Solid-State Thin-Film Supercapacitors with Ultrafast Charge

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Letter pubs.acs.org/NanoLett

Solid-State Thin-Film Supercapacitors with Ultrafast Charge/ Discharge Based on N‑Doped-Carbon-Tubes/Au-NanoparticlesDoped-MnO2 Nanocomposites Qiying Lv,† Shang Wang,†,‡ Hongyu Sun,§ Jun Luo,§ Jian Xiao,† JunWu Xiao,† Fei Xiao,† and Shuai Wang*,†,# †

School of Chemistry and Chemical Engineering, #Flexible Electronics Research Center (FERC), State Key Laboratory of Digital Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, and ‡School of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China § Beijing National Center for Electron Microscopy, School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Key Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: Although carbonaceous materials possess long cycle stability and high power density, their low-energy density greatly limits their applications. On the contrary, metal oxides are promising pseudocapacitive electrode materials for supercapacitors due to their high-energy density. Nevertheless, poor electrical conductivity of metal oxides constitutes a primary challenge that significantly limits their energy storage capacity. Here, an advanced integrated electrode for high-performance pseudocapacitors has been designed by growing N-dopedcarbon-tubes/Au-nanoparticles-doped-MnO 2 (NCTs/ ANPDM) nanocomposite on carbon fabric. The excellent electrical conductivity and well-ordered tunnels of NCTs together with Au nanoparticles of the electrode cause low internal resistance, good ionic contact, and thus enhance redox reactions for high specific capacitance of pure MnO2 in aqueous electrolyte, even at high scan rates. A prototype solid-state thin-film symmetric supercapacitor (SSC) device based on NCTs/ANPDM exhibits large energy density (51 Wh/kg) and superior cycling performance (93% after 5000 cycles). In addition, the asymmetric supercapacitor (ASC) device assembled from NCTs/ANPDM and Fe2O3 nanorods demonstrates ultrafast charge/discharge (10 V/s), which is among the best reported for solid-state thin-film supercapacitors with both electrodes made of metal oxide electroactive materials. Moreover, its superior charge/discharge behavior is comparable to electrical double layer type supercapacitors. The ASC device also shows superior cycling performance (97% after 5000 cycles). The NCTs/ANPDM nanomaterial demonstrates great potential as a power source for energy storage devices. KEYWORDS: Energy storage, metal oxides, electrical conductivity, all-solid thin-film supercapacitors, ultrafast charge/discharge

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hybrid nanocomposites for SCs have been the subject of intense interest because of the inherent limitations of the above single materials.12−20 Among various metal oxides, MnO2 has attracted considerable attention owing to its low cost, abundance in the Earth, and especially high theoretical capacity (1380 F/g).21,22 More significantly, MnO2-based nanomaterials have been widely employed in neutral aqueous electrolytes that can meet the environmental requirements of “green electrolytes” in SCs.23 However, the poor conductivity of MnO2 (10−5−10−6 S cm−1),24 dense morphology,25 and low mechanical strength26 lead to a very low percentage of Mn in MnO2 taking part in redox reactions, thus preventing its wide energy-storage

he rapidly growing demand for green and sustainable energy has triggered tremendous research efforts into the design and development of advanced energy storage devices.1,2 Supercapacitors (SCs), also known as electrochemical capacitors (ECs), are fascinating power source devices in this regard due to their high density, long life cycles, and high efficiency.3 So far, carbonaceous materials,4,5 metal oxides,6,7 hydroxides,8 and conducting polymers9,10 have been widely used in SCs. Each type of electrode material possesses its intrinsic advantages and disadvantages. For example, although carbonaceous materials have long cycle stability and high power density, their low energy density greatly limits their applications. On the other hand, transition metal oxides, hydroxides, and conducting polymers are being widely explored for their high pseudocapacitance together with low cost, low toxicity, and great flexibility in structure and morphology; however, they are usually kinetically unfavorable.11 Recently, © XXXX American Chemical Society

Received: June 23, 2015 Revised: October 13, 2015

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Figure 1. (a) Schematic illustration of the fabrication of NCTs/ANPDM nanocomposite on the carbon fabric substrate. (b−d) SEM images of ZnO nanorod arrays, ZnO/NC core−shell nanorod arrays, and NCTs/ANPDM nanocomposite arrays grown on the carbon fabric. (e) Low-magnification TEM image of NCTs/ANPDM nanocomposite. (f, g) TEM images of Au nanoparticles covered by the MnO2 and MnO2 besides the Au nanoparticles on the surface of the NCTs, respectively.

