Enhanced Electrical and MechanicalProperties of Chemically Cross-Linked Carbon-Nanotube-Based Fibers and Their Applicationin High-Performance SupercapacitorsGang Wang,†,▼ Sung-Kon Kim,‡,§,△,▼ Michael Cai Wang,∥,○ Tianshu Zhai,† Siddhanth Munukutla,⊥

Gregory S. Girolami,# Peter J. Sempsrott,# SungWoo Nam,∥ Paul V. Braun,†,‡,§,¶

and Joseph W. Lyding*,†,⊥

†Beckman Institute for Advanced Science and Technology, ‡Department of Materials Science and Engineering, §Materials ResearchLaboratory, ∥Department of Mechanical Science and Engineering, ⊥Department of Electrical and Computer Engineering, #School ofChemical Sciences, and ¶Department of Chemistry, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, UnitedStates△School of Chemical Engineering and School of Semiconductor and Chemical Engineering, Chonbuk National University, 567Baekje-Daero, Deokjin-gu, Jeonju 54896, Republic of Korea○Department of Mechanical Engineering, University of South Florida, Tampa, Florida 33620, United States

*S Supporting Information

ABSTRACT: The electrical conductivity and mechanicalstrength of fibers constructed from single-walled carbonnanotubes (CNTs) are usually limited by the weakinteractions between individual CNTs. In this work, wereport a significant enhancement of both of theseproperties through chemical cross-linking of individualCNTs. The CNT fibers are made by wet-spinning a CNTsolution that contains 1,3,5-tris(2′-bromophenyl)benzene(2TBB) molecules as the cross-linking agent, and the cross-linking is subsequently driven by Joule heating. Cross-linking with 2TBB increases the conductivity of the CNTfibers by a factor of ∼100 and increases the tensile strengthon average by 47%; in contrast, the tensile strength of CNTfibers fabricated without 2TBB decreases after the same Joule heating process. Symmetrical supercapacitors made fromthe 2TBB-treated CNT fibers exhibit a remarkably high volumetric energy density of ∼4.5 mWh cm−3 and a power densityof ∼1.3 W cm−3.KEYWORDS: carbon nanotube fiber, Joule heating, electrical conductivity, mechanical strength, supercapacitor

Single-walled carbon nanotubes (CNTs) are remarkableone-dimensional materials that have been the subject ofintense and increasing interest.1−3 CNTs have out-

standing electrical and mechanical properties at the molecularlevel, making them ideal candidates for electrical devices,mechanical load carrying, and other applications.4−6 Never-theless, the main obstacles that seriously limit the use of CNTsare the poor properties that result when they are assembledinto bundles at the macroscale. For instance, weak van derWaals interaction between CNTs leads to poor mechanicalstrength, elastic modulus, and electrical properties. Severalmethods have been investigated to enhance the interactionbetween CNTs to improve the electrical and mechanical

properties, such as twisting,7 shrinking,8,9 and applyingpressure.10,11

Recently, our group successfully demonstrated a method toenhance CNT interactions in electronic devices constructedfrom CNT networks: nanosoldering, in which a metal“nanosolder” is locally deposited at the CNT junctions.12,13

The nanosoldering is achieved by passing a current through aCNT network in the presence of a metal chemical vapordeposition (CVD) precursor; the current causes localized

Received: September 13, 2019Accepted: December 26, 2019Published: December 26, 2019

Artic

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heating at the more resistive CNT junctions, inducing selectivethermal decomposition of the precursor that encases the CNTjunctions with metal nanoparticles. In initial implementations,however, this method afforded a nanosolder that was inphysical contact with but not bonded covalently to the CNTs,and this lack of covalency limited the improvement in theelectronic and mechanical connection at the CNT junctions.To address this limitation, we developed a nanosoldering

process based on organic molecules that creates covalentlybonded links across CNT junctions. For these experiments1,3,5-tris(2′-bromophenyl)benzene (2TBB) was used as theorganic precursor. 2TBB is a halogenated aromatic hydro-carbon that can undergo dehalogenation and dehydrogenationprocesses upon heat treatment, resulting in covalent networksof carbon atoms.14,15 CNT network device performanceshowed that the ION/IOFF ratio was improved by a factor of∼40, with ION increased by nearly 2 orders of magnitude aftercross-linking by 2TBB.16,17

In this work, we report that treating CNT fibers with 2TBBmolecules followed by cross-linking can result in significantimprovements in the electrical and mechanical properties. Thecross-linking is effected by passing a current through a CNT/2TBB fiber; the resulting Joule heating at resistive CNTjunctions causes selective thermal dehalogenation of theorganic precursor leading to covalent bonding. After thisJoule heating treatment, there is a 100-fold increase inelectrical conductivity and on average a 47% improvement intensile strength. Furthermore, we also investigated theapplication of 2TBB cross-linked CNT fibers after treatmentfor energy storage. Microsupercapacitors made from the cross-linked fibers exhibit a volumetric energy density of ∼4.5 mWhcm−3 and a power density of ∼1.3 W cm−3.

RESULTS AND DISCUSSION

As shown in Figure 1a, continuous CNT/2TBB fibers werefabricated by wet-spinning from solutions containing CNTsmixed with 20 mg/mL 2TBB, a small molecule which can formsp2 bonds by dehalogenation and dehydrogenation pro-cesses.14,15 By co-injecting the CNT/2TBB solution into oneend of a quartz tube along with poly(vinyl alcohol) (PVA) asthe coagulation solution, meter-long continuous CNT/2TBBfibers could be drawn from the other end of the tube at a rateof 8−16 cm/min. The fiber is still wet immediately after exitingfrom the quartz tube and is allowed to dry in air for 2 min(corresponding to a travel path of 25 cm) before beingcollected on a spindle (inset of Figure 1a). Without this dryingstep, the wet fiber collected on the spindle would collapse andshow a ribbon-like structure instead of a cylindrical shape(Figure S1). During the wet-spinning process of the CNTfiber, the fiber is translated by the surrounding polymer fluid atthe center of the quartz tube, which applies a dragging forceand results in the alignment of CNTs in the as-prepared fiber.The magnified scanning electron microscopy (SEM) images ofthe CNT fibers clearly show the aligned CNT bundles alongthe fiber’s axis (Figure S2). Additionally, the fiber’s diameterand alignment can be easily modified by controlling theinjecting speeds of the CNT/2TBB solution and the PVAcoagulation solution (Figure S3). Under our typical extrusionconditions, the fiber has a diameter of 30 μm.Figure 1b,c shows representative SEM images of CNT/

2TBB fibers before and after Joule heating. Although the fiber’sdiameter and the surface morphology seem almost unchangedby the heating, further analysis reveals weight loss after Jouleheating (Table S1). Change of the fiber’s bulk density can bemainly attributed to the decomposition of PVA due to the hightemperature caused by Joule heating. According to the Ramanspectra shown in Figure 1d, the full width at half-maximum(fwhm) values of the G peak and the 2D peak due to sp2

Figure 1. (a) Schematic picture of the fabrication of CNT/2TBB fibers by wet-spinning. Inset is a photograph of a meter-long continuousCNT/2TBB fiber collected on a spindle. (b,c) SEM images of CNT/2TBB fiber (b) before and (c) after Joule heating treatment. Insets aremagnified images of CNT fibers with diameters of 30 μm. (d) Raman spectra of CNT fiber before and after Joule heating treatment.

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carbons have broadened after treatment. The fwhm of the Gpeak broadens from 85 to 117 cm−1, whereas the fwhm of the2D peak broadens from 69 to 94 cm−1, which can also beattributed to the amorphous carbon resulting from thedecomposition of PVA. In addition, the continued weaknessof the D peak indicates that the CNTs are undamaged andremain crystalline after the Joule heating process.Four-point probe measurements were used to measure the

fiber conductivity both during and after the Joule heatingtreatment in a vacuum. A typical current−voltage (I−V) curveobtained during the Joule heating process is shown as the blackcurve in Figure 2a. Four stages, I−IV, characterize theevolution of the I−V curve during heating. During stage I,the voltage applied to the fiber is less than 70 V and the fiberresistance is very stable. Upon increasing the voltage to 100 V,marked as stage II in Figure 2a, the fiber resistance decreases,which is consistent with fiber heating and development ofsemiconducting behavior. With a further increase in voltage to120 V in stage III, there is a sharp decrease in fiber resistance.During this stage, the fiber is hot enough to be visiblyincandescent in the vacuum chamber. After stage III, the fiberconductivity was a factor of 50−100 higher than its valuebefore treatment. At voltages higher than 120 V, the fiberresistance remains relatively constant, as seen in region IV in

Figure 2a. To quantify the Joule heating process, currentdensity is chosen because it can exclude the effect of samplesize. According to statistics of 15 samples (Table S1), we foundthat the current density during the rapid increase in electricalconductivity (i.e., during conversion from stage II to stage III)is ∼50 A/cm3.After this Joule heating sequence, the resistance of the fiber

remains permanently low. This finding was verified byperforming a second I−V measurement in a vacuum, whichis shown as the red curve in Figure 2a. This low resistance statepersists after the fiber is removed from the vacuum to a normalatmosphere: the black and red curves in Figure 2b represent I−V plots of the CNT/2TBB fiber before and after the Jouleheating treatment, respectively. Remarkably, the Joule heatingtreatment causes the fiber resistance to decrease by a factor ofabout 50 from 680 to 13.5 kΩ. Tens of CNT/2TBB fiberswere measured before and after the Joule heating treatment toconfirm the enhancement of conductivity (Figure 2c). Theoriginal conductivity of the CNT/2TBB fibers was 160 ± 40S/m before treatment but 10200 ± 1700 S/m afterward,indicating an average conductivity increase by a factor of 65.Some of the increase in conductivity can be attributed to the

Joule heating effect. To demonstrate this fact, we fabricated acontrol batch of CNT fibers without 2TBB using the same

Figure 2. (a) I−V curve of a CNT/2TBB fiber both during and after applying the Joule heating process in a vacuum chamber. Four stagesobserved during Joule heating are marked as I, II, III, and IV. (b) Four-point probe electrical measurement of a CNT/2TBB fiber before andafter Joule heating. (c) Electrical conductivities of a CNT fiber without 2TBB and a CNT/2TBB fiber (concentration of 2TBB is 20 mg/mL)before and after Joule heating treatment. (d) Dependence of electrical conductivity of CNT/2TBB fiber on the concentration of 2TBB afterJoule heating treatment.

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wet-spinning method (Figure S4) and treated them with thesame Joule heating process. The pretreated conductivity of thenon-2TBB fibers was 175 ± 40 S/m, which was similar to thevalues measured for the CNT/2TBB fibers (Figure 2c). AfterJoule heating, the conductivity of the non-2TBB CNT fibersincreased to 4900 ± 700 S/m, or about half of the value ofCNT/2TBB fibers after the same treatment. The increase ofnon-2TBB fiber’s electrical conductivity can be attributed tothe formation of amorphous carbon, resulting from thedecomposition of PVA during Joule heating treatment. Thisresult indicates that both Joule heating and 2TBB play a role inenhancing the electrical conductivity.The role of 2TBB was further explored by determining how

CNT/2TBB fiber conductivity depends on the amount of2TBB contained in the fibers. To carry out this study, 2TBBconcentrations of 0.4, 0.8, 4, 20, and 40 mg/mL were addedinto the CNT solution used in the wet-spinning process; theseconcentrations represent mass ratios of CNT to 2TBB of 10:1,5:1, 1:1, 1:5, and 1:10, respectively. Figure 2d shows theelectrical conductivities of these fibers after Joule heating. Forcomparison, the electrical conductivity of the CNT fiberwithout 2TBB is also exhibited in the figure. The conductivitiesincrease with 2TBB content, reaching a maximum for the fiberfabricated with 20 mg/mL 2TBB, consistent with thehypothesis that higher 2TBB concentrations produce morelinks between CNTs. Above 20 mg/mL 2TBB concentration,however, the conductivity drops off, very likely due to thepresence of a dilution effect that reduces the number anddensity of CNT−CNT junctions. It should be noted that2TBB acts as an organic precursor and nanosolder and isconsumed during the Joule heating process to enhance the

