Rational Design of Sandwiched NiâCo Layered Double Hydroxides

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9923-9934

pubs.acs.org/journal/ascecg

Rational Design of Sandwiched Ni−Co Layered Double Hydroxides Hollow Nanocages/Graphene Derived from Metal−Organic Framework for Sustainable Energy Storage Xue Bai,†,§ Qi Liu,*,†,‡,§ Zetong Lu,∥ Jingyuan Liu,†,‡,§ Rongrong Chen,†,‡ Rumin Li,†,§ Dalei Song,† Xiaoyan Jing,† Peili Liu,†,§ and Jun Wang*,†,‡ †

Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, ‡Institute of Advanced Marine Material, and College of Materials Science and Chemical Engineering, Harbin Engineering University, No.145 Nantong Street, Harbin 150001, People’s Republic of China ∥ Heilongjiang University of Science and Technology, No.2468 Puyuan Street, Harbin 150022, People’s Republic of China §

S Supporting Information *

ABSTRACT: In situ growth of Ni−Co layered double hydroxides on graphene nanosheets by virtue of metal− organic framework as a sacrifice template is reported, which yields hollow nanocages uniformly deposited on graphene nanosheets. The strong impact of graphene amount on the electrochemical performance of Ni−Co layered double hydroxides is illustrated. Controlling the mass of graphene (15 mg) leads to a maximum specific capacitance of 1265 F g−1, high rate capability (50% capacitance retention after increasing current density ten times), and good cycling life (92.9% capacitance retention after 2000 circles). The combination of battery-type Ni−Co LDH hollow nanocages/ graphene composite and active carbon allows for the excellent electrochemical performance measured in an asymmetric device. In detail, the assembled asymmetric supercapacitor is able to deliver maximum specific capacitance 170.9 F g−1 in a potential window of 0−1.7 V, high energy density (68.0 Wh kg−1), as well as excellent power output (4759 W kg−1). These electrochemical performances, in combination with its facile fabrication, render hollow Ni−Co LDH/graphene composite as a promising electrode material in a sustainable energy storage device. KEYWORDS: Sustainable, Supercapacitor, Metal organic frameworks, Layered double hydroxides, Graphene

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INTRODUCTION

Transition metal hydroxide, as an important part of batterytype materials, has been widely studied in asymmetric supercapacitors. Regarding metal hydroxide, more research attention has been attracted by layered double hydroxides.7 Layered double hydroxides are well-known anionic or hydrotalcite-like clays, whose structure is composed of positively charged brucite-like layers and interlayer charge compensating anions.8,9 Considering the special layered structure and anionexchange stability, recently, layered double hydroxides (LDH) have been widely investigated in various fields, such as catalysis, separation, and biotechnology.10 Particularly, transition metal doped layered double hydroxide hybrids have been conceived to be one of potential candidates in battery-type electrodes ascribed to their high redox activity, low cost, and long cycle life.11−13 However, LDH materials are still limited to the inherent low conductivity and severe aggregation, which greatly

With the aim of reducing the dependence for ever-decreasing demand of fossil fuels, various energy storage devices are employed.1−3 Supercapacitors, also called ultracapacitors, have attracted intense attention as sustainable energy storage devices by virtue of their outstanding properties, such as being environmentally friendly, advanced cycle stability, and, in particular, power delivery.4−6 Nevertheless, driven by the concern over the relative low energy density, the improvement of traditional supercapacitor technology with high performance appears to be vital in developing a supercapacitor. In this aspect, an asymmetric supercapacitor, utilizing battery-type material as the positive electrode and carbon material as the negative electrode, can result in an improvement in power and energy density simultaneously. The development of a high performance battery-type electrode has become an important factor for building such a device. Therefore, with the higher performance of the battery-type material, better properties of asymmetric supercapacitor could be obtained. © 2017 American Chemical Society

Received: June 11, 2017 Revised: August 24, 2017 Published: October 1, 2017 9923

DOI: 10.1021/acssuschemeng.7b01879 ACS Sustainable Chem. Eng. 2017, 5, 9923−9934

Research Article

ACS Sustainable Chemistry & Engineering impedes the rate capability and energy density.14 Rational design and fabrication of LDH materials in the nanostructure or incorporation of LDH with high conductive material to form a hybrid composite are supposed to be essential in improvement of electrochemical properties. Hollow micro/nanostructures with controllable size, shape, and architecture have attracted intense attention ascribed to their prominent features of distinguishable interior voids, low density, large specific area, and reduced transport lengths for both mass and charge transport.15 Until now, the researchers propose numerous methods to controllable synthesis hollow structure based on a soft or hard template.16−18 Rather complex fabrication approaches are involved, which result in increasing the difficulty in building a hollow structure. Therefore, seeking a facile method to construct a hollow structure still remains a huge challenge. Very recently, many researchers have reported various hollow architectures based on the metal−organic frame ZIF-67, and such a structure is feasible to convert to nanohollow structures without sacrificing the main shape of the polyhedron. Lou’s group reports a facile two-step diffusioncontrolled strategy to generate hierarchical hollow prisms composed of nanosized CoS2 bubble-like subunits for lithiumion batteries.19 This group also reports the synthesis of novel Co3O4/NiCo2O4 box-in-box nanocages (NCs) with different shell compositions, exhibiting enhanced pseudocapacitive and electrocatalytic properties.20 Such a structure shows exceptional electrochemical performance which may be due to the exposure of high specific surface for faradic reactions. Unfortunately, a severe issue associated with low conductivity significantly affects the improvement of electrochemical properties. Overcoming the conductivity of material in a designed method may bring more possibilities in modulating the properties of functional nanostructures for electrodes. In this aspect, various approaches to confine battery-type material with many carbonaceous materials have been used in energy storage applications, including carbon nanotubes, active carbon, and graphene.21−25 As an ideal matrix, graphene is commonly used for growth of functional nanomaterials. Specially, nanocomposites made by graphene and battery-type materials have been widely studied due to the synergistic effect arising from two materials.26 On the one hand, battery-type materials can provide many redox reaction sites which are the origin of high specific capacitance, and on the other hand, graphene can effectively enhance the conductivity of batterytype mterials. Meanwhile, the anchoring of nanomaterials on graphene can effectively reduce the restacking of graphene nanosheets, allowing a better utilization of the surface active sites.27 However, the hollow structure, as an important part of nanoparticles, has not been fabricated on graphene nanosheets widely, probably due to the sophisticated progress for constructing the hybrids. Herein, we report on the synthesis and characterization of Ni−Co LDH hollow nanocages deposited on commercial graphene nanosheets derived from ZIF-67/graphene pristine material via a structure-induced anisotropic chemical etching at elevated temperature. The overall process consists of a precipitation reaction to obtain metal organic frame/graphene composite and an acidic etching route to transform the template into a hollow structure. The as-prepared Ni−Co LDH/graphene composite exhibits superior electrochemical performance for the battery-type electrode. The conception of such a sandwich like hollow structure for the positive electrode has various advantages. Particularly, such a hollow structure

