Two dimensional nanomaterials for flexible supercapacitors.

Volume 43 Number 10 21 May 2014 Pages 3207–3812

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REVIEW ARTICLE Changzheng Wu et al. Two dimensional nanomaterials for flexible supercapacitors

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Two dimensional nanomaterials for flexible supercapacitors Xu Peng, Lele Peng, Changzheng Wu* and Yi Xie

Published on 11 March 2014. Downloaded on 30/04/2014 07:27:46.

Flexible supercapacitors, as one of most promising emerging energy storage devices, are of great interest owing to their high power density with great mechanical compliance, making them very suitable as power back-ups for future stretchable electronics. Two-dimensional (2D) nanomaterials, including the quasi-2D graphene and inorganic graphene-like materials (IGMs), have been greatly explored to providing huge potential for the development of flexible supercapacitors with higher electrochemical performance. This review article is devoted to recent progresses in engineering 2D nanomaterials for Received 11th November 2013

flexible supercapacitors, which survey the evolution of electrode materials, recent developments in 2D

DOI: 10.1039/c3cs60407a

nanomaterials and their hybrid nanostructures with regulated electrical properties, and the new planar configurations of flexible supercapacitors. Furthermore, a brief discussion on future directions,

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challenges and opportunities in this fascinating area is also provided.

1. Introduction The energy crisis in the 21st century has been for some time the most important current issue faced by researchers. In this regard, how to convert and store renewable energy efficiently still remains a significant and urgent problem for researchers to resolve. Among various energy storage devices, rechargeable batteries and electrochemical capacitors (ECs) are regarded typically as a suitable choice to store energy by transforming chemical energy into electrical energy.1–3 Rechargeable batteries, especially lithium ion batteries, have attracted much attention Hefei National Laboratory for Physical Sciences at the Microscale & Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China. E-mail: [email protected]

Xu Peng

Xu Peng received his BS in Materials Science and Engineering at Wuhan Institute of Technology in 2011. He is currently pursuing a PhD in the Department of Chemistry at the University of Science and Technology of China. His research is focused on the synthesis of polymer electrolytes and two-dimensional nanomaterials for electrode materials, especially their applications in flexible energy storage devices.

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due to their high energy density, yet they have sustained performance limitations in their short cycle life, relatively slow charging– discharging rates and consequently lower power densities. In order to compensate for the lower power densities of these batteries, supercapacitors, also known as electrochemical capacitors (ECs) or ultracapacitors, have emerged as considerable alternative candidates to batteries and offer a number of potential advantages in performance, including superior operating lifetimes, ultrafast charging–discharging rates, and high power densities and so on. For supercapacitors as energy storage devices, double layer supercapacitors store charges only via electrostatic ion absorption, resulting in a performance limitation by their energy density.4 Alternatively, pseudocapacitors undergo reversible redox reactions at the surface of their electrode materials, contributing more storage charges than double-layer supercapacitors, and thus a

Lele Peng

Lele Peng received his BS degree in Materials Science and Engineering at University of Science and Technology of China in 2012. He is currently pursuing his PhD in Department of Mechanical Engineering at The University of Texas at Austin. His current interests include the synthesis and characterization of nanomaterials for energy storage and conversion, especially for lithium ion batteries and high performance supercapacitors.

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superior energy density is presented by these.5,6 In this regard, pseudocapacitive electrode materials play a vital role in accomplishing both high power density and energy density, which is the target-goal for energy storage devices. Flexible or space-saving electronic devices such as displays,7 sensors,8 and mobile phones are desirable, but production constraints sometimes limit the serviceability of their practical use.9,10 To ‘power-up’ these portable electronics, the development of flexible energy storage and conversion devices are required to catch up with the rapid growth of the demand for flexible electronics. In this sense, flexible energy storage devices with higher energy density must be able to provide sustainable energy,11,12 and those with a higher power density are able to provide a high-rate current output. For such of energy storage devices, improving the electrochemical performance of supercapacitors with higher energy density has been proven to be a new avenue to meet this great demand. To this end, achieving flexible supercapacitors with the synergic advantages of improved electrochemical performance and ultrahigh flexibility is the greatest challenge. First of all, the primary challenge comes from flexibility, for most current conventional supercapacitors that are currently available in the industrial market are based on rigid electrodes, which significantly hinders the feasibility of making them flexible. Recently, much effort has been devoted to fabricating flexible and planar supercapacitors to meet the growing development and demand for small, flexible, thin, lightweight and even roll-up portable electronic devices.13–17 Flexible supercapacitors, a novel emerging branch of ECs, usually require thin-film electrode materials with high flexibility integrated on soft-matter substrates,18 requiring not only excellent electrochemical performance, but also high mechanical integrity upon bending, folding or even rolling with a compact lightweight design. In particular the choice of electrode materials for flexible supercapacitors is of great importance to all. To date, a number of nanostructured

electrode materials such as graphene,18–21 transition metal oxides (TMO)22–29,90 or hydroxides30,31,71 and conductive polymer materials32–37,65–67 have attracted a wide range of interest. These materials satisfy well the requirements of flexible supercapacitors which is attributed to their abundant electrochemical active sites and ability to entangle with each other. Among them, twodimensional nanomaterials become the best alternatives to achieve the highly flexible supercapacitors, owing to their high mechanical integrity that originates from the large overlapping areas when they are stacked layer-by-layer to form a thin film structure, as well as the good electrochemical properties that come from rich selection of atomic types in the 2D lattice plane, providing further possibilities for highly-active electrochemical sites. Moreover, two-dimensional (2D) nanomaterials, whose thickness in the horizontal direction is just a single or several atomic layers, forming confined-dimensional materials, have brought synergic advantages of controllable electrical properties and high specific surface areas as well as superior electrochemical activities.38–49 To summarise, besides the 1D (CNT, TMO nanowires etc.) and 3D (graphene network or foam) nanomaterials, 2D nanomaterials show promising signs for the construction of flexible supercapacitors owing to the synergic advantages of high mechanical strength as well as improved electrochemical performance (Fig. 1). In order to improve the flexibility and compact design to a higher level for flexible supercapacitors, further breakthroughs are required to optimise the structural design of the supercapacitor itself, rather than solely focusing on the electrode materials. Planar supercapacitors, as a new emerging branch of supercapacitors, enable the entire device to be much thinner and more flexible due to their unique structural design: the integration of three important components into a two-dimensional configuration in the same horizontal plane (Fig. 2).50–52,84,90 Planar supercapacitors significantly decrease the thickness in the vertical direction and rather expand along 2D horizontal planes, making

Changzheng Wu obtained his BS (2002) and PhD (2007) degrees in the Department of Chemistry at the University of Science and Technology of China. He has since worked as a postdoctoral fellow in the Hefei National Laboratory for Physical Sciences at Microscale. He is now a full professor of Department of Chemistry, University of Science and Technology of China. Dr Wu’s research is highly Changzheng Wu interdisciplinary. His current research interests focus on the synthesis and characterization of inorganic two dimensional nanomaterials and regulation of their intrinsic physical properties for wide applications in energy storage or energy conversion.

Yi Xie received her BS degree from Xiamen University (1988) and PhD from the University of Science and Technology of China (1996). She is now a Principal Investigator of Hefei National Laboratory for Physical Sciences at the Microscale and a full professor of the Department of Chemistry, USTC. She is a recipient of many awards, including the Chinese National Nature Science Award and the Yi Xie IUPAC Distinguished Women in Chemistry/Chemical Engineering Award. Her research interests are cutting-edge research at four major frontiers: solid state materials chemistry; nanotechnology; energy science; and theoretical physics.

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Fig. 1 From 2D nanomaterials to the electrodes of flexible supercapacitors. The scheme illustrates the structure design of different types of twodimensional nanomaterials which will lead to the optimal performance of flexible supercapacitors.

Fig. 2 Schematic comparison of the sandwich-type supercapacitor (left) and planar supercapacitor (right).

compact device design possible with superior flexibility and robust cyclability. In such devices, the electrolyte ions are transported two-dimensionally, which is suitable for 2D nanomaterials with 2D migration channels which shorten the ion travel distance by eliminating the necessity for a separator. 2D nanomaterials fit the needs of planar supercapacitors; due to their atomic-thickness ultrathin nanosheets can be stacked layer-by-layer which favors simultaneous ultra-flexibility and high mechanical properties as well as an intrinsic 2D configuration. Also, the inorganic lattice framework could act as more active sites to enhance the electrochemical performance of planar supercapacitors. In a word, ultrathin 2D nanomaterials represent a promising materials platform to realize high-performance flexible planar configuration devices as portable power backups and stretchable/flexible electronic devices (Table 1). Herein, we review the recent progress on flexible supercapacitors, in particular two-dimensional nanomaterials. In the second section, we describe the evolution of electrode materials for supercapacitors. 2D nanomaterials together with non-2D nanomaterials are discussed and their outlook described. In the next section, flexible supercapacitors with various kinds of


nanomaterial electrodes and diverse building principles are summarized, in particular for 2D nanomaterials. In the last section, the basic concepts and configurations of planar supercapacitors are introduced, and recent cases of typical planar supercapacitors are summarized in detail to highlight their significance, which is not only relevant to scientific research but also to practical industrial applications. The planar geometries of the 2D nanosheet materials are directly compatible with planar device designs, and their hybrid nanostructures with other functional materials have shown promising signs as the best candidates for planar supercapacitors. Finally, future tasks and existing problems for the development of flexible supercapacitors are reviewed.

2. Evolution of electrode materials for flexible supercapacitors A flexible supercapacitor is a super-fast rechargeable electrochemical energy storage device, combining the advantages of high storage capability and power output as well as high malleability without any significant performance loss. Thus, flexible supercapacitors require electrode materials not only with good electrochemical properties, but also with high mechanical integrity upon bending or folding, compact design and lightweight property. As an indispensable component of flexible supercapacitors, electrode materials have a significant influence on the performance of flexible supercapacitors. Originally, non-2D nanomaterials, especially 1D nanomaterials, were exploited to fabricate electrode materials for flexible supercapacitors for the convenient assembly of thin film electrodes with a conducting matrix and self-flexibility caused by intertwining with each other.

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Review Article Table 1 state

Comparison of specific capacitances, retention and flexibility of the reported flexible supercapacitors and planar supercapacitors in all-solid-

Active materials

Template materials

Capacitance

1D nanomaterials MnO2 nanowires

CNT paper

167.5 F g 210 F g

PANI PANI nanowires

Chemically converted graphene CNT Carbon cloth

PANI nanobars

CNT network

MnO2 nanorods

Carbon nanoparticles

PANI nanowire arrays V2O5 nanowire ZnO nanowires

Au-coated PET film CNT Kevlar fiber

PPy/MnO2 particles

Polypropylene fibrous films Graphene paper Cloth-supported SWCNTs

PANI nanofibers


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1

1

Retention

at 77 mA g

1

1

at 0.3 A g

1

16 F g at 0.16 mA cm 1079 F g 1, at 1.73 A g 1

2

Bacterial nanocellulose

WO3–x@Au@MnO2 core–shell nanowires

Flexible carbon fabric

588 F g

RuO2 TiC-CDC nano-felts

MWCNT Disordered carbon

Multilayered GO/Au nanoparticles RuO2

Multilayered GO

96 F g 1 at 1 A g 1 135 F g 1 in aqueous electrolyte 65 F g 1

3D nanomaterials MnO2-coated 3D graphene 2D nanomaterials PANI film

Carbon microfibers Carbon nanofiber H3PO4–PVA thin films Graphene paper

Co–Al hydroxide nanosheets VOPO4 nanosheet Ni(OH)2 nanosheet

1

Ionic liquid functionalized 175 F g chemically modified graphene

1

3D graphene network

130 F g

1

SWNT film on PET substrate Cotton cloth

55.0 F g

Graphene nanosheet– cotton cloth composite fabric Graphene Cellulose paper

Planar supercapacitors non-2D nanomaterials PET film Carbon/MnO2 (C/M) core shell fiber

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1

at 10 mV s

at 0.5 A g

50% after 200 cycles 86% after 2100 cycles

1

at 2.6 A g 1

326.8 F g aqueous

118.5 F g

1

2.5 F cm

3

1

68 99.5% at different bending angles

91 92 69

Subtle change at different bending angles

93 94 95 54

94% after 1500 cycles

96

74% after 1000 cycles 90% after 3000 cycles 98.8% after 1000 cycles

97 14 98 99

90% after 1000 cycles 96% after 500 cycles 96.15% after 2000 cycles

100 101 102

99.5% after 5000 cycles 110% after 5000 cycles

Maintained over 200 bending cycles to a radius of 3 mm No significant change under various bending conditions

103 26

No obvious deviation in the 106 CV curves under bending 95% after 2000 cycles of 107 twisted and bent states

1

1

in 6 M KOH

at 1 A g

64

104 105

2

81 mF cm

Ref.



Graphene multilayer films 880 F g 1 and 70 F m 2 at 5 mV s Au-doped PET film 8360.5 mF cm 2 Graphene 660.8 F cm 3

Functionalized reduced PET & Nafion films graphene oxide (FRGO) thin films

96% after 1000 cycles 88% after 3000 cycles 79% after 800 cycles at 3 A g

350 F g 1 for the electrode 91.9% after 1000 cycles materials 109 mF cm 2 obtained 97.3% after 10 000 cycles from electrode 588 F cm 3 57.3 F g 1 0.21 mF cm 2 in 1 M KNO3 2.4 mF cm 2 in PVA/H3PO4 110 F g 1 based on the active material 256 F g 1 at 0.5 A g 1 410 F g 1 at 0.5 A g 1 237 F g 1 at 0.01 V s 1 621.6 F g 1 (based on pristine MnO2) at 2 mV s 1 638 F g 1 at 2 A g 1 338.1 F g 1 at 0.5 mA cm 2 138 F g 1 at 10 mV s 1 in 6 M KOH 50.5 F g 1 (20.2 mF cm 2)

MnO2 nanoparticles PANI nanowire array PPy nanoparticles Ultrathin MnO2/ Zn2SnO4 nanowire PANI nanoparticles MnO2 nanoparticle Carbon black nanoparticles CNT

Flexibility

Almost constant after 500 cycles 93.8% after 1500 cycles 99.1% after 5000 cycles

1

Over 99% after 2000 cycles 96% after 2000 cycles 98.2% after 2000 cycles

1

92% after 200 bending actions

No significant changes after 1000 cycles

84% after 10 000 cycles

60

15 108 1000 repeated bend tests with only 6% increase of electrical resistivity

109 72

6 Negligible degradation 71 after 200 bending cycles Almost the same in CV 70 curve when bending radius of 2.2 mm

No apparent change of the 82 CV curves under five different bending angles


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(continued)

Active materials ZnCo2O4 nanowire arrays/carbon fibers 3D GeSe2 monoclinic structure


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Template materials

Capacitance 1

Retention at 1 A g

1

Flexibility

PET film

650 mF g

PET film

186 mF cm

2

99.3% after 2000 cycles

2D nanomaterials Graphene

PET film

394 mF cm

2

VS2 nanosheet

PET film

4760 mF cm

No degradation in performance after 1500 cycles Higher than 90% after 1000 cycles

Graphene

PET film

MnO2 nanosheet

Graphene

2.07 mF cm 2 at 1000 A g inorganic electrolyte 267 F g 1 at 0.2 A g 1

2

However, non-2D dimension active materials are not perfectly compatible with the electrode materials of flexible supercapacitors with thin film configurations. Very recently, 2D nanomaterials, such as graphene and inorganic graphene-like materials (IGM) have attracted much attention because of their high specific surface areas, high mechanical flexibility and extra electrochemical active sites which provide ideal material platforms for fabricating electrode materials for flexible supercapacitors. In this part, we focus on the evolution of electrode materials for flexible supercapacitors from the original non-2D nanomaterials to novel 2D nanomaterials, and further discussion was included to highlight the intrinsic merits of 2D nanomaterials for the fabrication of flexible supercapacitors. 2.1

Non-2D nanomaterials for flexible supercapacitors

To meet the demands of flexible supercapacitors, numerous kinds of material platforms are possible choices for flexible supercapacitor electrodes. Among them, one-dimensional nanostructures, including nanofibers (NFs)53–56 and nanowires (NWs),57 such as CNT, polymer NWs, TMO NWs, and core–shell hybrid NWs are the most common options to fabricate flexible supercapacitors, because they all have one-dimensional character, in which the formation behaviors tend to be entangled with each other. Meanwhile, on the macroscopic scale, due to strong interaction by entanglement with each other among 1D nanostructures, the possibility to assemble flexible composite matrixes has emerged, which are preferred to the fabrication of thin film electrodes of flexible supercapacitors. For example, Chen et al. made a prototype of a flexible supercapacitor based on an In2O3 nanowire–CNT composite electrode.58 The long nanowires show high flexibility, the dispersed assembly of which on transferred CNT films forms the In2O3 nanowire/CNT heterogeneous thin film structure. The pseudocapacitive flexible In2O3 nanowires are beneficial for the fabrication of high-efficiency supercapacitors which exhibits a higher specific capacitance of 64 F g 1 than pure CNT samples in nonaqueous electrolyte. It is the most general method to fabricate flexible supercapacitors using 1D nanomaterials as active materials and another component as the conducting material. However, 1D nanomaterials still have the limitation of a relatively smaller contact area with the matrix for



