Utilizing human hair for solid-state flexible fiber-based asymmetric supercapacitors

Journal Pre-proofs Full Length Article Utilizing human hair for solid-state flexible fiber-based asymmetric supercapacitors Jing Zhao, Junwei Gong, Chunliang Zhou, Chenxu Miao, Rong Hu, Kai Zhu, Kui Cheng, Ke Ye, Jun Yan, Dianxue Cao, Xianfa Zhang, Guiling Wang PII: DOI: Reference:

S0169-4332(20)30016-7 https://doi.org/10.1016/j.apsusc.2020.145260 APSUSC 145260

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

22 October 2019 9 December 2019 1 January 2020

Please cite this article as: J. Zhao, J. Gong, C. Zhou, C. Miao, R. Hu, K. Zhu, K. Cheng, K. Ye, J. Yan, D. Cao, X. Zhang, G. Wang, Utilizing human hair for solid-state flexible fiber-based asymmetric supercapacitors, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145260

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2020 Published by Elsevier B.V.

Utilizing human hair for solid-state flexible fiber-based asymmetric supercapacitors Jing Zhaoa, Junwei Gonga, Chunliang Zhoua,*, Chenxu Miaoa, Rong Hua, Kai Zhua, Kui Chenga, Ke Yea, Jun Yana, Dianxue Caoa, Xianfa Zhangb,*, Guiling Wanga,*

a

Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education,

Department of Materials Science and Engineering, Harbin Engineering University, Harbin, Heilongjiang, 150001, P. R. China b

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,

School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, PR China

*Correspondence to Guiling Wang: [email protected]; Chunliang Zhou: [email protected] Xianfa Zhang: [email protected]

1

Abstract The demands for wearable energy storage devices keep growing and novel flexible electrode materials and facile prepared methods for these devices are crucial and attractive. In this work, human hair fiber is employed as the flexible fiber-based material due to the high utilization values of low cost, controllable length and high elasticity capable. Herein, an effective and hazard-free microwave-assisted method is reported to develop superelastic reduced graphene oxide coating human hair (rGO@Hh) to achieve conductive and high-strength fibrous electrode. With the braided rGO@Hh fibers, nickel hydroxide nanoribbon-intercalated rGO nanosheets (Ni(OH)2/rGO@Hh) are further prepared through microwave radiation treatment, which shows an interconnected porous structure with outstanding electrochemical performances of high specific capacitance (316 F g-1 at 1 A g-1) and good cycling performance. Moreover, a solid-state fiber-based asymmetric supercapacitor (FASC) is assembled with rGO@Hh fibers and Ni(OH)2/rGO@Hh fibers as negative and positive electrodes, which demonstrates a high energy of 27.6 Wh kg−1 and a power density of 699 W kg-1, respectively. The FASC also can be further used for numerous electronic products, presenting its prospect for portable wearable devices with high tensile strength and excellent performances. Keywords: Human hair fibers; Reduced graphene oxide; Microwave-assisted method; Flexible fiber-based supercapacitor

2

1. Introduction Wearable and flexible energy storage devices are widely explored due to their promising applications in the field of smart fabrics and communication equipments. Stretchable and bendable supercapacitors are prospective candidates for developing the portable electronics [1, 2]. Among various flexible devices, the investigation of the high energy storage fiber-based supercapacitor (FBCs) is an attractive direction to meet the practical applicability in wearable electronics. Recently, fiber-based electrode materials have been used for the flexible wearable device. Lu et al. prepared the hybrid carbon nanotube (CNT)/graphene fiber with good elasticity capable of 800% strain [3]. Nagaraju and coworkers designed Cu fiber-based electrode material, which presents high specific capacity of 230.48 mA h g−1 and good cycle stability [4]. Commonly, assembling the hybrid system by combining battery-type materials as positive electrode and electric double layer capacitive materials as negative electrode can effectively enhance the energy density of the FBCs through extending the potential range and enhancing the specific capacitance [5, 6]. For improving the electrochemical performances, the suitable current collectors and electrode materials are the main components to fabricate the low cost and novel of hybrid FBCs. However, the complicated preparation process of the fibers limits their applications. On the other hand, biowaste or biomass-derived materials are greatly attractive due to their low cost and biodiversity [7-15], such as fish scale [16], bacterial cellulose (BC) [17] and paulownia sawdust [18], which present natural hierarchically structure. The waste/scrap human hairs which are virtually useless and difficult to clean up were thrown into trash in

