Aqueous Dispersion of Carbon Fibers and Expanded Graphite

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An Aqueous Dispersion of Carbon Fibers and Expanded Graphite Stabilized from the Addition of Cellulose Nanocrystals to Produce Highly Conductive Cellulose Composites Yuxin Liu, Bing Sun, Jianguo Li, Dong Cheng, Xingye An, Bo Yang, Zhibin He, Ryan Lutes, Avik Khan, and Yonghao Ni ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03456 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Sustainable Chemistry & Engineering

An Aqueous Dispersion of Carbon Fibers and Expanded Graphite Stabilized from the Addition of Cellulose Nanocrystals to Produce Highly Conductive Cellulose Composites Yuxin Liu †,‡, Bing Sun*†,‡, Jianguo Li‡,§, Dong Cheng‡,§, Xingye An‡,§, Bo Yang‡,

Zhibin He‡, Ryan Lutes‡, Avik Khan‡ and Yonghao Ni*‡,§ †Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650500, China ‡Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada §Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China * B. Sun. E-mail: [email protected] Tel: +86 15912424735., *Y. Ni. E-mail: [email protected]. Tel: +1 506 452 6084. ABSTRACT: Conductive cellulose composites have received much attention as emerging materials due to their unique properties, such as biodegradability and flexibility. However, the achievable levels of conductivity of these cellulose composites are generally low due to the intrinsically non-conductive properties of cellulose. In this study, cellulose nanocrystals (CNC) were applied for preparation of carbon fibers/expanded graphite (CF/EG) dispersion which was coated on cellulose paper to prepare highly conductive cellulose composites. Such composite materials were characterized based on TEM and SEM observation, FT-IR, XPS, XRD and by determining the zeta potential, particle size, contact angle and rheological behavior. It

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was found that the addition of CNC resulted in a significant improvement in the stability of the CF/EG dispersion. These results were explained by the facts: 1) CNC improved the surface charge of the CF and EG due to the presence of negatively charged groups on the CNC surface and 2) CNC increased the wettablity of the CF and EG due to large availability of hydroxyl groups from CNC. The as-prepared CF/EG/CNC dispersion was then applied by coating cellulose paper surfaces and the obtained papers presented superior conductivity and high flexibility. KEYWORDS:

Cellulose

nanocrystals,

Carbon

fibers,

Expanded

graphite,

Dispersant, Conductive paper

INTRODUCTION Flexible and highly conductive composites have numerous applications, including the manufacturing of modern electronics. Carbon material based composites have the characteristics of not only high conductivity, but also light weight and excellent flexibility, thus having particular advantages in aerospace electronics, flexible electronics and electromagnetic shielding.1-3 There are many different carbon materials including carbon fibers (CF), expanded graphite (EG), single wallcarbon nanotube (SWNT), graphene, graphene oxide (GO), carbon black and so on.4-6 Graphene nanosheets as one of the emerging two dimensional (2D) nanomaterials and carbon nanotubes as a tube-shaped material have a combination of excellent mechanical, electrical and thermal properties.7-8 Less expensive CF and EG are more commonly used as the conductive fillers for conductive composites.9 As the preferable carbon materials for manufacturing highly conductive composites,


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CF and EG are promising candidates due to their unique characteristics, such as high electro-conductivity, self-sensing, self-healing and electromagnetic reflection.10-13 However, CF and EG have poor wettability in water, which negatively effects the stability of the dispersion.14 Earlier studies15-17 have shown that surface treatment of carbon fibers improved the hydrophilicity of the carbon fibers, and thus improved its dispersion in water. Furthermore, the presence of some fine particles with typical particle size helps the fibers break loose from one another as mixing occurs.18 The dispersion can also be controlled by the length and concentration of carbon fibers in water19 or addition of polymers20. Expanded graphite dispersion could also be improved by mixing with the polymer solution to fill the pores of the expanded graphite sheet.21 However, the aqueous dispersion of CF and EG has generally poor stability due to their hydrophobic nature. Highly charged colloidal particles such as CNC can be good dispersing agents to improve the stability of CF/EG dispersion. Cellulose nanocrystals (CNC) have a high crystalline degree,22-24 and have attracted a considerable amount of attention due to its nanoscale dimensions, excellent mechanical characteristics and a large panel of interactions. CNC has also been used as a dispersant25-27, ideal reinforcement28, binder29 and template30-31 for composites and polymer matrices. For example, Olivier32 prepared luminescent SWNT for layer-by-layer assembled hybrid thin film and found that highly stable SWNT dispersion could be obtained by ultrasonicating the SWNT in CNC aqueous colloidal suspension. However, no information is available in the literature on the effect of CNC on the stability of CF and EG



