Highly Conductive PolypropyleneâGraphene Nonwoven Composite

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Highly Conductive Polypropylene-Graphene Nonwoven Composite via Interface Engineering Qin Pan, Eunkyoung Shim, Behnam Pourdeyhimi, and Wei Gao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01508 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Highly Conductive Polypropylene-Graphene Nonwoven Composite via Interface Engineering Qin Pan†‡, Eunkyoung Shim†‡, Behnam Pourdeyhimi†‡*, and Wei Gao†‡*

† The Nonwovens Institute, North Carolina State University, Raleigh, North Carolina 27606, United States ‡ Department of Textile Engineering, Chemistry & Science, College of Textiles, North Carolina State University, Raleigh, North Carolina 27695, United States

KEYWORDS: graphene oxide (GO), nonwoven, conductive polymer composites, interface engineering, polypropylene

*

E-mail: (B. P.) [email protected]. *E-mail: (W. G.) [email protected]

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Abstract

Here we report a highly conductive polypropylene-graphene nonwoven composite via direct coating of meltblown polypropylene (PP) nonwoven fabrics with graphene oxide (GO) dispersions in N, N-dimethyl formamide (DMF), followed by the chemical reduction of GO with hydroiodic acid (HI). GO as an amphiphilic macromolecule can be dispersed in DMF homogeneously at a concentration of 5 mg/mL, which has much lower surface tension (37.5 mN/m) than that of GO in water (72.9 mN/m, at 5 mg/mL). The hydrophobic PP nonwoven has a surface energy of 30.1 mN/m, close to the surface tension of GO in DMF. Therefore, the PP nonwoven can be easily wetted by the GO/DMF dispersion without any pre-treatment. Soaking PP nonwoven in a GO/DMF dispersion leads to uniform coatings of GO on PP-fiber surfaces. After chemical reduction of GO to graphene, the resulted PP/graphene nonwoven composite offers an electrical conductivity of 35.6 S m-1 at graphene loading of 5.2 wt.%, the highest among the existing conductive PP systems reported, indicating that surface tension of coating baths has significant impact on the coating uniformity and affinity. The conductivity of our PP/graphene nonwoven is also stable against stirring washing test. In addition, here we demonstrate a monolithic supercapacitor derived from the PP-GO nonwoven composite by using a direct laser-patterning process. The resulted sandwich supercapacitor shows a high areal capacitance of 4.18 mF/cm2 in PVA-H2SO4 gel electrolyte. The resulted highly conductive or capacitive PP/graphene nonwoven carries great promises to be used as electronic textiles.

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Introduction

Conductive polymers have received significant attention from both science and engineering communities, due to its widespread applications in antistatics, electromagnetic shielding, corrosion inhibitors, disposal electronic goods, light emitting diodes, energy storage devices, etc.1-4 So far, researchers have demonstrated three major methods to fabricate conductive polymers including coating, blending, and co-polymerization with conductive additives such as metallic or carbonaceous materials.5 Among them, coating turns out to be the most efficient, low-cost and easy-to-process protocol that requires relatively low loading of additives to achieve high electrical conductivity, more compatible with industrial practices. To ensure the uniform coating with good affinity, the wettability of the polymer matrix with the coating bath is of great significance, which is mostly determined by their surface free energy.6-7 To be wettable by a water-based coating solution, the polymer matrix should have high enough surface energy to overcome the surface tension of water.7 Water spreads easily on polar surfaces that may form dipole-dipole interactions or hydrophilic surfaces that form hydrogen bonds.8 However, many industrial polymers such as polypropylene (PP), polyethylene (PE), and polystyrene (PS) are hydrophobic, which poses huge challenges in surface coating. Therefore, conventional recipes to conductive PP/PE composites include melting/solution blending, and in-situ polymerization with conductive materials such as carbon black, multi-wall carbon nanotubes, graphite, or graphene oxide etc., all of which are quite complex in protocols and less effective in conductivity enhancement.9-11

For instance, Taipalus et al. blended carbon fibers with molten PP matrix, and obtained a 3


