PPy NTs composite

Journal Pre-proof Improved OER performance of Co3O4/N-CNTs derived from newly designed ZIF-67/PPy NTs composite

Sumbal Farid, Weiwei Qiu, Jialin Zhao, Xuedan Song, Qing Mao, Suzhen Ren, Ce Hao PII:

S1572-6657(19)31036-7

DOI:

https://doi.org/10.1016/j.jelechem.2019.113768

Reference:

JEAC 113768

To appear in:

Journal of Electroanalytical Chemistry

Received date:

29 October 2019

Revised date:

11 December 2019

Accepted date:

14 December 2019

Please cite this article as: S. Farid, W. Qiu, J. Zhao, et al., Improved OER performance of Co3O4/N-CNTs derived from newly designed ZIF-67/PPy NTs composite, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/j.jelechem.2019.113768

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© 2019 Published by Elsevier.

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Improved OER performance of Co3O4/N-CNTs derived from newly designed ZIF-67/PPy NTs composite Sumbal Farid, Weiwei Qiu, Jialin Zhao, Xuedan Song, Qing Mao, Suzhen Ren*, Ce Hao** State Key Laboratory of Fine Chemicals,Dalian University of Technology, Dalian 116024, Liaoning, China

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Corresponding authors:

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** E-mail address: [email protected] (Ce Hao).

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* Tel:+86 411 84986492; E-mail address: [email protected] (Suzhen Ren).

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Abstract In recent years, a noticeable shifting from conventional fossil fuels to renewable energy based systems has embarked electrochemical splitting of water as the most promising way to produce clean energy of hydrogen. Generally, the lack of active and stable electrocatalysts for oxygen evolution reaction (OER) limits the practicability of water splitting as a renewable source of energy. In this work, interconnected 3D structure of cobalt oxides nanoparticles (Co3O4 NPs) derived from ZIF-67 with nitrogen-doped carbon nanotubes (N-CNTs) of polypyrrole (PPy) origin is successfully synthesized as OER electrocatalysts. Impressively, the highly conductive

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N-CNTs that run through the Co3O4 NPs not only endow the resulting product Co3O4/N-CNTs

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with large active surface area and enhanced charge transfer between Co3O4 NPs, but also prevent Co3O4 NPs from aggregation. As a result, the as-synthesized Co3O4/N-CNTs electrocatalyst

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exhibits outstanding OER activity with a low onset potential of ~ 1.37 V (vs RHE), overpotential of only 200 mV to attain a stable current density of 10 mA cm–2 in basic media and very small

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Tafel slope of 40 mV dec–1. Moreover, the enhanced nitrogen-content also improves the OER

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kinetics by proficient charge transfer. The superb electrocatalytic performance and higher

Keywords:

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stability make the Co3O4/N-CNTs a proficient non-precious electrocatalyst for the OER.

ZIF-67/PPy NTs composites; Polypyrrole nanotubes; Metal organic framework;

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Cobalt oxide/carbon material; Oxygen evolution reaction.

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1. Introduction Currently, to resolve the environmental concerns of the increasing worldwide consumption of fossil fuels, intensive scientific efforts are underway in the exploration of advanced energy conversion devices [1]. The oxygen evolution reaction (OER) plays a pivotal role in watersplitting and many other energy-related technologies involving oxygen electrodes, which require highly efficient electrocatalysts to overcome large potential barrier of OER [2, 3]. Competing with the state-of-the-art noble metal electrocatalysts, earth-abundant transition metals-based compounds are making their place in OER catalysis due to their reasonable reactivity, low cost

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and high stability [4]. Among these, cobalt-based compounds with diverse structures and

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dimensions have sparked worldwide interest owing to the fact that they are earth-abundant, environmentally benign and cost-effective [5]. Nevertheless; the low electronic conductivity and

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durability in basic solutions hinder their further application [6, 7]. To ameliorate their catalytic

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activity, multiple strategies have been designed, e.g., the morphological modification to increase the surface area with higher number of the catalytic sites, higher porosity to favor mass and

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electron transport and the coupling of metal/carbon materials to perk up the electrical conductivity [8, 9]. At present, the prevalent strategy for solving above problem is to fabricate

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cobalt-based structures on conductive substrates (like graphene, carbon nanotubes etc.) owing to their remarkable physicochemical properties [10, 11]. However, these pristine carbon

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nanomaterials are chemicallyinert which usually require surface functionalization to activate

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their surface for the deposition of desired metal-based substances on them [12]. ZIF-67, a typical Co-based MOF, is one of the leading precursors for the fabrication of highly dynamic Co-based electrocatalysts due to its facile synthesis and high porosity with tunable pore structure [13-15]. The carbonization of ZIF-67 usually resulted incobalt/carbon materials holding good catalytic activities owing to the fast rate of mass transfer and the high exposure of the active sites [15-17]. Unfortunately, relatively poor electronic conductivity and lower graphitic degree-factors affect badly the electrocatalytic performance of these materials [18]. And polypyrrole (PPy) has been extensively investigated for adsorption of various metal ions because of its renowned nitrogen-containing species and the negative charges in solution. In view of this point, PPy nanostructures might be a wonderful substrate to grow ZIF-67 by adsorbing positively charged cobalt ions and later bridging them with organic linkers [1]. Moreover, nitrogen of PPy

Journal Pre-proof also serves as the heteroatoms in the N-doped carbon nanomaterials formed after carbonization [19]. Increasing the N-containing species and graphitic nature of the carbon matrix may lessen the overpotential, hence, improved catalytic activity [20]. To achieve the superior electrocatalytic performanceof ZIF-67-derived materials in OER, herein, we report a simple strategy to amplify their overall electrical conductivity by employing conductive PPy nanotubes for in situ growth of interconnected framework of ZIF-67 particles. Carbonization of this ZIF-67 threaded with PPy nanotubes (ZIF-67/PPy NTs) will result in

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cobalt oxides threaded with N-doped hollow carbon nanotubes of PPy origin (Co3O4/N-CNTs). The insertion of PPy into ZIF-67 is expected to increase the surface area and also provide an

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alternative electron pathway for fast electron transfer in the resulting Co3O4/N-CNTs. When evaluated for OER, such an easily obtained electrocatalyst (Co3O4/N-CNTs) shows high

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electrocatalytic activity; offering a small onset potential, large anodic currents and long-term

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stability in alkaline media. The low production cost and achieved higher electrocatalytic activity make the Co3O4/N-CNTs a preferred substituteto precious noble-metal based electrocatalysts in

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water splitting.

