Self-Assembly Method To Fabricate Reduced Graphene Oxide

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A self-assembly method to fabricate reduced graphene oxide aerogels loaded with nickel hydroxyl nanoparticles and their excellent properties in absorbing and supercapacitor Lei Zhang, Tao Wu, Heya Na, Cheng Pan, Xiaoyang Xu, Guanbo Huang, Yue Liu, Yu Liu, and Jianping Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03706 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 18, 2016

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Industrial & Engineering Chemistry Research

A self-assembly method to fabricate reduced graphene oxide aerogels loaded with nickel hydroxyl nanoparticles and their excellent properties in absorbing and supercapacitor Lei Zhang1, Tao Wu1,2, Heya Na1, Cheng Pan1, Xiaoyang Xu1, Guanbo Huang1, Yue Liu1, Yu Liu* 1, Jianping Gao* 1 1

Department of Chemistry, Tianjin University, Tianjin 300072, China

2

Department of Chemistry & Biochemistry, University of Maryland, College Park,

Maryland 20742, United States

Abstract A facile method for preparing nickel hydroxyl nanoparticles loaded graphene aerogels has been established. The prepared aerogels were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. Their applications as absorbents or electrode materials for supercapacitors were investigated. They showed excellent performance on the absorption of different dyes. The absorption capacities ranged from 202 mg g-1 to 513 mg g-1. They also displayed high absorption capacities toward oils and organic solvents. The aerogels demonstrated high capacitance and stability as electrode materials of supercapacitors. The specific capacitance reached 702 F g-1 at current densities of 1A g-1. Keywords: grapgene aerogel; nickel hydroxyl; absorption; supercapacitor *

Corresponding author. Tel.: +86 2227403475. Fax: +86 22-2740-3475. E-mail address: [email protected] (Y. Liu), [email protected] (J. Gao).

ACS Paragon Plus Environment


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1. Introduction Porous hydrogel or aerogel materials have attracted much attention because of their wide applications such as environmental protection and various biomaterials.1,2 Three-dimensional (3D) networks in gels are always constructed by weak crosslinking forces including hydrogen bonding and π-π interaction or chemical bonds.3,4 These interactions develop the gels with mechanical properties and also endow them with high susceptibility to external environment.5,6 Previously, hydrogels were usually synthesized by polymer and applied to tissue engineering7 and drug delivery.8 More recently, other kinds of inorganic constituents were applied for fabrication of hydrogels, such as clays9,10 and carbon nanotubes.11 However, because of the irregular crosslinking, the hydrogels exhibit poor mechanical strength, so their practical applications are limited. Graphene is a single or few layer sheet of graphitic carbon and has received much attention since it was reported in 2004.12 Its excellent mechanical,13,14 electronic15,16 and thermal properties17 makes it potentially applied in electronic devices, chemical and electrochemical catalyses, new energy resources, environmental protection, biotechnology and composite materials.18-22 Self-assembly is a simple method to prepare novel materials, such as one-dimensional (1D) graphene tube and graphene fiber23,24 and two-dimensional (2D) graphene films.25,26 Recently, 3D graphene hydrogels and areogels have been fabricated through self-assembly and they expanded the applications of grapgene.27,28


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At present, 3D graphene structures can be achieved by cross-linking29-32 and in situ reduction-assembly methods.30-33 Cross-linking method uses metal ions29,30 or polymers,31,32 called cross-linkers, to create graphene hydrogels by physical or chemical interactions, whereas in situ reduction-assembly usually realizes during chemical reduction or hydrothermal treatment,34-36 where reducing graphene oxide (GO) to graphene induces the self-assembly of graphene into hydrogels. In view of the low electrical conductivity by cross-linking method, great interest has been focused on in situ reduction-assembly method. This method facilitates the incorporation of nanoparticles in graphene hydrogels. For example, magnetic 3D graphene/Fe3O4 architectures have been prepared by chemical reduction of GO in the presence of Fe3O4 nanoparticles.37 Basic aluminum sulfate @ graphene hydrogels were prepared for removal of fluoride.38 Graphene hydrogels with nanoparticles are absolutely attractive due to their wide applications. However, the reducing agents are usually harmful to the environment. Thus, it is desired to develop an environmentally friendly and rapid method to fabricate graphene hydrogels. Herein, reduction-assembly method was used to fabricate graphene hydrogels. During the process, GO nanosheets were reduced by Ni nanoparticles and then they self-assembled to rGO (reduced graphene oxide) hydrogels. Meanwhile, Ni nanoparticles were oxidized to Ni(OH)2. The rGO hydrogels were transformed to aerogels via a freeze-drying method. The prepared graphene based aerogels showed excellent electrochemical performance as electrodes of supercapacitors. In addition, they also demonstrated good performances in removal of oils, organic solvents and



