2015 nanocarbon foam: fabrication, characterization and
TRANSCRIPT
Florida State University Libraries
2015
Nanocarbon Foam: Fabrication,Characterization and ApplicationTeng Liu
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FLORIDA STATE UNIVERSITY
THE GRADUATE SCHOOL
NANOCARBON FOAM: FABRICATION, CHARACTERIZATION AND APPLICATION
By
TENG LIU
A Thesis submitted to the
Materials Science and Engineering Program
in partial fulfillment of the
requirements for the degree of
Master of Science
2015
ii
Teng Liu defended this thesis on November 5, 2015.
The members of the supervisory committee were:
Eric Hellstrom
Professor Co-Directing Thesis
Mei Zhang
Professor Co-Directing Thesis
Richard Liang
Committee Member
Zhibin Yu
Committee Member
The Graduate School has verified and approved the above-named committee members, and
certifies that the thesis has been approved in accordance with university requirements.
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Dedicated to: my parents.
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ACKNOWLEDGMENTS
First and foremost, I offer my sincerest gratitude to my supervisors, Dr. Eric Hellstrom and
Dr. Mei Zhang, who have supported me throughout my thesis with their patience and knowledge
whilst allowing me the room to work in my own way. I attribute the level of my Masters degree to
their encouragement and effort and without them this thesis, too, would not have been completed
or written. One simply could not wish for a better or friendlier supervisor.
I would also like to thank my committee members, Dr. Zhiyong Liang and Dr. Zhibin Yu
for always providing necessary suggestions, necessary critique and intellectual comments and
recommendations to better this study.
Finally, I would like to thank my parents for their constant love, support and without whom
I would never have enjoyed so many wonderful opportunities.
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TABLE OF CONTENTS
List of Tables ................................................................................................................................ vii
List of Figures .............................................................................................................................. viii
Abstract ........................................................................................................................................... x
INTRODUCTION .......................................................................................................................... 1
1.1 Motivation ............................................................................................................................. 1
1.2 Problem Statement and Research Objectives ....................................................................... 3
LITERATURE REVIEW ............................................................................................................... 6
2.1 Fabrication of C3PMs ........................................................................................................... 6
2.1.1 Overview ........................................................................................................................ 6
2.1.2 Introduction to Growing C3PMs by Chemical Vapor Deposition ................................. 7
2.1.3 Fabrication of Self-assembled C3PMs through Solution - Based Route ..................... 10
2.1.4 Other Techniques for the Generation of C3PMs ......................................................... 15
2.2 Properties of C3PMs ........................................................................................................... 18
2.2.1 Introduction .................................................................................................................. 18
2.2.2 Physical Properties ....................................................................................................... 18
2.2.3 Mechanical Properties .................................................................................................. 20
2.2.4 Transport Properties ..................................................................................................... 23
2.2.5 Summary of Properties ................................................................................................ 27
2.3 C3PMs for Engineering Applications ................................................................................. 28
2.3.1 Energy Storage ............................................................................................................. 28
2.3.2 Sensors ......................................................................................................................... 33
2.3.3 Water Treatment .......................................................................................................... 35
2.3.4 Other Miscellaneous Applications ............................................................................... 36
METHODOLOGY ....................................................................................................................... 39
3.1 NCF Fabrication ................................................................................................................. 39
3.1.1 NCF Fabrication Principles .......................................................................................... 39
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3.1.2 Fabrication Process ...................................................................................................... 39
3.2 Characterization of NCF ..................................................................................................... 42
3.3 Engineering Device Characterization ................................................................................. 44
RESULTS AND DISCUSSIONS ................................................................................................. 45
4.1 Structure of NCF ................................................................................................................. 45
4.1.1 Morphology of NCF .................................................................................................... 45
4.1.2 Pore Characteristics of NCF ........................................................................................ 47
4.1.3 Volume Density ........................................................................................................... 50
4.2 Mechanical Properties of NCF ........................................................................................... 52
4.2.1 Characterization of Superelasticity .............................................................................. 52
4.2.2 Synergistic Mechanism ................................................................................................ 56
4.3 Electrical Conductivity ....................................................................................................... 59
4.4 Engineering Application ..................................................................................................... 59
4.4.1 Flexible Conductors and Strain-gauge Sensors ........................................................... 60
4.4.2 Cathode Materials for Lithium-air (Li-air) Battery...................................................... 63
SUMMARY AND CONCLUSIONS ........................................................................................... 67
5.1 Conclusions ......................................................................................................................... 67
5.2 Recommendation for Future Research ............................................................................... 68
5.2.1 NCF Structural Improvement and Modelling .............................................................. 68
5.2.2 NCF as a Skeleton to Fabricate Hybrid Systems ......................................................... 69
APPENDIX A. COPYRIGHT ...................................................................................................... 72
References ..................................................................................................................................... 97
Biographical Sketch .................................................................................................................... 109
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LIST OF TABLES
Table 1 Physical properties of C3PMs via versatile synthetic routes. .......................................... 19
Table 2 Pore characteristics of NCF with different level of pore sizes. ....................................... 50
Table 3 Electrical conductivity of NCF compared with C3PMs fabricated through versatile
techniques. .................................................................................................................................... 59
Table 4 Parameters of different cathodes for capacity study. ....................................................... 63
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LIST OF FIGURES
Figure 1 Improvement of density for carbon-based aerogel. .......................................................... 2
Figure 2 Improvement and development of C3PMs in the past decade. ........................................ 4
Figure 3 SEM images of SWNT forest. .......................................................................................... 8
Figure 4 Optical and SEM images of CNT sponges. ...................................................................... 9
Figure 5 Optical and SEM images of N-MWNT sponges. ........................................................... 10
Figure 6 Optical and SEM images of PVA-reinforced aerogel. ................................................... 13
Figure 7 Optical and SEM images of SWNT aerogel................................................................... 15
Figure 8 SEM images showing the morphology and structure of CNT foams (CNT/PMMA = 1/10)
after treatment at 1000 °C. ............................................................................................................ 16
Figure 9 Optical and SEM images of CNT-foam. ........................................................................ 17
Figure 10 SEM image and schematic model of a CNT forest. ..................................................... 21
Figure 11 Mechanical properties of graphene-coated aerogels.. .................................................. 23
Figure 12 Illustration of the electrical properties for C3PMs. ...................................................... 25
Figure 13 Thermal conductivities of C3PMs. ............................................................................... 27
Figure 14 Supercapacitor performance of sponges under compression.. ..................................... 29
Figure 15 Morphology and performance of SWNT/PANI nanoribbon composite aerogel – based
lithium storage device. .................................................................................................................. 31
Figure 16 Illustration and photoelectric conversion of the DSCs with CNT sponge. .................. 32
Figure 17 Illustration of the use of C3PMs in sensors. ................................................................. 34
Figure 18 Environmental application of CNT sponges. ............................................................... 35
Figure 19 Normalized resistance of SWNT-aerogel/PDMS composite films as a function of
uniaxial tensile strain along the direction of conduction. ............................................................. 36
Figure 20 Illustration of the use of C3PMs in desalination and other applications. ..................... 37
Figure 21 TGA spectrum for CNT and PMMA in (a) air environment; (b) N2 environment. ..... 40
Figure 22 Design, processing and porous architectures of NCF (ρ = 25.6 mg·cm3). ................... 41
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Figure 23 Morphology of NCF. .................................................................................................... 46
Figure 24 SEM images of microscopic architecture of NCFs with pore size of: (a) 4-8 m; (b) 8-
11 m; (c) 21-25 m; (d) 25-30 m; (f) 53-63 m. ..................................................................... 46
Figure 25 Nitrogen adsorption–desorption isotherms (a) and nano-level pore size distribution
curves (b) of NCF with different macro-level pore size. .............................................................. 48
Figure 26 SSA results by fitting a linear regression through the BET plots. ............................... 49
Figure 27 Plotting of volume density of NCF according to the weight ratio of polymer to CNT.
Inset of the figure shows lightweight of NCF. .............................................................................. 51
Figure 28 Typical compressive stress–strain curves of the NCF. ................................................. 52
Figure 29 Compressive performance of NCF. .............................................................................. 54
Figure 30 Compressive behavior of NCF with (a) different densities; (b) different PAN ratio. .. 55
Figure 31 In situ SEM characterization of NCF under compression and release process. ........... 56
Figure 32 TEM images and mechanical evaluation of crosslink effect. ....................................... 57
Figure 33 Schematic representation shows the mechanism by which it makes NCF networks
superelastic. ................................................................................................................................... 58
Figure 34 Piezoresistive behaviors of the NCF. ........................................................................... 61
Figure 35 Stability of NCF-based sensor. ..................................................................................... 62
Figure 36 Morphology and discharge performance of Li-air cells with NCF cathodes. .............. 64
Figure 37 Electrochemical impedance spectra of a Li-air cell. .................................................... 65
Figure 38 Morphology and compressive behavior of NCF-Epoxy composites. .......................... 69
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ABSTRACT
This thesis is a continuous effort contributed to the field of developing a new type of
functional porous materials - Nanocarbon Foam (NCF) by crosslinking multi-walled carbon
nanotubes (MWNTs) into networks in three-dimensional (3D). Synthetic routes and
characterizations of NCF, and their applications as strain-gauge sensors and electrode materials in
lithium-air (Li-air) battery are described.
In this research, the first accomplishment is proposing a robust methodology for creating
superealstic 3D macroscopic NCF with controlled cellular structure. The key contributions contain:
(1) understanding the premise of the design that gives the NCF with desired structure and porosity;
(2) designing fabrication protocol for NCFs with controlled densities and macroscopic structure;
(3) fabricating varied NCF with tunable porosity and structures, which in turn will endow the NCF
with different characteristics. This experimental methodology for systematic and quantitative
investigation of the processing-structure relationships provides a means for the fabrication
optimization of NCF with desired structures.
Though the mechanical, electronic, and thermal properties of CNTs have been extensively
studied, for NCF that is a mixture of pristine and functionalized CNTs, it will not only have the
collective behavior of the individual tubes, but will also have properties generated from the
interactions between the tubes and engineered components. To understand the structure-properties
relationship of NCF, the second accomplishment is studying the properties of obtained NCFs.
Density, specific surface area, porosity, compressive behavior, mechanical robustness, electrical
and electromechanical properties of NCF have been characterized in details. For comparison,
properties originated from cellular structures built of other materials, such as polymeric foam, fiber
aerogels, etc., are compared with that of NCF.
Moreover, some engineering applications of NCF have been discussed. With the unique
features of NCFs, my proposed future work will focus on understanding porous structure formation
and resulted unique properties by the means of scientific modelling. In addition, NCF will be
explored as the skeleton for fabricating hybrid systems.
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CHAPTER 1
INTRODUCTION
1.1 Motivation
Over the past several decades, interest in nanoscale materials has grown gradually, because
their unique properties relative to their bulky counterparts provide a good foundation or potential
applications for practical purposes. Recently, nanostructure carbon allotropes, one-dimensional
(1D) carbon nanotubes (CNTs) [1-3] and two-dimensional (2D) graphene [4-7] have drawn
unprecedented attention as a result of their novel properties，including high electrical conductivity
(up to 400,000 S cm-1) [8], excellent thermal conductivity (as high as 5000 W m-1) [9, 10], superior
mechanical properties with elastic moduli over 1 TPa [11] and outstanding applications in the
fields of electronics, sensors, catalysis, and energy related systems. It is of fundamental and
practical eminence to scale novel properties of individual CNT into macroscopic three-
dimensional (3D) porous structures, which take advantage of CNT’s flexibility, excellent electrical
and thermal properties, and mechanical integrity [12]. Therefore, tailored assembly of CNTs into
3D porous architectures is of scientific and technological significance to translate the intrinsic
features of individual CNT to a macroscopic level [13, 14], thereby allowing for some
unprecedented properties.
The emergent technologies, including energy storage, sensors, water treatment etc., have
driven great demand in synthesis of lightweight, elastic and robust materials with controlled
porosity to provide a variety of functionalities such as high surface area, low density, mechanical
integrity and great transport properties [15]. The scientific and engineering challenge is to design
and fabricate these structures in practical dimensions while maintaining accurate control over their
chemical and physical properties at the nanoscale. Carbon materials are extremely appealing with
their low density and versatility in many applications. Inspired by some natural structures [16],
creating 3D porous aerogels or foams with open or closed-cell cellular structure and high
continuity could be a strategy for achieving promising performance for widespread applications.
However, the inherent limits on the diversity of bulky carbon materials, combined with the lack of
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precise control of the physicochemical properties, present major challenges in the fabrication of
3D porous aerogels or foams that must be addressed before their extensive practical applications.
Carbon nanotubes, which are at the forefront of carbon-based nanomaterials, combine high
mechanical strength, low density, high flexibility, extremely high aspect ratio and high thermal
and electrical conductivity [8] [11]. CNTs hold great promise as an exceptional nanoscale building
block for constructing CNT-based 3D hierarchically porous materials (C3PMs). Integrating the
porous features and properties of CNTs is a feasible and highly efficient strategy to obtain
enhanced electrical and mechanical properties. First, porous structures can be designed to show an
increase of active material per predicted area, and demonstrate well-defined porosity. Second, the
diversity of synthetic routes for 3D porous structures aids the design and fabrication of multiple
porous structures, which are capable of exhibiting desirable properties for a range of functions.
Last but not least, 3D porous architectures made of CNTs possess a low weight advantage in many
applications [17].
Figure 1 Improvement of density for carbon-based aerogel.
The past several years have witnessed an increased number of works directed at fabricating
different C3PMs. One interesting phenomenon is that 70% of all the reviewed works correspond
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to the period of 2011-2015. The main reason for this growth trend is due to the superior
performance of the 3D architecture compared to 2D or 1D architectures, in which the enhancement
of the third dimension provides better load bearing for active materials. C3PMs have been and will
continue to be widely employed in various important applications such as sensors for detecting
pressure, gas and chemical vapor; as catalysts for chemical adsorption, photocatalysis, and water
treatment; as electrodes in supercapacitors, microbial fuel cells, lithium-ion batteries, and dye-
sensitized solar cells; as environmental materials for chemical removal, and capacitive
desalinization process; and as biomaterials for tissue engineering. Therefore, these high value
applications have driven the development of reliable routes for fabrication of C3PMs.
Researchers have done relevant work on the carbon-based porous materials over past three
decade, here carbon-based porous material refers to the carbon based-aerogel. Since for special
uses under many circumstances, the application always requires materials with low density when
being used for aerospace engineering and composite materials. However, the density of materials
always keeps at a high level while keeping other properties is needed. Luckily, researchers have
addressed this issue by using CNT and graphene as the building blocks to fabricate carbon based-
aerogel [17]. The obtained aerogel can have superior mechanical, electrical and thermal properties
at extremely low density level, that can be even smaller than the density of air. All those effort can
tell that new generation of those CNT and graphene give the hope to fabricate better C3PMs.
Therefore, the motivation to develop a new generation of C3PMs with controllable
structures based on CNT, graphene and other derivatives is to for emerging applications.
1.2 Problem Statement and Research Objectives
In the past decade, the understanding of C3Pεs’ structures and properties has been greatly
improved and C3PMs have penetrated many engineering fields and created considerable
applications (Figure 2). However, scaling the properties of individual CNTs (e.g. high specific
surface area, mechanical strength, and transport properties) to a macroscopic level—is still being
investigated. For example, the strength of most reported CNT-based 2D or 3D assemblies is
significantly lower than that of individual CNTs (up to 100 GPa). Two reasons may account for
this issue: the structure–property relationships of C3PMs have not been well demonstrated because
4
of the complexity of 3D macroscopic assemblies, and we do not yet have efficient fabrication
methodology to realize precise control over the structure.
Figure 2 Improvement and development of C3PMs in the past decade.
In combination with advanced characterization techniques, it is crucial to develop simple
methods for fabrication of well-defined CNT-based 3D hierarchical structures, so each class of
pores benefits from synergy effects. By carefully selecting synthetic methods, designing
appropriate crosslinking forces, and introducing functional ingredients, C3PMs with superior
properties can be readily produced. Currently, however, the field of C3PMs is far from mature,
some problems and issues are as followed. First, cutting down the cost of CNT precursors by
production technique development is required for their commercialization and competition with
other materials. Second, combining the advantages of both direct growth and solution-based
method for C3PMs is highly desired. In this way, one can facilely investigate correlations between
material performance and the precursors (e.g., different CNTs, other additives) to find general
rules for efficient design and desired functions, by the smart introduction of extra ingredients
(molecules or heteroatoms), and achieve optimal performance from high-quality individual CNTs.
Third, elegant fabrication approaches and relevant applications should be learned from other fields
and other materials, since the C3PMs field is a subject that requires interdisciplinary knowledge.
The goal of this research is to study experimentally the fabrication mechanisms and establish the
optimizing principles of the contribution factors on the overall performance of NCF, a unique
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C3PMs. By drawing on understanding of fabrication mechanisms, a novel NCF and NCF-based
system were realized. The tasks associated with the experimental and theoretical model
development are listed below:
Design an elegant approach to realize the tailored assembly of CNTs into 3D architecture
which can provide the possibility to translate individually superior properties to larger scale
with a significant increase of active materials per projected area.
Develop methodology for effective control of the porous architecture of NCFs to provide the
foundation to exhibit better properties.
Develop effective synthetic routes to have controllable flexibility and robustness for the new
generation of dynamic carbon systems.
Discover the potentials engineering applications of NCFs, such as sensors and energy storage
devices which show advanced performance compared with traditional form of CNT-based
materials.
Completion of this this research will advance the knowledge and understanding on how to
obtain well-designed NCF for various engineering applications.
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CHAPTER 2
LITERATURE REVIEW
New research based on the C3PMs may provide relatively efficient and inexpensive ways
for ubiquitously fabircating unique new mateiral. This chapter aims at providing an in-depth
understanding of available synthetic methodology and routes. It will evaluate the pertinent issues
on the development of CNT-based 3D porous materials to determine the knowledge that need to
be filled for viability. The application of C3PMs for energy storage, sensing system, composites,
etc have been explored. This chapter focuses on efforts and progress of C3PMs fabrication
methods, and explores the utilization of structures in these advanced applications. The methods of
fabricating different kinds of C3PMs are described in details. Additionally, a discussion of the
properties of C3PMs made through different synthetic routes is presented. This includes direct
growth, self assembly and other methods, highlighting their main features, advantages,
disadvantages and other related aspects. Applications in engineering fields and future prospects
are discussed in this review chapter.
2.1 Fabrication of C3PMs
2.1.1 Overview
C3PMs are 3D macroscopic CNT-based hierarchically porous structures, which are named
foams, sponges, or aerogels in various situations. In this thesis, sponge, aerogel, or foam will also
be with respect to the original papers. C3PMs have attracted significant attention as a means of
expanding the significance of CNTs, both in the fundamental sciences and in practical applications,
due to their high surface area, high mechanical strength, elasticity, excellent conductivity, and
significant adsorption characteristics at extremely low density. Generally, the fabrication of
C3PMs can be classified into two categories: direct growth route and solution-based route. The
following subsections will address recent progress on the rational design and preparation of
C3PMs.
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2.1.2 Introduction to Growing C3PMs by Chemical Vapor Deposition
CNT can be made directly by three major techniques: arc-discharge, laser ablation and
chemical vapor deposition (CVD) [18]. Among the synthesis techniques, CVD is the most
frequently used technique for the CNT preparation. The general procedure is to decompose the
carbon sources on the surface of catalyst particles at desired pressure and temperature, then the
CNTs grow from the catalyst particles based on the vapor-liquid-solid (VLS) mechanism [19].
CVD is also the most effective way for scalable manufacturing where a large number of aligned
and oriented CNTs having micro to macro lengths is desirable [20-24]. Therefore, CVD has been
applied to fabricate the C3PMs with high-quality and long individual CNTs. C3PMs formed by
either by vertically aligned or randomly interconnected CNTs porous structure, are both open-cell
foam system. Vertically aligned CNT arrays represents one of the open-cell C3PMs that inter-
nanotube space function as interconnected cells [25]. Hata et al. reported early work on preparation
of vertically aligned single-wall nanotube (SWNT) arrays using CVD method (Figure 3) [25].