dispersion. In this study, CNC was used as a dispersant for the preparation of CF and EG dispersion in water to improve the stability and rheology properties of CF/EG dispersion. The dispersion was then coated on cellulose paper to prepare highly conductive cellulose composites. The effect of CNC on the CF/EG dispersion was analyzed by zeta potential, particle size distribution, contact angle, rheological behavior and TEM, FT-IR, XPS, XRD, as well as the conductivity of the coated paper composites.

MATERIALS AND METHODS Materials. CNC was provided by Tianjin Woodelf Cellulose Co. Ltd., China, which was produced from dissolving pulp by the sulfuric acid-catalyzed (64 wt%) hydrolysis process according to the method described in the literature.33 EG was obtained by heating expendable graphite (particle size < 80 mesh) at 900°C for 30 s. CF with an average of 1 mm in length and 0.007 mm in diameter was purchased from Goodfellow and ASC material. Polyvinyl alcohol (PVA) of 31000-50000 Mw and 87-89% hydrolysis degree was supplied by Sigma-Aldrich. Commercial self-adhesive note paper was obtained from Highland and used as the substrates for the surface coating process. All materials were used as received and deionized water was used for all experiments. Figure 1 shows the TEM image of the CNC, and the SEM images of the EG and CF.


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Figure 1. Materials: (a) TEM image of CNC, (b) SEM image of EG and (c) SEM image of CF. Preparation of CF/EG/CNC dispersion. The CF was first dispersed and sonicated in a PVA solution with a Bransonic 3510R-DTH (USA) for 20 min at 100 W and 42 kHz in an ice water bath. The dosage of PVA was 2% based on the total dry weight of CF and EG. Subsequently, the as-prepared EG was added to the mixture at a weight ratio of EG/CF=3:1, and the mixture was mixed with a homogenizer (IKA T25, Germany) for 1 h at 10,000 rpm. After that, 2.0 to 10.0 wt% CNC (based on the total dry weight of CF/EG) was added, and the mixture was sonicated for 20 min at pH 7.5. The final dispersion had a total solid content of 5 wt%. Coating of the CF/EG/CNC dispersion onto cellulose paper. The prepared CF/EG/CNC dispersion was applied to the surface of the paper substrate with a Meyer bar on a lab coater (RK Print Coat Instruments Ltd K303, UK), to have a dry coating thickness of 10 ± 2 µm. The coated paper composites were dried at 70°C and conditioned for 24 hrs at 23°C and 50% RH before characterization. Characterization and measurements. The stability of the CF/EG/CNC dispersion was evaluated as the light absorption at 500 nm with a UV-vis spectrophotometer (Milton Roy, 1001 Plus), based on the principle that the light absorption value will



decrease accordingly when the black color CF and EG particles aggregate. The CF/EG/CNC dispersion was diluted with deionized water to 0.1 wt% concentration and pH adjusted to 7.5, and then stirred for 10 min at 10,000 rpm at room temperature. The resulting dispersion was left to stand at room temperature for 24 hrs before the light absorption measurements. A higher light absorption value indicated a better dispersion. Zeta potential and particle size distribution were determined at 25°C using a Brookhaven Eta-Plus Instrument (USA) in combination with a 90 plus/BI-MASS software. CF/EG/CNC dispersion with various amounts of CNC was diluted to 2.5 mg/mL and pH adjusted to 7.5, and then stirred for 10 min at 10,000 rpm. All experiments were carried out in triplicate, and the averages were reported. The rheological behavior of the CF/EG/CNC dispersion was characterized with a Brookfield viscometer (USA) at 25°C using a #3 spindle at a shear rate from 3-60 rpm. The CNC shape structure was observed using a transmission electron microscopy (TEM) (JEOL, Japan) with 200 kV acceleration voltages. The surface morphology of the conductive paper was investigated with a field emission scanning electron microscope (FE-SEM) (ZEISS Merlin, Germany). The CF/EG/CNC dispersion was further investigated by Fourier transform infrared spectroscopy in the spectral range from 4000 to 500 cm-1 with a Thermo Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA) on the pressed potassium bromide (KBr) discs as transparent pellets. The XPS analyses were carried out on an ESCAPlus Omicron spectrometer using a monochromatized Mg X-ray source (1253.6 eV). The XRD analyses were performed


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on a Bruker D8 Advance diffractometer using a Cu tube as X-ray source (λCu Kα = 1.54A°) a tube voltage of 40 kV, and a current of 40 mA. The electro-conductivity (S/m) of the coated paper was measured and calculated according to previous studies.29, 34-35

RESULTS AND DISCUSSION Process concept of CNC in stabilizing CF/EG dispersion. Our hypotheses are as follows: 1) CNC is strongly charged due to the presence of large amounts of sulfate groups. With the addition of CNC to the CF/EG system under strong shearing force, CNC nanoparticles are absorbed on the CF and EG surfaces based on the van der Waals interaction (repulsion and stabilization). 2) Steric hindrance (with CNC on the surface) would also lead to stable dispersion. 3) CF and EG have complementary functions in forming the conductive networks. 4) CNC improves the wettability of CF and EG surfaces which is important to their dispersing behavior in water. CNC is hydrophilic and has more negative charges. As illustrated in Figure 2, the CF and EG surfaces are covered with CNC after ultrasonication, resulting in increased electro negativity of the CF and EG particles in water and thus a more homogeneous and stable dispersion, which in turn leads to the higher conductivity of the coated paper composites. The resultant paper is highly electro-conductive (up to 2617 S/m conductivity was reached) and was used to form the circuit to light up a LED bulb.



Figure 2．Schematic of CNC assisted CF/EG dispersion and its use as conductive coating layer on a paper surface: (a) TEM of CF, EG, and CNC; (b) CNC/CF/EG dispersion via high shearing and sonication, which is stable: the charged CNC depositions onto the surfaces of CF and EG caused repulsion, responsible for the observed stable CNC/CF/EG dispersion; (c) CNC/CF/EG coating material applied onto paper surface. In the literature, the use of CNC as an effective dispersant/carrier for hydrophobic molecules has been reported. For example, CNC was used as a carrier for hydrophobic 20(R)-ginsenoside Rg3 to improve its bioavailability, dispersity, and antioxidation activity in aqueous media,36 and the results showed that the antioxidation efficiency was enhanced remarkably. In another study, CNC was used as a carrier for a hydrophobic spirooxazine dye to enhance its photochromic efficiency.30 This process concept is adopted here: upon the addition of CNC under ultrasonication, CNC would adsorb onto CF and EG surfaces, the negative charges of the CNC nanoparticles kept the CF/EG dispersion stable. CNC is strongly hydrated when dispersed in water, and at the same time CNC has


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a fairly high nonpolar contribution to its surface energy. For this reason, there would be significant van der Waals interactions between CNC and CF/EG particles. Moreover, both CNC and EG have large specific surface areas which are also important for the interaction.37 Therefore, it is reasonable to think that the addition of CNC can lead to significant improvement in the stability of the water dispersion of CF and EG, due to the improvement of wettability and electro negative repulsion.

Figure 3. TEM images of (a) EG and (b) EG with CNC.

Figure 4. Effect of CNC amount on the zeta potential and light absorption of the CF/EG/CNC dispersion. The light absorption was measured as absorbance at 500 nm after 24 hrs settlement of the 0.1 wt% dispersion; the zeta potential was measured and



plot as a function of the CNC amount. The stabilities of the dispersions were compared after 24 hrs and after 1 month.

Figure 5. The CF/EG/CNC composites were coated onto paper surface to make electric conductive paper. The conductivity and contact angle are plotted as a function of the CNC amount. The conductivity would decrease as folding times increasing. Characterization of CNC deposition onto CF and EG surface. Figure 3 (a) shows the thin layer structure of EG. EG particles have a relatively large specific surface area and are used as a porous adsorption substrate.37 Figure 3 (b) shows the rod shape of CNC nanoparticles absorbed on the EG surface after ultrasonication of EG in presence of CNC. The presence of PVA can enhance CNC adsorption onto the CF and EG surfaces. Sun demonstrated that CF coated with a thin layer of PVA had improved dispersibility and suspending stability in water.20 Pang reported that PVA molecules absorbed parallel onto the EG surface in the physical adsorption process.38


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Therefore, the hydrophilic nature of CNC combined with the water-soluble PVA molecules would facilitate the dispersion of the CF and EG, and the interactions of CNC with PVA, and fibers have been reported in the literature.39-41 Shown in Figure 4 is the effect of CNC addition on the zeta potential of the CF/EG dispersion. With the CNC addition, the zeta potential was more negative, indicating that the particles in the system had more negative charges on the surface. For example, with the CNC amount increased from none to 10.0 wt%, the zeta potential increased from -3.13 mV to -16.78 mV. The increased surface polarity of CF and EG due to the negative charges can improve the wetting behavior of CF and EG towards water. This finding is in agreement with those reported in the literature. Bismarck et al also reported that oxygen plasma treatment increased the electrokinetic and wetting properties of PAN-based carbon fibers.42 Tang et al investigated the impact of CNC on the

electro-conductive

cellulosic

paper

via

surface

coating

of

carbon

nanotube/graphene oxide nanocomposites, and found that the dispersion of carbon nanotube/graphene oxide nanocomposites improved with CNC addition due to increased zeta potential.29 The results on the contact angles of the CF/EG/CNC coating on paper are shown in Figure 5. The coating gave an advancing angle of 76.26 ° in the absence of CNC, while the advancing contact angle decreased to 66.78 ° with 2 wt% CNC. The advancing contact angle decreased further with the increase of the CNC amount. The results indicated that CNC addition improved the water wettability of CF/EG coating. This is understandable, as the CNC was rich in hydroxyl and sulfate groups which are



hydrophilic in nature. Valentini et al prepared CNC and GO composite films by drop casting water dispersion of GO in the presence of CNC. They found that the GO/CNC composite film had a hydrophilic behavior, and its water contact angle was smaller compared with that for the GO film in absence of CNC.43 The increased wettability was due to the abundant oxygen containing groups in the GO sheets to interact with the hydroxyl groups and oxygen atoms in the CNC chain by hydrogen bonds, which was beneficial to homogeneous dispersing of GO and CNC.

1.1 CF/CNC

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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EG/CNC

1.0

CF/EG/CNC

0.9

0.8 3437

2927

1386 1627 1062

4000 3500 3000 2500 2000 1500 1000

500

-1

Wavenumber (cm )

Figure 6. FT-IR spectra of CF/CNC, EG/CNC and CF/EG/CNC dispersion samples. The FT-IR spectrum of CF/EG/CNC dispersion is shown in Figure 6. Those at 3437, 2927, 1627, 1386, and 1062 cm-1 are attributed to O-H stretching, C-H stretching, C=O stretching, C-C skeletal vibration, and C-H bending vibrations, respectively, in agreement with those reported in the literature36. These results supported the conclusion that CNC was anchored onto the CF and EG surfaces. It is noted that the


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O-H stretching at 3437 cm-1 decreased slightly, and the C=O stretching at 1627 cm-1 and C-C skeletal vibration at 1386 cm-1 increased somewhat, likely due to the formations of hydrogen-bonding between CNC and CF or EG. 140000

(a) C-C

Intensity (CPS)

120000 100000 80000 60000 C-O

40000 20000 0

296

292

288

284

280

Binding Energy (eV) 100000

Intensity (CPS)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) C-C

80000 60000 C-O

40000 20000 0

C=O

296

292

288

284

280

Binding Energy (eV)

Figure 7. XPS of (a) CF/EG (i.e., without CNC) and (b) CF/EG/CNC dispersion (2 wt% CNC) samples. The XPS results are shown in Figure 7. For the CF/EG sample (without CNC) the deconvolution peaks at 284.37, 286.24 eV, confirm the presence of C-C and C-O/C-O-C (Figure 7(a)), which is due to the PVA coating on the surface, in agreement with those in the literature20 as PVA molecules were absorbed onto CF



surfaces. For the CF/EG/CNC dispersion sample (2 wt% CNC), these at 284.37, 286.24, 288.15 eV were assigned to C-C, C-O/C-O-C, and O-C=O (Figure 7(b)), functionalities. The presence of O-C=O at 288.15 eV confirmed the presence of CNC on the CF and EG surfaces. Zhang et al. also found that the presence of nanocellulose on the expandable graphite surface showed the O-C=O functionality in its XPS result, and the interactions of graphite with cellulose are based on hydrogen bonding, thus stabilizing the graphite dispersion.44 The XPS analyses support the presence of CNC in the CF/EG/CNC dispersion sample. In summary, the addition of CNC improved the dispersing of CF and EG in water, and increased the zeta potential and the water wettability of CF and EG. Hydrogen-bonds are likely the main interactions between CNC and CF/EG. As a result, the stability of CF/EG/CNC dispersion increased, which will be discussed in more detail below.


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1600 450

Viscosity (mPas)

400

1200

Viscosity (mPas)

350 300 250 200 150

800

15

30

45

60

Rotation (rpm)

0 wt% 2 wt% 4 wt% 6 wt% 8 wt%

400

0

0

10

20

30

40

50

60

Rotation (rpm) Figure 8. Effect of CNC amount on the rheological behavior of the CF/EG/CNC dispersion. 120 CF/EG/CNC 100

Distribution (-)

0 wt% 2 wt% 4 wt% 6 wt%

100

Distribution (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

EG/CNC

80 60 40 20

0

400 800 1200 1600 Particle size (nm)

60

40

20 0

400

800

1200

1600

Particle size (nm) Figure 9. Effect of the CNC on the particle size distribution of CF/EG/CNC dispersion. The inset showed that the particle size was significantly larger without CF. Effect of CNC addition on the stability of the CF/EG dispersion. Included in



Figure 4 also shows the effect of CNC amount on the stability of the CF/EG/CNC dispersion, in terms of visible light absorption at 500 nm. Since both CF and EG were black in color, the CF/EG/CNC dispersion had a strong absorption of visible light, and absorption intensity was proportional to the uniformity of the dispersion under the same concentration. When the dispersion lost stability due to aggregation of CF and EG particles, the light absorption decreased. As shown in Figure 4, in the absence of CNC, the light absorption of the CF/EG dispersion was only 0.210 (as absorbance); with 2 wt% CNC the absorbance increased to 2.557. With further increase of the CNC amount, the light absorption of the CF/EG/CNC dispersion increased, indicating that the stability of the dispersion improved with the increase of the CNC amount. The improved stability of the dispersion with addition of CNC was attributed to the increased zeta potential due to the anionic sulfate groups of CNC. Tang et al also found that CNC addition had a positive effect on the stability of carbon nanotube/graphene oxide dispersion, and they also observed similar effect of CNC on the zeta potential of the dispersion.29 Figure 8 shows the shear thinning behavior of the CF/EG/CNC dispersion. With the increase of the shear rate, the viscosity of the dispersion decreased, which was true for all cases. It is interesting to note that at different amounts CNC addition affected differently the viscosity of the dispersion at lower shear rate. With 2 wt% CNC addition, the viscosity decreased dramatically from about 1450 to about 600 mPas; when the CNC amount increased to 4, 6 or 8 wt%, the viscosity rebounded to 1000, 1050 and 1100 mPas, respectively. At a higher shear rate, the effect of CNC addition


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on the viscosity was much less pronounced. This phenomenon may be explained by the “separation and dispersion” concept in previous research work by King and Barbosa et al on the rheology of short fiber and nanoparticles suspension.45,46 With the addition of a small amount of CNC (e.g. 2 wt%), the stiff network structure of CF and EG started to separate under the shear force, and the negatively charged CNC particles led to repulsion of the particles. As a result, the viscosity decreased markedly. As the CNC amount increased, more CNC particles were available in the system to stabilize the fine CF and EG particles separated from the network fragments, and the fine particles remained dispersed in the system. The dispersion of the fine particles increased the crowding factor in the system and thus caused the viscosity of the dispersion to increase again. Figure 9 shows the effect of CNC addition on the particle size distribution of CF/EG/CNC dispersion. With the CNC amount increasing from 0 to 4.0 wt%, the particle size becomes smaller, due to the negative charges on the CNC surfaces. However, when the CNC amount increases further, the CF/EG/CNC dispersion after shear and ultrasonication exhibits an increase in its viscosity. As a result, the CF/EG/CNC dispersion would become less stable after a longer storage time (as shown in Figure 4). These results are in agreement with those reported Araki et al 47.



b

a

c

Figure 10. SEM images of the surface (a, b) and cross section of the paper coated with 10 µm of the CF/EG/CNC dispersion (24 wt% CF, 72 wt% EG, 2 wt% PVA, 2 wt% CNC).

20000 16000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12000 CF/EG/CNC 8000

CF/CNC CF/EG

4000

EG/CNC 0 15

20

25

30

35

22 (°) Figure 11. XRD of the CF/CNC, EG/CNC, CF/EG and CF/EG/CNC dispersion samples. Coating application of the CNC-enhanced CF/EG dispersion on cellulose paper. The CF/EG/CNC dispersion was applied onto the paper surface by rod coating. After drying, the thickness of the coating layer was about 10 µm, as indicated by the


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cross section SEM image in Figure 10(c). Examination of the coating surface by SEM (Figure 10(a) and (b)) revealed that the carbon fibers were distributed in the matrix of graphite flakes, functioning as the skeleton of the CF/EG network. This explains why CF and EG have a complementary function on the conductivity, as the CF fibers connected and strengthened the network. The XRD patterns of CF/EG/CNC dispersion are presented in Figure 11. A strong diffraction peak of CF/EG/CNC dispersion at 2θ=26.67° was assigned to the C(002) plane, owing to graphite crystals of CF and EG, in agreement with those reported by Xie48 and Yuan49. These results indicate that the presence of CNC on to the surface of both EG and CF does not change the structure of CF and EG. The broad diffraction peak of CF/EG/CNC sample is due to its smaller particle size, as shown in Figure 9. The effect of CNC amount on the conductivity of the coated cellulose composite was shown in Figure 5. The CNC amount had a significant effect on the conductivity: as the CNC amount increased from 0 wt% to 2 wt%, the conductivity increased from 1631 S/m to 2390 S/m. The highest conductivity was 2617 S/m, which was achieved with 4 wt% CNC. The improved conductivity was attributed to enhanced dispersing of CF/EG with CNC addition, which in turn improved the connectivity of the conductive network of the CF/EG/CNC coating layer on the paper surface. To the best of our knowledge, this is the highest conductivity ever reported for cellulose composite materials. The highest conductivity reported by Lara et al was 964 S/m for GP/CF/FB composites (56 % of graphite (GP), 24% of CF and 20% of cellulose fibers (FB)).11 Tang et al reported a carbon nanotube, OG and CNC composite with the



highest conductivity of 892 S/m. 29 The extremely high conductivity achieved in our present work was attributed to the enhanced complementary functions of CF and EG on the conductive network with the addition of CNC. However, at a too high CNC addition level (higher than 4 wt%), the presence of non-conductive CNC would ultimately have negative effects on the conductive CF/EG network, thus lowering the overall conductivity, as shown in Figure 5. Under optimal conditions (4 wt% CNC) the as-prepared CF/EG/CNC coated paper had much higher conductivity than those reported in literature in similar systems. The under lying mechanism could be that CF, EG and CNC had complementary functions on the conductivity and the mechanical strength of the coating layer. Good mechanical strength is necessary to maintain the integrity and connectivity of the conductive network which is crucial to the conductivity of the composite. Figure 12 (a) and (b) shows that the CF/EG/CNC coated cellulose composites maintained high conductivity after multiple times of folding, demonstrating good flexibility and mechanical properties. The circuit was assembled with a 4.5 V LED, three 1.5-volt button batteries in series, and two conductive paper strips of the same size (1.5 cm ×7.0 cm). Table 1 shows the effect of folding on the conductivity of the conductive paper composites. After 5 folds, the surface resistivity increased by only 3.7%, which increased to 36.4% after 15 folds. The folding endurance of conductive paper composites may be an important parameter for some applications such as flexible electronics. However, such data is rarely reported in literature.


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Figure 12. CF/EG/CNC coated conductive paper composite before (a) and after (b) folding. The circuit was assembled with a 4.5 V LED and three 1.5-volt button batteries in series conductive papers. The mechanical strength of the paper increased after coating with the CF/EG/CNC dispersion. As shown in Table 2, the tensile index of the paper increased from 57.5 to 61.1 N·m/g when it was coated with the EG/CF dispersion. The presence of CNC in the dispersion enhanced the strength improvement. The highest tensile index of 65.1 N·m/g was achieved when the paper was coated with the EG/CF/CNC-4 dispersion (4 wt% CNC amount). The strength enhancement was due to the migration of part of the CNC and PVA ingredients in the coating formula to the paper substrate to increase the inter fiber bonding. It is well known that PVA and CNC are able to improve paper strength. This is advantageous compared with those conductive composites prepared by the filler method wherein the tensile strength of the composite decreased dramatically when the conductive filler ratio was increased to improve the conductivity.11 Table 1. Effect of folding on the electric resistivity of the coated paper. Folding times (1)

Surface resistivity (Ω)



0

40.6

1

41.6

5

42.1

10

54.7

15

55.4

Table 2. Effect of CF/EG/CNC coating on the tensile strength of paper. Tensile index (N·m/g) Original paper

57.50

Paper coated with EG/CF/CNC-0

61.17


61.56


63.95


64.65


65.14

CONCLUSIONS Stable dispersion of carbon fibers (CF), expanded graphite (EG) and cellulose nanocrystals (CNC) were prepared using CNC as the dispersing agent. The CF/EG/CNC dispersion was applied to paper surface by rod coating method, and a highly conductive and flexible cellulose composite was obtained with conductivity of 2617 S/m, which was believed to be the highest level ever reported in the literature for similar cellulose composite materials. Moreover, the conductive composite had good folding endurance to demonstrate its good flexibility. Its conductivity decreased only


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3.7% after 5 times of back-and-forth folding. The composite also had excellent mechanical properties. Its tensile strength was even higher than the base paper due to the strengthening effect of the CNC and PVA. The CF/EG/CNC dispersion was characterized in terms of zeta potential, light absorption and rheology to investigate the effect of CNC addition on the stability of the dispersion. We found that CNC improved the dispersing of EG and CF thanks to the negative charges on the CNC surface, in addition CNC increased the wettability of the CF and EG surfaces. With the CNC addition, the stability of the CF/EG dispersion improved dramatically, which is responsible for the increased conductivity of the coated paper composite. AUTHORINFORMATION Corresponding Authors *E-mail：[email protected]. Tel: +86 15912424735. *E-mail：[email protected]. Tel: +1 506 452 6084. ORCID Yuxin Liu: 0000-0002-6516-9347 Yonghao Ni: 0000-0001-6107-6672 ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (grant no. 21466017, 31460176) as well as the Canada Research Chairs Program of the Government of Canada. REFERENCES (1) Chung, D. D. L. Electromagnetic interference shielding effectiveness of carbon materials. Carbon. 2001, 39 (2), 279-285. DOI: 10.1016/S0008-6223(00)00184-6.



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For Table of Contents Use Only.

Synopsis

Cellulose nanocrystals were used as dispersants for carbon fibers and expanded graphite to produce flexible and highly conductive green and sustainable cellulose composites.


Aqueous Dispersion of Carbon Fibers and Expanded Graphite

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