High-Performance Microsupercapacitors Based on Bioinspired

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Energy, Environmental, and Catalysis Applications

High-Performance Microsupercapacitors based on Bio-Inspired Graphene Microfibers Hui Pan, Da-Wei Wang, Qingfa Peng, Jun Ma, Xin Meng, Yaopeng Zhang, Yuning Ma, Shenmin Zhu, and Di Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01128 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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ACS Applied Materials & Interfaces

High-Performance Microsupercapacitors based on Bio-Inspired Graphene Microfibers Hui Pan,† Dawei Wang,§Qingfa Peng,‡ Jun Ma,ǁ Xin Meng,† Yaopeng Zhang,‡ Yuning Ma,┴ Shenmin Zhu†* and Di Zhang† †

State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering,

Shanghai Jiao Tong University, Shanghai 200240, China §

School of Chemical Engineering, UNSW Australia, UNSW Sydney, NSW 2052, Australia.

‡

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials

Science and Engineering, Donghua University, Shanghai 201620, China ǁ

School of Engineering, University of South Australia, Mawson Lakes SA 5095, Australia

┴

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,

China

Keywords: Biomimetric fabrication, Microfiber, Flexible, On-chip, Microsupercapacitor

ABSTRACT: The miniaturization of portable electronic devices has fueled the development of microsupercapacitors that hold great potential to complement or even to replace microbatteries and electrolytic capacitors. In spite of recent development taking advantage of printing and lithography, it remains a great challenge to attain high energy density without sacrificing power density. Herein, a new protocol mimicking the spider’s spinning process is developed to create highly oriented microfibers from graphene-based composite via a purpose-designed microfluidic chip. The orientation provides the microfibers with electrical conductivity ~3×104 S m-1, which leads to high power density; the energy 1

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density is sustained by nanocarbon and high-purity metallic molybdenum disulfide. The microfibers are patterned in-plane to fabricate asymmetric microsupercapacitors for flexible and on-chip energy storage. The on-chip microsupercapacitor with a high pattern resolution of 100 micrometer, delivers energy density up to the order of 10-2 Wh cm-3 and retains ultra-high power density exceeding 100 W cm-3 in aqueous electrolyte. This work provides new design of flexible and on-chip asymmetric microsupercapacitors based on microfibers. The unique biomimetic microfluidic fabrication of graphene microfibers for energy storage may also stimulate thinking of the bionic design in many other fields. INTRODUCTION The rapid development of mobile electronics has greatly stimulated the continuous miniaturization of energy storage devices. Supercapacitors, an alternative to batteries, has attracted great interests in the miniaturization and their integration on chips or flexible substrates for their distinct advantages — long lifetime and high power density.1-3 Consequently a broad range of techniques spanning from materials engineering to microdevice fabrication have been developed to improve microsupercapacitors.4-8 In spite of these respectable progress, challenges remain in the miniaturization of microsupercapacitors, and in general, it requires the device to attain high energy density without sacrificing power density and lifetime.9 To achieve a high device performance of microsupercapacitor, several tasks should be done: (i) to select active materials with high energy density; (ii) to increase the electrical conductivity and ion transfer rate to maintain power density; (iii) to assemble the device with high pattern resolution, consuming less current collector because it is included in calculation of device performance; (iv) to design an asymmetric configuration to enlarge the working potential.

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Figure 1. (a) Sandwich configuration. (b-d) Planar configuration. (b) The active material and current collector are simultaneously patterned. (c) Only the active material is patterned and it also acted as electrical transfer media. (d) The microfibers are arranged in parallel. The earliest microsupercapacitor inspired from microbattery is in sandwich configuration (Figure 1a), suffering from low specific capacitance and low power due to the poor ionic conductivity and extensive use of current collector.1 The subsequently developed planar configuration (Figure 1b) by patterning the active material in interdigitated fingers on insulating substrates, effectively improved the ion diffusion rate.10-12 However, if less current collector is wanted (Figure 1c), the electrical transport along the patterned fingers should be effectively improved as the active material should be acting simultaneously as electron transfer media in such situation.3, 13 High-conductive microfiber electrodes with high energy density may be competent when they are patterned in parallel on the substrate (Figure 3



