Controlled Hydrolysis of MetalâOrganic Frameworks: Hierarchical Ni

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Controlled Hydrolysis of Metal–Organic Frameworks: Hierarchical Ni/Co-Layered Double Hydroxide Microspheres for High Performance Supercapacitors Zhenyu Xiao, Yingjie Mei, Shuai Yuan, Hao Mei, Ben Xu, Yuxiang Bao, Lili Fan, Wenpei Kang, Fangna Dai, Rongming Wang, Lei Wang, Songqing Hu, Daofeng Sun, and Hong-Cai Zhou ACS Nano, Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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Controlled

Hydrolysis

of

Metal–Organic

Frameworks: Hierarchical Ni/Co-Layered Double Hydroxide Microspheres for High Performance Supercapacitors Zhenyu Xiao,†,§,∥Yingjie Mei,†,∥ Shuai Yuan,‡,∥ Hao Mei,† Ben Xu,† Yuxiang Bao,§ Lili Fan,† Wenpei Kang,† Fangna Dai,† Rongmign Wang,† Lei Wang,§ Songqing Hu,† Daofeng Sun,*† Hong-Cai Zhou*‡ †School

of Materials Science and Engineering, China University of Petroleum (East China),

Qingdao Shandong 266580, People’s Republic of China. ‡Department

of Chemistry, Materials Science and Engineering, Texas A&M University, College

Station, Texas 77842-3012, United States. §Key

Laboratory of Eco-chemical Engineering, Ministry of Education, Laboratory of Inorganic

Synthesis and Applied Chemistry, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China. *Corresponding Authors: [email protected]; [email protected]

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ABSTRACT: Pseudomorphic conversion of Metal-Organic Frameworks (MOFs) enables the fabrication of nanomaterials with well-defined porosities and morphologies for enhanced performances. However, the commonly reported calcination strategy usually requires high temperature to pyrolyze MOF particles and often results in uncontrolled growth of nanomaterials. Herein, we report the controlled alkaline hydrolysis of MOFs to produce layered double hydroxide (LDH) while maintaining the porosity and morphology of MOF particles. The preformed trinuclear M3(μ3-OH) (M = Ni2+ and Co2+) clusters in MOFs were demonstrated to be critical for the pseudomorphic transformation process. Isotopic tracing experiment revealed that the 18O labeled M3(μ3-18OH) participated in the structural assembly of LDH, which avoided the leaching of metal cations and the subsequent uncontrolled growth of hydroxides. The resulting LDHs maintain the spherical morphology of MOF templates, and possess hierarchical porous structure with high surface area (BET surface area up to 201 m2∙g-1), which is suitable for supercapacitor applications. As supercapacitor electrodes, the optimized LDH with the Ni:Co molar ratio of 7:3 shows the high specific capacitance (1652 F·g-1 at 1 A·g-1) and decent cycling performance, retaining almost 100% after 2000 cycles. Furthermore, the hydrolysis method allows the recycle of organic ligands and large-scale synthesis of LDH materials.

KEYWORDS: layered double hydroxide, metal-organic framework, supercapacitor, hierarchical microspheres, pseudomorphic conversion

Metal–organic frameworks (MOFs) are an emerging class of organic-inorganic hybrid materials with designable porous structures and tunable morphologies.1,2 They have been widely employed as precursors or templates to prepare metal oxides, porous carbons, and composite materials for a


