Energy-Saving Synthesis of MOF-Derived Hierarchical and Hollow Co

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Energy-saving Synthesis of MOF-Derived Hierarchical and hollow Co(VO3)2Co(OH)2 Composite Leaf Arrays for Supercapacitor electrode materials Yingxi Zhang, Hao Chen, Cao Guan, Yatao Wu, Chunhai Yang, Zhehong Shen, and Qichao Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05501 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Materials & Interfaces

Energy-saving Synthesis of MOF-Derived Hierarchical and hollow Co(VO3)2-Co(OH)2 Composite Leaf Arrays for Supercapacitor electrode materials Yingxi Zhang,a,b Hao Chen,b,c,* Cao Guan, c,* Yatao Wu,b Chunhai Yang,d Zhehong Shen, b,* and Qichao Zoua,* a

College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, PR

China.; b

School of Engineering, Zhejiang A&F University, Hangzhou 311300, PR China.;

c

Department of Materials Science and Engineering, National University of Singapore, 117574

Singapore.; d

School of Chemistry & Environment Engineering, Hubei University for Nationalities, Enshi,

445000, PR China.

ABSTRACT: A one-step and energy-saving method was proposed to synthesize hierarchical and hollow Co(VO3)2-Co(OH)2 composite leaf arrays on carbon cloth, which expressed high capacitance (522 mF cm–2 or 803 F g–1 at the current density of 0.5 mA cm–2), good rate capability (79.5% capacitance retention after a 30-fold increase of the current density) and excellent cycling stability (90% capacitance retention after 15 000 charge-discharge cycles) when tested as a supercapacitor electrode. KEYWORDS: metal-organic frameworks, supercapacitors, electrode materials, exchange reactions, hollow arrays

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Supercapacitor holds the advantages of easy fabrication, good compatibility and outstanding electrochemical performance, and thus is attracting growing interest from the energy material fields.1, 2 The electrode active material is the main contributor to the energy storage properties of a supercapacitor. Thereby the composition, structure, properties of an electrode, and its contact with the current collector directly affect the performance of supercapacitor.1,

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In order to

improve capacitance output and reduce the capacitance attenuation caused by the contact resistance, the in-situ growth of hierarchically structured electrode materials on conductive substrates has become a promising strategy for constructing self-supported supercapacitor electrodes with high performances.2, 6, 7 However, constructing hierarchically structured metal oxides/hydroxides arrays with excellent performances by simple methods is still a challenge. Since their discovery in the late 1990s, metal-organic frameworks (MOFs) have been widely applied in the gas storage, drug delivery, liquid phase separation, catalysis and other fields due to their wide variability in the terms of chemical composition, pore structure, and geometric morphology.8, 9 More interestingly, the MOFs formed by the coordination of inorganic metal ions and organic ligands possess a certain microscopic morphology, and can be employed as a sacrificial template to prepare various inorganic nanomaterials, such as porous carbons, metal oxides, sulfides, and phosphides, by utilizing heat treatments and/or chemical methods.8, 10-13 Moreover, these resulting products can retain the inherent micro-nano structures of the parent MOF, and own a rich porous structure and a high specific surface area, thereby hold excellent application potentials in electrochemical energy storage.10, 14, 15 Motivated by these interesting findings, very recently, we employed cobalt-based metal-organic framework (Co-MOF) nanowall arrays as the precursor to successfully fabricate hollow and porous NiCo2O4 nanowall arrays on carbon cloth (CC) current collector via an ion-exchange and etching reaction process


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followed by a thermal treatment.16 The as-fabricated NiCo2O4 nanowall electrode exhibited remarkable electrochemical performances with excellent rate capability and long cycle life, indicating that it’s a highly feasible path to utilize Co-MOF arrays supported on CC as the precursor to construct hierarchically structured metal oxide array electrodes for supercapacitor applications. However, this method still involves multiple steps and huge thermal energy consumption with annealing process. In this work, we report a simpler one-step method to synthesize hierarchical and hollow Co(VO3)2-Co(OH)2 (Co-V) composite leaf arrays supported on CC for supercapacitor electrodes. In this method, the exchange reaction between VO3– plus OH– and 2-methylimidazole (2-MIM) ligands of Co-MOF was utilized to in-situ grow Co-V composite leaf arrays on the surface of CC. Moreover, the whole reaction was conducted under room temperature, which is highly energysaving. To the best of our knowledge, no previous studies have reported such method to prepare self-supported supercapacitor electrodes. The as-prepared Co-V composites not only preserve the intrinsic leaf-like shape of Co-MOF, but also own a three dimensional (3D) hollow structure with irregular nanosheets on the outer surface. This unique hierarchical hollow+porous structure can allow intimate electrolyte penetration, promote fast ion/mass transport, shorten ion diffusion distance, and thus significantly enhance the electrochemical performance. As a demonstration, a CC supported Co-V-150 electrode prepared with a reaction time of 150 min exhibits high specific capacitance (Cs) of 522 mF cm–2 or 803 F g–1 at the current density of 0.5 mA cm–2, satisfactory rate capability (79.5% Cs retention after a 30-fold increase of the current density), and excellent cycling performance (90% Cs retention after 15 000 charge-discharge (CD) cycles).


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Figure. 1 Scheme of the formation mechanism of Co-MOF derived Co(VO3)2-Co(OH)2 composite.

The synthetic strategy for the hierarchical Co-V composite leaf arrays is schematically depicted in Figure 1 (see the Experimental Section in Supporting Information for details). First, the pre-prepared Co-MOF arrays on a CC substrate were inserted into a NaVO3 aqueous solution under 25 oC and kept stationary. Then, through a subsequent exchange between 2-MIM ligands and VO3–, the Co-MOF was etched through the removal of 2-MIM to release Co2+, and the Co(VO3)2 particles simultaneously deposited on the outer surface by a rapid binding between Co2+ and VO3–, resulting in the formation of a hollow structure. Meanwhile, the released 2-MIM was concentrated around the hollow structure and reacted with H2O to release OH–.17 Once the concentration reached a relatively high value, the OH– accompanied with VO3– to exchange with


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2-MIM ligands to produce Co-V composites on the surface of skeleton.

Figure. 2 SEM images of (a-c) Co-MOF@CC and (d-f) Co-V-150@CC. (g-i) TEM images, (j) EDXS spectrum, and (k) TEM elemental mapping of Co-V-150.

Since the Co-MOF leaf-like arrays (Figure 2a-c) served as the sacrifice templates to in-situ grow Co-V-150 composites, this product owns a similar microstructure of leaf-like arrays (Figure 2d-f). Because Co(OH)2 trends to form sheet-like structure,18 the structural unit of Co-V150 composites possesses an irregular sheet-like structure (Figure 2f). Furthermore, we also observe a hollow structure from the broken area of leaf-like structure (Figure S1). The results of


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nitrogen adsorption/desorption test further manifest a mesoporous feature of as-prepared Co-V150 (Figure S2). This porous feature can also be well confirmed by the transmission electron microscope (TEM) images (Figure 2g-i and S3). Based on these results, it can be inferred that the hierarchical hollow+porous leaf-like arrays supported on CC were successfully fabricated. In addition, all of the fabrication processes including the preparation of Co-MOF precursors don’t need heating and stirring, thereby are highly energy-saving.

Figure. 3 (a) XRD patterns, (b) XPS full spectra, and (f) Raman spectra of samples. (c) Co 2p XPS, (d) V 2p XPS, and (e) O 1s XPS spectra of Co-V-150.


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To investigate the elemental composition of Co-V-150, a TEM synchronous energy-dispersive X-ray spectroscopy (EDXS) measurement was conducted. Except for the C and Cu signals from the copper mesh covered with carbon film, the atomic ratio of V, Co, and O elements of Co-V150 is found to be 1.0: 2.0: 5.7 (Figure 2j). Based on this ratio, the molar ratio of Co(VO3)2 to Co(OH)2 is calculated to be near 1: 3. Moreover, the uniform distribution of these elements indicates the high homogeneity of two components in the Co-V-150 product (Figure 2k). The Xray diffraction (XRD) analysis was executed to study the crystalline state of samples. All diffraction peaks of Co-MOF disappeared after the exchange process, indicating the resulting Co-V-150 composite is amorphous (Figure 3a), which favours the penetration of aqueous electrolyte. Figure 3b shows the X-ray photoelectron spectroscopy (XPS) spectra of Co-MOF precursor and Co-V-150 composite, the signals of Co, V, and O appear as expected. The Co 2p and V 2p XPS spectra in Figure 3c and d illustrate the presentence of Co+2 and V+5 states,19 respectively, while the O 1s XPS spectrum (Figure 3e) of Co-V-150 indicate the possible coexistence of hydroxides and metal oxyacid salts.20 In the Raman spectra of samples (Figure 3f), the signal peaks of Co-N (220, 257, and 427 cm–1) and C-CH3 (677 cm–1) bonds of Co-MOF as well as absorbed NO3– (471 and 513 cm–1) are absent for the Co-V-150 product,21-24 while the new signals of Co-O and V-O bonds are clearly observed at 314 and 804 cm–1,25, 26 suggesting a complete consumption of Co-MOF after the exchange reactions. These results further confirm the successful synthesis of Co(VO3)2-Co(OH)2 composite.


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Figure. 4 (a) CV curves, (c) CD curves, and (e) cycling performances of the as-fabricated Co-V-150 electrode. Comparison of (b) CV curves and (d) Cs values of the electrodes based on Co-V composites prepared with different reaction time and Co(OH)2.

The electrochemical properties were investigated under a three-electrode cell configuration at 25 oC in an alkali electrolyte with CC supported samples as the working electrodes. The cyclic voltammetry (CV) curves of Co-V-150 electrode display a strong capacitance response with


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remarkable pseudocapacitive contribution (Figure 4a). Even at a higher scan rate of 50 mV s–1, the CV curve still has no obvious deformation, suggesting its superior capacitance output ability at high scan rates. Figure 4b provides a comparison of CV curves of the electrodes based on CoV composites obtained under different reaction time and pure Co(OH)2 prepared by a similar process,27 two pairs of visible redox peaks for all electrodes are associated with the faradaic redox transition related to Co2+/Co3+ and Co3+/Co4+.28,

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Notably, compared with the pure

Co(OH)2 electrode, the Co-V composite electrodes possess significantly enhanced redox peaks, indicating their better electrochemical activity. The hierarchical and hollow structure of Co-V composites can provide more active sites for efficient exposure to electrolyte for better electrochemical reactions, compared to the relatively smooth sheet-like structure of pure Co(OH)2 (Figure S4 vs. S5a-c). The comparison in Figure 4b further shows that the CV area of Co-V composite electrodes increases first and then decreases with the extension of synthesis time of composites, and reaches up to a maximum value at the reaction time of 150 min. This implies that the as-prepared Co-V-150 composite should own the best capacitance performance here. Similar to its CV curves, the CD curves of Co-V-150 composite electrode at different current densities also show a visible pseudocapacitive feature (Figure 4c). Figure 4d compares the Cs of various Co-V composite electrodes based on their CD curves (Figure 4c and S6). It displays that as the exchange time increased, the resulting Co-V composite exhibits higher Cs values. However, too long reaction time causes a decrease in Cs. This Cs variation tendency has a close relationship with the morphological evolution of Co-V composites as shown in Figure S4. In the early stage of exchange reaction, the exchange between VO3– and 2-MIM leaded to the formation of small Co(VO3)2 nanoparticles on the surface of Co-MOF skeleton (Figure S4a-b). Because only partial Co-MOF precursor was converted, the practical amount of active materials


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was very low, thus delivering a relatively low Cs. As the reaction time prolonged to 120 min, the released 2-MIM reacted with H2O to produce OH–, which combined with Co2+ to form sheet-like Co(OH)2. The co-growth of these Co(OH)2 nanosheets and Co(VO3)2 nanoparticles resulted in the formation of irregular sheet-like structure (Figure S4c-d and 2f). The increase in the amount of active materials accordingly elevated the Cs. With the further increase of exchange time, the size of sheet-like structure enlarged (Figure S4e-f). However, too long time leaded to a further exchange reaction between Co(OH)2 and VO3– to convert Co(OH)2 nanosheets to Co(VO3)2 nanoparticles (Figure S4g-h),30 and also caused the collapse of some hollow structures due to a poor structural stability of the aggregate of pure particles. The increasing of size, the degradation of nanosheets, and the destruction of hollow structures would reduce active sites for electrochemical reactions, thereby weaken the capacitance output. The Co-V-150 composite prepared with a moderate reaction time of 150 min possesses a well-defined, hierarchical, and hollow microarchitecture and sheet-like nanostructure units, thereby expresses the highest Cs values, with 522, 502, 488, 465, 431, and 415 mF cm–2 (or 803, 772, 751, 715, 663, and 638 F g– 1

) at current densities of 0.5, 4, 6, 8, 10, and 15 mA cm–2, respectively. Based on these values,

it’s found that 79.5% of the capacitance is retained from 0.5 to 15 mA cm–2 after a 30-fold elevation of the CD current density, indicating a satisfactory rate capability of Co-V-150 electrode. Moreover, the obtainable highest Cs (522 mF cm–2 or 803 F g–1 at 0.5 mA cm–2) of this electrode is significantly higher than those of the as-prepared pure Co(OH)2 electrode (111 mF cm–2 at 0.5 mA cm–2) and most previously reported electrodes based on similar active materials (Table S1). Such high capacitance performances are attributed to the unique structure features of the Co-V-150 composite leaf arrays we present here: (i) The hierarchical, hollow, and porous 3D structure can allow intimate electrolyte penetration and fast ion/mass transport, and shorten ion


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diffusion distance; (ii) The direct growth of active materials on the highly conductive CC current collector can ensure electron transport highway for fast reaction at high rates. In regard to cycling performances, Figure 4e further manifests that 90% of the initial capacitance can well retained after 15 000 CD cycles of charge–discharge at 10 mA cm–2, and the Coulombic efficiency maintains a value of about 99.5% after a few initial cycles, implying a more superior cycling stability of Co-V-150 composite electrode relative to some similar electrodes (Table S1). These excellent cycling performances should benefit from the well preservation of hierarchical hollow+porous arrays of this electrode during the cycling test, except for a slight coalescence of nanostructured units (Figure S7). The above results signify that the as-fabricated Co-V-150 electrode can be regarded as a hopeful candidate to fabricate practical supercapacitors. In summary, we reported a one-step and energy-saving synthesis of novel hierarchical Co-V composite leaf arrays supported on CC for supercapacitor electrodes. The optimal Co-V-150 composite electrode exhibited a high Cs of 522 mF cm–2 or 803 F g–1 at the current density of 0.5 mA cm–2, a satisfactory rate capability (79.5% Cs retention after a 30-fold increase of the current density), and excellent cycling performance (90% Cs retention after 15 000 CD cycles), indicating its good application potential for supercapacitors. Furthermore, this simple method can also be further extended to facilely synthesize other MOF derived metal compound arrays with excellent electrochemical properties for supercapacitors, electrocatalysts and metal-air batteries.

ASSOCIATED CONTENT Supporting Information Available. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, SEM images and electrochemical


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data of of different Co-V composites, TEM images of Co-V-150, and performances comparison table. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. Chen); [email protected] (C. Guan); [email protected] (Z. Shen); [email protected] (Q. Zou) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China (21561012, 51403190), Young Talent Cultivation Project of Zhejiang Association for Science and Technology (2016YCGC019), Youth Top-notch Talent Development and Training Program Foundation of Zhejiang A&F University, State Scholarship Fund of China Scholarship Council (CSC, 201708330113), 151 Talent Project of Zhejiang Province, and Zhejiang Province Society of Forestry.


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Energy-Saving Synthesis of MOF-Derived Hierarchical and Hollow Co

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