Functionalized Graphene Hydrogel‐Based High‐Performance Supercapacitors

Functionalized graphene hydrogels are prepared by a one-step low-temperature reduction process and exhibit ultrahigh specific capacitances and excellent cycling stability in the aqueous electrolyte. Flexible solid-state supercapacitors based on functionalized graphene hydrogels are demonstrated with superior capacitive performances and extraordinary mechanical flexibility. Electrochemical energy storage is a key enabling technology for clean and sustainable mobile energy supply.1-3 In this regard, electrochemical supercapacitors have recently received considerable attention for their potential applications in areas such as electric vehicles, mobile electronic products, and uninterrupted power supply.4-6 Rational design and synthesis of novel electrode materials with a precise control of their compositions, surface functionalization and hierarchical structures is of vital importance for the development of high-performance supercapacitors with large energy and power densities as well as a long cycle life. As a unique carbon nanomaterial, graphene has shown great potential in supercapacitor electrodes, mainly due to its large theoretical specific surface area (≈2630 m2 g−1), high conductivity, excellent mechanical flexibility and chemical stability.7-9 However, the strong π–π interaction between graphene sheets make them readily re-stack to form graphite-like powders or films when they are processed into bulk electrode materials, resulting in a severely decreased surface area and reduced diffusion rate and therefore unsatisfactory capacitive performance.10-12 Moreover, polymer binder and/or conductive additives are usually required to be mixed with graphene-based active materials to make supercapacitor electrodes. The inclusion of these electrochemically inert additives not only increases the complexity of electrode preparation but also imposes an adverse effect on the specific capacitive performance. With the flexible electronics becoming increasingly widespread in our daily lives, there is also an increasing demand for high-performance flexible solid-state supercapacitors for power supply.13-15 Recent studies have demonstrated the fabrication of solid-state supercapacitors based on graphene with higher specific capacitances (118–204 F g−1) than carbon nanotubes.16-18 However, the capacitive performance is still largely limited by the surface area of graphene electrodes with re-stacking of graphene sheets and their insufficient access to solid-state electrolytes. To address these challenges, graphene-based macrostructures with a three-dimensional (3D) porous network have recently attracted considerable interest.19-22 Among various 3D graphene materials, graphene hydrogels and aerogels have received particular attention.23-30 These unique graphene gels consist of interconnected 3D porous frameworks with large specific surface areas, allowing multidimensional electron transport and rapid electrolyte ions diffusion. They have been used as binder-free electrode materials and exhibited large specific capacitances (>200 F g−1), excellent rate capability and cycling stability in liquid electrolytes.23-30 These encouraging results demonstrate exciting potential of the 3D graphene macrostructures for energy storage applications and motivate efforts to further improve their specific capacitance without sacrificing the rate performance and cycle life. Herein, we report the synthesis of functionalized graphene hydrogels (FGHs) through a convenient one-step chemical reduction of graphene oxide (GO) using hydroquinones as the reducing and functionalizing molecules simultaneously. The mechanically strong FGHs are directly used as supercapacitor electrodes without adding any other binder or conductive additives with an impressive specific capacitance of 441 F g−1 at 1 A g−1 in the 1 M H2SO4 aqueous electrolyte, more than double the capacitance of the unfunctionalized graphene hydrogels (211 F g−1). Moreover, the FGHs exhibit excellent rate capability (80% capacitance retention at 20 A g-1) and cycling stability (86% capacitance retention over 10 000 cycles). Based on these results, we further fabricate FGHs-based flexible solid-state supercapacitors using H2SO4-polyvinyl alcohol (PVA) gel as the electrolyte. The integrated devices not only deliver excellent capacitive performances close to the ones in aqueous electrolyte (412 F g−1 at 1 A g-1, 74% capacitance retention at 20 A g−1 and 87% capacitance retention over 10 000 cycles), but also exhibit extraordinary mechanical flexibility and low self-discharge course. To the best of our knowledge, the specific capacitances of FGHs-based supercapacitors with aqueous and gel electrolytes are among the highest values achieved in chemically modified graphene electrode materials. The FGHs can be easily prepared by heating a homogeneous aqueous mixture of GO (2 mg mL−1) and hydroquinone (10 mg mL−1) without stirring at 100 °C for 12 hours (Figure 1a). The obtained FGHs are mechanically strong enough to allow for handling with tweezers (Figure 1a) and supporting a weight of as high as 7100 times its own dried-weight (Supporting Information, Figure