Bimetallic-MOF Derived Accordion-like Ternary Composite for High

Aug 23, 2018 - College of Science, China University of Petroleum (East China) ... Qingdao University of Science and Technology, Qingdao 266042 , P. R...
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Bimetallic-MOF Derived Accordion-like Ternary Composite for HighPerformance Supercapacitors Hao Mei,† Yingjie Mei,† Shiyu Zhang,† Zhenyu Xiao,*,‡ Ben Xu,† Haobing Zhang,† Lili Fan,† Zhaodi Huang,† Wenpei Kang,† and Daofeng Sun*,† †

College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, P. R. China 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, P. R. China

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ABSTRACT: Supercapacitors are regarded to be highly probable candidates for next-generation energy storage devices. Herein, a bimetallic Co/Ni MOF is used as a sacrificial template through an alkaline hydrolysis and selective oxidation process to prepare an accordion-like ternary NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 composite, which is composed of Co/Ni(OH)2 nanosheets with large specific surface as the frame and NiCo2O4 nanoparticles with high conductivity as the insertion, for supercapacitor application. This material exhibits both high specific capacitance (1315 F·g−1 at 5 A·g−1) and excellent cycle performance (retained 90.7% after 10 000 cycles). This hydrolysis−oxidation process, alkali hydrolysis followed by oxidation with H2O2, offers a novel approach to fabricate the Ni/ Co-based electrode materials with enhanced supercapacitor performance.

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INTRODUCTION With the increasingly serious massive fossil consumption, environmental destruction, and global warming issues, the research puzzles of high-efficiency, renewable, and secure energy storage devices for environmental protection have become the most critical problems faced by human beings.1−5 Among various energy storage devices, the supercapacitor wins the favor of researchers for its high power densities, fast charging, and excellent cycle performance.6−8 However, its relatively low energy density is an obstacle to commercial applications. The energy density is able to be improved through high specific capacitance and broadening of working voltage.9,10 Besides specific capacitance, cycling performance is another especially important factor that significantly determines the commercial applications of supercapacitors. To this end, design and fabrication of advanced energy storage materials with excellent cycling life and high specific capacitance is the key. Recently, transition metal hydroxides are considered to be probable electrode materials as a result of their high surface area and large theoretical capacitance (3500−4600 F·g−1).11,12 For example, binary Co/Ni hydroxides (Cox/Ni1−x(OH)2) show better electrochemical performance than either unitary cobalt hydroxide (Co(OH)2) or unitary nickel hydroxide (Ni(OH)2).13,14 Nonetheless, Cox/Ni1−x(OH)2 is still an unsatisfactory material due to the relatively poor conductivity and low cycling performance.15 To overcome these drawbacks, extensive efforts have been made to design multicomponent © XXXX American Chemical Society

composite materials combined with other well-known electrode materials which have high structural stability and electrical conductivity.16−20 For instance, Liu et al. successfully constructed a hierarchical NiCo2O4@Ni(OH)2 structure which presents a high capacitance of 464 F·g−1 nearly 1.6 times higher than layered Co3O4@Ni(OH)2 (291 F·g−1) and ∼18 times higher than NiCo2O4.21 Hence, to fabricate the composite of metal oxide and metal hydroxide should be a facile strategy to improve the supercpacitor performance. Although several such materials have been reported recently,21,22 the composite of bimetal oxides and bimetal hydroxides such as NiCo2O4/NixCo1−x(OH)2 have seldom been used for supercapacitors, probably due to the difficulty in achieving uniform distribution of the metal elements and the less controllable size, phase, and morphology.19 In this paper, we describe a hydrolysis−oxidation approach via a bimetallic metal−organic framework to stepwise fabrication of a NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite for high-performance supercapacitors. As is known, metal organic frameworks (MOFs), a novel sort of porous crystal material with well-defined pore structures, ultrahigh surface areas, rich metal centers, and periodic networks, have been regarded as talented precursors and sacrificial temples with which to construct porous electrode materials, such as carbon-based materials, metal Received: June 7, 2018

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DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry oxides, metal hydroxides, and other composite materials.23−25 Through reasonable control of the reaction process, these asobtained MOF-derived materials can inherit porous structures and open channels and often exhibit excellent electrochemical performances.26,27 To date, the reported MOF-derived nanomaterials have most concentrated on metal oxides and metal sulfides. As far as we know, there is only one sample of MOFderived Cox/Ni1−x(OH)2 that was applied in supercapacitors,28 and no composites of bimetal oxides and bimetal hydroxides with MOF as the precursor have been reported to date. Herein, a bimetallic Co/Ni-MOF [(NixCo3−x)3O(BTC)2(H2O)(DMF)]n was employed to prepare layered Cox/Ni1−x(OH)2 through an alkali hydrolysis strategy based on our previous work.29,30 The obtained cobalt nickel hydroxide with an optimized Co:Ni ratio of 0.63:1 displays an excellent capacitance of 1956 F·g−1 at 5 A·g−1 (15.37 times and 1.59 times compared with MOF-derived Co(OH)2 and Ni(OH)2), but relatively very low cycling performance (keeping 48% specific capacitance after 2000 cycles). After the layered Cox/ Ni1−x(OH)2 is further selectively oxidized to a NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite (Ni/ Co-TC), the cycling performance increases significantly with 90.7% retained after 10 000 cycles. Furthermore, the assembled NiCo2O4/β-NixCo 1−x(OH)2/α-Ni xCo1−x(OH)2//AC allsolid-state hybrid supercapacitor (named as Ni/Co-TC// AC) shows a high energy density of 36.98 Wh·kg−1 at 801.49 W·kg−1. This hydrolysis−oxidation strategy offers a new method to fabricate novel nanohybrid materials with enhanced supercapacitor performance through selective oxidation (Figure 1).

denoting the ratio of cobalt and nickel we added in the initial synthetic progress of MOF, rather than the actual ratio of cobalt and nickel. Synthesis of MOF and the Derived Hydroxide. Similar to the reported literature, for the synthesis of Co/Ni-MOFs-3:1, 1,3,5benzenetricarboxylic acid (0.099 g) and phthalic acid (0.0495 g) were dissolved in 15 mL of DMF. Then CoCl2·6H2O pretreated under vacuum at 333 K for 1 h (0.1868 g, 1.125 mmol) and NiCl2·6H2O (0.0891 g, 0.375 mmol) were added into the above solution, respectively. After that, the solution was heated at 120 °C for 2 days. After naturally cooled, the resulting products were collected by filtration. For comparison, Co/Ni-MOF-1:0, Co/Ni-MOF-5:1, Co/ Ni-MOF-1:1, and Co/Ni-MOF-0:1 were also prepared, and the only difference is the amount of cobalt chloride and nickel chloride (Table S2). In a typical synthesis, 0.1 g of MOF was immersed in 10 mL of potassium hydroxide solution (1 M KOH, 1 h), and washed two times with distilled water and ethanol, respectively. The products were dried in the air naturally. Synthesis of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2. In this experiment, the prepared Co/Ni(OH)2-3:1 (0.1 g) was placed in a 20 mL Teflon-lined autoclave containing 2.5 mL of deionized water, 1 mL of NH3·H2O, and 0.25 mL of H2O2 (30% for the mass fraction). Then the autoclave was heated to 140 °C for 12 h. The products were washed two times with deionized water and ethanol, respectively, and dried in the air naturally. To compare, the same steps were also used for Co/Ni(OH)2-1:0 and Co/Ni(OH)2-0:1. For comparison, the Co/Ni-MOF-3:1-600 was synthesized by the calcination of Co/Ni-MOF-3:1 in the air. 50 mg of Co/Ni-MOF-3:1 was put into a furnace at 600 °C for 5 h (air atmosphere, 5 °C/min). Similarly, the Co/Ni(OH)2-3:1-600 was synthesized under the same condition. Electrochemical Characterization. The electrochemical data were obtained from a CHI 760E instrument. In this experiment, we used platinum mesh (1.5 cm × 1.5 cm) as the counter electrode, calomel electrode (Hg/Hg2Cl2) as the reference electrode, and 6 M KOH as the electrolyte. For the working electrode, active materials (80%, 16 mg), carbon black (10%, 2 mg), and polytetrafluoroethylene (PTFE, 10%) were mixed into the ethanol; then the homogeneous slurry was heated at 60 °C in vacuum for at least 12 h. The solidified mixture (2.5 mg) was pressed on a piece of nickel foam (1 cm × 2 cm) under 1.0 MPa. The electrochemical tests for the as-prepared electrode were examined by cyclic voltammetry (CV) and galvanostatic charge/discharge. Electrochemical impedance spectra were conducted by applying a perturbation voltage of 5 mV in a frequency range of 0.01 to 106 Hz. The all-solid-state hybrid supercapacitor was assembled through using 2 mg of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 as the anode, 5.6 mg of carbon black as the cathode, and PVA/KOH hydrogel polymer as the bath solution. The carbon negative electrode materials were synthesized by mixing 18 mg of activated carbon with polytetrafluoroethylene solution (9:1 for mass ratio). For PVA/KOH gel electrolyte, 1 g of PVA was added into 40 mL of H2O and the cloudy solution was obtain, followed by heating and stirring enough time to get the clear solution. Then under a stirring condition, 6 g of KOH was put into the solution. The obtained colloidal solution was dried in air finally.

