Low-cost and high-performance of a vertically grown 3D Ni–Fe layered double hydroxide/graphene aerogel supercapacitor electrode material

Abstract

Developing low cost, highly efficient, stable and earth-abundant electrode materials for supercapacitors is critical for energy storage devices. In this work, a two-dimensional (2D) nickel-iron layered double hydroxide (Ni–Fe LDH) hybrid with 2D graphene was constructed into a three-dimensional (3D) aerogel by a facile one-step hydrothermal process assisted by freeze-drying treatment. Compared with the 2D structure, the 3D hybrid aerogel shows several advantages, including a unique porous framework, a multidimensional electron transport pathway and excellent electrical conductivity. When used as a supercapacitor electrode, benefiting from the above characteristics, the Ni–Fe LDH/graphene hybrid aerogel (Ni–Fe LDH/GHA) displays a high capacitance of 1196 F g⁻¹ at 1 A g⁻¹ and outstanding cycling stability with a capacitance retention of 80% after 2000 cycles. Furthermore, an asymmetric supercapacitor device with Ni–Fe LDH/GHA and active carbon as the positive and negative electrodes, respectively, achieved an energy density of 17.6 W h kg⁻¹ at a power density of 650 W kg⁻¹ and excellent long-term cycle stability (a specific capacitance of 91 F g⁻¹ at 1 A g⁻¹ after 3500 cycles with 87.2% retention). This work demonstrates that low cost, high performance Ni–Fe LDH/GHA has great potential for practical applications as a positive electrode in supercapacitors.

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Introduction

The development of energy storage technologies and devices is becoming more and more important in modern society because of the effective use of renewable energy.¹ Electrochemical capacitors (also known as supercapacitors (SCs)), as a new class of device for energy storage and conversion, have received considerable attention in the past few years owing to their high power density, long lifecycle and fast charging/discharging rate.² Currently, it is critical to explore cost-effective, highly efficient and earth-abundant advanced electrode materials for the next generation of SCs. Based on the energy storage mechanisms, SCs can be classified as electrochemical double-layer capacitors (EDLCs) or pseudocapacitors. EDLCs, for example, carbon materials (e.g. carbon nanotubes,³ graphene sheets⁴ and so on), store and supply energy through ion adsorption and desorption. Pseudocapacitors are more widely investigated according to their reversible redox reactions at the surface between the active materials and electrolyte, examples include metal oxides,⁵ hydroxides⁶ and conducting redox polymers.⁷ At present, there is widespread interest in using pseudocapacitor-based materials for SCs because the energy density associated with the faradaic reaction is substantially larger, by at least one or more orders of magnitude, than that of EDLCs. Therefore, significant effort has been focused on designing and preparing pseudocapacitor-based materials with unique structures and features, with the aim of improving their electrochemical performance for energy storage applications.

As a typical class of two-dimensional (2D) inorganic layered pseudocapacitor materials, layered double hydroxide (LDH) have received extensive attention in the electrochemistry field in recent years due to their easy-exchangeability, wide metal ion tunability and plentiful supply of redox active sites. Furthermore, their easily controlled facile chemical modification, low cost and lower environment pollution satisfies the requirements for sustainable development. In addition, due to the pre-intercalation of water molecules and the rapid expansion and contraction of their flexible structures, the 2D pseudocapacitor nanomaterials can store energy between nanosheets by a fast ion adsorption mechanism. Thus, the nature of 2D nanomaterials favours fast ionic transport through 2D channels that are free to expand and contract. Due to the above mentioned merits, over a long period of time, LDH-based pseudocapacitor materials have been extensively investigated, for instance, recent research has shown that Ni–Al,⁸ Ni–Co,⁹ Co–Al¹⁰ and Ni–Co–Al LDH¹¹ as SC positive electrode materials have advantages because mixed-metal hydroxide compounds have superior electrochemical performances than single metal compositions.^12,13

As a classic electroactive material, Ni–Al LDHs, formed by Al³⁺ partly replacing the Ni²⁺ of α-Ni(OH)₂, have demonstrated both improved cycle lives and high rate capacitance. However, the electrochemical characteristics of Ni–Al LDH are still largely limited because of their poor electric conductivity and aggregation issues. Moreover, during electrochemical testing, it is always observed that Al³⁺ ions dissolve in alkaline electrolyte in the form of Al(OH)⁴⁻ or AlO²⁻, which again drives the transformation of Ni–Al LDHs into the low capacitance of β-Ni(OH)₂.^14–17 To solve these issues, many other transition metal-based materials have been explored as replacements, because they are earth-abundant and stable relatively.^18,19 Recently, the material of Ni–Fe LDH is a hot topic for applications in various fields, such as non-precious metal catalysts in oxygen²⁰ and hydrogen evolution reactions,²¹ dye sorption²² and so on. As expected, in Ni–Fe LDH, it can be considered that the Fe³⁺ ions partly replace the Ni²⁺ of α-Ni(OH)₂ to form a LDH structure and Fe³⁺ ions thus enhance the electrical conductivity of NiOOH.^15,17 Very recently, a flower-like Ni–Fe LDH/carbon black composite was prepared and it exhibited good performance when serving as an alkaline battery cathode.¹⁴ However, as a pseudocapacitor-based electrode material, the Ni–Fe LDH is rarely studied.

Alternatively, building various carbon-based hybrid materials is an effective solution to improve the poor cycling lifespan of electrode materials, which results from their poor conductivity and structural stability. Among carbon-based hybrid materials, graphene-based functional materials have the priority owing to their large surface area, good conductivity and excellent structural stability. Generally, interconnected pores of three-dimensional graphene aim to prevent the restacking of graphene sheets. Compared with 2D graphene, the graphene aerogels offer more advantages, sparked by their unique architecture that integrates the advantages of multidimensional electron transport pathways.²³ Therefore, how to avoid the self-stacking of graphene and combine graphene with redox materials to improve the electrochemical performance of EDLCs remains an important scientific challenge.

In this work, a facile hydrothermal method was developed for the fabrication of a hybrid graphene aerogel based on Ni–Fe LDH nanosheets, assisted by freeze-drying treatment. This novel aerogel SC electrode material, in which Ni–Fe LDHs are grown and anchored on a 3D graphene interconnected nanosheet skeleton to create a unique porous structure, is expected to provides the open channels for electrolyte transportation, and also allow most of the Ni–Fe LDH sheets to be exposed to the electrolyte. Moreover, the interconnected mesopore channels can provide a more favorable path to shorten electrolyte diffusion pathways and facilitate ion transport. Therefore, the electrochemical performance of the Ni–Fe LDH/GHA can be significantly improved.

Experimental section

Materials synthesis

All chemicals were used as received without further purification. Firstly, graphene oxide (GO) was synthesised from KS-6 through the modified Hummers' method.²⁴ Secondly, the 3D Ni–Fe LDH/GHA composite was synthesised using Ni and Fe nitrates as metal sources via a one-step hydrothermal method. Typically, the as-prepared GO was exfoliated in 80 ml distilled water by ultrasonication for 2 h to make a homogenous GO aqueous dispersion. Next, Ni(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, CO(NH₂)₂ and C₆H₅Na₃O₇·2H₂O were dissolved into the above well-distributed solution (80 ml) with stirring for 10 min. Then, the mixed solution was transferred into a 100 ml Teflon-lined stainless steel autoclave, protected by N₂ for 1 h, sealed tightly and heated at 180 °C for 15 h. After the autoclave was naturally cooled to room temperature, the as-prepared hybrid hydrogel was taken out, underwent dialysis for 2 days in deionised water and finally freeze-dried for 24 h to obtain the 3D Ni–Fe LDH/GHA. For comparison, Ni–Fe LDH nanoplates without GO were prepared under the same conditions with a Ni/Fe molar ratio of 4 : 1. GO solution concentrations of 1, 1.25, 1.5, 1.75 and 2.0 mg ml⁻¹ were explored to form the aerogel. The influence of the mass of GO on the electrochemical properties was studied. On the basis of this (Fig. S1†), Ni₄–Fe₁ LDH/GHA (1.25 mg ml⁻¹) was the target sample to be explored.

