Solution Processable Holey Graphene Oxide and Its Derived

Jun 9, 2015 - Scalable preparation of solution processable graphene and its bulk materials with high specific surface areas and designed porosities is...
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Letter pubs.acs.org/NanoLett

Solution Processable Holey Graphene Oxide and Its Derived Macrostructures for High-Performance Supercapacitors Yuxi Xu,† Chih-Yen Chen,† Zipeng Zhao,‡ Zhaoyang Lin,† Chain Lee,† Xu Xu,† Chen Wang,‡ Yu Huang,‡ Muhammad Imran Shakir,§ and Xiangfeng Duan*,† †

Department of Chemistry and Biochemistry and ‡Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States § Sustainable Energy Technologies Center, College of Engineering, King Saud University, Riyadh 11421, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: Scalable preparation of solution processable graphene and its bulk materials with high specific surface areas and designed porosities is essential for many practical applications. Herein, we report a scalable approach to produce aqueous dispersions of holey graphene oxide with abundant in-plane nanopores via a convenient mild defect-etching reaction and demonstrate that the holey graphene oxide can function as a versatile building block for the assembly of macrostructures including holey graphene hydrogels with a three-dimensional hierarchical porosity and holey graphene papers with a compact but porous layered structure. These holey graphene macrostructures exhibit significantly improved specific surface area and ion diffusion rate compared to the nonholey counterparts and can be directly used as binder-free supercapacitor electrodes with ultrahigh specific capacitances of 283 F/g and 234 F/cm3, excellent rate capabilities, and superior cycling stabilities. Our study defines a scalable pathway to solution processable holey graphene materials and will greatly impact the applications of graphene in diverse technological areas. KEYWORDS: Solution processable, holey graphene, macrostructures, electrochemistry, supercapacitors

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plane nanopores via a convenient mild defect-etching reaction. The HGO sheets can not only be directly assembled into reduced HGO hydrogels (HGH) with a three-dimensional (3D) hierarchical porous network but also be chemically converted into solution processable reduced HGO (HG) which can be further assembled into HG papers (HGP) with a compact layered structure and interlayer porosity. We further show both HGO derived macrostructures (HGH and HGP) exhibit significantly improved specific surface areas and much better ion diffusion dynamics compared to the nonholey graphene counterparts and can function as binder-free supercapacitor electrodes with ultrahigh capacitive energy storage performances in various electrolytes. The solution processable HGO can be easily prepared by heating a homogeneous aqueous mixture of GO and H2O2 at 100 °C for 4 h under stirring. After removing residual H2O2 by centrifuging and washing the reaction mixture, the HGO can be easily redispersed in water to form a stable aqueous dispersion with a high concentration of 2 mg/mL (Figure 1a). The simplicity of the reaction makes the process readily scalable for large quantity production of HGO (Figure 1b). Transmission electron microscopy (TEM) studies revealed abundant in-plane pores with sizes of a few nanometers across the whole basal

raphene, a one-atom-thick carbon sheet, has been anticipated for revolutionizing a wide range of technological areas due to its multiple remarkable physic−chemical properties.1−4 However, pristine graphene or chemically converted graphene sheets with extended π-conjugation in the basal plane are prone to restack with each other via π−π stacking interaction and van der Waals force to form irreversible agglomerates, resulting in a significant deterioration of their properties including severely reduced specific surface area and much lower mass diffusion rate.5−9 In addition, the restacking of graphene sheets usually yields graphite-like powders, which requires additional processing procedures and/or inclusion of passive additives (e.g., binders are indispensable for electrochemistry applications) to fabricate the ultimate products, which can further degrade the overall performance. The free-standing monolithic graphene materials with the properties of individual graphene sheets well maintained are highly desired but a great challenge for many applications of graphene especially for electrochemical energy storage and conversion devices.10−16 To effectively exploit the unique attributes of graphene for many proposed applications, there are at least two major prerequisites. One is the availability of solution processable graphene and its chemical derivatives in large quantities. The other is to minimize the restacking induced property deterioration when the graphene sheets are processed into bulk materials. Herein, we report a scalable approach to produce solution processable holey graphene oxide (HGO) with abundant in© XXXX American Chemical Society

Received: March 30, 2015 Revised: May 21, 2015

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Figure 1. Preparation and characterization of HGO. (a) Photographs of aqueous dispersions of GO and HGO. (b) Photograph of massive production of HGO aqueous dispersion with a concentration of 2 mg/mL. (c) Low- and (d) high-magnification TEM images of HGO. (e) TEM image of cGO prepared in a control experiment. (f) C 1s XPS spectra normalized with respect to the C−C peak and (g) Raman spectra normalized with respect to the G peak for GO, cGO, and HGO. (h) Schematic illustration of formation mechanism of HGO.

oxygenated carbon species that are more chemically active than the graphitic area,17,18 we propose that the oxidative-etching reaction mainly initiate and propagate within the oxygenic defect regions, leading to the preferential removal of oxygenated carbon atoms and generation of carbon vacancies that gradually extend into nanopores in the basal plane (Figure 1h). Extending the reaction time would lead to a more aggressive etching of GO, enlarging the pore size, breaking the sheets, and even destroy them completely (Figure S3 in the Supporting Information). It should be noted that the etching reaction of GO by H2O2 needs high temperature and sufficient amount of H 2 O 2 . In contrast, during the synthesis of GO (see Experimental Methods in the Supporting Information), the addition of a moderate amount of H2O2 to the reaction mixture near room temperature was primarily used to reduce the residual permanganate and manganese dioxide to manganese sulfate. In this way, most H2O2 was consumed rapidly and would not react with GO to produce porous structures in the basal plane. Similar to GO, the solution processable HGO can not only be directly used for a wide range of applications such as nanocomposites, biomaterials, and environmental remediation4,19,20 but also provides a platform for rich solution chemistry including chemical reductions, covalent/noncovalent functionalizations, and supramolecular assembly.21−23 For example, the HGO dispersion can be readily used to prepare

plane of HGO (Figure 1c,d), in clear contrast to nanopore-free control GO (cGO) sheets prepared in a control experiment with no H2O2 added (Figure 1e), suggesting an efficient etching of carbon atoms of GO by H2O2. Nitrogen adsorption− desorption tests showed the HGO exhibited a much higher Brunauer−Emmett−Teller specific surface area of ∼430 m2/g than GO (∼180 m2/g) and a Barrett−Joyner−Halenda pore size distribution in the range of 2−70 nm (Figure S1 in the Supporting Information). Particularly, the HGO showed a more prominent pore size distribution in the range of 2−3 nm, which could be ascribed to the nanopores in the basal plane of HGO. For understanding the formation mechanism of HGO, we employed X-ray photoelectron spectroscopy (XPS) and Raman spectra to characterize the structures of GO, cGO, and HGO. The cGO shows partial deoxygenation (Figure 1f) (see Figure S2 in the Supporting Information for the deconvolution of each XPS spectrum) and increased intensity ratio of D peak to G peak (Figure 1g) in comparison with GO, which is ascribed to the solvent-assisted thermal reduction. In contrast, the HGO shows more significant deoxygenation than cGO and slightly decreased intensity ratio of D peak to G peak compared to GO, indicating that the HGO has fewer oxygen functionalities and fewer defects than the GO and cGO, which is unexpected due to the strong oxidation of H2O2. Given that the defect regions are distributed throughout the basal plane of GO and mainly consist of interconnected B

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Figure 2. Preparation and characterization of HGH. (a) Photographs of a HGO aqueous dispersion (2 mg/mL) and as-prepared HGH. (b) Photographs of a series of HGHs with different sizes and shapes. (c) Low- and (d) high-magnification SEM images of the interior microstructures of freeze-dried HGH. (e) Specific surface areas of HGH and GH determined by MB adsorption method.

monolithic mechanically strong HGHs (Figure 2a) via a reduction induced solution self-assembly process.24−29 As a control experiment, we have also prepared reduced GO hydrogels (GHs) under the same condition. XPS characterization indicated the HGO was sufficiently reduced with a significant deoxygenation during the synthesis of HGH (Figure S4 in the Supporting Information). With the flexible processability of HGO dispersion, the sizes and shapes of HGH can be easily tailored by changing the type of reactors. The freeze-dried HGH showed an interconnected 3D porous network with the pore size ranging from submicrometers to several micrometers and the pore walls consisting of thin layers of stacked HG sheets, as revealed by scanning electron microscopy (SEM) (Figure 2c,d). Methylene blue (MB) dye adsorption method was employed to determine the specific surface areas (SSAs) of HGH and GH.30 The HGH showed a very high accessible SSA of ∼1330 m2/g, ∼34% higher than that of GH (∼990 m2/g), indicating that the nanopores in the basal plane of HG can efficiently promote species to diffuse into the stacked graphene layers within the pore walls and significantly increase the accessible surface area. The unique hierarchical porosity of HGH combining the macropores arising from the 3D self-assembly of HG sheets with the nanopores in the individual HG sheets makes it a highly promising material for high-performance supercapacitor electrodes. We have next studied the electrochemical performances of the symmetric supercapacitors based on HGH and GH films as binder-free electrodes, which were prepared by mechanical pressing of the corresponding hydrogels (Figure S5 in the Supporting Information). Cyclic voltammetry (CV) studies showed a less oblique loop and larger current density for HGH compared to GH at a high scan rate of 1000 mV/s (Figure 3a), suggesting that the HGH has a higher ion accessible SSA and faster ion diffusion rate than GH. The nearly rectangular CV curves and the nearly symmetric triangle galvanostatic charge/discharge curves (Figure 3b,c) indicate a nearly ideal electrical-double-layer (EDL) capacitive character-

Figure 3. Electrochemical characterizations of HGH- and GH-based supercapacitors in 1.0 M H2SO4 aqueous electrolyte. (a) CV curves at a high scan rate of 1000 mV/s. (b,c) Galvanostatic charge/discharge curves at a current density of 1 A/g (b) and 100 A/g (c), respectively. (d) Specific capacitances versus current densities. (e) Nyquist plots with inset showing the close-up views of the high-frequency regime. (f) Cycling stability of HGH-based supercapacitor at a current density of 20 A/g.

istic for both HGH and GH. The HGH exhibited a specific capacitance of 283 F/g at a current density of 1 A/g, 38% higher than that of GH (205 F/g) (Figure 3b). When the current density was increased up to 100 A/g, the HGH could C

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Nano Letters retain as high as ∼75% of its initial value (212 F/g), while the GH only showed a ∼66% capacitance retention (136 F/g) (Figure 3d). Furthermore, the HGH showed a smaller voltage (IR) drop (0.14 V) than the GH (0.25 V) at the initial stage of the discharge curve at the current density of 100 A/g (Figure 3c), implying a lower equivalent series resistance (ESR) for HGH. The ion diffusion dynamics within the HGH and GH were further probed by electrochemical impedance spectroscopy (EIS) (Figure 3e). The Nyquist plots obtained over a frequency range from 100 kHz to 10 mHz showed a vertical line in the low-frequency regime, indicating a nearly ideal capacitive property for HGH and GH. A close-up view of the high-frequency regime of the Nyquist plots revealed a semicircle with smaller diameter and a short 45° Warburg region for HGH, confirming a lower charge transfer resistance and more rapid ion diffusion within the HGH due to its hierarchical porosity.31 By extrapolating the vertical portion of the plot to the real axis, an ESR could be derived to be 0.8 Ω for HGH, almost half of that of GH (1.4 Ω), which is consistent with the results of galvanostatic charge/discharge studies. Furthermore, the HGH also demonstrated excellent cycling stability with a ∼94% capacitance retention over 20 000 cycles at a high current density of 20 A/g (Figure 3f). The obvious capacitance decay in the initial 1000 cycles could be ascribed to the removal of a few oxygen-containing groups on HG sheets that contribute to pseudocapacitance and electrode/ electrolyte interface wetting.31 By chemical reduction of the HGO dispersion, we can also obtain well-dispersed solution processable HG (Figure 4a) through the electrostatic stabilization,32 which can be further assembled into large-area flexible HGP (Figure 4b) with a compact layered structure (Figure 4c) via a flow-directed selfassembly strategy.32−34 The HGP showed a SSA of 217 m2/g determined by the MB adsorption method, greatly higher than that of reduced GO paper (GP) (12 m2/g) prepared under the same condition, suggesting that the surface area of graphene within the HGP is highly accessible due to the nanopores in the HG sheets in spite of the compact stacking structure. Particularly, the effect of nanopores promoted ion diffusion and access to the graphene surface will be more significant in the compact papers than in the 3D hydrogels (Figure 4d), as confirmed by CV, galvanostatic charge/discharge, and EIS studies (Figure 4e−i). The HGP exhibited a specific capacitance of 209 and 157 F/g at a current density of 1 and 20 A/g, respectively, which are considerably higher than those of GP (116 and 65 F/g at 1 and 20 A/g, respectively) (Figure 4h). The greatly improved electrochemical performance of HGP was also evidenced by Nyquist plots that showed a much smaller semicircle and a much shorter 45° Warburg region for HGP (Figure 4i). With a high packing density of 1.12 g/cm3, the HGP could deliver an ultrahigh volumetric capacitance of 234 F/cm3, making it extremely promising for high-volumetricperformance supercapacitor electrode that is increasingly important for many applications with limited space such as miniaturized electronic devices. With the flexible and wearable electronics becoming increasingly widespread in our daily lives, there is also an rising demand for high-performance flexible solid-state supercapacitors for power supply.35−37 However, most of previous flexible solid-state devices use current collectors or supporting substrates for loading the electrode materials because of their poor mechanical strength and/or low electrical conductivity. Meanwhile, the mass loading and/or the packing density of the

Figure 4. Preparation and electrochemical characterizations of HGPand GP-based supercapacitors in 1.0 M H2SO4 aqueous electrolyte. (a) Photographs of aqueous dispersions of HGO and HG. (b) Photograph of a free-standing flexible HGP with a thickness of ∼9 μm. (c) SEM image of the cross-section of the HGP. (d) Schematic illustration of ion diffusion pathway across the GP and HGP. (e) CV curves at a scan rate of 200 mV/s. Galvanostatic charge/discharge curves at a current density of 1 A/g (f) and 20 A/g (g), respectively. (h) Specific capacitances versus current densities. (i) Nyquist plots.

electrode materials are usually low. All of these will greatly decrease the ratio of active electrode materials in the entire device and increase the mass or volume fraction of electrochemically passive components, leading to low specific capacitances when normalized by the total weight or volume of the entire device. In contrast, with a high electrical conductivity of 2030 S/m, the mechanically strong HGP can be used to fabricate flexible solid-state ultrathin film supercapacitors without any current collectors or supporting substrates (Figure 5). The entire device containing two 9 μm thick HGP electrodes only showed a total thickness of ∼30 μm, resulting in an ultrahigh volume ratio of 60% for the active electrode materials in the ultimate device. With an efficient infiltration of poly(vinyl alcohol) (PVA)-H2SO4 gel electrolyte into the porosity of HGP, the HGP-based solid-state supercapacitor exhibited a high specific capacitance of 201 and 140 F/g at a current density of 1 and 20 A/g, respectively, comparable to the device in aqueous electrolyte (Figure 5a,b). Because of the high packing density of 1.12 g/cm3 and high volume ratio of HGP electrodes, the entire device showed a superior volumetric capacitance of 34 F/cm3 at 1 A/g, which D

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For achieving higher energy density, we have evaluated the electrochemical performances of HGH- and HGP-based supercapacitors in organic electrolyte (2.0 M 1-ethyl-3methylimidazolium tetrafluoroborate (EMIMBF4) in acetonitrile (AN)) (Figure 6). With this electrolyte the operating voltage of the device can be extended from 1.0 to 3.5 V. With a highly accessible porosity arising from the holey graphene building block, the HGH and HGP could deliver a specific capacitance of 272 and 181 F/g respectively at a current density of 1 A/g, much higher than that of GH (198 F/g) and GP (97 F/g). Thus, a high energy density of 116 and 77 Wh/kg can be achieved for HGH and HGP, respectively. Moreover, the HGP could show a high volumetric capacitance of 203 F/cm3 and a high volumetric energy density of 86 Wh/L. It is worth noting that the capacitive energy storage performances of HGH- and HGP-based supercapacitors in aqueous, polymer gel, and organic electrolytes are comparable to or better than those of the best carbon nanomaterials reported to date (Table S1 and S2 in the Supporting Information), thus making these holey graphene macrostructures highly promising electrode materials for next-generation high-performance supercapacitors. In summary, we have reported a convenient and scalable defect-etching strategy to prepare solution processable HGO with abundant nanopores across the entire basal plane. The processable HGO can be directly self-assembled into monolithic HGHs with a hierarchical 3D porosity. Meanwhile, the reduction of HGO dispersion can produce solution processable HG which can be further assembled into flexible HGPs with a compact but porous layered structure. Because of the significantly enhanced ion diffusion and surface access enabled by nanopores in the holey graphene building block, both HGH and HGP show superior capacitive energy storage performance in various electrolytes, which is not only much better than those of nonholey graphene counterparts but also comparable to or better than those of the best carbon nanomaterials. This work defines a scalable pathway to solution processable holey graphene material and its derived macro-

Figure 5. Electrochemical characterization of HGP-based flexible solid-state supercapacitor. (a) Galvanostatic charge/discharge curves. (b) Specific capacitances versus current densities. (c) CV curves of the device at a scan rate of 200 mV/s at different bending radius. The inset shows the flexibility of the device with a total thickness of ∼30 μm. (d) Cycling stability of the device at a current density of 10 A/g at a bending radius of ∼2 mm. The inset shows the schematic illustration of the device configuration.

significantly outperforms that of laser scribed graphene-based device (0.55 F/cm3 at 1 A/g).35 Furthermore, the HGP-based solid-state supercapacitor showed excellent mechanical flexibility with almost the same electrochemical behavior even at a small bending radius of ∼2 mm (Figure 5c), and superior cycling stability with a ∼90% capacitance retention over 20 000 charge/discharge cycles at a high current density of 10 A/g under bending state (Figure 5d). Additionally, the solid-state device demonstrated a low self-discharge characteristic similar to that of commercial supercapacitors (Figure S6 in the Supporting Information), thus holding a great potential for powering the flexible and wearable electronic products.

Figure 6. Electrochemical characterizations of HGH- and HGP-based supercapacitors in organic electrolyte (2.0 M EMIMBF4 in AN) with GH and GP for comparison. (a,b) Galvanostatic charge/discharge curves of HGH- and GH-based supercapacitors at a current density of 1 A/g (a) and 20 A/ g (b), respectively. (c) Specific capacitances versus current densities for HGH- and GH-based supercapacitors. (d,e) Galvanostatic charge/discharge curves of HGP- and GP-based supercapacitors at a current density of 1 A/g (d) and 20 A/g (e), respectively. (f) Specific capacitances versus current densities for HGP- and GP-based supercapacitors. E

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structures with remarkable electrochemical performance, which can address the challenge for the applications of graphene in the electrochemical energy storage devices and beyond.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, additional characterization data, and comparison with previous results. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b01212.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Electron Imaging Center for Nanomachines (EICN) at the California NanoSystems Institute for the technical support of TEM. X.D. acknowledges partial support by a Dupont Young Professor Award (electrochemical characterization). I.S. would like to extend his sincere appreciation to the Deanship of Scientific Research at the King Saudi University for its funding of this research through the Research Prolific Research Group, Project No PRG-143625.

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