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Flexible and Binder-Free Hierarchical Porous Carbon Film for Supercapacitor Electrodes Derived from MOFs/CNT Yazhi Liu,† Gaoran Li,‡ Yi Guo,† Yulong Ying,† and Xinsheng Peng*,† †

State Key Laboratory of Silicon Materials, School of Material Science and Engineering, and ‡Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: Rational design of free-standing porous carbon materials with large specific surface area and high conductivity is a great need for ligh-weight suprecapacitors. Here, we report a flexible porous carbon film composed of metal−organic framework (MOF)-derived porous carbon polyhedrons and carbon nanotubes (CNTs) as binder-free supercapacitor electrode for the first time. Due to the synergistic combination of carbon polyhedrons and CNT, the obtained carbon electrode shows a specific capacitance of 381.2 F g−1 at 5 mV s−1 and 194.8 F g−1 at 2 A g−1 and outstanding cycling stability with a Coulombic effciency above 95% after 10000 cycles at 10 A g−1. The assembled aqueous symmetrical supercapacitor exhibits an energy density of 9.1 Wh kg−1 with a power density of 3500 W kg−1. The work opens a new way to design flexible MOF-based hierarchical porous carbon film as binder-free electrodes for high-performance energy storage devices. KEYWORDS: metal−organic frameworks, carbon nanotube, flexible carbon film, hierarchical porous structure, supercapacitor

1. INTRODUCTION The current increasing demand for portable and wearable electronics has promoted urgent needs for flexible and lightweight energy storage devices.1−5 Due to the high power density, fast charge/discharge rate, and excellent cycling stability, supercapacitors are considered as promising power sources for lightweight electronics.6−10 Carbon materials are the dominant electrode materials in supercapacitors owing to their low cost, wide availability, and excellent chemical stability.11−13 Nevertheless, fundamental improvements of carbon materials are needed to improve their electrochemical performance. On the basis of the energystorage mechanism of the electrical double-layer capacitor (EDLC), charge uptake is a key parameter that determines energy storage and output.14,15 Hence, much effort has been made to improve the specific surface area (SBET) and pore texture of the porous carbon.16−20 A universal approach is to design 3D hierarchical porous carbon nanoarchitecture with high surface area. This unique architecture can greatly enhance the wettability of the carbon material, facilitate ion immigration, and increase the accessible surface area of the electrode.21−26 Recently, metal−organic frameworks (MOFs), or porous coordination polymers (PCPs), have been explored as templates to prepare 3D hierarchical porous carbon and carrier to support capacitive active materials owing to their high SBET, tailoring pore structures and controlled channels.27−33 However, in order to further adjust the porous structure, the carbonized MOFs carbon needs post-chemical activation, such as HF etching or KOH activation, which is costly and complicated.34−37 Moreover, due to the powder nature of the © XXXX American Chemical Society

MOF-derived carbon, insulated polymer binder and black carbon are needed to fabricate the bulk electrode, which leads to extra weight, decreased accessible surface area, poor electron transfer, and increased inner resistance of the electrode. All of these disadvantages prevent the MOF-based carbon from fulfilling the requirements of lightweight electronics. Given the above considerations, fabricating a self-standing MOF-based carbon film with large SBET, hierarchical porous structure, and high electrical conductivity may be a promising approach to solve the problems.38 Here, we introduce a novel strategy for the development of a free-standing hierarchical porous carbon film (HPCF) consisting of MOF-derived porous carbon polyhedrons and carbon nanotubes (CNTs) for binder-free supercapacitor electrodes. In this construction, HKUST-1 (Cu3(BTC)2) is chosen as the carbon source, while CNT plays dual role, including the strings to interweave the carbon polyhedrons and the entire electrode into a free-standing film and the conductive agent to endow the electrode with excellent electronic conductivity. The obtained HPCFs show good flexibility, large SBET, interconnected porous structure, and outstanding conductivity owing to the synergistic effects of the porous carbon polyhedrons and CNT. The effects of the carbonization temperature, film thickness, and CNT content on the electrochemical behavior are systematically investigated. The results demonstrate that HPCF4 exhibits a specific capacitance of 381.2 F g−1 at 5 mV s−1, 194.8 F g−1 at 2 Received: March 9, 2017 Accepted: April 7, 2017 Published: April 7, 2017 A

DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Typical Fabrication Process of Flexible HPCF

A g−1, and outstanding cycling stability with a Coulombic effciency above 95% after 10000 cycles at 10 A g−1. The assembled symmetrical supercapacitor delivers an energy density of 9.1 Wh kg−1 with a power density of 3500 W kg−1 in 6 M KOH aqueous electrolyte.

2.3. Electrochemical Measurement. The as-prepared flexible HPCFs were directly used as working electrodes without any additives. To ensure that the electrode materials were thoroughly saturated with electrolyte solution, the working electrode was soaked in the electrolyte for 4 h before the electrochemical tests. Electrochemical experiments for individual electrode were performed in a threeelectrode system with platinum foil as counter electrode and Ag/AgCl as reference electrode in 6 M KOH electrolyte. The symmetric supercapacitor was fabricated by using two pieces of HPCF with a glass fiber separator in 6 M KOH solution. In the three-electrode configuration, the capacitance can be calculated according to the following equations

2. EXPERIMENTAL SECTION 2.1. Preparation of Flexible HPCF. First, positively charged copper hydroxide nanostrands (CHNs) and negatively charged CNT colloid solutions were prepared on the basis of our previous works.39,40 The mixture suspension of CHNs and CNT was filtered on a porous polycarbonate support to obtain a CHN/CNT hybrid thin film. Then, the CHN/CNT film was immersed in 20 mM H3BTC ethanol−water (1:1 in volume ratio) solution for 1 h to obtain a HKUST-1/CNT thin film. By controlling the volume ratio of CHNs to CNT, HKUST-1/ CNT thin films with different weight ratio was obtained. In this work, three samples with weight ratios (HKUST-1/CNT) of 20/1, 40/1, and 80/1 were fabricated, respectively. The transformation of HKUST-1/CNT thin film into flexible HPCF was achieved through a two-step morphology-preserved thermal conversion process. Typically, the HKUST-1/CNT thin film was first heated at 120 °C under nitrogen gas for 1 h and then calcinated at a certain temperature for 3 h. After being cooled to room temperature, the pyrolysis product was immersed in 0.7 M HNO3 aqueous solution for 48 h to remove the copper species resulting from the HKUST-1 pyrolysis. The freestanding HPCF was finally obtained after washing the precipitate with abundant water and drying at 80 °C overnight. To understand the influence of temperature on structure and subsequent electrochemical properties, the targeted temperatures were 400, 500, 600, 800, and 900 °C. The 40/1 HKUST-1/CNT sample calcinated at 400, 500, 600, 800, and 900 °C was designated accordingly as HPCF1, -2, -3, -4, and -5. For comparison, pure HKUST-1 powder was also calcinated under the same conditions and denoted as PCF1−5. Additionally, the 20/1 and 80/1 HKUST-1/CNT samples which calcinated at 800 °C were designated as HPCF6 and HPCF7, respectively. 2.2. Characterization. Scanning electronic microscopy (SEM) and transmission electron microscopy (TEM) tests were performed using a Hitachi SU-70 and Fei Tecnai G2 F20 S-TWIN, respectively. Fourier transform spectra (FTIR) were conducted on a Tensor 27 FTIR spectrometer (Bruker, Inc.) using KBr pellets. Raman spectra were recorded using laser Raman spectroscopy (Renishaw RM1000) under an excitation length of 532 nm. X-ray diffraction (XRD) was characterized using a Shimadzu XRD-6000 diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). The thermogravimetric analysis (TGA) was performed with a TA Q500 simultaneous thermal analyzer under N2 flow with a heating rate of 5 °C min−1. X-ray photoelectron spectroscopy (XPS) was obtained by an ESCALAB_250Xi X-ray photoelectron spectrometer using Al Kα X-ray as the excitation source. N2 sorption analysis was recorded by a Micromeritics specific area analyzer at 77 K using Brunauer−Emmett−Teller (BET) calculations for the specific surface area. The pore size distribution was obtained using the Barret−Joyner−Halenda (BJH) model and density-functional-theory (DFT) model on the adsorption branch of the isotherm. Electrical conductivity was recorded from a four-point probe resistivity measurement system (RTS-8 Four Probes Tech).

