Processing and Applications of CNT Sheets in Advanced Composite Materials

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PROCESSING AND APPLICATIONS OF CNT SHEETS IN ADVANCED COMPOSITE MATERIALS

Rachit Malik*, Colin McConnell†, Lu Zhang*, Ryan Borgemenke†, Richard Kleismit‡, Robert Wolf*, Mark R. Haase†, Yu-Yun Hsieh*, Ryan Noga†, Noe Alvarez†, David Mast‡, Vesselin Shanov*,† Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States* Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, United States† Department of Physics, University of Cincinnati, Cincinnati, OH, United States‡

CHAPTER OUTLINE 1 Introduction ....................................................................................................................................383 2 Techniques for Manufacturing of CNT Sheets .................................................................................... 384 2.1 “Buckypaper” or Dispersion/Filtration Approach ................................................................384 2.2 Dry/Solid-State Spinning From Vertically Aligned CNT Forests ............................................385 2.3 Direct Spinning and Winding From CNT Aerogel ................................................................386 3 Processing and Applications of CNT Sheets ...................................................................................... 387 3.1 Functionalization and Cross-Linking of CNTs for CNT/Polymer Composites ..........................387 3.2 Flexible, Nano-Structured Electrodes Based on CNT Sheets ...............................................405 4 Summary ........................................................................................................................................419 Acknowledgments ...............................................................................................................................419 References ......................................................................................................................................... 419 Further Reading ..................................................................................................................................429

1 INTRODUCTION Advances in technology and chemical engineering have enabled carbon nanotubes (CNTs) to evolve from powdery filler materials to actual building blocks for the development of sheets and yarns made entirely out of carbon nanotubes. For more than a decade after Iijima’s pioneering publication [1], CNTs were considered a nanofiller that could be used to enhance properties of other materials such as polymers. A wide range of studies focused on the dispersing of CNTs in a variety of polymer matrices were published. The extraordinary properties of individual nanotubes such as high strength [2] and high conductivity [3] were a key motivation for these studies. However, the published data so far Nanotube Superfiber Materials. https://doi.org/10.1016/B978-0-12-812667-7.00016-1 Copyright # 2019 Elsevier Inc. All rights reserved.

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were only partially successful in emulating properties of individual CNTs, because the fraction of CNTs that could be uniformly incorporated in a polymer matrix was limited due to difficulties in dispersion and coagulation of the nanotubes. Then, in 2002, Jiang et al. [4] made a breakthrough and reported the formation of a continuous thread made of CNTs by drawing a web of nanotubes from the edge of a CNT array produced by chemical vapor deposition (CVD). Later, in 2005, Zhang et al. [5] demonstrated that CNT sheets could also be made by “dry spinning” from CNT arrays. These reports eliminated the notion that CNTs were only used as powders and as fillers. CNTs could now be considered a material of construction, and macroscale assemblages could be made entirely out of CNTs. Li et al. [6] further bolstered this claim by reporting the development of a truly continuous method for production of CNTs in the form of fibers and sheets. This technique known as floating catalyst chemical vapor deposition (FCCVD) has been commercialized by Nanocomp Technologies, Inc. for mass production of CNT yarns and sheets [7]. CNTs in these assemblages are held together with van der Waals forces. These physical forces are strong enough to allow for the formation of macroscale CNT assemblages. However, van der Waals forces of attraction are still orders of magnitude weaker than covalent bonds. This poses an interesting challenge to the development of next generation of CNT assemblages, especially how to replace physical forces of attraction with stronger chemical bonds. This challenge is also unique in the way that it requires special processing of CNTs to alter their chemical nature from being typically inert to reactive with the help of functional groups. In this chapter, we provide an overview of the different processes for manufacturing CNT sheets along with a discussion of the different techniques explored in literature, for functionalization and cross-linking of CNT sheets. We also present an insight into research carried out at University of Cincinnati, wherein CNT sheets are explored for the development of high-strength composites and for lightweight, flexible supercapacitors.

2 TECHNIQUES FOR MANUFACTURING OF CNT SHEETS 2.1 “BUCKYPAPER” OR DISPERSION/FILTRATION APPROACH The first technique for making CNT sheets was inspired from the process of producing paper, namely, the formation of pulp, filtration, and drying. This process was a preferred choice at the time as the earliest forms of CNTs were powders deposited on walls of arc discharge [1] or laser ablation [8] reactors, using graphite as the carbon precursor. Deheer et al. [9] first dispersed powdered nanotubes in ethanol with the help of ultrasound and reported on a film made of CNTs by filtering the dispersion through a ceramic filter. This report was instrumental in creating interest in macroscopic assemblies of CNTs, and many researchers around the world started to explore the manufacturing of CNT films via the dispersion and filtration technique [10, 11]. The powder form of nanotubes naturally prompted many researchers to mix them with other matrix materials such as polymers and metals to advance the properties of the latter. However, achieving a uniform dispersion of CNTs in solution and in polymer matrices was particularly challenging as the CNTs tend to entangle and aggregate due to strong van der Waals interactions between individual nanotubes [12]. This required prior treatment of CNTs with strong acids and oxidizing agents to introduce functional groups that facilitated interaction of the nanotubes with the solvent and disperse aggregates [13]. The addition of surfactant also helps in aiding dispersion without requiring the treatment with strong acids [14]. CNT sheets produced via this approach

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constituted of randomly oriented CNTs in the final assembly [15, 16]. The random orientation of CNTs in buckypapers does not facilitate efficient load transfer between nanotubes, which in turn results in relatively poor properties [17, 18]. This in turn translates to less than expected improvement in mechanical properties of CNT buckypaper/polymer composites [19, 20]. Nevertheless, this technique has found viability in applications of CNTs wherein high electric conductivity is priority such as EMI shielding, deicing, and lightning protection in the aerospace industry [21]. General Nano LLC based in Cincinnati, Ohio, has furthered this technique and has developed the ability to continuously produce CNT buckypapers composed of randomly aligned CNTs. Fig. 1 illustrates the buckypaper manufacturing process along with SEM image showing random orientation of CNTs and highlighting the flexibility of the buckypaper.

2.2 DRY/SOLID-STATE SPINNING FROM VERTICALLY ALIGNED CNT FORESTS Jiang et al. [4], in 2002, made the discovery of spinning of continuous web of carbon nanotubes from vertically aligned arrays produced by CVD. This innovation transformed CNT research as it allowed for the creation of CNT sheets [22] with long, aligned nanotubes in a completely dry process. Zhang et al. [5] improvised Jiang’s method, and instead of twisting the ribbons pulled from arrays, they laid them flat to produce highly aligned, freestanding sheets of multiwalled carbon nanotubes. Sheets produced by this technique must be “densified” with volatile liquids to create more coherent, stronger, and more conductive sheets. The process of densification [5] involves wetting of single layer or multiple layers of dry-spun CNT web with volatile solvents such as acetone or ethanol. The evaporation of the solvent results in the creation of capillary forces that compact the sheet by bringing CNTs closer together. Dry-spun sheets treated with solvents and pressed to further improve compaction have been

FIG. 1 (A) Process for manufacturing buckypapers; (B) SEM image showing the buckypaper surface; (C) buckypaper origami airplane demonstrating their flexibility and mechanical robustness.
J. Schutz, € M. She, C. Huynh, S. Hawkins, M. Duke, S. Gray, Recent Reproduced with permission from K. Sears, L. Dumee, Developments in Carbon Nanotube Membranes for Water Purification and Gas Separation, Materials 3 (2010), https://doi.org/10.3390/ ma3010127.

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reported to reach
3 GPa in terms of tensile strength [23]. These sheets are much stronger than the strongest reported CNT films and buckypapers (0.2 GPa) produced via dispersion and filtration of nanotubes [17, 18, 24]. This solid-state process of making CNT sheets provides a viable alternative compared with the buckypaper approach as it eliminates the need for chemicals such as surfactants and acids for functionalization. Along with high strength, these sheets also possess high electric conductivity as shown by Di et al. [22]. The manufacturing of CNT sheets by “dry spinning” offers the key advantage of the alignment of CNTs in the direction of pulling. The alignment of nanotubes results in anisotropy in terms of the properties of the CNT sheet that can be observed by measuring electric conductivity and tensile strength along and against the CNT orientation. The ease of manufacturing coupled with properties such as high strength and conductivity makes the CNT sheet a strong candidate for combination with polymers. Liu et al. [25] first reported on the development of high-strength CNT/ polyvinyl alcohol (PVA) composite sheets by uniformly impregnating them with PVA from solution. Liu et al. used a spraying technique to infiltrate the PVA from solution during the manufacturing of the CNT sheet. Fig. 2 shows an illustration of the process. This innovation inspired other researchers to create high-strength composites of CNT sheet with polymers such as nylon [26], polyimide [27], bismaleimide (BMI) [28], and epoxy resin [29]. The development of the CNT sheet has led to a deluge of literature studies based around it, along with a plethora of applications such as in incandescent displays [30], anodes for Li-ion battery [31], banner-type sound wave generator [32], and transparent electrodes for LCD displays [33] demonstrating commercial applicability. Fig. 3 shows the different setups developed to utilize “spinnable” CNT forest or arrays to produce CNT threads and sheets by dry spinning at the UC Nanoworld. Fig. 3C and F displays examples of CNT thread and sheet produced by dry spinning.

2.3 DIRECT SPINNING AND WINDING FROM CNT AEROGEL The most modern method for producing CNTs is the FCCVD, first reported by Liu et al. [6] in 2004. This process was later commercialized by Lashmore et al. [7, 34] at Nanocomp Technologies, Inc. to mass produce CNT sheets and yarns. The most commonly used catalyst for FCCVD of CNTs is Fe in the form of ferrocene dissolved in an organic carbon precursor. The synthesis process has been described previously. FCCVD growth process, like arc discharge and laser ablation, provides very little control over diameter and length of the CNTs. However, the scale of the synthesis process coupled with the formation of an assembly of entangled CNTs or “sock” provides a key advantage over all other processes in terms of scalability as shown in the Fig. 4. FCCVD technique not only offers obvious advantages but also has certain drawbacks. A key drawback of the FCCVD process is the high amount of catalyst impurity. During synthesis, the iron nanoparticles not only sprout the CNTs but also tend to become embedded in the nanotube structure. CNT sheets as-produced by FCCVD have been reported to have a catalyst content greater than
15% [36]. Fig. 5 shows SEM and TEM images of catalyst nanoparticle aggregates and individual particles encapsulated in graphitic carbon cages, respectively. Additional processes involving hightemperature and acid treatment are needed for effective removal of catalyst impurities. These processes can also take a toll on the quality of the CNTs as acid treatment over long duration has shown to result defects in the CNT structure, which can negatively impact the properties of the nanotubes [37].

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Screen with slit CNT array CNT sheet

PVA solution spray

Rotating mandrel

(A) 2000

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1200 800

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0

(B)

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40 60 80 20 CNT weight fraction (%)

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FIG. 2 (A) Schematic view of spray winding. A CNT sheet is drawn out of a drawable array and continuously wound onto a rotating mandrel on which micrometer-sized droplets of PVA solution are deposited. A screen with a slit is used to control the spray area; (B) Tensile strength of the CNT/PVA composites as a function of the CNT percent weight fraction. The corresponding concentrations of PVA solutions are labeled. Reproduced with permission from W. Liu, X. Zhang, G. Xu, P.D. Bradford, X. Wang, H. Zhao, Y. Zhang, Q. Jia, F.G. Yuan, Q. Li, Y. Qiu, Y. Zhu, Producing superior composites by winding carbon nanotubes onto a mandrel under a poly(vinyl alcohol) spray, Carbon N. Y. 49 (2011) 4786–4791, https://doi.org/10.1016/j.carbon.2011.06.089.

3 PROCESSING AND APPLICATIONS OF CNT SHEETS 3.1 FUNCTIONALIZATION AND CROSS-LINKING OF CNTs FOR CNT/POLYMER COMPOSITES The manufacturing of macroscale CNT assemblages has garnered significant attention in recent years; however, the goal of emulating the properties of individual carbon nanotubes remains a challenge. CNTs in these assemblages are also held together via physical forces of attraction also known as

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FIG. 3 Facilities to make CNT thread and sheet from aligned CNT arrays by dry spinning: (A) and (B) CNT thread spinning machine, (C) SEM image of uniform CNT thread, (D) machine for drawing of CNT sheet, and (E) and (F) pure aligned CNT sheet formed by layering ribbon on a Teflon conveyor belt. Reproduced with permission from V. Shanov, W. Cho, R. Malik, N. Alvarez, M. Haase, B. Ruff, N. Kienzle, T. Ochmann, D. Mast, M. Schulz, CVD growth, characterization and applications of carbon nanostructured materials, Surf. Coatings Technol. 230 (2013) 77–86. https://doi.org/10.1016/j.surfcoat.2013.06.017.

van der Waals forces. These forces allow the nanotubes to slide within the assembly when uniaxial load is applied. The sliding of the nanotubes in the assemblage during mechanical testing is the most common proposed cause of failure [22, 38]. This sliding of nanotubes also causes a decline in modulus and yields a lower ultimate strength. A way to overcome this problem is by interconnecting the nanotubes through covalent bonds. Covalent bonds are created via sharing of electrons between individual atoms. Bond energies of covalent bonds are two orders of magnitude higher than that of van der Waals forces. The formation of covalent bonds between individual carbon nanotubes allow inter-CNT load transfer and enhance the mechanical properties of the CNT macrostructures. Theoretical and experimental studies have shown that cross-linking of CNTs has potential to circumvent the sliding of CNTs [39, 40]. Numerous experimental efforts to cross-link CNTs have been reported for powdered CNTs and recently for macroscale CNT assemblages [41–43]. Covalent functionalization involves chemical bonding in the form of covalent bonds that are formed between the carbon nanotubes and functional groups such as dOH and dCOOH. The earliest reports on covalent CNT functionalization involved treatment with concentrated acids and oxidizing agents [44]. These treatments result in disruption of the sp2-hybridized network of carbon atoms and lead to the creation of defect sites. The carbon atoms in the defect sites are converted to sp3 hybridization, and other atomic species such as oxygen, fluorine, and nitrogen become incorporated into the CNT structure. Extraneous atoms get integrated in the CNT structure in the form of functional groups that can then be utilized to make CNTs interact with other materials. However, the disruption in the translational symmetry of CNTs also affects the properties of CNT [44]. Therefore, CNT functionalization

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FIG. 4 (A) Long roll of CNT sheet produced by Nanocomp Technologies, Inc.; (B) a roll of CNT sheet being infiltrated with commercial resin to create prepreg on industrial machinery; (C) Miralon™ CNT tapes produced by Nanocomp Technologies, Inc. [35]. (B) Reproduced with permission from Large Sheets of Carbon Nanotubes Made by CVD, MRS Technol. Adv. 35 (2010) 179–181.

must be carried out in a controlled, optimized manner to enable covalent bonding with CNTs while still maintaining optimum CNT quality and structural properties. Due to the vast amount of literature available on covalent functionalization [44, 45] of powdered CNTs, it is not possible to include all efforts in this study while maintaining coherency. Therefore, we will specifically discuss research efforts of cross-linking CNT assemblages produced by dry spinning or floating catalyst CVD methods.

3.1.1 Wet/solution functionalization Cheng et al. [46] first reported on chemical functionalization of CNT sheets produced by Nanocomp Technologies, Inc. via FCCVD. They treated randomly oriented CNT sheets with m-chloroperoxybenzoic acid (mCPBA) to create epoxide groups on CNT sidewalls based on previously reported research [47]. Fig. 6 shows the reaction scheme proposed by the authors to interface epoxide-functionalized CNTs with BMI

FIG. 5 (A) SEM image of CNT sheet (arrows indicating some nanoparticulate aggregate impurities); (B) HR-TEM image of the nanoparticulate aggregate impurities, which are metallic iron catalyst particles encapsulated in graphitic carbon cages. Reproduced with permission from Y. Lin, J.W. Kim, J.W. Connell, M. Lebron-Colon, E.J. Siochi, Purification of carbon nanotube sheets, Adv. Eng. Mater. 17 (2015) 674–688, https://doi.org/10.1002/adem.201400306.

FIG. 6 Proposed reaction mechanism of functionalized CNTs and BMI resin. Reproduced with permission from Q. Cheng, B. Wang, C. Zhang, Z. Liang, Functionalized carbon-nanotube sheet/bismaleimide nanocomposites: mechanical and electrical performance beyond carbon-fiber composites, Small. 6 (2010) 763–767, https://doi.org/ 10.1002/smll.200901957.

