A ZIF-8-derived nanoporous carbon nanocomposite wrapped with Co3O4-polyaniline as an efficient electrode material for an asymmetric supercapacitor

Journal Pre-proof A ZIF-8-derived nanoporous carbon nanocomposite wrapped with Co3O4-polyaniline as an efficient electrode material for an asymmetric supercapacitor

Kisan Chhetri, Arjun Prasad Tiwari, Bipeen Dahal, Gunendra Prasad Ojha, Tanka Mukhiya, Minju Lee, Taewoo Kim, SuHyeong Chae, Alagan Muthurasu, Hak Yong Kim PII:

S1572-6657(19)30938-5

DOI:

https://doi.org/10.1016/j.jelechem.2019.113670

Reference:

JEAC 113670

To appear in:

Journal of Electroanalytical Chemistry

Received date:

14 October 2019

Revised date:

13 November 2019

Accepted date:

18 November 2019

Please cite this article as: K. Chhetri, A.P. Tiwari, B. Dahal, et al., A ZIF-8-derived nanoporous carbon nanocomposite wrapped with Co3O4-polyaniline as an efficient electrode material for an asymmetric supercapacitor, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/j.jelechem.2019.113670

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© 2019 Published by Elsevier.

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A ZIF-8-Derived Nanoporous Carbon Nanocomposite Wrapped with Co3O4-Polyaniline as an Efficient Electrode Material for an Asymmetric Supercapacitor Kisan Chhetri a, Arjun Prasad Tiwari a,b, Bipeen Dahal a, c, Gunendra Prasad Ojha a, Tanka Mukhiya

a,d

, Minju Lee a, Taewoo Kim a, Su-Hyeong Chae a, Alagan Muthurasu a, Hak Yong

Department of BIN Convergence Technology, Jeonbuk National University, Jeonju-si,

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a

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Kim a,e *

Carbon Nano Convergence Technology Center for Next Generation Engineers (CNN),

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b

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Jeollabuk-do, 561-756, Republic of Korea

d

e

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Central Department of Chemistry, Tribhuvan University, Nepal Department of Chemistry, Bhaktapur Multiple Campus, Tribhuvan University, Nepal

Department of Organic Materials and Fiber Engineering, Jeonbuk National University,

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c

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Jeonbuk National University, Jeonju-si, Jeollabuk-do, 54896, Republic of Korea

Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea *Corresponding author:

*Hak Yong Kim, Tel.: +82632702351, Fax: +82632704249, E-mail:[email protected]

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Abstract Zeolitic imidazolate framework-8 (ZIF-8)-derived nanoporous carbons (NPCs) have become more attractive in the energy storage sector due to their excellent characteristics, such as their porosity, available specific surface area, and charge storing ability. In this work, we further modify ZIF-8NPC by using Co3O4 nanoflakes together with polyaniline (PANI) in a controlled in situ polymerization on the surface of ZIF-8NPC to form a Co3O4-PANI@ZIF-

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8NPC nanocomposite. The as-prepared nanocomposite shows a high specific capacitance of

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1407 F g-1 at 1 A g-1 with remarkable rate capability and cyclic stability. An asymmetric

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supercapacitor (ASC) with Co3O4-PANI@ZIF-8NPC as a positive electrode and ZIF-8NPC

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as a negative electrode shows a high energy densities of 52.81 W h kg-1 (at a power density of 751.51 W kg-1) and 18.75 W h kg-1 (at a power density of 7500 W kg-1), along with a high

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cyclic stability as evidenced by retaining 88.43% of the initial capacitance after 10000 cycles

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at 10 A g-1. The remarkable electrochemical performance mentioned above is attributed to a synergistic effect between the high conductivity provided by PANI, the increased number of

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pseudocapacitive active sites provided by CO3O4, and the high surface area with a suitable pore size distribution provided by ZIF-8NPC. The results demonstrate that the as-prepared electrode materials can be a good alternative for applications in supercapacitors and other energy storage devices.

Keywords: Zeolitic imidazolate framework-8; Nanoporous carbon; Polyaniline; Cobalt oxide; Asymmetric supercapacitor

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1. Introduction There is a high demand for efficient energy storage and conversion devices to address the quickly exhausting supply of fossil fuels and their severe environmental impacts; thus, extensive research in related fields is ongoing. Currently, the applications of metal-organic frameworks (MOFs) and MOFs-derived materials in energy-related areas, such as water

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splitting, oxygen reduction reactions, hydrogen storage, photoinduced hydrogen evolution,

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methane storage, batteries, photovoltaics, and supercapacitors [1-9], are of tremendous

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interest among material scientists. In the field of supercapacitors (SCs), numerous studies

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have focused on various types of carbon and carbon nanocomposites with pseudocapacitive materials as efficient electrode materials [10-12]. One of the most promising sources of

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nanoporous carbon (NPC) material for SC application are MOFs due to their high surface

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area, tunable pore size, interconnected pores, and hierarchical nanostructures [13-15]. Additionally, MOFs have been used as a template to accommodate metal oxides,

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semiconductors and conducting polymers for the fabrication of various nanocomposite electrode materials in SC applications [16-18]. Out of more than 20,000 types of MOFs, there is a subclass denoted as MOF-zeolitic imidazolate frameworks (ZIFs) that have tunable porosity, structural flexibility, and functionalization of the internal surface; additionally, they have thermal, mechanical and chemical stability [19]. In ZIFs, imidazolate linkers connect metal centers to form threedimensional porous crystalline solids that are very similar in shape to other traditional inorganic zeolites [20]. From the wide variety of known ZIFs, in this study, we selected ZIF8 (Zn-based ZIF) powder as a precursor for the fabrication of an NPC after carbonization in 3

