantly enhanced and outperforms that of disordered and random microporous structures when the pores are ordered or in a regular manner due to the much faster ion diffusion in ordered structures compared to that in disordered and random porous structures. The construction of secondary mass transport pathways in wormhole-like mesoporous carbons appears to be effective in facilitating rapid diffusion in mesoporous structures.172 Optimization of the pore size and pore size distribution of porous carbons has been proven from both theory and experiment to be an effective way to improve their capacitive performance.162 It has been found that narrowing the pore size distribution results in an increase in the stored energy density, meaning that a monodispersed porous electrode with a suitable pore size would be ideal with respect to the energy storage of supercapacitors (Fig. 8c).162 Currently, various porous carbon materials are still the most widely used materials in supercapacitors. Considerable research efforts have been devoted to optimizing the pore structure to improve pore utilization, moving from disordered hierarchical pores to ordered mesopores, micropores and ordered hierarchical pores. An ideal pore structure for supercapacitors should meet the following requirements: a low pore volume, ensuring high particle density for high volumetric performance; an appropriate number of transport channels for fast ion delivery; a low pore aspect ratio to shorten the ion transport


Review

time and enhance ion transport behavior; and a narrow pore size distribution to reduce ion scattering and consequently improve the electrode kinetics. Hierarchical porous structures combining ordered mesopores and micropores have been proven to be effective in enhancing both energy and power density for supercapacitors. Thus it is highly desirable to design novel carbon materials and tune the carbon porosity to achieve high SSA, low pore volume, low pore aspect ratio, regular pore channels and narrow pore size distribution so that high gravimetric and volumetric performances can be achieved in future for porous carbon materials. 3.5.

Heteroatom doping

In addition to the density, morphology, volumetric SSA and pore structure, the surface heteroatoms (e.g. O, N, B, S and P) of carbon materials also play an important role in their volumetric performances. It has been demonstrated that various heteroatoms could remarkably increase the capacitance of carbon materials by contributing additional pseudocapacitance through surface faradaic reactions.134 Heteroatoms providing a pair of electrons can significantly change the electron donor–acceptor characteristics of carbon materials, which accordingly gives rise to a pseudocapacitive reaction. As a promising method to improve the supercapacitive performance, the introduction of oxygen-containing surface functionalities into carbon frameworks has attracted considerable attention in recent years.44,58,71,122,173,174 Commonly, these are generated through carbonization and activation of oxygenrich precursors,175 electrochemical oxidation,176 oxidation in O2177 or HNO3176 and oxygen-plasma treatment.178 For instance, a high-performance carbonaceous material for supercapacitors has been obtained through one-step carbonization of an oxygenrich seaweed biopolymer.108 Although the obtained material showed a low SSA, a volumetric capacitance of up to 180 F cm 3 was achieved in an aqueous electrolyte as a result of oxygen present in the carbon network participating in pseudofaradaic charge-transfer reactions. The oxygen-containing functionalities formed in the carbon materials are usually acidic in nature, consequently introducing electron-acceptor properties into the carbon surface.179 In an aqueous solution, they can remarkably increase the specific capacitance of carbon materials by inducing redox reactions that contribute pseudocapacitance to the overall capacitance as follows:180 4CQO + H+ + e " 4C–OH

(8)

–COO + H + e " –COOH

(9)

4CQO + e " 4C–O

(10)

+

In addition, oxygen-containing functionalities can improve the wettability between electrode materials and aqueous electrolytes, resulting in an increase in the electrolyte-accessible surface area. However, they would be detrimental in organic electrolytes due to irreversible reactions between oxygen and electrolyte ions, giving rise to decomposition of the electrolyte, high self-discharge rates, increase in the internal resistance of the electrode and leakage current and thereby poor cycle life.177,181


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Moreover, the presence of oxygen-containing functionalities can diminish the electrical conductivity of the electrode materials and consequently worsen their electrochemical performance. Among various heteroatoms, nitrogen has been the most widely investigated heteroatom in supercapacitor applications over the past few years.182 Commonly, N-rich functional groups include pyrrolic N, pyridinic N and quaternary N (Fig. 9a). Pyrrolic N atoms, incorporated into five-membered heterocyclic rings, are bonded to two carbon atoms and donate two p-electrons to the p system. Pyridinic N refers to N atoms incorporated by substituting a carbon atom in a C6 ring and each N atom is bonded to two carbon atoms, contributing one p-electron to the aromatic p system. Quaternary N atoms, doped inside the graphitic carbon plane and bonded with three sp2 carbon atoms, generate a positive charge and electron-acceptor properties.35,183,184 The pyridinic and pyrrolic N are always located at the edge of the graphitic carbon plane, while quaternary N can be located both at the edge and within the graphene layers. Pyrrolic N is thermally unstable and tends to be gradually converted to pyridinic and quaternary N above 600 1C, while pyridinic N transforms into quaternary N above 800 1C during heat treatment due to the much higher thermal stability of quaternary N.183,185,186 It has been found that the nitrogen content and nitrogen species present in N-doped carbon materials can be tailored by controlling the synthetic conditions.184 Pyrrolic and pyridinic N are believed to be electrochemically active in an alkaline aqueous solution, contributing additional pseudocapacitance as pyridinic N exhibits a much larger binding energy difference at basal planes and pyrrolic N displays large binding energies at both basal planes and edges (Fig. 9b).187,188 A quaternary N incorporated into the graphitic carbon plane and bonded to three carbon atoms can effectively benefit electron transfer and significantly enhance the electrical conductivity of carbon materials.35,46,184,189 Thus, this should be a promising method for improving the specific capacitance of carbon materials through controllable introduction of pyridinic N at the edge of a graphitic carbon plane. In recent years, the introduction of nitrogen has been achieved through two primary approaches, i.e. post-treatment and in situ methods.35,46,135,190 In the former, carbon materials are treated with N-rich reagents (e.g. ammonia and urea),


nitrogen plasma treatment or CVD to incorporate nitrogencontaining functionalities. In the latter, a variety of N-enriched carbon precursors (polyaniline (PANI), polypyrrole (PPy), polyacrylonitrile, melamine, dicyandiamide and cyanamide) have been used to prepare nitrogen-containing carbon materials.1 To date, ammonia has been the most widely-used reagent for doping carbon materials with nitrogen. The amount of nitrogen doped into carbons depends both upon the ammoxidation temperature and the flow rate as well as the pressure of ammonia gas. However, nitrogen-containing functional groups formed by this method are usually unstable and the content of introduced nitrogen is relatively low, since nitrogen-containing functional groups can only be incorporated on the surface of carbon materials. In contrast, due to the easy and facile operation, the in situ synthesis method will be a promising and popular approach to synthesize nitrogen-doped carbon materials in the near future. Nitrogen-containing functional groups have recently been proven to enhance the wettability of electrodes and improve the electrical conductivity and capacitance performance since N-doping generally provides basic characteristics giving rise to electron-donor properties, reduced work-function, high surface energy and surface reactivity as well as high pseudocapacitance through additional faradaic redox reactions.179,187,191–193 For example, nitrogen-doped porous CNFs were synthesized through carbonization of PPy-coated CNFs and a high volumetric capacitance of 200 F cm 3 with high rate-capability and a volumetric energy density of B7 W h L 1 were obtained in an aqueous electrolyte.95 It is believed that nitrogen-containing functional groups provide pseudocapacitance through the following faradaic reactions:191,194,195 C*H–NH2 + 2OH " C*QNH + 2H2O + 2e