3

our daily life. However, it is interesting that the human hair has high utilization value due to the low cost, controllable length and high elasticity capable, which is an ideal substrate for the fiber-based material. Human hair is made of α-keratin, including 15 wt% nitrogen and 5 wt% sulfur. Hence, there are abundant functional groups on its surface, which is beneficial for bonding other oxygen-containing groups. For the physical properties, the human hair shows a diameter of about 90 μm with light weight, which is an ideal elastic substrate for preparing ultralight flexible electronic devices. Furthermore, utilizing human hair to prepare fibrous electrode can both simplify the preparation process and reduce the fabrication cost, which is an appealing substrate of the practical applications and eco-friendliness of the flexible energy storage devices. But the human hair commonly has very poor electrical conductivity. Inspiring from this, combining the hair fiber with graphene is the crucial step to make it conductive. The typical methods for reducing graphene oxide containing the chemical reduction [19], high-temperature thermal reduction [20], and electrochemical reduction methods [21]. The chemical reduction and high-temperature thermal treatment among the typical reduction methods commonly possess hazardous reducing agents or severe energy loss. Moreover, the high temperature hydrothermal treatment will weaken the tensile strength and flexibility of the hair fibers. By contrast, the microwave irradiation is an ordinary, environmental friendly and rapid method using a small amount of ascorbic acid as the reductant, which can effectively shorten the preparation time and have little effect on the initial state of the hair fibers [22, 23]. The reduced graphene coating hair fiber is flexible, which has high tensile strength and can be weaved into various textiles. Overall, conductive hair fibers could be employed 4

as tiny electrodes to combine with skin-like electronics or multifunctional modules in portable wearable devices. Furthermore, the hybrid FBCs can deliver high energy density due to the high specific capacitance and coefficient between the positive and negative electrodes. The battery-type materials (Co3O4, Ni3Se2, NiO, etc.) used as the positive electrodes present good charge storage capability due to their excellent electrochemical activity and abundant redox sites for electrolyte ions [24]. Hence, massive efforts have been made to use the battery-type materials as positive electrode with capacitive materials as negative electrode to fabricate hybrid devices [25, 26]. In this work, for the first time, we prepared nickel hydroxide modified on reduced graphene oxide coating human hair (Ni(OH)2/rGO@Hh) fibers using a ultrafast and effective microwave-assisted method using a small amount of ascorbic acid as the reductant, which can effectively shorten the preparation time to generate Ni(OH)2 nanoribbons anchored reduced graphene nanosheets in ambient air. As the electrode for supercapacitor, the flowerlike Ni(OH)2/rGO@Hh fiber electrode presents a high specific capacitance (316 F g-1) and good cyclic stability (89% after 3000 cycles). Moreover, a solid-state fiber-based asymmetric supercapacitor (FASC) is fabricated with fibrous Ni(OH)2/rGO@Hh positive electrode and rGO@Hh negative electrode, which presents a high energy density of 27.6 Wh kg−1 and a power density of 699 W kg−1. The flexible FASC with impressive electrochemical performances can be widely employed for wearable and portable electronics. 2. Experiment Section 5

2.1. Synthesis of graphene oxide (GO): Graphene oxide (GO) was prepared by the Hummers' method [27]. Firstly, 3 g graphite powder was put into 500 mL solution of H2SO4 (70 ml), NaNO3 (0.5 g) and stirring strongly for 45 min. Then, 3 g KMnO4 was added slowly and the resultant temperature should keep to be below 10◦C in the ice-bath, then the mixture was continue to stir for 2 h. After that, the mixture was transfer to the 35℃ bath and stirred for 2 h. Then, 500 mL DI water was put in and the temperature was keep at 85 ℃ and stirred for 30 min. Then alternative 0.6 L of H2O and 18 mL of 25% H2O2 was put in sequence. The obtained luminous-yellow mixture was washed with 10 wt% HCl solution and DI water to remove impurities until the pH of the mixture became neutral. The obtained GO paste was dried in a freezer dryer and the GO solid was obtained. 2.2. Synthesis of reduced graphene oxide coating human hair (rGO@Hh) fibers: The rGO@Hh preparation process is illustrated in Figure 1a. GO solid (0.25 g) was put into 20 mL water with 20 mL dimethylformamide (DMF) and ultrasonication for 1 h to prepare the GO dispersion. Then 0.5 g ascorbic acid as the reductant was injected into 50 mL of 2 mg mL-1 GO dispersion with ultrasonication for 0.5 h to prepare the mixed dispersions. Subsequently, the hair fibers were put into the resultant and dipped for 45 min, the resultant was put into a microwave oven (PreeKem, APEX) and heated at 750 W for 15 min in ambient air. After a modified microwave radiation treatment, the rGO nanosheets gradually self-assembled onto hair closely with small accumulation. After that, the rGO@Hh coated hair fibers were dried at the room temperature, which was named as rGO@Hh. 6

2.3.

Synthesis

of

Ni(OH)2

nanoribbons

anchored

rGO@Hh

fiber

(Ni(OH)2/rGO@Hh): The Ni(OH)2/rGO@Hh fibers were synthesized with the microwave radiation method. Firstly, 1 g of rGO@Hh fibers, 0.5 g of nickel (II) sulfate hexahydrate and 2 g of urea were placed into 100 mL of DI water and stirring when the solution became clear. Subsequently, the obtained mixture was put into a microwave oven and reacted at 750 W for 15 min in ambient air. Finally, the collected sample was washed with DI water followed by drying at 60 ℃ in an oven for 12 h. 2.4. Materials characterizations The morphology of the as-prepared samples was identified using SEM (JEOL JSM-6480) and TEM (JEOL JEM2010). XPS measurement of the materials was carried out on Tryhermo ESCALAB 250 system with an Al Kα radiation (1485.6 eV). X-ray diffraction (XRD) system was conducted to explore the phase states of the samples using Rigaku TTR III system with Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV, 100 mA. 2.5. Preparation of working electrode The rGO@Hh fibers are twist together to prepare the fiber-based working electrodes. And the synthesis process of KOH/PVA gel electrolyte is similar to the previous report. 2 g PVA powder and 6 g KOH were added into a 60 mL of water solution and stirring under 85℃ until the solution was clear. Then the obtained gel was painted on the rGO@Hh and Ni(OH)2/rGO@Hh electrodes with 15 min standing in the air. Then two pieces of fibers were twisted together under 0.1 MPa for 5 min to obtain the solid-state device. Furthermore, the Ni(OH)2/rGO@Hh//rGO@Hh solid state supercapacitor was assembled 7

