A novel electrochemical procedure for grown of bimetallic metal–organic framework (Ni/Zn-MOF) and its derived hydroxide@C onto Ni foam as binder-free high performance battery-type electrodes for supercapacitors

Todays, metal-organic frameworks (MOFs) and their derived structures have been extensively investigated as the novel electrode materials in energy storage area due to their stable porous architectures and exceptionally large specic surface area. In this study, bimetallic Ni,Zn-MOF is synthesized onto Ni foam via a novel indirect cathodic electrodeposition method for the rst time. After that, the fabricated Ni,Zn-MOFs onto Ni foam was converted to corresponding bi-metal hydroxide@C/Ni foam through direct chemical treating with 6M KOH solution. The obtained Ni,Zn-MOFs/NF and Ni 2 − x Zn x (OH) 2 @C/NF electrodes are characterized through XRD, FT-IR, FE-SEM and EDS analyses. These analyses results conrmed deposition of well-dened crystalline porous sheet-like structures of Ni 3 − x Zn x (BTC) 2 deposited onto Ni foam, where the hydroxide@C electrode was also exhibited similar morphology. As the binder-free electrode, the as-prepared Ni,Zn-MOF@Ni foam exhibited the superior storage capacities of 356.1 mAh g − 1 and 255.5 mAh g − 1 as well as good cycling stabilities of 94.2 % and 84.5 % after 6000 consecutive charge/discharge cycles at the current densities of 5 and 15 A g − 1 , respectively. On the other hand, Ni,Zn-MOF derived hydroide@C/Ni foam presented the superior capacities of 545 mAh g − 1 and 406 mAh g − 1 as well as proper cycle lifes of 91.8 % and 78.3 % after 6000 cycling at the applied loads of 5 and 15 A g − 1 , respectively. Based on these ndings, both of these fabricated battery-type electrodes are introduced as the promising candidates for use in energy storage devices.


Introduction
In the recent years due to the growing demand for energy and problems related with fossil fuels such as the constant change in world prices, concerns about the scarcity of fossil fuels and global warming due to greenhouse gas emissions and with emerging the high power application that required high power and energy e ciency, reliability and long discharging cycle life, the development of renewable and environmentally friendly energy and, of course energy storage technologies for the practical applications, has received much attention since the early 1990s [1,2]. Improving the e ciency of energy storage systems, as an important part of energy consumption management, has always been the focus of research by scientists around the world. Among many energy storage systems, supercapacitors have received a lot of attention due to their high power density, long service life, fast charge/discharge, good cyclic stability, excellent intrinsic safety and wide operating temperature [3]. Possibility for design and development of new electrochemical capacitors with high e ciency are needed high performance electrode materials. Currently, there are many different types of capacitors that categorized by the dielectric properties and/or its physical state. Each type has its own unique characteristics and applications ranging from small capacitors for electronics devices to large power capacitors used in highvoltage power device [4]. Electrochemical capacitors (ECs) that categorized in the two main groups of electrochemical double layer capacitor (EDLC) and pseudocapacitor [4] are a special kind of capacitors based on charging and discharging at the electrode-electrolyte interface of high surface area materials, such as porous carbon derived materials [5,6], graphene-based structures, conducting polymers and some transition metal oxides and hydroxides [7][8][9].