Csp =

∫ I dV /νmV

(1)

where Csp (F g−1) is the specific capacitance from the closed cyclic voltammetry (CV) curve, I (A) is the current relating to the voltage V, V (V) is the voltage window, ν (mV s−1) is the scan rate, and m (g) is the mass of the active materials.

Cg = It /(mV )

(2)

Here, Cg is the specific galvanostatic capacitance f rom galvanostatic charge−discharge (GCD) curves and t (s) is the discharge time. For the two-electrode symmetric supercapacitors, the specific galvanostatic capacitance, the energy density (E, W h kg−1), and power density (P, W kg−1) were calculated by eqs 3−5

Cs = 2It /(mV )

(3)

E = CsV 2/(2 × 4 × 3.6)

(4)

P = E /t

(5)

where Cs is the specific galvanostatic capacitance of a single electrode and V (V) is the cell voltage after ohmic drop.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of HPCF. Scheme 1 illustrates the typical process to prepare the flexible HPCF. First, the CHN/CNT hybrid thin film was formed by filtering the homogeneous mixture of CHNs and CNT suspension. Then, the CHN/CNT thin film was immersed in 1,3,5benzenetricaboxylic acid (H3BTC) ethanol−water solution to prepare the HKUST-1/CNT film. Subsequently, the HKUST1/CNT thin film was calcinated and washed with HNO3 solution. After rinsing with abundant water and drying at 80 °C, the flexible HPCF film was obtained. Figure 1a reveals a well-matched XRD pattern with the reported results,41 indicating the successful preparation of HKUST-1/CNT film. The intense peak at ∼26° is assigned to the highly graphitic carbon wall of CNT. The thermalgravimetric analysis (Figure 1b) of HKUST-1/CNT shows two weight losses at around 100 °C and 315−382 °C. The former is due to the removal of adsorbed solvent in the pores of HKUST-1, while the latter is assigned to the pyrolysis of B

DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

HKUST-1, the shapes of the polyhedrons were well retained without decreasing electrical conductivity (Table S1). Moreover, it is noteworthy that through the acid treatment the Cu and Cu2O nanoparticles serve as the pore-forming agent to enrich the porous structure of the HPCFs by producing additional meso- and macropores in the carbon frameworks. However, previous studies of MOF-based porous carbon suffered from massive use of KOH activation or pore-forming additives to adjust the pore structure.36,42 The HPCF1, -2, -3, and -5 show similar morphology to that of the HPCF4 (Figure S1f−i). Owing to the CNT interweavement and the porous structure, the obtained HPCF4 shows excellent flexibility, which is able to restore its initial shape after bending (movie S1 and Figure S2). Based on the thickness and resistance, the electrical conductivity of HPCF1, -2, -3, -4, and -5 was calculated to be 532, 929, 1042, 1320, and 1240 S m−1, respectively (Figure S2b). The TEM images of the HPCF4@Cu and HPCF4 highlight the enriched hierarchical porous structure after acid treatment (Figure 3a and d). Magnification of the HPCF4 (Figure 3e)

Figure 1. (a) XRD pattern and (b) TGA curve of 40/1 HKUST-1/ CNT.

