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Design of ZIF-based CNTs wrapped porous carbon with hierarchical pores as electrode materials for supercapacitors
Journal of Physics and Chemistry of Solids 125 (2019) 57–63
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Design of ZIF-based CNTs wrapped porous carbon with hierarchical pores as electrode materials for supercapacitors
T
Lvye Yanga, Yi Fenga,∗, Dongbo Yub, Jianhao Qiua, Xiong-Fei Zhanga, Dehua Dongc, Jianfeng Yaoa,∗∗ a
College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agr-Forest Biomass, Nanjing Forestry University, Nanjing, Jiangsu, 210037, China School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, China c School of Material Science and Engineering, University of Jinan, Jinan, 250022, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoporous carbon N-doping Metal-organic framework Supercapacitor Hierarchical structure
ZIF-8 wrapped two-dimensional ZIF-L(Co) was prepared. By adjusting the Zn2+ concentration in precursor, ZIF8 layer with different thickness was coated onto sheet-like ZIF-L. After carbonization, hierarchical porous carbon with CNTs wrapping was formed. Proper ZIF-8 coating would result in the porous carbon with relative high BET surface area, N content, electrical conductivity and proper pore size distribution; thus showing higher supercapacitive performance than that of carbonized ZIF-8 or carbonized ZIF-L. The highest specific capacitance is 274 F/g for carbonized composite materials compared to that of 220 F/g for carbonized ZIF-8 and 57 F/g for carbonized ZIF-L. Moreover, such composite sample (CZ-6) showed high stability and the energy density reached 6.99 W h/kg at a power density of 0.262 kW/k, which is better than most of other ZIF-based porous carbon reported before.
1. Introduction Porous carbon materials have been widely used in energy-storage devices, especially in supercapacitors and batteries. Recently, metalorganic framework (MOF)-based carbon materials have used as promising candidates owing to their unique characterizations [1–5]. Zeolite imidazole frameworks (ZIFs) is one class of MOFs and is composed of Zn or Co as inorganic component linked with organic components [6,7]. Similar to MOFs, carbonization of ZIFs can form porous carbon, which is very promising to be used for supercapacitors. The first literature on carbonized ZIFs as electrode materials for supercapacitors was studied by Chaikittisilp et al. [1]. ZIF-8 was used as precursors and porous carbon was obtained by carbonization of ZIF-8. The resulting carbonized ZIF-8 showed a BET surface area of 24–1110 m2/g depending on the carbonization temperatures and the highest specific capacitance was achieved by ZIF-8 carbonized at 900 °C (214 F/g at 5 mV/s and 115 F/g at 100 mV/s). Though carbonized ZIF-8 showed good capacitive performance, lack of hierarchical pores (especially meso and macropores) and low electrical conductivity limited the further capacitance enhancement of such materials. Then, the following studies focused on the formation of hierarchical pores and enhancement of electrical conductivity of carbonized ZIF-8 [8,9]. For example, Jose ∗
et al. obtained ZIF-8-derived carbon with hierarchical pores via carbonization of assembled ZIF-8 nanoparticles and the specific capacitance was improved to 206 F/g at 100 mV/s [10]. Jiang et al. used CTAB as template to synthesized ZIFs@CTAB to obtain hierarchical porous carbon [11] and Huang used ZnO as template for ZIF particles coating to obtain porous carbon sheets with large pores were synthesized [12]. To enhance the electrical conductivity, either additional agents were added and coated onto ZIFs or coating ZIFs on conductive materials such as carbon nanotubes (CNTs) or rGO. For example, dopamine, melamine or urea were coated onto ZIFs and after carbonization, the degree of graphitization was improved due to the N-doping effects resulted from the second agents [13–15]. By coating ZIFs onto CNTs or rGO to form CNTs@ZIFs or rGO@ZIFs is another effective way to enhance the electrical conductivity because of the high conductive properties of CNTs or rGO [16–18]. Besides ZIF-8, ZIF-67 is another commonly used precursor to obtain porous carbon [19,20]. Due to the fact that ZIF-67 is Co-based, ZIF-67 derived carbon usually has high conductivity due to the catalysis of amorphous carbon by Co nanoparticles to form graphitic carbon. For example, Torad et al. carbonized ZIF-67 first and then removed Co to obtain porous carbon and such porous carbon showed large pore volume, high BET surface and high electrical conductivity; thus good
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Feng), [email protected] (J. Yao).
∗∗
https://doi.org/10.1016/j.jpcs.2018.10.012 Received 20 August 2018; Received in revised form 28 September 2018; Accepted 7 October 2018 Available online 08 October 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.
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as Z2-Z6, where 0.175 g of ZIF-L(Co) was mixed with 0.21, 0.42, 0.84, 1.68 and 3.36 g of zinc nitrate hexahydrate, respectively, while keeping the same molar ratio of Zn2+: Hmim: methanol and the preparation procedure as Z1. Porous carbons were obtained by carbonization of ZIF-L(Co)@ZIF-8 (Z1-Z6) under N2 at 800 °C for 4 h with a temperature increase rate of 1 °C/min. The carbonized products were etched in 37% HCl for 24 h to remove metal oxides and other impurities and then washed and dried at 80 °C. The final resulting carbons are named from CZ-1 to CZ-6. As comparison, ZIF-L(Co) and ZIF-8 were also carbonized with the same conditions of CZ-1.