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conductivity of 5 × 10 S m-1 for the composite when the loading of carbon fiber reaches 3.2 vol.%.9 Kalaitzidou et al. mixed PP powder with exfoliated graphite nanoplatelets under sonication in isopropyl alcohol, but a high loading of graphite as 40 vol.% is needed to achieve high conductivity of ~ 10 S m-1.10 Huang et al. firstly prepared PP/GO nanocomposites by in-situ Ziegler−Natta polymerization, which gives the electrical conductivity of 0.3 S m-1 at the GO loading of 4.9 wt.%.11 Obviously, these conventional blending and co-polymerization recipes usually require high loadings of conductive additives, which give rise to complex fabrication processes, inferior mechanical properties and high cost.12 Direct coating of PP with conductive carbon materials was rarely reported, probably due to its hydrophobic nature leading to the poor wetting and interface adhesion in between. Chemical and physical modifications can be used to make hydrophobic surfaces more hydrophilic, such as plasma treatment, atomic or molecular layer deposition, surface grafting, nanostructure engineering, etc.13-16 However, multiple steps of chemical or heat treatments are usually required, and some treatment will only result in temporary effects that fade away in a few hours.17

In this paper, we describe a simple method to direct coat PP nonwoven with graphene oxide (GO) dispersion in DMF. PP meltblown nonwovens have three-dimensional (3D) porous structures and can be engineered with randomly oriented fibers, and can have unique advantages over traditional woven/knitted fabrics, such as enhanced additive absorbency, better elasticity, lower cost etc. GO, a monolayer of honeycomb carbon atoms attached with various oxygenated functional groups, can be easily dispersed in water and some organic 4


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solvents, mainly due to its amphiphilic nature.18-20 It can be chemically or thermally converted back to graphene, which is proved to be an ideal additive for various conductive polymer composites owing to its high aspect ratio, extraordinary electrical conductivity, light weight and flexibility.21-23 Compared with the widely-used GO/H2O dispersion, GO/DMF dispersion with a much lower surface tension of 37.5 mN/m at 5 mg/mL is especially suitable for coating hydrophobic PP nonwoven, since PP with a surface free energy of 30.1 mN/m can partially overcome its surface tension and be wetted, leading to a uniform coating of GO with good affinity.7,

24

After chemical reduction with hydroiodic acid (HI), the

PP/graphene exhibited a conductivity of 35.6 S m-1, the highest among all the reported conductive PP system, at a relatively low graphene loading of 5.2 wt.%; meanwhile it maintains good wash fastness and mechanical properties. Furthermore, we also demonstrate a potential application of the PP/GO nonwoven as energy storage textiles, offering high areal capacitance as monolithic sandwich supercapacitors after surface laser treatment.

Experimental Section

Preparation of PP nonwoven fabric: polypropylene was made into nonwoven fabrics by meltblowing at the Nonwovens Institute Pilot Facility (North Carolina State University Raleigh, NC). Fabrics with an average fiber diameter of 3.3 µm were prepared at a constant die-to-collector distance of 300 mm, throughput of 0.3 ghm (gram per hole per minute) and air volume rates of 1000 m3 h-1m-1. The die had 1400 capillaries per meter with a diameter of 400 µm.

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Preparation of GO dispersion: GO was prepared by modified Hummer’s method as reported in the literature.25 Graphite was purchased (Micro 850) from Asbury Graphite Mills Inc., Kittanning, PA. GO/DMF dispersion was prepared by adding 2.5 g amount of GO into 500 mL DMF with vigorous sonication for 1 h until homogenous dispersion was formed.

Preparation of PP/GO and PP/graphene: PP nonwoven typically cut into pieces of 5 cm×5 cm, are directly dipped into GO/DMF dispersions under stirring for 5 mins, and taken out for drying in the oven at 70 ℃. This process is repeated for several times until the targeted loading of GO is achieved. Then the obtained PP/GO nonwovens are soaked in the hydroiodic acid solution (aq. 55 wt.%) and heated to 90 ℃ for 3 h to reduce GO sufficiently.

Control group design: the ozonated PP (o-PP) was dip-coated in the GO/H2O dispersion with the same concentration and reduced in the same way as the control group. Ozone was produced by an ozone generator (AtoZ ozone, Inc) with pure oxygen as input. PP nonwoven was put into a flask under continuous ozone flow at 25 ℃ for 1.5 hour.

