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Highly conductive three-dimensional MnO2– carbon nanotube–graphene–Ni hybrid foam as a binder-free supercapacitor electrode† Guoyin Zhu,a Zhi He,a Jun Chen,a Jin Zhao,a Xiaomiao Feng,a Yanwen Ma,*a Quli Fan,a Lianhui Wanga and Wei Huang*ab Carbon nanotube (CNT)–graphene hybrids grown on porous Ni foam are used as substrates to immobilize MnO2 nanoflakes, thus forming three-dimensional (3D) MnO2–CNT–graphene–Ni hybrid foam. The asprepared hybrid materials could be used as supercapacitor electrodes directly without any binder and conductive additives, and fully maintain the high conductivity and high surface-to-volume ratio of CNTs, large pseudocapacitance of MnO2 nanoflakes and high porosity provided by the framework of Ni foam. The conductivity of the 3D MnO2–CNT–graphene–Ni foam is as high as 117 S cm1 due to the seamless integration of MnO2 nanoflakes, CNTs, graphene and Ni foam among the 3D frameworks, which
Received 23rd August 2013 Accepted 28th October 2013
guarantee its low internal resistance (1.25 ohm) when compacted into supercapacitor devices. In aqueous electrolytes, the 3D MnO2–CNT–graphene–Ni based prototype supercapacitors show specific
DOI: 10.1039/c3nr04495e
capacitances of 251 F g1 with good cycling stability at a current density of 1.0 A g1. In addition, these
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3D hybrids also demonstrate their potential in all-solid-state flexible supercapacitors.
1. Introduction Supercapacitors are a type of key electrical energy storage device with a range of applications from high power uptake for sustainable and renewable energy of wind, sun and biomass to high power delivery for hybrid electric vehicles and laser beams. But to move towards advanced supercapacitors with higher operating voltage and higher energy without sacricing the power capability and cycle life, important further improvements in electrode materials are needed. Among supercapacitor electrode materials, MnO2, a pseudocapacitive transition-metal oxide, is considered as the most attractive candidate due to its high natural abundance, low cost, low toxicity, and excellent theoretical specic capacitance (1370 F g1).1,2 The theoretical value, however, is difficult to be reached in experiments mainly because MnO2 has a rather poor conductivity (106 to 105 S cm1).2 To resolve this issue, researchers have hybridized MnO2 nanostructures with conductive carbon materials including carbon nanotubes (CNTs) and graphene to increase the charge–discharge rate and cycling performance.3–7 The soa
Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210046, China. E-mail: [email protected]; [email protected]; Fax: +86 25 8586 6533; Tel: +86 25 8586 6396
b
Jiangsu-Singapore Joint Research Center for Organic/Bio-Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Technology, Nanjing 211816, China † Electronic supplementary 10.1039/c3nr04495e
information
(ESI)
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obtained MnO2–carbon composites were usually in the form of powder and needed to be pressed into together and then onto current collectors with the assistance of conductive additives and polymer binders. This will unavoidably result in contact resistance among the composite particles, additives, binders and collectors, which is an important part involved in the internal resistance (RS) of a supercapacitor device. The RS values of these MnO2 based composite electrodes were commonly in the range of 2–60 ohm. High RS will denitely cause instantaneous voltage drop and capacitive drop during the discharge pulse. Hence it is noteworthy and important to directly grow MnO2 nanostructures onto current collectors such as Ni foam, cellulose sponges and textiles in view of resistance reduction and process simplication.8–10 The emergence of graphene–Ni foam has given an alternative current collector for constructing 3D MnO2–based hybrid structures.11,12 Recently, He et al. have used the graphene–Ni foam to prepare freestanding and exible 3D graphene networks by removing the Ni templates, and then synthesized 3D MnO2–graphene composites with a capacitance of 130 F g1 (based on the entire electrode) for exible supercapacitor electrodes.13 In order to increase the specic surface area and functionality, we have improved the 3D graphene–Ni foam by growing CNT forests onto them.14,15 The interlayer graphene between CNTs and Ni metal could overcome the inadequate CNT–metal interfacial contact that hinders further applications.16 Moreover, graphene and CNTs are interconnected to each other through ohmic contact and even seamless junction.17,18 Hence CNT–graphene hybrids grown on Ni foam are expected to be ideal backbones for the
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immobilization of MnO2 species. In this research, graphene, CNTs and MnO2 nanostructures were grown on Ni foam one by one through two-step chemical vapor deposition (CVD) and a solution reaction. The so-obtained 3D MnO2–CNT–graphene–Ni hybrid structures that possessed a high conductivity were used to fabricate supercapacitors without any post-transfer process. The supercapacitors yielded a low RS of 1.25 ohm, a high gravimetric specic capacitance of 251 F g1 at a current density of 1.0 A g1 and high stability in aqueous electrolytes.
2.
Experimental section
2.1. Synthesis of 3D CNT–graphene–Ni hybrids The 3D CNT–graphene–Ni hybrids were synthesized by a twostep CVD process. Firstly, a piece of porous Ni foam (1 cm 2 cm, thickness 1.0 mm) was pressed into about 0.2 mm and then used as a template for the deposition of graphene with ethanol as the precursor at 900 C.14 Then the graphene–Ni substrate was immersed into 7% (w/w) polyethylene glycol (PEG, MW ¼ 20 000) ethanol solution containing 0.1 mol L1 Ni(NO3)2 for about 3 min and dried in air. Finally, CNTs were grown from ethanol by CVD at 750 C for 40 min.15
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temperature, with 1 mol L1 Li2SO4 as the electrolyte solution. The hybrid foam was directly used as an electrode without any post-treatment. The CV curves of the electrodes were measured between 0.2 V and 0.8 V at different scan rates. The galvanostatic cycling for each electrode was performed at a current density of 1.0 A g1. EIS was carried out over a frequency range of 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. The equation, C ¼ 2(I/m) (dt/dV), was used to calculate the specic capacitance from the slope of the charge–discharge curves (dV/dt), where I is the applied current and m is the mass of each electrode. The gravimetric specic capacitances of the electrodes were calculated by taking into account the total mass of MnO2, CNTs and graphene, i.e., subtracting the weight of Ni foam due to its surface covered by graphene (discussed in detail later). All-solid-state exible supercapacitors were demonstrated by compacting two symmetrical MnO2–CNT–graphene–Ni hybrid electrodes into two PET lms. The solid electrolyte used was the poly(vinyl alcohol)–H3PO4 gel electrolyte.
3.
Results and discussion
3.1. Morphological and structural characterization 2.2. Synthesis of 3D MnO2–CNT–graphene–Ni hybrids The 3D CNT–graphene–Ni foam composite was added to 10 mL of 2% (w/w) PEG aqueous solution and kept for 1 h in order to facilitate the adequate adsorption of PEG onto CNTs. Then 10 mL of 0.1 mol L1 KMnO4 aqueous solution was added and maintained at 70 C in an oil bath for 3 h. When the reaction solution was cooled down to room temperature naturally, the foam was taken out, washed with distilled water several times and then dried in an oven at 60 C for 12 h. The mass loading of MnO2 was 8 wt% by the examination of energy-dispersive X-ray spectroscopy (EDS, Thermo NORAN) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES, JY38S) (Fig. S1 and Table S1 in the ESI†).
Immobilizing MnO2 onto CNTs to form cable structures has been a widely adopted strategy to obtain high-performance supercapacitor electrodes.3,5,9 Here the one-dimensional (1D) features owned by MnO2–CNT cables were maintained by anchoring them onto Ni foam with the assistance of graphene interlayers (Fig. 1). In such a designed architecture, each component is connected with others, thus forming a complete and highly conductive network. The richly macroporous 3D structure of the Ni foam allows for the growth of CNT forest in every direction and a large mass loading of MnO2, superior to the at substrate that only leaves one side for CNT growth and MnO2 loading. Fig. 2a shows the SEM image of the graphene–Ni foam, where the features of graphene on the Ni skeleton, ripples and wrinkles
2.3. Characterization The products were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL-JEM-2100F at 200 kV) equipped with energydispersive X-ray spectroscopy (EDS, Thermo NORAN), X-ray diffraction (XRD, Philips X’pert Pro X-ray diffractometer with Cu ˚ X-ray photoelectron spectroscopy Ka radiation of 1.5418 A), (XPS, PHI5000 VersaProbe) and Raman spectroscopy (Renishaw inVia Raman Microscope with an argon-ion laser at an excitation wavelength of 514 nm). I–V curves were recorded by twopoint probe measurement (Keithley 2400 semiconductor parameter) at room temperature. 2.4. Electrochemical measurements Electrochemical performances of the supercapacitor cells were studied by cyclic voltammetry (CV), galvanostatic charge– discharge and electrical impedance spectroscopy (EIS) on the Autolab workstation. All electrochemical experiments were carried out using a two-electrode system at ambient
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Schematic illustration of the fabricated 3D MnO2–CNT–graphene–Ni hybrids. (a) Nickel foam substrate. (b) Graphene grown on the Ni foam by a CVD method. (c) CNT forest directly grown on graphene surfaces. (d) MnO2 nanostructures deposited onto CNT surfaces. Note: EtOH means ethanol. Fig. 1
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SEM images of the hybrid Ni foam. (a) Graphene–Ni. (b) CNT– graphene–Ni. (c and d) MnO2–CNT–graphene–Ni hybrid foam. Insets in (a) and (b) show the corresponding local 3D structures at low magnification. Fig. 2
generated from its different thermal expansion, are clearly visible. Aer the growth of CNTs, the surface of graphene is totally covered by randomly oriented, entangled CNT forest (Fig. 2b). The thickness of the CNT layer is about 5.0 mm according to the width increase of the graphene–Ni skeleton. Fig. 2c and d show the SEM images of the MnO2–CNT–graphene– Ni hybrids at low and high magnications, respectively. It is seen that the porous structure of the support is well maintained. MnO2–CNT composites show the 1D structure with diameters of 200 nm and rough surfaces, forming nanoake morphology similar to that reported by Lu et al.19 It is also noted that the MnO2–CNT lms still inherit the macroporous and quasi-aligned structures from the CNT forest. In addition, our MnO2–CNT composites could be easily scaled up because they are synthesized at ambient pressure and moderate temperature with the aid of PEG. Here PEG is an amphiphile to change CNTs from being hydrophobic to hydrophilic and also a reductant to promote the reduction reaction of KMnO4 (Fig. S2 in the ESI†), enabling us to feasibly prepare MnO2–CNT hybrids under a rather mild condition as compared with other reported methods.19–21 The morphology of MnO2–CNT hybrids was studied by TEM as shown in Fig. 3b. Here the typical TEM image of CNTs peeled off from CNT–graphene–Ni hybrids is also given for comparison (Fig. 3a). Aer heavily coated with MnO2, CNTs could not be distinguished anymore under TEM observation. The thickness of the MnO2 layer is estimated to be about 40–80 nm by comparing the diameter change between CNTs and MnO2–CNTs (Fig. 3a and b). These MnO2 nanoakes are typically less than 8 nm in thickness (Fig. 3c). The interplanar distance of lattice fringes is approximately 0.7 nm, corresponding to the (001) plane of birnessite-type MnO2 that prefers forming two-dimensional aky structures.19,22,23 The related EDS spectrum (Fig. 3d) reveals the Mn signal from MnO2 as expected. The Cu signal is derived from the Cu grid and the Ni signal is derived from the Ni catalysts.
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Fig. 3 TEM images of (a) CNTs and (b and c) MnO2–CNT hybrids. (d) EDS spectrum of the MnO2–CNT hybrids.
The Raman spectrum in Fig. 4a reveals that multi-layer graphene exists in graphene–Ni foam.14 In the spectrum of CNT– graphene–Ni hybrids, a pronounced peak at 1350 cm1 appears, attributed to the D band from the disordered carbon in CNTs. The intensity ratio of the D band to the G band (ID/IG) is 0.36 for CNT–graphene–Ni while it increases to 0.64 for MnO2– CNT–graphene–Ni, indicating that disorder in CNTs was enhanced aer their oxidation by KMnO4. In addition, a small peak (+) at a low frequency of 560–680 cm1 is a typical sign of MnO2 species existing in the MnO2–CNT–graphene–Ni foam.13 The XRD pattern of MnO2–CNT–graphene–Ni hybrids is shown in Fig. 4b. The diffraction peaks of (001), (110) and (020) that come from birnessite-type MnO2 can be observed, further supporting the TEM result (Fig. 3c). As shown in Fig. 4c and d, the chemical state of manganese in the hybrid was analyzed by XPS. The elemental signals of Mn, O and C can be observed in Fig. 4c. The Mn 2p peak is deconvoluted into the Mn 2p3/2 peak and Mn 2p1/2 peak with binding energy values of 641.1 eV and 652.8 eV, respectively. The spin-energy separation between these two peaks is 11.7 eV, matches well with the reported data for MnO2 (Fig. 4d).24
3.2. Electrical measurement and electrochemical properties According to the preceding experimental results, 3D MnO2– CNT–graphene–Ni hybrids with 1D quasi-aligned MnO2–CNT cable structures that tightly connected with graphene–Ni backbones have been successfully constructed, which is favorable for developing highly conductive and high pseudocapacitive electrodes. To the best of our knowledge, this is the rst report on the utilization of 3D CNT–graphene hybrid foam as a support to immobilize active nanomaterials. Since this modied 3D foam integrates the functionalities of current collectors and active species, it could be directly used as a supercapacitor electrode. Moreover, such routine CVD growth of graphene and
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Fig. 4 (a) Raman spectrum, (b) XRD pattern, (c) XPS survey scan and (d) Mn 2p XPS spectra of MnO2–CNT–graphene–Ni hybrid foam. The Raman spectra of graphene–Ni and CNT–graphene–Ni are also shown in (a) for comparison.
CNTs as well as the facile solution-based synthesis of MnO2 make this type of material easy to be reproduced and scaled up. To evaluate the electrical behaviour of MnO2–CNT–graphene– Ni hybrids, the J–V responses were measured at room temperature in Fig. 5a. The straight lines indicate the ohmic contact characteristics for graphene–Ni, CNT–graphene–Ni and MnO2– CNT–graphene–Ni samples as expected and the calculated average conductivities are 326 1, 174 2 and 117 4 S cm1, respectively. This means that the MnO2–CNT–graphene–Ni
hybrids exhibit conductivity as high as carbon black (2–200 S cm1),25 a commercial conductive additive for supercapacitors and batteries. The electrochemical capacitive performance for each type of electrode was evaluated by CV, galvanostatic charge– discharge and EIS tests. Fig. 5b shows the CV curves of the MnO2–CNT–graphene–Ni hybrid foam at different scan rates within an electrochemical window from 0.2 to 0.8 V. With increasing scan rate, the current response also increases without any obvious changes in the shape of the CV curve, indicating a
Fig. 5 (a) J–V responses of the graphene–Ni, CNT–graphene–Ni and MnO2–CNT–graphene–Ni hybrids. (b) CV curves of MnO2–CNT–graphene–Ni electrodes at different scan rates. (c) CV curves, (d) galvanostatic charge–discharge curves and (e) Nyquist plots of these three electrodes. (f) Specific capacitance and RS of MnO2 based composite electrodes (for details, see Table S3 in the ESI†). The inset in (e) shows the data in high frequency ranges.
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good rate performance. The rectangular and symmetric shape of the CV curve is also observed at a high scan rate of 100 mV s1 due to the low contact resistance of the electrodes. Fig. 5c presents CV curves of graphene–Ni, CNT–graphene–Ni and MnO2– CNT–graphene–Ni hybrid foam at a scan rate of 10 mV s1. The specic capacitances of graphene–Ni, CNT–graphene–Ni and MnO2–CNT–graphene–Ni electrodes are 23, 31 and 235 F g1, respectively. The Ni content in these three samples is 97, 89 and 84 wt%, respectively, and it is stable in the electrolyte due to graphene encapsulation and CNT covering (Fig. S3 in the ESI†). Moreover, the presence of oxygen-containing groups on CNTs aer MnO2 nanoparticle loading is not the major factor that affects the capacitance because their content changed slightly (Fig. S4 and Table S2 in the ESI†). Hence we can deduce that the CNT forest gives a limited while MnO2 provides the majority of contribution to the capacitance of MnO2–CNT–graphene–Ni foam. Fig. 5d shows galvanostatic charging–discharging curves for graphene–Ni, CNT–graphene–Ni and MnO2–CNT–graphene– Ni electrodes at 1.0 A g1. The longer discharging time of MnO2– CNT–graphene–Ni electrodes represents that their capacitance (251 F g1, other data are given in Fig. S5 in the ESI†) is much higher than those of graphene–Ni (25 F g1) and CNT–graphene– Ni electrodes (42 F g1). In addition, the nearly linear and almost symmetrical curve of MnO2–CNT–graphene–Ni electrodes indicates that they exhibit a good capacitive behavior. At the beginning of discharge, an unobvious IR drop is observed and the related resistance is calculated to be 1.35 ohm according to the equation R ¼ VIR/2I. Here VIR is the voltage range and I is the discharge current. EIS measurements were carried out in a frequency range of 100 kHz to 0.01 Hz to further evaluate the electrochemical and structural characteristics of the electrode material (Fig. 5e). In the low frequency region, the MnO2–CNT–
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graphene–Ni electrodes show a nearly vertical curve, indicating that their capacitive performance is close to that of the ideal capacitor. At very high frequencies (the inset in Fig. 5e), the le plot intersection at the real part (Z0 ) accounts for the combined resistance (RE) of the intrinsic resistance of electrodes and the ionic resistance of the electrolyte. The semicircle region in the plot curve corresponds to the charge transfer resistance (RCT), also called Faraday resistance.9 The tted equivalent circuit is given in Fig. S6 in the ESI.† In this case, the RE for MnO2–CNT– graphene–Ni electrodes is slightly larger than those for graphene–Ni and CNT–graphene–Ni electrodes. But only MnO2– CNT–graphene–Ni electrodes present RCT due to their Faraday capacitance. The RS is calculated to be 1.25 ohm by adding RE to RCT, well consistent with the data obtained from the charging– discharging analysis. Compared with the previously reported MnO2–CNT based electrodes, our MnO2–CNT–graphene–Ni electrodes signicantly reduce the RS, which is even lower than most of the MnO2–graphene and MnO2–CNT–graphene based electrodes (Fig. 5f and Table S3 in the ESI†).6–10,13,19,26–31 Obviously, the low RS of MnO2–CNT–graphene–Ni electrodes should come from their intrinsic high conductivity, rich porous 3D network and the nanoake morphology of MnO2 species. The former is benecial to decrease RE while the latter two are favourable for lowering RCT through providing a plenty of channels and surfaces for charge transportation. These results fully reect the merit of the 3D architecture cherished by the MnO2–CNT–graphene–Ni electrodes. The long-term stability of the MnO2–CNT–graphene–Ni electrodes was examined by galvanostatic charge–discharge cycling at a current density of 1.0 A g1, and the results are presented in Fig. 6a. The inset gure shows typical charge–discharge curves aer 382 000 s. It can be seen that all curves still remained in
Fig. 6 (a) Cycle performance of the MnO2–CNT–graphene–Ni electrodes measured at a current density of 1.0 A g1. The inset shows the galvanostatic charge–discharge curves after 382 000 s. (b) Ragone plot of the specific energy vs. specific power for MnO2–based supercapacitors. (c) Schematic structure of the all-solid-state supercapacitor.
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near linear and symmetrical shape, implying that the electrodes have favorable electrochemical reversibility and charge– discharge properties. The MnO2–CNT–graphene–Ni electrodes display long-term cycle stability, with an efficiency of 82% aer 3000 charge–discharge cycles,30,32,33 indicating that the repetitive charge–discharge cycles do not induce noticeable degradation of the microstructure.8,32 Fig. 6b presents the energy and power density for the MnO2–CNT–graphene–Ni hybrid electrodes, in which the reported data of some typical MnO2–carbon based supercapacitors are also given.7,29,33–37 At a current density of 2.4 A g1, the energy density is about 29 W h kg1 at a power density of 1200 W kg1, which can be comparable with other previously reported high-performance MnO2–carbon based supercapacitors.29,33 Moreover, the 3D hybrid foam could be feasibly assembled into all-solid-sate devices (Fig. 6c) and light a lightemitting diode (LED) aer charging (Fig. S7 in the ESI†). This device showed a specic capacitance of 107 F g1, demonstrating its potential in exible supercapacitors.
4. Conclusion A symmetric supercapacitor with low internal resistance has been constructed based on highly conductive 3D MnO2–CNT–graphene–Ni hybrid foam that can be directly used as an electrode. The conductivity of the hybrid electrodes (117 S cm1) is as high as that of carbon black and their internal resistance is less than most of the other previously reported MnO2-based supercapacitors. At a current density of 1.0 A g1, the as-assembled supercapacitor shows a specic capacitance of 251 F g1 and can be cycled reversibly in the voltage region of 0.2 to 0.8 V. It could maintain an energy density of 29 W h kg1 even at the power density of 1200 W kg1, reecting the merits of its low internal resistance. Moreover, the 3D hybrid foam can be easily assembled into all-solid-sate devices, demonstrating its potential in exible supercapacitors. The experimental progress in this study point out two factors to lower the internal resistance of MnO2 based supercapacitors in view of electrode design: (1) direct growth of MnO2 nanostructures onto highly conducive and high specic-surface-area collectors and (2) constructing a 3D network with rich pores to facilitate ion and charge transportation.
Acknowledgements This work was jointly supported by the National Basic Research Program of China (2009CB930600, 2012CB933301), the Ministry of Education of China (no. IRT1148), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Key Projects for International Cooperation (BZ2010043), and Jiangsu Provincial NSF (BK2011750).
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Highly conductive three-dimensional MnO2– carbon nanotube–graphene–Ni hybrid foam as a binder-free supercapacitor electrode† Guoyin Zhu,a Zhi He,a Jun Chen,a Jin Zhao,a Xiaomiao Feng,a Yanwen Ma,*a Quli Fan,a Lianhui Wanga and Wei Huang*ab Carbon nanotube (CNT)–graphene hybrids grown on porous Ni foam are used as substrates to immobilize MnO2 nanoflakes, thus forming three-dimensional (3D) MnO2–CNT–graphene–Ni hybrid foam. The asprepared hybrid materials could be used as supercapacitor electrodes directly without any binder and conductive additives, and fully maintain the high conductivity and high surface-to-volume ratio of CNTs, large pseudocapacitance of MnO2 nanoflakes and high porosity provided by the framework of Ni foam. The conductivity of the 3D MnO2–CNT–graphene–Ni foam is as high as 117 S cm1 due to the seamless integration of MnO2 nanoflakes, CNTs, graphene and Ni foam among the 3D frameworks, which
Received 23rd August 2013 Accepted 28th October 2013
guarantee its low internal resistance (1.25 ohm) when compacted into supercapacitor devices. In aqueous electrolytes, the 3D MnO2–CNT–graphene–Ni based prototype supercapacitors show specific
DOI: 10.1039/c3nr04495e
capacitances of 251 F g1 with good cycling stability at a current density of 1.0 A g1. In addition, these
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3D hybrids also demonstrate their potential in all-solid-state flexible supercapacitors.
1. Introduction Supercapacitors are a type of key electrical energy storage device with a range of applications from high power uptake for sustainable and renewable energy of wind, sun and biomass to high power delivery for hybrid electric vehicles and laser beams. But to move towards advanced supercapacitors with higher operating voltage and higher energy without sacricing the power capability and cycle life, important further improvements in electrode materials are needed. Among supercapacitor electrode materials, MnO2, a pseudocapacitive transition-metal oxide, is considered as the most attractive candidate due to its high natural abundance, low cost, low toxicity, and excellent theoretical specic capacitance (1370 F g1).1,2 The theoretical value, however, is difficult to be reached in experiments mainly because MnO2 has a rather poor conductivity (106 to 105 S cm1).2 To resolve this issue, researchers have hybridized MnO2 nanostructures with conductive carbon materials including carbon nanotubes (CNTs) and graphene to increase the charge–discharge rate and cycling performance.3–7 The soa
Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210046, China. E-mail: [email protected]; [email protected]; Fax: +86 25 8586 6533; Tel: +86 25 8586 6396
b
Jiangsu-Singapore Joint Research Center for Organic/Bio-Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Technology, Nanjing 211816, China † Electronic supplementary 10.1039/c3nr04495e
information
(ESI)
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available.
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obtained MnO2–carbon composites were usually in the form of powder and needed to be pressed into together and then onto current collectors with the assistance of conductive additives and polymer binders. This will unavoidably result in contact resistance among the composite particles, additives, binders and collectors, which is an important part involved in the internal resistance (RS) of a supercapacitor device. The RS values of these MnO2 based composite electrodes were commonly in the range of 2–60 ohm. High RS will denitely cause instantaneous voltage drop and capacitive drop during the discharge pulse. Hence it is noteworthy and important to directly grow MnO2 nanostructures onto current collectors such as Ni foam, cellulose sponges and textiles in view of resistance reduction and process simplication.8–10 The emergence of graphene–Ni foam has given an alternative current collector for constructing 3D MnO2–based hybrid structures.11,12 Recently, He et al. have used the graphene–Ni foam to prepare freestanding and exible 3D graphene networks by removing the Ni templates, and then synthesized 3D MnO2–graphene composites with a capacitance of 130 F g1 (based on the entire electrode) for exible supercapacitor electrodes.13 In order to increase the specic surface area and functionality, we have improved the 3D graphene–Ni foam by growing CNT forests onto them.14,15 The interlayer graphene between CNTs and Ni metal could overcome the inadequate CNT–metal interfacial contact that hinders further applications.16 Moreover, graphene and CNTs are interconnected to each other through ohmic contact and even seamless junction.17,18 Hence CNT–graphene hybrids grown on Ni foam are expected to be ideal backbones for the
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immobilization of MnO2 species. In this research, graphene, CNTs and MnO2 nanostructures were grown on Ni foam one by one through two-step chemical vapor deposition (CVD) and a solution reaction. The so-obtained 3D MnO2–CNT–graphene–Ni hybrid structures that possessed a high conductivity were used to fabricate supercapacitors without any post-transfer process. The supercapacitors yielded a low RS of 1.25 ohm, a high gravimetric specic capacitance of 251 F g1 at a current density of 1.0 A g1 and high stability in aqueous electrolytes.
2.
Experimental section
2.1. Synthesis of 3D CNT–graphene–Ni hybrids The 3D CNT–graphene–Ni hybrids were synthesized by a twostep CVD process. Firstly, a piece of porous Ni foam (1 cm 2 cm, thickness 1.0 mm) was pressed into about 0.2 mm and then used as a template for the deposition of graphene with ethanol as the precursor at 900 C.14 Then the graphene–Ni substrate was immersed into 7% (w/w) polyethylene glycol (PEG, MW ¼ 20 000) ethanol solution containing 0.1 mol L1 Ni(NO3)2 for about 3 min and dried in air. Finally, CNTs were grown from ethanol by CVD at 750 C for 40 min.15
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temperature, with 1 mol L1 Li2SO4 as the electrolyte solution. The hybrid foam was directly used as an electrode without any post-treatment. The CV curves of the electrodes were measured between 0.2 V and 0.8 V at different scan rates. The galvanostatic cycling for each electrode was performed at a current density of 1.0 A g1. EIS was carried out over a frequency range of 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. The equation, C ¼ 2(I/m) (dt/dV), was used to calculate the specic capacitance from the slope of the charge–discharge curves (dV/dt), where I is the applied current and m is the mass of each electrode. The gravimetric specic capacitances of the electrodes were calculated by taking into account the total mass of MnO2, CNTs and graphene, i.e., subtracting the weight of Ni foam due to its surface covered by graphene (discussed in detail later). All-solid-state exible supercapacitors were demonstrated by compacting two symmetrical MnO2–CNT–graphene–Ni hybrid electrodes into two PET lms. The solid electrolyte used was the poly(vinyl alcohol)–H3PO4 gel electrolyte.
3.
Results and discussion
3.1. Morphological and structural characterization 2.2. Synthesis of 3D MnO2–CNT–graphene–Ni hybrids The 3D CNT–graphene–Ni foam composite was added to 10 mL of 2% (w/w) PEG aqueous solution and kept for 1 h in order to facilitate the adequate adsorption of PEG onto CNTs. Then 10 mL of 0.1 mol L1 KMnO4 aqueous solution was added and maintained at 70 C in an oil bath for 3 h. When the reaction solution was cooled down to room temperature naturally, the foam was taken out, washed with distilled water several times and then dried in an oven at 60 C for 12 h. The mass loading of MnO2 was 8 wt% by the examination of energy-dispersive X-ray spectroscopy (EDS, Thermo NORAN) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES, JY38S) (Fig. S1 and Table S1 in the ESI†).
Immobilizing MnO2 onto CNTs to form cable structures has been a widely adopted strategy to obtain high-performance supercapacitor electrodes.3,5,9 Here the one-dimensional (1D) features owned by MnO2–CNT cables were maintained by anchoring them onto Ni foam with the assistance of graphene interlayers (Fig. 1). In such a designed architecture, each component is connected with others, thus forming a complete and highly conductive network. The richly macroporous 3D structure of the Ni foam allows for the growth of CNT forest in every direction and a large mass loading of MnO2, superior to the at substrate that only leaves one side for CNT growth and MnO2 loading. Fig. 2a shows the SEM image of the graphene–Ni foam, where the features of graphene on the Ni skeleton, ripples and wrinkles
2.3. Characterization The products were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL-JEM-2100F at 200 kV) equipped with energydispersive X-ray spectroscopy (EDS, Thermo NORAN), X-ray diffraction (XRD, Philips X’pert Pro X-ray diffractometer with Cu ˚ X-ray photoelectron spectroscopy Ka radiation of 1.5418 A), (XPS, PHI5000 VersaProbe) and Raman spectroscopy (Renishaw inVia Raman Microscope with an argon-ion laser at an excitation wavelength of 514 nm). I–V curves were recorded by twopoint probe measurement (Keithley 2400 semiconductor parameter) at room temperature. 2.4. Electrochemical measurements Electrochemical performances of the supercapacitor cells were studied by cyclic voltammetry (CV), galvanostatic charge– discharge and electrical impedance spectroscopy (EIS) on the Autolab workstation. All electrochemical experiments were carried out using a two-electrode system at ambient
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Schematic illustration of the fabricated 3D MnO2–CNT–graphene–Ni hybrids. (a) Nickel foam substrate. (b) Graphene grown on the Ni foam by a CVD method. (c) CNT forest directly grown on graphene surfaces. (d) MnO2 nanostructures deposited onto CNT surfaces. Note: EtOH means ethanol. Fig. 1
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SEM images of the hybrid Ni foam. (a) Graphene–Ni. (b) CNT– graphene–Ni. (c and d) MnO2–CNT–graphene–Ni hybrid foam. Insets in (a) and (b) show the corresponding local 3D structures at low magnification. Fig. 2
generated from its different thermal expansion, are clearly visible. Aer the growth of CNTs, the surface of graphene is totally covered by randomly oriented, entangled CNT forest (Fig. 2b). The thickness of the CNT layer is about 5.0 mm according to the width increase of the graphene–Ni skeleton. Fig. 2c and d show the SEM images of the MnO2–CNT–graphene– Ni hybrids at low and high magnications, respectively. It is seen that the porous structure of the support is well maintained. MnO2–CNT composites show the 1D structure with diameters of 200 nm and rough surfaces, forming nanoake morphology similar to that reported by Lu et al.19 It is also noted that the MnO2–CNT lms still inherit the macroporous and quasi-aligned structures from the CNT forest. In addition, our MnO2–CNT composites could be easily scaled up because they are synthesized at ambient pressure and moderate temperature with the aid of PEG. Here PEG is an amphiphile to change CNTs from being hydrophobic to hydrophilic and also a reductant to promote the reduction reaction of KMnO4 (Fig. S2 in the ESI†), enabling us to feasibly prepare MnO2–CNT hybrids under a rather mild condition as compared with other reported methods.19–21 The morphology of MnO2–CNT hybrids was studied by TEM as shown in Fig. 3b. Here the typical TEM image of CNTs peeled off from CNT–graphene–Ni hybrids is also given for comparison (Fig. 3a). Aer heavily coated with MnO2, CNTs could not be distinguished anymore under TEM observation. The thickness of the MnO2 layer is estimated to be about 40–80 nm by comparing the diameter change between CNTs and MnO2–CNTs (Fig. 3a and b). These MnO2 nanoakes are typically less than 8 nm in thickness (Fig. 3c). The interplanar distance of lattice fringes is approximately 0.7 nm, corresponding to the (001) plane of birnessite-type MnO2 that prefers forming two-dimensional aky structures.19,22,23 The related EDS spectrum (Fig. 3d) reveals the Mn signal from MnO2 as expected. The Cu signal is derived from the Cu grid and the Ni signal is derived from the Ni catalysts.
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Fig. 3 TEM images of (a) CNTs and (b and c) MnO2–CNT hybrids. (d) EDS spectrum of the MnO2–CNT hybrids.
The Raman spectrum in Fig. 4a reveals that multi-layer graphene exists in graphene–Ni foam.14 In the spectrum of CNT– graphene–Ni hybrids, a pronounced peak at 1350 cm1 appears, attributed to the D band from the disordered carbon in CNTs. The intensity ratio of the D band to the G band (ID/IG) is 0.36 for CNT–graphene–Ni while it increases to 0.64 for MnO2– CNT–graphene–Ni, indicating that disorder in CNTs was enhanced aer their oxidation by KMnO4. In addition, a small peak (+) at a low frequency of 560–680 cm1 is a typical sign of MnO2 species existing in the MnO2–CNT–graphene–Ni foam.13 The XRD pattern of MnO2–CNT–graphene–Ni hybrids is shown in Fig. 4b. The diffraction peaks of (001), (110) and (020) that come from birnessite-type MnO2 can be observed, further supporting the TEM result (Fig. 3c). As shown in Fig. 4c and d, the chemical state of manganese in the hybrid was analyzed by XPS. The elemental signals of Mn, O and C can be observed in Fig. 4c. The Mn 2p peak is deconvoluted into the Mn 2p3/2 peak and Mn 2p1/2 peak with binding energy values of 641.1 eV and 652.8 eV, respectively. The spin-energy separation between these two peaks is 11.7 eV, matches well with the reported data for MnO2 (Fig. 4d).24
3.2. Electrical measurement and electrochemical properties According to the preceding experimental results, 3D MnO2– CNT–graphene–Ni hybrids with 1D quasi-aligned MnO2–CNT cable structures that tightly connected with graphene–Ni backbones have been successfully constructed, which is favorable for developing highly conductive and high pseudocapacitive electrodes. To the best of our knowledge, this is the rst report on the utilization of 3D CNT–graphene hybrid foam as a support to immobilize active nanomaterials. Since this modied 3D foam integrates the functionalities of current collectors and active species, it could be directly used as a supercapacitor electrode. Moreover, such routine CVD growth of graphene and
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Fig. 4 (a) Raman spectrum, (b) XRD pattern, (c) XPS survey scan and (d) Mn 2p XPS spectra of MnO2–CNT–graphene–Ni hybrid foam. The Raman spectra of graphene–Ni and CNT–graphene–Ni are also shown in (a) for comparison.
CNTs as well as the facile solution-based synthesis of MnO2 make this type of material easy to be reproduced and scaled up. To evaluate the electrical behaviour of MnO2–CNT–graphene– Ni hybrids, the J–V responses were measured at room temperature in Fig. 5a. The straight lines indicate the ohmic contact characteristics for graphene–Ni, CNT–graphene–Ni and MnO2– CNT–graphene–Ni samples as expected and the calculated average conductivities are 326 1, 174 2 and 117 4 S cm1, respectively. This means that the MnO2–CNT–graphene–Ni
hybrids exhibit conductivity as high as carbon black (2–200 S cm1),25 a commercial conductive additive for supercapacitors and batteries. The electrochemical capacitive performance for each type of electrode was evaluated by CV, galvanostatic charge– discharge and EIS tests. Fig. 5b shows the CV curves of the MnO2–CNT–graphene–Ni hybrid foam at different scan rates within an electrochemical window from 0.2 to 0.8 V. With increasing scan rate, the current response also increases without any obvious changes in the shape of the CV curve, indicating a
Fig. 5 (a) J–V responses of the graphene–Ni, CNT–graphene–Ni and MnO2–CNT–graphene–Ni hybrids. (b) CV curves of MnO2–CNT–graphene–Ni electrodes at different scan rates. (c) CV curves, (d) galvanostatic charge–discharge curves and (e) Nyquist plots of these three electrodes. (f) Specific capacitance and RS of MnO2 based composite electrodes (for details, see Table S3 in the ESI†). The inset in (e) shows the data in high frequency ranges.
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good rate performance. The rectangular and symmetric shape of the CV curve is also observed at a high scan rate of 100 mV s1 due to the low contact resistance of the electrodes. Fig. 5c presents CV curves of graphene–Ni, CNT–graphene–Ni and MnO2– CNT–graphene–Ni hybrid foam at a scan rate of 10 mV s1. The specic capacitances of graphene–Ni, CNT–graphene–Ni and MnO2–CNT–graphene–Ni electrodes are 23, 31 and 235 F g1, respectively. The Ni content in these three samples is 97, 89 and 84 wt%, respectively, and it is stable in the electrolyte due to graphene encapsulation and CNT covering (Fig. S3 in the ESI†). Moreover, the presence of oxygen-containing groups on CNTs aer MnO2 nanoparticle loading is not the major factor that affects the capacitance because their content changed slightly (Fig. S4 and Table S2 in the ESI†). Hence we can deduce that the CNT forest gives a limited while MnO2 provides the majority of contribution to the capacitance of MnO2–CNT–graphene–Ni foam. Fig. 5d shows galvanostatic charging–discharging curves for graphene–Ni, CNT–graphene–Ni and MnO2–CNT–graphene– Ni electrodes at 1.0 A g1. The longer discharging time of MnO2– CNT–graphene–Ni electrodes represents that their capacitance (251 F g1, other data are given in Fig. S5 in the ESI†) is much higher than those of graphene–Ni (25 F g1) and CNT–graphene– Ni electrodes (42 F g1). In addition, the nearly linear and almost symmetrical curve of MnO2–CNT–graphene–Ni electrodes indicates that they exhibit a good capacitive behavior. At the beginning of discharge, an unobvious IR drop is observed and the related resistance is calculated to be 1.35 ohm according to the equation R ¼ VIR/2I. Here VIR is the voltage range and I is the discharge current. EIS measurements were carried out in a frequency range of 100 kHz to 0.01 Hz to further evaluate the electrochemical and structural characteristics of the electrode material (Fig. 5e). In the low frequency region, the MnO2–CNT–
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graphene–Ni electrodes show a nearly vertical curve, indicating that their capacitive performance is close to that of the ideal capacitor. At very high frequencies (the inset in Fig. 5e), the le plot intersection at the real part (Z0 ) accounts for the combined resistance (RE) of the intrinsic resistance of electrodes and the ionic resistance of the electrolyte. The semicircle region in the plot curve corresponds to the charge transfer resistance (RCT), also called Faraday resistance.9 The tted equivalent circuit is given in Fig. S6 in the ESI.† In this case, the RE for MnO2–CNT– graphene–Ni electrodes is slightly larger than those for graphene–Ni and CNT–graphene–Ni electrodes. But only MnO2– CNT–graphene–Ni electrodes present RCT due to their Faraday capacitance. The RS is calculated to be 1.25 ohm by adding RE to RCT, well consistent with the data obtained from the charging– discharging analysis. Compared with the previously reported MnO2–CNT based electrodes, our MnO2–CNT–graphene–Ni electrodes signicantly reduce the RS, which is even lower than most of the MnO2–graphene and MnO2–CNT–graphene based electrodes (Fig. 5f and Table S3 in the ESI†).6–10,13,19,26–31 Obviously, the low RS of MnO2–CNT–graphene–Ni electrodes should come from their intrinsic high conductivity, rich porous 3D network and the nanoake morphology of MnO2 species. The former is benecial to decrease RE while the latter two are favourable for lowering RCT through providing a plenty of channels and surfaces for charge transportation. These results fully reect the merit of the 3D architecture cherished by the MnO2–CNT–graphene–Ni electrodes. The long-term stability of the MnO2–CNT–graphene–Ni electrodes was examined by galvanostatic charge–discharge cycling at a current density of 1.0 A g1, and the results are presented in Fig. 6a. The inset gure shows typical charge–discharge curves aer 382 000 s. It can be seen that all curves still remained in
Fig. 6 (a) Cycle performance of the MnO2–CNT–graphene–Ni electrodes measured at a current density of 1.0 A g1. The inset shows the galvanostatic charge–discharge curves after 382 000 s. (b) Ragone plot of the specific energy vs. specific power for MnO2–based supercapacitors. (c) Schematic structure of the all-solid-state supercapacitor.
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near linear and symmetrical shape, implying that the electrodes have favorable electrochemical reversibility and charge– discharge properties. The MnO2–CNT–graphene–Ni electrodes display long-term cycle stability, with an efficiency of 82% aer 3000 charge–discharge cycles,30,32,33 indicating that the repetitive charge–discharge cycles do not induce noticeable degradation of the microstructure.8,32 Fig. 6b presents the energy and power density for the MnO2–CNT–graphene–Ni hybrid electrodes, in which the reported data of some typical MnO2–carbon based supercapacitors are also given.7,29,33–37 At a current density of 2.4 A g1, the energy density is about 29 W h kg1 at a power density of 1200 W kg1, which can be comparable with other previously reported high-performance MnO2–carbon based supercapacitors.29,33 Moreover, the 3D hybrid foam could be feasibly assembled into all-solid-sate devices (Fig. 6c) and light a lightemitting diode (LED) aer charging (Fig. S7 in the ESI†). This device showed a specic capacitance of 107 F g1, demonstrating its potential in exible supercapacitors.
4. Conclusion A symmetric supercapacitor with low internal resistance has been constructed based on highly conductive 3D MnO2–CNT–graphene–Ni hybrid foam that can be directly used as an electrode. The conductivity of the hybrid electrodes (117 S cm1) is as high as that of carbon black and their internal resistance is less than most of the other previously reported MnO2-based supercapacitors. At a current density of 1.0 A g1, the as-assembled supercapacitor shows a specic capacitance of 251 F g1 and can be cycled reversibly in the voltage region of 0.2 to 0.8 V. It could maintain an energy density of 29 W h kg1 even at the power density of 1200 W kg1, reecting the merits of its low internal resistance. Moreover, the 3D hybrid foam can be easily assembled into all-solid-sate devices, demonstrating its potential in exible supercapacitors. The experimental progress in this study point out two factors to lower the internal resistance of MnO2 based supercapacitors in view of electrode design: (1) direct growth of MnO2 nanostructures onto highly conducive and high specic-surface-area collectors and (2) constructing a 3D network with rich pores to facilitate ion and charge transportation.
Acknowledgements This work was jointly supported by the National Basic Research Program of China (2009CB930600, 2012CB933301), the Ministry of Education of China (no. IRT1148), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Key Projects for International Cooperation (BZ2010043), and Jiangsu Provincial NSF (BK2011750).
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