Graphene nanopetal wire supercapacitors with high energy density and thermal durability

Author’s Accepted Manuscript Graphene nanopetal wire supercapacitors with high energy density and thermal durability Guoping Xiong, Pingge He, Boyun Huang, Tengfei Chen, Zheng Bo, Timothy S. Fisher www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(17)30329-4 http://dx.doi.org/10.1016/j.nanoen.2017.05.050 NANOEN1991

To appear in: Nano Energy Received date: 6 April 2017 Revised date: 13 May 2017 Accepted date: 25 May 2017 Cite this article as: Guoping Xiong, Pingge He, Boyun Huang, Tengfei Chen, Zheng Bo and Timothy S. Fisher, Graphene nanopetal wire supercapacitors with high energy density and thermal durability, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.05.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Graphene nanopetal wire supercapacitors with high energy density and thermal durability Guoping Xionga,b1, Pingge Hea,b,c1, Boyun Huangc, Tengfei Chenc, Zheng Bod*, Timothy S. Fishera,b* a

Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA,

b

School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA,

c

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China,

d

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

[email protected] [email protected] * Corresponding authors.

Abstract Wire supercapacitors have recently elicited attention due to their potential to be woven into textiles as flexible power supplies for wearable electronic devices. However, contemporary wire supercapacitors generally suffer from low energy density and complicated fabrication and assembly processes. Here, we report a unique design of asymmetric wire supercapacitors in a wrap-twist architecture, with graphene nanopetals grown on carbon fiber tow as negative electrodes and MnO2 nanosheets electrodeposited on carbon nanotube paper as positive electrodes. The wrap-twist structure integrates both positive and negative electrodes with edgeenriched nanostructures, and a new assembly procedure greatly increases the contact area between the two electrodes, leading to significantly improved electrochemical performance. Such asymmetric wire supercapacitors exhibit ultrahigh capacitances of over 40 mF cm-1, 1

These authors contribute equally to this work. 1

outstanding energy and power densities, with energy densities up to 12 µWh cm-1 (approx. 50 µWh cm-2 on an area basis) and power densities up to approx. 3.7 mW cm-1 (approx. 15.4 mW cm-2), good cyclic stability and excellent flexibility. Moreover, the thermal performance of such wire supercapacitors is assessed by characterizing temperature effects on electrochemical performance over a temperature window ranging from 2 to 80 ˚C. The outstanding electrochemical and thermal performance of the wire supercapacitors demonstrates their promising potential as flexible power sources for wearable electronic devices.

Keywords: wire supercapacitor, wrap-twist, graphene nanopetals, edge-enriched nanostructures, high energy, thermal performance.

1. Introduction Wearable energy storage devices are becoming increasingly important to power electronic devices in modern life[1, 2]. Among these, wire supercapacitors represent a promising class of power supplies because of their high rate capability, fast power delivery, long cycle life, and high design versatility and flexible integration into woven fabrics or textiles[1, 3, 4]. Conventional wire supercapacitors are generally assembled by braiding two fiber electrodes together with a separator or solid state electrolyte that limits direct contact area between two electrodes, leading to unfavorable electrochemical performance in practical applications[4, 5]. To resolve the issue, a coaxial-fiber configuration consisting of a core fiber electrode, a separator or solid state electrolyte, and an outer electrode layer was recently proposed to increase the contact area between electrodes and to improve structural stability via layer-by-layer assembly[6]. Despite the 2

reported improvements in electrochemical performance[5, 7, 8], many issues such as complicated fabrication and assembly processes and relatively low energy density remain, hindering the practical feasibility and industrial scalability of these coaxial fiber supercapacitors[1, 4]. Thus, new designs of architectures for wire supercapacitors with high utilization efficiency of electrode materials and high controllability of the fabrication and assembly processes are needed to meet the increasing demand for high-performance wearable electronic devices. Meanwhile, the design of new nanomaterials for electrodes to achieve high energy density and high flexibility in wire supercapacitors has continued to gain attention[9-12]. Carbon fibers, carbon nanotubes, reduced graphene oxide and their related fiber composites have been widely adopted as electrode materials for wire supercapacitors because of their high aspect ratio, high conductivity, structural stability and mechanical flexibility[13-16]. However, the capacitance and energy density of carbon-based wire supercapacitors remain low compared to those of supercapacitors and batteries with conventional two-dimensional configurations. Graphene nanopetals (GPs), containing a few layers of vertically standing graphene nanosheets, with large surface area, high electrical conductivity and sharp edges show great promise as electrodes for electrochemical energy storage devices[17, 18]. In particular, our recent work has demonstrated the importance of edge effects in GPs for supercapacitor applications from both experimental studies and numerical simulations[19]. Moreover, GPs with sharp edges can significantly increase charge storage and facilitate ion transport to the electrode surface, leading to improved supercapacitor performance[20, 21]. Similarly, vertically standing MnO2 nanosheet arrays with sharp edges also exhibit outstanding electrochemical performance[22], and are widely used as pseudocapacitive materials in electrochemical energy storage systems. However, prior all-solid-

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state asymmetric wire supercapacitors have not fully exploited the unique edge effects by utilizing both positive and negative electrodes with edge-enriched nanostructures. Meanwhile, temperature significantly affects the performance of energy storage devices in practical applications, particularly those involving wearables. Issues such as capacitance decay, internal resistance increase, cycle life degradation and thermal runaway triggered by thermal stress from internal heat generation or external circumstances severely limit the application of energy storage devices in the real world[23, 24]. Thus, understanding thermal effects in wire supercapacitors is imperative to achieve high-performance and durable wearable electronic devices. Herein, we report a wrap-twist assembling strategy to fabricate all-solid-state asymmetric wire supercapacitors with ultrahigh energy density and high flexibility. The asymmetric wire supercapacitors consist of edge-enriched GPs grown on carbon fiber tow (CFT) as the negative electrode, vertically standing MnO2 nanosheets electrodeposited on carbon nanotube paper (Buckypaper, BP) as the positive electrode, and two types of polymer gel electrolytes (H3PO4/PVA and KOH/PVA) separately as a bifunctional electrolyte/separator. The wrap-twist configuration was realized by entangling a CFT/GPs negative fiber electrode with a BP/MnO2 positive paper electrode. Furthermore, temperature effects on the electrochemical performance were systematically investigated under an operating temperature window ranging from 2 to 80 ˚C to assess the thermal performance of such asymmetric wire supercapacitors. 2. Experimental section 2.1 Synthesis of GPs on CFT Commercial carbon fiber tow (CFT, HEXCEL Inc., USA) was used directly as substrate without further treatment for GP synthesis by MPCVD. CFT, with a length of 5 cm, elevated 15

