Novel synthesis of RGO/NiCoAl–LDH nanosheets on nickel foam for supercapacitors with high capacitance

Abstract

A novel nano-flower RGO/NiCoAl–LDH composite was fabricated through a smart binder-free hydrothermal process, for advanced electrochemical energy storage in supercapacitors. The morphology of the nano-flower structure was self-assembled using several nanosheets vertically interconnected with each flake, and the as-obtained nano-flower structure of the porous nanosheet morphology greatly enhanced the capacitance and the rate capacity of the electrode material for supercapacitors. When the composite was used in an electrode, the sample with 6.36 mg graphene oxide revealed surprising and optimized high performance. There was ultrahigh specific capacitance of 4732 F g⁻¹ at 1 A g⁻¹, and 4068 F g⁻¹ at 4 A g⁻¹, showing a good performance. The results exhibited the synergy and advantages of RGO with NiCoAl–LDH on a nickel foam carrier with nano-flower morphology. The results exhibit the superiority of the modified active metal substrate and elaborate graphene's influence on the material morphology. This will open up broad prospects for the future field of electrode materials for supercapacitors.

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1. Introduction

Over recent decades, because of serious environmental issues and reduction of fossil resources, energy-storage devices have been valued by many scholars.^1–5 Supercapacitors, as an emerging type of energy storage device, have received tremendous attention due to their many prominent advantages such as fast charging and discharging, great power density, super stability, as well as low-cost.^6,7 Hence, supercapacitors have been regarded as the most promising and potential energy-storage systems in today's world.^8–10 Based on the charge-storage mechanism, supercapacitors can be categorized broadly into two different types: one is electrical double-layer supercapacitor which is mainly determined by either electrostatic charge diffusion or ion adsorption; the other is pseudocapacitor which is subjected to faradaic redox reactions at the interface of the electrode.^3,11,12 In electrochemical field, among a wide range of explored oxide and hydroxide material, LDH containing transition metals are treated as a kind of promising material due to their enrich slabs, electrochemically active sites and versatility in the compositions.^13,14 However, the poor electric performance resulting from low electrical conductivity (∼10⁻⁷ S cm⁻¹) have been the inevitable shortcoming. Therefore, designing the outstanding composite, which is consisted of the doped conductive material and LDH, is a necessary work.

Graphene which has very high conductivity (26 000 S cm⁻¹) and high specific surface area (2630 m² g⁻¹, theoretical value) can provide good electron transfer paths and can facilitate the dispersing of LDHs to avoid agglomerating.^14–19 Furthermore, high conductivity with direct and reliable contact enhances the performance of LDH in supercapacitors and further develops its applications.^19–21 So far, graphene-based supercapacitors have shown a very good prospect in the application of supercapacitors. Qiu et al. doped different kinds of nano-carbon into NiCoAl–LDH and discussed the role of nanostructured carbon in terms of the pore structure and electrical conductivity. The results show that the existence of CNT and graphene is more beneficial to improve their electrochemical performance. The nanohybrid electrode (LDH-CNT/RGO) displays the highest specific capacitance of 1188 F g⁻¹ at a current density of 1 A g⁻¹ and 850 F g⁻¹ at 10 A g⁻¹ (72% of the initial capacitance).¹²

Nickel foam, as a commercial material with high electronic conductivity and excellent 3D structure, has become the remarkable substrate material for electrode. It would not only reduce the diffusion resistance of electrolyte but also facilitate ion transportation and maintain electron pathways smoothly during the very rapid charge–discharge reactions. When the nickel foam is used as active substrate material, RGO/NiCoAl–LDH composite film can be in situ formed on the nickel foam surface. Nickel foam pore induce hydrotalcite crystal grown oriently.^22,23 We believe that such combination not only simplifies the preparation process which is fussy and time-consuming, but also enhances the contact and adhesion between the LDH and RGO in the composite.^24,25 Guo et al. fabricated Ni(OH)₂/RGO on nickel foam though hydrothermal process and the hybrid has a super specific capacitance of 3328.7 F g⁻¹ at 2.2 A g⁻¹.²⁵

In recent years, our team members have done a lot of work in the field of supercapacitor, especially on the research of nano-carbon reinforced electrode material. Zhu et al. have prepared a novel Ni/NiO/graphene and it delivered high specific capacitance of 1027.27 F g⁻¹ at a current density of 2 A g⁻¹ and 720 F g⁻¹ at 20 A g⁻¹.³ Zhao et al. prepared the composite material consisting of carbon-coated LiFePO₄ and RGO sheets possesses a unique and effective three-dimensional “sheet-web” structure using an ultrasonic-assisted rheological phase method, the sample showed an excellent cycling stability with only about 10% capacity decay at 10C after 1000 cycles.² Peng et al. were synthesized active carbon with the high BET surface area (2457 m² g⁻¹) and its specific capacitance can remain 95% after 5000 cycles.²⁶

In this work, nickel foam was employed as an active substrate to prepare RGO/NiCoAl–LDH composite by a facile binder-free hydrothermal process for electrode material of capacitors, and the influence of the content of graphene oxide on the morphology and electrochemical performance were further discussed. As electrode materials for supercapacitors, the hybrids show a high specific capacitance and relatively good cycling stability, which suggest their potential application in supercapacitors.

2. Experimental

2.1 Materials preparation

Graphite oxide (GO) was purchased from Chengdu Organic Chemical Co., Ltd. Chinese Academy of Sciences.

The nickel foam (NF) (1 cm × 2 cm) was cleaned carefully by using an absolute ethyl alcohol solution with sonication for 2 h, then dried at 60 °C for 12 h in the oven. The synthesis of NiCoAl–LDH/RGO composite was carried out through a one-step hydrothermal process. The typical preparation process is clarified as follows: first, a certain amount of graphene oxide (GO) was uniformly dispersed in 40 mL of deionized water by ultrasonication for 2 h, then Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O (the metal ion molar ratio 2 : 1 : 1) were dissolved in above solution with string 1 h, finally urea was dissolved in the above solution to obtain a mixed solution with the total metal ion and urea concentrations of 12 and 36 mM. The NF substrate was then immersed in this aqueous solution. The mixture was loaded into a 50 mL Teflon-lined stainless steel autoclave for hydrothermal reaction at 100 °C for 24 h. The amount of added GO was 4.26 mg, 6.36 mg, 8.48 mg, 10.6 mg, the corresponding hybrids growing on the nickel foam were remarked as RGL-1@NF, RGL-2@NF, RGL-3@NF, RGL-4@NF respectively. The residual precipitant were also collected, labelled as RGL-2/NF. The final product was washed with water and ethanol several times. The as-prepared product was dried at 60 °C in oven for the whole night.

For comparison, the composite which doped 6.36 mg GO was also synthesized under the identical conditions but without NF and was noted RGL-2.

2.2 Characterization

The microstructure and surface morphology of RGO/NiCoAl–LDH@NF hybrid were characterized by transmission electron microscopy (TEM, JEOL, 2100F) and scanning electron microscopy (SEM, Hitachi, S3400, Japan). The crystallite structure was analyzed by X-ray diffraction (XRD, DX-2700, China) using Cu Kα radiation. IR spectra were obtained on a Fourier transform infrared (FT-IR, Bruker Tensor, Germany) spectrometer in the range from 500 to 4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was measured on a Kratos AXIS Ultra DLD spectrometer and the XPS spectra were corrected by the C 1s line at 284.6 eV.

2.3 Electrochemical characterization

The electrochemical measurements were conducted using a standard three-electrode system in 6 M KOH aqueous solution. The as-prepared composite powder was mixed with acetylene black (AB) and a binder (polyvinylidene fluoride) in a weight ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidone (NMP), and the suspension was uniformly coated on a NF substrate with an area of 1 × 1 cm² and then dried at 60 °C in an oven for the whole night. The other composite that with an approximate area of 1 × 2 cm² were directly used as working electrodes, while a Hg/HgO electrode as the reference electrode and graphite electrode as the counter electrode, respectively. Cyclic voltammograms (CV), galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS) measurements were carried out using an electrochemical workstation (CHI660e, Shanghai).

3. Results and discussion

3.1 XRD patterns, IR spectra results of samples

The typical XRD patterns of pure GO, the reduced GO, RGL-2 and RGL-2@NF are shown in Fig. 1. As is shown in Fig. 1(b), it is noticed that a conspicuous peak centred at 2θ = 10.2° is belonged to the (002) reflection of GO. Fig. 1(a) for RGO consists of a broad diffraction peak around at 2θ = 21°, which is corresponding to (002) reflection of RGO. While the GO diffraction peak completely disappeared after the reaction, demonstrating that the GO can be reduced in the presence of urea.¹² The interlayer spacing of 0.42 nm of RGO obtained by XRD results is higher than that of pristine graphite (0.34 nm). The increased d-spacing reveals the fact of existing oxygen functional groups, that is to say, GO is partial reduced to RGO.²⁷ The broad peak demonstrates the presence of single or few layer disorder graphene sheet.³ The graphene content in the RGL-2 calculated based on DTA and TG measurements (as shown in Fig. S5†) is about 14.8 wt%. Then we know the percentage of NiCoAl–LDH in the composite is 85.2 wt%.^27,58 The diffraction patterns of RGL-2 Fig. 1(c) and RGL-2@NF Fig. 1(d) represented a classic hydrotalcite-like structure, exhibiting reflections of (003), (006), (012) and (110) crystal face of hydrotalcite.^12,28 The basal spacing value of as-prepared sample is 0.765 nm, which is ascribed to the carbonate-intercalated LDH materials.^8,29 For RGL-2 Fig. 2(c) and RGL-2@NF composites Fig. 2(d), no characteristic (002) peak of graphene are observed. This phenomenon indicates that RGO exists on the hybrid without agglomeration or the content of grapheme is too low to be detected. Compared to RGL-2, the powder which scraped from RGL-2@NF exhibits better crystallinity. The XRD results suggest that urea can promote the conversion of GO into RGO and nickel foam have a positive effect on crystallinity of hydrotalcite.

Fig. 1 XRD patterns of (a) RGO powder, (b) pure GO, (c) RGL-2 composite, and (d) RGL-2@NF.

Fig. 2 FT-IR spectra of GO, RGO, NiCoAl–LDH, RGO/NiCoAl–LDH.

The surface functional groups of the corresponding samples and the effect of urea in the hydrothermal process are further confirmed by the Fourier transform IR (FT-IR) spectra. For GO, the bands at 1056 cm⁻¹ and 1733 cm⁻¹ are belonged to the stretching vibrations of C–O in alkoxy and C=O in carboxylic acid.^3,30 The bands at 1216 cm⁻¹ and 1623 cm⁻¹ are the bending vibration from hydroxyl groups and the C=C stretching vibrations.^31,32 Compared with the peak intensities of GO, the stretching vibration of C–O, C–O–C and C=O become weaker in RGO/LDH, it demonstrates that the most of GO in the hybrid have been reduced to RGO. At the same time, the peak intensities of –OH (around 3402 cm⁻¹) in the RGO/LDH is stronger than RGO and the shape of peak is consistent with LDH's, indicating urea both has played a vital role in the reduction reaction of GO, and acts as the precipitant in the reaction process. Characteristic bands of the –OH stretching vibrations (the –OH groups are in the water molecules and the hydrogen bonding among the hydroxyl groups in the sample) located at around 3400 cm⁻¹, companied with the bending vibrations at 1633 cm⁻¹.³³ The strong and sharp bands (1366 cm⁻¹ and 823 cm⁻¹) in NiCoAl–LDH can be attributed to the bending vibrations of carbonate ions intercalated in the lamellar structure.⁷ The bands below 1000 cm⁻¹ are assigned to the translational modes of Co–OH, Al–OH and M–O–M bands.^34–37 With the addition of RGO, the peak of 1366 cm⁻¹ become weak, implying carbonate ions in the partial interlayer space are replaced by graphene flakes.

3.2 Ultrahigh capacitance of the samples

CV curves of RGL-2@NF and RGL-2 recorded at a scan rate of 5 mV s⁻¹ are shown in Fig. 3(a). Obviously, it can be seen that each CV curve exhibits a pair of strong and symmetric redox peaks, indicating that the capacitance feature is determined by pseudocapacitance behavior. As is well known, the integral area of the CV curves can be used to estimate the capacitance of an electrode material.^27,38,39 It is evident that the area under the CV curves for RGL-2@NF film is much larger than that of composite without NF, demonstrating the higher specific capacitance of RGL-2@NF. Furthermore, it is observed that the start potential of the oxidation peak of the RGL-2@NF hybrid shifts towards the negative direction compared to that of the RGL-2. The shifts of the onset potential of oxidation peak exhibits that a decreased redox energy barrier for the Ni(OH)₂/NiOOH and Co(OH)₂/CoOOH reactions, which indicates the improvement in the reaction kinetics and the better electrochemical activities of the hybrid.⁴⁰ Fig. 3(b) shows the galvanostatic charge–discharge characteristics of RGL-2 and RGL-2@NF that performed within a potential range of 0 to 0.45 V at constant current density of 1 A g⁻¹. The capacitance of the RGL-2@NF is thus calculated to be 4732 F g⁻¹, higher than that of the powder without NF (893 F g⁻¹). This capacitance is one of the higher value reported up to now. The ever reported the higher capacitance value was 4172.5 F g⁻¹ for α-Ni(OH)₂ by using microwave-assisted method. Li et al. were synthesized α-Ni(OH)₂ nanosheets, it exhibits a specific capacitance of 4172.5 F g⁻¹ at 1 A g⁻¹ which are larger than theoretical value. Mainly due to pseudocapacitance and high specific surface area supply an additional boost to the value.⁴¹ Guo et al. design a unique structure of Ni(OH)₂, it delivered a specific capacitance of 3328.7 F g⁻¹ at 1.5 A g⁻¹. The great enhancement is ascribed to the vertically aligned scaffold and porous channels of the Ni(OH)₂ nanosheets which make it possible to expose almost all of the surface, providing plentiful electroactive. In our work, large specific capacitances were contributions from the synergistic effect among the NiCoAl–LDH composite, graphene and nickel foam in the hybrid electrode. Besides graphene with high conductivity as a conducting building or scaffold can further increase the electrical conductivity of the nanohybrids, thus promoting the fast charge transfer.²⁵

Fig. 3 CV curves of (a) RGL-2 and RGL-2@NF hybrid at a scan rate of 5 mV s⁻¹. (b) Charge–discharge curves of RGL-2 and RGL-2@NF at 1 A g⁻¹.

The CV curves of the RGL-2@NF at different potential sweep rates are shown in Fig. 4(a). A symmetric shape during the charge–discharge process is observed at a low scan rate. Curves are out of shape as the scan rate increases, which is explained by the mechanism of electrochemical reactions that at low scan rates, the movement of OH⁻ ions is slower than that at high scan rates, and the active materials have a higher utilization ratio. Most of the CV curves are symmetric which suggest the fine reversibility of the oxidation and reduction processes. Co(OH)₂ and Ni(OH)₂ may happen during the electrochemical reactions according to the following reactions.^5,16,30–44

Co(OH)₂ + OH⁻ → CoOOH + H₂O + e⁻

CoOOH + OH⁻ → CoO₂ + H₂O + e⁻

Ni(OH)₂ + OH⁻ → NiOOH + H₂O + e⁻

Fig. 4 (a) CV curves of the RGL-2@NF nanosheets at various scan rates; (b) galvanostatic charge–discharge curves of RGL-2@NF electrode measured at various current densities.

Fig. 4(b) shows the galvanostatic charge–discharge property of RGL-2@NF composites. CV and GCD curves of the other hybrids are presented in Fig. S1–S4.† The specific capacitance C_s (F g⁻¹) can be calculated as the following equation.^16,44,45

C_s = C/m = (I × Δt)/(m × ΔV)

where ΔV (V) is the potential window; I (A) is the charge–discharge current; Δt (s) is the discharge time; and m (g) is the weight of active material. The result are plotted in Fig. 5. Form its histogram, we can clearly see that the value of capacitance of RGL-2@NF is much higher than that of RGL-2 at various current density. What’s more, the RGL-2@NF nanosheets show 84.3% capacitance retention from 0.5 A g⁻¹ to 4 A g⁻¹. More details about the two samples' capacitance at different current density are listed in Table S1.† The data of the residual precipitate is listed in Table S2.†

Fig. 5 Histogram that marked with the specific capacitance and the capacitance retention.

In order to investigate the impact of GO content, we have an electrochemical activity test for RGL-1@NF, RGL-2@NF, RGL-3@NF and RGL-4@NF. Galvanostatic charging–discharging at constant current density of 1 A g⁻¹ is an effective method for exploring the influence of different graphene oxide content on their electrochemical properties. All the curves display a wide discharge plateau corresponding to the Faraday process. Fig. 6(a) shows that RGL-2 has a lower charge plateau, suggesting less electrochemical polarization.⁴⁶ It can be clearly seen that the charge/discharge time varies with GO content at the same current density, and the RGL-2@NF (the mass of GO is 6.36 mg) display the longest discharge time, implying the highest specific capacitance. This is corresponding to the result of CV curve which has the largest integral area.

Fig. 6 (a) Galvanostatic charge–discharge curves of RGL-1@NF, RGL-2@NF, RGL-3@NF and RGL-4@NF. (b) CV curves of RGL-1@NF, RGL-2@NF, RGL-3@NF and RGL-4@NF.

3.3 SEM/TEM results and formation mechanism

Fig. 7(a) exhibits that the nickel foam has a 3D, cross-linked grid structure, which would offer a rich porosity structure and a high specific surface area. Fig. 7(b) shows the growth of nanosheets on nickel foam edge at low magnification. As is shown in Fig. 7(c) and (d), we can observe that the NiCoAl–LDH flakes efficiently and uniformly grow on the NF and the RGO covers its surface to form an ordered and diverse structure. We can clearly see that the RGL-2 powder displays a random, uneven distribution and quite fat flakes, which mainly attribute to the aggregated and crumpled RGO nanosheets Fig. 7(e) and (f). While the RGL-2@NF composite owns extremely homogeneous porous structure, so it is apparent that the presence of NF significantly influences the surface morphology of the RGO/NiCoAl–LDH flakes and facilitates the formation of conductive network structures. Associated with the favourable morphology changing, the electrochemical capacity performance of RGL-2@NF is improved greatly. The conclusions are consistent with CV and GCD test results.

Fig. 7 Scanning electron microscope (SEM) images of (a–d) the RGL-2@NF, and (e and f) RGL-2.

We further compared the difference of the hybrids with different GO content. The typical scanning electron microscopy (SEM) images Fig. 8(a–d) can be clearly seen that well-defined nanosheets perpendicularly grown on the RGO surface. This morphology is in line with the other results reported in the literature. Song et al. prepared NiCoAl–LDH@RGO with a sandwich-like structure,⁴⁵ but the difference is that we can obtain a flower-like structure by controlling the content of GO. The morphologies of the staggered accumulation of nanosheets vary with the addition of different GO content. When the addition of GO amount is 4.26 mg, big and random sheets intersect in different directions. So it only has the small specific surface area Fig. 8(a). Upon increasing GO amount to 6.36 mg, uniform vertical nanosheets can be fabricated. As increasing the GO content, GO can induce LDH sheets jumping to the same direction and develop flower-like structure by the type of side by side in each area. This result was observed serendipitously and underscored that GO content could profoundly affect the morphology, structure and electrochemical activity of the electrode materials.

Fig. 8 SEM images of (a) RGL-1@NF composite, (b) RGL-2@NF composite, (c) RGL-3@NF composite, (d) RGL-4@NF composite.

The morphology and structure of NiCoAl–LDH deposited on 3D graphene are shown in Fig. 9. The TEM images Fig. 9(a) and (b) show interconnected dark lines on graphene, which suggests the formation of vertically aligned interconnected NiCoAl–LDH microflakes on the graphene surface clearly. In TEM image Fig. 9(b), homogeneous and thin nanosheets of NiCoAl–LDH depict the formation of LDH over the graphene sheets. However, RGL-2 of nanosheets which didn't dope nickel foam in the preparation process display disorder and thick. The phenomenon is corresponding to the results of SEM Fig. 7(c) and (d). When there is abundant amount of GO sheets in the solution, LDH nanosheets tend to be vertical to the GO plate to blanket its surfaces, which presents a 3D structure.⁴⁴ Favourable morphology with evenly distributed and good porosity would lead to a larger specific surface area, which provides the structural foundation for the high specific capacitance Fig. 9(b). The 3D structure of NiCoAl–LDH together with graphene could provide a large accessible surface area, which play a vital role in enhancing the supercapacitor performance. The HR-TEM images of the RGL-2@NF reveal the combing effect between NiCoAl–LDH nanoparticles and graphene. The visible lattice fringes with a spacing of about 0.38 nm, attributing to the (006) planes of the NiCoAl–LDH phase, and 0.23 nm corresponding to the (009) planes. Furthermore, the NiCoAl–LDH nanoparticles targeted grow on the graphene which were observed in Fig. 9(d), indicative of a plate-like structure for small-sized nanoparticles. In addition, the scanning TEM (STEM) image of the RGL-2@NF hybrids and the corresponding elemental mapping from the rectangle region marked in Fig. 9(d) further reveal NiCoAl–LDH vertical loading on graphene surfaces and intimate interaction between them.^47,48

Fig. 9 TEM and HR-TEM images of samples: (a and b) TEM images of the RGL-2 and RGL-2@NF. (c) HR-TEM image of the RGL-2@NF. (d) STEM image of the RGL-2@NF hybrid and the corresponding elemental mapping of C, Ni, Co from the square region marked in (d).

In order to describe the fabrication process of the porous nano-sheet RGL@NF, the detailed preparation process of the composite is illustrated in Scheme 1. First, GO was dispersed in water by sonication uniformly, then a reasonable amount of Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O were slowly added into the GO suspension. Due to abundant negatively charged oxygen-containing functional groups attaching on the GO surface, such as –OH, –COOH, which can absorb metal ions depending on the electrostatic force.^49,50 When the metal ions were added into the GO suspension, metal ions were adsorbed onto the surface of GO.⁵¹ Second, the above solution was heated to 100 °C and kept for 24 h in oven. During this process, the urea was decomposed gradually to provide OH⁻ and alkaline environment for the formation of NiCoAl–LDH crystal on the GO surface. Subsequently, the initial LDH crystal seeds on the GO surface grew gradually to nanosheets and GO was simultaneously reduced to graphene. With reaction time increasing, hundreds of self-assembled micro-petals interconnected with several flakes, resulting in the 3D flower-like nanosheets morphology. Obviously, the reduced graphene oxide plays a vital role in self-assembling the vertically aligned LDH nanoflakes. Meanwhile nickel foam cleverly offered a directional template to make for uniform anchoring of the RGL-2 array onto the surface of the NF, thereby fabricating a three-dimensional (3D) porous diverse structure.

Scheme 1 Illustration for the possible formation of the RGL@NF.

The surprising performance of aligned growth RGO/NiCoAl–LDH nanosheets on nickel foam can be attributed to the following aspects: ahead, the nanosheets with network structures can make the electrode material expose more inner active sites;³⁷ second, compared with the thick petal-like RGO/NiCoAl–LDH, the interconnected network of RGL@NF directly oriented growing among nickel foam framework or bone, exhibits the uniform and unagglomerate nanoparticles and leads to closer electric contact of active material with conductive substrates and high utilization of active sites on the enlarged surface area.³⁶ Third, due to the 3D cross-link-grid structure, the pore fabric of electrode material is more regular and the pore number also can increase largely, the fact can be confirmed from SEM.

More details about the surface electronic states of the compositions are obtained by X-ray photoelectron spectroscopy (XPS) Fig. 10(a)–(d). The survey spectrum was performed at the region of 0–1100 eV Fig. 10(a). It mainly shows carbon (C 1s), oxygen (O 1s), nickel and cobalt species. It can be definitely observed that the sample is composed of Ni, Co, Al, C, O elements. The C 1s spectra can be divided into four peaks which are assigned to four different kinds of oxygen-containing functional groups: the carbon and oxygen single-bond (C–O, 285.5 eV), the carbon and oxygen single-bond (C–C, 284.5 eV), the carbonyl carbon (C=O, 286.6 eV), and the carboxylate carbon (O–C=O, 288.9 eV) derived from carbonate of the interlayer in the NiCoAl–LDH/RGO/NF.³⁸ In the high-resolution C 1s spectra of RGL-2 Fig. 10(b), the absorbance band intensity of C–C is strong with little existence of oxygen-containing functional groups, demonstrating the high quality graphene.⁹ This phenomenon is mainly attributed to considerable deoxygenation in the hydrothermal process.^52,53 The Ni 2p spectra shows two main peaks at 855.6 eV and 873.4 eV which are assigned to Ni 2p_3/2 and Ni 2p_1/2 respectively. Its spin-energy separation is 17.8 eV and the satellite peaks (Ni 2p_3/2, satellite: 861.7 eV, Ni 2p_1/2, satellite: 879.8 eV) Fig. 10(c). This is typical of Ni(OH)₂ structure, many previously reported data also confirmed this point.^19,37 A similar case is shown in Fig. 10(d), the binding energy at 780.5 eV, 796.5 eV are corresponding to the Co 2p_3/2, Co 2p_1/2, and its spin-energy separation is 16.0 eV. The obvious shake-up satellite peak are observed, thus confirmed the existence of Co(OH)₂. We all believe that the coexisting of these elements will improve the electrochemical performance which can be affirmed by the electrochemical test later.

Fig. 10 XPS spectra of RGL-2@NF composite: (a) survey spectrum, (b) C 1s spectrum, and (c) Ni 2p spectrum, (d) Co 2p spectrum of the RGL-2@NF composite.

So as to confirm the chemical performance of the hybrids and further explain the role of nickel foam, electrochemical impedance spectroscopy (EIS) was carried out on CHI660e. Fig. 11 shows the impedance spectra of RGL-2 and RGL-2@NF. The Nyquist plots recorded in the frequency range from 10⁵ Hz to 10⁻² Hz with an open circuit potential of 0.3 V.^16,24 The equivalent circuit for the fitting of the EIS data is obtained by Zsimp Win software. All of the plots have three regions: the x-intercept of the plot at high frequency represents the equivalent resistance (R_s);^54,55 the semicircle in high frequency region be perceived as the charge transfer resistance (R_ct) at the contact interface of the electrode–electrolyte and the nearly straight line reflect the diffusion of OH⁻ ions on the active materials.^14,56–59 As can be seen from the Fig. 11 inset, the hybrid which is aligned attracted on the nickel foam displays a smaller R_s value (0.64 Ω) and R_ct value (0.003 Ω) than the hybrid without nickel foam (R_s value: 0.64 Ω, and R_ct value: 0.003 Ω). Generally, the low charge transfer resistance of the hybrid electrode will be favourable for providing a convenient or fast pathway for ion and electron transport,^60,61 thus exhibiting the excellent electrochemical property. This is attributed to the existence of RGO.^61,62 In addition, we also observed obviously the RGL-2@NF hybrid exhibits a line that is similar to vertical in the low frequency region, demonstrated the excellent electro-chemical capacitance of the RGL-2@NF in the 6 M KOH solution. Then it directly shows that the nickel foam plays a key role in the charge–discharge process. In conclude, the nickel foam play a significant role in building a diverse structure and improving the performance in electrochemistry.

Fig. 11 EIS spectra of RGL-2 and RGL-2@NF composite electrode. The inset shows the impedance in the high frequency regions of two samples.

Table 1 lists the specific capacitance of the samples prepared at different content of GO. Obviously, its value increased when increased GO amount. However, because of large amount of GO covering its active sites, it shows bad electrochemical performance when the content of GO is excessive.

Table 1 Specific capacitance of RGL-1@NF, RGL-2@NF, RGL-3@NF, RGL-4@NF at different current density

Sample	Capacitance (F g^−1)
	0.5 A g^−1	1 A g^−1	2 A g^−1	4 A g^−1	8 A g^−1
RGL-1@NF	2013	1773	1424	897	300
RGL-2@NF	4849	4732	4462	4068	3262
RGL-3@NF	4243	4050	4068	3281	2583
RGL-4@NF	3776	3107	2292	1595	693

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4. Conclusion

The ultrahigh capacitance and good rate capability of RGO/NiCoAl–LDH@NF composite has been developed successfully by a facile, low-cost and one-step hydrothermal approach. Because of the addition of 3D structure of the nickel foam, the unique pore structure among composite facilitates electrolyte electrolytic penetration and also shortens the proton diffusion distance during charge–discharge process,^63,64 leading to a big and outstanding pseudocapacitance of 4732 F g⁻¹ at a current density of 1 A g⁻¹ and 4068 F g⁻¹ at 4 A g⁻¹. Furthermore, the morphology of RGO/NiCoAl–LDH@NF can be controlled by adjusting graphene oxide content. Therefore, large specific capacitance value and high rate capability of the RGO/NiCoAl–LDH@NF composite is a very great spot for the developing of electrode material in supercapacitors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (# 21476145). The authors thank Wei Min and Zhu Peng for the discussions and helps.