(11)

C*–NH2 + 2OH " C*–NHOH + H2O + 2e

(12)

where C* stands for the carbon network. However, an increase in the nitrogen content does not always lead to an increase in specific capacitance.196 The specific capacitance was found to be proportional to the nitrogen content in both acidic and alkaline electrolytes, whereas it was almost constant in an organic aprotic electrolyte, highlighting the important role of

Fig. 9 (a) Schematic types of N-doping of carbon. (b) The binding energies between potassium ions and N-configurations at basal planes and edges, which were calculated by first-principles density functional theory.187 Reproduced with permission.187



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protons in pseudocapacitive effects. Nevertheless, the relationship between nitrogen content, SSA and specific capacitance is still not fully understood. In addition, a very high nitrogen content will give rise to an adverse effect on the electrochemical performance due to the decomposition of the organic electrolytes.197 In addition, other heteroatoms, such as boron, phosphorus, sulfur and fluorine, could also introduce pseudocapacitance and increase the EDLC of carbon materials.154,198–200 Boron is electron-deficient with three valence electrons and can substitute carbon at the trigonal sites, giving rise to oxidation resistance and a shift in the Fermi level of the conduction band and correspondingly modifying the electronic structure of carbon.201 Similar to nitrogen-doping, boron-doping could also improve electrical conductivity, increase electrochemical activity and produce additional functional groups on the carbon surface, thus greatly enhancing the electrochemical performances of carbon materials. In addition, it has been demonstrated that boron could improve the thermal stability and increase the degree of disorder of carbon materials. Moreover, boron-doping could also increase the hydrophobicity and wettability of carbon materials in organic electrolytes, resulting in an increase in the surface area accessible to electrolyte ions.202 Phosphorus could enhance the charge delocalization of carbon atoms and lead to a carbon morphology with many open edged sites.203 Sulfur, similar to oxygen, nitrogen and phosphorous, has lone-pair electrons and acts as an electron donor, giving rise to pseudocapacitance in carbon. Sulfur-doping could modify the electronic structure of carbon, due to the sulfur atoms affecting p electrons in the carbon lattice, resulting in a significant improvement in electrical conductivity, surface reactivity and wettability.99 The electronic structure of carbon can be tuned depending upon the amount of doped sulfur atoms. Theoretical studies indicate that the band gap of graphene is opened after sulfur-doping, thus the doped sheet can be a small-bandgap semiconductor or it could exhibit enhanced metallic properties compared to the non-doped graphene sheets.204 Fluorine may significantly improve the electrochemical performance of carbon electrodes thanks to the increased electrical conductivity as a consequence of the semiionic bonding features in non-aqueous electrolytes and enhanced polarization from the highly electronegative fluorine functional groups as well as the refinement of pore structures/surfaces.154 In summary, incorporating heteroatoms into carbon materials has been demonstrated to be an effective approach to increase the electrochemical performances of carbon materials. However, it should be noted that the side effects of heteroatoms should be carefully considered such as irreversible reactions with electrolyte ions, decomposition of electrolyte, high selfdischarge rate, and enhanced internal resistance of the electrode. In addition, the mechanism of how the introduced heteroatoms affect the performance of supercapacitors has not been fully understood. 3.6.

Electrical conductivity

The electrical conductivity of carbon materials is also crucial with respect to their electrochemical performances. It is closely related to the internal resistance, rate capability, power


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Fig. 10 The trade-off relationship between electrical conductivity and pore structure.

delivery, and energy efficiency of carbon materials. There is usually a trade-off relationship between electrical conductivity and pore structure (Fig. 10). On the one hand, a highly porous structure can provide a large SSA and favorable diffusion channels for electrolyte ions but usually gives rise to poor electrical conductivity due to the low graphitization degree, which is unfavorable for the transfer of electrons. On the other hand, high electrical conductivity is usually associated with a high graphitization degree, resulting in a small SSA and an undeveloped pore structure due to the collapse of the pore structure, which will suppress the transport and diffusion of electrolyte ions. Therefore, we should rationally optimize the transport/diffusion of electrolyte ions and transfer of electrons through nano-engineering or pore structure design of carbon materials to retain the balance between electrical conductivity and pore structure in order to maximize the electrochemical performance. As is commonly known, 1D carbon nanomaterials with a large aspect ratio such as CNTs and CNFs usually have excellent electrical conductivity. It has been reported that the volumetric performance and rate capability of graphene could be significantly enhanced after hybridizing with CNTs to form graphene composites.5,65,69 Therefore, the construction of 3D structured carbon nanocomposites through the introduction of 1D carbon nanomaterials (CNTs and CNFs) into other dimensional carbon materials should be a highly promising approach to achieve a balance between electrical conductivity and pore structure, resulting in the improvement of both gravimetric and volumetric capacitance. There are some reviews in which the effect of electrical conductivity on the performances of electrode materials has been discussed in detail,1,205,206 so this will not be discussed further in this section to avoid unnecessary overlap.

4. Methods to improve the volumetric performances of carbon materials During the past few decades, numerous efforts have been made to improve the energy density without sacrificing the high power density and long cycle life while using environmentally benign materials in the field of supercapacitors. Various porous carbon materials with high SSAs are most widely investigated as


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electrode materials for supercapacitors. Even though they exhibit high gravimetric capacitance and thus high gravimetric energy or power density, the volumetric performance of porous carbons is relatively inferior due to their low bulk density (o0.8 g cm 3). Notably, low packing density is responsible for an increased amount of electrolyte being needed to fill the large empty spaces in the electrode, leading to the device having a higher weight as a whole. As a consequence, the gravimetric energy density with regard to the entire supercapacitor cell will be significantly diminished, greatly inhibiting large-scale commercial application. Therefore, it is imperative that novel carbon materials with high volumetric performances are developed to meet demands for compact and portable energy storage devices from an application standpoint. More recently, several promising approaches have gradually emerged and considerable significant research breakthroughs have been made with respect to improving the volumetric performance of carbon materials.63,66,67,112,122 In this section, we will give an in-depth summary of significant research breakthroughs achieved through various promising approaches in recent years. 4.1.

Mechanical compression

One of the most straightforward methods to densify carbon materials is mechanical compression. In mechanics, compression is the application of balanced inward forces to different points on a material or structure to reduce its size in one or more directions. When put under compression, every material will suffer some permanent or transient deformation (even if imperceptible), which causes the average relative positions of its atoms and molecules to change. Porous carbon materials are highly porous and compressible. Therefore, it is possible to employ sufficient force to compress carbon materials in order to adjust their physical structure (Fig. 11). Within this context, mechanical compression is the most commonly and easily used method to densify carbon materials,86,112,120,126,207,208 which are typically compressed in a mold using a hydraulic press. For instance, Ruoff and co-workers employed simple mechanical compression to densify aMEGO.112 After compression with a force of 25 t, the density of the aMEGO sample increased to 0.75 g cm 3 with a SSA of 707 m2 g 1 compared to that of the uncompressed sample (0.34 g cm 3), leading to a significant improvement in the volumetric capacitance from 54 to 110 F cm 3 in an organic electrolyte.112 As yet another example, Xu and coworkers reported a 3D free-standing holey graphene framework (HGF) film with a hierarchical porous structure and

Fig. 11 Schematic of mechanical compression.