S1). Scanning electron microscopy (SEM) images reveal that the freeze-dried FGH (aerogel) has an interconnected 3D macro-porous network with pore sizes ranging from sub-micrometers to several micrometers and pore walls consisting of ultrathin layers of stacked graphene sheets (Figure 1b,c). X-ray diffraction (XRD) patterns confirm the efficient de-oxygenation of GO to form graphene framework of FGHs upon hydroquinone reduction (Figure 1d). The interlayer distance of freeze-dried FGH is calculated to be 3.56 Å, which is much lower than that of GO precursor (7.50 Å) while slightly higher than that of graphite (3.35 Å), suggesting the existence of π–π stacking between graphene sheets in the FGHs. The broad XRD peak of the freeze-dried FGH indicates the poor ordering of graphene sheets along their stacking direction and reflects that the framework of FGHs is composed of few-layer stacked graphene sheets, which is consistent with the SEM and TEM studies (Figure S2). The hierarchical porous structure of FGHs was confirmed by the nitrogen adsorption and desorption measurements. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analysis reveal that the freeze-dried FGH has a high specific surface area of ≈297 m2 g−1 with a pore volume of ≈0.95 cm3 g−1 and the pore sizes in the range of 2–70 nm (Figure S3). It should be noted that the BET measurement of the freeze-dried FGHs could substantially underestimate the specific surface area of FGHs because of partial re-stacking of some graphene layers and the fusing of mesopores within FGHs during the freeze-drying process. Considering the FGHs were used directly as the supercapacitor electrodes without freeze-drying, we have also adopted an alternative approach to determine the intrinsic surface area of the wet FGHs by employing the methylene blue (MB) dye adsorption method. With this approach a high specific surface area of ≈1380 m2 g−1 is achieved for FGHs, which is about half of theoretical surface area of single-layer graphene (≈2630 m2 g−1). The formation of FGHs was found to be highly concentration dependent. When we used a low-concentration aqueous mixture of GO (0.1 mg mL−1) and hydroquinone (0.5 mg mL−1) with the same feeding ratio for reaction, we obtained a black dispersion of functionalized graphene (FG) that could be centrifuged and redispersed in water (inset of Figure 1e). This dispersion allowed us to confirm the surface functionalization of graphene by hydroquinone molecules using UV-vis and atomic force microscopy (AFM) characterizations. As shown in Figure 1e, the main absorption peak at 230 nm for GO red-shifts to 266 nm for FG, indicating the recovery of electronic conjugation within FG sheets upon hydroquinone reduction.31 Another shoulder peak at 220 nm for FG is ascribed to the absorption of hydroquinone,32 indicating the existence of hydroquinone molecules adsorbed on the surface of reduced GO. AFM images show a clear height change between single-layer GO (0.9 nm) and FG (1.5 nm) sheets (Figure 1f,g). Considering the height of reduced GO is a little lower than that of GO due to de-oxygenation31 and the distance of π–π interaction between aromatic molecules is about 0.35 nm,33 we believe both sides of FG sheets are covered by hydroquinone molecules via π–π interaction, which has also been observed in similar systems of graphene with benzene or other π-conjugated molecules.34, 35 The surface chemistry of freeze-dried FGHs was further characterized by X-ray photoelectron spectroscopy (XPS). Compared with GO, the FGHs have deceased but still with notable oxygen functional groups, most of which can be attributed to the adsorbed hydroquinone molecules with two hydroxyl groups (Figure S4). This is consistent with UV-vis and AFM results. We have also prepared unfunctionalized graphene hydrogels (GHs) using a hydrothermal reduction method.23 The specific surface area of GHs was measured to be ≈1260 m2 g−1 using the MB adsorption technique, which is largely similar to that of FGHs. Compared with GHs, we estimated the mass loading of hydroquinone on FGHs was about 17 wt% (Supporting Information, experimental methods). The FGHs were cut into self-supported slices with a thickness of ≈3 mm and a dried-weight of ≈2 mg, which were further pressed on two platinum foils and used directly as supercapacitor electrodes for assembling symmetric supercapacitors in 1 M H2SO4 aqueous electrolyte. For comparison, GHs were also tested under the same conditions. Figure 2a shows the cyclic voltammetry (CV) curves of the FGHs- and GHs-based symmetric supercapacitors. It can be seen that the CV curve of GHs exhibit a typical rectangular shape, implying pure electrical double-layer capacitive behavior. In contrast, the CV curve of FGHs displays a box-like shape superimposed with a pair of Faradaic peaks in the potential range of 0.1–0.3 V, which is caused by the reversible redox reaction of the adsorbed hydroquinone molecules (hydroquinone ↔ quinone +2 H+ +2 e−) and indicates the coexistence of both the electrical double-layer capacitance and pseudocapacitance. Another weak couple of redox peaks at about 0.5 V are associated with the remaining oxygen-containing groups on the graphene sheets of FGHs. The Faradaic peaks of hydroquinone have nearly symmetrical wave-shapes and a small peak separation (≈60 mV), indicating that the surface confined redox reaction have a good reversibility and a fast charge transfer process. Even when the scan rate increases to 100 mV s−1, the CV curve of FGHs basically maintains the Faradaic peaks-incorporated rectangular shape, similar to that observed at 5 mV s−1 (Figure S5a), which is indicative of a quick charge-propagation capability of both the electrical double-layer capacitance and the pseudocapacitance in the FGHs electrodes. The galvanostatic charge/discharge curves of FGHs show a deviation from the ideal triangle shape exhibited by GHs, especially in the potential range of 0–0.35 V (Figure 2b). This result also confirms the significant contribution of pseudocapacitance. The specific capacitance values were derived from the galvanostatic charge/discharge curves of FGHs (Figure S5b) and are shown in Figure 2c. The FGHs-based supercapacitor shows an impressive specific capacitance of 441 F g−1 at a current density of 1 A g−1, more than double that of GH-based one (211 F g−1). Assuming that the FGHs hold an electrical double-layer capacitance of approximately 232 F g−1 based on the specific surface areas of FGHs and GHs, the pseudocapacitance contributed by hydroquinone is calculated to be 1461 F g−1, about 83% of its theoretical value (1751 F g−1), indicating a highly efficient utilization of adsorbed hydroquinone in FGHs. Furthermore, upon increasing the current density up to 20 A g-1, the specific capacitance of FGHs remains at 352 F g−1, 80% of that at 1 A g−1 and still more than double that of GHs (172 F g−1), highlighting the excellent rate capability of FGHs. A long cycle life is another important concern for practical application of supercapacitors containing pseudocapacitance. Importantly, the FGHs electrodes show excellent electrochemical stability with 86% of its initial capacitance retained after 10 000 charge/discharge cycles at a high current density of 10 A g−1 (Figure 2d). Meanwhile, there is only a small change in the CV curves before and after 10 000 charge/discharge cycles (inset of Figure 2d), indicating the non-covalent interactions between hydroquinone and graphene are strong enough to sustain a long cycle life. Furthermore, the few remaining oxygen functionalities on the graphene sheets of FGHs which survive the hydroquinone reduction are also highly stable for achieving excellent cyclability. It is worth noting that the specific capacitances achieved in FGHs are significantly higher than those obtained from other chemically modified graphene (Table S1), such as heteroatoms doped graphene,18, 36-38 porous graphene,17, 28, 30, 39 and oxygen-containing surface-group-functionalized graphene.40-42 Moreover, the FGH electrodes exhibit higher specific capacitances and better cycling stability than graphene/polymer43-47 and graphene/MnO248, 49 composites electrodes (Table S2). There are several factors that can contribute to the outstanding performance of FGHs electrodes. First, the graphene sheets of FGHs provide a large surface area for accommodating a large amount of hydroquinone molecules, which can greatly enhance the contribution of pseudocapacitance. Second, all the hydroquinone molecules were directly attached on graphene sheets via π–π interaction. This intimate contact makes full use of the pseudocapacitive component and affords rapid electron transfer from graphene substrate to hydroquinone for the fast Faradaic reaction. Third, the interconnected meso- and macro-porous structure of FGHs can facilitate ions diffusion into the pores as well as electron transport throughout the entire graphene framework. In order to demonstrate the superior performances of FGH electrodes for electrochemical energy storage in flexible electronics, we further fabricated flexible solid-state supercapacitors based on FGHs. FGHs can function as an ideal electrode for flexible solid-state supercapacitor devices due to the exceptional mechanical and electrical robustness of the highly interconnected 3D network.24 First, a free-standing FGH with a thickness of ≈3 mm was cut into rectangular strips with a dried-weight of ≈2 mg, which were pressed on the gold-coated polyimide substrates to form flexible thin film electrodes with an areal mass of ≈1 mg cm−2 (Figure 3a). Although the graphene framework became flat and crumpled upon pressing, the 3D continuous porous network was well maintained (Figure 3b,c), which is beneficial for the gel electrolyte infiltration and ions diffusion. Next, a H2SO4-PVA aqueous solution (≈10 wt% for both H2SO4 and PVA) was slowly poured onto two separate FGH films and air-dried for 12 hours to evaporate excess water. The two FGH electrodes were then pressed together under a pressure of ≈1 MPa for 30 min, which allowed the polymer gel electrolyte on each electrode to combine into one thin separating layer. The resulting solid-state supercapacitor was highly flexible and robust (Figure 3d,e). The FGH-based flexible solid-state supercapacitor shows almost the same CV curve with the one in 1 M H2SO4 aqueous electrolyte, where the Faradaic peaks indicates the presence of pseudocapacitance of hydroquinone (Figure 4a). The accurate specific capacitances were obtained from galvanostatic charge/discharge measurements (inset of Figure 4b). It is found that the specific capacitance of the solid-state supercapacitor is 412 F g−1 at 1 A g−1, only 6.6% lower than that (441 F g−1) of the one in aqueous electrolyte, which is consistent with the CV analysis and reflects the efficient infiltration of polymer gel electrolyte into the 3D network of FGHs. As the current density increases from 1 to 20 A g−1, the solid-state supercapacitor still exhibits a high specific capacitance of 304 F g−1, 74% of that at 1 A g−1. The difference of rate performance between the solid-state supercapacitor and the one in aqueous electrolyte can be ascribed to higher internal resistance and slower ions diffusion in solid-state devices with gel electrolyte.24 The specific capacitances of FGH-based solid-state supercapacitor here is substantially higher than previous reported solid-state devices made of carbon nanotubes and their composites,50, 51 graphene films,16-18 and conducting polymers (Table S3).52 The FGH-based solid-state supercapacitors also exhibit extraordinary mechanical flexibility in bending tests. As shown in Figure 4c, the CV curves of the device measured at various bending angles show almost the same electrochemical behavior even at a large bending angle of 150°. The performance durability of the device was further characterized by galvanostatic charge/discharge tests up to 10 000 cycles at a high current density of 10 A g−1 under 150° bending angle (Figure 4d). Only 13% decay in specific capacitance was observed, highlighting the excellent mechanical and electrical robustness of the interconnected 3D network of FGHs and its favorable interfacial compatibility with polymer gel electrolyte. For practical application, the leakage current and self-discharge characteristics of the device are important factors to consider, which, unfortunately, typically received insufficient attention in many studies. The leakage current of our FGH-based solid-state supercapacitor was ≈12 μA (Figure S6a) (0.015 μA mF−1, normalized by capacitance), greatly lower than that of carbon nanotube/polyaniline composite supercapacitor (17.2 μA and 0.034 μA mF−1).51 The self-discharge of the solid-state supercapacitor was also tested (Figure S6b). The device underwent a rapid self-discharge process in the first half hour, however, the self-discharge course was very slow after several hours. Finally, the device showed a stable output voltage of ≈0.5 V after 4 hours and 41% of the initial charged potential was well retained even after one day, which is significantly higher than polypyrrole-based solid-state supercapacitors (≈0.2 V).52 The advantage of low self-discharge course is highly desirable for the applications of the devices in flexible electronics. To further demonstrate the practical usage of the highly flexible solid-state supercapacitors based on FGHs, we connected three supercapacitor units in series to create a tandem device. Each supercapacitor unit has the same mass loading of FGHs (≈2 mg for one electrode). CV and galvanostatic charge/discharge measurements show the potential window is extend from 1.0 V for one unit to 3.0 V for the tandem device (Figure 4e,f). Meanwhile, the tandem device shows almost unchanged charge/discharge time compared with individual units at the same current density (Figure 4f), suggesting the performance of each supercapacitor unit is well retained in the tandem device. After charged at 3.0 V, the tandem device can light up a green LED (the lowest working potential is about 2.0 V) (inset of Figure 4f), demonstrating the practical potential of the fabricated flexible supercapacitors for mobile energy supply. In summary, we have developed a convenient one-step strategy to prepare FGHs incorporating hydroquinones into the high-surface-area 3D graphene framework via π–π interaction as a pseudocapacitive component. This methodology not only allows efficient loading of pseudocapacitive hydroquinone molecules and fast charge transfer between graphene and hydroquinone, but also ensures rapid ion diffusion and electron transport throughout the entire porous network. The as-prepared FGHs exhibit outstanding electrochemical performances, including ultrahigh specific capacitances and excellent cycling stability in the aqueous electrolyte. The mechanically strong FGHs have been further assembled into flexible solid-state supercapacitors using H2SO4-PVA gel as the electrolyte. The as-fabricated devices show superior capacitive performances and exceptional mechanical flexibility. These results clearly reveal that the electrochemical performances of graphene-based electrodes can be greatly promoted by elaborated combination of surface functionalization and hierarchical structures of graphene sheets. This study demonstrates an exciting pathway to the rational design and fabrication of functionalized 3D graphene materials for electrochemical energy storage and flexible electronics. X.D. acknowledges partial financial support by a Dupont Young Professor Award. 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