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Fabricating 3D Macroscopic Graphene-Based Architectures with Outstanding Flexibility by the Novel Liquid Drop/Colloid Flocculation Approach for Energy Storage Applications Meng Han,† Anjali Jayakumar,§ Zongheng Li,† Qiannan Zhao,† Junming Zhang,§ Xiaoping Jiang,† Xiaolong Guo,† Ronghua Wang,‡ Chaohe Xu,*,†,∥ Shufeng Song,† Jong-Min Lee,*,§ and Ning Hu*,†

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College of Aerospace Engineering, and The State Key Laboratory of Mechanical Transmissions, Chongqing University, Chongqing 400044, China ‡ College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China § School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore ∥ Key Laboratory of Low-Grade Energy Utilization Technologies and Systems of the Ministry of Education of China, Chongqing 400044, China S Supporting Information *

ABSTRACT: Inspired by “water ripples” in nature and the flocculation phenomenon in colloid chemistry, a novel liquid drop/ colloid flocculation approach is developed to fabricate an extremely flexible and compressible 3D macroscopic graphene-based architecture (hydrogels or aerogels), via a new coagulation-induced self-assembly mechanism. This facile and universal technique can be achieved in a neutral, acidic, or basic coagulation bath, producing microsized hydrogels with various structures, such as mushroom, circle, disc shapes, etc. The method also allows us to introduce various guest materials in the graphene matrix using transition metal salts as the coagulating bath. A mushroom-shaped NiCo oxide/GS hybrid aerogel (diameter: 3 mm) is prepared as an example, with ultrathin NiCo oxide nanosheets in situ grown onto the surface of graphene. By employing as binder-free electrodes, these hybrid aerogels exhibit a specific capacitance of 858.3 F g−1 at 2 A g−1, as well as a good rate capability and cyclic stability. The asymmetric supercapacitor, assembling with the hybrid aerogels as cathode and graphene hydrogels as anode materials, could deliver an energy density of 21 Wh kg−1 at power density of 4500 W kg−1. The ease of synthesis and the feasibility of obtaining highly flexible aerogels with varied morphologies and compositions make this method a promising one for use in the field of biotechnology, electrochemistry, flexible electronics, and environment applications. KEYWORDS: graphene, hydrogel, NiCo oxides, flocculation, supercapacitors

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surface area (theoretical value: 2630 m2 g−1),6 and excellent mechanical strength (Young’s modulus ∼1 TPa).7 Despite all the advantages of graphene, the complete exploitation of these extremely useful properties has been hindered by some drawbacks in the preparation and subsequent applications of graphene. For example, graphene

INTRODUCTION

Graphene is a single atomic layer of sp2-bonded carbon atoms arranged in honeycomb lattices. It was first exfoliated from highly oriented pyrolytic graphite using a micromechanical technique by Novoselov and Geim in 2004.1 Ever since its successful preparation and characterization, graphene has attracted tremendous attention owing to its various unprecedented properties, such as ultrahigh carrier mobility (∼1000 cm2 V−1 s−1),1 high optical transmittance (∼97.7%),2 good thermal conductivity (∼5000 W m−1 K−1),3−5 high specific © XXXX American Chemical Society

Received: February 23, 2018 Accepted: June 13, 2018

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DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

neutral, alkaline, or acidic medium for a coagulating bath increases the practicability of the method. It also gives researchers the opportunity to play around with the method parameters to produce various shapes for hydrogels, such as mushroom, circle, disc shapes, etc. as demonstrated in this study. Most importantly, the method makes it possible to produce microsized hydrogels, showing great promise in applications in the field of microdevices. As an example, 3D mushroom-shaped NiCo oxide/graphene hybrid aerogels (defined as NiCo/GA, diameter: 3 mm) were prepared using this liquid drop/colloid flocculation approach, with ultrathin NiCo oxide nanosheets uniformly in situ anchored onto the surface of graphene. Benefiting from the porous structure, high conductivity, and synergistic effect of ultrathin nanosheets, the hybrid aerogels exhibited high specific capacitance, good rate capability, and excellent cyclic performance when employed as binder-free electrode of supercapacitors. Moreover, we assembled an asymmetric supercapacitor by using our designed hybrid aerogels and pure graphene hydrogels in this work. The energy densities as high as 24.6 and 21 Wh kg−1 were achieved at 804.5 and 4500 W kg−1, respectively. The hybrid device also exhibits remarkable cyclic capabilities with almost no capacity loss even extending to more than 5000 cycles, suggesting that the novel method is quite effective to prepare 3D macroscopic graphene-based architectures with potential applications in energy storage and conversion devices.