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resin. They also stretched the functionalized CNT sheets to induce the alignment of CNTs in the sheet and claimed significant improvement in mechanical properties for functionalized CNT/BMI composite sheets. Further, this group described the achievement of an optimum degree of functionalization at 4% but did not provide any experimental details on the measurement of the degree of functionalization. Acid functionalization is a relatively less complicated chemical treatment to carry out on CNTs, and CNT yarns produced by dry spinning and by FCCVD technique have been treated with acids to improve their properties [48–50]. It is particularly popular for CNT assemblages produced by FCCVD technique to remove the large amount of metal catalyst impurities [36] incorporated into the assemblage due to the nature of the synthesis process. Im et al. [51] and Tran et al. [52] used CNT threads produced by FCCVD technique and treated them with acids or acid mixtures. They used a mixture of concentrated HNO3 and H2SO4 (3:1) for 1 h to create oxygen-functionalized CNTs. The acid-treated threads were then soaked with a solution of 1,5-pentanediol in sulfuric acid for 1 h at 200°C in Ar atmosphere. The objective of this treatment was to carry out an esterification reaction between carboxylic acid groups produced on CNTs by the prior acid treatment and the hydroxyl groups of 1,5-pentanediol. Im et al. reported improvement in mechanical properties of CNT fibers treated with acid and crosslinked with diol and attributed the improvement to the formation of ester linkages between CNTs and the diol resulting in cross-linking of CNTs. Chemical functionalization has also been utilized to modify CNT yarns produced by solid-state spinning technique. Cai et al. [53] employed aryldiazonium chemistry to cross-link multiwalled nanotubes in a dry-spun yarn by infiltrating different aryldiazonium compounds (Fig. 7A) during the dry spinning process as illustrated in Fig. 7B. The long, multistep procedure as per the authors involved the application of pH-controlled solution to the CNT yarn during solid-state spinning from CNT forests. Then, the “wet” yarn was suspended in the same treatment solution with gentle agitation for 30 min. A buffer solution containing 0.1 M Na2CO3 and 0.1 M NaHCO3 was then added dropwise to the treatment solution, to gradually raise the pH to 9. The CNT yarn sample was kept in the gently agitated solution for 8 h at room temperature before being washed thoroughly with water and then with acetone. Additionally, for CNT yarns treated with carboxyl-functionalized aryldiazonium salts, a final treatment with diluted hydrochloric acid (pH < 3) was applied for 60 min at room temperature to convert the carboxylic acid groups to their protonated form. Functionalization of the CNTs in the thread had a positive impact on its mechanical properties (
25% improvement over the untreated thread). The authors stated that the improved strength should be attributed to the enhanced interactions between the CNTs, produced by an increase in friction between the modified CNT surfaces. The improved properties could also be attributed to possible cross-linking as shown in the illustration in Fig. 7C. Min et al. [54] built upon the aryldiazonium functionalization of CNTs described by Cai et al. to further create a cross-linked CNT yarn by using a commercial cross-linking resin compound hexa(methoxymethyl)melamine (HMMM). They first functionalized the CNT yarns with aryldiazonium salt A (Fig. 7A) following the same procedure as Cai et al. [53] and then infiltrated the functionalized CNT yarn with the cross-linking resin compound. HMMM reacts with carboxyl groups via its six methoxy end groups to yield an ester linkage and methanol by-product. Usually, a mild acid is required to catalyze the esterification albeit the acid can also catalyze the reaction between HMMM molecules to form a polymer [55, 56]. The tensile strength of the cross-linked thread reached a maximum 2.5 GPa, exhibiting a 40% enhancement over the control (nonfunctionalized) samples. Cross sections of thread prepared by focused ion beam (FIB) and viewed under SEM showed that the functionalized

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FIG. 7 (A) The two types of aryldiazonium salts (A and B) used for functionalization; (B) schematic illustration of infiltration of the chemical solution during the spinning of the CNT threads; (C) sidewall modification of CNTs and aryldiazonium compounds showing different possibilities of reaction and cross-linking. Reproduced with permission from J.Y. Cai, J. Min, J. McDonnell, J.S. Church, C.D. Easton, W. Humphries, S. Lucas, A.L. Woodhead, An improved method for functionalisation of carbon nanotube spun yarns with aryldiazonium compounds, Carbon N. Y. 50 (2012) 4655–4662, https://doi.org/10.1016/j.carbon.2012.05.055.

cross-linked thread had a relatively denser internal structure and therefore improved mechanical properties (Fig. 8A and B). The authors also presented SEM images of the fractured ends of threads demonstrating different failure modes for different threads and elucidating the effect of cross-linking in Fig. 8C–E. Similar studies on infiltration of different chemicals [43, 57, 58] into CNT yarns have been reported, which typically involve a postinfiltration processing/curing step to induce cross-linking. However, it is important to note that the compact structure of CNT yarns makes it difficult to achieve uniform infiltration and as highlighted by the need to use extended impregnation time and vacuum [53, 54] to allow for proper infiltration. These techniques, though having shown promise for cross-linking CNT yarns, will be difficult to apply to CNT sheets. Additionally, solution functionalization by default involves wetting of the CNT assemblage in a solution and take anywhere between minutes or hours to achieve the desired extent of functionalization. And after functionalization in solution, one must carry out disposal of by-products and residual chemicals. Thus, there is an active interest in the exploration of dry techniques that can facilitate CNT functionalization. The ideal dry technique should work in a short

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FIG. 8 FIB-SEM images of (A) untreated and (B) cross-linked CNT/HMMM composite yarns; fractured ends of (C) pristine CNT yarn, (D) diazonium salt-treated CNT yarns, and (E) aryldiazonium salt-treated CNT/HMMM composite yarns. Reproduced with permission from J. Min, J.Y. Cai, M. Sridhar, C.D. Easton, T. R. Gengenbach, J. McDonnell, W. Humphries, S. Lucas, High performance carbon nanotube spun yarns from a crosslinked network, Carbon N. Y. 52 (2013) 520–527, https://doi.org/10.1016/ j.carbon.2012.10.004.

amount of time, without leaving residual chemicals, and most importantly have minimal impact on the structure and of CNT assemblages such as yarns and sheets.

3.1.2 Dry functionalization techniques Dry, energy-intensive techniques that have been explored for modifying CNTs include irradiation with electron beam [59] and γ-rays [41]. Banhart et al. [60], in 2001, first showed the possibility of creating connections between CNTs by irradiating the junctions in a scanning electron microscope (SEM). Peng et al. [42] achieved a mean fracture strength >100 GPa by cross-linking individual multiwalled carbon nanotubes by irradiation with electron beam in a TEM. Similarly, numerous studies on individual CNTs and CNT bundles carried out inside electron microscopes have demonstrated high strength via cross-linking. However, it is important to note that these studies are done inside an electron microscope that greatly limits the length of the sample that can be irradiated. Also, a high irradiation dose can

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900

30

800

25

600 600

4.2 kGy/h 1.5 kGy/h

500 400

Modulus (GPa)

Breaking stress (MPa)

be achieved in these microscopes in a matter of seconds or minutes as they operate at high-current densities
A/cm2, whereas industrial-scale electron irradiation machine typically operates at low current densities of the order of
μA/cm2 [59]. At such low current densities, it can take days or months of exposure to achieve the desired dosage. Also, the penetration depth of 200 keV electrons is <1 μm, and thus, it will not be able to penetrate the entire cross section of a 100-layer CNT sheet (
4 μm thickness) produced by dry spinning. Thus, the scalability of e-beam for cross-linking macroscale CNT assemblages is greatly limited. Miller et al. [61] recently reported on cross-linking of CNT yarns using a combination of wet/chemical functionalization and e-beam irradiation. They attempted to overcome the need for high-current density radiation by chemically functionalizing CNTs prior to e-beam exposure for varying amount of time (20–90 min). CNT yarns and sheets produced by FCCVD method showed improvements in mechanical properties after chemical functionalization and e-beam irradiation. The improvement in properties was much greater for samples that had undergone both the treatments compared with samples treated with one technique. Cross-linking CNTs by e-beam in the presence of polymer can be detrimental to the mechanical properties as discovered by Hiremath et al. [62]. They irradiated CNT yarns in air, which were previously infiltrated with polystyrene (PS), and found that the mechanical properties of the CNT/PS yarns declined compared with that of pristine CNT (PCNT) yarn, irrespective of the dosage of the radiation. Hiremath et al. attributed the decline to the creation of defects as the irradiation was carried out in air. Ionizing γ-irradiation from Co60 source has also been explored as means to crosslink CNT yarns produced by dry spinning technique [59]. CNT yarns typically showed an improvement in tensile stress after irradiation. However, they also observed a slight decrease in diameter after irradiation. Miao et al. [41] found that a low radiation of 100 kGy was sufficient to induce changes in the CNT yarn, as further increase in radiation dosage did not enhance or degrade the mechanical properties any further as seen in Fig. 9. It is important to note that the highest radiation dose rate that they applied was 4.2 kGy/h, which suggests that it would take an exposure time of almost a day (23.8 h) to achieve a total dosage of 100 kGy. Cai et al. [63] built upon these results and studied the effects of γ-irradiation on CNT yarns infiltrated with epoxy. The objective was to induce reaction and cross-linking between CNTs and the infiltrated epoxy resin to improve the mechanical properties of the composite yarns. They subjected the yarns to a total irradiation dose of about 200 kGy in an oxygen-rich environment. Such a γ-irradiated CNT yarn and γ-irradiated CNT/epoxy composite yarn showed improvement in

20 15 10

4.2 kGy/h 1.5 kGy/h

5 0

0 100 200 300 400 500 600 700 800 Dose (kGy)

0 100 200 300 400 500 600 700 800 Dose (kGy)

FIG. 9 Strength and elastic modulus of CNT yarns exposed to different levels of gamma-irradiation [41].

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mechanical properties over their nonirradiated counterparts. Cai et al. also observed a raise in percent oxygen content by XPS and an increase in the ID/IG ratio by Raman spectroscopy for the irradiated yarns. The authors speculated about the formation of defects and cross-links formed between CNTs and epoxy due to irradiation, but did not provide conclusive evidence. While γ-irradiation has been shown to positively impact the mechanical properties of CNTs and CNT/polymer composites, the mechanism for this improvement in properties is still unclear. Miao et al. and Cai et al. both utilized the same source for γ-irradiation, and it was calculated that it would take days to achieve the desired dosage to impact the properties of CNTs that greatly limits industrial application. Another dry technique that enables functionalization of carbon nanotubes is plasma treatment. One of the earliest reports on CNT functionalization by plasma was published in 2002 by Khare et al. [64] wherein they treated SWNTs with a cold H2 plasma in a glow-discharge process. Subsequently, they also explored functionalization with NH3 [65], CF4 [66], and N2 [67] to introduce different functional groups on CNTs. There have been multiple studies utilizing low-pressure plasma to functionalize powdered CNTs for a range of applications. However, the generation of low-pressure plasma necessitates the use of a vacuum chamber, which makes it not pliable for the functionalization of macroscale CNT assemblages such as sheets and yarns. Hence, atmospheric pressure plasma systems based on dielectric barrier discharge have become an attractive alternative for rapid functionalization of carbon nanotube assemblages. Atmospheric pressure plasma functionalization of CNTs was first reported by Okpalugo et al. [68]. They used dielectric barrier discharge of atmospheric “ordinary” air to functionalize CNT buckypapers with oxygen functional groups within 5 s of treatment. Thereafter, several studies utilizing powdered CNTs have shown the potential of plasma treatment as a fast and efficient method to create a variety of functional groups by varying the active gas and parameters such as power and treatment time [69–72]. Plasma treatment allows the user to exercise greater control over the extent of functionalization of the nanotubes. The process can be tailored by controlling plasma parameters such as plasma power, pressure, and gas flow rates. This extent of control enables to prevent overfunctionalization, which can help preserve the structure of CNTs and thus the electric and mechanical properties. Kolacyak et al. [73] observed that powdered CNTs functionalized by atmospheric pressure oxygen plasma showed improved dispersibility in water. The authors attributed this property to the creation of oxygenbased functional groups such as hydroxyl and carbonyl onto CNT sidewalls. The improved dispersibility of plasma-functionalized CNTs is ascribed to the intermolecular interaction of the functional groups from the CNTs with the water or solvent molecules. Naseh et al. [37] did a comparative study on the effectiveness of plasma functionalization over the conventional chemical treatments to introduce carboxyl (COOH) functionality to the nanotubes. Using temperature-programmed desorption (TPD) technique, they reported that acid-treated nanotubes have more functional groups produced on the surface; however, plasma makes the process shorter and cleaner and results in less damage to the CNT structure. Fig. 10 shows the extent of damage to CNTs by plasma and acid treatments via SEM micrographs. The ability to uniformly disperse plasma-functionalized powdered CNTs in water and other polar solvents has been utilized to create CNT/polymer composites with improved mechanical properties [74, 75]. However, majority of available literature is composed of studies on plasma functionalization of powdered CNTs and CNT arrays for a variety of applications. The available literature on plasma functionalization of CNT assemblages such as sheets and yarns is scarce. Wei et al. [76] reported on the development of high-strength CNT/PVA composite yarns using the dry spinning approach by carrying out oxygen plasma treatment on the CNTs prior to infiltration. Park et al. [77] explored

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FIG. 10 SEM micrograph of (A) as-prepared, (B) annealed, (C) plasma-functionalized, and (D) acid-treated MWCNTs. Plasma conditions: power ¼ 34.1 W and treatment time ¼ 3 min. Reproduced with permission from M.V. Naseh, A.A. Khodadadi, Y. Mortazavi, F. Pourfayaz, O. Alizadeh, M. Maghrebi, Fast and clean functionalization of carbon nanotubes by dielectric barrier discharge plasma in air compared to acid treatment, Carbon N. Y. 48 (2010) 1369–1379, https://doi.org/10.1016/j.carbon.2009.12.027.

oxygen plasma treatment of CNT yarns produced by FCCVD by using an atmospheric pressure plasma system. They studied the effect of oxygen flow rate during the plasma treatment on the mechanical properties of CNT yarns while keeping all other parameters such as time of exposure and plasma power constant. Park et al. did not use any polymer to infiltrate the CNT yarns. Interestingly, they reported an increase in tensile strength of CNT yarns functionalized with oxygen plasma over that of PCNT yarns with maximum strength observed for yarns treated with 100 sccm oxygen plasma. This result is contradictory to that reported by Yu et al. [78] who demonstrated a decline in mechanical properties of CNT yarns treated with atmospheric pressure oxygen plasma irrespective of exposure time. Park et al. claim that plasma functionalization produces oxygen functional groups on CNTs that can then lead to improved hydrogen bonding between CNTs resulting in improved mechanical properties. However, this claim is questionable as it is known that plasma functionalization can create defects and even remove extraneous carbon material [79]. The latter can create voids between CNTs and thus reduce

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possibility of hydrogen bonding. There is a need for further studies to better understand the effect of plasma on the interaction between CNTs in macroscale assemblages. It is also vital to note that the duration of plasma treatment needed to induce chemical changes to CNTs is relatively very short as compared with that with wet/solution techniques. Table 1 presents a compilation of results from literature on the functionalization and cross-linking of CNT assemblages produced by FCCVD and by dry spinning from aligned CNT arrays. CNT thread is the chosen substrate in majority of studies in literature due to the relatively more sturdy and robust structure of yarns compared with CNT sheets. Recently, research at Nanoworld Labs, University of Cincinnati, has shown that CNT sheets can be

Table 1 Results From Literature on Functionalization and Cross-Linking of CNT Assemblages Produced by FCCVD and by Dry Spinning From Aligned CNT Arrays

CNT Form Direct spinning to sheets (FCCVD)

Direct spinning to fibers (FCCVD)

Dry spinning to fiber

Dry spinning to sheet

Functionalization and CrossLinking Pristine/BMI composite Functionalized/BMI composite Functionalized 40% stretched/BMI composite Pristine dOH functionalization with 2-azidoethanol dOH functionalized and crosslinked with e-beam Pristine Acid functionalized and cross-linked with epoxy Pristine Cross-linking with PEG-bis(azide) via [2 + 1] cycloaddition Pristine Oxygen plasma functionalized Pristine Aryldiazonium functionalized and cross-linked with HMMM Pristine Gamma-irradiated Pristine/PVA Oxygen plasma functionalized/PVA Pristine Cross-linked with bis(perfluoroazide) via [2 + 1] cycloaddition

Tensile Stress (MPa)

Tensile Modulus (GPa)

700 1437 3081

50 124 350

[46]

90 MPa/g. cm3 145

0.6

[61]

220

2

408 1132

14.6 62

[52]

200 1400

– –

[43]

553 404 1300 2300

– – 47.2 121

[78]

665 846 1600 2200 35.9 144.5

13.9 20.6 100 200 1.6 32.9

References

1

[54]

[41] [76] [58]

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effectively functionalized by plasma and combined with epoxy to produce high-strength, lightweight composites [80]. The details of this study are presented in the next section along with other research work carried out on CNT sheets at Nanoworld Labs, University of Cincinnati.