composed of multiple LDH nanosheets interconnected each other not only provides more active sites for reacting with electrolyte but also effectively accommodates the volume expansion and contraction during long-term charge−discharge tests. Moreover, graphene nanosheets, which are regarded as substrates for immobilizing hollow LDH nanocages, provide efficient paths for electron transportation. Thanks to the special sandwich like structure, the hollow nanocages grow uniformly on graphene nanosheets, and graphene nanosheets are sandwiched with hollow cages, which are feasible for fast electron transfer and reduce the ion-diffusion path. The effect of graphene mass on the electrochemical performance is also investigated and compared to the individual Ni−Co LDH nanocages, demonstrating the vital role of graphene. Therefore, we propose a Ni−Co LDH nanocages/graphene composite as a viable and potential candidate for a high performance asymmetric supercapacitor.

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EXPERIMENTAL SECTION

Synthesis of ZIF-67/Graphene Composite. The commercial graphene powder with different masses (8, 15, and 25 mg) is dispersed in 50 mL of methanol under ultrasonication for 1 h. A total of 0.498 g of cobalt nitrate hexahydrate is dissolved in the above solution and treated under ultrasonication for 1 h. Then 0.656 g of 2methylimidazole is dispersed in 50 mL of methanol and mixed with GO/Co(NO3)2 solution. The mixture is stirred for 0.5 h and kept static at room temperature for 24 h. The product is collected by centrifugation, washed with ethanol three times, and dried at 60 °C for 24 h. ZIF-67 monomer is synthesized via the same method without adding graphene powder. Synthesis of Ni−Co LDH Nanocages/Graphene Composite. The as-fabricated ZIF-67/graphene composite (40 mg) is redispersed in 50 mL of ethanol containing 0.2 g of Ni(NO3)2·6H2O and refluxed for 1 h under stirring. The purple powder changes to light green. The product is collected by centrifugation, rinsed with ethanol three times, and dried in an oven at 60 °C for 24 h. According to chemical treatment by weighting mass change before and after acid etching, which can remove Ni−Co LDH, the mass ratio with different graphene addition can be obtained. The mass ratios of Ni−Co LDH with 8, 15, and 25 mg of graphene are calculated to be 81%, 59%, and 25%, respectively. For contrast, pure Ni−Co LDH nanocages and the graphene composite fabricated by mixture are also prepared and denoted as Ni−Co LDH/graphene M. The mass ratio of Ni−Co LDH/graphene M is the same as that of Ni−Co LDH/15 mg graphene (59%). Characterization. The crystalline structure of the fabricated samples is determined with a powder XRD system (Rigaku TTRIII) equipped with Cu Ka radiation (λ = 0.15406 nm) and Raman spectroscopy (Jobin-Yvon HR800 micro-Raman system). The morphologies of the hybrids are investigated with the aid of a scanning electron microscope (SEM, JEOL, JSM-6480) and transmission electron microscope (TEM, JEOL 2010) with a field emission gun operating at 200 kV. The energy dispersive X-ray spectrometry (EDS, JEOL JSM-6480A microscope) is used to investigate the elemental composition of the samples. The spectra of materials are determined by X-ray photoelectron spectroscopy (XPS; PHI5700 ESCA spectrometer with Al KR radiation (hυ = 1486.6 eV)). Nitrogen adsorption−desorption isotherms are measured to obtain surface area and porosity distribution using a micromeritics ASAP 2010 instrument. Electrochemical Characterization. The electrochemical performance of the Ni−Co LDH composite is performed on an electrochemical workstation CHI660I (Chenhua, Shanghai, China) via a traditional three-electrode system in which as prepared electrode hybrids (1.0 cm × 1.0 cm), Pt foil, and saturated calomel electrode (SCE) are used as the working electrode, counter electrode, and reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance 9924


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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the synthesis procedure of the Ni−Co LDH hollow nanocages/graphene composite.

