Bimetallic-MOF Derived Accordion-like Ternary Composite for …dfsun.sci.upc.edu.cn/_upload/article/files/52/32/a1839b... · 2018. 8. 28. · Co-based electrode materials with enhanced

Bimetallic-MOF Derived Accordion-like Ternary Composite for High-Performance SupercapacitorsHao 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

*S Supporting Information

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

■ INTRODUCTIONWith the increasingly serious massive fossil consumption,environmental destruction, and global warming issues, theresearch puzzles of high-efficiency, renewable, and secureenergy storage devices for environmental protection havebecome the most critical problems faced by human beings.1−5

Among various energy storage devices, the supercapacitor winsthe favor of researchers for its high power densities, fastcharging, and excellent cycle performance.6−8 However, itsrelatively low energy density is an obstacle to commercialapplications. The energy density is able to be improvedthrough high specific capacitance and broadening of workingvoltage.9,10 Besides specific capacitance, cycling performance isanother especially important factor that significantly deter-mines the commercial applications of supercapacitors. To thisend, design and fabrication of advanced energy storagematerials with excellent cycling life and high specificcapacitance is the key.Recently, transition metal hydroxides are considered to be

probable electrode materials as a result of their high surfacearea and large theoretical capacitance (3500−4600 F·g−1).11,12For example, binary Co/Ni hydroxides (Cox/Ni1−x(OH)2)show better electrochemical performance than either unitarycobalt hydroxide (Co(OH)2) or unitary nickel hydroxide(Ni(OH)2).

13,14 Nonetheless, Cox/Ni1−x(OH)2 is still anunsatisfactory material due to the relatively poor conductivityand low cycling performance.15 To overcome these drawbacks,extensive efforts have been made to design multicomponent

composite materials combined with other well-knownelectrode materials which have high structural stability andelectrical conductivity.16−20 For instance, Liu et al. successfullyconstructed a hierarchical NiCo2O4@Ni(OH)2 structurewhich presents a high capacitance of 464 F·g−1 nearly 1.6times higher than layered Co3O4@Ni(OH)2 (291 F·g

−1) and∼18 times higher than NiCo2O4.21 Hence, to fabricate thecomposite of metal oxide and metal hydroxide should be afacile strategy to improve the supercpacitor performance.Although several such materials have been reportedrecently,21,22 the composite of bimetal oxides and bimetalhydroxides such as NiCo2O4/NixCo1−x(OH)2 have seldombeen used for supercapacitors, probably due to the difficulty inachieving uniform distribution of the metal elements and theless controllable size, phase, and morphology.19 In this paper,we describe a hydrolysis−oxidation approach via a bimetallicmetal−organic framework to stepwise fabrication of a NiCo2-O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary compositefor high-performance supercapacitors.As is known, metal organic frameworks (MOFs), a novel

sort of porous crystal material with well-defined porestructures, ultrahigh surface areas, rich metal centers, andperiodic networks, have been regarded as talented precursorsand sacrificial temples with which to construct porouselectrode materials, such as carbon-based materials, metal

Received: June 7, 2018

Article

pubs.acs.org/ICCite This: Inorg. Chem. XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.8b01574Inorg. Chem. XXXX, XXX, XXX−XXX

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oxides, metal hydroxides, and other composite materials.23−25

Through reasonable control of the reaction process, these as-obtained MOF-derived materials can inherit porous structuresand open channels and often exhibit excellent electrochemicalperformances.26,27 To date, the reported MOF-derived nano-materials have most concentrated on metal oxides and metalsulfides. As far as we know, there is only one sample of MOF-derived Cox/Ni1−x(OH)2 that was applied in supercapacitors,

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and no composites of bimetal oxides and bimetal hydroxideswith 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)2through an alkali hydrolysis strategy based on our previouswork.29,30 The obtained cobalt nickel hydroxide with anoptimized Co:Ni ratio of 0.63:1 displays an excellentcapacitance of 1956 F·g−1 at 5 A·g−1 (15.37 times and 1.59times 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/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite (Ni/Co-TC), the cycling performance increases significantly with90.7% retained after 10 000 cycles. Furthermore, the assembledNiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2//AC all-solid-state hybrid supercapacitor (named as Ni/Co-TC//AC) shows a high energy density of 36.98 Wh·kg−1 at 801.49W·kg−1. This hydrolysis−oxidation strategy offers a newmethod to fabricate novel nanohybrid materials with enhancedsupercapacitor performance through selective oxidation(Figure 1).

