Hydrophilicity improvement of graphene fibers for high-performance

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Materials and Interfaces

Hydrophilicity improvement of graphene fibers for high-performance flexible supercapacitor Tuxiang Guan, Liming Shen, and Ningzhong Bao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02504 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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Industrial & Engineering Chemistry Research

Hydrophilicity improvement of graphene fibers for high-performance flexible supercapacitor

Tuxiang Guan,1 Liming Shen,1* and Ningzhong Bao1,2* 1. College of Chemical Engineering, State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, P. R. China 2. Institute of Graphene, Jiangsu Industrial Technology Research Institute, Nanjing, Jiangsu 210009, P. R. China

*Corresponding author: E-mail: [email protected] (L. Shen); [email protected] (N. Bao) Tel. & Fax: +86 25 83172244

ORCID identifiers: Liming Shen: 0000-0001-7028-6794 Ningzhong Bao: 0000-0001-5617-2467

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Abstract Super flexible graphene fiber (GF) has great potential for yarn supercapacitors (YSCs) used in wearable devices. The performance of YSCs can be improved by increasing both the specific surface area and the hydrophilicity of GFs. Here, we report on the continuous fabrication of hydrophilic GF by wet spinning technique. The hydrothermal treatment of GF in KOH aqueous solution is the crucial step for the hydrophilicity improvement. The synthesized K-GF possesses a porous ruffle structure, excellent hydrophilicity, and good conductivity. The K-GF based YSC shows good flexibility while folding from 0 to 180º and exhibits the capacitance of 145.6 mF/cm2 at the energy density of 3.23 µWh/cm2 and power density of 0.017 mW/cm2.

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1. Introduction In recent years, wearable electronic devices such as smart health devices and implantable medical devices, have attracted great attention due to their tremendous potential for applications.1-4 In order to create sustainable power supply for these wearable devices, flexible energy storage devices, particularly yarn supercapacitors (YSCs) become increasingly important because of their excellent flexibility and long service life.5 Thus far, metal wire composite6-8 and polymer9,10 have been widely used as electrodes in YSCs. While compared with rechargeable battery systems, the capacitance of these YSCs is still quite low due to their limited specific surface area. Therefore, conductive carbon materials such as carbon fibers11 and carbon nanotube fibers12-14 have been investigated as electrode materials. Among potential alternatives, graphene-based fibers have also been intensively studied due to the excellent flexibility, conductivity, and high capacitance attributed from their porous and ordered structure.1517

Graphene fibers (GF) are generally prepared by using wet spinning approach.18-20

The capacitance of GF can be further enhanced through increasing the specific surface area by modifying the preparing process of GF such as the non-liquid-crystal spinning21 or the thermal reduction of graphene oxide fibers.22 However, the natural hydrophobicity of graphene is a common issue that makes it difficult for the electrode to be sufficiently wet by electrolyte.23,24 As a result, the surface of graphene fibers cannot be effectively utilized for further improvement of capacitance. To reduce the hydrophobicity of graphene fibers, the most investigated method is to add hydrophilic additives. But the hydrophilic fillers are always insulating materials and appear to impair the connection between graphene sheets and thus degrade the conductivity of graphene fibers. For example, the addition of insulating polyvinyl acetate (PVA) to graphene fibers reduced its electrical conductivity from around 5003



5000 S/m (generally reported value range) to only 110 S/m, significantly limiting the further improvement of electrode performance.25 Cellulose is another type of filler that can significantly improve the hydrophilicity of graphene,23 but it is also insulating and unstable under acidic conditions. As the general preparation method of graphene fibers, a typical wet spinning process includes three steps: first, injecting graphene oxide solution through a narrow channel into coagulating bath to form fibrous gels; second, drying out the fibrous gels to graphene oxide fibers; finally, reducing the graphene oxide fibers to graphene fibers. During the preparation of graphene fibers by the wet spinning approach, we noticed that there are a tremendous amount of oxygen-containing functional groups in the graphene oxide fibers, including epoxide, hydroxyl, and carboxyl groups. After the reduction by hydroiodic acid (HI), most of these functional groups are reduced except for the epoxide groups.26 We thus wondered if the hydrophobic problem of graphene fibers can be solved by simply modifying the existing preparation process, instead of adding insulating additives. If these epoxide groups are converted to hydroxyl groups, the hydrophilicity of graphene fibers can be significantly improved while retaining the excellent conducting network of graphene sheets. In the present study, we prepared the graphene oxide fibers (GOF) by the wet spinning technique and then chemically reduced the as-synthesized GOF to graphene fibers (GF) by HI. The special structure with ruffles and pores is also observed on the surface of GF. This structure feature boosts the specific surface area. A subsequent hydrothermal treatment in KOH solution resulted in the formation of KOH-treated GF (K-GF) with significantly improved hydrophilicity. The excellent hydrophilicity endows the K-GF of more effective active areas, which enables the electrolyte to efficiently wet the electrodes. Meanwhile, since no insulating fillers are introduced, the 4


