Nylon-Graphene Composite Nonwovens as Monolithic Conductive or

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Nylon-Graphene Composite Nonwovens as Monolithic Conductive or Capacitive Fabrics Qin Pan, Eunkyoung Shim, Behnam Pourdeyhimi, and Wei Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00471 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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ACS Applied Materials & Interfaces

Nylon-Graphene Composite Nonwovens as Monolithic Conductive or Capacitive Fabrics Qin Pan†‡, Eunkyoung Shim†‡, Behnam Pourdeyhimi†‡*, and Wei Gao†‡*

† The Nonwovens Institute, College of Textiles, North Carolina State University, Raleigh, North Carolina 27695, 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, nylon-6, conductive polymer composites, monolithic supercapacitors

ABSTRACT

Here we describe a nylon-graphene nonwoven (NGN) composite, prepared via melt-blowing of nylon-6 into nonwoven fabrics and infiltrate those with graphene oxide (GO) in aqueous dispersions, which were further chemically reduced into graphene to offer electrical conductivity. The correlation between the conductivity and the graphene loading is described 1 ACS Paragon Plus Environment


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by the percolation scaling law σ = ( − ) , with an exponent of 1.2 and a critical concentration of 0.005 wt%, the lowest among all the nylon composites reported. Monolithic supercapacitors have been further developed on the nylon-GO nonwoven composites (NGO), via a programed CO2-laser patterning process. The nylon nonwoven works as an efficient matrix, providing high capacity to GO and ensuring enough electrode materials generated via the subsequent laser patterning processes. Our best monolithic supercapacitors exhibited an areal capacitance of 10.37 mF cm-2 in PVA-H2SO4 electrolyte, much higher than the 1~3 mF cm-2 reported for typical microsupercapacitors. Moreover, our supercapacitors were able to retain a capacitance density of 5.07 mF cm-2 at an ultrahigh scan rate (1 V s-1), probably due to the facilitated ion migration within the highly porous nonwoven framework. This is the first report of highly functional nylon-6 nonwovens, fabricated via industrially scalable pathways into low-cost conductive polymer matrices and disposable energy storage systems.

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Introduction

Conductive polymeric fabrics, which retains the original features of polymers such as lightweight, flexible, tough and easy to process, can find versatile applications such as embedded antennas, antistatic fabrics, electromagnetic shielding, wearable electronics etc.1-4 Such conductive polymers can be fabricated via the incorporation of additive materials into polymeric matrices. For example, graphene has been shown as an additive for relevant systems such as polystyrene, epoxy and polyurethane.5-10 At certain critical concentration (percolation threshold), graphene can form an electron-transport pathway, resulting in the transition from insulators to electrical conductors.11 So far, the lowest threshold reported for graphene-polymer composites is 0.0174 wt.% by Wang et al.

10

However, most polymeric

matrices involved so far are bulk polymer films, electrospun nets, woven or knitted polymeric fabrics, etc. Nonwoven fabrics, on the other hand, have rarely been reported as the basic host of conductive additives. Nonwoven systems, usually as 3D porous matrices made from molten polymers or entangled fibers, have notable advantages over traditional woven or knitted fabrics as the matrices of conductive textiles, due to their better absorbency of additives, higher porosity, more stretchiness and lower cost.12-13 Melt-blowing and spun bounding, both as novel and economic techniques recently developed to mass produce nonwoven fabrics, have received enormous attention in the textile industry in the past decade.13 Polyamide 6, commonly known as nylon-6, is widely used in apparel industry due to its good processability, hydrophilicity and excellent mechanical strength. It has been chosen as 3 ACS Paragon Plus Environment


