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Ultrathin MnO2 nanoflakes deposited on carbon nanotube networks for symmetrical supercapacitors with enhanced performance
Journal of Power Sources 341 (2017) 27e35
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Ultrathin MnO2 nanoflakes deposited on carbon nanotube networks for symmetrical supercapacitors with enhanced performance Peng Sun, Huan Yi, Tianquan Peng, Yuting Jing, Ruijing Wang, Huanwen Wang, Xuefeng Wang* Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
MnO2@CNTs/Ni mesh electrode exhibits a high specific capacitance. A symmetric supercapacitor (SSC) was assembled with enhanced performance. The SSC delivers a wide working voltage and superior energy density and power density.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2016 Received in revised form 15 November 2016 Accepted 29 November 2016
Manganese dioxide is a promising electrode material for electrochemical supercapacitors, but its poor electronic conductivity (105~106 S cm1) limits the fast charge/discharge rate for practical applications. In the present work, we use the chemical vapor deposition (CVD) method to grow highly conductive carbon nanotube (CNT) networks on flexible Ni mesh, on which MnO2 nanoflake layers are deposited by a simple solution method, forming a hierarchical core-shell structure. Under the optimized mass loading, the as-fabricated MnO2 nanoflake@CNTs/Ni mesh electrode exhibits a high specific capacitance of 1072 F g1 at 1 A g1 in three-electrode configuration. Due to advantageous features of these core-shell electrodes (e.g., high conductivity, direct current path, structure stability), the as-assembled symmetric supercapacitor (SSC) based on MnO2@CNTs/Ni mesh has a wide working voltage (2.0 V) in 1 M Na2SO4 aqueous electrolyte. Finally an impressive energy density of 94.4 Wh kg1 at 1000 W kg1 and a high power density of 30.2 kW kg1 at 33.6 Wh kg1 have been achieved for the as-assembled SSC, which exhibits a great potential as a low-cost, high energy density and attractive wearable energy storage device. © 2016 Published by Elsevier B.V.
Keywords: Carbon nanotubes MnO2 nanoflakes Ni mesh Core-shell structure Symmetric supercapacitor
1. Introduction Supercapacitors with superior power density, fast chargedischarging rate and long cycle life for energy storage devices have attracted intense research attention in recent years [1,2]. It is
* Corresponding author. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.jpowsour.2016.11.112 0378-7753/© 2016 Published by Elsevier B.V.
noted that electrode materials play a key role towards the development of high performance supercapacitors in terms of the morphology, porosity, size, and electronic conductivity. Pseudocapacitive RuO2$xH2O has been studied extensively as an attractive electrode material for supercapacitors owing to its high energy density and large charge transfer-reaction pseudocapacitance which is based on fast and reversible redox reactions at the electrode surface [3,4]; however, the high cost prevents its large-scale
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applications. Among the pseudocapacitive electrode materials, MnO2 is one of the most attractive candidate because of low cost, high natural abundance, and excellent theoretical specific capacitance (1370 F g1) [5]. However, it is difficult to achieve the theoretical value in most experiments due to its low electronic conductivity (106 to 105 S cm1) [6], low ion diffusion constant and poor stability [5]. For MnO2-based supercapacitors, several effective strategies have been carried out to address these issues. One way is to create various nanostructures, such as nanorods [7], nanowires [8], nanospheres [9], nanoflowers [10], nanotubes [11] and nanoflakes [12], which exhibit higher specific capacitance and rate/ cycling performance than their bulk counterparts. Another way is combining MnO2 with highly conductive materials (especially for various carbons) to form a hybrid nanostructure [13e15]. For instance, nanoporous gold was used as substrates to deposit nanocrystalline MnO2, resulting in a specific capacitance of the constituent MnO2 (~1145 F g1) [6]. Unfortunately, the high cost of Au has restricted its potential applications. Recently, to solve low energy density problem, asymmetric supercapacitors (ASCs) have been explored by combining MnO2 as energy source and porous carbon as power source to increase the operation voltage. For example, an asymmetric electrochemical capacitor based on graphene as negative electrode and a MnO2 nanowire/graphene composite as positive electrode was developed in neutral aqueous Na2SO4 electrolyte, which exhibited a superior energy density of 30.4 Wh kg1 in the high-voltage region of 0e2.0 V [8]. Similar energy density has been obtained based on activated carbon nanofibers as negative electrode and MnO2/carbon nanofiber composites as positive electrode [16]. In spite of these progresses, the energy density of most MnO2-based ASCs cannot exceed that of lead acid batteries (20e40 Wh kg1) and their power densities inevitably decrease due to the discrepancy in kinetics and specific capacities between the two electrodes [9,17,18]. Therefore, for MnO2-based supercapacitor device it remains a huge challenge to increase specific capacitance and to realize high energy density with high power density at the same time. In this work, we report ultrathin MnO2 nanoflakes deposited on carbon nanotube networks/Ni mesh with a well-designed coreshell nanostructure (MnO2@CNTs/Ni), which gives a high specific capacitance of 1072 F g1 at 1 A g1 in three-electrode system. In addition, a symmetric supercapacitor (SSC) has been assembled from two pieces of MnO2@CNTs/Ni electrodes using 1 M Na2SO4 solution as electrolyte, which exhibits wide working voltage of 2.0 V and high power density of 30.2 kW kg1 at 33.6 Wh kg1. Meanwhile, such a supercapacitor device shows very good stability and long cycle life.
naturally to room temperature. Generally, ~0.3 mg CNTs (average) is loaded to Ni substrate (1 cm 1 cm) after CVD process. 2.2. Synthesis of the MnO2@CNTs/Ni In the second step, the as-synthesized CNTs/Ni was added into 10 mL of 5% (w/w) polyethylene glycol (PEG) aqueous solution with magnetic stirring for 1 h (ensure the adequate adsorption of PEG). Then 10 mL of 0.05 M KMnO4 (A.R., Sinopharm Chemical Reagent Co., Ltd.) aqueous solution was added and heated at 75 C in an oil bath with continuous magnetic stirring for 2 h. The mesh was taken out, washed repeatedly with deionized water after the solution was cooled down to room temperature. After that, the product was dried in a vacuum at 60 C for 12 h to obtain the MnO2@CNTs/Ni composite. The mass loading of MnO2 was confirmed by a highly sensitive balance with a precision down to ±0.01 mg, which was directly obtained by subtracting the substrate (CNTs/Ni) weight from the total weight of the substrate and the MnO2 onto its surface. Different mass loadings of MnO2 can be obtained by changing the amount of KMnO4. 2.3. Materials characterization The morphology and microstructure of the products were characterized by field emission scanning electron microscopy (FESEM; Hitachi S-4800) and transmission electron microscopy (TEM; JEOL, JEM-2100), X-ray diffraction (XRD; Bruker Focus D8 with Cu Ka radiation), Raman spectroscopy (Renishaw In via, 514 nm laser under ambient conditions) and BrunnerEmmetTeller (BET, NOVA 2200e, Quanta-chrome, America). 2.4. Electrochemical measurement
2. Experimental
The electrochemical measurements including cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were conducted in an electrochemical workstation (CHI660D, Chenhua, Shanghai, China) at room temperature. In the three-electrode system, the MnO2@CNTs/Ni was directly used as working electrode, while platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively, in 1 M Na2SO4 electrolyte. For comparison, CNTs/Ni synthesized by CVD method was tested with CV and GCD in 1 M Na2SO4 electrolyte without any treatment. Further to explore the advantages of this material for real applications, a symmetric supercapacitor was assembled from two pieces of MnO2@CNTs/Ni using 1 M Na2SO4 solution as electrolyte, which were separated by a common filter paper. The specific capacitances can be calculated from galvanostatic tests by the equation [19]:
2.1. Synthesis of the CNTs/Ni
C ¼ IDt=mDV
MnO2@CNTs/Ni was synthesized by a facile two-step method. The shiny and flexible Ni mesh (mesh number: 200, 1 cm 1 cm) was firstly immersed into 1 M H2SO4 solution with sonication for 15 min to remove the NiO oxidation layer on the surface. In the first step, carbon nanotubes (CNTs) were grown on Ni mesh (CNTs/Ni) via a chemical vapor deposition (CVD). The as-pretreated Ni mesh was soaked in 30 mL ethanol solution with 0.1 M polyethylene glycol (PEG) (C.P., Sinopharm Chemical Reagent Co., Ltd.) and 0.1 M Ni(NO3)2 (A.R., Sinopharm Chemical Reagent Co., Ltd.) for 3 h and then dried in air. After that, the Ni mesh with catalyst was placed into a Al2O3 tubular furnace. C2H2 gas as the carbon source was pumped into the tube, which was heated up to 550 C at a heating rate of 5 C min1 and held at 550 C for 1 h, and then cooled down
(1)
where I is the discharging current, t is the discharge time, DV is the potential window, and m is the mass of active material in the working electrode for three-electrode system (For symmetric cell system, m is the total mass of the active electrode materials in two electrodes). Energy density (E) and power density (P) can be calculated from galvanostatic tests by the following equations [19]:
i. h E ¼ CðDVÞ2 2
(2)
P ¼ E=Dt
(3)
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Fig. 1. The fabrication steps of the MnO2@CNTs/Ni.
where E, C, DV, P and Dt are the specific energy, specific capacitance, working voltage, specific power and discharge time, respectively.
3. Results and discussion 3.1. Morphological and structural characterizations The schematic diagram (Fig. 1) exhibits the preparation process of the MnO2 nanoflake@CNTs/Ni mesh, which involves two major steps. In the first step, CNT networks are grown on highly conductive and flexible Ni mesh in a CVD tube furnace using Ni(NO3)2 and C2H2 as catalyst and carbon source, respectively. In the second step, MnO2 nanoflakes are coated on the network of CNTs based on the spontaneous redox reaction given in equation (4) [20], eventually to form a MnO2 nanoflake@CNT core-shell structure on Ni mesh. 2 4MnO 4 þ 3C þ H2O ¼ 4MnO2 þ CO3 þ 2HCO3
(4)
In such a hybrid structure, MnO2 nanoflakes overspread on CNTs and connect with each other on Ni mesh, forming a highly conductive three-dimensional (3D) network. In the process, PEG is used as an amphiphile for promoting the reduction reaction of KMnO4 under a rather mild condition (75 C, 1 atm) in comparison with other reported methods [21,22]. Fig. 2 shows low- and high magnification FESEM and TEM images of the as-synthesized CNTs/Ni mesh. It was observed that CNTs with high density are grown uniformly on Ni mesh, forming a 3D hierarchical structure (Fig. 2A, B). The CNTs/Ni keeps the ordered two dimensional woven structure of the Ni mesh substrate. Meanwhile, CNTs/Ni can be readily rolled up, which is appropriate for flexible device applications (Fig. S1 in Supporting Information). Higher-magnification FESEM images (Fig. 2B) provide clearer information about the Ni mesh growing CNT nanotubes. It can be seen that every CNTs/Ni fiber has the uniform diameter of approximately 63 mm, which is larger than that of pristine Ni fiber diameter (ca. 45 mm) (Fig. S2 in Supporting Information). These nanotubes are interconnected to form a 3D networks and the
Fig. 2. (A, B, C) FESEM and (D) TEM images of the CNTs grown on Ni mesh.
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average diameter of CNTs is ~20 nm (Fig. 2C). The tubular structure can be further confirmed from the TEM image (Fig. 2D). The FESEM and TEM images of as-synthesized MnO2@CNTs on Ni mesh are shown in Fig. 3. In the low magnification FESEM image (Fig. 3A) the MnO2@CNTs/Ni keeps two dimensional structure uniformly on Ni mesh substrate. With high magnification (Fig. 3B) the cross-linked MnO2 nanoflakes and CNTs with a hierarchical core-shell structure was clearly observed on the surface of CNTs, which could provide abundant space for electrolyte ions. In such a special structure, MnO2 nanoflakes on CNTs can increase the effective contact area and supply fast paths for the insertion and extraction of electrolyte ions, which are beneficial to the Faraday reaction between MnO2 and electrolyte ions [9,10,32]. Fig. 3C and D are the TEM images of MnO2@CNTs peeled off from Ni mesh. After coated with MnO2 nanoflakes, CNTs could not be distinguished under TEM observation. The thickness of the MnO2 layer is approximately 40e50 nm calculated by the diameter difference between MnO2@CNTs/Ni and CNTs/Ni (Figs. 3C, 2D). These MnO2 nanoflakes are generally less than 4 nm in thickness (Fig. 3D), and the ultrathin MnO2 nanoflakes are advantageous to the insertion and extraction of electrolyte ions without any obstruction. Fig. 3E
shows the selected-area electron diffraction (SAED) pattern of MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2), indicating that the coated MnO2 is amorphous. The elemental spatial distributions of MnO2@CNTs/Ni (0.5 mg cm2 mass loading) were characterized by energy-dispersive spectroscopy (EDS) of individual elements Mn, O, C and Ni, as shown in Fig. 3F. Raman spectrum and XRD pattern are shown in Fig. 4. Two most intense peaks D (disordered) and G (graphite-like) are obtained at 1360 cm1 and 1600 cm1 (Fig. 4A), which can be attributed to the characteristic peaks of CNTs [23]. The G mode is assigned to the E2g phonon of C sp2 atoms, and the D modes are caused by the Raman double resonant scattering from nonzero-center phonon modes which are originated from amorphous disorder and defects within the carbon lattice [24,25]. The intensity ratio of the D band to the G band (ID/IG) is 0.68 for CNTs/Ni and 0.82 for MnO2@CNTs/Ni, indicating the disorder degree of CNTs is increased in MnO2@CNTs/Ni because of the KMnO4 oxidation during the preparation process. It is generally accepted that the ID/IG ratio defines the defect density of carbon materials and electrochemical reactions are most likely to occur at the sites of the defects [26]. In addition, a pronounced peak centered at 638 cm1 in low frequency range arises from MnO2
Fig. 3. (A, B) FESEM and (C, D) TEM images of MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2). (E) SAED pattern and (F) EDS analysis of MnO2@CNTs/Ni (0.5 mg cm2 mass loading).
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species in the MnO2@CNTs/Ni. In the XRD pattern of MnO2@CNTs/ Ni (Fig. 4B), the strong peaks at 2q ¼ 29 , 44 , 52 , 77 are attributed to Ni mesh. However, only the unobvious peak (121) at 2q ¼ 37 and the peak (211) at 2q ¼ 43 for MnO2 (JCPDS File Card No. 14e0644) are faintly spotted, suggesting small mass loading and the amorphous phase of MnO2 in the composite (corresponds to the result of SAED pattern) [9,18]. BrunauerEmmettTeller (BET) analysis reveals the specific surface areas of CNTs/Ni, MnO2@CNTs/Ni (0.5 mg cm2 and 5.0 mg cm2 mass loading). The N2 adsorptiondesorption curves of CNTs/Ni, MnO2@CNTs/Ni (0.5 mg cm2 and 5.0 mg cm2) are shown in Support Information (Fig. S6). As shown in Table S1, the specific surface areas of CNTs/Ni, MnO2@CNTs/Ni (0.5 mg cm2 and 5.0 mg cm2) are 28.319, 45.836, 6.589 m2 g1, respectively. 3.2. Electrochemical characterization The CV comparison for the MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2), CNTs/Ni and Ni mesh in 1 M Na2SO4 electrolyte at 20 mV s1 is shown in Fig. 5A, with a potential window from 0.2e0.8 V. Generally, the integral area of CV curves is proportional to specific capacitance at the same scan rate. It can be observed that the CV loop of the MnO2@CNTs/Ni is much larger than that of CNTs/Ni and Ni. Therefore, the capacitance value of the composite is much higher than that of CNTs/Ni and Ni, which is attributed to the positive synergistic effect between conductive CNTs and pseudocapacitive MnO2. The charge storage mechanism is based on surface adsorption of electrolyte cations Naþ as well as proton incorporation according to the reaction [27]: MnO2 þ xNaþ þ yHþ þ (x þ y)ee 4 MnOONaxHy
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specific capacitance of 1072 F g1 is obtained for the electrode with a mass loading of 0.5 mg cm2 at a current density of 1 A g1. The capacitance retention is approximately 60% from 1 to 10 A g1. Such superior capacitive behavior may be attributed to: (1) the core-shell structure of MnO2@CNTs and ultrathin MnO2 nanoflakes (less than 4 nm in thickness), which are beneficial to the insertion and extraction of electrolyte ions; (2) the ordered two dimensional woven structure of the Ni mesh substrate and the 3D network structure of interconnected CNTs, which results in lower contact resistance (illustrated in Fig. S3 in Supporting Information). When the mass loading is 0.85, 2.22, 3.50 and 5.00 mg cm2, the specific capacitance values are 684.0, 340.1, 305.9 and 288.0 F g1 at 1 A g1, respectively. The decrease of the specific capacitance with increasing MnO2 mass loading is due to: (1) the additional MnO2 generates a low proton diffusion; (2) the electrical conductivity of the MnO2@CNT composite is decreasing at high mass loading [28]. As shown in Table S2 in support information, the specific capacitance values obtained in this work are much better than previous works [29e31]. To evaluate the charge transfer and electrolyte diffusion in CNTs/ Ni and MnO2@CNTs/Ni (0.5 and 5.0 mg cm2 mass loading) electrodes, electrochemical impedance tests were carried out at 0.4 V with a frequency range from 0.1 to 105 Hz. Fig. S4 shows the Nyquist
(5)
In order to evaluate the contribution of MnO2 to the capacitance of the MnO2@CNTs/Ni electrodes, we investigated the effect of the mass loading on the capacitance value, which will be useful for practical application. Different mass loading of MnO2 from 0.5 to 5.0 mg cm2 have been loaded by changing the amount of KMnO4 during the preparation process. Fig. 5B and C shows the CV curves of MnO2@CNTs/Ni with different mass loadings of MnO2 at the scan rate of 10 mV s1 and 50 mV s1, respectively. The CV curves of low mass loading of MnO2 show much more rectangular and symmetric than that of high loaded sample. Meanwhile, with increase of scan rates from 10 to 50 mV s1, the CV curves deviate from ideal capacitor due to the large resistance in high mass loading MnO2. Fig. 5D shows the CV curves of the MnO2@CNTs/Ni (0.5 mg cm2 mass loading) at different scan rates. With increasing scan rate from 5 to 200 mV s1, the current density also increases without any obvious changes in the shape of the CV curves, indicating a very good rate performance. The rectangular and symmetric shape of the CV curves is observed at high scan rates, implying the low contact resistance of the electrodes. Fig. 6A shows the galvanostatic charge-discharge (GCD) curves of MnO2@CNTs/Ni (0.5 mg cm2 mass loading) and CNTs/Ni in the potential range from 0.2e0.8 V (vs SCE) at a current density of 1 A g1. The much longer discharging time of the MnO2@CNTs/Ni electrode than that of the CNTs/Ni electrode indicates the increase of capacitance mainly from the MnO2 component. GCD curves of the MnO2@CNTs/Ni electrode at different mass loadings and current densities are shown in Fig. 6B, C. The nearly linear and almost symmetrical curves of MnO2@CNTs/Ni indicate that the electrode exhibits a good capacitive behavior. According to equation (1), we can calculate the capacitance values at different current densities from the discharging time. Fig. 6D shows the specific capacitance values at different current densities for MnO2@CNTs/Ni with different mass loadings of MnO2. It is noted that an ultrahigh
Fig. 4. Raman spectra (A) and XRD patterns (B) of the samples. The mass loading of MnO2 (MnO2@CNTs/Ni) is 0.5 mg cm2.
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Fig. 5. Electrochemical properties of the samples in three electrode system. (A) CV curves for MnO2@CNTs/Ni, CNTs/Ni and bare Ni mesh at a scan rate of 20 mV s1. (B, C) CV curves of MnO2@CNTs/Ni with different mass loadings for MnO2. The scan rates are (B) 10 mV s1 and (C) 50 mV s1. (D) CV curves of the MnO2@CNTs/Ni at different scan rates. The mass loading of MnO2 is 0.5 mg cm2.
Fig. 6. Electrochemical properties of the samples in three electrode system. (A) Galvanostatic charge-discharge curves for MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2) and CNTs/Ni at 1 A g1. (B) Galvanostatic charge-discharge curves of MnO2@CNTs/Ni with different mass loadings of MnO2 at 1 A g1. (C) Galvanostatic charge-discharge curves of the MnO2@CNTs/Ni at different current densities. The mass loading of MnO2 is 0.5 mg cm2. (D) Specific capacitance values versus current density for MnO2@CNTs/Ni at different mass loadings of MnO2.
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Fig. 7. (A) Schematic structure of the flexible MnO2@CNTs/Ni based symmetric supercapacitor (SSC) in 1 M Na2SO4 aqueous solution. (B) CV curves at different voltage windows. (C) CV curves of the SSC device at different scan rates. (D) Specific capacitances of the SSC device at different scan rates. (E) Cycling performance of the SSC device at 100 mV s1 for 1000 cycles. (F) Galvanostatic charge-discharge curves of the SSC device at various current densities. (G) Specific capacitances of the SSC device at different current densities.
plots of CNTs/Ni and MnO2@CNTs/Ni. For each electrode, a semicircle and a straight line can be observed in high-frequency region and low-frequency region, respectively. Compared with
MnO2@CNTs/Ni electrodes, the almost vertical slope of CNTs/Ni impedance plot reflects the ideal capacitive behavior. As shown in the enlarged view of the high-frequency region in the inset, the
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diameter of the semicircle for each electrode corresponds to the charge-transfer resistance caused by the Faradaic reaction on the electrode surface. The charge-transfer resistance of the CNTs/Ni and MnO2@CNTs/Ni (0.5 and 5 mg cm2 mass loading) electrodes are about 0.18, 0.20 and 0.43 U, respectively. The electrode with low mass loading of MnO2 has higher electronic conductivity, which is beneficial to the charge transfer during the Faradaic reaction. As shown in the enlarged view of the high-frequency region, the intercept along the x axis is the internal resistance, including the electrolyte resistance, the intrinsic resistance of the active material and the contact resistance at the active material/current collector interface. The CNTs/Ni electrode shows the smallest internal resistance of 3.56 U; however, MnO2@CNTs/Ni (5 mg cm2 mass loading) electrode shows the largest resistance of 3.66 U. To further explore the advantages of this material for practical application, a symmetric supercapacitor (SSC) has been assembled from two pieces of MnO2@CNTs/Ni with the mass loading of 0.5 mg cm2 using 1 M Na2SO4 solution as electrolyte. Schematic structure of the flexible SSC is illustrated in Fig. 7A. We perform CV measurements on the MnO2@CNTs/Ni based SSC using a twoelectrode system at different working voltage at 10 mV s1 (Fig. 7B). It is noted that the working voltage can be extended to 2.0 V without obvious polarizations, which is higher than that of other MnO2-based SSCs [7,18,32]. It is well known that water decomposition is the key factor that determines the stable voltage (normally about 1 V) of a capacitive device in aqueous electrolyte system. For the our SSC high working voltage (2.0 V) can be reached, indicating as-prepared MnO2@CNTs/Ni electrode has high oxygen and hydrogen evolution over-potentials in 1 M Na2SO4 electrolyte. The CV curves of our SSC (the working voltage is 2.0 V) at different scan rates are shown in Fig. 7C. The rectangular shape and symmetry of the CV scans can be observed even at the high scan rate of 500 mV s1, indicating its good electrochemical performance. Fig. 7D shows specific capacitance of our SSC at different scan rates. The capacitance retention from 10 to 500 mV s1 is up to 43.1% (140.2 F g1 at 10 mV s1 and 60.4 F g1 at 500 mV s1). As presented in Fig. 7E, the long-term stability of the SSC was examined by CV cycling at a scan rate of 100 mV s1 and the SSC keeps the capacitance retention of 83.4% after 1000 cycles. Fig. 7F shows the GCD curves of the MnO2@CNTs/Ni based SSC at different current densities over the working voltage of 0e2.0 V. The
symmetrical charge-discharge characteristics represents a good capacitive characteristic for our supercapacitor. The specific capacitance of the SSC at different current densities are shown in Fig. 7G. Calculated from Fig. 7F according equation (1), the specific capacitances are 170.0 F g1 at 1 A g1 and 91.0 F g1 at 20 A g1. The capacitance retention from 1 to 20 A g1 is up to 53.5%. According to equation (2) and equation (3), we can calculate energy density and power density from specific capacitances in Fig. 7C and F. As shown in Fig. 8A, the highest energy density of 78.0 Wh kg1 (at a power density of 1402 W kg1) and the highest power density of 30.2 kW kg1 (at an energy density of 33.6 Wh kg1) have been obtained from CV curves. According to GCD curves, the highest energy density of 94.4 Wh kg1 (at a power density of 1000 W kg1) and the highest power density of 20.0 kW kg1 (at an energy density of 55.6 Wh kg1) are achieved at a working voltage of 2.0 V. These results are much higher than that of other reported MnO2-based SSCs or ASCs, such as CNPs-MnO2//CNPs-MnO2 (CNPs: carbon nanoparticles) SSC (4.8 Wh kg1,14.0 kW kg1) [7], graphene-MnO2//graphene-MnO2 SSC (6.8 Wh kg1, 62.0 W kg1) [28], graphene-MnO2//ACN (ACN: activated carbon nanofiber) ASC (51.1 Wh kg1, 102.2 W kg1) [20], AC//MnO2-CNTs ASC (13.3 Wh kg1, 600.0 W kg1) [32], Ni(OH)2-MnO2-RGO//FRGO ASC (54.0 Wh kg1, 392.0 W kg1) [33], AGMn//aMEGO ASC (20.8 Wh kg1, 32.3 kW kg1) [34], UPMNFs//FMCNTs ASC (47.4 Wh kg1, 200.0 W kg1) [35] and AC//K0.27MnO2$0.6H2O ASC (25.3 Wh kg1, 140.0 W kg1) [36] (see Table S3 in Supporting Information). To further demonstrate the practical application, our SSC device can successfully power a green light-emitting diode (LED, working voltage: 1.5 V) for 5 min, after being charged at 10 A g1 for 24 s as shown in Fig. 8B. Impressively, two SSC devices in series can even power 12 LEDs for more than 1 min (Fig. 8B). 4. Conclusion The MnO2@CNTs/Ni composite is prepared by a facile two-step method on a flexible and metallic Ni mesh under a mild reaction condition, in which MnO2 nanoflakes and CNTs forming a hierarchical core-shell structure. In such a unique structure, the electrode exhibits a high specific capacitance of 1072 F g1 at 1 A g1 in 1 M Na2SO4 electrolyte. The as-assembled MnO2@CNTs/Ni based SSC exhibits a wide working voltage (2.0 V) in 1 M Na2SO4 electrolyte,
Fig. 8. (A) Ragone plot for the MnO2@CNTs/Ni based SSC with other MnO2-based supercapacitors reported previously. (B) One device can power a green light-emitting (LED, working voltage: 1.5 V) and two SSC devices in series can power 12 LEDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
P. Sun et al. / Journal of Power Sources 341 (2017) 27e35
high power density (30.2 kW kg1 at an energy density of 33.6 Wh kg1), high energy density (94.4 Wh kg1 at a power density of 1000 W kg1) and long-term cycle stability (keeps a retention of 83.4% after 1000 cycles). For practical application, one SSC was assembled to power 1 LED for 5 min and two SSC devices in series can power 12 LEDs for more than 1 min. In conclusion, the MnO2@CNTs/Ni composite is a very promising electrode material for assembling capacitor with both high power density and high energy density. Acknowledgements The authors gratefully acknowledge the financial support offered by NSFC Grants (21373152). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.11.112. References [1] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520e2531. [2] C.B.E., Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer Science & Business Media, 2013. [3] C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Design and tailoring of the nanotubular arrayed architecture of hydrous RuO2 for next generation supercapacitors, Nano Lett. 6 (2006) 2690e2695. [4] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities, Adv. Energy Mater. 4 (2014) 1300816. langer, Charge storage mechanism of MnO2 electrode used [5] B.T. Toupin M, D. Be in aqueous electrochemical capacitor, Chem. Mater. 16 (2004) 3184e3190. [6] X. Lang, A. Hirata, T. Fujita, M. Chen, Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors, Nat. Nanotechnol. 6 (2011) 232e236. [7] L. Yuan, X.H. Lu, X. Xiao, T. Zhai, J. Dai, F. Zhang, B. Hu, X. Wang, L. Gong, J. Chen, C. Hu, Y. Tong, J. Zhou, Z.L. Wang, Flexible solid-state supercapacitors based on carbon nanoparticles/MnO2 nanorods hybrid structure, ACS Nano 6 (2012) 656e661. [8] Z.S. Wu, W. Ren, D.W. Wang, F. Li, B. Liu, H.M. Cheng, High-energy MnO2 nanowire/graphene and graphene asymmetric electrochemical capacitors, ACS Nano 4 (2010) 5835e5842. [9] Y. Jin, H. Chen, M. Chen, N. Liu, Q. Li, Graphene-patched CNT/MnO2 nanocomposite papers for the electrode of high-performance flexible asymmetric supercapacitors, ACS Appl. Mater. interfaces 5 (2013) 3408e3416. [10] L. Hu, W. Chen, X. Xie, N. Liu, Y. Yang, H. Wu, Y. Yao, M. Pasta, H.N. Alshareef, Y. Cui, Symmetrical MnO2-carbon nanotube-textile nanostructures for wearable pseudocapacitors with high mass loading, ACS Nano 5 (2011) 8904e8913. [11] W. Xiao, H. Xia, J.Y.H. Fuh, L. Lu, Growth of single-crystal a-MnO2 nanotubes prepared by a hydrothermal route and their electrochemical properties, J. Power Sources 193 (2009) 935e938. [12] T.M. Higgins, D. McAteer, J.C. Coelho, B. Mendoza Sanchez, Z. Gholamvand, G. Moriarty, N. McEvoy, N.C. Berner, G.S. Duesberg, V. Nicolosi, J.N. Coleman, Effect of percolation on the capacitance of supercapacitor electrodes prepared from composites of manganese dioxide nanoplatelets and carbon nanotubes, ACS Nano 8 (2014) 9567e9579. [13] K.W.S. An K H, Y.S. Park, et al., Electrochemical properties of high-power supercapacitors using single-walled carbon nanotube electrodes, Adv. Funct. Mater. 11 (2001) 387e392. [14] B.F. Frackowiak E, Carbon materials for the electrochemical storage of energy
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in capacitors, Carbon 39 (2001) 937e950. [15] C.C.K. Kaempgen M, J. Ma, et al., Printable thin film supercapacitors using single-walled carbon nanotubes, Nano Lett. 9 (2009) 1872e1876. [16] J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, A high-performance asymmetric supercapacitor based on carbon and carboneMnO2 nanofiber electrodes, Carbon 61 (2013) 190e199. [17] H. Gao, F. Xiao, C.B. Ching, H. Duan, High-performance asymmetric supercapacitor based on graphene hydrogel and nanostructured MnO2, ACS Appl. Mater. interfaces 4 (2012) 2801e2810. [18] P. Tang, L. Han, L. Zhang, Facile synthesis of graphite/PEDOT/MnO2 composites on commercial supercapacitor separator membranes as flexible and highperformance supercapacitor electrodes, ACS Appl. Mater. interfaces 6 (2014) 10506e10515. [19] H. Wang, H. Yi, X. Chen, X. Wang, Asymmetric supercapacitors based on nanoarchitectured nickel oxide/graphene foam and hierarchical porous nitrogendoped carbon nanotubes with ultrahigh-rate performance, J. Mater. Chem. A 2 (2014) 3223. [20] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Asymmetric supercapacitors based on graphene/MnO2 and activated carbon nanofiber electrodes with high power and energy density, Adv. Funct. Mater. 21 (2011) 2366e2375. [21] Q. Lv, S. Wang, H. Sun, J. Luo, J. Xiao, J. Xiao, F. Xiao, S. Wang, Solid-state thinfilm supercapacitors with ultrafast charge/discharge based on N-doped-carbon-tubes/Au-nanoparticles-doped-MnO2 nanocomposites, Nano Lett. 16 (2016) 40e47. [22] Q. Li, X.F. Lu, H. Xu, Y.X. Tong, G.R. Li, Carbon/MnO2 double-walled nanotube arrays with fast ion and electron transmission for high-performance supercapacitors, ACS Appl. Mater. interfaces 6 (2014) 2726e2733. [23] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (3) (1970) 1126e1130. [24] L. Xu, S. Li, Z. Wu, H. Li, D. Yan, C. Zhang, Growth and field emission properties of nanotip arrays of amorphous carbon with embedded hexagonal diamond nanoparticles, Appl. Phys. A 103 (1) (2011) 59e65. [25] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (18) (2006) 187401. [26] J. Shen, T. Li, W. Huang, Y. Long, N. Li, One-pot polyelectrolyte assisted hydrothermal synthesis of RuO2-reduced graphene oxide nanocomposite, Electrochim. Acta 95 (2013) 155e161. [27] G.Y. Simon P, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845e854. [28] Yongmin He, Wanjun Chen, Xiaodong Li, Zhenxing Zhang, Jiecai Fu, Changhui Zhao, A.E. Xie, Freestanding three-dimensional graphene/MnO2 composite networks as ultralight and flexible supercapacitor electrodes, ACS Nano 7 (2013) 174e182. [29] L. Bao, J. Zang, X. Li, Flexible Zn2SnO4/MnO2 core/shell nanocable-carbon microfiber hybrid composites for high-performance supercapacitor electrodes, Nano Lett. 11 (2011) 1215e1220. [30] Z. Lei, F. Shi, L. Lu, Incorporation of MnO2-coated carbon nanotubes between graphene sheets as supercapacitor electrode, ACS Appl. Mater. interfaces 4 (2012) 1058e1064. [31] W. Chen, R.B. Rakhi, L. Hu, X. Xie, Y. Cui, H.N. Alshareef, High-performance nanostructured supercapacitors on a sponge, Nano Lett. 11 (2011) 5165e5172. [32] L. Li, Z.A. Hu, N. An, Y.Y. Yang, Z.M. Li, H.Y. Wu, Facile synthesis of MnO2/CNTs composite for supercapacitor electrodes with long cycle stability, J. Phys. Chem. C 118 (2014) 22865e22872. [33] H. Chen, S. Zhou, L. Wu, Porous nickel hydroxide-manganese dioxide-reduced graphene oxide ternary hybrid spheres as excellent supercapacitor electrode materials, ACS Appl. Mater. interfaces 6 (2014) 8621e8630. [34] Z.L. Zhao X, S. Murali, et al., Incorporation of manganese dioxide within ultraporous activated graphene for high-performance electrochemical capacitors, ACS nano 6 (2012) 5404e5412. [35] H. Jiang, C. Li, T. Sun, J. Ma, A green and high energy density asymmetric supercapacitor based on ultrathin MnO2 nanostructures and functional mesoporous carbon nanotube electrodes, Nanoscale 4 (2012) 807e812. [36] Q. Qu, L. Li, S. Tian, W. Guo, Y. Wu, R. Holze, A cheap asymmetric supercapacitor with high energy at high power: activated carbon// K0.27MnO2$0.6H2O, J. Power Sources 195 (2010) 2789e2794.
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Ultrathin MnO2 nanoflakes deposited on carbon nanotube networks for symmetrical supercapacitors with enhanced performance Peng Sun, Huan Yi, Tianquan Peng, Yuting Jing, Ruijing Wang, Huanwen Wang, Xuefeng Wang* Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
MnO2@CNTs/Ni mesh electrode exhibits a high specific capacitance. A symmetric supercapacitor (SSC) was assembled with enhanced performance. The SSC delivers a wide working voltage and superior energy density and power density.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2016 Received in revised form 15 November 2016 Accepted 29 November 2016
Manganese dioxide is a promising electrode material for electrochemical supercapacitors, but its poor electronic conductivity (105~106 S cm1) limits the fast charge/discharge rate for practical applications. In the present work, we use the chemical vapor deposition (CVD) method to grow highly conductive carbon nanotube (CNT) networks on flexible Ni mesh, on which MnO2 nanoflake layers are deposited by a simple solution method, forming a hierarchical core-shell structure. Under the optimized mass loading, the as-fabricated MnO2 nanoflake@CNTs/Ni mesh electrode exhibits a high specific capacitance of 1072 F g1 at 1 A g1 in three-electrode configuration. Due to advantageous features of these core-shell electrodes (e.g., high conductivity, direct current path, structure stability), the as-assembled symmetric supercapacitor (SSC) based on MnO2@CNTs/Ni mesh has a wide working voltage (2.0 V) in 1 M Na2SO4 aqueous electrolyte. Finally an impressive energy density of 94.4 Wh kg1 at 1000 W kg1 and a high power density of 30.2 kW kg1 at 33.6 Wh kg1 have been achieved for the as-assembled SSC, which exhibits a great potential as a low-cost, high energy density and attractive wearable energy storage device. © 2016 Published by Elsevier B.V.
Keywords: Carbon nanotubes MnO2 nanoflakes Ni mesh Core-shell structure Symmetric supercapacitor
1. Introduction Supercapacitors with superior power density, fast chargedischarging rate and long cycle life for energy storage devices have attracted intense research attention in recent years [1,2]. It is
* Corresponding author. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.jpowsour.2016.11.112 0378-7753/© 2016 Published by Elsevier B.V.
noted that electrode materials play a key role towards the development of high performance supercapacitors in terms of the morphology, porosity, size, and electronic conductivity. Pseudocapacitive RuO2$xH2O has been studied extensively as an attractive electrode material for supercapacitors owing to its high energy density and large charge transfer-reaction pseudocapacitance which is based on fast and reversible redox reactions at the electrode surface [3,4]; however, the high cost prevents its large-scale
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applications. Among the pseudocapacitive electrode materials, MnO2 is one of the most attractive candidate because of low cost, high natural abundance, and excellent theoretical specific capacitance (1370 F g1) [5]. However, it is difficult to achieve the theoretical value in most experiments due to its low electronic conductivity (106 to 105 S cm1) [6], low ion diffusion constant and poor stability [5]. For MnO2-based supercapacitors, several effective strategies have been carried out to address these issues. One way is to create various nanostructures, such as nanorods [7], nanowires [8], nanospheres [9], nanoflowers [10], nanotubes [11] and nanoflakes [12], which exhibit higher specific capacitance and rate/ cycling performance than their bulk counterparts. Another way is combining MnO2 with highly conductive materials (especially for various carbons) to form a hybrid nanostructure [13e15]. For instance, nanoporous gold was used as substrates to deposit nanocrystalline MnO2, resulting in a specific capacitance of the constituent MnO2 (~1145 F g1) [6]. Unfortunately, the high cost of Au has restricted its potential applications. Recently, to solve low energy density problem, asymmetric supercapacitors (ASCs) have been explored by combining MnO2 as energy source and porous carbon as power source to increase the operation voltage. For example, an asymmetric electrochemical capacitor based on graphene as negative electrode and a MnO2 nanowire/graphene composite as positive electrode was developed in neutral aqueous Na2SO4 electrolyte, which exhibited a superior energy density of 30.4 Wh kg1 in the high-voltage region of 0e2.0 V [8]. Similar energy density has been obtained based on activated carbon nanofibers as negative electrode and MnO2/carbon nanofiber composites as positive electrode [16]. In spite of these progresses, the energy density of most MnO2-based ASCs cannot exceed that of lead acid batteries (20e40 Wh kg1) and their power densities inevitably decrease due to the discrepancy in kinetics and specific capacities between the two electrodes [9,17,18]. Therefore, for MnO2-based supercapacitor device it remains a huge challenge to increase specific capacitance and to realize high energy density with high power density at the same time. In this work, we report ultrathin MnO2 nanoflakes deposited on carbon nanotube networks/Ni mesh with a well-designed coreshell nanostructure (MnO2@CNTs/Ni), which gives a high specific capacitance of 1072 F g1 at 1 A g1 in three-electrode system. In addition, a symmetric supercapacitor (SSC) has been assembled from two pieces of MnO2@CNTs/Ni electrodes using 1 M Na2SO4 solution as electrolyte, which exhibits wide working voltage of 2.0 V and high power density of 30.2 kW kg1 at 33.6 Wh kg1. Meanwhile, such a supercapacitor device shows very good stability and long cycle life.
naturally to room temperature. Generally, ~0.3 mg CNTs (average) is loaded to Ni substrate (1 cm 1 cm) after CVD process. 2.2. Synthesis of the MnO2@CNTs/Ni In the second step, the as-synthesized CNTs/Ni was added into 10 mL of 5% (w/w) polyethylene glycol (PEG) aqueous solution with magnetic stirring for 1 h (ensure the adequate adsorption of PEG). Then 10 mL of 0.05 M KMnO4 (A.R., Sinopharm Chemical Reagent Co., Ltd.) aqueous solution was added and heated at 75 C in an oil bath with continuous magnetic stirring for 2 h. The mesh was taken out, washed repeatedly with deionized water after the solution was cooled down to room temperature. After that, the product was dried in a vacuum at 60 C for 12 h to obtain the MnO2@CNTs/Ni composite. The mass loading of MnO2 was confirmed by a highly sensitive balance with a precision down to ±0.01 mg, which was directly obtained by subtracting the substrate (CNTs/Ni) weight from the total weight of the substrate and the MnO2 onto its surface. Different mass loadings of MnO2 can be obtained by changing the amount of KMnO4. 2.3. Materials characterization The morphology and microstructure of the products were characterized by field emission scanning electron microscopy (FESEM; Hitachi S-4800) and transmission electron microscopy (TEM; JEOL, JEM-2100), X-ray diffraction (XRD; Bruker Focus D8 with Cu Ka radiation), Raman spectroscopy (Renishaw In via, 514 nm laser under ambient conditions) and BrunnerEmmetTeller (BET, NOVA 2200e, Quanta-chrome, America). 2.4. Electrochemical measurement
2. Experimental
The electrochemical measurements including cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were conducted in an electrochemical workstation (CHI660D, Chenhua, Shanghai, China) at room temperature. In the three-electrode system, the MnO2@CNTs/Ni was directly used as working electrode, while platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively, in 1 M Na2SO4 electrolyte. For comparison, CNTs/Ni synthesized by CVD method was tested with CV and GCD in 1 M Na2SO4 electrolyte without any treatment. Further to explore the advantages of this material for real applications, a symmetric supercapacitor was assembled from two pieces of MnO2@CNTs/Ni using 1 M Na2SO4 solution as electrolyte, which were separated by a common filter paper. The specific capacitances can be calculated from galvanostatic tests by the equation [19]:
2.1. Synthesis of the CNTs/Ni
C ¼ IDt=mDV
MnO2@CNTs/Ni was synthesized by a facile two-step method. The shiny and flexible Ni mesh (mesh number: 200, 1 cm 1 cm) was firstly immersed into 1 M H2SO4 solution with sonication for 15 min to remove the NiO oxidation layer on the surface. In the first step, carbon nanotubes (CNTs) were grown on Ni mesh (CNTs/Ni) via a chemical vapor deposition (CVD). The as-pretreated Ni mesh was soaked in 30 mL ethanol solution with 0.1 M polyethylene glycol (PEG) (C.P., Sinopharm Chemical Reagent Co., Ltd.) and 0.1 M Ni(NO3)2 (A.R., Sinopharm Chemical Reagent Co., Ltd.) for 3 h and then dried in air. After that, the Ni mesh with catalyst was placed into a Al2O3 tubular furnace. C2H2 gas as the carbon source was pumped into the tube, which was heated up to 550 C at a heating rate of 5 C min1 and held at 550 C for 1 h, and then cooled down
(1)
where I is the discharging current, t is the discharge time, DV is the potential window, and m is the mass of active material in the working electrode for three-electrode system (For symmetric cell system, m is the total mass of the active electrode materials in two electrodes). Energy density (E) and power density (P) can be calculated from galvanostatic tests by the following equations [19]:
i. h E ¼ CðDVÞ2 2
(2)
P ¼ E=Dt
(3)
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Fig. 1. The fabrication steps of the MnO2@CNTs/Ni.
where E, C, DV, P and Dt are the specific energy, specific capacitance, working voltage, specific power and discharge time, respectively.
3. Results and discussion 3.1. Morphological and structural characterizations The schematic diagram (Fig. 1) exhibits the preparation process of the MnO2 nanoflake@CNTs/Ni mesh, which involves two major steps. In the first step, CNT networks are grown on highly conductive and flexible Ni mesh in a CVD tube furnace using Ni(NO3)2 and C2H2 as catalyst and carbon source, respectively. In the second step, MnO2 nanoflakes are coated on the network of CNTs based on the spontaneous redox reaction given in equation (4) [20], eventually to form a MnO2 nanoflake@CNT core-shell structure on Ni mesh. 2 4MnO 4 þ 3C þ H2O ¼ 4MnO2 þ CO3 þ 2HCO3
(4)
In such a hybrid structure, MnO2 nanoflakes overspread on CNTs and connect with each other on Ni mesh, forming a highly conductive three-dimensional (3D) network. In the process, PEG is used as an amphiphile for promoting the reduction reaction of KMnO4 under a rather mild condition (75 C, 1 atm) in comparison with other reported methods [21,22]. Fig. 2 shows low- and high magnification FESEM and TEM images of the as-synthesized CNTs/Ni mesh. It was observed that CNTs with high density are grown uniformly on Ni mesh, forming a 3D hierarchical structure (Fig. 2A, B). The CNTs/Ni keeps the ordered two dimensional woven structure of the Ni mesh substrate. Meanwhile, CNTs/Ni can be readily rolled up, which is appropriate for flexible device applications (Fig. S1 in Supporting Information). Higher-magnification FESEM images (Fig. 2B) provide clearer information about the Ni mesh growing CNT nanotubes. It can be seen that every CNTs/Ni fiber has the uniform diameter of approximately 63 mm, which is larger than that of pristine Ni fiber diameter (ca. 45 mm) (Fig. S2 in Supporting Information). These nanotubes are interconnected to form a 3D networks and the
Fig. 2. (A, B, C) FESEM and (D) TEM images of the CNTs grown on Ni mesh.
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average diameter of CNTs is ~20 nm (Fig. 2C). The tubular structure can be further confirmed from the TEM image (Fig. 2D). The FESEM and TEM images of as-synthesized MnO2@CNTs on Ni mesh are shown in Fig. 3. In the low magnification FESEM image (Fig. 3A) the MnO2@CNTs/Ni keeps two dimensional structure uniformly on Ni mesh substrate. With high magnification (Fig. 3B) the cross-linked MnO2 nanoflakes and CNTs with a hierarchical core-shell structure was clearly observed on the surface of CNTs, which could provide abundant space for electrolyte ions. In such a special structure, MnO2 nanoflakes on CNTs can increase the effective contact area and supply fast paths for the insertion and extraction of electrolyte ions, which are beneficial to the Faraday reaction between MnO2 and electrolyte ions [9,10,32]. Fig. 3C and D are the TEM images of MnO2@CNTs peeled off from Ni mesh. After coated with MnO2 nanoflakes, CNTs could not be distinguished under TEM observation. The thickness of the MnO2 layer is approximately 40e50 nm calculated by the diameter difference between MnO2@CNTs/Ni and CNTs/Ni (Figs. 3C, 2D). These MnO2 nanoflakes are generally less than 4 nm in thickness (Fig. 3D), and the ultrathin MnO2 nanoflakes are advantageous to the insertion and extraction of electrolyte ions without any obstruction. Fig. 3E
shows the selected-area electron diffraction (SAED) pattern of MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2), indicating that the coated MnO2 is amorphous. The elemental spatial distributions of MnO2@CNTs/Ni (0.5 mg cm2 mass loading) were characterized by energy-dispersive spectroscopy (EDS) of individual elements Mn, O, C and Ni, as shown in Fig. 3F. Raman spectrum and XRD pattern are shown in Fig. 4. Two most intense peaks D (disordered) and G (graphite-like) are obtained at 1360 cm1 and 1600 cm1 (Fig. 4A), which can be attributed to the characteristic peaks of CNTs [23]. The G mode is assigned to the E2g phonon of C sp2 atoms, and the D modes are caused by the Raman double resonant scattering from nonzero-center phonon modes which are originated from amorphous disorder and defects within the carbon lattice [24,25]. The intensity ratio of the D band to the G band (ID/IG) is 0.68 for CNTs/Ni and 0.82 for MnO2@CNTs/Ni, indicating the disorder degree of CNTs is increased in MnO2@CNTs/Ni because of the KMnO4 oxidation during the preparation process. It is generally accepted that the ID/IG ratio defines the defect density of carbon materials and electrochemical reactions are most likely to occur at the sites of the defects [26]. In addition, a pronounced peak centered at 638 cm1 in low frequency range arises from MnO2
Fig. 3. (A, B) FESEM and (C, D) TEM images of MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2). (E) SAED pattern and (F) EDS analysis of MnO2@CNTs/Ni (0.5 mg cm2 mass loading).
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species in the MnO2@CNTs/Ni. In the XRD pattern of MnO2@CNTs/ Ni (Fig. 4B), the strong peaks at 2q ¼ 29 , 44 , 52 , 77 are attributed to Ni mesh. However, only the unobvious peak (121) at 2q ¼ 37 and the peak (211) at 2q ¼ 43 for MnO2 (JCPDS File Card No. 14e0644) are faintly spotted, suggesting small mass loading and the amorphous phase of MnO2 in the composite (corresponds to the result of SAED pattern) [9,18]. BrunauerEmmettTeller (BET) analysis reveals the specific surface areas of CNTs/Ni, MnO2@CNTs/Ni (0.5 mg cm2 and 5.0 mg cm2 mass loading). The N2 adsorptiondesorption curves of CNTs/Ni, MnO2@CNTs/Ni (0.5 mg cm2 and 5.0 mg cm2) are shown in Support Information (Fig. S6). As shown in Table S1, the specific surface areas of CNTs/Ni, MnO2@CNTs/Ni (0.5 mg cm2 and 5.0 mg cm2) are 28.319, 45.836, 6.589 m2 g1, respectively. 3.2. Electrochemical characterization The CV comparison for the MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2), CNTs/Ni and Ni mesh in 1 M Na2SO4 electrolyte at 20 mV s1 is shown in Fig. 5A, with a potential window from 0.2e0.8 V. Generally, the integral area of CV curves is proportional to specific capacitance at the same scan rate. It can be observed that the CV loop of the MnO2@CNTs/Ni is much larger than that of CNTs/Ni and Ni. Therefore, the capacitance value of the composite is much higher than that of CNTs/Ni and Ni, which is attributed to the positive synergistic effect between conductive CNTs and pseudocapacitive MnO2. The charge storage mechanism is based on surface adsorption of electrolyte cations Naþ as well as proton incorporation according to the reaction [27]: MnO2 þ xNaþ þ yHþ þ (x þ y)ee 4 MnOONaxHy
31
specific capacitance of 1072 F g1 is obtained for the electrode with a mass loading of 0.5 mg cm2 at a current density of 1 A g1. The capacitance retention is approximately 60% from 1 to 10 A g1. Such superior capacitive behavior may be attributed to: (1) the core-shell structure of MnO2@CNTs and ultrathin MnO2 nanoflakes (less than 4 nm in thickness), which are beneficial to the insertion and extraction of electrolyte ions; (2) the ordered two dimensional woven structure of the Ni mesh substrate and the 3D network structure of interconnected CNTs, which results in lower contact resistance (illustrated in Fig. S3 in Supporting Information). When the mass loading is 0.85, 2.22, 3.50 and 5.00 mg cm2, the specific capacitance values are 684.0, 340.1, 305.9 and 288.0 F g1 at 1 A g1, respectively. The decrease of the specific capacitance with increasing MnO2 mass loading is due to: (1) the additional MnO2 generates a low proton diffusion; (2) the electrical conductivity of the MnO2@CNT composite is decreasing at high mass loading [28]. As shown in Table S2 in support information, the specific capacitance values obtained in this work are much better than previous works [29e31]. To evaluate the charge transfer and electrolyte diffusion in CNTs/ Ni and MnO2@CNTs/Ni (0.5 and 5.0 mg cm2 mass loading) electrodes, electrochemical impedance tests were carried out at 0.4 V with a frequency range from 0.1 to 105 Hz. Fig. S4 shows the Nyquist
(5)
In order to evaluate the contribution of MnO2 to the capacitance of the MnO2@CNTs/Ni electrodes, we investigated the effect of the mass loading on the capacitance value, which will be useful for practical application. Different mass loading of MnO2 from 0.5 to 5.0 mg cm2 have been loaded by changing the amount of KMnO4 during the preparation process. Fig. 5B and C shows the CV curves of MnO2@CNTs/Ni with different mass loadings of MnO2 at the scan rate of 10 mV s1 and 50 mV s1, respectively. The CV curves of low mass loading of MnO2 show much more rectangular and symmetric than that of high loaded sample. Meanwhile, with increase of scan rates from 10 to 50 mV s1, the CV curves deviate from ideal capacitor due to the large resistance in high mass loading MnO2. Fig. 5D shows the CV curves of the MnO2@CNTs/Ni (0.5 mg cm2 mass loading) at different scan rates. With increasing scan rate from 5 to 200 mV s1, the current density also increases without any obvious changes in the shape of the CV curves, indicating a very good rate performance. The rectangular and symmetric shape of the CV curves is observed at high scan rates, implying the low contact resistance of the electrodes. Fig. 6A shows the galvanostatic charge-discharge (GCD) curves of MnO2@CNTs/Ni (0.5 mg cm2 mass loading) and CNTs/Ni in the potential range from 0.2e0.8 V (vs SCE) at a current density of 1 A g1. The much longer discharging time of the MnO2@CNTs/Ni electrode than that of the CNTs/Ni electrode indicates the increase of capacitance mainly from the MnO2 component. GCD curves of the MnO2@CNTs/Ni electrode at different mass loadings and current densities are shown in Fig. 6B, C. The nearly linear and almost symmetrical curves of MnO2@CNTs/Ni indicate that the electrode exhibits a good capacitive behavior. According to equation (1), we can calculate the capacitance values at different current densities from the discharging time. Fig. 6D shows the specific capacitance values at different current densities for MnO2@CNTs/Ni with different mass loadings of MnO2. It is noted that an ultrahigh
Fig. 4. Raman spectra (A) and XRD patterns (B) of the samples. The mass loading of MnO2 (MnO2@CNTs/Ni) is 0.5 mg cm2.
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Fig. 5. Electrochemical properties of the samples in three electrode system. (A) CV curves for MnO2@CNTs/Ni, CNTs/Ni and bare Ni mesh at a scan rate of 20 mV s1. (B, C) CV curves of MnO2@CNTs/Ni with different mass loadings for MnO2. The scan rates are (B) 10 mV s1 and (C) 50 mV s1. (D) CV curves of the MnO2@CNTs/Ni at different scan rates. The mass loading of MnO2 is 0.5 mg cm2.
Fig. 6. Electrochemical properties of the samples in three electrode system. (A) Galvanostatic charge-discharge curves for MnO2@CNTs/Ni (0.5 mg cm2 mass loading of MnO2) and CNTs/Ni at 1 A g1. (B) Galvanostatic charge-discharge curves of MnO2@CNTs/Ni with different mass loadings of MnO2 at 1 A g1. (C) Galvanostatic charge-discharge curves of the MnO2@CNTs/Ni at different current densities. The mass loading of MnO2 is 0.5 mg cm2. (D) Specific capacitance values versus current density for MnO2@CNTs/Ni at different mass loadings of MnO2.
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Fig. 7. (A) Schematic structure of the flexible MnO2@CNTs/Ni based symmetric supercapacitor (SSC) in 1 M Na2SO4 aqueous solution. (B) CV curves at different voltage windows. (C) CV curves of the SSC device at different scan rates. (D) Specific capacitances of the SSC device at different scan rates. (E) Cycling performance of the SSC device at 100 mV s1 for 1000 cycles. (F) Galvanostatic charge-discharge curves of the SSC device at various current densities. (G) Specific capacitances of the SSC device at different current densities.
plots of CNTs/Ni and MnO2@CNTs/Ni. For each electrode, a semicircle and a straight line can be observed in high-frequency region and low-frequency region, respectively. Compared with
MnO2@CNTs/Ni electrodes, the almost vertical slope of CNTs/Ni impedance plot reflects the ideal capacitive behavior. As shown in the enlarged view of the high-frequency region in the inset, the
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P. Sun et al. / Journal of Power Sources 341 (2017) 27e35
diameter of the semicircle for each electrode corresponds to the charge-transfer resistance caused by the Faradaic reaction on the electrode surface. The charge-transfer resistance of the CNTs/Ni and MnO2@CNTs/Ni (0.5 and 5 mg cm2 mass loading) electrodes are about 0.18, 0.20 and 0.43 U, respectively. The electrode with low mass loading of MnO2 has higher electronic conductivity, which is beneficial to the charge transfer during the Faradaic reaction. As shown in the enlarged view of the high-frequency region, the intercept along the x axis is the internal resistance, including the electrolyte resistance, the intrinsic resistance of the active material and the contact resistance at the active material/current collector interface. The CNTs/Ni electrode shows the smallest internal resistance of 3.56 U; however, MnO2@CNTs/Ni (5 mg cm2 mass loading) electrode shows the largest resistance of 3.66 U. To further explore the advantages of this material for practical application, a symmetric supercapacitor (SSC) has been assembled from two pieces of MnO2@CNTs/Ni with the mass loading of 0.5 mg cm2 using 1 M Na2SO4 solution as electrolyte. Schematic structure of the flexible SSC is illustrated in Fig. 7A. We perform CV measurements on the MnO2@CNTs/Ni based SSC using a twoelectrode system at different working voltage at 10 mV s1 (Fig. 7B). It is noted that the working voltage can be extended to 2.0 V without obvious polarizations, which is higher than that of other MnO2-based SSCs [7,18,32]. It is well known that water decomposition is the key factor that determines the stable voltage (normally about 1 V) of a capacitive device in aqueous electrolyte system. For the our SSC high working voltage (2.0 V) can be reached, indicating as-prepared MnO2@CNTs/Ni electrode has high oxygen and hydrogen evolution over-potentials in 1 M Na2SO4 electrolyte. The CV curves of our SSC (the working voltage is 2.0 V) at different scan rates are shown in Fig. 7C. The rectangular shape and symmetry of the CV scans can be observed even at the high scan rate of 500 mV s1, indicating its good electrochemical performance. Fig. 7D shows specific capacitance of our SSC at different scan rates. The capacitance retention from 10 to 500 mV s1 is up to 43.1% (140.2 F g1 at 10 mV s1 and 60.4 F g1 at 500 mV s1). As presented in Fig. 7E, the long-term stability of the SSC was examined by CV cycling at a scan rate of 100 mV s1 and the SSC keeps the capacitance retention of 83.4% after 1000 cycles. Fig. 7F shows the GCD curves of the MnO2@CNTs/Ni based SSC at different current densities over the working voltage of 0e2.0 V. The
symmetrical charge-discharge characteristics represents a good capacitive characteristic for our supercapacitor. The specific capacitance of the SSC at different current densities are shown in Fig. 7G. Calculated from Fig. 7F according equation (1), the specific capacitances are 170.0 F g1 at 1 A g1 and 91.0 F g1 at 20 A g1. The capacitance retention from 1 to 20 A g1 is up to 53.5%. According to equation (2) and equation (3), we can calculate energy density and power density from specific capacitances in Fig. 7C and F. As shown in Fig. 8A, the highest energy density of 78.0 Wh kg1 (at a power density of 1402 W kg1) and the highest power density of 30.2 kW kg1 (at an energy density of 33.6 Wh kg1) have been obtained from CV curves. According to GCD curves, the highest energy density of 94.4 Wh kg1 (at a power density of 1000 W kg1) and the highest power density of 20.0 kW kg1 (at an energy density of 55.6 Wh kg1) are achieved at a working voltage of 2.0 V. These results are much higher than that of other reported MnO2-based SSCs or ASCs, such as CNPs-MnO2//CNPs-MnO2 (CNPs: carbon nanoparticles) SSC (4.8 Wh kg1,14.0 kW kg1) [7], graphene-MnO2//graphene-MnO2 SSC (6.8 Wh kg1, 62.0 W kg1) [28], graphene-MnO2//ACN (ACN: activated carbon nanofiber) ASC (51.1 Wh kg1, 102.2 W kg1) [20], AC//MnO2-CNTs ASC (13.3 Wh kg1, 600.0 W kg1) [32], Ni(OH)2-MnO2-RGO//FRGO ASC (54.0 Wh kg1, 392.0 W kg1) [33], AGMn//aMEGO ASC (20.8 Wh kg1, 32.3 kW kg1) [34], UPMNFs//FMCNTs ASC (47.4 Wh kg1, 200.0 W kg1) [35] and AC//K0.27MnO2$0.6H2O ASC (25.3 Wh kg1, 140.0 W kg1) [36] (see Table S3 in Supporting Information). To further demonstrate the practical application, our SSC device can successfully power a green light-emitting diode (LED, working voltage: 1.5 V) for 5 min, after being charged at 10 A g1 for 24 s as shown in Fig. 8B. Impressively, two SSC devices in series can even power 12 LEDs for more than 1 min (Fig. 8B). 4. Conclusion The MnO2@CNTs/Ni composite is prepared by a facile two-step method on a flexible and metallic Ni mesh under a mild reaction condition, in which MnO2 nanoflakes and CNTs forming a hierarchical core-shell structure. In such a unique structure, the electrode exhibits a high specific capacitance of 1072 F g1 at 1 A g1 in 1 M Na2SO4 electrolyte. The as-assembled MnO2@CNTs/Ni based SSC exhibits a wide working voltage (2.0 V) in 1 M Na2SO4 electrolyte,
Fig. 8. (A) Ragone plot for the MnO2@CNTs/Ni based SSC with other MnO2-based supercapacitors reported previously. (B) One device can power a green light-emitting (LED, working voltage: 1.5 V) and two SSC devices in series can power 12 LEDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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high power density (30.2 kW kg1 at an energy density of 33.6 Wh kg1), high energy density (94.4 Wh kg1 at a power density of 1000 W kg1) and long-term cycle stability (keeps a retention of 83.4% after 1000 cycles). For practical application, one SSC was assembled to power 1 LED for 5 min and two SSC devices in series can power 12 LEDs for more than 1 min. In conclusion, the MnO2@CNTs/Ni composite is a very promising electrode material for assembling capacitor with both high power density and high energy density. Acknowledgements The authors gratefully acknowledge the financial support offered by NSFC Grants (21373152). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.11.112. References [1] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520e2531. [2] C.B.E., Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer Science & Business Media, 2013. [3] C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Design and tailoring of the nanotubular arrayed architecture of hydrous RuO2 for next generation supercapacitors, Nano Lett. 6 (2006) 2690e2695. [4] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities, Adv. Energy Mater. 4 (2014) 1300816. langer, Charge storage mechanism of MnO2 electrode used [5] B.T. Toupin M, D. Be in aqueous electrochemical capacitor, Chem. Mater. 16 (2004) 3184e3190. [6] X. Lang, A. Hirata, T. Fujita, M. Chen, Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors, Nat. Nanotechnol. 6 (2011) 232e236. [7] L. Yuan, X.H. Lu, X. Xiao, T. Zhai, J. Dai, F. Zhang, B. Hu, X. Wang, L. Gong, J. Chen, C. Hu, Y. Tong, J. Zhou, Z.L. Wang, Flexible solid-state supercapacitors based on carbon nanoparticles/MnO2 nanorods hybrid structure, ACS Nano 6 (2012) 656e661. [8] Z.S. Wu, W. Ren, D.W. Wang, F. Li, B. Liu, H.M. Cheng, High-energy MnO2 nanowire/graphene and graphene asymmetric electrochemical capacitors, ACS Nano 4 (2010) 5835e5842. [9] Y. Jin, H. Chen, M. Chen, N. Liu, Q. Li, Graphene-patched CNT/MnO2 nanocomposite papers for the electrode of high-performance flexible asymmetric supercapacitors, ACS Appl. Mater. interfaces 5 (2013) 3408e3416. [10] L. Hu, W. Chen, X. Xie, N. Liu, Y. Yang, H. Wu, Y. Yao, M. Pasta, H.N. Alshareef, Y. Cui, Symmetrical MnO2-carbon nanotube-textile nanostructures for wearable pseudocapacitors with high mass loading, ACS Nano 5 (2011) 8904e8913. [11] W. Xiao, H. Xia, J.Y.H. Fuh, L. Lu, Growth of single-crystal a-MnO2 nanotubes prepared by a hydrothermal route and their electrochemical properties, J. Power Sources 193 (2009) 935e938. [12] T.M. Higgins, D. McAteer, J.C. Coelho, B. Mendoza Sanchez, Z. Gholamvand, G. Moriarty, N. McEvoy, N.C. Berner, G.S. Duesberg, V. Nicolosi, J.N. Coleman, Effect of percolation on the capacitance of supercapacitor electrodes prepared from composites of manganese dioxide nanoplatelets and carbon nanotubes, ACS Nano 8 (2014) 9567e9579. [13] K.W.S. An K H, Y.S. Park, et al., Electrochemical properties of high-power supercapacitors using single-walled carbon nanotube electrodes, Adv. Funct. Mater. 11 (2001) 387e392. [14] B.F. Frackowiak E, Carbon materials for the electrochemical storage of energy
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