increase the active site of ANPDM material, we introduce the cyclic voltammetry (CV) method using different scan rates instead of the traditional reported approach35,39 for the electrodeposition of MnO2 and Au. There are another two advantages of the alternant cyclic voltammograms method used here. (1) The MnO2 nanothin film can be doped with Au simultaneously during the redox reaction.36,40 (2) The weight of the Au or MnO2 can be well-controlled by adjusting the applied scan rates. The NCTs are chosen since they are conductive and can serve as three-dimensional scaffolds for binder-free electrodes and provide a larger surface area and shorter ion diffusion. The fabrication process for an advanced hybrid electrode by growing ANPDM on the NCTs scaffold is presented in Figure 1, panel a. Briefly, ZnO nanorod arrays template was applied to construct the three-dimensional NCTs/ANPDM grown on the carbon fabric. The fabrication route shows as follows. First, free-standing ZnO nanorod arrays as templates were grown on the carbon fabric by a hydrothermal synthesis method (Step I in Figure 1a).41 As shown in Figure 1, panel b, the carbon fibers are uniformly covered with ZnO nanorods. Second, a thin amorphous N-doped carbon layer was formed on the surface of ZnO nanorods via the polymerization of dopamine and subsequently thermal carbonization processes (Step II in Figure 1a, and scanning electron microscopy (SEM) image in

applications. Therefore, enhancing the conductivity of MnO2 appears to be crucial to realize the high capacitance of MnO2 in practical applications. To enhance the conductivity of MnO2, considerable research efforts have been made to synthesize various MnO2/conductive matrix hybrid materials such as MnO2@SnO2,27 MnO2/short-MWCNT (multiwalled carbon nanotube),28 and C/MnO2 nanomaterials.29 In addition, the C/ MnO2 hybrid materials exhibit great potential for the nextgeneration SCs because carbon is an ultralight material and has high conductivity, chemical inertness, and biocompatibility.30 However, the poor electric conductivity of MnO2 is still difficult to be resolved in hybrid electrodes.31−33 Incorporation of metal cations or nanoparticles, such as Ag,34 Au,35 Co,36 Al,37 and Cu,38 through hydrothermal approaches or the physical vapor deposition (PVD) method can improve the specific capacitance of various types of Mn oxides. Nevertheless, the reported methods require long time or high cost. Moreover, the resultant improvement in electronic conductivity and capacitance is still obscure. In this work, to further improve the application of MnO2 in advanced energy storage devices, an N-doped-carbon-tubes/ Au-nanoparticles-doped-MnO2 (NCTs/ANPDM) coating on carbon fabric electrode has been designed and fabricated for recording the electrochemical performance at charge/discharge status and obtaining detailed information. Furthermore, to B

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Figure 2. XPS in the region of (a) Mn 2p, (b) O 1s, and (c) Au 4f orbits (“black” represents the raw data, and the red line is the total fitted curve) for NCTs/ANPDM nanocomposite.

contributions: the peak at 398.9 eV belongs to pyridinic nitrogen (N1), the peak at 399.9 eV belongs to pyrrolic nitrogen (N2), and the peak at 400.7 eV indicates the presence of quaternary nitrogen (N3).42 The XPS spectrum of C 1s (Figure S3b) can be divided into three peaks. The main peaks at 285.0 eV, 286.2 eV, and 288.3 eV are featured as sp2hybridized graphite-like carbon (C−C sp2), C−N, and a certain number of OC−O groups, respectively.42 These findings indicate that polydopamine transformed to N-doped carbon after carbonization, and the literature reports that this is beneficial for cation transport in the interface due to defects in the carbon caused by N-doping.43,44 According to the Mn 2p spectrum (Figure 2a), the 2p1/2 peak of Mn is centered at 653.6 eV, while the Mn 2p3/2 peak can be deconvoluted into two peaks as Mn3+ (641.6 eV)45 and Mn4+ (643.1 eV),46 respectively. The O 1s spectrum can be deconvoluted into four components, which are related to the Mn−O−Mn bond at 529.8 eV for the tetravalent oxide, the Mn−OH bond at 530.3 eV for a hydrated trivalent oxide, the H−O−H bond at 531.2 eV for residual water,22 the O−C−O bond at 532.0 eV, and a OC−O bond at 533.3 eV in the materials (Figure 2b).42 The relative fraction of each O bonding state is given in Table S1. In Figure 2, panel c, we can observe two Au 4f binding-energy peaks at 84.1 and 87.9 eV, which are consistent with the previous report for Au metal.47,48 The results are in good agreement with the analysis of XRD and TEM. Before examining the electrochemical performance of the NCTs/ANPDM electrode, the influence of the weight ratio of Au to MnO2 in the NCTs/ADPDM nanocomposite was studied. The weight ratio of Au to MnO2 was controlled by changing the electrodeposition charge through adjusting the scan rate. To understand the electrochemical performance characteristics, the ion transport property within the NCTs/ ANPDM electrode was investigated using electrochemical impedance spectroscopy (EIS). A frequency response analysis over the frequency range from 1 MHz to 10 mHz yields the Nyquist plots. The Nyquist plots were fitted based on an equivalent circuit model consisting of bulk solution resistance Rs, charge-transfer Rct, Warburg resistance W0, and double-layer capacitance Cdl on the grain surface. The EIS investigation indicates that the Rct of the NCTs/ANPDM electrodes decreases from 3.7 to 1.2 Ω when the weight ratio of Au to MnO2 increases from 0:100 to 6:100, and it decreases slightly when further increasing the Au doping so that the weight ratio of Au to MnO2 increases from 10:100 to 28:100 (Figure S4a). The corresponding specific capacitance of the electrodes first increases with Au doping to 22:100 and then decreases sharply (Figure S4b). The result implies that although the conductivity

Figure 1c). Third, the carbon fabric coated with ZnO/Ndoped-C nanorods was used as a working electrode (1 × 1 cm2), and ANPDM is successfully electrodeposited on the surface of the ZnO/N-doped-C heterostructured nanorod arrays through an alternant cyclic voltammograms method in an aqueous solution containing 0.05 M Mn(Ac)2 and 0.1 M Na2SO4 and an aqueous solution containing 3 mM HAuCl4 and 0.1 M KCl, respectively (Step III and Step IV in Figure 1a). Finally, the prepared ZnO/N-doped-C/ANPDM electrode was immersed in 3.0 M KOH solution to remove the ZnO nanorods template and form the NCTs/ANPDM electrode (Step V in Figure 1a). A typical SEM image of the prepared NCTs/ANPDM core−shell structure is presented in Figure 1, panel d. The X-ray diffraction (XRD) pattern (Figure S1) of the as-prepared ZnO-nanorods/N-doped-C/ANPDM electrode exhibits the peaks of ZnO and Au. However, no peaks of MnO2 exist, as shown in Figure S1, indicating that the MnO2 is amorphous or its nanocrystallines are too small by the CV method with very weak diffraction intensity. The peak of C is also nonobvious due to its amorphous structure under the thermal process and simultaneously covered by the peak of the substrate. After etching of the ZnO nanorod templates, the diffraction peaks ascribed to ZnO are not found from the XRD pattern in Figure S1, revealing that the ZnO nanorod templates were completely dissolved in 3.0 M KOH solution. For comparison, the NCTs/MnO2 electrode (Step VI and VII in Figure 1a) was also prepared with the same method. More structure details are examined by transmission electron microscope (TEM), and Figure 1, panel e shows the typical TEM image of NCTs/ANPDM, which confirms that the asprepared MnO2 nanothin films with distributed Au nanoparticles completely covered the surface of the carbon tube as intended. The lattice fringe of these nanoparticles covered by a fluffy layer with an interlayer distance of 2.35 Å in Figure 1, panel f is in agreement with the lattice fringe of the (111) crystal planes of Au (JCPDS 00−001−1172), which is also consistent with the XRD pattern. Moreover, the fluffy thin layer with fringe spacing of 2.44 Å is indexed not as any of Au, but as a tetragonal phase of MnO2, according to the standard diffraction pattern of MnO2 (JCPDS 44−0141). Obviously, the MnO2 crystal sheets obtained through the CV method are so thin that their XRD intensity is too weak to be observed. To further elucidate the structure and chemical composition of the NCTs/ANPDM electrode material, X-ray photoelectron spectroscopy (XPS) measurement was performed, and the results are shown in Figures 2, S2, and S3. From the highresolution XPS spectra of N 1s in NCTs/ANPDM, it can be seen that the N 1s region (Figure S3a) exhibits three main C

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Figure 3. Electrochemical performance of the 3D NCTs/ANPDM electrode. (a, b) CV curves at different scan rates in 1.0 M Na2SO4 electrolyte. (c) Galvanostatic charge−discharge curves at different current densities. (d) Gravimetric capacitance calculated from GCD curves as a function of current densities. (e) Cycling stability of NCTs/MnO2 and NCTs/ANPDM electrodes with the same mass of MnO2 as a function of cycle number. The measurement of the capacitance retention was carried out during galvanostatic charge/discharge at a current density of 30 A/g. (f) EIS Nyquist plots for the NCTs/ANPDM and NCTs/MnO2 electrodes.

density (Figure S7d). The gravimetric capacitance of the NCTs/ANPDM electrode is better than previous reports such as MnO2/s-MWNT (392.1 F/g at 2 mV/s),28 MnO2@SnO2 (367.5 F/g at 50 mV/s),27 CNT/MnO2 (944 F/g at 1 mV/ s),49 Co3O4@Pt@MnO2 (539 F/g at 1 A/g),50 Ag/MnO2/ PANI (800 F/g at 1 mA/cm2),40 and Au-doped MnO2 films (626 F/g at 5 mV/s).35 When the current density increased to 30 A/g, the NCTs/ANPDM electrode retained a high mass capacitance of 806 F/g, while the NCTs/MnO2 electrode only showed a gravimetric capacitance of 548 F/g. Moreover, the gravimetric capacitance retention (73%) of the former at high current density discharge is higher than most previous reported materials such as WO3−x@Au@MnO2 (58% at 15 A/g).48 The electrochemical performance of the NCTs/ANPDM electrode presented here is obviously attributable to several aspects. (1) The N-doped-CTs that uniformly cover the carbon fabric fiber can serve as three-dimensional scaffolds for binder-free electrodes and provide a larger surface area and shorter ion diffusion. (2) MnO2 obtained through CV at different scan rates has a more distorted octahedral structure, which provides more active sites for manganese ions so that the deposited film has higher active specific surface area. This has been confirmed by Ashassi-Sorkhabi and co-workers in 0.02 M KMnO4 and 0.15 M K2SO4 mixture.39 (3) The distributed gold nanoparticles in MnO2 nanothin layers can provide abundant Au/ MnO2 interfaces and direct and stable pathways for rapid electron transport. Thus, the conductivity of the nanohybrid electrode can be improved greatly without sacrificing the electrochemical performance of any pure material. As a result, fast electrosorption of Na+ cations and the subsequent quick reversible faradaic process proceeding from Na+ cations can occur when the NCTs/ANPDM electrode is measured in Na2SO4 aqueous solution, which produces high capability. In the three-electrode system, the NCTs/ANPDM electrode also showed excellent long-term cycle life, which is another key parameter in relation to the performance of an SCs. Figure 3, panel e shows the specific capacitance variation of the NCTs/ ANPDM electrode as a function of cycle number at a high current density of 30 A/g. The specific capacitance of the NCTs/ANPDM electrode gradually increased to 108%, then fluctuated slightly, and finally remained almost constant at ∼103% over 5000 cycles. On the contrary, the NCTs/MnO2

of the electrode has not apparently changed, the effective Au/ MnO2 interfaces obviously decreased when the weight ratio of Au to MnO2 increases to 28:100, resulting in the reduction of electrochemical performance. Actually, the SEM images (Figure S5) of the NCTs/ANPDM electrodes with different Au/MnO2 weight ratio indicate that too much Au doping gives rise to an agglomerated morphology, which prevents Au nanoparticles from penetrating into the MnO2 nanothin films. Thus, the electrochemical performance of the electrode is reduced. To evaluate the electrochemical performance benefits of the NCTs/ANPDM (weight ratio of Au/MnO2, 22:100) electrode, electrochemical measurements were then performed with a three-electrode system in 1.0 M Na2SO4 electrolyte. For comparison, bare NCTs and NCTs/MnO2 electrodes were also intensively studied (Figures S6 and S7). Both the increased current density (Figure S6a) and the decreased IR drop at the starting point of the discharge curves (Figure S6b) demonstrate that the electrochemical behavior of NCTs/ANPDM is greatly enhanced for the MnO2 doped with Au nanoparticles. CV curves of the NCTs/ANPDM electrode at various scan rates are shown in Figure 3, panels a and b. The rectangular shapes of the CV curves, even at a high scan rate of 500 mV/s, were retained, indicating their excellent rate performance. At a scan rate of 5 mV/s, the calculated gravimetric capacitance of the NCTs/ANPDM electrode achieved 1091 F/g, and the gravimetric capacitance still remained at 341 F/g at a high scan rate of 500 mV/s. Rate capability constitutes a key factor in determing the SC electrodes for practical applications, which can be seen from the symmetry and resistance drop value of the galvanostatic charge/discharge (GCD) curves. Typical GCD curves of the NCTs/ANPDM electrode collected at different current densities are shown in Figure 3, panel c. The GCD curves show a nearly symmetric triangular shape with small voltage drops at the initial point of the discharge curve. Obviously, the NCTs/ANPDM electrode possesses a good rate capability characteristic and superior reversible redox reaction. The specific capacitance performance derived from GCD curves is shown in Figure 3, panel d. The gravimetric capacitance of the NCTs/ANPDM electrode achieved 1106 F/g at a current density of 5 A/g. In contrast, the gravimetric capacitance of the NCTs/MnO2 electrode was only 740 F/g at the same current D

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and plotted on the Ragone diagram are shown in Figure 4, panel d. A maximum gravimetric energy density and power density of 51 Wh/kg and 28 kW/kg were obtained, respectively, based on the total weight of the active materials. Such performance in this work is not only superior to that of many other MnO2-based SSCs,17,52 but also superior to those MnO2-based composite ASCs,53−59 as given in the same plot for comparison. The volumetric power and energy densities against the device stack, as well as a more comprehensive comparison with the previously reported devices, is also provided in Figure S8b. The maximum volumetric energy density is up to 0.021 mWh/cm3 (Figure S8b), which is higher than that of MWCNTs-based SSCs (0.008 mWh/cm3),60 single-walled carbon nanotubes-based SSCs (0.01 mWh/ cm3),61 and carbon/MnO2-based SSCs (0.009 mWh/cm3).62 Moreover, the SSC exhibits excellent cycling stability with high capacitance retention of 93% after 5000 charge/discharge cycles at a current density of 20 A/g (Figure 4e). This value is better than the values of H-TiO2@MnO2//H-TiO2@C (91.2% after 5000 cycles)63 and much higher than those reported for other aqueous and solid-state ASCs such as MnO2 NW/graphene// graphene (79% after 1000 cycles)64 and MnO2//graphene (83.4% after 5000 cycles).55 The excellent rate performance of the SSC could be attributed to the lower resistance and faster charge transport as revealed by the electrochemical impedance (EIS) in Figure 4, panel f. The Nyquist plots display typical diffusion-controlled Warburg capacitive behavior with a straight line in the low-frequency region. It is worth noting that there is nearly no semicircle in the high-frequency region, indicating an ultrasmall transfer resistance in SSC that ensures fast charge transfer between electrolyte and electrodes. The ASC device was fabricated with NCTs/ANPDM electrode (positive) and Fe2O3 nanorods electrode (negative). The Fe2O3 nanorods were chosen here for their suitable window in negative potential. To maximize the performance of the ASC device, the charge between the positive and negative electrodes should be balanced. The preparation, characterization (Figures S9−S12), and electrochemical performance (Figure S13) of the Fe2O3 electrode and the calculated weight ratio between the MnO2 and Fe2O3 electrodes are shown in the Supporting Information. The CV and GCD curves of the ASC at various scan rates and current densities were collected to evaluate its electrochemical performance and are shown in Figure 5, panels a−c. The rectangular-like shapes of these CV curves demonstrate their ideal capacitive behavior at scan rates from 5−100 mV/s (Figure 5a). Additionally, the CV retangular shape is well-maintained in the charge and discharge processes over a wide range of scan rates from 1000−10 000 mV/s (Figure 5b), which indicates a substantially high rate performance of the ASC. The ultrafast charge/discharge rate constitutes a breakthrough in the development of transition-metal-oxidebased thin-film pseudocapacitors, which is 10-times faster than PEDOT−PSS/RuO2·xH2O//PEDOT−PSS/RuO2·xH2O SSC (1 V/s),65 20-times faster than MnO2//rGO ASC (500 mV/ s),66 and five-times faster than MnO2//Fe2O3 ASC (2 V/s).67 The rate capability of the ASC is comparable to electrical double layer type SCs such as 3D rGO//rGO SSC (10 V/s).68 Particularly noteworthy is that the specific capacitance of the ASC was estimated as 168 F/g at 5 mV/s and still can be retained at 37 F/g even though the scan rate increased to 10 000 mV/s, further demonstrating the very good high-rate performance of the ASC device. The GCD curves at different current densities are shown in Figure 5, panel c. Discharge

Scheme 1. Schematic Illustration of the Fabrication of SolidState Thin-Film SC Devices

The performance of the SSC (NCTs/ANPDM//NCTs/ ANPDM) device was analyzed through both CV and GCD experiments, as shown in Figure 4, panels a and b. CV curves present a symmetric shape during forward and backward scans at different scan rates, strongly suggesting the capacitive ability of the NCTs/ANPDM SC. GCD measurements further confirmed the superior performance of the solid-state SSC device. As shown in Figure 4, panel b, the discharge curves are almost symmetrical to the corresponding charge curves, indicating the good capacitive behavior of the SSC device. The gravimetric capacitance calculated based on the discharging curves could reach 576 F/g at a current density of 1 A/g and still retain 284 F/g at a higher current density of 20 A/g (Figure 4c). The corresponding volumetric capacitance (volume is based on the whole device) was calculated to be 240 mF/cm3 at a current density of 1 A/g, and 119 mF/cm3 at a current density of 20 A/g, respectively (Figure S8a). Energy density and power density constitute two important parameters for the SC device. The energy density (E) and power density (P) of the solid-state SSC calculated from GCD E

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Figure 4. (a) CV curves of the assembled solid-state SSC device collected at different scan rates. (b) GCD curves of SSC device collected at different current densities. (c) Gravimetric capacitance calculated from the discharge curves as a function of current density. (d) Ragone plots of the solidstate SSC device. (e) Cycle performance of the SSC device measured at a current density of 20 A/g. (f) EIS Nyquist plots for the SSC device.

Figure 5. (a, b) CV curves of the assembled solid-state ASC device collected at different scan rates. (c) GCD curves of ASC device collected at different current densities. (d) Volume-specific capacitance and gravimetric capacitance calculated from the discharge curves as a function of current density. (e) Cycle performance of the ASC device measured at a current density of 8.40 mA/cm3. (f) EIS Nyquist plots for the ASC device.

higher than the values reported for other solid-state SCs such as TiO2@PPy-SC device (0.013 mWh/cm3)71 and TiO2@Cbased SSCs (0.011 mWh/cm3).72 Significantly, the solid-state ASC device exhibits cycling stability with 97% retention of its initial capacitance even after 5000 charge/discharge cycles at a current density of 8.40 mA/cm3 (Figure 5e). The retention rate is even better than that of SSC. To elucidate the electrochemical performance characteristics, an EIS test was further conducted. As shown in Figure 5, panel f, a nearly straight line together with a ultrasmall ESR (0.7 Ω), in agreement with GCD, again verified that the ASC behaves as an ideal capacitor. The ultrafast charge/discharge behavior of the ASC, as compared with the SSC, should be due to the fact that the Fe2O3 nanomaterials are more suitable for the negative electrode for their appropriate working window in negative potential. When compared with the last reports for MnO2// Fe2O3-ASCs,70,73 the faster charge/discharge behavior of the ASC device is obviously attributable to the following unique features. (1) The NCTs scaffold possesses better wettability and electric conductivity for facilitating fast electron transport

curves are almost symmetrical to the corresponding charge curves, indicating good capacitive behavior for the ASC device. In addition, these GCD curves show only an ultrasmall voltage drop (