High‐Performance Biscrolled MXene/Carbon Nanotube Yarn … · 2018. 8. 8. · Small

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High-Performance Biscrolled MXene/Carbon Nanotube Yarn Supercapacitors

Zhiyu Wang, Si Qin, Shayan Seyedin, Jizhen Zhang, Jiangting Wang, Ariana Levitt, Na Li, Carter Haines, Raquel Ovalle-Robles, Weiwei Lei, Yury Gogotsi, Ray H. Baughman, and Joselito M. Razal*

Z. Wang, Dr. S. Qin, Dr. S. Seyedin, J. Zhang, Dr. J. Wang, Dr. W. Lei, Dr. J. M. RazalInstitute for Frontier MaterialsDeakin UniversityGeelong Waurn Ponds Campus, Geelong, Victoria 3216, AustraliaE-mail: [email protected]. Levitt, Prof. Y. GogotsiDepartment of Materials Science and Engineering and A. J. Drexel Nanomaterials InstituteDrexel UniversityPhiladelphia, PA 19104, USADr. N. Li, Dr. C. Haines, Prof. R. H. BaughmanAlan G. MacDiarmid NanoTech InstituteUniversity of Texas at DallasRichardson, TX 75080, USADr. R. Ovalle-RoblesNano-Science and Technology CenterLintec of America, Inc.Richardson, TX 75081, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201802225.

DOI: 10.1002/smll.201802225

1. Introduction

The evolution of electronics toward small, soft, and wearable devices has created new paradigms in flexible energy storage solutions.[1–6] Although still in its infancy, high energy density supercapacitors (SCs) with a form factor of fabrics provide promising energy storage solutions to the increasing demand

Yarn-shaped supercapacitors (YSCs) once integrated into fabrics provide promising energy storage solutions to the increasing demand of wearable and portable electronics. In such device format, however, it is a challenge to achieve outstanding electrochemical performance without compromising flexibility. Here, MXene-based YSCs that exhibit both flexibility and superior energy storage performance by employing a biscrolling approach to create flexible yarns from highly delaminated and pseudocapacitive MXene sheets that are trapped within helical yarn corridors are reported. With specific capacitance and energy and power densities values exceeding those reported for any YSCs, this work illustrates that biscrolled MXene yarns can potentially provide the conformal energy solution for powering electronics beyond just the form factor of flexible YSCs.

MXene Yarn Supercapacitors

of electronics used in diverse applica-tions, such as personal health monitoring devices, body sensors, implanted devices, and intelligent fabrics.[7–10] Wearable applications in particular require that SCs deliver high specific performance in volumetric, aerial, and gravimetric terms, and be reliable and comfortable to use for extended periods.[11,12] One way in meeting such high requirements is to make SC in the form of conformable yarns.[13–16] This conversion from bulky and rigid conven-tional SCs into a flexible fiber-shaped device also provides the potential for tex-tile integration.

The challenge in this regard is making SC electrodes with high electrical con-

ductivity and high capacitance. In conventional SCs, that is, in coin cells, these electrode attributes are achieved through the use of metal current collectors and the addition of conductive binders to active materials with low conductivity. Some fiber-shaped SCs have adopted the use of metal or metal-coated wires[11,12,17,18] as current collectors, where active materials are applied either by coating or by hydrothermal methods.[19,20] Their low specific performance, due to the weight and volume of the metal wire and the limited loading of active material, has motivated the development of various methods for spinning electroactive or conductive materials into yarns and fibers, such as conductive polymers,[21,22] carbon fibers,[23] carbon nano-tubes (CNTs),[7,18,21,24–26] graphene,[27–29] and various combina-tions of these materials.

Each of these materials has its own merits, and the type of fiber spinning method employed influences fiber characteris-tics. For example, wet-spun conductive polymer fibers (such as polyaniline and polypyrrole) have high pseudocapacitance, but low conductivity and cyclic stability.[1] Carbon-based materials, in comparison, have outstanding electrical conductivity, higher surface area, and excellent stability. In particular, CNT-based yarns are promising due to the high electrical conductivity and strength.[30]

Spinning conductive fibers by biscrolling CNT sheets into highly flexible yarns allows for straightforward integration of active materials into CNTs without the need for a binder and laborious procedure.[8,30] Active material loading in excess of 90 wt% can be realized, and both yarn diameter and porosity can be controlled, making active materials readily accessible to