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mm above a 55-mm-diameter Mo puck by ceramic spacers, was subjected to the MPCVD system (the schematic diagram of the MPCVD chamber is shown in Fig. S1) with a condition of H2 (50 sccm) and CH4 (10 sccm) as the primary feed gases at 30 Torr total pressure. The GP growth duration was 30 mins and the plasma power is 600 W during the growth process. This plasma is sufficient to heat the samples from room temperature up to ~1100 °C, as measured by a dualwavelength pyrometer (Williamson PRO 92). 2.2 Electrodeposition of MnO2 on Buckypaper Commercial Buckypaper (BP, Nano Tech Lab, Inc., USA), was used as substrate to electrodeposit MnO2. The electrodeposition process was conducted in a three-electrode configuration consisting of BP as the working electrode, Pt mesh as the counter electrode and Ag/AgCl as the reference electrode. The MnO2 was electrodeposited on BP at a constant potential of -1 V vs. Ag/AgCl in an aqueous solution containing 0.1 M MnSO4 and 0.1 M Na2SO4 at ambient temperature. The electrodeposition duration was 90 s. 2.3 Fabrication of wrap-twist asymmetric wire supercapacitors To demonstrate the versatility of wrap-twist asymmetric wire supercapacitors, two types of solid state polymers including KOH/PVA and H3PO4/PVA were separately used as a bifunctional electrolyte/separator. The fabrication process of a typical asymmetric wire supercapacitor based on KOH/PVA electrolyte is as follows: a BP/MnO2 paper electrode and a CFT/GPs fiber electrode were both immersed in KOH/PVA electrolyte (preparation details of the electrolyte are provided in Supporting Information) for a few minutes to be fully soaked and penetrated by dilute gel solution. The electrodes with the electrolyte solution coating on were placed in a fume hood at room temperature to evaporate the excess water. After the gel was

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solidified at room temperature, the CFT/GPs surface was further coated with a thin layer of gel solution to increase the surface viscosity. Subsequently, the CFT/GPs fiber electrode was wraptwisted with the BP/MnO2 paper electrode. The wrap-twist asymmetric wire supercapacitor was achieved after the gel electrolyte was solidified. The asymmetric wire supercapacitors based on H3PO4/PVA electrolyte were fabricated in a similar process using H3PO4/PVA as electrolyte. 2.4 Material Characterization. The surface morphology of electrodes was characterized by field emission scanning electron microscope (SEM, Hitachi S-4800) operated at 5 kV. Raman characterization was performed with LabRAM HR spectrometer (HORIBA Scientific) with a fixed laser excitation wavelength of 532 nm, power of 5 mW, spot size of approx. 1 μm, and magnification of 50×. 2.5 Electrochemical measurements The electrochemical performance was tested using a Gamry Echem Testing System, Gamry Instruments, Inc., USA. Electrochemical characterizations of electrodes were conducted in a three-electrode configuration at room temperature using 1 M KOH as aqueous electrolyte. The BP/MnO2 and CFT/GPs electrodes served directly as working electrodes, separately. Pt mesh and SCE were used as counter electrode and reference electrode, respectively. The wrap-twist asymmetric wire supercapacitor devices were electrochemically characterized in a two-electrode configuration. EIS measurements were carried out with an alternating current perturbation amplitude of 5 mV in the frequency ranging from 1 MHz to 0.1 Hz. The methods to calculate length-based and areal capacitances, energy and power densities are provided in Supporting Information. 3. Results and discussion 6

The fabrication process of the asymmetric wire supercapacitors in a wrap-twist configuration is schematically illustrated in Fig. 1a, with detailed fabrication procedures described in the Methods section. Briefly, GPs were grown on CFT as a negative fiber electrode by microwave plasma chemical vapor deposition (MPCVD). Pure carbon microfibers in CFT exhibit a relatively smooth surface (see Fig. 1b) and possess an average diameter of approx. 7 µm (see Fig. 1c). After GP growth, the carbon fiber surface is coated with a uniform, thin layer of GPs, making it appear fluffy (see Fig. 1d). A close-up of the GP-coated carbon microfibers in Fig. 1e shows that GPs with sharp edges uniformly and densely decorate the carbon fiber surface, and the thickness of a typical GP layer is approx. 500 nm (see Fig. S2). A high-magnification SEM image in Fig. 1e inset clearly shows thin and sharp edges of GPs, and the span of individual petal is in a range of several hundred nanometers. Meanwhile, MnO2 electrodeposited on BP was used as a positive paper electrode. Commercial BP is a flat film composed of randomly interconnected carbon nanotubes (see Fig. 1f and 1g), providing a lightweight, flexible, and mechanically robust substrate for MnO2 deposition.[25] After an electrodeposition duration of 90 seconds, vertically standing MnO2 nanosheets uniformly cover the surface of individual carbon nanotubes within the BP (see Fig. 1h and Fig. 1i). As shown in the high-magnification SEM inset of Fig. 1i, such MnO2 nanosheets with sharp edges exhibit a lateral size of tens of nanometers. Raman spectra of BP/MnO2 provided in Fig. S3 further confirm the existence of MnO2 deposited on the BP surface.

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Fig. 1. (a) Schematic of the wrap-twist asymmetric wire supercapacitor fabrication process. SEM images showing the electrode morphologies: (b) Bare CFT at low magnification. (c) Carbon microfibers with smooth surface. (d) Large-scale uniform coverage of GPs on CFT. (e) GPs grown on carbon fibers at high magnification (the inset displays GPs with sharp edges). (f) Bare BP at low magnification. (g) Randomly interconnected CNTs within BP. (h) MnO2 electrodeposited on BP at low magnification. (i) Vertically standing MnO2 nanosheets coated on CNTs at high magnification (the inset shows a close-up of a single CNT decorated with MnO2 nanosheets).

For asymmetric supercapacitors, charge balance between positive and negative electrodes is crucial to achieve comprehensively favorable electrochemical performance. Therefore, before assembly, the electrochemical properties of both positive and negative electrodes were measured in a three-electrode configuration using BP/MnO2 and CFT/GPs electrodes as working electrodes separately, 1 M KOH as the aqueous electrolyte, SCE as the reference electrode, and Pt mash as the counter electrode. Fig. 2a shows cyclic voltammetry (CV) curves of the BP/MnO2 electrode measured at scan rates from 5 to 100 mV s-1 in a voltage window from 0 to 0.5 V vs. SCE. The

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CV curves display a deviation from a perfect rectangular shape that could be associated with possible reactions between MnO2 and KOH electrolyte[26]. The BP/MnO2 electrode exhibits electrochemical stability in the potential window from 0 to 0.5 V vs. SCE. Moreover, galvanostatic charge/discharge curves of a typical BP/MnO2 electrode at different current densities are provided in Fig. S4a. CV profiles of a typical CFT/GPs electrode tested at scan rates from 5 to 100 mV s-1 in a voltage window from -1 to 0 V vs. SCE are shown in Fig. 2b. Because carbon materials are generally hydrophobic in nature, we employed an electrochemical activation treatment[27] to improve the electrochemical activity and surface hydrophilicity of CFT/GPs electrodes. The details of the activation process are provided in Fig. S5. The CFT/GPs electrodes present relatively more rectangular CV shapes, indicating predominantly capacitive behavior and low internal resistance. The CFT/GPs electrode exhibits an electrochemically stable potential window over a range from -1 to 0 V vs. SCE. Moreover, galvanostatic charge/discharge curves of a typical CFT/GPs electrode at different current densities are provided in Fig. S4b. Capacitances of the CFT/GPs and BP/MnO2 electrodes can be calculated from CV loops at different scan rates (detailed calculations are provided in Supporting Information), and corresponding capacitance retentions of these two electrodes are plotted as a function of scan rate in Fig. 2c. When the scan rate increases from 5 to 100 mV s-1, the capacitances of both electrodes display a gradual attenuation, with a capacitance retention of 76% for CFT/GPs electrode and 63% for BP/MnO2 electrode, indicating good rate capabilities for both electrodes. To balance charge between the positive and negative electrodes, comparative CV curves of BP/MnO2 and CFT/GPs electrodes tested at 20 mV s-1 are plotted in Fig. 2d. The current

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densities in CV areas are determined based on the contact area between cathode and anode. The operating voltage for the asymmetric wire supercapacitor is expected to reach 1.5 V, because the total device voltage can be expressed as the sum of the potential ranges of positive (BP/MnO2) and negative (CFT/GPs) electrodes. The charge balance between two electrodes follows the relationship Q+ = Q-, in which the stored charge by each electrode is determined by the areal capacitance (Ca), the operating voltage window (V), and the surface area of each electrode (A) according to: Q=Ca×A×V[28, 29]. Integrating the area of the CV curve loop in Fig. 2d, (Ca+×V+): (Ca-×V-)= 2:3. Thus, the optimal areal ratio of BP/MnO2 electrode (A+) to CFT/GPs electrode (A-) is 3:2. When designing the wire supercapacitors, we chose a BP/MnO2 electrode: CFT/GPs electrode area ratio of 3:2 (corresponding to a length-based ratio of 16:9) to optimize the electrochemical performance for asymmetric wire supercapacitors. Long-term cycle life tests of BP/MnO2 and CFT/GPs electrodes over 5000 cycles were conducted using galvanostatic charge/discharge cycling, and the capacitance retentions of the BP/MnO2 and CFT/GPs electrodes as a function of charge/discharge cycle numbers are shown in Fig. S4c and Fig. S4d, respectively, indicating high cyclic stabilities and high charge transfer efficiencies of these electrodes over long-term cycling. The internal resistance of electrode materials is also a critical factor in practical applications. The electrochemical impedance spectra (EIS) of BP/MnO2 and CFT/GPs electrodes recorded from 0.1 Hz to 1 MHz with an alternating current (AC) perturbation amplitude of 5 mV are provided in Fig. S6 and Fig. S7. Measured from the impedance spectra, the equivalent series resistances (Re) of BP/MnO2 and CFT/GPs electrodes are 4.9 Ω cm-1 and 3.45 Ω cm-1, respectively, indicating the relatively low internal resistance of the electrodes.

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Fig. 2. Electrochemical characterization of electrodes in 1 M KOH aqueous solution in a three-electrode configuration. (a) CV curves of the BP/MnO2 electrode measured at scan rates from 5 to 100 mV s -1 in a voltage window from 0 to 0.5 V vs. SCE. (b) CV curves of the CFT/GPs electrode measured at scan rates from 5 to 100 mV s-1 in a voltage window from -1 to 0 V vs. SCE. (c) The relationship between capacitance retention and scan rate for the CFT/GPs and BP/MnO2 electrodes. (d) Comparative CV curves of BP/MnO2 and CFT/GPs electrodes measured at a scan rate of 20 mV s-1.

All-solid-state asymmetric wire supercapacitors using BP/MnO2 as the positive electrode, CFT/GPs as the negative electrode, and KOH/PVA as the bifunctional electrolyte/separator were fabricated, and their electrochemical performance was systematically characterized. Fig. 3a displays CV curves of a typical asymmetric wire supercapacitor at scan rates from 5 to 100 mV s−1 with a voltage window from 0 to 1.5 V. Mild redox peaks observed in CV curves are attributed to the pseudocapacitive behavior of the wire supercapacitor resulting from the redox reactions between MnO2 and KOH[26, 30].

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Fig. 3b provides galvanostatic charge/discharge profiles of the asymmetric device at different current densities ranging from 0.4 to 1.4 mA cm-1 (profiles at higher current densities are provided in Fig. S8). For wire-shaped supercapacitors, length-based and areal capacitances are more suitable as criteria to evaluate electrochemical performance for practical applications. Based on the galvanostatic charge/discharge curves in Fig. 3b, the wire supercapacitor exhibits a high length-based capacitance, reaching up to 40 mF cm-1 (corresponding to an areal capacitance of 163.5 mF cm-2) at a current density of 0.4 mA cm-1. These metrics are significantly higher than those of contemporary wire supercapacitor devices (see Table S1)[5, 13, 30-36]. The observed ultrahigh capacitance is attributed to increased contact area between two electrodes assembled in the unique wrap-twist architecture and to the edge-enriched nanostructures in the electrode materials. Capacitances normalized by the length and area of the wire supercapacitor are plotted as a function of current density, as shown in Fig. 3c (detailed calculation methods are provided in the Supporting Information). As the current density increases from 0.4 to 2 mA cm-1, capacitance (per unit length and area) of the wire supercapacitor displays a gradual attenuation from 40 mF cm-1 (corresponding to 163.5 mF cm-2) at a current density of 0.4 mA cm-1 to 29.5 mF cm-1 (corresponding to 122.5 mF cm-2) at a current density of 2 mA cm-1, with a corresponding capacitance retention of 75% (see Fig. S9) indicating a high rate capability. Nyquist plots for the asymmetric wire supercapacitor recorded from 0.1 Hz to 1 MHz shown in Fig. S10 can be fitted by an equivalent circuit consisting of a bulk electrolyte resistance, a charge transfer resistance, a pseudocapacitive element from redox reactions of the electrode materials an d electrolyte, and a constant phase element to represent the double-layer

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capacitance[37]. Measured from the impedance spectrum, the real-axis intercept of the impedance spectrum at high frequencies corresponding to the bulk electrolyte resistance Re is 11 Ω cm-1. The semicircle in the high frequency region corresponds to a charge transfer resistance Rct is 0.3 Ω cm-1, indicating a low charge transfer resistance of the present wire supercapacitor.

Fig. 3. The electrochemical performance of a typical two-terminal asymmetric wire supercapacitor based on KOH/PVA electrolyte. (a) CV profiles at different scan rates of 5, 10, 20, 50, 100 mV s -1 in a voltage window from 0 to 1.5 V. (b) Galvanostatic charge/discharge curves at different constant current densities from 0.4 to 1.4 mA cm-1 in a voltage range between 0 and 1.5 V. (c) Length- and area-based capacitances of the asymmetric wire supercapacitor as a function of discharge current densities calculated from galvanostatic charge/discharge curves. (d) Cyclic stability test at a current density of 3 mA cm-1 from 0 to 1.5 V and corresponding coulombic efficiencies over 5,000 cycles.

Stable long-term cycle life of supercapacitors is an important requirement for practical applications. Fig. 3d shows the cyclic stability of a typical asymmetric wire supercapacitor using galvanostatic charge/discharge over 5000 cycles at a current density of 3 mA cm−1. The capacitance of the wire supercapacitor increases slightly during the first 1000 cycle and then

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undergoes a gradual attenuation until the 5000th cycle, ending with 90% retention as compared to the first cycle. This initial gradual increase of capacitance can be hypothetically explained by the structural activation [38-40]. Moreover, coulombic efficiencies of the wire supercapacitor are over 92% during 5000 cycles, indicating high charge transfer efficiencies over the long-term cycling test. To demonstrate the versatility of as-prepared wrap-twist wire power supplies, similar asymmetric wire supercapacitors with H3PO4/PVA as the bifunctional electrolyte/separator were prepared. The electrochemical performance of such asymmetric wire supercapacitors based on H3PO4/PVA electrolyte is provided in Fig. 4. Fig. 4a shows typical CV profiles at scan rates from 5 to 100 mV s-1 in a potential window from 0 to 1.5 V. Two obvious redox peaks appear in the CV curve, associated with the redox reactions between MnO2 and H3PO4 electrolyte[41, 42]. Fig. 4b provides galvanostatic charge/discharge curves of the asymmetric wire supercapacitors at different current densities ranging from 0.4 to 1.4 mA cm-1 (profiles at higher current densities are shown in Fig. S11). Voltage plateaus are observed in the charge/discharge curves, consistent with the potentials of the redox peaks in the CV curves. Based on the galvanostatic charge/discharge profiles, the length-based capacitance of the wire supercapacitor reaches up to 37.7 mF cm-1 (corresponding to an areal capacitance of 156.4 mF cm-2) at a current density of 0.4 mA cm-1, comparable to that of wire supercapacitors based on KOH/PVA electrolyte and substantially higher than those of the state-of-the-art wire supercapacitors (see Table S1)[5, 13, 30-34, 43]. Capacitances normalized by the length and area of the wire supercapacitor are plotted as functions of discharge current density in Fig. 4c. As current density increases from 0.4 to 2 mA

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cm-1, capacitances (per unit length and area) of the wire supercapacitor display a slight decrease from 37.7 mF cm-1 (corresponding to156.4 mF cm-2) at a current density of 0.4 mA cm-1 to 31.8 mF cm-1 (corresponding to 132.1 mF cm-2) at a current density of 2 mA cm-1, with a capacitance retention of 84.5% (see Fig. S12). Such a high rate capability is comparable to or higher than those of reported wire supercapacitors [5, 44, 45]. Nyquist plots for the asymmetric wire supercapacitor recorded from 0.1 Hz to 1 MHz are provided in Fig. S13. The bulk electrolyte resistance Re value is approx. 8 Ω cm-1, and the charge transfer resistance Rct is 0.5 Ω cm-1. Long-term cycle life tests over 5000 cycles at a current density of 3 mA cm−1 were conducted using galvanostatic charge/discharge cycling in a potential window of 0 to 1.5 V. Fig. 4d shows the capacitance retention of the wire supercapacitor as a function of charge/discharge cycle number. The capacitance undergoes a slight increase before a gradual attenuation during 5000 cycles, similar to that of wire supercapacitors based on KOH/PVA electrolyte, with an overall capacitance retention of approx. 102% compared to the first cycle (97% compared to the maximum peak), which is significantly better than the state-of-the-art wire supercapacitors reported in prior work[5, 30, 35, 36]. Moreover, the calculated coulombic efficiencies of the wire supercapacitor are higher than 93% during all 5000 cycles, indicating high charge transfer efficiencies during the cyclic stability tests. The ultrahigh capacitances, high rate capability and excellent cyclic stability of wire supercapacitors using both KOH/PVA and H3PO4/PVA as a bifunctional electrolyte/separator further confirm the structural and performance superiority of the wrap-twist asymmetric wire supercapacitors, indicating great promise for practical wearable electronic device applications.

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Fig. 4. The electrochemical performance of a typical two-terminal asymmetric wire supercapacitor based on H3PO4/PVA electrolyte. (a) CV profiles measured at scan rates of 5, 10, 20, 50, 100 mV s -1 in a voltage window from 0 to 1.5 V. (b) Galvanostatic charge/discharge curves at different constant current densities from 0.4 to 1.4 mA cm-1 in a voltage range between 0 and 1.5 V. (c) Length-based and areal capacitances of the wire supercapacitor as a function of discharge current densities calculated from galvanostatic charge/discharge curves. (d) Cyclic stability test at a current density of 3 mA cm-1 from 0 to 1.5 V and corresponding coulombic efficiencies over 5,000 cycles.

For wearable electronic device applications, energy storage devices are generally required to possess a high degree of mechanical flexibility. As shown in Fig. 5a, a single wrap-twist asymmetric wire supercapacitor can lift objects with weights above 400 g, demonstrating outstanding mechanical robustness. Moreover, the wire supercapacitor can be easily wrapped around a tube (see Fig. 5b), confirming its high flexibility. To further demonstrate the structural flexibility and stability of the wrap-twist wire supercapacitors, CV curves of the wire supercapacitor based on H3PO4/PVA electrolyte were recorded at different applied bending angles (α) of 45˚, 90˚, 135˚ and 180˚ (the flexibility of the

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wire supercapacitors based on KOH/PVA electrolyte is demonstrated and shown in Fig. S14). As shown in Fig. 5c (inset), under various bending states, CV curves show little change as compared to the original state (without bending), even when the device is bent up to 180˚. After releasing the bending force, the CV curve still maintains its original shape. Fig. 5c displays almost 100% capacitance retention of the wire supercapacitor under different bending states, indicating the excellent flexibility of the present wire supercapacitor. Moreover, to meet the deformation stability requirements for practical applications, the wire supercapacitor was further subjected to a consecutive bending situation, and the cycling test result is provided in Fig. 5d. The CV curves of the wire supercapacitor under different cyclic bending times are shown in Fig. 5d (inset), and based on the CV loops, the present wire supercapacitor displays a slight decrease in capacitance with a final capacitance retention of 93% after 100 bending cycles as compared to the original state. All the results reveal that the wire supercapacitor possesses outstanding structural flexibility and stability, suggesting that it is a promising candidate for flexible power supplies in wearable electronic devices. For practical application demonstration, leakage current and self-discharge characteristics of a typical asymmetric wire supercapacitor based on H3PO4/PVA electrolyte are shown in Fig. 5e and 5f, respectively. Leakage current was measured by keeping the wire supercapacitor at a constant voltage of 1.5 V for 24 h. The results in Fig. 5e indicates that leakage current drops quickly at an initial stage and gradually stabilizes, reaching 0.0013 mA cm-1 after maintaining a constant voltage for 24 h. Such a low leakage current indicates good capacitor performance, which could be attributed to few shuttle reactions caused by the impurities in electrode materials[46, 47]. Self-discharge of a supercapacitor refers to the gradual decrease in the voltage

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across the capacitor that occurs when the capacitor is left unconnected to either a charging circuit or an electrical load, reflecting a loss of efficiency for the capacitor as an energy storage device[48]. Fig. 5f displays the self-discharge profile of the wire supercapacitor under such an open-circuit voltage condition. The asymmetric wire supercapacitor maintains approx. 53% of the initial potential after 24 h of self-discharge. The leakage current and self-discharge behavior of the asymmetric wire supercapacitor demonstrates significant promise as power supplies for high-performance wearable electronic devices. A comparison of energy and power densities (Ragone plot) normalized by the length of the present asymmetric wire supercapacitors with various wire supercapacitors reported in prior literature is given in Fig. 5g. For the wire supercapacitor based on H3PO4/PVA electrolyte, the average energy density, calculated by the methods described in the Supporting Information, reaches up to 11.8 µWh cm-1, and the device delivers a maximum average power density of approx. 3.7 mW cm-1. Moreover, for the wire supercapacitor based on KOH/PVA electrolyte, the maximum average energy density can reach approx. 12 µWh cm-1, and the maximum average power density is approx. 1.5 mW cm-1. The energy and power densities of the as-prepared asymmetric wire supercapacitors are significantly higher than those of contemporary wire supercapacitors (see Table S1)[5, 30, 35, 36, 49, 50].

The energy and power densities

normalized by the active area of wire supercapacitors have also been provided in Fig. S15, which indicates a maximum average energy density of the present wire supercapacitors of 50 µWh cm-2 (corresponding to a volumetric energy density of 2.6 mWh cm-3), and the devices deliver a maximum average power density of 15.4 mW cm-2 (corresponding to a volumetric power density of 802 mW cm-3).

The energy and power densities normalized by the volume of wire

supercapacitors have also been provided in Fig. S16. The areal and volumetric energy and power

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densities are also higher than those of state-of-the-art wire supercapacitors (see Table S1)[32, 43, 44, 51, 52], indicating outstanding overall performance of the present wire supercapacitors. To test the feasibility and functional performance of the present wire supercapacitors, three asprepared wire supercapacitor devices based on H3PO4/PVA electrolyte (with a length of 2.5 cm for each device) were connected in series and initially charged to 4.5 V to light a red lightemitting-diode (LED, Velleman, Inc., with a turn-on voltage of 1.5 V) (see Fig. 5h).

