Self-assembled reduced graphene hydrogels by facile chemical reduction using acetaldehyde oxime for electrode materials in supercapacitors

Abstract

Self-assembled three-dimensional (3D) reduced graphene hydrogels (RGHs) were fabricated by the facile chemical reduction of a graphene oxide (GO) dispersion with ammonia using acetaldehyde oxime as reducing and doping agent. The chemical reduction of GO was confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy. The hierarchical porosity and structures of the resulting RGHs can be demonstrated by field emission scanning electron microscopy (FESEM) and N₂ sorption experiments. Benefiting from the developed porosity with micro-meso hierarchical pore texture, the specific capacitance of RGHs exhibited high specific capacitances of 230.4, 155.3, 234.2, 155.1 and 191.8 F g⁻¹ at 0.3 A g⁻¹ for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15 in 6 M KOH electrolyte, respectively. More importantly, the RGHs maintained high capacitances of 167.1, 110.4, 142.3, 106.9 and 142.3 F g⁻¹ (the retention rates are 72.5, 71.1, 60.8, 68.9 and 74.2% for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15) at a very high current density of 20 A g⁻¹, indicating good electrochemical stability and a high degree of reversibility in the repetitive charge/discharge cycling test.

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1 Introduction

Because of their several advantages such as high power density, long cycle life, pulse power supply, fast charge/discharge rates, simple principles, and low maintenance cost,^1–4 supercapacitors, which are also named electrochemical capacitors and ultracapacitors, have attracted widespread attention in recent years as a promising candidate for alternative energy storage devices. Actually, there are two kinds of supercapacitors based on the different energy storage mechanisms, electrochemical double-layer capacitors (EDLCs) and pseudo-capacitors. EDLCs store energy based on the electrostatic adsorption of both anions and cations in which the capacitance is stored as a build-up of charge in the layers of the electrical double-layer formed at the interface between a high-surface area electrode and an electrolyte; while pseudo-capacitors store energy through fast and reversible surface redox reactions, in which the capacitance is stored through a faradaic process, involving fast and reversible redox reactions between electrolyte and electro-active materials on the electrode surface.

Among the many materials used in supercapacitors, graphene has attracted considerable interest since its multiple appealing features such as high specific surface area, excellent electrical conductivity, and extraordinary chemical/electrochemical stability and mechanical flexibility, which have shown potential applications in electrochemical energy storage devices such as supercapacitors,^5–15 batteries,^16–20 fuel cells.^21–24 However, the obtained two-dimensional (2D) graphene tend to interact with each other to form irreversible aggregates or overlapping to graphitic structure due to noncovalent interactions such as π–π stacking and hydrophobic interactions between the intersheet of graphene, resulting in a dramatic decrease of the surface area.^25–36 Consequently, the intrinsic properties of the obtained graphene sheets can not be exploited thoroughly, thus inefficient utilization of graphene layers for electrochemical energy storage. In order to fully utilize the high intrinsic surface area and further explore the new functions of graphene, self-assembly of nanoscale graphene into monolithic macroscopic materials with 3D porous networks can largely translate the properties of individual graphene into the resulting macrostructures. The formation of 3D graphene network can effectively prevent graphene from restacking and retain the high specific surface area that is necessary for high specific capacitance. Additionally, with a highly interconnected graphene network for excellent electron transport and interpenetrated porous network for rapid ion transport, the 3D graphene macrostructures are ideally suited for supercapacitor electrodes. The methods to prepare 3D structures especially reduced graphene hydrogels or aerogels generally include hydrothermal reduction, hydrothermal combined with chemical reduction, chemical reduction, N-doped hydrothermal reduction, solvothermal reduction, electrochemical reduction and so on,^25–36 in which chemical reduction by using various reducing agents, such as ferrous ions,³⁷ NaHSO₃,³⁸ mercaptoacetic acid,³⁹ HI,^40,41 ammonium sulfide,⁴² hydrazine,^43,44 phenolic acids,⁴⁵ polydopamine,⁴⁶ hydroquinones,⁴⁷ paraphenylene diamine,^48,49 L-phenylalanine,⁵⁰ sodium ascorbate,^51–54 thiourea dioxide,⁵⁵ thiocarbohydrazide⁵⁶ and so on, has attracted extensive attention due to its lower temperature and shorter time in the process of preparation in contrast to other methods.

