Download - Carbon Nanofibers Prepared via Electrospinning

Transcript
Page 1: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

Michio Inagaki , * Ying Yang , and Feiyu Kang

Carbon Nanofi bers Prepared via Electrospinning

Carbon nanofi bers prepared via electrospinning and following carboniza-tion are summarized by focusing on the structure and properties in relation to their applications, after a brief review of electrospinning of some poly-mers. Carbon precursors, pore structure control, improvement in electrical conductivity,and metal loading into carbon nanofi bers via electrospinning are discussed from the viewpoint of structure and texture control of carbon.

1. Introduction

Fibrous carbon materials have attracted the attention of scien-tists and engineers. [ 1 , 2 ] In the 1960s carbon fi bers were developed as one of the important industrial materials for modern science and technology, and which have been produced from various carbon precursors via the melt-spinning process. Polyacryloni-trile (PAN) has been used as the principal precursor associated with different modifi cations in processing, such as the use of additives, oxidative stabilization of as-spun PAN fi bers at a low temperature, and stretching during stabilization and carboniza-tion. Isotropic and anisotropic mesophase pitches and phenolic resins have also been precursors for carbon fi bers. A catalytic chemical vapor deposition (CVD) process has also produced carbon fi bers with a structure and properties that are different from those produced via melt-spinning, which is called vapor-grown carbon fi ber (VGCF). In the center of VGCFs, thin tubes consisting of straight carbon layers were found, [ 3 ] which were later reported to be formed via arc-discharging [ 4 ] and named carbon nanotubes (CNTs). [ 5 , 6 ] The process used for the produc-tion of VGCFs was successfully applied to synthesize CNTs. [ 7 ] The diameters of CNTs are in the nanometer range, e.g., 7 nm for single-wall CNTs, in contrast to the micrometer-range

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Prof. M. Inagaki Professor Emeritus of Hokkaido UniversitySapporo 060-8628, Japan E-mail: [email protected] Prof. Y. Yang Department of Electrical EngineeringTsinghua UniversityBeijing 100084China, State Key Laboratory of Control and Simulation of Power System and Generation EquipmentsTsinghua UniversityBeijing 100084, China Prof. F. Kang Department of Materials Science and EngineeringTsinghua UniversityBeijing 100084, China

DOI: 10.1002/adma.201104940

Adv. Mater. 2012, 24, 2547–2566

diameters of carbon fi bers. CNTs are con-sidered to be an important material for the development of modern nanotech-nology in the 21st century, because carbon fi bers have supported the development of modern technology since the 1960s.

Electrospinning has been used to pro-duce nanofi bers of various polymers with diameters from a few tens of nanometers to a few micrometers in different forms

such as nonwoven mats (webs), yarns, etc. It is a relatively simple and low-cost strategy to produce continuous nanofi bers from polymer solutions or melts. Over the past decade, electro-spun nanometer- to sub-micrometer-sized polymer nanofi bers have attracted much attention in both research and commerce. A few companies started to develop electrospun nanofi ber prod-ucts on the basis of large-scale electrospinning setups. This technique was reviewed from different viewpoints, focusing on polymer nanofi bers. [ 8–17 ]

In this review, we focuse on carbon nanofi bers prepared via electrospinning and carbonization by summarizing on their structure and properties in relation to their applications, after a brief explanation on the setup and conditions for electrospin-ning, and some polymer nanofi bers. Carbon precursors, control in pore structure, improvement in electrical conductivity, metal loading of carbon nanofi bers are discussed from the viewpoint of control of structure and texture in the resultant carbon nanofi bers. Although one of the applications of electrospun carbon nanofi bers is the support for catalysts and electrochemi-cally active materials, loading and deposition of these materials on carbon nanofi bers are not included here.

2. Electrospinning of Polymers

2.1. Setup and Conditions for Electrospinning

A fundamental setup for electrospinning of polymers is sche-matically shown in Figure 1 . A viscoelastic solution of polymers is charged by a DC or AC high voltage due to potential differ-ence between the syringe and grounded target. The repulsion among the charges on the surface of the drop at the tip of the syringe (spinneret) competes with the surface tension, which tends to stabilize the drop. Once a critical condition is reached at which surface charge repulsion dominates, a jet is drawn from the spinneret under a constant fl ow rate . The accelerating jet decreases in diameter with increasing external applied fi eld and the surface charge repulsion continually draw on it, until a point is reached where the axis of the jet bends, and the jet begins whipping. As the solvent evaporates, the jet solidifi es to

2547wileyonlinelibrary.com

Page 2: Carbon Nanofibers Prepared via Electrospinning

254

www.advmat.dewww.MaterialsViews.com

REV

IEW Michio Inagaki is the pro-

fessor emeritus of Hokkaido University and Toyohashi University of Technology. He received his PhD from the Nagoya University, Japan, in 1963. He worked in different universities, Nagoya University, Toyohashi University of Technology, Hokkaido University, and Aichi Institute of Technology.

His research activities are in the fi elds of science and engi-neering on carbon materials, and are highlighted by The SGL Carbon Award from American Carbon Society in 2004.

Ying Yang is the professor of Tsinghua University. She received her PhD from Tsinghua University, Beijing, China in 2007. She was a post doctoral associate in Department of Chemical Engineering at Massachusetts Institute of Technology from 2008 to 2009.

Feiyu Kang was born in 1962, received doctoral degree from Hong Kong University of Science and Technology. He now is a full professor in Department of Materials Science and Engineering, and also a dean in Graduate School at Shenzhen, Tsinghua University. His research is focusing on nanocarbon materials, graphite, thermal

conductive materials, lithium ion battery, super-capacitors, electric vehicles, porous carbon and adsorption, indoor air clearing and water purifi cation.

Figure 1 . Scheme of fundamental setup for electrospinning.

form thin fi bers, which are deposited on the grounded target (or collector). The diameters of polymer fi bers are around nanom-eters (nanofi bers), from few tens nanometers to micrometers an example of polymer nanofi bers, although it is still diffi cult to electrospin polymer into uniform nanofi bers with diameter as small as several nanometers up to now.

More than 100 kinds of polymers have been used to produce their nanofi bers via electrospinning in the past 20 years. Elec-trospinning process of various polymers was discussed on the condition to control fi ber diameter in nanometer scale. [ 8 , 11 , 13–15 ] The electrospun polymer fi bers are usually smooth solid fi bers. By using a spinneret consisting of two coaxial capillaries and different polymer solution in each capillary, nanofi bers were successfully prepared with core-shell structure [ 18–20 ] or with hollow structure. [ 21 ] The use of volatile solvent resulted in porous nanofi bers. [ 22–24 ] The mechanical and thermodynamic properties of electrospun polymer nanofi bers were discussed from the electrospinning conditions. [ 17 ]

The electrospun polymer fi bers can be collected in the form of a random nonwoven mat (web) by using fl at metallic plate or mesh as a collector. By using a round collector rotating in a con-stant speed the continuous fi bers can be collected in an aligned form. By using a spinneret with single nozzle and immiscible two polymer solutions, side-by-side bicomponent nanofi bers were prepared. [ 25 ] The use of multi-needle spinneret can enhance the production rate of polymer nanofi bers. [ 26 ] Recent developments in designs for the collection of spun nanofi bers, in particular, the mass production of nanofi bers and the fabrica-tion of various forms, from nonwoven form to yarn, 3D assem-blies and patterned structures, were reviewed. [ 15 ]

The spinnability of a polymer solution and the morphology of resulting fi bers are known to depend strongly on three fac-tors; properties of polymer solution, processing condition and

8 wileyonlinelibrary.com © 2012 WILEY-VCH Verlag

atmosphere condition. The fi rst factor, solution properties, includes surface tension, electrical conductivity, dielectric con-stant, viscosity of the polymer solution, depending strongly on the polymer (its concentration, molecular weight and architec-ture), solvent (vapor pressure, diffusivity in air, etc) and addi-tives (surfactants, salts, etc). In electrospinning of poly(ethylene oxide) PEO, a marked solvent effect was reported; a mixed sol-vent of ethanol with deionized water in either 1/1 or 3/1 volume

GmbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 3: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

Figure 2 . Dependences of diameter of polyacrylonitrile fi ber on concen-tration of the precursor solution and applied voltage for spinning. [ 33 ] Reprinted with permission from Elsevier.

ratio had to be used. [ 27 ] The second factor, processing condition, includes fl ow rate of the polymer solution controlled by syringe pump, strength of electric fi eld controlled by power supply, and distance between the spinneret tip and the collector. The last factor, atmosphere condition, includes the gas used (such as air, N 2 , Ar, vacuum) and its fl ow rate, and humidity in the electrospinning hood. Humidity in spinning atmosphere was reported to affect the morphology and porosity of electrospun fi bers of various polymers, polystyrene PS, poly(vinyl chloride) PVC and poly(methyl methacrylate) PMMA, poly(vinyl chloride) PVC. [ 28–30 ] However, most of the works on electrospinning of polymer nanofi bers focused on the control of the fi rst two fac-tors and ignored the atmosphere effect. Concentration and fl ow rate of a combination of precursor polymer and solvent, addi-tives in the solution, power voltage applied for spinning, and distance between the spinneret tip and the collector are usually reported as parameters to be controlled. Although a wide variety of polymers have been studied for electrospinning, there are no universal ranges for these parameters to synthesize homoge-neous thin fi bers found until now.

2.2. Polymer Nanofi bers

The polymer nanofi bers which have been converted to carbon nanofi bers are rather limited, as polyacrylonitrile (PAN), poly-imide (PI), poly(vinyl alcohol) (PVA), poly(vinyliden fl uoride) (PVDF) and pitch, although so many kinds of polymers have been electrospun.

PAN has been commonly electrospun into high-quality fi bers with various diameters [ 31 ] and also converted to carbon nanofi bers, as explained in the following sections. Morphology (i.e., diameter and uniformity) of the electrospun fi bers is sen-sitive to three factors mentioned, but the solution properties are found to be dominant. [ 32–36 ] Generally, the diameter of the elec-trospun fi bers decreases dramatically with decreasing poly mer concentration and with increasing electrical conductivity of solution. Up to now, it is still diffi cult to electrospin PAN pol-ymer into uniform nanofi bers with diameter as small as sev-eral nanometers. [ 37 ] A special alignment can be also achieved by manipulating the distribution of electric fi eld. [ 38 ] Moreover, the carbon nanotubes are embedded into the electrospun PAN fi bers in order to improve the mechanical properties, thermal stability and electrical conductivity, since the pure fi bers are not as strong as desired. [ 39–42 ]

On a N,N-dimethylformamide (DMF) solution of PAN, effects of the concentration of the polymer solution and the applied DC voltage between the electrode immersed into the solution and the target on the diameter of PAN nanofi bers are studied. [ 33 ] Solution concentration governs the diameter of resultant fi bers, as shown in Figure 2 a). Effect of PAN concen-tration in DMF solution on diameter of electrospun nanofi bers was also reported to depend on molecular weight of PAN [ 43 ] and entanglement density of the polymeric solution. [ 44 ] Applied voltage did not appreciably affect on the fi ber diameter, [ 33 ] as shown in Figure 2 b).

Recently, PI fi bers are prepared by many researchers through electrospinning. [ 45–54 ] Generally, three-step method is adopted, which include polymerization of polyamic acid (PAA),

© 2012 WILEY-VCH Verlag GAdv. Mater. 2012, 24, 2547–2566

electrospinning of PAA solution and then imidization of the electrospun PAA fi bers. Concentration or viscosity of the PAA solution was found to be one of the most effective variables on the control of morphology of electrospun PI fi bers. [ 45 ] Diameter of the PI fi bers synthesized ranges from several tens nanom-eter to several hundreds nanometer. The applied voltage and the feeding rate of the precursor solution may have an infl u-ence on the diameter, even though the effect is not drastic. By adding salts to PAA solution and controlling humidity condi-tions ultrafi ne uniform nanofi bers were prepared with a narrow range of nanoscale diameters (33 ± 5 nm). [ 46 ]

PVDF is a thermoplastic polymer which can be easily proc-essed into various forms. PVDF was electrospun from its solution of a mixed solvent of acetone and DMAc under sev-eral concentrations from 12 to 18 wt%. [ 55 ] Webs of electrospun PVDF fi bers were prepared from its mixed solutions of DMF and acetone, [ 56 ] fi ber diameter ranging from 50 to 300 nm from a 15% PVDF solution of 8/2 DMF/acetone. Its porous fi bers were prepared via electrospinning accompanied by phase sepa-ration during electrospinning process, which was induced by poly(ethylene oxide) and water mixed into PVDF solution. [ 57 ]

Poly(vinyl alcohol) (PVA) is a water-soluble polyhydroxy polymer and has been used as a carbon precursor in funda-mental researches, even though it is easily decomposed at a high temperature and gives a low carbon yield. PVA nanofi bers

2549wileyonlinelibrary.commbH & Co. KGaA, Weinheim

Page 4: Carbon Nanofibers Prepared via Electrospinning

2550

www.advmat.dewww.MaterialsViews.com

REV

IEW

with various diameters (50–250 nm) were easily electrospun

from its 7–15 wt% aqueous solution. [ 58 , 59 ] From a 12 wt% PVA aqueous solution containing 1.0 wt% MWCNT, the composite nanofi bers were prepared by electrospinning. [ 60 ] Effect of pH of PVA aqueous solution was discussed on the morphology and diameter of its fi bers electrospun from a 7 wt% solution. [ 61 ] The average diameter of PVA nanofi bers electrospun at pH 7.2 was 290 nm and it became thinner with increasing pH under basic conditions. Under acidic conditions, however, the electrospin-ning of PVA solution was not continuous and PVA with beads-on-string structures was obtained due to the protonation of PVA.

In contrast to PVA, pitch has several advantages as the pre-cursor for carbon nanofi bers; high carbon yield and low cost. For pitches, however, a proper solvent, which dissolves the pitch to high enough concentration and has a proper vapori-zation point, has to be selected for its electrospinning. The DMF-insoluble fraction of a petroleum-derived isotropic pitch in THF solution and a binary solvent of DMF and THF were used for the spinning of pitch to convert to carbon nanofi bers, as explained in the next section. Molecular structure of the pre-cursors was shown to have an effect on spinnability of their THF solution by using two pitches with different weight-aver-aged molecular weights of 2380 and 556. [ 62 ] THF-soluble com-ponent of the pitch with the lower molecular weight gave better spinnability on its 40 wt% THF solution, but the pitch with the higher molecular weight resulted in more microscopic carbon nanofi bers

3. Carbon Nanofi bers Synthesized Via Electrospinning

In Table 1 , the papers reporting on the preparation of carbon nanofi bers via electrospinning process are listed in the order of published year by summarizing key points on precursor solution, spinning condition, treatments to convert to carbon nanofi bers and their characteristics. [ 63–141 ] In Table 1 , the main purpose of the papers is also indicated together with the ref-erence number. As carbon precursors, PAN and pitches were frequently used, probably because both of them are also used in the production of commercial carbon fi bers. In addition, poly(vinyl alcohol) (PVA), polyimides (PIs), polybenzimidazol (PBI) poly(vinylidene fl uoride) (PVDF), phenolic resin and lignin were used.

