Nickel-Doped Ultrathin K-Birnessite Manganese Oxide Nanosheet As

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Nickel-doped ultrathin K-birnessite manganese oxide nanosheet as pseudocapacitor electrode with excellent cycling stability for high-power pesudocapacitors Junming Chen, You Liu, Guiling Wang, Jiahao Guo, and Xuchun Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02363 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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ACS Sustainable Chemistry & Engineering

Nickel-doped ultrathin K-birnessite manganese oxide nanosheet as pseudocapacitor electrode with excellent cycling stability for high-power pesudocapacitors Junming Chen a, You Liu a, Guiling Wang a, *, Jiahao Guo a, Xuchun Wang a a

College of Chemical and Materials Engineering, Anhui Science and Technology University, Donghua Road No.9, Fengyang, 233100, China

Corresponding

author:

Guiling

Wang,

Tel.:

0086-550-6732385.

E-

mail:[email protected]

Abstract We herein report a kind of nickel-doped ultrathin δ-MnO2 nanosheets prepared using a facile chemical bath deposition method. The obtained δ-MnO2 materials have 2D ultrathin nanosheet structures with a few atomic layers. Electrochemical measurements indicate that appropriate amount of nickel doping can remarkably improve the specific capacitance of the δ-MnO2, and that 1.0 mol% nickel-doped δ-MnO2 nanosheets display the best specific capacitance of 337.9 F g-1 at 1 A g-1. The specific capacitance can maintain at 158 F g-1 even as the current density increases to 20 A g-1, demonstrating the electrode material possesses good rate performance. In addition, the discharge capacity fading from 160.9 to 158.8 F g-1 is slight after 4000 cycles, and the corresponding capacitance retention is as high as 98.6 %. The good rate capacity and stability

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of the δ-MnO2 nanosheets can be attributed to the ultrathin structure of a few atomic layers which provides large surface areas and lots of reactive active sites. Moreover, the appropriate amount of nickel ion doping at atomic level improves the conductivity of the δ-MnO2 material. Keywords: Manganese dioxide; Ultrathin nanosheet; Cycling stability; Rate performance; Supercapacitor 1. Introduction Recently, supercapacitors have attracted tremendous interest of researchers because of their high power density, excellent cycle life and fast charge-discharge rate.1,

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Generally,

supercapacitor materials can be classed into pseudocapacitor materials (mainly including transition metal oxides) and electric double-layer capacitor (EDLC) materials (carbon material is regarded as the most common) based on their electrochemistry reaction mechanisms. The specific capacitance of carbon materials is limited by the fact that their energy storage only depends on ion adsorption of double layers. However, transition metal oxides, such as MnOx ,3−5 Co3O4 ,6−8 NiO 9, 10 etc. show a more preponderant energy density because they store energy not only through ion adsorption but also through redox reactions. Thus more and more researchers set their sights on transition metal oxides as supercapacitor electrode materials. Among the transition metal oxides electrode materials, MnO2 has been extensively investigated as one of the most promising pesudocapacitor materials because of its high theoretical capacity (1370 F g−1 as the oxidation state of Mn ion changing from 4+ to 3+ over a potential window of 0.8 V), natural abundance, low cost and environmental friendliness.11


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However, the rate capacity and cycling stability of manganese oxides are undesirable for practical applications due to some major drawbacks including sluggish ionic transport rate and poor electrical conductivity.12 Numerous efforts have been devoted to overcome the disadvantages thus improving the electrochemical performance of manganese oxides.13-17 Up to now, one popular approach to introduce conductive materials (such as carbon, conductive polymers etc)

18–20

is to fabricate MnO2/conductive material nanostructures. However, most of

the conductive materials in these nanostructures are difficult to be incorporated to the structures of the manganese oxides, leading to the restriction of the enhancement in conductivity because the conductive amelioration for the bulk MnO2 is only limited to the interfaces between MnO2 and conductive materials.21 To increase the electrochemical utilization ratio of MnO2, doping with metal ions is one of the most effective methods.21, 22, 34 The electronic structures of bulk MnO2 can be optimized by metal ions doping, which can enhance the ionic/electrical conductivity as well as improve the rate capacity of MnO2 electrodes.21 In addition, with the rapid development of nanotechnology and wide application in the field of electrochemical energy storage, the different dimensional nano materials electrode has showed remarkably electrochemical reaction active.23-25 Ultrathin nanosheets materials was widely application in high performance electrochemical supercapacitors due to its large surface area.25-27 In fact, graphene materials firstly reported by Geim et al have demonstrated outstanding mechanical and electronic character.28 The discovery promotes the development of the ultrathin inorganic materials beyond grapene.