efficient ion transport shortcuts but a high packing density through mechanical compression of hydrothermally synthesized HGF (Fig. 12).126 The compressed HGF films exhibited a specific surface area of B810 m2 g 1, close to that of the uncompressed HGFs (B830 m2 g 1). With a large ionaccessible surface area, efficient electron and ion transport pathways as well as a high packing density, the compressed HGF films could deliver a gravimetric capacitance of 298 F g 1 and a volumetric capacitance of 212 F cm 3 in an organic electrolyte. More importantly, the fully packaged device stack achieved gravimetric and volumetric energy densities of 35 W h kg 1 and 49 W h L 1, respectively, approaching those of lead acid batteries. In order to achieve high volumetric energy and power densities, Zhou and co-workers employed a mechanical densification method to densify aligned CNTs (A-CNTs).86 In this method, A-CNT forests were initially placed in a specially designed sample holder, whose height was the same as the height of the A-CNT forest. The A-CNT forest was densified along one direction to a fixed amount using a mechanical bar and then another mechanical bar in the orthogonal direction was used to press the A-CNT forest to the final density (Fig. 13). Through varying the inter-CNT distance in the densification process, A-CNT forests with different volume fractions can be achieved. As a consequence, the volumetric fraction of A-CNTs could be precisely tuned from 1% to more than 40%. Compared with other methods, the mechanical compression method has the following merits: (i) it can be easily and widely used to densify all carbon materials, ranging from activated carbons, templated carbons and carbon xerogels to recently-emerged novel nanocarbons such as CNTs and graphene. (ii) The density of carbon materials can be precisely tuned by adjusting the pressure. (iii) This method can be easily employed in large scale commercial applications. However, this method generally needs high pressure and specific equipment. 4.2.

Capillary compression

Unlike mechanical compression, which relies on an external force, compression by capillary pressure through controlled evaporation of solvents is a novel and effective liquid-mediated method to increase the packing density of carbon materials. Porous yet densely-packed carbon electrodes with a high ionaccessible surface area and low ion transport resistance are crucial to high-density electrochemical capacitive energy storage devices. In order to tackle the low packing density of graphene materials, Yang and co-workers recently reported that RGO hydrogel films with a metastable and adaptive pore structure can be compressed irreversibly by capillary pressure to increase the packing density through controlled removal of volatile solvent trapped in the gel (Fig. 14).67 This simple approach, taking advantage of RGO’s intrinsic microcorrugated 2D configuration and self-assembly behavior, enabled subnanometer scale integration of graphene sheets with electrolytes to form highly compact carbon electrodes with a continuous ion transport network. By changing the ratio of volatile and nonvolatile liquids, the packing density of the RGO flexible


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Fig. 12 (a) Schematic of the preparation process of HGFs and HGF films. (b) An image showing HGFs before and after mechanical compression with the flexibility of the compressed HGF film shown in the inset. (c) Cross-sectional scanning electron microscopy (SEM) image of the compressed HGF film. (d) Transmission electron microscopy (TEM) image of holey graphene sheets in HGFs. (e) Specific capacitances versus different current densities. (f) Cycling stability of a HGF film. (g) Ragone plots of volumetric energy density versus volumetric power density. Reproduced with permission.126

Fig. 13

Optical images, schematic of mechanical densification process, and SEM images of 1% Vf and 40% Vf A-CNTs. Reproduced with permission.86



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films was controlled from 0.13 to 1.33 g cm 3, leading to exceptionally high volumetric capacitance and energy density. More importantly, the fabrication of RGO gel films and subsequent compression was essentially compatible with the traditional cost-effective paper-making process and could be readily scaled up. With regard to the capillary compression method, the zipping effect of liquids is also employed to densify carbon materials during the evaporation process. When liquids are introduced into carbon materials, the surface tension of the liquids and the strong van der Waals interactions effectively zip the carbon materials during the evaporation and drying process. Within this context, Futaba et al. reported the fabrication of densely packed and aligned SWNT materials through adopting the zipping effect of liquids to draw tubes together in a pioneering work (Fig. 15).87 This process could be completed in two steps. First, the SWNTs were drawn together through liquid capillary forces and the forest was decreased by B20% in the lateral dimension. Then, van der Waals forces adhered the tubes near to ideal packing, transforming a forest into a small and rigid single body during liquid evaporation (Fig. 15a). As a result, a 4.5-fold decrease in the two lateral dimensions but no detectable change in the height could be observed, leading to a B20-fold increase in mass density (Fig. 15c). It can be noted that collapses were observed with all tested liquids, including water, alcohols, acetone and hexane, demonstrating the generality of this approach. More importantly, the densely packed SWNT


solid retains the intrinsic properties of SWNTs such as high SSA, flexibility and electrical conductivity. Apart from these, the shape could be engineered by controlling the fabrication process, which is beneficial with regard to applications from 100% binder-free SWNT electrodes for supercapacitors to flexible heaters. It is believed that the alignment of the as-grown forest is of significant importance in triggering efficient liquid-induced collapse (Fig. 15b), an understandable point because the alignment provides the ideal conditions for optimum van der Waals overlap, resulting in well-ordered packing of tubes and structural soundness.87 This approach represents substantial progress towards producing macroscopic-scale high-density SWNT materials engineered for both shape and structure and opens diverse functionality that is advantageous for numerous applications, thus it can be extended further to make new and sophisticated CNT forms using more complex SWNT forests or introducing different materials into the system. Following the seminal work by Futaba and coworkers, Jiang and Lin demonstrated a modified method involving a twostage, self-aligned liquid densification process combining mechanical bending and liquid densification for CNT forests for supercapacitor applications with enhanced energy density (Fig. 15d).209 In this process, the CNT film was gently bent by a mechanical force to the position marked ‘‘2’’ in the figure using an elastomer roller to generate a downward tendency in the CNTs. Subsequently, the structure was dipped into liquid and then dried in air. When liquids were introduced into the sparse

Fig. 14 (a) An image showing the flexibility of the film. (b and c) SEM images of cross sections of the obtained EM-RGO films containing (b) 78.9 vol% and (c) 27.2 vol% of H2SO4. (d) Volumetric capacitance and (e) energy density as a function of the areal mass loading of EM-RGO film (r = 1.25 g cm 3) and the dried RGO film (r = 1.49 g cm 3). Reproduced with permission.67



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Fig. 15 (a) SEM image of SWNT-forest structural collapse from a single drop of liquid.87 (b) Schematic of the collapse of the aligned low-density as-grown forest (above) to the highly densely packed SWNT solid (below).87 (c) Overlaid images illustrating the decrease in lateral dimensions before (grey) and after (black) collapse.87 (d) Schematic of the two-stage, self-aligned vertical densification process for as grown CNT forests.209 Reproduced with permission.87,209

CNT forests, the tubes were drawn together through liquid capillary forces; then, strong van der Waals forces adhered the CNTs near to ideal packing, transforming a sparse forest into a small and rigid ‘‘CNT solid’’ during liquid evaporation. The initial deformation of the CNT forest as result of the mechanical force in step 1 helped the surface tension force in step 2 to shrink CNTs in the vertical direction. In this case, the as-grown CNT forest on top of a silicon substrate was reduced from 320 to 21 mm and the densified CNT forests exhibited an enhanced volumetric capacitance from 1.07 to 10.7 F cm 3. Interestingly, the densified CNTs maintained their film continuity and self-alignment configuration but the alignment changed into the lateral direction of the growth substrate, while the CNT silicon bottom contacts were preserved. In contrast to the densification process mentioned above, the capillary compression method has the following advantages: (1) no holding covers are necessary in the densification process. (2) It can achieve self-aligned vertical densification. (3) It is versatile and can be widely used to densify various carbon materials with low packing density. (4) Breakage of as-grown CNT-substrate contacts can be effectively avoided during the densification process. 4.3.