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2.1 Materials

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2. Experimental

Cobalt nitrate hexahydrate was purchased from Kermel. 2-methylimidazole was purchased from

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Tianjin Guangfu Fine Chemical Research Institute. Pyrrole was purchased from Aladdin. Methyl orange and Iron chloride hexahydrate were purchased from Tianjin Damao Fine Chemical Co., Ltd. Ammonia solution and ethanol were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. All these chemical reagents are of analytical grade and have been used without further purification. 2.2 Synthesis of PPy nanotubes PPy NTs were prepared at room temperature employing a method reported previously with little modification [21]. Briefly, a specific amount of FeCl3 (1.5 mmol) was added into the aqueous solution (30 mL) of methyl orange (MO) (5 mmol). The fibrous flocculant, MO-FeCl3 precipitated out immediately. After 30 min, pyrrole monomer (1.5 mmol) was added slowly and

Journal Pre-proof the mixture was then stirred for 24 h at room temperature. The resulted black precipitates were collected and washed with deionized water and ethanol several times until the filtrate turned out to be colorless and then dried under vacuum at 60 °C. 2.3 Synthesis of ZIF-67 ZIF-67 was synthesized at room temperature using a previously reported procedure with modification [22]. First, a homogeneous solution of cobalt nitrate hexahydrate (1.1641 g) was formed in 30 mL methanol under continuous stirring; 2-methylimidazole (1.3122 g) was dissolved in 30 mL methanol and continuously stirred for 30 min. Next, the latter clear solution

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of 2-methylimidazole was added drop wise into the former pink solution. The resulting solution

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was stirred for next 30 min and then kept static for 24 h at room temperature. The purple precipitate was collected and washed with ethanol several times and dried at 60 °C in a vacuum

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oven. 2.4 Synthesis of ZIF-67/PPy nanotubes

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To prepare ZIF-67/PPy NTs, PPy nanotubes (0.02 g) were dispersed in 20 mL of methanol via

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ultrasonication for 1 h and then cobalt nitrate (1.1641 g) was added into above dispersion and stirred for 1 h to finely disperse the Co2+ ions on PPy nanotubes. Next, a methanolic solution (30

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mL) of 2-methylimidazole (1.3122 g) was poured slowly into the above solution. After being stirred for 30 min, the above solution was kept static for 24 h. The obtained product ZIF-67/PPy

vacuum.

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NTs was washed using ethanol and water several times and dried overnight at 60 °C under

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2.5 Synthesis of Co3O4/N-CNTs

Fig. 1 illustrates the typical fabrication process of Co3O4/N-CNTs from ZIF-67/PPy NTs. The assynthesized ZIF-67/PPy NTs was laden in a ceramic boat and putt into a tube furnace having a heating rate of 5 °C min–1 and maintained at 600 °C for 2 h in the air to get the final product, labeled as Co3O4/N-CNTs. For comparison, ZIF-67 and PPy nanotubes were also carbonized under same conditions to get ZIF-67-H and N-CNTs. It is noteworthy that the selected calcination temperature is thoroughly based on careful thermogravimetric analysis. 2.6 Synthesis of pristine Co3O4 As a control catalyst, pristine Co3O4 was prepared following a method reported previously with minor modification [23]. First, cobalt nitrate (0.58 g) was dissolved in 20 mL of water and stirred for 30 min. After that, 20 mL of ammonia solution (25%) was added into the above solution and

Journal Pre-proof stirred for the next 30 min. Then, the mixture was transferred into a Teflon-lined stainless steel autoclave and followed by heating at 180 °C for 12 h. Afterward, it was cooled down naturally and the obtained product was thoroughly washed with water and ethanol for several times. Asprepared product was then dried at 60 °C for 24 h in the oven and subsequently annealed at 500 °C for 2 h in the air to obtain final black product Co3O4. 2.7 Characterization The morphology and structural characterization of various samples were studied via Scanning electron microscopy (SEM), Thermogravimetric analysis (TGA), Fourier transform infrared

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spectroscopy (FT-IR), Raman spectroscopy, X-ray diffraction (XRD) and X‐ray photoelectron

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spectroscopy (XPS). To check the morphology, SEM images of different samples were obtained using QUANTA 450 at 20 kV. And to inspect the chemical composition of samples, Energy

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dispersive spectroscopy (EDS) was carried out together with SEM. TGA analysis was done on TGA-Q50 in the air (40 mL min–1) with a linear heating rate of 10 °C min–1 to examine the

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thermal stability of prepared samples. Further, the FT-IR spectroscopy was performed on

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Bruker-Tenson 27 FT-IR spectrometer in the 4000–400 cm–1 range to find out typical functional groups of the various samples. XPS data were obtained using a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer with a monochromic Al X‐ray source. XRD

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analysis was performed using PANalyticals X’pert power diffractometer, Netherlands. To

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evaluate the porosity and pores size distribution of all samples,N2 adsorption-desorption measurements were also done. The specific surface area was concluded by the Brunauer-

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Emmett-Teller (BET) method via a micromeritics JWBK 122 W instrument. The pore size distribution was measured using the Barrett-Joyner-Halenda (BJH) method. 2.8 Electrocatalytic measurements Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were performed on CHI 760E (Chenhua, China) with a standard three‐electrode system in 1.0 M KOH solution at room temperature. A glassy carbon electrode (GCE, geometric area of 0.197 cm2, 5 mm in diameter) was employed as the working electrode mounted on a rotating apparatus. A Pt wire electrode was used as the counter electrode, and the Ag/AgCl was used as the reference electrode. Proceeding to each experiment, oxygen was bubbled into the KOH electrolyte for 15 minute and its flow was uphold during the whole measurement to ensure O2 saturation. The CV was carried out at various scan rates while LSV measurement was carried out at a scan rate of

Journal Pre-proof 5mV s–1. A resistance test was done before each measurement and the iR compensation was made via the CHI software. The modified-GCE (working electrode) was prepared as follow: GCE laden with as‐synthesized Co3O4/N-CNTs was utilized as the working electrode. Before use, the GCE was polished with 0.05 mm alumina slurries and then ultrasonicated in acetone solution for 5 min prior to the electrochemical measurements. After that, 10 μL of catalyst ink suspension, synthesized by dispersing 2 mg of the catalytic material into 1 mL of ethanol including 10 μL 5% Nafion, was drop-casted on the GCE to obtain different catalyst loading (0.10, 0.15, 0.20, 0.25 mg cm–2). Next, the electrode left at room temperature to dry for 10 h and

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ready to use.

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2.9 Electrical double layered capacitance (Cdl): To calculate Cdl, the CV measurements were done in a non-Faradic region (1.01–1.14 V vs RHE) in 1.0 M KOH solution at different scan

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rates of 2, 4, 6, 8, 10 mV s–1. After that, the capacitive current density (j|ja-jc|) was calculated at fixed potential of 1.10 V vs RHE and plotted verses the CV scan rates to obtain a straight line

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3. Results and discussion

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curve, the slope of which is the equivalent to the Cdl (mF cm–2) [19, 23].