sheets easily restack into tight aggregates due to large surface energy and π−π interactions, finally resulting in sharp reduction of its effective specific surface area.8−10 In electrochemical systems, the serious issue of restacking of graphene sheets could restrict ion diffusion from the electrolyte solution to the surface of electrode materials and reduce the energy storage performances.11−14 This makes it difficult for graphene to be employed in stable and efficient electrochemical systems, which usually demand porous structure and high surface area for increasing active sites and free diffusion of ions. It has been a great challenge for researchers worldwide to find ways to overcome these obstacles and find solutions to fully tap the potential of graphene-based materials.15,16 To solve these problems, the development of three-dimensional (3D) architectures has attracted immense attention17−19 because 3D structures can effectively prevent the restacking of graphene sheets and maintain their ultrahigh specific surface area.20 Among these, graphene hydrogels, which are water-rich 3D structures with an extremely porous framework, exhibit specific surface area as high as ∼1000 m2 g−1 and have demonstrated extraordinary properties when used as supercapacitors, electrocatalysts, etc.17−19,21,22 Generally, the most conventional approaches to prepare graphene hydrogels were the hydrothermal/solvothermal method, or chemical reduction technique, wherein the graphene sheets are self-assembled via the π−π stacking forces during the reduction process to form a water-rich continuous 3D network.23−25 Apart from these, hybrid graphene-based hydrogels, combining guest materials with graphene, can also be prepared by the above methods and have been designed to synergistically improve the comprehensive properties.26−28 However, the methods have a few limitations when it comes to producing highly flexible and compressible aerogels, and the final shape of the hydrogel/ aerogel formed is usually the shape of the reaction container, the most common being the cylinder shape and some very rare systems forming triangular shape, gear shape, etc. Moreover, these approaches are always complex and time-consuming, and if considering mass production, expensive equipment is further required. Thus, searching for efficient, scalable, and costeffective techniques is crucial to both scientific research and practical application for graphene-based hydrogels and aerogels. In this work, we successfully designed a novel approach called a liquid drop/colloid flocculation approach to synthesize 3D graphene hydrogels and/or aerogels (obtained after freeze drying of hydrogels), which can be achieved in an acidic, neutral, or alkaline coagulating bath (Figure S1). This method also lets us incorporate guest molecules and compounds in situ, like NiCo-oxide in our study, and this is performed with ease by employing transition metal salt solutions as the coagulating bath. Unlike the hydrothermal/solvothermal or chemical reduction method, the formation of hydrogels here is based on a new coagulation-induced self-assembly mechanism: the coagulation starts when the colloidal solution meets an electrolyte in the coagulation bath, giving way to an agglomerated mass in the bath, which forms the hydrogel. Benefiting from that, the obtained aerogels exhibited outstanding flexibility and can recover their original shape after dozens of compression and decompression cycles. The method makes it very easy to prepare pure or hybrid graphene hydrogels (and/or aerogels), and it also has enormous potential to be modified in various ways to prepare other 3D materials apart from graphene. The possibility of using a

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EXPERIMENTAL SECTION Synthesis of 3D Graphene-Based Architectures. All chemicals were purchased from Aladdin Industrial Corporation without further purification. A coagulating bath was prepared by dissolving 1.0 mmol of nickel nitrate and 2.0 mmol of cobalt nitrate in 20 mL of absolute ethanol, defined as solution A. Then, 5.0 mmol of hexamethylenetetramine (HMT) was dissolved into 12 mL of distilled water under stirring (defined as solution B). Afterward, these two solutions were mixed together under vigorous magnetic stirring, and the coagulating bath containing the bimetallic (Ni, Co) nitrate precursor was obtained. GO aqueous dispersion (∼10 mg mL−1) was injected into this coagulating bath drop by drop under rotation. Afterward, the intermediate products were heated at 95 °C for 4 h in an oven. The products obtained at this step were thoroughly washed, freeze-dried, and calcined at 300 °C for an hour to obtain the novel 3D mushroom-like NiCo oxide/graphene aerogels. Pure graphene hydrogels can be synthesized by washing the obtained NiCo oxide/graphene hydrogels with 0.1 M HCl. In this study, we employed these graphene hydrogels as anode materials for asymmetric supercapacitors. Materials Characterizations. Microstructures were studied using field emission scanning electron microscopy (FESEM, JEOL, JSM-6700F) and transmission electron microscopy (TEM, JEOL, JEM-2010F). BET was determined by micromeritics ASAP 2020 V3.04 G. X-ray photoelectron spectroscopy (XPS) was performed on a KRATOS AXIS DLD spectrometer. Raman spectroscopy was performed by LabRAM HR Evolution of HORIBA (Jobin Yvon S.A.S, France). Fourier transform infrared (FT-IR) spectra were recorded on a NICOLET iN10 (Thermo Fisher Scientific, America) infrared spectrophotometer. Electrochemical Measurements. The 3D NiCo/GA and pure graphene hydrogels were used as working electrodes B

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the preparation process of the 3D macroscopic graphene-based hybrid aerogels or hydrogels and as electrode materials of ASC.

