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Flexible Black-Phosphorus Nanoflakes/Carbon Nanotubes Composite Paper for High-Performance All-Solid-State Supercapacitors Bingchao Yang, Chunxue Hao, Fusheng Wen, Bochong Wang, Congpu Mu, Jianyong Xiang, Lei Li, Bo Xu, Zhisheng Zhao, Zhongyuan Liu, and Yongjun Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13572 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Applied Materials & Interfaces

Flexible Black-Phosphorus Nanoflakes/Carbon Nanotubes Composite Paper for HighPerformance All-Solid-State Supercapacitors Bingchao Yang†, #, Chunxue Hao†, #, Fusheng Wen †,*, Bochong Wang§, Congpu Mu§, Jianyong Xiang†,*, Lei Li , Bo Xu†, Zhisheng Zhao†, Zhongyuan Liu†,*, Yongjun Tian† ⊥

†State

Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, People’s Republic of China §School of Sciences, Yanshan University, Qinhuangdao 066004, People’s Republic of China Northwest Institute for Non-ferrous Metal Research, Xian 710016, People’s Republic of China ⊥

ABSRTACT: We proposed a simple route for fabrication of the flexible BP nanoflakes/CNTs composite paper as flexible electrodes in all-solid-state supercapacitors. The highly conductive CNTs not only play a role as active materials, but also increase conductivity of the hybrid electode, enhance electrolyte shuttling and prevent the restacking between BP nanoflakes. The fabricated flexible all-solid-state supercapacitor (ASSP) device at the mass proportion of BP/CNTs 1:4 was found to deliver the highest volumetric capacitance of up to 41.1 F/cm3 at 0.005V/s, superior to the ASSP based on the bare graphene or BP. The BP/CNTs (1:4) device delivers a rapid charging/discharging up to 500 V/s, which exhibits the characteristic of a high power density of 821.62 W/cm3, while having outstanding mechanical flexibility and high cycling stability over 10,000 cycles (91.5% capacitance retained). Moreover the BP/CNTs (1:4) ASSP device still retains large volumetric capacitance (35.7 F/cm3 at the scan rate of 0.005 V/s) even after 11 months. In addition, the ASSP of BP/CNTs (1:4) exhibits high energy density of 5.71 mWh/cm3 and high power density of 821.62 W/cm3. As indicated in our work, the strategy of assembling stacked-layer composites films will open up novel possibility for realizing BP and CNTs in new-concept thin-film energy storage devices. KEYWORDS: two dimensional layered material; black phosphorous; carbon nanotubes; flexible energy storage devices; all-solid-state supercapacitor 1 ACS Paragon Plus Environment

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1. INTRODUCTION With the fast development of mobile electronics, energy storage technology has been attracting much attention, which plays an important role in energy sustainability that results in the energy and cost savings.1-6 Compared with the widely used Li-ion batteries,1,2,5,7-9 supercapacitors have many advantages in long cycle life, high power density and fast chargedischarge capabilities.10-12 Graphene supercapacitors as electrical double-layer capacitors (EDLCs) can achieve a high capacitance at fast speed charge/discharge rate because of the large specific surface areas of electrode materials and electrical conductivity.10,11,13,14 In order to further improve the supercapacitor performance, two-dimensional (2D) layered materials have been extensively researched, such as metallic 1T MoS2 single layers,12 SnSe2,15 Mxenes,14 and black phosphorous (BP),16 which benefit from the efficient intercalation and electrosorption of ions. As a p-type direct bandgap semiconducting layered material, BP has been widely investigated in the past three years because of its tunable bandgap between 0.30 and 2.2 eV by controlling the number of layers,17-20 the high carrier mobility up to ~1,000 cm2/Vs17 for building (opto)electronic technologies,17,19,21-24 and the high surface area and chemical activities for application in the fields of energy storage, absorbents and catalytic agents.16,25-28 High volumetric capacitance of 13.75 F/cm3 has been achieved for flexible all-solid-state supercapacitor (ASSP) using liquid-exfoliated BP nanoflakes as electrode materials, which is larger than that of restacked graphene (~1.0 F/cm3).11,29 Moreover, this flexible ASSP can reach a large charge/discharge rate of 10 V/s and exhibits cycling stability over 30,000 cycles.16 The higher volumetric capacitance and faster charge/discharge would be expected if the efficient route was explored to raise the conductivity and prevent the restacking effect of liquid-exfoliated BP nanoflakes. Recently, highly conductive carbon nanotubes (CNTs) have been used to improve the supercapacitor performance, such as CNTs/graphene,30 2 ACS Paragon Plus Environment

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CNTs/MoS2,31 CNTs/Mxene,32 and so on. Therefore, an effective hierarchical structure of liquid-exfoliated BP nanoflakes and CNTs with high conductivity is also expected to further enhance the performance of BP-based supercapacitors. The 3D network constructed by CNTs also contributes to suppress the restacking effect of BP nanosheets. In this work, we proposed a simple route for fabrication of the flexible BP nanoflakes/CNTs composite paper electrodes through filtration of the liquid-exfoliated BP nanoflakes and CNTs mixed dispersions with different mass proportions. The fabricated flexible hybrid ASSP device at the mass proportion of BP:CNTs=1:4 was found to deliver the highest volumetric capacitance of up to 41.1 F/cm3 at 0.005V/s, superior to the ASSP based on the bare graphene11,29 and BP.16 In this supercapacitor electrode, the highly conductive CNTs not only play a role as active materials, but also increase conductivity of the hybrid electode, enhance electrolyte shuttling and prevent the restacking between BP nanoflakes.30,33

2. EXPERIMENTAL SECTION 2.1 Prepare of the solution of BP and CNTs Bulk black phosphorous (BP) was prepared as reported in our previous work.16 In the exfoliation process, BP powders (20 mg) was added to anhydrous acetone (20 ml) and ultrasonicated for 10 hours at 300 W using an ultrasonic cell crusher noise isolating thamber (HN-1000Y, HANU, Shanghai) in iced water environment. The BP nanoflakes dispersion was centrifuged at 5000 rpm for 60 min after exfoliated by anhydrous acetone. CNTs (diameter: about 20 nm) were purchased from Beijing DK nanotechnology Co., Ltd., China. CNTs were treated as following: 0.4 g of prepared CNTs was activated in a mixture of HNO3 (50 ml) and H2SO4 (100 ml) for 5 hours, and then washed using deionized water until neutrality. The functional CNTs were ultrasonic dispersed in deionized water for 1 hour and then extracted after centrifugation at 5000 rpm for 60 min. 2.2 Fabrication of flexible BP nanoflakes/CNTs composite paper ASSP Devices 3 ACS Paragon Plus Environment


The mixture solutions of BP and CNTs with different mass proportions were mixed up by adjusting the volume of CNTs and BP solution. Suspensions of CNTs solution was filtered by vacuum filtration through a porous alumina membrane filter (Whatman, 20 nm pore size). The mixture solutions of BP and CNTs were filtered by vacuum filtration in sequence after 1 hour bath sonication. After few hours of drying, the filtered papers were transferred directly in 3 M NaOH solution. When the porous alumina membrane was completely dissolved in the NaOH solution, the solution was replaced repeatedly with deionized water until neutral. Then the floating papers were salvaged by the PET substrate coated with 20 nm Au film (Au-PET) and closely attached onto the Au-PET substrate after 30 min of drying at 50 ℃. The packing density of BP nanoflakes/CNTs composite paper was measured as follows: the mass of AuPET substrate was measured before and after the transformation of BP/CNTs composite paper, and the thickness of BP/CNTs composite paper was obtained by measuring its cross-section via the scanning electron microscopy (SEM). The preparation of PVA/H3PO4 electrolyte and BP/CNTs composite devices were performed as our previous report.16 2.3 Characterization The X-ray diffraction (XRD) patterns were collected on SmartLab with Cu-Kα radiation. The scanning electron microscopy (SEM) images were taken by a scanning electron microscopy (S-4800, Japan). The Raman measurement was collected with a laser radiation of 514 nm. The TEM images of the BP nanoflakes were obtained in a transmission electron microscopy (JEM-2010, Japan). The electrochemical measurements were carried out on CHI660E electrochemical workstation. The capacitance values of the devices were calculated according to the CV curves as following equations29 =

1 2∆

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The energy density (E) and power density (P) in the Ragone plot were evaluated under the dynamic conditions of cyclic voltammetry from the following equations29 1 1 E = ∆ 2 2 3600 P =

E × 3600 3 ∆

where CV refers to volumetric stack capacitance of the device. And V (cm3) is the combined volume of the current collector, electrodes materials, solid state electrolyte and separator coat, ν is the potential scan rate (V/s), ∆ is the potential window (V), and ∆ is the discharging time (s), respectively.

3. RESULTS AND DISCUSSION BP crystal was synthesized by using red phosphorus under high pressure of 2 GPa and high temperature of 800 oC (Figure S1a). A well-defined layered structure is observed by SEM (Figure S1b). The XRD pattern (Figure S1c) indicates the good quality of as-prepared BP crystal. The dispersion of liquid-exfoliated BP nanoflakes and CNTs were acquired by ultrasonic treatment in acetone and deionized water, respectively, and then the BP nanoflakes and CNTs were mixed and sonicated to achieve uniform dispersion. As shown in Figure 1a, the uniform colloidal states of BP nanoflakes dispersion, CNTs dispersion, and BP/CNTs (1:4) mixed dispersion are confirmed using the Tyndall scattering effect by passing a red laser beam through the solution. The diameter of CNTs is about 20 nm (Figure S2a), and the lateral sizes of liquid-exfoliated BP nanoflakes range from several tens to several hundreds of nanometers (see TEM images in Figure S2b). After filtration and film casting, a large area self-assembly flexible BP/CNTs composite paper with the diameter of about 38 mm was obtained (Figure 1b and Figure S3a). The lateral size of fabricated flexible BP/CNTs composite paper depends on that of the used porous alumina membrane filter, and one suitable for the industrial manufacture can be used in the fabrication. As shown in Figure 1d, 5 ACS Paragon Plus Environment


the conductivity of flexible BP/CNTs composite paper proportionally increases with the content of CNTs. The experimental I-V curves of BP/CNTs (4:1) and BP/CNTs (1:4) composite papers were compared in the voltage window from -1 V to +1 V (Figure 1c). The flexible BP/CNTs (1:4) composite paper has an electric conductance of 1362 S m-1, much higher than 633 S m-1 of BP/CNTs (4:1) composite paper (Figure1d and Table S1). As shown in Table S1, the BP/CNTs composite devices exhibit larger packing density than the bare BP nanoflakes film by drop-casting. A high volumetric capacitance can be obtained for the BP/CNTs (1:4) composite paper with a large packing density of 0.889 g cm-3. The high areal and volumetric capacitances are essential for the BP/CNTs composite papers with less empty pores. Figure 1e-g show top-view SEM images of flexible BP/CNTs (1:4) composite paper (see also Figure S3d). As-fabricated film consists of porous structure with the lateral sized interstices of hundreds of nanometers. In high-magnification SEM images, the surface roughness with interstices is significantly noticeable, which is conducive to the immersion of electrolytes.30 In addition, EDS mapping (Figure S3e-f) for flexible BP/CNTs (1:4) composite paper indicates the liquid-exfoliated BP nanoflakes are uniformly dispersed in CNTs network. The cross-section SEM images in Figure 1h-j show that the continuous and uniform composite films are readily produced, and the thickness of flexible BP/CNTs (1:4) paper is determined to be around 1.13 µm. The Raman and XRD results also prove that the flexible composite papers consist of BP nanoflakes and CNTs (Figure S3b, c). Raman spectrum of flexible BP/CNTs (1:4) composite paper exhibits five characteristic vibration modes, which are assigned to , and modes of BP,34 and D, G modes of CNTs. We fabricated the flexible ASSP by using BP/CNTs composite papers and a gel electrolyte of polyvinyl alcohol/H3PO4 (PVA/H3PO4). PVA/H3PO4 gel electrolyte was sandwiched between two as-prepared flexible BP/CNTs composite papers. Compared with the ASSP of bare BP nanoflakes electrode that we previously reported,16 the use of CNTs greatly improves 6 ACS Paragon Plus Environment

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the electrochemical performances of BP as electrode materials. As shown in Figure 2a-c (see also Table S1), the volumetric capacitance at different scan rates of 0.1, 1 and 10 V/s consistently shows the first increase and then decrease with the increase in quantity of CNTs. Obviously, the ASSP of BP/CNTs (1:4) delivers the highest volumetric capacitance. This dependence of volumetric capacitance on the used quantity of CNTs could be schematically understood in Figure 2d-f. At the low level of CNTs, the serious restacking effect of BP nanoflakes is not efficiently suppressed, and the conductivity of flexible BP/CNTs is not sufficiently raised, and thus the intercalation and electrosorption of cation and anion are not significantly improved. With the increasing addition of CNTs, the restacking of BP nanoflakes can be gradually suppressed, and the conductivity is also gradually raised, leading to the gradually improved intercalation and electrosorption of ions. By increasing addition of CNTs to a much higher level, however, the BP nanoflakes as active electrode material can be also reduced to a much lower level, which becomes unfavorable for the performance of BP/CNTs composite paper. As a result, the best performance of BP/CNTs composite paper is actually obtained by increasing addition of CNTs to an appropriate level, just as observed in ASSP of BP/CNTs (1:4). The electrochemical performances of BP/CNTs ASSP devices were evaluated by using cyclic voltammetry (CV) at scan rates from 0.005 to 500 V/s. For all the tested ASSP devices of BP/CNTs (4:1, 1:1, 1:4, 1:8) and pure CNTs at a low scan rate of 0.005 V/s, the CV curves maintain quasi-rectangular rectangular shape, indicating the dominant electrochemical double-layer capacitance behavior (Figure 3a and Figure S4-S7). For the ASSP device of pure BP nanoflakes reported in our previous study,16 the CV curve shows obvious deviation from a rectangular shape at low scan rate. These observed quasi-rectangular CV curves at low scan rate should be attributed to addition of highly conductive CNTs, which favor the electric double layer capacitor behavior.35,36 For the ASSP devices of BP/CNTs, the CV curves retain a quasi-rectangular shape at the increasing scan rate from 0.005 to 100 V/s, exhibiting typical 7 ACS Paragon Plus Environment


EDLC characteristic of superior rate capability (Figure 3b, c and Figure S4-S7). At further increasing rate of above 100 V/s, the CV curve obviously deviates from the rectangular shape. At 500 V/s, the ASSP devices display rapid charge/discharge and the characteristic of a high instantaneous power. As shown in Figure S8a-c, the ASSP device of BP/CNTs (1:4) always delivers the highest volumetric capacitance than the other fabricated ones at the increasing scan rate from 0.005 to 500 V/s. For the ASSP device of BP/CNTs (1:4), as shown in Figure 3d, the calculated volumetric capacitance displays a fast drop from 41.1 F/cm3 to 35.8 F/cm3 and then to 34.4 F/cm3 with the increase of scan rate from 0.005 to 0.008 and then to 0.01 V/s. At the large scan rates of 200 V/s and 500 V/s, the volumetric capacitances of 4.9 F/cm3 and 3.3 F/cm3 are still remained, respectively, much higher than those of graphene-based superapacitor.11,29 Galvanostatic charging/discharging tests were carried out at various current densities (Figure 3e). The charge/discharge curves display nearly ideal triangular capacitive behavior, which also indicates the dominant electrochemical double-layer capacitance behavior of BP/CNTs composites electrodes. With the ASSP device of BP/CNTs (1:4) being bent towards nearly fold, the measured CV curves at 1 V/s as shown in Figure 3f demonstrate excellent flexibility. The ASSP device of BP/CNTs (1:4) exhibits high cyclability. Capacitive retention of 91.5% is obtained after 10,000 cycles (Figure 4a), which is significantly improved in contrast to 84.5% for the device of bare BP nanoflakes.16 For comparison, cycling tests have been performed on the other ASSP devices of BP/CNTs (4:1, 1:1, 1:8) and bare CNTs (Figure S9S12), and the capacitive retention of 74.0%, 87.2%, 87.9% and 96.1% have been observed, respectively, after 10,000 cycles. Clearly, the CNTs network plays an important role in cyclability of the BP/CNTs ASSP devices. The ASSP device of BP/CNTs (1:4) was unpacked after 10,000 cycles to check the morphologies of BP nanoflakes and CNTs by the TEM measurements, as demonstrated in Figure 4c. Compared with as-prepared BP/CNTs (1:4) composite paper (Figure 4b), BP nanoflakes can still maintain two-dimensional structure 8 ACS Paragon Plus Environment

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feature after long cycling test. Amazingly, the BP/CNTs (1:4) ASSP device still delivers large volumetric capacitance (35.7 F/cm3 at the scan rate of 0.005 V/s) after 11 months, which indicates that our BP/CNTs (1:4) ASSP device possesses the outstanding stability of capability (Figure S13). The Nyquist plot of BP/CNTs (1:4) ASSP device was provided to understand the electron and ion transport properties (Figure S14). The BP/CNTs (1:4) ASSP device shows a low equivalent series resistance (ESR) of ~3.4 Ω, much lower than that of the ASSP device of BP nanoflakes (~40 Ω)16 and also that of reduced graphene sandwich solid state device (~20 Ω).29 Moreover, in the low frequency range, the low resistance for ion diffusion and the good capacitive behavior can be demonstrated by the straight line of large slope close to 90º. Figure 5a shows the galvanostatic charge/discharge curves at a current density of 10 A/cm3 for a single and four connected devices of BP/CNTs (1:4) in series. The four connected devices in series after charging to 4.0 V were applied to light a green LED (working at voltage range of 3.0-3.2 V). The lighting was observed to last for more than 6 minutes. A Ragone plot is shown in Figure 5b to compare the performance of BP/CNTs (1:4) ASSP device with those of aluminum electrolytic capacitor (3 V/300 μF), lithium thin-film battery (4 V/500 μAh)37 and excellent supercapacitor devices based on 2D material electrodes. The BP/CNTs (1:4) ASSP device shows a ultrahigh power density of 821.62 W/cm3 and a ultrahigh energy density of 5.71 mWh/cm3. Compared with reduced graphene film sandwich solid state device (a power density of 1.6 W/cm3 and an energy density of 0.15 mWh/cm3),29 laser-scribed graphene (LSG) sandwich solid state supercapacitor (a power density of 3.8 W/cm3 and an energy density of 0.11 mWh/cm3),11 three-dimensional hierarchical nanocomposites MoS2@Ni(OH)2 solid state supercapacitor (a power density of 11 W/cm3， and an energy density of 5.2 mWh/cm3),38 MoS2-reduced graphene oxide/multi-walled CNTs solid state supercapacitor (a power density of 1.07 W/cm3, and an energy density of 0.54 9 ACS Paragon Plus Environment


mWh/cm3),39 and MXene-graphene solid state supercapacitor (a power density of 1.61 W/cm3, and an energy density of 3.37 mWh/cm3),14 the BP/CNTs (1:4) ASSP device has the larger energy and power densities. Compared to the flexible ASSP of pure liquid-exfoliated BP nanoflakes (a power density of 8.83 W/cm3, and an energy density of 2.47 mWh/cm3),16 the energy and power densities have been significantly improved by addition of CNTs. The introduction of highly conductive CNTs into BP nanoflakes electrodes favors the improvement of conductivity (1362 S m-1). As reported in our previous study,16 the low packing density in flexible ASSP of pure liquid-exfoliated BP nanoflakes (0.30 g cm-3) limits the achievement of a high volumetric capacitance. By vacuum filtration, a higher packing density of 0.889 g cm-3 is achieved in the BP/CNTs (1:4) ASSP device (see Table S1). Thereby, the greatly enhanced performance of BP/CNTs (1:4) ASSP device can be attributed to the improved conductivity by addition of CNTs and packing density by vacuum filtration. Additionally, the introduced CNTs is favorable for the efficient intercalation and electric adsorption of ions and thus the enhanced performance of supercapacitor based on BP nanoflakes.30 The excellent electrochemical performance of the flexible BP/CNTs (1:4) ASSP device relies on the densely-packed hybrid with an integrated synergetic effect of ultrathin liquidexfoliated BP nanoflakes and highly conductive CNTs, the good contact between the CNTs and BP nanoflakes. Firstly, the facial intercalation and fast diffusion of ions between the adjacent puckered layers owing to the large spacing for BP,16 which is more suitable as electrode material of electrical double-layer capacitor (EDLC) compared with graphene. Secondly, the 3D network structure formed by introduction of CNTs not only plays a key role of fast conductive bridge between BP nanoflakes and current collector but also efficiently suppresses the restacking effect of BP nanoflakes which is favorable for the efficient intercalation and electric adsorption of ions. Moreover, the characteristic structure and mechanical ﬂexibility of CNTs is beneficial to the enhancement of ﬂexibility and mechanical 10 ACS Paragon Plus Environment

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strength for the fabricated BP/CNTs composite paper which is critical for integrated transfer of BP/CNTs composite paper from the filter to Au-PET. In addition, our flexible BP/CNTs (1:4) ASSP device can still keep excellent electrochemical performance even after 11 months in the air, and exhibits excellent environmental stability.

4. CONCLUSION In summary, we demonstrate that BP nanoflakes/CNTs composite paper as flexible electrodes in all-solid-state supercapacitors shows excellent electrochemical energy storage performance. The fabricated ASSP of BP/CNTs (1:4) has been found to deliver large volumetric capacitance (41.1 F/cm3 at the scan rate of 0.005 V/s), outstanding mechanical flexibility and high cycling stability (91.5% capacitance retained after 10,000 cycles). The BP/CNTs (1:4) ASSP device still deliver large volumetric capacitance (35.7 F/cm3 at the scan rate of 0.005 V/s) even after 11 months esposure in air, suggesting excellent environmental stability. Moreover, the ASSP of BP/CNTs (1:4) exhibits an energy density ranging from 0.46 to 5.71 mWh/cm3 and a maximum power density of 821.62 W/cm3. As indicated in our work, the strategy of assembling stacked-layer composites films will open up novel possibility for realizing 2D BP and 1D CNTs in new-concept thin-film energy storage devices.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The optical, SEM images and XRD spectrum of BP crystal (Figure S1). Thickness, density, conductivity, volumetric capacitance at 0.1 V/s, and volumetric capacitance at 1.0 V/s of BP, CNTs and BP/CNTs composite papers as all-solid-sate supercapacitor 11 ACS Paragon Plus Environment


(Table S1). The TEM image of CNTs, and the TEM and selected area electron diffraction images of liquid-exfoliated BP nanoflakes (Figure S2). The optical image, Raman spectrum, XRD spectra, SEM image and energy dispersive spectral (EDS) mapping images of BP/CNTs composite paper (Figure S3). Electrochemical properties of a BP/CNTs (4:1) ASSP device, a BP/CNTs (1:1) ASSP device, a BP/CNTs (1:8) ASSP device and a bare CNTs paper ASSP device (Figures S4, S5, S6, and S7). Cyclic voltammograms curves at 1 V/s, galvanostatic charging/discharging curves at a current density of 1 A/cm3, stack capacitances, and Ragone plot of BP/CNTs ASSP devices with different mass proportion (Figure S8). Cycle stability of a CNTs paper ASSP device, a BP/CNTs (1:8) ASSP device, a BP/CNTs (1:1) ASSP device, and a BP/CNTs (4:1) ASSP device at a scan rate of 0.5 V/s (Figures S9, S10, S11, and S12). Stack capacitances of a BP/CNTs (1:4) ASSP device at different scan rates after 11 months in the air (Figure S13). EIS measurements of a BP/CNTs (1:4) ASSP device (Figure S14). ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (F. Wen). *E-mail: [email protected] (J. Xiang). *E-mail: [email protected] (Z. Liu). Author Contributions #

B. C. Yang and C. X. Hao contributed equally to this work.

Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Grant No. 51672240, 51571172, 51571171, 51421091), Natural Science Fundation for Distinguished Young Scholars of Hebei Province (Grant No. E2017203095).


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Figure 1. . Fabrication and characterization of BP/CNTs papers. (a) Schematic illustration and photographs of BP nanoflakes, CNTs solution, and the mixture dispersion. (b) Photograph of flexible BP/CNTs paper. (c) The I-V curves of BP/CNTs papers with the mass proportions of 1:4 and 4:1. (d) A comparison of electrical conductivity values for different mass proportion between BP nanoflakes and CNTs. The different magnification SEM images of (e-g) the top view and (h-j) cross section of BP/CNTs (1:4) paper.


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Figure 2. .Statistics results from different mass proportion of BP/CNTs paper ASSP devices at different scan rates, (a) 0.1 V/s, (b) 1 V/s, and (c) 10 V/s. Schematic illustration for the mechanism of electrosorption in different cases, (d) more BP nanoflakes, (e) appropriate proportion, and (f) more CNTs.



Figure 3. .Electrochemical properties of a BP/CNTs (1:4) ASSP device with ~1.13 µm thick paper electrodes. (a-c) Cyclic voltammograms at various scan rates reach up to 500 V/s. (d) Stack capacitances calculated from the CV curves at different scan rates. (e) Galvanostatic charging/discharging curves at different current densities. (f) CV curves at a scan rate of 1 V/s with differently bent configurations.


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Figure 4. .(a) Cycle stability of a BP/CNTs (1:4) ASSP device at a scan rate of 0.5 V/s. (b) The TEM morphology of BP/CNTs paper electrode (b) before charging and (c) after 10,000 cycles. The bar is 100 nm.



Figure 5. .(a) Galvanostatic charging/discharging curves for a single ASSP BP/CNTs (1:4) device and four connected devices in series at a current density of 10 A/cm3. The right photographs are four connected devices series lighting green LED (3.0-3.2 V) for 6 min. (b) Ragone plot of ASSP BP/CNTs (1:4) device, compared with several commercial energystorage systems37 and reported devices.11,14,16,29,38,39


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TOC


Carbon Nanotube Composite

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