■ EXPERIMENTAL SECTIONStructure Characterizations. The chemical reagents were

directly used without any purification. X-ray powder diffractionspatterns were used to test the crystallinity and phase purity through aBruker AXS D8 Advance instrument (Cu−Kα, λ = 0.15418 nm). Themorphology and structure of prepared samples were obtained throughscanning electron microscopy (SEM, JSM-7500F). FTIR spectra wereconducted using a Nexus FT-IR spectrometer (under the 4000−600cm−1 region). The N2 adsorption−desorption isotherms and pore sizedistributions were tested by a surface area analyzer ASAP-2020 (thesamples were degassed for 5 h at 100 °C before measurements). TGAmeasurements 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/Ni-

MOF-a:b, MOF derived Co/Ni(OH)2 as Co/Ni(OH)-a:b, with a:b

denoting the ratio of cobalt and nickel we added in the initialsynthetic progress of MOF, rather than the actual ratio of cobalt andnickel.

Synthesis of MOF and the Derived Hydroxide. Similar to thereported literature, for the synthesis of Co/Ni-MOFs-3:1, 1,3,5-benzenetricarboxylic acid (0.099 g) and phthalic acid (0.0495 g) weredissolved in 15 mL of DMF. Then CoCl2·6H2O pretreated undervacuum 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 byfiltration. 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 onlydifference is the amount of cobalt chloride and nickel chloride (TableS2). In a typical synthesis, 0.1 g of MOF was immersed in 10 mL ofpotassium hydroxide solution (1 M KOH, 1 h), and washed two timeswith distilled water and ethanol, respectively. The products were driedin the air naturally.

Synthesis of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2. Inthis experiment, the prepared Co/Ni(OH)2-3:1 (0.1 g) was placed ina 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 werewashed two times with deionized water and ethanol, respectively, anddried in the air naturally. To compare, the same steps were also usedfor 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 thecalcination of Co/Ni-MOF-3:1 in the air. 50 mg of Co/Ni-MOF-3:1was 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 samecondition.

Electrochemical Characterization. The electrochemical datawere obtained from a CHI 760E instrument. In this experiment, weused platinum mesh (1.5 cm × 1.5 cm) as the counter electrode,calomel electrode (Hg/Hg2Cl2) as the reference electrode, and 6 MKOH 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 homogeneousslurry was heated at 60 °C in vacuum for at least 12 h. The solidifiedmixture (2.5 mg) was pressed on a piece of nickel foam (1 cm × 2cm) under 1.0 MPa. The electrochemical tests for the as-preparedelectrode were examined by cyclic voltammetry (CV) andgalvanostatic charge/discharge. Electrochemical impedance spectrawere conducted by applying a perturbation voltage of 5 mV in afrequency range of 0.01 to 106 Hz.

The all-solid-state hybrid supercapacitor was assembled throughusing 2 mg of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 as theanode, 5.6 mg of carbon black as the cathode, and PVA/KOHhydrogel polymer as the bath solution. The carbon negative electrodematerials were synthesized by mixing 18 mg of activated carbon withpolytetrafluoroethylene solution (9:1 for mass ratio). For PVA/KOHgel electrolyte, 1 g of PVA was added into 40 mL of H2O and thecloudy solution was obtain, followed by heating and stirring enoughtime to get the clear solution. Then under a stirring condition, 6 g ofKOH was put into the solution. The obtained colloidal solution wasdried in air finally.

■ RESULTS AND DISCUSSIONCharacterizations. The synthesis process of NiCo2O4/β-

NixCo1−x(OH)2/α-NixCo1−x(OH)2 is demonstrated schemati-cally in Figure 1. A μ3-OH bridged trinuclear second buildingunit (TN-SBU) and trimesic acid ligands coexist in thestructure of Co/Ni-MOF. MOF precursors with different Co/Ni ratios show similar PXRD patterns and FTIR spectra (asshown in Figures S2 and S3). The Co/Ni molar ratios of Co/Ni-MOFs can be easily tuned by the selection of raw materialswith different ratios, and the actual Co/Ni ratios have beenensured by ICP-OES (Tables S1 and S3). When the Co/Ni-

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

Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.8b01574Inorg. Chem. XXXX, XXX, XXX−XXX

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MOFs are soaked in the alkaline solution, the Cox/Ni1−x-(OH)2 nanoparticles are gained through the reaction of TN-SBU and OH− and the BTC3− ligands are released. The freeBTC3− ions may act as the surfactant to facilitate the formationof cobalt nickel hydroxide hexagonal sheets (Figure S12).Subsequently, layered Co/Ni(OH)2 was used as precursor toconstruct the NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2ternary composite through a selective oxidation process usingH2O2. In the oxidation process, a portion of Co

2+ was oxidizedto Co3+, resulting in the production of NiCo2O4 and α-NixCo1−x(OH)2.

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The transformation process of MOF to NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 composite is illustratedby XRD patterns. As displayed in Figure S4, after soaking inalkaline solution, the characteristic diffraction peaks of MOFparent disappeared and the diffraction peaks of Co/Nihydroxides presented, demonstrating that the MOF wastransformed to cobalt nickel hydroxide. The as-obtainedsamples of Co/Ni(OH)2-0:1 and Co/Ni(OH)2-1:1 (Co:Niratios of 0.22:1) were identified as β-Ni(OH)2 phase (JCPDScard: 14-0117), while the samples of Co/Ni(OH)2-5:1 (Co:Niratios 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 highersupercapacitor performance.13 Then, after a selective oxidationprocess using H2O2 as the oxidant, a series of new diffractionpeaks 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 character-istic diffraction peaks of 11.34°, 22.74°, 59.98° belong to α-NixCo1−x(OH)2 (JCPDS card: 38-0715), as shown in Figure 2.

The (003) interplanar d-spacing of α-NixCo1−x(OH)2 is 0.78nm, which indicates that the BTC3− anions may stack into abilayer-like structure.32−34 The characteristic bands of BTC3−

anions (1357 and 1608 cm−1) are also observed in the FTIRspectrum (Figure S6), further confirming the existence ofBTC3−.35,36 All the major diffraction peaks of NiCo2O4, β-NixCo1−x(OH)2, and α-NixCo1−x(OH)2 can be observed inNiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 patterns, andno extra peaks are detected, indicating the successful synthesisof the ternary material through a selective phase trans-formation.The microstructures and surface morphologies of the as-

obtained MOF derived nanomaterials were obtained from

SEM and TEM (Figure 3 and Figure S12). Compared with theMOF parent, the Co/Ni(OH)2-3:1 sample still retains ananoplate morphology but exhibits sharper edges with ahexagonal shape (as shown in Figure S12). After oxidation byH2O2, the as-obtained hybrid sample presents an accordion-like 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 S9and S10). Furthermore, the TEM is further used to confirmthe hybrid structure (Figure 3d,e). The Co/Ni(OH)2nanosheets present an obvious packed sheetlike structure,and the NiCo2O4 particles are disorderly distributed on thesurface and interior of the nanosheets without aggregation.The hybrid structure of oxidation products is also confirmedby high-resolution TEM (HRTEM) images (Figure 3f). Thefringe spacings of 0.28, 0.23, and 0.267 nm are resulted fromthe (220) lattice spacing of NiCo2O4, the (101) lattice spacingof β-NixCo1−x(OH)2, and the (101) lattice spacing of α-NixCo1−x(OH)2, respectively. These results are also supportedby 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-Nix-Co1−x(OH)2, Brunauer−Emmett−Teller (BET) measure-ments were determined. As shown in Figure S9, the obvioushysteresis within the limits of ca. 0.6−1.0 P/P0 demonstratesthat the existence of mesopores resulted from the accordion-like structure. The BET surface area of the porous NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 is 178.3 m

2·g−1. Inaddition, from the Barrett−Joyner−Halenda (BJH) pore sizedistribution (Figure S10), the pore size distribution is around12 nm, which is corresponding to SEM results (Figure 3c).These mesopores not only offer enough electrochemicalactivity center but also provide excellent electrolyte accessfor 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, thecorresponding PXRD patterns are in good accordance withβ-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 suggestedthat only Co2+ can be oxidized by H2O2, and the coexistence ofNi and Co is key to the synthesis of α-NixCo1−x(OH)2. TheCo/Ni-MOF-3:1 and Co/Ni(OH)2-3:1 samples are alsooxidized through a traditional postcalcination method, and ahybrid phase of NiCo2O4/NiO can be obtained, as shown inFigure S8.To understand better the chemical states change of bonded

elements during the selective oxidation process, X-rayphotoelectron spectroscopy (XPS) spectra were further usedto determine the obtained Co/Ni(OH)2-3:1 and NiCo2O4x-Co1−x(OH)2/α-NixCo1−x(OH)2. As shown in Figure 4a,c, it isobvious that the Ni 2p regions of Co/Ni(OH)2-3:1 andNiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 are very sim-ilar. The peaks around 855.9 and 873.6 eV are assigned to theNi 2p3/2 and Ni 2p1/2 of Ni

