Graphene Interface with

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 37645-37654

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Charge Transfer in Ultrafine LDH Nanosheets/Graphene Interface with Superior Capacitive Energy Storage Performance Yingchang Jiang,† Yun Song,† Yanmei Li,† Wenchao Tian,† Zhichang Pan,† Peiyu Yang,† Yuesheng Li,† Qinfen Gu,‡ and Linfeng Hu*,† †

Department of Materials Science, Fudan University, Shanghai 200433, P. R. China Australia Synchrotron, 800 Blackburn Road, Clayton, 3168, Australia

‡

S Supporting Information *

ABSTRACT: Two-dimensional LDH nanosheets recently have generated considerable interest in various promising applications because of their intriguing properties. Herein, we report a facile in situ nucleation strategy toward in situ decorating monodispersed Ni−Fe LDH ultrafine nanosheets (UNs) on graphene oxide template based on the precise control and manipulation of LDH UNs anchored, nucleated, grown, and crystallized. Anion-exchange behavior was observed in this Ni− Fe LDH UNs@rGO composite. The Ni−Fe LDH UNs@rGO electrodes displayed a significantly enhanced specific capacitance (2715F g−1 at 3 A g−1) and energy density (82.3 Wh kg−1 at 661 W kg−1), which exceeds the energy densities of most previously reported nickel iron oxide/hydroxides. Moreover, the asymmetric supercapacitor, with the Ni−Fe LDH UNs @rGO composite as the positive electrode material and reduced graphene oxide (rGO) as the negative electrode material, exhibited a high energy density (120 Wh kg −1) at an average power density of 1.3 kW kg −1. A charge transfer from LDH layer to graphene layer, which means a built in electric field directed from LDH to graphene can be established by DFT calculations, which can significantly accelerate reaction kinetics and effectively optimize the capacitive energy storage performance. KEYWORDS: layered double hydroxides, ultrafine nanosheets, charge transfer, interface, energy storage

1. INTRODUCTION Inorganic two-dimensional (2D) layered compounds have been intensively studied in recent years because of their intriguing properties, such as swelling and delamination behavior, intrinsically charged surface, and potential applications in energy storage, optoelectronic devices, and thin-film fabrication.1,2 The atomically thin LDH nanosheets exfoliated from bulk 3D precursor generally exhibit maximized surface area, and the atomic thickness makes the number of the active sites comparable with their total atoms.3−5 On the other hand, the characteristic 2D morphology of these materials provides an ideal template to grow or assemble various nanostructures to realize synergistic coupling effects and enhanced functionalities.6,7 It has been demonstrated that combinations of electrochemically active nanostructures with graphene materials can not only provide conducting networks but also buffer the large volume changes during charge/discharge process.8,9 Furthermore, the as-grafted nanostructures with second © 2017 American Chemical Society

building blocks on 2D template were highly desirable to provide more active sites for fast electrochemical reaction, as well as shorten the diffusion length of the charge carriers. For example, ultradispersed anatase TiO2 nanoparticles loaded on Graphene showed ultrahigh rate capabilities for lithium storage.10 Bimetallic layered double hydroxides (LDHs) with a general formula of [M2+1−xM3+x (OH)2]x+ [An−x/n·mH2O]x− (M2+ and M3+, the bivalent and trivalent metal cations, respectively; An−, the charge-balancing anion of valence n; x = M3+/(M2+ + M3+)) is an important member of the 2D inorganic family, which have attracted increasing interest due to their wide applications in catalysis, separation, biotechnology, electrochemistry.11−15 Recently, LDH ultrafine nanosheets (UNs) with lateral size Received: June 29, 2017 Accepted: October 9, 2017 Published: October 9, 2017 37645

DOI: 10.1021/acsami.7b09373 ACS Appl. Mater. Interfaces 2017, 9, 37645−37654

Research Article

ACS Applied Materials & Interfaces

with 1.5 M sodium nitrate (NaNO3) in a flask when remaining other conditions. 2.4. Fabrication of Ni−Fe LDH UNs@rGO Electrodes, rGO Electrodes, and Asymmetric Supercapacitor. The Ni−Fe LDH UNs@rGO electrodes assumed as the positive of the asymmetric supercapacitor was prepared by mixing 80 wt % of activated materials, 15 wt % of acetylene black and 5 wt % of polytetrafluoroethylene and then spread on to a 10 mm × 40 mm Ni foam. For rGO electrodes, GO was first prepared by a modified Hummers method as reported previously.20 The GO suspension was then freeze-dried overnight to obtain the freeze-dried GO. This product was reduced to the freezedried rGO by 150 °C thermal treatment for 1 h in vacuum. Because of thermal treatment of solid samples, the formed three-dimensional porous structure of the rGO electrodes were fabricated using the method as follows: a mixture of rGO, 15 wt % of acetylene black (as an electrical conductor), 5 wt % of polytetrafluorene−ethylene (as a binder), and a small amount of ethanol was prepared by milling to produce a homogeneous paste. This paste was then pressed onto nickel foam current collectors to produce the rGO electrodes. To fabricate the asymmetric supercapacitor, the above two electrodes were then pressed and combined with each other with PVA gel of KOH (3 M) as the solid electrolyte to assemble the full cell. The cell was encapsulated by flexible Kapton film with two pieces of copper wires connected to the edges of the two electrodes. The specific capacitance (Csp), energy and power densities of the asymmetric supercapacitor were all calculated based on the total mass of both the negative and positive electrodes excluding the current collector. 2.5. Electrochemical Measurements. The electrochemical properties of the as-obtained electrodes were investigated under a three-electrode cell configuration at 25 °C in 1 M KOH. The nickel foam supporting active materials acted directly as the working electrodes and were soaked in a 1 M KOH solution and degassed in a vacuum for 1 h before the electrochemical test. Platinum foil and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrochemical properties of asymmetric supercapacitor were investigated under a two-electrode cell configuration with Ni−Fe LDH UNs@rGO electrode as the positive electrode and rGO as the negative electrode in 1 M KOH electrolyte solution. The CV, galvanostatic charge−discharge, and electrochemical impedance spectroscopy (EIS) measurements were conducted on a Gamary 90I electrochemical workstation (Gamary Instrument Company, USA). 2.6. Characterization. The morphologies of as-prepared samples were observed by a FEI Nova Nano SEM 450 field-emission scanning electron microscopy. The SAED pattern and TEM images were obtained on a Philips CM200FEG field emission microscope. For TEM observation, an aqueous droplet consisting of Ni−Fe LDH UNs@rGO composite was dropped on a Cu grid. The crystal structure characteristics were studied using a Bruker D8-A25 diffractometer using Cu Kα radiation (λ = 1.5406 Å). The elemental composition and chemical state of the as-prepared Ni−Fe LDH UNs@rGO composites were measured using a PHI 5000C EACA system X-ray photoelectron spectroscopy (XPS), with a C 1s peak at 284.6 eV as the standard signal. Raman spectra were obtained by a Horiba Jobin Yvon XploRA confocal Raman microscopy system at a laser wavelength of 532 nm and a spot size of 0.5 mm. Thermogravimetric (TGA) measurements were carried out using a SDT Q600 instrument in a temperature range of room temperature to 900 °C at a heating rate of 10 °C/min under air flow. 2.7. DFT (Density Functional Theory) Calculations. Single graphene layer containing 32 carbon atoms and single LDH layer with 9 metal, 18O, 18H atoms were used to build the supercell (9.85 × 9.85 × 26.1 Å3) for calculation. The distance of vacuum between repeating layers was set to 20 Å to avoid unphysical interactions between supercells. Metal positions were set as Ni:Fe = 2:1 ratio. To obtain the most stable interfacial configuration, various initial LDH-graphene distances (1.0−2.8 Å) were optimized with the lowest energy. DFT calculations were performed by using CASTEP code with ultrasoft pseudopotentials based on the exchange and correlation in the Perdew−Burke−Ernzerhof (PBE). A Hubbard-like localized term was added as GGA+U method. DFT-D semiempirical correction method

less than 10 nm has emerged and generally show superior physical/chemical properties because of the further improved active sites and much higher surface area. Tor example, Zhang successfully grown Ni−Fe LDH UNs on nitrogen-doped graphene and the as-designed new structure shows interesting cooperative Iinterface for highly efficient Lithium−Sulfur batteries.16 The Zn−Cr LDH UNs grafted on graphene displays significantly enhanced visible-light-induced photocatalytic performance.17 Our previous work delivers that large-sized transition metalbased LDHs nanoplates are ideal anode materials for supercapacitors becuase of the variable oxidation states and fast and successive redox reactions.18,19 It is rational that highperformance supercapacitor for energy storage could be realized in LDH UNs system benefited from the small sizeeffect and enhanced surface area. However, the study of LDH UNs on solid-state supercapacitor have still been scarcely developed up to date. Inspired of this consideration, herein, we report a novel and facile one-step precipitation strategy to in situ nucleation of NiFe-LDH UNs on reduced Graphene oxide (rGO). Anion-exchange behavior was observed in this Ni−Fe LDH UNs@rGO composite. The as-obtained LDH UNs@ rGO composite exhibited a high specific capacitance (2715 F g −1 at 3 A g −1) and energy density (82.3 Wh kg −1 at 661 W kg −1 ) because of their highly exposed active surface, charge teansfer at the inherent interface, the synergistic effects of the LDH UNs and rGO. The as-obtained electrochemical performance generally exceeds that of most previously reported nickel iron oxide/hydroxides.

2. EXPERIMENTAL SECTION 2.1. Materials. Natural graphite powder with a purity of 99.99% was purchased from Sigma-Aldeich. Other chemical reagents (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd., and directly used without further purification. 2.2. Synthesis of Ni2−Fe1 LDH UNs@rGO. First, a designed mass (100−300 mg) of GO was put into 1000 mL of deionized water and ultrasonicated at room temperature for 1h to form a stable GO dispersion. Then 4 mmol of NiCl2·6H2O, 2 mmol of FeCl2·4H2O, anthraquinone-2-sulfonic acid sodium salt monohydrate (AQS, 4 mmol) and hexamethylenetetramine (HMT, 48 mmol) were dissolved in above dispersion under ultrosonication for another 5 min. The dispersion was refluxed under continuous magnetic stirring and nitrogen gas protection at 100 °C for 6 h. After being cooled to room temperature, the precipitate was collected by centrifugation, then washed with deionized water and ethanol several times, and finally dried at 60 °C. Various Ni−Fe LDH UNs@rGO with different Ni: Fe ratios (e.g., 3:1 and 4:1) were synthesized just by refluxing the GO dispersed aqueous solution of NiCl2−FeCl2-AQS−HMT at varied Ni:Fe ratios (3:1 and 4:1) under the same experimental conditions. When keeping other experimental conditions unchanged, bare Ni−Fe LDH nanosheets were obtained in the absence of GO. 2.3. Anion Exchange of Ni−Fe LDH UNs@rGO. Ni−Fe LDHs intercalated with various anions were prepared via an ethanol-assisted anion-exchanged approach to minimize the carbonate contamination. For instance, to prepared the dodecyl sulfate anion (DS―)intercalated Ni−Fe LDHs, a 0.2 g sample of as-synthesized Ni−Fe LDH UNs@rGO intercalated with AQS2― was dispersed into 200 mL of water/ethanol (1:1, v/v) binary solution containing 1 M sodium dodecyl sulfate (SDS, C12H25OSO3Na) and 2 mM HCl after purging with nitrogen gas. The molar ratio (LDH/DS―) used in the ion exchange reaction was about 1.3:200. The vessel was tightly capped and mechanically shaken for 1 week to complete the exchange into the DS― form. Similarly, the nitrate (NO3―) intercalated Ni−Fe LDH can be obtained just by mixing 0.2 g of AQS2― intercalated Ni2Fe2 LDH UNs@rGO into 200 mL of the same ethanol/water binary liquid 37646


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Figure 1. (a) Schematic illustration of the in situ solution growth of LDH UNs on GO. (b) The projected atomic configuration of LDH and rGO top-surface. The basic primitive cell of both LDH and rGO can be regarded as hexagonal atomic ring with size of 2.05 and 1.43 Å, respectively.

Figure 2. (a−c) Typical TEM images and corresponding EDX spectra. (d) The corresponding SAED pattern and (e) HRTEM image of the Ni2− Fe1 LDH UNs@rGO composite. (f) The top-view atomic confriguation of Ni2−Fe1 LDH UNs (001) plane. (g) Size-distribution diagram of the asgrown LDH UNs on rGO matrix obtained from the TEM images. (h) XRD patterns of the prinstine Ni2−Fe1 LDH nanosheets, Ni2−Fe1 LDH UNs@rGO composite, and the products after ion-exchange with DS− and NO3−. (i) Schematic illustration of anion exchange process in the Ni2−Fe1 LDH UNs. calculations and Monkhorst Pack k-point mesh 5 × 5 × 1 was used in the calculation to ensure the total energy value convergence within 1 meV/atom. During the geometry optimization process, the tolerance

represented by the Tkatchenko-Scheffler (TS) scheme was used to account for hydrogen bonding and van der Waals (VdW) interactions. The cutoff energy of the plane wave basis-set was 400 eV in all 37647


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Figure 3. (a, b) XPS spectra and atomic concentration spectrum of Ni2−Fe1 LDH UNs @rGO composite, respectively. The S signal was detected from the AQS2− ions in the interlayer of LDH UNs. (c, d) High-resolution XPS survey scan of Ni 2p and Fe 2p regions, respectively. convergence accuracy was set when the total energy was less than 1.0 × 10−6 eV/atom, the maximum force on per atom was less than 0.01 eV/nm, the maximum stress was less than 0.05 GPa, and the maximum displacement between cycles was less than 1 × 10−5 eV.

Ni(OH)2 or NiOOH. Two sets of selected area electron diffraction (SAED) pattern respectively from the LDH and rGO matrix can be distinguished clearly in Figure 2d. The asobserved lattice spacing in the HRTEM image was calculated to be 0.25 nm, matching well with the (001) plane of hexagonal Ni−Fe LDH,23 indicating that the (001) facet of LDH UNs interacted with the Graphene (Figure 2e−f). The average size of the LDH UNs was around 6.5 nm (Figure 2g). One can see that the (001) surface of LDH phase and top-surface of rGO generally shows the same atomic configuration and chemical coordination environment with a hexagonal ring as the primitive cell (Figure 1b). However, the size (r) of the LDH/rGO hexagonal rings presents distinct size of 2.05 and 1.43 Å, respectively. Therefore, the in-plane lattice mismatch between LDH and rGO can be estimated to be as large as 43.3% from eq 1, and the large lattice mismatch suggests that the LDH UNs is not likely to grow as a planar epitaxy mechanism.24

3. RESULTS AND DISCUSSION 3.1. In-situ nucleation of NiFe-LDH 2D ultrafine nanosheets on rGO. As shown in Figure 1, in situ facile growth of Ni−Fe LDH UNs@rGO (Ni:Fe = 2:1, atom ratio) composites was induced by the heterogeneous nucleation of Ni−Fe LDH on graphene oxide. The XRD pattern of the product confirms the formation a rhombohedral structure of LDH (Figure S1). The Raman spectra of the products are presented in Figure S2. The bands below 700 cm−1 were associated with metal−oxygen stretching and metal-hydroxide bending vibrations. The strong vibration band observed at around 1679 cm−1 was assigned to CO stretching mode in the AQS2− (anthraquinone-2-sulfonate anions, C14H7O5S2−) guests. The bands at 1598 cm−1 corresponded aromatic CC stretching mode and the other at 1333 and 1295 cm−1 to aromatic C−H stretching modes, respectively.21 The peaks at 1185 (broad), 1089, and 1038 cm−1 were attributed to asymmetrical and symmetrical stretching modes of interlayer sulfonate groups, respectively. In contrast to bare Ni2−Fe1 LDH, the LDH UNs@rGO sample presents another two characteristic peaks at 1353 and 1584 cm −1, corresponding to the D band and the G band of carbon, respectively. The high D/G intensity ratio demonstrates the existence of rGO and the reduction of GO during the growth process.22 The typical transmission electron microscopy (TEM)/scanning electron microscopy (SEM) images in Figure 2a, c and Figure S3 demonstrate fairly dense coverage of a large number of Ni2−Fe1 LDH UNs on rGO template, the corresponding energy dispersive X-ray spectroscop (EDX) point analysis result in Figure 2b confirms ultrafine nanosheets contains Fe an Ni two elements rather than a single element compounds, like

rLDH − rgraphene rgraphene

× 100% = 43.3% (1)

This type of ultrafast growth profile is likely to be related to the nature and size of the heterogeneity triggering nucleation, which could be explained to be that grains nucleated at higher undercooling, the growth rate increases abruptly from start, to eventually reach the constant value. Tiny particles attached on GO sheets were pinned by the high concentration oxygen groups and defects on GO without continuous growing into larger and well-defined hexagonal nanoplates. The thickness of the LDH nanosheets is estimated to be ∼3.8 nm by the atomic force microscopy (AFM) image in the Figure S4, indicating the ultrathin feature. Further confirmed by the SEM observation in Figure S5, the LDH UNs nucleated and grew on graphene oxide supports quite quickly and almost into this scale after about 3 h. With the reaction time extending, the particles grew 37648


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Figure 4. (a) Comparison of CV curves of Ni−Fe LDH UNs@rGO hybrid-, and Ni−Fe LDH-electrodes at a scan rate of 10 mV s −1. (b) Galvanostatic charge−discharge curves of Ni2−Fe1 LDH UNs@rGO hybrid electrode at different current densities. (c) Comparison of Cs of the hybrid electrodes prepared with different Ni/Fe atomic mole ratios. (d) Comparison of Nyquist plots of pristine Ni2−Fe1 LDH and Ni2−Fe1 LDH UNs@rGO hybrid electrodes.

electron Spectroscopy (XPS) characterization in Figure 3 was used to examine chemical states of nickel and iron in the composite. By using a Gaussian fitting, two obvious shakeup satellites (indicated as “Sat”) close to two spin−orbit doublets at 875.4 and 858.2 eV can be identified as Ni 2p1/2 and Ni 2p3/2 signals of Ni2+, respectively. The remarkable peak intensity difference in the corresponding two satellite peaks (noted as “Sat”.) suggests the main presence of Ni2+. Two main peaks at binding energies of 714.4 and 728.0 eV are respectively assigned to the Fe 2p3/2 and Fe 2p1/2, and the absence of the corresponding two satellite peaks indicates the main presence of Fe3+ by a possible oxidation of AQS.23 The oxidation of Fe rather than Ni in the present system should be attributed to the slightly higher M(III)/M(II) redox potentials of Ni than Fe. AQS plays as an in situ oxidizer (Scheme S1) to transform Fe2+ into Fe3+ and simultaneously intercalated into the interlayer as anions to balance the positive charge of host layers.27 Apparently, such a one-step precipitation method is much facile than the previously reported two-step assembly strategy including exfolication of positively charged LDHs and heteroassembly with negatively charged graphene.28,29 Note that 2D nanosheet morphology of Ni2−Fe1 LDH, with a later size of ∼600 nm and a thickness about 19.2 nm (Figure S7), was obtained with the absence of GO under the same conditions in the present study. The yellow-green color of the product was also quite different with the black color for the LDH UNs@ GO (Figure S8). Such a drastic morphological difference highlights the formation of rGO. The dependence of the morphology on the amount of GO was further studied. The bare LDH nanosheets could be easily observed in the sample with too little amount of GO (Figure S9). However, excessive GO amount could reduce the adhesion density of Ni2Fe1 LDH UNs on GO. Generally, in a classical model of crystal nucleation, the specific driving force (Δμnuc), required for a

up without further except for more uniform particle size. In addition, various Ni−Fe LDH UNs@rGO with different Ni: Fe ratios (e.g., 3:1 and 4:1) were also characterized by the transmission electron microscopy (TEM) and energy dispersive spectrum (EDS) shown in the Figure S6. There is no significant difference on morphology and structure among three samples with different ratio. The ion-exchange behavior of our LDH UNs@rGO was subsequently determined in an aqueous process. Figure 2h shows the X-ray diffraction (XRD) patterns of the pristine Ni2−Fe1 LDH, Ni2−Fe1 LDH UNs@rGO composite, and the samples after ion-exchange with DS− (dodecyl sulfate, C12H25OSO3−) and NO3− (nitrate), respectively. For pristine Ni2−Fe1 LDH sample, all the diffraction peaks are readily indexed as a hydrotalcite-like phase similar to those in α-Co-AQS2−-LDH phase.25 The LDH phase was confirmed and the refined lattice parameters of such a rhombohedral structure were c = 60.3 Å and a = b = 3.08 Å. The XRD patterns indicate the characteristic reflections of a well-oriented LDH structure due to the series of high order (00l) reflections. No peaks of impurities were discerned, indicating the high purity of the product. The XRD pattern of the Ni2−Fe1 LDH UNs@rGO composite was almost the same as that of pristine Ni2−Fe1 LDH. Interestingly, after ionexchange process, the basal spacing, 2.0 nm for the pristine AQS2− form, shifted to 2.4 nm for the DS− form and 0.8 nm for the NO3− form, which are calculated by Bragg eq (2dsin θ = nλ). The sharp diffraction peaks of the DS− form demonstrate that the high crystallinity of the LDH UNs can be well maintained during the ion-exchange process. Such an ionexchange behavior is generally characteristic for inorganic layered compounds, indicating that the basal interlayer spacing of the LDH UNs@rGO can be stretched and compressed in atomic scale easily.26 Accordingly, our LDH UNs@rGO composite can be regarded as microreactors for various anions in a scale of several nanometres (Figure 2i). X-ray Photo37649


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Figure 5. (a) CV curves of the Ni2−Fe1 LDH@rGO//rGO asymmetric supercapacitor at different scan rates. (b) Galvanostatic charge−discharge curves and (c) Cs of this asymmetric supercapacitor at different current densities. (d) Cycling performance of the asymmetric supercapacitor at a current density of 5 A g −1. (e) Images of the red LED indicator at different stages powered by the charged LDH@rGO//rGO asymmetric supercapacitors. (f) A picture showing that two supercapacitors in series can lighten up a green LED indicator.

M2+ (M = Ni, Fe) reacted with the gradual release of OH− deriving from the decomposition of HMT to form Ni−Fe LDH nuclei, that were tightly anchored onto GO sheets with monodispersity. And then they began to crystallize and grow, gradually forming LDH UNs@GO composites. During this process, bivalent iron was converted to trivalent iron by AQS2−. The as-obtained anion AQS2− as counteranions intercalated between the neighboring crystal layers of layered crystal structure of Ni−Fe LDH. Meanwhile, GO sheets were reduced to rGO sheets with excess HMT as a mild reductant.22 According to above result of TGA data (Figure S10), the chemical composition of as-prepared LDH UNs was estimated as Ni2/3Fe1/3(OH)2(AQS)1/6·0.5H2O. Thus, the general reactions for the formation of LDH UNs@GO can be simply written as follows:

stable nucleus is inversely related to the square root of the activation energy, ΔGnuc, for nucleation as30 Δμnuc = 4V

πα 3f 3ΔGnuc

(3)

where α is the surface free energy gained by the formation of a new solid/liquid interface rather than being dissolved in the solvent, V is the solute molar volume, Cnuc is the solute concentration at the state of nucleation, and f is a wetting term describing the interaction of the solvent and solid precipitate. For homogeneous nucleation, f = 1, however, for heterogeneous nucleation, f < 1. For the present process, GO provided large amounts of defects/functional groups as heterogeneous nucleation sites for in situ nucleation of Ni−Fe LDH UNs. The presence of abundant carboxyl and hydroxyl surface functional groups on the GO sheets lead to the metal cation grafted on the GO sheets by electrostatic force to become the aggregation cores. The activation energy of LDH, ΔGnuc, should be significantly decreased due to f ≪ 1. The dramatically decreased activation energy should greatly increase the nucleation rate of LDH nuclei. As a consequence, the absorbed

2/3Ni 2 + + 1/3Fe3 + + 2OH− + 1/6AQS2 − + 0.5H 2O rGO

⎯⎯⎯→ Ni 2/3Fe1/3(OH)2 (AQS)1/6 · 0.5H 2O@rGO

(4)

3.2. Electrochemical Performance of Ni−Fe LDH UNs@rGO. The typical cyclic voltammetry (CV) curve of the as-obtained hybrid composites (Figure 4a) exhibited reversible 37650


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Figure 6. (a) 3D charge-density difference for the interface of LDH UNs and rGO by DFT calculation studies. Blue and green regions represent charge accumulation and depletion, respectively. (b) Electron localization function (ELF) and (c) a 2D slice view of the ELF of Ni−Fe LDH UNs@ rGO interfaces. (d) Total and (e) partial calculated density of states (DOS) of Ni−Fe LDH UNs@rGO composites.

oxides of a single metal.33,34 The hybrid formed at 2:1 of Ni: Fe showed the Cs values 2715, 2489, 2311, 2050, and 1756 F g−1 at current densities of 3, 5, 10, 15, and 20 A g−1, respectively, based on active materials. The highest Cs (2715 F g−1 at 3 A g−1) and the maximum energy density 82.3 Wh kg−1 at 661 W kg −1) based on our LDH UNs@rGO were found to greatly exceed those of most reported nickel−iron oxide/hydroxide based pseudocapacitive materials (Table S1). Additionally, electrochemical impedance spectra (EIS) was also used to compare the electrochemical properties of Ni2−Fe1 LDH UNs@rGO composite and bare Ni2−Fe1 LDH UNs electrode. Figure 4d shows the typical Nyquist plots, which consist of a compressed semicircle a semicircle at a higher frequency region and a spike at lower frequency. At the high frequency, the intersection of the curve at the real part indicates the resistance of the electrochemical system (Rs, which includes the inherent resistance of the electroactive material, ionic resistance of electrolyte, and contact resistance at the interface between electrolyte and electrode) and the semicircle diameter reflects the charge-transfer resistance (Rct). Apparently, Ni2−Fe1 LDH UNs@rGO possesses a much lower Rs and Rct value compared with that of bare Ni2−Fe1 LDH UNs electrode, which can be attributed to the conducting effect of rGO. Moreover, the relatively steeper low-frequency tail of Ni2−Fe1 LDH UNs@ rGO suggests higher electrons diffusivity. Therefore, all of these are beneficial for electrons storage properties of Ni2−Fe1 LDH UNs@rGO electrode. An asymmetric supercapacitor using Ni2−Fe1 LDH UNs@ rGO and porous freeze-dried rGO as anodes was further fabricated. The freeze-dried rGO displayed excellent electric double-layer capacitance property at −1.0 to 0.0 V.35

redox peaks within 0.0−0.5 V, corresponded to the reversible reactions of Ni(II) ↔ Ni(III) and Fe(II) ↔ Fe(III).31 This indicates the strong pseudocapacitive nature of the as-obtained electrodes. As shown in the Figure S11a, those overlapping representative CV curves of the Ni−Fe LDH UNs@rGO electrode for the first to hundredth cycles at a scan rate of 20 mV s−1 illustrated the steady reversible redox reaction. The average specific capacitance of this Ni2−Fe1 LDH UNs@rGO composite was calculated to be 2561 F/g at a scan rate of 20 mV/s from CV curve, which was much higher than that of bare Ni2−Fe1 LDH electrode (1535 F/g at a scan rate of 20 mV/s). By comparingthe cycling performance of Ni−Fe LDH UNs@ rGO and NiFe-LDH electrodes in Figrure S11b, the capacity of composite material was much more stable than bare NiFeLDH. The TEM of Ni−Fe LDH UNs@rGO after 1000 cycles was shown in Figure S12, The morphology of active material has no obvious change compared with initial. Figure 4b shows the galvanostatic charge−discharge curves of Ni2−Fe1 LDH UNs@rGO hybrid electrode at different current densities. Figure 4c demonstrates the Cs of the hybrid film-electrodes at various Ni: Fe feeding mole ratios. As more nickel was embedded, Cs increased. The finding that a higher Fe content in bimetallic Ni−Fe hydroxides plays a favorable role is consistent with previous reports.27,32 It was proposed that an increased Fe content in bimetallic Ni−Fe composites may result in partial-charge transfer from Fe sites to activate Ni centers, and enhance the electrochemical activity. Moreover, the higher conductive Fe exposed on the surface of hydroxide layers contributed to an improved conductivity, which was also beneficial in increasing the reaction efficiency. However, too much Fe may cause the contaminant of hydroxide or hydrous 37651


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solids. As shown in Figure 6b and c, the high ELF value around metal and O atoms indicates strongly paired electrons. The ELF volume at the metal site with distorted square distribution shape indicate that the bonding interaction between metal and O atoms is partially ionic. It shows that there is some charge transfer from metal to O atoms. The ELF distribution at the H sites in the bottom of LDH layer is not spherically symmetric and it is polarized toward the graphene layer indicating the influence of the interaction between H atoms and Graphene. The low ELF value between LDH and Graphene layers indicates weak van der Waals (VdW) interactions dominate between layers. These results may enrich the interfacial behavior for LDH and rGO heterostructures, and might be useful to understand the formation mechanism for this unique structure. More interestingly, theoretical calculations show the large local density of states (DOS) across the Fermi level (Figure 6d and e, Figure S14), which reveals the metallic behavior in the present LDH UNs@GO system. Accordingly, the excellent performance on capacitive energy storage should be attributed to the following factors:38,39 (1) Considering that the capacitive energy storage is a representative surface chemical process including charge transfer and adsorption, our LDH UNs with low-crystallinity and highly exposed active surface provides more active sites for charge storage, and also greatly improves the electron transportation from active materials to the current collector, which was confirmed by the decreasing equivalent series resistance. (2) The ultrasmall size of the LDH UNs presents very short ion diffusion paths, which are favorable for fast redox reactions. (3) The synergistic effects of the LDH UNs and rGO: the monodispersed LDH UNs in situ decorating on rGO and the resultant well-defined hybrid net-structure therefrom should favor the electronic transport and accessibility of the electrolytes to the active compounds. The charge transfer and possible built-in electric field at the LDH UNs@ rGO interface could significantly accelerate reaction kinetics.

Consequently, Its CV curve exhibits double contribution of electric double-layer capacitance and pseudocapacitance at 0− 1.6 V (Figure 5a). The Cs shown in Firuge 5c of Ni2−Fe1 LDH UNs@rGO//rGO was calculated based on active materials from its galvanostatic charge−discharge curves in Figure 5b, and the energy density of the Ni2−Fe1 LDH UNs@rGO//rGO was calculated to be 120 Wh kg−1 at average power densities of 1.3 kW kg−1, respectively. Compared with individual Ni2−Fe1 LDH plates and rGO, the asymmetric supercapacitor showed further enhancement in both energy density and power density, which should benefit from the high energy contribution of Ni2−Fe1 LDH and the enough power support of rGO. The asobtained asymmetric supercapacitor can retain about 80% of its original capacitance after 5000 cycles, which was comparable to those of some other asymmetric supercapacitors (Figure 5d). As shown in Figure 5e, the red light-emitting diode (LED) remained very bright after 10 min and even effective enough for indication after 15 min. We also assembled two supercapacitors in series, and the device could power white LED indicator efficiently (Figure 5f). 3.3. DFT Calculations. The interfacial state of chargedensity for the present system was further illustrated by density functional theory (DFT) calculations as shown in Figures 6 and S13. Note that such a DFT calculation on this LDH@rGO system is highly complicated, and a simplified model of only considering one layer of LDH crystal was adopted to get a convergent result. The imulation results indicates a strong interaction between LDH and Graphene layers. A strong charge accumulation can be found above the Graphene layer, whereas the charge depletion regions appear near the bottom of H atoms (facing the Graphene layer) and surrounding metal atoms. This indicates that there is a charge transfer from LDH layer to graphene layer, which means a built in electric field directed from LDH to Graphene can be established. The charge transfer was usually observed in atomically thin heterolayers, which may possess better electrochemical properties.36,37 In the present system, unlike conventional heterogeneous interfaces constructed by assembling disparate bulk materials together, the inherent atomic interfaces formed in the ultrasmall LDH UNs and ultrathin 2D graphene can well tune the electron distribution and maximizing the interaction at the interface. More importantly, the sufficient hybridization of atomic orbitals at the inherent atomic interfaces could allow carrier delocalization, which promotes charge transfer across the interface and leads to band bending to form a built-in electric field. The transfer of electrons between LDH layer and 2D graphene sheet can be illustrated in terms of the Mulliken population analysis. A further Mulliken charge analysis of the simplified model with a monolayer of LDH on 2D graphene shows each C atom on graphene averagely obtains 0.03 e. The positive value of Mulliken charge of C atoms represents the electrons are transfer from LDH to rGO; whereas the negative values show that each Ni atom averagely loses 0.77 e, each Fe atoms averagely loses 0.94 e. The forming energy (ΔEform) was calculated to be −7.246 eV by following formula 5, which indicates an energy favorable process for LDH forming on graphene layer. ΔEform = Etotal − (E LDH + Egraphene)

4. CONCLUSION In summary, highly dispersed Ni−Fe LDH 2D UNs in situ decorated on reduced graphene oxide have been successfully obtained using a novel one-step precipitation method, which is much facile than the previously reported two-step assembly strategy. The composited Ni2−Fe1 LDH UNs@rGO electrode can be regarded as microreactors for various anions in a scale of several nanometres, and also exhibited high specific capacitance (2715 F g −1 at 3 A g −1) and energy density (82.3 Wh kg −1 at 661 W kg −1), which exceeds the that of most previously reported nickel iron oxide/hydroxides. The excellent energy storage performance should be attributed to the highly exposed active surface, the synergistic effects of the LDH UNs and rGO and the charge transfer at the LDH UNs@ rGO interface. Such a novel structure might also show excellent performance on splitting of water to generate oxygen (oxygen evolution reaction, OER) which is underway. Moreover, the synthetic strategy developed in the present study might be extended to other bimetallic LDH UNs@rGO, such as Ni−Co LDH, Fe− Co LDH UNs, etc.

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ASSOCIATED CONTENT

S Supporting Information *

The electronic structure was further studied by examining the valence electron localization function (ELF). ELF can be used as an indicator for understanding the chemical bonding in

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09373. 37652


Research Article


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XRD patterns of NiFe LDH UNs@GO, reduction and oxidation of AQS, Raman spectra of bare Ni2-Fe1 LDH nanosheets, GO, and Ni2-Fe1 LDH UNs@rGO composite, SEM images of the as-prepared and as-grown Ni2Fe1 LDH UNs@rGO sample, AFM image and crosssectional profile of LDH CNs on the rGO, TEM images and EDS of as-grown Ni-Fe LDH UNs@rGO, SEM and TEM images of the Ni2-Fe1 LDH nanosheets prepared with the absence of GO, one-pot synthesis of Ni2-Fe1 LDH UNs@rGO sample, color differences between the bare Ni2-Fe1 LDH nanosheet suspension and Ni-Fe LDH UNs@rGO composite, SEM images of as-obtained samples with different mass ratio between Ni2-Fe1 LDH and rGO, TGA curves of different samples, cycling performance of NiFe-LDH UNs@rGO and NiFe-LDH electrodes, CV curve of the Ni-Fe LDH UNs@rGO electrode, TEM amages of The Ni-Fe LDH UNs@rGO, top-view and side-view of the charge-density difference, projected density of states, and maximum Cs, energy and corresponding average power densities (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Linfeng Hu: 0000-0002-0640-508X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51372040, 51701042, 51601040), the Shanghai Rising-Star Program (16QA1400700), and the Science and Technology Commission of Shanghai Municipality (Nos. 15YF1401300).

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