Notes and references

1. Q. Wang, J. Yan and Z. J. Fan, Energy Environ. Sci., 2016, 9, 729.
2. B. Wang, D. L. Wang and X. S. Zhao, J. Mater. Chem. A, 2013, 1, 135.
3. Y. C. Zhu, W. Chu and T. Lin, RSC Adv., 2015, 5, 77985.
4. H. Jiang, T. Zhao and J. Ma, J. Mater. Chem., 2011, 21, 3818.
5. J. Pu, Y. Tong and Z. H. Wang, J. Power Sources, 2014, 25, 250.
6. J. T. Zhang, S. Liu and X. P. Gao, J. Mater. Chem. A, 2014, 2, 1524.
7. Y. Li, L. Cao and Y. Zhang, J. Mater. Chem. A, 2014, 2, 6540.
8. Y. W. Zhu, S. T. Murali, M. D. Stoller and R. S. Ruoff, Science, 2013, 332, 1537.
9. J. Y. Ji, L. L. Zhang and R. S. Ruoff, ACS Nano, 2013, 7, 6237.
10. B. Wang, Q. Liu, Z. Y. Qian, X. F. Zhang and L. H. Liu, J. Power Sources, 2014, 246, 747.
11. H. B. Li, M. H. Yu and G. W. Yang, Nat. Commun., 2013, 4, 1894.
12. C. Yu, J. Yang and J. S. Qiu, Nanoscale, 2014, 6, 3097.
13. P. Syedvali, G. Rajeshkhanna and P. Justin, RSC Adv., 2015, 5, 38407.
14. Y. Bai, W. Q. Wang and L. Gao, J. Mater. Chem. A, 2015, 3, 12530.
15. Z. P. Sun and X. M. Lu, Ind. Eng. Chem. Res., 2012, 51, 9973.
16. S. D. Min, C. J. Zhao and X. Z. Qian, Electrochim. Acta, 2014, 115, 155.
17. F. Zhang, T. F. Zhang, X. Yang, Y. Huang and Y. S. Chen, Energy Environ. Sci., 2013, 6, 1623.
18. S. Bose, T. Kuila, A. K. Mishra, R. Rajasekar, N. H. Kimc and J. H. Lee, J. Mater. Chem., 2012, 22, 767.
19. T. Zhu, J. S. Chen and X. W. Lou, J. Mater. Chem., 2010, 20, 7015.
20. Y. B. Tan and J. M. Lee, J. Mater. Chem. A, 2013, 1, 14814.
21. X. H. Cao, Z. Y. Yin and H. Zhang, Energy Environ. Sci., 2014, 7, 1850.
22. C. H. Wu, Q. Shen, R. Mi and H. Wang, J. Mater. Chem. A, 2014, 2, 15987.
23. Y. H. Li, L. J. Cao, L. Qiao and Y. H. Zhang, J. Mater. Chem. A, 2014, 2, 6540.
24. C. Z. Yuan, L. Yang and X. W. Lou, Energy Environ. Sci., 2012, 5, 7883.
25. S. D. Min, C. J. Zhao and Z. P. Guo, J. Mater. Chem. A, 2015, 3, 3641.
26. Z. Peng, Z. L. Guo, W. Chu and M. Wei, RSC Adv., 2016, 6, 42019.
27. M. D. Zhang, C. Yu, C. T. Zhao, X. D. Song, X. T. Han, S. H. Liu, C. Hao and J. S. Qiu, Energy Storage Materials, 2016, 5, 223.
28. J. Wang, J. You, Z. S. Li, P. P. Yang and M. L. Zhang, Solid State Sci., 2008, 10, 1093.
29. Y. Wimalasiria, R. Fan and L. D. Zou, Electrochim. Acta, 2014, 134, 127.
30. X. Bai, Q. Liu, H. S. Zhang, J. Y. Liu, Z. S. Li, X. Y. Jing, Y. Yuan, L. H. Liu and J. Wang, Electrochim. Acta, 2016, 215, 492.
31. H. L. Wang, H. S. Casalongue and H. J. Dai, J. Am. Chem. Soc., 2010, 132, 7472.
32. Z. C. Huang, S. L. Wang and R. Li, Electrochim. Acta, 2015, 152, 117.
33. Y. F. Dong, S. H. Liu, Z. Y. Wang, Y. Liu, Z. B. Zhao and J. S. Qiu, RSC Adv., 2015, 5, 8929.
34. J. T. Kloprogge, D. Wharton and R. L. Frost, Am. Mineral., 2002, 87, 623.
35. L. H. Su and X. G. Zhang, J. Power Sources, 2007, 172, 999.
36. Z. P. Liu, R. Z. Ma and T. Sasaki, J. Am. Chem. Soc., 2006, 128, 4872.
37. T. Zhai, F. X. Wang and Y. X. Tong, Nanoscale, 2013, 5, 6790.
38. K. C. Liu and M. A. Anderson, J. Electrochem. Soc., 1996, 143, 124.
39. G. F. Cai, J. P. Tu and X. L. Wang, Nanoscale, 2012, 4, 5724.
40. M. L. Huang, C. D. Gu and J. P. Tu, J. Power Sources, 2014, 259, 98.
41. Y. Q. Zhu, C. B. Cao, S. Tao, W. S. Chu, Z. Y. Wu and Y. D. Li, Sci. Rep., 2016, 4, 5787.
42. L. J. Xie, Z. G. Hu and K. X. Li, Electrochim. Acta, 2012, 78, 205.
43. Z. R. Chang, Y. J. Zhao and Y. C. Ding, J. Power Sources, 1999, 77, 69.
44. C. Z. Yuan, L. Yang and X. W. Lou, Energy Environ. Sci., 2012, 22, 4592.
45. P. P. Huang, C. Y. Cao and W. G. Song, J. Mater. Chem. A, 2015, 3, 10858.
46. F. L. Lai, Y. P. Huang, Y. E. Miao and T. X. Liu, Electrochim. Acta, 2015, 174, 456.
47. J. Y. Hu, G. Lei, Z. G. Lu and H. T. Liu, Chem. Commun., 2015, 51, 9983.
48. J. Yang, C. Yu, X. M. Fan, S. X. Liang, S. F. Li, H. W. Huang, Z. Ling, C. Hao and J. S. Qiu, Energy Environ. Sci., 2016, 9, 1299.
49. J. Wang, Y. C. Song and Z. H. Jiang, Energy Fuels, 2010, 24, 6463.
50. J. Wu, P. Guo and L. M. Liu, J. Mater. Chem. A, 2015, 3, 15331.
51. J. Yang, C. Yu, X. M. Fan, C. T. Zhao and J. S. Qiu, Adv. Funct. Mater., 2015, 25, 2109.
52. S. D. Min, C. J. Zhao and X. Z. Qian, Electrochim. Acta, 2015, 3, 3641.
53. S. Biswas and L. T. Drzal, Chem. Mater., 2010, 22, 5667.
54. X. Chen, X. H. Chen and S. M. Huang, J. Power Sources, 2013, 243, 555.
55. Y. F. Dong, S. H. Liu, Z. Y. Wang, Y. Liu, Z. B. Zhao and J. S. Qiu, Chem. Mater., 2016, 28, 5855.
56. S. E. Chun, S. I. Pyun and G. J. Lee, Electrochim. Acta, 2006, 51, 6479.
57. Y. Y. Horng, Y. C. Lu and K. H. Chen, J. Power Sources, 2010, 195, 4418.
58. M. Kim, Y. Hwang and J. Kim, J. Power Sources, 2013, 239, 225.
59. A. D. Jagadale and C. D. Lokhande, Electrochim. Acta, 2013, 98, 32.
60. K. S. Novoselov and A. K. Geim, Rev. Mod. Phys., 2009, 81, 109.
61. X. M. Fan, C. Yu, J. Yang, Z. Ling, C. Hu, M. D. Zhang and J. S. Qiu, Adv. Energy Mater., 2014, 7, 1401761.
62. C. H. Wang, X. Zhang, X. Z. Sun and Y. W. Ma, Electrochim. Acta, 2016, 191, 329.
63. J. Xu, S. L. Gai, F. He, N. Niu and P. P. Yang, Dalton Trans., 2014, 43, 11667.
64. G. J. Liu, B. Wang, L. Wang, Y. H. Yuan and D. L. Wang, RSC Adv., 2016, 6, 7129.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23863g

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