Herein, ZIF-67 was chosen as a typical MOF to synthesize the cobalt-based OER electrocatalysts

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since it can be prepared on a large scale via a low cost, simple and green approach. The presynthesized PPy nanotubes were served as a substrate to successfully grow ZIF-67 by adding

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Co2+ and 2-methylimidazole sequentially. The free Co2+ ions would be easily adsorbed onto the

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PPy NTs via electrostatic attraction to serve as nucleation hubs for ZIF-67 growth. During the carbonization process at 600 °C in the air for 2 h, the cobalt ions of ZIF-67 were transformed into Co3O4, while the nearby N-containing PPy NTs were pyrolyzed to N-doped graphitic carbon, resulting in well dispersed Co3O4 nanoparticles embedded in the N-doped graphitic carbon [24]. Fig. 1 depicts the complete procedure of Co3O4/N-CNTs synthesis from ZIF-67/PPy NTs.

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Fig. 1. Schematic design for the synthesis of the Co3O4/N-CNTs

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3.1 Morphology and structural characterization

As shown clearly by SEM images in Fig. 2(a,b), hollow PPy NTs penetrated and intertwined

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with the ZIF-67 polyhedra displaying a reticular structure, which can not only protect the ZIF-67 polyhedra from aggregation but also rope the ZIF-67 polyhedra jointly. After heat treatment, in

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the resulting product Co3O4/N-CNTs, the ZIF-67 transformed into Co3O4 still shows polyhedral

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morphology but with irregular facets and a bit shrunk size seen in Fig. 2(c,d). Further, PPy NTs converted into N-doped porous carbon nanotubes (N-CNTs). The roughness of Co3O4/N-CNTs surface may initiate from C, N or H elements vaporization during the carbonization process [17]. This shows the transformation of PPy components into the N-doped graphitic carbon and cobalt ionsfrom ZIF-67 into evenly distributed Co3O4, which in situ encapsulated in the porous carbon matrix. A catalyst with rough surface usually means high surface area; exposing more surface active sites accessible for oxygen containing intermediates, thus leading to the considerable improvement in electrocatalytic performances [25]. Elemental analysis (EDS) was additionally performed to inspect the elemental composition of ZIF-67/PPy NTs and Co3O4/N-CNTs as given in Fig. 2(e,f), which confirmed the presence of C, N, O, and Co elements. Moreover, the EDS analysis data also revealed that after carbonization process of the as-prepared ZIF-67/PPy NTs

Journal Pre-proof precursor, the Co metal content lifted up from 9.44 wt% of ZIF-67/PPy NTs to 13.27 wt% of Co3O4/N-CNTs. The increase of Co content can be ascribed to the loss of lighter components during pyrolysis of ZIF-67/PPy NTs, but the heavier Co element enduring in the final product [22, 26].

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5 mm

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1 mm

(d)

(e)

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(c)

3 mm

5 mm

Element C-K

Weight (%) 66.06

Atomic (%) 76.17

N-K

7.42

6.41

O-K

17.08

Co-K

9.44

Total

100.00

(f)

Element C-K

Weight (%) 56.08

Atomic (%) 69.72

N-K

9.43

6.48

15.22

O-K

21.12

20.21

2.20

Co-K

13.27

3.59

Total

100.00

Journal Pre-proof Fig. 2. SEM images of (a,b) ZIF-67/PPy NTs, (c,d) Co3O4/N-CNTs, and EDS spectrum of (e) ZIF-67/PPy NTs, (f) Co3O4/N-CNTs FT-IR spectra were observed to illustrate the different components of as-prepared samples and to verify the formation of Co3O4/N-CNTs as shown in Fig. 3(a). For ZIF-67, the FT-IR band at 1567 cm–1 is assigned to the C=N stretching vibrationin imidazole ring. Meanwhile, the FT-IR band at 1455 cm–1 corresponds to C–C stretching vibration between the methyl group and carbon in imidazole ring. At 1425 cm–1, “out of plane” bending mode of the methyl groups can be

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observed. Furthermore, the bands in the range of 1013–1430 cm–1 are ascribed to the entire ring

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stretching mode of imidazole ring. The FT-IR bands of variable intensities at 995, 757, 694 cm–1 belong to the vibration mode of imidazole rings, which were generated by the C–H bonds

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bending vibrations of benzene. The presence of a strong band at 425 cm –1 further confirms the formation of Co–N bond [22]. For PPy NTs, a broad FT-IR band at 1554 cm–1 is attributed to C–

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C stretching modes in the pyrrole ring, while the band at 1476 cm–1 corresponds to C–N stretching mode in the ring. In the range of 1250–1000 cm–1 for the C–H and N–H in-plane

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vibrations, a broad band with maximum at 1181 cm–1 is well detected in the spectrum of PPy NTs. A characteristic band at 1041 cm–1 corresponds to the C–H and N–H in-plane vibrations

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and of the C–C out-of-plane ring vibrations mode appears at 969 cm–1. A band at about 917 cm–1 is attributed to out-of-plane C–H deformation vibrations of the pyrrole ring in the FT-IR

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spectrum of the PPy NTs [27, 28]. In the FT-IR spectrum of ZIF-67/PPy NTs, distinct bands of

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ZIF-67 and PPy NTs both are retained. Noticeably, resonance peaks in the range of 694–1567 cm–1 completely vanish in the spectrum of the Co3O4/N-CNTs, signifying that the imidazole rings are entirely collapsed during the calcination process and new distinct bands appear at 560 cm–1 and 658 cm–1 assigning to Co–O vibrations of cobalt oxide [29]. Moreover, for both the Co3O4/N-CNTs and N-CNTs, a broad band appears with maximum value at about 1635 cm–1 for N-CNTs formed from PPy NTs attributing to C=C stretching vibrations which indicates the presence of graphitic carbon [30, 31]. FTIR spectrum of the pristine Co3O4 shows two strong peaks at 663 cm−1 and 569 cm−1 corresponding to stretching and bending frequencies of Co–O, respectively [29].

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(a)

Co3O4/N-CNTs

Co3O4

80 Removal of solvent

Weight loss (%)

Transmittance (%)

(b)

100

ZIF-67-H ZIF-67/PPy NTs N-CNTs ZIF-67

60

40

ZIF-67/PPy NTs PPy NTs ZIF-67

20

Degradation of organic components

PPy NTs 1000

1500

100

2000

-1 Wavenumber (cm )

*

Co3O4 *

*

*

*

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ZIF-67-H

Intensity (a.u.)

