Activated Carbon Textile via Chemistry of Metal Extraction for

Nov 12, 2016 - Particularly, the combined effects of mechanical buckling and fracture that occurred between ZnO and cotton turned out to play an impor...
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Activated Carbon Textile via Chemistry of Metal Extraction for Supercapacitors Do Van Lam,†,‡ Kyungmin Jo,§ Chang-Hyun Kim,†,‡ Jae-Hyun Kim,†,‡ Hak-Joo Lee,†,‡ and Seung-Mo Lee*,†,‡ †

Department of Nanomechanics, Korea Institute of Machinery and Materials (KIMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, South Korea ‡ Nano Mechatronics, Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, South Korea § Center for Inorganic Analysis, Division of Metrology for Quality of Life, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, South Korea S Supporting Information *

ABSTRACT: Carbothermic reduction in the chemistry of metal extraction (MO(s) + C(s) → M(s) + CO(g)) using carbon as a sacrificial agent has been used to smelt metals from diverse oxide ores since ancient times. Here, we paid attention to another aspect of the carbothermic reduction to prepare an activated carbon textile for high-rate-performance supercapacitors. On the basis of thermodynamic reducibility of metal oxides reported by Ellingham, we employed not carbon, but metal oxide as a sacrificial agent in order to prepare an activated carbon textile. We conformally coated ZnO on a bare cotton textile using atomic layer deposition, followed by pyrolysis at high temperature (C(s) + ZnO(s) → C′(s) + Zn(g) + CO(g)). We figured out that it leads to concurrent carbonization and activation in a chemical as well as mechanical way. Particularly, the combined effects of mechanical buckling and fracture that occurred between ZnO and cotton turned out to play an important role in carbonizing and activating the cotton textile, thereby significantly increasing surface area (nearly 10 times) compared with the cotton textile prepared without ZnO. The carbon textiles prepared by carbothermic reduction showed impressive combination properties of high power and energy densities (over 20-fold increase) together with high cyclic stability. KEYWORDS: carbothermic reduction, mechanical buckling, opening mode fracture, atomic layer deposition, ALD, supercapacitor, energy storage power density and good cyclability.1,2 Unlike lithium-ion batteries, a supercapacitor has relatively low energy density, which limits its wide applications. Carbon materials, among widely used electrode materials in supercapacitors, usually possess higher power density and longer lifespan but lower energy density, while transition metal oxides and conductive polymers possess higher energy density but poorer cyclic life

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nergy is lifeblood constantly circulating through the arteries of our modern civilization. Seeking sustainable and clean alternative energy sources has become one of the most urgent scientific and technological challenges nowadays due to the threat of depleting the planet’s fossil fuel. However, it seems that the challenge is not energy sources per se but devices with high performance to store and deliver efficiently the volatile energy where required. Supercapacitors, one of emerging energy storage devices, have been regarded as a promising device for next-generation electronics and electric vehicles because of their excellent properties, such as high © 2016 American Chemical Society

Received: September 30, 2016 Accepted: November 12, 2016 Published: November 12, 2016 11351

DOI: 10.1021/acsnano.6b06608 ACS Nano 2016, 10, 11351−11359

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Figure 1. Fabrication of activated carbon textile via carbothermic reduction. (a) Schematic illustration showing preparation procedure of activating carbon out of cotton via carbothermic reduction. (b) TGA analysis results of raw CT and ZnO/cotton textile with different ALD cycles (different ZnO thicknesses). “a250CT” denotes a cotton textile on which ZnO ALD of 250 cycles is applied. In our ALD setup, the growth rate of ZnO ALD was ∼1.6 Å/cycle, which means that ZnO with a thickness of 160 nm was able to be deposited by 1000 ALD cycles. In the profile, three distinct phase changes were observed. See the text for details. (c) EDX elemental analysis data. It was shown that reaction byproducts such as Zn are completely evaporated at 1000 °C.

