Ultrathin porous nickel–cobalt hydroxide nanosheets for high-performance supercapacitor electrodes

Abstract

In this work, ultrathin porous nickel–cobalt layered double hydroxide (Ni–Co LDH) hybrid nanosheets on metal nickel sheets are synthesized via a facile hydrothermal method without any adscititious surfactant. The fabricated Ni–Co LDH hybrid nanosheet-based electrodes for supercapacitors in aqueous electrolyte exhibit a significantly enhanced specific capacitance (2184 F g⁻¹ at 1 A g⁻¹) and energy density (91.76 W h kg⁻¹ at 825.84 W kg⁻¹) due to the pronounced synergistic effect between Ni²⁺ and Co²⁺. Meanwhile, the Ni–Co LDH hybrid nanosheets as electrode materials have excellent long-life cycling stability, retaining 88.5% of the initial capacitance after 2000 cycles. Thereby, the Ni–Co nanocomposites are promising electrode materials for high-energy-density long-life cycling supercapacitors.

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1. Introduction

Supercapacitors, also known as electrochemical capacitors, could meet the urgent requirements in the modern electronics industries by virtue of characteristics such as a high energy density, rapid charge–discharge rate and long cycle life in comparison with traditional secondary batteries and conventional dielectric capacitors.^1–4 They occupy a significant position in electric vehicles, portable electronics and industrial scale products.

However, to become primary devices for power supply, supercapacitors must be developed further to improve their abilities to deliver high energy and power simultaneously. Carbon materials, including carbon nanotubes and graphenes, are currently being investigated as electrochemical supercapacitors.^5–8 Due to the limitations of reversible ion absorption at the electrode/electrolyte interface as a form of energy storage, pure carbon-material-based capacitors do not exhibit high enough specific capacitance or energy density to meet the ever-growing need for peak-power assistance in electric vehicles. In this concern, many efforts have been absorbed in the investigation of pseudocapacitive transition-metal oxides or hydroxides, such as RuO₂, MnO₂, NiO, Ni(OH)₂, Co₃O₄, Co(OH)₂, Fe₃O₄, and their binary systems.^9–17 Among many metal oxides/hydroxides, Co(OH)₂ and Ni(OH)₂ are the best candidates as a pseudocapacitive electrode material with very high specific capacitances, owing to their layered structures with large interlayer spacing and characteristic redox reaction.^18,19

Recently, metallic layered double hydroxides (LDHs) are well-known anionic or hydrotalcite-like clays with a general chemical formula of [M_1−x²⁺M_x³⁺(OH)₂]^x+[A_xⁿ⁻/n·mH₂O]^x−, in which M²⁺ and M³⁺ are the bivalent and trivalent metal cations respectively and Aⁿ⁻ is the charge-balancing anion of valence n and x = M³⁺/(M²⁺ + M³⁺).²⁰ In addition to divalent and trivalent cations, other valent states such as Li⁺, Sn⁴⁺, Ti⁴⁺, etc., may be embedded in the octahedral sites of closely packed configuration in the layers.^21–23 LDHs containing transition metal elements have been reported to be promising electrode materials for supercapacitors because of their relatively high redox activity, environmentally friendly nature and effective utilization of homogeneous dispersion transition metal atoms.^24,25 For example, Huang et al. fabricated Al–Ni LDH nanosheets on nickel foam by a hydrothermal process.²⁶ The as-prepared Al–Ni LDH supported on Ni foam as supercapacitor electrode exhibited very high specific capacitance (2123 F g⁻¹ at 1 A g⁻¹) and long cycle life. Wang et al. has fabricated a continuous CoAl-LDH thin film electrode by drying a nearly transparent colloidal solution of cobalt-based LDH nanosheets on an indium tin oxide (ITO)-coated glass plate substrate, which has a large specific capacitance of 833 F g⁻¹.²⁷ Although LDHs have been investigated widely, their electrochemical behavior is usually not good enough to meet the demand of new energy storage devices.