supercapacitive performance was achieved (238 F/g at 20 mV/s) [21]. Porous carbon resulted from carbonization of ZIF-8 usually has high BET surface area and N content but less stability and relative low conductivity whereas porous carbon derived from ZIF-67 have good conductivity and stability but relative low surface area and nitrogen content [1,3,21]. Moreover, ZIF-67 and ZIF-8 are both particle-like. The dimensions of carbon materials are significant to the contact between electrolyte and active materials and ion or electron diffusion [15,22]. For instance, two-dimensional (2D) carbon has a high electrochemical active area in the application of supercapacitor, and provides a convenient channel for the transfer of charges [23–25]. Therefore, there is a need to design a new structure to combine the merits of Co-based and Zn-based ZIFs and take dimension effects into consideration. ZIF-L including ZIF-L(Zn) and ZIF-L(Co), has a leaf-like structure [26,27]. Due to the 2D structure, such ZIF-L is an interesting precursor to obtain porous 2D carbon [28]. In this study, ZIF-8-embedded ZIF-L (Co) was prepared. After carbonization, 3D porous nitrogen-doped carbons based on 2D porous carbons was obtained. Such structure would bring the following advantages: (a) 2D ZIF-L with high aspect ratio would allow more ZIF-8 crystals coating; (b) Carbonized ZIF-8 would bring in more micropores and improve the surface area whereas the Co nanoparticles from ZIF-L(Co) facilitate the graphitization of carbon, thus increasing the conductivity of materials and (c) hierarchical pores with combination from macropores to micropores would be formed; thus favoring the accessibility of ion-/electron-transport. Moreover, through this study, we found that carbon nanotubes (CNTs) were also formed and wrapped around the porous 2D carbon, which can act as conducting binder to further enhance the conductivity and facilitate ion-/electron-transport [18,29–31]. Here, we offer new carbon materials derived from ZIF-8-embedded ZIF-L(Co) with self-formed CNTs, and the capacitive behavior of such materials is studied and discussed.
2.3. Characterization X-ray diffraction (XRD) patterns were examined using a Rigaku MiniFlex II diffractometer. Raman spectra were recorded using Themor DXR530. Scanning electron microscopy (SEM) (JEOL, Japan) and transmission electron microscope (TEM) (Model 2100F, JEOL, Japan) were performed to character the morphology of the materials. Energydispersive spectroscopy (EDS) spectrum was obtained at the conjunction with TEM instrument. X-ray photoelectron spectroscopy (XPS) was conducted by using a Daojin AXIS UltraDLD spectrometer. Nitrogen adsorption–desorption analysis was conducted by a Micromeritics ASAP 2020 at 77 K. The electrochemical performance of samples was conducted in a three-electrode system and two-electrode system. Cyclic voltammetry (CV), galvanostatic charge-discharge properties (GCD), and electrochemical impedance spectroscopy (EIS) tests were tested on an electrochemical working station (CHI 760e, Chenhua Instrument Company, Shanghai, China). The detailed preparation of working electrodes and electrochemical measurements were given in supporting information. 3. Results and discussion
2. Experimental The illustration of the preparation of ZIF-L(Co)@ZIF-8–based porous carbon is shown in Fig. 1. Firstly, ZIF-8 particles in-situ grew onto the two-dimensional ZIF-L as coating layer. In this study, different amount of Zn2+ was added into precursors and it is reasonable to expect that higher amount of Zn2+ addition would result in the thicker layer of ZIF-8 wrapping 2D ZIF-L. The 2D ZIF-L, on the other hand, functioned as a good support for the growth of ZIF-8 particles and can effectively avoid the agglomeration of ZIF-8 particles so that the gaps among the particles facilitates the formation of large pores during carbonization. As described in introduction, hierarchical porous carbon can be obtained by carbonization of proper assembly of ZIF-8 particles because large pores were created from the gaps among particles [10]. Similar effects are expected in our case. Moreover, due to the catalytic effect of cobalt nanoparticles in ZIF-L, higher degree of graphitization of carbons is expected and CNTs might form and wrap around the 2D nanosheets considering the fact that ligands such as Hmin can form CNTs under the catalysis of Co at proper carbonization conditions [29,30,35,36]. Fig. 2 gives the XRD patterns of pure ZIF-L(Co), pure ZIF-8 and ZIFL@ZIF-8 (Z1-Z6) before and after carbonization. According to the XRD patterns in Fig. 2a, all characteristic peaks of ZIF-8 and ZIF-L are shown in Z1-Z6 samples, indicating the successful ZIF-8 coating onto ZIF-L. After carbonization, porous carbons were formed. The peak at 26° (002)
2.1. Chemicals 2-Methylimidazole (Hmim, purity 98%), Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and cobalt nitrate hexahydrate (Co(NO3)2·6H2O) were purchased from Aladdin. Hydrochloric acid (HCl, 37 wt%) and methanol were obtained from Nanjing Reagent. All of the reagents were used without further refinement. 2.2. Sample preparation Pristine ZIL-L(Co) and ZIF-8 were synthesized by the same procedure reported in literature [32–34]. ZIF-L(Co) was prepared by using Co (NO3)2·6H2O, and Hmim with a molar ratio of 1 Co2+:8 Hmim: 2211 water. ZIF-L(Co)@ZIF-8 was synthesized by the following procedure: 0.175 g of as-synthesized ZIF-L(Co) and 0.105 g of Zn(NO3)2·6H2O were added in 3.85 g of methanol under stirring, followed by the addition of Hmin solution (0.231 g Hmin in 3.85 g methanol). The ZIF-L(Co) and ZIF-8 synthesis solution (Zn2+: Hmim: methanol molar ratio of 1:8:681) was held at room temperature for 24 h and then filtered and washed by methanol to obtain the solid-state samples (ZIF-L(Co)@ZIF-8). It was then dried at 55 °C overnight and noted as Z1. The ZIF-L@ZIF-8 composites prepared with different amount of ZIF-8 precursors were noted
Fig. 1. Illustration of the fabrication of ZIF-L(Co)@ZIF-8 derived CNTs wrapped porous carbon with hierarchical pores.
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Fig. 2. XRD patterns of ZIF-8, ZIF-L(Co) and ZIF-L(Co)@ZIF-8 (Z1-Z6) before (a) and after (b) carbonization. Table 1 Pore information of the carbonized samples. Sample
SBET (m2/g)
Smicro (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmicro/Vtotal
Carbonized ZIF-8 Carbonized ZIF-L CZ-1 CZ-4 CZ-6
1205 175 325 475 1007
915 84 48 276 762
1.003 0.097 0.270 0.329 0.763
0.412 0.037 0.019 0.124 0.344
41.2% 38.1% 7.0% 37.7% 45.1%
acid (Fig. S1). From SEM images, it can be seen that ZIF-8 is a cubic-like structure with an average particle size of approximately 100 nm (Fig. 3a) whereas ZIF-L is leaf-like with a length of several micronmeter and thickness of ca. 300 nm (Fig. 3b). Fig. 3c–h shows the SEM images of ZIF-L@ZIF-8 (Z1, Z4 and Z6) before and after carbonization. For composite materials, more than one layer of ZIF-8 particles wrapped ZIF-L and thicker ZIF-8 layers (Fig. 3d and e) were formed around ZIF-L when more ZIF-8 precursor solution (higher Zn2+ concentration) was added during the synthesis. High-resolution SEM images of Z1, Z4 and Z6 are shown in Fig. S2. After carbonization, the original structure was almost maintained. Considering the fact the BET surface area of carbonized ZIF-8 is much higher than that of carbonized ZIF-L, it might be advantageous for samples with thicker ZIF-8 coating because more micropores and higher BET surface area might be achieved in final carbonized samples. Fig. 4 shows TEM images of carbonized ZIF-L@ZIF-8. From Fig. 4a, the porous carbon with CNTs wrapping can be clearly observed. It is interesting to note that CNTs were formed during
Fig. 3. SEM images of ZIF-8 (a), ZIF-L(Co) (b), Z1 (c), Z4 (d), Z6 (e), and CZ-1 (f), CZ-4 (g) and CZ-6 (h).
indicates the formation of graphitized carbon [37] and the peaks at 44° (111) and 51° (200) showing in carbonized ZIF-L and carbonized ZIFL@ZIF-8 (Fig. 2b) indicate the existence of Co crystals [38,39]. The TEM image also indicates the existence of Co crystals, which were enclosed in the carbon matrix and difficultly removed by the etching of
Fig. 4. TEM images of carbonized ZIF-L(Co)@ZIF-8. 59
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Fig. 5. Cyclic voltammograms of carbonized ZIF-8 (a), carbonized ZIF-L (b), CZ-1 (c), CZ-4 (d), CZ-6 (e) at scan rates of 5–200 mV/s, and the specific capacitance variations according to CV tests (f).
specific capacitance is expected in CZ-6. The supercapacitive performance of carbonized ZIF-L, ZIF-8 and ZIF-L(Co)@ZIF-8 were firstly examined in a three-electrode system. Fig. 5 gives the specific capacitance of all samples at scan rates of 5–200 mV/s. CZ-6 shows the highest specific capacitance than the other CZ samples and carbonized ZIF-8 and carbonized ZIF-L. Though the BET surface area of CZ-6 is lower than that of carbonized ZIF-8, it shows better supercapacitive performance. In order to explain the abovementioned phenomenon, Raman spectra and XPS spectra of CZ-6, carbonized ZIF-8 and ZIF-L were performed. Fig. 6a shows the Raman spectra of carbonized ZIF-L(Co), carbonized ZIF-8 and CZ-6. The peaks at around 1350 and 1580 cm−1 ascribe to the D and G bands of carbon, respectively and the values of IG/ID can indicate the degree of graphitization of carbon [41]; thus reflecting conductivity of samples. The values of IG/ID of carbonized ZIF-L,
carbonization probably due to the catalytic graphitization effects of Co nanoparticles on Hmin ligands during carbonization process [35,40]. The formation of CNTs might play a positive role in enhancing the electrical conductivity of obtained porous carbons; thus improving the supercapacitive performance. Table 1 shows the pore information of synthesized porous carbons. Carbonized ZIF-8 has higher BET surface area (1205 m2/g) than carbonized ZIF-L (175 m2/g). Therefore, as discussed above, due to the fact that higher Zn2+ concentration in precursor resulted in thicker ZIF-8 particle coating, CZ-6 has the highest BET surface area compared to other CZ samples. Moreover, more micropores were observed in CZ-6 and such micropores mainly came from the carbonization of ZIF-8 in ZIF-L(Co)@ZIF-8 composites (N2 adsorption-desorption isotherms in Fig. S3). As a general trend, high BET surface area usually results in better capacitance performance, especially for EDLCs; thus higher 60
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Fig. 6. Raman spectra (a) and XPS (b) of carbonized ZIF-L, carbonized ZIF-8 and CZ-6.
Fig. 7. Galvanostatic charge-discharge curves of carbonized ZIF-8 (a), carbonized ZIF-L (b) and CZ-6 (c) at different current densities, Nyquist plots and equivalent circuit model with two constant phase elements CPE1 and CPE2 (d).
whereas carbonized ZIF-L usually has higher degree of graphitization, thus better electrical conductivity. With the enhancement of Zn2+ concentration in precursors, more ZIF-8 particles were formed and thicker ZIF-8 layer were formed. With proper ratio of ZIF-8 and ZIF-L, balance of BET surface area, N content and conductivity was adjusted and optimized, so that improved specific capacitance can be achieved compared to that of carbonized ZIF-L and ZIF-8. CZ-6 has relative high BET surface area (1007 m2/g, which is lower than that of carbonized ZIF-8), N content (9%, which is lower than carbonized ZIF-8) and degree of graphitization (which is lower than carbonized ZIF-L). Moreover, CNTs were formed, which could offer high electronic conductivity and afford the channel for the transport of ions [30,44]. Therefore CZ-6 has the highest specific capacitance.
carbonized ZIF-8 and CZ-6 are 1.035, 0.814, 0.947, respectively, indicating that carbonized ZIF-L and CZ-6 has higher graphitic degree than carbonized ZIF-8. This can be explained by the fact that the Co nanoparticles from ZIF-L facilitates the graphitization process of carbon [42]. Fig. 6b shows the XPS spectra of carbonized ZIF-L, carbonized ZIF8 and CZ-6. The N content of carbonized ZIF-L, carbonized ZIF-8 and CZ-6 are 0.7%, 12% and 9%, respectively, indicating that N-doping carbon was mainly attributed by carbonization of ZIF-8. N-doping is another contribution to the specific capacitance because it can improve conductivity and improve extra pseudocapacitance, as has been proven in literature [43–45]. From the above analysis, it can be seen that carbonized ZIF-8 facilitates the formation of micropores, enhances the BET surface area and improves the N content in carbon framework
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Fig. 8. Electrochemical performance of CZ-6 in two-electrode cell: the CV (a), GCD curves (b), the specific capacitance variations at different current densities according to CP tests (c), Ragone plot of the supercapacitor (d), and cycling stability at 20 A/g (e).
and N content whereas carbonized ZIF-L increased the graphitic degree (conductivity) of composite samples. Electrochemical analysis shows that such carbonized ZIF-L(Co)@ZIF-8 (CZ-6) has higher capacitance performance than carbonized ZIF-L and ZIF-8 due to the advantages of combination of ZIF-8 and ZIF-L because CZ-6 has a compromised high BET surface area, N content and conductivity. Moreover, due to the catalytic character of Co formed from carbonized ZIF-L, CNTs were formed and wrapped the porous carbon, which can further promote the conductivity of carbonized ZIF-L(Co)@ZIF-8.
Galvanostatic charge-discharge curves of carbonized ZIF-L, carbonized ZIF-8 and CZ-6 are shown in Fig. 7a–c. All samples exhibit nearly symmetric situation with CZ-6 having the largest area, indicating the best capacitive performance among all samples, which are consistent with the CV results (Fig. 5). Fig. 7d presents the Nyquist plots with the equivalent circuit. Both carbonized ZIF-L and CZ-6 show a smaller value crossing with the Z’ axis in the high frequency, suggesting the lower equivalent serial internal resistance (Rs) of 0.64 and 1.31 Ohm than that of carbonized ZIF-8 (2.34 Ohm). The better conductivity of carbonized ZIF-L and CZ-6 were further confirmed. The charge transfer resistance (Rct) could be calculated by the diameter of semicircle (inset in Fig. 7d), and the fitted values of carbonized ZIF-L, CZ-6 and carbonized ZIF-8 are 0.02, 0.10 and 0.15 Ohm, respectively. In the low frequencies, carbonized ZIF-L and CZ-6 show the steeper curve slope, indicating the better ion diffusion capability [46,47]. Considering the good performance in a three-electrode system, CZ-6 was further evaluated by a symmetric two-electrode system. The CV curves maintained quasi-rectangular shape at scan rates from 5 to 200 mV/s (Fig. 8a). The specific capacitance according to CP result is calculated to be 204 F/g at 1 A/g and 170 F/g at 20 A/g with the capacitance retention of 83%. The Ragone plot of the CZ-6//CZ-6 symmetric supercapacitor (Fig. 8d) shows that the energy density reached 6.99 and 4.58 W h/kg at power densities of 0.262 and 4.711 kW/kg, respectively. The charge–discharge cycle stability was tested at a high current density of 20.0 A/g, as shown in Fig. 8e. After 10000 cycles, the capacitance of CZ-6 remained at 96% of the initial capacitance. All these results indicate that CZ-6 has high supercapacitive performance and is superior to other MOF-based materials studied elsewhere [4,48–51]; thus showing the potential of CZ-6 for high-performance supercapacitors.
Acknowledgements The authors are grateful for the financial support of National Key R& D Program of China (2017YFD0601006), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Youth Fund of Natural Science Foundation of Jiangsu Province (BK20170919) and Scientific Research Foundation for Returned Scholars from Nanjing Forestry University (GXL2018014). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2018.10.012. References [1] W. Chaikittisilp, M. Hu, H. Wang, H.-S. Huang, T. Fujita, K.C.W. Wu, L.-C. Chen, Y. Yamauchi, K. Ariga, Nanoporous carbons through direct carbonization of a zeolitic imidazolate framework for supercapacitor electrodes, Chem. Commun. 48 (2012) 7259–7261 https://doi.org/10.1039/c2cc33433j. [2] C. Young, R.R. Salunkhe, J. Tang, C.-C. Hu, M. Shahabuddin, E. Yanmaz, M.S.A. Hossain, J.H. Kim, Y. Yamauchi, Zeolitic imidazolate framework (ZIF-8) derived nanoporous carbon: the effect of carbonization temperature on the supercapacitor performance in an aqueous electrolyte, Phys. Chem. Chem. Phys. 18 (2016) 29308–29315 https://doi.org/10.1039/c6cp05555a. [3] J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi, Thermal conversion of core-shell metal-organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon, J. Am. Chem. Soc. 137 (2015)
4. Conclusions In this study, ZIF-L@ZIF-8 was prepared and carbonized to obtain hierarchical porous carbon. Carbonized ZIF-8 enhanced the BET surface 62
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Design of ZIF-based CNTs wrapped porous carbon with hierarchical pores as electrode materials for supercapacitors
T
Lvye Yanga, Yi Fenga,∗, Dongbo Yub, Jianhao Qiua, Xiong-Fei Zhanga, Dehua Dongc, Jianfeng Yaoa,∗∗ a
College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agr-Forest Biomass, Nanjing Forestry University, Nanjing, Jiangsu, 210037, China School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, China c School of Material Science and Engineering, University of Jinan, Jinan, 250022, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoporous carbon N-doping Metal-organic framework Supercapacitor Hierarchical structure
ZIF-8 wrapped two-dimensional ZIF-L(Co) was prepared. By adjusting the Zn2+ concentration in precursor, ZIF8 layer with different thickness was coated onto sheet-like ZIF-L. After carbonization, hierarchical porous carbon with CNTs wrapping was formed. Proper ZIF-8 coating would result in the porous carbon with relative high BET surface area, N content, electrical conductivity and proper pore size distribution; thus showing higher supercapacitive performance than that of carbonized ZIF-8 or carbonized ZIF-L. The highest specific capacitance is 274 F/g for carbonized composite materials compared to that of 220 F/g for carbonized ZIF-8 and 57 F/g for carbonized ZIF-L. Moreover, such composite sample (CZ-6) showed high stability and the energy density reached 6.99 W h/kg at a power density of 0.262 kW/k, which is better than most of other ZIF-based porous carbon reported before.
1. Introduction Porous carbon materials have been widely used in energy-storage devices, especially in supercapacitors and batteries. Recently, metalorganic framework (MOF)-based carbon materials have used as promising candidates owing to their unique characterizations [1–5]. Zeolite imidazole frameworks (ZIFs) is one class of MOFs and is composed of Zn or Co as inorganic component linked with organic components [6,7]. Similar to MOFs, carbonization of ZIFs can form porous carbon, which is very promising to be used for supercapacitors. The first literature on carbonized ZIFs as electrode materials for supercapacitors was studied by Chaikittisilp et al. [1]. ZIF-8 was used as precursors and porous carbon was obtained by carbonization of ZIF-8. The resulting carbonized ZIF-8 showed a BET surface area of 24–1110 m2/g depending on the carbonization temperatures and the highest specific capacitance was achieved by ZIF-8 carbonized at 900 °C (214 F/g at 5 mV/s and 115 F/g at 100 mV/s). Though carbonized ZIF-8 showed good capacitive performance, lack of hierarchical pores (especially meso and macropores) and low electrical conductivity limited the further capacitance enhancement of such materials. Then, the following studies focused on the formation of hierarchical pores and enhancement of electrical conductivity of carbonized ZIF-8 [8,9]. For example, Jose ∗
et al. obtained ZIF-8-derived carbon with hierarchical pores via carbonization of assembled ZIF-8 nanoparticles and the specific capacitance was improved to 206 F/g at 100 mV/s [10]. Jiang et al. used CTAB as template to synthesized ZIFs@CTAB to obtain hierarchical porous carbon [11] and Huang used ZnO as template for ZIF particles coating to obtain porous carbon sheets with large pores were synthesized [12]. To enhance the electrical conductivity, either additional agents were added and coated onto ZIFs or coating ZIFs on conductive materials such as carbon nanotubes (CNTs) or rGO. For example, dopamine, melamine or urea were coated onto ZIFs and after carbonization, the degree of graphitization was improved due to the N-doping effects resulted from the second agents [13–15]. By coating ZIFs onto CNTs or rGO to form CNTs@ZIFs or rGO@ZIFs is another effective way to enhance the electrical conductivity because of the high conductive properties of CNTs or rGO [16–18]. Besides ZIF-8, ZIF-67 is another commonly used precursor to obtain porous carbon [19,20]. Due to the fact that ZIF-67 is Co-based, ZIF-67 derived carbon usually has high conductivity due to the catalysis of amorphous carbon by Co nanoparticles to form graphitic carbon. For example, Torad et al. carbonized ZIF-67 first and then removed Co to obtain porous carbon and such porous carbon showed large pore volume, high BET surface and high electrical conductivity; thus good
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Feng), [email protected] (J. Yao).
∗∗
https://doi.org/10.1016/j.jpcs.2018.10.012 Received 20 August 2018; Received in revised form 28 September 2018; Accepted 7 October 2018 Available online 08 October 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.
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as Z2-Z6, where 0.175 g of ZIF-L(Co) was mixed with 0.21, 0.42, 0.84, 1.68 and 3.36 g of zinc nitrate hexahydrate, respectively, while keeping the same molar ratio of Zn2+: Hmim: methanol and the preparation procedure as Z1. Porous carbons were obtained by carbonization of ZIF-L(Co)@ZIF-8 (Z1-Z6) under N2 at 800 °C for 4 h with a temperature increase rate of 1 °C/min. The carbonized products were etched in 37% HCl for 24 h to remove metal oxides and other impurities and then washed and dried at 80 °C. The final resulting carbons are named from CZ-1 to CZ-6. As comparison, ZIF-L(Co) and ZIF-8 were also carbonized with the same conditions of CZ-1.
supercapacitive performance was achieved (238 F/g at 20 mV/s) [21]. Porous carbon resulted from carbonization of ZIF-8 usually has high BET surface area and N content but less stability and relative low conductivity whereas porous carbon derived from ZIF-67 have good conductivity and stability but relative low surface area and nitrogen content [1,3,21]. Moreover, ZIF-67 and ZIF-8 are both particle-like. The dimensions of carbon materials are significant to the contact between electrolyte and active materials and ion or electron diffusion [15,22]. For instance, two-dimensional (2D) carbon has a high electrochemical active area in the application of supercapacitor, and provides a convenient channel for the transfer of charges [23–25]. Therefore, there is a need to design a new structure to combine the merits of Co-based and Zn-based ZIFs and take dimension effects into consideration. ZIF-L including ZIF-L(Zn) and ZIF-L(Co), has a leaf-like structure [26,27]. Due to the 2D structure, such ZIF-L is an interesting precursor to obtain porous 2D carbon [28]. In this study, ZIF-8-embedded ZIF-L (Co) was prepared. After carbonization, 3D porous nitrogen-doped carbons based on 2D porous carbons was obtained. Such structure would bring the following advantages: (a) 2D ZIF-L with high aspect ratio would allow more ZIF-8 crystals coating; (b) Carbonized ZIF-8 would bring in more micropores and improve the surface area whereas the Co nanoparticles from ZIF-L(Co) facilitate the graphitization of carbon, thus increasing the conductivity of materials and (c) hierarchical pores with combination from macropores to micropores would be formed; thus favoring the accessibility of ion-/electron-transport. Moreover, through this study, we found that carbon nanotubes (CNTs) were also formed and wrapped around the porous 2D carbon, which can act as conducting binder to further enhance the conductivity and facilitate ion-/electron-transport [18,29–31]. Here, we offer new carbon materials derived from ZIF-8-embedded ZIF-L(Co) with self-formed CNTs, and the capacitive behavior of such materials is studied and discussed.
2.3. Characterization X-ray diffraction (XRD) patterns were examined using a Rigaku MiniFlex II diffractometer. Raman spectra were recorded using Themor DXR530. Scanning electron microscopy (SEM) (JEOL, Japan) and transmission electron microscope (TEM) (Model 2100F, JEOL, Japan) were performed to character the morphology of the materials. Energydispersive spectroscopy (EDS) spectrum was obtained at the conjunction with TEM instrument. X-ray photoelectron spectroscopy (XPS) was conducted by using a Daojin AXIS UltraDLD spectrometer. Nitrogen adsorption–desorption analysis was conducted by a Micromeritics ASAP 2020 at 77 K. The electrochemical performance of samples was conducted in a three-electrode system and two-electrode system. Cyclic voltammetry (CV), galvanostatic charge-discharge properties (GCD), and electrochemical impedance spectroscopy (EIS) tests were tested on an electrochemical working station (CHI 760e, Chenhua Instrument Company, Shanghai, China). The detailed preparation of working electrodes and electrochemical measurements were given in supporting information. 3. Results and discussion
2. Experimental The illustration of the preparation of ZIF-L(Co)@ZIF-8–based porous carbon is shown in Fig. 1. Firstly, ZIF-8 particles in-situ grew onto the two-dimensional ZIF-L as coating layer. In this study, different amount of Zn2+ was added into precursors and it is reasonable to expect that higher amount of Zn2+ addition would result in the thicker layer of ZIF-8 wrapping 2D ZIF-L. The 2D ZIF-L, on the other hand, functioned as a good support for the growth of ZIF-8 particles and can effectively avoid the agglomeration of ZIF-8 particles so that the gaps among the particles facilitates the formation of large pores during carbonization. As described in introduction, hierarchical porous carbon can be obtained by carbonization of proper assembly of ZIF-8 particles because large pores were created from the gaps among particles [10]. Similar effects are expected in our case. Moreover, due to the catalytic effect of cobalt nanoparticles in ZIF-L, higher degree of graphitization of carbons is expected and CNTs might form and wrap around the 2D nanosheets considering the fact that ligands such as Hmin can form CNTs under the catalysis of Co at proper carbonization conditions [29,30,35,36]. Fig. 2 gives the XRD patterns of pure ZIF-L(Co), pure ZIF-8 and ZIFL@ZIF-8 (Z1-Z6) before and after carbonization. According to the XRD patterns in Fig. 2a, all characteristic peaks of ZIF-8 and ZIF-L are shown in Z1-Z6 samples, indicating the successful ZIF-8 coating onto ZIF-L. After carbonization, porous carbons were formed. The peak at 26° (002)
2.1. Chemicals 2-Methylimidazole (Hmim, purity 98%), Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and cobalt nitrate hexahydrate (Co(NO3)2·6H2O) were purchased from Aladdin. Hydrochloric acid (HCl, 37 wt%) and methanol were obtained from Nanjing Reagent. All of the reagents were used without further refinement. 2.2. Sample preparation Pristine ZIL-L(Co) and ZIF-8 were synthesized by the same procedure reported in literature [32–34]. ZIF-L(Co) was prepared by using Co (NO3)2·6H2O, and Hmim with a molar ratio of 1 Co2+:8 Hmim: 2211 water. ZIF-L(Co)@ZIF-8 was synthesized by the following procedure: 0.175 g of as-synthesized ZIF-L(Co) and 0.105 g of Zn(NO3)2·6H2O were added in 3.85 g of methanol under stirring, followed by the addition of Hmin solution (0.231 g Hmin in 3.85 g methanol). The ZIF-L(Co) and ZIF-8 synthesis solution (Zn2+: Hmim: methanol molar ratio of 1:8:681) was held at room temperature for 24 h and then filtered and washed by methanol to obtain the solid-state samples (ZIF-L(Co)@ZIF-8). It was then dried at 55 °C overnight and noted as Z1. The ZIF-L@ZIF-8 composites prepared with different amount of ZIF-8 precursors were noted
Fig. 1. Illustration of the fabrication of ZIF-L(Co)@ZIF-8 derived CNTs wrapped porous carbon with hierarchical pores.
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Fig. 2. XRD patterns of ZIF-8, ZIF-L(Co) and ZIF-L(Co)@ZIF-8 (Z1-Z6) before (a) and after (b) carbonization. Table 1 Pore information of the carbonized samples. Sample
SBET (m2/g)
Smicro (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmicro/Vtotal
Carbonized ZIF-8 Carbonized ZIF-L CZ-1 CZ-4 CZ-6
1205 175 325 475 1007
915 84 48 276 762
1.003 0.097 0.270 0.329 0.763
0.412 0.037 0.019 0.124 0.344
41.2% 38.1% 7.0% 37.7% 45.1%
acid (Fig. S1). From SEM images, it can be seen that ZIF-8 is a cubic-like structure with an average particle size of approximately 100 nm (Fig. 3a) whereas ZIF-L is leaf-like with a length of several micronmeter and thickness of ca. 300 nm (Fig. 3b). Fig. 3c–h shows the SEM images of ZIF-L@ZIF-8 (Z1, Z4 and Z6) before and after carbonization. For composite materials, more than one layer of ZIF-8 particles wrapped ZIF-L and thicker ZIF-8 layers (Fig. 3d and e) were formed around ZIF-L when more ZIF-8 precursor solution (higher Zn2+ concentration) was added during the synthesis. High-resolution SEM images of Z1, Z4 and Z6 are shown in Fig. S2. After carbonization, the original structure was almost maintained. Considering the fact the BET surface area of carbonized ZIF-8 is much higher than that of carbonized ZIF-L, it might be advantageous for samples with thicker ZIF-8 coating because more micropores and higher BET surface area might be achieved in final carbonized samples. Fig. 4 shows TEM images of carbonized ZIF-L@ZIF-8. From Fig. 4a, the porous carbon with CNTs wrapping can be clearly observed. It is interesting to note that CNTs were formed during
Fig. 3. SEM images of ZIF-8 (a), ZIF-L(Co) (b), Z1 (c), Z4 (d), Z6 (e), and CZ-1 (f), CZ-4 (g) and CZ-6 (h).
indicates the formation of graphitized carbon [37] and the peaks at 44° (111) and 51° (200) showing in carbonized ZIF-L and carbonized ZIFL@ZIF-8 (Fig. 2b) indicate the existence of Co crystals [38,39]. The TEM image also indicates the existence of Co crystals, which were enclosed in the carbon matrix and difficultly removed by the etching of
Fig. 4. TEM images of carbonized ZIF-L(Co)@ZIF-8. 59
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Fig. 5. Cyclic voltammograms of carbonized ZIF-8 (a), carbonized ZIF-L (b), CZ-1 (c), CZ-4 (d), CZ-6 (e) at scan rates of 5–200 mV/s, and the specific capacitance variations according to CV tests (f).
specific capacitance is expected in CZ-6. The supercapacitive performance of carbonized ZIF-L, ZIF-8 and ZIF-L(Co)@ZIF-8 were firstly examined in a three-electrode system. Fig. 5 gives the specific capacitance of all samples at scan rates of 5–200 mV/s. CZ-6 shows the highest specific capacitance than the other CZ samples and carbonized ZIF-8 and carbonized ZIF-L. Though the BET surface area of CZ-6 is lower than that of carbonized ZIF-8, it shows better supercapacitive performance. In order to explain the abovementioned phenomenon, Raman spectra and XPS spectra of CZ-6, carbonized ZIF-8 and ZIF-L were performed. Fig. 6a shows the Raman spectra of carbonized ZIF-L(Co), carbonized ZIF-8 and CZ-6. The peaks at around 1350 and 1580 cm−1 ascribe to the D and G bands of carbon, respectively and the values of IG/ID can indicate the degree of graphitization of carbon [41]; thus reflecting conductivity of samples. The values of IG/ID of carbonized ZIF-L,
carbonization probably due to the catalytic graphitization effects of Co nanoparticles on Hmin ligands during carbonization process [35,40]. The formation of CNTs might play a positive role in enhancing the electrical conductivity of obtained porous carbons; thus improving the supercapacitive performance. Table 1 shows the pore information of synthesized porous carbons. Carbonized ZIF-8 has higher BET surface area (1205 m2/g) than carbonized ZIF-L (175 m2/g). Therefore, as discussed above, due to the fact that higher Zn2+ concentration in precursor resulted in thicker ZIF-8 particle coating, CZ-6 has the highest BET surface area compared to other CZ samples. Moreover, more micropores were observed in CZ-6 and such micropores mainly came from the carbonization of ZIF-8 in ZIF-L(Co)@ZIF-8 composites (N2 adsorption-desorption isotherms in Fig. S3). As a general trend, high BET surface area usually results in better capacitance performance, especially for EDLCs; thus higher 60
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Fig. 6. Raman spectra (a) and XPS (b) of carbonized ZIF-L, carbonized ZIF-8 and CZ-6.
Fig. 7. Galvanostatic charge-discharge curves of carbonized ZIF-8 (a), carbonized ZIF-L (b) and CZ-6 (c) at different current densities, Nyquist plots and equivalent circuit model with two constant phase elements CPE1 and CPE2 (d).
whereas carbonized ZIF-L usually has higher degree of graphitization, thus better electrical conductivity. With the enhancement of Zn2+ concentration in precursors, more ZIF-8 particles were formed and thicker ZIF-8 layer were formed. With proper ratio of ZIF-8 and ZIF-L, balance of BET surface area, N content and conductivity was adjusted and optimized, so that improved specific capacitance can be achieved compared to that of carbonized ZIF-L and ZIF-8. CZ-6 has relative high BET surface area (1007 m2/g, which is lower than that of carbonized ZIF-8), N content (9%, which is lower than carbonized ZIF-8) and degree of graphitization (which is lower than carbonized ZIF-L). Moreover, CNTs were formed, which could offer high electronic conductivity and afford the channel for the transport of ions [30,44]. Therefore CZ-6 has the highest specific capacitance.
carbonized ZIF-8 and CZ-6 are 1.035, 0.814, 0.947, respectively, indicating that carbonized ZIF-L and CZ-6 has higher graphitic degree than carbonized ZIF-8. This can be explained by the fact that the Co nanoparticles from ZIF-L facilitates the graphitization process of carbon [42]. Fig. 6b shows the XPS spectra of carbonized ZIF-L, carbonized ZIF8 and CZ-6. The N content of carbonized ZIF-L, carbonized ZIF-8 and CZ-6 are 0.7%, 12% and 9%, respectively, indicating that N-doping carbon was mainly attributed by carbonization of ZIF-8. N-doping is another contribution to the specific capacitance because it can improve conductivity and improve extra pseudocapacitance, as has been proven in literature [43–45]. From the above analysis, it can be seen that carbonized ZIF-8 facilitates the formation of micropores, enhances the BET surface area and improves the N content in carbon framework
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Fig. 8. Electrochemical performance of CZ-6 in two-electrode cell: the CV (a), GCD curves (b), the specific capacitance variations at different current densities according to CP tests (c), Ragone plot of the supercapacitor (d), and cycling stability at 20 A/g (e).
and N content whereas carbonized ZIF-L increased the graphitic degree (conductivity) of composite samples. Electrochemical analysis shows that such carbonized ZIF-L(Co)@ZIF-8 (CZ-6) has higher capacitance performance than carbonized ZIF-L and ZIF-8 due to the advantages of combination of ZIF-8 and ZIF-L because CZ-6 has a compromised high BET surface area, N content and conductivity. Moreover, due to the catalytic character of Co formed from carbonized ZIF-L, CNTs were formed and wrapped the porous carbon, which can further promote the conductivity of carbonized ZIF-L(Co)@ZIF-8.
Galvanostatic charge-discharge curves of carbonized ZIF-L, carbonized ZIF-8 and CZ-6 are shown in Fig. 7a–c. All samples exhibit nearly symmetric situation with CZ-6 having the largest area, indicating the best capacitive performance among all samples, which are consistent with the CV results (Fig. 5). Fig. 7d presents the Nyquist plots with the equivalent circuit. Both carbonized ZIF-L and CZ-6 show a smaller value crossing with the Z’ axis in the high frequency, suggesting the lower equivalent serial internal resistance (Rs) of 0.64 and 1.31 Ohm than that of carbonized ZIF-8 (2.34 Ohm). The better conductivity of carbonized ZIF-L and CZ-6 were further confirmed. The charge transfer resistance (Rct) could be calculated by the diameter of semicircle (inset in Fig. 7d), and the fitted values of carbonized ZIF-L, CZ-6 and carbonized ZIF-8 are 0.02, 0.10 and 0.15 Ohm, respectively. In the low frequencies, carbonized ZIF-L and CZ-6 show the steeper curve slope, indicating the better ion diffusion capability [46,47]. Considering the good performance in a three-electrode system, CZ-6 was further evaluated by a symmetric two-electrode system. The CV curves maintained quasi-rectangular shape at scan rates from 5 to 200 mV/s (Fig. 8a). The specific capacitance according to CP result is calculated to be 204 F/g at 1 A/g and 170 F/g at 20 A/g with the capacitance retention of 83%. The Ragone plot of the CZ-6//CZ-6 symmetric supercapacitor (Fig. 8d) shows that the energy density reached 6.99 and 4.58 W h/kg at power densities of 0.262 and 4.711 kW/kg, respectively. The charge–discharge cycle stability was tested at a high current density of 20.0 A/g, as shown in Fig. 8e. After 10000 cycles, the capacitance of CZ-6 remained at 96% of the initial capacitance. All these results indicate that CZ-6 has high supercapacitive performance and is superior to other MOF-based materials studied elsewhere [4,48–51]; thus showing the potential of CZ-6 for high-performance supercapacitors.
Acknowledgements The authors are grateful for the financial support of National Key R& D Program of China (2017YFD0601006), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Youth Fund of Natural Science Foundation of Jiangsu Province (BK20170919) and Scientific Research Foundation for Returned Scholars from Nanjing Forestry University (GXL2018014). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2018.10.012. References [1] W. Chaikittisilp, M. Hu, H. Wang, H.-S. Huang, T. Fujita, K.C.W. Wu, L.-C. Chen, Y. Yamauchi, K. Ariga, Nanoporous carbons through direct carbonization of a zeolitic imidazolate framework for supercapacitor electrodes, Chem. Commun. 48 (2012) 7259–7261 https://doi.org/10.1039/c2cc33433j. [2] C. Young, R.R. Salunkhe, J. Tang, C.-C. Hu, M. Shahabuddin, E. Yanmaz, M.S.A. Hossain, J.H. Kim, Y. Yamauchi, Zeolitic imidazolate framework (ZIF-8) derived nanoporous carbon: the effect of carbonization temperature on the supercapacitor performance in an aqueous electrolyte, Phys. Chem. Chem. Phys. 18 (2016) 29308–29315 https://doi.org/10.1039/c6cp05555a. [3] J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi, Thermal conversion of core-shell metal-organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon, J. Am. Chem. Soc. 137 (2015)
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