- Email: [email protected]
The growth of nickel-manganese and cobalt-manganese layered double hydroxides on reduced graphene oxide for supercapacitor
Electrochimica Acta 206 (2016) 108–115
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The growth of nickel-manganese and cobalt-manganese layered double hydroxides on reduced graphene oxide for supercapacitor M. Lia , J.P. Chenga,* , J. Wangb , F. Liua , X.B. Zhanga a State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Zhejiang University, Hangzhou 310027, China b School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China
A R T I C L E I N F O
Article history: Received 1 March 2016 Received in revised form 15 April 2016 Accepted 18 April 2016 Available online 26 April 2016 Keywords: Layered double hydroxide Reduced graphene oxide Electrochemical performance Supercapacitors
A B S T R A C T
Pure Ni-Mn layered double hydroxide (LDH), Co-Mn LDH with a flower-like morphology and sandwichlike Ni-Mn LDH/reduced graphene oxide (rGO), Co-Mn LDH/rGO hybrids are fabricated via a simple coprecipitation method. In the hybrids, Ni-Mn and Co-Mn hydroxide nanoflakes are tightly anchored on the both surfaces of rGO, leading to the composites with high specific surface areas. Electrochemical measurements prove that rGO can improve the capacitance and cyclic stability of the hybrid materials and that Ni-Mn LDH delivers a much higher specific capacitance but a worse cycling performance than Co-Mn LDH. A high specific capacitance of 1635 F g1 at 1 A g1 and a high rate retention of 71% at 10 A g1 are achieved for Ni-Mn LDH/rGO. A hybrid capacitor with Ni-Mn LDH/rGO as positive electrode and activated carbon as negative electrode is assembled. It possesses a specific capacitance of 84.26 F g1 at 1 A g1 and an energy density of 33.8 Wh kg1 within a potential window of 1.7 V. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, tremendous efforts have been devoted to exploring renewable and high-performance electrode materials for electrochemical capacitors (ECs)/supercapacitors. Layered double hydroxides (LDHs) are unusual layered materials consisting of positively-charged hydrotalcite-like layers, weakly bound, charge compensating anions and water molecules in the interlayer region. Owing to the layered structure, they usually possess high specific surface area, which can facilitate fast ion transfer. This offers them great superiority in electrochemical fields [1–6]. The LDHs containing Ni, Co, Mn, etc, have proved themselves to be one of the most promising electrode materials for ECs due to their high specific capacitance, long life span, low cost and easy synthesis. Many investigations have been performed to synthesize Ni-Mn LDH and Co-Mn LDH and measure their electrochemical performances. Various synthesis methods have been thus developed. For example, Anandan’s group reported the sonochemical synthesis of Ni-Mn LDH [7]. Sim et al. used a reverse micelle method to synthesize colloidal Ni-Mn LDH [8]. Hierarchical Ni-Mn LDH nanosheets could be grown on Ni foam by a one-step method at
* Corresponding author. Tel.: +86 571 87951411; fax: +86 571 87951411. E-mail address: [email protected] (J.P. Cheng). http://dx.doi.org/10.1016/j.electacta.2016.04.084 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
80 C, as reported by Guo and co-workers [9]. Electrodeposition method was applied by Jagadale to fabricate Co-Mn LDH [10]. Some above methods are complex, unsuitable for large scale fabrication or resulting in the impurity. Thus, a facile time-/energy-saving method to synthesize pure Ni-Mn LDH and Co-Mn LDH is urgently needed. We know that co-precipitation, hydrothermal method, anion exchange and calcination-rehydration are common methods to synthesis LDHs. Among them, co-precipitation is much more energy-saving, environmentally friendly, and easily carried out. It is well-known that the electrochemical performance of LDHs can be further improved if they are combined with carbon materials to form a composite [11–14]. Carbon materials are usually used as electrical double-layer capacitive electrode materials or as additives in transition metal oxides/hydroxides. As additives, carbon materials can provide large surface areas, prevent oxides/hydroxides from agglomeration, facilitate electrical conductivity, and enhance mechanical stability. Zhao and Wang deposited Co-Mn LDH onto carbon fibers and carbon cloth, respectively [15,16]. The results revealed that the hierarchical configuration of LDHs/carbon could improve the exposure of active sites and enable a fast charge transfer to the electrode/electrolyte interface. Zhao also reported a hierarchical structure composed of Ni-Mn LDH microcrystals grafted on carbon nanotubes (CNTs) backbone with tunable Ni/Mn ratios by an in situ growth route [15].
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
The electrochemical investigation showed that Ni3Mn1 LDH/CNTs electrode was rather active with the maximum specific capacitance of 2960 F g1 [5]. Our group has also fabricated a hierarchical NiCoMn LDH/rGO composite, which could deliver a high capacitance of 912 F g1, much higher than that of pure NiCoMn LDH (772 F g1)[17]. Some other Ni-Mn LDH/carbon composites, such as, ternary NiCo2S4 nanotube/Ni-Mn LDH/3D graphene sponge [18], Ni-Mn LDH/graphene superlattice [19], and sulfidation Ni-Mn LDH/graphene oxide [20] have been recently reported. All these composites exhibited outstanding capacitance and rate performance, indicating their promising potential as electrode materials of ECs. Herein, we attempt to synthesize pure Ni-Mn LDH, Co-Mn LDH, and their composites with reduced graphene oxide by simple coprecipitation method. In the composites, LDH nanosheets are anchored onto the surfaces of reduced graphene oxide (rGO) to form a hierarchical architecture. Electrochemical tests are carried out to reveal their capacitive performances as electrode materials for ECs. It is found that Ni-Mn LDH/rGO composite can deliver a high capacitance (1635 F g1), a fair rate capability and a long life span. In addition, an asymmetric capacitor is designed with Ni-Mn LDH/rGO and activated carbon (AC) as positive and negative materials, respectively. 2. Experimental
109
composite was evaluated to be 4.6%, determined by weighing the final product. Pure Ni-Mn LDH could be also prepared by the same process but without using any GO. Similarly, pure Co-Mn LDH and Co-Mn LDH/rGO composite could be also fabricated by the same procedure using 4 mmol Co (NO3)2. 2.2. Materials characterization The structure and morphology of the products were characterized by an X-ray diffractometer (XRD, using Cu-Ka radiation at 40 kV, 40 mA, 3 min1, Shimadzu, LabX XRD-6000) with a secondary graphite monochromator, scanning electron microscope (SEM, Hitachi S-4800), and transmission electron microscope (TEM, Philip-CM200). Fourier transformed infrared spectroscopy (FT-IR, Bruker spectrometer, Tensor 27) was obtained by dispersing the sample in KBr and pressing it into transparent pellets. The specific surface area and pore distribution of the materials were tested by an ASAP 2020 analyzer (Micromeritics Instrument Corporation) by Brunauer-Emmett-Teller (BET) method at 77 K in N2 with a pre-treatment at 180 C for over 10 h degassing. X-ray photoelectron spectroscopy (XPS, Thermo, Escalab 250Xi) was measured and the XPS spectra were calibrated by the C 1s line at 284.8 eV. Thermogravimetric (TG) analysis was carried out on an SDT Q600 instrument from 25 to 800 C with the heating rate of 10 C min1 in air.
2.1. Preparation of materials 2.3. Preparation of electrodes and electrochemical characterization Natural flake graphite (500–600 mm, Qingdao Xinghe Graphite Co., Ltd) was used to prepare graphene oxide (GO). By a modified Hummers method, as reported elsewhere [21], oxidation, exfoliation and washing process were carried out on the flake graphite. After centrifugation and standardization, 6 g L1 GO suspension was obtained. Then, a specific volume of above GO suspension containing 15 mg GO was dispersed into 250 mL deionized water to form a transparent suspension, followed by the addition of 4 mmol Ni(NO3)2, 2 mmol Mn(NO3)2 and 18 mmol NH4F. Under strong stirring, 18 mL ammonia (1 mol L1) was added dropwise into the above suspension at room temperature in 2 h for the fabrication of LDHs. After being washed and dried at 80 C, Ni-Mn LDH/rGO was thus obtained. The mass content of rGO in Ni-Mn LDH/rGO
For a three-electrode configuration, the electrode containing as-prepared materials, Ni plate and an Ag/AgCl electrode were used as work electrode, counter electrode and reference electrode, respectively. The work electrode was prepared by mixing the electrode material, acetylene black, polyvinylidene fluoride in a mass ratio of 80:10:10 in N-methyl-2-pyrrolidone, which was painted onto a Ni foam substrate (1.5 cm 1.5 cm) and completely dried at 80 C in vacuum. The active material loaded on each electrode was about 4 mg. 2 M KOH solution was acted as electrolyte. A salt-bridge was used to connect Ag/AgCl electrode and the electrolyte. Cyclic voltammograms (CV), galvanostatic charge-discharge curves and electrochemical impedance
Fig. 1. (a, b) SEM image of pure Ni-Mn LDH, (c, d) SEM image of Ni-Mn LDH/rGO, (e) SEM image of pure Co-Mn LDH, and (f) SEM image of Co-Mn LDH/rGO.
110
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
spectroscopy (EIS) were measured on a CHI660D electrochemical workstation. The cycle life test was conducted on a LAND CT2001A system by galvanostatic charge-discharge techniques. The electrochemical measurement for the hybrid capacitor was conducted by a two-electrode configuration in 2 M KOH solution. 3. Results and discussion In the composites, GO was acted as the backbone for the in situ growth of Ni-Mn LDH and Co-Mn LDH crystals. The oxygencontaining groups on GO sheets could act as nucleation sites for the LDH crystals. Ni2+ or Co2+ and Mn2+ in the solution could form coordination complexes with carboxyl and hydroxyl groups on the GO nanosheets through electrostatic attraction. At the same time, GO could be reduced to rGO by Mn2+. When the precipitating agent (ammonia) is added to the solution, LDH crystals are formed first on the coordination sites. The formed positively charged lattices of LDH crystals attach to the negatively charged carboxylic groups on the surfaces of rGO [22]. As the LDH crystals grow to nanosheets, the rGO surface is completely covered with vertical LDH, greatly improved the utility of the two components. Thus, 3D sandwichlike Ni-Mn LDH/rGO material with a high specific surface area is obtained. The morphology and structure described above can be directly proved by the SEM images of Ni-Mn LDH/rGO hybrid. As shown in
Fig. 2. TEM images of (a) Ni-Mn LDH and (b) Ni-Mn LDH/rGO.
Fig. 1a and 1b, pure Ni-Mn LDH represents a flower-like morphology. LDH nanosheets are laminated and slit-shaped assembled. Fig. 1c and 1d show the sandwich-like Ni-Mn LDH/ rGO. The surfaces of rGO are fully covered with LDH nanosheets. Besides, the agglomeration of LDH sheets is effectively prevented by the heterogeneous structure. Both Co-Mn LDH and Co-Mn LDH/ rGO present the similar morphology and architecture as Ni-Mn LDH and Ni-Mn LDH/rGO hybrid, as shown in Fig. 1e and 1f. The TEM images of Ni-Mn LDH and Ni-Mn LDH/rGO are presented in Fig. 2. As we can see, flower-like (Fig. 2a) and sandwich-like (Fig. 2b) structures are presented again. The selected-area electron diffraction (SAED) pattern of Ni-Mn LDH reveals hexagonally arranged spots, confirming the single crystallinity of Ni-Mn LDH nanosheet and its good crystallinity. The SAED pattern of the hybrid shows clear diffraction rings, caused by the growth of LDH on the surfaces of rGO. The XRD patterns of GO, pure LDHs, and their composites are all shown in Fig. 3a. Except a hexagonal phase with a hydrotalcite-like structure, no other crystalline phase is detected in pure Ni-Mn LDH or Co-Mn LDH, which indicates a high purity of the final products. The interlayer spacing of plane (003) (d003) is 0.772 and 0.747 nm for Ni-Mn LDH and Co-Mn LDH, respectively. The strong diffraction peak at 9.3 is ascribed to the (001) plane of GO for the GO pattern. The absence of the diffraction peak at around 26 indicates the
Fig. 3. (a) XRD patterns and (b) FTIR spectra of GO, Ni-Mn LDH, Co-Mn LDH, Ni-Mn LDH/rGO, and Co-Mn LDH/rGO.
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
complete oxidation of graphite. However, the absence of the diffraction peaks of rGO in the hybrids reveals that the restacking of rGO nanosheets is effectively prevented by the complete exfoliation of graphite in the hybrids. This further indicates that thin LDH nanoflakes are located on the surfaces of rGO nanosheets [23]. Compared with pure LDHs, the diffraction peaks of plane (003) of the hybrids shift towards the low angle region. To be precise, the diffraction peaks of plane (003) for pure Ni-Mn LDH and Co-Mn LDH are at 11.45 and 11.84 , while the corresponding hybrids are at 11.04 and 11.18 , respectively. The peak shift of LDHs leads to a large interlayer spacing of 0.801 and 0.791 nm for Ni-Mn LDH and Co-Mn LDH in the hybrids. This can be ascribed to the weakened electrostatic interactions between the hydrotalcite-like sheets and the species in the interlayer space, which probably involves the driving force of LDHs grown on the rGO surfaces [24,25]. The larger interlayer spacing of the hybrids may also lead to a better electrochemical performance than the pure counterparts. The FT-IR spectra of GO, LDHs, and LDH/rGO hybrids are shown in Fig. 3b. The bands at 3440 and 1630 cm1 are assigned to the O-H stretching vibrations of water molecules and the hydrogen bonding among the hydroxyl groups in all LDH samples [26]. Weak absorption bands around 1729 and 1200 cm1 in the GO spectrum are corresponding to the vibration of C¼O and C-O functional groups. These oxygen-containing functional groups on GO were nucleation sites for LDH crystals to grow. The bands at 1384 cm1 in pure LDHs and the hybrids are attributed to the vibration of CO32 anion in the interlayer space [27]. These bands below 800 cm1 come from the stretching and bending vibrations of metal-oxygen (M-O) in the hydrotalcite-like lattice [24,28]. The only difference between LDHs and corresponding LDH/rGO hybrids is the band at 1108 cm1 coming from rGO. TG measurement was carried out on Ni-Mn LDH and the corresponding hybrid. We can see that their weight loss can be divided into two stages in Fig. S1. The first stage below 120 C is due to the removal of adsorbed water and interlayer water molecules.
111
The second one is the dehydroxylation, the removal of interlayer anions (CO32) and the decomposition of rGO (only for the hybrid), leading to the formation of Ni-Mn oxides [27]. The weight loss of pure LDH and the hybrid is about 40.4% and 53.8%, respectively, at this stage. Thus the final Ni-Mn oxides occupy about 59.6% and 46.2% for pure LDH and the hybrid, respectively. Because of the difference of adsorbed water and interlayer water in the two samples, the weight loss of stage one in the hybrid is about 8.9% more than that of pure LDH. So the weight percentage of rGO in NiMn LDH/rGO can be thus calculated to be 4.5%, well consistent with the result measured by the weights of initial GO and the final composite product. XPS measurements were further carried out on the Ni-Mn LDH/rGO hybrid and GO. The corresponding results are presented in Fig. 4. The survey spectrum indicates the presence of Ni, Mn, C and O, in Fig. 4a. The measured molar ratio of Ni to Mn is 1.41. The Ni 2p spectrum is well fitted with one spin-orbit doublet and two shakeup satellites. The peaks at the binding energy of 855.5 and 873.1 eV, with a spin-energy separation of 17.6 eV, are assigned to Ni2+ state in Ni(OH)2 in Fig. 4b [11,29–32]. The peaks of Mn 2p3/ 2 and Mn 2p1/2 located at 641.7 and 653.2 eV in Fig. 4c suggest the presence of Mn3+ in the sample [5,15]. The O 1s spectrum in Fig. 4d decomposes into three constituents, M-O-M and M-O-H in the M(OH)6 octahedron at 528.8 and 530.7 eV, respectively, and H-O-H at 532.7 eV [22,33]. The C 1s spectrum in Fig. 4e originates from both rGO and CO32 anions in the LDH interlayer space. The peak at 284.8 eV represents non-oxygenated carbon (C¼C) from rGO nanosheets. The carbonyl carbon at 286.2 eV and the carboxylate carbon (O C¼O) at 288.7 eV may come from CO32 or rGO [34,35]. For comparison, the C 1s spectrum of asprepared GO is exhibited in Fig. 4f. Except non-oxygenated carbon (C¼C), three oxidation states can be clearly observed, the carbon in C-O bond at 285.2 eV, the carbonyl carbon at 286.9 eV and the carboxylate carbon (OC¼O) at 288.6 eV [34,36]. By comparison with the XPS spectrum of GO, we can find that the rGO in the hybrid is well reduced by Mn2+ during the synthesis process.
Fig. 4. (a) XPS wide spectrum (b) Ni 2p, (c) Mn 2p, (d) O 1s, (e) C 1s of Ni-Mn LDH/rGO composite, and (f) C 1s spectra of GO.
112
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
The N2 adsorption/desorption measurements were carried out to measure the specific surface area and pore volume of Ni-Mn LDH/rGO composite and pure Ni-Mn LDH. As shown in Fig. 5a, the N2 adsorption/desorption curves of both samples represent typical IV isotherms with distinct hysteresis loops indicating a mesoporous structure. They also exhibit an H3-type hysteresis loop due to the existing mesopores of absorbents having slit-shaped pores [37]. Mesopores and macropores with a relatively wide distribution within the range from 20 to 100 nm can be observed from the pore size distribution plots in Fig. 5b. Calculated by BET method, pure Ni-Mn LDH has a specific surface area of 45.5 m2 g1, while NiMn LDH/rGO exhibits a much higher specific surface area of 91.9 m2 g1, about 2 times of pure LDHs. The corresponding pore volume is 0.25 and 0.42 m3 g1 for pure Ni-Mn LDH and the hybrid, respectively. It is reasonable to believe that the introduction of rGO in the composite depresses the aggregation of LDH platelets, thus leading to a high surface area [22], which is beneficial to the kinetics of electrochemical reaction. Electrochemical measurements in a three-electrode configuration were carried out to evaluate the electrochemical performances of the as-prepared materials. The CV curves of Ni-Mn LDH, Co-Mn LDH and their hybrids at the scan rate of 20 mV s1 are shown in Fig. 6a. The redox peaks during the anodic and cathodic sweeps can be clearly observed for each electrode material, showing their typical pseudocapacitive behavior. The CV curves of Ni-Mn LDH and Ni-Mn LDH/rGO represent the same shape, indicating an identical electrochemical process, similarly as CoMn LDH and Co-Mn LDH/rGO. In comparison to pure LDHs, the LDH/rGO hybrids possess larger enclosed CV areas, signifying better capacitive behavior. Their galvanostatic charge-discharge
Fig. 6. (a) CV curves of the four samples at scan rate of 20 mV s1, (b) chargedischarge curves of them at 1 A g1 (c) the dependence of specific capacitance on the current density of Ni-Mn LDH, Co-Mn LDH, Ni-Mn LDH/rGO, and Co-Mn LDH/ rGO.
