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Advanced Review

Review of nanostructured carbon materials for electrochemical capacitor applications: advantages and limitations of activated carbon, carbide-derived carbon, zeolite-templated carbon, carbon aerogels, carbon nanotubes, onion-like carbon, and graphene Wentian Gu and Gleb Yushin∗ Electric double layer capacitors, also called supercapacitors, ultracapacitors, and electrochemical capacitors, are gaining increasing popularity in high power energy storage applications. Novel carbon materials with high surface area, high electrical conductivity, as well as a range of shapes, sizes and pore size distributions are being constantly developed and tested as potential supercapacitor electrodes. This article provides an overview of the electrochemical studies on activated carbon, carbide derived carbon, zeolite-templated carbon, carbon aerogel, carbon nanotube, onion-like carbon, and graphene. We discuss the key performance advantages and limitations of various nanostructured carbon materials and provide an overview of the current understanding of the structure–property relationships related to the transport and adsorption of electrolyte ions on their surfaces, specific and volumetric capacitance, self-discharge, cycle life, electrolyte stability, and others. We discuss the impact of microstructural defects, pore size distribution, pore tortuosity, chemistry and functional groups on the carbon surface, nanoscale curvature, and carbon-electrolyte interfacial energy. Finally, we review state-of-the art commercial large scale applications of supercapacitors, including their use in smart grids and distributed energy storage, hybrid electric and electric vehicles, energy efficient industrial equipment, ships, wind power stations, uninterruptible power supplies, power backup, and consumer devices. © 2013 John Wiley & Sons, Ltd. How to cite this article:

WIREs Energy Environ 2013. doi: 10.1002/wene.102

INTRODUCTION ∗

Correspondence to: [email protected]

Georgia Institute of Technology, School of Materials Science and Engineering, Atlanta, GA, USA Conflict of interest: The authors have declared no conflicts of interest for this article.

E

lectric double-layer capacitors (EDLCs) represent a unique type of high-power electrochemical energy storage devices, where the capacitance arises from the charge separation at an electrode–electrolyte

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developed. As a result, the range of commercial applications of electrochemical capacitor technology is expanding (Figure 2) and novel materials are being constantly developed and tested as potential EDLC electrodes. Maxwell Technologies (the largest US manufacturer of EDLC cells and modules) has reported nearly six times increase in the EDLC sales in the last 5 years, although from a relatively small base.

100 000

Specific power (W kg–1)

(a)

10 000

1000

100

10 1

10

100

Specific energy (Wh kg–1) 100 000

Volumetric power density (W L–1)

(b)

10 000

1000

100

10 1

10

100

Volumetric energy density (Wh L–1)

FIGURE 1 | Schematic energy characteristics of various types of commercially available electrochemical capacitors in comparison with lead-acid, nickel metal hydride and Li-ion batteries as a simplified Ragone plot: (a) specific (mass-normalized) power versus specific energy and (b) volumetric power versus energy density.

interface. The energy storage in EDLCs is based on the adsorption of electrolyte ions on the large specific surface area of electrically conductive porous electrodes, most commonly porous carbons. The key advantages of EDLCs over batteries include much higher power density (Figure 1), much lower internal resistance, broader temperature window of a stable operation, very rapid charging (in seconds or less), higher round-trip efficiencies and significantly longer cycle life possible (millions of cycles). The main limitation of EDLCs is their lower energy density than batteries (Figure 1) combined with a high cost per unit energy. The cost of EDLCs in the last decade has been decreasing significantly faster than that of batteries. The progress in the performance improvements of the EDLCs has been noticeably more rapid as well. In addition, other types of electrochemical capacitors (such as asymmetric capacitors where only one electrode is of an EDLC-type) with higher energy density (Figure 1) and lower cost have recently been

GENERAL PRINCIPLES AND CELL CONSTRUCTION The simplest building block of an EDLC consists of two high surface area porous conductive electrodes (commonly based on carbon) immersed into electrolyte and separated by an ion-conducting and electron-insulating separator membrane. When a voltage is applied across these electrodes the electrolyte ions of the opposite sign accumulate on the surface of each electrode. The areal concentration of such ions is commonly proportional to the applied voltage. The charge separation between an electrode space charge and a layer of electro-adsorbed ions is called an electric double layer.1–3 During charging and discharging of a pure EDLC no charge transfer takes place across the electrode/electrolyte interface. However, many carbon electrode materials contain functional groups on their surfaces, which may exhibit chemical and electrochemical interactions with selected electrolytes. In case when such interactions involve fast and reversible charge transfer reactions between the carbon surface and the electrolyte ions, a battery-like Faradaic charge storage process on the electrode surface (often called pseudocapacitance) may complement previously described electrostatic ion adsorption of an ideal EDLC. Somewhat similar to a gas adsorption, the adsorption of ions under the application of an electric field could be described by an adsorption isotherm, which describes the ion surface coverage at constant temperature as a function of their concentration, controlled by the applied voltage.4–6 In contrast to gas sorption, however, the ion adsorption has a very weak dependence on temperature. The balance between the changes in entropy and enthalpy of a system controls the ion concentration on the electrode surface, which minimizes the total Gibbs free energy of the system. When ion adsorption sites have a very broad distribution of enthalpies, the concentration of electroadsorbed ions follows a linear dependence on the applied voltage for a wide range of concentrations, as typically observed in some pseudocapacitive materials and in EDLCs.7,8 However, if the enthalpy of ionelectrode interactions deviates weakly from its average

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Nanostructured carbon materials for electrochemical capacitor applications

(a)

(b)

(c)

(d)

(e)

(f)

FIGURE 2 | Examples of the large-volume applications of electrochemical capacitors: (a) an electric bus with a small distance range but fast (1 min) charging (Yaxing, China) (Reproduced from China Buses.com), (b) a hybrid energy-efficient forklift (Still, Germany) (Reproduced with permission from Still GmbH), (c) off-shore variable speed wind turbines with enhanced reliability and efficiency (Reproduced with permission, Copyright: F. Schmidt/Fotolia), (d, e) hybrid energy-efficient automated stacking and harbor cranes (Gottwald, Germany) (Reproduced with permission of Gottwald Port Technology GmbH), (f) an electric ferry with rapid charging and low vibration operation (STX Europe, South Korea). (Reproduced with permission from STX Europe)

value, the pseudocapacitance will exhibit a sharp peak at certain voltage values.8 Energy density of an EDLC is determined by the capacitance of positive, C+ , and negative, C− , EDLC electrodes and the maximum voltage, Vmax , at which the device can be operated:    2 C− · C+ EDLC EDLC = . (1) E · Vmax C− + C+ When the capacitance of both electrodes is the same, the EDLC energy is maximized: EEDLC =

1  EDLC 2 . C· Vmax 2

(2)

To achieve high energy density, one shall maximize the volumetric capacitance of each electrode and increase the maximum allowed operational voltage of EDLCs. The maximum voltage is generally determined by the stability window (from the lowest stable stable stable potential Vlow to the highest stable potential Vhigh ) of the selected electrolyte (Figure 3(a)). Impurities and functional groups on carbon, however, may stable catalyze electrolyte decomposition and increase Vlow stable or decrease Vhigh . The potential of each electrode

in a fully discharged symmetric EDLC is the same stable stable and Vhigh (Figure 3(a)). and located between Vlow When an EDLC is galvanostatically charged (by a constant current), the potential of each electrode changes linearly with time. In an ideal case, the stable at exactly the negative electrode lowers to Vlow same time as the potential of the positive electrode stable raises to Vhigh . In this case, the maximum EDLC EDLC voltage Vmax can approach the maximum electrolyte electrolyte stable stable = Vhigh − Vlow . In a nonideal stability Vmax case, one of the electrodes reaches the boundary of the allowed potential window faster than the other (Figure 3(b)). In this case the highest voltage applicable electrolyte EDLC will be lower than Vmax . to the EDLC Vmax By doping one of the electrodes, however, one may change their corresponding potential in the EDLC fully discharged state in such a way as to counterbalance the difference in their rates of potential change (Fig. 3c). Alternatively, by using different thicknesses of the electrodes the slopes of the potential change could be adjusted. Asymmetric electrochemical capacitors utilize different types of materials for positive and negative electrodes. In most cases one of the double layer porous carbon electrodes is replaced with a battery-type higher capacitance electrode capable of

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(a)

EDLC

(b)

electrolyte

Ideal EDLC: Vmax ≈ ΔV max Electrolyte oxidation

EDLC

Non-ideal EDLC: Vmax Electrolyte oxidation

stable

V high

Potential of an EDLC cathode

Potential of an EDLC cathode Electrolyte stability window

ΔV

electrolyte

< ΔV max

Initial potential of EDLC anode and cathode

electrolyte max

Electrolyte stability window

Max voltage of an EDLC

Initial potential of EDLC anode and cathode Potential of an EDLC anode

electrolyte

ΔV max

EDLC

V max

Potential of an EDLC anode

Max voltage of an EDLC EDLC

V max

stable

Electrolyte reduction Energy

(c)

V low

Electrolyte reduction

Charge

Hold

Energy

Discharge

EDLC electrolyte Non-ideal after doping one of the electrode: Vmax ≈ ΔV max

Charge

Hold

Discharge

(d) 5.0

Electrolyte oxidation

Organic electrolyte oxidation

Electrolyte stability window

Initial potential of an EDLC cathode

Max voltage of an EDLC

electrolyte

ΔVmax

Initial potential of a doped EDLC anode

EDLC

V max

Potential of doped EDLC anode

Potential vs. Li/Li+

4.0 Potential of an EDLC cathode

3.0

2.0

1.0 Electrolyte reduction

Potential of EDLC electrodes

Potential of an ‘EDLC’-like cathode of an asymmetric capacitor

Potential of a graphite anode of an asymmetric capacitor

Vmax of an EDLC ~ 2.7 V

Vmax of an asymmetric cell ~ 4 V

Organic electrolyte reduction

0.0 Energy

Charge

Hold

Charge

Discharge

Charge

Hold Hold

Discharge Discharge

FIGURE 3 | Schematic energy diagrams of (a and b) regular and (c) doped EDLC electrodes as well as (d) electrodes in an asymmetric capacitor upon charging and discharging. In (b) doping of one of the electrodes compensates for the difference in the capacitance between positive and electrodes, thereby increasing the maximum operational voltage of an EDLC. In (d) a battery-like graphitic negative electrode operates at the potential below the decomposition limit of the organic electrolyte. However, the layer of decomposed electrolyte (called SEI) is electrically isolative and is stable, which prevents further electrolyte decomposition. The area within the discharge curves (colored in slightly darker green or darker blue) is proportional to the energy stored in each device.

Faradaic ion storage. Such designs increase maximum device voltage and the total energy density. For example, in case of electrochemical capacitors with organic electrolytes, lowering a potential of the negative electrode (anode) below the stability value will trigger electrolyte reduction. In some cases, though, reduced electrolyte may produce stable electrically isolative solid products that uniformly coat the electrode surface, forming a so-called solidelectrolyte interphase (SEI) layer, which remain impermeable to electrolyte solvent and thus prevents further electrolyte reduction. Unfortunately, the SEI disrupts formation of a double-layer and induces resistance for the ion transport. However, if a double layer negative electrode is replaced with a battery-type electrode material capable of Faradaic ion storage higher maximum device voltage can be achieved in such an asymmetric capacitor (Figure 3(d)). In one

example, a porous carbon EDLC negative electrode can be replaced with an intercalation-type graphitic negative electrode in a Li-ion containing electrolyte. In this case one can gain over 1.2 V of additional device voltage (Figure 3(d)). Meeting the SEI stability and high current performance requirements with graphite, however, is challenging. The bonus advantage of hybrid devices is significantly higher volumetric capacity of battery-type electrodes over porous carbon EDLC electrodes, which further increases the overall device energy density. We will briefly review several types of asymmetric capacitors in the end of this review.

ELECTROLYTES Electrolytes used in EDLCs may be divided into three classes: (1) aqueous (solutions of acids, bases and

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Nanostructured carbon materials for electrochemical capacitor applications

salts), (2) organic (for pure EDLCs—most commonly solutions of tetraethylammonium tetrafluoroborate (TEATFB) salt either in anhydrous acetonitrile (AN) or propylene carbonate (PC) solvents), and (3) ionic liquids (IL). Several recent publications provide a comprehensive review of the room temperature IL properties and compositions.9–11 Aqueous electrolytes are typically stable to 0.6–1.4 V in symmetric EDLCs, organic electrolytes to 2.2–3.0 V, and ILs to 2.6–4.0 V. As the energy density of an EDLC strongly depends on the maximum voltage applied to the device (Eqs. (1) and (2)), the choice of electrolyte may significantly affect the energy density of EDLC. For example, one of the key advantages of IL-based EDLCs is their very high energy density. In addition, ILs are nonflammable, which is important for many mobile applications, including their potential use in hybrid vehicles. Serious shortcomings of ILs are their very high (often prohibitively high) current cost and relatively low ionic mobility at room temperature and below, which limits the charge/discharge rate of IL-based EDLCs. Novel methods of IL synthesis may potentially make them more affordable. The advantages of using aqueous electrolytes include their very low cost, safety and high ionic conductivity. Their disadvantages, however, include their low maximum applied voltage and, in some cases, corrosion of EDLC electrodes observed at higher temperatures and voltages (particularly for acid-based electrolytes, such as H2 SO4 solutions), which limits the cycle life of the EDLCs and contributes to self-discharge. Organic electrolytes are somewhat in between aqueous ones and ILs in terms of the price, voltage and charge–discharge time. Organic electrolyte-based EDLCs offer cycle life in excess of 500,000, are used in the majority of commercial EDLCs. In addition, EDLCs with organic electrolytes are much less flammable than Li-ion batteries.

ELECTRODE MATERIALS This review largely focuses only on pure carbon electrodes with only a few examples describing the use of other materials in asymmetric capacitors. Current technologies allow for commercial production of various types of carbon materials with high surface area, high electrical conductivity, as well as a range of shapes, sizes, and pore size distributions (see selected examples in Figures 4 and 5). Our goal is to provide an overview of the published studies of the common types of such carbons and, more importantly, to share our view about their potential advantages and limitations in EDLC applications. We

further aim to provide an overview of the current understanding of the structure–property relationships related to the transport and adsorption of ions within these carbon materials. We would like to emphasize that the extensive list of references provided in this overview is not meant to be comprehensive. We shall note that while volumetric capacitance of capacitor electrodes is significantly more important for applications than specific capacitance, the majority of publications do not report volumetric capacitance of the studied materials. In addition, because volumetric capacitance could be affected by calendaring (post-fabrication electrode densification) and because precise estimation of electrode thickness is more difficult than precise estimation of electrode mass, reporting volumetric capacitance could be prone to errors. As such, we mostly report specific capacitance in our review.

Activated Carbons Activated carbon (AC) is a form of disordered carbon with small pores and large specific surface area (SSA). ACs are prepared by thermal decomposition and partial oxidation (activation) of organic compounds, including a very wide selection of natural and synthetic precursors. AC is the oldest and the most common type of porous carbons. The use of AC in Egypt was described as early as in 1550 BC.20 Industrial production of ACs in the U.S. started in 1913.21 Although the first industrial production of activated carbons started nearly a century ago, achieving precise control over the pore structure has been very challenging despite extensive studies and improvements in activation processes. Most of the pores in activated carbons are in the 0.4–4 nm range, and the pore size distribution is generally relatively wide, which may limit their performance in some EDLC applications. The common natural precursors for activated carbon synthesis include coal, petroleum coke, pitch, wood, nutshells, peat, lignite, and more exotic ones include starch, sucrose, corn grain, leaves, coffee grounds, and straw.22–42 More advanced ACs with better developed porosity, reproducible properties and more uniform microstructure and pores are produced from synthetic polymers, such as polyacrylonitrile (PAN), polyvinylidene chloride (PVDC), polyfurfuryl alcohol (PFA), polyvinyl chloride (PVC), polypyrrole (PPy), polyaniline (PANI), polydivinylbenzene (PDVB),22,43–49 to mention a few. In fact, most organic materials rich in carbon that do not fuse upon thermal decomposition can be used as precursors. The activation process is generally divided into two categories: (1) thermal (also called physical) and

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(a)

SEM

5 μm

(d)

CDC SEM

500 nm

(b)

SEM

2 μm

(e)

SEM

1 μm

ZTC

CDC

Graphene

(c)

SEM

5 μm

AC

(f)

TEM

5 nm

Carbon onions

FIGURE 4 | Electron microscopy images of high surface area carbon materials: (a) scanning electron microscopy (SEM) of micron-scale micro-/meso-/macro-porous CDC (Reproduced with permission from Ref 12. Copyright 2006, Elsevier), (b) SEM of mesoporous silica-templated CDC (Reproduced with permission from Ref 13. Copyright 2011, John Wiley and Sons, Ltd), (c) SEM of AC particles, (d) SEM of ZTC (Reproduced with permission from Ref 14. Copyright 2010, American Chemical Society), (e) SEM of multilayer graphene flakes (Reproduced from Ref 15. Copyright 2011, Wiley-VCH), (f) transmission electron microscopy (TEM) of carbon onions. (Reproduced with permission from Ref 16. Copyright 2007, Elsevier)

500 μm

AC fabric

40 μm

CNT forest

2 μm

CNT fabric

300 nm

CNT paper

AC fibers

(e)

(d)

0.4 mm

(c)

(b)

(a)

(f)

500 nm

CNT forest

FIGURE 5 | SEM micrographs of high surface area carbon materials: (a) AC fabrics (Reproduced with permission from Ref 17. Copyright 2013, John Wiley and Sons, Ltd), (b) AC fibers (Reproduced with permission from Ref 17. Copyright 2013, John Wiley and Sons, Ltd), (c) CNT fabric, (d and e) vertically aligned CNT forest (Reproduced with permission from Ref 18. Copyright 2013, John Wiley and Sons, Ltd), (f) randomly oriented CNTs within CNT paper mats. (Reproduced with permission from Ref 19. Copyright 2012, Royal Society of Chemistry)

(2) chemical. Production of ACs by physical activation involves carbonization of a precursor (removal of noncarbon species by thermal decomposition in inert atmosphere) and gasification (development of porosity by partial etching of carbon during the annealing with an oxidizing agent, such as CO2 , H2 O, or

a mixture of both).50,51 Production of ACs by chemical activation generally involves the reaction of a precursor with a chemical reagent (such as KOH,52–56 H3 PO4, 57 ZnCl2, 58 H2 SO4, 59 among a few) at elevated temperatures and generally results in smaller pores, more uniform pore size distribution

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and higher specific capacitance in both aqueous and organic electrolytes.