3.1.3 Plasma functionalized CNT sheet/epoxy composites Owing to their unique properties, carbon nanotubes make an ideal material for reinforcing polymer matrices. However, the use of powdered CNTs mixed in a polymer matrix has not shown significant improvement in the properties of the composite over the neat polymer. The % wt. CNT content of powdered CNTs incorporated in a polymer typically varies between 0.05% and 5% as high CNT tends to create aggregates and bundles of CNTs within the polymer matrix. Recent progress made by Zhu et al. [25, 26, 81] indicates that dry drawn CNT sheets and sheets produced via floating catalytic CVD method [46] yield significantly stronger composites than those made from powdered CNTs. CNT sheet/polymer composites with greater than 50 wt% nanotubes have been developed, indicating a “role reversal” in which the polymer matrix is used to reinforce the CNT sheet. The approach employed by Zhu et al. involves spraying of polymer solution during the manufacturing of the CNT sheet, thereby allowing uniform impregnation of polymer within the CNT sheet. At Nanoworld Labs, researchers succeeded to combine this approach with atmospheric pressure plasma functionalization of CNT sheet to manufacture strong, lightweight CNT/epoxy composites. Plasma functionalization of buckypapers [82] and its application for improving interaction with polymer matrices has been reported previously. However, functionalization of buckypapers after manufacturing limits the penetration of the plasma flux leading to nonuniform functionalization and limited improvement in properties of resulting composites [83]. We demonstrated functionalization of CNTs during the sheet formation that allowed uniform functionalization and interaction with the epoxy throughout the thickness of the composite. Majority of existing literature sources in studying the interaction between functionalized CNTs and epoxy utilize powdered CNTs and the maximum wt% CNT content achieved in these reports is significantly below 50% with average ranging between 0.1% and 5%. There are multiple reports on the development of CNT sheet/polymer composites composed of aligned CNT produced by dry spinning method [25, 29, 84, 85]. However, most of these reports focus on improving techniques by mechanical methods such as stretching [23] and pressing [86]. None of them has explored the functionalization of CNTs within the sheet and its effect on the mechanical properties of CNT/polymer composites. Recently, there have been studies carried out on the effect of functionalization of CNT fibers produced by FCCVD technique [52]. This study and others involving treatment of CNT sheets [87] and yarns [54] in a solution require disposal of the chemicals after use. Also, the treatment is carried out after the CNT assemblage has already been formed, which can cause issues pertaining to penetration of functionalizing solution and the resin afterward. This results in the use of vacuum techniques to better infiltrate the polymer resin into the bulk of the CNT assemblage [54]. However, to the best of our knowledge, there are no studies reported on the functionalization of CNT sheets by atmospheric pressure plasma and its effect on the mechanical properties of CNT sheet/ polymer composites. Hang et al. [78] used a similar plasma system to explore oxygen functionalization of CNT sheets and yarns produced by the FCCVD technique. However, their study was only focused on functionalization, and they did not explore the combination of functionalized CNT sheets/yarns and polymer resin to produce high-strength composites. We previously explored plasma functionalization of CNT sheets and their application in CNT/PVA composites [88]. The following study helps in

3 PROCESSING AND APPLICATIONS OF CNT SHEETS

399

FIG. 11 (A) Schematic of the manufacturing plasma-functionalized CNT sheets and composites. (B) Image of single layer of CNT ribbon passing under He/O2 plasma torch before getting wound on the cylinder. The mirror image illustrates the plasma flux passing through a torch grid. Reproduced with permission from R. Malik, C. McConnell, N.T. Alvarez, M. Haase, S. Gbordzoe, V. Shanov, Rapid, in situ plasma functionalization of carbon nanotubes for improved CNT/epoxy composites, RSC Adv. 6 (2016) 108840–108850, https://doi.org/10. 1039/C6RA23103A.

creating greater understanding of the effect of plasma functionalization on the properties of CNT sheets and CNT/epoxy composites. In this study, CNT sheets were manufactured by wrapping 10 layers of CNT web drawn from a vertically aligned CNT array, on a rotating cylinder. Functionalized CNT sheets were manufactured by positioning the plasma torch between the CNT array and the rotating cylinder, 4.0 mm above the ribbon. The ribbon was functionalized at a rate of 1 cm/s as it passed under the plasma torch. Fig. 11A illustrates a schematic of the process that shows the manufacturing process. Each layer of the CNT sheet is functionalized with plasma as it gets wound on the rotating cylinder. A Surfx Atomflo 400D RF plasma system was used in this study. The plasma was generated with a mixture of helium and oxygen. The effective extent of functionalization was varied by changing the plasma power in conjunction with the oxygen flow rate, as per Table 2. After this, the CNT web was detached from the rotating cylinder, and the 10-layer CNT sheet was sprayed with a solution of epoxy in a solvent mixture of 66:34 toluene/DMF, respectively. The gradual evaporation of the solvent enables the “densification” of the

Table 2 Varied Plasma Power (W) Employed for Functionalization in Accordance With Changing Oxygen Flow Rate (L/min) Plasma Power (W)

Helium Flow Rate (L/min)

Oxygen Flow Rate (L/min)

0W 80 W 100 W 120 W 140 W

15.0 15.0 15.0 15.0 15.0

– 0.05 0.1 0.15 0.2

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CHAPTER 16 CNT SHEET COMPOSITES

CNT ribbons into a compact CNT sheet. After drying the 10-layer sheet, the process was repeated 10 times to prepare a final CNT sheet composed of 100 layers. After the accumulation of 100 layers of CNT ribbon, the resulting sheet was removed from the cylinder by detaching from the underlying Teflon™ film. The manufacturing process was completed by curing the CNT/epoxy sheet composites in a vacuum bag under 10 MPa pressure applied in a hydraulic hot press. The CNT/epoxy composite was cured by heating at 107.22°C (225°F) for 1 h followed by heating at 176.66°C (350°F) for 2 h and finally by cooling down to room temperature. PCNT sheets without epoxy resin were processed under similar conditions and used as control samples. PCNT sheet/epoxy composites with varying wt% CNT content were manufactured by changing the amount of dissolved epoxy in solution. The wt% CNT content was determined via thermogravimetric analysis (TGA) in N2 atmosphere. Table 3 shows variation in CNT percent content in composites made with varying concentration of epoxy in solution. Fig. 12 displays mechanical properties of CNT/epoxy composites composed of pristine (0 W) CNTs as a function of wt% CNT content. The increase in the concentration of epoxy in solution results in increase in load to failure of the composites. However,

Table 3 Variation in wt% CNT Content Determined by TGA in CNT/Epoxy Sheet Composites Produced From Varying Concentration of Epoxy Solutions Concentration of Epoxy Solution (w/v, %)

Wt% CNT Content

0 0.1 0.25 0.5 1.0

100 82 63 42 24

FIG. 12 Mechanical properties of pristine CNT/epoxy sheet composites as a function of wt% CNT content.

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401

increasing epoxy content increases the thickness of the composite and decreases the wt% CNT content. Therefore, composites with 42 wt% CNT content demonstrate a higher load to failure but lower tensile stress when compared with that of composites with 63 wt% CNT content. Interestingly, composites with CNT content greater than 63% demonstrate lower load to failure values. This could be attributed to increased chances of CNT sliding past one another due to the absence of epoxy glue to hold them together. Through these experiments, it was determined that CNT/epoxy composites with 63 wt% of CNT demonstrate the best mechanical properties. CNT/epoxy composites with plasma-functionalized CNTs were fabricated, and the wt% CNT content was set at 63%. The degree of plasma functionalization was controlled by adjusting the plasma power and oxygen flow rate. Fig. 13 shows the effect of plasma functionalization on the mechanical properties of functionalized CNT/epoxy composites with 63 wt% CNT content. Tensile stress of CNT/ epoxy composites initially increases with the increase in plasma power, reaching the highest value for

FIG. 13 (A) Mechanical properties of CNT/epoxy composite (with 63 wt% CNTs) as a function of plasma power; (B) representative stress–strain curves of composites composed of pristine and plasma-functionalized CNTs compared with CNT sheet alone.

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CHAPTER 16 CNT SHEET COMPOSITES

100 W plasma-functionalized CNT/epoxy composites. High degree of functionalization achieved by using plasma power greater than 100 W results in significant damage to the structure of the individual nanotubes, thereby negatively affecting the mechanical properties of the resulting composite. The extent of functionalization that can be controlled by the level of power and oxygen gas flow rate is critical toward the development of high-strength composites. The improvement in properties can be attributed to the reaction and cross-linking between dOH groups on functionalized CNTs and the epoxy resin. The interaction between functionalized CNTs and the epoxy resin was confirmed by performing a controlled experiment involving the reaction of functionalized CNT sheet with epoxy resin in solution. The functionalized CNT sheets were produced by in situ plasma treatment at 100 W. Tetrafunctional epoxy resin (4,40 -methylenebis(N,N-diglycidylaniline)) was dissolved in toluene to form a 5 wt% solution. The functionalized CNT sheet was immersed in the epoxy/toluene solution in a sealed beaker and was heated on a hot plate to 90°C with stirring for 4 h. Upon the completion of this procedure, the CNT sheet was removed from the epoxy/toluene solution and rinsed thrice with excess of hot toluene at 80°C to remove any unreacted epoxy. After washing with toluene, the CNT sheet was dried in a vacuum oven at 80°C for 8 h. The dried CNT sheet was expected to react with epoxy, and the resulted sample was studied by XPS. Fig. 14A shows the survey scan from the functionalized CNT sheet derivatized with epoxy that reveals 3.6 at.% N content. However, the survey scan of functionalized CNT sheet (Fig. 14B) that was not treated with epoxy showed the absence of any N peak. The epoxy resin in this derivatization experiment was used without any additional curing agent. It is important to note that the tetrafunctional epoxy molecule used in this study contains nitrogen in the form of tertiary amines, thus serving as a source of nitrogen. Tertiary amine is known to be able to catalyze the ring opening of the epoxide group and thereby allow reaction with hydroxyl groups to take place [89]. Further, we observed an increase in at.% O content for the derivatized CNT sheet, which is attributed to the oxygen atoms in the epoxy resin attached to the CNT sheet. Cross-linking within the composite can be classified into three linkages, namely, CNT-epoxy, CNT-epoxy-CNT, and epoxy-epoxy. When the composite contains functionalized CNTs and the wt% content of CNTs in the composite is high (82 and 63 wt%), CNT-epoxy and CNT-epoxy-CNT linkages can be considered dominant. Thus, the tensile strength and modulus of functionalized CNT-epoxy composites are greater than that of composites with pristine (0 W) CNTs and CNT sheet alone (Fig. 13B). However, when the wt% epoxy content in the final composite is increased, epoxy-epoxy linkages can be expected to dominate. This results in increased overall cross-linking, which causes an increase in modulus but coupled with a decrease in tensile strength. The introduction of epoxy increases the density of the composite by filling voids between CNTs that are held together with van der Waals forces. On increasing epoxy content, there is an increased probability of epoxyepoxy bonding. The latter results in a relatively higher degree of cross-linking leading to a raise in modulus with increase in epoxy fraction and decrease in CNT content from 63% to 42%. However, CNTs are significantly stronger than epoxy, and an increase in epoxy fraction causes a reduction in overall CNT content, which can lead to a relatively lower tensile stress for composites with lower CNT content (42 wt%). Here, it is important to note that the increase in modulus for 100 W plasmafunctionalized CNT sheets is greater than that for PCNT sheet when comparing composite with 63 and 42 wt% CNTs. This dissimilarity can be attributed to the increased interaction between the plasma-treated carbon nanotubes and the resin via the functional groups. Mechanical properties of CNT sheets without epoxy show a decreasing trend after plasma functionalization with strength going down with increasing plasma power. These results are expected as the

3 PROCESSING AND APPLICATIONS OF CNT SHEETS

403

FIG. 14 (A) XPS survey scan of plasma-functionalized CNT sheets treated with epoxy solution in toluene in the absence of curing agent; (B) XPS survey scan of plasma-functionalized CNT sheet.

number of defects increases with increase in plasma power also shown via Raman spectroscopy. Similar results have been reported previously showing negative effect of plasma treatment on mechanical properties of CNT threads and buckypapers in the absence of any polymer matrix. However, on combining functionalized CNTs with epoxy and other polymers [90], numerous researchers have observed an improvement in the mechanical properties of the final composite over those made with nonfunctionalized/PCNTs. Therefore, it is essential to achieve an optimum degree of functionalization of CNTs combined with the appropriate amount of reactive polymer resin in order to produce a composite with superior mechanical properties. Specific strength (stress normalized with density) is an important metric for evaluating structural aerospace materials. Composites with high wt% CNT content possess light weight, which makes them good candidates for aerospace applications. The uniform functionalization and infiltration achieved in this work while maintaining a high wt% CNT content allow us to create

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CHAPTER 16 CNT SHEET COMPOSITES

FIG. 15 (A) Specific strength of pristine and plasma-functionalized CNT/epoxy composites as a function of wt% CNT content; SEM images of fractured ends of (B) PCNT-epoxy composite and (C) 100 W CNT-epoxy composite.

composites with better properties than those obtained from CNT buckypaper/polymer composites, reported previously [91]. Fig. 15A shows the specific strength of CNT/epoxy composites with and without plasma-functionalized CNTs. The formation of covalent cross-links between functionalized CNTs and epoxy enables better load transfer along the length of the composite. Fig. 15B and C shows SEM images of fractured composite ends made with pristine and functionalized CNTs, respectively. PCNT/epoxy composites fracture unevenly with loose CNTs hanging out from the broken end possibly a result of sliding of nanotubes past each other (Fig. 15B). The clean, relatively sharper fracture typically observed in Fig. 15C for functionalized CNT/epoxy composite is an evidence of the formation of covalent bonds between CNTs and epoxy resin. These results demonstrate the synergy effect of reactive polymer resins combined with functionalized CNTs. In summary, CNTs functionalized with 100 W plasma combined with
37 wt% epoxy resin demonstrated 43% improvement in tensile strength and 78% improvement in modulus over composites made with PCNTs. The improvement in properties obtained by addition of epoxy resin and plasma functionalization is about 70% in tensile strength and over 171% in modulus over PCNT sheets.

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405

3.2 FLEXIBLE, NANO-STRUCTURED ELECTRODES BASED ON CNT SHEETS Carbon nanotubes (CNTs) have been a popular choice as electrode material due to their high electric conductivity, excellent mechanical strength, and very good chemical stability [92]. These properties also make CNTs ideal for use as electrodes in electric double-layer capacitors (EDLCs) with high-rate capability and long life cycle. EDLCs store energy by the accumulation of charge on the surface of the electrodes, which is a nonfaradaic process. EDLCs rely heavily on the available surface area of the electrodes and therefore are limited in terms of the energy density. Materials such as some transition metal oxides [93] and conductive polymers [94] are called pseudocapacitive materials as they store charge by undergoing redox reactions or faradaic processes. On their own, these materials provide high specific capacitance; however, due to their relatively poor conductivity and mechanical properties, they are often combined with CNTs and other powdered carbon nanomaterials to form electrodes for supercapacitors (SCs). SCs offer the advantage of fast charge speed, high power density and long-term cycling stability that can bridge the gap between traditional capacitors and batteries [95]. The performance of SCs strongly depends on the properties of the electrode materials used, which need to be electrically conductive, have a high surface area, and are chemically stable. The development of flexible and wearable energy storage devices also demands the electrode materials to be mechanically stable and light weight. CNTs are a viable material for the manufacturing of flexible electrodes; however, the use of powdered CNTs leads to the creation of random networks that add more obstacles to a fast charge transfer [96–98]. Further, the creation of electrodes from powdered materials often requires conductive additives and binders [99, 100] to be added followed by casting on a current collector that is typically made of metal. The use of a separate metallic current collector adds a tremendous amount of “parasitic weight” to the SC, thereby limiting its application in wearable and flexible electronics. CNT sheets have been utilized as scaffolds and current collectors in combination with pseudocapacitive materials such as MnO2 [101] and polyaniline (PANI) [102, 103] to produce lightweight, flexible SCs. Vertically aligned CNT arrays, typically grown on metal foils [104–106], have been shown to facilitate better mobility of ions due to their high porosity and specific surface area. The increased porosity of CNT arrays allows greater loading of pseudocapacitive materials while still maintaining uniform diffusion of the electrolyte throughout the electrode area. The use of the CNT growth substrate (metal foil) as current collector allows rapid electron transport to and from the individual nanotubes, which in turn results in an overall improved charge transfer allowing the SCs to maintain high energy densities at very high power densities. However, the use of metal foils also limits the flexibility of the electrode and adds additional weight to the SC or battery [107]. Researchers have attempted to overcome this problem by using carbonaceous substrates for CNT growth and as current collectors. Carbon cloth or fabric [108, 109] have been used as substrates for CNT synthesis and as scaffolds for pseudocapacitive materials [110, 111] to make flexible SCs. The relatively low electric conductivities of these materials [112], however, tend to slow down the rate of electron transfer resulting in lower overall charge transfer rate and thus reduced capacitance at high-current densities. Weng et al. [113] utilized CNT sheet as substrate for growing CNTs in an application such as electrodes in Li-ion batteries. The CNT sheet was coated with Si and Al2O3 buffer layer prior to catalyst deposition. Silicon can be beneficial in Li-ion batteries as a Li reservoir during the lithiation process, but in this case, it could also act as a barrier to electron transfer between the newly grown CNT array and the CNT sheet. We believe that by avoiding the Si buffer layer, better bonding and the formation of covalent junctions between the nanotubes in the array and the nanotubes in the sheet [114, 115] can be achieved. Here, we employ an approach described below to realize this strategy.

