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Asymmetric carbon nanotube–MnO2 two-ply yarn supercapacitors for wearable electronics
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 135401 (http://iopscience.iop.org/0957-4484/25/13/135401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.153.184.170 This content was downloaded on 03/07/2014 at 10:30
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 135401 (8pp)
doi:10.1088/0957-4484/25/13/135401
Asymmetric carbon nanotube–MnO2 two-ply yarn supercapacitors for wearable electronics Fenghua Su1,2 and Menghe Miao2 1
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China 2 CSIRO Materials Science and Engineering, PO Box 21, Belmont, Victoria 3216, Australia E-mail: [email protected] Received 10 December 2013, revised 12 January 2014 Accepted for publication 29 January 2014 Published 28 February 2014
Abstract
Strong and flexible two-ply carbon nanotube yarn supercapacitors are electrical double layer capacitors that possess relatively low energy storage capacity. Pseudocapacitance metal oxides such as MnO2 are well known for their high electrochemical performance and can be coated on carbon nanotube yarns to significantly improve the performance of two-ply carbon nanotube yarn supercapacitors. We produced a high performance asymmetric two-ply yarn supercapacitor from as-spun CNT yarn and CNT@MnO2 composite yarn in aqueous electrolyte. The as-spun CNT yarn serves as negative electrode and the CNT@MnO2 composite yarn as positive electrode. This asymmetric architecture allows the operating potential window to be extended from 1.0 to 2.0 V and results in much higher energy and power densities than the reference symmetric two-ply yarn supercapacitors, reaching 42.0 Wh kg−1 at a lower power density of 483.7 W kg−1 , and 28.02 Wh kg−1 at a higher power density of 19 250 W kg−1 . The asymmetric supercapacitor can sustain cyclic charge–discharge and repeated folding/unfolding actions without suffering significant deterioration of specific capacitance. The combination of high strength, flexibility and electrochemical performance makes the asymmetric two-ply yarn supercapacitor a suitable power source for flexible electronic devices for applications that require high durability and wearer comfort. Keywords: asymmetric supercapacitor, carbon nanotube, manganese oxide, two-ply yarn (Some figures may appear in colour only in the online journal)
1. Introduction
electronic textiles due to lack of strength, flexibility, durability and human comfort (for example, mobility and breathability, i.e., passages for air and moisture). Strong and flexible threadlike (fibre or yarn) supercapacitors present attractive options for producing flexible and breathable woven and knitted electronic textile architectures. There is a recent surge of research activity on threadlike supercapacitors which may be broadly classified into two types of architecture, two-ply yarn and coaxial yarn [5, 12, 19, 20, 22, 25, 29, 36, 40, 41]. In comparison with batteries, supercapacitors have high power density and can be charged and discharged quickly
Integration of miniaturized electronic devices with textiles can lead to the development of many interesting applications; for example, electronic textiles provide a platform for on-body sensing to support people in various situations and activities [7, 11]. Supercapacitors can provide the power requirements for these wearable electronics due to their high power density and reliability [31, 36]. Bulk and film-like supercapacitors, although showing excellent electrochemical performance, cannot meet the requirements for production and end use of such 0957-4484/14/135401+08$33.00
1
c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 135401
F Su and M Miao
MnO2 ) was proposed as a pertinent strategy to overcome the limited voltage window of symmetrical MnO2 supercapacitors [4, 13]. Since then, the concept has been widely validated and the feasibility of such a device is well recognized. Because of the enhanced operating voltage, the AC(−) k MnO2 (+) combination provides energy densities up to nearly one order of magnitude higher than those of symmetrical MnO2 devices, and comparable to conventional symmetric carbon/carbon supercapacitors that utilize non-aqueous electrolytes [3]. In this study, we produced an asymmetric two-ply yarn supercapacitor in which the negative electrode was an as-spun CNT yarn and the positive electrode was a CNT yarn coated with MnO2 /polymer composite. The asymmetric two-ply yarn supercapacitor demonstrated significantly higher energy density than the aforementioned AC k MnO2 bulk asymmetric supercapacitor and symmetric supercapacitors constructed from CNT yarn coated with MnO2 /polymer composite. The asymmetric two-ply yarn supercapacitor also showed high cycle stability, strength and flexibility as required in wearable electronic applications.