and lower power density.3−5 Beyond doubt, developing a method to increase active surface area of the electrode materials is a key sector to improve the electrochemical performance of supercapacitors. Meanwhile, the current boom of wearable energy storage has brought about keen competition for developing high-performance electrode materials suitable for integration into wearable electronics.6−12 Supercapacitors using carbon nanotubes or graphene-based electrodes have emerged to possess high power density and mechanical robustness.12−18 However, their expensive production cost, complex preparation, and relatively low energy density seem daunting for commercialization. A textile-based electrode has been recently regarded to be one of the promising alternatives, which is prepared by either coating with the functional materials, modifying the textile chemistry, electrospinning, or carbonizing under high temperature.18−26 Although carbonization enables conductive and porous materials for electrochemical capacitors, proper structural engineering for high porosities still remains challenging. Several studies have shown that porosities in carbonized fibers can be tuned by chemical activation using various etchants such as KOH, H3PO4, NH3, and ZnCl2.27−32 However, it shows multistepwise procedures mainly caused by the used etchant. Besides, the as-prepared electrode requires the use of polymer binder and additive, which inevitably sacrifices overall energy storage capacity and greatly reduces the electrical conductivity of the electrode. Hence, it is of significant importance to develop a facile etchant-free activation method to produce an electrode possessing a combination of high active surface area and high energy/power density with cyclic stability. Herein, we report that the carbothermic reduction commonly used for metal extraction can be an effective way to fulfill aforementioned requirements. Namely, via atomic

layer deposition (ALD), a uniform ZnO layer was conformally deposited on the cotton textile, followed by pyrolysis at high temperature in order to evaporate Zn and O elements (C(s) + ZnO(s) → C′(s) + Zn(g) + CO(g)). The process led to formation of a micrometer-sized open pore network on the carbon fiber wall, which enabled swift transport of electrolyte ions for high-performance supercapacitors. We found that the mechanical buckling and opening mode fracture that frequently occur in layered materials are initiated by the differences in thermal stress, which highly increase surface area. Moreover, the evaporation of the reaction products of Zn and CO additionally created nanopores of various sizes on the remaining carbon textile with numerous wrinkles and cracks. Consequently, the resulting activated carbon textile exhibited nearly 10-fold and 20-fold increase in surface area and in energy density compared to that of the cotton textile pyrolyzed without ZnO.

RESULTS AND DISCUSSION General Idea for Activating Carbon via Carbothermic Reduction. Historically, carbothermic reduction has been widely used to extract metal from diverse oxide ores, where carbon is the most widely used reducing agent (MO(s) + C(s) → M(s) + CO(g)). We took note of the chemical formula and thought differently. Namely, it was conceived that it is likely possible to use not carbon, but metal oxides as a sacrificial agent (C(s) + MO(s) → C′(s) + M(g) + CO(g)) in order to activate carbon, once carbon is not decomposed and metal oxide is fully evaporated at a certain temperature range. It was expected that the reduction of the metal oxide and subsequent boiling/ evaporation of metal can give rise to serious surface roughening of carbon, as long as the metal oxide can be conformally coated on the carbon. A long time ago, Ellingham33 reported 11352

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Figure 2. Phase and structure identification after carbothermic reduction. (a) XRD spectra show crystal phase changes before and after carbothermic reduction, i.e., pyrolysis of CT with ZnO coating. The raw CT shows typical peaks from cellulose. Compared to CT undergoing pyrolysis without ZnO (p-CT), CT pyrolyzed with ZnO (p-aCT) displayed a sharper peak related to graphitization. (b) Raman spectra showed changes in the relative intensity ratio of G to D bands (D/G ratio). (c) Variations in electrical sheet resistances of pyrolyzed cotton textiles with different ZnO thickness. See the text for details.

roughening by evaporating Zn- and O-related materials would naturally occur, which creates simple grooves, craters, depressions, or steps at most on the carbon surface, like the soil surface roughened by snow melting. However, in the following, we show that our results do not end there. We found that the carbon was activated chemically as well as mechanically. Activation via Chemical Method. In a conventional way to produce activated carbon out of diverse source materials with chemical activation, the source materials are impregnated with a chemical activator (such as KOH, H3PO4, and ZnCl2), and then carbonization and activation are simultaneously proceeded by high-temperature pyrolysis.30−32 This reaction transforms carbon atoms into gaseous molecules and generated micro/ mesoporous structures on the products. Consequently, it leads to changes in surface area, maintaining analogous structure as the pristine materials. In our approach, similar effects with conventional chemical activation occurred. The resulting carbon textile maintained mechanical flexibility and macroscopic morphology of the raw cotton textile (Figure S1). The changes in phase structure were also similar to the changes by widely used carbonization/activation (Figure 2). In X-ray diffraction (XRD) spectra (Figure 2a), the carbon textile produced by carbothermic reduction (pyrolyzed with ZnO) displayed a broad graphitic stacking carbon peak at 2θ = 22.3° and a weak peak at 2θ = 43.8° due to the formation of higher degrees of intralayer condensation.32 The disappearance of ZnO and cellulose diffraction peaks in the ZnO-coated cotton textiles (aCTs) after carbothermic reduction (p-aCTs) indicated the complete conversion of cellulose into graphitic carbon. As the number of ALD cycles (thickness of ZnO) increased, these two peaks became broader. The Raman spectra (Figure 2b) exhibited two peaks at 1320 and 1590 cm−1, which are assigned to the characteristic defective D band and graphitic G band of carbon materials, respectively.40 The relative intensity ratio of D/G implies the degree of structural disorder with respect to a perfect graphitic structure. The ratio of ALD cycles to D/G was 0:250:500:1000 = 0.844:0.780:0.820:0.834, which was well matched with the XRD data. These XRD and Raman results indicated that the ZnO shell could lead to improvement in degree of graphitization and structural order of the resulting carbon textile, which also partly accounted for the changes in electrical resistance (Figure 2c) together with the

thermodynamic reducibility of metal oxides in metallurgical processes. It says that several metal oxides (e.g., SiO2, Cr2O3, Al2O3, TiO2, NiO, FeO, and MnO) can be converted into metal via carbothermic reduction, and the onset reduction temperature via carbon varies depending on oxides (e.g., TSiO2 = ∼1620 °C, TTiO2 = ∼1650 °C). On the basis of his report and general boiling points of diverse metals, we chose ZnO among numerous oxides because the carbothermic reduction of ZnO can spontaneously occur at a temperature (∼1000 °C) relatively lower than that of the other oxides; the metallic Zn boils at low temperature (∼907 °C), and ZnO can be readily deposited with extreme conformality via a low-temperature ALD process. For the selection of an exemplary material to demonstrate the feasibility of our activation approach utilizing the carbothermic reduction, several kinds of carbon materials were considered. Among them, a cotton textile (CT) made by weaving natural cotton fibers was chosen because it is currently of considerable interest for future wearable electronics,19,34−36 and approaches to functional carbon and textiles via ALD has long been established.37,38 Swatches of the textile were loaded into the ALD chamber, followed by ZnO ALD (aCT). Subsequent annealing under vacuum at 1000 °C for 1 h was performed in order to drive carbothermic reduction (C(s) + ZnO(s) → C′(s) + Zn(g) + CO(g)) and finally to obtain an activated carbon textile (p-aCT) (Figure 1a). Thermodynamic phase changes in the ZnO/cotton composite that occurred during reaction were monitored by thermogravimetric analysis (TGA). As can be seen from TGA profiles of raw cotton and ZnO/cotton with different ALD cycles (different ZnO thicknesses), three distinct phase changes were observed (Figure 1b). First, we were able to realize that in the region of RT < T < ∼350 °C, water is evaporated, thereby causing a slight decrease in cotton weight.39 Second, it was noticed that in the region of ∼350 °C < T < ∼850 °C, cellulose fibers were pyrolyzed, and above ∼850 °C, the solid-state etching of carbon by carbothermic reduction occurs. Additionally, the energydispersive X-ray spectroscopy (EDX) analysis indicated that reaction byproducts like Zn are completely evaporated at 1000 °C (Figure 1c and Table S1), consequently leaving roughened carbon fibers. Considering the basic mechanism of our approach, one can easily expect that during pyrolysis surface 11353

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Figure 3. Surface roughening by mechanical buckling and fracture. Carbothermic reduction, i.e., pyrolysis with ZnO, generated both wrinkles and cracks on the resulting carbon textile. Depending on the ZnO thickness, the buckling and fracturing behaviors varied considerably, which increased surface area and pore volumes of the carbon textiles. See the text for details. (a−e) SEM images of representative cotton (a) and carbon textiles (b−e) together with schematic models depicting changes in surface morphology of the individual cotton (a) and carbon (b−e) fiber. (a) Raw cotton textile. (b) Pyrolyzed cotton textile without ZnO (p-CT). (c,d) Pyrolyzed cotton textile with ZnO deposited for 250, 500, and 1000 ALD cycles (p-aCT). (f,g) Nitrogen adsorption/desorption isotherms and pore distribution of textiles.

