Carbon nanofibers by pyrolysis of self-assembled perylene diimide derivative gels as supercapacitor electrode materials

Article abstract of DOI:10.1039/c5ta03546e

Carbon nanofibers (CNFs) have a wide range of applications and are commonly prepared under harsh conditions with small quantities. A green method for the preparation of CNFs under mild conditions with the scalable potential is highly challenging. Here, we

Full text of DOI:10.1039/c5ta03546e

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DOI: 10.1039/C5TA03546E
Journal of Materials Chemistry A
ARTICLE
Carbon nanofibers by pyrolysis of self-assembled perylene diimide
derivative gels as supercapacitor electrode materials†
Xia Liu,a,b,c Aled Robertsc,d, Adham Ahmed,c Zhenxin Wang,*a Xu Li,d and Haifei Zhang*c
Received 00th January 20xx,
Accepted 00th January 20xx
Carbon nanofibers (CNFs) have a wide range of applications and are commonly prepared under harsh conditions with
small quantities. A green method for the preparation of CNFs under mild conditions with the scalable potential is highly
challenging. Here, we describe a facile water-based room-temperature approach to producing gel fibers from a perylene
diimide derivative via self-assembly, gelation, freeze-drying, and then to generating CNFs by subsequent pyrolysis under
Ar. The entangled CNFs, with relatively uniform diameters around 20-50 nm, can be prepared as thin films or monoliths.
The CNFs doped with elements N and Sn can be readily produced. The CNFs are characterized by various techniques and
particularly evaluated as electrode materials for supercapacitor by cyclic voltammetry and galvanostatic discharge. A non-
ionic surfactant (Pluronics F-127) can be easily incorporated into the self-assembly process, which produces the CNFs with
a higher surface area 520 m2 g-1 and a specific capacitance of 346 F g-1 at the scan rate of 1 mV s-1 or 192 F g-1 at the current
density of 1 A g-1. The CNF electrodes are highly stable. Quite surprisingly, the specific capacitance increases with repeated
testing, achieving an impressive 226 F g-1 for 1000 charge/discharge cycles at 4 A g-1.
DOI: 10.1039/x0xx00000x
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1. Introduction
processes where the carbonaceous materials are directly
Carbon nanostructures, such as fullerenes, carbon nanotubes, formed, CNFs are usually prepared by decomposition of small
graphene, and carbon nanofibers (CNFs), have attracted great molecules or carbonization of carbon-rich polymers. For
interests because of their remarkable physical and chemical templating methods, the templates will have to be removed to
properties and wide range of applications.1 CNFs can be generate the CNFs, normally by washing or etching
prepared by different approaches on a relatively large scale procedures.
and hence are potentially cost-effective. CNF-based materials
The vapor-growth approach involves the use of high
have been applied in areas such as hydrogen storage, catalyst temperature and high vacuum. As a result, the carbon
support,2 gas sensing,3 electrochemical biosensors,4 materials are usually prepared in small quantities.
supercapacitors,5 and water treatment.6 CNFs may substitute Electrospinning is a highly versatile method for preparation of
for the more expensive carbon nanotubes or graphenes for fibers from different types of polymers, but is generally slow
some applications.7,8 CNFs can be prepared either by (restricted by the injection rate). Volatile organic solvents or
templating or template-free approaches. In the template-free harsh conditions are commonly used to produce the fibers of
methods, CNFs are conventionally formed by vapor growth or desired polymers by electrospinning. CNFs are obtained after
catalytic decomposition/growth of carbon precursors in the carbonization of the polymer nanofibers. The preparation of
gas phase and also by the widely used electrospinning CNFs by carbonization normally requires the pre-formed
method.2,9-11 In the templating approach, anodic aluminium carbon-rich nanofibers of polymers or other organic materials.
oxide (AAO) membranes are widely used as hard templates,12 A green (e.g., use of water rather than organic solvents) and
whilst other materials such as crab shell can also be used.13 template-free approach (to avoid the procedures/costs of
Preformed nanowires or in-situ formed and assembled producing and removing templates) with the ability for easy
nanowires may be used as templates to form CNFs in scale-up (essential for applications) and under mild conditions
solvothermal processes.14,15 Apart from the solvothermal (e.g., room temperature synthesis) is highly desirable for the
synthesis of CNFs.
Perylene diimides are multi-aromatic molecules, have an
a. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of
extended quadrupolar π system, and are utilized in many
Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun
fields, including biosensing, light emitting diodes, field effect
130022, P. R. China. E-mail address: wangzx@ciac.ac.cn
transistors, photovoltaic cells, and pigments.16,17 Due to their
strong hydrophobicity, self-assembly of perylene diimides can
form nanostructures in various organic solvents via π-π
interaction/stacking and interaction of side chains. For
example, 0-dimensional or 1-dimensional nanostructures are
b. University of Chinese Academy of Sciences, Beijing 100039, P. R. China.
c. Department of Chemistry, University of Liverpool, Oxford Street, Liverpool L69
7ZD, United Kingdom. E-mail address: zhanghf@liv.ac.uk
d. Institute of Materials Research and Engineering, A*STAR, 3 Research Link 117602,
Singapore.