As low-toxicity, high efficiency and versatile reagents, oximes were used as oxygen scavenger in boiler water usually. Guo and coworkers have firstly prepared a highly stable graphene suspension by using dimethyl ketoxime (DMKO) as reductant, in which DMKO could be hydrolyzed in aqueous solution and slowly generates hydroxylammonium and acetone under alkaline condition at elevated temperature.⁵⁷ Therefore, GO was reduced into graphene by in situ generated hydroxylammonium, which was an appropriate reductant for the GO reduction in aqueous suspension as reported recently.^58,59 As a same reducing oxime, acetaldehyde oxime can also be hydrolyzed to hydroxylammonium and acetaldehyde, which has been also used to be oxygen scavenger in boiler water. Based on the above analysis, it was reasonably considered that the strong reducing capacity of acetaldehyde oxime in alkaline solution was attributed to the synergic effect of hydroxylammonium and acetaldehyde after hydrolysis. In present work, self-assembled 3D reduced graphene hydrogels (RGHs) were prepared in a facile chemical reduction process in GO suspension with acetaldehyde oxime as reducing agents. In the preparation process of RGHs (Scheme 1), acetaldehyde oxime and ammonia are added into the GO suspension under sonication to get a brown opaque colloidal solution. Then, the mixtures are placed in the oil bath at 90 °C without stirring for 4 h to get cylindrical RGHs. Benefiting from the abundant micro-meso hierarchical pore texture, the supercapacitors based on RGHs exhibited a high specific capacitance of 230.4, 155.3, 234.2, 155.1 and 191.8 F g⁻¹ at 0.3 A g⁻¹ for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15 in 6 M KOH electrolyte, respectively. More importantly, the RGHs can maintain a high capacitance up to 167.1, 110.4, 142.3, 106.9 and 142.3 F g⁻¹ at a very high current density of 20 A g⁻¹, the retention rate are 72.5, 71.1, 60.8, 68.9 and 74.2% for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15, respectively. It also showed that the electrode based on RGHs has good stability and high reversibility in the charge/discharge cycling test. The specific capacitance of the supercapacitor were maintained at 199.3, 133.1, 167.2, 127.4 and 163.9 F g⁻¹ (capacitance retention ∼95.3, 91.5, 91.3, 88.9 and 91.2%) after 5000 cycles at 1 A g⁻¹.

Scheme 1 Illustration of the preparation of reduced graphene hydrogels (RGHs).

2 Experimental

2.1 Materials

Graphite powder was purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. All other chemicals including acetaldehyde oxime were purchased from Sinopharm Chemical Reagent Co. Ltd. and used directly without further purification.

2.2 Preparation of GO and RGHs

GO was synthesized from natural graphite by using modified Hummer's method according to our previous work.^{55,56,60–64} The concentration of GO suspension was determined by drying small amount of suspension in a vacuum oven at 50 °C for 24 h and then weighing the dried GO. In a typical procedure, acetaldehyde oxime (30, 60, 150, 300 and 450 mg) and ammonia solution (200 μL) were added to GO suspension (15 mL) with a concentration of 2 mg mL⁻¹. Subsequently, the mixture was sonicated to form a homogeneous solution and then placed in an oil bath at 90 °C for 4 h without stirring to get the reduced graphene hydrogels, which were labelled as RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15, respectively. After the solution was naturally cooled down to room temperature, the resulting cylindrical structure of the hydrogels were washed with water for several times to remove residual impurities until a neutral pH of the filtrate was obtained and then the wet hydrogels were lyophilized to get aerogels for characterization.