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our nonwoven material, mainly due to its wide industrial relevance and high affinity with GO.14-19 So far, most of conductive nylon-6/graphene composites reported were prepared via in-situ polymerization or solution blending. For instance, Zheng et al. polymerized ε-caprolactam monomer in the presence of GO, followed by thermal reduction of GO within the composite. They obtained a low percolation threshold of ~ 0.75 wt.% and a high electrical conductivity of ∼ 0.028 S m-1 at ~ 2.97 wt.% of GO.14 Pant et al. blended nylon-6 with GO in the solution of CH3COOH/HCOOH, and employed the electrospinning and hydrothermal treatment to obtain nylon-6/graphene spider-wave-like nano-nets. However, their highest electrical conductivity was only 125 mS m-1 at the graphene loading of 2.2 wt%.15 Compared with these complicated recipes, engineering nylon-nonwoven structures and infiltrating them with GO in aqueous dispersion turns out to be a much simpler process, which is also more efficient and compatible with industrial practices.20-23 Energy-storage textiles, mainly in forms of wearable supercapacitors and batteries, can not only provide power for wearable electronics, but also store electrical energy harvested by a thermoelectric, a piezoelectric or a solar panel.24-25 For the later cases, an aqueous/gel-based supercapacitor (also named as electrochemical double-layer capacitor (EDLC)) on a wearable substrate is favoured over batteries, since fast and multiple charge/discharge cycles, low toxicity, structural integrity, and fire safety are all indispensable in those applications.21, 26 Despite plenty of research on wearable energy storage systems, the nonwoven-based devices have been rarely reported.27-28 Here we carefully engineer our nonwoven matrices, which have large porosity and structural integrity to allow enough mass loading of active materials, 4 ACS Paragon Plus Environment

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to facilitate the migration of electrolyte ions, and to retain the flexible, stretchable, and tough features of the pristine fabrics, offering integrated wearable supercapacitors with high energy and power densities at a relatively low cost.23 Hu et al.20, Pasta et al.29, and Jost et al.26 have reported woven-fabric-based supercapacitors with high performance and easy fabrication process, but still with several unresolved problems, such as the uncomfortable thicknesses, sealing of electrolytes, and biosafety. Different from these existing devices, progresses in micro-fabrication technologies have enabled the integration of electrodes, current collectors and electrolyte into a monolithic device, which minimizes the energy storage units and allows the shaping of whole system with digital designs for on-chip integrations.30-34 All-solid-state micro-supercapacitors have been reported via patterning of GO on PET films with a standard LightScribe DVD burner or a commercialized CO2-laser cutter.33-34 Their planar micro-supercapacitors achieved an area capacitance of 2.32 mF cm-2 at a current density of 16.8 mA cm-3 in solid-state electrolyte PVA-H2SO4. However, the PET-supported systems are incompatible with wearable applications, mainly due to the lack of wearability and stretchability. In this paper, we report a conductive nylon-6 nonwoven fabricated via melt-blowing nylon-6 into nonwoven films and dip-coating of the nonwoven matrices with GO dispersions in water. After chemical reduction, the NGN composite showed an ultra-low threshold of 0.005 wt% and a high electrical conductivity of 9.2 S m-1 at the graphene loading of 6.5 wt%. Direct laser writing technique was employed to fabricate all-solid-state supercapacitors on nylon-GO nonwovens (NGO) in both concentric-circular (CC) and sandwich geometries. 5 ACS Paragon Plus Environment


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The sandwich supercapacitor shows a superior areal capacitance of 10.37 mF cm-2 at the scan rate of 10 mV s-1 in PVA-H2SO4 electrolyte, which is the highest among all the micro-EDLCs reported in literature.32-35 It retains 50% of the capacitance density when the scan rate rises to 1 V s-1, which is still higher than the typical areal capacitance values between 1~3 mF cm-2 for other micro-capacitors at much lower scan rates.32-34 Due to its high capacitance density and low equivalent series resistance (ESR), our NGO sandwich supercapacitor offers an areal energy density of 1.44 µWh cm-2 and power density of 78.37 mW cm-2, the highest among the similar carbon-based wearable supercapacitors.32-33, 36-37 Compared with other functional textiles, our NGN fabric exhibits excellent electrical conductivity and energy-storage capabilities, holding great promises as portable/wearable power systems for a variety of applications.

Results and Discussion

Synthesis and Characterization of Composite Nonwoven. The nylon-6 nonwoven fabrics were prepared via the melt-blowing method at air-flow rates of 500, 1000 and 1500 m3 h-1, respectively. With the increasing air-flow rates, the average fiber diameter decreases from 10.5 µm, to 6.3 µm and eventually to 3.2 µm (see Figure S1 in supporting information), since the faster air flow applies higher shear force to the fibers during its flight from the spinneret to the collector. Finer fibers, due to their higher specific surface area, have been proven to take up more GO under the same conditions (See Figure S2). Therefore, nylon


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nonwovens with the finest fibers (average fiber diameter of 3.2 µm) were used for all the following experiments. Scheme 1 illustrates the dip-coating process used to fabricate NGO composites and the two subsequent derivations into conductive textiles and monolithic energy-storage devices. Since nylon is hydrophilic, the hydrogen bonding between nylon fibers and GO flakes ensures the good affinity between them. The mass loading of GO can be controlled through the number of coating cycles. After chemical reduction with 50 wt.% hydrazine, the NGN with different loadings of reduced graphene oxide (rGO) can be obtained.

