- Email: [email protected]
Fabrication of manganese dioxide nanoplates anchoring on biomass-derived cross-linked carbon nanosheets for high-performance asymmetric supercapacitors
Journal of Power Sources 300 (2015) 309e317
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Fabrication of manganese dioxide nanoplates anchoring on biomassderived cross-linked carbon nanosheets for high-performance asymmetric supercapacitors Yiju Li a, Neng Yu b, Peng Yan a, Yuguang Li c, Xuemei Zhou c, Shuangling Chen c, Guiling Wang a, *, Tong Wei a, Zhuangjun Fan a, ** a Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China c 101 Institute of the Ministry of Civil Affairs, Beijing 100070, China
h i g h l i g h t s The CCNs was derived from a common biomass of Willow catkin. The MnO2@CCNs composite shows good capacitive performance. The assembled asymmetric cell shows high energy density.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2015 Received in revised form 29 August 2015 Accepted 19 September 2015 Available online xxx
In this paper, MnO2 nanoplates loading on biomass-derived cross-linked carbon nanosheets have been prepared by a two-step synthesis. At first, the cross-linked carbon nanosheets derived from willow catkin are synthesized by one-step pyrolysis and activation method, then the MnO2 anchored cross-linked carbon nanosheets is prepared via in-situ hydrothermal deposition. The asymmetric supercapacitor with terrific energy and power density is assembled by employing the MnO2 anchored cross-linked carbon nanosheets as the positive electrode and the cross-linked carbon nanosheets as the negative electrode in a 1 M Na2SO4 electrolyte. The asymmetric supercapacitor displays a high energy density of 23.6 Wh kg1 at a power density of 188.8 W kg1 within a wide voltage rage of 0e1.9 V. In addition, the asymmetric supercapacitor exhibits excellent cycling stability with only 1.4% capacitance loss after 10000 cycles at 1 A g1. These discoveries open up the prospect of biomass/biowaste derived carbon-based composites for high-voltage asymmetric supercapacitors with superb energy and power density performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Asymmetric supercapacitor Biomass Carbon nanosheets Manganese dioxide Composite
1. Introduction Over the past decades, with the excessive consumption of fossil energy and the increasingly severe environment pollution problem, exploring green, high-efficiency, and sustainable energy sources, as well as advanced energy conversion and storage technologies is an urgent task needed to be solved. Supercapacitor, as a sort of typical
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), fanzhuangjun@hrbeu. edu.cn (Z. Fan). http://dx.doi.org/10.1016/j.jpowsour.2015.09.077 0378-7753/© 2015 Elsevier B.V. All rights reserved.
energy storage device with excellent power density and long lifespan, has attracted growing attention [1e5]. However, the inferior energy density of conventional electrochemical capacitor has limited their applications. To solve the problem, construction of asymmetric systems has been extensively explored by combining a Faradic pseudocapacitor-like electrode (positive electrode) and an electric double layer capacitive electrode (negative electrode) to increase the operation voltage, which can significantly improve the energy density [6e10]. At present, activated carbon (AC) particulates with a high specific surface area are the most common carbon material for the negative electrode of asymmetric supercapacitor [11,12],
310
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
Nevertheless, the power characteristics of AC are limited due to the minuscule pores and blocked pore texture, which is hardly accessible to hydrate ions. As a rising star of carbon material, graphene could be a candidate for the negative electrode of asymmetric supercapacitor for its high surface area and superb electrical conductivity [13e17]. But most of methods for synthesis of graphene often require complicated synthesis conditions or special facilities, leading to the high cost. Therefore, it is a preponderant way for facile direct pyrolysis carbon precursors, such as biomass or biowaste, into graphene-like carbon nanosheets [18e22]. As a common biomass, willow catkin containing abundant cellulose is inedible and has no economic value. Fortunately, the willow catkin with hollow and multilayer morphology can be facilely converted into value-added thin carbon nanosheets via simple pyrolysis and alkali activation. This kind of carbon nanosheets, which possess relatively flat adsorption surfaces and interconnected networks as well as relatively high in-plane electrical conductivity, has superior electrochemical performance and is able to serve as conductive carbon substrates to anchor metal oxide. Among the multifarious pseudocapactive metal oxides, MnO2 is one of the most promising positive electrode materials for low-cost and environment friendly asymmetric supercapacitor devices. Especially, a combination of MnO2 pseudocapactive electrode with carbon material electrode can reach up to a high voltage of 2 V, far beyond theoretical water decomposition voltage (1.23 V). However, the intrinsically poor electronic conductivity of the MnO2 immensely confines the improvement of specific capacitance. Hence, it is an alternative route to anchor MnO2 on 3D cross-linked carbon nanosheets (CCNs) to enhance the electrical conductivity of the electrode and thus potentially improve the capacitive performance of asymmetric supercapacitor. Here, we reported the MnO2 nanoplates anchoring on biomassderived CCNs (MnO2@CCNs) have been prepared by a two-step synthesis and the asymmetric supercapacitor assembled by employing MnO2@CCNs as positive electrode and CCNs as negative electrode exhibits a superb energy density of 23.6 Wh kg1, significantly superior than those of symmetrical supercapacitor based on pure carbon materials [23e26] and higher than those of other MnO2-based asymmetric supercapacitors [27e31]. 2. Experimental 2.1. Synthesis of CCNs Briefly, the collected willow catkin was dispersed in acetone and then under ultrasound for 30 min. Next, the willow catkin was washed by DI water and dealt with violent stirring for getting rid of seeds and impurities. Then the willow catkin with brief cutting was poured into 100 mL potassium hydroxide solution and heated at 100 C until all of the DI water was evaporated. The remnant mixture was heated up to 400 C at a heating rate of 5 C min1 for 3 h and then continually heated up to 850 C at a heating rate of 10 C min1 for 1 h in a tubular furnace under argon atmosphere. 2.2. Synthesis of MnO2@CCNs composite MnO2@CCNs composite was synthesized by in-situ hydrothermal deposition method. In a typical process, 0.1 g of as-prepared CCNs was added into 90 mL of 0.04 M KMnO4 solution. Subsequently, the mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated up to 120 C from room temperature at a heating rate of 2 C min1 for 1 h in an electric furnace. Then, the mixture was naturally cooled down to room temperature. The reaction during the hydrothermal process is assumed to be as follows:
4KMnO4 þ 3C þ H2O ¼ 4MnO2 þ K2CO3 þ 2KHCO3
(1)
Next, the composite was through vacuum filtration and washed with DI water. Finally, the obtained material was dried at 60 C for 12 h in vacuum oven. The brief flow diagram of preparation process was illustrated in Fig. 1. 2.3. Materials characterizations The crystalline phase of as-prepared materials were analyzed using X-ray diffractometer (XRD, Rigaku TTR III) with Cu Ka radiation (l ¼ 0.1514178 nm). Netzsch STA 449C analyzer was employed to analyze the weight percent of MnO2 from room temperature to 800 C at a heating rate of 5 C min1 under N2 atmosphere (TGA). Scanning electron microscopy (SEM, JEOL JSM-6480) and transmission electron microscopy (TEM, FEI Teccai G2 S-Twin, Philips) were used to characterize the microstructure. Nitrogen adsorptionedesorption analysis was conducted via utilizing Micromeritics ASAP 2020 physisorption analyzer at 77 K. The specific surface area was calculated by the BrunauereEmmetteTeller (BET) method using the adsorption branch of nitrogen isotherm. The information of surface chemical species was revealed by X-ray photoelectron spectroscopy with Al Ka radiation (XPS, Thermo ESCALAB 250). 2.4. Electrochemical measurements All electrochemical measurements were conducted in 1 M NaSO4 electrolyte. In a typical three-electrode set-up, the asprepared electrode (1 1 cm2) served as the working electrode, a platinum foil (2 1 cm2) acted as the counter electrode, and an Ag/ AgCl electrode was selected as the reference electrode. In the threeelectrode system, the mass loading of active materials on the working electrode is about 6.3 mg. The cyclic voltammetry (CV) tests, galvanostatic chargeedischarge (GCD) measurements and electrochemical impedance spectroscopy (EIS) were carried out by using a computerized potentiostat (VMP3/Z Bio-Logic) controlled by the EC-lab software. CV and GCD tests in the three-electrode system were conducted between 0 and 0.9 V and EIS measurements were performed by applying an alternating voltage with 5 mV amplitude in a frequency scope from 10 mHz to 100 kHz at open circuit potential. The asymmetric two-electrode cells were assembled using MnO2@CCNs as positive electrode and CCNs as negative electrode and were conducted in the voltage range of 0e1.9 V. The tests of cycle life were performed on a NEWARE battery program-control test system (CT-3008W). The mass loading ratio of active materials (MnO2@CCNs/CCNs) was determined according to the follow equation:
mþ C DE ¼ m Cþ DEþ
(2)
where m is mass loading of the active material in the electrode, C is the specific capacitance under a specific current or scan rate and DE is the potential range of each electrode. The subscripts þ and e represent the positive and negative electrodes, respectively. The specific capacitance measured by the galvanostatic chargeedischarge method can be calculated based on the following equation:
Cm ¼
Id Dt DV m
(3)
where Cm (F g1) is specific capacitance, Id (mA) is the discharge current, Dt (s) is the discharge time and DV (V) is the discharge potential or voltage range. In three-electrode system, m (mg)
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
311
Fig. 1. The schematic illustration of the preparation of MnO2@CCNs composite.
represents the mass loading of active material in a single electrode. In two-electrode cell, m (mg) is the total mass loading of active materials based on both electrodes. The energy density E (Wh kg1) and power density P (W kg1) of an asymmetric supercapacitor in the Ragone plots were calculated according to the following equation:
E¼
1 1 Cm DV 2 2 3:6
(4)
P¼
E 3600 Dt
(5)
where Cm (F g1) is the specific capacitance of asymmetrical capacitor which is calculated based on total mass loading of active materials in both electrodes, DV (V) is the discharge voltage range (excluding the IR drop) and Dt (s) is the discharge time. As we all known, the capacitance C(w) based on impedance measurements can be defined as follows: 00
C ¼ C 0 ðwÞ jC ðwÞ
(6)
00
C 0 ðwÞ ¼
C 00 ðwÞ ¼
Z ðwÞ wjZðwÞj2 Z 0 ðwÞ wjZðwÞj2
(7)
(8)
where w is the angular frequency, C 0 ðwÞ is the real part and C 00 ðwÞ is the imaginary part of the cell capacitance. Z 0 ðwÞ and Z 00 ðwÞ are the real and imaginary parts of the complex impedance Z(w), respectively. 3. Results and discussion 3.1. Characterization of materials The CCNs sample was synthesized by one-step pyrolysis and activation treatment of natural abundant willow catkin (Fig. 2A). One interesting finding is that the structure of willow catkin is hollow and thin-walled (Fig. 2B), which contributes to the KOH permeation and carbon nanosheets exfoliation during the pyrolysis process. Treated with pyrolysis and KOH activation, the willow catkin was successfully converted into interconnected thin carbon nanosheets (Fig. 2C). Such 3D cross-linked carbon nanosheets can offer conducted networks for fast electrons transfer and serve as an ideal platform for anchoring active materials. Moreover, it can be easily discovered that the MnO2 is uniformly dispersed on the CCNs
surfaces after hydrothermal deposition (Fig. 2D). The TEM images, shown in the inset of Fig. 2D, further confirm the homogeneous distribution of MnO2 nanoplates. Fig. 3A shows the XRD patterns of CCNs and MnO2@CCNs composite. For CCNs, the diffraction peak centered at ~25 can be ascribed to the (002) plane of the partially ordered CCNs [32]. Except for the diffraction peak of (002), another two diffraction peaks appearing at 37.1 and 65.6 for MnO2@CCNs composite are indexed to the diffractions of (311) and (440) planes of a-MnO2, respectively (JCPDS card No. 42-1169). The broad peaks demonstrate the poor crystallinity and almost amorphous nature of the MnO2 nanoplates [33]. The information on the surface chemical composition was obtained from XPS. The wide-scan XPS spectrum of MnO2@CCNs composite confirms the existence of C, O, and Mn (Fig. 3B). The high resolution spectrum of O 1s band is presented in Fig. 3C. The XPS peaks of 529.5 and 531.6 eV can be assigned to MneOeMn and MneOeH, respectively. For Mn 2p pattern, two main peaks located at 653.6 eV and 642.0 eV correspond to the Mn 2p1/2 and Mn 2p3/2 with a spin-energy separation of 11.6 eV, further confirming the presence of a-MnO2 in the composite (Fig. 3D) [34]. The specific surface area of the CCNs and MnO2@CCNs composite is determined by the nitrogen adsorptionedesorption isotherms (Fig. 3E). For the CCNs derived from willow catkin, its specific surface area is up to 1507 m2 g1. Therefore, the carbon nanosheets with such a high specific surface area are suitable for working as the backbone materials to combine with the active materials of MnO2. With the sacrifice of carbon and formation of MnO2 nanoplates, the MnO2 nanoplates homogenously anchor on the surface of CCNs and will block some nanopores and micropores of CCNs. Therefore, the specific surface of MnO2@CCNs composite will be undoubtedly smaller than pure CCNs and the pore distribution will be changed. But the specific surface area of the MnO2@CCNs composite can still reach up to 234 m2 g1. Such an acceptable specific surface area of MnO2@CCNs composite contributes to the sufficient contact between active material and electrolyte. Fig. 3F shows the pore distributions calculated by density functional theory (DFT) model. The pore size distribution of MnO2@CCNs composite mainly focuses on ~1.6 nm. The pore texture of such large micropores is favorable for the ions access [19]. In order to accurately ascertain the weight percent of MnO2 in MnO2@CCNs composite, TGA measurement was conducted and the curve is exhibited in Fig. 3G. The TGA plot shows two weight loss stages. The first step of apparent mass loss (about 8.5%) from room temperature to ~200 C can be attributed to the loss of adsorbed water and crystalline water in the MnO2@CCNs composite. The second step of mass loss (about 22.5%) is assigned to the thermal decomposition of the CCNs [35]. The residual substance did not show obvious weight loss until the terminal of the test. Accordingly, the percentage of MnO2 in the composite is calculated
312
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
Fig. 2. The photograph of willow catkin (A). The SEM images of willow catkin (B), CCNs (C) and MnO2@CCNs composite (D). The TEM image of MnO2@CCNs composite (Inset in Fig. 2D).
to be ~75.4%. 3.2. Electrochemical measurements Fig. 4 demonstrates the electrochemical performance of CCNs and MnO2@CCNs composite measured in a three-electrode system. Both of the CV and GCD tests were conducted in 1 M NaSO4 electrolyte within potential windows ranging from 0 to 0.9 V. The CV curves and GCD plots of CCNs are displayed in Fig. 4A and C, respectively. The nearly rectangular CV curves suggest the ideal capacitive behavior of the CCNs material. Even at a higher scan rate of 100 mV s1, the shapes of CV curves remain quasi rectangle, signifying the intrinsic good conductivity of CCNs and low contact resistance in the electrode. The GCD curves of CCNs electrode at current density of 0.2, 0.5, 1, 2 and 5 A g1 were shown in Fig. 4C. The nearly symmetric chargeedischarge curve reveals excellent reversibility and high coulombic efficiency. For MnO2@CCNs composite (Fig. 4B), it can be observed that the CV curves show relatively rectangular shape for scan rates up to 20 mV s1, and at the same scan rate the resultant current density from MnO2@CCNs composite is much higher than that of pure CCNs owing to the loading of the pseudocapacitive MnO2 nanoplates. With the scan rate increasing (50 and 100 mV s1), the current densities increase and the shape of CV curves displays some distortions from an ideal rectangle, indicating a deviation from the perfect capacitive behavior. This is probably due to increasing overpotentials from ion transport between the electrolyte and MnO2 nanoplates. The GCD curves of MnO2@CCNs composite (Fig. 4D) at various current densities show longer chargeedischarge times than those of pure CCNs, which is consistent with the results of CV curves. In addition, the specific capacitance versus current density is summarized in Fig. 4E. Apparently, the MnO2@CCNs composite demonstrate the improved specific capacitance than pure CCNs. The specific capacitances are 262, 217, 189, 162 and 133 F g1 when the current
densities increase to 0.2, 0.5, 1, 2 and 5 A g1, respectively, which is larger than those of CCNs and comparable to those of other reported MnO2-based composites [19,36,37]. The results of comparison are listed in Table 1. However, it is worth mentioning that the capacitance retention of CCNs (60.2%) is superior to that of MnO2@CCNs composite (50.7%) with current density enlarging from 0.2 A g1 to 5 A g1. EIS was employed for further investigating the electrochemical performance of the as-prepared samples (Fig. 4F). The steep linear curve in the low frequency region suggests nearly ideal capacitive performance and the semi-circle in the high frequency region indicates the charge transfer resistance. It is clear that the CCNs electrode shows a nearly vertical line along the imaginary coordinates in the low frequency region, which demonstrates its ideal double electric layer capacitive behavior. However, the MnO2@CCNs composite electrode exhibits a more inclined curve in the same low frequency region, implying the major performance of pseudo-capacitance. In the high frequency region, the charge transfer resistance of CCNs (1.47 U) is a little less than that of MnO2@CCNs composite (1.89 U) through the fitting of equivalent circuit diagram. This is expected since the charge transfer of MnO2@CCNs composite involves redox processes that are more sluggish than simple surface adsorption and disadsorption of double electric layer capacitance. Especially, the ion diffusion process can be expressed by the length of the Warburg-type line (the slope of the 45 part of the Nyquist plots in the middle frequency region) [38]. The Warburg-type line of CCNs is much short. That means the rapid ion diffusion in the 3D interconnected conductive networks of carbon nanosheets. Moreover, the bode phase curve of CCNs (Fig. 4G) show that the phase angle is ~83 and is approaching to that of an ideal capacitor of 90 [39]. As a comparison, the phase angle of MnO2@CCNs composite is only ~69 , which is much less than that of ideal capacitor. This is because the MnO2@CCNs composite primarily displays the pseudocapacitive performance.
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317 Fig. 3. The XRD patterns of CCNs and MnO2@CCNs composite (A). The wide-scan XPS spectrum of MnO2@CCNs composite (B). The high-resolution O 1s XPS spectrum (C) and Mn 2p XPS spectrum (D) of MnO2@CCNs composite. The N2 adsorptionedesorption isotherms (E) and pore size distributions (F) of CCNs and MnO2@CCNs composite. The TGA curve of the MnO2@CCNs composite (G).
313
314 Y. Li et al. / Journal of Power Sources 300 (2015) 309e317 Fig. 4. The electrochemical performance conducted in three-electrode setup. The CV curves of CCNs (A) and MnO2@CCNs composite (B). The galvanostatic chargeedischarge curves of CCNs (C) and MnO2@CCNs composite (D). The specific capacitance versus current density (E). The Nyquist plots (F) and bode phase diagrams of CCNs (C) and MnO2@CCNs composite (G).
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
315
Table 1 Performance comparison of the MnO2ecarbon composite based electrodes. Samples
Sa
Vrb (V)
Elc
Cd (F g1)
Ref.
MnO2/rGO hydrogel MnO2-GAs MnO2@R-GO@Ni-foam GW-MnO2 MnO2$xH2O/carbon aerogel CMG15 Birnessite-type MnO2/AC Carbon@MnO2 coreeshell nanosphere Amorphous MnO2/microporous carbon sphere MnO2/C@CNT MnO2@CCNs
2 A g1 0.5 A g1 1 A g1 1 A g1 2 mV s1 1 A g1 1 A g1 2 mV s1 2 mV s1 0.2 A g1 0.2 A g1
0e1 0e0.7 0e0.8 0e1 0e1 0e1 0.2e0.8 0e0.8 0e0.8 0.1e0.8 0e0.9
1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 6 M KOH 1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 0.5 M Na2SO4 1 M Na2SO4
214 210 225 194 226 111.1 210 252 218 227 262
[15] [31] [32] [16] [36] [17] [35] [30] [37] [38] This work
a b c d
Scan rate or current density. Voltage range. Electrolyte type. Specific capacitance.
Asymmetric energy storage devices consisting of diverse types of electrodes have been extensively investigated recently owing to the wide operating voltage thus exhibiting high energy density. We constructed an asymmetric supercapacitor by employing MnO2@CCNs composite as the positive electrode and CCNs as the negative electrode in 1 M Na2SO4 solution. Fig. 5 displays the schematic diagram of the assembled structure for such an asymmetric cell. CV measurements were first utilized to estimate the electrochemical potential ranges of individual electrode in a threeelectrode set-up. The stable potential window is 1.0-0 V for the CCNs electrode while it is 0e0.9 V for MnO2@CCNs composite electrode (Fig. 6A). The specific capacitances calculated from the CV curves are 188 F g1 and 141 F g1 for MnO2@CCNs composite and CCNs electrode, respectively. So the mass loading ratio between the electrodes (mþ/m-) is about 1:1.2 according to the equation (2). The mass loadings of active materials on negative electrode (CCNs) and positive electrode (MnO2@CCNs composite) are about 5.4 mg and 4.5 mg, respectively. As shown in Fig. 6B, the MnO2@CCNs//CCNs asymmetric supercapacitor exhibits a stable voltage window up to 1.9 V and presents an ideal capacitive behavior with relatively rectangular CV curves even at a scan rate up to 100 mV s1. The GCD plots of the asymmetric supercapacitor under various current
densities are shown in Fig. 6C. The inset demonstrates the charge/ discharge curves of the individual electrodes vs. the Ag/AgCl reference electrode and the as-assembled asymmetric supercapacitor at a current density of 2 A g1. The current densities are determined based on the total weight of MnO2@CCNs positive electrode and CCNs negative electrode. It is apparent that the voltage is almost proportional to the chargeedischarge time, demonstrating a typical capacitive performance. Moreover, the charge time is nearly equal to the discharge time, which indicates super-high coulombic efficiency. Especially, the low IR drop for MnO2@CCNs//CCNs asymmetric supercapacitor manifests that the assembled asymmetric cell has low internal resistance. The specific capacitance changed with current density can intuitively reveals the rate performance (Fig. 6D). It is can be observed that the MnO2@CCNs//CCNs asymmetric supercapacitor can exhibit the specific capacitance of 47.4 F g1 at 0.2 A g1. Even at a high chargeedischarge rate of 5 A g1, the asymmetric cell still has a specific capacitance of 32.1 F g1, suggesting excellent rate performance. The cycle life test was used to evaluate the long-term cycling stability of the asymmetric supercapacitor and was measured by the consecutive galvanostatic charge and discharge at current density of 1.0 A g1 in a voltage window of 0e1.9 V. It is exciting that the asymmetric supercapacitor has superb cycling stability (Fig. 6E).
Fig. 5. The schematic of the assembled asymmetric supercapacitor based on MnO2@CCNs composite as positive electrode and CCNs as negative electrode.
316 Y. Li et al. / Journal of Power Sources 300 (2015) 309e317 Fig. 6. The electrochemical performance of MnO2@CCNs//CCNs asymmetric supercapacitor. Comparative CV curves of CCNs and MnO2@CCNs composite electrodes in a three-electrode set-up at a scan rate of 10 mV s1 (A). The CV curves at different scan rates (B). The galvanostatic chargeedischarge profiles at different current densities. The inset shows the charge/discharge curves of the individual electrodes vs. the Ag/AgCl reference electrode and the asassembled asymmetric supercapacitor at a current density of 2 A g1. (C). The specific capacitance versus current density (D). The specific capacitance and columbic efficiency versus the number of cycles tested at 1 A g1 (E). The normalized real (F) and imaginary (G) part capacitance versus frequency. The Ragone plots of the asymmetric cell (H). Performance comparison of the MnO2@CCNs//CCNs asymmetric cell versus previously reported MnO2-based asymmetric supercapacitors in aqueous electrolyte (I).
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
Only 1.4% capacitance loss is observed after 10000 cycles at 1 A g1, and the coulombic efficiency remains above 95%. This cycling performance is much superior than that of some reported asymmetric cells, such as rGO//MnO2/rGO (89.4% retention after 1000 cycles) [15], graphene//MGC (79% of the initial capacitance after 1000 cycles) [40], graphene/MnO2-textile//CNT-textile (~95% retention over 5000 cycles) [41] and GH//MnO2eNF (83.4% retention over 5000 cycles) [34]. Fig. 6FeG shows the frequency response of normalized C 0 ðwÞ and C 00 ðwÞ before cycle and after 10000 cycles, respectively. As we all known, the operating frequency (fo) and characteristic relaxation time constant (to) are the important quantitative indexes of how fast the cell can be charged and discharged reversibly [19]. Through 10000 cycles, the MnO2@CCNs// CCNs asymmetric cell still has an operating frequency of 0.21 Hz (corresponding to to ¼ 4.76 s), which is little inferior than that of 0.29 Hz before cycle (corresponding to to ¼ 3.44 s). The result shows that the MnO2@CCNs//CCNs asymmetric cell remains fast response time despite undergoing ultra-long cycle life test. Fig. 6H shows the Ragone plot of power density (P) and energy densities (E) considering the total loading mass of both positive and negative electrode materials and excluding the IR drop. It is worth noting that the maximum energy density for the asymmetric supercapacitor is 23.6 Wh kg1 at a power density of 188.8 W kg1, which is comparable or higher than those of other MnO2-based asymmetric cells in aqueous electrolyte solutions (Fig. 6I) [9,28,29,40,42]. Furthermore, this assembled asymmetric cell displays a good power characteristic, and it keeps an acceptable energy density of 12.1 Wh kg1 when the power density reaches up to 3570.5 W kg1. All of the above results suggest that this sort of lowcost, eco-friendly and high-performance asymmetric supercapacitor is expected to be a promising energy storage device in portable electronics and electric vehicles.
[22]
4. Conclusions
[23] [24]
In this work, we successfully prepared the cross-linked carbon nanosheets derived from the common biomass of willow catkin using one-step facile pyrolysis and activation method. The interconnected thin carbon nanosheets serve as 3D conductive networks for fast electrons transfer and offer a fine platform for anchoring pseudocapacitive active materials. The MnO2 nanoplates were uniformly distributed on the CCNs by in-situ hydrothermal deposition. When employing the MnO2@CCNs composite as the positive electrode and the CCNs as the negative electrode, the assembled asymmetric supercapacitor exhibits a remarkable energy density of 23.6 Wh kg1 with a wide voltage range of 0e1.9 V. Moreover, the asymmetric cell exhibits superb cycling stability with only 1.4% capacitance loss after 10000 cycles at 1 A g1. Therefore, we hold a positive attitude that such low-cost and highperformance asymmetric supercapacitor realized by earthabundant and eco-friendly materials can offer great promise in the application of high-performance energy storage devices. Notes The authors declare no competing financial interest. Acknowledgments We gratefully acknowledge the financial support of this research
317
by the National Natural Science Foundation of China (No. 51572052).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797e828. S. Liu, S. Sun, X.-Z. You, Nanoscale 6 (2014) 2037e2045. L.L. Zhang, X. Zhao, Chem. Soc. Rev. 38 (2009) 2520e2531. G.A. Snook, P. Kao, A.S. Best, J. Power Sources 196 (2011) 1e12. V. Augustyn, P. Simon, B. Dunn, Energy Environ. Sci. 7 (2014) 1597e1614. Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Adv. Funct. Mater. 21 (2011) 2366e2375. J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi, F. Wei, Adv. Funct. Mater. 22 (2012) 2632e2641. H. Chen, S. Zhou, M. Chen, L. Wu, J. Mater. Chem. 22 (2012) 25207e25216. J. Cao, Y. Wang, Y. Zhou, J.-H. Ouyang, D. Jia, L. Guo, J. Electroanal. Chem. 689 (2013) 201e206. C. Zhou, Y. Zhang, Y. Li, J. Liu, Nano Lett. 13 (2013) 2078e2085. Q. Qu, L. Li, S. Tian, W. Guo, Y. Wu, R. Holze, J. Power Sources 195 (2010) 2789e2794. V. Ganesh, S. Pitchumani, V. Lakshminarayanan, J. Power Sources 158 (2006) 1523e1532. L. Deng, G. Zhu, J. Wang, L. Kang, Z.-H. Liu, Z. Yang, Z. Wang, J. Power Sources 196 (2011) 10782e10787. Y. Jin, H. Chen, M. Chen, N. Liu, Q. Li, ACS Appl. Mater. Interfaces 5 (2013) 3408e3416. S. Wu, W. Chen, L. Yan, J. Mater. Chem. A 2 (2014) 2765e2772. J. Zhu, J. He, ACS Appl. Mater. Interfaces 4 (2012) 1770e1776. S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, ACS Nano. 4 (2010) 2822e2830. H. Wang, Z. Xu, A. Kohandehghan, Z. Li, K. Cui, X. Tan, T.J. Stephenson, C.K. King'ondu, C.M. Holt, B.C. Olsen, ACS Nano. 7 (2013) 5131e5141. H. Wang, Z. Li, J.K. Tak, C.M. Holt, X. Tan, Z. Xu, B.S. Amirkhiz, D. Harfield, A. Anyia, T. Stephenson, Carbon 57 (2013) 317e328. ~ ero, F. Leroux, F. Be guin, Adv. Mater. 18 (2006) 1877e1882. E. Raymundo-Pin M. Sevilla, W. Gu, C. Falco, M. Titirici, A. Fuertes, G. Yushin, J. Power Sources 267 (2014) 26e32. X. Liu, M. Zheng, Y. Xiao, Y. Yang, L. Yang, Y. Liu, B. Lei, H. Dong, H. Zhang, H. Fu, ACS Appl. Mater. Interfaces 5 (2013) 4667e4677. Z. Lei, N. Christov, L.L. Zhang, X.S. Zhao, J. Mater. Chem. 21 (2011) 2274e2281. T.E. Rufford, D. Hulicova-Jurcakova, Z. Zhu, G.Q. Lu, Electrochem. Commun. 10 (2008) 1594e1597. B.G. Choi, M. Yang, W.H. Hong, J.W. Choi, Y.S. Huh, ACS Nano. 6 (2012) 4020e4028. W. Xing, S. Qiao, R. Ding, F. Li, G. Lu, Z. Yan, H. Cheng, Carbon 44 (2006) 216e224. H. Xia, Y. Wang, J. Lin, L. Lu, Nanoscale Res. Lett. 7 (2012) 1e10. C.-C. Liu, D.-S. Tsai, W.-H. Chung, K.-W. Li, K.-Y. Lee, Y.-S. Huang, J. Power Sources 196 (2011) 5761e5768. T. Cottineau, M. Toupin, T. Delahaye, T. Brousse, D. Belanger, Appl. Phys. A 82 (2006) 599e606. Y. Zhao, Y. Meng, P. Jiang, J. Power Sources 259 (2014) 219e226. C.-C. Ji, M.-W. Xu, S.-J. Bao, Z.-J. Lu, C.-J. Cai, H. Chai, R.-Y. Wang, F. Yang, H. Wei, New J. Chem. 37 (2013) 4199e4205. Y. Li, D. Cao, Y. Wang, S. Yang, D. Zhang, K. Ye, K. Cheng, J. Yin, G. Wang, Y. Xu, J. Power Sources 279 (2015) 138e145. C. Long, D. Qi, T. Wei, J. Yan, L. Jiang, Z. Fan, Adv. Funct. Mater. 24 (2014) 3953e3961. H. Gao, F. Xiao, C.B. Ching, H. Duan, ACS Appl. Mater. Interfaces 4 (2012) 2801e2810. J. Han, L.L. Zhang, S. Lee, J. Oh, K.-S. Lee, J.R. Potts, J. Ji, X. Zhao, R.S. Ruoff, S. Park, ACS Nano. 7 (2012) 19e26. J. Li, X. Wang, Q. Huang, S. Gamboa, P.J. Sebastian, J. Power Sources 160 (2006) 1501e1505. W. Wei, X. Huang, Y. Tao, K. Chen, X. Tang, Phys. Chem. Chem. Phys. 14 (2012) 5966e5972. Z. Li, L. Zhang, B.S. Amirkhiz, X. Tan, Z. Xu, H. Wang, B.C. Olsen, C. Holt, D. Mitlin, Adv. Energy Mater. 2 (2012) 431e437. S. Uppugalla, U. Male, P. Srinivasan, Electrochim. Acta 146 (2014) 242e248. Z.-S. Wu, W. Ren, D.-W. Wang, F. Li, B. Liu, H.-M. Cheng, ACS Nano. 4 (2010) 5835e5842. G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z. Bao, Nano. Lett. 11 (2011) 2905e2911. L. Li, Z.A. Hu, N. An, Y.Y. Yang, Z.M. Li, H.Y. Wu, J. Phys. Chem. C 118 (2014) 22865e22872.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Fabrication of manganese dioxide nanoplates anchoring on biomassderived cross-linked carbon nanosheets for high-performance asymmetric supercapacitors Yiju Li a, Neng Yu b, Peng Yan a, Yuguang Li c, Xuemei Zhou c, Shuangling Chen c, Guiling Wang a, *, Tong Wei a, Zhuangjun Fan a, ** a Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China c 101 Institute of the Ministry of Civil Affairs, Beijing 100070, China
h i g h l i g h t s The CCNs was derived from a common biomass of Willow catkin. The MnO2@CCNs composite shows good capacitive performance. The assembled asymmetric cell shows high energy density.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2015 Received in revised form 29 August 2015 Accepted 19 September 2015 Available online xxx
In this paper, MnO2 nanoplates loading on biomass-derived cross-linked carbon nanosheets have been prepared by a two-step synthesis. At first, the cross-linked carbon nanosheets derived from willow catkin are synthesized by one-step pyrolysis and activation method, then the MnO2 anchored cross-linked carbon nanosheets is prepared via in-situ hydrothermal deposition. The asymmetric supercapacitor with terrific energy and power density is assembled by employing the MnO2 anchored cross-linked carbon nanosheets as the positive electrode and the cross-linked carbon nanosheets as the negative electrode in a 1 M Na2SO4 electrolyte. The asymmetric supercapacitor displays a high energy density of 23.6 Wh kg1 at a power density of 188.8 W kg1 within a wide voltage rage of 0e1.9 V. In addition, the asymmetric supercapacitor exhibits excellent cycling stability with only 1.4% capacitance loss after 10000 cycles at 1 A g1. These discoveries open up the prospect of biomass/biowaste derived carbon-based composites for high-voltage asymmetric supercapacitors with superb energy and power density performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Asymmetric supercapacitor Biomass Carbon nanosheets Manganese dioxide Composite
1. Introduction Over the past decades, with the excessive consumption of fossil energy and the increasingly severe environment pollution problem, exploring green, high-efficiency, and sustainable energy sources, as well as advanced energy conversion and storage technologies is an urgent task needed to be solved. Supercapacitor, as a sort of typical
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), fanzhuangjun@hrbeu. edu.cn (Z. Fan). http://dx.doi.org/10.1016/j.jpowsour.2015.09.077 0378-7753/© 2015 Elsevier B.V. All rights reserved.
energy storage device with excellent power density and long lifespan, has attracted growing attention [1e5]. However, the inferior energy density of conventional electrochemical capacitor has limited their applications. To solve the problem, construction of asymmetric systems has been extensively explored by combining a Faradic pseudocapacitor-like electrode (positive electrode) and an electric double layer capacitive electrode (negative electrode) to increase the operation voltage, which can significantly improve the energy density [6e10]. At present, activated carbon (AC) particulates with a high specific surface area are the most common carbon material for the negative electrode of asymmetric supercapacitor [11,12],
310
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
Nevertheless, the power characteristics of AC are limited due to the minuscule pores and blocked pore texture, which is hardly accessible to hydrate ions. As a rising star of carbon material, graphene could be a candidate for the negative electrode of asymmetric supercapacitor for its high surface area and superb electrical conductivity [13e17]. But most of methods for synthesis of graphene often require complicated synthesis conditions or special facilities, leading to the high cost. Therefore, it is a preponderant way for facile direct pyrolysis carbon precursors, such as biomass or biowaste, into graphene-like carbon nanosheets [18e22]. As a common biomass, willow catkin containing abundant cellulose is inedible and has no economic value. Fortunately, the willow catkin with hollow and multilayer morphology can be facilely converted into value-added thin carbon nanosheets via simple pyrolysis and alkali activation. This kind of carbon nanosheets, which possess relatively flat adsorption surfaces and interconnected networks as well as relatively high in-plane electrical conductivity, has superior electrochemical performance and is able to serve as conductive carbon substrates to anchor metal oxide. Among the multifarious pseudocapactive metal oxides, MnO2 is one of the most promising positive electrode materials for low-cost and environment friendly asymmetric supercapacitor devices. Especially, a combination of MnO2 pseudocapactive electrode with carbon material electrode can reach up to a high voltage of 2 V, far beyond theoretical water decomposition voltage (1.23 V). However, the intrinsically poor electronic conductivity of the MnO2 immensely confines the improvement of specific capacitance. Hence, it is an alternative route to anchor MnO2 on 3D cross-linked carbon nanosheets (CCNs) to enhance the electrical conductivity of the electrode and thus potentially improve the capacitive performance of asymmetric supercapacitor. Here, we reported the MnO2 nanoplates anchoring on biomassderived CCNs (MnO2@CCNs) have been prepared by a two-step synthesis and the asymmetric supercapacitor assembled by employing MnO2@CCNs as positive electrode and CCNs as negative electrode exhibits a superb energy density of 23.6 Wh kg1, significantly superior than those of symmetrical supercapacitor based on pure carbon materials [23e26] and higher than those of other MnO2-based asymmetric supercapacitors [27e31]. 2. Experimental 2.1. Synthesis of CCNs Briefly, the collected willow catkin was dispersed in acetone and then under ultrasound for 30 min. Next, the willow catkin was washed by DI water and dealt with violent stirring for getting rid of seeds and impurities. Then the willow catkin with brief cutting was poured into 100 mL potassium hydroxide solution and heated at 100 C until all of the DI water was evaporated. The remnant mixture was heated up to 400 C at a heating rate of 5 C min1 for 3 h and then continually heated up to 850 C at a heating rate of 10 C min1 for 1 h in a tubular furnace under argon atmosphere. 2.2. Synthesis of MnO2@CCNs composite MnO2@CCNs composite was synthesized by in-situ hydrothermal deposition method. In a typical process, 0.1 g of as-prepared CCNs was added into 90 mL of 0.04 M KMnO4 solution. Subsequently, the mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated up to 120 C from room temperature at a heating rate of 2 C min1 for 1 h in an electric furnace. Then, the mixture was naturally cooled down to room temperature. The reaction during the hydrothermal process is assumed to be as follows:
4KMnO4 þ 3C þ H2O ¼ 4MnO2 þ K2CO3 þ 2KHCO3
(1)
Next, the composite was through vacuum filtration and washed with DI water. Finally, the obtained material was dried at 60 C for 12 h in vacuum oven. The brief flow diagram of preparation process was illustrated in Fig. 1. 2.3. Materials characterizations The crystalline phase of as-prepared materials were analyzed using X-ray diffractometer (XRD, Rigaku TTR III) with Cu Ka radiation (l ¼ 0.1514178 nm). Netzsch STA 449C analyzer was employed to analyze the weight percent of MnO2 from room temperature to 800 C at a heating rate of 5 C min1 under N2 atmosphere (TGA). Scanning electron microscopy (SEM, JEOL JSM-6480) and transmission electron microscopy (TEM, FEI Teccai G2 S-Twin, Philips) were used to characterize the microstructure. Nitrogen adsorptionedesorption analysis was conducted via utilizing Micromeritics ASAP 2020 physisorption analyzer at 77 K. The specific surface area was calculated by the BrunauereEmmetteTeller (BET) method using the adsorption branch of nitrogen isotherm. The information of surface chemical species was revealed by X-ray photoelectron spectroscopy with Al Ka radiation (XPS, Thermo ESCALAB 250). 2.4. Electrochemical measurements All electrochemical measurements were conducted in 1 M NaSO4 electrolyte. In a typical three-electrode set-up, the asprepared electrode (1 1 cm2) served as the working electrode, a platinum foil (2 1 cm2) acted as the counter electrode, and an Ag/ AgCl electrode was selected as the reference electrode. In the threeelectrode system, the mass loading of active materials on the working electrode is about 6.3 mg. The cyclic voltammetry (CV) tests, galvanostatic chargeedischarge (GCD) measurements and electrochemical impedance spectroscopy (EIS) were carried out by using a computerized potentiostat (VMP3/Z Bio-Logic) controlled by the EC-lab software. CV and GCD tests in the three-electrode system were conducted between 0 and 0.9 V and EIS measurements were performed by applying an alternating voltage with 5 mV amplitude in a frequency scope from 10 mHz to 100 kHz at open circuit potential. The asymmetric two-electrode cells were assembled using MnO2@CCNs as positive electrode and CCNs as negative electrode and were conducted in the voltage range of 0e1.9 V. The tests of cycle life were performed on a NEWARE battery program-control test system (CT-3008W). The mass loading ratio of active materials (MnO2@CCNs/CCNs) was determined according to the follow equation:
mþ C DE ¼ m Cþ DEþ
(2)
where m is mass loading of the active material in the electrode, C is the specific capacitance under a specific current or scan rate and DE is the potential range of each electrode. The subscripts þ and e represent the positive and negative electrodes, respectively. The specific capacitance measured by the galvanostatic chargeedischarge method can be calculated based on the following equation:
Cm ¼
Id Dt DV m
(3)
where Cm (F g1) is specific capacitance, Id (mA) is the discharge current, Dt (s) is the discharge time and DV (V) is the discharge potential or voltage range. In three-electrode system, m (mg)
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
311
Fig. 1. The schematic illustration of the preparation of MnO2@CCNs composite.
represents the mass loading of active material in a single electrode. In two-electrode cell, m (mg) is the total mass loading of active materials based on both electrodes. The energy density E (Wh kg1) and power density P (W kg1) of an asymmetric supercapacitor in the Ragone plots were calculated according to the following equation:
E¼
1 1 Cm DV 2 2 3:6
(4)
P¼
E 3600 Dt
(5)
where Cm (F g1) is the specific capacitance of asymmetrical capacitor which is calculated based on total mass loading of active materials in both electrodes, DV (V) is the discharge voltage range (excluding the IR drop) and Dt (s) is the discharge time. As we all known, the capacitance C(w) based on impedance measurements can be defined as follows: 00
C ¼ C 0 ðwÞ jC ðwÞ
(6)
00
C 0 ðwÞ ¼
C 00 ðwÞ ¼
Z ðwÞ wjZðwÞj2 Z 0 ðwÞ wjZðwÞj2
(7)
(8)
where w is the angular frequency, C 0 ðwÞ is the real part and C 00 ðwÞ is the imaginary part of the cell capacitance. Z 0 ðwÞ and Z 00 ðwÞ are the real and imaginary parts of the complex impedance Z(w), respectively. 3. Results and discussion 3.1. Characterization of materials The CCNs sample was synthesized by one-step pyrolysis and activation treatment of natural abundant willow catkin (Fig. 2A). One interesting finding is that the structure of willow catkin is hollow and thin-walled (Fig. 2B), which contributes to the KOH permeation and carbon nanosheets exfoliation during the pyrolysis process. Treated with pyrolysis and KOH activation, the willow catkin was successfully converted into interconnected thin carbon nanosheets (Fig. 2C). Such 3D cross-linked carbon nanosheets can offer conducted networks for fast electrons transfer and serve as an ideal platform for anchoring active materials. Moreover, it can be easily discovered that the MnO2 is uniformly dispersed on the CCNs
surfaces after hydrothermal deposition (Fig. 2D). The TEM images, shown in the inset of Fig. 2D, further confirm the homogeneous distribution of MnO2 nanoplates. Fig. 3A shows the XRD patterns of CCNs and MnO2@CCNs composite. For CCNs, the diffraction peak centered at ~25 can be ascribed to the (002) plane of the partially ordered CCNs [32]. Except for the diffraction peak of (002), another two diffraction peaks appearing at 37.1 and 65.6 for MnO2@CCNs composite are indexed to the diffractions of (311) and (440) planes of a-MnO2, respectively (JCPDS card No. 42-1169). The broad peaks demonstrate the poor crystallinity and almost amorphous nature of the MnO2 nanoplates [33]. The information on the surface chemical composition was obtained from XPS. The wide-scan XPS spectrum of MnO2@CCNs composite confirms the existence of C, O, and Mn (Fig. 3B). The high resolution spectrum of O 1s band is presented in Fig. 3C. The XPS peaks of 529.5 and 531.6 eV can be assigned to MneOeMn and MneOeH, respectively. For Mn 2p pattern, two main peaks located at 653.6 eV and 642.0 eV correspond to the Mn 2p1/2 and Mn 2p3/2 with a spin-energy separation of 11.6 eV, further confirming the presence of a-MnO2 in the composite (Fig. 3D) [34]. The specific surface area of the CCNs and MnO2@CCNs composite is determined by the nitrogen adsorptionedesorption isotherms (Fig. 3E). For the CCNs derived from willow catkin, its specific surface area is up to 1507 m2 g1. Therefore, the carbon nanosheets with such a high specific surface area are suitable for working as the backbone materials to combine with the active materials of MnO2. With the sacrifice of carbon and formation of MnO2 nanoplates, the MnO2 nanoplates homogenously anchor on the surface of CCNs and will block some nanopores and micropores of CCNs. Therefore, the specific surface of MnO2@CCNs composite will be undoubtedly smaller than pure CCNs and the pore distribution will be changed. But the specific surface area of the MnO2@CCNs composite can still reach up to 234 m2 g1. Such an acceptable specific surface area of MnO2@CCNs composite contributes to the sufficient contact between active material and electrolyte. Fig. 3F shows the pore distributions calculated by density functional theory (DFT) model. The pore size distribution of MnO2@CCNs composite mainly focuses on ~1.6 nm. The pore texture of such large micropores is favorable for the ions access [19]. In order to accurately ascertain the weight percent of MnO2 in MnO2@CCNs composite, TGA measurement was conducted and the curve is exhibited in Fig. 3G. The TGA plot shows two weight loss stages. The first step of apparent mass loss (about 8.5%) from room temperature to ~200 C can be attributed to the loss of adsorbed water and crystalline water in the MnO2@CCNs composite. The second step of mass loss (about 22.5%) is assigned to the thermal decomposition of the CCNs [35]. The residual substance did not show obvious weight loss until the terminal of the test. Accordingly, the percentage of MnO2 in the composite is calculated
312
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
Fig. 2. The photograph of willow catkin (A). The SEM images of willow catkin (B), CCNs (C) and MnO2@CCNs composite (D). The TEM image of MnO2@CCNs composite (Inset in Fig. 2D).
to be ~75.4%. 3.2. Electrochemical measurements Fig. 4 demonstrates the electrochemical performance of CCNs and MnO2@CCNs composite measured in a three-electrode system. Both of the CV and GCD tests were conducted in 1 M NaSO4 electrolyte within potential windows ranging from 0 to 0.9 V. The CV curves and GCD plots of CCNs are displayed in Fig. 4A and C, respectively. The nearly rectangular CV curves suggest the ideal capacitive behavior of the CCNs material. Even at a higher scan rate of 100 mV s1, the shapes of CV curves remain quasi rectangle, signifying the intrinsic good conductivity of CCNs and low contact resistance in the electrode. The GCD curves of CCNs electrode at current density of 0.2, 0.5, 1, 2 and 5 A g1 were shown in Fig. 4C. The nearly symmetric chargeedischarge curve reveals excellent reversibility and high coulombic efficiency. For MnO2@CCNs composite (Fig. 4B), it can be observed that the CV curves show relatively rectangular shape for scan rates up to 20 mV s1, and at the same scan rate the resultant current density from MnO2@CCNs composite is much higher than that of pure CCNs owing to the loading of the pseudocapacitive MnO2 nanoplates. With the scan rate increasing (50 and 100 mV s1), the current densities increase and the shape of CV curves displays some distortions from an ideal rectangle, indicating a deviation from the perfect capacitive behavior. This is probably due to increasing overpotentials from ion transport between the electrolyte and MnO2 nanoplates. The GCD curves of MnO2@CCNs composite (Fig. 4D) at various current densities show longer chargeedischarge times than those of pure CCNs, which is consistent with the results of CV curves. In addition, the specific capacitance versus current density is summarized in Fig. 4E. Apparently, the MnO2@CCNs composite demonstrate the improved specific capacitance than pure CCNs. The specific capacitances are 262, 217, 189, 162 and 133 F g1 when the current
densities increase to 0.2, 0.5, 1, 2 and 5 A g1, respectively, which is larger than those of CCNs and comparable to those of other reported MnO2-based composites [19,36,37]. The results of comparison are listed in Table 1. However, it is worth mentioning that the capacitance retention of CCNs (60.2%) is superior to that of MnO2@CCNs composite (50.7%) with current density enlarging from 0.2 A g1 to 5 A g1. EIS was employed for further investigating the electrochemical performance of the as-prepared samples (Fig. 4F). The steep linear curve in the low frequency region suggests nearly ideal capacitive performance and the semi-circle in the high frequency region indicates the charge transfer resistance. It is clear that the CCNs electrode shows a nearly vertical line along the imaginary coordinates in the low frequency region, which demonstrates its ideal double electric layer capacitive behavior. However, the MnO2@CCNs composite electrode exhibits a more inclined curve in the same low frequency region, implying the major performance of pseudo-capacitance. In the high frequency region, the charge transfer resistance of CCNs (1.47 U) is a little less than that of MnO2@CCNs composite (1.89 U) through the fitting of equivalent circuit diagram. This is expected since the charge transfer of MnO2@CCNs composite involves redox processes that are more sluggish than simple surface adsorption and disadsorption of double electric layer capacitance. Especially, the ion diffusion process can be expressed by the length of the Warburg-type line (the slope of the 45 part of the Nyquist plots in the middle frequency region) [38]. The Warburg-type line of CCNs is much short. That means the rapid ion diffusion in the 3D interconnected conductive networks of carbon nanosheets. Moreover, the bode phase curve of CCNs (Fig. 4G) show that the phase angle is ~83 and is approaching to that of an ideal capacitor of 90 [39]. As a comparison, the phase angle of MnO2@CCNs composite is only ~69 , which is much less than that of ideal capacitor. This is because the MnO2@CCNs composite primarily displays the pseudocapacitive performance.
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317 Fig. 3. The XRD patterns of CCNs and MnO2@CCNs composite (A). The wide-scan XPS spectrum of MnO2@CCNs composite (B). The high-resolution O 1s XPS spectrum (C) and Mn 2p XPS spectrum (D) of MnO2@CCNs composite. The N2 adsorptionedesorption isotherms (E) and pore size distributions (F) of CCNs and MnO2@CCNs composite. The TGA curve of the MnO2@CCNs composite (G).
313
314 Y. Li et al. / Journal of Power Sources 300 (2015) 309e317 Fig. 4. The electrochemical performance conducted in three-electrode setup. The CV curves of CCNs (A) and MnO2@CCNs composite (B). The galvanostatic chargeedischarge curves of CCNs (C) and MnO2@CCNs composite (D). The specific capacitance versus current density (E). The Nyquist plots (F) and bode phase diagrams of CCNs (C) and MnO2@CCNs composite (G).
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
315
Table 1 Performance comparison of the MnO2ecarbon composite based electrodes. Samples
Sa
Vrb (V)
Elc
Cd (F g1)
Ref.
MnO2/rGO hydrogel MnO2-GAs MnO2@R-GO@Ni-foam GW-MnO2 MnO2$xH2O/carbon aerogel CMG15 Birnessite-type MnO2/AC Carbon@MnO2 coreeshell nanosphere Amorphous MnO2/microporous carbon sphere MnO2/C@CNT MnO2@CCNs
2 A g1 0.5 A g1 1 A g1 1 A g1 2 mV s1 1 A g1 1 A g1 2 mV s1 2 mV s1 0.2 A g1 0.2 A g1
0e1 0e0.7 0e0.8 0e1 0e1 0e1 0.2e0.8 0e0.8 0e0.8 0.1e0.8 0e0.9
1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 6 M KOH 1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 1 M Na2SO4 0.5 M Na2SO4 1 M Na2SO4
214 210 225 194 226 111.1 210 252 218 227 262
[15] [31] [32] [16] [36] [17] [35] [30] [37] [38] This work
a b c d
Scan rate or current density. Voltage range. Electrolyte type. Specific capacitance.
Asymmetric energy storage devices consisting of diverse types of electrodes have been extensively investigated recently owing to the wide operating voltage thus exhibiting high energy density. We constructed an asymmetric supercapacitor by employing MnO2@CCNs composite as the positive electrode and CCNs as the negative electrode in 1 M Na2SO4 solution. Fig. 5 displays the schematic diagram of the assembled structure for such an asymmetric cell. CV measurements were first utilized to estimate the electrochemical potential ranges of individual electrode in a threeelectrode set-up. The stable potential window is 1.0-0 V for the CCNs electrode while it is 0e0.9 V for MnO2@CCNs composite electrode (Fig. 6A). The specific capacitances calculated from the CV curves are 188 F g1 and 141 F g1 for MnO2@CCNs composite and CCNs electrode, respectively. So the mass loading ratio between the electrodes (mþ/m-) is about 1:1.2 according to the equation (2). The mass loadings of active materials on negative electrode (CCNs) and positive electrode (MnO2@CCNs composite) are about 5.4 mg and 4.5 mg, respectively. As shown in Fig. 6B, the MnO2@CCNs//CCNs asymmetric supercapacitor exhibits a stable voltage window up to 1.9 V and presents an ideal capacitive behavior with relatively rectangular CV curves even at a scan rate up to 100 mV s1. The GCD plots of the asymmetric supercapacitor under various current
densities are shown in Fig. 6C. The inset demonstrates the charge/ discharge curves of the individual electrodes vs. the Ag/AgCl reference electrode and the as-assembled asymmetric supercapacitor at a current density of 2 A g1. The current densities are determined based on the total weight of MnO2@CCNs positive electrode and CCNs negative electrode. It is apparent that the voltage is almost proportional to the chargeedischarge time, demonstrating a typical capacitive performance. Moreover, the charge time is nearly equal to the discharge time, which indicates super-high coulombic efficiency. Especially, the low IR drop for MnO2@CCNs//CCNs asymmetric supercapacitor manifests that the assembled asymmetric cell has low internal resistance. The specific capacitance changed with current density can intuitively reveals the rate performance (Fig. 6D). It is can be observed that the MnO2@CCNs//CCNs asymmetric supercapacitor can exhibit the specific capacitance of 47.4 F g1 at 0.2 A g1. Even at a high chargeedischarge rate of 5 A g1, the asymmetric cell still has a specific capacitance of 32.1 F g1, suggesting excellent rate performance. The cycle life test was used to evaluate the long-term cycling stability of the asymmetric supercapacitor and was measured by the consecutive galvanostatic charge and discharge at current density of 1.0 A g1 in a voltage window of 0e1.9 V. It is exciting that the asymmetric supercapacitor has superb cycling stability (Fig. 6E).
Fig. 5. The schematic of the assembled asymmetric supercapacitor based on MnO2@CCNs composite as positive electrode and CCNs as negative electrode.
316 Y. Li et al. / Journal of Power Sources 300 (2015) 309e317 Fig. 6. The electrochemical performance of MnO2@CCNs//CCNs asymmetric supercapacitor. Comparative CV curves of CCNs and MnO2@CCNs composite electrodes in a three-electrode set-up at a scan rate of 10 mV s1 (A). The CV curves at different scan rates (B). The galvanostatic chargeedischarge profiles at different current densities. The inset shows the charge/discharge curves of the individual electrodes vs. the Ag/AgCl reference electrode and the asassembled asymmetric supercapacitor at a current density of 2 A g1. (C). The specific capacitance versus current density (D). The specific capacitance and columbic efficiency versus the number of cycles tested at 1 A g1 (E). The normalized real (F) and imaginary (G) part capacitance versus frequency. The Ragone plots of the asymmetric cell (H). Performance comparison of the MnO2@CCNs//CCNs asymmetric cell versus previously reported MnO2-based asymmetric supercapacitors in aqueous electrolyte (I).
Y. Li et al. / Journal of Power Sources 300 (2015) 309e317
Only 1.4% capacitance loss is observed after 10000 cycles at 1 A g1, and the coulombic efficiency remains above 95%. This cycling performance is much superior than that of some reported asymmetric cells, such as rGO//MnO2/rGO (89.4% retention after 1000 cycles) [15], graphene//MGC (79% of the initial capacitance after 1000 cycles) [40], graphene/MnO2-textile//CNT-textile (~95% retention over 5000 cycles) [41] and GH//MnO2eNF (83.4% retention over 5000 cycles) [34]. Fig. 6FeG shows the frequency response of normalized C 0 ðwÞ and C 00 ðwÞ before cycle and after 10000 cycles, respectively. As we all known, the operating frequency (fo) and characteristic relaxation time constant (to) are the important quantitative indexes of how fast the cell can be charged and discharged reversibly [19]. Through 10000 cycles, the MnO2@CCNs// CCNs asymmetric cell still has an operating frequency of 0.21 Hz (corresponding to to ¼ 4.76 s), which is little inferior than that of 0.29 Hz before cycle (corresponding to to ¼ 3.44 s). The result shows that the MnO2@CCNs//CCNs asymmetric cell remains fast response time despite undergoing ultra-long cycle life test. Fig. 6H shows the Ragone plot of power density (P) and energy densities (E) considering the total loading mass of both positive and negative electrode materials and excluding the IR drop. It is worth noting that the maximum energy density for the asymmetric supercapacitor is 23.6 Wh kg1 at a power density of 188.8 W kg1, which is comparable or higher than those of other MnO2-based asymmetric cells in aqueous electrolyte solutions (Fig. 6I) [9,28,29,40,42]. Furthermore, this assembled asymmetric cell displays a good power characteristic, and it keeps an acceptable energy density of 12.1 Wh kg1 when the power density reaches up to 3570.5 W kg1. All of the above results suggest that this sort of lowcost, eco-friendly and high-performance asymmetric supercapacitor is expected to be a promising energy storage device in portable electronics and electric vehicles.
[22]
4. Conclusions
[23] [24]
In this work, we successfully prepared the cross-linked carbon nanosheets derived from the common biomass of willow catkin using one-step facile pyrolysis and activation method. The interconnected thin carbon nanosheets serve as 3D conductive networks for fast electrons transfer and offer a fine platform for anchoring pseudocapacitive active materials. The MnO2 nanoplates were uniformly distributed on the CCNs by in-situ hydrothermal deposition. When employing the MnO2@CCNs composite as the positive electrode and the CCNs as the negative electrode, the assembled asymmetric supercapacitor exhibits a remarkable energy density of 23.6 Wh kg1 with a wide voltage range of 0e1.9 V. Moreover, the asymmetric cell exhibits superb cycling stability with only 1.4% capacitance loss after 10000 cycles at 1 A g1. Therefore, we hold a positive attitude that such low-cost and highperformance asymmetric supercapacitor realized by earthabundant and eco-friendly materials can offer great promise in the application of high-performance energy storage devices. Notes The authors declare no competing financial interest. Acknowledgments We gratefully acknowledge the financial support of this research
317
by the National Natural Science Foundation of China (No. 51572052).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797e828. S. Liu, S. Sun, X.-Z. You, Nanoscale 6 (2014) 2037e2045. L.L. Zhang, X. Zhao, Chem. Soc. Rev. 38 (2009) 2520e2531. G.A. Snook, P. Kao, A.S. Best, J. Power Sources 196 (2011) 1e12. V. Augustyn, P. Simon, B. Dunn, Energy Environ. Sci. 7 (2014) 1597e1614. Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Adv. Funct. Mater. 21 (2011) 2366e2375. J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi, F. Wei, Adv. Funct. Mater. 22 (2012) 2632e2641. H. Chen, S. Zhou, M. Chen, L. Wu, J. Mater. Chem. 22 (2012) 25207e25216. J. Cao, Y. Wang, Y. Zhou, J.-H. Ouyang, D. Jia, L. Guo, J. Electroanal. Chem. 689 (2013) 201e206. C. Zhou, Y. Zhang, Y. Li, J. Liu, Nano Lett. 13 (2013) 2078e2085. Q. Qu, L. Li, S. Tian, W. Guo, Y. Wu, R. Holze, J. Power Sources 195 (2010) 2789e2794. V. Ganesh, S. Pitchumani, V. Lakshminarayanan, J. Power Sources 158 (2006) 1523e1532. L. Deng, G. Zhu, J. Wang, L. Kang, Z.-H. Liu, Z. Yang, Z. Wang, J. Power Sources 196 (2011) 10782e10787. Y. Jin, H. Chen, M. Chen, N. Liu, Q. Li, ACS Appl. Mater. Interfaces 5 (2013) 3408e3416. S. Wu, W. Chen, L. Yan, J. Mater. Chem. A 2 (2014) 2765e2772. J. Zhu, J. He, ACS Appl. Mater. Interfaces 4 (2012) 1770e1776. S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, ACS Nano. 4 (2010) 2822e2830. H. Wang, Z. Xu, A. Kohandehghan, Z. Li, K. Cui, X. Tan, T.J. Stephenson, C.K. King'ondu, C.M. Holt, B.C. Olsen, ACS Nano. 7 (2013) 5131e5141. H. Wang, Z. Li, J.K. Tak, C.M. Holt, X. Tan, Z. Xu, B.S. Amirkhiz, D. Harfield, A. Anyia, T. Stephenson, Carbon 57 (2013) 317e328. ~ ero, F. Leroux, F. Be guin, Adv. Mater. 18 (2006) 1877e1882. E. Raymundo-Pin M. Sevilla, W. Gu, C. Falco, M. Titirici, A. Fuertes, G. Yushin, J. Power Sources 267 (2014) 26e32. X. Liu, M. Zheng, Y. Xiao, Y. Yang, L. Yang, Y. Liu, B. Lei, H. Dong, H. Zhang, H. Fu, ACS Appl. Mater. Interfaces 5 (2013) 4667e4677. Z. Lei, N. Christov, L.L. Zhang, X.S. Zhao, J. Mater. Chem. 21 (2011) 2274e2281. T.E. Rufford, D. Hulicova-Jurcakova, Z. Zhu, G.Q. Lu, Electrochem. Commun. 10 (2008) 1594e1597. B.G. Choi, M. Yang, W.H. Hong, J.W. Choi, Y.S. Huh, ACS Nano. 6 (2012) 4020e4028. W. Xing, S. Qiao, R. Ding, F. Li, G. Lu, Z. Yan, H. Cheng, Carbon 44 (2006) 216e224. H. Xia, Y. Wang, J. Lin, L. Lu, Nanoscale Res. Lett. 7 (2012) 1e10. C.-C. Liu, D.-S. Tsai, W.-H. Chung, K.-W. Li, K.-Y. Lee, Y.-S. Huang, J. Power Sources 196 (2011) 5761e5768. T. Cottineau, M. Toupin, T. Delahaye, T. Brousse, D. Belanger, Appl. Phys. A 82 (2006) 599e606. Y. Zhao, Y. Meng, P. Jiang, J. Power Sources 259 (2014) 219e226. C.-C. Ji, M.-W. Xu, S.-J. Bao, Z.-J. Lu, C.-J. Cai, H. Chai, R.-Y. Wang, F. Yang, H. Wei, New J. Chem. 37 (2013) 4199e4205. Y. Li, D. Cao, Y. Wang, S. Yang, D. Zhang, K. Ye, K. Cheng, J. Yin, G. Wang, Y. Xu, J. Power Sources 279 (2015) 138e145. C. Long, D. Qi, T. Wei, J. Yan, L. Jiang, Z. Fan, Adv. Funct. Mater. 24 (2014) 3953e3961. H. Gao, F. Xiao, C.B. Ching, H. Duan, ACS Appl. Mater. Interfaces 4 (2012) 2801e2810. J. Han, L.L. Zhang, S. Lee, J. Oh, K.-S. Lee, J.R. Potts, J. Ji, X. Zhao, R.S. Ruoff, S. Park, ACS Nano. 7 (2012) 19e26. J. Li, X. Wang, Q. Huang, S. Gamboa, P.J. Sebastian, J. Power Sources 160 (2006) 1501e1505. W. Wei, X. Huang, Y. Tao, K. Chen, X. Tang, Phys. Chem. Chem. Phys. 14 (2012) 5966e5972. Z. Li, L. Zhang, B.S. Amirkhiz, X. Tan, Z. Xu, H. Wang, B.C. Olsen, C. Holt, D. Mitlin, Adv. Energy Mater. 2 (2012) 431e437. S. Uppugalla, U. Male, P. Srinivasan, Electrochim. Acta 146 (2014) 242e248. Z.-S. Wu, W. Ren, D.-W. Wang, F. Li, B. Liu, H.-M. Cheng, ACS Nano. 4 (2010) 5835e5842. G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z. Bao, Nano. Lett. 11 (2011) 2905e2911. L. Li, Z.A. Hu, N. An, Y.Y. Yang, Z.M. Li, H.Y. Wu, J. Phys. Chem. C 118 (2014) 22865e22872.