and repeatedly for a long time without degradation [31]. On the other hand, supercapacitors have a relatively low energy density in comparison with conventional batteries. The energy density of a supercapacitor is proportional to the specific capacitance (C) and the square of the operating voltage (U ) of the supercapacitor, i.e., E = CU 2 /2. The use of high performance pseudocapacitive electrodes such as metal oxides, conducting polymers and reduced graphene oxides can increase the capacitance and thus the energy density of supercapacitors. To improve the energy density of supercapacitors, considerable research effort has been focused on developing non-polarizable electrodes using redox-active materials such as the transition metal oxides RuO2 , NiO2 , CoO2 and MnO2 [15, 23, 35, 38]. In particular, MnO2 has received a lot of attention as supercapacitor electrode material due to its high theoretical pseudocapacitance, low cost and environmental compatibilities [21, 43]. However, its relatively low electrical conductivity (10−5 –10−6 S cm−1 ) results in a poor rate capability. The electrical conductivity and rate capability of MnO2 can be significantly improved by compositing MnO2 with carbon materials such as carbon nanotubes and graphenes [10, 15, 30, 33]. Besides, MnO2 is a brittle material and has not been made into fibre or threadlike supercapacitors for use in wearable electronics. Carbon nanotubes are attractive electrode materials for the development of high performance supercapacitors due to their high charge transport capability (low resistivity) and high electrolyte accessibility (primarily mesoporosity) [6, 9, 24]. Carbon nanotubes on their own are electrical double layer capacitor (EDLC) materials and have lower energy storage capabilities than redox materials (pseudocapacitors) such as conducting polymers and metal oxides. Research has been performed to develop different types of CNT electrode materials and combine them with various electrolytes to improve their performance, safety and lifespan for supercapacitors [7, 18, 37]. Carbon nanotubes can be made into dry-spun yarns and polymer composite fibres. Carbon nanotube yarns have excellent textile properties (strong and flexible) and much higher electrical conductivity [26] than pseudocapacitance materials and are an attractive choice for current collectors in threadlike supercapacitors [36]. Despite the increase in cell capacitance, the cell voltage limitation in oxide-based symmetrical devices is still a problem for maximization of the energy density of supercapacitors. Another approach is therefore to construct a hybrid or asymmetric supercapacitor, where the negative electrode of a symmetrical redox-active device is replaced by an EDLC electrode to reach a more negative potential. As a result, the operating voltage window for asymmetric supercapacitors can be significantly widened [4, 13, 17]. Aqueous-based supercapacitors possess a number of advantages in commercial applications, such as their high ionic conductivity for achieving high power density. The maximum operating voltage for aqueous-based supercapacitors is about 1.2 V. The operating voltage can be increased to more than 2 V for asymmetric supercapacitors incorporating an EDLC electrode and a non-polarizable electrode made from a transition metal oxide [15, 23, 35, 38]. The assembly of activated carbon/MnO2 asymmetric devices (AC k
2. Experimental details 2.1. Preparation of as-spun carbon nanotube yarn
CNT forests were grown on silicon wafer substrates bearing a thermal oxide layer and iron catalyst coating using chemical vapour deposition (CVD) of acetylene in helium [14]. The resulting CNTs had 7 ± 2 walls with an outer diameter of 10 nm and an inner diameter of 4 nm approximately. The length of the CNTs was found to be approximately 350 µm by measuring the height of the forests. The CNT forests were spun into yarns using an ‘up-spinner’ machine described previously [28]. The CNT yarns were spun to a twist level of 5000 turns per metre using a spindle speed of 5000 rpm. 2.2. Preparation of MnO2 –PVA and PVA–H3 PO4
99.9% purity manganese dioxide (MnO2 ) powder with particle size less than 5 µm (Sigma-Aldrich) and 98–99% hydrolysed polyvinyl alcohol (PVA) with average molecular weight of 57 000–66 000 (Alfa Aesar) were used. 2.0 g MnO2 powder and 0.5 g PVA powder were added to 10 ml deionized water with vigorous stirring. The solution was heated up steadily to 90 ◦ C under vigorous stirring until a uniform MnO2 –PVA paste was formed. 0.8 g H3 PO4 was added to 10 ml deionized water with stirring and then 1 g PVA powder was added. The solution was heated up steadily to 90 ◦ C under vigorous stirring until the solution became clear to form the required PVA–H3 PO4 gel. The PVA–H3 PO4 gel was used as both electrolyte and separator in the supercapacitors. 2.3. Preparation of the supercapacitors
A schematic of the preparation procedures for the two-ply yarn asymmetric supercapacitor is shown in figure 1. Coating of MnO2 –PVA paste and PVA–H3 PO4 gel was carried out using a home-built continuous dip-coating line as shown in 2
Nanotechnology 25 (2014) 135401
F Su and M Miao
Figure 1. Preparation procedures for the two-ply yarn supercapacitors.
Figure 2. Continuous coating of MnO2 /PVA composite and PVA–H3 PO4 gel.
containing MnO2 which is determined by a high precision balance with an accuracy of 0.1 µg (Mettler Toledo). The energy density (E) and power density (P) of the supercapacitor cell were calculated using the equations E = C1V 2 /2 and P = E/1t, respectively [32]. The EIS was used to explore the electrochemical behaviour of the electrode materials with an amplitude of 5 mV in the frequency range from 0.01 Hz (10 mHz) to 500 kHz. The impedance was normalized to the yarn surface area to take the supercapacitor dimensions into consideration [34].
figure 2. The dip-coating line included a pig-tail shaped wire loop for holding liquid, an electrical heating unit for drying and an electrical motor for controlling yarn throughput speed. The liquid take-up rate could be controlled by adjusting the motor speed. A twist tester was used to assemble the CNT yarn and CNT–MnO2 composite yarn into two-ply yarn supercapacitors. One end of the two constituent yarns was twisted while the other end was held still by a fixed gripper. A CNT yarn coated with MnO2 @ PVA (positive electrode) and a CNT yarn coated with PVA (negative electrode) were used to make the asymmetric two-ply yarn supercapacitor. Two symmetric two-ply supercapacitors were prepared from two CNT@PVA k CNT@PVA yarns and two CNT@MnO2 @PVA k CNT@MnO2 @PVA yarns, respectively, for use as references.
