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Fabricating graphene hydrogels with controllable pore structure via one-step chemical reduction process
Carbon 109 (2016) 673e680
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Carbon journal homepage: www.elsevier.com/locate/carbon
Fabricating graphene hydrogels with controllable pore structure via one-step chemical reduction process Yuhui Xie, Xinxin Sheng, Delong Xie**, Zixian Liu, Xinya Zhang*, Li Zhong School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2016 Received in revised form 19 August 2016 Accepted 24 August 2016 Available online 25 August 2016
The porous graphene hydrogels (GHs) are of particular interest in various applications, such as energystorage devices, catalyst and sensors. In this paper, GHs with controllable pore structure are prepared from graphene oxide (GO) aqueous dispersion by a facile chemical reduction method. The pore size distribution (PSD) and specific surface area (SSA) of GHs can be regulated by adjusting the pH value of GO dispersion. It is found that both the pore size and SSA of GHs gradually increase with the pH value of GO dispersion. For the GH prepared at pH ¼ 1.65, the PSD has one peak at 1.83 nm with a SSA of 723.35 m2/g. As the pH value increases to 11.73, the peak moves to 3.2 nm and the SSA keeps rising to 1107.24 m2/g. Rheological measurements show that both the storage modulus and yield stress of the GHs decrease with the increasing pH value. Electrochemical evaluation of GHs as the electrodes of supercapacitors reveals that the specific capacitance reaches the highest value for GH prepared at pH ¼ 5.25, which has a specific pore structure, showing the coexistence of large (3.2 nm) and small pores (1.89 nm). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction During the last decades, graphene has aroused increasing interest considering its extraordinarily electrical, thermal and mechanical properties [1e3]. But the severe agglomeration and restacking of graphene sheets, as a result of the p-p interactions between neighbouring graphene sheets, hinders its practical application [4,5]. Fabricating 2D graphene nano-sheets into 3D macroscopic materials with porous network, can partly preserve the inherent structure and properties of graphene sheets, thus has attracted considerable interest. Due to the synergistic combination of the graphene characteristics and the unique micro-structure, 3D macroscopic graphene materials display super performance in versatile applications such as: energy storage [6e8], environmental systems [9e11], catalysis [12,13] and sensors [14,15]. For these applications, the pore characteristics, such as specific surface area (SSA), pore structure and pore size distribution (PSD) of these graphene materials have great influence on the performance [16,17]. In supercapacitors, it is widely accepted that larger pores are better for the penetration/distribution of electrolyte and thus preferable for enhanced performance [18e20]. But some other
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhang).
(D.
http://dx.doi.org/10.1016/j.carbon.2016.08.079 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
Xie),
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reports revealed that the performance could be significantly improved when the 3D micro-structure presented appropriate PSD [21e23]. Mechanical strength is another property that affects the application of 3D graphene materials. Recently, a kind of 3D macroporous graphene materials with different morphology were prepared using hexane droplet as the template [24]. The morphology of these materials could be modulated by adjusting the concentration of graphene oxide (GO) dispersion and then greatly influenced the mechanical properties. Apart from numerous methods that have been developed to prepare graphene hydrogels (GHs), self-assembling technique, especially using GO dispersion as the precursor, attracts particular interest because it is low cost, controllable and large scalable [6,25,26]. GO, containing a large number of oxygenated functional groups, can be easily dispersed in water and interact with each other to construct GHs [27,28]. Xu et al. first reported that GO dispersed in aqueous could be hydrothermally reduced and then self-assembled into GHs, which were mechanical strong, electrical conductive and thermal stable [29]. After that, many reducing agents have been applied to assist the preparation of GHs in milder condition, such as vitamin C [26,30], FeSO4 [31], HI [32] and hydroquinone [20]. The pore size of the as-prepared GHs was always randomly distributed, ranging from submicrometer to several micrometers. Many efforts have been paid to optimize the synthesis method to prepare GHs with preferable internal structure [17,19,33]. Incorporating metal oxides into GHs, for example, not only brings GHs with novel properties but also weakens the
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interactions between graphene sheets and enlarges the pore size [7,34]. There also has been reported that the self-assembling process could be optimized by changing the fraction of KMnO4 and concentration of GO, allowing to the formation of GHs with different pore size and pore wall structure [25]. Another interesting work is that combined pre-reduction process with hydrothermal reduction can suppress the shell formation and thus prepared GHs with different architecture [35]. However, the reported methods of manipulating the pore structure of GHs are of multi-step and complicated with limited regulating capability. Herein, we describe a facile one-step method in which GHs with continuous changing pore characteristics can be fabricated from GO dispersion via a modified chemical reduction process. Prior to chemical reduction, different mass of H2SO4 or NaOH solution is added into GO dispersion to regulate the pH, achieving the formation of GHs with tunable pore structure. As the pH value of GO dispersion increases, both the size and the pore diameter of the GHs gradually increase, while the thickness of pore wall decreases. The changing behaviour of the resulting GHs is systematically investigated and a possible mechanism is proposed. Based on these GHs with various pore structure, the rheological and electrochemical measurements are conducted to investigate the relationship between pore structure and properties of GHs. The GHs prepared at lower pH range have higher mechanical strength as a result of the thicker pore walls. While the best capacitance performance is obtained at the middle pH value, i.e. pH ¼ 5.25, where the pore structure of the hydrogel demonstrates an appropriate PSD with the coexistence of large and small pores. 2. Experimental 2.1. Materials Graphite powder (325 mesh, 99.95%) and sodium ascorbate (99%) were supplied by Aladdin (Shanghai, China) and used as received. Concentrate sulfide acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30 wt%) and hydrochloric acid were obtained from Kaixin Chemical Reagent Co., Ltd. (China). Sodium nitrate (NaNO3) and potassium permanganate (KMnO4) were all of analytic grade and purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (China). All chemicals were used without further purification. 2.2. Preparation of GO GO was prepared from graphite powder by modified Hummers' method [36,37]. In brief, 5 g graphite powder was first mixed with 2.5 g NaNO3 in a 2000 mL round flask, 115 mL concentrate sulfide acid was then poured into the flask in an ice bath with vigorous agitation for 2 h. After that, 25 g potassium permanganate was slowly added into the flask in 30 min while the system was kept in ice bath to maintain the temperature under 20 C. The ice bath was then removed, followed by heating the suspension to 35 C for another 1 h reaction, forming a thick paste. At the end of the reaction, 230 mL water was slowly added into the flask with an increasing temperature to 95 C, and the suspension stayed at this temperature for another 15 min. Next, the suspension was further diluted by 700 mL hot water together with 20 mL H2O2 (30%). The mixture was then filtered and washed by 1:10 HCl solution followed by washing with hot water and centrifugation for three times. Later the resulting solid was dispersed in 700 mL water with the help of ultrasonication for 1 h and then submitted to centrifugation at 8000 rpm for 10 min to remove the undissolvable particles. Finally, the GO aqueous dispersion (about 7 mg/mL) was obtained as a result of further purification by dialysis for 1 week.