Co3O4/N-CNTs

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*

Intensity (a.u.)

(d)

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*

300

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C

*Co3O4 *

200

400

500

600

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D

G

ID/IG = 0.93

N-CNTs

ID/IG = 0.99

PPy NTs ZIF-67-H ZIF-67/PPy NTs

ID/IG = 0.74

Co3O4@N-CNTs

PPy NTs

ZIF-67

ZIF-67 30

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50

60

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20

2 (degree)

800



ID/IG = 1.04

ZIF-67/PPy NTs

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Temperature ( C)

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500

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1000

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-1 Raman shift (cm )

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Fig. 3. (a) FTIR spectra of the ZIF-67, PPy NTs, ZIF-67/PPy NTs, ZIF-67-H, N-CNTs, Co3O4, and Co3O4/N-CNTs, (b) TGA curves of the ZIF-67, PPy NTs and ZIF-67/PPy NTs in the air, (c) XRD pattern of the ZIF-67, PPy NTs, ZIF-67/PPy NTs, ZIF-67-H, Co3O4, and Co3O4/N-CNTs, (d) Raman spectra of the ZIF-67, PPy NTs, ZIF-67/PPy NTs, ZIF-67-H, N-CNTs, and Co3O4/NCNTs

To estimate the optimum carbonization temperature for the fabrication of Co3O4/N-CNTs, TGA measurements were performed in the air as shown in Fig. 3(b). For ZIF-67, gradual weight loss of about 8 % is observed below 300°C related to the removal of unreacted species and guest molecules. Followed by a slow weight loss of about 12 % up to 400 °C where organic ligand begins to decompose. As the temperature increased, ZIF-67 precursors decompose sharply up to

Journal Pre-proof 600 °C into Co and C. TGA curve of the PPy NTs first shows the sudden weight loss of about 8 % below 100 °C related to the loss of trapped solvent molecules and unreacted species. Followed by gradual weight loss of about 24 % up to 800 °C, PPy NTs decomposes into N-CNTs [27]. A very small weight loss of only 2 % is observed for ZIF-67/PPy NTs up to 400 °C involving loss of solvent molecules. It demonstrates that ZIF-67/PPy NTs is quite stable up to 400 °C in the air. As the temperature increases from 400 °C, the weight of ZIF-67/PPy NTs declined quickly. It shows the decomposition of ZIF-67 precursors into Co3O4 and transformation of PPy NTs into

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N-CNTs. XRD analysis of the as-prepared samples was performed for structural characterization as shown

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in Fig. 3(c).The XRD pattern of the as-prepared ZIF-67 is in good concord with previously reported results [22], demonstrating that the ZIF-67 polyhedra are highly crystalline and phase-

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pure. A distinct broad peak of PPy NTs with a maximum value at 21.3° indicates the amorphous

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nature of PPy [32]. ZIF-67-H shows broadened peak corresponding to amorphous carbon and less fitted peaks of cobalt oxides [33]. ZIF-67/PPy NTs spectrum displays intense diffraction

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peaks at the same positions as ZIF-67, confirming the presence of highly crystallized and phasepure ZIF-67 in ZIF-67/PPy NTs [34]. The incorporation of PPy NTs does not disturb the ZIF-67

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crystal structure. As the diffraction peaks intensity of PPy NTs is too little compared with that of the ZIF-67, the diffraction pattern of PPy NTs is not observed clearly in ZIF-67/PPy NTs [1]. For

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Co3O4/N-CNTs, the major diffraction peaks at 2 of 36.5°, 42.3°, 61.4°, 73.6° and 77.4° are attributed to Co3O4 phase in agreement with previous reports and literature [22]. Moreover, a

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relatively sharp peak related to graphitic carbon appears at 26.1° in Co3O4/N-CNTs [35-37]. This result suggests the higher graphitization degree in Co3O4/N-CNTs, which is further supported by the lower ID/IG value of Co3O4/N-CNTs than that ofPPy NTs and ZIF-67/PPy NTs as shown in Raman spectra given below. So, these results strongly advocate the complete chemical transformation of ZIF-67/PPy NTs into Co3O4/N-CNTs. In order to further explore the graphitic level of the as-synthesized samples, Raman spectra were obtained as shown in Fig. 3(d). Raman spectra of Co3O4/N-CNTs, PPy NTs, N-CNTs, and ZIF67/PPy NTs revealed two characteristic bands at 1344 cm–1 and 1579cm–1 that are assigned to D and G bands, respectively, which are absent from the spectra of ZIF-67 and ZIF-67-H. The G band corresponds to graphitic in-plane stretching vibrations of sp2 carbon atoms and the D band

Journal Pre-proof indicates the defective or disordered graphitic carbon [22]. The intensity ratio of D and G bands (ID/IG) is a useful index to evaluate the structural defects and edge exposure in carbon-based materials [20]. For the Co3O4/N-CNTs, the ID/IG value is about 0.74, which is lower than that of PPy NTs (0.99), N-CNTs (0.93) and ZIF-67/PPy NTs (1.04). This indicates the presence of more long-range disordered graphitic carbon in Co3O4/N-CNTs than other samples [38], which is also in agreement with the characteristic diffraction peak at 26.1° in the XRD pattern. To analyze the nature of the porosity, pores size distribution and the specific surface area of the

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as-prepared materials, N2 adsorption-desorption isotherms were recorded at 77 K. It is observed that Co3O4, N-CNTs, ZIF-67/PPy NTs, and Co3O4/N-CNTs displayed distinct type IV adsorption

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isotherm with the specific surface areas of 136, 699, 917 and 1213 m2 g–1, respectively as shown in Fig. 4(a) [39]. Among them, the sample Co3O4/N-CNTs possesses a distinctive H3 hysteresis

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loop, which is a characteristic feature of mesoporous materials [40]. Notably, the mesoporous

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structure with a large number of holes can also offer abundant ion-diffusion paths that endow fast mass/charge transfer. In addition, Co3O4/N-CNTs have a fairly larger pore volume of 0.180

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m3 g–1 and larger pore size of 1.7, 4.3 and 16.4 nm (reveals hierarchical porous structure) as compared to N-CNTs (0.159 m3 g–1, 0.5 nm), Co3O4 (0.029 m3 g–1, 2–10 nm) and ZIF-67/PPy

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NTs (0.168 m3 g–1, 0.2–10 nm) (Fig. 4(b). The presence of a hierarchical porous structure of Co3O4/N-CNTs might increase the interfacial contact between the catalyst and KOH electrolyte,

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which may provide efficient penetration of the electrolyte into the active materials. Hence, the diffusion kinetics within the electrode greatly enhances, which in turn favors the catalysis.

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Moreover, larger pore size and high pore volume are beneficial for the movement of reactants and products leading to high catalytic activity [41]. Besides, the specific surface area of the Co3O4/N-CNTs upon carbonization was increased as compared to the precursor ZIF-67/PPy NTs. So, the catalytic performance improves, which possibly due to the more Co3O4 exposed on the outer surface of the Co3O4/N-CNTs, thus resulting in enhanced catalytic performance [22].

Journal Pre-proof 0.18

(a)

N-CNTs ZIF-67/PPy NTs Co3O4

(b)

0.14

Co3O4/N-CNTs

-1

400

dV/dD (cm3 g

300 200 100

0.12

Co3O4

0.10

Co3O4/N-CNTs

0.08

N-CNTs ZIF-67/PPy NTs

0.06 0.04 0.02 0.00

0 0.0

0.2

0.4

0.6

0.8

1.0

0

Relative pressure (P/Po)

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20

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3 -1 Volume adsorbed (cm g )

500

0.16

-1 nm )

600

30

40

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Pore size (nm)

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Fig. 4. (a) N2 adsorption-desorption isotherms of Co3O4, ZIF-67/PPy NTs, N-CNTs and

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Co3O4/N-CNTs, (b) Corresponding BJH pore size distribution curves of Co3O4, ZIF-67/PPy

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NTs, N-CNTs and Co3O4/N-CNTs

Co3O4

Total pore volume

(m2 g–1)

(nm)

(cm3g–1)

699

0.5

0.159

136

2–10

0.029

917

0.2–10

0.168

1213

1.7, 4.3, 16.4

0.180

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ZIF-67/PPy NTs

Pore size

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N-CNTs

BET surface area

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Sample

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Table 1. Microstructural properties of N-CNTs, Co3O4, ZIF-67/PPy NTs, and Co3O4/N-CNTs

Co3O4/N-CNTs

XPS characterization was done to further investigate the chemical bonding and surface electronic characteristics of the as-synthesized Co3O4/N-CNTs. The full XPS elemental survey confirmed the existence of C, N, O, and Co by their noticeably observed peaks as shown in Fig. 5(a). Fig. 5(b) shows the deconvoluted C1s spectrum fitted by four peaks at 284.8, 285.7 and 287.8 eV, which are attributed to graphitic carbon (C–C/C=C), surface nitrogen and oxygen groups (denoted as C–N, and C–O/C=O, respectively). The additional peak at 291.4 eV represents nitrogen-doped carbon materials [20]. Recent findings reveal that in addition to graphitic carbon,

Journal Pre-proof ketonic C=O species in carbon matrix can also modify the electronic structures of the nearby carbon atoms and promotes the adsorption of intermediates during water oxidation, thus influence the OER activity positively [42]. Fig. 5(c) shows that the N 1s spectrum is deconvoluted into three peaks centered at 398.7, 399.6 and 400.8 eV, which are assigned to the pyridinic-N, pyrrolic-N, and graphitic-N, respectively [15]. Pyridinic nitrogen is a key indicator of sp2-nitrogen coordinated to two carbon atoms [18], which is also bonded to a cobalt ion. Notably, the occurrence of graphitic N is an indication of the in situ growth of N‐doped graphitic carbon. These two kinds of nitrogen provide active sites to promote OER. Since Co coordinated

of

to N can change the electronic state of carbon in Co-N-C moieties via a strong electron

ro

withdrawing effect. This results in the decreased electron density of adjoining C atoms, hence facilitating the adsorption of various OER intermediates and boost up the electron transfer with

-p

improved kinetics. Moreover, the graphitic-N species in carbon matrix may interrupt the electroneutrality of their adjoining carbon atoms, and trigger these carbon atoms as additional active

re

sites for the adsorption of OER intermediates and thus facilitate the OER [43]. The

lP

deconvolution of O 1s spectrum is attained with the three fitted peaks as shown in Fig. 5(d). The fitting peak at 529.4 eV is characteristic of the cobalt–oxygen bonds [20]. The deconvoluted

na

peak at 530.9 eV assigns to the chemisorbed oxygen atoms on or within the surface. Moreover, the well resolved peak at 531.6 eV corresponds to the low oxygen coordination with large number of defect sites [14, 16]. In the deconvoluted Co 2p spectra, the two major peaks at 781.2

ur

and 796.9 eV are assigned to Co 2p3/2 and Co 2p1/2 as shown in Fig. 5(e). The energy difference

Jo

of 15.7 eV between Co 2p3/2 and Co 2p1/2 spin-orbit splitting demonstrates the presence of low spin Co2+ [22]. The fitted peak at 781.2 eV with its satellite peak at 786.2 eV is in accordance with Co 2p3/2 binding energy of Co3O4; whereas the peak at 796.9 eV with its satellite peak at 802.6 eV is in accordance with Co2p1/2 binding energy of Co3O4 [44]. This result further verifies that Co3O4 is the only type of the cobalt element in the as-prepared catalyst.

Journal Pre-proof

(a)

(b)

Co 2p3/2

Co3O4/N-CNTs

C 1s

C-C/C=C C-N

Co 2p1/2 C 1s

Intensity (a.u.)

O 1s

C-O/C=O N 1s

C=N

Co 3p Co 3s

200

400

600

800

1000

282

Binding energy (eV)

286

288

290

292

Binding energy (eV)

ro

(c)

284

of

0

N 1s

(d)

O (chemisorbed) O-C

re

Pyrrolic N

O 1s

Co-O

-p

Pyridinic N

394

396

398

na

lP

Graphitic N

400

402

404

406 525

(e)

ur

Binding energy (eV)

Co

3+

Co

2+

531

534

537

540

Binding energy (eV)

Co 2p

Jo

Co 2p3/2

528

Co 2p1/2

Co

3+

Co

2+ Sat.

Sat.

775

780

785

790

795

800

805

810

Binding energy (eV)

Fig. 5. XPS survey scan of (a) Co3O4/N-CNTs and high resolution XPS spectra of (b) C1s (c) N1s (d) O 1s (e) Co 2p

Journal Pre-proof

3.2 Electrocatalytic measurements To evaluate the electrocatalytic OER performance of the as-prepared Co3O4/N-CNTs electrocatalyst, LSV is carried out in a standard three electrode system with Co3O4/N-CNTs deposited carbon electrode as the working electrode, Ag/AgCl as the reference electrodeand platinum wire as the counter electrode. For comparison, LSV curves of Co3O4, N-CNTs, ZIF67/PPy NTs, and commercial RuO2 are also assessed under similar conditions. The polarization curves of various samples obtained in 1.0 M KOH at a scan rate of 5 mV s–1 are shown in Fig.

of

6(a). The polarization curve of Co3O4/N-CNTs affords a smaller onset potential (~ 1.37 V) and the highest OER activity than that of others by holding a very small overpotential of 200 mV at

ro

10 mA cm–2 as shown in Table 2. Tafel plots of the different electrodesare obtained by fitting

-p

their corresponding polarization curves, shown in Fig. 6(b). The Co3O4/N-CNTs displays a Tafel slope of 40 mV dec–1, which is much smaller than that of N-CNTs (165 mV dec–1), Co3O4 (147

re

mV dec–1), ZIF-67/PPy NTs (124 mV dec–1) and commercial RuO2 (57 mV dec–1), which

lP

suggests faster OER kinetics on the surface of Co3O4/N-CNTs electrode [45]. Another critical property for OER electrocatalysts is their long-term stability [46]. The stability test for Co3O4, N-

na

CNTs, ZIF-67/PPy NTs, RuO2, and Co3O4/N-CNTs was performed by a chronoamperometric technique. After continuous testing for 24 h at constant potential of 200 mV in 1 M KOH, the Co3O4/N-CNTs exhibits a limited current attenuation (6 %), whereas, the current loss for other

ur

samples is more than 20 %. These results suggest that the sample Co3O4/N-CNTs possesses

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excellent stabilities during the OER, as shown in Fig. 6(c). In addition, the Co3O4/N-CNTs also displays higher stability by upholding OER activity after 1000 cycles of potential sweep in 1 M KOH solution as shown in Fig. 6(c) inset. EIS was additionally carried out to look deep into the conductive properties and the electron transfer kinetics of as-synthesized materials as it plays a vital role in the OER performance. The data obtained from EIS can be explained through a Nyquist plot, which is the plot of imaginary portion of the impedance (Z') versus realportion of the impedance (Z"). More significantly, the semicircular part in the higher frequency region describes the charge transfer resistance (Rct), which arises mainly due to the charge transfer processes of the electrode materials [47, 48]. From the Nyquist curves given in Fig. 6(d), it can be clearly seen that Co3O4 and ZIF-67/PPy NTs display semicircular region whereas the N-CNTs and Co3O4/N-CNTs show almost no semicircular region, representing low Rct and thus high

Journal Pre-proof electronic conductivity. Amongst all, the Co3O4/N-CNTs electrocatalyst show straighter up line at low frequency region indicating low diffusion resistance for ions [31, 49]. This may arise due to the significant synergistic effect between cobalt oxides and N-doped carbon matrix suggesting the enhanced electron transport and fast electrocatalytic kinetics in the OER, thus partly accounting for the better OER performance of Co3O4/N-CNTs [46, 50].

Co3O4/N-CNTs

200

Co3O4 N-CNTs RuO2

150

-1

0.45 0.40 0.35

N-CNTs

0.30

RuO2

165 mV dec

-1

124 mV dec

-1

Co3O4

0.25

re

50

0

57 mV dec

ZIF-67/PPy NTs Co3O4/N-CNTs

-1

40 mV dec

-1

0.20

1.2

1.4

1.6

1.8

Potential (V vs RHE) 100

2.0

2.2

lP

1.0

40

1st Cycle 1000th Cycle

150

100

Jo

60

ur

200

50

0.6

0.8

1.0

1.2

1.4

RuO2 N-CNTs Co3O4 ZIF-67/PPy-NTs

1.8

(d) Co3O4/N-CNTs N-CNTs ZIF-67/PPy NTs Co3O4

5

Co3O4/N-CNTs

1.6

-2 Log j (mA cm )

6

na

80

0.4

7

(c)

Current density (mA cm-2)

Normalized current (%)

147 mV dec

-p

ZIF-67/PPy NTs

100

(b)

ro

Overpotential (V vs RHE)

250

- Z'' (Ohm)

Current density (mA cm-2)

0.50

of

(a)

300

4

CPE

Rs

3

Wo

Rct

2

Co3O4/N-CNTs 0 1.0

1.2

1.4

1.6

1.8

1

2.0

Potential (V vs RHE)

20 0

6

12

Time (h)

18

24

0 0

1

2

3

4

5

6

Z' (Ohm)

Fig. 6. (a) Polarization curves of N-CNTs, Co3O4, RuO2, ZIF-67/PPy NTs, and Co3O4/N-CNTs, in 1.0 M KOH solution with a scan rate of 5 mV s–1 at 1600 rpm, (b) Tafel plot of N-CNTs, Co3O4, RuO2, ZIF-67/PPy NTs, and Co3O4/N-CNTs, (c) Chronoamperometric responses of Co3O4/N-CNTs, Co3O4, RuO2, N-CNTs, and ZIF-67/PPy NTs at the fixed potential of 200 mV,

Journal Pre-proof inset(c), Cyclic stability of Co3O4/N-CNTs from LSV, (d) Nyquist plots of the Co3O4,N-CNTs, ZIF-67/PPy NTs, and Co3O4/N-CNTs Table 2. OER activity of different samples Sample

Onset potential

Overpotential (mV)

(V)

1.55

ZIF-67/PPy NTs

1.43

Co3O4/N-CNTs

1.37

of

Co3O4

57

440

165

ro

1.57

400

147

360

124

200

40

-p

N-CNTs

340

re

1.54

at 10 mA cm

(mV dec–1)

lP

RuO2

–2

Tafel slope

It is well known that the high electrocatalytic activity of various electrode materials may be due

na

to their higher electrochemical surface areas (ECSAs). As ECSAs can beindirectly described by double-layer capacitance, we obtained the ECSAs of N-CNTs, Co3O4, RuO2, ZIF-67/PPy NTs,

ur

and Co3O4/N-CNTs by calculating their double-layer capacitances (Cdl) [23, 51]. Cdl was

Jo

obtained through cyclic voltammetry (CV) at various scan rates (2, 4, 6, 8, 10 mV s–1) in nonFaradaic region in the range of 1.01 to 1.14 V vs. RHE, Fig. 7(a,b,c,d,e). For different samples, double layer charging current was calculated from the CV curves at a fixed potential of 1.10 V vs. RHE and then plotted verses the scan rates to get a straight line as shown in Fig. 7(f). The slope of current~scan rate is the value of Cdl. As shown in Fig. 7(f), values of Cdl for ZIF-67/PPy NTs, N-CNTs, RuO2, Co3O4, and Co3O4/N-CNTs are 1.67, 1.45, 3.46, 0.49, and 7.21 mF cm–2, respectively. Hence, ECSAs of Co3O4/N-CNTs is larger than that of others, being linked with larger accessible surface area and more available active sites, which could improve the OER performance [52].

