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Changsoon Choi, Jae Ah Lee, A Young Choi, Youn Tae Kim,* Xavier Lepró, Marcio D. Lima, Ray H. Baughman, and Seon Jeong Kim* There has been a lot of interest in flexible, lightweight, and high-power energy devices for wearable smart cloth and miniaturized electronic applications. To meet the demands for such applications, recent research has focused on dimension conversion of energy devices from three- or two-dimensional (3D, 2D) types to one-dimensional (1D) fibrous structure.[1] Such a trend is well demonstrated in energy generation or conversion fields, for example, fiber photovoltaic cells,[2–8] fiber piezoelectric generators,[9,10] fiber thermoelectric generators,[11] and fiber biofuel cells.[12] As for supercapacitors, one of the next-generation energy storage media for a high level of electrical power and long lifetime,[13] nanowire/microfiber hybrid-structure supercapacitors,[14,15] a pen ink decorated metal wire supercapacitor,[16] and a self-powered system integrated supercapacitor[17] have recently been reported. Nevertheless, such fiber supercapacitors still suffer from complicated fabrication methods, complex structures, and rigidity or relatively low flexibility, which could be obstacles to scaling-up and wearable electronics applications. Meanwhile, realizing high electrochemical performance of supercapacitors is another important issue. Especially for supercapacitors based on manganese oxide (MnO2) — a promising transition metal oxide as a pseudo-capacitive material with high theoretical capacitance, low cost, natural abundance, and environmental friendliness[18] —overcoming the poor electrical conductivity (10−5–10−6 S cm–1) of the MnO2 still remains an unavoidable challenge[19] to be addressed for optimization of its charge storage performance. Accordingly, several research groups have introduced some structural strategies for electrode design in order to enhance the electrical conductivity and facilitate the full utilization of MnO2 by incorporating metal oxide or metal-based nanostructures as an effective electron pathway.
C. Choi, J. A. Lee, Prof. S. J. Kim Center for Bio-Artificial Muscle and Department of Biomedical Engineering Hanyang University Seoul, 133-791, Republic of Korea E-mail: [email protected] A. Y. Choi, Prof. Y. T. Kim IT Fusion Technology Research Center and Department of IT Fusion Technology Chosun University Gwangju, 501-759, Republic of Korea E-mail: [email protected] Dr. X. Lepró, Dr. M. D. Lima, Prof. R. H. Baughman The Alan G. MacDiarmid NanoTech Institute University of Texas at Dallas Richardson, TX, 75083, USA
DOI: 10.1002/adma.201304736
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For example, a variety of nanowires, such as SnO2,[20] ZnO,[14] Zn2SnO4,[15] Co3O4,[21] and WO3,[22] have been grown on the surface of current collectors and nanoscopic MnO2 deposited on them to fabricate core/shell-structured hybrid electrodes. In addition, nanotube arrays of Mn have been synthesized and the tube surface oxidized to make a MnO2/Mn/MnO2 sandwichstructured electrode.[23] High electrolyte surface area and a fast charge storage process have been effectively achieved by such uniquely designed architectures, resulting in high specific capacitance and rate capability. However, these electrodes require complex multistep fabrication processes for growing nanostructures and they can be sensitive to mechanical deformation such as folding or twisting in real applications owing to the delicate or fragile nature of metal oxide–based nanostructures. In the study presented in this Communication, in order to overcome the structural drawbacks of these reported electrodes without impairing their high electrochemical performance, we developed a yarn electrode with the novel design of poretrapped MnO2. The multi-walled carbon nanotube (MWNT) yarn electrode is prepared by twisting MWNT sheets to assign high internal porosity and MnO2 is deposited on it to make a CNT/MnO2 composite yarn (CMY) supercapacitor. Owing to the 3D porosity inside the yarn, MnO2 can be effectively trapped in the pores during deposition, resulting in formation of a well-blended zone that consists of nanoscopic MnO2 and aligned CNT bundles. This CMY blend zone is expected to have several positive effects on electrochemical performance as follows: first, it enables an enlarged electrolytic surface area of MnO2, providing more active sites for cations during faradic reaction. Moreover, short ion-diffusion length is achieved by nanopore-trapped MnO2, which will facilitate full utilization of MnO2 even at high scan rate. Finally, effective electron delivery between the MnO2 (active material) and uniaxially aligned adjacent CNT bundles of yarn (current collector) is accomplished, leading to low resistance of the electrode. Consequently, high values of specific capacitance (25.4 F cm–3 at 10 mV s–1) and energy and average power densities (3.52 mWh cm–3 and 127 mW cm–3, repectively) have been achieved in micrometerdiameter all-solid-state yarn supercapacitors. This functionalized yarn is so strong, lightweight, and exceptionally flexible that it is expected to be applicable to wearable devices[24] or microelectromechanical systems (MEMSs) where high performance is necessary.[25] A bare CNT yarn was used as a current collector and a substrate simultaneously in this work because of its highly ordered and internally porous structure, extraordinary mechanical properties,[26] and good electrical conductivity.[27] The MWNT sheets were drawn from a MWNT forest made using the chemical
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Flexible Supercapacitor Made of Carbon Nanotube Yarn with Internal Pores
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Figure 1. a) Overview SEM image of CMY. The as-calculated volume is about 2.1 × 10–6 cm3, assuming that the yarn is cylindrical. b,c) Magnified surface SEM images of bare yarn (b) and CMY (c). Well-coated flower-shaped MnO2 particles can be seen along the CNT bundles. d,e) Results of XPS analysis of MnO2 coating layer for oxidation state determination. Binding energies of Mn 2p (d) and O 1s (e) components are shown.
vapor deposition (CVD) method previously reported.[27] Two- or three-layer sheets were stacked and twisted to make the flexible, electrically conductive current collector yarn, affording internal porosity. Deposition of the MnO2 film onto the yarn was followed by a one-step potentiostatic method that does not include additional vacuum or annealing processes. We believe that the simply structured electrode and easy fabrication processes for the CMY are suitable for a mass-production system. The overview morphology of CMY is shown in Figure 1a. From the scanning electron microscopy (SEM) image, the bias angle α, which is the angle between the longitudinal direction of the yarn and the orientation direction of the CNT bundle, is about 30° and the diameter of the CMY can be controlled to be about 16–28 μm. The magnified surface image of the bare yarn (Figure 1b) shows uniaxially aligned and tightly packed CNT bundles in one direction. Such well-aligned bundles provide a highly effective electron pathway,[28,29] which can be advantageously used as a current collector for flexible 1D electronic devices. Deposition of MnO2 onto the yarn’s surface results 2060
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in formation of a flower-like patterned deposit,[30] as shown in Figure 1c. X-ray photoelectron spectroscopy (XPS) analysis was carried out to determine the oxidation state of the MnO2. The binding-energy separation between the Mn 2p3/2 and Mn 2p1/2 doublet peaks is 11.8 eV (Figure 1d), which exactly matches the reported energy separation in MnO2.[31] From the oxygen 1s orbital analysis, intensity ratios of three overlapping peaks, Mn–O–Mn, Mn–O–H, and H–O–H, are confirmed to be 1, 0.18, and 0.21, respectively (Figure 1e). A more accurate oxidation state can be mathematically calculated using Equation (1) with S being the peak intensity[32] Ox state =
4(SMn−O−Mn − SMn−OH ) + 3SMn−OH SMn−O−Mn
(1)
According to this equation, the oxidation state of our MnO2 deposits is confirmed to be 3.82, which shows good agreement with the Mn peak analysis. For structural investigation, the CMY was cut using a focused ion beam. Figure 2a shows a cross-sectional SEM image of
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COMMUNICATION Figure 2. a) Cross-sectional SEM image of CMY electrode (inset, scale bar is 5 μm) and its magnified image (scale bar in inset represents 5 μm), showing high internal porosity. b) Magnified edge part of the CMY. The core/shell (CNT/MnO2) structure and direction of gradual concentration decrement of MnO2 are shown. c) Elemental mapping by EDS over the image (b). d) EDS line-scan data along the dashed line shown in (b). It can be confirmed that the Mn’s atomic percentage starts to decrease from the yarn surface in the direction of the core, having a concentration gradient to a depth of approximately 2–3 μm.
CMY. It can be confirmed that mesopores (2–50 nm in diameter) and macropores (larger than 50 nm) are densely formed among the CNT bundles. The porosity of the bare MWNT yarn was calculated using
=
Vt − Vc Vc
(2)
where is porosity, Vt the total volume of the MWNT yarn, and Vc the volume of MWNT. The MWNT volume was calculated by dividing the MWNT mass by the density of MWNT bundles (ca. 1.67 g cm–3). The porosity of bare MWNT yarn was calculated to be about 60%. Because inherently porous CNTs and the aligned bundles build up a high-order porous CNT yarn electrode, hierarchical porosity and good electrical conductivity were simultaneously achieved. In Figure 2b, a MnO2 layer about 200–300 nm thick is observed to cover the core CNT yarn. For further investigation, energy dispersive spectroscopy (EDS) and elemental mapping analysis were performed as shown in Figures 2c and d. The maximum atomic percentage of Mn at the yarn’s surface starts to decrease in the direction towards the center of the yarn, having a concentration gradient to a depth of approximately 2–3 μm. It can be stated, therefore, that the MnO2 shell overlaps with the MWNTs, rather than being discretely deposited on the surface of the collector, forming a blended
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or hybridized CNT/MnO2. A similar Mn infiltration tendency is also observed at the other side of the yarn (Figure S1, Supporting Information) and no Mn peak is found in the core part of the yarn (Figure S2, Supporting Information). In this CNT/MnO2 blended zone, because of the self-limited growth of MnO2—limited by the well-developed pores—not only do ions from the electrolyte have great accessibility to the enlarged electrochemically active area of nanostructured MnO2, but also the solid-state ion diffusion length in the metal oxide is dramatically shortened. Moreover, electrons generated according to[33] MnIII (x + y) MnIV 1−(x + y) OOCx H y → MnIV O2 + xC + + yH + + (x + y)e−
(3)
during the discharging process can be effectively collected by adjacent CNT bundles, enabling short electron diffusion length and small contact resistance. Although the volume fraction of MnO2 is extremely small compared to the total volume, after the MnO2 deposition a cyclic voltammogram (CV) area approximately five times that before the deposition was obtained (Figure S3, Supporting Information). This high contribution of MnO2 to energy storage capacity is notable compared with other MnO2-based wire-shaped
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Figure 3. a,b) CV curves of CMY (a) and MCF (b) electrodes with various scan rates (three-electrode system in 0.1M Na2SO4). c) Plot of CV area ratios to compare the retention performance at high scan rates. d–f) Performance of solid-state CMY supercapacitor (symmetric, PVA-KOH gel electrolyte): volumetric capacitance as a function of scan rate (d), CV curves with various scan rates (e), and Nyquist curve (f). All performances of the solid-state CMY supercapacitor are normalized by total volume containing MWNT yarn and MnO2. Inset in (d): Optical image of the CMY supercapacitor (scale bar represents 30 μm). Inset in (f): High frequency region (100 kHz~) of the Nyquist curve.
micro-supercapacitors (areal capacitance of 3.01 mF cm–2 for the bare electrode is increased to 3.707 mF cm–2 after MnO2 deposition).[34] The weight of deposited MnO2 was measured by electrochemical quartz-crystal microbalance (EQCM). The mass of MnO2 per area is about 20.4–27.6 μg cm–2, which converts to 4.44–6 wt% of total mass. The relatively small weight percentage of the MnO2 loaded on the yarn can be explained by our 1D electrode’s morphology characteristic that only the surface of the MWNT yarn is utilized as a MnO2 loading site and the core does not participate in MnO2 loading, just acting as an electrical pathway. To confirm the importance of the internally trapped MnO2, we deposited MnO2 onto carbon fiber (MCF) as a control electrode, which has no internal porosity (detailed comparison information is presented in Figure S4 in the Supporting Information). Electrochemical performances of the CMY and MCF electrodes were measured using a three-electrode system in 0.1 M Na2SO4. In Figure 3a, cyclic voltammetry (CV) graphs of CMY at different scan rates are shown. The rectangular CV shape at such a high scan rate of 3000 mV s–1 represents a very small equivalent series resistance (ESR) of the CMY and fast ion diffusion into the porous structured composite yarn. On the other hand, CV of MCF in Figure 3b shows significantly dented CVs at high scan rates. Their rate capability can be calculated by CV area ratio, which is the CV area normalized by initial CV area (at 100 mV s–1), and scan rates (Figure 3c). The CMY retains its CV area up to 62% at 1000 mV s–1, and 28.3% at 3000 mV s–1, while MCF shows only 11.5% and 2.6% retention, respectively. These capacitance drops at high scan rate can be explained by assuming that the ion-accessible area of the metal
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oxide decreases as the scan rate increases because charge diffusion in MnO2 with poor electrical conductivity is interrupted owing to the time constraint for fast charging/discharging. Therefore, this leads to only the outer surface of the MnO2 being electrochemically activated,[35] resulting in low performance at high scan rate. This kind of problem can be alleviated by incorporating nanostructured MnO2 for short ion diffusion length.[36–38] In this research, as one kind of solution for such a research trend, high rate capability of the CMY electrode is achieved by the advanced strategic design of nanostructured MnO2, which is realized inside the highly porous yarn. Therefore, fast charge transfer, enlarged electrochemical surface area, and short ion diffusion length are effectively achieved, while electrochemical and structural stabilities of the whole electrode are well maintained under applied deformation without significant performance degeneration, as discussed later. For a real application, we fabricated a solid-state and totally flexible CMY supercapacitor by placing two symmetric CMY electrodes on top of each other and coating them with poly(vinyl alcohol)–potassium hydroxide (PVA-KOH) gel electrolyte. The resulting volumetric capacitance (normalized by total volume of active materials including MWNY yarn and MnO2) of the solidstate CMY supercapacitor is shown in Figure 3d. The highest volumetric capacitance was about 25.4 F cm–3, which is about 10 times the literature value (2.5 F cm–3) of the maximum volumetric capacitance of MnO2/carbon fiber.[17] The CV graphs of the solid-state CMY supercapacitor are presented in Figure 3e. A pseudo-capacitive rectangular CV shape is observed up to 1000 mV s–1 scan rate. In electrochemical impedance spectroscopy (EIS) measurement, normalized ESR measured at
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COMMUNICATION Figure 4. a) Optical image of hand-made CMY textile consisting of 15 yarns. Inset: A CMY electrode woven into a commercial textile (scale bar represents 1 mm). b) CV graphs of not deformed, wound, and knotted CMY electrodes at 100 mV s–1. Insets: Images of wound (scale bar represents 1 mm) and knotted CMY electrodes (scale bar represents 25 μm; diameter of the glass tube for the winding test was about 1.5 mm). c) Comparison of CV plots of solid-state CMY supercapacitor before and after 1000th bending test. Inset: Image of bent CMY supercapacitor with 90° bending angle; scale bar represents 1 mm. d) Volumetric energy and average power densities for solid-state CMY supercapacitor and other carbon fiber supercapacitors.[17,40] The highest values of energy and average power densities of our work are 3.52 mWh cm–3 and 127 mW cm–3, respectively.
1 kHz for the CMY supercapacitor is as small as 2.12 mΩ cm3. In addition, the high slope of the Nyquist curve in the highfrequency region also implies a good capacitive characteristic[39] for our CMY supercapacitor. Because of the high flexibility and mechanical properties of the CMY, it is expected that it will make a great building block for smart cloth. A textile was fabricated using 15 CMY electrodes, and one thread of CMY was woven into a commercial textile, as shown in Figure 4a. Moreover, once energy is captured from the environment, it needs to be stored for later use. Therefore, the CMY can be an ideal energy storage medium for integrated or packed energy systems with already reported 1D fibrous energy conversion or generation devices[2–12]. For further investigation of flexibility, the CMY electrode was wound around a glass tube, and a knot was even made in the middle of it by hand, as shown in the inset of Figure 4b. Although some partially peeled-off brittle MnO2 spots are observed at the knot, the areas of CV plots from knotted and wound yarns were almost identical to that of the non-deformed one. In addition, we performed a bending cycling test with a solid-state CMY supercapacitor to figure out the effect of stress on it. After the 1000th bending with 90° bending angle, no significant capacitance drop was observed, as shown in Figure 4c. These results from Figures 4b and c imply that CMY’s performance could be quite well maintained under mechanically harsh conditions in real applications. We believe that such a novel capacitance preservation
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ability against mechanical deformation mainly originates from both the CNT yarn’s unique mechanical properties[27] and good adhesion of MnO2 to the collector in our case. Volumetric energy and average power densities of the solid-state CMY supercapacitor and values for carbon fiber supercapacitors from the literature are shown in Figure 4d. In the plots, the highest values of energy and average power densities of the CMY supercapacitor are 3.52 mWh cm–3 and 127 mW cm–3, respectively. The values are significantly higher than for the MnO2/carbon fiber supercapacitor (0.22 mWh cm–3 and 8 mW cm–3)[17] and for the MnO2/ZnO nanowire grown carbon fiber fabric supercapacitor (0.04 mWh cm–3 and 2.44 mW cm–3 ).[40] In summary, we have fabricated a flexible, micrometer-diameter yarn supercapacitor. Internally porous, uniaxially aligned CNT yarn was used as the current collector and MnO2 was electrochemically deposited, constructing a hybridized CNT/MnO2 structure in the yarn electrode. In such a hybridized zone, entrapped MnO2 in the inner nanopores provides high pseudocapacitive surface area and short ion diffusion length. Therefore, the uniquely designed composite yarn shows high specific capacitance, high capacitance retention ability at fast charge/ discharge rate, and enhanced energy and average power densities. It also maintains its structure and electrochemical performance under mechanical deformation. We believe that the CMY micrometer-sized supercapacitor can be utilized in MEMS where a high-performance energy source is required or under
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particularly harsh conditions where the use of a conventional planar-type supercapacitor is definitely limited. More research on integration with fiber energy generation is in progress for a fiber-type one-body energy generation and storage system.
CNT yarn electrode preparation and cross-sectional analysis: MWNT sheets were drawn from a CNT forest fabricated using the CVD method.[19] Two layers of 3 mm × 75 mm-sized sheets were stacked and densified using ethanol. Then they were twisted about 10000 times per meter to make a very dense and compact yarn. The lengths of CMY for the three-electrode experiments and CMY device were fixed at 1 cm. Cross sections of CMY were prepared by cutting it along its diameter using a Ga ion beam (7 nA beam current) in a focused ion beam instrument (FIB, FEI Nova 200) operated at 30 kV. The cleaned cut yarn was next transferred to a scanning electron microscope (Zeiss Supra 40) to perform the microscopy (at 15 kV) and elemental energy-dispersive X-ray spectroscopy (EDAX) mapping analysis (at 20 kV). SEM images other than the cross sections were obtained using an FE SEM-S4700 from Hitachi. MnO2 film deposition and electrochemical performance measurement: Electrochemical deposition of MnO2 and performance measurements for the CMY and MCF were conducted with a three-electrode system (CHI 627b, CH Instruments, Austin, TX), using Ag/AgCl as the reference electrode and Pt mesh as the counter electrode. MnO2 was deposited onto the bare yarn using the potentiostatic method. A potential of 1.3 V was applied for 3–6 s in an electrolyte of 0.02 M MnSO4·5H2O and 0.2 M Na2SO4 for instant deposition. The deposits’ oxidation state was characterized by XPS (VG Multilab ESCA 2000 system) analysis. CVs for CMY and MCF were measured in 0.1 M Na2SO4 solution. Solid-state CMY supercapacitor fabrication: PVA-KOH gel electrolyte was prepared by mixing 3 g PVA (Sigma Aldrich) with 1.62 g KOH (J. T. Baker) in 30 mL deionized water and heating it at 90 °C for several hours until it become transparent. Two prepared CMYs were coated with PVA-KOH several times and dried at room temperature for 5 h. Then, two PVA-KOH coated CMYs were carefully placed on top of each other, so as not to cause an electrical short circuit, and finally coated by PVA-KOH again to make a solid-state CMY supercapacitor. Calculation of volumetric capacitance, MnO2 weight, energy, and power densities: The volumetric capacitance was calculated from the CV data using Q(CNT /MnO2 yarn) V × volume(CNT /MnO2 yarn)
(4)
where Q is the charge and ΔV is the width of the voltage window. With the EQCM measurement, oscillation frequency differences between the working and reference crystals were transformed into deposited MnO2 mass using the Sauerbrey equation:[41] f =
−2 f 02 M A(:D )0.5
(5)
where f0 is the resonant frequency of the reference crystal, and μ and ρ are the shear parameter and density of quartz, respectively. With electrode area (A), the mass change (ΔM) of a deposit can be calculated by recording the frequency change (Δf) between the working crystal and the reference one. From the slope shown in Figure S5 (Supporting Information), the mass of as-deposited MnO2 per unit charge transferred was determined to be approximately 5.41 × 10−4 g C–1. For a given constant scan rate v [V s−1], the average power during discharge (Pav [W]) was calculated by integrating the current density (I) versus voltage (V) curve:[42] 1 Pav = I dV (6) 2
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E =
Vi × Pav v × 3600
(7)
where Vi is the initial voltage during discharge.
Experimental Section
C volumetric =
Integration to obtain the average power during charge and discharge provided nearly identical results. The discharged energy (E [W h]) was obtained using
Supporting information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Creative Research Initiative Center for Bio-Artificial Muscle of the Ministry of Science, ICT & Future Planning (MSIP), the MSIP–US Air Force Cooperation Program (NRF2013K1A3A1A32035592), and the Industrial Strategic Technology Development Program (10038599) in Korea; Air Force Grant AOARD10-4067, Air Force Office of Scientific Research grant FA9550-09-1-0537, and Robert A. Welch Foundation grant AT-0029 in the USA. Received: September 21, 2013 Published online: December 18, 2013
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Changsoon Choi, Jae Ah Lee, A Young Choi, Youn Tae Kim,* Xavier Lepró, Marcio D. Lima, Ray H. Baughman, and Seon Jeong Kim* There has been a lot of interest in flexible, lightweight, and high-power energy devices for wearable smart cloth and miniaturized electronic applications. To meet the demands for such applications, recent research has focused on dimension conversion of energy devices from three- or two-dimensional (3D, 2D) types to one-dimensional (1D) fibrous structure.[1] Such a trend is well demonstrated in energy generation or conversion fields, for example, fiber photovoltaic cells,[2–8] fiber piezoelectric generators,[9,10] fiber thermoelectric generators,[11] and fiber biofuel cells.[12] As for supercapacitors, one of the next-generation energy storage media for a high level of electrical power and long lifetime,[13] nanowire/microfiber hybrid-structure supercapacitors,[14,15] a pen ink decorated metal wire supercapacitor,[16] and a self-powered system integrated supercapacitor[17] have recently been reported. Nevertheless, such fiber supercapacitors still suffer from complicated fabrication methods, complex structures, and rigidity or relatively low flexibility, which could be obstacles to scaling-up and wearable electronics applications. Meanwhile, realizing high electrochemical performance of supercapacitors is another important issue. Especially for supercapacitors based on manganese oxide (MnO2) — a promising transition metal oxide as a pseudo-capacitive material with high theoretical capacitance, low cost, natural abundance, and environmental friendliness[18] —overcoming the poor electrical conductivity (10−5–10−6 S cm–1) of the MnO2 still remains an unavoidable challenge[19] to be addressed for optimization of its charge storage performance. Accordingly, several research groups have introduced some structural strategies for electrode design in order to enhance the electrical conductivity and facilitate the full utilization of MnO2 by incorporating metal oxide or metal-based nanostructures as an effective electron pathway.
C. Choi, J. A. Lee, Prof. S. J. Kim Center for Bio-Artificial Muscle and Department of Biomedical Engineering Hanyang University Seoul, 133-791, Republic of Korea E-mail: [email protected] A. Y. Choi, Prof. Y. T. Kim IT Fusion Technology Research Center and Department of IT Fusion Technology Chosun University Gwangju, 501-759, Republic of Korea E-mail: [email protected] Dr. X. Lepró, Dr. M. D. Lima, Prof. R. H. Baughman The Alan G. MacDiarmid NanoTech Institute University of Texas at Dallas Richardson, TX, 75083, USA
DOI: 10.1002/adma.201304736
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For example, a variety of nanowires, such as SnO2,[20] ZnO,[14] Zn2SnO4,[15] Co3O4,[21] and WO3,[22] have been grown on the surface of current collectors and nanoscopic MnO2 deposited on them to fabricate core/shell-structured hybrid electrodes. In addition, nanotube arrays of Mn have been synthesized and the tube surface oxidized to make a MnO2/Mn/MnO2 sandwichstructured electrode.[23] High electrolyte surface area and a fast charge storage process have been effectively achieved by such uniquely designed architectures, resulting in high specific capacitance and rate capability. However, these electrodes require complex multistep fabrication processes for growing nanostructures and they can be sensitive to mechanical deformation such as folding or twisting in real applications owing to the delicate or fragile nature of metal oxide–based nanostructures. In the study presented in this Communication, in order to overcome the structural drawbacks of these reported electrodes without impairing their high electrochemical performance, we developed a yarn electrode with the novel design of poretrapped MnO2. The multi-walled carbon nanotube (MWNT) yarn electrode is prepared by twisting MWNT sheets to assign high internal porosity and MnO2 is deposited on it to make a CNT/MnO2 composite yarn (CMY) supercapacitor. Owing to the 3D porosity inside the yarn, MnO2 can be effectively trapped in the pores during deposition, resulting in formation of a well-blended zone that consists of nanoscopic MnO2 and aligned CNT bundles. This CMY blend zone is expected to have several positive effects on electrochemical performance as follows: first, it enables an enlarged electrolytic surface area of MnO2, providing more active sites for cations during faradic reaction. Moreover, short ion-diffusion length is achieved by nanopore-trapped MnO2, which will facilitate full utilization of MnO2 even at high scan rate. Finally, effective electron delivery between the MnO2 (active material) and uniaxially aligned adjacent CNT bundles of yarn (current collector) is accomplished, leading to low resistance of the electrode. Consequently, high values of specific capacitance (25.4 F cm–3 at 10 mV s–1) and energy and average power densities (3.52 mWh cm–3 and 127 mW cm–3, repectively) have been achieved in micrometerdiameter all-solid-state yarn supercapacitors. This functionalized yarn is so strong, lightweight, and exceptionally flexible that it is expected to be applicable to wearable devices[24] or microelectromechanical systems (MEMSs) where high performance is necessary.[25] A bare CNT yarn was used as a current collector and a substrate simultaneously in this work because of its highly ordered and internally porous structure, extraordinary mechanical properties,[26] and good electrical conductivity.[27] The MWNT sheets were drawn from a MWNT forest made using the chemical
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Flexible Supercapacitor Made of Carbon Nanotube Yarn with Internal Pores
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Figure 1. a) Overview SEM image of CMY. The as-calculated volume is about 2.1 × 10–6 cm3, assuming that the yarn is cylindrical. b,c) Magnified surface SEM images of bare yarn (b) and CMY (c). Well-coated flower-shaped MnO2 particles can be seen along the CNT bundles. d,e) Results of XPS analysis of MnO2 coating layer for oxidation state determination. Binding energies of Mn 2p (d) and O 1s (e) components are shown.
vapor deposition (CVD) method previously reported.[27] Two- or three-layer sheets were stacked and twisted to make the flexible, electrically conductive current collector yarn, affording internal porosity. Deposition of the MnO2 film onto the yarn was followed by a one-step potentiostatic method that does not include additional vacuum or annealing processes. We believe that the simply structured electrode and easy fabrication processes for the CMY are suitable for a mass-production system. The overview morphology of CMY is shown in Figure 1a. From the scanning electron microscopy (SEM) image, the bias angle α, which is the angle between the longitudinal direction of the yarn and the orientation direction of the CNT bundle, is about 30° and the diameter of the CMY can be controlled to be about 16–28 μm. The magnified surface image of the bare yarn (Figure 1b) shows uniaxially aligned and tightly packed CNT bundles in one direction. Such well-aligned bundles provide a highly effective electron pathway,[28,29] which can be advantageously used as a current collector for flexible 1D electronic devices. Deposition of MnO2 onto the yarn’s surface results 2060
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in formation of a flower-like patterned deposit,[30] as shown in Figure 1c. X-ray photoelectron spectroscopy (XPS) analysis was carried out to determine the oxidation state of the MnO2. The binding-energy separation between the Mn 2p3/2 and Mn 2p1/2 doublet peaks is 11.8 eV (Figure 1d), which exactly matches the reported energy separation in MnO2.[31] From the oxygen 1s orbital analysis, intensity ratios of three overlapping peaks, Mn–O–Mn, Mn–O–H, and H–O–H, are confirmed to be 1, 0.18, and 0.21, respectively (Figure 1e). A more accurate oxidation state can be mathematically calculated using Equation (1) with S being the peak intensity[32] Ox state =
4(SMn−O−Mn − SMn−OH ) + 3SMn−OH SMn−O−Mn
(1)
According to this equation, the oxidation state of our MnO2 deposits is confirmed to be 3.82, which shows good agreement with the Mn peak analysis. For structural investigation, the CMY was cut using a focused ion beam. Figure 2a shows a cross-sectional SEM image of
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COMMUNICATION Figure 2. a) Cross-sectional SEM image of CMY electrode (inset, scale bar is 5 μm) and its magnified image (scale bar in inset represents 5 μm), showing high internal porosity. b) Magnified edge part of the CMY. The core/shell (CNT/MnO2) structure and direction of gradual concentration decrement of MnO2 are shown. c) Elemental mapping by EDS over the image (b). d) EDS line-scan data along the dashed line shown in (b). It can be confirmed that the Mn’s atomic percentage starts to decrease from the yarn surface in the direction of the core, having a concentration gradient to a depth of approximately 2–3 μm.
CMY. It can be confirmed that mesopores (2–50 nm in diameter) and macropores (larger than 50 nm) are densely formed among the CNT bundles. The porosity of the bare MWNT yarn was calculated using
=
Vt − Vc Vc
(2)
where is porosity, Vt the total volume of the MWNT yarn, and Vc the volume of MWNT. The MWNT volume was calculated by dividing the MWNT mass by the density of MWNT bundles (ca. 1.67 g cm–3). The porosity of bare MWNT yarn was calculated to be about 60%. Because inherently porous CNTs and the aligned bundles build up a high-order porous CNT yarn electrode, hierarchical porosity and good electrical conductivity were simultaneously achieved. In Figure 2b, a MnO2 layer about 200–300 nm thick is observed to cover the core CNT yarn. For further investigation, energy dispersive spectroscopy (EDS) and elemental mapping analysis were performed as shown in Figures 2c and d. The maximum atomic percentage of Mn at the yarn’s surface starts to decrease in the direction towards the center of the yarn, having a concentration gradient to a depth of approximately 2–3 μm. It can be stated, therefore, that the MnO2 shell overlaps with the MWNTs, rather than being discretely deposited on the surface of the collector, forming a blended
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or hybridized CNT/MnO2. A similar Mn infiltration tendency is also observed at the other side of the yarn (Figure S1, Supporting Information) and no Mn peak is found in the core part of the yarn (Figure S2, Supporting Information). In this CNT/MnO2 blended zone, because of the self-limited growth of MnO2—limited by the well-developed pores—not only do ions from the electrolyte have great accessibility to the enlarged electrochemically active area of nanostructured MnO2, but also the solid-state ion diffusion length in the metal oxide is dramatically shortened. Moreover, electrons generated according to[33] MnIII (x + y) MnIV 1−(x + y) OOCx H y → MnIV O2 + xC + + yH + + (x + y)e−
(3)
during the discharging process can be effectively collected by adjacent CNT bundles, enabling short electron diffusion length and small contact resistance. Although the volume fraction of MnO2 is extremely small compared to the total volume, after the MnO2 deposition a cyclic voltammogram (CV) area approximately five times that before the deposition was obtained (Figure S3, Supporting Information). This high contribution of MnO2 to energy storage capacity is notable compared with other MnO2-based wire-shaped
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Figure 3. a,b) CV curves of CMY (a) and MCF (b) electrodes with various scan rates (three-electrode system in 0.1M Na2SO4). c) Plot of CV area ratios to compare the retention performance at high scan rates. d–f) Performance of solid-state CMY supercapacitor (symmetric, PVA-KOH gel electrolyte): volumetric capacitance as a function of scan rate (d), CV curves with various scan rates (e), and Nyquist curve (f). All performances of the solid-state CMY supercapacitor are normalized by total volume containing MWNT yarn and MnO2. Inset in (d): Optical image of the CMY supercapacitor (scale bar represents 30 μm). Inset in (f): High frequency region (100 kHz~) of the Nyquist curve.
micro-supercapacitors (areal capacitance of 3.01 mF cm–2 for the bare electrode is increased to 3.707 mF cm–2 after MnO2 deposition).[34] The weight of deposited MnO2 was measured by electrochemical quartz-crystal microbalance (EQCM). The mass of MnO2 per area is about 20.4–27.6 μg cm–2, which converts to 4.44–6 wt% of total mass. The relatively small weight percentage of the MnO2 loaded on the yarn can be explained by our 1D electrode’s morphology characteristic that only the surface of the MWNT yarn is utilized as a MnO2 loading site and the core does not participate in MnO2 loading, just acting as an electrical pathway. To confirm the importance of the internally trapped MnO2, we deposited MnO2 onto carbon fiber (MCF) as a control electrode, which has no internal porosity (detailed comparison information is presented in Figure S4 in the Supporting Information). Electrochemical performances of the CMY and MCF electrodes were measured using a three-electrode system in 0.1 M Na2SO4. In Figure 3a, cyclic voltammetry (CV) graphs of CMY at different scan rates are shown. The rectangular CV shape at such a high scan rate of 3000 mV s–1 represents a very small equivalent series resistance (ESR) of the CMY and fast ion diffusion into the porous structured composite yarn. On the other hand, CV of MCF in Figure 3b shows significantly dented CVs at high scan rates. Their rate capability can be calculated by CV area ratio, which is the CV area normalized by initial CV area (at 100 mV s–1), and scan rates (Figure 3c). The CMY retains its CV area up to 62% at 1000 mV s–1, and 28.3% at 3000 mV s–1, while MCF shows only 11.5% and 2.6% retention, respectively. These capacitance drops at high scan rate can be explained by assuming that the ion-accessible area of the metal
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oxide decreases as the scan rate increases because charge diffusion in MnO2 with poor electrical conductivity is interrupted owing to the time constraint for fast charging/discharging. Therefore, this leads to only the outer surface of the MnO2 being electrochemically activated,[35] resulting in low performance at high scan rate. This kind of problem can be alleviated by incorporating nanostructured MnO2 for short ion diffusion length.[36–38] In this research, as one kind of solution for such a research trend, high rate capability of the CMY electrode is achieved by the advanced strategic design of nanostructured MnO2, which is realized inside the highly porous yarn. Therefore, fast charge transfer, enlarged electrochemical surface area, and short ion diffusion length are effectively achieved, while electrochemical and structural stabilities of the whole electrode are well maintained under applied deformation without significant performance degeneration, as discussed later. For a real application, we fabricated a solid-state and totally flexible CMY supercapacitor by placing two symmetric CMY electrodes on top of each other and coating them with poly(vinyl alcohol)–potassium hydroxide (PVA-KOH) gel electrolyte. The resulting volumetric capacitance (normalized by total volume of active materials including MWNY yarn and MnO2) of the solidstate CMY supercapacitor is shown in Figure 3d. The highest volumetric capacitance was about 25.4 F cm–3, which is about 10 times the literature value (2.5 F cm–3) of the maximum volumetric capacitance of MnO2/carbon fiber.[17] The CV graphs of the solid-state CMY supercapacitor are presented in Figure 3e. A pseudo-capacitive rectangular CV shape is observed up to 1000 mV s–1 scan rate. In electrochemical impedance spectroscopy (EIS) measurement, normalized ESR measured at
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COMMUNICATION Figure 4. a) Optical image of hand-made CMY textile consisting of 15 yarns. Inset: A CMY electrode woven into a commercial textile (scale bar represents 1 mm). b) CV graphs of not deformed, wound, and knotted CMY electrodes at 100 mV s–1. Insets: Images of wound (scale bar represents 1 mm) and knotted CMY electrodes (scale bar represents 25 μm; diameter of the glass tube for the winding test was about 1.5 mm). c) Comparison of CV plots of solid-state CMY supercapacitor before and after 1000th bending test. Inset: Image of bent CMY supercapacitor with 90° bending angle; scale bar represents 1 mm. d) Volumetric energy and average power densities for solid-state CMY supercapacitor and other carbon fiber supercapacitors.[17,40] The highest values of energy and average power densities of our work are 3.52 mWh cm–3 and 127 mW cm–3, respectively.
1 kHz for the CMY supercapacitor is as small as 2.12 mΩ cm3. In addition, the high slope of the Nyquist curve in the highfrequency region also implies a good capacitive characteristic[39] for our CMY supercapacitor. Because of the high flexibility and mechanical properties of the CMY, it is expected that it will make a great building block for smart cloth. A textile was fabricated using 15 CMY electrodes, and one thread of CMY was woven into a commercial textile, as shown in Figure 4a. Moreover, once energy is captured from the environment, it needs to be stored for later use. Therefore, the CMY can be an ideal energy storage medium for integrated or packed energy systems with already reported 1D fibrous energy conversion or generation devices[2–12]. For further investigation of flexibility, the CMY electrode was wound around a glass tube, and a knot was even made in the middle of it by hand, as shown in the inset of Figure 4b. Although some partially peeled-off brittle MnO2 spots are observed at the knot, the areas of CV plots from knotted and wound yarns were almost identical to that of the non-deformed one. In addition, we performed a bending cycling test with a solid-state CMY supercapacitor to figure out the effect of stress on it. After the 1000th bending with 90° bending angle, no significant capacitance drop was observed, as shown in Figure 4c. These results from Figures 4b and c imply that CMY’s performance could be quite well maintained under mechanically harsh conditions in real applications. We believe that such a novel capacitance preservation
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ability against mechanical deformation mainly originates from both the CNT yarn’s unique mechanical properties[27] and good adhesion of MnO2 to the collector in our case. Volumetric energy and average power densities of the solid-state CMY supercapacitor and values for carbon fiber supercapacitors from the literature are shown in Figure 4d. In the plots, the highest values of energy and average power densities of the CMY supercapacitor are 3.52 mWh cm–3 and 127 mW cm–3, respectively. The values are significantly higher than for the MnO2/carbon fiber supercapacitor (0.22 mWh cm–3 and 8 mW cm–3)[17] and for the MnO2/ZnO nanowire grown carbon fiber fabric supercapacitor (0.04 mWh cm–3 and 2.44 mW cm–3 ).[40] In summary, we have fabricated a flexible, micrometer-diameter yarn supercapacitor. Internally porous, uniaxially aligned CNT yarn was used as the current collector and MnO2 was electrochemically deposited, constructing a hybridized CNT/MnO2 structure in the yarn electrode. In such a hybridized zone, entrapped MnO2 in the inner nanopores provides high pseudocapacitive surface area and short ion diffusion length. Therefore, the uniquely designed composite yarn shows high specific capacitance, high capacitance retention ability at fast charge/ discharge rate, and enhanced energy and average power densities. It also maintains its structure and electrochemical performance under mechanical deformation. We believe that the CMY micrometer-sized supercapacitor can be utilized in MEMS where a high-performance energy source is required or under
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particularly harsh conditions where the use of a conventional planar-type supercapacitor is definitely limited. More research on integration with fiber energy generation is in progress for a fiber-type one-body energy generation and storage system.
CNT yarn electrode preparation and cross-sectional analysis: MWNT sheets were drawn from a CNT forest fabricated using the CVD method.[19] Two layers of 3 mm × 75 mm-sized sheets were stacked and densified using ethanol. Then they were twisted about 10000 times per meter to make a very dense and compact yarn. The lengths of CMY for the three-electrode experiments and CMY device were fixed at 1 cm. Cross sections of CMY were prepared by cutting it along its diameter using a Ga ion beam (7 nA beam current) in a focused ion beam instrument (FIB, FEI Nova 200) operated at 30 kV. The cleaned cut yarn was next transferred to a scanning electron microscope (Zeiss Supra 40) to perform the microscopy (at 15 kV) and elemental energy-dispersive X-ray spectroscopy (EDAX) mapping analysis (at 20 kV). SEM images other than the cross sections were obtained using an FE SEM-S4700 from Hitachi. MnO2 film deposition and electrochemical performance measurement: Electrochemical deposition of MnO2 and performance measurements for the CMY and MCF were conducted with a three-electrode system (CHI 627b, CH Instruments, Austin, TX), using Ag/AgCl as the reference electrode and Pt mesh as the counter electrode. MnO2 was deposited onto the bare yarn using the potentiostatic method. A potential of 1.3 V was applied for 3–6 s in an electrolyte of 0.02 M MnSO4·5H2O and 0.2 M Na2SO4 for instant deposition. The deposits’ oxidation state was characterized by XPS (VG Multilab ESCA 2000 system) analysis. CVs for CMY and MCF were measured in 0.1 M Na2SO4 solution. Solid-state CMY supercapacitor fabrication: PVA-KOH gel electrolyte was prepared by mixing 3 g PVA (Sigma Aldrich) with 1.62 g KOH (J. T. Baker) in 30 mL deionized water and heating it at 90 °C for several hours until it become transparent. Two prepared CMYs were coated with PVA-KOH several times and dried at room temperature for 5 h. Then, two PVA-KOH coated CMYs were carefully placed on top of each other, so as not to cause an electrical short circuit, and finally coated by PVA-KOH again to make a solid-state CMY supercapacitor. Calculation of volumetric capacitance, MnO2 weight, energy, and power densities: The volumetric capacitance was calculated from the CV data using Q(CNT /MnO2 yarn) V × volume(CNT /MnO2 yarn)
(4)
where Q is the charge and ΔV is the width of the voltage window. With the EQCM measurement, oscillation frequency differences between the working and reference crystals were transformed into deposited MnO2 mass using the Sauerbrey equation:[41] f =
−2 f 02 M A(:D )0.5
(5)
where f0 is the resonant frequency of the reference crystal, and μ and ρ are the shear parameter and density of quartz, respectively. With electrode area (A), the mass change (ΔM) of a deposit can be calculated by recording the frequency change (Δf) between the working crystal and the reference one. From the slope shown in Figure S5 (Supporting Information), the mass of as-deposited MnO2 per unit charge transferred was determined to be approximately 5.41 × 10−4 g C–1. For a given constant scan rate v [V s−1], the average power during discharge (Pav [W]) was calculated by integrating the current density (I) versus voltage (V) curve:[42] 1 Pav = I dV (6) 2
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E =
Vi × Pav v × 3600
(7)
where Vi is the initial voltage during discharge.
Experimental Section
C volumetric =
Integration to obtain the average power during charge and discharge provided nearly identical results. The discharged energy (E [W h]) was obtained using
Supporting information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Creative Research Initiative Center for Bio-Artificial Muscle of the Ministry of Science, ICT & Future Planning (MSIP), the MSIP–US Air Force Cooperation Program (NRF2013K1A3A1A32035592), and the Industrial Strategic Technology Development Program (10038599) in Korea; Air Force Grant AOARD10-4067, Air Force Office of Scientific Research grant FA9550-09-1-0537, and Robert A. Welch Foundation grant AT-0029 in the USA. Received: September 21, 2013 Published online: December 18, 2013
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