Three-Dimensional Networked Metal–Organic Frameworks with

Oct 30, 2017 - Metal–organic frameworks (MOFs) with high porosity and a regular porous structure have emerged as a promising electrode material for ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38737-38744

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Three-Dimensional Networked Metal−Organic Frameworks with Conductive Polypyrrole Tubes for Flexible Supercapacitors Xingtao Xu,†,‡ Jing Tang,*,‡ Huayu Qian,‡ Shujin Hou,† Yoshio Bando,‡,§ Md. Shahriar A. Hossain,‡,§ Likun Pan,*,† and Yusuke Yamauchi*,‡,§,⊥ †

Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials Science, East China Normal University, 3663 N. Zhongshan Rd., Shanghai 200-062 China ‡ International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Australian Institute for Innovative Materials (AIIM), University of Wollongong, Squires Way, North Wollongong, New South Wales 2500, Australia ⊥ School of Chemical Engineering & Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane QLD 4072, Australia S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) with high porosity and a regular porous structure have emerged as a promising electrode material for supercapacitors, but their poor electrical conductivity limits their utilization efficiency and capacitive performance. To increase the overall electrical conductivity as well as the efficiency of MOF particles, threedimensional networked MOFs are developed via using preprepared conductive polypyrrole (PPy) tubes as the support for in situ growth of MOF particles. As a result, the highly conductive PPy tubes that run through the MOF particles not only increase the electron transfer between MOF particles and maintain the high effective porosity of the MOFs but also endow the MOFs with flexibility. Promoted by such elaborately designed MOF−PPy networks, the specific capacitance of MOF particles has been increased from 99.2 F g−1 for pristine zeolitic imidazolate framework (ZIF)-67 to 597.6 F g−1 for ZIF−PPy networks, indicating the importance of the design of the ZIF−PPy continuous microstructure. Furthermore, a flexible supercapacitor device based on ZIF−PPy networks shows an outstanding areal capacitance of 225.8 mF cm−2, which is far above other MOFs-based supercapacitors reported up to date, confirming the significance of in situ synthetic chemistry as well as the importance of hybrid materials on the nanoscale. KEYWORDS: metal−organic frameworks, conductive polymer, supercapacitor, flexibility, in situ synthesis

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MOFs in a wide variety of fields, including gas storage, separation and adsorption, catalysis, and drug delivery.3−5 Recently, several pioneering studies have used pristine MOFs as electrode active materials for supercapacitors.6−8 Such MOFs used for supercapacitors are typically microporous and show the potential of excellent capacitive performance.9,10 Addition-

INTRODUCTION The past decade has witnessed explosive growth in the preparation, characterization, and study of the materials known as metal−organic frameworks (MOFs). These materials are generally constructed by joining metal ions with organic linkers, using strong bonds to engineer open crystalline frameworks with permanent porosity.1,2 Careful selection of MOF constituents, that is, the metal ions and organic linkers, can yield crystals with ultrahigh porosity, regular pores, and high chemical stability, thus giving rise to potential uses of © 2017 American Chemical Society

Received: July 9, 2017 Accepted: October 9, 2017 Published: October 30, 2017 38737

DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the procedure for the fabrication of (a) ZIF-67 and (b) ZIF−PPy.

