Porous nanotubes derived from a metal-organic framework as high-performance supercapacitor electrodes

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CERAMICS INTERNATIONAL

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Porous nanotubes derived from a metal-organic framework as high-performance supercapacitor electrodes Hui Li, Fan Yue, Chao Yang, Peng Qiu, Peng Xue, Qian Xu, Jide Wangn Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education & Xinjiang Uygur Autonomous Region, College of Chemistry and Chemical Engineering of Xinjiang University, Urumqi 830046, China Received 2 September 2015; received in revised form 16 October 2015; accepted 19 October 2015

Abstract In this work, porous Co3O4 nanotubes were prepared by calcining nanoscale Co-MOF-74 crystals at an optimized temperature. The shell of the Co3O4 nanotubes consisted of small interconnected nanoparticles. Supercapacitors were successfully constructed using the resulting porous Co3O4 nanotubes as electrode materials. Electrochemical data demonstrated that the Co3O4 nanotubes exhibited good capacitive behavior with a specific capacitance of 647 F g
1 at a current density of 1 A g
1. Apart from high capacitance, the Co3O4 nanotubes also demonstrated excellent cycling stability with no obvious decrease after 1500 cycles at 2 A g
1, which indicated that it can be promising electroactive materials for supercapacitors. The remarkable performance of the nanotubes could be attributed to their special morphology and appropriate pore-size distribution. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.

Keywords: Co3O4 nanotubes; Co-MOF-74; Metal-organic framework; Supercapacitors

1. Introduction The depletion of fossil fuels, the long-term effects of greenhouse gas, and increasing environmental pollution warrant the urgent exploration of renewable energy resources and related energy generation, storage, and conservation technologies [1–5]. Supercapacitors can store high energy and transport high power within an extremely short period; therefore, they are considered highly suitable candidates for portable electronic devices and hybrid electric vehicles. These favorable attributes have attracted considerable attention toward supercapacitors in recent years [5–8]. To become primary devices for power supply, supercapacitors must be developed further to enable them to deliver high energy and high power simultaneously. Numerous efforts have been exerted to investigate supercapacitors, such as porous carbon n

Corresponding author. Tel./fax: þ 86 991 8582807. E-mail address: [email protected] (J. Wang).

materials and transition metal oxides [9–11]. Among transition metal oxides, Co3O4 is deemed as an ideal electrode for supercapacitors because of its low cost, low environmental footprint, high redox activity, and massive theoretical specific capacitance (ca. 3560 F/g
1) [9,12–14]. The capacitance of Co3O4 electrode materials for supercapacitors is mainly attributed to fast and reversible Faradaic redox reactions. To increase specific capacitance and achieve superior cycle life for Co3O4 electrodes, porous Co3O4 electrode materials with rational morphology design and pore size are developed. These materials provide sufficient space for volume changes and accessible channels that facilitate ion diffusion and transfer, which are critical for enhancing the electrochemical activity and lifetime of supercapacitors [15–18]. Several attempts have been made to prepare porous Co3O4-based electrode materials. Recently, porous Co3O4 nanocrystals with diverse structures, including nanowalls [19], nanowires [20], hollow boxes [21], spheres [12,22,23], and nanotubes [24], have been successfully synthesized and used to construct

http://dx.doi.org/10.1016/j.ceramint.2015.10.101 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.

Please cite this article as: H. Li, et al., Porous nanotubes derived from a metal-organic framework as high-performance supercapacitor electrodes, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.101

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supercapacitors. Other solutions involve coating/supporting Co3O4 with various carbon materials because the introduced carbon materials do not only provide continuous electron transport channels but also allow the formation of abundant pore structures [25–27]. Although the aforementioned approaches have been proven to be relatively effective, considerable improvement can still be achieved. Notably, some metal-organic frameworks (MOFs) with exceptional specific surface areas and pore volumes have been utilized to generate porous carbon and metal oxide nanomaterials through the solid-state thermolysis of as-prepared MOF crystals [28–31]. This strategy is interesting because carbon materials or metal oxides with extremely porous structures can be obtained directly from MOFs without extra templates. Zeolitic imidazolate frameworks (ZIF-8, ZIF-67) [32,33], Prussian blue analogs [34], and HKUST-1 [35] were recently selected as templates to obtain hollow polyhedrons; they exhibited enhanced electrochemical performance as anode materials for supercapacitors. However, the search for new materials remains an urgent task given that further improvement of this type of electrode is highly desirable. Isostructural materials known as M-MOF-74 (M ¼ Co, Ni, Fe, Zn, Mg) have received notable attention because of their stable structures and abundant pores [36–38]. In this study, porous Co3O4 nanotubes with hexagonal cuboid structures were prepared by calcining nanoscale Co-MOF-74 crystals at an optimized temperature. The electrochemical capacitance behavior of the Co3O4 nanotube electrodes was investigated through cyclic voltammetry (CV) and galvanostatic charge– discharge studies. The electrochemical data demonstrated that the Co3O4 nanotubes displayed good capacitive behavior with specific capacitances of 647, 619, 571, 515, and 498 F g
1 at current densities of 1, 2, 4, 6, and 10 A g
1, respectively. Furthermore, the obtained electrode showed no obvious capacitance decay even after over 1500 charge–discharge cycles at a current density of 2 A g
1.

tightly and heated to 110 1C in an oil bath for 6 h with magnetic stirring. After the solution was naturally cooled down to room temperature, the product was collected through centrifugation and washed several times with deionized water and ethanol.

2. Experimental

2.5. Electrochemical measurements

2.1. Materials used

For electrochemical measurements, the working electrode was fabricated by mixing active materials (Co3O4), conductive carbon black, and PTFE at a weight ratio of 8:1:1. The resultant mixture was then coated onto a piece of nickel foam of approximately 1 cm2, and pressed into a thin foil at a pressure of 10.0 MPa. The electron was then dried at 120 1C overnight under vacuum. CV and capacitive performance assessment were conducted using a CHI 660D electrochemical workstation (Shanghai Chenhua Instrument, Inc.) in an aqueous KOH electrolyte (2.0 M) with a three-electrode cell. In this setup, the platinum foil served as the counter electrode and a saturated calomel electrode served as the reference electrode. The mass loading of Co3O4 on the nickel foam was approximately 2.4 mg cm
2. Specific capacitance was based on the total mass of the Co3O4 nanostructure. Galvanostatic charge– discharge tests were conducted on a LAND battery programcontrol test system. Electrochemical impedance spectrometry (EIS) was conducted using a Zennium IM6 electrochemical

Cobalt acetate and 2,5-dihydroxyterephthalic acid were purchased from Alfa Aesar. Tetrahydrofuran (THF) was purchased from Sinopharm Chemical Reagent Co. Ltd. Conductive carbon black was obtained from Cabot Corporation, whereas polytetrafluoroethylene (PTFE) was acquired from Sigma-Aldrich. All reagents were used without further purification. 2.2. Synthesis of hexagonal cuboid Co-MOF-74 nanocrystals Nanoscale Co-MOF-74 with hexagonal cuboid structures were synthesized through the following procedures: 2,5dihydroxyterephthalic acid (3 mmol) was dissolved in 30 mL THF, whereas cobalt acetate (3 mmol) was dissolved in 30 mL water. The two aforementioned solutions were then mixed under vigorous stirring for 10 min. Then, the bottle was capped

2.3. Synthesis of the porous Co3O4 nanotubes Co-MOF-74 hexagonal cuboid crystals were heated in a furnace in air from room temperature to 350 1C with a slow heating rate of 1 1C min
1 and maintained at 350 1C for 2 h. The sample was slowly cooled to room temperature, and a black Co3O4 sample was obtained and named Co3O4-350. Similarly, compounds Co3O4-400 and Co3O4-500 were obtained by heating Co-MOF-74 at 400 1C and 500 1C, respectively. 2.4. Instruments and characterization Powder X-ray diffraction (PXRD) was performed on a Rigaku X-ray diffractometer (Rigaku Rint, Japan). Thermogravimetric analysis (TGA) was performed on a Shimadzu DTG-50 thermal analyzer from room temperature to 800 1C at a heating rate of 10 1C min
1. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were conducted on LEO 1450VP. Meanwhile, high-resolution transmission electron microscopy (HR-TEM) investigations were performed using a Titan G2 60-300 (FEI, USA). X-ray photoelectron spectroscopy (XPS) was conducted using an ESCA-3400 spectrometer (Shimadzu, Japan). Nitrogen sorption isothermals were measured using an automatic volumetric adsorption equipment (Quantachrome, USA). The surface areas of Co3O4 were calculated using the Brunauer–Emmett– Teller (BET) method, and the pore volume and average pore diameter were determined by applying the Barrett–Joyner– Halenda (BJH) method to the desorption branches of nitrogen isotherms.

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Fig. 1. SEM images of (a) Co-MOF-74, in which the insets are enlarged hexagonal cuboid morphology images of crystals; (b) Co3O4-350; (c) Co3O4-400; and (d) Co3O4-500.

workstation (Zahner, Germany) at a frequency ranging from 100 kHz to 50 mHz. 3. Results and discussions Our experiments were based on nanoscale Co-MOF-74 frameworks with exceptional chemical/thermal stability and high surface area; the average cross-sectional channel dimensions are 11.08  11.08 Å2 [37]. The morphology of the asprepared precursor was examined by SEM. The results showed nanoscale hexagonal cuboid crystals with smooth surfaces (Fig. 1a). The PXRD patterns of the precursors were obtained, which indicated the phase of the as-prepared samples (Fig. S1). All diffraction peaks corresponded well to pure Co-MOF-74 and matched well with the simulation results. The TG curve of the as-prepared nanoscale Co-MOF-74 is presented in Fig. S2. Mass loss was shown to terminate at approximately 318 1C. Based on the TGA result, we set the calcination temperature to 350 1C initially. Porous nanotubes were successfully prepared after calcining the as-prepared precursor at 350 1C in air, and the PXRD patterns indicated that all the diffraction peaks could be indexed to standard Co3O4 (JCPDS no. 74-1656) (Fig. 2). No diffraction peak from other

Fig. 2. PXRD patterns of compounds Co3O4-350, Co3O4-400, and Co3O4500, as well as the standard patterns of Co3O4.

impurities was observed, which indicated the high purity of the Co3O4 products. Furthermore, morphology was successfully maintained during the calcination process at 350 1C. In

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Fig. 3. (a, b) TEM images of Co3O4-350 at different magnifications. (c, d) HR-TEM images of Co3O4-350. The inset of (d) shows the SAED pattern of the HRTEM image.

particular, the resultant Co3O4 exhibited a hexagonal cuboid morphology (Fig. 1b). Based on the EDS pattern (Fig. S3), the atomic ratio of Co:O was approximately 1:1.5, which matched with Co3O4 (1:1.3). The porous structure could be confirmed by the transmission electron microscopy (TEM) images (Fig. 3a–c). The considerable contrast difference between the darker marginal region and the brighter central region further suggested the porous structure of the Co3O4 nanotubes. Moreover, the walls of the Co3O4 nanotubes comprised nanoparticles and exhibited highly rough surfaces. The crystalline nature of the porous Co3O4 nanotube structure was confirmed by HR-TEM and the selected-area electron diffraction (SAED) pattern, as shown in Fig. 3d. The HR-TEM image displayed distinct lattice fringes with d spacing of 0.46 nm, which was consistent with the (111) lattice planes of Co3O4. The SAED pattern indicated the polycrystalline characteristics of the Co3O4 nanotube structure. The pores in the Co3O4 structures might have formed from the original pore channels of Co-MOF-74 and the decomposition of 2,5-dihydroxyterephthalic acid. The Co3O4 nanotubes began to burst when calcination temperature was increased to 400 1C (Fig. 1c). Once the temperature reached 500 1C, the products were no longer nanotubes but nanoparticle aggregates (Fig. 1d). This finding

could be possibly attributed to the stress action under a high temperature. To investigate the oxidation status of cobalt further, XPS was conducted to examine Co3O4-350, Co3O4-400, and Co3O4-500 (Fig. 4). The Co 2p XPS peaks of the three Co3O4 samples corresponded to their position and distribution (Fig. 4a–c). All the XPS spectra presented two major peaks at 795.170.1 eV and 780.070.1 eV with a spin energy separation of approximately 15 eV, which corresponded to the Co 2p1/2 and Co 2p3/2 spin– orbit peaks of Co3O4, respectively [39,40]. The fitting peaks at 781.270.1 eV and 796.570.1 eV were indexed to Co(II). The peaks at 795.170.1 eV and 780.070.1 eV corresponded to Co (III). Furthermore, the O1s XPS peaks of the three Co3O4 samples were asymmetrical with a visible shoulder at a high binding energy. As shown in Fig. 4d–f, each asymmetric O1s peak could be coherently fitted by three components. The O1 component of the O1s spectra at 530.170.1 eV was attributed to the lattice oxygen in the Co3O4 phase. The O2 component at medium binding energy (531.270.1 eV) was associated with the O2
ions in the oxygen-deficient regions within the matrix of Co3O4 (oxygen vacancies). By contrast, the O3 component at approximately 53370.3 eV was usually attributed to the chemisorbed and dissociated oxygen species [8,22]. Both PXRD and XPS

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Fig. 4. XPS spectra of Co3O4-350, Co3O4-400, and Co3O4-500. (a–c) Co2p and (d–f) O1s.

characterizations verified the complete conversion of the precursor into Co3O4. The surface area and pore distribution of Co3O4 were analyzed using N2 adsorption and desorption isotherms (Fig. 5). All the samples displayed a typical type IV adsorption isotherm with an H3-type hysteresis loop, which indicated the presence of a mesoporous structure [41] that might originated from the

inheritance of porous Co-MOF-74 and the decomposition of the samples during heat treatment. Co3O4-350 yielded a high BET specific surface area of approximately 45.9 m2 g
1. However, at high treatment temperatures, such as 400 1C and 500 1C, the samples experienced a decrease in specific surface area to 36.2 m2 g
1 and 21.6 m2 g
1. With the increase in calcination temperature, a shift in the adsorption step and hysteresis loop

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Fig. 5. N2 adsorption–desorption isotherm (a) and the corresponding pore-size distribution (b) of Co3O4-350, Co3O4-400, and Co3O4-500.

toward higher relative pressure was observed. This result indicated that the pore size of the synthesized Co3O4 product increased. The sharp ascent in the high-pressure region should be ascribed to the multilayer adsorption of N2 in the pores of the Co3O4 particles. The differences among the isotherms of the three samples suggested the significant differences in pore structures. The pore-size distribution calculated using the BJH method showed that Co3O4-350 was composed of hierarchical porous composites with a narrow distribution centered at approximately 3 nm and a wide distribution centered at approximately 17 nm (Fig. 5b). The presence of pores smaller than 20 nm was beneficial in improving the capacitive properties of the materials. The number of mesoporous structures apparently decreased, whereas the number of macroporous structures increased with increasing calcination temperature. The electrochemical performance of the as-prepared Co3O4 as electrode material for supercapacitors was evaluated using 2 M KOH aqueous solution as the electrolyte. Fig. 6a shows the CV curves of the Co3O4-350 electrode with various sweep rates ranging from 5 mV s
1 to 40 mV s
1 in the potential window of 0–0.5 V. Two pairs of redox peaks were observed, which corresponded to the conversion between different cobalt oxidation states (Co3O4/CoOOH and CoOOH/CoO2) [8,12,18]. The peak currents also increased with the increase in scan rate, but the shape of the CV curve generally remained with distinct current peaks. This observation implied the good reversibility of the fast charge–discharge response of the materials. The slight peak shift was attributed to the polarization effect of the electrode [42,43]. Fig. 6b shows the chronopotentiometry curves of Co3O4-350 measured at different discharge current densities within the potential window of 0–0.4 V in 2 M KOH solution. The specific capacitance was calculated using the following formula [44]: C ¼ I Δ t=ðm Δ VÞ where I is the discharge current, Δt is the discharge time, m is the mass of the active materials, and ΔV is the voltage window. All the electrochemical measurements were performed at room temperature. As shown in Figs. 6b and S4,

the discharge time of these electrodes was in the order of Co3O4-350 4 Co3O4-400 4 Co3O4-500, which indicated that Co3O4-350 possessed a larger specific capacitance than Co3O4400 and Co3O4-500. This finding could be attributed to the unique pore-size distribution and larger surface area of Co3O4350. From the correlation of the specific capacitances with different current densities, the decrease in capacitances with the increase in current densities might be ascribed to limited ion migration into the interior of the active materials. At the current density of 1 A g
1, the specific capacitance of Co3O4350 (647 F g
1) was considerably higher than those of Co3O4400 (439.5 F g
1) and Co3O4-500 (129 F g
1). Although specific capacitance decreased gradually with increasing current density because of insufficient active materials involved in the redox reaction at high current densities, the specific capacitance of Co3O4-350 still reached 515 F g
1 at 6 A g
1. This capacitance was superior to those of Co3O4-400 and Co3O4-500 at 1 A g
1. Long-term cycling stability has long been considered crucial in the practical application of supercapacitors. As shown in Fig. 6d, the galvanostatic charge–discharge measurements were conducted at the current density of 2 A g
1 in 2 M KOH electrolyte. The Co3O4-350 electrode continuously increased until the 1300th cycle, at which point full activation at the electrode–electrolyte interface was achieved. Compared with the first charge–discharge, the specific capacitance exhibited no obvious decrease after 1500 cycles. By contrast, the specific capacitance of Co3O4-400 began to decline after approximately the 550th cycle. Compared with that of Co3O4350, the specific capacitance of Co3O4-500 continued to have a larger gap, although it rose gradually. Notably, the porous nanotube structure may provide sufficient space for changes in volume during the charge–discharge process, which considerably improves the cycle life of the electrode. To understand the fundamental behavior of supercapacitor electrodes further, EIS measurements were performed by applying an AC voltage of 5 mV amplitude within a frequency range of 50 mHz to 100 kHz at an open circuit potential. Two major characteristic features observed in the high- and low-frequency

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Fig. 6. (a) Cyclic voltammogram curves at various scan rates ranging from 5 mV s
1 to 40 mV s
1 and (b) charge–discharge curves measured at different current densities of Co3O4-350. (c) Average specific capacitance at various current densities, (d) cycling performance at a current density of 2 A g
1, and (e) Nyquist plots of the EIS of Co3O4-350, Co3O4-400, and Co3O4-500.

regions were attributed to various resistance phenomena during different interfacial processes in Faradaic reactions. The fitting circuits of these Nyquist plots revealed that the charge-transfer resistance values (semicircle in high frequency) of Co3O4-350, Co3O4-400, and Co3O4-500 were 0.41, 0.54, and 0.63 Ω, respectively. The results suggested that Co3O4-350 exhibited a faster charge transport than the others. The nearly linear EIS plots of Co3O4 in the low-frequency region were characteristic of Warburg impedance, which represented the electrolyte diffusion in active

materials [20,45]. In the plots, Co3O4-350 presented more vertical lines leaning toward the imaginary axis than Co3O4-400 and Co3O4-500, which implied that Co3O4-350 produced a more facile electrolyte diffusion on the surface because of appropriate pore size. 4. Conclusions In summary, porous Co3O4 nanotubes were prepared by simply calcining hexagonal cuboid Co-MOF-74 crystals. The

Please cite this article as: H. Li, et al., Porous nanotubes derived from a metal-organic framework as high-performance supercapacitor electrodes, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.101

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resulting porous Co3O4 nanotube structures were used as electrode materials for supercapacitors. The electrochemical results showed that porous Co3O4-350 exhibited a specific capacitance as high as 647 F g
1 at 1 A g
1. In addition, the nanostructure displayed excellent long-time cycling stability with no obvious decrease after 1500 cycles at 2 A g
1 in 2 M KOH electrolyte. The good supercapacitor performance is related to the unique porous nanotube structure and pore-size distribution of Co3O4-350. MOFs are promising precursors that can provide advanced materials with unique structures and appropriate pore-size distribution. Acknowledgments Financial support from the National Natural Science Foundation of China (Nos. 21162027 and 21261022), the Graduate Student Research Innovation Project of Xinjiang (No. XJGRI2013016), and the Outstanding Doctoral Innovation Project of Xinjiang University (XJUBSCX-2012020) is gratefully acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint. 2015.10.101.

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Please cite this article as: H. Li, et al., Porous nanotubes derived from a metal-organic framework as high-performance supercapacitor electrodes, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.101