Scheme 1 The fabrication process of NGN for conductive and capacitive fabrics: a) dip coating of nylon-6 nonwoven fabric into 0.5 wt.% of GO dispersion in DI water to obtain NGO; b) chemically reduce NGO by 50 wt.% N2H4; (c) direct laser writing to fabricate NGO supercapacitors with concentric circular (CC) and sandwich geometry; (d) photograph



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of the all-solid-state NGO supercapacitor with PVA-H2SO4 gel electrolyte and current collectors.

Figure 1 shows the morphology of NGN with increasing rGO loadings from 0.1 wt.% to 6.5 wt.%. Pure nylon fibers exhibit quite smooth surfaces, with obvious entanglement and localized alignment between individual fibers (see Figure S1). This is one of the key characteristics of nonwoven fabrics, which enhances the capillary flow of GO dispersion through the inter-fiber spaces during dip coating, also known as the wicking ability, and thus improves the coating effectiveness.38 When the rGO loading is as low as 0.1 wt.%, the rGO flakes form ultra-thin layers covering the surfaces of nylon fibers as shown in Figure 1 (a). These thin layers turn out to be larger sheets that either wrap around the buddle of nylon fibers or spread out in the gaps between individual fibers, when the loading rises to 1.3 and 2.8 wt.% shown in Figure 1 (b) and (c). At the highest loading of 6.5 wt.%, the rGO itself forms a continuous network inside the matrix and the fibers penetrate through the network of rGO as shown in Figure 1 (d). The loading-dependent morphology in NGN samples indicates the formation of different conductive pathways inside the nonwoven fabric, which significantly influence the electrical conductivity of the NGN.


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Figure 1 SEM images of NGN with different rGO mass loading of 0.1 wt.% (a), 1.3 wt.% (b), 2.8 wt.% (c) and 6.5 wt.% (d).

Electrical Conductivity of NGN. Previous studies have proved that chemically-reduced GO shows very high electrical conductivity, indicating its potential as an effective additive to impart conductivity.39-40 Figure 2 (a) shows the increase in the conductivity of NGN as a function of rGO weight fraction. The electrical conductivity was measured by a four-point-probe method (see Figure S3). Our NGN sample offers the conductivity of 0.03 9 ACS Paragon Plus Environment


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S m-1 at the low loading of 0.06 wt.%, which exceeds the required value (10 S m-1)

for

antistatic applications of thin films.41 This also indicates that an infinite network of connected pathways of rGO starts forming at an ultra-low threshold below 0.06 wt.%. The electrical conductivity of the composite can be analyzed with the critical concentration of the conductive filler by the following scaling law:11 σ = ( − )

Equation (1)

where is the weight fraction of rGO in our case. The exponent in Equation (1), also known as the universal critical exponent, generally depends on the dimensionality of the conductive fillers, with typical values ranging from ca. 1 to 1.3 for two-dimensional (2-D) systems and 1.6 to 2 for three-dimensional (3-D) systems.42 The inset figure shows the logarithmic plot of the electrical conductivity as a function of − in weight fraction according to Equation (1), with a good linear fit ( = 0.99) depicted. Based on this analysis, the percolation threshold of the NGN composite occurs when the weight fraction of rGO is ca. 0.005 wt.%. This percolation threshold is around two magnitude lower than those reported for the similarly graphene-coated textiles such as graphene on polyamide 66 electrospun web (0.1 wt.%) and cotton fabrics (0.8 wt.%).43-44 To the best of our knowledge, it is also the lowest among all the conductive nylon systems reported so far.19 Such a low percolation threshold is probably due to the unique structure of the NGN fabric. Firstly, the nylon nonwoven, which consists of randomly-oriented and self-connected fibers, forms a skeleton for the future electrically conductive pathways. We estimated the weight fraction of rGO required to cover all the fiber surfaces with a continuous monolayer of 10 ACS Paragon Plus Environment

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graphene, which turned out to be 0.024 wt.%, higher than the 0.005 wt.% percolation threshold we observed experimentally. This is totally reasonable since conductivity should be achieved before all the surfaces are wrapped with graphene-like structures. In other words, as long as the conductive rGO sheets form certain continuous pathway in the skeleton, an electron-conducting route is generated. Thus, our pre-constructed nylon skeleton provides a more efficient way to utilize the conductive 2D additives, as compared to the directly dispersion of them into a polymer matrix.10 Secondly, the effective chemical reduction via hydrazine is key to obtain high conductivity. Thermal treatment can also remove the oxygenated groups of GO and convert it to conductive rGO, but usually gives an inferior electrical conductivity.45 Thirdly, the interconnected skeleton of nylon nonwovens with abundant internal pores allows the coated GO sheets to be reduced more effectively, due to the full exposure to the reducing reagent.10 As a result, the rGO sheets, with the excellent electrical conductivity and high aspect ratio, provide the fabric with quite high electrical conductivity at an ultra-low threshold. The analysis also gives a scaling exponent t of ca. 1.2, identical with the previously reported value for graphene, which also implies the formation of 2-D conductive networks.42 Above the threshold, the electrical conductivity rapidly rises with the increase of rGO loadings, which yields the value of 0.07 S m-1 at 0.1 wt.%, 1.1 S m-1 at 1.3 wt.%, 2.6 S m-1 at 2.8 wt.%, and 9.2 S m-1 at 6.5 wt.%.



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Figure 2 (a) Electrical conductivity of NGN composites as a function of weight percentage of rGO loadings (wt.%). The inset shows a logarithm plot of the conductivity as a function of − , with an exponent t of 1.2 and a critical weight percentage (threshold) of 0.005 wt.%. (b) Comparison of the electrical conductivity of NGN before and after the wash in DI water with stirring for 10 minutes at 25°C. Data of NGN with different rGO loadings of 0.1 wt.%, 1.3 wt.%, 2.8 wt.% and 6.5 wt.% are presented.

The affinity between the coated rGO and nylon-6 nonwoven fabric is of great importance, especially for the potential application in wearable electronics. We characterized the affinity by the wash fastness tests according to a standard procedure depicted in ISO 105C-106.46 Figure 2 (b) compares the electrical conductivity of the NGN with different rGO loadings at 0.1 wt.%, 1.3 wt.%, 2.8 wt.% and 6.5 wt.%, respectively, before and after the wash in 400 mL of DI water with stirring for 10 mins at 25 °C. For the medium level of rGO loading (1.3 wt.% and 2.8 wt.%), the weight loss of rGO sheets during washing is suppressed due to the hydrophobic nature of rGO and the tight contact between rGO/rGO 12 ACS Paragon Plus Environment

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sheets and rGO/nylon fibers. As a result, the minimum weight loss rarely influences the conductivity of the composite. However, when the rGO loading goes higher to 6.5 wt.% and forms a continuous network, the rGO sheets at the outer layer are loosely stacked and easier to fall off during washing, which will compromise the rGO network and lower the conductivity. For the fabrics with low loading of rGO (0.1 wt.%), every rGO sheet counts for the conductive pathway, so even minimal rGO loss will result in the obvious decrease in conductivity. The SEM images of the four NGN samples after washing show slightly morphology change that some rGO sheets wrinkle up due to turbulent water flow (see Figure S4). The wash fastness of the composite can be improved by crosslinking the polymer and rGO with some chemical reagents, e.g. glutaraldehyde, which is still under our investigation. In summary, the combination of dip-coating with GO and chemical reduction via hydrazine gives our NGN an ultra-low threshold for conductivity, while higher conductivity and good wash fastness were also demonstrated at higher loadings, offering great promises to be used as conductive textiles. Fabrication and Characterization of Monolithic Supercapacitors on NGO. In addition to conductive fabrics, the NGO has also been directly fabricated into free-standing supercapacitors via a laser-patterning process. In our study, a CO2-laser patterning machine (Epilog Laser Mini, 18 inch × 12 inch, 40 W) was used to treat the GO layers on nylon nonwoven surfaces, converting GO into laser-scribed GO (LGO), which is electrically conductive and porous. As compared to the hydrazine reduction previously used for conductive NGNs, the laser treatment actually induces the chemical decomposition of GO, 13 ACS Paragon Plus Environment