Large-scale synthesis of SWNTs used to suffer from the production of impurities that must be
removed through purification steps, which can damage the SWNTs. To address this problem,
water–assisted CVD approach was used to remove the amorphous carbon from the catalyst surface,
which helped form vertically aligned SWNT arrays in the millimeter scale. It was demonstrated
that the activity and lifetime of the catalysts were improved by the addition of a controlled amount
of water, balancing the relative levels of precursors and water was also crucial to maximize
catalytic lifetime. In this respect, the best result was 2.5 mm high SWNT forest grown in 10 min
[25]. Since these pioneering works, the CVD technique has been generally applied and modified
for the fabrication of C3PMs. As another typical example, Dai and co-workers presented a
molecular oxygen-assisted plasma enhanced CVD (PE-CVD) approach to prepare high yields of
vertically aligned SWNT arrays [26]. Ci et al. applied the water-assisted CVD route to grow multi-
walled, freestanding CNT arrays, which gave the extension for the CVD techniques to build 3D
CNT arrays [27]. In general, tube length and substrate choice always limit the potential
applications. Thus, it will be beneficial to grow longer tubes based on new substrates for diverse
situations. Ajayan and co-workers advanced the floating catalyst CVD technique to grow
freestanding CNTs arrays ca. 1mm high with supercompressibility [28]. They employed the same
technique to fabricate CNT arrays on metallic substrates, which exhibited enhanced electron field
8
emission properties [29]. This approach started from a solution made by dissolving 1 g of ferrocene
in 100 ml xylene, which was injected into a steel bottle with a syringe pump. The main idea behind
this approach is to take advantage of oxidation resistance of Inconel, which benefits both for
growth and high-temperature applications. In this study, every individual nanotube is electrically
end-contacted and metallized for realistic devices. The same authors also created a structurally
tunable 3D substrate based on vertically aligned CNT arrays, which exhibited outstanding surface
enhanced Raman scattering performance [30]. Recently, Shanov et al. reported the growth of
record-long (21.7 mm) vertically aligned MWNT arrays [31]. In this case, the ratio of ethylene
and H2 as well as water concentration were well optimized to prolong the catalyst lifetime.
Yaglioglu and co-workers recently reported that tuning the CNT diameter and density of
vertically aligned CNTs grown by fixed catalyst or floating catalyst CVD techniques resulted in
profound differences in mechanical properties [32]. This investigation demonstrated the fixed
catalysts method yielded low density CNT forests, and the floating catalyst method yielded high
density CNT forests.
Figure 3 SEM images of SWNT forest. (a) SEM image of a 2.5 mm-tall SWNT forest. Scale bar
is 1 mm. (b) SEM image of the SWNT forest ledge. Scale bar is 1 mm. Hata, K., et al, 2004.
Science. 306: 1362-1364. Copyright © 2004, American Association for the Advancement of
Science.
Alternatively, randomly interconnected CNTs arrays can also be prepared through the
CVD technique. For example, Gui et al. demonstrated the great effort in preparation of the
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macroscopic CNT sponge in which CNTs were randomly self-assembled into a 3D framework
with high flexibility and robustness (Figure 4) [33]. They accomplished this by employing a
modified floating catalyst CVD technique. It should be emphasized that the introduction of
different carbon sources (dichlorobenzene) was used to interfere with the aligned growth of CNTs,
and therefore the CNTs were randomly stacked into an interconnected structure. The obtained
CNT sponge consisting of self-assembled CNT frameworks showed high mechanical flexibility
and robustness. In another case, Hata and co-workers created a viscoelastic C3PM made of a
random network of long CNTs [34]. The main strategy to make this viscoelastic CNT material was
to combine reactive ion etching (RIE) to the catalyst film with water-assisted CVD in order to
lower CNT area density as well as assemble traversing long CNTs with a very high density of
intermittent physical interconnections, analogous to aggregating hair. RIE exposure reduced the
catalyst density to construct randomly oriented CNTs. Simultaneously, water-assisted CVD
technique synthesized long and clean CNTs. Final compression process increased material density.
The as-prepared CNT material exhibited temperature-invariant viscoelasticity from -196°C to
1000°C.
Figure 4 Optical and SEM images of CNT sponges. (a) Optical image of carbon nanotube
sponges. (b) Cross-sectional SEM image of the sponge. (c) Illustration of the sponge. Gui, X., et
al, 2010. Adv Mater 22: 617-621. Copyright © 2010, WILEY-VCH Verlag GmbH & Co. KGaA,
Weinheim.
Moreover, the CVD approach is the most effective technique to achieve the doping of
heteroatoms into C3PMs. Theoretical and experimental studies have demonstrated that
heteroatoms, including boron, nitrogen and sulfur can induce tubular morphology changes in CNT,
such as nanojuctions and enhance specific capacitance, cycle life and power capability of the
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assembled electrochemical devices [35, 36]. For example, following the fabrication of pristine
CNT sponges, Hashim et al. fabricated covalently bonded 3D boron-doped MWNT (B-MWNT)
solids via boron induced nanojunctions using a CVD method [37]. The boron dopant was believed
to play a key role in creating “elbow-like” junctions and covalent nanojunctions, which endowed
the 3D B-MWNT excellent mechanical flexibility. Shan et al. reported the facile synthesis of 3D
sponge-like N-doped MWNT (N-MWNT) sponges [38]. The N and S are synergistically
promoting the formation of “elbow” and “welded” junctions between CNTs. It was interesting to
find that the microstructures of N-MWNTs can be easily controlled by adjusting thiophene
concentration, which in turn enhances the mechanical and electrical properties of N-MWNT
sponge (Figure 5) [38].
Figure 5 Optical and SEM images of N-MWNT sponges. (a) Photo of N-MWNT sponges. (b)
SEM images of N-MWNT sponges. Shan, C., et al, 2013. Nano Letters 13: 5514-5520. Copyright
© 2013, American Chemical Society.
2.1.3 Fabrication of Self-assembled C3PMs through Solution - Based Route
The technological promise of ordered 3D CNTs networks comes from the fact that their
structure, porosity, and thus their properties can be well controlled through the proper approach.
Several parameters of the solution-based technique have a significant impact on the formation of
C3PMs in a controlled manner, and in turn the solution-based technique provides one ideal
pathway to fabricate C3PMs. The starting material for the fabrication of C3PMs is the CNT powder
which needs to be dispersed into solution, with or without the aid of support agents under the right
conditions. This step can be performed with the addition of crosslinking agents or more
11
environmentally friendly dispersion aids. In this case, it is required to disperse the CNTs into a
certain solution, which is either easy for dispersion aids being removable or transferable into
“useful parts” (binders or cross-linkers) to further improve the mechanical and electrical properties
of the formed C3PMs. After the formation of CNT dispersions, it is always followed by a common
drying and/or pyrolysis process for keeping the desired porosity in the final product. Critical point
drying and freeze drying techniques are two of the major drying methods. In addition, for each
method, different dispersing species, additives and temperatures should be adjusted to get final
products. Compared to direct growth of C3PMs, a solution-based route has several advantages: it
is a process which helps to form the CNTs dispersion homogeneously; it is a process which allows
milder synthetic conditions to easily tune the structure and property of final products during the
process. The interactions between CNTs are strong enough that solutions could be directly frozen
to produce the porous structure effectively.
As the dispersion process for CNTs has been well-developed [39-42], the following
subsections will give some detailed discussions on these sol-gel transition methods including direct
drying and some other novel preparation routes.
2.1.3.1 Direct Drying for C3PMs Preparation with Dispersion Agents
The starting material to generate C3PMs is always a hydrogel or organogel from the
dispersion which provides a matrix to form the C3PMs with tailored structures and properties. The
gel commonly consists of a solid network which interacts through chemical or physical bonds. To
produce the C3PMs by this method, drying the hydrogels or organogels is the key step for the
formation of freestanding C3PMs. However, during the drying process, one big issue is the
capillary force caused by the evaporation of water, which will collapse the porous structure.
Utilizing critical point drying and freeze drying techniques can avoid the structure collapse
significantly. Generally speaking, the freeze drying method would control the capillary stress by
going around the triple point boundary of water, which would lower the temperature of water and
make the phase go from solid to the gas phase under low pressure. However, during the freeze
drying process, water crystallization is a crucial issue, which should be controlled to prevent
disrupting the sample [43]. Regarding this, critical point drying technique provides the advantage
to avoid any phase boundary. In the case of water, its high critical point (374°C) may damage the
porous structures during the critical point drying process. Therefore, acetone is always chosen for
12
exchanging of water and washed away with liquid CO2. Because CO2 is an ideal supercritical fluid
candidate that exhibits its critical point at only 1000 psi and 31°C [44]. After, assembly, the
structure was dried to prepare C3PMs. Actually, various dispersion agents for C3PMs formed by
the direct drying method can be found in the literature in which surfactants and crosslinkers are
used.
2.1.3.2 Surfactants Enhanced Methods for Direct Drying
Triton X-100, sodium dodecylbenzene sulfonate (NaDDBS), and sodium dodecyl sulfate
(SDS) are the most extensively used surfactants [45]. During the dispersion process, the surfactants
adsorb on the nanotubes surface. An additional ultra-sonication treatment of aqueous dispersions
may help to debundle nanotubes by providing high local shear, particularly to the nanotubes end.
For example, a notable achievement in this effort has been the use of the ionic surfactant NaDDBS
by the Yodh’s group [46, 47]. The authors measured the small-angle neutron scattering (SANS)
intensity profiles for NaDDBS stabilized SWNT suspensions as a function of SWNT concentration.
Rigid rod behavior was observed at large Q and a crossover to network behavior was observed in
stable nanotube suspensions and gels. To make the SWNT suspensions, elastic gels prepared from
suspensions with different concentrations of CNTs were soaked in an aqueous polyvinyl alcohol
(PVA) bath for network reinforcement (Figure 6). The samples were then subjected to critical point
drying to create the CNT aerogel [46]. The comparison between PVA-reinforced gels and non-
reinforced gels shows that the reinforcement contributed to exceptional mechanical properties
(Figure 6b, better rigidity, able to hold 8000 times its own weight) albeit at the cost of conductivity
(10-5 S cm-1). Recently, Ostojic reported a fabrication method based on SDS surfactant aided
SWNTs solution via novel DNA/protein complex-assisted assembly to induce SWNT gelation
[48]. The fabricated SWNT aerogel resulted from ethanol and ethanol–water solvent exchange and
subsequent critical point–drying exhibited a previously unobserved peak at 1.3 eV that corresponds
to the phonon-assisted photoluminescence emission. In another interesting example,
Tanthapanichakoon and co-workers dispersed MWNTs in a carboxymethyl cellulose (CMC)
sodium salt solution and then fabricated the macro-porous solid foam by freeze-drying [49]. It was
found that the prepared solid foams displayed hybrid characteristics from the original MWNTs,
and the resulting bulk matrix with a controlled macro-pore system. The solid foams can also
13
recover to their original shapes after being repeatedly squeezed, indicating good elastic and
structure memory properties. Moreover, the electrical resistivity of a bulk sample responded
rapidly and sensitively to a change in the ambient pressure, which shows potential for the strain-
gauge sensor application.
Figure 6 Optical and SEM images of PVA-reinforced aerogel. (a) Three PVA-reinforced aerogel
pillars (total mass = 13.0 mg) supporting 100 g, or ca. 8000 times their weight. (b) SEM
micrograph revealing the macroporous architecture of PVA-reinforced aerogel (0.5 wt % PVA,
10 mg mL-1 CNT content). Bryning, M.B., et al, 2007. Adv Mater 19: 661-664. Copyright © 2007,
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
2.1.3.3 Crosslinking Enhanced Methods for Direct Drying
It is noteworthy that in most cases, it is difficult to realize the formation of gel by pure
CNTs because of weak inter-tube π-π stacking forces. Therefore the “cross-linkers” were
introduced to facilitate constructing the porous structure, these “linkers” are primarily made of
organic solvents and polymers. Sol–gel polymerization of the precursors in the presence of the
additives leads to the formation of composite structures in which the additive becomes a part of
the primary carbon network structure. This approach was used to enhance the electrical, thermal
and mechanical properties of the composite material relative to pure carbon aerogel (CA) as
recently demonstrated for CNT additives. For example, organic solvents of resorcinol-
formaldehyde and furfuryl alcohol (FA) provide promising results. Worsley and co-workers have
extensively studied the formation of 3D SWNT and Double-Walled CNT (DWNT) 3D porous
architectures prepared by resorcinol-formaldehyde polymerization [50]. By adopting relatively
14
low concentration of organic sol gel precursor to a suspension of highly purified SWNTs, the
process was used to form the conductive glue which could hold the CNT assembly together. The
subsequent supercritical drying and pyrolysis process converted the organic ingredients to carbon
as binder. The resulting CNT foam exhibited elastic behavior up to strains as large as ~ 80%. The
same authors also fabricated the CNT aerogel by varying the SWNT loading from 0 wt% to 55
wt% [51]. Electrical conductivities were improved by an order of magnitude for the SWNT-CA
(55 wt% nanotubes) compared to those of foams with lower weight percentage of nanotubes.
Shrinkage of the aerogel monoliths during carbonization and drying was also almost completely
eliminated.
Chen and co-workers formed a 3D assembly of CNTs in solution to induce gelation by
using two chemical cross-linkers: ferrocene-grafted poly(pphenyleneethynylene) (Fc-PPE) and
ferrocene-grafted poly[(p-phenyleneethynylene)-alt-(2,5-thienyleneethynylene)] (Fc-PPETE),
followed by CO2 supercritical drying process (Figure 7) [52]. The obtained thermally annealed
aerogel exhibited large specific area (up to 680 m2 g-1) and excellent electrical conductivity. Tasis
and co-workers prepared a porous multi-walled CNT – based aerogel by using PVA as the
structural binder [53]. The obtained aerogel showed thermal incandescence below 200 °C and
enhanced catalytic activity for the oxidation of CO. PVA reinforcement can also be combined with
surfactant – based dispersion aids to form CNT aerogel as cited above [46]. More recently, with
the aid of different polymers, for instance, the homogeneous aqueous suspension of HNO3–treated
MWNTs can be achieved by using chitosan as a binder [54]. Regarding this, well aligned, cellular
CNT macrostructures were fabricated by employing an ice-segregation-induced self-assembly
(ISISA) process. The structural features of ordered 3D lamellar microstructures could be controlled
by changing the rate of dipping of the CNT suspensions and the molecular mass of the chitosan
sample [55]. In another interesting example, Francisco del Monte and co-workers dispersed long
MWNTs (LN) and short MWNTs (SN) in three different polymer (i.e., chitosan, chondroitin
sulphate and gelation) solutions [56]. The suspension was then treated with ISISA methodologies
to prepare modified tridimensional CNT-based scaffolds [56]. Chitosan and chondroitin 3D
scaffolds were presented as more favorable substrates for mammalian cell growth. In some elegant
work, post treatment to achieve the crosslinking effect was developed to enhance the mechanical
properties of pre-formed CNT aerogel. To address the problem that CNT-based structures will
undergo structural collapse or plastic deformation with a reduction in compressive strength when
15
they are subjected to cyclic strain, Islam and co-workers reported a graphene coated SWNT
aerogels by coating pre-formed CNT aerogels with Polyacrylonitrile (PAN) polymer, which were
converted into several layers of graphene through a two-step pyrolysis procedure [57]. As a result,
the PAN enhancement does not affect the structural integrity of the CNT aerogel or the porosity
of the nanotube network, furthermore, crosslinking effect transformed the inelastic aerogel into a
superelastic material.
Figure 7 Optical and SEM images of SWNT aerogel. (a) Optical images of annealed SWNT
aerogel. (b) SEM images of the SWNT aerogel. Scale bar: 200 nm. Kohlmeyer, R.R., et al, 2011.
Carbon 49: 2352-2361. Copyright © 2011. Published by Elsevier Ltd. All rights reserved.
Elegant design of the direct drying process can also lead to the formation of C3PMs with
a tailored alignment structure [54, 58-60]. For example, Zou and co-workers successfully
synthesized MWNT aerogels by using Poly (3-(trimethoxysilyl) propylmethacrylate) (PTMSPMA)
as crosslinkers [60]. In this case, hydrolysis and condensation of PTMSPMA resulted in strong
and permanent chemical bonding between percolated MWNTs. The MWNT aerogels made using
the unidirectional freezing method exhibited honeycomb-like macro-pores with a surface area of
580 m2 g−1, anisotropic mechanical properties and good electrical conductivity
2.1.4 Other Techniques for the Generation of C3PMs
Pore former can also be used through the dispersion enhanced technique to impart specific
structural features to generate desired properties. After gel formation, these pore former will be
16
removed to yield an ordered network of pores within the C3PMs. Using this approach, materials
with hierarchical structure can be fabricated in which large cavities formed by pore former are
interconnected, while meso- and micro- pores are generated by interconnecting of CNTs. Forming
the soft pore templates followed by the heat treatment has also been used to create free-standing
C3PMs. In general, well-controlled macro- and meso-porous structures have been obtained
through the utilization of soft templates. Zhang and co-workers reported that they could facilitate
crosslinking CNTs networks by novel method through infiltrating CNT sheets and yarns with PAN,
followed by heat treatment under certain atmosphere [61]. The SEM and HRTEM images
indicated the PAN polymer converted into graphitic structures and crosslinked the CNTs after
carbonization. The PAN-based crosslinking method was further extended by using
polymethylmethacrylate (PMMA) microspheres as a template to fabricate 3D porous CNT foam
(Figure 8) [62]. The porosity and the pore size of the CNT foam can be tuned easily by adjusting
the concentrations and particle sizes of PMMA spheres. Zhang and co-workers also demonstrated
that PAN could induce better mechanical compressibility in their hybrid foam.
Figure 8 SEM images showing the morphology and structure of CNT foams (CNT/PMMA =
1/10) after treatment at 1000 °C. (a, b) Surface of CNT foam. Cui, Y. and M. Zhang, 2013. J of
Mater Chem A, 1: 13984-13988. Copyright © 2013, Royal Society of Chemistry.
Recently, one interesting organic solvent furfuryl alcohol (FA) has been deeply explored
by Gutierrez’s group to apply in deep eutectic solvents (DES). DES played a double role as catalyst
in the FA condensation and structure-directing agent (soft pore former) during the process. In the
condition without CNTs, the presence of DES induced the formation of monolithic carbons with
hierarchical porosity structure comprising micro- up to macro-pores [63]. The morphology of the
17
resulting foams showed a bi-continuous porous network built of highly cross-linked clusters that
assembled into a stiff, interconnected structure. In the case with the MWNTs, FA condensated on
the walls of the MWNTs so that the resulting porous structure represented a 3D network composed
of MWNTs wrapped with carbon from the FA condensation and pyrolysis [64]. Recently, Liu and
co-workers tailored the macroscopic shaping by suspending CNTs with alginate, followed by
calcination processes. The obtained porous CNT materials exhibited controlled shape and full
accessibility. The macroscopic shaping of CNT leads to a drastic pressure drop across the solid
bed compared to the one without shaping [65].
Figure 9 Optical and SEM images of CNT-foam. (a and b) SEM images of the CNT-foam
obtained from calcination in air flow at 450 °C for 3 h. (c) Optical photos of representative CNT-
foams with various shapes. (d) Optical photo that shows three CNT-foam cylinders supporting
500 g weight. Liu, Y., et al, 2013. J of Mater Chem A, 1: 9508-9516. Copyright © 2013, Royal
Society of Chemistry.
The low temperature chemical fusion (LTCF) method was also applied to synthesize
porous CNTs foam with controlled size and shape. To address this, Huu and coworkers used CNT
as a skeleton, dextrose as a carbon source, and citric acid as a carboxyl group donor to react with
hydroxyl groups present in dextrose (Figure 9) [66]. An ammonium carbonate was used as a pore
formation agent during the LTCF treatment, which undergoes decomposition into gaseous product
during thermal treatment. Meso-/macroporosity of the CNT foam can be tuned by the amount of
ammonia carbonate. The obtained CNT foam exhibited a 3D porous structure with a high
18
accessible surface area (> 350 m2/g) and tunable meso- and macro-porosity. The CNT foams also
possessed a relatively high mechanical strength which facilitates its handling and transport.
