Siru
Chen
,
Ming
Xue
*,
Yanqiang
Li
,
Ying
Pan
,
Liangkui
Zhu
,
Daliang
Zhang
,
Qianrong
Fang
and
Shilun
Qiu
*
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail: sqiu@jlu.edu.cn; xueming@jlu.edu.cn
First published on 26th December 2014
Cobalt-based oxides have been shown to be promising materials for application in high-energy-density Li-ion batteries and supercapacitors. In this paper, we report a new and simple strategy for the synthesis of a mixed-metal spinel phase (ZnCo2O4) from a zinc and cobalt mixed-metal organic framework (JUC-155). It is important to rationally design a MOF with a precise ratio (Co/Zn) and a synthetic process that is simple and time saving. After solid-state annealing of the mixed-metal MOF precursor at 400 °C for two hours, a pure ZnCo2O4 phase with a high surface area (55 cm2 g−1) was obtained. When used as electrode materials for supercapacitors, an exceptionally high specific capacitance of 451 F g−1 was obtained at the scan rate of 5 mV s−1. The capacitance loss after 1500 cycles was only 2.1% at a current density of 2 A g−1, indicating that this phase has an excellent cycling stability. These remarkable electrochemical performances suggest that this phase is potentially promising for application as an efficient electrode in electrochemical capacitors.
Spinel zinc cobaltite (ZnCo2O4) is isomorphic to the Co3O4 crystal structure and seems to be a promising alternative owing to its higher conductivity and richer redox reactions, caused by the coupling of the two metal species.13,14 Various approaches have been used to synthesise ZnCo2O4 nanomaterials, for example, co-precipitation, the sol–gel method and synthesis on a particular support.15–20 The supercapacitive property of ZnCo2O4 was investigated in prior efforts that showed that ZnCo2O4 is an excellent electrode material for supercapacitors. For example, ZnCo2O4 synthesized by co-precipitation shows a specific capacitance of 77 F g−1 at 5 mV s−1 scan rate.17 Using an epoxide driven, sol–gel method, Davis et al. obtained zinc cobaltite nanocrystals with exceptional capacitance and excellent cycle durability.18 In addition, there are some literature reports about ZnCo2O4 grown on Ni foam, which also show good capacitance properties.19,20 Although there have been several successful attempts to synthesise ZnCo2O4 nanomaterials, it is difficult to control the structure and porosity of the obtained mixed-metal oxides. Thus it is important to develop a simple method for producing spinel structured metal oxides with a high surface area and well-defined composition and morphology.
Metal–organic framework (MOFs) materials have attracted considerable attention in recent years due to their high surface area and porosity. These materials have been investigated for use in gas storage and separation, catalysis, ion exchange, and chemical sensors.21–27 Recently, they have been also used as a template or precursor to obtain different nanomaterials, including porous carbon and metal oxides.28–32 For example, Xu et al. first used the direct thermolysis method to prepare porous carbon with MOF-5 as a carbon precursor.33 You et al. reported the large-scale fabrication of multi-walled carbon nanotubes (MCNTs) through one-step, direct thermolysis of a metal–organic framework [Ni3(btc)2·12H2O] (btc = benzene-1,3,5-tri-carboxylate) precursor under the protection of an inert gas.34 Cho et al. synthesised spindle-like mesoporous Fe2O3 for lithium batteries through the calcination of MIL-88-Fe.35 Fe2O3 microboxes with hierarchical shell structures were synthesised by Lou et al. from Prussian blue microcubes and these structures display a high specific capacity of 950 mA h−1 at 200 mA g−1 in lithium ion batteries.36 The synthesis of metal oxides from MOFs has obvious advantages compared to other methods owing to their high surface area and unique structure. The resulting metal oxides usually have a high surface area and high conductivity, properties that are highly desirable for supercapacitors. Although many metal oxides have been developed using this method, the most reported are single-metal oxides and the synthesis of mixed-metal oxides from mixed-metal MOFs has rarely been seen. To the best of our knowledge, there have not yet been reports of using MOFs to synthesise a mixed-metal oxide for supercapacitors.37–40 Usually, mixed-metal MOFs can be constructed by both pre- and post-synthetic methods. In the past decade, some mixed-metal MOFs have been reported, particularly as magnetic MOF materials.41 It is common to construct these MOFs by using various metal ions in the reaction. Recently, post-synthetic ion metathesis (PSIM) was also found to be a simple method to construct mixed-metal MOFs with special properties.42,43 It is worth noting that some important functional mixed-metal MOFs were constructed by metalloligands, which have a variety of different functional sites, such as open metal sites, catalytic active metal sites etc.44,45 Furthermore, mixed-metal oxides with richer redox reactions show higher capacitance.14 So, mixed-metal MOFs constructed with Ni, Co, Zn, and Mn metals will be good precursors for the synthesis of new mixed-metal oxide materials as supercapacitor electrodes.
