Article pubs.acs.org/cm
Facile Fabrication of Three-Dimensional Graphene and Metal− Organic Framework Composites and Their Derivatives for Flexible All-Solid-State Supercapacitors Xilian Xu,†,‡ Wenhui Shi,∥,‡ Peng Li,† Shaofeng Ye,† Chenzeng Ye,∥ Huijian Ye,† Tiemei Lu,† Aiai Zheng,† Jixin Zhu,§ Lixin Xu,† Mingqiang Zhong,† and Xiehong Cao*,† †
College of Materials Science and Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310014, China Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310014, China § Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzu Road, Nanjing 211816, P. R. China ∥
S Supporting Information *
ABSTRACT: A facile and general method for the large-scale preparation of various three-dimensional (3D) graphene oxide/metal−organic framework (GO/MOF) composites is developed through a simple mixing process using MOFs and graphene oxide. This preparation method is able to rapidly produce GO/MOF composite hydrogels with controllable composition in only several minutes, which is also suitable to a series of different MOFs. The obtained GO/MOF composites are severed as the precursors for the subsequent preparation of MOF-derived composite aerogels, e.g., rGO/Fe2O3 and rGO/ NiO/Ni composite aerogels, through freeze-dry and calcination processes. When used as a supercapacitor electrode, the rGO/Fe2O3 composite shows a good rate capability with high specific capacitances of 869.2 and 289.6 F·g−1 at the current densities of 1 and 20 A·g−1, respectively, as well as a long cycle life without obvious decrease of capacitance after 5000 cycles. Moreover, the flexible all-solid-state supercapacitor device is also fabricated based on the obtained rGO/Fe2O3 composite aerogel, which exhibits a high volumetric capacitance of 250 mF·cm−3 at 6.4 mA·cm−3 and a capacity retention of 96.3% after 5000 cycles at 50.4 mA·cm−3, as well as an excellent mechanical flexibility.
1. INTRODUCTION The fast depletion of fossil fuel in the past decades has brought severe energy crises and environmental issues. Development of renewable energy sources as well as high-efficient energy storage/conversion devices has emerged as one of the most urgent and important tasks currently. Electrochemical energy storage devices, especially lithium-ion batteries and supercapacitors, have played an essential role in portable devices and electric vehicles in recent years.1−3 Supercapacitors are widely recognized as promising energy storage devices for portable electronics owing to their long cycle life, high power density, and fast charge/discharge rate.4,5 Electrode materials are one of the key factors determining the performance of electrochemical energy storage devices. Various functional electrode materials with well-defined structures have been developed, including carbon materials,6 metal oxides,7 conducting polymers,8 graphene-based composites,9 and so on. Among a variety of reported electrode materials, graphene-based composites have been regarded as one of the most promising candidates for supercapacitors.10−14 Especially, those electrodes constructed by three-dimensional (3D) graphene architectures have © 2017 American Chemical Society
demonstrated excellent performances in energy-storage applications due to their large specific surface area, high electrical conductivity, and excellent mechanical properties.15 More importantly, 3D graphene architectures can be used as templates for the deposition of numerous active nanomaterials, which could generate synergistic effects to achieve enhanced supercapacitor performances.16 Recently, metal−organic frameworks (MOFs), an important type of inorganic−organic porous crystal, have drawn intensive attention in a wide range of research fields.17,18 Benefiting from the unique structural and compositional features, MOFs are also versatile precursors/templates for the preparation of various nanostructured materials, including metal−carbon, metallic compound−carbon, and so on.19−22 To further improve the electron transport rate of MOF-derived electrode materials, incorporation of graphene into MOF-derived materials is an effective way.23,24 For example, our group Received: May 11, 2017 Revised: June 21, 2017 Published: June 22, 2017 6058