3. Results and discussion 3.1. Yarn structure
SEM and optical images of the CNT and composite yarns and the flexible asymmetric supercapacitor of CNT@MnO2 @ PVA k CNT@PVA are shown in figure 3. The diameter of the as-spun CNT yarn was around 15 µm (figure 3(a)) and increased to about 21 µm after coating with PVA–H3 PO4 (figure 3(c)). The CNT@MnO2 –PVA yarn diameter was about 20 µm (figure 3(b)). When the PVA–H3 PO4 coating was applied, the yarn diameter increased to about 28 µm (figure 3(d)). The resulting asymmetric two-ply yarn supercapacitor had a diameter of about 50–60 µm, as shown in figure 3(e) and (f). The two-ply yarn supercapacitor is significantly thinner than fine count conventional textile yarns, which are typically hundreds of micrometres (µm) in diameter. The two-ply yarn supercapacitor retains the strength and flexibility of the CNT yarn so that the supercapacitor can be processed into textile fabrics using conventional textile processes, such as weaving, knitting and braiding [27]. The spaces between the supercapacitors in the fabric structure allow moisture and air to pass, providing comfort for the wearer.
2.4. Characterization
The morphologies of the CNT yarn and CNT@MnO2 composite yarn were characterized by scanning electron microscopy (SEM, Hitachi S-4800). Energy-dispersive spectroscopy (EDS) was performed on the specimens inside the SEM to provide elemental mapping information. Cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were performed on 10 mm supercapacitor specimens using a potential static electrochemical workstation (600 D, CH Instruments) set in two-electrode configuration. 1.0 mol l−1 aqueous H3 PO4 was used as the electrolyte. The specific capacitance of a supercapacitor cell (Cm ) was calculated using the equation Cm = 2I 1t/m1V , where I is the constant discharge current, 1t is the discharging time, m is the mass of the yarn in the overlap portion and 1V is the voltage drop upon discharging [21]. In the two-ply yarn asymmetric supercapacitor based on CNT@MnO2 k CNT, m is the mass of the yarn 3
Nanotechnology 25 (2014) 135401
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Figure 4. SEM images and EDS spectrum. (a) CNT yarn; (b) and (c)
CNT@MnO2 yarn; (d) EDS spectrum taken from the marked region in (c).
The asymmetric two-ply yarn supercapacitor exhibited substantially larger current density than the two symmetric supercapacitors at the same level of potential. The current density of the symmetric supercapacitor CNT@MnO2 k CNT@MnO2 was larger than that of the symmetric supercapacitor CNT k CNT based on the as-spun CNT yarn. All the galvanostatic charge–discharge curves of the three two-ply yarn supercapacitors in figure 5(b) show typical triangular shapes for supercapacitors. The asymmetric supercapacitor CNT@MnO2 k CNT exhibits much longer charge–discharge time than the two symmetric supercapacitors CNT k CNT and CNT@MnO2 k CNT@MnO2 . The specific capacitance calculated from the CV curve was used to evaluate the charge storage capacity of the supercapacitors. The dependence of the gram capacitance on the current density for the three two-ply yarn supercapacitors is shown in figure 5(c). The specific capacitance of the symmetric supercapacitor based on as-spun yarn (i.e., CNT k CNT) was relatively low. The coating of MnO2 @PVA composite resulted in a significant improvement (CNT@MnO2 k CNT@MnO2 ). However, a far greater improvement in gram capacitance over the whole range of current density was achieved by forming an asymmetric supercapacitor (CNT@MnO2 k CNT) from the as-spun CNT yarn and the CNT@MnO2 yarn. At
Figure 3. CNT, CNT/composite yarns and electrodes. (a) SEM image of CNT yarn; (b) SEM image of CNT@MnO2 yarn; (c) CNT@PVA yarn; (d) SEM image of CNT@MnO2 @PVA yarn; (e) SEM image of the asymmetric two-ply yarn supercapacitor CNT@MnO2 @PVA k CNT@PVA; (f) optical image of the asymmetric two-ply yarn supercapacitor CNT@MnO2 @PVA k CNT@PVA.
Figure 4(a) shows that the carbon nanotubes are well aligned in the as-spun CNT yarn. After being coated with the MnO2 @PVA (figure 4(b)), the yarn surface is covered by bright particles (figure 4(c)) of sizes from submicron to around 2 µm. Energy-dispersive x-ray spectroscopy (figure 4(d)) confirmed that the elemental composition of the bright particles was MnO2 . 3.2. Symmetric versus asymmetric two-ply yarn supercapacitors
The CV curves of the three two-ply yarn supercapacitors at the same scanning rate of 0.05 V s−1 are shown in figure 5(a).