approach is attractive and can achieve a high areal capacitance, it raises the production cost, prolongs the synthesis time, and has limited universality for producing new MOFs. Thus, it is still a challenge to explore a novel and simple method for the preparation of MOF-based electrode materials that possess good electrical conductivity and effective porosity for supercapacitors. Polypyrrole (PPy), a typical conjugated polymer in the form of nanotubes, has been considered as a promising pseudocapacitive electrode material for supercapacitors because of its low cost, high electrical conductivity in doped states (100−10 000 S m−1), and ease of large-scale fabrication.14−16 Although PPy exhibits excellent specific capacitance (400−3400 F g−1),17 its large volumetric swelling and shrinking during the charge/ discharge process often leads to structural breakdown and thus fast capacitance decay,18 which limit its application in supercapacitors. In addition, PPy has been widely investigated for adsorption of heavy metal ions because of its well-known nitrogen-containing functional groups and the negative charges in solution.19−21 Considering this point, PPy tubes might be a surprising substrate for the growth of MOFs by adsorbing positively charged metal ions and sequentially bridging them with specific organic linkers. To accelerate the wide application of MOFs in supercapacitors, herein, we report a new and simple strategy to increase the overall electrical conductivity as well as the efficiency of MOF particles for practical use by using conductive PPy tubes as the interconnected frameworks for in situ growth of MOF particles (Figure 1b). In this hybrid structure, each MOF particle can be effectively dispersed and connected by PPy tubes without sacrificing any porosity in the MOFs themselves. PPy tubes serve as both the glue and the dispersant for MOF particles, forming an “MOF-to-PPy-toMOF” conducting network. The MOF−PPy hybrid materials are expected to display high electrical double layer capacitance from the MOFs and provide extra pseudocapacitance from the PPy. Furthermore, the mechanical properties of MOFs can also be strengthened through networking MOF particles by PPy tubes, which could stabilize the microstructure of the MOFbased hybrids and maintain fast electron/ion diffusion under bending conditions,22 thus guaranteeing a stable energy output for wearable devices.

ally, electron transfer in such MOF-based electrodes usually relies on the contact between MOF particles, that is, a “MOFto-MOF” conducting pathway (Figure 1a). However, the poor electrical conductivity of MOFs limits electron transfer between MOF particles. Furthermore, the poor electrical conductivity of MOFs also hinders the electron transfer between an MOFbased electrode and current collectors.7 Thus, the utilization of MOFs as electrode materials in supercapacitors is low, as they lead to poor capacitance. To address this issue, various strategies have been developed so far and can be generally categorized into two classes. One strategy is using a conductive binder for “bridges” to construct an “MOF-to-conducting binder-to-MOF” conducting pathway. This concept was first reported by the Yaghi group.11 In their work, they first physically mixed graphene, a star twodimensional carbon material, with pristine MOF particles. In the resultant materials, graphene sheets could not only act as “bridges” between MOF particles for constructing an “MOF-tographene-to-MOF” conducting pathway, but also help to disperse the MOF particles as well. The contact between MOF particles and graphene was poor, however, because of the limitations of the physical mixing strategy, which would lead to a large loss of electron transfer. Additionally, the simple mixing method cannot ensure that every MOF particle is wellsurrounded by graphene and cannot prevent the aggregation of MOF particles. Thus, the low efficiency of the “MOF-to-MOF” electron transfer pathway still remains, reducing the efficiency of MOFs for practical use. Inspired by the pioneering work of Yaghi’s group, Wang et al. recently reported another conductive binder for MOFs by electrochemically interweaving MOFs with the conductive polymer polyaniline (PANI).12 In this structure, the electrochemically deposited conductive PANI not only increases the electron transfer between MOFs by effectively linking up the isolated MOF particles and forming an “MOFto-PANI-to-MOF” conducting pathway, but also provides pseudocapacitance. Nevertheless, the MOF particles were preprepared and coated on carbon cloth in Wang’s work, so that only superficial MOF particles could be linked by PANI, and the aggregation of MOFs was not prevented. Moreover, the blocking of pores in MOFs with surfaces that were covered by either graphene or PANI was unavoidable, which would hamper the diffusion of ions and their accommodation in MOFs. Another approach is chemically synthesizing new-type MOFs with good electrical conductivity.13 Although this 38738

DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744

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ACS Applied Materials & Interfaces

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chemical impedance spectroscopy (EIS) in the frequency range from 10 mHz to 100 kHz. For the gel electrolyte preparation, Na2SO4 (1 M, 6 g) and PVA powder (6 g) were added into 60 mL of deionized water, which was heated to 85 °C under stirring until the mixture became clear. In the case of flexible supercapacitors, two identical pieces of ZIF−PPy-2 flexible graphite electrodes (4 × 1.5 cm2) were directly immersed in a hot PVA/Na2SO4 gel electrolyte for 5 min. Afterward, the two asprepared electrodes were symmetrically integrated into one flexible supercapacitor device. Calculation of specific capacitances derived from the GCD curves: In general, the gravimetric specific capacitance (Cg, F g−1) is calculated using the following equation:

EXPERIMENTAL SECTION

Chemicals. Pyrrole, methyl orange (MO), FeCl3, Co(NO3)2· 6H2O, Na2SO4, 2-methylimidazole (2-MeIM), polyvinyl alcohol (PVA, Mw = 89 000 to 98 000), poly(vinylidene difluoride) (PVDF), methanol, and N-methyl 2-pyrrolidinone (NMP) were purchased from Wako Pure Chemical Industries and used without further purification. Synthesis of PPy Tubes. The synthesis of PPy tubes was established according to the procedure reported by Yang et al.23 The first step was the complete dispersion of 0.05 g MO in 60 mL of deionized water. Under intense stirring (300 rpm) in an ice bath, 0.243 g of FeCl3 was dispersed in an aqueous solution of MO with posterior inclusion of 0.105 mL of pyrrole. The mixture acquired a blackish aspect with high viscosity and was maintained under stirring (in the dark) for 24 h. The resulting powder was washed with ethanol and water (for MO elimination) several times and dried. Synthesis of ZIF−PPy. Typically, a certain amount of PPy tubes (20−100 mg) was dispersed in 20 mL of methanol solution with the assistance of ultrasonication for 1 h. The resultant solution was mixed with Co(NO3)2·6H2O (454 mg) and then stirred for 1 h to form solution A. Subsequently, 513 mg of 2-MeIM was dissolved in 20 mL of methanol to form a clear solution B, which was subsequently slowly added to solution A under stirring for 0.5 h. After being kept still for 24 h, zeolitic imidazolate framework (ZIF)−PPy was collected by centrifugation, washed thoroughly with methanol several times, and finally dried at 60 °C for 24 h. ZIF−PPy samples obtained from 20, 40, 60, and 100 mg PPy tubes are denoted as ZIF−PPy-1, ZIF−PPy-2, ZIF−PPy-3, and ZIF−PPy-5, respectively. The mass ratio of PPy in the ZIF−PPy is obtained by dividing the mass of the added PPy tubes by the total mass of the obtained ZIF−PPy product, and is estimated to be about 17, 28, 37, and 49% for ZIF−PPy-1, ZIF−PPy-2, ZIF− PPy-3, and ZIF−PPy-5, respectively. For comparison, ZIF-67 was synthesized by a similar process without the addition of PPy tubes. Material Characterization. Powder X-ray diffraction (XRD) patterns were obtained on an Ultima RINT 2000 X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation (40 kV, 40 mA, 2° min−1 scan rate). The Fourier transform infrared (FTIR) spectrum was recorded with a Thermo Scientific Nicolet 4700 FTIR spectrometer using a deuterated triglycine sulfate detector in the wavenumber range from 400 to 4000 cm−1. N2 adsorption/desorption isotherms were measured using a BELSORP-mini (BEL, Japan) at 77 K. The surface areas of all samples were estimated by the Brunauer−Emmett−Teller method by using the adsorption branch data in the relative pressure (P/P0) range of 0.05−0.5 based on the adsorption data. The overall morphology of the samples was characterized on a Hitachi SU8000 field-emission scanning electron microscope (FESEM) operating at 5 kV. Before the FESEM observations, a platinum coating was applied to ensure clear FESEM images. The specific morphology was obtained on a JEOL JEM-2100 field-emission transmission electron microscope (TEM). Electrochemical Performance Measurements. For electrode preparation, an 80 wt % sample was mixed with 10 wt % carbon black and 10 wt % PVDF in an NMP solvent. After ultrasonication for 15 min, a certain volume of the mixture was dropped onto flexible graphite paper (thickness: 1 mm) and dried at 60 °C. Typically, each working electrode comprises an exposed area of 1 × 1 cm2 with a mass loading of 1 mg. In addition, we choose flexible graphite paper as the current collector because of its excellent electrical conductivity as well as its negligible capacitive performance (usually below 1 F g−1), which would contribute little to the total capacitance of the prepared working electrode. All electrochemical measurements were carried out on a CHI 660E instrument. In a three-electrode system, the electrochemical performances of PPy, ZIF-67, and ZIF−PPy electrodes were tested by cyclic voltammetry (CV) and galvanostatic-charge−discharge (GCD) methods in 1 M Na2SO4 as the aqueous electrolyte, with a platinum wire as the counter electrode and an Ag/AgCl electrode as the reference electrode. The Nyquist plots were studied by electro-

Cg = I × t /(m × ΔV )

(1)

where I is the discharge current (A), t is the discharge time (s), m is the mass of active materials (g), and ΔV is the potential change during the discharge process (V). For areal specific capacitance (Ca, mF cm−2), eq 1 can be changed to the following equation: Ca = I × t /(S × ΔV )

(2)

where S is the apparent area of the electrodes (cm2). The specific capacitance of the flexible supercapacitor device derived from GCD curves can also be calculated by eq 2 with some changes, where S is the representative of the whole apparent area of electrodes (cm2). The energy (E, mW h cm−2) and power density (P, mW cm−2) of the flexible supercapacitor are the crucial parameters for practical applications, which can be calculated following these equations:

E = 0.5Ca(ΔV )2

P = E /t where Ca is the total areal specific capacitance of the flexible supercapacitor (mF cm−2), ΔV is the potential change during the discharge process (V), E is the energy, and t is the discharge time (s).

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RESULTS AND DISCUSSION In this work, we selected the famous ZIF-67 as a typical example of MOFs, and the obtained hybrids are denoted as ZIF−PPy. The synthetic strategy is schematically illustrated in Figure 1b, and the detailed synthetic processes are described in the Experimental Section. First, the presynthesized PPy tubes were homogeneously dispersed in a Co2+-containing methanol solution via ultrasonication. The free Co2+ ions would be adsorbed onto the negatively charged PPy tubes via electrostatic attraction.24 Then, the 2-MeIM methanolic solution was added drop by drop to induce the heterogeneous nucleation of ZIF-67 seed crystals on the surfaces of the PPy tubes, which would serve as crystal nuclei for the growth of large ZIF-67 polyhedra in the reaction process. Finally, the ZIF−PPy networks were obtained after 24 h of reaction. The PPy tubes were prepared first according to the established method23 and were well-characterized before further use. As shown in the FESEM image (Figure S1a), the as-prepared PPy tubes interconnect with each other and form a network, having a diameter of 200−400 nm and a length of several micrometers. The hollow structure and the rough surface of PPy tubes were detected from a random cracked PPy tube (Figure S1b). The FTIR (Figure S2) of our PPy tubes matches well with a previous report,25 implying the high purity of the as-prepared PPy. N2 adsorption/desorption measurements (Figure S3) show that PPy tubes possess a specific surface area of 16.9 m2 g−1. 38739

DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744

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and ZIF−PPy-5, respectively. All ZIF−PPy hybrids display a network consisting of uniform PPy tubes intertwined with ZIF polyhedra. The ZIF-67 particles can be well-dispersed and connected by PPy tubes. Moreover, an increased ratio of PPy tubes will lead to a decreased size of the ZIF-67 crystals, from 2.0 μm for ZIF−PPy-1 to 0.6 μm for ZIF−PPy-5 (Figures 2b,d,f,h) because there are more nucleation sites from the increased PPy tubes while the addition of the cobalt and 2MeIM precursors is fixed. To give further insight into the morphology and structure of the as-synthesized ZIF−PPy, TEM analysis was carried out for the representative ZIF−PPy-2. Figure 3a shows the overall

As shown in Figure 2, ZIF-67 particles have been grown successfully around the PPy tubes by using the PPy tubes as the

Figure 3. (a,b) TEM images of ZIF−PPy-2. (c) XRD patterns and (d) N2 adsorption/desorption isotherms of ZIF-67 and ZIF−PPy hybrids.