Figure 2. XRD patterns of as fabricated (a) ZIF-67 and simulated lines, (b) graphene and Ni−Co LDH/25 mg grphene composite, and (c) Ni−Co LDH/graphene composite with different graphene mass. To obtain q+ = q−, the mass balancing follows eq 2:

spectroscopy (EIS) measurements are performed in 1 M KOH aqueous at room temperature. Asymmetric Supercapacitors. An aqueous asymmetric supercapacitor device is fabricated with Ni−Co LDH/3D RGO NF, commercial activated carbon (AC), and cellulose acetate membrane as the positive electrode, negative electrode, and separator, respectively. The negative electrode is prepared by mixing the AC, acetylene black, and polytetrafluoroethylene (PTFE) in a weight ratio of 85:10:5 in ethanol under ultrasonic conditions to produce a homogeneous paste. Then the paste is coated onto a piece of 1 × 1 cm2 nickel foam. At last, the negative electrode is pressed and dried under vacuum at 60 °C overnight. The positive electrode is prepared by the same approach with mixing the Ni−Co LDH/graphene composite. The charge balance theory is used to determine the mass of negative electrode (AC), as shown in eq 1: q = C ΔVm

m+ C ΔV = − − m− C+ΔV+

(2)

C+ and C−, calculated by discharge curves, are the specific capacitances of the Ni−Co LDH/graphene electrode and the AC electrode, respectively. ΔV+ and ΔV− are the potential range of Ni− Co LDH/graphene and AC electrodes, respectively. The optimum weight ratio between the Ni−Co LDH/graphene and AC electrodes is calculated to be m+/m− ≈ 0.25. The active mass of the positive electrode is 3.47 mg; thus, the active mass of the negative electrode is calculated to be 13.86 mg. The specific capacitance (Cs) is calculated by the following equation derived from GCD curves.2839

Cs = I Δt /ΔVm

(1) 9925

(3) DOI: 10.1021/acssuschemeng.7b01879 ACS Sustainable Chem. Eng. 2017, 5, 9923−9934

Research Article

ACS Sustainable Chemistry & Engineering The corresponding energy density E (Wh kg−1) and power density P (W kg−1) are calculated from the following eqs 4 and 5:

E = 0.5CsΔV 2

(4)

P = E3600/Δt

(5)

and Ni−Co LDH/graphene all exhibit two obvious peaks located at 1347 and 1579 cm−1 which are the characteristic peaks of D and G bands, respectively. The G band is related to the graphitic order, and the D band is ascribed to the degree of disorder in the structure.34 Obviously, the commercial graphene displays a high degree of graphitization, which is due to the processing technique without involving chemical oxidation reaction. Additionally, two peaks at 470 and 520 cm−1 can be identified in Ni−Co LDH and Ni−Co LDH/graphene composites, which are due to the Ni−OH/Co−OH and Ni− O/Co−O stretching modes, respectively.35−37 The results indicate the successful fabrication of the Ni−Co LDH/ graphene composite. For comparison, SEM images of ZIF-67/graphene composite and Ni−Co LDH/graphene composite are investigated. As depicted in Figure 3a,b, the as-fabricated ZIF-67 particles are

−1

where Cs (F g ) is the specific capacitance, ΔV (V) is the potential window of the assembled device, m (g) is the active mass of electrode, and Δt (s) is the discharge time derived from charge/discharge measurement.

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RESULTS AND DISCUSSION A schematic illustration for the synthesis of the hierarchical sandwiched Ni−Co LDH hollow nanocages/graphene composite is displayed in Figure 1. The graphene nanosheets are designed to serve as the deposition substrate and electron transportation scaffold. During the synthesis procedure, the cobalt ions are gradually adsorbed on the surface of graphene nanosheets and subsequently react with 2-methylimidazole. The intermediate ZIF-67/graphene composite is collected and dispersed in ethanol. In the next step, the ZIF-67 polyhedrons are converted to hollow nanocages constructed from Ni−Co LDH nanosheets with the addition of Ni(NO3)2. The restriction of low conductivity for LDH can be easily overcome in our rational design by the addition of graphene nanosheets. After reaction, both hollow LDH nanocages and graphene nanosheets can remain simultaneously in the composite with hierarchical structures, which can greatly boost the electrochemical performance of the material. The crystallographic structure of ZIF-67, graphene and various Ni−Co LDH/graphene composites in terms of different graphene mass, are determined by X-ray diffraction measurement (XRD). As depicted in Figure 2a, all of the diffraction peaks of as-prepared ZIF-67 can be well assigned to the simulated peaks published in previous reports, indicating the high purity of ZIF-67 templates.24,29 After reacting with Ni(NO3)2, all of the diffraction peaks are well indexed to the Ni−Co LDH phase (JCPDS no. 40-0416).25,30 No additional reflections are detected, revealing that the ZIF-67 templates are completely converted to Ni−Co LDH under a mild reaction condition (Figure 2b). It is worth mentioning that a sharp diffraction peak located at about 26° with high intensity is observed, which is assigned to the (002) plane of graphene, suggesting the high graphitization degree of graphene nanosheets.31 Except for the high intensity peaks, a broad peak located at the same place is characteristic of exfoliated nature of graphene nanosheets.32 A series of XRD patterns of Ni−Co LDH with different graphene mass are further investigated in Figure 2c. All samples exhibit diffraction peaks of Ni−Co LDH, confirming the successful transformation of ZIF-67 to Ni−Co LDH. The peaks derived from graphene nanosheets are revealed when the additive mass is more than 8 mg. This phenomenon is speculated due to the result that hierarchical Ni−Co LDH hollow nanocages are uniformly deposited on the surface of graphene, effectively avoiding the restacking of graphene. Increasing the amount of graphene nanosheets, to some extent, results in graphene agglomerating together, which is reflected in the enhanced intensity of XRD peaks indexed to graphene. Note that all of the diffraction peaks of the Ni−Co LDH are broadened in width and weakened in intensity, suggesting that the as-obtained Ni−Co LDH hollow nanocages are essentially composed of numerious LDH nanocrystallites.33 The Raman spectra of commercial graphene, Ni−Co LDH, and Ni−Co LDH/graphene are shown in Figure S1. Graphene

Figure 3. SEM images of (a and b) ZIF-67/graphene, (c and d) Ni− Co LDH hollow nanocages/graphene composite, and (e−g) mapping result of Ni−Co LDH hollow nanocages/graphene composite.

deposited uniformly on graphene nanosheets, exhibiting a sandwich structure. The ZIF-67 particles have a homogeneous polyhedron shape with smooth surface and an average particle size of about 500 nm. When the ZIF-67 react with Ni(NO3)2 at an appropriate amount, the solid polyhedrons converted into hollow nanocages which can be recognized by the change of contrast (Figure 3c,d). Obviously, the as-fabricated Ni−Co LDH hollow nanocages with rough surface retain the main structure of ZIF-67 without obvious fracture, indicating the successful fabrication of a hollow structure. It is speculated that the formation process involves mild reaction conditions depicted as a diffusion-controlled ion exchange process, which is used largely to construct a hollow structure from MOF materials.38 To further detect element distribution of the Ni− Co LDH/graphene composite, energy-dispersive X-ray spectroscopy (EDS) is performed, as shown in Figure S2. The elements of selected area are C, O, Ni, and Co. The C is derived from graphene nanosheets. The other elements come from Ni−Co LDH. The element ratio of Ni and Co is calculated to be 3.2:1.8. SEM mapping results reveal that nickel and cobalt are the main elements in the composite and uniformly distributed in the composite (Figure 3e−g). SEM and XRD patterns of Ni−Co LDH/graphene M and Ni−Co LDH/15 mg graphene are also carried out to discuss the morphology and structure change. As shown in Figure S3, XRD patterns of Ni−Co LDH/graphene M and Ni−Co LDH/15 mg graphene are almost the same with obvious diffraction peaks derived from LDH and graphene. As shown in Figure S4a-b, 9926


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hollow nanocages which are attached on ultrathin graphene, with interlinked petals on the bottom and hollow nanocages on the top (Figure 4b). The phenomenon may be due to the nucleation process involved in ion nucleation on the surface of graphene and nuclear growth to form polyhedron as time goes on. Subsequently, the small petals and hollow nanocages occur after the ion-exchange reaction. Such a hierarchical structure is feasible to expose a more active surface for reacting with electrolyte, which can store numerous electric energy for applications. After meticulous observation of structure from the edge of nanocages, the shell of the hollow structure is composed of interconnected nanosheets with ultrathin thickness (Figure 4c,d). Nevertheless, the main shape of Ni−Co LDH remains intact, consistent with SEM results. The hierarchical sandwich like structure can expose a more active surface for reaction with electrolyte. The surface areas of Ni−Co LDH with different graphene mass are investigated by N2 adsorption−desorption measurements. Typical IV isotherms with H3-type hysteresis loops (P/P0 > 0.4) can be obtained in all composites, which is the characteristic of mesopores (Figure S5). These mesopores derive from interconnected Ni−Co LDH hollow nanocages. The Brunauer−Emmett−Teller surface area of Ni−Co LDH/graphene are estimated to be 126.8, 142.8, 218.9, and 151.3 m2g−1 for various graphene masses of 0, 8, 15, and 25 mg, respectively. It is obvious that the surface area of Ni−Co LDH/15 mg graphene is much higher than that of Ni−Co LDH. This is because Ni−Co LDH grows on both sides of commercial grpahene, preventing agglomeration of both Ni−Co LDH nanocages and graphene. When the mass of graphene increases higher than 15 mg, the Ni−Co LDH nanocages are insufficient to deposite on the surface of graphene, resulting in agglomeration of graphene itself, leading to a significant decrease of surface areas.39 Furthermore, according to the pore size distribution of four samples, four samples all show mesopores centered at 3.7 nm which may derive from LDH

the composite prepared by mixing Ni−Co LDH and graphene shows obvious distribution separately. The graphene nanosheets exhibit an ultrathin layer with smooth surface, indicating that no materials distributed on graphene. However, the Ni− Co LDH/graphene composite fabricated via in situ growth exhibits that nanocages deposited on both surfaces of graphene nanosheets without obvious aggregation. During the design of material, the graphene is regared as a substrate which provides high conductivity of electrode. Moreover, the synergestic effect between Ni−Co LDH and graphene may enhance the electrochemical performance of materials. More detailed morphological information on Ni−Co LDH/ graphene composite is performed by TEM. Apparently, hierarchical hollow nanocages on the graphene nanosheet are detected as illustrated in Figure 4. It is easy to distinguish

Figure 4. TEM images of Ni−Co LDH hollow nanocages/graphene composites with different magnification.

Figure 5. High resolution XPS spectra of (a) Co 2p in ZIF-67 and (b and c) Co 2p and Ni 2p in Ni−Co LDH. (d) Schematic illustration of the possible mechanism reaction involved in forming Ni−Co LDH. 9927


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Figure 6. Electrochemical performance of ZIF-67 and the Ni−Co LDH/graphene composite, (a) CV curves measured at a constant scan rate of 5 mV s−1, (b) charge−discharge curves at a current density of 1 A g−1, and (c) Nyquist plots measured in the frequency range from 0.01 to 105 Hz.

leaving the hollow interior behind. The released Co2+ ions from ZIF-67 and Ni(NO3)2 may be partially oxidized by dissolved O2 and NO3− ions in the solution quickly. At last, the Ni2+/Ni3+ and Co2+/Co3+ could coprecipitate with OH− to form Ni−Co LDH nanosheets on the surface.46,47 Thus, hollow nanocages composed of Ni−Co LDH nanosheets on the graphene nanosheet were obtained via such a facile method. The electrochemical properties of ZIF-67 and Ni−Co LDH/ graphene are initially investigated by cyclic voltammetry (CV), galvonostatic charge−discharge curves (GCD), and EIS measurements in 1 M KOH electrolyte. Figure 6a presents the CV curves of ZIF-67 and Ni−Co LDH/15 mg graphene composite at a constant sweep rate of 5 mV s−1. Compared with ZIF-67 with relatively low current response, Ni−Co LDH/15 mg graphene material displays obvious redox peaks which can be expressed by the faradic reactions below:48

nanosheets. When the graphene mass increases more than 15 mg, other mesopores located at about 5−20 nm occur, which may due to the interconnected graphene nanosheets. Although the characteristic results confirm the successful fabrication of the LDH composite, however, to our best knowledge, LDH has a general chemical formula [MII1−xMIIIx(OH)2]x+[An‑]x/n·mH2O, where MII and MIII are divalent and trivalent cations, and An‑ can be almost any organic or inorganic anion.40,4130,31 Thus, the essential factor in constructing the LDH structure is elements valences, which is of great importance in fabrication of Ni−Co LDH. In this aspect, we propose the probably reactions that occurred during the ion exchange process. For a detailed comparison, the valence changes before and after the ion-exchange reaction are measured by XPS measurements. As illustrated in Figure 5a, the main peaks at 780.98 and 796.80 eV belong to Co 2p3/2 and Co 2p1/2, with a spin-energy separation of about 16 eV, that can be well assigned to Co2+ in ZIF-67. After the ion-exchange reaction, the Co 2p spectrum of Ni−Co LDH has been altered significantly, accompanying with two types of Co species clearly detected, which is attributed to the mixed valence of Co2+ (binding energy located at 782.92 and 798.91 eV) and Co3+ (binding energy at 780.34 and 795.14), suggesting the coexistence of Co2+ and Co3+.32,33,42,43 More importantly, the high-resolution XPS spectrum of Ni 2p is also best fitted by considering two spin−orbit doublets characteristic of Ni2+ (binding energy at 855.64 and 873.31 eV) and Ni3+ (binding energy at 859.19 and 876.49 eV) and two shakeup satellites (Figure 5c).44 Such an observation is important, which could well explain the probably mechanism during the synthesis procedure (Figure 5d). At the initial stage, the main valence of Co in ZIF-67 is +2; however, a substantial change of valence involved in the ion-exchange reaction with Ni(NO3)2 is observed, mainly representing that the main valences of Co in Ni−Co LDH are Co2+ and Co3+.45 During this procedure, it is supposed that protons derived from the hydrolysis of Ni2+ ions can gradually etch ZIF-67 templates,

Co(OH)2 + OH ↔ CoOOH + H 2O + e−

(6)

CoOOH + OH ↔ CoO2 + H 2O + e−

(7)

Ni(OH)2 + OH ↔ NiOOH + H 2O + e−

(8)

It is noteworthy that the integrated area of CV curves of the Ni−Co LDH/graphene composite is apparently larger than that of pristine ZIF-67, suggesting Ni−Co LDH/graphene possesses ultrahigh specific capacitance. Such significantly improved electrochemical performance is further proved by galvanostatic charge−discharge curves. Figure 6b exhibits the comparison of charge−discharge curves for ZIF-67 and the Ni− Co LDH/graphene composite which are measured at the same current density of 1 A g−1. Obviously, the discharge time of the Ni−Co LDH/graphene (1265 F g−1) composite is much longer than that of ZIF-67 (210 F g−1), again confirming the enhanced specific capacitance of Ni−Co LDH/graphene, which is in accordance with CV results. 9928


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Figure 7. (a) CV curves of Ni−Co LDH/graphene with different graphene mass at a constant scan rate of 5 mV s−1 and (b) GCD curves of Ni−Co LDH/graphene with different graphene mass at a current density of 1 Ag−1.

Such superior electrochemical performance may be ascribed to the low resistance of the electrode. To certify the hypothesis, EIS measurements are carried out in the frequency range from 10 kHz to 0.01 Hz. Notably, the equivalent series resistance and charge transfer resistance of Ni−Co LDH/graphene are all smaller in comparison with ZIF-67, suggesting the enhanced electrochemical properties of Ni−Co LDH/graphene. Next, we evaluate the electrochemical performance of the asprepared Ni−Co LDH/graphene composite with different graphene amount. Figure 7a displays a comparison of CV curves of the Ni−Co LDH/graphene composite recorded at the same scan rate of 5 mV s−1 with 0, 8, 15, and 25 mg of graphene. At a first glance, all of the curves exhibit obvious redox peaks, reflecting the pseudocapacitive control of all of the material. However, both materials show a similar profile with redox peaks located at different voltage, which may be ascribed to the difference in the polarization behavior and the Ohmic resistance of the electrodes during the CV test.49 In more detail, large potential ranges between redox peaks are detected in CV curves, probably due to the high resistance of material with little graphene mass (0 and 8 mg). Furthermore, when the graphene mass is higher than 15 mg, a significant redox peak shift associated with a small redox potential range is revealed in CV curves, resulting from the synergistic effect between Ni−Co LDH and graphene nanosheets. The Ni−Co LDH/15 mg graphene composite exhibits the largest CV integrated area in all of the composites, suggesting the high specific capacitance at this optimal graphene mass. Continuing to increase graphene mass results in a decrease in capacitance, probably assigned to the low Ni−Co LDH loading content, which will decrease the faradic reaction sites of material to react with electrolyte. However, Ni−Co LDH with 25 mg of graphene also displays obvious redox peaks with small potential range, indicating the high conductivity of this composite with excellent reversibility. As shown in Figure 7b, GCD curves are performed to further evaluate the properties of all composites at a constant current density of 1 A g−1. The potential range applied in charge− discharge curves is 0−0.6 V, which is smaller than that in CV curves (0−0.8 V), probably due to the oxidation evolution reaction. With the increase of graphene amount, the internal resistance (IR drop) reduces dramatically, demonstrating that the graphene nanosheets can effectively reduce the resistance of the composite, as well as improve the charge transport and electron collection rates. Notably, the internal resistance, obtained from GCD curves, of 25 mg of graphene is much smaller than others, which may be ascribe to the excess amount of graphene reducing the resistance of composite. All of the

curves have a typical plateau in accordance with CV curves, which is the characteristic of pseudocapacitance. By calculating the discharge time, the specific capacitances of Ni−Co LDH with different graphene mass are 787.6 (0 mg), 963.2 (8 mg), 1265.2 (15 mg), and 1020.3 F g−1 (25 mg), respectively. The high performance of the Ni−Co LDH/graphene composite is tightly connected with the synergistic effect between Ni−Co LDH and graphene. For comparison, the electrochemical performances of Ni−Co LDH, Ni−Co LDH/15 mg graphene and Ni−Co LDH/graphene M are performed, as shown in Figure S6. Ni−Co LDH/15 mg graphene exhibits the highest specific capacitance with lowest resistance in the three materials. The synergistic effect between graphene and Ni− Co LDH is very important to improve the performance of the material. That is why the composite synthesized by a mixture of Ni−Co LDH and graphene exhibited unsatisfied performance in comparison with Ni−Co LDH/graphene fabricated by in situ growth. To prove the existence of the synergistic effect between graphene and Ni−Co LDH, a detailed comparison of C 1s, Ni 2p, and Co 2p XPS spectra is illustrated in Figure S7. According to the results of XPS spectra before and after hybrid, the shifts of Co 2p, Ni 2p, and C 1s peaks in binding energies confirm strong electron interactions between Ni−Co LDH and graphene nanosheets in the Ni−Co LDH/graphene composite. These interactions will change electronic states of Co, Ni, and C atoms and will enhance the electrochemical performance of Ni−Co LDH/graphene.50,51 The true electrochemical surface area of the Ni−Co LDH/ graphene composite and pristine Ni−Co LDH electrodes is determined by a mature method which is widely utilized in oxygen evolution reaction catalyst.52−57 All of the electrodes are assumed as pure double layer type, in whose CV curves the current density at a specific voltage should be in proportion with the scan rate as illustrated in eq 9.58 I = vC DL

(9)

Figure S8a-d show the CV curves of the Ni−Co LDH/ graphene composite with different graphene mass recorded in different potential regions at various sweep rates in 1 M KOH solution. The double layer charging current is measured from the CV curves at −0.55, −0.52, −0.45, and −0.45 V for Ni−Co LDH, Ni−Co LDH/8 mg graphene, Ni−Co LDH/15 mg graphene, and Ni−Co LDH/25 mg graphene, respectively. The anodic charging current densities are plotted against different scan rates, which give a linear relation (Figure S9). The slope of the liner is equal to the double layer capacitance. The measured double layer capacitances of Ni−Co LDH and Ni−Co LDH/ 9929


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Figure 8. (a) CV curves of the Ni−Co LDH/15 mg graphene composite at different scan rates ranging from 2 to 20 mV s−1, (b) GCD curves of the Ni−Co LDH/15 mg graphene composite at various current densities, (c) specific capacitance calculated from discharge curves at differernt current densities, (d) Nyquist plots and (e) cycling stability tests of the Ni−Co LDH/graphene composite with different graphene mass, and (f) schematic of the electrons transportation of the as-prepared electrode.

graphene compsite electrodes are 1.4, 2.1, 2.7, and 2.7 mF cm2, respectively. Ni−Co LDH with 15 mg and 25 mg of graphene exhibit high ECSA in comparison with the other two electrodes. The results indicate that graphene can effectively increase ECSA of electrodes. Detailed CV profiles of the Ni−Co LDH/15 mg graphene composite, measured at various scan rates ranging from 2 to 20 mV s−1 in the potential range between −0.2 and +0.8 V, are plotted in Figure 8a. As observed from CV curves, no significant change is detected in the shapes of CV curves, thus suggesting a good rate capability of the Ni−Co LDH/15 mg graphene electrode. The scan rates gradually increase accompanying with a redox peaks shift to more positive and negative potentials, which is due to the internal resistance of the electrode.41,59 Figure 8b displays the GCD curves of Ni−Co LDH/15 at various current densities from 1 to 10 A g−1. Observation of the obvious voltage plateaus during charge/ discharge profiles further confirms the faradic behavior of the material, consistent with the CV results. The specific capacitance as a function of current densities of Ni−Co LDH with different graphene masses is plotted in Figure 8c. For the Ni−Co LDH/15 mg graphene composite, the maximum specific capacitance can reach 1265 F g−1 and still retain 637.3 F g−1 (about 50.1% capacitance retention) after current density increase from 1 to 10 A g−1. Additionally, such capacitance retention can increase to 52.3% for 25 mg of graphene composite when the current density increases ten times, and at this graphene mass, the maximum specific capacitance is 1020 F g−1. However, for the samples with low graphene amount and without graphene, the capacitance is so limited such as 59% and 48% capacitance retention for Ni−Co LDH with 8 mg and 0 mg of graphene when the current density increases only 3.33 times, respectively. The results demonstrate that the graphene amount plays an important role in promoting the electrochemical performance of Ni−Co LDH,

including conductivity and rate capability. The rate capability can be improved with the increase of graphene mass, presumably due to the high conductivity of graphene nanosheets. The addition of graphene can reduce the electron diffusion path and thus decrease the overall resistance of the composite. The enhanced electrochemical performance depending on the graphene mass is also confirmed by EIS measurements as shown in Figure 8d. All of the plots are composed of a semicircle in the high frequency, followed by a straight line in the low frequency region. The intercepts on the real axis representing equivalent series resistance (ESR) of four samples are 10.24 (0 mg of graphene), 7.6 (8 mg of graphene), 6.1 (15 mg of graphene), and 4.9 Ω (25 mg of graphene), respectively. The result indicates that the greater the graphene mass, the lower the internal resistance that can be obtained. Moreover, the diameter of semicircles gradually decreases with an increase of adding graphene mass, which implies feasible electron transportation. The more vertical straight plots at the low-frequency region with high graphene mass indicates that Ni−Co LDH/graphene possesses ideal capacitive and fast ion diffusion behaviors. As a result, the electrochemical resistance of Ni−Co LDH can be greatly reduced by the addition of graphene, leading to the enhanced electrochemical performance of materials. To validate the importance of the graphene mass regarding the cycling stability, continuous charge−discharge 2000 circles are performed to Ni−Co LDH with various graphene masses at a constant current density of 3.33 A g−1 (Figure 8e). Obviously, the activation of electrode and electrolyte gradually penetrating into the hollow nanocages during cycling occurs as evidenced by the gradual increase in specific capacitance during the initial 200 circles. After 200 circles, the capacitance presents a gradual decay in varying degrees, supposed to be caused by the instability of Ni−Co LDH and the fracture of the hollow structure during the long-term charge−discharge test. With 9930


Research Article


Figure 9. (a) Schematic of the assembled asymmetric supercapacitor with the Ni−Co LDH/15 mg graphene composite as a positive electrode and active carbon as a negative electrode, (b) CV curves of Ni−Co LDH/15 mg graphene and active carbon at a constant sweep rate of 2 mV s−1, (c) CV curves of assembled supercapacitor devices measured at different scan rates ranging from 2 to 40 mV s−1, (d) galvanostatic charge−discharge curves of the fabricated asymmetric supercapacitor at various current densities, (e) cycling performance of the assembled supercapacitor during 2500 circles at a constant current density of 1.4 A g−1, inset: charge−discharge curves of assembled device, and (f) Ragone plot of the assembled supercapacitor device, inset: the photo of assembled supercapacitor and the lighted up LED.

curves of Ni−Co LDH/15 mg graphene and active carbon, respectively. As shown in Figure 9b, the CV curve of active carbon, measured in the potential range between −1 and 0 V, presents a quasi-rectangle without redox peaks resulting from the characteristic of double layer capacitive behavior. On the contrary, obvious redox peaks of the Ni−Co LDH/15 mg graphene composite are observed with a potential range from −0.2 to +0.8 V derived from faradic reactions. Due to the obvious polarization phenomenon that occurred when the potential exceed 0.7 V, the optimal potential range of the asymmetric supercapacitor is determined to be 0−1.7 V. Such a potential range is higher than previous reports, which is competitive to increase energy and power densities. Figure 9c displays the CV curves of the assembled device measured at various sweep rates ranging from 5 to 40 mV s−1. A combination of both pseudocapacitance in high voltage and EDLC types in low voltage of capacitance have been observed at all sweep rates. Furthermore, the redox peaks shift to more positive and negative potential with the increase of scan rates. However, the shape of CV curves does not exhibit obvious distortion although the scan rate is as high as 40 mV s−1, indicating the high rate capability and rapid I−V response. Galvanostatic charge−discharge curves are also performed to evaluate the specific capacitance of the asymmetric device. As shown in Figure 9d, the shapes of charge−discharge curves tend to be quasi-rectangle, and charge curves are nearly symmetric with the discharge parts, suggesting excellent capacitive behavior of the assembled device. Based on the discharge time of the charge−discharge curves, the maximum specific capacitance of the asymmetric device can reach 170.9 F g−1 at current density of 0.7 A g−1, although the current density increases eight times, the specific can still retain 60.2 F g−1. Moreover, the assembled supercapacitor device also exhibits exceptional cycling stability which is measured by repeating charge−discharge curves. After 2500 circles, the electrode can still maintain 94.2% of the initial capacitance value, again confirming the potential of such a material in the supercapacitor. The asymmetric supercapacitor still has some

respect to the four samples, Ni−Co LDH without graphene only achieves 42.1% capacitance retention after cycling 2000 circles, which is much smaller than those of 8 (64.5%) and 25 mg (91.8%). Especially, the capacitance can still be retained at 92.9% when the graphene adding mass is 15 mg. As can be seen, the cycling stability of Ni−Co LDH is largely dependent on the mass of graphene, which is considering that the Ni−Co LDH manifests low conductivity that is not feasible to iondiffusion. However, more graphene nanosheets can restack together when the adding mass is ultrahigh. The restacking graphene can hardly decrease the contact surface for reacting with electrolyte, thus reducing the specific capacitance. In this case, 15 mg is an optimal amount in the design of the Ni−Co LDH/graphene composite for a high performance battery-type electrode with high specific capacitance, rate capability, and excellent cycling stability. The reason for such a high electrochemical performance of the composite can be concluded as follows. First, Ni−Co LDH with a hollow nanocages structure can provide more active sites for faradic reactions. Moreover, such a hollow structure can also accommodate the volume expansion and contraction arising from long-term charge−discharge tests.42,62 Second, graphene nanosheets used as a substrate for immobilization of the Ni− Co LDH hollow structure can effectively restrict the aggregation of Ni−Co LDH nanocages and restacking of graphene nanosheets. Such a design can expose more active sites for reacting with the electrolyte. Third, graphene, with a high conductivity, used as a scaffold, can transport the electrons at a high rate and thus decrease the overall resistance of the electrode. The suitability of as-prepared material in practical utilization is also evaluated in the two-electrode asymmetric supercapacitor consisting of Ni−Co LDH/15 mg graphene as the positive electrode and active carbon as the negative electrode. As illustrated in Figure 9a, the PTFE membrane is sandwiched with two electrodes and 1 M KOH is used as the electrolyte. Before testing the electrochemical performance of the device, the operating voltage range is surmised by measuring the CV 9931



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drawbacks (high IR drop from charge−discharge curves) derived from capacitance mismatch of positive and negative electrodes. However, the assembled device can deliver a maximum energy density of 68.0 Wh kg−1 at a power density of 594.9 W kg−1 and still retains 23.96 Wh kg−1 at a power density of 4759.1 W kg−1, as illustrated in the Ragone plot which reveals the relationship between energy and power densities. The detailed electrochemical performance is also listed in Table 1. Such energy and power output are much

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01879. Raman spectra of graphene, Ni−Co LDH, and Ni−Co LDH/graphene; EDS spectrum of Ni−Co LDH/ graphene composite; XRD patterns and SEM images of Ni−Co LDH/graphene M and Ni−Co LDH/15 mg graphene; nitrogen adsorption−desorption isotherms and BJH pore size distributions of Ni−Co LDH with different graphene mass; electrochemical performance of Ni−Co LDH, Ni−Co LDH/graphene mix, and Ni−Co LDH/15 mg graphene; XPS spectra of graphene and Ni−Co graphene composite; CV curves of Ni−Co LDH with different grapheen mass; current density as a function of the scan rate for all prepared electrodes. (PDF)

Table 1. Comparison of Electrochemical Performances for Various Supercapacitors positive materials// negative materials

energy density (Wh kg−1)

power density (W kg−1)

cycle no.

retention %

Ni−Co DH//AC rGO/CoAlLDH//RGO MnCo-LDH@ Ni(OH)2//AC NiCo2O4@ NiO//AC CoNiFe-LDH/ CNFs-0.5//AC Ni−Co LDH from ZIF-67// AC HCNs@NiCoLDH// graphene [email protected]//CBC-N2 Ni−Co LDH/ graphene//AC

42.5 22.6

400.0 90.0

3000 5000

80.7 94.0

61 50

47.9

750.7

5000

90.9

62

31.5

215.2

3000

89.0

63

30.2

800.1

2000

82.7

65

27.5

375.0

1000

89.3

66

*E-mail: [email protected]. Fax:+86 451 8253 3062. Tel: +86 451 8253 3062. *E-mail: [email protected].

47.04

699.7

3000

93.5

67

ORCID

Qi Liu: 0000-0002-2195-8707 Jun Wang: 0000-0002-5192-0574

ref

36.3

800.2

2500

89.3

67

68.0

594.9

2500

94.2

this work

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AUTHOR INFORMATION

Corresponding Authors

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC 61473095), Fundamental Research Funds of the Central University (HEUCFM), Natural Science Foundation of Heilongjiang Province (B2015021), International Science & Technology Cooperation Program of China (2015DFR50050), and the Major Project of Science and Technology of Heilongjiang Province (GA14A101).

higher than previous reports about asymmetric supercapacitors, such as Ni−Co DH//AC, 61 rGO/CoAl-LDH//RGO,60 MnCo-LDH@Ni(OH)2//AC,62 NiCo2O4@NiO//AC,63 CoNiFe-LDH/CNFs-0.5//AC,64 Ni−Co LDH from ZIF-67//AC,65 HCNs@NiCo-LDH//graphene,66 and [email protected]// CBC-N2.67 Moreover, the asymmetric supercapacitor device with a size of 1 × 1 cm2, can light up an LED as illustrated in the inset of Figure 9f, again demonstrating the potential for utilization in sustainable energy-storage conversion devices.

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REFERENCES

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CONCLUSION In summary, hierarchical Ni−Co LDH hollow nanocages incorporated with graphene nanosheets via facile in situ precipitate followed by ion-exchange reaction is reported, which is easy to control and suitable to large-scale production. The as-synthesized composite shows a hollow structure composed from interconnected LDH nanopetals, homogeneously deposited on graphene nanosheets. The special hierarchical hollow structure associated with high conductive graphene facilitates the transmission of electrolytic ions and guarantees a more efficient charge and higher redox capacitance. Moreover, the composite with optimal graphene mass of 15 mg reveals excellent electrochemical results as a battery-type electrode. The assembled asymmetric supercapacitor based on composite and active carbon exhibits excellent electrochemical performance, including high specific capacitance, energy density and power density, and good cycling stability. The electrochemical results indicate that Ni− Co LDH/graphene is a promising material for a battery-type electrode in sustainable energy devices. 9932


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