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All-in-One, Compact Architecture towards Wearable, AllSolid-State, High-Volumetric-Energy-Density Supercapacitors Tingting Gao, Zhan Zhou, Jianyong Yu, Dianxue Cao, Guiling Wang, Bin Ding, and Yiju Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06143 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Materials & Interfaces

All-in-One, Compact Architecture towards Wearable, All-Solid-State, High-Volumetric-Energy-Density Supercapacitors

Tingting Gao1, a, Zhan Zhou3, a, Jianyong Yu1, *, Dianxue Cao2, Guiling Wang2, Bin Ding1, Yiju Li2, * 1

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, P. R. China 2 Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, P. R. China 3 College of Materials Science and Engineering, Donghua University, Shanghai 201620, P. R. China

a

These authors contributed equally to this work. * E-mail: [email protected] (Jianyong Yu); [email protected] (Yiju Li)

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Abstract High-performance flexible energy storage devices are an important prerequisite to the utilization of various advanced wearable electronics, such as healthcare sensors and smart textiles. In this work, we design a wearable, all-solid-state, all-in-one asymmetric supercapacitor by integrating current collectors, separator, negative and positive electrodes into a thin, flexible and porous polyamide nanofiber film. The positive and negative electrodes are respectively electrodeposited into each side of the carbon nanotubes (CNT) modified porous polyamide nanofiber film to form the integrated and compact asymmetric cell. The all-in-one, thin-film asymmetric supercapacitor is binder-, additives- and metal current collector-free, which can effectively decrease the cost, simplify the assembly procedures and increase the energy density. The assembled flexible all-in-one asymmetric supercapacitor with a compact structure shows high gravimetric and volumetric specific capacitance of 70 F g-1 and 3.1 F cm-3 under a current density of 0.5 A g-1 in a neutral PVA/LiCl gel electrolyte, respectively. Additionally, the all-in-one asymmetric cell displays a favorable volumetric energy density of 1.1 Wh L-3, which is among the highest compared with other reported flexible solid-state supercapacitors. Notably, multiple cell units can be integrated in one piece of polyamide nanofiber film and connected in series to satisfy the need of high output voltage.

Keywords: All-In-One; Compact Structure; Volumetric Capacitance; Wearable; Supercapacitor


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Introduction Developing versatile portable and wearable electronics gradually becomes one of the trends in pursuit of multifunctionality and miniaturization. The fast development of personal electronics demands for safe, cost-effective

and

high-performance

energy

storage

devices.1-6

Supercapacitors have been extensively investigated as promising candidates in energy storage systems due to their high power density and long-term cycle durability.7-12 However, the conventional aqueous electrochemical supercapacitors suffer from inferior assembly, poor flexibility and low energy density.13-15 The supercapacitors are usually assembled with slurry-coated electrodes based on the metal current collectors, liquid electrolyte and porous separator. Owing to the additional weight and volume induced by the metal-based current collectors, additives and binders, the actual gravimetric and volumetric energy density of the supercapacitors are greatly reduced.16-18 Moreover, the utilization of liquid electrolyte makes the device encapsulation of supercapacitor a tricky issue, especially in terms of wearable electronics. Therefore, to meet the rapid development of the flexible wearable electronics, tremendous efforts have been devoted to developing the safe, wearable and high-performance supercapacitors.19-25 Recently, some flexible, solid-state planar supercapacitors assembled with the self-supported flexible electrodes based on the metal mesh, carbon cloth and conductive agents modified cellulose etc. have been successfully prepared and achieved favorable electrochemical performance.26-33 Nevertheless, there are still some issues needed to be urgently addressed. For the self-supported metal mesh based flexible electrodes, even though they possess high conductivity and good mechanical properties, the heavy weight of metal 3 ACS Paragon Plus Environment