and conducted with the voltage window in the range of 0-1.4 V. All the electrochemical characterization methods were carried out on the electrochemical working station (CHI 600A). CV and GCD characterizations were conducted within the potential window range from 0 to 1.4 V and EIS test was presented by using an operating potential with 5 mV amplitude in frequency between 10 mHz and 100 kHz. The cycle life was characterized on a NEWARE battery test system (CT-3008W). The specific capacitances (C) at varied current densities are evaluated from the GCD curves based on the following equation: C=

IΔt ΔV

(1)

where I (A g-1) presents the current density, △t (s) represents the discharging time and △V (V) is the discharging voltage. Herein , the mass loading of active materials is calculated to be 1.2 mg cm-2. 3. Results and Discussion

8

Scheme 1. Schematic illustration of the preparation procedure of the Ni(OH)2/rGO@Hh fiber and the fabrication of the asymmetric supercapacitor.

The Ni(OH)2/rGO@Hh fiber is prepared using facile and fast microwave method, the preparation process is illustrated in Scheme 1. The human hair is considered as one of the abundantly available biowaste. Utilizing waste human hair with high toughness for fiber-based supercapacitors can decrease the cost and improve the electrochemical performances. The human hair (Hh) fibers are used as flexible substrate material for the follow-up growth of graphene and Ni(OH)2. Firstly, the reduced graphene coated Hh fibers (rGO@Hh) are prepared using the microwave-assistant method, which can make the Hh fibers highly conductive without little effect on their tensile strength. Subsequently, the Ni(OH)2 was anchored on rGO@Hh fibers with solution including NiSO4·6H2O and NH2CONH2 with further microwave-assistant method. 9

Figure 1a shows the SEM image of the gold-sprayed human hair fiber, presenting a radius of ≈45 µm with a squamaceous surface. Subsequently, the surface of these fibers became rough and uniformly coated with rGO nanosheets after the microwave treatment, as presented in Figure 1b and c. Herein, the rGO nanosheets primarily improve the electronic conductivity of the Hh fibers. The SEM images of the Hh fibers (Figure 1d-f) show fibrous structure with more hierarchical nanosheets, presenting that the fibers have high flexibility under curved condition without any break after the coverage of Ni(OH)2, indicating the suitability for flexible and wearable supercapacitors. This hierarchical structure brings abundant accessible transport channels for electrolyte penetration and enhances their specific capacitance and rate performance.

Figure 1. (a) SEM image of the gold-sprayed Hh fiber, showing fibrous structure with smooth surface. (b, c) SEM images of the rGO@Hh fiber, showing fibrous structure with rough surface. (d-f) SEM images of the Ni(OH)2/rGO@Hh fibers, showing an interconnected nanosheets network.

10

The morphology of the samples was further identified by transmission electron microscope (TEM). The TEM images of rGO are displayed in Figure 2a and b, which present porous thin layered structure. The TEM images (Figure 2c, d) show that the rGO nanosheets was attached uniformly with interconnected cross-linked Ni(OH)2 nanoribbons network under the effect of the microwave radiation. The close connection between Ni(OH)2 nanoribbons network and rGO nanosheets is beneficial for ensuring rapid electron transport and good electric conductivity of the electrode. Additionally, the high-resolution TEM (HR-TEM) image (Figure 2e) of Ni(OH)2/rGO shows a lattice fringe distance of about 0.7 nm, corresponding to the (003) plane of Ni(OH)2. Furthermore, the selective area electron diffraction (SAED) pattern of the composite demonstrates polycrystalline characteristic (Figure 2f). In addition, the diffraction rings can be indexed to the (003), (006), (101) and (100) planes of Ni(OH)2, respectively, which is in accordance with the XRD results and demonstrates the successful synthesis of Ni(OH)2 nanoribbons.

11

Figure 2. (a) TEM and (b) HRTEM images of rGO. (c-e) TEM images of the Ni(OH)2/rGO, which show the interconnected Ni(OH)2 nanoribbons anchored rGO nanosheets. (f) SEAD pattern of the Ni(OH)2/rGO.