Figure 1. Synthetic process of NiCo2O4/β-NixCo1−x(OH)2/αNixCo1−x(OH)2.

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EXPERIMENTAL SECTION

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Structure Characterizations. The chemical reagents were directly used without any purification. X-ray powder diffractions patterns were used to test the crystallinity and phase purity through a Bruker AXS D8 Advance instrument (Cu−Kα, λ = 0.15418 nm). The morphology and structure of prepared samples were obtained through scanning electron microscopy (SEM, JSM-7500F). FTIR spectra were conducted using a Nexus FT-IR spectrometer (under the 4000−600 cm−1 region). The N2 adsorption−desorption isotherms and pore size distributions were tested by a surface area analyzer ASAP-2020 (the samples were degassed for 5 h at 100 °C before measurements). TGA measurements were tested by the Mettler Toledo TGA instrument (N2 atmosphere, 10 °C/min, 40−900 °C). For convenience, the prepared MOFs were termed as Co/NiMOF-a:b, MOF derived Co/Ni(OH)2 as Co/Ni(OH)-a:b, with a:b

RESULTS AND DISCUSSION Characterizations. The synthesis process of NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2 is demonstrated schematically in Figure 1. A μ3-OH bridged trinuclear second building unit (TN-SBU) and trimesic acid ligands coexist in the structure of Co/Ni-MOF. MOF precursors with different Co/ Ni ratios show similar PXRD patterns and FTIR spectra (as shown in Figures S2 and S3). The Co/Ni molar ratios of Co/ Ni-MOFs can be easily tuned by the selection of raw materials with different ratios, and the actual Co/Ni ratios have been ensured by ICP-OES (Tables S1 and S3). When the Co/NiB

DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

SEM and TEM (Figure 3 and Figure S12). Compared with the MOF parent, the Co/Ni(OH)2-3:1 sample still retains a nanoplate morphology but exhibits sharper edges with a hexagonal shape (as shown in Figure S12). After oxidation by H2O2, the as-obtained hybrid sample presents an accordionlike structure that is constructed by Co/Ni(OH)2 nanosheets, and some NiCo2O4 nanoparticles distributed on these sheets (Figure 3a). From the lateral top view of the nanosheets (Figure 3c), the average interlayer spacing is about 12 nm, which corresponds to the pore size distribution (Figures S9 and S10). Furthermore, the TEM is further used to confirm the hybrid structure (Figure 3d,e). The Co/Ni(OH) 2 nanosheets present an obvious packed sheetlike structure, and the NiCo2O4 particles are disorderly distributed on the surface and interior of the nanosheets without aggregation. The hybrid structure of oxidation products is also confirmed by high-resolution TEM (HRTEM) images (Figure 3f). The fringe spacings of 0.28, 0.23, and 0.267 nm are resulted from the (220) lattice spacing of NiCo2O4, the (101) lattice spacing of β-NixCo1−x(OH)2, and the (101) lattice spacing of αNixCo1−x(OH)2, respectively. These results are also supported by XRD results (Figure 2). To gain further insight into the pore structure and specific surface of the prepared NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2, Brunauer−Emmett−Teller (BET) measurements were determined. As shown in Figure S9, the obvious hysteresis within the limits of ca. 0.6−1.0 P/P0 demonstrates that the existence of mesopores resulted from the accordionlike structure. The BET surface area of the porous NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2 is 178.3 m2·g−1. In addition, from the Barrett−Joyner−Halenda (BJH) pore size distribution (Figure S10), the pore size distribution is around 12 nm, which is corresponding to SEM results (Figure 3c). These mesopores not only offer enough electrochemical activity center but also provide excellent electrolyte access for high supercapacitor performance (see below). To compare, the MOF derived Co(OH)2 or Ni(OH)2 were also oxidized by H2O2. As shown in Figure S7, the corresponding PXRD patterns are in good accordance with β-Ni(OH)2 (JCPDS card no: 14-0117) or Co3O4/β-Co(OH)2 (JCPDS card nos: 42-1003 and 30-0443); it was also suggested that only Co2+ can be oxidized by H2O2, and the coexistence of Ni and Co is key to the synthesis of α-NixCo1−x(OH)2. The Co/Ni-MOF-3:1 and Co/Ni(OH)2-3:1 samples are also oxidized through a traditional postcalcination method, and a hybrid phase of NiCo2O4/NiO can be obtained, as shown in Figure S8. To understand better the chemical states change of bonded elements during the selective oxidation process, X-ray photoelectron spectroscopy (XPS) spectra were further used to determine the obtained Co/Ni(OH)2-3:1 and NiCo2O4xCo1−x(OH)2/α-NixCo1−x(OH)2. As shown in Figure 4a,c, it is obvious that the Ni 2p regions of Co/Ni(OH)2-3:1 and NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 are very similar. The peaks around 855.9 and 873.6 eV are assigned to the Ni 2p3/2 and Ni 2p1/2 of Ni2+, indicating the Ni2+ was not oxidized by H2O2. In the Co 2p region of the oxide sample, the peaks at 780.5 and 796 eV are assigned to Co3+, while the peaks at 782.4 and 797.7 eV are assigned to Co2+, which means that both Co2+ and Co3+ exist in NiCo2O4/β-NixCo1−x(OH)2/ α-NixCo1−x(OH)2 (Figure 4d). As a contrast, the two peaks in the Co 2p region of Co/Ni(OH)2-3:1 are attributed to the Co 2p1/2 (797.3 eV) and Co 2p3/2 (781.5 eV) of Co2+, indicating

