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Cite this: DOI: 10.1039/c3nr06650a
One-step synthesis of graphene nanoribbon–MnO2 hybrids and their all-solid-state asymmetric supercapacitors† Mingkai Liu,a Weng Weei Tjiu,b Jisheng Pan,b Chao Zhang,a Wei Gaoa and Tianxi Liu*a Three-dimensional (3D) hierarchical hybrid nanomaterials (GNR–MnO2) of graphene nanoribbons (GNR) and MnO2 nanoparticles have been prepared via a one-step method. GNR, with unique features such as high aspect ratio and plane integrity, has been obtained by longitudinal unzipping of multi-walled carbon nanotubes (CNTs). By tuning the amount of oxidant used, different mass loadings of MnO2 nanoparticles have been uniformly deposited on the surface of GNRs. Asymmetric supercapacitors have been fabricated with the GNR–MnO2 hybrid as the positive electrode and GNR sheets as the negative electrode. Due to the desirable porous structure, excellent electrical conductivity, as well as high rate capability and specific capacitances of both the GNR and GNR–MnO2 hybrid, the optimized GNR//GNR– MnO2 asymmetric supercapacitor can be cycled reversibly in an enlarged potential window of 0–2.0 V. In addition, the fabricated GNR//GNR–MnO2 asymmetric supercapacitor exhibits a significantly enhanced maximum energy density of 29.4 W h kg1 (at a power density of 12.1 kW kg1), compared
Received 16th December 2013 Accepted 18th January 2014
with that of the symmetric cells based on GNR–MnO2 hybrids or GNR sheets. This greatly enhanced energy storage ability and high rate capability can be attributed to the homogeneous dispersion and
DOI: 10.1039/c3nr06650a
excellent pseudocapacitive performance of MnO2 nanoparticles and the high electrical conductivity of
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the GNRs.
1. Introduction Along with the gradual depletion of fossil fuels and the increasing severity of environmental pollution problems, the task of developing efficient and clean energy conversion and storage devices has become increasingly urgent in order to meet future demand.1–3 Supercapacitors, due to their good cycling stability, high power density and low maintenance cost along with high rate of charge propagation, are one of the most promising energy storage systems and have attracted a great deal of interest in recent years.4–8 Based on their energy storage mechanisms, supercapacitors can be classied into two categories, i.e. electric double layer capacitors (EDLCs) and pseudocapacitors. Today, with the development of industry and productivity, advanced supercapacitors with higher energy density and operating voltage are urgently needed for practical applications. Among the present clean-energy systems,
a
State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai, 200433, P. R. China. E-mail: [email protected]; Fax: +86-21-65640293; Tel: +86-21-55664197
b
Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore, 117602, Singapore † Electronic supplementary 10.1039/c3nr06650a
information
(ESI)
available.
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See
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developing and fabricating asymmetric supercapacitors is an efficient way to solve the currently existing problems. This is because such a capacitor can utilize different potential windows of two different kinds of electrodes (i.e. battery-type Faradic electrode and capacitor-type electrode),9,10 resulting in greatly enlarged potential windows and signicantly improved energy density. Transition-metal oxides, such as nickel hydroxide, manganese oxide and ruthenium oxide, are commonly used as active electrode materials due to their excellent pseudocapacitive effect depending on their fast and reversible redox reactions of the electrolyte ions at the electrode surface for charge storage. However, the drawbacks of transition-metal oxides, for example, the tendency of aggregation, lower operating voltages and poor cycle capability, have caused a major bottleneck in their performance and seriously limited their further applications. Recently, carbonic materials with porous structure and good conductive capability have been used to hybridize with transition-metal oxides in order to realize the synergistic effects of pseudocapacitive and EDLC performance,11 which gives a promising way to solve the currently existing problems of supercapacitors for practical applications. Graphene nanoribbons (GNRs) are novel carbon materials in the family of carbon-based nanomaterials which also includes C60, carbon nanotubes (CNTs), graphene, and so on. GNRs have attracted considerable attention for theoretical research and potential applications in the eld of materials science owing to
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their exotic properties.12–16 Recently, several techniques have been explored to synthesize GNRs with reduced defects, such as chemical vapor deposition (CVD),17 chemical,18 lithographic,19,20 and solution-based oxidative methods.21,22 Compared with the graphene prepared from oxidized graphite, GNR has a characteristic geometry with widths varying from a few to several hundreds of nanometers and a length in the micrometer scale, which provides it with particular interest for the design and development of carbon-based nanomaterials or hybrids.23,24 Due to their extraordinary electrical, optical, thermal, and mechanical properties resulting from their unique geometry, GNRs have been conveniently designed for various nanoscale device applications, such as supercapacitors,25 photovoltaics26 and eld effect transistors.27,28 Nanostructured transition metal oxides, such as NiO, MnO2, and RuO2, are also excellent electrode materials for supercapacitors because of their exceptional pseudocapacitive properties.29–33 Among these commonly used metal oxide materials, MnO2 is regarded as one of the most attractive electrode materials due to its low cost, high abundance, environmental benignity, and satisfactory specic capacitance.34,35 However, the poor electrical conductivity and typically densely packed structure of MnO2 has largely hindered its wide application in energy storage systems, including supercapacitors. Most recently, considerable efforts have been devoted to hybridize these metal oxide materials with conductive materials in order to minimize the equivalent series resistance and further improve the power density of the devices. Although some works on graphene–MnO2 composites have been reported,36–39 nonetheless, the investigation to develop much simpler synthesis methods for preparing these kind of hybrid materials with unique structure and high performance is still an urgent task and a great challenge. In this work, a novel hybrid material (GNR–MnO2) of GNR and MnO2 nanoparticles has been easily synthesized by a one-step method. By tuning the amount of oxidant (KMnO4) used in the synthesis process, not only have the pristine CNTs been totally unzipped resulting in the formation of GNR, but also MnO2 nanoparticles have been deposited simultaneously and controllably on the surface or interlayer of GNRs. In addition, a novel all-solid-state asymmetric supercapacitor sandwiched by neutral gel electrolyte (PAAK/KCl) with GNR–MnO2 hybrid as the positive electrode and pure GNR materials as the negative electrode has been fabricated. Electrochemical characterization indicates that this asymmetric supercapacitor not only has a wide potential window of 0–2.0 V which results in a greatly enhanced energy density (29.4 W h kg1), but also exhibits an excellent cycling performance of 88% capacitance retention over 5000 cycles. This excellent performance makes the prepared GNR–MnO2 hybrid a promising electrode material for wide applications in energy storage and conversion systems in the future.
2. 2.1
Experimental Preparation of pure GNRs and GNR–MnO2 hybrids
GNRs were prepared by longitudinal unzipping of the pristine multi-walled CNTs (diameter of 40–60 nm, from Chengdu
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Organic Chemicals Co. Ltd.). Typically, 200 mg of CNTs was dissolved in 50 mL of concentrated sulfuric acid (H2SO4, 98%) and stirred for 1 h. Subsequently, 5 mL phosphoric acid (H3PO4, 30%) was added to the mixture and the reaction temperature was gradually increased to 75 C. 100 mg potassium permanganate (KMnO4) was then added into the reaction system every 30 min until 1.0 g KMnO4 was used up. The reaction mixture was naturally cooled to room temperature and poured into 1 L ice-water containing 6 mL H2O2 (30 wt%). Aer being coagulated for 24 h, the obtained precipitate was sonicated for 30 min and dialyzed against DI water for 1 week, resulting in the formation of GNRs. To prepare the GNR– MnO2 hybrids, the same amount of pristine CNTs, H2SO4 and H3PO4 was added to the reaction ask. By adding different amounts of KMnO4 (more than 1.0 g) into the reaction mixture, GNR–MnO2 hybrids with different contents of MnO2 were obtained. It is noted that during the preparation of GNR– MnO2 hybrids, the reaction temperature began to drop to room temperature aer 1.0 g KMnO4 was added, aer which more KMnO4 (0.1–0.4 g) was continuously added into the reaction system to react with the GNRs to form MnO2 nanoparticles on the surface or in the interlayer of GNRs. No H2O2 was used throughout the whole preparation process of the GNR–MnO2 hybrids. Similar dialysis treatment was used for GNR–MnO2 hybrids. The obtained pure GNRs and GNR–MnO2 hybrids were then dispersed in the solution of N,N-dimethyl formamide (DMF) to form a stable dispersion with a concentration of 1 mg mL1. 2.2
Characterization
Thermogravimetric analysis (TGA) was performed with a Perkin Elmer Pyris-1 TGA under a nitrogen atmosphere with a heating rate of 10 C min1. Transmission electron microscopy (TEM) images were observed on a JEOL JEM-2100 TEM instrument with an accelerating voltage of 200 kV. A scanning electron microscope (SEM, VEGA TS 5136MM) equipped with energydispersive spectroscopy (EDS) was used to study the morphologies and microstructures of the samples. Raman spectra were measured on an Avalon Instruments Raman Station using a He– Ne laser (l ¼ 632.8 nm). X-Ray photoelectron spectroscopy (XPS) analyses were conducted with a VG ESCALAB 220l-XL device. XPS curve tting and background subtraction were accomplished using XPS peak41 soware. The mass loading of each electrode was measured by an electric balance with a precision of 0.01 mg (Sartorius BT 125D). The electrical conductivities of GNR and GNR–MnO2 samples were measured on a 4-Point Probes resistivity measurement system (RTS-8). 2.3
Fabrication of all-solid-state supercapacitors
The prepared GNRs and GNR–MnO2 hybrid were spin-coated on the surface of uorine-doped tin oxide (FTO) and then thermally treated at 500 C in an argon atmosphere for 2 h, followed by fabrication of the electrodes. The symmetric supercapacitors were prepared in a two-electrode system by direct assembly of the obtained GNRs or GNR–MnO2 electrodes sandwiched by a potassium polyacrylate/KCl
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(PAAK/KCl) gel electrolyte, which was prepared by adding 1.0 g of PAAK into 10 mL of KCl solution (1.0 M) as previously reported.2 Here, the mass of the active materials on each electrode is about 0.6–1.0 mg for symmetric devices. The corresponding asymmetric supercapacitor was also fabricated in such a two-electrode system with PAAK/KCl as the electrolyte, where GNRs spin-coated on FTO act as the negative electrode and GNR–MnO2 hybrids spin-coated on FTO act as the positive electrode with mass loadings ranging from 0.5–1.5 mg.
2.4
Electrochemical measurements
All the electrochemical tests were carried out on a CHI 660D electrochemical workstation (Shanghai Chenhua, China) at room temperature. In order to investigate the electrochemical properties of the prepared electrode, cyclic voltammetry (CV) tests with various scan rates, galvanostatic charge–discharge tests at various current densities, electrochemical impedance spectroscopy (EIS) and cycling stability of the fabricated cells were performed. The gravimetric specic capacitance was calculated using the following eqn (1): C¼
4I mdV =dt
(1)
where I (A) is the constant current of discharge, m (g) is the total mass of the two electrodes, and dV/dt (V s1) is the slope obtained by tting a straight line to the discharge curve. The energy density, E (W h kg1), was calculated by eqn (2). The maximum power density, P (kW kg1), was calculated from the constant current discharge curves and normalized with the total weight of two electrodes, according to eqn (3) as follows: E ¼ CSPV2/8 P¼
Scheme 1
V2 4RESR m
(2) (3)
Illustration of the fabrication process of GNR–MnO2 hybrid.
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where the RESR is the equivalent series resistance of the cells, and V is the discharge voltage excluding the intial voltage drop.
3.
Results and discussion
The preparation process of the GNR–MnO2 hybrid is illustrated in Scheme 1. As reported previously,40,41 CNTs can be controllably unzipped by an appropriate amount of oxidant (KMnO4). Here, both the opening of the pristine CNTs and the deposition of MnO2 nanoparticles on the surface of GNRs have been simultaneously achieved by a simple one-step method. For all of the KMnO4 used in the reaction, the amount being consumed to unzip the pristine CNTs is about 5 times the CNT weight; and the remaining amount is utilized for the redox reaction with GNRs, generating MnO2 nanoparticles on the surface or the interlayer of GNRs. The proposed reaction mechanism based on the electrostatic interaction between GNRs and KMnO4 and can be illustrated by the following reaction:42,43 4MnO4 + 3C + H2O / 4MnO2 + CO32 + 2HCO3 Here, the carbon substrate serves as the sacricial reductant for converting aqueous permanganate (MnO4) to MnO2 nanoparticles, which are further deposited homogeneously on the surface of GNRs. The weight ratios of MnO2 nanoparticles in the GNR–MnO2 hybrid materials, determined by thermogravimetric analysis (TGA) (Fig. S1†), are about 5%, 11%, 18%, and 23%, respectively, and are produced by electrostatic interactions between 1.1, 1.2, 1.3 and 1.4 g of the oxidant (KMnO4) with the pristine CNTs (200 mg). The increased mass ratios of the MnO2 nanoparticles are ascribed to the gradually increasing amount of KMnO4 used in the experiments. Fig. 1 shows the representative transmission electron microscopy (TEM) images of the obtained GNR sheets and GNR–MnO2 hybrids in comparison with pristine CNTs. Compared with the pristine CNTs (Fig. 1a and b), it is clear that the prepared GNR sheets (Fig. 1c) show layer structures, with no CNTs being observed, indicating that the pristine CNTs are totally opened or unzipped, resulting in the formation of GNRs. This can be further conrmed by the X-ray diffraction (XRD) results in the following section. The opened GNR sheets have a comparable length to the pristine CNTs even aer sonication, which further endows the GNR sheets with a geometric structure with higher aspect ratio and reduced defects (as seen in Fig. 1d). Furthermore, the obtained GNR sheets are crosslinked and inter-connected with each other, which increase efficiently the contact area between different GNRs and facilitates the formation of a conductive pathway. These features may have a signicant effect on the electrochemical performance of the thus-fabricated supercapacitor devices using GNRs and GNRbased hybrid materials. Here, the MnO2 nanoparticles produced by electrostatic interactions between KMnO4 and GNRs have intimate contact with the GNR sheets due to their extremely small size (with diameter of 3–5 nm). In addition, the obtained MnO2 nanoparticles can be distributed
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Fig. 1 TEM images of (a and b) pristine CNTs, (c and d) pure GNR sheets at low and high magnifications. GNR–MnO2 hybrid materials with (e) 5% and (f) 23% mass loadings of MnO2 nanoparticles.
homogeneously on the surface of GNRs, as seen in Fig. 1e. No distinct aggregates of MnO2 nanoparticles were observed even at a high mass loading of 23% (Fig. 1f). With the formation of a hierarchical structure of this hybrid material, the synergistic effect of the electrochemical performance of GNR sheets and MnO2 nanoparticles within the hybrid can be well realized with a reasonable design and assembly of the devices. The morphologies of the synthesized GNR sheets and GNR– MnO2 hybrids are further examined by scanning electron
Fig. 2 SEM images of (a) pristine CNTs, (b) GNR sheets, and GNR– MnO2 (23%) hybrid at (c) low and (d) high magnification.
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microscopy (SEM). As seen in Fig. 2, compared with the tubelike structure of pristine CNTs (Fig. 2a), the GNR sheets show clearly lamellar architecture coupled with microporous structure, with an average pore size of about 50 nm, distributed uniformly throughout the material. This unique structure will promote effectively the electrochemical performance of GNR sheets by providing an efficient and shortened pathway for the transport of electrolyte ions. Moreover, the presence of porous structure throughout the GNR material conrms the absence of serious stacking between the GNR sheets, which will benet the EDLC effect of the electrodes based on GNR sheets. It is also noted that the MnO2 nanoparticles cannot be clearly observed on the exterior surface of the GNR–MnO2 hybrid by SEM (Fig. 2c and d), probably due to the low mass loading and extremely small size of the MnO2 nanoparticles. Therefore, the uniform distribution of MnO2 nanoparticles is further conrmed by energy-dispersive spectroscopy (EDS) elemental mapping. An SEM image coupled with EDS mapping images made from the K-line energy densities of C, Mn and O of the same area are shown in Fig. S2.† In comparison with the C element, the mapping images of O and Mn show a lower distribution intensity, further conrming the uniform distribution and low mass loading of MnO2 nanoparticles in the hybrid material. Fig. S3† shows the XRD patterns of pristine CNTs, GNR sheets, and GNR–MnO2 hybrids with different mass loadings of MnO2 nanoparticles. It can be seen that pristine CNTs show an intense sharp peak located at 2q ¼ 26.1 , corresponding to the characteristic diffraction peak of graphite or CNTs. The XRD pattern of GNRs exhibits a broad peak at 2q ¼ 25.4 , corre˚ which is consistent with the sponding to a d-spacing of 3.5 A, XRD results of graphene, further conrming that the pristine CNTs have been totally unzipped.44 However, for GNR–MnO2 hybrids, the XRD patterns show inconspicuous, new and broad diffraction peaks at 2q ¼ 12.3 and 37.5 , which can be indexed to the (001) and (211) crystal planes of birnessite-type MnO2, respectively.45 The broad features and relatively low intensity of the peak proles suggest that the obtained MnO2 phase in the hybrid materials is amorphous and very small, which is in accordance with the previous reports on redox-deposited MnO2 on carbon materials.46 Raman spectroscopy is performed to investigate the vibrational properties of the as-prepared samples, as seen in Fig. 3. For pristine CNTs and GNR sheets, the G band at 1585 cm1, which corresponds to the E2g phonon of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, and the D band at 1350 cm1, which corresponds to the breathing mode of the rings or the K-point phonons of A1g symmetry, can be obviously observed. The intensity ratio (ID/IG) of D band and G band is usually used to reect the defect density of carbonaceous materials. The ID/IG ratio of GNR–MnO2 (23%) hybrid materials has increased to 1.36 compared with that (1.18) of GNRs, indicating an enhanced level of disorder within the GNR sample and the formation of sp3 carbon by functionalization of the GNR sheets. It is also noted that for the GNR–MnO2 hybrid, apart from D and G bands, apparent peaks at about 575 cm1 and 649 cm1 in the low wavenumber region can be observed. These peaks are commonly attributed to the Mn–O stretching
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Raman spectra of pristine CNTs, GNR sheets, GNR–MnO2 hybrids with varying MnO2 mass loadings of 5%, 11%, 18% and 23%, respectively. Fig. 3
vibration in the basal plane of MnO6 and the symmetric stretching vibration (Mn–O) of the MnO6 groups, respectively.47,48 The enhanced characteristic vibration intensity indicates the increase in the mass loading of MnO2 in the prepared hybrid with the amount of oxidant used increasing, which is in accordance with the TGA results (Fig. S1†). XPS analysis is employed in order to provide further evidence for the structural information of the GNR–MnO2 hybrid. As seen in Fig. 4, the peaks of Mn (2p1/2, 2p3/2), O 1s and C 1s can be clearly observed in the survey spectrum, illustrating the presence of MnO2 nanoparticles on the surface of GNRs. The inset spectrum shows the high resolution spectrum of Mn 2p in the GNR–MnO2 hybrid. The binding energy peaks of Mn 2p3/2 and Mn 2p1/2 are located at 642.1 eV and 653.8 eV, respectively, indicating a spin-energy separation of 11.7 eV. These results are
Fig. 4 XPS survey spectra of the GNR–MnO2 (23%) hybrid. The inset illustrates the narrow spectra of the Mn 2p peaks of the hybrid.
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in good accordance with the data of Mn 2p3/2 and Mn 2p1/2 in MnO2 reported previously,49,50 which further conrm the successful assembly of MnO2 nanoparticles on the surface of GNRs. In order to explore the GNR sheets and GNR–MnO2 hybrids as electrochemical electrodes in asymmetric supercapacitors, the specic capacitance, which is a crucial factor in designing and fabricating asymmetric devices, of the prepared materials needs to be characterized rstly in a symmetric system. Here, the electrochemical properties of pristine CNTs, GNR sheets and GNR–MnO2 hybrids with different mass loadings were characterized by cyclic voltammogram (CV), galvanostatic charge–discharge, and frequency response measurements in symmetric two-electrode systems with PAAK/KCl as the electrolyte, as seen in Fig. 5. Fig. 5a shows the CV curves of pristine CNTs, GNR sheets and GNR–MnO2 hybrid materials with various mass loadings of MnO2 nanoparticles at a scan rate of 10 mV s1. It can be seen that the CV curves of pristine CNTs and GNR sheets show an approximately rectangular shape, which is the characteristic for EDLC performance.51 In contrast, the CV curves of GNR–MnO2 hybrids show relatively apparent redox peaks at about 0.3–0.4 V, which can be ascribed to typical pseudocapacitive performance of MnO2 nanoparticles. As the gravimetric current density increases with the loading level of MnO2 nanoparticles, coupled with the apparent redox peaks, it is clear that MnO2 nanoparticles in the hybrids in fact play the role of pseudocapacitors. This can also be conrmed from the skewed triangle shape of the charge–discharge curves of GNR– MnO2 hybrids and the increased discharge time, as compared with the symmetrical triangle of the charge–discharge curves with limited discharge time for both pristine CNTs and GNR sheets (Fig. 5b). However, as seen in Fig. 5c, it is worthwhile to note that the specic capacitance (287 F g1) of the GNR–MnO2 (23%) hybrid is lower than that (305 F g1) of GNR–MnO2 (18%) hybrid. This decrease of specic capacitance with MnO2 mass loading above 18 wt% can be ascribed to that the additional MnO2 nanoparticles, especially for those located far away from the GNR surface, do not participate actively in pseudocapacitive reactions due to a proton diffusion constant. Also, the increased MnO2 mass loading may decrease the electrical conductivity of the GNR–MnO2 hybrid due to the poor electrical conductivity of MnO2 itself (106 S cm1).52 Furthermore, the electrical conductivities of the samples have been measured using a 4-point system. GNR–MnO2 (23%) shows a signicantly reduced conductivity of 1 S cm1 compared with 40 S cm1 for GNR– MnO2 (5%) and 63 S cm1 for GNR. Fig. 5d shows the specic capacitance of pristine CNTs, GNR sheets and GNR–MnO2 hybrids as a function of discharge current density from 0.5 A g1 to 10 A g1. A sample GNR–MnO2 (18%) hybrid shows a specic capacitance of 305 F g1 at a discharge current density of 0.5 A g1, as well as 277 F g1 at 10 A g1, meaning that 90% of its capacitance was retained even with such a huge current span. This excellent rate performance of the prepared GNR–MnO2 hybrid can be ascribed to the high electrical conductivity of GNR sheets as a supporting substrate and the nanosize of the MnO2 particles, as well as the hierarchical GNR–MnO2 hybrid
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Electrochemical performance of pristine CNTs, GNR sheets and the prepared GNR–MnO2 hybrids with different mass loadings of MnO2 nanoparticles. (a) Cycle voltammograms (at 10 mV s1), and (b) charge–discharge curves (at 0.5 A g1) for pristine CNTs, GNR sheets, GNR–MnO2 hybrids with various MnO2 mass loadings of 5%, 11%, 18%, and 23%, respectively. (c) Specific capacitance of pristine CNTs, GNR sheets and the prepared GNR–MnO2 hybrids calculated from the discharge time at a current density of 0.5 A g1. (d) Specific capacitances of pristine CNTs, GNR sheets and GNR–MnO2 hybrids as a function of discharge current density from 0.5 A g1 to 10 A g1. Fig. 5
with porous structure and ne dispersion of MnO2 nanoparticles. In order to obtain more information on the capacitive performance of the as-prepared materials, the electrodes of GNRs, GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids are selected to be further measured by electrochemical impedance spectroscopy (EIS) and for cycling performance (Fig. 6). With a complex nonlinear least-squares (CNLS) system, the equivalent circuit for tting the EIS spectra has been established (Fig. S4†). As seen in Fig. 6a, all these four materials clearly show vertical lines in the low-frequency region, indicating an ideal capacitive property. The x-intercept at the real part (Z0 ) of the Nyquist plot, which is a combined resistance (Re) from the ionic resistance of the electrolyte, intrinsic resistance of the active materials, and contact resistance at the active material/current collector interface,53 has a signicant inuence on the capacitive performance of the electrode materials. Here, the magnitudes of the equivalent series resistances of GNRs, GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids are about 2.7, 3.4, 3.8 and 3.8 U, respectively, suggesting that the prepared samples are good electrode materials with very small series resistances. The increased resistance of
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GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids as compared with that of GNRs can be attributed to the resistive effect of MnO2 with a low electrical conductivity. Besides, the charge-transfer and ion diffusion resistance (Rct) are important features for the electrode materials. Here, the charge-transfer resistances of GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids, which also can be calculated from the diameters of the semicircles, are determined to be about 1.1, 1.7 and 2.5 U, respectively, endowing them with desirable facilitated charge transfer properties. The long-term stability performance of GNRs, GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids as electrode materials are evaluated by galvanostatic charge–discharge tests up to 5000 cycles at a constant current density of 1 A g1. As seen in Fig. 6b, GNR–MnO2 (18%) hybrids and GNRs show capacitance retentions of 87.5% and 95.7%, respectively, aer 5000 cycles, indicating that both GNR–MnO2 hybrids and GNRs possess good stability as supercapacitor electrodes. Such an excellent stability coupled with the low resistance and 3D hierarchical structure of the GNR–MnO2 hybrid material ensure that the GNR sheets together with novel GNR–MnO2 hybrid materials are ideal electrode candidates for energy storage applications.
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(a) Nyquist plots of the impedance spectra for GNRs, GNR– MnO2 (5%) and GNR–MnO2 (18%) hybrids in a frequency range of 0.01 Hz to 100 kHz. The inset shows the enlarged impedance spectra at the high-frequency region. (b) Cycling performance of GNRs, GNR– MnO2 (5%) and GNR–MnO2 (18%) hybrids measured at a current density of 1 A g1. Fig. 6
To explore further the advantages of the GNR–MnO2 hybrids, asymmetric supercapacitors of GNR//GNR–MnO2 (18%) based on GNRs (negative electrode) and GNR–MnO2 (18%) hybrid (positive electrode) with PAAK/KCl gel as the electrolyte have been fabricated, as shown in Fig. 7a. In order to obtain the optimized operating potential window for the asymmetric supercapacitor, it is necessary to equalize the charges stored at the positive and negative electrodes, which are related to the specic capacitance (Csp), the potential range for the charge– discharge processes (DV), and the mass (m) of the electrode material. In this work, the optimal mass ratio between the GNRs and GNR–MnO2 (18%) hybrid is about 1 : 2.1, which can be calculated using the following equation: m Csp DV ¼ mþ Cspþ DVþ As seen in Fig. 7b, the CV curves of the GNR//GNR–MnO2 (18%) asymmetric supercapacitor exhibit rectangular-like
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shapes with inconspicuous redox peaks which were measured from 0 to 2.0 V at different scan rates of 5, 10, 20, 50, 100 and 200 mV s1 respectively, conrming the ideal capacitive behavior of both the GNR and GNR–MnO2 electrodes. Here, the less conspicuous redox peak compared with that of the symmetric devices may be explained as follows: the negative electrode of asymmetric supercapacitor consists of pure GNR sheets and can only provide EDLC contribution. With the increase of scan rate, an obvious increase of current density is achieved, conrming a good rate capability of the asymmetric GNR//GNR–MnO2 (18%) supercapacitor. Furthermore, compared with the potential window (from 0 to 1.0 V) of GNRs and GNR–MnO2 (18%) electrodes, the enlarged potential window can give a great enhancement of the energy density for the prepared asymmetric supercapacitor since the energy density is proportional to the square of the potential. Galvanostatic charge–discharge tests were also performed with different current densities in a potential window from 0 to 2.0 V, as shown in Fig. 7c. The relatively symmetric charge–discharge curves and the linear potential–time prole in the whole potential range further conrm the excellent capacitive behavior of the GNR//GNR– MnO2 (18%) asymmetric supercapacitor. The specic capacitance of the GNR//GNR–MnO2 (18%) asymmetric supercapacitor calculated from the slope of the discharge curves is 212 F g1 at a current density of 0.5 A g1, and also retains 157 F g1 even at a high current density of 10 A g1 based on the total mass of the active materials in the two electrodes. Such an excellent electrochemical property can be ascribed to the synergistic effect of the EDLC performance of GNR sheets and the pseudocapacitance effect of the MnO2 nanoparticles. Cycling life, as one of the most important requirements in practical applications, has been tested at a constant current density of 1 A g1 for 5000 cycles. As shown in Fig. 7d, 88% of the initial capacitance is still retained even aer 5000 cycles, indicating a good long-term stability of the asymmetric GNR// GNR–MnO2 (18%) supercapacitor. The charge–discharge curves (the inset in Fig. 7d) show a normative multi-cycle triangular wave with tiny curvature for both the charging and discharging processes, indicating the pseudocapacitive effect of MnO2 nanoparticles along with the double layer contribution from the GNR sheets. This superior electrochemical performance of the GNR–MnO2 (18%) hybrid material as the positive electrode can be ascribed to the combined effect of the excellent electrical conductivity of the GNR sheets, good contact between MnO2 nanoparticles and GNR sheets, as well as the facilitated transfer of electrolyte ions and charges through the porous and hierarchical structure of the hybrid materials. EIS analysis is one of the most reliable and principal methods for examining the fundamental behavior of electrode materials for supercapacitors. In our study, in order to demonstrate the resistance change before and aer 5000-cycle tests, EIS has been measured in the frequency range from 0.01 Hz to 100 kHz at an open circuit potential with an ac amplitude of 10 mV (Fig. 8a). At the low-frequency region, the two impedance spectra show oblique straight lines, indicating the excellent pure capacitive behavior of the electrodes. The arc shapes at the high-frequency region can further demonstrate a long-term electrochemical
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Fig. 7 (a) Schematic diagram of the structure of the fabricated all-solid-state asymmetric supercapacitor (GNR//GNR–MnO2 (18%)) with GNRs as the negative electrode and the GNR–MnO2 (18%) hybrid as the positive electrode. (b) CV curves and (c) galvanostatic charge–discharge curves measured with different scan rates or current densities for the GNR//GNR–MnO2 (18%) supercapacitor. (d) Cycle performance of the GNR// GNR–MnO2 (18%) supercapacitor with a discharge current density of 1 A g1. The inset is the galvanostatic charge–discharge curve of the cell.
Fig. 8 Nyquist plots of GNR//GNR–MnO2 (18%) asymmetric supercapacitor in the range from 0.01 Hz to 100 kHz during the life cycle tests.
stability of the GNR//GNR–MnO2 (18%) hybrid material. The obtained impedance spectra were also analyzed by equivalent circuits as shown in Fig. S4.† It can be seen that, even aer 5000 cycles, the Re value is almost unchanged with only a slight increase of Rct observed (from 0.9 to 1.8 U). These EIS results Nanoscale
clearly demonstrate the exceptional stability of the asymmetric GNR//GNR–MnO2 (18%) supercapacitor. In order to evaluate the device performance of the fabricated asymmetric supercapacitor based on GNR sheets and the GNR– MnO2 (18%) hybrid, the energy density (E) and power density (P) calculated from galvanostatic discharge curves are plotted on the Ragone diagram (Fig. 9). Compared with symmetric supercapacitors fabricated based on GNR sheets or GNR–MnO2 hybrids with different mass loadings of MnO2 nanoparticles (i.e. GNR//GNR, GNR–MnO2 (5%)//GNR–MnO2 (5%) and GNR– MnO2 (18%)//GNR–MnO2 (18%)), the asymmetric supercapacitor of GNR//GNR–MnO2 (18%) shows greatly superior electrochemical properties with greatly enhanced power density and energy density. The maximum energy density of 29.4 W h kg1 (at a power density of 12.1 kW kg1) and power density of 25.9 kW kg1 (at an energy density of 22.5 W h kg1) were achieved at an operating voltage of 2.0 V, indicating that the asymmetric supercapacitor can provide high energy density without much sacrice in power density. More interestingly, the maximum energy density (29.4 W h kg1) of the asymmetric GNR//GNR–MnO2 (18%) supercapacitor in our work is higher than or comparable to other manganese oxides-based symmetric or asymmetric supercapacitors reported previously, e.g. MnO2//MnO2 (
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Cite this: DOI: 10.1039/c3nr06650a
One-step synthesis of graphene nanoribbon–MnO2 hybrids and their all-solid-state asymmetric supercapacitors† Mingkai Liu,a Weng Weei Tjiu,b Jisheng Pan,b Chao Zhang,a Wei Gaoa and Tianxi Liu*a Three-dimensional (3D) hierarchical hybrid nanomaterials (GNR–MnO2) of graphene nanoribbons (GNR) and MnO2 nanoparticles have been prepared via a one-step method. GNR, with unique features such as high aspect ratio and plane integrity, has been obtained by longitudinal unzipping of multi-walled carbon nanotubes (CNTs). By tuning the amount of oxidant used, different mass loadings of MnO2 nanoparticles have been uniformly deposited on the surface of GNRs. Asymmetric supercapacitors have been fabricated with the GNR–MnO2 hybrid as the positive electrode and GNR sheets as the negative electrode. Due to the desirable porous structure, excellent electrical conductivity, as well as high rate capability and specific capacitances of both the GNR and GNR–MnO2 hybrid, the optimized GNR//GNR– MnO2 asymmetric supercapacitor can be cycled reversibly in an enlarged potential window of 0–2.0 V. In addition, the fabricated GNR//GNR–MnO2 asymmetric supercapacitor exhibits a significantly enhanced maximum energy density of 29.4 W h kg1 (at a power density of 12.1 kW kg1), compared
Received 16th December 2013 Accepted 18th January 2014
with that of the symmetric cells based on GNR–MnO2 hybrids or GNR sheets. This greatly enhanced energy storage ability and high rate capability can be attributed to the homogeneous dispersion and
DOI: 10.1039/c3nr06650a
excellent pseudocapacitive performance of MnO2 nanoparticles and the high electrical conductivity of
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the GNRs.
1. Introduction Along with the gradual depletion of fossil fuels and the increasing severity of environmental pollution problems, the task of developing efficient and clean energy conversion and storage devices has become increasingly urgent in order to meet future demand.1–3 Supercapacitors, due to their good cycling stability, high power density and low maintenance cost along with high rate of charge propagation, are one of the most promising energy storage systems and have attracted a great deal of interest in recent years.4–8 Based on their energy storage mechanisms, supercapacitors can be classied into two categories, i.e. electric double layer capacitors (EDLCs) and pseudocapacitors. Today, with the development of industry and productivity, advanced supercapacitors with higher energy density and operating voltage are urgently needed for practical applications. Among the present clean-energy systems,
a
State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai, 200433, P. R. China. E-mail: [email protected]; Fax: +86-21-65640293; Tel: +86-21-55664197
b
Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore, 117602, Singapore † Electronic supplementary 10.1039/c3nr06650a
information
(ESI)
available.
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See
DOI:
developing and fabricating asymmetric supercapacitors is an efficient way to solve the currently existing problems. This is because such a capacitor can utilize different potential windows of two different kinds of electrodes (i.e. battery-type Faradic electrode and capacitor-type electrode),9,10 resulting in greatly enlarged potential windows and signicantly improved energy density. Transition-metal oxides, such as nickel hydroxide, manganese oxide and ruthenium oxide, are commonly used as active electrode materials due to their excellent pseudocapacitive effect depending on their fast and reversible redox reactions of the electrolyte ions at the electrode surface for charge storage. However, the drawbacks of transition-metal oxides, for example, the tendency of aggregation, lower operating voltages and poor cycle capability, have caused a major bottleneck in their performance and seriously limited their further applications. Recently, carbonic materials with porous structure and good conductive capability have been used to hybridize with transition-metal oxides in order to realize the synergistic effects of pseudocapacitive and EDLC performance,11 which gives a promising way to solve the currently existing problems of supercapacitors for practical applications. Graphene nanoribbons (GNRs) are novel carbon materials in the family of carbon-based nanomaterials which also includes C60, carbon nanotubes (CNTs), graphene, and so on. GNRs have attracted considerable attention for theoretical research and potential applications in the eld of materials science owing to
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their exotic properties.12–16 Recently, several techniques have been explored to synthesize GNRs with reduced defects, such as chemical vapor deposition (CVD),17 chemical,18 lithographic,19,20 and solution-based oxidative methods.21,22 Compared with the graphene prepared from oxidized graphite, GNR has a characteristic geometry with widths varying from a few to several hundreds of nanometers and a length in the micrometer scale, which provides it with particular interest for the design and development of carbon-based nanomaterials or hybrids.23,24 Due to their extraordinary electrical, optical, thermal, and mechanical properties resulting from their unique geometry, GNRs have been conveniently designed for various nanoscale device applications, such as supercapacitors,25 photovoltaics26 and eld effect transistors.27,28 Nanostructured transition metal oxides, such as NiO, MnO2, and RuO2, are also excellent electrode materials for supercapacitors because of their exceptional pseudocapacitive properties.29–33 Among these commonly used metal oxide materials, MnO2 is regarded as one of the most attractive electrode materials due to its low cost, high abundance, environmental benignity, and satisfactory specic capacitance.34,35 However, the poor electrical conductivity and typically densely packed structure of MnO2 has largely hindered its wide application in energy storage systems, including supercapacitors. Most recently, considerable efforts have been devoted to hybridize these metal oxide materials with conductive materials in order to minimize the equivalent series resistance and further improve the power density of the devices. Although some works on graphene–MnO2 composites have been reported,36–39 nonetheless, the investigation to develop much simpler synthesis methods for preparing these kind of hybrid materials with unique structure and high performance is still an urgent task and a great challenge. In this work, a novel hybrid material (GNR–MnO2) of GNR and MnO2 nanoparticles has been easily synthesized by a one-step method. By tuning the amount of oxidant (KMnO4) used in the synthesis process, not only have the pristine CNTs been totally unzipped resulting in the formation of GNR, but also MnO2 nanoparticles have been deposited simultaneously and controllably on the surface or interlayer of GNRs. In addition, a novel all-solid-state asymmetric supercapacitor sandwiched by neutral gel electrolyte (PAAK/KCl) with GNR–MnO2 hybrid as the positive electrode and pure GNR materials as the negative electrode has been fabricated. Electrochemical characterization indicates that this asymmetric supercapacitor not only has a wide potential window of 0–2.0 V which results in a greatly enhanced energy density (29.4 W h kg1), but also exhibits an excellent cycling performance of 88% capacitance retention over 5000 cycles. This excellent performance makes the prepared GNR–MnO2 hybrid a promising electrode material for wide applications in energy storage and conversion systems in the future.
2. 2.1
Experimental Preparation of pure GNRs and GNR–MnO2 hybrids
GNRs were prepared by longitudinal unzipping of the pristine multi-walled CNTs (diameter of 40–60 nm, from Chengdu
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Organic Chemicals Co. Ltd.). Typically, 200 mg of CNTs was dissolved in 50 mL of concentrated sulfuric acid (H2SO4, 98%) and stirred for 1 h. Subsequently, 5 mL phosphoric acid (H3PO4, 30%) was added to the mixture and the reaction temperature was gradually increased to 75 C. 100 mg potassium permanganate (KMnO4) was then added into the reaction system every 30 min until 1.0 g KMnO4 was used up. The reaction mixture was naturally cooled to room temperature and poured into 1 L ice-water containing 6 mL H2O2 (30 wt%). Aer being coagulated for 24 h, the obtained precipitate was sonicated for 30 min and dialyzed against DI water for 1 week, resulting in the formation of GNRs. To prepare the GNR– MnO2 hybrids, the same amount of pristine CNTs, H2SO4 and H3PO4 was added to the reaction ask. By adding different amounts of KMnO4 (more than 1.0 g) into the reaction mixture, GNR–MnO2 hybrids with different contents of MnO2 were obtained. It is noted that during the preparation of GNR– MnO2 hybrids, the reaction temperature began to drop to room temperature aer 1.0 g KMnO4 was added, aer which more KMnO4 (0.1–0.4 g) was continuously added into the reaction system to react with the GNRs to form MnO2 nanoparticles on the surface or in the interlayer of GNRs. No H2O2 was used throughout the whole preparation process of the GNR–MnO2 hybrids. Similar dialysis treatment was used for GNR–MnO2 hybrids. The obtained pure GNRs and GNR–MnO2 hybrids were then dispersed in the solution of N,N-dimethyl formamide (DMF) to form a stable dispersion with a concentration of 1 mg mL1. 2.2
Characterization
Thermogravimetric analysis (TGA) was performed with a Perkin Elmer Pyris-1 TGA under a nitrogen atmosphere with a heating rate of 10 C min1. Transmission electron microscopy (TEM) images were observed on a JEOL JEM-2100 TEM instrument with an accelerating voltage of 200 kV. A scanning electron microscope (SEM, VEGA TS 5136MM) equipped with energydispersive spectroscopy (EDS) was used to study the morphologies and microstructures of the samples. Raman spectra were measured on an Avalon Instruments Raman Station using a He– Ne laser (l ¼ 632.8 nm). X-Ray photoelectron spectroscopy (XPS) analyses were conducted with a VG ESCALAB 220l-XL device. XPS curve tting and background subtraction were accomplished using XPS peak41 soware. The mass loading of each electrode was measured by an electric balance with a precision of 0.01 mg (Sartorius BT 125D). The electrical conductivities of GNR and GNR–MnO2 samples were measured on a 4-Point Probes resistivity measurement system (RTS-8). 2.3
Fabrication of all-solid-state supercapacitors
The prepared GNRs and GNR–MnO2 hybrid were spin-coated on the surface of uorine-doped tin oxide (FTO) and then thermally treated at 500 C in an argon atmosphere for 2 h, followed by fabrication of the electrodes. The symmetric supercapacitors were prepared in a two-electrode system by direct assembly of the obtained GNRs or GNR–MnO2 electrodes sandwiched by a potassium polyacrylate/KCl
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(PAAK/KCl) gel electrolyte, which was prepared by adding 1.0 g of PAAK into 10 mL of KCl solution (1.0 M) as previously reported.2 Here, the mass of the active materials on each electrode is about 0.6–1.0 mg for symmetric devices. The corresponding asymmetric supercapacitor was also fabricated in such a two-electrode system with PAAK/KCl as the electrolyte, where GNRs spin-coated on FTO act as the negative electrode and GNR–MnO2 hybrids spin-coated on FTO act as the positive electrode with mass loadings ranging from 0.5–1.5 mg.
2.4
Electrochemical measurements
All the electrochemical tests were carried out on a CHI 660D electrochemical workstation (Shanghai Chenhua, China) at room temperature. In order to investigate the electrochemical properties of the prepared electrode, cyclic voltammetry (CV) tests with various scan rates, galvanostatic charge–discharge tests at various current densities, electrochemical impedance spectroscopy (EIS) and cycling stability of the fabricated cells were performed. The gravimetric specic capacitance was calculated using the following eqn (1): C¼
4I mdV =dt
(1)
where I (A) is the constant current of discharge, m (g) is the total mass of the two electrodes, and dV/dt (V s1) is the slope obtained by tting a straight line to the discharge curve. The energy density, E (W h kg1), was calculated by eqn (2). The maximum power density, P (kW kg1), was calculated from the constant current discharge curves and normalized with the total weight of two electrodes, according to eqn (3) as follows: E ¼ CSPV2/8 P¼
Scheme 1
V2 4RESR m
(2) (3)
Illustration of the fabrication process of GNR–MnO2 hybrid.
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where the RESR is the equivalent series resistance of the cells, and V is the discharge voltage excluding the intial voltage drop.
3.
Results and discussion
The preparation process of the GNR–MnO2 hybrid is illustrated in Scheme 1. As reported previously,40,41 CNTs can be controllably unzipped by an appropriate amount of oxidant (KMnO4). Here, both the opening of the pristine CNTs and the deposition of MnO2 nanoparticles on the surface of GNRs have been simultaneously achieved by a simple one-step method. For all of the KMnO4 used in the reaction, the amount being consumed to unzip the pristine CNTs is about 5 times the CNT weight; and the remaining amount is utilized for the redox reaction with GNRs, generating MnO2 nanoparticles on the surface or the interlayer of GNRs. The proposed reaction mechanism based on the electrostatic interaction between GNRs and KMnO4 and can be illustrated by the following reaction:42,43 4MnO4 + 3C + H2O / 4MnO2 + CO32 + 2HCO3 Here, the carbon substrate serves as the sacricial reductant for converting aqueous permanganate (MnO4) to MnO2 nanoparticles, which are further deposited homogeneously on the surface of GNRs. The weight ratios of MnO2 nanoparticles in the GNR–MnO2 hybrid materials, determined by thermogravimetric analysis (TGA) (Fig. S1†), are about 5%, 11%, 18%, and 23%, respectively, and are produced by electrostatic interactions between 1.1, 1.2, 1.3 and 1.4 g of the oxidant (KMnO4) with the pristine CNTs (200 mg). The increased mass ratios of the MnO2 nanoparticles are ascribed to the gradually increasing amount of KMnO4 used in the experiments. Fig. 1 shows the representative transmission electron microscopy (TEM) images of the obtained GNR sheets and GNR–MnO2 hybrids in comparison with pristine CNTs. Compared with the pristine CNTs (Fig. 1a and b), it is clear that the prepared GNR sheets (Fig. 1c) show layer structures, with no CNTs being observed, indicating that the pristine CNTs are totally opened or unzipped, resulting in the formation of GNRs. This can be further conrmed by the X-ray diffraction (XRD) results in the following section. The opened GNR sheets have a comparable length to the pristine CNTs even aer sonication, which further endows the GNR sheets with a geometric structure with higher aspect ratio and reduced defects (as seen in Fig. 1d). Furthermore, the obtained GNR sheets are crosslinked and inter-connected with each other, which increase efficiently the contact area between different GNRs and facilitates the formation of a conductive pathway. These features may have a signicant effect on the electrochemical performance of the thus-fabricated supercapacitor devices using GNRs and GNRbased hybrid materials. Here, the MnO2 nanoparticles produced by electrostatic interactions between KMnO4 and GNRs have intimate contact with the GNR sheets due to their extremely small size (with diameter of 3–5 nm). In addition, the obtained MnO2 nanoparticles can be distributed
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Fig. 1 TEM images of (a and b) pristine CNTs, (c and d) pure GNR sheets at low and high magnifications. GNR–MnO2 hybrid materials with (e) 5% and (f) 23% mass loadings of MnO2 nanoparticles.
homogeneously on the surface of GNRs, as seen in Fig. 1e. No distinct aggregates of MnO2 nanoparticles were observed even at a high mass loading of 23% (Fig. 1f). With the formation of a hierarchical structure of this hybrid material, the synergistic effect of the electrochemical performance of GNR sheets and MnO2 nanoparticles within the hybrid can be well realized with a reasonable design and assembly of the devices. The morphologies of the synthesized GNR sheets and GNR– MnO2 hybrids are further examined by scanning electron
Fig. 2 SEM images of (a) pristine CNTs, (b) GNR sheets, and GNR– MnO2 (23%) hybrid at (c) low and (d) high magnification.
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microscopy (SEM). As seen in Fig. 2, compared with the tubelike structure of pristine CNTs (Fig. 2a), the GNR sheets show clearly lamellar architecture coupled with microporous structure, with an average pore size of about 50 nm, distributed uniformly throughout the material. This unique structure will promote effectively the electrochemical performance of GNR sheets by providing an efficient and shortened pathway for the transport of electrolyte ions. Moreover, the presence of porous structure throughout the GNR material conrms the absence of serious stacking between the GNR sheets, which will benet the EDLC effect of the electrodes based on GNR sheets. It is also noted that the MnO2 nanoparticles cannot be clearly observed on the exterior surface of the GNR–MnO2 hybrid by SEM (Fig. 2c and d), probably due to the low mass loading and extremely small size of the MnO2 nanoparticles. Therefore, the uniform distribution of MnO2 nanoparticles is further conrmed by energy-dispersive spectroscopy (EDS) elemental mapping. An SEM image coupled with EDS mapping images made from the K-line energy densities of C, Mn and O of the same area are shown in Fig. S2.† In comparison with the C element, the mapping images of O and Mn show a lower distribution intensity, further conrming the uniform distribution and low mass loading of MnO2 nanoparticles in the hybrid material. Fig. S3† shows the XRD patterns of pristine CNTs, GNR sheets, and GNR–MnO2 hybrids with different mass loadings of MnO2 nanoparticles. It can be seen that pristine CNTs show an intense sharp peak located at 2q ¼ 26.1 , corresponding to the characteristic diffraction peak of graphite or CNTs. The XRD pattern of GNRs exhibits a broad peak at 2q ¼ 25.4 , corre˚ which is consistent with the sponding to a d-spacing of 3.5 A, XRD results of graphene, further conrming that the pristine CNTs have been totally unzipped.44 However, for GNR–MnO2 hybrids, the XRD patterns show inconspicuous, new and broad diffraction peaks at 2q ¼ 12.3 and 37.5 , which can be indexed to the (001) and (211) crystal planes of birnessite-type MnO2, respectively.45 The broad features and relatively low intensity of the peak proles suggest that the obtained MnO2 phase in the hybrid materials is amorphous and very small, which is in accordance with the previous reports on redox-deposited MnO2 on carbon materials.46 Raman spectroscopy is performed to investigate the vibrational properties of the as-prepared samples, as seen in Fig. 3. For pristine CNTs and GNR sheets, the G band at 1585 cm1, which corresponds to the E2g phonon of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, and the D band at 1350 cm1, which corresponds to the breathing mode of the rings or the K-point phonons of A1g symmetry, can be obviously observed. The intensity ratio (ID/IG) of D band and G band is usually used to reect the defect density of carbonaceous materials. The ID/IG ratio of GNR–MnO2 (23%) hybrid materials has increased to 1.36 compared with that (1.18) of GNRs, indicating an enhanced level of disorder within the GNR sample and the formation of sp3 carbon by functionalization of the GNR sheets. It is also noted that for the GNR–MnO2 hybrid, apart from D and G bands, apparent peaks at about 575 cm1 and 649 cm1 in the low wavenumber region can be observed. These peaks are commonly attributed to the Mn–O stretching
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Raman spectra of pristine CNTs, GNR sheets, GNR–MnO2 hybrids with varying MnO2 mass loadings of 5%, 11%, 18% and 23%, respectively. Fig. 3
vibration in the basal plane of MnO6 and the symmetric stretching vibration (Mn–O) of the MnO6 groups, respectively.47,48 The enhanced characteristic vibration intensity indicates the increase in the mass loading of MnO2 in the prepared hybrid with the amount of oxidant used increasing, which is in accordance with the TGA results (Fig. S1†). XPS analysis is employed in order to provide further evidence for the structural information of the GNR–MnO2 hybrid. As seen in Fig. 4, the peaks of Mn (2p1/2, 2p3/2), O 1s and C 1s can be clearly observed in the survey spectrum, illustrating the presence of MnO2 nanoparticles on the surface of GNRs. The inset spectrum shows the high resolution spectrum of Mn 2p in the GNR–MnO2 hybrid. The binding energy peaks of Mn 2p3/2 and Mn 2p1/2 are located at 642.1 eV and 653.8 eV, respectively, indicating a spin-energy separation of 11.7 eV. These results are
Fig. 4 XPS survey spectra of the GNR–MnO2 (23%) hybrid. The inset illustrates the narrow spectra of the Mn 2p peaks of the hybrid.
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in good accordance with the data of Mn 2p3/2 and Mn 2p1/2 in MnO2 reported previously,49,50 which further conrm the successful assembly of MnO2 nanoparticles on the surface of GNRs. In order to explore the GNR sheets and GNR–MnO2 hybrids as electrochemical electrodes in asymmetric supercapacitors, the specic capacitance, which is a crucial factor in designing and fabricating asymmetric devices, of the prepared materials needs to be characterized rstly in a symmetric system. Here, the electrochemical properties of pristine CNTs, GNR sheets and GNR–MnO2 hybrids with different mass loadings were characterized by cyclic voltammogram (CV), galvanostatic charge–discharge, and frequency response measurements in symmetric two-electrode systems with PAAK/KCl as the electrolyte, as seen in Fig. 5. Fig. 5a shows the CV curves of pristine CNTs, GNR sheets and GNR–MnO2 hybrid materials with various mass loadings of MnO2 nanoparticles at a scan rate of 10 mV s1. It can be seen that the CV curves of pristine CNTs and GNR sheets show an approximately rectangular shape, which is the characteristic for EDLC performance.51 In contrast, the CV curves of GNR–MnO2 hybrids show relatively apparent redox peaks at about 0.3–0.4 V, which can be ascribed to typical pseudocapacitive performance of MnO2 nanoparticles. As the gravimetric current density increases with the loading level of MnO2 nanoparticles, coupled with the apparent redox peaks, it is clear that MnO2 nanoparticles in the hybrids in fact play the role of pseudocapacitors. This can also be conrmed from the skewed triangle shape of the charge–discharge curves of GNR– MnO2 hybrids and the increased discharge time, as compared with the symmetrical triangle of the charge–discharge curves with limited discharge time for both pristine CNTs and GNR sheets (Fig. 5b). However, as seen in Fig. 5c, it is worthwhile to note that the specic capacitance (287 F g1) of the GNR–MnO2 (23%) hybrid is lower than that (305 F g1) of GNR–MnO2 (18%) hybrid. This decrease of specic capacitance with MnO2 mass loading above 18 wt% can be ascribed to that the additional MnO2 nanoparticles, especially for those located far away from the GNR surface, do not participate actively in pseudocapacitive reactions due to a proton diffusion constant. Also, the increased MnO2 mass loading may decrease the electrical conductivity of the GNR–MnO2 hybrid due to the poor electrical conductivity of MnO2 itself (106 S cm1).52 Furthermore, the electrical conductivities of the samples have been measured using a 4-point system. GNR–MnO2 (23%) shows a signicantly reduced conductivity of 1 S cm1 compared with 40 S cm1 for GNR– MnO2 (5%) and 63 S cm1 for GNR. Fig. 5d shows the specic capacitance of pristine CNTs, GNR sheets and GNR–MnO2 hybrids as a function of discharge current density from 0.5 A g1 to 10 A g1. A sample GNR–MnO2 (18%) hybrid shows a specic capacitance of 305 F g1 at a discharge current density of 0.5 A g1, as well as 277 F g1 at 10 A g1, meaning that 90% of its capacitance was retained even with such a huge current span. This excellent rate performance of the prepared GNR–MnO2 hybrid can be ascribed to the high electrical conductivity of GNR sheets as a supporting substrate and the nanosize of the MnO2 particles, as well as the hierarchical GNR–MnO2 hybrid
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Electrochemical performance of pristine CNTs, GNR sheets and the prepared GNR–MnO2 hybrids with different mass loadings of MnO2 nanoparticles. (a) Cycle voltammograms (at 10 mV s1), and (b) charge–discharge curves (at 0.5 A g1) for pristine CNTs, GNR sheets, GNR–MnO2 hybrids with various MnO2 mass loadings of 5%, 11%, 18%, and 23%, respectively. (c) Specific capacitance of pristine CNTs, GNR sheets and the prepared GNR–MnO2 hybrids calculated from the discharge time at a current density of 0.5 A g1. (d) Specific capacitances of pristine CNTs, GNR sheets and GNR–MnO2 hybrids as a function of discharge current density from 0.5 A g1 to 10 A g1. Fig. 5
with porous structure and ne dispersion of MnO2 nanoparticles. In order to obtain more information on the capacitive performance of the as-prepared materials, the electrodes of GNRs, GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids are selected to be further measured by electrochemical impedance spectroscopy (EIS) and for cycling performance (Fig. 6). With a complex nonlinear least-squares (CNLS) system, the equivalent circuit for tting the EIS spectra has been established (Fig. S4†). As seen in Fig. 6a, all these four materials clearly show vertical lines in the low-frequency region, indicating an ideal capacitive property. The x-intercept at the real part (Z0 ) of the Nyquist plot, which is a combined resistance (Re) from the ionic resistance of the electrolyte, intrinsic resistance of the active materials, and contact resistance at the active material/current collector interface,53 has a signicant inuence on the capacitive performance of the electrode materials. Here, the magnitudes of the equivalent series resistances of GNRs, GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids are about 2.7, 3.4, 3.8 and 3.8 U, respectively, suggesting that the prepared samples are good electrode materials with very small series resistances. The increased resistance of
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GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids as compared with that of GNRs can be attributed to the resistive effect of MnO2 with a low electrical conductivity. Besides, the charge-transfer and ion diffusion resistance (Rct) are important features for the electrode materials. Here, the charge-transfer resistances of GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids, which also can be calculated from the diameters of the semicircles, are determined to be about 1.1, 1.7 and 2.5 U, respectively, endowing them with desirable facilitated charge transfer properties. The long-term stability performance of GNRs, GNR–MnO2 (5%), GNR–MnO2 (18%) and GNR–MnO2 (23%) hybrids as electrode materials are evaluated by galvanostatic charge–discharge tests up to 5000 cycles at a constant current density of 1 A g1. As seen in Fig. 6b, GNR–MnO2 (18%) hybrids and GNRs show capacitance retentions of 87.5% and 95.7%, respectively, aer 5000 cycles, indicating that both GNR–MnO2 hybrids and GNRs possess good stability as supercapacitor electrodes. Such an excellent stability coupled with the low resistance and 3D hierarchical structure of the GNR–MnO2 hybrid material ensure that the GNR sheets together with novel GNR–MnO2 hybrid materials are ideal electrode candidates for energy storage applications.
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(a) Nyquist plots of the impedance spectra for GNRs, GNR– MnO2 (5%) and GNR–MnO2 (18%) hybrids in a frequency range of 0.01 Hz to 100 kHz. The inset shows the enlarged impedance spectra at the high-frequency region. (b) Cycling performance of GNRs, GNR– MnO2 (5%) and GNR–MnO2 (18%) hybrids measured at a current density of 1 A g1. Fig. 6
To explore further the advantages of the GNR–MnO2 hybrids, asymmetric supercapacitors of GNR//GNR–MnO2 (18%) based on GNRs (negative electrode) and GNR–MnO2 (18%) hybrid (positive electrode) with PAAK/KCl gel as the electrolyte have been fabricated, as shown in Fig. 7a. In order to obtain the optimized operating potential window for the asymmetric supercapacitor, it is necessary to equalize the charges stored at the positive and negative electrodes, which are related to the specic capacitance (Csp), the potential range for the charge– discharge processes (DV), and the mass (m) of the electrode material. In this work, the optimal mass ratio between the GNRs and GNR–MnO2 (18%) hybrid is about 1 : 2.1, which can be calculated using the following equation: m Csp DV ¼ mþ Cspþ DVþ As seen in Fig. 7b, the CV curves of the GNR//GNR–MnO2 (18%) asymmetric supercapacitor exhibit rectangular-like
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shapes with inconspicuous redox peaks which were measured from 0 to 2.0 V at different scan rates of 5, 10, 20, 50, 100 and 200 mV s1 respectively, conrming the ideal capacitive behavior of both the GNR and GNR–MnO2 electrodes. Here, the less conspicuous redox peak compared with that of the symmetric devices may be explained as follows: the negative electrode of asymmetric supercapacitor consists of pure GNR sheets and can only provide EDLC contribution. With the increase of scan rate, an obvious increase of current density is achieved, conrming a good rate capability of the asymmetric GNR//GNR–MnO2 (18%) supercapacitor. Furthermore, compared with the potential window (from 0 to 1.0 V) of GNRs and GNR–MnO2 (18%) electrodes, the enlarged potential window can give a great enhancement of the energy density for the prepared asymmetric supercapacitor since the energy density is proportional to the square of the potential. Galvanostatic charge–discharge tests were also performed with different current densities in a potential window from 0 to 2.0 V, as shown in Fig. 7c. The relatively symmetric charge–discharge curves and the linear potential–time prole in the whole potential range further conrm the excellent capacitive behavior of the GNR//GNR– MnO2 (18%) asymmetric supercapacitor. The specic capacitance of the GNR//GNR–MnO2 (18%) asymmetric supercapacitor calculated from the slope of the discharge curves is 212 F g1 at a current density of 0.5 A g1, and also retains 157 F g1 even at a high current density of 10 A g1 based on the total mass of the active materials in the two electrodes. Such an excellent electrochemical property can be ascribed to the synergistic effect of the EDLC performance of GNR sheets and the pseudocapacitance effect of the MnO2 nanoparticles. Cycling life, as one of the most important requirements in practical applications, has been tested at a constant current density of 1 A g1 for 5000 cycles. As shown in Fig. 7d, 88% of the initial capacitance is still retained even aer 5000 cycles, indicating a good long-term stability of the asymmetric GNR// GNR–MnO2 (18%) supercapacitor. The charge–discharge curves (the inset in Fig. 7d) show a normative multi-cycle triangular wave with tiny curvature for both the charging and discharging processes, indicating the pseudocapacitive effect of MnO2 nanoparticles along with the double layer contribution from the GNR sheets. This superior electrochemical performance of the GNR–MnO2 (18%) hybrid material as the positive electrode can be ascribed to the combined effect of the excellent electrical conductivity of the GNR sheets, good contact between MnO2 nanoparticles and GNR sheets, as well as the facilitated transfer of electrolyte ions and charges through the porous and hierarchical structure of the hybrid materials. EIS analysis is one of the most reliable and principal methods for examining the fundamental behavior of electrode materials for supercapacitors. In our study, in order to demonstrate the resistance change before and aer 5000-cycle tests, EIS has been measured in the frequency range from 0.01 Hz to 100 kHz at an open circuit potential with an ac amplitude of 10 mV (Fig. 8a). At the low-frequency region, the two impedance spectra show oblique straight lines, indicating the excellent pure capacitive behavior of the electrodes. The arc shapes at the high-frequency region can further demonstrate a long-term electrochemical
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Fig. 7 (a) Schematic diagram of the structure of the fabricated all-solid-state asymmetric supercapacitor (GNR//GNR–MnO2 (18%)) with GNRs as the negative electrode and the GNR–MnO2 (18%) hybrid as the positive electrode. (b) CV curves and (c) galvanostatic charge–discharge curves measured with different scan rates or current densities for the GNR//GNR–MnO2 (18%) supercapacitor. (d) Cycle performance of the GNR// GNR–MnO2 (18%) supercapacitor with a discharge current density of 1 A g1. The inset is the galvanostatic charge–discharge curve of the cell.
Fig. 8 Nyquist plots of GNR//GNR–MnO2 (18%) asymmetric supercapacitor in the range from 0.01 Hz to 100 kHz during the life cycle tests.
stability of the GNR//GNR–MnO2 (18%) hybrid material. The obtained impedance spectra were also analyzed by equivalent circuits as shown in Fig. S4.† It can be seen that, even aer 5000 cycles, the Re value is almost unchanged with only a slight increase of Rct observed (from 0.9 to 1.8 U). These EIS results Nanoscale
clearly demonstrate the exceptional stability of the asymmetric GNR//GNR–MnO2 (18%) supercapacitor. In order to evaluate the device performance of the fabricated asymmetric supercapacitor based on GNR sheets and the GNR– MnO2 (18%) hybrid, the energy density (E) and power density (P) calculated from galvanostatic discharge curves are plotted on the Ragone diagram (Fig. 9). Compared with symmetric supercapacitors fabricated based on GNR sheets or GNR–MnO2 hybrids with different mass loadings of MnO2 nanoparticles (i.e. GNR//GNR, GNR–MnO2 (5%)//GNR–MnO2 (5%) and GNR– MnO2 (18%)//GNR–MnO2 (18%)), the asymmetric supercapacitor of GNR//GNR–MnO2 (18%) shows greatly superior electrochemical properties with greatly enhanced power density and energy density. The maximum energy density of 29.4 W h kg1 (at a power density of 12.1 kW kg1) and power density of 25.9 kW kg1 (at an energy density of 22.5 W h kg1) were achieved at an operating voltage of 2.0 V, indicating that the asymmetric supercapacitor can provide high energy density without much sacrice in power density. More interestingly, the maximum energy density (29.4 W h kg1) of the asymmetric GNR//GNR–MnO2 (18%) supercapacitor in our work is higher than or comparable to other manganese oxides-based symmetric or asymmetric supercapacitors reported previously, e.g. MnO2//MnO2 (