morphology of the ZIF−PPy network. Enlarged TEM image shown in Figure 3b reveals that ZIF−PPy consists of welldistributed ZIF-67 solid polyhedra which are penetrated and interlinked by hollow PPy tubes. The PPy tubes that run throughout the ZIF crystals not only act as a dispersant to prevent the aggregation of ZIF-67 crystals, but also act as the glue for networking the ZIF-67 particles, which would promote electron transfer between ZIF-67 particles. Notably, this work is the first report on the synthesis of an MOF-conducting polymer hybrid architecture. It is also expected that such a continuous hybrid structure of an MOF-conducting polymer should combine the advantages of good conductivity and good mechanical properties, and thus have potential as electrode materials for practical application in flexible supercapacitor devices. The phase identification of pure ZIF-67 and the ZIF− PPy hybrids was conducted by powder XRD. As shown in Figure 3c, the typical intense diffraction peaks of ZIF−PPy match well with those of pure ZIF-67, indicating the presence of highly-crystallized ZIF-67 in ZIF−PPy hybrids. The diffraction pattern of PPy is not observed because the diffraction intensity of PPy tubes is too weak compared with that of the ZIF crystals. The porosities of ZIF-67 and ZIF−PPy hybrids were investigated by N2 adsorption−desorption measurements (Figure 3d). The results show that the specific

Figure 2. FESEM images of (a,b) ZIF−PPy-1, (c,d) ZIF−PPy-2, (e,f) ZIF−PPy-3, and (g,h) ZIF−PPy-5.

substrate and adding Co2+ ions and 2-MeIM sequentially. It is noteworthy that the ratio of ZIF to PPy and the size of ZIF particles in the final ZIF−PPy hybrids can be easily adjusted by fixing the ZIF precursors while tuning the additional amount of the PPy substrate. To obtain an optimized sample, various amounts of PPy tubes (20, 40, 60, and 100 mg) were used for the synthesis of ZIF−PPy, and the obtained ZIF−PPy samples are denoted as ZIF−PPy-1, ZIF−PPy-2, ZIF−PPy-3, and ZIF− PPy-5, respectively. The actual weight ratio of PPy is obtained via dividing the mass of the added PPy tubes by the total mass of the obtained ZIF−PPy product, and is estimated to be 17, 28, 37, and 49% in the ZIF−PPy-1, ZIF−PPy-2, ZIF−PPy-3, 38740

DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744

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ACS Applied Materials & Interfaces surface area of ZIF−PPy decreases from 1545.2 m2 g−1 for ZIF−PPy-1 to 518.8 m2 g−1 for ZIF−PPy-5 (Table S2) because of the increased proportion of PPy tubes, which have only a low specific surface area of 16.9 m2 g−1. Generally, high specific surface area, regular pore size (especially sub-1 nm), and good electrical conductivity of porous materials usually lead to high capacitance. Considering that the prepared ZIF−PPy will combine the advantages of each component, including the high porosity and abundant micropores (sub-1 nm)26 of ZIF-67 and the good electrical conductivity from the famously conductive PPy tubes, ZIF− PPy would be a promising electrode material for electrochemical capacitors. The electrochemical performances of the ZIF−PPy hybrids, pure ZIF-67, and PPy tubes were first investigated in a three-electrode system. The CV curves shown in Figure S4a indicate that all samples display rectangle-like curves, and ZIF−PPy-2 exhibits the highest current density, suggesting that ZIF−PPy-2 has the best capacitive properties among all samples. The specific capacitances at 0.5 A g−1 (denoted as C0.5) and 20 A g−1 (denoted as C20) were calculated by GCD measurements (Figures 4a, S4b) and are