2.3. Preparation of GHs The GHs were prepared by a modified chemical reduction method [29,38]. Typically, 10 mL GO dispersion (3 mg/mL) was firstly submitted to ultrasonication for 20 min to further exfoliate the GO sheets followed by adding 66 mg sodium ascorbate under ultrasonication to form homogeneous dispersion. And then the pH of the GO dispersion was adjusted by adding NaOH or H2SO4 solution. Successfully, the mixture was sealed in a glass vial and heated at 90 C for 3 h to form the GHs. Finally, the GHs were purified by dialysis for following experiments. 2.4. Characterization The micromorphology of the freeze-dried GHs was investigated by scanning electron microscopy (SEM) using a Zeiss Merlin SEM. The zeta potential of graphene oxide dispersion was measured by Zetasizer Nano-ZS instrument (Malvern Instruments). Powder Xray diffraction (XRD) analysis of freeze-dried GHs was carried out on Bruker D8 ADVANCE diffractometer with Cu Ka radiation generator. The surface chemistry of the GHs was studied on Thermo Scientific Escalab 250Xi with an exciting source of Al. Fourier transform infrared spectra (FTIR) of GO and GHs was obtained using a VERTEX 33 instrument, Bruker, German. Raman spectra were taken by a LabRAM Aramis (HORIBAJobinYvon, France) with lexc ¼ 532 nm. Pore size distribution was measured from N2 absorption at 77 k using the TriStar II 3020 (Micromeritics Inc.) system and calculated according to the BarreteJoynereHalenda (BJH) equation. The samples were applied to degas at 150 C for 12 h before the analysis. The water content of the GHs (Cw) was measured according to Cw ¼ (MGH MD)/MGH 100% [29], where MGH is the original weight of GHs and MD is the weight of the freeze-dried GHs. The rheological investigation of GHs was performed on MCR 300 (Paar Physica) Rheometer at 25 C. A 25 mm parallel-plates geometry was chosen with the fixed gape distance of 2 mm. The dynamic frequency sweep experiments were measured from 1 to 100 rad/s and the oscillatory strain was set to be 0.2% followed by the oscillatory strain sweep measurements. The electrochemical properties of the hydrogels were measured by a conventional three-electrode system using 1 mol/L H2SO4 aqueous solution as the electrolyte. In a typical procedure, GHs were first cut into slices with the thickness of 2 mm, and then immersed in 1 mol/L H2SO4 aqueous solution for 12 h to ensure the complete penetration of electrolyte. After that, the as-prepared slices of GHs were fixed by a platinum clip to make the working electrode. The saturated calomel electrode (SCE) and platinum plate were used as the reference and counter electrode, respectively. Both electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried out on the electrochemical workstation (PGSTAT 302N, Metrohm AG, Switzerland). CV measurements with different scan rates were performed in the potential window of 0e0.8 V. The specific capacitance values were calculated from the CV curves according to the formulation, C ¼ A/2msDV, where A is the integration of CV curve, m is the mass of electrode, s is the scan rate and DV is the potential window. 3. Results and discussion 3.1. Structure of GHs The preparation procedure of GHs with different pore structure is illustrated in Fig. 1a. A series of GO/sodium ascorbate suspensions are first mixed with NaOH or H2SO4 solution to adjust the pH to be
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1.65, 3.47, 5.25, 7.39, 9.51 and 11.73, respectively, and then treated at 90 C for 3 h to fabricate GHs. The as-prepared GHs are respectively denoted as GH-1.65, GH-3.47, GH-5.25, GH-7.39, GH-9.51 and GH-11.73, as shown in Fig. 1b. It is clear that the size and exterior appearance of the GHs are different. The size of GHs exhibits a gradual expansion, and the exterior appearance of GH-n (n 5.25) is much more compact comparing to GHs prepared at higher pH values. Specifically, the diameter and water content of the GHs have been measured and the results are presented in Fig. 2, where we can see that both the diameter and water content of GHS increase with the increasing pH value, e.g. the size increases from 7.995 mm of GH-1.65 to 10.75 mm of GH-11.75, while the water content rises from 97.6% to 98.6%. The interconnected porous microstructure of dried GHs is revealed by SEM, and the results are shown in Fig. 3 (left). It can be seen that the internal structure of the GHs prepared from precursors with different pH value shows obvious difference. The GHs prepared at high pH value are loosely porous, and the pore wall is consisted of fewer layers of graphene sheets. While the GHs prepared at low pH value, by contrast, render much compact structure (Fig. 3 and Fig. S1) and the pore wall is consisted of thicker layers of stacked graphene sheets. Furthermore, as shown in SEM images, GHs prepared at high pH value have more cracks compared to that prepared at low pH value, which may be attributed to the loose structure consisted of fewer layers of graphene sheets. During the freeze drying process, the water in the GHs is first frozen to ice, as a result, the pore volume is expanded to squeeze the graphene layers and collapse the pore walls [18,39]. Thus, GHs prepared at high pH value suffer from more severe collapse as a result of thinner pore walls. Higher water content, in addition, resulting in more ice formation, may also contribute to the cracks. But the dried GHs prepared at lower pH value, by contrast, are consisted of continuous graphene layers with much less fractures. Nitrogen isothermal adsorption analysis is employed to further investigate the pore characteristics of dried GHs. The PSD of dried GHs is calculated by the BarretteJoynereHalenda (BJH) model and the results are presented in Fig. 3 (right), demonstrating the obvious existence of mesopores. The dried GH-1.65 shows only one peak at 1.83 nm. The pore size gradually shifts toward larger as the
Fig. 1. (a) Diagram for synthesis of GHs by reduction of GO dispersion with different pH value; (b) Effect of pH value of GO dispersion on the formation of GHs, the size of GHs gradually increased with the increasing pH value. (A colour version of this figure can be viewed online.)
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Fig. 2. Diameter and water content of GHs as a function of pH value. (A colour version of this figure can be viewed online.)
pH value increases. The peak of the dried GH-11.73 is of 3.2 nm. At middle pH range, the PSD curve of GH-5.25 presents double-peak feature, which means the co-existence of small and large pores. The PSD results further confirm the different internal structure of GHs prepared at different pH value, as observed on SEM images. The methylene blue absorption method is adopted to evaluate the SSA of the wet GHs, since the BET method can not reveal the real SSA of the as-prepared GHs due to the collapse during the drying process [24,40,41]. The results of the SSA measurements are presented in Fig. 4, SSA gradually increases with the increasing pH value, which shares the similar varying tendency to the size of GHs (Fig. 2). The minimum SSA is 723.35 m2/g for GH-1.65, while the SSA of GH-11.73 is up to 1107.24 m2/g. As discussed above, GHs with controlled pore structure can be successfully prepared by simply adjusting the pH value of GO precursor. Therefore, the role of pH in the preparing procedure requires to be further investigated to better understand the mechanism. In general, GO sheets bring a large amount of oxygenated groups such as epoxide, hydroxyl and carboxyl. Thus, stable GO aqueous dispersion can be formed due to the hydrophilic groups and the electrostatic repulsion force between GO sheets as a result of the ionization of carboxylic acid [28,42,43]. Recent research, however, shows that it is the electrostatic repulsion force between GO sheets that plays the key role in stabling GO aqueous dispersion [28,44,45]. Herein, pH may change the ionization behaviour of carboxyl groups, which in return affects the stability of the electrostatically stabilized GO dispersion. As it can be seen from Fig. 5, the absolute value of zeta (z) potential increases with the increasing pH value over the whole range, indicating the increasing electrostatic repulsion force between GO sheets [28]. It is reported that with the decreasing of pH value, the electrostatic repulsion force, calculated based on the DLVO theory, is significantly decreased [46]. The reduced GO (rGO) sheets are also influenced by pH value when dispersed in water. The rGO sheets can be stably dispersed in water at alkaline condition, as a result of the ionization of residual carboxyl groups in rGO sheets [28,47]. In other words, the pH value can affect the ionization of carboxyl groups, rendering GO/rGO sheets with different intensity of electrostatic repulsion force, and thus influence the dispersing or self-assembling behaviour of GO/rGO sheets. It is widely accepted that the restored p-p interactions between rGO sheets cause the formation of physical crosslinking sites and drive the self-assembling process [29,31]. During the reduction process, oxygenated functional groups on GO sheets are eliminated
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Fig. 4. SSA of GHs based on the methylene blue absorption method.
Fig. 5. Zeta potential of GO in aqueous dispersion with different pH value.
Fig. 3. SEM images (left) and PSD curves (right) of GHs, the red circles indicate the existence of cracks. (A colour version of this figure can be viewed online.)
to partially restore the p conjugate structure, as confirmed by X-ray diffraction (XRD) patterns (Fig. 6). The interlayer space of freezedried GH-5.25 is calculated to be 3.69 Å. This value is much lower than that of graphite oxide (7.96 Å) and slightly higher than that of graphite (3.35 Å), suggesting the existence of p-p stacking between rGO sheets and the residual oxygenated functional groups on the rGO sheets. Furthermore, the diminished oxygenated groups cause the transformation from hydrophilicity of GO sheets to hydrophobicity of rGO sheets, which also contributes to the self-assembling process. Then, the question remaining to be clarified is that whether altering the pH value would affect the chemical structure of rGO sheets as well as the driving forces of the self-assembling process.
Therefore, FTIR, XPS and Raman measurements are carried out to reveal more detail information before and after the reduction. Fig. 7a is the FTIR spectra of freeze-dried GO precursor and GHs. The peaks for the oxygenated functional groups of GHs dramatically decrease compared to that of GO, and the curves of different GHs show no obvious difference. The results of FTIR agree well with XPS analysis, as displayed in Fig. 7b. The XPS survey scan shows that all GHs contain C and O with almost equal intensity, indicating the similar surface chemical structure of all GHs. Furthermore, C1s spectra of all GHs (Fig. S2) can be fitted into four component peaks located at 284.8, 286.6, 288.0 and 289.2 ev, which can be identified to CeC, CeO, C]O, and OeC]O, respectively. Fig. 7c shows the typical Raman spectra of pristine graphite, GO and GHs. It can be seen that the ID/IG of GO is only 0.82, while that of GHs is almost the same, increasing to 1.10, 1.11 and 1.09, respectively. This phenomenon indicates that GHs prepared within different pH value have the similar electronic conjugation state. Thus, the FTIR, XPS and Raman measurements confirm that the pH value presents little influence on the chemical structure of GHs and all GHs have nearly the same chemical compositions. Thus, changing the pH value of GO precursors would not change the driving forces of the self-
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Fig. 6. XRD pattern of graphite, GO and GH-5.25. (A colour version of this figure can be viewed online.)
assembling process. It is the difference in electrostatic repulsion force between GO/rGO sheets, as a result of varied pH value, that contributes to the formation of GHs with different structure. Accordingly, the pH value can affect the electrostatic repulsion forces between colloid particles, which in here specifically refer to GO/rGO sheets. At lower pH value, repulsion force between rGO sheets is weaker, thus the driving force of the self-assembling conquers the resistance between rGO sheets more easily and compels the rGO sheets to approach to each other and overlap, resulting in a more compact layered structure. While at higher pH value, stronger repulsion force between rGO sheets holds greater resistance to the driving force and repels the crosslinking of rGO sheets, leading to a more loose porous structure. Furthermore, hydrogels prepared at higher pH value, which possess larger pores with higher SSA, have greater interaction areas for water molecules, resulting in higher water content [48]. 3.2. Rheology of GHs The rheological measurements are applied to investigate the structure and property of the as-prepared GHs. Fig. 8a shows the result of small-deformation oscillatory measurement of GH-1.65 at 25 C. Clearly, as a function of angular frequency, storage modulus (G0 ) of GH-1.65 is approximately 1 order of magnitude larger than loss modulus (G00 ) over the entire tested range (1e100 rad/s), indicative of an elastic rather than viscous material with the
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permanent cross-linked network. Moreover, G0 is essentially independent of frequency, while G00 exhibits slightly sensitive to frequency but without crossing over each other. Such rheological behaviour is the characteristic of hydrogel materials with high covalent or noncovalent cross-linking. The other GHs prepared at different pH value also show the similar behaviour (Fig. S3), confirming the solid like gel characteristic of all the as-prepared samples. The G0 values of all GHs recorded at 10 rad/s are evaluated (Fig 8b). The G0 value, decreasing with the increasing pH value, shows the opposite changing trend of size, water content and SSA of GHs. G0 can be seen as a parameter to evaluate the rigidity of hydrogel network. Thus, higher G0 value, at this point, means stronger gel intensity [49]. As mentioned above, the partial overlapping or coalescing of rGO sheets via p-p stacking interactions is the main driving force for self-assembling process. GHs prepared at lower pH value have a more compact structure with the pore walls consisting of thicker stacked layers of graphene sheets. In other words, more cross-linking sites are formed for GHs prepared at lower pH value. Such structure renders GHs prepared at lower pH value with stronger resistance to deformation, resulting in higher G0 value. Yield stress (sc), another important rheological parameter, is measured by oscillatory strain sweep measurements (Fig. 8c and d). During the measurements, the stress first increases linearly with the strain amplitude to reach the maximum value and then deviates from linearity and declines sharply. The stress gets its maximum value at the highest point, which is defined as sc. Further increasing the strain, the hydrogels start to break and flow, and hence sc can be used to evaluate the mechanical strength of hydrogels. Obviously, the sc also decreases with the increasing pH value (Fig. 8d), which further confirms the relationship between mechanical strength and internal structure of GHs. The GHs prepared at lower pH value have a more compact structure with more crosslinking sites and stronger p-p interactions, leading to the formation of more rigid structure. 3.3. Electrical properties of GHs Graphene based three-dimensional materials have been widely applied to fabricate supercapacitor for utilizing their extraordinary high SSA, electrical conductivity and mechanical stability. The porous structure of the as-prepared GHs has been effectively regulated by changing the pH value of the pre-treated GO dispersion. Electrochemical measurements are then carried out to investigate the influence of pore structure on the capacitive performance of GHs. Fig. 9 a & b show the CV curves of GH-1.65 and GH-11.73 in the range of 0e0.8 V vs SCE at the scan rates from 5 to 100 mV/s. Quasi-rectangular shapes can be observed for all GHs at
Fig. 7. (a) FTIR spectral of GO and GHs; (b) XPS survey scan of GHs; (c) Raman spectra of graphite, GO and GHs. (A colour version of this figure can be viewed online.)
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Fig. 8. Rheological properties: (a) G0 and G00 plots as a function of angular frequency of GH-1.65; (b) G0 values (recorded at 10 rad/s) of GHs prepared at different pH; (c) oscillatory strain sweep curve of GH-1.65 and (d) sc values of GHs prepared at different pH. (A colour version of this figure can be viewed online.)
low scan rates (Fig. 9, Fig. S4), indicating the ideal electrical doublelayer capacitive behaviour. The GHs prepared at higher pH value (GH-11.73 and GH-9.51), which possess bigger pores, can keep the rectangle-like shape even at high scan rate of 50 mV/s, while the curves of the other GHs are distorted at this high scan rate. These results confirm the better charge propagation within these electrode materials consisted of larger pores. Another evidence of this better charge propagation is the highest capacitance retention (CR) of the GH-11.73, which occupies the biggest pores, as shown in Fig. 9c. The CR is defined as the ratio of capacitance measured at 5 mV/s and 100 mV/s, respectively. It can be seen that the CR increases along with the pH value. Since pores can be seen as the reservoirs of ions, larger pores can provide sufficient ions in the internal of the electrode and thus shorten the diffusion distance of the ions [50]. And that facilitates the charge/discharge process during the test and helps to maintain the capacitance even at high scan rates. Therefore, larger pores have advantage on optimising the performance of capacitors. The specific capacitance (SC) calculated based on CV measurements at 5 mv/s of all GHs is presented in Fig. 9d. The SC, however, increases with the increasing pH value before reaching the turning point at pH ¼ 5.25 and then decreases, which is not simply increasing along with SSA of GHs as reported elsewhere [51]. Generally, higher SSA means more sites are available for ions to be absorbed on the surface of electrode, resulting in improved capacitance. But the relationship between SSA and capacitance becomes more complicated when pore size of electrode materials is taken into account [50]. As shown in Fig. 3, the PSD curve of GH5.25 has two peaks, representing the cooperation of large and small pores. Larger pores are more preferable for the permeation and storage of ions but less efficient in electroabsorption of electrolyte ions comparing to smaller pores [50]. The special pore structure of GH-5.25 can thus utilize the advantage of both large
and small pores at the same time and contributes to the higher specific capacitance. EIS analysis, another powerful method, is performed to further investigate the role of PSD in supercapacitor performance. Nyquist plots of GHs measured in the frequency range of 100 kHz to 10 mHz are presented in Fig. 10, which show a typical response of porous electrode. The plots can be divided into three parts, the semicircle at high frequency range, the 45 Warburg line at middle frequency range and the near vertical line in low frequency range, respectively. The semicircle represents the interfacial charge transfer resistance. Clearly, diameter of the semicircle gradually decreases with the increasing pH value, indicating that the GHs prepared at higher pH value possess lower charge transfer resistance. Furthermore, the 45 Warburg region in the middle frequency domain, as a result of the ion diffusion/transportation in the electrolyte, becomes shorter, indicating the faster ion movement for the formation of electrical double layer [52]. As for the third part, the near vertical line is one of the symbols of capacitor. The GH-1.65 shows a significant difference with other GHs, that is the line deviates from vertical obviously. Such feature proves that the GH-1.65 has a poor capacitive behaviour. The EIS measurements further confirm the advantage of lager pores for developing excellent capacitors. But combining the results of CV and EIS measurements, conclusion can be drawn that the capacitive performance is much more complicated than the simply linear correlation between pore size and capacitance. The cooperation of large and small pores can balance the merits and shortcomings of them and best suit the designation of high performance capacitors. 4. Conclusions GHs with controllable pore structure are achieved by adjusting the pH value of GO dispersion prior to chemical reduction. The
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Fig. 9. CV curves of GH-1.65 (a) and GH-11.73 (b) at different scan rates; (c) capacitance retention of electrodes made from GHs; (d) specific capacitance of GHs calculated from the CV measurements at the scan rate of 5 mv/s. (A colour version of this figure can be viewed online.)
consisted of more layers of graphene sheets. The rheological tests show that both the G0 and sc values decrease with the increasing pH value. The electrochemical measurements based on the asprepared GHs reveal that larger pores are preferable for the penetration/diffusion of electrolyte ions, but it is the cooperation of large and small pores with suitable PSD that can obtain enhanced capacitive performance. Acknowledgement The authors gratefully acknowledge the financial support of the Science and Technology Planning Project of Guangdong Province, China (Grant NO. 2015A010105008) and the Science and Technology Planning Project of Guangzhou Science Technology & Innovation Commission (201607010049). Thanks also to Professor Yuxia Pang and Jianhao Chen for the measurements of rheology. Appendix A. Supplementary data Fig. 10. Nyquist plots for different hydrogel electrodes. The inset shows the expanded high frequency region of the plots. (A colour version of this figure can be viewed online.)
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2016.08.079. References
absolute value of zeta potential of GO dispersion gradually increases with the pH value, rendering increasing electrostatic repulsion forces between GO/rGO sheets, which is utilized to regulate the self-assembling process. As the pH increases, the peak of PSD curves gradually shifts to larger, from 1.83 to 3.2 nm, and at the middle pH range, the GHs present two peaks indicating the coexistence of large and small pores. Furthermore, GHs prepared at lower pH value are much more compact and the pore walls are
[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. [2] P.C. Sherrell, C. Mattevi, Mesoscale design of multifunctional 3D graphene networks, Mater. Today (2016), http://dx.doi.org/10.1016/ j.mattod.2015.12.004. [3] G. Tai, T. Zeng, H. Li, J. Liu, J. Kong, F. Lv, Temperature and pH effect on reduction of graphene oxides in aqueous solution, Mater. Res. Express 1 (3) (2014) 035605.
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[4] X. Yang, J. Zhu, L. Qiu, D. Li, Bioinspired effective prevention of restacking in multilayered graphene films: towards the next generation of highperformance supercapacitors, Adv. Mater. 23 (25) (2011) 2833e2838. [5] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (10) (2008) 3498e3502. [6] J. Hu, Z. Kang, F. Li, X. Huang, Graphene with three-dimensional architecture for high performance supercapacitor, Carbon (2014) 67221e67229. [7] J. Ye, Y. An, E. Montalvo, P.G. Campbell, M.A. Worsley, I.C. Tran, et al., Solventdirected sol-gel assembly of 3-dimensional graphene-tented metal oxides and strong synergistic disparities in lithium storage, J. Mater. Chem. A 4 (11) (2016) 4032e4043. [8] M. Jana, S. Saha, P. Khanra, P. Samanta, H. Koo, N. Chandra Murmu, et al., Noncovalent functionalization of reduced graphene oxide using sulfanilic acid azocromotrop and its application as a supercapacitor electrode material, J. Mater. Chem. A 3 (14) (2015) 7323e7331. [9] H. Bi, X. Xie, K. Yin, Y. Zhou, S. Wan, L. He, et al., Spongy graphene as a highly efficient and recyclable sorbent for oils and organic solvents, Adv. Funct. Mater. 22 (21) (2012) 4421e4425. [10] S. Barg, F.M. Perez, N. Ni, P. do Vale Pereira, R.C. Maher, E. Garcia-Tunon, et al., Mesoscale assembly of chemically modified graphene into complex cellular networks, Nat. Commun. (2014) 54328. [11] H.H. Bay, D. Patino, Z. Mutlu, P. Romero, M. Ozkan, C.S. Ozkan, Scalable multifunctional ultra-thin graphite sponge: free-standing, superporous, superhydrophobic, oleophilic architecture with ferromagnetic properties for environmental cleaning, Sci. Rep. (2016) 621858. [12] J. Li, C. Liu, Y. Liu, Au/graphene hydrogel: synthesis, characterization and its use for catalytic reduction of 4-nitrophenol, J. Mater. Chem. 22 (17) (2012) 8426e8430. [13] B.F. Machado, P. Serp, Graphene-based materials for catalysis, Catal. Sci. Technol. 2 (1) (2012) 54e75. [14] L.T. Hoa, J.S. Chung, S.H. Hur, A highly sensitive enzyme-free glucose sensor based on Co3O4 nanoflowers and 3D graphene oxide hydrogel fabricated via hydrothermal synthesis, Sens. Actuators B (2016) 22376e22382. [15] V. Georgakilas, J.N. Tiwari, K.C. Kemp, J.A. Perman, A.B. Bourlinos, K.S. Kim, et al., Noncovalent functionalization of graphene and graphene oxide for energy materials, biosensing, catalytic, and biomedical applications, Chem. Rev. 116 (9) (2016) 5464e5519. [16] C. Cheng, G. Jiang, C.J. Garvey, Y. Wang, G.P. Simon, J.Z. Liu, et al., Ion transport in complex layered graphene-based membranes with tuneable interlayer spacing, Sci. Adv. 2 (2) (2016) e1501272. [17] J. Li, S. Zhao, G. Zhang, Y. Gao, L. Deng, R. Sun, et al., A facile method to prepare highly compressible three-dimensional graphene-only sponge, J. Mater. Chem. A 3 (30) (2015) 15482e15488. [18] Z. Wang, Z. Tang, Z. Han, S. Shen, B. Zhao, J. Yang, Effect of drying conditions on the structure of three-dimensional N-doped graphene and its electrochemical performance, RSC Adv. 5 (26) (2015) 19838e19843. [19] X. Jia, C. Zhang, J. Liu, W. Lv, D.-W. Wang, Y. Tao, et al., Evolution of the effect of sulfur confinement in graphene-based porous carbons for use in Li-S batteries, Nanoscale 8 (8) (2016) 4447e4451. [20] Y. Xu, Z. Lin, X. Huang, Y. Wang, Y. Huang, X. Duan, Functionalized graphene hydrogel-based high-performance supercapacitors, Adv. Mater 25 (40) (2013) 5779e5784. [21] C. Zheng, L. Qi, M. Yoshio, H. Wang, Cooperation of micro- and meso-porous carbon electrode materials in electric double-layer capacitors, J. Power Sources 195 (13) (2010) 4406e4409. [22] D. Wang, F. Li, M. Liu, G. Lu, H. Cheng, 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angew. Chem. Int. Ed. Engl. 47 (2) (2008) 373e376. [23] Z. Wu, Y. Sun, Y. Tan, S. Yang, X. Feng, K. Mullen, Three-dimensional graphene-based macro- and mesoporous frameworks for high-performance electrochemical capacitive energy storage, J. Am. Chem. Soc. 134 (48) (2012) 19532e19535. [24] Y. Li, J. Chen, L. Huang, C. Li, J.-D. Hong, G. Shi, Highly compressible macroporous graphene monoliths via an improved hydrothermal process, Adv. Mater 26 (28) (2014) 4789e4793. [25] W. Lv, Y. Tao, W. Ni, Z. Zhou, F. Su, X. Chen, et al., One-pot self-assembly of three-dimensional graphene macroassemblies with porous core and layered shell, J. Mater. Chem. 21 (33) (2011) 12352e12357. [26] W. Chen, L. Yan, In situ self-assembly of mild chemical reduction graphene for three-dimensional architectures, Nanoscale 3 (8) (2011) 3132e3137.
[27] W. Gao, L.B. Alemany, L. Ci, P.M. Ajayan, New insights into the structure and reduction of graphite oxide, Nat. Chem. 1 (5) (2009) 403e408. [28] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nat. Nanotechnol. 3 (2) (2008) 101e105. [29] Y. Xu, K. Sheng, C. Li, G. Shi, Self-assembled graphene hydrogel via a one-step hydrothermal process, ACS Nano 4 (7) (2010) 4324e4330. [30] Z. Sui, X. Zhang, Y. Lei, Y. Luo, Easy and green synthesis of reduced graphite oxide-based hydrogels, Carbon 49 (13) (2011) 4314e4321. [31] H. Cong, X. Ren, P. Wang, S. Yu, Macroscopic multifunctional graphene-based hydrogels and aerogels by a metal ion induced self-assembly process, ACS Nano 6 (3) (2012) 2693e2703. [32] L. Zhang, G. Shi, Preparation of highly conductive graphene hydrogels for fabricating supercapacitors with high rate capability, J. Phys. Chem. C 115 (34) (2011) 17206e17212. [33] L. Zhang, F. Zhang, X. Yang, G. Long, Y. Wu, T. Zhang, et al., Porous 3D graphene-based bulk materials with exceptional high surface area and excellent conductivity for supercapacitors, Sci. Rep. (2013) 31408. [34] J. Yuan, J. Zhu, H. Bi, X. Meng, S. Liang, L. Zhang, et al., Graphene-based 3D composite hydrogel by anchoring Co3O4 nanoparticles with enhanced electrochemical properties, Phys. Chem. Chem. Phys. 15 (31) (2013) 12940e12945. [35] K. Hu, X. Xie, M. Cerruti, T. Szkopek, Controlling the shell formation in hydrothermally reduced graphene hydrogel, Langmuir 31 (20) (2015) 5545e5549. [36] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958), 1339e1339. [37] H. Hu, Z. Zhao, Q. Zhou, Y. Gogotsi, J. Qiu, The role of microwave absorption on formation of graphene from graphite oxide, Carbon 50 (9) (2012) 3267e3273. [38] K. Sheng, Y. Xu, C. Li, G. Shi, High-performance self-assembled graphene hydrogels prepared by chemical reduction of graphene oxide, New Carbon Mater. 26 (1) (2011) 9e15. [39] Z. Han, Z. Tang, P. Li, G. Yang, Q. Zheng, J. Yang, Ammonia solution strengthened three-dimensional macro-porous graphene aerogel, Nanoscale 5 (12) (2013) 5462e5467. [40] X. Yang, C. Cheng, Y. Wang, L. Qiu, D. Li, Liquid-mediated dense integration of graphene materials for compact capacitive energy storage, Science 341 (6145) (2013) 534e537. [41] S. Wu, W. Chen, L. Yan, Fabrication of a 3D MnO2/graphene hydrogel for highperformance asymmetric supercapacitors, J. Mater. Chem. A 2 (8) (2014) 2765e2772. [42] J. Kim, L.J. Cote, F. Kim, W. Yuan, K.R. Shull, J. Huang, Graphene oxide sheets at interfaces, J. Am. Chem. Soc. 132 (23) (2010) 8180e8186. [43] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (6) (2008) 1679e1682. [44] C.J. Shih, S. Lin, R. Sharma, M.S. Strano, D. Blankschtein, Understanding the pH-dependent behavior of graphene oxide aqueous solutions: a comparative experimental and molecular dynamics simulation study, Langmuir 28 (1) (2012) 235e241. [45] L. Wu, L. Liu, B. Gao, R. Munoz-Carpena, M. Zhang, H. Chen, et al., Aggregation kinetics of graphene oxides in aqueous solutions: experiments, mechanisms, and modeling, Langmuir 29 (49) (2013) 15174e15181. [46] M.M. Gudarzi, Colloidal stability of graphene oxide: aggregation in two dimensions, Langmuir 32 (20) (2016) 5058e5068. [47] X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, et al., Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation, Adv. Mater. 20 (23) (2008) 4490e4493. [48] F.A. Aouada, B.-S. Chiou, W.J. Orts, L.H.C. Mattoso, Physicochemical and morphological properties of poly(acrylamide) and methylcellulose hydrogels: effects of monomer, crosslinker and polysaccharide compositions, Polym. Eng. Sci. 49 (12) (2009) 2467e2474. [49] S. Nesrinne, A. Djamel, Synthesis, characterization and rheological behavior of pH sensitive poly(acrylamide-co-acrylic acid) hydrogels, Arab. J. Chem. (2013), http://dx.doi.org/10.1016/j.arabjc.2013.11.027. [50] A. Ghosh, Y.H. Lee, Carbon-based electrochemical capacitors, ChemSusChem 5 (3) (2012) 480e499. [51] H. Teng, Y.-J. Chang, C.-T. Hsieh, Performance of electric double-layer capacitors using carbons prepared from phenoleformaldehyde resins by KOH etching, Carbon 39 (13) (2001) 1981e1987. [52] B.G. Choi, J. Hong, W.H. Hong, P.T. Hammond, H. Park, Facilitated ion transport in all-solid-state flexible supercapacitors, ACS Nano 5 (9) (2011) 7205e7213.
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Fabricating graphene hydrogels with controllable pore structure via one-step chemical reduction process Yuhui Xie, Xinxin Sheng, Delong Xie**, Zixian Liu, Xinya Zhang*, Li Zhong School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2016 Received in revised form 19 August 2016 Accepted 24 August 2016 Available online 25 August 2016
The porous graphene hydrogels (GHs) are of particular interest in various applications, such as energystorage devices, catalyst and sensors. In this paper, GHs with controllable pore structure are prepared from graphene oxide (GO) aqueous dispersion by a facile chemical reduction method. The pore size distribution (PSD) and specific surface area (SSA) of GHs can be regulated by adjusting the pH value of GO dispersion. It is found that both the pore size and SSA of GHs gradually increase with the pH value of GO dispersion. For the GH prepared at pH ¼ 1.65, the PSD has one peak at 1.83 nm with a SSA of 723.35 m2/g. As the pH value increases to 11.73, the peak moves to 3.2 nm and the SSA keeps rising to 1107.24 m2/g. Rheological measurements show that both the storage modulus and yield stress of the GHs decrease with the increasing pH value. Electrochemical evaluation of GHs as the electrodes of supercapacitors reveals that the specific capacitance reaches the highest value for GH prepared at pH ¼ 5.25, which has a specific pore structure, showing the coexistence of large (3.2 nm) and small pores (1.89 nm). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction During the last decades, graphene has aroused increasing interest considering its extraordinarily electrical, thermal and mechanical properties [1e3]. But the severe agglomeration and restacking of graphene sheets, as a result of the p-p interactions between neighbouring graphene sheets, hinders its practical application [4,5]. Fabricating 2D graphene nano-sheets into 3D macroscopic materials with porous network, can partly preserve the inherent structure and properties of graphene sheets, thus has attracted considerable interest. Due to the synergistic combination of the graphene characteristics and the unique micro-structure, 3D macroscopic graphene materials display super performance in versatile applications such as: energy storage [6e8], environmental systems [9e11], catalysis [12,13] and sensors [14,15]. For these applications, the pore characteristics, such as specific surface area (SSA), pore structure and pore size distribution (PSD) of these graphene materials have great influence on the performance [16,17]. In supercapacitors, it is widely accepted that larger pores are better for the penetration/distribution of electrolyte and thus preferable for enhanced performance [18e20]. But some other
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhang).
(D.
http://dx.doi.org/10.1016/j.carbon.2016.08.079 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
Xie),
[email protected]
reports revealed that the performance could be significantly improved when the 3D micro-structure presented appropriate PSD [21e23]. Mechanical strength is another property that affects the application of 3D graphene materials. Recently, a kind of 3D macroporous graphene materials with different morphology were prepared using hexane droplet as the template [24]. The morphology of these materials could be modulated by adjusting the concentration of graphene oxide (GO) dispersion and then greatly influenced the mechanical properties. Apart from numerous methods that have been developed to prepare graphene hydrogels (GHs), self-assembling technique, especially using GO dispersion as the precursor, attracts particular interest because it is low cost, controllable and large scalable [6,25,26]. GO, containing a large number of oxygenated functional groups, can be easily dispersed in water and interact with each other to construct GHs [27,28]. Xu et al. first reported that GO dispersed in aqueous could be hydrothermally reduced and then self-assembled into GHs, which were mechanical strong, electrical conductive and thermal stable [29]. After that, many reducing agents have been applied to assist the preparation of GHs in milder condition, such as vitamin C [26,30], FeSO4 [31], HI [32] and hydroquinone [20]. The pore size of the as-prepared GHs was always randomly distributed, ranging from submicrometer to several micrometers. Many efforts have been paid to optimize the synthesis method to prepare GHs with preferable internal structure [17,19,33]. Incorporating metal oxides into GHs, for example, not only brings GHs with novel properties but also weakens the
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interactions between graphene sheets and enlarges the pore size [7,34]. There also has been reported that the self-assembling process could be optimized by changing the fraction of KMnO4 and concentration of GO, allowing to the formation of GHs with different pore size and pore wall structure [25]. Another interesting work is that combined pre-reduction process with hydrothermal reduction can suppress the shell formation and thus prepared GHs with different architecture [35]. However, the reported methods of manipulating the pore structure of GHs are of multi-step and complicated with limited regulating capability. Herein, we describe a facile one-step method in which GHs with continuous changing pore characteristics can be fabricated from GO dispersion via a modified chemical reduction process. Prior to chemical reduction, different mass of H2SO4 or NaOH solution is added into GO dispersion to regulate the pH, achieving the formation of GHs with tunable pore structure. As the pH value of GO dispersion increases, both the size and the pore diameter of the GHs gradually increase, while the thickness of pore wall decreases. The changing behaviour of the resulting GHs is systematically investigated and a possible mechanism is proposed. Based on these GHs with various pore structure, the rheological and electrochemical measurements are conducted to investigate the relationship between pore structure and properties of GHs. The GHs prepared at lower pH range have higher mechanical strength as a result of the thicker pore walls. While the best capacitance performance is obtained at the middle pH value, i.e. pH ¼ 5.25, where the pore structure of the hydrogel demonstrates an appropriate PSD with the coexistence of large and small pores. 2. Experimental 2.1. Materials Graphite powder (325 mesh, 99.95%) and sodium ascorbate (99%) were supplied by Aladdin (Shanghai, China) and used as received. Concentrate sulfide acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30 wt%) and hydrochloric acid were obtained from Kaixin Chemical Reagent Co., Ltd. (China). Sodium nitrate (NaNO3) and potassium permanganate (KMnO4) were all of analytic grade and purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (China). All chemicals were used without further purification. 2.2. Preparation of GO GO was prepared from graphite powder by modified Hummers' method [36,37]. In brief, 5 g graphite powder was first mixed with 2.5 g NaNO3 in a 2000 mL round flask, 115 mL concentrate sulfide acid was then poured into the flask in an ice bath with vigorous agitation for 2 h. After that, 25 g potassium permanganate was slowly added into the flask in 30 min while the system was kept in ice bath to maintain the temperature under 20 C. The ice bath was then removed, followed by heating the suspension to 35 C for another 1 h reaction, forming a thick paste. At the end of the reaction, 230 mL water was slowly added into the flask with an increasing temperature to 95 C, and the suspension stayed at this temperature for another 15 min. Next, the suspension was further diluted by 700 mL hot water together with 20 mL H2O2 (30%). The mixture was then filtered and washed by 1:10 HCl solution followed by washing with hot water and centrifugation for three times. Later the resulting solid was dispersed in 700 mL water with the help of ultrasonication for 1 h and then submitted to centrifugation at 8000 rpm for 10 min to remove the undissolvable particles. Finally, the GO aqueous dispersion (about 7 mg/mL) was obtained as a result of further purification by dialysis for 1 week.
2.3. Preparation of GHs The GHs were prepared by a modified chemical reduction method [29,38]. Typically, 10 mL GO dispersion (3 mg/mL) was firstly submitted to ultrasonication for 20 min to further exfoliate the GO sheets followed by adding 66 mg sodium ascorbate under ultrasonication to form homogeneous dispersion. And then the pH of the GO dispersion was adjusted by adding NaOH or H2SO4 solution. Successfully, the mixture was sealed in a glass vial and heated at 90 C for 3 h to form the GHs. Finally, the GHs were purified by dialysis for following experiments. 2.4. Characterization The micromorphology of the freeze-dried GHs was investigated by scanning electron microscopy (SEM) using a Zeiss Merlin SEM. The zeta potential of graphene oxide dispersion was measured by Zetasizer Nano-ZS instrument (Malvern Instruments). Powder Xray diffraction (XRD) analysis of freeze-dried GHs was carried out on Bruker D8 ADVANCE diffractometer with Cu Ka radiation generator. The surface chemistry of the GHs was studied on Thermo Scientific Escalab 250Xi with an exciting source of Al. Fourier transform infrared spectra (FTIR) of GO and GHs was obtained using a VERTEX 33 instrument, Bruker, German. Raman spectra were taken by a LabRAM Aramis (HORIBAJobinYvon, France) with lexc ¼ 532 nm. Pore size distribution was measured from N2 absorption at 77 k using the TriStar II 3020 (Micromeritics Inc.) system and calculated according to the BarreteJoynereHalenda (BJH) equation. The samples were applied to degas at 150 C for 12 h before the analysis. The water content of the GHs (Cw) was measured according to Cw ¼ (MGH MD)/MGH 100% [29], where MGH is the original weight of GHs and MD is the weight of the freeze-dried GHs. The rheological investigation of GHs was performed on MCR 300 (Paar Physica) Rheometer at 25 C. A 25 mm parallel-plates geometry was chosen with the fixed gape distance of 2 mm. The dynamic frequency sweep experiments were measured from 1 to 100 rad/s and the oscillatory strain was set to be 0.2% followed by the oscillatory strain sweep measurements. The electrochemical properties of the hydrogels were measured by a conventional three-electrode system using 1 mol/L H2SO4 aqueous solution as the electrolyte. In a typical procedure, GHs were first cut into slices with the thickness of 2 mm, and then immersed in 1 mol/L H2SO4 aqueous solution for 12 h to ensure the complete penetration of electrolyte. After that, the as-prepared slices of GHs were fixed by a platinum clip to make the working electrode. The saturated calomel electrode (SCE) and platinum plate were used as the reference and counter electrode, respectively. Both electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried out on the electrochemical workstation (PGSTAT 302N, Metrohm AG, Switzerland). CV measurements with different scan rates were performed in the potential window of 0e0.8 V. The specific capacitance values were calculated from the CV curves according to the formulation, C ¼ A/2msDV, where A is the integration of CV curve, m is the mass of electrode, s is the scan rate and DV is the potential window. 3. Results and discussion 3.1. Structure of GHs The preparation procedure of GHs with different pore structure is illustrated in Fig. 1a. A series of GO/sodium ascorbate suspensions are first mixed with NaOH or H2SO4 solution to adjust the pH to be
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1.65, 3.47, 5.25, 7.39, 9.51 and 11.73, respectively, and then treated at 90 C for 3 h to fabricate GHs. The as-prepared GHs are respectively denoted as GH-1.65, GH-3.47, GH-5.25, GH-7.39, GH-9.51 and GH-11.73, as shown in Fig. 1b. It is clear that the size and exterior appearance of the GHs are different. The size of GHs exhibits a gradual expansion, and the exterior appearance of GH-n (n 5.25) is much more compact comparing to GHs prepared at higher pH values. Specifically, the diameter and water content of the GHs have been measured and the results are presented in Fig. 2, where we can see that both the diameter and water content of GHS increase with the increasing pH value, e.g. the size increases from 7.995 mm of GH-1.65 to 10.75 mm of GH-11.75, while the water content rises from 97.6% to 98.6%. The interconnected porous microstructure of dried GHs is revealed by SEM, and the results are shown in Fig. 3 (left). It can be seen that the internal structure of the GHs prepared from precursors with different pH value shows obvious difference. The GHs prepared at high pH value are loosely porous, and the pore wall is consisted of fewer layers of graphene sheets. While the GHs prepared at low pH value, by contrast, render much compact structure (Fig. 3 and Fig. S1) and the pore wall is consisted of thicker layers of stacked graphene sheets. Furthermore, as shown in SEM images, GHs prepared at high pH value have more cracks compared to that prepared at low pH value, which may be attributed to the loose structure consisted of fewer layers of graphene sheets. During the freeze drying process, the water in the GHs is first frozen to ice, as a result, the pore volume is expanded to squeeze the graphene layers and collapse the pore walls [18,39]. Thus, GHs prepared at high pH value suffer from more severe collapse as a result of thinner pore walls. Higher water content, in addition, resulting in more ice formation, may also contribute to the cracks. But the dried GHs prepared at lower pH value, by contrast, are consisted of continuous graphene layers with much less fractures. Nitrogen isothermal adsorption analysis is employed to further investigate the pore characteristics of dried GHs. The PSD of dried GHs is calculated by the BarretteJoynereHalenda (BJH) model and the results are presented in Fig. 3 (right), demonstrating the obvious existence of mesopores. The dried GH-1.65 shows only one peak at 1.83 nm. The pore size gradually shifts toward larger as the
Fig. 1. (a) Diagram for synthesis of GHs by reduction of GO dispersion with different pH value; (b) Effect of pH value of GO dispersion on the formation of GHs, the size of GHs gradually increased with the increasing pH value. (A colour version of this figure can be viewed online.)
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Fig. 2. Diameter and water content of GHs as a function of pH value. (A colour version of this figure can be viewed online.)
pH value increases. The peak of the dried GH-11.73 is of 3.2 nm. At middle pH range, the PSD curve of GH-5.25 presents double-peak feature, which means the co-existence of small and large pores. The PSD results further confirm the different internal structure of GHs prepared at different pH value, as observed on SEM images. The methylene blue absorption method is adopted to evaluate the SSA of the wet GHs, since the BET method can not reveal the real SSA of the as-prepared GHs due to the collapse during the drying process [24,40,41]. The results of the SSA measurements are presented in Fig. 4, SSA gradually increases with the increasing pH value, which shares the similar varying tendency to the size of GHs (Fig. 2). The minimum SSA is 723.35 m2/g for GH-1.65, while the SSA of GH-11.73 is up to 1107.24 m2/g. As discussed above, GHs with controlled pore structure can be successfully prepared by simply adjusting the pH value of GO precursor. Therefore, the role of pH in the preparing procedure requires to be further investigated to better understand the mechanism. In general, GO sheets bring a large amount of oxygenated groups such as epoxide, hydroxyl and carboxyl. Thus, stable GO aqueous dispersion can be formed due to the hydrophilic groups and the electrostatic repulsion force between GO sheets as a result of the ionization of carboxylic acid [28,42,43]. Recent research, however, shows that it is the electrostatic repulsion force between GO sheets that plays the key role in stabling GO aqueous dispersion [28,44,45]. Herein, pH may change the ionization behaviour of carboxyl groups, which in return affects the stability of the electrostatically stabilized GO dispersion. As it can be seen from Fig. 5, the absolute value of zeta (z) potential increases with the increasing pH value over the whole range, indicating the increasing electrostatic repulsion force between GO sheets [28]. It is reported that with the decreasing of pH value, the electrostatic repulsion force, calculated based on the DLVO theory, is significantly decreased [46]. The reduced GO (rGO) sheets are also influenced by pH value when dispersed in water. The rGO sheets can be stably dispersed in water at alkaline condition, as a result of the ionization of residual carboxyl groups in rGO sheets [28,47]. In other words, the pH value can affect the ionization of carboxyl groups, rendering GO/rGO sheets with different intensity of electrostatic repulsion force, and thus influence the dispersing or self-assembling behaviour of GO/rGO sheets. It is widely accepted that the restored p-p interactions between rGO sheets cause the formation of physical crosslinking sites and drive the self-assembling process [29,31]. During the reduction process, oxygenated functional groups on GO sheets are eliminated
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Fig. 4. SSA of GHs based on the methylene blue absorption method.
Fig. 5. Zeta potential of GO in aqueous dispersion with different pH value.
Fig. 3. SEM images (left) and PSD curves (right) of GHs, the red circles indicate the existence of cracks. (A colour version of this figure can be viewed online.)
to partially restore the p conjugate structure, as confirmed by X-ray diffraction (XRD) patterns (Fig. 6). The interlayer space of freezedried GH-5.25 is calculated to be 3.69 Å. This value is much lower than that of graphite oxide (7.96 Å) and slightly higher than that of graphite (3.35 Å), suggesting the existence of p-p stacking between rGO sheets and the residual oxygenated functional groups on the rGO sheets. Furthermore, the diminished oxygenated groups cause the transformation from hydrophilicity of GO sheets to hydrophobicity of rGO sheets, which also contributes to the self-assembling process. Then, the question remaining to be clarified is that whether altering the pH value would affect the chemical structure of rGO sheets as well as the driving forces of the self-assembling process.
Therefore, FTIR, XPS and Raman measurements are carried out to reveal more detail information before and after the reduction. Fig. 7a is the FTIR spectra of freeze-dried GO precursor and GHs. The peaks for the oxygenated functional groups of GHs dramatically decrease compared to that of GO, and the curves of different GHs show no obvious difference. The results of FTIR agree well with XPS analysis, as displayed in Fig. 7b. The XPS survey scan shows that all GHs contain C and O with almost equal intensity, indicating the similar surface chemical structure of all GHs. Furthermore, C1s spectra of all GHs (Fig. S2) can be fitted into four component peaks located at 284.8, 286.6, 288.0 and 289.2 ev, which can be identified to CeC, CeO, C]O, and OeC]O, respectively. Fig. 7c shows the typical Raman spectra of pristine graphite, GO and GHs. It can be seen that the ID/IG of GO is only 0.82, while that of GHs is almost the same, increasing to 1.10, 1.11 and 1.09, respectively. This phenomenon indicates that GHs prepared within different pH value have the similar electronic conjugation state. Thus, the FTIR, XPS and Raman measurements confirm that the pH value presents little influence on the chemical structure of GHs and all GHs have nearly the same chemical compositions. Thus, changing the pH value of GO precursors would not change the driving forces of the self-
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Fig. 6. XRD pattern of graphite, GO and GH-5.25. (A colour version of this figure can be viewed online.)
assembling process. It is the difference in electrostatic repulsion force between GO/rGO sheets, as a result of varied pH value, that contributes to the formation of GHs with different structure. Accordingly, the pH value can affect the electrostatic repulsion forces between colloid particles, which in here specifically refer to GO/rGO sheets. At lower pH value, repulsion force between rGO sheets is weaker, thus the driving force of the self-assembling conquers the resistance between rGO sheets more easily and compels the rGO sheets to approach to each other and overlap, resulting in a more compact layered structure. While at higher pH value, stronger repulsion force between rGO sheets holds greater resistance to the driving force and repels the crosslinking of rGO sheets, leading to a more loose porous structure. Furthermore, hydrogels prepared at higher pH value, which possess larger pores with higher SSA, have greater interaction areas for water molecules, resulting in higher water content [48]. 3.2. Rheology of GHs The rheological measurements are applied to investigate the structure and property of the as-prepared GHs. Fig. 8a shows the result of small-deformation oscillatory measurement of GH-1.65 at 25 C. Clearly, as a function of angular frequency, storage modulus (G0 ) of GH-1.65 is approximately 1 order of magnitude larger than loss modulus (G00 ) over the entire tested range (1e100 rad/s), indicative of an elastic rather than viscous material with the
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permanent cross-linked network. Moreover, G0 is essentially independent of frequency, while G00 exhibits slightly sensitive to frequency but without crossing over each other. Such rheological behaviour is the characteristic of hydrogel materials with high covalent or noncovalent cross-linking. The other GHs prepared at different pH value also show the similar behaviour (Fig. S3), confirming the solid like gel characteristic of all the as-prepared samples. The G0 values of all GHs recorded at 10 rad/s are evaluated (Fig 8b). The G0 value, decreasing with the increasing pH value, shows the opposite changing trend of size, water content and SSA of GHs. G0 can be seen as a parameter to evaluate the rigidity of hydrogel network. Thus, higher G0 value, at this point, means stronger gel intensity [49]. As mentioned above, the partial overlapping or coalescing of rGO sheets via p-p stacking interactions is the main driving force for self-assembling process. GHs prepared at lower pH value have a more compact structure with the pore walls consisting of thicker stacked layers of graphene sheets. In other words, more cross-linking sites are formed for GHs prepared at lower pH value. Such structure renders GHs prepared at lower pH value with stronger resistance to deformation, resulting in higher G0 value. Yield stress (sc), another important rheological parameter, is measured by oscillatory strain sweep measurements (Fig. 8c and d). During the measurements, the stress first increases linearly with the strain amplitude to reach the maximum value and then deviates from linearity and declines sharply. The stress gets its maximum value at the highest point, which is defined as sc. Further increasing the strain, the hydrogels start to break and flow, and hence sc can be used to evaluate the mechanical strength of hydrogels. Obviously, the sc also decreases with the increasing pH value (Fig. 8d), which further confirms the relationship between mechanical strength and internal structure of GHs. The GHs prepared at lower pH value have a more compact structure with more crosslinking sites and stronger p-p interactions, leading to the formation of more rigid structure. 3.3. Electrical properties of GHs Graphene based three-dimensional materials have been widely applied to fabricate supercapacitor for utilizing their extraordinary high SSA, electrical conductivity and mechanical stability. The porous structure of the as-prepared GHs has been effectively regulated by changing the pH value of the pre-treated GO dispersion. Electrochemical measurements are then carried out to investigate the influence of pore structure on the capacitive performance of GHs. Fig. 9 a & b show the CV curves of GH-1.65 and GH-11.73 in the range of 0e0.8 V vs SCE at the scan rates from 5 to 100 mV/s. Quasi-rectangular shapes can be observed for all GHs at
Fig. 7. (a) FTIR spectral of GO and GHs; (b) XPS survey scan of GHs; (c) Raman spectra of graphite, GO and GHs. (A colour version of this figure can be viewed online.)
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Fig. 8. Rheological properties: (a) G0 and G00 plots as a function of angular frequency of GH-1.65; (b) G0 values (recorded at 10 rad/s) of GHs prepared at different pH; (c) oscillatory strain sweep curve of GH-1.65 and (d) sc values of GHs prepared at different pH. (A colour version of this figure can be viewed online.)
low scan rates (Fig. 9, Fig. S4), indicating the ideal electrical doublelayer capacitive behaviour. The GHs prepared at higher pH value (GH-11.73 and GH-9.51), which possess bigger pores, can keep the rectangle-like shape even at high scan rate of 50 mV/s, while the curves of the other GHs are distorted at this high scan rate. These results confirm the better charge propagation within these electrode materials consisted of larger pores. Another evidence of this better charge propagation is the highest capacitance retention (CR) of the GH-11.73, which occupies the biggest pores, as shown in Fig. 9c. The CR is defined as the ratio of capacitance measured at 5 mV/s and 100 mV/s, respectively. It can be seen that the CR increases along with the pH value. Since pores can be seen as the reservoirs of ions, larger pores can provide sufficient ions in the internal of the electrode and thus shorten the diffusion distance of the ions [50]. And that facilitates the charge/discharge process during the test and helps to maintain the capacitance even at high scan rates. Therefore, larger pores have advantage on optimising the performance of capacitors. The specific capacitance (SC) calculated based on CV measurements at 5 mv/s of all GHs is presented in Fig. 9d. The SC, however, increases with the increasing pH value before reaching the turning point at pH ¼ 5.25 and then decreases, which is not simply increasing along with SSA of GHs as reported elsewhere [51]. Generally, higher SSA means more sites are available for ions to be absorbed on the surface of electrode, resulting in improved capacitance. But the relationship between SSA and capacitance becomes more complicated when pore size of electrode materials is taken into account [50]. As shown in Fig. 3, the PSD curve of GH5.25 has two peaks, representing the cooperation of large and small pores. Larger pores are more preferable for the permeation and storage of ions but less efficient in electroabsorption of electrolyte ions comparing to smaller pores [50]. The special pore structure of GH-5.25 can thus utilize the advantage of both large
and small pores at the same time and contributes to the higher specific capacitance. EIS analysis, another powerful method, is performed to further investigate the role of PSD in supercapacitor performance. Nyquist plots of GHs measured in the frequency range of 100 kHz to 10 mHz are presented in Fig. 10, which show a typical response of porous electrode. The plots can be divided into three parts, the semicircle at high frequency range, the 45 Warburg line at middle frequency range and the near vertical line in low frequency range, respectively. The semicircle represents the interfacial charge transfer resistance. Clearly, diameter of the semicircle gradually decreases with the increasing pH value, indicating that the GHs prepared at higher pH value possess lower charge transfer resistance. Furthermore, the 45 Warburg region in the middle frequency domain, as a result of the ion diffusion/transportation in the electrolyte, becomes shorter, indicating the faster ion movement for the formation of electrical double layer [52]. As for the third part, the near vertical line is one of the symbols of capacitor. The GH-1.65 shows a significant difference with other GHs, that is the line deviates from vertical obviously. Such feature proves that the GH-1.65 has a poor capacitive behaviour. The EIS measurements further confirm the advantage of lager pores for developing excellent capacitors. But combining the results of CV and EIS measurements, conclusion can be drawn that the capacitive performance is much more complicated than the simply linear correlation between pore size and capacitance. The cooperation of large and small pores can balance the merits and shortcomings of them and best suit the designation of high performance capacitors. 4. Conclusions GHs with controllable pore structure are achieved by adjusting the pH value of GO dispersion prior to chemical reduction. The
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Fig. 9. CV curves of GH-1.65 (a) and GH-11.73 (b) at different scan rates; (c) capacitance retention of electrodes made from GHs; (d) specific capacitance of GHs calculated from the CV measurements at the scan rate of 5 mv/s. (A colour version of this figure can be viewed online.)
consisted of more layers of graphene sheets. The rheological tests show that both the G0 and sc values decrease with the increasing pH value. The electrochemical measurements based on the asprepared GHs reveal that larger pores are preferable for the penetration/diffusion of electrolyte ions, but it is the cooperation of large and small pores with suitable PSD that can obtain enhanced capacitive performance. Acknowledgement The authors gratefully acknowledge the financial support of the Science and Technology Planning Project of Guangdong Province, China (Grant NO. 2015A010105008) and the Science and Technology Planning Project of Guangzhou Science Technology & Innovation Commission (201607010049). Thanks also to Professor Yuxia Pang and Jianhao Chen for the measurements of rheology. Appendix A. Supplementary data Fig. 10. Nyquist plots for different hydrogel electrodes. The inset shows the expanded high frequency region of the plots. (A colour version of this figure can be viewed online.)
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2016.08.079. References
absolute value of zeta potential of GO dispersion gradually increases with the pH value, rendering increasing electrostatic repulsion forces between GO/rGO sheets, which is utilized to regulate the self-assembling process. As the pH increases, the peak of PSD curves gradually shifts to larger, from 1.83 to 3.2 nm, and at the middle pH range, the GHs present two peaks indicating the coexistence of large and small pores. Furthermore, GHs prepared at lower pH value are much more compact and the pore walls are
[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. [2] P.C. Sherrell, C. Mattevi, Mesoscale design of multifunctional 3D graphene networks, Mater. Today (2016), http://dx.doi.org/10.1016/ j.mattod.2015.12.004. [3] G. Tai, T. Zeng, H. Li, J. Liu, J. Kong, F. Lv, Temperature and pH effect on reduction of graphene oxides in aqueous solution, Mater. Res. Express 1 (3) (2014) 035605.
680
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[4] X. Yang, J. Zhu, L. Qiu, D. Li, Bioinspired effective prevention of restacking in multilayered graphene films: towards the next generation of highperformance supercapacitors, Adv. Mater. 23 (25) (2011) 2833e2838. [5] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (10) (2008) 3498e3502. [6] J. Hu, Z. Kang, F. Li, X. Huang, Graphene with three-dimensional architecture for high performance supercapacitor, Carbon (2014) 67221e67229. [7] J. Ye, Y. An, E. Montalvo, P.G. Campbell, M.A. Worsley, I.C. Tran, et al., Solventdirected sol-gel assembly of 3-dimensional graphene-tented metal oxides and strong synergistic disparities in lithium storage, J. Mater. Chem. A 4 (11) (2016) 4032e4043. [8] M. Jana, S. Saha, P. Khanra, P. Samanta, H. Koo, N. Chandra Murmu, et al., Noncovalent functionalization of reduced graphene oxide using sulfanilic acid azocromotrop and its application as a supercapacitor electrode material, J. Mater. Chem. A 3 (14) (2015) 7323e7331. [9] H. Bi, X. Xie, K. Yin, Y. Zhou, S. Wan, L. He, et al., Spongy graphene as a highly efficient and recyclable sorbent for oils and organic solvents, Adv. Funct. Mater. 22 (21) (2012) 4421e4425. [10] S. Barg, F.M. Perez, N. Ni, P. do Vale Pereira, R.C. Maher, E. Garcia-Tunon, et al., Mesoscale assembly of chemically modified graphene into complex cellular networks, Nat. Commun. (2014) 54328. [11] H.H. Bay, D. Patino, Z. Mutlu, P. Romero, M. Ozkan, C.S. Ozkan, Scalable multifunctional ultra-thin graphite sponge: free-standing, superporous, superhydrophobic, oleophilic architecture with ferromagnetic properties for environmental cleaning, Sci. Rep. (2016) 621858. [12] J. Li, C. Liu, Y. Liu, Au/graphene hydrogel: synthesis, characterization and its use for catalytic reduction of 4-nitrophenol, J. Mater. Chem. 22 (17) (2012) 8426e8430. [13] B.F. Machado, P. Serp, Graphene-based materials for catalysis, Catal. Sci. Technol. 2 (1) (2012) 54e75. [14] L.T. Hoa, J.S. Chung, S.H. Hur, A highly sensitive enzyme-free glucose sensor based on Co3O4 nanoflowers and 3D graphene oxide hydrogel fabricated via hydrothermal synthesis, Sens. Actuators B (2016) 22376e22382. [15] V. Georgakilas, J.N. Tiwari, K.C. Kemp, J.A. Perman, A.B. Bourlinos, K.S. Kim, et al., Noncovalent functionalization of graphene and graphene oxide for energy materials, biosensing, catalytic, and biomedical applications, Chem. Rev. 116 (9) (2016) 5464e5519. [16] C. Cheng, G. Jiang, C.J. Garvey, Y. Wang, G.P. Simon, J.Z. Liu, et al., Ion transport in complex layered graphene-based membranes with tuneable interlayer spacing, Sci. Adv. 2 (2) (2016) e1501272. [17] J. Li, S. Zhao, G. Zhang, Y. Gao, L. Deng, R. Sun, et al., A facile method to prepare highly compressible three-dimensional graphene-only sponge, J. Mater. Chem. A 3 (30) (2015) 15482e15488. [18] Z. Wang, Z. Tang, Z. Han, S. Shen, B. Zhao, J. Yang, Effect of drying conditions on the structure of three-dimensional N-doped graphene and its electrochemical performance, RSC Adv. 5 (26) (2015) 19838e19843. [19] X. Jia, C. Zhang, J. Liu, W. Lv, D.-W. Wang, Y. Tao, et al., Evolution of the effect of sulfur confinement in graphene-based porous carbons for use in Li-S batteries, Nanoscale 8 (8) (2016) 4447e4451. [20] Y. Xu, Z. Lin, X. Huang, Y. Wang, Y. Huang, X. Duan, Functionalized graphene hydrogel-based high-performance supercapacitors, Adv. Mater 25 (40) (2013) 5779e5784. [21] C. Zheng, L. Qi, M. Yoshio, H. Wang, Cooperation of micro- and meso-porous carbon electrode materials in electric double-layer capacitors, J. Power Sources 195 (13) (2010) 4406e4409. [22] D. Wang, F. Li, M. Liu, G. Lu, H. Cheng, 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angew. Chem. Int. Ed. Engl. 47 (2) (2008) 373e376. [23] Z. Wu, Y. Sun, Y. Tan, S. Yang, X. Feng, K. Mullen, Three-dimensional graphene-based macro- and mesoporous frameworks for high-performance electrochemical capacitive energy storage, J. Am. Chem. Soc. 134 (48) (2012) 19532e19535. [24] Y. Li, J. Chen, L. Huang, C. Li, J.-D. Hong, G. Shi, Highly compressible macroporous graphene monoliths via an improved hydrothermal process, Adv. Mater 26 (28) (2014) 4789e4793. [25] W. Lv, Y. Tao, W. Ni, Z. Zhou, F. Su, X. Chen, et al., One-pot self-assembly of three-dimensional graphene macroassemblies with porous core and layered shell, J. Mater. Chem. 21 (33) (2011) 12352e12357. [26] W. Chen, L. Yan, In situ self-assembly of mild chemical reduction graphene for three-dimensional architectures, Nanoscale 3 (8) (2011) 3132e3137.
[27] W. Gao, L.B. Alemany, L. Ci, P.M. Ajayan, New insights into the structure and reduction of graphite oxide, Nat. Chem. 1 (5) (2009) 403e408. [28] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nat. Nanotechnol. 3 (2) (2008) 101e105. [29] Y. Xu, K. Sheng, C. Li, G. Shi, Self-assembled graphene hydrogel via a one-step hydrothermal process, ACS Nano 4 (7) (2010) 4324e4330. [30] Z. Sui, X. Zhang, Y. Lei, Y. Luo, Easy and green synthesis of reduced graphite oxide-based hydrogels, Carbon 49 (13) (2011) 4314e4321. [31] H. Cong, X. Ren, P. Wang, S. Yu, Macroscopic multifunctional graphene-based hydrogels and aerogels by a metal ion induced self-assembly process, ACS Nano 6 (3) (2012) 2693e2703. [32] L. Zhang, G. Shi, Preparation of highly conductive graphene hydrogels for fabricating supercapacitors with high rate capability, J. Phys. Chem. C 115 (34) (2011) 17206e17212. [33] L. Zhang, F. Zhang, X. Yang, G. Long, Y. Wu, T. Zhang, et al., Porous 3D graphene-based bulk materials with exceptional high surface area and excellent conductivity for supercapacitors, Sci. Rep. (2013) 31408. [34] J. Yuan, J. Zhu, H. Bi, X. Meng, S. Liang, L. Zhang, et al., Graphene-based 3D composite hydrogel by anchoring Co3O4 nanoparticles with enhanced electrochemical properties, Phys. Chem. Chem. Phys. 15 (31) (2013) 12940e12945. [35] K. Hu, X. Xie, M. Cerruti, T. Szkopek, Controlling the shell formation in hydrothermally reduced graphene hydrogel, Langmuir 31 (20) (2015) 5545e5549. [36] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958), 1339e1339. [37] H. Hu, Z. Zhao, Q. Zhou, Y. Gogotsi, J. Qiu, The role of microwave absorption on formation of graphene from graphite oxide, Carbon 50 (9) (2012) 3267e3273. [38] K. Sheng, Y. Xu, C. Li, G. Shi, High-performance self-assembled graphene hydrogels prepared by chemical reduction of graphene oxide, New Carbon Mater. 26 (1) (2011) 9e15. [39] Z. Han, Z. Tang, P. Li, G. Yang, Q. Zheng, J. Yang, Ammonia solution strengthened three-dimensional macro-porous graphene aerogel, Nanoscale 5 (12) (2013) 5462e5467. [40] X. Yang, C. Cheng, Y. Wang, L. Qiu, D. Li, Liquid-mediated dense integration of graphene materials for compact capacitive energy storage, Science 341 (6145) (2013) 534e537. [41] S. Wu, W. Chen, L. Yan, Fabrication of a 3D MnO2/graphene hydrogel for highperformance asymmetric supercapacitors, J. Mater. Chem. A 2 (8) (2014) 2765e2772. [42] J. Kim, L.J. Cote, F. Kim, W. Yuan, K.R. Shull, J. Huang, Graphene oxide sheets at interfaces, J. Am. Chem. Soc. 132 (23) (2010) 8180e8186. [43] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (6) (2008) 1679e1682. [44] C.J. Shih, S. Lin, R. Sharma, M.S. Strano, D. Blankschtein, Understanding the pH-dependent behavior of graphene oxide aqueous solutions: a comparative experimental and molecular dynamics simulation study, Langmuir 28 (1) (2012) 235e241. [45] L. Wu, L. Liu, B. Gao, R. Munoz-Carpena, M. Zhang, H. Chen, et al., Aggregation kinetics of graphene oxides in aqueous solutions: experiments, mechanisms, and modeling, Langmuir 29 (49) (2013) 15174e15181. [46] M.M. Gudarzi, Colloidal stability of graphene oxide: aggregation in two dimensions, Langmuir 32 (20) (2016) 5058e5068. [47] X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, et al., Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation, Adv. Mater. 20 (23) (2008) 4490e4493. [48] F.A. Aouada, B.-S. Chiou, W.J. Orts, L.H.C. Mattoso, Physicochemical and morphological properties of poly(acrylamide) and methylcellulose hydrogels: effects of monomer, crosslinker and polysaccharide compositions, Polym. Eng. Sci. 49 (12) (2009) 2467e2474. [49] S. Nesrinne, A. Djamel, Synthesis, characterization and rheological behavior of pH sensitive poly(acrylamide-co-acrylic acid) hydrogels, Arab. J. Chem. (2013), http://dx.doi.org/10.1016/j.arabjc.2013.11.027. [50] A. Ghosh, Y.H. Lee, Carbon-based electrochemical capacitors, ChemSusChem 5 (3) (2012) 480e499. [51] H. Teng, Y.-J. Chang, C.-T. Hsieh, Performance of electric double-layer capacitors using carbons prepared from phenoleformaldehyde resins by KOH etching, Carbon 39 (13) (2001) 1981e1987. [52] B.G. Choi, J. Hong, W.H. Hong, P.T. Hammond, H. Park, Facilitated ion transport in all-solid-state flexible supercapacitors, ACS Nano 5 (9) (2011) 7205e7213.