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organic dyes. 2. Experimental 2.1 Materials Natural graphite powder was bought from Qingdao Graphite Factory. The nickel (II) nitrate, hydrazine hydrate (85 wt%), ethylene glycol, hydrogen peroxide (30%), sodium borohydride, 4-nitrophenol (4-NP) and other reagents were all obtained from Tianjin Chemical Reagent Co. All chemicals were of analytical grade and used as received. 2.2 Preparation of GO Graphene oxide was prepared from purified natural graphite by a modified Hummer’s method.39 2.3 Preparation of Ni nanoparticles A typical preparation procedure is described as follows: NiCl2 solution (1 mol L-1) of 5 mL was added into 15 mL ethylene glycol. Then 5 mL hydrazine hydrate (85 wt%) was added and the color of the solution turned to purple from green. The solution was then heated at 120 °C for 20 min to obtain Ni nanoparticles. The Ni nanoparticles were easily separated from the solution with a magnet, purified by washing with distilled water and absolute ethanol successively, and dried in a vacuum at 40 °C for 6 h. 2.4 Preparation of graphene aerogels Ni nanoparticles (0.015 g) and 15 mL GO aqueous suspension (2 mg mL-1) were mixed and sonicated for 1 h to form a homogenous solution (Unless specified, the


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rGO aerogel with this Ni/GO ratio was used in the following applications). The mixed solution was heated at 90 °C for 24 h in a bath without stirring, during which the hydrogel was formed. The hydrogel was separated from the solution and washed with water before it was freeze-dried to form an aerogel. In order to investigate the pore structure, different hydrogels have been fabricated with changing the mass of Ni nanoparticles by the same method. 2.5 Characterazition Photographs of the samples and experimental procedures were taken with a digital camera (COOLPIX S620, Nikon, Japan). The X-ray diffraction (XRD) patterns of the samples were recorded with a BDX3300 diffractomerter employing Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 4° min-1 with a 2θ range of 20° to 90°. Transmission electron microscopy (TEM) was performed using a Philips Tecnai G2F20 microscope at 200 kV. The morphology of the samples were observed by a scanning electron microscope (SEM). The samples were gold coated using a sputter coater (Desk-II; Denton Vacuum) and put onto the stage in the chamber of SEM (JEOL-6700F ESEM, Japan) for observation. A Perkin-Elmer Paragon-1000 Fourier transform infrared spectroscopy (FT-IR) spectrometer was used to study the composition of the GO. The GO powders were pulverized with KBr and pressed into pellets. Spectra were obtained in range of 750-4000 cm-1 by averaging 16 scans at a resolution of 4 cm-1 with 1 min intervals to minimize the effects of dynamic scanning. Raman spectroscopy analysis was performed from 500 to 2000 cm-1 at room temperature using an Raman spectrometer (DXR, USA) with a Nd:YAG laser