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and the patterning process is more controllable in shape, dimension and design, thanks to the localized heating capability of the laser beam. In addition, it could allow patterning of complex built-in circuitry involving various elements such as capacitor, inductance, resistance, etc. to control desired charge/discharge properties of the functional textiles. Figure 3 (a) and (b) show two different supercapacitors with concentric-circular (CC) and sandwich geometries, respectively. The LGO, shown as the dark black parts, acts as the active electrode material, while the NGO acts as the matrix and the separator. In sandwich supercapacitors, LGO layers exist on both sides of the fabric. Electrolyte can be directly casted onto its surface and gets rapidly absorbed into the matrix due to good hydrophilicity and wicking ability of the nylon nonwoven. Figure 3 (c) shows the interface between an LGO electrode and the original GO layer on the top surface of a CC supercapacitor. LGO is shown to be fluffier and highly porous, which enhances its contact with the electrolyte. Figure 3 (d) shows the cross-sectional image of the sandwich supercapacitor, where the interface between the NGO matrix and the LGO electrode at one side is highlighted by the red dashed line. LGO electrodes with sufficient thickness is key to the high energy-storage capacity. Its thickness was estimated to be ca. 32 μm for sandwich supercapacitors at each side, while ca. 58 μm for the CC ones (see Figure S5 (b)). The reason for thinner LGO layer in sandwich supercapacitors is that a lower power was applied during the laser-patterning process to prevent short-circuiting. The thicker LGO electrodes in our systems than other laser-scribed GO supercapacitors33 are attributed to: 1) the repeated laser writing process induced more GO on the top surface of NGO decomposed sufficiently; 2) absorbent and porous nylon 14 ACS Paragon Plus Environment

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nonwoven can achieve a high mass loading of ca. 50 wt.% and ensure a thick enough GO substrate. The GO sheets absorbed onto nylon fibers mainly due to Van der Waals' force and hydrogen bonding.47 With more coating cycles, the GO sheets not only exist on the fiber surfaces and in the inter-fiber spaces, but also self-assemble and stack on the top surface of the fabric via hydrogen bonding, forming a relatively thick layer of ca. 15 µm (see Figure S5 (a)) at a high loading of ca. 50 wt.%.48 This outer-layer GO serves as the precursor for laser treatment, after which the LGO layer turns out much darker and fluffier than the original GO, indicating GO decomposition occurs and the specific surface area increases. Due to such unique LGO electrodes, the CC and sandwich supercapacitors exhibited excellent performances.



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Figure 3 Morphologies of NGO supercapacitors via direct laser patterning: photographs of (a) CC supercapacitor, with darker part as the LGO electrodes and grayish part as the NGO separator and matrix, and (b) sandwich supercapacitor with LGO electrodes at both side of NGO; SEM images of (c) the interface between LGO and GO on the surface of CC supercapacitor, and (d) the cross-sectional image of a sandwich NGO-based supercapacitor.

Electrochemical Performance of NGO supercapacitors. Figure 4 compares the electrochemical performance of the two supercapacitors with different geometries in two different electrolytes. We choose an aqueous elelctrolyte of 1 M Na2SO4 since neutral 16 ACS Paragon Plus Environment

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electrolyte is more suitable for wearble elelctronics, and compare with a gel electrolyte of 1 M H2SO4 in 10 wt.% PVA, which is widely-used to fabricate all-solid energy storage devices. The cyclic voltammograms (CV) curves at a scan rate of 200 mV s-1 are shown in Figure 4 (a), which are nearly rectangle, indicating good ionic diffusion within the electrodes even at high scan rate. For both CC and sandwich structures, higher areal capacitance in the solid electrolyte of PVA-H2SO4 were obtained, mainly due to the faster diffusion of protons with smaller size than that of Na+ ions. Here the areal capacitance was calculated based on the area of the whole device, including the total area of two electrodes and the gap in-between, since the areal capacitance aligns better with the true performance of a supercapacitor, especially in the case of micro-devices. The specific capacitance also depends on the geometry. The sandwich structure gives much higher specific capacitance than the CC structure, due to its stacking configuration. Among the four cases, the sandwich supercapacitor in PVA-H2SO4 electrolyte offers the highest areal capacitance of 7.16 mF cm-2, more than twice of that reported in literature for the laser-scribed graphene micro-supercapacitors at much lower scan rate.32-33 CC supercapacitors also shows high areal capacitance of 3.85 mF cm-2 in PVA-H2SO4 electrolyte and 2.97 mF cm-2 in Na2SO4 electrolyte, respectively. These values were obtained at the scan rate of 200 mV s-1, which means it takes only 5 s to be charged to 1 V.



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Figure 4 Electrochemical performance of the CC and sandwich supercapacitors in aqueous electrolyte (1 M Na2SO4) and solid electrolyte (1 M H2SO4 in 10 wt.% PVA). (a) cyclic voltammograms (CV) curves at the scan rate of 200 mV s-1; (b) Nyquist plots from 1 M Hz to 0.01 Hz; inset shows the zoom-in Nyquist plots at the high frequency domain for different devices in different electrolytes.

The electrochemical interfaces within the four supercapacitors are also analyzed via electrochemical impedance spectroscopy (EIS). Figure 4 (b) shows their Nyquist plots with the imaginary components (−Z’’) against the real component (Z’) of the impedance. Each data point was obtained at a specific frequency of the AC signal, with the highest frequency of 1 MHz to the lowest frequency of 0.01 Hz. The inset is the zoom-in figure at the leftmost part of the EIS spectrum at high frequencies. Each EIS spectrum consists of three parts: 1) a depressed semicircle, though not obvious in all the spectra, indicating the faradic charge transfer processes that typically refer to the redox reactions at the carbon-electrode surfaces; 2) a short Warburg region reflecting inhomogeneous ionic diffusion pathways; 3) a tilted


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straight line corresponding to the double-layer capacitor behavior at lower frequencies.49 An equivalent circuit (see Figure S6) is used to fit each EIS spectrum and the quantitative analysis results of each supercapacitor are shown in Table 1. The equivalent circuit consists of four parts: 1) a series resistance (R ( ), which is the intercept of the curve with the x-axis, comprised of the resistance from electrolyte, electrodes and the external circuits; 2) a charge transfer resistance (R ), which comes from the resistance at the electrode/electrolyte interfaces; 3) a generalized finite length Warburg (GFLW) element-open circuit terminus (W* ), referring to the non-uniform diffusion and reflective boundary;50 4) two constant phase elements, the CPE and the CPE.// , corresponding to the charge transfer capacitance and the double-layer capacitance at the electrode/electrolyte interfaces, respectively. The W* element is described by Equation (2):

012 = 1 ×

*3(4×5×67 )8 (4×5×67 )8

Equation (2)

Where 1 is the Warburg resistance, representing the frequency dependence of ion diffusion in the electrolyte, 91 is the Warburg time constant, and the exponent α ranges between 0~1 in real case.51

Each CPE is described by the Equation (3):

0:;< =

=

Equation (3)

>? (4×5)8

Where @* is the capacitance.



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Table 1. Fitting Results for Nyquist Plot in Figure 4 (b) with Equivalent Circuit in Figure S6 CPEct Fitting Parameter

Ω Rs/Ω

Wo

CPEdll

Rct/Ω Ω Y0/mF

α

Rw/Ω

Tw/s

α

Y0/F

α

CC

PVA-H2SO4

40.46

5.10

0.00022

0.69

43.72

0.03

0.57

0.001

0.84

Structure

1 M Na2SO4 (aq)

139.90

17.05

0.00017

0.75

384.30

0.76

0.38

0.0003

0.90

Sandwich

PVA-H2SO4

3.19

0.57

0.0038

0.70

5.46

0.20

0.73

0.0024

0.86

Structure

1 M Na2SO4 (aq)

7.68

0.61

0.0099

0.52

8.93

2.90

0.43

0.0016

0.89

As shown in Table 1, the sandwich supercapacitor shows much lower ( than CC supercapacitor in both electrolytes. This is mainly due to shorter ionic diffusion distance in the sandwich structure, which is even less than the thickness of the fabric (0.2 mm), much shorter than that in the CC case (ca. 1 mm). For both of them, ( is lower when working in the solid electrolyte PVA-H2SO4 than the aqueous electrolyte Na2SO4, probably due to two reasons: 1) H+ with smaller size than Na+ can migrate faster leading to lower ionic resistance; 2) the inferior contact between the hydrophobic LGO electrode and the aqueous electrolyte of Na2SO4 versus that in the case of the sticky gel electrolyte PVA-H2SO4. ABC.// is revealed by the rightmost straight line at low-frequency domain in the EIS spectra. The exponent α is given to be 1 in the ideal double-layer supercapacitors. However, due to the roughness and porosity of the LGO layers after the laser treatment, the straight line is a little tilted with α smaller than 1.52 In the Warburg region, sandwich structure shows much lower 5 than that 20 ACS Paragon Plus Environment

