A hybrid aerogel of Co–Al layered double hydroxide/graphene with three-dimensional porous structure as a novel electrode material for supercapacitors

Abstract

A three-dimensional (3D) porous hybrid aerogel with a robust interconnected network, which is constructed from cobalt–aluminum layered double hydroxide (Co–Al LDH) nanosheets and graphene, is reported here. The unique character of this hybrid aerogel is that the 3D graphene aerogel (GA) skeleton is incorporated with two-dimensional (2D) Co–Al LDH nanosheets to assemble macroscopic graphene monoliths. Furthermore, the 3D GA in the hybrid aerogel is a scaffold and support for the dispersion of the Co–Al LDH nanosheets, resulting in a relatively loose and open structure within the electrode matrix. When used as a cathode material for supercapacitors, the as-obtained porous Co–Al LDH/graphene hybrid aerogel (Co–Al LDH/GHA) exhibits favorable capacitance and excellent cycling performance, and demonstrates a maximum specific capacitance of 640 F g⁻¹ at a low current density of 1 A g⁻¹. This remains at 530 F g⁻¹ when the current density is increased to 10 A g⁻¹, and even remains at 305 F g⁻¹ at a much higher current density of 20 A g⁻¹. The capacitance keeps at around 97% after 10 000 cycles, demonstrating that the hybrid aerogel has excellent high-current capacitive properties. The enhanced high-current capacitance of this composite benefits from its unique nano-scaled Co–Al LDH with a short diffusion pathway and the excellent electrical conductivity of 3D GA.

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Introduction

Supercapacitors are believed to be a competitive type of auxiliary power supply equipment, due to their high power density, long cycle life, fast charging/discharging within seconds, high reliability and low maintenance cost.^1–6 Based on their charge-storage mechanisms, supercapacitors can be classified as two main types: electric double-layer capacitors (EDLCs) and faradaic supercapacitors (FSs).⁷ The materials studied for capacitors have been mainly of three types: carbon-based materials, metal oxides and conducting polymers. High surface area carbon materials (activated carbon black, carbon aerogels, and carbon nanotubes) are typical electric double-layer capacitors, whereas metal oxides and conducting polymers are pseudocapacitors.⁸

Nowadays, the fabrication and development of novel electrode materials play a decisive role in the improvement of the electrochemical performance of supercapacitors. Recently, numerous efforts have been extensively devoted to explore optimum EDLC electrode materials that can be utilized practically as power sources to supply a large pulsed current and high energy density. Among various available candidates, carbon materials are mostly investigated due to their remarkable advantages such as excellent chemical stability, relatively low price, desirable electric conductivity,⁹ etc. To date, many novel carbon materials have attracted extensive attention, such as activated carbons,¹⁰ graphene fibers,¹¹ carbon nanotubes,^12,13 and carbon aerogels.^14,15 Graphene, an unusual material made of atomically thick layers of carbon, has attracted a great deal of attention due to its high accessible surface area, good electrical conductivity, chemical stability and mechanical strength.¹⁶ Therefore, based on these advantages, it is a good candidate for the electrode material of supercapacitors. However, the carbon layers of graphene tend to stack together, which largely limits its application and has negative effects on its electrochemical performance. In order to alleviate this influence, novel graphene aerogels, which have the building blocks of a 3D network structure and possess abundant interconnected micropores or mesopores, have been designed and synthesized to avert the obvious aggregation of graphene. Graphene aerogels are a large class of gel consisting of 3D solid networks and have a lot of open pores,^17–19 thus inducing their many unique properties, such as low density, large specific surface area and excellent electrical conductivity. Benefiting from their unique structure and excellent properties, they have been studied extensively in supercapacitors, both as electrolyte and electrode materials. For example, Yamazaki and coworkers fabricated novel composite hydrogels containing two kinds of natural material, cellulose and chitin, and two kinds of ionic liquid.²⁰ Hybrid hydrogels immersed in H₂SO₄ aqueous solution were applied as an electrolyte for supercapacitors. The gel electrolyte based supercapacitor displayed an enhanced capacitance of 155 F g⁻¹ with excellent high-rate discharge capability and durability. Graphene paper, made by mechanically pressing a graphene aerogel, demonstrated a specific capacitance of 172 F g⁻¹ at 1 A g⁻¹ when it acted as a supercapacitor electrode.²¹ It is noted that, although these 3D GAs with high surface areas and rich pore structures have been extensively tested for EDLCs, their specific capacitance is still unsatisfactory due to the low theoretical capacitance of carbon-based materials.²² To further enhance their energy density without a sacrifice of power density, carbon incorporated with high energy density electrode materials, such as transition metal oxides and conductive polymers, has also been extensively explored to meet the requirements of high energy storage devices.²³

Currently, pseudocapacitive positive electrode materials made of nanoplate-shaped transition-metal compounds, such as metal oxides/hydroxides and layered double hydroxides (LDHs), have been shown to hold great potential for advanced supercapacitors.²⁴ LDHs have been reported as an electrode material for supercapacitors because of their unique structural anisotropy, low cost, high specific capacitance, environmentally friendly nature and effective utilization of transition metal atoms.²⁵ However, owing to their poor electric conductivity, LDHs cannot display perfect supercapacitive performance. Therefore, in order to improve the capacitive performance of LDHs, combining them with conducting materials is a normal and effective approach. For instance, Co–Al LDH/MWCNT composites exhibit higher specific capacitance and better long-life performance at high current density than pure LDH electrodes.²⁶ Moreover, activated carbon/LDH composite electrodes have also been reported to retain their value of specific capacitance after long cycling at high current density in supercapacitor electrode applications.²⁷ Also, some work has been performed combining LDHs with graphene to increase the specific capacitance as an electrode material.^20,28,30

Herein, we report a facile method for the fabrication of a flexible architecture material (3D Co–Al LDH/GHA) made of Co–Al LDH nanosheets grown in situ on graphene aerogel using a one-step hydrothermal treatment. During the hydrothermal process assisted by a freeze-drying treatment, the 3D structure of the Co–Al LDH/GHA can be completely maintained through the formation of physical cross-links between graphene sheets. The unique 3D structure of the composite is composed of 2D LDH nanosized lamellae and 3D graphene aerogel. When it is used as an electrode material, on one hand, this novel hybrid aerogel allows most of the Co–Al LDH sheets to be exposed to the electrolyte and also provides open channels for electrolyte transportation, on the other hand, it can offer a continuous electron pathway to ensure good electrical contact and to facilitate ion transport by shortening diffusion pathways. Therefore, it is highly expected that an improved electrochemical performance from 3D Co–Al LDH/GHA may be achieved.

Experimental

Materials synthesis

The graphene oxide (GO) in this work was synthesized from KS-6 by a modified Hummers method.²⁹ The 3D Co–Al LDH/GHA was synthesized by using a hydrothermal method assisted by a freeze-drying process. In a typical procedure, a specific amount of GO was exfoliated in 40 mL distilled water by ultrasonication to make a GO aqueous dispersion. At the same time, Co(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O were dissolved in another 40 mL of water. After this, the above metal ion solution with urea was slowly added to the GO aqueous dispersion solution with strong stirring. In order to produce a suitable content of GA in the Co–Al LDH/GHA composite, the content of GO in the above solution was 10.8, 13.7 or 15.2%. Co–Al LDH/GHA composites with different GO percentages are thus referred to as Co–Al LDH/GHA-10.6%, Co–Al LDH/GHA-13.7% and Co–Al LDH/GHA-15.2%, respectively. Subsequently, these mixtures were sealed in a 100 mL Teflon-lined autoclave and hydrothermally treated at 180 °C for 18 h. Finally, the resulting hybrid hydrogel underwent dialysis for 2 days in deionized water to remove the residual reducing agents, and were freeze-dried (5 × 10⁻² mbar at T ≤ −46 °C) for 24 h to obtain 3D Co–Al LDH/GHA. For comparison, we have synthesized pure GA and Co–Al LDH. Typically, GO was dispersed in 80 mL of water by sonication, reaching a concentration of up to 1 mg mL⁻¹. The resulting brown dispersion was sealed in an autoclave and heated at 180 °C for 18 h. Co(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O and urea were dissolved in another 80 mL of water, and the resulting dispersion was sealed in an autoclave and heated at 180 °C for 18 h. The morphologies and electrochemical properties of Co–Al LDH/GHA-10.6% and Co–Al LDH/GHA-15.2% were inspected in detail and are compared in Fig. S1 and S2.† Based on the compared results of the various Co–Al LDH/GHA composites, a GO percentage of 13.7% is the most suitable amount; therefore, Co–Al LDH/GHA-13.7% is the target composite and is referred to as 3D Co–Al LDH/GHA in this work.

Materials characterization

The morphology and crystalline structure of the samples were investigated by field emission scanning electron microscopy (FE-SEM, Zeiss Supra 55) and high resolution transmission electron microscopy (HRTEM, JEM-2100F). The specific surface areas and the pore-size distributions were obtained at 77 K using the Brunauer–Emmett–Teller (BET) method with a Micromeritics ASAP2020 surface area and porosity analyzer. Before measurements, the samples were degassed at 80 °C for 12 h. The N₂ adsorption–desorption isotherm is fitted based on the Brunauer–Emmett–Teller (BET) model. The pore size distribution is determined by the Barrett–Joyner–Halenda (BJH) method. X-ray diffraction (XRD) patterns were measured on a Shimadzu XRD-6000 diffractometer using Cu-Kα radiation (λ = 0.15406 nm). Raman spectra were taken with a Renishaw RM2000 confocal Raman spectrometer with a 532 nm excitation laser. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo Scientific ESCALAB 250 instrument equipped with a Mg-Kα X-ray source (1253.6 eV) at a pass energy of 30.0 eV. The binding energies obtained in the XPS analysis were calibrated for specimen charging by referencing the C 1s to 284.6 eV.

Electrochemical testing

The working electrode was prepared by mixing the active material, acetylene black and polyvinylidene fluoride (PVDF) in a mass ratio of 80 : 10 : 10 in methyl-2-pyrrolidone to produce a homogeneous paste. Then, the paste was coated onto the Ni foam substrate and the electrode was dried at 60 °C in vacuum. The active material load on each electrode was about 3 mg. Electrochemical measurements were performed under a three-electrode system in 6 M KOH aqueous solution. The Ni foam loaded with active material, platinum foil and a saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode. Cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) were measured using a CHI650B electrochemical workstation. Cycle life studies were conducted on an Abrin 2001A test system by the galvanostatic charge–discharge technique. The C_s of the hybrid electrodes were calculated from the 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.

Results and discussion

Formation mechanism

For the sake of pinpointing the formation process of 3D Co–Al LDH/GHA composites, the detailed preparation process of these composites is illustrated in Fig. 1a: firstly, graphene oxide was synthesized according to the procedure reported elsewhere.²⁶ After that, graphene oxide was dispersed in water by sonication, then Co(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O were slowly added to the GO dispersion to form a stable suspension. Next, the mixture underwent hydrothermal self-assembly at 180 °C for 18 h to generate 3D macroscopic graphene-based hybrid hydrogels. The hydrothermal process involves the reduction of graphene oxide to form a highly cross-linked hydrogel. Meanwhile, Co–Al LDH grew on the graphene surface to form nanosheets, and GO was simultaneously reduced to graphene and self-assembled to construct a 3D porous network. After the subsequent freeze-drying process, 2D LDH nanoplates were uniformly dispersed on the 3D network of graphene aerogel, and as a result, a 3D structure of Co–Al LDH/GHA can be completely maintained. Fig. 1b shows the amplified partial microstructure of 3D structured Co–Al LDH/GHA; in such a hybrid structure, the 2D Co–Al LDH nanosized lamellae act as an effective spacer to rationally design hybrid aerogels in order to take advantage of both graphene and 2D Co–Al LDH. It is believed that the simultaneous assembly of layers on the surface of the graphene aerogel can help to prevent restacking of the GNSs. On the other hand, the presence of graphene aerogel with a high electrical conductivity is quite favorable to facilitate electron transport in composite materials. Namely, once the electrons reach the GNS, they can quickly access the Co–Al LDH (Fig. 1b), and thereby the conductivity of the composite can be improved. As expected, the as-prepared three-dimensional 3D Co–Al LDH/GHA is able to perform with a higher capacitance than either of the individual components (GA and Co–Al LDH).

Fig. 1 (a) Schematic illustration of our designed synthesis route of 3D porous Co–Al layered double hydroxide/graphene hybrid aerogel, (b) ideal electron-transfer pathway for the prepared 3D porous Co–Al layered double hydroxide/graphene hybrid aerogel.

Structural and morphological characterizations

With the purpose of directly confirming and comparing the microstructure of 3D Co–Al LDH/GHA, pure Co–Al LDH and GA, the morphology and porous attributes of the as-fabricated 3D Co–Al LDH/GHA, pure Co–Al LDH and GA were characterized by SEM and are presented in Fig. 2. Fig. 2a and b show typical SEM images of the GA after the freeze-drying process, and it can be clearly seen that the morphology of the aerogel is quite uniform on a large scale. Closer observation shows that the aerogel exhibits a 3D network of randomly oriented sheet-like structures with wrinkled texture, and is rich in hierarchical pores with a wide size distribution. Fig. 2c is the SEM image of pure Co–Al LDH and it can be observed that it consists of a compacted lamellar structure with a regular hexagonal shape, several nanometers in thickness. Fig. 2d–f reveal panoramic SEM images of the as-fabricated 3D Co–Al LDH/GHA (Co–Al LDH/GHA-13.7%) composite material. It is noteworthy that the hybrid aerogel shows an interconnected and porous 3D architecture, in which the pore sizes are in the range from submicrometer to several micrometers. Moreover, as noticed, the Co–Al LDH nanosheets occupy most of the available surface of the graphene, giving much higher loading of Co–Al LDH nanosheets in the composite aerogels. The pore walls of the 3D porous structure consist of thin layers of stacked graphene sheets, which are very important for effective electrolyte transport and active-site accessibility. A close observation of the composite is shown in Fig. 2e and shows that the Co–Al LDH nanosheets are uniformly grown on both sides of the graphene, indicating efficient assembly between the Co–Al LDH nanosheets and graphene sheets. Co–Al LDH nanosheets bridge the defects for electron transfer as well as increase the layer spacing between graphene sheets, resulting in improved electrical conductivity of the pure Co–Al LDH. In addition, with the purpose of confirming the effects of GA content on the morphologies of the Co–Al LDH/GHA composites, SEM images of Co–Al LDH/GHA-15.2% and Co–Al LDH/GHA-10.6% have also been inspected and the corresponding images are shown in Fig. S1.† As shown in Fig. 2d–f and S1,† with a decrease in GA content, the 3D structure of the Co–Al LDH/GHA composites becomes more and more unobvious, and with an increase in Co–Al LDH content, obvious aggregation appeared.

Fig. 2 SEM images: (a and b) GA, (c) pure Co–Al LDH, (d–f) 3D Co–Al LDH/GHA.

To further check the possibility of a porous structure and to determine the surface area of various samples, the N₂ adsorption–desorption isotherm and the Barrett–Joyner–Halenda pore size distribution plots of 3D Co–Al LDH/GHA, pure Co–Al LDH and GA are shown in Fig. 3. It can be clearly seen that there are a lot of mesopores in the 3D Co–Al LDH/GHA and the pore size determined by the BHJ plots lies in the range of 15–80 nm. Moreover, the BET surface area for the 3D Co–Al LDH/GHA composite was 80 m² g⁻¹, lower than 200 m² g⁻¹ for the GA, but much higher than 20 m² g⁻¹ for the pure Co–Al LDH, which indicated that after the freeze-drying process, more mesopores exist in the 3D Co–Al LDH/GHA and its surface area is larger than other similar materials.²⁷ Furthermore, it is noticeable that these pores allow most of the graphene sheets to be exposed to the electrolyte and provide open channels for electrolyte transport, hence benefiting the electrochemical performance of the 3D Co–Al LDH/GHA.

Fig. 3 The gas (N₂) adsorption–desorption isotherm loop and the pore size distribution data for (a) graphene aerogels, (b) pure Co–Al LDH, (c) 3D Co–Al LDH/GHA.

XRD is a powerful and essential apparatus for evaluating the structure of 3D Co–Al LDH/GHA. Fig. 4 displays typical XRD patterns of various samples. For the as-fabricated 3D Co–Al LDH/GHA and pure Co–Al LDH, the XRD patterns are identical to the report elsewhere, and all the diffraction peaks can be indexed as a rhombohedral structure (LDH).³⁰ The broad XRD peaks of the resulting GA indicate the poor ordering of the graphene sheets along their stacking direction and reflect that the framework of the aerogel is composed of few-layer stacked graphene sheets. It is worth noting that for 3D Co–Al LDH/GHA, no apparent diffraction peak could be identified at 20–30°, indicating that Co–Al LDH was efficiently deposited on the graphene surface, suppressing the stacking of graphene layers.

Fig. 4 A comparison of the XRD patterns: (a) 3D Co–Al LDH/GHA, (b) pure Co–Al LDH and (c) GA.

Raman spectroscopy is a nondestructive approach for characterizing graphitic materials, therefore, to further characterize and determine the ordered and disordered crystal structures of the as-prepared samples, Raman spectroscopy of 3D Co–Al LDH/GHA, pure Co–Al LDH, GA and graphite oxide were measured and are shown in Fig. 5. The Raman spectrum of GO (Fig. 5a) shows two main peaks: a D band at about 1352 cm⁻¹ assigned to a breathing mode of k-point phonons of A_1g symmetry and a G band at about 1590 cm⁻¹ ascribed to the E_2g phonon of the C sp² atoms.³¹ The D-band corresponds to the defects induced in the graphitic structure and functional groups attached to the surface of the graphene sheets, while the G-band corresponds to the in-plane vibrations of the graphitic structure. Compared to graphite oxide, an increased I_D/I_G intensity ratio for the GA is observed; the higher I_D/I_G ratio for the GA is indicative of the removal of the oxygen functional groups from GO, and also indicates a decrease in the size of the in-plane sp² domains. As displayed in Fig. 5c, two strong peaks in the Raman spectra are located at 497 cm⁻¹and 572 cm⁻¹ which are typically assigned to Co–Al LDH.³² Raman spectroscopy of the as-fabricated 3D Co–Al LDH/GHA (Fig. 5d) reveals that the D-band and the G-band located at around 1340 cm⁻¹ and 1587 cm⁻¹ exhibit a ratio of integrated peak intensities (I_D/I_G) of 1.034.

Fig. 5 A comparison of Raman spectra: (a) graphite oxide, (b) GA, (c) pure Co–Al LDH, (d) 3D Co–Al LDH/GHA.

Fig. 6 shows the XPS spectra of various as-prepared samples, and as indicated in Fig. 6a, in contrast to GO and GA, the obvious peaks of Co 2p and Al 2p appear in the full XPS spectrum of 3D Co–Al LDH/GHA, suggesting the presence of Co–Al LDH. The C 1s XPS spectrum of the graphene oxide powder is presented in Fig. 6b and it can be decomposed into four components of carbon atoms in different functional groups: the aromatic linked carbon (C=C, 284.7 eV), the C in oxygen single bonds (C–O, 286.6 eV) the carbonyl C (C=O, 288.4 eV) and the carboxylate carbon (O–C=O, 289.1 eV).³³ For comparison, Fig. 6c displays the C 1s spectrum of GA; it is noticeable that GA exhibits the same oxygen functionalities in the C 1s spectrum as GO. However, compared with GO, the intensities of the peaks for GA centered at 286.0, 288.9 and 289.3 eV, especially for the intensity of the peak centered at 286.0 eV, are much smaller, whereas for the peak at 284.8 eV, the intensity is higher than that observed for GO (Fig. 6b). This implies that during the hydrothermal process, most of the oxygen-containing functional groups in graphene oxide, especially the epoxy and alkoxy groups, were removed, and most of the conjugated bonds were restored. The C 1s spectrum recorded for the 3D Co–Al LDH/GHA composite as shown in Fig. 6d has similar peak intensities to GA, suggesting that the reduction of graphene oxide is not affected by the presence of Co–Al LDH. The Co 2p core level spectrum of the hybrid aerogel is presented in Fig. 6e; the peaks located at 781.96 and 798.1 eV are well assigned to Co 2p_3/2 and Co 2p_1/2 of Co²⁺, which is further confirmed by its shake-up satellite peaks at 785.81 and 803.75 eV for Co²⁺ ions.³⁴

Fig. 6 (a) XPS survey spectrum of GA, Co–Al LDH and Co–Al LDH/GHA: (b–d) high-resolution XPS spectra of C 1s in the GO, GA and Co–Al LDH/GHA, and (e) XPS core level spectra of Co 2p.

It has been reported that graphene aerogels can be obtained by several alternative approaches, such as sol–gel, chemical reduction, hydrothermal method, etc. In the present work, the 3D Co–Al LDH/GHA composite was also obtained via a hydrothermal approach, and under such hydrothermal conditions, the graphene hybrid shows an interconnected macroporous framework of graphene sheets with a uniform deposition of Co–Al LDH, which was simultaneously formed during the reaction. It is apparent that the formation of Co–Al LDH among the graphene aerogel interlayer is a key factor allowing the GNSs to be prevented from re-stacking successfully. The above argument is strongly supported by TEM and high resolution TEM (HRTEM) observations. Fig. 7a and b are low magnification TEM images of graphene aerogel; it is found that the graphene sheets stacked densely to form a wrinkled shell surface, and graphene sheets interconnected to form a porous part. The ribbon-like graphene sheets in Fig. 7c show that the porous walls of GA consist of stacked graphene sheets in few layers. Fig. 7d–f show TEM images of a typical nanosized Co–Al LDH sample; it can be clearly observed that the LDH is in nearly hexagonal platelets with a lateral size. The clear lattice fringes and the corresponding SAED patterns in Fig. 7f demonstrate a higher crystallinity of Co–Al LDH. Fig. 7g–i present the TEM images of the as-fabricated 3D Co–Al LDH/GHA; both Co–Al LDH and graphene nanosheets are clearly observed and the 3D GA has been decorated with many Co–Al LDH nanosheets. These Co–Al LDH nanosheets have different orientations on the graphene surface: some Co–Al LDH nanosheets grow with the crystallites parallel to the graphene surface (Fig. 7g), whereas some LDH nanosheets grow with the crystallites perpendicular to the graphene surface (Fig. 7h). It can also be clearly seen in Fig. 7g that the nanosized Co–Al LDH platelets are randomly laid on the graphene nanosheets and the Co–Al LDH platelets deposited on the graphene sheets still have hexagonal shapes. It should be noted that the noticeable lattice fringes and the corresponding SAED patterns in Fig. 7i also indicate the high crystalline degree of 3D Co–Al LDH/GHA.

Fig. 7 TEM images of (a–c) GA, (d–f) pure Co–Al LDH, (g–i) 3D Co–Al LDH/GHA.

Electrochemical characterization

Cyclic voltammogram and galvanostatic discharge–charge measurements at various current densities for 3D Co–Al LDH/GHA and pure Co–Al layered double hydroxide have been investigated and compared with each other. Fig. 8 presents CV curves of 3D Co–Al LDH/GHA and pure Co–Al LDH electrodes at various scan rates ranging from 1 to 10 mV s⁻¹ in the potential window of 0.1 to 0.5 V (vs. SCE). As shown in Fig. 8, both electrodes exhibit a couple of redox peaks at the potential of 403 mV (vs. Hg/HgO) for the positive sweep process and at the potential of 336 mV (vs. Hg/HgO) for the negative sweep process, indicating the reversible conversion of Co(II)/Co(III). The potential difference between the cathodic peak and anodic peak increases with the growth of sweep rate, which is attributed to the fact that electrolyte ions are fully utilized by both the outer and inner actives sites at lower scan rates but only by outer actives sites at higher scan rates in the redox reactions.³⁵ However, it should be noted that the integral areas of the CV curves at different sweep rates for 3D Co–Al LDH/GHA are much larger than that for the pure Co–Al LDH, suggesting the higher electrochemical capacitance of 3D Co–Al LDH/GHA (Fig. 8a and b). This phenomenon is more obvious at the high sweep rate of 10 mV s⁻¹, indicating the higher rate ability of 3D Co–Al LDH/GHA. This point will be further confirmed by later charge–discharge investigation. The success of the high rate capability may be attributed to the fast mass-transfer within the mesoporous support enabled by the large pore volume and pore size of 3D Co–Al LDH/GHA, and to the fast charge-transfer and transport provided by the well-connected 3D conductive structure of GA.

Fig. 8 Cyclic voltammogram: (a) pure Co–Al LDH, (b) 3D Co–Al LDH/GHA.

Fig. 9a depicts the first cycle charge–discharge profiles of the pure Co–Al LDH electrode at different densities; the first discharge specific capacitances are 320, 200, 180, and 80 F g⁻¹ at 1, 2, 4, and 10 A g⁻¹. For comparison, the first charge–discharge profiles of 3D Co–Al LDH/GHA at different densities are depicted in Fig. 9b. Impressively, the hybrid aerogel electrode exhibits excellent pseudocapacitances of 640, 624, 580, 530 and 305 F g⁻¹ at 1, 2, 4, 10 and 20 A g⁻¹, implying that the specific capacitance of the hybrid material is remarkably enhanced compared with the corresponding pure Co–Al LDH and demonstrates an excellent retention rate of 82.8% for 10 A g⁻¹. If the weight percentage of graphene of 13.7% is deducted from the hybrid aerogel and only the contribution of the redox-active phase of LDH is considered, the maximum corrected capacitance is ca. 742 F g⁻¹ at 1 A g⁻¹ and 614 F g⁻¹ at 10 A g⁻¹. At even higher 20 A g⁻¹, it can still reach 353 F g⁻¹, far superior to that of pure LDH (Fig. 9a). As exhibited in Fig. 9c, the specific capacitances of both samples gradually decrease with the increase of the current density. However, the capacitances of the 3D hybrid material are much higher than pure Co–Al LDH at various constant current densities, especially the high rate performance. In order to ensure the effects of GA content on the electrochemical performance of the Co–Al LDH/GHA composites, the charge–discharge curves of Co–Al LDH/GHA-15.2% and Co–Al LDH/GHA-10.6% at various current densities have also been measured and are exhibited in Fig. S2.† When compared with the target 3D Co–Al LDH/GHA, further increase or decrease of GA loading both result in a decrease of the specific capacitance. When the loading content of GA in the Co–Al LDH/GHA is too much (Co–Al LDH/GHA-15.2%), the pseudocapacitance, which is produced from the Co–Al LDH, of the composite obviously decreases; as a result, the Co–Al LDH/GHA shows a low specific capacitance. When the loading content of GA in the Co–Al LDH/GHA is too little (Co–Al LDH/GHA-10.6%), it does not favor the formation of effectively dispersed Co–Al LDH layers on the GA and the 3D structure becomes unobvious, which results in the low utilization of Co–Al LDH. That is to say, if the loading content of GA in the Co–Al LDH/GHA is suitable, the structural advantage of the composite is apparent, and as a result, the electrochemical properties of the composite will obviously improved. Therefore, the superior performance of Co–Al LDH/GHA may benefit from its open and porous 3D network structure which obviously increases the contact area between the electrode materials and the electrolyte and consequently makes full use of the contribution of the electrochemically active material to the overall capacitance. Furthermore, the cycling properties of the 3D Co–Al LDH/GHA electrode were carried up to over 10 000 cycles at 20 A g⁻¹, as shown in Fig. 9d. It is found that the present hybrid aerogel exhibits an excellent cycle life over all cycle numbers; the specific capacitance loss after 10 000 cycles is only ca. 3%, which further reflects the outstanding cycling stability of the 3D Co–Al LDH/GHA electrode.

Fig. 9 The first cycle discharge–charge profiles of (a) pure Co–Al LDH, (b) 3D Co–Al LDH/GHA, (c) the specific capacitance of the sample at various discharge current densities, (d) cyclic performance of 3D Co–Al LDH/GHA at 20 A g⁻¹.

To further investigate the electrochemical performance of 3D Co–Al LDH/GHA, the electrochemical impedance of the Co–Al LDH and 3D Co–Al LDH/GHA were measured in the frequency range of 100–0.1 kHz at open circuit potential with an AC perturbation of 5 mV (Fig. 10). Measured impedance spectra were analyzed using the CNLS fitting method based on the equivalent circuit,^36,37 which is given in the inset of Fig. 10. In the equivalent circuit, R_s is the resistance related to the ionic conductivity of the electrolyte and electronic conductivity of the electrodes and current collectors, C_DL is the double-layer capacitance on the grain surface, and R_F in parallel with C_DL is mainly associated with the faradic reactions. C_L is the limit capacitance. The values of R_s, C_DL, R_F, and C_L calculated from CNLS fitting of the experimental impedance spectra for pure Co–Al LDH and 3D Co–Al LDH/GHA are presented in Table 1. Obviously, the value of various kinds of capacitance calculated from the equivalent circuit for 3D Co–Al LDH/GHA are higher than those of pure Co–Al LDH. This may be attributed to the graphene aerogel in 3D Co–Al LDH/GHA overlapping to form a conductive network through sheet plane contact, which facilitates the fast electron transfer between the active materials and the charge collector.

Fig. 10 Nyquist plots of pure Co–Al LDH and 3D Co–Al LDH/GHA.

Table 1 Calculated values of R_s, C_DL, R_F, and C_L through CNLS fitting of the experimental impedance spectra based upon the proposed equivalent circuit in Fig. 10

	Rs	CDL	RF	CL
Co–Al LDH	0.715	0.001913	0.5336	0.2789
Co–Al LDH/GHA	0.7138	0.00259	0.2712	0.3284

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Conclusions

In summary, we have initially fabricated a 3D Co–Al layered double hydroxide/graphene hybrid aerogel with a robust interconnected 3D network. Advantageous structural features including a good 3D mesoporous network, high specific surface area, high pore volumes, and high electric conductivities of the GA make it an excellent host for Co–Al LDH. The hybrid aerogel exhibits high discharge capacity, excellent rate capability, and good cycling stability, due to its improved electrical conductivity and fast charge transport, that all benefit from its novel 3D porous network. We believe that our present synthetic strategy can be further extended to develop other 3D layered double hydroxide/graphene aerogel materials for various applications, such as sensors, batteries, and supercapacitors.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (no. 51102010, 21336003, 21371021), the Program for New Century Excellent Talents in University of China (NCET-12-0758) and Science and Technology Commission of Shanghai Municipality (no. 14DZ2261000).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00103j

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