†
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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ARTICLE
Journal of Materials Chemistry A
formed by self-assembly via the modification of side chains at room temperature. The samples were denoted VaieswXAr+ticGledOLnlioner
DOI: 10.1039/C5TA03546E
and the change of solubility with “poor” or “good” X + MM + GdL, where X (N, K, or T) indicates the base in the PI
solvents.18,19 Water soluble perylene diimides are investigated solution.
by modification of perlylene diimide with ionic substituents at
The formed gels could be washed using water, acetone and
the imide positions and/or bay positions. The aggregation of cyclohexane in order. The gels were soaked in water for 30
the molecules is prevented in water due to the electrostatic minutes, replaced with fresh water 3 times; then washed with
repulsion and steric hindrance.20 In order to get biocompatible acetone using the simiar procedure and soaked in acetone
hydrogels, perylene diimide derivatives have been produced overnight. Finally the hydrogels were soaked in cyclohexane
by reaction of perylene-3,4:9,10-tetracarboxylic dianhydride for 5 hours before freeze-drying. All the hydrogels with and
with amino acids. Once dissolved in water, the gelation/self- without washing treatment were frozen in liquid nitrogen and
assembly of such perylene diimide derivatives may be initiated then freeze-dried in a freeze-dryer (Vis Advantage) for 48 h.
conveniently by change of pH or crosslinking with Finally, the as-obtained dry samples were pyrolysed in a
melamine.17,21-24 These hydrogels consist of entangled Nabertherm furnace (model R 50/500/12) under an Ar
nanofibers, showing semiconducting and photo-switching atmosphere at 2
℃ ℃ for 2 h, 3 ℃
min-1 to 200 min-1 to 800
behavior.25 The dried gels may maintain the nanofibrous
℃ for 3 h to form the black product. 1.0 g dry samples could
structure and exhibit air-stable photoconductivity.23 These yield approximately 0.23 g of carbon materials. The CNFs were
nanofibers have recently been used as templates for the prepared from both washed and un-washed gels.
synthesis of hybrid and supermicroporous nanotubules.26
Preparation of the F-127 templated CNFs: The F-127-
By utilizing the properties that perylene diimides are templated gels were formed by mixing 0.5 cm3 of PI in TEA
molecules with high carbon content and can form nanofibrous solution with 0.5 cm3 aqueous 40 mM MM solution containing
hydrogels in water under mild conditions, we describe here a 0.05 mg cm-3 F-127. GdL was subsequently added (13 mg cm-3
highly efficient method to produce CNFs via the self-assembly based on the mixed solution) to initiate the gelation at room
of perylene diimide derivatives (PI) and subsequent temperature. The gels were washed, freeze-dried, and
carbonization. Further surfactant templating is introduced to carbonized using the procedures described above.
form the CNFs with higher surface area. The potential of using
Characterization: The fiber structure was observed by a
these CNFs as high performance supercapacitor electrodes is Hitachi-S4800 scanning electron microscope (SEM) with an
demonstrated.
energy dispersive X-ray (EDX) microanalysis detector
(INCA7200, Oxford Instrument). A small piece was cut from the
monolith using a blade and then adhered to a stud using
2. Experimental section
Materials and reagents: Perylene-3,4,9,10-tetracarboxylic double-sided carbon tape. The CNFs were imaged directly. The
dianhydride (PTCDA, 97%), 5-aminoisophthalic acid (94%), dry gels were coated with gold using a sputter-coater
imidazole (ACS reagent, ≥99%), triethylamine (TEA, ≥99%), (EMITECH K550X) for 2 minutes at 25 mA before SEM imaging.
potassium stannate trihydrate (K2SnO3, 99.9%), NaOH, The Brunauer-Emmett-Teller (BET) surface area and pore
glucono-δ-lactone (GdL), melamine (MM, 99%), and non-ionic volume by N2 sorption at 77 K were determined using a
surfactant Pluronic F-127 were purchased from Sigma-Aldrich Micromeritics ASAP 2020 adsorption analyzer. Samples were
and used without further purification. Standard analytical degassed for 10 h at 150
℃ before N2 sorption analysis.
grade solvents and de-ionized water were used routinely. Powder X-ray diffraction (PXRD) patterns were collected on a
Synthesis of perylene diimide derivative (PI): The perylene Panalytical X’ Pert Pro Multi-Purpose Diffractometer in high-
diimide derivative with four carboxylic acid groups (PI) was throughput transmission geometry. Cu anode was operated at
synthesized as reported previously.17 Briefly, PTCDA (0.20 g), 5- 40 kV and 40 mA. Samples were pressed into the well of
aminoisophthalic acid (0.23 g), and imidazole (1.60 g) were aluminium plate. XRD patterns were measured over 5-50°2θ
heated at 127
℃ for 6 hours under nitrogen atmosphere. Then with a scan time of 60 minutes. Thermal stability of PI
ethanol (20 cm3) was poured into the hot mixture, refluxed for nanofibers was assessed using a thermalgravimetric analyzer
6 hours and then cooled down and left overnight to precipitate (TGA, TA instruments, Q5000IR). The samples were heated to
out the solid. The precipitate was filtered and washed with 800
℃ at a rate of 5 ℃
min-1 under N2 atmosphere. Elemental
ethanol. The product was dried at 60
get 0.32 g deep red powder (yield: 88%).