Self-assembly

Self-assembly is a type of process in which a disordered system of pre-existing components, either separate or linked, spontaneously forms an organized structure or pattern as a


consequence of specific, local interactions (usually noncovalent) among the components themselves without human intervention.210 It can occur with components whose sizes range from molecular to macroscopic as long as appropriate conditions are satisfied. Self-assembly processes commonly exist throughout nature and technology. Within this context, self-assembly has been another widely used method to densify carbon materials during recent years. For instance, Lee and coworkers reported the preparation of MWNT thin films through layer-by-layer (LBL) assembly of surface functionalized MWNTs (Fig. 16a and b).88 In this process, negatively and positively charged MWNTs were functionalized with carboxylic acid and amine groups, respectively, allowing the incorporation of MWNTs into highly tunable thin films via the LBL technique (Fig. 16c). It is believed that the surface charge density of MWNTs could play an important role in controlling film thickness and roughness by adjusting the charge reversal mechanism as well as the degree of interpenetration. In this case, the thickness of the MWNT films could range from several tens of nanometers to several micrometers. As a result, volumetric specific capacitances of 132 and 180 F cm 3 were obtained for additive-free and densely packed films in aqueous and organic electrolytes, respectively,88,89 due to the high CNT densities and well developed nanopores in the LBL MWNT thin films, making these MWNTs assemblies promising for supercapacitor electrodes. In addition, the authors believed that these novel MWNT thin films, which fully utilized the


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Fig. 16 (a) Layer-by-layer assembled MWNT thin film with positively and negatively charged MWNTs.88 (b) Digital image and (c) SEM cross-sectional of representative MWNT electrodes on ITO-coated glass slides.89 (d) Synthesis scheme depicting the reaction between a graphene oxide (GO) layer and a CNT and a view of the fabricated r[GO–CNT] film.65 (e) Cross-sectional SEM of a r[GO–CNT] film.65 Reproduced with permission.65,88,89

intrinsic superior properties of functionalized MWNTs and the precise control of the LBL system, could be used to design ideal electrode materials for fuel cells, photoelectrochemical cells, batteries, supercapacitors and sensors.88 In the subsequent work by the same group, they demonstrated the fabrication of RGO sheets separated by MWNTs through alternate LBL assembly for electrochemical microcapacitor applications.131 In this case, submicron thin films of amine-functionalized MWCNTs and RGO derived from GO were shown to be cross-linked with amide bonds with high packing densities of B70%. As a consequence, these carbon-only electrodes were found to deliver a high volumetric capacitance of up to 160 F cm 3 in an acidic electrolyte. Alternatively, Jung et al. synthesized a graphene/CNT composite with a lamellar structure by an amidation-reaction-assisted selfassembly followed by filtration (Fig. 16d).65 During this process, amine-functionalized CNTs were bonded with NHS-activated GO by amidation, preventing restacking between graphene layers and the agglomeration of CNTs (Fig. 16e). In addition, it was found that CNTs were attached along the edges and the basal plane of graphene. These CNTs not only acted as spacers to increase the electrolyte-accessible SSA but also provided excellent 3D paths for electrons. The average density was calculated as 1.5 g cm 3, almost 70% that of graphite. Thus, the volumetric capacitance of the composite was as high as 165 F cm 3 in an organic electrolyte, considerably superior to other carbon-based electrodes. Although the LBL assembly process enables the development of conformal thin coatings with highly controllable thicknesses ranging from the nanometer to the micrometer scale, it is very time-consuming and tedious. In addition, the thickness of the resultant films is commonly limited to several micrometers. In order to address the drawbacks of LBL assembly, other promising alternative strategies have been successfully developed during recent years.124,127,130 For instance, Wu and coworkers prepared graphene film through vacuum filtration of


an RGO dispersion.124 The prepared compact graphene film has a layered structure with a density of 1.53 g cm 3, leading to a volumetric capacitance of 87 F cm 3 in 1 M H2SO4. Activated microwave exfoliated graphene oxide (aMEGO) can also be densified by vacuum-assisted filtration. Zhou’s group synthesized an aligned nano-porous aMEGO film with a thickness of 90–100 mm through this method, resulting in a tremendous bulk density of 1.15 g cm 3.127 In this case, the aMEGO layers were pulled towards the surface of a filter paper and these layers of nano-porous aMEGO stacked successively on top of each other. Interestingly, the aligned aMEGO film provided efficient packing of the sheets while preserving the concentration and distribution of nano-sized pores. Consequently, a very high volumetric capacitance of up to 177 F g 1 was achieved in an organic electrolyte, which was attributed to its highly dense and well-ordered structure. Following the same synthetic approach, very recently, Byon et al. demonstrated a self-standing, binder-free functionalized MWNT/graphene film with a hierarchical structure, a thickness of up to tens of micrometers and a relatively high density through vacuum filtration of a mixed dispersion of negatively charged functionalized MWNTs and GO followed by heat treatment.130 The resulting film showed a pillared structure composed of parallel and densely-packed graphene sheets with uniformly interspersed functionalized MWNTs, resulting in an unusually high mass density of 1.94 g cm 3. As a consequence, the composite film delivered a high volumetric capacitance (307 F cm 3) and energy density (450 W h L 1) in an organic electrolyte. In addition to carbon/carbon composites, other types of carbonbased composites (such as carbon/metal oxides, carbon/polymers and carbon/metal carbides) can also be prepared through vacuum filtration.70,124 For example, Wu and coworkers synthesized selfstanding and highly flexible composite films of graphene/PANI through vacuum filtration of mixed dispersions of RGO and PANI nanofibers.124 It has been found that PANI nanofibers were sandwiched between graphene layers with interspaces between


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graphene layers of 10–200 nm. Despite the much lower density (0.76 g cm 3) compared to graphene film, the composite film achieved a much higher volumetric capacitance of 160 F cm 3 due to the synergistic effect of both components. More recently, Zhao and coworkers proposed the fabrication of a flexible and sandwich-like MXene/CNT composite film through alternating vacuum-assisted filtration of MXene and CNT dispersions.70 In this case, the resulting composite film exhibited a high density of 2.9 g cm 3 and high electrical conductivity (385 S cm 1) as well as excellent volumetric capacitance (390 F cm 3). In addition to the facile and simple operation as well as the low cost, the vacuum filtration method avoids the use of high temperature and organic solvents and allows for control of the thickness, density and composition of resultant films by changing the filtration volume and dispersion composition, providing a facile synthesis of multicomponent carbon-based nanostructures with tunable properties. 4.4.