In addition, assembling SWNTs onto a sacrificial metal foam is another appealing method
for the formation of surfactant free, binder-free freestanding SWNT foams [67]. It was
demonstrated by utilizing SWNT dispersion in organic n-methylpyrrolidone solvents and
subsequent electrophoretic assembly onto a nickel (Ni) foam sacrificial template, in which the
binder-free freestanding SWNT foam material was achieved by etching Ni foam in HCl. Owing to
the high electrochemical performance, the SWNT foam could serve as a universal platform for
high performance electrochemical energy storage.
2.2 Properties of C3PMs
2.2.1 Introduction
The mechanical, electronic, and thermal properties of CNTs have been extensively studied.
However, for C3PMs with a mixture of pristine and functionalized tubes, it will not only have the
collective behavior of the individual tubes, but also have properties generated from the interactions
between the tubes and engineered components. This section will focus on the physical, mechanical
and transport properties of C3PMs, including density, specific surface area, porosity, compressive
behavior, robustness, flexibility, electrical and thermal transport properties.
2.2.2 Physical Properties
Table 1 summarizes the basic physical properties of some typical C3PMs. Low density and
the pore structure (pore size distribution, specific surface area (SSA), porosity) of C3PMs are
critical for future application performance. The synthetic principles and fabrication methods rule
the basic physical properties of C3PMs. For example, for the isotropic CNT sponge synthesized
by Gui et al., the density of CNT sponge was carefully controlled in the range of 5.8 to 25.5 mg
cm-3 by varying the injection rate of the carbon source [33]. However, compared with other types
of C3PMs, the low SSA of 62.8 m2 g-1 might be because of the MWNTs possessing large diameters.
From the standpoint of enhancing the SSA of C3PMs, few-walled or even single-walled CNT may
better serve as building blocks because of the correlation between the SSA and number of walls.
19
In this case, great effort has been made to create C3PMs with large SSA [46, 52, 68]. By using
SWNTs, the SSA of the final aerogel converted from supercritical drying reached 1291 m2 g-1,
which is approaching the theoretical limit for SWNTs with closed-ends [69]. For another
interesting example, the obtained SWNT aerogel exhibited very low density (2.7 mg·cm-3) and
high SSA (1011 m2·g-1) [70]. Forming a wet gel is always the first step for the supercritical drying.
However, for freeze drying, it can directly give the formation of freestanding aerogel. The pore
structure can also be controlled during the drying process. It was reported that, the chitosan (CHI)-
dispersed MWNT suspension can be directly converted into macro-porous foam through freeze
drying [71]. The cooling rate during the freezing step clearly affected the pore size. Mean pore
sizes decreased as the cooling rate increased. Another extraordinary case is the MWNT/graphene
hybrid aerogel synthesized by Gao et al. with ultralow density (0.16 mg·cm-3) [17], in which the
aerogels were formed by freeze drying. As indicated in Table 1, from the standpoint of tailoring
pore structure, elaborate control of the soft template can give the designed macro-porous system,
rendering the C3PMs with low density and high SSA [62, 64, 66].
Table 1 Physical properties of C3PMs via versatile synthetic routes.
20
Porosity is another important aspect for evaluating C3PMs. For the C3PMs obtained from
the synthetic routes above-mentioned, all those low-density materials feature porosities up to 95-
99.9%.
2.2.3 Mechanical Properties
Due to its small diameter and strong bonds, an individual CNT exhibits excellent
mechanical flexibility and can be repeatedly bent through large angles (e.g., buckling) and strains
without structural failure under large strain and load [72]. The extraordinary structural properties
of individual CNTs have been extensively studied and well documented [73]. The remarkable
flexibility and resilience of nanotubes potentially make them ideal low weight components once
they are established into C3PMs. The mechanical properties of C3PMs discussed here are mainly
focused on their compressive behavior. The compressive properties for C3PMs are categorized
into three major parts with different synthetic routes to form a 3D framework: (1) elastic CNT
sponges or arrays directly grown using the CVD technique; (2) stiff and rigid self-assembled CNT
aerogel synthesized from the solution-based route; (3) C3PMs hybrids with tailored mechanical
behavior.
Porous CNT sponges grown through the floating catalyst CVD technique shows high
structural flexibility and robustness. The compressibility can reach more than 95% volume
reduction ( > 95%) at low stress values (< 0.25 MPa) due to their high porosity and structural
flexibility [74]. Experiments have also confirmed that the sponges could elastically recover most
of their original volume by free expansion, and resist structural fatigue under cyclic stress
conditions in both air and aqueous solution. Additionally, it was reported that compressive strength
consistently increased by 20 fold when the sponge density changed from 5.8 to 25.5 mg·cm–3. In
addition, the CNT sponge showed virtually elastic recovery to their original volume (>98%) in
the presence of oil, compared to other media such as air or water [75]. Tensile and compressive
testing of CNT sponge/polymer hybrids showed that the hybrids exhibit 10-60% enhancement in
yielding strengths (about 16.1 MPa at 1.77 wt % of CNTs) compared with the control sample (pure
epoxy, 10.3 MPa), with the yielding strength increaseing linearly with the weight fraction of CNTs.
The hybrid sample also exhibited good flexibility and could be compressed within a modest strain
range (<7%) and recovered to its original volume upon unloading [76]. Another extraordinary case
for elastic C3PMs is the vertically aligned MWNT arrays grown by CVD, in which nanotubes act
21
as an open cell foam structure, exhibiting super-compressible behavior. For strains below 20% on
the first cycle, the elastic modulus of the materials is just over 50 MPa [28]. For strains between
20 and 70%, the elastic modulus equals 12 MPa. The compressive strength of this obtained foam-
like film (12 to 15 MPa) is much higher than other low-density flexible foam that are capable of
sustaining large strains [28]. Yaglioglu et al. investigated how the diameter and packing density of
these structures governed the mechanical properties of the CNT forest. The mechanical behavior
of CNT forests grown by the fixed catalyst CVD method and the floating catalyst method have
been compared. Floating catalyst method derived high-density CNT columns exhibit significant
elastic recovery after deformation, whereas fixed catalyst method derived low-density CNT
columns exhibit large plastic deformation. By analogy with open-cell materials, a simple model
was proposed to represent the elastic behavior of CNT forests. The calculations based on this
model show general dependencies and sensitivities of the elastic properties to the structural
attributes of the CNT forests (floating catalyst). This model can also be used as a design guideline
for proper mechanical properties and selection of a synthetic method (Figure 10) [32].
Figure 10 SEM image and schematic model of a CNT forest. (a) SEM image of the CNT forest.
(b) Schematic 2D model of a CNT forest. (c) Effective modulus values for a range of CNT
diameters and CNT–CNT spacing. Yaglioglu, O., et al, 2012. Adv Funct Mater 22: 5028-5037.
Copyright © 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
22
The mechanical evaluation has also been applied to study the C3PMs prepared from
solution processed CNT, in which supercritical drying always endows the CNT aerogel with higher
mechanical stiffness and strength. For instance, Islam and co-workers fabricated freestanding
aerogel from purified and isolated SWNTs [68], and mechanical characterization tests show that
these aerogels have open-cell structures and their Young’s moduli are higher than other aerogels
(alumina, carbon, silica etc.,) with comparable density [77-80]. However, unlike CNT sponge from
CVD, the CNT aerogel was fragile because of the increased π-π interactions. In another study, the
CNT was dispersed with the assistance of ferrocene-grafted polymers, giving an aerogel
mechanically stable and stiff [52]. The CNT aerogel could support over 1000 times of its own
weight. This was because of polymer-modified CNTs forming wet gels between ferrocenyl groups
through the π-π stacking interactions. Although the properties of elasticity, mechanical resilience
and low density offered by CNTs have been exploited in the building of supercritical induced CNT
aerogels, all these aerogels have so far undergone structural collapse under heavy load or large
strain [46]. To address this issue, Islam et al. fabricated graphene-coated SWNT aerogels to
transform the original inelastic aerogel into a superelastic material, thereby breaking through the
stagnation associated with this area of research [57]. The graphene coating not only maintains the
structural integrity, compressibility and porosity of the nanotube network but also increases the
Young’s modulus (E) and energy storage modulus by a factor of ~ 6, and the loss modulus by a
factor of ~3 (Figure 11). The graphene coating plays an important role as the ‘nodes’ to contribute
the superelasticity and complete fatigue resistance. The above mechanical behavior of C3PMs may
give us some inspiration that in the system of organogel (CNT with binders), there always exists
one optimal ratio between CNT/binders to make the final C3PMs transform from flexibility to
rigidity. Thus to better tune the ratio will give more accurate control over the mechanical properties
of C3PMs. Stimulated by numerous previous efforts to modify the mechanical properties,
Charnvanichborikarn et al. studied the mechanical properties of CNT aerogels prepared by an
organic sol-gel method (resorcinol-formaldehyde polymerization) under bombardment at room
temperature with energetic heavy ions (2 MeV 129Xe) [81]. Experimental results demonstrated
heavy-ion irradiation which caused an increase in Young’s modulus (E) and failure stress. The
authors attributed the irradiation-induced improvement in E largely to changes in the monolith
density. In addition, it could also be due to the formation of new cross-links between adjacent
ligaments as a result of irradiation [82].
23
The synergistic effect from hybrid aerogels also contributes to their enhanced mechanical
properties. Independently, the ultra-flyweight aerogels (UFAs) composed of CNT and graphene
exhibit temperature-invariant (-190–900°C) superelasticity [17]. Compression experiments show
a nearly complete recovery after 50–82 % compression of the UFAs. The stress-strain curve of the
1000th cycle is nearly identical to that of the first cycle. It has been investigated that the uniform
and tight covering of CNTs favors the load transfer from graphene to elastic CNTs, resulting in
super-elasticity for the hybrids aerogel. The cooperative effect is directly linked to the van der
Waals attraction between CNTs and graphene.
Figure 11 Mechanical properties of graphene-coated aerogels. (a) Nanotube aerogels collapse
and graphene-coated aerogels recover their original shape after substantial compression. (b)
Stress versus strain curves for nanotube aerogels along the loading direction and for graphene-
coated aerogels during loading-unloading cycles. Kim, K.H., Y. Oh, and M.F. Islam, 2012. Nat
Nano 7: 562-566. Copyright © 2012, Rights Managed by Nature Publishing Group.
2.2.4 Transport Properties
There has been tremendous work relevant to the electrical transport properties of individual
CNTs. Generally speaking, SWNTs have extremely high mobility (>105 cm2/ (V·s)) and current
24
carrying capacity. MWNTs have similar transport properties, however, transport coupling between
the walls of individual tubes or bundles leads to some differences [83-85]. In addition to intrinsic
impedance of inter-tubes in networks of C3PMs, the properties of the junctions between nanotubes
and collective effects can critically determine the transport performance of the entire network.
This section firstly focuses on the electrical transport properties of C3PMs with respect to
various parameters: temperature, current pulse, external force (or pressure), and particle irradiation.
These four parameters can be categorized into issue of energy scales that affect the electrical
transport properties. For instance, Zhang et al. did pioneering research on the temperature
dependence of the transport properties of as-grown SWNTs aerogels and graphitic layer coated
(Gr-coated) SWNTs samples [86]. (Figure 12a) Electrical conductivities of all samples increased
with temperature. Tunneling at the junctions is the suggested transport mechanism for SWNT
aerogels [87]. Similar observations in the temperature dependencies of electrical conductance for
both SWNT aerogels junctions and isolated SWNT junctions also confirmed the electrical ideality
of the SWNT aerogel junction [88]. The lower electrical conductivity and greater temperature
dependence of Gr-coated aerogel relative to those of as grown aerogel, single SWNT, single-layer
graphene, bilayer graphene, and a graphene nanoribbon is more likely caused by a perturbation of
the structural symmetry and band structure of the SWNTs by the graphitic coating [88-90]. Within
the context of improving the electrical conductivity performance, thermal annealing followed by
certain procedures has been proven to be an effective way for enhancing properties of both SWNTs
and DWNTs aerogel [52]. After the annealing treatment, the electrical conductivity of CNT
aerogels was substantially increased by a factor of 6-13.
Current dependent conductance in a 3D percolating system with randomly distributed
CNTs will provide a new approach to modify the transport properties of C3PMs. Islam and co-
workers discovered that short, high-current pulses applied to PVA-reinforced SWNT aerogel
sample created stepwise, irreversible increases in network electrical conductivity [46]. Figure 12b
shows repeated pulsing at a constant current level does not significantly change the resistance of
the sample, however, using the current over 100 mA would probably result in higher electrical
conductivity. The same technique has also been applied to improve electrical conductivity of
MWNT aerogels [60]. By applying 15 ms 100 mA current pulses at an interval of 30 s for 20
pulses, a corresponding increase of the conductivity from 7.9×10-3 S·cm-1 to 0.67 S·cm-1 was
observed.
25
Figure 12 Illustration of the electrical properties for C3PMs. (a) Electrical properties of SWNT
aerogel vs temperature. Zhang, K.J., et al, 2013. Adv Mater 25: 2926-2931. Copyright © 2013,
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Discrete current pulses, Ip, applied
across a sample, as shown in this schematic, improved the electrical conductivity of PVA-
reinforced CNT aerogels. Bryning, M.B., et al, 2007. Adv Mater 19: 661-664. Copyright © 2007,
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c,d,e,f) Electromechanical properties of
CNT sponge and composites. Gui, X, et al, 2010. ACS Nano, 4: 2320-2326. Copyright © 2010,
American Chemical Society.
Mechano-responsive conductivity is the strain-dependent conductance that exists in robust
C3PMs. Such a property reflects the electrical contacts between nanotubes, which has also been
applied in the application of sensors. Camilli et al. investigated the relationship between the
electrical conductivity and compression of a CNT-based percolating network. The conductivity
linearly scales with the applied compressive loads and increases up to 615% for compression of
75%. This sensing mechanism resides in the increase of contact points among adjacent CNTs
which is caused by the squeezing of the inter-tube pores under compression [91]. In another case,
two-probe electrical resistivity was monitored during in situ compression for a CNT sponge [74].
The sponge resistivity showed reversible change in the cycle, decreasing by about 20% (from 2.1
down to 1.7 Ω·cm) during compression to strain of 60% and returning to almost the initial value
upon unloading. The sponge also maintained a resistivity changing between 2.2 and 2.4 Ω·cm after
300 cycles. It has also been confirmed that CNT sponge nanocomposites with direct infiltration of
26
epoxy fluids exhibited the same exciting properties [76]. Figure 12c shows there was a drop of
resistivity when the composites were under compression to a strain of 7%, and the resistivity
returned to the original value at the end of the cycle. Increasing tunneling points between CNTs
embedded in the epoxy matrix is the main reason responsible for this. Electrical conductivity of
some CNT aerogel-based nanocomposites also remains constant at a high value after many cycles
of mechanical deformation. The polymer composite based on graphene/MWNT aerogel
maintained its electrical conductivity at a constant level (2.8 S·cm-1) after 100 cycles of repeated
stretching by 20% and 5000 bending cycles [92]. The same striking characteristic has also been
observed in SWNT aerogel based elastic conductors [93]. The electrical conductivity of elastic
conductor could reach a high level (70–108 S·cm-1), and maintained their values under repeated
cycling of stretch release and bending.
Charnvanichborikarn et al. investigated the electrical properties of cross-linked CNT-based
nanofoams exposed to ion irradiation at room temperature over a wide range of ion masses and
fluencies. The initial increase of electrical resistance for nanofoams was attributed to the buildup
of defects in graphitic nanoligaments. The decrease in electrical resistance of nanofoams when
exposed to Ne and heavier ions was attributed to radiation-induced foam densification. These
results have demonstrated that ion bombardment can be used to tailor electrical properties and
ligament morphology of CNT-based foams [94].
From the standpoint of geometric scaling, percolation in CNT-based 3D porous structure
or engineering hybrid systems needs to be taken into considerations. For the 2D percolation system
of CNTs, there are many experimental studies about the percolation behavior with conducting
objects of various shapes and geometries [95-98]. Inspired from the percolation behavior of 2D
systems, some novel work has attempted to investigate the C3PMs. During the process of
exploring the reasons that caused the electrical conductivity of liquid nitrogen freeze-dried
nanofibrillated cellulose (NFC)/few-walled CNT (FWNT) aerogel with the sheet like structure,
which was higher than that of the liquid propane freeze-drying aerogel with fibrillar structure [99],
the higher conductivity behavior was tentatively assigned by the authors for a subtle hierarchical
percolation. At a smaller length scale, the FWNTs percolated within the 2D aggregate sheets
within the NFC matrix. These essentially 2D conducting sheets percolating in the overall 3D
aerogel volume on a larger scale, indicated, the whole 3D structure was connected by fibrillar NFC
and FWNT.
27
In addition, CNT networks also offer significant technological promise towards
understanding the thermal properties of nanotubes on a macroscopic size scale. In uncoated CNT
networks, nanotube junctions are the primary bottleneck for heat transfer of thermal applications
[100-102]. Zhang et al. reported temperature-dependent (100–300 K) thermal properties of SWNT
and graphene-coated (Gr) SWNT aerogels [86]. Figure 13 illustrates that thermal conductivities of
both aerogels have similar value compared to the conventional carbon aerogels of higher density
[103-105]. Furthermore, the temperature dependence of their thermal conductivity is similar to
isolated SWNT-SWNT junctions in the range of 100-300K [103]. In particular, the junction
thermal conductance of the as-grown SWNT aerogel approaches the theoretical maximum value
for a van der Waals bonded SWNT junction. The proposed interpretation is the reduction of
phonon mean free path.
Figure 13 Thermal conductivities of C3PMs. (a) Thermal conductivities of SWNT aerogels and
carbon aerogels (CAs) with various densities at room temperature. (b-c) Thermal conductivities
of (b) Gr-coated and (c) as-grown SWNT aerogels as a function of temperature (excluding
thermal radiation within the aerogel). Zhang, K.J., et al, 2013. Adv Mater 25: 2926-2931.
Copyright © 2013, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
2.2.5 Summary of Properties
In order to develop and improve the properties of C3PMs, we need to understand the
structures that develop from different fabrication methods. Here, fabrication process is the key
issue affecting the whole performance of C3PMs. Currently, fabrication strategies can be divided
28
into two main classes: direct CVD growth, and solution-based route (either a sol–gel process or
direct freeze drying). The CVD method can create high quality aerogels constructed from long
individual CNTs in one step, suitable for applications requiring exceptional electrical and
mechanical properties. The solution-based method enjoys more flexibility in both precursor
selection (e.g., different types of CNTs, various additives) and synthetic approaches, which is
promising for tailoring desirable functions of materials. Diverse potential applications (e.g., energy
storage, catalysis, water treatment, pressure/gas sensors, energy absorption) have also been
reviewed, confirming that the unique combination of CNTs and aerogels has enormous practical
value.
2.3 C3PMs for Engineering Applications
As discussed above, highly porous structures endow C3PMs with large accessible SSA,
interconnected conductive networks, superior mechanical properties and a special
microenvironment. As a result, they have been widely explored for applications in the fields of
energy storage, sensors, absorbers, flexible and conductive polymers, etc., and exhibit improved
performance with respect to other forms of CNT materials. In this section, some recent engineering
applications of C3PMs are summarized.
2.3.1 Energy Storage
2.3.1.1 Supercapacitor
The major type of energy storage applications for C3PMs is using the porous structure with
high surface area for electrochemical capacitor uses, also known as a supercapacitor due to its high
power densities, long life and high rate capability [106]. Electrochemical properties based on neat
CNT sponges prepared through the floating catalyst CVD technique have been explored by Wu
and co-workers [107]. The CNT sponge, composed of interconnected conducting networks, can
be easily compressed and recover most of its original scale. The most striking characteristic is the
ability to maintain more than 70% of the original specific capacity of the device under repeated
cycling of 80% strain (Figure 14). Such a property makes it possible to graft pseudocapacitive
layers onto the surface of C3PMs build flexible high performance supercapacitor electrodes [108-
29
110]. C3PMs deposited with a second phase of redox active polymer are also studied as the
supercapacitor electrodes materials. Zhong et al. fabricated the supercapacitor electrodes by
depositing polyaniline (PANI) on CNT sponge [111]. The corresponding self-supported, free-
standing CNT sponge/PANI hybrid material did not require any binders and delivered excellent
area capacitance (1.85-1.62 F/cm2) coupled with high cycle stability. To improve the device
characteristics, hybrid supercapacitors have also been investigated [112] [113]. The MnO2-CNT-
sponge supercapacitor cell exhibited high specific capacitance (1230 F g-1), good cycling stability,
high power and energy density, thereby making them potentially promising candidates for energy
storage.