In this paper, we present a facile one-step method of growing binder-free nickel–cobalt layered double hydroxide (Ni–Co LDH) hybrid films with ultrathin porous nanostructures on Ni sheet for supercapacitors. Unlike previous synthetic methods of LDHs,^28,29 it needs neither any adscititious surfactant for controlling the morphology nor oxidants for producing trivalent cations, respectively. Accordingly, the as-obtained hybrid films have the following advantages over previous pseudocapacitive materials: (i) up to now, there are few reports about Ni–Co LDHs grown on metal nickel sheet for supercapacitors. With the existence of metal substrate, the electrical conductivity of the electrode materials are improved effectively. (ii) The porous Ni–Co LDH nanosheets are aligned quasi vertically on Ni sheet, which allows the effortless access of electrolyte to the entire nanosheets. Besides, no binder is needed, which enhanced the electrical conductivity of the electrode materials which can be caused by the electrical resistance of the binder. (iii) The single crystalline bimetallic hydroxides own stronger layered orientation than unitary hydroxides, which can enhance the electron transportation between active materials and current collector, and permit nickel and cobalt hydroxides to devote their double pseudocapacitance more efficiently. In addition, this Ni–Co LDH hybrid film was used as the positive electrode materials and activity carbon as the negative electrode materials to fabricate the asymmetric supercapacitor.

2. Experimental

2.1 Reagents and materials

The nickel(II) chloride hexahydrate (NiCl₂·6H₂O), the cobalt(II) chloride hexahydrate (CoCl₂·6H₂O), sodium hydroxide (NaOH), hydrochloric acid (HCl), and acetone were purchased from Sigma Chemical Corp. Absolute ethanol was purchased from Chinasun Specialty Products Co. Ltd. All of chemical reagents were used as received without further purification.

The hybrid porous electrode material was synthesized by a simple one-step method as follows. In a typical procedure, all of the nickel sheets in this study were cut from a roll of nickel sheet with a thickness of 0.15 mm. Pieces of nickel sheet with the size of 1 × 2 cm² were carefully cleaned using a 1 M HCl solution in an ultrasonic bath for 10 min in order to remove the surface oxide layer,²⁸ and then they were cleaned in acetone, absolute ethanol and deionized water for 15 min in sequence by ultrasonication. Drying and weighing of the nickel sheets were performed after all of these cleaning processes were completed. Then 10 ml of NiCl₂·6H₂O (1 mmol) and CoCl₂·6H₂O (2 mmol) solution were slowly added into 10 ml of NaOH (6 mmol) solution under vigorous stirring to form a homogeneous solution. The mixed solution and the clean nickel sheets were transferred into a 40 ml Teflon-lined stainless-steel autoclave, following by heating the autoclave in an oven at 80 °C for 8 h to allow the growth of Co_xNi_1−x(OH)₂ nanosheets on nickel sheet. Fig. S1† shows the contrastive photograph of a bare Ni sheet (see Fig. S1a†) and nickel–cobalt hydroxide nanosheets/Ni (see Fig. S1b†). For the sake of comparison, the hybrid nanosheets with different Ni : Co feeding mole ratios were also fabricated using the same procedure.

2.2 Material characterization

The morphologies were observed by scanning electron microscope (SEM, S-4800, Hitachi). Energy dispersive X-ray spectroscopy (EDX) measurements were employed to investigate the elemental compositions of the samples. The samples were coated with gold prior to observation. The crystalline structure was characterized by X-ray diffraction (XRD, Philips X'pert PRO MPD diffractometer) with Cu Kα radiation at a scanning speed of 8° min⁻¹ over the 2θ range from 10 to 80°.

2.3 Electrochemical measurements

The electrochemical properties of the as-obtained electrodes were investigated under a three-electrode cell configuration at room temperature in 1 M KOH. The nickel sheet supporting hybrid nanosheets acted directly as the working electrodes, platinum (Pt) and Ag/AgCl were used as the counter and reference electrodes, respectively. The cyclic voltammetry (CV) and galvanostatic charge–discharge measurements were conducted on a CHI 760D electrochemical workstation (Shanghai Chenhua, China). The C_s of hybrid film-electrodes at various Ni : Co feeding mole ratios were calculated from galvanostatic charge–discharge curves as follows: C_s = I × Δt/(ΔV × m), where C_s (F g⁻¹) is the specific capacitance, I (A) is the discharge current, Δt (s) is the discharge time, ΔV (V) is the potential change during the discharge, and m (g) is the mass of the active material in the electrode. The energy and power densities were calculated as follows:^30–32 E = 0.5 × C_s × V², P_ave = E/Δt, where E (W h kg⁻¹) is the energy density, V (V) is the cell voltage excluding IR drop, P_ave (W kg⁻¹) is the average power density, and Δt (s) is the discharge time.

3. Results and discussion

3.1 Morphology and composition of LDHs

Fig. 1 shows a typical scanning electron microscopy (SEM) image of the as-obtained hybrid nanosheets on the nickel sheet substrate with different Ni/Co ratio changed from 1 : 1 to 1 : 10. The pure Ni(OH)₂ products are nano-thin films (see Fig. S2a†), while the pure Co(OH)₂ products are nanosheets (see Fig. S2c†). As shown in Fig. 1c, these nanosheets are aligned vertically on the nickel sheet. This vertical array facilitates charge transport and ion diffusion without any blocks of binder. The cobalt amount played an important role in the morphological control of the hybrid films. As the concentration of Co²⁺ increased from 1 : 1, 1 : 2, 1 : 5, 1 : 8 to 1 : 10, Co(OH)₂ was incorporated into the single nickel hydroxide phase, and the mean thickness of the hybrid film continued to increase (Fig. 1a, c, e, g and i). Meanwhile, the pore-structure also increased. But with addition of Co²⁺, the as-obtained hybrid film had a worse nanostructure. This could be attributed to the collapse of the pore structures. This means that Co²⁺ has an obvious influence not only on the thickness of the nanosheets but also on the porous structure of the films. When the Ni : Co ratio reaches 1 : 2, it can induce the formation of not only the ultrathin nanosheets but also the well-defined porous nanostructures (Fig. 1c). This favored the ion diffusion in the active materials.

Fig. 1 SEM images and EDX spectrum of the as-obtained nanosheets with different Ni : Co feeding mole ratios: (a and b) 1 : 1, (c and d) 1 : 2, (e and f) 1 : 5, (g and h) 1 : 8, (i and j) 1 : 10; (k) color changes with different Ni : Co feeding mole ratio of 1 : 0, 1 : 1, 1 : 2, 1 : 5, 1 : 8, 1 : 10 and 0 : 1 (from left to right).

Energy dispersive X-ray (EDX) spectroscopy analysis was performed to evaluate the availability of this method to accurately control the composition. The peaks representing Ni, Co and O are present, which is in keeping with the composition of nickel–cobalt hydroxide nanosheets. The peaks representing Cu and C are from the carbon-supported Cu grid for EDX measurements (Fig. 1b, d, f, h and j). More importantly, the ratio from EDX analysis is very close to the feeding mole ratio. The high accuracy in the control of the contents is due to the fast and complete co-deposition of precursor. In addition, this is a versatile approach for the fabrication and surveying of many other multicomponent metal hydroxides with precisely controlled proportions of components.

Fig. 2 demonstrates the typical TEM image, HRTEM image and X-ray diffraction (XRD) pattern of the hybrid nanosheets. Structural details of the Co_0.66Ni_0.33(OH)₂ nanosheets were further characterized by TEM and HRTEM. Fig. 2a shows the morphological characteristics which is in accordance with the SEM result. Fig. 2b exhibits the HRTEM image taken from the Co_0.66Ni_0.33(OH)₂ nanosheet, a lattice spacing of around 0.23 nm, which is in agreement with the (101) lattice plane. In Fig. 2c, the XRD pattern comprises nine diffraction peaks appearing at 2θ values of 11.3°, 19.3°, 33.0°, 38.5°, 52.1°, 59.0°, 62.7°, 69.2° and 71.5°, which can be successfully indexed to (003), (001), (100), (101), (102), (110), (111), (103) and (201) plane reflections of both Co(OH)₂ and Ni(OH)₂ phase. It is difficult to differentiate between cobalt and nickel hydroxide phases, because they have similar structures and their diffraction peaks are very close.³³ But at a low angle and a high angle, the comparison of XRD patterns of the hybrid nanosheets indicates a gradual transformation of crystal structure from Ni(OH)₂ phase (black line) to Ni–Co LDH phase and then to Co(OH)₂ phase (yellow line), as the feeding Ni : Co mole ratio increased (Fig. 2d). Along with the basal plan reflection, all diffraction peaks within the range of 10–80° (2θ) attributed to the characteristic reflections of Co_xNi_1−x(OH)₂ materials. Also, the good crystallinity of the deposited films of Co_xNi_1−x(OH)₂ materials can be assured by the high and sharper diffraction peaks.

Fig. 2 TEM image (a) and HRTEM image (b) of the Co_0.66Ni_0.33(OH)₂ nanosheets, XRD patterns of hybrid nanosheets (c) with 1 : 2 of Ni : Co feeding mole ratio, (d) with different Ni : Co feeding mole ratios (black line: pure Ni(OH)₂ phase, yellow line: pure Co(OH)₂ phase).

3.2 The growth mechanism of LDHs on nickel sheet

The explore the possible formation mechanism of the Co_xNi_1−x(OH)₂ LDHs on metal nickel sheet, we have carried out the time-dependent experiments at different reaction stages. After the initial 2 h reaction, only a few nanosheet particles appear on the substrate (Fig. S3a†). With continuous extending reaction time, nanosheet arrays based on the nucleus appeared and aligned (Fig. S3b†). When the reaction time up to 8 h, nanosheets grow with a high density on the substrate (Fig. S3c†). Further increasing the reaction time to 12 h, the nanosheets get extremely dense and tanglesome (Fig. S3d†).

To thoroughly investigate the formation of Co_xNi_1−x(OH)₂ LDHs on metal nickel sheet, we present the schematic of the growth mechanism in Scheme 1. The crystal formation process of the precursor can be classified as heterogeneous nucleation and subsequent crystal growth. If only crystal seeds are produced on the substrate, the subsequent growth of porous films will be viable. Ni²⁺ and Co²⁺ first reacted with OH⁻ to produce nickel and cobalt hydroxide monomers,^34,35 which precipitated as nuclei and quickly grew into the primary particles. In this process, partial bivalent cobalt was converted to trivalent cobalt by high temperature oxidation, instead of adscititious halogen oxidant.^36,37 These primary particles aggregated into chains which partly deposited on the surface of nickel substrate to become the aggregation cores. As the nickel and cobalt (including trivalent cobalt) hydroxide primary particles continued to aggregate, they underwent olation reactions with each other due to random dispersion of nickel and cobalt hydroxide primary particles. And then they began to crystallize and grow along the c-axis, gradually forming a Ni–Co LDH nanosheet layer.³⁸ Because the aggregation cores of primary particles might distribute on nickel substrates along different directions, the Ni–Co LDH hybrid nanosheets were intersected, producing films with highly well-define porous structures on the nickel sheet. In this way, the hybrid nanosheet-based films enjoyed a well-defined porous nanostructure and good ions access availability.

Scheme 1 The possible formation mechanism of Ni–Co LDH hybrid nanosheets supported on nickel sheet.

3.3 Electrochemical properties of hybrid film electrodes

In order to identify which architecture is favorable for high-rate capacitive energy storage, the specific capacitance values of the samples have been determined by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements in a three electrode configuration, in which the Co_xNi_1−x(OH)₂ LDHs electrode was used as a working electrode, platinum as a counter and Ag/AgCl as a reference electrode. The CV curves obtained at the scan rate of 5, 10, 20, 50, 70 and 100 mV s⁻¹ and galvanostatic charge–discharge curves obtained at different current densities of 1, 2, 5, 10 and 20 A g⁻¹ for the hybrid nanosheets electrode with different Ni : Co feeding mole ratios: 1 : 0 (Fig. S4a and b†), 1 : 1 (Fig. 3a and b), 1 : 2 (Fig. 3c and d), 1 : 5 (Fig. 3e and f), 1 : 8 (Fig. 3g and h), 1 : 10 (Fig. 3i and j), 0 : 1 (Fig. S4c and d†). By comparing the CV curves, it can seen obviously that the pure Ni(OH)₂ nanosheet electrode has the minimum areas (Fig. S4a†) and the Co_0.66Ni_0.33(OH)₂ LDHs electrode has the maximum areas within 0–0.6 V (Fig. 3c). When the Ni/Co ratios increased from 1 : 0 to 1 : 2, the CV curve areas also increased accordingly. But when the Ni/Co ratios continue to increase to 1 : 5, the CV curve areas were significantly reduced. The CV curve areas of the sample Ni : Co at 1 : 8, 1 : 10 and 0 : 1 also less than the sample Ni : Co at 1 : 2.

Fig. 3 CV curves obtained at scan rate of 5, 10, 20, 50, 70 and 100 mV s⁻¹ and galvanostatic charge–discharge curves obtained at different current densities of 1, 2, 5, 10 and 20 A g⁻¹ for the hybrid nanosheets electrode with different Ni : Co feeding mole ratios: (a and b) 1 : 1, (c and d) 1 : 2, (e and f) 1 : 5, (g and h) 1 : 8, (i and j) 1 : 10.

The voltammetric responses of the Co_0.66Ni_0.33(OH)₂ LDHs electrode at different scan rates, from 5 to 100 mV s⁻¹, are shown in Fig. 3c. It can be seen from Fig. 3c there are two pairs of well-defined redox peaks on the CV curves within 0–0.6 V. The redox peaks correspond to the conversion between different oxidation states of nickel and cobalt according to eqn (1)–(3).^39,40

Ni(OH)₂ + OH⁻ → NiOOH + H₂O + e⁻ (1)

Co(OH)₂ + OH → CoOOH + H₂O + e⁻ (2)

CoOOH + OH → CoO₂ + H₂O + e⁻ (3)

It is observed that the current under curve is gradually increased with a higher scan rate and hence signifies an ideally capacitive behavior.

The Co_xNi_1−x(OH)₂ LDHs electrode is used for GCD cycling between 0 and 0.55 V in 1 M KOH electrolyte at different current densities, which showing in Fig. 3. From the galvanostatic charge–discharge curves, the nonlinear charge–discharge profile of the Co_xNi_1−x(OH)₂ electrode arises from the redox reaction, which deviates from usual linear behavior of voltage with time, normally exhibited by EDLCs.⁴¹ Obviously, the largely discharge curve can be divided into two sections, a sudden potential drop due to internal resistance and a slow potential decay due to the Faradic redox reaction. The result shows that with the increase in current density, the discharge time reduces. As shown in Fig. 3d, an increase in the IR drop with an increase in the charging current density for Co_0.66Ni_0.33(OH)₂ LDHs electrode is clearly observed.

The redox behaviours of the binary materials with different Ni : Co feeding mole ratios were characterized in details by CV measured at 5 mV s⁻¹. As shown in Fig. 4a, the CV curves showed shifts in the peaks with the compositions of the Co_xNi_1−x(OH)₂ are changed. Except the pure nickel sample, the other six materials displayed evident reversible redox couples. The comparison of CV curve average areas further indicates that the hybrid film-electrode possesses a significantly higher specific capacitance than unitary nickel hydroxide or cobalt hydroxide electrodes.^41,42 The redox peaks shifted to positive potential and became broadened with increased cobalt contents. It has been reported that the redox potential of Co(OH)₂ to CoOOH transition is more negative than that of Ni(OH)₂ to NiOOH transition.^43,44 Therefore, the peak shift of these materials can be attributed to the increased cobalt content, which exhibited more like cobalt hydroxide redox behaviour. The peak broadening is possibly due to the fact that multiple phases exist at intermediate Ni/Co ratios and may display broadened redox features.⁴⁵ The more feasible redox peaks might be contributed by both metal ions as follows: firstly, compared with Ni(OH)₂ or Co(OH)₂ phases of unitary component electrode, Ni–Co LDH phase of the hybrid nanosheets owned the layered crystal structure with enlarged interlayer spacing, which can enhance the ions diffusion within active materials. Secondly, the as-obtained hybrid film was composed of much thinner nanosheets with better-defined porous nanostructures than pure nickel hydroxide or cobalt hydroxide films. This can promote the electrolyte access and the exposure of active sites to electrolyte. Furthermore, Ni–Co LDH with highly oriented layered single crystal structure can improve the electron transportation from active materials to the current collector, which can be confirmed by the electrochemical impedance spectra (EIS) (Fig. S5†). As shown in Fig. S4,† the Nyquist plots of the Co_0.66Ni_0.33(OH)₂ electrode have an almost vertical line in the low frequency region, indicating a good capacitive behavior. In the high frequency region suggested a small electrode resistance and high charge-transfer rate between the electrolyte and the active material. In contrast, the other two electrodes showed a large diffusion resistance in the high frequency region, indicating a poor charge-transfer rate. Fig. 4b shows the galvanostatic charge–discharge curves of the different Ni : Co feeding mole ratios electrode within potential window of 0 to 0.55 V at a current density of 1 A g⁻¹. In Fig. 4, the maximal integral area and redox current is obtained for Co_0.66Ni_0.33(OH)₂ electrode. The total specific capacitance was calculated by the discharge curve, which summarized in Fig. 5a. As more cobalt was embedded, C_s increased. However, too much cobalt, such as 1 : 5, 1 : 8 and 1 : 10 of mole ratios for Ni : Co, caused a decreasing C_s due to the increasing ratio of Co(OH)₂ (lower C_s). The specific capacitances of 1398, 1991, 2184, 1305, 1473, 1488 and 699 F g⁻¹ can be achieved at 1 A g⁻¹ for samples of Ni : Co is 1 : 0, 1 : 1, 1 : 2, 1 : 5, 1 : 8, 1 : 10 and 0 : 1, respectively. This C_s variation could be also well reflected by the gradual changes of crystal structure and morphology of hybrid films at various Ni : Co feeding mole ratios. The thinner nanosheets with pure LDH phase seemed to be more effective for contributing pseudocapacitance.^46–48 The sample Ni : Co at 1 : 2 has the highest specific capacitance, at 2184, 1942, 1866, 1664 and 1494 F g⁻¹ at current densities of 1, 2, 5, 10 and 20 A g⁻¹, respectively. The highest C_s (2184 F g⁻¹ at 1 A g⁻¹) was significantly higher than the specific capacitance of other six nickel–cobalt hydroxide composite, indicating the most effective charge transfer and high energy storage. This high C_s should be attributed by the unique nanostructure features of the Ni–Co LDH hybrid nanosheets: highly oriented layered single crystal thin nanosheets aligning quasi vertically on the nickel sheet and the resultant well-defined porous nanostructure therefrom.

Fig. 4 Electrochemical characterization of the Co_xNi_1−x(OH)₂ electrode with different Ni : Co feeding mole ratios in 1 M KOH: (a) CV curves obtained at a scan rate of 5 mV s⁻¹, (b) galvanostatic charge–discharge curves obtained at constant charging current of 1 A g⁻¹.

Fig. 5 (a) Comparison of specific capacitances of the Co_xNi_1−x(OH)₂ electrodes prepared with different Ni : Co feeding mole ratios. (b) Ragone plot for Co_0.66Ni_0.33(OH)₂ electrode at different current densities. Inset shows graph of specific capacitance with different current densities. (c) Cycling performance of the Co_0.66Ni_0.33(OH)₂ electrode at a current density of 1 A g⁻¹. (d) Schematic illustration of the asymmetric supercapacitor configuration and digital image of 28 round light-emitting diodes (LED) connected in parallel lighted by two supercapacitors in series.

The relation between current density dependent the specific capacitance with Co_0.66Ni_0.33(OH)₂ electrode shows a decrease in specific capacitance from 2184 to 1494 F g⁻¹ with increases in current density (shown in inset of Fig. 5b). It is observed that a rise in charging current density decreases specific capacitance values. Significantly, the specific capacitance of Co_0.66Ni_0.33(OH)₂ composite electrode still retain a value of as high as 1494 F g⁻¹ at 20 A g⁻¹. This is a consequence of the diffusion effect of the proton within the electrode, where inner active sites cannot precede the redox transitions completely at a higher current density.⁴⁹ What's more, the Co_0.66Ni_0.33(OH)₂ nanosheets electrode shows 68.41% capacitance retention from 1 to 20 A g⁻¹, indicating the highly stability. Based on the as-obtained C_s values, the energy and power densities of the Co_0.66Ni_0.33(OH)₂ nanosheets electrode can be calculated, and are used in the Ragone plot shown in Fig. 5b. The maximum energy density and power density for Co_0.66Ni_0.33(OH)₂ nanosheets electrode are found to be 91.76 W h kg⁻¹ and 825.84 W kg⁻¹. Besides, these capacitance values, energy density and power density are higher than those of previously reported LDH-like pseudocapacitive materials, such as NiCo LDH, CoAl LDH, NiAl LDH, CoNiAl LDH and their composites with graphene (Table S1, ESI†).

Moreover, cyclic performances of the Co_0.66Ni_0.33(OH)₂ nanosheets electrode were also examined by galvanostatic charge–discharge test for 2000 cycles, as shown in Fig. 5c. It is noteworthy that the capacitance retained at least about 88.5% after 2000 cycles at current density of 1 A g⁻¹, which is higher than those of some other reported composites.^50,51 As revealed in Fig. S6,† the SEM images of Co_0.66Ni_0.33(OH)₂ nanosheets electrode show the changes in morphology between the states of pre-measurement and the 2000th cycle. The ultrathin porous structure of Ni–Co LDH hybrid nanosheets was still maintained. The results demonstrated that the Co_0.66Ni_0.33(OH)₂ electrode offers an excellent pseudocapacitance performance, including high specific capacitance and rate capability, good charge–discharge stability and long-term cycling life.^52–54

To future evaluate the Ni–Co hybrid electrode for real application, a 4 × 2 cm² asymmetric supercapacitor device was made by using the Co_0.66Ni_0.33(OH)₂ nanosheets electrode as the positive electrode and the activated carbon (AC) on Ni surface as the negative electrode in 3 M KOH with one piece of cellulose paper as the separator.^55,56 The electrochemical performance of the supercapacitor device was characterized by galvanostatic charge–discharge measurements at various current densities (Fig. S7a†).

The specific capacitance of the Co_0.66Ni_0.33(OH)₂//AC was calculated from its galvanostatic charge–discharge curves, as shown in ESI Fig. S7b.† The specific capacitance value of the capacitor is up to 513 F g⁻¹ at 1 A g⁻¹ and it can remain 338 F g⁻¹ when the current density increases to 20 A g⁻¹. On the basis of these data, Ragone plot of the device describing the relation between energy density and power density was obtained and shown in Fig. S7c.† Moreover, cyclic performance of the capacitor was also examined by galvanostatic charge–discharge tests for 2000 cycles. Fig. S7d† reveals the outstanding cycling life of our supercapacitor device, the capacitance keeps at least about 82.3% after 2000 cycles at current density of 1 A g⁻¹. Significantly, two such 4 × 2 cm² asymmetric supercapacitors connected in series can efficiently power 3 mm diameter 28 round light-emitting diodes (LED) indicators connected in parallel (Fig. 5d), suggesting that this supercapacitor has the merit of application potentials.

4. Conclusion

In summary, highly porous hybrid films composed of ultrathin Ni–Co LDH nanosheets composites supported on the mental nickel sheet have been successfully synthesized using a facile one-step hydrothermal method without any surfactant and oxygenant. The Ni–Co LDH hybrid material electrodes deliver a greatly enhanced specific capacitance (2184 F g⁻¹ at 1 A g⁻¹) and energy density (91.76 W h kg⁻¹ at 825.84 W kg⁻¹) due to the pronounced synergistic effect between Ni²⁺ and Co²⁺. Simultaneously, the excellent long-time cycling stability can be also attributed to the highly oriented layered crystal structure and ultrathin nature of hybrid nanosheets as well as the well-define porous nanostructure. These excellent electrochemical performances indicate the further potential applications of the Ni–Co LDHs flexible electrode material in energy storage field for small and light-weight electronic devices. Tandem configuration of these Ni–Co LDHs based pseudocapacitors was employed to demonstrate the lighting of the LED. Moreover, the optimization of electrochemical properties by changing the composition in the final products could be a very useful strategy in improving the electrochemical performances, which could provide further insights in the material design.

Acknowledgements

This work was financially supported by the projects (no. 21371007) from National Natural Science Foundation of China, Anhui Provincial Natural Science Foundation (1208085QB28), Anhui Provincial Natural Science Foundation for Distinguished Youth (1408085J03), Natural Science Foundation of Anhui (KJ2012A139) and the Program for Innovative Research Team at Anhui Normal University.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c5ra01294e

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