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Many layered inorganic materials, such as NiO,27


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MnO230-32,

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MoO2,33 Co3O4,29 etc, have weak chemical bonding between layers, thus, it is

possible that the layer materials can be exfoliated into single layer. In this work, we reported a facile method for the synthesis of nickel-doped ultrathin manganese oxide nanosheets. Firstly, ultrathin δ-MnO2 nanosheets are prepared using a liquid phase precipitation method. The ultrathin δ-MnO2 nanoflakes can provide a large amount of reactive active sites and high external surface areas. It is beneficial for the process of charge transfer during charge-discharge. Additionally, we adopt the nickel ion doping to optimize the electronic structure of the ultrathin MnO2 nanosheets, which is expected to improve the intrinsic conductivity, and meanwhile contributes to the enhancement in the cycling stability and high rate performance for supercapacitors. 2. Experiment section 2.1. Material Synthesis All the chemical reagents were of analytically pure grade and used as received. Nickeldoped ultrathin manganese oxide nanosheets were synthesized using a facile chemical bath deposition method. Typically, proper amount of manganese acetate acid and nickel acetate acid in different molar ratios of 0%, 0.5%, 1%, 2.5%, 5% (relative to Mn(Ac)2, and the total amount of metal ions is 2.8 mmol), EDTA (ethylene diamine tetraacetic acid disodium salt, to chelate Mn2+ ion) of 1.16 g and proper amount of SDS (sodium dodecyl sulfonate, as surface active agent) were dissolved in 50 mL distilled water under stirring. Subsequently, 0.25 M NaOH aqueous solutions (50 mL) were added to the above solution. Then, 0.12 M 50 mL K2S2O8


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aqueous solution was added dropwise into the above solution to obtain the precipitation product through chemical deposition reaction. The solution was maintained at 30 ºC in a water bath for 12 h to gain the nickel-doped ultrathin manganese oxide nanosheets which were marked as NMO-1, NMO-2, NMO-3, NMO-4, NMO-5, respectively. Afterwards, the obtained deposition products were filtered and washed with water and ethanol solution, and dried at 80 ºC. 2.2. Materials characterization The XRD patterns were tested on a RigakuSmart Lab X-ray diffractometer operated at 40 kV using a Cu Kα radiation at a scanning rate of 5

o

min−1. The morphologies and

microstructures of the samples were observed using field-emission scanning electron microscopy (FE-SEM, Carl Zeiss Super55 operated at 5 kV) and field-emission transmission electron microscope (TEM, Hitachi HT7700 operated at 120 kV), respectively. Atomic force microscopy (AFM, Bruker Multimode 8) was used to observe the surface morphologies at room temperature in nitrogen. 2.3. Electrochemical properties All the electrochemical properties were tested in a three-electrode system consisting of a working electrode, a counter electrode, and a reference electrode using the saturated Hg/HgO electrode in 6 M KOH aqueous electrolyte. The working electrodes were composed of 80 wt% as-prepared samples, 10 wt% acetylene black as conductive additive, and 10 wt% PTFE (1 wt % suspension in water) as binder. The mixed slurry was coated on nickel foam with an area of 1 cm × 1 cm and dried at 80 °C for 12 h, and then pressed under a pressure of 10 MPa for 10 min. The


