Cobalt terephthalate MOF-templated synthesis of porous nano-crystalline Co3O4 by the new indirect solid state thermolysis as cathode material of asymmetric supercapacitor

Author’s Accepted Manuscript Cobalt terephthalate MOF-templated synthesis of porous nano-crystalline Co3O4 by the new indirect solid state thermolysis as cathode material of asymmetric supercapacitor Hadise Bigdeli, Morteza Moradi, Shaaker Hajati, Mohammad Ali Kiani, Jozsef Toth www.elsevier.com/locate/physe

PII: DOI: Reference:

S1386-9477(17)31141-4 http://dx.doi.org/10.1016/j.physe.2017.08.005 PHYSE12885

To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 31 July 2017 Accepted date: 5 August 2017 Cite this article as: Hadise Bigdeli, Morteza Moradi, Shaaker Hajati, Mohammad Ali Kiani and Jozsef Toth, Cobalt terephthalate MOF-templated synthesis of porous nano-crystalline Co3O4 by the new indirect solid state thermolysis as cathode material of asymmetric supercapacitor, Physica E: Low-dimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2017.08.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Cobalt terephthalate MOF-templated synthesis of porous nano-crystalline Co3O4 by the new indirect solid state thermolysis as cathode material of asymmetric supercapacitor a

a

a

b

Hadise Bigdeli , Morteza Moradi *, Shaaker Hajati , Mohammad Ali Kiani , Jozsef Toth a

c

Department of Semiconductors, Materials and Energy Research Center, P.O. Box 31787-316, Tehran, Iran

b

Chemistry & Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran

c

Institute for Nuclear Research, Hungarian Academy of Sciences (MTA ATOMKI), P.O. Box 51, H4001 Debrecen, Hungary [email protected] [email protected] *

To whom correspondence should be addressed: Tel.: +98-912-2467683;

Abstract In this work, two different types of Co3O4 nano-crystals were synthesized by (i) conventional direct solid state thermolysis of cobalt terephthalate metal-organic framework (MOF-71) and (ii) new indirect solid state thermolysis of Co(OH)2 derived by alkaline aqueous treatment of MOF-71. The products were then characterized by X-ray diffraction technique (XRD), Fourier transforms infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Reflection electron energy loss spectroscopy (REELS), Brunauer, Emmett, and Teller (BET),

scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques. By REELS analysis the energy band gap of MOF-71 was determined to be 3.7 eV. Further, electrochemical performance of each Co3O4 nanostructure was studied by the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in a three-electrode system in KOH electrolyte. An asymmetric supercapacitor was fabricated using indirect Co3O4 nanoparticles as cathode and electrochemically reduced graphene oxide as anode, and the electrochemical properties were studied and showed a high energy density of 13.51 Wh kg-1 along with a power density of 1

9775 Wkg-1 and good cycling stability with capacitance retention rate of 85% after 2000 cycles. Graphical Abstract

Keywords: Supercapacitor, Metal-organic framework, Solid state thermolysis, Co3O4 nanocrystals.

1. Introduction Severe decrement of fossil fuel resources calls for development of advanced ecofriendly energy storage devices, e.g. electrochemical systems. Electrochemical energy storage systems include batteries, fuel cells, and supercapacitors; these can be used in all types of electronic devices. In these systems storage and energy conversion are performed via 2

different mechanisms, but there are similarities in energy production. The electrochemical capacitors (supercapacitors), which have been widely studied because of their high power density and rapid rate of charge-discharge beyond all kinds of the devices, offer a large number of life cycles and perfect biocompatibility, making them special devices [1-6]. Recently, porous metal-organic frameworks (MOFs) with their large surface area and specific