Construction of dense carbon assemblies

As is commonly known, graphene sheets as building blocks can form novel nanostructures and can even directly form 3D macroform materials with desired properties through a bottom-up assembly approach.64,211–214 To date, most graphene assemblies formed from interlinked graphene sheets have been demonstrated to be promising electrode materials due to their high SSA, excellent electrical conductivity and open ion pathways.212,215 Unfortunately, they usually possess a low packing density with plenty of empty space, similar to other carbon nanomaterials, giving rise to inferior volumetric performances. Therefore, one can expect to obtain a high volumetric capacitance through the synthesis of highly dense carbon assemblies on the premise of preserving a high porosity. Nevertheless, achieving highly dense graphene assemblies with a high porosity is still a significant challenge with respect to preparing novel carbon nanomaterials with high volumetric performances. Evaporation-induced drying is a viable approach to prepare highly dense graphene assemblies due to its facile operation, low energy, low cost and easy industrialization. More recently, Tao and coworkers reported the synthesis of a highly dense but porous graphene macroassembly (HPGM) constructed of compactly interlinked graphene nanosheets through evaporationinduced drying of a graphene hydrogel (Fig. 17).64 The HPGM

Fig. 17

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assembly balances two opposing characteristics, i.e. high density and high porosity. The as-prepared HPGM has a porous microstructure with a SSA of 370 m2 g 1 and a large density of up to 1.58 g cm 3. As a result, it achieves a volumetric capacitance of up to 376 F cm 3 with a maximum energy density as high as 13.1 W h L 1 in aqueous electrolytes. More importantly, carbon is conductive and moldable and thus it can be directly used as an electrode without any additives. The drying process is believed to be crucial to the formation of HPGM. During evaporation-induced drying, the evaporation of water exerts a pulling force on graphene layers, leading to shrinkage of the 3D network. As a consequence, the robust but elastic 3D network constructed of interlinked and flexible graphene sheets shrinks but maintains a porous structure, which is inherited from the parent hydrogel during the evaporation of water. Finally, an HPGM consisting of curved and compactly packed graphene sheets with interconnected small cylindrical pores is formed.64 Fabrication of dense but porous graphene films has been a very hot topic in recent years in the field of energy storage. Recently, Yoon et al. reported a simple method to fabricate a highly dense and vertically-aligned graphene film electrodes through simple hand-rolling and cutting processes (Fig. 18).63 During this process, by virtue of the isotropic-nematic transition of GO, the authors firstly rolled the as-made GO film with a little solvent on its surface to facilitate good adhesion between film surfaces. Then, they cut the rolled GO film at 40 1C using a rotary cryomicrotome before drying, which was helpful in avoiding damage to the cross-section during cutting. Due to the vertically aligned and open-edged graphene structure, the aligned RGO film displayed a high packing density (1.33 g cm 3) and a high volumetric capacitance of 171 F cm 3 as well as a high energy density of 7.43 W h L 1 in an aqueous electrolyte. Han and coworkers reported a scalable method to synthesize holey graphene in a single step without using any catalysts or special chemicals.125 They firstly heated graphene materials in air for a given period of time, followed by vacuum filtration with subsequent drying and pressing. The holey graphene film with a high mass density of 1.2 g cm 3 displayed a high volumetric capacitance of 54 F cm 3 and an energy density of B12 W h L 1 in an organic electrolyte. Alternatively, Yan and coworkers developed a novel strategy to synthesize rationally functionalized graphene sheets through

Schematic of the formation of graphene-based 3D porous macroforms with different drying processes. Reproduced with permission.64



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Fig. 18 (a) Schematic of the fabrication of VArGO electrodes. (b) Electrolyte ions could not easily enter because of the very close stacking of the horizontally aligned RGO film. (c) Electrolyte ions homogeneously and directly diffused into the VArGO sheets. (d and e) Cross sectional morphologies with inset images of an RGO film (d) and VArGO electrodes (e). Reproduced with permission.63

Fig. 19 Schematic of the fabrication of functionalized graphene nanosheets.122 Reproduced with permission.122

low-temperature thermal reduction of GO with a slow heating rate using an Mg(OH)2 template (Fig. 19).122 Owing to its dented sheet with a high surface area, numerous oxygen-rich functional groups and low pore volume, the as-prepared graphene materials yielded ultrahigh gravimetric (456 F g 1) and volumetric capacitances (470 F cm 3), almost 3.7 and 3.3 times higher than those of RGO, respectively. More importantly, the assembled symmetric device exhibited an ultrahigh volumetric energy density of 27.2 W h L 1, which was among the highest values for carbon materials in aqueous electrolytes. It is believed that such a graphene material with an impressive electrochemical performance and a green and facile fabrication method should be of significant importance in both academia and industry. 4.5.

Incorporation of pseudocapacitive components

As is commonly known, supercapacitors can be divided into two categories according to their energy storage mechanism,


i.e. EDLCs and pseudocapacitors.1 In the former, capacitance mainly comes from pure electrostatic charges accumulated at the electrode/electrolyte interface and electrodes consist of various porous carbon materials. In the latter, fast and reversible redox reactions or faradaic processes of the electroactive species take place on the surface of the electrode and electrodes consist of various transition metal oxides and electrically conducting polymers. Although a variety of porous carbon materials are widely used as electrode materials for supercapacitors, the specific capacitance of pseudocapacitive materials can be 10–100 times higher than those of carbon materials. In addition, the packing density of transition metal oxides/ hydroxides is generally much higher than that of carbon materials. Within this context, the volumetric capacitance of pseudocapacitive materials is usually much higher than that of carbon materials. However, they commonly have poor electrical conductivity, which seriously plagues their power delivery. Therefore, it is feasible to increase the volumetric performance


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of porous carbon materials through incorporation of pseudocapacitive materials, making full use of the merits of different components. During recent years, various pseudocapacitive materials, such as MnO2, MoOx, Ni(OH)2, VN, Nb2O5 and PANI, have been introduced into porous carbon materials to enhance their volumetric capacitance.138–140,144,148,216–221 For instance, Fischer and coworkers demonstrated the incorporation of homogeneous, nanoscale MnO2 deposits into the tortuous interiors of carbon nanofoams through self-limiting reactions with permanganate.216 The MnO2 coating contributed additional capacitance to the carbon nanofoam while maintaining the favorable high-rate electrochemical performance inherent to the ultraporous carbon structure of the nanofoam. As a result, the composite produced a high volumetric capacitance of 90 F cm 3. Following the same synthetic approach, Ruoff et al. recently reported the deposition of MnO2 nanoparticles (2–3 nm) onto a porous aMEGO carbon scaffold.144 The resultant 3D composite architecture facilitated transport of both electrolyte ions and electrons to the electrode surface. In addition, the uniform size and spatial distribution of MnO2 nanoparticles also enhanced their electrochemical utilization, giving the composite a high specific capacitance (640 F cm 3), good rate capability and long cycle life. More recently, Li and coworkers fabricated a flexible solid-state asymmetric supercapacitor based on bendable film electrodes with a 3D expressway-like graphene architecture and Ni(OH)2 nanoparticles intercalated in between the densely stacked

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graphene through an extended filtration-assisted method.217 In the composite, Ni(OH)2 nanoplates acted both as effective space inhibitors to prevent graphene restacking and pseudocapacitors to improve the overall capacitance, while the highly conductive graphene nanosheets served as an expressway for efficient electronic transportation. Moreover, the graphene nanosheets also maintained the overall integrity of the film and offered sufficient flexibility. As a consequence, the 3D electrode exhibited a superior supercapacitive performance, including high gravimetric capacitance (573 F g 1), high volumetric capacitance (655 F cm 3), excellent rate capability and superior cycling stability. Electrically conducting polymers can also be introduced into carbon materials to improve the volumetric capacitance due to their super-high specific capacitance and good electrical conductivity.148 For example, Hyder’s group demonstrated thin film electrodes of PANI-NFs and functionalized CNTs created by LBL assembly for high-power and high-energy supercapacitors (Fig. 20a).141 The composite film was composed of a nanoscale interpenetrating network structure with well-developed nanopores that could achieve a high volumetric capacitance (238 F cm 3) and a high volumetric energy density (220 W h L 1). Wu and coworkers prepared RGO/PANI composite films through vacuum filtration of mixed dispersions of both components.124 The flexible composite film had a layered structure with PANI nanofibers (PANI-NFs) sandwiched between RGO layers (Fig. 20b and c), resulting in a large volumetric capacitance of 160 F cm 3. More recently, Xiong et al. reported the fabrication