℃
under vacuum oven to analysis data was obtained from a Thermo FlashEA1112 series
CHNSO elemental analyzer. The surface of the samples was
Preparation of the CNFs: To form the gel, 10.0 cm3 stock studied on a VG ESCA LAB-220i XL X-ray photoelectron
solutions of PI (20 mM) were prepared by dissolving PI in 0.06 spectrometer (XPS) with an exciting source of Al. A hand-held
M NaOH solution (denoted as N), 12 mg cm-3 K2SnO3 solution pH meter (HANNA instruments) was used to monitor the pH
(denoted as K), or TEA solution with a molar ratio of PI:TEA of change during the course of gelation. The pH meter was
1:6 (denoted as T). 0.5 cm3 of these solutions and 0.5 cm3 calibrated using pH 4 and pH 7 buffer solutions, and then
water or 40 mM MM solution in water were added to a test rinsed with de-ionized water. The probe was inserted into the
tube, followed by addition of 13 mg GdL (2 molar equivalent). solution and pH readings were recorded.
Mixing was achieved by shaking the test tubes for
approximately 10 seconds. After this, the samples were prepared by casting
allowed to stand, with gelation taking place over 30~120 min impregnated sample onto a glass carbon electrode with a
Electrochemical measurements: The working electrode was
a
polytetrafluoroethene (PTFE)
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JouPrlneaasleofdoMnaotetraiadljsusCthmeamrgisintrsy A
Journal of Materials Chemistry A
diameter of 3 mm. Typically, a slurry of the active material
ARTICLE
We firstly investigated how the bases and acids could be
View Article Online
DOI: 10.1039/C5TA03546E
(90%) with PTFE (10%) in N-methylpyrrolidone (NMP) with a used to form the gels. The organic base TEA and inorganic base
concentration of 5 mg cm-3 was made, then an accurate NaOH were initially chosen. This was to see if the use of
volume (2 µL) was pipetted onto the surface of the glass different bases would have any impact on gel fibers. K2SnO3
carbon electrode, before allowing the NMP to evaporate in an was also selected because it was a weak base after hydrolysis
oven overnight. Due to the very low bulk density of the CNFs, a in water and the element Sn might be incorporated into the
small mass of the CNFs could form a dense film and cover the CNFs after pyrolysis.28 The presence of Sn/SnO2 nanoparticles
surface of the working electrod to give reliable test data.27
A
in CNFs can be very useful as electrode materials for lithium-
three-electrode cell system was used to evaluate the ion batteries.29,30
electrochemical performance by cyclic voltammetry (CV, CHI
660D electrochemical analyzer) at room temperature. The
electrolyte used was 2 M H2SO4 aqueous solution. The voltage
cycled from 0.1 V to 0.8 V.A platinum wire (or sheet) and an
Ag/AgCl electrode were used as the counter and the reference
electrodes, respectively. The capacitance value was obtained
, N2, 127 o
C
PI
from the CV curves according to the equation: Cavg
=
pH ↓
Pyrolysis
/2(ΔVvm), where is calculated by integrating the
base
GdL
total area in the CV plot, i is the response current density, ΔV is
the potential by measuring its voltage span from the CV plot, v
is the potential sweep rate, m is the mass of active materials,
the value divided by 2 because both the charge and discharge
process exist in a cycle.
Bases used to dissolve PI in water:
Triethylamine(T)
K2SnO3(K)
NaOH(N)
Acid to initiate gelation:
For the F-127 templated CNFs, in order to improve the
stability, the working electrode was also prepared by casting a
Nafion-impregnated sample onto a glassy carbon electrode.
Typically, the CNFs were dispersed in an ethanol solution
containing a Nafion solution (5 wt% in water) by sonication for
20 min, with a concentration of 5 mg cm-3. The slurry (2 µL)
was then deposited onto the glassy carbon electrode and dried
in an oven, giving a loading of CNFs on the electrode at 0.14
mg cm-2. The reliability of the test results was further
investigated by increasing the CNF loading to 1.13 mg cm-2,
achieved by depositting 4 µl of the CNF dispersion in Nafion
solution (5 mg cm-3) and repeating 4 times. A three-electrode
cell system was used to evaluate the electrochemical
performance by CV and the galvanostatic charge-discharge
method on a CHI 660D electrochemical workstation (Shanghai
CH Instrument Co., China). The specific capacity was calculated
by the equation: Cg = (IΔt)/(mΔV) where I is the current loaded
(A), Δt is the discharge time (s), ΔV is the potential change
during the discharge process, and m is the mass of active
material in a single electrode (g). The Ragone plot was
obtained using the galvanostatic charge-discharge results from
the 3-electrode system. The energy density (E) was calculated
by E=(CΔV2)/2 while the power density (P) was calculated by
P=(QΔV)/(2t)=E/t.
glucono-δ-lactone(GdL)
Crosslinker:
Melamine(MM)
Scheme 1. Top: synthesis and chemical structure of PI
molecule; Central: the process of fabricating carbon materials;
Bottom: chemical structures of bases, acid and crosslinker
used.
