Controlled Hydrolysis of Metal–Organic Frameworks: Hierarchical Ni

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Controlled Hydrolysis of Metal−Organic Frameworks: Hierarchical Ni/Co-Layered Double Hydroxide Microspheres for HighPerformance 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,*,† and 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 S Supporting Information *

ABSTRACT: Pseudomorphic conversion of metal−organic frameworks (MOFs) enables the fabrication of nanomaterials with welldefined 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. An 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 a 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 a 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 recycling of organic ligands and large-scale synthesis of LDH materials. KEYWORDS: layered double hydroxide, metal−organic framework, supercapacitor, hierarchical microspheres, pseudomorphic conversion 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;

M

etal−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 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 © 2019 American Chemical Society

Received: March 18, 2019 Accepted: May 23, 2019 Published: May 23, 2019 7024

DOI: 10.1021/acsnano.9b02106 ACS Nano 2019, 13, 7024−7030

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Figure 1. Schematic illustration of the synthetic strategy of the Ni/Co-LDH. Digital pictures of the 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.

MOFs, the morphology of MOF particles and the Ni/Co ratios can be adjusted. By judicious control, well-defined Ni/CoLDH 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 conditions, the LDH hierarchical microspheres with Ni:Co ratios of 7:3 show 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.

therefore they 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 results 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 toward large-scale 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 mesopores within the microporous frameworks to facilitate substrate diffusion.12−15 However, many MOFs completely lose their 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 an MOF template for supercapacitor applications. 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 tracing experiments 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

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 1,4-benzenedicarboxylate (BDC), and half of an isonicotinate (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 hypothesize that the preformed cluster in the MOF will facilitate the pseudomorphic conversion toward LDH. As expected, MOFs based on μ3-OH-bridged clusters were converted into LDH by alkaline hydrolysis without obvious change of particle morphology. To further reveal the role of μ3-OH, an isotope tracer technique was employed to monitor the position of μ3-OH during the hydrolysis process. As shown in Figure 2, two 7025

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LDHs with maintained morphology. For comparison, MOFs without μ3-OH-bridged SBUs usually lose their 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 the powder X-ray diffraction (PXRD) patterns, in line with the decreased particle sizes (Figure S4). The XPS results demonstrate that the Ni-MOF crystals and nanospheres present similar coordination motifs, further verifying the maintained framework structure (Figure S8a and b).15,21 Since Ni2+ and Co2+ have a 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 (Figures S5 and S6) and X-ray photoelectron spectroscopy (XPS) (Figure S8c−f). 1H NMR of a digested MOF microsphere shows a BDC:Ina ratio of 1.1:2 (Figure S9), matching well with the 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 coordinate with free OH− anions in solution. The adjacent clusters are bridged by OH− anions 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

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

parallel experiments were designed. First, the 18O-labeled MOF was prepared by adding H218O during MOF synthesis, generating 18O-Ni-MOF with 8.9% of the μ3-OH in the SBU replaced by 18O. After soaking 18O-Ni-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 16O-Ni-MOF was soaked in an 18O-labeled 1 M KOH aqueous solution. The 18O abundance in the Ni(OH)2-LDH product indicates that 32% of the OH− group from Ni-MOF was maintained in the Ni(OH)2-LDH. The experiment and calculation are detailed in Table S3. In summary, although the oxygen exchange between the OH− group and aqueous solutions is inevitable, around 28% to 32% of μ3-OH from the Ni3(μ3-OH) cluster of MOFs is still maintained during hydrolysis. These results are consistent with our hypothesis that the preformed Ni3(μ3-OH) cluster acts as a template to facilitate the direct formation of Ni(OH)2-LDH, therefore avoiding 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

Figure 3. (a) SEM image of the Ni/Co-MOF-7:3 precursor. (b) SEM image of the Ni/Co-LDH-7:3. (c and e) TEM images of the Ni/CoLDH-7:3 at different magnification. (d, 1−3) SEM mapping of the Ni/Co-LDH-7:3. 7026

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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: 14-0117). No apparent diffraction peak is observed for the Ni/Co-LDH7:3, 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/Co-LDH-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 calculated from Barrett−Joyner−Halenda indicates the existence of both micropores and mesopores with diameters of 1.5 and 4 nm, respectively. Within the hierarchical nanostructure of Ni/Co-LDH-7:3, the micropores significantly enhance the surface area while the mesopores allow free OH− migration in the charge/discharge process, which makes this material suitable for supercapacitor applications. The CV curves of the 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 capacitances 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 the Ni/Co-LDH-7:3 electrode is higher than other Ni/Co-LDH electrodes with 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/CoMOF-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 were also attempted, and the results are shown in Figure S20. The steel mesh and carbon cloth electrodes 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 the electrode surface, Figure S20f). After an activated process of acid treatment, the active cloth presents an improved hydrophilicity but a poor mechanical stability (inset in Figure S20i), and the electrochemical performance of the activated carbon electrode is also lower than the nickel foam electrode. To eliminate the possible background capacity resulting 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 (maintaining almost 100% of the initial capacitance), which is among the

in large scale (23 g per run, as shown in Figure 1). After alkaline hydrolysis, the ligand can be easily precipitated 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 S7). The transformation from MOFs to LDH was monitored by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 3). Taking Ni/CoMOF-7:3 as an example (7:3 indicates the Ni/Co ratio in the starting material), uniform microspheres with sizes around 1.5 μm were observed (Figure 3a). After soaking the Ni/CoMOF-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 energy-dispersive 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 was found to affect the morphology of Ni/Co-LDH (Figure S10). Generally, with the decrease of the Ni/Co ratio, the size of nanosheet subunits increases, while the verticalgrowth 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

Figure 4. (a) XRD patterns of Ni/Co-LDH with different Ni:Co ratios, derived from MOFs. (b) N2 adsorption/desorption isotherms and pore size disribution of MOF-derived Ni/CoLDH-7:3. 7027

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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) Corresponding specific capacitance of Ni/Co-LDH electrodes at current densities from 1 to 25 A·g−1. (e) Cycling performance of the Ni/CoLDH electrodes at a current density of 5 A·g−1. (f) Comparison of Nyquist plots of Ni/Co-LDH electrodes.

energy density of 1817.3 W·kg−1 at a power density of 26.0 Wh·kg−1 (Figure S22f).

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 the hierarchical microsphere play important roles in improving the 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 decrease by only 19% (Figure S22d). After 10 000 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 a high energy density of 32.9 Wh· kg−1 at a power density of 74.3 W·kg−1, while retaining a high

CONCLUSION In summary, an alkaline hydrolysis method has been employed to prepare Ni/Co-LDH in large scale from an 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-LDH7:3 hierarchical microspheres with optimized morphology, porosity, and composition were 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, maintaining almost 100% of the initial capacitance after 2000 cycles. An all-solid-state Ni/Co-LDH-7:3//AC asymmetric supercapacitor was fabricated30 and achieved a high energy density of 32.9 Wh·kg−1 at 74.3 W·kg−1 with good cycling 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 7028

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× 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 polytetrafluoroethylene (PTFE) with a weight ratio of 8:1:1 in EtOH was first prepared, followed by heating at 60 °C in vacuo for at least 12 h. About 2.5 mg of the solidified mixture 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:

bimetallic LDHs from MOF templates. This facile, controllable, and scalable strategy will be widely applied in MOFderived 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 SEM (JSM7500F; 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 analyzer, ASAP-2020. Prior to the BET measurements, the samples were degassed for 5 h at 100 °C. FTIR spectra were measured using a PerkinElmer 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 °C·min−1 in the range of 40−700 °C under nitrogen. Preparation of MOF Precursors. Commercially available reagents were used as received without further purification. A mixture of Ni(NO3)2·6H2O (95 g, 317 mmol), BDC (60 g, 360 mmol), and Ina (36 g, 300 mmol) was dissolved in N,N-dimethylformamide (DMF, 2400 mL), and 2400 mL of ethylene glycol (EG) was added to the mixture. The solution was heated to 140 °C for 3 days in a 5 L 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 °C. 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 molar ratios of 7:3, 5:5, 3:7, and 0:10. At the same time, Ni/CuMOF-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 a 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 MOFderived NixCo1−x(OH)2 compounds were obtained and named Ni/ CoLDH-10:0, Ni/CoLDH-7:3, Ni/CoLDH-5:5, Ni/LDH-3:7, and Ni/LDH-0:10. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to indicate the actual atomic ratio, and the resuls were summarized in Table S4. Synthesis of Ni0.7Cu0.3(OH)2. Ni/Cu-MOF-7:3 (0.15 g) was immersed in 10 mL 1M KOH aqueous solution 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 °C 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 min and remained for 24 h at room temperature; then the resulting green precipitates were collected by centrifuging, washed with water subsequently three times, and finally vacuum-dried at 100 °C 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 was 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

Csp = (I Δt )/(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 to 105 Hz. An asymmetric supercapacitor was fabricated using the MOFderived NixCo1−x(OH)2 as the positive electrode and an 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 a 6 M KOH aqueous solution.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02106. Additional SEM/TEM images, XRD, XPS, 1H NMR, FT-IR, BET, CV, and TGA figures (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shuai Yuan: 0000-0003-3329-0481 Wenpei Kang: 0000-0001-6550-9287 Fangna Dai: 0000-0002-5300-5388 Rongmign Wang: 0000-0002-5445-541X Lei Wang: 0000-0001-7275-4846 Songqing Hu: 0000-0001-5004-6259 Hong-Cai Zhou: 0000-0002-9029-3788 Author Contributions ∥

Z.X., Y.M., and S.Y. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the NSFC (Grant Nos. 21875285, 21805155) and Taishan Scholars Program (ts201511019). The gas adsorption−desorption studies of this research were supported by 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 (DESC0001015). Structural analyses were supported by the Robert 7029

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ACS Nano

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

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DOI: 10.1021/acsnano.9b02106 ACS Nano 2019, 13, 7024−7030