Activated Carbon Powders Nearly all commercial EDLCs utilize high surface area AC powders due to their well-developed manufacturing technologies, easy production in large quantities, relatively low cost and great cycle stability.3 During electrode formation, AC powders are mixed with polymer binders (often polytetrafluoroethylene, PTFE), casted on current collector foil (for EDLCs with organic and IL electrolytes, thin Al foil is commonly used as a current collector) and dried. Commercial ACs commonly offer SSA in the range of 700–2200 m2 /g and moderately high specific capacitance in the range of 70–200 F/g in aqueous and 50–120 F/g in organic electrolytes60–64 (Table 1). Furthermore, the recent developments in the synthesis of ACs having greatly enhanced specific capacitance (up to 250–300 F/g in aqueous, organic, and IL-based electrolytes) demonstrate that for a significant portion of EDLC applications ACs may remain the material of choice27,43,65,75,77,83,84,89,95–97 (Table 1). Novel methods of AC synthesis offer better control of their chemistry and pore size distribution. In one recent paper, for example, direct activation of synthetic polymers (such as PPy) was demonstrated to produce ACs exhibiting pore volume in the range of 1.1–2.39 cm3 /g and surface area in the range of 2100–3430 m2 /g, as estimated by nitrogen sorption technique. When tested in symmetric EDLC with 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImTFB) IL electrolyte, these carbons demonstrated specific capacitance of up to 300 F/g, which is a more than two-fold improvement compared to state-of-the-art commercial ACs (Figure 6(a)). Furthermore, charge–discharge tests performed at 60 ◦ C showed no visible degradation and even a small increase in the specific capacitance after 10,000 galvanostatic cycles. As readers may have noticed, some of the reported surface area values in these novel ACs exceed theoretical surface area of a perfect single-walled graphene. However, formation of vacancies or agglomeration of vacancies within individual graphene layers can allow for the nitrogen (gas molecule) adsorption on the edges of graphene segments, leading to such incredibly high (>2700 m2 /g) measured specific surface areas. We shall also warn the readers that there are no tools capable of estimating the surface area of microporous materials precisely. Gas sorption techniques rely on

theories that simplify a gas sorption process and shape of the pores in carbon. As a result, the reported surface areas should be considered for estimating purposes only. In order to achieve higher surface areas and eliminate bottle-neck pores (which may exist in ACs and slow down transport of ions) while uniformly enlarging the smallest micropores produced in the course of carbonization of organic precursors, several promising routes were proposed. According to one method an equilibrium content of oxygen-containing functional groups are uniformly formed on the porous carbon surface during room temperature treatment in acids.46 These groups together with the carbon atoms are later removed via heat treatment at 900 ◦ C. Repetition of the process of uniform formation of chemisorbed oxygen functional groups and subsequently removing them, allows for the uniform pore broadening needed to achieve the optimum PSD.46 In another study, an environmentally friendly low-temperature hydrothermal carbonization was utilized in order to introduce a network of uniformly distributed oxygen within the carbon structure in one step.27 This material, produced from natural precursors (such as wood dust and potato starch), was then transformed into microporous carbons with high SSA of 2100–2450 m2 /g via simultaneous heat-treatment (and thus uniform removal of the oxygen-containing functional groups from the internal material surface) and opening of closed and bottle-neck pores by activation. A very high specific capacitance of 140–210 F/g was demonstrated in a TEATFB-based organic electrolyte.27

Activated Carbon Films and Monoliths Formation of EDLC electrodes from films and monoliths of ACs allows for a significant increase in their electrical conductivity (due to the elimination of both the nonconductive binder and the high resistance particle-to-particle point contacts) and volumetric capacitance (due to the elimination of the large macropores between the particles).47,72,79,82,87,88 For example, meso/microporous AC monoliths (1 mm thickness) produced by chemical activation (KOH) of mesophase pitch precursor and exhibiting SSA of up to 2650 m2 /g and surface area of micropores of up to 1830 m2 /g showed an outstanding initial capacitance of up to 334 F/g (307 F/cm3 ) in 1 M H2 SO4 ,79 which is one of the highest volumetric capacitance reported for carbon materials. Nitrogen-doped macro/meso/microporous AC monoliths (cylindrical shape with up to 17 mm diameter) having a very moderate SSA of 772 m2 /g were recently shown to exhibit high specific capacitance of up to ∼200 F/g

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TABLE 1 Electrochemical Performance of Activated Carbon in EDLCs Reported Activation Precursor

BET-SSA

Capacitance

(m2 g−1 ) (F g−1 )/Cell Type

Method



Physical-H2 O

2240

240/2 symm



Physical-CO2

1050

Melamine mica

30% HNO3 then ammonia treatment

Wood sawdust Pitch CF Poly(vinylidene chloride) (PVDC)

Chemical-KOH



Chemical-KOH

2500

Polyvinyl alcohol

Chemical-KOH

2218

Polyvinyl alcohol

Chemical-KOH

2218

Phenol formaldehyde resin

Chemical-KOH/ZnCl2

2387

Electrolyte Organic electrolyte

Ref

1.2 M TEABF4 in acetonitrile (AN)

50

52/2 symm

1M TEABF4 in polycarbonate (PC)

51

3487

148/2 symm

1M TEABF4 in PC

65

Chemical-KOH

2967

236/2 symm

1M TEABF4 in AN

27

Chemical-KOH

770

46/2 symm

TEABF4 in PC

66

2050

38/2 symm

1M TEABF4 in PC

54

110/2 symm

1M TEABF4 in AN

64

115/2 symm

1M Et3 MeNBF4 in PC

49

147/2 symm

1M LiPF6 in EC-DEC

49

142/2 symm

1M Et3 MeNBF4 in PC

67

Polybenzimidazol

Chemical-N2

1220

23/2 symm

0.8M TEABF4 in PC

68

Lignocellulosic materials



2300

95/2 symm

1.5 M TEABF4 in AN

61

Lignocellulosic materials



2315

125/2 symm

1.7M N(C2 H5 )4 CH3 SO3 in AN

62

Polyacrylonitrile (PAN)



1340

66/2 symm

1 M LiPF6 in EC-DEC

69

PAN



1340

90/2 symm

1 M TEABF4 in PC

70

Coconut shell



1692

22/2 symm

1 M LiClO4 in PC

71

Pitch CF



1000

21/2 symm

1 M LiClO4 in PC

71

Pitch CF



1500

24/2 symm

1 M LiClO4 in PC

71

Phenol resin



1232

3/2 symm

1 M LiClO4 in PC

71

Phenol resin



1542

18/2 symm

1 M LiClO4 in PC

71

Pitch



1016

1/2 symm

1 M LiClO4 in PC

71

Pitch



1026

2/2 symm

Sulfonated poly(divinylbenzene)

physical-CO2

2420

206/3

Rubber wood sawdust

physical-CO2

913

138/2 symm

Poly (amide imide)

Physical-CO2

1360

196/3

Aqueous electrolyte

1 M LiClO4 in PC

71

2M H2 SO4 aq. sol.

47

1M H2 SO4 aq. sol.

72

6M KOH aq. sol.

73

Pitch fiber

Physical-H2 O

880

28/2 symm

1M KCl aq. sol.

74

Coconut shell

Chemical-Melamine and urea

804

230/2 symm

1M H2 SO4 aq. sol.

75

Melamine mica

Chemical-30% HNO3 then ammonia treatment

86

115/2 symm

1M H2 SO4 aq. sol.

65

Phenolic resin

Chemical- 2M HNO3



60/2 symm

6M KOH aq. sol.

76

graphite

Chemical- HNO3 /H2 SO4 (1:1)



1071/3

0.1M KOH aq. sol.

59

Rice husk

Chemical- H2 SO4



175/3

6M KOH aq. sol.

40

Wood sawdust

Chemical-KOH

2967

143/2 symm

6M KOH aq. sol.

27

Eggshell

Chemical-KOH

221

297/3

6M KOH aq. sol.

41

polystyrene

Chemical-KOH

2350

258/3

6M KOH aq. sol.

77

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Nanostructured carbon materials for electrochemical capacitor applications

TABLE 1 Continued Reported Precursor

Activation

BET-SSA

Capacitance

Method

(m2 g−1 )

(F g−1 )/Cell Type

Electrolyte

Ref

Viscose fibers

Chemical-KOH

1304

270/3

7M KOH aq. sol.

78

Celtuce leaves

Chemical-KOH

3404

273/2 symm

2M KOH aq. sol.

36

Polyvinyl alcohol

Chemical-KOH

2218

218/2 symm

30 wt.% KOH aq. sol.

49

Petroleum residue (ethylene tar)

Chemical-KOH

2652

334/2 symm

1M H2 SO4 aq. sol.

79

Phenol-formaldehyde resin

Chemical-KOH

1900

100/2 symm

1M H2 SO4 aq. sol.

52

Pitch

Chemical-KOH

1611

220/2 symm

2M H2 SO4 aq. sol.

55

Pitch

Chemical-KOH

3160

295/2 symm

2M H2 SO4 aq. sol.

56

Pitch

Chemical-KOH

2860

130/2 symm

1M H2 SO4 aq. sol.

53

Seed shell

Chemical-KOH

2100

355/3

1M H2 SO4 aq. sol.

39

Viscose fibers

Chemical-KOH

1304

340/3

4M H2 SO4 aq. sol.

78

Beer lees

Chemical-KOH

3557

188/3

0.1M H2 SO4 aq. sol.

37

glucose

Chemical-KOH



220/3

1M Na2 SO4 aq. sol.

42

Styrene-divinylbenzene (SC)

Chemical-H3 PO4

434

210/2 symm

1M H2 SO4 aq. sol.

57

PAN

Chemical- ZnCl2

824

174/2 symm

6M KOH aq. sol.

58

Camellia oleifera shell

Chemical- ZnCl2

2080

184/3

6M KOH aq. sol.

38

Camellia oleifera shell

Chemical- ZnCl2

2080

230/3

1M H2 SO4 aq. sol.

38

PAN

Chemical-O2

1290

150/2 symm

1M H2 SO4 aq. sol.

80



Cold-plasma treated

1270

142/3

0.5M H2 SO4 aq. sol.

81

Resorcinol and formaldehyde (RF)



603

198/2 symm

2M H2 SO4 aq. sol.

82

Seaweed



746

264/2 symm

1M H2 SO4 aq. sol.

83

Seaweed



270

198/3

1M H2 SO4 aq. sol.

84

PAN



302

202/2 symm

1M H2 SO4 aq. sol.

85

Coconut shell



1692

36/2 symm

1M H2 SO4 aq. sol.

71

Pitch CF



1000

32/2 symm

1M H2 SO4 aq. sol.

71

Pitch CF



1500

36/2 symm

1M H2 SO4 aq. sol.

71

Phenol resin



1232

40/2 symm

1M H2 SO4 aq. sol.

71

Phenol resin



1542

33/2 symm

1M H2 SO4 aq. sol.

71

Pitch



1016

1/2 symm

1M H2 SO4 aq. sol.

71

Pitch



1026

28/2 symm

1M H2 SO4 aq. sol.

71

Poly(vinylidene chloride)



700

64/2 symm

1M H2 SO4 aq. sol.

86

PAN



302

126/2 symm

6M KOH aq. sol.

85

Poly-aniline (PANI)



400

125/3

6M KOH aq. sol.

48

RF



722

199/3

6M KOH aq. sol.

87





1151

150/2 symm

6M KOH aq. sol.

88





2500

275/3

6M KOH aq. sol.

89

6M KOH aq. sol.

90

30 wt.% KOH aq. sol.

91

Li2 SO4 aq. sol.

89

Phenolic resin/PVA



416

171/3

Dimethyl acetamide/ polybenzimidazol



1220

178/2 symm





2500

210/3

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TABLE 1 Continued Reported Precursor

Activation

BET-SSA

Capacitance

Method

(m2 g−1 )

(F g−1 )/Cell Type

Pitch fiber

Chemical-KOH

2436

224/2 symm

Electrolyte Ionic liquid electrolyte

N,N-diethyl-N-methyl(2methoxyethyl)ammonium tetrafluoroborate (DEME-BF4 )

Ref 92

PAN

Chemical-NaOH

3291

196/2 symm

LiN(SO2 CF3 )2 + C3 H5 NO2

93



Oxygen-plasma treated

2103

204/3

N,N-diethyl-N-methyl(2-methoxyethyl) ammonium bis(trifluoromethyl sulfonyl)imide (DEME-TFSI)

94





1428

60/3

N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14-TFSI)

95

in 6 M KOH electrolyte.87 In another recent study, S-containing macro/meso/microporous AC monoliths produced by physical (CO2 ) activation of carbonized PDVB also demonstrated specific capacitance of up to ∼200 F/g in 2 M H2 SO4 electrolyte47 (volumetric capacitance was not provided in the last two studies, but it is not expected to exceed ∼40 F/cm3 due to the presence of high content of macropores). A recent publication on patterned thin (1–3 μm) AC films reported specific capacitance over 300 F/g in 1 M H2 SO4 electrolyte.101

Activated Carbon Fibers Activated carbon fibers/fabrics (Figure 5(a)) similarly do not commonly require polymer binder for the formation of EDLC electrodes and exhibit high electrical conductivity.66,68–71,73,74,78,81,85,86,92–94,102,103 In contrast to the monolithic electrodes, however, AC fabric electrodes could offer very high mechanical flexibility. Their often higher power characteristics originate from the smaller electrode thickness and the presence of high volume of macro/mesopores between the individual fibers. Depending on the fiber diameter and the activation process utilized, the ion transport and the overall specific power of AC fiber-based EDLC may vary in a broad range. Apart from catalyst-grown carbon fibers and CNTs (to be discussed in a subsequent section), the smallest diameter AC fibers are produced by carbonization and activation of electrospun polymer solutions.67,68,85,90,91,104–106 The produced AC nanofiber electrodes exhibit outstanding rate capability, but suffer from low density.68,85 In fact, the density of high-power AC fiber electrodes is often noticeably lower than that of AC powder electrodes, which leads to their lower volumetric capacitance. However, several studies have demonstrated very

promising performance of dense AC fiber-based EDLCs. For example, pitch-derived carbon fiber electrodes (individual fiber diameter in the range of 2–30 μm) physically activated in an H2 O stream to moderately high SSA of 880 m2 /g while retaining high density (up to 0.8 g/cc) exhibited specific capacitance of up to 112 F/g (90 F/cm3 ) in 1 M KCl electrolyte.74 Chemical activation parameters can similarly be used to control the density and porosity of carbon fibers. For example, chemically (KOH) activated mesophase pitch based carbon fibers showed SSA increase from 510 to 2436 m2 /g upon increase in the KOH-to-C ratio from 1.5 to 4.92 The highest gravimetric capacitance in both ILs and TEATFBbased organic electrolytes (up to ∼180 F/g) was achieved in the sample with the highest SSA, while the highest volumetric capacitance (up to ∼88 F/cm3 ) was achieved in moderately activated fibers with SSA of 1143 m2 /g.92 Oxygen-containing plasma treatment of AC fibers was found to increase their SSA and specific capacitance in aqueous (0.5 M H2 SO4 ) electrolyte.81,94 Interestingly, treatment in a pure O2 atmosphere at moderate temperatures (∼250 ◦ C) did not significantly change the SSA, but introduced higher content of C=O functional groups, which resulted in an increase of specific capacitance from 120 to 150 F/g in 1 M H2 SO4 electrolyte, presumably due to improved wetting and a higher contribution from pseudocapacitance produced by the introduced functional groups.80 In spite of multiple recent improvements, the key challenge with traditional AC technology is how to independently control the SSA, pore volume, pore size and shape in these materials. Thus, other synthesis techniques were developed to address these limitations.

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Electrolyte: EMImTFB (IL)

(a)

(b)

Tests performed at +60 °C

Activation at 600 °C At 650 °C At 700 °C 250 200 150 100 50 10 µm

0

AC electrodes (250 μm)

1

10

100 Scan rate (mV/s)

Tests performed at −70 °C

100 0

Produced at 800 °C

−100 −200

At 900 °C At 700 °C

−300 −400 −2.0

1000

ZTC electrodes (200 μm) −1.5

100

Specific capacitance (F/g)

Contact pads Carbon onions electrodes

10–1 3.5V/25 mF supercapacitor

63V/220 μF Electrolytic capacitor

Substrate

20

40

60

80

1.5

2.0

100

120

140

160

180

Activated graphene electrodes (20 μm) 100 mV/s 200 mV/s 500 mV/s

100

0 −100 −200

10–4 0

1.0

Tests performed at + 20 °C 200

10–3

−0.5 0.0 0.5 Voltage (V)

Electrolyte: BMIMTFB in AN

Tests performed at +20 °C Carbon onions Microsupercapacitor

Activated carbon microsupercapacitor

10–2

−1.0

500 nm

(d)

Electrolyte: TEATFB in AN 101

Stack capacitance (F/g)

Specific capacitance (F/g)

Specific capacitance (F/g)

300

(c)

Electrolyte: SBPTFB in mixed organic solvents 200

200

250 nm

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Potential (V)

Scan rate (V/s)

FIGURE 6 | Performance of some of the advanced high surface area carbons in EDLC applications: (a) specific capacitance of polymer-derived activated carbons in an ionic liquid electrolyte at elevated temperature (Reproduced with permission from Ref 43. Copyright 2011, John Wiley and Sons, Ltd), (b) low temperature cyclic voltammetry of ZTC recorded at the rate of 1 mV/second in an organic electrolyte (Reproduced with permission from Ref 98. Copyright 2012, John Wiley and Sons, Ltd), (c) volumetric capacitance of micro-EDLC with OLC electrodes in an organic electrolyte compared to that of typical electrolytic capacitor, EDLC and micro-EDLC with AC electrode recorded at ultra-high scan rates (Reproduced with permission from Ref 99. Copyright 2010, Nature), (d) room temperature cyclic voltammetry of activated graphene in an organic electrolyte. (Reproduced with permission from Ref 100. Copyright 2011, The American Association for the Advancement of Science)

Carbide Derived Carbons Carbon produced by extraction of metals from metal carbides is commonly termed carbide derived carbon (CDC). Treatments in supercritical water or halogens as well as vacuum decomposition have been explored to etch metal atoms from various carbides, producing carbon coatings and bulk porous material as well as porous carbon powders, fibers and membranes. Several recent publications review the progress in CDC materials.107,108 The first manufacturing process reporting mass production of CDC was developed in 1918109 and was used until the 1960s as a method for the production of SiCl4 from SiC by treating SiC in a dry Cl2 gas etchant at temperatures above 1000 ◦ C. The produced SiCl4 was collected in a condenser,

while CDC was commonly discarded or fully oxidized during the chloride manufacturing process. In 1959, the CDC itself drew some attention because in contrast to carbon produced from organic precursors, CDC was found to contain a minimal content of hydrogen and other contaminants. In the last decade, it was further discovered that CDC may offer other unique technological advantages and additionally serve as a good model material for systematic studies of ion and gas adsorption.108,110–114 For EDLC applications, CDC is generally produced by treatment of carbides in a chlorine gas. In the case of binary carbides a simplified reaction can be written as: MeCx + 0.5Cl2 = MeCly + xC, where Me could be Al,115 B,116–119 Ca,120 Cr,121 Mo,122,123 Si,13,114,124–128 Ti,114,129–132 V,133 W,134 Zr,111,135

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or other carbide-forming metals. In addition, CDC with enhanced pore volume could be produced using ternary carbides, such as Ti2 AlC, Ti3 AlC2 , and Ti3 SiC2. 136–138 For thin films of less than several microns the reaction is typically not limited by diffusion139,140 and thus large size metal carbide powders or thick films could be transformed into CDC. While various carbon structures can be found in CDC, including nanodiamond,141 carbon onions,141 graphene,142 graphite,142 and very dense vertically aligned carbon nanotubes (CNTs),142,143 simple disordered carbon was found to be the most attractive and abundant material for EDLC applications.13,110,111,119,122,126,127,132,133 By selecting different carbide precursors and changing the chlorination temperature one can tune the average pore size in CDC and study the effect of the pore size on the electroadsorption of electrolyte ions of different size.110,111,113

Carbide Derived Carbon Powders and Nanopowders The specific capacity of CDC in aqueous electrolytes is moderately high. Depending on the preparation conditions and the selection of the initial carbide precursor it commonly ranges from 60 to 220 F/g111,119,132,144 (Table 2). In a 1 M H2 SO4 electrolyte, CDC produced from micro-size particles of SiC (often termed SiC-CDC) demonstrated specific capacitance up to 153 F/g,132 B4 C-CDC up to 147 F/g,119 ZrC-CDC up to 160 F/g,111 Ti2 AlC-CDC up to 175 F/g,119 TiC-CDC up to 196 F/g,111,132 and CDCs derived from a TiC/TiO2 mixture—up to 217 F/g.132 In basic electrolytes, CDC may demonstrate higher capacitance. For example, in 6 M KOH the specific capacitance of B4 C-CDC and SiC-CDC could be increased up to 178 F/g and 200 F/g, respectively.116 While some of these values appear relatively large, with the exception of Al4 C3 -CDC in 6 M KOH, CDC cannot match the gravimetric performance of some of the activated carbons in comparable electrolytes. The surface chemistry of CDC may partially explain such a moderate performance. The high area-normalized capacity and frequency response of aqueous electrolytes often correlates with the degree of wetting of electrolyte on the carbon surface.149 Unfortunately, Cl-terminated surface is highly hydrophobic. Removal of Cl from CDC by treatment in H2 or NH4 112,114 does not allow one to achieve the desired high hydrophilicity of carbon surface.

Because of the significantly reduced contribution of the pseudocapacitance, the total specific and volumetric capacitance of CDC in organic electrolytes is slightly smaller than in aqueous electrolytes. Yet, in these high-voltage EDLC applications CDCs outperform most of the nanostructured carbon materials, significantly exceeding the capacitance of some commercial activated carbons, particularly if normalized by the electrode volume.13,110,111,115,119,122,123,127,129,132,146,150,151 TiC-CDC demonstrates one of the highest specific capacitance in TEATFB, in excess of 160 F/g Al4 C3 -CDC showed specific (110 F/cm3 ).110 capacitance of up to 114 F/g in a triethylmethylammonium tetrafluoroborate (EMATFB)—based organic electrolyte.115 VC-CDC133 and Mo2 CCDC122 showed capacitance up to 133 F/g and 140 F/g in EMATFB electrolyte, respectively. Activation of CDC may allow one to enhance its ability for adsorption of gas molecules and ions.152 Indeed, after activation of CDCs, their surface chemistry could be modified and their pore volume and specific capacitance could be significantly enhanced. For example, the specific capacitance of TiC-CDC in organic electrolyte was observed to increase by up to 30% with chemical (KOH) activation and reach 180 F/g.148 Decreasing the particle size of CDC to submicron range was also shown to increase its surface area and specific capacitance. A systematic study on SiC CDC with varying particle size and synthesis temperature showed up to a 200% increase in pore volume and up to a 30% increase in BET SSA by decreasing the size of CDC particles to 20 nm.145 The specific capacitance in TEATFB—based electrolyte was found to increase with increasing synthesis temperature and decreasing particle size with the highest value of 135 F/g achieved for 20 nm SiC-CDC synthesized at 800 ◦ C. The rate performance of the produced electrode, however, was not significantly improved in spite of the dramatically shorter ion diffusion path within individual particles. The nanoparticle agglomeration within the electrode may have reduced its ion migration capability. In addition, significantly higher porosity of the EDLC electrode resulted in inferior volumetric capacitance compared to that of conventional CDC.

Carbide Derived Carbon Films Formation of dense CDC films maximizes the volume occupied by the active material within the EDLC electrode and thus maximizes its volumetric capacitance.150,151 Unfortunately, however, processing CDC films makes it difficult to efficiently remove

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TABLE 2 Electrochemical Performance of Carbide Derived Carbons in EDLCs BET-SSA Type of Carbide

2 −1

(m g )

Reported Capacitance (F g−1 )/Cell Type

Electrolyte Organic electrolyte

Ref

Al4 C3

1470

114/3

1M (C2 H5 )3 CH3 NBF4 in PC

115

Mo2 C

1855

143/2 symm

1M (C2 H5 )3 CH3 NBF4 in AN

122

Mo2 C

2082

132/3

1M (C2 H5 )3 CH3 NBF4 in PC

123

SiC



170/2 symm

1M TEABF4 in AN

107

SiC

2250

170/2 symm

1M TEABF4 in AN

127

SiC

2400

170/2 symm

1M TEABF4 in AN

126

SiC

2400

1853/2 symm

1M TEABF4 in AN

126

SiC

1300

135/2 symm

1.5M TEABF4 in AN

145

SiC

1190

105/2 symm

1.2M triethylmethylammonium tetrafluoroborate (TEMA) in AN

128

TiC

1450

130/3

1M (C2 H5 )3 CH3 NBF4 in AN

146

TiC

1260

160/2 symm

1.5M TEABF4 in AN

110

TiC

1700

130/2 symm

1.5M TEABF4 in AN

130

TiC

1390

65/2 symm

1.5M TEABF4 in AN

147

TiC

1588

120/2 symm

1.2M TEMA in AN

129

TiC, KOH activated

1700

180/2 symm

1.5M TEABF4 in AN

148

VC



133/2 symm

1M (C2 H5 )3 CH3 NBF4 in AN

107

VC

1305

133/2 symm

1M (C2 H5 )3 CH3 NBF4 in AN

133

VC

1305

150/2 symm

1M TEABF4 in AN

133

B4 C

1400

147

1M H2 SO4 aq. sol.

119

B4 C

1461

177/3

6M KOH aq. sol.

116

CaC2

658

196/3

6M KOH aq. sol.

120

Mo2 C

1855

120/2 symm

6M KOH aq. sol.

144

Aqueous electrolyte

SiC

2729

200/2 symm

1M H2 SO4 aq. sol.

13

SiC

2430

153/2 symm

1M H2 SO4 aq. sol.

132

SiC

2380

200/3

6M KOH aq. sol.

116



175

1M H2 SO4 aq. sol.

119

TiC

1500

150/2 symm

1M H2 SO4 aq. sol.

111

TiC

1400

196/2 symm

1M H2 SO4 aq. sol.

132

TiC

1390

110/2 symm

1M H2 SO4 aq. sol.

147

TiC



150/2 symm

H2 SO4 aq. sol.

107

Ti2 AlC

ZrC



190/2 symm

H2 SO4 aq. sol.

107

ZrC

1350

190/2 symm

1M H2 SO4 aq. sol.

111

TiC

1269

160/2 symm

Ethyl-methylimmidazoliumbis(trifluoro-methanesulfonyl)imide (EMI-TFSI)

113

the chloride contaminants during or after the synthesis and retain high rate capability. A recent study provided a proof of concept for the fabrication of EDLC electrodes with volumetric capacity in excess of 175 F/cm3 in TEATFB electrolyte (the highest volumetric capacitance of carbon in organic electrolyte

IL

reported to date) and 160 F/cm3 in 1 M H2 SO4 for 2 μm thick films.151 The decrease in capacitance by up to six times produced by increasing the film thickness to 200 μm could be explained by the collapse of the internal porosity caused by interface stresses and by electrolyte starvation.

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Carbide Derived Carbon Fibers Formation of carbide nanofibers or nanofiber mats has opened the opportunity for the formation of CDC (nano)fibers153 with enhanced power characteristics. CDC nanofibers produced from SiC precursor fibers demonstrated a BET SSA of 1050 m2 /g and a pore volume of 0.53 cm3 /g.128 Their specific capacitance in TEATFB electrolyte reached only 78 F/g, which was 25% lower than that of micron-sized SiCCDC powder tested at identical conditions.128 TiC nanofibers prepared using a combination of electrospinning and carbo-thermal reduction allowed formation of TiC-CDC nanofibers with 100–200 nm diameters, BET SSA of up to 1390 m2 /g and pore volume of up to 1.5 cm3 /g.147 While their specific capacitance was found not to exceed 65 and 100 F/g in TEATFB and H2 SO4 -based electrolytes, respectively, they showed improved capacity retention at faster sweep rate and higher current densities than TiC-CDC produced from conventional powders.147 A likely reason for the small capacitance values observed is the presence of amorphous carbon in the initial TiC precursor fibers.

Template-Assisted Synthesis of Carbide Derived Carbon In EDLC electrodes composed of micron-size particles, the ion transport within individual particles may largely govern the EDLC’s overall power storage and the charge–discharge time.1 While regular CDC powder allows one to achieve high gravimetric and volumetric capacitance in organic electrolytes,110,115,122,133,148 the slow ion transport in small curved micropores leads to a moderate rate of charge and discharge. Decreasing the size of the particles to 20–200 nm dimensions by synthesizing CDC nanopowder or felt128,145,147 makes the electrode preparation more challenging, reduces the volumetric capacitance and electrical conductance of the electrode and at the same time does not provide a break-through improvement in power characteristics of the EDLCs because small curved mesopores between individual nanoparticles impede the ion transport within the electrode. A more promising approach for power enhancement may include the formation of CDC powder with a combination of straight mesopore channels for fast ion transport and subnanometer pores in the volume between the large channels for high specific capacitance.13,126,127 So far, formation of such CDC has been demonstrated from mesoporous SiC precursor, prepared using mesoporous SiO2 template and polycarbosilane precursor13,126,127 (Figure 7). While CDC produced from nonporous

Infiltration with polycarbosilane and pyrolysis SBA-15: mesoporous SiO2 SiO2 etching in HF

Microporous CDC nanorods

Chlorination of SiC

CDC: inverse replica of SBA-15 Large mesopores between the nanorods for rapid electrolyte transport

Mesoporous SiC: inverse replica of SBA-15

FIGURE 7 | Schematic of the formation of CDC with dual pore size distribution—ordered mesopores for rapid ion transport and rapid charging of EDLCs and small disordered micropores for high volumetric capacitance. (Reproduced with permission from Ref 127. Copyright 2010, American Chemical Society)

SiC micropowder require at least 900 ◦ C for complete chlorination108,154 mesoporous SiC powder can be completely transformed into CDC at temperatures below 700 ◦ C, and exhibits BET SSA in the range of 2250–2729 m2 /g and pore volume in the range of 1.4–2.0 cm3 /g.13,126,127 The produced SiC-CDC exhibits impressive capacitance of up to 170 F/g (64 F/cm3 ) in TEATFB-based organic electrolyte,127 up to 180 F/g (and up to 51 F/cm3 ) in 1-ethyl-3methylimidazolium tetrafluoroborate (EMITFB) ionic liquid electrolyte at room temperature and up to 200 F/g (68 F/cm3 ) in 1 M H2 SO4. 13 More importantly, the preparation and application of templated CDC overcame the present limitations of nonaccessible surface because of bottle necks and poor control over the pore size distribution in the carbons currently used, and showed a route for further performance enhancement. The ordered mesoporous channels in SiC-CDC allow for very fast ionic transport into the bulk of the CDC particles, thus leading to an excellent frequency response and outstanding capacity retention at high current densities. The enhanced transport led to 85 and 90% capacity retention at current densities increasing from 0.1 to ∼20 A/g in organic127 and aqueous13 electrolytes, respectively. This is even higher than what was observed in CDC nanofiber felts.

Zeolite-Templated Carbons Microporous zeolite-templated carbons (ZTCs) are produced by carbon deposition on zeolite templates followed by the removal of zeolite. A first publication

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TABLE 3 Electrochemical Performance of Zeolite-Templated Carbons in EDLCs BET-SSA Type of Zeolite

Activation Method

2 −1

(m g )

Reported Capacitance (F g−1 )/Cell Type

Electrolyte Organic electrolyte

Ref

13X

Chemical-KOH

2970

176/2 symm

1.5M TEABF4 in AN

159

13X

Chemical-KOH

2864

160/2 symm

1.5M TEABF4 in AN

160

Y



1941

146/2 symm

1.5M TEABF4 in AN

161

FAU (diamond-like framework)



1693

190/3

1.5M TEABF4 in AN

162

X



3040

168/3

1M TEABF4 in PC

163

Y



1820

300/2 symm

1M H2 SO4 aq. sol.

14

Y



1680

340/2 symm

1M H2 SO4 aq. sol.

164

Chemical-KOH

2970

259/2 symm

6M KOH aq. sol.

159

13X

reporting formation of porous carbon using sacrificial zeolite templates in 1997 describes some of the unique advantages of this technology.155 Zeolites have uniform three-dimensional pore channels. By depositing graphene monolayers uniformly on the internal surface area of zeolites and by etching the zeolite using a combination of acids, one may produce porous carbon replicas with an ordered array of micropores. The initial studies using propylene, polyacrylonitrile (PAN) or polyfurfuryl alcohol (PFA) as carbon precursors reported BET SSA in the range of 1320–2260 m2 /g, micropore volume in the range of 0.54–1.11 cm3 /g and total pore volume in the range of 0.96–1.87 cm3 /g.155 At the time of publication such surface area and well developed porosity was the highest among carbons produced without any activation. In the follow up studies a combination of PFA infiltration and carbonization with CVD carbon deposition using propylene allowed the ZTC to retain the pore ordering of the original zeolite template.156,157 BET SSA up to 3600 m2 /g was reported.157 Further studies showed that using lowpressure vapor deposition could be sufficient for the formation of uniform ZTCs.14,98 It was also found that HF dissolution of zeolite template may leave small cryolite (Na3 AlF6 ) residues at the end of the process, which should be further dissolved using concentrated H2 SO4 or other acids.14 A recent model proposes that at the atomic level ideal ZTCs are made up of single wall carbon nanotube (SWCNT) segments interconnected into a three-dimensional regular network with aligned pore channels.158 The important characteristics of the ZTC for EDLC include the very high uniformity of pores, as in the zeolite template, high surface area, pore alignment and the opportunity to dope carbon without distortion of the pore size distribution. The large variety of zeolites with more than 300 types available

Aqueous electrolyte

allows one to select an appropriate template for the desired pore size in a ZTC. Very high specific capacitance of ZTC was reported in both aqueous and organic electrolytes (Table 3). By using an acrylonitrile (AN) precursor, nitrogen-doped ZTC with a moderate BET SSA of 1680 m2 /g demonstrated an initial specific capacitance of 340 F/g in 1 M H2 SO4 due to the pseudocapacitance effect of the nitrogen-containing functional groups.164 The produced EDLCs retained more than 75% capacity after 10,000 cycles. By using a low pressure CVD from acetylene, ZTC was produced with a BET SSA of up to 1820 m2 /g and a specific capacitance in excess of 300 F/g in 1 M H2 SO4. 14 The broad peak in the cyclic voltammetry curve revealed pseudocapacitance reactions, which originate from surface oxides unintentionally introduced at room temperature during zeolite etching and purification. In contrast to nitrogen-related pseudocapacitance, which shows noticeable fading after 10,000 cycles,164 these ZTCs showed excellent stability as well as a 5% increase in capacitance after 10,000 cycles. More importantly, this study demonstrated that ZTCs with aligned pores may offer excellent rate capabilities to EDLCs with the highest operating frequency (the frequency at which 50% of the capacitance can be accessed) approaching 10 Hz for 250 μm thick electrodes.14 In organic electrolytes the performance of ZTC is still very good but noticeably slower, which could be related to the undesirable interactions between the functional groups on the ZTC surface and electrolyte solvents and possibly salts as well as larger ion size, which may slow down the ion transport. ZTC produced using ethylene or nitrogen saturated with acetonitrile as carbon precursors and exhibiting BET SSA in the range of 853–1941 m2 /g and total pore volume in the range of 0.6–1.08 cm3 /g showed specific

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capacitance in the range of 87–146 F/g in a TEATFBbased electrolyte.161 No positive effect of nitrogen doping on the specific capacitance was observed and, in contrast to many activated carbons165 and CDCs,110 the specific capacitance linearly increased with SSA, as would be expected for materials having the same pore size and similar microstructure. The pores in the produced samples were not well-aligned and the capacitance decreased to 50–75% when the current density was increased from 0.01 to 0.2 A/g.161 The highest decrease was observed in nitrogendoped ZTC having the highest nitrogen content.110 ZTC produced using acetylene CVD in different types of zeolites (zeolite Y, zeolite X, and zeolite β) and having aligned pores with a BET SSA in the range of 1950–3040 m2 /g and pore volume in the range of 0.97–1.59 cm3 /g showed specific capacitance of 131–164 F/g in a TEATFB-based electrolyte.163 The specific capacitance was found to linearly depend on the BET SSA.163 The 180 μm thick ZTC electrodes showed impressive retention of specific capacitance (up to 96%) when the current density was increasing from 0.05 to 2 A/g.163 Further improved ZTC synthesis by the same group resulted in achieving very impressive specific capacitance of up to 190 F/g (83 F/cm3 ) and up to 80% capacitance retention for current density increasing to 20 A/g in a TEATFBbased electrolyte.162 In addition to a high rate capability at room temperature, ZTCs are promising for high-performance electrodes in EDLCs operating at temperatures as low as −60 ◦ C98 (Figure 6(b)). This ability to quickly store and deliver a significant amount of electrical energy at ultra-low temperatures is critical for the energy-efficient operation of high altitude aircraft and spacecraft, as well as the exploration of the natural resources available in Polar Regions and mountains. According to conventional wisdom, mesoporous electrochemical capacitor electrodes with pores large enough to accommodate fully solvated ions are needed for sufficiently rapid ion transport at lower temperatures. However, strictly microporous ZTC produced using low pressure CVD with acetylene precursor and having a moderate BET SSA of 1405 m2 /g demonstrated 146 F/g (90 F/cm3 ) at room temperature and 123 F g−1 (76 F/cm3 ) at −70 ◦ C with a proper organic electrolyte.98 At −60 ◦ C selected cells based on 200 μm electrodes exhibited characteristic charge–discharge time constants of less than 9 seconds, which is comparable to that of commercial devices operating at room temperature and having smaller electrode thickness.98 Two studies reported the effect of activation on the ZTC performance. ZTC produced using PFA

infiltration and carbonization followed by CVD using nitrogen saturated with acetonitrile as a precursor with an optional KOH activation showed BET SSA of up to 2864 m2 /g.160 The activation was found to increase BET SSA and improve both the ZTC’s H2 gas storage capability and specific capacitance.160 Selected samples showed virtually no decrease in specific capacitance (up to 160 F/g in TEATFB-based electrolyte) when the current density was increased from 0.25 to 2 A/g.160 In another study on ZTC produced using ethylenediamine and carbon tetrachloride as carbon precursors, a KOH activation was found to significantly improve the capacitance in both aqueous and organic electrolytes.159 Interestingly, physical activation with CO2 did not significantly improve the performance of ZTCs. The KOH-activated ZTC with BET SSA up to 2970 m2 /g and micropore volume up to 1.04 cm3 /g, showed specific capacitance up to 259 F/g in 6 M KOH electrolyte and 176 F/g in 1.5 M TEATFB-based electrolyte at the current density of 0.25 A/g.159 EDLC performance in an organic electrolyte, however, was not very stable—the sample demonstrated 20% capacity fading after 900 cycles.159

Carbon Nanotubes Carbon nanotubes (CNTs) are carbon allotropes with cylindrical 1-D structure. CNTs are consisted of either one rolled-up graphitic sheet (single-walled CNT) or several coaxial ones (multiwalled CNT). CNTs (Figure 5(c)–(f)) are commonly grown via catalystassisted chemical vapor deposition (CVD) techniques using hydrocarbon-based gaseous precursors, such as methane, acetylene, propylene, and others. In contrast to advanced ACs, CDCs, and ZTCs, CNTs have relatively low SSA, low density and are typically difficult to process into thick electrodes using conventional electrode preparation methods. Novel fabrication techniques, however, allow for formation of vertically (relative to the substrate or current collector) (Figure 5(d) and (e)) or horizontally (Figure 5(c)) aligned CNT-based electrodes which do not require any polymeric binder. While the low CNT density and SSA limit volumetric capacitance and energy density of CNT-based EDLCs’, the high electrical conductivity and open porosity of CNTs may allow for a fast ion transport and thus high power characteristics of CNTbased EDLCs. Table 4 shows the broad range of capacitance values reported for CNT-based EDLC electrodes. One of the earliest studies showing the potential of thin (25 μm) CNT-based electrodes to provide high

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TABLE 4 Electrochemical Performance of Carbon Nanotubes in EDLCs BET-SSA Activation Method

2 −1

(m g )

Reported Capacitance (F g−1 )/cell Type

Chemical-KOH

1050

65/2 symm

Chemical-KOH

510

50/2 symm

Chemical functionalization then LbL



250/half Li cell

Organic electrolyte

Electrolyte

Ref

1.4M TEABF4 in AN

166

1M LiClO4 in EC/DEC (1:1)

167

1M LiPF6 in EC/DMC (3:7 vol.)

168



985

90/2 symm

1.5M TEABF4 in AN

169



1064

79/2 symm

1M TEABF4 in AN

170



1000

80/2 symm

1M TEABF4 in PC

171





83/3

1M TEABF4 in PC

172





24/3

1M TBAPF6 in AN

173





79/3

1M LiClO4 in PC

174



782

75/3

Chemical-HNO3 ; Mixed with PF then thermal

150

91/2 symm

Chemical-HNO3 ; Mixed with PF then thermal & chemical

120

21/2 symm

Chemical-HNO3

430

104/2 symm

38 wt.% H2 SO4 aq. sol.

178

Chemical-HNO3

411

80/2 symm

6M KOH aq. sol.

179

Chemical-KOH

1050

90/2 symm

6M KOH aq. sol.

166

Ammonia plasma

86.5

207/2 symm

6M KOH aq. sol.

180

Chemical-HNO3

430

49/2 symm

38 wt% H2 SO4 aq. sol.

181

Chemical functionalization then LbL



160/3

1M H2 SO4 aq. sol.

182

Aqueous electrolyte

1M LiClO4 in PC

175

38 wt.% H2 SO4 aq. sol.

176

38 wt.% H2 SO4 aq. sol.

177





153/3

1M H2 SO4 aq. sol.

174



357

140/2 symm

7.5N KOH aq. sol.

183





18/2 symm

6M KOH aq. sol.

184





20/2 symm

6N KOH aq. sol.

185





20/2 symm

6M KOH aq. sol.

186



410

68

6M KOH aq. sol.

187





22

6M KOH aq. sol.

188





22.5/3

1M NaCl aq. sol.

189

∼400

440/2 symm

1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl)imide [EMIM][Tf2 N]

178

70

27/3

1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4 )

190

Oxygen plasma



power and high rate performance was reported in 1997.181 In this study multiwalled CNTs (MWCNTs) with an average diameter of ∼8 nm were treated in nitric acid, processed into electrodes and tested in concentrated (38 wt. %) H2 SO4 -based aqueous electrolyte. While the CNTs showed moderately high specific capacitance of 102 F/g, it offered a very high operating frequency of up to 100 Hz.181

Ionic liquid

Another study performed in 1999 showed MWCNTs with a diameter of 15–45 nm and BET SSA in the range of 128–410 m2 /g having a specific capacitance of up to 68 F/g in 6 M KOH electrolyte when measured in three-electrode cells.187 A follow-up study by the same group investigated electroadsorption of ions from aqueous (6 M KOH) electrolyte on MWCNTs with 20–30 nm outer

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diameter, BET SSA in the range of 128–411 m2 /g and large pore volume in the range of 269–634 cm3 /g.179 Depending on the type of MWCNTs and the acid treatments used, capacitance in the range of 4–80 F/g was detected, with the highest values obtained in MWCNTs with a herringbone morphology (edge graphene planes exposed to the surface), the highest BET SSA and a significant (up to 10.8 wt.%) amount oxygen.179 The additional treatment of the best MWCNT sample in concentrated (69%) nitric acid at 80 ◦ C for 1 h increased the amount of functional groups on its surface as well as its BET SSA to 475 m2 /g, leading to an increase in its capacitance to 137 F/g.179 It was noticed, however, that the oxygencontaining functional groups responsible for high pseudocapacitance values often cause self-discharge in the capacitors and thus the highly oxidized MWCNTs may have limited applications.179 Carbon-MWCNT composite electrodes were studied in 1999. After treatment in nitric acid MWCNTs of 20–30 nm in diameter were mixed with a phenol-formaldehyde (PF) resin, processed into thin films and carbonized at 850 ◦ C, resulting in a porous C-MWCNT composite with a BET SSA of 150 m2 /g and pore volume of 0.45 cm3 /g.176 In spite of the low SSA, a large capacitance of 90.8 F/g (95.3 F/cm3 ) was reported in concentrated (38 wt. %) H2 SO4 electrolyte.176 However, another study by the same group reported much more modest capacitance values (15–25 F/cm3 ) using very similar processing technique: nitric acid—purified MWCNTs of 20–30 nm in diameter, mixed with PF were processed into thin films, carbonized at 850 ◦ C and boiled in a mixture of sulfuric and nitric acids.177 These smaller capacitance values are more consistent with the research results later reported by others on MWCNTs (1–20 F/cm3 ). Single-walled CNTs (SWCNTs) offer higher SSA than MWCNTs, but tend to bundle and may contain semiconducting tubes, which reduces SSA available for ion storage. A 2000 report was one of the first to describe the performance of SWCNT bundles (1.2–1.4 nm tube diameter, 10 nm bundle diameter) in EDLCs.189 The SWCNT bundles were processed into a paper with a density of 0.3–0.4 g/cm3 and thickness of 25–40 μm, where tubes hold together by van der Waals interactions. BET SSA of the samples was not provided. In three-electrode cells with aqueous electrolytes (1 M NaCl, 7 M H2 SO4 , and 5 M KOH) most SWCNT paper samples showed capacitance ranging from 19 to 26 F/g for scan rates up to 500 mV/second, with several samples exhibiting capacitance as high as 30–41 F/g in 1 M aqueous solution of NaCl.189

A small capacitance decrease (less than 15%) was observed when the sweep rate was increased from 5 to 500 mV/second.189 Broad redox/pseudocapacitive peaks observed in most SWCNT electrodes due to the presence of oxygen-containing functional groups were eliminated by annealing the SWCNT electrode at 900 ◦ C in Ar.189 A follow-up study reported the performance of SWCNT electrodes in 0.1 M aqueous solutions of NaCl, CsCl, benzyltriethylammonium chloride (BTEAC), LiNO3 , NaNO3 , Ca(NO3 )2 , and trimethylammonium nitrate (TMAN), as well as in organic solvent based electrolytes, such as LiClO4 and TBATFB in acetonitrile (AN) solvent.191 While no well-defined Faradaic peaks were observed with TBATFB-based electrolyte, a LiClO4 -based electrolyte clearly demonstrated the presence of Faradaic oxidation–reduction processes.191 Capacitance values similar to that of Ref 189 were achieved. This study was also one of the first to utilize electrochemical quartz crystal microbalance (EQCM), and which detected electrode mass changes of up to ∼1% in aqueous electrolytes and in a TEATFB-based organic electrolyte and up to ∼4% in a 1 M LiClO4 organic solvent based electrolyte upon charging.191 When used in IL electrolytes with different anions [such as bis(trifluoromethanesulphonyl) amide (BFSA), dicyanamide (DCA), hexafluorophosphate (HFP) and p-tolunesulphonate(PTS)] and cations (such as R,R0imidazolium and R,R0-pyrrolidinium) SWCNT paper demonstrated capacitance ranging from 14 to 24 F/g, but selected IL electrolytes offer a very high voltage stability window of up to 5.5 V, tested with Pt electrodes.173 Highly purified SWCNTs with BET SSA 462–782 m2 /g were tested in a 1 M LiClO4 -based organic electrolyte in another study and exhibited specific capacitance of 45–75 F/g, demonstrating good rate capability, with less than 10% capacitance decrease for current density increasing from 0.01 to 0.16 A/g.175 Significantly higher specific capacitance (up to 180 F/g in 7 M KOH electrolyte) of SWCNTs bundles (with a bundle diameter of 10–20 nm) was soon reported by another group.183 The heat-treatment of SWCNT samples at temperatures up to 1000 ◦ C increased their BET SSA from 210 to 350 m2 /g and decreased the average mesopore size from 6.5 to below 3 nm.183 The higher volume of smaller pores was offered as a possible explanation for the unusually high capacitance observed.183 Chemical activation was proposed in 2002 to increase electrochemical capacitance of MWCNTs while retaining high electrical conductivity.166 Depending on the starting MWCNTs, the burnoff ranged from 20% to 45%, but the activation

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allowed BET SSA increase up to 1050 m2 /g. The specific capacitance in 6 M KOH aqueous electrolyte reached 90 F/g, and in 1.4 M TEATFB-based organic electrolyte reached 65 F/g.166 A similarly positive effect of KOH activation of MWCNTs was also observed in another study, which reported an increase in BET SSA from 194 to 510 m2 /g after chemical activation and a corresponding increase in specific capacitance from 25 to 50 F/g in an organic electrolyte based on LiClO4 salt.167 Composite electrodes composed of high surface area porous carbons and CNTs may offer a combination of high energy and power densities, unattainable in one-component electrodes. For example, the addition of 15 wt.% of CNTs to ‘regular’ ELDC electrodes based on activated carbons was found to improve their power characteristics in organic electrolytes without significant decrease in specific capacitance.169 The first study of MWCNTs grown directly on conductive graphite foil substrates was reported in 2002. Misaligned MWCNTs with a diameter of 50 nm showed an unusually high specific capacitance of 116–153 F/g in 1 M aqueous solution of H2 SO4 , and up to 13–79 F/g in 0.1 M LiClO4 organic solvent based electrolyte.174 The calculation of MWCNT capacitance in this study assumed invariance in the capacitance and mass of graphitic foil substrates after the CNT growth, which may be incorrect and explain the unusually high capacitance observed. Development of methods to grow CNTs on bulk metal substrates (such as an Inconel alloy) improved the current collector/CNT contact resistance and rate capability.184 In this study a more moderate specific capacitance of only 18 F/g was achieved in 6 M KOH electrolyte.184 In another study, however, vertically aligned MWCNTs grown to 0.15 mm on Inconel foils showed specific capacitance as high as 83 F/g at 1 mV/second and 47 F/g at 1000 mV/second in 1 M TEATFB organic solvent based electrolyte.172 Various MWCNT electrodes (including those grown directly on Al foil) with BET SSA ranging from 47 to 1064 m2 /g and a density ranging from 0.1 to 0.9 g/cm3 , were also investigated in 1 M TEATFB organic solvent based electrolyte.170 As their capacitance values were quite modest (8–16 F/cm3 , over three times smaller than that of commercial activated carbons) the authors concluded that this technology would unlikely be acceptable for mainstream EDLC applications.170 Aligned CNT forests (Figure 5(d) and (e)) may provide higher rate capability than misaligned CNTs having similar thickness and mass loading due to the reduced electrical resistance (no high resistance

point contacts which increase electrode resistance if the individual CNTs are connected in a series) of the electrode combined with a smaller pore tortuosity and thus their lower ionic resistance. However, the as-grown CNT forests commonly exhibit low density (0.08–0.3 g/cc, which corresponds up to 98% porosity) and thus low volumetric capacitance. A simple method to produce electrodes with greatly enhanced density was proposed in 2006.171 This method described the introduction of liquid into 0.1–1.5 mm long aligned SWCNT forest (2.8 nm average tube diameter), followed by the evaporation of the liquid, which causes the electrode densification by over 20 times (to an average tube separation distance of less than 1 nm) and preserves the very high degree of tube alignment and high BET SSA of 1000 m2 /g.171 The specific capacitance of these high-density SWCNT electrode reached 80 F/g in TEATFB-based organic electrolyte.171 Plasma treatment induces defects and function groups on the surface of CNTs and other carbon materials, which greatly enhance their specific capacitance, as shown in Ref 180. Aligned ∼50 nm diameter MWCNT electrodes were grown on Ni foil substrates and studied in 6 M KOH electrolyte solution. The ammonia plasma etching increased the BET SSA from 9.6 to 86.5 m2 /g, while increasing the specific capacitance from 37 to 207 F/g.180 These values are unusually high for the modest surface area of these samples (up to 239 μF/cm2 ). We shall note that even if carbons are treated in plasma of oxygenfree or inert gases, their subsequent exposure to air (which contains O2 , H2 O, CO2 and other species) shall result in the satisfaction of the plasma-induced dangling bonds in carbon with oxygen-containing surface functionalities. It is likely that the introduced functional groups provide high pseudocapacitance, but the exact origin of such high area-normalized capacitance remains unclear. Similarly high values of specific capacitance of plasma-activated MWCNTs were reported in the studies by another research group.178 The use of oxygen plasma on 0.1–0.2 mm tall vertically aligned MWCNT electrodes (5–10 nm in diameter) resulted in opening of the CNT tips, increasing the SSA to 400 m2 /g, as well as the number of defects and functional groups on the tubes’ outer walls, which, in turn, lead to outstanding specific capacitance (>400 F/g) in IL electrolytes.178 This is significantly higher than that of similar vertically aligned MWCNTs that were not exposed to plasma etching/activation (27 F/g in ILs,190 and 11–22 F/g in 6 M KOH188 ). Unfortunately, plasma activated samples decorated with functional groups may potentially have issues with low cycle stability

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and high leakage current, but such studies were not presented in either of the articles.178,180 In addition to growing vertically aligned CNTs on metal foil substrates170,171,174,180,184 or transferring vertically aligned CNTs from ceramic substrates to metal foils,192,193 the simplest and the most conventional method for the preparation of CNT electrodes for EDLC includes oxidation or functionalization and dispersion of CNTs in aqueous solutions followed by filtering and drying.189 This method, however, is not compatible with commercial EDLC preparation techniques, does not permit formation of thick (0.1–0.3 mm) and dense (0.5–1 g/cm3 ) electrodes and suffers from low precision and poor control over the electrode thickness and porosity. One may argue that due to their low specific surface area, pure CNT-based electrodes will likely only find use in ultra-high power/low energy applications anyway, which may, in turn, only require thin (0.001–0.01 mm) electrodes. However, a recent progress in the large-scale fabrication of ultrastrong CNT fabrics may open some opportunities for the fabrication of lightweight, multifunctional and flexible EDLCs with energy storage, electromagnetic shielding and load-bearing functionalities, as recently demonstrated for Li-ion batteries.194 Two common (for research laboratories) methods for the formation of well-controlled dense ultra-thin CNT films include layer-bylayer (LBL) deposition168,182 and electrophoretic deposition.185,195 The repeated, sequential immersion of a substrate into different aqueous solutions of functionalized CNTs, some having only a positive charge (such as amine functional groups, CNT–NH2 ) and others having only a negative charge (such as carboxylic functional groups, CNT–COOH) on their surface produces CNT-based LBL thin film assembly.168,182 Ultra-thin (

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