Fig. 5. (a) N2 adsorption/desorption isotherms and (b) pore size distribution plots of Ni-Mn LDH and Ni-Mn LDH/rGO.
curves are shown in Fig. 6b. The hybrids show a lower potential window (0.45 V) than the pure LDHs (0.5 V). This is due to the lower charge-carrying property of rGO than LDHs. The discharge time at 1 A g1 in the sequence from long to short is Ni-Mn LDH/ rGO, Ni-Mn LDH, Co-Mn LDH/rGO, and Co-Mn LDH. The CV curves and galvanostatic charge-discharge curves of the four samples at various scan rates and current densities are shown in Fig. S2 and S3 in the supplementary material. The calculated specific capacitance of each sample is presented in Fig. 6c. Co-Mn LDH delivers a low specific capacitance of 160 F g1 (or 80C g1) at 1 A g1, while Ni-Mn LDH exhibits a much high value (1010 F g1 or 505C g1). This is due to the higher capacitance of Ni than that of Co. Ni(OH)2 usually presents better capacitive performance
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
than Co(OH)2 under the same crystal structure. Thus, Ni-Mn LDH can deliver higher capacitance than Co-Mn LDH here. This can be further illustrated by the comparison of the specific capacitances of Ni-Mn LDH, Co-Mn LDH and NiCoMn LDH in Fig. S4 [17]. The specific capacitance of Ni-Mn LDH is the highest, which can be assigned to highly active Ni2+. On the other hand, during the synthesis process, GO could oxidize some metal ions in the solution. Co2+ and Mn2+ could be readily oxidized to be trivalent ions, but Ni2+ is more stable than them. It is reasonable to believe that partial Co ions in Co-Mn LDH/rGO are Co3+, thus leading to a low specific capacitance. Then Ni-Mn LDH/rGO has a higher capacitance than Co-Mn LDH/rGO. Meanwhile, we can see that both Ni-Mn LDH/rGO and Co-Mn LDH/rGO hybrids show higher specific capacitances than the pure LDH counterparts (1635 and 290 F g1) at 1 A g1. Here, the contribution from rGO is well presented by improving the specific capacitance. The specific capacitances of the four samples are displayed in Table S1 in supplementary information. The capacitance of Ni-Mn LDH/rGO at 10 A g1 can still be 71% of that at 1 A g1, but only 68% for NiMn LDH, indicating the better rate capability of Ni-Mn LDH/rGO composite. In addition to improving the specific capacitance and rate capability, the influence of rGO can be further understood by the following cycling tests and EIS analyses. Fig. 7a shows the cycle-life data of the four samples at the current density of 10 A g1 for 10,000 cycles. Compared to Ni-Mn LDH, Co-Mn LDH displays better charge-discharge stability after 10,000 cycles. Meanwhile, the LDH/rGO hybrids present prior
Fig. 7. (a) cycle-life data under the current density of 10 A g1 and (b) Nyquist plots of the four samples.
113
performances to the pure LDHs. Ni-Mn LDH/rGO has a better stability (74.1% retention) than that of pure Ni-Mn LDH (67.7% retention). The fast capacitance decay in the first 2000 cycles is observed in both samples, followed by a very stable state. The relatively rapid capacitance deterioration is probably attributed to the structure collapse, the reduce of active surface area, the phase transformation and an increase of the resistance during chargedischarge process in alkali environment [24,38]. Similarly, Co-Mn LDH/rGO is more stable than Co-Mn LDH. Among the four samples, Co-Mn LDH/rGO has the best performance, about 100% capacitance retention even after 10,000 cycles. EIS tests were carried out over a frequency range from 102 to 105 Hz [39,40]. Fig. 7b shows the Nyquist plots of the LDH/rGO composites and LDHs electrodes. Each plot represents the characteristic depressed semicircle in high- and medium-frequency region and extended tail in the low frequency region. The intercepts of the Nyquist curves on the real axis are about 0.3 0.5 V, indicating the good conductivity of the electrolyte and low internal resistance for the electrodes. According to the equivalent circuits, the charge transfer resistances (Rct) of CoMn LDH, Ni-Mn LDH, Co-Mn LDH/rGO and Ni-Mn LDH/rGO, are calculated to be 1.0, 1.7, 0.7 and 1.2 V respectively. The low Rct of Co-Mn LDH/rGO means its fast electron transport, leading to a higher capacitance retention of Co-Mn LDH/rGO. Meanwhile, studies have proved that Co3+ has better rate capability and cycling stability than those of Co2+ and Ni2+. As a summary of the performances of LDHs and LDH/rGO hybrids in the three-electrode system, we can find that element Co with better conductivity enhances the cycling stability as well as facilitates the reaction kinetics, and that Ni-containing LDHs usually conduct a high capacitive value. The hierarchical structure of LDH/rGO has several advantages. First, a direct contact between LDH nanosheets and the highly conductive rGO via chemical bonding builds up an electron transfer channel, which facilitates the reaction kinetics. Second, owing to its mechanically robust property, rGO may serve as a structural buffer layer to the internal strain during charge-discharge cycling. Finally, the unique 3D LDH/ rGO architecture, resulting in a high specific surface area, can provide sufficient exposure of active centers with the interconnected porous network, which enables fast ion migration and efficient redox reaction. Thus, rGO in the composites can be beneficial to the specific capacitance, rate capability and cycling performance. An asymmetric capacitor was assembled to investigate the practical application of Ni-Mn LDH/rGO. As schematically shown in Fig. 8a, the picture shows the structure of the asymmetric capacitor that consists of Ni-Mn LDH/rGO as positive electrode, activated carbon (AC) as negative electrode, nitrocellulose film as the separator. Fig. 8b shows the CV curves of the hybrid capacitor from 5 to 100 mV s1 in a potential of 1.8 V. The galvanostatic chargedischarge curves and the specific capacitances at different current densities are shown in Fig. 8c (based on the total mass of the electrode materials, about 20 mg). The specific capacitance of the hybrid capacitor is 84.26 F g1 at 1 A g1. The Ragone plot of the capacitor is shown in Fig. 8d. The maximum energy density of NiMn LDH/rGO//AC can reach 33.8 W h kg1 at the power density of 0.85 kW kg1, and it can still maintain 21.29 W h kg1 at the power density of 8.5 kW kg1. We also used the charged asymmetric capacitor to connect a small lamp (driving voltage, 1.5 V). It could light for 34 s, after being charged up to 1.7 V at the charge current density of 1 A g1, and decreased to 29 s at the charge current density of 10 A g1. When we connected two lamps simultaneously, they could work for 14 s after being charged to 1.7 V at 1 A g1. All above results prove that Ni-Mn LDH/rGO composite is a potential candidate material for high-performance capacitors.
114
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
Fig. 8. (a) a schematic image of the asymmetric capacitor, (b) CV curves of Ni-Mn LDH/rGO//AC capacitor, (c) dependence of specific capacitances on current densities and charge-discharge curves of the capacitor, and (d) Ragone plot of the asymmetric capacitor and a photograph of two lights connected to the charged asymmetric capacitor.
4. Conclusions
Appendix A. Supplementary data
In summary, co-precipitation method could be used to synthesize flower-like pure Ni-Mn LDH and Co-Mn LDH, sandwich-like Ni-Mn LDH/rGO and Co-Mn LDH/rGO hybrids. These LDH nanoflakes were deposited on the surfaces of rGO in the hybrid, which improved the specific surface area. Electrochemical measurements proved that the capacitive performances could be improved by addition of rGO. Ni-Mn LDH/rGO had a high specific capacitance of 1635 F g1 at 1 A g1 due to its high specific surface area and high electrochemical activity. Co-Mn LDH/rGO had a stable performance because of its highly-conductive nature. An asymmetric capacitor was assembled with Ni-Mn LDH/rGO and activated carbon as positive and negative electrode, respectively. It had a high specific capacitance and an energy density within the potential window of 1.7 V.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.04.084.
Acknowledgements This work is financially supported by the Public Projects (Analysis and Test) of Zhejiang Province (No. 2015C37027) and Zhejiang Provincial Natural Science foundation of China (Grant NO. LY13E020002).
References [1] Q. Wang, D. O'Hare, Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets, Chem. Rev. 112 (2012) 4124–4155. [2] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828. [3] H. Chen, L. Hu, M. Chen, Y. Yan, L. Wu, Nickel-cobalt layered double hydroxide nanosheets for high-performance supercapacitor electrode materials, Adv. Funct. Mater. 24 (2014) 934–942. [4] L. Wang, D. Wang, X.Y. Dong, Z.J. Zhang, X.F. Pei, X.J. Chen, B. Chen, J. Jin, Layered assembly of graphene oxide and Co-Al layered double hydroxide nanosheets as electrode materials for supercapacitors, Chem. Comm. 47 (2011) 3556–3558. [5] J. Zhao, J. Chen, S. Xu, M. Shao, Q. Zhang, F. Wei, J. Ma, M. Wei, D.G. Evans, X. Duan, Hierarchical NiMn layered double hydroxide/carbon nanotubes architecture with superb energy density for flexible supercapacitors, Adv. Funct. Mater. 24 (2014) 2938–2946. [6] J.P. Cheng, J. Zhang, F. Liu, Recent development of metal hydroxides as electrode material of electrochemical capacitors, RSC Adv. 4 (2014) 38893–38917. [7] S. Anandan, C.Y. Chen, J.J. Wu, Sonochemical synthesis and characterization of turbostratic MnNi(OH)2 layered double hydroxide nanoparticles for supercapacitor applications, RSC Adv. 4 (2014) 55519–55523.
M. Li et al. / Electrochimica Acta 206 (2016) 108–115 [8] H. Sim, C. Jo, T. Yu, E. Lim, S. Yoon, J.H. Lee, J. Yoo, J. Lee, B. Lim, Reverse micelle synthesis of colloidal nickel-manganese layered double hydroxide nanosheets and their pseudocapacitive properties, Chem. Eur. J. 20 (2014) 14880–14884. [9] X.L. Guo, X.Y. Liu, X.D. Hao, S.J. Zhu, F. Dong, Z.Q. Wen, Y.X. Zhang, Nickelmanganese layered double hydroxide nanosheets supported on nickel foam for high-performance supercapacitor electrode materials, Electrochim. Acta 194 (2016) 179–186. [10] A.D. Jagadale, G. Guan, X. Li, X. Du, X. Ma, X. Hao, A. Abudula, Ultrathin nanoflakes of cobalt–manganese layered double hydroxide with high reversibility for asymmetric supercapacitor, J. Power Sources 306 (2016) 526–534. [11] J. Yang, C. Yu, X. Fan, Z. Ling, J. Qiu, Y. Gogotsi, Facile fabrication of MWCNTdoped NiCoAl-layered double hydroxide nanosheets with enhanced electrochemical performances, J. Mater. Chem. A 1 (2013) 1963–1968. [12] L. Zhang, J. Wang, J. Zhu, X. Zhang, K.S. Hui, K.N. Hui, 3D porous layered double hydroxides grown on graphene as advanced electrochemical pseudocapacitor materials, J. Mater. Chem. A 1 (2013) 9046–9053. [13] M. Li, F. Liu, J.P. Cheng, J. Ying, X.B. Zhang, Enhanced performance of nickel– aluminum layered double hydroxide nanosheets/carbon nanotubes composite for supercapacitor and asymmetric capacitor, J. Alloy. Compd. 635 (2015) 225–232. [14] J. Fang, M. Li, Q. Li, W. Zhang, Q. Shou, F. Liu, X. Zhang, J. Cheng, Microwaveassisted synthesis of CoAl-layered double hydroxide/graphene oxide composite and its application in supercapacitors, Electrochim. Acta 85 (2012) 248–255. [15] J. Zhao, J. Chen, S. Xu, M. Shao, D. Yan, M. Wei, D.G. Evans, X. Duan, CoMnlayered double hydroxide nanowalls supported on carbon fibers for highperformance flexible energy storage devices, J. Mater. Chem. A 1 (2013) 8836–8843. [16] D. Wang, J. Li, D. Zhang, T. Liu, N. Zhang, L. Chen, X. Liu, R. Ma, G. Qiu, Layered Co–Mn hydroxide nanoflakes grown on carbon cloth as binder-free flexible electrodes for supercapacitors, J. Mater. Sci. 51 (2016) 3784–3792. [17] M. Li, J.P. Cheng, F. Liu, X.B. Zhang, 3D-architectured nickel–cobalt–manganese layered double hydroxide/reduced graphene oxide composite for highperformance supercapacitor, Chem. Phys. Lett. 640 (2015) 5–10. [18] H. Wan, J. Liu, Y. Ruan, L. Lv, L. Peng, X. Ji, L. Miao, J. Jiang, Hierarchical configuration of NiCo(2)S(4) nanotube@Ni-Mn layered double hydroxide arrays/three-dimensional graphene sponge as electrode materials for highcapacitance supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 15840–15847. [19] W. Quan, Z.L. Tang, S.T. Wang, Y. Hong, Z.T. Zhang, Facile preparation of freestanding rGO paper-based Ni-Mn LDH/graphene superlattice composites as a pseudocapacitive electrode, Chem. Comm. 52 (2016) 3694–3696. [20] J. Chen, X. Wang, J. Wang, P.S. Lee, Sulfidation of NiMn-layered double hydroxides/graphene oxide composites toward supercapacitor electrodes with enhanced performance, Adv. Energ. Mater. 6 (2016), doi:http://dx.doi. org/10.1002/aenm.201501745. [21] Q. Shou, J. Cheng, L. Zhang, B.J. Nelson, X. Zhang, Synthesis and characterization of a nanocomposite of goethite nanorods and reduced graphene oxide for electrochemical capacitors, J. Solid State Chem. 185 (2012) 191–197. [22] 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. [23] N. Yulian, L. Ruiyi, L. Zaijun, F. Yinjun, L. Junkang, High-performance supercapacitors materials prepared via in situ growth of NiAl-layered double
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39] [40]
115
hydroxide nanoflakes on well-activated graphene nanosheets, Electrochim. Acta 94 (2013) 360–366. J. Xu, S. Gai, F. He, N. Niu, P. Gao, Y. Chen, P. Yang, A sandwich-type threedimensional layered double hydroxide nanosheet array/graphene composite: fabrication and high supercapacitor performance, J. Mater. Chem. A 2 (2014) 1022–1031. H. Wang, X. Xiang, F. Li, Facile synthesis and novel electrocatalytic performance of nanostructured Ni–Al layered double hydroxide/carbon nanotube composites, J. Mater. Chem. 20 (2010) 3944–3952. B. Wang, Q. Liu, Z. Qian, X. Zhang, J. Wang, Z. Li, H. Yan, Z. Gao, F. Zhao, L. Liu, Two steps in situ structure fabrication of Ni–Al layered double hydroxide on Ni foam and its electrochemical performance for supercapacitors, J. Power Sources 246 (2014) 747–753. J.P. Cheng, L. Liu, J. Zhang, F. Liu, X.B. Zhang, Influences of anion exchange and phase transformation on the supercapacitive properties of a-Co(OH)2, J. Electroanal. Chem. 722–723 (2014) 23–31. R. Xie, G. Fan, Q. Ma, L. Yang, F. Li, Facile synthesis and enhanced catalytic performance of graphene-supported Ni nanocatalyst from a layered double hydroxide-based composite precursor, J. Mater. Chem. A 2 (2014) 7880–7889. L. Gu, L. Qian, Y. Lei, Y. Wang, J. Li, H. Yuan, D. Xiao, Microwave-assisted synthesis of nanosphere-like NiCo2O4 consisting of porous nanosheets and its application in electro-catalytic oxidation of methanol, J. Power Sources 261 (2014) 317–323. B. Liu, Y. Zhang, L. Tang, X-ray photoelectron spectroscopic studies of Ba0.5Sr0.5Co0.8Fe0.2O3-d cathode for solid oxide fuel cells, Int. J. Hydrogen Energ. 34 (2009) 435–439. M. Shao, F. Ning, J. Zhao, M. Wei, D.G. Evans, X. Duan, Preparation of Fe3O4@SiO2@layered double hydroxide core-shell microspheres for magnetic separation of proteins, J. Am. Chem. Soc. 134 (2012) 1071–1077. W. Chen, Y. Yang, H. Shao, Cation-exchange induced high power electrochemical properties of core–shell Ni(OH)2@CoOOH, J. Power Sources 196 (2011) 488–494. M. Li, J.P. Cheng, F. Liu, X.B. Zhang, In situ growth of nickel-cobalt oxyhydroxide/oxide on carbon nanotubes for high performance supercapacitors, Electrochim. Acta 178 (2015) 439–446. M. Li, J.P. Cheng, J.H. Fang, Y. Yang, F. Liu, X.B. Zhang, NiAl-layered double hydroxide/reduced graphene oxide composite: microwave-assisted synthesis and supercapacitive properties, Electrochim. Acta 134 (2014) 309–318. Z. Gao, J. Wang, Z. Li, W. Yang, B. Wang, M. Hou, Y. He, Q. Liu, T. Mann, P. Yang, M. Zhang, L. Liu, Graphene nanosheet/Ni2+/Al3+ layered double hydroxide composite as a novel electrode for a supercapacitor, Chem. Mater. 23 (2011) 3509–3516. H. Zhang, X. Zhang, D. Zhang, X. Sun, H. Lin, C. Wang, Y. Ma, One-step electrophoretic deposition of reduced graphene oxide and Ni(OH)2 composite films for controlled syntheses supercapacitor electrodes, J Phys. Chem. B 117 (2013) 1616–1627. M. Sawangphruk, P. Srimuk, P. Chiochan, A. Krittayavathananon, S. Luanwuthi, J. Limtrakul, High-performance supercapacitor of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper, Carbon 60 (2013) 109–116. J.T. Zhang, S. Liu, G.L. Pan, G.R. Li, X.P. Gao, A 3D hierarchical porous a-Ni(OH)2/ graphite nanosheet composite as an electrode material for supercapacitors, J. Mater. Chem. A 2 (2014) 1524–1529. Y. Li, L. Jiang, J. Li, Y. Liu, Novel phosphorus-doped lead oxide electrode for oxygen evolution reaction, RSC Adv. 4 (2014) 5339–5342. Y. Li, L. Jiang, F. Liu, J. Li, Y. Liu, Novel phosphorus-doped PbO2-MnO2 bicontinuous electrodes for oxygen evolution reaction, RSC Adv. 4 (2014) 24020–24028.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The growth of nickel-manganese and cobalt-manganese layered double hydroxides on reduced graphene oxide for supercapacitor M. Lia , J.P. Chenga,* , J. Wangb , F. Liua , X.B. Zhanga a State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Zhejiang University, Hangzhou 310027, China b School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China
A R T I C L E I N F O
Article history: Received 1 March 2016 Received in revised form 15 April 2016 Accepted 18 April 2016 Available online 26 April 2016 Keywords: Layered double hydroxide Reduced graphene oxide Electrochemical performance Supercapacitors
A B S T R A C T
Pure Ni-Mn layered double hydroxide (LDH), Co-Mn LDH with a flower-like morphology and sandwichlike Ni-Mn LDH/reduced graphene oxide (rGO), Co-Mn LDH/rGO hybrids are fabricated via a simple coprecipitation method. In the hybrids, Ni-Mn and Co-Mn hydroxide nanoflakes are tightly anchored on the both surfaces of rGO, leading to the composites with high specific surface areas. Electrochemical measurements prove that rGO can improve the capacitance and cyclic stability of the hybrid materials and that Ni-Mn LDH delivers a much higher specific capacitance but a worse cycling performance than Co-Mn LDH. A high specific capacitance of 1635 F g1 at 1 A g1 and a high rate retention of 71% at 10 A g1 are achieved for Ni-Mn LDH/rGO. A hybrid capacitor with Ni-Mn LDH/rGO as positive electrode and activated carbon as negative electrode is assembled. It possesses a specific capacitance of 84.26 F g1 at 1 A g1 and an energy density of 33.8 Wh kg1 within a potential window of 1.7 V. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, tremendous efforts have been devoted to exploring renewable and high-performance electrode materials for electrochemical capacitors (ECs)/supercapacitors. Layered double hydroxides (LDHs) are unusual layered materials consisting of positively-charged hydrotalcite-like layers, weakly bound, charge compensating anions and water molecules in the interlayer region. Owing to the layered structure, they usually possess high specific surface area, which can facilitate fast ion transfer. This offers them great superiority in electrochemical fields [1–6]. The LDHs containing Ni, Co, Mn, etc, have proved themselves to be one of the most promising electrode materials for ECs due to their high specific capacitance, long life span, low cost and easy synthesis. Many investigations have been performed to synthesize Ni-Mn LDH and Co-Mn LDH and measure their electrochemical performances. Various synthesis methods have been thus developed. For example, Anandan’s group reported the sonochemical synthesis of Ni-Mn LDH [7]. Sim et al. used a reverse micelle method to synthesize colloidal Ni-Mn LDH [8]. Hierarchical Ni-Mn LDH nanosheets could be grown on Ni foam by a one-step method at
* Corresponding author. Tel.: +86 571 87951411; fax: +86 571 87951411. E-mail address: [email protected] (J.P. Cheng). http://dx.doi.org/10.1016/j.electacta.2016.04.084 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
80 C, as reported by Guo and co-workers [9]. Electrodeposition method was applied by Jagadale to fabricate Co-Mn LDH [10]. Some above methods are complex, unsuitable for large scale fabrication or resulting in the impurity. Thus, a facile time-/energy-saving method to synthesize pure Ni-Mn LDH and Co-Mn LDH is urgently needed. We know that co-precipitation, hydrothermal method, anion exchange and calcination-rehydration are common methods to synthesis LDHs. Among them, co-precipitation is much more energy-saving, environmentally friendly, and easily carried out. It is well-known that the electrochemical performance of LDHs can be further improved if they are combined with carbon materials to form a composite [11–14]. Carbon materials are usually used as electrical double-layer capacitive electrode materials or as additives in transition metal oxides/hydroxides. As additives, carbon materials can provide large surface areas, prevent oxides/hydroxides from agglomeration, facilitate electrical conductivity, and enhance mechanical stability. Zhao and Wang deposited Co-Mn LDH onto carbon fibers and carbon cloth, respectively [15,16]. The results revealed that the hierarchical configuration of LDHs/carbon could improve the exposure of active sites and enable a fast charge transfer to the electrode/electrolyte interface. Zhao also reported a hierarchical structure composed of Ni-Mn LDH microcrystals grafted on carbon nanotubes (CNTs) backbone with tunable Ni/Mn ratios by an in situ growth route [15].
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
The electrochemical investigation showed that Ni3Mn1 LDH/CNTs electrode was rather active with the maximum specific capacitance of 2960 F g1 [5]. Our group has also fabricated a hierarchical NiCoMn LDH/rGO composite, which could deliver a high capacitance of 912 F g1, much higher than that of pure NiCoMn LDH (772 F g1)[17]. Some other Ni-Mn LDH/carbon composites, such as, ternary NiCo2S4 nanotube/Ni-Mn LDH/3D graphene sponge [18], Ni-Mn LDH/graphene superlattice [19], and sulfidation Ni-Mn LDH/graphene oxide [20] have been recently reported. All these composites exhibited outstanding capacitance and rate performance, indicating their promising potential as electrode materials of ECs. Herein, we attempt to synthesize pure Ni-Mn LDH, Co-Mn LDH, and their composites with reduced graphene oxide by simple coprecipitation method. In the composites, LDH nanosheets are anchored onto the surfaces of reduced graphene oxide (rGO) to form a hierarchical architecture. Electrochemical tests are carried out to reveal their capacitive performances as electrode materials for ECs. It is found that Ni-Mn LDH/rGO composite can deliver a high capacitance (1635 F g1), a fair rate capability and a long life span. In addition, an asymmetric capacitor is designed with Ni-Mn LDH/rGO and activated carbon (AC) as positive and negative materials, respectively. 2. Experimental
109
composite was evaluated to be 4.6%, determined by weighing the final product. Pure Ni-Mn LDH could be also prepared by the same process but without using any GO. Similarly, pure Co-Mn LDH and Co-Mn LDH/rGO composite could be also fabricated by the same procedure using 4 mmol Co (NO3)2. 2.2. Materials characterization The structure and morphology of the products were characterized by an X-ray diffractometer (XRD, using Cu-Ka radiation at 40 kV, 40 mA, 3 min1, Shimadzu, LabX XRD-6000) with a secondary graphite monochromator, scanning electron microscope (SEM, Hitachi S-4800), and transmission electron microscope (TEM, Philip-CM200). Fourier transformed infrared spectroscopy (FT-IR, Bruker spectrometer, Tensor 27) was obtained by dispersing the sample in KBr and pressing it into transparent pellets. The specific surface area and pore distribution of the materials were tested by an ASAP 2020 analyzer (Micromeritics Instrument Corporation) by Brunauer-Emmett-Teller (BET) method at 77 K in N2 with a pre-treatment at 180 C for over 10 h degassing. X-ray photoelectron spectroscopy (XPS, Thermo, Escalab 250Xi) was measured and the XPS spectra were calibrated by the C 1s line at 284.8 eV. Thermogravimetric (TG) analysis was carried out on an SDT Q600 instrument from 25 to 800 C with the heating rate of 10 C min1 in air.
2.1. Preparation of materials 2.3. Preparation of electrodes and electrochemical characterization Natural flake graphite (500–600 mm, Qingdao Xinghe Graphite Co., Ltd) was used to prepare graphene oxide (GO). By a modified Hummers method, as reported elsewhere [21], oxidation, exfoliation and washing process were carried out on the flake graphite. After centrifugation and standardization, 6 g L1 GO suspension was obtained. Then, a specific volume of above GO suspension containing 15 mg GO was dispersed into 250 mL deionized water to form a transparent suspension, followed by the addition of 4 mmol Ni(NO3)2, 2 mmol Mn(NO3)2 and 18 mmol NH4F. Under strong stirring, 18 mL ammonia (1 mol L1) was added dropwise into the above suspension at room temperature in 2 h for the fabrication of LDHs. After being washed and dried at 80 C, Ni-Mn LDH/rGO was thus obtained. The mass content of rGO in Ni-Mn LDH/rGO
For a three-electrode configuration, the electrode containing as-prepared materials, Ni plate and an Ag/AgCl electrode were used as work electrode, counter electrode and reference electrode, respectively. The work electrode was prepared by mixing the electrode material, acetylene black, polyvinylidene fluoride in a mass ratio of 80:10:10 in N-methyl-2-pyrrolidone, which was painted onto a Ni foam substrate (1.5 cm 1.5 cm) and completely dried at 80 C in vacuum. The active material loaded on each electrode was about 4 mg. 2 M KOH solution was acted as electrolyte. A salt-bridge was used to connect Ag/AgCl electrode and the electrolyte. Cyclic voltammograms (CV), galvanostatic charge-discharge curves and electrochemical impedance
Fig. 1. (a, b) SEM image of pure Ni-Mn LDH, (c, d) SEM image of Ni-Mn LDH/rGO, (e) SEM image of pure Co-Mn LDH, and (f) SEM image of Co-Mn LDH/rGO.
110
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
spectroscopy (EIS) were measured on a CHI660D electrochemical workstation. The cycle life test was conducted on a LAND CT2001A system by galvanostatic charge-discharge techniques. The electrochemical measurement for the hybrid capacitor was conducted by a two-electrode configuration in 2 M KOH solution. 3. Results and discussion In the composites, GO was acted as the backbone for the in situ growth of Ni-Mn LDH and Co-Mn LDH crystals. The oxygencontaining groups on GO sheets could act as nucleation sites for the LDH crystals. Ni2+ or Co2+ and Mn2+ in the solution could form coordination complexes with carboxyl and hydroxyl groups on the GO nanosheets through electrostatic attraction. At the same time, GO could be reduced to rGO by Mn2+. When the precipitating agent (ammonia) is added to the solution, LDH crystals are formed first on the coordination sites. The formed positively charged lattices of LDH crystals attach to the negatively charged carboxylic groups on the surfaces of rGO [22]. As the LDH crystals grow to nanosheets, the rGO surface is completely covered with vertical LDH, greatly improved the utility of the two components. Thus, 3D sandwichlike Ni-Mn LDH/rGO material with a high specific surface area is obtained. The morphology and structure described above can be directly proved by the SEM images of Ni-Mn LDH/rGO hybrid. As shown in
Fig. 2. TEM images of (a) Ni-Mn LDH and (b) Ni-Mn LDH/rGO.
Fig. 1a and 1b, pure Ni-Mn LDH represents a flower-like morphology. LDH nanosheets are laminated and slit-shaped assembled. Fig. 1c and 1d show the sandwich-like Ni-Mn LDH/ rGO. The surfaces of rGO are fully covered with LDH nanosheets. Besides, the agglomeration of LDH sheets is effectively prevented by the heterogeneous structure. Both Co-Mn LDH and Co-Mn LDH/ rGO present the similar morphology and architecture as Ni-Mn LDH and Ni-Mn LDH/rGO hybrid, as shown in Fig. 1e and 1f. The TEM images of Ni-Mn LDH and Ni-Mn LDH/rGO are presented in Fig. 2. As we can see, flower-like (Fig. 2a) and sandwich-like (Fig. 2b) structures are presented again. The selected-area electron diffraction (SAED) pattern of Ni-Mn LDH reveals hexagonally arranged spots, confirming the single crystallinity of Ni-Mn LDH nanosheet and its good crystallinity. The SAED pattern of the hybrid shows clear diffraction rings, caused by the growth of LDH on the surfaces of rGO. The XRD patterns of GO, pure LDHs, and their composites are all shown in Fig. 3a. Except a hexagonal phase with a hydrotalcite-like structure, no other crystalline phase is detected in pure Ni-Mn LDH or Co-Mn LDH, which indicates a high purity of the final products. The interlayer spacing of plane (003) (d003) is 0.772 and 0.747 nm for Ni-Mn LDH and Co-Mn LDH, respectively. The strong diffraction peak at 9.3 is ascribed to the (001) plane of GO for the GO pattern. The absence of the diffraction peak at around 26 indicates the
Fig. 3. (a) XRD patterns and (b) FTIR spectra of GO, Ni-Mn LDH, Co-Mn LDH, Ni-Mn LDH/rGO, and Co-Mn LDH/rGO.
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
complete oxidation of graphite. However, the absence of the diffraction peaks of rGO in the hybrids reveals that the restacking of rGO nanosheets is effectively prevented by the complete exfoliation of graphite in the hybrids. This further indicates that thin LDH nanoflakes are located on the surfaces of rGO nanosheets [23]. Compared with pure LDHs, the diffraction peaks of plane (003) of the hybrids shift towards the low angle region. To be precise, the diffraction peaks of plane (003) for pure Ni-Mn LDH and Co-Mn LDH are at 11.45 and 11.84 , while the corresponding hybrids are at 11.04 and 11.18 , respectively. The peak shift of LDHs leads to a large interlayer spacing of 0.801 and 0.791 nm for Ni-Mn LDH and Co-Mn LDH in the hybrids. This can be ascribed to the weakened electrostatic interactions between the hydrotalcite-like sheets and the species in the interlayer space, which probably involves the driving force of LDHs grown on the rGO surfaces [24,25]. The larger interlayer spacing of the hybrids may also lead to a better electrochemical performance than the pure counterparts. The FT-IR spectra of GO, LDHs, and LDH/rGO hybrids are shown in Fig. 3b. The bands at 3440 and 1630 cm1 are assigned to the O-H stretching vibrations of water molecules and the hydrogen bonding among the hydroxyl groups in all LDH samples [26]. Weak absorption bands around 1729 and 1200 cm1 in the GO spectrum are corresponding to the vibration of C¼O and C-O functional groups. These oxygen-containing functional groups on GO were nucleation sites for LDH crystals to grow. The bands at 1384 cm1 in pure LDHs and the hybrids are attributed to the vibration of CO32 anion in the interlayer space [27]. These bands below 800 cm1 come from the stretching and bending vibrations of metal-oxygen (M-O) in the hydrotalcite-like lattice [24,28]. The only difference between LDHs and corresponding LDH/rGO hybrids is the band at 1108 cm1 coming from rGO. TG measurement was carried out on Ni-Mn LDH and the corresponding hybrid. We can see that their weight loss can be divided into two stages in Fig. S1. The first stage below 120 C is due to the removal of adsorbed water and interlayer water molecules.
111
The second one is the dehydroxylation, the removal of interlayer anions (CO32) and the decomposition of rGO (only for the hybrid), leading to the formation of Ni-Mn oxides [27]. The weight loss of pure LDH and the hybrid is about 40.4% and 53.8%, respectively, at this stage. Thus the final Ni-Mn oxides occupy about 59.6% and 46.2% for pure LDH and the hybrid, respectively. Because of the difference of adsorbed water and interlayer water in the two samples, the weight loss of stage one in the hybrid is about 8.9% more than that of pure LDH. So the weight percentage of rGO in NiMn LDH/rGO can be thus calculated to be 4.5%, well consistent with the result measured by the weights of initial GO and the final composite product. XPS measurements were further carried out on the Ni-Mn LDH/rGO hybrid and GO. The corresponding results are presented in Fig. 4. The survey spectrum indicates the presence of Ni, Mn, C and O, in Fig. 4a. The measured molar ratio of Ni to Mn is 1.41. The Ni 2p spectrum is well fitted with one spin-orbit doublet and two shakeup satellites. The peaks at the binding energy of 855.5 and 873.1 eV, with a spin-energy separation of 17.6 eV, are assigned to Ni2+ state in Ni(OH)2 in Fig. 4b [11,29–32]. The peaks of Mn 2p3/ 2 and Mn 2p1/2 located at 641.7 and 653.2 eV in Fig. 4c suggest the presence of Mn3+ in the sample [5,15]. The O 1s spectrum in Fig. 4d decomposes into three constituents, M-O-M and M-O-H in the M(OH)6 octahedron at 528.8 and 530.7 eV, respectively, and H-O-H at 532.7 eV [22,33]. The C 1s spectrum in Fig. 4e originates from both rGO and CO32 anions in the LDH interlayer space. The peak at 284.8 eV represents non-oxygenated carbon (C¼C) from rGO nanosheets. The carbonyl carbon at 286.2 eV and the carboxylate carbon (O C¼O) at 288.7 eV may come from CO32 or rGO [34,35]. For comparison, the C 1s spectrum of asprepared GO is exhibited in Fig. 4f. Except non-oxygenated carbon (C¼C), three oxidation states can be clearly observed, the carbon in C-O bond at 285.2 eV, the carbonyl carbon at 286.9 eV and the carboxylate carbon (OC¼O) at 288.6 eV [34,36]. By comparison with the XPS spectrum of GO, we can find that the rGO in the hybrid is well reduced by Mn2+ during the synthesis process.
Fig. 4. (a) XPS wide spectrum (b) Ni 2p, (c) Mn 2p, (d) O 1s, (e) C 1s of Ni-Mn LDH/rGO composite, and (f) C 1s spectra of GO.
112
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
The N2 adsorption/desorption measurements were carried out to measure the specific surface area and pore volume of Ni-Mn LDH/rGO composite and pure Ni-Mn LDH. As shown in Fig. 5a, the N2 adsorption/desorption curves of both samples represent typical IV isotherms with distinct hysteresis loops indicating a mesoporous structure. They also exhibit an H3-type hysteresis loop due to the existing mesopores of absorbents having slit-shaped pores [37]. Mesopores and macropores with a relatively wide distribution within the range from 20 to 100 nm can be observed from the pore size distribution plots in Fig. 5b. Calculated by BET method, pure Ni-Mn LDH has a specific surface area of 45.5 m2 g1, while NiMn LDH/rGO exhibits a much higher specific surface area of 91.9 m2 g1, about 2 times of pure LDHs. The corresponding pore volume is 0.25 and 0.42 m3 g1 for pure Ni-Mn LDH and the hybrid, respectively. It is reasonable to believe that the introduction of rGO in the composite depresses the aggregation of LDH platelets, thus leading to a high surface area [22], which is beneficial to the kinetics of electrochemical reaction. Electrochemical measurements in a three-electrode configuration were carried out to evaluate the electrochemical performances of the as-prepared materials. The CV curves of Ni-Mn LDH, Co-Mn LDH and their hybrids at the scan rate of 20 mV s1 are shown in Fig. 6a. The redox peaks during the anodic and cathodic sweeps can be clearly observed for each electrode material, showing their typical pseudocapacitive behavior. The CV curves of Ni-Mn LDH and Ni-Mn LDH/rGO represent the same shape, indicating an identical electrochemical process, similarly as CoMn LDH and Co-Mn LDH/rGO. In comparison to pure LDHs, the LDH/rGO hybrids possess larger enclosed CV areas, signifying better capacitive behavior. Their galvanostatic charge-discharge
Fig. 6. (a) CV curves of the four samples at scan rate of 20 mV s1, (b) chargedischarge curves of them at 1 A g1 (c) the dependence of specific capacitance on the current density of Ni-Mn LDH, Co-Mn LDH, Ni-Mn LDH/rGO, and Co-Mn LDH/ rGO.
Fig. 5. (a) N2 adsorption/desorption isotherms and (b) pore size distribution plots of Ni-Mn LDH and Ni-Mn LDH/rGO.
curves are shown in Fig. 6b. The hybrids show a lower potential window (0.45 V) than the pure LDHs (0.5 V). This is due to the lower charge-carrying property of rGO than LDHs. The discharge time at 1 A g1 in the sequence from long to short is Ni-Mn LDH/ rGO, Ni-Mn LDH, Co-Mn LDH/rGO, and Co-Mn LDH. The CV curves and galvanostatic charge-discharge curves of the four samples at various scan rates and current densities are shown in Fig. S2 and S3 in the supplementary material. The calculated specific capacitance of each sample is presented in Fig. 6c. Co-Mn LDH delivers a low specific capacitance of 160 F g1 (or 80C g1) at 1 A g1, while Ni-Mn LDH exhibits a much high value (1010 F g1 or 505C g1). This is due to the higher capacitance of Ni than that of Co. Ni(OH)2 usually presents better capacitive performance
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
than Co(OH)2 under the same crystal structure. Thus, Ni-Mn LDH can deliver higher capacitance than Co-Mn LDH here. This can be further illustrated by the comparison of the specific capacitances of Ni-Mn LDH, Co-Mn LDH and NiCoMn LDH in Fig. S4 [17]. The specific capacitance of Ni-Mn LDH is the highest, which can be assigned to highly active Ni2+. On the other hand, during the synthesis process, GO could oxidize some metal ions in the solution. Co2+ and Mn2+ could be readily oxidized to be trivalent ions, but Ni2+ is more stable than them. It is reasonable to believe that partial Co ions in Co-Mn LDH/rGO are Co3+, thus leading to a low specific capacitance. Then Ni-Mn LDH/rGO has a higher capacitance than Co-Mn LDH/rGO. Meanwhile, we can see that both Ni-Mn LDH/rGO and Co-Mn LDH/rGO hybrids show higher specific capacitances than the pure LDH counterparts (1635 and 290 F g1) at 1 A g1. Here, the contribution from rGO is well presented by improving the specific capacitance. The specific capacitances of the four samples are displayed in Table S1 in supplementary information. The capacitance of Ni-Mn LDH/rGO at 10 A g1 can still be 71% of that at 1 A g1, but only 68% for NiMn LDH, indicating the better rate capability of Ni-Mn LDH/rGO composite. In addition to improving the specific capacitance and rate capability, the influence of rGO can be further understood by the following cycling tests and EIS analyses. Fig. 7a shows the cycle-life data of the four samples at the current density of 10 A g1 for 10,000 cycles. Compared to Ni-Mn LDH, Co-Mn LDH displays better charge-discharge stability after 10,000 cycles. Meanwhile, the LDH/rGO hybrids present prior
Fig. 7. (a) cycle-life data under the current density of 10 A g1 and (b) Nyquist plots of the four samples.
113
performances to the pure LDHs. Ni-Mn LDH/rGO has a better stability (74.1% retention) than that of pure Ni-Mn LDH (67.7% retention). The fast capacitance decay in the first 2000 cycles is observed in both samples, followed by a very stable state. The relatively rapid capacitance deterioration is probably attributed to the structure collapse, the reduce of active surface area, the phase transformation and an increase of the resistance during chargedischarge process in alkali environment [24,38]. Similarly, Co-Mn LDH/rGO is more stable than Co-Mn LDH. Among the four samples, Co-Mn LDH/rGO has the best performance, about 100% capacitance retention even after 10,000 cycles. EIS tests were carried out over a frequency range from 102 to 105 Hz [39,40]. Fig. 7b shows the Nyquist plots of the LDH/rGO composites and LDHs electrodes. Each plot represents the characteristic depressed semicircle in high- and medium-frequency region and extended tail in the low frequency region. The intercepts of the Nyquist curves on the real axis are about 0.3 0.5 V, indicating the good conductivity of the electrolyte and low internal resistance for the electrodes. According to the equivalent circuits, the charge transfer resistances (Rct) of CoMn LDH, Ni-Mn LDH, Co-Mn LDH/rGO and Ni-Mn LDH/rGO, are calculated to be 1.0, 1.7, 0.7 and 1.2 V respectively. The low Rct of Co-Mn LDH/rGO means its fast electron transport, leading to a higher capacitance retention of Co-Mn LDH/rGO. Meanwhile, studies have proved that Co3+ has better rate capability and cycling stability than those of Co2+ and Ni2+. As a summary of the performances of LDHs and LDH/rGO hybrids in the three-electrode system, we can find that element Co with better conductivity enhances the cycling stability as well as facilitates the reaction kinetics, and that Ni-containing LDHs usually conduct a high capacitive value. The hierarchical structure of LDH/rGO has several advantages. First, a direct contact between LDH nanosheets and the highly conductive rGO via chemical bonding builds up an electron transfer channel, which facilitates the reaction kinetics. Second, owing to its mechanically robust property, rGO may serve as a structural buffer layer to the internal strain during charge-discharge cycling. Finally, the unique 3D LDH/ rGO architecture, resulting in a high specific surface area, can provide sufficient exposure of active centers with the interconnected porous network, which enables fast ion migration and efficient redox reaction. Thus, rGO in the composites can be beneficial to the specific capacitance, rate capability and cycling performance. An asymmetric capacitor was assembled to investigate the practical application of Ni-Mn LDH/rGO. As schematically shown in Fig. 8a, the picture shows the structure of the asymmetric capacitor that consists of Ni-Mn LDH/rGO as positive electrode, activated carbon (AC) as negative electrode, nitrocellulose film as the separator. Fig. 8b shows the CV curves of the hybrid capacitor from 5 to 100 mV s1 in a potential of 1.8 V. The galvanostatic chargedischarge curves and the specific capacitances at different current densities are shown in Fig. 8c (based on the total mass of the electrode materials, about 20 mg). The specific capacitance of the hybrid capacitor is 84.26 F g1 at 1 A g1. The Ragone plot of the capacitor is shown in Fig. 8d. The maximum energy density of NiMn LDH/rGO//AC can reach 33.8 W h kg1 at the power density of 0.85 kW kg1, and it can still maintain 21.29 W h kg1 at the power density of 8.5 kW kg1. We also used the charged asymmetric capacitor to connect a small lamp (driving voltage, 1.5 V). It could light for 34 s, after being charged up to 1.7 V at the charge current density of 1 A g1, and decreased to 29 s at the charge current density of 10 A g1. When we connected two lamps simultaneously, they could work for 14 s after being charged to 1.7 V at 1 A g1. All above results prove that Ni-Mn LDH/rGO composite is a potential candidate material for high-performance capacitors.
114
M. Li et al. / Electrochimica Acta 206 (2016) 108–115
Fig. 8. (a) a schematic image of the asymmetric capacitor, (b) CV curves of Ni-Mn LDH/rGO//AC capacitor, (c) dependence of specific capacitances on current densities and charge-discharge curves of the capacitor, and (d) Ragone plot of the asymmetric capacitor and a photograph of two lights connected to the charged asymmetric capacitor.
4. Conclusions
Appendix A. Supplementary data
In summary, co-precipitation method could be used to synthesize flower-like pure Ni-Mn LDH and Co-Mn LDH, sandwich-like Ni-Mn LDH/rGO and Co-Mn LDH/rGO hybrids. These LDH nanoflakes were deposited on the surfaces of rGO in the hybrid, which improved the specific surface area. Electrochemical measurements proved that the capacitive performances could be improved by addition of rGO. Ni-Mn LDH/rGO had a high specific capacitance of 1635 F g1 at 1 A g1 due to its high specific surface area and high electrochemical activity. Co-Mn LDH/rGO had a stable performance because of its highly-conductive nature. An asymmetric capacitor was assembled with Ni-Mn LDH/rGO and activated carbon as positive and negative electrode, respectively. It had a high specific capacitance and an energy density within the potential window of 1.7 V.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.04.084.
Acknowledgements This work is financially supported by the Public Projects (Analysis and Test) of Zhejiang Province (No. 2015C37027) and Zhejiang Provincial Natural Science foundation of China (Grant NO. LY13E020002).
References [1] Q. Wang, D. O'Hare, Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets, Chem. Rev. 112 (2012) 4124–4155. [2] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828. [3] H. Chen, L. Hu, M. Chen, Y. Yan, L. Wu, Nickel-cobalt layered double hydroxide nanosheets for high-performance supercapacitor electrode materials, Adv. Funct. Mater. 24 (2014) 934–942. [4] L. Wang, D. Wang, X.Y. Dong, Z.J. Zhang, X.F. Pei, X.J. Chen, B. Chen, J. Jin, Layered assembly of graphene oxide and Co-Al layered double hydroxide nanosheets as electrode materials for supercapacitors, Chem. Comm. 47 (2011) 3556–3558. [5] J. Zhao, J. Chen, S. Xu, M. Shao, Q. Zhang, F. Wei, J. Ma, M. Wei, D.G. Evans, X. Duan, Hierarchical NiMn layered double hydroxide/carbon nanotubes architecture with superb energy density for flexible supercapacitors, Adv. Funct. Mater. 24 (2014) 2938–2946. [6] J.P. Cheng, J. Zhang, F. Liu, Recent development of metal hydroxides as electrode material of electrochemical capacitors, RSC Adv. 4 (2014) 38893–38917. [7] S. Anandan, C.Y. Chen, J.J. Wu, Sonochemical synthesis and characterization of turbostratic MnNi(OH)2 layered double hydroxide nanoparticles for supercapacitor applications, RSC Adv. 4 (2014) 55519–55523.
M. Li et al. / Electrochimica Acta 206 (2016) 108–115 [8] H. Sim, C. Jo, T. Yu, E. Lim, S. Yoon, J.H. Lee, J. Yoo, J. Lee, B. Lim, Reverse micelle synthesis of colloidal nickel-manganese layered double hydroxide nanosheets and their pseudocapacitive properties, Chem. Eur. J. 20 (2014) 14880–14884. [9] X.L. Guo, X.Y. Liu, X.D. Hao, S.J. Zhu, F. Dong, Z.Q. Wen, Y.X. Zhang, Nickelmanganese layered double hydroxide nanosheets supported on nickel foam for high-performance supercapacitor electrode materials, Electrochim. Acta 194 (2016) 179–186. [10] A.D. Jagadale, G. Guan, X. Li, X. Du, X. Ma, X. Hao, A. Abudula, Ultrathin nanoflakes of cobalt–manganese layered double hydroxide with high reversibility for asymmetric supercapacitor, J. Power Sources 306 (2016) 526–534. [11] J. Yang, C. Yu, X. Fan, Z. Ling, J. Qiu, Y. Gogotsi, Facile fabrication of MWCNTdoped NiCoAl-layered double hydroxide nanosheets with enhanced electrochemical performances, J. Mater. Chem. A 1 (2013) 1963–1968. [12] L. Zhang, J. Wang, J. Zhu, X. Zhang, K.S. Hui, K.N. Hui, 3D porous layered double hydroxides grown on graphene as advanced electrochemical pseudocapacitor materials, J. Mater. Chem. A 1 (2013) 9046–9053. [13] M. Li, F. Liu, J.P. Cheng, J. Ying, X.B. Zhang, Enhanced performance of nickel– aluminum layered double hydroxide nanosheets/carbon nanotubes composite for supercapacitor and asymmetric capacitor, J. Alloy. Compd. 635 (2015) 225–232. [14] J. Fang, M. Li, Q. Li, W. Zhang, Q. Shou, F. Liu, X. Zhang, J. Cheng, Microwaveassisted synthesis of CoAl-layered double hydroxide/graphene oxide composite and its application in supercapacitors, Electrochim. Acta 85 (2012) 248–255. [15] J. Zhao, J. Chen, S. Xu, M. Shao, D. Yan, M. Wei, D.G. Evans, X. Duan, CoMnlayered double hydroxide nanowalls supported on carbon fibers for highperformance flexible energy storage devices, J. Mater. Chem. A 1 (2013) 8836–8843. [16] D. Wang, J. Li, D. Zhang, T. Liu, N. Zhang, L. Chen, X. Liu, R. Ma, G. Qiu, Layered Co–Mn hydroxide nanoflakes grown on carbon cloth as binder-free flexible electrodes for supercapacitors, J. Mater. Sci. 51 (2016) 3784–3792. [17] M. Li, J.P. Cheng, F. Liu, X.B. Zhang, 3D-architectured nickel–cobalt–manganese layered double hydroxide/reduced graphene oxide composite for highperformance supercapacitor, Chem. Phys. Lett. 640 (2015) 5–10. [18] H. Wan, J. Liu, Y. Ruan, L. Lv, L. Peng, X. Ji, L. Miao, J. Jiang, Hierarchical configuration of NiCo(2)S(4) nanotube@Ni-Mn layered double hydroxide arrays/three-dimensional graphene sponge as electrode materials for highcapacitance supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 15840–15847. [19] W. Quan, Z.L. Tang, S.T. Wang, Y. Hong, Z.T. Zhang, Facile preparation of freestanding rGO paper-based Ni-Mn LDH/graphene superlattice composites as a pseudocapacitive electrode, Chem. Comm. 52 (2016) 3694–3696. [20] J. Chen, X. Wang, J. Wang, P.S. Lee, Sulfidation of NiMn-layered double hydroxides/graphene oxide composites toward supercapacitor electrodes with enhanced performance, Adv. Energ. Mater. 6 (2016), doi:http://dx.doi. org/10.1002/aenm.201501745. [21] Q. Shou, J. Cheng, L. Zhang, B.J. Nelson, X. Zhang, Synthesis and characterization of a nanocomposite of goethite nanorods and reduced graphene oxide for electrochemical capacitors, J. Solid State Chem. 185 (2012) 191–197. [22] 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. [23] N. Yulian, L. Ruiyi, L. Zaijun, F. Yinjun, L. Junkang, High-performance supercapacitors materials prepared via in situ growth of NiAl-layered double
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39] [40]
115
hydroxide nanoflakes on well-activated graphene nanosheets, Electrochim. Acta 94 (2013) 360–366. J. Xu, S. Gai, F. He, N. Niu, P. Gao, Y. Chen, P. Yang, A sandwich-type threedimensional layered double hydroxide nanosheet array/graphene composite: fabrication and high supercapacitor performance, J. Mater. Chem. A 2 (2014) 1022–1031. H. Wang, X. Xiang, F. Li, Facile synthesis and novel electrocatalytic performance of nanostructured Ni–Al layered double hydroxide/carbon nanotube composites, J. Mater. Chem. 20 (2010) 3944–3952. B. Wang, Q. Liu, Z. Qian, X. Zhang, J. Wang, Z. Li, H. Yan, Z. Gao, F. Zhao, L. Liu, Two steps in situ structure fabrication of Ni–Al layered double hydroxide on Ni foam and its electrochemical performance for supercapacitors, J. Power Sources 246 (2014) 747–753. J.P. Cheng, L. Liu, J. Zhang, F. Liu, X.B. Zhang, Influences of anion exchange and phase transformation on the supercapacitive properties of a-Co(OH)2, J. Electroanal. Chem. 722–723 (2014) 23–31. R. Xie, G. Fan, Q. Ma, L. Yang, F. Li, Facile synthesis and enhanced catalytic performance of graphene-supported Ni nanocatalyst from a layered double hydroxide-based composite precursor, J. Mater. Chem. A 2 (2014) 7880–7889. L. Gu, L. Qian, Y. Lei, Y. Wang, J. Li, H. Yuan, D. Xiao, Microwave-assisted synthesis of nanosphere-like NiCo2O4 consisting of porous nanosheets and its application in electro-catalytic oxidation of methanol, J. Power Sources 261 (2014) 317–323. B. Liu, Y. Zhang, L. Tang, X-ray photoelectron spectroscopic studies of Ba0.5Sr0.5Co0.8Fe0.2O3-d cathode for solid oxide fuel cells, Int. J. Hydrogen Energ. 34 (2009) 435–439. M. Shao, F. Ning, J. Zhao, M. Wei, D.G. Evans, X. Duan, Preparation of Fe3O4@SiO2@layered double hydroxide core-shell microspheres for magnetic separation of proteins, J. Am. Chem. Soc. 134 (2012) 1071–1077. W. Chen, Y. Yang, H. Shao, Cation-exchange induced high power electrochemical properties of core–shell Ni(OH)2@CoOOH, J. Power Sources 196 (2011) 488–494. M. Li, J.P. Cheng, F. Liu, X.B. Zhang, In situ growth of nickel-cobalt oxyhydroxide/oxide on carbon nanotubes for high performance supercapacitors, Electrochim. Acta 178 (2015) 439–446. M. Li, J.P. Cheng, J.H. Fang, Y. Yang, F. Liu, X.B. Zhang, NiAl-layered double hydroxide/reduced graphene oxide composite: microwave-assisted synthesis and supercapacitive properties, Electrochim. Acta 134 (2014) 309–318. Z. Gao, J. Wang, Z. Li, W. Yang, B. Wang, M. Hou, Y. He, Q. Liu, T. Mann, P. Yang, M. Zhang, L. Liu, Graphene nanosheet/Ni2+/Al3+ layered double hydroxide composite as a novel electrode for a supercapacitor, Chem. Mater. 23 (2011) 3509–3516. H. Zhang, X. Zhang, D. Zhang, X. Sun, H. Lin, C. Wang, Y. Ma, One-step electrophoretic deposition of reduced graphene oxide and Ni(OH)2 composite films for controlled syntheses supercapacitor electrodes, J Phys. Chem. B 117 (2013) 1616–1627. M. Sawangphruk, P. Srimuk, P. Chiochan, A. Krittayavathananon, S. Luanwuthi, J. Limtrakul, High-performance supercapacitor of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper, Carbon 60 (2013) 109–116. J.T. Zhang, S. Liu, G.L. Pan, G.R. Li, X.P. Gao, A 3D hierarchical porous a-Ni(OH)2/ graphite nanosheet composite as an electrode material for supercapacitors, J. Mater. Chem. A 2 (2014) 1524–1529. Y. Li, L. Jiang, J. Li, Y. Liu, Novel phosphorus-doped lead oxide electrode for oxygen evolution reaction, RSC Adv. 4 (2014) 5339–5342. Y. Li, L. Jiang, F. Liu, J. Li, Y. Liu, Novel phosphorus-doped PbO2-MnO2 bicontinuous electrodes for oxygen evolution reaction, RSC Adv. 4 (2014) 24020–24028.