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CHAPTER 16 CNT SHEET COMPOSITES

3.2.1 Three-dimensional, free-standing polyaniline/carbon nanotube composite-based electrode for high-performance supercapacitors We report the development of a novel three-dimensional nanostructured electrode architecture, which combines the high porosity and better ion diffusion properties of a CNT array with the superior mechanical properties and high electric conductivity of CNT sheets. Vertically aligned carbon nanotube arrays were synthesized by plasma-enhanced chemical vapor deposition (PECVD) directly on a CNT sheet substrate composed of horizontally aligned carbon nanotubes using Ni catalyst. Thus, a freestanding structure (N-doped carbon nanotube, NCNT) consisted of the CNT sheet acting as substrate and also as current collector was developed. The CNT array provides increased porosity and higher surface area to the electrode. This electrode scaffold was then coated with PANI via an easy electrodeposition process. The result was a truly freestanding electrode manufactured without any metal support, conductive additives, or binders [116]. NCNT/PANI electrodes with PVA/H2SO4 gel electrolyte were used to create a symmetrical supercapacitor device and evaluated for energy storage. The first step toward the fabrication of NCNT sheets is the manufacturing of CNT sheets by dry drawing from spinnable arrays. CNT sheets produced in this manner [117, 118] are composed of CNTs aligned in the direction of drawing or “spinning” as can be shown in the SEM image in Fig. 16B. The CNT sheet substrates used in this study were made by winding 100 layers of CNT web while densifying the deposited webs with acetone. The evaporation of acetone creates capillary forces between the nanotubes and brings them closer together resulting in a “densified” sheet material. After densification, the CNT sheets had a thickness of
4 μm and possess an areal density of 0.24 mg/cm2. As-produced CNT sheets composed of aligned carbon nanotubes were coated with 7 nm Ni catalyst by e-beam evaporation. Ni-coated sheets were annealed in a hydrogen atmosphere at 500°C prior to growth of CNT array. Annealing at these conditions allows uniform Ni particle formation for CNT synthesis as shown in Fig. 16C. Annealed Ni-coated CNT sheets were employed as substrates for growth of vertically aligned CNTs using acetylene gas as carbon precursor in an ammonia plasma atmosphere by PECVD in an Aixtron Black Magic™ reactor. The growth was carried out for 10 min at 700°C temperature and 3.7 mBar pressure in an ammonia (NH3) plasma environment with acetylene (C2H2) as the carbon precursor. NH3 and C2H2 were fed at 160 and 40 sccm, respectively, to maintain a 4:1 ratio considered ideal for CNT growth [119]. The DC plasma electric field also enables vertical alignment of CNTs during growth [120] creating a three-dimensional structure composed of vertically aligned CNTs extending from horizontally aligned CNTs in the CNT sheet substrates. The resulting threedimensional structure is shown in the SEM images in Fig. 16D and E. The latter is composed of vertically aligned CNT arrays grown perpendicularly to horizontally aligned CNTs in the sheet substrate. This novel, three-dimensional structure is called NCNT, and PCNT sheets used for comparison are called PCNT or PCNT sheets. To the best of our knowledge, the synthesis of aligned CNT directly (without Si/Al2O3) on CNT sheet substrate by PECVD is reported here for the first time. We also characterize the specific surface area (SSA) and pore-size distribution of the NCNT and PCNT sheet. The NCNT materials show an SSA of
312 m2/g with greater pore-size distribution in the range of 5–10 nm compared with PCNT that exhibits an SSA of 174 m2/g. This increased SSA and the pore-size volume in the range of 5–10 nm can be attributed to the vertically aligned CNT array, and those increments are beneficial in terms of PANI loading and a fast transfer of electrolyte ions. CNT sheets were employed to create freestanding electrodes by attaching a copper wire using electrically conductive silver epoxy. The connection point was then sealed with conventional epoxy. NCNT or PCNT electrodes were assigned as working electrode in a three-electrode configuration

3 PROCESSING AND APPLICATIONS OF CNT SHEETS

Ni coating by e-beam CNT sheet

407

Ni-coated CNT sheet PECVD

(A)

PANI/NCNT sheet

PANI deposition

(B)

(C)

(D)

(E)

NCNT sheet

FIG. 16 (A) Schematic illustration along with SEM images of (B) pristine CNT sheet; (C) Ni-coated CNT sheet; (D) vertically aligned CNT array grown on CNT sheet (NCNT); (E) NCNT sheet coated with PANI (30 cycles).

connected to a Gamry Interface 1000 Potentiostat/Galvanostat with a Pt wire and Ag/AgCl electrode used as the counter and reference electrode, respectively. Prior to deposition of PANI, the NCNT electrodes were treated with 4 M H2SO4 for 30 min to remove Ni catalyst from the tips of the CNT array synthesized in a tip growth mode. PANI was deposited on the NCNT electrode by cyclic voltammetry (CV). A solution of 0.018 M aniline in 1 M H2SO4 was used for deposition, and the electrodes were cycled between 0.2 and 0.8 V at a scan rate of 25 mV/s. The amount of PANI deposited was controlled by changing the number of cycles of CV. The electrodes in this work are named after the number of cycles of PANI deposition by CV as follows: 10PANI/NCNT, 20PANI/NCNT, 30PANI/NCNT, 40PANI/NCNT, and 50PANI/NCNT for electrodes coated with 10, 20, 30, 40, and 50 cycles of CV,

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CHAPTER 16 CNT SHEET COMPOSITES

respectively. NCNT sheet electrode without PANI coating was also prepared for comparative testing and is termed as NoPANI/NCNT electrode. After coating, the electrodes were rinsed three times with deionized (DI) water and dried overnight under vacuum at room temperature. PCNT sheet without any CNT array grown on it was also coated with PANI and tested for comparison. PCNT sheet electrode coated with 30 cycles of PANI deposition is termed as 30PANI/PCNT. A symmetrical supercapacitor device was assembled by combining two PANI/NCNT electrodes with PVA/H2SO4 solid gel electrolyte sandwiched in between. PVA/H2SO4 gel electrolyte was prepared by dissolving 1 g PVA in 10 mL water and 2 mL concentrated H2SO4. The mixture was stirred at 90°C until all the PVA dissolved and the solution became clear. The gel, which remains a viscous liquid, was cooled down to 50°C and then applied as a thin layer on the NCNT/PANI electrodes. The gel-coated electrodes were dried overnight at room temperature. After drying, the two electrodes were gently pressed together with an additional thin layer of PVA/H2SO4 gel applied in between. The device was dried in air overnight. The PVA/ H2SO4 gel acts as an electrolyte and separator for the device. Electrochemical measurements were carried out using a Gamry Potentiostat (Interface 1000) at room temperature in a three-electrode configuration for the NCNT and NCNT/PANI electrodes and in a two-electrode configuration for the symmetrical SCs. All electrodes tested in this study had a working area of 0.64 cm2. The half-cell testing in three-electrode configuration was carried out using 1 M H2SO4 as electrolyte. The electrodes were evaluated by cyclic voltammetry at different scan rates between a potential window of 0.2 and 0.8 V. Galvanostatic charge–discharge experiments for all electrodes were carried out between a potential window of 0 and 0.8 V at different current densities. Electrochemical impedance spectroscopy measurements were conducted over a frequency range from 105 to 101 Hz at a sinusoidal voltage amplitude of 10 mV. Mass measurements were made using a Sartorius ME5 microbalance with a precision of 0.1 μg. The mass of electrodes used for calculations was obtained as a sum of the mass of the CNT sheets and the mass of the PANI coating (calculated by measuring the mass of the electrodes before and after PANI coating). The performance of NCNT/PANI electrodes was further evaluated for charge storage and capacitance retention in a two-elecrode “device” configuration. Equations used for calculations of capacitance, energy, and power density are also as follows. The specific capacitance (gravimetric) (Csp) in the three-electrode system was calculated by using the equation Csp ¼

It ΔVm

where I is the discharge current, t is the discharge time, ΔV is the operating voltage window, and m is the mass of the active materials (PANI only) or total mass of the electrode (PANI and CNT sheet). Similarly, areal capacitance (Car) and volumetric capacitance (Cv) of the electrode are calculated using the equations Car ¼

It ΔVa

Cv ¼

It ΔVv

where a and v are the area and volume of each electrode.

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409

The specific capacitance of the supercapacitor device (CSC) is calculated using the equation CSC ¼

It ΔVm

where m is the total mass of the PANI coating on both electrodes or the total mass of both electrodes (CNT sheets and PANI coating). The energy density (ESC, Wh/kg) and power density (PSC, kW/kg) of the supercapacitor device are calculated using the following equations: ESC ¼

Csc ðΔV Þ2 2  3:6

PSC ¼

Esc  3:6 t

where t is the time of discharge for the supercapacitor device. CNTs fabricated in a NH3 plasma atmosphere have also been shown to become incorporated with nitrogen in the graphitic structure [121]. Such a phenomenon can alter the electronic properties. The resulted N-doped CNTs have also demonstrated better electrochemical activities than their non-Ndoped counterparts [122, 123], which invites application in fuel cells [124], batteries [125], and supercapacitors [126]. Lim et al. [127, 128] demonstrated that polymerization of PANI can be initiated from N-doped sites on carbon nanotubes. This motivated us to perform N-doping of the PECVD-grown CNT array and to prove nitrogen incorporation by XPS. The results are presented in Fig. 17. Fig. 17A shows an XPS survey scan of NCNT sheet revealing over 6% atomic nitrogen content after 10 min of growth time in NH3/C2H2 plasma atmosphere. High-resolution scan and curve fit of N1 s spectra of NCNT sheets displayed in Fig. 17B reveals the presence of pyridinic (
398.5 eV) and pyrrolic (
401 eV) nitrogen incorporated in the CNT structure. The obtained results are similar to that reported in the literature for nanotubes produced by PECVD [129, 130]. The presence of Ni peaks is attributed to the Ni catalyst contained in the tips of the nanotubes in the CNT array grown on CNT sheet. Haq et al. [128] demonstrated that polymerization of PANI can be initiated from N-doped sites on carbon nanotubes. Their results suggest that the presence of nitrogen incorporated in the NCNT sheets could help create better bonding with the PANI coating. Electrochemical deposition of PANI and other conductive polymers is a viable method to produce these materials in a controlled and reproducible manner. NCNT electrodes for PANI deposition were prepared by adhering the NCNT sheet on adhesive Kapton™ tape to allow exposure only on the side of the CNT sheet with the grown CNT array. The electric contacts are made only from top of the CNT sheet with a copper foil and conductive silver epoxy. The contact point between the Cu foil and the CNT sheet is then sealed with conventional, electrically insulating epoxy. The amount of conductive polymer deposited is controlled by adjusting the time of electrodeposition. In this report, PANI electrodeposition was carried out by using cyclic voltammetry (CV) at a constant scan rate of 25 mV/s from a 0.018 M aniline solution in 1 M H2SO4. Fig. 18A–E shows SEM images of NCNT samples coated with increasing amount of PANI after 10, 20, 30, 40, and 50 cycles of CV here labeled as 10PANI/ NCNT, 20PANI/NCNT, 30PANI/NCNT, 40PANI/NCNT, and 50PANI/NCNT, respectively. A thin coating of PANI forms on individual CNT when the number of cycles of CV is small. As the number of cycles rises, the thickness of the PANI coating also tends to increase, as seen by the increasing diameters of the nanotube-like structures. Raising the number of cycles of CV further (40 and 50) eventually

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FIG. 17 (A) XPS survey scan of NCNT sheet; (B) high-resolution N1s spectrum obtained from NCNT sheet after 10 min of growth in the PECVD reactor.

leads to the beginning of the formation of a network of PANI nanowires on top of PANI-coated CNT array. Fig. 18F shows a coating of PANI after 30 cycles of CV on PCNT sheets for comparison. The absence of the CNT array leads to the formation of a PANI nanowire network directly on the flat CNT sheet surface. Mass loading of PANI was accurately determined using a microbalance as follows: 0.234, 0.274, 0.316, 0.364, and 0.416 mg/cm2 for electrodes coated with 10, 20, 30, 40, and 50 cycles of CV, respectively. It is known that the rate of PANI electrodeposition is also dependent on the concentration of the aniline monomer in solution [131]. In this study, the use of low concentration of aniline (0.018 M) allows for the uniform coating of PANI on the individual CNT, which results in a core-shell morphology as seen in TEM images in Fig. 19A and B. Cyclic voltammogram plots of NCNT sheet and PCNT sheets in 1 M H2SO4 without any PANI coating are shown in Fig. 20A. NCNT sheets demonstrate a higher areal capacitance (11.43 mF/cm2)

FIG. 18 (A)–(E) SEM images of PANI/NCNT after 10, 20, 30, 40, and 50 cycles of PANI deposition; (F) SEM image of PANI/PCNT sheet (PANI/PCNT) after 30 cycles of PANI deposition.

FIG. 19 (A), (B) TEM images of 30PANI/NCNT electrodes showing core-shell morphology composed of PANI shell on CNT core.

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FIG. 20 (A) Cyclic voltammograms (CV) of NCNT and PCNT electrodes at 50 mV/s scan rate. (B) Cyclic voltammograms (CV) of PANI/NCNT electrodes at 50 msV/s coated with different cycles of CV of PANI deposition. (C) Nyquist plots of NCNT electrodes coated with different PANI loadings. The inset displays comparison between Nyquist plots of 40PANI/NCNT and NCNT electrode with no PANI coating. (D) Areal and volumetric capacitances of different PANI/NCNT electrodes measured at a 1.56 mA/cm2 current density. (E) Charge–discharge curves obtained from NCNT electrodes with different PANI loadings at a current density of 1.56 mA/cm2. (F) Specific capacitance of the different PANI/NCNT electrodes as a function of areal current density used for the conducted measurement.

as calculated from the area under the curve of CV scan at 50 mV/s compared with that obtained from PCNT sheets (
3.7 mF/cm2). The increased capacitance can be attributed to the greater electrochemical activity due to the nitrogen incorporated or N-doping of NCNT sheets compared with the inert chemical nature of PCNT sheets. Fig. 20B shows different cyclic voltammograms obtained from PANI/NCNT electrodes at 50 mV/s with different amounts of PANI electrodeposited with 10, 20, 30, 40, and 50 CV cycles. The area under the CV curve increases with increasing cycles CV and PANI coating thickness. This observation reaffirms that fact that PANI is the most significant contributor to the overall capacitance of the electrode. It is the combination of a highly pseudocapacitive material like