Fig. 20 (a) Schematic of LBL assembly of the PANI/CNT thin film.141 (b and c) Cross-sectional SEM images of the graphene/PANI film prepared by vacuum filtration.124 (d) Schematic of the synthesis process of the ternary composite.222 (e) Schematic of the synthesis and (f) volumetric capacitance of the high-density graphene/PANI composite monoliths.148 Reproduced with permission.124,141,148,222



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of a nanometer-thick PANI film conformal coating on graphitic petals grown on a conductive carbon cloth through electropolymerization of aniline monomers (Fig. 20d).222 The hybrid electrodes yielded a maximum capacitance of 2000 F g 1 (based on PANI mass), a high rate capability and a large volumetric capacitance of 230 F cm 3. More recently, Prof. Yang and coworkers reported a compact graphene/PANI monolith with a density of over 1.5 g cm 3, achieved by controlling ANI monomer adsorption in graphene monoliths, confining its polymerization to the graphene surface, followed by shrinkage of the graphene network (Fig. 20e).148 Due to the high density of the monolith and the pseudocapacitance of PANI, a volumetric capacitance of 802 F cm 3 was achieved, which was significantly higher than those of other carbon materials and conducting polymers. Notably, taking whole electrode components into consideration, a high volumetric capacitance of 590 F cm 3 could be achieved with much less dependence on electrode thickness than for layered electrodes. However, the rate capability is still far from being at a satisfactory level for practical application due to the intrinsic poor rate performance of conducting polymers. Since oxygen-containing functional groups and some heteroatoms (such as N, S, B and P) can contribute tremendous pseudocapacitance through redox reactions during the charge/ discharge process, they have attracted considerable attention and have been introduced into carbon materials to improve capacitive performance in recent years. For instance, Tian et al. prepared 3D functionalized multilayered graphene with controllable interconnected pores and oxygen-containing surface functional groups by combining hydrochloric acid-assisted ultrasonic exfoliation and a thermal reduction approach.134 Thanks to the strong synergistic effect of the interconnected pores and surface functional groups, the resultant graphene materials showed an amazingly high gravimetric (508 F g 1) and volumetric capacitance (218 F cm 3) as well as a high volumetric energy density (28 W h L 1). Electrochemical oxidization (EO) treatment is a facile and suitable alternative to generate oxygencontaining functional groups on the surface of carbon materials. For example, Xiong and coworkers investigated the electrochemical performances of graphitic petal electrodes before and after EO treatment; these were prepared using microwave chemical vapor deposition (CVD) and patterned by optical lithography.223 They found that the volumetric capacitance was improved by two orders of magnitude from 2.1 to 270 F cm 3 after EO treatment while maintaining high charge/discharge rates. In addition, the EO-treated electrode exhibited a high volumetric energy density of up to 10 W h L 1 with good cycling stability. The intriguing volumetric performance could be attributed to the unique structure of the graphitic petals: (i) the sharp graphitic petal edges enhanced the local electric field, allowing more charge to be stored along the edges and also accelerating ion diffusion because of low energy barriers. (ii) The EO treatment only improved the surface wettability of the electrode to the electrolyte but also provided excellent pseudocapacitance from the oxygen-rich functional groups. However, rapid degradation of capacitance and increased series resistance was also observed as a result of decomposition of organic electrolytes due



to the presence of numerous surface oxygen-rich functional groups. Singhal et al. recently demonstrated a novel and simple method of incorporating pseudocapacitive surface functionalities on free-standing CNFs using sodium chloride.224 In this study, the presence of NaCl led to increased oxygen on the surface of CNFs, particularly in the form of carboxyl groups, which then facilitated the adsorption of sulfur functional groups during acid treatment. As a consequence, despite a low surface area (24 m2 g 1), the synthesized material exhibited high gravimetric (204 F g 1) and volumetric capacitances (63 F cm 3), attributed to the surface functional groups participating in the redox reactions. Nitrogen atoms not only enhanced the surface wettability to electrolytes but also improved the conductivity of the electrode materials. Within this context, Hulicova-Jurcakova and coworkers reported nitrogen-enriched nonporous carbon materials prepared with an ammonia treatment of N-containing carbon derived from melamine.191 Due to the small volume of the carbon materials, a high volumetric capacitance of 152–280 F cm 3 was achieved in aqueous electrolytes. More recently, Yan and coworkers demonstrated a facile approach to synthesize porous disordered carbon layers as energy storage units coating graphene sheets to form interconnected sandwiched graphene/carbon (SGC) frameworks through one-step pyrolysis of a mixture of GO/PANI composite and KOH (Fig. 21).21 As effective energy storage units, these porous carbon layers played an important role in enhancing electrochemical performances. The porous carbon material obtained exhibited a high specific surface area (2927 m2 g 1), hierarchical interconnected pores, moderate pore volume (1.78 cm3 g 1), short ion diffusion paths and high nitrogen level (6 at%). Consequently, it displayed unparalleled gravimetric (481 F g 1) and outstanding volumetric capacitance (212 F cm 3) as well as a high volumetric energy density (11.3 W h L 1) in an aqueous electrolyte. In summary, although considerable achievements have been obtained through incorporation of pseudocapacitive materials, functional groups and/or heteroatoms into porous carbon materials to improve the volumetric performance of carbon materials during the past few years, some significant obstacles have to be conquered. (i) The introduction of transition metal oxides will unavoidably diminish the overall electrical conductivity of the system owing to their intrinsically poor conductivity, consequently leading to poor rate capability. There should therefore be a compromise between electrical conductivity and the loading of metal oxides to achieve a high volumetric capacitance without increasing the charge-transfer resistance or blocking the transport of electrolyte ions within the composite electrode. In addition, intimate contact between the carbon materials and the current collector is also a promising solution to minimizing the interfacial resistance. (ii) Poor cycling stability of pseudocapacitive materials is still a major challenge due to large volume changes or swelling/shrinking of metal oxides and polymers during long-term charge/discharge processes. Therefore, we should take advantage of the synergistic effect between the individual components in the composites to buffer the large volume change or swelling/shrinking of


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Fig. 21

Schematic of the preparation of SGC samples.21 Reproduced with permission.21

metal oxides and polymers during long-term charge/discharge processes to protect these electroactive materials from mechanical degradation, thereby improving the cycling stability. (iii) The agglomeration of pseudocapacitive materials and inferior interfacial contact between pseudocapacitive materials and carbon substrates should be efficiently tackled. Thus, an important approach would be to synthesize nanomaterials or well-designed nanostructured materials because nanostructured materials possessing a high specific surface area can provide short transport/diffusion path lengths for both ions and electrons, greatly mitigating the agglomeration of pseudocapacitive materials and significantly increasing the interfacial area between metal oxides/polymers and carbon materials, resulting in faster kinetics and excellent cycling stability. (iv) Oxygen-rich functional groups are detrimental in organic electrolytes owing to irreversible reactions between oxygen and electrolyte ions, which cause decomposition of electrolyte and high self-discharge rates and leakage current and thereby an inferior cycle life. 4.6.

Review

Other methods

In addition to the methods mentioned above, some other promising approaches have also been proposed to tackle the low volumetric performances of carbon materials in recent years. The hydrothermal process is a widely used method to synthesize materials in a high-temperature aqueous solution at high vapor pressures using autoclaves. Fused silica capillary columns are flexible with good high-temperature and pressure tolerance and thus are commonly used for chromatographic separation.5 Enlightened by this, Yu and coworkers recently developed a scalable method to continuously produce hierarchically structured carbon microfibers using a fused silica capillary column as a hydrothermal microreactor (Fig. 22a).5 The microfibers, which were made of interconnected networks of aligned SWNTs with interposed nitrogen-doped RGO sheets, have a mesoporous structure with a large surface area (396 m2 g 1) and high electrical conductivity (102 S cm 1). During the synthesis process, the authors took advantage of the following synergistic effects. First, GO acted as a good


surfactant to disperse SWNTs, while SWNTs hindered the restacking of GO30, resulting in a homogeneous aqueous suspension. Second, heteroatom doping not only improved surface conductivity and wettability but also induced high pseudocapacitance in carbon nanomaterials. Finally, ethylenediamine with its two –NH2 end groups could function as a molecular end-anchoring reagent to effectively bind acidoxidized SWNTs and GO to create 3D pillared vertically aligned architectures, oriented along the fiber length (Fig. 22d–f) by shear forces while flowing through the fused-silica capillary column. As a result, the resultant fibers showed a volumetric capacitance as high as 305 F cm 3 and the all-solid-state microsupercapacitors exhibited a volumetric energy density of 6.3 W h L 1, comparable to that of 4 V–500 mA h thin-film lithium batteries, while maintaining a high power density and a long cycle life. The packing density of graphene can also be improved through a precipitation-assisted method. Following this strategy, Li’s group recently prepared RGO with a high apparent density of 1.40 g cm 3 (Fig. 23).123 In this process, the low pressure created by the decomposition of the functional groups into gases was not believed to be enough to overcome the van der Waals interactions between the graphene layers, resulting in restacking between graphene layers during thermal heating. On the other hand, the growth of ZnO during heat treatment might hinder the exfoliation of the graphene layer. As a result, the volumetric capacitance of RGO could reach 255 F cm 3 along with excellent cycle stability. This enhanced electrochemical performance could be attributed to its unique structure and high density. Considering the high density and the excellent electrochemical performance, the as-prepared RGO will be a promising candidate for further application in compact energy storage devices. Incorporation of other non-carbon materials without pseudocapacitive behavior has also been proposed recently to improve the volumetric capacitance of carbon materials.70 Unlike other materials, MXenes are a relatively young class of 2D solids produced by selective etching of the A-group layers from the MAX phases and have already been proved to be


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Fig. 22 (a) Schematic of the synthesis of carbon hybrid microfibers. (b) Image of the as-prepared fibers collected in water. (c) A dry fiber with a diameter of B50 mm and a length of B0.5 m (B20 inches). (d–f) SEM images of the cross-section of the as-prepared fiber. Reproduced with permission.5

Fig. 23

The preparation process of high density RGO. Reproduced with permission.123

promising candidates for supercapacitors, exhibiting volumetric capacitances that exceed those of most previously reported materials.225 Gogotsi and coworkers recently demonstrated a method of producing MXene using a solution of lithium fluoride and hydrochloric acid (Fig. 24).225 Interestingly, the resulting hydrophilic material swelled in volume when hydrated and could be shaped like clay as well as dried into a highly conductive solid or rolled into films tens of micrometers thick. As a consequence, the additive-free clay films possessed extraordinary volumetric capacitances of up to 900 F cm 3 in an aqueous electrolyte with excellent cyclability and rate performances. It is believed that these non-oxide 2D materials will push electrochemical energy storage devices to new heights in the near future. Furthermore, they fabricated flexible and sandwich-like MXene/CNT composite paper electrodes through an alternating filtration method.70 A high volumetric capacitance of 390 F cm 3 was achieved with excellent cycling stability for the MXene/CNT paper electrode.


A crumpled graphene structure is desirable for restricting irreversible stacking between individual nanosheets, which is a major hurdle with respect to its widespread application. Recently, Lee and coworkers demonstrated a novel sea-urchin-shaped hard-template approach to fabricate highly crumpled graphene balls in bulk quantities via simultaneous chemical etching and reduction of GO@Fe2O3 particles (Fig. 25).118 During this process, the outer GO layers are spontaneously transformed into crumpled balls composed of numerous folds and wrinkles based on their plastic deformations, along with a decrease in total volume as a result of etching. Thanks to its large electrolyteaccessible surface area and good water-dispersion stability with high electrical conductivity as well as relatively high packing density, the pelletized crumpled graphene electrode exhibits a high gravimetric specific capacitance of 396 F g 1 and a particularly high volumetric capacitance of 330 F cm 3 in an aqueous electrolyte. More importantly, the specific capacitance of the pelletized crumpled graphene electrode barely changes


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Review

Fig. 24

Schematic of MXene clay synthesis and electrode preparation. Reproduced with permission.225

Fig. 25

Schematic illustrating the template-guided formation of a 3D crumpled graphene ball. Reproduced with permission.118

as the current density increases from 0.5 to 100 A g 1 because the pelletizing process can additionally provide considerable and stable interfacial contacts and low-electrical-resistance pathways resulting in fast interfacial charge transfer. Such a template-guided method for synthesizing crumpled balls does not require complex processing conditions and post-treatments, thus it is regarded as a useful approach for large-scale preparation with a high yield. In addition, the pelletized crumpled graphene electrode is recognized as an ideal electrode structure for high-performance supercapacitor devices. In summary, various promising approaches proposed recently to enhance the volumetric performances of carbon


materials for supercapacitors have been demonstrated in detail. In general, mechanical compression, liquid-mediated methods and self-assembly could effectively improve the volumetric performance by increasing the density of various carbon materials while preserving a high ion-accessible surface area and unimpeded transport paths for electrolyte ions. Despite these important achievements, the gravimetric and volumetric performances of carbon materials are restricted with respect to future development because of their intrinsically low specific capacitance and energy density. Incorporation of pseudocapacitive components could significantly improve the volumetric performances of carbon materials. On the one hand, the


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pseudocapacitive components can contribute much higher pseudocapacitance to the total capacitance. On the other hand, they could increase the packing density of carbon materials due to their relatively high density. Nevertheless, the cyclability and electrical conductivity of carbon materials are substantially decreased by the addition of redox-active functional components, due to their intrinsically poor conductivity and significant volume change during the charge/discharge process. Therefore, it is crucial that interfacial interactions between carbon materials and pseudocapacitive components are optimized and that carbon-based composite structures are painstakingly designed to ease the abovementioned problems. Due to the explosion of publications in this exciting field, we do not claim that this review includes all the published work on the methods emerging in recent years to improve the volumetric performances of carbon materials.

