Towards Textile Energy Storage from Cotton T-Shirts

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Lihong Bao and Xiaodong Li *

Towards Textile Energy Storage from Cotton T-Shirts

Cotton, consisting mainly of cellulose fi bers, is the most widely used natural fi bers for producing soft and breathable textiles and clothing. [ 1 ] Emerging research on cotton fabrics with ver-satile functionalities [ 1–6 ] revolutionized the use of cotton fab-rics as a wearable platform for tremendous applications. [ 1 , 4 , 6 , 7 ] Moreover, recently demonstrated prototypes of textile super-capacitors [ 8–11 ] and fi ber supercapacitors [ 12 , 13 ] boosted textile-based energy storage applications. Such supercapacitors were constructed either by coating the textiles with functional thin fi lm layers [ 8–11 ] or by modifying the fi bers with chemicals. [ 12 , 13 ] However, direct conversion of cotton textiles into electrically active textiles for constructing supercapacitors remains a great challenge. Enabling textile energy storage requires that the textiles have good electrical conductivity. A well established method is polymer coating, in which individual fi bers are uni-formly coated with a smooth and coherent layer of conduc-tive polymers. [ 6 , 14 , 15 ] The drawback of this method is that the obtained conductive fabrics show aged decay of electrochem-ical stability. [ 16 ] Recently, Cui et al. developed a solution-based technique to convert cotton textiles into conductive textiles by conformally coating the cellulose fi bers with carbon nanotubes (CNTs) [ 11 ] or graphene [ 10 ] thin fi lms. The three-dimensional (3D) high-surface-area characteristic of such textiles facilitates the access of electrolytes, enabling high electrochemical per-formance of textile supercapacitors. [ 10 , 11 ] However, the employ-ment of organic surfactant for preparing CNT “ink” is not enviromentally benign. The other drawback is that the use of CNTs increases the cost of the device, which more or less deters their technological applications.

Inspired by traditional activation of cellulose fi bers into acti-vated carbon fi bers (ACFs) with a simple chemical route, [ 17–19 ] we converted cotton T-shirt textiles into activated carbon textiles (ACTs) for energy storage applications. After such functionali-zation, the textile features were well reserved and the obtained ACTs are highly conductive and fl exible, enabling an ideal electric double layer capacitor (EDLC) performance. The con-structed MnO 2 /ACT hybrid composite by integrating pseudo-capacitive MnO 2 into the ACTs showed a remarkably enhanced electrochemical perfromance. Finally, asymmetric supercapac-itors were assembled with the ACTs as the negative electrode and the hybrid MnO 2 /ACT comosite as the positive electrode. The textile-based asymmetric supercapacitors exhibited superior electrochemical characteristics. Such design opens up a new way for the functionalization of cotton textiles as supercapacitor

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Dr. L. H. Bao , Prof. X. D. Li Department of Mechanical Engineering University of South Carolina 300 Main Street, Columbia, SC 29208, USA E-mail: [email protected]

DOI: 10.1002/adma.201200246

building blocks and provides a low-cost and green solution for textile-based energy-storage devices.

