Graphene Hydrogel

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Green Synthesis of Three-Dimensional MnO2/Graphene Hydrogel Composite as a High-Performance Electrode Material for Supercapacitors Xiaoyi Meng, Liang Lu, and Chunwen Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02354 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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ACS Applied Materials & Interfaces

Green Synthesis of Three-Dimensional MnO2/Graphene Hydrogel Composite as a High-Performance Electrode Material for Supercapacitors Xiaoyi Meng,1,2 Liang Lu,1 and Chunwen Sun1,2* 1

CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, P. R. China. 2 School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, 100049, P. R. China. Email: [email protected]

ABSTRACT Graphene hydrogels (GHs) and their composite have attracted wide attention due to the special structure of graphene assembly and exceptionally electrochemical performance as electrodes for energy storage devices. Here, we report GH with three-dimensional (3D) architecture prepared by a hydrothermal method via a self-assembled process in glucose and ammonia system as well as subsequent freeze-drying. The δ-MnO2/GH composite was then obtained by immersing GH in KMnO4 solution with a certain concentration under heat treatment. The asymmetric supercapacitor-MnO2/GH//GH

consisting

of

pseudocapacitive

nanosheets-like

δ-MnO2/GH as cathode and electric double-layer capacitive GH as anode provides high energy density of 34.7 Wh/kg at a power density of 1.0 kW/kg. Importantly, it is found that the psedocapacitive behavior of MnO2 have great effects on the rate performance of the supercapacitor, which is indentified by kenetic analysis.

KEYWORDS: graphene hydrogel; hydrothermal sythesis; MnO2; supercapacitor; kinetics characteristics.

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1. Introduction Today, with the development of technology and civilization, people pay more attention to effective energy use and the way of energy utilization. Therefore, developing high-efficiency energy storage systems to meet the requirements of various applications becomes increasingly urgent and significant. Supercapacitors storing energy using electrochemical double layer effect or/and fast surface redox rections, show high power density and long cycle life, which are considered as one of the most promising electrochemical energy storage devices,1 or even a supplement device to batteries. Supercapacitors have been widely used as power sources for portable electronics, hybrid electric vehicles, and stand-by power systems.2 Recently, intensive efforts have been made to develop two type of supercapacitor electrode materials: electric double layer materials (activated carbon,3 graphene,4 CNTs,5 etc.) and pseudocapacitive materials (RuO2,6 MnO2,7 polypyrrole,8 etc.). Besides, materials like Co3O4 and Ni(OH)2 showing non-capacitive or battery-like behavior also have been used as electrode materials in supercapacitors.9 Graphene, a well-known carbon material, has been widely investigated in sensors, energy storage and other fields due to its unique properties, such as ultrahigh surface area, superior electronic conductivity and electrochemical stability.10 However, two-dimensional (2D) graphene sheets have a tendency to form graphite-like agglomerates through the strong van der Waals interactions, making them difficult to prepare in large scale and use effectively in practice. In recent years, three-dimensional (3D) graphene macrostructures assembled by graphene sheets have 2


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attracted great interests since their highly porous networks are capable of minimizing the re-stacking of 2D graphene sheets.11,12 Meanwhile, 3D graphene-based materials can be prepared relatively easy by various strategies, including one-step hydrothermal reduction, chemical reduction or template method.13-15 For example, graphene hydrogel (GH) as a member of 3D graphene family can be prepared by reducing the graphene oxide (GO) solution with a suitable concentration. To date, several reducing reagents have been developed to synthesize GHs with various morphologies and properties, such as N2H4·H2O,16 HI,17 NaHSO3 and Na2S,18 CO(NH2)2,19 H2C2O4 and NaI,20 ascorbic acid,21 and phenolic acids,22 etc. Particularly, glucose can also be a low-cost and environment-friendly reducing medium for fabricating 3D GH.23 Tang et al. reported that three-dimensional reduced graphene oxide (3D-rGO) can be prepared by one step hydrothermal method using glucose as the reducing agent and CaCO3 as the template. The as-prepared 3D-rGO electrode exhibited a specific capacitance of 88.9 F/g at 1 A/g.24 Ji et al. synthesized 3D interconnected graphene-based aerogels using glucose both as morphology oriented agent and reductant, which showed a high specific capacitance of 161.6 F/g at 0.5 A/g.25 However, a more efficient and milder strategy is still needed for realizing syntheis of 3D graphene macrostructures in large scale. Metal oxides have been studied as electrode materials for pseudocapacitors due to their higher energy and power densities compared with that of carbon materials. In particular, manganese dioxide (MnO2) is the most attractive electrode material for supercapacitors because of its abundance, low-cost, environmental friendliness, and 3