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loading mass of the active material is about 1-2 mg cm-2. The prepared process of the counter electrode was the same as that of the working electrode. The homogenous slurry obtained by mixing activated carbon, acetylene black, and PTFE in a weight ratio of 80:10:10 were smeared onto nickel foam of 2.5 cm × 2.5 cm. The supercapacitors were assembled and tested by galvanostatic charge/discharge testing system (NEWARE, Shenzhen, China) at different current densities within the potential window of −0.1 to 0.55 V vs. Hg/HgO. The cyclic voltammetry (CV) measurement was performed from − 0.1 to 0.55 V at different scanning rates. The electrochemical impedance spectroscopy (EIS) was measured within a frequency range of 0.01 to 100000 Hz. Both CV and EIS were performed on a CHI 660E electrochemical workstation. The specific capacitance of the electrode materials was estimated from the charge-discharge test according to the following equation: Csp=I×∆t/(∆V×m ), where Csp is the specific capacitance (F g−1 ), I is the charge-discharge current (A), ∆t is the discharge time (s), ∆V is the chargedischarge potential window (V), and m is the mass of the active materials (g). 3. Results and discussion X-ray diffraction (XRD) test was performed to study the phase structure of the as-obtained samples, as shown in Figure 1a. It can be seen that all the observed peaks of the nickel-doped MnO2 nanosheets are well indexed as layered delta-phase MnO2 (JCPDS card no. 43-1456) and can be clearly identified without any impurity peaks. In addition, all the diffraction peaks intensities of the samples are relatively weak and the peak are broad, suggesting that the structure


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of the prepared materials is poorly crystallized. The poor crystallization of supercapacitor materials can easily offer access for ion penetration through the bulk of the active materials.35 The morphologies of the ultrathin δ-MnO2 nanosheets with different nickel percentages at low and high magnifications are shown in Figure 1b-f. It can be seen from a typical SEM image (Figure 1b) that the pure δ- MnO2 nanoflakes have an ultrathin structure and lamellar morphology with graphene-like folds. Moreover, the lateral size of the interwoven ultrathin δMnO2 nanosheets is ca. several micrometers. After doping 0.5 mol% nickel, the change of the product morphologies is small as shown in Figure 1c. With further increasing nickel content to 1.0 mol% (Figure 1d), the samples still possess graphene-like structure with the size of several micrometers. However, when nickel percentage reaches 5.0 mol% (Figure 1f), well-formed graphene-like nanosheets no longer exist; instead, nanosheets with indistinct packed structure with larger size are obtained. The morphology evolution of the products manifest that the graphene-like structure of ultrathin δ-MnO2 nanosheets with proper particle size and high surface area can be maintained if the amount of nickel ion doping is in a proper range. Energy-dispersive X-ray spectrometry (EDS) maps were performed to analyze the elemental compositions of the the nickel-doped ultrathin δ-MnO2 nanosheets in Figure 2. Figure 2a-e show the typical images of the NMO-1, NMO-2, NMO-3, NMO-4 and NMO-5 samples respectively, as well as the corresponding recorded elemental signals. It can be clearly seen that the K−K signal is also evenly distributed in the five products. We infer that K+ ion exists in the layered crystal structure of δ-MnO2. It can stabilize the crystal structure of δ-MnO2 in process of


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charge-discharge. In addition, to further illustrate the nickel ion doping at atomic level, the TEM in-situ elemental mapping was employed to analyse the nickel element distribution in a single nanosheets, as illstrated in Figure S1 (See Supporting Information). It is clearly indicated from Figure S1 that the Ni-K signal could be detected all over the entire nanosheets for the doped samples, suggesting that the nickel has really got into the crystal lattice of the ultrathin δ-MnO2 nanosheets instead of existing as a separate phase, which is consistent with the XRD result. Moreover, ICP analysis is used to investigate the nickel content of sample powder in Table S1 (See Supporting Information). It showed that the obtained the nickel content of final sample powder is nearly consistent with that of the starting materials. A typical transmission electron microscope (TEM) image of the ultrathin δ-MnO2 nanosheets are showed in Figure 3a. It indicates that the ultrathin δ-MnO2 nanosheets with the size of several micrometers present the graphene-like lamellar morphology, and it is nearly transparent to the electron beam. Moreover, the SAED pattern (inset of Figure 3a) displays two diffraction rings with polycrystal characteristic, which is consistent with the (110) and (200) reflections of layered δ-MnO2. To further investigate the microstructure of the ultrathin δ-MnO2 nanosheets, high resolution TEM (HR-TEM) image is captured, as shown in Figure 3b. It can be seen that the interplanar spacing between the adjacent lattice planes are approximately 0.49 nm, corresponding to the (200) crystal plane of δ-MnO2 (Figure 3k). Figure 3c and Figure 3e show the transmission electron microscope (TEM) images of NMO-2 and NMO-3 samples, respectively. It is clearly observed that the samples are still the graphene-like structure with


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several micrometer size. However, well-formed graphene-like nanosheets no longer exist with further increasing nickel content (Figure 3g and i); instead, nanosheets with indistinct packed structure and larger size are obtained. The TEM images are consistent with SEM result. In addition, the nickel-doped ultrathin δ-MnO2 nanosheets tend to get wrinkled and fold, so that we directly tested the thickness of the nanosheets from the vertical edges, and the thickness is only 2-3 atomic layers with ca. 2.4 nm as shown in Figure 3d, f and j. It suggests that the lattice fringes interplanar spacing is ca. 0.71 nm, corresponding to the (001) crystal planes of δ-MnO2 crystal (Figure 3d). The ultrathin δ-MnO2 nanosheets with a thickness of a few atomic layers are expected to possess some extraordinary chemical and physical characteristics. In addition, the atomic force microscopy (AFM) images are shown in Figure 4, from the AFM images, there are only nanoparticles with the size of dozens of nanometers can be observed, which is resulted from the agglomeration of ultrathin nanosheets. However, the test results for the fine nanoparticles indicate that the height of all the nickel doped ultrathin δ-MnO2 nanosheet samples along the blue line is approximately 2.6 nm, matching well with the measured thickness of 2.4 nm for ultrathin δ-MnO2 nanosheets along the direction of the vertical edges in TEM images. Moreover, the nitrogen adsorption/desorption curves test are performed to investigate BET surface areas of the NMO-1 and NMO-3 samples in Figure S2 (See Supporting Information). The surface area of the nickel doping ultrathin δ-MnO2 nanosheets (NMO-3) is calculated to be 106.4 m2g-1, which is higher than that of ultrathin δ-MnO2 nanosheets (81.9 m2g-1). The large surface area of nickel-


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doped ultrathin δ-MnO2 nanosheets can provide more active sites and fast charge transfer during charge-discharge, thus enhancing the high rate and cycling capability of surpercapacitors. The electrochemical properties of the nickel-doped ultrathin δ-MnO2 nanosheets were investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charging-discharging techniques. Figure 5a shows the typical charge-discharge curves of the five samples at 0.5 A g-1. It can be seen that the NMO-3 exhibits the highest specific capacitance of 394.8 F g-1. The superior electrochemical performance of NMO-3 is further studied by galvanostatic charging-discharging test at different current densities, as illustrated in Figure 5b. The large distortion of the charge-discharge curves is caused by the Faradic pseudocapacitance contribution of the sample. Figure 5c displays the calculated specific capacitance of the NMO-3 sample as a function of discharge current density. It can be seen that the sample has a high specific capacitance of 337.9 F g-1 at 1 A g-1. The specific capacitance can maintain at 158 F g-1 as the current density increases to 20 A g-1, demonstrating good rate performance. In addition, the cycle life test of the NMO-3 sample at the high current density 20 A g-1 is also performed in Figure 5d. The discharge capacitance fading from 160.9 to 158.8 F g-1 is slight, and the corresponding capacitance retention is as high as 98.6 % after 4000 cycles, which is better than that of NMO-1(87.5% after 4000cycles in Figure S3). It indicates that the δ-MnO2 nanosheets with Ni doping has enhanced cycling stability and wide potential applications. Moreover, the SEM test was performed to observe the possible transformation of morphology for the ultrathin nanosheets after the long-term cycling, as shown in Figure S4 (See


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Supporting Information). It can be seen that the ultrathin nanosheets still retain their original structural characteristics. Therefore, the excellent rate performance and superior cycling stability of the NMO-3 sample is ascribed to the ultrathin two-dimensional structure and nickel ion doping at atomic level (doping Ni2+ tends to generate p-type dope and enhances the electron conductivity of the ultrathin MnO2 nanosheets). For ultrathin δ-MnO2 nanosheets, the 2D nanostructure can provide a large number of active sites for the electrochemical reaction during charge-discharge. The cyclic voltammetry (CV) curves of nickel-doped ultrathin δ-MnO2 nanosheets at a low scan rate of 2 mV s-1 are displayed in Figure 6a. It can be clearly seen that the area of CV curves for the NMO-3 sample is larger than those of other samples, suggesting superior pseudocapacitive character. Meanwhile, a pair of reversible redox peaks can be observed at around 0.35/0.5 V vs. Hg/HgO in 6 M KOH solution, corresponding to K+ ions insertion/extraction at the electrode/electrolyte interface. Furthermore, the CV curve of the NMO-3 sample reveals a nearly symmetrical shape as the scan rate increases to 500 mV s1

(Figure 6b), indicating excellent reversibility. To further investigate the electrochemical

behavior of different nickel doped ultrathin δ-MnO2, Figure 6c depicts the Nyquist plots of the five products. Those curves are all composed of a single semicircle at high frequency and a straight line at low frequency. The diameter of the semicircle for NMO-3 is smaller than that of others samples (the inset of Figure 6c), indicating a lower charge transfer resistance. This may be owing to the conductivity enhancement of ultrathin δ-MnO2 nanosheets with appropriate


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amount of nickel ion doping at atomic level.36 Meanwhile, the steeper slope of the straight line for NMO-3 than those of other samples demonstrate a better capacitance characteristic of NMO3. Figure 6d shows the Bode plots of the five samples. It is clearly seen that the phase angle for NMO-3 is closer to the ideal value of -90° compared to those of others, meaning the superior capacitance behavior with faster electrolyte ion response of NMO-3. To evaluate the potential applications of the nickel-doped ultrathin δ-MnO2 nanosheets, we fabricated a two-electrode asymmetric supercapacitor using the NMO-3 sample as the positive electrode, the commercialized porous carbon as the negative electrode, and 6 M KOH aqueous as the electrolyte. The energy density was calculated using the following equation: E =1/2CV2

(1)

Where C is the total cell discharge specific capacitance (F g-1) and V is the cell voltage window (v). The power density was calculated using the following equation: P=E/∆t

(2)

Where E is the energy density (Wh kg-1) and ∆t is the discharge time (s). Figure 7b displays the typical charge-discharge curves for the asymmetric supercapacitor at different current densities. It can be seen that the asymmetric supercapacitor exhibits high specific capacitances of 226.4, 190.9, 122.2, 100.2 and 70.5 F g-1 at the current densities of 0.2, 0.5, 1, 2, and 5 A g−1, respectively. The cycling performance of the asymmetric supercapacitor was tested at a current density of 10 A g-1 in Figure S5 (See Supporting Information). It shows that the corresponding capacitance retention is as high as 86.7 % after 5000 cycles. Moreover,


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the asymmetric supercapacitor shows excellent rate performance at various discharge current densities, as illustrated in Figure 7c. Figure 7d depicts the Ragone plot of the asymmetric supercapacitor. A maximum energy density of 75.5Wh kg-1 and a maximum power density of 8.2 kW kg-1 are calculated based on the total mass of NMO-3 and porous carbon, which demonstrates that the appropriate amount of nickel doped ultrathin δ-MnO2 nanosheets is a very promising electrode material. 4. Conclusions The nickel-doped ultrathin δ-MnO2 nanosheets have been synthesized through a facile chemical bath deposition approach. The different doping content of nickel does not affect the morphology of ultrathin δ-MnO2 nanosheets, but changes the electron conductivity. When the doping nickel content is 1.0 mol%, the ultrathin δ-MnO2 nanosheet electrode exhibits desirable rate capacity and superior cycling stability. The excellent electrochemical performances of the δMnO2 nanosheets are attributed to the ultrathin two-dimensional structure which offers large surface area and lots of active sites during electrochemical reaction. Additionally, the improvement of conductivity of bulk materials is realized by the appropriate amount of nickel ion doping at atomic level. ASSOCIATED CONTENT Supporting Information TEM in-situ elemental mapping images of NMO-3, Nitrogen adsorption/desorption curves, Long-term cycle performance of NMO-1 and SEM images of NMO-3 electrode after charge-


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discharge for 4000 cycles. This information is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgment We are grateful for the financial support from the Natural Science Foundation of Anhui Province (Grant No. KJ2013A079), Talent foundation of Education Department of Anhui (Grant No. gxfx ZD2016176), Natural Science Foundation of Anhui Science and Technology University (ZRC20 16486), Materials Science and Engineering Key Discipline Foundation (AKZDXK2015A01), Na tural Science Foundation of Anhui Province (Grant No. KJ2016A171) and Talent Introduction Pr oject of Anhui Science and Technology University.