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of CC, indicating much more uniform pathways for ion diffusion. In addition, 5 is also lower in the gel electrolyte of PVA-H2SO4 than the aqueous electrolyte of Na2SO4 for both supercapacitors. This is probably because protons (H+) with smaller sizes can access more conducting pathways and transport much faster via the Grotthuss (hopping) mechanism than Na+ ions in water which can only transport together with water molecules via the vehicle mechanism.53 In Figure 5 we characterize the kinetic analysis of the sandwich supercapacitor. The CV curves of a typical sandwich supercapacitor working in PVA-H2SO4 at different scan rates from 10 mV s-1 to 1000 mV s-1 were overlapped in Figure 5 (a). Unlike most supercapacitors, the shape of CV curves retains the rectangular shape even when the scan rate reaches 400 mV s-1, indicating the excellent ionic diffusion within the matrix. In Figure 5 (b), the specific capacitance is plotted versus the inverse square root of the scan rate. Common supercapacitors usually show a linear decrease of capacitance in the region 2 starting from 20 mV s-1, due to the diffusion-control processes.54 However, for the sandwich supercapacitor, it indicated a linear trend starting from ~ 60 mV s-1, corresponding to the good rate capability observed before. At the scan rate lower than 60 mV s-1 (region 1), the capacitance is almost independent of the scan rate, indicating that the electrolyte diffusion is not the limiting factor for charge storage within this time domain. The extrapolated y-intercept yields the infinite sweep-rate capacitance. The capacitance started to deviate far from it when the scan rate is higher than 60 mV s-1. It reaches a highest areal capacitance of



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10.37 mF cm-2 at the scan rate of 10 mV s-1, which is higher than all the electrochemical double-layer micro-supercapacitors reported, as far as we know.

Figure 5 Kinetic analysis of the sandwich supercapacitor working in PVA-H2SO4. (a) overlapped CV curves at various scan rates from 10 to 1000 mV s-1. (b) a plot of areal capacitance vs. scan rate-1/2. The separation of diffusion-controlled region and capacitive-controlled region occurs at ~ 60 mV s-1, indicating excellent ion diffusion in our supercapacitors.

Figure 6 (a) shows the areal capacitance of our CC and sandwich supercapacitors in aqueous and solid-state electrolyte compared with the reported laser-scribed graphene supercapacitors. Our supercapacitors have exhibited 2~4 times higher areal capacitance than those reported in literature.33 The major reasons for this enhancement include: 1) the absorptive nature of nylon nonwoven leading to the high loading of GO, laying foundation for the thicker LGO electrodes; 2) the repeated scanning of the laser beam allowing the


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surface GO layers to decompose more sufficiently; 3) the porous structure of NGO improving the contact between LGO and electrolyte. The Ragone plot of their areal energy density vs. power density based on the total area of the device was shown in Figure 6 (b). The sandwich supercapacitor in PVA-H2SO4 electrolyte offers both the highest areal energy density of 1.44 µWh cm-2 and power density of 78.37 mW cm-2 due to its high capacitance and low ESR. This monolithic supercapacitor demonstrates superior areal energy and power density as compared with existing carbon-based textile supercapacitors in literature,32-33, 36-37 showing great potential to be used in wearable energy storage systems.

Figure 6 (a) comparison of areal capacitance density between our supercapacitors and reported laser-scribed rGO supercapacitors33. (b) Ragone plot of areal energy density versus power density of our supercapacitors, as compared with similar systems reported (data from ref [32-33, 36-37]). * denotes the data converted from the reported values of volumetric density in the corresponding references, based on the reported thickness. Reproduced with permission from ref. 32. Copyright 2011 Nature Publishing Group. Reproduced with 23 ACS Paragon Plus Environment


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permission from ref. 33. Copyright 2013 Nature Publishing Group. Reproduced with permission from ref. 36. Copyright 2014 Royal Society of Chemistry. Reproduced with permission from ref. 37. Copyright 2013 John Wiley and Sons.