We fabricated highly conductive and fl exible activated carbon textiles (ACTs) by direct conversion of cotton T-shirt textiles through a traditional dipping, drying and curing process, which involved dipping cotton textiles with 1 M NaF solution, drying the pre-treated textiles at 120 ° C for 3 h in a pre-heated oven, and annealing the dried textiles in a horizontal tube furnace at 800−1000 ° C for 1 h under vacuum and inert atmosphere con-ditions. Figure 1 shows the functionalization of cotton T-shirt textiles into activated carbon textiles (ACTs). A photograph of a commercial cotton T-shirt is shown in Figure 1 a. Scanning electron microscopy (SEM) characterization of the cotton textile reveals that the textile consists of interwoven cellulose fi bers with diameters ranging from 5 to 10 μ m (Figure 1 d). Figure 1 b shows a picece of ACT. The ACT is mechanically fl exible even under folding condition (Figure 1 c) and highly conductive (sheet resistance ∼ 10-20 Ω sq − 1 ). SEM images of the ACT (Figure 1 e) show that the textile feature is well reserved and the diamters of the ACT fi bers are comparable to those of cellulose fi bers in the cotton textile (inset of Figure 1 d). Fourier transform infrared (FTIR) analysis was performed to validate the func-tionalization of cellulose fi bers into activated carbon fi bers, as shown in Figure 1 f. In the FTIR spectrum of cotton textile, the broad adsorption bands at 3335 and 3276 cm − 1 are attributed to ν (O − H) stretching mode of hydrogen-bonded hydroxyl group, [ 20 ] while the bands at 2915 and 2848 cm − 1 can be assigned to sym-metric and asymmetric stretching mode of ν (C − H) in alkane groups, respectively. [ 21 , 22 ] The band at 2028 cm − 1 is ascribed to ν (C ≡ C) stretching in alkyne groups, and the band located at 1619 cm − 1 comes from ν (C = C) vibrations in alkene groups. A series of adsorption bands at 1428, 1370, 1336 and 1315 cm − 1 are attributed to ν (C − O) stretching in carboxylic groups. [ 17 ] The bands at 1282, 1199, and 1161 cm − 1 are ascribed to ν (C − O) vibrations in ester, ether or phenol groups, while the bands at 1110, 1053, and 1030 cm − 1 are assigned to ν (C − O) vibrations in alcohol (R − OH) groups. [ 17 ] The bands at 999 and 902 cm − 1 are due to ν (C − H) stretching in alkene groups. Compared with the spectrum of cotton textiles, the distinct adsorption characteris-tics of ACT lie in the absence of adsorption bands for hydrogen-bonded hydroxyl groups in frequency range of 3200−3600 cm − 1 and alkane groups in frequency range of 2850-3000 cm − 1 , and formation of adsorption bands for ν (C = C) aromatic stretching vibrations (1555, 1499, 1464 and 1415 cm − 1 ) [ 17 ] and γ (C − H) out-of plane bending vibrations (720 and 792 cm − 1 ) in benzene derivatives, indicating dehydration process and aromatic car-bonization of cotton textiles in activation. [ 17 , 20 ] Other adsorption bands of the FT-IR spectrum of ACT similar to those of cotton textile are assigned as follows: The band at 1682 cm − 1 is attrib-uted to ν (C = O) vibrations in carbonyl groups, while a series of adsorption bands at 1384, 1358, and 1304 cm − 1 result from ν (C − O) vibrations in carboxylic groups. The band at 1262 cm − 1

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Figure 2 . (a) Cyclic voltammetry (CV) curves of ACT at different scan rates in 1 M aqueous Na 2 SO 4 solution. (b) Specifi c capacitances at dif-ferent scan rates of ACT derived from CV curves.

Figure 1 . Optical photograph of (a) a commercial cotton T-shirt (b) a piece of ACT and (c) a piece of ACT under folding condition, showing its highly fl exible nature. (d) and (e) SEM images of cotton T-shirt textile and ACT, and insets are SEM images of individual cellulose fi ber and activated carbon fi ber, respectively. (f) FT-IR spectra of cotton textile and ACT, showing the conversion of cellulose fi bers into activated carbon fi bers. Scale bars, 1 mm (d); 2 μ m (inset of (d)); 1 mm (e) and 5 μ m (inset of (e)).

is due to ν (C − O) vibrations in ester, ether or phenol groups. The bands at 1144 and 1063 cm − 1 are ascribed to ν (C − O) vibrations in alcohol (R − OH) groups, and the band at 936 cm − 1 comes from ν (C − H) vibrations in alkene groups. The concurrence of aromatic structure in ACT is attributed to its continuous regen-eration as consumption of intermediate matter proceeded by

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gasifi cation of cellulose fi bers, [ 23 ] during which solid char con-sisting of element carbon was formed and simultaneously the char was activated with porous surface (Figure 1 e) by preloaded NaF.

Cyclic voltammetry (CV) characterization of the ACT as the supercapacitor electrode is shown in Figure 2 . The ACT shows stable electrochemical performance at the potential window ranging from −1.0 to 0 V vs Ag/AgCl in 1 M Na 2 SO 4 aqueous solution. The quasi-rectangular shape of cyclic voltammetry (CV) curves (Figure 2 a) indicates the ideal electrical double layer capacitive behavior. The calculated specifi c capacitances (see Equation (1) in Supporting Information (SI)) from CV are shown in Figure 2 b. The ACT electrode can achieve specifi c capacitances of 70.2, 54.5, 45.4, 36.5, 26.1 F g − 1 at scan rates of 2, 5, 10, 20, 50 mV s − 1 , respectively, which is comparable to that of solution-processed textiles. [ 10 ] The decay of specifi c capaci-tance with increasing scan rate is due to that at low scan rates, the current accumulating process was slow, which enabled full access of active pores/sites on the electrode, thereby resulting