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excitation source (532 nm). Elemental composition analyses were carried out using a X-ray photoelectron spectroscopy (XPS, PHI1600 ESCA System, Perkin Elmer, USA) with Al Kα radiation (hν = 1486.6 eV). Ultraviolet-visible (UV-vis) absorption spectra were recorded with a TU-1901 UV-vis spectrcphotometer. The porosity was calculated by a “density bottle method” using water as the medium to fill the pores. The “true density” (ρ) and porosity of the aerogel are measured according to the formula:

where, W1 and W2 are the weights of the density bottle filled with water and the density bottle containing the sample and water, respectively; m is the weight of the sample; Vt and V are the volume of the material and the volume of the material without pore, respectively. 2.6 Dye-absorption by rGO areogels Five organic dyes, methyl orange (MO), congo red (CR), methyl blue (MB), methylene blue (MEB) and rhodamine B (RB) were used in the dye-absorption test. The dyes were dissolved in water to form the dye solutions with an initial concentration of 2 mmol L-1. Then the aerogels were added into the dye solutions for adsorption. The concentrations of the dye solutions were measured by UV-vis spectroscopy to determine the absorption of the aerogels at 25 °C. The dye-absorption performance of the aerogel was performed as the equilibrated absorption capacity of the dye per unit mass of the aerogel:


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where, Qeq (mg g-1) is the absorption capacity, Ci (mmol mL-1) and Ceq (mmol mL-1) are the initial and final concentration of the dye solution, respectively, V (mL) is the volume of the dye solution, Md (mg mmol-1) is the molecular weight of the dye, and m is the weight of the aerogel. 2.7 Absorption of oils and organic solvents by rGO aerogels The absorption capacities of oils and organic solvents by rGO aerogels were investigated. The aerogels were weighed and the put into different kinds of oils and organic solvents. After the absorption was complete, the aerogels were removed from the oils and organic solvents and weighed again. The absorption capacity (Q) was calculated using the following equation: Q = (M- M0)/M0 Where, M0 and M are the weights of the rGO aerogel before and after absorption, respectively. 2.8 Electrochemical measurements of aerogel supercapacitor All the electrochemical measurements were carried out on a CHI 660D electrochemical workstation (Shanghai Chen Hua). The electrochemical tests were performed in a three-electrode system in 1 mol L-1 KOH aqueous solution with Pt foil as counter electrode and Hg/HgO electrode as reference electrode. Electrochemical performance of the aerogel was examined through cyclic voltammetry (CV), galvanostatic charge/discharge (GV), cycling performance and electrochemical impedance spectroscopy (EIS) tests. CV tests were done with 10, 20, 50 and 100 mV



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s-1 scan rates in a potential window of 0.1-0.7 V vs Hg/HgO reference electrode. Galvanostatic charge/discharge measurement was performed for various current densities for different mass loadings. 3. Results and discussion 3.1 Structural and morphological characterization The Ni nanoparticles were prepared by the reduction of NiCl2 solution. As shown in Figure 1a, the Ni nanoparticles are about 30nm. The XRD pattern of the Ni nanoparticles in Figure 1b has peaks at 44.06°, 51.48° and 76.10° which correspond to the (111), (200) and (220) fcc structures of Ni (JCPDS, 04-0850). The XRD result indicates that the Ni2+ ions have been reduced to Ni by hydrazine hydrate.

(a)

(b)

Figure 1. TEM image of Ni nanoparticles (a); XRD pattern of Ni nanoparticles (b). The GO was prepared by a modified Hummer’s method. An aqueous suspension of GO was obtained via ultrasonication and it would be stable for several months with no precipitation, implying that the GO nanosheets are hydrophilic and thin so that they can be suspended in numerous polar solvents. These stable GO aqueous suspensions are crucial for preparing rGO aerogels. As GO and Ni nanoparticles mixed solution


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was heated without stirring, a hydrogel was gradually formed (Figure 2a). The hydrogel was dried by a freeze-drying method to form an aerogel. The surface (Figure 2b) and cross section (2c) of the rGO aerogel were observed with SEM. It can be observed that the rGO aerogel is highly porous. The rGO nanosheets assembled and connected each other to from porous structure once GO was reduced by Ni nanoparticles. The results can explain why the hydrogel can float on the water and the aerogel is so light.

(a)

(b)

(c)

Figure 2. Photographs of the GO suspension before and after the reduction (a); SEM images of the surface (b) and cross-section (c) of the rGO aerogel The porosity of the rGO aerogels with different Ni/GO ratio has been also researched and the results are shown in Figure 3. As the Ni/GO ratio increased, the porosity of the rGO aerogels decreased. The results indicate the important role of Ni



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nanoparticles in adjusting the porous structure of rGO aerogels.

Figure 3. Porosity of rGO aerogels prepared by different mass ratios of Ni nanoparticles and GO (GO concentration was 2 mg mL-1) The internal structure of the rGO aerogel was also observed by TEM (Figure 4a-b). Figure 4a demonstrates that there are many nanoparticles on the rGO walls of the rGO aerogel. These particles may be Ni nanoparticles or the reaction product of Ni nanoparticles with GO. In order to identify the nanoparticles, XRD analysis was conducted. The XRD diagram of the rGO areogel in Figure 4c exhibits six peaks, among which, the peaks at 32.86°, 38.21°, 51.27° and 59.10° is well indexed to the (100), (101), (102) and (110) planes of nickel hydroxide (JCPDS, 14-0117). It proves that Ni reacted with GO and generated Ni(OH)2.

(a)


(b)

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(c) Figure 4. TEM images (a-b) and XRD pattern (c) of the rGO aerogel FT-IR and Raman are widely used tools to investigate the structure of the graphene-based materials. The FT-IR spectra of GO and aerogel were shown in Figure 5a. For GO, the peaks at 1730 cm-1 (νC=O) and 1055 cm-1 (νC-O) confirms the presence of oxygen-containing functional groups. The peak at 1730 cm-1 is almost disappears in the aerogel spectrum. It proves that some oxygen-containing functional groups disappeared in the rGO aerogel. The Raman spectra of GO and rGO aerogel were shown in Figure 5b Graphene-based materials have two main features in the Raman spectrum. They are G band (the E2g mode of sp2 carbon atoms) at 1612 cm-1 and D band (the symmetry A1g mode) at 1365 cm-1. Changes in the ratio of the areas of the D and G bands (D/G) indicate the changes of the electronic conjugation state of the GO during the reaction.40,41 As shown in Figure 4c, the D/G ratio of the rGO aerogel is larger than that of GO. Since the Raman D/G intensity ratio is inversely proportional to the average size of the sp2 domains, the increase of the D/G intensity ratio demonstrates that smaller in-plane sp2 domains were formed during the reaction.40,42 This also indicates that GO was reduced to rGO by Ni nanoparticles in the process of



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preparing rGO aerogel.

(a)

(b)

Figure 5. FT-IR spectra (a) and Raman spectra (b) of GO and aerogel The extent of the GO reduction, as well as the composition of GO and the aerogel, was further characterized by XPS. As shown in Figure. 6a-b, the atomic percentages of C and O in GO are about 72.6% (C) and 27.4% (O), whereas they are 77.3% (C) and 17.7% (O) in the aerogel. Because there are many Ni(OH)2 nanoparticles in the aerogel, the O is from the O in both Ni(OH)2 and rGO. After subtracting the O from Ni(OH)2, the actual C/O ratio in the aerogel was calculated, which is 7.0. This result is higher than GO (a ratio of 2.6) and GO reduced by sodium borohydride (a C/O ratio of 5.3).43 Figure 6c-d show the C1s XPS spectra of GO and the rGO aerogel, respectively. The spectrum of GO can be deconvoluted into three different peaks: the alkyl C and sp2-bonded carbon network (C-C/C=C, 284.44 eV), the C-O of the hydroxyl and epoxy groups (286.55 eV), and the carbonyl C (C=O, 288.47 eV).44,45 The C-C/C-O peak intensity ratio in the aerogel spectrum obviously increased compared to GO. The above XPS results indicates that many of the oxygen-containing functional groups were removed in the rGO aerogel, indicating


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reduction of GO by Ni nanoparticles.

(a)

(b)

(c)

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Figure 6. The XPS spectra for GO (a) and rGO aerogel (b); the C1s XPS spectra for GO (c) and rGO aerogel (d) The process of the rGO hydrogel formation was speculated and illustrated in Figure 7. The sonication treatment made Ni nanoparticles deposit on the surface of GO nanosheets. As heating at 90 °C, GO was reduced to rGO by Ni nanoparticles while Ni nanoparticles were oxidized to form Ni(OH)2 on rGO nanosheets. The rGO



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nanosheets became hydrophobic and they self-assembled into a stable rGO hydrogel. The Ni(OH)2 nanoparticles played a significant role to prevent the stacking of rGO nanosheets and to direct the growth of the hydrogel.46

Figure 7. The process of self-assembly of rGO hydrogels 3.3 Absorption performances of rGO aerogels to organic dyes, oils and organics There are some pollutants like organic dyes in the wastewater released from the industries and they are toxic to microorganisms. Nanoparticles and porous materials are often used as absorbents to remover the dyes from wastewaters. Here, five organic dyes are selected to investigate the absorption ability of rGO aerogels. The large porosity and the π–π interactions between the rGO and the dyes make the absorption easy. When the aerogels were soaked in the dye solutions, the colors of the solutions became light gradually (Figure 8a-b). The absorption capacities of the aerogel for the different dyes ranged from 202 mg g-1 (0.62 mmol g-1) to 681 mg g-1 (0.85 mmol g-1) (as shown in Figure 8c), which is better than those previous reported (0.0113 or 0.6 mmol g-1),47,48 owing to the high porosity of rGO aerogels. Among the five dyes, the absorption of congo red and methyl blue are more efficient. The absorption results


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indicate that the rGO aerogel is an efficient absorbent for the removal of dyes from water.

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(c) Figure 8. Photographs of dye solutions before (a) and after absorption by the rGO aerogels (b); absorption capacities of the aerogels for different organic dyes (c) Since the rGO aerogel is hydrophobic and has a high porosity, it would have excellent absorption capacities for oils and organic solvents.49-51 As shown in Figure 9a-b, the aerogel can rapidly and selectively absorb the oil on the water. Figure 9c shows the absorption capacities of the aerogel toward a series of oils and organic solvents, including some commercial oils (corn oil, bean oil, olive oil, lubricating oil, pump oil and diesel fuel) and common organic solvents (CCl4, DMF, DMSO, THF and CH2Cl2). The aerogels showed the largest absorption capacity (106.97 g g-1) for



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CCl4, and the absorption capacity data for other oils and solvents were in the range of 29.53-82.17g g-1, depending on their densities. As shown in Figure S1, we compare the absorption capacities of CCl4 by the aerogels of different porosity. It can be observed that the aerogel with higher porosity has much higher absorption capacity. And the absorption capacities of the aerogel for oils and solvents were as good as other graphene materials50,51 and better than other kinds of materials,52-54 which results from high hydrophobicity and porosity of aerogel. These results show that the rGO aerogel has an excellent performance for the separation of oils and organic solvents from water.

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Figure 9. Photographs of the absorption of diesel oil by the aerogel over time (Sudan III was used to dye the oil for clear observation) (a-b); absorption capacities of oils and organic solvents by the aerogels (c) 3.4 Electrochemical activity of the rGO aerogel electrodes As an important energy storage device, supercapacitors have attracted tremendous attention

because

of

their

high

power

density,

long

cycle

life,

fast

charging/discharging within seconds, high reliability and low maintenance cost.55,56 Active pseudocapacitive materials, like metal oxide and metal hydroxide,57 were most widely investigated over the past few years. Traditional active electrodes are composed of active materials and additives, including carbon black and PTFE. In this work, the rGO aerogel was directly pressed into the nickel foam without using any additives. Figure. 10a shows the CV curves of the rGO aerogel in 1 mol L-1 KOH aqueous solution at different scan rates. The corresponding specific capacitance (SC) was calculated from the following equation:

C=

∫ idv s × m × ∆v

where C (F g-1) is the SC, i (A) is the discharge current, ∆v (V) represents the potential windows, s (V s-1) is the scan rate, and m (g) is the mass of the active material. As shown in Figure 10b, the specific capacitances (SCs) of the rGO aerogel are 1092, 860, 593 and 407 F g-1 respectively at scan rates of 10, 20, 50 and 100 mV s-1. The decrease in SC with increasing scan rate is attributed to the fact that the electrolyte ions are unable to fully access the interior surfaces of the active materials