DOI: 10.1021/acs.chemmater.7b01947 Chem. Mater. 2017, 29, 6058−6065
Article
Chemistry of Materials
centrifugation, washed with methanol several times, and dried in a vacuum oven at 40 °C. Synthesis of Other MOF Crystals. The preparation processes of ZIF-8, MOF-5, Sn-MOF, and Co-MOF crystals were conducted according to the previously reported methods with slight modifications. (1) ZIF-8:35 a solution of Zn(NO3)2·6H2O (18.9 g) in 15 mL of methanol was mixed with a solution of 2-methylimidazole (8.2 g) in 30 mL of methanol. The ZIF-8 crystals were obtained after stirring the mixture for 12 h. (2) MOF-5:36 0.357 g of Zn(NO3)2·6H2O and 0.066 g of 1,4-benzedicarboyxlic acid were dissolved in 12 mL of DMF and then transferred to an autoclave. After the reaction at 120 °C for 24 h, MOF-5 crystals were obtained. (3) Sn-MOF:37 0.128 g of SnSO4, 0.099 g of 1,4-benzedicarboyxlic acid, and 0.056 g of KOH were dissolved in 10 mL of deionic water and then stirred continuously for 1 h. The mixture was heated in an autoclave at 180 °C for 12 h and cooled to room temperature. The Sn-MOF crystals were collected by washing with ethanol and dried in a vacuum oven. (4) Co-MOF:38 0.725 g of Co(NO3)·6H2O and 0.821 g of 2-methylimidazole were separately dissolved in a mixture of methanol and ethanol (40 mL, volume ratio = 1:1). Then, the two solutions were mixed and reacted for 24 h. The resultant Co-MOF crystals with purple color were collected by centrifugation, washed with methanol several times, and dried in a vacuum oven. Preparation of GO/MOF Composite Aerogels. The GO/FeMOF composite aerogel was prepared through a simple mix method (Scheme 1, steps 1−2). Briefly, the prepared Fe-MOF crystals (20 mg)
recently reported the rGO/MoO3 electrode materials for flexible energy storage application, which were prepared by wrapping Mo−MOFs with GO followed by a calcination process to converted the Mo−MOFs to porous MoO 3 structures.25 Generally, preparation of graphene−MOF composite is the prior step for the preparation of MOF-derived nanomaterials. Up to now, many methods have been developed to produce graphene−MOF composite, such as in situ growth, hydrothermal method, direct mixing method, Pickering emulsion polymerization, and atomic layer deposition method.26−30 However, those reported methods still have the limitations in scalability and controllability of the prepared graphene−MOF composites. In situ growth of MOFs on graphene is the most widely used method for the preparation of graphene−MOF composites,31,32 but the synthetic process normally suffers from the following problems. (1) The addition of excess MOF precursor (i.e., metal salts and organic ligands) may result in the aggregation of GO nanosheets, which normally leads to a low concentration of MOFs in the obtained graphene−MOF composite. Moreover, it is difficult to control the structure of the composite and the distribution of each of the components within produced graphene−MOF composite. (2) Since many MOFs cannot be synthesized in aqueous solution, a pretreatment of GO sheets to enable their dispersion in corresponding solvent is necessary. This greatly restricts the types of MOFs that are suitable for the preparation of graphene−MOF composite through the in situ method. (3) The obtained graphene−MOF composite is normally in powder form with highly restacked graphene layers. Although our group24 and other researchers33,34 have demonstrated presynthesized 3D graphene structures can be used as templates for the preparation of graphene−MOF composite with hierarchical structures, complicated synthetic steps are normally required. Therefore, it is still a challenge for the low-cost, scalable and convenient preparation of graphene−MOF composite with 3D structures. Here, we report a general, simple, and low-cost approach, called the “mixing” method, for large-scale preparation of various 3D graphene/MOF composites, which can be used as precursors for the preparation of MOF-derived composite materials. The prepared 3D rGO/Fe2O3 composite, derived from the composite of Fe-MOF and GO (GO/Fe-MOF), exhibited excellent electrochemical performance in the application of the all-solid-state flexible supercapacitor.