The X-ray diffraction (XRD) patterns of the as-prepared materials are displayed in Figure 3a. The diffraction peaks observed at 12.1°, 24.6°, 33.3°, and 59.4° correspond to (003), (006), (101), and (110) planes, respectively. All of the characteristic peaks of the Ni(OH)2/rGO@Hh are perfectly matched with the Ni(OH)2 (JCPDF No. 38-0715) phase[2]. Notably, the broad diffraction peaks of C located at about 26.5° may cause by a uniform distribution of the rGO nanosheets in the obtained fibers. Additionally, no other peaks were observed indicates that the high purity of the composite, presenting that the Ni(OH)2/rGO@Hh fibers have been integrally prepared. Furthermore, the chemical states

12

of the samples are determined by the X-ray photoelectron spectroscopy (XPS) analysis. Accordingly, the full survey spectra of the Ni(OH)2/rGO@Hh fibers primarily displays C, O and Ni species (Figure 3b). For the high resolution Ni 2p XPS spectrum of the Ni(OH)2/rGO@Hh fibers (Figure 3c), two distinct peaks located at around 872.6 and 854.8 eV may correspond to Ni 2p1/2 and Ni 2p3/2. Notably, there are two additional peaks centered at about 879.5 eV and 861.3 eV, which are fitted to the satellites, respectively. The C 1s XPS spectra of the composite (Figure 3d) shows four groups, which correspond to different functional groups of carbon atoms including the C-C (284.7 eV), C-O (286.7 eV), C=O (287.8 eV) and O=C-O (288.9 eV) bondings, presenting the existence of remanent oxygen-containing functional groups on the rGO nanosheets, which can generate more chemical electroactive sites for the growth of Ni(OH)2.

13

Figure 3. (a) XRD patterns of rGO@Hh and Ni(OH)2/rGO@Hh. (b) XPS survey spectrum, high resolution (c) Ni 2p spectra and (d) C 1s spectra of the Ni(OH)2/rGO@Hh fibers.

The electrochemical performances of the electrodes are ulteriorly explored by the three-electrode system. The CV curves of the rGO@Hh electrode at a varied scan rates in 6 M KOH electrolyte (Figure 4a) present a relatively quasi-rectangular in shape with no obvious distortion, indicating typical capacitive behavior and good electrochemical reversibility. Correspondingly, the galvanostatic charge/discharge (GCD) curves of rGO@Hh present symmetrical and triangle shape at a varied current densities (1 to 20 A g-1) during the charging/discharging processes, demonstrating its capacitive contribution 14

(Figure 4b). Negligible iR drop in the GCD curves present that the low internal resistance of the material. Figure 4c presents the CV curves of the Ni(OH)2/rGO@Hh electrode, which is distinguished from that of the rGO@Hh, presenting the pseudocapacitive characteristic of Ni(OH)2. All the CV curves present a pair of redox peaks located at around 0.10 V and 0.27 V, corresponding to the cathodic and anodic peaks. The slightly shift of the redox peaks demonstrate the good reversibility of the Ni(OH)2/rGO@Hh electrode. Additionally, the CV curves show negligible change along with the enlarged scan rates, demonstrating that the rapid mass transportation and charge transfer, which can decrease the polarization of the material. Correspondingly, the GCD curves of Ni(OH)2/rGO@Hh electrode (Figure 4d) show a triangle in shape with two plateaus of the GCD curves, indicating the reversible pseudocapacitive characteristics of the electrodes. Figure 4e presents the specific capacitance (Cs) of the two electrodes at a series of current densities, which could be evaluated based on the GCD curves. The specific capacitance of the Ni(OH)2/rGO@Hh fibers is 316 F g-1 at 1 A g-1, which is significantly improved compared to the rGO@Hh (149 F g-1) resulting from the high pseudocapacitance provided by Ni(OH)2. The vertical interconnected nanosheets ensure the accessibility for electrolyte ions, hence maintaining a high specific capacitance of 138 F g-1 even at a higher current density (20 A g-1). This significant enhancement is generated from the whole capacitance of the Ni(OH)2/rGO@Hh fibers of the assorted pseudocapacitive contribution offered by Ni(OH)2 and the EDLC provided by rGO in the fibers, respectively. The electrochemical impedance spectroscopy (EIS) is adopted to explore the essential electrochemical performances of the electrodes (Figure 4f). The slop of the straight line portion of the 15

rGO@Hh curve is a steep slope, which presents an ideal capacitive behavior [28, 29]. Meanwhile, the EIS curve of rGO@Hh shows a small semicircle (2.01 Ω) in the high-frequency region, demonstrating that the small charge transfer impedance and rapid ion transfer, as shown in the inset of Figure 4f [30]. Furthermore, the relative small equivalent series resistance (3.48 Ω) indicates low internal resistance and intimate interfacial contact between the conductive rGO and the Ni(OH)2 nanoribbons. As displayed in Figure 4g, the Ni(OH)2/rGO@Hh electrode presents an impressive cycle property with 89% retention after 3000 cycles, which mainly attributes to the stable rGO@Hh substrate and its open vertical layered structure. According to the high performances of the Ni(OH)2/rGO@Hh fibers, we match the rGO@Hh as the negative electrode material to fabricate fiber-based asymmetric supercapacitors [31-34].

16

Figure 4. (a) CV curves tested at a series of scan rates and (b) GCD curves tested under various current densities of the rGO@Hh. (c) CV measurement tested at various scan rates and (d) GCD curves measured at various current densities of the Ni(OH)2/rGO@Hh. (e) Specific capacitances versus current densities, (f) Nyquist plots (the insert presents the magnified semicircles in the high-frequency region) and (g) cycling life tested at 1 A g-1 for 3000 cycles of the rGO@Hh and Ni(OH)2/rGO@Hh.

Furthermore, the tensile stress-strain of the fiber-based electrodes was tested to investigate their mechanical properties, as shown in Figure 5a. It was shown that the strain of the fiber-based electrode reaches 56.8% at the maximum stress. After the hair fibers were coated with rGO and Ni(OH)2, the strain of the fibers still maintains 56.4% and 53.8% at 17

the maximum stress, respectively. These results indicate that the fiber-based electrodes can achieve high strain under high stress with good toughness after the microwave treatment. To ascertain the relationship between stretchability and electrochemical stability of the electrode, CV at a scan rate of 50 mV s-1 was performed under various tensile strains (0%, 10%, 20%, 30%, 40% and 50%) (Figure 5b). Obviously, the CV curves maintain the original shape with negligible changes, indicating the good structural stability of the electrode. Consequently, the fibrous electrode maintains capacitance retention of 99.1% at 50% strain, presenting its high tensile strength. The flexibility of the fibers tends to woven into various fabrics. Moreover, the textile knitted with Ni(OH)2/rGO@Hh fibers demonstrates their promising applications in wearable and portable electronic devices due to the good conductivity and flexibility (Figure 5c and d).

Figure 5 (a) Tensile stress-strain curves of the electrodes. (b) CV curves and capacitance

18

retention of the stretchable fibers with various tensile strains at 50 mV s-1. (c-d) Photographs of the knitted fabrics of Ni(OH)2/rGO@Hh fibers.

The flexible fiber-based asymmetric supercapacitor (FASC) is assembled with the Ni(OH)2/rGO@Hh fibers and the rGO@Hh electrodes (Ni(OH)2/rGO@Hh//rGO@Hh) to further explore the electrochemical performances of the materials. For supercapacitors, the mass ratio between the both electrodes is analyzed by the following equation [35]: m1 C 2 V2  m 2 C1V1

(2)

where m is mass of the electroactive constituent, the subscripts of 1 and 2 present the anode and cathode. C presents the capacitance and V is the potential window of respective electrode. Based on the voltage ranges of 0-1.4 V and specific capacitance values for the Ni(OH)2/rGO@Hh fibers and rGO@Hh, the balancing mass ratio of the positive and negative electrodes was optimized to be m(Ni(OH)2/rGO@Hh)/m(rGO@Hh) = 0.58. Figure 6a presents the CV curves of the FASC at a series of scan rate (5-100 mV s-1). The CV curves of the device shows rectangular shape even at a high scan rate of 100 mV s-1, corresponding to the ideal capacitive behavior. As shown in Figure 6b, the GCD curves of the device are tested at varied current densities. The symmetric triangular-shaped GCD curves demonstrate the ideal capacitive behavior and good Coulombic efficiency. The specific capacitances (C) tested at varied current densities of the FASC are shown in Figure 6c. The device delivers a high specific capacitance of 102 F g-1 at 1 A g-1 and

19

outstanding rate performance with high capacitance retention of 64% even at a high current density (20 A g-1). Figure 6d presents the cycling stability of the FASC, which delivers good cyclic stability with 81% capacitance retention at 1 A g-1 for 10000 cycles. The GCD curves remain symmetric triangle shape during the long-time cycles, which presents the outstanding cycling stability and high electrochemical performance (the inset in Figure 6d). The Nyquist plots and the equivalent circuit of the device before and after 10000 cycles are shown in Figure 6e and the inset. The equivalent circuit model including an internal resistance (Rs), a charge-transfer resistance (Rf), a constant phase element (Cdl) and a Warburg resistance (W) [36]. The Rs including internal resistance of active material, resistance of electrolyte and contact resistance between the active material and electrolyte. The Rs are evaluated to be about 3.48 and 3.67 V before and after 10000 cycles, respectively. The slightly change of Rs mainly results from the increasing contact resistance due to the separation between the active material after the long cycle. The Rf can be calculated by the semicircle diameter, which has no obvious change after 10000 cycles, indicating the good structural stability and the fast charge transfer of the electrode. The straight line at low frequency region is attributed to the ion resistance of the electrolyte into the internal electrode, indicating its ideal capacitive behavior [37]. For practical applications, the power density (P, W kg-1) and energy density (E, Wh kg-1) are prime parameters to represent the electrochemical properties of FASC, which are evaluated according to the GCD curves at varied current densities based on the equations as follows [38]:

20

E

1  C  △V 2 7.2

(3)

P

E  3600 △t

(4)

where C (F g-1) indicates the capacitance calculated by the total mass of the electrodes, △ t (s) signifies the discharging time and △ V (V) presents the discharging potential. Ragone plots of the FASC characterized at varied current densities are depicted in Figure 6f. The FASC delivers a high energy density of 27.6 Wh kg-1 and a power density of 699 W kg-1. Even at a power density of 14000 kW kg-1, the energy density still maintains 17.5 Wh kg-1, which is better than that of the Ni3S4/CC//AC device,[39] Ni-Co oxides//AC [40], Mn3O4@GR//AC [41], Ni3S2@AC [42], Ni-MOF//AC [43], NiS HNPs//AC [44] and V2O5/rGO//AC [45]. These outstanding electrochemical performances of the FASC may result from the mutual effects between the two electrodes. On one hand, the human hair serves as a suitable substrate for the active materials due to its strong toughness and superb flexibility. On the other hand, the rGO nanosheets serve as the conductive supporter for the Ni(OH)2 nanoribbons as well as keep the high electronic conductivity of the fiber-based electrode. The impressive electrochemical performances of the FASC can thus be caused by the mutual effects between the Ni(OH)2/rGO@Hh fibers and rGO@Hh electrodes, where Ni(OH)2 nanoribbons uniformly and closely anchor in rGO nanosheets to form interconnected porous structure, which can bring larger effective contact area and expose more electroactive sites for electrolyte ions.

21

Figure 6. Electrochemical performances of the FASC device. (a) CV measurement tested at various scan rates. (b) GCD measurement tested at various current densities. (c) Specific capacitances versus varied current densities. (d) Cycle performance performed at 1 A g-1 with 10000 cycles (the insert presents the GCD curves for the cell). (e) Nyquist plots at first and after 10000 cycles (the insert is the equivalent fitting model). (f) Ragone plots compared with the other representative reports.

4. Conclusion We have successfully employed the human hair fiber as the flexible fiber-based substrate due to the low cost, high tensile strength and high elasticity capable. Conductive and superelastic reduced graphene oxide coating human hair (rGO@Hh) high-strength fibrous electrode was achieved using an effective and hazard-free microwave-assisted method. Moreover, the rGO nanosheets are served as the supporter for the Ni(OH)2 nanoribbons, which can provide the high electrical conductivity and further enhance the electrochemical performances of the electrode. Moreover, Ni(OH)2/rGO@Hh fibers are further synthesized

22

via microwave radiation method, which shows an interconnected porous structure with high specific capacitance of 316 F g-1 at 1 A g-1 and outstanding rate capability. Consequently, the fiber-based

asymmetric

supercapacitor

was

assembled

with

Ni(OH)2/rGO@Hh fibers and rGO@Hh fibers as the two electrodes, which delivers high energy density of 27.6 Wh kg-1 and impressive cycle performance. These results can reveal the prospect of this fiber-based supercapacitor for portable wearable device with high tensile strength and high electrochemical performances. Notes The authors declare no competing financial interest. Acknowledgements We gratefully acknowledge the financial support of this research from the National Natural Science Foundation of China (51572052) and Ph.D. Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities (3072019GIP1010).

References: [1] F. Béguin., V. Presser., A. Balducci., E. Frackowiak., Carbons and electrolytes for advanced supercapacitors, Adv. Mater. 26 (2014) 2219–2251. [2] J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi, F. Wei., Advanced asymmetric supercapacitors based on Ni(OH)2/Graphene and porous graphene electrodes with high energy density, Adv. Funct. Mater. 22 (2012) 2632–2641. [3] Z. Lu, J. Foroughi, C. Wang, H. Long, G.G. Wallace, Superelastic Hybrid CNT/Graphene Fibers for Wearable Energy Storage, Adv. Energy Mater. 8 (2017) 1702047. [4] G. Nagaraju, S.C. Sekhar, J.S. Yu, Utilizing waste cable wires for high-performance fiber-based hybrid supercapacitors: an effective approach to electronic-waste management, Adv. Energy Mater. 8 (2017) 1702201.

23

[5] J. Shen, C. Yang, X. Li, G. Wang, High-performance asymmetric supercapacitor based on nanoarchitectured polyaniline/graphene/carbon nanotube and activated graphene electrodes, ACS Appl. Mater. Interfaces 5 (2013) 8467-8476. [6] H. Gao, F. Xiao, C.B. Ching, H. Duan, High-performance asymmetric supercapacitor based on graphene hydrogel and nanostructured MnO2, ACS Appl. Mater. Interfaces 4 (2012) 2801−2810. [7] C. Long, X. Chen, L. Jiang, L. Zhi, Z. Fan, Porous layer-stacking carbon derived from in-built template in biomass for high volumetric performance supercapacitors, Nano Energy 12 (2015) 141–151. [8] F. Lai, Y. Miao, L. Zuo, H. Lu, Y. Huang, T. Liu, Biomass-derived nitrogen-doped carbon nanofiber network: a facile template for decoration of ultrathin nickel-cobalt layered double hydroxide nanosheets as high-performance asymmetric supercapacitor electrode, Small 12 (2016) 3235–3244. [9] M. Zhang, C. Wang, H. Wang, M. Jian, X. Hao, Y. Zhang, Carbonized cotton fabric for high-performance wearable strain sensors, Adv. Funct. Mater. 27 (2017) 1604795. [10] C. Peng, X. Yan, R. Wang, J. Lang, Y. Ou, Q. Xue, Promising activated carbons derived from waste tea-leaves and their application in high performance supercapacitors electrodes, Electrochim. Acta 87 (2013) 401-408. [11] L. Qie, A. Manthiram, A facile layer-by-layer approach for high-areal-capacity sulfur cathodes, Adv. Mater. 27 (2015) 1694-1700. [12] L. Qie, W. Chen, X. Xiong, C. Hu, F. Zou, P. Hu, Y. Huang, Sulfur-doped carbon with enlarged interlayer distance as a high-performance anode material for sodium-ion batteries, Adv. Sci. 2 (2015) 1500195. [13] Z. Gao, C. Bumgardner, N. Song, Y. Zhang, J. Li, X. Li, Cotton-textile-enabled flexible self-sustaining power packs via roll-to-roll fabrication, Nat. Commun. 7 (2016) 11586. [14] C. Liang, J. Bao, C. Li, H. Huang, C. Chen, Y. Lou, H. Lu, H. Lin, Z. Shi, S. Feng, One-dimensional hierarchically porous carbon from biomass with high capacitance as supercapacitor materials, Micropor. Mesopor. Mater. 251 (2017) 77-82. [15] C. Chen, Y. Zhang, Y. Li, J. Dai, J. Song, Y. Yao, Y. Gong, I. Kierzewski, J. Xie, L. Hu, All-wood, low tortuosity, aqueous, biodegradable supercapacitors with ultra-high capacitance, Energy Environ. Sci. 10 (2017) 538-545. [16] W. Chen, H. Zhang, Y. Huang, W. Wang, A fish scale based hierarchical lamellar porous carbon material obtained using a natural template for high performance electrochemical capacitors, J. Mater. Chem. 20 (2010) 4773-4775. [17] Z. Hu, S. Li, P. Cheng, W. Yu, R. Li, X. Shao, W. Lin, D. Yuan, N,P-co-doped carbon nanowires prepared from bacterial cellulose for supercapacitor, J. Mater. Sci. 51 (2016) 2627–2633. [18] X. Liu, M. Zheng, Y. Xiao, Y. Yang, L. Yang, Y. Liu, B. Lei, H. Dong, H. Zhang, H. Fu, Microtube bundle carbon derived from paulownia sawdust for hybrid supercapacitor electrodes, ACS Appl. Mater. Interfaces 5 (2013) 4667-4677. [19] J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Reduction of graphene oxide via L-ascorbic acid, Chem. Commun. 46 (2010) 1112-1114. [20] Y. Qiu, F. Guo, R. Hurt, I. Kulaots, Explosive thermal reduction of graphene oxide-based materials: mechanism and safety implications, Carbon 72 (2014) 215-223.

24

[21] S.Y. Toh, K.S. Loh, S.K. Kamarudin, W.R.W. Daud, Graphene production via electrochemical reduction of graphene oxide: Synthesis and characterisation, Chem. Eng. J. 251 (2014) 422–434. [22] R. Liu, Y. Zhang, Z. Ning, Y. Xu, A catalytic microwave process for superfast preparation of high-quality reduced graphene oxide, Angew Chem. Int. Ed. Engl. 56 (2017) 15677-15682. [23] D. Voiry, J. Yang, J. Kupferberg, R. Fullon, C. Lee, H.Y. Jeong, H.S. Shin, M. Chhowalla, High-quality graphene via microwave reduction of solution-exfoliated graphene oxide, Science 353 (2016) 1413-1416. [24] H. Ma, D. Kong, Y. Xu, X. Xie, Y. Tao, Z. Xiao, W. Lv, H.D. Jang, J. Huang, Q.H. Yang, Disassembly-reassembly approach to RuO2/Graphene composites for ultrahigh volumetric capacitance supercapacitor, Small 13 (2017). [25] P. Yu, G.J. Cao, S. Yi, X. Zhang, C. Li, X.Z. Sun, K. Wang, Y.W. Ma, Binder-free 2D titanium carbide (MXene)/carbon nanotube composites for high-performance lithium-ion capacitors, Nanoscale 10 (2018) 5906-5913. [26] D. Yang, Q. Zhao, L. Huang, B. Xu, N.A. Kumar, X.S. Zhao, Encapsulation of NiCo2O4 in nitrogen-doped reduced graphene oxide for sodium ion capacitors, J. Mater. Chem. A 6 (2018) 14146-14154. [27] S. Guo, W. Wang, C.S. Ozkan, M. Ozkan, Assembled graphene oxide and single-walled carbon nanotube ink for stable supercapacitors, J. Mater. Res. 28 (2013) 918-926. [28] H. Fan, W. Shen, Gelatin-based microporous carbon nanosheets as high performance supercapacitor electrodes, ACS Sustainable Chem. Eng. 4 (2016) 1328-1337. [29] Y. Li, N. Yu, P. Yan, Y. Li, X. Zhou, S. Chen, G. Wang, T. Wei, Z. Fan, Fabrication of manganese dioxide nanoplates anchoring on biomass-derived cross-linked carbon nanosheets for high-performance asymmetric supercapacitors, J. Power Sources 300 (2015) 309-317. [30] S. Gao, Y. Sui, F. Wei, J. Qi, Q. Meng, Y. Ren, Y. He, Dandelion-like nickel/cobalt metal-organic framework based electrode materials for high performance supercapacitors, J Colloid Interface Sci, 531 (2018) 83-90. [31] Z. Fan, J. Yan, T. Wei, G. Ning, T. Li, F. Wei, Asymmetric supercapacitors based on graphene/MnO2 and activated Carbon nanofiber electrodes with high power and energy density, Adv. Funct. Mater. 21 (2011) 2366–2375. [32] Q. Li, R. Jiang, Y. Dou, Z. Wu, T. Huang, D. Feng, J. Yang, A. Yu, D. Zhao, Synthesis of mesoporous carbon spheres with a hierarchical pore structure for the electrochemical double-layer capacitor, Carbon 49 (2011) 1248-1257. [33] Z. Ling, Z. Wang, M. Zhang, C. Yu, G. Wang, Y. Dong, S. Liu, Y. Wang, J. Qiu, Sustainable Synthesis and Assembly of Biomass-Derived B/N Co-Doped Carbon Nanosheets with Ultrahigh Aspect Ratio for High-Performance Supercapacitors, Adv. Funct. Mater. 26 (2016) 111-119. [34] D. Wang, F. Li, Z. Chen, G. Lu, H. Cheng, Synthesis and Electrochemical Property of Boron-Doped Mesoporous Carbon in Supercapacitor, Chem. Mater. 20 (2008) 7195–7200. [35] L. Sheng, L. Jiang, T. Wei, Z. Fan, High volumetric energy density asymmetric supercapacitors based on well-balanced graphene and graphene-MnO2 electrodes with densely stacked architectures, Small 12 (2016) 5217-5227. [36] H. Fang, S. Zhang, T. Jiang, R. Lin, Y. Lin, One-step synthesis of Ni/Ni(OH)2@multiwalled carbon nanotube coaxial nanocable film For high performance supercapacitors, Electrochimica Acta 125 (2014) 427-434.

25

[37] W. Zhou, X. Cao, Z. Zeng, W. Shi, Y. Zhu, Q. Yan, H. Liu, J. Wang, H. Zhang, One-step synthesis of Ni3S2 nanorod@Ni(OH)2 nanosheet core–shell nanostructures on a three-dimensional graphene network for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 2216-2221. [38] C. Long, D. Qi, T. Wei, J. Yan, L. Jiang, Z. Fan, Nitrogen-doped carbon networks for high energy density supercapacitors derived from polyaniline coated bacterial cellulose, Adv. Funct. Mater. 24 (2014) 3953–3961. [39] F. Huang, Y. Sui, F. Wei, J. Qi, Q. Meng, Y. He, Ni 3S4 supported on carbon cloth for high-performance flexible all-solid-state asymmetric supercapacitors, J. Mater. Sci.-Mater. El 29 (2017) 2525-2536. [40] C. Tang, Z. Tang, H. Gong, Hierarchically porous Ni-Co oxide for high reversibility asymmetric full-cell supercapacitors, J. Electrochem. Soc. 159 (2012) A651-A656. [41] Y. Xiao, Y. Cao, Y. Gong, A. Zhang, J. Zhao, S. Fang, D. Jia, F. Li, Electrolyte and composition effects on the performances of asymmetric supercapacitors constructed with Mn3O4 nanoparticles–graphene nanocomposites, J. Power Sources 246 (2014) 926-933. [42] J. Wen, S. Li, K. Zhou, Z. Song, B. Li, Z. Chen, T. Chen, Y. Guo, G. Fang, Flexible coaxial-type fiber solid-state asymmetrical supercapacitor based on Ni3S2 nanorod array and pen ink electrodes, J. Power Sources 324 (2016) 325-333. [43] L. Kang, S. Sun, L. Kong, J. Lang, Y. Luo, Investigating metal-organic framework as a new pseudo-capacitive material for supercapacitors, Chinese Chem. Lett. 25 (2014) 957-961. [44] Z. Li, X. Yu, A. Gu, H. Tang, L. Wang, Z. Lou, Anion exchange strategy to synthesis of porous NiS hexagonal nanoplates for supercapacitors, Nanotechnology 28 (2017) 065406. [45] B. Saravanakumar, K.K. Purushothaman, G. Muralidharan, Fabrication of two-dimensional reduced graphene oxide supported V2O5 networks and their application in supercapacitors, Mater. Chem. Phys. 170 (2016) 266-275.

26

The samples were prepared and the paper was written by Dr. Jing Zhao in Harbin Engineering University. The tensile stress-strain characterization was provided by Mr. Junwei Gong in Harbin Engineering University. Thanks to Dr. Guiling Wang, Mr. Chunliang Zhou, Mr. Chenxu Miao, Ms. Rong Hu, Dr. Kai Zhu, Dr. Kui Cheng, Dr. Ke Ye, Dr. Jun Yan and Dr. Dianxue Cao for their assistance with the discussion and revision of this paper in Harbin Engineering University. Thanks to Dr. Xianfa Zhang with the help of XPS analyses of the samples in Heilongjiang University.

27

Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Highlights 1. The human hair with high elasticity capable can serve as a promising flexible supporter for the electrode. 2. The reduced graphene oxide nanosheets can effectively improve the electrical conductivity of the electrode. 3. The Ni(OH)2 nanoribbon-intercalated rGO nanosheets shows an interconnected porous structure. 4. The solid-state flexible supercapacitor delivers high energy density and good cycling stability.

28