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variety of applications, including catalysis, sensing, and energy storage.3-6 Such derived materials commonly inherit the porous structures of their parent MOFs, and possess enhanced electrochemical activity for applications in supercapacitors and electrocatalysis.7,8 For example, porous Co3O4 rhombic dodecahedral nanostructures have been synthesized by the thermal pyrolysis of ZIF-67. Compared with traditional Co3O4, the 3D hierarchical nanostructures exhibit higher surface area and richer channels, therefore provide a highway for both electrons and electrolytes for efficient energy storage.9 Although thermal conversions of MOFs offer a promising way to synthesize metal oxide nanomaterials that are difficult to obtain using conventional methods, this strategy inevitably result in the decomposition of expensive organic ligands, which limit the large-scale applications.10,11 The alkaline hydrolysis strategy seems to be an alternative path towards largescale fabrication of MOF derived materials. By replacing the organic ligands with OH-, metal oxide/hydroxide materials can be obtained while the organic ligands can be subsequently recycled. Moreover, by removing the organic fragments, this process generates mesoporous within the microporous frameworks to facilitate substrate diffusion.12-15 However, many MOFs completely lose the porosity and morphology under alkaline conditions,16 possibly due to the leaching of metal cations from MOFs and the subsequent uncontrolled growth of metal hydroxides. Therefore, understanding the conversion mechanism is important to design MOF precursors for the synthesis of nanomaterials with desired physicochemical properties. Herein, we report a facile and large-scale synthesis of Ni-Co layered double hydroxide (LDH) hierarchical microspheres via MOF template for supercapacitors application. MOFs with preformed M3(μ3-OH) (M = Ni2+ and Co2+) clusters were selected as templates to produce LDH nanostructures17 through a pseudomorphic conversion process under alkaline conditions. Isotopic


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tracing experiment revealed the μ3-OH groups in the M3(μ3-OH) clusters of MOF parents play an important role in the pseudomorphic transformation process, which avoided the leaching of metal cations to maintain the MOF morphology. Taking advantage of the high tunability of MOFs, the morphology of MOF particles and the Ni/Co ratios can be adjusted. By judicious control, well-defined Ni/Co-LDH hierarchical microspheres with ultrathin nanosheet subunits were obtained. These microspheres possess a 3D architecture with a large accessible surface area for high active sites and mesoporous channels for fast electrolyte and electron transport. Under optimized condition, the LDH hierarchical microspheres with Ni:Co ratios of 7:3 shows a high capacitance value of 1652 F·g-1 and outstanding long-cycling performance. Moreover, the assembled hybrid all-solid-state supercapacitor displays a high energy density of 32.9 Wh·kg-1 at a power density of 74.3 W·kg-1. RESULTS AND DISCUSSION A MOF named [Ni3(OH)(Ina)3(BDC)1.5] was selected as the template, because it possesses a Ni3(μ3-OH) cluster similar to the building unit of LDH (Figure S1).18 The asymmetric unit of [Ni3(OH)(Ina)3(BDC)1.5] consists of one hexacoordinated Ni, one μ3-OH, half of a BDC and half of an Ina. The μ3-OH connects three adjacent Ni atoms to form a trinuclear second building unit (SBU) which is further bridged by BDC and Ina to construct the final porous framework (Figure 1). Since the Ni3(μ3-OH) SBU can also be viewed as the building unit of LDH, we hypothesis that the preformed cluster in the MOF will facilitate the pseudomorphic conversion towards LDH. As expected, MOFs based on μ3-OH bridged clusters were converted into LDH by alkaline hydrolysis without obvious change of particle morphology.


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To further reveal the role of μ3-OH, isotope tracer technique was employed to monitor the position of μ3-OH during the hydrolysis process. As shown in Figure 2, two parallel experiments were designed. First, the 18O labeled MOF was prepared by adding H218O during MOF synthesis, generating 18O-Ni-MOF with a 8.9% of μ3-OH in the SBU replaced by 18O. After soaking 18ONi-MOF in common KOH solution, 28 % of the μ3-OH from Ni-MOF was maintained in the produced Ni(OH)2-LDH after hydrolysis. In an alternative route, the regular soaked in the

18O

labeled 1 M KOH aqueous solution. The

18O

16O-Ni-MOF

was

abundance in Ni(OH)2-LDH

product indicates that 32 % OH- group from Ni-MOF maintained in the Ni(OH)2-LDH. The experiment and calculation are detailed in Table S3. In summary, although the oxygen exchange between OH- group and aqueous solutions is inevitable, around 28% to 32% of μ3-OH from Ni3 (μ3-OH) cluster of MOFs are still maintained during hydrolysis. These results is consistent with our hypothesis that the preformed Ni3(μ3-OH) cluster act as a template to facilitate the direct formation of Ni(OH)2-LDH, therefore avoid the leaching of Ni2+ and the uncontrolled growth of metal hydroxides. This result highlights the importance of μ3-OH, which can be widely applied to guide the selection of MOF templates beyond this work. Guided by this design principle, a series of MOFs without μ3-OH bridged SBUs were tested as templates, which give rise to expected LDHs with maintained morphology. For comparison, MOFs without μ3-OH bridged SBUs usually lose the morphology after alkaline hydrolysis (Figure S2 and Table S5). Taking advantage of the high tunability of MOFs, the morphology and metal species in the material can be readily tuned, giving rise to LDH with desired morphology and composition.19 The morphology of MOFs can be tuned from block crystals to uniform microspheres by the addition of different amounts of glycol (Figure S3).20 Broadening of diffraction peaks were observed in PXRD patterns, in line with the decreased particle sizes (Figure S4). The XPS


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results demonstrate that the Ni-MOF crystals and nanospheres present similar coordination motifs, further verifying the maintained framework structure (Figure S8a and 8b).15,

21

Since

Ni2+ and Co2+ have similar ionic radius and coordination environment, the Ni2+ in MOFs can be partially replaced by Co2+ during MOF synthesis. By varying the Ni/Co ratios of starting materials (i.e. Ni(NO3)2 and Co(NO3)2), Ni/Co-bimetallic crystals and microspheres can be obtained while maintaining the same framework structure, as indicated by PXRD (Figure S5 and S6) and XPS (Figure S8c-f). 1H-NMR of digested MOF microsphere shows a BDC:Ina ratio of 1.1:2 (Figure S9), matching well with theoretical composition of MOFs. Subsequently, through a facile and controllable alkaline hydrolysis strategy,22 the target LDH hierarchical microspheres were constructed. During the alkaline hydrolysis process, the coordinated ligands (BDC and Ina) are removed from the skeleton of MOFs, and the trinuclear metal clusters are released to coordinated with free OH- ions in solution. The adjacent clusters are bridged by OH- ions to form initial NixCo1-x(OH)2 nuclei, which further grow within the lowest energy (101) plane to form thin hexagonal sheets (Figure S10). Both large single crystals and spherical nanoparticles of MOF precursors can readily undergo pseudomorphic transformation by alkaline hydrolysis. Under optimized conditions, the Ni/Co-LDH hierarchical microspheres can be synthesized in large scale (23 g per run, as shown in Figure 1). After alkaline hydrolysis, the ligand can be easily precipitate out by HCl acidification and 90% of organic ligands can be recycled by filtration. Moreover, this alkaline hydrolysis method can be readily extended to fabricate other bimetallic LDHs such as NixCu1-x(OH)2 (Figure S6). The transformation from MOFs to LDH was monitored by scanning electron microscopes (SEM) and transmission electron microscopy (TEM) (Figure 3). Taking Ni/Co-MOF-7:3 as an example (7:3 indicates the Ni/Co ratio in the starting material), uniform microspheres with sizes


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around 1.5 μm were observed (Figure 3a). After soaking the Ni/Co-MOF-7:3 in 1 M KOH solution for 6 h at room temperature, the size of each microsphere remains unchanged (Figure 3b). The microspheres present obviously hierarchical architectures, consisting of sheet-like subunits. The TEM images also confirmed the pseudomorphic conversion process, with a maintained particle size of 1.5 μm (Figure 3c and Figure S11a). Elemental mapping by energydispersive X-ray spectroscopy (EDX) (Figure 3d) reveals that the Ni/Co-LDH-7:3 consists of homogeneously distributed Co and Ni elements. The Ni/Co ratio were found to affect the morphology of Ni/Co-LDH (Figure S10). Generally, with the decrease of Ni/Co ratio, the size of nanosheet-subunits increases, while the vertical-growth subunits translate to hexagon nanosheets growing along the surface of microspheres. The complete phase transformation was also verified by IR (Figure S12), TGA (Figure S13) and XPS (Figure S14).23 The crystallinity of the Ni/Co-LDH products was characterized by XRD (Figure 4a). The Ni/Co-LDH with the Ni:Co ratios of 5:5, 3:7 and 0:10 can be identified as a Co(OH)2 phase (JCPDS: 74-1057), and the product lacking Co2+ is found to be pure Ni(OH)2 phase (JCPDS: 140117). No apparent diffraction peak is observed for the Ni/Co-LDH-7:3, indicating an intermediate phase between Ni(OH)2 and Co(OH)2. The Ni/Co ratio also controls the porosity of Ni/Co-LDH materials. To gain further insight into the pore structure and the specific surface area, the N2 adsorption/desorption isotherms were tested (Figure S15). By gradually replacing Ni with Co, a decrease of micropore volume and BET surface areas was observed. This is in line with the observation by TEM that Ni favors the formation of fine nanosheet-subunits. Ni/CoLDH-7:3 showed a distinct hysteresis in a range of 0.4-0.8 P/P0 (Figure 4b), indicating the presence of mesopores formed by nanosheet-subunits. The BET surface area and total pore volume are calculated to be 201 m2∙g-1 and 0.30 cm3∙g-1 respectively. The pore size distribution


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calculated from Barrett-Joyner-Halenda indicates the existing of both micropores and mesopores with diameters of 1.5 nm and 4 nm respectively. Within the hierarchical nanostructure of Ni/CoLDH-7:3, the micropores significantly enhance the surface area while the mesopores allow the free OH- migration in charge/discharge process, which makes this material suitable for supercapacitor applications. The CV curves of Ni/Co-LDH-7:3 based electrode show symmetric and gradually enhanced redox peaks with scan rate increasing from 1 to 10 mV·s-1, indicating a typical pseudocapacitive performance and outstanding reversibility (Figure 5b).24 The Galvanostatic Charge–Discharge (GCD) curves involve an obvious plateau of faradic reactions during charging and discharging (Figure 5c).25 The specific capacitance of 1652, 1626.4, 1588, 1516, 1434, 1380 and 1300 F·g-1 are achieved at current densities of 1, 2, 5, 10, 15, 20, and 25 A·g-1 respectively (Figure 5d), demonstrating an outstanding rate capability of 78.7% from 1 to 25 A·g-1. The capacity value of Ni/Co-LDH-7:3 electrode is higher than other Ni/Co-LDH electrodes with the Ni:Co mole ratios of 10:0, 5:5, 3:7 and 0:10 (specific capacities of 1203.8, 1041.4, 711.8 and 323.4 F·g-1 at a current density of 1 A·g-1 respectively). The value is also higher than Ni/Co-MOF-7:3 without hydrolysis (807.6 F·g-1, Figure S17), calcinated Ni/Co-MOF-7:3 at 350 °C (161.2 F·g-1, Figure S18) and Ni0.7Co0.3(OH)2 prepared by chemical precipitation (1190 F·g-1, Figure S19). Current collectors including stainless steel mesh, carbon cloth and activated carbon cloth was also attempted, and the results are shown in (Figure S20). The steel mesh and carbon cloth electrode demonstrate a low specific capacity, due to the weak contact between steel mesh and active materials (inset in Figure S20c) as well as the weak contact between carbon cloth and electrolyte (bubble on the surface of electrode surface, Figure S20f). After activated process of acid treatment, active cloth presents an improved hydrophilia but a poor mechanical stability (inset in