Figure 5. Electrochemical properties of the symmetric and asymmetric two-ply yarn supercapacitors. (a) Cyclic voltammograph at 50 mV s−1 ; (b) galvanostatic charge/discharge curves at 0.267 A g−1 ; (c) gram capacitance as a function of charge–discharge current density. 4
Nanotechnology 25 (2014) 135401
F Su and M Miao
Figure 6. Electrochemical impedance spectroscopy (EIS) of symmetric and asymmetric two-ply yarn supercapacitors. (a) full range; (b) magnified plot at the high frequency end.
a current density of 2.67 A g−1 , the gram capacitance of the asymmetric two-ply yarn supercapacitor was over five times greater than that of the symmetric supercapacitor CNT k CNT. This improvement could be attributed to the utilization of the pseudocapacitive MnO2 particles in the asymmetric supercapacitor architecture [15, 16]. The asymmetric supercapacitor CNT@MnO2 k CNT showed a capacitance of 12.5 F g−1 at a current density of 0.14 A g−1 , which is higher than that of previously reported single-walled carbon nanotube (CNT) fibres containing (salmon) DNA (7.2 F g−1 ) [1] but lower than that of graphene fibre-based supercapacitors (25–40 F g−1 ) [25]. The electrochemical behaviour of the two symmetric and one asymmetric two-ply yarn supercapacitors was further analysed using electrochemical impedance spectroscopy (EIS). In the complex-plane impedance plot (Nyquist plot) in figure 6(a), each data point was measured at a different frequency (0.01 Hz–500 kHz) and the lower left portion of the plot corresponds to higher frequencies. The intersection of the Nyquist plot at the Z 0 -axis represents the equivalent series resistance (ESR) of the corresponding two-electrode supercapacitor [2, 34]. As shown in the magnified figure (figure 6(b)), the incorporation of MnO2 particles in the symmetric supercapacitor resulted in a significant reduction of ESR, but the asymmetric supercapacitor CNT@MnO2 k CNT showed similar ESR to the symmetric supercapacitor CNT k CNT. At the lower frequency end (figure 6(a)), the curves of the two symmetric supercapacitors were closer to the Z 00 -axis than the asymmetric supercapacitor, indicating more capacitive behaviour [2].
measured in the same voltage window, 0–2.0 V, but at different scan rates from 0.05 to 2.0 V s−1 . The CV curves maintained their near-rectangular shapes at even the highest scan rate of 2.0 V s−1 . Figure 7(c) shows the galvanostatic charge– discharge curves in different potential windows measured from the asymmetric two-ply yarn supercapacitor at a constant current density of 0.267 A g−1 . Both the charge and the discharge curves remained in reasonably good symmetry at all voltages up to 2.0 V. The charge time and discharge time were nearly proportional to the charge–discharge potential, indicating rapid I –V response and ideal capacitive characteristics. The charge–discharge characteristics of the asymmetric supercapacitor in the potential window of 0–2.0 V were further evaluated at different current densities ranging from 0.067 to 5.33 A g−1 , all exhibiting near-to-triangular charge–discharge curves, as shown in figure 7(d). The main advantage of the asymmetric supercapacitor came from the extension of the electrochemical potential window from 1.0 to 2.0 V. The specific capacitance and energy density of the asymmetric two-ply yarn supercapacitor are plotted against the potential window span in figure 7(e). The higher operating voltage significantly improved the energy storage capacity of the asymmetric supercapacitor. The gram capacitance increased by 54% (from 11.4 to 17.5 F g−1 ) and the energy density increased more than six-fold (from 5.66 to 35.0 Wh kg−1 ) due to the extension of operating potential from 1.0 to 2.0 V. Ragone plots are commonly used for performance comparison of different energy storing devices. The energy and power densities of the supercapacitors were calculated from the discharge curves measured at different current densities. Figure 7(f) compares the Ragone plot of the asymmetric two-ply yarn supercapacitor (CNT@MnO2 k CNT) with that of the symmetric two-ply yarn supercapacitor CNT k CNT. At equivalent charging conditions, the asymmetric supercapacitor CNT@MnO2 k CNT demonstrated nearly 20 times higher energy density and two times higher power density than the symmetric supercapacitor CNT k CNT. The symmetric supercapacitor had a low energy density between 1 and 2.12 Wh kg−1 over the range of power density from 241.8 to 10 000 W kg−1 . In contrast, the energy density of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k
3.3. Characteristics of the asymmetric two-ply yarn supercapacitor
Figure 7 displays the electrochemical characteristics of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT. The CV curves in figure 7(a) were measured at different potential windows up to 2.0 V at a constant scan rate of 0.1 V s−1 . These curves show basically rectangular shapes with approximately mirror images with respect to the zerocurrent line, demonstrating the pseudocapacitive behaviour of the MnO2 electrode. Figure 7(b) displays another series of CV curves of the asymmetric two-ply yarn supercapacitor 5
Nanotechnology 25 (2014) 135401
F Su and M Miao
Figure 7. Electrochemical properties of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT. (a) CV curves in different potential windows measured at a constant scan rate of 0.1 V s−1 ; (b) CV curves in the potential window from 0 to 2.0 V measured at different scan rates; (c) galvanostatic charge–discharge curves in different potential windows measured at a constant current density of 0.267 A g−1 ; (d) galvanostatic charge–discharge curves in the same potential window from 0 to 2.0 V measured at different current densities; (e) capacitance and energy density window at 0.267 A g−1 as a function of the operating potential span; (f) Ragone plot showing the relation between energy density and power density for the symmetric two-ply yarn supercapacitor CNT k CNT and asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT.
Figure 8. Stability of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT derived from galvanostatic charge–discharge curves conducted at 0.533 A g−1 . (a) capacitance retention after cyclic charge–discharge; (b) capacitance retention at different bending angles; (c)
capacitance retention after cyclic folding and unfolding.