Materials characterisation

X-ray diffraction (XRD) patterns of the GHA, Ni–Fe LDH and Ni–Fe LDH/GHA samples were collected on a BRUKER-D8 X-ray diffractometer using Cu-Kα radiation (λ = 1.5418 Å). The morphologies of the samples were characterised with field emission scanning electronic microscopy (FE-SEM, JEOL-2700). Further information about the microstructure of the samples was obtained from transmission electron microscopy (TEM, JEM-2100F). The Brunauer–Emmett–Teller (BET) method was used to estimate the specific surface area and the Barrett–Joyner–Halenda (BJH) method (Autosorb-iQASIQ) was used to calculate the pore-size distribution data. Raman spectra were acquired using a LabRAM HR operating at 532 nm under ambient conditions. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo Scientific ESCALAB 250 instrument equipped with an Mg-Kα X-ray source (1253.6 eV) at energy of 30.0 eV.

Electrochemical measurements

The active material was mixed with polyvinylidene difluoride (PVDF) and acetylene black in a mass ratio of 80 : 10 : 10 in methyl-2-pyrrolidone to make slurry that was subsequently coated on Ni foam (1 × 2 cm) and dried in an oven at 60 °C for 12 h. In a three-electrode system, 3D Ni–Fe LDH/GHA on Ni foam was used as the working electrode, the reference and counter electrodes were the saturated Hg/HgO electrode and platinum foil, respectively. Every working electrode contained about 1.5–2.0 mg of active material on 1 cm².

For a two electrode asymmetric SC system, Ni–Fe LDH/GHA served as the positive electrode and active carbon (AC) served as the negative electrode. The cyclic voltammetry (CV), galvanostatic charge and discharge (GCD) measurements were carried out using a CHI660E electrochemical workstation and cycle life studies were measured by an Autolab electrochemical workstation in a 6 M KOH aqueous solution. All of the experiments were performed at room temperature with N₂ protection.

The specific capacitance (C_s) values of the working electrodes were calculated in the three-electrode system from charge–discharge profiles using the following equation:

C_s = (I × Δt)/(ΔV × m) (1)

For the two electrode asymmetric system, the following equation was used to calculate C_s:

C = I × Δt/M × ΔV (2)

where C (F g⁻¹) is the capacitance of the device, 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 total weight of the device.

The energy and power density of the SC device were calculated using the following equations:²⁵

E = 1/2 × C × ΔV² (3)

P = E/Δt (4)

where C (F g⁻¹) is the capacitance of the device, I (A) represents the discharge current, ΔV refer to the potential changes within the discharge time Δt (s), E (W h kg⁻¹) corresponds to the energy density and P (W kg⁻¹) is the power density.

Results and discussion

Formation mechanism

An overview of the synthesis process for the 3D Ni–Fe LDH/GHA is illustrated in Fig. 1. Primarily, GO was fabricated by an improved Hummers' method. Then, the homogenous GO aqueous dispersion was mixed with Ni²⁺, Fe³⁺ nitrate, urea and Na₃C₆H₅O₇·2H₂O. Next, the solution was strongly stirred for 20 min followed by a self-assembly process under hydrothermal conditions that enabled the formation of a 3D Ni–Fe LDH hybrid aerogel. At this stage, the hexagonal morphology of the Ni²⁺–Fe³⁺ LDH material is thought to be developed naturally as a result of the crystallographic habit of the LDH material. Their uniformity and high crystallinity could be attributed to the slow hydrolysis of the tri-sodium citrate and urea. Citrate has a strong affinity to M²⁺ and M³⁺ cations, i.e. Ni²⁺ and Fe³⁺ rapidly form the complex and it is beneficial to create a regular morphology of the LDH.¹⁴ What is more, Fe³⁺ ions enhanced the electrical conductivity of Ni(OH)₂ ²⁶ and the doping of α-Ni(OH)₂ with Fe³⁺ ions formed a Ni–Fe LDH structure.²⁷ The concentration of reactive ions is extremely crucial to obtain a separation of the nucleation and growth steps for high-quality crystals. At the beginning of reaction, the GO was reduced and formed into a highly cross-linked hydrogel. Meanwhile, the anion on the surface of GO was interacted with metal ions to form the Ni–Fe LDH nuclear grains. With the hydrothermal time increasing, the hydrolysis of urea was accelerated and more OH⁻ ions were released in the solution. These nanoflakes further grew on the 3D structure.

Fig. 1 Schematic illustration of our designed synthesis route of 3D porous Ni–Fe LDH/GHA.

For comparison, bare graphene aerogels (GAs) and Ni–Fe LDH were also prepared. The morphology of the GAs, pure Ni–Fe LDH and Ni–Fe LDH/GHA composites were directly proved by the SEM images displayed in Fig. 2a–d. Among them, Fig. 2a is the image of GAs and the 3D network structure can be clearly observed. Such a 3D network structure provides hierarchical porous features with a wide scale distribution. However, the pure Ni–Fe LDH exhibits a compacted lamellar structure and a somewhat aggregated phenomenon in Fig. 2b. In order to prevent the LDH nanoflakes from self-aggregation and the loss of effective active area, it is necessary to construct a robust and rigid structure to support them. It is noteworthy that a panoramic SEM image of the as-fabricated 3D Ni–Fe LDH/GHA (Fig. 2c) shows an interconnected and porous 3D architecture. Moreover, the Ni–Fe LDH nanosheets occupy most of the available surface of the graphene, giving much higher loadings of LDH nanosheets in the hybrid aerogels. It is clearly seen that most of the Ni–Fe LDHs in the composite material have regular hexagon shapes and are several tens of nanometers in thickness in Fig. 2d. These Ni–Fe LDH nanosheets are interlacedly grown on the graphene skeleton surface, which prevent the agglomeration of LDH. That is important for effective electrolyte transport and active-site accessibility.²⁸ The element mapping images of the Ni–Fe LDH/GHA sample illustrate the distribution of very elements in Fig. 2e. It is clear that Ni, Fe and C were overlapped with the same shape, demonstrating that Ni–Fe LDH was distributed evenly on the surface of graphene. From all above, it can be observed that the morphology of Ni–Fe LDH/GHA is a uniformly distributed 3D structure.

Fig. 2 SEM images of (a) GA; (b) pure Ni–Fe LDH; (c) 3D porous Ni–Fe LDH/GHA, inset: freeze-drying aerogel; (d) high-resolution SEM image of Ni–Fe LDH/GHA; (e) the TEM mapping images of Ni–Fe LDH/GHA.

The above points were strongly confirmed by the following results. In order to further determine the porous structure and check the surface area of various samples, N₂ adsorption desorption measurements were carried out (Fig. S2†) and it is verified that the GAs, pure Ni–Fe LDH and Ni–Fe LDH/GHA have specific surface areas of 113.6, 37.2 and 101.3 m² g⁻¹, respectively. Even with amounts of Ni–Fe LDH nanosheets grown on the surface of graphene, the specific surface area of the hybrid aerogels was still retained well. Compared with pure LDHs, the 3D structure of hybrid aerogels has higher BET surface area which would allow the materials and electrolyte to have good contact. The porosity of materials is highly important for superior performance as electrodes for SCs. Therefore, BJH analysis was carried out to measure the pore size and distribution of the Ni–Fe LDHs/GHA. Its average pore diameters were mainly distributed a narrow size between 3.6 nm and 10.3 nm, which are classified as mesopores. The above evaluation confirms that the 3D sample has a porous nanostructure. It is clear that the Ni–Fe LDH/GHA almost has as high as a specific surface area as the GAs and these mesopores will be favourable for improvement of electrochemical performance because of the benefit to fast ion transfer.

The crystal structures of the obtained GAs, pure Ni–Fe LDHs and Ni–Fe LDH/GHA were investigated by XRD, as displayed in Fig. 3. The powder XRD pattern of the composite (Fig. 3a and b) exhibited the typical signature of hexagonal lattices with diffraction peaks which are attributed to the (003), (006), (012), (015), (018), (110) and (013) planes with R3m rhombohedral symmetry (JCPDS card 22-0700),²⁹ similar to the diffraction patterns of the α-Ni(OH)₂ phase, as reported respectively.²⁶ As shown in Fig. 3c, the GO was turned into GHAs after the hydrothermal reduction, the typical peak at 2θ = 25.2, its broad feature indicates the distortion from ordered arrangement of graphene sheets along their stacking direction.

Fig. 3 XRD patterns of material: (a) 3D Ni–Fe LDH/GHA, (b) pure Ni–Fe LDH and (c) GAs.

XRD was used to identify the phases. Further information about the microstructure of the samples was obtained from TEM. Fig. 4a and b show TEM images of the GAs, it is observed that the graphene sheets stacked densely to form a wrinkled shell surface and graphene nanosheets interconnected to form a porous structure. The TEM images of Ni–Fe LDH with different magnifications reveal an uniform nano-size distribution of Ni–Fe LDH particles in Fig. 4c and d. It can be clearly observed that the LDH is in nearly hexagonal platelets with a lateral size. Fig. 4e and f further reveal that the 3D structure of the hybrid aerogels was constructed with graphene sheets and multiple overlapping Ni–Fe LDHs nanosheets with an interlayer separation of 0.26 nm, in agreement with its (012) plane.³⁰ A typical HRTEM image of the hybrid proves the intimate interfacial contact among two components (Fig. 4f). Ni–Fe LDH platelets are randomly laid or vertical grown on the graphene nanosheets and the Ni–Fe LDH platelets deposited on the graphene sheets still have hexagonal shapes.

Fig. 4 TEM and HRTEM images of various samples: (a and b) GAs; (c and d) Ni–Fe LDH; (e and f) Ni–Fe LDH/GHA.

Raman spectroscopy is a non-destructive method for characterising graphitic materials, therefore, to further characterise and determine the ordered and disordered crystal structures of the as-prepared samples. Raman spectroscopy of the 3D Ni–Fe LDH/GHA, pure Ni–Fe LDHs, GA and GO were measured and shown in Fig. S3.† For Ni–Fe LDHs (Fig. S3b†), besides the peaks at 477 and 684 cm⁻¹, which are ascribed to NiFe-LDHs,^22,23 Raman shift samples at 1344 and 1583 cm⁻¹ verify the formation of GAs. The peak at 1344 cm⁻¹ is ascribed to disordered carbon which is induced by the graphitic structure and functional groups attached to the surface of the graphene sheets, while the peak at 1583 cm⁻¹ is ascribed to graphite in-plane vibrations,²³ which are induced by sp²-bond carbon atoms in a 2D hexagonal graphitic layer. The I_D/I_G ratios for GAs and hybrid aerogel are 1.3, respectively, demonstrating a decrease in the average size of the sp² domains due to the partial removal of oxygen functional groups. Raman spectroscopy of the 3D Ni–Fe LDH/GHA (Fig. S3a†) reveals that the D-band and G-band located at around 1344 and 1583 cm⁻¹, respectively, exhibit a ratio of integrated peak intensities.

In order to further identify the chemical state of the C, Ni and Fe elements, high-resolution XPS spectra of the Ni–Fe LDH/GHA and the GO composite were analysed (Fig. 5) and the full XPS spectra of various as-prepared samples were investigated in Fig. S4.† Four components of carbon atoms in different functional groups are indicated in Fig. 5b. The peaks at 284.6, 285.8, 287.4 and 288.7 eV are designated as C=C, C–O, C=O and –COO, respectively.²³ The C elements are compared in GO and the hybrid aerogel in Fig. 5a and b, it is noticeable that the hybrid aerogel exhibits the same oxygen functionalities in the C 1s spectrum as GO. However, compared with GO, the intensities of the peaks for hybrid aerogel centred at 284.6, 286.3, 288.4 and 289.3 eV, especially for the intensity of the peak centred at 286.3 eV, are much smaller, whereas for the peak at 284.6 eV, the intensity is higher than that observed for GO. In order to further identify the chemical state of the Ni and Fe elements, the high-resolution XPS spectra of the Ni–Fe LDHs and Ni–Fe LDH/GHA composites were analysed. The distance of Ni 2p_1/2 and Ni 2p_3/2 is 17.6 eV, moreover, the Ni 2p core level spectrum of the Ni–Fe LDHs (Fig. 5c) shows the Ni 2p_1/2 (∼874.0 eV) and Ni 2p_3/2 (∼856.3 eV) spin–orbit doublets accompanied by satellite peaks (∼880.2 and ∼861.2 eV), suggesting the divalent Ni²⁺ state.^23,26,27 The Fe 2p core-level spectrum of the Ni–Fe LDHs (Fig. 5d) presented Fe 2p_1/2 (∼726.0 eV) and Fe 2p_3/2 (∼712.7 eV) spin–orbit peaks, indicating the existence of Fe³⁺ species.^31,32

Fig. 5 High-resolution XPS spectra of (a) C 1s (GO), (b) C 1s, (c) Ni 2p, (d) Fe 2p of Ni–Fe LDH/GHA.

The attractive morphologies of the Ni–Fe LDH/GHA show promise for energy storage applications. Hence, the electrochemical behaviour of the Ni–Fe LDH and Ni–Fe LDH/GHA nanostructures were further characterised by CV (Fig. 6a and b) in a three electrode system using a 6 M KOH aqueous electrolyte. In a potential range of 0.0 to 0.50 V, CV curves show typical pseudocapacitor behaviour at different sweep rates. A pair of redox peaks in CV curves due to the faradaic reaction of redox reaction of Ni²⁺ ↔ Ni³⁺, which refers to LDH-Ni(OH)₂ + OH ↔ LDH-NiOOH + H₂O + e⁻.³³ Notably, the Ni–Fe LDH/GHA electrode demonstrated a larger redox current density than the Ni–Fe LDH, implying more efficient utilisation of the electroactive components, owing to the 3D structure of the Ni–Fe LDHs with graphene nanoparticles. The potential of oxidation peaks presented a tendency to positively shift while the reduction peaks moved somewhat which suggested a more facile conversion of Ni³⁺ to Ni²⁺ than the reverse process for Ni–Fe LDH materials under fast charge/discharge conditions.

Fig. 6 (a and b) CV curves of Ni–Fe LDH and Ni–Fe LDH/GHA over a potential range from 0 V to 0.5 V at various scan rates; (c and d) GCD curves of Ni–Fe LDH and Ni–Fe LDH/GHA in a three-electrode system; (e) specific capacitance of the samples at various discharge current densities, (f) cyclic performance of 3D Ni–Fe LDH/GHA at 10 A g⁻¹.

The galvanostatic charging/discharging profiles for the pure Ni–Fe LDH and Ni–Fe LDH/GHA electrodes at different current densities are shown in Fig. 6c and d. Compared with the corresponding pure Ni–Fe LDHs, the hybrid aerogel electrode exhibits excellent specific capacitances (1196 F g⁻¹) at 1 A g⁻¹ and excellent retention rate even at a rather high current density (72% capacitance retention at 10 A g⁻¹). The specific capacitances of both samples gradually decrease with the increase of the current density. Apparently, the capacitances of the 3D hybrid material are much higher than pure Ni–Fe LDH at various constant current densities, especially the high rate performance in Fig. 6e. This performance is superior to many of the similar single active materials of LDHs, such as Ni–Al LDH,³⁴ Co–Al LDH¹⁰ and Co–Fe LDH³⁵ electrode materials (Table S1†^36–39). However, it is noted that when compared with some two or more active components of LDHs, for example, Ni–Co LDH, Ni–Mn LDH and Ni–Co–Al LDH, the present Ni–Fe LDH/GHA do not represent the best performance, it is also the aim to optimise its performance in the future. To further evaluate the durability of our material, the cycling stability of the SC was tested through a cyclic charge/discharge process at a current density of 10 A g⁻¹. As exhibited in Fig. 6f, it is observed that the specific capacity of the Ni–Fe LDH/GHA decreases slightly from 698 to 558 F g⁻¹ after 2000 cycles, with an excellent capacity retention of 80% of initial value. As validated by SEM and TEM investigation (Fig. S5†), after a prolonged charge–discharge process, the regular hexagon shapes structure of Ni–Fe LDH is still maintained and without apparent damage. The 3D graphene structure also can be seen and to support the LDH to release the mechanical stress during charge–discharge process, and therefore it is beneficial in maintaining the stability of the electrode structure.

To further investigate the samples for practical application, two electrode SCs device Ni–Fe LDH/GHA//AC were fabricated, in which pressed Ni foam substrate was used as the current collector, employing AC as negative electrode and Ni–Fe LDH/GHA as the positive electrode (Fig. S6†). It illustrates the CV curves of the Ni–Fe LDH/GHA//AC asymmetric SCs at various sweep rates from 5 to 50 mV s⁻¹ with a potential window of 1.3 V in Fig. 7a. The CV curves of the device exhibit an efficient charge storage behaviour, which still retain a primary shape even at a sweep rate of 50 mV s⁻¹. To evaluate the electrochemical performance further, GCD tests were conducted at various current densities from 0.5 to 5 A g⁻¹, as shown in Fig. 7b. It is noted that the asymmetric SCs deliver a specific capacitance of 91 F g⁻¹ at 0.5 A g⁻¹ and still maintain 40 F g⁻¹ at 5 A g⁻¹. For SCs, energy density and power density are two key parameters in evaluating the performances of SCs. Fig. 7c displays the relation between energy and power density of the device calculated by eqn (3) and (4). The energy density can reach as highly as 21.3 W h kg⁻¹ at a power density of 356 W kg⁻¹, and still maintains an energy density of 9.3 W h kg⁻¹ at a power density of 3451 W kg⁻¹. Compared with other LDH materials, the performance of the Ni–Fe/GHA material in alkaline supercapacitor is beneficial to the development of the LDH supercapacitors (more details in Table S2, ESI†).^40–44 The long term cycling performance is an important criterion for SCs. Fig. 7d display the stability of the device. It is obtained that the capacitance retention of the Ni–Fe LDH/GHA//AC device is 87.2% after 3500 cycles. The GCD curve of the device at 1 A g⁻¹ after 3500 cycles (Fig. S5d†) verifies the stability of the device. All the results of this work compare favourably with earlier related reports.^34–39

Fig. 7 (a) CV curves, (b) GCD curves of Ni–Fe LDH/GHA//AC in a two-electrode system, (c) Ragone plot related to energy densities and power densities, inset showing the specific capacitances at different current densities, and (d) cycling stability measured at a current density of 1 A g⁻¹, inset showing the GCD curves of Ni–Fe LDH/GHA//AC in the first 10 cycles.

Conclusions

We have successfully synthesised a strongly coupled ternary architecture Ni–Fe LDH/GHA by a one-step hydrothermal process into a 3D porous interconnected network for SCs. Such a 3D mesoporous structure favours the improvement of electrical conductivity and fast charge transport with the electrolyte. Therefore, the hybrid aerogel showed a high discharge capacity for a specific capacitance of 1196 F g⁻¹ at a current density of 1 A g⁻¹ and excellent cycling stability at 10 A g⁻¹. In addition, an asymmetric SC device is optimised using AC as the negative electrode and Ni–Fe LDH/GHA as the positive electrode. This device achieves a remarkable performance with a specific capacitance of 75 F g⁻¹ at 1 A g⁻¹ and an energy density of 17.6 W h kg⁻¹ at a power density of 650 W kg⁻¹, and still retains 40 F g⁻¹ at 5 A g⁻¹ and 9.3 W h kg⁻¹ at a power density of 3451 W h kg⁻¹, indicating outstanding energy storage performance. We believe that our present synthetic strategy can be further extended to develop other 3D layered double hydroxide/graphene aerogel materials for SCs. These results could benefit the development of 3D layered aerogel-based SC electrode materials for energy storage device applications.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (No. 51102010, 21336003, 21371021), the National Key Research Program of China (No. 2016YFB0901501) and Science and Technology Commission of Shanghai Municipality (No. 14DZ2261000).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19495h

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