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1d). In this design, the ion diffusion path is perpendicular to the fiber’s axis while the electrons transfer along the fiber’s axis. Consequently, the ion diffusion rate and electrical conductivity are both promoted. More importantly, it seems much more accessible using two kinds of microfibers to achieve an asymmetric layout, which can enlarge the potential window of the device and finally increase the device performance.14 Nature provides a great imitation for scientific research to design and fabricate new materials with extraordinary performance.15-17 For example, spiders produce silky web with high tensile strength and ductility.18 The secret of spiders’ success originates from their benign liquid crystal spinning process to produce highly oriented fibers.19-20 Therefore in this work, a microfluidic chip mimicking the spider’s spinneret was developed to spin highly oriented graphene-based microfibers. The high degree of orientation of the microfibers generates high electrical conductivity of ~3×104 S m-1, which contributes to retaining power density although the fibers contain either carbon or MoS2 to improve energy density. The micro-size of the fibers in combination with the high conductivity, enable us to assemble asymmetric microsupercapacitor having a high pattern resolution of 100 µm and using less current collector. Our on-chip microsupercapacitor delivers exceptional device performance, an energy density of up to the order of 10-2 Wh cm-3 and a power density of 1.01×102 W cm-3 in aqueous electrolyte. EXPERIMENTAL SECTION Materials preparation Graphene oxide (GO) and cellulose nanocrystals (CNCs) were prepared according to references.2123

1T phase MoS2 was prepared by a sonication exfoliation method. MoS2 was dispersed in a mixture of

isopropanol and water (1:1 vol%) and sonicated for 4 h, followed by centrifugation at 10000 r min-1. Whilst the precipitate was removed, the upper suspension was further sonicated interruptedly, with the

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total sonication time of over 48 h. This produced upper clear grey suspension containing 0.1 wt% 1T MoS2 for the following applications, which was found to be stable for at least three months. The suspension were mixed with CNC/GO at a mass ratio of 1:9 and MoS2/GO at 1:19. Cross-linked polyacrylate hydrogel was employed to concentrate the composite colloids at low concentration. The obtained suspension was further concentrated by centrifugation to create highly concentrated spinning dopes (~3 wt%) for the following application. A polydimethylsiloxane (PDMS) replica of a channel was built by rapid prototyping of PDMS and SU-8 2050 negative photoresist. The PDMS replica was then sealed to a flat PDMS whose surfaces had been treated by oxygen plasma. A micro-injection pump was used to inject the spinning dope into the microfluidic channel, followed by extrusion into a coagulation bath containing aqueous solution of isopropanol (80 vol%), ethanol (15 vol%) and 0.1 M Cu (NO3)2 (5 vol%). The as-spun microfibers were immersed in deionized water for 2 h to remove the metal ions. The microfibers were reduced by fumigating with 50% hydrazine hydrate at 120 °C for 1 h and were further carbonized in N2 at 450 °C for 2 h. Devices assembly Polyvinyl alcohol (PVA)-H3PO4 was used to prepare all-solid-state supercapacitors. 1 g of PVA powder was dissolved in 10 mL deionized water by vigorous stirring at 90 °C. 1 mL of H3PO4 was dropped into PVA solution to obtain gel electrolyte. Two fibers were fixed on a Kapton substrate in parallel. Copper wires were connected with the fibers by silver paste. After the silver paste was solidified (60 °C for 2 h), the fibers were covered with the gel electrolyte, followed by solidification overnight. On a rectangular Kapton substrate (1×2.5 cm), 10 rGO-C and 10 rGO-MoS2 fibers with length of 8 mm were alternatively fixed in parallel and perpendicular to the long side of the substrate. The