interactions between CNT junctions. Energy-dispersive X-rayspectroscopy analysis confirms the disappearance of the Brpeak, which is consistent with conversion of 2TBB to acrosslinked form (Figure S5).In addition to investigations of the electrical conductivities

of the CNT/2TBB fibers before and after Joule heating, wealso studied their mechanical properties. Before Joule heating,the CNT/2TBB fiber has a tensile strength of 280 MPa with abreaking elongation of 24% (black curve of Figure 3a). Aftertreatment, the fiber tensile strength increases to 440 MPa andthe breaking elongation drops to 19% (red curve of Figure 3a).For comparison, CNT fibers without 2TBB were alsomeasured as a control experiment. Unlike the CNT/2TBBfiber, the tensile strength of a CNT fiber without 2TBBdecreases after Joule heating treatment (Figure 3b), which isconsistent with previous reports.18−20 These results indicatethat 2TBB plays a direct role in enhancing the mechanicalstrength of CNT-based fibers. Figure 3c,d shows thedistributions in tensile strength and breaking elongation forCNT fibers with and without 2TBB and before and after Jouleheating treatment. After treatment, the CNT/2TBB fibersshow a 47% higher tensile strength on average with littlechange in breaking elongation. In contrast, after treatment, theCNT fibers without 2TBB exhibited a 24% drop in tensilestrength and a 61% decrease in breaking elongation.We also investigated the effect of 2TBB cross-linking on the

mechanical strength of the CNT fibers by adjusting theconcentration of 2TBB during fiber fabrication, and the resultsare shown as Figure S6. 2TBB concentrations of 0.4, 0.8, 4, 20,and 40 mg/mL were added into the CNT solution used in thewet-spinning process. After treatment, the tensile strength of

Figure 3. (a) Typical stress−strain curves of CNT/2TBB fiber before and after Joule heating treatment. (b) Typical stress−strain curves ofCNT fiber, without 2TBB, before and after Joule heating treatment. (c) Comparison of stress and (d) breaking elongation for CNT fibers,with and without 2TBB, before and after Joule heating treatment.

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the fiber first increases with 2TBB content, reaches a maximumfor the fiber with 20 mg/mL 2TBB, and finally decreasesslightly for the fiber with the highest content of 2TBB; thisdependence on the amount of added 2TBB is the same as thatseen for the fiber strength.We also investigated whether the alignment of the CNTs

affects the electrical and mechanical properties of the fibers.Because the alignment of the CNTs along the fiber axis isenhanced by the dragging force of the surrounding polymerfluid during the fabrication process, it is possible to adjust thealignment of the CNTs in the fiber by controlling the flowspeed of the polymer fluid. Thus, we prepared two kinds ofCNT fibers, one in which the PVA polymer was injected at 60mL/h, and the other at 30 mL/h (the injection speed of theCNT solution was 0.25 mL/h for both groups). Before Jouleheating, the electrical conductivity and tensile strength are 39%and 18% higher, respectively, for the fibers fabricated at thegreater injection speed (Figure S7). After Joule heating,however, the electrical conductivity and tensile strength areonly 7% and 4% higher, respectively, for the fibers fabricated at

the greater injection speed. We conclude that, before Jouleheating, the electrical and mechanical properties of the fiberare sensitive to the degree of alignment of the CNTs but that,after Joule heating, these properties are dominated by theeffects of cross-linking (and the degree of alignment is of lesserimportance).We next prepared and studied microsupercapacitors (mSCs)

by parallel assembly of the CNT fiber electrodes with asolvent-cast poly(vinyl alcohol)/phosphoric acid solid electro-lyte. When prepared in this way, the fiber mSCs aremechanically robust and flexible. The electrochemical proper-ties of the mSCs were measured by two-electrode cyclicvoltammetry (CV) and galvanostatic charge−discharge(GCD) measurements over the voltage window of 0 to 0.8V. Figure 4a shows the CV curves at a constant scan rate of 50mV s−1 for two types of mSCs: those made from fibers beforeJoule heating and those made from fibers after Joule heating.The CV curve of the cross-linked fiber mSC exhibits a nearlyrectangular-shaped current−voltage sweep, which is character-istic of an ideal SC, demonstrating the effective formation of an

Figure 4. (a) CV curves (at a scan rate of 50 mV s−1), (b) GCD profiles (at current densities ranging from 0.05 to 0.8 A cm−3), (c) rate-dependent capacitance, (d) Nyquist plots, (e) real (C′) and imaginary parts (C″) of the capacitance vs frequency, and (f) Ragone plot(including other reported energy and power density values) for fiber mSCs.

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electrical double layer at the electrode−electrolyte interfaceand good charge propagation across the electrodes.21,22

Additional CV curves at different scan rates are shown inFigure S8. A fiber mSC consisting of CNT/2TBB fibers beforeJoule heating shows a distorted CV curve and significantlysmaller cathodic/anodic current values, indicating that they aremore resistive. An enlarged CV is shown in Figure 4a and anelectrochemical impedance spectroscopy (EIS) curve meas-ured from the CNT/2TBB fiber before Joule heating is shownin Figure S9. The poor signal-to-noise ratio in the EISspectrum is due to the poor electrical conductivity of the fiber.The larger integrated current exhibited by treated fiberelectrodes can be mainly attributed to their larger electricalconductance. Rate-dependent capacitances of fiber mSC werecalculated from the GCD profiles measured at different currentdensities ranging from 0.05 to 0.8 A cm−3 (Figure 4b,c). Thevolumetric capacitance, calculated based on the total electrodevolume excluding the electrolyte, is as high as ∼50 F cm−3 at acurrent of 0.05 A cm−3 and is comparable to or greater thanthe value of other mSCs previously reported.18,23−29 At highcurrent densities (0.8 A cm−3), the fiber mSC retains 42% of itsinitial capacitance (at 0.05 A cm−3); this encouraging resultcan be attributed to the low internal electrode resistance andgood accessibility of the electrolyte into the electrode.Additionally, we investigated the dependence of specificcapacitance on the length of the fiber supercapacitor (FigureS10). Our fiber supercapacitor retains 81% specific capacitancewith increasing length from 2 to 14 cm.To examine the flexibility of the mSCs and their electro-

chemical performance when subjected to bending, wemeasured the electrochemical properties of the fibers atbending angles ranging from 0 to 150° (Figure S11). Over thisrange, the CV curves and Nyquist plots are essentiallysuperimposable and the capacitance varies by only ~4%. Theresults indicate that the fiber supercapacitors are highly flexibleand that their electrochemical properties remain excellent evenwhen strongly bent.EIS measurements were performed to understand the

frequency-dependent capacitance over the frequency range of106 to 10−2 Hz, measured at equilibrium open-circuit potential(∼0 V) (Figure 4d,e). The intercept on the real axis (Z′) of theNyquist plot at high frequencies is associated with theequivalent series resistance (ESR), which indicates thecombined series resistance of the electrolyte, electrode, currentcollectors, and electrode/current collector contact (Figure4d).30 The cross-linked fiber mSC exhibits a small ESR (∼2kΩ) owing to the good electrical conductivity of theelectrode.29 No semicircle is observed in the high-frequencyregion to mid-frequency region, from which we conclude thatthere is little or no interfacial charge resistance (RCT) betweenthe electrode and electrolyte. A nearly vertical line in the low-frequency region of the Nyquist plot reveals that the responseof the electrode at low frequencies is purely capacitive. Thecharge−discharge rate was estimated from the characteristicrelaxation time constant (τ0), defined as the reciprocal of thefrequency at the maximum imaginary capacitance (C″).23,31 τ0is also determined by the complex form of capacitance, C(ω),which can be obtained from frequency-dependent impedanceZ(ω) in eqs 1, 2, and 3:31

ω ω ω=Z j C( ) 1/ ( ) (1)

ω ω ω= ′ + ″Z Z jZ( ) ( ) ( ) (2)

ω ωω ω

ω ωω

′ = − ″| |

″ = ′| |

CZZ

CZ

w Z( )

( )( )

, ( )( )( )2 2 (3)

where ω is the angular frequency (2πf) and C′(ω) and C″(ω)are the real and imaginary parts of C(ω), respectively. τ0 = 1/f 0where f 0 is the frequency at the half-maximum of C′(ω) or thefrequency of the peak maximum of C″(ω). The peak maximumof C″(ω) for our cross-linked fiber mSCs appears at a relativelyhigh frequency, 0.1 Hz, that corresponds to 7 s for τo. This fastfrequency response supports the finding that the cross-linkedfiber mSCs undergo fast charge−discharge due to the efficiention transport and the high electrical conductivity of theelectrode. More importantly, the Ragone plot shows that thecross-linked fiber mSCs are characterized by a volumetricenergy density of ∼4.5 mWh cm−3 and a power density of ∼1.3W cm−3 (Figure 4f). These values are comparable to or exceedthose of current state-of-the-art mSCs.25,26,32−42

CONCLUSION

In this study, we employed a wet-spinning process to createCNT fibers that also contained the halogenated aromatichydrocarbon 2TBB. Subsequent passage of current through thefibers locally heats the CNT junctions, which triggers cross-linking through the formation of covalent bonds between theCNTs and 2TBB molecules. This cross-linking results insignificant improvements in the electrical and mechanicalproperties of the fiber. In addition, microsupercapacitorsprepared from the cross-linked fibers exhibit volumetric energydensities of ∼4.5 mWh cm−3. The nanosoldering techniquetriggered by the Joule heating process could be generallyapplicable to improve the performance of CNT macro-materials.

EXPERIMENTAL METHODSMaterials and Characterization. Single-walled carbon nano-

tubes (SWCNTs) synthesized by a high-pressure carbon monoxide(HiPco) method were purchased from Carbon Nanotechnologies Inc.PVA powder and sodium deoxycholate (SDC) powder werepurchased from Alfa Aesar. All of these materials and chemicalswere used as received, if not otherwise specified. 2TBB moleculeswere synthesized by a literature method.14,15 The microstructures ofCNT fibers before and after Joule heating treatment were imaged on aFEI Quanta FEG 450 environmental scanning electron microscope inits high-vacuum mode. A Horiba Raman confocal imaging microscopewas used to collect the Raman spectra of the CNT fibers. A Keithley4200 semiconductor characterization system and a T.A. SystemsQ800 dynamic mechanical analyzer were used to measure theelectrical conductivity and mechanical properties of the CNT fibersbefore and after Joule heating. Electrochemical performance testswere carried out on a Bio-Logic VMP3 multichannel potentiostat.

Wet-Spinning Fabrication of a CNT Fiber and the JouleHeating Process. To make the CNT solution, as-received HiPcoSWCNT powders were mixed with SDC in a mass ratio of 1.5:1, andthen deionized water was added to adjust the SWCNT concentrationin the mixing solution to 4 mg/mL. A detailed description of thespinning process can be found in the Supporting Information.

For the Joule heating process, individual fibers were mounted on aglass slide and connected by copper wires coated with silver paint atfour different points along the fiber. The fiber device was placed on asubstrate holder, loaded into a turbomolecular pumped vacuumchamber, and electrically connected for external four-point probemeasurements. After the chamber was evacuated to ∼10−5 Torr,voltages were applied, starting at 1 V and increasing in 5 Vincrements, typically up to 140 V. For each increment, the voltage andthe corresponding current were recorded.

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Fiber mSC Fabrication and Characterization. Nanosolderedfiber mSCs were fabricated by the parallel assembly of the fiberelectrodes with a solvent-cast poly(vinyl alcohol)/phosphoric acid(PVA/H3PO4) solid electrolyte, which was made as follows. First, 1.0g of PVA (Mw ∼ 95 000, 95% hydrolyzed) was dissolved in 15 mL ofdeionized water at 90 °C with vigorous stirring until the solutionbecame transparent. After the solution was cooled to roomtemperature, 0.8 g of H3PO4 (85 wt % aqueous solution, Aldrich)was added and the resulting solution was stirred for 12 h at roomtemperature. Two parallel nanosoldered fiber electrodes were placedfacing each other ∼1 mm apart on a flat poly(ethylene terephthalate)substrate. The PVA/H3PO4 solution was cast on it and then dried in aventilated oven at room temperature for 12 h.Electrochemical characterizations, including cyclic voltammetry,

galvanostatic charge−discharge, and electrochemical impedancespectroscopy, were performed using a VMP3 multichannel potentio-stat (VMP3, Bio-Logic, USA) in the two-electrode mode at roomtemperature. EIS was measured over a frequency range of 106−10−2Hz at a sinusoidal voltage amplitude of 10 mV. In the GCD profiles,the specific capacitance can be estimated from the following equation:

= [ Δ Δ ]C I V t V4 / ( / ) (4)

where I is the current applied, ΔV/Δt is the slope of the dischargecurve after the IR drop at the beginning of the discharge curve, and Vis the volume of the electrode.The volumetric power (P) and energy (E) were calculated from the

following equations:

= Δ = ΔP V R V E C V( ) /4 , 0.5 ( ) /36002ESR

2 (5)

where ΔV, RESR, and V are the voltage window obtained fromdischarge curve after the IR drop, the internal resistance from the IRdrop, and the volume of the electrode, respectively, and the factor of3600 converts from seconds to hours. The internal resistance wascomputed from the voltage drop at the beginning of each dischargecycle:

= ΔR V i/2iRESR (6)

where ΔViR and i are the voltage drop between the first two points inthe voltage drop at the top cutoff and applied current, respectively.

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsnano.9b07244.

SEM images of collapsed ribbon-like fiber, aligned CNTbundles and CNT fibers without 2TBB; photographs ofwet-spinning process under various injecting conditions;energy-dispersive X-ray spectroscopy analysis of as-prepared fiber before and after treatment; dependenceof fiber’s mechanical strength on 2TBB’s concentration;comparison of tensile strength and electrical conductiv-ity of the CNT fibers fabricated with different injectingspeed; CV curves of the fiber supercapacitor; CV curvesand EIS spectrum of the fiber supercapacitor beforeJoule heating; dependence of supercapacitor’s specificcapacitance on fiber’s length; electrochemical propertiesof the fiber supercapacitor under various bending angles;and comparison of the electrical and mechanicalproperties of our CNT fiber with those of other CNT-based composite fibers (PDF)

AUTHOR INFORMATIONCorresponding Author*E-mail: lyding@illinois.edu.

ORCIDGregory S. Girolami: 0000-0002-7295-1775SungWoo Nam: 0000-0002-9719-7203Paul V. Braun: 0000-0003-4079-8160Joseph W. Lyding: 0000-0001-7285-4310Author Contributions▼G.W. and S.-K.K. contributed equally.

NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTS

Funding for this project was provided in part by a seed fundinggrant from the Beckman Institute for Advanced Science andTechnology at the University of Illinois. G.S.G. acknowledgessupport from the National Science Foundation under GrantNo. CHE 1665191. S.N. acknowledges support from theNational Science Foundation under Grant No. DMR 1708852.P.V.B. and J.W.L. acknowledge support from the NationalScience Foundation Engineering Research Center for PowerOptimization of Electro Thermal Systems (POETS) underGrant No. EEC 1449548. G.W. would like to thank Dr. Subedifor the use of the microbalance, and Dr. Meng Qinghai fordiscussions and help with the wet-spinning fabrication method.This work was conducted in part at the Microscopy Suite ofthe Beckman Institute for Advanced Science and Technologyat the University of Illinois at UrbanaChampaign (UIUC-BI-MS). Material characterization and mechanical measure-ment were carried out in part in the Materials ResearchLaboratory Central Research Facilities, University of Illinois.

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