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 [10][11][12]. MOFs have also employed for chemical separations, drug delivery, electrocatalysis, uorescence and electrochemical studies as well as energy storage purposes (supercapacitors, lithium-ion batteries), photocatalyst, gas sensing, water treatment, solar cells, and carbon dioxide capture [11]. Because of the unique properties such as tunable pore sizes, high porosity, extremely high surface area and ordered crystalline structures, MOFs have attracted great attention in the last two decades [2,11]. However, the direct application of MOFs in electrode materials is limited by the poor electroconductibility, large steric hindrance and the chemical/mechanical stability [13][14][15]. Hence, it is essential to explore new strategy to enhance the electrochemical performance of MOFs. MOFs materials with single metallic element always exhibit poor performance for the inactivated sites and undesirable structure [16]. Yang et al. synthesized a layered structure Ni-based MOF with capacitances of 1127 and 668 F g − 1 at rates of 0.5 and 10 A g − 1 , respectively [17]. Bimetallic MOFs with two different kinds of central metal ions could provide improved electrochemical performance for SC [16,18]. The synergistic effect of the mixed metal ions can effectively enhance the conductivity and speci c area of MOFs, which makes the MOF-electrode exhibit superior speci c capacitance and preferable cycling life [19]. Yang et al. [20] reported synthesis of a Zndoped Ni-MOF with capacitances of 1620 and 860 F g − 1 were achieved at 0.25 and 10 A g − 1 , respectively, and the retention was kept at 91% even after 3000 cycles for MOF sample with a Zn/Ni of 0.26. Chen et al. [21] have prepared bimetallic Ni/Co-MOFs with nano-sheet assembled ower-like morphology with high speci c capacitance of 1220.2 F g − 1 at 1 A g − 1 and high capacitance retention of 87.8% after 3000 cycles. Xia et al. have developed a kind of Ni/Co-MOF with ake-assembled spherical microstructures, which had a capacitance of 530.4 F g − 1 at 0.5 A g − 1 and almost no capacity attenuation after 2000 charge/discharge cycles [22]. Bimetallic MOFs with homogeneous distribution of two kinds of metal nodes could be utilized as ideal templates to construct bimetallic oxides with controllable composition. Micro-and/or nanopores in MOFs can be retained after post treatment, which bene ts improving ion mobility and high speci c capacitance of supercapacitors. MOF-derived oxides structures are the promising candidate for supercapacitor electrode material due to its layered structure, low cost and environmental friendly physicochemical feature [23].
In most of these studies, solvothermal and hydrothermal methods have been used for synthesis of MOFs materials. However, these methods often contain multi-step and complex processing procedures or highcost techniques, leading to low reproducibility and they often are time and resource-consuming [24][25].
Compared with the conventional methods, electrochemical method is considered as one of the most promising method to prepare MOF thin lms and coatings because this method does not require complex preparation steps and has a short growth time and the possibility of the controlling the phase, morphology and size of the MOF products by altering the synthesis parameters. Also electrochemical synthesis allows the large-scale production of MOFs in powder form [26].
In this paper, we report simple deposition of a well-structured bimetallic Ni,Zn-MOF though indirect cathodic electrochemical procedure onto Ni-foam support and its chemical treating in basic solution to convert bimetal hydoxide@C active material. The fabricated electrodes were directly used as binder-free capacitor electrode, which have advantageous such as additive-free, possibility of bulk production and porous texture.

Sample preparation
For fabrication of the Ni,Zn-MOF onto Ni-foam cathode, an indirect cathodic electrodeposition procedure was employed. In this deposition protocol, a typical two-electrode electrochemical cell composed of Ni foam (SA = 2 cm 2 ) as the cathode side and two-parallel graphite plates as the anode side (SA = 6 cm 2 ) were used. The electrolyte solution was prepared with dissolving the 0.25 g BTC, 0.15 g Ni(NO 3 ) 2 , 0.1 g Zn(NO 3 ) 2 and 0.2g NaNO 3 in 100 mL of double distilled water. The electrodeposition process was carried out at the current load of 25 mA/cm 2 for 10 min. The deposition temperature and pH of the electrolyte solution were 25 o C and 5.5, respectively. After depositions, the Ni-foam cathode was washed several times with the double distilled H 2 O and heat-treated at 80 o C for 6h. The fabricated Ni,Zn-MOF/Ni foam was then applied as ready-to-use binder-free electrode in the charge storage tests. To prepare MOFderived hydroxide electrode, the Ni,Zn-MOF onto Ni foam was directly putted in 6M KOH solution for 12h, where the solution was agitated during the reaction time. After that, the Ni-foam electrode was immerged into deionized water for 24 h and then heat-treated at 80 o C for 4h. The powder form of Ni,Zn-MOF was also synthesized by the similar cathodic procedure using Ni plate as the cathode side. In which, the lm formed onto Ni foil was removed and heat-treated at the 80 o C for 6 h. The synthesized sample was named as Ni,Zn-MOF powder.

Sample characterizations
FE-SEM (model Mira 3-XMU) was applied to determine the morphology characteristics and elemental mapping of the fabricated Ni,Zn-MOF/NF and its derived hydroxide/NF electrodes. X-ray diffraction (XRD) analyses was performed through X'Pert PRO MRD instrument. FT-IR spectra of pure BTC and the synthesized MOF powder were taken via a Bruker Vector 22 FT-IR spectrometer (in the range of 400-4000 cm − 1 ).

Electrochemical tests
Electrochemical tests were carried out using potentiostat/galvanostat Eco Chemie instrument with speci cations AUTOLAB®, Model: PGSTAT 30. In the electrochemical set-up, the fabricated Ni,Zn-MOF/NF or MOF-derived hydroxide/Ni foam was connected to working line of PGSTAT 30. The Pt rod and Ag/AgCl were also connected to the counter and reference positions, respectively. The electrolyte of the electrochemical cell was selected to be 2M potassium hydroxide. CV graphs were recorded within the potential domain of -0.1 and 0.6 V vs. Ag/AgCl (for the fabricated Ni,Zn-MOF/Ni foam electrode) and − 0.3 and 0.7 V vs. Ag/AgCl (for MOF-derived hydroxide/Ni foam). Notably, mass loads of Ni,Zn-MOF and MOF-derived hydroxide formed onto Ni-foam cathodes were respectively found to be 5.6 mg and 4.1 mg. The capacity values delivered by both fabricated working electrodes were calculated from the CV diagrams using the following expression [24]: In this relation, C (mAh/g) is the speci c capacity, Q is total charge (Coulombs), ΔV is the applied potential window in volt unit, m is mass of the Ni,Zn-MOF and MOF-derived hydroxide (g), and I(V) is the measured currents (A). The GCD digrams were also recorded at the current loads between 1 to 30 A g ─1 in the potential window of -0.1 to 0.45 V and the speci c capacities of each electrode were obtained via relation (2) [25]: where C is the stored charge by WE electrode in mAh/g unit, I is the current load in A unit, ΔV is the potential window (0.55 V), Δt is the time of a discharge branch in second unit and m is mass of the electrodeposited active material (g). Electrochemical impedance spectroscopy (EIS) pro les were obtained at the frequencies ranging from 100 to 0.001 kHz at OCP potential of 5 mV. Fig. 1 shows XRD patterns of the Ni,Zn-BTC/Ni foam electrode and the Ni,Zn-MOF powder. Nickel foam (NF) with 3D macro-porous structure, high mechanical strength, exibility, high electrical conductivity and relatively high speci c surface areas is proper support as current collector for the capacitor electrode. Its macro-porous channels with several hundred micrometers pore size could provide enhanced mass transport for the involved electrochemical processes [26][27]. As seen in Fig 200) and (220) planes of Ni foam. With respect to high intensity of diffraction peaks of nickel-foam substrate, the diffraction related to MOF deposited onto Nifoam have disappeared as the background peaks. However, Ni,Zn-MOF deposited onto Ni-foam presents four main peaks located at 8.53°, 11.41° and 13.56° (Fig. 1a) which are analogous to those previously reported for Ni,Zn-MOF [28][29]. Fig. 1b shows the powder XRD diffraction pattern of the prepared Ni,Zn-MOF powder. From the XRD pattern, it is obvious that the Ni,Zn-MOF powder exhibited crystalline structure with peaks position similar to those observed for the Ni,Zn-MOF/NF electrode. This indicates that the prepared Ni,Zn-MOF in both forms of powder and thin lm shows the similar topological structure, which has formed by connection of the metal ions with BTC ligands [30][31].
Fourier transformation infrared (FT-IR) spectroscopy was also employed to characterize the prepared Ni,Zn-MOF sample. FTIR spectra of the pure benzene,3-tricarboxylic acid (H 3 BTC) and Ni,Zn-MOF powders are shown in Fig. 2. For both samples, the intensive band at 3417 cm -1 is due to the stretching vibrations of water molecules (OH vibrations) [32]. The IR spectrum of H 3 BTC (Fig. 2a) showed peaks at 1722 cm −1 and 1277 cm −1 corresponding to stretching vibrations of C=O and C-O, respectively, indicating the presence of carboxylic acid groups in BTC structure [32]. The symmetric and antisymmetric stretching vibrations of O-C-O groups are in the range of 1350-1620 cm −1 [33]. More precisely, the bands at 1620, 1577, 1448 and 1382 cm -1 are respectively related to the asymmetric and symmetric stretching vibrations of the carboxylate groups in BTC [32]. Compared with IR spectrum of pure BTC in Fig. 2a, it was found that although there is no signi cant changes in the peak positions for the Ni,Zn-MOF powder sample (Fig. 1b), several new IR bands are appeared at the low wavenumbers of 1000 cm -1 . This is due to the adsorption of metal ions to the structure of BTC through its carboxylic functional groups. As a result, the peaks related to Ni-O are observed at 830 and 689 cm -1 and Zn-O bond vibration is found at 520 cm -1 [34]. Therefore, it was concluded that Ni and Zn cations have been successfully introduced into the framework of BTC linkers to form Ni,Zn-MOF structure. The Ni/Zn-MOF deposit exhibits a ower-like morphology with three-dimensional spatial structure like as petals (Fig. 3b). The observed petals are consisting of multilayered nanosheets and irregular distribution with thickness of ~500 nm, where their lateral surfaces become relatively smooth (Fig. 3d). This morphology can be strongly in uenced by the method of electrochemical synthesis and the simultaneous presence of nickel and zinc metallic ions, because the structure observed in this paper is signi cantly different from the structures reported in other articles [31,35]. It is well known that the Ni,Zn-BTC coordination complex can form 3D polymeric chains via the strong π-π interaction or hydrogen bonding between the ligands could hold the chains together to form 3D network [36]. EDX results showed that the ratio of Ni/Zn in the fabricated MOF sample is 3:2 (Fig. 4e).
The Ni,Zn-MOF derived hydroxide onto Ni-foam electrode was also analyzed by FE-SEM technique to observe its morphology. Fig. 5 presents FE-SEM images of the MOF-derived hydroxide onto Ni-foam support. In FE-SEM image of Fig. 5a, the Ni-foam framework and open pore is easily seen and it is also observed that the derived hydroxide is present at all surfaces of Ni foam (Fig. 5b). Similar to pristine MOF, the derived hydroxide has three-dimensional ower-like morphology (Fig. 5b). The observed 3D ower has composed of the plates with several micrometers in sizes and relative porous texture (Figs. 5c,d). The carbon presence is also give that the hydroxide formed onto Ni foam has been carbon particles on its backbone. Hence, the chemical formula of the Ni 2-x Zn x (OH) 2 @C could be ascribed to the MOF-derived hydroxide onto Ni-foam. EDAX data in Fig. 6f indicates that the fabricated material onto Ni-foam has the atomic percentages of 45.9 % carbon, 37.38% oxygen, 10.13 % nickel and 6.59 % zinc atoms (Fig. 6f).
These atomic percentages indicated that the ratio of Ni/Zn in the fabricated MOF sample is about 3:2 ( Fig. 6f) and hence the chemical formula of Ni 1.5 Zn 0.5 (OH) 2 @C is predicted for the MOF-derived hydroxide onto Ni-foam.
After characterization of the electrodeposited lms of Ni,Zn-MOF and its derived hydroxide onto the conductive nickel-foam support, their electrochemical performances as binder-free capacitive electrode were evaluated through CV, GCD and EIS tests. Fig. 7a shows CV plots of the binder-free pristine Ni,Zn-MOF/Ni foam and its derived Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam electrodes at the scan rate of 5 mV/s. Notably, the applicable potential window for the fabricated Ni,Zn-MOF/Ni foam and Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam electrodes were 0-0.55V and -0.2-0.6 V vs. Ag/AgCl, respectively. In CV curve of an ideally reversible system, the oxidation and reduction peaks have same potential difference at various scan rates based on the number of electrons involved in the electrochemical reaction. Thus, in quasi-reversible systems the oxidation peak moves to more positive potentials and reduction peak moves to more negative potentials due to the presence of kinetic barriers in charge transfer processes at the electrode/electrolyte interface when the scan rate is increase. For both fabricated electrodes (Fig. 7a), the observed pair of redox peaks within the CV curves was ascribed to Ni(II)↔Ni(III) and Zn(II) ↔Zn(III) (i.e. M-O/M-O-OH, M=Zn or Ni) during anodic scan which were reversed during cathodic potential sweep [37][38][39]. Additionally, it is seen that by increasing the scan rate, the speci c capacitances are decreased (Figs. 7b,c) due to the inaccessibility of some active sites within the fabricated active material onto Ni-foam for electrolyte ion diffusion. electrodes, which manifesting that the currents are linearly related with the square root of scan rate for both fabricated electrodes. The peak current of a battery-type electrode material is linearly related to square root of the scanning rate (i p ∼ν 1/2 ), whereas a capacitor-type material exhibited linear current response relation on the scan rate (i∼ν). So, it can be said that both fabricated electrodes i.e. Ni,Zn-MOF/Ni foam Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam store charges through typical battery-type process [40,41]. The capacity values of the fabricated electrodes were calculated through Eq. (1) and the results are given in Fig. 7d mV/s, respectively. These data indicated that the prepared bimetallic Ni,Zn-MOF/NF electrode retained more than 52 % of its primary capacity by increasing the scan rate from 5 to 300 mV/s, which signi ed a proper high-rate capability of the fabricated MOF electrode. Furthermore, Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam electrode served its 54.59% of its initial storage at the high applied scan rate of 300 mV/s, verifying its proper high-rate capacitive performance. foam. In all GCD curves, one couple of charge/discharge plateau is observed, which is originated from Faradic reactions of conversion of Ni (II) to Ni (III) and Zn (II) to Zn (III), which implicated a battery-like characteristics of prepared electrodes (Figs. 9b,c). From the GCD curves, columbic e ciency of both electrodes were calculated and plotted in Fig. 9d. The calculations showed that the columbic e ciencies change from 99.7% to 79.1 for Ni,Zn-MOF/NF electrode and from 98.9% to 76.5% for Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam electrode by increasing the applied current load from 1 A/g to 30 A/g (Fig.   9d). These results veri ed the proper battery-type storage performances of both prepared electrodes.
GCD curves were also utilized to estimate the speci c capacities of the bimetallic MOF and hydroxide material at the different current densities, as presented in Fig. 9e respectively. These data veri ed that MOF-derived hydroxide active material onto Ni foam has larger charge storage than those of pristine MOF onto Ni foam, which maybe related to the high density of active sites in the hydroxide electrode as compared with those of pristine MOF electrode, where most weight of the pristine electrode is composed by organic linker. Furthermore, these data indicate that the capacity values were gradually falls by increasing of the applied current density. The major reason for this reduction trend is that the electrolyte ions cannot fully access to the active sites of the electrode bulk material under high current densities and only outer parts/surfaces of the active electrode material are accessible for electrolyte ions diffusion at high current densities. Hence, the electrochemical contribution of both Ni 2+ and Zn 2+ centers into the faradic reactions for both electrodes is limited at the high-rate discharging conditions, resulting the reduced speci c capacities delivered by the Ni,Zn-MOF/NF electrode [42][43][44]. However, the high-rate performances of 50.46% and 49.18% were respectively observed for Ni,Zn-MOF/NF and Ni,Zn-hydroxide@C/NF electrodes with increasing the applied current load from 1 to 30 A/g. For comparison of the capacitive performance of our prepared MOF and derived hydroxide active material with those reported in literature, some data are listed in Table 1. Comparison of the capacitive performance of our fabricated bimetal Ni,Zn-MOF/NF with those reported for single metal MOF electrode (i.e. Ni-MOF with Cs=334 mAh/g at 1 A/g, rate capability=49% and capacity retention=42.8 % after 2500 cycles at 10 A g -1 [37]) proved the better charge storage ability of the mixed Ni 2+ /Zn 2+ -based MOF electrode as a result of the synergetic effects within the fabricated MOF structure [38][39][40][41]. In fact, the incorporation or doping of zinc within Ni-MOF affects both the ionic and electronic conductivity, which results in better electrode utilization and the improvement of reaction kinetics, which is vital factor for any supercapacitor electrode material [42][43][44][45]. Furthermore, large operational potential window is another key factor for a supercapacitor, which lead to obtain high energy density values [44][45][46]. In fact, the wider potential window will cause an incredible enhancing the delivered energy densities.  [46]) reported up now, the potential window of our fabricated Ni,Zn-MOF electrode (ΔV=0.6 V) was expanded, further indicating its better electrochemical performance as supercapacitor electrode. As seen in Table 1, the Ni,Zn-MOF electrode fabricated by our simple electrochemical method exhibits better speci c capacity, high-rate capability and capacity retention as compared with those reported for Ni-based bimetal MOF electrodes in literature [37][38][39][40][41][42][43][44][45][46], which can be related to its layered three-dimensional spatial petal-like structure (Figs. 3,5), proper distribution of metal cation centers (Figs. 4,6) and suitable contact to Ni-foam support (Fig. 12).
The cycling performance is also a critical parameter to evaluate the charge storage performance of a supercapacitor. The cyclability of the fabricated pristine MOF electrode was also tested at the current loads of 5 and 15 A g -1 and the obtained results are presented in Fig. 10. The rst ten cycles within the 6000 GCD cycling of the fabricated MOF/Ni foam electrode at the current load of 5 and 15 A/g are given in Figs. 10a,c. It is seen that the fabricated electrode exhibited a well-de ned battery-type performance during the 6000 cycling tests. Through the discharge times obtained from Figs. 10a,c and using Eq. (2), the speci c capacities of the Ni,Zn MOF/NF electrode were calculated at the cycle numbers of 1, 100, 200, 300, …. and 6000. The obtained capacity values were plotted against the cycle number, which are represented in Figs. 10b,d. The capacity retention or stability values at the applied charge/discharge currents were also calculated using the obtained capacity data at the above mentioned cycles (i.e. capacity retention (%)=[C n /C 1 ]×100, where C n is the capacity of electrode in n th cycle). Figs. 10b,d show the capacity retentions (%) versus the cycle number at both current load of 5 and 15 A/g. Based on the calculated capacity data, it was found that the speci c capacity of the fabricated electrode is reduced from 356.5 mAh/g (for 1 th cycle) to 334.9 mAh/g (for 6000 th cycle) when current load was 5 A/g, as seen in Fig. 10b. Furthermore, it was calculated that the Ni,Zn MOF/NF electrode is exhibited 255.6 mAh/g at the st cycle of GCD test with current load of 15 A/g, and 215.9 mAh/g at the end of 6000 cycling, as given in Fig. 10d. Also, it was revealed that our prepared electrode is capable to serve 94.1% and 84.5% of its initial capacity values after 6000 cycling at the current loads of 5 and 15 A/g, respectively (Figs. 10b,d). The cycling data of the fabricated MOF-derived hydroxide/Ni foam electrode is also presented in Fig. 11. Based on the GCD curves of this electrode at the current loads of 5 and 15 A/g (Fig. 11a,c), the two important parameters of capacitance values in each cycle and also capacity retention during the 6000 cycling process were calculated and the obtained data were plotted in Fig. 11b,d. It was observed that the Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam electrode exhibits capacity values of 501.2 mAh/g and 318.1 mAh/g after 6000 cycles at the currents of loads of 5 and 15 A/g (Figs. 11b,d). Also, the capacity retentions or cycle lives of 91.9% and 78.3 % were respectively calculated for Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam at these applied charge-discharging loads (Figs. 11b,d). Comparing these capacity retentions with those reported for other MOF-based electrodes listed in Table 1 indicated that our fabricated electrode has a superior cycling ability. This good cycle life at the high current densities could be related to its appropriate electrochemical stability and also its good physical contact to the current collector support (nickel foam), which maybe originate from the direct electrochemical growth of active electrode material (i.e. Ni,Zn-MOF and Ni 1.5 Zn 0.5 (OH) 2 @C) as well as its unique morphology, self-assembling, and proper distribution onto the surface of Ni-foam (Figs. 3-6).
To further veri cation of the observed high-capacitive performance of the fabricated electrodes, electrochemical impedance spectroscopy (EIS) at the frequency of 0.01-10 5 Hz was recorded and its result is shown in Fig. 12. As presented in inset of Fig. 12, the EIS curve could be well-tted with an equivalent circuit consisting of solution resistance (R s ), faradaic capacitance (C F ), Warburg resistance (W) and charge transfer resistance (R ct ). In the low-frequency range, both electrodes exhibit nearly straight line, ascribing to the diffusive resistance for the electrodes. Furthermore, diameter of the semicircle in the medium frequency could be used to estimate charge transfer resistance of the fabricated electrodes. In the high frequency region, intercept of the real impedance is equal to the equivalent series resistance (ESR), which are sum of the internal resistance of the electrode, the solution resistance as well as resistances of all contacts through the impedance measurement. This parameter determines the rate of charging/discharging of the electrode. R s and R ct quantities were measured to be 1.4 Ω and 0.72 Ω, respectively. The low ESR and R ct quantities further manifested the high performance of the fabricated electrodes for supercapacitor applications.

Conclusion
In summary, a bimetallic MOF/Ni foam electrode consisting of nickel and zinc elements was successfully synthesized by a facile electrochemical deposition route. Then, it was converted to Ni 1.5 Zn 0.5 (OH) 2 @C/Ni foam through chemical treating in 6M KOH solution. The fabricated materials were applied as the binderfree supercapacitor electrodes. The as-prepared Ni,Zn-MOF and its derived hydroxide material had selfassembly petal-like structures, which respectively showed a battery-type charge storage behavior with high speci c capacity of 442 mAh g -1 and 637 mAh g -1 at 1 A g -1 and excellent capacity retentions of 94.2 % and 91.9 % after 6000 cycles at 5 A g -1 . The facile electrochemical route followed by chemical treating used here could be readily used for fabrication of other mixed metal MOFs and derived hydrixude@C electrodes for supercapacitor applications. More generally, by selecting of appropriate metal cations, it would be possible to synthesize various bi-metal MOFs onto any conductive support as binder-free ready-to-use electrode for energy storage aims.