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fiber retains high electrical conductivity (1256 S/m). Under the synergistic effect of porous ruffle structure, hydrophilicity, and electrical conductivity, the K-GF electrode exhibits an excellent electrochemical performance: a measured specific capacitance of 145.6 mF/cm2 at the current density of 0.086 mA/cm2, which is among the top level of relevant works. This study exhibits a simple and efficient way for solving the hydrophobicity problem of graphene fibers and provides a new direction for further heightening the performance of yarn supercapacitors. 2. Methods and characterization 2.1 Materials The materials used in this study are listed as follows: crystalline flake graphite (100 mesh, Qingdao Meizhen Co.), KMnO4 (AR, Sinopharm Chemical Reagent Co.), H3PO4 (AR, Sinopharm Chemical Reagent Co.), acetic acid (AR, Shanghai Shenbo Chemical Co.), hydroiodic acid (HI, 47%, AR, Shanghai Macklin Biochemical Co.), H2O2 (30%, AR, Tianjin Fuqi Chemical Co.), polyvinyl alcohol (PVA, MW 1750±50, AR, Sinopharm Chemical Reagent Co.), KOH (AR, Sinopharm Chemical Reagent Co.), and ethanol (AR, Wuxi City Yasheng Chemical Co.). All chemicals were used without further purification. 2.2 Fabrication of graphene fibers Graphite oxide (GO) was synthesized by a modified Hummers method.27, 28 The GO solution was concentrated to 8 g/L using a membrane concentration process.29 The GOFs were fabricated on a home-made wet spinning apparatus with the mixture of ethanol and acetic acid used as the coagulation bath (1:1, in volume, Movie S1). The as-made GOFs were reduced to GFs in the mixture of HI and acetic acid (1:1 in volume) at 60 °C. The GFs were immersed in KOH solution (2 M) and sealed in a 35 mL Teflon-

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lined autoclave, before heated at 110 °C for 8 h to form K-GF. The obtained K-GF was washed with DI water for three times and then immersed in DI water for 24 h to remove residual KOH. For comparison, the hydrothermal treatment of GFs in deionized water (W-GF), instead of KOH, was conducted under the same conditions. In order to measure the contact angle, the GO solution (8 g/L) was uniformly dropped on a glass slide, and dried at 60 °C for 24 h to prepare a graphene oxide film (GOM). The GOM was reduced by HI to become graphene film (GM). The GM was then hydrothermally treated in KOH solution or DI water in the same way as described above, for preparing K-GM and W-GM. 2.3 Fabrication of yarn supercapacitors (YSCs) The electrolyte was first prepared by adding 3 g of polyvinyl alcohol (PVA) and 3 g of H3PO4 into 30 g of DI water, followed by heating at 90 °C with magnetic stirring for 3 h. Two fibers (1.5 cm in length) were placed parallel to each other on a glass substrate (Figure S1a), with one end of each fiber fixed to a copper current collector using conductive silver glue. After the electrolyte was uniformly applied on the surface of fibers, the fiber electrode was dried at room temperature until the gel electrolyte solidified. For comparison, three types of YSCs were prepared by using K-GFs, W-GFs, and GFs. 2.4 Characterizations The morphology and surface structure of fibers were observed by field emission scanning electron microscopy (FESEM, HITACHI S-4800). The X-ray diffraction (XRD) was carried out using a D8-Advance, Bruker AXS diffractometer (Cu-K a radiation, k=1.5418 Å) operating at 40 kV and 40 mA. FT-IR was carried on RQUINOX55, Bruker. The contact angle was measured on SDC-350, Sindin. The

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conductivity was measured with a multimeter. The electrochemical performance of YSCs was tested with a two-electrode system on an electrochemical workstation (Autolab PGSTAT302N, Metrohm). The test procedure for coefficient of water absorption was as follows：a certain amount of graphene fibers (weight recorded as m1, about 50 mg) was immersed in DI water for 1 min and then taken out. Gently tapped away any visible moisture from fiber surface with facial tissues and then measured the weight of wet fibers (m2). The coefficient of water absorption was calculated as 𝜂=(m2- m1)/ m1. The capacitance was calculated from galvanostatic charge–discharge (GCD)30 curves using Equation 1. IΔt

(1)

C = ΔU

where C is the total capacitance of device; I and Δt are the discharging current and time, respectively; and ΔU is the potential window. The area specific capacitance (Cs) and the volume specific capacitance (Cv) of fiber supercapacitor device, were calculated using Equation 2 and 3, respectively. C

(2)

Cs = 2S C

(3)

Cv = 2V

where S is the area of a single electrode (S=πDL) and V is the volume of a single electrode (V=1/4πD2L); and D and L are the diameter and the length of GF, respectively. The area specific capacitance CSE and the volume specific capacitance CVE of a single electrode were calculated by using Equation 4 and 5, respectively.31 (4)

C𝑆𝐸 = 4Cs

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(5)

C𝑉𝐸 = 4Cv

The area energy (Es) and the power (Ps) density of YSCs were calculated using Equation 6 and 7, respectively. 1

Es = 2Cs∆U2

(6)

Ps = Es/tdis

(7)

where tdis is the discharge time. 3. Results and discussion Figure 1a illustrates the preparation process of K-GF. First, GOF was prepared with the wet spinning method from GO solution with a typical schlieren texture.32 The average size of GO sheets is about 1 µm and the thickness is about 1.5 nm (Figure S2), manifesting the single layer state of GO.33 After reduced by HI, GF was obtained. The GF was then treated hydrothermally in KOH aqueous solution, resulting in the K-GF with improved hydrophilicity. The length of GOF can reach meters by the continuous preparation method, as shown in Movie S1 and Figure S1b. The diameter of fiber can be easily controlled by changing the diameter of needle and the concentration of GO solution, as shown in Figure S1c. The GF of 50 µm in diameter (the corresponding needle size is 0.33 mm and the dimeter of GOF fiber is 100 µm) was selected for preparing YSCs. As a comparison, hydrothermally treated GF in pure water (W-GF) was also prepared.

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Figure 1. Schematic illustrations of (a) the fabrication process of K-GF using GO liquid crystal (8 g/L) as precursor, and (b) the microstructure and surface functional group changes of the fiber during fabrication process. 3.1 Microstructure and morphology Figure 1b illustrates the structural evolution of fiber during the preparation process of K-GF. Stage I: the initial formation of GOF with circular cross-section right after injecting GO solution into the coagulating bath. Stage II: the formation of ruffles and pores on the GOF during the exchange of solvents and the drying process, respectively. Stage III: the chemical reduction of GOF to GF with greatly improved conductivity and surface epoxy groups. Stage IV: the conversion of epoxy groups to hydroxyl groups by hydrothermal treatment in KOH aqueous solution, forming the KGF with significantly improved hydrophilicity. The microstructure and morphology of K-GF can be more clearly observed in Figure 2a-c. The K-GF has uniform diameter of about 50 µm (Figure 2a, Figure S3) and circular cross-section (Figure 2c). The enlarged surface SEM image (Figure 2b) shows that the entire fiber exhibits a ruffle structure and the graphene sheets within the fiber are closely stacked together, forming a conducting network. This type of structure was formed before the hydrothermal treatment in KOH solution. As shown in Figure 9