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Figure S20i), and the electrochemical performance of activated carbon electrode is also lower than nickel foam electrode. To eliminate the possible background capacity resulted from nickel foam, the performance of bare nickel foam was measured, which shows a low capacitance of 32 F·g-1 at a current density of 5 A·g-1 (Figure S21). Furthermore, the Ni/Co-LDH-7:3 electrode exhibits an unexpected cycling capability. After 2000 cycles of charge/discharge at 5 A·g-1, a specific capacitance of 1641.6 F·g-1 was observed (maintain almost 100 % of the initial capacitance), which is among the highest of reported Ni/Co-LDH materials in the literature (Table S2). The initial increase in specific capacity is due to the activation of Ni/Co-LDH material, which has been observed in other LDH materials.26,27 As shown in Figure 5f, the Ni/Co-LDH-7:3 electrode possesses a low charge-transfer resistance and a higher line slope in the low frequency region for high capacity performance (supported by the simulated equivalent circuit results, Table S6). Those results demonstrate that the synergistic effect of Ni/Co elements and the pore-structure of hierarchical microsphere play important role on improving supercapacitor performance of Ni/Co-LDH. Meanwhile, an all-solid-state asymmetric supercapacitor28 was fabricated using Ni/Co-LDH-7:3 as the positive electrode and activated carbon (AC) as the negative electrode, with a mass ratio of 1:3.29 The specific capacitance reaches up to 106.2 F·g-1 at 0.1 A·g-1 (Figure S22c). When increasing the current density to 2.5 A·g-1, the specific capacitances only decrease by 19% (Figure S22d). After 10000 galvanostatic charge/discharge cycles at a current density of 1.5 A·g-1, the capacitance maintains 83.50% of its initial value. The asymmetric device shows high energy density of 32.9 Wh·kg-1 at a power density of 74.3 W·kg-1, while retaining a high energy density of 1817.3 W·kg-1 at a power density of 26.0 Wh·kg-1 (Figure S22f). CONCLUSION


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In summary, an alkaline hydrolysis method has been employed to prepare Ni/Co-LDH in large-scale from MOF-template. The μ3-OH bridged cluster in the MOF-template was demonstrated to be critical to maintain the morphology of particles during hydrolysis. By judicious design, Ni/Co-LDH-7:3 hierarchical microspheres with optimized morphology, porosity and composition was obtained. As supercapacitor electrodes, Ni/Co-LDH-7:3 shows a high specific capacitance of 1652 F·g-1 at 1 A·g-1 and excellent cycling stability of maintaining almost 100% of the initial capacitance after 2000 cycles. An all-solid-state Ni/Co-LDH-7:3//AC asymmetric supercapacitor was fabricated,30 and achieved a high energy density of 32.9 Wh·kg-1 at 74.3 W·kg-1 with good cyclic stability. Moreover, the hydrolysis method allows the recycling of organic ligands and large-scale fabrication of metal hydroxide materials. Besides providing a series of tunable Ni/Co-LDH materials, our work further suggests that the alkaline hydrolysis method can be readily extended to other bimetallic LDHs from MOF-templates. This facile, controllable and scalable strategy will be widely applied in MOF derived functional materials for electrochemical applications and beyond. METHODS Material Characterization. X-ray powder diffraction patterns of the prepared samples were collected on a Bruker AXS D8 Advance instrument with Cu-Kα radiation (λ = 1.5418 Å). The morphology and structure of the prepared samples were examined by scanning electron microscopy (SEM, JSM-7500F; TEM, Tecnai G2 F20). The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area of samples by N2 adsorption-desorption measurements employing a surface area analyser, ASAP-2020. Prior to the BET measurements, the samples were degassed for 5 h at 100 ℃. FTIR spectra were measured using Perkin Elmer


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Frontier FT-IR Spectrometer in the 4000-400 cm-1 region. Thermogravimetric analysis (TGA) experiments were carried out on a Mettler Toledo TGA instrument with a heating rate of 10 ℃·min-1 in the range of 40-700 ℃ under nitrogen. Preparation of MOF precursors. Commercially available reagents were used as received without further purification. A mixture of Ni(NO3)2•6H2O (95.25 g, 317.34 mmol), 1,4benzenedicarboxylic acid (BDC, 60 g, 360 mmol) and isonicotinate (Ina, 36 g, 300 mmol) were dissolved in N,N-dimethylformamide (DMF, 2400 mL) and 2400 mL ethylene glycol (EG) was added to the mixture. The solution was heated to 140 ℃ for 3 days in a 5 liter Silk mouth bottle. After cooling to room temperature, the resulting precipitate was washed several times with EtOH, and then dried in an oven at 60 ℃. For comparison, Ni/Co-MOF-7:3, Ni/Co-MOF-5:5, Ni/Co-MOF-3:7 and Co-MOF were also synthesized by a similar method, in which Ni(NO3)2•6H2O was replaced by Co(NO3)2•6H2O with the molar ratios of 7:3, 5:5, 3:7, and 0:10. At the same time, Ni/Cu-MOF-7:3 were also synthesized by Ni(NO3)2•6H2O and Cu(NO3)2•3H2O. Synthesis of MOF-NixCo1-x(OH)2. The obtained MOFs (60 g) were added to 3000 ml KOH aqueous solution (1 M) at room temperature and held for 6 h. The resulting precipitate was washed with ultrapure water and EtOH (three times, 200 mL each time) to remove excess KOH and residual ligands. After drying, the MOF derived NixCo1-x(OH)2 compounds were obtained and named as Ni/Co(OH)2-10:0, Ni/Co(OH)2-7:3, Ni/Co(OH)2-5:5, Ni/Co(OH)2-3:7 and Ni/Co(OH)2-0:10. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to indicate the actual atomic ratio.


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Synthesis of Ni0.7Cu0.3(OH)2. Ni/Cu-MOF-7:3 (0.15 g) were immersed in 10 mL of an aqueous solution of KOH with 1 M concentrations for 6 h, and the products were obtained after rinsing with deionized water and ethanol three times in order to remove the residual KOH and the organic ligands. The products were then left at 60 ℃ for 12 h. Direct synthesis of Ni(OH)2 and Ni0.7Co0.3(OH)2. In a typical synthesis, Ni(NO3)2•6H2O (2.91 g, 10 mmol) was dissolved in 30 mL of deionized (DI) water, then KOH (1.12g, 20 mmol) was dissolved in 30 mL of DI water. Those two solutions were mixed and stirred for 10 minutes and remained 24 h at room temperature, then the resulting green precipitates were collected by centrifuging, washed with water subsequently for 3 times, and finally vacuum dried at 100 ℃ for 24 h. At the same time, the Ni(NO3)2•6H2O was replaced by Co(NO3)2•6H2O with the molar ratios of 7:3. The Ni0.7Co0.3(OH)2 were also synthesized by a similar method. Preparation of PVA/KOH hydrogel polymer electrolyte. PVA (2 g) was mixed with 80 ml of water. The mixture was heated and stirred until a clear solution was obtained. Subsequently, KOH (12 g) was added to the stirred solution. The prepared gel solution was poured into a watch-glass and left in ambient conditions to allow the evaporation of excess water. Electrochemical characterization. All electrochemical measurements were carried out under the same ambient conditions using a CHI 760E electrochemical workstation. In a three-electrode system, 6 M KOH was used as the aqueous electrolyte, a platinum net (1.5 cm × 1.5 cm) as the counter electrode, and a saturated calomel electrode (Hg/HgO) as the reference electrode. For the working electrode, a homogeneous slurry containing active materials, carbon black and polytetra-ﬂuoroethylene (PTFE) with a weight ratio of 8:1:1 in EtOH was first prepared, and was followed by heating at 60 ℃ in vacuo for at least 12 h. About 2.5 mg of the solidiﬁed mixture


