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Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor
Materials Chemistry and Physics xxx (2015) 1e5
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor Guo-Chang Li, Xiu-Ni Hua, Peng-Fei Liu, Yi-Xin Xie, Lei Han* Institute of Inorganic Materials, School of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A MOF microflowers were prepared from Co2þ and nicotinic acid. Porous Co3O4 microflowers were obtained by calcinations the MOF precursors. Porous Co3O4 electrode exhibits a specific capacitance of 240.2 F g1 at a current density of 0.625 A g1. The specific capacitance of Co3O4 remains more than 96.3% after 2000 cycles at 3.75 A g1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2015 Received in revised form 16 October 2015 Accepted 5 November 2015 Available online xxx
Porous Co3O4 microflowers were successfully synthesized by calcinations of novel flower-like metalorganic framework microcrystals. A supercapacitor was conducted based on this porous Co3O4 microflowers as the electrode material, which shows that the porous Co3O4 electrode exhibit a specific capacitance of 240.2 F g1 at the current density of 0.625 A g1 and remains more than 96.3% after 2000 cycles. The results demonstrate that porous Co3O4 microflowers can be acted as a promising electrode candidate in supercapacitors. © 2015 Elsevier B.V. All rights reserved.
Keywords: Inorganic compounds Microporous materials Electrochemical properties Electron microscopy
1. Introduction Supercapacitors, known as electrochemical capacitors or ultracapacitors, are considered as one of the newest innovations in energy storage devices and have attracted considerable attention because of its flexible operating temperature, high power capabilities, and long lifespan [1e3]. Based on the energy storage mechanism, supercapacitors are divided into pesudocapacitors and
* Corresponding author. E-mail address: [email protected] (L. Han).
electric double layer capacitors (EDLCs). In contrast, pesudocapacitors making use of reversible redox reactions on the surface of the electrode materials can show much higher specific capacitance than EDLCs, which store energy by ion adsorption [4,5]. In order to enhance the specific capacitance of supercapacitors, massive effort has been center on investigating the pesudocapacitors made of low-cost transition metal oxides (Co3O4, NiO, MnO2, etc) or hydroxides (Co(OH)2 and Ni(OH)2) [6e8]. Among these candidates, Co3O4 is more attractive due to its high theoretical specific capacitance (3560 F/g) and good chemical and thermal stability for energy storage [9,10]. Recently, metal-organic frameworks (MOF) have been utilized
http://dx.doi.org/10.1016/j.matchemphys.2015.11.011 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
2
G.-C. Li et al. / Materials Chemistry and Physics xxx (2015) 1e5
as precursors to generate porous metal oxide micro/nanostructure materials [11e14]. For instance, MIL-88-Fe was used as a template for fabricating spindle-like porous a-Fe2O3 [11], porous hollow Co3O4 with rhombic dodecahedral structures were prepared by the calcination of ZIF-67 microcrystals [12]. From the crystallography viewpoint and growth mechanism, the MOF precursor always has micro-or nanostructure [15e18] and it may be an effective and reasonable method to generate micro/nanostructure metal oxides. More particularly, a lot of new pores were generated due to the decomposition of organic ligands, finally resulting in a porous metal oxide structures. Despite many efforts has been made, a lot of work to be done if we want fully take advantage of this precursorsdrive-to-oxides approach. It has great challenge how to receive well-defined nano/micro precursors and maintain the morphology during calcining process. Herein, we successfully fabricated a novel flower-like MOF precursor through a facile solvothermal method. To further control the calcination conditions, porous Co3O4 microflowers can be obtained. Electrochemical properties of the Co3O4 electrode are tested by cyclic voltammetry (CV) and chronopotentiometry (CP) measurements in 3.0 M KOH electrolyte, which shows that the porous Co3O4 electrode exhibits a specific capacitance of 240.2 F g1 at a current density of 0.625 A g1 and remained more than 96.3% after 2000 cycles at 3.75 A g1. 2. Experimental 2.1. Materials preparation All chemicals in the experiment were analytical grade and used without further purification. In a typical synthesis, firstly, 0.20 g nicotinic acid, 0.035 g NaOH, 15.0 mL ethylene glycol and 15.0 mL alcohol were mixed together and were magnetic stirred for 1 h at room temperature. Then 0.30 g Co(NO3)2$6H2O were added into the above mixture. After stirring for 1 h, the solution was transferred into a Teflon-lined stainless steel autoclave maintained at
