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A ‘‘skeleton/skin’’ strategy for preparing ultrathin free-standing single-walled
carbon nanotube/polyaniline films for high performance supercapacitor
electrodes
Zhiqiang Niu,
ab
Pingshan Luan,
ad
Qi Shao,
b
Haibo Dong,
ad
Jinzhu Li,
ad
Jun Chen,
c
Duan Zhao,
ad
Le Cai,
ad
Weiya Zhou,
*
a
Xiaodong Chen
*
b
and Sishen Xie
a
Received 25th April 2012, Accepted 30th July 2012
DOI: 10.1039/c2ee22042c
One of the most critical aspects in the preparation of single-walled carbon nanotubes (SWCNTs)/
conducting polymer hybrid electrodes is to improve the energy density without seriously deteriorating
their high power capability. Here, we report a ‘‘skeleton/skin’’ strategy for the preparation of free-
standing, thin and flexible SWCNT/polyaniline (PANI) hybrid films by a simple in situ electrochemical
polymerization method using directly grown SWCNT films with a continuous reticulate structure as
template. In situ electrochemical polymerization can achieve effective deposition of PANI onto the
surface of SWCNT bundles in the films and control the morphology and microstructure of the
SWCNT/PANI hybrid films. In a SWCNT/PANI hybrid film, the directly grown SWCNT film with
continuous reticulate architecture acts as the skeleton and PANI layers act as the skin. This unique
continuous ‘‘skeleton/skin’’ structure ensures that these hybrid films have much higher conductivity
compared to SWCNT/PANI composite films based on post-deposition SWCNT films. Flexible
supercapacitors have been fabricated using the SWCNT/PANI hybrid films as both electrodes and
charge collectors without metallic current collectors. High energy and power densities (131 W h kg
1
and 62.5 kW kg
1
, respectively) have been achieved for the optimized assembly. The high electrical
conductivity and flexibility, in combination with continuous porous architecture, suggests that the as-
prepared ultrathin free-standing SWCNT/PANI hybrid films have significant potential as promising
electrode materials for thin, lightweight and flexible energy storage devices with high performance.
a
Beijing National Laboratory for Condensed Matter Physics, Institute of
Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail:
wyzhou@iphy.ac.cn; Fax: +86-10-82640215; Tel: +86-10-82649381
b
School of Materials Science and Engineering, Nanyang Technological
University, 50 Nanyang Avenue, Singapore 639798. E-mail: chenxd@
ntu.edu.sg
c
Intelligent Polymer Research Institute, ARC Centre of Excellence for
Electromaterials Science, Australian Institute of Innovative Materials,
Innovation Campus, University of Wollongong, Northfields Avenue,
Wollongong, NSW 2522, Australia
d
Graduate School of the Chinese Academy of Sciences, Beijing 100039,
China
Broader context
The hybrid electrodes of SWCNT/conducting polymer display high energy density due to pseudocapacitance originating from the
conducting polymer. However, their power density is dramatically reduced in comparison with pure SWCNT-based electrodes, due
to the poor electrical conductivity of PANI layers and overlapped PANI–PANI contact. Therefore, one of the most critical aspects in
the development of SWCNT/conducting polymer supercapacitors is to optimize the energy density without deteriorating their high
power capability as these two parameters determine concomitantly the ultimate performance of the supercapacitor. In this work, we
report a ‘‘skeleton/skin’’ strategy to prepare free-standing, thin and flexible SWCNT/PANI hybrid films by a simple in situ elec-
trochemical polymerization method using directly grown SWCNT films with continuous reticulate structure as template. The high
electrical conductivity and flexibility, in combination with continuous porous architecture, suggest that as-prepared ultrathin free-
standing SWCNT/PANI hybrid films have significant potential as promising electrode materials for thin, lightweight and flexible
energy storage devices with high performance. The flexible supercapacitors based on the SWCNT/PANI hybrid films achieve high
energy and power densities.
8726 | Energy Environ. Sci., 2012, 5, 8726–8733 This journal is ª The Royal Society of Chemistry 2012
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Introduction
Supercapacitors, also known as electrochemical capacitors, have
attracted a lot of attention because of their high power and
energy density as well as long cycle life.
1–4
As energy storage
devices, supercapacitors can be applied to many fields, such as
electric vehicles, pulsed power applications and portable
devices.
2
For such applications, it is extremely important to
develop supercapacitors with both higher power density and
higher energy density than those currently available. The
performance characteristics of supercapacitor devices are
fundamentally determined by the structures and electrochemical
properties of electrode materials. Carbon-based materials are
widely used as supercapacitor electrodes because of their desir-
able physical and chemical properties.
3,5–7
Among these carbon-
based materials, single-walled carbon nanotubes (SWCNTs)
have attracted a great deal of attention owing to their high
conductivity, low mass density, large specific surface area and
high mechanical strength.