Scheme 1. Preparation Processes for GO/MOF and rGO/ MOF-Derived Composite Aerogels
were rapidly added into an aqueous dispersion of GO (4 mg·mL−1, 1 mL) under vigorous vibration using a Q2−1 vortex mixer to form a GO/Fe-MOF composite hydrogel. After that, the prepared composite hydrogel was subjected to a freeze-dry process to remove the water and obtain GO/Fe-MOF composite aerogel. The other GO/MOF composite aerogels, including GO/Ni-MOF, GO/ZIF-8, GO/MOF-5, GO/Sn-MOF, GO/Co-MOF, and GO/Fe-MOF/Ni-MOF, were prepared according to the aforementioned process for GO/Fe-MOF aerogel. Preparation of rGO/MOF-Derived Composite Aerogels. The prepared GO/MOF composite aerogels were placed in the center of a quartz tube and annealed under N2 protection at 450 °C for 1 h, followed by another thermal treatment in air at 380 °C for 1 h to obtain rGO/MOF-Derived composite aerogels (Scheme 1, step 3). Fabrication of All-Solid-State Supercapacitor. The gel electrolyte was first prepared by mixing 1 g of PVA (molecular weight: 88000−97000 g/mol) and 1 g of KOH in 10 mL of distilled water at 80 °C with stirring until forming a transparent solution. The gel electrolyte was then deposited on a rGO/Fe2O3 composite electrode,
2. EXPERIMENTAL SECTION Synthesis of Fe-MOF Crystals. To synthesize Fe-MOF crystals, a mixture of ferric chloride (FeCl3, 243 mg) and terephthalic acid (H2BDC, C8H6O4, 166 mg) was first prepared in N,N-dimethylformamide (DMF, 10 mL). The mixture was heated in an oil bath at 100 °C for 12 h and then cooled down naturally to room temperature. The Fe-MOF crystals with orange color were collected by centrifugation, washed with ethanol several times, and dried in a vacuum oven at room temperature. Synthesis of Ni-MOF Crystals. An aqueous dispersion of nickel chloride hexahydrate (NiCl2·6H2O, 1.3 g) and potassium oxalate (K2C2O4, 3 g) was first prepared by dissolving them in 20 mL of deionic water (refer to as A). Then, 1.5 mL of ethylenediamine (C2H8N2) was added dropwisely and reacted with 1.3 g of NiCl2· 6H2O to form a mixture B. After that, mixture B was added into dispersion A and kept stirring for 48 h at room temperature. The resultant Ni-MOF crystals with blue color were collected by 6059
DOI: 10.1021/acs.chemmater.7b01947 Chem. Mater. 2017, 29, 6058−6065
Article
Chemistry of Materials which was fabricated by coating a slury of rGO/Fe2O3, acetylene black, and polyvinylidene difluoride (weight ratio is 8:1:1) on a flexible carbon paper. After that, another rGO/Fe2O3 composite electrode was placed on the top of the formed gel electrolyte film and an all-solidstate supercapacitor was obtained. Characterizations. FESEM was performed on a HITACHI S4700 (Hitachi). TEM images were obtained by using a JEM-100CX II (FEI). The XRD patterns were collected using a X’Pert PRO (PNAlytical) with a high-intensity Cu Kα irradiation (λ = 1.54 Å). Raman spectra were recorded by using a LabRAM HR800 (Horiba JobinYvon) with a 531.95 nm laser. XPS results were recorded with a Kratos Axis Ultra-DLD system (Shimadzu Co., Ltd., Hongkong). Measurement of Electrochemical Performance. CV, galvanostatic charge/discharge, and electrochemical impedance spectroscopy was carried out on a CHI 760E electrochemistry workstation. The specific capacitance (Cs), energy density (de), and power density (dp) of the fabricated electrodes measured in the three-electrode system are calculated according to the following equations: Cs =
∫ I dt mΔV
(1)
Cs =
I Δt mΔV
(2)
de =
1 C(ΔV )2 2
(3)
dp =
de Δt
(4) Figure 1. Photographs of GO dispersion (a), GO/Fe-MOF hydrogel (b), GO/Fe-MOF aerogel (c), and other GO/MOF composite aerogels (d−i, scale bar = 0.35 cm).
I is the current, dt is the time differential, m is the mass of the active material, V is the potential range of a scanning region. Δt and are the discharge time and discharge potential range. The electrochemical performance of all-solid-state supercapacitor device was calculated according to the following equations:
IΔt Cs = V ΔV
The morphologies of the prepared GO/Fe-MOF composite aerogel (weight ratio of Fe-MOF to GO is 5:1) and its derived rGO/Fe2O3 composite aerogel are shown in Figure 2. The obtained GO/Fe-MOF composite aerogel exhibits highly porous 3D structures with microsized pores, which are
(5)
de =
C(ΔV )2 7200
(6)
dp =
de × 3600 Δt
(7)
Δt and ΔV are the discharge time and potential range, respectively. V is the volume of the device including carbon paper, gel electrolyte, and active material.
3. RESULTS AND DISCUSSION The preparation processes for 3D GO/MOF composite and their derived nanostructures are schematically illustrated in Scheme 1. Briefly, GO aqueous solution and the presynthesized MOF crystals (e.g., Fe-MOF, Figure S1) are mixed by vigorously stirring for several minutes. GO sheets and MOFs are spontaneously self-assembled into 3D porous structures, and GO/MOF composite hydrogels are obtained (Step 1). After that, a freeze-dry process is conducted to produce GO/ MOF composite aerogels (Step 2), which are then subjected to a two-step annealing process under N2 and air atmospheres, respectively, leading to the formation of rGO/MOF-derived composite aerogels (Step 3). Noted that the two-step annealing process can prevent the prepared rGO/MOF-derived composite aerogels from collapse and keep the 3D porous structure. On the basis of this simple and general method (Scheme 1), various composite aerogels composed of GO with Fe-MOF, NiMOF, ZIF-8, MOF-5, Sn-MOF, Co-MOF, and even a FeMOF/Ni-MOF mixture were successfully obtained (Figure 1).
Figure 2. SEM and TEM images of the prepared GO/Fe-MOF composite aerogel before (a−c) and after the two-step annealing process (d−f). (a) SEM images of the GO/Fe-MOF composite aerogel. Inset: High-magnification SEM image of the GO/Fe-MOF composite aerogel. (b) TEM image of the GO/Fe-MOF composite. Inset: SAED pattern of the GO/Fe-MOF composite. (c) TEM images of a Fe-MOF crystal. Inset: High-magnification TEM image of the FeMOF crystal. (d) SEM and (e) TEM images of the rGO/Fe2O3 composite aerogel. Inset: High-magnification SEM image (d) and SAED pattern (e) of the rGO/Fe2O3 composite. (f) TEM images of a Fe2O3 nanostructure. Inset: HRTEM image of Fe2O3 NPs. 6060
DOI: 10.1021/acs.chemmater.7b01947 Chem. Mater. 2017, 29, 6058−6065
Article
Chemistry of Materials constructed by interconnected GO sheets (Figure 2a). Fe-MOF crystals were uniformly coated on the surface of GO sheets with a similar morphology to that of the pristine Fe-MOF crystals (Figure S1). The TEM image of a GO/Fe-MOF composite aerogel shows some wrinkled and folded sheets (Figure 2b), indicating the presence of GO sheets. In addition, Fe-MOF crystals with size of ∼600 nm can be clearly observed (Figure 2b,c), which is consistent with the observation shown in Figure 2a, further confirming the formation of the GO/Fe-MOF composite. The rGO/Fe2O3 composite aerogel was obtained after the two-step annealing process (Scheme 1, step 3), in which GO was thermally reduced to rGO and Fe-MOFs were converted to Fe2O3 nanostructures. Figure 2d indicates that the rGO/Fe2O3 composite aerogel possesses a porous 3D structure with many Fe2O3 nanostructures decorated on rGO sheets (inset of Figure 2d). TEM image of a rGO/Fe2O3 composite aerogel in Figure 2e clearly indicates the presence of rGO sheets, and the Fe2O3 porous nanostructures are constructed by numerous Fe2O3 nanoparticles (NPs) with size of