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Figure 3 . (a) SEM image of MnO 2 /ACT hybrid composite fabricated by electrochemical deposition MnO 2 for 120 min using ACT as a supporting substrate, showing that all the ACT fi bers are uniformly coated by MnO 2 . (b) SEM image of an individual ACT fi ber coated with a thin fi lm of MnO 2 nanofl owers. (c) TEM image of the fl ower-like MnO 2 nanostructure and the inset is the corresponding SAED pattern, confi rming its amorphous nature. (d) and (e) Mn 2p and Mn 3s XPS spectra of fl ower-like MnO 2 nanostructure. Scale bars, 10 μ m (a); 1 μ m (b) and 100 nm (c), respectively.

in high specfi c capacitances. [ 24 , 25 ] It was found that 48% of the capacitance could be achieved when the scan rate was increased by a factor of 10 (5 mV s − 1 to 50 mV s − 1 ), showing a good rate capability of the ACT electrode (Figure 2 b). These features high-light the successful functionalization of cotton textile as super-capacitor electrode.

The coupled superior capacitive characteristic and high porosity make the ACTs unique supporting backbones for con-trolled deposition of nanostructured pseudo-capacitive MnO 2 to construct a hybrid composite for enhanced electrochemical performance. [ 10 , 11 , 25 , 26 ] ACTs were immersed into a mixed solu-tion of Mn(CH 3 COO) 2 and Na 2 SO 4 , followed by controlled elec-trochemical deposition using a three-electrode setup. The mass loading of MnO 2 was controlled by adjusting the deposition cur-rent and deposition time. It was found that the deposition mass is linearly dependent on the deposition time at an applied current of 1 mA cm − 2 , and the average deposition rate is ∼ 40 μ g min − 1 (see Figure S1, SI). Figure 3 shows the structural characteriza-tion of MnO 2 /ACT hybrid composite. A representative SEM image of MnO 2 /ACT hybrid composite is shown in Figure 3 a, which was fabricated by electrochemical depostion of MnO 2 for 120 min using ACTs as supporting substrates. Nanostruc-tured MnO 2 was uniformly coated onto the surface of ACT microfi bers over almost the entire network of porous ACTs and has a nanofl ower-like morphology (Figures 3 a and b), which is consistent with previous studies. [ 10 , 11 ] Transimission electron microscopy (TEM) characterization on an individual MnO 2

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nanofl ower (Figure 3 c) is consistent with the SEM results and the corresponding selected-area electron diffraction (SAED) pattern indicates that the MnO 2 nanofl owers are amorphous. X-ray photoelectron spectroscopy (XPS) characterization was performed to identify the oxidation state and elemental composi-tion of deposited manganese oxide nanostructures. The binding energy peaks of Mn 2p 3/2 and Mn 2p 1/2 are centered at 642.2 eV and 654.1 eV in Mn 2p spectrum, respectively (Figure 3 d), which is in good agreement with the previously reported peak binding energy separation (11.8 eV) between Mn 2p 3/2 and Mn 2p 1/2 . [ 27 , 28 ] As reported, the average oxidation state of Mn in manganese oxides can be determined by the separation of peak energies ( Δ E) of the Mn 3s peaks caused by multiplet splitting, where the Δ E data of MnO, Mn 3 O 4 , Mn 2 O 3 and MnO 2 are 5.79, 5.50, 5.41 and 4.78 eV, respectively. [ 28 , 29 ] The as-prepared hybrid textile shows a separated energy of 4.76 eV for the Mn 3s dou-blet (Figure 3 e), indicating that the coated manganese oxide is MnO 2 .

CV tests were performed to explore the electrochemical per-formance of the hybrid MnO 2 /ACTs composite, as is shown in Figure 4 . The CV curves keep the quasi-rectangular shape at the scan rate up to 20 mV s − 1 (Figure 4 a). Due to the overpo-tential on MnO 2 and the high surface area of activated carbon at the high voltage limit (1 V), the shapes of CV curves at the scan rate of 50 mV s − 1 (Figure 4 a) show somewhat distortion from ideal capacitive behavior. [ 10 , 30 , 31 ] The specifi c capacitances calculated from the CV curves at different scan rates are shown



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Figure 4 . (a) Cyclic voltammetry (CV) curves of MnO 2 /ACT hybrid composite at different scan rates in 1 M aqueous Na 2 SO 4 solution. (b) Comparative CV curves of ACT and MnO 2 /ACT hybrid composite at a scan rate of 2 mV s − 1 . (c) Specifi c capacitances at different scan rates of MnO 2 /ACT hybrid composite derived from CV curves.