HKUST-1. Since no apparent weight loss is observed after 382 °C, we assume that the majority of functional groups in HKUST-1/CNT film have been pyrolyzed before 382 °C. On the basis of this assumption, the HKUST-1/CNT thin film was calcinated at 400, 500, 600, 800, and 900 °C to investigate the effect of calcination temperature on electrochemical performance. Parts a−i of Figure 2 clearly show the morphology evolution from 40/1 HKUST-1/CNT to HPCF. The hybrid HKUST-1/

Figure 3. (a, d) TEM images of HPCF4@Cu and HPCF4. (b, e) High-magnification TEM images of (a) and (d). (c, f) Cu, C, and O element mapping results of (a) and (c).

further confirms CNTs penetrate into the carbon polyhedrons. Simultaneously, the CNT are stretching out from the porous carbon polyhedrons interior and stringing the adjacent polyhedrons together, providing the HPCFs with excellent mechanical properties, fully interconnected porous structure and conductive networks (Figure S3). The elemental mapping results of C, O, and Cu of HPCF4 and HPCF4@Cu demonstrate the absence of copper species after acid treatment (Figure 3c,f). The TEM and XPS results of PCF4 show similar results (Figure S4). In order to avoid the signal interference of CNT, the XRD, FTIR, and Raman analysis of the PCFs rather than HPCFs are performed (Figure 4). For all of the samples, the weak and broad peaks at around 24.5° in the XRD pattern suggest that the graphitic structure was not well developed; however, it was improved by raising the temperature (Figure 4a). In addition, after acid treatment, the Cu and Cu2O peaks disappear, which coincides with the TEM and XPS result of PCF4. The FTIR spectra of the HKUST-1 and PCF4 are shown in Figure 4b. For HKUST-1, the peaks at 732, 761, 938 cm−1 are related to C− CO2 stretching,43 1171 cm−1 for C−O stretching, and 1371, 1443, 1631 cm−1 for COO−Cu2 stretching.44 Nevertheless, these peaks become weak or disappear after calcination at 400

Figure 2. SEM images of (a) surface and (b) cross section of 40/1 HKUST-1/CNT film. (d) Surface and (e) cross section of HPCF4@ Cu. (g) Surface and (h) cross section of HPCF4. (c), (f), and (i) are the corresponding high magnification of (b), (e), and (h), respectively. The inset of (g) is the digital image of HPCF4.

CNT thin film exhibits a typical lamellar structure, with an average HKUST-1 particle size of 1 μm and a thickness of 35 μm (Figure 2a and b), far surpassing that of the CHNs/CNT precursor (17 μm, Figure S1d,e). Because of the uniform interweavement in CHNs/CNT film, the CNT penetrates into the HKUST-1 polyhedrons and strings them together after in situ growth of the crystals, contributing to a flexible and freestanding thin film (Figure 2c). After the calcination, the carbon polyhedrons become relatively rough due to the parasitic Cu and Cu2O nanoparticles formed during the thermal treatment (Figure S1a,b). These copper species can be easily removed after acid treatment as confirmed in the SEM images (Figure 2g−i), and though the thickness of HPCF4 (25 μm) decreased after the thermal and acid treatment due to the pyrolysis of C

DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) XRD patterns, (b) FTIR spectra, and (c) Raman spectra of PCF1−PCF5.