5. Conclusion and outlook With the rapid depletion of fossil fuels and increasingly worsening environmental pollution, there is an increasingly urgent demand for sustainable and renewable energy and environmental protection, which has triggered intensive research into efficient, clean energy conversion and storage devices from alternative energy sources to meet urgent future energy requirements worldwide. Supercapacitors are widely regarded as one of the most promising candidates for green energy storage devices due to their high power density, ultra-fast charge and discharge rate, long cycling life and safe operation. Nevertheless, the energy storage ability of supercapacitors is remarkably lower than that of batteries, hence they are significantly far from fulfilling the ever-growing energy demands. Thus, in recent years, tremendous research efforts have been devoted to increasing the energy density of supercapacitors without diminishing their high power output and cycling life. Despite significant achievements being made, regrettably, only the gravimetric performances have been the focus in most cases, while the volumetric performances were usually overlooked, which is unfortunate since volumetric performance is a much more reliable and precise metric for evaluating the charge-storage capacity of supercapacitors compared with the gravimetric performance. In particular, the volumetric performance is of significant importance from an application viewpoint where supercapacitors are employed in a limited space such as mobile electronics, electric vehicles and other compact electronic devices. However, the bulk density of carbon materials, the most widely used materials for supercapacitors, is typically lower than 0.8 g cm 3 in most cases, resulting in relatively low volumetric capacitance. In addition, carbon materials with a low packing density usually have abundant empty spaces that could be flooded by electrolyte, giving rise to an increase in the total mass of the device and consequently lowering the energy density based on the total mass of the device. To this end, achieving a high volumetric performance for a highly compact carbon material with a high porosity remains a significant challenge.



In this review, we pointed out the significant importance of volumetric performance for future design and development of supercapacitors and have reviewed recent advances in the rational design and development of novel carbon-based materials with high volumetric performance. Despite these tremendous achievements, there are still many challenges that need to be overcome on the road to the design and synthesis of novel carbon materials with high gravimetric and volumetric performances such as the intrinsically low bulk density of various carbon nanomaterials, the rational design and construction of pore structures, low electrical conductivity and the remarkable volume change in pseudocapacitive components as well as poor interfacial interactions between pseudocapacitive components and carbon materials. In addition, particular emphasis is focused on discussing the factors influencing the volumetric performance of carbon materials from a structural design point of view. Ideal carbon electrode materials should possess a high ion-accessible surface area, hierarchical interconnected pores for ion transport, excellent electrical conductivity for electron transfer, high thermal and chemical stability and a relatively high packing density for a high volumetric performance. However, none of the respective carbon structures can meet all the requirements. Moreover, there is usually a trade-off relationship between the SSA and the packing density for most material designs. On the one hand, a highly porous material can provide a large SSA and favorable channels for ion diffusion, enabling high gravimetric capacitance; however, this usually leads to a low volumetric capacitance owing to its relatively low packing density. On the other hand, a denser material can enhance the volumetric capacitance but significantly decreases the ion-accessible surface area and transport rate of electrolyte ions, giving rise to a relatively low gravimetric capacitance and a poor rate capability. Therefore, it is advisable to design and construct novel 3D carbon architectures by combining the advantages of different carbon building blocks to take full advantage of the synergistic effects of individual building blocks. The pore structure of carbon materials, including pore shape and structure, pore size distribution and regularity of pores, also plays an important role in their ultimate electrochemical performance. It is closely related to the processes of ion transport and electron transfer and therefore is a crucial aspect influencing their specific capacitance and rate capability. An ideal pore structure for supercapacitors should meet the following requirements: low pore volume, ensuring high particle density for a high volumetric performance; an appropriate number of transport channels for fast ion delivery; a low pore aspect ratio to shorten the ion transport time and enhance ion transport behavior; and a narrow pore size distribution to reduce ion scattering and consequently improve the electrode kinetics. Hierarchical porous structures have been proven to be effective in enhancing both energy and power density for supercapacitors. Thus, it is highly desirable to design novel carbon materials with high SSAs, low pore volumes, low pore aspect ratios, regular pore channels and narrow pore size distributions so that good gravimetric and volumetric performances can be achieved in future porous carbon materials.


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This review also provides an in-depth summary of significant research breakthroughs achieved through various approaches for enhancing the volumetric performances of carbon materials in recent years. Mechanical compression, capillary compression, self-assembly and construction of dense carbon assemblies may effectively improve the volumetric performance through increasing the density of various carbon materials while maintaining a high ion-accessible surface area and unimpeded ion transport channels. However, the gravimetric and volumetric performances of carbon materials are also restricted to some extent as a result of their intrinsically low specific capacitance and energy density. Incorporation of pseudocapacitive components could greatly improve the volumetric performances of carbon materials by contributing a large additional pseudocapacitance and increasing the packing density of carbon materials. Nevertheless, the cyclability and electrical conductivity of carbon materials are substantially decreased due to the intrinsically poor conductivity and significant volume changes in redox-active functional components during charge/discharge processes. Therefore, interfacial interactions between carbon materials and pseudocapacitive components should be optimized and carbon-based composite structures should also be painstakingly designed to ease these problems. To develop novel carbon-based materials with outstanding gravimetric and volumetric performances, two interesting and important research directions in the area of material synthesis and structural design for supercapacitor electrodes are proposed as follows: (1) combining 2D graphene with other dimensional carbon blocks to construct 3D carbon materials with optimal porosity, effectively removing superfluous and ion-inaccessible pores and ensuring the necessary SSA for charge accumulation; (2) incorporating nanoscale pseudocapacitive components into carbon nanomaterials and optimizing the loading and interfacial interactions to take full advantage of the synergistic effects between the individual components. It should be pointed out that volumetric performance could be seriously affected by the fabrication procedure of electrodes and precise estimation of the thickness of electrodes is much more difficult than precise estimation of electrode weights, which may result in significant calculation errors for volumetric performance. Thus, scientists and engineers around the world should work together to develop more advanced equipment/ technology to estimate electrode thickness more precisely in order to reduce errors. In addition, test methods and instrumentation for electrochemical performances are various but not unified and there is a lack of internationally accepted universal standards, such as the fabrication processes of electrodes/devices, electrochemical workstations with various brands, measurement of electrochemical performances of materials and electrodes and calculation methods of packing density and specific capacitance (gravimetric/volumetric). This may bring huge data errors and be unfavorable for the comparison of performances. Therefore, international standards for testing methods and instrumentation, as well as unified and clear rules for reporting the performances of materials/devices should be established as soon as possible to help scientists and


Review

engineers in this field as well as helping the public to evaluate the performances of materials/devices more accurately. Moreover, gravimetric and volumetric performances are only two of the crucial criteria for electrode materials and devices. Other important measures, such as cycle lifetime, charge/discharge efficiency, self-discharge, temperature range of operation and production cost, should not be overlooked. Notably, with regard to some specific applications such as microdevices and thin-film devices, the areal performances are actually more important than both gravimetric and volumetric performances because the weight of active materials on a chip or a smart fabric and the volume of microdevices and thin-film devices are generally negligible. Despite some existing challenges, the impressive research advances achieved during the past few years have reflected the bright future of high-volumetricperformance carbon materials. We firmly believe that advanced carbon nanomaterials with both high gravimetric and volumetric performances will be eventually achieved if scientists and researchers worldwide work together unremittingly in this fascinating field.

Acknowledgements The authors acknowledge financial support from the National Science Foundation of China (21571040, 51202043), the Fundamental Research Funds for the Central Universities (HEUCFQ20151014) and the China Scholarship Council (CSC).

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