8–22
Pure SWCNT film-based super-
capacitor electrodes are usually regarded as a competitive
material for high-power electrodes because of their good elec-
trical conductivity and readily accessible surface area.
11,14,15
However, the specific surface area of the pure SWCNT film-
based electrodes is generally low because SWCNTs exist in the
form of bundles, leading to a low energy density.
3,23
Much effort
has gone into improving the energy density of SWCNT-based
electrodes, such as adding conducting polymers into SWCNT-
based electrodes.
17,24–34
The SWCNT/conducting polymer elec-
trodes display high energy density due to pseudocapacitance
originating from the conducting polymer, but, their power
density is dramatically reduced in comparison to the pure
SWCNT-based electrodes, due to the poor electrical conductivity
of conducting polymers. Therefore, one of the most critical
aspects in the development of SWCNT/conducting polymer
supercapacitors is to optimize the energy density without dete-
riorating their high power capability, as these two parameters
determine the ultimate performance of the supercapacitors.
Polyaniline (PANI) is considered to be one of the most prom-
ising electrode materials because of its relatively high conductivity
and lower cost compared to many other conducting polymers.
35–40
Recently, significant research efforts have focused on preparation
of CNT/PANI composites, wherein the PANI is coated onto the
surface of CNTs or bundles of CNTs in the form of powders or
films by various approaches, such as electropolymerization and
chemical oxidation polymerization.
29–32,41
Electrodes based on
SWCNT/PANI powders have usually been achieved by pressing
the composites into tablets or by mixing SWCNT/PANI
composite with conductive binders and coating it onto collector
electrodes.
17,42,43
In these electrodes, the connections between
SWCNT/PANI nanowires or nanorods overlap with the PANI–
PANI contact. Compared to SWCNT–SWCNT contact, the
overlapped PANI–PANI contact has a higher sheet resistance,
leading to a lower power density of such SWCNT/PANI
composite electrodes. Besides, the brittle mechanical nature of
SWCNT/PANI composite electrodes hinders their practical
applications. For the electrodes of mixed SWCNT/PANI
composites and binders, the addition of binders degraded the
electrical and electrochemical properties of SWCNT/PANI elec-
trodes.
3
These SWCNT/PANI composite electrodes cannot meet
the future demands of supercapacitors, which are required to be
thin, lightweight, cheap and flexible. Several groups have inves-
tigated the fabrication of flexible CNT/PANI films by different
methods with the assistance of CNT film templates.
30,31,41
For
instance, Fan et al. reported that the free-standing CNT/PANI
film can be achieved by chemical oxidation polymerization using
CNT Bucky paper and freestanding CNT networks with
randomly entangled individual CNTs and CNT bundles as
template.
31,41
They assembled all-solid-state paperlike polymer
(30 mm in thickness for either electrode component) super-
capacitors with high specific capacitance.
41
Sun et al. demon-
strated the preparation of free-standing CNT/PANI film by
electropolymerization using CNT Bucky paper as template.
30
However, the conductivity of these free-standing CNT/PANI
films was generally lower than 150 S cm
1
.
30,31
In other words,
although the energy densities of these free-standing CNT/PANI
films was improved due to pseudocapacitance originating from
PANI, the power densities of these free-standing CNT/PANI film
electrodes was generally lower than 2.2 kW kg
1
.
31,41
Further-
more, the thickness of CNT/PANI film is still tens of micrometers.
Therefore, fabricating large-area, thin, lightweight and flexible
SWCNT/PANI composite film electrodes with high conductivity
would satiate a large demand. Here, we report a ‘‘skeleton/skin’’
strategy to prepare thin and flexible SWCNT/PANI hybrid films
with high conductivity, in which directly grown SWCNT film with
continuous reticulate architecture acts as the skeleton and PANI
acts as the skin. Thin, lightweight and flexible supercapacitors
have been fabricated using as-prepared SWCNT/PANI hybrid
films as both electrodes and charge collectors without metallic
current collectors. Excellent performance in terms of high energy
and power densities has been achieved.
Experimental
Preparation of SWCNT/PANI hybrid films
The SWCNT films used as template to electrodeposit PANI were
directly prepared by a floating chemical vapor deposition method,
as reported previously.
45
Electrodeposition of PANI was performed
in a traditional three-electrode cell, in which a platinum plate, a
saturated Calomel electrode (SCE) and a SWCNT film were used as
the counter, reference and working electrodes, respectively, as
shown in Fig. 1a. PANI was electrodeposited at a constant current
of 1 mA cm
2
versus SCE in an electrolyte of 0.5 M H
2
SO
4
,0.5M
Na
2
SO
4
and 0.05 M aniline. All chemicals were analytical grade.
The electrodeposition was carried out at room temperature.
Preparation of supercapacitors based on SWCNT/PANI hybrid
films
The freestanding hybrid films were cut into the desired shape and
mass as supercapacitor electrodes. To calculate and compare
specific capacitance conveniently, each SWCNT/PANI film
supercapacitor electrode for CV and charge/discharge measure-
ments has same mass (10 mg). The electrolyte was 1 M nonaqueous
LiClO
4
in a mixture of ethylene carbonate (EC), diethyl carbonate
(DEC), and dimethylene carbonate (DMC) in a volume ratio of
EC/DEC/DMC ¼ 1 : 1 : 1. The separator (Celgard 2325) and
electrolyte were sandwiched by the SWCNT/PANI hybrid films on
the PET substrates.
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Characterization
Cyclic voltammetry (CV) of the SWCNT/PANI hybrid film
supercapacitors was measured by CHI 660C (CHI Instruments).
The galvanostatic charge–discharge of the SWCNT film super-
capacitors at a high operation voltage range (0 to 2.0 V) was
carried out on a supercapacitor test system (BT2000 Arbin). The
morphology and microstructure of the SWCNT films or
SWCNT/PANI hybrid films were characterized by scanning
electron microscopy (SEM, Hitachi S-5200) and transmission
electron microscopy (TEM, JEOL JEM-2010). The thickness of
the SWCNT films and the SWCNT/PANI hybrid films was
measured by atomic force microscopy (AFM, Dimension
Icon) with the NanoScope V Controller. The Raman spectra
were recorded with a spectrophotometer (WITec alpha 300 R)
with operating wavelengths of 633 and 532 nm. FTIR spectra
were recorded on a FT-IR system (Perkin Elmer).
Results and discussion
In situ electrochemical polymerization is an effective process to
deposit PANI onto templates. It can sensitively adjust the
morphology and microstructure of the PANI according to the
template. In situ electrochemical polymerization of PANI was
generally performed in a traditional three-electrode cell, as depicted
in Fig. 1a. Here, we used free-standing directly grown SWCNT
films as a template to prepare SWCNT/PANI hybrid films because
of their high conductivity and unique structure. Free-standing
directly grown SWCNT films possess homogeneity in about 50 cm
2
(Fig. 1b), which provides an opportunity to tailor such SWCNT
films into any desired shape as a template to electrodeposit PANI in
accordance with the device requirement. Besides, the directly grown
SWCNT film is highly conductive with a very low sheet resistance
in the range of 5–50 Ohm per square for the film thickness from 500
to 100 nm. This indicates that directly grown SWCNT films would
be a good candidate as a template to deposit PANI. However, since
the free-standing directly grown SWCNT films are thin (less than
several hundred nanometers) and easily aggregated in electrolyte,
they cannot be used directly as templates to deposit PANI. To
overcome this problem, we spread out and fixed the film onto a
holder with a hole (Fig. 1a), such that two sides of the film were in
direct contact with the electrolyte. Although the directly grown
SWCNT films are thin, these free-standing SWCNT films exhibits a
high tensile strength of 360 MPa, which is 30 times higher than that
ofatypicalbulkypaper.
44,45
Good strength and toughness ensure
that the SWCNT films can be easily handled and can maintain its
structure and integrity during the electrodeposition process, and act
like a skeleton to support the deposited PANI layers. The SEM
image of a SWCNT film (Fig. 1c) shows a nanoporous architecture
without visible impurities, which would be of great significance for
its use as a template to deposit PANI. The thinness and porous
structure of the SWCNT films enable aniline molecules to infiltrate
into the porous films easily and deposit onto the walls of SWCNT
bundles more efficiently.
Fig. 1d displays a typical SEM image of single layer SWCNT
bundles peeling off from a thick film. It clearly illustrates that
the SWCNT bundles in the film are firmly connected with each
other and form a continuous 2D reticulate structure in the plane
parallel to the surface of the film. This unique architecture plays
a key role in the higher strength and conductivity of the directly
Fig. 1 (a) Sketch of PANI electrodeposition using the directly grown SWCNT film as template. (CE: counter electrode, RE: reference electrode, W E:
working electrodes). (b) Optical image of a directly grown SWCNT film with about 200 nm thickness. (c) SEM image of a directly grown SWCNT film.
(d) SEM image of a single layer of SWCNT bundles peeled off from a thick film. (e) TEM image of a SWCNT junction in the directly grown film. (f)
SEM image of a single layer of SWCNT/PANI bundles peeled off from a thick SWCNT/PANI film. (g) TEM image of SWCNT/PANI in the hybrid film.
(h) Optical image of SWCNT/PANI hybrid film (size: 2 cm 2 cm, thickness: 240 nm, deposition time: 30 s).
8728 | Energy Environ. Sci., 2012, 5, 8726–8733 This journal is ª The Royal Society of Chemistry 2012
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