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an inert atmosphere [21, 22]. However, having only ZIF-8-derived NPC (ZIF-8NPC) as the SC electrode material showed a lower value of electrochemical double layer capacitance (EDLC) owing to their intrinsically weak electroactivity and electrical conductivity properties [23, 24]. Fortunately, the low conductivity and electroactivity problem can be overcome by making ZIF-8NPC-based nanocomposites with other carbon materials, such as graphene [25], CNTs [26], CNFs [27, 28], graphene oxides [29] and pseudocapacitive

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materials, such as metal oxides [30-32], metal oxide nanoparticles [33, 34], metal hydroxides

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[35, 36] and conducting polymers [37, 38]. The novel addition of pseudocapacitive materials demonstrates a synergistic effect on the electrochemical performance of the ZIF-8NPC-based

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nanocomposite electrodes [39]. The EDLC is because of the adsorption of coulombic charge

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close to the electrode-electrolyte interface, and the pseudocapacitance is because of surface

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redox reactions related to individual charge transfer mechanisms [40]. The surface characteristics of the electrode material have a substantial influence on the capacitance

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(pseudocapacitance and double-layer capacitance) value during the charging-discharging

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process [41]. Pseudocapacitors have a higher capacitance than double-layer capacitors, but pseudocapacitive materials with weak electrical conductivity lead to a low cyclic stability and low power density [42]. Among the many metal oxides, Co3O4 is one of the best materials for pseudocapacitor applications because of its tremendous theoretical specific capacitance (3560 F g-1), low price, and eco-friendliness [43]. However, typical Co3O4 exhibits poor conductivity, low specific capacitance, and poor rate capability, which frequently results in poor cycle stability over a long charge-discharge process [44]. To overcome these demerits, an in situ polymerization of aniline with Co3O4 on the surface of ZIF-8NPC is a good solution. The polyaniline (PANI) can facilitate electrolyte diffusion and promote the rate performance of the electrode via multiple redox reactions during transitions 4

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between various oxidation states [45]. The hybrid methodology also allows the synergetic nanocomposite to act in ways that are impossible by any single component on its own, and this leads to additional charge storage through enhanced charge transportation and kinetic behavior with effective contact between the electrolyte and electroactive materials. Hence, the blend of two charge storage mechanisms (EDLC and pseudocapacitance) with a good capacitance value has shown great potential for use as electrode materials [9, 37, 46].

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To date, many studies have been performed on the development of different metal oxides and

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conducting polymers with carbon templates for supercapacitor applications. For example, an

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MOF-derived nanoporous carbon and a conducting polymer showed specific capacitance values of up to 1100 F g-1 with a specific energy density of 21 W h kg-1 at a specific power

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density of 1200 W kg-1 [39]. In other work, polypyrrole@UIO-66@cotton fabric electrodes

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for flexible supercapacitors showed a specific capacitance of 565 F g-1 at a current density of

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0.8 mA cm-2 [47], and PANI-ZIF-67-CC exhibited an areal capacitance of 2146 mF cm-2 at 10 mV s-1 [48]. More interestingly, the hierarchical porous PANI/MIL-101 electrode had a

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specific capacitance of 1197 F g-1 at 1 A g-1 [45], and the use of Zn-MOF/PANI exhibited values as high as 477 F g-1 at a current density of 1 A g-1 [49]. Similar to that of PANI/ZnO/ZIF-8/graphene/polyester, a textile electrode exhibited an areal capacitance of 1378 mF cm-2 at 1 mA cm-2 [50], and a flexible PANI-CNT@ZIF-67-CC electrode with a hierarchical porous structure showed an areal capacitance of 3511 mF cm-2 [51]. In this context, a ZnO@MOF@PANI composite, exhibited a specific capacitance of 340.7 F  g-1 at a current density of 1 A g-1 [52]. Min et al. reported another interesting work in which crosslinked PANI- and ZIF-derived N-doped porous carbon composites showed a specific capacity of 755 F g-1 at 1 A g-1 [53]. Furthermore, ZIF-67-derived hierarchical Co3O4-encapsulated 5

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PANI hollow nanocages exhibited specific capacitances of 1301 F g-1 at 1 A g-1 with an energy density of 41.5 W h kg-1 at a power density of 0.8 kW kg-1 in an asymmetric device [54]. Based on the above research, to overcome the low specific capacitance value and short life cycles of MOF-derived NPC-based nanocomposites, we designed nanocomposites comprising of ZIF-8NPC that is wrapped with a mixture of Co3O4 NFs and PANI by a

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controlled in situ polymerization. Interestingly, the as-prepared Co3O4-PANI@ZIF-8NPC

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hybrid nanocomposite exhibits high specific capacitance, good rate capability, and long

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cyclic stability. We expect that this hybrid nanocomposite design will be quintessential and applicable to other types of MOF-derived nanoporous carbon with various combinations of

2.1 Materials Aniline

(C6H5NH2,

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2. Experimental Section

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metal oxides and conductive polymer-based supercapacitor electrodes.

≥99.5%),

2-methylimidazole

(CH3C3H2N2H,

99%)

and

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poly(vinylenedifluoride) (PVDF) were obtained from Sigma–Aldrich, Germany. Potassium hydroxide (KOH, ≥99.5%), hydrochloric acid (HCl, 35-37%), cobalt nitrate hexahydrate (Co(NO3)2⋅6H2O, 97%) and methanol (CH3OH, 99.5%) were obtained from Samchun, Republic of Korea. Zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O, 99%) was purchased from Junsei Chemicals, Japan. Ammonium persulfate ((NH4)2S2O8, 98%) and N-methyl-2pyrrolidinone (NMP) were purchased from Showa Chemicals, Japan. All the chemicals were of analytical grade and used without additional purification except for aniline, which was double distilled before use. All aqueous solutions were prepared with double distilled water. 2.2 Synthesis of ZIF-8 and nanoporous carbon (ZIF-8NPC) 6

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Zn(NO3)2⋅6H2O (2.34 g) and 2-methylimidazole (22.7 g) were dissolved individually in 100 mL methanol by stirring for 30 min. Then, these two solutions were quickly mixed and stirred for 50 min. The final solution was aged for 24 h at room temperature, and after 24 h, a white precipitate settled at the bottom. The precipitated white powder was collected by centrifugation (8000 rpm, 20 min), followed by repeated washings with ethyl alcohol. Last, the collected product was dried at 80 °C for 12 h and named ZIF-8. ZIF-8NPC was obtained

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by a one-step carbonization of ZIF-8 at 900 °C in a nitrogen atmosphere. The temperature of

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the tube furnace was increased at a heating rate of 3 °C min-1 and the sample was carbonized for 3.5 h. Then, the black powder was carefully washed with 15 wt% hydrofluoric acid. The

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main purpose of HF washing was to remove the trace amount of zinc present in the

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carbonized sample [6]. Finally, the black powder was washed many times with double

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distilled water and then dried overnight at 80 °C.

2.3 Synthesis of the Co3O4 nanoflakes (Co3O4 NFs)

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First, Co(NO3)2⋅6H2O (17.46 g) and potassium hydroxide (KOH) (17.95 g) were individually

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dissolved in 100 mL of double distilled water. Then, 100 mL of aqueous KOH was added dropwise to the Co(NO3)2⋅6H2O solution. Instantly, a pink precipitate appeared that was oxidized by air to dark brown Co(OH)3 at 80 °C. The dark brown precipitate was separated and washed with double distilled water and dried in an oven at 120 °C for 24 h. Finally, cobaltic-cobaltous oxide (Co3O4) NFs were synthesized by heating the cobaltic hydroxide in a muffle furnace at 350 °C for 3.5 h with a heating rate of 3 °C min-1. Afterward, they were stored for future use. 2.4 Synthesis of the Co3O4 NFs with PANI (Co3O4-PANI) and Co3O4-PANI encapsulated ZIF-8NPC nanocomposite (Co3O4-PANI@ZIF-8NPC) Twenty milligrams of Co3O4 NFs and ZIF-8NPC each were well dispersed in a solution of 7

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aniline (0.2 M) in 30 mL aqueous HCl (1 M). A controlled in situ oxidative polymerization of the Co3O4-ZIF-8NPC-aniline-HCl solution was carried out in the presence of ammonium persulfate ((NH4)2S2O8) (0.25 M) in 30 mL aqueous HCl (1 M) as an oxidant at a temperature below 5 °C. The optimal ratio of aniline to oxidant (1:1.25) was kept constant during the reaction. The as-synthesized Co3O4-PANI@ZIF-8NPC was washed with 0.1 M HCl and double distilled water until the filtrate was a neutral pH and then the sample was dried

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overnight at 60 °C. Similarly, Co3O4-PANI was prepared by using the above synthesis route

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without ZIF-8NPC. 2.5 Materials characterization

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Powder X-ray diffraction was used for the structural and geometrical analysis of the prepared

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samples along with an X-ray diffractometer (XRD, Rigaku Corporation, Tokyo, Japan) using

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Cu Kα (λ = 1.54056 Å) radiation over a 2θ range of 5° to 80°. Thermal analysis of the ZIF-8 powder was performed using a thermogravimetric analyzer (PerkinElmer, Pyris 1 TGA). The

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surface morphology, microstructural analysis, and elemental analysis of the fabricated

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samples were described using field emission scanning electron microscopy (FE-SEM, SUPRA40VP, Carl Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2100 plus, JEOL Ltd, Japan). The N2 adsorption-desorption isotherms, Brunauer–Emmett–Teller (BET) specific surface area, and pore size distribution were recorded at 77 K with a Micromeritics instrument (3Flex 5.01). The carbon content in the samples was determined by using Raman spectroscopy (RFS-100S, Bruker, Germany). The surface chemistry and the binding energy of different electronic states of the samples were examined by XPS with a Kalpha X-ray photoelectron spectrometer (Thermo Scientific, Nexsa XPS system). Fourier transform infrared (FTIR) spectra were obtained using an ABB Bomen MB 100 spectrometer. 2.6 Electrochemical measurement 8

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Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements of the samples were analyzed by using a VersaSTAT-4 electrochemical analyzer (AMETEK Inc., USA). The working electrodes were fabricated by mixing 80% of the active material with 10% carbon black and 10% PVDF with NMP as a solvent to make a slurry. After grinding in a mortar pestle, a homogeneous slurry was formed, and the slurry was drop cast on nickel foam (1x1 cm2) and dried at 70 °C for 20

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h to evaporate the NMP. The mass of the electrode materials was 5.0 mg each and they were

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pressed at 6 MPa for 25 s before being used as the electrode material for supercapacitor applications.

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The electrochemical measurements were performed in a three-electrode system, and

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fabricated samples on nickel foam were used as the working electrode, Ag/AgCl (3 M KCl

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saturated) was used as the reference electrode and platinum wire was used as the counter electrode with a 3 M KOH aqueous electrolyte. CV and GCD measurements were recorded at

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-0.2 V to 0.6 V at different scan rates and -0.2 V to 0.5 V at different current densities. EIS

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measurements were carried out in a range from 106 Hz to 10-2 Hz and displayed on Nyquist plots. The specific capacitance values from the GCD curves were calculated using the following equation:

𝐶=

𝐼 × 𝑡𝑑 𝑚×𝑉

(1)

where ‘C’ is the specific capacitance (F g-1) based on the mass of the electrode material, ‘I’ is the discharge current, ‘td’ is the discharge time (s), ‘V’ is the discharge potential range (V) and ‘m’ represents the mass of the electrode material (g). 2.7 Asymmetric supercapacitor (ASC) device fabrication 9

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A split test cell (2GxA18011 MTI Corporation) was used to design the ASC. Both the CV and GCD were carried out in a potential range between 0.0 V and 1.5 V. The CV and GCD were tested at scan rates of 10, 30, 50, 70, and 100 mV s-1 and current densities of 1, 2, 3, 5, 7, and 10 A g-1, respectively. The impedance of the ASC was taken in a range between 106 Hz and 10-2 Hz and displayed on Nyquist plots. The ASC device was fabricated using the Co3O4PANI@ZIF-8NPC nanocomposite as a positive electrode and ZIF-8NPC as a negative

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electrode. The use of ZIF-8NPC as a negative electrode material could be an alternative to

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activated carbon (AC) and nitrogen-doped graphene hydrogels (NGHs) due to ZIF-8NPC exhibiting identical electrochemical results [23, 55]. The CV and GCD measurements of ZIF-

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8NPC in the 3-electrode system were recorded from -1 V to 0 V at different scan rates and

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current densities. Based on the electrochemical data of the negative and positive electrodes in

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the 3-electrode system, the optimal mass ratio of positive to negative electrode material for

𝑚+

=

𝐶− × 𝑉− 𝐶+ × 𝑉+

(2)

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𝑚−

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the asymmetric supercapacitor device was calculated with the following equation:

where m+, m-, C+, C-, V+, and V- are the mass of the electrode material (g), specific capacitance (F g-1), and potential window (V) of both electrodes, respectively. The negative electrode material was prepared by mixing 90% as-prepared ZIF-8NPC and 10% PVDF in NMP to form a homogeneous slurry. ZIF-8NPC does not require any conductive agent during the electrode fabrication procedure [23]. The obtained slurry was deposited homogeneously onto nickel foam, and electrochemical tests were conducted. At the time of ASC assembly, the split cell device, the separator, and the electrodes were soaked in 3 M KOH. The energy density, power density, and coulombic efficiency were calculated by using the following formulas: 10

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𝐸= 𝑃=

𝑛=

𝐶×𝑉 2

(3)

2×3.6 3600 ×𝐸

(4)

𝑡𝑑 𝑡𝑑

(5)

𝑡𝑐

where ‘tc’ is the charging time (s), ‘E’ is the energy density (W h kg-1), ‘P’ is the power

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density (W kg-1) and ‘𝑛’ is the Coulombic efficiency (%).

3.1 Structural and morphological analysis

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3. Results and discussion

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A schematic representation of the detailed synthesis process of the Co3O4-PANI@ZIF-8NPC

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nanocomposite is presented in Fig. 1. A simple synthesis using zinc nitrate hexahydrate and 2-methylimidazole produced consistent rhombic dodecahedron-shaped nanocrystals of ZIF-8

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that were used as a template. First, ZIF-8 was carbonized at 900 °C in a N2 atmosphere to get

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ZIF-8NPC. Then, the obtained ZIF-8NPC was soaked in an aniline-HCl aqueous solution with Co3O4 NFs. At last, a controlled chemical oxidative polymerization of aniline on the surface of ZIF-8NPC was carried out to obtain the Co3O4-PANI@ZIF-8NPC nanocomposite.

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Fig. 1. Graphic outline of the Co3O4-PANI@ZIF-8NPC nanocomposite synthesis.

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Fig. 2 (a) and (b) show the FE-SEM images of the rhombic dodecahedron shape of ZIF-

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8NPC at a low and high magnification, respectively. The average size of the ZIF-8NPC is 150±50 nm. Fig. 2(c) and (d) shows the PANI nanofiber synthesized from a controlled

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chemical oxidative polymerization of aniline and the porous Co3O4 NFs, respectively. The nanocomposite of PANI and porous Co3O4 NFs is shown in Fig. 2(e). It is clearly observed

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from Fig. 2(f) that the Co3O4-PANI@ZIF-8NPC nanocomposite is composed of ZIF-8NPC heterogeneously wrapped with the PANI nanofiber and Co3O4 NFs to form a hybrid nanocomposite.

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Fig. 2. FE-SEM images of (a, b) ZIF-8NPC, (c) PANI, (d) Co3O4 NFs, (e) Co3O4-PANI, and

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(f) the Co3O4-PANI@ZIF-8NPC nanocomposite.

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Fig. 3 displays the TEM, HR-TEM, and SAED pattern of the Co3O4-PANI@ZIF-8NPC

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nanocomposite. The TEM image of the as-fabricated Co3O4-PANI@ZIF-8NPC also shows a heterogeneous wrapping of ZIF-8NPC by the Co3O4 NFs and PANI. Additionally, the HRTEM also confirms the wrapping of ZIF-8NPC by the Co3O4 and PANI to form the Co3O4PANI@ZIF-8NPC nanocomposite (Fig. 3 (a) and (b)), and the shape of the nanostructure is modified from a rhombic dodecahedron to an irregular sphere. The elemental composition of the as-fabricated nanocomposite shows the presence of ‘C’, ‘Co’, ‘N’ and ‘O’ without impurities (Fig. 3c). The TEM images of the Co3O4-PANI nanocomposite with an elemental mapping of C, Co, O, and N are shown in Fig. S1. The Co3O4 NFs and PANI nanostructures wrapped around the ZIF-8NPC help to enhance the pseudocapacitive behavior, which is in good agreement with the electrochemical performance results of the nanocomposite. The 13

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lattice spacing of the Co3O4-PANI@ZIF-8NPC nanocomposite is measured to be approximately 0.243 and 0.204 nm, which corresponds to the (311) and (400) planes of Co3O4, respectively, as shown in the HRTEM image of Fig. 3(d). To determine the nature of crystallinity, SAED was conducted. The presence of dark and bright fringes in the Fig. 3d inset shows the polycrystalline nature of the nanocomposite in the SAED patterns. These crystal planes were identical with the XRD result and Fig. 3 (a) and (b) are in good agreement

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with the morphology of the Co3O4-PANI@ZIF-8NPC nanocomposite as shown in Fig. 2(f).

Fig. 3. TEM images of the Co3O4-PANI@ZIF-8NPC nanocomposite: (a, b) Images at different magnifications; (c) elemental mapping of C, Co, N, and O; and (d) the HR-TEM image with an inset showing the SAED pattern.

The N2 adsorption-desorption isotherms and pore size distributions are shown in Fig. 4. For 14

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the Co3O4-PANI@ZIF-8NPC nanocomposite, the adsorption-desorption isotherms exhibited typical type-IV isotherms with a hysteresis loop indicating the presence of mesopores throughout the samples (Fig. 4 (a)). This result is further established by the corresponding pore size distribution (PSD) curves as depicted in Fig. 4 (b). The PSD curves show that a high concentration of pores is in a diameter range of 15-40 nm, which is an appropriate range for the diffusion of ions during electrochemical tests. The coexistence of a microporous and

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mesoporous structure enhances the transport and diffusion of electrolytic ions on the surface

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of the electrode material during the charge-discharge process, which enhances the electrochemical performance [54]. From the BET and BJH analyses, the specific surface area

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and total pore volume were found to be 52.29 m2 g-1 and 0.27 cm3 g-1, respectively. From the

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adsorption-desorption isotherms and the pore size distribution of ZIF-8NPC, it is clear that

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there is a type-I isotherm indicating ZIF-8NPC is highly microporous (Fig. S2). Moreover, the isotherm at a relative pressure below 0.1 shows a higher nitrogen adsorption capacity,

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which further supports the high concentration of micropores in comparison to the

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concentration of mesopores. The BET specific surface area was found to be 698.87 m² g-1, which provides more active sites for redox reactions and promotes the exploitation of the active material.

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Fig. 4. (a and b) N2 adsorption-desorption isotherm and pore size distribution of the Co3O4-

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Raman spectra.

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PANI@ZIF-8NPC nanocomposite, respectively, along with the (c) XRD pattern and (d)

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The XRD patterns of the as-synthesized samples are shown in Fig. 4 (c) to study their crystallinity and phase purity. The peaks at 2θ values of 18.81, 31.15, 36.76, 38.51, 44.72, 55.61, 59.35, and 65.21° are the characteristic peaks of Co3O4 NFs with no diffraction peaks of other impurities being present, which indicates the successful synthesis of Co3O4 NFs in accordance with those of previous reports [44]. The XRD pattern of ZIF-8 powder (Fig. S3(b)) is due to the ordered porous structure of the particles between 2θ values of 5 and 40° and indicates the formation of nanosized crystals as shown in Fig. S3 (a). The XRD diffractogram of PANI and ZIF-8NPC shows a broad peak at 2θ values of 24.84° and 25.46°, respectively (Fig. S4 (a) and (b)), which indicates the ZIF-8NPC is amorphous in nature. The XRD 16

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diffractogram of PANI-Co3O4 shows a broad peak at a 2θ value of 24.86° due to the (200) plane of carbon along with the respective peaks of Co3O4 NFs, which supports the conclusion that the material is a nanocomposite of PANI and Co3O4. Similarly, the XRD pattern of PANI-Co3O4@ZIF-8NPC shows corresponding peaks of Co3O4 with a combined broad peak of PANI and ZIF-8NPC at a 2θ value range from 20° to 30° due to the (200) and (002) plane, respectively. Which confirms that the material is a nanocomposite of PANI, Co3O4, and ZIF-

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8NPC. The degree of structural defects found in the nanocomposite can also be analyzed by

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Raman spectroscopy (Fig. 4 (d)). Two peaks, equivalent to the D band (1343 cm-1) and G band (1580 cm-1), can be detected in Co3O4-PANI@ZIF-8NPC due to ZIF-8NPC. The G

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band and D band indicate the graphitic order and the disorder in the lattice structure,

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respectively. Apart from the G and D band some other characteristic peaks at 189, 475, 658,

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and 2427 cm-1 are due to the presence of Co3O4 [56] and peaks at 1215, 1512, and 2930 cm-1 are due to the presence of PANI [57]; thus supporting the existence of Co3O4 and PANI in the

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nanocomposite. The ID/IG ratio of the Co3O4-PANI@ZIF-8NPC nanocomposite is 1.01

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whereas Co3O4-PANI and Co3O4 don’t show such peaks due to the absence of graphitic ZIF8NPC. Fig. S5 (b) shows the Raman spectra of ZIF-8NPC. The increased ID/IG ratio indicates that the degree of defects in the nanocomposite is higher than the degree of graphitization [13].

The FTIR spectra of the Co3O4-PANI@ZIF-8NPC nanocomposite with Co3O4-PANI and Co3O4 are shown in Fig. 5 (a). For Co3O4 NFs, two characteristic peaks positioned at 640 and 565 cm-1 are related to the Co-O stretching [58]. As a consequence of the C=C stretching modes of the quinoid (Q) ring and benzenoid ring, two peaks at 1565 and 1484 cm-1 are observed, respectively [16]. The peaks at 1294 , 1116 , and 819 cm-1 are due to the C-N 17

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stretching vibration of an aromatic amine, classic N=Q=N stretching mode and C-H out of plane bending mode of PANI, respectively [59]. For the Co3O4-PANI nanocomposite, some respective peaks of PANI and Co3O4 are observed. The Co3O4-PANI@ZIF-8NPC nanocomposite shows the above described characteristic peaks of Co3O4 at 563 and 660 cm-1, and the peaks for PANI at 800, 1131, 1294, 1492, and 1565 cm-1, suggesting that the ZIF8NPC is wrapped by PANI and Co3O4. The valence state of the cobalt and the surface

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chemistry of the Co3O4-PANI@ZIF-8NPC nanocomposite was analyzed by XPS. Fig. 5(b)

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shows the wide scan XPS survey spectra of the Co3O4-PANI@ZIF-8NPC nanocomposite and display the main elements: C 1s, N 1s, O 1s and Co 2p with peak positions at 284.79, 399.51,

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530.38, and 780.18 eV, respectively. Details can be obtained from the high-resolution XPS

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spectrum of each elements. The XPS spectrum of Co 2p shows two main spin-orbit lines and

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weak satellite signals, representing the existence of Co2+ and Co3+ in the nanocomposite as shown in Fig. 5(c). The deconvoluted peaks at 796.3 and 781.3 eV along with the peaks at

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794.7 and 779.8 eV are respectively related to the Co 2p1/2 and Co 2p3/2 signals of Co2+ and

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the Co 2p1/2 and Co 2p3/2 signals of Co3+ [54, 60]. The deconvoluted peaks of the O 1s corelevel at 529.9, 531.4, 532.2, and 533.4 eV respectively correspond to the O2- derived from the Co3O4, O-H group, carboxyl group, and adsorbed H2O found on the nanocomposite as shown in Fig. 5(d) [45]. The XPS spectrum of the N 1s core-level as shown in Fig. 5(e) contains four typical peaks at 398.4, 399.6, 401.2, and 401.9 eV that are related to the pyridinic-N, pyrrolic-N, quaternary-N, and oxidized nitrogen, respectively [27]. The pyrrolic-N has the highest peak exhibiting the best protonation level (N+/N) of PANI, which enhances the electrical properties of the sample [61]. The C 1s spectrum (Fig. 5(f)) shows the main peak at 284.6 eV relating to the C-C bonds. Moreover, other representative peaks at 285.7, 286.8, and 288.2 eV correspond to the C-N, C-O and C=O bond from some carboxyl groups or oxygen18

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containing groups [62]. The composite shows atomic percentages of 64.06% C 1s, 9.12% N

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2p, 20.36% O 1s, and 6.47% Co 2p found on the surface of the nanocomposite.

Fig. 5. (a) FTIR spectra, (b) XPS spectra of Co3O4-PANI@ZIF-8NPC and (c, d, e, f) highresolution XPS spectra of Co 2p, O 1s, N 1s and C 1s of the Co3O4-PANI@ZIF-8NPC nanocomposite, respectively.

3.2. Electrochemical measurements To evaluate and compare the electrochemical properties of the prepared materials as a 19

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supercapacitor electrode material, CV, GCD and EIS tests were conducted with a standard three-electrode configuration in a 3 M KOH electrolyte solution. The electrochemical performance of the Co3O4-PANI@ZIF-8NPC nanocomposite electrode was systematically measured and evaluated. Fig. 6 (a) shows the CV curves of the electrode measured at scan rates of 10, 30, 50, 70, and 100 mV s-1 within a potential range of -0.2 to 0.6 V. The welldefined redox peaks at slower scan rates are noticeable in the CV curves due to a reversible

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Faradaic reaction [63]. The reduction peak shifts to a lower potential as the scan rate

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increases. GCD curves were measured at potential ranges of -0.2 to 0.5 V at current densities of 1, 2, 3, 5, 7, and 10 A g-1, as shown in Fig. 6 (b). During the charge-discharge process, the

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discharging curves are asymmetrical to their corresponding charging curves due to the

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faradaic pseudocapacitance occurring at different current densities. Fig. 6 (c) exhibits the

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specific capacitance versus the current density calculated from the GCD profiles. The specific capacitances of the Co3O4-PANI@ZIF-8NPC electrode in the three electrode systems

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are 1407, 1160, 1119, 921, 850, and 742 F g-1, corresponding to current densities of 1, 2, 3, 5,

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7, and 10 A g-1, respectively. To study the cyclic stability of the as-prepared electrode, the GCD test was carried out for 10000 cycles at a current density of 20 A g-1. The Co3O4PANI@ZIF-8NPC nanocomposite electrode has a specific capacitance value of 87.7% of the initial capacitance, showing that the electrode has a good cyclic stability after 10000 chargedischarge cycles (Fig. 6 (d)). Furthermore, the electrode shows a specific capacitance of 1302 F g-1 at a current density of 1 A g-1, indicating that the nanocomposite has a 92.7% retention of the initial specific capacitance, even after 10000 charge-discharge cycles (Fig. 6 (e)). EIS was fitted by an equivalent circuit, as shown in Fig. S6. The Nyquist plot before and after the 10000 cycle stability test is shown in Fig. 6 (f). This shows that before the stability test, EIS is more inclined toward the vertical Z” axis, indicating better capacitive performance with a 20

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lower value of charge-transfer resistance (Rct). The slope in the low-frequency region is related to ion diffusion on the electrode surface, the steeper slope of the line before the stability test represents faster ion diffusion compared to that after the stability test. Comparative studies of similar types of materials are listed in Table S1, which shows that the

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Co3O4-PANI@ZIF-8NPC nanocomposite has the best electrochemical performance.

Fig. 6. Electrochemical measurements of the Co3O4-PANI@ZIF-8NPC nanocomposite: (a) CV curves at different scan rates, (b) GCD curves at different current densities, (c) specific 21

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capacitance at different current densities, (d) cyclic stability at a current density of 20 A g-1 with the inset showing the last 10 cycles of the stability test, (e) GCD curves at a current density of 1 A g-1, and (f) Nyquist plots with the inset showing a magnified section of the Nyquist plot.

The comparative electrochemical performances of the Co3O4, Co3O4-PANI, and Co3O4PANI@ZIF-8NPC nanocomposites are shown in Fig. 7. The inconsistency of the current

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response in the CV curves at a scan rate of 50 mV s-1 (Fig. 7 (a)) of the three samples is

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notable and obviously shows pseudocapacitive features. Among these three electrodes, the

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Co3O4-PANI@ZIF-8NPC nanocomposite showed a better current response. Fig. 7 (b) shows

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comparative GCD profiles of the three samples measured at a current density of 1 A g-1. A longer discharge time of the GCD curve was observed for the Co3O4-PANI@ZIF-8NPC

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nanocomposite. Individual electrochemical performances of Co3O4 and Co3O4-PANI are

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shown in Fig. S7 and S8, respectively. The specific capacitance value of the Co3O4PANI@ZIF-8NPC nanocomposite electrode (1407 F g-1) is superior to those of the Co3O4-

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PANI (1003 F g-1, Fig. S8) and Co3O4 (867 F g-1, Fig. S7) electrodes at a current density of 1 A g-1 (Fig. 7 (c)). Nyquist plots are composed of a semicircle in the high-frequency region, and the diameter of the semicircle specifies the charge-transfer resistance (Rct). The slope in the low-frequency region is associated with ion diffusion at the electrode-electrolyte interface [64]. The length of EIS is shorter for the Co3O4-PANI@ZIF-8NPC electrode compared to those of the other two electrodes, and the line is more inclined toward the vertical Z” axis, indicating that it has a better capacitive performance (Fig. 7 (d)). The above mentioned results show that the Co3O4-PANI@ZIF-8NPC nanocomposite is a good electrode material for supercapacitor applications. The pseudocapacitive effect can be attributed to the synergy 22

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between PANI and Co3O4; they provide the nanocomposite with increased surface conductivity, fast ion transport, and a sufficient number of active sites, which enhances the adsorption of ions and improves the electrochemical performance [65]. Comparative data from some recently published studies on MOF-derived nanoporous carbons and their

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composite-based electrode materials for supercapacitors are shown in Table S1.

Fig. 7. (a) Comparison of CV curves at a scan rate of 50 mV s-1, (b) GCD curves at a current density of 1 A g-1, (c) specific capacitance at different current densities and (d) Nyquist plots with the inset showing a magnified section of the Nyquist plot.

Further testing for the application of the Co3O4-PANI@ZIF-8NPC nanocomposite as a supercapacitor electrode was conducted. An asymmetric supercapacitor was made by using ZIF-8NPC as the negative electrode, Co3O4-PANI@ZIF-8NPC as the positive electrode, and 23

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3 M KOH as the electrolyte with a filter paper separator. The electrochemical performances of ZIF-8NPC as a negative electrode are shown in Fig. S9 along with the XRD (Fig. S4 (b)) and Raman (Fig. S5 (b)) measurements. The GCD curves of ZIF-8NPC show that the specific capacitance is 190 F g-1 at a current density of 1 A g-1. The charge between both electrodes was balanced based on equation (2) to obtain the best electrochemical performance. Fig. 8 (a) represents the individual CV curves of the negative and positive electrodes at a scan rate of

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50 mV s-1 in the same plot. The CV curves of the Co3O4-PANI@ZIF-8NPC//ZIF-8NPC

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supercapacitor in a potential window of 1.5 V at various scan rates from 10 to 100 mV s-1 are shown in Fig. 8 (b). Furthermore, as a result of fast rates of ionic and electronic transport, the

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CV curves of the device do not show significant peaks. The CV curves of the asymmetric

. Even when the potential window is increased up to 1.5 V, the shape of the CV curves shows

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device in an increasing potential window are displayed in Fig. 8 (c) at a scan rate of 50 mV s-

good capacitive behavior without any oxygen evolution reaction. Fig. 8 (d) presents the GCD

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curves at different current densities for the evaluation of the electrochemical performance of

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the ASC device. The GCD curves show a symmetrical shape, signifying the charge balance between both electrodes. According to the GCD curves from Fig. 8 (d), the specific capacitance values are 169, 139, 112, 97, 74 and 60 F g-1 at current densities of 1, 2, 3, 5, 7, and 10 A g-1, respectively. The GCD profiles show an increase in charge-discharge time with increasing applied potential, representing the increase of specific capacitance at an energy density of 2 A g-1 (Fig. 8 (e)), which coincides with the CV results. The cyclic stability test is an important factor for estimating the applied performance of the ASC device. The cyclic stability was investigated at a constant current density of 10 A g-1, as shown in Fig. 8 (f). After the test, 88.43% of the initial specific capacitance remains after 10000 cycles, displaying a long cycling life and a good stability of the electrode material. Even after the 24

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10000 cycle stability test at 10 A g-1, we checked the charge-discharge behavior of the device at 1 A g-1 and compared it with the same GCD value before the stability test, as shown in Fig. 8 (g); the value from the device was 151 F g-1, which was 89.34% of the initial value (169 F g-1 at 1 Ag-1). The coulombic efficiencies of the ASC device at current densities of 1, 2, 3, 5, 7, and 10 A g-1 are 92.1, 93.2, 94.1, 94.5, 94.7, and 95.1%, respectively, when using equation (5). The rate capabilities of the device are 82, 66, 57, 44, and 36% at current densities of 2, 3,

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5, 7 and 10 A g-1, respectively, as presented in Fig. 8(h). Compared to the enhanced

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coulombic efficiency of the Co3O4-PANI@ZIF-8NPC//ZIF-8NPC device, increasing the current density shows a characteristic increase in kinetic reversibility with an uninterrupted

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charge-discharge route [66], as shown in Fig. 8 (h). Nyquist plot of the device before and

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after the stability test is shown in Fig. 8 (i). A comparison of the charge transfer resistance

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(Rct) value is a good method for analyzing the electrochemical performance of the system. While intrinsic resistance (Rs) is constant and independent of the current density, the

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Warburg diffusion resistance (Rw) is frequency reliant [16]. The Co3O4-PANI@ZIF-

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8NPC//ZIF-8NPC device showed excellent supercapacitor performance by exhibiting notable conductivity and fast ion diffusion.

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Fig. 8. (a) CV curves of the negative and positive electrode at a scan rate of 50 mV s-1, (b)

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CV curves at different scan rates, (c) CV curves with an increasing potential window (from 0.9 to 1.5 V at 50 mV s-1), (d) GCD curves at different current densities, (e) GCD curves with an increasing potential window (from 0.9 to 1.5 V at 2 A g-1), (f) cyclic stability at a current density 10 A g-1 with the inset showing the last 10 cycles of the stability test, (g) GCD curves before and after the stability test at 1 A g-1, (e) specific capacitance and coulombic efficiency at different current densities, and (f) Nyquist plots with the inset showing a magnified section of the Nyquist plot for the ASC device.

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The energy densities and power densities of the device were calculated from equations (3) and (4). The maximum energy density is 52.18 W h kg-1 at a power density of 751.51 W kg-1 and an increased power density of 7500 W kg-1 gives an energy density of 18.75 W h kg-1. A schematic illustration of the device is shown in Fig. 9 (b), and a digital photograph is shown in Fig. S10. The energy density and power density of the Co3O4-PANI@ZIF-8NPC//ZIF8NPC supercapacitor are better than those of other related electrode materials (Table S2). For

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example, hierarchical Co3O4/PANI hollow nanocages (Co3O4/PANI//AC) show an energy

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density of 41.5 W h kg-1 at a power density of 800 W kg-1 [54], the ZIF-8NPC-PANI composite symmetric supercapacitor (NPC-PANI//NPC-PANI) shows an energy density of

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21 W h kg-1 at a power density of 1200 W kg-1 [39], the ZIF-67-derived CoOx//NG-A has an

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energy density of 33.89 W h kg-1 at a power density of 500 W kg-1 [67] and ZIF-67 derived

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nanoporous Co3O4//C shows an energy density of 36 W h kg-1 at a power density of 1600 W kg-1 [68]. Fig. 9 (a) presents the Ragone plots of the Co3O4-PANI@ZIF-8NPC//ZIF-8NPC

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supercapacitor, which shows high energy density values over a wide range of power densities

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(751.51 to 7500 W kg-1). These results make the Co3O4-PANI@ZIF-8NPC//ZIF-8NPC ASC device a good alternative for practical applications in the field of energy storage by using special microstructures that enhance structural stability and improve electrical conductivity.

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Fig. 9. (a) Ragone plot and (b) schematic diagram of the as-fabricated ASC device.

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4. Conclusions

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In summary, a Co3O4-PANI@ZIF-8NPC nanocomposite was fabricated by controlling an in situ polymerization of aniline with Co3O4 NFs on the surface of ZIF-8NPC. The combination

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of PANI and Co3O4 with a nanoporous carbon featuring good conductivity and fast ion

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diffusion, provides the Co3O4-PANI@ZIF-8NPC nanocomposite with good electrochemical energy storage ability. The as-prepared nanocomposite electrode shows a specific capacitance

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of 1407 F g-1 at 1 A g-1 which is higher than the values recently reported for similar types of MOF-derived nanocomposite materials. An ASC device provides a high energy densities of 52.81 W h kg-1 (power density of 751.51 W kg-1) and 18.75 W h kg-1 (power density of 7500 W kg-1) with good cyclic stability as evidenced by retaining 89.34% of the initial capacitance after 10000 cycles at 10 A g-1. The high electrochemical performance demonstrates that with controlled engineering, the Co3O4-PANI@ZIF-8NPC nanocomposite can be potentially used as an electrode material for energy storage applications.

Acknowledgments 28

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This work was supported by a program of the National Research Foundation (NRF) of Korea, which was funded by the Ministry of Science, ICT (No. 2017H1D8A2030449) for fostering next-generation researchers in engineering. This work was also supported by a grant from the National Research Foundation (NRF) of Korea, which was funded by the Korean government (MSIT) (No. 2019R1F1A1051574).

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GO/MOFs:

Synthesis,

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Prof. Hak Yong Kim and Kisan Chhetri conceived and designed the experiment. Kisan Chhetri performed the experiments, data collection, data analysis, and manuscript writing. Dr. Arjun Prasad Tiwari and Dr. Alagan Muthurasu were involved in co-supervising of this work. Bipeen Dahal, Tanka Mukhiya and Gunendra Prasad Ojha helped in the experimental data analysis and interpreting the results. Taewoo Kim, Su-Hyeong Chae, and Minju Lee assisted to design the figures and characterization of the samples. All authors discussed the results and

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commented on the manuscript.

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2019.10.14 To The Editor-in-Chief, Journal of Electroanalytical Chemistry The authors have no conflicts of interest to declare.

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Yours sincerely,

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Professor Hak Yong Kim

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Department of BIN Convergence Technology

Department of Organic Materials and Fiber Engineering

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Jeonbuk National University, Jeonju, 561-756, South Korea,

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Email: [email protected].

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Graphical abstract

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Highlights  A unique Co3O4-PANI@ZIF-8NPC nanocomposite is designed for supercapacitor applications.  An asymmetric supercapacitor, Co3O4-PANI@ZIF-8NPC//ZIF-8NPC, was assembled and tested.

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 The synergetic effect between the Co3O4 and PANI-enhanced ZIF-8NPC delivered

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excellent electrochemical performance.

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 The ASC showed a high energy density of 52.81 W h kg-1 at a power density of 751.51 W

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kg-1.

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