Therefore, we designed and synthesised a new mixed-metal MOF ZnCo2O(BTC)2(DMF)·H2O with a three-dimensional structure, named JUC-155 (JUC = Jilin University China), using the traditional solvent thermal method. Using JUC-155 as the precursor, pure phase ZnCo2O4 nanoparticles were prepared via solid-state transformations at three different temperatures. The optimal sample with a high surface area was obtained at a low temperature and showed promise as a potential electrode material for supercapacitors. Electrochemical measurements demonstrated that the as-prepared ZnCo2O4 materials possessed a large specific capacitance and a high electrochemical stability.
The powder X-ray diffraction (PXRD) data were collected on a SHIMADZU LabX XRD-6000 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) at room temperature.
The Fourier transform infrared (FTIR) spectra (KBr, Aldrich) were measured using a Shimadzu IRAFFINITY-1 Fourier transform infrared spectrometer. Samples were packed firmly to obtain transparent films.
The thermogravimetric analysis (TGA) was performed using a SHIMADZU DTG-60 thermal analyser system at a heating rate of 10 °C min−1 to 600 °C in dried air with a flow rate of 30 mL min−1. The sample was loaded on an alumina pan.
Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analyses were performed on a JEOS JSM-6510.
A transmission electron microscope, JEM-2010, was used for the transmission electron microscopy studies. A Gatan 794 CCD camera was used for recording transmission electron microscopy images.
Nitrogen adsorption/desorption measurements were carried out at 77 K with an Autosorb iQ MP (Quantachrome Instruments) to determine the surface area. Before the sorption analysis, the sample was evacuated at 200 °C for 10 hours using a turbo molecular vacuum pump. Specific surface areas were calculated from nitrogen adsorption data by multipoint BET analysis.
Parameter | ZnCo2O(BTC)2(DMF)(H2O)·H2O (JUC-155) |
---|---|
a R = ∑||Fo| − |Fc||/∑|Fo|. b R w = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. | |
Empirical formula | C21H7N1Zn1Co2O15 |
Formula weight | 696.51 |
Crystal system | Tetragonal |
Space group | I4cm |
a [Å] | 20.796(3) |
b [Å] | 20.796(3) |
c [Å] | 17.869(4) |
α [°] | 90 |
β [°] | 90 |
γ [°] | 90 |
V [Å3] | 7728(2) |
Z | 8 |
T (K) | 293 |
λ (Å) | 0.71073 |
ρ calcd (Mg m−3) | 1.197 |
μ (mm−1) | 1.515 |
GOF on F2 | 1.009 |
R [I > 2σ(I)] | 0.0787 |
R w | 0.1679 |
The formation of JUC-155 was confirmed by the PXRD results shown in Fig. 2a. The pattern is in agreement with the simulated one, confirming the structure of JUC-155, that is, isostructural with MOF-38 (Zn-btc).47 It should be noted that the relative intensities of the as-synthesized PXRD are slight different compared with the simulation because of the orientation of JUC-155 crystals. As shown in SEM images, the JUC-155 shows quadrangular rod morphology with a length of about 20 μm and a width of about 6 μm (Fig. S1a†). The energy dispersive spectroscopy (EDS) analysis shows that JUC-155 is composed of cobalt and zinc elements and that the molar ratio is about two (Fig. S2†). The elemental distribution analysis of the selected region further confirms that the zinc and cobalt elements are homogeneously distributed in the whole samples, as shown in Fig. S1b.†
Fig. 2 (a) PXRD patterns of the simulated single crystal crystallography data and as-synthesized JUC-155; (b) TG analysis curve of the as-synthesized JUC-155 crystal under air flow. |
A Fourier transform infrared (FTIR) spectrum was used to study the composition of JUC-155. As shown in Fig. S3,† after the coordination of carboxyl to Co and Zn there are two strong bands at 1574 cm−1 and 1373 cm−1 assigned to the asymmetric and symmetric stretching modes of the coordinated (COO−) group. The broad and strong band centred at 3425 cm−1 produced from H2O of hydration appears, suggesting the existence of coordinated and free solvent H2O molecules in the MOF. The characterisation peak of a protonated carboxylate group at 1730–1680 cm−1 does not appear, indicating that the BTC ligands are completely deprotonated.
The thermal stability of the as-synthesised JUC-155 was investigated by the TG analysis technique. Fig. 2b shows the TGA curves recorded from 50 °C to 600 °C. Three obvious weight-loss steps were observed. The first occurred in the range of 50 to 104 °C corresponding to a 2.47% weight loss, which is related to the elimination of one water guest molecule (calculated at 2.49%). The second weight loss (of 13.45%) occurs between 110 °C and 260 °C, owing to the loss of one coordinated water molecule and one coordinated DMF molecule (calculated at 12.60%). The final step starting at 350 °C indicates the decomposition of BTC species in the MOF at the same time as the framework begins to collapse.
Based on the TGA result, the JUC-155 was calcinated at three different temperatures (400, 450, and 500 °C) for two hours to obtain ZnCo2O4 materials 1, 2 and 3, respectively. As shown in Fig. 3, all of the diffraction peaks in the sample can be readily assigned to the spinel ZnCo2O4 phase (JCPDS card no. 23-1390). No characterisation peaks from other impurities were observed, indicating the high purity of the decomposed product. The morphologies of the three compounds were investigated by SEM and TEM. Compared to the morphology of JUC-155, the ZnCo2O4 materials showed a coarse shape and their size decreased a little because of the decomposition of the organic species (Fig. S4a–c†). The uniform distribution of Zn and Co species was confirmed by elemental mapping (Fig. S4d–f†). The molar ratio of Co and Zn elements is about two, as confirmed by EDS analysis (Fig. S5†). TEM was performed to learn the exact particle size and the shape of the ZnCo2O4 samples. As can be seen in Fig. 4, the TEM results indicated that the bulk ZnCo2O4 is composed of numerous ZnCo2O4 nanoparticles with size less than 20 nm and these nanoparticles can further aggregate to form a porous structure. Furthermore, the lattice fringes from HRTEM can be readily indexed to the 111 and 400 crystal planes of the ZnCo2O4 phase. The selected area electron diffraction (SAED) pattern shown as the inset can also be indexed to the spinel ZnCo2O4 phase, which is in agreement with the PXRD results.
Fig. 4 TEM images of porous ZnCo2O4 particles: (a, b) 1, (c, d) 2, and (e, f) 3. Insets (in b, d and f) are corresponding SAED patterns. |
A nitrogen adsorption–desorption analysis was performed at 77 K to investigate the porous features of ZnCo2O4 (Fig. S6†). From the adsorption studies, it is clear that the surface area of the three samples decreases with the increase in the decomposition temperature, which may be caused by an increase in crystal size at higher temperatures. The specific Brunauer–Emmett–Teller (BET) surfaces were 55.0 cm2 g−1, 45.9 cm2 g−1, and 20.4 cm2 g−1 for compounds 1, 2 and 3, respectively, and these values are comparable to the other metal oxides obtained from MOFs.
To evaluate the electrochemical properties of zinc cobalt oxides in supercapacitors, the performance of the as-prepared ZnCo2O4 nanoparticles were studied by cyclic voltammetry (CV), galvanostatic charge–discharge and cycling life measurements with a three-electrode system in a 6.0 M KOH solution. Fig. S7† shows the representative cyclic voltammetry (CV) curves of the three samples at various scan rates (5 mV s−1, 10 mV s−1, 20 mV s−1, 50 mV s−1, and 100 mV s−1) in a potential window from 0.05 to 0.45 V (vs. Ag/AgCl). A pair of well-defined redox peaks is visible in all of the CV curves as a result of the Faradic capacitive behaviour related to Co(OH)2/CoOOH. With increasing scan rate, the shapes of the CV curves do not significantly change except for a little shift in the peak position. This reveals the ideal capacitive behaviour of the ZnCo2O4 samples. Fig. 5a shows the comparison of the CV curves for electrodes 1, 2 and 3 at a scan rate of 5 mV s−1. From the three curves, we can see that the CV internal area for electrode 1 is the largest one and that for electrode 3 is the least one, which can be attributed to the higher surface area of compound 1 because this favours fast ionic transport in the electrodes. The capacitance of the ZnCo2O4 electrode at various scan rates can be calculated from the following equation:48
(1) |
To further evaluate the application potential of the as-synthesised ZnCo2O4 samples as electrodes for electrical capacitors, a galvanostatic charge–discharge test was conducted in a stable potential window between 0.05 and 0.4 V (vs. Ag/AgCl) at various current densities ranging from 1 to 40 A g−1, as shown in Fig. S9.† A linear variation parallel to the vertical axis indicates pure double-layer capacitance behaviour; the slope variation caused by redox reactions indicates a typical pseudocapacitive behaviour. The specific capacitance of the electrodes at different current densities can be calculated using the following equation:48
(2) |
The long-term cycling stability of the ZnCo2O4 samples (electrodes 1, 2, and 3) is an important quality required for practical applications. Measured under galvanostatic charge–discharge at a current density of 2 A g−1, only a small capacitance drop is observed after 1500 cycles, as shown in Fig. 5d. The specific capacitances remain at about 425 F g−1 (97.9%), 313 F g−1 (96.8%), and 212 F g−1 (94.6%) for electrodes 1, 2 and 3, respectively. Fig. S10† shows the first and last 15th galvanostatic charge–discharge cycles for electrodes 1, 2 and 3, respectively. From the curves, we can see that the shapes of the first and the last 15th curves remain almost the same, revealing its excellent cycling capability.
Footnote |
† Electronic supplementary information (ESI) available: Crystal data and additional figures. CCDC 1018100. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00167b |
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