CNT was up to 42.0 Wh kg−1 at the low power density of 483.7 W kg−1 , and 28.02 Wh kg−1 at the high power density of 19 250 W kg−1 . These values were much higher than the energy density of the symmetric film CNT supercapacitor (
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Asymmetric carbon nanotube–MnO2 two-ply yarn supercapacitors for wearable electronics
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 135401 (http://iopscience.iop.org/0957-4484/25/13/135401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.153.184.170 This content was downloaded on 03/07/2014 at 10:30
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 135401 (8pp)
doi:10.1088/0957-4484/25/13/135401
Asymmetric carbon nanotube–MnO2 two-ply yarn supercapacitors for wearable electronics Fenghua Su1,2 and Menghe Miao2 1
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China 2 CSIRO Materials Science and Engineering, PO Box 21, Belmont, Victoria 3216, Australia E-mail: [email protected] Received 10 December 2013, revised 12 January 2014 Accepted for publication 29 January 2014 Published 28 February 2014
Abstract
Strong and flexible two-ply carbon nanotube yarn supercapacitors are electrical double layer capacitors that possess relatively low energy storage capacity. Pseudocapacitance metal oxides such as MnO2 are well known for their high electrochemical performance and can be coated on carbon nanotube yarns to significantly improve the performance of two-ply carbon nanotube yarn supercapacitors. We produced a high performance asymmetric two-ply yarn supercapacitor from as-spun CNT yarn and CNT@MnO2 composite yarn in aqueous electrolyte. The as-spun CNT yarn serves as negative electrode and the CNT@MnO2 composite yarn as positive electrode. This asymmetric architecture allows the operating potential window to be extended from 1.0 to 2.0 V and results in much higher energy and power densities than the reference symmetric two-ply yarn supercapacitors, reaching 42.0 Wh kg−1 at a lower power density of 483.7 W kg−1 , and 28.02 Wh kg−1 at a higher power density of 19 250 W kg−1 . The asymmetric supercapacitor can sustain cyclic charge–discharge and repeated folding/unfolding actions without suffering significant deterioration of specific capacitance. The combination of high strength, flexibility and electrochemical performance makes the asymmetric two-ply yarn supercapacitor a suitable power source for flexible electronic devices for applications that require high durability and wearer comfort. Keywords: asymmetric supercapacitor, carbon nanotube, manganese oxide, two-ply yarn (Some figures may appear in colour only in the online journal)
1. Introduction
electronic textiles due to lack of strength, flexibility, durability and human comfort (for example, mobility and breathability, i.e., passages for air and moisture). Strong and flexible threadlike (fibre or yarn) supercapacitors present attractive options for producing flexible and breathable woven and knitted electronic textile architectures. There is a recent surge of research activity on threadlike supercapacitors which may be broadly classified into two types of architecture, two-ply yarn and coaxial yarn [5, 12, 19, 20, 22, 25, 29, 36, 40, 41]. In comparison with batteries, supercapacitors have high power density and can be charged and discharged quickly
Integration of miniaturized electronic devices with textiles can lead to the development of many interesting applications; for example, electronic textiles provide a platform for on-body sensing to support people in various situations and activities [7, 11]. Supercapacitors can provide the power requirements for these wearable electronics due to their high power density and reliability [31, 36]. Bulk and film-like supercapacitors, although showing excellent electrochemical performance, cannot meet the requirements for production and end use of such 0957-4484/14/135401+08$33.00
1
c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 135401
F Su and M Miao
MnO2 ) was proposed as a pertinent strategy to overcome the limited voltage window of symmetrical MnO2 supercapacitors [4, 13]. Since then, the concept has been widely validated and the feasibility of such a device is well recognized. Because of the enhanced operating voltage, the AC(−) k MnO2 (+) combination provides energy densities up to nearly one order of magnitude higher than those of symmetrical MnO2 devices, and comparable to conventional symmetric carbon/carbon supercapacitors that utilize non-aqueous electrolytes [3]. In this study, we produced an asymmetric two-ply yarn supercapacitor in which the negative electrode was an as-spun CNT yarn and the positive electrode was a CNT yarn coated with MnO2 /polymer composite. The asymmetric two-ply yarn supercapacitor demonstrated significantly higher energy density than the aforementioned AC k MnO2 bulk asymmetric supercapacitor and symmetric supercapacitors constructed from CNT yarn coated with MnO2 /polymer composite. The asymmetric two-ply yarn supercapacitor also showed high cycle stability, strength and flexibility as required in wearable electronic applications.