Journal Pre-proof 12.5

(a)

20

(b) N-CNTs

10.0

Co3O4 7.5

-2

Current density (mA cm )

10 5 0

-1 2 mV s -1 4 mV s -1 6 mV s -1 8 mV s -1 10 mV s

-5 -10 -15 1.00

1.02

1.04

1.06

1.08

1.10

1.12

5.0 2.5

-1 2 mV s -1 4 mV s -1 6 mV s -1 8 mV s -1 10 mV s

0.0 -2.5 -5.0 -7.5

1.14

1.00

1.02

Potential (V vs RHE) 20

RuO2

0

-1 2 mV s -1 4 mV s -1 6 mV s -1 8 mV s -1 10 mV s

-10 -20

-40 1.00

1.02

1.04

1.06

na

-30

10

1.14

ZIF-67/PPy NTs

1.08

1.10

1.12

0

-1 2 mV s -1 4 mV s -1 6 mV s -1 8 mV s -1 10 mV s

-5

-10 -15 -20 1.00

1.14

1.02

1.04

100

Co3O4/N-CNTs

-2

0 -20 2 mV s

-40

4 mV s

-60

6 mV s 8 mV s

-80 1.00

-1 -1 -1

1.04

1.06

1.08

1.10

1.12

1.10

Co3O4/N-CNTs

1.12

1.14

-1

1.14

Potential (V vs RHE)

-2 7.21 mF cm

60

RuO2

40

ZIF-67/PPy NTs

20

-1

10 mV s

1.02

j (mA cm )

20

1.08

(f)

80

Jo

40

1.06

Potential (V vs RHE)

ur

(e)

60 -2

1.12

5

Potential (V vs RHE)

Current density (mA cm )

1.10

re

10

80

1.08

-p -2

20

lP

-2

Current density (mA cm )

15

(d)

ro

(c)

30

1.06

Potential (V vs RHE)

Current density (mA cm )

40

1.04

of

-2

Current density (mA cm )

15

0 2

4

-2 3.46 mF cm -2 1.67 mF cm

N-CNTs

-2 1.45 mF cm

Co3O4

-2 0.49 mF cm

6

-1 Scan rate (mV s )

8

10

Fig. 7. CV curves of (a) N-CNTs, (b) Co3O4, (c) RuO2, (d) ZIF-67/PPy NTs, and (e) Co3O4/NCNTs in 1.0 M KOH solution at various scan rates, (f) Cdl measurement at1.10 V of ZIF-67/PPy NTs, N-CNTs, RuO2, Co3O4, and Co3O4/N-CNTs

Journal Pre-proof

In addition, to investigate the effect of different catalyst loading on the OER activity of Co3O4/NCNTs, LSV is performed with the different amount of Co3O4/N-CNTs deposited on a carbon electrode as the working electrode, Ag/AgCl as the reference electrode and platinum wire as the counter electrode in standard three electrode system and the results are shown in Fig. 8(a). Tafel plots obtained from the corresponding polarization curves are also given Fig. 8(b). The obtained data clearly showed that with the higher amount of catalyst loading, higher electrocatalytic activity can be obtained as seen in Table 3. This may arise due to the fact that large amount of

(a)

(b)

Catalyst's loading 0.25 mg cm 0.20 mg cm

150

-2 -2

lP

-2

0.15 mg cm

-2 0.10 mg cm

100 50 0 1.2

1.4

1.6

1.8

ur

1.0

2.0

47 mV dec

0.35

re

200

Overpotential (V vs RHE)

-p

0.40

250

na

Current density (mA cm-2)

300

ro

of

catalytic material provides more available active sites for OER performance.

45 mV dec

Catalyst's loading

-1

-1

-2

0.10 mg cm

0.30

-2

0.15 mg cm

44 mV dec

-2 0.20 mg cm -2 0.25 mg cm

0.25

40 mV dec

-1

-1

0.20

2.2

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

-2 Log j (mA cm )

Potential (V vs RHE)

Jo

Fig. 8. (a) Polarization curves of Co3O4/N-CNTs with different loading in 1.0 M KOH solution with a scan rate of 5 mV s–1 at 1600 rpm, (b) Tafel plot of Co3O4/N-CNTs with different loading Table 3. Comparison of electrocatalytic performance of Co3O4/N-CNTs towards OER with different catalyst loading Co3O4/N-CNTs –2

(mg cm ) 0.10

Onset potential (V)

1.49

Overpotential (mV) –2

at 10 mA cm 320

Tafel slope (mV dec–1)

47

Journal Pre-proof 0.15

1.47

260

45

0.20

1.42

220

44

0.25

1.37

200

40

In our catalyst, interconnected structure of Co3O4 and N-CNTs is expected to combine the advantages of large surface area and abundant active sites, which could prop up the

of

mass/electron transfer at the interface by providing an alternative way, and hence improve the conductivity as proved by our experiment. Moreover, the insertion of metal into carbon materials

ro

could modify their electronic behavior, thereby increasing the adsorption of OER intermediates

Jo

ur

na

lP

re

-p

and improving the electrocatalytic activity as shown in Fig. 9.

Fig. 9. Schematic illustration of Co3O4/N-CNTs for OER To evaluate the performances of the as-prepared catalyst objectively, an inclusive comparison of the as-prepared Co3O4/N-CNTs catalyst with previously reported Co-based catalysts is presented in Table 4, which evidently shows that the OER performance of Co3O4/N-CNTs is much better or even lower than various previously reported catalysts in basic media.

Journal Pre-proof Table 4. Comparison of electrocatalytic performance of Co3O4/N-CNTs towards OER with recent literature works Catalysts

Operating

Overpotential

Tafel slope

media

(mV) at 10 mA

(mV dec–1)

References

cm–2 NiCo2O4 3-D

1 M KOH

383

137

[53]

1 M KOH

536

57

[54]

Co3O4-C/rGO-W

0.1 M KOH

382

62

[55]

Co3O4/C-T

1 M NaOH

402

70

[23]

NC@Co-NGC

0.1 M KOH

387

91

[56]

400

61

[57]

420

107

[50]

Co3O4 hollow

na

1 M KOH

ur

nanosheets

1 M KOH

Jo

NiCo-C

-p re

lP

nanocages Co@N-carbon

ro

polyhedrons

of

nanoflowers

NixCoyO4/Co-NG

0.1 M KOH

399

77

[58]

CoNPs-N-GR

1 M KOH

400

65

[59]

FeNi@OCNF

1 M KOH

281

36

[60]

CoOx

1 M KOH +

309

27.6

[61]

450

35.8

[62]

3+

0.3 mM Fe

Co and Mn-based

1 M KOH

Journal Pre-proof oxides

CoS2-

1 M KOH

272

62.8

[63]

1 M KOH

200

40

This work

NiCo2S4/NSG Co3O4/N-CNTs

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The aforementioned results evidently demonstrate that the as-prepared Co3O4/N-CNTs possesses excellent OER performance as well as higher stability ascribed to the following aspects: (1) The

ro

higher amount of cobalt species (CoIII species) in the Co3O4/N-CNTs largely favors the electrocatalytic OER by the adsorption of comparatively large number of water oxidation

-p

intermediates like OH– ions; (2) More efficiently exposed active sites are provided; (3) A

re

synergistic effect between cobalt species and N-CNTs in the case of our present Co3O4/N-CNTs electrocatalyst could reinforce the electrocatalytic OER performance, by providing surface high-

lP

valence-state Co species susceptible to more easy oxidationinto CoOOH species as actual active sites. Moreover, N-CNTs species, which may serveas conductive carriers, may appreciably speed

na

up the entire OER process due to the efficient electron pathway to the surface active metal oxo/hydroxide sites; (4) The interconnected structure of N-CNTs with Co3O4 component can not

ur

only improve the mass/charge transfer efficiency and conductivity but also cushion the structural

Jo

collapse tendency in the electrocatalytic process of oxides, so as to stabilize the present structure.

4. Conclusion

We have prepared an in-grown structure of cobalt oxides and N-CNTs by first weaving ZIF-67 with PPy NTs via a simple method. After simple carbonization, the ZIF-67/PPy NTs precursor completely converted into the Co3O4/N-CNTs architectures with plenteous porosity. As OER electrocatalyst, Co3O4/N-CNTs exhibits superb activity and stability, with a small overpotential (200 mV at 10 mA cm-2) and an extremely low Tafel slope (40 mV dec–1). In conclusion, interconnected 3D structure of cobalt oxides and N-CNTs strongly favors the mass/charge transfer at the interface of electrode material leading to high electrocatalytic activities towards OER.

Journal Pre-proof

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant No. 21677029).

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[2] W.J. Xu, F.L. Lyu, Y.C. Bai, A.Q. Gao, J. Feng, Z.X. Cai, Y.D. Yin, Porous cobalt oxide nanoplates enriched with oxygen vacancies for oxygen evolution reaction, Nano Energy 43

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(2018) 110–116.

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[3] S. Farid, S.Z. Ren, C. Hao, MOF-derived metal/carbon materials as oxygen evolution reaction catalysts, Inorg. Chem. Commun. 94 (2018) 57–74.

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[4] Y. Zhao, J. Zhang, W. Wu, X. Guo, P. Xiong, H. Liu, G. Wang, Cobalt-doped MnO2 ultrathin nanosheets with abundant oxygen vacancies supported on functionalized carbon

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nanofibers for efficient oxygen evolution, Nano Energy 54 (2018) 129–137. [5] M. Zhang, Q. Dai, H. Zheng, M. Chen, L. Dai, Novel MOF-derived Co@N-C bifunctional

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catalysts for highly efficient Zn-Air batteries and water splitting, Adv. Mater. 30 (2018)

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[6] Y. Hao, Y. Xu, J. Liu, X. Sun, Nickel-cobalt oxides supported on Co/N decorated graphene

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as an excellent bifunctional oxygen catalyst, J. Mater. Chem. A 5 (2017) 5594–5600. [7] J. Du, C. Li, X. Wang, X. Shi, H.-P. Liang, Mesoporous ultrathin cobalt oxides nanosheets grown on carbon cloth as a high-performance electrode for oxygen evolution reaction, ACS Appl. Energy Mater. 2 (2019) 1977–1987. [8] H. Han, S. Chao, X. Yang, X. Wang, K. Wang, Z. Bai, L. Yang, Ni nanoparticles embedded in N doped carbon nanotubes derived from a metal organic framework with improved performance for oxygen evolution reaction, Int. J. Hydrogen Energy 42 (2017) 16149– 16156. [9] K. Prabakaran, M. Lokanathan, B. Kakade, Three dimensional flower like cobalt sulfide (CoS)/functionalized MWCNT composite catalyst for efficient oxygen evolution reactions, Appl. Surf. Sci. 466 (2019) 830–836.

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[17] S. Liu, T. Deng, X. Hu, X. Shi, H. Wang, T. Qin, X. Zhang, J. Qi, W. Zhang, W. Zheng, Increasing surface active Co2+ sites of MOF-derived Co3O4 for enhanced supercapacitive performance via NaBH4 reduction, Electrochim. Acta 289 (2018) 319–323. [18] M. Li, C. Bao, Y. Liu, J. Meng, X. Liu, Y. Cai, D. Wuu, Y. Zong, T.-P. Loh, Z. Wang, Reduced graphene oxide-supported cobalt oxide decorated N-doped graphitic carbon for efficient bifunctional oxygen electrocatalysis, RSC Adv. 9 (2019) 16534–16540. [19] S. Liu, X. Chen, S. Wang, Z. Yang, J. Gao, P. Zhu, X. Zhao, G. Wang, 3D CNTs-threaded N-doped hierarchical porous carbon hybrid with embedded Co/CoOx nanoparticles as efficient bifunctional catalysts for oxygen electrode reactions, Electrochim. Acta 292 (2018) 707–717.

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Journal Pre-proof Highlights • Hierarchically porous interconnected network of N-doped carbon nanotubes threaded with cobalt oxide (Co3O4/N-CNTs) was prepared. • Large electrochemical active surface area owned by Co3O4/N-CNTs led to high electrocatalytic properties. • High electrical conductivity of Co3O4/N-CNTs architectures were evaluated as OER electrocatalysts in alkaline media.

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Co3O4/N-CNTs architectures exhibited amazing OER activity and stability.

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Journal Pre-proof Author Contributions Section

Individuals claiming authorship meet all 3 of the following conditions: 1) Authors make substantial contributions to conception and design, and/or acquisition of data, and/or analysis and interpretation of data; 2) Authors participate in drafting the article or revising it critically for important intellectual content;

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3) Authors give final approval of the version to be submitted and any revised version.

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Acquisition of data: Farid; Zhao; Hao; Mao; Ren

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Study conception and design: Farid; Qiu; Mao; Zhao; Song

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Each author’s LAST NAME is typed next to the appropriate category:

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Analysis and interpretation of data: Farid; Qiu; Song; Zhao; Hao Drafting of manuscript: Farid; Ren; Qiu; Zhao

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Critical revision: Farid; Song; Mao; Ren; Hao

Journal Pre-proof 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.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: