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Facile Synthesis of Mixed Metal Organic Frameworks: Electrode Materials for Supercapacitor with Excellent Areal Capacitance and Operational Stability Sayed Habib Kazemi, Batoul Hosseinzadeh, Hojjat Kazemi, Mohammad Ali Kiani, and Shaaker Hajati ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04502 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018
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Facile Synthesis of Mixed Metal Organic Frameworks: Electrode Materials for Supercapacitor with Excellent Areal Capacitance and Operational Stability
Sayed Habib Kazemi a *, Batoul Hosseinzadeh a, Hojjat Kazemi b, Mohammad Ali Kianic, Shaaker Hajatid a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan
45137-66731, Iran b
Analytical Chemistry Research group, Research Institute of Petroleum Industry (RIPI), Tehran,
Iran c
Chemistry & Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran-
Iran d
Department of Semiconductors, Materials and Energy Research Center, P.O. Box 31787-316,
Tehran, Iran * Corresponding Author: Sayed Habib Kazemi (
[email protected])
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Abstract Electrode materials with high surface area, tailored pore size and efficient capability for ion insertion and enhanced transport of electrons and ions are needed for advanced supercapacitors. In the present study, a mixed metal organic framework (cobalt and manganese based MOF) was synthesized through a simple one pot solvothermal method and employed as the electrode material for supercapacitor. Notably, Co-Mn MOF electrode displayed a large surface area and excellent cycling stability (over 95% capacitance retention after 1500 cycles). Also, superior pseudocapacitive behavior was observed for Co-Mn MOF electrode in KOH electrolyte with an exceptional areal capacitance of 1.318 F cm-2. Moreover, an asymmetric supercapacitor was assembled using Co-Mn MOF and activated carbon electrode as positive and negative electrodes, respectively. The fabricated supercapacitor showed specific capacitances of 106.7 F g-1 at a scan rate of 10 mV s-1 and delivered maximum energy density of 30 Wh kg-1 at 2285.7 W kg-1. Our studies suggest the Co-Mn MOF as promising electrode materials for supercapacitor applications. Keywords: Metal-organic frameworks; Nanostructures, Areal Capacitance; Asymmetric Supercapacitor; Energy Storage
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Introduction Today, with increasing population and consumption of fossil fuels along with the lack of fuels, development of sustainable and economical energy has become an important area of interest for both scientific and technological researchers
1-4
. Utilization of conversion and energy storage
devices such as fuel cells, batteries and supercapacitors suggested as a favorable solution to reduce these problems 5-9. Amongst the different energy storage systems, supercapacitors known as devices having high power density, long-life operational stability, high-rate capability and wide operation temperature range
8, 10-14
. Based on the mechanism of charge storage processes,
large accessible surface area, considerable electrical conductivity and tailored pore size are required for electrode materials employed for supercapacitors applications. During the last years, two main categories of materials have been employed for supercapacitor applications. The first class includes carbonaceous materials such as carbon nanotubes, carbon fibers, graphene and mesoporous carbons which mainly store charges based on the non-Faradaic double layer charge storage or EDLC supercapacitors. Second class of supercapacitor materials are transition metal oxides/hydroxides (such as MnO2, RuO2, Co(OH)2, NiO) and conductive polymers which store the charge via redox reactions occurred at the surface of electrode materials; also called pseudocapacitors 2-4, 9, 15-27. To achieve higher performance of supercapacitors, development of new electrode materials, especially nanostructured materials, with exceptional charge storage properties and rate capability is essential. Among all the electrode materials used, metal-organic frameworks (MOFs) are a new class of porous materials prepared by chemical interaction of organic linkers and metal ions or clusters
28
. Because of the exciting properties such as tunable pore sizes,
extremely high surface area, remarkable permanent porosity, and ordered crystalline structures,
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MOFs have attracted great attention in the last two decades
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. MOFs have also employed for
chemical separations, drug delivery, catalysis, fluorescence and electrochemical studies as well as energy storage purposes 7, 35-42. Due to promotion of charge transfer inside the MOFs structure and redox behavior of their metal cations, these nanostructures successfully applied as electrode materials in energy storage device, especially Lithium ion batteries and supercapacitors 6, 43-52. Recently, several works have done for energy storage purposes, for instance Fe-MOF, Co-MOF, and Co-Zn MOF have reported as electrode materials for supercapacitors 53-55. However, from an overall perspective, low electrical conductivity and hindered ion entrance usually limit the practical applications of much of the MOFs for supercapacitor applications
56-58
. Fortunately, by
using appropriate linker and metal cations and tuning the resultant structures, it is possible to facilitate the ion insertion/extraction inside the MOFs framework likewise enhancing the transport pathways for electrons 59. In this work, we report a facile one-pot solvothermal synthesis of mixed metal MOF (Co-Mn MOF) and its application as supercapacitor electrode. Our results demonstrate that the Co-Mn MOF supercapacitor preserves its excellent specific capacitance in alkaline aqueous electrolyte compared to the previously reported works in this area, in addition to exceptional stability.
