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Three-Dimensional Shape Engineered, Interfacial Gelation of Reduced Graphene Oxide for High Rate, Large Capacity Supercapacitors Uday Narayan Maiti, Joonwon Lim, Kyung Eun Lee, Won Joon Lee, and Sang Ouk Kim* Assembly of graphene into functional macroscopic objects, such as films,[1] sheets,[2] fibers,[3] foams,[4,5] and other complex architectures,[6] is of enormous research interest. How to attain desired structures in a cost effective and manufacturable manner is crucial for energy harvest/storage, catalysis, sensors and so on. Unlike fullerene or carbon nanotubes, whose assembly generally relies on weak van der Walls force or chemical modification, two-dimensional graphene may straightforwardly exploit strong interlayer π–π stacking. Unfortunately, such a strong and directional interaction frequently results in graphitic stacking with minimal surface area.[7,8] Gelation is a straightforward route to macroscopic functional materials from graphene. Taking advantage of high electrical conductivity, large surface area, and soft hydrated character, graphene gel possesses enormous potentials for supercapacitor electrode,[9–12] catalytic support,[13,14] cell growth scaffold[15] and so on.[16,17] To date, several graphene gelation principles have been developed, including reduction of graphene oxide (GO) dispersion,[9,10,18] flow directed interfacial assembly[19] and template assembly.[20] Nevertheless, arbitrary large scale production of optimized porous structures via minimal processing steps remains formidable technological challenge. We present a surprisingly simple and versatile graphene gelation principle capable of three-dimensional shape engineering of micrometer thick hydrogels without any practical size limit. Simple immersion of arbitrary shaped Zn objects in aqueous GO dispersion spontaneously generates graphene hydrogel films at Zn surfaces. This site specific gelation enables a wide range controllability of three-dimensional gel structures in porous morphology as well as macroscopic object scale according to customized purposes. Significantly, this gelation principle has been exploited for high rate, large capacity supercapacitor electrodes. In general, fast charging/discharging rate (or power density) is hardly compatible with large areal capacity[21–23] (or energy density) for energy devices. While thin supercapacitor electrodes with facile electrolyte transports are favorable for high rate capacity, thick electrodes are desired for large areal capacity.[24–26] In this work, three-dimensional controllability of graphene gel morphology optimized the Dr. U. N. Maiti, J. Lim, K. E. Lee, Dr. W. J. Lee, Prof. S. O. Kim Center for Nanomaterials and Chemical Reactions Institute for Basic Science (IBS) Materials Science & Engineering KAIST, Daejeon 305–701, Republic of Korea E-mail: [email protected]
DOI: 10.1002/adma.201303503
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aqueous electrolyte transport within sufficiently thick gel structures. Consequently, fundamental challenge to attain large areal capacity without sacrificing rate capability is successfully addressed. Synthetic scheme of graphene hydrogel is presented in Figure 1a. While Zn foils are immersed in mild acidic dispersion of GO, black graphene hydrogels spontaneously grow at Zn surface. The grown gel thickness is roughly tunable with immersion time. Typically, one hour deposition produced 78-μm-thick gel films in 10−3 M hydrochlorid acid (HCl) containing GO dispersion (Supporting information, Figure S1a). Unreacted physisorbed GO sheets are washed with pure water. The remaining gel films were detached from Zn substrate by mild acid etching and dialyzed to remove acidic impurities. After dialysis, hydrated dark graphene hydrogel films were obtained, whose water content is typically 89 wt%. The surface area of graphitic gel framework, measured by methylene blue absorption (see Experimental Section for details), showed a large value of 614.9 m2 g−1. One hour deposited gel (78 μm thick) withstands the maximum stress of 0.9 MPa (Figure S1b), which is comparable to the typical strengths of polymer or graphene hydrogels.[19,27] This interfacial gelation is arbitrary scalable, as Zn substrate size determines the gelation area (Figure 1b). Cross-sectional SEM inspection of freeze-dried aerogel (Figures 1c,d)) reveals that graphene sheets are interconnected in a quasi-parallel manner to form an open porous morphology. The pore size ranges from tens of nanometers to several micrometers scale. It is obvious that freeze-drying significantly reduces the pore volume, while the spacer water molecules are evaporated. Typically, the thickness of aerogel is approximately reduced by 30% from hydrogel. The chemical structure of aerogel was characterized by XPS and Raman spectroscopy. The deconvoluted C1s XPS spectrum (Figure 1e) shows the three peaks for graphitic structure (C–C/ C=C at 284.6), hydroxyl/epoxy groups (C–O at 286.6 eV), and carbonyl group (O–C=O at 288 eV), respectively. In contrast to the XPS C1s spectrum of GO (Figure S1c), sharp decreases of the peak intensities for oxygen functional groups indicate a high level reduction of GO during gelation.[28,29] In the Raman spectrum (Figure 1f), the G and D peaks appear at 1596 cm−1 and 1352 cm−1 for GO, whereas they appear at 1582 cm−1 and 1352 cm−1 for graphene gel, respectively. The downshifted G peak position matches well with that of pristine graphene. Additionally, a significant increase in the intensity ratio of G to D peaks (ID/IG) from 0.94 to 1.45 is observed due to the increase of sp2 domains.[30] XPS survey for Zn2p (Figure S1d) does not give any detectable peaks for Zn impurity. The graphene hydrogel consisting of highly reduced graphene interconnected
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gelation mechanism, a strong acidic condition increases the reduction potential of GO[32] and thus increases the gelation rate. For one hour gelation, the gel film thickness increases from 145 to 278 to 477 μm, as the HCl concentration is increased by 3, 5 and 10 folds, respectively. This variation of acidity also influences the graphene sheet arrangement within gel films. Graphene sheets are found to be progressively entangled with acidity, as obviously shown in Figures 2d–2f (see large field views in Figure S2). A faster gelation causes less ordered reduction of GO and yields such a randomized network morphology. Accordingly, the specific surface area increases from 623.2 m2 g−1, 666.7 m2 g−1 to 778.5 m2 g−1 with acidity. Our gelation can create layered nanocomposites for synergistic material properties. Irrespective of the shape, size and chemical character, any nanomaterials dispersible in Figure 1. (a) Graphene gelation procedure. (b) Scalability of gelation. (c-d) FESEM images of aqueous GO solution, can be fished within graphene layers during layer-by-layer gelafreeze-dried graphene aerogels. (e) XPS C1s spectrum of aerogel. (f) Raman spectra of GO tion. Figure 2g shows the cross sectional and graphene aerogel. SEM image of nanocomposites including Si nanoparticles. The composite film thickness is tunable up to a few tens of micrometers (Figures S3a–S3b) framework shows a high electrical conductivity. 78 μm thick gel with gelation time. Similar processes can be employed for nanoshows the surface resistivity of 124 Ω/䊐 and bulk conductivity composites, including, for instances, pristine carbon nanotubes of 103.3 S m−1, which correspond to the highest values ever (CNTs) (Figure 2h) and TiO2 particles (Figures S3c–S3d). Owing reported for conducting hydrogels thus far.[19,27,31] to the percolated graphene networks, the nanocomposites mainThe reduction potential of reduced GO (rGO)/GO (−0.4 V tain excellent electrical conductivity (Figure 2i). We note that the vs SHE at pH 4) is higher than that of Zn/Zn2+ (−0.76 V vs composites with CNTs exhibit significantly enhanced conducSHE).[32] Thus, the interfacial gelation follows the reaction tivity, which is attributed to the reduction of inter-sheet resistscheme in an acidic condition, as below. ance between rGO sheets in the presence of highly conducting CNT networks. G O + Zn + H+ → r G O + Zn2+ + H2 O Introduction of various template structures at Zn surface This reaction can be decomposed into two half reactions. facilitates a straightforward route for three-dimensional macroscopic shape engineering of graphene gel. Figures 3a–3c shows ZN → Zn2+ + 2e− (−0.76 V vs SHE) the gels infiltrated into cotton fabric and Ni foam templates. Highly flexible conducting gel fabric is potentially useful for G O + H+ + e − → r G O + H2 O (−0.4 V vs SHE at pH 4) wearable or implantable devices. The three-dimensional gel infiltrated into macroporous Ni form (Figure 3c) is advantaBased on the reduction mechanism the spontaneous gelageous for high rate energy storage devices.[20] Figure 3d demontion can be understood, as illustrated in Figures 2a–2c. Due to strates the site specificity of our gelation process. The gel film the lower reduction potential (or higher oxidation potential) shape exactly follows the prepatterned Zn substrate. The conof Zn, electrons are readily transferred to the GO platelets at formability and site specificity can be further exploited for arbimetal surface, while Zn surface is ionized. The resultant Zn2+ trary shaping into three-dimensional macroscopic structures. attracts negatively charged GO sheets and mediates tight conGraphene gel tube could be directly grown at Zn wire surface tact at Zn surface (Figure 2a). Simultaneously, the reduction (Figure 3e and Figure S4a for larger diameter). Subsequent of GO sheets (Figure 2b) occurs, following the above reaction etching of Zn core generates hollow tube structures. Knitting scheme. Subsequent electron transfer from bottom Zn subZn wires into a net structure yields continuous monolithic grastrate to the additional GO sheets via reduced rGO layer leads phene gel net (Figure 3f). Besides, three-dimensional shaping to the layer-by-layer interfacial deposition of rGO (Figure 2c). of Zn frame (Figure S4b) can generate monolithic graphene gel Strong face to face stacking between rGO layers is restrained with complex morphologies (Figures 3g– 3h). Cross-sectional by the water molecules associated to the residual oxygen SEM investigation of vertically oriented tubes (Figure 3i) functional groups at GO or rGO basal planes.[19] Gelation discloses the quasi-parallel arrangement of graphene sheets gradually slows down with time due to the screening effect retaining the porous morphology. from the previously deposited gel layers. In this electroless 616
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Figure 3. (a) Schematic illustration of templated gelation. (b) Digital images of graphene gels infiltrated into cotton fabric and Ni foam, respectively. (c) SEM images of the graphene gel within Ni foam. (d) Local gelation at prepatterned Zn substrate. (e) SEM image of graphene gel tube grown from Zn wire. (f) Monolithic graphene gel fiber net grown from interwoven Zn wire net. (g,h) 3D graphene gel architecture grown from Zn wires-plates assembly. (i) Cross sectional SEM image of vertical gel tube.
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Figure 2. (a-c) Schematic illustration of interfacial gelation mechanism. (d-f) FESEM images of freeze-dried aerogels prepared from GO dispersion with HCl concentrations of 0.003 M, 0.005 M and 0.01 M, respectively. Cross-sectional FESEM images of (g) graphene-Si particle nanocomposites and (h) graphene-CNT nanocomposites, respectively. (i) Electro-conductivity data of air-dried graphene gel sheet (G-paper) and nanocomposites.
Our graphene gel films consisting of highly electro-conductive rGO framework surrounding open pores is an excellent candidate material for supercapacitor electrode.[33] The supercapacitor performance was analyzed in a symmetric two-electrode configuration with two gel films (thickness: 152 μm, mass loading: 0.44 mg cm−2). Figure 4a displays the cyclic voltammograms (CV) maintaining rectangular shape even at the high scan rate of 2 V s−1. The rectangular shape is well-maintained upto 10 V s−1 (Figure S5a), indicating the low electrochemical internal series resistance. The charge-discharge (CC) profile exhibits perfect triangles even at a high current density of 10 mA cm−2 (Figure 4b), confirming electrical double layer type storage mechanism.[21] Figure 4c presents areal capacitance values (measured from CC curves) for the current densities from 1 to 140 mA cm−2. (The corresponding gravimetric specific capacitances are displayed in Figure S5b). Significantly, the areal capacity does not show significnat reduction at high discharge rates. The areal capacity at 1 mA cm−2 is 33.8 mF cm−2, which slightly decreases to 29.89 mF cm−2 at 10 mA cm−2. A still high value of 26.4 mF cm−2 is measured even at the ultrafast discharge rate of 100 mA cm−2. This indicates that the capacitor can be charged and discharged within 200 ms without sacrificing its areal capacity. Notably, at this ultrafast charge/discharge rate, the CC curve (Figure S5c and S5d) still maintains nearly perfect triangular shape with a nominal voltage drop (0.11 V) at the beginning of discharge. This is attributed to the stable charge double layer formation and fluent ion transport through open porous structure. The areal capacitance values are among the best reported values to date.[11,20,23,34–37] It is noteworthy that the observed high rate capability of our gel films is far better than those of previous bulk graphene gels with random network morphology.[9,10,38,39] Bulk graphene gels usually show acceptable performance upto a low charge/discharge rate below 100 mV s−1. Long electrolyte diffusion pathway via random network morphology principally limits the rate performance. Moreover, due to the intrinsic rigidity and fragility of graphene random network, it is hard to prepare thin and uniform gel films from bulk gelation. By contrast, our interfacial gel films with quasi parallel graphene stacking are inherently flexible and free standing with micrometer scale thickness. They also possess an ideal combination of high areal graphene loading and open porosity. This
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Figure 4. (a) CV curves of graphene gel measured in 1 M H2SO4 solution. (b) CC curves measured at current rate of 10 mA cm−2. (c) Areal capacitance measured at different discharging current rates. (d) Capacity retention during 4000 cycles of charging and discharging (current rate: 10 mA cm−2). (e) Electrochemical impedance spectrum. Inset shows magnified view at low frequency range. (f) Ragone plot.
optimized morphological characteristic results in such an excellent supercapacitor performance achieving both high rate performance and large areal capacitance, simultaneously. Supercapacitors based on our graphene gel films possess excellent stability (Figure 4d) under repeated charging and discharging cycles. When the capacitor is charged and discharged for 4000 cycles at a fast rate of 10 mA cm−2, it retains 97.8% of its initial capacity. From electrochemical impedance analysis from 10 mHz to 100 kHz with 10 mV sinusoidal voltage input (Figure 4e), electrochemical series resistance, estimated from the intercept of low frequency impendence spectrum with real axis (inset of Figure 4e), was measured to be 1.95 Ω. Electrolyte diffusion characteristics within gel electrode can be evaluated by the low frequency spectrum. For an ideal supercapacitor without diffusion barrier, the impedance spectrum at low frequency is perpendicular to the real axis.[40] As shown in the magnified impedance spectrum, it nearly follows the ideal perpendicular characteristic confirming fluent ion transport via hydrated open porous structures. The overall performance of graphene gel based supercapacitor is presented in the form of Ragone plot in Figure 4f. Supercapacitors are commonly regarded as the energy device for high power density rather than high energy density. By contrast, our graphene gel based supercapacitor demonstrates the energy density as high as 2.73 μWh cm−2 as well as high areal power of 369.8 mW cm−2. The maximum attainable areal energy density is 4.66 μWh cm−2 at the power density of 124 mW cm−2. The maximum areal power density of 369.8 mW cm−2 is comparable or even higher than state of the art graphene based supercapacitors.[11,21,23] In summary, we have demonstrated versatile tailored graphene gelation relying on interfacial electroless reduction of GO. Straightforward solution processing enables threedimensional shape-engineering of graphene gels in the level of porous network morphology as well as macroscopic object level. Owing to the high electrical conductivity and fluent ion transport, graphene gel is employed for high performance electrical double layer supercapacitor electrodes. Electrically conductive,
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quasi parallel interconnected graphene framework surrounding open porosity attains not only high rate capability but also large areal capacity, which have rarely been achieved simultaneously. Moreover, customized shape engineering of graphene gel can be further exploited for various other applications, such as biomimetic nanocomposites, tissue engineering, actuators, shape memory materials and so on.
Experimental Section Interfacial Graphene Gelation: Aqueous GO dispersion (6 mg mL−1) was prepared from graphite (Bay Carbon) following a modified Hummers method. The dispersion is diluted to 3 mg mL−1 with predetermined amount of HCl (10−3–10−2 M). Zn foil is immersed into the acidified GO dispersion for interfacial gelation for a desired period (1–5 h). Interfacial gel grown at Zn surface is thoroughly washed with D. I. water and immersed in water (20 min) to remove physisorbed GO platelets. Following this washing step, the gel film is detached from Zn surface by immersion in 20 fold diluted HCl solution. The free standing gel is collected and immersed in aqueous HCl (20 fold diluted) solution for another 4 h to remove Zn impurities. Finally, the prepared gel is kept in D. I. water to remove acidic impurities. For shape engineered gelation, commercially available zinc wires (100 μm diameter and 4 cm length, Alfa Esser) or hand-made Zn wire-sheet assembly were used. After conformal gelation at the desired frame surface, the core Zn frame is etched away with 10% HCl solution. Synthesis of Graphene Based Nanocomposites: TiO2 and Si nanoparticles (Aldrich) are dispersed in D. I. water (2 mg mL−1) by ultrasonication. They are settled down for 10 min and top homogeneous dispersion is selectively taken with a pipette. Equal volume of the nanoparticle dispersion and GO dispersion (6 mg mL−1) were mixed. For CNT composites, single wall CNTs are directly dispersed in 3 mg mL−1 GO solution. In the presence of the nanomaterials, gelation rate slowed down. A higher HCl concentration (0.005 M) is employed to compensate the gelation rate decrease. Other gelation conditions are similar to the pure graphene gelation. The co-assembled nanocomposites are dried at ambient condition to obtain freestanding graphene nanocomposites. Templated Gelation: To obtain a gel fabric, a thin fabric is tightly bound at Zn foil surface before its immersion in acidified GO solution (3 mg mL−1 GO, 0.005 M HCl). After 3 h deposition, the gelled fabric is detached from Zn foil by acid treatment, as described above. Ni foam is infiltrated with 3 mg mL−1 GO solution by 5 min sonication. This wet Ni
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work is financially supported by Institute for Basic Science (IBS). Received: July 27, 2013 Revised: September 4, 2013 Published online: October 18, 2013
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foam is placed at Zn foil surface and subsequently immersed in acidified GO solution (3 mg mL−1 GO, 0.005 M HCl). Gelation is continued for 8 h. Finally the gelled Ni foam is detached from zinc foil by dilute HCl etching. Characterizations: For the measurement of water content in a hydrogel, the weights of hydrogel and corresponding aerogel films with 3 cm × 3 cm size are compared. Average thickness of gel film is estimated by screw gauge measurement. Specific surface area of the gel is determined by methylene blue absorption, which is widely used for graphitic materials.[21,41] Graphene gel sheets with an area of 4 cm2 are mechanically stirred in 10 ml methylene blue solution (4 × 10−4 M) at 200 rpm for 3 days to reach maximum absorption. The concentrations of methylene blue in the solution before and after the absorption by gel is determined by UV-Abs spectroscopy at 665 nm wavelength. Each absorbed methylene blue molecule corresponds to 1.35 nm2 of surface area. Sheet resistance of gel film was measured by a four-wire sensing system (Keithley 2635 system). XPS measurements were carried out with a Sigma Probe with monochromatic X-ray source (Thermo VG Scientific, Inc.). Raman spectroscopic measurement was performed with ARAMIS, Horiba Jobin Yvon, France using 514.5 nm laser source. The mechanical property of gel was measured by Instron 5566 for 1 cm × 1 cm films. The strain rate was 2 mm/min and preload was 0.02 N. Electrochemical Measurements: Electrochemical capacitor performance was tested by sandwiching two gel films (1 cm × 0.5 cm) between two platinum foils. Regular filter papers soaked with electrolyte (1 M H2SO4) were placed between the gel films, which serve as separator and ion porous membrane, simultaneously. The sandwiched structure was kept between two glass slides and the entire assembled system was placed within a narrow channel within a polystyrene block to make sure that the Pt collectors are tightly contact with the gel electrodes, the entire system is partially immersed into electrolyte. The Pt current collectors were connected to electrochemical analysis instrument (VersaSTAT3 electrochemical workstation, Princeton Applied Research, USA). For the measurement of active mass of electrode, graphene gel electrodes were dialyzed for 3 days and dried at 60 °C for 24 h after electrochemical measurements. The mass of dried sample was measured with microbalance, which was used to calculate the specific capacitance. The relations used for the electrochemical analysis are given in the Supporting Information.
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Three-Dimensional Shape Engineered, Interfacial Gelation of Reduced Graphene Oxide for High Rate, Large Capacity Supercapacitors Uday Narayan Maiti, Joonwon Lim, Kyung Eun Lee, Won Joon Lee, and Sang Ouk Kim* Assembly of graphene into functional macroscopic objects, such as films,[1] sheets,[2] fibers,[3] foams,[4,5] and other complex architectures,[6] is of enormous research interest. How to attain desired structures in a cost effective and manufacturable manner is crucial for energy harvest/storage, catalysis, sensors and so on. Unlike fullerene or carbon nanotubes, whose assembly generally relies on weak van der Walls force or chemical modification, two-dimensional graphene may straightforwardly exploit strong interlayer π–π stacking. Unfortunately, such a strong and directional interaction frequently results in graphitic stacking with minimal surface area.[7,8] Gelation is a straightforward route to macroscopic functional materials from graphene. Taking advantage of high electrical conductivity, large surface area, and soft hydrated character, graphene gel possesses enormous potentials for supercapacitor electrode,[9–12] catalytic support,[13,14] cell growth scaffold[15] and so on.[16,17] To date, several graphene gelation principles have been developed, including reduction of graphene oxide (GO) dispersion,[9,10,18] flow directed interfacial assembly[19] and template assembly.[20] Nevertheless, arbitrary large scale production of optimized porous structures via minimal processing steps remains formidable technological challenge. We present a surprisingly simple and versatile graphene gelation principle capable of three-dimensional shape engineering of micrometer thick hydrogels without any practical size limit. Simple immersion of arbitrary shaped Zn objects in aqueous GO dispersion spontaneously generates graphene hydrogel films at Zn surfaces. This site specific gelation enables a wide range controllability of three-dimensional gel structures in porous morphology as well as macroscopic object scale according to customized purposes. Significantly, this gelation principle has been exploited for high rate, large capacity supercapacitor electrodes. In general, fast charging/discharging rate (or power density) is hardly compatible with large areal capacity[21–23] (or energy density) for energy devices. While thin supercapacitor electrodes with facile electrolyte transports are favorable for high rate capacity, thick electrodes are desired for large areal capacity.[24–26] In this work, three-dimensional controllability of graphene gel morphology optimized the Dr. U. N. Maiti, J. Lim, K. E. Lee, Dr. W. J. Lee, Prof. S. O. Kim Center for Nanomaterials and Chemical Reactions Institute for Basic Science (IBS) Materials Science & Engineering KAIST, Daejeon 305–701, Republic of Korea E-mail: [email protected]
DOI: 10.1002/adma.201303503
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aqueous electrolyte transport within sufficiently thick gel structures. Consequently, fundamental challenge to attain large areal capacity without sacrificing rate capability is successfully addressed. Synthetic scheme of graphene hydrogel is presented in Figure 1a. While Zn foils are immersed in mild acidic dispersion of GO, black graphene hydrogels spontaneously grow at Zn surface. The grown gel thickness is roughly tunable with immersion time. Typically, one hour deposition produced 78-μm-thick gel films in 10−3 M hydrochlorid acid (HCl) containing GO dispersion (Supporting information, Figure S1a). Unreacted physisorbed GO sheets are washed with pure water. The remaining gel films were detached from Zn substrate by mild acid etching and dialyzed to remove acidic impurities. After dialysis, hydrated dark graphene hydrogel films were obtained, whose water content is typically 89 wt%. The surface area of graphitic gel framework, measured by methylene blue absorption (see Experimental Section for details), showed a large value of 614.9 m2 g−1. One hour deposited gel (78 μm thick) withstands the maximum stress of 0.9 MPa (Figure S1b), which is comparable to the typical strengths of polymer or graphene hydrogels.[19,27] This interfacial gelation is arbitrary scalable, as Zn substrate size determines the gelation area (Figure 1b). Cross-sectional SEM inspection of freeze-dried aerogel (Figures 1c,d)) reveals that graphene sheets are interconnected in a quasi-parallel manner to form an open porous morphology. The pore size ranges from tens of nanometers to several micrometers scale. It is obvious that freeze-drying significantly reduces the pore volume, while the spacer water molecules are evaporated. Typically, the thickness of aerogel is approximately reduced by 30% from hydrogel. The chemical structure of aerogel was characterized by XPS and Raman spectroscopy. The deconvoluted C1s XPS spectrum (Figure 1e) shows the three peaks for graphitic structure (C–C/ C=C at 284.6), hydroxyl/epoxy groups (C–O at 286.6 eV), and carbonyl group (O–C=O at 288 eV), respectively. In contrast to the XPS C1s spectrum of GO (Figure S1c), sharp decreases of the peak intensities for oxygen functional groups indicate a high level reduction of GO during gelation.[28,29] In the Raman spectrum (Figure 1f), the G and D peaks appear at 1596 cm−1 and 1352 cm−1 for GO, whereas they appear at 1582 cm−1 and 1352 cm−1 for graphene gel, respectively. The downshifted G peak position matches well with that of pristine graphene. Additionally, a significant increase in the intensity ratio of G to D peaks (ID/IG) from 0.94 to 1.45 is observed due to the increase of sp2 domains.[30] XPS survey for Zn2p (Figure S1d) does not give any detectable peaks for Zn impurity. The graphene hydrogel consisting of highly reduced graphene interconnected
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gelation mechanism, a strong acidic condition increases the reduction potential of GO[32] and thus increases the gelation rate. For one hour gelation, the gel film thickness increases from 145 to 278 to 477 μm, as the HCl concentration is increased by 3, 5 and 10 folds, respectively. This variation of acidity also influences the graphene sheet arrangement within gel films. Graphene sheets are found to be progressively entangled with acidity, as obviously shown in Figures 2d–2f (see large field views in Figure S2). A faster gelation causes less ordered reduction of GO and yields such a randomized network morphology. Accordingly, the specific surface area increases from 623.2 m2 g−1, 666.7 m2 g−1 to 778.5 m2 g−1 with acidity. Our gelation can create layered nanocomposites for synergistic material properties. Irrespective of the shape, size and chemical character, any nanomaterials dispersible in Figure 1. (a) Graphene gelation procedure. (b) Scalability of gelation. (c-d) FESEM images of aqueous GO solution, can be fished within graphene layers during layer-by-layer gelafreeze-dried graphene aerogels. (e) XPS C1s spectrum of aerogel. (f) Raman spectra of GO tion. Figure 2g shows the cross sectional and graphene aerogel. SEM image of nanocomposites including Si nanoparticles. The composite film thickness is tunable up to a few tens of micrometers (Figures S3a–S3b) framework shows a high electrical conductivity. 78 μm thick gel with gelation time. Similar processes can be employed for nanoshows the surface resistivity of 124 Ω/䊐 and bulk conductivity composites, including, for instances, pristine carbon nanotubes of 103.3 S m−1, which correspond to the highest values ever (CNTs) (Figure 2h) and TiO2 particles (Figures S3c–S3d). Owing reported for conducting hydrogels thus far.[19,27,31] to the percolated graphene networks, the nanocomposites mainThe reduction potential of reduced GO (rGO)/GO (−0.4 V tain excellent electrical conductivity (Figure 2i). We note that the vs SHE at pH 4) is higher than that of Zn/Zn2+ (−0.76 V vs composites with CNTs exhibit significantly enhanced conducSHE).[32] Thus, the interfacial gelation follows the reaction tivity, which is attributed to the reduction of inter-sheet resistscheme in an acidic condition, as below. ance between rGO sheets in the presence of highly conducting CNT networks. G O + Zn + H+ → r G O + Zn2+ + H2 O Introduction of various template structures at Zn surface This reaction can be decomposed into two half reactions. facilitates a straightforward route for three-dimensional macroscopic shape engineering of graphene gel. Figures 3a–3c shows ZN → Zn2+ + 2e− (−0.76 V vs SHE) the gels infiltrated into cotton fabric and Ni foam templates. Highly flexible conducting gel fabric is potentially useful for G O + H+ + e − → r G O + H2 O (−0.4 V vs SHE at pH 4) wearable or implantable devices. The three-dimensional gel infiltrated into macroporous Ni form (Figure 3c) is advantaBased on the reduction mechanism the spontaneous gelageous for high rate energy storage devices.[20] Figure 3d demontion can be understood, as illustrated in Figures 2a–2c. Due to strates the site specificity of our gelation process. The gel film the lower reduction potential (or higher oxidation potential) shape exactly follows the prepatterned Zn substrate. The conof Zn, electrons are readily transferred to the GO platelets at formability and site specificity can be further exploited for arbimetal surface, while Zn surface is ionized. The resultant Zn2+ trary shaping into three-dimensional macroscopic structures. attracts negatively charged GO sheets and mediates tight conGraphene gel tube could be directly grown at Zn wire surface tact at Zn surface (Figure 2a). Simultaneously, the reduction (Figure 3e and Figure S4a for larger diameter). Subsequent of GO sheets (Figure 2b) occurs, following the above reaction etching of Zn core generates hollow tube structures. Knitting scheme. Subsequent electron transfer from bottom Zn subZn wires into a net structure yields continuous monolithic grastrate to the additional GO sheets via reduced rGO layer leads phene gel net (Figure 3f). Besides, three-dimensional shaping to the layer-by-layer interfacial deposition of rGO (Figure 2c). of Zn frame (Figure S4b) can generate monolithic graphene gel Strong face to face stacking between rGO layers is restrained with complex morphologies (Figures 3g– 3h). Cross-sectional by the water molecules associated to the residual oxygen SEM investigation of vertically oriented tubes (Figure 3i) functional groups at GO or rGO basal planes.[19] Gelation discloses the quasi-parallel arrangement of graphene sheets gradually slows down with time due to the screening effect retaining the porous morphology. from the previously deposited gel layers. In this electroless 616
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Figure 3. (a) Schematic illustration of templated gelation. (b) Digital images of graphene gels infiltrated into cotton fabric and Ni foam, respectively. (c) SEM images of the graphene gel within Ni foam. (d) Local gelation at prepatterned Zn substrate. (e) SEM image of graphene gel tube grown from Zn wire. (f) Monolithic graphene gel fiber net grown from interwoven Zn wire net. (g,h) 3D graphene gel architecture grown from Zn wires-plates assembly. (i) Cross sectional SEM image of vertical gel tube.
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Figure 2. (a-c) Schematic illustration of interfacial gelation mechanism. (d-f) FESEM images of freeze-dried aerogels prepared from GO dispersion with HCl concentrations of 0.003 M, 0.005 M and 0.01 M, respectively. Cross-sectional FESEM images of (g) graphene-Si particle nanocomposites and (h) graphene-CNT nanocomposites, respectively. (i) Electro-conductivity data of air-dried graphene gel sheet (G-paper) and nanocomposites.
Our graphene gel films consisting of highly electro-conductive rGO framework surrounding open pores is an excellent candidate material for supercapacitor electrode.[33] The supercapacitor performance was analyzed in a symmetric two-electrode configuration with two gel films (thickness: 152 μm, mass loading: 0.44 mg cm−2). Figure 4a displays the cyclic voltammograms (CV) maintaining rectangular shape even at the high scan rate of 2 V s−1. The rectangular shape is well-maintained upto 10 V s−1 (Figure S5a), indicating the low electrochemical internal series resistance. The charge-discharge (CC) profile exhibits perfect triangles even at a high current density of 10 mA cm−2 (Figure 4b), confirming electrical double layer type storage mechanism.[21] Figure 4c presents areal capacitance values (measured from CC curves) for the current densities from 1 to 140 mA cm−2. (The corresponding gravimetric specific capacitances are displayed in Figure S5b). Significantly, the areal capacity does not show significnat reduction at high discharge rates. The areal capacity at 1 mA cm−2 is 33.8 mF cm−2, which slightly decreases to 29.89 mF cm−2 at 10 mA cm−2. A still high value of 26.4 mF cm−2 is measured even at the ultrafast discharge rate of 100 mA cm−2. This indicates that the capacitor can be charged and discharged within 200 ms without sacrificing its areal capacity. Notably, at this ultrafast charge/discharge rate, the CC curve (Figure S5c and S5d) still maintains nearly perfect triangular shape with a nominal voltage drop (0.11 V) at the beginning of discharge. This is attributed to the stable charge double layer formation and fluent ion transport through open porous structure. The areal capacitance values are among the best reported values to date.[11,20,23,34–37] It is noteworthy that the observed high rate capability of our gel films is far better than those of previous bulk graphene gels with random network morphology.[9,10,38,39] Bulk graphene gels usually show acceptable performance upto a low charge/discharge rate below 100 mV s−1. Long electrolyte diffusion pathway via random network morphology principally limits the rate performance. Moreover, due to the intrinsic rigidity and fragility of graphene random network, it is hard to prepare thin and uniform gel films from bulk gelation. By contrast, our interfacial gel films with quasi parallel graphene stacking are inherently flexible and free standing with micrometer scale thickness. They also possess an ideal combination of high areal graphene loading and open porosity. This
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Figure 4. (a) CV curves of graphene gel measured in 1 M H2SO4 solution. (b) CC curves measured at current rate of 10 mA cm−2. (c) Areal capacitance measured at different discharging current rates. (d) Capacity retention during 4000 cycles of charging and discharging (current rate: 10 mA cm−2). (e) Electrochemical impedance spectrum. Inset shows magnified view at low frequency range. (f) Ragone plot.
optimized morphological characteristic results in such an excellent supercapacitor performance achieving both high rate performance and large areal capacitance, simultaneously. Supercapacitors based on our graphene gel films possess excellent stability (Figure 4d) under repeated charging and discharging cycles. When the capacitor is charged and discharged for 4000 cycles at a fast rate of 10 mA cm−2, it retains 97.8% of its initial capacity. From electrochemical impedance analysis from 10 mHz to 100 kHz with 10 mV sinusoidal voltage input (Figure 4e), electrochemical series resistance, estimated from the intercept of low frequency impendence spectrum with real axis (inset of Figure 4e), was measured to be 1.95 Ω. Electrolyte diffusion characteristics within gel electrode can be evaluated by the low frequency spectrum. For an ideal supercapacitor without diffusion barrier, the impedance spectrum at low frequency is perpendicular to the real axis.[40] As shown in the magnified impedance spectrum, it nearly follows the ideal perpendicular characteristic confirming fluent ion transport via hydrated open porous structures. The overall performance of graphene gel based supercapacitor is presented in the form of Ragone plot in Figure 4f. Supercapacitors are commonly regarded as the energy device for high power density rather than high energy density. By contrast, our graphene gel based supercapacitor demonstrates the energy density as high as 2.73 μWh cm−2 as well as high areal power of 369.8 mW cm−2. The maximum attainable areal energy density is 4.66 μWh cm−2 at the power density of 124 mW cm−2. The maximum areal power density of 369.8 mW cm−2 is comparable or even higher than state of the art graphene based supercapacitors.[11,21,23] In summary, we have demonstrated versatile tailored graphene gelation relying on interfacial electroless reduction of GO. Straightforward solution processing enables threedimensional shape-engineering of graphene gels in the level of porous network morphology as well as macroscopic object level. Owing to the high electrical conductivity and fluent ion transport, graphene gel is employed for high performance electrical double layer supercapacitor electrodes. Electrically conductive,
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quasi parallel interconnected graphene framework surrounding open porosity attains not only high rate capability but also large areal capacity, which have rarely been achieved simultaneously. Moreover, customized shape engineering of graphene gel can be further exploited for various other applications, such as biomimetic nanocomposites, tissue engineering, actuators, shape memory materials and so on.
Experimental Section Interfacial Graphene Gelation: Aqueous GO dispersion (6 mg mL−1) was prepared from graphite (Bay Carbon) following a modified Hummers method. The dispersion is diluted to 3 mg mL−1 with predetermined amount of HCl (10−3–10−2 M). Zn foil is immersed into the acidified GO dispersion for interfacial gelation for a desired period (1–5 h). Interfacial gel grown at Zn surface is thoroughly washed with D. I. water and immersed in water (20 min) to remove physisorbed GO platelets. Following this washing step, the gel film is detached from Zn surface by immersion in 20 fold diluted HCl solution. The free standing gel is collected and immersed in aqueous HCl (20 fold diluted) solution for another 4 h to remove Zn impurities. Finally, the prepared gel is kept in D. I. water to remove acidic impurities. For shape engineered gelation, commercially available zinc wires (100 μm diameter and 4 cm length, Alfa Esser) or hand-made Zn wire-sheet assembly were used. After conformal gelation at the desired frame surface, the core Zn frame is etched away with 10% HCl solution. Synthesis of Graphene Based Nanocomposites: TiO2 and Si nanoparticles (Aldrich) are dispersed in D. I. water (2 mg mL−1) by ultrasonication. They are settled down for 10 min and top homogeneous dispersion is selectively taken with a pipette. Equal volume of the nanoparticle dispersion and GO dispersion (6 mg mL−1) were mixed. For CNT composites, single wall CNTs are directly dispersed in 3 mg mL−1 GO solution. In the presence of the nanomaterials, gelation rate slowed down. A higher HCl concentration (0.005 M) is employed to compensate the gelation rate decrease. Other gelation conditions are similar to the pure graphene gelation. The co-assembled nanocomposites are dried at ambient condition to obtain freestanding graphene nanocomposites. Templated Gelation: To obtain a gel fabric, a thin fabric is tightly bound at Zn foil surface before its immersion in acidified GO solution (3 mg mL−1 GO, 0.005 M HCl). After 3 h deposition, the gelled fabric is detached from Zn foil by acid treatment, as described above. Ni foam is infiltrated with 3 mg mL−1 GO solution by 5 min sonication. This wet Ni
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work is financially supported by Institute for Basic Science (IBS). Received: July 27, 2013 Revised: September 4, 2013 Published online: October 18, 2013
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foam is placed at Zn foil surface and subsequently immersed in acidified GO solution (3 mg mL−1 GO, 0.005 M HCl). Gelation is continued for 8 h. Finally the gelled Ni foam is detached from zinc foil by dilute HCl etching. Characterizations: For the measurement of water content in a hydrogel, the weights of hydrogel and corresponding aerogel films with 3 cm × 3 cm size are compared. Average thickness of gel film is estimated by screw gauge measurement. Specific surface area of the gel is determined by methylene blue absorption, which is widely used for graphitic materials.[21,41] Graphene gel sheets with an area of 4 cm2 are mechanically stirred in 10 ml methylene blue solution (4 × 10−4 M) at 200 rpm for 3 days to reach maximum absorption. The concentrations of methylene blue in the solution before and after the absorption by gel is determined by UV-Abs spectroscopy at 665 nm wavelength. Each absorbed methylene blue molecule corresponds to 1.35 nm2 of surface area. Sheet resistance of gel film was measured by a four-wire sensing system (Keithley 2635 system). XPS measurements were carried out with a Sigma Probe with monochromatic X-ray source (Thermo VG Scientific, Inc.). Raman spectroscopic measurement was performed with ARAMIS, Horiba Jobin Yvon, France using 514.5 nm laser source. The mechanical property of gel was measured by Instron 5566 for 1 cm × 1 cm films. The strain rate was 2 mm/min and preload was 0.02 N. Electrochemical Measurements: Electrochemical capacitor performance was tested by sandwiching two gel films (1 cm × 0.5 cm) between two platinum foils. Regular filter papers soaked with electrolyte (1 M H2SO4) were placed between the gel films, which serve as separator and ion porous membrane, simultaneously. The sandwiched structure was kept between two glass slides and the entire assembled system was placed within a narrow channel within a polystyrene block to make sure that the Pt collectors are tightly contact with the gel electrodes, the entire system is partially immersed into electrolyte. The Pt current collectors were connected to electrochemical analysis instrument (VersaSTAT3 electrochemical workstation, Princeton Applied Research, USA). For the measurement of active mass of electrode, graphene gel electrodes were dialyzed for 3 days and dried at 60 °C for 24 h after electrochemical measurements. The mass of dried sample was measured with microbalance, which was used to calculate the specific capacitance. The relations used for the electrochemical analysis are given in the Supporting Information.
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