Journal Pre-proof

Self-assembled bimetallic cobalt–manganese metal–organic framework as a

highly efficient, robust electrode for asymmetric supercapacitors

Youngho Seo, Pragati A. Shinde, Sehong Park, Seong Chan Jun

PII: S0013-4686(19)32199-1
DOI: https://doi.org/10.1016/j.electacta.2019.135327
Reference: EA 135327

To appear in: Electrochimica Acta

Received Date: 05 August 2019

Accepted Date: 17 November 2019

Please cite this article as: Youngho Seo, Pragati A. Shinde, Sehong Park, Seong Chan Jun, Self-
assembled bimetallic cobalt–manganese metal–organic framework as a highly efficient, robust
electrode for asymmetric supercapacitors, Electrochimica Acta (2019), https://doi.org/10.1016/j.
electacta.2019.135327

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© 2019 Published by Elsevier.

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Self-assembled bimetallic cobalt–manganese metal–organic framework as a

highly efficient, robust electrode for asymmetric supercapacitors

Youngho Seo‡, Pragati A. Shinde‡, Sehong Park, Seong Chan Jun*

Nano-Electro Mechanical Device Laboratory, School of Mechanical Engineering, Yonsei

University, Seoul 120-749, South Korea

Corresponding Author

*Prof. Seong Chan Jun

‡ Authors contribute equally for this publication.

E-mail: scj@yonsei.ac.kr

Fax: +82-2-312-2159,

Tel: +82-2-2123-5817.

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ABSTRACT

To obtain highly efficient supercapacitors with extraordinary capacitance, energy density,

and cycle life, the design and development of new electrode materials having numerous

active sites on the surface are essential. Not only the kind of material but also the surface

morphology and microporous structure of the electrode plays an important role in the

electrochemical performance of the supercapacitor. In the present work, a bimetallic Co/Mn-

metal–organic framework (MOF) has been prepared using a simple hydrothermal method for

supercapacitor application. This bimetallic MOF mainly produces a synergistic effect by

connecting two kinds of metal ions through an organic ligand. The resulting bimetallic

Co/Mn MOF electrode shows a high specific capacitance of 1176.59 F g-1 (2.76 F cm–2) at a

current density of 3 mA cm–2 and excellent cycling stability over 5000 cycles. The

constructed Co/Mn MOF//AC hybrid supercapacitor exhibits a specific energy of 57.2 Wh

kg-1 at a specific power of 2000 W kg-1 and excellent capacity retention of 93.51% over 5000

cycles. These outstanding results are compared with those of similar devices in the literature,

thus revealing that our approach is a desirable direction for future research on high-

performance energy storage devices as well as hybrid supercapacitors.

KEYWORDS: Metal organic framework, Hybrid supercapacitor, Synergistic effect,

Organic linker, N-doping.

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1. INTRODUCTION

The rising global environmental problems have attracted significant research attention to

the development of sustainable and renewable energy storage technologies. As a more

efficient energy storage system, supercapacitors are of considerable interest because of their

outstanding features such as high energy density, excellent charge–discharge rates, high

power density, good long-term cycle life, and eco-friendliness. Nevertheless, the capacitance

value and energy storage capability of present supercapacitors can scarcely satisfy the rising

energy requirements for modern electronics [1, 2]. As a result, improving the energy density

of present SCs without sacrificing their other features such as, power density and capacitance

are highly desirable.

According to the equation E = ½ CV2, the energy density (E) is directly related to the

capacitance (C) and cell voltage (V) [3, 4]. Meanwhile, asymmetric or hybrid supercapacitors

have been proposed as highly efficient energy storage devices with a high energy storage

capability because a wide cell voltage is achieved by the exactly opposite potential windows

of the positive and negative electrodes. In contrast to carbon-based materials, which have a

low capacitance and energy density, pseudocapacitive materials (PCMs) such as transition

metal oxides/hydroxides and their composites have been widely used as positive electrodes

for supercapacitors [5, 6]. However, PCMs usually experience low electrical conductivity and

poor cycling stability, which hamper their large-scale commercial application. Presently,

binary/ternary metal oxides are drawing increasing interest because of their multiple valence

states with highly active redox sites, which offer much higher electrical conductivity and

specific capacitance than individual metal oxides [7]. In this regard, various binary/ternary

metal oxides such as NiO@NiCo2O4 [8], MnCo2O4@NiO [9], MnMoO4@NiCo2O4 [10] have

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been prepared for high-performance SCs. Although great improvements have been attained,

additional progress to enhance the electrochemical performance of these binary/ternary metal

oxides, specifically their specific capacitance, energy density, and long-term cycling stability,

yet leftovers a tentative slab, perhaps due to the semiconducting nature of most of the metal

oxides, which cannot provide high conductivity for effective charge transfer.

Recent successful research studies have established a new class of electrode materials

known as metal–organic frameworks (MOFs), which serve as more efficient conductive

scaffold materials that not only improve the conductivity of electrode but can also be utilized

as highly capable active electrode materials for various applications. MOFs have garnered

significant interest among the scientific community because of their high surface areas,

tunable microstructure, excellent porosity, and high number of active metal centers [11].

Usually, MOFs can be formed by the chemical coordination between a metal ion and an

organic linker, and they have been employed as potential materials to accumulate electrolyte

ions via static adsorption on their surface as well as to promote faradaic reactions of the

attached metal species [12]. Nevertheless, the poor electrical conductivity and unsystematic

arrangement of MOFs are unsuitable for large-scale application as they cause low

electrochemical performance and chemical stability owing to the structural instability during

regular cycling [13]. As a result, more research has been devoted to minimizing the barriers

related to poor conductivity and structural arrangement of MOFs [14]. Although enormous

progress has been attained in the fabrication of MOF-based materials for energy storage

applications, the direct utilization of MOF-based materials for efficient supercapacitor

remains challenge.

Herein, we report the synthesis of self-assembled bimetallic Co/Mn-MOF nanoparticles on

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Ni foam for high-performance supercapacitor. Aminoterephthalic acid is used as the organic

linker, which itself serves as a rich nitrogen source. To the best of our knowledge, this is the

first report directly employing a bimetallic MOF with aminoterephthalic acid and Co and Mn

ions as an active electrode material for supercapacitor. The bimetallic Co/Mn-MOF

nanoparticles aligned homogeneously on the surface of Ni foam, which could present a high

specific surface area and sufficient porosity. Moreover, hybrid supercapacitor was prepared

using Co/Mn-MOF nanoparticles and activated carbon as the positive and negative

electrodes, respectively, and it delivered a high energy density and good chemical stability in

a KOH electrolyte.

2. EXPERIMENTAL SECTION

2.1 Synthesis of bimetallic Co/Mn-MOF

All chemicals were used as received from Sigma-Aldrich without further purification. The

bimetallic Co/Mn-MOFs were directly prepared on a Ni foam substrate using a single-step

hydrothermal method. Before synthesis, the Ni foam substrate was separately cleaned with 3

M HCl, ethanol and deionized (DI) water for 30 min each and then dried in an oven at 60 °C

for 10 h. To prepare bimetallic Co/Mn-MOF, 0.05 M aminoterephthalic acid was first

dissolved in dimethylformamide (DMF)/ethanol solution (7:1) by continuous magnetic

stirring for 20 min. Subsequently, 0.05 M Co(NO3)2, 6H2O and 0.05 M Mn(NO3)2, 4H2O

were added to this solution. Then, the prepared solution was transferred to a Teflon-lined

stainless-steel autoclave with the well-cleaned Ni foam and placed in an oven at 140 °C for

12 h. After the autoclave naturally cooled to room temperature, the as-prepared bimetallic

Co/Mn-MOF coated Ni foam was removed, rinsed several times with DI water, and finally

dried at 60 °C overnight. For comparison, Co-MOF and Mn-MOF were prepared separately

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following the same reaction procedure. The active materials on the Ni foam substrate possess

a mass of 2.27, 2.31 and 2.35 mg cm–2, for Co-MOF, Mn-MOF and Co/Mn-MOF,

respectively.

2.2 Material characterization

The surface morphology of the prepared samples was analyzed using a field emission

scanning electron microscope (FESEM, JEOL-7610F-Plus, JEOL Ltd.) equipped with an

energy dispersive X-ray spectrometer (EDS) and a transmission electron microscope (TEM,

JEOL JEM-2010). The specific surface area and pore size of the samples were measured via

N2 adsorption/desorption measurements (Autosorb-iQ 2ST/MP, Quantachrome). The

crystallographic study was performed using X-ray diffraction (XRD, Rigaku Ultima

diffractometer using Cu-Kα radiation). Lattice vibration and structure of MOF were analyzed

via Raman spectroscopy (LabRam Aramis, Horriba Jovin Yvon) excitation at 532 nm, 2.33

eV). To determine the surface oxidation states, X-ray photoelectron spectroscopy (XPS)

measurements were performed using an X-ray photoelectron spectrometer (Thermo Scientific

Inc., UK) with Kα radiation (hν = 1486.6 eV).

2.3 Electrochemical measurements

The electrochemical properties of the prepared electrodes were measured in two and three-

electrode configurations using cyclic voltammetry (CV), galvanostatic charge–discharge

(GCD), and electrochemical impedance spectroscopy (EIS) techniques on a ZIVE SP1

electrochemical workstation. A three-electrode system was used for the half-cell test with the

Co/Mn-MOF as the working electrode, platinum (Pt) wire as the counter electrode, Hg/HgO

as the reference electrode and 1 M KOH electrolyte as an electrolyte. The EIS test was

conducted in the frequency range of 0.01 Hz to 100 kHz at an open circuit voltage of 10 mV.
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Two-electrode measurements were performed for hybrid supercapacitor, which was

fabricated using bimetallic Co/Mn-MOF electrode as the positive electrode, activated carbon

as the negative electrode, cellulose filter paper as the separator, and 1 M KOH as an

electrolyte. The activated carbon electrode was prepared as follows: activated carbon, carbon

black (conductive agent), and Nafion (as a binder) were mixed at an 80:10:10 ratio to form a

homogeneous slurry with ethanol as a solvent. The prepared slurry was coated on Ni foam

and dried at 70 °C overnight to form a well-adhered activated carbon electrode.

The specific capacitance, areal capacitance, specific energy, and specific power for the

three-electrode and hybrid supercapacitor were calculated according to the following

equations:

𝐼 × ∆t
𝐶𝑆 = 𝑚 × ∆𝑉 (1)

𝐼 × ∆𝑡
𝐶𝐴 = 𝐴 × ∆𝑉 (2)

where CS (F g–1) and CA (F cm–2) are the specific, areal, and volumetric capacitances,

respectively. I is the current (A), ∆V is the potential window (V), ∆t is the discharge time (s),

m is the mass of the active material (g), A is the area (cm2), and V is the volume (cm3) of the

device.

The specific energy and specific power of the hybrid supercapacitor were calculated using the

following equations:

0.5 × 𝐶𝑆 × ∆𝑉2
𝐸= 3.6
(3)

E × 3600
𝑃= ∆t
(4)

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Where, E is the specific energy (Wh kg–1), P is the specific power (W kg–1).

3. RESULTS AND DISCUSSION

The overall synthesis approach for the preparation of bimetallic Co/Mn-MOF

nanoparticles on Ni foam and the hybrid supercapacitor assembly is schematically

illustrated in Figure 1. The Co/Mn-MOF nanoparticles are formed on the Ni foam via a

single-step hydrothermal route, during which the organic linker aminoterephthalic acid

itself acts as a source of nitrogen. The prepared Co/Mn-MOF nanoparticles electrode is

appropriate for supercapacitor as it offers abundant active sites on its surface, superior

conductivity, and intermediate spaces for the electrochemical reactions, which no doubt

helps to improve the electrochemical performance of supercapacitors.

The FESEM and TEM analyses were performed to determine the surface morphological

features of the as-prepared samples. Figure 2 reveals FESEM images of Co-MOF, Mn-

MOF, and bimetallic Co/Mn-MOF at two different magnifications as well as their

corresponding TEM images. Figure 2a and 2b demonstrates the surface morphology of the

Co-MOF, which is composed of nanosheets with a highly porous structure. The

corresponding TEM image for the Co-MOF is shown in Figure 2c, which supported the

FESEM analysis, as the formation of nanosheets is clearly observed. For Mn-MOF, as

shown in Figure 2d and 2e, the surface morphology shows the formation of branched

nanoflakes assembled on the surface of the Ni foam, providing large open spaces for the

easy access of electrolyte ions into the interior of active material. A TEM image of the

Mn-MOF is shown in Figure 2f, which also reveals a branched nanoflake morphology.

Figure 2g and 2h shows the surface morphology of the Co/Mn-MOF, indicating the
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formation of a forest of nanoparticles. The FESEM images for Co/Mn-MOF shows the

formation of porous network with numerous open spaces or voids. These voids are helpful

for the easy electrolyte accession into the interior of active material, which increases the

energy storage capability of electrode. TEM image of Co/Mn-MOF reveals the porous

network which is surrounded by carbon shell. This nanostructure is more useful for

supercapacitor as it provide plenty of surface active sites and ion diffusion pathways to

facilitate the charge insertion/desertion process more easily. Such surface morphology

composed of nanoparticles is very helpful for energy storage applications, especially for

supercapacitors. Furthermore, the elemental composition of Co/Mn-MOF is mapped using

EDS analysis. Figure 3a-e exhibits elemental mapping images of Co/Mn-MOF, which

clearly show the uniform distribution of Co, Mn, N, and C throughout the surface. Figure

3f shows the EDS spectrum, and the inset table shows the At% of elements present in the

sample. C is 77.43%, N is 15.73%, Mn is 3.28%, Co is 3.57% on the surface of Ni foam.

The crystal structure and phase purity of the as-obtained samples were analyzed by

XRD and Raman techniques, and the obtained results are illustrated in Figure 4a. The

XRD results for all the samples match well with those in previous reports of Co-MOF and

Mn-MOF structures. Co-MOF shows three dominant diffraction peaks at 2θ of 10.76, 19.5,

26.22° [15]. The Mn-MOF illustrates v peaks at 2θ of 5.52, 19.6, 25.72° [16]. The

obtained results are well matched with the earlier reports on Co-MOF and Mn-MOFs.

Moreover, Co/Mn-MOF possesses diffraction peaks at 2θ of 5.48, 10.96, 19.72, 26.04°.

The characteristic diffraction peaks of Co/Mn-MOF include peaks of both Co-MOF and Mn-

MOF, suggesting that Co/Mn-MOF is successfully prepared and it has the properties of both

Co-MOF and Mn-MOF at the same time.

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Additionally, Raman spectroscopy was performed for Co/Mn-MOF, as shown in Figure

4b. As seen in figure, two broad peaks appear at 1412.52 and 1517.72 cm–1, which

correspond to the D and G band of carbon and agree well with the earlier report [17].

Moreover, the large specific surface area and appropriate pore size distribution are very

important to attaining a high energy storage capacity for supercapacitors. Therefore, the N2

adsorption/desorption isotherms were performed to evaluate the specific surface area and

porosity of the prepared samples. Figure 4c depicts the N2 adsorption/desorption isotherms

for the Co-MOF, Mn-MOF, and bimetallic Co/Mn-MOF. The isotherm depicts typical

type-III hysteresis in a wide relative pressure range of 0.45–1.0 ρ/ρ0, which indicates the

presence of a mesoporous structure. The specific surface area was calculated according to

a Brunauer–Emmett–Teller (BET) analysis, and Co-MOF, Mn-MOF, and Co/Mn-MOF

demonstrated specific surface areas of 53.045, 42.24, and 79.321 m2 g–1, respectively. In

addition, the pore-size distribution for all samples were examined using the BJH method,

as shown in Figure 4d. All samples show a wide pore-size distribution with maximum pore

diameters in the range of 2–20 nm, thus signifying the existence of mesopores. The Co-

MOF, Mn-MOF, and Co/Mn-MOF demonstrated average pore volumes of 0.084, 0.028

and 0.292 cm3 g–1, respectively. Thus, the Co/Mn-MOF possesses a high specific surface

area and superior pore-size distribution. The high surface area provides more

electrochemical active sites and stores more electrolyte ions during electrochemical

reactions, which is beneficial to the high electrochemical performance and energy density

of supercapacitors.

The XPS is the basic tool for collecting information on the compositional and surface

electronic structure of the elements in the sample. The XPS survey scan spectrum of

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bimetallic Co/Mn-MOF in Figure 5a clearly shows that the sample contains Co, Mn, N,

and C. The XPS core-level spectrum of Co 2p is divided into two major peaks at binding

energies of 781.2 and 796.9 eV, corresponding to Co 2p3/2 and Co 2p1/2 (Figure 5b), which

are features of the Co2+ and Co3+ states, respectively. Figure 5c shows the XPS core-level

spectrum for Mn 2p, which splits into two broad peaks at binding energies of 642.0 and

653.8 eV, assigned to the Mn 2p3/2 and Mn 2p1/2 levels, characteristic of the Mn2+ and

Mn3+ states [18]. The presence of satellite peaks in Co 2p and Mn 2p spectra might be due

to the energy loss during the excitation of valence electrons. The high-resolution spectrum

of N 1s (Figure 5d) consists of three peaks corresponding to pyridinic N, pyrrolic N, and

graphitic N at binding energies of 398.08, 399.68, and 400.78 eV, respectively. The

presence of nitrogen improves the electronic conductivity of the compound and promotes

the ultimate electrochemical performance of the electrode. Figure 5e shows the XPS

spectrum for C 1s, which consists of three peaks at binding energies of 284.58 eV, which

belongs to C–C bonds, 285.08 eV, which belongs to C–O and C=N bonds, and 288.68 eV,

which belongs to C–N and O–C–O bonds [19-21]. The presence of C–N bonds indicates

that nitrogen has been effectively doped into the carbon structure.

3.1 Electrochemical performance of the bimetallic Co/Mn-MOF

In order to confirm the feasibility of as-prepared MOFs for supercapacitor applications, an

electrochemical analysis was performed using a three-electrode system in a 1 M KOH

electrolyte via CV, GCD, and EIS measurements. The comparative CV curves of Co-MOF,

Mn-MOF, and bimetallic Co/Mn-MOF electrodes at the same scan rate of 100 mV s-1 in the

potential window of 0–0.8 V (vs Hg/HgO) are shown in Figure 6a. All the CV curves clearly

show well-defined redox pairs in both the cathodic and anodic regions, indicating the

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capacitive feature of the electrodes due to faradic redox reactions. Notably, the Co/Mn-MOF

electrode exhibits a larger area under the CV curve and the highest cathodic and anodic

current responses, which means that it can store more charge than Co-MOF and Mn-MOF

electrodes. In the same way, GCD measurements were carried out at the same current density

of 10 mA cm−2 in the potential range of 0–0.6 V (vs Hg/HgO) (Figure 6b). The GCD curves

follow an analogous tendency to that of the CV analysis. The Co/Mn-MOF electrode exhibits

longer charging and discharging times than the other two electrodes, suggesting that it can

offer a high specific capacitance and energy storage capability. Furthermore, EIS analysis

was performed to investigate the reaction kinetics and resistance of the electrode in further

detail. Figure 6c shows the Nyquist plots for Co-MOF, Mn-MOF, and Co/Mn-MOF in the

frequency range of 0.01 Hz to 100 kHz at a bias potential of 10 mV. Nyquist plots generally

show two separate regions corresponding to high and low frequencies. The high-frequency

region shows a semicircular arc that relates to the faradaic charge-transfer resistance (Rct),

and the low-frequency region shows a straight line, which reveals the diffusion resistance of

the electrode. The Nyquist plot intersects the real axis in the high-frequency region, which is

associated with equivalent series resistance (Rs). The Rs is a combination of the electrolyte

resistance, the intrinsic resistance of the electrode, and the contact resistance between the

active material and current collector. The obtained values of Rs for Co-MOF, Mn-MOF, and

Co/Mn-MOF electrodes are 1.62, 1.36, and 1.27, respectively. The low resistance of the

Co/Mn-MOF electrodes indicates the improved electrical conductivity of the electrode owing

to the synergistic effect between the two metal species and the presence of a good interface

between the active electrode material and electrolyte. The above results confirm that the

bimetallic Co/Mn-MOF electrode offers a higher charge storage capacity than individual Co-

MOF and Mn-MOF electrodes. To further check the rate capability of the Co/Mn-MOF
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electrode, CV and GCD measurements were performed at various scan rates. Figure 6d

shows the CV curves of the Co/Mn-MOF electrode at different scan rates ranging from 5 to

100 mV s–1 in the potential window of 0-0.8 V (vs Hg/HgO). All the CV curves show well-

defined redox peaks in both the cathodic and anodic regions for all scan rates, indicating the

dominance of charge storage via faradic reactions of the active materials. In addition, the

shape of the CV curves remained the same for all scan rates, suggesting the good rate

capability of the electrode. The GCD curves of the Co/Mn-MOF electrode at different current

densities are shown in Figure 6e. The nonlinear performance of GCD curves well supports

the assumptions made based on the CV analysis. The low IR drop indicates the good

electrical conductivity of the Co/Mn-MOF electrode, which is favorable for attaining

enhanced electrochemical performance. The specific and areal capacitances were calculated

for the Co/Mn-MOF electrode from the GCD curves, and the results are plotted in Figure 6f.

The Co/Mn-MOF electrode exhibits a high specific capacitance of 1176.59 F g–1 (2.76 F cm–
2) at a current density of 3 mA cm-2, which is superior to those of previously reported MOF-

based electrodes (Table 1). In contrast, at a high current density of 20 mA cm-2,

the electrode shows a specific capacitance of 737.5 F g–1 (1.73 F cm–2), signifying good

rate capability. Furthermore, to check the contributions of the capacitive and diffusion-

controlled processes to the total charge storage, some additional investigations were

performed. Generally, the peak current measured from the CV curves at different scan rates

follows the following power law equation:

𝑖𝑝 = 𝑎𝑣𝑏 (5)

where a and b are adjustable parameters, and b is always 0.5 or 1. When b = 0.5, the

current is relative to the square root of the scan rate, and the major of the charge is stored by a
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diffusion-controlled process. Meanwhile, when b = 1, the major of charge is stored by a

capacitive process. The slope of the plot of log (i) versus log (υ) (Figure 6g) gives the b

value, which is 0.582 for the Co/Mn-MOF electrode. This figure shows that the peak current

and scan rate have a linear relation, indicating that charge was stored mainly by a diffusion-

controlled process. Furthermore, to calculate the amount of charge from the diffusion-

controlled process, we assume that the total charge at some fixed potential is the sum of the

charge from the capacitive process (𝑠1𝑣) and diffusion-controlled process (s2𝑣1/2).

Therefore, the power law equation can be modified to Equation (7): [22,23].

𝑖𝑝 = 𝑠1𝑣 + 𝑠2𝑣1/2 (6)

which can be converted into

𝑖𝑝/𝑣1/2 = 𝑠1𝑣1/2 + s2 (7)

where 𝑖𝑝 is the peak current (A), and 𝑠1 v and s2 v1/2 are the current contributions of the

capacitive and diffusion-controlled processes. The slope and y-intercept of the plot of 𝑖𝑝 /v1/2

versus v1/2 give values of 𝑠1 and s2. Using the values of 𝑠1 and s2 in Equation (7), the

capacitive and diffusion-controlled charges can be separately obtained. Figure 6h shows the

contributions of the capacitive and diffusion-controlled processes to the total charge for the

Co/Mn-MOF electrode at different scan rates, indicating the dominance of diffusion-

controlled charge storage in the total charge. Furthermore, it is essential to check the cycling

stability of electrode for practical applications. Here, the cycling stability of Co/Mn-MOF

electrode is tested via GCD tests for 5000 cycles in a 1 M KOH electrolyte. The Co/Mn-MOF

electrode exhibits excellent cycling stability, maintaining 85.3% of its original capacitance

after 5000 cycles (Figure 6i). The overall results indicate that the Co/Mn-MOF electrode

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demonstrates exceptional electrochemical performance in terms of cycling stability and

specific capacitance. The enhanced electrochemical performance of the Co/Mn-MOF

electrode could be due to the following features: i) the nanoparticles assisted surface of

Co/Mn-MOF provides large surface area, excellent porosity, and plenty of surface active sites

for electrochemical reactions, ii) the presence of nitrogen in the Co/Mn-MOF nanoparticles

could effectively enhances the electrical conductivity, allows rapid electron flow and

increases rate capability of the electrode, iii) both the Co and Mn species contribute

synergistically for the enhancement of electrochemical performance providing multiple

valance states and reaction kinetics and iv) lastly, the binder-free, self-assembled synthesis of

MOF electrodes on Ni foam restrict the use of additives and binders that decreases electrical

conductivity and unnecessary dead surface area of electrode.

3.2 Electrochemical Performance of Hybrid Supercapacitor

As the above results illustrates that bimetallic Co/Mn-MOF electrode exhibited superior

electrochemical performance, therefore, to check its practical application hybrid

supercapacitor was constructed employing bimetallic Co/Mn-MOF as the positive electrode

and activated carbon (AC) as the negative electrode. Figure 7 shows the electrochemical

analysis results of Co/Mn-MOF//AC hybrid supercapacitor. Figure 7a demonstrates the CV

curves of Co/Mn-MOF//AC hybrid supercapacitor at various scan rates (5-300 mV s–1)

within the potential window of 0 to 1.4 V. Remarkably, the CV curves maintain their same

shape at all scan rates, signifying the excellent reversibility of prepared Co/Mn-MOF//AC

hybrid supercapacitor. As scan rate increases the area under the CV curve also increases

indication excellent capacitive features. The GCD curve of Co/Mn-MOF//AC hybrid

supercapacitor at different current densities (10-30 mA cm–2) are shown in Figure 7b. The
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GCD curves exhibits the non-linear behavior, indicating good capacitive features.

Furthermore, the low IR drop even at high current densities, indicates the high conductivity

and excellent electrochemical reversibility of the hybrid supercapacitor. Moreover, the

specific and real capacitances for the Co/Mn-MOF/AC hybrid supercapacitor are evaluated

from GCD curves and plotted in Figure 7f. The Co/Mn-MOF/AC hybrid supercapacitor

delivered 210.2 F g-1 (0.74 F cm–2) of specific capacitance at 10 mA cm-2. The higher

capacitance for Co/Mn-MOF/AC hybrid supercapacitor could be consigned to the fast and

reversible redox reactions of active materials, which donate to both the charge storage and the

good kinetic balance of electrodes. Moreover, practical applications of the supercapacitor

device depend mainly on its energy and power performances. The specific energy and

specific power for the Co/Mn-MOF//AC hybrid supercapacitor are evaluated form GCD

curves and plotted in Figure 7c. The hybrid supercapacitor delivered a maximum specific

energy of 57.2 Wh kg-1 at a specific power of 2000 W kg-1 at current density of 10 mA cm-2,

which is superior compared to the literature reports in Table 2. Furthermore, the cycling

stability of Co/Mn-MOF//AC hybrid supercapacitor is tested by repeating charge-discharge

test for 5000 cycles. In the subsequent cycling test, the hybrid supercapacitor maintained

93.51% of their initial performance over 5000 cycles. The electrochemical performance of

Co/Mn-MOF//AC hybrid supercapacitor is compared to previously reported MOF based

devices (Table 2). Figure 6f shows the Nyquist plots for the Co/Mn-MOF//AC hybrid

supercapacitor in the frequency range of 0.01 Hz to 100 kHz at a bias potential of 10 mV

before and after cycling. The Rs values before and after cycling for the Co/Mn-MOF//AC

hybrid supercapacitor are 1.61 and 1.92 Ω cm-2, respectively. Again the lower value of

resistance indicates the good conductivity of prepared hybrid supercapacitor and excellent

feasibility with electrolyte.

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4. CONCLUSIONS

In summary, bimetallic Co/Mn-MOF nanoparticles has been prepared on Ni foam by a

simple and scalable hydrothermal approach as electrode material for supercapacitor. The

self-assembled Co/Mn-MOF nanoparticles possess many merits of high surface area, good

porosity, multiple valance states and more importantly these nanoparticles have own

contact with the current collector and provide interspaces for the easy diffusion of ions.

The bimetallic Co/Mn-MOF was directly employed as electrode for supercapacitor.

Consequently, the maximum specific capacitance of 1176.59 F g-1, (2.76 F cm-2) at a

current density of 3 mA cm-2, good rate capability and capacity retention of 85.3% over

5000 cycles were achieved for Co/Mn-MOF nanoparticles. Moreover, the hybrid capacitor

prepared using Co/Mn-MOF and activated carbon delivered a high specific energy of 57.2

Wh kg-1 at 2000 W kg-1, and maintained 93.51% of their initial capacitance over 5000

charge-discharge cycles. These out comings provide desirable direction for future research

on direct utilization of metal-organic frameworks for hybrid supercapacitors.

Acknowledgements

This research was supported by the Nano·Material Technology Development Program

(NRF-2017M3A7B4041987) through the National Research Foundation of Korea (NRF)

funded by the Ministry of Science.

Conflict of Interest
The authors declare no conflict of interest

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Figure 1. Schematic illustration of preparation of bimetallic Co/Mn-MOF (cathode) and

activated carbon (anode).

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Figure 2. FESEM (left, center) and TEM (right) images of (a-c) Co-MOF (d-f) Mn-MOF (g-i)

Bimetallic Co/Mn-MOF.

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1 μm 1 μm

1 μm 1 μm

1 μm

Figure 3. EDS elemental mapping images of Co/Mn-MOF: (a) FESEM image, (b) cobalt, (c)

manganese, (d) nitrogen, and (e) carbon mappings, and the (f) EDS spectrum for Co/Mn-

MOF.

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Figure 4. Comparisons of the (a) XRD patterns, (b) Raman spectra, (c) nitrogen

adsorption/desorption, and (d) the pore size distribution of bimetallic Co/Mn-MOF, Mn-MOF

and Co-MOF.

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Figure 5. XPS spectra of bimetallic Co/Mn-MOF. (a) Survey, (b) Co 2p, (c) Mn 2p, (d) N 1s,

(e) C 1s

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Figure 6. Electrochemical performance of bimetallic Co/Mn-MOF in a three-electrode

configuration: Comparisons of the (a) CV, (b) GCD, and (c) EIS curves of the Co-MOF, Mn-

MOF, and Co/Mn-MOF. (d) CV curves of Co/Mn-MOF at different scan rates, (e) GCD

curves of Co/Mn-MOF at different current densities. (f) Specific/areal capacitance as a

function of current density. (g) Power-law dependence of the redox peak current on the scan

rate for the Co/Mn-MOF electrode. (h) Contribution ratios of diffusion-controlled and

capacitive charge at different scan rates. (i) Cycling stability over 5000 cycles at 10 mA mA

cm ―2

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Figure 7. Electrochemical characterization of bimetallic Co/Mn-MOF//AC device: (a) CV

curves at different scan rates (b) GCD curves at different current densities (c) Specific/Areal

capacitance as a function (d) Ragone plot of the device (e) Cycle stability over 5000 cycles at

10 mA cm ―2 (f) Compare EIS before/after cycling

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Sr. Material Morphology Organic linker Capacitance Stability Ref.

No

1. Ni-Co-S- 1,4-benzenedicarboxylic 1377.5 F g−1 91% after 3000 [24]

Nanosheets
MOF acid at 1 A g−1 cycles
2. Hexagonal- 13.6 F cm–2 at 79.9% after 3000 [25]
Co-MOF terephthalic acid
microblocks 2 mA cm–2 cycles
3. 1127 F g–1 90% after 3000 [26]
Ni-MOF Nanosheets p-benzenedicarboxylic acid
at 0.5 A g–1 cycles
4. benzene-1,3,5-tricarboxylic 1057 F g–1 70% after 2500 [27]
Ni-MOF Nanosheets
acid at 1 A g–1 cycles
5. Polycrystalline 833 C g–1 95.6% after 5000 [28]
Ni-Co MOF isophthalic acid
flower at 0.5 A g–1 cycles
Nanosheets
6. 2,3,5,6- 1098 F g–1 92.6% after 2000 [29]
Mn-MOF and
tetrafluoroterephthalic acid at 1 A g–1 cycles
nanoparticles
7. Zn- Layered 477 F g–1 [30]
8-hydroxyquinoline -
MOF/PANI structure at 1 A g–1
Spherical and
8. Ni/Co- 4,4’-oxybis-benzoic 860 F g–1 [31]
rod shaped -
MOF/rGO acid at 1 A g–1
particles
9. Co-Ni/ 1.318 F cm–2 86% after 3000 [32]
Nanoparticles p-benzenedicarboxylic acid
MOF at 1 mA cm–2 cycles
10. 1,3,5-benzenetricarboxylic 758 F g–1 75% after 5000 [33]
Ni/Co- MOF Dandelion-like
acid, at 1 A g–1 cycles
11. Layered 1049 F g−1 97.4% after 5000 [34]
Ni/Co–MOF terephthalic acid
structure at 1 A g−1 cycles
12. 875 C g–1 90.1% after 5000 [35]
Ni-Co MOF honeycomb 1,4-benzenedicarboxylic
at 1 A g–1 cycles
13. 2354.3 F g−1 83.3% after 1000 [36]
Zn-Co-S/NF Nanosheets 2-methylimidazole
at 0.5 A g−1 cycles
14. Co/Mn 5.65 F cm–2 85.3% after 5000 This
Nanoparticles terephthalic acid
MOF/NF at 10 mA cm–2 cycles work

Table 1. Comparison of electrochemical properties of fabricated electrode and recently

reported data in literature

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Sr. Specific energy; specific

Device Specific capacitance Cycling stability Ref.
No. power
87.8% after 3000
1. Ni-Co-S//AC ASC 135.6 F g–1 at 1 A g−1 36.9 Wh kg–1 at 1066.4 W kg–1 [24]
cycles
92.6% after 4000
2. Ni/Co-MOF//AC 172.7 F g–1 at 0.5 A g–1 77.7 Wh kg–1 at 450 W kg–1 [28]
cycles
Ni/Co-MOF- 91.6% after 6000
3. 159.8 F g–1 at 1 A g–1 72.8 Wh kg–1 at 850 W kg–1 [31]
rGO//AC cycles
4. Ni-MOF//AC 88 F g–1 at 1 A g–1 31.5 Wh kg–1 at 800 W kg–1 - [33]
5. Co/Mn-MOF//AC 106.7 F g–1 at 10 mV s–1 30 Wh kg–1 at 2285.7 W kg–1 - [37]
CNT@Mn-MOF// 88% after 3000
6. 50.3 F g–1 at 0.5 A g–1 6.9 Wh kg–1 at 122.6 W kg–1 [38]
CNT@Mn-MOF cycles
Co/Ni-MOF//CNTs- 86% after 5000
7. 211.1 F g–1 at 1 A g–1 61.8 Wh kg–1 at 725 W kg–1 [39]
COOH cycles
210.2 F g–1 at 10 mA 93.51% after 5000 This
8. Co/Mn MOF//AC 57.2 Wh kg–1 at 2000 W kg–1
cm−2 cycles work

Table 2. Comparison of electrochemical properties of fabricated device and recently reported

data in literature

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Declaration of interests

☑ The authors declare that they have no known competing financial interests or personal

relationships that could have appeared to influence the work reported in this paper.