Interwoven Carbon Nanotube Wires for High-Performing

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Interwoven carbon nanotube wires for high performing, mechanically robust, washable and wearable supercapacitors Mihir Kumar Jha, Kenji Hata, and Chandramouli Subramaniam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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ACS Applied Materials & Interfaces

Interwoven Carbon Nanotube Wires for High Performing, Mechanically Robust, Washable and Wearable Supercapacitors Mihir Kumar Jha†, Kenji Hata‡, Chandramouli Subramaniam†*

†Department

of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai

400076, India ‡National

Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,

Ibaraki 305-8560, Japan KEYWORDS: wearable supercapacitor; energy density; power density; washable; carbon nanotube wire; interweaving

ABSTRACT: Energy storage system with large storage capacity, rapid power release and simultaneous tolerance to harsh mechanical stresses, is a major bottleneck for

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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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realizing self-sustaining, wearable electronics. Addressing this, we demonstrate carbon nanotube-wire (CNT-wire) interwoven solid-state supercapacitive energy storage devices (sewcaps) exhibiting superior storage capacity (30 Wh/kg, compared to electrochemical capacitors ~10 Wh/kg) and fourteen-fold higher power density (3511 W/kg), compared to Li-ion batteries (~250 W/kg). While the high specific surface area, electrical conductivity of CNT-wires and high ionic conductivity of the electrolyte enable high energy density, the device design enables the combination of planar and radial diffusive pathways for ultra-low interface resistance (~0.2 mΩ per sewcap) and rapid charging-discharging ability (τ=1.16 ms). Thus, this versatile approach of interweaving to form functional devices provides tunable power delivery across six orders of magnitude (2 µW-2 W) through reconfiguration of the interweaving pattern and density. Importantly, such textile-integrated sewcaps exhibit unaltered performance (>95% retention

across

4000

charge-discharge

cycles)

under

extreme

mechanical

punishments such as repeated laundering, flexing (~68°), rolling (360°) and crushing (~21.8 kPa) implying direct interfacing with wearable platforms.


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1. INTRODUCTION:

Textiles constitute the primary interface between skin and surroundings, offering physical comfort and protection. Continued miniaturization and extensive ubiquity of electronic gadgets have resulted in emergence of smart e-textiles and intelligent clothing,1,2 for wearable computing, personalised healthcare,3 fitness,4,5 and strategic sectors6,7. Systems for storing and delivering electrical power form the heart of such rapidly-emerging technologies aimed at augmenting human-machine interfacing and therefore poses stringent scientific and technological challenges. A critical, scientific challenge is to achieve both high energy density (as-in Li-ion battery) and high power density (as-in supercapacitors),8-10 to accumulate energy from intermittent and sporadic voltage sources such as piezoelectric energy harvesters and thereby realize selfsustaining, wearable devices.11 Fundamentally, this translates to minimal internal resistance and rapid cycling capabilities that are challenging to realize in conventional Li-ion batteries. In addition, mandatory technological demands include ultra-portability and seamless integrability with textiles. Importantly, mechanical robustness of the


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energy storage system is a critical parameter for ensuring its lifetime and reliability. ‘Mechanical robustness’, as defined for wearable applications by International Technology Roadmap for Semiconductors (ITRS) stipulates comprehensive tolerance to all forms of mechanical stress such as bending, twisting and impact, the harshest of these being washability and launderability. Finally, it is highly desirable to employ nonlithographic processes to achieve such high-performing, mechanically robust devices for facile and scalable integration onto clothing with minimal modifications of existing processes. Strategies attempted earlier include device fabrication on flexible, polymeric substrates such as elastomers,12 polyethylene terephthalate,13 and polyimide,14 that afford high mechanical robustness but suffer from poor integrability on textiles. Cheng et. al. devised a flexible supercapacitor-electrode comprising of graphene/MnO2/CNT nanocomposite film.15 Rogers and group has demonstrated several functional devices such as a micro-fluidic device for colorimetric sensing of sweat,16 visual sensor17 on wearable platforms. A parallel approach of employing functional yarns for easier integration on textiles has been gaining prominence in the recent years. The inherent geometry of a yarn enables fabrication of coaxial cables, sandwiched flexible sheets


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and lithographically fabricated interdigitated electrodes.18-25 Le and co-workers reported a multiwalled carbon nanotube based coaxial supercapacitor.26 Although several such devices exhibit tolerance to uni-dimensional flexibility, combining comprehensive mechanical robustness with high performance metrics has remained a significant challenge to power autonomous wearable devices. Thus, a singular energy storage unit with high energy density and high power density with tunable performance, long cyclability and extreme mechanical robustness (bending, twisting, crushing and laundering in hot water with detergents under high torque) has never been realized. Such a device is therefore expected to provide complete independent operability and thereby produce paradigm shifts in wearable technologies.

Addressing this demand, we demonstrate a wearable, micro-scale, electrochemical, energy storage device exhibiting superior energy density (~30 Wh/kg) and power density (~3511 W/kg) unrivalled by several previous reports, with demonstrated ruggedness to extreme mechanical and environmental duress. The device is termed ‘sewcap’, signifying its non-lithographic fabrication approach (by sewing) and


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supercapacitive operational principle. Fabrication of sewcap involves interweaving electrically conductive, carbon nanotube immobilized yarns (CNT-wires) through an alkali-polymer electrolyte sheet to form orthogonally intersecting junctions (Figure 1) that act as energy storage units. The high porosity (pore volume ~0.16 cm3/g, pore diameter ~0.5 nm), high specific surface area (~619 m2/g), light weight, mechanical tenacity (linear density=0.125 mg/cm, Young’s modulus ~160 MPa) and electrical conductivity (~65 S/cm) of CNT-wire (Figure S1 and Figure S2) is synergistically combined with the solid electrolyte to realize seamless electrode-electrolyte interface. This is reflected in low iRdrop (~0.04 V), and furthermore, enables rapid cyclability (relaxation time constant, τ=1.1 ms) at a discharge rate~1.9 A/g (Figure 2c).

Conventional sewing and manual interweaving to realize micro-scale device architectures presents a paradigm shift in conceptual device designing that enables realization of versatile architectures providing for tunable capacitance (0.26 mF to 0.9 mF) (Figure S3). Thus, extending the sewcaps in horizontal and vertical directions can give rise to a high volumetric capacitance (~440 F/cm3). A horizontally interwoven


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device of 21 sewcaps, exhibits excellent energy density (8.2 Wh/kg @ 0.9 A/g), power density (2667 W/kg @ 2 A/g) (Figure 3a, 3c and Figure S4) and cyclability (Figure S5).


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Figure 1. (a) Schematic representation of various steps such as sewing and packaging to fabricate the sewcap with CNT-wires and solid electrolyte, culminating in integration onto clothing. Scanning electron microscope (SEM) image of the CNT-wire and photograph of the sewcap worn on a T-shirt is also provided. (b) SEM image of the interconnected CNT network on the polyester wire. (c) SEM image of the cross-junction created by sewing indicating the device architecture. (d) Cyclic voltammograms of sewcap at different scan rates. (e) Gravimetric Ragone plot comparing the performance of sewcap with other reported literature. (f) Areal Ragone plot.

Such cumulatively enhanced and tunable performance (10.2 Wh/kg; 6097.8 W/kg @ 3.8 A/g in a single sewcap to 12.04 Wh/kg; 6227.6 W/kg @ 3.8 A/g in two vertically-stacked sewcaps) is achieved without compromising on functional flexibility and mechanical ruggedness. Accordingly, we demonstrate that sewcaps can withstand extreme punishments and rough handling such as laundering (using detergents, hot water and high torque~2000 rpm), bending (~360°) and smashing with a hammer (~21.8 kPa), retaining 95% energy over 4000 cycles (Figure 2e and Figure 4). Conceptually, this approach is directly amenable to assembly-line technology existing for textile manufacturing. Finally, the elegant packaging of the device through lamination provides it with the requisite mechanical robustness and conformability for direct integration with clothing. Such textile-integrated sewcaps can seamlessly blend with clothing and power


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a LED. Thus, we believe that the functional diversity and mechanical robustness demonstrated here, by using such a facile fabrication strategy, have not been reported for any energy storage system.

2. EXPERIMENTAL SECTION:

2.1. Synthesis of CNTs & CNT ink: Single walled carbon nanotubes were synthesized using a water assisted chemical vapor deposition technique termed as ‘super-growth’ method,27 resulting in mono-dispersed single walled carbon nanotubes with high purity (~ 99 %, specific surface area~ 1000 m2/g), uniformity (length ~ 350 μm, diameter ~ 3 nm) and high aspect ratio (>105). The electrical conductivity of CNTs in the form of a bucky paper is estimated to be ~30 S/cm through four-probe measurements. Different weights of the as-prepared nanotubes were dispersed in water along with a surfactant and probe-sonicated in small aliquots to form highly stable CNT dispersions, which we call ‘CNT ink’.

2.2. CNT-ink sample preparation for TEM: A TEM copper-grid was brought in contact with the upper meniscus of CNT-ink, kept in a vial, with the aid of cross-tweezers, and


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held for 2 hours. The CNT-ink wets the surface of the grid almost instantaneously due to capillary action. The grid is then dried overnight in ambient conditions for evaporation of the solvent. This procedure neither dilutes the CNTs nor subjects them through ultrasonication and thereby represents their true dispersed state in the CNT-ink. A schematic of the set-up has been provided in Figure S6.

2.3. Preparation of CNT-wire & alkali polymer electrolyte: Thoroughly cleaned synthetic yarns were dipped in 0.1% & 0.5% (w/v) CNT ink successively to form a uniform, conformal coating of CNTs along its surface. Each CNT dipping was followed by a subsequent dipping in ethanol. The number of dipping was in accordance with the percolation threshold plot (Figure S7). Further enhancement in conductivity was effected by exposure of the yarns to sulfuric acid (1 M). The yarns thus prepared exhibited a finite electrical conductivity indicating a strong continuously interpenetrating network of CNTs immobilized onto the matrix. The resulting yarns coated with CNT ink were referred to as ‘CNT-wire’. A solid electrolyte sheet was prepared by mixing polyvinyl alcohol and potassium hydroxide in appropriate proportions (PVA-KOH ~8:3).


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The solution was cast into thin films of thickness ~200 μm, using a doctor-blade and dried at 60°C for 15 minutes to result in a free-standing film of 150 μm thickness.

2.4. Fabrication of supercapacitor: CNT-wires were sewn across the solid-electrolyte with the threads traversing each other orthogonally to yield junctions that function as capacitors. The sheet electrolyte layer in between the electrodes (CNT-wires) serves as the dielectric of a typical double layer capacitor configuration. Different versions of the capacitor were fabricated by changing the number of CNT-wires acting as electrodes (uni-ply and multi-ply CNT wire).

To achieve effective and extensive interfacing of the electrode and electrolyte, and to bring out the importance of seamless interfaces for high performance, the CNT-wires were interwoven across the PVA-KOH film in its gel-state. Subsequent annealing at 60°C for 15 minutes results in the formation of sewcaps with seamless electrodeelectrolyte interfacing. Such a procedure facilitates enhanced percolation and infiltration of the electrolyte into the micro and meso-pores of the CNT-wire electrode.


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2.5. Performance enhancement: The device was exposed to varying and controllable levels of humidity using a custom-built, adiabatically isolated chamber, equipped with a hygrometer and thermometer. The humidity in this chamber was controlled by varying the temperature of a volume of water kept inside it. The sewcap was kept inside the chamber for about an hour in order to equilibrate with the prevailing humidity levels. The sewcap was then taken out and almost instantaneously packaged with polyethylene terephthalate (PET) at a temperature of 70 °C using a laminator. A PET boundary was inserted while packaging to- (i) restrict humidity from flowing out of the electrolyte and (ii) to construct a seamless integration of the laminating sheets with the capacitor in between.

2.6. Capacitance tunability: Multiple capacitor units were created by increasing the number of orthogonal junctions both horizontally across an electrolyte sheet and vertically by stacking another electrolyte sheet with a thread sewn across it. Both series and parallel configuration could be achieved this way in a single supercapacitor device. A cuboid configuration, comprising of both horizontally and vertically placed sewcap


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junctions, was also fabricated in order to showcase the three-dimensional enhancement of capacitance.

2.7. Mechanical aspects: The device integrated on a T-shirt was washed in a washing machine with detergent and warm water (60 °C) to access its washability. A bending jig, with controllable arms, was used to bend the device and thereby impart compressive stress. A hammer with a circular impact area of 3.14 cm2 was used for impact tests.

2.8. LED demonstration: An USB-powered Arduino micro-controller board is used to supply an input voltage of ~5 V. The fabricated device is made to discharge through a resistor connected parallel to it. The series diode guarantees that the capacitor does not discharge through the Arduino board. The potential difference created at the two terminals of the capacitor is fed as an input voltage to the Arduino.

2.9. Integration to clothing: The energy storage patch was integrated to clothing using two different modes of attachment- (i) a mechanical fastener (Velcro) was affixed on one side of the fabricated device. The complementary texture of the fastener was stitched onto the fabric, (ii) the fabricated device was directly stitched onto the fabric in


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such a way that the active part of the device i.e. the electrolyte and the junction, remains unaffected.

2.10. Characterizations: Raman spectra and spectral mapping was carried out in a WiTec micro-Raman spectrometer equipped with 532 nm Nd-YAG laser source, in a confocal back-scattering geometry with a notch filter. The signal was dispersed in a 300 grooves/mm grating on to a Peltier-cooled CCD detector. Spectral mapping was carried out by collecting 10000 spectra over 20 µm x 20 µm area. Malvern: ZEN 1600 particle size analyzer and Physica MCR 301, Anton Paar rheometer were used for rheological measurements. Transmission electron microscopy image of the CNT ink was recorded using FEI Technai G2, F30 FEG-TEM 300 kV instrument. Weighing of CNT threads involved the use of a Sartorius microbalance. Field emission scanning electron microscopy

images

were

taken

in

JEOL

JSM-7600F

FE-SEM

instrument.

Quantachrome Autosorb surface area analyzer and Perkin Elmer (USA) Diamond TG/DTA instrument provides us with surface and material properties of the thread. A compact tape casting film coater MSK-AFA-III was used for uniform casting of PVA


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KOH film over a Teflon sheet substrate. All electrochemical characterizations were carried out in Biologic SP-300 electrochemical workstation. The performance of the sewcap at low temperature was estimated by equilibrating the sewcap at 273 K for 30 minutes followed by electrochemical measurements. The temperature of the sewcap during measurements was monitored with a digital thermocouple. Variation of capacitance with frequency was measured in a Novocontrol Technologies’ Broadband dielectric spectrometer. Time evolution of contact angle was measured using GBX Digidrop Contact Angle Meter. Infrared (IR) spectra were recorded in Perkin Elmer FTIR-spectrometer. Thermo-gravimetric analysis (TGA) of CNTs, CNT-wire and PVA-KOH coated CNT-wire was recorded in TG 209 F1 Libra of Netzsch Instruments. All the TGAs are recorded in air at a scan rate of 5 oC/min for a direct comparison.

3. RESULTS AND DISCUSSION:

The most important obstacle to realize truly wearable devices lies in blending the chemical nature of raw materials with fabrication techniques and processability, all of which are diametrically conflicting for textiles and conventional Si-based devices. Since,


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this forms a critical demand for wearable applications, we adopt a multi-step strategy to achieve this. The first step in the strategy, involves the homogeneous stabilisation of hydrophobic single walled carbon nanotubes (CNTs) in aqueous medium using sodium deoxycholate (NaDOC) as surfactant. Sodium deoxycholate is chosen due to its similarity in solubility parameter with CNTs. Such non-specific electrostatic stabilisation of CNTs with NaDOC facilitates the removal of the surfactant at a later stage. The uniform CNT dispersions with concentrations ranging from 0.1 mg/ml (CNT-ink) to 0.5 mg/ml (CNT-paste) exhibit no aggregation, as shown by their high surface charge stabilization (ζ= -41 mV and -32 mV, respectively) (Figure S8). Importantly, the structural framework and chemical nature of the CNTs is completely preserved as confirmed from vibrational spectroscopy (radial breathing mode ~190 cm-1 and IG/ID ratio ~2) (Figure S9) and retention of specific surface area after dispersion. This is further supported by TEM images of the CNT-ink exhibiting uniform and well-separated CNTs sheathed by amorphous NaDOC (Figure S10). Such aqueous, conductive inks are an optimal intermediary phase for uniformly immobilizing the CNTs on arbitrary substrates. This is achieved in second step involving the dip-coating of multi-ply, polyester synthetic


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yarns through the CNT ink. The choice of substrate is dictated by its inherent porosity, pliability (linear density ~0.125 g/cm), tenacity (160 MPa) and compatibility for further processing. Subsequent washing in ethanol/water mixture removes NaDOC, leaving behind a conformal, uniform coating of CNT on the yarn. The complete removal of surfactants is confirmed from (a) enhancement in electrical conductivity (by 30 %) of the CNT-wire after ethanol-washing, due to improvement in inter-CNT contacts (Figure S1a), and (b) clean, one-step thermal degradation behaviour of the CNT-wire (Figure S2b). The individual fibrils of the multi-ply yarn are at least ten times smaller (10-15 µm) than the length of CNTs (350 µm) employed. This results in the CNTs sheathing the individual fibrils and simultaneously forming a criss-crossing network interconnecting multiple such fibrils (Figure 1b). This results in establishment of highly interconnected electrical pathways with extended CNT network bridging and binding the micro-fibrils together to result in diameter constriction of the fibre by ~43% to 170 µm. Although the observed conductivity is lower than conventional metals (~105 S/cm), it is one of the highest conductivity reported for similar materials employing nanocarbons such as CNTs and graphene.28,29 The CNT-wire outperforms several other similar composite


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fibers in terms of both conductivity (Table S1) and fiber resistivity (Table S2). Importantly, the conductivity is ohmic across the entire length scale (500 μm to 10 cm) and potential window over which the device is fabricated and tested in this work (Figure S1b). Moreover, this report demonstrates a scalable fabrication strategy for CNT-wires through a facile dip-coating procedure. The electrical resistance of CNT-wires, thus fabricated, does not vary with mechanical deformations (Figure S1c).


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Figure 2. (a) CVs of the sewcap at different scan rates at ambient humidity levels. (b) Capacitance versus voltage curves at varying humidity levels. (c) GCD curves of the final device at different current densities, fabricated by interweaving of CNT-wires across the gelstate of the electrolyte. (d) Capacitive retention and specific capacitance for ~1000 chargedischarge cycles at RH 38 % and RH 89 %. (e) Capacitive retention and specific capacitance of the final packaged device for 4000 cycles. CVs of the 1st and 4000th cycle measured on the same sewcap (Inset). (f) Nyquist plot of the sewcap recorded at different humidity levels, and with the device fabricated by interweaving CNT-wires across the gel-state of the electrolyte.

The dimensionality and uniformity of the CNT network in CNT-wire is determined from the percolation polynomial to be 3.6, indicating extensive, conductive three-dimensional networking (Figure S7). Such interpenetrating and continuous network of CNTs formed on the cellulose thread is responsible for achieving uniform electrical conductivity (65 S/cm) across a wide length scale. The hole-type conduction mechanism of the CNTwire is confirmed by increase in its electron work function (ϕ) upon acid treatment (from 4.72 eV to 5.47 eV) (Figure S11). Significantly, the one-dimensional fiber resistivity of the CNT-wire is substantially lower than several other composite fiber materials (Table S2). Symbiotic integration of the polyester matrix with CNTs is confirmed by high specific surface area (~619 m2/g), enhanced pore volume (0.16 cc/g) and porosity (< 2 nm). The sub-nanometer pores are critical in achieving high capacitance due to distortion of the ion-solvation shell.8-10 Thus the CNT immobilization produces a 1015


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fold enhancement in electrical conductivity and ~3.5 fold increase (0.4 GPa to 1.9 GPa) in mechanical strength without compromising on the tenacity and flexibility. This conductivity of CNT-wires is unchanged in the sewcap. Further, the process adopted yields a meter of CNT-wire within 10 seconds utilizing about 0.5 mg of CNTs (linear density=0.125 mg/cm) enabling rapid scale-up and processing. Moreover, the initial contact angle of electrolyte on the surface of CNT-wire is estimated to be 80°. However, the contact angle reduces drastically to less than 30° within 25 seconds of contact, thereby confirming the hydrophilicity of the electrode and a facile percolation of electrolyte (Figure S12). Such rapid reduction of contact angle with residence time is attributed to two main reasons. Firstly, the CNTs are immobilized on porous and intrinsically hydrophilic cellulose scaffold to form the CNT-wire electrode. The porosity of the scaffold causes wicking of the electrolyte in to the CNT-wire due to capillary forces. Secondly, acid-treatment of the CNT-wire during its fabrication is established to create p-doped CNT-wire (Figure S11) with –COOH and –OH surface functionalities resulting in increased hydrophilicity. This is also supported by appearance of strong peaks corresponding to O-H stretching (~3433 cm-1), C=O stretching (1633 cm-1 and 1780 cm-


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in the FT-IR spectra of acid-washed CNTs (Figure S11b). Accordingly, a seamless

interface between the CNT-wire and electrolyte is created, as observed in Movie S3 provided with Supporting Information.

The final sewcap fabrication step involves interweaving of the CNT-wire electrodes across a solid-electrolyte sheet made of polyvinyl alcohol-potassium hydroxide (PVAKOH~8:3 by weight) (Figure 1a). The solid-electrolyte is cast into free-standing films of varying thickness using a doctor-blade coater. Cross-weaving multiple such CNT-wire electrodes establishes perpendicular junctions with the solid electrolyte trapped between them. Thereby, a non-lithographically fabricated micro-scale supercapacitor is realized at each junction (Figure 1c and Figure S13). The SEM images (Figure S13) indicate complete penetration of the electrolyte onto CNT-wire. To further confirm the infiltration of electrolyte into individual micro-fibrils of the CNT-wire, a confocal, microRaman spectral mapping of its cross-section was performed. (details in Experimental Section 2.10). Such a cross-sectional mapping (Figure S13e, 13f) exhibits uniform presence of vibrational features of both CNTs (1350 cm-1 and 1597 cm-1 corresponding


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to D-band and G-band, respectively) and PVA-KOH electrolyte (2850-2920 cm-1 for symmetric and asymmetric C-H stretching vibration, 1150 cm-1 for C-O and 1145 cm-1 corresponding to CH2 bending). Thus, the absolute penetration of CNT-wire by the electrolyte is confirmed and is important for formation of extensive electrochemical double layer. Application of a voltage between the two CNT-wires polarizes the solid electrolyte (PVA-KOH) creating an electrical double layer and thereby a supercapacitive energy storage unit (Figure 1a). Since device fabrication involves interweaving the CNTwire electrodes through the solid-electrolyte, the thickness of the electrolyte plays a critical role in controlling the capacitance and mechanical robustness of the final device. While a thicker electrolyte (300 μm) is mechanically resilient for interweaving, the capacitance of the device decreases due to increase in the distance between the electrodes, as is expected for a conventional capacitor. In contrast, thin electrolyte sheets ( 95 %). This is ~5-10 % lower than several other reports employing solid-state electrolytes and is directly attributable to the greater polarizability of the electrolyte. Such significantly lower iRdrop originates from both the greater ionic conductivity of the electrolyte sheet (~10-3 S/cm) due to hydration and reduction in the resistance across the electrode-electrolyte interface. The conductivity pertains to its intrinsic ionic conductivity, in contrast to other reports employing PVA-based hydrogels, which operate on a combination of electronic conductivity from the nanofillers and the ionic conductivity of the PVA-based hydrogel.33 Since ionic conductivity directly relates to the polarizability of the electrolyte, the value of ionic conductivity obtained in this report (~1 mS/cm) is critical for the high-performance


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of sewcaps. The discharge profiles exhibit two different slopes, attributed to the radial and edge diffusion and the planar diffusive pathways and concur with the CVs (Figure S19 and Figure 2c). This observation is substantiated by the ~44.4 % drop in equivalent resistance of the electrolyte as seen in the Nyquist plot (Figure 2f). Fabrication of device in the gel-state of the polymer electrolyte ensures lowering of the charge transfer resistance to ~5 Ω with a steep slope indicating facile EDL formation. Hydration of the electrolyte sheet results in fundamentally modifying the charge transport from a Grotthus-mediated pathway to a vehicular pathway,34 and significantly enhances the performance of the sewcap. Another primary reason for lowered iRdrop is the uniform conductivity of the CNT-wire electrode across greater length scales (Figure S1b). Since the length of CNT-wire used for device fabrication is small (0.5 cm), it acts as an equipotential surface and therefore results in minimal voltage fluctuations due to internal resistance. Thus, a combinatorial synergism driven by the hole-doped CNT-wire and enhanced ionic conductivity of the electrolyte enable achieving lower internal resistance of the sewcap. This also indicates the possibility of perspiration-induced wearable energy storage devices in future – a domain inaccessible to alkali-ion based energy


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storage systems.35 The energy density determined from such sewcaps is enhanced by 12.5 times to ~30 Wh/kg (Figure 1e, 1f). The micro-scale dimensions of the CNT-wire electrode and the corresponding device implies that a small segment of the CNT-wire is used in the sewcap junction. This leads to enhanced electric flux density, which contributes towards higher performance metrics. Correspondingly, the specific, areal and volumetric capacitance, energy and power density are determined to be 261.2 F/g, 521.0 F/cm2, 113.0 F/cm3, 30.2 Wh/kg and 3511.0 W/kg, respectively. Such a performance from one isolated sewcap far exceeds several previous reports.36-38 The performance of sewcaps originates from a unique combination of high specific surface area (~619 m2/g) and mesoporous (

Interwoven Carbon Nanotube Wires for High-Performing

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