and repeatedly for a long time without degradation [31]. On the other hand, supercapacitors have a relatively low energy density in comparison with conventional batteries. The energy density of a supercapacitor is proportional to the specific capacitance (C) and the square of the operating voltage (U ) of the supercapacitor, i.e., E = CU 2 /2. The use of high performance pseudocapacitive electrodes such as metal oxides, conducting polymers and reduced graphene oxides can increase the capacitance and thus the energy density of supercapacitors. To improve the energy density of supercapacitors, considerable research effort has been focused on developing non-polarizable electrodes using redox-active materials such as the transition metal oxides RuO2 , NiO2 , CoO2 and MnO2 [15, 23, 35, 38]. In particular, MnO2 has received a lot of attention as supercapacitor electrode material due to its high theoretical pseudocapacitance, low cost and environmental compatibilities [21, 43]. However, its relatively low electrical conductivity (10−5 –10−6 S cm−1 ) results in a poor rate capability. The electrical conductivity and rate capability of MnO2 can be significantly improved by compositing MnO2 with carbon materials such as carbon nanotubes and graphenes [10, 15, 30, 33]. Besides, MnO2 is a brittle material and has not been made into fibre or threadlike supercapacitors for use in wearable electronics. Carbon nanotubes are attractive electrode materials for the development of high performance supercapacitors due to their high charge transport capability (low resistivity) and high electrolyte accessibility (primarily mesoporosity) [6, 9, 24]. Carbon nanotubes on their own are electrical double layer capacitor (EDLC) materials and have lower energy storage capabilities than redox materials (pseudocapacitors) such as conducting polymers and metal oxides. Research has been performed to develop different types of CNT electrode materials and combine them with various electrolytes to improve their performance, safety and lifespan for supercapacitors [7, 18, 37]. Carbon nanotubes can be made into dry-spun yarns and polymer composite fibres. Carbon nanotube yarns have excellent textile properties (strong and flexible) and much higher electrical conductivity [26] than pseudocapacitance materials and are an attractive choice for current collectors in threadlike supercapacitors [36]. Despite the increase in cell capacitance, the cell voltage limitation in oxide-based symmetrical devices is still a problem for maximization of the energy density of supercapacitors. Another approach is therefore to construct a hybrid or asymmetric supercapacitor, where the negative electrode of a symmetrical redox-active device is replaced by an EDLC electrode to reach a more negative potential. As a result, the operating voltage window for asymmetric supercapacitors can be significantly widened [4, 13, 17]. Aqueous-based supercapacitors possess a number of advantages in commercial applications, such as their high ionic conductivity for achieving high power density. The maximum operating voltage for aqueous-based supercapacitors is about 1.2 V. The operating voltage can be increased to more than 2 V for asymmetric supercapacitors incorporating an EDLC electrode and a non-polarizable electrode made from a transition metal oxide [15, 23, 35, 38]. The assembly of activated carbon/MnO2 asymmetric devices (AC k
2. Experimental details 2.1. Preparation of as-spun carbon nanotube yarn
CNT forests were grown on silicon wafer substrates bearing a thermal oxide layer and iron catalyst coating using chemical vapour deposition (CVD) of acetylene in helium [14]. The resulting CNTs had 7 ± 2 walls with an outer diameter of 10 nm and an inner diameter of 4 nm approximately. The length of the CNTs was found to be approximately 350 µm by measuring the height of the forests. The CNT forests were spun into yarns using an ‘up-spinner’ machine described previously [28]. The CNT yarns were spun to a twist level of 5000 turns per metre using a spindle speed of 5000 rpm. 2.2. Preparation of MnO2 –PVA and PVA–H3 PO4
99.9% purity manganese dioxide (MnO2 ) powder with particle size less than 5 µm (Sigma-Aldrich) and 98–99% hydrolysed polyvinyl alcohol (PVA) with average molecular weight of 57 000–66 000 (Alfa Aesar) were used. 2.0 g MnO2 powder and 0.5 g PVA powder were added to 10 ml deionized water with vigorous stirring. The solution was heated up steadily to 90 ◦ C under vigorous stirring until a uniform MnO2 –PVA paste was formed. 0.8 g H3 PO4 was added to 10 ml deionized water with stirring and then 1 g PVA powder was added. The solution was heated up steadily to 90 ◦ C under vigorous stirring until the solution became clear to form the required PVA–H3 PO4 gel. The PVA–H3 PO4 gel was used as both electrolyte and separator in the supercapacitors. 2.3. Preparation of the supercapacitors
A schematic of the preparation procedures for the two-ply yarn asymmetric supercapacitor is shown in figure 1. Coating of MnO2 –PVA paste and PVA–H3 PO4 gel was carried out using a home-built continuous dip-coating line as shown in 2
Nanotechnology 25 (2014) 135401
F Su and M Miao
Figure 1. Preparation procedures for the two-ply yarn supercapacitors.
Figure 2. Continuous coating of MnO2 /PVA composite and PVA–H3 PO4 gel.
containing MnO2 which is determined by a high precision balance with an accuracy of 0.1 µg (Mettler Toledo). The energy density (E) and power density (P) of the supercapacitor cell were calculated using the equations E = C1V 2 /2 and P = E/1t, respectively [32]. The EIS was used to explore the electrochemical behaviour of the electrode materials with an amplitude of 5 mV in the frequency range from 0.01 Hz (10 mHz) to 500 kHz. The impedance was normalized to the yarn surface area to take the supercapacitor dimensions into consideration [34].
figure 2. The dip-coating line included a pig-tail shaped wire loop for holding liquid, an electrical heating unit for drying and an electrical motor for controlling yarn throughput speed. The liquid take-up rate could be controlled by adjusting the motor speed. A twist tester was used to assemble the CNT yarn and CNT–MnO2 composite yarn into two-ply yarn supercapacitors. One end of the two constituent yarns was twisted while the other end was held still by a fixed gripper. A CNT yarn coated with MnO2 @ PVA (positive electrode) and a CNT yarn coated with PVA (negative electrode) were used to make the asymmetric two-ply yarn supercapacitor. Two symmetric two-ply supercapacitors were prepared from two CNT@PVA k CNT@PVA yarns and two CNT@MnO2 @PVA k CNT@MnO2 @PVA yarns, respectively, for use as references.
3. Results and discussion 3.1. Yarn structure
SEM and optical images of the CNT and composite yarns and the flexible asymmetric supercapacitor of CNT@MnO2 @ PVA k CNT@PVA are shown in figure 3. The diameter of the as-spun CNT yarn was around 15 µm (figure 3(a)) and increased to about 21 µm after coating with PVA–H3 PO4 (figure 3(c)). The CNT@MnO2 –PVA yarn diameter was about 20 µm (figure 3(b)). When the PVA–H3 PO4 coating was applied, the yarn diameter increased to about 28 µm (figure 3(d)). The resulting asymmetric two-ply yarn supercapacitor had a diameter of about 50–60 µm, as shown in figure 3(e) and (f). The two-ply yarn supercapacitor is significantly thinner than fine count conventional textile yarns, which are typically hundreds of micrometres (µm) in diameter. The two-ply yarn supercapacitor retains the strength and flexibility of the CNT yarn so that the supercapacitor can be processed into textile fabrics using conventional textile processes, such as weaving, knitting and braiding [27]. The spaces between the supercapacitors in the fabric structure allow moisture and air to pass, providing comfort for the wearer.
2.4. Characterization
The morphologies of the CNT yarn and CNT@MnO2 composite yarn were characterized by scanning electron microscopy (SEM, Hitachi S-4800). Energy-dispersive spectroscopy (EDS) was performed on the specimens inside the SEM to provide elemental mapping information. Cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were performed on 10 mm supercapacitor specimens using a potential static electrochemical workstation (600 D, CH Instruments) set in two-electrode configuration. 1.0 mol l−1 aqueous H3 PO4 was used as the electrolyte. The specific capacitance of a supercapacitor cell (Cm ) was calculated using the equation Cm = 2I 1t/m1V , where I is the constant discharge current, 1t is the discharging time, m is the mass of the yarn in the overlap portion and 1V is the voltage drop upon discharging [21]. In the two-ply yarn asymmetric supercapacitor based on CNT@MnO2 k CNT, m is the mass of the yarn 3
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Figure 4. SEM images and EDS spectrum. (a) CNT yarn; (b) and (c)
CNT@MnO2 yarn; (d) EDS spectrum taken from the marked region in (c).
The asymmetric two-ply yarn supercapacitor exhibited substantially larger current density than the two symmetric supercapacitors at the same level of potential. The current density of the symmetric supercapacitor CNT@MnO2 k CNT@MnO2 was larger than that of the symmetric supercapacitor CNT k CNT based on the as-spun CNT yarn. All the galvanostatic charge–discharge curves of the three two-ply yarn supercapacitors in figure 5(b) show typical triangular shapes for supercapacitors. The asymmetric supercapacitor CNT@MnO2 k CNT exhibits much longer charge–discharge time than the two symmetric supercapacitors CNT k CNT and CNT@MnO2 k CNT@MnO2 . The specific capacitance calculated from the CV curve was used to evaluate the charge storage capacity of the supercapacitors. The dependence of the gram capacitance on the current density for the three two-ply yarn supercapacitors is shown in figure 5(c). The specific capacitance of the symmetric supercapacitor based on as-spun yarn (i.e., CNT k CNT) was relatively low. The coating of MnO2 @PVA composite resulted in a significant improvement (CNT@MnO2 k CNT@MnO2 ). However, a far greater improvement in gram capacitance over the whole range of current density was achieved by forming an asymmetric supercapacitor (CNT@MnO2 k CNT) from the as-spun CNT yarn and the CNT@MnO2 yarn. At
Figure 3. CNT, CNT/composite yarns and electrodes. (a) SEM image of CNT yarn; (b) SEM image of CNT@MnO2 yarn; (c) CNT@PVA yarn; (d) SEM image of CNT@MnO2 @PVA yarn; (e) SEM image of the asymmetric two-ply yarn supercapacitor CNT@MnO2 @PVA k CNT@PVA; (f) optical image of the asymmetric two-ply yarn supercapacitor CNT@MnO2 @PVA k CNT@PVA.
Figure 4(a) shows that the carbon nanotubes are well aligned in the as-spun CNT yarn. After being coated with the MnO2 @PVA (figure 4(b)), the yarn surface is covered by bright particles (figure 4(c)) of sizes from submicron to around 2 µm. Energy-dispersive x-ray spectroscopy (figure 4(d)) confirmed that the elemental composition of the bright particles was MnO2 . 3.2. Symmetric versus asymmetric two-ply yarn supercapacitors
The CV curves of the three two-ply yarn supercapacitors at the same scanning rate of 0.05 V s−1 are shown in figure 5(a).
Figure 5. Electrochemical properties of the symmetric and asymmetric two-ply yarn supercapacitors. (a) Cyclic voltammograph at 50 mV s−1 ; (b) galvanostatic charge/discharge curves at 0.267 A g−1 ; (c) gram capacitance as a function of charge–discharge current density. 4
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Figure 6. Electrochemical impedance spectroscopy (EIS) of symmetric and asymmetric two-ply yarn supercapacitors. (a) full range; (b) magnified plot at the high frequency end.
a current density of 2.67 A g−1 , the gram capacitance of the asymmetric two-ply yarn supercapacitor was over five times greater than that of the symmetric supercapacitor CNT k CNT. This improvement could be attributed to the utilization of the pseudocapacitive MnO2 particles in the asymmetric supercapacitor architecture [15, 16]. The asymmetric supercapacitor CNT@MnO2 k CNT showed a capacitance of 12.5 F g−1 at a current density of 0.14 A g−1 , which is higher than that of previously reported single-walled carbon nanotube (CNT) fibres containing (salmon) DNA (7.2 F g−1 ) [1] but lower than that of graphene fibre-based supercapacitors (25–40 F g−1 ) [25]. The electrochemical behaviour of the two symmetric and one asymmetric two-ply yarn supercapacitors was further analysed using electrochemical impedance spectroscopy (EIS). In the complex-plane impedance plot (Nyquist plot) in figure 6(a), each data point was measured at a different frequency (0.01 Hz–500 kHz) and the lower left portion of the plot corresponds to higher frequencies. The intersection of the Nyquist plot at the Z 0 -axis represents the equivalent series resistance (ESR) of the corresponding two-electrode supercapacitor [2, 34]. As shown in the magnified figure (figure 6(b)), the incorporation of MnO2 particles in the symmetric supercapacitor resulted in a significant reduction of ESR, but the asymmetric supercapacitor CNT@MnO2 k CNT showed similar ESR to the symmetric supercapacitor CNT k CNT. At the lower frequency end (figure 6(a)), the curves of the two symmetric supercapacitors were closer to the Z 00 -axis than the asymmetric supercapacitor, indicating more capacitive behaviour [2].
measured in the same voltage window, 0–2.0 V, but at different scan rates from 0.05 to 2.0 V s−1 . The CV curves maintained their near-rectangular shapes at even the highest scan rate of 2.0 V s−1 . Figure 7(c) shows the galvanostatic charge– discharge curves in different potential windows measured from the asymmetric two-ply yarn supercapacitor at a constant current density of 0.267 A g−1 . Both the charge and the discharge curves remained in reasonably good symmetry at all voltages up to 2.0 V. The charge time and discharge time were nearly proportional to the charge–discharge potential, indicating rapid I –V response and ideal capacitive characteristics. The charge–discharge characteristics of the asymmetric supercapacitor in the potential window of 0–2.0 V were further evaluated at different current densities ranging from 0.067 to 5.33 A g−1 , all exhibiting near-to-triangular charge–discharge curves, as shown in figure 7(d). The main advantage of the asymmetric supercapacitor came from the extension of the electrochemical potential window from 1.0 to 2.0 V. The specific capacitance and energy density of the asymmetric two-ply yarn supercapacitor are plotted against the potential window span in figure 7(e). The higher operating voltage significantly improved the energy storage capacity of the asymmetric supercapacitor. The gram capacitance increased by 54% (from 11.4 to 17.5 F g−1 ) and the energy density increased more than six-fold (from 5.66 to 35.0 Wh kg−1 ) due to the extension of operating potential from 1.0 to 2.0 V. Ragone plots are commonly used for performance comparison of different energy storing devices. The energy and power densities of the supercapacitors were calculated from the discharge curves measured at different current densities. Figure 7(f) compares the Ragone plot of the asymmetric two-ply yarn supercapacitor (CNT@MnO2 k CNT) with that of the symmetric two-ply yarn supercapacitor CNT k CNT. At equivalent charging conditions, the asymmetric supercapacitor CNT@MnO2 k CNT demonstrated nearly 20 times higher energy density and two times higher power density than the symmetric supercapacitor CNT k CNT. The symmetric supercapacitor had a low energy density between 1 and 2.12 Wh kg−1 over the range of power density from 241.8 to 10 000 W kg−1 . In contrast, the energy density of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k
3.3. Characteristics of the asymmetric two-ply yarn supercapacitor
Figure 7 displays the electrochemical characteristics of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT. The CV curves in figure 7(a) were measured at different potential windows up to 2.0 V at a constant scan rate of 0.1 V s−1 . These curves show basically rectangular shapes with approximately mirror images with respect to the zerocurrent line, demonstrating the pseudocapacitive behaviour of the MnO2 electrode. Figure 7(b) displays another series of CV curves of the asymmetric two-ply yarn supercapacitor 5
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Figure 7. Electrochemical properties of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT. (a) CV curves in different potential windows measured at a constant scan rate of 0.1 V s−1 ; (b) CV curves in the potential window from 0 to 2.0 V measured at different scan rates; (c) galvanostatic charge–discharge curves in different potential windows measured at a constant current density of 0.267 A g−1 ; (d) galvanostatic charge–discharge curves in the same potential window from 0 to 2.0 V measured at different current densities; (e) capacitance and energy density window at 0.267 A g−1 as a function of the operating potential span; (f) Ragone plot showing the relation between energy density and power density for the symmetric two-ply yarn supercapacitor CNT k CNT and asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT.
Figure 8. Stability of the asymmetric two-ply yarn supercapacitor CNT@MnO2 k CNT derived from galvanostatic charge–discharge curves conducted at 0.533 A g−1 . (a) capacitance retention after cyclic charge–discharge; (b) capacitance retention at different bending angles; (c)
capacitance retention after cyclic folding and unfolding.
CNT was up to 42.0 Wh kg−1 at the low power density of 483.7 W kg−1 , and 28.02 Wh kg−1 at the high power density of 19 250 W kg−1 . These values were much higher than the energy density of the symmetric film CNT supercapacitor (