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was then painted between two pieces of nickel foam (1 cm × 2 cm) and pressed face-to-face with a pressure of 1.0 MPa. The mass loading of the active material was accurately determined by the mass difference of the nickel foams before and after the loading of active materials. The electrochemical performance of the electrode prepared in this way was examined by cyclic voltammetry (CV) and galvanostatic charge/discharge. The discharge specific capacitance (Csp) was calculated according to the following Equation. Csp=(IΔt)/(mΔU) where I [A] is the applied current, ΔU [V] is the tested potential range, Δt [s] is the discharge time, and m [g] is the mass of the tested active material within the electrodes. Electrochemical impedance spectra were collected by applying a perturbation voltage of 5 mV in a frequency range of 0.01 Hz to 105 Hz. An asymmetric supercapacitor was fabricated using the MOF derived NixCo1-x(OH)2 as the positive electrode and an activated carbon (AC) electrode as the negative electrode. Activated carbon (90 wt%) and polytetrafluoroethylene (10 wt%) were mixed and spread on two pieces of nickel foam (1 cm × 2 cm), then an AC electrode was fabricated by pressing the nickel foams under 1.0 MPa face-to-face. Electrochemical measurements of the asymmetric supercapacitors with optimized mass ratio were conducted using the CHI 760E workstation, and included cyclic voltammetry and galvanostatic charge/discharge in 6M KOH aqueous solution.


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Figure 1. Schematic illustration of the synthetic strategy of the Ni/Co-LDH. Digital pictures of Ni/Co-MOF template (60 g) and the resultant Ni/Co-LDH (23 g) prepared in one reaction. During the alkaline hydrolysis process, 90% of the main organic ligands can be recollected and reused in the subsequent reaction.


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Figure 2. Two ways to monitor the μ3-OH groups by isotopic tracing experiment.

Figure 3. (a) The SEM image of the Ni/Co-MOF-7:3 precursor. (b) The SEM image of the Ni/Co-LDH-7:3. (c) and (e) The TEM images of the Ni/Co-LDH-7:3 at different magnification. (d 1-3) The SEM mapping of the Ni/Co-LDH-7:3.


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Figure 4. (a) The XRD patterns of Ni/Co-LDH with different Ni:Co ratios, derived from MOFs. (b) N2 adsorption/desorption isotherm curves of MOF-derived Ni/Co-LDH-7:3.


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Figure 5. (a) CV curves of blank Ni net and Ni/Co-LDH derived from MOFs with different Ni/Co ratios at a scan rate of 2 mV·s-1. (b) CV curves of the LDH-7:3 electrode at different scan rates. (c) GCD curves of the LDH-7:3 electrode at different current densities. (d) The corresponding specific capacitance of Ni/Co-LDH electrodes at current densities from 1 to 25 A·g-1. (e) Cycling performance of the Ni/Co-LDH electrodes at a current density of 5 A·g-1. (f) Comparison of Nyquist plots of Ni/Co-LDH electrodes.


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ASSOCIATED CONTENT Supporting Information Additional SEM/TEM images, XRD, XPS, 1H-NMR, FT-IR, BET, CV, and TGA figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. Conflict of Interest: The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. E-mail: [email protected] Author Contributions ∥These

authors contributed equally.

ACKNOWLEDGMENT This work was supported by the NSFC (Grant Nos. 21875285, 21805155), Taishan Scholars Program (ts201511019). The gas adsorption-desorption studies of this research were supported by the the Center for Gas Separations, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0001015). Structural analyses were supported by the Robert A. Welch Foundation through a Welch Endowed Chair to H.J.Z. (A-0030). The authors also acknowledge the financial support of the U.S. Department of Energy Office of Fossil Energy National Energy Technology Laboratory


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(DEFE0026472) and National Science Foundation Small Business Innovation Research (NSFSBIR) program under Grant No. (1632486). This paper is dedicated to Professor Xin-Tao Wu on the occasion of his 80th birthday.

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Table of Contents Graphic


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Controlled Hydrolysis of MetalâOrganic Frameworks: Hierarchical Ni

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