1

96.5% after 10 000 cycles 92% after 7000 cycles

Ref. 83

Similar to that of its initial 110 state after 1000 cycles 84 Very small degradation 89 even after 200 bending cycles 5% loss after 1000 bending 18 cycles Slight differences after 90 thousands of times

its nanoscale diameter and thus the surface areas of 1D nanomaterials are not utilized effectively. Besides, the flexibility of 1D nanomaterials-based supercapacitors is mainly attributed to their flexible matrix; degradation may appear when repeated bending/folding occurs in such supercapacitors. Another alternative type of material platform for the fabrication of flexible supercapacitors is 3D nanomaterials. 3D nanomaterials display obvious differences from 1D and 2D nanomaterials owing to their infinite growth from three dimensional features in their spatial structure. Owing to their three-dimensional active sites, the porous-and-loose features of 3D building blocks provide more benefits for supercapacitors with high capacitance performance. As is known, to construct the flexible supercapacitors a thin-film structure of electrode materials is usually required.59 From this viewpoint, 3D building blocks have intrinsic spatial disadvantages. For example, they are ill-suited for compression into thinner and lighter structures. When 3D nanostructures are under a compressive force, or even one obviously outstripping the limit stress of the crystal lattice, they break up the 3D nanostructures such as tunnels or highly active surfaces, and thus destroy the adsorption/ desorption passageway of the electrolyte ions. In this case, compressing 3D nanostructures into a thin film structure would further decrease rapidly their capacitive performance. An exceptional case comes from the 3D graphene-based nanostructures. Due to their structural flexibility from the light-weight atoms, the emergence of 3D graphene network structures provides a novel platform to fabricate flexible devices. To be specific, a 3D graphene structure with a remarkably high specific surface area was produced by chemical vapor deposition (CVD) growth using Ni foam as a sacrificial template, which offered both high flexibility and conductivity, coming from its network and porous structure.60 These 3D graphene network structures with the ability to sustain large deformation show promising signs for the fabrication of flexible energy devices. 2.2 Two-dimensional nanomaterials for flexible supercapacitors Recently, two-dimensional nanomaterials, with their distinct properties and high specific surface areas, have emerged as a novel material platform to fabricate flexible energy devices.

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Since the discovery of the exotic properties of graphene, 2D layered materials such as transition metal dichalcogenides (TMDs),46 metal chalcogenides, transition metal oxides (TMO)89 and other 2D compounds have attracted renewed interest in application as energy storage and conversion devices. Their two dimensional nature has brought great advantages in achieving both high capacitive performance and excellent high flexibility. For IGMs, with their various options for alternative elements, one could imagine anything beyond pure-carbon graphene. The atomic thickness of 2D graphene-like materials would introduce lots of active sites for extra electrochemical reaction, which is of great advantage in enhancing the capacitive performance of the supercapacitors. Also, 2D nanomaterials give great promise to show fascinating mechanical properties, including bending/folding strength as well as excellent flexibility. For 2D nanomaterials, the large overlapping area between each nanosheet has the great benefit to allow the physicochemical properties to remain unaffected during bending or folding, contributing to ultrahigh flexibility. Moreover, because the planar geometries of the 2D nanosheets are directly compatible with planar device design, the extensible ability of 2D nanosheet materials also caters for the achievement of ultra-thin electrodes and the formation of ion migration. To sum up, because of their distinct properties and high specific surface areas, two-dimensional nanomaterials are of significant importance in the application of flexible supercapacitors as promising energy storage devices. Electrode materials with high surface areas, superior mechanical flexibility and abundant electrochemical active sites were favorable to fabricate flexible supercapacitors, involving nanostructures in the form of one-dimensional (nanowires, nanofibers, nanotubes, etc.), two-dimensional (graphene, inorganic ultrathin nanosheets, etc.), and three-dimensional (graphene network or foam structures). Specially, 2D nanomaterials with their two dimensionalities have brought great advantages in high flexibility, achieved by layer-by-layer stacking of single sheets to form a flexible thin film, in which the overlapping areas facilitate the mechanical stability and robust flexibility. The high specific surface areas of the 2D materials introduce lots of active sites for electrochemical reactions and electron transport, which are of great advantage in enhancing the capacitive performance of the supercapacitors. Moreover, for IGMs, i.e. ultrathin nanosheets and their derivates, with a variety options for alternative elements in 2D inorganic lattices, we could reach even higher capacitive performance with their pseudocapacitive properties arising from the physicochemical properties of various inorganic elements. Based on these above considerations, 2D nanomaterials provide an ideal materials platform for flexible supercapacitors with high performance and superior flexibility, deserving intensive attentions.

3. Flexible supercapacitors 3.1

Non-2D nanomaterials based flexible supercapacitors

3.1.1 1D carbon nanomaterials based flexible supercapacitors. As long as electrode materials can be assembled into thin film structures with macroscopic mechanical flexibility, they have the possibility to be applied as thin film electrodes for the construction

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Fig. 3 (a) SEM image of nanocup film; inset is the scheme of the growth template. (b) SEM image of the as-obtained nanocup film in crosssectional view. (c) The capacitance change as a function of temperature change from 20 1C to 80 1C. (d) Digital photographs of large-scale CNC supercapacitor films demonstrating optical transparency and mechanical flexibility. Adapted with permission from ref. 61, copyright Nature Publishing Group 2012.

of flexible supercapacitors. One-dimensional nanostructures are easy to interconnect with each other to form thin film structures with high mechanical strength. In detail, fabricating flexible supercapacitors, using 1D nanomaterials like nanofibers, nanowires, nanorods and nanotubes as electrode materials, have been demonstrated widely and received considerable attention. Generally speaking, investigation of flexible supercapacitors on 1D nanomaterials is divided into two categories according to the structural configuration of the electrodes: (1) 2D planar thin films assembled by the entanglement of 1D nanostructures; (2) screwing into a 1D fiber or 1D nanocup array using 1D nanostructures as the building blocks. To form a 2D planar thin film, 1D nanomaterials need to be well dispersed in the matrix before forming the self-assembled thin film. In addition, the formation of a 1D fiber or 1D nanocup was often accomplished through core–shell structural design or the use of a fabrication template, respectively. Jung et al. presented carbon nanocups (CNCs), which consisting of arrays of periodic and interconnected nano-cup morphologies to fabricate flexible supercapacitors (Fig. 3a and b).61 When a CNC with a few-layer graphitic structure is used as a thin film electrodes and a PVA–H3PO4 gel electrolyte is sandwiched between the two separated CNC electrode films, the as-assembled supercapacitor represents excellent flexibility with CV and CD properties retained even after being helically rolled up to 7201 (Fig. 3d). The flexible supercapacitor produced by nano-engineered carbon electrodes interconnected with 1D carbon nanocups presents sufficient mechanical strength and areal capacitance. 3.1.2 1D hybrid structure based flexible supercapacitors. A hybrid structure of nanocomposites with one component providing accessibility to high electrochemical rate capacity and the other component possessing a fast faradaic reaction


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are imperative for achieving both high power density and energy density.62,63 Hybridising the conducting materials with the one-dimensional materials opens new opportunities for further improving the electrochemical performance of flexible supercapacitors. Currently, nanocomposites containing carbon nanotubes and 1D redox active materials have attracted much attention as electrode materials. For example, Chou et al. used CNT paper as the matrix, prepared by a simple filtration method, and MnO2 nanowires was electrodeposited onto the CNT paper to obtain a free-standing, flexible electrode with good mechanical toughness and high capacitance and cycling stability for supercapacitors.64 In such a composite electrode, the CNT paper acted as a conductive and flexible substrate for flexible electrodes in supercapacitors, and the nanowire structure of the MnO2 as the pseudocapacitive material facilitated the contact of the electrolyte, and thus increased the capacitance. Apart from the TMO 1D nanomaterials, recent years have witnessed the development of conducting polymers for flexible energy device applications. Since the conducting polymers exhibited excellent pseudocapacitive performance resulting from redox reactions during the charging–discharging process as well as their high processability in dispersion, they are promising candidates for electrodes as active materials. For example, PANI has attracted particular interest mainly because of its high theoretical specific capacitance (about 2000 F g 1) and high degree of processability, which means it can be easily synthesized into different morphologies such as nanoparticles, nanofibers and nanowires, and these nanostructures could be easily deposited on various substrates for application as electrodes in supercapacitors.65–67 Unfortunately, the relatively low conductivity of PANI still limits electron transport during the charge–discharge processes when applied in supercapacitors. To address this problem, PANI is often coated on conductive templates with a high specific surface area to fully exploit its advantageous electrochemical properties. For an instance, Wu et al. reported a novel method for preparing stable aqueous dispersions of chemically converted graphene (CCG)–PANInanofiber composites by vacuum filtration.68 The composite film has a layered structure in which PANI-NFs are sandwiched between CCG layers (Fig. 4a), and thus the conductivity of the composite film is increased compared to pure PANI-NF film. Furthermore, it can be bent into large angles or be shaped into various desired structures due to its mechanical stability and high flexibility (Fig. 4c). The conductivity of the composite film containing 44% CCG (5.5 102 S m 1) is about 10 times that of a pure PANI-NF film. Flexible supercapacitors based on this conductive flexible composite film exhibited a large electrochemical capacitance (210 F g 1) at a discharge rate of 0.3 A g 1 and this capacitance can be maintained for about 94% at 3 A g 1 which was significantly higher than that of a pure PANI sample. This increase was attributed to the synergic effects of these two components. CCG sheets act as the template to sustain PANI-NFs, preventing the fibers from degradation during charge–discharge. Thus, the pseudocapacitive performance of PANI could be enhanced. Similarly, Meng et al. demonstrated a kind of ultrathin all-solid-state supercapacitor configuration


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Fig. 4 (a) SEM image of a flexible G-PNF30 film in cross-sectional view. (b) Galvanostatic charge–discharge curves of the supercapacitors based on G-PNF30 (blue), as-formed PANI-NF (green), and CCG films (black) at current density of 0.3 A g 1. (c) Digital photograph of a flexible G-PNF30 film. (d) Schematic illustration of supercapacitor cell. Adapted with permission from ref. 68, copyright American Chemical Society 2010.

with an extremely simple process using two slightly separated polyaniline-based electrodes well solidified in the PVA/H2SO4 gel electrolyte.69 As is known, incorporating conducting carbonbased materials with pseudocapacitive materials is a promising approach to improve further the electrochemical performance of devices. In this work, pseudocapacitive PANI was uniformly coated on the freestanding CNT networks made of randomly entangled individual CNTs and CNT bundles through an in situ polymerization process. After that, these hybridized thin films were immersed in PVA/H2SO4 and then fabricated as electrodes by pressing them together under 10 MPa after vaporizing the excess water. The thickness of the as-fabricated device is largely comparable to that of a piece of commercial standard A4 print paper, showing the ideal flexibility of the as-obtained thin film electrodes (Fig. 5). In its highly flexed (twisted) state, the integrated device shows a high specific capacitance of 350 F g 1 for the electrode materials, good cycle stability after 1000 cycles with about 91.9% retention and a leakage current as small as 17.2 mA. Furthermore, due to its polymer-based hybrid structure, it has a specific capacitance of as high as 31.4 F g 1 for the entire device, which is more than 6 times that of the high-level commercial supercapacitor products. This approach to assemble highly flexible and all-solid-state paperlike polymer supercapacitors may bring new designs for energy-storage devices in the future wearable electronics area. The examples above represent some facile methods to fabricate electrodes for flexible supercapacitors by compositing 1D nanostructures onto 2D thin film templates. It is apparently that the flexibility of 1D nanomaterial-based flexible supercapacitors arises primarily from the method of assembly ways of the active material and flexible matrix. From this viewpoint, 1D nanomaterials still are limited by a relatively smaller contact area due to their nanoscale diameter. To this end, 2D nanomaterials, such as ultrathin nanosheets, would be more promising for a tight contact with the matrix due to their near-perfect geometry matching

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Fig. 5 (a) Schematic illustration of the PANI–CNT nanocomposite electrodes well dispersed in the polymer gel electrolyte. (b) Digital photographs of the all-solid-state device (size B0.5 cm 2.0 cm) under normal conditions (top) and its highly flexed state (bottom) under electrochemical measurements. Adapted with permission from ref. 69, copyright American Chemical Society 2010.

with 2D planar substrates. As for mechanical properties, 2D active nanomaterials are naturally coated on 2D template nanomaterials without any incompatibility, and thus more flexibility was shown from the micro- to the macro-scale. Also, the atomic thickness of the 2D nanomaterials (nanosheets) provides more active surface area, which leads to superior electrochemical performance. 3.2 Two-dimensional-nanomaterial-based flexible supercapacitors 3.2.1 Graphene-based flexible supercapacitors. Twodimensional nanomaterials have been widely employed to fabricate high performance flexible supercapacitors because of their high specific surface areas and high mechanical stability. 2D graphene, as one of the most representative carbon allotropes, was discovered experimentally in 2004,38 and consists of a flat monolayer of sp2 bonded carbon atoms in a two-dimensional (2D) honeycomb lattice. Graphene has a high theoretical specific surface area (2630 m2 g 1),21 and superior mechanical strength, thus graphene-based flexible energy devices have been extensively explored since its discovery. However, for the majority of graphenebased supercapacitors, the values of specific capacitance, energy density, and power density have remained lower than the expected

values because of the restacking of graphene sheets. In this regard, regulating strong sheet-to-sheet van der Waals interaction to a reasonable degree is urgently needed. Recently, El-Kady et al. used the direct laser from a standard LightScribe DVD optical drive to reduce graphite oxide films to graphene on flexible substrate.18 In this work, a simple method to avoid the restacking of graphene sheets is presented. Irradiation of the film with an infrared laser reduced the GO to laser-scribed graphene (LSG) (Fig. 6c). Analysis of cross sections of the film with scanning electron microscopy showed that the initially stacked GO sheets were converted into well-exfoliated LSG sheets (Fig. 6e). Based on these LSG sheets, the as-obtained supercapacitors represent the best electrochemical performance among graphene-based flexible supercapacitors, and show excellent cycling stability. The realization of highly flexible and all-solid-state energystorage devices depends strongly on both the electrical properties and mechanical integrity of the controlled assembly of the electrode along with the solid electrolyte. However, how to improve further the performance of supercapacitors by optimizing their ion transport remains a challenge. Here a reported method could alleviate this problem to some extent. Choi et al. assembled a graphenebased flexible supercapacitor by easy assembly of functionalized

Fig. 6 Schematic illustration of the fabrication of laser-scribed graphene-based flexible supercapacitor. (a to d) A GO film supported on a flexible substrate is placed on top of a LightScribe-enabled DVD media disc, and then a computer image is laser-irradiated on the GO film in a computerized LightScribe DVD drive. Finally the LSG film was peeled off for device fabrication. (e) As shown in the photograph, the color of the GO film changes from golden brown to black since it was reduced to laser-scribed graphene. As shown in the cross-sectional SEM images, the low-power infrared laser changes the stacked GO sheets (left) immediately into a well-exfoliated few-layer LSG film (right). (f) A symmetric supercapacitor is fabricated from two LSG film electrodes and a flexible substrate, ion-porous separator, and electrolyte. Adapted with permission from ref. 18, copyright AAAS 2012.

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reduced graphene oxide (f-RGO) thin films (as electrode) and solvent-cast Nafion electrolyte membranes (as electrolyte and separator).70 It is possible to demonstrate improved ion and charge transport at the interface for high ionic conductivity of Nafion. Also, the Nafion polymer acted as an electrochemical binder for the adhesion of the Nafion-coated electrode and Nafion membrane in the process of assembly into all-solid-state supercapacitors (Fig. 7a and c). As a result, the RCT of f-RGO was lower than that of RGO, which was attributed to close contact between the electrode and the electrolyte, and thus the enhanced electrochemical performance displayed. As a consequence, the f-RGO-based SCs (f-RGO-SCs) showed almost 2 times higher specific capacitance (118.5 F g 1 at 1 A g 1) and rate capability (retention rate B90% at 30 A g 1) compared to those of all-solidstate graphene SCs (62.3 F g 1 at 1 A g 1 and 48% retention at 30 A g 1). Besides, the f-RGO-SCs clearly exhibited more capacitive and less resistive behavior compared to the RGO-SCs, and the facilitated ion diffusion at the electrical double layer is proven by the 4-fold faster relaxation of the f-RGO-SCs than that of the RGO-SCs and the greater capacitive behavior of the former at the low-frequency region where the ion diffusion occurs. This improved electrochemical performance is mainly due to the following reasons: (1) The interconnected networks of f-RGO boost fast charge transport and construct continuous transport pathways, which is beneficial to transportation of electrolyte

Fig. 7 (a) Photograph and schematic illustration of all-solid-state flexible thin f-RGO-SC. (b) SEM image of f-RGO films. (c) SEM images of f-RGOSC in a cross-sectional view (the inset is a high-magnification SEM image). (d) CV curves of f-RGO-SC and bent f-RGO-SC at a 100 mV s 1 scan rate, (e) CV curves of RGO-SC and bent RGO-SC at a 100 mV s 1 scan rate. Adapted with permission from ref. 70, copyright American Chemical Society 2011.


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ions; (2) the close contact between the electrode and the electrolyte caused by the Nafion polymer decreases resistance and an enhanced performance is presented. In a bending test to demonstrate flexibility, the CV curves of the f-RGO-SCs displayed almost the same shape compared to the clearly different shape of RGO-SCs (Fig. 7d). This proves obviously the interpenetrating network structures of f-RGO and that good adhesion between the electrode and electrolyte was achieved. Moreover, the cycling stability of all-solid-state flexible f-RGO-SCs demonstrated no significant changes after 1000 cycles of charging and discharging due to the interconnected network structures assembled by f-RGO and Nafion. Therefore, this general method to introduce the Nafion polymer will pave a simple and useful way to improve the performance of all-solid-state flexible energy-storage devices. Self-supporting carbon-based papers or films have shown great potential as flexible electrodes with excellent mechanical strength. However, in most cases graphene nanosheets (GNS) were assembled into macroscopic paper-like structures in a way that reduced the large accessible surface area, caused by irreversible agglomeration and restacking of the individual GNS. This disadvantage must be solved before application to electrode materials. More recently, freestanding 3D graphene nanostructures constructed by GNS which have a cross-linked 3D porous structure have been reported with high specific surface area and more electrochemical active sites. He et al. fabricated a type of MnO2-coated 3D graphene network composite structure for flexible supercapacitor by using pressed Ni foam.60 Due to the superior mechanical strength and flexibility of the Ni foam, this supercapacitor showed free-standing, flexible, lightweight (0.75 mg cm 2), and highly conductive (55 S cm 1) properties, of which the remarkably high specific surface area (392 m2 g 1) allows an extremely large mass loading of 92.9% MnO2 (measured content for the entire electrode). These as-fabricated supercapacitors exhibit a high areal capacitance of 1.42 F cm 2 and a high specific capacitance of 130 F g 1 (calculated for the entire electrode other than MnO2). In a word, owing to the strength and ductility of Ni metal and the highly optimized composite structures, this type of simple supercapacitor possesses remarkable electrochemical performance and excellent mechanical properties accomplished with relatively high flexibility (Fig. 8). In conclusion, tailoring the distance between each stacked graphene sheet, broadening the pathway of ion transportation and constructing the 3D porous network the structures demonstrated above are effective ways to utilize the high intrinsic surface areas of graphene, and furthermore to enhance the performance of graphene-based flexible supercapacitors. Consequently, the limited structural disadvantages of pure graphene could be regulated through these approaches, but it is still hard to control precisely the electrical properties of graphene. Moreover, the purecarbon atoms of graphene have also hampered the further improvement of electrochemical performance. With the above considerations, many efforts have been made on IGMs, in that an inorganic lattice framework in 2D confinement conditions would give further advantages for enhancing the electrochemical performance of flexible supercapacitors.

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Fig. 8 (a) SEM images of samples with mass loadings of 9.8 mg cm 2. The inset shows high-magnification SEM image. (b) Schematic illustration of the structure of flexible supercapacitors consisting of two symmetrical graphene–MnO2 composite electrodes, a polymer separator, and two flexible PET membranes. The two digital photographs display the flexible supercapacitors when bent. Adapted with permission from ref. 60, copyright American Chemical Society 2013.

3.2.2 2D hybrid structure for flexible supercapacitors. As a new branch of 2D materials, 2D TMO and 2D transition metal hydroxide (TMH) ultrathin nanosheets with atomic thickness, that is, IGMs, have received considerable attention. For IGMs, the 2D inorganic lattice would lead to an extra contribution of abundant active pseudocapacitive centers from various kinds of inorganic atoms or ions with unique physicochemical properties. In this regard, the improved electrochemical performance comes from not just their high specific surface area, but also the chemical activity of the lattice framework. 2D inorganic nanomaterials show promising potential as flexible supercapacitor electrodes, but the intrinsically disadvantageous properties of the TMO and TMH hinder their practical application as flexible supercapacitors: (1) The lower electrical conductivity of TMO and TMH greatly hinders electron transport during the electrode reactions. (2) The presence of strain in pure TMO and TMH causes cracking of the electrode during the charge–discharge process, and thus poor cyclic performance emerges. (3) The 2D nanomaterials are hard to access due to their diverse coordination styles. Most of the active materials do not have layered structures, which hampers their exfoliation into ultrathin nanosheets with atomic thickness. To mitigate these practical problems, composite electrodes from hybrid structures have been used intensively instead of a single ingredient.62,63 For example, a layer-by-layer strategy was adopted to assemble a

pseudomaterial–graphene hybrid film, leading to enhanced electrochemical performances in a flexible supercapacitor. Xie et al. demonstrated a facile one-step approach via a convenient solvothermal route by reacting Ni(acac)2, graphene oxide (GO) and an amount of water in benzyl alcohol, and thus the selfassembled b-Ni(OH)2/graphene nanohybrids with a layer-by-layer structure were prepared.71 Furthermore, by using a gold-coated flexible PET sheet as the counter electrode and PVA/KOH gel as both the electrolyte and separator, an all-solid-state thin-film pseudocapacitor with superior flexibility was successfully fabricated (Fig. 9a). This formation of the novel LBL b-Ni(OH)2/graphene nanohybrids provides a prototype for investigating the electrochemical performance of LBL pseudocapacitive materials for an all-solid-state thin film supercapacitor. The as-fabricated device possesses a high specific capacitance of 660.8 F cm 3 with negligible degradation even after 2000 cycles, which was obviously higher than that of flexible supercapacitors based on 1D nanomaterials (Fig. 9b). Dong et al. also fabricated layer-bylayer (LBL) assembled multilayer films of Co–Al layered double hydroxide nanosheets (Co–Al LDH-NS) and graphene oxide (GO) electrodes for supercapacitors.72 As it is known, good electrical conductivity of substrate materials and high pseudocapacitance materials are two important elements for a high-performance pseudocapacitor. In their work, positively charged Co–Al LDH-NS was successfully exfoliated into single sheets of Co–Al layered

Fig. 9 (a) Schematic illustration of the flexible all-solid-state thin-film supercapacitor with the advantages of the layer-by-layer (LBL) structure built with pseudocapacitive b-Ni(OH)2 as the active material and graphene layers as the conducting material. (b) Cycling stability of the flexible all-solid-state thinfilm supercapacitor measured under repeated bending/extending deformation. Adapted with permission from ref. 71, copyright Elsevier 2013.

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Fig. 10 (a) Schematic illustration of the LBL assembly process for fabricating LBL multilayer films of positively charged Co–Al LDH-NS and negatively charged GO nanosheets. (b) Digital photograph of a 40-bilayer Co–Al LDH-NS/GO film deposited on a flexible PET substrate. (c) Area and specific capacitances of a 40-bilayer Co–Al LDH-NS/GO film before and after reduction. Adapted with permission from ref. 72, copyright American Chemical Society 2011.

double hydroxide (Co–Al LDH) with a thickness of about 1.0 nm. The controlled chemical groups on the GO surface result in its electronegativity, and further reduction of GO enhances the overall conductivity. It has been reported that only the surface area or a very thin layer of the active surface plays a key role during pseudo-capacitive processes. Thanks to the charge matching of R-GO and Co–Al LDH-NS, the LBL assembly, through electrostatic interaction, is beneficial to make use of the surface area of graphene, and further utilization of the pseudocapacitance of Co–Al LDH-NS (Fig. 10a). After assembling a high quality multilayer film, the electrochemical performances of the films were investigated using a three-electrode system which was used in flexible supercapacitor devices. The supercapacitors with a 40-bilayer film exhibited a high specific capacitance of 880 F g 1 and area capacitance of 70 F m 2 under a scan rate of 5 mV s 1, and the film exhibited no obvious decrease in capacitance over 2000 cycles. After treating the multilayer films at 200 1C for 2 h in an H2 atmosphere, the specific capacitance and area capacitance of the above supercapacitor were greatly increased up to 1204 F g 1 and 90 F m 2, due to partial reduction of GO (Fig. 10c). A flexible electrode made by depositing a Co–Al LDH-NS/GO multilayer film on a PET substrate showed potential for flexible energy-storage devices (Fig. 10b). The reversible redox Faradaic reactions occurring on the surface of layered Ni(OH)2 and Co–Al layered double hydroxide nanosheets both exhibit potential windows of only about 0.5 V, which is much lower than the electrochemical potential


window of aqueous solution (1.23 V). That is to say, the energy density of TMH-based flexible supercapacitors is still much lower. In this regard, it is essential to find a new graphene-like layered pseudocapacitive material with a higher redox potential, power density and energy density in order to satisfy the practical applications of power supply. Layered TMDs, such as MoS2, TiS2, WS2, and VS2, have been studied extensively as chemically active electrocatalysts, and energy storage and electronics devices for their versatile chemical properties.73–75,89 In the area of energy storage, for example, MoS2 is an excellent anode material for LIB for its two redox peaks at 2.1 V and 2.5 V74 which refer to the redox reaction occurring on the surface of MoS2, described as follows: MoS2 + 4Li+ + 4e 2 Mo + 2Li2S. However, MoS2 nanosheets are rarely applied to the electrode material of supercapacitors due to their intrinsically low electrical conductivity. To fabricate high performance supercapacitors, there are three major problems for TMDs nanosheets, especially for MoS2 nanosheets, to overcome and resolve.76,77 Firstly, TMDs can be easily oxidized by O2 dissolved in the aqueous electrolyte which is essential for the construction of supercapacitors. Secondly, due to their easily oxidized characteristics, they would be unstable in a high rate charging– discharging process, which is the main characteristic of supercapacitors. Lastly, in the TMDs’ crystallographic structure, the transition metal atomic layer is sandwiched between two sulfur layers via covalent bonding, and van der Waals forces emerge between their multi-layers, allowing easy intercalation of

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foreign ions (H+, Li+). However, pseudocapacitive reactions always occur during charging–discharging processes, and thus the monolayer structure might be destroyed, owing to the valence change of transition metal ion, eventually affecting the cyclability of the supercapacitors. Therefore, much more efforts must be made to apply layered TMDs nanosheets to flexible supercapacitors, especially pseudocapacitors or supercapacitors in an aqueous electrolyte. 3.2.3 2D transition metal carbides and vanadyl phosphate based flexible supercapacitors. Graphene established a new milestone in the rapid development of flexible devices due to its intrinsic advantages in electrical, thermal and mechanical properties; since then has emerged a flood of scientific reports or industrial updates about graphene-based devices, and even IGMs-based devices. With the variety of options for alternative inorganic elements, the diverse IGMs could possess further fantastic properties and functionalities beyond the graphene paradigm. For the construction of flexible supercapacitors, it is of great advantage to use 2D IGMs because of their comparable good electrical conductivity and ability for the effective electrosorptions of the electrolyte ions, or greater probability of an extra contribution from the abundant active pseudocapacitive centers from inorganic frameworks, thereby producing the excellent electrochemical performance of 2D-IGM-based flexible supercapacitors. A recent work presented a well-defined proof of this point. Lukatskaya et al. demonstrated high performance EDLCs based on two-dimensional Ti3C2 MXene which combined

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good electrical conductivities with a hydrophilic surface, as well as the electrochemical intercalation of a variety of cations.78 To be specific, MXenes are 2D layered nanomaterials synthesized by the extraction of ‘‘A’’ layers from MAX phases, which are known as layered carbides or carbonitrides. In this work, the Ti3C2 MXene layer has shown promise as an electrode material for its ability to be intercalated by some cations of Li+, Na+, Mg2+, etc., making up for its ‘bottleneck’ of a low specific surface area. Intercalation capacitance can far exceed double-layer capacitances, and enhanced electrosorption of ions was observed on the Ti3C2 MXene layer. Benefiting from the above chemical mechanisms of both intercalation and double layer capacitance, flexible Ti3C2Tx paper electrodes were fabricated with superior performances where T represents surface terminal group (O, OH and/or F) and x is the number of terminal groups. In KOH aqueous electrolyte, the volumetric capacitance reached 340 F cm 3 which is much higher than activated graphene. The Ti3C2 MXene layer, with flexibility, high electric conductivity and unique intercalation character, opens a new way of developing high performance flexible energy storage devices (Fig. 11). Another representative example is not too far away: layered phosphates. Wu et al. introduced a1-vanadyl phosphate (VOPO4) ultrathin nanosheets with less than six atomic layers, exhibiting an extremely high redox voltage of about 1.0 V as the cathode material in an aqueous energy storage device, approaching the limited electrochemical window of an aqueous solution.6 Although the exfoliated VOPO4 nanosheet shows great potential

Fig. 11 Electrochemical performance of binder-free Ti3C2Tx paper electrodes: (a) schematic illustration of electrode fabrication. Firstly, the multilayer Ti3C2Tx powders are delaminated to produce few-layer MXene flakes; the resulting colloidal solution is filtered through a porous membrane, producing binder and additive-free Ti3C2Tx paper electrodes. (b) SEM images of paper electrode. Inset shows a digital photograph of the flexibility of the paper electrode. (c) CV curve of Ti3C2Tx paper in a KOH electrolyte. (d) EIS data in KOH for the Ti3C2Tx electrode (KOH, solid symbols) and Ti3C2Tx paper (p-KOH). (e) Rate performance of the Ti3C2Tx paper (open symbols) compared with multilayer exfoliated Ti3C2Tx electrode (solid symbols) in KOH-, MgSO4-, and NaOAc containing electrolytes. (f) Capacitance retention test of Ti3C2Tx paper in KOH. Inset: Galvanostatic charging–discharging test collected at 1 A g 1. Adapted with permission from ref. 78, copyright AAAS 2013.

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Fig. 12 (a) TEM image of the 2D VOPO4–graphene hybrid structure, scale bar, 80 nm. (b) SEM image of the hybrid thin film in a cross-sectional view with a thickness of B90 nm. Scale bar: 500 nm; the inset shows the surface of the hybrid film, in which the graphene lies under the 2D VOPO4 layers, as indicated by the white arrows. Inset scale bar: 100 nm. (c) Schematic illustration of the layer-by-layer structure of the assembled 2D VOPO4/graphene hybrid thin film. (d) Schematic illustration of the as-fabricated flexible pseudocapacitor in an all-solid-state system, in which the 2D VOPO4/graphene hybrid layers are the working electrode on the gold-coated PET sheet and PVA/LiCl gel is the electrolyte. (e) CV curves of the 2D VOPO4/graphene hybrid film in a PVA/LiCl gel electrolyte when the working voltages ranges from 0 V to 1.2 V at 0.1 mV s 1. (f) Galvanostatic charge–discharge curves at different current densities of 0.2, 0.4, 0.6 and 1 A m 2. Adapted with permission from ref. 6, copyright Nature Publishing Group 2013.

for application in flexible supercapacitors owing to its 2D layer structure and high potential window, the poor electrical conductivity of the VOPO4 nanosheet hinders the performance of the as-fabricated supercapacitors. A layer-by-layer VOPO4/ graphene hybrid film with a thickness of about 90 nm was assembled with both high conductivity and pseudocapacitive performance (Fig. 12a and b). In this case, the formation of the ordered hybrid structure relies on the existence of hydrogen bonds as the interaction force, and the order of stacking by which the final VOPO4/graphene hybrid films were achieved (Fig. 12c). Moreover, PVA/LiCl was introduced as both the solid electrolyte and the separator, further preventing the potential dissolution of the active material of the VOPO4 nanosheets because of its neutral pH (Fig. 12d).79 Notably, reversible a pseudocapacitive reaction of VOPO4 nanosheets with Li+ can be represented: VOPO4 + xLi+ + xe 2 LixVOPO4. Consequently, the design of a hybrid structure brings high performance with a high specific capacitance (8360.5 mF cm 2) and redox voltage (up to 1.0 V) which leads to an ultra-high energy density of 1.7 mW h cm 2 and a power density of 5.2 mW cm 2. These values are the highest among the flexible pseudocapacitors in all-solid-state systems, with negligible degradation of the specific capacitance even after 400 bending cycles. This superior pseudocapacitive performance is ascribed to the 2D design of the hybrid thin film which consists of highly pseudocapacitive 2D materials composite on a highly conductive graphene matrix.


The principles of construction of flexible supercapacitors have been widely investigated in recent years. The flexible electrode materials involve both pure materials such as CNT, carbon nanocups, graphene, transition metal carbides and hybrid nanostructures such as graphene/1D nanomaterials or graphene/IGM composites. However, the fundamental structures and intrinsic mechanisms have not been fully understood. In fact, great challenges still remain for further great enhancement of the energy densities of flexible supercapacitors, catering to the great demands for power supply in practical applications. Therefore, future-generation electrode materials with high voltage windows and superior cycling stability should be explored urgently.

4. Planar supercapacitors in all-solidstate (micro-supercapacitors) Recent advances in flexible supercapacitors have led to the trend of searching for new alternative materials with superior electrochemical performance and excellent mechanical properties of high flexibility. However, in addition to the role of the electrode material, the design of a of supercapacitor device configuration including electrode materials, the electrolyte, and the current collector is also required. Various methods have been developed on decreasing thickness of the supercapacitors to improve their flexibility and specific capacitance, but there is no guarantee to

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Fig. 13 Schematic illustration of the planar supercapacitor using aligned 3D CNT forest electrodes. Adapted with permission from ref. 80, copyright IEEE 2009.

prevent short circuits when there is no separator in the device. Moreover, when a supercapacitor works at different bending states, it still requires the ions to be transported smoothly under the effects of mechanical strain. Consequently, how to enhance effectively the performance of a flexible supercapacitor still remains a big challenge. In order to push the performance of flexible supercapacitors to a higher level, further breakthroughs are required for new configuration design of supercapacitor itself. Herein, planar supercapacitors are proven to be a reasonable option. By optimizing the ion transport mechanism in the charging–discharging processes, planar supercapacitors would be a possible way to relieve the existing problems. Planar supercapacitors, also referred to microcapacitors, are regarded as a novel branch of supercapacitors, requiring three requisite components integrated onto the same plane, including the electrode material, electrolyte, and current collector.50–52,84,90 The planar configuration design offers planar channels for electrolyte ions, facilitating fast ion transport in the two-dimensional direction. Furthermore, the planar configuration would not affect the ion transport when the supercapacitors are under different bending states, such as folded, rolled and so on. For example, Jiang et al. reported a simple, yet versatile planar MEMS supercapacitor using porous CNT forests as electrodes with low contact resistance without using the top/bottom electrode architecture.80 Unlike the conventional sandwich-type structure, the two symmetric electrodes were placed on the same plane (Fig. 13). The major advantages of this structural design included size reduction and simplification of the fabrication process. Furthermore, the transport distance of ions in the electrolyte could be well controlled and shortened while eliminating the necessity of a separator. However, the CNT forest electrodes are not fully employed due to their dense 3D structures, which blocks the access of the ions to the inside surfaces and interfaces of the CNT forests. Although it turned out that the utilization of surface areas was only about 60%, it was still meaningful that this novel design became the prototype of planar supercapacitors and was widely used in the following investigations.

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To understand further the inner mechanism on electrode materials for planar supercapacitors, Chmiola et al. used carbide-derived carbon (CDC) as an electrode which was about 10 mm thick in 2D thin-film micro-supercapacitors with several square centimeter footprints.81 Interestingly, there was a huge increase in volumetric capacitance as the film thickness decreased from 200 to B2 mm. For example, for a CDC film of B50 mm in TEABF4, the volumetric capacitance (B60 F cm 3) is similar to that was observed in a B300 mm traditionally processed electrode from carbon powder (B50 F cm 3). When CDC films thicker than B100 mm were placed in a 1 M H2SO4 electrolyte, the volumetric capacitance is similar to that measured on the 300 mm traditionally processed electrode. As the coating thickness decreases to B2 mm, the volumetric capacitance increases greatly to nearly 180 F cm 3 in TEABF4 electrolyte and B160 F cm 3 in 1 M H2SO4. It can be noted that the thinner film electrodes gave respectable gravimetric and volumetric specific capacitance. Conversely, the decrease in capacitance with thicker films is most likely due to microstructural rearrangement from relaxation of surface stresses, which leads to collapse of the porosity and perturbation of the interconnected structure that facilitates electron conduction. The effects of electrolyte starvation caused by thicker films coupled to structural collapse may also aggravate the drop in capacitance with thicker films which require a larger number of ions to migrate from the bulk electrolyte into the pore structure. We can summarize that there are three key points to achieve the maximum performance of planar supercapacitors: (1) reducing the thickness of electrode materials to attain high gravimetric capacitance; (2) increasing the utilization of surface areas to provide more reaction sites; (3) optimizing ion transport channels for the transmission of electrolyte ions. Hence, 2D nanomaterials with all the above characteristics are expected to be promising candidates as thin film electrode materials in planar supercapacitors. 4.1 1D nanomaterials based flexible planar supercapacitors in all-solid-state The in-plane structure developed above provides a significant extension to flexible supercapacitors, yet the supercapacitors used above are still on rigid substrates. To achieve high flexibility, all-solid-state flexible supercapacitors based on a carbon/MnO2 (C/M) core shell fiber structure were fabricated onto a flexible PET substrate.82 It is well known that using serial and parallel assemblies would be an effective way to meet high voltage window and high energy density. Xiao et al. connected the three SCs in series in their study exhibiting a 2.4 V charge–discharge voltage window with almost the same discharge time compared with a single SC, which operates at 0.8 V (Fig. 14c). Besides, high electrochemical performance such as high rate capability with a scan rate up to 20 V s 1, high volume capacitance of 2.5 F cm 3, and an energy density of 2.2 104 W h cm 3 were achieved in these cases. Moreover, high flexibility and easy scale-up ability were also achieved, demonstrating potential applications for different operation voltages and meeting energy demand (Fig. 14a and b). Liu et al. demonstrated recently a flexible all-solid-state planar-integrated fiber supercapacitor based on hierarchical ZnCo2O4 nanowire


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Fig. 14 (a) Digital photographs of flexible supercapacitor bent at different angles. (b) CV curves of flexible supercapacitor bent at different angles. (c) Galvanostatic charge–discharge curves of single SC and three SCs connected in series. The inset shows the schematic illustration of the three SCs connected in series. Adapted with permission from ref. 82, copyright American Chemical Society 2012.

Fig. 15 (a) Digital photograph of the flexible planar-integrated fiber supercapacitors with several ZnCo2O4 nanowire–carbon composite fiber electrodes. (b) Galvanostatic charge–discharge curves of the first cycle at 1 A g 1 ranging from 0–1.5 V. (c) The analysis of the equivalent circuit of planar supercapacitors. (d) Enhanced distributed-capacitance effect on capacity for the flexible planar-integrated fiber supercapacitors. Adapted with permission from ref. 83, copyright Wiley-VCH 2013.

arrays/carbon fibers electrodes on a flexible substrate.83 In this work, various flexible fiber supercapacitors were fabricated by planar integration of 2, 6, 10, 14, 20, and 30 composite fibers and an enhanced distributed-capacitance effect was found. The planar supercapacitors presented here can be either scaled up by integrating more ZnCo2O4 nanowires–carbon composite fibers electrodes on the same substrate to meet higher power and energy requirements, or decrease to a small size to fit for micro-power sources with lower capacity and smaller dimensions in specific


applied fields (Fig. 15a and c). This feasible and meaningful work provides a simple method to regulate the performance of planar supercapacitors for practical use. 4.2 Two-dimensional nanomaterials for flexible planar supercapacitors in the all-solid-state 4.2.1 Conductive 2D nanomaterials for flexible planar supercapacitors in the all-solid-state. The earlier focus on flexible planar supercapacitors was exclusively limited to 1D

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Fig. 16 (a) Schematic illustration of the stacked geometry used for the fabrication of sandwich-type supercapacitor devices (left) and the in-plane geometry used for planar supercapacitor devices (right); (b) cyclic stability of the RMGO supercapacitor by performing charge–discharge over 1500 cycles. (c) A digital photograph of a planar supercapacitor device with high flexibility based on RMGO developed using the new in-plane geometry. Adapted with permission from ref. 84, copyright American Chemical Society 2011.

nanomaterials. Considering that planar supercapacitor design calls for electrodes with 2D permeable channels, the use of 2D nanomaterials has been motivated to a large degree from its potential of multiple advantages. 2D nanomaterials are promising material platforms to fabricate flexible energy storage devices due to their unique 2D morphologies, high specific surface areas and the high mechanical stability (microscopic compressibility and macroscopic extension). With the advent of atomically thin and flat layers of conducting 2D nanomaterials such as graphene, part of a TMO nanosheet/nanoflake and TMD nanosheets, new designs for thin-film supercapacitors with good performance have become possible. A wide variety of 2D nanomaterials are adopted to fabricate planar supercapacitors. As the most prototypical 2D nanomaterial, graphene was studied by many researchers as an electrode material in the initial stages for its high electron conductivity and high surface area. For example, Yoo et al. reported an ‘‘in-plane’’ fabrication approach for ultrathin supercapacitors based on electrodes comprised of multilayer reduced graphene oxide.84 As mentioned above, the stacked graphene sheets hinder the performance of graphene-based energy storage devices. In this work, in-plane design is straightforward to implement and exploits efficiently the surface of each graphene layer; the movements of the ions are favored along the planar graphene sheet rather than perpendicular to it (Fig. 16a). Besides, the layer-bylayer (LBL) method was used to create MGO films which were then chemically reduced by hydrazine to form RMGO films. The electrostatic interactions between the cationic polymer (poly ethyleneimine) (PEI) and graphene oxide (GO) allow for the creation of multi-layered graphene oxide (MGO) films rather than stacked geometries. Since the thickness of the RMGO sample is 10 nm, the 2D in-plane design takes advantage of the atomic layer thicknesses and flat morphology of graphene and is ideal for 2D devices to reach specific capacities up to 80 mF cm 2, while much higher (394 mF cm 2) specific capacities are observed on multilayer reduced graphene oxide electrodes. The favorable in-plane design demonstrated here offers new opportunities for the electrolyte ions to enhance interaction with

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2D nanomaterials, leading to a full utilization of the high surface area offered by the 2D thin-film layers. The demonstrated planar supercapacitors in the all-solid-state and provide a prototype for 2D thin-film based energy storage devices. As well as graphene nanosheets, IGMs are also regarded as a promising electrode material platform for planar supercapacitors with high flexibility. However, TMDs are ill-suited for fabricating flexible supercapacitors with a sandwich structure, due to their properties of easy oxidization and poor pseudocapacitive performance.76,77 Planar supercapacitors may appear as a favorable candidates to mitigate these problems. Conducting IGMs hold promising potential for the construction of flexible supercapacitors, but it is relatively difficult to gain conducting IGMs because most of the reported IGMs are either semiconducting (MoS2 and WS2) or insulating (BN and BCN).85–87 Before the investigation of conducting 2D VS2,88,89 few works on metallic IGMs were reported. As a typical conducting TMD, the VS2 ultrathin nanosheet is made up of metal V layers which were sandwiched between two S layers, and thus stacked in several layers by weak van der Waals interactions (Fig. 17). Since 2D electron–electron correlations among V atoms would preferably induce more complicated planar electric transportation properties, it is likely to be a preferable choice for planar

Fig. 17 Side-view of the atomic structure of the bulk VS2 crystal (left). The atomic structure of the bulk VS2 crystal projected along c axis, showing the notable quasi-two-dimensional structural characteristics, in that all vanadium atoms are in the same plane (right). Adapted with permission from ref. 89, copyright American Chemical Society 2011.


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Fig. 18 (a) Schematic illustration of planar ion migration pathways for the in-plane supercapacitor. (b) Schematic illustration of the in-plane configuration of the as-fabricated supercapacitor. (c) CV curves of the as-fabricated supercapacitor at different scanning rates of 20, 100, and 200 mV s 1. (d) Galvanostatic charge–discharge curves and IR drop illustration of the as-fabricated supercapacitor. (e) Galvanostatic charge–discharge curves of the as-fabricated in-plane supercapacitor at current densities of 0.1, 0.2, 0.4 A m 2. (f) Cyclic stability of the supercapacitor, showing negligible degradation in the coulombic efficiency (blue) and specific capacitance (green). Adapted with permission from ref. 89, copyright American Chemical Society 2011.

supercapacitors. To avoid degradation of the VS2 nanosheet electrodes, Feng et al. used a solid-state polymer electrolyte prepared by mixing the PVA gel with a water-soluble ionic liquid of BMIMBF4.89 More importantly, the VS2 ultrathin nanosheet represents a promising planar metallic behavior and high specific surface area which is an ideal electrode material for electrical double layered capacitors (EDLC). As expected, a specific capacitance of 4760 mF cm 2 was realized in a 150 nm planar configuration, of which no obvious degradation was observed even after 1000 charge–discharge cycles, offering a new planar supercapacitor with high performance based on quasi-two-dimensional nanomaterials (Fig. 18). 4.2.2 Pseudo-capacitive 2D nanomaterials for flexible planar supercapacitors in the all-solid-state. Although only one conducting IGMs has shown great promise for planar supercapacitors, their electrochemical performance is still far from that of traditional pseudocapacitors. In this case, how to involve additional redox reactions in the pseudocapacitive behavior has been a significant way to enhance further the electrochemical performance of planar supercapacitors. From this viewpoint, 2D pseudocapacitive nanomaterials are a promising platform for creating high performance planar supercapacitors. However, most pseudocapacitive nanomaterials have poor electron conductivity in the insulating state, which greatly hinders the electrochemical performance of the supercapacitors. In order to achieve the synergic advantages of high electrochemical activity and enhanced electrical conductivity, Peng et al. presented a novel hybrid nanostructure composed of distinct quasi-2D ultrathin nanosheets to enhance the supercapacitor performance. Specifically, a 2D d-MnO2 nanosheet was integrated on graphene sheets as the electrode of the planar structures (Fig. 19).90 The integrated d-MnO2


nanosheets served as active centers for the pseudocapacitive reactions contributing to great enhancement of the specific capacitance. The redox reaction during this process can be described as follows: MnO2 + xH+ + xe 2 MnOOHx. Meanwhile, the d-MnO2 integrated on graphene could potentially tailor the distance between each layer of the densely stacked graphene, and open up the interlayer space to allow for more electrolyte ions to penetrate efficiently into the hybridized film. This rational design can introduce more electrochemically active surfaces for the absorption–desorption of the electrolyte ions, and thus bring an extra interface to the hybridized interlayer areas to facilitate charge transport during the charging– discharging processes. Consequently the hybrid 2D nanostructure design enhances the electrochemical performance of the as-fabricated planar supercapacitors with a high specific capacitance of 267 F g 1 at a current density of 0.2 A g 1, excellent rate capability and cycling performance with a capacitance retention of 92% after 7000 charge–discharge cycles. More importantly, given the high microscopic compressibility and macroscopic extensibility of the hybridized planar structure, the as-fabricated planar supercapacitors display extraordinary mechanical flexibility and stability under various bending states from curving and folding to rolling, with 490% capacitance retention after being folded for thousands of times (Fig. 20). In addition, hybridization of distinct quasi-2D ultrathin nanosheets provides a novel model to improve the electrochemical performance of flexible supercapacitors. These optimized 2D hybrid thin film structures represent a promising direction for building future-generation high-performance, flexible energy storage devices. In general, the literature reported above summarized the recent progress in planar supercapacitors. Specially, conductive

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to see that conductive 2D nanomaterials lack pseudocapacitance while pseudocapacitive 2D nanomaterials are short of electron conductivity. In order to solve this dilemma, the suggested material should have high electron conductivity, a high voltage window, excellent pseudocapacitive performance and a simple way to provide a 2D ion migration channel. Further works should concentrate on the search for new 2D nanomaterials with the above merits. What’s more, increasing the capacitance of the planar supercapacitors to a high level is also necessary in future work.


5. Conclusions and outlooks

Fig. 19 (a–d) Schematic illustration of hybrid 2D d-MnO2/graphene structure based planar supercapacitors. (a) Schematic illustration of the flexible planar supercapacitor, constructed with the hybrid thin film as the working electrode, the current collector, and gel electrolyte on a flexible PET substrate. (b) Ion migration channel of the planar supercapacitor showing the 2D hybrid thin film as two symmetric working electrodes. (c) The hybrid thin film was made of layers of chemically integrated quasi-2D d-MnO2 nanosheets on graphene. (d) Schematic illustration of 2D planar ion transport within the 2D d-MnO2/ graphene hybrid structures. (e) TEM image of the 2D hybrid structure with d-MnO2 nanosheets integrated on the graphene surface. (f) SEM images of the surface of the hybrid thin film. (g) SEM images of the hybrid thin film in a crosssectional view with an average film thickness of B40 nm. Adapted with permission from ref. 90, copyright American Chemical Society 2013.

Fig. 20 (a) CV curves for the planar supercapacitor under three different bending states. (b) Digital photograph of the folded/rolled sample and the comparison of specific capacitance between the MnO2–graphene sample and pure graphene samples at different current density. Adapted with permission from ref. 90, copyright American Chemical Society 2013.

2D nanomaterials and pseudocapacitive 2D nanomaterials as ultrathin electrode materials were reviewed. However, it is easy

3320 | Chem. Soc. Rev., 2014, 43, 3303--3323

In this review article, we have summarized recent progress in flexible supercapacitors by applying 2D nanomaterials as electrode materials. 2D nanomaterials have shown the promising advantages of high specific surface area, excellent mechanical performance with high compliance and potential pseudocapacitive behavior, providing an ideal platform for the assembly of thin film structures that is vital for the construction of flexible supercapacitors. In this regard, finding new 2D nanomaterials with higher redox reactions with better pseudocapacitive behavior would be the next essential step for flexible supercapacitors with further improved electrochemical performances. Apart from the selection of electrode materials, designed of the configuration of the thin film electrode shows next performance growth points for flexible supercapacitors, for example, in-plane supercapacitors. In planar supercapacitors, all the three ingredients of collector, electrode, electrolyte, are all in the same horizon, in which an assembled thin film of quasi-2D graphene and inorganic graphene-like materials acted as an excellent material platform for planar configurations. Planar supercapacitors could optimize the 2D ion migration channels and enhance performances in the construction of hybrid planar energy storage devices. To sum up, 2D nanomaterials pave a new way for the construction of next-generation flexible supercapacitors with improved electrochemical performance. Although considerable progresses have been made in flexible supercapacitors regarding the discovery of novel materials, technical improvements and planar configuration design, there are still real problems and obstacles at the current stage. For example, developing 2D nanomaterials with both high electric conductivity and pseudocapacitive performance remains a challenge. Most electrode materials with pseudocapacitive behavior are insulators, that do not satisfy the requirements of electron and ion transport during the electrochemical processes. Although hybridizing insulating pseudocapacitive materials with conducting graphene have received considerable attention, heterostructural interface problems still exist due to the weak interaction forces in the hybrid structure, not the crystalline bonding heterostructure. For conducting 2D IGMs, a way to introduce further electrochemical active sites is urgently needed, in order to promote electrochemical behavior under high rate conditions. Moreover, to realize higher flexibility and better performance, other experimental factors such as the selection of substrate and electrolyte also have an influence in the electrochemical properties of 2D


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nanomaterials. In effect, how to deposit effectively the electrode materials in a flexible plastic substrate with ohmic contact is still a great challenge. For the electrolyte in flexible supercapacitors, although the gel electrolytes, such as PVA–LiCl, PVA–H3PO4, PVA–KOH, etc., work well in the flexible supercapacitors, more efforts are needed to develop new types of polymer electrolytes with higher ionic conductivity and lower electronic conductivity, catering for the construction of flexible supercapacitors with superior performance.


Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21222101, 11132009, 21331005, 11321503, J1030412), the Chinese Academy of Science (XDB01010300), the Program for New Century Excellent Talents in University, and the Fundamental Research Funds for the Central Universities (No. WK2060190027 and WK2310000024).

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Chem. Soc. Rev., 2014, 43, 3303--3323 | 3323

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