To determine the origin of the high capacitance in ZIF−PPy, we tried to separate the capacitive contributions from ZIF and PPy in the ZIF−PPy hybrid electrode. As we all know, PPy tubes store charges via Faradaic electron charge transfer (i.e. pseudocapacitance),39,40 while ZIF particles store charges via forming electrical double-layer capacitance.12 In our ZIF−PPy hybrid structure, ZIF particles do not change the structural and intrinsic properties of PPy tubes because PPy tubes just serve as the support for the growth of ZIF particles, and there is no strong chemical interfacial interaction between PPy tubes and ZIF particles. Thus, the pseudocapacitance of PPy tubes (CPPy = 443.3 F g−1) would be hardly influenced before and after interweaving the ZIF particles. On the basis of this assumption, the total capacitance of ZIF−PPy can be described by the equation CZIF−PPy × (mZIF + mPPy) = CZIF × mZIF + CPPy × mPPy, where CZIF−PPy is the specific capacitance of ZIF−PPy (F g−1), mZIF and mPPy are the masses of ZIF and PPy in ZIF−PPy, respectively, CZIF is the specific capacitance of ZIF in ZIF−PPy (F g−1), and CPPy is the specific capacitance of PPy (F g−1). Because we already know the mass ratio of PPy to ZIF in the ZIF−PPy hybrids (Table S3), CZIF in different ZIF−PPy samples would be calculated after excluding the capacitive contribution from the PPy tubes. Thus, the CZIF in each ZIF− PPy is estimated to be 412.2, 597.6, 556.3, and 497.2 F g−1 for ZIF−PPy-1, -2, -3, and -5, respectively (Table S3). As plotted in Figure 4b, it is clear that the specific capacitance of ZIF can be increased remarkably after integrating with PPy tubes, and the value can be maximized by adjusting the proportion of the PPy substrate and exceeds all those obtained from the previous research.11,12,41−43 The electrical conductivities of the ZIF−PPy electrodes were investigated by EIS (Figure 4c and Table S3). After incorporation of PPy into ZIF-67, the charge transfer resistance (Rct) of ZIF−PPy gradually decreases with the increase of PPy, indicating that the introduction of PPy is favorable for accelerating the charge transfer in the ZIF−PPy electrodes. Additionally, ZIF−PPy electrodes also show steeper linear gradients in the sloped region of the Nyquist plot (which is known as the Warburg region), providing evidence for the lower resistance in the ZIF−PPy electrode than in the pure ZIF-67 electrode.44 The increased capacitive performance of ZIF can be ascribed to the novel “ZIF-to-PPy-to-ZIF” conducting network in the ZIF−PPy continuous microstructure. As mentioned above, PPy tubes serve as both the glue and the dispersant for ZIF particles in this hybrid structure, so that electrons can be transferred efficiently through the thus-formed “ZIF-to-PPy-to-ZIF” conducting network during electrochemical application. In addition, the PPy tubes also help to promote electron transfer between the ZIF−PPy hybrid and the current collector. As a result, ZIF particles can be used more effectively for charge storage, which leads to superior capacitance as well as improved capacitance retention. When the proportion of PPy further increases, however, the value of CZIF decreases slightly, possibly because of the following reasons: (i) the increased ratio of PPy will cause serious agglomeration of PPy tubes in the ZIF−PPy hybrid (see Figure 2e−h). Thus, lots of PPy tubes cannot be utilized efficiently as the support for growing ZIF particles, leading to a poor use of PPy for promoting the charge transfer between ZIF particles and PPy tubes. Thus, the electron transfer pathways of “ZIF-to-PPy-to-ZIF” are not further improved. (ii) As shown in Figure 2, the size of ZIF particles becomes smaller as the proportion of PPy is increased, resulting

Figure 4. (a) GCD curves at 0.5 A g−1 of PPy, ZIF-67, and ZIF−PPy hybrids. (b) CZIF and capacitance retention of ZIF-67 and ZIF−PPy hybrids. (c) Nyquist impedance spectra of PPy, ZIF-67, and ZIF−PPy hybrids. (d) Comparison of the cycling performance of PPy, ZIF-67, and ZIF−PPy-2 at a current density of 20 A g−1.

summarized in Table S3. The C0.5 of ZIF−PPy-2 is 554.4 F g−1, which is much higher than those of the other ZIF−PPy hybrids (417.5−514.5 F g−1) and that of pure PPy (443.3 F g−1), and is almost 5 times higher than that of pure ZIF-67 (99.2 F g−1). To the best of our knowledge, this value is also much higher than those of activated carbons (74−260 F g−1),27−29 carbon nanotubes (80−228 F g−1),30−32 graphene (154−484 F g−1),33−35 MOF-derived carbons (188−332 F g−1),36−38 and pristine MOF-based electrodes (0.3−552 F g−1), as summarized and compared in Table S4. The corresponding capacitance retentions (C20/C0.5) are shown in Figure 4b and Table S3. Clearly, ZIF−PPy-2 exhibits the best capacitance retention capability. 38741

DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744

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ACS Applied Materials & Interfaces in serious aggregation between small-sized ZIF particles. Moreover, a large proportion of small ZIF particles would not be inserted by PPy tubes (see Figure 2g,h), further limiting the utilization of ZIF particles for charge storage. In addition, the capacitance retention of the ZIF−PPy electrode also first increases from ZIF-67 to ZIF−PPy-1 and to ZIF−PPy-2, and then reduces from ZIF−PPy-3 to ZIF−PPy-4 because of the poor capacitance retention of PPy. Therefore, the ZIF−PPy-2 with the optimized ratio of ZIF to PPy exhibits the best specific capacitance and capacitance retention and should be a promising electrode material for supercapacitors. The long-term cycling stability of ZIF−PPy-2 was investigated by GCD measurements at a current density of 20 A g−1 (Figure 4d). ZIF−PPy-2 shows much higher capacitance retention compared with the PPy tubes and ZIF-67. The SEM images of the ZIF−PPy-2 electrode shown in Figure S5 reveal that the morphology of our ZIF−PPy still remains well even after 10 000 cycles, indicating the good reversibility of ZIF−PPy-2. To further demonstrate the superiority of our ZIF−PPy structure produced by in situ growth of ZIF-67 around PPy tubes, we simply mixed ZIF-67 and PPy tubes in the same proportions as ZIF−PPy-2. The mixed sample was denoted as ZIF/PPy, and the proportion of PPy was ∼28 wt %. The capacitive performance of ZIF/PPy was measured by GCD measurements in a three-electrode system (Figure S6). The C0.5 of ZIF/PPy is 196.7 F g−1, which is much lower than that of ZIF−PPy-2 (554.4 F g−1), demonstrating the importance of the design of our ZIF−PPy continuous microstructure. Flexible supercapacitors have gained worldwide attention as an emerging energy storage device for wearable and miniaturized electronics.45 As is well-known, areal capacitance is an important factor for evaluating the performance of flexible supercapacitors. Thus, the relationship between the areal capacitance of the ZIF−PPy-2 electrode and its mass loading was investigated by GCD measurements with an areal current density of 0.4 mA cm−2, and the results are shown in Figure S7. It can be seen that when the optimized mass loading is 10 mg cm−2, a maximal areal specific capacitance of 2.33 F cm−2 can be achieved. On the basis of this remarkable electrochemical performance, a prototype flexible supercapacitor device based on ZIF−PPy-2 was built with an electrode−electrolyte− electrode integrated structure (Figure 5a). A PVA/NaSO4 gel electrolyte, which serves as both the solid-state electrolyte and a thin separator, can be easily impregnated into the ZIF−PPy-2 network, and in this way, the thickness of the entire device can be minimized.46 According to the CV curves shown in Figure 5b, no obvious redox peaks of the ZIF−PPy-2-based flexible supercapacitor are observed. The areal capacitances were investigated by GCD measurements at various current densities (0.4−4.8 mA cm−2). As shown in Figure 5c, all GCD curves exhibit a symmetric linear profile, revealing the good capacitive performance of ZIF−PPy-2. The corresponding capacitances versus current densities are shown in Figure S8. It can be seen that the ZIF−PPy-2-based flexible supercapacitor exhibits a maximum areal specific capacitance of 225.8 mF cm−2 at a current density of 0.4 mA cm−2, and to the best of our knowledge, this value is far above those achieved for the MOFbased flexible supercapacitors reported up to date (5.1−175 mF cm−2).11−13,47−49 According to the Ragone plots shown in Figure 5d, the highest energy density is 0.0113 mW h cm−2 with a power density of 0.12 mW cm−2, with 0.0076 mW h cm−2 retained at 1.44 mW cm−2, which surpass the values

Figure 5. (a) Schematic representation of a flexible supercapacitor device based on ZIF−PPy-2 electrodes and gel electrolyte. (b) CV curves at various scan rates, (c) GCD curves at various current densities, and (d) corresponding Ragone plots and comparisons of ZIF−PPy-2 flexible supercapacitor.