2.3 Materials characterizations

X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific Escalab 250 instrument equipped with Al Kα source (10 mA, 14 kV) and operating at 1486.8 eV during the measurements. The base pressure in the spectrometer analyzer chamber was lower than 2 × 10⁻⁹ mbar. The charge neutralizer filament was used during the experiment to control charging of the samples. To determine the surface element molar ratio, the atom sensitivity factors are 0.296, 0.711, and 0.477 for C 1s, O 1s, and N 1s, respectively. FTIR spectra of the samples were measured on a Nicolet 5700 spectrometer (Perkin Elmer, USA). The samples were prepared by grinding the aerogels with KBr together and then compressed into thin pellets. X-ray diffraction (XRD) was used to characterize the crystal structure of GO and RGHs on a Brucker D8 Advance diffraction with Cu Kα radiation (λ = 1.5418 Å). The aerogels were placed on the glass plates fixed on a sample holder and subjected to XRD analysis at room temperature. Raman spectroscopy was performed using a LabRAM HR800 (JY Horiba) with a 435 nm wavelength laser. The samples were prepared by putting the aerogels smoothly on a glass slide. The morphology of RGHs was observed with a S-4800 field emission scanning electron microscope (SEM) (Hitachi, Japan), in which the aerogels (fractured surface) were placed on conductive adhesive pasting on the sample stage. The specific surface area was measured on an ASAP 2020 apparatus using the Brunauer Emmett Teller (BET) method. The specific surface area and pore size distributions (PSDs) of the hierarchical carbon materials were measured by nitrogen sorption test using ASAP 2020 equipment (Micrometitics USA). Brunauer–Emmett–Teller (BET) surface area (S_BET) was calculated using the N₂ adsorption isotherm data within the relative pressure of 0.05–0.25. Total pore volume (V_T) was obtained at p/p⁰ = 0.995. Microspore volume (V_micro) was determined by t-plot method. Mesopore volume (V_meso) was calculated by subtracting the micropore volume from the total pore volume. PSDs were determined by applying the nonlocal density functional theory (NLDFT) model on the adsorption isotherms and assuming a slit-shape pore. Elemental analysis (CHON) was performed at FLASH EA1112 Elemental Analyzer.

2.4 Electrochemical measurements

The electrochemical properties and capacitance measurements of supercapacitor were studied on a CHI660D electrochemical workstation (Chenhua Instruments Co. Ltd., Shanghai) at ambient temperature in a three-electrode system by using RGHs as working electrode, a platinum film as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode, in which cyclic voltammetry (CV), galvanostatic charge–discharge measurement and electrical impedance spectroscopy (EIS) were performed. A potential window of −0.9–0 V vs. SCE reference electrode was applied to the electrochemical measurements. Working electrodes were prepared by pressing 2 mg of RGHs onto nickel foam under 10 MPa. In the electrochemical test, 6 M KOH solution was used as electrolyte. In order to ensure the electrode materials thoroughly wetted with the electrolyte of 6 M KOH solution, the working electrode was vacuum-impregnated with the electrolyte for 30 min and then steeped for 12 h before electrochemical tests.

The specific capacitance is calculated by the following equation:

C_m = (It)/(ΔVm)

where C_m (F g⁻¹) is the gravimetric specific capacitance of RGHs, I (A) is the discharge current, t (s) is the discharge time, ΔV (V) is the potential window (0.9 V in this study), and m (g) is the mass of active material in working electrode.

3 Results and discussion

N₂ adsorption and desorption measurement was carried out to obtain the information of surface areas and pore size distributions of RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15. As shown in Fig. 1a, all the curves show typical IV isotherms with H4 hysteresis loop, suggesting the existence of abundant mesopores in the five samples. In the patterns of pore size distribution as shown in Fig. 1b, all the samples are mesoporous materials and show broad pore size distribution. It can be seen that RGHs-1, RGHs-5 and RGHs-15 show much larger pore volume of the pore width ranging from 2 nm to 8 nm than that of RGHs-2 and RGHs-10, then lead to a larger surface areas, which is consistent with the nitrogen adsorption–desorption isotherms. As shown in Table 1, the specific surface area RGHs are 152, 82, 139, 86 and 132 m² g⁻¹ while the pore volumes are 0.21, 0.14, 0.22, 0.22 and 0.68 cm³ g⁻¹ for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15, respectively.

Fig. 1 (a) Pore size distribution and (b) N₂ sorption isotherms of RGHs.

Table 1 Porosity parameters of RGHs

Sample	SBET (m^2 g^−1)	VT (cm^3 g^−1)	Vmeso (cm^3 g^−1)	Vmicro (cm^3 g^−1)	D (nm)
RGHs-1	152	0.21	0.19	0.02	3.7
RGHs-2	82	0.14	0.10	0.04	5.3
RGHs-5	139	0.22	0.20	0.02	4.4
RGHs-10	86	0.22	0.21	0.01	7.1
RGHs-15	132	0.68	0.61	0.07	16.4

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Field emission scanning electron microscopy (FESEM) was applied to observe the interior microstructures of the obtained RGHs. As observed from the SEM images shown in Fig. 2 and S1–S4,† a typical 3D network with pore structures formed by cross-linking of graphene sheets are quite uniform in the large scale, and the hierarchical pore with the wide size distribution ranges from micropores to mesopores, which is in agreement with N₂ sorption experiment. As described in the introduction section, the 3D porous structures can be formed through the partial overlapping or aggregating of flexible graphene sheets via π–π stacking and hydrophobic interactions in the reduction process by using acetaldehyde oxime in an aqueous suspension of GO with ammonia.

Fig. 2 SEM images of RGHs-5 (a–d) of the interior microstructures with different magnifications.

The XRD patterns of GO and RGHs are shown in Fig. 3a. The feature diffraction peak of GO appeared at 9.43°, giving an interlayer spacing (d-spacing) of 9.37 Å. This interlayer space was much higher than that of pristine graphite (3.4 Å) as a result of the introduction of oxygenated functional groups on graphene sheets. For the resulting RGHs (Fig. 3a), the peak located at 9.43° disappeared, while small broad bumps near 23.82, 25.16, 25.35, 23.68 and 25.76° appeared which could be attributed to an interlayer spacing of 3.73, 3.54, 3.51, 3.75 and 3.46 for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15, indicating the stacking of graphene sheets during the process of chemical reduction of GO. The broad XRD peaks of the resulting freeze-dried RGHs samples can be explained by the exfoliation of the layered GO and destruction of the regular stacks of graphite or graphite oxide. The structural changes after the reduction process are further reflected in the Raman spectra of GO and as-prepared RGHs. In Raman spectra as shown in Fig. 3b, the intensity ratios of the well-documented D bands (the A_1g symmetry mode) in the vicinity of 1345 cm⁻¹ and G bands (the E_2g mode of the sp² carbon atoms) in the vicinity of 1596 cm⁻¹ of RGHs were enhanced after the chemical reduction process, compared with that of GO, indicating the improvement of the disordered graphene sheets. The intensity ratio of I_D/I_G is 0.89 in GO^{55,56,60–64} and increases to 1.08, 1.00, 1.06, 1.02 and 1.06 for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15, demonstrating that the chemical reduction altered the structure of GO. FT-IR spectra is employed to further prove the removal of oxygenated groups by acetaldehyde oxime with ammonia (Fig. S5†). The broad and intense band at 3000–3500 cm⁻¹ related to hydroxyl groups and peak at 1728 cm⁻¹ related to carbonyl and carboxyl groups decrease, while a new peak at 1665 cm⁻¹ attributed to the stretching and bending vibrations from C=C appears, corresponding to the remaining sp² character from the unoxidized graphitic domains. Furthermore, new peaks at 1548 and 1100 cm⁻¹ corresponding to the C=N and C–N bending vibration appear, implying the doping of nitrogen in the reducing process. The element compositions of the prepared samples are determined by XPS measurement (Fig. S6†). Based on the XPS results, nitrogen doping of RGHs is further confirmed, which are mainly from the hydrolysed component of hydroxylammonium. The N content of RGHs is in the range of 2.46–5.13 atm%, while the O content of RGHs decreases from 29.06 to 8.28, 9.39 and 13.01 atm% for RGHs-1, RGHs-2 and RGHs-5, respectively. Because the XPS analysis only give the relative atomic ratio on the surface of RGHs, we further employ elemental analysis to analyse the C, H, O and N in RGHs. As shown in Table S1,† the C, H and O contents increase dramatically after reduction of acetaldehyde oxime, but have no obvious changes with the increase of the addition of acetaldehyde oxime. However, the N content increases obviously from 7.65 to 8.37 mass% with increasing of acetaldehyde oxime. This can be attributed to the increase of the reduction agents of hydrolyzed hydroxylammonium in the reduction process.

Fig. 3 XRD (a) and Raman (b) spectra of RGHs.

High resolution XPS measurements were further performed to analyze atom binding states of the prepared materials (Fig. 4), which showed the C 1s, O 1s and N 1s deconvolution spectra of RGHs-1 (a and b), RGHs-2 (c and d) and RGHs-5 (e and f). In case of C 1s of RGHs-1, four peaks at 284.4, 286.3, 287.4 and 288.8 eV corresponding to C=C/C–C in sp²-hybridized domains, C–O (epoxy and hydroxyl), C=O (carbonyl) and O=C–O (carboxyl) groups were observed. Besides, the appearance of the new characteristic peak corresponding to C–N bond (285.2 eV) indicates the successful doping of nitrogen. The N 1s of RGHs-1 are determined by 398.1, 399.6 and 401.2 eV (Fig. S7†), which can be attributed to pyridinic N, pyrrolic N and graphitic N, respectively. In case of O 1s of RGHs-1, three peaks of C–O (carbonyl), C=O (epoxy and hydroxyl) and O=C–O (carboxyl) groups are determined by the peaks at 531.5, 532.7 and 533.6 eV, respectively. Compared to that of GO,^{55,56,60–64} the peaks of C–O, C=O and O=C–O of RGHs became much weaker, indicating the removal of oxygenated groups. Moreover, the C 1s, N 1s and O 1s spectra in RGHs-2 and RGHs-5 also gave the same phenomenon and results as RGHs-1. All the high-resolution XPS spectra demonstrate the effective removal of oxygen-containing groups and doping of nitrogen by acetaldehyde oxime with ammonia in the reduction process compared to GO.

Fig. 4 High-resolution XPS spectra of C 1s and O 1s peaks for RGHs-1 (a and b), RGHs-2 (c and d) and RGHs-5 (e and f).

The electrochemical performance of the obtained RGHs as electrode materials for supercapacitors were evaluated by cyclic voltammograms (CV) and galvanostatic charge/discharge test in a three-electrode system (Fig. 5). Fig. 5a shows the CV curves of RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15 at scan rate of 10 mV s⁻¹. The CV curves at different scan rates from 5 to 150 mV s⁻¹ (Fig. S8†) of RGHs all show rectangular-like shape with obvious current enlargement spread over a wide range of −0.4–0 V, indicating excellent capacitive behavior and the presence of pseudo-capacitance effect, which could be attributed to the complex and overlapped redox reactions introduced by N, O-doped species. These results can be further confirmed by galvanostatic charge/discharge measurements. As shown in Fig. 5b and S9,† the galvanostatic charge/discharge curves at different current density exhibit nearly triangular shapes with a small deviation from linearity, implying good capacitive behaviors originating from the combination of electric double layer capacitance and pseudocapacitance. It is considered that the galvanostatic charge/discharge measurement is a more accurate technique to determine the specific capacitance of electrode materials, especially for the one involving pseudocapacitance.

Fig. 5 (a) Cyclic voltammograms of the supercapacitor based on RGHs at 10 mV s⁻¹; and (b) galvanostatic charge/discharge curves of RGHs at 0.3 A g⁻¹.

The specific capacitance of the RGHs electrodes are evaluated to be 230.4, 155.3, 234.2, 155.1 and 191.8 F g⁻¹ at a discharge current density of 0.3 A g⁻¹ for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15, respectively (Fig. 5b). As shown in Fig. S9† and 6a–e, the specific capacitances of RGHs are further investigated with the increasing of charge/discharge current density from 0.3 to 20 A g⁻¹. It can be seen that the specific capacitances decrease at high charging/discharging current density. This is a common feature of real supercapacitor since there is no enough time for the electrolyte ions to diffusion into the entire pore surface, especially at higher charge current density. However, RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15 can still maintain the specific capacitances of 167.1, 110.4, 142.3, 106.9 and 142.3 F g⁻¹ at a very high current density of 20 A g⁻¹, the retention rate are 72.5, 71.1, 60.8, 68.9 and 74.2%, respectively (Fig. 6f). The excellent performance of RGHs can be attributed to the good combination of the hierarchical porous structure increasing the effective surface area and surface accessibility and the nitrogen-containing functional groups inducing the additional pseudocapacitance as well as improving the wettability of the material.

Fig. 6 Galvanostatic charge/discharge curves of RGHs at different charging/discharging current density of RGHs-1 (a), RGHs-2 (b), RGHs-5 (c), RGHs-10 (d) and RGHs-15 (e); and relationship of the specific capacitance with respect to the current density from 0.3 to 20 A g⁻¹ for RGHs (f).

EIS data in a typical Nyquist plots for RGHs at frequencies ranging from 10 kHz to 1 mHz are given in Fig. 7a. For ideal porous electrodes, Nyquist plot is a vertical straight line perpendicular to the horizontal coordinate. However, for real porous electrodes, Nyquist plot can be separated into 3 parts: high frequency region, middle frequency region, low frequency region. At very high frequencies, the imaginary part (Z′) of the impedance is near to zero and the real part of resistance (Z′′) is derived from the electrolyte and the contact between the electrode and the current collector (R_s). The uncompleted semicircle loop at high frequency demonstrates the charge transfer resistance (R_ct) at the interface between the electrolyte and electrode which is related to pseudocapacitive performance. The 45° slope region at middle frequency can be attributed to the ions diffusion/transport from the electrolyte to the pore on the surface of samples. In the low-frequency region, the Nyquist plot is a straight line, representing the dominance of ideal double-layer charge/discharge behaviors. The more vertical the line is, the more ideal the capacitor is. It is shown that all RGHs have almost vertical lines, indicating ideal capacitive behavior. This ideal capacitive behavior should also be ascribed to the micro-meso hierarchical pore texture of RGHs that favors fast ionic diffusion. Fig. 7b gives the cycle durability of RGHs electrodes investigated by galvanostatic charge/discharge measurement at a current density of 1 A g⁻¹ for 5000 cycles. It is clear that the specific capacitance still remains at 199.3, 133.1, 167.2, 127.4 and 163.9 F g⁻¹ after 5000 cycles at 1 A g⁻¹, which are ∼95.3, 91.5, 91.3, 88.9 and 91.2% of the initial capacitance value after 5000 cycles, indicating the as-prepared RGHs display an outstanding cycling stability as electrode materials for supercapacitors.

Fig. 7 (a) Electrochemical impedance spectra; and (b) long-term cycle test of RGHs measured at a current density of 1 A g⁻¹ within the potential range from 0 to −0.9 V.

4 Conclusions

In conclusion, 3D porous reduced graphene hydrogels (RGHs) with micro-meso hierarchical pore structures have been prepared in a facile chemical reduction method by using acetaldehyde oxime as reducing and doping agents under alkaline condition. Electrochemical measurements demonstrate that RGHs have moderate capacitive performance of 230.4, 155.3, 234.2, 155.1 and 191.8 F g⁻¹ at 0.3 A g⁻¹ for RGHs-1, RGHs-2, RGHs-5, RGHs-10 and RGHs-15, respectively. More importantly, RGHs maintain high capacitances of 167.1, 110.4, 142.3, 106.9 and 142.3 F g⁻¹ at a very high current density of 20 A g⁻¹, highlighting the promise of the as-prepared RGHs for high power applications. It also showed that the electrode based on RGHs has good stability and high reversibility in the charge/discharge cycling test. These results demonstrate that the as-prepared RGHs presented here might be a promising candidate for the electrode materials of high-performance supercapacitors.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (21402108, 21476132, 51302156 and 21576158), Shandong Natural Science Foundation (ZR2014BQ036), and Young Teacher Supporting Fund of Shandong University of Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04348h

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