75 C for 12 h and cooled down to room temperature naturally. The pink precipitates were collected by centrifugation and washed with absolute ethanol several times, and then dried in a vacuum at 50 C overnight. Finally, the pink precipitates were calcinated at 500 C for 20 min in air and black Co3O4 can be obtained. 2.2. Characterization The morphology of the as-prepared samples was studied by Hitachi S-4800 field-emission scanning electron microscope (FESEM) at an acceleration voltage of 5.0 kV. The phase structures of the samples was characterized by a Bruker D8 advance X-ray powder diffractometer using Cu Ka radiation (l ¼ 0.15418 nm). The thermal behavior of the precursors was examined by employing thermogravimetric (TG) on SII TG/DTA7300 thermal analyzer. N2 adsorption-desorption was determined by BrunauereEmmetteTeller (BET) measurements using V-Sorb 2800 TP surface area and pore size analyzer. The electrochemical tests were performed by an electrochemical analyzer system, CHI660E (Chenhua, Shanghai, China) using a three-electrode cell in 3.0 M KOH aqueous solution. 2.3. Electrode preparation The working electrodes was prepared by mixing 80 wt.% Co3O4, 10 wt.% acetylene black and 10 wt.% polytetrafluoroethylene (PTFE), the slurries were coated on the nickel foam substrates of about 1 cm2, then pressed at 10 MPa and then further dried at 60 C overnight. The mass loading of active material is 2.0 mg. The electrochemical tests were performed by an electrochemical analyzer system, CHI660E (Chenhua, Shanghai, China) using a three-electrode cell in 3.0 M KOH aqueous solution. The Co3O4electrode, a platinum plate and a saturated calomel electrode (SCE) were respectively used as working electrode, counter electrode and reference electrode.
Fig. 1. (aec) SEM images of the as-prepared precursors; (d) Perspective view of the host structure of MOF precursor.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
G.-C. Li et al. / Materials Chemistry and Physics xxx (2015) 1e5
Fig. 2. XRD patterns of the obtained Co3O4 microflowers.
3. Results and discussion The pink MOF precursors were obtained by a facile solvothermal method from the coordination of nicotinic acid and Co2þ, resulting into novel multilayer microflowers. The XRD pattern of the precursor can be described as the compound [Co2(H2O)(C6H4O2N)4]$ sol (Fig. S1), which matches well with the crystal structure of the reported compound [19]. This MOF structure is robust and thermally stable open-framework based on rigid dimetallic carboxylate clusters as the basic building unit. The host framework possesses an effective channel area with the dimensions of 10.8 4.5 Å, as shown in Fig. 1d. In Fig. 1a and b, we can see that the flower-like precursor has
3
uniform size and shape with lengths of 5e6 mm and widths of 3e4 mm. It shows that the sample almost completely made up of many nanoplates structures. From images of Fig. 1c, it can be observed that many nanoplates have assembled as a novel multilayer flower-like structure and the surface of the precursor is relatively smooth. Moreover, there are many nanogaps formed between each layer. The TGA curve of precursor is shown in Fig. S2, the MOF precursor decomposes with increasing temperature in air below 500 C, thus the calcinations temperature was set as 500 C. The XRD patterns of as-prepared black powders are shown in Fig. 2. All the diffraction peaks can be in agreement with the standard spectrum of Co3O4 (JCPDS card no. 65-3103). No impurity diffraction peaks were observed, which indicates that the precursor has completely transformed into Co3O4 at 500 C in air. The morphology of the MOF precursor was well maintained after calcination in air, as displayed in low magnification FE-SEM image (Fig. 3a and b). Compared with the precursor, the size of the Co3O4 microflowers has little shrinked and the surface has changed roughness. From magnified SEM image Fig. 3c and d, we can see Co3O4 maintained nanogaps and generated many new nanopores, which might due to the release of gas in the process of the precursor decomposition. These nanogaps and nanopores might provide many channels to let ions go through, which can shorten the diffusion path of ions and thus enhance the specific capacitances. To further investigate the specific surface areas and the pore size distribution of the porous Co3O4microflowers, BrunauereEmmetteTeller (BET) adsorption/desorption measurements were performed. The nitrogen adsorption isotherm is a typical IV type curve and a distinct hysteresis loop can be observed in the range of ca.0.8e1.0 P/P0 (Fig. 4), which indicates the existence of a mesoporous structure in the sample [20]. The BET surface area of the as-prepared Co3O4 was measured to be 16.7 m2 g1. In addition, according to the corresponding BarretteJoynereHalenda (BJH) pore size distribution curve, the mean pore diameter is about 20 nm (inset in Fig. 4). Such porosity of Co3O4 microflowers can
Fig. 3. SEM images of porous Co3O4 microflowers.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
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and Co3þ/Co4þ [23], according to the following equation [24,25].
Fig. 4. Nitrogen adsorption/desorption isotherm and the corresponding pore size distribution (inset) of the porous Co3O4 microflowers.
facilitate the transfer of ions and electrons at the electrode/electrolyte interface and provide sufficient active sites for Faraday reaction [21,22]. To evaluate the electrochemical properties of as-prepared porous Co3O4 microflowers electrodes, CV and CP measurements are performed in 3M KOH aqueous solution. As shown in Fig. 5a, the CV curves of the porous Co3O4 microflowers electrodes within a potential window of 0e0.5 V at different scan rates. Two pairs of well-dened redox peaks were observed, this peaks can be respectively ascribed to the redox reaction on the surface of the porous Co3O4 microflowers based on the redox mechanism of Co2þ/Co3þ
Co3 O4 þ OH þ H2 O43CoOOH þ e
(1)
CoOOH þ OH 4CoO2 þ H2 O þ e
(2)
Obviously, the shapes are different from that of electric double layer capacitance, manifesting the capacitance mainly results from pseudocapacitive property of porous Co3O4 microflowers electrodes. Fig. 5b depicts the CP curves in a potential range of 0e0.45 V at different current densities. The chargingedischarging curves show well symmetry, demonstrating the excellent electrochemical capacitance behavior and the reversible redox process. The specific capacitance of the porous Co3O4 microflowers electrodes at different current densities can be calculated from the CP curves according to the equation C ¼ iDt/(mDV), where i (A) is the discharge current, Dt (s) is the total discharge time, m (g) is the mass of Co3O4 materials and DV (V) is the potential range of charge/ discharge. Based on the above equation, the specific capacitance of the porous Co3O4 microflowers electrode is calculated to be 240.2, 233.7, 224.9, 214.6 and 202.1 F g1 at the current densities of 0.625, 1.25, 2.50, 3.75 and 6.25 A g1(Fig. 5c). Respectively, with the increase of the current densities from 0.625 to 6.25 A g1, the specific capacitances of 240.2 F g1 decrease to 202.1 F g1with 84.1% of capacitance is retained, indicating that good rate capability of the porous Co3O4 microflowers electrodes. Compared to those previously reported for Co3O4 materials (Supporting information Table S1), the capacitance of our synthesized samples ranks among the best [13,26e29]. For example, porous Co3O4 materials prepared by solid-state thermolysis of a novel Co-MOF crystal precursor in air can displays a specific capacitance of 150 F g1 at a current density of 1 A g1 in 2 M KOH electrolyte [13]. In addition, mesoporous Co3O4 nano-cubes were formed by further thermal treatment can
Fig. 5. (a) CV curves of porous Co3O4 microflowers electrode at different scan rates; (b) CP curves of porous Co3O4 microflowers electrode at different current densities; (c) The corresponding specific capacitance of porous Co3O4 microflowers electrode calculated by the CP curves and current densities; (d) Cycle performance of porous Co3O4 microflowers electrode at the current density of 3.75 A g1 after 2000 cycles.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
G.-C. Li et al. / Materials Chemistry and Physics xxx (2015) 1e5
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240.2 F g1 at a current density of 0.625 A g1and offers a lower specific capacitance decay of ca. 3.7% after 2000 cycles in 3.0 M aqueous KOH solution. The results demonstrate that the porousCo3O4 microflowers could be used as electrode material for high performance supercapacitor applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21471086), the Social Development Foundation of Ningbo (No. 2014C50013), the projects for Graduate Student Research and Innovation Fund of Ningbo University (G15007), and the K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data
Fig. 6. Electrochemical impedance spectra (EIS) of porous Co3O4 microflowers electrode in 3 M KOH solution and a corresponding equivalent circuit (inset) for modeling the measured impedance spectroscopy.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2015.11.011. References
deliver a specific capacitance of 220 F g1 at a current density of 0.6 A g1 in 6 M KOH electrolyte [28]. The good electrochemical performance could be ascribed to the porous characteristic and the multilayer structure of the Co3O4 microflowers. The porous characteristic can serve as a robust reservoir for ions and the multilayer structure can shorten the ion transport/diffusion path within the electrodeeelectrolyte interface, thus enhancing the redox kinetics [30]. Cyclic stability is one of the most important factors for electrode materials in practical applications. The cyclic stability of the porous Co3O4 microflowers electrodes examined by chronopotentiometry at a current density of 3.75 A g1 for 2000 cycles is display in Fig. 5d. It can be seen that after 2000 cycles the specific capacitance has little degradation and retained 96.3% of its initial capacity. The remarkable cycling performance of the porous Co3O4 microflowers electrodes might be attributed to the robust multilayer structure and nanopores, which can maintain the morphology, prevent the collapse of the structure, shorten the diffusion path of ions and thus enhance the specific capacitances. To further investigate the electrochemical characteristics of the porous Co3O4 microflowers electrodes, the electrochemical impedance spectroscopy (EIS) has been performed in the frequency range from 0.01 Hz to 100 kHz and the corresponding Nyquist plot are shown in Fig. 6. This Nyquist plots mainly consisted of a semicircle at the high-frequency region and followed by a straight line at the low-frequency region. The inset in Fig. 6 shows an equivalent circuit, where Rs is the bulk solution resistance, Rct is the charge-transfer resistance, Cp is the faradaic pseudocapacitor, CPE represents the constant phase element accounting for a double-layer capacitance. The Rs and Rct values of the porous Co3O4 microflowers is found to be 0.67 U and 1.82 U, respectively. These values are much lower than the same (Co3O4) obtained with different morphologies by other conventional methods reported in the literature [29,31e33]. It implies that the porous Co3O4 microflowers electrodes have good capacitive behavior. 4. Conclusions In summary, porous Co3O4 microflowers assembled by nanoplates have been easily fabricated by calcination of a novel flowerlike MOF precursor. The porous Co3O4 microflowers can be applied to supercapacitor, and it exhibits a specific capacitance of
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Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor Guo-Chang Li, Xiu-Ni Hua, Peng-Fei Liu, Yi-Xin Xie, Lei Han* Institute of Inorganic Materials, School of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A MOF microflowers were prepared from Co2þ and nicotinic acid. Porous Co3O4 microflowers were obtained by calcinations the MOF precursors. Porous Co3O4 electrode exhibits a specific capacitance of 240.2 F g1 at a current density of 0.625 A g1. The specific capacitance of Co3O4 remains more than 96.3% after 2000 cycles at 3.75 A g1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2015 Received in revised form 16 October 2015 Accepted 5 November 2015 Available online xxx
Porous Co3O4 microflowers were successfully synthesized by calcinations of novel flower-like metalorganic framework microcrystals. A supercapacitor was conducted based on this porous Co3O4 microflowers as the electrode material, which shows that the porous Co3O4 electrode exhibit a specific capacitance of 240.2 F g1 at the current density of 0.625 A g1 and remains more than 96.3% after 2000 cycles. The results demonstrate that porous Co3O4 microflowers can be acted as a promising electrode candidate in supercapacitors. © 2015 Elsevier B.V. All rights reserved.
Keywords: Inorganic compounds Microporous materials Electrochemical properties Electron microscopy
1. Introduction Supercapacitors, known as electrochemical capacitors or ultracapacitors, are considered as one of the newest innovations in energy storage devices and have attracted considerable attention because of its flexible operating temperature, high power capabilities, and long lifespan [1e3]. Based on the energy storage mechanism, supercapacitors are divided into pesudocapacitors and
* Corresponding author. E-mail address: [email protected] (L. Han).
electric double layer capacitors (EDLCs). In contrast, pesudocapacitors making use of reversible redox reactions on the surface of the electrode materials can show much higher specific capacitance than EDLCs, which store energy by ion adsorption [4,5]. In order to enhance the specific capacitance of supercapacitors, massive effort has been center on investigating the pesudocapacitors made of low-cost transition metal oxides (Co3O4, NiO, MnO2, etc) or hydroxides (Co(OH)2 and Ni(OH)2) [6e8]. Among these candidates, Co3O4 is more attractive due to its high theoretical specific capacitance (3560 F/g) and good chemical and thermal stability for energy storage [9,10]. Recently, metal-organic frameworks (MOF) have been utilized
http://dx.doi.org/10.1016/j.matchemphys.2015.11.011 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
2
G.-C. Li et al. / Materials Chemistry and Physics xxx (2015) 1e5
as precursors to generate porous metal oxide micro/nanostructure materials [11e14]. For instance, MIL-88-Fe was used as a template for fabricating spindle-like porous a-Fe2O3 [11], porous hollow Co3O4 with rhombic dodecahedral structures were prepared by the calcination of ZIF-67 microcrystals [12]. From the crystallography viewpoint and growth mechanism, the MOF precursor always has micro-or nanostructure [15e18] and it may be an effective and reasonable method to generate micro/nanostructure metal oxides. More particularly, a lot of new pores were generated due to the decomposition of organic ligands, finally resulting in a porous metal oxide structures. Despite many efforts has been made, a lot of work to be done if we want fully take advantage of this precursorsdrive-to-oxides approach. It has great challenge how to receive well-defined nano/micro precursors and maintain the morphology during calcining process. Herein, we successfully fabricated a novel flower-like MOF precursor through a facile solvothermal method. To further control the calcination conditions, porous Co3O4 microflowers can be obtained. Electrochemical properties of the Co3O4 electrode are tested by cyclic voltammetry (CV) and chronopotentiometry (CP) measurements in 3.0 M KOH electrolyte, which shows that the porous Co3O4 electrode exhibits a specific capacitance of 240.2 F g1 at a current density of 0.625 A g1 and remained more than 96.3% after 2000 cycles at 3.75 A g1. 2. Experimental 2.1. Materials preparation All chemicals in the experiment were analytical grade and used without further purification. In a typical synthesis, firstly, 0.20 g nicotinic acid, 0.035 g NaOH, 15.0 mL ethylene glycol and 15.0 mL alcohol were mixed together and were magnetic stirred for 1 h at room temperature. Then 0.30 g Co(NO3)2$6H2O were added into the above mixture. After stirring for 1 h, the solution was transferred into a Teflon-lined stainless steel autoclave maintained at
75 C for 12 h and cooled down to room temperature naturally. The pink precipitates were collected by centrifugation and washed with absolute ethanol several times, and then dried in a vacuum at 50 C overnight. Finally, the pink precipitates were calcinated at 500 C for 20 min in air and black Co3O4 can be obtained. 2.2. Characterization The morphology of the as-prepared samples was studied by Hitachi S-4800 field-emission scanning electron microscope (FESEM) at an acceleration voltage of 5.0 kV. The phase structures of the samples was characterized by a Bruker D8 advance X-ray powder diffractometer using Cu Ka radiation (l ¼ 0.15418 nm). The thermal behavior of the precursors was examined by employing thermogravimetric (TG) on SII TG/DTA7300 thermal analyzer. N2 adsorption-desorption was determined by BrunauereEmmetteTeller (BET) measurements using V-Sorb 2800 TP surface area and pore size analyzer. The electrochemical tests were performed by an electrochemical analyzer system, CHI660E (Chenhua, Shanghai, China) using a three-electrode cell in 3.0 M KOH aqueous solution. 2.3. Electrode preparation The working electrodes was prepared by mixing 80 wt.% Co3O4, 10 wt.% acetylene black and 10 wt.% polytetrafluoroethylene (PTFE), the slurries were coated on the nickel foam substrates of about 1 cm2, then pressed at 10 MPa and then further dried at 60 C overnight. The mass loading of active material is 2.0 mg. The electrochemical tests were performed by an electrochemical analyzer system, CHI660E (Chenhua, Shanghai, China) using a three-electrode cell in 3.0 M KOH aqueous solution. The Co3O4electrode, a platinum plate and a saturated calomel electrode (SCE) were respectively used as working electrode, counter electrode and reference electrode.
Fig. 1. (aec) SEM images of the as-prepared precursors; (d) Perspective view of the host structure of MOF precursor.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
G.-C. Li et al. / Materials Chemistry and Physics xxx (2015) 1e5
Fig. 2. XRD patterns of the obtained Co3O4 microflowers.
3. Results and discussion The pink MOF precursors were obtained by a facile solvothermal method from the coordination of nicotinic acid and Co2þ, resulting into novel multilayer microflowers. The XRD pattern of the precursor can be described as the compound [Co2(H2O)(C6H4O2N)4]$ sol (Fig. S1), which matches well with the crystal structure of the reported compound [19]. This MOF structure is robust and thermally stable open-framework based on rigid dimetallic carboxylate clusters as the basic building unit. The host framework possesses an effective channel area with the dimensions of 10.8 4.5 Å, as shown in Fig. 1d. In Fig. 1a and b, we can see that the flower-like precursor has
3
uniform size and shape with lengths of 5e6 mm and widths of 3e4 mm. It shows that the sample almost completely made up of many nanoplates structures. From images of Fig. 1c, it can be observed that many nanoplates have assembled as a novel multilayer flower-like structure and the surface of the precursor is relatively smooth. Moreover, there are many nanogaps formed between each layer. The TGA curve of precursor is shown in Fig. S2, the MOF precursor decomposes with increasing temperature in air below 500 C, thus the calcinations temperature was set as 500 C. The XRD patterns of as-prepared black powders are shown in Fig. 2. All the diffraction peaks can be in agreement with the standard spectrum of Co3O4 (JCPDS card no. 65-3103). No impurity diffraction peaks were observed, which indicates that the precursor has completely transformed into Co3O4 at 500 C in air. The morphology of the MOF precursor was well maintained after calcination in air, as displayed in low magnification FE-SEM image (Fig. 3a and b). Compared with the precursor, the size of the Co3O4 microflowers has little shrinked and the surface has changed roughness. From magnified SEM image Fig. 3c and d, we can see Co3O4 maintained nanogaps and generated many new nanopores, which might due to the release of gas in the process of the precursor decomposition. These nanogaps and nanopores might provide many channels to let ions go through, which can shorten the diffusion path of ions and thus enhance the specific capacitances. To further investigate the specific surface areas and the pore size distribution of the porous Co3O4microflowers, BrunauereEmmetteTeller (BET) adsorption/desorption measurements were performed. The nitrogen adsorption isotherm is a typical IV type curve and a distinct hysteresis loop can be observed in the range of ca.0.8e1.0 P/P0 (Fig. 4), which indicates the existence of a mesoporous structure in the sample [20]. The BET surface area of the as-prepared Co3O4 was measured to be 16.7 m2 g1. In addition, according to the corresponding BarretteJoynereHalenda (BJH) pore size distribution curve, the mean pore diameter is about 20 nm (inset in Fig. 4). Such porosity of Co3O4 microflowers can
Fig. 3. SEM images of porous Co3O4 microflowers.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
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G.-C. Li et al. / Materials Chemistry and Physics xxx (2015) 1e5
and Co3þ/Co4þ [23], according to the following equation [24,25].
Fig. 4. Nitrogen adsorption/desorption isotherm and the corresponding pore size distribution (inset) of the porous Co3O4 microflowers.
facilitate the transfer of ions and electrons at the electrode/electrolyte interface and provide sufficient active sites for Faraday reaction [21,22]. To evaluate the electrochemical properties of as-prepared porous Co3O4 microflowers electrodes, CV and CP measurements are performed in 3M KOH aqueous solution. As shown in Fig. 5a, the CV curves of the porous Co3O4 microflowers electrodes within a potential window of 0e0.5 V at different scan rates. Two pairs of well-dened redox peaks were observed, this peaks can be respectively ascribed to the redox reaction on the surface of the porous Co3O4 microflowers based on the redox mechanism of Co2þ/Co3þ
Co3 O4 þ OH þ H2 O43CoOOH þ e
(1)
CoOOH þ OH 4CoO2 þ H2 O þ e
(2)
Obviously, the shapes are different from that of electric double layer capacitance, manifesting the capacitance mainly results from pseudocapacitive property of porous Co3O4 microflowers electrodes. Fig. 5b depicts the CP curves in a potential range of 0e0.45 V at different current densities. The chargingedischarging curves show well symmetry, demonstrating the excellent electrochemical capacitance behavior and the reversible redox process. The specific capacitance of the porous Co3O4 microflowers electrodes at different current densities can be calculated from the CP curves according to the equation C ¼ iDt/(mDV), where i (A) is the discharge current, Dt (s) is the total discharge time, m (g) is the mass of Co3O4 materials and DV (V) is the potential range of charge/ discharge. Based on the above equation, the specific capacitance of the porous Co3O4 microflowers electrode is calculated to be 240.2, 233.7, 224.9, 214.6 and 202.1 F g1 at the current densities of 0.625, 1.25, 2.50, 3.75 and 6.25 A g1(Fig. 5c). Respectively, with the increase of the current densities from 0.625 to 6.25 A g1, the specific capacitances of 240.2 F g1 decrease to 202.1 F g1with 84.1% of capacitance is retained, indicating that good rate capability of the porous Co3O4 microflowers electrodes. Compared to those previously reported for Co3O4 materials (Supporting information Table S1), the capacitance of our synthesized samples ranks among the best [13,26e29]. For example, porous Co3O4 materials prepared by solid-state thermolysis of a novel Co-MOF crystal precursor in air can displays a specific capacitance of 150 F g1 at a current density of 1 A g1 in 2 M KOH electrolyte [13]. In addition, mesoporous Co3O4 nano-cubes were formed by further thermal treatment can
Fig. 5. (a) CV curves of porous Co3O4 microflowers electrode at different scan rates; (b) CP curves of porous Co3O4 microflowers electrode at different current densities; (c) The corresponding specific capacitance of porous Co3O4 microflowers electrode calculated by the CP curves and current densities; (d) Cycle performance of porous Co3O4 microflowers electrode at the current density of 3.75 A g1 after 2000 cycles.
Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011
G.-C. Li et al. / Materials Chemistry and Physics xxx (2015) 1e5
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240.2 F g1 at a current density of 0.625 A g1and offers a lower specific capacitance decay of ca. 3.7% after 2000 cycles in 3.0 M aqueous KOH solution. The results demonstrate that the porousCo3O4 microflowers could be used as electrode material for high performance supercapacitor applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21471086), the Social Development Foundation of Ningbo (No. 2014C50013), the projects for Graduate Student Research and Innovation Fund of Ningbo University (G15007), and the K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data
Fig. 6. Electrochemical impedance spectra (EIS) of porous Co3O4 microflowers electrode in 3 M KOH solution and a corresponding equivalent circuit (inset) for modeling the measured impedance spectroscopy.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2015.11.011. References
deliver a specific capacitance of 220 F g1 at a current density of 0.6 A g1 in 6 M KOH electrolyte [28]. The good electrochemical performance could be ascribed to the porous characteristic and the multilayer structure of the Co3O4 microflowers. The porous characteristic can serve as a robust reservoir for ions and the multilayer structure can shorten the ion transport/diffusion path within the electrodeeelectrolyte interface, thus enhancing the redox kinetics [30]. Cyclic stability is one of the most important factors for electrode materials in practical applications. The cyclic stability of the porous Co3O4 microflowers electrodes examined by chronopotentiometry at a current density of 3.75 A g1 for 2000 cycles is display in Fig. 5d. It can be seen that after 2000 cycles the specific capacitance has little degradation and retained 96.3% of its initial capacity. The remarkable cycling performance of the porous Co3O4 microflowers electrodes might be attributed to the robust multilayer structure and nanopores, which can maintain the morphology, prevent the collapse of the structure, shorten the diffusion path of ions and thus enhance the specific capacitances. To further investigate the electrochemical characteristics of the porous Co3O4 microflowers electrodes, the electrochemical impedance spectroscopy (EIS) has been performed in the frequency range from 0.01 Hz to 100 kHz and the corresponding Nyquist plot are shown in Fig. 6. This Nyquist plots mainly consisted of a semicircle at the high-frequency region and followed by a straight line at the low-frequency region. The inset in Fig. 6 shows an equivalent circuit, where Rs is the bulk solution resistance, Rct is the charge-transfer resistance, Cp is the faradaic pseudocapacitor, CPE represents the constant phase element accounting for a double-layer capacitance. The Rs and Rct values of the porous Co3O4 microflowers is found to be 0.67 U and 1.82 U, respectively. These values are much lower than the same (Co3O4) obtained with different morphologies by other conventional methods reported in the literature [29,31e33]. It implies that the porous Co3O4 microflowers electrodes have good capacitive behavior. 4. Conclusions In summary, porous Co3O4 microflowers assembled by nanoplates have been easily fabricated by calcination of a novel flowerlike MOF precursor. The porous Co3O4 microflowers can be applied to supercapacitor, and it exhibits a specific capacitance of
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Please cite this article in press as: G.-C. Li, et al., Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.011