2+, indicating the Ni2+ was notoxidized by H2O2. In the Co 2p region of the oxide sample, thepeaks at 780.5 and 796 eV are assigned to Co3+, while thepeaks at 782.4 and 797.7 eV are assigned to Co2+, which meansthat 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 inthe Co 2p region of Co/Ni(OH)2-3:1 are attributed to the Co2p1/2 (797.3 eV) and Co 2p3/2 (781.5 eV) of Co

2+, indicating

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



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that only Co2+ exists (Figure 4b). These results further confirmthat only Co2+ can be selectively oxidized in the H2O2oxidation process, as is supported by the XRD result ofunitary oxidation products (Figure S7). In the C 1s spectrumof NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 (FigureS16d), the binding energies of carbon atoms in the aromaticsand carboxylic appear at 284.5 and 288.3 eV, providing theevidence for the existence of BTC3− in α-NixCo1−x(OH)2,which is also supported by XRD (Figure 2) and FTIR (FigureS6).Electrochemical Performances. For better studying the

electrochemical characteristics of the prepared nanomaterialsfor electrochemical energy storage application, a three-electrode electrolytic bath with 6 M KOH as aqueouselectrolyte was used to conduct the related tests ((CV,GCD, and EIS). Figure 5a displays the CV curves of MOFs-

derived Cox/Ni1−x(OH)2 at 10 mV·s−1. Obviously, the

oxidation potential of bimetal hydroxide is lower than that ofthe unitary products, suggesting that the bimetal hydroxide canbe charged with ease, which may result from the synergisticeffects of Co2+ and Ni2+ ions.14,37 In addition, the anodic peakof Co/Ni(OH)2-3:1 is stronger than the other samples, whichimplies that Co/Ni(OH)2-3:1 may show the best capacityperformance.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 ofoxidation and reduction peaks suggests the high Coulombeffect. Corresponding to the CV peaks, the GCD curves of theCo/Ni(OH)2-3:1 (Figure 5c) present well-defined charge anddischarge platforms. Specific capacitances of 2335, 2220, 2123,1956, 1760, 1567, and 1413 F·g−1 were observed at currentdensities of 0.5, 1, 2, 5, 10, 15, 20 A·g−1, respectively. As shown

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) Co2p of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2.



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in Figure 5d, the MOF-derived Co/Ni(OH)2 shows betterperformance than unitary hydroxide samples, a fact which isattributed to the synergistic effects of cobalt and nickel ions.Obviously, Co/Ni(OH)2-3:1 shows better capacitance per-formance than that of others. Then, the cycling performance ofCo/Ni(OH)2-3:1 was tested at 5 A·g

−1 (Figure 6d). However,it shows very low cycling performance and 52% specificcapacitance are lost in 2000 cycles, significantly limiting itspractical application.

To improve the cycling performance, a hybrid NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composite wasconstructed through a selective oxidation using H2O2 andCo/Ni(OH)2-3:1 as the precursor, and the electrochemicalperformances are presented in Figure 6. As shown in Figure 6a,apparently, the location of redox peaks slowly changes with theincreasing scan rates, implying excellent reversibility and rateperformance. Figure 6b displays the GCD curves of theNiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 electrode.

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/β-NixCo1−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/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2.



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http://dx.doi.org/10.1021/acs.inorgchem.8b01574

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-Nix-Co1−x(OH)2/α-NixCo1−x(OH)2 selectively oxidized by H2O2via Co/Ni(OH)2-3:1 as the precursor, Co/Ni-MOF-3:1-600and Co/Ni(OH)2-3:1-600 obtained by direct calcination ofCo/Ni-MOF and Co/Ni(OH)2-3:1, respectively. It can beobserved that NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2shows a significantly higher specific capacitance than thedirectly calcined products (Co/Ni-MOF-3:1-600 and Co/Ni(OH)2-3:1-600). Figure 6d displays the cycling perform-ance tests of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2and Co/Ni(OH)2-3:1. The NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 ternary composites are able to keep 90.7%specific capacitance after 10 000 charge−discharge cycles at 5A·g−1, which is significantly better than the Co/Ni(OH)2-3:1precursor. At the initial 1000 cycles,38 the specific capacitanceis gradually rising instead of decreasing, indicating fullactivation at the initial 1000 cycles. The excellent capacitanceand cycling performance should attribute to the multilevelhierarchical structure of NiCo2O4/β-NixCo1−x(OH)2/α-Nix-Co1−x(OH)2 ternary composite. Its well-designed accordion-like structure with nanoscale thickness of Co/Ni(OH)2nanosheets is able to efficiently shorten the diffusion pathand promote the migration rate of electrolyte ions during therapid charge/discharge process. The channel and voidsbetween adjacent nanosheets can also provide enough volumefor cycling test to protect the hierarchical structure.13 Inaddition, the NiCo2O4 not only serves as scaffolds to supportthe formation of the accordion-like structure but also promotesthe migration of electrons. The synergetic contribution of theunique NiCo2O4 nanoparticles and Co/Ni(OH)2 nanosheets

and the novel accordion-like architecture guarantees that theternary NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 com-posites exhibit an excellent capacitance and cycling perform-ance.To further study the practical applications of prepared

NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2, an all-solid-state hybrid supercapacitor was fabricated with 5.6 mg ofactivated carbon as the negative electrode, 2 mg of NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 as the positive elec-trode, and polyvinyl alcohol (PVA)/KOH as the gel electrolyte(denoted as Ni/Co-TC//AC). Figure 7a shows CV curves ofthe Ni/Co-TC//AC all-solid-state device at the voltagewindow of 0−1.6 V with different scan rates. Galvanostaticcharge/discharge curves of the Ni/Co-TC//AC device presenttypical analogous triangular shapes. The calculated specificcapacitances are 104, 95, 89, 81, 75, and 68 F·g−1 at a currentdensity of 1, 2, 3, 5, 7, and 10 A·g−1, respectively. After 10 000cycles, 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-statedevice are obtained by the following equations:

∫=E Im V tdtt

1

2

=Δ

PE

t

By calculation, the energy density is 36.98 Wh·kg−1 at801.49 W·kg−1 (Figure 7d). Furthermore, this all-solid-statedevice can light up a red light-emitting diode (operatingvoltage 1.6−3 V, 20 mA), indicating that the Ni/Co-TC//AChybrid supercapacitor exhibits promising potential for practicalapplications.

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) Thespecific capacitance of Ni/Co-TC//AC all-solid-state device. (d) The Ragone plot of Ni/Co-TC//AC all-solid-state device.



F


■ CONCLUSIONSIn summary, by using a bimetallic Co/Ni MOF as sacrificialtemplate, a NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2ternary composite with an accordion-like nanosheets structurewas successfully synthesized via an alkaline hydrolysis and thenselective oxidation process. The as-obtained NiCo2O4/β-NixCo1−x(OH)2/α-NixCo1−x(OH)2 consists of Co/Ni(OH)2nanosheets with high surface areas and NiCo2O4 nanoparticleswith high conductivity, thus exhibiting high specific capaci-tance of 1315 F·g−1 at 5 A·g−1 and outstanding cyclingperformance of retaining 90.7% specific capacitance in 10 000cycles. Our work presented here may provide a new strategy ofa hydrolysis−oxidation process via a bimetallic MOF templatefor preparation of multicomponent electrode materials withwell-defined morphologies and excellent properties.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.inorg-chem.8b01574.

XRD patterns, IR curves, SEM image, TEM image, andelectrochemical experiments (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected] (D.S.).*E-mail: [email protected] (Z.X.).ORCIDWenpei Kang: 0000-0001-6550-9287Daofeng Sun: 0000-0003-3184-1841FundingThis work was supported by the NSFC (Grant Nos. 21371179,21571187), Taishan Scholar Foundation (ts201511019),Shandong Provincial Natural Science Foundation (ZR2017-BB038), and the Fundamental Research Funds for the CentralUniversities (13CX05010A, 14CX02150A, 15CX02069A,15CX06074A, 16CX02016A).NotesThe authors declare no competing financial interest.

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