Continuous carbon nanofiber bundles with tunable pore structures and functions for weavable fibrous supercapacitors

Energy Storage Materials 5 (2016) 43–49

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Energy Storage Materials journal homepage: www.elsevier.com/locate/ensm

Continuous carbon nanofiber bundles with tunable pore structures and functions for weavable fibrous supercapacitors Lin Shi a,c, Xianglong Li a, Yuying Jia a, Debin Kong a, Haiyong He a, Manfred Wagner b, Klaus Müllen b, Linjie Zhi a,n a

National Center for Nanoscience and Technology, No. 11, Beiyitiao, Zhongguancun, Beijing 100190 PR China Max-Planck-Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany c China Building Materials Academy, Guanzhuang Dongli 1#, Chaoyang District, Beijing 100024, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 30 November 2015 Received in revised form 9 May 2016 Accepted 25 May 2016 Available online 26 May 2016

Fiber supercapacitors weavable into smart textiles have attracted great attention. The key to fabricating such energy storage devices is to develop flexible fiber-like electrodes with tunable functionalities and low production cost. Here, we present an efficient strategy for the roll-to-roll mass production of carbonaceous nanofiber yarns with tunable pore structures and functions. Our protocol combines a modified electrospinning technique with a pyrolysis procedure, thereby using economic polymers as precursors. The as obtained fiber supercapacitors exhibit a remarkably high length-specific capacitance and excellent rate capability, demonstrating their great potential as smart textiles with energy storage functions. & 2016 Elsevier B.V. All rights reserved.

Keywords: Carbon nanofiber yarns One-dimensional supercapacitors Electrospinning Pyrolysis

1. Introduction Smart textiles [1–3] integrated with versatile functions such as energy-conversion, environment-monitoring, and informationprocessing, have attracted great attention for applications in military, public safety, customer fitness, and medical fields. Given the great potential to revolutionize the way wearers see, feel, think, and act, such smart textiles must be integrated with energy storage functionality as well, aiming to ensure their stable, sustained, and safe performance. To achieve this integration, flexible and miniaturized energy storage gadgets are emerging; in particular, weavable fiber shaped supercapacitors with coaxial [4,5] and/or two-ply [6,7] configurations have attracted considerable attention. The key to fabricating fiber supercapacitors is to develop flexible fiber like electrodes. Diverse strategies have thus been pioneered for realizing such electrode paradigms, most of which involve the conformal deposition of active electrode materials on an electrically conductive one dimensional (1D) scaffold, such as carbon microfibers [8], carbon nanotube yarns [9–12], conductive polymer [13], all-graphene core-sheath microfibers [14], and/or metallic substance-enabled wires [15,16]. However, for practical applications, continuous long fiber electrodes with tunable functionalities and low production cost are required. With this regard, n

Corresponding author. E-mail address: [email protected] (L. Zhi).

http://dx.doi.org/10.1016/j.ensm.2016.05.009 2405-8297/& 2016 Elsevier B.V. All rights reserved.

carbon nanotube yarns have been tested instead of other previously reported fiber-like electrode materials, since the large scale production of CNT arrays has been accomplished based on typical chemical vapor deposition (CVD) techniques and the continuous spinning of CNT yarns is technologically possible. One of the critical issues for CNT yarns is the difficulty for nanoscale pore structure engineering, which has been proven to play a key role in the electrochemical performance of electrical double layer supercapacitor [17]. Graphene-based fiber materials are another interesting choice for mass production of 1D electrodes [14,18,19]. However, techniques for continuous long fiber production with controllable pore structure fabrication remain a challenge. Electrospinning has been widely adopted for the fabrication of carbon-based electrode materials for supercapacitors due to its simple, cost-effective and scalable merits [20]. However, all the resultant supercapacitors demonstrated so far are of a conventional two-dimensional (2D) plate-like structure, because such a sophisticated technique generally enables the formation of twodimensional webs and/or three-dimensional bulk materials [21– 24]. The question thus arises whether 1D polymer nanofiber yarns and subsequent carbonaceous nanofiber yarns could become accessible by electrospinning. Experiments have been proposed to modify the electrospinning methods to generate successfully onedimensional (1D) yarns consisting of aligned polymeric nanofibers, by for example employing static or dynamic liquid receiver [25,26], or physically manipulating collectors and tuning the electric field [27]. Unfortunately, no report has been made to