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interspace of the fibers was set as 2.5 mm. Silver paste was used to connect the fibers with a copper tape. Then PVA-H3PO4 gel electrolyte was covered on the fibers. Characterization Polarizing microscopy (VHM-3201 OLYMPUS) was used to study the structure of liquid crystals. The liquid crystals were further investigated by synchrotron radiation small-angle X-ray scattering (SAXS) on BL16B1 beamline with a wavelength of 0.124 nm and sample-to-detector distance of 2500 mm at Shanghai Synchrotron Radiation Facility (SSRF). The morphologies of fibers were studied by a scanning electron microscopy (SEM, JSM-6360LV JEOL Inc.) and a transmission electron microscopy (TEM, JEM-2010HT JEOL Inc.). The phases of the pure materials and their composites were determined by a on a Rigaku D/max2550VL/PC system. The orientation of the composite fibers were further analyzed by wide-angle X-ray diffraction (WAXD) with sample-to-detector distance of 83.425 mm at BL16B1. Cyclic voltammetry (CV) tests and EIS measurements were performed on an electrochemical workstation (CHI660D). EIS measurements were recorded over a frequency range of 1 to 100 kHz with an amplitude of 5 mV at the open-circuit potential. Galvanostatic charge-discharge tests were performed using a LAND (LANHE Inc.) battery testing system. The measurements of the devices were performed at room temperature. The size information of the fibers were measured by a thickness tester (AICE Inc.), in combination of SEM. RESULTS AND DISCUSSION Biomimetic fabrication of graphene-based microfibers Of all the silks that a spider can produce, the dragline silk from major ampullate gland is mostly studied for its excellent mechanical properties.24 The gland (Figure 2a) consists of a long, winding and narrow tail (Segment 1) and a wider and shorter ampulla or sac (Segment 2) for the secretion and 6


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physiological regulation of spidroin into liquid crystal. The ampulla transports the liquid crystal into a truncated conical funnel (Segment 3) that marks the start of a highly elongated, progressively narrowing S-shaped duct (Segment 4). The moderate reduction in the duct diameter should generate a slow elongational flow rate that prevents the premature crystallization of the feedstock (secretion), before it reaching the high-shearing drawdown taper. Generally, spiders use internal shearing of liquid crystal dope via sophisticated apparatuses to produce silk threads with high toughness.20, 25

Figure 2. (a) Major ampullate gland of a Nephila clavipes with its winding tail (Segment 1), central ampulla (Segment 2), conical funnel (Segment 3) and S-shaped duct (Segment 4). The initial (Wi) and terminal (Wo) inner diameters of the funnel are ~350 µm and 40 µm while the elongational Segment 4 is 18 mm. (b) The biomimetic microfluidic chip (MC) with an initial width (Wi') of 2 mm, terminal (Wo') of 92 µm and a elongational Segment 4' of 20 mm. (c-f) Liquid crystals formed in the MC channel under shearing: (c,d) POM and (e,f) SAXS patterns. (g) Schematic spinning process via a MC starting from the 7



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high-viscosity spinning dopes. (h) Ribbon-like microfibers with a width of 30 µm and a thickness of 4 µm. Herein, we designed a new microfluidic device (Figure 2b, Figure S1), including an injection pipe (Segment 1'), a cushion chamber (Segment 2'), a hyperbolic-type funnel (Segment 3') and a linear shearing channel (Segment 4') to simulate the liquid spinning process of spiders. Compared to previous works mimicking animals’ spinning to fabricate artificial silks, the device in this work achieved channel size down to ~92 µm. Due to the small size of the microchannel, our as-prepared fibers are also extremely thin down to 1 µm (Figure S2), which is thinnest among those reported graphene fibers. The small size of the fibers enables us to realize the purpose-designed new microsupercapacitor. Colloidal liquid crystals consisting of GO and CNCs (Figure 2c,d, and Figure S3) were prepared by a single-particle-level mixing method;26-27 GO-MoS2 was similarly made by replacing CNC with 1T MoS2 (Figure 2e, f, and Figure S3). The colloidal liquid crystals with extremely high viscosity were pumped into the microchannel, and turned into micro-scale fibers at the outlet (Figure 2g). An isopropanol based solution was used to accelerate the fiber formation. The GO in as-prepared fibers (Figure S2) were further reduced to reduced graphene oxide (rGO) by fumigating in hydrazine atmosphere at 120 °C, followed by carbonization at 450 °C which converted CNC into carbon. Finally, highly conductive rGO-carbonized CNC (denoted as rGO-C for anode) and rGO-MoS2 (for cathode) microfibers were obtained (Figure 2h). As a comparison, the conventional wet-spinning method was conducted by using a spinneret with an inner diameter of 0.5 mm and L/D ratio of 22 to fabricate rGO-C fibers. The incorporation of nanocarbon and MoS2 nanosheets can prevent rGO sheets from restacking; on the other hand, the nano-additives would significantly increase the specific capacitance as well as energy density. Furthermore, the two kinds of rGO based composite ultrafine fibers can be assembled

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into an asymmetric capacitor — rGO-C as anode and rGO-MoS2 as cathode; the asymmetric configuration enlarges the operating potential window which finally increase the energy density. In this work, we developed a new method to fabricate high-quality 1T phase MoS2 by using a unique exfoliation agent composed water and isopropanol with volume ratio of 1/1. Fractional centrifugation and sonication were used to assist exfoliation. Figure 3a contains XRD patterns of the raw materials and their composites. Only (002) peak can be found in 1T MoS2 indicating high purity of the phase. GO only shows a characteristic peak of (001). However, when they are composited, the (001) of GO shifts to smaller angle which demonstrating the MoS2 nanosheets are insertd in the layers of GO homogeneously and enlarge the interlayer space of GO. The XRD of the rGO-MoS2 microfibers was collected from synchrotron radiation X-ray wide angle diffraction (Figure 3b). The rGO-MoS2 microfibers have two groups of obvious equatorial streaks in equatorial direction. The assignment of these streaks was completed by radial integration. The inner streaks (6.15 Å) can be assigned to (002) plane of 1T phase MoS2 while the outer streaks (3.45 Å) is assigned to (001) plane of rGO.28

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Figure 3. (a) XRD patterns of 2H MoS2, 1T MoS2, GO and its composite GO-1T MoS2. (b) Syncrotron radiation WAXD of rGO-1T MoS2. (c) TEM image and (d) EDS mapping of rGO-MoS2 microfiber. Because the dispersion of nanoparticles in matrix is important to the electron transport, TEM was employed to study the morphology of rGO-MoS2 microfiber. In Figure 3c, the MoS2 nanoplates of ~20 nm in lateral dimension are uniformly dispersed on rGO sheets with no aggregation. The inset clearly shows the MoS2 nanoplates well surrounded by the corrugated fringes corresponding to graphene layers, which refers to intimate contact between MoS2 and rGO. The dispersion of nanoparticles is further demonstrated by EDS mapping (Figure 3d). The uniform dispersion of MoS2 nanoplates and their intimate contact with rGO would lead to high electrical conductivity and thus exceptional electrochemical performance.

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Orientation and electric conductivity of graphene-based microfibers

Figure 4. Two dimensional WAXD patterns of (a) rGO-C conventional fibers, (b) rGO-C microfibers and (c) rGO-MoS2 microfibers, respectively. (d) Azimuth patterns integrated from rGO (001) plane. Synchrotron radiation wide-angle X-ray diffraction was employed to analyze the orientation degree of composite fibers. The rGO-C control fibers show an almost completely dispersive diffraction ring (Figure 4a), indicating a low degree of orientation. By contrast, the microfibers exhibit two strong diffraction patterns in the equatorial direction (Figure 4b,c), demonstrating a significantly high orientation. Azimuthal profiles integrated from the rGO (001) plane were used to quantitatively analyze the orientation degree of rGO sheets in composite fibers, as shown in Figure 4d. The orientation degree can be characterized by Herman’s orientation factor frGO.29-30 The rGO-C and rGO-MoS2 microfibers show orientation degrees of 0.58 and 0.66, respectively, being twice the control fibers. This leads to a conclusion that our microfluidic spinning method can produce graphene composite microfibers having an exceptional orientation degree. The cross section of microfiber reveals highly ordered structure under

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SEM (Figure S2), forming a strong contrast with the irregular and disordered structure for the conventional fibers.

Figure 5. (a) Conductivity of control and microfibers. (b) RESR as a function of current density. (c) Complex plane plot of the impedance. (d) Impedance phase angle as a function of frequency. In general, high degree orientation benefits good electrical conductivity of graphene fibers.30-31 The electrical conductivity was measured by voltammetry method (Figure S4) and the conductivity are presented in Figure 5a. The microfibers show ultra-high conductivity (3.12×104 S m-1 for rGO-C and 3.27×104 S m-1 for rGO-MoS2), which is much higher than those of rGO-CNT fibers (1.02×104 S m-1),32 laser-scribed rGO (2.35×103 S m-1),33 and rGO fibers fabricated by conventional method not only in this work but also in literature (4.42×103 S m-1).30 Electrical and ionic conduction directly dominate the power density of the device according to P = E2/(4VRESR), where E is the operating voltage window, V is the stack volume and RESR is the overall volumetric resistance of the device.33 Xin et al. proved that the well aligned sp2 graphene sheets along 12


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the fiber axis are conducive to the electrical transport.31 Chen et al. demonstrated that the electrochemical performance can be improved by increase the orientation of rGO fiber due to the elevated conductivity.30 In this work, to investigate the RESR, the microfibers were patterned in parallel on a Kapton film and then covered by polyvinyl alcohol (PVA)-H3PO4 hydrogel electrolyte to assemble a flexible all-solidstate supercapacitor. The voltage drop at the beginning of discharge curve, known as the iR drop (Vdrop), is usually adopted to estimate the RESR via RESR = Vdrop/2i, where i is the current density. The iR drop was extracted from galvanostatic charge/discharge curve (Figure S5). As shown in Figure 5b, the microfibers have RESR values of ~4×10-3 Ω cm3, which is two orders lower than that (0.3 Ω cm3) of the control fiber. Such a low electronic resistance should highly lift up the powder density of the final devices. Electrochemical impedance spectroscopic (EIS) was employed to further understand the electrical resistance of the microfibers. The intercept at the Zreal axis at high frequency represents the RESR, corresponding to the total resistance of the device, including electrolyte resistance, the internal resistance of the electrode and the contact resistance.34 Figure 5c shows the areal RESR of 2.3 Ω cm2 for rGO-C microfibers and 1.6 Ω cm2 for rGO-MoS2 microfibers, both of which are smaller than that (5.6 Ω cm2) of the control fibers. The RC time constant τ0, indicative of the power of the device, is the minimum time needed to discharge all the energy with an efficiency of more than 50%.10 It is obtained from characteristic frequency f0 at the phase angle of -45° according to τ0 = 1/f0. The dependence of the phase angle on the frequency for microfiber supercapacitors is shown in Figure 5d. The τ0 is only 20 ms for rGO-C microfibers and more interestingly, 14.7 ms for rGO-MoS2 microfibers, in contrast to 27 ms for rGO-C control fibers. The scan rate ability also depends on τ0 — the lower the τ0, the higher the scan rate ability.1 Therefore, the maximum scan rate at which a rectangular shape is still retained reaches up to 30 13



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V s-1 for rGO-MoS2 microfibers, due to its exceptional electrical transfer behavior. It is worth to note that the time constants of the microfibers fabricated by microfluidic chip are comparable to onion-like carbon (26 ms)13 and laser-scribed rGO (20 ms).33 Data from recent state-of-the-art reports, as well as a commercial supercapacitor and aluminum capacitors are included for comparison (Table S1). Electrochemical

performance

of

microsupercapacitors

assembled

from

graphene-based

microfibers A flexible asymmetric microsupercapacitor was assembled as illustrated in Figure 6a. It is composed of 20 microfibers in parallel: 10 rGO-C microfibers as the anode and 10 rGO-MoS2 microfibers as the cathode. The device can be easily bent or twisted showing high flexibility in Figure 6b,c. Furthermore it can be used to light a microwatt LED as shown in Figure 6d. From the CVs of the two kinds of fiber supercapacitors (Figure S5), we know that the rGO-C composite can be operated at 0.5-0 V while the rGO-MoS2 can be operated at 0-0.8 V. Conceivably, the operation potential window of the asymmetric device should be 0-1.3 V. This is further confirmed by the CVs collected using different potential windows shown in Figure 6e. The CVs of the device at different scan rates are shown in Figure 6f, from which we calculated the specific and stack capacitances and plotted it against with scan rate shown in Figure 6g. The asymmetric device can deliver specific capacitance over 121 F cm-3 below 1 V s-1, and it remains at 40 F cm-3 at 10 V s-1. Comparison of the specific volumetric capacitance (Csp) of different materials is presented in Table S2. The cycling performance under bending and twisting is shown in Figure 6h, which indicates high durability. Figure 6h also contains the flexibility measurement by collecting its charge/discharge curves at 40 A cm-3 under various deformation, and this shows no or little effect of mechanical deformation on the device performance.

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Figure 6. (a) Schematic assembly of a flexible asymmetric supercapacitor. (b,c) Photographs of supercapacitor under twisting and bending. (d) The supercapacitor lights a LED under flexible state. (e) CVs collected from different potential windows at scan rate of 2 V s-1. (f) CVs collected from different scan rate at potential windows of 0-1.3 V. (g) Evolution of specific and stack capacitances versus scan rate. (h) Cycling durability under twisting and bending, inset of the charge/discharge curves collected under various deformation at 40 A cm-3. An on-chip microsupercapacitor was assembled on a Si wafer in size of 1 cm2 (Figure 7a). Noteworthy is that our device has pattern resolution of 100 µm (Figure 7b), which is smaller than most reported works.35 A comparison of pattern resolution for different on-chip microsupercapacitors are also presented in Table S3. The cycling performance in Figure 7c reveals 97% of the capacitance remained 15



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after 10,000 cycles, indicating excellent durability. The CV of the device contained in Figure 7c exhibits similar characteristics to the flexible device introduced above. Detailed electrochemical properties of a single device are contained in Figure S6. In order to meet the demand for practical applications, several devices usually are connected together in series and/or parallel combinations, just like batteries. Compared with a single device with an operating voltage of 1.3 V, the two devices connected in series exhibits a 2.6 V charge/discharge operating potential window with similar discharge time (Figure 7d). For two connected in parallel, the output current increases by a factor of two, and the discharge time doubles a single device when operated at the same current density (Figure 7e). Figure 7f shows a Ragone plot comparing the performance of our on-chip microsupercapacitor with different energy-storage systems and other reported state-of-the-art microsupercapacitors using aqueous solution systems. Notably, our device delivers a volumetric energy density up to the order of 10-2 Wh cm-3, being two orders higher than the energy densities of commercially available supercapacitors (2.75 V/44 mF).8 The maximum volumetric power density for our device reaches up to 1.01×102 W cm-3, comparable to the commercially available supercapacitors (3 V/300 µF).28 Moreover, the comprehensive performance of our device made from microfibers are better than existing graphene based systems, rGOCNT,32 LSG,33 and LSG/MnO2.36 Performance per device area is a key metric for the integration of onchip energy storage devices. In Figure S7 containing the area-normalized Ragone plot of various on-chip microsupercapacitors, our device shows the best comprehensive performance with areal energy density of ~100 µWh cm-2 and power density of ~ 102 mWh cm-2.

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Figure 7. (a) Schematic assembly of an on-chip asymmetric microsupercapacitor. (b) SEM of microfibers aligned in parallel on a Si wafer. (c) Cycling performance. Inset the CV curves collected from the first and 200th cycles. (d,e) Galvanostatic charge/discharge curves for two devices connected in in series (d) and parallel (e). (f) Energy and power densities of the asymmetric microdevice compared with commercially available energy-storage systems and other reported microsupercapacitors. AC refers to activated carbon, LSG to laser scribed graphene, rGO-CNT to nitrogen-doped rGO and single-walled carbon nanotubes. Data for the Li battery and aluminum electrolytic capacitor are reproduced from ref. 33.

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CONCLUSIONS In this work, we have developed a biomimetic approach where an artificial microfluidic chip was designed and fabricated to prepare graphene-based composite microfibers for flexible and on-chip microsupercapacitors. The liquid crystal spinning through the chip produced highly orientated microfibers, which markedly increased the microfiber electrical conductivity. Meanwhile either nanocarbon or 1T MoS2 was incorporated into the fibers for promotion of energy density. The high electrical conductivity and small cross section of the fibers enabled us to design and fabricate on-chip microsupercapacitors having high pattern resolution (100 µm) with a less quantity of current collector used. This led to exceptional device performance — energy density up to the order of 10-2 Wh cm-3 and power density to ×102 W cm-3. This work built up a new platform based on microfluidic manufacture of high-performance microfibers for energy storage microsystems. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The size information of microfluidic chip, morphology of as-spun fibers, liquid crystal structures of composite colloids, electrical conductivity of fibers, electrochemical properties of fibers and on-chip microsupercapacitors, and properties comparison for different capacitors. AUTHOR INFORMATION Corresponding Author: [email protected] Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Key R&D Program of China (No. 2016YFA0202900) and NSFC (51672173). We also thank Shanghai Synchrotron Radiation Facility (SSRF) and Instrumental Analysis Center of Shanghai Jiao Tong University (SJTU) for the measurements. The authors greatly appreciate Profs. Deyue Yan and Zhiping Zhou for the discussion on the materials. REFERENCES (1) Kyeremateng, N. A.; Brousse, T.; Pech, D. Microsupercapacitors as miniaturized energy-storage components for on-chip electronics. Nat. Nanotechnol. 2017, 12, 7-15. (2) Wang, D.; Xiao, Y.; Luo X.; Wu Z.; Wang Y.; Fang B. Swollen ammoniated MoS2 with 1T/2H hybrid phases for high-rate electrochemical energy storage. ACS Sustainable Chem. Eng. 2017, 5, 25092515. (3) Gao, W.; Singh, N.; Song, L.; Liu, Z.; Reddy, A. L. M.; Ci, L.; Vajtai, R.; Zhang, Q.; Wei, B.; Ajayan, P. M. Direct laser writing of micro-supercapacitors on hydrated graphite oxide films. Nat. Nanotechnol. 2011, 6, 496-500. (4) Liu, L.; Niu, Z.; Chen, J. Design and integration of flexible planar micro-supercapacitors. Nano Res. 2017, 1-21. (5) Qi, D.; Liu, Y.; Liu, Z.; Zhang, L.; Chen, X. Design of architectures and materials in in-plane microsupercapacitors: Current Status and Future Challenges. Adv. Mater. 2017, 29, 201602802. (6) Wang, K.; Zou, W.; Quan, B.; Yu, A.; Wu, H.; Jiang, P.; Wei, Z. An All-solid-state flexible microsupercapacitor on a Chip. Adv. Energy Mater. 2011, 1, 1068-1072.

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High-Performance Microsupercapacitors Based on Bioinspired

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