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PANI with the open, 3-D structure of the NCNT sheets, which leads to the development of superior electrodes as discussed further in this report. The electrodes are also studied by electrochemical impedance spectroscopy (EIS) and Nyquist plots from all samples, as shown in Fig. 20C. A typical Nyquist plot is composed for three regions selected based on frequency. At high frequency, one can obtain the solution resistance (Rs) determined as the point of intersection of the plot with the real axis (x-axis). Rs is defined as the series resistance obtained from the internal resistance of the electrode and the ohmic contact resistance at the interface of the electrode and the electrolyte [132, 133]. The high-frequency region shows a characteristic semicircle that diameter can provide an indication of the electron transfer rate, also known as the charge transfer resistance (Rct) within the electrode [134, 135]. At middle and low frequencies, Nyquist plots composed of a “Warburg line” with a slope of
45 degrees followed by an almost vertical straight line. The slope of the straight line determines the resistance to diffusion of the electrolyte ions into the regions of the electrode. As seen in Fig. 20C, the Rs of the different electrodes increases with the rising in the number of PANI deposition cycles. This is because more PANI is deposited when increasing the number of CV cycles, which causes an increase of the overall electrode resistance. A similar trend of increase in Rs with increased loading of other pseudocapacitive materials has been reported previously by Lv et al. [132]. Interestingly, Rs for NCNT sheets with no PANI coating is higher than that for 10, 20, and 30 PANI/NCNT samples but lower than those for 40 and 50 PANI/NCNT. This behavior could be attributed to the fact that the electric conductivity of NCNT sheets is lower than that of metals, and the NCNT sheets are not supported by a metal foil or grid serving as current collector. On the addition of PANI however, the electric conductivity of the composite electrode is increased as PANI coats individual nanotubes. Thus, a 10PANI/NCNT shows lower Rs than NoPANI/NCNT electrode. This behavior is contradictory to what was observed by Huang et al. [135] who used a metal foil as a current collector (relatively lower internal resistance) but similar to that reported by Cherusseri et al. [112] who employed a carbon fiber fabric (relatively high internal resistance) as a current collector. 10PANI/NCNT, 20PANI/NCNT, and 30PANI/NCNT barely show the formation of the semicircle described above, which indicates very low Rct or resistance to electron transfer within the electrode. 40PANI/NCNT and 50PANI/NCNT start to indicate the formation of the semicircle as Rct resistance increases with the amount of PANI deposited on the electrode. The capacitive performance of the electrodes was evaluated using Galvanostatic charge-discharge. Fig. 20D shows the charge-discharge curves obtained by testing different electrodes at same current density of 1.56 mA/cm2. From a practical perspective, areal capacitance (mF/cm2) and volumetric capacitance (mF/cm2) are important metrics along with specific (gravimetric) capacitance (F/g) for evaluating the performance of lightweight, flexible supercapacitors. Fig. 20E displays areal capacitance (mF/cm2) and volumetric capacitance (mF/cm3) for the PANI/NCNT electrodes, which shows a similar trend of increased values with rising amount of PANI coating. 50PANI/NCNT electrode demonstrates areal and volumetric capacitances of 151 mF/cm2 and 216.6 F/cm3, respectively, at a current density of 1.56 mA/cm2. All the different electrodes used in this study are of the same surface area (0.64 cm2) but possess different mass because of the different amount of PANI coating. Therefore, the observed capacitance is plotted as a function of areal current density (mA/cm2), as opposed to gravimetric current density (A/g) for a meaningful comparison. It is noted that the specific capacitance increases with an increase in number of PANI deposition cycles up to 40 cycles; however, on increasing the PANI growth cycles to 50, the specific capacitance goes down. This is because the addition of PANI beyond 30 cycles results in a shift in the structural configuration of the newly deposited polymer. When the cycles are

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increased to 40, we noticed the beginning of a network formation of PANI nanowires on top of the CNT array (Fig. 18D). When further increasing the number of cycles of deposition to 50, the density of the PANI network is increased, as seen in SEM image in Fig. 18E. The increased density of the PANI nanowire network could slow down the diffusion of ions within the PANI coating, thereby reducing the expected increase in capacitance from additional mass of PANI [135]. This analysis is also supported by the decreased slope of the Nyquist plot for 50PANI/NCNT electrode (Fig. 20C) in the low-frequency region. The latter is an indicator of created diffusion resistance of ions to and from the electrode, since higher slope indicates faster diffusion. For the PECVD-grown CNT arrays with nanotube length of about 2 μm, the volumetric capacitance calculated based on the total active volume of 30PANI/NCNT electrode (excluding the CNT sheet current collector) reaches 566 F/cm3. This value is one of the highest seen for PANI-based electrodes and better than other reported composites of PANI with different CNT assemblages, such as sheets/films on conductive glass [96], graphite paper [136] substrates, and with other nanomaterials such as graphene [137, 138]. The PANI-CNT core-shell morphology maintained in the 10PANI/NCNT, 20PANI/NCNT, and 30PANI/NCNT can be identified as one of the key reasons for the superior electrode performance in this work. Fig. 20F shows the areal capacitance of the different CNT/PANI electrodes as a function of areal current density. PANI/NCNT electrodes coated with 10, 20, and 30 cycles of PANI retain more than 80% of their capacitance even when the current density is increased 30 times. CNT sheet acts as a lightweight current collector, and the CNT array growing directly on the sheet facilitates a fast electron transfer from PANI to the CNT sheet. The high-rate performance of PANI/NCNT electrodes is an improvement over that of other examples from literature of PANI-based, freestanding electrodes [139, 140] and significantly better than the performance of PANI-based electrodes employing metallic or carbonaceous current collectors [141, 142]. The performance at high-current densities of PANI/NCNT sheets with CNT sheet current collectors is comparable with that reported by Zhang et al. [143, 144], wherein they have used aligned CNT arrays grown on metal foil current collectors and coated with PANI. The above results showed that 30PANI/NCNT electrode delivers the highest specific capacitance calculated based on mass of PANI. This electrode also has better capacitance retention when subjected to a 30-fold increase in current density, as compared with the 40PANI/NCNT and 50PANI/NCNT electrodes. However, one of the most important metrics used to judge the performance of a capacitor is its cycling stability, which illustrates the ability of the device to retain charge over many cycles of charge– discharge. The cycling stability of 30PANI/NCNT electrode was evaluated by charging and discharging the electrode in a potential window of 0–0.8 V for 5000 cycles at a high-current density of 15.6 mA/cm2 or 49.5 A/g (28.1 A/g based on total mass of electrode). Fig. 21A shows percent capacitance retained by 30PANI/NCNT and 30PANI/PCNT electrodes as a function of the number of cycles of charge-discharge. The 30PANI/NCNT electrode retains 80% of its original capacitance after 5000 cycles in comparison with 30PANI/PCNT electrode, which retains only 44% of its original capacitance. The charge-discharge plots from the 1st to 5000th cycle for the 30PANI/NCNT electrode are shown in Fig. 21B. The performance of 30PANI/NCNT composite electrode can be attributed to a series of factors working in tandem. Firstly, the formation of a CNT core and PANI shell allows rapid electron transport between the nanotube and conducting polymer [145, 146]. Secondly, the CNT arrays in the core-shell morphology are growing directly on CNT sheet current collectors, which further reduces resistance to electron movement within the electrode [143]. Thirdly, in the case of 30PANI/ NCNT electrode, the amount of PANI coating is optimum in such a way that the electrode can provide sufficiently high capacitance while still maintaining an ordered nanowire array-like structure. This

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FIG. 21 (A) Capacitance retention between 30PANI/NCNT and 30PANI/PCNT electrodes over 5000 cycles of chargedischarge at 49.5 A/g current density. (B) 1st and 5000th charge-discharge plots for 30PANI/NCNT electrode at 49.5 A/g.

“open” porous array structure allows fast diffusion of ions maintaining high ionic conductivity within the electrode [147–149]. Any additional PANI coating (40 and 50 cycles) tends to reduce the porosity of the electrode, which can slow the diffusion of ions to the interior PANI surfaces [144]. Finally, CNTs in the core provide a stable mechanical framework, which delivers reinforcement to the PANI shell as the latter undergoes volumetric changes during cycles of charge-discharge [102, 150]. In case of NCNT sheets, the PANI shell is additionally reinforced by possible covalent bonding with the CNTs in the array via N-doped sites [128] created during CNT growth [130]. PANI deposited on the 30PANI/PCNT electrode exists as a network of interconnected-independent nanowires on the flat CNT sheet surface. The lack of reinforcement of the PANI nanowires by CNT cores leads to a steep decline in percent capacitance retention, especially when undergoing charge-discharge at a high-current density of 49.5 A/g. The metal-free, binder-free, and freestanding NCNT electrode design allows the creation of lightweight, flexible, and durable energy storage devices of the future. One of the key components of this design is the CNT sheet substrate. The high strength and high conductivity of CNT sheets enables freedom in the design of a supercapacitor device based on CNT sheet electrodes. We used two 30PANI/ NCNT composite electrodes to fabricate a symmetrical supercapacitor device with a PVA/H2SO4 gel employed as separator and as solid electrolyte. Fig. 22A and B shows an illustration and digital image of the actual capacitor, respectively. PANI/NCNT jj PANI/NCNT capacitor is electrochemically analyzed with CV and charge–discharge techniques. Fig. 23A displays CV curves obtained by cycling the supercapacitor between 0 and 0.8 V at different scan rates. The characteristic PANI peaks seen in the CV curves of individual electrodes are less pronounced in the case of the supercapacitor device, which has been observed previously [151] when the test setup is changed from a three-electrode setup to twoelectrode symmetrical setup. Fig. 23B shows the charge–discharge curves of the supercapacitor made with 30PANI/NCNT electrodes tested at different current densities. The discharge portion of the charge-discharge curves is used to determine the capacitance of the supercapacitor device. PANI/ NCNT jj PANI/NCNT capacitor exhibits a capacitance of 128 F/g at a current density of 2.47 A/g when calculated based on the mass of the PANI loading. Since the current collector used to fabricate the capacitor is a lightweight CNT sheet, the specific capacitance when calculated based on the total mass

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(A)

NCNT sheets

PVA/H2SO4 gel electrolyte

Cu foil for electrical contact

PANI/NCNT||PANI/NCNT supercapacitor

(B)

FIG. 22 (A) Illustration of the PANI/NCNT jj PANI/NCNT capacitor (SC) assembly; (B) picture of the assembled SC in this work.

of both electrodes (including mass of CNT sheet current collector) quantifies to 74 F/g at 1.4 A/g. This value of specific capacitance based on the overall mass of the supercapacitor (SC) is higher than previously reported data for flexible devices based on PANI with CNTs [152] and graphene [153]. The ultrathin profile of the PANI/NCNT electrodes (2 μm tall CNT array on 4 μm thick CNT sheet) translates to a volumetric capacitance of 67 F/cm3 for the entire device at a current density of 2.47 A/g (1.56 mA/cm2). This value is almost 4.5 times greater than that reported recently [112] for supercapacitors with carbon nanotube/polypyrrole brushlike electrodes, in which CNTs are grown on carbon fiber substrates. PVA/H2SO4 gel, prior to drying, uniformly penetrates the PANI/NCNT structure, which allows fast exchange of ions between the PANI shell and the gel electrolyte, thus translating to an excellent high-rate performance, as seen in Fig. 23C. The ability of the capacitor to retain a high capacitance with increase in current density also enables maintaining high energy density with increase in power density. Fig. 23D shows the variation of energy density with power density of the

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FIG. 23 (A) CV curves of SC at different scan rates. (B) Charge-discharge curves of SC at different current densities (calculated based on mass of PANI loading). (C) Percent rate of capacitance retention of SC when chargeddischarged at different current densities. (D) Ragone plot of SC showing energy density as a function of power density. (E) Percent capacitance retention of SC during 10,000 cycles of charge–discharge at 24.7 A/g current density.

PANI/NCNT jj PANI/NCNT capacitor compared with other examples of flexible supercapacitors based on PANI from the literature. Energy density and power density have been calculated based on the mass of PANI loading. The capacitor reveals an energy density of
11.1 Wh/kg and power density of 0.98 kW/kg, when tested at a current density of 2.47 A/g. The overall goal of a supercapacitor is to bridge the gap between the high energy density of the batteries and high power density of traditional capacitors. Rapid charge transfer achieved by PANI/NCNT jj PANI/NCNT SC enables the capacitor to retain more than 64% of its energy density (7.16 Wh/kg), even when the power density is increased

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more than 27 times (0.98–26.83 W/kg). These values of energy and power densities are better than those reported for flexible devices based on PANI combined with carbon nanotubes [152, 154–156]. An ideal supercapacitor also needs to have a long life cycle. PANI/NCNT jj PANI/NCNT SC was evaluated for its cycling ability by carrying out charge-discharge at a current density of 24.7 A/g (calculated based on PANI loading of the two electrodes or 15.63 mA/cm2) for 10,000 cycles. The percent capacitance retained is plotted as a function of the number of cycles in Fig. 23E, and the PANI/ NCNT jj PANI/NCNT SC retains more than 92% of its original capacitance after 10,000 cycles of charge-discharge. Such a performance at a high-current density is significantly better than that achieved by other flexible supercapacitors based on PANI [153–162]. PANI/NCNT jj PANI/NCNT capacitor is made of flexible components, and the performance of the device was examined by carrying out cyclic voltammetry while bending the SC at various angles. Fig. 24A shows the comparison of cyclic voltammograms of the SC at 50 mV/s scan rate while bending at 90 degrees, 180 degrees, and in flat state (0°). The area under the CV plots shows negligible change, while the device is tested bent at 90 degrees and 180 degrees. The practical applicability of the PANI/ NCNT jj PANI/NCNT SC is also demonstrated by using three devices connected in series to power a red LED, as seen in Fig. 24B. Although, the absolute value of energy density for PANI/NCNT jj PANI/ NCNT SC is not very high, the electrode design of our device shows promise, especially in terms of retaining high energy density with increasing power densities. Energy density can be further enhanced by increasing the potential window of the device via employing organic electrolytes and ionic liquid as electrolyte. The CNT/PANI core/shell morphology is also critical for high charge-rate performance. It is reasonable to anticipate that the greater the length of the CNTs in the array, the higher the PANI loading that can be deposited on the electrode while still maintaining core-shell morphology. Other studies have explored the combination of manganese oxide and vertically aligned CNT arrays grown on metal substrates [105, 163] and on carbon cloth [108] as electrodes for high charge-rate supercapacitors. To the best of our knowledge, the present work demonstrates for the first time an SC device based on vertically aligned CNTs grown directly on lightweight, aligned, highly conductive, and flexible CNT sheets. This opens a scope for further improvements in the development of flexible supercapacitors with high capacitance and high charge-rate capabilities.

FIG. 24 (A) Cyclic voltammograms of the PANI/NCNT jj PANI/NCNT SC bent at different angles conducted at 50 mV/s; (B) picture of three capacitors connected in series used to power a red LED (1.8–2.0 V rated).

REFERENCES

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4 SUMMARY Carbon nanotubes (CNT) have evolved from being powdered fillers used to enhance properties of polymers to constituent building blocks used to create macroscale assemblages such as sheets and yarns. In this chapter, we presented an overview of the CNT sheet making process starting from buckypaper to dry spinning and floating catalyst CVD (FCCVD). The continuous nature of the FCCVD process has enabled the production of CNT assemblages on industrial scale. Dry spinning allows for the creation of aligned, catalyst-free, high quality CNT sheets. In this chapter, we discussed studies on the functionalization and cross-linking of CNTs in macroscale assemblages with the objective of emulating properties of individual nanotubes. Dry techniques like plasma and e-beam have shown promise toward fast process of creating functional groups and cross-links on CNTs and without disturbing the overall structure of the assembly. We also discussed results from studies carried out at the UC Nanoworld Labs, on the application of atmospheric pressure plasma functionalization during the manufacturing of the CNT sheet. CNT sheets functionalized with oxygen plasma and cross-linked with epoxy were used to create composites, which demonstrated 70% greater tensile strength over PCNT sheets. CNT/epoxy composites with greater than 60 wt% CNT content have been developed steering in a “role reversal” wherein a minor amount of polymer has been used to reinforce a CNT sheet. Further, we highlight the versatility of the CNT sheet via its application as nanostructured electrodes in the development of efficient, flexible supercapacitors. CNT sheets have been employed for the first time as substrates for CNT array growth in a PECVD process. This resulted in the creation a three-dimensional structure supported with increased surface area of a CNT array combined with the flexibility of the CNT sheet. NCNT electrodes designed in this study are combined with the conductive polymer PANI to create symmetrical, flexible supercapacitor devices. The latter demonstrated more than 80% capacitance retention when operated at an extremely high-current density of 74 A/g. These devices also retained greater than 90% of original capacitance after 10,000 cycles of charge-discharge. Thus, the CNT sheet material emerges as an ideal candidate for the processing and development of a variety of applications from composites to energy storage.

ACKNOWLEDGMENTS The authors would like to acknowledge the financial support and expertise of the team at NASA Langley Research Center through the NASA NNX13AF46A grants. This work was partially funded by the National Science Foundation (NSF) through the following grants: CMMI-0727250, SNM-1120382, and ERC-0812348. The authors also appreciate the support of DURIP-ONR N00014-15-1-2473, ARMY W911NF-16-2-0026, and NASA Grant NNC16CA17C.

REFERENCES [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58, https://doi.org/ 10.1038/354056a0. [2] M. Yu, B. Files, S. Arepalli, R. Ruoff, Tensile loading of ropes of single wall carbon nanotubes and their mechanical properties, Phys. Rev. Lett. 84 (2000) 5552–5555, https://doi.org/10.1103/PhysRevLett.84.5552.

420

CHAPTER 16 CNT SHEET COMPOSITES

[3] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature 382 (1996) 54–56, https://doi.org/10.1038/382054a0. [4] K. Jiang, Q. Li, S. Fan, Nanotechnology: Spinning continuous carbon nanotube yarns, Nature 419 (2002) 801, https://doi.org/10.1038/419801a. [5] M. Zhang, S. Fang, A.A. Zakhidov, S.B. Lee, A.E. Aliev, C.D. Williams, K.R. Atkinson, R. H. Baughman, Strong, transparent, multifunctional, carbon nanotube sheets, Science 309 (2005) 1215–1219, https://doi.org/10.1126/science.1115311. [6] Y.-L. Li, I.A. Kinloch, A.H. Windle, Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis, Science 304 (2004) 276–278, https://doi.org/10.1126/science.1094982. [7] D. Lashmore, J.J. Brown, J.K. Chaffee, B. Rescnicoff, P.L. Antoinette, Systems and Methods for Formulation and Harvesting of Nanofibrous Materials, in: US Patent Number 20160250823A1, 2016. [8] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Catalytic growth of single-walled manotubes by laser vaporization, Chem. Phys. Lett. 243 (1995) 49–54, https://doi.org/10.1016/0009-2614(95)00825-O. [9] W.A. DeHeer, W.S. Bacsa, A. Ch^atelain, T. Gerfin, R. Humphrey-Baker, L. Forro, D. Ugarte, Aligned carbon nanotube films: production and optical and electronic properties, Science 268 (1995) 845–847, https:// doi.org/10.1126/science.268.5212.845. [10] H.E. Unalan, G. Fanchini, A. Kanwal, A. Du Pasquier, M. Chhowalla, Design criteria for transparent SingleWall carbon nanotube thin-film transistors, Nano Lett. 6 (2006) 677–682, https://doi.org/10.1021/nl052406l. [11] F. Hennrich, S. Lebedkin, S. Malik, J. Tracy, M. Barczewski, H. Rosner, M. Kappes, Preparation, characterization and applications of free-standing single walled carbon nanotube thin films, Phys. Chem. Chem. Phys. 4 (2002) 2273–2277, https://doi.org/10.1039/B201570F. [12] Y.Y. Huang, E.M. Terentjev, Dispersion and rheology of carbon nanotubes in polymers, Int. J. Mater. Form. 1 (2008) 63–74, https://doi.org/10.1007/s12289-008-0376-6. [13] V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis, C. Galiotis, Chemical oxidation of multiwalled carbon nanotubes, Carbon N. Y. 46 (2008) 833–840, https://doi.org/10.1016/j. carbon.2008.02.012. [14] H. Wang, Dispersing carbon nanotubes using surfactants, Curr. Opin. Colloid Interface Sci. 14 (2009) 364–371, https://doi.org/10.1016/j.cocis.2009.06.004. [15] S. Manivannan, I.O. Jeong, J.H. Ryu, C.S. Lee, K.S. Kim, J. Jang, K.C. Park, Dispersion of single-walled carbon nanotubes in aqueous and organic solvents through a polymer wrapping functionalization, J. Mater. Sci. Mater. Electron. 20 (2009) 223–229, https://doi.org/10.1007/s10854-008-9706-1. [16] K. Sears, L. Dum
ee, J. Sch€utz, M. She, C. Huynh, S. Hawkins, M. Duke, S. Gray, Recent developments in carbon nanotube membranes for water purification and gas separation, Materials 3 (2010), https://doi.org/ 10.3390/ma3010127. [17] J. Zhang, D. Jiang, H.-X. Peng, F. Qin, Enhanced mechanical and electrical properties of carbon nanotube buckypaper by in situ cross-linking, Carbon N. Y. 63 (2013) 125–132, https://doi.org/10.1016/j. carbon.2013.06.047. [18] M.B. Jakubinek, B. Ashrafi, J. Guan, M.B. Johnson, M.A. White, B. Simard, 3D chemically cross-linked single-walled carbon nanotube buckypapers, RSC Adv. 4 (2014) 57564–57573, https://doi.org/10.1039/ C4RA12026D. [19] A.M. Dı´ez-Pascual, D. Gasco´n, Carbon nanotube Buckypaper reinforced acrylonitrile–butadiene–styrene composites for electronic applications, ACS Appl. Mater. Interfaces 5 (2013) 12107–12119, https://doi. org/10.1021/am4039739. [20] Z. Wang, Z. Liang, B. Wang, C. Zhang, L. Kramer, Processing and property investigation of single-walled carbon nanotube (SWNT) buckypaper/epoxy resin matrix nanocomposites, Compos. Part A Appl. Sci. Manuf. 35 (2004) 1225–1232, https://doi.org/10.1016/j.compositesa.2003.09.029. [21] J. Han, H. Zhang, M. Chen, D. Wang, Q. Liu, Q. Wu, Z. Zhang, The combination of carbon nanotube buckypaper and insulating adhesive for lightning strike protection of the carbon fiber/epoxy laminates, Carbon N. Y. 94 (2015) 101–113, https://doi.org/10.1016/j.carbon.2015.06.026.

REFERENCES

421

[22] J.T. Di, D.M. Hu, H.Y. Chen, Z.Z. Yong, M.H. Chen, Z.H. Feng, Y.T. Zhu, Q.W. Li, Ultrastrong, foldable, and highly conductive carbon nanotube film, ACS Nano 6 (2012) 5457–5464, https://doi.org/10.1021/ Nn301321j. [23] Q. Liu, M. Li, Y. Gu, Y. Zhang, S. Wang, Q. Li, Z. Zhang, Highly aligned dense carbon nanotube sheets induced by multiple stretching and pressing, Nanoscale 6 (2014) 4338–4344, https://doi.org/10.1039/ c3nr06704a. [24] I.W. Chen, R. Liang, H. Zhao, B. Wang, C. Zhang, Highly conductive carbon nanotube buckypapers with improved doping stability via conjugational cross-linking, Nanotechnology 22 (2011) 485708, https://doi. org/10.1088/0957-4484/22/48/485708. [25] W. Liu, X. Zhang, G. Xu, P.D. Bradford, X. Wang, H. Zhao, Y. Zhang, Q. Jia, F. G. Yuan, Q. Li, Y. Qiu, Y. Zhu, Producing superior composites by winding carbon nanotubes onto a mandrel under a poly(vinyl alcohol) spray, Carbon N. Y. 49 (2011) 4786–4791, https://doi.org/10.1016/j. carbon.2011.06.089. [26] X. Wang, P.D. Bradford, W. Liu, H. Zhao, Y. Inoue, J.P. Maria, Q. Li, F.G. Yuan, Y. Zhu, Mechanical and electrical property improvement in CNT/nylon composites through drawing and stretching, Compos. Sci. Technol. 71 (2011) 1677–1683, https://doi.org/10.1016/j.compscitech.2011.07.023. [27] Q. Jiang, X. Wang, Y. Zhu, D. Hui, Y. Qiu, Mechanical, electrical and thermal properties of aligned carbon nanotube/polyimide composites, Compos. Part B Eng. 56 (2014) 408–412, https://doi.org/10.1016/j. compositesb.2013.08.064. [28] Y.-N. Liu, M. Li, Y. Gu, Y. Zhang, Q. Li, Z. Zhang, Ultrastrong carbon nanotube/bismaleimide composite film with super-aligned and tightly packing structure, Compos. Sci. Technol. 117 (2015) 176–182, https:// doi.org/10.1016/j.compscitech.2015.06.014. [29] Y.N. Liu, M. Li, Y. Gu, K. Wang, D. Hu, Q. Li, Z. Zhang, A modified spray-winding approach to enhance the tensile performance of array-based carbon nanotube composite films, Carbon N. Y. 65 (2013) 187–195, https://doi.org/10.1016/j.carbon.2013.08.013. [30] P. Liu, L. Liu, Y. Wei, K. Liu, Z. Chen, K. Jiang, Q. Li, S. Fan, Fast high-temperature response of carbon nanotube film and its application as an incandescent display, Adv. Mater. 21 (2009) 3563–3566, https://doi. org/10.1002/adma.200900473. [31] K. Fu, O. Yildiz, H. Bhanushali, Y. Wang, K. Stano, L. Xue, X. Zhang, P.D. Bradford, Aligned carbon nanotube-silicon sheets: A novel nano-architecture for flexible lithium ion battery electrodes, Adv. Mater. 25 (2013) 5109–5114, https://doi.org/10.1002/adma.201301920. [32] K. Jiang, C. Feng, Z. Chen, L. Liu, S. Fan, Q. Li, Q. Li, Flexible thermoacoustic device, US20100086150A1; 2010, (2010). [33] W. Fu, L. Liu, K. Jiang, Q. Li, S. Fan, Super-aligned carbon nanotube films as aligning layers and transparent electrodes for liquid crystal displays, Carbon N. Y. 48 (2010) 1876–1879, https://doi.org/10.1016/j. carbon.2010.01.026. [34] J. Chaffee, D. Lashmore, D. Lewis, J. Mann, M. Schauer, B. White, Direct synthesis of CNT yarns and sheets, in: Technical Proceedings of the 2008 NSTI Nanotechnology Conference and Trade Show, NSTINanotech, Nanotechnology, vol. 3, 2008, pp. 118–121. http://www.nsti.org/publications/Nanotech/2008/ pdf/319.pdf. [35] Nanocomp Technologies Inc. Miralon Sheets/Tape, (n.d.). [36] Y. Lin, J.W. Kim, J.W. Connell, M. Lebron-Colon, E.J. Siochi, Purification of carbon nanotube sheets, Adv. Eng. Mater. 17 (2015) 674–688, https://doi.org/10.1002/adem.201400306. [37] M.V. Naseh, A.A. Khodadadi, Y. Mortazavi, F. Pourfayaz, O. Alizadeh, M. Maghrebi, Fast and clean functionalization of carbon nanotubes by dielectric barrier discharge plasma in air compared to acid treatment, Carbon N. Y. 48 (2010) 1369–1379, https://doi.org/10.1016/j.carbon.2009.12.027. [38] X. Zhang, Q. Li, Y. Tu, Y. Li, J.Y. Coulter, L. Zheng, Y. Zhao, Q. Jia, D.E. Peterson, Y. Zhu, Strong carbonnanotube fibers spun from long carbon-nanotube arrays, Small 3 (2007) 244–248, https://doi.org/10.1002/ smll.200600368.

422

CHAPTER 16 CNT SHEET COMPOSITES

[39] M. Huhtala, A.V. Krasheninnikov, J. Aittoniemi, S.J. Stuart, K. Nordlund, K. Kaski, Improved mechanical load transfer between shells of multiwalled carbon nanotubes, Phys. Rev. B 70 (2004) 45404. https://doi. org/10.1103/PhysRevB.70.045404. [40] C.F. Cornwell, C.R. Welch, Very-high-strength (60-GPa) carbon nanotube fiber design based on molecular dynamics simulations, J. Chem. Phys. 134 (2011), https://doi.org/10.1063/1.3594197. [41] M. Miao, S.C. Hawkins, J.Y. Cai, T.R. Gengenbach, R. Knott, C.P. Huynh, Effect of gamma-irradiation on the mechanical properties of carbon nanotube yarns, Carbon N. Y. 49 (2011) 4940–4947, https://doi.org/ 10.1016/j.carbon.2011.07.026. [42] B. Peng, M. Locascio, P. Zapol, S. Li, S.L. Mielke, G.C. Schatz, H.D. Espinosa, Measurements of nearultimate strength for multiwalled carbon nanotubes and irradiation-induced crosslinking improvements, Nat Nano. 3 (2008) 626–631, https://doi.org/10.1038/nnano.2008.211. [43] H. Kim, J. Lee, B. Park, J.-H. Sa, A. Jung, T. Kim, J. Park, W. Hwang, K.-H. Lee, Improving the tensile strength of carbon nanotube yarn via one-step double [2+ 1] cycloadditions, Korean J. Chem. Eng. 33 (2016) 299–304, https://doi.org/10.1007/s11814-015-0140-9. [44] K. Balasubramanian, M. Burghard, Chemically functionalized carbon nanotubes, Small 1 (2005) 180–192, https://doi.org/10.1002/smll.200400118. [45] P. Abiman, G.G. Wildgoose, R.G. Compton, A mechanistic investigation into the covalent chemical derivatisation of graphite and glassy carbon surfaces using aryldiazonium salts, J. Phys. Org. Chem. 21 (2008) 433–439, https://doi.org/10.1002/poc.1331. [46] Q. Cheng, B. Wang, C. Zhang, Z. Liang, Functionalized carbon-nanotube sheet/bismaleimide nanocomposites: mechanical and electrical performance beyond carbon-fiber composites, Small 6 (2010) 763–767, https://doi.org/10.1002/smll.200901957. [47] D. Ogrin, J. Chattopadhyay, A.K. Sadana, W.E. Billups, A.R. Barron, Epoxidation and deoxygenation of single-walled carbon nanotubes: Quantification of epoxide defects, J. Am. Chem. Soc. 128 (2006) 11322–11323, https://doi.org/10.1021/ja061680u. [48] F. Meng, J. Zhao, Y. Ye, X. Zhang, Q. Li, Carbon nanotube fibers for electrochemical applications: effect of enhanced interfaces by an acid treatment, Nanoscale 4 (2012) 7464–7468, https://doi.org/10.1039/ C2NR32332J. [49] K. Wang, M. Li, Y.-N.N. Liu, Y. Gu, Q. Li, Z. Zhang, Effect of acidification conditions on the properties of carbon nanotube fibers, Appl. Surf. Sci. 292 (2014) 469–474, https://doi.org/10.1016/j.apsusc.2013.11.162. [50] Y. Li, H. Li, A. Petz, S. Kunsa´gi-Ma´t
e, Reducing structural defects and improving homogeneity of nitric acid treated multi-walled carbon nanotubes, Carbon N. Y. 93 (2015) 515–522, https://doi.org/10.1016/j. carbon.2015.05.068. [51] Y.-O. Im, S.-H. Lee, T. Kim, J. Park, J. Lee, K.-H. Lee, Utilization of carboxylic functional groups generated during purification of carbon nanotube fiber for its strength improvement, Appl. Surf. Sci. 392 (2017) 342–349, https://doi.org/10.1016/j.apsusc.2016.09.060. [52] T.Q. Tran, Z. Fan, A. Mikhalchan, P. Liu, H.M. Duong, Post-treatments for multifunctional property enhancement of carbon nanotube fibers from the floating catalyst method, ACS Appl. Mater. Interfaces (2016) acsami.5b09912. https://doi.org/10.1021/acsami.5b09912. [53] J.Y. Cai, J. Min, J. McDonnell, J.S. Church, C.D. Easton, W. Humphries, S. Lucas, A.L. Woodhead, An improved method for functionalisation of carbon nanotube spun yarns with aryldiazonium compounds, Carbon N. Y. 50 (2012) 4655–4662, https://doi.org/10.1016/j.carbon.2012.05.055. [54] J. Min, J.Y. Cai, M. Sridhar, C.D. Easton, T.R. Gengenbach, J. McDonnell, W. Humphries, S. Lucas, High performance carbon nanotube spun yarns from a crosslinked network, Carbon N. Y. 52 (2013) 520–527, https://doi.org/10.1016/j.carbon.2012.10.004. [55] C. Perruchot, M.-L. Abel, J.F. Watts, C. Lowe, J.T. Maxted, R.G. White, High-resolution XPS study of crosslinking and segregation phenomena in hexamethoxymethyl melamine–polyester resins, Surf. Interface Anal. 34 (2002) 570–574, https://doi.org/10.1002/sia.1362.

REFERENCES

423

[56] M.ł. Mami
nski, M. Czarzasta, P. Parzuchowski, Wood adhesives derived from hyperbranched polyglycerol cross-linked with hexamethoxymethyl melamines, Int. J. Adhes. Adhes. 31 (2011) 704–707, https://doi.org/ 10.1016/j.ijadhadh.2011.06.012. [57] O.-K. Park, W. Lee, J.Y. Hwang, N.-H. You, Y. Jeong, S.-M. Kim, B.-C. Ku, Mechanical and electrical properties of thermochemically cross-linked polymer carbon nanotube fibers, Compos. Part A Appl. Sci. Manuf. 91 (2016) 222–228, https://doi.org/10.1016/j.compositesa.2016.10.016. [58] Y. Inoue, K. Nakamura, Y. Miyasaka, T. Nakano, G. Kletetschka, Cross-linking multiwall carbon nanotubes using PFPA to build robust, flexible and highly aligned large-scale sheets and yarns, Nanotechnology 27 (2016) 115701, https://doi.org/10.1088/0957-4484/27/11/115701. [59] T. Filleter, H.D. Espinosa, Multi-scale mechanical improvement produced in carbon nanotube fibers by irradiation cross-linking, Carbon N. Y. 56 (2013) 1–11, https://doi.org/10.1016/j.carbon.2012.12.016. [60] F. Banhart, The formation of a connection between carbon nanotubes in an Electron beam, Nano Lett. 1 (2001) 329–332, https://doi.org/10.1021/nl015541g. [61] S.G. Miller, T.S. Williams, J.S. Baker, F. Sola´, M. Lebron-Colon, L.S. McCorkle, N.G. Wilmoth, J. Gaier, M. Chen, M. a Meador, Increased tensile strength of carbon nanotube yarns and sheets through chemical modification and Electron beam irradiation, ACS Appl. Mater. Interfaces 6 (2014) 6120–6126, https://doi.org/10.1021/am4058277. [62] N. Hiremath, X. Lu, M.C. Evora, A. Naskar, J. Mays, G. Bhat, Effect of solvent/polymer infiltration and irradiation on microstructure and tensile properties of carbon nanotube yarns, J. Mater. Sci. 51 (2016) 10215–10228, https://doi.org/10.1007/s10853-016-0249-1. [63] J.Y. Cai, J. Min, M. Miao, J.S. Church, J. McDonnell, R. Knott, S. Hawkins, C. Huynh, Enhanced mechanical performance of CNT/polymer composite yarns by γ-irradiation, Fibers Polym. 15 (2014) 322–325, https://doi.org/10.1007/s12221-014-0322-9. [64] B.N. Khare, M. Meyyappan, A.M. Cassell, C.V. Nguyen, J. Han, Functionalization of carbon nanotubes using atomic hydrogen from a glow discharge, Nano Lett. 2 (2002) 73–77, https://doi.org/ 10.1021/nl015646j. [65] B.N. Khare, P. Wilhite, R.C. Quinn, B. Chen, R.H. Schingler, B. Tran, H. Imanaka, C.R. So, C. W. Bauschlicher, M. Meyyappan, Functionalization of carbon nanotubes by Ammonia glow-discharge: Experiments and modeling, J. Phys. Chem. B 108 (2004) 8166–8172, https://doi.org/10.1021/jp049359q. [66] B.N. Khare, P. Wilhite, M. Meyyappan, The fluorination of single wall carbon nanotubes using microwave plasma, Nanotechnology 15 (2004) 1650, https://doi.org/10.1088/0957-4484/15/11/048. [67] B. Khare, P. Wilhite, B. Tran, E. Teixeira, K. Fresquez, D.N. Mvondo, C. Bauschlicher, M. Meyyappan, Functionalization of carbon nanotubes via nitrogen glow discharge, J. Phys. Chem. B 109 (2005) 23466–23472, https://doi.org/10.1021/jp0537254. [68] T.I.T. Okpalugo, P. Papakonstantinou, H. Murphy, J. Mclaughlin, N.M.D. Brown, Oxidative functionalization of carbon nanotubes in atmospheric pressure filamentary dielectric barrier discharge (APDBD), Carbon N. Y. 43 (2005) 2951–2959, https://doi.org/10.1016/j.carbon.2005.06.033. [69] J. Lee, D. Lim, W. Choi, S. Dimitrijev, Influence of various plasma treatment on the properties of carbon nanotubes for composite applications, J. Nanosci. Nanotechnol. 12 (2012) 1507–1512, https://doi.org/ 10.1166/jnn.2012.4598. [70] J.Y. Yook, J. Jun, S. Kwak, Amino functionalization of carbon nanotube surfaces with NH3 plasma treatment, Appl. Surf. Sci. 256 (2010) 6941–6944, https://doi.org/10.1016/j.apsusc.2010.04.075. [71] V.K. Abdelkader, S. Scelfo, C. Garcı´a-Galları´n, M.L. Godino-Salido, M. Domingo-Garcı´a, F.J. Lo´pezGarzo´n, M. P
erez-Mendoza, Carbon tetrachloride cold plasma for extensive chlorination of carbon nanotubes, J. Phys. Chem. C 117 (2013) 16677–16685, https://doi.org/10.1021/jp404390h. [72] G. Kalita, S. Adhikari, H.R. Aryal, D.C. Ghimre, R. Afre, T. Soga, M. Sharon, M. Umeno, Fluorination of multi-walled carbon nanotubes (MWNTs) via surface wave microwave (SW-MW) plasma treatment, Phys. E Low-Dim. Syst. Nanostruct. 41 (2008) 299–303, https://doi.org/10.1016/j.physe.2008.07.015.

424

CHAPTER 16 CNT SHEET COMPOSITES

[73] D. Kolacyak, J. Ihde, C. Merten, A. Hartwig, U. Lommatzsch, Fast functionalization of multi-walled carbon nanotubes by an atmospheric pressure plasma jet, J. Colloid Interface Sci. 359 (2011) 311–317, https://doi. org/10.1016/j.jcis.2011.03.069. [74] C.H. Tseng, C.C. Wang, C.Y. Chen, Functionalizing carbon nanotubes by plasma modification for the preparation of covalent-integrated epoxy composites, Chem. Mater. 19 (2007) 308–315, https://doi.org/ 10.1021/cm062277p. [75] W.J. Zhang, X.Y. Li, X. Wang, D.G. Yu, W.H. Qian, Y.T. Ye, Z.Y. Wang, Improvement effect of functionalized Cnts on mechanical and thermal properties of epoxy matrix, Appl. Mech. Mater. 217–219 (2012) 272–275, https://doi.org/10.4028/www.scientific.net/AMM.217-219.272. [76] H. Wei, Y. Wei, Y. Wu, L. Liu, S. Fan, K. Jiang, High-strength composite yarns derived from oxygen plasma modified super-aligned carbon nanotube arrays, Nano Res. 6 (2013) 208–215, https://doi.org/ 10.1007/s12274-013-0297-7. [77] O.-K. Park, W. Young Kim, S. Min Kim, N.-H. You, Y. Jeong, H. Su Lee, B.-C. Ku, Effect of oxygen plasma treatment on the mechanical properties of carbon nanotube fibers, Mater. Lett. 156 (2015) 17–20, https://doi.org/10.1016/j.matlet.2015.04.141. [78] H. Yu, D. Cheng, T.S. Williams, J. Severino, I.M. De Rosa, L. Carlson, R.F. Hicks, Rapid oxidative activation of carbon nanotube yarn and sheet by a radio frequency, atmospheric pressure, helium and oxygen plasma, Carbon N. Y. 57 (2013) 11–21, https://doi.org/10.1016/j.carbon.2013.01.010. [79] C. Feng, K. Liu, J.S. Wu, L. Liu, J.S. Cheng, Y. Zhang, Y. Sun, Q. Li, S. Fan, K. Jiang, Flexible, stretchable, transparent conducting films made from superaligned carbon nanotubes, Adv. Funct. Mater. 20 (2010) 885–891, https://doi.org/10.1002/adfm.200901960. [80] R. Malik, C. McConnell, N.T. Alvarez, M. Haase, S. Gbordzoe, V. Shanov, Rapid, in situ plasma functionalization of carbon nanotubes for improved CNT/epoxy composites, RSC Adv. 6 (2016) 108840–108850, https://doi.org/10.1039/C6RA23103A. [81] Q. Jiang, L. Wu, Property enhancement of aligned carbon nanotube/polyimide composite by strategic prestraining, J. Reinf. Plast. Compos. 35 (2016) 287–294, https://doi.org/10.1177/0731684415614086. [82] P. P€otschke, N.P. Zschoerper, B.P. Moller, U. Vohrer, Plasma functionalization of multiwalled carbon nanotube bucky papers and the effect on properties of melt-mixed composites with polycarbonate, Macromol. Rapid Commun. 30 (2009) 1828–1833, https://doi.org/10.1002/marc.200900286. [83] Q. Jiang, Y. Li, J. Xie, J. Sun, D. Hui, Y. Qiu, Plasma functionalization of bucky paper and its composite with phenylethynyl-terminated polyimide, Compos. Part B Eng. 45 (2013) 1275–1281, https://doi.org/ 10.1016/j.compositesb.2012.06.017. [84] L. Zhang, X. Wang, W. Xu, Y. Zhang, Q. Li, P.D. Bradford, Y. Zhu, Strong and conductive dry carbon nanotube films by microcombing, Small (2015) 3830–3836, https://doi.org/10.1002/smll.201500111. [85] W. Liu, H. Zhao, Y. Inoue, X. Wang, P.D. Bradford, H. Kim, Y. Qiu, Y. Zhu, Poly(vinyl alcohol) reinforced with large-diameter carbon nanotubes via spray winding, Compos. Part A Appl. Sci. Manuf. 43 (2012) 587–592, https://doi.org/10.1016/j.compositesa.2011.12.029. [86] T.H. Nam, K. Goto, Y. Yamaguchi, E.V.A. Premalal, Y. Shimamura, Y. Inoue, S. Arikawa, S. Yoneyama, S. Ogihara, Improving mechanical properties of high volume fraction aligned multi-walled carbon nanotube/epoxy composites by stretching and pressing, Compos. Part B Eng. 85 (2016) 15–23, https://doi. org/10.1016/j.compositesb.2015.09.012. [87] P. Liu, Y.F. Tan, D.C.M. Hu, M. Hu, D. Jewell, H.M. Duong, Multi-property enhancement of aligned carbon nanotube thin films from floating catalyst method, Mater. Des. 108 (2016) 754–760, https://doi.org/ 10.1016/j.matdes.2016.07.045. [88] R. Malik, Y. Song, N. Alvarez, B. Ruff, M. Haase, B. Suberu, A. Gilpin, M. Schulz, V. Shanov, Atmospheric pressure plasma functionalization of dry-spun multi-walled carbon nanotubes sheets and its application in CNT-polyvinyl alcohol (PVA) composites, Symp. UU—Plasma Low-Energy Ion-Beam-Assisted Process. Synth. Energy-Related Mater, 2013: p. mrss13-1574-uu03-04 (8 pages). https://doi.org/10.1557/opl.2013.701.

REFERENCES

425

[89] L. Shechter, J. Wynstra, Glycidyl ether reactions with alcohols, phenols, carboxylic acids, and acid anhydrides, Ind. Eng. Chem. 48 (1956) 86–93, https://doi.org/10.1021/ie50553a028. [90] N.G. Sahoo, S. Rana, J.W. Cho, L. Li, S.H. Chan, Polymer nanocomposites based on functionalized carbon nanotubes, Prog. Polym. Sci. 35 (2010) 837–867, https://doi.org/10.1016/j.progpolymsci.2010.03.002. [91] J.W. Kim, G. Sauti, E.J. Siochi, J.G. Smith, R.A. Wincheski, R.J. Cano, J.W. Connell, K.E. Wise, Toward high performance thermoset/carbon nanotube sheet nanocomposites via resistive heating assisted infiltration and cure, ACS Appl. Mater. Interfaces 6 (2014) 18832–18843, https://doi.org/10.1021/am5046718. [92] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520–2531, https://doi.org/10.1039/B813846J. [93] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, Nanostructured carbon-metal oxide composite electrodes for supercapacitors: a review, Nanoscale 5 (2013) 72–88, https://doi.org/10.1039/C2NR32040A. [94] K. Wang, H. Wu, Y. Meng, Z. Wei, Conducting polymer nanowire arrays for high performance supercapacitors, Small 10 (2014) 14–31, https://doi.org/10.1002/smll.201301991. [95] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854, https://doi. org/10.1038/nmat2297. [96] M.N. Hyder, S.W. Lee, F.C ¸ . Cebeci, D.J. Schmidt, Y. Shao-Horn, P.T. Hammond, N. Hyder, S.W. Lee, C. Fevzi, P.T. Hammond, Layer-by-layer assembled polyaniline nanofiber/multiwall carbon nanotube thin film electrodes for high-power and high-energy storage applications, ACS Nano 5 (2011) 8552–8561, https://doi.org/10.1021/nn2029617. [97] J. Ge, G. Cheng, L. Chen, Transparent and flexible electrodes and supercapacitors using polyaniline/singlewalled carbon nanotube composite thin films, Nanoscale 3 (2011) 3084–3088, https://doi.org/10.1039/ C1NR10424A. [98] C. Meng, C. Liu, S. Fan, Flexible carbon nanotube/polyaniline paper-like films and their enhanced electrochemical properties, Electrochem. Commun. 11 (2009) 186–189, https://doi.org/10.1016/j.elecom.2008.11.005. [99] J. Yan, T. Wei, Z. Fan, W. Qian, M. Zhang, X. Shen, F. Wei, Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors, J. Power Sources 195 (2010) 3041–3045, https://doi.org/10.1016/j.jpowsour.2009.11.028. [100] C. Huang, P.S. Grant, One-step spray processing of high power all-solid-state supercapacitors, Sci. Rep. 3 (2013)2393https://doi.org/10.1038/srep02393. [101] J.-H. Kim, K.H. Lee, L.J. Overzet, G.S. Lee, Synthesis and electrochemical properties of spin-capable carbon nanotube sheet/MnOx composites for high-performance energy storage devices, Nano Lett. 11 (2011) 2611–2617, https://doi.org/10.1021/nl200513a. [102] Z. Niu, P. Luan, Q. Shao, H. Dong, J. Li, J. Chen, D. Zhao, L. Cai, W. Zhou, X. Chen, S. Xie, A “skeleton/ skin” strategy for preparing ultrathin free-standing single-walled carbon nanotube/polyaniline films for high performance supercapacitor electrodes, Energy Environ. Sci. 5 (2012) 8726–8733, https://doi.org/10.1039/ C2EE22042C. [103] H. Lin, L. Li, J. Ren, Z. Cai, L. Qiu, Z. Yang, H. Peng, Conducting polymer composite film incorporated with aligned carbon nanotubes for transparent, flexible and efficient supercapacitor, Sci. Rep. 3 (2013) 1353https://doi.org/10.1038/srep01353. [104] G.A. Malek, E. Brown, S.A. Klankowski, J. Liu, A.J. Elliot, R. Lu, J. Li, J. Wu, Atomic layer deposition of Al-doped ZnO/Al 2O3 double layers on vertically aligned carbon nanofiber arrays, ACS Appl. Mater. Interfaces 6 (2014) 6865–6871, https://doi.org/10.1021/am5006805. [105] S.A. Klankowski, G.P. Pandey, G. Malek, C.R. Thomas, S.L. Bernasek, J. Wu, J. Li, Higher-power supercapacitor electrodes based on mesoporous manganese oxide coating on vertically aligned carbon nanofibers, Nanoscale 7 (2015) 8485–8494, https://doi.org/10.1039/c5nr01198a. [106] G.P. Pandey, S.A. Klankowski, Y. Li, X.S. Sun, J. Wu, R.A. Rojeski, J. Li, Effective infiltration of gel polymer electrolyte into silicon-coated vertically aligned carbon nanofibers as anodes for solid-state Lithium-ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 20909–20918, https://doi.org/10.1021/acsami.5b06444.

426

CHAPTER 16 CNT SHEET COMPOSITES

[107] S.A. Klankowski, R.A. Rojeski, B.A. Cruden, J. Liu, J. Wu, J. Li, A high-performance lithium-ion battery anode based on the core–shell heterostructure of silicon-coated vertically aligned carbon nanofibers, J. Mater. Chem. A 1 (2013) 1055, https://doi.org/10.1039/c2ta00057a. [108] X.-J. Li, Y. Zhao, W.-G. Chu, Y. Wang, Z.-J. Li, P. Jiang, X.-C. Zhao, M. Liang, Y. Liu, Vertically aligned carbon nanotube@MnO2 nanosheet arrays grown on carbon cloth for high performance flexible electrodes of supercapacitors, RSC Adv. 5 (2015) 77437–77442, https://doi.org/10.1039/c5ra15146e. [109] J. Cherusseri, K.K. Kar, Hierarchical carbon nanopetal/polypyrrole nanocomposite electrodes with brushlike architecture for supercapacitors, Phys. Chem. Chem. Phys. 18 (2016) 8587–8597, https://doi.org/ 10.1039/C6CP00150E. [110] Z.H. Huang, Y. Song, X.X. Xu, X.X. Liu, Ordered Polypyrrole nanowire arrays grown on a carbon cloth substrate for a high-performance Pseudocapacitor electrode, ACS Appl. Mater. Interfaces 7 (2015) 25506–25513, https://doi.org/10.1021/acsami.5b08830. [111] T. Kaewsongpol, M. Sawangphruk, P. Chiochan, M. Suksomboon, P. Suktha, P. Srimuk, A. Krittayavathananon, S. Luanwuthi, P. Iamprasertkun, J. Wutthiprom, N. Phattharasupakun, P. Sirisinudomkit, T. Pettong, J. Limtrakul, High-performance supercapacitor of electrodeposited porous 3D polyaniline nanorods on functionalized carbon fiber paper: Effects of hydrophobic and hydrophilic surfaces of conductive carbon paper substrates, Mater. Today Commun. 4 (2015) 176–185, https://doi.org/ 10.1016/j.mtcomm.2015.08.005. [112] J. Cherusseri, K.K. Kar, Ultra-flexible fibrous supercapacitors with carbon nanotube/polypyrrole brush-like electrodes, J. Mater. Chem. A 4 (2016) 9910–9922, https://doi.org/10.1039/C6TA02690G. [113] W. Weng, H. Lin, X. Chen, J. Ren, Z. Zhang, L. Qiu, G. Guan, H. Peng, Flexible and stable lithium ion batteries based on three-dimensional aligned carbon nanotube/silicon hybrid electrodes, J. Mater. Chem. A 2 (2014) 9306, https://doi.org/10.1039/c4ta00711e. [114] Y. Zhu, L. Li, C. Zhang, G. Casillas, Z. Sun, Z. Yan, G. Ruan, Z. Peng, A.-R.O. Raji, C. Kittrell, R. H. Hauge, J.M. Tour, A seamless three-dimensional carbon nanotube graphene hybrid material, Nat. Commun. 3 (2012)1225https://doi.org/10.1038/ncomms2234. [115] R. Rao, G. Chen, L.M.R. Arava, K. Kalaga, M. Ishigami, T.F. Heinz, P.M. Ajayan, A. R. Harutyunyan, Graphene as an atomically thin interface for growth of vertically aligned carbon nanotubes, Sci. Rep. 3 (2013)1891https://doi.org/10.1038/srep01891. [116] R. Malik, L. Zhang, C. McConnell, M. Schott, Y.-Y. Hsieh, R. Noga, N.T. Alvarez, V. Shanov, Threedimensional, free-standing polyaniline/carbon nanotube composite-based electrode for high-performance supercapacitors, Carbon N. Y. 116 (2017) 579–590, https://doi.org/10.1016/j.carbon.2017.02.036. [117] V. Shanov, W. Cho, R. Malik, N. Alvarez, M. Haase, B. Ruff, N. Kienzle, T. Ochmann, D. Mast, M. Schulz, CVD growth, characterization and applications of carbon nanostructured materials, Surf. Coatings Technol. 230 (2013) 77–86, https://doi.org/10.1016/j.surfcoat.2013.06.017. [118] Y. Koo, R. Malik, N. Alvarez, L. White, V.N. Shanov, M. Schulz, B. Collins, J. Sankar, Y. Yun, Aligned carbon nanotube/copper sheets: A new electrocatalyst for CO2 reduction to hydrocarbons, RSC Adv. 4 (2014) 16362, https://doi.org/10.1039/C4RA00618F. [119] M. Meyyappan, A review of plasma enhanced chemical vapour deposition of carbon nanotubes, J. Phys. D. Appl. Phys. 42 (2009)213001https://doi.org/10.1088/0022-3727/42/21/213001. [120] M. Meyyappan, L. Delzeit, A. Cassell, D. Hash, Carbon nanotube growth by PECVD: A review, Plasma Sources Sci. Technol. 12 (2003) 205–216, https://doi.org/10.1088/0963-0252/12/2/312. [121] D.H. Lee, W.J. Lee, S.O. Kim, Highly efficient vertical growth of wall-number-selected, N-doped carbon nanotube arrays, Nano Lett. 9 (2009) 1427–1432, https://doi.org/10.1021/nl803262s. [122] X.Y. Chen, C. Chen, Z.J. Zhang, D.H. Xie, X. Deng, J.W. Liu, Nitrogen-doped porous carbon for supercapacitor with long-term electrochemical stability, J. Power Sources 230 (2013) 50–58, https://doi.org/ 10.1016/j.jpowsour.2012.12.054.

REFERENCES

427

[123] D. Hulicova-Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Combined effect of nitrogen- and oxygencontaining functional groups of microporous activated carbon on its electrochemical performance in supercapacitors, Adv. Funct. Mater. 19 (2009) 438–447, https://doi.org/10.1002/adfm.200801236. [124] Z. Chen, D. Higgins, H. Tao, R.S. Hsu, Z. Chen, Highly active nitrogen-doped carbon nanotubes for oxygen reduction reaction in fuel cell applications, J. Phys. Chem. C 113 (2009) 21008–21013, https://doi.org/ 10.1021/jp908067v. [125] J. Hou, C. Cao, F. Idrees, X. Ma, Hierarchical porous nitrogen-doped carbon Nanosheets derived from silk for ultrahigh-capacity battery anodes, ACS Nano 9 (2015) 2556–2564, https://doi.org/10.1021/nn506394r. [126] S. He, H. Hou, W. Chen, 3D porous and ultralight carbon hybrid nanostructure fabricated from carbon foam covered by monolayer of nitrogen-doped carbon nanotubes for high performance supercapacitors, J. Power Sources 280 (2015) 678–686, https://doi.org/10.1016/j.jpowsour.2015.01.159. [127] J.W. Lim, A.U. Haq, S.O. Kim, Direct growth of polyaniline chains from nitrogen site of N-doped carbon nanotubes for high performance supercapacitor, Adv. Sci. Technol. 93 (2014) 164–167, https://doi.org/ 10.4028/www.scientific.net/AST.93.164. [128] A.U. Haq, J. Lim, J.M. Yun, W.J. Lee, T.H. Han, S.O. Kim, Direct growth of polyaniline chains from N-doped sites of carbon nanotubes, Small 9 (2013) 3829–3833, https://doi.org/10.1002/smll.201300625. [129] S. Hussain, R. Amade, E. Jover, E. Bertran, Nitrogen plasma functionalization of carbon nanotubes for supercapacitor applications, J. Mater. Sci. 48 (2013) 7620–7628, https://doi.org/10.1007/s10853-0137579-z. [130] J.-B. Kim, S.-J. Kong, S.-Y. Lee, J.-H. Kim, H.-R. Lee, C.-D. Kim, B.-K. Min, Characteristics of nitrogendoped carbon nanotubes synthesized by using PECVD and thermal CVD, J. Korean Phys. Soc. 60 (2012) 1124–1128, https://doi.org/10.3938/jkps.60.1124. [131] Y. Cao, T.E. Mallouk, Morphology of template-grown polyaniline nanowires and its effect on the electrochemical capacitance of nanowire arrays, Chem. Mater. 20 (2008) 5260–5265, https://doi.org/ 10.1021/cm801028a. [132] T. Lv, Y. Yao, N. Li, T. Chen, Highly stretchable supercapacitors based on aligned carbon nanotube/molybdenum disulfide composites, Angew. Chemie - Int. Ed. 55 (2016) 9191–9195, https://doi.org/10.1002/ anie.201603356. [133] J. Zhang, X.S. Zhao, On the configuration of supercapacitors for maximizing electrochemical performance, ChemSusChem 5 (2012) 818–841, https://doi.org/10.1002/cssc.201100571. [134] L. Zhang, D. DeArmond, N.T. Alvarez, D. Zhao, T. Wang, G. Hou, R. Malik, W.R. Heineman, V. N. Shanov, Beyond graphene foam, a new form of three-dimensional graphene for supercapacitor electrodes, J. Mater. Chem. A 4 (2016) 1876–1886, https://doi.org/10.1039/C5TA10031C. [135] F. Huang, D. Chen, Towards the upper bound of electrochemical performance of ACNT@polyaniline arrays as supercapacitors, Energy Environ. Sci. 5 (2012) 5833, https://doi.org/10.1039/c1ee01989a. [136] J. Zhang, J. Jiang, H. Li, X.S. Zhao, A high-performance asymmetric supercapacitor fabricated with graphene-based electrodes, Energy Environ. Sci. 4 (2011) 4009, https://doi.org/10.1039/c1ee01354h. [137] D.-W. Wang, F. Li, J. Zhao, W. Ren, Z.-G. Chen, J. Tan, Z.-S. Wu, I. Gentle, G.Q. Lu, H.M. Cheng, Fabrication of graphene/polyaniline composite paper via in situ anodic Electropolymerization for high-performance flexible electrode, ACS Nano 3 (2009) 1745–1752, https://doi.org/10.1021/ nn900297m. [138] X. Lu, H. Dou, S. Yang, L. Hao, L. Zhang, L. Shen, F. Zhang, X. Zhang, Fabrication and electrochemical capacitance of hierarchical graphene/polyaniline/carbon nanotube ternary composite film, Electrochim. Acta 56 (2011) 9224–9232, https://doi.org/10.1016/j.electacta.2011.07.142. [139] S.B. Kulkarni, U.M. Patil, I. Shackery, J.S. Sohn, S. Lee, B. Park, S. Jun, High-performance supercapacitor electrode based on a polyaniline nanofibers/3D graphene framework as an efficient charge transporter, J. Mater. Chem. A 2 (2014) 4989–4998, https://doi.org/10.1039/C3TA14959E.

428

CHAPTER 16 CNT SHEET COMPOSITES

[140] S.K. Simotwo, C. DelRe, V. Kalra, Supercapacitor electrodes based on high-purity electrospun polyaniline and polyaniline–carbon nanotube nanofibers, ACS Appl. Mater. Interfaces 8 (2016) 21261–21269, https:// doi.org/10.1021/acsami.6b03463. [141] M. Hassan, K.R. Reddy, E. Haque, S.N. Faisal, S. Ghasemi, A.I. Minett, V.G. Gomes, Hierarchical assembly of graphene/polyaniline nanostructures to synthesize free-standing supercapacitor electrode, Compos. Sci. Technol. 98 (2014) 1–8, https://doi.org/10.1016/j.compscitech.2014.04.007. [142] F. Guo, H. Mi, J. Zhou, Z. Zhao, J. Qiu, Hybrid pseudocapacitor materials from polyaniline@multi-walled carbon nanotube with ultrafine nanofiber-assembled network shell, Carbon N. Y. 95 (2015) 323–329, https://doi.org/10.1016/j.carbon.2015.08.052. [143] H. Zhang, G. Cao, Z. Wang, Y. Yang, Z. Shi, Z. Gu, Tube-covering-tube nanostructured polyaniline/carbon nanotube array composite electrode with high capacitance and superior rate performance as well as good cycling stability, Electrochem. Commun. 10 (2008) 1056–1059, https://doi.org/10.1016/j. elecom.2008.05.007. [144] H. Zhang, G. Cao, W. Wang, K. Yuan, B. Xu, W. Zhang, J. Cheng, Y. Yang, Influence of microstructure on the capacitive performance of polyaniline/carbon nanotube array composite electrodes, Electrochim. Acta 54 (2009) 1153–1159, https://doi.org/10.1016/j.electacta.2008.09.004. [145] Y. Liao, C. Zhang, X. Wang, X.-G. Li, S.J. Ippolito, K. Kalantar-zadeh, R.B. Kaner, Carrier mobility of single-walled carbon nanotube-reinforced polyaniline nanofibers, J. Phys. Chem. C 115 (2011) 16187–16192, https://doi.org/10.1021/jp2053585. [146] Y. Shi, L. Peng, Y. Ding, Y. Zhao, G. Yu, Nanostructured conductive polymers for advanced energy storage, Chem. Soc. Rev. 44 (2015) 6684–6696, https://doi.org/10.1039/C5CS00362H. [147] M. Yu, Y. Ma, J. Liu, S. Li, Polyaniline nanocone arrays synthesized on three-dimensional graphene network by electrodeposition for supercapacitor electrodes, Carbon N. Y. 87 (2015) 98–105, https://doi.org/ 10.1016/j.carbon.2015.02.017. [148] Y. Liu, Y. Ma, S. Guang, F. Ke, H. Xu, Polyaniline-graphene composites with a three-dimensional arraybased nanostructure for high-performance supercapacitors, Carbon N. Y. 83 (2015) 79–89, https://doi.org/ 10.1016/j.carbon.2014.11.026. [149] J. Huang, K. Wang, Z. Wei, Conducting polymer nanowire arrays with enhanced electrochemical performance, J. Mater. Chem. 20 (2010) 1117–1121, https://doi.org/10.1039/B919928D. [150] J. Benson, I. Kovalenko, S. Boukhalfa, D. Lashmore, M. Sanghadasa, G. Yushin, Multifunctional CNTpolymer composites for ultra-tough structural supercapacitors and desalination devices, Adv. Mater. 25 (2013) 6625–6632, https://doi.org/10.1002/adma.201301317. [151] E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. B
eguin, Supercapacitors based on conducting polymers/nanotubes composites, J. Power Sources 153 (2006) 413–418, https://doi.org/10.1016/j. jpowsour.2005.05.030. [152] C. Meng, C. Liu, L. Chen, C. Hu, S. Fan, Highly flexible and all-solid-state paperlike polymer supercapacitors, Nano Lett. 10 (2010) 4025–4031, https://doi.org/10.1021/nl1019672. [153] Y. Wang, S. Tang, S. Vongehr, J. Ali Syed, X. Wang, X. Meng, High-performance flexible solid-state carbon cloth supercapacitors based on highly Processible N-graphene doped Polyacrylic acid/polyaniline composites, Sci. Rep. 6 (2016)12883https://doi.org/10.1038/srep12883. [154] L.-J. Bian, F. Luan, S.-S. Liu, X.-X. Liu, Self-doped polyaniline on functionalized carbon cloth as electroactive materials for supercapacitor, Electrochim. Acta 64 (2012) 17–22, https://doi.org/10.1016/j. electacta.2011.12.012. [155] Q. Liu, M.H. Nayfeh, S.-T. Yau, Brushed-on flexible supercapacitor sheets using a nanocomposite of polyaniline and carbon nanotubes, J. Power Sources 195 (2010) 7480–7483, https://doi.org/10.1016/j. jpowsour.2010.06.002. [156] R. Wang, Q. Wu, X. Zhang, Z. Yang, L. Gao, J. Ni, O.K.C. Tsui, Flexible supercapacitors based on a polyaniline nanowire-infilled 10 nm-diameter carbon nanotube porous membrane by in situ electrochemical polymerization, J. Mater. Chem. A 4 (2016) 12602–12608, https://doi.org/10.1039/C6TA03957J.

FURTHER READING

429

[157] a. M. Khosrozadeh, Q. Xing, Wang, a high-capacitance solid-state supercapacitor based on free-standing film of polyaniline and carbon particles, Appl. Energy (2014)https://doi.org/10.1016/j. apenergy.2014.08.046. [158] K. Chi, Z. Zhang, J. Xi, Y. Huang, F. Xiao, S. Wang, Y. Liu, Freestanding graphene paper supported threedimensional porous graphene-polyaniline nanocomposite synthesized by inkjet printing and in flexible allsolid-state supercapacitor, ACS Appl. Mater. Interfaces 6 (2014) 16312–16319, https://doi.org/10.1021/ am504539k. [159] J. Ma, S. Tang, J.A. Syed, X. Meng, Asymmetric hybrid capacitors based on novel bearded carbon fiber cloth–pinhole polyaniline electrodes with excellent energy density, RSC Adv. 6 (2016) 82995–83002, https://doi.org/10.1039/C6RA16291F. [160] Y.Y. Horng, Y.C. Lu, Y.K. Hsu, C.C. Chen, L.C. Chen, K.H. Chen, Flexible supercapacitor based on polyaniline nanowires/carbon cloth with both high gravimetric and area-normalized capacitance, J. Power Sources 195 (2010) 4418–4422, https://doi.org/10.1016/j.jpowsour.2010.01.046. [161] Y. Xie, Y. Liu, Y. Zhao, Y.H. Tsang, S.P. Lau, H. Huang, Y. Chai, Stretchable all-solid-state supercapacitor with wavy shaped polyaniline/graphene electrode, J. Mater. Chem. A 2 (2014) 9142–9149, https://doi.org/ 10.1039/C4TA00734D. [162] Q. Liu, S. Jing, S. Wang, H. Zhuo, L. Zhong, X. Peng, R. Sun, Flexible nanocomposites with ultrahigh specific areal capacitance and tunable properties based on a cellulose derived nanofiber-carbon sheet framework coated with polyaniline, J. Mater. Chem. A 4 (2016) 13352–13362, https://doi.org/10.1039/ C6TA05131F. [163] R. Amade, E. Jover, B. Caglar, T. Mutlu, E. Bertran, Optimization of MnO2/vertically aligned carbon nanotube composite for supercapacitor application, J. Power Sources 196 (2011) 5779–5783, https://doi.org/ 10.1016/j.jpowsour.2011.02.029.

FURTHER READING [164] Large Sheets of Carbon Nanotubes Made by CVD, MRS Technol. Adv. 35 (2010) 179–181.