(pyrolysis with ZnO, i.e., ZnO/CT → p-aCT), in contrast, created striking wrinkles and cracks. We have so far witnessed that with increasing thickness of the ZnO layer, mechanical buckling of individual carbon fibers occurs first (Figure 3c) and then concurrent mechanical buckling and cracking follow (Figure 3d,e). In particular, the mechanical buckling was identified to break out when the ZnO thickness was around 40 nm (250 ALD cycles, i.e., p-a250CT). When the thickness was around 160 nm (1000 ALD cycles, i.e., p-a1000CT), the buckling and fracture in the carbon fibers appeared to be saturated. The mechanical buckling and fracture produced numerous wrinkles and cracks on the carbon textiles, which led to formation of open pore networks in the carbon fibers, thereby greatly increasing surface area. In addition, the chemical activation by evaporation of Zn- and O-related materials allowed further surface area increase owing to the formation of

amount of contaminated oxygen likely derived from the reduction process (Table S1).41 Activation via Mechanical Buckling and Fracture. The chemical changes of the cotton textile by carbothermic reduction were analogous to the changes by the conventional activation methods. It was, however, observed that the carbothermic reduction led to noticeable morphology disparity compared to the conventional activation. As can be recognized from scanning electron microscope (SEM) images of representative carbon textiles prepared by carbothermic reduction (Figure 3), these textile fibers exhibited noticeable differences in morphology, which hinted that the ZnO layer plays a critical role in those morphological changes. The simple carbonization process (pyrolysis without ZnO, i.e., CT → pCT) resulted in minor variations in surface morphology of the carbon textile (Figure 3a,b). The carbothermic reduction 11354

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Figure 4. Proposed mechanism for the formation of wrinkles and cracks during carbothermic reduction. We simplified the cotton/ZnO fiber as a cylinder-shaped fiber with two layers (a). Based on cylindrical symmetry, we defined two elemental sections to explain the origin of wrinkle (b) and crack (c) formation. (d) Resulting cotton/ZnO fiber with wrinkles and cracks. (e) Final carbon fiber that experienced mechanical activation together with chemical activation. It is believed that the surface area in the carbon fiber increases further by the formation of a nanopores after chemical activation. See the text for details.

= ∼310 GPa)45 consequently leads to compressive stress to cotton (σr,co < 0 and σθ,co < 0) on the r−θ plane. However, on the r−z plane, cotton is subjected to tensile stress due to Poisson’s ratio (σz,co > 0).46 Currently, a thin and stiff ZnO layer rests on thick and less stiff cotton. Under the compressive thermal stress, the ZnO film buckles into a sinusoidal shape with wavelength of λ to release the applied stress. The wavelength is proportional to TZnO(EZnO/Eco)1/3, where TZnO is thickness of the ZnO.47 This relation indicates that the thinner the ZnO is, the shorter the wavelength is. Our results regarding the wrinkle generated in the r−θ plane by carbothermic reduction are largely in accord with this relation (Figure 3c−e). Regarding the generation of an array of cracks (red arrows in Figure 3d,e), the answer was found in the study about fracture spacing in layered system like cracks in sedimentary rocks. It has been reported that the fractures in layered materials are confined by layer boundaries and developed roughly perpendicular to the layer boundaries and parallel with each other if the stress state in the fractured layer is uniaxially tensile.43 Moreover, the spacing of these fractures (S) is often proportional to the thickness of the fractured layer (Tco in Figure 4c).43 Until now, numerous studies have been performed, which agreed that in general the fracture spacing (S) is a function of the thickness ratio between the fractured layer and the neighboring layer (TZnO/Tco), that is, S ∼ Tco/ TZnO. Our observation appears to correspond well with this relation. For instance, as can be seen in Figure 3d,e, with increasing ZnO thickness, the spacing S becomes closer.

nanopore networks. The specific surface area and pore volume of the carbon textile (p-aCT, pyrolyzed with ZnO) determined by Brunauer−Emmett−Teller (BET) method showed increases of 10- and 7-fold over those of the carbon textile (p-CT, pyrolyzed without ZnO), respectively (Figure 3f,g and Table S2). Origin of Wrinkles and Cracks. The involved mechanism in the wrinkle and crack formation was able to be interpreted with the theory regarding the thin film mechanics42 and opening-mode fractures in layered materials.43 The underlying principles leading to the formation of wrinkles on the thin film resting on a certain foundation was investigated long ago. The brittle fracture leading to an array of cracks in layered materials was also studied by engineers and geoscientists in the past few decades. In order to understand our mechanical activation behaviors, we invoked a simple model which enables us to explain both phenomena concurrently. We assumed a single cotton fiber coated with ZnO as a cylinder (Figure 4a) covered with a uniform ZnO layer. Based on the cylindrical symmetry (Figure S2), we defined two elemental sections, that is, the first element in the r−θ plane (Figure 4b) and the second in the r−z plane (Figure 4c). Because the cotton/ZnO multilayered composite undergoes high-temperature annealing, the thermal stress (σT) is naturally generated in the cotton due to the difference in the coefficient of thermal expansion (α) of cotton (αco = 13−30 ppm)44 and ZnO (αZnO = 2−4 ppm).45 It can be intuitively recognized that the huge difference in Young’s modulus between cotton (Eco = 7−12 GPa)44 and ZnO (EZnO 11355

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Figure 5. Mechanical stability of the textile electrodes. (a) Digital image showing mechanical stability of the cotton textile activated with ZnO (p-a1000CT). See Movie S1 in the Supporting Information. (b) Representative stress and strain curve of the raw cotton textile and the cotton textile activated with ZnO (p-a1000CT). (c) Changes in electrical resistance difference ratio (R − R0)/R0 of the electrodes under cyclic bending test for 1000 cycles.

Figure 6. Electrochemical performance of the carbon textiles prepared by carbothermic reduction. (a) Galvanostatic charge and discharge curves collected at a loading current of 1 A/g. (b) Calculated specific capacitance based on galvanostatic charge and discharge. (c) Cyclic voltammetry curves at a scan rate of 0.5 V/s. (d) Nyquist plot of the supercapacitor at open-circuit potential. The inset shows the magnified high-frequency region of the plot. (e) Cycling stability measured at a scan rate of 1 V/s for 10 000 cycles. (f) Ragone plot. Carbon textile prepared by pyrolysis with ZnO shows nearly 20-fold increase in energy density compared to that of the carbon textile prepared by pyrolysis without ZnO.

Our interpretation regarding the wrinkle and crack generation is simply based on one of the well-known mechanical models. Therefore, the relevant mechanism could be explained by any other interpretation. As can be noticed from TGA analysis (Figure 1b), the carbon textile coated with ZnO likely underwent volumetric shrinkage during pyrolysis. Considering that the fiber with a high aspect ratio is constrained by the ZnO layer, it can be assumed that the fiber shrinkage leads to tensile stress in the fiber and compressive stress in the ZnO layer to the longitudinal direction of the fiber (r−z plane). Thus, in the r−z plane, the compressive stress (σz,ZnO < 0) and the tensile stress (σz,co > 0) could also bring about wrinkles in the ZnO layer and cracks in the fiber, respectively (Figure S3). Mechanical Stability and Electrochemical Performances of the Carbon Textile. Prior to electrochemical performance evaluation of our carbon textile, we paid special

attention to the change in macroscopic mechanical properties of the textile before and after carbothermic reduction because, for practical application, the mechanical stability is of utmost importance (Figure 5a). It has been reported that the carbon textile often becomes stiffer or, in general, loses the mechanical flexibility after pyrolysis.27−29 Our textiles revealed similar mechanical degradation (Figure 5b). Because pyrolysis led to serious cracking of individual carbon fibers, the decrease in both elastic modulus (ECT = 59.2 ± 15.2 MPa, Ep‑a1000CT = 7.3 ± 1.5 MPa) and ultimate tensile strength (σCT = ∼2.1 MPa, σp‑a1000CT = ∼0.2 MPa) after pyrolysis was natural. However, surprisingly, the ductility (εCT = ∼0.36) was observed to be nearly unchanged. It was so far observed that the pyrolysis induces serious microscopic changes in the textile, whereas the change in the macroscopic woven structure of the textile is less. Considering these observations, it is believed that the pyrolysis exerted a minor influence over the macroscopic ductility of the 11356

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we showed that the combined effects of mechanical buckling and fracture that occurred between ZnO and cotton play an important role in activating carbon. We believe that our strategy using a bifunctional carbothermic reduction for chemical and mechanical activation can be an ingenious and efficient way for nanostructuring and enhancing porosity of the ubiquitous raw materials for various applications including wearable energy storage.

resulting textile. Additionally, changes in resistance difference ratio of the electrodes under cyclic bending tests were nearly unchanged, indicating good stability of electromechanical performance under mechanical disturbance (Figure 5c). Until now, we showed that the carbothermic reduction (pyrolysis with ZnO) activates the cotton textile chemically as well as mechanically (Figure 4,e). Particularly, mechanical buckling and fracture led to noteworthy changes in surface morphology of carbon and a great increase in surface area. The resulting carbon textiles were observed to have open pore networks on the individual carbon fiber wall, which was believed to enable swift transport of electrolyte ions for highperformance supercapacitors. Electrochemical performances of the carbon textiles were first examined by galvanostatic charge and discharge (GCD) measurements using a two-electrode setup with 1 M H2SO4 aqueous electrolyte at room conditions. The GCD curves showed nearly triangular shapes with a significantly low voltage (IR) drop, indicating good ionic conductivity and excellent electric double-layer behavior (Figure 6a). With increasing ZnO thickness, capacitance constantly increased from 6.4 mF/cm2 (2.2 F/g) to 121.5 mF/cm2 (63.2 F/g) at a loading current of 0.1 A/g (Figure 6b). Interestingly, the areal capacitance is highest among activated carbons obtained by various activation tools (Table S3). Moreover, the p-a1000CT showed impressive capacitance retention (∼72%) when the loading current increased from 0.1 to 3 A/g, which is superior to other textile-based supercapacitors.20−22 Cyclic voltammetry (CV) curves of the prepared supercapacitors measured at 0.5 V/s exhibited nearly rectangular shape (Figure 6c). A Nyquist plot measured by electrochemical impedance spectroscopy (EIS) showed a quasivertical profile with a remarkable reduction in equivalent series resistance for the p-a1000CT compared to the p-CT, suggesting that the p-a1000CT supercapacitors have nearly ideal capacitive behavior and excellent ionic conductivity (Figure 6d). More importantly, the supercapacitors exhibited excellent cycling stability without obvious reduction in capacitance after 10 000 loading cycles at the scan rate of 1 V/s (Figure 6e). The electrodes were further fabricated into symmetric flexible solid-state supercapacitors. The electrochemical−mechanical stability under bending tests exhibited reasonable changes in capacitance under mechanical deformations (Figure S4). The Ragone plot for the symmetrical pa1000CT supercapacitors showed great improvements in energy densities of over 20-fold compared to the p-CT (Figure 6f). Besides, our fabricated devices also showed better energy density compared to other textile- and graphene-based supercapacitors in literature.20,48,49 More importantly, it was tested that the carbon textile prepared by carbothermic reduction exhibits electrochemical performances superior to the carbon textile prepared by conventional chemical activation using KOH (Figure S5).

METHODS Preparation of ZnO-Coated Cotton Textile Using ALD. The commercially available natural cotton textile (TX304, TexWipe) was used as a starting material with the mass density of ∼18 mg/cm2. Initially, the textile was cut into rectangular pieces of 6 × 8 cm2, which were loaded into an ALD reactor (S200, Savannah, Cambridge NanoTech Inc.). The deposition was performed at the working temperature of 80 °C and pressure of ∼0.1 Torr using diethylzinc (Zn(C2H5)2, DEZ, Sigma-Aldrich) and H2O as precursors. The ALD condition was set in exposure mode with a 0.02 s pulse, 20 s exposure, and 30 s purge of DEZ, followed by 0.1 s pulse, 20 s exposure, and 30 s purge of H2O for each ALD cycle. The ALD process was repeated for 100, 250, 500, 750, and 1000 cycles for each sample. Si substrates were also loaded into the ALD chamber for reference. Growth rate of ZnO on Si substrates was ∼1.6 Å /cycle. Preparation of a Carbon Textile via Carbothermic Reduction. The as-prepared ZnO-deposited cotton textile samples were inserted into a quartz tube. The tube was evacuated to ∼0.1 Torr and heated to 1000 °C with a ramp rate of 10 °C/min. The samples were then annealed for 1 h under an Ar flow of 100 sccm. In order to prevent the tube from cracking, the furnace was kept closed until the temperature reached 600 °C. Then, the furnace was opened to quickly cool the annealed samples to room temperature. Preparation of a Carbon Textile via KOH-Based Activation. Cotton textile pieces of 3 × 8 cm2 were dipped into 50 mL aqueous solutions of 0.5, 1, and 2 M KOH for 3 h. The samples were then dried naturally overnight before being placed on a hot-plate heated to 100 °C in 1 h for further drying. The KOH-based activation procedure is similar to the preparation of carbon textile via carbothermic reduction. Unlike the ZnO-activated carbon textile, the KOH-activated carbon textile contained a large amount of KOH residue and contaminant. For further processing, the as-prepared samples were dipped in 1 M H2SO4 solution for 1 h to remove the residue and contaminant and then rinsed several times in H2O to neutralize their pH. Mechanical Characterizations of the Textile Electrodes. Uniaxial tensile tests were carried out at room conditions using a microtester (Deben, N200) with a constant displacement rate of 1.7 μm/s and with a gauge length of 12 mm. Several samples of 2.6 × 30 mm2 in size were used. After the test was finished, the changes in elastic modulus, ultimate tensile strength, and maximum tensile strain of each sample were carefully monitored and compared. Two representative curves were selected for demonstration. In addition, cyclic electromechanical bending tests were implemented by our home-built electromechanical system. It consisted of a commercial mechanical tester (Tytron 250, MTS) for doing the cyclic bending test, a laser sensor (LJ-V7020, Keyence) for calculating bending radius, and a source meter (Keithley source meter 2400, Keithley Instruments) for measuring variation in electrical resistance during the cyclic bending test. An electrode of 5 × 30 mm2 was attached on a PET substrate of 30 × 100 mm2, which was then gripped by the two ends of the mechanical tester. The cyclic bending test was carried out at frequency of 1 Hz up to 1000 cycles. The bending radius ranged from 160 to 25 mm. Fabrication of Supercapacitors. A round cell puncher (WCH125, Wellcos Corp.) was used to fabricate electrodes for aqueous supercapacitors. A 1 M H2SO4 electrolyte solution was prepared to wet the electrodes and a filter paper separator. The separator was then sandwiched between the two electrodes inserted inside a test cell for electrochemical measurements. In the case of solid-state supercapacitors, 1 g of poly(vinyl alcohol) (Mw = 89 000−98 000, PVA,

CONCLUSIONS In this study, we introduced a chemical and mechanical aspect of the carbothermic reduction that remained unnoticed until now. Thermodynamic reducibility of ZnO enabled us to prepare carbon textile electrodes for supercapacitors, with good combination properties of high power and energy densities together with high cyclic stability. Notably, through a series of experiments, we figured out that the carbothermic reduction of ZnO deposited on cotton textile leads to carbonization/ activation in a chemical as well as mechanical way. Particularly, 11357

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ACS Nano Sigma) was added to the 10 mL solution of 1 M H2SO4. The solution was then heated to 90 °C in a hot bath with constant stirring using a magnetic bar for 30 min to form a clear jelly-like solution. Two identical electrodes of 1 cm2 in area were immersed into the gel electrolyte for 30 min to allow the electrolyte to uniformly wet the electrodes. A symmetrical gel supercapacitor device was fabricated by inserting an electrolyte-wetted filter paper between the two electrodes. Electrochemical Measurements. The electrochemical performances of the supercapacitors were evaluated by an electrochemical workstation (CHI 600E, CH instruments) with a two-electrode setup. CV, GCD, and EIS measurements were sequentially carried out. CV profiles were collected at 0.01, 0.025, 0.05, 0.1, 0.2, 0.5, 1.0, 2.5, 5.0, and 10 V/s with a voltage window of 1 V. Galvanostatic current was cycled at 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 A/g with the same voltage window. EIS was measured in the range of 10 mHz and 100 kHz with peak to peak amplitude of 10 mV at an open-circuit potential. The cyclic performance was performed by a CV measurement at a scan rate of 1 V/s for 10 000 cycles. All devices were subjected to five precycles at a low scan rate of 0.01 V/s in order to stabilize measurements before recording test data. Areal capacitances of the devices (F/cm2) were calculated from the cyclic voltammetry curves according to the following equation: Q C= 2ΔV × S

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Do Van Lam: 0000-0001-5067-1783 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We would like to acknowledge the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of the Republic of Korea (Grant CAP-13-2-ETRI), and the internal research program of the Korea Institute of Machinery and Materials (SC1170). REFERENCES (1) Salunkhe, R. R.; Lee, Y. H.; Chang, K. H.; Li, J. M.; Simon, P.; Tang, J.; Torad, N. L.; Hu, C. C.; Yamauchi, Y. Nanoarchitectured Graphene-Based Supercapacitors for Next-Generation Energy-Storage Applications. Chem. - Eur. J. 2014, 20, 13838−13852. (2) Miller, J. R.; Burke, A. F. Electrochemical Capacitors: Challenges and Opportunities for Real-World Applications. Electrochem. Soc. Interface 2008, 17, 53−57. (3) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (4) Zhang, L. L.; Zhao, X. S. Carbon-Based Materials As Supercapacitor Electrodes. Chem. Soc. Rev. 2009, 38, 2520−2531. (5) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797− 828. (6) Li, L.; Wu, Z.; Yuan, S.; Zhang, X. B. Advances and Challenges for Flexible Energy Storage and Conversion Devices and Systems. Energy Environ. Sci. 2014, 7, 2101−2122. (7) Cai, X.; Peng, M.; Yu, X.; Fu, Y.; Zou, D. Flexible Planar/FiberArchitectured Supercapacitors for Wearable Energy Storage. J. Mater. Chem. C 2014, 2, 1184−1200. (8) Raccichini, R.; Varzi, A.; Passerini, S.; Scrosati, B. The Role of Graphene for Electrochemical Energy Storage. Nat. Mater. 2015, 14, 271−279. (9) Wang, X.; Lu, X.; Liu, B.; Chen, D.; Tong, Y.; Shen, G. Flexible Energy-Storage Devices: Design Consideration and Recent Progress. Adv. Mater. 2014, 26, 4763−4782. (10) Shao, Y.; El-Kady, M. F.; Wang, L. J.; Zhang, Q.; Li, Y.; Wang, H.; Mousavi, M. F.; Kaner, R. B. Graphene-Based Materials for Flexible Supercapacitors. Chem. Soc. Rev. 2015, 44, 3639−3665. (11) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors. Science 2012, 335, 1326−1330. (12) Ghosh, D.; Kim, S. O. Chemically Modified Graphene Based Supercapacitors for Flexible and Miniature Devices. Electron. Mater. Lett. 2015, 11, 719−734. (13) Wu, Z. S.; Parvez, K.; Feng, X.; Müllen, K. Graphene-Based InPlane Micro-Supercapacitors with High Power and Energy Densities. Nat. Commun. 2013, 4, 2487. (14) Hu, L.; Choi, J. W.; Yang, Y.; Jeong, S.; La Mantia, F.; Cui, L. F.; Cui, Y. Highly Conductive Paper for Energy-Storage Devices. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 21490−21494. (15) Lim, J.; Maiti, U. N.; Kim, N. Y.; Narayan, R.; Lee, W. J.; Choi, D. S.; Oh, Y.; Lee, J. M.; Lee, G. Y.; Kang, S. H.; et al. Dopant-Specific Unzippping of Carbon Nanotubes for Intact Crystalline Graphene Nanostructures. Nat. Commun. 2015, 7, 10364. (16) Sun, H.; Xie, S.; Li, Y.; Jiang, Y.; Sun, X.; Wang, B.; Peng, H. Large-Area Supercapacitor Textiles with Novel Hierarchical Conducting Structures. Adv. Mater. 2016, 28, 8431−8438.

(1)

where Q (C) is the average charge during the charging and discharging process, ΔV (V) is the potential window, and S (cm2) is the projected area of the electrodes. Alternatively, areal capacitances of the devices were measured from the galvanostatic charge−discharge curves followed by the following equation:

C=

I S × dV /dt

(2)

where I(A) is the constant discharging current, dV/dt is the slope of the discharging curve calculated in the range of Vmax to (1/2)Vmax. The energy density (E) and power density (P) were calculated with the following equations: E=

C × (ΔV )2 2

(3)

E Δt

(4)

and

P=

where C is taken from the eq 2 and Δt is the discharging time. Characterizations. A field emission scanning electron microscopy (JSM-700F, JEOL) was used to analyze the surface morphology of the samples. EDX (JSM-700F, JEOL) was used for elemental analysis. XRD (Empyrean, PANalytical) was used to study the crystallinity of the samples. Raman measurements were done by a Raman spectrometer (inVia Raman microscope, Renishaw) equipped with 514 nm laser line and an objective lense (50×). TGA was performed using a TGA/DSC 1 Mettler-Toledo instrument with a scan rate of 10 °C/min. Specific surface areas were measured by N2 adsorption BET (Micromeritics Tristar 3000). The sheet resistance was measured using a four-point probe system (MCP-T360 Loresta-EP, Mitsubishi Chemical).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06608. Figures S1−S5 and Tables S1−S3 (PDF) Movie S1 (AVI) 11358

DOI: 10.1021/acsnano.6b06608 ACS Nano 2016, 10, 11351−11359

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DOI: 10.1021/acsnano.6b06608 ACS Nano 2016, 10, 11351−11359