MOFs are soaked in the alkaline solution, the Cox/Ni1−x(OH)2 nanoparticles are gained through the reaction of TNSBU and OH− and the BTC3− ligands are released. The free BTC3− ions may act as the surfactant to facilitate the formation of cobalt nickel hydroxide hexagonal sheets (Figure S12). Subsequently, layered Co/Ni(OH)2 was used as precursor to construct the NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite through a selective oxidation process using H2O2. In the oxidation process, a portion of Co2+ was oxidized to Co3+, resulting in the production of NiCo2O4 and αNixCo1−x(OH)2.31 The transformation process of MOF to NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2 composite is illustrated by XRD patterns. As displayed in Figure S4, after soaking in alkaline solution, the characteristic diffraction peaks of MOF parent disappeared and the diffraction peaks of Co/Ni hydroxides presented, demonstrating that the MOF was transformed to cobalt nickel hydroxide. The as-obtained samples of Co/Ni(OH)2-0:1 and Co/Ni(OH)2-1:1 (Co:Ni ratios of 0.22:1) were identified as β-Ni(OH)2 phase (JCPDS card: 14-0117), while the samples of Co/Ni(OH)2-5:1 (Co:Ni ratios of 1.19:1) and Co/Ni(OH)2-1:0 (Co:Ni ratios of 1:0) were β-Co(OH)2 phase (JCPDS card: 30-0443). Furthermore, the product of Co/Ni(OH)2-3:1 (Co:Ni ratios of 0.63:1) presents an amorphous feature, which is beneficial for higher supercapacitor performance.13 Then, after a selective oxidation process using H2O2 as the oxidant, a series of new diffraction peaks appear. The characteristic peaks at 31.15°, 36.69°, 44.62°, 64.98° can be indexed to (200), (311), (400), (440) planes of NiCo2O4 (JCPDS card: 20-0781), and the characteristic diffraction peaks of 11.34°, 22.74°, 59.98° belong to αNixCo1−x(OH)2 (JCPDS card: 38-0715), as shown in Figure 2.

Figure 2. XRD pattern of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2.

The (003) interplanar d-spacing of α-NixCo1−x(OH)2 is 0.78 nm, which indicates that the BTC3− anions may stack into a bilayer-like structure.32−34 The characteristic bands of BTC3− anions (1357 and 1608 cm−1) are also observed in the FTIR spectrum (Figure S6), further confirming the existence of BTC3−.35,36 All the major diffraction peaks of NiCo2O4, βNixCo1−x(OH)2, and α-NixCo1−x(OH)2 can be observed in NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 patterns, and no extra peaks are detected, indicating the successful synthesis of the ternary material through a selective phase transformation. The microstructures and surface morphologies of the asobtained MOF derived nanomaterials were obtained from C

DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a−c) SEM of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2; (d−f) TEM of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2. NiCo2O4 particles are marked by red squares.

Figure 4. (a) Ni 2p of Co/Ni(OH)2-3:1; (b) Co 2p of Co/Ni(OH)2-3:1; (c) Ni 2p of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2; (d) Co 2p of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2.

derived Cox/Ni1−x(OH)2 at 10 mV·s−1. Obviously, the oxidation potential of bimetal hydroxide is lower than that of the unitary products, suggesting that the bimetal hydroxide can be charged with ease, which may result from the synergistic effects of Co2+ and Ni2+ ions.14,37 In addition, the anodic peak of Co/Ni(OH)2-3:1 is stronger than the other samples, which implies that Co/Ni(OH)2-3:1 may show the best capacity performance. The CV curves of Co/Ni(OH)2-3:1 in the range of 1−100 mV·s−1 are shown in Figure 5b. The good symmetry of oxidation and reduction peaks suggests the high Coulomb effect. Corresponding to the CV peaks, the GCD curves of the Co/Ni(OH)2-3:1 (Figure 5c) present well-defined charge and discharge platforms. Specific capacitances of 2335, 2220, 2123, 1956, 1760, 1567, and 1413 F·g−1 were observed at current densities of 0.5, 1, 2, 5, 10, 15, 20 A·g−1, respectively. As shown

that only Co2+ exists (Figure 4b). These results further confirm that only Co2+ can be selectively oxidized in the H2O2 oxidation process, as is supported by the XRD result of unitary oxidation products (Figure S7). In the C 1s spectrum of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 (Figure S16d), the binding energies of carbon atoms in the aromatics and carboxylic appear at 284.5 and 288.3 eV, providing the evidence for the existence of BTC3− in α-NixCo1−x(OH)2, which is also supported by XRD (Figure 2) and FTIR (Figure S6). Electrochemical Performances. For better studying the electrochemical characteristics of the prepared nanomaterials for electrochemical energy storage application, a threeelectrode electrolytic bath with 6 M KOH as aqueous electrolyte was used to conduct the related tests ((CV, GCD, and EIS). Figure 5a displays the CV curves of MOFsD

DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) CV curves of MOF-derived Co/Ni(OH)2 at 10 mV·s−1. (b) CV curves of Co/Ni(OH)2-3:1. (c) GCD curves of Co/Ni(OH)2-3:1. (d) Comparison of Cs of MOF-derived Co/Ni(OH)2..

Figure 6. CV curves (a) and GCD curves (b) of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2. (c) The specific capacitance of NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2, Co/Ni-MOF-3:1-600, Co/Ni(OH)2-3:1-600. (d) Cycling-life tests of Co/Ni(OH)2-3:1 and NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2.

To improve the cycling performance, a hybrid NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite was constructed through a selective oxidation using H2O2 and Co/Ni(OH)2-3:1 as the precursor, and the electrochemical performances are presented in Figure 6. As shown in Figure 6a, apparently, the location of redox peaks slowly changes with the increasing scan rates, implying excellent reversibility and rate performance. Figure 6b displays the GCD curves of the NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 electrode.

in Figure 5d, the MOF-derived Co/Ni(OH)2 shows better performance than unitary hydroxide samples, a fact which is attributed to the synergistic effects of cobalt and nickel ions. Obviously, Co/Ni(OH)2-3:1 shows better capacitance performance than that of others. Then, the cycling performance of Co/Ni(OH)2-3:1 was tested at 5 A·g−1 (Figure 6d). However, it shows very low cycling performance and 52% specific capacitance are lost in 2000 cycles, significantly limiting its practical application. E

DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) CV curves of the Ni/Co-TC//AC all-solid-state device. (b) GCD curves of the Ni/Co-TC//AC all-solid-state device. (c) The specific capacitance of Ni/Co-TC//AC all-solid-state device. (d) The Ragone plot of Ni/Co-TC//AC all-solid-state device.

and the novel accordion-like architecture guarantees that the ternary NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 composites exhibit an excellent capacitance and cycling performance. To further study the practical applications of prepared NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2, an all-solidstate hybrid supercapacitor was fabricated with 5.6 mg of activated carbon as the negative electrode, 2 mg of NiCo2O4/ β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 as the positive electrode, and polyvinyl alcohol (PVA)/KOH as the gel electrolyte (denoted as Ni/Co-TC//AC). Figure 7a shows CV curves of the Ni/Co-TC//AC all-solid-state device at the voltage window of 0−1.6 V with different scan rates. Galvanostatic charge/discharge curves of the Ni/Co-TC//AC device present typical analogous triangular shapes. The calculated specific capacitances are 104, 95, 89, 81, 75, and 68 F·g−1 at a current density of 1, 2, 3, 5, 7, and 10 A·g−1, respectively. After 10 000 cycles, the prepared hybrid supercapacitor can maintain 79% specific capacitance at 3 A·g−1 (Figure 7c). Energy density (E) and power density (P) of the Ni/Co-TC//AC all-solid-state device are obtained by the following equations:

The specific capacitances are 1646, 1572, 1490, 1315, 1160, 996, 896 F·g−1 at the current densities from 0.5 to 20 A·g−1, respectively. Figure 6c displays the specific capacitance of the samples with different oxidation process, including NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 selectively oxidized by H2O2 via Co/Ni(OH)2-3:1 as the precursor, Co/Ni-MOF-3:1-600 and Co/Ni(OH)2-3:1-600 obtained by direct calcination of Co/Ni-MOF and Co/Ni(OH)2-3:1, respectively. It can be observed that NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 shows a significantly higher specific capacitance than the directly calcined products (Co/Ni-MOF-3:1-600 and Co/ Ni(OH)2-3:1-600). Figure 6d displays the cycling performance tests of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 and Co/Ni(OH)2-3:1. The NiCo2O4/β-NixCo1−x(OH)2/αNixCo1−x(OH)2 ternary composites are able to keep 90.7% specific capacitance after 10 000 charge−discharge cycles at 5 A·g−1, which is significantly better than the Co/Ni(OH)2-3:1 precursor. At the initial 1000 cycles,38 the specific capacitance is gradually rising instead of decreasing, indicating full activation at the initial 1000 cycles. The excellent capacitance and cycling performance should attribute to the multilevel hierarchical structure of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite. Its well-designed accordionlike structure with nanoscale thickness of Co/Ni(OH) 2 nanosheets is able to efficiently shorten the diffusion path and promote the migration rate of electrolyte ions during the rapid charge/discharge process. The channel and voids between adjacent nanosheets can also provide enough volume for cycling test to protect the hierarchical structure.13 In addition, the NiCo2O4 not only serves as scaffolds to support the formation of the accordion-like structure but also promotes the migration of electrons. The synergetic contribution of the unique NiCo2O4 nanoparticles and Co/Ni(OH)2 nanosheets

E=

I m

P=

E Δt

t2

∫t1

V dt

By calculation, the energy density is 36.98 Wh·kg−1 at 801.49 W·kg−1 (Figure 7d). Furthermore, this all-solid-state device can light up a red light-emitting diode (operating voltage 1.6−3 V, 20 mA), indicating that the Ni/Co-TC//AC hybrid supercapacitor exhibits promising potential for practical applications. F

DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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CONCLUSIONS In summary, by using a bimetallic Co/Ni MOF as sacrificial template, a NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite with an accordion-like nanosheets structure was successfully synthesized via an alkaline hydrolysis and then selective oxidation process. The as-obtained NiCo2O4/βNixCo1−x(OH)2/α-NixCo1−x(OH)2 consists of Co/Ni(OH)2 nanosheets with high surface areas and NiCo2O4 nanoparticles with high conductivity, thus exhibiting high specific capacitance of 1315 F·g−1 at 5 A·g−1 and outstanding cycling performance of retaining 90.7% specific capacitance in 10 000 cycles. Our work presented here may provide a new strategy of a hydrolysis−oxidation process via a bimetallic MOF template for preparation of multicomponent electrode materials with well-defined morphologies and excellent properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01574. XRD patterns, IR curves, SEM image, TEM image, and electrochemical experiments (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.S.). *E-mail: [email protected] (Z.X.). ORCID

Wenpei Kang: 0000-0001-6550-9287 Daofeng Sun: 0000-0003-3184-1841 Funding

This work was supported by the NSFC (Grant Nos. 21371179, 21571187), Taishan Scholar Foundation (ts201511019), Shandong Provincial Natural Science Foundation (ZR2017BB038), and the Fundamental Research Funds for the Central Universities (13CX05010A, 14CX02150A, 15CX02069A, 15CX06074A, 16CX02016A). Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01574 Inorg. Chem. XXXX, XXX, XXX−XXX