Experimental section Synthesis of Co-Mn MOF All chemical and solvents were purchased from Sigma-Aldrich and Acros companies and were used without further purification. In a typical synthesis of Co-Mn MOF, 0.08g of pbenzenedicarboxylic acid (Terephthalic acid, H2BDC) and 0.04g cobalt(II) chloride were dissolved in 10 ml of N,N-dimethylformamide (DMF) under vigorous stirring at room
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temperature. Subsequently, 0.04 g manganese (II) nitrate was added to this solution and the resultant reaction mixture transferred into a Teflon-lined stainless steel autoclave. Also, a Nifoam electrode (1cm × 1cm) was introduced in the solution (in autoclave), and then autoclave temperature was fixed at 120 °C for 24 h. After cooling the autoclave at room temperature, the violet color product and electrode washed three times with ethanol, finally dried at 120 °C for 12h in an oven. Additionally, to compare the mixed metal MOF with single metal based MOF, Co-based MOF, Mn-based MOF and Co-Mn MOF with different amount of cobalt(II) chloride were prepared by the same procedure. It should be noted that the Co-Mn MOFs prepared with different amount of cobalt(II) chloride (0.02, 0.04, 0.08g) are denoted as MOF-1, MOF-2, MOF3. Instruments and methods Galvanostatic charge/discharge and cyclic voltammetry experiments were carried out on an Autolab-101 potentiostat-galvanostat (Eco Chemie, The Netherlands) and electrochemical impedance spectroscopy tests were carried out using a Zahner/Zennium potentiostat-galvanostat (Zahner, Germany). Scanning electron microscopy (SEM, TESCAN VEGA3) equipped with EDX analyzer and transmission electron microscopy (TEM, Hitachi 200 kV electron beam energy) were used to study the morphology of samples. Analysis of surface area for prepared samples was performed by porosimetry analysis (BET and BJH analysis) on a Beslorp instrument (BELMAX, Japan) and X-ray photoelectron spectroscopy was performed by XPS, VG-Microtech Multilab 3000. Fourier transform infrared (FT-IR) transmission spectra were recorded using a Bruker Vector 22 spectrophotometer. Also, XRD analysis was carried out using a Philips PC-APD instrument. Electrochemical measurements
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Electrochemical experiments were carried out in both two and three-electrode cells. For threeelectrode studies, an Ag/AgCl electrode used as reference electrode and a platinum wire employed as counter electrode. Also, different MOF/Ni-Foam electrodes (MOF/NF) were used as working electrodes. For all measurements a 2 M KOH solution was used as electrolyte. Electrochemical impedance spectroscopy (EIS) measurement were recorded in the range of 100 kHz to 0.1 Hz at open circuit potential (OCP) by applying a perturbation signal of 10 mV. Two electrode tests were performed by using an asymmetric configuration in which an activated carbon electrode was used as negative and MOF/NF electrode as positive electrodes (MOF/NF//AC).
Result and discussion Morphological and structural studies The morphology of the Co-Mn based MOF nanostructures was investigated using SEM and TEM measurements and the results are provided in Figures 1A and 1B. As seen, SEM images present a well-defined and layered structure for MOF structure with the layer size of few hundred nanometers to micrometer scale. Also, higher magnification SEM image, Figure 1B, shows that the micro-structured layers consist of many uniform units of almost 100 nm or smaller size. However, some deviations from very crystalline and uniform distribution of deposited MOF materials, which is expected to see for powdered and crystallized MOF materials, may be occurred due to the direct deposition of Co-Mn MOF on Ni foam substrate with non-smooth surface. Figure S-1 provides more SEM images recorded with different magnifications. To better realize the smaller nanostructured units, TEM images are provided as Figures 1C and 1D. As seen in Figure 1C, the Co-Mn-MOF presents a well distributed, unique
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shaped and ordered nanoparticles with average particle size of 2 nm. Higher resolution image was also confirmed that the nanocomposite maintains a high surface area (Figure 1D); providing an excellent sites for charge storage. The crystalline structures of samples were examined by X-ray diffraction (XRD). Figure 2 shows the X-ray diffraction patterns of (a) Mn-based MOF (measured), (b) Mn-based MOF (simulated), (c) Co-based MOF (measured), (d) Co-based MOF (simulated), and (e) Mn-Co-based MOF (measured). As seen in figure 2, the measured patterns are partly in good agreement with the simulated ones. The relatively wide peaks in the measured patterns indicate that the domains of crystallinity corresponding to different pore sizes (as seen in BJH plot) are limited, while the simulated patterns are based on the assumption of having sharp pore size distribution and wide domain of crystallinity, which thus results in sharper peaks. In addition, the trace of peaks corresponding to Mn- and Co-based MOFs are observed in Mn-Co-based MOF. However, in addition to the above-mentioned reasons, there are expectedly some discrepancies between the measured and simulated patterns, because of the structure dependence on the solvent used, while this is not included in the simulation. Overall, the successful synthesis of Mn-Co-MOF by hydrothermal method is confirmed. 49, 54 Moreover, energy dispersive X-ray analysis (EDX) was carried out for MOF samples and its result represented the presence of both Co and Mn in an almost equal atomic percent in Co-Mn MOF (Figure 1E).
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Figure 1. (A) and (B) SEM images in different magnifications, (C) and (D) TEM images, and (E) typical EDX plot for Co-Mn MOF 8 ACS Paragon Plus Environment
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Figure 2. X-ray diffraction patterns of (a) Mn-based MOF (measured), (b) Mn-based MOF (simulated), (c) Co-based MOF (measured), (d) Co-based MOF (simulated), and (e) Mn-Cobased MOF (measured).
Fourier transformation Infrared (FT-IR) spectroscopy was also employed to characterize the CoMn MOF. Figure 3 and Figure S-2 show FT-IR spectra of Co-Mn MOFs. The FT-IR spectra illustrated that these Co-Mn MOFs, displayed similar peaks in general. As seen in Figure 3, the 9 ACS Paragon Plus Environment
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strong bands at 1390 cm-1 and 1556 cm-1 are attributed to the symmetric and asymmetric stretching modes of coordinated (-COO-) group, respectively, which are in agreement with the previously reported results60. The band at 3400 cm-1 can be assigned to stretching vibration of OH group and peaks with lower intensities in the range of 750 to 1200 cm-1 is related to paraaromatic C–H stretching vibration bands. The absorption bands at 611 and 516 cm-1 is the characteristic stretching vibration of Co-O and Mn-O modes, respectively61. Therefore, the results of XRD analysis and FT-IR spectra confirmed the formation of Co-Mn MOFs.
Figure 3. FT-IR spectrum recorded for Co-Mn MOF (MOF-2) The surface area and pore-size distribution are two key factors for the electroactive materials in electrical storage applications. Isothermal N2 adsorption-desorption measurements were used to obtain the information on the specific surface area and pore size of Co-Mn MOF nanocomposite. Figure 4 depicts the N2 adsorption/desorption, BJH and BET plots recorded for MOF-2. Adsorption/desorption isotherm plot in Figure 4 revealed the type IV for adsorption on MOF-2. It should be noted that the loop is almost similar to H3 type. Such a hysteresis loop confirming the formation of mesoporous structure of MOF-2. The surface area of the MOF-2 is 15.8 m2 g-1 based on the adsorption/desorption isotherms, also the average pore size of MOF-2 was 10 ACS Paragon Plus Environment
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estimated as 12.4 nm using BJH method. As seen in BJH plot (Figure 4), the size of the major part of pores is less than 20 nm with an average of 12.4 nm. Also, a large pore volume of 0.19 cm3 g-1 was estimated for MOF-2; providing considerable space for charges/electron insertiondeinsertion into the texture of MOF electrode material. The large surface area should be due to well-defined and layered structure of MOF to form the porous structure as shown in SEM and TEM images.
Figure 4. N2 adsorption and desorption isotherm (a), BET (b) and BJH (c) plots recorded for CoMn MOF
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Additionally, thermogravimetric analysis was carried out for Co-Mn MOF synthesized in the present work (provided as Figure S-3, supporting information). The TGA curves showed that the first weight loss of about 15% between 20 °C and 200 °C, can be attributed to the loss of crystalline solvent (DMF) and adsorbed solvent. Then, Co-Mn MOF behaved unchanging up to at least 480 °C; indicating that the framework is stable up to this high temperature. A main weight loss of 40% was observed between 480 and 550 °C, which was assigned to the decomposition of organic linkers. Furthermore, Survey (Figure 5a) and narrow (Figures 5b-5e) XPS spectra were taken from the obtained MOF to determine its elemental composition and chemical states. The XPS spectra were first smoothed and calibrated to the C 1s binding energy (284.6 eV) followed by the implementation of Shirley algorithm to subtract their background
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. Fig. 5a shows the well
contribution of carbon and oxygen as indicators of the linker used, as well as Mn and Co in the MOF-2 structure. In addition, the narrow spectra corresponding to C 1s, O 1s, Mn 2p and Co 2p were deconvoluted to determine the extent to which the different states of each element are present in the MOF. As shown in Figures 5b and 5c, the presence of C 1s at 284.6 eV (CC/C=C), 285.6 eV (C-O) and 288.5 eV (C=O), as well as the presence of O 1s at 531.6 eV (O=C) and 533 eV (O-C) confirm that the BDC linker has successfully contributed in the structure of MOF-2. As seen in Figure 5d, the peaks observed at 642.14 and 653.74 eV correspond to Mn 2p3/2 and Mn 2p1/2, respectively, which are attributed to manganese coordinated with oxygen of the linker. Satellites shown are originated from valence excitation losses. Furthermore, similar coordination was found between cobalt and oxygen confirmed by peaks at 781.35 and 797.2 eV corresponding to Co2p3/2 and Co2p1/2, respectively (Figure 5e). Co 2p electrons energy loss during the excitation of valence electrons causes several satellites, as
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shown in Figure 5e. After considering the relative surface sensitivity of the elements detected, the percentages of carbon, oxygen, manganese and cobalt were determined to be 69.57, 26.28, 2.01 and 2.14 %. Overall, it was concluded that the MOF-2 has been successfully synthesized.
Figure 5. XPS spectra taken from the MOF-2, (a) survey, (b) C 1s, (c) O 1s, (d) Mn 2p and (e) Co 2p. Electrochemical properties of Co-Mn MOF as electrode materials for supercapacitor In order to evaluate the applicability of as-synthesized Co-Mn MOFs as electrode materials for electrochemical supercapacitors purposes, the electrochemical studies were investigated in both three and two electrode systems. Figure 6a shows cyclic voltammograms (CVs) at different scan
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rates for Co-Mn based MOF in the potential window of -0.2 V to 0.6 V (vs. Ag/AgCl) in KOH electrolyte. As seen, the shape of the CVs demonstrates typically pseudocapacitive characteristics. Considering the incorporation of metallic species in the structure of Co-Mn MOF, appearance of redox peaks in voltammograms can be attributed to the redox reactions of cobalt species and the following reactions can be proposed:
Co2+ + OH-
Co2+(OH)ads + e-
Co2+(OH)ads
Co3+(OH)ads + e-
Moreover, the insertion-deinsertion of alkali cations (K+) into the structure of Co-Mn MOF can be explained for charge storage mechanism through Mn species4. It should be noted that the electrochemical behavior of Mn species is mainly pseudocapacitive which increase the charge storage capacitance similar to carbon based electrode materials. The pseudocapacitive behavior of Mn species in the MOF based electrode may be considered as following4: O
Mn
O + K + + e-
O
Mn
OK
Also, galvanostatic charge-discharge experiments were carried out at different current densities from 1 to 5 mA cm-2 (Figure 6b). As seen, discharge branch of the curves displayed two different regions of voltage drop. The first region is attributed to nonlinear potential drop due to pseudocapacitive behavior of MOF electrode and the second linear drop with sharper decay can be described by double layer mechanism of charge storage. To further study the electrochemical performance and capacitive behavior of Co-Mn MOF, charge/discharge plots at various current densities were employed to estimate the capacitance and obtained results are presented as Figure 6c. As evidenced by Figure 6b and 6c, charge/discharge curves show smoothly sloping potential variations against time in good agreement with voltammograms; indicating the pseudocapacitive feature of Co-Mn MOF electrode material. It should be mentioned that the decrease in specific 14 ACS Paragon Plus Environment
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capacitance by increasing the currents attributed to the limited ions migration inside the active materials in shorter times. A maximum specific capacitance of 1.318 mF cm-2 was reached at 1 mA cm-2 for Co-Mn MOF nanostructure (MOF-2).
Figure 6. Electrochemical performances of MOF-2 electrode: (a) CV curves at different scan rates (5, 10, 25, 50 and 75 mV s-1), (b) charge/discharge profiles at different currents, (c) specific capacitances at different current estimated from charge/discharge curves
Figure 7 presents the compared electrochemical results of different Co-Mn MOF electrodes (MOF-1, MOF-2 and MOF-3). As clearly seen in Figure 7a and Figure 7b, MOF-2 electrode 15 ACS Paragon Plus Environment
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shows the best electrochemical performance and specific capacitance. The specific capacitance was calculated from the cyclic voltammograms and discharge curves using the following equations9:
C=
1 i V dV 2mv∆V
C=
i A × ∆t m × ∆V
(1)
(2)
where C (F g-1) is the specific capacitance of the MOF based electrode, ν (mV s-1) is the potential scan rate, i (A) is voltammetric current and m (g) is the mass loading. Also, i (A) is the constant discharge current, ∆t (s) is discharge time, and ∆V (V) is the potential drop at a constant discharge current. By replacing m (g) with surface of the electrode (A (cm2)) in equations (1) and (2) the areal capacitances were calculated in F cm-2. A maximum specific capacitance of 2.375 F cm-2 at a scan rate of 5 mV s-1
was observed for MOF-2. Also, Maximum specific capacitance estimated from charge/discharge plots is almost 1.318 F cm-2 at a current density of 1 mA cm-2. These parameters for MOF-1 and MOF-3 are considerably lower than MOF-2 capacitance at the same conditions. Electrochemical result of three electrode system revealed that the increasing in cobalt content of the MOF structure resulted in an increasing trend in capacitance up to 0.04 gr. However, specific capacitance was decreased at higher cobalt amounts. We also examined the performance of pure Mn-based and Co-based MOFs as electrode materials (the result shown in Figure 8). Moreover, cyclic voltammetry and charge/discharge analysis indicated that the electrochemical and pseudocapacitive behavior of mixed metal Co-Mn based MOF-2 was superior compared to Cobased and Mn-based MOFs (Figure 8a and 8b). The electrochemical impedance spectroscopy 16 ACS Paragon Plus Environment
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was also used to examine the electrochemical behavior of the MOF-2 electrode in 2 M of KOH and at open circuit potential (Figure 8c). Obtained Nyquist plots were analyzed with an equivalent circuit of Ru(CPE [Rct Zd]), where Ru referred to the electrochemical series resistance, Rct is the charge transfer resistance, CPE was the constant phase element used to model the capacitive behavior of the MOF-2 electrode interface, and Zd is diffusion impedance coupled to Rct. As seen, the value of Ru was considerably small (almost 0.65 Ω) and Rct is about 0.17 Ω; implying that the kinetics of electron/charge transfer at MOF-2 electrode is significantly facilitated which is very important for supercapacitor purposes. The results of EIS experiment are in good agreement with CV and charge/discharge tests; indicating the viability of the MOF-2 electrode for energy storage applications.
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Figure 7. Electrochemical performances of three MOF electrodes: (a) cyclic voltammograms of three synthesized MOFs at scan rate 5mV s-1, (b) charge-discharge profiles, and (c) specific capacitances at different current densities
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Figure 8. (a) Cyclic voltammograms of Co-MOF, Mn-MOF and Co-Mn based MOF electrodes at the scan rates of 5mV s-1 in KOH 2M, (b) corresponding charge-discharge profiles for three electrodes at the current density of 1mA, and (c) Nyquist plot representation of MOF-2 in KOH 2M at open circuit potential, and (d) cycle life stability of Co-Mn based MOF (MOF-2) electrode
A critical parameter which plays very important role in performance of a supercapacitor is the operational long cycle life of electrode materials; hence the cycle life of the Co-Mn MOF electrode (MOF-2) was investigated by cyclic voltammetry measurement at 100 mV s-1 in 2 M of KOH electrolyte and the capacitive retention as a function of cycle number plotted (shown in Figure 8d). In fact, the capacitance retention was observed to increase from during the first 700th, perhaps due to the gradual activation of the MOF-2 electrode surface and thus improved access of the electrolyte ions into the smaller size pores (micropores) with increasing CV cycles. It was observed that the MOF-2 electrode preserves 96% of its initial specific capacitance after 1500 successive cycles. Moreover, by increasing the cycle number up to 3000, the MOF-2 electrode maintains 86% of its initial capacitance; indicating the excellent long cycle life and electrochemical stability of the Co-Mn MOF electrode. It was also found that after thousands of
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potential cycling test in alkaline solution, there is no peeling off or break in the Co-Mn MOF electrode; confirming the stability of the electrode material in strong alkaline media.
Two electrode MOF-2/NF//AC asymmetric supercapacitor studies To evaluate the practical advantage of MOF-2 electrode for supercapacitor purposes, an asymmetric supercapacitor was constructed using MOF-2/NF as positive and activated carbon electrode as the negative electrode in a sandwich-type two-electrode configuration. To find the electrochemical potential windows of each electrode, firstly cyclic voltammetry tests were performed in a three electrode system (Figure 9a). As seen, stable potential windows in the ranges of −1.0 up to 0.0 V for AC electrode and -0.2 to +0.7 V for MOF-2 electrode were obtained. Thus, the operational cell voltage can be considered as the sum of these two voltage ranges or 0.0 to 1.7 V. Figure 9b shows the voltammetric response of the MOF-2/NF/AC supercapacitor at different applying cell voltages, also the variation of the specific capacitance against the voltage window was exhibited in Figure 9c. It is clear that the the stored energy enhanced with increasing the voltage window, however applying voltages higher than 1.7 volt will result in the oxygen evolution due to electrolyte oxidation. Thus, the voltage window of 0.0 to 1.7 V was preferred as optimum operational cell voltage.
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Figure 9. a) Cyclic voltammograms of AC and MOF-2/NF electrodes in a three electrode cell in 2 M KOH at a scan rate of 50 mV s-1, b) voltammograms of MOF-2/NF//AC asymmetric cell at different voltage windows at a scan rate of 20 mV/s, and c) specific capacitances of the asymmetric supercapacitor against applied cell voltages
To further check the electrochemical performance of the MOF-2/NF/AC supercapacitor, voltammograms at different voltage scan rates in the voltage range of 0-1.7 V in KOH electrolyte was recorded (Figure 10a). Furthermore, capacitive performance of the cell was investigated by galvanostatic charge-discharge curves at various currents (Figure 10b). Also, Figure 10c shows
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the charge-discharge plots recorded at various potential windows at 0.2 A g-1. Maximum specific capacitances of 106.7 F g-1 and 86.8 F g-1 were achieved for MOF-2/NF//AC supercapacitor at a scan rate of 10 mV s-1 and discharge current of 0.8 A g-1, respectively. It should be mentioned that specific capacitance as a functions of scan rate, applied current densities and various applied voltages to the asymmetric cell are provided as Figure S-4 in supporting information. Additionally, specific energy and specific power of the MOF-2/NF//AC cell which are important factors for evaluating power application of electrochemical supercapacitor were estimated, and the corresponding Ragone plot for asymmetric cell is shown as Figure 10d. It is obvious that the specific energy was decreased slightly with increasing specific power of the supercapacitor. From Ragone plot, it was confirmed that MOF-2/NF//AC supercapacitor possess an excellent energy density (30.85) and satisfactory power density (2285.7). The asymmetric MOF-2/NF//AC c supercapacitor can deliver a maximum energy density of 30.85 W h kg-1 at a power density of 685 Wkg-1, even remaining at 22.8 Wh kg-1 at a power density of 2285.7 kW kg-1, which originates from the remarkable specific capacitance contribution of MOF-2/NF and the excellent power support of AC electrode. Furthermore, our findings confirm that the values of energy and power density are comparable or even better than those of the recently reported asymmetric supercapacitors, as shown in Table 164-68.
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Table 1. Energy and Power densities calculated for MOF-2/NF//AC asymmetric supercapacitor Energy density / W h kg-1 Power density / W Kg-1 Reference 26
450
64
20
800
65
16
4000
66
10
6000
67
27
2600
68
30
2285.7
This work
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Figure 10. a) Voltammograms recorded for MOF-2/NF//AC asymmetric cell at different scan rates, b) and c) are galvanostatic charge-discharge curves for various operational voltages at a constant current
of 3 A g-1 and different current densities, respectively, and d) Ragone plot for MOF-2/NF//AC supercapacitor
Our findings showed that 74% of initial value of maximum specific capacitance was retained when the current density increased from 0.8 to 2.8 A g-1. Moreover, the cycle life stability was examined over 3000 successive charge-discharge cycles for the asymmetric supercapacitor (Figure 11a). Figure 11b illustrates the voltammograms recorded for MOF-2/NF//AC supercapacitor before and after applying 3000 operational cycles. All electrochemical findings revealed that the MOF-2/NF can be used as supercapacitor electrode for highly stable supercapacitors with improved capacitance and energy density.
Figure 11. a) Capacitance retention ratio during 3000 successive cycle numbers and b) voltammograms at before and after applying 3000 charge-discharge cycles
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Conclusions In summary, we introduce the potential application of synthesized bimetallic MOF containing Co and Mn using solvothermal method, for supercapacitor purposes. Remarkably, MOF-2 nanocomposite was used to construct a high performance supercapacitor electrode and it was found that the electrode maintains excellent areal capacitance and considerable cycling stability. A maximum specific capacitance of 2.375 F cm-2 was achieved for MOF-2/NF electrode at 5 mV s-1, in addition to capacitance retention of more than 85% after 3000 successive charge-discharge cycles. Also, high rate capability was observed for the electrode in which the supercapacitor was retained more than 50% of its initial capacitance when the applied current was changed from 1 to 5 A g-1. Furthermore, an asymmetric supercapacitor was assembled using MOF-2 electrode and activated carbon electrode as positive and negative electrodes, with a high specific capacitances of 106.7 F g-1 at a scan rate of 10 mV s-1 and delivered maximum energy density of 30 Wh kg-1 at 2285.7 W kg-1. Thus, this work demonstrates that MOF-2/NF electrode could find potential applications for supercapacitive purposes.
Acknowledgements The financial supports of the work by Institute for Advanced Studies in Basic Sciences and Iranian National Science Foundation Grant (INSF-95849280) are appreciated. Supporting Information SEM images for MOF-2, FT-IR spectra for MOF-1 and MOF-3, Thermogravimetric curves (TG and DTG) for MOF-2, Plots of specific capacitance against voltage scan rates, operation voltages at a constant discharge current and current densities.
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