Gelation of PI solutions with TEA was investigated by adding
HCl solution. The gels formed were inhomogeneous and brittle
(see ESI, Fig. S1†). Glucono-δ-lactone (GdL) can hydrolyze
slowly in water, which produces gluconic acid and results in a
decrease in pH. GdL has been demonstrated to produce
homogeneous and reproducible hydrogels from low molecular
weight hydrogelators because of its controlled hydrolysis.24,31
In this study, it was found that homogeneous gels could be
formed with GdL if the PI concentrations > 0.5 mM (see ESI,
Fig. S1d†). Below this concentration, the gels were highly
fragile. The optimal conditions to form homogeneous and
strong gels were found to be 0.06 M for NaOH, 12 mg cm-3 for
K2SnO3, and PI:TEA = 1:6 respectively. GdL was added at 13 mg
cm-3 based on the PI solution. In addition to the monolithic gel,
a thin film (thickness around 50 µm) could be formed simply
by spreading PI + GdL solution between two glass slides,
followed by gelation with time (Fig.S2 ). The thickness of the
film could be easily adjusted by using a spacer between the
slides to adjust the thickness of the precursor solution.
3. Results and discussion
3.1 Preparation of CNFs from PI gels
Scheme 1 shows the synthesis of PI and the process to
produce CNFs. PI has four carboxylic acid groups. However, the
solubility is low in neutral water. Different bases, including
K2SnO3 (denoted as K), NaOH (denoted as N), and
triethylamine (TEA, denoted as T), were used to dissolve PI in
aqueous solution. With the reduced solution pH by adding
acid, the dark red solution gradually formed into a stable gel.
The formed gels were freeze-dried to produce dry red
porous monoliths. Entangled but mostly discrete nanofibers
were observed for T + GdL gels (Fig. S3†), while it was difficult
to see separated fibers for the other systems. After
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ARTICLE
Journal of Materials Chemistry A
carbonization under Ar, the nanofibers appeared to uniform. The CNF film could be formed similarly but it was very
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DOI: 10.1039/C5TA03546E
fragile for handling. This study has then focused on the
aggregate/fuse for all the PI gels formed (Fig. S4
†
).
For applications with CNFs, it is desirable to have uniform characterization and use of the CNF monoliths.
and discreet nanofibers, giving rise to higher and accessible
Nitrogen doped carbon has been reported to be a promising
fiber surface. It is postulated that the loss of nanofibers during material for enhanced capacity and electronic conductivity by
carbonization could be due to presence of precursor and/or donation of its lone pair of electrons to the extended
base/GdL in the freeze-dried samples. A sequential washing conjugated system.34,35 Melamine (MM), a N-rich compound,
procedure with water/acetone/cyclohexane was employed to was used to crosslink PI in water to form gels.17 We
wash the as-formed gels. The final solvent was the volatile hypothesized that carbonization of MM-crosslinked fibers
hydrophobic cyclohexane. This was to reduce the surface could produce doped CNFs with higher N content. Hence, the
tension (limiting the impact on nanofiber morphology) and gelation of PI solutions was performed with MM by mixing
o
facilitate the removal of solvent (melting point 6 C) by freeze- equal volume of PI solution and MM solution. The gels formed
drying.32,33
were very weak and brittle (Fig. S5†). In order to prepare
strong and uniform gels, MM crosslinking was combined with
the acid-triggered gelation. The PI solutions (20 mM, in 0.06 M
NaOH, 12 mg cm-3 K2SnO3 or PI:TEA = 1:6) were mixed with
equal volumes of 40 mM MM solution. Subsequently, GdL (13
mg cm-3) was added into the mixture solution. Uniform gels
were formed for all the PI solutions.
The PI gels crosslinked with MM were subjected to the same
washing procedure. After freeze-drying and carbonization, gel
monoliths (red) and carbon monoliths (black) with discrete
nanofibrous structures were obtained for T + MM + GdL and K
+ MM + GdL (Fig. 1c-f). The nanofibers were fused after
carbonization for the N + MM + GdL fibers. For the K + MM +
GdL sample, the Energy-dispersive X-ray (EDX) data showed
the presence of Sn (15%) (Fig. 1g/h). Thermal gravimetric
analysis (TGA) under N2 was performed on the samples T +
MM + GdL and K + MM + GdL (Fig. S6†). The initial mass loss
could be attributed to the residual solvent or relatively volatile
o
compounds until 150 C. There was then continuous mass loss
across the heating region until 800 oC. Perylene derivatives can
be stable up to 400 oC.36,37 MM can completely sublime at a
temperature of ~330 oC in an open system, which can be
suppressed to a large extent if MM is heat-treated by the
substances with a capacity to bridge the MM molecules. When
MM was added as a cross-linker for PI, a large part of MM did
not sublime. It is possible for MM to polymerize to give a
carbon nitride allotrope (g-C3N4) at the high temperature.38
The mass loss at 150-450 oC might be ascribed to the pyrolysis
of the organic moiety of the hydrolysis product of GdL. When
the carbonization temperature increased from 500 oC to 800
oC, the gradual weight loss indicated that N-doped carbon was
produced slowly by the reaction between the semi-carbonized
PI and the reactive nitrogen-containing species generated
during the decomposition of carbon nitride.39.40 This test
revealed a mass loss of about 79.27% for sample T + MM +
GdL, and a mass loss of 76.71% for sample K + MM + GdL on
Fig. 1 SEM images of freeze-dried samples T + GdL (a), T + MM
+ GdL (c), K + MM + GdL (e), and the carbonized samples T +
GdL (b), T + MM + GdL (d), K + MM + GdL (f). The EDX spectrum
(g) and the corresponding element composition (h) of the
calcined sample K + MM + GdL. The insets in (a-f) show the
photos of the relevant monoliths. All samples were washed
using water, acetone and cyclohexane in order. The scale bar
of inset photos represents 5 mm.
heating to 800
elemental analysis found the carbon contents were higher (>
℃ at the rate 5 ℃
min-1 under N2. The
After washing the as-formed gels and subsequent freeze- 84%) with T+GdL and T+MM+GdL system (Table S1
drying, there was no visible volumetric shrinkage. Randomly contents were in the range of 1.7 – 6.8 %.
positioned and discrete nanofibers were observed (Fig. 1a).
†). The N
After carbonization, although there was small shrinkage of the 3.2 Characterization and electrochemical evaluation of the
monoliths, the nanofiber structures were retained (Fig. 1b). CNFs
Discrete nanofibers with the diameters ranging from 20 to 50 Fig. 2 shows the powder X-ray diffraction (PXRD) patterns of
nm were produced, which were continuous and relatively the gel fibers and the CNFs formed with MM in different base
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ARTICLE
solutions. Before carbonization, the PXRD patterns were the diffraction from the surface Sn4+ is negligible. The atomic
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similar, showing two diffraction peaks at 2θ = 24 and 26° (Fig. percentage of Sn determined by XPS wasDaOpI:p1r0o.1x0im39a/Cte5TlyA043.15406%E
2a). These results indicated a well-defined and compact π-π (calculated with element peak areas with relevant atomic
o
stacking of perylene cores in freeze-dried gel fibers.17,41 After sensitivity factors for x-ray sources at 54.7 C). The first major
carbonization, the PXRD patterns showed a broad peak at 2θ = peak in the XPS spectra (Fig. 3a, b) was for C1s, which may be
16°-27°, indicating amorphous carbon materials (Fig. 2b). For devolved into a big peak at 284.8 eV (C-C bonds) and three
the carbonized K + MM + GdL, there were sharp and strong minor peaks at around 286.0, 287.4 and 290.1 eV for the
peaks at 2θ = 30.66°, 32.01° and 43.87°, corresponding to the carbon atoms attached to oxygen atoms in the three different
(200), (101) and (220) planes of metallic Sn, respectively.42 This environment such as C-OH, C=O and COOH (Fig. S8
†
). The
peak at around 531.5 eV
), which
is a result of Sn4+ reduced to form metallic Sn by carbon during oxygen spectrum gave
a
the high temperature carbonization process in Ar corresponding to the double bonded oxygen (Fig. S8
†
atmosphere.43,44 All the carbon materials formed from K2SnO3 is contributed from SnO2.46 Washed K + MM + GdL improved
solution showed the presence of metallic Sn by PXRD (Fig. S7 the nanofiber morphology but there was minimal impact on
a/b
†
).
element status (Fig. S8† and Fig. S9†).
Fig. 2 PXRD patterns of samples K + MM + GdL, N + MM + GdL,
T + MM + GdL, T + MM + GdL through washing treatment
before (a) and after (b) carbonization.
XPS analysis of carbonized K + MM + GdL and N + MM + GdL
showed the presence of C, N, and O, with the elements K and
Sn specifically for the sample K + MM + GdL (Fig. 3a, b). The
N1s spectrum can be fitted into four component peaks, which
are located at 398.0, 400.01, 401.6 and 403.0 eV for the
sample K + MM + GdL (Fig. 3c). These peaks are identified as
pyridinic nitrogen (49.62%), pyrrolic nitrogen (31.17%),
quaternary nitrogen (9.13%) and oxidized nitrogen (10.08%),
respectively. Pentagonal pyrrolic nitrogen could be formed at
low temperature, and converted to pyridinic and quaternary
nitrogen as the carbonization temperature increased.34,35 The
N content of 5.7% (atomic percentage) confirmed that the N
doped carbon materials were produced successfully. The
presence of elemental Sn was evidenced by two strong peaks
of Sn 3d5/2 and 3d3/2 around 486.8 eV and 495.2 eV, which
were attributed to ionic tin (Sn4+), with a minor peak of Sn
3d5/2 at 485.1 eV attributed to metallic tin (Sn0) (Fig. 3d). The
oxide component dominated with a relative intensity of
approximately 96%.45 This seems contradictory to the finding
of the PXRD pattern (Fig. 2b) where the strong diffraction
peaks for metallic Sn are observed. Because the XPS only
measures the surface of the materials with little penetration (5
nm), the Sn particles on the material surface were exposed to
air during handling and could be readily oxidized to Sn4+ while
they could maintain the metallic status inside the bulk
material. As the PXRD analyzes the bulk powder sample, the
strong diffractions from metallic Sn are thus observed while
Fig. 3 XPS spectra of the calcined samples K + MM + GdL (a), N
+ MM + GdL (b), and the corresponding closer view and peak
fitting for N1s (c) and Sn3d (d) of the calcined K + MM + GdL.
N2 sorption analysis was employed to investigate the
porosity of the representative carbon nanofiber materials. The
BET surface areas of K + GdL, K + MM + GdL, N + MM + GdL
and T + MM + GdL CNFs were 40, 123, 206, and 124 m2 g-1,
respectively. Such surface areas are typical for CNFs.2 The N2
isothermal curves showed the characteristics of micropores
and some mesopores (Fig. S10†), with most of the pores in the
micropore region formed during the carbonization process.
Table 1. Specific capacitance by CV testing for the CNFs with
PTFE as glue to prepare the working electrode, Pt wire as
counter electrode, Ag/AgCl as reference electrode, and a scan
rate of 1 mV s-1.
sample
K + GdL
Capacitance (F g-1)
25
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Journal of Materials Chemistry A
(a)
(b)
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DOI: 10.1039/C5TA03546E
K + MM + GdL
59
K + MM + GdL (washed)
N + GdL
115
14
N + GdL (washed)
N + MM + GdL
35
45
N + MM + GdL (washed)
T + GdL
109
11
53
Fig. 4 Cyclic voltammograms recorded in 2 M H2SO4 by using
different CNFs coated glassy carbon electrodes as working
electrode with polytetrafluoroethene (PTFE) as glue, Pt wire as
counter electrode, and Ag/AgCl electrode as reference electrode.
The scan rate is 1 mV s-1. (a) Comparison of the CNFs prepared from
K2SnO3 (K) solution; (b) Comparison of the CNFs prepared from TEA
(T) solution. The CNFs prepared from both washed and un-washed
gels were tested.
T + MM + GdL
T + MM + GdL (washed)
T + MM (+ F-127) + GdL (washed)
80
346
CNFs have been widely used as electrode materials.1,5 The
most definite test of an electrode materials is to perform the
test in a fully packaged cell. This is, however, not always
practical. The specific capacitance can be satisfactorily
evaluated by cyclic voltammetry (CV) and galvanostatic
discharge, with the latter method correlating more closely to
how a load is typicaly applied to a supercapacitor in the
majority of applications.47 The electrochemical cells can be a
three-electrode or two-electrode system. The values obtained
can be considerably different.48 In this work, the potential of
the CNFs as electrode material for supercapacitor was
evaluated with a 3-electrode cell and 2 M H2SO4 solution as
electrolyte.49 The cyclic voltammograms of the electrodes
coated with different CNF samples were recorded in a
potential window from 0.1 to 0.8 V (vs Ag/AgCl) (Fig. 4). In
order to see how the CNF morphology may influence the
supercapacitor performance, the CNFs prepared from both
washed and un-washed gels were tested. The specific
capacitance of K + GdL, N + GdL, T + GdL CNFs were quite low
at 25, 14 and 11 F g-1, respectively. For the gelation initiated in
K2SnO3 and TEA solution, the incorporation of MM into the gel
by crosslinking PI molecules led to the CNFs with higher
capacity (Fig. 4a, b, and Table 1). This was the case as well for
gelation in NaOH solution (Table 1). When the as-formed gels
were washed with water/acetone/cyclohexane, individually
separated nanofibers were formed after carbonization (Fig. 1).
The surface area of the CNFs with washing procedure was
higher than that of the CNFs prepared without washing the
gels (Fig. S11). Uniform CNFs structure and large specific
surface area can facilitate penetration of the electrolyte
solution through the electrode materials. The washing
procedure seemed to affect the internal resistance of the
electrode materials as well. For example, the charge/discharge
curves of samples N+MM+GdL with and without washing at 10
A g-1 showed that the samples without washing had a relative
high internal resistance (IR drop) (Fig. S12). This resulted in
enhanced capacitance for all the samples, particularly 115 F g-1
for the washed K + M + GdL carbon sample.
3.3 Nonionic surfactant F-127 templated CNFs and improved
supercapacitor performance
As the gel nanofibers are formed in a self-assembly process, it
is easy to incorporate other molecules during self-assembly.
The use of K2SnO3 as base to dissolve PI was an example for
the preparation of Sn-incorporated CNFs, as described above.
The micropores/mesopores present in the CNFs were formed
during carbonization, but at a small quantity and hence a low
surface area (Fig. S10†). CNFs with a high surface area may be
highly beneficial for different applications.1-6 Non-ionic
surfactant (e.g., Pluronics F-127) templating is a facile way to
fabricate mesoporous CNFs and other type of mesoporous
materials.12,50 Here, in order to produce surfactant templated
CNFs, F-127 was dissolved in MM solution and then mixed with
the PI-TEA solution. The gelation was initiated by adding GdL
at 13 mg cm-3 based on the solution volume. The formed gel
was washed, freeze-dried, and then carbonized in Ar. The
template F-127 was removed simultaneously during
carbonization.
Fig. 5a shows the randomly distributed but separated
nanofibers for the F-127 templated material. N2 sorption gave
a BET surface area at 520 m2 g-1. The isothermal curve in Fig.
5b is typical of a microporous material. Indeed, the density
functional theory (DFT) analysis indicates the pore sizes are
mainly in the micropore region with the main peak centred
around 0.8 nm (Fig. 5b inset). This is different from previous
reports,51 where F-127 templating would usually result in the
formation of mesopores.50-51 We postulate that the
micropores produced in the CNFs here may be the result that
F-127 did not form the micelles (which is a pre-requisite for
preparation of mesoporous materials) when participating in
the PI gelation process. It is likely that F-127 molecular
templating rather than the common micellar templating
occurred in this preparation.
The F-127 templated CNFs produced
a significantly
improved specific capacity of 346 F g-1, compared to a
capacitance of 80 F g-1 for the non-templated CNFs prepared
under the similar condition (Table 1). The CV curve was nearly
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rectangular in shape, indicating good charge propagation results may be doubtful. To address these conceVrinews,Atrthicele Oanrleinae
-2
DOI: 10.1039/C5TA03546E
within the electrode (Fig. S13
†
). Aqueous electrolyte solution loading of the CNFs was increased to 1.13 mg cm . As can be
was used in this study for the CV test. The electrochemical seen in Fig. 5c and 5d, the shapes of the CV curves are nearly
window for aqueous electrolytes is from 0 V to approximately rectangular and the specific capacitance results are
1 V.48 The voltage range used in the CV tests in this study was comparable to the electrode with 0.14 mg cm-2 CNFs. At low
0.1 – 0.8 V49 because there was a significant hump in current scan rates (< 20 mV s-1) or low current density (2 A g-1), the
after 0.9 V, corresponding to irreversible chemical reactions specific capacitance from 1.13 mg cm-2 CNF electrode was
(Fig. S14
from 2 M to 1 M to see how this would impact the capacity by capacitance was higher than the electrode with 0.14 mg cm-2
CV. The two curves were very similar (Fig. S15 ). The specific CNFs when the scan rates or current density increased,
capacitance (204 F g-1) using 1 M H2SO4 aqueous solution as indicating a better rate performance. The specific capacitance
electrolyte solution was slightly less than that (215 F g-1) of achieved at 20 A g-1 was 121 F g-1 (Table S3
). The specific
capacitance of the F-127 templated CNFs is among the very
† & Table S3 † ). However, the specific
).52 The concentration of H2SO4 solution was changed lower (Table S2
†
†
using 2 M H2SO4 solution at the scan rate of 10 mV s-1.
Due to its improved electrode stability, a Nafion solution high values for CNFs.5,15,35 F-127 is a water-soluble poly
was used to impregnate the CNF sample and prepare the (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide)
electrode. A specific capacitance of 320 F g-1 was recorded for triblock copolymer with high molecular weight (MW =12 600).
the rate of 2 mV s-1. The specific capacitance decreased with The self-assembled F-127 acts as sacrificial templates during
the increase of scan rate but still reaching 118 F g-1 at a scan carbonization process. This has led to the formation of CNFs
rate of 200 mV s-1 (Table S2
curves were nearly rectangular (Fig. S16a
†
); whilst the shapes of all the with higher surface area and more interconnected
). The symmetric micropores/mesopores. Particularly, a relatively wide pore size
†
galvanostatic charge-discharge curves were obtained from the distribution from 2-5 nm was observed. In this work, aqueous
same working electrode with current densities ≥ 1 A g-1 (Fig. H2SO4 solution was used as aqueous electrolyte, and the size
S16c
†
). A high capacitance of 192 F g-1 was obtained at the of the hydrated ion [SO42-(H2O)12] is about 5.33 Å.53 The
current density of 1 A g-1. The value was reduced to 105 F g-1 at improved surface area and pore size distribution have
the current density of 20 A g-1 (Table S3
†). The area loading of contributed positively to the movement of the ions, leading to
the CNFs on the working electrode was 0.14 mg cm-2. This improved capacitance.
loading is thought to be low and may not be meaningful for
real supercapacitor applications. The reliability of the test
(a)
(b)
500 nm
(d)
(c)
Fig. 5 F-127 templated CNFs (prepared from the washed gel) and the characterization data. (a) The SEM image showing the
nanofiber morphology. (b) N2 gas sorption isothermal curve and the pore size distribution (inset) calculated by DFT method. (c)
Cyclic voltammograms at different scanning rates. (d) Constant current charge-discharge curves at different current density. The
Nafion-glued CNFs as working electrode with a CNF area loading of 1.13 mg cm-2, Pt sheet as counter electrode, and Ag/AgCl
electrode as reference electrode.
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DOI: 10.1039/C5TA03546E
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ARTICLE
The overall performance of the F-127 templated CNFs gelation process, which produced the CNFs (diameters around
elelctrode is shown in a Ragone plot (Fig. 6a), obtained from 50 nm) with much improved surface area at 520 m2 g-1 and
the galvanostatic charge-discharge data at different current specific capacitance of 346 F g-1 at the scan rate of 1 mV s-1 or
densities.52 The highest gravimetric energy density achieved 192 F g-1 at the current density of 1 A g-1 with a CNF area
was around 13 Wh kg-1. As a trend, the energy density loading of 0.14 mg cm-2. Increasing the CNF loading (1.13 mg
decreased with the increase of power density. To investigate cm-2) produced comparable capacitance results. The specific
the electrochemical stability of the F-127 templated CNF capacitance was lower at low current density but became
electrode
materials,
galvanostatic
charge/discharge higher at the current density > 2 A g-1, with a specific
current capacitance of 121 F g-1 at 20 A g-1. The stability test of the
measurements were performed repeatedly at
a
density of 4 A g-1. Interestingly, the specific capacitance CNFs electrode showed a specific capacitance of 226 F g-1 for
increased slightly with more test cycles (Fig. 6b). This 1000 charge/discharge cycles at 4 A g-1. Although porous
phenomenon perhaps resulted from the activation process. At carbons can be prepared by carbonizing easily available
the initial stage, active materials had not been fully used. After biomaterials and demonstrate some excellent applications, there
repetitive charge/discharge cycles, the electrochemically are very limited controls on porosity, composition and the form
active materials would be fully exposed to the electrolyte. The of the materials (e.g., thin films).54 The method developed here
specific capacitance of the F-127 templated CNF materials may be easily modified and optimized to prepare various
could reach a high value of 226 F g-1 at 1000 charge/discharge doped and functional nanofiberous materials with desired
cycles, indicating that the capacitance of F-127 templated CNF fiber diameter, pore structure, and material format, which are
materials was very stable. This is among the highest specific highly important for a range of applications.
performance for a high current density at 4 A g-1 (Table S4
†
and the relevant referenes). What’s more, the previously
reported methods for fabricating CNFs (such as
electrospinning, templating, and vapor growth) require the use
of harsh conditions or volatile organic solvents. Instead, our
method has used water as solvent and form gel fibers under
mild conditions, which is green and sustainable. Because the
gel fibers can be formed in any vessel with the change of pH in
aqueous solutions, this method may be easily scaled up to
produce a large quantity of CNFs.
4. Conclusions
In summary, we have developed a water-based approach to
prepare CNFs from a perylene diimide derivative via a simple
gelation and carbonization procedure. Triethylamine was used
to help to dissolve and could produce the best discrete CNFs
compared to NaOH and K2SnO3. However, the use of K2SnO3 as
base resulted in the incorporation of Sn/SnO2 into the CNFs.
Melamine was employed to crosslink the precursor molecules
and produce N-doped CNFs after carbonization. It was
necessary
to
wash
the
as-formed
gels
with
water/acetone/cyclohexane in order to produce CNFs with
relatively uniform separated CNFs with the diameters around
20-50 nm. The CNFs showed BET surface areas in the region of
100–200 m2 g-1. The specific capacitance measured by CV was
quite low for the CNFs prepared from un-washed gels
(aggregated carbon nanofibers), but could be improved to over
100 F g-1 for the CNFs prepared from washed gels (separated
carbon nanofibers). More interestingly, a non-ionic surfactant
Pluronics F-127 could be easily incorporated during the
Fig. 6 (a) Ragone plots of the prepared supercapacitor for the
electrode prepared from F-127-templated CNFs. (b) The graph
showing the stability of the F-127-templated CNF electrode for
up to 1000 charge/discharge cycles at current density of 4 A g-1
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with a CNF area loading of 0.14 mg cm-2. The other test
conditions were the same as used in Fig. 5.
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Acknowledgements
XL acknowledges the joint PhD studentship from China
Scholarship Council. The authors are grateful for the access to
the facilities in the Centre for Materials Discovery at the
University of Liverpool. We thank Emily Draper for providing
initial gel samples for carbonization, Prof Dave Adams for
commenting on the manuscript, and Thomas Whittles for
helping with XPS measurement. ZW thanks the NSFC (Grant
No.21127010) and Jilin Provincial Science and Technology
Department (Grant No. 20100701) for financial supports.
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Journal of Materials Chemistry A
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A water-based approach to fabricating CNFs from a perylene diimide derivative (PI) via a simple gelation process
and a carbonization procedure is described. The CNFs materials can be easily produced with doped N or Sn/SnO2.
Importantly, the use of non-ionic surfactant Pluronics F-127 as templates can generate CNFs with a high surface
area at 520 m2 g-1 and high capacitance as electrode materials for supercapacitors.
Keyword: carbon nanofiber, self-assembly, perylene diimide, pyrolysis, capacitance
Authors: Xia Liu, Aled D. Roberts, Adham Ahmed, Zhenxin Wang*, Xu Li, and Haifei Zhang*
Title: Carbon nanofibers by pyrolysis of self-assembled perylene diimide derivative gels as supercapacitor
electrode materials
200
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