In order to convert electrospun polymer nanofi bers to carbon nanofi bers, carbonization process at around 1000 ° C has to be applied. In principle, any polymer with a carbon backbone can potentially be used as a precursor. For the carbon precursors, such as PAN and pitches, so-called stabilization process before carbonization is essential to keep fi brous morphology, of which the fundamental reaction is oxidation to change resultant car-bons diffi cult to be graphitized at high temperatures as 2500 ° C. During stabilization and carbonization of polymer nanofi bers, they showed signifi cant weight loss and shrinkage, resulting in the decrease of fi ber diameter.

Here, the results obtained in these papers are reviewed by dividing into the sections based on the purposes of the research works; fundamental structure and properties of the

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag

carbon nanofi bers, their performance in energy storage devices, lithium-ion rechargeable batteries and electrochemical capaci-tors, and composite nanofi bers with carbon nanotubes.

3.1. Structure and Properties

PAN nanofi bers were prepared by electrospinning from DMF solution, on which structure and electromagnetic properties were studied. [ 63–65 ] Structural analysis was performed on carbon nanofi bers, which were prepared from PAN/DMF solution by carbonization at 750 ° C followed by 1100 ° C. [ 77 ] The resultant carbon nanofi bers had average diameter of 110 nm, interlayer spacing d 002 of 0.368 nm and Raman band intensity ratio I D /I G of 0.93. From SEM and TEM observations, the fi ber was con-cluded to have skin-core heterogeneity; in the skin carbon layers being oriented predominantly parallel to the fi ber surface. PAN-based carbon nanofi ber bundles, which were prepared from 10 wt% PAN/DMF solution added 5 wt% acetone and 0.01 wt% dodecylethyldimethylammonium bromide, and collected on the rim of the rotating disc covered with Al foil, were subjected to the heat treatment at 1400, 1800, and 2200 ° C for 1 h. [ 107 ] The diameter of nanofi bers composing the bundles was approxi-mately 330 nm for as-spun, 250 nm for 1000 ° C-treated and 220 nm for 1800 ° C-treated. TEM images are shown in Figure 3 on 1000 ° C-treated and 2200 ° C-treated nanofi bers, the latter having d 002 of 0.344 nm and I D /I G of larger than 1.0. Aiming to have better alignment of basic structural units of hexagonal carbon layers along the fi ber axis, multi-walled carbon nano-tubes (MWCNTs) were embedded into electrospun PAN-based carbon nanofi bers, although the improvement was observed just around MWCNTs. [ 97 ] TEM observation on MWCNTs-embedded PAN-based nanofi bers by in-situ heating up to 750 ° C showed only a local orientation of carbon layers. [ 142 ] On PAN-based nanofi bers web after the activation by steam at 800 ° C, adsorp-tion behavior of benzene vapor was studied at a temperature of 343–423 K under a pressure up to 4.0 kPa, confi rming a high adsorption in comparison with activated carbon fi ber A-10. [ 82 ] By focusing on PAN, electrospinnability, environmentally benign nature and commercial viability were recently reviewed. [ 143 ]

Core-shell polymeric nanofi bers were electrospun through a doubled capillary, PAN/DMF solution in the outer capillary and poly(methyl methacrylate) PMMA in the inner capillary, and converted to hollow carbon nanofi bers by carbonization up to 1100 ° C. [ 81 ] Similar hollow nanofi bers were synthesized by electrospinning of emulsion-like DMF solution of PAN and PMMA in different ratios through a single capillary of 0.5 mm diameter, followed by carbonization at 1000 ° C and heat treatment up to 2800 ° C, [ 84 ] as shown in Figure 4 . By changing PAN/PMMA ratio, mesopore volume could be controlled; mes-opore volume V meso changed from 0.18 cm 3 g − 1 for 9/1 ratio to 0.47 cm 3 g − 1 for 5/5 ratio, although micropore volume V micro was almost constant of 0.34 cm 3 g − 1 . [ 84 ]

Mixing of poly(vinylpyrrolidone) (PVP) into PAN was also employed to control pore structure in the nanofi bers. [ 103 ] PVP was dissolved out from as-spun fi bers at 100 ° C under hydro-thermal condition and the resultant PAN nanofi bers were car-bonized at 1000 ° C after stabilization. The change of PAN/PVP ratio in the precursor solution from 0.8/0.2 to 0.8/1.0 resulted

GmbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 5: Carbon Nanofibers Prepared via Electrospinning

2551

www.advmat.dewww.MaterialsViews.com

wileyonlinelibrary.com© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

REV

IEW

Table 1 . Preparation of carbon nanofi bers via electrospinning.

Ref. (Purpose a) ) Precursor and additives/solvent

Spinning conditions b) Stabilization, carbonization and activation Carbon nanofi bers synthesized and functionalities

[ 63–65 ] (Structure) PAN/DMF 15 kV/–/15 cm/– no stabilization, carbonized at 600, 800,

1000, 1200 ° C for 1/2h in vacuum92.5 × 164 nm 2 , σ = 490 S m − 1 , magnetoresis-

tance (1000 ° C) of -0.75 at 1.9 K and 9T

[ 66 ] (Composite) [PAN + SWCNT]/DMF 25 kV/–/15 cm/0.9 mm stabilized at 200 ° C, carbonized at 1100 o C homogeneous and straight SWCNTs in PAN,

stiffening of PAN

[ 67 ] (EDLCs) 10 wt%PAN/DMF 10–25 kv/–/–/– stabilized at 280 ° C, carbonized & activated

at 700-800 ° C in N 2 + steam

200–400 nm dia., S BET 1230 m 2 g − 1 , V total 0.55cc g − 1 ,

173 F g − 1 at 10 mA g − 1 in 30vol% KOH

[ 68 ] (Properties) 12 wt%PI/(THF + MeOH) 13–15 kV/20 g h − 1 /6–7

cm/0.41 mm/–

imidization at 150–250 ° C, carbonized at

700–1000 and 2200 o C

electrical conductivity: from 2.5 and

5.3 S cm − 1 , tensile strength: from 5.0

and 73.9 MPa by 1000 and 2200 o C

[ 69 ] (Properties) Pitch/THF – carbonization & activation S BET 2060–2220 m 2 g − 1 , microporous

[ 70 , 71 ] (EDLCs) PBI/DMAc 10–25 kV/–/–/– 700–850 ° C for 1/2h in N 2 + 30vol% steam 100-500nm dia., 800 ° C-treated S BET 1220 m 2

g − 1 , V micro 0.71mL g − 1 , 175F g − 1 in 30wt%KOH

[ 72 ] (Properties) DMF-insoluble pitch/THF 18 kV/3mL h − 1 /10 cm/

0.88 mm

stabilized at 300 ° C, carbonized at 700,

1000, 1200 ° C for 1h

dumb-bell like cross-section with (4–6) × (2–3) µ m, electrical conductivity 83S/cm

[ 73 ] (Composite) [6.7 wt%PAN + 3.3 wt%

Fe-acac]/DMF

30 kV/–/30 cm/– stabilized at 250 ° C, Fe reduction at 500–

550 ° C in H 2 , carbonized at 1100 o C

MWCNTs grown by CVD of hexane at 700 ° C

on nanofi bers

[ 74 ] (EDLCs) 10 wt%PAN/DMF 20 kV/–/–/–/300 rpm stabilized at 280 ° C, carbonized & activated

at 800 ° C for 1/4–1h in N 2 + steam

200–350 nm dia., S BET = 1160 m 2 g − 1 , V total =

0.64 cc g − 1 , 134 F g − 1 in 6N KOH with

1 mA cm − 2

[ 75 ] (EDLCs) PI/(THF + MeOH) 13–15 kV/–/–/– imidized at 350 ° C, carbonized at 1000 ° C and

activated at 650–800 ° C in N 2 + steam

S BET = 941–1450 m 2 /g, V micro = 0.37–0.56cc g − 1

750 ° C-activated: 175F g − 1 in 30wt% KOH

[ 76 ] (Composite) [7 wt%PAN + 2–35 wt%

MWCNT]/DMF

30 kV/–/30 cm/–/10 m/s stabilized at 220 ° C for 2h in air, carbonized

at 850 ° C for 1/2h in Ar

100–300nm dia., 5% MWCNT composite

shows max. strength 80MPa

[ 77 ] (Properties) 8 wt%PAN/DMF 0.9 kV/cm/–/–/0.1 mm/

5 m s − 1 stabilized at 250 ° C, carbonized at

1100 ° C 1h

110 nm dia., 89.4%C, d 002 = 0.368 nm, I D /I G =

0.9, weaker than commercially available fi bers

[ 78 ] (EDLCs) 20 wt%PBI/DMAc –/–/–/0.5 mm/300 rpm carbonized & activated at 700, 750, 800,

850 ° C in N 2 + 30% steam for 1/2h800 ° C-treated: S BET = 1220m 2 g − 1 , V micro =

0.71 cc g − 1 , 202F g − 1 in 1M H 2 SO 4 with

1 mA cm − 2 , good rate performance

[ 79 ] (Properties) [20 wt%PI + Fe-acac

(0–3.0 wt%)]/DMAc

6–15 kV/20–50 µ m

min − 1 /–/–/–

carbonized at 400–1200 ° C for 1h α -Fe and Fe 3 O 4 are formed above 800 ° C, d 002

= 0.34 nm, I D /I G = 1.8 by 3wt% atfter 1200 o C

[ 80 ] (LIBs) PAN/DMF 25 kV/–/300 rpm/–/

0.5 mmstabilize at 280 ° C for 1h, carbonized at

700–2800 ° C

1000 ° C-treated C dis = 450 mAh g − 1 but C irr =

ca. 500 mAh g − 1 in 1M LiClO 4 EC/DEC with

30 mA g − 1

[ 81 ] (Structure) PAN/DMF(shell) and

PMMA/Acetone(core)0.3 kV cm − 1 /–/18 cm/

0.3–1.65 mm

stabilized at 250 ° C carbonized at 1100 ° C

for 1h

hollow carbon nanofi bers

[ 82 ] (Properties) 10 wt% PAN/DMF 20 kV/–/18 cm/–/– stabilized at 280 oC, carbonized at 80 ° C,

activated at 800 ° C in 30% steam for 1hS BET = 1193m 2 g − 1 , V micro = 0.455 cc g − 1 high

adsorptivity for benzene vapor

[ 83 ] (EDLCs) [PAN + Ru-acac]/DMF 20 kV/–/18 cm/

0.5 mm/300 rpmstabilized at 280 ° C, carbonized & activated

at 800 ° C in N 2 + 30 vol% steam

200-900nm dia., Ru metal (2–15 nm)-

embedded, 7.31wt% Ru: 391F g − 1 (261F g − 1

pseudocapactitance)

[ 84 ] (Properties) [PAN + PMMA(5/5, 7/3,

9/1)]/DMF

20 kV/–/–/0.5 mm/

300 rpmstabilized at 280 ° C for 1h, carbonized at

1000 & 2800 o C

PAN/PMMA = 5/5 and 1000 ° C-carbonized:

S BET = 940 m 2 g − 1 , V meso = 0.47 & V micro =

0.35 cc g − 1

[ 85 ] (EDLCs) [10wt%PAN + 1–55 wt%

ZnCl 2 ]/DMF

25 kV/–/–/–/300 rpm stabilized at 280 ° C, carbonized at 800 ° C

for 1h in Ar, repeatedly wash by HCl

S BET = 310–550 m 2 g − 1 , V total = 0.17–0.34cc g − 1 ,

140 F g − 1 in 6M KOH

[ 86 ] (Properties) [PI + 1, 3, 5wt%TEA]/DMF 19–23 kV/–/15 cm/– stabilized by oxidation, carbonized

at 1000 ° C for 1 h in Ar

80 nm dia. from 18wt%PAA solution, conduc-

tivity of the mat under 22,000Pa pressure was

16S/cm

[ 87 ] (Properties) [9.1wt%PAN + SiO 2 ]/ 18 kV/1.0 mL h − 1 /–/1.27

mm/300 rpm

stabilized at 250 ° C for 8h, carbonized at

1050 ° C for 1h in N 2 , washed out Si by HF

fi ber diameter increases, bead, bead-on-string

and fi ber structures, S BET = 340 m 2 g − 1 , V total =

0.472 cc g − 1

[ 88 ] (Properties) [PAN + PI (10/0, 7/3, 5/5,

3/7, 0/10)]/DMF

20 kV/–/–/–/ca. 300 rpm stabilized at 280 ° C for 1 h, carbonized at

1000 ° C in Ar

carbon yield increases, σ decreases and I D /I G

increases with increasing PI content

Adv. Mater. 2012, 24, 2547–2566

Page 6: Carbon Nanofibers Prepared via Electrospinning

2552

www.advmat.dewww.MaterialsViews.com

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

REV

IEW

Ref. (Purpose a) ) Precursor and additives/solvent

Spinning conditions b) Stabilization, carbonization and activation Carbon nanofi bers synthesized and functionalities

[ 89 ] (Properties) [Ph + PVB]/MeOH 15 kV/0.01–0.20 mL

min − 1 /10-20cm/1.0 mm

cured by adding formaldehyde, carbonized at

900 ° C for 2 h in N 2

fl exible carbon nanofi ber fabric by adding PVB,

S BET = 495 m 2 g − 1

[ 90 ] (LIBs) [PAN + Co(OAc) 2 ]/DMF 10–11 kV/2.5 µ L m − 1 /

12 cm/0.6 mm

stabilized at 230 ° C, carbonized at

600 ° C for 10h

100–300 nm dia., 40wt%Co loaded, high dis-

charge capacity > 750 mAh/g & good cyclability

[ 91 ] (LIBs) [PAN + Fe-acac]/DMF 12–14 kV/4 µ L min − 1 /

12 cm/–

stabilize at 240 ° C, carbonized at 500–700 ° C

for 10h

Fe 3 O 4 -included, interesting cycle performance

[ 92 ] (Properties) [8 wt%PAN + 4.8 wt%

Pd(OAc) 2 ]/DMF

30 kV/–/30 cm/– stabilized at 230 ° C for 3 h, reduced Pd 2 + at

300 ° C, carbonized at 1100 ° C in Ar

200–500 nm dia., Pd ( ∼ 37nm size) dispersed,

high sensitivity for H 2 O 2 and NADH

[ 93 ] (Fuel cells) PAN/DMF 13.7 kV/–/12 cm/– stabilized at 280 ° C in air, carbonized at

1200 ° C for 1h in Ar

∼ 150 nm dia., σ = 50S/cm, electrochemical

deposition of Pt(50-200 nm), high electro-

catalytic activity and stability

[ 94 ] (EDLCs) [10 wt%PAN +

3 wt%MWCNT]/DMF

20 kV/1 mL h − 1 / 15 cm/

0.5 mm/300 rpm

stabilized at 280 ° C, carbonized and

activated at 800 ° C, coated by polypyrole

PAN-based: S BET = 742m 2 g − 1 , 141 F g − 1 , PAN/

CNT: S BET = 1170m 2 g − 1 , 180 F g − 1 , PPy/PAN/

CNT: 333 F g − 1

[ 95 ] (EDLCs) [10 wt%PAN + 10 wt%

V 2 O 5 ]/DMF

20 kV/1 mL h − 1 /

130 rpm/17 cm/1.27 mm

stabilized at 250 ° C for 8h, carbonized and

activated with KOH at 800 ° C in Ar

V(5wt%)-embedded, S BET = ∼ 2800 m 2 g − 1 ,

107F g − 1 in 1M (C 2 H 5 ) 4 NBF 4 /PC with 20 A g − 1

[ 96 ] (Properties) [10 wt%PAN + V 2 O 5 ]/DMF 15 kV/1 mL h − 1 /

17 cm/1.27 mm/130 rpm

stabilized at 250 ° C for 8h, carbonized at

1050 ° C for 1h, activated by KOH at 750 ° C

V(25wt%, 80nm)-embedded, S BET =

2780 m 2 g − 1 , H 2 adsorption = 2.41wt% at

303K &10 MPa

[ 97 ] (Structure) [PAN + 2.5, 5, 25 wt%

MWCNTs]/DMF

12 kV/–/15 cm/

0.5 mm/1000 rpmstabilized at 250 ° C 1h in air, carbonized at

750, 1200 ° C for 1h in N 2

little improvement in carbon layer alignment

around MWCNTs

[ 98 ] (Properties) [10 wt%PAN + 0.5–

1.5 wt%Mn(OAc)]/DMF

20 kV/1 mL h − 1 / 18 cm/

0.5 mm/300 rpm

stabilized at 280 ° C, carbonized at

1000 ° C, activated at 800 ° C in N 2 +

30 vol%H 2 O

Mn(0.23wt%)-embedded: S BET =

1230 m 2 g − 1 , V micro = 0.42 cc g − 1 , toluene

adsorption = 68g/100g

[ 99 ] (LIBs) [8 wt%PAN + Mn(OAc) 2 ]/

DMF

14 kV/0.75 mL h − 1 /

15 cm/–

stabilized at 280 ° C in air, 700 ° C for 1h MnO + Mn 3 O 4 -loaded porous nanofi bers:

800–450mAh g − 1

[ 100 ] (LIBs) [PAN + (poly- L -lactic acid) +

Si(17:3:6)]/DMF

21 kV/0.75 mL h − 1 /

15 cm/–

stabilize at 280 ° C for 8h in air, 700 ° C for 1h

in Ar

Si nanoparticle-loaded, C dis = 1100 and C irr =

240 mAh g − 1

[ 101 ] (Properties) [PAN + pitch]/(DMF + THF) 25 kV/–/–/– Carbonization at 1000 ° C, steam activation at

700-1000 ° C for 1h

Improvement in spinnability by PAN, S BET =

732-1877 m 2 g − 1 , V total = 0.3-1.1 cc g − 1

[ 102 ] (EDLCs) [PAN + CA]/DMF – 800 ° C for 0h without CA: S BET = 742 m 2 g − 1 , pore 2.0 nm,

141F g − 1 with 15wt%CA: S BET = 1160 m 2 g − 1 ,

pore 2.8 nm, 245F g − 1

[ 103 ] (Structure) [PAN + PVP]/DMF 10 kV/–/–/10–15 cm/

800 µ m

dissolved PVP by hot water, stabilized at

270 ° C and carbonized at 1000 o CS BET 330-570 m 2 g − 1 ,

[ 104 ] (LIBs) PAN/DMF, TBT/mineral oil,

(PAN/TBT = 1/1)20 kV/15 & 5 µ L min − 1 /

15 cm/1.2 & 0.5 mm

dissolved out mineral oil in n-octane,

carbonized at 1000 ° C for 5 h

Sn@carbon nanotube, 200 nm dia. with ca.

100 nm Sn, reversible capacity 737 mAh g − 1

[ 105 ] (EDLCs) [7.2 wt% PAN + 0.8 wt%

MWCNT]/DMF

30 kV/–/30 cm/– stabilized at 240 ° C, carbonized at

700 ° C, activated at 650 ° C with H 2 O 2 vapor

PAN: S BET = 930 m 2 g − 1 , σ = 0.86S cm − 1 ,

170F g − 1 in H 2 SO 4 PAN/MWCNT: S BET =

810 m 2 g − 1 , σ = 5.32S cm − 1 , 310F g − 1

[ 106 ] (LIBs) [PAN + PMMA + tin octoate]/

DMF

stabilized at 250 ° C, multichannel tubular with ∼ 2 µ m dia. & ∼ 150 nm

channels, reversible capacity 648 mAh g − 1

[ 107 ] (Properties) 10 wt%PAN/(DMF +

acetone + bromide)

20 kV/1.5 mL/h − 1 / 22 cm/

0.55 mm/ 1500 rpm

stabilized at 280 ° C, carbonized at

1000 ° C in vacuo, heated to 1400–2200 ° C

for 1h

bundles, 2200 ° C-treated: d 002 = 0.344 nm, I D /

I G > 1, σ // = 840, strength = 542 MPa, Young’s

modulus = 58GPa

[ 108 ] (EDLCs) [PAN + 0, 35, 100, 135 wt%

Ni(OAc)]/DMF

35 kV/0.5 mL h − 1 /

20 cm/0.6 mm

stabilized at 280 ° C in air, carbonized at

1000 ° C for 1h

22.4 wt%Ni: 164 F/g in 6 M KOH at 2 mV s − 1

scan

[ 109 ] (EDLCs) [10 wt%PAN + 1, 3,

5 wt%Ni(NO 3 ) 2 ]/DMF

25 kV/–/20 cm/–/350 rpm stabilized at 280 ° C in air, carbonized at

1000 ° C, leached excess Ni in 4M NaOH

S BET = 480–682 m 2 g − 1 , V meso =

0.21–0.32cc − 1 , V micro = 0.19–0.23 cc g − 1

for 1–5 wt%Ni-salt added,

[ 110 ] (LIBs) [7 wt%PAN + 5, 10,

15 wt%ZnCl 2 ]/DMF

14 kV/0.5 mL h − 1 / 15 cm/– stabilized at 280 ° C for 2.5 h, carbonized at

700 ° C for 1h in N 2 , washed out Zn by HCl

15 wt% ZnCl 2 -mixed gives S BET = 438 m 2 g − 1 ,

V total = 0.29 and V micro = 0.24 cc g − 1 , C dis =

400 mAh g − 1 at 10th cycle

Table 1. Continued.

Adv. Mater. 2012, 24, 2547–2566

Page 7: Carbon Nanofibers Prepared via Electrospinning

2553

www.advmat.dewww.MaterialsViews.com

wileyonlinelibrary.com© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

REV

IEW

[ 111 ] (LIBs) [10 wt%PAN + (0–30 wt%)

Si]/DMF

17 kV/0.75 mL h − 1 /

15 cm/–

stabilized at 280 ° C in air for 5.5 h,

carbonized at 700 ° C for 1h in Ar

C dis = 855 and C irr = 321 mAh g − 1 with

100 mA g − 1 in 1M LiPF 6 /(EC + EMC)

[ 112 ] (LIBs) [PVA + Si[/(H 2 O + Na

alkyl- benzenesulfonate)

20 kV/1 mL h − 1 / 11cm/– dried on copper current collector and then

heated at 500 ° C for 2 h in Ar

Si nanoparticles (40 nm) embedded in

irregular pores, C dis increase with cycles, C dis =

892 mAh g − 1 after 50 cycles

[ 113 ] (LIBs) 8 wt%PAN + (15, 30,

50 wt%) Si( > 70 nm)]/DMF

21 kV/0.75 mL h − 1 /

15 cm/–/–

stabilized at 280 ° C in air for 5 h, carbonized

at 700 ° C for 1 h in N 2

30 wt%Si precursor: C dis = 1281mAh g − 1 , C irr =

260 mAh g − 1 , for 1st cycle, but C dis =

318 mAh g − 1 for 50th.

[ 114 ] (Properties) [10 wt%PAN + 1, 2, 3 wt%

(FeSO 4 + FeCl 3 )]/DMF

25 kV/–/20 cm/–/– stabilized at 280 ° C in air for1h, immersed

into 4M NaOH, carbonized at 1000 ° C

containing 3wt% iron oxide: S BET = 550 m 2 g − 1

[ 115 ] (Properties) [10 wt%PAN + 1–5 wt%

SiWA or SiMoA]/DMF

25 kV/–/20 cm/–/– stabilized at 280 ° C in air, carbonized at

1000 ° C, sashed in 4 M NaOH

A3-sized webs consisting of 100-300 nm fi bers,

σ for SiMoA-contained fi ber: 1.7 × 10 − 3 S cm − 1

[ 116 ] (LIBs) [7 wt%PAN + 0, 5, 10,

20 wt% fumed SiO 2 ]/DMF

21 kV/0.8 mL h − 1 /

15 cm/–

stabilized at 280 ° C for 2.5 h, carbonized at

700 or 1000 ° C, washed out SiO 2 by HF

C irr = 593 mAh g − 1 for 20wt% SiO 2 , large

irreversible capacity

[ 117 ] (LIBs) [10 wt%PAN + 30 wt%

Ni(OAc) 2 ]/DMF

12.5 kV/0.5 mL h − 1 /

15 cm/–

stabilized 280 ° C for 6 h in air, carbonized at

600 ° C for 8h in Ar

dispersed Ni with 20nm size, C dis =

795 mAh g − 1 and C irr = 225 mAh g − 1 in LiPF 6

electrolyte

[ 118 ] (LIBs) [8 wt%PAN + 15, 30, 50 wt%

Mn(OAc) 2 ]/DMF

21 kV/–/–/– stabilized at 280 ° C for 5 h, carbonized at

700 ° C for 1 h in Ar

MnO x ( ∼ 10 nm)-embeddedwith, C dis =

785 mAh g − 1 in 1M LiPF 6 (EC/EMC = 1/1)

with 50 mA g − 1

[ 119 ] (Properties) [15 wt%PAN-itaco acid

copolymer]/DMF

25kV/–/40cm/ 0.4 mm/

200 rpm

stretched in hot water, stabilized under

tension, carbonized at 1000–1500 o C

aligned carbon nanofi ber bundles, expecting

high mechanical strength

[ 120 ] (LIBs) [8 wt%PAN + poly- L -lactic

acid]/DMF

17 kV/0.5 mL h − 1 / 15 cm/– stabilized at 280 ° C for 5.5 h in air,

carbonized at 800 ° C for 1 h

S BET = 235 m 2 g − 1 , V total = 0.114 cc g − 1 , V micro =

0.086 cc g − 1 , C dis = 435 mAh g − 1 after 50 cycles

[ 121 ] (Properties) [Ph + PVB, piridine and/or

Na 2 CO 3 ]/MeOH

15 kV/0.01 mL min − 1 /

15 cm/0.5 mm

cured by adding formaldehyde, carbonized

at 900 ° C for 2 h in N 2

addition of PVB improves spinnability, addition

of electrolyte (pyridine and Na 2 CO 3 allows

thinner fi bers

[ 122 ] (LIBs) [PVA + SnCl 2 ]/H 2 O 25 kV/1.0 mL h − 1 / 15 cm/

1 mm

carbonized at 500 ° C for 3 h in Ar/H 2 including ca. 1nm Sn, C dis = 382 mAh g − 1 at

20th cycle

[ 123 ] (LIBs) [PVA + SnCl 2 ]/H 2 O 35 kV/1.0 mL h − 1 / 20 cm/

1 mm

carbonized at 550 ° C for 3 h in Ar/H 2 Sn/SnO x containing fi bers, 735 and

510 mA h − 1 in 1st and 40th cycle.

[ 124 ] (LIBs) [8 wt%PAN + 50 wt%

Cu(OAc) 2 ]/DMF

10.5 kV/0.5 mL h − 1 /

15 cm/–

stabilized at 280 ° C 5.5 h, carbonized at

600 ° C for 8 h in Ar

fcc-Cu particles embedded in fi bers, C dis = 617

and C irr = 253 mAh g − 1 in LiPF 6 (EC/EMC =

1/1) with 50 mA g − 1

[ 125 ] (Properties) 50 wt%Polycarbosilane/THF 10 kv/4 mL h − 1 /10 cm/

0.8 mm

carbonized in Ar + Cl 2 at 800 or 90 ° C for 2

or 3 h,

S BET = 3116 m 2 g − 1 , high storage capacity of H 2

as 3.86% at 17bar and 77K

[ 126 ] (Properties) [10 wt%PAN + 7wt%

CoFe 2 O 4 ]/DMF

20 kV/5 mL h − 1 /15 cm/

0.8 mm/800 rpm

stabilized at 25 ° C for 2 h in air, carbonized

at 100 ° C for 4 h in Ar

Superparamagnetic, saturation magnetization

of 63emu/g

[ 127 ] (Properties) [Lignin + Pt-(acac)]/ethanol 12 kV/–/20–25 cm/– stabilized at 20 ° C for 36 h in air fl ow,

carbonized at 600–1000 ° C400 nm ∼ 1 µ m, 0.6wt%Pt-loaded: S BET =

1200 m 2 g − 1 , V micro = 0.52 cc g − 1 ,

[ 128 ] (EDLCs) 10 wt%PAN/DMF 25 kV/–/–/– Stabilized at 28 ° C, 1h, carbonized & activated

by steam at 800 ° Cca. 900 nm dia., microporous, S BET =

550 m 2 g − 1 , V micro = 0.35cc g − 1 , Na-ion

capture = 3.2 mg g − 1

[ 129 ] (LIBs) [PAN + Si]/DMF 15 kV/1 mL/h/25 cm/– carbonized at 800, 1000, 120 ° C for 1 h in Ar Si(50–80 nm)-embedded, 120 ° C-treated: C dis =

500 mAh g − 1 and better capacity retention of

78% after 50th cycle.

[ 130 ] (Properties) 9 wt%PAN/DMF 15–25 kV/–/15–25 cm/– stabilized at 30 ° C, carbonized at 800–140 ° C, 380-530nm, tensile strength = 3.5 GPa,

Young’s modulus = 172 GPa after 140 ° C,

[ 131 ] (EDLCs) [PAN + PVP]/DMF PAN/

DMF and PVP/DMF

20 kV/–/15 cm/– stabilized at 30 ° C, carbonized to 97 ° C, acti-

vated by CO 2 at 850 ° C

blend & side-by-side fi bers of PAN/PVP,

interconnected membranes, the latter

ca. 220F g − 1 in 1M H 2 SO 4

[ 132 ] (Properties) 30 wt% Ph (novolac-type)/

MeOH

25 kV/–/20 cm/– cured in formaldehyde + HCl, carbonized at

80 ° C for 2 h

100–450 nm dia., narrow pore size in

0.4–0.7 nm, V tota l = 0.91cc g − 1 , I D /I G = 0.88

Table 1. Continued.

Ref. (Purposea)) Precursor and additives/solvent

Spinning conditionsb) Stabilization, carbonization and activation Carbon nanofi bers synthesized and functionalities

Adv. Mater. 2012, 24, 2547–2566

Page 8: Carbon Nanofibers Prepared via Electrospinning

2554

www.advmat.dewww.MaterialsViews.com

REV

IEW Table 1. Continued.

Ref. (Purposea)) Precursor and additives/solvent

Spinning conditionsb) Stabilization, carbonization and activation Carbon nanofi bers synthesized and functionalities

[ 57 ] (Properties) [PVDF + PEO]/(DMF + water) 22.5 kV/–/40 cm/– stabilized and defl uorized with DBU at 5 ° C,

carbonized at 1000 ° C for 1h2.3 µ m dia., pores with ca. 100 nm in

4.3 pores µ m − 2 , high electrode performance

for redox reaction

[ 133 ] (LIBs) [4 wt%PAN + 8 wt% LiFePO 4

precursor]/DMF

15 kV/–/15 cm/ 0.012 inch stabilized 28 ° C for 5h in air, carbonized at70

° C 18h in ArLiFePO 4 /C composite nanofi bers + beads,

160 mAh g − 1 in 1M LiPF 6 EC/EMC

[ 134 ] (Properties) PAN/DMF 17 kV/–/–/1.27 mm stabilized at 26 ° C, carbonized at 105 ° C for

1h, activated by H 3 PO 4 at 75 ° C for 3hactivated by using 80 wt% H 3 PO 4 : S BET =

621 m 2 g − 1 , V total = 0.501cc g − 1 , V micro =

0.071cc g − 1 , NO gas sensitive

[ 62 ] (Propeties) 40 & 45 wt% pitches (M w =

2380 & 556)/THF

25 kV/3 mL h − 1 /10cm/

0.88 mm

carbonized at 100 ° C for 1h, activated by

steam/N 2 at 700, 800, 90 ° C for 1hM w had a strong effect on spinnability, S BET =

2053 m 2 g − 1 after 90 ° C activation of pitch A

with M w = 2380

[ 135 ] (Properties) 10 wt% PAN/DMF 30 kV/1.5 mL h − 1 /

35 cm/ —

hot-pressed at 20 ° C under 400kPa, stabilized

at 27 ° C, carbonized at 1000 ° C

the mats consisting of carbon nanofi bers with

380 nm diameter and having the bulk density

of 1.21–1.23 g cc − 1

[ 136 ] (EDLC) 10 wt%[PAN + TEOS (7/3)]/

DMF

20 kV/–/25 cm/–/– stabilized at 28 ° C in air, carbonized at 800,

900, 1000 ° C

80 ° C-treated: 150nm diameter, microporous

with S BET = 1200 m 2 g − 1 , w micro = 0.6 nm,

160–130F/g in 6M KOH

[ 137 ] (EDLC) 10 wt%[PAN + β -CD] + 3 wt%

AgNO 3 /DMF

25 kV/–/–/–/– stabilized at 28 ° C for 1h, activated at 80 ° C in

steam/N 2 ,PAN/30% β -CD: ∼ 350 nm diameter, SBET =

1096 m 2 g − 1 , microporous, 150F g − 1 in 6 M

KOH, < 20 nm Ag dispersed

[ 138 ] (EDLC) [20–50 wt%Pitch/THF ] +

PAN/DMF (7/3)

20 kV/–/25 cm/–/– sabilized and carbonization at 1000 ° C S BET = 966 m 2 g − 1 , V pore = 0.38cc g − 1 , w micro =

1.6 nm, 130 F g − 1 , 100 kW kg − 1 , 15 Wh kg − 1

[ 139 ] (Properties) [10wt%PAN + (1, 5,

10 wt %Fe 3 O 4 + X–100]/

DMF

11–12kV/2mL

h − 1 /17cm/–/–

stabilized at 25 ° C for 20 min, carbonized at

700, 90 ° C for 1 h

290–270 nm diameter, Fe 3 O 4 nanoparticles

dispersed, σ = 9.2 S cm − 1 and M s = 16 emn g − 1

(10 wt%, 90 ° C)

[ 140 ] (Properties)10 wt%PAN/DMF 20kV/1mL h − 1 /

15cm/0.5 mm/–

stabilized at 28 ° C, carbonized & activated in

30 vol% steam at 80 ° CS BET = 710 m 2 g − 1 , V micro = 0.345 cc g − 1 , > 60%

removal of NO, no (NO + NO 2 ) detected

[ 141 ] (Properties)

30 wt%Ph + 0.9 wt%PVB +

0.1 wt%Na 2 CO 3 /MeOH

25kV/1mL h − 1 /20 cm/–/– immersed into formaldehyde and HCl

solution, carbonized at 80 ° C for 2 h100-450nm diameter, S BET = 812 m 2 /g − 1 ,

V micro = 0.91cc g − 1 , w micro = 0.4-0.7 nm, H 2 O &

EtOH vapor adsorption

a) Classifi ed into structure, properties, electrochemical capacitors (EDLCs), lithium-ion rechargeable batteries (LIBs), fuel cells and composites with carbon nanotubes. b) Voltage/fl ow rate/distance between nozzle and collector/needle diameter, and collector rotation if any. PAN: poly(acrylonitrile, PI: polyimide. PBI: poly(benzimidazole), PMMA: poly(methyl methacrylate), PVB: poly(vinyl butyral), PVP: poly(vinyl pyrrolidone), CA: cellulose acetate, Ph: phenolic resin, SWCNT: single-wall carbon nanotube, MWCNT: multiwalled carbon nanotube, DMF: N,N-dimethylformamide, THF: tetrahydrofurane, DMAc: dimethylacetamide, EtOH: ethanol, MeOH: methanol, TEA: triethy-lamide, TBT: tributyltin, acac: acetylacetonate, OAc: acetate. SiWA: silicotungstic acid, SiMoA: silicomolybdic acid, TEOS: tetraethoxy orthosilicate, β -CD: β -cyclodextrin, X-100: surfactant Triton X-100, σ : electrical conductivity, S BET : BET surface area, V total : total pore volume, V micro : micropore volume, V meso : mesopore volume, w micro : micro-pore size, d 002 : interlayer spacing, I D /I G : intensity ratio of D-band to G-band in Raman spectrum, C dis : reversible discharge capacity, C irr : irreversible capacity, NADH: nicoti-namide adenine dinucleotide, M w : weight-averaged molecular weight, Ms: saturation magnetization

in the carbon nanofi bers with S BET from 237 to 571 m 2 g − 1 and total pore volume V total from 0.10 to 0.19 cm 3 g − 1 . PVP/PAN solutions were separately fed into the spinneret to form side-by-side bicomponent nanofi bers and compared with PAN/PVP blend nanofi bers after carbonization up to 970 ° C and their activation at 850 ° C in CO 2 . [ 131 ] Side-by-side bicomponent nanofi bers changed the cross-section morphology from round to cocoon-like shape by PVP extraction.

The DMF-insoluble fraction of a petroleum-derived isotropic pitch in THF solution (40 wt% pitch) was successfully elec-trospun to form the web of carbon fi bers with 2-6 µ m diam-eter. [ 69 , 72 ] The diffi culty to prepare thinner fi ber was pointed out to be resulted from the low boiling point (65–67 ° C) of the solvent THF, the viscosity of the jet increasing due to the volatilization of THF during electrospinning. After activation,

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag G

the webs were microporous, showing very high S BET as 2200 m 2 g − 1 . By mixing PAN with a pitch, spinnability using a binary solvent DMF + THF (1/1) was improved, resulting in the fi bers with the diameter of 750 nm. [ 101 ] After activation by using steam at 900 ° C, S BET of 1877 m 2 g − 1 and V total of 1.11 cm 3 g − 1 , consisting of both micro- and meso-pores, were obtained. THF-soluble component of the pitch with a low molecular weight of 556 gave better spinnability on its 40 wt% THF solution, but the carbon nanofi bers prepared from a high molecular weight pitch as 2380 showed higher development of micropores, giving S BET of 2053 m 2 g − 1 , after carbonization at 1000 ° C and activated at 900 ° C in steam/N 2 fl ow. [ 62 ] Highly porous carbon nanofi bers were obtained by electrospinning of THF solution of polycarbosilane, followed by pyrolysis at different temperatures and chlorination to extract Si. [ 125 ] The nanofi bers pyrolyzed at

mbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 9: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

Figure 3 . TEM images of PAN-based carbon nanofi bers after heat treatment at 1000 ° C (a) and 2200 ° C (b). [ 107 ] Reprinted with permission from Elsevier.

900 ° C and chlorinated at 850 ° C had very high S BET as 3116 m 2 g − 1 and V total of 1.66 cm 3 g − 1 , which were reported to have a high storage capacity for hydrogen as 3.86 wt% at 17 bar and 77 K.

Polyimide (PI) was also spun to prepare carbon nanofi bers. [ 68 , 75 , 79 , 86 , 88 ] Carbon nanofi bers prepared from a PI of PMDA/ODA with the diameter less than 2-3 µ m could give relatively high tensile strength as 74 MPa and electrical con-ductivity of 5.3 S cm − 1 after the heat treatment at 2200 ° C. [ 68 ] A thermotropic PI (Matimid 5218) dissolved into dimethy-lacetamide (DMAc) together with 0.3–3.0 wt% iron(III) acety-lacetnate (AAI) was spun to nanofi bers in the atmosphere with 24% humidity and then carbonized at 400–1200 ° C. [ 79 ] It was declared that AAI worked as a catalyst after decomposing to a-Fe and Fe 3 O 4 during carbonization, although the struc-ture parameters changed a little, d 002 decreasing from 0.37 to 0.34 nm, Lc(002) increasing from 1.0 to 4.2 nm. The addition of PAN in PI solution improved spinnability and decreased the diameter of resultant carbon fi bers. [ 88 ]

Poly(vinylidene fl uoride) (PVDF) nanofi bers were spun from the solution of DMF with poly(ethylene oxide) (PEO) and water, of which the webs were dehydrofl uorized by using 1,8-diazabi-cyelo[5,4,0]undec-7-ene at 90 ° C, followed by carbonization at 1000 ° C for 1 h in N 2 to convert to carbon nanofi ber webs. [ 57 ] They contained three kinds of pores; the largest pores were the interstices among nanofi bers, intermediate-sized pores with 100-300 nm size were formed on the fi ber surface due to

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Mater. 2012, 24, 2547–2566

liquid-liquid phase separation and the micro-pores were due to the decomposition of PEO during carbonization. Dehydrofl uorination process was found to be the key to retain the pore morphology in as-spun fi bers during carbonization.

For electrospinning of phenolic resin (novolac type), its concentration had to be selected, the solution with more than 65 wt% was diffi cult to be spun and that with less than 50 wt% gave fi bers having beads. [ 89 ] Spinnability of phenolic resin solution was improved by the addition of a small amount of poly(vinyl butyral) (PVB) with a molecular weight (M w ) of 110,000, 1 to 3 wt%. The resultant carbon nanofi ber fabrics prepared at 900 ° C were fl exible and had S BET of ca. 500 m 2 /g. Addition of a high molecular weight PVB (M w of 340,000) markedly improved the spinnability of phenolic resin solution due to decreasing solution viscosity and the addi-tion of an electrolyte (pyridine or Na 2 CO 3 ) allowed to give thinner fi bers because of increasing electrical conductivity of the pre-cursor solution; 0.1 wt% Na 2 CO 3 resulting in the carbon nanofi bers with an average diam-eter of 110 nm and S BET of 790 m 2 g − 1 . [ 121 ] Microporous carbon nanofi bers having V micro of 0.9 cm 3 g − 1 were prepared from novolac-type phenol-formaldehyde by adding PVB and Na 2 CO 3 , followed by carbonization at 800 ° C without activation. [ 141 ] Electrical conductivity

of these carbon nanofi ber fabrics was 5.29 S cm − 1 . Carbon nanofi bers with a narrow pore size distribution of 0.4–0.7 nm were prepared from novolac-type phenolic resin via electros-pinning of its methanol solution, curing in formaldehyde/HCl and carbonization at 800 ° C. [ 132 ] Even though no activation was applied, the nanofi bers had S BET of 812 m 2 g − 1 , V total of 0.91 cm 3 g − 1 and relatively low I D /I G of 0.88.

Electrical conductivity was measured to be 4.9 S cm − 1 on 800 ° C-carbonized PAN-based carbon nanofi bers [ 63 ] and the 1000 ° C-treated nanofi bers showed a large negative magne-toresistance, -0.75 at a temperature of 1.9 K under a magnetic fi eld of 9 T. [ 65 ] Although the papers have said that fi brous mor-phology was survived under the heat treatment, it has to take into account that these nanofi bers are not stabilized before car-bonization when these properties were compared with those of commercial PAN-based carbon fi bers. The PAN-based carbon nanofi ber bundles prepared by carbonization at 2200 ° C showed the conductivity of 840 S cm − 1 in parallel with fi ber axis, but 61 S cm − 1 in perpendicular to the fi ber axis. [ 107 ] Electrospun PAN-based carbon nanofi bers showed electrical conductivity changes sensitive to NO gas after activation through immersion into 80 wt% H 3 PO 4 aqueous solution for 12 h and then heat-treatment at 750 ° C in Ar. [ 134 ]

Mechanical property measurements on single fi bers were carried out on PAN-based carbon nanofi bers. [ 77 ] Averaged bending modulus was measured to be 63 GPa by mechanical resonance method and Weibull fracture stress was 640 MPa

2555wileyonlinelibrary.comeim

Page 10: Carbon Nanofibers Prepared via Electrospinning

2556

www.advmat.dewww.MaterialsViews.com

REV

IEW

Figure 4 . SEM images as-spun (a), 1000 ° C-carbonized (b) and 2800 ° C-treated (c) fi bers and TEM image of 2800 ° C-treated fi ber (d) prepared from the mixture of PAN/PMMA = 5/5. [ 84 ] Reprinted with permission from Elsevier.

with a failure probability of 63%. Tensile strength and Young’s modulus measured on the bundles of electrospun PAN-based carbon nanofi bers were 542 MPa and 58 GPa, respectively. [ 107 ] These mechanical properties reported on electrospun carbon nanofi bers are much inferior to commercially available PAN-based carbon fi bers. Since oxidative stabilization of PAN fi bers has been known to be the most important unit-process for PAN-based carbon fi bers, optimization of stabilization con-dition for electrospun PAN nanofi bers has to be studied in detail. Relatively high tensile strength and Young’s modulus were reported on electrospun single nanofi bers prepared from 9 wt% PAN/DMF solution, followed by stabilization at 300 ° C for 1 h in air and carbonized at 800, 1100, 1400 and 1700 ° C in N 2 . [ 130 ] Tensile strength depended strongly on heat treatment temperature, showing the maximum of 2.30 GPa at 1400 ° C,

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag G

and Young’s modulus increased with increasing temperature, giving 181 GPa at 1700 ° C, as shown in Figure 5 . These changes in mechanical properties with heat treatment were explained by the growth of crystallite in nanofi bers. However, it has to be pointed out that orientation of crystallites in the nanofi bers is randomly oriented, not axial. Stretching on as-spun PAN nanofi ber bundles was applied before and during their stabili-zation, expecting well-developed nanotexture and high mechan-ical properties, [ 119 , 143 ] although no detailed experimental data were presented regrettably.

Loading of nanoparticles of various metals and metal oxides were performed via electrospinning process. Various transition metal nanoparticles were loaded to carbon nanofi bers in order to improve the performance of electrochemical capacitors and lith-ium-ion rechargeable batteries, and also loading of platinum to

mbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 11: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

Figure 5 . Dependences of tensile strength and Young’s modulus of carbon nanofi bers on car-bonization temperature. [ 130 ] Reprinted with permission from Elsevier.

carbon nanofi ber webs was carried out for fuel cell applications, as described separately in the following sections. By electros-pinning of lignin/ethanol solution containing 0.2 and 0.4 wt% Pt acetylacetonate, followed by stabilization at 200 ° C in air and carbonization at 600–1000 ° C, microporous Pt-loaded carbon nanofi bers were obtained. [ 127 ] Pd-loaded carbon nanofi bers were prepared from 8 wt% PAN/DMF solution containing 4.8 wt% Pd acetate Pd(OAc) 2 by electrospinning, accompanying by the stabilization in steps from 230 to 300 ° C and carbonization at 1100 ° C. [ 92 ] The resultant nanofi bers showed high electrocata-lytic activity toward the reduction of H 2 O 2 . Magnetic CoFe 2 O 4 nanoparticles were embedded in PAN-based carbon nanofi bers via electrospinning of PAN/DMF solution with dispersed oleic acid-modifi ed CoFe 2 O 4 nanoparticles with 5 nm size, followed by stabilization and carbonization. [ 126 ] CoFe 2 O 4 -embedded nanofi bers were superparamagnetic because of nanosized mag-netic particles and saturation magnetization increased from 45 to 63 emu g − 1 by carbonization. SiO 2 -embedded carbon nanofi bers were prepared by electrospinning of PAN/DMF solutions containing different amounts of SiO 2 , followed by stabilization and carbonization. [ 87 ] SiO 2 particles embedded in carbon nanofi bers were washed out by HF, but S BET and V total increased only to 340 m 2 g − 1 and 0.472 cm 3 g − 1 . Vanadium-embedded carbon nanofi bers were prepared by electrospin-ning of PAN/DMF solutions containing different amounts of V 2 O 5 . [ 96 ] After activation by using KOH at 750 ° C, nanoporous nanofi ber were obtained, S BET reaching to 2780 m 2 g − 1 , V total to 2.67 cm 3 g − 1 and V micro to 1.52 cm 3 g − 1 , which gave a hydrogen storage capacity of 2.41 wt% at 303 K and 10 MPa. Mn-loaded carbon nanofi bers, which were activated by steam at 850 ° C and had V micro of 0.42 cm 3 g − 1 , gave relatively high adsorption capacity for toluene at 289 K. [ 98 ]

3.2. Electrode Materials for Electrochemical Capacitors

Porous carbon materials are essential for the electrodes of elec-trochemical capacitors, including electric double-layer capaci-tors. In commercially available capacitors, activated carbons are commonly used. For the improvement of capacitor perform-ance, various carbon materials have been proposed, activated carbon fi bers, templated carbons, carbon nanotubes etc. [ 144 ] Application of electrospun carbon nanofi bers to electrochemical

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, WeinAdv. Mater. 2012, 24, 2547–2566

capacitors was studied either by control-ling pore structure in the nanofi bers as the electrode for electric double-layer capacitors (EDLCs) or by loading fi ne metal particles to the nanofi bers via electrospinning in order to add some pseudocapacitance. Advantage of electrospinning is that it can produce webs, which make their activation and formation of electrode for capacitors easy.

Webs of PAN-based carbon nanofi bers, which were prepared by electrospinning, sta-bilization, carbonization and activation at 700, 750 and 800 ° C in a fl ow of N 2 containing 30 vol% steam, were used as electrodes of EDLC using 30 wt% KOH aqueous solution. [ 5 ] The 700 ° C-activated webs gave a high capaci-

tance as 173 F g − 1 at low discharge current density as 10 mA g − 1 , but at high current density as 1000 mA g − 1 the 800 ° C-acti-vated webs gave a high capacitance as 120 F g − 1 : the former had S BET of 1230 m 2 g − 1 consisting of micropores but the latter had S BET of 850 m 2 g − 1 consisting of mesopores. Similar results were obtained in other papers. [ 74 ] The importance of the pres-ence of mesopores was reported in many papers using different carbon materials. [ 145 ] PAN-based carbon nanofi bers prepared by mixing PAN with 15 wt% cellulose acetate (CA) gave S micro of 919 m 2 g − 1 and S meso of 241 m 2 g − 1 , and consequently showed the capacitance of 245 F g − 1 in 6M KOH aqueous solution with a current density of 1 mA cm − 2 . [ 102 ] Electrospun PAN-based carbon nanofi bers prepared from PAN/DMF solution con-taining PVP by carbonization at 970 ° C in N 2 and activation at 800 ° C in CO 2 were tested in 1 M H 2 SO 4 aqueous solution. [ 131 ] Side-by-side bicomponent carbon nanofi bers prepared by using 8 wt% PVP gave the capacitance of 221 F g − 1 , higher than that measured on carbon nanofi bers prepared from PAN solution blended with PVP.

PAN/DMF solution dispersed multiwalled carbon nanotubes (MWCNTs) was successfully spun and carbonized to get webs consisting of carbon nanofi bers which gave improved EDLC per-formance in aqueous electrolytes. [ 94 , 105 ] The addition of 3 wt% MWCNTs in precursor PAN increased S BET to 1170 m 2 g − 1 and electrical conductivity to 0.98 S cm − 1 , consequently increased EDLC capacitance to 180 F g − 1 in 6M KOH. [ 94 ] Coating of polypyrole (PPy) on these MWCNT-embedded nanofi bers led to further increase in capacitance to 333 F g − 1 . For MWCNT-embedded carbon nanofi bers, electrical conductivity and capaci-tance in 1 M H 2 SO 4 increased to 5.32 S cm − 1 and 310 F g − 1 , respectively, from 0.86 S cm − 1 and 170 F g − 1 for the nanofi bers without MWCNTs. [ 105 ] Polybenzimidazol (PBI) was also selected as carbon precursor for electrospinning. [ 70 , 71 , 78 ] Carbon nanofi ber webs were prepared by electrospinning of dimethyl acetamide (DMAc) solution of PBI (20 wt%), carbonization and activation at 700–850 ° C in a fl ow of N 2 containing 30 vol% steam. Dependences of EDLC capacitance in 1 M H 2 SO 4 aqueous electrolyte on discharge current density are shown on the webs activated at different temperatures in Figure 6 . The capacitance of about 202 F g − 1 was measured on the webs acti-vated at 800 ° C with discharge current density of 1 mA cm − 2 , which had S BET of 1220 m 2 g − 1 , V micro of 0.71 cm 3 g − 1 and V meso of 0.20 cm 3 g − 1 .

2557wileyonlinelibrary.comheim

Page 12: Carbon Nanofibers Prepared via Electrospinning

2558

www.advmat.dewww.MaterialsViews.com

REV

IEW

Figure 6 . Dependences of electric double-layer capacitance on discharge current density for polybenzimidazol-derived carbon nanofi ber webs heat-treated at different temperatures. [ 78 ] (a) 700 o C, (b) 750 o C, (c) 800 o C and (d) 850 ° C. Reprinted with permission from Elsevier.

Figure 7 . Electrochemical adsorption capacity of Na ion in NaCl solution. [ 128 ]

Polyimide (PI)-derived carbon nanofi ber webs were also tested as electrode materials of EDLC using 30 wt% KOH aqueous solution and showed a relatively high capacitance of 175 F g − 1 with a high current density of 1000 mA cm − 2 . [ 75 ] The precursor for spinning was pyromellitic dianhydride (PMDA) and 4,4’-oxydianiline (ODA) dissolved in a mixed solution of THF and methanol. The PI webs obtained after spinning were thermally imidized by heating up to 350 ° C and carbonized at 1000 ° C. All webs activated in air fl ow containing 40 vol% steam at a temperature of 650–800 ° C gave a peculiar behavior: capacitance increased with increasing current density up to 10 mA cm − 2 and then decreased.

Addition of inorganic salts into PAN infl uenced on the pore structure of the resultant carbon nanofi bers. [ 85 , 108–110 ] Electros-pinning from PAN/DMF solutions containing 1-15 wt% ZnCl 2 resulted in microporous carbon nanofi bers without any activa-tion process after removing Zn compounds by HCl. [ 85 , 110 ] Addi-tion of 5 wt% Ni(NO 3 ) 2 4H 2 O into PNA/DMF solution resulted in the increase in S BET , mainly due to the increase in mes-opores, without any activation process after carbonization. [ 109 ] Pore structure control in microporous carbon nanofi bers was tried by mixing of tetraethoxy orthosilicate and AgNO 3 into PAN [ 136 , 137 ] and by changing concentration of carbon precursor pitch/PAN. [ 138 ]

The principle of electrochemical capacitors, i.e. , physical adsorption/desorption of electrolyte ions in solution, was applied for water purifi cation (capacitive de-ionization) by using different carbon materials. [ 146 ] Electrochemical adsorption capacity of Na + of electrospin PAN-based carbon fi ber webs, which was measured in NaCl aqueous solution, was compared for various carbon materials in Figure 7 . They showed the

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag Gm

capacity of 3.2 mg g − 1 , which was comparable with those for carbon nanotube fi lm and carbon aerogel.

In order to add pseudo-capacitance to electric double-layer capacitance, loading of Ru on electrospun carbon nanofi bers was carried out. [ 83 ] By the addition of Ru acetylacetonate into the 10 wt% PAN/DMF solution, metallic Ru particles with 2–15 nm size were embedded in the nanofi bers. Nanofi bers loaded by 7.31 wt% Ru showed the capacitance of 391 F g − 1 in 6M KOH aqueous solution, although the nanofi bers without Ru loading gave 140 F g − 1 . Carbon nanofi bers containing nanoparticles of metallic Ni were prepared through electrospinning of PAN/DMF solution added 35, 100 or 135 wt% nickel acetate. [ 117 ] The fi bers obtained had a diameter of about 150 nm and fi ne Ni particles were deposited on their surfaces. The nanofi ber webs with 22.4 wt% Ni loading showed the capacitance of 164 F g − 1 in 6M KOH with a current density of 250 mA g − 1 , which was much larger than about 50 F g − 1 for the nanofi ber webs without Ni loading. Imbedding of metallic V into PAN-based carbon nanofi bers were performed through electrospinning of PAN/DMF solution containing V 2 O 5 . [ 95 ] After activation using KOH at 800 ° C, S BET reached 2800 m 2 g − 1 , due to micropores with 0.7–1.2 nm and mesopores with 2–4 nm.

3.3. Anode Materials for Lithium-Ion Rechargeable Batteries

Carbon nanofi ber webs prepared through electrospinning and following carbonization have an advantage for anode of lith-ium-ion rechargeable batteries (LIBs), because of no necessity to use an electric conductive additive, such as acetylene black, and an organic binder, such as PTFE. However, they can not be expected to give low irreversible capacity, as natural graphite which is currently used in commercially available batteries, because most of carbon nanofi bers have low graphitization degree. Few papers reported the results of their application to anode of LIBs, but a high irreversible capacity could not be avoided. On 1000 ° C-treated PAN-based carbon nanofi ber webs, reversible discharge capacity C dis of 450 mAh g − 1 was obtained with a current density of 30 mA g − 1 , a little higher than natural graphite, but irreversible capacity C irr was as high

bH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 13: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

Figure 8 . Charge-discharge curves and cyclic performance of the carbon nanofi ber webs prepared from 15 wt% Si-dispersed PAN/DMF solu-tion. [ 111 ] Reprinted with permission from Royal Society of Chemistry.

as 500 mAh g − 1 . [ 80 ] High temperature treatment of the webs could not improve LIB performance. Pores were introduced into carbon nanofi bers by adding poly- L -lactic acid (PLLA) in PAN/DMF solution. [ 100 , 120 ] Addition of PLLA created micro-pores without activation process, V micro being 0.086 cm 3 g − 1 , and increased C dis to 435 mAh g − 1 after 50th cycle with a cur-rent density of 50 mA g − 1 . [ 120 ] Addition of fumed SiO 2 with minute particles in PAN/DMF solution was carried out in order to create pores after leaching out of SiO 2 , but it showed a low S BET as 92 m 2 g − 1 and very high C irr , more than 1000 mAh g − 1 , at the fi rst cycle. [ 116 ]

Many works applying electrospinning are aiming to load some electrochemically active metallic particles to carbon nanofi bers in order to improve LIB performance. Si-loaded carbon nanofi bers were prepared from PAN/DMF solution with Si nanoparticles (ca. 70 nm) [ 100 , 111 , 113 , 129 ] and also from PVA/H 2 O solution dispersed Si nanoparticles (ca. 40 nm) with a surfactant. [ 112 ] Galvanostatic charge-discharge curves and cyclic performance with a current density of 100 mA g − 1 in 1 M LiPF 6 /EC + EMC solution are shown on the webs prepared from 15 wt% Si mixed PAN/DMF solution by carbonization at 700 ° C in Figure 8 . C dis for the 1st cycle is very high as 855 mAh g − 1 with a relatively large C irr of 312 mAh g − 1 . C dis decreases slowly with cycling, 781 mAh g − 1 after 10th cycle and 773 mAh g − 1 after the 20th cycle, which is much higher than the theoretical capacity for graphite and also than carbon nanofi ber webs pre-pared without Si addition.

Sn-loading via electrospinning was performed by adding a Sn compound into the precursor solution. [ 104 , 106 , 122 , 123 ] Sn-contained carbon nanofi ber webs were prepared from PVA/H 2 O solution dissolved SnCl 2 by electrospinning and following carbonization in Ar/H 2 (95/5 v/v). [ 122 , 123 ] The diameter of the resultant nanofi bers was about 4 µ m and contained Sn/SnO x particles of about 20-40 nm size, most of which were located in pores of carbon nanofi bers, as shown in Figure 9 a). The webs show relatively high C dis as 735 mAh g − 1 but relatively large C irr for the 1st cycle (457 mAh g − 1 ), although C irr becomes smaller after the 2nd cycle (Figure 9 b). Sn-encapsulated carbon nanofi bers were prepared through electrospinning of PAN nanofi bers containing tributyltin (TBT) dissolved into a mineral oil, followed by extracting the mineral oil, most of which located at the core of the fi ber, in n -octane and by heating at 1000 ° C in Ar/H 2 atmosphere to carbonize the outer PAN sheath and to decompose the TBT core to metallic Sn. [ 104 ] The resultant nanofi bers showed high C dis as 737 mAh g − 1 (91% of the theo-retical capacity), even after 200 cycles. Sn-encapsulated carbon nanofi bers were also prepared from PAN/DMF containing PMMA and tin octoate. [ 106 ] The nanofi bers had the diameter of about 2 µ m and consisted of hollow channels with about 100 nm diameter containing metallic Sn nanoparticles. Sn content in these nanofi bers was 66 wt% and its particles were covered by carbon layer with a thickness of about 5 nm. The nanofi bers showed a high C dis as 648 mAh g − 1 with a current density of 100 mA g − 1 even after 140 cycles.

Loading of Co, Fe, Mn, Ni and Cu was carried out mostly by adding their acetates into PAN/DMF solu-tion. [ 90 , 91 , 99 , 114 , 115 , 117 , 118 , 124 ] Carbon nanofi bers containing either metallic Co, Ni or Cu nanoparticles delivered relatively high C dis in 1 M LiPF 6 /EC + DEC electrolyte. [ 90 , 117 , 124 ] When Mn acetate

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, 24, 2547–2566

(Mn(OAc) 2 ) was used, carbon nanofi bers containing crystalline MnO and Mn 3 O 4 nanoparticles were obtained after carboniza-tion at 700 ° C, which had steady cyclic performance. [ 99 , 118 ] The resultant carbon nanofi bers include MnO x nanoparticles in their matrix with pores, as shown in Figure 10 a), and give high capacity with good cyclic performance, the nanofi bers prepared by adding 50 wt% Mn(OAc) 2 giving a steady capacity value of about 600 mAh g − 1 , much higher than the nanofi bers prepared without Mn(OAc) 2 , as shown in Figure 10 b). When ferric acety-lacetonate was added into PAN/DMF solution, Fe 3 O 4 -loaded carbon nanofi bers were obtained. [ 91 ] After the carbonization at 600 ° C, the resultant carbon fi bers included crystalline Fe 3 O 4 particles with about 20 nm size, of which content was calcu-lated as 31 wt%. C dis of the nanofi bers decreases rapidly during fi rst few cycles and then tends to increase with increasing cycle, giving higher discharge capacity than that without Fe 3 O 4 .

Nanofi bers of LiFePO 4 /C composite were obtained as nanofi bers via electrospinning of PAN/DMF solution

2559wileyonlinelibrary.combH & Co. KGaA, Weinheim

Page 14: Carbon Nanofibers Prepared via Electrospinning

2560

www.advmat.dewww.MaterialsViews.com

REV

IEW

Figure 9 . TEM image of Sn/SnO x -containing carbon nanofi bers (a) and cyclic performance in 1 M LiPF 6 /EC + DMC electrolyte solution with a cur-rent density of 30 mA g − 1 . [ 123 ] Reprinted with permission from Elsevier. Figure 10 . TEM image of MnO x -loaded carbon nanofi ber (a) and cyclic

performance in 1 M LiPF 6 EC/DEC electrolyte (b) for carbon nanofi bers prepared by adding 0. 10, 30 and 50 wt% Mn(OAc) 2 into PAN/DMF solu-tion. [ 118 ] Reprinted with permission from Elsevier.

containing phosphate precursor (LiCO 2 CH 3 , Fe(CO 2 CH 3 ) 2 and

H 3 PO 4 in equal mole ratio) with a wide range of diameters of 10–200 nm, together with beads. [ 133 ] The nanofi bers carbonized at 700 ° C gave the highest C dis of 160 mAh g − 1 in 1 M LiPF 6 /EC + EMC solution as cathode. Nanofi bers of Li 2 ZnTi 3 O 8 /C composites were synthesized via electrospinning of an ethanol solution of tetrabutyl titanate, zinc acetate and lithium acetate with PVP, followed by calcination at 750 ° C in air. [ 147 ]

3.4. Composite With Carbon Nanotubes

Various carbon nanotubes (CNTs) were included in carbon nanofi bers via electrospinning for the reinforcement of their mechanical properties. [ 66 , 76 , 107 ] Carbon nanofi bers were pre-pared by electrospinning of PAN/DMF solution, in which 1–4 wt% single-wall carbon nanotubes (SWCNTs) were dis-persed, stabilizing in air and carbonizing at 1100 ° C. [ 66 ]

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag G

SWCNTs were distributed in parallel to the fi ber axis, main-taining their straight shape, in PAN nanofi bers with the diam-eter of 50–200 nm. Elastic modulus of SWCNT/PAN nanofi bers, which was evaluated using AFM, increased with increasing SWCNT content, from 60 GPa for PAN fi ber to 140 GPa for that containing 4 wt% SWCNT. Multi-walled carbon nano-tubes (MWCNTs) were also successfully included into electro-spun carbon nanofi bers prepared from PAN/DMF solution, in which the content of MWCNTs were 2-35 wt%. [ 76 ] In as-spun nanofi bers, most MWCNTs are aligned well along the fi ber axis, but slightly-curved MWCNTs resulted in curved nanofi bers and winding or helical MWCNTs could not be embedded within the nanofi bers. The PAN nanofi bers containing 5 wt% MWCNTs showed tensile strength of 80 MPa and tensile mod-ulus of 3.1 GPa. The fi brous morphology was maintained after

mbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 15: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

carbonization at 850 ° C, even though PAN shrunk largely but MWCNTs did not.

Carbon nanotubes were grown on the surface of electrospun carbon nanofi bers. [ 73 ] Fe-loaded carbon nanofi bers were pre-pared from PAN/DMF solutions containing 3.3 and 6.7 wt% Fe acetyleacetonate Fe(acac) 3 by electrospinning, stabilization, reduction of Fe 3 + to metallic Fe at 500–550 ° C in H 2 and car-bonization at 1100 ° C in Ar. Metallic Fe particles with 10–20 nm size dispersed in the nanofi bers. On the surface of the carbon nanofi bers thus prepared, MWCNTs were grown by catalytic CVD of hexane, of which the length depended on the period of catalytic CVD.

4. Discussions

4.1. Carbon Precursors

For the production of carbon materials, the selection of pre-cursor is the most crucial, because it determines the nanote-xture of the resultant carbon under ordinary carbonization conditions, such as the heat treatment under atmospheric pres-sure. The nanotexture formed during carbonization process, which is classifi ed into random, planar, axial and point orienta-tions on the basis of the scheme of preferred orientation of the basic structural units of carbon layers, is known to govern the structural development by further treatment at high tempera-tures and, as a consequence, to infl uence on various properties, such as electrical, mechanical and chemical properties. [ 1 , 148 ] Also, carbon materials prepared from various precursors have been classifi ed into two groups, based on whether the struc-tural change to graphite (three-dimensionally ordered struc-ture) is easy or not at high temperatures above 2500 ° C, calling graphitizing and non-graphitizing carbons, [ 1 ] even though some of them are known to show different behaviors.

The diameter of electrospun carbon nanofi bers is 100 to 1000 nm, which might give some infl uence on the formation of nanotexture during their carbonization process and also on structural modifi cation at high temperatures. Size of carbon layers evaluated as La by X-ray diffraction was experimentally shown to depend strongly on the particle size of spherical carbon materials. [ 148 ] PAN-derived carbon fi bers prepared via melt-spinning, stabilization and carbonization have a random nanotexture and are non-graphitizing (called general-purpose grade). In order to improve nanotexture to axial orientation, stretching of the as-spun fi bers is known to be necessary during further heat treatments, and the resultant PAN-based carbon fi bers give higher strength and Young’s modulus (high-perform-ance grade). [ 2 ] The carbon prepared in the gallery of layer-struc-tured montmorillonite (template carbonization) is easily con-verted to graphite at a high temperature as 2800 ° C, because the pyrolysis of PAN is performed in monolayer of carbon atoms between template layers. [ 149 ] Therefore, the formation of nano-texture in nano-sized fi brous carbons (carbon nanofi bers) and their change in structure and properties with high temperature treatment might be a little different from our knowledge based on large-sized particles of carbons with different nanotextures. PAN-based continuous nanofi bers were prepared under tension

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, 24, 2547–2566

during stabilization and carbonization by expecting superior mechanical properties. [ 119 ] However, no structural studies had been reported yet and the heat treatment had been done only up to 1500 ° C.

Stabilization of PAN fi bers with micrometer-sized diameters is known to be the most important process on the production of carbon fi bers. [ 2 ] In most literatures working on PAN-based carbon nanofi bers, however, as-electrospun PAN fi bers were stabilized at 280 ° C in air and carbonized at a temperature of 600–1100 ° C in inert atmosphere. It might be needed to explore the optimum condition for stabilization and carbonization from structural view points, such as the size of basis structural units of carbon layers and their orientation in nanofi bers, car-bonization yield, content of foreign atoms, hydrogen, nitrogen and oxygen, which remain even after carbonization, and also changes in diameter and its distribution.

Aromatic polyimides (PIs) are interesting carbon precursors because they give a wide range of structure, from well-crystallized graphite fi lm to amorphous carbon with random nanotexture, by selecting a combination of the starting dianhydride and the diamine. [ 150 ] However, one kind of polyimide (a combination of pyromellitic anhydride and 4,4 ′ -oxydianiline, PMDA/ODA) was mostly used for electrospinning. [ 75 , 86 ] Even when starting from PMDA/ODA, the thickness of its fi lm is known to govern the graphitizability: thin fi lms with less than 25 µ m thickness can be converted to highly crystalline graphite by high temperature treatment, but thicker fi lms can not, which is explained by the orientation of precursor molecules in the fi lm. [ 150 ] Studies on different PIs by coupling with heat treatment at high temper-atures and detailed structural analyses may give a more wide range of nanotextures in the resultant carbon nanofi bers and, consequently, may open more wide applications, because of the possibility to have a controlled structure from graphite to amor-phous. For applications requiring high electrical conductivity, some of PIs are worthwhile to be tested as a precursor.

Pitch is less costly than other carbon precursors, such as PAN and PI, but not many reports discuss its use for electrospin-ning. Since pitch is a mixture of various hydrocarbons, certain components might be needed to be selected for electrospinning in relation to the solvent selected, as the DMF-insoluble part of the pitch with THF. [ 72 ] Mesophase pitch, which is used as a pre-cursor for commercial mesophase-pitch-based carbon fi bers, is worthwhile to be used for electrospinning, even though a stabi-lization process is needed to get pitch-based carbon nanofi bers, which is expected to lower its graphitizability.

Although poly(vinyl alcohol) PVA has a very low carboni-zation yield (few wt%), it might be an interesting precursor because water can be used as a solvent. [ 112 , 122 , 123 ]

Phenolic resins are often used as carbon precursor, which gives non-graphitizing carbon in a conventional carbonization process with the advantage of a relatively high carbonization yield. However, it is rarely used in studies on carbon nanofi bers via electrospinning. [ 89 , 121 , 132 ]

4.2. Pore Structure Control

In most cases, electrospun carbon nanofi bers were prepared in the form of webs (mats), which is an advantage for various

2561wileyonlinelibrary.combH & Co. KGaA, Weinheim

Page 16: Carbon Nanofibers Prepared via Electrospinning

2562

www.advmat.dewww.MaterialsViews.com

REV

IEW

applications, such as electrodes for EDLCs and LIBs as men-

tioned above. In these webs consisting of carbon nanofi bers, different kinds of pores are formed; large pores (maybe better to call them spaces), macropores ( > 50 nm size), mesopores (2–50 nm size) and micropores ( < 2 nm size).

Large spaces are formed due to entangling of different nanofi bers and disperse widely in their size and morphology. These pores are very diffi cult to control and characterize mainly because they are fl exible (fl exible interparticle pores). [ 1 ] The bulk density or porosity of the webs is the parameter that is easy to determine and effective for characterizing these large spaces. However, information on the bulk density or porosity has not been presented in most literature reporting on electro-spun carbon nanofi ber webs. It has to be emphasized that their presence in the webs is very important for applications: they are pathways for electrolytes in energy storage devices to diffuse into smaller pores to be adsorbed, but a too large amount of pores makes the volumetric storage capacity small. When elec-trospinning conditions are not appropriate, bead-like particles are formed together with fi brous particles and rigidly bound interconnections between nanofi bers are formed after carboni-zation, which may result in a rigid macropore system.

Macro-, meso- and micropores are formed in each nanofi ber and are rigid (rigid intraparticle pores). Most micropores and some mesopores are intrinsically formed in fi brous carbon par-ticles during the carbonization process due to the evolution of different gaseous species, such as hydrocarbons of various sizes, CO, CO 2 , etc., and thus strongly depends on the carbon pre-cursor used and carbonization conditions. To increase and con-trol the amount of micropores, a conventional activation process has been applied in many works by using either steam, KOH, or H 3 PO 4 as an activation reagent on electrospun nanofi bers after their carbonization (Table 1 ). The addition of ZnCl 2 , which was one of reagents for conventional activation, into a PAN/DMF solution before electrospinning was also adopted, but no pronounced improvement of S BET was obtained. [ 85 ] For appli-cation to the electrode of electrochemical capacitors, the acti-vation process was applied on electrospun carbon nanofi bers derived from PAN, [ 67 , 74 , 128 ] PBI, [ 70 , 71 , 78 ] and PI. [ 75 ] By the activa-tion of PAN-based carbon nanofi bers in a fl ow of N 2 containing 30 wt% steam at 800 ° C, S BET increased to 1160 m 2 g − 1 , V total to 0.64 cm 3 g − 1 and capacitance to 134 F g − 1 in 6M KOH aqueous electrolyte with a current density of 1 mA cm − 2 . [ 75 ] By the activa-tion of PBI-derived carbon nanofi bers at 800 ° C, capacitance of 202 F g − 1 in 1 M H 2 SO 4 aqueous electrolyte was obtained with good rate performance. [ 78 ] Also the addition of cellulose acetate into PAN was tried to control pore structure without activation process, giving certain increases in S BET from 740 to 1160 m 2 g − 1 and in capacitance from 141 to 245 F g − 1 in 6M KOH with a current density of 1 mA cm − 2 . [ 102 ] By the activation of carbon nanofi bers, micropores are supposed to be formed on their surface, which is expected to be advantageous for adsorption/desorption. For commercially available activated carbon fi bers (ACFs) their advantages for adsorption/desorption of gases due to the presence of micropores on their surface are now well known. [ 1 ] Good rate performance in electrochemical capacitors was reported on electrospun carbon nanofi ber webs, [ 74 , 78 ] prob-ably due to the formation of micropores on the nanofi ber sur-face, but its experimental evidence is not reported yet.

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag G

Mesopores can be increased by applying a conventional acti-vation process on the electrospun carbon nanofi bers, but it is possible only by sacrifi cing micropores. It might be interesting to add a template, either surfactants or MgO precursors into electrospinning precursor solution, which can leave mesopores after carbonization. [ 151 ]

Most of the macropores and some of the mesopores have to be introduced into fi brous particles intentionally, by using either an additional carbon precursor, which gives pores after carboni-zation because of its low carbonization yield, such as PMMA in PAN to introduce tubular macropores, [ 84 ] or an additional sol-vent for electrospinning solution, such as water in DMF. [ 57 ]

Not only micro- and mesopores of the electrodes in EDLCs are important for adsorption of electrolyte ions, but also the diffu-sion of ions to micro- and/or meso-pore surfaces to be adsorbed is known to give a strong infl uence on capacitor performance, particularly charge/discharge process with high rates. The pres-ence of mesopores, in addition to micropores, has been pointed out to be important for getting good rate performance, in other words, high retention in capacitance with high rate charge/dis-charging. Obtaining webs consisting of carbon nanofi bers may be one of the merits of the process via electrospinning, because any binder and electroconductive additives (commonly carbon blacks) are not needed to form electrode sheets and the presence of large spaces between nanofi bers may be advantageous for the diffusion of electrolyte ions to the micro- and meso-pores in nanofi bers. However, it has to be mentioned that the webs usu-ally have a low bulk density, which may lead to low volumetric capacitance.

The optimum conditions for electrode carbon, including its pore structure, is not yet clearly understood and they might be different for charge/discharge conditions and also for electro-lyte ions. It also has to be pointed out that direct comparison among the capacitance values reported is very diffi cult because their determination procedure is not standardized. [ 144 ] In order to show the merits of using carbon nanofi bers prepared via elec-trospinning clearly under the present situation, more detailed studies are strongly demanded on pore structure control, not only the control of meso- and micropores in nanofi bers, but also proper characterization and the control of large spaces in the webs. In addition, wettability of the resultant carbon nanofi ber webs with electrolyte solutions, either aqueous or non-aqueous, has to be studied, which might depend on precursor and car-bonization temperature.

4.3. Improvement in Electrical Conductivity

High electrical conductivity of electrospun carbon nanofi bers is required in various applications, electrode materials for energy storage devices, EDLCs and LIBs, and also catalyst supports.

As the anode of LIBs, carbon nanofi ber webs are required to have both high electrical conductivity and well-developed graphite structure. High temperature treatment is known to be preferred to improve these two factors. However, graphitiz-ability of a carbon depends strongly on carbon precursor used, for examples, PAN-based carbon fi bers can not get high degree of graphitization but mesophase-pitch-based carbon fi bers can

mbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 17: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

be graphitized. [ 1 ] Since the stabilization process for the prepara-tion of carbon nanofi bers from PAN and pitch is essential to keep fi brous morphology after carbonization, the possibility to have highly graphitized nanofi bers is supposed to be low-ered, although detailed study is needed. PAN-derived carbon nanofi bers could not be graphitized even by the heat treatment at 2800 ° C, giving d 002 of 0.341 nm and electrical conductivity of 20 S cm − 1 , of which C dis became low as about 120 mAh g − 1 in 1 M LiClO 4 /EC + DEC electrolyte. [ 80 ] Since the nanofi bers have low graphitization degree, a large C irr could not be avoided; 1000 ° C-treated nanofi bers showed C dis of ca. 450 mAh g − 1 and C irr of ca. 500 mAh g − 1 for the 1st charge/discharge cycle. On the other hand, some of polyimides (PIs) is known to have very high graphitizability after high temperature treatment in very thin fi lms without stabilization and to get thin graphite fi lm. [ 150 ] Carbon nanofi bers prepared from one of PIs, PMDA/ODA, and heat-treated at a high temperature as 3000 ° C are worthwhile to be tested as the anode of LIBs.

High temperature treatment is known to improve conduc-tivity not only due to the development of graphitic structure but also due to exclusion of foreign atoms in carbon materials. However, it is also known to reduce meso- and micropores easily, [ 151 , 152 ] which is not desirable for EDLC applications. In order to increase electrical conductivity of electrospun carbon nanofi bers, embedding of MWCNTs was performed in the relation to EDLC application. [ 94 , 105 ] Embedding of 0.8 wt% MWCNTs led to the increase in electrical conductivity of the webs from 0.86 to 5.32 S cm − 1 , accompanied by the increase in EDLC capacitance from 170 to 310 F g − 1 in 1 M H 2 SO 4 aqueous electrolyte. [ 105 ]

In order to control the electrical conductivity of carbon nanofi bers prepared via electrospinning, therefore, funda-mental studies on changes in structure and nanotexture with heat treatment in a wide range of temperature are needed on various carbon precursors, including PAN, pitch and PI.

4.4. Loading of Metallic Species

One of the requirements for electrospun nanofi bers to be applied to energy storage devices is uniform loading of metallic nanoparticles. There have been proposed different techniques, as some of them were described in the previous section and also reviewed in a reference. [ 16 ] The process for loading via elec-trospinning may be divided into two, mixing either metallic nanoparticles or metal precursors into carbon precursor before electrospinning and loading of metal particles on electrospun fi bers either before or after carbonization. By the former process, metallic nanoparticles are embedded and immobilized into the resultant carbon nanofi bers, but they have to survive as active species under the temperature for carbonization and some of nanoparticles might not be active because they are placed in the center of the fi bers. By the latter process, however, they are preferentially deposited on the surface of the nanofi bers and so the substrate nanofi bers can be treated under different condi-tions, such as high temperature treatment for their graphitiza-tion to improve electrical conductivity, but immobilization and grain growth inhibition of the deposited metallic nanoparticles has to be taken into consideration.

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, 24, 2547–2566

For LIB applications, the loading of nanoparticles of various metallic species, which are electrochemically active for lithium storage, is carried out by mixing their precursors with or without organic additives into a carbon precursor solution before elec-trospinning, as described in the previous section. Most Sn/SnO x nanoparticles formed in carbon nanofi bers electrospun from PVA/H 2 O solution containing SnCl 2 are located in the pores, as shown in Figure 9 a, which is the reason why the nanofi ber webs show a stable cycle performance (Figure 9 b) because the space neighbored to Sn/SnO x particles in the pore may absorb a large volume expansion due to lithium alloying. [ 123 ] Carbon nanofi bers encapsulated Sn nanoparticles (ca. 100 nm size) via electrospinning using coaxial spinneret showed a steady cyclic performance at high C dis of 737 mAh g − 1 (91% of theoretical capacity). [ 104 ] It has to be mentioned that electrospinning of carbon precursor with Sn precursor can be done easily and effectively more than the deposition of Sn after carbonization, because Sn and/or SnO x nanoparticles included into carbon matrix can be active in LIBs, in other words, lithium ions in LIBs can penetrate into carbon nanofi bers to react with metals. Cathode material for LIBs, LiFePO 4 , was loaded into carbon nanofi bers via electrospinning, [ 133 ] but it did not show an advan-tage for the electrospinning process because the appropriate spinning conditions were not selected.

In order to add pseudo-capacitance due to redox reaction to the EDLC of the substrate carbon nanofi ber webs, loading of the metallic species onto carbon nanofi bers via electrospinning was carried out by using Ru acetylacetonate Ru(acac) 3 [ 83 ] and V 2 O 5 . [ 95 ] By addition of 20 wt% Ru(acac) 3 into a PAN/DMF solution, nanometer-sized Ru particles (2–15 nm) were embedded and, as a consequence, the capacitance in 6M KOH electrolyte increased from 140 to 391 F g − 1 by embedding 7.31 wt% Ru, about 250 F g − 1 being added as pseudo-capacitance. [ 83 ] In contrast to LIBs, metallic species have to be located on the surface by expo-sition to an electrolyte solution and the species included into the carbon matrix is diffi cult to work with because most electrolyte ions in capacitors cannot penetrate into the carbon matrix, which may reduce the utilization effi ciency of loaded metal.

5. Conclusions

Carbon nanofi bers prepared via electrospinning and following carbonization are summarized by focusing on the structure and properties in relation to their applications. The precursor, pore structure, electrical conductivity, and metal loading of carbon nanofi bers were discussed from the viewpoint of structure and nanotexture of carbons.

Electrospinning is an interesting and promising technique to prepare carbon nanofi bers, having intermediate diameters between carbon nanotubes and carbon fi bers. There have been many reports on the preparation of carbon nanofi bers via electrospinning, as summarized in Table 1 . However, more detailed studies still remain to be carried out, such as the selec-tion of carbon precursors from the viewpoint of structure and nanotexture control in nanofi bers, optimization of electros-pinning conditions and further treatment, including stabiliza-tion, carbonization, activation and further heat treatment at high temperatures. Important factors to be controlled through

2563wileyonlinelibrary.combH & Co. KGaA, Weinheim

Page 18: Carbon Nanofibers Prepared via Electrospinning

2564

www.advmat.dewww.MaterialsViews.com

REV

IEW

electrospinning are pore structure, degree of graphitization,

electrical conductivity and metallic species loading of carbon nanofi bers, which depend strongly on the selection of precur-sors, including carbon precursor, metal precursor and additives, and preparation conditions of carbon nanofi bers, including electrospinning and carbonization.

Acknowledgements The authors acknowledge the fi nancial supports from National Natural Science Foundation of China (No. 51102143), New Teachers Fund for Doctor Stations (20100002120006, Ministry of Education) (Y.Y.) and also from Guangdong Province Innovation R&D Team Plan.

Received: December 27, 2011 Revised: January 31, 2012

Published online: April 17, 2012

[ 1 ] M. Inagaki , F. Kang , Carbon Materials Science and Engineering -From fundamentals to applications . Tsinghua Univ. Press. , Beijing, China , 2006

[ 2 ] J.-B. Donnet , T. K. Wang , J. C. M. Peng , S. Rebouillat , Carbon Fibers , 3rd Edition . Marcel Dekker , New York, USA , 1998 .

[ 3 ] A. Oberlin , M. Endo , T. Koyama , J. Crystal Growth 1976 , 32 , 335 . [ 4 ] S. Iijima , Nature 1991 , 354 , 56 . [ 5 ] S. Iijima , T. Ichihashi , Nature 1993 , 363 , 603 . [ 6 ] D. S. Bethune , C. H. Kiang , M. S. Devries , G. Gorman , R. Savoy ,

J. Vazquez , R. Beyers , Nature 1993 , 363 , 605 . [ 7 ] M. Endo , H. Muramatsu , T. Hayashi , Y. A. Kim , M. Terrones ,

M. S. Dresselhaus , Nature 2005 , 433 , 476 . [ 8 ] Z. Huang , Compos. Sci. Technol. 2003 , 63 , 2223 . [ 9 ] D. Li , Y. N. Xia , Adv. Mater. 2004 , 16 , 1151 . [ 10 ] I. Chronakis , J. Mater. Process. Technol. 2005 , 167 , 283 . [ 11 ] T. Subbiah , G. S. Bhat , R. W. Tock , S. Parameswaran ,

S. S. Ramkumar , J. Appl. Polym. Sci. 2005 , 96 , 557 . [ 12 ] A. Greiner , J. H. Wendorff , Angew. Chem. Int. Ed. 2007 , 46 , 5670 . [ 13 ] D. H. Reneker , A. L. Yarin , E. Zussman , H. Xu , Adv. Appl. Mechan.

2007 , 41 , 43 . [ 14 ] D. H. Reneker , A. L. Yarin , Polymer 2008 , 49 , 2387 . [ 15 ] W.-E. Teo , R. Inai , S. Ramakrishna , Sci. Technol. Adv. Mater. 2011 ,

12 , 013002 . [ 16 ] Z. Dong , S. J. Kennedy , Y. Wu , J. Power Sources 2011 , 196 , 4886 . [ 17 ] A. Arinstein , E. Zussman , J. Polym. Sci. B 2011 , 49 , 691 . [ 18 ] Z. Sun , E. Zussman , A. L. Yarin , J. H. Wendorff , A. Greiner , Adv.

Mater. 2003 , 15 , 1929 . [ 19 ] Y. Zhang , Z. M. Huang , X. Xu , C. T. Lim , S. Ramakrishna , Chem.

Mater. 2004 , 16 , 3406 . [ 20 ] H. Chen , N. Wang , J. Di , Y. Zhao , Y. Song , L. Jiang . Langmuir 2010 ,

26 , 1 1291 . [ 21 ] D. Li , Y. Xia , Nano Lett. 2004 , 4 , 933 . [ 22 ] M. Bognitzki , W. Czado , T. Frese , A. Schaper , M. Hellwig ,

M. Steinhart , A. Greiner , J. H.Wendorff , Adv. Mater. 2001 , 13 , 70 . [ 23 ] S. Megelski , J. S. Stephens , D. B. Chase , J. F. Rabolt , Macromol-

ecules 2002 , 35 , 8456 . [ 24 ] Z. Qi , H. Yu , Y. Chen , M. Zhu , Mater. Lett. 2009 , 63 , 415 . [ 25 ] T. Lin , H. Wang , X. Wang , Adv. Mater. 2005 , 17 , 2699 . [ 26 ] Y. Yang , Z. Jia , Q. Li , L. Hou , J. Liu , L. Wang , Z. Guan , M. Zahn ,

IEEE Trans. 2010 , 17 , 1592 . [ 27 ] Y. Yang , Z. Jia , Q. Li , Z. Guan , IEEE Trans. 2006 , 13 , 580 . [ 28 ] C. L. Casper , J. S. Stephens , N. G. Tassi , D. B. Chase , J. F. Rabolt ,

Macromolecules 2004 , 37 , 573 .

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag

[ 29 ] L. H. C. Mattoso , R. D. Offeman , D. F. Wood , W. J. Orts , E. S. Medeiros , Can. J. Chem. 2008 , 86 , 590 .

[ 30 ] C. L. Pai , M. C. Boyce , G. C. Rutledge , Macromolecules 2009 , 42 , 2102 .

[ 31 ] H. Fong , D. H. Reneker , Munich: Hanser 2001 , 225 . [ 32 ] S. Borhani , S. A. Hosseini , S. G. Etemad , J. Militky , J. Appl. Polymer

Sci. 2008 , 108 , 2994 . [ 33 ] S. Y. Gu , J. Ren , G. J. Vancso , Euro. Polymer J. 2005 , 41 , 2559 . [ 34 ] S. H. Tan , R. Inai , M. Kotaki , S. Ramakrishna , Polymer 2005 , 46 ,

6128 . [ 35 ] X. H. Qin , E. L. Yang , N. Li , S. Y. Wang , J. Appl. Polymer Sci. 2007 ,

103 , 3865 . [ 36 ] S. V. Fridrikh , J. H. Yu , M. P. Brenner , G. C. Rutledge , Phys. Rev.

Lett. 2003 , 90 , 144502 . [ 37 ] O. S. Yordem , M. Papila , Y. Z. Menceloglu , Mater. Design 2008 , 29 ,

34 . [ 38 ] K. Acatay , E. Simsek , C. Ow-Yang , Y. Z. Menceloglu , Angew. Chem.

Int. Ed. 2004 , 43 , 5210 . [ 39 ] J. J. Ge , H. Q. Hou , Q. Li , M. J. Graham , A. Greiner , D. H. Reneker ,

F. W. Harris , S. Z. Cheng , J. Am. Chem. Soc. 2004 , 126 , 15754 . [ 40 ] X. L. Xie , Y. W. Mai , X. P. Zhou , Mater. Sci. Eng. R 2005 , 49 , 89 . [ 41 ] F. Dabiriana , Y. Hosseinib , S. A. Hosseini Ravandia , J. Textile Inst.

2007 , 98 , 237 . [ 42 ] M. Naebe , T. Lin , M. P. Staiger , L. Dai , X. Wang , Nanotechnology

2008 , 19 , 305702 [ 43 ] T. Wang , S. Kumar , J. Appl. Polym. Sci. 2006 , 102 , 1023 . [ 44 ] S. Basu , A. K. Agrawal , M. Jassal , J. Appl. Polym. Sci. 2011 , 122 ,

856 . [ 45 ] S. Fukushima , Y. Karube , H. Kawakami , Polym. J. 2010 , 2010 , 1 . [ 46 ] F. Chen , X. Peng , T. Li , S. Chen , X.-F. Wu , D. H. Rebeker , H. Hou , J.

Phys. D 2008 , 41 , 025308 . [ 47 ] D. Chen , T. Liu , X. Zhou , W. C. Tjiu , H. Hou , J. Phys. Chem. B 2009 ,

113 , 9741 . [ 48 ] Y. Y. Lv , J. Wu , L.-S. Wan , Z.-K. Xu , J. Phys. Chem. C 2008 , 112 ,

10609 . [ 49 ] W. J. Kim , J. Y. Chang , Mater. Lett. 2011 , 65 , 1388 . [ 50 ] J. Zhu , S. Wei , X. Chen , A. B. Karki , D. Rutman , D. P. Young ,

Z. Guo , J. Phys. Chem. C 2010 , 114 , 8844 . [ 51 ] Y. Karubea , H. Kawakami , Polym. Adv. Technol. 2010 , 21 , 861 . [ 52 ] Q. Zhang , D. Wu , S. Qi , Z. Wu , X. Yang , R. Jin , Mater. Lett. 2007 ,

61 , 4027 . [ 53 ] C. Qin , J. Wang , S. Cheng , X. Wang , L. Dai , G. Chen , Mater. Lett.

2009 , 63 , 1239 . [ 54 ] C. Nah , S. H. Han , M.-H. Lee , J. S. Kim , D. S. Lee , Polym. Int. 2003 ,

52 , 429 . [ 55 ] J. R. Kim , S. W. Choi , S. M. Jo , Electrochim. Acta. 2004 , 50 , 69 . [ 56 ] Z. Zhao , J. Li , X. Yuan , X. Li , Y. Zhang , J. Sheng , J. Appl. Polym. Sci.

2005 , 97 , 466 . [ 57 ] Y. Yang , A. Centrone , L. Chen , F. Simeon , T. A. Hatton ,

G. C. Rutledge , Carbon 2011 , 49 , 3395 . [ 58 ] B. Ding , H.-Y. Kim , S.-C. Lee , D.-R. Lee , K.-J. Choi , Fibers Polym.

2002 , 3 , 73 . [ 59 ] S. Nagamine , S. Ishimaru , K. Taki , M. Ohshima , Mater. Lett. 2011 ,

65 , 3027 . [ 60 ] M. J. Kim , J. Lee , D. Jung , S. E. Shim , Synth. Met. 2010 , 160 ,

1410 . [ 61 ] W. K. Son , J. H. Youk , T. S. Lee , W. H. Park , Mater. Lett. 2005 , 59 ,

1571 . [ 62 ] B.-H. Kim , A. H. Wazir , K. S. Yang , Y. H. Bang , S. R. Kim , Carbon

Lett. 2011 , 12 , 70 . [ 63 ] Y. Wang , S. Serrano , J. J. Santiago-Aviles , J. Mater. Sci. Lett. 2002 ,

21 , 1055 . [ 64 ] Y. Wang , S. Serrano , J. J. Santiago-Aviles , Synth. Met. 2003 , 38 ,

423 . [ 65 ] Y. Wang , J. J. Santiago-Aviles , J. Appl. Phys. 2003 , 94 , 1721 .

GmbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566

Page 19: Carbon Nanofibers Prepared via Electrospinning

www.advmat.dewww.MaterialsViews.com

REV

IEW

[ 66 ] F. Ko , Y. Gogotsi , A. Ali , N. Naguib , H. Ye , G. Yang , C. Li , P. Willis , Adv. Mater. 2003 , 15 , 1161 .

[ 67 ] C. Kim , K. S. Yang , Appl. Phys. Lett. 2003 , 83 , 1216 . [ 68 ] K. S. Yanga , D. D. Edieb , D. Y. Limc , Y. M. Kimd , Y. O. Choia ,

Carbon 2003 , 41 , 2039 . [ 69 ] S. H. Park , C. Kim , Y. O. Choi , K. S. Yang , Carbon 2003 , 41 , 2655 . [ 70 ] C. Kim , S. H. Park , W. J. Lee , K. S. Yang , Electrochim. Acta 2004 , 50 ,

877 . [ 71 ] C. Kim , J. S. Kim , S. J. Kim , W. J. Lee , K. S. Yang , J. Electrochem.

Soc. 2004 , 151 , A769 . [ 72 ] S. H. Park , C. Kim , K. S. Yang , Synth. Met. 2004 , 143 , 175 . [ 73 ] H. Hou , D. H. Reneker , Adv. Mater. 2004 , 16 , 69 . [ 74 ] C. Kim , K. S. Yang , W. J. Lee , Electrochem. Solid State Lett. 2004 , 7 ,

A397 . [ 75 ] C. Kim , Y. O. Choi , W. J. Lee , K. S. Yang , Electrochim. Acta 2004 , 50 ,

883 . [ 76 ] H. Q. Hou , J. J. Ge , J. Zeng , Q. Li , D. H. Reneker , A. Greiner ,

S. Z. D. Cheng , Chem. Mater. 2005 , 17 , 967 . [ 77 ] E. Zussman , X. Chen , W. Ding , L. Calabri , D. Dikin , J. Quintana ,

R. S. Ruoff , Carbon 2005 , 43 , 2175 . [ 78 ] C. Kim . J. Power Sources 2005 , 142 , 382 . [ 79 ] G. S. Chung , S. M. Jo , B. C. Kim , J. Appl. Polym. Sci. 2005 , 97 , 165 . [ 80 ] C. Kim , K. S. Yang , M. Kojima , K. Yoshida , Y. J. Kim , Y. A. Kim ,

M. Endo , Adv. Func. Mater. 2006 , 16 , 2393 . [ 81 ] E. Zussman , A. L. Yarin , A. V. Bazilevsky , R. Avrahami , M. Feldman ,

Adv. Mater. 2006 , 18 , 348 . [ 82 ] W. G. Shim , C. Kim , J. W. Lee , J. J. Yun , Y. I. Jeong , H. Moon ,

K. S. Yang , J. Appl. Polym. Sci. 2006 , 102 , 2454 . [ 83 ] Y. W. Ju , G. R. Choi , H. R. Jung , C. Kim , K. S. Yang , W. J. Lee , J.

Electrochem. Soc. 2007 , 154 , A192 . [ 84 ] C. Kim , Y. Jeong , B. Ngoc , K. S. Yang , M. Kojima , Y. A. Kim ,

M. Endo , J. W. Lee , Small 2007 , 3 , 91 . [ 85 ] C. Kim , B. T. N. Ngoc , K. S. Yang , M. Kojima , Y. A. Kim , Y. J. Kim ,

M. Endo , S. C. Yang , Adv. Mater. 2007 , 19 , 2341 . [ 86 ] N. T. Xuyen , E. J. Ra , H.-Z. Geng , K. K. Kim , H. K. An , Y. H. Lee , J.

Phys. Chem. B 2007 , 111 , 1 1350 . [ 87 ] J. S. Im , S.-J. Park , Y.-S. Lee , J. Colloid Interface Sci. 2007 , 314 , 32 . [ 88 ] C. Kim , Y. J. Cho , W. Y. Yun , B. T. N. Ngoc , K. S. Yang , D. R. Chang ,

J. W. Lee , M. Kojima , Y. A. Kim , M. Endo , Solid State Commun. 2007 , 142 , 20 .

[ 89 ] K. Suzuki , H. Matsumoto , M. Minagawa , M. Kimura , A. Tanioka , Polymer J. 2007 , 39 , 1128 .

[ 90 ] L. Wang , Y. Yu , P. C. Chen , C. H. Chen , Scripta Mater. 2008 , 58 , 405 .

[ 91 ] L. Wang , Y. Yu , P. C. Chen , D. W. Zhang , C. H. Chen , J. Power Sources 2008 , 183 , 717 .

[ 92 ] J. Huang , D. Wang , H. Hou , T. You , Adv. Funct. Mater. 2008 , 18 , 441 .

[ 93 ] M. Y. Li , G. Y. Han , B. S. Yang , Electrochem. Commun. 2008 , 10 , 880 .

[ 94 ] Y. W. Ju , G. R. Choi , H. R. Jung , W. J. Lee , Electrochim. Acta 2008 , 53 , 5796 .

[ 95 ] J. S. Im , S.-W. Woo , M.-J. Jung , Y.-S. Lee , J. Coll. Interf. Sci. 2008 , 327 , 115 .

[ 96 ] J. S. Im , O. Kwon , Y. H. Kim , S.-J. Park , Y.-S. Lee , Micropo. Mesopo. Mater. 2008 , 115 , 514 .

[ 97 ] S. Prilutsky , E. Zussman , Y. Cohen , Nanotechnology 2008 , 19 , 165603 .

[ 98 ] G. Y. Oh , Y. W. Ju , H. R. Jung , W. J. Lee , J. Anal. Appl. Pyrolysis 2008 , 81 , 211 .

[ 99 ] L. W. Ji , A. J. Medford , X. W. Zhang , J. Mater. Chem. 2009 , 19 , 5593 .

[ 100 ] L. W. Ji , X. W. Zhang , Electrochem. Commun. 2009 , 11 , 1146 . [ 101 ] N.-N. Bui , B.-H. Kim , K. S. Yang , M. E. Dela Cruz , J. P. Ferraris ,

Carbon 2009 , 47 , 2538 .

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, 24, 2547–2566

[ 102 ] Y. W. Ju , S. H. Park , H. R. Jung , W. J. J. Lee , J. Electrochem. Soc. 2009 , 156 , A489 .

[ 103 ] Z. Y. Zhang , X. H. Li , C. H. Wang , S. W. Fu , Y. C. Liu , C. L. Shao , Macromol. Mater. Eng. 2009 , 294 , 673 .

[ 104 ] Y. Yu , L. Gu , C. L. Wang , A. Dhanabalan , P. A. V. Aken , J. Maier , Angew. Chem. Int. Ed. 2009 , 48 , 6485 .

[ 105 ] Q. H. Guo , X. P. Zhou , X. Y. Li , S. L. Chen , A. Seema , A Greiner , H. Q. Hou , J. Mater. Chem. 2009 , 19 , 2810 .

[ 106 ] Y. Yu , L. Gu , C. B. Zhu , P. A. V. Aken , J. Maier , J. Am. Chem. Soc. 2009 , 131 , 15984 .

[ 107 ] Z. Zhou , C. Lai , L. Zhang , Y. Qian , H. Hou , D. H. Reneker , H. Fong , Polymer 2009 , 50 , 2999 .

[ 108 ] J. Li , E. H. Liu , W. Li , X. Y. Meng , S. T. Tan , J. Alloys Compd. 2009 , 478 , 371 .

[ 109 ] S. K. Nataraj , B. H. Kim , J. H. Yun , D. H. Lee , T. M. Aminabhavi , K. S. Yang , Mater. Sci. Eng. B 2009 , 162 , 75 .

[ 110 ] L. W. Ji , X. W. Zhang , Electrochem. Commun. 2009 , 11 , 684 . [ 111 ] L. W. Ji , K. H. Jung , A. J. Medford , X. W. Zhang , J. Mater. Chem.

2009 , 19 , 4992 . [ 112 ] X. Fan , L. Zou , Y. P. Zheng , F. Y. Kang , W. C. Shen , Electrochem.

Solid State Lett. 2009 , 12 , A199 . [ 113 ] L. Ji , X. Zhang , Carbon 2009 , 47 , 3219 . [ 114 ] S. K. Nataraj , B. H. Kim , M. Dela Cruz , J. Ferraris , T. M. Aminabhavi ,

K. S. Yang , Mater. Lett. 2009 , 63 , 218 . [ 115 ] S. K. Nataraj , B. H. Kim , J. H. Yun , D. H. Lee , T. M. Aminabhavi ,

K. S. Yang , Synth. Met. 2009 , 159 , 1496 . [ 116 ] L. W. Ji , Z. Lin , A. J. Medford , X. W. Zhang , Carbon 2009 , 47 , 3346 . [ 117 ] L. W. Ji , Z. Lin , A. J. Medford , X. W. Zhang , Chem.-A Eur. J. 2009 ,

15 , 10718 . [ 118 ] L. W. Ji , X. W. Zhang , Electrochem. Commun. 2009 , 11 , 795 . [ 119 ] J. Liu , Z. Yue , H. Fong , Small 2009 , 5 , 536 . [ 120 ] L. W. Ji , X. W. Zhang , Nanotechnology 2009 , 20 , 155705 . [ 121 ] S. Imaizumi , H. Matsumoto , K. Suzuki , M. Minagawa , M. Kimura ,

A. Tanioka , Polymer J 2009 , 41 , 1124 . [ 122 ] L. Zou , L. Gan , F. Y. Kang , M. X. Wang , W. C. Shen , Z. H. Huang , J.

Power Sources 2010 , 195 , 1216 . [ 123 ] L. Zou , L. Gan , R. Lv , M. Wang , Z. Huang , F. Kang , W. Shen , Carbon

2011 , 49 , 89 . [ 124 ] L. W. Ji , Z. Lin , R. Zhou , Q. Shi , O. Toprakci , A. J. Medford ,

C. R. Millns , X. W. Zhang , Electrochim. Acta 2010 , 55 , 1605 . [ 125 ] M. Rose , E. Kockrick , I. Senkovska , S. Kaskel , Carbon 2010 ,

48 , 403 . [ 126 ] I.-H. Chen , C.-C. Wang , C.-Y. Chen , Carbon 2010 , 48 , 604 . [ 127 ] R. Ruiz-Rosas , J. Bedia , M. Lallave , I. G. Loscertales , A. Barrero ,

J. Rodríguez- Mirasol , T. Cordero , Carbon 2010 , 48 , 696 . [ 128 ] M. Wang , Z.-H. Huang , L. Wang , M.-X. Wang , F. Kang , H. Hou ,

New J. Chem. 2010 , 34 , 1843 . [ 129 ] H. S. Choi , J. K. Lee , H. Y. Lee , S. W. Kim , C. R. Park , Electrochim.

Acta 2010 , 56 , 790 . [ 130 ] S. N. Arshad , M. Naraghi , I. Chasiotis , Carbon 2011 , 49 , 1710 . [ 131 ] H. Niu , J. Zhang , Z. Xie , X. Wang , T. Lin , Carbon 2011 , 49 , 2380 . [ 132 ] M.-X. Wang , Z.-H. Huang , F. Kang , K. Liang , Mater. Lett. 2011 , 65 ,

1875 . [ 133 ] O. Toprakci , L. Ji , Z. Lin , H. A. K. Toprakci , X. Zhang , J. Power

Sources 2011 , 196 , 7692 . [ 134 ] S. C. Kang , J. S. Im , Y.-S. Lee , Carbon Lett. 2011 , 12 , 21 . [ 135 ] Y. Yang , F. Simeon , T. A. Hatton , G. C. Rutledge , J. Appl. Polym. Sci.

( in press ). [ 136 ] B.-H. Kim , K. S. Yang , H.-G. Woo , Electrochem. Commun. 2011 , 13 ,

1042 . [ 137 ] B.-H. Kim , K. S. Yang , H.-G. Woo , J Nanosci. Nanotech. 2011 , 11 ,

7193 . [ 138 ] B.-H. Kim , K. S. Yang , Y. A. Kim , Y. J. Kim , B. An , K. Oshida , J.

Power Sources 2011 , 196 , 10496 . [ 139 ] M. Bayat , H. Yang , F. Ko , Polymer 2011 , 52 , 1645 .

2565wileyonlinelibrary.combH & Co. KGaA, Weinheim

Page 20: Carbon Nanofibers Prepared via Electrospinning

2566

www.advmat.dewww.MaterialsViews.com

REV

IEW

[ 140 ] M.-X. Wang , Z.-H. Huang , T. Shimohara , F. Kang , K. Liang . Chem.

Eng. J. 2011 , 170 , 505 . [ 141 ] M.-X. Wang , Z.-H. Huang , F. Kang , K. Liang , Mater. Lett. 2011 , 65 ,

1875 . [ 142 ] S. Prilutsky , E. Zussman , Y. Cohen , J. Polym. Sci. B 2010 , 48 ,

2121 [ 143 ] S. K. Nataraj , K. S. Yang , T. M. Aminabhavi , Prog. Polym. Sci. 2012 ,

37 , 487 . [ 144 ] W. Zhang , J. Liu , G. Wu , Carbon 2003 , 14 , 2805 . [ 145 ] M. Inagaki , H. Konno , O. Tanaike , J. Power Sources 2010 , 195 ,

7880 .

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag

[ 146 ] Y. Oren , Desalination 2008 , 228 , 10 . [ 147 ] L. Wang , L. Wu , Z. Li , G. Lei , Q. Xiao , P. Zhang , Electrochim. Acta

2011 , DOI: 10.1016/j-3l3ctacta.2011.03.122. [ 148 ] M. Inagaki , TANSO 1985 , 1985 No. 122, 114 (in Japanese). [ 149 ] T. Kyotani , N. Sonobe , A. Tomita , Nature 1988 , 331 , 331 . [ 150 ] M. Inagaki , T. Takeichi , Y. Hishiyama , A. Oberlin , Chemistry and

Physics of Carbon Vol 26 , (Eds: P. A. Thrower , L. R. Radovic ), Marcel Dekker, Inc. , New York, USA 1999 , Ch. 3.

[ 151 ] M. Inagaki , H. Orikasa , T. Morishita , RSC Adv. 2011 , 1 , 1620 . [ 152 ] Z. Li , M. Jaroniec , Y.-J. Lee , L. R. Radovic , Chem. Commun. 2002 ,

1346 .

GmbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 2547–2566