Ni-Al layered double hydroxide with regulated

Chemical Engineering Journal 368 (2019) 905–913

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Ni-Al layered double hydroxide with regulated interlayer spacing as electrode for aqueous asymmetric supercapacitor

T

Heng Zhanga,b, Muhammad Usman Tahirb, Xiuling Yanc, Xueming Liua, Xintai Sua, ⁎ Lijuan Zhanga, a The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), School of Environment and Energy, South China University of Technology, Guangzhou 510006, China b Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China c School of Chemistry and Environmental Science, Yili Normal University, Yining, 835000, China

H I GH L IG H T S

NA-LDH-OA ultrathin nanosheets • The were synthesized by two-phase sol-

G R A P H I C A L A B S T R A C T

The NA-LDH-OA-2//AC ASC device deliver a high energy density of 40.26 Wh kg−1 at a power density of 943 W kg−1, and good cycling performance.

vothermal method.

cycling stability of 94.5% • Excellent capacitance retention after 5000 cycles.

asymmetric supercapacitor • The achieved a high energy density of 40.26 Wh kg−1 at a power density of 943 W kg−1.

A R T I C LE I N FO

A B S T R A C T

Keywords: Layered double hydroxide Sodium oleate Electrochemical energy storage Asymmetric supercapacitors

Using sodium oleate as surfactant and intercalating agent, a regulated Ni-Al layered double hydroxide (NA-LDHOA) nanosheets with a high interlayer space were prepared by two-phase method combined with a short reflux process and mild solvothermal reaction. In the process of electrochemical reaction, the electron transport is accelerated by the higher base spacing of the NA-LDH-OA ultrathin nanosheets. The as-prepared material exhibited a high specific capacity (1.040 C cm−2 at a current density of 1.68 mA cm−2) and good cycling performance (capacity retention of 88.25% after 2000 cycles) in a three-electrode system. Moreover, an aqueous asymmetric supercapacitor (ASC) was assembled using the synthesized NA-LDH-OA nanosheets as the positive electrode and activated carbon (AC) as the negative electrode. The aqueous ASC device (NA-LDH-OA-2//AC) achieved a high energy density of 40.26 Wh kg−1 at a power density of 943 W kg−1, and a good cycling performance of 94.5% retention after 5000 cycles. These results demonstrated that the NA-LDH-OA nanosheets possess the potential for upcoming energy storage devices.

⁎

Corresponding author. E-mail address: [email protected] (L. Zhang).

https://doi.org/10.1016/j.cej.2019.03.041 Received 28 November 2018; Received in revised form 26 February 2019; Accepted 6 March 2019 Available online 07 March 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.


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1. Introduction

[24]. This strategy plays an important role in promoting electron transfer together with accelerating mass transfer, and thus possessing good electrochemical properties. However, most of strategies for expanding interlayer space involve multi-step synthetic processes. And controllable the preparation of pristine Ni-Al-LDHs with high interlayer space for high-performance SCs electrodes materials still remains a challenge. To our best knowledge, there is no report about using oleate ligand as an intercalating agent to synthesis electrode materials for supercapacitor. Our group had been devoted to the synthesis of diverse nanocrystals with two-dimensional (2D)/three-dimensional (3D) structure such as metal oxides/hydroxide using a variety of oleic acid, sodium oleate and oleylamine as surfactants by solvothermal method over the last few years [25,26]. Based on the above research work, for the first time, sodium oleate is selected to assist the synthesis of various NA-LDH-OAs with enlarged interlayer spacing by two-phase solvothermal method, which used in supercapacitor positive electrodes. The as-obtained NALDH-OAs displayed an enlarged interlayer d-spacing of 43.7 Å, which are conducive to increased ion storage and rapid ion diffusion along the interlayer channels. Consequently, ultrathin NA-LDH-OA-2 achieved the ultrahigh capacitance (1.040 C cm−2 at 1.68 mA cm−2), the superior capacitance retention (79.42% when the current density increased from 1.68 to 16.80 mA cm−2) in three-electrode system. In addition, the assembled NA-LDH-OA-2//AC ASC device works synergistically to achieve superior performance in terms of its working potential (1.6 V), energy density 40.26 Wh kg−1) with a power density (943 W kg−1) and cycle life (capacity retention of 94.5% retention after 5000 cycles). Moreover, this facile method can fabricate other LDHs, which has a wide range of applications in the fields of photocatalysis, electrocatalysis, light response and so on.

The demand for sustainable and renewable energy is increasing in the application of modern digital communications, electric vehicles and renewable energy storage devices. Therefore, it is urgent to explore efficient electrochemical energy storage and conversion devices [1–6]. Supercapacitors (SCs), called electrochemical capacitors, have become a promising energy storage device because of their high power density, fast charge and recharge capability, along with long cycle life. However, the practical application of SCs still hindered by the relatively low energy density while preserving a high power density. Therefore, it is necessary to modify the SCs electrode materials to make them have the advantages of ultra-high energy density and ultra-long life. Asymmetric supercapacitors (ASC) is largely characterized by taking advantages of a battery-type (usually pseudocapacitance) electrode and a capacitortype (usually electrochemical double layer) electrode, thereby enhancing the energy density and power density [7–12]. Recently, numerous researchers have devoted to explore the electrode materials with high performance for SCs. Metallic layered double hydroxides (LDHs), a kind of two-dimensional inorganic layered materials, where divalent and trivalent metal cations in the host layers and charge-balancing anions in the interlayer, have aroused great interest in adsorption [13], catalysis [14], biotechnology [15], photocatalysis [16] and electrochemistry [17]. In addition, LDHs have been demonstrated to hold enormous potential for advanced SCs owing to their high electrochemical activity, relatively low cost and tunable composition [18,19]. For example, Hong et al. demonstrated the nickel-aluminium LDHs (Ni-Al LDHs) hollow microspheres for SCs by an in-situ growth method using Al2O3 hollow microspheres as hard template, which exhibits superior specific capacitance and super-long cycle life. [20]. Xiao et al. have synthesized nickel-cobalt-aluminium LDHs (Ni-Co-Al LDHs) hexagonal nanosheets for SCs through a traditional hydrothermal method [21]. Wang et al. demonstrated an in-situ growth approach to fabricate NiAl-LDHs on zeolitic imidazole frameworks (ZIF-8) derived carbon with excellent electrochemical performance [22]. Despite of various superior supercapacitor performance LDHs-based composite electrode materials have been reported, however, the pristine LDHs display relatively low specific capacitance and inferior rate capability, owing to their faradaic redox reactions limited greatly by irreversible reaction kinetics. The surface redox process of LDHs nanosheets is also limited by solidstate diffusion. A promising and effective strategy for acceleration and reinforcement the kinetics is to enlarge interlayer space of nanoplates. Meaningfully, Feng altered inserted anion through the introduction of additive sodium dodecylsulfonate (SDS) into Ni-Fe-LDHs interlayer, achieving a higer interlyer distance (2.49 nm) and application in oxygen evolution reaction [23]. The sodium dodecylbenzene sulfonate (SDBS) expanded inerlayer space of Ni-Co-LDH is capable of delivering a reversible specific capacitance of 1094 F g−1 at 5 A g−1

2. Experimental section 2.1. Chemicals and reagents Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) and Aluminum (III) nitrate nonahydrate (Al(NO3)3·9H2O) were purchased from Aladdin biochemical technology Co., Ltd (Shanghai, China). Sodium oleate (NaOA) (97%) was purchased from Tokyo Chemical Industry Development Co., Ltd (Shanghai, China). Sodium hydroxide (NaOH) and absolute n-hexane were purchased from Baishi Chemical Reagents Co., Ltd (Tianjin, China). Absolute ethanol (C2H5OH) was purchased from Yongsheng fine chemical Co., Ltd (Tianjin, China). 2.2. Synthesis of NA-LDH-OA All chemicals were of pure analytical grade and used without any further purification. The schematic diagram of the synthesis of NA-LDHOA is illustrated in Fig. 1. For a typical run, 2 mmol of Ni(NO3)2·6H2O,

Fig. 1. Schematic illustration of the synthesis of the NA-LDH-OA. 906


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1 mmol of Al(NO3)3·9H2O and 7 mmol of Sodium oleate (C18H35COONa or NaOA) were dissolved in ethanol (20 mL) and n-hexane (30 mL) mixture solution and stirred at 70 °C for 1 h. Then added 10 mL of sodium hydroxide (0.28 g) aqueous solution stirred for another hour. The obtained slurry was transferred into a 100 mL autoclave and heated at 120, 160 and 200 °C for 24 h, the final green product is denoted as NALDH-OA-1, NA-LDH-OA-2, NA-LDH-OA-3, respectively. The precipitates were washed with anhydrous alcohol and n-hexane several times, and dried in vacuum oven at 70 °C for 12 h.

Cmc =

2.5. Asymmetric assembly and measurements For asymmetric supercapacitor (ASC) device, the as-synthesized NALDH-OA-2 and active carbon (AC) used as positive and negative electrodes, respectively, using cellulose paper as the separator was investigated in 6 M KOH. The negative electrode was prepared by using a similar procedure with positive electrode. The energy density (E, Wh kg−1) and power density (P, W kg−1) were calculated through equations [27]:

The crystal structure and composition of the products were determined by a Rigaku D/max-ga X-ray diffraction (XRD) at a scanning rate of 2° min−1 using Cu Kα radiation (λ = 1.54178 Å). The morphologies and microstructures of the samples were characterized with a field emission scanning electron microscope (FESEM Hitachi SU8000) at 20 kV, transmission electron microscopy (TEM, Hitachi H-600) and high-resolution transmission electron microscope (HRTEM JEM 2100). ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS) measurements were examined with an Al Kα (1486.8 eV) X-ray source. Raman spectra was conducted on a Bruker spectrometer with 532 nm. The thickness of nanosheets was collected with an atomic force microscopy (AFM) (Bruker multimode 8). Nitrogen adsorption–desorption isotherms were performed by a Micromeritics ASAP 2020 physisorption instrument at 77 K. The specific surface area via the Brunauer–Emmett–Teller (BET) method and pore size distribution derived from the Barrett-Joyner-Halenda (BJH) and Density Functional Theory (DFT) methods. Fourier transform IR (FT-IR) measurements were performed on a Bruker EQUINOX-55 FT-IR spectrophotometer with a resolution of 2 cm−1 using the KBr pellet technique at room temperature.

E = P=

C=

1 2Av

∫V

Vt

o

I(V)dV

Vt

o

I(V)dV

The morphological formation of the NA-LDH-OA samples were clarified by FE-SEM and TEM in Fig. 2. As shown in Fig. 2a, it is distinct that NA-LDH-OA-2 display a uniform ultrathin sheets structure. It also demonstrates that the nanosheets of NA-LDH-OA-2 are collections of some nanoflakes array which are accumulated with each other to form a layered structure. This conformation not only afford an express path for electronic/ionic diffusion but also provide abundant electrochemical active sites for full redox reactions, thus potentially increasing the capacitive properties. Different resolution SEM images for NA-LDHOA-1 and NA-LDH-OA-3 and are presented in Fig. S1. Fig. S1a shows the low magnification SEM image of NA-LDH-OA-1 composed of amorphous nanosheets. From the high-magnification SEM image (Fig. S1b), amorphous nanoflakes are tightly stacked together. Moreover, Fig. S1c-d shows various magnification SEM images of NA-LDH-OA-3. As can be seen from Fig. S1c and d images, NA-LDH-OA-3 assembled by nanoplates has a smooth surface. The obvious nanosheets can accelerate both electrolyte permeation and sufficient ion transition through electrochemical process, which is extremely important for the electrochemical properties. In addition, the ultrathin feature of the NA-LDHOA-2 can be further examined by TEM images. And Fig. S2 display the TEM images of NA-LDH-OA-1 and NA-LDH-OA-3, which are consistent with the SEM images. The low resolution TEM images in Fig. 2b show the NA-LDH-OA-2 nanosheets planes are uniformly distributed and displays a quasi-hexagonal morphology. The high-resolution TEM (HRTEM) image of NA-LDH-OA-2 in Fig. 2c exhibits the observed and measured lattice fringe of 0.15 nm corresponding to the (1 1 0) planes of Ni-Al-LDH nanocrystals, which is in accordance with the XRD results [28]. The selected area electron diffraction (SAED) pattern exhibits some distinct spots in the inset of Fig. 2c, demonstrating the polycrystalline nature of NA-LDH-OA-2. The crystallographic features of the as-synthesized samples were studied by XRD and the results of NA-LDHOA-2 as shown in Fig. 2d. For NA-LDH-OA-2, NA-LDH-OA-1 and NALDH-OA-3 samples (Fig. S3a), their major peaks are located at approximately 11.73°, 23.58°, 35.12°, 39.71°, 47.28°, 61.21° and 62.59°. These peaks can be assigned to the (0 0 3), (0 0 6), (0 1 2), (0 1 5), (0 1 8), (1 1 0) and (1 1 3) planes of Ni6Al2(OH)16CO3(OH)·4H2O (JCPDS 15-0087). The XRD data (Fig. S3b) exhibits a low angle broad

(2)

where I (A) represents the current, Vt (V) and Vo (V) are the highest and lowest potential, v (mV s−1) is the scan rate, and A (cm2) is the geometrical area of the electrode. The areal capacity (Cac, C cm−2), areal-specific capacitance (Ca, F cm−2) and mass-specific capacitance (Cmc, F g−1) of electrode from galvanostatic charge and discharge (GCD) curves were calculated through equation:

I ·Δt A

(3)

Ca =

I ·Δt A·ΔV

(4)

(7)

3.1. Structure and morphology characterizations

(1)

Cac =

E × 3600 Δt

(6)

3. Results and discussion

All electrochemical tests were conducted in alkaline electrolyte (6.0 M KOH) on a CHI 660E electrochemical working station. Platinum foil and standard calomel electrode (SCE) as the counter and reference electrodes, respectively. To prepare the working electrode, 80 wt% NALDH-OA, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder and N-methyl 2-pyrrolidinone (NMP) as solvent were mixed under ultrasonic for thirty minutes. Then, the obtained sticky material was smeared on a nickel foam (1 cm × 1 cm), the loading mass was 1.84, 1.68 and 1.76 mg cm−2 for NA-LDH-OA-1, NA-LDH-OA-2 and NA-LDH-OA-3, respectively, after drying in vacuum oven at 120 °C overnight. The specific capacitance (Cs, F cm−2) and specific capacity (C, C cm−2) computed from the CV curves was calculated through equation:

∫V

∫ I V (t )dt 3.6

where I is current density, V(t) is device voltage, dt is time differential and Δt is discharge time.

2.4. Electrochemical measurements

1 2Av (Vt − Vo)

(5)

where I (A) represents the discharge current, and Δt (s), m (g), ΔV (V) and A (cm2) are the discharge time, the mass of active materials, potential drop during discharge, and the surface area of the device, respectively.

2.3. Material characterizations

Cs =

I ·Δt m ·ΔV

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Fig. 2. (a) SEM image, (b) TEM image and (c) HR-TEM image of NA-LDH-OA-2 (inset: the corresponding SAED pattern); (d) XRD pattern of NA-LDH-OA-2; (e) HAADF-STEM image of the NA-LDH-OA-2 and the corresponding EDS mapping of Ni, Al and O elements.

Ni2+ in the various NA-LDH-OAs samples [29]. Fig. S5b exhibited the C1s XPS spectra, the three peaks at 284.80, 286.85 and 288.57 eV are corresponding to the components of the carbon-carbon (CeC), oxygencarbon (CeO) and carboxylate carbon (OeC]O) bonds, respectively. These bonds derived from oleate ligands of the surface in the nanosheets are displayed in various NA-LDH-OAs samples, which are also consistent with the XRD results as remarked above. Moreover, FT-IR spectra of various NA-LDH-OAs are shown in Fig. 3d. All of samples exhibit broad absorptions bands in the range of 3700–3100 cm−1. The prominent absorption band around 3455 cm−1 can be attributed to the interlayer water and stretching vibration of O–H groups of inorganic layers. In all cases, the typical adsorption bands of eCH3 asymmetric stretching was observed at 2956 cm−1 and two eCH2 stretching vibrations were observed at 2926 (asymmetric stretching vibration) and 2856 cm−1 (symmetric stretching vibration). The adsorption bands at 1557 cm−1 is assigned to the OeCeO stretching vibration. Furthermore, the intensities of the absorption bands at 1364 cm−1 confirms the existence of CO32− anions in the inter-layer region. Furthermore, the Raman spectrum of various NA-LDH-OAs samples are exhibited in Fig. S6. The absorption peak was observed at 560 cm−1 corresponding to v (Ni-OOH) and v(Ni-OH) in three samples [30,31]. The peak assigned to v(O-OH) in Ni(III)eOOH linkage [32,33] and Ni(III) hydroperoxide complexes stretches at 705 cm−1 [34]. The presence of relative

Bragg diffraction at 2θ = 2.0° for NA-LDH-OA-1, NA-LDH-OA-2 and NA-LDH-OA-3. This peak corresponds to a d value of 43.7 Å in a layered structure., which is bigger than twice the length of an oleate ligand (∼1.97 nm × 2). As shown in Fig. 2e, the HAADF-STEM image and the corresponding elemental mapping of NA-LDH-OA-2 show that the distribution of Ni, Al and O elements is uniform in the layered structure. The thickness of the NA-LDH-OA-2 sample was further investigated by atomic force microscopy (AFM). AFM inspection (Fig. 3a, b) revealed that the thickness of the as-synthesized NA-LDH-OA-2 sheets was determined to be ≈4.3 nm. The value is slightly consistent with the crystallographic thicknesses of 4.37 nm. As shown in Fig. S4, the distance is caused by the balance between the steric hindrance and the electrostatic attraction. And the steric hindrance is attribute to the oleate ligands on the basal planes of thin sheets, the electrostatic attraction is caused by a polar basal oxygen plane and a polar basal nickel and aluminum planes of adjacent nanosheets. The elemental compositions and valences of NA-LDH-OA-1, NA-LDH-OA-2 and NA-LDH-OA-3 were carried out by XPS technique. Fig. S5a display a full survey spectrum, which is composed of Ni 2p, Al 2p, O 1s and C 1s peaks. The Ni 2p spectrum (Fig. 3c) are associated with two spin-orbit splitting peaks, conforming to Ni 2p1/2 and Ni 2p3/2, and two shakeup satellites (indicated as “Sat.”). In addition, two main peaks around 872.96 and 853.25 eV with a spin-energy separation of 17.6 eV are characteristic of 908


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Fig. 3. (a) AFM image and (b) the corresponding height profiles of NA-LDH-OA-2; (c) Ni 2p binding energy and (d) FT-IR spectra of NA-LDH-OA-1, NA-LDH-OA-2, NA-LDH-OA-3.

process. More importantly, numerous mesopores can offer more electroactive sites for full redox reactions and abundant channels for ionic and electronic transport, thus leading to increase of electrochemical performance.

intensities of C]CeH out-of-plane and C]CeH in-plane stretch at 933 cm−1 and 1261 cm−1, respectively. The result can be attributed to the interaction of C]C with Ni-OOH in oleate ligands. Nitrogen adsorption/desorption isotherm profiles of various NALDH-OAs were performing to study the specific surface area and pore size distributions. The isotherms in Fig. S7 have been shifted for reasons of clarity. As shown in Fig. S7a, the isotherms of NA-LDH-OA-1, NALDH-OA-2 and NA-LDH-OA-3 exhibit Type-IV isotherms with a prominent hysteresis loop in the relative pressure range of 0.42–0.95, which manifest a typical mesopores structure. Additionally, the specfic values of the BET surface area and textural properties of three samples were summerized in Table S1. From Table S1, it could be found that the surface areas of NA-LDH-OA-1, NA-LDH-OA-2 and NA-LDH-OA-3 are 70.24, 45.69 and 86.37 m2 g−1, and their total volumes are 0.17, 0.40 and 0.37 cm3 g−1, respectively. Furthermore, as presented in Fig. S7a and Table S1, the isotherms of NA-LDH-OA-2, NA-LDH-OA-3 confirmed the existence of small amount of microporous under relative pressure stage (> 0.95). Hence, the total volumes of the obtained NA-LDH-OA was further analyzed by DFT method. As shown in Table S1, it can be found that the trend of result obtained by DFT method is consistent with that of BJH method (Table S1) and the total volumes of NA-LDHOA-1, NA-LDH-OA-2 and NA-LDH-OA-3 is 0.155, 0.170 and 0.168 cm3 g−1, respectively. In addition, the pore size distribution was calculated by the BJH method and presented in Fig. S7b. It can be seen that the presence of mesopores in three samples with average pore sizes centered at 14.75, 17.64 and 8.47 nm, respectively. From the above results, it can be clearly discerned that NA-LDH-OA-2 showed the smallest specific surface area, while its average pore size reached the maximum. Hence, NA-LDH-OA-2 with the largest pore volume is of importance to promote the diffusion of electrolyte, transfer of electron and buffer the volume expansion during the charging-discharging

3.2. Electrochemical performance of the samples To evaluate the electrochemical performance of NA-LDH-OA-1, NALDH-OA-2 and NA-LDH-OA-3 samples, CV, GCD and electrochemical impedance spectroscopy (EIS) were measured. The typical CV curves of the NA-LDH-OA-1, NA-LDH-OA-2, NA-LDH-OA-3 and nickel foam electrodes at 20 mV s−1 are shown in Fig. S8a. For nickel foam electrode, the CV area is negligible, indicating the capacitor contribution of the substrate can be overlooked. The CV curves of three samples owns similar shapes and display two pairs of redox peaks. And their shapes are distinct from electrical double layer capacitors (EDLCs) shapes [35]. The CV curves of obtained samples display a set of intense redox peaks, implying the typical battery-type electrochemical characteristics of a various of NA-LDH-OAs electrodes [36–38]. These results are primarily attributed to the rapid and reversible faradaic redox reactions related to M(OH)2/MOOH (M represents Ni ions). The appropriate areal capacity of NA-LDH-OA-1, NA-LDH-OA-2 and NA-LDH-OA-3 computed from the CV curves at various scan rates is displayed in Fig. S8b. NA-LDH-OA-2 exhibits a higher specific capacity (0.980 C cm−2) than NA-LDH-OA-1 (0.685 C cm−2) and NA-LDH-OA-3 (0.780 C cm−2). The CV curves of the NA-LDH-OA-2 electrode at various scan rates ranging from 10 to 100 mV s−1 are given in Fig. 4a. The CV curves of all samples have similar shapes, and the integral area increase with increasing of scan rates. Moreover, the oxidation peaks and reduction peaks shifted with the increase of the scan rates, which can be ascribed to the NA-LDH-OA2 electrode polarization and quasi-reversible feature. Moreover, the 909


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Fig. 4. (a) CV curves of NA-LDH-OA-2 at various scan rate from 10 to 100 mV s−1 and (b) specific capacity of NA-LDH-OA-2 from CV measurements of (a); (c) the galvanostatic charge-discharge curves of NA-LDH-OA-2 at various current densities and (d) specific capacity of NA-LDH-OA-2 from the discharging curves of (c).

stability of these samples in Fig. S11, which exhibit cycling stability of about 26.2, 22.2 and 25.2% loss after 5000 cycles. In addition, compared with a range of reported LDH-based electrode materials for SCs in three electrode system (present in Table S2), the as-obtained NA-LDHOA-2 exhibits superior electrochemical properties, benefiting from the large interlayer structure, are highly competitive in most of reported LDH-based materials. Electrochemical impedance spectroscopy (EIS) was adopted to investigate the NA-LDH-OA-1, NA-LDH-OA-2, and NA-LDH-OA-3 electrode kinetic properties. Fig. 5b shows the Nyquist plots, the intercept on real axis in a high frequency is the combined resistance (Rs), and the semicircle in the high frequency represents the charge-transfer resistance (Rct) [44]. The inset of Fig. 5b compares the Nyquist plots of three electrodes, the NA-LDH-OA-2 electrode display smaller semicircle diameter, demonstrating its lower Rs and Rct. The slope of the lines at low frequencies is related to the diffusive resistance of the OH− ion within the electrolyte in the electrode pores and proton diffusion in the electrode materials. For an in-depth understanding of the impedance spectra, the Nyquist plots were conducted on an equivalent circuit inset in Fig. 5b. Where, Rs is the electrolyte solution resistance and the contact resistance between the active material and substrate; Zw, Cdl, and Cps values are listed in Table S3. Among which, NA-LDH-OA-2 shows lower Rs (0.185) and Rct (0.502), revealing fast electrolyte ion diffusion and superior electrochemical properties. All results well prove that NA-LDH-OA-2 is much fitter for an SCs than NA-LDH-OA-1 and NALDH-OA-3. These impressive electrochemical properties of NA-LDH-OA-2 can be attributed to the reasonable construction of the electrode: (1) the ultrathin nanosheets structure can provide additional internal accessible active sites, shorten the diffusion distance and accelerate mass transfer kinetics. (2) A large interlayer spacing in the 2D structure act as “ion-buffering reservors”, which effectively regulates the volume change during the redox reaction, which contributes to good cycling stability. (3) The nanosheets with optimized pore size not only accelerate the ionic mobility of electrolyte but also retain more electrolyte in voids during the reversible charging-discharging process.

appropriate area-specific capacity from CV curves of NA-LDH-OA-2 are shown in Fig. 4b. It can be seen when the scan rate increases from 10 to 100 mV s−1, the specific capacity decreases from 1.075 to 0.515 C cm−2. This phenomenon indicates that the capacity is primarily attributed to the redox reaction and can be ascribed to the intrinsically electro-active sites, which cannot be oxidized or reduced absolutely at fast scan rate, causing inadequate utilization of electrode materials. Fig. S9 shows the relationship between the corresponding peak currents and the square root of the scan rate (mV s−1)1/2 of NA-LDH-OA-1, NA-LDHOA-2 and NA-LDH-OA-3. The result shows the good linear relationship, which highlights the diffusion behavior of various NA-LDH-OAs during the redox reaction process [39,40]. In other words, various NA-LDHOAs could be considered as the battery-type materials [41–43]. The GCD curves of the NA-LDH-OA-2 electrodes are displayed in Fig. 4c. Fig. S10a shows the GCD curves of the NA-LDH-OA-1, NA-LDHOA-2, and NA-LDH-OA-3 at 3.68, 3.36, and 3.52 mA cm−2, respectively. It can be clearly seen that the curves exhibit nonlinear shape, and voltage platform at 0.15 and 0.25 V, suggesting typical battery-type behavior, which are in agreement with the CV tests. Fig. S10b display the areal-specific capacity from the appropriate GCD curves. The obtained samples of NA-LDH-OA-1, NA-LDH-OA-2, and NA-LDH-OA-3 exhibit areal-specific capacity of 0.788, 0.987, and 0.795 C cm−2 at 3.68, 3.36, and 3.52 mA cm−2, respectively. In Fig. 4d, NA-LDH-OA-2 shows the areal-specific capacitys of 1.040, 0.987, 0.907, 0.865, 0.851 and 0.826 C cm−2 at current density ranging from 1.68, 3.36, 6.72, 10.08, 13.44, 16.80 mA cm−2, respectively. When the current density of NA-LDH-OA-2 electrode is increased from 1.68 to 16.80 mA cm−2, 79.42% (0.826 C cm−2/1.040 C cm−2) of the initial capacity retains. This indicates that NA-LDH-OA-2 electrode has high rate capability. Moreover, the cycling stability of the NA-LDH-OA-1, NA-LDH-OA-2, and NA-LDH-OA-3 electrode is examined by successive galvanostatic tests at current density of 7.36, 6.72, 7.04 mA cm−2 respectively for 2000 cycles. In Fig. 5a, the NA-LDH-OA-1, NA-LDH-OA-2, NA-LDH-OA3 can retain 85.53%, 88.25%, and 82.20% of initial capacity respectively, up to 2000 cycles, which demonstrates the excellent electrochemical durability for all samples. And we also checked the long cycle 910


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Fig. 5. (a) Cycling performance of the samples at 120, 160, 200 °C for 24 h, (b) Nyquist plots (the inset of the high-frequency region data).

Fig. 6. (a) Schematic illustration of the of the NA-LDH-OA-2//AC asymmetric supercapacitor, (b) CV curves of NA-LDH-OA-2 and AC electrode at the scan rate of 20 mV s−1 in 6 M KOH, (c) CV curves at the scan rate of 20 mV s−1 in different voltage range, (d) charge-discharge curves at current density of 6.62 mA cm−2 in different voltage range from 1.1 to 1.7 V, (e) CV curves at scan rates of 10–100 mV s−1, (f) charge–discharge curves at current densities of 2.21–44.10 mA cm−2, (g) the Ragone plots of the asymmetric supercapacitor, and (h) cyclic stability performance at a current density of 44.10 mA cm−2, (i) LED indicator lighted up by two ASCs.

between 0.0 V and 0.5 V. The AC electrode shows rectangular shapes which suggests an obvious double-layer capacitor behavior ranging from −1.0 to 0.0 V. Moreover, to obtain charge balance of positive and negative electrodes, the loading mass ratio between the NA-LDH-OA-2 and AC electrodes were estimated by the equation: m+/ m− = (C − × ΔV −)/(C + × ΔV +) . According to formulae (1), the optimum loading mass ratio between the positive and the negative electrode is 1.41:3.0. Fig. 6c illustrate the CV curves of the NA-LDH-OA-2// AC under a scan rate of 20 mV s−1 at different voltage windows, which indicates that the ideal operation voltage of the ASC device can afford

3.3. Electrochemical characterization of NA-LDH-OA-2//AC ASC cell To investigate the practical application of NA-LDH-OA-2 ultrathin nanosheets, the positive electrode of NA-LDH-OA-2 and the negative electrode of AC were assembled into an asymmetric supercapacitor (ASC) in Fig. 6a. The CV, GCD profiles and the corresponding specific capacitance for AC in a three-electrode system in 6.0 M KOH electrolyte are displayed in Fig. S12. Fig. 6b exhibits the CV curves of NA-LDH-OA2 electrode and AC electrode acquired at scan rates of 20 mV s−1. The NA-LDH-OA-2 electrode indicates that the battery-type behavior is 911


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up to 1.7 V. The GCD curves under a current density of 5.16 mA cm−2 at various voltage ranges were tested in Fig. 6d. The result shows that the voltage window of the NA-LDH-OA-2//AC ASC device can also be enhanced to 1.7 V. The shape of CV curves of the NA-LDH-OA-2//AC in Fig. 6e exhibit a couple of redox peaks, which indicates phenomenal battery-type behavior at 0–1.6 V. Moreover, the shape of CV curves with a slight change at scan rate of 100 mV s−1, suggesting good rate profile and good battery characteristic of NA-LDH-OA-2//AC. The typical GCD plots at current densities from 2.21 to 44.10 mA cm−2 in Fig. 6f demonstrate that the NA-LDH-OA-2//AC ASC device have a commendable nonlinear relationship in potential-time curves, indicating the as-fabricated NA-LDH-OA-2//AC ASC show a tremendous battery behavior. Fig. 6g manifest a Ragone plots, which expounds the relationship between the energy and power density of ASC device. It can be seen that the energy density of the NA-LDH-OA-2//AC ASC device decreased from 42.95 to 17.85 Wh kg−1 when the power density increased from 0.47 to 9.4 kW kg−1. The energy density and power density of the NA-LDH-OA-2//AC ASC device are much better than some reported values in the literature, such as Ni-Al LDH-NF//GNS-NF (30.2 Wh kg−1 at 800 W kg−1) [45], Ni0.34Co0.66(OH)2//AC (20.6 Wh kg−1 at 3930 W kg−1) [46], Ni-Co oxide//AC (10.2 Wh kg−1 at 470 W kg−1) [47], Ni-Co sulfide//AC (25 Wh kg−1 at 447 W kg−1) [48], and NiCo2S4//AC (22.8 Wh kg−1 at 160 W kg−1) [49]. Moreover, the cycling performance of NA-LDH-OA-2//AC ASC device was evaluated at current density of 44.10 mA cm−2 for 5000 cycles shown in Fig. 6h. It is worth to be noted that the NA-LDH-OA-2//AC ASC exhibits a superior cyclic stability of 5.5% loss even after 5000 cycles. Fig. 6i exhibts that two tandem NA-LDH-OA-2//AC ASC devices could easily lighted up one white round light-emitting diode (LED). The good physical and chemical results demonstrate that the sodium oleate regulated Ni-Al-LDH ultrathin sheets hold great promise in high-performance ASC devices.

Energy Environ. Sci. 7 (2014) 14–18. [3] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, P.L. Taberna, S.H. Tolbert, H.D. Abruna, P. Simon, B. Dunn, High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance, Nat. Mater. 12 (2013) 518–522. [4] Y. Zhao, X. He, R. Chen, Q. Liu, J. Liu, D. Song, H. Zhang, H. Dong, R. Li, M. Zhang, J. Wang, Hierarchical NiCo2S4@ CoMoO4 core-shell heterostructures nanowire arrays as advanced electrodes for flexible all-solid-state asymmetric supercapacitors, Appl. Surf. Sci. 437 (2018) 73–82. [5] Y. Zhao, X. He, R. Chen, Q. Liu, J. Liu, J. Yu, J. Li, H. Zhang, H. Dong, M. Zhang, J. Wang, A flexible all-solid-state asymmetric supercapacitors based on hierarchical carbon cloth@ CoMoO4@ NiCo layered double hydroxide core-shell heterostructures, Chem. Eng. J. 352 (2018) 29–38. [6] G. Wang, F. Yue, L. Zhang, B. Yan, J. Luo, X. Su, Oxygen vacancy-rich anatase TiO2 hollow spheres via liquid nitrogen quenching process for enhanced photocatalytic hydrogen evolution, ChemCatChem 11 (2019) 1057–1063. [7] G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z. Bao, Solution-processed graphene/MnO2 nanostructured textiles for high-performance electrochemical capacitors, Nano Lett. 11 (2011) 2905–2911. [8] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Asymmetric supercapacitors based on graphene/MnO2 and activated carbon nanofiber electrodes with high power and energy density, Adv. Funct. Mater. 21 (2011) 2366–2375. [9] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater. 22 (2010) E28–62. [10] X. He, Q. Liu, J. Liu, R. Li, H. Zhang, R. Chen, J. Wang, High-performance all-solidstate asymmetrical supercapacitors based on petal-like NiCo2S4/Polyaniline nanosheets, Chem. Eng. J. 325 (2017) 134–143. [11] X. He, R. Li, J. Liu, R. Chen, D. Song, J. Wang, Hierarchical FeCo2O4@NiCo layered double hydroxide core/shell nanowires for high performance flexible all-solid-state asymmetric supercapacitors, Chem. Eng. J. 334 (2018) 1573–1583. [12] X. Bai, J. Liu, H. Zhang, Z. Li, X. Jing, P. Liu, J. Wang, R. Li, Hierarchical Co3O4@ Ni (OH)2 core-shell nanosheet arrays for isolated all-solid state supercapacitor electrodes with superior electrochemical performance, Chem. Eng. J. 315 (2017) 35–45. [13] H. Zhang, H. Chen, S. Azat, Z.A. Mansurov, X. Liu, J. Wang, X. Su, R. Wu, Super adsorption capability of rhombic dodecahedral Ca-Al layered double oxides for Congo red removal, J. Alloys. Compd. 768 (2018) 572–581. [14] S. He, Z. An, M. Wei, D.G. Evans, X. Duan, Layered double hydroxide-based catalysts: nanostructure design and catalytic performance, Chem. Commun. 49 (2013) 5912–5920. [15] K. Ladewig, Z.P. Xu, G.Q. Lu, Layered double hydroxide nanoparticles in gene and drug delivery, Expert Opin. Drug Del. 6 (2009) 907–922. [16] C.G. Silva, Y. Bouizi, V. Fornés, H. García, Layered double hydroxides as highly efficient photocatalysts for visible light oxygen generation from water, J. Am. Chem. Soc. 131 (2009) 13833–13839. [17] W. Yang, Z. Gao, J. Wang, J. Ma, M. Zhang, L. Liu, Solvothermal one-step synthesis of Ni-Al layered double hydroxide/carbon nanotube/reduced graphene oxide sheet ternary nanocomposite with ultrahigh capacitance for supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 5443–5454. [18] P. Vialat, C. Mousty, C. Taviot-Gueho, G. Renaudin, H. Martinez, J.C. Dupin, E. Elkaim, F. Leroux, High-performing monometallic cobalt layered double hydroxide supercapacitor with defined local structure, Adv. Funct. Mater. 24 (2014) 4831–4842. [19] M. Shao, R. Zhang, Z. Li, M. Wei, D.G. Evans, X. Duan, Layered double hydroxides toward electrochemical energy storage and conversion: design, synthesis and applications, Chem. Commun. 51 (2015) 15880–15893. [20] W. Wang, N. Zhang, Z. Shi, Z. Ye, Q. Gao, M. Zhi, Z. Hong, Preparation of Ni-Al layered double hydroxide hollow microspheres for supercapacitor electrode, Chem. Eng. J. 338 (2018) 55–61. [21] Y. Xiao, D. Su, X. Wang, S. Wu, L. Zhou, Z. Sun, Z. Wang, S. Fang, F. Li, Ultrahigh energy density and stable supercapacitor with 2D NiCoAl Layered double hydroxide, Electrochim. Acta 253 (2017) 324–332. [22] B. Han, G. Cheng, E. Zhang, L. Zhang, X. Wang, Three dimensional hierarchically porous ZIF-8 derived carbon/LDH core-shell composite for high performance supercapacitors, Electrochim. Acta 263 (2018) 391–399. [23] H. Zhong, X. Cheng, H. Xu, L. Lin, D. Li, P. Tang, N.A. Vante, Y. Feng, Carbon fiber paper supported interlayer space enlarged Ni2Fe-LDHs improved OER electrocatalytic activity, Electrochim. Acta 258 (2017) 554–560. [24] Y. Lin, X. Xie, X. Wang, B. Zhang, C. Li, H. Wang, L. Wang, Understanding the enhancement of electrochemical properties of NiCo layered double hydroxides via functional pillared effect: an insight into dual charge storage mechanisms, Electrochim. Acta 246 (2017) 406–414. [25] Y. Li, C. Yang, J. Ge, C. Sun, J. Wang, X. Su, A general microwave-assisted twophase strategy for nanocrystals synthesis, J. Colloid Interf. Sci. 407 (2013) 296–301. [26] X. Liu, C. Niu, X. Zhen, J. Wang, X. Su, Novel approach for synthesis of boehmite nanostructures and their conversion to aluminum oxide nanostructures for remove Congo red, J. Colloid Interf. Sci. 452 (2015) 116–125. [27] J. Lin, X. Zheng, Y. Wang, H. Liang, H. Jia, S. Chen, J. Qi, J. Cao, W. Fei, J. Feng, Rational construction of core–shell Ni3S2@Ni(OH)2 nanostructures as battery-like electrodes for Supercapacitors, Inorg. Chem. Front. 5 (2018) 1985–1991. [28] J. Memon, J. Sun, D. Meng, W. Ouyang, M.A. Memon, Y. Huang, S. Yan, J. Geng, Synthesis of graphene/Ni–Al layered double hydroxide nanowires and their application as an electrode material for supercapacitors, J. Mater. Chem. A 2 (2014) 5060–5067. [29] J.W. Lee, T. Ahn, D. Soundararajan, J.M. Ko, J.D. Kim, Non-aqueous approach to the preparation of reduced graphene oxide/alpha-Ni(OH)2 hybrid composites and their high capacitance behavior, Chem. Commun. 47 (2011) 6305–6307.

4. Conclusions In summary, we reported an innovative Sodium oleate modified NiAl-LDH derived from two phase solvothermal method. It is noteworthy that this NA-LDH-OAs electrodes with short electronic and ionic diffusion distance, expanded interlayer space and 2D frame structure can provide an effective pathway of ion and electron transport, thus enhancing electrochemical activity. Therefore, the NA-LDH-OA-2 achieves high areal-specific capacity of 1.040 C cm−2 at 1.68 mA cm−2, excellent rate capability (maintain 79.42% at a current density of 16.80 mA cm−2) and good cycling stability (88.25% of capacity retention after 2000 cycles at 6.72 mA cm−2). Furthermore, the assembled NA-LDH-OA-2//AC asymmetric supercapacitor delivered a high energy density of 40.26 Wh kg−1 at 943 W kg−1. These good results make the NA-LDH-OA-2 underlying in high-performance energy-storage devices. Moreover, this finding will pave the way to synthesize a series of LDHbased materials for high performance energy storage. Acknowledgement The authors gratefully acknowledge the financial support by National Natural Science Foundation of China (No. 51561030). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.03.041. References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19–29. [2] J.B. Goodenough, Electrochemical energy storage in a sustainable modern society,

912


H. Zhang, et al.

[40] J. Li, T. Pu, B. Huang, X.H. Hou, C.L. Zhao, L. Xie, L.Y. Chen, Scalable synthesis of two-dimensional porous sheets of Ni-glycine coordination complexes: a novel highperformance energy storage material, J. Colloid Interf. Sci. 531 (2018) 360–368. [41] J. Lin, H. Wang, Y. Yan, X. Zheng, H. Jia, J. Qi, J. Cao, J. Tu, W. Fei, J. Feng, Corebranched CoSe2/Ni0.85Se nanotube arrays on Ni foam with remarkable electrochemical performance for hybrid supercapacitors, J. Mater. Chem. A 6 (2018) 19151–19158. [42] J. Lin, Z. Zhong, H. Wang, X. Zheng, Y. Wang, J. Qi, J. Cao, W. Fei, Y. Huang, J. Feng, Rational constructing free-standing Se doped nickel-cobalt sulfides nanotubes as battery-type electrode for high-performance supercapattery, J. Power Sources 407 (2018) 6–13. [43] S. Liu, Y. Yin, K.N. Hui, S.C. Lee, S.C. Jun, High-performance flexible quasi-solidstate supercapacitors realized by molybdenum dioxide@nitrogen-doped carbon and copper cobalt sulfde tubular nanostructures, Adv. Sci. 5 (2018) 1800733. [44] J. Zhu, Y. Jiang, Z. Lu, C. Zhao, L. Xie, L. Chen, J. Duan, Single-crystal Cr2O3 nanoplates with differing crystalinities, derived from trinuclear complexes and embedded in a carbon matrix, as an electrode material for supercapacitors, J. Colloid Interf. Sci. 498 (2017) 351–363. [45] L. Zhang, K.N. Hui, K. San Hui, H. Lee, High-performance hybrid supercapacitor with 3D hierarchical porous flower-like layered double hydroxide grown on nickel foam as binder-free electrode, J. Power Sources 318 (2016) 76–85. [46] Y. Tang, Y. Liu, S. Yu, W. Guo, S. Mu, H. Wang, Y. Zhao, L. Hou, Y. Fan, F. Gao, Template-free hydrothermal synthesis of nickel cobalt hydroxide nanoflowers with high performance for asymmetric supercapacitor, Electrochim. Acta 161 (2015) 279–289. [47] C. Tang, Z. Tang, H. Gong, Hierarchically porous Ni-Co oxide for high reversibility asymmetric full-cell supercapacitors, J. Electrochem. Soc. 159 (2012) A651–A656. [48] Y.H. Li, L.J. Cao, L. Qiao, M. Zhou, Y. Yang, P. Xiao, Y.H. Zhang, Ni–Co sulfide nanowires on nickel foam with ultrahigh capacitance for asymmetric supercapacitors, J. Mater. Chem. A 2 (2014) 6540–6548. [49] W. Kong, C. Lu, W. Zhang, J. Pu, Z. Wang, Homogeneous core–shell NiCo2S4 nanostructures supported on nickel foam for supercapacitors, J. Mater. Chem. A 3 (2015) 12452–12460.

[30] H.L. Jing Yang, Wayde N. Martens, Ray L. Frost, Synthesis and characterization of cobalt hydroxide, cobalt oxyhydroxide, and cobalt oxide nanodiscs, J. Phys. Chem. C 114 (2010) 923–937. [31] H.B. Li, M.H. Yu, X.H. Lu, P. Liu, Y. Liang, J. Xiao, Y.X. Tong, G.W. Yang, Amorphous cobalt hydroxide with superior pseudocapacitive performance, ACS Appl. Mater. Interfaces 6 (2014) 745–749. [32] J.R.K. Cynthia Rajani, David H. Petering, Resonance raman studies of HOO-Co(III) bleomycin and Co(III) bleomycin: identification of two important vibrational modes, ν(Co-OOH) and ν(O-OH), J. Am. Chem. Soc. 126 (2004) 3829–3836. [33] B.W.K. Nakamoto, Chichester, Brisbane, Toronto, Singapore 1986. 484 Seiten, Berichte der Bunsengesellschaft für physikalische Chemie, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4. Auflage, John Wiley & Sons, New York, 1988, p. 561-561. [34] C.C. Wang, H.C. Chang, Y.C. Lai, H. Fang, C.C. Li, H.K. Hsu, Z.Y. Li, T.S. Lin, T.S. Kuo, F. Neese, S. Ye, Y.W. Chiang, M.L. Tsai, W.F. Liaw, W.Z. Lee, A structurally characterized nonheme cobalt-hydroperoxo complex derived from its superoxo intermediate via hydrogen atom abstraction, J. Am. Chem. Soc. 138 (2016) 14186–14189. [35] Y. Liu, G. Jiang, S. Sun, B. Xu, J. Zhou, Y. Zhang, J. Yao, Growth of NiCo2S4 nanotubes on carbon nanofibers for high performance flexible supercapacitors, J. Electroanal. Chem. 804 (2017) 212–219. [36] S. Liu, D. Ni, H. Li, K.N. Hui, C.Y. Ouyang, S.C. Jun, Effect of cation substitution on the pseudocapacitive performance of spinel cobaltite MCo2O4 (M =Mn, Ni, Cu, and Co), J. Mater. Chem. A 6 (2018) 10674–10685. [37] J. Lin, Y. Yan, X. Zheng, Z. Zhong, Y. Wang, J. Qi, J. Cao, W. Fei, Y. Huang, J. Feng, Designing and constructing core-shell NiCo2S4@Ni3S2 on Ni foam by facile one-step strategy as advanced battery-type electrodes for supercapattery, J. Colloid Interf. Sci. 536 (2019) 456–462. [38] J. Lin, H. Jia, H. Liang, S. Chen, Y. Cai, J. Qi, C. Qu, J. Cao, W. Fei, J. Feng, In situ synthesis of vertical standing nanosized NiO encapsulated in graphene as electrodes for high performance supercapacitors, Adv. Sci. 5 (2018) 1700687. [39] S. Sun, G. Jiang, Y. Liu, Y. Zhang, J. Zhou, B. Xu, Growth of MnO2 nanoparticles on hybrid carbon nanofibers for flexible symmetrical supercapacitors, Mater. Lett. 197 (2017) 35–37.

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