water ripple on a static water surface. It is by drawing inspiration from these simple, yet useful flocculation and water-ripple propagation phenomena that we developed our liquid drop/colloid flocculation method for the preparation of hydrogels. The flocculation phenomenon on one hand creates an inward aggregation tendency in the GO drop, whereas the water-ripple effect gives it a counteracting force to spread and propagate when a GO drop falls on a static electrolyte solution. It is the effect of these counter-acting forces which leads to interesting shapes like the mushroom, circle, disc shapes, etc. in the hydrogels produced by this method (Figure S1, as shown in Supporting Information). Based on the above description, the liquid drop/colloid flocculation approach makes use of a new coagulation-induced self-assembly mechanism: the coagulation starts when the colloidal solution meets an electrolyte in the coagulation bath, giving way to an agglomerated mass in the bath, which forms the hydrogel. This is totally different from the conventional hydrothermal/solvothermal or chemical reduction method, where the formation mechanism involves the π−π stacking interactions between graphene nanosheets to form the 3D continuous structure. Learning from colloidal chemistry, the coagulation solutions can be a wide range of electrolytes, such as acidic (HCl, H2SO4, etc.), neutral (KCl, NaCl, Na2SO4, etc.), and alkaline (KOH, NaOH, etc.) solutions and soluble transition metal salts (FeCl3, FeSO4, CuCl2, etc.). In this work, pure graphene hydrogels can be obtained in acidic, neutral, alkaline, or soluble transition metal salt solutions (Figure S1), demonstrating a facile, effective, and universal characteristic of this method and exhibiting great promise in practical applications. As shown in Figure S2, GO aqueous dispersion was dropped into pure H2O and NaOH aqueous solutions with different concentrations. In the case of pure H2O, GO aqueous dispersion will partially diffuse into liquid in 1−2 min and hardly assemble into a well-defined 3D structure. However, by employing NaOH aqueous solution as a coagulation bath, we can produce GO hydrogels with different shapes. The difference was that the as-formed 3D structures in a low concentration were not strong enough and could be destroyed easily when touched gently by a toothpick. However, they can keep the original shape in a high concentration bath. The reason was that the flocculation effect of GO colloid solution became stronger in a high concentration electrolyte solution, thus leading to the final 3D GO structures with high strength. Moreover, the developed method also showed

directly. No polymer binders or carbon additives were added in this work. Several pieces of small mushroom-like active materials were pressed into nickel foams as an electrode. The mass loading of the active materials was about ∼1.5 mg. The 3D NiCo/GA and pure graphene hydrogels were first studied in a three-electrode setup. The counter electrode, reference electrode, and electrolyte are a piece of Pt foil, saturated calomel electrode (SCE), and 2.0 M KOH aqueous solution. The electrochemical tests for asymmetric supercapacitors (ASC) were evaluated by a two-electrode configuration, where the 3D NiCo/GA and graphene hydrogels were used as the cathode and anode materials, respectively. All electrochemical data were recorded by a CHI760E electrochemical workstation at room temperature. For NiCo/GA, the current densities and specific capacitances were determined based on the total active mass including graphene. For ASC, the total mass of cathode materials and anode materials was used to calculate the operating current densities. Electrochemical calculations were performed according to equations listed in our previous works.28

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RESULTS AND DISCUSSION The 3D macroscopic graphene-based architectures with novel shapes and outstanding flexibility were fabricated by a newly developed liquid drop/colloid flocculation approach followed by some simple postprocessing steps, as illustrated in Figure 1. We make use of a commonly observed flocculation phenomenon, where the addition of electrolytes or soluble salts into a stable colloidal system kick-starts agglomeration or coagulation of solids. As clarified in previous works, GO dispersion is a well-known colloidal solution, which is validated by the fact that it shows a profound Tyndall effect, an intrinsic characteristic of colloids.29,30 Thus, it is clear that flocculation would happen on introducing an electrolyte solution into GO suspension; the converse is also true. The difference here on adding the GO solution into the electrolyte solution is in the enormous volume and concentration of the electrolyte solution surrounding the drop of the added GO solution. This leads to more intense aggregation and coagulation compared to the coagulation effect observed on adding the electrolyte into the GO solution, and this is due to the stronger electrostatic attraction acting on the GO drop from the surrounding electrolyte. Another interesting observation of a natural phenomenon that inspired us to develop our method came from perceiving how a disturbance created and propagated a C

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a−d) SEM images of the as-synthesized 3D NiCo/GA hybrid and (e) TEM and (f) HRTEM images of the 3D NiCo/GA hybrid.

significant advantages in mass production, where a large quantity of hydrogels or aerogels can be easily prepared at a time, as shown in Figure S3a. Excitingly, the obtained graphene-based aerogels (after freeze-drying and calcination) were super lightweight and exhibited outstanding flexibility and compressibility and can bear dozens of compression− decompression cycles by tweezers (Figure S3b and Supplementary Video 1). Except for the production of pure graphene hydrogels, the designed approach could also let us incorporate guest materials in situ. It is performed with ease by employing transition metal salt solutions as the coagulating bath. For example, 3D NiCo/ GA is obtained in the coagulating bath consisting of a mixed Ni nitrate and Co nitrate precursor, followed by hydrothermal and calcination treatments. Beyond that, we also employed Co(NO3)2, FeCl3, and MnSO4 ethanol solutions as coagulating baths and successfully prepared different kinds of hybrid hydrogels and aerogels (Figure S4), verifying that this novel synthesis technique could be extended to other materials systems. The ease of synthesis and the feasibility of obtaining highly flexible aerogels with varied shapes and compositions make this method a promising one for use in a broad range of material preparations for lithium ion batteries, supercapacitors, sensors, catalysts, and so on. To gain insight into the microstructures and morphology of the obtained macroscopic hydrogels, we employed FE-SEM and TEM techniques. For NiCo/GA, it can be observed that the morphology resembled a three-dimensional mushroom-like structure, with the surface full of wrinkles and corrugations, as depicted in Figure 2a. More detailed information was collected

from the high-resolution SEM and TEM images. As illustrated in Figure 2b−2e, sheet-like structures are observed with thickness of several nanometers, which were in situ grown onto the surface of graphene. Elemental analysis via EDS mapping proved that the ultrathin nanosheets were composed of Ni, Co, and O with a uniform distribution, as displayed in Figures S6 and S7. HRTEM results demonstrated that the lattice distance of the nanosheets was 0.156 and 0.212 nm, respectively, matched well with (311) planes of NiCo2O4 and (200) planes of NiO (Figure 2f and Figure 3), respectively, indicating that the ultrathin nanosheets were composed of NiCo2O4 and NiO nanocrystals. The nitrogen adsorption−desorption isotherms of NiCo/GA exhibited a typical IV hysteresis loop at a relative pressure of 0.4−0.8 (Figure S8a), indicating multimodal pore size characteristics. The BET surface area of 57.85 m2 g−1 with

Figure 3. XRD patterns of NiCo/GA. D

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Survey XPS spectra of the 3D NiCo/GA hybrid. Core level (b) C 1s, (c) N 1s, (d) O 1s, (e) Co 2p, and (f) Ni 2p XPS spectra.

deformation vibrations (1397 cm−1), C−O−C (1226 cm−1), and alkoxy C−O stretching vibrations (1057 cm −1 ), respectively. After reduction, the peak intensities of oxygen functional groups were reduced significantly, demonstrating the considerable deoxygenation nature of GO (green curve, Figure S10b). Additionally, the skeletal vibration of rGO was observed at about 1575 cm−1 on the FT-IR spectrum of pure GA (the inset).35 The spectrum of NiCo/GA also shows two metal−oxygen vibration peaks at 550 cm−1 (Co−O) and 639 cm−1 (Ni−O) (Figure S10c). The vibrational band at 1384 cm−1 was assigned to carboxy C−O deformation vibrations. Apart from this, the band at about 1578 cm−1 was also observed in NiCo/GA, similar to pure GA.36 XPS was used to characterize the surface composition, electronic structure, and valence states of the hybrid NiCo/ GA. The full XPS spectrum exhibits that the NiCo/GA is composed of C, N, O, Ni, and Co elements (Figure 4a). By using a Gaussian/Lorentz fitting, the C 1s peak can be divided into five peaks at 284.71 (C−C/CC), 285.28 (C−OH), 286.26 (C−O−C), 287.16 (CO), and 288.7 eV (C(O)−O), respectively (Figure 4b). Compared with pure GO in our previous work,28 the obvious reduction in peak intensities indicates that GO was reduced to GS successfully during the

an average pore size of 5.05 nm (Figure S8c) was detected and thus could further improve the electrochemical properties of the aerogels by enhancing ion immersion and diffusion. Raman results were used to analyze the structural features and composition of GO, pure GA, and NiCo/GA. As shown in Figure S9, the D band and G band of GO and GA were centered at ∼1341 and ∼1578 cm−1; however, for NiCo/GA, these two bands were centered at ∼1354 and ∼1595 cm−1, respectively. There was a blue shift compared with pure GA special for the G band, revealing the p-type doping effect. This indicates significant electronic interactions between GA and NiCo oxides. The ID/IG values in the case of GO were found to be 1.0095, but in pure GA and NiCo/GA, the ID/IG ratio was 1.1529 and 1.0616, demonstrating the decreases of the in-plane sp2 domain sizes and removal of oxygen functional groups of GO.31 Additionally, the obvious peak at 540.09 cm−1 in the Raman spectrum of the NiCo/GA corresponds to Ni−O and Co−O vibrations.32−34 FT-IR spectra were recorded to determine the surface functional groups of GO, pure GA, and NiCo/GA, as seen in Figure S10. As for GO (blue curve), the vibrational bands could index as CO stretching vibration of the carboxyl group (1727 cm−1), aromatic CC (1620 cm−1), carboxy C−O E

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) CV and (b) galvanostatic charge−discharge curves of the 3D NiCo/GA hybrid. (c) The calculated specific capacitance at different current densities and (d) the cyclic performances of this NiCo/GA.

Figure 6. (a, b) SEM images of the dried graphene hydrogels synthesized by the diluted HCl etching process, (c) EDS mapping, and (d) the corresponding EDS spectra of the dried graphene hydrogels.

(Figure 4e) can be divided into two spin−orbit characteristics of Co2+ and Co3+ (780.25 and 795.5 eV) and their respective shakeup satellite peaks (786.5 and 803.75 eV). The Ni 2p emission peaks (Figure 4f) can also be fitted into two spin− orbits, typical of Ni2+ and Ni3+ (855.28 and 872.98 eV), and two shakeup satellite peaks (860.63 and 880.23 eV). These

reactions (Figure 4b). As displayed in Figure 4c, the N 1s spectrum can be fitted into pyridinic N (398.9 eV), pyrrolic N (400.1 eV), and graphitic N (401.1 eV), respectively. Apparently, nitrogen doping was caused by the chemical decomposition of HMT during hydrothermal reaction, which will further enhance the electrochemical properties of graphene, as clarified by Huang’s work.37 The Co 2p curve F

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. (a) CV curves of the pure GS hydrogels and (b) the calculated specific capacitances. (c) Galvanostatic charge−discharge plots and (d) the corresponding specific capacitances. (e) Cyclic performances of the designed pure GS hydrogels.

results show that the sample contains Co2+, Co3+, Ni2+, and Ni3+, in good accordance with the literature.38−41 Electrodes were fabricated from the 3D NiCo/GA hybrids (∼1.5 mg) by directly sandwiching the hybrid aerogels between two pieces of nickel foam (1 cm × 1 cm) without any conductive additives or polymer binders. We first employed a three-electrode setup to study the electrochemical performances of the hybrid aerogels in a 2 M KOH aqueous electrolyte. The CV curves of the 3D NiCo/GA hybrid electrodes were recorded in the potential window of 0−0.5 V at different scan rates. As shown in Figure 5a, the electrode possesses excellent reversibility as observed from the symmetric anodic and cathodic peaks. The anodic and cathodic peaks appeared at 0.2 and 0.3 V at a low scan rate of 2 mV s−1, respectively, mainly indexed to Faradaic redox reactions related to M−O/M−O−OH (M is Ni or Co).42 To well study the electrochemical performances, we further performed a series of galvanostatic charge−discharge tests. The curves are quite symmetric, suggesting excellent reversibility, as seen in Figure 5b. Furthermore, the charge−discharge curves display an obvious voltage plateau between 0.2 and 0.3 V at all current density ranges, according well with CV results. The

calculated capacitance of the electrode is about 858.3, 791.2, 740.5, 709, and 684 F g−1 at 2, 4, 6, 8, and 10 A g−1, respectively, demonstrating superior rate performances, as shown in Figure 5c. Additionally, our materials also possess good cyclic properties, with no observable degradation found after 2000 cycles but a surprising 26% enhancement of the specific capacitance due to the gradual electrochemical activation process (Figure 5d), as observed in most of the Ni−Co based electrode materials.43−47 The excellent capacitance performances of the designed electrode could be ascribed to the 3D structures of NiCo/GA hybrid: (i) the ultrathin layered structure of NiCo oxide nanosheets favors the fast electrochemical reactions and increases the utilization of active materials; (ii) the porous nature of the hybrid aerogels ensures fast electrolyte ion diffusion; (iii) the in situ nucleation and growth combined with 3D continuous conductive network endow a desirable transport path for electrons; (iv) the binderfree NiCo/GA hybrid is also greatly favorable for the fast energy storage. To clarify the possibility in practical applications, we further characterize the capacitance performances of this 3D NiCo/ GA in ASC, by using graphene hydrogels as anode materials. G

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 8. (a) CV curves and (b) charge−discharge curves of the NiCo/GA//graphene ASCs. (c) The corresponding rate performance plots of ASCs. (d) Ragone plot of the NiCo/GA//graphene ASCs and comparison with results in the literature. (e) The cyclic properties of ASCs.

were 169 (2 mV s−1), 162 (5 mV s−1), 156 (10 mV s−1), 152 (20 mV s−1), and 143 F g−1 (50 mV s−1) at different scan rates, respectively, as shown in Figure 7b. In galvanostatic charge− discharge tests (Figure 7c and 7d), the specific capacitance retained about 89.8%, even increasing the current density from 2 to 10 A g−1, demonstrating its excellent rate performance. Furthermore, after 1000 cycles, pure graphene hydrogels also deliver capacitance retention as high as ∼99.5%, showing superior cycling stability (Figure 7e). Thus, the delivered specific capacitance, rate performance, and cycling stability make graphene hydrogels an ideal candidate for use as anode materials in ASC. The ASC was assembled using the 3D mushroom-like porous NiCo/GA as the cathode electrode and pure graphene hydrogels as the anode electrode. The masses and charges of two opposite electrodes were optimized according to our previous works. The optimized mass ratio was 0.40 based on CV results at 10 mV s−1. As displayed in Figure 8a, CV results of our NiCo/GA//graphene ASC device possess a stable voltage window from 0.8 to 1.6 V. Therefore, the operational voltage range of ASC was set as 0.0−1.6 V. Figure 8b shows the charge−discharge curves at various current densities. The

Notice: herein, the pure graphene hydrogels were collected from NiCo oxide/graphene hydrogels by diluted HCl etching method. As demonstrated in Figure 6a and 6b, the freeze-dried pure graphene hydrogels also show a mushroom-like structure with some changes of the morphology during HCl etching, owing to the continuous Ni and Co dissolution. EDS mapping and spectra clearly demonstrated that all of Ni and Co were etched out by the diluted HCl solution (Figure 6c and 6d). Thus, there are no influences of metallic impurities on the final capacitance performances of graphene hydrogels. The freezedried pure graphene hydrogels were further characterized by BET measurements. It is shown that the freeze-dried graphene hydrogels have a BET value of ∼76.6 m2 g−1, average pore sizes of ∼10.0 nm, and pore volume of 0.148 cm3 g−1. The relatively low specific surface value may be caused by the inevitable partial restacking of graphene layers during the freeze-drying process. Figure 7a shows the CV curves of the pure graphene hydrogels between −1.0 and 0.0 V at various scan rates. Quasirectangular shapes were observed at scan rates of 2 to 50 mV s−1, demonstrating an electrical double-layer capacitance mechanism. The calculated capacitances based on CV results H

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces high symmetry of the curves suggests a great reversibility of this hybrid device. The cell capacitances can reach up to 69, 62, 57, 52, and 48 F g−1 at 2, 4, 6, 8, and 10 A g−1, respectively (Figure 8c). The corresponding energy and power densities were also studied, as depicted in Ragone plots. The energy density can reach to 24.6 Wh kg−1 at power density of 804.5 W kg−1, demonstrating a good electrochemical performance (Figure 8d). This superior electrochemical performance is better than some of the previous values reported in the literature for similar materials, such as Co(OH)2 nanoflakes// AC,48 RuO2-GS//PANi-GS,49 MnO2-GS//GS,50 and RuO2GS//GS.51 The excellent cyclic properties were also evaluated at 8 A g−1 to verify their ability for practical applications. As shown in Figure 8e, the calculated specific capacitance did not decrease even when the cycle number reached 5000 times (enhanced 20% compared to initial value), demonstrating the predominant cyclic performances. From the above results, superior electrochemical properties are identified for our hybrid aerogels obtained using our newly developed liquid drop/ colloid flocculation approach, combined with different postprocessing. Considering the highly practicable concept and its easy operation, the developed technique for graphenebased aerogels and hydrogels is promising for larger-scale applications pertaining to energy.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C. Xu). *E-mail: [email protected] (N. Hu). *E-mail: [email protected] (J.-M. Lee). ORCID

Chaohe Xu: 0000-0002-1345-1420 Shufeng Song: 0000-0001-9049-8305 Jong-Min Lee: 0000-0001-6300-0866 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21503025, 21603019), Fundamental Research Funds for the Central Universities (No. 0903005203377, 106112016CDJXY130001, and 106112016CDJZR325520), Key Program for International Science and Technology Cooperation Projects of Ministry of Science and Technology of China (No. 2016YFE0125900), Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA1059), and Hundred Talents Program of Chongqing University.

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CONCLUSIONS In summary, we developed a completely new liquid drop/ colloid flocculation approach to synthesize 3D macroscopic graphene-based hybrid hydrogels or aerogels, learning from a simple flocculation phenomenon occurring at a colloidal/ electrolyte interface and a water-ripple propagation phenomenon at a static liquid surface. The porous and highly conducting aerogels prepared from this method showed outstanding flexibility and compressibility and retained their structures without collapse even after dozens of compression and decompression cycles. The method also gives the feasibility to scalably produce hydrogels/aerogels of varied size, shape, and composition, for applications in various fields. Taking the mushroom-like porous NiCo/GA electrodes as an example, the delivered specific capacitance is as high as 858 F g−1, with capacitance retention of up to 86.5%, indicating superior rate capability. The assembled ASC by using 3D porous NiCo/GA as cathode materials and graphene hydrogels as anode materials can deliver energy density of 24.6 and 21 Wh kg−1 at 804.5 and 4500 W kg−1, respectively, and remarkable cyclic performances even extending to 5000 cycles. Our work thus brings out a totally new concept which is facile, effective, and universal to produce 3D macroscopic graphenebased architectures of varied sizes and shapes, giving us a lot of promise for engineering applications.

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compressibility during compression−decompression cycles by tweezers (AVI) Video (AVI)

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REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306 (5696), 666−669. (2) De, S.; Coleman, J. N. Are There Fundamental Limitations on the Sheet Resistance and Transmittance of Thin Graphene Films? ACS Nano 2010, 4 (5), 2713−2720. (3) Nika, D. L.; Balandin, A. A. Two-dimensional phonon transport in graphene. J. Phys.: Condens. Matter 2012, 24 (23), 233203. (4) Calizo, I.; Balandin, A. A.; Bao, W.; Miao, F.; Lau, C. N. Temperature dependence of the Raman spectra of graphene and graphene multilayers. Nano Lett. 2007, 7 (9), 2645−2649. (5) Balandin, A. A. Thermal properties of graphene and nanostructured carbon materials. Nat. Mater. 2011, 10 (8), 569−581. (6) Stoller, M. D.; Park, S. J.; Zhu, Y. W.; An, J. H.; Ruoff, R. S. Graphene-Based Ultracapacitors. Nano Lett. 2008, 8 (10), 3498− 3502. (7) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321 (5887), 385−388. (8) Xin, G. Q.; Sun, H. T.; Hu, T.; Fard, H. R.; Sun, X.; Koratkar, N.; Borca-Tasciuc, T.; Lian, J. Large-Area Freestanding Graphene Paper for Superior Thermal Management. Adv. Mater. 2014, 26 (26), 4521−4526. (9) Luo, Z. T.; Lu, Y.; Somers, L. A.; Johnson, A. T. C. High Yield Preparation of Macroscopic Graphene Oxide Membranes. J. Am. Chem. Soc. 2009, 131 (3), 898−899. (10) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and characterization of graphene oxide paper. Nature 2007, 448 (7152), 457−460. (11) Luo, J. Y.; Jang, H. D.; Huang, J. X. Effect of Sheet Morphology on the Scalability of Graphene-Based Ultracapacitors. ACS Nano 2013, 7 (2), 1464−1471. (12) Weng, Z.; Su, Y.; Wang, D. W.; Li, F.; Du, J. H.; Cheng, H. M. Graphene-Cellulose Paper Flexible Supercapacitors. Adv. Energy Mater. 2011, 1 (5), 917−922.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02942. Experimental section and additional figures, such as photo graphics, elemental mapping, EDS spectra, BET, and pore size distribution curves (PDF) Video of graphene-based aerogels (after freeze-drying and calcination) exhibiting outstanding flexibility and I

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces (13) Xiong, Z. Y.; Liao, C. L.; Han, W. H.; Wang, X. G. Mechanically Tough Large-Area Hierarchical Porous Graphene Films for High-Performance Flexible Supercapacitor Applications. Adv. Mater. 2015, 27 (30), 4469−4475. (14) Yu, A. P.; Roes, I.; Davies, A.; Chen, Z. W. Ultrathin, transparent, and flexible graphene films for supercapacitor application. Appl. Phys. Lett. 2010, 96 (25), 253105−253105−3. (15) Wang, R. H.; Xu, C. H.; Sun, J.; Gao, L. Three-Dimensional Fe2O3 Nanocubes/Nitrogen-doped Graphene Aerogels: Nucleation Mechanism and Lithium Storage Properties. Sci. Rep. 2015, 4, 7171. (16) Wang, R. H.; Xu, C. H.; Du, M.; Sun, J.; Gao, L.; Zhang, P.; Yao, H. L.; Lin, C. C. Solvothermal-Induced Self-Assembly of Fe2O3/ GS Aerogels for High Li-Storage and Excellent Stability. Small 2014, 10 (11), 2260−2269. (17) Tai, Z. X.; Yan, X. B.; Xue, Q. J. Three-Dimensional Graphene/ Polyaniline Composite Hydrogel as Supercapacitor Electrode. J. Electrochem. Soc. 2012, 159 (10), A1702−A1709. (18) Chen, S.; Duan, J. J.; Tang, Y. H.; Qiao, S. Z. Hybrid Hydrogels of Porous Graphene and Nickel Hydroxide as Advanced Supercapacitor Materials. Chem. - Eur. J. 2013, 19 (22), 7118−7124. (19) Xu, Y. X.; Lin, Z. Y.; Huang, X. Q.; Liu, Y.; Huang, Y.; Duan, X. F. Flexible Solid-State Supercapacitors Based on Three-Dimensional Graphene Hydrogel Films. ACS Nano 2013, 7 (5), 4042−4049. (20) Liu, F.; Song, S. Y.; Xue, D. F.; Zhang, H. J. Folded Structured Graphene Paper for High Performance Electrode Materials. Adv. Mater. 2012, 24 (8), 1089−1094. (21) Wang, R. H.; Xu, C. H.; Lee, J. M. High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels. Nano Energy 2016, 19, 210−221. (22) Chen, S.; Duan, J. J.; Jaroniec, M.; Qiao, S. Z. ThreeDimensional N-Doped Graphene Hydrogel/NiCo Double Hydroxide Electrocatalysts for Highly Efficient Oxygen Evolution. Angew. Chem., Int. Ed. 2013, 52 (51), 13567−13570. (23) Bi, H. C.; Yin, K. B.; Xie, X.; Zhou, Y. L.; Wan, N.; Xu, F.; Banhart, F.; Sun, L. T.; Ruoff, R. S. Low Temperature Casting of Graphene with High Compressive Strength. Adv. Mater. 2012, 24 (37), 5124−5129. (24) Sui, Z. Y.; Meng, Q. H.; Zhang, X. T.; Ma, R.; Cao, B. Green synthesis of carbon nanotube-graphene hybrid aerogels and their use as versatile agents for water purification. J. Mater. Chem. 2012, 22 (18), 8767−8771. (25) Chen, W. F.; Yan, L. F. In situ self-assembly of mild chemical reduction graphene for three-dimensional architectures. Nanoscale 2011, 3 (8), 3132−3137. (26) Xu, C. H.; Xu, B. H.; Gu, Y.; Xiong, Z. G.; Sun, J.; Zhao, X. S. Graphene-based electrodes for electrochemical energy storage. Energy Environ. Sci. 2013, 6 (5), 1388−1414. (27) Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors. Nat. Mater. 2008, 7 (11), 845−854. (28) Wang, R. H.; Han, M.; Zhao, Q. N.; Ren, Z. L.; Xu, C. H.; Hu, N.; Ning, H. M.; Song, S. F.; Lee, J. M. Construction of 3D CoO Quantum Dots/Graphene Hydrogels as Binder-Free Electrodes for Ultra-high Rate Energy Storage Applications. Electrochim. Acta 2017, 243, 152−161. (29) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008, 3 (2), 101−105. (30) Xu, C. H.; Lu, M. H.; Yan, B. G.; Zhan, Y.; Balaya, P.; Lu, L.; Lee, J. Y. Electronic Coupling of Cobalt Nanoparticles to NitrogenDoped Graphene for Oxygen Reduction and Evolution Reactions. ChemSusChem 2016, 9 (21), 3067−3073. (31) Wang, L.; Wang, X. H.; Xiao, X. P.; Xu, F. G.; Sun, Y. J.; Li, Z. Reduced graphene oxide/nickel cobaltite nanoflake composites for high specific capacitance supercapacitors. Electrochim. Acta 2013, 111, 937−945. (32) Jacintho, G. V. M; Brolo, A. G.; Corio, P.; Suarez, P. A. Z.; Rubim, J. C. Structural Investigation of MFe2O4 (M = Fe, Co) Magnetic Fluids. J. Phys. Chem. C 2009, 113 (18), 7684−7691.

(33) Lazarevic, Z. Z.; Jovalekic, C.; Milutinovic, A.; Sekulic, D.; Ivanovski, V. N.; Recnik, A.; Cekic, B.; Romcevic, N. Z. Nanodimensional spinel NiFe2O4 and ZnFe2O4 ferrites prepared by soft mechanochemical synthesis. J. Appl. Phys. 2013, 113 (18), 187221. (34) Liu, Z. Q.; Xiao, K.; Xu, Q. Z.; Li, N.; Su, Y. Z.; Wang, H. J.; Chen, S. Fabrication of hierarchical flower-like super-structures consisting of porous NiCo2O4 nanosheets and their electrochemical and magnetic properties. RSC Adv. 2013, 3 (13), 4372−4380. (35) Park, S. H.; Bak, S. M.; Kim, K. H.; Jegal, J. P.; Lee, S. I.; Lee, J.; Kim, K. B. Solid-state microwave irradiation synthesis of high quality graphene nanosheets under hydrogen containing atmosphere. J. Mater. Chem. 2011, 21 (3), 680−686. (36) Das, A. K.; Layek, R. K.; Kim, N. H.; Jung, D.; Lee, J. H. Reduced graphene oxide (RGO)-supported NiCo2O4 nanoparticles: an electrocatalyst for methanol oxidation. Nanoscale 2014, 6 (18), 10657−10665. (37) Lin, T. Q.; Chen, I. W.; Liu, F. X.; Yang, C. Y.; Bi, H.; Xu, F. F.; Huang, F. Q. Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage. Science 2015, 350 (6267), 1508−1513. (38) Choudhury, T.; Saied, S. O.; Sullivan, J. L.; Abbot, A. M. Reduction of Oxides of Iron, Cobbalt, Titanium and Niobium by Low-Energy Ion-Bombardment. J. Phys. D: Appl. Phys. 1989, 22 (8), 1185−1195. (39) Kim, J. G.; Pugmire, D. L.; Battaglia, D.; Langell, M. A. Analysis of the NiCo2O4 spinel surface with Auger and X-ray photoelectron spectroscopy. Appl. Surf. Sci. 2000, 165 (1), 70−84. (40) Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rios, E.; Berry, F. J. Characterization of the nickel cobaltite, NiCo2O4 prepared by several methods: An XRD, XANES, EXAFS, and XPS study. J. Solid State Chem. 2000, 153 (1), 74−81. (41) Thissen, A.; Ensling, D.; Madrigal, F. J. F.; Jaegermann, W.; Alcantara, R.; Lavela, P.; Tirado, J. L. Photoelectron spectroscopic study of the reaction of Li and Na with NiCo2O4. Chem. Mater. 2005, 17 (20), 5202−5208. (42) Zhang, G. Q.; Wu, H. B.; Hoster, H. E.; Chan-Park, M. B.; Lou, X. W. Single-crystalline NiCo2O4 nanoneedle arrays grown on conductive substrates as binder-free electrodes for high-performance supercapacitors. Energy Environ. Sci. 2012, 5 (11), 9453−9456. (43) Wang, C. H.; Zhang, X.; Zhang, D. C.; Yao, C.; Ma, Y. W. Facile and low-cost fabrication of nanostructured NiCo2O4 spinel with high specific capacitance and excellent cycle stability. Electrochim. Acta 2012, 63, 220−227. (44) Wang, H. W.; Hu, Z. A.; Chang, Y. Q.; Chen, Y. L.; Wu, H. Y.; Zhang, Z. Y.; Yang, Y. Y. Design and synthesis of NiCo2O4-reduced graphene oxide composites for high performance supercapacitors. J. Mater. Chem. 2011, 21 (28), 10504−10511. (45) Jiang, H.; Ma, J.; Li, C. Z. Hierarchical porous NiCo2O4 nanowires for high-rate supercapacitors. Chem. Commun. 2012, 48 (37), 4465−4467. (46) Chang, J.; Sun, J.; Xu, C. H.; Xu, H.; Gao, L. Template-free approach to synthesize hierarchical porous nickel cobalt oxides for supercapacitors. Nanoscale 2012, 4 (21), 6786−6791. (47) Wei, T. Y.; Chen, C. H.; Chien, H. C.; Lu, S. Y.; Hu, C. C. A Cost-Effective Supercapacitor Material of Ultrahigh Specific Capacitances: Spinel Nickel Cobaltite Aerogels from an Epoxide-Driven SolGel Process. Adv. Mater. 2010, 22 (3), 347−351. (48) Kong, L. B.; Liu, M.; Lang, J. W.; Luo, Y. C.; Kang, L. Asymmetric Supercapacitor Based on Loose-Packed Cobalt Hydroxide Nanoflake Materials and Activated Carbon. J. Electrochem. Soc. 2009, 156 (12), A1000−A1004. (49) Zhang, J. T.; Jiang, J. W.; Li, H. L.; Zhao, X. S. A highperformance asymmetric supercapacitor fabricated with graphenebased electrodes. Energy Environ. Sci. 2011, 4 (10), 4009−4015. (50) Zhou, G. M.; Wang, D. W.; Li, F.; Zhang, L. L.; Li, N.; Wu, Z. S.; Wen, L.; Lu, G. Q.; Cheng, H. M. Graphene-Wrapped Fe3O4 Anode Material with Improved Reversible Capacity and Cyclic Stability for Lithium Ion Batteries. Chem. Mater. 2010, 22 (18), 5306−5313. J

DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (51) Wu, Z. S.; Wang, D. W.; Ren, W.; Zhao, J.; Zhou, G.; Li, F.; Cheng, H. M. Anchoring Hydrous RuO2 on Graphene Sheets for High-Performance Electrochemical Capacitors. Adv. Funct. Mater. 2010, 20 (20), 3595−3602.

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DOI: 10.1021/acsami.8b02942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX