Three-Dimensional Nanoarchitecture of Carbon Nanotube-Interwoven

Jul 17, 2019 - The exploration of a new family of materials with superior performance for capacitive deionization (CDI) compared to commercial activat...
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Three-Dimensional Nanoarchitecture of Carbon Nanotubes-Interwoven Metal#Organic Frameworks for Capacitive Deionization of Saline Water Xingtao Xu, Chenglong Li, Chen Wang, Lu Ji, Yusuf Valentino Kaneti, Huajie Huang, Tao Yang, Kevin Wu, and Yusuke Yamauchi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b02367 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Three-Dimensional Nanoarchitecture of Carbon Nanotubes-Interwoven Metal‒Organic Frameworks for Capacitive Deionization of Saline Water

Xingtao Xu,a Chenglong Li,a Chen Wang,a Lu Ji,a Yusuf Valentino Kaneti,b,c Huajie Huang,*d Tao Yang,*a Kevin C.-W. Wue and Yusuke Yamauchi*b,c,f,g

[a]

State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, 1 N. Xikang Rd., Nanjing 210-098, China.

[b]

Key Laboratory of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China

[c]

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

[d]

College of Mechanics and Materials, Hohai University, 1 N. Xikang Rd., Nanjing 210-098, China

[e]

Department of Chemical Engineering, National Taiwan University (NTU), No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan

[f]

School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia

[g]

Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea

E-mails: [email protected]; [email protected]; [email protected]

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Abstract The exploration of a new family of materials with superior performance for capacitive deionization (CDI) compared to commercial activated carbons is of significant interest. In this work, a three-dimensional (3D) nanoarchitecture composed of metal-organic frameworks (MOFs) interwoven by carbon nanotubes (CNTs) has been synthesized by utilizing CNTs to bridge the neighboring MOF nanocrystals. The resulting MOF/CNT hybrid not only shows improved electrical conductivity and 3D hierarchical structure, but also retains the high porosity of MOFs. Consequently, the MOF/CNT hybrid exhibits a high desalination capacity of 16.90 mg g-1, highlighting the potential of non-carbonized MOF-based materials as CDI electrodes.

Keywords: Capacitive deionization; Water desalination; Metal-organic frameworks; Carbon nanotubes; In situ synthesis

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Introduction Due to the climate change, population growth, and environmental pollution, the supply for affordable clean water has become a serious global issue in the 21st century.1-9 The purification of saline water provides a significant solution to address this issue. Conventional methods such as reverse osmosis, electrodialysis, multistage flash distillation typically require high energy consumption with low purification efficiency and occasional secondary pollution.10-13 In recent years, capacitive deionization (CDI) which is based on electrical double layer (EDL) theory, has emerged as an alternative desalination method for water purification.14-19 When saline water flows through the CDI cell, the dissolved ions could be easily removed from saline water and stored within the pores of carbon electrodes by forming EDL (see Figure S1). Consequently, this process leads to the large-scale and environmentally friendly production of clean water with low energy consumption.20-23 In the past few years, carbon materials are undoubtedly the most widely-studied materials in CDI field due to their abundance and outstanding CDI properties.24-27 Different types of carbon-based materials, such as graphene, carbon nanotubes (CNTs), carbon nanofibers, mesoporous carbons, and biomass-derived carbons have shown good performance for CDI.17,

28-34

In addition to the great

advances in carbon materials, the exploration of a new family of materials with superior performance for CDI compared to commercial activated carbons is also of significant interest. Recently, a pioneering work has demonstrated the direct utilization of metal-organic frameworks (MOFs) for CDI.35 As highly porous materials, MOFs have the potential to replace conventional carbon materials in CDI, due to their higher porosity compared to many carbon materials.36-41 However, the poor conductivity of most MOFs has limited their application for CDI and other electrochemical applications.42-47

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Our recent work has shown that hybridization of poorly conductive MOFs with polypyrrole nanotubes could effectively improve the CDI performance.35 However, the complicated synthetic procedures combined with the high production cost of polypyrrole render this method unfavorable for large-scale production.48 Therefore, in this work, commercially available CNTs are used as an alternative to polypyrrole nanotubes. It is worth noting that these CNTs were firstly oxidized in concentrated nitric acid (HNO3) in order to impart abundant oxygen-containing groups on their surface, which are favourable for attracting free Co2+ via electrostatic interaction,49 and serve as nucleation sites for MOF growth. A three-dimensional (3D) nanoarchitecture composed of MOF particles with interwoven CNTs has been successfully synthesized via in situ growth MOF particles on CNTs (Figure 1). In such 3D nanoarchitecture, CNTs bridge the neighboring MOF particles by forming “MOF-CNT-MOF” electron conducting pathways, which consequently lead to reduced electrical impedance and enhanced electrochemical performance. As a proof of concept, the representative ZIF-67/CNT hybrid was prepared and directly used as a CDI electrode for the first time. Afterwards, the CDI cell built with two pairs of identical electrodes was applied for purification of saline water. The CDI test results reveal the high CDI performance of the ZIF-67/CNT hybrid electrode with a high desalination capacity of 16.90 mg g-1.

Figure 1. Schematic illustration showing the synthetic process of the ZIF-67/CNT hybrid.

Experimental Section

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Chemicals Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, 99.5%), 2-methylimidazole (2-MeIM, 99%), HNO3, ethanol (C2H6O, 99.5%), methanol (CH4O, 99.5%), sodium chloride (NaCl, ≥99%), poly(vinylidenefluoride) (PVDF) and N-methyl-2-pyrrolidinone (NMP) were purchased from Sinopharm Chemical Reagent Co., Ltd. Carbon nanotubes (CNTs) were purchased from Shenzhen Nanotech Port Co., Ltd and Vulcan XC 72 was obtained from Cabot Corporation. Synthesis of ZIF-67/CNT hybrid CNTs with functionalized carboxylic groups were obtained by treating 500 mg of CNTs in HNO3 for 24 h. Then, solution A was prepared by dispersing an optimized amount of the functionalized CNTs into a methanolic solution of Co(NO3)2 (20 mL) (the weight of Co(NO3)2·6H2O is 454 mg), followed by ultrasonication for 1 h. The optimized amount of CNTs was determined to be 50 mg, and the detailed optimization procedure is described in Figure S2. Solution B was prepared by dissolving 513 mg of 2-MeIM into 20 mL of methanol. Then, solution B was added dropwise into solution A under continuous stirring for 0.5 h. After 24 h reaction, the ZIF-67/CNT hybrid was collected by centrifugation, washed thoroughly with methanol several times, and finally dried at 60 ºC for 24 h. Electrode preparation Each electrode with a mass loading of 10 mg cm-2 was prepared by pasting a mixture of 80 wt.% sample, 10 wt.% carbon black (Vulcan XC 72), and 10 wt.% PVDF onto the graphite paper (thickness of 1 mm). Characterization X-ray diffractometer (Ultima Rint 2000, RIGAKU, Japan) equipped with Cu Kα radiation (40 kV, 40 mA) was used to characterize the phase composition of the samples. A BELSORP-mini (BEL, Japan) system was used to obtain the nitrogen (N2) adsorption-desorption isotherms. Morphological characterization of the samples were carried out by using field-emission scanning electron microscope (FESEM, Hitachi SU8000)

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and transmission electron microscope (TEM, JEOL JEM-2100). Electrochemical measurements The electrochemical measurements were conducted by using an electrochemical workstation (CHI-660E) with a three-electrode system consisting of 1 M NaCl aqueous electrolyte, a platinum wire counter electrode and an Ag/AgCl reference electrode. Cyclic voltammetry (CV) and gravimetric charge-discharge (GCD) measurements were carried out in the potential range of 0 to 1 V. The Nyquist plots were obtained from electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 10 mHz to 100 kHz . The specific capacitances were calculated from the discharge curves by using the following equation: (1) where i is the discharge current density (A g-1), t is the discharge time (s), and ΔV is the voltage window (V). Desalination tests Prior to the desalination tests, a CDI cell was constructed with two pairs of identical electrodes. Anion- and cation- exchange membranes were further used to alleviate the co-ion effect. Next, the desalination performance was studied by batch-mode electrosorption experiments with a continuous recycling system, which includes a CDI cell, a peristaltic pump, a power source, and a tank. In each experiment, the concentration variation of the NaCl solution was continuously monitored and measured at the outlet of the CDI cell by using an ion conductivity meter. The volume of the solution was 50 mL, the flow rate was 20 mL min-1, and the operating voltage was 1.2 V. The desalination capacity (Λ, mg g-1) and rate (v, mg g-1 min-1) at t min were calculated as follows: (2) (3) where C0 and Ct represent the concentrations of NaCl at initial stage and t min, respectively (mg L-1), V

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represents the volume of the NaCl solution (L), and m represents the total mass of the electrode material (g). Results and discussion The crystal structure of the ZIF-67/CNT hybrid was analyzed by powder XRD (Figure 2). Compared with pristine ZIF-67, the ZIF-67/CNT hybrid shows similar diffraction peaks as the simulated peaks of ZIF-67, indicating the highly crystalline structure of the ZIF-67/CNT hybrid.40 In addition, no obvious diffraction peaks of CNTs are observed in the XRD pattern of the ZIF-67/CNT hybrid, possibly due to the low content of CNTs in the hybrid.26

Figure 2. XRD patterns of ZIF-67/CNT hybrid, pristine ZIF-67, and pristine CNTs.

The morphology of the ZIF-67/CNT hybrid was investigated by FESEM and TEM. As shown in Figure 3a-c, the ZIF-67/CNT hybrid exhibits a 3D network-like structure composed of ZIF-67 polyhedra

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with inserted CNTs. The size range of the ZIF-67 polyhedra is around 200-300 nm. Further analysis by TEM reveals that these CNTs bridge the neighboring ZIF-67 polyhedra (Figure 3d) to form a “ZIF-67-CNT-ZIF-67” electron conducting pathway. This novel hybrid structure can increase the electrical conductivity of ZIF-67 electrode due to the good electrical conductivity of CNTs. Furthermore, the porosity of the ZIF-67/CNT hybrid was studied by N2 adsorption-desorption isotherm (Figure S3). The specific surface area (SSA) of the ZIF-67/CNT hybrid is 1585.8 m2 g-1, close to that of pure ZIF-67 (1689.7 m2 g-1; Figure S4), indicating that the high porosity of the ZIF-67 particles is retained in the final ZIF-67/CNT hybrid even after the introduction of CNTs. In addition, this SSA value is also much higher than that of pristine CNTs (134.0 m2 g-1; Figure S5).

Figure 3. (a-c) FESEM and (d) TEM images of the ZIF-67/CNT hybrid. The electrochemical performance of the ZIF-67/CNT hybrid was investigated by CV, GCD and EIS, respectively. In each test, a three-electrode system composed of a working electrode (active

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material), a counter electrode (Pt wire), and a reference electrode (Ag/AgCl) was used with 1 M NaCl solution as the electrolyte. As shown in Figure 4a, all samples display a rectangular-shaped CV curve without any obvious redox peaks, indicating a capacitive behaviour.50 The specific capacitances calculated by the discharge time at 1 A g-1 are 149.2, 36.3, and 74.2 F g-1 for ZIF-67/CNT hybrid, ZIF-67, and CNTs, respectively (Figure 4b). Furthermore, regardless of the current density, the ZIF-67/CNT hybrid always shows the highest specific capacitance (Figure 4c). The superior electrochemical performance of the hybrid can be ascribed to several reasons. Firstly, the introduction of highly conductive CNTs into poorly conductive ZIF-67 particles leads to the improvement in the electrical conductivity of the ZIF-67 electrode, thereby leading to the effective utilization of the ZIF-67 particles. Secondly, the high porosity of the ZIF-67 particles is retained in the final ZIF-67/CNT hybrid even after the introduction of CNTs.

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Figure 4. Electrochemical characterizations of ZIF-67/CNT hybrid, ZIF-67 and CNTs. (a) CV curves at 20 mV s-1. (b) Discharge curves at 1 A g-1. (c) Specific capacitances vs. current density. (d) Nyquist impedance spectra.

EIS was further utilized to evaluate the charge transfer properties of the different electrodes. The charge transfer resistance (Rct, Ω) can be determined from the diameter of the semicircle at high frequency region,51 The Rct values of ZIF-67/CNT hybrid, ZIF-67, and CNTs are 1.15, 3.82, and 0.52 Ω, respectively (Figure 4d). Obviously, the ZIF-67/CNT hybrid shows a lower Rct value than pristine ZIF-67, demonstrating that the introduction of CNTs indeed contributes to the enhanced electrical conductivity of the ZIF-67 electrode.

Figure 5. (a) Variations in saline water concentration and (b) CDI Ragone plots for ZIF-67/CNT hybrid, ZIF-67, and CNTs. CDI performances of the ZIF-67/CNT hybrid in solutions with (c) various concentrations of saline water and (d) various saline species (5 mM).

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To evaluate the potential of the ZIF-67/CNT hybrid for water purification, a CDI cell was fabricated with two pairs of identical ZIF-67/CNT hybrid electrodes and this cell was tested in saline water (5 mM) at a fixed potential of 1.2 V (Figure 5). Figure 5a compares the desalination performances of ZIF-67/CNT hybrid, ZIF-67, and CNTs. It is obvious that the ZIF-67/CNT hybrid shows the best desalination performance with a high desalination capacity of 10.30 mg g-1. This value is much higher than those of commercial activated carbons (3.99 mg g-1; Figure S6) and the physical mixture of ZIF-67 and CNTs with the same content of CNTs (4.33 mg g-1; Figure S7). In addition, the ZIF-67/CNT hybrid electrode also shows superior desalination performance compared to many other reported CDI electrodes (Table S1). The corresponding Ragone plots shown in Figure 5b reveal that the Ragone plot of the ZIF-67/CNT hybrid shifts more toward upper right than those of ZIF-67 and CNTs, suggesting the higher desalination capacity and rate. The potential of the ZIF-67/CNT hybrid for water purification was further investigated by varying the saline concentrations (e.g. 5, 10, 15 and 20 mM; Figure 5c) and the saline species (e.g. NaCl, KCl, MgCl2, CaCl2; Figure 5d). As shown in Figure 5c, the desalination capacity of the ZIF-67/CNT hybrid increases from 10.30 mg g-1 for 5 mM to 16.90 mg g-1 for 20 mM, revealing the good desalination capability of the ZIF-67/CNT hybrid even at a high saline concentration. In addition, the desalination tests for various saline solutions (e.g., NaCl, KCl, MgCl2, CaCl2) indicate that the electrosorption ability is the order of CaCl2>MgCl2>KCl>NaCl for the ZIF-67/CNT hybrid (Figure 5d), possibly due to the combined effects of ionic charge and hydrated radius. Further cycling electrosorption/desorption experiments confirm the excellent cycling stability of the ZIF-67/CNT hybrid (Figure S8), revealing the good structural stability of our ZIF-67/CNT hybrid under repeated use.

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Conclusions In summary, this work has demonstrated the successful synthesis of ZIF-67/CNT hybrid with a novel 3D nanoarchitecture by interweaving ZIF-67 particles with CNTs for CDI of saline water. In the 3D hybrid nanoarchitecture, CNTs bridge the neighboring ZIF-67 particles by forming “ZIF-67-CNT-ZIF-67” electron conducting pathways, giving rise to improved electrical conductivity. Consequently, the as-prepared ZIF-67/CNT hybrid exhibits a high desalination capacity of 16.90 mg g-1, which is among the best reported CDI electrodes to date. It is expected that this work will promote future research of non-carbonized MOF-based materials as CDI electrodes. Notes The authors declare no competing financial interest. Acknowledgements This work was jointly supported by the Fundamental Research Funds for the Central Universities (B18020057), China Postdoctoral Science Foundation funded project (419082), Advanced Foundation of Science and Technology Innovation Projects for Returned Scholars in Nanjing (B19118), and the grants from the National Natural Science Foundation of China (51879068, 41561134016), and National Key Research and Development Program (2018YFC0407900). This work was performed in part at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for Australia's researchers. Supporting Information Schematic illustration of the desalination process using CDI; Variations in saline concentration for ZIF-67/CNTs with various contents of CNTs; N2 adsorption-desorption isotherms of ZIF-67/CNT hybrid, pristine ZIF-67, and pristine CNTs; Variations in saline concentration for commercial activated carbons and

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for physical mixture of ZIF-67 and CNTs with similar content of CNTs; cycling electrosorption/desorption performance of the ZIF-67/CNT hybrid; and comparison of desalination performance of the as-prepared ZIF-67/CNT hybrid electrode with other reported CDI electrodes.

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For Table of Contents Use Only

ZIF-67/CNT hybrid was synthesized by using CNTs to interweave ZIF-67 nanocrystals and applied as a capacitive deionization electrode without any carbonization.

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