High Density of Free-Standing Holey Graphene ... - ACS Publications

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High Density of Free-Standing Holey Graphene/PPy Films for Superior Volumetric Capacitance of Supercapacitors Zhimin Fan,†,# Jianpeng Zhu,†,‡,# Xinghui Sun,† Zhongjun Cheng,§ Yuyan Liu,*,† and Youshan Wang*,∥ †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P.R. China ‡ Hongdou Group, Jiangsu General Science Technology Co., Ltd., Wuxi, Jiangsu 214000, P.R. China § Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, Heilongjiang 150090, P.R. China ∥ National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, Heilongjiang 150090, P.R. China S Supporting Information *

ABSTRACT: The volumetric performance is a vitally important metric for portable electronic and wearable devices with limited space. However, it is contradictory for the most supercapacitors in the connection between the volumetric and gravimetric capacitances. Herein, we report a simple strategy to prepare a free-standing and binder-free holey graphene/PPy film that possesses a dense microstructure but still high gravimetric capacitances. The holey graphene/PPy film own high-efficiency ion transport channels and big ion-accessible surface area to achieve high-powered supercapacitor electrodes, which have a superior volumetric capacitance (416 F cm−3) and high gravimetric capacitance (438 F g−1) at 1.0 A g−1 in 6 M KOH electrolyte. Meanwhile, it possesses high rate capability and good cycling performance (82.4% capacitance retention even after 2000 cycles). Furthermore, the volumetric energy density of assembled holey graphene/PPy film symmetric supercapacitor can show high as 22.3 Wh L−1. Such densely packed free-standing holey graphene/PPy film is a very significant electrode material for compact and miniaturized energy storage equipment in the further. KEYWORDS: holey graphene, polypyrrole, high density, supercapacitor, high volumetric performance

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to have high volumetric energy density.6−9 High volumetric capacitance requires the gravimetric capacitance and density to achieve the best. However, the relationship between the gravimetric and density is contradictory for most supercapacitors.10,11 High density of electrode not only can inevitably reduce the electrolyte ion-accessible effective surface areas but also impede electrolyte ion transport channels and bring about a poor gravimetric capacitance. Usually with a large effective surface area of electrode materials are able to achieve ideal gravimetric capacitance, while the volumetric capacitance

INTRODUCTION Supercapacitors are regarded as a kind of fascinating green energy storage equipment attribute to their excellent cyclability, superior power density, environmental friendliness, and low maintenance cost.1−5 Although remarkable gravimetric capacitances have been gained in many published reports, the pore volume of nanomaterials of supercapacitors is usually relatively large, resulting in low packing density and poor volumetric capacitance. The volumetric capacitance property of supercapacitors has become a paramount metric in real practical applications of small equipment, such as very popular portable electronic equipment, wearable devices, electric vehicles, and mobile electronics, so it is very significant to devise and fabricate miniaturized energy storage devices that are necessary © 2017 American Chemical Society

Received: March 10, 2017 Accepted: June 12, 2017 Published: June 12, 2017 21763

DOI: 10.1021/acsami.7b03477 ACS Appl. Mater. Interfaces 2017, 9, 21763−21772

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ACS Applied Materials & Interfaces

g−1, this ascribed to its low density (40 mg cm−3).12 If it can assemble holey graphene and PPy into a compact composite with high specific capacitance, that would obtain high volumetric property. Herein, we report a simple strategy to prepare a free-standing and binder-free holey graphene/PPy film that possesses a dense microstructure but still high gravimetric capacitance. The synergistic effect between holey graphene nanosheets and PPy with a dense interconnected pore connectivity structure could provide efficient electrolyte ion transport channel to obtain high-powered supercapacitor electrodes and cycling performance. The symmetrical supercapacitor could achieve high volumetric energy density of 22.3 Wh L−1 at 189.5 W L−1.

is weak attribute to its lower density.12 Therefore, it is still a challenge to design excellent electrode materials for supercapacitors that possess a high volumetric capacitance and superior volumetric energy density. Graphene, as an important energy storage material for the next generation of high-powered supercapacitors on account of its excellent ultrahigh inherent electrical conductivity (∼16 000 s m−1), high surface area (∼2630 m2 g−1), high chemical stability, superior mechanical properties and low cost.13−16 However, pristine graphene with extended π-conjugation in the basal plane is liable to restack with each other by means of intense van der Waals force, as well as π−π stacking interaction to form irreversible agglomerates, leading to a low specific surface area and poor supercapacitor performance.17,18 Nevertheless, because of these properties, chemically converted graphene can be assembled into three-dimensional monolith.19 The three-dimensional graphene monolith electrode has a large effective specific area and unimpeded electrolyte ion transport channel, which can easily achieve high gravimetric capacitance, while most possess poor volumetric capacitance because of its lower density. To settle the matter above, it seems to be promising to prepare dense graphene block with aggregation morphology formed through the accumulation of grapheme nanosheets to increase the density and volumetric energy density of supercapacitors.20−24 It can employ a threedimensional porous of graphene hydrogel to assemble compact graphene foam that rely on capillary compression, which bring about the concentration of a three-dimensional compact structure since an intense interaction between graphene nanosheets and water, thus the volumetric capacitance of this dense graphene can reach up to 376 F cm−3 because of its high density (1.58 g cm−3).25 Currently, holey graphene as novel graphene derivatives, has caused tremendous interest in energy storage areas on account of its outstanding high inherent electrical conductivity, high surface area and abundant efficient edge active sites, which can shorten electrolyte ion transmission between different graphene layers.11,26−29 Qu et al. reported a compact N-doped holeygraphene monolith microstructure, which has an excellent volumetric capacity of 1052 mAh cm−3 for lithium ion battery. Unfortunately, the electrolyte ion transport channel and ionaccessible effective surface area are gravely impaired because of the closely stacked graphene agglomerate electrodes, so usually need to crush this high density of material and mix with binder to make electrodes, while the gravimetric and volumetric capacitance are decreased inevitably.20,21 It is well-known that the theory of graphene gravimetric capacitance is only about 550 F g−1. An extremely effectual strategy to enhance the capacitance of graphene is to compound it with pseudocapacitive materials.30−34 Polypyrrole (PPy) is a vitally significant material of pseudocapacitors due to its relatively good conductivity, simple synthesis, high redox pseudocapacitive storage and low preparation cost.5,35−38 Assembling holey graphene and PPy into a compact composite with high volumetric capacitance is a challenge. This requires holey graphene/PPy composite to not only have a dense assembly framework but also not sacrifice density. Zhao et al. prepared a three-dimensional graphene by hydrothermal treatment of homogeneous graphene oxide aqueous dispersion at 180 °C, then electrochemical polymerization and the three-dimensional graphene/PPy foam were obtained. Unfortunately, the volumetric capacitance of foam structure achieved only 14 F cm−3 despite the specific capacitance could reach up to 350 F

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EXPERIMENTAL SECTION

Materials. Pyrrole, phosphoric acid (H3PO4), hydrogen peroxide (H2O2), sulfuric acid (H2SO4), potassium hydroxide (KOH), potassium permanganate (KMnO4), and sodium sulfate (Na2SO4) were supplied by Sinopharm Chemical Reagent Company. Natural flake graphite (200 meshes) was acquired from Nanjing XFNANO Material Tech. Co., Ltd. All chemicals could be used directly without additional purification except that pyrrole (Py) needed distillation under reduced pressure. Preparation of High Density of Free-Standing Holey Graphene/Polypyrrole (HHG-PPy). Graphene oxide (GO) was obtained through an improved Hummers’ method.39 HG-PPy-20 was prepared on the basis of following procedure. Typically, a 5 mL of 0.3 wt % H2O2 and Py (20 μL) were mixed with 50 mL GO (1 mg mL−1) suspension under stirring. The as-obtained mixtures were sealed and hydrothermal treated at 180 °C for 6 h in a 100 mL Teflonlined autoclave, and subsequently spontaneously cooling to room temperature. Then, the achieved holey graphene/polypyrrole hydrogel (HGPPy-20) was purified with ultrapure water to remove redundant residual impurities. Afterward, use a blade to cut as-prepared cylindrical HG-PPy-20 into slice, and immersed subsequently in aqueous electrolyte (6.0 M KOH) for 10 h to ensure the internal water was exchanged by the electrolyte. Finally, the HG-PPy-20 slice was pressed under 50 MPa for 20 min to form high density of free-standing holey graphene/polypyrrole film (HHG-PPy-20) with a thickness of 10 μm, and this film can be directly applied to working electrode. According to the same preparation method, the HHG-PPy-10 and HHG-PPy-30 were prepared, respectively. For comparison, the high density of holey graphene film (HHG) was obtained. Briefly, 50 mL of GO (2 mg mL−1) suspension was mixed with a 4 mL 0.3 wt % H2O2 under stirring. Then, the mixtures were sealed and hydrothermal treated at 180 °C for 6 h in a 100 mL Teflonlined autoclave, and subsequently spontaneously cooling to room temperature. Finally, the obtained holey graphene hydrogel (HG) was purified with ultrapure water to remove redundant residual impurities and other steps were similar to current method of HHG-PPy-20. Characterization Methods. Atomic force microscopy image was collected in a Dimension Icon using a tapping mode. The microstructures of the as-acquired materials were studied using transmission electron microscope (JEOL JEM-2100), as well as scanning electron microscope (SUPRA 55 SAPPHIRE). The Brunauer−Emmett−Teller analysises were applied to measure the specific surface areas. In addition, the distribution curves of pore size were acquired via density functional theory method. X-ray diffraction analyses were studied through a D8 Advance (Bruker) using Cu Ka radiation. A Thermo ESCALAB 250 X-ray was utilized to perform the X-ray photoelectron spectroscopy dates using Al Kα radiation. Raman spectroscopy (Renishaw Invia, UK) and Fourier transform infrared spectroscopy (Thermo Nicolet5700, USA) were applied to analyze the chemical structure of samples. Electrochemical Measurements. The as-achieved working electrodes were performed using cyclic voltammetry (CV), constant current charge/discharge, and the electrochemical impedance spectroscopy (EIS) in the 6 M KOH aqueous electrolyte on a CHI 760D 21764


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Figure 1. (a) Schematic illustration of the preparation process of free-standing HHG-PPy film. TEM images of the samples: (b) HG-PPy-0, (c) HGPPy-10, and (d) HG-PPy-20. (The inset shows the selected-area electron diffraction pattern.) (e) TEM images of the HG-PPy-30. SEM images of the cross-section of (f) HHG-PPy-20 and (g) HG-PPy-20.

Pv = Ev /t

workstation (Shanghai Chenhua) using a three electrode system. Furthermore, the prepared high density of free-standing HHG-PPy films was directly applied to the working electrodes without any additional additives. Meanwhile, the counter and reference electrodes were employed by Pt foil and Ag/AgCl, respectively. CV and constant current charge/discharge measurements were carried out between −1 and 0 V. In addition, the EIS dates were performed using a frequency range from 0.01 HZ to 100 kHZ at an alternating voltage with 5 mV amplitude. The gravimetric and volumetric capacitances were estimated on the basis of

Cs (Fg −1) =

where Cv presents the volumetric capacitance of HHG-PPy-20 based on single electrode material. ΔV (V) and t (s) present the voltage range and discharge time, respectively.

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RESULTS AND DISCUSSION As we can see in Figure S1a, the prepared GO is snuff-colored. The solubility of GO was tested and shown in the lower left corner of Figure S1a. The concentration of GO aqueous dispersion was 2 mg mL−1 for 30 min under sonication. It is obvious seen that the light dark brown dispersion of the GO in water contains no much visible precipitate and can exist stably for 30 d. As shown in Figure S1b, atomic-force microscopy image displays that the thickness of GO nanosheets is about 1.0 nm, and lengths in the range of 1−5 μm. It is worth pointing out that the van der Waals thickness of original graphene nanosheets is only ∼0.34 nm,13 while the GO nanosheets become thicker because of the function of the sp3-hybridized carbon atoms as well as the existence of oxygen functional groups.40,41 TEM and SEM display crumpled nanosheets, similar to the previous reports.39,42,43 The formation procedure for high density of free-standing HHG-PPy films is illustrated in Figure 1a, and the photographs of HG-PPy-10, HG-PPy-20, and HG-PPy-30 are shown in

It mΔV

Cv (Fcm−3) = ρ × Cs where Cs (Fg−1) and Cv (Fcm−3) present the gravimetric and volumetric capacitance of the as-achieved electrodes, respectively, and m(g) presents the mass of the active material, I (A) presents the constant discharge current, ΔV (V) presents the potential windows in the charge−discharge curve, t (s) presents the discharge time, and ρ (g cm−3) presents the density of the as-achieved electrode materials. Moreover, to further explore the real application of sample electrodes, the symmetric supercapacitor was fabricated with 1 M Na2SO4 aqueous electrolyte to evaluate the energy density. Volumetric energy density (Ev) and volumetric power density (Pv) were estimated as

Ev = 1/8Cv ΔV 2 21765


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Figure 2. (a) N2 adsorption−desorption isotherms of samples HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30, the pore size distribution is inserted. (b) XRD spectra of the prepared of samples. (c) Raman spectra of GO, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30. (d) FTIR spectra of the prepared of samples. C 1s XPS spectra of (e) HHG and (f) HHG-PPy-20.

the surface of holey graphene to yield a HG-PPy hydrogel.20 Then the HG-PPy was compressed to a high density of film with flexibility. It is well-known that the Py monomer possesses a typical conjugated structure, which may facilely link upon the surfaces of GO nanosheets via π−π interaction or H-bonding. Furthermore, the self-stacked behavior of GO nanosheets can be efficiently impeded by introducing Py during the hydrothermal treatment, and simultaneously enhance the usable GO nanosheets to form a huge volume of three-dimensional graphene hydrogel with thin connection walls.12 So, the

Figure S2a. A controlled quantity of H2O2 aqueous solution and Py monomer were added into GO nanosheets aqueous dispersion. The mixtures were disposed using hydrothermal reaction at 180 °C for 6 h. It is noteworthy that during the hydrothermal process, the carbon atoms on the GO are surrounded by a large number of defective active sites that could be partially oxidized and etched through controlling the amount of H2O2, and the leaving carbon vacancies slowly extended into nanopores.11,26,28,44 At the same time, the H2O2 can make Py polymerize into PPy, and PPy uniform coating on 21766


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enhance the density of electrodes, while reduce the electrolyte ion diffusion rate and ion-accessible effective surface area, leading to a poor volumetric property. Although the sample HHG-PPy-20 has a densely stacked structure, it still owns abundant porous structure. The pore-size distribution curve of Figure 2a (inset) shows that the HHG-PPy-20 possesses numerous wildly pore sizes of 0.9 to 3.5 nm. This unique holey structure can effectively facilitate electrolyte ion to proliferate into the different stacked graphene layers.11,20 The XRD patterns were used to reveal the crystal structure. In Figure 2b, GO shows an intense and sharp diffraction peak at 9.4° corresponds to an interplanar distance of 0.9 nm, indicating the successful oxidation of natural flake graphite.9,39 When the GO is etched by only using H2O2 aqueous solution without the addition of Py at 180 °C, the typical diffraction peak of GO located at 9.4° disappears, but two peaks are detected for graphite-like structure at 2θ = 24.5° and 43°, which correspond to (002) and (100) planes, respectively, demonstrating the successful preparation of holey graphene.29 After a controlled quantity of H2O2 aqueous solution and Py monomer were added into GO aqueous dispersion at 180 °C and the obtained 3D holey graphene with PPy composite foam was compressed to form HHG-PPy-10 and HHG-PPy-20, respectively. The peak at about 2θ = 43° disappeared, and the broad peak shifted from 2θ = 24.5° to 2θ = 25°, suggesting that the holey graphene and PPy have been completely interacted. It is noteworthy that compared with HHG-PPy-10 and HHG-PPy-20, the sample HHG-PPy-30 shows diffraction peaks at 43°, which is ascribed to the characteristic peak of graphene corresponding plane. For HHG-PPy-30, spherical PPy attached to the graphene nanosheets and no holes formed on the graphene nanosheets, which are similar to mechanically mix graphene with PPy, and thus the bonding force between of PPy and holey graphene is relatively weak. To further understand the structure of the as-obtained samples, the Raman spectroscopy test was performed. As we can see the Raman spectrum of GO demonstrates two typical remarkable peaks at 1602 and 1351 cm−1 (Figure 2c), which correspond to the G band and D band, standing for the vibration sp2hybridized graphitic carbon atoms as well as the transform of sp2-hybridized to sp3-hybridized, respectively. It is worth pointing out that the Raman spectrum of HHG-PPy-10 shows a series of weak peaks except the D and G band of holey graphene, revealing the coexistence of holey graphene and PPy. For sample HHG-PPy-20 and HHG-PPy-30, more intense characteristic peaks of PPy were observed, evidencing the successful polymerization of Py around holey graphene nanosheets. To confirm the interactions between PPy and holey graphene, the FTIR absorption spectra of GO, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 were shown in Figure 2d. For the pattern of GO, the bands at 1735, 1226, and 1047 cm−1 that represent the stretching vibrations of CO, C−OH, and carboxy C−O, respectively. Moreover, the bands at 1620 and 1405 cm−1 represent the adsorbed H2O bending as well as C− O deformation, respectively. The results are in correspondence with the previous reports, indicating the success of the GO preparation.39,45−47 For HHG-PPy-10, the bands located at 1549 and 1472 cm−1 may on account of typical ring vibrations of PPy, and the strong band at 1176 cm−1 corresponding to  C−H band in plane vibrations, suggesting that PPy was indeed loaded onto the surface of holey graphene through interactions hydrogen banding and π−π stacking between them. The FTIR

amount of Py monomer is an important determinant of the structure and performance. When not added Py under the same conditions, it can be seen that there are many big holes existing in the graphene nanosheets (Figure 1b), this is because immoderate H2O2 would generate an aggressive etching of graphene oxide, as well as expanding the pore size.11,18 The selected area electron diffraction pattern of top left corner of Figure 1b displays the hexagonal crystalline structure, indicating that the HG-PPy-0 is severely etching of graphene. When added a low content of Py (10 μL), it is obviously seen that the sample still has a lot of big holes on the surface of graphene (Figure 1c), but the size of holes significantly less than HGPPy-0 and the graphene nanosheets becomes thick and rough. When added a content of Py (20 μL), it can be seen that a uniform pores sizes distribute in the entire basal plane of graphene with a few nanometers and PPy layers are homogeneously coated along holey graphene nanosheets (Figure 1d). This would be further demonstrated by the uniform distribution of C, O and N atoms in the elemental mapping analysis (Figure S2c). At the same time, as show in the inset of Figure 1c and Figure 1d, it can be observed clearly that the electron diffraction spots of graphene have disappeared for HG-PPy-10 and HG-PPy-20 due to the presence of amorphous PPy. Most important of all, Py was imported into the 3D holey graphene foam for in situ polymerization that prevents possible self-stacking of holey-graphene hydrogel. When further increasing the amount of Py (Figure 1e, 30 μL and Figure S2b, 40 μL), it can be clearly observed that spherical PPy attached to the graphene nanosheets, and the holes were not markedly observed on the graphene nanosheets. These results suggest that in the process of hydrothermal reaction, the H2O2 will first induce Py into PPy, and the residual H2O2 will etch the graphene to form holey graphene. It is worth pointing out that during the hydrothermal process, only adding graphene oxide and Py without the addition of H2O2, the achieved 3D hydrogel chiefly consists of the Py monomer and graphene, which Py primarily physically distributes within the 3D graphene structure, and Py did not polymerize into PPy.12 Therefore, the efficient H2O2 etching process generates throughout entire basal plane of graphene and makes Py polymerize into PPy. When the HG-PPy-20 is compressed to form free-standing film (HHG-PPy-20), the density of HHG-PPy-20 can reach 0.95 g cm−3, while the HG-PPy-20 is only 0.05 g cm−3. Figure 1f shows the cross section of HHG-PPy-20 and before it is compressed (Figure 1g, the interconnected three-dimensional microporous network of HG-PPy-20 has much thinner connection and the pore size is ranged from submicrometer to several micrometers), the PPy and holey graphene were tightly cumulated to develop a very dense framework and no obvious nanopores can be observed. Figure S3 shows the surface of the HHG-PPy-20, and it is shown that HHG-PPy-20 possesses a compact microstructure with high density of 0.95 g cm−3, but still has abundant pore structure, which can efficiently facilitate electrolyte ion to diffuse among the different stacked graphene layers. Nitrogen adsorption−desorption tests displayed a large specific surface area (312 m2 g−1) for HHG-PPy-20, and the N2 isotherms demonstrate a recombination isotherm connecting Type I and IV adsorption, differing from that of HHG-PPy10 and HHG-PPy-30. The specific surface areas of HHG-PPy10 and HHG-PPy-30 are 228 and 135 m2 g−1, respectively, which are distinctly less than that of HHG-PPy-20. Many studies have effectively proved that the compact structure can 21767


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Figure 3. (a) CV curves of HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 at a scan rate of 5 mV s−1. (b) CV curves of HHG-PPy-20 at different scan rates. (c) Galvanostatic charge−discharge curves of HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 at 1.0 A g−1. (d) Capacitance retention of HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 at different current densities. (e) Cycling performance of HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 at 1.0 A g−1 for 2000 cycles. (f) EIS of HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30.

and 284.5 eV (C−C), respectively.48 For HHG-PPy-20, a new peak was observed at 285.5 eV, indicating the presence of C− N.49 It is worth noting that the oxygen content of holey graphene and PPy composites gradually reduce with the increase of content of Py, which is because the Py monomer can partially reduce the graphene oxide.46 The electrochemical properties of HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 electrodes are surveyed by CV, constant current charge−discharge, as well as EIS,

results of HHG-PPy-20 and HHG-PPy-30 are similar to the HHG-PPy-10. The surface chemistry of the obtained materials was further testified via XPS. For HHG, only the O 1s and C 1s peaks are detected, but the XPS spectra of HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 show additional N 1s peak because of the existence of PPy (Figure S4). Figure 2e and 2f describe the C 1s spectra of HHG and HHG-PPy-20, respectively. For HHG, the XPS peaks of C 1s are exploded into three component peaks at 287.7 (CO), 286.3 (C−O), 21768


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Figure 4. Electrochemical performance of symmetric supercapacitor based on HHG-PPy-20 film electrode: (a) CV curves operated in different potential windows at 50 mV s−1. (b) Galvanostatic charge−discharge curves at various current densities. (c) Volumetric capacitances at different current densities, the schematic of ion-diffusion pathway across the HHG-PPy-20 and HHG-PPy-30 are inserted. (d) The Ragone plots relating volumetric energy density to volumetric power density in comparison with some previous studies.

exhibit nonsymmetric triangular shape, indicating that it possesses the existence of pseudocapacitance except for the electric double layer capacitances.46 The capacitance of the sample electrodes is presented at different current densities (1−20 A g−1) in Figure 3d. It is noteworthy that specific capacitance of HHG-PPy-20 electrode remains up to 328 F g−1, as well as a high capacity retention rate of 74%, even though the current density reach as high as 20 Ag1−, which is significantly better than HHG-PPy-10 (62%) and HHG-PPy-30 (50%), suggesting that the sample HHGPPy-20 with a suitable pore structure and PPy can be uniformly loaded on holey graphene. The electrochemical stability of HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 were studied with long-term charge−discharge cycle measurements at 1.0 A g−1 (Figure 3e). It is obviously seen that the sample HHG-PPy-20 electrode exhibits capacitance retention of about 82.4%, superior to the HHG-PPy-10 (74.8%) and HHG-PPy30 (61.5%), showing good electrochemical stability of the HHG-PPy-20, this is because of high-efficiency ion transport channels and big ion-accessible surface area in the densely stacked holey structure. EIS was performed to investigate the variation of chargetranfer kinetics and the ion diffusion rate of the prepared HHG, HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30. As we can see in Figure 3f, all of the Nyquist plots exhibit similar curves,

respectively. The CV curves of HHG, HHG-PPy-10, HHGPPy-20, and HHG-PPy-30 electrodes are exhibited in Figure 3a at 5 mV s−1. For HHG electrode, the CV curve shows rectangular shape, demonstrating a double layer electric capacitive behavior, while the CV curves of HHG-PPy-10, HHG-PPy-20, and HHG-PPy-30 electrodes are similar quasirectangular shapes, suggesting that this three electrode materials exhibit a double layer electric capacitive and pseudocapacitance behavior. In addition, it is obviously seen that the HHG-PPy-20 shows the highest CV integration area, followed by that of HHG-PPy-30 and HHG-PPy-10, respectively, indicating that the electrochemical performances of HHG-PPy-20 are higher than those of HHG-PPy-30 and HHG-PPy-10, this owing to the best synergy of an appropriate pore structure of holey graphene and PPy. It is obviously seen that the current response of HHG-PPy-20 increases as the scanning rate increases (Figure 3b). Moreover, the CV curves can hold rectangular shape, which signifies that the HHG-PPy20 electrode has good rate stability and high capacitive performance with good ion response. Figure 3c shows the constant current charge−discharge curves of HHG, HHG-PPy10, HHG-PPy-20, and HHG-PPy-30 electrodes at 1.0 A g−1. We can clearly observe that the HHG exhibited linear charge− discharge profile because of its idea double layer electric capacitive. Unlike the HHG, the other three sample electrodes 21769



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which include an incomplete semicircle in the high-frequency region and a straight line in the low-frequency region. Usually, the interfacial charge-transfer resistance (Rct) was estimated via the diameter of the semicircle in the high-frequency.50,51 The Rct of HHG-PPy-20 is 1.2 ohm, smaller than that of the HHGPPy-10 (2.5 ohm) and HHG-PPy-30 (5.1 ohm), respectively, indicating that the HHG-PPy-20 possesses a low resistance with a fast ion response at the high-frequency ranges. Meanwhile, the Warburg-type line of HHG-PPy-20 electrode is as shorter as that of HHG-PPy-10 and HHG-PPy-30 electrode in the inset of Figure 3f, demonstrating low electronic resistance and rapid ion diffusion for HHG-PPy-20 electrode, which is because the unique special hole structure can facilitate ion enter into the graphene surface and rapid diffusion rate in spite of the dense packing structure.11,18,20 To further explore the real application of HHG-PPy-20 electrode, the symmetric supercapacitor was fabricated with 1 M Na2SO4 aqueous electrolyte to evaluate the energy density. Figure 4a displays the CV curves of symmetric supercapacitor within various potential windows at 50 mV s−1. We can distinctly observe the anodic current does not display any obvious increase even at 1.4 V, indicating that the electrode is stable. In addition, the entire charge−discharge curves of the symmetric supercapacitor reveal triangular shapes (Figure 4b), demonstrating that its capacitive behavior is very good.52 Meanwhile, as can be seen in Figure 4c, the HHG-PPy-20 could achieve a maximum capacitance of 328 F cm−3 at 0.5 A g−1. Schematic graphs of the ion-diffusion channel across the HHGPPy-30 and HHG-PPy-20 are shown in the inset of Figure 4c. For HHG-PPy-20, the nanoporous in graphene nanosheets of film can shorten ion diffusion distance to immensely accelerate the ion transport across the entire HHG-PPy-20 between neighboring layers of graphene, but the ions in HHG-PPy-30 need to go through longer paths to seek out the broken graphene nanosheets edges to bypass each layer of graphene nanosheets in the film.11 This unique holey structure and packing density of HHG-PPy-20 increase the ion diffusion rate and ion-accessible effective surface area to generate a high volumetric capacitance. The cycling stability of the symmetric supercapacitor is further conducted at 1.0 A g−1 for 2000 cycles and it is clearly seen that the supercapacitor retains 80.2% of the initial capacitance, indicating its good long-term cyclic durability (Figure S5). Furthermore, the maximum volumetric energy density of HHG-PPy-20 symmetrical supercapacitor can reach 22.3 at 189.5 W L−1, which is obviously superior to many other previously reported symmetric supercapacitors in aqueous electrolyte.9,25,53−57

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03477. Morphology of graphene oxide, digital photos, and SEM images of the as-obtained samples, the morphology of surface for HHG-PPy-20, XPS spectra, and the cycling stability of symmetric supercapacitor (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (+86)045186413711. *E-mail: [email protected]. Tel: (+86)045186402377. ORCID

Zhongjun Cheng: 0000-0001-5550-2989 Yuyan Liu: 0000-0003-3030-8551 Author Contributions #

Z.F. and J.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by the National Natural Science Foundation of China (Grant 51573035).

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REFERENCES

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CONCLUSIONS In summary, a free-standing and binder-free holey graphene/ PPy film has been prepared by a simple strategy, and the density can reach 0.95 g cm−3. The films can be directly applied to working electrodes. On account of its dense packing, highefficiency ion transport channels and big ion-accessible effective surface area, it revealed a superior volumetric capacitance (416 F cm−3) and high gravimetric capacitance (438 F g−1) at 1.0 A g−1. Furthermore, the symmetrical supercapacitor could achieve high volumetric energy density of 22.3 Wh L−1 at 189.5 W L−1. Such densely packed holey graphene/PPy films are very promising candidate for compact and miniaturized energy storage equipment in the further. 21770


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