°C, indicating that 400 °C is enough to decompose the HKUST-1, which agrees well with the TGA result. Furthermore, the ID/IG ratio increases with the rising calcination temperature, suggesting the increasing defects of the products, which is in line with the enhanced surface area and pore volume. The composition of the HPCFs was measured by XPS. The overview XPS spectra of HPCF4 and HPCF4@Cu (Figure S5a) show signals from C 1s and O 1s, while the Cu 2p signals are only detected in HPCF4@Cu (Figure S5b), reconfirming the removal of Cu species after acid treatment. Figure S5c represents the detailed high-resolution XPS data of C 1s after deconvolution. The peaks at 284.4, 285.2, 286.0, and 287.7 eV correspond to CC, C−C, C−O, and CO, and the additional small peak at 290.6 eV is assigned to the π−π* electronic transitions.45 The atomic ratios of C, O, and Cu elements according to the XPS analysis are listed in Table S1. The C content increases while the O content decreases gradually with the increasing calcination temperature, indicating deeper carbonization of the precursor. The N2 sorption isotherms were performed to determine the detailed porous characteristics of the HPCFs. The BET calculations, BJH model, and DFT model are adopted to analyze the SBET and the pore size distribution, as summarized in Figure S6 and Table S1. After acid treatment, both the SBET and the pore volume show a remarkable enhancement from 190.2 m2 g−1 and 0.19 cm3 g−1 (HPCF4@Cu) to 620.1 m2 g−1 and 0.63 cm3 g−1 (HPCF4), indicating the contribution made by Cu and Cu2O elimination to the surface area of the products. Additionally, the meso- and macropores increased greatly after acid treatment (Figure S6a,c,e), which is in good agreement with the TEM results. Besides, the HPCF1−5 show a typical type IV hysteresis loop with a steep uptake at low pressure, suggesting the coexistence of micro-, meso-, and macropores. The SBET values of the HPCFs display a positive relation with the calcination temperature, increasing from 330.2 (HPCF1) to 620.1 m2 g−1 (HPCF4). From the BJH analysis, it is noticeable that the contents of meso- and macropores were enhanced with the calcination temperature, which can be ascribed to the accelerated gas release and the faster growth of copper nanoparticles at higher calcination temperature. In the DFT analysis, when the temperature exceeds 800 °C, the dominant ∼3.8 nm pores are absent and the SBET of the sample shows a slight decrease, which is attributed to the collapse of the nanoporous structure. All these results indicate that we have successfully prepared the HPCFs with adjustable porous structures. 3.2. Electrochemical Characterization of Porous Carbon Film. Figure 5a shows the CV curves of the HPCFs and PCF4 at 200 mV s−1. The CV curves of HPCF1, -2, and -3 are distorted with a pair of obvious redox peaks at ∼−0.6 V,

Figure 5. Electrochemical performance of the HPCFs. (a) CV curves and (b) GCD profiles of the HPCFs and PCF4 at 200 mV s−1 and 10 A g−1, respectively. (c) CV curves and (d) GCD profiles of HPCF4 at different current densities. (e, f) Specific capacitance of the samples as a function of scan rate and current density, respectively.

while the HPCF4 and -5 possess an ideal quasi-rectangular shape without any redox peaks, indicating the additional pseudocapacitance of HPCF1, -2, and -3 comes from the oxygen-containing groups (Table S1). The specific capacitance of HPCF1, -2, -3, -4, and -5 at 200 mV s−1 was calculated to be 63.9, 192.9, 213.8, 253.9, and 248.0 F g−1, respectively. As for PCF4, the specific capacitance is 172.9 F g−1, remarkably lower than that of HPCF4. Figure 5b illustrates the GCD profiles of the samples at 10 A g−1. All of the samples display a good symmetric triangle shape, revealing good reversibility during the charge−discharge process. The capacitance of HPCF1, -2, -3, -4, and -5 is 32.0, 81.9, 117.3, 144.2, and 133.3 F g−1, respectively. As can be seen, the specific capacitance increases with the temperature when it is lower than 800 °C. This is due to the incomplete pyrolysis under the lower carbonization temperature, resulting in a higher oxygen groups content, lower graphitic degree, and lower electron conductivity, as confirmed by the XPS, XRD, and electrical conductivity results. However, when the temperature exceeds 800 °C, the capacitance shows a slight decrease, which is due to the collapse of the porous structure. D

DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

S8a−c). Obviously, the closed area of the CV curve decreases with the increase in film thickness, which can be attributed to the increase inner ionic diffusion resistance, as confirmed by the EIS results (Figure S8d). To explore the effect of CNT content, we prepared HPCF6 and HPCF7, of which the weight ratio of HKUST-1 to CNT was 20/1 and 80/1, respectively. The carbon nanotube treated under the same conditions was named CNT. Figure 7 shows the morphology evolution to prepare

Figure 5c presents the CV curves of the HPCF4 from 10 to 200 mV s−1. Upon the increase of the scan rate, such a quasirectangular shape is well maintained without obvious change, suggesting the excellent rate performance of HPCF4. Figure 5d shows the GCD profiles of HPCF4 at different current densities. The specific capacitance is calculated to be 194.8, 144.2, and 120.9 F g−1 at 2, 10, and 100 A g−1. The high electrical conductivity and efficient ion transport networks endows HPCF4 with superior rate capability (larger than 60% retention from 2 to 100 A g−1), exceeding most other flexible or self-standing carbon electrodes (Table S2). Besides, these values are higher than other reported MOF-based carbon electrodes and comparable to other carbon electrodes, such as biomass-derived carbons, activated carbons, and graphene, for aqueous electrolyte-based supercapacitors (Table S3). The specific capacitance as a function of both scan rate and current density is shown in Figure 5e,f (see detailed information in Tables S4 and S5). To further understand the electrochemical performance of the as-prepared electrodes, electrochemical impedance spectroscopy (EIS) was carried out with a frequency ranging from 10−2 to 105 Hz (Figure 6a). In the Nyquist plots, the diameter

Figure 7. SEM images of HKUST-1/CNT films with different weight ratios. (a), (c), (e) and (g) Cross sections of 20/1 HKUST-1/CNT films, HPCF6, 80/1 HKUST-1/CNT films, and HPCF7, respectively. (b), (d), (f), and (h) Corresponding high magnification SEM images of (a), (c), (e), and (g), respectively.

Figure 6. (a) Nyquist plots of the HPCFs. Inset: high-ffrequency region. (b) Long cycle performance and Coulombic efficiency of HPCF4 at 10 A g−1.

of the small semicircle from the high- to mid-frequency region indicates the charge-transfer resistance (R ct ), and the intersection of the real axis represents the electrolyte resistance (Rs), which comprises electrode ohmic resistance and solution resistance.46 Obviously, HPCF1 displayed the largest Rct, while HPCF4 displayed the smallest one, indicating the highest ionic conductivity of the HPCF4. In addition, judging from the intersection of the real axis, the HPCF4 and -5 present a smaller Rs than that of HPCF1, -2, and -3. The lowest equivalent internal resistance, coupling with the excellent electrical conductivity, highest pore volume, and largest SBET value result in the best electrochemical performance of HPCF4. The long-term cycling test of HPFC4 was carried out at 10 A g−1 for 10000 cycles (Figure 6b). Though slight capacitance decay is observed at the first 250 cycles, it was able to retain 95% of its initial capacitance after 10000 cycles, demonstrating the superior cycling durability and high ion mobility of the HPCF4. Besides, after the cycling tests, the hierarchical porous structures, specific surface area as well as the pore size distribution of HPCF4 (Figure S7a,b) are almost reserved (Figure S7), indicating the superior structural stability of the asprepared carbon materials during the electrochemical process. It is clear that 800 °C is the optimal pyrolysis temperature for the weight ratio of HKUST-1 to CNT 40/1. However, the film thickness and the content of CNT are also important parameters that affect the capacitive behavior. The HPCF4s with different thicknesses were prepared, and their electrochemical performance at 200 mV s−1 was measured (Figure

HPCF6 and 7. Figure 8 displays the electrochemical tests of the HPCF4, -6, and -7 and CNT electrode. In Figure 8a, all of the samples show a quasi-rectangular shape at 200 mV s−1; however, the integrated area of the CNT and HPCF6 and -7 are smaller than that of HPCF4. Besides, the capacitance of CNT, HPCF4, and PCF4 at 200 mV s−1 is 74, 253.9, and 172.9

Figure 8. Electrochemical performance of HPCF4, -6, and -7. (a) CV curves and (b) GCD curves of HPCF4, -6, and -7 and CNT at 200 mV s−1 and 10 A g−1. (c, d) Specific capacitance calculated from scan rate and current density, respectively. E

DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces F g−1, demonstrating the synergistic effects of the two component as well as the major-capacitance-contributor role of the carbon polyhedrons. Figure 8b shows the GCD test of these samples at 10 A g−1. The charge−discharge profiles present good symmetric triangular shapes, suggesting the ideal electrical double layer capacitance. The specific capacitances of these samples as a function of scan rate and current density are displayed in parts c and d, respectively, of Figure 8. Obviously, the HPCF4 achieves the highest capacitance among the samples, implying that 40/1 is the optimal mass ratio of HKUST-1 to CNT. This can be explained as follows: the CNT acts as the electrical conductive network, while the carbon polyhedrons serve as the major contributor to capacitance. Therefore, for HPCF7, the low capacitance is caused by the low CNT content, as confirmed by the largest Rct and Rs in the EIS spectra (Figure S9). On the other hand, for HPCF6, the decreased capacitance is caused by the lower carbon polyhedron content. We also assembled a symmetric supercapacitor cell based on HPCF4 for better understanding of its electrochemical performance. The GCD curves in Figure 9b maintain perfect

supercapacitor, the HPCF4 displays 381.2 F g−1 at 5 mV s−1, 194.8 F g−1 at 2 A g−1, and outstanding cycling stability at 10 A g−1. The assembled aqueous symmetrical supercapacitor exhibits an energy density of 9.1 Wh kg−1 with a power density of 3500 W kg−1. This work opens up a new pathway to design free-standing and flexible porous carbon thin films for binderfree and high-performance energy storage devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03368. SEM images of HPCF1, -2, -3, and -5; tensile strength− strain curves and mechanical data of the HPCFs; TEM images of PCF4@Cu and PCF4; XPS spectra of HPCF4@Cu and HPCF4; N2 adsorption/desorption isotherms and pore size of distribution of the HPCFs; physical and electrochemical properties of the samples; SEM and N2 adsorption/desorption isotherms of HPCF4 after cycling tests; CV and EIS results of HPCF4 with different thickness and comparison of electrochemical performance between HPCF4 with other carbon-based materials (PDF) Movie showing the flexibility of HPCF4 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 571 87952625. Tel: +86 571 87951958. E-mail: [email protected]. ORCID

Xinsheng Peng: 0000-0002-5355-4854 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program (2016YFA0200204), the National Basic Research Program of China 973 Program (2015CB655302), the Natural Science Foundation for Outstanding Young Scientist of Zhejiang Province, China (LR14E020001), and the National Natural Science Foundations of China (NSFC 21671171).

Figure 9. (a) CV curves and (b) GCD curves of HPCF4-based symmetric supercapacitor cell at different scan rates and current densities. (c) Specific capacitance for a single electrode at different current densities. (d) Ragone plots and performance comparison of the as-prepared symmetric cell vs previously reported carbon symmetric supercapacitors in aqueous electrolyte.

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symmetric triangular shapes at different current density with little IR drop, implying the ideal capacitive properties with good reversibility. The specific capacitance was 157.9 F g−1 at 1 A g−1 and retains 128.9 F g−1 at 10 A g−1, demonstrating its excellent rate capability. The Ragone plot of the HPCF4//HPCF4 symmetric supercapacitor cell (Figure 9d) shows an energy density of 9.1 Wh kg−1 at a power density of 3500 W kg−1, comparable to that of other carbon-based aqueous symmetric supercapacitors.2,11,47−50

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4. CONCLUSIONS In summary, we have prepared a flexible hierarchical porous carbon film (HPCF) as binder-free supercapacitor electrode via simple calcination of HKUST-1/CNT thin film. The porous structure of the HPCF can be easily tuned by the calcination temperature. The meticulous structural design endows the HPCF4 with the largest SBET (620.1 m2 g−1), hierarchical porous structure, highest electrical conductivity (1320 S m−1), and excellent flexibility. When used as binder-free electrode for F

DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX