Flexible and Wearable Fiber Microsupercapacitors Based on Carbon

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Flexible and Wearable Fiber Micro-Supercapacitors Based on Carbon Nanotube-Agarose Gel Composite Electrodes Sung Kon Kim, Hyung-Jun Koo, Jinyun Liu, and Paul V. Braun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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ACS Applied Materials & Interfaces

Flexible and Wearable Fiber Micro-Supercapacitors Based on Carbon Nanotube-Agarose Gel Composite Electrodes Sung−Kon Kim,†, §,‡ Hyung−Jun Koo,ǁ,‡ Jinyun Liu, † and Paul V. Braun†,⊥,* †

Department of Materials Science and Engineering, Frederick Seitz Materials Research

Laboratory, Beckman Institute for Advanced Science and Technology, ⊥Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States §

School of Chemical Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu,

Jeonju-si, Jeollabuk-do 54896, Republic of Korea ǁDepartment

of Chemical & Biomolecular Engineering, Seoul National University of Science

and Technology, 232 Gongneung-ro, Nowon-gu, Seoul, 139-743, Republic of Korea

KEYWORDS: supercapacitors, electrodes, fibers, carbon nanotubes, agarose

ACS Paragon Plus Environment

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ABSTRACT Fiber electrodes provide interesting opportunities for energy storage by providing both mechanical flexibility and the opportunity to impart multifunctionality to fabrics. We show here carbon nanotube (CNTs)-embedded agarose gel composite fiber electrodes with a diameter of ~120 µm consisting of 60 wt% CNTs that can serve as the basis for flexible and wearable fiber micro-supercapacitors (mSCs). Via an extrusion process, CNT bundles are induced to align in an agarose filament matrix. Due to the shear alignment of the CNT bundles, the dehydrated filaments have an electrical conductivity as high as 8.3 S cm-1. The composite fiber electrodes are mechanically stable, enabling formation of twisted two-ply fiber mSCs integrated with a solid electrolyte. The fiber mSC shows a high capacitance (~1.2 F cm-3), good rate retention (~90%) at discharge current densities ranging from 5.1 to 38 mA cm-3, long cycle life under repeated charging/discharging (10% fade after 10000 cycles) and good performance after at least 1000 cycles of deformation with a radius of curvature of 12.3 mm (90° bend). After coating with a thin layer of PDMS, the fiber mSCs could be cycled over 10000 times under water. Impedance studies indicate the superior performance is due to the high electrical conductivity along the aligned CNTs and the large electrode surface area that is accessible through the ion conducting agarose.


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INTRODUCTION Advances in carbon nanomaterials and fabrication methods have enabled realization of supercapacitors (SCs) of diverse sets of shapes and sizes. Fundamentally, SCs are based on two porous conductive electrodes separated by an ion, but not electrically conductive electrolyte.1 While SCs do not have the energy density of secondary batteries, they can exhibit exceptional cycle lives and provide high peak powers, which has led to interest in using them for applications such as grid-scale energy storage, hybrid electric vehicles, consumer electronics, and military devices.2-5 One particular opportunity for SCs are applications where their form factor flexibility gives them a particular advantage over secondary batteries, e.g. portable electronics, roll-up displays, smart skins, and wearable electronics.6-7 It would be particularly interesting to directly incorporate SCs into textiles with other electronic components, but it remains a significant challenge to fabricate them in fiber form, the focus of our work here, without degrading their performance.8 Robust fiber micro-SCs (mSCs) are attracting considerable attention because they provide a path for direct integration of energy storage into textiles.9-11 Potential electrode components for fiber mSCs include graphene,4, 12-13 carbon nanotubes (CNTs),8, 14-16 metal oxides,17-20 composites,21-32 and commercial carbon fiber, natural fibers, and metal wires coated with active materials.33-38 However, the microscopic surface area of the fiber electrodes, commonly 1000 m2 g-1),39-41 resulting in a lower capacitance. 1D mSC electrode designs which provide the required good ion-accessibility as well as high electrical conductance also remain challenging. In one example of a promising starting point, Gao et al. fabricated cellulose derivative-wrapped graphene/CNT core-sheath wires.30 The cellulose sheath is ionically conductive and electrically


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insulating, allowing ions to access the graphene/CNT core while preventing an electrical short formation. This concept of encapsulation of the electrically conductive constituent with an ion conductive layer holds significant promise for SC designs. Three classes of fiber SCs designs have been reported: two parallel fibers,12, 19-21, 23, 35-37 two twisted fibers,4, 14, 17, 22, 24-31, 34 and single coaxial fiber.8, 13, 15, 18, 30, 32-33, 38 The parallel fiber designs are supported by a flat substrate. It should be noted that designs where the fiber is placed on a substrate cannot be integrated into fabrics while the twisted and coaxial fiber SCs are freestanding, offering potential for their being integrated into textiles and other macro-scale devices. The coaxial fiber provides a large interfacial area between the electrodes and is produced by layer-by-layer assemblies of a core fiber electrode, a separator or solid electrolyte, and an outer electrode layer. The twisted fiber designs are perhaps the easiest to fabricate, and are compatible with many design concepts (e.g. can be braided with varying numbers of electrodes). Here, using agarose gel and CNTs, we create highly flexible and mechanically robust composite fiber mSC electrodes. 120~250 µm diameter electrically conductive fibers are first prepared by drying-induced size reduction of CNT-embedded agarose hydrogel filaments. Agarose, a hydrophilic natural polymer, forms a highly moldable hydrogel due to its thermal solgel transition characteristics. Here specifically, the combination of its good mechanical properties and ion conducting nature make it a good matrix for the CNTs. The CNTs are oriented in the axial direction as a result of shear force induced by the extrusion process. The diameter of the fibers is modulated by tuning the diameter of the extrusion orifice. We note the smaller diameter agarose-CNT composite fibers exhibit both good ion-accessibility and higher electric conductance due to the enhanced shear alignment of the CNTs. When assembled into solid-state


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two-ply (twisted) fiber mSCs, the resulting mSCs show high volumetric capacitance and long cycle life and are flexible enough for stable operation under repeated bending.

EXPERIMENTAL SECTION Agarose-CNT Fiber Electrodes. Agarose-CNT (aCNT) composite fibers were prepared starting from our previous protocol.42 Briefly, 2.0 g of a multiwalled carbon nanotube (MWNT) suspension (3 wt% in water, Nanostructure & Amorphous Materials, Inc.) was heated to ~100 °C with constant stirring for 5 min. 40 mg of agarose (Acros) was added to the solution and stirred for 30 min while maintaining the temperature. The hot liquid suspension of MWNT and agarose was transferred to a 5 mL of syringe equipped with a needle and the hydrogel filament was formed during extrusion through tubes with 0.5 or 1 mm diameter orifices (Tygon®, SaintGobain Corp.). The hydrogel filaments were dried at room temperature for 12 h forming the aCNT composite fibers. Agarose-CNT Fiber mSC Fabrication. The PVA/H3PO4 electrolyte solution was prepared as follows. 1.0 g of PVA (Mw~95 000 g mol-1, 95% hydrolyzed, Acros) was dissolved in 15 mL of deionized water at 90 °C with vigorous stirring until the solution became transparent. After cooling to room temperature, 0.8 g of H3PO4 (85 wt% aqueous solution, Aldrich) was added to the solution and stirred for 12 h at room temperature forming a homogeneous solution. Asprepared aCNT fibers were fully soaked in the PVA/H3PO4 solution for ~1 min and removed. The fibers were dried at room temperature resulting in their being coated with a solid polymer electrolyte. Two aCNT fibers were again soaked in the PVA/H3PO4 solution for forming short circuit barrier and improving adhesion between them and promptly removed, and twisted


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together before drying. For better electric contact with probes of the measurement analyzer, the electrode edge of the aCNT were extended using a metal wire with help of silver paint and insulating adhesive tape. The length of two fiber electrodes used is ~6 cm. PDMS-Coated aCNT Fiber mSC Fabrication. aCNT mSC was immersed in a well-mixed PDMS precursor solution (Sylgard 184, Dow Corning Corp., silicone elastomer base and curing agent in a 10:1 weight ratio) and removed, followed by curing in an oven at 80 °C for 1 h 30 min. Characterization. Optical images were taken by a stereo microscope (Amscope ZM-4TNZ) equipped with a digital camera (Cannon EOS Rebel T3). Composite fibers were potted in epoxy in preparation for ultra-microtoming, followed by transfer onto copper grids. The microstructures were imaged using a Hitachi S-4700 scanning electron microscope (SEM), equipped with energy dispersive spectroscopy (EDS, Oxford Instruments) capabilities and JEOL 2100 transmission electron microscope (TEM). Electrochemical characterization, including CV, GCD, and EIS, was performed using a VMP3 multichannel potentiostat (VMP3, Bio-Logic, USA) in the twoelectrode mode at room temperature. EIS measurements were performed over a frequency range of 106 to 10-2 Hz at a sinusoidal voltage amplitude of 10 mV. In the GCD profiles, the volumetric capacitance can be estimated using the following equation: C = 2I/[(⊿E/⊿t)V]

(1)

where I is the current applied, ⊿E/⊿t is the slope of the discharge curve after a IR drop at the beginning of the discharge curve, and V is the volume of electrodes (in cm3).

RESULTS AND DISCUSSION The agarose-CNT composite, abbreviated as aCNT in this study, fibers were formed by extrusion of a heated aqueous mixture of agarose and CNT through tubes with 0.5 and 1.0 mm diameter


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orifices, following our previous study.42 The mixture cooled and solidified during extrusion to form continuous long fibers. The diameter of the extruded hydrogel fiber is nearly equivalent to the diameter of the tube, offering a practical route for diameter control. The as-prepared hydrogel fibers were dried in air for 12 h, leading to approximately 4-fold shrinkage in diameter to 126 µm for the fiber drawn from the 0.5 mm tube and 248 µm for the one drawn from the 1.0 mm tube. They will be referred to as aCNT1 (126 µm diameter) and aCNT2 (248 µm diameter) (Figures 1a-d). The dried composite fibers have a fibril surface morphology, consistent with our previous study and other reports,26, 42 and can be readily used as electrodes in a twisted fiber mSC. 20 µm diameter aCNT fibers (drawn from a 0.19 mm inside diameter tube) were also formed (Supporting Information Figure S1) but we were unable to produce twisted fiber mSC using these fibers (the fibers mechanically failed because of their small size). Both individual CNTs and CNT bundles are observed in the cleaved edge of aCNT1. The CNTs are oriented along the fiber length, presumably by shear forces during extrusion (Figure 1e and see Supporting Information Figure S2).26 Transmission electron microscopy (TEM) analysis of aCNT1 ultra-microtomed in the longitudinal direction of fiber agrees with this finding (Figure 1f). Figure S3 (see Supporting Information) presents the current-voltage (I-V) plots of the aCNT1 and aCNT2 electrodes. The electrical conductivity derived from the slopes of the I-V plots is as high as ~8.3 S cm-1 for aCNT1 electrode, twice the ~4.7 S cm-1 determined for the aCNT2 electrode and comparable to values for carbon-based fibers previously reported.4, 21 The higher electrical conductivity of aCNT1 is attributed to the presumably higher degree of sheer alignment of the CNTs relative to the larger diameter aCNT2 (Figures 1e and 1f).31 The electrical conductivity of aCNT1 is over 2 orders of magnitude greater than our previous agarose-graphene


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composite fiber (0.018 S cm-1) due to the higher loading of active carbon materials.42 In our previous study, the graphene loadings in suspension were limited to 0.1 wt%, but, in this study, the CNT loadings in suspension were increased to 3 wt% using highly concentrated CNT suspension which resulted in aCNT fiber containing 60 wt% of CNTs once the fiber was dried. The aCNT fiber electrodes are mechanically robust and can be twisted, knotted, rolled up, and even woven into a cloth (Figures 1g-i). An all-solid-state two-ply aCNT fiber mSCs was formed by bonding and twisting one aCNT fiber with another one with the help of a PVA/H3PO4 solid electrolyte (Figures 2a-c and see Supporting Information Figure S4 and S5). aCNT mSCs were also woven into a fabric (see Supporting Information Figure S6). When exposed to the PVA/H3PO4 aqueous solution, the aCNT1 fiber swells due to swelling of the hydrophilic agarose hydrogel (Figure 2d). After drying, the solid electrolyte separates the two aCNT electrodes, preventing short formation. The electrolyte ions are present in the ion-conducting agarose after drying, as shown by the ionic conductivity of agarose being ~5.6 × 10-4 S cm-1 after immersion in a 85 wt% of H3PO4 solution, and drying for 12 h. Figure 2c shows the solid electrolyte forms a coating on tightly twisted two fiber electrodes without any gaps and voids, which is important for reliable performance and minimization of internal resistance and leakage currents.30 Electrochemical characterization of aCNT mSCs was conducted by two-electrode cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements over the voltage window of -0.5 to 0.5 V. Figure 3a presents CV curves of the aCNT mSCs at a constant scan rate of 10 mV s-1. The aCNT1 mSC shows a nearly rectangular-shaped CV curve, which is a typical of an electrical double layer device, during voltage sweeps as fast as 100 mV s-1 (see Supporting Information Figure S7), while the CV curve of the aCNT2 mSC exhibits considerable distortion and decreased current values at the same scan rate, due to its more resistive behavior.43-44 Figure


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3b shows the GCD profiles of the aCNT mSCs at a constant current of 7.7 mA cm-3. For the aCNT1 mSC, symmetric triangular-shaped voltage-time responses during charging/discharging are observed, indicating excellent reversibility and good charge propagation across the electrodes.45 The small IR drop, the voltage drop between the first two points in the voltage drop at the top cutoff, confirm the good electrical conductivity of the fiber electrode combined with good accessibility of the internal surface by electrolyte ions.46 The aCNT2 mSC shows smaller charging/discharging durations and a larger IR drop than aCNT1 mSC at the same current density, which is consistent with CV results. Not surprisingly, the magnitude of the IR drop and the differences between aCNT1 and aCNT2 mSCs increase with increasing applied current densities in the GCD measurements (Figure 3c). The internal resistance values (R), given by R = ⊿ViR / 2i, where ⊿ViR and i are the IR drop and applied current, respectively, are ~7 kΩ for aCNT1 mSC and ~9 kΩ for aCNT2 mSC. The volumetric capacitance at a current density of 7.7 mA cm-3, calculated from the discharge curves and the total electrodes volume excluding the electrolyte, of aCNT1 mSC is 1.2 F cm-3, twice the 0.6 F cm-3 of aCNT2 mSC. 1.2 F cm-3 is comparable to or exceeds values reported for other mSCs (see Supporting Information Table S1).19, 47-51 The length-normalized capacitance, another important evaluation criterion for fiber SCs,28 is ~0.11 mF cm-1, much higher than those of most reported carbon-based fiber electrodes.4, 15, 25, 32, 37 The volumetric capacitances at current densities ranging from 5.1 to 38 mA cm-3 are plotted in Figure 3d (see Supporting Information Figure S8). The aCNT1 mSC preserves 90% of its 1.2 F cm-3 (at 5.1 mA cm-3) capacitance at a current of 38 mA cm-3 (1.1 F cm-3 capacitance). Notably, the rate retention capacity of aCNT1 is 20% higher than that of conventional sandwich-typed CNT SC.49 The high rate retention capacity of aCNT1 mSC arises


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from primarily from the lower internal resistance of electrode and good accessibility of the electrolyte to the embedded CNTs.51-52 To better understand which electrochemical aspects of aCNT1 mSC enable its high performance, electrochemical impedance spectroscopy (EIS) over a frequency range of 106 to 102

Hz was employed (Figure 4). The intercept of the real axis (Z’) of the Nyquist plots provides an

equivalent series resistances (ESRs) including electrode and electrolyte resistances and contact resistance between the current collector and electrode (Figures 4a and 4b).41, 53 The aCNT1 mSC shows a smaller ESR (~7.0 kΩ) than that of the aCNT2 mSC (~9.9 kΩ), both of which are consistent with what is observed in the internal resistance values from the GCD curves, mostly due to the good electrical conductivity of aCNT1 electrode. The absence of a high frequency loop (related to charge transfer resistance (RCT)), implies negligible interfacial charge transfer between electrode and electrolyte.54 The nearly vertical slope of aCNT1 mSC at low frequency indicates almost purely capacitive behavior, while the smaller slope of aCNT2 mSC indicates a more resistive behavior.55 The low frequency response is strongly related to the diffusional (or mass transport) behavior.56 Ion diffusion is also particularly important for high rate capacity retention and power density.57 The impact of ion diffusion was studied via the impedance in the medium-low frequency region (1~0.01 Hz). The slope of plot of Zˊ against ω-1/2 (so-called Randles plot, Figure 4c) corresponds to the Warburg coefficient (kw), which is related to mass transfer of ions and is expressed as follows:58-59

= √ ⁄ ∗

(2)

where R is the gas constant, T is the absolute temperature in Kelvin, n is the charge transfer number, A is the area of electrode surface, D is the diffusion coefficient of ions and C* is the


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ionic concentration. Notably, aCNT1 mSC shows smaller slope with respect to ω-1/2 than aCNT2 mSC, indicating that the ion diffusion of aCNT1 mSC is faster than that of aCNT2 mSC. The faster ion diffusion of aCNT1 mSC is not completely understood, but it might be a function of aCNT1’s smaller fiber diameter. Figure 5 shows SEM-energy dispersive spectroscopy (EDS) phosphorous line profiles for the aCNT electrodes, after exposure to the PVA/H3PO4 solid electrolyte. Phosphorous is present evenly throughout the cross-sectional area of aCNT1 but not for aCNT2. We speculate the H3PO4–containing electrolyte may not completely reach the fiber center of the larger dimeter aCNT2, resulting in an inhomogeneous distribution of ions.28 Good wetting of the aCNT electrode by electrolyte is important for fast ion diffusion as ion diffusion is proportional to the square of ion concentration, as given by equation (2). The differences between aCNT1 and aCNT2 is supported by the characteristic relaxation time constant (τo), which is defined as the time where the resistive and capacitive behaviors are equal (Figure 4d).48, 60

τo is determined from the complex form of capacitance, C(ω), which can be obtained from the

frequency-dependent impedance Z(ω) in equations (2-4):60 = 1/

(3)

Z = ′ + "

(4)

%&"'

& ′ '

C$ = '|&'| , " = |&'|

(5)

ω is the angular frequency (= 2πf) and C′(ω) and C″(ω) are the real part and imaginary parts of C(ω), respectively. τo=1/fo where fo is the frequency at the half maximum of C′(ω) or peak maximum of C"(ω). The peak maximum of C"(ω) for aCNT1 mSC appears at a high frequency, reflecting the 10 s τo, while that for aCNT2 mSC is below minimum frequency (< 0.01 Hz),


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indicating a much slow frequency response. We attribute the faster frequency response of aCNT1 mSC to efficient ion transport through the well-aligned CNTs, good wetting of the hydrophilic agarose matrix by electrolyte, and high electrical conductivity of electrode. The electrochemical performance and cycle life of supercapacitors commonly decrease when deformed which is attributed to mechanical damage leading to loss of electrically connectivity within the device. Figures 6a shows the CV curves of straight and bent aCNT1 mSC at a constant scan rate of 10 mV s-1 (see Supporting Information Figure S9) and figure 6b presents an image of the bent aCNT1 mSC. The CV curves are similar for both the straight and curved electrodes, indicating that the aCNT-based mSC retains its performance even when deformed. Figure 6c presents the cycle life of aCNT1 mSC over 10000 GCD cycles at a constant current of 13 mA cm-3. It retains over 90% of the initial capacitance after 10000 GCD cycles, and coulombic efficiency is essentially 100% during cycling. The capacitance drop is less than 1% after 1000 bending cycles with a radius of curvature of 12.3 mm (90° bend) (Figure 6d). To demonstrate the practical potential of the fabricated aCNT fiber mSCs, the possibilities in terms of tandem configuration in series and in parallel (Figure 7a) and waterresistant property (Figure 7b) are needed to explore. Not surprisingly, the aCNT1 mSCs could be assembled either in series or in parallel to the meet power and energy required for specific applications (Figure 7a). Two 1V aCNT1 mSCs linked in series show a 2 V charge/discharge voltage window, and when linked in parallel, two 1V aCNT1 mSCs prove power for twice as long as for a single aCNT1 mSC at the same current over a voltage window of 1 V. For wearable applications, the aCNT mSCs are required to be stable under high humidity, or even water immersion conditions where the PVA/H3PO4 solid electrolyte could swell or


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dissolve. To provide water stability, aCNT1 mSCs were coated with a thin (~6 µm thick) layer of hydrophobic poly(dimethylsiloxane) (PDMS) (see Supporting Information Figure S10). PDMS is known to be an effective water barrier,61 and we suspect it is also a good barrier to prevent the solid electrolyte from leaving. The aCNT1 mSCs with and without PDMS coating were submerged in deionized water, and evaluated over 10000 GCD cycles at a constant current of 12 mA cm-3 (Figure 7b). The deionized water was changed every 6 h during cycling. The capacitance of the PDMS-free aCNT1 mSC initially increases for 50 GCD cycles due to the water uptake of solid electrolyte. This is not surprising, as ion diffusion in a gel electrolyte is commonly faster than in a solid electrolyte, and fast ion diffusion may result in an increased capacitance.26, 62-64 The capacitance then decreases significantly with increasing number of GCD cycles, falling to nearly 0 after 2000 GCD cycles, as a consequence of dissolution of H3PO4 and PVA out of the mSC. In contrast, a PDMS-coated aCNT1 mSC retains ~91% of initial capacitance under water for at least 10000 GCD cycles.

CONCLUSION In summary, we demonstrate high-performance agarose and CNT composite fiber electrodes and integrate the electrodes into water-resistant twisted fiber flexible mSCs. The composite fiber electrodes exhibit both high electrical conductivity and flexibility. The narrower diameter fibers provide better electrolyte ion accessibility to the CNTs and have better alignment of the CNTs along the fiber axis relative to wider diameter electrodes, resulting in better ion and electron transport properties. Thus the fiber electrodes employed mSCs show good volumetric capacitance, rate performance, and cycle stability under deformation (bending) and repeated


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charging/discharging which are attributed to the excellent mechanical and electrochemical properties of the aCNT electrodes and toughness of the agarose matrix. We envisage that the fibrous mSCs formed in this study will be suitable for integration into various fabrics, providing a route to for wearable and flexible energy storage systems.


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FIGURES

Figure 1. Optical micrographs (a,b) and SEM images (c,d) of (a,c) aCNT1 and (b,d) aCNT2. (e) Cross-sectional SEM image of aCNT1. (f) TEM fiber-direction view of microtomed aCNT1. Optical micrographs of aCNT1 being (g) knotted and (h) wound around a glass rod. (i) Photograph of a aCNT1 fiber woven into a cloth (the aCNTa fiber is the thin black line between the two dotted lines).


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Figure 2. (a) Schematic illustration (Inset: ion and electron transport pathways in aligned CNTs), (b) photograph, and (c) optical micrograph of twisted aCNT1 mSC. (d) In the optical image, the lower region is a dry aCNT1 electrode and the upper region was immersed and drawn out of the PVA/H3PO4 solution.


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(b)

10

0.6

aCNT1

IR

0.4

5 0

aCNT2

-5

Voltage (V)

Current (mA cm-3)

(a)

IR

0.2

aCNT1

0.0

aCNT2

-0.2 -0.4

@10 mV s-1

-0.6

-10 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

0

50

100

150

Time (s)

Voltage (V)

(c) aCNT2

0.6 0.4

aCNT1 0.2 0.0

0

5 10 15 20 25 30 35 40 -3

Current (mA cm )

Capacitance (F cm-3)

(d) 0.8

IR drop (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2 1.0

aCNT1

0.8 0.6

Sandwich

0.4

aCNT2

0.2 0.0

0

5 10 15 20 25 30 35 40

Current (mA cm-3)

Figure 3. (a) CV curves (at a constant scan rate of 10 mV s-1), (b) GCD curves (at a constant current of 7.7 mA cm-3), (c) IR drop as a function of currents, and (d) rate dependent capacitance of aCNT1 and aCNT2 mSCs. In (d), sandwich denotes conventional sandwich type CNT SCs (see Supporting Information for details on the sandwich type SC).


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(b)

80

40

aCNT2 20 0

aCNT1

4

aCNT1

60

-Z'' (kΩ)

5

-Z'' (kΩ)

(a)

aCNT2

3 2 1

0

10

20

30

40

0

50

0

5

10

15

Z' (kΩ)

Z' (kΩ)

(c)

(d)

40000

1.5

20000

C'' (F cm-3)

aCNT2

Z' (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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aCNT1

aCNT1

1.0

aCNT2

0.5 0.0

0 0

1

2 -1/2

ω

(Hz

3 -1/2

4

10-2

10-1

)

100

101

102

f (Hz)

Figure 4. (a) Nyquist plots of aCNT1 and aCNT2 mSCs over the frequency range of 100 kHz to 0.01 Hz measured at equilibrium open circuit potential (~0 V). (b) High frequency region of (a). (c) Randles plot and (d) imaginary part of the capacitance (C″) vs. frequency (f) for aCNT1 and aCNT2 mSCs. Dotted line indicates the frequency at the peak.


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Figure 5. SEM-EDS phosphorous line profiles for the cross-sectional area of (a) aCNT1 and (b) aCNT2, both of which were nominally filled with the PVA/H3PO4 solid electrolyte.

Figure 6. (a) CV curves of aCNT1 mSC in straight and bent states at a constant scan rate of 10 mV s-1. (b) A photograph of aCNT1 mSC in a bent state. (c) Stability and coulombic efficiency of aCNT1 mSC over 10000 GCD cycles at 13 mA cm-3. (d) Capacitance of aCNT1 mSC over 1000 straight-bending cycles with a radius of curvature of 12.3 mm (90° bend). C0 is the initial capacitance and C is the capacitance at the indicated number of cycles.


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(a)

2.0

Voltage (V)

1.5 1.0 0.5 0.0 -0.5 0

100

200

300

Time (s)

̶

Tandem serial

(b)

+ ̶

+

Tandem parallel

1.5

1.0

C/C0

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PDMS-coated 0.5

Uncoated 0.0 0

2000 4000 6000 8000 10000

Cycle number

Figure 7. (a) GCD curves of single aCNT1 mSC (solid) and tandem ones linked in series (dashed) and in parallel (dotted) at a current of 3 µA. (b) Cycle lives of an uncoated aCNT mSC and a PDMS-coated aCNT mSC in deionized water bath over 10000 GCD cycles at 12 mA cm-3 (10000 GCD cycles for the PDMS-coated mSC took about 100 hours). Deionized water was changed every 6 h during cycling.


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ASSOCIATED CONTENT Supporting Information. Additional experimental information, SEM, I-V curves, the fiber mSC fabrication process, a photograph of mSCs woven into a fabric, CV and GCD curves, optical micrograph of PDMS-coated aCNT1 and a comparison of the capacitances of various mSCs reported in the literature. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG0207ER46471, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign.


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Flexible and Wearable Fiber Microsupercapacitors Based on Carbon

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