mesh will greatly decrease the gravimetric energy density of the entire supercapacitor. The freestanding carbon cloth based electrodes also have excellent conductivity, but, their mechanical properties need to be further enhanced to meet the requirements of flexible wearable supercapacitors. As for the flexible electrodes based on the conductive agents decorated cellulose, the inferior stability and durability in aqueous electrolyte greatly hinder their practical application in wearable energy storage devices. Moreover, nearly all the reported flexible supercapacitors are assembled with separated components, which suffer from intricate fabrication procedures and incompact construction. Electrospun porous polyamide nanofiber film with light weight, excellent mechanical and thermal properties, and favorable hydrophilicity is an ideal choice as the separator. At the same time, the conductive additives modified porous polyamide nanofibers can serve as light-weight current collector and three-dimensional (3D) substrate for loading active materials. Inspired by these concepts, in this work, we constructed a wearable, all-in-one, all-solid-state supercapacitor, which is integrated into a porous polyamide nanofiber film without using binder, additives and metal current collector. The electrode materials of the hydroxyl oxidize iron nanosheets (FeOOH NSs) and manganese dioxide nanowires (MnO2 NWs) with high theoretical specific capacitance, nontoxicity and high chemical stability were directly electrodeposited on each side of the CNT-decorated polyamide nanofiber film to build an integrated and compact thin-film supercapacitor with a thickness of 90 µm (Figure 1). The porous and hydrophilic polyamide nanofiber film contributes to unimpeded electrolyte ions migration and the continuous CNT networks on both sides of the porous polyamide nanofiber film can facilitate the electron transport. As a result, the assembled all-in-one and thin-film 4 ACS Paragon Plus Environment

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supercapacitor has a high specific capacitance of 70 F g-1 at a current density of 0.5 A g-1. Even at a high current density (10 A g-1), the supercapacitor still has a specific capacitance 40.1 F g-1. More importantly, benefiting from the compact and all-in-one structure, the flexible solid-state asymmetric supercapacitor has a high volumetric specific capacitance of 3.1 F cm-3 and a favorable volumetric energy density of 1.1 Wh L-1. Additionally, multiple cell units can be integrated in a piece of polyamide nanofiber film and stacked in series to gain the high output voltage. The all-in-one architecture design based on the flexible and porous nanofiber film offers a promising strategy for the development of high-performance wearable energy storage devices.

Figure 1. Schematic showing the all-in-one structure of the asymmetric supercapacitor. All components of the asymmetric supercapacitor are integrated into a piece of thin porous polyamide nanofiber film.



Experimental section

Preparation of polyamide nanofiber film The polyamide solution (18wt.%) was prepared by adding 1.8 g dried polyamide 6,6 powders into 8.2 g formic acid. The mixture was stirred under room temperature for about 6 h. A plastic syringe with a metal needle (18 gauge) was used as a reservoir. The electrospinning process was operated with a flow rate of 0.5 mL h-1 controlled by a syringe pump, a high voltage of 18 kV applied by a high voltage power supply, and a distance of 15 cm between the needle tip and collector. The obtained polyamide nanofiber film was dried at 80°C for at least 24 h in a vacuum oven.

Fabrication of CNT-modified polyamide nanofiber film The electrospun polyamide nanofiber film was put on a sand core funnel to act as a filter membrane. 0.2 mg mL-1 CNT solution was then added onto one side of the polyamide nanofiber film to filter the water. After dry under 60oC, the same operation procedures were applied to the other side of the polyamide nanofiber film.

Fabrication of flexible, all-in-one, all-solid-state asymmetric supercapacitor The CNT-modified polyamide nanofiber film was first immersed into a mixed solution of 0.1 mol L-1 Fe(NH4)2(SO4)2·6H2O, 0.2 mol L-1 CH3COONa and 0.1 mol L-1 Na2SO4. The FeOOH nanosheets were electrodeposited on the one side of the CNT-modified polyamide nanofiber film (0.785 cm2) at a constant current density of 0.125 mA cm-2, using a platinum plate (1 cm×2 cm) as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. After washed with deionized water, the treated CNT-modified polyamide nanofiber film was then dipped into a mixed solution containing 0.1 mol L-1 Mn(CH3COO)2·4H2O and 0.1 mol L-1 6 ACS Paragon Plus Environment

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Na2SO4. The MnO2 nanowires were electrodeposited on the other side of the CNT-modified polyamide nanofiber film (0.785 cm2) at a constant current density of 0.25 mA cm-2, using a platinum plate (1 cm×2 cm) as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. The active material ratio of the negative to positive electrodes was controlled by the electrodeposition time. After electrodeposition, the all-in-one asymmetric supercapacitor was washed with deionized water and dried under room temperature. The mass loading of active materials is determined using an electronic calibrated electronic balance (accuracy: 0.01 mg). The polyvinyl alcohol/lithium chloride (PVA/LiCl) gel electrolyte was prepared by adding 2.1 g LiCl and 1 g PVA into 10 mL deionized water and keeping vigorous stirring for 3 h on hotplate. The all-in-one asymmetric cell was then sufficiently soaked in the PVA/LiCl gel electrolyte through vacuum perfusion in sealed plastic container. After the solidification of gel electrolyte at room temperature and encapsulation, the flexible, all-in-one, all-solid-state asymmetric supercapacitor is obtained.

Materials characterizations The morphologies of the samples were studied by using a scanning electron microscope (SEM) (JEOL JSM-6480) and transmission electron microscope (TEM) (FEI Teccai G2 S-Twin) with an energy-dispersive X-ray (EDX) analyzer. The phase structures of the samples were investigated using an X-ray diffractometer (XRD, Rigaku TTR III) with Cu Kα radiation (λ=0.1514178 nm). The surface information about chemical species was investigated by X-ray photoelectron spectroscopy equipped with Al Kα radiation (XPS, Thermo ESCALAB 250).

Electrochemical measurements 7 ACS Paragon Plus Environment


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To study the electrochemical performance of single electrode, a three-electrode test system in 2 mol L-1 LiCl solution with a Pt plate (1 cm×2 cm) used as the counter electrode, a saturated Ag/AgCl electrode as the reference electrode and the as-prepared FeOOH NSs or MnO2 NWs electrode as the work electrode was utilized. For the all-in-one asymmetric supercapacitor, its electrochemical performance was investigated in a PVA/LiCl gel electrolyte within an operated voltage range of 0-1.6 V. To determine the mass loading ratio of positive to negative electrodes, we calculated it according to the following equation:

m+ C− × ∆E− = m− C+ × ∆E+

(1)

Where m+ and m- (mg) represent the mass loadings of active materials for the positive and negative electrodes, respectively. C+ and C- (F g-1) are the specific capacitances of the positive and negative electrodes, respectively, ∆E+ and ∆E- (V) are the potential ranges of the positive and negative electrodes, respectively. The mass loadings of the FeOOH and MnO2 electrode are 0.19 and 0.21 mg cm-2, respectively. The CV and GCD measurements were conducted using a Bio-Logic potentiostat. The cycling test was carried out using a LAND battery program controlled test system. The gravimetric specific capacitance Cm (F g-1) of the electrodes and supercapacitors based on the GCD curves were calculated according to the following equation:

Cm =

I d × ∆t ∆V × m

(2)

Where Id (mA) represents the discharge current, ∆V (V) stands for the discharge potential or voltage window and ∆t (s) is the discharge time. For the asymmetric supercapacitor, m (mg) is the total mass of active materials of negative and positive electrodes. For the three-electrode 8 ACS Paragon Plus Environment

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test system, m (mg) represents the active mass of negative or positive electrode; The volumetric specific capacitance Cv (F cm-3) of the asymmetric cell is calculated from the gravimetric specific capacitance Cm (F g-1) and mass loading of active materials Dm (mg cm-3). The Dm (mg cm-3) is based on the volume of the entire supercapacitor. The volumetric energy density E (mWh cm-3) of the asymmetric supercapacitor was evaluated according to the following equation:

E=Cv×∆V2×1/(2×3.6)

(3)

Where ∆V (V) stands for the discharge voltage range excluding the IR drop.

Results and discussion Figure 2a shows the schematic of the sandwich-like CNT-modified polyamide nanofiber film. Both sides of the porous polyamide nanofiber film are decorated with CNT, which can directly act as current collectors and 3D substrates for loading active materials. The porous polyamide nanofiber layer in the middle serves as the separator. Figure 2b displays the morphology of the CNT on the polyamide nanofibers. The CNT is interwoven to form continuous conductive networks, which can facilitate the electron transport (Figure S1). The morphology of the electrospun polyamide nanofibers is shown in Figure 2c. Numerous polyamide nanofibers are connected with each other and densely stacked, contributing to preventing the CNT from penetrating to the other side of the polyamide nanofiber film during the filtration. The active materials were loaded on the 3D CNT-decorated polyamide nanofibers using one-step electrodeposition method. Figure 2d and S2 show that the FeOOH NSs was uniformly distributed on the one side of the CNT-modified polyamide nanofiber film. The TEM image 9 ACS Paragon Plus Environment


reveals the FeOOH has the morphology of 3D interconnected thin nanosheets (Figure 2e). The crystalline texture of the FeOOH NSs was further investigated using the XRD, high-resolution TEM (HRTEM) and selected area electron diffraction (SAED). The two weak diffraction peaks located at the 26.3o and 36.6o are attributed to the (120) and (111) lattice planes of the goethite α-FeOOH (JCPDS #29-0713) (Figure S3). The HRTEM image demonstrates the FeOOH NSs has some local order and plenty of disorder in the long range (Figure 2f). The lattice fringe spacings of 2.31 and 2.43 Å are corresponding to the (200) and (111) lattice planes of the α-FeOOH, respectively.34 The SAED pattern indicates the FeOOH NSs has an inferior polycrystal nature (inset in Figure 2f), which is consistent with the HRTEM and XRD results. XPS was further used to investigate the chemical composition of the FeOOH NSs. The XPS spectrum of Fe 2p displays two main peaks located at 711.8 (Fe 2p3/2) and 725.2 eV (Fe 2p1/2) together with two shake-up satellite peaks at 719.1 and 733 eV (Figure S4a). The O 1s peak can be deconvoluted into two peaks of Fe-O-H (531.8 eV) and Fe-O-Fe (530.1 eV) (Figure S4b). These results agree well with the literature reports for FeOOH.35, 36

Figure 2g shows that the electrodeposited MnO2 NWs is uniformly distributed on the CNT-decorated polyamide nanofibers. The MnO2 nanowire with a diameter of ~10 nm has a rough surface, contributing to increasing the active sites (Figure S5). The diffraction peaks at 25.7o and 52.8o are assigned to the (220) and (440) crystal faces of the tetragonal α-MnO2 (JCPDS #44-0141) (Figure S3). The MnO2 NWs presents poor polycrystalline nature, which is also confirmed by the HRTEM and SAED pattern (Figure 2h). The lattice fringe spacings of 1.82, 2.40 and 2.46 Å are corresponding to the (411), (211) and (400) lattice planes of the 10 ACS Paragon Plus Environment

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tetragonal α-MnO2, respectively, which is in accordance with other reported literatures.37 Figure 2i-l show the high-angle annular dark field scanning TEM (HAADF-STEM) image and elemental mappings of the MnO2 NWs. Mn and O elements are uniformly distributed throughout the entire nanowire. The electron energy-loss spectroscopy (EELS) was also used for confirming the chemical composition. Figure S6a and b show the HAADF-STEM image and corresponding EELS spectrum of the MnO2 NWs, respectively. There are two typical peaks related to O K-edge and Mn L-edge of MnO2.38 The relative composition ratio of the Mn L-edge to O K-edge is about 1:2, verifying the formation of MnO2 (Figure S6c, d). XPS analysis was conducted to further study the surface chemical bonding nature of the MnO2 NWs. For the XPS spectrum of O 1s, the peaks at 531.6 and 529.9 eV are assigned to the Mn-O-H and Mn-O-Mn bonds, respectively (Figure S7a).39 The Mn 2p XPS spectrum displays two characteristic peaks located at 654.3 and 642.4 eV, which are corresponding to the Mn 2p1/2 and Mn 2P3/2 spin-orbit peaks of α-MnO2 (Figure S7b).40 It is noteworthy that, even the porous CNT-decorated polyamide nanofibers are loaded with the active materials of MnO2 NWs and FeOOH NSs, there remain numerous open nanopores in the electrodes, which contribute to adequate electrolyte penetration and unimpeded ions transport (Figure S8).



Figure 2. Morphology characterization of each component. (a) Schematic illustration showing the sandwich-like CNT-modified porous polyamide nanofiber film. (b) SEM image of the CNT on the polyamide nanofibers. (c) SEM image showing the dense and porous polyamide nanofibers. SEM (d) and TEM images (e) of the FeOOH NSs. (f) HRTEM image of the FeOOH NSs. Inset is the SAED pattern of the FeOOH NSs. SEM (g) and TEM images (h) of the MnO2 NWs. Inset in Figure 2h is the SAED pattern of the MnO2 NWs. (i-l) HAADF-STEM image and corresponding elements mapping of the MnO2 NWs.

The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were taken advantage to investigate the electrochemical performance of the single electrode (FeOOH NSs and MnO2 NWs electrodes) in three-electrode test system. The CV curves of the FeOOH NSs electrode at different scan rates show a distorted rectangular shape especially at high scan rates (Figure 3a). The distortion of the CV curves at high scan rates is attributed to the polarization of the FeOOH NSs electrode. Figure 3b shows the GCD curves of the FeOOH NSs


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electrode within a potential range of -0.8-0 V (vs. Ag/AgCl electrode) at various current densities. The GCD curves of the FeOOH NSs electrode show a quasi-triangle shape without obvious charge and discharge plateaus. The CV curves of the MnO2 NWs electrode at scan rates from 5 to 100 mV s-1 are displayed in Figure 3c. The current density increases with the increase of scan rate. It suggests the redox reaction of the MnO2 NWs electrode is not limited by the kinetics at least within the scan rate range of 5-100 mV s-1. The GCD curves of the MnO2 NWs electrode show a relatively symmetric triangular shape within a potential range of 0-0.8 V (vs. Ag/AgCl electrode) at the current densities from 0.5 to 10 A g-1, implying favorable capacitive performance (Figure 3d). The gravimetric specific capacitances of the FeOOH NSs and MnO2 NWs electrodes are summarized in Figure 3e. The MnO2 NWs (FeOOH NSs) electrode has gravimetric specific capacitances of 301 (315), 256 (287), 210 (241), 180 (207), 156 (194) F g-1 at the current densities of 0.5, 1, 2, 5 and 10 A g-1, respectively. When the current density increases from 0.5 to 10 A g-1, the specific capacitance retentions of the FeOOH NSs and MnO2 NWs electrodes are about 51.8% and 61.6%, respectively. Figure 3f show CV curves of the FeOOH NSs and MnO2 NWs electrodes with the scan rate of 30 mV s-1. The stable potential ranges of the FeOOH NSs and MnO2 NWs electrodes are -0.8 V-0 V and 0-0.8 V, respectively. The gravimetric specific capacitances calculated from the CV curves are 203 and 184 F g-1 for FeOOH NSs and MnO2 NWs electrodes, respectively. Therefore, to optimize the performance of the asymmetric cell, the mass loading ratio of the FeOOH NSs to MnO2 NWs active materials (m-/m+) is determined to be about 1:1.1 according to the equation (1).



Figure 3. Electrochemical performance of the FeOOH NSs and MnO2 NWs electrodes in three-electrode test set-up. (a) CV curves of the FeOOH NSs electrode at different scan rates within a potential widow of -0.8-0 V vs. Ag/AgCl. (b) GDC curves of the FeOOH NSs electrode under various current densities from 0.5 to 10 A g-1. (c) CV curves of the MnO2 NWs electrode at different scan rates within a potential range of 0-0.8 V vs. Ag/AgCl. (d) GDC curves of the MnO2 NWs electrode under various current densities from 0.5 to 10 A g-1. (e) Rate performance of the FeOOH NSs and MnO2 NWs electrodes. (f) CV curves of the FeOOH NSs and MnO2 NWs electrodes at a scan rate of 30 mV s-1.