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the electrolyte. Hence, various guest materials have been made into various forms of flexible CNT-based yarn-shaped super-capacitors (YSCs), such as sheath–core, twisted, and coiled configurations, which effectively capture the capacitive or pseu-docapacitive properties of the guest active material.[8,17,22,30–35]

Recent literature suggests that MXenes, a large family of 2D transition metal carbides and nitrides, might be attrac-tive active guest materials for YSCs. MXenes have the gen-eral formula Mn+1XnTx, where M is a transition metal, X is carbon and/or nitrogen with n = 1, 2, or 3, and Tx denotes the surface termination groups (F, O, and OH).[36] The volumetric capacitance and electrical conductivity of Ti3C2TX MXene, the most studied material in the family to date, reach 1500 F cm−3 and 9880 S cm−1, respectively,[37–40] which exceed most SC electrode materials and approach the volumetric capacitance of some metal oxides such as MnO2 and RuO2 (>1000 F cm−3).[41,42] MXene-based fiber and yarn SCs have been prepared by coating and by wet-spinning methods,[43,44] but the realized performance is low because of the low MXene loading (coating) or the low conductivity (wet-spinning) of the resulting fiber.

Here, we fabricate flexible yarn electrodes by biscrolling MXene with CNTs referred to as “BMX yarns.” The highest MXene loading of ≈98 wt% displayed record specific volumetric, aerial, gravimetric, and linear capacitance of 1083 F cm−3, 3188 mF cm−2, 428 F g−1, and 118 mF cm−1, respectively, at a cur-rent density of 2 mA cm−2. When the BMX yarns are made into

freestanding asymmetric YSC prototypes by pairing with bis-crolled RuO2 yarns, the respective maximum energy and power densities of 61.6 mWh cm−3 (168 µWh cm−2 and 8.4 µWh cm−1) and 5428 mW cm−3 (14.8 mW cm−2 and 741 µW cm−1) were achieved. The device displayed high capacitance retention after repeated bending. Proof-of-concept textile integration of the BMX YSCs demonstrates their applications as conformal energy yarns for powering electronic devices.

2. Results and Discussion

2.1. Biscrolled MXene-CNT Yarns

MXene was synthesized by a minimally intensive layer delami-nation (MILD) method (Figure S1, Supporting Information). We fabricated the BMX yarns using a biscrolling method,[25,30] where a MXene dispersion is drop-cast on stacked CNT sheets prior to twisting the sheet stack into a yarn using ≈2000 turns m−1 of inserted twist (Figure 1). Here, we focus on stud-ying the BMX yarns with MXene loading above 50 wt%, since neat CNT yarns possess low specific capacitance (≈4.5 F g−1 and ≈32 F cm−3)[31,35,45] compared to MXene (up to 450 F g−1 and 1500 F cm−3).[37,38] The amount of MXene in the yarn was tuned by varying the MXene dispersion concentration to achieve a MXene loading as high as ≈97.4 wt% (Table S1, Supporting Information).

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Figure 1. a) Schematic of the fabrication process for BMX yarns. MXene dispersion was coated on CNT sheets and biscrolled to form a composite BMX yarn in which MXene is evenly distributed within the CNT framework. SEM images showing the b) surface and c) cross-section morphologies of the BMX yarn containing 97.4 wt% MXene. d) High-magnification colored SEM image of the marked section in (c) with enhanced contrast illustrating how the MXene sheets are trapped within CNT yarn corridors. e) SEM image showing a BMX yarn containing 90 wt% MXene tied into a reef knot.

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Similar to neat CNT yarns, the BMX yarns are flexible and have circular diameter (Figure 1b,c) suggesting that the MXene sheets are homogeneously dispersed in the CNT network. Scan-ning electron microscope (SEM) images reveal that thin layers of MXene sheets are intermingled within the CNT network and trapped within helical yarn corridors, creating pores and voids of varying sizes (Figure 1c,d, and Figure S2, Supporting Infor-mation). The X-ray diffraction (XRD) spectra also confirm that MXene remained in its delaminated state after yarn fabrication, as shown by the sharp diffraction signal at ≈7° for the (002) plane of MXene (Figure S3, Supporting Information). The yarn diameter increases with MXene loading from 42 µm for the neat CNT yarn to ≈120 µm for BMX yarn with 97.4 wt% MXene (Table S1 and Figure S4, Supporting Information). Unlike the hydrophobic neat CNT yarns, the BMX yarns easily wet when immersed in aqueous electrolyte, owing to the hydrophilic nature of MXene. This porosity and hydrophilicity facilitate the infiltration of electrolyte within the pores and fast access of ions, as shown and discussed later. Also, despite the increase in diameter and porosity, the BMX yarns remain pliable, as demon-strated by the absence of cracks and distortion to the yarn struc-ture after tying into complex knots (Figure 1e, and Figure S5, Supporting Information). The electrical resistance remains ≈340 Ω cm−1 irrespective of MXene loading, reflecting that of the CNT framework (Figure S6, Supporting Information). The electrical conductivity of the yarns varies with MXene loading due to the increase in yarn diameter. The measured conductivi-ties (26 S cm−1 for the BMX yarn and 235 S cm−1 for the neat CNT yarn) are comparable with those of previously reported guest-loaded and neat CNT yarns.[15,46,47]

2.2. Electrochemical Properties

We characterized the electrochemical properties of BMX yarns using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) tests, where a single yarn is used as the working electrode in a three-electrode setup. A 3 m H2SO4 electrolyte was used to enable direct comparison with litera-ture reports on MXene that show very high capacitances.[37,48] The CV curves of all three yarns with different MXene load-ings display the two prominent broad peaks attributed to the intercalation/deintercalation of H+ and surface redox reac-tions (Figure 2a).[36,38,49] The area of the CV curves increases with MXene loading, which indicates increased capacitance due to accessible MXene sheets in the yarn. These results agree with the GCD curves, which show the increasing dis-charge time with MXene loading (Figure 2b). As with the CV curves, the intercalation/deintercalation of H+ at ≈−0.35 V in the GCD curves are also observed.

The BMX yarn with the highest MXene loading of 97.4 wt% exhibits the highest capacitance (Figure 2c). Its specific capac-itance values (volumetric, aerial, and gravimetric) exceed those of any freestanding fiber-shape electrode reported to date (Table 1, and Figure S7, Supporting Information). Its areal capacitance is ≈3188 mF cm−2 at a current density of 2 mA cm−2, corresponding to volumetric, gravimetric, and linear capacitances of 1083 F cm−3, 532 F g−1, and 118 mF cm−1, respectively. When the current density is increased ten times (from 2 to 20 mA cm−2), the BMX yarn still delivers a very high specific areal capacitance 2506 F cm−2. Moreover, the BMX yarns demonstrate excellent cyclic stability in 3 m H2SO4.

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Figure 2. a) CV curves obtained at 5 mV s−1, b) GCD curves obtained at 2 mA cm−2, c) specific areal capacitance of the BMX yarn electrodes from the GCD curves, and d) Nyquist plot of the EIS data. All data were obtained from the BMX yarn electrodes with different MXene loadings using 3 m H2SO4 electrolyte. Inset in (d) shows the high-frequency area.

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At 97.4 wt% MXene loading, the capacitance retention after 10 000 cycles at 20 mA cm−2 is ≈100% (Figure S8, Supporting Information). SEM images (Figure S9, Supporting Information) of BMX yarns (97.4 wt%) after 10 000 cycles show that the shape of the yarn and the MXene morphology largely remained unchanged. It is worth noting that the amount of CNT was kept the same when fabricating BMX yarns with different loadings. Therefore, the actual mass of MXene in the 97.4 wt% BMX yarns is about five times higher than that of 86.2 wt% yarn, while the 86.2 wt% BMX yarn has 2.3 times more MXene than the 69.2 wt% BMX yarn. There-fore, the capacitance of these yarns reflects the amount of MXene within the yarns.

Similar to thick MXene electrodes,[37,38] the observed inter-calation/deintercalation process becomes diffusion-limited at high scan rates and at high current densities (Figures S7 and S10, Supporting Information). This behavior is apparent for BMX yarn with high MXene loading (97.4 wt%); yarns with low MXene loading (69.2 wt%) are highly capacitive even at high scan rate (Figure 2c). The Nyquist plots of the BMX yarns (Figure 2d) show that the equivalent series resistance (ESR) increases with MXene loading. These results suggest that as the yarn becomes denser when MXene loading increases, the ion diffusion resistance increases, similar to the behavior of thick planar MXene electrodes.[37,38]

2.3. Freestanding Yarn Supercapacitors and Prototype Demonstrations

We fabricated freestanding yarn supercapacitors that adopted the symmetric device configuration of conventional superca-pacitors where the two electrodes are identical (referred to as sYSC) by twisting together two MXene/CNT yarns separated by a polyvinyl alcohol (PVA)-H2SO4 gel electrolyte (see the Experimental Section for fabrication details). Both the CV and GCD tests were used to assess their performance at different scan rates and current densities, respectively (Figure S11, Supporting Information). This symmetric configuration

resulted in a stable cell voltage of ≈0.8 V reflecting the stable potential window of MXene. It was found that the highest energy density and power density of 8.54 mWh cm−3 and 530 mW cm−3, respectively, are higher than those of other CNT-based yarns such as MoS2/CNT yarn (3.0 mWh cm−3),[53] MnO2/CNT yarn (3.52 mWh cm−3),[54] and CNT/rGO yarn (2.4 mWh cm−3).[52]

To extend the cell voltage and further increase the energy density, we also adopted the asymmetric device configuration (referred to as aYSCs) by pairing the BMX yarn (as anode) with a biscrolled RuO2/CNT yarn (BRU yarn) as cathode. The BRU yarns were made by biscrolling RuO2 nanoparticles (Figure S12, Supporting Information) onto CNT following the same procedure employed in making the BMX yarns. Since RuO2 operates at a potential window of 0–1 V versus Ag/AgCl, whereas the potential window of MXene is from −0.6 to 0.1 V versus Ag/AgCl (Figure 3a), this electrode pair enabled an effective cell voltage of up to 1.5 V, compared to only 0.8 V for the sYSC (Figure 3b). The specific capacitance values of aYSC (203 F cm−3, 554 mF cm−2, 123 F g−1, and 27.8 mF cm−1 at 2 mA cm−2) (Figure 3c,d) are also higher than for sYSC and the capacitance retention is ≈90% of its initial capaci-tance after 10 000 charge/discharge cycles at 20 mA cm−2 (Figure 3e). Insets in Figure 3e display the minimal changes in the GCD curves after 10 000 cycles.

The Ragone plot shown in Figure 3f demonstrates the excel-lent energy and power densities of the aYSC compared to lit-erature reports. Figure S13 (Supporting Information) shows the Ragone plot in terms of aerial and linear densities. The aYSC delivers a volumetric energy density of 61.6 mWh cm−3 at a power density of 358 mW cm−3 (168 µWh cm−2 at 975 µW cm−2, and 8.4 µWh cm−1 at 48.8 µW cm−1). Even at the highest power density of 5428 mW cm−3, the asymmetric device has an energy density of 11.9 mWh cm−3 (32.4 µWh cm−2 at 14.8 mW cm−2, and 1.63 µWh cm−1 at 741 µW cm−1). These values are higher than previously reported fiber-shaped supercapacitors.[19,23,33,52–57]

We summarized in Table S2 (Supporting Information) these performance values along with the employed fabrication methods, device design, and active electrode materials. This literature snapshot shows that the presently reported aYSCs provide a higher energy density than any fiber-shaped superca-pacitors in the literature. It is also worth noting that this energy density is comparable to many conventional planar supercapac-itors[58–60] and much higher than that of typical commercially available supercapacitors (1.7–9.3 mWh cm−3).[61] As an esti-mate, 1 cm2 of aYSC woven into a 100 thread count plain weave fabric could provide ≈0.44 mWh of energy to power low energy devices, such as a 19 µW sensor[62] or a 34 µW transmitter,[63] for up to 1 day without recharging. Since YSCs can rapidly store the electrical energy, in the future they could be paired with other flexible and miniature energy generating devices (e.g., twistron devices[64] and triboelectric nanogenerators[65]) to make autonomously powered devices.

The freestanding aYSCs demonstrate excellent performance even when subjected to repeated bending cycles at various bending angles (Figure 4a). The CV curves before and after 1000 bending cycles to 90° overlap, and capacitance reten-tion is nearly 100%, which demonstrates that the electrodes

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Table 1. Comparison of specific capacitance of various fiber-shaped electrodes for supercapacitors.

Materials CA [mF cm−2] CV [F cm−3] CL [mF cm−1] CG [F g−1] Ref.

CNT/PEDOT 134.8 [7]

VN@CNT fiber 715 [14]

PEDOT@PNC 404.1 80.8 [50]

MnO2-

PEDOT:PSS-

CNT

478.6 411.6 [21]

OMC@CNT 121.4 [21]

NiCo2S4@

graphene fiber

388 [51]

CNT/RGO 59.8 [52]

Biscrolled

MnO2-CNT

888.2 154.7 [15]

BMX yarn 3188 1083 118 523 This work

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endure the repeated deformations imposed during the test (Figure 4b,c).

By having the aYSC diameter close to that of cotton yarn, it was possible to weave them into a cotton yarn fabric. The resulting “energy textile” prototype was used to power portable electronics. In one example, two aYSCs (≈4 cm each in length) were woven into the fabric (≈100 thread count), replacing two manually extracted cotton strands, and then electrically con-nected in series to achieve the voltage needed for powering a red light-emitting diode (LED) (Figure 5b). Video S1 (Sup-porting Information) shows its operation for over 60 s. This energy textile remained flexible, strong, and functional after bending and twisting (Video S2, Supporting Information). In another example, the energy textile was used to power a digital timer for over 10 min (Figure 5c, and Video S3, Supporting Information) and a digital watch for ≈3 h (Figure 5d, and Video S4, Supporting Information).

3. Conclusion

This work reports the convenient fabrication of mechani-cally and electrically durable, flexible energy-storing yarns and textile prototypes. The employed biscrolling technique enabled the spinning of BMX yarns containing predominantly MXene nanosheets (up to ≈98 wt%) that are trapped within CNT yarn scrolls. Importantly, this BMX yarn provided a specific capaci-tance as high as 1083 F cm−3 (3188 mF cm−2), which exceeds the previously recorded performance for any yarn supercapac-itor electrode. When fabricated into a yarn supercapacitor, the asymmetric electrode configuration reached a maximum energy density (61.6 mWh cm−3) and a power density (5428 mW cm−3), which are useful for powering small electronic devices using small quantities of supercapacitor that are woven into a tex-tile. Moreover, the energy-storing textile prototype provides high mechanical durability and outstanding performance over

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Figure 3. a) CV curves measured at a scan rate of 5 mV s−1 showing the working potential range of the BMX and BRU yarn electrodes. CV curves of a freestanding aYSC device measured at b) different potential window at a scan rate of 10 mV s−1 and c) at different scan rates at 1.5 V. d) GCD curves of the aYSC device measured at different charge–discharge current density. e) Capacitance retention of the freestanding aYSC device. f) Ragone plot of the specific volumetric energy density and volumetric power density of the aYSC device compared with other reports.

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Figure 4. a) Photographs showing the freestanding aYSC device under different bending angles. Capacitance retention of the aYSC b) under different bending angles and c) during 1000 bending cycles. Inset in (c) shows the CV curves obtained before and after the 1000 bending cycles.

Figure 5. a) Schematic illustration of the energy textile prototype containing the aYSC device. Some examples showing that the energy textile prototype can power b) an LED, c) a digital timer, and d) a digital watch.

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many charge–discharge cycles, thereby indicating promise for integration with flexible and textile-based electronics.

4. Experimental SectionMXene Preparation: Ti3C2Tx MXene was synthesized following previously

reported methods.[37,66] Briefly, the MAX phase (Ti3AlC2) was etched in a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF) to yield multilayered Ti3C2Tx. LiF (2 g) was added to 40 mL of 9 m HCl, followed by the slow addition of 2 g of Ti3AlC2. The sample was kept at 35 °C and etched for 24 h under stirring. The multilayer Ti3C2Tx was washed with deionized water by centrifugation until the solution reached a pH value ≈6. The MXene dispersion was sonicated for 15 min while purging with Ar. The dispersion was centrifuged for 10 min at 1500 rpm (≈244 × g) and the supernatant was collected and transferred to dimethylformamide (DMF) by further centrifugation for the fabrication of the BMX yarns.

Preparation of BMX and BRU Yarns: CNT sheets (≈1 cm width) were drawn from a CNT forest of vertically aligned nanotubes that are ≈350 µm in height and ≈9 nm in diameter with ≈6 walls.[30,67] For biscrolling BMX yarns, five layers of CNT sheets were stacked on a glass substrate with one end fixed to an electric motor. MXene dispersion in DMF (2–30 mg mL−1) was dropped on the CNT sheets. The CNT sheets were twisted to ≈2000 turns m−1 by a motor. The yarns were densified by evaporating the DMF naturally in air. The weight of CNT sheets (m1) used for biscrolling and the BMX yarn (m2) were weighed on a Perkin Elmer AD-6 microbalance. The mass loading of the yarn was calculated using the following equation: η = (m2 − m1)/m2 × 100%. BRU yarns containing ruthenium oxide (RuO2) were prepared by following the same procedure described above. RuO2 was prepared following previously reported method.[68] Briefly, 100 mg of ruthenium chloride hydrate (RuCl3 · xH2O) was dissolved in 50 mL water. Sodium hydroxide solution (≈0.1 m) was added dropwise under stirring until pH ≈ 7. The precipitate was washed, freeze dried and heated in air at 150 °C for 2 h.

Characterization: The weight of the yarn was measured using a PerkinElmer AD-6 microbalance. Yarn diameters were measured using an Olympus DP71 optical microscope from an average of ten different locations. SEM images were taken on a Zeiss Supra VP55 with an accelerating voltage of 5 kV. XRD patterns were obtained on a PANalytical X’pert Powder diffractometer with a Cu Kα radiation source operated at 40 kV (λ = 1.5406 Å) and a 2θ range of 4°–60°. The electrical resistance was measured using a Keysight 34461A multimeter using a 4-point probe setup. Conductivity (σ) was calculated by σ = l/RAc, where l, R, and Ac are the length, resistance, and the cross-sectional area of the yarn, respectively.

Fabrication and Characterization of Yarn Electrodes and YSCs: The electrochemical performance of the yarns was first measured in a three-electrode configuration. It was found that commonly used current collectors such as titanium foil introduce large current background during electrochemical test which often mask the signal from yarn electrode of fine diameters. In order to obtain more reliable results, it is best to avoid using additional current collector, and only expose the yarn to the electrolyte during electrochemical test. In a typical experiment, the working electrode was prepared by attaching an ≈10–15 mm long BMX yarn to a fine copper wire using conductive silver paste on one end. The connection and the copper wire were sealed in epoxy glue to avoid contact with the 3 m H2SO4 electrolyte. The counter and the reference electrodes were graphite rod and Ag/AgCl in 3 m KCl, respectively. The Ag/AgCl reference electrode was separated from the H2SO4 electrolyte by using a salt bridge. PVA-H2SO4 electrolyte was prepared by dissolving 1 g of PVA (Sigma-Aldrich, MW = 89 000–98 000) in 10 mL of water at 80 °C under vigorous stirring. After cooling down to room temperature, 1 g of sulfuric acid (H2SO4) was added. For preparation of YSCs, ≈50 mm of yarn electrode was coated with PVA-H2SO4 electrolyte and dried in air overnight. Two coated yarn electrodes were twisted together and coated twice with PVA-H2SO4 electrolyte to ensure a complete coating. For the symmetric configuration, two yarns with the same length were used. For the asymmetric device, the length and active material loadings

of the two yarn electrodes were adjusted so that they have a matching capacitance. All electrochemical tests were carried out using a Biologic SP-300 electrochemical station at room temperature. For the three-electrode setup, the current density presented in the CV and GCD curves were normalized to the external surface area of the yarn electrode. For the two-electrode setup, the current applied in the GCD tests was calculated from the outer surface area (A) and volume (V) of the BMX yarn electrode. Cycling stability was measured by repeating the GCD test for 10 000 cycles at a current density of 20 A cm−2. The capacitance was calculated by integrating the GCD data using the following Equation (1)

Ci t

V

td

o∫= ∆ (1)

where i is the current, and ΔV is the effective voltage window (minus the iR drop). The specific areal, volumetric, and linear capacitance of the electrode were obtained by normalizing the capacitance to the outer surface area, volume, and length of the yarn electrode (for three-electrode configuration), while those of the supercapacitor device were normalized to the total area, total volume, and length of whole device including both electrodes (for two-electrode device including sYSC and aYSC). The outer surface area (A) and volume (V) of the yarn electrode were calculated using the following Equation (2) and (3), respectively

A l dArea : π= (2)

Volume : /2 2V l dπ ( )= (3)

where l and d are the length and diameter of the yarn electrode. The electrochemical impedance spectroscopy was performed using a three-electrode configuration at open-circuit potential within a frequency range from 100 kHz to 10 mHz at an amplitude of 5 mV. For comparison purposes, the specific volumetric energy density of a commercial supercapacitor was calculated by normalizing the energy density to its nominal size.[61]

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

AcknowledgementsZ.W. and S.Q. contributed equally to this work. The authors acknowledge the Australian Research Council (FT130100380, IH140100018, and DP170102859) and the Institute for Frontier Materials (REGS) at Deakin University for funding. Z.W. acknowledge financial support from China Scholarship Council (File No. 201606930013). R.H.B. thanks Robert A. Welch Foundation Grant No. AT-0029 for support. A.L. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1646737.

Conflict of InterestThe authors declare no conflict of interest.

Keywordsbiscrolling, carbon nanotubes, MXene, yarn supercapacitors

Received: June 9, 2018Revised: July 6, 2018

Published online:

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