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for charge-storage because of the reduced diffusion time at a high scan rate.58 The capacitance retention of the aerogel at a scan rate of 100 mV s-1 is 62.7% of that at 10 mV s-1. As shown in Figure S2, the specific capacitance of the aerogel is much higher than pure rGO and Ni(OH)2. And the capacitance of the aerogel electrode is higher than that of other graphene material.59

(a)

(b)

Figure 10. CV curves (a) and SCs (b) of the aerogel electrode at different scan rates The GV curves of the aerogel electrode measured at different current densities are shown in Figure. 11a. The SC was calculated from the following equation: C=

i × ∆t ∆v × m

where C (F g-1) is the SC, i (A) is the discharge current, ∆t (s) is the discharge time, ∆v (V) represents the potential windows, and m (g) is the mass of the active material. The SCs of the electrodes are 1033, 797, 654 and 561 F g-1 at current densities of 0.5, 1, 1.5 and 2 A g-1, respectively. The decrease in the SC values with increasing current density is due to the slow diffusion/migration of protons in the electrodes at high current density.60 The result is better than the similar materials like nanoporous Ni(OH)2 thin film on 3D ultrathin-graphite foam (119 F g-1 at a current density of 1 A


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g-1) 61 and porous graphene (245 F g-1 at a current density of 1 A g-1).62

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Figure 11. GV curves (a) and SCs (b) of the rGO aerogel electrode at different current densities To exploit the application of the aerogel as a supercapacitor electrode material, the capacitance retention was measured for 2000 charge/discharge cycles at a current density of 2 A g-1 as shown in Figure 12. Capacitance loss with extended cycling is a common phenomenon in supercapacitors because of damage of the electrode structure. After 2000 charge/discharge cycles, the electrode retained about 95.07%.

Figure 12. Capacitance retention of the rGO aerogel electrode at a constant current density of 2 A g-1 For a further understanding of the properties for the aerogel electrode materials,



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impedance measurements were performed in 0.1 mol L-1 KOH aqueous solution. Figure 13a shows the Nyquist spectrum of the aerogel taken in the frequency range from 0.01 to 100 kHz. An intercept at high frequency region with real part (Z´) is attributable to combinational resistance (R1), including ionic resistance of electrolyte, intrinsic resistance of active material and contact resistance of active material and current collector. And the distorted semicircle is observed at higher frequencies, which results from a parallel combination of the charge-transfer resistance (R2) and the constant phase element (CPE) arising due to inhomogeneity in the electrode surface due to random distribution of nanoparticles.63 However, the diameter of the semicircle is small. It is indicative of low interfacial R2 for this electrode, which can be attributed to the high electrical conductivity of this electrode. At lower frequencies, the slope of a curve about 45° with Z´ has appeared because of the diffusion of counter ions between the electrolyte and electrode material.64 This is represented by Warburg element (W). The fitting of the Nyquist spots by a suitable circuit is shown in Figure 13b and the spectrum could be nicely fit with the equivalent circuit.

(a)


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CPE R1 R2

W

(b) Figure 13. EIS in Nyquist plots of the aerogel electrode (a) and the equivalent circuit for ﬁtted curve (b). 4

Conclusion In summary, the rGO aerogels were fabricated by a simple method. During the

process, GO was reduced to RGO and Ni nanoparticles were oxidized to Ni(OH)2. The prepared rGO aerogel has unique porous structure with high porosity. The rGO aerogel has excellent catalytic performance for removing the organic dyes from water, but also possesses good absorption abilities for the oils and organic solvents. These results demonstrated that the aerogel can be widely used for sewage treatment and environmental protection. Moreover, the rGO aerogel displayed good capacitance and capacitance retention, so it will be potential electrode materials for supercapacitors. 5

Acknowledgements This work was supported by the National Science Foundation of China (21202115).

6

Associated Content

Supporting Information Available: The absorption capacities by the aerogels of different porosity, cyclic voltammetry curves and specific capacitance of rGO, Ni(OH)2 and the aerogel. This material is available free of charge via the Internet at http://pubs.acs.org.



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Self-Assembly Method To Fabricate Reduced Graphene Oxide

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