in Figure 4 c, indicating remarkably improved capacitance per-formance with almost 3 times increase in specifi c capacitance compared to that of ACTs. At the scan rate of 2 mV s − 1 , the spe-cifi c capacitance can achieve 269.5 F g − 1 , which exceeds that of CNT/MnO 2 cotton textiles [ 11 ] and is comparable to that of solu-tion-processed graphene/MnO 2 textiles. [ 10 ] It was observed that the hybrid composite with 120 min MnO 2 deposition achieved the best electrochemical performance (see Figure S2, SI) com-pared to other composites with different deposition time periods (30 min, 60 min, 240 min). This is probably due to that for rela-tively shorter time deposition (30 min and 60 min), the surface of activated carbon fi bers was not fully coated with MnO 2 thin layers, and for longer time deposition (240 min), the depos-ited MnO 2 layers were too thick for effi cient utilization of their capacity. Figure 4 b shows the comparative CV curves of the respective ACT and hybrid MnO 2 /ACT composite electrodes at a scan rate of 2 mV s − 1 , in which the potential range of ACT was from −1.0 V to 0.2 V while that of MnO 2 /ACT was from 0.0 V to 1.0 V (both vs Ag/AgCl). The shape of the CV curve of MnO 2 /ACT is very different from that of ACT because the total capacitance comes from the combined contribution mainly from the redox pseudo-capacitance of MnO 2 and partly from the electric double layer capacitance of ACT in the hybrid com-posite. Both ACT and MnO 2 /ACT hybrid composite have stable electrochemical performance at their corresponding potential windows, therefore it is expected that the operation voltage can be extended to 2 V in mild aqueous solution if asymmetric supercapacitors were constructed by using MnO 2 /ACT with positive polarization as positive electrode and ACT with nega-tive polarization as negative electrode, respectively.

Figure 5 shows the electrochemical characterization of asym-metric supercapacitors which were assembled with MnO 2 /ACT as the positive electrode and ACT as the negative electrode, respectively, using 1 M Na 2 SO 4 aqueous solution as electrolyte (Schematically shown in Figure 5 a). Both CV and galvanostatic (GV) constant-current charging/discharge tests were used to evaluate the electrochemical performance of the entire device. To avoid unexpected damage of the device under high-voltage level at the early cycling stage, each electrode was polarized at the same potential before cycling the asymmetric superca-pacitor. [ 31 , 32 ] Figure 5 b shows the CV curves of the asymmetric supercapacitor measured at different scan rates of 2, 5, 10, 20, 50, 100 mV s − 1 with a potential window ranging from 0 to 2 V.

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These CV curves show ideal capacitive behavior with nearly rectangular shape and retain the rectangular shape without apparent distortion with increasing scan rate up to 100 mV s − 1 , indicating stable and high electrochemical performance of the asymmetric supercapacitor. The specifi c capacitances at dif-ferent scan rates calculated from the CV curves (see Equation (1), SI) are presented in Figure 5 c. The specifi c capacitance gradually decreases with increasing scan rate and achieves only 20% capacitance retention when the scan rate reaches 100 mV s − 1 , due to the fact that diffusion limits the movement of cations with time constraint, in which only the most outer surface was utilized for charge storage. [ 26 ] Figure 5 d shows the GV curves of the asymmetric supercapacitor at a current density of 1 A g − 1 . These GV curves have a very symmetric nature, indi-cating a rapid current-voltage response and good electrochem-ical capacitive behavior. The specfi c capacitances derived from the discharge curves at different charge/discharge rates (cur-rent densities) are shown in Figure 5 e. The specfi c capacitance of the ACT//MnO 2 /ACT asymmetric supercapacitor at the cur-rent density of 1 mA/cm 2 was calculated to be 120 F g − 1 , which exceeds that of graphene/MnO 2 //AC [ 26 ] and AC//MnO 2 [ 30 ] asymmetric supercapacitors. Energy density and power den-sity are two key factors for evaluating the power applications of supercapacitors. A good supercapacitor is expected to provide high energy density or high specifi c capacitance at high charge-discharge rates (current densities). Figure 5 f shows the Ragone plots for the MnO 2 /ACT//ACT asymmetric supercapacitor. The energy density decreases from 66.7 down to 17.8 Wh kg − 1 , while the power density increases from 0.8 up to 4.97 kW kg − 1 as the current density increases from 1 to 20 mA cm − 2 . Com-pared to that of individual ACT and MnO 2 /ACT electrodes, the asymmetric supercapacitor shows remarkably improved energy density and power density (Figure 5 f). These values are much higher than those of conventional supercapacitors in Ragone plot. [ 33 ] The maximum energy density of 66.7 Wh kg − 1 is much higher than that of MnO 2 -based asymmetric supercapacitors with aquesous solutions as electrolytes, such as MnO 2 //AC, [ 30 , 34 ] graphene/MnO 2 //CNT, [ 10 ] and MnO 2 /graphene//graphene. [ 32 ] Another important requirement for supercapacitor applications is cycling capability. The GV cycling tests over 1000 cycles for the MnO 2 /ACT//ACT asymmetric supercapacitor at a current density of 1 A g − 1 were carried out. Figure 5 g shows the spe-cifi c capacitance retention of the MnO 2 /ACT//ACT asymmetric

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Figure 5 . (a) Schematic of the assembled architecture of asymmetric supercapacitor with MnO 2 /ACT as positive electrode and ACT as negative elec-trode, respectively. (b) CV curves of the asymmetric supercapacitor at different scan rates with a potential window of 2 V in 1 M Na 2 SO 4 aquesous electrolyte. (c) Specifi c capacitances of the asymmetric supercapacitor at different scan rates. (d) GV constant-current charge/discharge curves of the asymmetric supercapacitor at a current density of 1 A g − 1 . (e) Specifi c capacitances of the asymmetric supercapacitor at different current densities. (f) Ragone plots of the MnO 2 /ACT electrode, ACT electrode, and as-assembled MnO 2 /ACT//ACT asymmetric supercapacitor. (g) Charge/discharge cycle performance of the MnO 2 /ACT//ACT asymmetric supercapacitor with a potential window of 2 V at a current density of 1 A g − 1 . (h) Comparative Nyquist plots of electrochemical impedance spectra of MnO 2 /ACT//ACT asymmetric supercapacitor in the frequency range of 100 kHz to 1 Hz before and after cycle tests.

supercapacitor as a function of charge/discharge cycling numbers with only 2.5% loss in the specifi c capacitance after 1000 charge-discharge cycles, which is comparable to that of graphene/MnO 2 //activated carbon fi ber (97.3% retention after 1000 cycles) [ 26 ] and that of graphene/MnO2//CNT (95% reten-tion after 5000 cycles). [ 10 ] Electrochemical impedance spectro-scopy (EIS) was carried out for an in-depth understanding of the electrochemical behavior of the asymmteric supercapacitors. Figure 5 h shows the Nyquist plots of the asymmetric superca-pacitor before cycling and after 1000 cycles. It is shown that the spectra are similar in terms of the curve shape except that the small change of equivalent series resistance (ESR) at high fre-quency (100 kHz), which represents a combined resistance of the electrolyte resistance and electrical resistance of textile elec-trodes. [ 10 ] The measured ESRs of the asymmetric supercapacitor

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before cycling and after 1000 cyclres are 7.3 and 8.5 Ω , respec-tively, further demonstrating the excellent electrochemical cyling stability of the asymmetric supercapacitor.

The superior electrochemical performance of the MnO 2 /ACT//ACT asymmetric supercapacitor can be attributed to the following features of the design: 1) activated carbon (AC) micofi bers serve as highly conductive backbones with high surface area for depositing nanostructured MnO 2 and provide excellent interfacial contact between MnO 2 and ACT micofi bers for high accessibility of electrolytic ions; 2) 3D porous micro-structure of the ACTs facilitates conformal coating of nanostruc-tured MnO 2 and the MnO 2 nanofl ower architectures shorten the diffusion paths of electrolyte ions during fast charge/dis-charge process, allowing for high electrochemical utilization of MnO 2 ; 3) The high surface area of ACT negative electrode is



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favorable for forming electric double layers and facilitates the electrolytic ion transport, providing enhanced energy density and power density. With controlled high-mass loading of MnO 2 nanomaterials and optimized mass balance of the electrodes for stable and higher potential windows in ionic liquid electrolyte, further improvement in energy density can be achieved.

In summary, cotton textiles were successfully functionalized into activated carbon textiles (ACTs) by a simple chemical acti-vation route. The ACTs exhibited an ideal electric double layer capacitive behavior and the constructed MnO 2 /ACT hybrid composite by integrating nanostructured MnO 2 into ACTs remarkably enhanced electrochemical performance. The key point of the design lies in activation of cotton T-shirt textiles into porous, mechanically fl exible, and highly conductive ACTs with high electrolytically accessible surface area. These charac-teristics offer the MnO 2 /ACT//ACT asymmetric supercapacitor the excellent electrochemical performance, such as high spe-cifi c capacitance, good charge/discharge stability, long-term cycling life, high energy density, and high power density. These fi ndings demonstrate that such low cost and environmentally friendly textiles can be a low-cost and green solution for textile-based energy-storage devices.

Experimental Section Activation of cotton T-shirt textile: Activation of cotton T-shirt textile was

performed by a simple dipping, drying and annealing process. Firstly, a piece of commercial cotton T-shirt made of woven cellulose fi bers was dipped into 1 M NaF solution and kept for 1 h. Then the ‘wet’ textile was dried in a pre-heated oven at 120 ° C for 3 h to remove the water. The annealing of the cotton textile was done in a horizontal tube furnace (diameter: 41 mm, length: 1200 mm). The NaF-treated cotton textile was inserted into the center of the tube furnace and heated at 800−1000 ° C for 1 h with 300 sccm (standard cubic centimeter) continuous fl ow of argon gas. After cooling, the as-activated cotton textile was washed with distilled water to remove the remained NaF and dried in a pre-heated oven at 120 ° C for 3 h to remove the water.

Electrochemical deposition of MnO 2 nanofl owers: Typically, a piece of activated carbon textile (ACT) was cut with size of 1 cm × 2 cm and then immersed into the mixed of 0.1 M Mn(CH 3 COO) 2 (Sigma-Aldrich) and 0.1 M Na 2 SO 4 (Sigma-Aldrich). Electrochemical deposition of MnO 2 nanomaterials was achieved with a three electrode setup, where the conductive ACT was used as the working electrode, platinum foil as the counter electrode, and Ag/AgCl electrode as the reference electrode. A small piece of Cu foil was used to connect the textile and the crocodile clips to avoid potential side reactions during deposition. A constant current 1 mA cm − 2 was applied for a range of deposition time 30 ∼ 240 min to conformally coat MnO 2 nanostructures on ACT fi bers. After electrodeposition, the textiles were taken out and carefully washed with DI water to remove excessive electrolyte, and then dried in a pre-heated oven at 100 ° C for 3 h. The mass of MnO 2 nanostrucutres can be obtained by the weight difference before coating and after drying of the textiles.

Structural characterization of nanostructured MnO 2 : The ACT and/or MnO 2 /ACT hybrid composite were characterized by scanning electron microscopy (SEM, Zeiss Ultra Plus FESEM), transmission electron microscopy (TEM, Hitachi H8000), Fourier transform infrared (FTIR) analysis (Perkin Elmer Spectrum 100 FTIR spectrometer fi tted with a Diamond ATR attachment), and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD equipped with a monochromated Al K α X-ray source and hemispherical analyzer capable of an energy resolution of 0.5 eV).

Electrochemical characterization: All electrochemical characterization was carried out using a CHI 760D electrochemical workstation

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(CH Instruments Inc.). For ACT and MnO 2 /ACT hybrid as supercapacitor electrodes, the electrochemical tests were performed with a standard three-electrode set-up, in which Ag/AgCl was used as reference electrode, Pt foil as counter electrode and the synthesized sample as working electrode, respectively. A 1 M Na2SO4 solution served as electrolyte at room temperature. Cyclic voltammetry (CV) was performed at scan rates of 2, 5, 10, 20, 50, and 100 mV s − 1 , respectively. Galvanostatic (GV) charge/discharge curves were obtained at various current densities to evaluate the specifi c capacitance. Electrochemical impedance spectra (EIS) were measured in the frequency range from 10 000 to 1 Hz with 0 V mean voltage and amplitude 5 mV using the same setup as CV and GV tests. Asymmetric supercapacitor was assembled by using as-made MnO 2 /ACT as the positive electrode, ACT as the negative electrode, copper foil attached to the textiles by silver paste as current collectors, and Whatman fi lter paper soaked with 1 M Na 2 SO 4 aqueous solution as the separator sandwiched in between two electrodes.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the US Army Research Offi ce under Agreement/Grant W911NF-07-1-0320, the National Science Foundation (CMMI-1129979 and CMMI-0968843), and the University of South Carolina NanoCenter. The authors thank Dr. Douglas Blom at the University of South Carolina EM Center for TEM technical support and stuff members at the University of South Carolina EM Center for SEM technical support.

Received: January 17, 2012 Published online: May 16, 2012

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