2d-e, the GF without hydrothermal treatment possesses the similar microstructure and morphology. The specific surface area of GF, K-GF and W-GF are 4.456, 4.836, and 5.088 m2/g, respectively, and the corresponding average pore size are 3.72, 3.29, and 3.29 nm (Figure S4). The similar surface area and pore size demonstrate the structure stability of fibers. The interesting ruffle structure is common in fibers prepared by wet spinning technique,34 but it has rarely been discussed in graphene fibers-related works. The formation of ruffles is attributed to the difference in coagulating rates between the sheath and the core of GOFs, as suggested by previous studies on other types of fibers.35,36 The coagulation process of GOF took less than 10 s (Movie S1). When the GO solution was injected into the coagulating bath from a needle, the outer layer of the stream immediately coagulates to form a gel sheath. The initially formed GOF has relatively smooth surface and circular shaped cross-section (Figure 1b, stage I). With the coagulation proceeding, the water inside fiber continuously diffuses out which causes the shrinkage of fiber volume. Since the stiff sheath cannot shrink simultaneously with the core of fiber, ruffles are resulted (Figure 1b, Stage II). The following drying process of the gel state GOF also affects the formation of ruffles. As depicted in Figure S5, the diameter of GOF continues to decrease during the drying progress due to the evaporation of solvent from GOF, and GO sheet will be compressed due to the capillary action of solvent, which will promote the formation of ruffles.37-39 The existence of ruffles is beneficial to the specific surface area of fibers. But the number of ruffles needs to be controlled because too many ruffles will lead to serious structural defects and eventually cause the failure of fiber strength. Changing the exchange rate of solvents or extending the drying time may efficiently control the formation of ruffles.

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Besides the ruffles, narrow pores are also observed (Figure 2a and b). We think these pores were also formed during the drying process of the gel state fibers in air, where not only the diameter but also the length of the fiber decreases. As shown in Figure 3, the initial length of a gel state fiber is about 13 cm. After drying at room temperature for 360 s, the fiber length is reduced to 3.3 cm, only 25% of its original length. Such a large shrinkage in length is likely due to the strong van der Waals force between graphene sheets and the capillary motion of solvent.38 Under the shrinkageinduced compression force, as depicted in Figure 1b, the GO sheets stacked parallel to the radical direction are squeezed and the pores are thus formed between detached layers. It is very interesting that the shrunk ruffle fiber (3.3 cm) can be stretched to a length of 4.1 cm (elongation of 24.2%). Under a suitable pulling force, the ruffled fiber can be repeatedly pulled and then automatically return to 3.3 cm after removing the pulling force. We think the elasticity of our fibers is attributed to the pores and ruffles. Different from the commercial carbon fibers, our fiber has porous ruffle structure. As depicted in the inset of Figure 3, the pores are more rounded among relaxed graphene sheets, and they deform under a pulling force; ruffles can also elongate under a pulling force, functioning like a spring. In the fully straightened state, the strength of GOF and GF are 60.2 and 83.7 MPa, respectively (Figure S6). This mechanical enhancement originates from the decreased interlayer spacing between graphene sheets and the restoration of the structural integration of graphene units.40,41 After hydrothermal treatment, the strength of K-GF and W-GF are 76.2 and 90.6 MPa, respectively. The small difference implies the structure stability of GF.

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Figure 2. SEM images of (a, d) side view, (b, e) surface ruffles and pores, and (c, f) cross-section of K-GF and GF, respectively.

Figure 3. The time-dependent fiber length during the drying process. The insert illustrates the elongation of ruffles and pores when a graphene fiber is subjected to a pulling force. The XRD was used to characterize the crystal structure change during the fiber preparation. As seen in Figure 4a, GOF exhibits a characteristic peak at 9.3°, corresponding to the (002) diffraction of GO,42 but GF, K-GF, and W-GF exhibit a characteristic diffraction peak at 24°. The right shift of diffraction peak from 9.3 to 24° is due to the removal of oxygen-containing groups and the recovery of conjugated structure. As seen in Figure 4b, there are three types of carbon bonds observed in the C1s spectrum of K-GF: C-C bond at 284.5 eV, C-O bond at 286.2 eV, and C=O at 288.2 12


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eV. Both GF and W-GF show similar carbon bonds (Figure S7). The total oxygen atom content in GF, K-GF, and W-GF are 10.26%, 12.38%, and 9.9%, respectively (Figure 4c). The oxygen atom content in K-GF has improved 20% compared with that in GF. We think it is due to the epoxy ring-opening reaction. The primary functional groups on GO include carboxyl, hydroxyl, and epoxy groups. It was found that epoxy groups could still remain on the surface of graphene (Figure 1, stage III) after the GO was reduced to graphene by I-.26 In the alkaline environment such as KOH aqueous solution, the epoxy group undergoes a nucleophilic reaction: the OH- attacks the �-carbon atom of the epoxy group, making the epoxy group open and forming two hydroxyl groups (Figure 1b stage IV).43 As a result, the oxygen atom content in the K-GF increases, and the generated hydroxyl groups are beneficial to the hydrophilicity of materials. The WGF contains less oxygen atom than GF, indicating a higher reduction level after the hydrothermal treatment in DI water. FT-IR spectra also confirm the increased content of oxygen atoms in K-GF. In Figure 4d, the peak at 3433 cm-1 and 1385 cm-1 can be assigned to νOH and βOH (in COH), respectively; and the peak at 1632 cm-1 and 1575 cm-1 belong to δH2O and νC=C, respectively.43-46 The peak of νOH and βOH in K-GF is significantly stronger than in GF, indicating more -OH groups existing on K-GF.

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Figure 4. (a) XRD patterns of different fibers, (b) C1s XPS spectra of K-GF, (c) widescan XPS spectra of different fibers, and (d) FT-IR spectra of different fibers. 3.2 Conductivity and hydrophilicity Figure 5a shows the length-dependent resistance graph of various fibers. With the increase of fiber length, the resistance also increases linearly, indicating the physical and chemical uniformity of the prepared fibers. The conductivity of K-GF, GF, and WGF are 1256, 1860, and 2440 S/m, respectively. Compared with previously reported works,30,31,47 the K-GF without any insulating filler, remains a high level of conductivity credited to the relatively good conducting network between graphene sheets. The decrease of conductivity from GF to K-GF is caused by the change of surface functional groups. The W-GF exhibits the highest conductivity due to its higher reduction lever. Contact angle test is a direct way to characterize the hydrophilicity of materials. However, it is difficult to directly measure the contact angle of fibers because the diameter of the fibers is only 50 µm. So, three films, K-GM, GM, and W-GM, were prepared for this purpose. Since the electrolyte infiltration is a gradual process, we measured the contact angle as a function of time. Figure 5b shows a time-dependent 14


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change of contact angle, while using the PVA/H3PO4 electrolyte as medium. At 0 s, the contact angle of W-GM, GM, and K-GM are 123°, 120°, and 84°, respectively. The contact angles decrease for short time (< 20 s) and then stabilize at 106°, 100°, and 63° for W-GM, GM, and K-GM, respectively. The contact angle of K-GM is smaller than the other two, indicating that the K-GM has the best hydrophilicity. We also used water as medium to measure the contact angle. The measured contact angles for the three films are smaller in general because of the lower viscosity of water, but they still show the similar trend as in the PVA/H3PO4 electrolyte. The water absorption test was carried out to further confirm the hydrophilicity of K-GF. As shown in Figure 5c, the water absorption coefficient of K-GF, GF, and W-GF are 74.1%, 45.9%, and 40.5%, respectively. The K-GF shows the highest water absorption coefficient, while the WGF shows the lowest. This agrees well with the results from the contact angle measurements. As we have mentioned above, ruffles and pores are observed on the surface of graphene fibers, we thus simplify the ruffles and pores structure to a groove to demonstrate the electrolyte infiltration into grooves. As shown in Figure 5d, at the hydrophobic situation, the contact angle between the droplet and the material surface is large. The inherent hydrophobicity of graphene will prevent electrolyte from wetting the groove. But, with the improvement of hydrophilicity, the contact angle between the droplet and the material surface is smaller. As the infiltration time increases, the bottom of the groove can be fully infiltrated. Therefore, with the best wetting property in aqueous solutions, K-GF can absorb more electrolyte and shows higher water absorption coefficient, leading to a higher effective active area, which is important to the capacitance performance; while the W-GF has the worst hydrophilicity due to its high reduction level.

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Figure 5. (a) Fiber length-dependent resistance of different fibers; (b) time-dependent change of contact angle of K-GM, GM, and W-GM. Solid legends and hollow legends correspond to measurements in PVA/H3PO4 electrolyte and DI water, respectively; (c) water absorption coefficient of K-GF, GF, and W-GF; and (d) schematic illustration of electrolyte infiltration on the ruffle surface. 3.3 Electrochemical performance To reflect the capacitance of three kinds of graphene fibers (K-GF, GF, and WGF), we performed electrochemical characterization using two electrode system. Figure 6a shows the cyclic voltammograms (CV) curves of K-GF tested at different scanning rates. The good symmetry indicates that the fabricated YSC has great reversibility. At low scan rates (0.01 and 0.02 V/s), the curve shape is close to a rectangle, reflecting that the YSC has ideal characteristics of an electrochemical capacitor and the charging/discharging process is basically reversible. With the scanning rate increasing, it becomes harder for ions in electrolyte to quickly migrate to the electrode surface. As a result, the response times become longer and the curve gradually develops into a spindle type, agreeing with other reported results.48 As for the GF, the CV curves (Figure S8a) slightly deviates from the rectangular shape; and the shape deviation for

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W-GF (Figure S8b) is much more severe due to the inferior contact between the electrode and electrolyte caused by the worse hydrophilicity. Figure 6b shows the comparison of CV curves of K-GF, GF, and W-GF. The K-GF curve encloses the largest area, indicating that it has the largest capacitance. In contrast, the W-GF has the lowest capacitance and the GF is in between.

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Figure 6. (a) CV curves of K-GF at different scanning rates; (b) CV curves of K-GF, GF, and W-GF at 0.01 V/s; GCD tests of (c) K-GF and (d) GF at different current densities; (e) Ragone plots: 1. all-graphene core-sheath microfibers,49 2. core-shell nano-cables on carbon cloth,50 3. 4V/500µAh Li thin-film battery,51 4. metal oxide nanorods/rGO,52 5. graphene fibers,46 and 6. Graphene fibers.53 (f) CV curves of YSC with different bending angles; (g) the cycling performance of K-GF supercapacitor with different bending angles at 0.344 mA/cm2 (left and bottom axes) and the capacitance retention with up to 1000 bending cycles from 0º to 180º (right and top axes), and (h) Nyquist plots of K-GF and GF. The inset shows the high frequency behavior. We also conducted galvanostatic charge discharge (GCD) measurements of three YSCs made of the three kinds of fibers. As shown in Figure 6c, the K-GF’s GCD curve is close to the isosceles triangle structure with a small IR drop of 0.015 V at the current density of 0.086 mA/cm2. The GCD curve of GF (Figure 6d) slightly deviates from the isosceles triangle shape with a higher IR drop of 0.052 V at the current density of 0.087 mA/cm2 due to the limited improvement of hydrophilicity. Because of the dreadful contact between the electrolyte and the fiber surface due to the worst hydrophilicity, the shape of the GCD curve of the W-GF severely deviates from the isosceles triangle structure (Figure S8c). Although the W-GF has the best conductivity, it shows a poor electrochemical property (1.4 mF/cm2, 1.1 F/cm3). As shown in Figure 6c and Figure S8d, when the current density is 0.086 mA/cm2, the specific capacitance of K-GF is 145.6 mF/cm2 (117.2 F/cm3). It is much larger than that of GF. The corresponding energy density and power density of the YSC device is 3.23 µWh/cm2 (2.6 mWh/cm3) and 0.017 mW/cm2 (0.013 W/cm3), respectively. Benefit from the good conductivity of K-GF, when the current density is further increased to 0.86 mA/cm2, the specific capacitance becomes 94.6 mF/cm2 (76.2 F/cm3), and the 19



capacity retention is 65.0%. The GF’s specific capacitance is 70.4 mF/cm2 (56.8 F/cm3) at the current density of 0.087 mA/cm2. With the current density increasing to 0.87 mA/cm2, the specific capacitance becomes 30.7 mF/cm2 (24.7 F/cm3) and the corresponding rate capability is 42% (Figure 6d and Figure S8d). It is worth mentioning that in order to directly demonstrate the effect of hydrophilicity and the outstanding conductivity of graphene, we used pure graphene fibers as the electrodes. Figure 6e visually reflects that our YSC still has higher capacitance, even when compared with the electrodes that contain pseudo capacitance materials or the electrodes that were modified by mitigating stacking. More comparisons are listed in Tables S1 and S2. Finally, we assembled flexible supercapacitors using PET as a flexible substrate. The supercapacitors can be easily bent to different angles (the curvature radius for 60º, 90º, 120º, 180º are 6.11, 3.13, 1.69, 0.91 cm, respectively). As shown in Figure 6f, at different bending angles, the CV curve of the device remains basically the same. In Figure 6g, the red dot-line shows the capacitance retention with up to 1000 bending cycles from 0º to 180º. The slight value change of each testing point may arise from the sensitive contact between the solid electrolyte and fiber. After bending 1000 times, the capacitance retention is 95.3%. The colorful dotted line segments in Figure 6g show the capacitance retention with 500 charge-discharge cycles at different bending angle (0, 90, 120, 180, and 0º). When the bending angle is restored to 0º, the capacitance retention is 91.1% after 2500 charge-discharge cycles. These results show that our supercapacitor has good flexibility and cycling life. Figure 6h shows the Nyquist plots of K-GF and GF. In the high-frequency region, the X-intercept of the Nyquist plot represents the equivalent series resistance (ESR). The diameter of the semicircle represents the charge transfer resistance (Rct). The straight line of 45º in the mid-low frequency region, known as the Warburg resistance,

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is a result of ion diffusion. As we can see, K-GF shows a smaller semicircle, representing a lower Rct, and the straight line which represents the ion diffusion is barely observable, implying fast ion diffusion. This proves that K-GF does have a better contact with the electrolyte and the fiber surface is fully utilized, leading to the excellent performance of K-GF. On the other hand, although GF has similar structure with KGF, it has lower hydrophilicity. As a result, the contact between electrolyte and electrode will definitely be worse, leading to a smaller effective active area, which causes a higher charge transfer resistance. Meanwhile, in the low frequency range, the plot of GF is more inclined, while the plot of K-GF is nearly vertical to the Z’ axis, indicating that K-GF could provide more channel for the ions diffusion,23 which also leads to the better performance of K-GF. The Nyquist plot of W-GF, on the other hand, shows a much larger semicircle, which corresponds to the worst performance (Figure S8e). 4. Conclusion In summary, we have successfully prepared hydrophilic graphene fibers for high performance yarn supercapacitors with the wet spinning technique and subsequent hydrothermal treatment in KOH aqueous solution. The major findings and results are listed as follows: 1. The ruffles and pores on the surface of graphene fibers are investigated. The formation mechanism of ruffles involves mainly the difference of solvent exchange rates between the fiber core and sheath. The volume shrinkage of fiber during drying process leads to the formation of pores.

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2. The hydrophilicity of fibers can be dramatically enhanced via hydrothermal treatment in KOH solution. The reason is attributed to the ring-opening reaction of epoxy groups that convert to hydroxyl groups in alkaline environment. 3. The outstanding conductivity of graphene fibers is retained as much as possible, since no insulating nanofillers are involved during the preparation of GF and the 3D conducting networks of graphene sheets are maintained. 4. Profiting from the synergistic effect of porous ruffle structure, improved hydrophilicity, and good conductivity, the K-GF based YSC shows excellent electrochemical performance and flexibility. Acknowledgements This research was supported by the Natural Science Foundation of China (No. 51425202, No. 51772150), the Natural Science Foundation of Jiangsu Province (No. BK20160093), the Key Research and Development Program of Jiangsu Province (No. BE2016006-1), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Additional Information Supporting Information: schematic illustration of a yarn supercapacitor and the images of GOF; AFM characterization of GOF sheets; SEM images and statistical distribution of the diameter of K-GF; nitrogen adsorption-desorption isotherms and pore size distribution of fibers; the time-dependent diameter of GOF during the drying process; tensile stress curves of fibers; C1s XPS spectra of GF and W-GF; and electrochemical characterization of GF and W-GF. Competing financial interests: the authors declare no competing financial interests. 22


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Table of Content 84x47mm (300 x 300 DPI)


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Hydrophilicity improvement of graphene fibers for high-performance

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