Reduced Graphene

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Functional Nanostructured Materials (including low-D carbon)

Three-Dimensional Porous Carbon Nanotubes/ Reduced Graphene Oxide Fiber from Rapid Phase Separation for High-Rate All-Solid-State Supercapacitor Wujun Ma, Min Li, Xing Zhou, Jihang Li, Yanmao Dong, and Meifang Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19359 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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ACS Applied Materials & Interfaces

Three-Dimensional Porous Carbon Nanotubes/Reduced Graphene Oxide Fiber from Rapid Phase Separation for High-Rate All-SolidState Supercapacitor Wujun Ma,*, † Min Li, ‡ Xing Zhou, † Jihang Li†, Yanmao Dong†, Meifang Zhu*,§

†

School of Chemistry, Biology and Material Engineering, Suzhou University of Science and

Technology, Suzhou 215009, China ‡

College of Textiles and Clothing, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, P.

R. China §

College of Materials Science and Engineering, Donghua University, Shanghai 201620, China

*

E-mail: [email protected] (W.J. Ma); [email protected] (M. F. Zhu)

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ABSTRACT Graphene fiber-based supercapacitors are rising as one of the greatest potentials for portable/ wearable energy-storage devices. However, their rate performance is not well-pleasing, which greatly impedes their broad practical applications. Herein, 3D porous CNT/rGO fibers were prepared by a non-solvent induced rapid phase separation method followed by hydrazine vapor reduction. Benefit from their three-dimensional porous structure, large specific surface area and high conductivity, the fabricated supercapacitor exhibits a high volume capacitance of 54.9 F cm-3, and a high energy and power density (4.9 mWh cm-3 and 15.5 W cm−3). Remarkably, the supercapacitor works well at a high scan rate of 50 V s-1 and shows a fast frequency response with a short time constant of 78 ms. Furthermore, the fiber-shaped supercapacitor also exhibits very stable electrochemical performances when it is subjected to mechanical bending and succeeding straightening process, indicating its great potential application in ﬂexible electronic devices. KEYWORDS: wet spinning, rapid phase separation, graphene fiber, high rate, flexible supercapacitors


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INTRODUCTION Fiber-shaped supercapacitors are rising as one of the greatest potentials for portable and wearable energy storage devices, as they are flexible and miniature and can be easily integrated or woven into variously shaped structures.

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In most cases, fiber-shaped supercapacitors are assembled from

fibrous electrodes.7-8 For this purpose, it is very critical to find proper fibrous electrodes with outstanding properties of being flexible, mechanical strong and electrically conductive.9-11 Recently, a tremendous amount of efforts have been dedicated to developing graphene fibers into flexible electrodes for supercapacitors owing to their advantages of flexibility, lightweight and high strength.12-18 Extensive progress has attained in the past few years on the enhancement of specific capacitance and energy densities of graphene fiber-based supercapacitors through optimizing the pore structure 13, 19 or combining with pseudocapacitive materials such as MnO2,20-22 MoO3, 23 MoS2 24-25 and polythiophene.

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However, their rate performance is not well-pleasing, which greatly impedes

their broad practical applications. Therefore, to extend the applications of graphene fiber-based supercapacitor for the progress of wearable electronic, it is urgent to improve their rate capability. Poor rate capability can be mainly ascribed to a limited ion-diffusion rate and large resistance of electrode materials.27-28 For graphene fiber, the aggregation of graphene nanosheets resulting from ππ interaction during the spinning process seriously restricts the electrolyte transfer, reduces the ionaccessible surface area and increases the long path of ion diffusion, which severely deteriorates their rate capability. To obtain high rate graphene fiber-based supercapacitors, shorter ion-diffusion path and lower electron-transfer resistance are the two most significant factors for the fabrication of graphene fiber.29-30 Rational design and fabricating of graphene fiber with optimized 3D architectures is an effective method to avoid this kind of restacking and shorten the ion-diffusion path.31 By injecting GO gel into liquid nitrogen then freeze-drying, Gao et al. manufactured graphene aerogel ACS Paragon Plus Environment


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fiber with a high specific surface area (SSA) of 884 m2 g-1. 32 The ice-templating strategy could bring the merits of high SSA and ordered structures, yet in the same breath limits the scalability of resulting aerogel fibers because of the complicated fabrication process. Another way to inhibit the aggregation is to cooperate graphene fibers with other materials that lay between the graphene nanosheets such as carbon black33 and CNT.12, 15 Incorporating highly conductive CNT can not only efficiently inhibit the aggregation of graphene nanosheets, but also improve their conductivity. As an example, the recent research on fabrication of graphene fibers applied this strategy can improve its SSA from 24 to 396 m2 g-1 and conductivity from 12 to 102 S cm-1 after CNT was added to rGO.15 However, these fabricated CNT/rGO fibers lack adequate open hierarchical pores, which is a vital factor for achieving high rate capability and short charging times. To solve the challenge and achieve high rate capability in graphene fiber-based supercapacitors, we demonstrate here the scalable fabrication of high conductive, 3D porous CNT/rGO fibers by a non-solvent induced rapid phase separation method followed by hydrazine vapor reduction. Such porous structure with sufficient open hierarchical pores, high SSA and conductivity offers a facile pathway for efficient transport and diffusion of electrolytes and ions throughout the electrode, and provide a consecutive conductive network for electron-expedited transfer. Based on these advantages, the hybrid fibers exhibit attractive electrochemical performances in terms of great rate capability and high energy and power density when applied in a flexible supercapacitor.

RESULTS AND DISCUSSION Among various methods developed for the fabrication of graphene fiber, wet-spinning has gained considerable attention, because it is scalable and inexpensive.34-36 Generally, the wet-spinning technique is a non-solvent induced phase separation process.37-38 The phase separation rate is a vital factor in the formation of porous structure during the fiber manufacturing process, which can be ACS Paragon Plus Environment

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controlled by coagulation bath.13, 35 In our previous work, acetic acid has been proven to be an effective coagulation bath for the phase separation of GO.19,

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Compared with acetic acid,

trifluoroacetic acid (TFA) can promote the phase separation of GO at much higher rates, which can be attributed to the stronger acidity and hydrogen bonds that formed with GO sheets (see Figure S2 and Supporting Information for more details). Therefore, acetic acid and TFA were used as coagulation bath here to demonstrate the effect of phase separation rate on the porous structure of graphene fibers.

Figure 1. (a) Digital images of CNT/GO fiber produced using a home-made apparatus. (b) A two hundred-meter long porous CNT/GO fiber wound on a roller. (c-h) Cross-sectional SEM images of porous CNT/GO fiber (c-e) and compact CNT/GO fiber (f-h). GO sheets with sizes of a few hundred nanometers (Figure S1) were synthesized by a modified Hummers' method.

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MWNTs were employed as conductive fillers to enhance the electric

conductivity and as pillars to restrain rGO nanosheets from aggregating during the reduction process.



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The CNT/GO fibers were fabricated by wet-spinning method employing acetic acid and TFA as coagulation bath, respectively (see Experimental Section and Figure 1a). When using TFA as coagulation bath, the CNT/GO fiber can be fabricated continuously (Figure 1b, Movie S1), while for acetic acid, the obtained fiber stuck together. TFA's advantage in this respect, compared to acetic acid, lies in two abilities: i) promoting the phase separation of GO with water at much higher rates and ii) high-volatility (When the CNT/GO gel fiber left the coagulation bath, TFA volatilized from the fibers quickly). The microstructure of CNT/GO fibers obtained using both coagulation bath is shown in Figure 1(c-h). It is obviously that the phase separation rate affects the porous structure which is formed in the solidification process. TFA derived CNT/GO fiber exhibits a well-defined interconnected porous structure composed of macropores and mesopores: the macropores with the diameter in the range of dozens to hundreds nanometers are constructed by vertically aligned CNT and rGO sheets and a lot of mesopores in the range of several tens of nanometres enclosed by parallelly aligned CNT and rGO sheets (Figure 1e). In contrast, the CNT/GO fiber formed with acetic acid as coagulation bath shows a compact structure (Figure 1f-h). The mechanism of forming a 3D porous CNT/GO fiber may be as following. When the dispersion of CNT/GO is injected into the coagulation bath, the initially homogeneous CNT/GO dispersion becomes unstable with the rapid access of TFA, and it instantaneously separates into a CNT/GO-rich phase and a CNT/GO-lean phase. When the phase separation of the CNT/GO is completed, the CNT/GO-rich phase develops into the framework of the CNT/GO fiber while the CNT/GO-lean phase mainly consisting of the solvent resulting in porous channels within the CNT/GO fiber.


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Figure 2. (a) XRD patterns of the porous and compact CNT/GO fibers. (b) Nitrogen-sorption isotherms of the porous and compact CNT/GO fibers (c) Pore-size distribution curves of the porous and compact CNT/GO fibers.

Figure 2a gives the XRD patterns of porous and compact CNT/GO fiber. The compact CNT/GO fiber showed a broad (002) peak at 2θ = 11.7° with an interlayer spacing of 0.851 nm. And this peak almost disappeared in porous CNT/GO fiber, which confirmed that the restacking of graphene was effectively prevented.

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Figure 2(b) shows nitrogen sorption isotherms of the porous and compact

CNT/GO fibers. Both the porous and compact CNT/GO fibers exhibit typical type-IV isotherm with an evident adsorption hysteresis loop, suggesting that there are a relatively large amounts of mesoand macropores in both the porous and compact CNT/rGO fiber (Figure 2b).42 The Brunauer Emmett Teller analyses reveal that the SSA of porous CNT/GO fiber is 404.1 m2 g-1, which is almost two times of the compact CNT/rGO fiber (208.2 m2 g−1). The pore-size distribution lies in the 2–200 nm range is given in Figure 2(c), which shows two peaks at 2.5 nm and 32.6 nm for the porous CNT/GO fiber and 3.8 nm for the compact CNT/rGO fiber, which is consistent with SEM observation (Figure 1e).



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Figure 3. (a–c) Cross-sectional and (d–f) surface SEM images of the porous CNT/rGO hybrid fiber.

However, the CNT/GO hybrid fibers without reduction lack electrical conductivity, which impedes their practical application in energy storage. To obtain highly conductive CNT/rGO hybrid fiber, the as-spun porous CNT/GO hybrid fibers were reduced through a modified hydrazine vapor reduction method (see Experimental Section). Raman spectra (Figure S3) shows that the intensity ratio (ID/IG) of D band and G band of porous CNT/GO fiber is about 0.89, while the ID/IG of CNT/rGO fiber is 1.37, indicating the oxygen-containing functional groups in GO was almost removed. 43 The hybrid fiber with 20% CNT after reduction has a conductivity of 38.7 S cm-1, which is more than twice of the neat rGO fiber (17.1 S cm-1) (Figure S4 and Table S2), and this suggesting that CNT incorporation can significantly improve the electrical conductivity of the graphene fiber. The impact of bending on the electrical resistance of the porous CNT/rGO fiber was studied, and it exhibits a negligible variation in either the bent or straight state over 1000 cycles (Figure S5). The microscopic morphology of the porous CNT/rGO hybrid fiber was characterized using SEM. It can be obviously seen in Figure 3(a-c) that the fiber resembles a freeze-dried structure and the 3D porous architecture is well preserved because large amount of vertically aligned CNT is filled between the GO sheets and


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offer rigid support for the GO sheets, thus avoiding shrinking when the fiber is reduced. Such a 3D porous structure diminished the aggregation of rGO sheets, and favored the high SSA and conductivity, hence it could supply adequate space for charge storage and fast pathway for ions and electrons transfer. Practically, the interconnected meso-macropores is particularly attractive for the convenient ion diffusion and adsorption which is decisive for electrochemical performance.31 There are many grooves formed on the CNT/rGO fibers (Figure 3d-f). And the grooves can act as fast channels for ion exchange which could reduce the diffusion length and kinetic difficulties of the electrolyte ions into the fiber. Despite being highly porous, the porous CNT/rGO fiber renders excellent mechanical strength (Figure S6 and Table S2), which is a valuable feature of CNT/rGO fiber particularly required for flexible supercapacitors.



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Figure 4. (a) Pictures of porous CNT/rGO fiber-based supercapacitor. (b-f) Cyclic voltammetry curves of porous and compact CNT/rGO fibers at different scan rates. (g) Discharge current of porous CNT/rGO fiber at different scan rate.

Due to the high mechanical strength and conductivity, the 3D porous CNT/rGO fibers can be employed as fiber electrodes for supercapacitor directly, and no additional current collector is needed. All-solid-state supercapacitors were assembled by twisting two identical fibers, which were coated by a thin layer of PVA/H3PO4 gel electrolyte (Figure 4a). The supercapacitor possesses great flexibility and toughness, that can be woven into a glove with different angles. A two-electrode system was employed to test their electrochemical performances, Figure 4b-f show their CV curves at different scan rates ranging from 2 to 50 V s-1. Compared with compact CNT/rGO fiber-based supercapacitor, the porous CNT/rGO fiber-based supercapacitor demonstrates prominently improved electrochemical performance. The CV curves of porous CNT/rGO fiber-based supercapacitor maintains near-ideal rectangular shapes at a scan rate of up to 50 V s-1 (Figure 4b-f), indicating nearperfect formation of electrical double layer.44 It can be seen that the discharge current of the CV curves gradually increased as the increasing of scan rate (Figure 4b-f). In contrast, the compact CNT/rGO fiber-based supercapacitor shows almost a straight line at 2 V s-1 (Figure 4b), indicating that charging/discharging of compact CNT/rGO fiber did not occur at high scan rates because of limited ion mobility.45 Notably, the porous CNT/rGO fiber-based supercapacitor possesses ultrafast charging and discharging capability, the discharge current of the porous CNT/rGO fiber-based supercapacitor increased linearly with the scan rate until it approached 40 V s-1 (Figure 4f), demonstrating that the charging-discharging process was significantly fast in the porous CNT/rGO fiber electrode, even though no metal current collector was used. To the best of our knowledge, in


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fact, this scan rate of 50 V s-1 is the highest achieved for any graphene fiber-based supercapacitors to date.

Figure 5. Electrochemical performance of the porous and compact CNT/rGO fiber-based supercapacitors. (a) Volumetric capacitances at different scan rates. (b) Nyquist plots for the porous and compact CNT/rGO fiber-based supercapacitors. Inset: magnified diagram for high frequencies. (c,

d) Frequency dependence of the real and imaginary parts (C´ and C") of the volumetric capacitance of the porous and compact CNT/rGO fiber-based supercapacitor. High capacitance retention capability during ultrafast charging/discharging process is of vital importance for high-performance supercapacitors. The volumetric capacitance of the porous and compact CNT/rGO fiber SCs at various scan rates from 0.002 to 50 V s-1 is compared in in Figure 5a. Here and elsewhere, capacitances are normalized with respect to the volume of two fibers. Notably, the volumetric capacitances of porous CNT/rGO fiber-based SC recorded at 0.01 V s-1 was 39.3 F cm-3, a little higher than compact CNT/rGO fiber-based SC (32.8 F cm-3). With the scan rate increased from 0.01 to 50 V s-1, the capacitance of porous CNT/rGO fiber-based SC dropped to about ACS Paragon Plus Environment


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12.2 F cm-3, which is about 31% of the capacitance at the scan rate of 0.01 V s-1, higher than the capacitance retention (0.9%) of compact CNT/rGO fiber-based SC, indicating that the 3D porous structure with a short diffusion pathway and high electrical conductivity avails fast charge transport. The ultrahigh rate capability of the porous CNT/rGO fiber-based SC was ulteriorly verified through galvanostatic charge-discharge (GCD) tests (Figure S8). The GCD curves are symmetric, and no obvious voltage drop was observed at various current densities. Electrochemical impedance spectroscopy (EIS) was used to study the kinetics of ion diffusion in the fiber electrodes. The Nyquist plots of the CNT/rGO fiber-based SCs show typical features of EDCL supercapacitors (Figure 5b). The porous CNT/rGO fiber-based SC at low-frequency regions exhibits a nearly perpendicular line, verifying its perfect capacitive behavior.46 The much shorter Warburg region of porous CNT/rGO fiber-based SC suggests that the ions can easily diffuse into the bulk of the porous CNT/rGO fiber electrode compared to the compact CNT/rGO fiber electrode. The enlarged view of the highfrequency range (Inset of Figure 5b) shows that the porous CNT/rGO fiber-based SC has an equivalent series resistance (ESR) of 115.3 Ω, which is approximately one-tenth of the compact CNT/rGO fiber-based SC (1050.4 Ω). An equivalent circuit was fitted by the Nyquist plots (Figure S9), and the values of different circuit elements were summarized in Table S3. The rate capability of the SCs was further evaluated from relaxation time constant τ0 (τ0 is the minimum time needed to discharge all the energy from a device with an efficiency of greater than 50%).

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The τ0 of porous

CNT/rGO fiber-based SC is 78 ms (Figure 5c), which is much lower than that of the compact CNT/rGO fiber-based SC (8264 ms) (Figure 5d). This finding strongly suggests that porous CNT/rGO fiber-based SC possesses tremendous potential to deliver ultrahigh power and energy instantaneously. The excellent electrochemical performances may be explained by the following two points. They are ( i ) 3D porous structure in the fiber inhibited the restacking of rGO sheets and


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therefore reduce the migration length and kinetic difficulties of the electrolyte ions during rapid charge-discharge processes, ( ii ) the CNT in the fiber formed a conductive network for the fast electrons transport, thus reducing the ESR of the porous CNT/rGO fiber electrode. The above two points are both favorable for rate capability at high charge-discharge rates.

Figure 6. (a) Ragone plot of the porous CNT/rGO fiber-based SC in comparison with commercially available devices. (b) Cycling stability of the SC. (c) CV curves of the SC bent with different angles. (d) Capacitance retention of the SC after 1000 cycles up to 90° bending angle. A comparison of our device with different energy storage devices is presented in Figure 6a. The porous CNT/rGO fiber-based SC can deliver a high energy density of 4.9 mWh cm-3, which is about eight times higher than the commercially available SCs and comparable to the thin-film lithium battery. The energy density is also superior to some previously reported fiber-shaped SCs (Table S4). The maximum power density of the porous CNT/rGO fiber-based supercapacitor is 15.5 W cm-3, which is about 5-500 times of previously reported all-solid-state fiber-based SCs reported so far



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(Table S4), and comparable to the electrolytic capacitor. As far as we know, there are no other rGO fiber-based SCs available that show the same performance characteristics. Importantly, the porous CNT/rGO fiber-based SC exhibits remarkable cycling stability and no obvious capacitance loss after 10000 cycles with a scan rate of 5 V s-1 (Figure 6b). The flexibility of the SC was studied by CV at a scan rate of 500 mVs-1 in various bending angles. The almost overlapping CV curves at different bending angles suggest the prominent mechanical stability and flexibility of the fiber-based SC (Figure 6c). The capacitance of the fiber-based SC upon bending numbers at 90° is given in Figure 6d. As shown, the capacitance exhibits only a slight loss after 1000 bending numbers, suggesting ultraflexibility and highly stable electrochemical performance. The results signify that the as-fabricated SC can be used in flexible electronics.

CONCLUSIONS In this work, we have developed a method combining rapid phase separation and hydrazine vapor reduction to fabricate 3D porous CNT/rGO fibers. This method is facile and scalable, which could become a common method for the fabrication of 3D porous fibers, not just graphene fibers. An allsolid-state SC was assembled using these 3D porous CNT/rGO fiber as flexile electrode. Due to its highly porous structure and excellent conductivity, the SC exhibits high rate performance, fast frequency response, and high energy and power density.

METHODS Preparation of Spinning Dopes. Graphene oxide was fabricated using a modified Hummers' method.40 0.8 g GO was sonicated in 80 ml DI water for 1 h using a probe ultrasonication (Misonix Sonicator 3000). Then appropriate amounts of CNT (Nanjing JCNANO Technology Co., Ltd.,


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Nanjing, China) was added to the GO solution to prepare CNT/GO dispersion with GO:CNT ratio of 90:10 and then the mixed dispersions were sonicated for another 1 h after the addition of CNT.

Wet-Spinning of CNT/rGO Fibers. The dispersion was filled into a syringe and then injected into a rotating coagulation bath (acetic acid or TFA) through a syringe needle by a syringe pump, the injection rate of the CNT/GO dispersion was 30 μl /min and the coagulation bath rotated at speeds 6 rpm, the obtained CNT/GO fiber was dried at room temperature. To obtain CNT/rGO fibers, the obtained CNT/GO fibers were put into a teflon vessel, 2 mL of hydrazine solution was dropped into the vessel. Then the vessel was sealed in an autoclave and maintained at 85 °C for 24 h. Finally, the obtained fibers were washed with DI water for three times and dried at room temperature.

Fabrication of Wire-Shaped Solid-State Supercapacitor. H3PO4 (1.0 g), Polyvinyl alcohol (1.0 g) and DI water (8.0 g) were added into a vial, then heated up to 95 °C and maintained for 2h under magnetic stirring. Two fibers were immersed in the hot H3PO4/PVA gel solution for 1 h, after drying at room temperature the same two fibers were carefully twisted, and finally coated with H3PO4-PVA along the twisted part to obtain an all-solid-state FSSCs. Materials Characterization. The microstructure of the fibers was carried out with a scanning electron microscope (SEM, HITACHI, S4800). The conductivity was executed by a twoprobe resistance tester. Nitrogen sorption isotherms were measured using Micromeritics ASAP2020. Raman spectroscopy was obtained from a Renishaw inVia Raman system.

Electrochemical Characterization. The electrochemical properties of the fiber-based SCs were studied under a two-electrode configuration. The CV, GCD and EIS measurements were carried out on an electrochemical workstation (CHI 660e, CH Instruments, Inc.). Bending stability was measured using a homemade bending-recovery device. ACS Paragon Plus Environment


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ASSOCIATED CONTENT

Supporting Information

Additional detailed calculation, figures and tables about materials characterization and electrochemical characterization.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W. J. Ma)

*E-mail: [email protected] (M. F. Zhu)

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work is financially supported from Natural Science Foundation of China (51673038, 21702143), Suzhou Key Industrial Technology Innovation Project (SYG201814), Science and Technology Commission of Shanghai Municipality (16JC1400700), the NSF of Jiangsu Province (BK20170377), and the University Scientific Research Project of Jiangsu Province (17KJB150035).


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Reduced Graphene

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