microporous

volume

have

found

applications

in

gas

adsorption

and

storage/separation, drug delivery and heterogeneous catalysis. Applications of MOFs have increased because of their simple synthesis process and diverse molecular structures [7, 8]. The porosity of MOFs makes it possible for electrolytes to easily pass thorough them and generate large capacitance. The production of MOF-based supercapacitors has been reported only recently. As a pioneer in this scope, Diaz et al. studied Zn-MOF (Co) for supercapacitor electrodes; the results showed a gravimetric capacitance of only 2 F g-1 [9]. Kang et al. synthesized a Ni-based MOF and fabricated an asymmetric supercapacitor as where the MOF served as positive electrode, commercial activated carbon was used as negative electrode, and KOH was applied as electrolyte, with an energy density of 16.5 Wh kg-1 [10]. Unfortunately, MOFs are mostly nonconductive, and as a result, it is a challenging task to build effective supercapacitors based on pure MOFs. Most importantly, by taking advantage of their thermal behavior and chemical reactivity, various porous carbons and metal oxides can be achieved easily through heat treatment of MOFs [11, 12]. Heat treatment improves physical, chemical, mechanical and, in particular, metallurgical properties of materials. Han et al. calcined the bimetallic organic frameworks in air to produce doubleshelled NiO/ZnO hollow spheres to be used as electrodes of a supercapacitor. They reported the specific capacitance of NiO/ZnO as 497 F g-1 at a current density of 1.3 A g-1 [13]. Also, Tang et al. investigated different heat treatment conditions. Accordingly, untreated ZIF-8 and 3

calcined ZIF-8 electrodes under air and nitrogen atmosphere exhibited specific capacitances of 96, 156, and 185 F g-1, respectively, at a scan rate of 5 mV s-1, and good stability beyond 1500 cycles [14]. Although different types of MOF calcinations have been studied in supercapacitor applications, but to the best of our knowledge, there are no reports comparing the heat treatment methods with the water decomposition of MOFs. One of the major concerns in using MOFs in aqueous media, especially in strong alkaline or acidic conditions, is their chemical stability and crystal-to-crystal transformation to inorganic materials. In this work, a cobalt-based MOF (MOF-71) was prepared under standard solvothermal conditions using Co(NO3)6 as metal precursor, 1,4-benzenedicarboxylate(BDC) as organic ligand, and N,Ndimethylformamide (DMF) as solvent. Two different types of cobalt-based active materials, namely Co3O4 and Co(OH)2, with uniform morphologies can be obtained by thermal and alkaline aqueous treatments, respectively. Also, a new concept of MOF heat treatment based on the solid state thermolysis (SST) of MOF-71-derived Co(OH)2 was studied. Further, a novel asymmetric supercapacitor device was developed by employing the MOF-71-derived active materials as cathode and electrochemically reduced graphene as anode in a 6 M KOH aqueous electrolyte. High specific capacity, power density, energy density and cycling stability of the as-prepared asymmetric supercapacitor device demonstrates its potential applications in energy storage and conversion systems. 2. Experimental 2.1. Synthesis and solid state thermolysis of MOF-71 The MOF-71(Co) was prepared via a simple solvothermal reaction where cobalt nitrate hexahydrate (Co(NO3)2·6H2O) (0.75 g) and 1,4-benzenedicarboxylic acid (H2BDC) (0.428 g) 4

were dissolved in ethanol (12 mL) and N,N-dimethylformamide (DMF) (48 mL) and poured into a Teflon-lined autoclave where the solution was heated at 100 °C for 12 h. Once the autoclave was cooled down to room temperature, we were left with a pink/purple crystalline powder precipitated in the vessel. To remove the solvent, the precipitate was washed with DMF and ethanol three times. Then, the product was dried in a vacuum oven at 110 °C for 12 h. Two types of MOF-71 thermolysis were designed to produce MOF-derived Co3O4: (i) MOF-71 was added to 0.1 M NaOH solution and stirred for 1 h ([Co(bdc)(DMF)] + 2OH- → [Co(OH)2] + bdc2-+ DMF), so that the color of the precipitate changed to brown. The product was then cleaned thoroughly using deionized water as solvent via centrifugation for three times. The collected powder was dried in a vacuum oven at 100 °C for 24 h; this sample is designated as Co(OH)2. After SST of cobalt hydroxide at 350 °C for 6 hat a heating rate of 5 °C min-1, a porous cobalt oxide was produced (named Indirect/Co3O4 (In-CO3O4)). (ii) In order to investigate the effects of direct heat treatment on the MOF, another type of SST process was implemented, wherein MOF-71 was loaded into a ceramic crucible and then heated in a tube furnace to 450 °C at a heating rate of 5 °C min-1 for 1.5 h; finally, it turned into the black powder called Direct/Co3O4 (Di-Co3O4). The working electrodes were prepared by mixing active material, carbon black, and polyvinylidene fluoride (PVDF) (10% solution in NMP) with a mass ratio of 75:20:5. A 5% solution of the mixture in isopropanol was sprayed on Ni foam (NF) as the current collector. The level of active material specific loading on NF was in milligram (ca. 1-2 mg) per a square centimeter scale. The foam was dried in vacuum oven at 100 °C for 24 h. The tests were undertaken by employing the MOF-71derived active materials as cathode and electrochemically reduced graphene as anode in a 6 M KOH aqueous electrolyte. Overall, synthetic process for active material and supercapacitor construction is illustrated in Scheme 1.

5

2.2. Reduction of GO Graphene oxide (GO) was prepared by the modified Hummers’ method wherein GO film was deposited at nickel foam by spraying the solution of GO (active material) and PVDF (binder) at a weight ratio of 90:10 into 2 ml of NMP (solvent). To enhance the capacity, the GO was reduced to rGO by a cyclic voltammetry (CV) method known as electrochemical reduction of graphene oxide (ERGO) in 6 M KOH solution. This electrochemical method offers low cost, high sensitivity and no use of any toxic solvent. The reduction was performed at a potential within the range -1.1 – 0 V vs. calomel at ascan rate of 50 mV s-1 at 30 cycles. 2.3. Electrochemical test All the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were carried out on an electrochemical workstation (Model OGF500, Origalys) in a standard three-electrode cell system at the room temperature. A platinum electrode and a saturated calomel electrode (SCE) served as the counter and reference electrodes, respectively, in 6 M KOH electrolyte. The working electrodes were prepared by mixing active material, carbon black, and polyvinylidene fluoride (PVDF) (10% solution in NMP) at a mass ratio of 75:20:5. A 5% solution of the mixture in isopropanol was sprayed on Ni foam (NF) as the current collector. The foams were dried in vacuum oven at 100 °C for 24 h. For the two-electrode tests, the In-Co3O4was used as cathode and electrochemically reduced graphene served as anode. CV curves were figured out at various scan rates ranging from 5 to 100 mV s-1 with the potential within range -0.05 – 0.5 V. The galvanostatic chargedischarge tests were measured within the potential window 0 –0.52 V at various current densities within 1 – 10 A g-1. An Autolab PGSTAT30 was employed to measure electrochemical impedance spectroscopy (EIS) and the measurements were tested ata

6

frequency ranging from 100 to 0.1 Hz. All of the electrochemical measurements were taken at the room temperature. 2.4. Materials characterization The prepared samples were structurally characterized using powder X-ray diffraction (XRD, Philips X´pert diffractometer with CuKα radiation (λ = 0.154 nm) generated at 40 kV and 30 mA with a step size of 0.04 °s-1). Specific surface areas of the products were measured by so-called Brunauer-Emmett-Teller (BET) method using nitrogen adsorption and desorption isotherms on a Micromeritics Instrument Corporation sorption analyzer (TriStar II 3020). Pore size and size distribution of the materials were calculated by the Barrett- JoynerHalenda (BJH) method, and Fourier transform infrared spectroscopy (FT-IR, Bruker Vector 33) was recorded on an infrared spectrometer. Morphology analysis was accomplished using a Tescan Vega3 scanning electron microscope (SEM) equipped with an energy dispersive Xray detector (Oxford Instruments INCAx-act), so as to determine elemental composition of the sample. XPS spectra were recorded by an ATOMKI ESA-31 (Hungary) instrument. C 1s (284.5 eV) was used as binding energy reference for handling the surface charging effects. This instrument was also used for the reflection electron energy loss spectroscopy (REELS) to acquire the spectrum at electron primary energy of 4000 eV incident at 50° and reflected at 0° with respect to the sample surface normal. 3. Results and discussion 3.1. Characterization Scanning electron microscopy (SEM) was used to observe the morphology of MOF-71 (Fig. 1a), direct Co3O4 (Fig. 1b), Co(OH)2 (Fig. 1c) and indirect Co3O4 (Fig. 1d). As seen, MOF-71 looks like a pile-up of about 50-100 nm thick nanosheets in a compressed form 7

which leads to small surface area of this metal-organic framework, as will be further supported by BET study (Fig. 2a). After the SST of MOF-71, the as-directly-prepared cobalt oxide was shown to be like agglomerated particles which are expected to have small surface areas, as supported in below by BET analysis (Fig. 2b). The NaOH-processed MOF-71 led to cobalt hydroxide as if the sheets were separated from one another, thereby resulting in larger surface areas, which will be confirmed by BET results (Fig. 2c). However, calcination of this compound, which indirectly ended up with cobalt oxide production, showed a surface area not as large as that of Co(OH)2, though its sheet-like structure has almost been kept (Fig. 1d). The electrochemically grown rGO on nickel foam looks like a shrinked leaf, confirming well reduction of GO (not shown here). EDX spectra of MOF-71 (Fig. 1e), direct Co3O4 (Fig. 1f), Co(OH)2 (Fig. 1g) and indirect Co3O4 (Fig. 1h) were acquired to undertake elemental analysis of the prepared materials. As expected, the numerous carbon and cobalt peaks observed in Fig. 1e may indicate the successful synthesis of MOF-71, which is additionally supported by XRD results. The disappearance of carbon peak in Figs. 1f-1h confirms the successful preparation of direct Co3O4, Co(OH)2 and indirect Co3O4, which would also be also validated by XRD studies. N2 adsorption and desorption behavior of MOF-71 (Fig. 2a), direct Co3O4 (Fig. 2b), Co(OH)2 (Fig. 2c) and indirect Co3O4 (Fig. 2d) were studied, from which the BET surface areas of 10.43, 5.34, 129.46 and 33.47 m2 g-1 were obtained, respectively. As seen, the surface area of indirect Co3O4 is enhanced by a factor of six, as compared to the direct one. From Figs. 3a-3d, the isotherms of all four materials are observed to be of type III, which generally corresponds to non-porous, or possibly macro-porous materials with low adsorption energy. The small surface area obtained for these materials confirms such characteristics. Taken by Li et al. [15], XRD pattern of MOF-71 is shown in Fig. 3a; it is in good agreement 8

with the XRD pattern of the MOF-71 sample prepared here (Fig. 3b). The XRD taken from Co3O4 directly synthesized by the SST of MOF-71 is shown in Fig. 3c; it conforms well with reference card 00-043-1003. Fig. 3d presents the XRD pattern of the Co(OH)2 which was prepared by putting the MOF-71 into NaOH. Comparing this pattern to the card 00-001-0357 confirms successful synthesis of cobalt hydroxide. By annealing the cobalt hydroxide prepared from MOF-71, Co3O4 was prepared, as confirmed by XRD patterns (Fig. 3e) according to the reference card 00-043-1003. FTIR spectra were taken to further investigate the formation of MOF-71 (Fig. 3f), direct Co3O4 (Fig. 3g), Co(OH)2 (Fig. 3h), indirect Co3O4 (Fig. 3i). Li et al. [15] demonstrated that the FTIR of MOF-71 presents a series of bands at 752, 1113, 1375, 1545 and 1656 cm-1, which are in good consistency with this work (Fig. 3f). The disappearances of the above-mentioned peaks in Figures 3g and 3i as well as the appearance of the peaks at 570 cm-1 and 660 cm-1 well indicate the evolution of MOF-71 to cobalt oxide. In Figures 3g and 3i, the peaks observed at 570 cm-1 and 660 cm-1 are attributed to Co-O stretching vibration mode and the bridging vibration of O-Co-O bond, respectively, which together confirm the formation of cobalt oxide [16]. The peak appeared around 3413 cm-1 indicates the presence of hydrogen-bonded hydroxyl groups. In Fig. 3h, non-hydrogen bonded hydroxyl groups are presented by peak at 3430 cm-1 that confirms the formation of Co(OH)2. The additional peak observed at 526 cm-1 is assigned to Co-OH bending vibrations of Co(OH)2. Narrow and survey XPS spectra of the MOF71, cobalt hydroxide and direct cobalt oxide were measured (Fig. 4a-4l) [17,18]. To compensate surface charging, C 1s was considered as reference to be appeared at binding energy 284.5. Shirley algorithm, implemented in user-friendly software by us, was applied for the background subtraction. The peak deconvolution was performed using Origin Pro version 9.0 (See Fig. 4). C 1s, O 1s and 9

Co 2p taken from MOF-71 (Figs. 4a-4c) confirm the successful formation of this metalorganic framework. The C 1s and O 1s peaks corresponding to O-C=O (288.5 eV) and C-OCo (530.4 eV), respectively, shown in Figs. 4e and 4f indicate the presence of a trace of MOF71 after converting this material to Co(OH)2. Multiplet splitting observed in Fig. 4k caused by the unpaired electrons in the valence band of direct Co3O4 confirms the successful preparation of this material. In addition, after the direct SST of MOF-71, the C 1s corresponding to O-C=O (288.5 eV) as an indication of the MOF-71 disappeared which confirm the high conversion of this material to Co3O4. In Fig. 4i, the C 1s appeared at 284.5 eV is attributed to surface adsorbed carbon, while in any of Figs. 4a and 4e, it is originated from both surface contamination and C-C in the structures of MOF-71 and Co(OH)2. Based on our knowledge, no information are available on the electronic properties and band gap of MOF-71. Therefore, as a trigger of providing such information, the reflection electron energy loss spectrum of MOF71 was taken at primary energy of 4000 eV (Fig. 5). Then, the energy band gap was determined to be 3.7 eV (see inset of Fig. 5). It is well known that the bang gap plays a key role in the electronic properties of materials. For instance, the breakdown electric field is related to the band gap as EBF  1.36( Eg 4)3 which is the case for semiconductors of band gap less than 4.0 eV [19]. The breakdown electric field of MOF-71 was determined to be 1.08 V/nm, which is stable enough where it is used in strong electric field. Dielectric function of such material as a key function in energy storage applications can also be determined from REELS using dielectric response theory (not provided here) [20, 21]. 3.2. Electrochemical measurements The electrochemical properties of the electrode materials were studied by the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS), and cycling life measurements in a three-electrode cell in 6 M KOH 10

electrolyte. The CV curves for Di-Co3O4 (Fig. 6a) and Co(OH)2 (Fig. 6b) were measured at various scan rates ranging from 5 to 100 mV s-1 within the potential window of -0.05 to 0.50 V (vs. SCE). These contain the redox peaks, suggesting that the main charge storage mechanism is pseudo-capacitive. In the case of Co3O4 and Co(OH)2, a pair of redox peaks were observed due to the following reversible reaction of Co3O4and Co(OH)2 with OHanions [22, 23]. Co3O4 + OH- ↔ 3CoOOH + e-

(1)

Co(OH)2 + OH- ↔ CoOOH + H2O + e-

(2)

CoOOH + OH- ↔ CoO2 + H2O + e-

(3)

The area under the CV curve is equal to the charge stored during the reaction. The specific capacitance can be calculated using Equation (4) [24]: Cs = ʃ I(V) dV/ m.υ.ΔV

(4)

Where ʃI(V)dV is the integration of the current during discharge, m is the loading mass of the active material, υ is the scan rate and ΔV is potential window. Using Equation (4), specific capacitance values of Di-Co3O4 at the different scan rates of 5, 10, 25, 50, and 100 mV s -1 were obtained as 280, 227, 168, 130, and 102 F g-1, respectively; corresponding values for Co(OH)2 were 225, 199, 176, 150, and 130 F g-1, respectively. In order to evaluate applicability of the samples as supercapacitor electrode materials, GCD measurements were conducted at different current densities. Real-time GCD curves for Di-Co3O4 and Co(OH)2 at various current densities(1, 2, 3, 5, and 10 A g-1) and the potential range of 0 to 0.52 V are shown in Fig. 6d and Fig. 6e, respectively. The gravimetric specific capacitances (Cm) were calculate from the GCD data [25]: Cs = I ∆t / m ∆V

(5)

11

where I is discharge current, Δt is discharge time, m is mass of the active material and ΔV is potential window. Specific capacitances of Di-Co3O4 at different current densities of 1, 2, 3, 5, and 10 A g-1 were 276, 214, 162, 128, and 92 F g-1, respectively, and those of Co(OH)2 were 220, 193, 175, 130, and 115 F g-1, respectively. According to the literature, metal oxides have a high capacitance value which makes them perfect material for electrode cells [26-30]. Interestingly, in the case of Di-Co3O4, at low current densities, low concentration of system polarization allows charging process to be completed in a long time. In this case, the sufficient number of ions available near the electrode and the redox reaction of cobalt oxide provide high capacity. Coverage of nanoparticles on the Ni foam leads to new paths and facilitates electron transfer, thus improving electrical conductivity and behavior of the active substance. At higher scan rates, Di-Co3O4 has lower specific capacitance (as depicted in Fig. 6g) while Co(OH)2 represents higher specific capacitance due to higher porosity, as found out from BET data (Fig. 2c) and SEM images of Co(OH)2 (Fig. 1c). The cobalt hydroxide nanoparticles with large specific surface area provided the electrolytes with more access to active material and better faradaic reactions in higher scan rates. This facilitated the migration of ions and decreased the distances of ionic diffusion; hence, large specific surface area of nanoporous Co(OH)2 leads to fast response time at higher current densities, thereby achieving higher capacities and better efficiency than those of Di-Co3O4 (Fig. 6h). According to the disharmonious results in capacity at different scan rates, porous Co(OH)2 was calcined at 350 oC under the air atmosphere, with the product named indirect Co3O4 (In-Co3O4), so as to study the effect of porosity and pore size distribution on specific capacitance and electrochemical performance. The BET results showed that the specific surface area of In-Co3O4 is more than that of Di-Co3O4 because of large specific surface area of MOF-71-derived cobalt hydroxide. The specific capacitance values of In-Co3O4 at the 12

different scan rates of 5, 10, 25, 50, and 100 mV s-1 were found as 320, 265, 252, 217, and 197 F g-1, respectively (Fig. 6c), and those at current densities of 1, 2, 3, 5, and 10 A g-1 were 302, 246, 228, 211, and 190 F g-1, respectively (Fig. 6f). According to the above description about surface area and porosity, an increase in BET results of the In-Co3O4 justifies the higher specific capacitance of these samples. In order to demonstrate the advantages of the In-Co3O4, Cs values are plotted versus current densities in Fig. 6i. In this figure, once the current density is increased by a factor of 10, the capacitance is still at 190 F g-1 (63% retention) with better properties than Di-Co3O4 and Co(OH)2 at either of low or high current densities. EIS measurements were also carried out to evaluate the charge transfer and electrolyte diffusion in the electrode/electrolyte interface (Nyquist plot), as shown in Fig. 7. The semicircle in the high-medium frequency region is related to the charge-transfer in faradaic reactions, while the straight line refers to the diffusion of ions in electrodes, and the Warburg slope in the low frequency region is related to the ion diffusion/transport in the electrolyte. The lower Warburg resistance will facilitate the diffusion of electrolyte ions (OH) into the electrode materials [31]. Accordingly, a facile and reversible faradaic redox would be taken, and a good electrochemical property could be realized for the In-Co3O4 electrode [32-36]. The impedance data was fitted with an equivalent electrical circuit presented in Fig. 7 and Table 1. The circuit model is composed of a solution resistance (Rs), charge-transfer resistance (Rct), double-layer capacitance (Cdl), and Warburg diffusion resistance (W). At very high frequencies, the real part (Z′) is the equivalent series resistance (Rs), which consists of intrinsic resistance of the electrode, the bulk electrolyte, and the resistance at the electrolyte/electrode interface [37]. According to the equivalent circuit, the Rs for In-Co3O4 was found to be 0.806 Ω, which is slightly lower than that of the Di-Co3O4 (0.826 Ω) and

13

Co(OH)2 (0.864 Ω). The Warburg section of In-Co3O4 is the shortest one which suggests the fastest ion diffusion thereby contributing to enlarged capacitance. One advantage of the asymmetric supercapacitor (ASC) is the use of the pseudocapacitive positive electrode to enhance the specific capacitance of the cell. On the other hand, the operating potential of the asymmetric cell can be extended due to the over potential of reversible hydrogen electro sorption in a nanoporous carbon-based negative electrode (and use the electric-double layer capacitance) [38-41]. In the asymmetric cell, the working potential depends on the capacitance of electrodes. The electrochemical potential windows of the electrodes were estimated by CV measurements in three electrode configurations (Fig. 8a). As can be seen from the SEM image on Fig. 1d, the positive electrode of the ASC is In-Co3O4 with a particular morphology that provides easy access to large number of ions to penetrate deep inside the porous material, giving proper utilization of the effective mass. On the other extreme, electrochemically reduced GO with higher electrical conductivity than bare GO was used as negative electrode. Thus, this specially designed ASC will link the advantages of these two materials to provide high specific energy and specific power. The rGO electrode operates within a potential range of -1.1 to 0 V, while the In-Co3O4 electrode displayed a symmetric CV shape between -0.05 and 0.5 V, indicating a stable potential range between 0 and 1.6 V for the ASC. Mass ratio of the positive and negative electrodes for an ACS full cell can be calculated based on the principle of charge balance as follows: m+/ m- = ( C- × ∆V- )/ ( C+ × ∆V+ )

(6)

Where m is the mass loading of the material on electrodes, ΔV is the working potential window and C is the specific capacitance of the electrode material. So, an optimal mass ratio (m+/m- ≈ 1.1) was determined for the electrodes from the values of specific capacitances and the potential windows of electrodes. The capacitive performance of ASC based on rGO and 14

In-Co3O4 was evaluated using CV and GCD methods (Figs. 8b and c). Fig. 8d presents specific capacitances of 38, 34, 31, 25 and 11 F g-1 at current densities of 1, 2, 3, 5, and 10 A g-1, respectively. The specific energy (E) and power (P) of the asymmetric device were evaluated using Equations (7) and (8): E =1/2 C × ∆V2

(7)

P=E/t

(8)

Ragone plot of the In-Co3O4//rGO derived from discharge curve based on Equations (7) and (8) are displayed and compared with previously reported devices (Fig. 9a) [42-48]. The maximum energy density of the supercapacitor is 13.51 W h Kg-1 which decreases to 3.91 W h kg-1 as the power density increases from 844 to 9775 W kg-1. Moreover, obtained results are comparable or better than those of the recently reported asymmetric supercapacitors. As an important factor for practical applications, stability of the supercapacitor was examined at a current density of 3 A g-1 for up to 2000 charge/discharge cycles (Fig. 9b). Accordingly, it was observed that the specific capacitance of this device slightly increased as the number of cycles increased to up to 500 cycles, beyond which it gradually decreased and reached 85% of the initial capacitance after 2000 cycles. The slight increase in specific capacitance in early cycles can be ascribed to the gradual access of electrolyte ions to deeper active sites into the electrode materials. To demonstrate practical application of In-Co3O4//rGO supercapacitor, two home-designed plexiglass cells assembled in series succeeded to lighten up 18 red light emitting diodes (LED) for 2 minutes after charging to 3.2 V (Fig. 9b). 4. Conclusions In summary, cobalt-based MOF (MOF-71) was prepared under standard solvothermal conditions. Two different types of cobalt-based active materials were obtained by thermal and alkaline aqueous treatments, named Di-Co3O4 and Co(OH)2, respectively. The Di-Co3O4 15

had larger capacitance, while higher scan rates Co(OH)2 represented higher specific capacitance due to its higher porosity. Therefore, porous hydroxide was thermolyzed (InCo3O4) to study and compare the effects of porosity and pore size distribution on specific capacitance and electrochemical performance. After optimization, a novel asymmetric supercapacitor device was developed where in the MOF-71-derived active materials served as cathode and electrochemically reduced graphene was used as anode in a 6 M KOH aqueous electrolyte. The asymmetric supercapacitor operated at 1.6 V and exhibited a specific capacitance of 38 F g-1 at a current density of 1 A g-1. It was seen to offer excellent maximum energy density and power density (13.51 Wh kg-1 and 9775 W kg-1, respectively), with a specific capacitance loss of less than 15% after 2000 charge-discharge cycles.

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Fig. 1. SEM images of (a) MOF-71, (b) direct Co3O4, (c) Co(OH)2, (d) indirect Co3O4, and EDX spectra of (e) MOF-71, (f) direct Co3O4, (g) Co(OH)2 and (h) indirect Co3O4. Fig. 2. N2 adsorption and desorption isotherms of (a) MOF-71, (b) direct Co3O4, (c) Co(OH)2 and (d) indirect Co3O4. Fig. 3. XRD patterns of (a) MOF-71 taken by Li et al, (b) MOF-71 synthesized here, (c) direct Co3O4, (d) Co(OH)2, (e) indirect Co3O4 and FTIR spectra of (f) MOF-71, (g) direct Co3O4, (h) Co(OH)2 and (i) indirect Co3O4. Fig. 4. XPS spectra of (a-d) MOF71, (e-h) Co(OH)2 and (i-l) direct Co3O4. Fig. 5. REELS spectrum of MOF-71 and its band gap (inset). Fig. 6. Cyclic voltammetry curves at different scan rates of (a) Di-Co3O4, (b) Co(OH)2 and (c) In-Co3O4 in a 6 M KOH. Galvanostatic charge-discharge curves of (d) Di-Co3O4, (e) Co(OH)2 and (f) In-Co3O4 at different current densities. Cycling performance of samples at various current densities of 1, 2, 3, 5 and 10 A g-1 during 50 cycles; (g) Di-Co3O4, (h) Co(OH)2, (i) In-Co3O4. Fig. 7. Nyquist plots of three samples in 6 M KOH. Fig. 8. Electrochemical measurements of the asymmetric supercapacitor. (a) Cyclic voltammetry (CV) curves of the Di-Co3O4 and rGO as working electrodes in three-electrode system (b) cyclic voltammetry (CV) curves of the asymmetric supercapacitor at various scan rates (c), galvanostatic charge-discharge (GCD) curves of the asymmetric supercapacitor at different current densities and (d) capacitance of the asymmetric supercapacitor at different current densities. Fig. 9. (a) Ragone plot of the asymmetric supercapacitor at various current densities. (b) Cycle performance of the rGO/Co3O4 asymmetric supercapacitor with a voltage of 1.6 V at current density 3 A g-1 for 2000 cycles in 6 M KOH aqueous solution and photographs of two supercapacitors in series which could lighten up 18 red LED during 2 min.

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Table 1. Electrical Parameters for three samples from EIS Measurements at OCP.

Circuit elements

Di-Co3O4

Co(OH)2

In-Co3O4

Rs

0.826

0.864

0.806

Rct

0.344

0.016

0.042

Cdl

0.0001

0.0002

0.002

Cps

0.002

0.148

0.239

W

0.418

0.186

0.812

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Scheme 1. Schematic illustration of the synthesis of MOF-71 and formation of active material

21

Fig. 1.

22

Fig. 2.

23

Fig. 3.

24

Fig. 4.

25

Fig. 5.

26

Fig. 6.

27

Fig. 7.

28

Fig. 8.

29

Fig. 9.

30

Highlights



Two different types of Co3O4 were synthesized by direct and indirect solid stete thermolysis of MOF-71.



Among studied active materials, indirect MOF-71 derived Co3O4 represented higher specific capacitance of 320 F g-1 at a current density of 1 A g-1.



An asymmetric supercapacitor was fabricated using indirect Co3O4 nanoparticles as cathode and electrochemically reduced graphene oxide as anode



A high energy density of 13.51 Wh kg-1 along with a power density of 9775 Wkg-1 and good cycling stability with capacitance retention rate of 85% after 2000 cycles were obtained

31