Conclusion

In summary, a simple dip-coating recipe was used to fabricate NGO composites, which have been further developed into conductive fabrics and monolithic textile supercapacitors. Hydrazine reduction can effectively reduce GO and offer high electrical conductivity in the final NGN composite. An ultra-low conductive threshold of 0.005 wt.% was observed, mainly due to high effectiveness of rGO coating that forms the conductive pathways. At relatively high loadings of rGO, these 2D sheets form a continuous network inside the nylon nonwoven, enhancing the electrical conductivity significantly. At the rGO loading of 6.5 wt.%, the conductivity reaches ~ 10 S m-1, much higher than the reported electrical conductivity of similar nylon systems. The NGN can retain its original conductivity after standard washing cycles, offering the potential applications in smart garment. On the other hand, the NGO composites are converted into textile supercapacitors in different geometries via a direct laser-patterning process. Both CC and sandwich supercapacitors offer superior areal capacitance density over that of the existing systems. The sandwich supercapacitor gives the highest areal capacitance of 10.37 mF cm-2 at the scan rate of 10 mV s-1 in PVA-H2SO4 electrolyte, higher than all the reported micro-EDLCs working in aqueous or 24 ACS Paragon Plus Environment

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gelled electrolyte. It can retain ca. 50% of that capacitance when the scan rate reaches 1 V s-1, showing excellent rate capability. Due to the high capacitance density and low ESR, it offers an areal energy density of 1.44 µWh cm-2 and power density of 78.37 mW cm-2, higher than the similar micro-EDLCs based on carbon/polymer composites. As conductive fabrics or monolithic textile supercapacitors with superior performance, our nylon-graphene nonwoven composites do carry great promises for applications in the wearable electronics.

Experimental Section

Preparation of nylon-6 nonwoven fabric: Nylon-6, Ultramid® B27E (supplied by BASF), was made into nonwoven fabrics by melting-blow method at the Nonwovens Institute Pilot Facility (North Carolina State University Raleigh, NC). Fabrics with different fiber diameters were prepared by setting constant die-to-collector distance of 200 mm and throughput of 0.3 ghm (gram per hole per minute) but different air flow rates of 500, 1000 and 1500 m3 h-1. Preparation of NGO: GO dispersion in deionized (DI) water was prepared by adding a certain amount of GO (0.5 g, 1.25 g, and 2.5 g) into 500 mL deionized (DI) water with vigorous sonication for 1 h until homogenous dispersion was formed. GO was prepared by modified Hummer’s method as reported in the literature.55 Graphite was purchased (Micro 850) from Asbury Graphite Mills Inc., Kittanning, PA. Nylon-6 nonwovens, typically cut into 2 inch × 2 inch, were dipped into the as-prepared GO dispersions under stirring for 5 mins 25 ACS Paragon Plus Environment


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at 25 °C, and taken out for drying in the oven of 70 °C. This process will be repeated for several times until obtaining the target loading of GO. Preparation of NGN: NGN was prepared by soaking NGO in the 50 wt.% hydrazine aqueous solution at 25 °C overnight. After the chemical reduction, the as-prepared NGN was washed in DI water extensively and dried in the oven of 70 °C. Preparation of NGO supercapacitors: the as-prepared NGO was laser reduced by Epilog Laser Mini (18 inch × 12 inch, 40 W) with power of 4 W and scanning speed of 15% repeated four times in CC geometries, and power of 3.2 W and scanning speed of 15% repeated twice on each side in sandwich geometries. Several drops of the aqueous electrolyte (1 M Na2SO4) or solid-state electrolyte (1 M H2SO4 in 10 wt.% of PVA aqueous solution) was directly dropped onto its surface. Gold-coated vinyl films were used as current collectors. Materials characterization: scanning electron microscopy (SEM) pictures were collected with a Phenom ProX desktop SEM. Samples were sputter coated with palladium for 45 seconds before use. Electric 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. Electrochemical characterization: Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) measurements were used to characterize the performance of the supercapacitors using an Autolab workstation (Metrohm, USA). CV was recorded at different scan rates of 10 − 1000 mV s-1 in the window of 0 − 1 V. EIS were measured in 26 ACS Paragon Plus Environment

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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 A.EFGE (F) =

G (.I ⁄.)

, where i refers to the

mean current in CV curves and dV/dt is the scan rate. The specific areal capacitance (A( ) is the capacitance per unit area of the whole device, AN = A.EFGE /O. For CC structure, S equals the area of the outer circle of 0.45 cm2; while for sandwich structure, S equals 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 =

:Q ×R< S ×TUU

, in which CN is the areal capacitance obtained

from A.EFGE ⁄O . The maximum areal power density was calculated by BVWX =

R< S Y×

Nylon-Graphene Composite Nonwovens as Monolithic Conductive or

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