high specific capacitance.26,27 Herein, GH with interconnected porous architecture is synthesized through a eco-friendly and facile hydrothermal method utilizing glucose as a sort of green and effective reductant. In addition, MnO2 nanosheets are deposited on the surface of GH in the process of redox reaction between KMnO4 and carbon material of GH itself. As an electrode for the asymmetric supercapacitor MnO2/GH//GH, it shows high energy density of 34.7 Wh/kg at a power density of 1.0 kW/kg. Importantly, the kinetics characteristics of supercapacitors are analyzed. It is found that the psedocapacitive behavior of MnO2 has great effects on the rate performance of the supercapacitor.

2. Experimental section Materials: All chemicals used in the experiment were of analytical grade without further purification. Synthesis of Graphene Oxide: GO was prepared from graphite by a modified Hummers method as previously reported.28 In a typical procedure, 1 g of graphite and 28 mL of H2SO4 (98 wt%) were mixed in a beaker under magnetic stirring continuously in an ice-water bath for 2h. Then，0.5 g of NaNO3 and 3 g of KMnO4 were slowly added, respectively, and stirred continuously for another 2 h. Then the mixture was heated to 35 °C and kept for 4 h with stirring, followed by adding 60 mL deionized water and heated at 95 °C for 30 min. Lastly, 20 mL of H2O2 was added into the mixture to remove residual KMnO4. The mixture was collected and washed with 1 M HCl and deionized water. The products was then freezed for 12 h at -2 °C，

4


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followed by freeze-dring for 24 h in vaccum. Synthesis of Graphene Hydrogel: 3D graphene hydrogel (GH) was synthesized through a facile one-pot hydrothermal process. Typically, 7 mL GO suspension with a concentration of 3 mg/mL was added into a 10 mL vial. Then, 42 mg glucose was added and ultrasonic treatment for 5 min until completely dissolved. After that, 1 mL of ammonia solution (25 wt% in water) was added and dispersed uniformly by ultrasonic treatment. Subsequently, the mixture was incubated at 95 °C for 4 h. The as-prepared 3D GH was immersed in deionized water for three days to remove residual glucose and ammonia. The obtained GH was freeze-dried under vacuum for further characterization. Synthesis of

MnO2/GH and MnO2: The GH was immersed in 1 mg/mL KMnO4

solution and then heated at 80°C for 6 h, and the obtained mixture was named MnO2/GH. In addtion, the effects of different reaction time (3 h and 9 h) on the content of MnO2 and GH in the hybrids have been studied. For comparing the difference of electrochemical properties between MnO2/GH and MnO2, pure phase δ-MnO2 was deposited on nickel foam (NF) by a simple hydrothermal method. In a typical process, a piece of NF was immersed in 60 mL of 0.1 M KMnO4 solution and transferred to a Teflon-lined stainless steel autolave, and then heated up to 160 °C and kept at this temperature for 24 h. The mass loading of δ-MnO2 on NF was about 2 mg/cm2. Characterization Methods: X-ray diffraction (XRD) patterns of the samples were collected on a PANalytical X’Pert3 Powder diffractometer (Cu Kα, λ = 1.54 Å). The 5



morphologies were characterized by field-emission scanning electron microscope (SEM, SU8020). TEM images and and energy dispersive X-ray elemental mappings were conducted by a transmission electron microscope (Tecnai G2 F20). Fourier-transform infrared (FTIR) spectra were recorded on a spectrometer (Vertex 80, Bruker) with KBr pellets. Raman spectra were measured using a Confocal Raman Spectrometer (Horiba, France) with an excitation length of 538 nm. X-ray photoelectron spectra (XPS) were performed on a Perkin-Elmer PHI-5700 ESCA System with a monochromated Al Kα X-ray source (1486.6 eV). Thermo-gravimetric analysis (TGA) was carried out on a TA instrument TGA-2050 at a heating rate of 10 °C/min in air. Brunauer–Emmett–Teller (BET) specific surface areas were determined from nitrogen sorption isotherms that were measured with a V-Sorb2800P analyzer at 77 K. Electrochemical Measurements: The electrochemical capacitive performances of the GH or MnO2/GH were performed in 1 M Na2SO4 aqueous electrolyte solution by a conventional three-electrode configuration where a platinum plate and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. For the preparation of working electrode, a slice of the as-synthesized GH or MnO2/GH was cut and directly pressed onto nickel foam under 8 MPa without using any binding agents or conducting additives, and then vacuum-impregnated with the electrolyte solution for 6 h before tests. The mass loading of each electrode was about 0.5 mg/cm2. Two-electrode configurations tested in 1 M Na2SO4 solution were performed to 6


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evaluate its electrochemical performance. Two types of symmetric supercapacitor (GH//GH,

MnO2/GH//MnO2/GH)

and

one

asymmetric

supercapacitor

(MnO2/GH//GH) were assembled. The mass loading of GH and MnO2 electrode was about 0.3 and 0.7 mg/cm2, respectively. All the electrochemical measurements were carried out on an Autolab electrochemical workstation (PGSTAT302N). The specific capacitance (Cs) values of the electrode can be calculated using the following Equation: ∆

C =

(1)

∆

where I is the discharge current (A), ∆t is the discharge time (s), m is the mass of the active material in the electrode (g), and ∆V is the potential window (V). The specific capacitance values of supercapacitors can be calculated based on CV measurements based on the following Equation:

C =

(2)

where V is the voltage window of supercapacitor (V), I is current in CV curves (A), m is the total mass of active material on two electrodes (g), v is scan rate (V/s). The energy density and power density of supercapacitors can be calculated by means of the following Equations:

E = . C ∆ P=

(3)

.

(4)

where Cs is the specific capacitance of the supercapacitor (F/g), ∆V is the voltage window of supercapacitor (V), and t is the discharge time (s).

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3. RESULTS AND DISCUSSION 3.1 Characterization of GH and MnO2/GH The GO sample was prepared by a modified Hummer method. The synthesis process of GH and MnO2/GH is illustrated schematically in Figure 1. GO can be well dispersed and formed a stable aqueous suspension due to the hydrophilic oxygenated functional groups on GO sheets. In the process of reaction, GO sheets were partially reduced in the presence of both glucose and ammonia, inducing the reduced GO (rGO) sheets to cross-link each other via π−π stacking interactions to self-assemble into 3D GH.29 As previously reported,30 glucose can be a mild agent for reducing GO into rGO but only when ammonia solution exists. We also found that 3D GH cannot be formed in the absence of ammonia. Subsequently, MnO2/GH is prepared by immersing a bulk of GH in KMnO4 solution and heated for another 6 h. In this preparation procedure, MnO2 nanosheets were deposited on the graphene nanosheets based on the redox reaction as follows:31 4 ! 3# ! $ → 4 ! # ! 2$#

(5)

Figure 1. Schematics of the synthesis process of GH and MnO2/GH

It is noteworthy that the reaction between KMnO4 solution and GH is sensitive to the experimental conditions. As shown in Figure S1a (Supporting Information), the 8


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reaction process in acidic condition with 0.1 M HCl is obviously different from that of the solution without addition of acid. It is observed that the color of KMnO4 solution was faded within 3 h while the reaction in neutral aqueous media proceeds slowly. It indicates that KMnO4 solution has a higher activity and faster reaction kinetics. In addition, as shown in Figure S1b and 1c the MnO2 product obtained in acidic condition forms α crystalline phase with flowerlike morphology. On the contrary, MnO2 prepared in the solution without addition of acid tends to form δ phase with sheet-like morphology. It may be attributed to the differnt reaction mechanisms under different pH value. In this work, we only focuses on the MnO2 prepared in solution without addition of acid and study its electrochemical performance. X-ray diffraction (XRD) patterns of the obatined GO, GH and MnO2/GH are shown in Figure 2a. The characteristic diffraction peak of the GO is observed at 10.7°. After freeze-drying treatment, the GH and MnO2/GH was further examined by XRD. In the XRD patterns of both GH and MnO2/GH, a broad and strong peak at 25° and a week peak at 44°correspond to the (002) and (100) planes, respectively, indicating that GO was largely reduced by glucose. Moreover, four planes of (001), (002), (111) and (020) in the MnO2/GH sample reveals that the obtained product belongs to δ-MnO2 phase or birnessite MnO2 (PDF no. 42-1317).32,33 As shown in Figure S2, the δ-MnO2 has a layered structure which is benifical for electrolyte ion insertion/extraction.34 Figure 2b shows the Raman spectra of the GO, GH and MnO2/GH. The two distinct peaks at about 1343.6 and 1587.7 cm-1 are assigned to a disorder-introduced feature (D band) and the E2g mode of graphite (G band), respectively.35 The intensity ratios of D versus 9



G band (ID/IG) of GO, GH and MnO2/GH are 1.45, 1.70 and 1.37, respectively, which means the structural disorder of graphitic sp2 domains of carbon materials. The weak peaks at about 573 and 640 cm-1 of the MnO2/GH sample can be ascribed to the symmetric stretching vibration (Mn–O) of the MnO6 groups.36,37 The structure characteristics of GO, GH and MnO2/GH was further characterized by Fourier transform infrared (FT-IR) spectroscopy as shown in Figure 2c. For GO sample, the absorption peaks at 3430 and 1638 cm-1 represent the O-H stretching vibration of water molecules, the skeletal vibration of aromatic C=C and the O-H bending vibration of intercalated water molecules, respectively. The intensity of these peaks in GH and MnO2/GH spectra decreases obviously. In addition, The spectrum of GO shows the strong peak at 1725 cm-1 corresponds to C=O stretching vibrations from the carbonyl and carboxylic groups, and the peaks at 1206 cm-1 and 1055 cm-1 owing to alkoxy C-O stretching vibrations, which almost disappear in the case of GH and MnO2/GH, illustrating that the GO was successfully reduced by glucose.38,39 Moreover, the week peaks around 428~590 cm-1 in the spectra of MnO2/GH can be attributed to the Mn-O and Mn-O-Mn vibrations. The absorption bands at 1564, 1412 and 1172 cm-1 can be referred to the vibration from the intercalation of the solvated K+.32 The surface elemental composition and valenvce of the MnO2/GH composite was analyzed by X-ray photoelectron spectroscopy (XPS). The peaks of Mn (2p1/2, 2p3/2), O 1s and C 1s can be observed in Figure 2d. The C 1s XPS spectrum of the GH (Figure 2e) exhibits strong C-C peak and weak C-O, O-C=O and C=O peaks, suggesting the considerable deoxygenation during the reduction process.40 The two 10


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peaks at 654.3 eV and 642.7 eV are ascribed to

Mn2p1/2 and Mn2p3/2, respectively,

which demonstrates Mn valence is +4 (Figure 2f).41

Figure 2. Structural and composition characterizations of the GO, GH and MnO2/GH: (a) XRD patterns, (b) Raman spectra and (c) IR spectra; (d-f) XPS spectra: (d) A typical survey spectrum, (e) C1s spectrum, and (f) Mn 2p spectrum of the MnO2/GH sample.

Figure 3a shows the scanning electron microscopy (SEM) image of the GH after freeze-drying treatment. The 3D architecture of GH was constructed by rGO nanosheets randomly during the GO reduction process. On the other hand, GH with cross-linking macroporous structure can provide large surface area and numerous channels for electrolyte ion transfer, which is favourable for forming high ion absorption capacitance. Moreover, flake-like MnO2 was anchored on the rGO nanosheets after immersed in KMnO4 solution and heated for some time. (Figure 3b). In the reaction process, although a part of graphene carbon was reduced, the robust

11



3D GH can still be a conductive matrix for supporting MnO2 nanosheets and transferring electrons. The MnO2/GH composite was futher characterized by transmission electron microscopy (TEM) (Figure 3c,d). It can be seen that nanosheet-like MnO2 is incorporated into thin rGO sheets (Figure 3d). The TEM mapping images and energy dispersive X-ray (EDX) spectrum of the MnO2/GH//GH is shown in Figure S3, and it is further demonstrates that MnO2 is deposited on the GH nanosheets successfully to form a stable composite.

Figure 3. SEM images of the GH (a) and MnO2/GH (b); TEM images of the MnO2/GH sample (c, d).

3.2 Electrochemical performances of the GH and MnO2/GH supercapacitors The electrochemical performance of the GH and MnO2/GH materials were tested in 1 M Na2SO4 electrolyte under three-electrode system. Figure 4a shows the cyclic 12


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voltammetry (CV) curves of the GH and MnO2/GH electrodes at a sweep rate of 10 mV/s in a potential range from 0.0 to 1.0 V, and CV curves at different scan rates are shown in Figure S4a,c. GH electrode displays typically rectangular shape, indicating its ideal double layer capacitive behavior. MnO2 electrode shows a pair of weak and broad characteristic peaks marked with asterisk corresponding to the Faradaic reaction process of Na+ intercalation to MnO2, indicating slow reaction kinetics feature and the psedocapacitive property of MnO2. It is generally acknowledged that the psedocapacitive machanism for MnO2 in Na2SO4 electrolyte mainly consists of two parts: surface Faradaic reaction involves the surface adsorption of Na+ and bulk Faradaic reaction related to the intercalation or deintercalation of Na+ in the bulk of MnO2. The Faradaic process can be expressed as the following equations :42

'()*+, ! -./ ! 0 ⇋ -.'()*+,

(6)

! -./ ! 0 ⇋ -.

(7)

In addition, the specific capacitances of the GH and MnO2/GH versus current density are plotted in Figure 4b according to the galvanostatic charge-discharge (GCD) curves in Figure S4b,d. The specific capacitance values of the GH and MnO2/GH electrode are estimated to be 113.0 F/g and 200.6 F/g at a current density of 1 A/g, respectively. Furthermore, The electrochemical impedance spectra (EIS) of the GH and MnO2/GH electrode were measured in the frequency range between 100 kHz and 0.1 Hz with 10 mV amplitude (Figure S4e). The curves of the GH electrode at low frequency region is approximately parallelled to the imaginary part due to its capacitive behavior. On the other hand, it can be seen obviously that MnO2/GH 13



electrode possesses a larger Warburg impedance than that of GH, which is mainly attributed to low Na+ diffusion in MnO2 active material. Thus, the cycling performance of the MnO2/GH is also worse than the GH (Figure S4f). Moreover, the MnO2 electrode was prepared by depositing MnO2 nanosheets onto NF homogeneously (Figure S5a,b,c). The XRD pattern of MnO2/NF suggests that the deposited MnO2 have the same crystalline phase as that of the MnO2 in MnO2/GH composite (Figure S5d). Although the δ-MnO2/NF shows well-defined morphology, the MnO2/NF electrode exhibits lower capacitance and larger resistance compared with that of the MnO2/GH electrode (Figure S5e,f), indicating that GH as a good conductive substrate is beneficial for the fast charge transfer and electrolyte ion diffusion due to its porous structure. The MnO2/GH composites with different ratios of MnO2 and GH were also prepared by controlling different reaction time. Figures S6a and S6b show the comparison of CV and GCD results of the three hybrids, respectively. The obtained MnO2/GH exhibits better capacitive performance when the reaction time is 6 h. The content of MnO2 in the composites was determined by TGA measurements (Figure S6d). The GH was burnt up and MnO2 was decomposed into Mn2O3 when it was heated up to 800 °C. The values of weight remaining in MnO2/GH composites obtained at 3h, 6h and 9h are 19.2, 21.8 and 42.6 wt.%, respectively. Therefore, the mass ratios of GH and MnO2 in the above three composites are 3.74, 3.16 and 1.32, which indicates the amount of MnO2 increases with reaction time prolonging. From the EIS spectra (Figure S6c), it can be concluded that the products have more large 14


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charge transfer resistance with longer reaction time, which is mainly due to poor conductivity

of

the

composites

with

more

MnO2.

In

addition,

N2

adsorption-desorption isotherms of the GH and MnO2/GH are shown in Figure S6e,f. The surface areas of the GH and MnO2/GH samples are 172.8 and 54.7 m2/g, respectively. However, their average pore sizes are similar (about 4 nm). This porous structure can provide efffective pathways for electrolyte diffusion. Therefore, the MnO2/GH composite obtained under 6 h reaction exhibits high capacitive performance owing to its appropriate pore size distribution and high psedocapacitive behavior of MnO2. The feasibility of the obtained GH and MnO2/GH as electrodes for supercapacitors was examined. The symmetric supercapacitors GH//GH and MnO2/GH//MnO2/GH were assembled with the same material as both anode and cathode, while the asymmeric supercapacitor MnO2/GH//GH was fabricated with MnO2/GH as cathode and GH as anode. The electrochemical performances of these devices were first measured by CV tests (Figure S7a,c,e). Note that GH electrode can also exhibit a stable potential window of -1.0~0 V (Figure S8). Consequently, the voltage window of the MnO2/GH//GH supercapacitors can be extended to 2 V, while the two symmetric supercapacitors only exhibit a voltage window of only 1 V. The CV and charge-discharge curves of MnO2/GH//GH operated at different working voltages with a constant scan rate of 20 mV/s and charge-discharge current density of 1 A/g, respectively (Figure 7c,d), indicating an excellent electrochemical stability. The CV curves of the GH//GH and MnO2/GH//GH can keep stable shapes with increasing 15



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scan rate; nevertheless, the MnO2/GH//MnO2/GH presents obvious deformation at high scan rates, suggesting relatively inferior rate performance. Additionally, galvanostatic charge-discharge (GCD) tests were also conducted to further investigate the rate capability of the supercapatiors (Figures S7b, d, f). As shown in Figure 8e, the GH//GH, MnO2/GH//GH and MnO2/GH//MnO2/GH deliver a specific capacitance of 19.4, 62.5 and 132.0 F/g at a current density of 1 A/g, respectively. When the test current density increases to 20 A/g, the specific capacitance of the MnO2/GH//GH device still reaches 28.8 F/g, but the capacitance of MnO2/GH//MnO2/GH only remains 25.0 F/g. Therefore, the rate performance difference of the three devices is consistent with the CV observations. 3.3 Analysis on the kinetics characteristics of supercapacitors To futher vertify the rate performance and charge storage kinetics characteristics of the asymmetric supercapacitor MnO2/GH//GH, the CV curves at different scan rates were analyzed quantitatively based on the following equation:43,44

i = 2 3 ! 2 3 /

(8)

In Eq. (8), i is the current at a fixed potential (A), v is the scan rate in CV measurements (V/s), k1 and k2 are constants. The total stored charge can be separated into two components: k1v is ascribed to capacitive contribution from electric double-layer effect, and k2v1/2 is corresponded to pseudocapacitive contribution from faradaic reaction occurring on the surface and in the bulk of δ-MnO2 material. As shown in Figure S9, the capacitive contribution to the device is dominant, especially at high scan rates. Specifically, the capacitive contribution to the device is almost the 16


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same as the pseudocapacitive contribution at low scan rate. Nevertheless, the pseudocapacitance part attenuates dramatically (from 48.1 to 17.9%) when the scan rate is increased from 10 to 300 mV/s, while the capacitive contribution is barely changed (only 3% reduction) (Figure 4f). It can be confirmed that, firstly, the psedocapacitance is derived from the surface Faradaic reaction of δ-MnO2 and the Na+ intercalation into the layered δ-MnO2, as shown in Figure 4a. Secondly, the intercalation or deintercalation of electrolyte cation is a diffusion-controlled kinetics process. In consequence, the decay of rate performance could be attribute to sluggish kenetic behavior of electrolyte ion in the process of intercalation or deintercalation to MnO2. For the same reason, the MnO2/GH//MnO2/GH device shows worse rate capability (Figure 4e). For the GH//GH supercapacitor, in contrast, relatively decent rate capability mainly originates from stable electric double-layer capacitive characteristic due to fast electrolyte ion adsorption and desorption process. Consequently, the asymmetric supercapacitor exhibits good performance, which can be attributed to the synergistic effect of high pseudocapacitance of MnO2 material and stable rate feature of graphene macroporous assembly.

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Figure 4. (a) The CV curves of the GH and MnO2/GH electrodes at a scan rate of 10 mV/s; (b) Specific capacitance of the GH and MnO2/GH electrodes at different current densities; (c) CV curves of the MnO2/GH//GH at different voltages at a scan rate of 20 mV/s; (d) The typical charge/discharge plots of the asymmetric supercapacitor MnO2/GH//GH at different voltages at a current density of 1 A/g; (e) The specific capacitance of the three devices at different current densities; (f) Contribution of capacitive and diffusion-controlled in the asymmetric supercapacitor MnO2/GH//GH at different scan rates.

The energy density and power density of the three supercapacitors were caculated according to the equations (3) and (4), and compared with the results reported in literatures previously, as shown in Figure S10. As expected, the asymmetric supercapacitor presents the maxmuim energy density of 34.7 Wh/kg with a power density of 1.0 kW/kg. Even at high power density 20.0 kW/kg, the supercapacitor still achieves an energy density of 16.0 Wh/kg. Moreover, the MnO2/GH//GH device shows higher energy density and power density than some reported similar devices (Figure 5a). The long-term cyclic measurements of the supercapacitors were evaluated 18


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by GCD at a current density of 5 A/g (Figure 5b). The equivalent series resistance (Rs) and maximum power density (Pmax) of the three supercapacitor are calculated from ∆ViR of the charge-discharge data according to the following equations: ∆56 = . ! 78 9*: =

< ;

6=

=

(9)

*

The dependence of ∆ViR on curent densities of the supercapacitors is displayed in Figure S11 and specfic values are displayed in Table S1. The Rs values of GH//GH, MnO2/GH//GH and MnO2/GH//MnO2/GH devices are 6.01, 6.28 and 18.94 Ω, respectively. Apparently, the Rs values of devices increase with increasing the amount of MnO2. The MnO2/GH//GH device displays a maximum power density of 149.85 kW/kg, which demonstrates potential application in energy storage field.

Figure 5. Ragon plot of MnO2/GH//GH (a) and comparison with reported several MnO2-based supercapacitors (the references are listed in Supporting Information); Long-term cycle performance of the supercapacitor at a current density of 5 A/g (b). The inset in (b) shows a photograph of a LED light powered by MnO2/GH//GH device.

4. CONCLUSION

19



In summary, a green glucose and ammonia reduction system was developed to prepare 3D GH macroporous framework in large scale as a simple and effective method. As an electrode material for supercapacitor, MnO2 nanosheets were then deposited on the GH surface via wet chemical reaction process. The electrochemical performances of both GH and MnO2/GH were investigated by three-electrode system in 6 M KOH solution. The MnO2/GH composite displays a high specific capacitance of 200.6 F/g, much higher than that of the GH, which is mainly originated from the pseudocapacitive effect of MnO2. Futhermore, three kinds of supercapacitors were fabricated successfully. It was found that the MnO2/GH//GH device exhibited a wide working voltage window (2 V) and high energy density of 34.7 Wh/kg at a power density of 1.0 kW/kg. In term of the kinetic characteristics of the device, the combinaton of high specific capacitance of MnO2 and excellent rate capability of the GH is responsible for the good electrochemical performance of the obtained the asymmetric supercapacitor. This work demonstrates a promising strategy for design of high-performance next-generation supercapacitors.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Photographs of KMnO4 solution reacted with GH in acidic and neutral conditions at different reaction time, Schematics of the crystal structure of δ-MnO2, TEM elemental mappings, XRD pattern, SEM image, CV curves, GCD curves, 20


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Nyquist plots of EIS, the cycle performance, Capacitive contribution of the asymmetric supercapacitor, The Ragon plots of the three supercapacitors, The ∆ViR-Curent density plots for the supercapacitors, and The calculated Rs and Pmax values of three supercapacitors. The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work was funded by the National Science Foundation of China (Nos. 51672029 and 51372271) and the National Key R & D Project from Ministry of Science and Technology, China (2016YFA0202702). This work was also supported by the Thousands Talents Program for the pioneer researcher and his innovation team in China.

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Table of contents (TOC)

27


Graphene Hydrogel

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