Preparation of monolithic sandwich supercapacitors on PP-GO nonwoven: the as-prepared PP-GO was laser reduced by Epilog Laser Mini (18 inch×12 inch, 40 W) with power of 3.2 W and scanning speed of 15% on each side. Several drops of the gel electrolyte (1M H2SO4 in 10 wt% of PVA) was directly dropped onto its surface. Gold-coated vinyl films were used as current collectors.

Materials characterization: scanning electron microscopy (SEM) pictures were collected 6


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with a Phenom ProX desktop SEM. Samples were sputter coated with palladium for 45 seconds before use. Electrical conductivity test was measured by a standardized four-point probe setup with probe spacing of 0.3 cm. Each sample was cut into 2 cm × 7 cm rectangular shape and measured three times to obtain an average value. Wash fastness of PP/graphene composites was tested following the standards in ISO 105C-106.26 In detail, PP/ graphene composites with graphene loading of 1.2 wt.%, 2.6 wt.%, 3.0 wt.% and 5.2 wt.% respectively were washed in 400 mL of DI water under stirring for 10 mins at 25 ℃. After wash, the fabrics were dried in the oven of 70 ℃ and then measured their electrical conductivity again. FT-IR spectra (ATR) were collected with a Thermo Nicolet Nexus 470 instrument. Tensile strength was measured by MTS tensile tester ATSM D5035. Contact angle was measured with the OCA 15 contact angel goniometer. Liquid interfacial tension of GO/H2O and GO/DMF dispersion was measured via pendant drop method using the same goniometer. The surface tension of the hanging liquid was calculated by Bashforth and Adam equation:

+

"

= −! + 2,

equation (1)

#

where B is:

B=

#% &∆(

equation (2)

)*+

where ,-. is the liquid surface tension, ∆ρ is the difference between the density of the drop and its environment (air or another liquid), g is the gravitational acceleration, a is the 7


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radius of curvature at the apex, x, z and φ are the coordinates of the drop as shown in Scheme 1.

Scheme 1 Geometrical component of a pendant drop

Cyclic voltammograms (CV) and electrochemical impedance

spectroscopy (EIS)

measurements were conducted using an Autolab workstation (Metrohm, USA). CV was recorded at a scan rate of 40 mV s-1 in the window of 0 − 1 V. EIS were measured in the frequency range from 0.01 Hz to 1 M Hz with amplitude of 10 mV. The capacitance (C) was calculated according to the formula:

56789:7 ;F= =

9

equation (3)

;6.⁄6> =

where i refers to the mean current in CV curves and dV/dt is the scan rate. The specific areal capacitance (5E = is the capacitance per unit area of the whole device: 8


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5F =

GHIJKLI

equation (4)

F

with S equalling to the area of single-side electrode of 1 cm2. The areal energy density E was calculated also based on the areal of the whole device:

E=

GO ×PQ %

equation (5)

R×STT

The maximum areal power density was calculated by:

UV#W =

PQ %

equation (6)

X×QFY×F

where ΔE is the potential window, ESR is the equivalent series resistance measured by EIS.

Results and Discussion

Disadvantages of Hydrophilic Pre-treatment on PP Nonwoven: With a large number of oxygenated groups in its structure, GO exhibits the best dispersibility in water than in other solvents. Therefore, GO/H2O dispersions are often used to coat GO onto various polymer matrices.27-29 However, PP nonwoven fabric cannot be wetted by the GO/H2O dispersion, since its surface free energy (30.1 mN/m, at 20 ℃) is too low to overcome the high surface tension of GO/H2O dispersion (> 70 mN/m). Figure 1 (a) shows the correlation between the concentration of GO/H2O dispersion and its surface tension. The lowest surface tension of GO/H2O is 72.21 mN/m at a concentration of 20 mg/mL, quite close to that of water (72.80 mN/m),30 indicating that GO has little influence on surface tension of water. 9


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Therefore, improving the hydrophilicity of PP nonwoven by surface modification seems more feasible. Plasma, as the most common surface modification methods, allows instant but temporary increase of surface hydrophilicity, requiring the coating process to follow immediately. To achieve long-term enhancement of surface hydrophilicity, ozone was used to add more oxygenated groups onto PP surface to render higher and more durable hydrophilicity. Figure 1 (b) shows the photograph of the PP nonwoven before and after ozonation for 1.5 h at 20 ℃, with no visible difference in the appearance. However, upon dipped into the GO/H2O dispersion of 5 mg/mL, the ozonated PP (o-PP) nonwoven shows much better affinity with GO which forms a continuous coating layer as shown in Figure 1 (c). This is mainly due to the partial oxidization of PP by ozone, which enhances the wettability of PP nonwoven. In contrast, the GO/H2O dispersion can’t spread out on the original PP nonwoven but only aggregate into small droplets due to its hydrophobic nature. The contact angle θ between PP nonwoven with water further confirms the improvement of wettability, which decreases from 125° to 120° after ozonation (see Figure S1 in supporting information). However, ozone treatment will only modify the top surface of the PP nonwovens, with the inner fiber surfaces remaining quite hydrophobic. Therefore, instead of penetrating into the PP nonwoven thoroughly, the GO dispersion only spreads out on the top surface of the fabric and forms a sheath-like layer. In addition, ozonation also compromise the mechanical strength of the PP nonwovens due to the damage caused to the individual polymer chains. Since the high-energy ozone will probably break some PP chains and thereby cause polymer degradation, the o-PP shows significantly decreased strain at break from 95.3% to 25.1% (Figure 4 (b)). 10


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Figure 1 (a) Correlation between the surface tension of GO/H2O dispersion and GO/DMF dispersion with concentration (C, mg/mL) of GO. Photograph of (b) PP nonwoven before and after ozonation and (c) PP and ozonated PP (o-PP) nonwoven coated with GO/H2O dispersion.

Interfacial Properties of GO Dispersion. Therefore, to develop a new method for coating PP nonwovens with GO uniformly without sacrificing its mechanical strength, we used GO/DMF dispersion to replace the aqueous dispersion for coating. Pure DMF has much lower surface tension of 37.6 mN/m at 20 ℃. As shown in Figure 1 (a), the introduction of GO also has very slight influence on it. Although GO is reported as an amphiphilic surfactant capable of forming stable Pickering emulsions,31 its ability to lower the liquid surface tension is not obviously detected as with those common surfactant systems (e.g. sodium 11


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dodecylsulfate). Surfactant molecules need to lie at the liquid-air interfaces to reduce the surface tension of water (to replace the hydrogen bonding of water molecules, which is the origin of surface tension, with the hydrophobic interactions between the aliphatic carbon chains). However, GO has difficulties diffusing onto the surface of the water droplet, mainly due to the large lateral size and its bifacial hydrophilicity.32 This slow migration can be accelerated by air flotation, only after which can GO reduce the liquid surface tension significantly.32 Therefore, GO offers minimum impact on the surface tension of water or DMF during the pendant drop experiment. But GO’s amphiphilicity still helps forming a stable and homogenous dispersion in DMF at a relatively high concentration, which facilitates its coating with a variety of polymer matrices. At the concentration of 5 mg/mL, GO/DMF dispersion shows a surface tension of 37.5 mN/m, thus the PP nonwoven with surface energy of 30.1 mN/m can partially overcome it and be wetted homogenously as shown in Figure 2 (a). Thanks to the high porosity of the PP nonwoven, the GO/DMF dispersion can wick into the fabric due to capillary forces rather than only form a top layer on the fabric surfaces, leading to uniform coating throughout the whole nonwoven. The deposition of GO onto PP fibers can be described as self-assembly. Initially, GO sheets got absorbed onto the PP fiber surface mainly due to Van der Waals' force. With more coating cycles, GO with plenty of oxygenated groups were stacked onto each other via hydrogen bonding and π-π interaction, leading to continuous GO layers with certain thickness that cover the entire fiber surface. With different number of coating cycles and followed with chemical reductions via HI, the PP/graphene nonwoven composites with different graphene loadings can be obtained. Figure 2 (b), (c) and (d) shows the morphology of pure PP 12


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nonwoven, PP/ graphene composite with low loading of 1.2 wt.% and high loading of 5.2 wt.%, respectively. Prepared via melt blowing method, all PP fibers show quite smooth surfaces, with average fiber diameter of 3.3 µm. At low loading of 1.2 wt.%, graphene forms thin sheets that covers the fibers. In some region, it aggregates into small flakes attached onto PP fibers, as shown in Figure 2 (c). While at high loading (5.2 wt.%), the graphene forms larger and continuous sheets, which not only wrap around PP fibers but also occupy the inter-fiber spaces shown in Figure 2 (d). Different morphology of the PP/graphene reveals different conducting pathways formed in the composites, which greatly influence the resultant electrical conductivity.

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Figure 2. A photographic image of PP nonwoven coated with the GO/DMF dispersion (a). SEM images of PP nonwoven with average fiber diameter of 3.3 µm (b), PP/graphene with loading of 1.2 wt.% (c) and PP/graphene with loading of 5.2 wt.% (d).

Electrical Conductivity and Wash Fastness of PP/Graphene: The electrical conductivity of the resultant PP/graphene composites with different graphene loadings has been measured via a four-point-probe method. Figure 3 (a) shows the correlation between the electrical conductivity of composites and the graphene loading. Apparently, the conductivity increases 14


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with higher graphene loading. At a low loading of 1.2 wt.%, an electrical conductivity of 2.4 S m-1 was observed, indicating the percolation threshold is far below 1 wt.%. With the graphene content of 5.2 wt.%, the conductivity reaches 35.6 S m-1, the highest among all the reported conductive PP systems. Such a good performance is partially attributed to our unique PP nonwoven matrix. Firstly, it serves as the skeleton for the conductive pathways. Once the graphene sheets form any continuous pathway inside the skeleton, the insulator-to-conductor conversion occurs. Thus, it provides a more efficient way to utilize the conductive 2D additives, compared with traditional blending. Secondly, the highly porous PP nonwoven matrix facilitate GO’s exposure to the reducing agent, leading to more sufficient reduction. In addition, the effective chemical reduction via HI is also the key to obtain high conductivity. Reduction of the PP/GO composite with hydrazine (aq, 50 wt.%) was also conducted, and a lower conductivity of 9.2 S m-1 was observed at a higher loading of 6.5 wt.%, as compared with 35.6 S m-1 at 5.2 wt.% for HI reduction. Moreover, the HI-reduced o-PP/graphene composite at the loading of 6.6 wt.% gives a conductivity of 11.6 S m-1, only one third of that of PP/graphene via GO/DMF coating (at 5.2 wt.%). This is probably due to the fully wetted PP nonwoven by the GO/DMF dispersion with more uniform coating layers that form continuous and smooth conductive pathways afterwards.

Apart from high conductivity, the affinity between the graphene coating layer and PP nonwoven fabric is also crucial, especially for the potential applications in wearable electronics. The wash fastness was characterized by a washing test according to a standard procedure depicted in ISO 105C-106.26 In detail, PP/graphene composites with loading of 15


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1.2 wt.%, 2.6 wt.%, 3.0 wt.% and 5.2 wt.% respectively were washed in 400 mL of DI water under stirring for 10 mins at 20 ℃. After wash, the composite nonwovens were dried in the oven of 70 ℃ and then measured their electrical conductivity again. Figure 3 (b) compares the conductivity of the four composites before and after wash. In general, all of them exhibited good stability in water. For composite nonwoven with low loading (1.2 wt.%), every graphene sheet counts for the conductive pathway, so even minimal graphene loss will result in obvious decrease in conductivity. For composites with medium and high level of graphene loading, minimum drop in conductivity was observed. The stability and wash fastness of the graphene coating is originated from: 1) the hydrophobic nature of graphene and PP fiber; 2) the strong contact between graphene/graphene sheets due to π − π stacking; 3) the good affinity between graphene/PP fibers as the result of the complete wetting. In summary, the combination of dip-coating with GO and chemical reduction with HI renders the PP nonwoven the highest conductivity and superior wash fastness when compared with all the existing conductive PP systems, demonstrating great promises to be used in conductive or electronic textiles.

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Figure 3 (a) Electrical conductivity of PP/graphene composites as a function of weight percentage of graphene loadings, compared with o-PP/graphene and PP/exfoliated graphite composite in the ref [10]. (b) Comparison of the electrical conductivity of PP/graphene before and after the wash in DI water with stirring for 10 minutes at 25 ℃. Data of PP/graphene with different loadings of 1.2 wt.%, 2.6 wt.%, 3 wt.% and 5.2 wt.% are presented.

Characterization of PP, PP/GO, and PP/Graphene. Since the conductivity of PP/graphene is largely dependent on the extent of GO reduction, FT-IR spectra were used to further investigate the level of GO reduction. Chemical and thermal reduction can only partially remove the oxygenated groups on GO. Figure 4 (a) displays the FT-IR spectra of PP, PP/GO and PP/graphene. The PP/GO has two characteristic peaks at around 1620 cm-1 (ν HOH bending in water)

and 1720 cm-1 (ν C=O), corresponding with the adsorbed water molecules between

GO sheets and the carbonyl groups from carboxyl, ester and lactol groups attached on the basal plane of GO.21 After chemical reduction with HI, most of the intercalated water and oxygenated groups on GO were removed. Thus, these two peaks were significantly 17


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suppressed, indicating GO is partially reduced. The inset shows the spectra from 1000 cm-1 to 3800 cm-1. PP/GO shows a broad bump from ~ 3600 cm-1 to ~3100 cm-1, mainly due to the stretching of -OH groups in the form of alcohols/phenols, as well as the -COOH groups in GO. This broad bump also diminished in the PP/graphene composite, proving those oxygenated groups on GO were reduced.

Figure 4 (a) FT-IR spectra of PP, PP/GO and PP/graphene. The inset is the wider-range spectra from 1000 cm-1 to 3800 cm-1. (b) Load-strain curve for PP nonwoven, PP/GO, PP/Graphene, and o-PP in the tensile test.

We further investigated the mechanical properties of PP/graphene composites via tensile test. Figure 4 (b) shows the load-strain curve of PP, PP/GO, PP/graphene and o-PP nonwoven, respectively. As mentioned before, the ozonated PP nonwoven shows inferior stretchiness due to the polymer degradation caused by ozonation. Its peak load decreases from 14.2 N to 8.0 N, while the strain at break decreases significantly from 95.3% to 25.1%. In contrast, 18


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directly coating GO into PP nonwoven resulted in the much better preservation of the original stretchiness. Its strain at break reaches 74.3%, a bit lower than the pure PP, probably due to the interactions between GO sheets and the polymer fibers that restrict the mobility of the polymer chains.33 After chemical reduction, the PP/graphene shows strain at break of 70.3%, indicating the moderate impact of reduction and heating on the mechanical properties. Therefore, our recipe of making PP/graphene nonwoven without ozonation or plasma treatment ensures the uniform coating, good wash fastness, and reasonable mechanical strength and stretchiness of the resulted composites.

Laser-writing of supercapacitors on PP-GO nonwoven. In addition to conductive polymers, polymer/graphene composites with excellent electrochemical properties have also shown great potential as active materials in supercapacitors and batteries.34-35 Our PP-GO nonwoven can also be directly fabricated into monolithic sandwich supercapacitors via a laser-patterning process. Here we used a CO2-laser patterning machine (Epilog Laser Mini, 18 inch × 12 inch, 40 W) to treat the GO layers on both sides of PP-GO nonwoven as shown in Figure 5 (a), resulting in the conversion of GO into fluffy, porous and conductive laser-scribed GO (LGO). Figure 5 (b) shows the photograph of our monolithic sandwich supercapacitor on PP-GO nonwoven, with the device thickness of c.a. 250 µm. The dark black LGO parts act as the active electrode material, while the PP-GO in between works as the matrix and the separator. PVA-H2SO4 gel electrolyte (1 M H2SO4 in 10 wt% PVA aqueous solution) was directly casted onto its surface and penetrated into the matrix. Figure 5 19


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(c) shows the cyclic voltammograms (CV) curves at a scan rate of 40 mV s-1. Its areal capacitance

reaches

4.18

mF/cm2,

higher

than

the

laser-scribed

GO/PET

micro-supercapacitor in the same electrolyte.36 This is probably due to the relatively high loading of GO on PP nonwoven (~20 wt%) ensuring enough LGO electrode materials and the optimized laser patterning process. Figure 5 (d) shows the Nyquist plots of the imaginary components (-Z’’) vs. the real component (Z’) of the impedance with the highest frequency of 1 MHz to the lowest frequency of 0.01 Hz. The inset is the zoom in figure of the EIS spectrum in the high frequency domain. The intercept of the slanted straight line with the Z′ axis indicates the ESR is c.a. 17 Ω. The straight line tilted little in the rightmost part suggested fast ionic diffusion in PVA-H2SO4 electrolyte. Its areal energy density is calculated based on the whole device area to be 0.58 µWh cm-2, with the power density of 14.1 mW cm-2, showing promising potential to be used in wearable energy storage systems.

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Figure 5 (a) Scheme of laser-patterning process to fabricate monolithic sandwich supercapacitor on PP-GO nonwoven. (b) photograph of the directly laser-scribed PP-GO supercapacitor. (c) cyclic voltammograms (CV) curves at the scan rate of 40 mV s-1 in PVA-H2SO4 gel electrolyte; (b) Nyquist plots from 1 M Hz to 0.01 Hz; inset shows the zoom-in Nyquist plots at the high frequency domain.

Conclusions

In summary, a novel comparison between the surface tensions of GO dispersions in water and in an organic solvent (DMF) has been discussed, with the clarification on how GO impacts 21


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the surface tension of different solvents. Subsequently, direct dip-coating of PP nonwovens with GO/DMF dispersions has been demonstrated to incorporate GO into the hydrophobic PP nonwovens. Thanks to the low surface tension of GO/DMF (37.5 mN/m), the PP nonwoven can be wetted directly, eliminating the conventional pretreatment processes for PP, such as O2-plasma and ozonation (O3). Compared with ozone/plasma pretreatment to improve the hydrophilicity of PP, our coating recipe not only provides the long-term affinity between GO and PP nonwoven, but also avoids possible polymer degradation during pretreatment at a lower cost. The obtained PP/graphene composites show much higher conductivity than other conductive PP systems reported so far, owing to the unique nonwoven matrix that provides skeletons for percolated conductive pathways and enhances the exposure to reductants. Such high electrical conductivity can be well maintained in the wash test, probably due to PP nonwoven fully wetted by GO/DMF leading to good affinity between PP and graphene sheets. In addition, with graphene incorporation, peak load of the PP nonwoven increases due to the reinforcement effect of these high-modulus graphene sheets. However, the strain at break decreases a little, because of the decline in the polymer-chain mobility caused by the 2D-sheet additives. A potential application as disposable energy-storage systems was demonstrated. The monolithic sandwich supercapacitor on PP-GO nonwoven shows a high areal capacitance of 4.18 mF/cm2 in PVA-H2SO4 gel electrolyte, with areal energy and power density of 0.58 µWh cm-2 and 14.1 mW cm-2, respectively. Overall, the PP/graphene nonwoven composites prepared via the one-step dip coating offer high electrical conductivity, good wash fastness, and great mechanical strength. Upon proper laser treatment, it turns into a monolithic supercapacitor offering high energy and power density, carrying 22


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great promises as low-cost, light-weight, disposable and flexible electronic textiles.

ASSOCIATED CONTENT

Supporting Information.

Photographs and results of contact angle measurement; Tensile strength of PP nonwoven after ozone treatment; Photographs and schemes of pendant drop measurement.

AUTHOR INFORMATION

Corresponding Author

*E-mail: (B. P.) [email protected] *E-mail: (W. G.) [email protected];

Funding Sources

Funding from project C-15-180, the Nonwoven Institute, NC State University

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ACKNOWLEDGMENT

The authors acknowledge the funding support from the Nonwoven Institute (North Carolina State University) and the technical support from the Nonwovens Institute Pilot Facility for the melting blowing process.

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Highly Conductive PolypropyleneâGraphene Nonwoven Composite

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