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engineer these polymeric yarns further into aligned functional carbonaceous materials with tailored pore structure, although they possess advantageous features such as low cost, up-scalability, ease of synthesis, and more importantly, nanofiber-involved 1D macroscopic nature, particularly for fiber supercapacitors. Herein, we present a novel and efficient strategy for the roll-toroll mass production of continuous and flexible carbon nanofiber yarns (CNF yarns), by firstly using a modified electrospinning technique to prepare 1D polymeric nanofiber bundles and secondly thermal cross-linking and pyrolysis treatment to obtain CNF yarns. The mechanically robust fiber, when explored as a freestanding electrode for a fibe-shaped flexible all-solid-state supercapacitor, can exhibit stable electrochemical performances with 100% capacitance retention even under continuous dynamic operations of forceful bending (180°). More importantly, the pore structure of CNF yarns can be facilely tailored via adjusting the polymer precursors, thus forming a multi-channelled CNF yarns. The multi-channel structure not only significantly enlarges the accessible surface area for ion adsorption/desorption but also greatly decreases the ion transport distance. Hence, the multichannelled CNF yarns exhibit remarkably high length-specific capacitance with 92 mF/cm2 at a current density of 10 μA/cm2 and an impressive rate capabilities with a capacitance of 55 μA/cm2 at a current density of 210 μA/cm2.

2. Experimental 2.1. Preparation of CNF yarn The CNF yarns were produced by liquid receiver (that is, water)-mediated electrospinning and subsequent pyrolysis as schematically illustrated in Fig. 1a. Polymeric yarns were first produced by the water-mediated electrospinning. The electrospinning solution was prepared by adding 0.9 g polyacrylonitrile (PAN; SigmaAldrich) to 9 g dimethylformamide (DMF) under continuous mechanical stirring, and then injected to the syringe with a blunt tip needle at an injection velocity of 1 ml h 1. A voltage of 15 kV was applied to the solution to start the spinning process by a high voltage source (SL50P60, Spellman High Voltage Electronics Corporation). A grounded petri dish containing deionized water placed at a distance of 15 cm away from the needle was used as an

intermediate collecting media and a motorized take-up roller as a final collector. The formed polymeric nanofibers on the water surface were carefully drawn, with the aid of an electrically insulating rod, and directed to the roller, during which polymeric yarns were formed. The resultant yarns were continuously drawn by the revolving roller at a desired take-up velocity that was set by simply adjusting the rotary speed of the motor. After air-drying, the as-produced polymeric yarns were stabilized at 280 °C in air for 1 h and then heated at 900 °C for 2 h in argon atmosphere, thus obtaining the carbon nanofiber (CNF) yarns. The same procedure was used to produce multi-channelled CNF (MC-CNF) yarns except for the use of a different electrospinning solution (9 g DMF containing 0.9 g PAN and 0.9 g polymethyl methacrylate (PMMA)). 2.2. Fabrication of fiber supercapacitors The gel electrolyte was prepared by dissolving 10 g polyvinyl alcohol (PVA; Alfa Aesar) in a mixed solution of 10 g phosphoric acid (H3PO4) and 100 g deionized water under vigorous stirring at 80 °C. Two CNF yarns or MC-CNF yarns were coated with the thusprepared gel electrolyte and then twisted to form a fiber supercapacitor. 2.3. Characterization The morphologies of the CNF yarns and MC-CNF yarns were characterized by scanning electron microscopy (SEM) (Hitachi S-4800). The microstructure and element composition analysis of the CNF yarns and MC-CNF yarns were performed using transmission electron microscopy (TEM, Tecnai G2 F20 U-TWIN) and energy dispersive X-ray spectroscopy (EDX). The Brunauer–Emmett–Teller (BET) surface area was tested by nitrogen cryosorption (Micromeritics, ASAP 2020). 2.4. Electrochemical measurements All cyclic voltammetry (CV) and galvanostatic charge-discharge measurements were performed using a static potential electrochemical workstation (VSP, Bio-Logic, France) within the voltage range from 0 to 1 V. Considering the unique one-dimensional configuration of the fiber supercapacitors, the capacitance was evaluated on the basis of the effective length of the device, and calculated based on galvanostatic discharge curves by using the

Fig. 1. Fabrication of CNF yarns. (a) Schematic illustration of the fabrication process. (b, c) Photographs of (b) a CNF yarn precursor (namely, PAN yarn) and (c) a CNF yarn collected on insulating rods.

L. Shi et al. / Energy Storage Materials 5 (2016) 43–49

following formula:

CL =

I × Δt L × ΔV

where CL (mF cm 1) is the length-specific capacitance, I (mA) is the discharge current, Δt (s) is the discharge time, L (cm) is the effective length of the fiber supercapacitors, and ΔV (V) is the voltage change during discharge.

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3. Results and discussion A typical fabrication process of CNF yarns is schematically shown in Fig. 1a. First, a polyacrylonitrile (PAN) solution is electrospun into a flat web of randomly looped PAN nanofibers, not on the surface of the collecting electrode but rather on the surface of water [25]. Second, a PAN nanofiber bundle is drawn out from the floating PAN nanofiber web and directed across the water surface

Fig. 2. Characterization and diameter-control of CNF yarns. (a, b, c) SEM images of a CNF yarn with the mean diameter of 46 μm at different magnifications. Scale bar: (a) 100 μm, (b) 2 μm, and (c) 200 nm. (d, e) TEM images of CNFs involved. Scale bar: (d) 500 nm, (e) 200 nm. (f) CNF yarn diameter versus take-up velocity. A linear dependence is observed in the measured range. (g) Diameter distribution of CNF yarns. Insets schematically show the cross sections of the yarns with different quantities of CNFs.

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to a rotating take-up roller. This leads to a continuous alignment of the PAN nanofibers and continuous elongation of the nanofiber bundles. In the successive drawing/winding process at a fixed take-up rotating rate, the drawing force of the rotator and the surface tension of the remaining water within the bundle further pull the polymeric nanofibers together into a yarn structure. (See Movie in Supporting information) Subsequently, the continuous

white PAN nanofiber yarn (Fig. 1b) is pyrolyzed at 900 °C under inert atmosphere to result in a black CNF yarn (Fig. 1c). In comparison with the existing current collector/active material coresheath structured yarns, the obtained carbon nanowire yarns are completely of electrochemically active (Fig. S1). It should be noted that the procedure developed here for PAN nanofiber yarn formation and for CNF yarn production is technically fittable to a roll-

Fig. 3. CNF yarn-based fiber supercapacitors. (a) Schematic illustration (up) and typical SEM image (down) of a fiber supercapacitor made of two twined CNF yarns coated with gel electrolyte. Scale bar: 500 μm. (b, c) CV curves (b) with a scan rate of 50 mV s 1 and galvanostatic charge-discharge curves (c) at the current density of 1 μA cm 1, the numbers labelled represent the diameter of a single yarn electrode in the fiber supercapacitors. (d) Length-specific capacitance versus current density for fiber supercapacitors with different diameters of CNF yarn electrodes. (e) Relationship between the length-specific capacitance (CL) and the CNF yarn diameter (D) at the current densities of 0.6 μA cm 1 (orange) and 1 μA cm 1 (blue), the corresponding fitting curves are superposed. (f) Length-specific capacitance under different bending states. Inset: schematic illustration of a CNF yarn-based fiber supercapacitor with the bending degree defined as θ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

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to-roll process. Hence, given the continuous supply of a polymer precursor solution, the take-up velocity of 30 rpm set of one drawing/winding process can enable the production of polymeric yarns at a reasonably high rate of 94.2 cm per minute and thus 1357 m per day for a single pillow, suggesting a production efficiency of over 1000 m per day of CNF yarns even including a 25% length shrinking of polymer nanofiber yarns during pyrolysis. Scanning electron microscopy (SEM) (Fig. 2b and c) and transmission electron microscopy (TEM) (Fig. 2d and e) characterizations demonstrate that the thus-produced CNF yarns are quite uniform (Figs. 2a and S2) and consist of purely carbon nanofibers being aligned along the yarn axis. Interestingly, the diameter of the CNF yarns can be easily managed by the take-up velocity of the PAN nanofibers that is controlled by the rotating roller. Fig. 2f shows the diameter change of CNF yarn along with the change in the take-up velocity. It becomes evident that the yarn diameter is perfectly inversely proportional to the take-up velocity, which is also verified by linear fitting of the data points (R2 of 0.98), according to the equation:

D = − 6.384 × VT + 335.6 where D (μm) is the diameter of CNF yarns, VT (rpm) is the take-up velocity. This linear correlation thus greatly facilitates the preparation of the CNF yarns with a predefined diameter across the range from several micro-meter to a few hundred micro-meter. In addition, the diameter of CNF yarn is highly uniform along the whole yarn length, which is of significant importance for largescale applications. To disclose this merit, numerous sections along the yarns with lengths up to 10 m were randomly cut by a doctor blade. SEM characterizations of these yarn sections demonstrate a very narrow diameter distribution along the whole yarn length (Fig. 2g), reflecting the excellent uniformity of the yarn diameter at the macroscopic scale. For weavable fiber supercapacitors, two as-prepared CNF yarns with the same diameters are coated with PVA-H3PO4 gel electrolyte and intertwined into all-solid-state fiber supercapacitors (Fig. 3a) [11,28]. The electrochemical properties of the as-prepared fiber supercapacitors were characterized by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. Fig. 3b shows the CV curves of the fiber supercapacitors composed of CNF yarns of different diameters, recorded with a voltage window from 0 to 1 V at a scan rate of 50 mV s 1. Apparently, all the measured supercapacitors exhibit CV curves of nearly the same shape, consistently reflecting a capacitive behavior. This observation implies that the CNFs constituting the yarns are electrically conductive in nature as well as highly accessible to the electrolyte regardless of the yarn diameter. This is also the case for the galvanostatic charge-discharge curves (Fig. 3c). It is noteworthy that all the curves present a quasi-symmetrical triangular shape, further suggesting that the capacitance of the thus-prepared supercapacitors is mainly originating from the electric double layer at the CNF yarn electrode–electrolyte interface [29]. For practical applications, length-specific capacitance is more useful than weight-specific capacitance, particularly for one-dimensional fiber supercapacitors [4]. The length-specific capacitances of the devices at various current densities are summarized in Fig. 3d. Notably, at all the measured current densities, the length-specific capacitance of the fiber supercapacitors increases with the diameter of the CNF yarns. A reasonable explanation is that the large-diameter electrode contains much more active material (CNFs) than that involved in the small-diameter electrode. Interestingly, the length-specific capacitance (CL) of the fiber supercapacitors with 119 μm CNF yarn electrodes is 0.38 mF cm 1 at 2.61 μA (Figs. 3d and S3), which is nineteen times higher than that (0.02 mF cm 1) of the previously reported all-graphene fiber

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supercapacitors [14] with a comparable electrode diameter of 117 μm. Since the yarn is constructed with a number of aligned CNFs, understanding the relationship between length-specific capacitance (CL) and the yarn diameter is helpful for the rational design of fiber supercapacitors. Fig. 3e indicates the correlations between the CL and D at two representative current densities. Surprisingly, the consistent non-linear fitting of the experimental data indicates that the CL is proportional to the square of D, which matches perfectly with the theoretically predicted relationship between CL and D, specifically by assuming that the capacitance of a CNF yarn is the sum of capacitances of individual CNFs (See Theoretical Derivation in Supporting information). This result suggests that the capacitance of the fiber supercapacitor can be tailored by simply tuning the diameter of the CNF yarns. These CNF yarns can work as free-standing electrode with no need to add any supplementary conductive or electrochemically inactive cores that adversely impact the lightweight, wearable, and energy storage feature of the textiles. Further experimental investigations demonstrate that a significantly expanded range of length-specific capacitance from 0.004 mF cm 1 to 4.27 mF cm 1 (Fig. 3d and e) has been achieved on the present fiber capacitors. It is worth noting as well that the measured capacitance of the fiber capacitor shows an approximately linear increase with increasing length of the yarn electrode (Fig. S4), which can be attributed to the high uniformity of the yarn as discussed above. This linear correlation is crucial especially when considering the large-scale fabrication and implementation of such devices. The fiber supercapacitors composed of CNF yarns also possess high stability during long-lasting cycling test. As exemplified in Supporting information Fig. S5, the capacitance of a 46 μm yarnbased supercapacitor retains more than 80% of its original value even after 10,000 consecutive charge-discharge cycles. In addition, as a building block of wearable textiles, the fiber supercapacitors must exhibit excellent flexibility. The CNF yarn-based supercapacitors were therefore deformed and evaluated further under different bending states (Fig. S6). Fig. 3f provides the length-specific capacitance values of the supercapacitors with 46 μm yarn electrodes at various bending angles. Apparently, the capacitance changes very little even when the bending angle reaches 180°, suggesting the excellent flexibility of the as-prepared fiber supercapacitors, which is undoubtedly desirable for flexible and wearable textiles. Similar performance characteristics are also observed for the fiber supercapacitors with other diameters of CNF yarn electrodes. The excellent flexibility of the device can be attributed to the high mechanical flexibility of the CNF yarn electrodes and the existence of certain amount of solid gel electrolyte on the yarn. It is well known that the capacitance of carbon electrode-based supercapacitor is strongly dependent on the surface area of carbon materials accessible to the electrolyte ions, since it mainly comes from the electrostatic charge accumulated at the electrode/electrolyte interface [30–32]. The rational design of the hierarchical pore structure of the electrode material is therefore crucial for advancing the supercapacitor performance [33]. With the strategies developed in this work, we can tailor the pore structure of individual CNF by simply adjusting the polymer precursors. By simply introducing PMMA into the PAN-based electrospinning solution [34], PAN@PMMA nanofiber bundles can be obtained with the same electrospinning process. More interestingly, subsequent pyrolysis of the PAN@PMMA nanofiber bundles results in CNF yarns in which every CNF is composed of multiple hollow channels with an average channel diameter of ca. 40 nm (Fig. 4a and b). The formation of these hollow channels is attributed to the decomposition of PMMA at 900 °C. Furthermore, the chemical composition of these multi-channelled CNFs (MC-CNFs) has no obvious change in comparison to PAN-based CNFs as revealed by energy

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Fig. 4. Boosting fiber supercapacitors. (a, b) SEM (a) and TEM (b) images of MC-CNFs. Scale bar: 200 nm. (c) Length-specific capacitance versus discharge current density for MC-CNF yarn-based fiber supercapacitors, in comparison with those for plain CNF yarn-based fiber supercapacitors. Both capacitance and rate capability are remarkably improved after tuning the microstructure of CNF yarns involved.

dispersive X-ray (EDX) analyses (Supporting information Fig. S7). The porous structure of CNFs and MC-CNFs are characterized by the nitrogen adsorption/desorption isotherms (Fig. S8). The nitrogen adsorption/desorption isotherm of MC-CNFs exhibits combined characteristic of type I/IV isotherm. The hysteresis loop at high relative pressure suggests the existence of mesopores. The pore size distribution curves (Fig. S8c) calculated by the BarrettJoyner-Halenda (BJH) indicate that MC-CNFs have well-developed mesopores (2–50 nm). Meanwhile, the specific surface area of the CNF yarns and MC-CNF yarns were obtained from the Brunauer– Emmett–Teller (BET) method, implying a significant increase from 10.1 m2 g 1 to 204.8 m2 g 1 after introducing multi-channels. Considering the basic unit of the samples with different diameters is single carbon nanofiber or multi-channeled carbon nanofiber, the specific area of different-diameter yarns are similar. The MC-CNF-based supercapacitors show significantly enhanced electrochemical performances against PAN-derived CNFbased supercapacitors. CV and galvanostatic charge/discharge tests (Fig. S9) of the MC-CNF-based supercapacitors consistently deliver significantly higher specific capacitance at various current density in all the cases with different yarn diameters, demonstrating the greatly enhanced rate capability (Fig. 4c). Remarkably, when converted to area specific capacitance as exhibited in Fig. S10, the multi-channeled-CNF-119 μm yarns (MC-CNF yarn-119 μm)

demonstrated both a high area-specific capacitance up to 92 mF/ cm2 at a current density of 10 μA/cm2 and an impressive rate capabilities with a capacitance of 55 μA/cm2 even at a current density of 210 μA/cm2, which is a pretty good result among previous works especially in pure carbon fiber based supercapacitors (Table S1). Given the same CNF yarn diameter and a similar yarn configuration, this remarkable improvement in performance could be closely associated with the tailoring of the microstructure of CNFs by introducing multiple nanoscale channels into the fiber matrix. As a result, the creation of multi-channels in the CNFs not only significantly enlarges the accessible surface area for ion adsorption/desorption, thus enhancing the capacitance of the electrode, but also greatly decreases the ion transport distance and also ion transport resistance.

4. Conclusions In summary, we have demonstrated a new and efficient chemical protocol for the continuous production of one-dimensional flexible carbon nanofiber yarns by electrospinning a rationally designed polymer solution onto a liquid surface followed with continuous drawing/winding and heat treatment. The method not only provides the capability for industrial-scale production of

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uniform carbon nanofiber yarns through a roll-to-roll procedure but also enables controlled modulation of the yarn diameter and facile tuning of the microstructures and functions of the individual CNF at the same time. The thus obtained CNF yarns can therefore be used to fabricate fiber supercapacitors with no need of current collectors and any other additives but with remarkably high length-specific capacitance and excellent rate capability. The technique demonstrated here thus opens up a new avenue for the facile, low-cost, and continuous chemical fabrication of high-performance flexible fiber supercapacitors, suitable for real applications in the field of smart textiles.

Acknowledgements The authors acknowledge support from the Ministry of Science and Technology of China (No. 2012CB933403), the National Natural Science Foundation of China (Grant Nos. 21173057 and 51425302), the Max Planck Society through the program of Partner Group, and the Chinese Academy of Sciences.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ensm.2016.05.009.

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