Figure 4a and b show the photo images of the all-in-one MnO2 NWs//FeOOH NSs asymmetric supercapacitor, which is integrated into a thin polyamide nanofiber film. The active materials of FeOOH NSs and MnO2 NWs are electrodeposited respectively on either side of the CNT-modified electrospun polyamide nanofiber film. The all-in-one MnO2 NWs//FeOOH NSs asymmetric cell exhibits excellent flexibility. Figure 4c displays the cross-section of the all-in-one MnO2 NWs//FeOOH NSs asymmetric cell. The thickness of the whole all-in-one asymmetric supercapacitor is only about 90 µm. The dense texture of the electrospun polyamide nanofibers can effectively prevent the short circuit, which could be caused by the CNT penetration throughout the porous polyamide nanofiber film during the filtration process. 14 ACS Paragon Plus Environment

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Figure 4d-g show the SEM image and corresponding elemental mappings of the cross-section of the asymmetric supercapacitor. The well-defined element distributions confirm the successful preparation of sandwich-like all-in-one asymmetric supercapacitor. Note that although the thicker anode and cathode layers lead to higher energy density, the risk of short circuit increases as well. Therefore, taking the performance and safety issues into account, the proposed compact all-in-one structure can be further optimized by adjusting parameters, such as the porosity of the polyamide nanofiber film and the penetration thickness of CNT etc..

Figure 4. Assembly of the flexible all-in-one MnO2 NWs//FeOOH NSs asymmetric supercapacitor. (a, b) Photo images showing the flexible, all-in-one MnO2 NWs//FeOOH NSs asymmetric supercapacitor, which is integrated into a thin and porous polyamide nanofiber film. (c) SEM image demonstrating the cross-section of the all-in-one asymmetric supercapacitor. The thickness is about 90 µm. (d-g) SEM image and corresponding element mappings of the cross-section of the all-in-one asymmetric cell, displaying the sandwich-like structure.

The all-in-one, all-solid-state asymmetric cell integrated into the polyamide nanofiber film demonstrates favorable flexibility and wearability (Figure 5a). Inset shows the assembly schematic of the all-in-one asymmetric cell, which is sealed by two polymer membranes. Before investigating the electrochemical performance of the all-in-one asymmetric cell, we first 15 ACS Paragon Plus Environment


explored the stability of the polyamide nanofiber film in aqueous electrolyte. The tensile strength of the wet polyamide nanofiber film only decreases 2% compared with the dry one, suggesting the excellent stability under work situation (Figure S9). We also confirm the capacitance contribution of the CNT in the porous polyamide nanofiber film. As shown in Figure S10, the area of CV curve of the CNT-modified polyamide nanofiber film without loading active materials is ignorable, which indicates the capacitance of the CNT-decorated film is negligible. Figure S11 displays the CV curves of the MnO2 NWs//FeOOH NSs asymmetric supercapacitor with different scan rates. The distortion of the CV curves at high scan rates is ascribed to the polarization of the MnO2 NWs and FeOOH NSs electrodes during the redox reaction. The CV curves of the flexible, all-in-one, all-solid-state asymmetric supercapacitor with various bending angles are shown in Figure 5b. The shape of CV curves remains nearly the same under different bending angles of 0o, 45o and 90o, which exhibits excellent foldability. We also tested the specific capacitance of the MnO2 NWs//FeOOH NSs asymmetric supercapacitor after different bending times. As shown in Figure S12, there is no apparent performance decay even after 5000 bending times (before: 70 F g-1 at 0.5 A g-1; 5000th: 68 F g-1 at 0.5 A g-1), which further confirms excellent folding stability. Figure 5c displays the GCD curves of the all-in-one asymmetric supercapacitor at different current densities within an operation voltage window of 0-1.6 V. The symmetric triangle shape of the GCD curves manifests the favorable charge/discharge reversibility. The gravimetric (volumetric) specific capacitances of the all-in-one asymmetric cell at the current densities of 0.5, 1, 2, 5 and 10 A g-1 are 70 (3.1), 61 (2.7), 50 (2.2), 43 (1.9) and 39 (1.7) F g-1 (F cm-3), respectively (Figure 5d). Moreover, the assembled all-in-one MnO2 NWs//FeOOH NSs asymmetric cell exhibits 16 ACS Paragon Plus Environment

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excellent cycling stability. The specific capacitance retention remains 96.5% after 10,000 cycles and the Coulombic efficiency keeps at about 100% during the cycling test (Figure 5e). We also measured the electrochemical impedance spectroscopy (EIS) curves of the MnO2 NWs//FeOOH NSs asymmetric supercapacitor after 1 and 10000 cycles. After 10000 cycles, the ohmic (Rs) and charge transfer resistance (Rct) slightly increases, further confirming the favorable cycling stability (Figure S13). Figure 5f shows the Ragone plots of various solid-state asymmetric/symmetric supercapacitors.41-45 Our all-in-one MnO2 NWs//FeOOH NSs asymmetric cell possesses a high volumetric energy density of 1.1 Wh L-1 at a current density of 22.2 mA cm-3, which is among the best compared with other solid-state supercapacitors (Table 1). Note that, the relatively low power density may be attributed to be the usage of gel electrolyte and the relatively inferior electronic conductivity of the active materials of FeOOH and MnO2, which can be further optimized. To gain high voltage output, multiple cell units can be connected in series. Inset in Figure 5g displays multiple all-in-one asymmetric cells can be integrated in a thin and porous polyamide nanofiber film. Figure 5g shows the GCD curves of the single all-in-one asymmetric supercapacitor and tandem devices which are composed of two and three cell units connected in series. The output voltage of the tandem device can be greatly enhanced and reach to 4.8 V. Inset in Figure 5e shows the green light-emitting diode (LED) can be powered by the tandem all-in-one asymmetric supercapacitors which contain three cell units connected in series. It is expected that the performance of the all-in-one asymmetric supercapacitor can be further enhanced by increasing mass loading and voltage window, as well as using other advanced electrode materials.



Figure 5. Electrochemical performance of the flexible, all-in-one, all-solid-state MnO2 NWs//FeOOH NSs asymmetric supercapacitor. (a) Photograph showing the wearable, all-solid-state MnO2 NWs//FeOOH NSs asymmetric supercapacitor. Inset is the assembly schematic of the all-in-one asymmetric cell. (b) CV curves of the flexible, all-in-one, all-solid-state asymmetric cell at different curvatures of 0o, 45o and 90o with a scan rate of 20 mV s-1. (c) GDC curves of the asymmetric cell at various current densities. (d) Rate performance of the flexible, all-in-one, all-solid-state MnO2 NWs//FeOOH NSs asymmetric supercapacitor. (e) Cycling performance of the asymmetric supercapacitor at a current density of 2 A g-1. (f) Ragone plots of our flexible, all-in-one, all-solid-state MnO2 NWs//FeOOH NSs asymmetric supercapacitor and other reported solid-state supercapacitors. (g) GDC curves of the supercapacitors assembled with multiple cell units in series (current density: 1 A g-1). Inset showing all the cell units are integrated into one piece of polyamide nanofiber film.


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Table. 1 Summary of the volumetric specific capacitance and energy density obtained from various flexible solid-state symmetric and asymmetric supercapacitors. Symmetric (SSCs)/asymmetric supercapacitors (ASCs)

(a)

C(a) (F cm-3)

Ed(b) (Wh L-3)

R(c)

Ref.

TiN-SSCs ZnO@C@MnO2-SSCs VOx-SSCs VN/CNTs-SSCs TiO2@C-SSCs MnO2//Fe2O3-ASCs MnO2//FeOOH-ASCs

0.33 0.325 2.5 7.9 0.125 1.2 1.1

0.05 0.04 0.12 0.54 0.011 0.41 0.48

2.5 mA cm-3 0.5 mA cm-2 20 mA cm-2 25 mA cm-3 0.1 mA cm-2 10 mV s-1 20 mA cm-3

41 19 20 21 42 43 22

α-Fe2O3@PANI//PANI-ASC

1.5

0.35

10 mV s-1

23

ZnO@MnO2//RGO-ASCs MnO2 NWs//Fe2O3 NTs-ASCs Co9S8//Co3O4@RuO2-ASCs VOx//VN-ASCs Ni/MnO2-FP//Ni/AC-FP-ASCs H-TiO2@MnO2//H-TiO2@C-ASCs MnO2 NWs//FeOOH NSs-ASCs

0.52 1.5 4.28 1.35 2.0 0.71 3.1

0.23 0.32 1.44 0.61 0.50 0.3 1.1

10 mV s-1 2 mA cm-2 2.5 mA cm-2 0.5 mA cm-2 5 mV s-1 0.5 mA cm-2 22.2 mA cm-3

44 26 27 28 29 30 This work

Volumetric specific capacitance;

(b)

Volumetric energy density;

(c)

Current density or scan rate. The volumetric specific

capacitance and energy density are based on the volume of the entire supercapacitors.



Conclusion In this work, we demonstrated a wearable, all-solid-state, all-in-one thin-film asymmetric supercapacitor, which is integrated into a flexible and porous polyamide nanofiber film without using binder, conductive additives and metal current collector. The FeOOH NSs and MnO2 NWs electrodeposited on each side of the CNT-modified porous polyamide nanofiber film act as the negative and positive electrodes, respectively. The porous polyamide nanofiber film in the middle without CNT decoration directly serves as the separator. The flexible, all-in-one asymmetric supercapacitor with a compact structure shows high gravimetric and volumetric specific capacitances of 70 F g-1 and 3.1 F cm-3 at a current density of 0.5 A g-1 in a noncorrosive PVA/LiCl gel electrolyte, respectively. Moreover, the integrated thin-film asymmetric supercapacitor displayed a favorable volumetric energy density of 1.1 Wh L-3 within a wide voltage range of 1.6 V. Noteworthily, multiple cell units can be integrated in a polyamide nanofiber film and stacked in series to obtain high output voltage. The compact all-in-one structure design of the thin-film flexible supercapacitor can meet the requirement of high volumetric energy density for the wearable energy storage devices.


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Acknowledgements We acknowledge the support of the National Natural Science Foundation of China (51572052).

Conflict of Interest The authors declare no conﬂict of interest.

Supporting Information Figure S1. TEM image of the CNT film. Figure S2. SEM image and corresponding element mappings of the FeOOH NSs. Figure S3. XRD patterns of the CNT@MnO2 NWs and CNT@FeOOH NSs. Figure S4. Fe 2p and O 1s XPS spectra of the FeOOH NSs. Figure S5. TEM images of the MnO2 NWs. Figure S6. HAADF-STEM image and corresponding EELS spectra of the MnO2 NWs. Figure S7. O 1s and Mn 2p XPS spectra of the MnO2 NWs. Figure S8. SEM images of the MnO2 NWs and FeOOH NSs. Figure S9. Typical stress-strain curves of the electrospun polyamide nanofiber film in dry and wet states. Figure S10. CVs curves of the bare CNT//CNT symmetric supercapacitor and all-in-one MnO2 NWs//FeOOH NSs asymmetric supercapacitor at a scan rate of 20 mV s-1. Figure S11. CVs curves of the all-in-one MnO2 NWs//FeOOH NSs asymmetric cell at different scan rates. Figure S12. Specific capacitances of the MnO2 NWs//FeOOH NSs asymmetric supercapacitor after different bending times. Figure S13. Nyquist plots of the MnO2 NWs//FeOOH NSs asymmetric supercapacitor after 1 and 10000 cycles.



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