Figure 14 Supercapacitor performance of sponges under compression. (a) Comparison between
CV curves from the original sponge and the sponge compressed by 50% and 80% at a scan rate
of 1000 mV s-1. (b) Calculated specific capacitances of the three samples at different scan rates.
(c) Volume-normalized capacitances (Cvolume) of the three samples. (d) CV curves of the sponge
recorded at 1000 mV s-1 before and after 1000 compression cycles [107]. Li, P., et al, 2013.
Nanoscale 5: 8472-8479. Copyright © 2013, Royal Society of Chemistry.
2.3.1.2 Battery Electrodes
C3PMs have also been modified to serve as the electrode material for lithium-ion and
lithium-air battery applications [114] [115]. For the Li-ion battery (LIB) application, graphitic
30
carbon has been one of the anode materials of choice in commercial lithium-ion batteries, but
graphitic carbon is limited to low specific capacity and rate capability. To further improve the LIB
performance, a new generation of anode materials is required to overcome these limitations.
Porous CNT aerogel or sponge has a number of desirable characteristics which make them
attractive candidates as the LIB anode materials. The advantages over powder like graphitic
materials include: (i) a 3D interconnected CNT structure to improve electrical conductivity by
facilitating electron percolation, (ii) a high surface area for good electrode-electrolyte contact and
a hierarchy of pores to improve rate capability, (iii) no need to use an electrochemically-inactive
binder that increases the weight and reduces overall specific capacity [116]. In an effort to further
improve the capacity of electrode materials in LIB, Chen et al. reported one MWNT sponge
network, coated with atomic layer deposition (ALD) V2O5, resulting a well-defined composite
sponge. The core/shell MWNT/V2O5 sponge delivers a stable high areal capacity of 816 μAh/cm2
for 2 Li/V2O5 at 1C rate (1.1 mA/cm2), 450 times that of a planar V2O5 thin film cathode [114].
The cycling stability was improved using a larger discharge cutoff voltage. Compared with V2O5,
silicon (Si) even shows higher theoretical capacity (≈ 4200 mAh g-1). However, significant
challenges still remain for single-component Si anodes, which originates from their poor structural
stability and cycle life. Regarding this, the combination of Si with CVD - CNT sponge has been
proposed to be used as an electrode material [117, 118]. The obtained CNT/Si sponge materials
showed a high specific capacity of 2800 mAh g-1 with excellent cycling performance. The CNT
sponge here served as a conductive path and platform to accommodate the volume expansion of
Si. In another case, Ge et al. prepared a SWNT/PANI nanoribbon aerogel that showed high
capacity (185 mAh/g) and good cycle performance (Figure 15) [119]. The improved performance
could be attributed to effects of efficient ion/electron transport within the 3D CNT network,
shortened ion diffusion distance, and effective penetration of electrolyte within the 3D porous
framework.
The lithium air battery is receiving world-wide interest because of the potential to store
much more energy than the best LIB. However, the biggest problem in Li-air batteries always
occurs at the cathode side because lithium peroxide is continuously generated, which blocks
electrolyte and oxygen pathways. Therefore, high performance carbon cathodes are required. To
meet this requirement, Shen et al. produced a lithium-air battery equipped with a Pd-modified
CNT sponge cathode material, which effectively improved catalytic reactivity of the oxygen
31
reduction reaction [115]. In addition, the lithium-air battery can tolerate regular air with any
humidity level and delivers a specific capacity as high as 9092 mA h g-1.
Lithium-sulfur (Li-S) rechargeable batteries are promising next-generation energy storage
devices because of their large capacity and low cost. However, rapid capacity fading arising from
the polysulfide shuttle and limited sulfur volume in the cathode make current Li-S batteries not
practically viable. To address these issues, Pu et al. developed one method to use a liquid-type
polysulfide catholyte soaked in CNT sponges, achieving excellent cycling performances, 81.4%
capacity retention, and 0.023% capacity fading per cycle in 800 cycles [120]. Furthermore, the
energy density is 5 times higher than that of commercial LiCoO2-based batteries.
Figure 15 Morphology and performance of SWNT/PANI nanoribbon composite aerogel – based
lithium storage device. (a) Photograph of thin SWNT/PANI nanoribbon composite aerogel (~100
um). (b) A flexible lithium ion half-cell fabricated from the thin composite film (~100 um) as
electrode lighting a red LED. (c) Capacity and Coulombic efficiency of coin cells with
SWNT/PANI nanoribbon composite aerogel electrode at a current density of 30 mA/g. (d) The
comparison of lithium storage of different CNT/PANI composites as electrodes. The composite
electrodes all had a CNT content of ~33 wt %. Ge, D., et al, 2014. Chemistry of Materials 26:
1678-1685. Copyright © 2014, American Chemical Society.
2.3.1.3 Fuel Cells and Dye Sensitized Solar Cell
A proton exchange membrane fuel cell (PEMFC) is constructed from a proton exchange
layer, catalyst layers, and gas diffusion layers (GDLs). The performance of PEMFC is related to
32
electrochemical reactions. The GDL plays an important role being a medium which allows electron
conduction, gas diffusion, and mass-transfer control generated from catalyst layers. Previous
results show that water flow, temperature gradient, thermal conductivity and surface nature are
critical parameters in fuel cell performance [121-123]. Therefore, the biggest challenge is to
prepare a GDL material with these superior characteristics: high electrical and thermal
conductivity, low mass-transfer resistance and controlled surface nature. In this regard, Nakagawa
et al. employed CNT foams that were prepared from an aqueous suspension of CNTs dispersed by
chitosan as GDLs [71]. It was demonstrated that PEMFC assembled with the CNTs foams
displayed good fuel cell performance. The impedance measurement also showed the CNT foams
were crucial in reducing the ohmic resistance in PEMFC assembly. C3PM can be used as GDL in
PEMFC assembly and can also be applied as a catalyst support for platinum. Francisco and co-
workers demonstrated microchannelled 3D architectures composed of MWNTs decorated with Pt
nanoparticles and chitosan (CHI) using the ISISA process [54]. The macroporous 3D architectures
exhibit excellent electron conductivity (2.5 S·cm-1) thanks to the interconnection of MWNTs.
Specifically, the Pt/MWNT/CHI 3D porous structures provide remarkable performance as anodes
for a direct methanol fuel cell (DMFC).
Figure 16 Illustration and photoelectric conversion of the DSCs with CNT sponge. (a) Schematic
illustration of the counter electrodes fabrication using the CNT sponge. (b) Photoelectric
conversion performance of the DSCs with CNT sponge/FTO, CNT sponge/glass and Pt/FTO
counter electrodes. (c) Nyquist plots of DSCs with counter electrode of sponge/FTO, with the
inset showing the expanded range of the ordinate and abscissa. Chen, J., et al, 2012. Carbon, 50:
5624-5627. Copyright © 2013. Published by Elsevier Ltd. All rights reserved.
33
Typically, a dye-sensitized solar cell (DSSC) comprises three major components including:
a dye-sensitized titanium dioxide (TiO2) anode, iodide electrolyte, and a counter electrode [124].
Pt is always used as the catalytic layer material, which makes the reduction reaction fast enough
[124]. As discussed, C3PM provides a large surface area for catalytic conversion and well-defined
porosity for electrolyte diffusion. To demonstrate this, Chen et al. applied CNT sponge as the
catalytic layer of counter electrode for a DSSC (Figure 16) [125]. The CNT-sponge showed high
catalytic activity, and a photoelectric conversion efficiency of 6.21% has been achieved, which is
comparable to the 7.63% efficiency achieved with a platinum counter electrode.
2.3.1.4 Latent Heat Storage Device
Latent heat storage of phase change materials (PCM) is a promising way to effectively
utilize environmental heat and waste heat. Cao et al. reported a multifunctional phase change
hybrid composed of paraffin, which was infiltrated into a porous carbon nanotube sponge [126].
The CNT sponge here plays a key role in not only acting as a flexible scaffold for wax but also
maintaining a high thermally conductive network. The energy storage of the hybrids can be driven
by small voltage or light illumination with high electro-to-heat or photo-to-thermal storage
efficiencies (40%-60%).
2.3.2 Sensors
C3PMs that possess high conductivity, high strength coupled with high surface area have
been demonstrated to provide an ideal platform for electrochemical, gas and pressure, and
electromechanical sensors, due to changing their electrical conductance, shape, and volume. Chen
et al. reported a 3D CNT and graphene hybrid foam, which could be further used as
electrochemical electrodes for sensing, such as for H2O2, which is the widely-detected intermediate
of biological oxidase enzyme reactions [127]. Specifically, the hybrid foam exhibited a high
sensitivity (~ 470.7 mA M-1 cm-2) and low detection limit for dopamine detection. Modified with
horseradish peroxidase and Nafion, the hybrid foam were also used to detect H2O2 with high
sensitivity (~ 137.9 mA M-1 cm-2), low detection limit and wide linear detection range.
MWNT aerogel has been used for the fabrication of chemiresistor-type sensors, which are
capable of detecting environmental pollution gases with high sensitivity in the ppm range (Figure
34
17a) [60]. Their hierarchically porous structures not only provide high surface area, but also
stimulate analyte diffusion within the structure [128, 129]. As listed in Figure 17(b-c), the
performance of CNT aerogels as the active materials for a sensing device was evaluated by
monitoring its resistance change (resistance decrease ΔR/ original resistance value R) as function
of time using different concentrations of the analyte. Alternating exposure of the MWNT aerogels
to chloroform vapor and air demonstrated a reproducible response (increase of the resistance to a
saturated value) and recovery. In addition, the sensory response of MWNT aerogel showed a
significantly faster response compared with a MWNT thin film control sample, indicating the
contributions of the unique porous structure [60].
Moreover, the observed reversible and controllable resistance change in the C3PMs or
hybrids under compression at modest strains facilitates the development of electromechanical
sensors [17, 57, 76] [74]. Furthermore, the unique characteristics for CNT sponge-based hybrid
materials are reversible resistance decrease (ΔR) relative to original value (R0) over many cycles,
and roughly linear relationship versus strain in every cycle. These properties provide one
alternative route for hybrid fabrication with uniform filler dispersion and reliable inter-filler
connection [57] [74].
Figure 17 Illustration of the use of C3PMs in sensors. (a) The resistance change of MWNT
aerogel and upon exposure to chloroform vapor. The bias voltage is fixed at 0.1 V [60]. Zou, J.,
et al, 2010. ACS Nano 4: 7293-7302. Copyright © 2010, American Chemical Society. (b) Testing
of a sponge with density of 7.3 mg/cm3 by a preloading cycle to remove the permanent
deformation. The plots show the change of compressive stress and resistivity during a
compression cycle ( = 60%). (c) Reversible change of resistivity (dash) over five stress (solid) cycles. Gui, X, et al, 2010. ACS Nano, 4: 2320-2326. Copyright © 2010, American Chemical
Society.
35
2.3.3 Water Treatment
There has been a growing demand for recyclable organic absorbents that can eliminate oil
pills or organic pollutants from water [130]. C3PMs-based adsorbents have been proven as
effective materials for removing different kinds of contaminants in water. For example, Gui et al.
found that CNT sponge grown by CVD can absorb 80 to 180 times their own weight for a wide
range of solvents and oils. The CNT sponge in pristine or densified form can both actively absorb
and remove different type of oils spreading on water surface (Figure 18). It is worth noting that a
small particle of a densified CNT sponge can remove a diesel oil slick with an area of 227 cm2 in
short time, the oil area is about 800 times that of the projected sponge area [33]. Based on mass
and volume, CNT sponge has better absorption of oil from oil slicks than woolen felt and
polypropylene fiber fabric. The maximum oil sorption capacity of CNT sponge (Qm) was 92.30
g/g, which was over 10 times greater than that of woolen felt and polypropylene fiber fabric [131].
Moreover, by applying the method in ref. 38 and increasing in the concentration of ferrocene, a
magnetic CNT sponge with Fe encapsulated in it was obtained [132]. The magnetic CNT sponge
showed application as sorbents for spilled oil recovery with a high mass sorption capacity (up to
56 g/g) and excellent recyclability (more than 1000 times).
Figure 18 Environmental application of CNT sponges. (a) Absorption capacity (Q) measured for
a range of oils (solid symbols) and organic solvents. The dashed line indicates increasing
absorption capacity for higher-density liquids. b) Summary of the absorption capacity for diesel
oil measured from CNT sponges, natural products (cotton, loofah), polymeric sponges
(polyurethane, polyester) and activated carbon. c) Snapshots showing the absorption of a 28 cm2
mm-thick vegetable oil film (dyed with Oil Blue) distributed on a water bath by a small spherical
sponge. Gui, X., et al, 2010. Adv Mater 22: 617-621. Copyright © 2010, WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim.
36
Within the context of C3PMs based hybrids, in some recent elegant studies, Dong et al.
confirmed that monolithic 3D hybrid CNT and graphene foam with superhydrophobic and
superoleophilic properties could selectively remove oils and organic solvents from water with high
absorption capacity and improved recyclability compared with neat CNT sponges [133]. Gao et al.
reported that the ultra-flyweight aerogel (UFA) exhibited striking characteristics: less than 3.5 kg
UFA could absorb 1 ton of petroleum, promising great potential in pollution treatment [17]. Liu et
al. also demonstrated a novel CNT-foam with tunable meso- and macroporosity, by adding
ammonium carbonate (pore former) in optimal amounts, which resulted in high absorption
capacity of organic solvent weight (up to 650 kg·m-3) and extremely high regeneration capacity
[66]. Moreover, Li et al. fabricated nanoparticles (CdS) – CNT hybrids sponge, which exhibited
high efficiency in removing organic contaminants from water [134].
2.3.4 Other Miscellaneous Applications
Novel flexible conducting materials have attracted more interest due to their potential
application in flexible electronics [135]. For example, Islam et al. demonstrated this with
composites of SWNT-aerogel/ Polydimethylsiloxane (PDMS) by backfilling SWNT aerogels of
various shape and size with the elastomer PDMS to make excellent stretchable conductors. The
3D interconnected network of SWNT carries the electricity, and PDMS provides flexibility (Figure
20) [93].
Figure 19 Normalized resistance of SWNT-aerogel/PDMS composite films as a function of
uniaxial tensile strain along the direction of conduction. Transparent films for (a) first two
stretch-release cycles. (b) The same measurements with non-transparent films show similar
behavior. (c) Normalized resistance of non-transparent SWNT-aerogel/PDMS composite films
under uniaxial tensile strain perpendicular to conduction direction for 5th and 20th stretching
cycles. Kim, K.H., M. Vural, and M.F. Islam, 2011. Adv Mater 23: 2865-2869. Copyright © 2011,
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
37
The SWNT-aerogel/PDMS composite materials reached a stable R/R0 after 4 stretch-
release cycles and demonstrated excellent reversibility afterwards. The resistance remained almost
the same during the 5th and 20th stretching cycles for tensile strain up to 100%. As illustrated in
Figure 19, the most attractive property of SWNT-aerogel/PDMS composite material was the
negligible influence on their electrical conductivity from large angle bending deformations. There
was less than 0.1% change in R/R0 during the change of curvature.
More recently, Chen et al. reported a MWNT/graphene aerogel (MGCA) – PDMS
composite material formed by synergistic graphene/MWNT interaction and backfilling of PDMS.
The conductivity of this composite material could reach as high as 2.8 S cm-1 and remain constant
after 100 times repeated stretching by 20%, and 5000 times bending. This makes MGCA/PDMS
composite material very attractive in many applications that demand bending and stretchable
durability [92].
Figure 20 Illustration of the use of C3PMs in desalination and other applications. (a)
Desalination curve for CNT sponges in a CDI cell. Inset is the schematic of the flow-through
CDI cell. Wang, L., et al, 2011. J of Mater Chem 21: 18295-18299. Copyright © 2011. Published
by Elsevier Ltd. All rights reserved. (b) Illustration of the process in which a CNT sponge is
directly converted into a GNR aerogel by unzipping MWNTs and the morphology of 3D GNR
aerogels. Peng, Q., et al, 2014. Advanced Materials 26: 3241-3247. Copyright © 2014, WILEY-
VCH Verlag GmbH & Co. KGaA, Weinheim.
Capacitive deionization (CDI) is a promising method for desalination of brackish water by
removing salt from an aqueous solution with a capacitor-like device [136]. Wang et al. reported
CNT sponge prepared from CVD, which can be used as a promising electrode material for
capacitive deionization (Figure 20a) [137]. The maximum adsorption capacity is 40 mg g-1, much
higher than other carbon-based materials in the literature. C3PMs have also been applied to the
application of biomaterials. Francisco del Monte et al. explored the interactions of three types of
38
MWNT-based 3D scaffolds with three types of mammalian cells displaying different sizes and
adhesion patterns [56]. It was shows that scaffolds with both a pore size that closely matched that
of cells and lower surface roughness revealed higher viability values. In addition, it was first
reported with highly viable cultures of porcine endothelial cells derived from peripheral blood
progenitors (ECPC) on 3D CNT-based scaffolds, thus opening up their application in stem cell
research as other types of progenitors cells. Those research results bring novel insights into the
fields of biomedicine.
C3PMs can also be the platform to generate other types of 3D porous structure. Peng et al.
directly converted the CNT sponges into GNR aerogels by in-situ unzipping with KMnO4 in
concentrated H2SO4 [138]. The most exciting part is the obtained graphene aerogel keeps the
original structure of the CNT sponge (Figure 20b). This transformation method can provide an
alternative approach for fabricating 3D porous materials.
Generally speaking, C3PMs hold the fascinating properties such as ultralight, low density,
high surface area, and conductive. In the following chapters, the fabrication of various shapes and
sizes of NCF with varying density from MWNTs is reported, and their structure, mechanical,
electrical properties and engineering applications are introduced.
39
CHAPTER 3
METHODOLOGY
3.1 NCF Fabrication
In this chapter, the design principles and fabrication of various thicknesses and sizes of
NCF with various densities and pore characteristics from MWNT are reported. The methods used
to characterize their structure, mechanical, electrical and other properties are introduced.
3.1.1 NCF Fabrication Principles
We designed the NCF based on four criteria: (1) the CNTs must assemble into a 3D foam
with an open-cell cellular architecture, (2) the cellular structure of the NCF must not collapse
during the synthetic process; (3) the CNTs in the foam must be isotropically crosslinked with each
other to form elastic interconnected networks rather than simply stacking together; (4) the porosity
and macroscopic morphology must be uniform and easily tuned. To satisfy the first three criteria,
the formation of 3D, stable, and elastic CNT networks, we combined the unique properties of
nanocarbon as crosslinkers (easy to form bonding) with CNTs to yield networks of NCF with
robust elasticity and stability. The last requirement can be satisfied by using polymer templates,
which hold the uniformity in shape and are easily removed during the thermal treatment under a
relatively lower temperature range that do not have impact on the properties of CNTs.
3.1.2 Fabrication Process
MWNT was purchased from General Nano Inc, and used as received. Previous studies have
shown that the estimated average length of the MWNTs is about 1~2 mm and with an average
diameter of 10 nm. Dimethylformamide (DMF) and all solvents were purchased from Sigma-
Aldrich and used as received. Polyacrylonitrile (PAN) (Mw = 150,000) and poly(methyl
methacrylate) (PMMA) (Mw = 15,000) were purchased from Sigma-Aldrich. Conductive silver
paint was purchased from Alfa Aesar.
40
The hybrid foam fabrication process is simple and low-cost, utilizing PAN as a precursor
to crosslink MWNTs and PMMA microspheres as templates [62]. The reason to choose PAN as
the precursor for crosslinking is that PAN has been used as the precursor for manufacturing high
performance carbon fibers. It can be carbonized to form strong C–C bonding when it is treated in
an inert gas over 1000 °C. This guarantees the robustness of the final NCF. The relatively low
pyrolysis temperature (~ 300°C) of PMMA proves its feasibility as the template and will not
disrupt the CNTs’ properties and frameworks during the pyrolysis process. To design the synthetic
route, the thermal stability of CNT and PMMA need to be evaluated in order to provide accurate
control of the temperature. Achieving the crosslinking effect from PAN precursor, requires
ramping the temperature over 1000 °C in an inert gas. The basic principle is to effectively remove
the PMMA sphere template, while keeping the 3D CNT architecture intact and achieving the
crosslinking.
Figure 21 TGA spectrum for CNT and PMMA in (a) air environment; (b) N2 environment.
Thermogravimetric analysis (TGA Q50, TA Instruments) was employed to study the
thermal stability of CNTs and PMMA. Air was first used for the analysis with a flow of 60 mL
min-1. The temperature was increased at the rate of 10 °C min-1. The results showed that CNTs
started to oxidize at around 550 °C, PMMA started to be decomposed and evaporated at around
270 °C and decomposed to 2% of its original weight while keeping the temperature at 300 °C in
minutes (Figure 21a). Thus, we can set 300 °C as the temperature to remove the PMMA in air,
which is much lower than the oxidation temperature of CNT. Another reason to use 300 °C is that,
41
at 300 °C PAN can be stabilized and transformed into a condensed heterocyclic ring structure
[139]. Figure 21b shows the thermal stability of CNT and PMMA under N2 with increasing
temperature. CNT only showed a weight drop around 1.8% after the temperature approached
1000 °C, while PMMA was totally removed. This result also guarantees no impact of CNTs and
removal of PMMA during the carbonization process of PAN.
Figure 22 Design, processing and porous architectures of NCF (ρ = 25.6 mg·cm3). (a) Schematic
showing the synthetic steps. (b-d) Microscopic architecture of NCFs at various magnifications,
showing the hierarchical cellular structure. (e) SEM image of cell wall for pores of NCF.
Figure 22a describes the synthesis pathway. The fabrication process started by making
PAN/DMF solution. 1 weight percent PAN was mixed into DMF with a stir bar at 60 °C for one
day. Appropriate amount of CNTs were added to isopropyl alcohol (IPA) solution followed by
dropping the PAN/DMF into the solution to make CNT/PAN/IPA mixture. The mixture was then
well dispersed by high power sonication (Misonix Sonicator 3000) for 30 min. Then, PMMA
spheres were added to the CNT/PAN/IPA suspension. The followed 10 min bath sonication helped
the mixture to reach a uniform dispersion of PMMA spheres in the CNT/PAN/IPA suspension.
After this, the mixture was put into vacuum filtration system to make the CNT/PAN/PMMA solid
gel. The CNT/PAN/PMMA was then pressed by a mesh-like block at room temperature to ensure
complete removal of solvents (IPA and DMF). The weight ratio of CNT to PMMA (CNT/PMMA)
was chosen to make the NCF with different density. For the suspension with the weight of CNT
42
as 20 mg and CNT:PMMA weight ratio as 1:17, a solid CNT/PAN/PMMA sample with 3.4 cm in
diameter and thickness in 3 mm was obtained. The NCF was obtained through two heat treatments
[140], at 300 °C for 3 h in air and then at 1200 °C for 1 h in nitrogen with 80 ml/min flow rate.
During the 1st heat treatment, the PMMA microspheres were depolymerized and expelled while
the PAN precursor was stabilized. In the 2nd high temperature treatment, the stabilized PAN
precursor was carbonized to form graphitic features among CNTs. The PAN-derived conjugated
structure was converted into graphitic structures by dehydrogenation and denitrogenation during
this stabilization step, endowing the resultant NCFs with elastic resilience. The PMMA spheres
were depolymerized and expelled during the first step. The CNTs in NCF are kept intact during
the fabrication process.
The most straightforward way to control the density of sol-gel-based foams is adjusting the
weight fraction of solid constituents in solution. NCFs with varying density were fabricated by
adjusting the fabrication process: CNT content, additive content (PAN), pore creator content
(PMMA spheres) and post-treatment process. Adjusting the weight ratio of CNT/PAN or
CNT/PMMA will vary the NCF’s density.
3.2 Characterization of NCF
Morphology: The NCF morphology was characterized using a scanning electron
microscope (SEM) (JEOL 7400) at 10 kV. SEM is a convenient and useful tool to study carbon
nanotubes because it easily produces good topological contrasts with secondary electrons. The
theoretical resolution limit of SEM is known to be 1 nm, and the real resolution is typically limited
to a few nanometer, which makes the imaging of NCF easily. For SEM images of NCF, it is
recommended to use a high acceleration voltage, short working distance, and small spot size to
have clear surface structures and cause little damage to the samples. Each foam sample was
carefully cut in half using a laser machine (VLS2.30, Universal Laser Systems Inc.) and the cutting
surface was imaged. High-resolution transmission electron microscopy (HRTEM, JEM-
ARM200F, 200 kV) was used to observe the structures around CNT joints.
Mechanical characterization: compression tests, cyclic tests, and sample dynamic property
tests (storage modulus, loss modulus and damping ratio) were performed using Q800 (TA
43
Instruments) at multi-frequency mode with 1% strain amplitudes with 0.1 N preload at frequencies
from 0.01 Hz to 20 Hz. Rectangular NCF samples with width of ~ 3 mm and thickness of 2 mm
were used for compression test. The stress-strain (σ- ) curves with of 30, 50, and 80% were
measured at a strain rate of 10% strain rate per minute and a 0.01 N preload was applied to make
a uniform flat contact between the compression heads and the sample to prevent slipping of the
sample. A 200-cycle loading-unloading fatigue cyclic test was performed by measuring σ versus
50 % at a strain rate of 10% strain rate per minute. The tensile test stress-strain curves of the
NCF samples were obtained on the Q800. Rectangular samples with length of 30 mm, width of 3
mm, thickness of 0.2 mm were used, and the tensile loading rate was 1N per minute.
Viscoelasticity of materials is characterized by the storage modulus (E') and loss modulus
(E"), which account for elasticity and viscosity, respectively. The origins of the storage modulus
in foams are usually the stretching and bending of the cell walls while the loss modulus is due to
energy dissipation typically in the form of heat. Viscoelastic properties of materials, often
represented by elastomeric polymers, are inherently temperature dependent due to their
characteristic glass transition temperatures and crystallinity of polymer chains. These temperature-
sensitive physical features of the polymeric material stem from the thermally activated molecular
motion. The E’ and E” of NCFs were measured by Q800 DMA using multi-frequency mode with
1% strain amplitudes, 0.1 N preload, and at frequencies from 0.01 Hz to 20 Hz. In the test, the E’
and E” are obtained by using the following equations. First to determine , the phase lag between
stress and strain, by fitting the sinusoidal curves with
= 0 sin (tω) (3.1)
σ = σ0 sin (tω+ ) (3.2)
where ω is frequency of strain oscillation and t is time. Then, E' and E" are calculated by plugging
the obtained values of in the equations of
E' = ε0 σ0 cos δ (3.3)
E" = ε0 σ0 sinδ (3.4)
E' and E" can be calculated.
Pore Characteristics: The surface area and porosity analysis is performed through nitrogen
adsorption experiments. The nitrogen absorption isotherms were obtained using the Micromeretics
TriStar 3000 Surface Area and Porosity Analyzer at 77.3 K. The NCF samples were degassed and
heated in N2 atmosphere at 300 °C for 12 hours prior to the measurements to remove all traces of
44
moisture. For each sample, 50 points each were taken for the adsorption and desorption curves.
The data analysis, including the Brunauer-Emmett-Teller (BET) surface area, t-plot micropore area,
and BJH pore size distribution, were performed using the TriStar 3000 software. Duplicate samples
were tested and the average values are reported here.
Electrical conductivity: two-probe and four-probe methods were used to measure the
resistance of the NCF. For two-probe measurement, a layer of silver paint and copper wire were
uniformly attached onto the ends of a rectangular NCF sample (3 mm × 3 mm × 2 mm), and for
the two-probe measurements, the resistance reading was recorded by connecting the copper wires
to a multimeter (Fluke 89IV) using alligator clamps. The contact resistance between the two
clamps was measured (0.15 Ω) and subtracted the measured resistance of NCFs. For four-probe
measurements, the electrode were attached to the ends of the rectangular NCF, supplying the
measurement current of 50 mA, while the voltage drop across the sample was measured with a
second set of electrodes, thus isolating the sample resistance from the electrode contact resistance.
3.3 Engineering Device Characterization
Lithium air (Li-air) Battery: A round-shape NCF with a density of ~30 mg/cm3, 1.8 cm in
diameter and ~150 um in thickness was used as air electrode (cathode). A Li-air cell was built in
an argon atmosphere glovebox by stacking a Li-metal anode (Alfa Aesar, 99.9%, 0.75 mm
thickness), a piece of glass fiber separator (EL-CELL GmbH, Co, Germany, 1.55 mm thickness)
and an air electrode in sequence into an electrochemical testing cell (ECC-AIR, EL-CELL GmbH,
Co, Germany). The organic electrolyte was made with 1 M LiCF3SO3 (Sigma Aldrich, 99.995%)
in Tetraethylene glycol dimethyl ether (TEGDME, TEGDME, Sigma Aldrich, 99%).
Discharge measurements were carried out in 1 atm oxygen gas at room temperature using
an Arbin Instruments (Arbin-010 MITS pro 4.0-BT2000) controlled by a computer. The
electrochemical impedance spectrum of the Li-air battery was recorded over a frequency sweep of
0.1-106 Hz using a Gamry Instruments (Reference 3000). It was measured using a 10 mV
amplitude sine wave performed at open circuit voltage in potentiostatic mode. The resulting
spectrum was analyzed by the Gamry Echem Analyst program. Then a fresh Li-air cell was
discharged to 2 V at a fixed discharge current density (0.1mA/cm2).
45
CHAPTER 4
RESULTS AND DISCUSSIONS
4.1 Structure of NCF
4.1.1 Morphology of NCF
Due to the simplicity and inherent flexibility of the fabrication process in our design
methodology, great versatility in controlling the shapes of NCFs was possible. Integrated hybrid
foams with desired shapes, such as thin NCF sponge, cubes, films can be prepared with the laser
cutting technique and shaping mold. As illustrated in Figure 23, PMMA spheres with diameter in
8-11 um were used to fabricate NCF, it is also noteworthy that NCF consists of CNTs self-
assembled into hierarchically porous structure with major ordered cellular pores surrounded by
thin membrane-like cell walls consisting of CNT bundles. The overall structure of NCF resembles
closed-cell polymer foams; however, the cell walls with permeability and accessible internal
surfaces are slightly corrugated and have a thickness of tens to hundreds of nanometers. Such a
high ratio of pore width to wall thickness endows NCF with low density and extra-high porosity.
Figures 23 b - d presents typical SEM images of NCF with a density of 25.6 mg/cm3. Figure 23 c
demonstrates that the thickness of the cell wall is around 1 um when the weight ratio of CNT to
PMMA spheres is 1/17, the most important feature is the structure of NCF did not collapse and
maintained well. It was because of the reason that during 1200 °C treatment (carbonization), the
PAN derived condensed heterocyclic ring structure changed into a graphitic structure [141]. The
cellular structures of NCF exhibited the geometry with a major cellular pore size of 8-11 um, and
these cells were interconnected with each other through junctions of cell walls (Figure 23b).
Zooming in on the cell wall revealed that the cell walls consisted of numerous minor cellular pores
with sizes of 200 - 500 nm (Figure 23d). The significant structural difference between NCFs and
other aerogels is the unique NCF cell walls that consist of similar minor cellular pores, even though
the thickness of the cell wall is ~ 10 times thinner than that of typical cellular foams made of
polymer [142, 143].
46
Figure 23 Morphology of NCF. (a) Optical image of NCF. (b-d) Microscopic architecture of
NCFs at various magnifications, showing the hierarchically cellular structure.
More importantly, SEM observation suggest the feasibility of controlling microstructures
Figure 24 SEM images of microscopic architecture of NCFs with pore size of: (a) 4-8 m; (b) 8-
11 m; (c) 21-25 m; (d) 25-30 m; (f) 53-63 m.
47
of the NCFs. In Figures 24 a - f, the pore widths become bigger with an increase in the diameter
of PMMA spheres, while the cell wall thickness becomes thicker as PMMA sphere size increases.
The diameter of the PMMA spheres used ranges from 4-8 m to 53-63 m. Therefore, the
morphology of NCFs can be easily modulated by adjusting the diameter of PMMA spheres while
keeping weight ratio of PMMA/CNT as the same value. This is of scientific significance for the
potential applications that require hierarchically porous structure could exhibit different size of
pores.
4.1.2 Pore Characteristics of NCF
Large pores inside NCFs can be easily characterized by SEM, but relatively small ones,
such as micropores (< 2nm) and mesopores (2-50 nm), are studied by the nitrogen adsorption-
desorption measurements. In order to correctly characterize the NCF in terms of microstructure, a
non-destructive BET surface area technique was used to determine the SSA [144], pore volumes
(V), and the pore size distributions (PSD) of NCF with density of ~ 25 mg/cm3. For nitrogen
adsorption experiments, the nitrogen absorption isotherms were obtained using the Micromeretics
TriStar 3000 Surface Area and Porosity Analyzer. The samples were degassed and heated at
300 °C for 12 hours in N2 atmosphere prior to the measurements to remove all traces of moisture.
For each sample, 50 points each were taken for the adsorption and desorption curves. The data
analysis, including the BET surface area, t-plot micropore area, and BJH pore size distribution,
were performed using the TriStar 3000 software.
Figure 25a is a group of nitrogen isotherms at 77.2 K upon adsorption. The presented
isotherms were constructed by plotting experimentally measured specific adsorbed volume versus
relative pressure P/P0 for NCF with different macro-scale pore size in same density level (~ 25
mg/cm3), which means the NCFs were fabricated with same CNT/PMMA/PAN ratio but different
diameter in PMMA spheres. Here, P represents the equilibrium pressure and P0 represents the
saturation pressure of the adsorbates (nitrogen) at the adsorption temperature 77.2 K. The total
pore volume, average pore diameter, and SSA of the NCFs are summarized in Table 2. For a clear
representation of changes in specific adsorbed volumes at different P, the adsorption and
desorption curves were kept from the main plots of the isotherms. The full isotherm of the NCFs
at ρ = 25 mg/cm3 with different pore size, which include the similar characteristic features for all
48
the isotherms. The specific adsorbed volume of nitrogen slightly decreased with an increase in
macro-scale pore size, possibly from some bundling between MWNTs and increased
entanglements at bigger macro-level pore size.
Figure 25 Nitrogen adsorption–desorption isotherms (a) and nano-level pore size distribution
curves (b) of NCF with different macro-level pore size.
With the increase in the macro-pore diameter of NCFs at the same density level (~ 25
mg/cm3) , the nitrogen adsorption-desorption isotherms of hybrid foams, given in Figure 25a,
displays the features of type IV IUPAC (International Union of Pure and Applied Chemistry) of
NCFs with a very small hysteresis. The figure also suggests that the foams contain both micropores
(< 2 nm) and meso- or macropores (> 50 nm) consistent with the hierarchical structure observed
under SEM (Figure 24). Generally, a steep increase of nitrogen adsorption at low P/P0 is ascribed
to the existence of micropores. According to this point, there is a small amount of micropores in
NCFs. Compared with other drying method, for example, the freezing drying method at -20 °C
always produces relatively large pores, and it will cause the loss of micropores inevitably [145].
The desorption hysteresis loops mainly occur at high relative pressure above 0.8, showing there
are abundant mesopores and macropores, which is consistent with type IV under IUPAC
classification [146]. The mesopores may offer a large specific surface area for energy storage
devices. Furthermore, from the pore size distribution curves in Figure 25b, it can be seen that the
pores with diameter > 10 nm contribute to most of the total pore volume. Another interesting
49
phenomenon is the peak of pore size distribution shifts from 70 nm to 40 nm while increasing the
PMMA sphere size from 4-8 m to 25-30 m. This can be attributed to the densification of the
CNT networks on cell walls that accompanies N2 adsorption isotherms. Thus, the hierarchically
interconnected pore distributions of NCF are beneficial for engineering applications.
The SSA of NCF with various pore size is determined by analyzing the measured nitrogen
adsorption isotherms using the BET theory. We generated BET plots of 1/(Q[(P0/P)-1] versus P/P0
based on measured adsorption isotherms (Figure 26) and then fit a linear regression over a P/P0
range of 0.04 - 0.20 to obtain the y-intercept
i = 1/(QmC) (4.1)
and the slope
s = (C — l)/(QmC) (4.2)
where, Q is the mass of adsorbed nitrogen at a relative pressure P/P0 per unit mass, Qm is the mass
of the adsorbed nitrogen monolayer per unit mass and C is the BET constant. Note that the choice
of the linearity regime in the BET plot varies depending on types of samples – usually within the
P/P0 of 0.05 – 0.30. This P/P0 could be attributed to an energetically homogeneous surface having
high adsorption energies based on highly graphitized carbon [147].
Figure 26 SSA results by fitting a linear regression through the BET plots.
50
The SSA is calculated from Qm:
SSA = (Qm NAAcs) / Mw (4.3)
Qm = l / (i+s) (4.4)
where, NA was the Avogadro’s number, Acs was the cross-sectional area of nitrogen atoms, and
Mw was the molecular weight of nitrogen. As shown in Table 2, SSA increased with a decrease in
pore size because of smaller specific adsorbed volume of nitrogen at bigger pore size area.
Table 2 Pore characteristics of NCF with different level of pore sizes
Pore size (um) BET surface area
(m2/g)
External surface area
(m2/g)
BJH Pore volume
(cm3/g)
4-8 283 201 1.42
8-11 245 185 1.25
17-21 233 170 1.23
21-25 221 155 1.25
25-30 223 154 1.29
Although the SSA of the obtained NCF is not as large as that of C3PMs fabricated by
critical-point drying or freeze drying [60] [52, 148], it is still of higher values than some other
carbon-based materials [149]. More significantly, the hierarchically porous structures and tunable
SSA of NCFs can provide a potential skeleton for fabricating novel composite materials.
4.1.3 Volume Density
The C3Pεs’ density is generally in the range from several to tens of mg/cm3, and some
related results in the literature have been summarized in Table 3 [19] [22] [26] [29]. The density
value is associated with the fabrication process, CNT content, additive content (dispersing aids or
binders), post-treatment process, etc. While the volume density of NCF is related with the weight
ratio of each component, mainly determined by the weight ratio of CNT/PMMA. Figure 27 shows
the actual volume density of our NCF made with different weight ratios of CNT/PMMA while
51
keeping the PAN/CNT weight ratio as 0.5. The NCF with the polymer/CNT ratio of 0.3, 3, 5.8, 10,
17, 20, 30, 35, 40, and 45 correspond to NCF-0.3R, NCF-3R, NCF-5.8R, NCF-10R, NCF-17R,
NCF-20R, NCF-30R, NCF-35R, NCF-40R and NCF-45R, respectively. The NCF-0.3R has the
highest density of ~150 mg·cm3. The introduction of PMMA spheres significantly reduces the
density value of NCFs. With the increase of the weight ratio between polymer and CNTs, the NCF
density becomes smaller. The minimum can be ~12.5 mg·cm3, yielding a porosity of ~99% if the
densities of CNT, PAN and PMMA are 2.1, 1.18, and 1.2 mg/cm3, respectively. The ultralight
weight allows the NCF to stand on hair as shown in the inset of Figure 27. By contrast, the CNT
aerogel or foam prepared by a normal dry-pressing method shows a much higher density of ~350
mg·cm3 [145], which is related with its dense microstructure. If the PMMA is completely removed
and PAN transforms to the graphitic structure as 0.5 of its original weight, the real density values
of NCFs fabricated by our novel approach were close to calculated density (theoretical density) of
the NCF which will be around ~10 mg/cm3, which ensures the feasibility of our approach to give
good control of the density of NCFs.
Figure 27 Plotting of volume density of NCF according to the weight ratio of polymer to CNT.
Inset of the figure shows lightweight of NCF.
52
4.2 Mechanical Properties of NCF
4.2.1 Characterization of Superelasticity
The high porosity of NCFs allows compression of ~ 90%. We hypothesize that the
synergistic mechanism (crosslinking effect) and ordered porous structure are responsible for the
outstanding mechanical properties of NCFs. The NCF hybrid foams resemble closed-cells at the
macroscopic scale with open-cell walls. Therefore, their loading curves share similarities with
those of closed-cell cellular polymeric foams. However, those NCFs can be compressed to the
strain ( ) as high as 90% and can recover to the original volume after the release of compressive
stress (σ). As demonstrated in Figure 28, the σ is plotted as a function of of the 3 loading-
unloading cycles for NCF, the compressive σ- curves for the NCF were set at maxima of 30, 50
and 80% respectively.
Figure 28 Typical compressive stress–strain curves of the NCF. The black, red, and blue solid
lines present the viscoelastic properties of the NCF at different maximum strain of 30 %, 50 %,
and 80 %, respectively.
The volume and morphology of the NCF after cycles of compressions keep almost the
same without a discernible variation compared to the original NCF. The curves during loading
process show three characteristic regions, typically observed in a cellular foam [150]: a Hookean
53
or linear elastic regime that the stress increases linearly with the strain when compressive strain is
less than 12%, where the elastic modulus was calculated to be about 530.6 kPa and a relative
plateau region at the strain (12% < < 65%) from which most of the absorbed energy dissipated,
and the densification region at strain larger than 65% marked by the rapid increase of stress due to
the continuous decrease of pore volume (Figure 28). The NCF exhibits a much broader first linear
region than that of a typical closed-cell cellular aerogels or foams (<5%) [151]. To provide the
insight into the mechanism of superelasticity, we evaluated the compressive performance of CNT
foam without PAN crosslinking effect for comparison. The NCF which is with crosslinking
unfolds almost completely once the external pressure is removed, while the CNT foam which is
without crosslinking recovers only partially. While in the unloading process, the decreasing stress
is accompanied by timely recovery of the NCF and the cell structures are totally restored when the
stress is fully removed. These SEM observations corresponded well with the three regions σ-
regime (Figure 31).
The NCF does not show any plastic deformation over a very large range of strain rates of
10% strain min-1 (normalized to the uncompressed foam thickness) to 50% strain min-1. The NCF
was subjected to a cyclic compression test with 200 loading-unloading fatigue cycles at the of
50% with a loading rate of 10% strain min-1, and it exhibited near-zero plastic deformation (Δ ≈
0.1%) and no degradation in compressive strength, highlighting their structural robustness (Figure
29a). In comparison, the polymeric foams always exhibit a plastic deformation of 20% at ≈ 50%
[150], plastic deformation and degradation of compressive strength are obvious in other C3PMs
[28, 74] . From the 50th cycle, the loading curves were nearly overlapped with each other. This
phenomenon indicates that no fatigue of NCF occurred in these cycles. The maximum stress
decreased for only about 2.3 %, and the total strain was lost for about 0.1%. All the unloading
curves, including the first cycle, were nearly identical, indicating the foams holding the unchanged
energy stored in each cycle. Energy absorption is another key parameters of cellular solids. For
the first cycle, we calculated the work performed in the compression to be 4.84 mJ·cm-3 and the
energy dissipation to be 2.03 mJ cm-3, yielding an energy loss coefficient (ΔU/U) of 0.42. After
100 cycles, a nearly constant energy loss efficient of ~ 0.32 was calculated (Figure 29b).
Interestingly, our hybrid foams can reversibly undergo large-strain deformation (~90%) in liquids
(acetone, ethanol, DMF, oil, and electrolytes) in which liquid medium are absorbed and discharged
reversibly under cyclic compressions. Remarkably, there is no obvious change in compressive
54
stress needed to extrude a low-viscosity liquid from the foam, even when the strain rate was
increased to 50% strain min-1.
Figure 29 Compressive performance of NCF. (a) 200 cyclic compressive fatigue test with of
50%. (b) History of the Young’s modulus, maximum stress and energy loss coefficient as a function of the compressive test cycles. (c) Dynamic rheological compressive behaviors with
oscillatory of ± 1% during the temperature ramping process. (d). Dynamic rheological
compressive behaviors with oscillatory of ± 1% during the frequency sweep process.
Notably, further dynamic compressive viscoelastic measurements also revealed the NCF
demonstrating rather stable viscoelastic frequency- and temperature-invariant stability, and
reversible deformation. We performed a temperature ramp on the NCF using the DMA equipped
with a forced convection oven (FCO) from room temperature to 400 °C at a ramp rate of 5 °C/min,
oscillatory of ± 1%, in order to characterize the temperature-dependent structural properties of
NCF within the linear viscoelastic regime. No characteristic temperatures (Figure 29c) such as a
glass transition temperature (Tg) or a melting temperature (Tm) were observed from the test.
Because the presence of Tg and Tm is illustrated with sharp peaks in the damping ratio, that is
55
always observed in the polymeric materials [34]. DMA measurement in air during temperature
ramping process revealed the NCF’s storage and loss modulus depending little on temperature (25
to 400°C in air for Figure 29c) compared with the conventional elastomers like silicone rubber
[34], which showed the advanced mechanical properties of NCF. DMA measurements in air also
showed that storage modulus (E ) and loss modulus (E ) were nearly stable and were independent
of the frequency. E was one order of magnitude higher than E which implied that the elastic
response was predominant (Figure 29d).
Figure 30 Compressive behavior of NCF with (a) different densities; (b) different PAN ratio.
The mechanical properties of NCFs depend strongly on their densities; this is mainly due
to the thickness of pore walls increasing with the NCFs’ density as illustrated in Figure 30. Thicker
cell walls will provide pores with higher stiffness and membrane tensile strength to bear more
compressive strengths. As shown in Figure 30a, the stress-strain curves of 4 NCFs samples with
different densities are similar in shape. However, the compressive modulus increased from 50.7
KPa for NCF with density of 10.8 mg/cm3 to 530.6 KPa for 25.6 mg/cm3 according to the first
region of loading curves. The maximum stress at a strain of 50% increased from 22 KPa for NCF
with density of 10.8 mg/cm3 to198 KPa for 25.6 mg/cm3. According to obtained data, the
compressive modulus or the maximum stress at a strain of 50% has a nearly liner relationship with
the weight density of NCFs. As in Figure 30b, crosslinking agent (PAN) / CNT weight ratio will
also determine the mechanical properties of foams. The PAN/CNT 2.0 NCF is elastic and stronger.
56
The uncomplete curve is because of the limit of the DMA. This tunable mechanical properties
provide hybrid foams with potential applications in shock damping and energy cushioning [150,
152].
4.2.2 Synergistic Mechanism
Because the C3PMs are generally brittle. They have low mechanical resilience and
permanent deformation could usually be observed after severe compression. In context, the
exceptional static and dynamic mechanical performance exhibited in our NCFs is a unique feature.
Previous studies have suggested that when C3PMs or foam made of graphene is severely
compressed, damage to the local microstructure from the collapsing and overwhelming elastic
energy due to inter-wall adhesion, such as van der Waals interactions, would prevent elastic
recovery, cause the loops of elasticity and strength [151]. We thus demonstrate that in our NCF,
the highly ordered, hierarchical architectures with the synergistic effects at different levels
contribute to this remarkable mechanical performance. By monitoring the deformation process of
the NCF, the contributions of every structural level to the improved stability can be illustrated.
There are as follows:
Figure 31 In situ SEM characterization of NCF under compression and release process.
First, the structural regularity endows the NCF with homogeneous structural reorganization
under increasing compressive strain. When uniaxial stress is applied to NCF, the edge of cell walls
transfer the force, the structure deformation was activated, and the walls in each cell began bending
(Figure 31). Due to their ordered cellular structure, the porous structure in NCF exhibit uniform
57
deformation, while some non-uniform distribution of cell size in randomly structured foams or
aerogels leads to inhomogeneous strain and severe local deformation. In addition, such
honeycomb-like porosity, in which the nanostructured carbon based walls are organized, was also
considered to be a crucial aspect to improve the modulus and strength [151].
Second, the cell walls in NCF are comprised of intersecting nanotubes coated with
graphitic flakes, as shown in Figure 32a. As expected, the nanotubes behave like long and
entangled threads, while the graphitic structures act like a jacket to “lock” the CNTs. The sliding
of CNTs will be confined by the graphitic structures under high loading to dissipate energy. Since
the nanotubes in the NCF are long and one nanotube belongs to many joints, the “locking” and
sliding increase the robustness of the NCF. For comparison, the CNT foam without PAN
crosslinking effects was prepared through the same fabrication process. The dashed line in Figure
33b shows the result of compression testing of the foam without PAN crosslinking. The NCF with
crosslinks unfolds almost completely once the external pressure is removed, while the CNT foam
without crosslinks recovers only partially. The crosslinks created by PAN (Figure 32a) tightly
bonds the CNTs together, such crosslinking effect not only enhanced the mechanical strength but
also improved the compressibility of the NCFs.
Third, the mechanical stability of cell edges (walls) was crucially important for assuring
the structural compressibility and elasticity under severe compression.
Figure 32 TEM images and mechanical evaluation of crosslink effect. (a) TEM image shows the
structure after carbonization. (b) The compressive stress–strain curves of the NCF with
crosslinks (solid line) and the CNT foam without PAN crosslinks (dashed line).
58
We propose mechanisms by which graphitic flakes imparts superelasticity and fatigue
resistance to NCF by considering the mechanical transport properties under strain. For CNT foam
without crosslinking, the struts can bend and freely rotate about the existing nodes when
compressed by 10-15%. As NCF is compressed further, the struts align and further increase the
contact area between existing nodes and form ‘new’ nodes, causing a gradual decrease in resistance.
However, when the load is removed, there will not be restorative force to destabilize these
additional nodes, and CNT foam without crosslinking cannot recover their original shapes. (A
large van der Waals attraction is required for node removal [153].)
The graphitic flake coating significantly strengthens the nodes and the struts, hindering free
rotation of the struts about the nodes, and generates an approximately six fold increase in both the
Young’s modulus (Figure 32b). Thus, the increase in contact area between the nanotubes is
significantly smaller under small compression (10-15%). Simultaneously, the graphitic structures
at the nodes will be deformed significantly, as the coated struts approach close to one another to
start forming ‘new’ nodes at higher deformation level. Because the number of struts in both
nanotube and coated NCF is the same, the number of new nodes at higher compression should be
similar. Once the load is removed, coated nodes return to their pre-compressed configurations,
allowing the coated NCF to spring back to their original shape. We show our illustration
schematically in Figure 33.
Figure 33 Schematic representation shows the mechanism by which it makes NCF networks
superelastic.
59
4.3 Electrical Conductivity
Excellent electrical behavior of the CNT itself and effective connections between CNTs in
NCF result in high electrical conductivity of our NCFs in spite of the ultrahigh porosity. As shown
in Table 3, the electrical conductivity of the CNT ranges from 3 to 8 S/cm, which is much higher
than the values obtained in C3PMs fabricated by flash-freeze-drying or critical-point-drying
method with similar CNT fractions [15]. More research results of the electrical conductivity of
various carbon-based aerogels or foams are summarized in Table 3.
Table 3 Electrical conductivity of NCF compared with C3PMs fabricated through versatile
techniques.
4.4 Engineering Application
Due to the highly efficient open cell structure, high specific surface area, good electrical,
and mechanical properties, CNT foams and their derivatives (NCFs) find applications in many
fields including electric double-layer capacitors, compressive sensors, dampers, and electrodes for
60
energy storage devices. In this section, results of feasibility studies with a focus on flexible
conductors and strain-gauge sensors, flexible electrodes for lithium-air (Li-air) battery and double-
layer capacitors are presented. NCFs showed an enhancement in compressive sensing system as
evidenced by the stability and repeatability under cycles of compression. NCF films with
controllable structures as electrodes for Li-air battery showed enhanced discharge capacity
compared with other traditional carbon-based electrodes materials, which could further be
significantly improved by system optimization of the structures. NCFs also showed potentials to
be used as flexible electrodes for double-layer capacitors.
4.4.1 Flexible Conductors and Strain-gauge Sensors
Owing to NCF’s excellent compression recovery under a wide strain range and good
electrical conductivity, the as-prepared NCF shows potential applications as sensors or flexible
conductors in the smart material field. Figure 34a shows the apparent relative resistance change in
the NCF as a function of the applied compressive strain. The resistance change of the NCF was
recorded using the two-probe testing method in situ during the cycles of loading-unloading process.
The variation in the resistances for NCF in response to repeated compressive cycles resulted in an
instant resistance decrease with the loading process, whereas a complete and fast recovery was
recorded during the unloading process. Such reversible behavior is consistent over hundreds or
more cycles both in a relatively narrow strain region (0% to 30%) and in a wider strain region (30%
to 80%) at a speed of 10% min-1. Apparently, no creep behavior in resistance was observed which
in turn proved the structural robustness of NCF. Considering that NCF exhibits relatively stable
viscoelastic frequency- and temperature-invariant stability, frequency-dependence of the strain
sensor is also one of the major characteristics when utilized as a strain-gauge sensor. Herein, we
investigated the piezoresistivity behavior of the NCF at different frequencies ranging from 0.01
Hz to 0.4 Hz, as shown in Figure 34b. Interestingly, no apparent frequency dependence was
observed at the applied strain levels, which further indicates the fast and stable response of the
NCF based sensors. Moreover, a sinusoidal strain was applied to the NCF to directly measure the
delay time as illustrated in Figure 34c. The calculated electric delay time was approximately 300
ms, which approaches the level of some previously studied film-based sensor [154]. Our strain
sensors could operate in a relatively high-strain region of up to 6×105 μ , which is at least three
61
orders of magnitudes higher than that of the CNT buckypaper-based sensor (400 μ ). The gauge
factor (GF) defined as (ΔR/R0)/ was calculated to be 1.5. Although some carbon nanomaterials
based strain sensors exhibit a higher GF of up to 2000, while their small working strain range
always limit the practical applications [155]. For example, the graphene based strain gauge sensors
with a GF of 300 is less than 0.4% [156]. Compared to conventional strain sensors made of metal
with a GF up to 2.0 (<5% working strain range), the NCF-based sensor has the comparable GF
value, while maintaining the working strain range even 15 times higher [157].
Figure 34 Piezoresistive behaviors of the NCF. (a) Relative resistance change with applied
compression strain. (b) Resistance response at different frequencies under 20–30% strain. (c)
Resistance change recorded for a number of cycles at compressive strains between 5% and 45%
(with 500 ms inteval). (d) Resistance change recorded for a number of cycles at compressive
strains between 5% and 55%.
The variation in resistance of the compressed NCF was mainly determined by the contact
resistance and conducting paths between the nearby CNT cell walls. Considering the high porosity
of the NCF, the Poisson's ratio of the NCF was experimentally verified to be close to zero. Thus,
62
the density of the NCF linearly increased with the applied strain. During the deformation process,
the density of NCF increased while being applied with compressive strain, the surrounding cell
walls started to form contacts with one another. Thus, the formation of more conducting pathways
was increasing, yielding an increase in conduction and a decrease in resistance. Once the
compressive deformation was released gradually, such changes in conduction and resistance
recovered in the opposite manner. More importantly, the relative resistance change was almost
linear to the compressive strain even at high compressive strain levels.
Considering the potential application as large strain-gauge sensors, sensing stability and
robustness are major parameters regarded. To evaluate the sensing stability of the materials, the
NCF was subjected to over 300,000 cycles of fatigue testing using a micro-step controller. Figure
35a shows after 140,000 cyclic loading, the resistance of the tested NCF increased only 0.9 %.
These results suggest the very good and comparable resistance stability over continuous
deformation cycles. Meanwhile, the NCF could still maintain its mechanical stability and electrical
conductivity without apparent deformation.
Figure 35 Stability of NCF-based sensor. (a) Electro-mechanical test of NCF under cyclic strain
deformation. (b) Resistance change recorded for a number of cycles at compressive strains
between 5% and 55% at 200 °C.
In addition, the as-prepared NCF-based strain sensor can also operate in a relatively wide
temperature range from room temperature to 200 °C, as shown in Figure 35b. The resistance
changes with the applied strain levels of 0% to 50% at 200 °C were similar to those at room
temperature.
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4.4.2 Cathode Materials for Lithium-air (Li-air) Battery
The Li-air battery possesses a very high theoretical specific energy (gravimetric energy
density) originating from the use of lithium metal as the negative electrode and the consumption
of gaseous oxygen from the atmosphere at the positive electrode. Lithium is the lightest metal and
it provides a specific capacity of 3861 mAh/g, which is much greater than that of graphite or other
commercially available anodes [158]. The cathode oxidant, oxygen, is not stored in the cathode
and is supposed to be easily obtained from the surrounding environment, which is similar in a
sense to a gasoline engine. The theoretical capacity of a Li-air battery is limited by the pore volume
in the cathode available for the formation of Li2O2. Accordingly, the theoretical specific energy of
Li-air batteries using organic electrolyte is estimated up to be of 3000 Wh/kg which is 3-5 times
higher than that of Li-ion batteries.
Hence, in this study, we further investigated the capacity performance of NCF as cathode
materials for Li-air battery. Various techniques including electrochemical impedance spectroscopy
(EIS) and SEM were employed to study the discharge capacity mechanism. Also, the influence of
NCF pore size on the discharge capacity of Li-air battery was studied, attempting to form the
correlate pore structures with capacity.
Table 4 Parameters of different cathodes for capacity studies
64
For the study of the influence of the pore size in the cathode on the discharge capacity, five
different cathodes were fabricated. Technical information of the cathodes is summarized in Table
4. Their SEM images are shown in Figure 36 (a).
The multiple-discharge curves of Li-air cells with NCF electrode of different pore size
exposed to 1 atm oxygen atmosphere are depicted in Figure 36b. The first discharge capacity shows
a monotonic decrease with increasing pore size. And the capacity values are much larger than those
with buckypaper as cathode [159]. The high capacity results from the extreme high porosity of the
CNT foam sample. The capacity decreases with increasing pore size can be explained in multiple
ways:
Figure 36 Morphology and discharge performance of Li-air cells with NCF cathodes. (a)
Morphology of NCFs with different pore sizes (from left to right, the pore size is 4-8 m, 8-11
m, 17-21 m, 21-25 m, and 25-30 m. (b) Discharge curves of Li-air cells with different
cathodes at the current densities of 0.1 mA/cm2.
1. As described in various recent papers [160-163], mesopores are more effective than
micropores for Li2O2 decomposition. Large macropores contribute to an improved rate capability
and large capacity, which are likely due, to deeper oxygen diffusion into the electrode, and to
larger pore bottlenecks. In our case, since pore size is sufficiently large, the surface area also plays
65
a role. The discharge capacity of cells may simply be proportional to the externally accessible
surface area of the electrodes [164].
2. The NCF sample is highly porous thus easy to be compressed during the cell assembly.
Samples with large pore size are compressed more severely, leading to the larger reduced capacity.
Despite the similar pore volume results from BET measurement, the actual pore volume including
the macropore varies. The NCF with smaller pore size, such as 4-8 m, yields larger macro pore
volume and thus contributes to larger discharge capacity.
3. It is necessary to have some open pores for the oxygen supply. When the pore is too
large, those large pores are easily flooded by the electrolyte, reducing the generation of triple
junctions and increasing the weight of lithium air batteries. Therefore large pore size would not be
helpful in this case and the NCFs with pore size less than 4-8 m is possible to yield even larger
discharge capacity.
The electrochemical impedance spectroscopy (EIS) of a Li-air cell were recorded before
and after discharge at the current densities of 0.1 mA/cm2 as shown in Figure 37. An equivalent
electric circuit is used to simulate the EIS as shown in the inset of Figure 37. The high-frequency
intercept of the semicircle on the real axis is reflected by an ohmic resistance (Rs). The semicircle
in the high- and medium-frequency regions represents a parallel combination of charge-transfer
resistance (Rct) and constant phase element (ZQ). An inclined line in the low-frequency region is
related to a finite length Warburg element (Zw) arising from a diffusion-controlled process.
Figure 37 Electrochemical impedance spectra of a Li-air cell. The inset is the equivalent electric.
66
From the fitted results, the Rs of the Li-air cell before the first discharge is 95 Ω and the Rct
is 30 Ω. However, the Rs and the Rct of the Li-air cell after the first discharge at 0.1 mA/cm2
increase to 120 and 150 Ω, respectively. Rct is inversely proportional to the rate coefficient of the
chemical reaction, the porosity of the air electrode, and the oxygen concentration in the air
electrode [165]. During discharge, the porosity and oxygen concentration decreases due to pore
blocking, passivation of insoluble discharge product. Reduced porosity and oxygen concentration
in the air electrode are responsible for the growth of Rct. The discharge product, Li2O2, has the
electronic properties of an insulator [158].
The studies showed that NCF is an excellent cathode skeleton for Li–air batteries due to its
porous, conductive, freestanding nature. The battery also delivers a capacity as high as 9566
mAh/gc. The NCF also provides the opportunity to study the discharge mechanism based on the
hierarchically porous structure.
67
CHAPTER 5
SUMMARY AND CONCLUSIONS
5.1 Conclusions
Various networks of C3PMs have been extensively studied with different research focuses
that often yield case-specific results. Therefore, there have not been universal methods to create
CNT networks in 3D, which can be utilized for diverse applications from high strength to flexible
devices. Possible applications include absorbents, filters, tissue engineering, heat sink,
electrochemical devices etc. In this thesis work, networks of MWNT with crosslinking effect in
3D was created, so-called Nanocarbon Foam (NCF). Their structural, porous, mechanical, and
electrical characteristics were identified in order to explore the possibility of using the porous NCF
for diverse engineering applications.
In Chapter 3 and 4, we described creating NCF and characterizing their structural,
mechanical, and electrical properties. We created graphitic flakes in the CNT foam to introduce
crosslinking that enhanced flexibility and robustness. In this way, we were able to create
superelastic NCF with a full recovery from a large compressive strain (> 90%) and excellent
fatigue resistance under extreme conditions such as over 300,000 compression/release cycles. The
corsslinking was done by converting PAN polymer into graphitic flakes on MWNT surface using
two-step pyrolysis. The superelastic and robust NCFs yielded a porosity higher than 99%.
Interestingly, though somewhat expected, NCFs showed temperature-invariant complete recovery
in the temperature range from room temperature to 400 °C. This superior property originated from
the inherent strength of CNT springs based on stable carbon-carbon bonds in the graphitic walls
of CNTs, van der Waals interactions within the linear viscoelastic regime. Therefore, the van der
Waals interaction-driven graphitic crosslinking allowed for the temperature-invariant mechanical
properties. We also observed that NCF showed a high specific surface area of ~ 400 m2 g-1. The
corresponding porosity was higher than 99%. The highest electrical conductivity obtained in NCF
as 8 S cm-1 was in a sample with the density of 150 mg/cm3. The NCF displays a hierarchically
porous structure, which includes different level of pores.
68
The engineering application side of this thesis work on NCFs is discussed in Chapter 4.
We first determined the potential of NCF to be used as strain gauge sensors by examining its
electrical response under cycles of compression/release. It was found that the NCF-based sensor
showed repeatability in resistance change, fast response, high stability and robustness.
Furthermore, NCF sheets with controllable structures were tested as electrode materials for Li-air
battery application. The discharge capacity of a Li-air battery with a NCF cathode showed higher
values compared with buckypaper or carbon fiber paper as cathode materials.
All the interpretation of the data, results, and engineering applications presented in this
thesis would be able to provide enough evidence on the application level that the NCFs have
intriguing potential to be used for many applications that require hierarchically porous structure,
high surface area, large porosity, and a great degree of tunability of mechanical properties.
5.2 Recommendation for Future Research
5.2.1 NCF Structural Improvement and Modelling
In order to interpret the relationship between porous structure and diverse properties,
different models should be established. The proposed work mainly includes three parts: (1) the
relationship between the amount of each building block (CNT, PAN, PMMA) with real dimensions
and densities; (2) the relationship between pressure and elastic deformation mechanism; (3) the
relationship between pressure and resistance change. For convenience, it is assumed that the
pressure applied on the top surface is the same everywhere. Therefore, the focus of this task is to
build up the multi-physical model that simulates the NCF structure and model it under different
conditions.
Professional modelling software such as Abaqus should be performed to correlate those
relationship. For Abaqus, the preformed module includes: Part, Property, Assembly Step,
Interaction Load, Mesh Creating, Job and Visualization. Especially for the compressive process
simulation, accurate visualized models from Solid Works can be easily embedded into Abaqus to
simulate how the pore structure of NCF deforms during the compressive process. The simulated
results will also give hints to better control the structure during the fabrication process.
69
5.2.2 NCF as a Skeleton to Fabricate Hybrid Systems
Phase 1: NCF-polymer composites through foam infiltration
The porosity and permeability of the NCF open a new way to create NCF-polymer
composite materials through direct polymer infiltration of the dry NCF foam samples. Future work
could fabricate model composites using common polymers spanning high impact strength, rubber-
like elastomeric characteristics, and energy damping capabilities: epoxy, polydimethylsiloxane
(PDMS), and PMMA. Traditional CNT composites are usually fabricated by directly mixing CNTs
with the polymer, attaining percolation of the CNT structure is problematic because it requires
either different mixing to sufficiently disperse CNTs into high-viscosity polymer melts or solutions,
or days of waiting time to achieve controlled evaporation. In contrast, our fabrication process
involves simply introducing the polymer solution drop-by-drop into the foam samples or
immersing the foam samples into polymer solution aided by vacuum, followed by heat curing or
other post-treatment. The resulting composites are truly multi-functional materials, where the
polymer matrix dominates the mechanical properties, but the composite retains the high
conductivity of the original NCF.
Figure 38 Morphology and compressive behavior of NCF-Epoxy composites. (a-b) Morphology
of NCF-Epoxy composites. (c) Compressive behavior of NCF-Epoxy composites (the strength of
the sample is over the measurement capability of the test equipment).
70
For example, infiltration with epoxy increases the compressive modulus by two orders of
magnitude (Figure 38c), while infiltration with PDMS produces a composite with ideally elastic
behavior in compression. In all cases, the final composite retained over 50% of the conductivity
of the NCF before infiltration. This is in agreement with the results reported for CNT/graphene-
polymer composites also made by direct polymer infiltration, using foams produced via CVD [76,
166].
Phase 2: NCF as the Backbone for Electrochemical Devices
For the Li-air battery, the following research should be focused at evaluating how different
parameters would govern the whole device’s performance. New approaches to address the issues
are listed below:
Samples with different porosity (densities), different PAN/CNT ratio should be tested. A
proposed layered structure with different density or pore size in each layer in the NCF is promising
to render better performance. A balance between the mesopore and the macropore structures of the
air electrode is required to realize the full performance potential of Li-air battery. Ideally, an
electrode with a channel opening that increases gradually from inside to outside will minimize
electrode blocking during discharge.
Another important aspect is to search for a catalyst that is soluble in the organic electrolyte.
As illustrated from the calculation, despite the large discharge capacity, most of the voids are not
filled in the cathode. If we can find a catalyst that enables Li2O2 to grow and decompose without
direct contact with carbon, the electrochemical performance can be greatly enhanced. A solution-
phase bifunctional catalyst is reported and may apply to this scenario [167].
Lithium ion (Li ion) batteries are the dominant power source for portable electronic devices.
Among the anode materials, silicon (Si) attracts lots of attention due to its high theoretical specific
capacity of ~ 4200 mAhg-1 with mechanical integrity [168]. However, significant challenges still
remain for single-component Si anodes, which originate from their poor structural stability and
low electrical conductivity. Notably, integrating nano-Si with a conductive carbon matrix is
considered a promising strategy for improving device performance [169]. Therefore, it’s critical
to build robust and elastic substances to encapsulate Si nano-particles. In addition, maintaining the
electrical connection among the nanostructured Si particles, conductive additives, and current
collectors is another challenge considering the large volume change from even stabilized Si. A
71
robust system built from NCF with nanosized particles should be beneficial for the performance
of Li ion battery. First, the porous structure of NCF allows the volume expansion of Si, while the
conductive carbon scaffold facilitates electron transport. Second, the elasticity and flexibility of
the structure not only ensures intimate contact between Si and conductive networks, which enables
easy electron transport and limits the solid electrolyte interfaces (SEI) formation on individual Si
particles, but also effectively confines Si during volume variation and improves the electrode
integrity. Such a unique design will endow the electrodes with high power and long cycling
stability.
72
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REFERENCES
1. Endo, M., M.S. Strano, and P.M. Ajayan, Potential applications of carbon nanotubes, in
Carbon nanotubes. 2008, Springer. p. 13-61.
2. Harris, P.J. and P.J.P.J.F. Harris, Carbon nanotubes and related structures: new materials
for the twenty-first century. 2001: Cambridge university press.
3. Baughman, R.H., A.A. Zakhidov, and W.A. de Heer, Carbon Nanotubes--the Route Toward
Applications. Science, 2002. 297(5582): p. 787-792.
4. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. Science, 2004.
306(5696): p. 666-669.
5. Rao, C.e.N.e.R., et al., Graphene: The New Two‐Dimensional Nanomaterial. Angewandte
Chemie International Edition, 2009. 48(42): p. 7752-7777.
6. Meric, I., et al., Current saturation in zero-bandgap, top-gated graphene field-effect
transistors. Nature nanotechnology, 2008. 3(11): p. 654-659.
7. Bai, J. and Y. Huang, Fabrication and electrical properties of graphene nanoribbons.
Materials Science and Engineering: R: Reports, 2010. 70(3–6): p. 341-353.
8. Javey, A., et al., Ballistic carbon nanotube field-effect transistors. Nature, 2003. 424(6949):
p. 654-657.
9. Pop, E., et al., Thermal Conductance of an Individual Single-Wall Carbon Nanotube above
Room Temperature. Nano Letters, 2005. 6(1): p. 96-100.
10. Balandin, A.A., et al., Superior Thermal Conductivity of Single-Layer Graphene. Nano
Letters, 2008. 8(3): p. 902-907.
11. Yu, M.-F., et al., Tensile Loading of Ropes of Single Wall Carbon Nanotubes and their
Mechanical Properties. Physical Review Letters, 2000. 84(24): p. 5552-5555.
12. Wang, X., G. Sun, and P. Chen, Three-dimensional porous architectures of carbon
nanotubes and graphene sheets for energy applications. Frontiers in Energy Research,
2014. 2.
13. Lee, S.H., et al., Tailored Assembly of Carbon Nanotubes and Graphene. Advanced
Functional Materials, 2011. 21(8): p. 1338-1354.
14. Nardecchia, S., et al., Three dimensional macroporous architectures and aerogels built of
carbon nanotubes and/or graphene: synthesis and applications. Chemical Society
Reviews, 2013. 42(2): p. 794-830.
98
15. Du, R., et al., Macroscopic Carbon Nanotube-based 3D Monoliths. Small, 2015. 11(27): p.
3263-3289.
16. Cranford, S.W., et al., Nonlinear material behaviour of spider silk yields robust webs.
Nature, 2012. 482(7383): p. 72-76.
17. Sun, H., Z. Xu, and C. Gao, Multifunctional, Ultra-Flyweight, Synergistically Assembled
Carbon Aerogels. Advanced Materials, 2013. 25(18): p. 2554-2560.
18. Mubarak, N.M., et al., An overview on methods for the production of carbon nanotubes.
Journal of Industrial and Engineering Chemistry, 2014. 20(4): p. 1186-1197.
19. Chen, Y. and J. Zhang, Chemical Vapor Deposition Growth of Single-Walled Carbon
Nanotubes with Controlled Structures for Nanodevice Applications. Accounts of
Chemical Research, 2014. 47(8): p. 2273-2281.
20. Li, W.Z., et al., Large-Scale Synthesis of Aligned Carbon Nanotubes. Science, 1996.
274(5293): p. 1701-1703.
21. Ren, Z.F., et al., Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass.
Science, 1998. 282(5391): p. 1105-1107.
22. Fan, S., et al., Self-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission
Properties. Science, 1999. 283(5401): p. 512-514.
23. Wei, B.Q., et al., Microfabrication technology: Organized assembly of carbon nanotubes.
Nature, 2002. 416(6880): p. 495-496.
24. Andrews, R., et al., Continuous production of aligned carbon nanotubes: a step closer to
commercial realization. Chemical Physics Letters, 1999. 303(5–6): p. 467-474.
25. Hata, K., et al., Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled
Carbon Nanotubes. Science, 2004. 306(5700): p. 1362-1364.
26. Zhang, G., et al., Ultra-high-yield growth of vertical single-walled carbon nanotubes:
Hidden roles of hydrogen and oxygen. Proceedings of the National Academy of Sciences
of the United States of America, 2005. 102(45): p. 16141-16145.
27. Ci, L., et al., Ultrathick Freestanding Aligned Carbon Nanotube Films. Advanced Materials,
2007. 19(20): p. 3300-3303.
28. Cao, A., et al., Super-Compressible Foamlike Carbon Nanotube Films. Science, 2005.
310(5752): p. 1307-1310.
29. TalapatraS, et al., Direct growth of aligned carbon nanotubes on bulk metals. Nat Nano,
2006. 1(2): p. 112-116.
99
30. Lee, S., et al., Utilizing 3D SERS Active Volumes in Aligned Carbon Nanotube Scaffold
Substrates. Advanced Materials, 2012. 24(38): p. 5261-5266.
31. Cho, W., M. Schulz, and V. Shanov, Growth and characterization of vertically aligned
centimeter long CNT arrays. Carbon, 2014. 72(0): p. 264-273.
32. Yaglioglu, O., et al., Wide Range Control of Microstructure and Mechanical Properties of
Carbon Nanotube Forests: A Comparison Between Fixed and Floating Catalyst CVD
Techniques. Advanced Functional Materials, 2012. 22(23): p. 5028-5037.
33. Gui, X., et al., Carbon Nanotube Sponges. Advanced Materials, 2010. 22(5): p. 617-621.
34. Xu, M., et al., Carbon Nanotubes with Temperature-Invariant Viscoelasticity from –196° to
1000°C. Science, 2010. 330(6009): p. 1364-1368.
35. Sumpter, B.G., et al., A theoretical and experimental study on manipulating the structure
and properties of carbon nanotubes using substitutional dopants. International Journal of
Quantum Chemistry, 2009. 109(1): p. 97-118.
36. Sharifi, T., et al., Nitrogen doped multi walled carbon nanotubes produced by CVD-
correlating XPS and Raman spectroscopy for the study of nitrogen inclusion. Carbon,
2012. 50(10): p. 3535-3541.
37. Hashim, D.P., et al., Covalently bonded three-dimensional carbon nanotube solids via boron
induced nanojunctions. Sci. Rep., 2012. 2.
38. Shan, C., et al., Three-Dimensional Nitrogen-Doped Multiwall Carbon Nanotube Sponges
with Tunable Properties. Nano Letters, 2013. 13(11): p. 5514-5520.
39. Vaisman, L., H.D. Wagner, and G. Marom, The role of surfactants in dispersion of carbon
nanotubes. Advances in Colloid and Interface Science, 2006. 128–130(0): p. 37-46.
40. Wang, Y., Z. Iqbal, and S. Mitra, Rapidly Functionalized, Water-Dispersed Carbon
Nanotubes at High Concentration. Journal of the American Chemical Society, 2005.
128(1): p. 95-99.
41. Datsyuk, V., et al., Chemical oxidation of multiwalled carbon nanotubes. Carbon, 2008.
46(6): p. 833-840.
42. Green, M.J., Analysis and measurement of carbon nanotube dispersions: nanodispersion
versus macrodispersion. Polymer International, 2010. 59(10): p. 1319-1322.
43. Worsley, M.A., et al., Synthesis of Graphene Aerogel with High Electrical Conductivity.
Journal of the American Chemical Society, 2010. 132(40): p. 14067-14069.
44. Pierre, A.C. and G.M. Pajonk, Chemistry of Aerogels and Their Applications. Chemical
Reviews, 2002. 102(11): p. 4243-4266.
100
45. Islam, M.F., et al., High Weight Fraction Surfactant Solubilization of Single-Wall Carbon
Nanotubes in Water. Nano Letters, 2003. 3(2): p. 269-273.
46. Bryning, M.B., et al., Carbon Nanotube Aerogels. Advanced Materials, 2007. 19(5): p. 661-
664.
47. Hough, L.A., et al., Structure of Semidilute Single-Wall Carbon Nanotube Suspensions and
Gels. Nano Letters, 2006. 6(2): p. 313-317.
48. Ostojic, G.N., Optical Properties of Assembled Single-Walled Carbon Nanotube Gels.
ChemPhysChem, 2012. 13(8): p. 2102-2107.
49. Thongprachan, N., et al., Preparation of macroporous solid foam from multi-walled carbon
nanotubes by freeze-drying technique. Materials Chemistry and Physics, 2008. 112(1): p.
262-269.
50. Worsley, M.A., et al., Mechanically robust and electrically conductive carbon nanotube
foams. Applied Physics Letters, 2009. 94(7): p. -.
51. Worsley, M.A., et al., Properties of single-walled carbon nanotube-based aerogels as a
function of nanotube loading. Acta Materialia, 2009. 57(17): p. 5131-5136.
52. Kohlmeyer, R.R., et al., Preparation of stable carbon nanotube aerogels with high electrical
conductivity and porosity. Carbon, 2011. 49(7): p. 2352-2361.
53. Skaltsas, T., G. Avgouropoulos, and D. Tasis, Impact of the fabrication method on the
physicochemical properties of carbon nanotube-based aerogels. Microporous and
Mesoporous Materials, 2011. 143(2–3): p. 451-457.
54. Gutiérrez, M.C., et al., Macroporous 3D Architectures of Self-Assembled MWCNT Surface
Decorated with Pt Nanoparticles as Anodes for a Direct Methanol Fuel Cell. The Journal
of Physical Chemistry C, 2007. 111(15): p. 5557-5560.
55. Park, S.-H., et al., Morphology control of three-dimensional carbon nanotube
macrostructures fabricated using ice-templating method. Journal of Porous Materials,
2013. 20(5): p. 1289-1297.
56. Nardecchia, S., et al., Modulating the cytocompatibility of tridimensional carbon nanotube-
based scaffolds. Journal of Materials Chemistry B, 2013. 1(24): p. 3064-3072.
57. Kim, K.H., Y. Oh, and M.F. Islam, Graphene coating makes carbon nanotube aerogels
superelastic and resistant to fatigue. Nat Nano, 2012. 7(9): p. 562-566.
58. Gutiérrez, M.C., M.L. Ferrer, and F. del Monte, Ice-Templated Materials: Sophisticated
Structures Exhibiting Enhanced Functionalities Obtained after Unidirectional Freezing
and Ice-Segregation-Induced Self-Assembly†. Chemistry of Materials, 2008. 20(3): p.
634-648.
101
59. Kwon, S.-M., H.-S. Kim, and H.-J. Jin, Multiwalled carbon nanotube cryogels with aligned
and non-aligned porous structures. Polymer, 2009. 50(13): p. 2786-2792.
60. Zou, J., et al., Ultralight Multiwalled Carbon Nanotube Aerogel. ACS Nano, 2010. 4(12): p.
7293-7302.
61. Cui, Y. and M. Zhang, Cross-links in Carbon Nanotube Assembly Introduced by Using
Polyacrylonitrile as Precursor. ACS Applied Materials & Interfaces, 2013. 5(16): p.
8173-8178.
62. Cui, Y. and M. Zhang, Fabrication of cross-linked carbon nanotube foam using
polymethylmethacrylate microspheres as templates. Journal of Materials Chemistry A,
2013. 1(44): p. 13984-13988.
63. Carriazo, D., et al., Resorcinol-Based Deep Eutectic Solvents as Both Carbonaceous
Precursors and Templating Agents in the Synthesis of Hierarchical Porous Carbon
Monoliths. Chemistry of Materials, 2010. 22(22): p. 6146-6152.
64. Gutiérrez, M.C., et al., Deep-Eutectic-Solvent-Assisted Synthesis of Hierarchical Carbon
Electrodes Exhibiting Capacitance Retention at High Current Densities. Chemistry – A
European Journal, 2011. 17(38): p. 10533-10537.
65. Liu, Y., et al., Macroscopic shaping of carbon nanotubes with high specific surface area
and full accessibility. Materials Letters, 2012. 79(0): p. 128-131.
66. Liu, Y., et al., Synthesis of porous carbon nanotubes foam composites with a high accessible
surface area and tunable porosity. Journal of Materials Chemistry A, 2013. 1(33): p.
9508-9516.
67. Carter, R., et al., Solution Assembled Single-Walled Carbon Nanotube Foams: Superior
Performance in Supercapacitors, Lithium-Ion, and Lithium–Air Batteries. The Journal of
Physical Chemistry C, 2014. 118(35): p. 20137-20151.
68. Kim, K.H., Y. Oh, and M.F. Islam, Mechanical and Thermal Management Characteristics
of Ultrahigh Surface Area Single-Walled Carbon Nanotube Aerogels. Advanced
Functional Materials, 2013. 23(3): p. 377-383.
69. Yang, C.-M., et al., Effect of Purification on Pore Structure of HiPco Single-Walled Carbon
Nanotube Aggregates. Nano Letters, 2002. 2(4): p. 385-388.
70. Jung, S.M., et al., A facile route for 3D aerogels from nanostructured 1D and 2D materials.
Sci. Rep., 2012. 2.
71. Nakagawa, K., et al., Freeze-dried solid foams prepared from carbon nanotube aqueous
suspension: Application to gas diffusion layers of a proton exchange membrane fuel cell.
Chemical Engineering and Processing: Process Intensification, 2011. 50(1): p. 22-30.
102
72. Yakobson, B.I., C.J. Brabec, and J. Bernholc, Nanomechanics of Carbon Tubes: Instabilities
beyond Linear Response. Physical Review Letters, 1996. 76(14): p. 2511-2514.
73. Iijima, S., et al., Structural flexibility of carbon nanotubes. The Journal of Chemical Physics,
1996. 104(5): p. 2089-2092.
74. Gui, X., et al., Soft, Highly Conductive Nanotube Sponges and Composites with Controlled
Compressibility. ACS Nano, 2010. 4(4): p. 2320-2326.
75. Gui, X., et al., Elastic shape recovery of carbon nanotube sponges in liquid oil. Journal of
Materials Chemistry, 2012. 22(35): p. 18300-18305.
76. Gui, X., et al., A Facile Route to Isotropic Conductive Nanocomposites by Direct Polymer
Infiltration of Carbon Nanotube Sponges. ACS Nano, 2011. 5(6): p. 4276-4283.
77. Gross, J. and G. Scherer, Structural Efficiency and Microstructural Modeling of Wet Gels
and Aerogels. Journal of Sol-Gel Science and Technology, 1998. 13(1-3): p. 957-960.
78. Woignier, T., et al., Different kinds of structure in aerogels: relationships with the
mechanical properties. Journal of Non-Crystalline Solids, 1998. 241(1): p. 45-52.
79. Bryning, M.B., et al., Thermal conductivity and interfacial resistance in single-wall carbon
nanotube epoxy composites. Applied Physics Letters, 2005. 87(16): p. -.
80. Hsieh, W.H., et al., Experimental investigation of heat-transfer characteristics of aluminum-
foam heat sinks. International Journal of Heat and Mass Transfer, 2004. 47(23): p. 5149-
5157.
81. Charnvanichborikarn, S., et al., Heavy-ion-induced modification of structural and
mechanical properties of carbon-nanotube aerogels. Carbon, 2013. 57(0): p. 310-316.
82. Peng, B., et al., Measurements of near-ultimate strength for multiwalled carbon nanotubes
and irradiation-induced crosslinking improvements. Nat Nano, 2008. 3(10): p. 626-631.
83. Stahl, H., et al., Intertube Coupling in Ropes of Single-Wall Carbon Nanotubes. Physical
Review Letters, 2000. 85(24): p. 5186-5189.
84. Salvato, M., et al., Charge Transport and Tunneling in Single-Walled Carbon Nanotube
Bundles. Physical Review Letters, 2008. 101(24): p. 246804.
85. Kwon, Y.-K., S. Saito, and D. Tománek, Effect of intertube coupling on the electronic
structure of carbon nanotube ropes. Physical Review B, 1998. 58(20): p. R13314-
R13317.
86. Zhang, K.J., et al., Thermal and Electrical Transport in Ultralow Density Single-Walled
Carbon Nanotube Networks. Advanced Materials, 2013. 25(21): p. 2926-2931.
103
87. Itkis, M.E., et al., Thermal Conductivity Measurements of Semitransparent Single-Walled
Carbon Nanotube Films by a Bolometric Technique. Nano Letters, 2007. 7(4): p. 900-
904.
88. Zhou, C., J. Kong, and H. Dai, Intrinsic Electrical Properties of Individual Single-Walled
Carbon Nanotubes with Small Band Gaps. Physical Review Letters, 2000. 84(24): p.
5604-5607.
89. Morozov, S.V., et al., Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer.
Physical Review Letters, 2008. 100(1): p. 016602.
90. Han, M.Y., J.C. Brant, and P. Kim, Electron Transport in Disordered Graphene
Nanoribbons. Physical Review Letters, 2010. 104(5): p. 056801.
91. Camilli, L., et al., Pressure-dependent electrical conductivity of freestanding three-
dimensional carbon nanotube network. Applied Physics Letters, 2013. 102(18): p. -.
92. Chen, M., et al., Highly conductive and stretchable polymer composites based on
graphene/MWCNT network. Chemical Communications, 2013. 49(16): p. 1612-1614.
93. Kim, K.H., M. Vural, and M.F. Islam, Single-Walled Carbon Nanotube Aerogel-Based
Elastic Conductors. Advanced Materials, 2011. 23(25): p. 2865-2869.
94. Charnvanichborikarn, S., et al., Tailoring properties of carbon-nanotube-based foams by ion
bombardment. Applied Physics Letters, 2012. 101(10): p. -.
95. Martin, C.A., et al., Formation of percolating networks in multi-wall carbon-nanotube–epoxy composites. Composites Science and Technology, 2004. 64(15): p. 2309-2316.
96. Grunlan, J.C., et al., Water-Based Single-Walled-Nanotube-Filled Polymer Composite with
an Exceptionally Low Percolation Threshold. Advanced Materials, 2004. 16(2): p. 150-
153.
97. Sandler, J.K.W., et al., Ultra-low electrical percolation threshold in carbon-nanotube-epoxy
composites. Polymer, 2003. 44(19): p. 5893-5899.
98. Kymakis, E., I. Alexandou, and G.A.J. Amaratunga, Single-walled carbon nanotube–polymer composites: electrical, optical and structural investigation. Synthetic Metals,
2002. 127(1–3): p. 59-62.
99. Wang, M., et al., Modifying Native Nanocellulose Aerogels with Carbon Nanotubes for
Mechanoresponsive Conductivity and Pressure Sensing. Advanced Materials, 2013.
25(17): p. 2428-2432.
100. Prasher, R.S., et al., Turning Carbon Nanotubes from Exceptional Heat Conductors into
Insulators. Physical Review Letters, 2009. 102(10): p. 105901.
104
101. Chalopin, Y., S. Volz, and N. Mingo, Upper bound to the thermal conductivity of carbon
nanotube pellets. Journal of Applied Physics, 2009. 105(8): p. -.
102. Nirmalraj, P.N., et al., Electrical Connectivity in Single-Walled Carbon Nanotube Networks.
Nano Letters, 2009. 9(11): p. 3890-3895.
103. Lu, X., et al., Thermal and electrical conductivity of monolithic carbon aerogels. Journal of
Applied Physics, 1993. 73(2): p. 581-584.
104. Feng, J., J. Feng, and C. Zhang, Thermal conductivity of low density carbon aerogels.
Journal of Porous Materials, 2012. 19(5): p. 551-556.
105. Bock, V., et al., Thermal properties of carbon aerogels. Journal of Non-Crystalline Solids,
1995. 185(3): p. 233-239.
106. Winter, M. and R.J. Brodd, What Are Batteries, Fuel Cells, and Supercapacitors? Chemical
Reviews, 2004. 104(10): p. 4245-4270.
107. Li, P., et al., Highly deformation-tolerant carbon nanotube sponges as supercapacitor
electrodes. Nanoscale, 2013. 5(18): p. 8472-8479.
108. Simon, P. and Y. Gogotsi, Materials for electrochemical capacitors. Nat Mater, 2008. 7(11):
p. 845-854.
109. Winter, M. and R.J. Brodd, What Are Batteries, Fuel Cells, and Supercapacitors? (Chem.
Rev. 2003, 104, 4245−4269. Published on the Web 09/28/2004.). Chemical Reviews,
2005. 105(3): p. 1021-1021.
110. Arico, A.S., et al., Nanostructured materials for advanced energy conversion and storage
devices. Nat Mater, 2005. 4(5): p. 366-377.
111. Zhong, J., et al., Carbon nanotube sponges as conductive networks for supercapacitor
devices. Nano Energy, 2013. 2(5): p. 1025-1030.
112. Chen, W., et al., High-Performance Nanostructured Supercapacitors on a Sponge. Nano
Letters, 2011. 11(12): p. 5165-5172.
113. Chen, W., R.B. Rakhi, and H.N. Alshareef, High energy density supercapacitors using
macroporous kitchen sponges. Journal of Materials Chemistry, 2012. 22(29): p. 14394-
14402.
114. Chen, X., et al., MWCNT/V2O5 Core/Shell Sponge for High Areal Capacity and Power
Density Li-Ion Cathodes. ACS Nano, 2012. 6(9): p. 7948-7955.
115. Shen, Y., et al., A high-capacity lithium–air battery with Pd modified carbon nanotube
sponge cathode working in regular air. Carbon, 2013. 62(0): p. 288-295.
105
116. Zhou, G., F. Li, and H.-M. Cheng, Progress in flexible lithium batteries and future
prospects. Energy & Environmental Science, 2014. 7(4): p. 1307-1338.
117. Hu, L., et al., Silicon–Carbon Nanotube Coaxial Sponge as Li-Ion Anodes with High Areal
Capacity. Advanced Energy Materials, 2011. 1(4): p. 523-527.
118. Wang, X., et al., Building sponge-like robust architectures of CNT-graphene-Si composites
with enhanced rate and cycling performance for lithium-ion batteries. Journal of
Materials Chemistry A, 2015. 3(7): p. 3962-3967.
119. Ge, D., et al., In Situ Synthesis of Hybrid Aerogels from Single-Walled Carbon Nanotubes
and Polyaniline Nanoribbons as Free-Standing, Flexible Energy Storage Electrodes.
Chemistry of Materials, 2014. 26(4): p. 1678-1685.
120. Pu, X., G. Yang, and C. Yu, Liquid-Type Cathode Enabled by 3D Sponge-Like Carbon
Nanotubes for High Energy Density and Long Cycling Life of Li-S Batteries. Advanced
Materials, 2014. 26(44): p. 7456-7461.
121. Chen, J., T. Matsuura, and M. Hori, Novel gas diffusion layer with water management
function for PEMFC. Journal of Power Sources, 2004. 131(1–2): p. 155-161.
122. Kawase, M., T. Inagaki, and K. Miura, Nonisothermal Through-Plane Transport Model of
PEMFC with Local VLE Assumption. Meeting Abstracts, 2008. MA2008-02(11): p. 834.
123. Ziegler, C. and D. Gerteisen, Validity of two-phase polymer electrolyte membrane fuel cell
models with respect to the gas diffusion layer. Journal of Power Sources, 2009. 188(1): p.
184-191.
124. Shao, F., et al., Growth of Various TiO2 Nanostructures for Dye-Sensitized Solar Cells. The
Journal of Physical Chemistry C, 2010. 115(5): p. 1819-1823.
125. Chen, J., et al., The application of a three dimensional CNT-sponge as the counter electrode
for dye-sensitized solar cells. Carbon, 2012. 50(15): p. 5624-5627.
126. Chen, L., et al., Electro- and Photodriven Phase Change Composites Based on Wax-
Infiltrated Carbon Nanotube Sponges. ACS Nano, 2012. 6(12): p. 10884-10892.
127. Dong, X., et al., Synthesis of graphene-carbon nanotube hybrid foam and its use as a novel
three-dimensional electrode for electrochemical sensing. Journal of Materials Chemistry,
2012. 22(33): p. 17044-17048.
128. Sizgek, G.D., et al., Mesoporous Zirconium Titanium Oxides. Part 2: Synthesis, Porosity,
and Adsorption Properties of Beads. Langmuir, 2008. 24(21): p. 12323-12330.
129. Sizgek, G.D., et al., Mesoporous Zirconium Titanium Oxides. Part 3. Synthesis and
Adsorption Properties of Unfunctionalized and Phosphonate-Functionalized
Hierarchical Polyacrylonitrile-F-127-Templated Beads. Langmuir, 2009. 25(19): p.
11874-11882.
106
130. Yuan, J., et al., Superwetting nanowire membranes for selective absorption. Nat Nano,
2008. 3(6): p. 332-336.
131. Zhu, K., et al., Oil spill cleanup from sea water by carbon nanotube sponges. Frontiers of
Materials Science, 2013. 7(2): p. 170-176.
132. Gui, X., et al., Magnetic and Highly Recyclable Macroporous Carbon Nanotubes for Spilled
Oil Sorption and Separation. ACS Applied Materials & Interfaces, 2013. 5(12): p. 5845-
5850.
133. Dong, X., et al., Superhydrophobic and superoleophilic hybrid foam of graphene and
carbon nanotube for selective removal of oils or organic solvents from the surface of
water. Chemical Communications, 2012. 48(86): p. 10660-10662.
134. Li, H., et al., Photocatalytic, recyclable CdS nanoparticle-carbon nanotube hybrid sponges.
Nano Research, 2012. 5(4): p. 265-271.
135. Rogers, J.A., T. Someya, and Y. Huang, Materials and Mechanics for Stretchable
Electronics. Science, 2010. 327(5973): p. 1603-1607.
136. Oren, Y., Capacitive deionization (CDI) for desalination and water treatment — past,
present and future (a review). Desalination, 2008. 228(1–3): p. 10-29.
137. Wang, L., et al., Capacitive deionization of NaCl solutions using carbon nanotube sponge
electrodes. Journal of Materials Chemistry, 2011. 21(45): p. 18295-18299.
138. Peng, Q., et al., Graphene Nanoribbon Aerogels Unzipped from Carbon Nanotube Sponges.
Advanced Materials, 2014. 26(20): p. 3241-3247.
139. Fitzer, E., W. Frohs, and M. Heine, Optimization of stabilization and carbonization
treatment of PAN fibres and structural characterization of the resulting carbon fibres.
Carbon, 1986. 24(4): p. 387-395.
140. Luo, X. and P.T. Mather, Conductive shape memory nanocomposites for high speed
electrical actuation. Soft Matter, 2010. 6(10): p. 2146-2149.
141. Chae, H.G., et al., Stabilization and carbonization of gel spun polyacrylonitrile/single wall
carbon nanotube composite fibers. Polymer, 2007. 48(13): p. 3781-3789.
142. Wang, X., N. Feng, and S. Chang, Effect of precured degrees on morphology, thermal, and
mechanical properties of BR/SBR/NR foams. Polymer Composites, 2013. 34(6): p. 849-
859.
143. Gu, R., S. Konar, and M. Sain, Preparation and Characterization of Sustainable
Polyurethane Foams from Soybean Oils. Journal of the American Oil Chemists' Society,
2012. 89(11): p. 2103-2111.
107
144. Brunauer, S., P.H. Emmett, and E. Teller, Adsorption of Gases in Multimolecular Layers.
Journal of the American Chemical Society, 1938. 60(2): p. 309-319.
145. Dong, L., et al., Facile preparation of carbon nanotube aerogels with controlled
hierarchical microstructures and versatile performance. Carbon, 2015. 90: p. 164-171.
146. Brantley, S.L. and N.P. Mellott, Surface area and porosity of primary silicate minerals.
American Mineralogist, 2000. 85(11-12): p. 1767-1783.
147. Sing, K.S.W., et al., Reporting Physisorption Data for Gas/Solid Systems, in Handbook of
Heterogeneous Catalysis. 2008, Wiley-VCH Verlag GmbH & Co. KGaA.
148. Worsley, M.A., et al., Carbon Scaffolds for Stiff and Highly Conductive Monolithic Oxide–Carbon Nanotube Composites. Chemistry of Materials, 2011. 23(12): p. 3054-3061.
149. Menna, C., C.E. Bakis, and A. Prota, Effect of nanofiller length and orientation distributions
on Mode I fracture toughness of unidirectional fiber composites. Journal of Composite
Materials, 2015.
150. Gibson, L.J. and M.F. Ashby, Cellular solids: structure and properties. 1997: Cambridge
university press.
151. Qiu, L., et al., Biomimetic superelastic graphene-based cellular monoliths. Nat Commun,
2012. 3: p. 1241.
152. Hu, H., et al., Ultralight and Highly Compressible Graphene Aerogels. Advanced Materials,
2013. 25(15): p. 2219-2223.
153. Hough, L.A., et al., Viscoelasticity of Single Wall Carbon Nanotube Suspensions. Physical
Review Letters, 2004. 93(16): p. 168102.
154. Xiao, X., et al., High-Strain Sensors Based on ZnO Nanowire/Polystyrene Hybridized
Flexible Films. Advanced Materials, 2011. 23(45): p. 5440-5444.
155. Stampfer, C., et al., Nano-Electromechanical Displacement Sensing Based on Single-Walled
Carbon Nanotubes. Nano Letters, 2006. 6(7): p. 1449-1453.
156. Zhao, J., et al., Ultra-sensitive strain sensors based on piezoresistive nanographene films.
Applied Physics Letters, 2012. 101(6): p. 063112.
157. Window, A.L. and G.S. Holister, Strain gauge technology. 1982: Applied science
publishers.
158. Christensen, J., et al., A Critical Review of Li/Air Batteries. Journal of The Electrochemical
Society, 2011. 159(2): p. R1-R30.
159. Li, Y., K. Huang, and Y. Xing, A hybrid Li-air battery with buckypaper air cathode and
sulfuric acid electrolyte. Electrochimica Acta, 2012. 81(0): p. 20-24.
108
160. Chervin, C.N., et al., Carbon Nanofoam-Based Cathodes for Li–O2 Batteries: Correlation
of Pore–Solid Architecture and Electrochemical Performance. Journal of The
Electrochemical Society, 2013. 160(9): p. A1510-A1516.
161. Ma, S.B., et al., Effect of porosity on electrochemical properties of carbon materials as
cathode for lithium-oxygen battery. Journal of Power Sources, 2013. 244(0): p. 494-498.
162. Olivares-Marin, M., et al., Effects of architecture on the electrochemistry of binder-free
inverse opal carbons as Li-air cathodes in an ionic liquid-based electrolyte. Journal of
Materials Chemistry A, 2013. 1(45): p. 14270-14279.
163. Kang, J., O.L. Li, and N. Saito, Hierarchical meso–macro structure porous carbon black as
electrode materials in Li–air battery. Journal of Power Sources, 2014. 261(0): p. 156-
161.
164. Meini, S., et al., Effect of Carbon Surface Area on First Discharge Capacity of Li-O2
Cathodes and Cycle-Life Behavior in Ether-Based Electrolytes. Journal of The
Electrochemical Society, 2012. 159(12): p. A2135-A2142.
165. Zhang, G.Q., et al., α-MnO2/Carbon Nanotube/Carbon Nanofiber Composite Catalytic Air
Electrodes for Rechargeable Lithium-air Batteries. Journal of The Electrochemical
Society, 2011. 158(7): p. A822-A827.
166. Shin, M.K., et al., Elastomeric Conductive Composites Based on Carbon Nanotube Forests.
Advanced Materials, 2010. 22(24): p. 2663-2667.
167. Sun, D., et al., A Solution-Phase Bifunctional Catalyst for Lithium–Oxygen Batteries.
Journal of the American Chemical Society, 2014. 136(25): p. 8941-8946.
168. Chan, C.K., et al., High-performance lithium battery anodes using silicon nanowires. Nature
Nanotechnology, 2008. 3(1): p. 31-35.
169. Wang, B., et al., Intertwined Network of Si/C Nanocables and Carbon Nanotubes as
Lithium-Ion Battery Anodes. ACS Applied Materials & Interfaces, 2013. 5(14): p. 6467-
6472.
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BIOGRAPHICAL SKETCH
The author was born in Xi’an, Shaanxi in China in 1989. He earned his Bachelor of
Engineering degree in Materials Science and Engineering with from Northwestern Polytechnical
University (Xi’an, China) in July 2011. In 2015, he received his εaster of Science degree in
Material Science and Engineering from Florida State University. His current research interests are
the nanoscale materials fabrication and application.