achieved for MOFs-based supercapacitors reported previously.11,12,47,49,50 To evaluate the feasibility of ZIF−PPy-2 as a flexible electrical energy storage component for flexible/wearable electronics, the electrochemical performance of the ZIF−PPy2 flexible supercapacitor was investigated under various bending conditions. Because of the excellent mechanical properties of our ZIF−PPy-2, a little change in the CV curves was observed under various bending conditions (Figure 6a), demonstrating the superior flexibility of the device. Because a single device has a limited working potential window and energy storage capacity, using serial and parallel assemblies would be a simple and viable way to exercise control over the operating voltage and energy storage. Therefore, a single device and three devices connected in series and parallel, respectively, were tested by GCD measurements at the same current of 2.4 mA (Figure 6b). The charge/discharge voltage of the three devices connected in series was 1.8 V, with almost the same discharge time compared with a single device, whereas the discharge time for three devices connected in parallel was about 2.7 times that for a single device, which approaches to the theoretical value of 3, conforming to the theorem for series and parallel connections of capacitors.

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CONCLUSIONS In summary, we demonstrated a general and effective strategy to design and prepare a flexible MOFs-based supercapacitor device by fabricating an in situ network of ZIF-67 particles connected by conductive PPy tubes. The obtained ZIF−PPy hybrids exhibit continuous microstructures comprising uniform PPy tubes inserted into and through ZIF-67 particles, high specific surface area, and good electrical conductivity, thus resulting in an unexpectedly high gravimetric specific capacitance of 554.4 F g−1 at a current density of 0.5 A g−1 and excellent stability in a three-electrode system. Moreover, a high areal specific capacitance of 2.33 F cm−2 was also achieved 38742

DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744

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ACS Applied Materials & Interfaces ORCID

Likun Pan: 0000-0001-9294-1972 Yusuke Yamauchi: 0000-0001-7854-927X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.T. is an overseas researcher under Postdoctoral Fellowship of the Japan Society for the Promotion of Science (JSPS project number 17F17080). As a jointly supervised PhD candidate from East China Normal University, X.X. was also partially supported by the China Scholarship Council (CSC) and the Outstanding Doctoral Dissertation Cultivation Plan (PY2015040). This work was partially supported by the service of The Pore Fabrication Pty Ltd. (Australia), The Australian Research Council (ARC) Future Fellow (FT150100479), The AIIM-MANA 2016 grant, and JSPS KAKENHI Grant Number 17H05393 (Coordination Asymmetry).

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Figure 6. (a) CV curves collected at 5 mV s−1 for the ZIF−PPy-2 flexible supercapacitor device subjected to different bending angles. The inset is a digital photograph of a ZIF−PPy-2 flexible supercapacitor device (4 × 1.5 cm2), showing its good flexibility. (b) GCD curves for flexible supercapacitors with different device configurations at a fixed current of 2.4 mA.

in the three-electrode system. In terms of the practical supercapacitor application, the flexible supercapacitor device based on ZIF−PPy-2 exhibits stable performance between 0 and 0.6 V and a high areal specific capacitance of 225.8 mF cm−2. To the best of our knowledge, this value is far above those achieved for previously reported MOF-based flexible supercapacitors. With the benefits of its high flexibility, environmental friendliness, and ease of connection in series and parallel, the ZIF−PPy-2 flexible supercapacitor device has high potential for applications in flexible/wearable electronics.

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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.7b09944. FESEM images, FTIR spectrum, and N2 adsorption− desorption isotherms of PPy tubes; the specific surface areas of ZIF-67 and ZIF−PPy hybrids; FESEM images of the ZIF−PPy-2 electrode through 10 000 cycles; the electrochemical performance of ZIF−PPy hybrids; the electrochemical performance of a ZIF−PPy-2-based flexible supercapacitor; and comparisons of capacitance for ZIF−PPy-2 and other MOF-based electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.T.). *E-mail: [email protected] (L.P.). *E-mail: [email protected] (Y.Y.). 38743

DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744

Research Article

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DOI: 10.1021/acsami.7b09944 ACS Appl. Mater. Interfaces 2017, 9, 38737−38744