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Figure Captions Figure 1 XRD patterns (a) and SEM images of NMO-1 (b), NMO-2 (c), NMO-3 (d), NMO-4 (e), and NMO-5 (f) samples. Figure 2 EDS mappings of NMO-1 (a), NMO-2 (b), NMO-3 (c), NMO-4 (d), and NMO-5 (e) samples. Figure 3 TEM and HR-TEM images of NMO-1 (a, b), NMO-2 (c, d), NMO-3 (e, f), NMO-4 (g, h), and NMO-5 (i, j) samples. Crystal structure images of δ-MnO2 (k). The inset of a, c, e, g and i are the corresponding SAED patterns. Figure 4 AFM images of NMO-1 (a), NMO-2 (b), NMO-3 (c), NMO-4 (d), and NMO-5 (e) samples. Figure 5 Galvanostatic charge-discharge curves of NMO-1, NMO-2, NMO-3, NMO-4, and NMO-5 samples (a). Galvanostatic charge-discharge curves (b) and specific capacitance of the NMO-3 sample at different current densities (c). Long-term cycle performance of the NMO-3 samples at 20 A g-1 (d). Figure 6 CV curves (a), Nyquist plots (c) and Bode plots (d) of NMO-1, NMO-2, NMO-3, NMO-4, and NMO-5 samples. CV curves of NMO-3 sample at various scan rates (b). Figure 7 The schematic illustration of the asymmetric supercapacitor configuration (a). Galvanostatic charge-discharge curves (b), cycling performance (c) and Ragone plot (d) of the asymmetric supercapacitor at various current densities.


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Figure 1 XRD patterns (a) and SEM images of NMO-1 (b), NMO-2 (c), NMO-3 (d), NMO-4 (e), and NMO-5 (f) samples.


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Figure 2 EDS mappings of NMO-1 (a), NMO-2 (b), NMO-3 (c), NMO-4 (d), and NMO-5 (e) samples.


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Figure 3 TEM and HR-TEM images of NMO-1 (a, b), NMO-2 (c, d), NMO-3 (e, f), NMO-4 (g, h), and NMO-5 (i, j) samples. Crystal structure images of δ-MnO2 (k). The inset of a, c, e, g and i are the corresponding SAED patterns.


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Figure 4 AFM images of NMO-1 (a), NMO-2 (b), NMO-3 (c), NMO-4 (d), and NMO-5 (e) samples.


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Figure 5 Galvanostatic charge-discharge curves of NMO-1, NMO-2, NMO-3, NMO-4, and NMO-5 samples (a). Galvanostatic charge-discharge curves (b) and specific capacitance of the NMO-3 sample at different current densities (c). Long-term cycle performance of the NMO-3 samples at 20 A g-1 (d).


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Figure 6 CV curves (a), Nyquist plots (c) and Bode plots (d) of NMO-1, NMO-2, NMO-3, NMO-4, and NMO-5 samples. CV curves of NMO-3 sample at various scan rates (b).


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Figure 7 The schematic illustration of the asymmetric supercapacitor configuration (a). Galvanostatic charge-discharge curves (b), cycling performance (c) and Ragone plot (d) of the asymmetric supercapacitor at various current densities.


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Table of Contents

Synopsis: The nickel-doped δ-MnO2 nanosheets obtained by a facile chemical bath deposition method. have 2D ultrathin nanosheet structures with few atomic layers. The specific capacitance can maintain at 158 F g-1at 20 A g-1 and the corresponding capacitance retention is as high as 98.6 % after 4000 cycles.


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Nickel-Doped Ultrathin K-Birnessite Manganese Oxide Nanosheet As

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