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Fig. 5. (a) A typical wire supercapacitor being loaded with objects of increasing weights. (b) A wire supercapacitor wrapping around a tube. (c) Capacitance retentions of a typical wire supercapacitor (based on H3PO4/PVA electrolyte) under different bending angles of 45˚, 90˚, 135 ˚ and 180˚ (the inset shows its

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CV curves at a scan rate of 30 mV s-1 under different bending angles). (d) Capacitance retention of the wire supercapacitor under different cyclic bending times (the inset shows its CV curves at a scan rate of 50 mV s-1 under different bending cycles). (e) Leakage current of the asymmetric wire supercapacitor charged to a potential of 1.5 V and kept at the voltage for 24 h. (f) Self-discharge curve of the asymmetric wire supercapacitor under an open-circuit condition for 24 h after being charged at 1.5 V. (g) Comparative energy and power densities normalized by the length of the asymmetric wire supercapacitors and other contemporary wire supercapacitor devices cited from refs. [5], [30], [35], [36] [49] and [50]. (h) Photograph of a charged wire supercapacitor group consisting of three individual wire supercapacitor devices in series to light a red LED.

For practical applications in wearable power supplies, wire supercapacitors are also required to operate over a wide temperature window. Therefore, fundamental understanding of how temperature affects the performance of wire supercapacitors is imperative. Temperature effects on the performance of the present wire supercapacitors (based on H3PO4/PVA electrolyte) were systematically investigated by electrochemically testing the wire supercapacitors over a temperature window ranging from 2 to 80 ˚C (experimental details are provided in Supporting Information), the results of which are provided in Fig. 6. As shown in Fig. 6a, as operating temperature increases, the area of the CV curve loop significantly increases, indicating an enhancement in capacitance with increasing temperatures. Meanwhile, redox peaks in CV curves associated with the electrochemical reactions between electrodes and electrolyte become more pronounced. This phenomenon is believed to be attributed to the improved ionic conductivity of electrolytes and enhanced electron mobility in the electrodes caused by elevated operating temperatures[23, 53]. Fig. 6b shows galvanostatic charge/discharge profiles of the wire supercapacitor at a current of 6 mA and different operating temperatures, in which apparent voltage plateaus are observed in all charge/discharge curves, consistent with the redox peaks in the CV curves of Fig. 6a. The discharge time significantly increases with increasing operating temperatures, further confirming that the wire supercapacitor exhibits higher capacitances at elevated operating temperatures.

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Moreover, the initial voltage drop associated with the device internal resistance (known as IR drop) in the discharge curves decreases with increasing operating temperature (see Fig. S17), indicating a decrease in the internal resistance of the wire supercapacitor. Furthermore, the charge/discharge curves of wire supercapacitors become more asymmetric with increasing temperatures. The highly asymmetric shapes of charge/discharge curves at higher temperatures (see Fig. 6b) indicate the occurrence of more side reactions during the charge/discharge process, which can be explained by the fact that high temperatures facilitate ion mobility and thus improve ionic conductivity [23, 54], making impurities in electrode materials more reactive with electrolyte and leading to imbalanced kinetics during charge/discharge process and low coulombic efficiency (close to 13% decrease in coulombic efficiency at 80 ˚C compared to that at 2 ˚C, estimated from the charge/discharge curves). For a deeper understanding of the temperature-dependent electrochemical behavior of the wire supercapacitor, the relationship between capacitance and operating temperature was thoroughly investigated. The kinetics involved can be expressed by an Arrhenius-type equation: C = C0 exp (-Q/RT)

(1)

where C is the amount of charge accumulated at the electrode-electrolyte interfacial zone governed by the molecular or ionic motion mechanism, C0 is a pre-exponential constant, Q is the activation energy, T is absolute temperature, and R is the universal gas constant[55]. In this work, the Arrhenius plot of capacitance as a function of inverse temperature is plotted in Fig. 6c, in which the experimental data can be linearly fitted with a fixed slop of -Q/R. Consequently, the activation energy Q is calculated to be 32.8 kJ mol-1, indicating that charge storage in the wire supercapacitor is a faradaically controlled process[23, 56]. The high activation energy achieved

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here suggests a high ionic mobility for the electrolyte, revealing that the ionic conductivity of electrolyte is a strong function of operating temperature[23, 57]. Comparative Nyquist plots for the wire supercapacitor at various temperatures are provided in Fig. 6d. The bulk electrolyte resistance, reflected by the real-axis intercept of the impedance spectrum at high frequency, significantly decreases from approx. 70 to 7 Ω as operating temperature increases from 2 to 80°C (see the inset of Fig. 6d). This nearly tenfold decrease in the bulk electrolyte resistance demonstrates that the operating temperature strongly affects the ionic conductivity of the electrolyte over a wide temperature range. Moreover, all the Nyquist plots show negligible semicircles, indicating a low charge transfer resistance of the wire supercapacitor. After being tested at a high temperature of 80 ˚C, the wire supercapacitor was cooled naturally to room temperature and then electrochemically measured at room temperature to test the thermal stability and repeatability. As shown in Fig. S18, the charge/discharge curves and Nyquist plots measured at room temperature before and after high-temperature measurements nearly overlap, indicating outstanding thermal stability and repeatability of the wire supercapacitor.

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Fig. 6 Electrochemical performance of wire supercapacitors at different temperatures of 2 ˚C, room temperature (RT), 30, 40, 50, 60, 70 and 80 ˚C. (a) CV curves at a scan rate of 20 mV s −1 in a voltage window from 0 to 1.5 V. (b) Galvanostatic charge/discharge profiles at a current of 0.6 mA in a voltage window from 0 to 1.4 V. (c) Arrhenius plot of capacitance as a function of inverse temperature. (d) Nyquist plots (the inset shows the impedance spectrum in the high-frequency region) recorded from 0.1 Hz to 1 MHz of the wire supercapacitor at various temperatures.

Leakage behavior of the wire supercapacitor at various operating temperatures of 2 ˚C, room temperature, 30, 40, 50, 60, 70 and 80 ˚C have been provided in Fig. S17. After keeping the wire supercapacitor at a constant voltage of 1.5 V for more than 20 h at room temperature, leakage current stabilizes at approx. 0.0012 mA (see Fig. S19a) and drops to approx. 0.00025 mA when the temperature decreases to 2 ˚C (see Fig. S19b). When the operating temperature increases from 2 to 40 ˚C, leakage current rises to 0.0105 mA. Subsequently, as the operating temperature increases from 40 to 60 ˚C, leakage current undergoes a continuous increase, reaching up to 0.06 mA at an operating temperature of 60 ˚C (see Fig. S19b). This result indicates that high temperatures facilitate the ion mobility and improve the ionic conductivity[23, 54], making impurities in electrode materials more reactive with electrolyte (i.e., more shuttle reactions), 24

which consequently causes a higher leakage current with increasing operating temperatures[46, 47].

4. Conclusions All-solid-state asymmetric wire supercapacitors were fabricated in a unique wrap-twist architecture consisting of GPs grown on CFT by MPCVD process as a negative electrode, MnO 2 nanosheets electrodeposited on BP as a positive electrode and all-solid-state polymer (KOH/PVA and H3PO4/PVA) as a bifunctional electrolyte/separator. Such CFT/GPs//BP/MnO2 asymmetric wire supercapacitors exhibit ultrahigh capacitance, good flexibility, excellent cyclic stability, outstanding energy and power densities, which can be attributed to increased effective contact area between two electrodes by a paper-wrap-twist-fiber architecture, newly developed electrode materials with edge-enriched nanostructures, and optimized asymmetric electrode configuration. Moreover, thermal performance of the wire supercapacitors was systematically characterized by electrochemically at various temperatures ranging from 2 to 80 ˚C. The results reveal that temperature significantly influences the capacitance and internal resistance of the wire supercapacitors by affecting the ionic conductivity of PVA-abased polymer electrolytes, electron mobility in electrodes and faradaic reaction rates at the electrode surface, providing an understanding of the thermal performance of the wire supercapacitors in practical applications. The unique wrap-twist asymmetric wire supercapacitors enhanced by edge-enriched nanostructures presented in this work is expected to promote the development of next-generation textile electronic devices and wearable energy storage systems.

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Acknowledgements The authors gratefully acknowledge support from the US Air Force Office of Scientific Research under the MURI program on Nanofabrication of Tunable 3D Nanotube Architectures (PM: Dr. Joycelyn Harrison, Grant: FA9550-12-1-0037), the US National Science Foundation's Scalable Nanomanufacturing Program (Grant: 1344654), Zhejiang University open projects (Grant No. ZJUCEU 2015014), and the National Natural Science Foundation of China, (PI: Dr. Boyun Huang, Grant: No. 51021063). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at XXXX.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

W. Zeng, L. Shu, Q. Li, S. Chen, F. Wang and X. M. Tao, Adv. Mater. 26 (2014) 5310-5336. M. Stoppa and A. Chiolerio, Sensors 14 (2014) 11957-11992. Z. Yang, J. Deng, X. Chen, J. Ren and H. Peng, Angew. Chem. Int. Edit. 52 (2013) 13453-13457. D. Yu, Q. Qian, L. Wei, W. Jiang, K. Goh, J. Wei, J. Zhang and Y. Chen, Chem. Soc. Rev. 44 (2015) 647-662. V. T. Le, H. Kim, A. Ghosh, J. Kim, J. Chang, Q. A. Vu, D. T. Pham, J.-H. Lee, S.-W. Kim and Y. H. Lee, ACS nano 7 (2013) 5940-5947. Q. Kang, J. Zhao, X. Li, G. Zhu, X. Feng, Y. Ma, W. Huang and J. Liu, Nano Energy 32 (2017) 201-208. J. Sun, Y. Huang, C. Fu, Z. Wang, Y. Huang, M. Zhu, C. Zhi and H. Hu, Nano Energy 27 (2016) 230237. A. Lamberti, A. Gigot, S. Bianco, M. Fontana, M. Castellino, E. Tresso and C. F. Pirri, Carbon 105 (2016) 649-654. G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev. 41 (2012) 797-828. L. L. Zhang and X. Zhao, Chem. Soc. Rev. 38 (2009) 2520-2531. P. He, L. Liu, W. Song, G. Xiong, T. S. Fisher and T. Chen, RSC Advances 5 (2015) 31837-31844. L. Hao, X. Li and L. Zhi, Adv. Mater. 25 (2013) 3899-3904. X. Xiao, T. Li, P. Yang, Y. Gao, H. Jin, W. Ni, W. Zhan, X. Zhang, Y. Cao and J. Zhong, Acs Nano 6 (2012) 9200-9206. D. Zhang, M. Miao, H. Niu and Z. Wei, ACS nano 8 (2014) 4571-4579. W. Zhou, K. Zhou, X. Liu, R. Hu, H. Liu and S. Chen, J. Mater. Chem. A 2 (2014) 7250-7255.

26

[16] D. Yu, S. Zhai, W. Jiang, K. Goh, L. Wei, X. Chen, R. Jiang and Y. Chen, Adv. Mater. 27 (2015) 48954901. [17] G. Xiong, K. Hembram, D. N. Zakharov, R. G. Reifenberger and T. S. Fisher, Diam. Relat. Mater. 27 (2012) 1-9. [18] Y. Wu, B. Yang, G. Han, B. Zong, H. Ni, P. Luo, T. Chong, T. Low and Z. Shen, Adv. Funct. Mater. 12 (2002) 489-494. [19] H. Yang, J. Yang, Z. Bo, S. Zhang, J. Yan and K. Cen, J. Power Sources 324 (2016) 309-316. [20] G. Xiong, P. He, L. Liu, T. Chen and T. S. Fisher, J. Mater. Chem. A 3 (2015) 22940-22948. [21] C. Uthaisar and V. Barone, Nano lett. 10 (2010) 2838-2842. [22] Y.-C. Chen, Y.-K. Hsu, Y.-G. Lin, Y.-K. Lin, Y.-Y. Horng, L.-C. Chen and K.-H. Chen, Electrochim. Acta 56 (2011) 7124-7130. [23] G. Xiong, A. Kundu and T. Fisher. Springer-Thermal Engineering and Applied Science 2015. [24] S.-K. Kim, H. J. Kim, J.-C. Lee, P. V. Braun and H. S. Park, ACS nano 9 (2015) 8569-8577. [25] G. Xiong, K. P. S. S. Hembram, R. G. Reifenberger and T. S. Fisher, J. Power Sources 227 (2013) 254259. [26] H.-Q. Wang, G.-f. Yang, Q.-Y. Li, X.-X. Zhong, F.-P. Wang, Z.-S. Li and Y.-h. Li, New J. Chem. 35 (2011) 469-475. [27] G. Xiong, C. Meng, R. G. Reifenberger, P. P. Irazoqui and T. S. Fisher, Energy Technol. 2 (2014) 897905. [28] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li and F. Wei, Adv. Funct. Mater. 21 (2011) 2366-2375. [29] J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi and F. Wei, Adv. Funct. Mater. 22 (2012) 2632-2641. [30] P. Xu, B. Wei, Z. Cao, J. Zheng, K. Gong, F. Li, J. Yu, Q. Li, W. Lu and J.-H. Byun, ACS nano 9 (2015) 6088-6096. [31] F. Su and M. Miao, Nanotech. 25 (2014) 135401. [32] L. Kou, T. Huang, B. Zheng, Y. Han, X. Zhao, K. Gopalsamy, H. Sun and C. Gao, Nat. Commun. 5 (2014) 3754. [33] Q. Chen, Y. Meng, C. Hu, Y. Zhao, H. Shao, N. Chen and L. Qu, J. Power Sources 247 (2014) 32-39. [34] J. Bae, M. K. Song, Y. J. Park, J. M. Kim, M. Liu and Z. L. Wang, Angew. Chem. Int. Edit. 50 (2011) 1683-1687. [35] S. Senthilkumar and R. K. Selvan, Chem. Electro. Chem. 2 (2015) 1111-1116. [36] G. K. Veerasubramani, K. Krishnamoorthy, P. Pazhamalai and S. J. Kim, Carbon 105 (2016) 638-648. [37] E. Supercapacitors. Plenum Publishing Co., New York 1999. [38] S. K. Meher and G. R. Rao, J. Phys. Chem. C 115 (2011) 15646-15654. [39] X. Zhang, W. Shi, J. Zhu, W. Zhao, J. Ma, S. Mhaisalkar, T. L. Maria, Y. Yang, H. Zhang and H. H. Hng, Nano Research 3 (2010) 643-652. [40] C. Yuan, X. Zhang, L. Su, B. Gao and L. Shen, J. Mater. Chem. 19 (2009) 5772-5777. [41] L. Wu, R. Li, J. Guo, C. Zhou, W. Zhang, C. Wang, Y. Huang, Y. Li and J. Liu, AIP Advances 3 (2013) 082129. [42] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan and X. W. D. Lou, Adv. Mater. 24 (2012) 5166-5180. [43] G. Qu, J. Cheng, X. Li, D. Yuan, P. Chen, X. Chen, B. Wang and H. Peng, Adv. Mater. 28 (2016) 36463652. [44] C. Choi, S. H. Kim, H. J. Sim, J. A. Lee, A. Y. Choi, Y. T. Kim, X. Lepró, G. M. Spinks, R. H. Baughman and S. J. Kim, Sci. Rep. 5 (2015) 9387. [45] F. Su, X. Lv and M. Miao, Small 11 (2015) 854-861.

27

[46] G. Xiong, C. Meng, R. G. Reifenberger, P. P. Irazoqui and T. S. Fisher, Adv. Energy Mater. 4 (2014) 1300515-1300524. [47] H. Wada, K. Yoshikawa, S. Nohara, N. Furukawa, H. Inoue, N. Sugoh, H. Iwasaki and C. Iwakura, J. Power Sources 159 (2006) 1464-1467. [48] B. W. Ricketts and C. Ton-That, J. Power Sources 89 (2000) 64-69. [49] J. Ren, W. Bai, G. Guan, Y. Zhang and H. Peng, Adv. Mater. 25 (2013) 5965-5970. [50] K. Krishnamoorthy, P. Pazhamalai, G. K. Veerasubramani and S. J. Kim, J. Power Sources 321 (2016) 112-119. [51] Y. Meng, Y. Zhao, C. Hu, H. Cheng, Y. Hu, Z. Zhang, G. Shi and L. Qu, Adv. Mater. 25 (2013) 23262331. [52] K. Guo, Y. Ma, H. Li and T. Zhai, Small 12 (2016) 1024-1033. [53] G. Xiong, P. He, L. Liu, T. Chen and T. S. Fisher, Frontiers in Energy Research 3 (2015) 39. [54] M. Ayadi, A. Eddahech, O. Briat and J.-M. Vinassa. Power Engineering, Energy and Electrical Drives (POWERENG), 2013 Fourth International Conference on: IEEE 2013, 1466-1470. [55] C. Liu, Z. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett. 10 (2010) 4863-4868. [56] B. E. Conway, W. Pell and T. Liu, J. Power Sources 65 (1997) 53-59. [57] S. Mitra, A. Shukla and S. Sampath, J. Power Sources 101 (2001) 213-218.

Highlights

   

A unique design of asymmetric wire supercapacitors in a wrap-twist architecture was proposed. Edge-enriched nanostructures were designed as both positive electrode (MnO2 nanosheets) and negative electrode (graphene nanopetals). Wire supercapacitors exhibit outstanding electrochemical performance, high mechanical robustness and high functional flexibility. A deep understanding of thermal behavior of wire supercapacitors was achieved.

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Graphical abstract

1