Hierarchical MnO2 Spheres Decorated by Carbon-Coated Cobalt

Subscriber access provided by KUNGL TEKNISKA HOGSKOLAN

Article

Hierarchical MnO2 spheres decorated by carbon coated cobalt nanobeads: low cost and high performance electrode materials for supercapacitors Jian Zhi, Oliver Reiser, and Fuqiang Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12779 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Hierarchical MnO2 spheres decorated by carbon coated cobalt nanobeads: low cost and high performance electrode materials for supercapacitors Jian Zhia,b*, Oliver Reisera and Fuqiang Huangb,c* a

Institute of Organic Chemistry, University of Regensburg, Universitätsstr.31, 93053

Regensburg, Germany. b

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics,

Chinese Academy of Sciences, Shanghai 200050, P.R. c

Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth

Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China. Email: [email protected], [email protected]. KEYWORDS: MnO2 spheres, cobalt nanobeads, low cost, high performance, symmetrical supercapacitor

ABSTRACT: MnO2 is a promising electrode material for supercapacitors, because it exhibits high theoretical specific capacitance (1380 Fg-1) for electrical charge while also being inexpensive and environmentally benign. However, owing to its low electrical conductivity, the intrinsic pseudocapacity of MnO2 is not fully utilized. In this work, hierarchically structured spheres composed of MnO2 nanoplatelets and carbon coated cobalt nanobeads (MnO2-

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

NPs@Co/C) are chosen as electrode materials for supercapacitor. With a Co/C mass loading of 19 wt%, the electrical conductivity of the hybrid is 122 folds larger than that of pristine MnO2, showing a specific capacitance of the constituent MnO2 as high as 1240 Fg-1, being close to the theoretical value. Such improved specific capacitance of MnO2-NPs@Co/C electrode is largely contributed from the enhanced double-layer charging and Faradaic pseudocapacity of MnO2. Moreover, the fabricated symmetrical supercapacitor also exhibits excellent cycling stability with 89.1% capacitance retention over 10000 cycles, as well as high energy densities in both aqueous and organic electrolyte (24 Wh kg-1 and 33 W kg-1, respectively). Compared with frequently used noble metals to enhance the electrochemical performance of MnO2, the utilization of low cost Co/C nanobeads is proven to be more efficient and thus showing great potential for commercial application.

1. INTRODUCTION Supercapacitors have received considerable attention as promising candidates for energy storage owing to the high capacity, power density and long cycle stability compared to traditional batteries.

1-2 3-4

They have been extensively applied in pulse laser techniques, energy

management systems and hybrid electric vehicles. The high electrical performance of supercapacitors mainly results from a large ion-penetrable area and reversible reactions of the active materials.5 Carbon based materials with a porous structure, transition-metal oxides and conjugated polymers are quintessential active materials for the electrode of supercapacitors. 6-7 Among the transition-metal oxides, MnO2 is a promising pseudocapacitive materials due to the low fabrication cost and large theoretical specific capacitance.8 However, owing to its semiconducting character, the electrical conductivity of MnO2 is relatively low (10-5-10-6 S/cm). As a result, the reported specific capacitance of MnO2 (normally 200-400 Fg-1) is far lower than


2

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


its theoretical value (1380 Fg-1). hybrid

materials,

such

as

9-12

To effectively use MnO2, various MnO2/conductive matrix

MnO2/Zn2SnO4,13

MnO2/graphene14,

MnO2/Cu,15

and

MnO2/conducting polymers16 have been used for supercapacitors, achieving remarkable cycling and rate performance. However, in these cases, the weight mass loading of MnO2 is relatively low, and the composites exhibit only little improvement of capacitance compared with pristine MnO2. Thus, the high pseudocapacitive potential of MnO2 is still not fully developed. Incorporating highly conductive noble metals, such as Au, Ag and Pt into MnO2 matrix was also proven to be an efficient strategy to increase the specific capacitance by reducing chargetransfer resistances.17-21 For example, with a silver content of only 5±0.6%, the specific capacitance of Ag-MnO2 hybrid films can reach up to 770 Fg-1 under 2 mVs-1.22 In addition, a Au/MnO2 core-shell nanowire hybrid electrode was reported showing a capacitance of 1145 Fg-1 under 50 mVs-1, which is the highest value for a metal/MnO2 electrode to date.23 However, incorporating expensive noble metals in MnO₂ is economically not viable. Therefore, low cost alternatives are needed to increase electrical conductivity of MnO2 in order to provide an ideal hybrid electrode for supercapacitors with excellent capacitance and rate stability. Recently, Stark et al. reported ferromagnetic carbon coated cobalt nanobeads (denoted as Co/C NBs), using reduced flame-spray pyrolysis technology on large scale (>30 g h-1).24-25 The deposition of a thin carbon layer (less than 2 nm) over the pyrophoric metal core enables a remarkable thermal stability,26 without adversely affecting the magnetization (MSbulk = 158 emu g-1). In our previous work, we have reported the synthesis of three-dimensional (3D) hierarchical spheres emplying MnO2 nanoplatelets (NPs) as building blocks, in which Co/C NBs are homogeneously distributed.27 These hybrid particles (denoted as MnO2-NPs@Co/C) were


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

evaluated as catalysts towards oxidation of alcohols, showing high activity with excellent recyclability, which can simply be achieved by magnetic decantation. The unique hybrid structure inspired us to search for different applications of such MnO2NPs@Co/C composites. Besides their high magnetization, Co/C nanocomposites also have exceptional electrical conductivity being comparable to Pt, Pd and Ag.28-30 We therefore questioned if in combination with pseudocapacitive MnO2, the resulting composites MnO2NPs@Co/C would have potential for energy storage devices by evaluating its electrochemical properties and thus being applicable as electrode materials in a symmetrical supercapacitor.

2. EXPERIMENTAL SECTION 2.1 Synthesis of MnO2-NPs@Co/C Carbon coated cobalt nanobeads (Co/C NBs, 11wt%-13wt% carbon) were purchased from Turbobeads Llc, Switzerland. MnO2-NPs@Co/C (see SI for details) was prepared as follows. Firstly, 10 mL of water suspension of Co/C NBs (15 mg mL−1) was subjected to ultrasonic vibration for 10 min. Then KMnO4 (1.1 g) were added. The obtained suspension was sealed in silica tubes and heated in a microwave reactor to 140 °C for 10 min, and the obtained powders were obtained by a magnet and washed with water. The black powders were dried at 100 °C for 12 h in an oven.

Finally, the obtained powers were mechanical milled for 10 min, and

transferred to 50 ml distilled water in a flask under continuous stirring. After that, MnSO4•H2O (0.254g, 1.5 mmmol) and KMnO4 (0.158 g, 1.0 mmol) was added to the mixture under stirring. Then 10 ml concentrated H2SO4 was added, and the colour of the suspension changed to black, confirming the reaction between KMnO4 and MnSO4 occurred.31 The solution was stirred for 45 min at a temperature of 60°C, and the obtained precipitate was collected by external magnet,


4

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


washed with ethanol. The black powders were dried in an oven at 70 °C overnight and MnO2NPs@Co/C was finally obtained. The synthesis of MnO2-NPs@Co/C with different mass loadings (5, 10, 15, 19, 30 wt%) of Co/C NBs were synthesized by changing the employed amount of Co/C NBs. The mass loading (19 wt%) of Co/C in the composite was determined by ICP-AES (inductively coupled plasma-atomic emission spectroscopy, ICPS-7500). 2.2 Characterization SEM (Scanning electron microscopy) were taken with a Zeiss Gemini 1530 FEG microscope at 5 kV. TEM (Transmission electron microscopy) measurements were performed on a FEI Tecnai F30 FEG microscope. N2 sorption-desorption isotherms were plotted using a MicromeriticsModel Tristar 3000 analyzer at 77 K. The BET (Brunauer-Emment-Teller) method was employed to calculate the corresponding surface areas. XPS experiments were performed on a PHI-5000C ESCA system (Perkin-Elmer). The electrical conductivity was measured using an Accent HL5500 Hall System. 2.3 Electrochemical Characterization Electrodes used for a symmetrical super were fabricated as follows: MnO2 materials (MnO2NP@Co/C or MnO2-NP; 90 wt%) were mixed with a NMP solution of PVDF (polyvinylidene fluoride, 10 wt%) to obtain a slurry. Then the slurry was coated onto graphite paper, which acts as current collector. After drying at 100 ° C, supercapacitor electrode was obtained. Symmetrical cells were built by assembling the fabricated electrode with a polypropylene film as separator, in a glass cell containing 1 M Na2SO4 aqueous solution or 1M LiPF6 in EC-DEC (ethylene carbonate (EC) and diethyl carbonate (DEC), volume ratio is 1:1) as the electrolyte. All the electrochemical measurements were performed using CHI 660B (CH Instruments Inc.).


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

The specific capacitance (based on one electrode in symmetrical supercapacitor), as well as energy and power density of the whole cell were obtained using the equations in supporting information. 3. RESULTS AND DISCUSSION

Figure 1. (a) TEM image of Co/C NBs. (b) SEM image of MnO2-NPs@Co/C. (c-d) TEM and HAADF-STEM images of MnO2-NPs@Co/C.

Figure 1a shows TEM image of Co/C NBs, in which aggregated nanobeads with different diameters (20-70 nm) can be clearly observed. Utilizing the carbon layer of Co/C nanobeads as the reducing agent and KMnO4 as starting materials, MnO2-NPs@Co/C hierarchical spheres (81 wt% MnO2 determined by ICP-AES, figure 1b) were obtained, using a mechanochemical assisted protocol as previously described.27 In this process, the transformation of the MnO2


6

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


morphology to a highly porous structure occurred,.27 TEM and HAADF-STEM (high-angle annular dark field scanning transmission electron microscopy) images of MnO2-NPs@Co/C (Figure 1c and d) show that many nanoplatelets assembled into sphere-like MnO2 architectures. Such interconnected small nanoplatelets create abundant space, which ensures a facile electrolyte ion transport and more electroactive species on the surface of the electrode. Furthermore, the black areas in Figure 1c clearly show the embedded Co/C NBs inside the MnO2 spheres, which are confirmed by EDX line scanning spectra (Figure S1). From the EDX maps, Mn and Co are seen in green and red to overlap of the composites (STEM, Figure S2). For comparison, nanoplatelets assembled pristine MnO2 spheres (denoted as MnO2-NPs), without embedding of Co/C NBs were also synthesized through a similar protocol, and the corresponding TEM and SEM images are shown in Figure S3 and Figure S4, demonstrating that the existence of Co/C NBs doesn't change the morphology of the s. X-ray diffraction pattern of MnO2-NPs@Co/C was shown in Figure S5. The 2 θ peaks at 38.3°, 42.2° and 57.2° 2 belong to γ-MnO2 (JCPDS 14644).27 XRD patterns of MnO2-NPs@Co/C show quite low crystallinity, which might be of advantage since it is reported that defects or amorphous phase of MnO2 may offer much higher electrochemical performance.32


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

Figure 2. (a) Full XPS spectra of MnO2-NPs@Co/C. (b,c) XPS spectra of Co 2p and Mn 2p region of MnO2-NPs@Co/C. (d) N2 isotherms of MnO2-NPs@Co/C and corresponding pore size distribution.

The metal oxidation states of MnO2-NPs@Co/C are measured by XPS (X-ray photoelectron spectroscopy). Figure 2a shows the complete spectrum of MnO2-NPs@Co/C nanocomposites, the peaks of Mn 2p, Co 2p, O1s and C 1s are all included. At a binding energy of at 778.2 eV and 794.2 eV, the two peaks are consistent with the Co 2p3/2 and Co 2p1/2 of metallic Co species (Figure 2b).27, 33 Owing to the carbon shell on the surface of the Co nanobeads, no satellite peaks of Co 2p3/2 and Co 2p1/2 can be found in the spectra, indicating that there is no phase formation of Co3O4 on the particles.34-35 The two peaks at binding energies of 641.9 and 652.8 eV correspond to the Mn 2p3/2 and Mn 2p1/2 spin-orbit peaks (Figure 2c). The high resolution XPS spectra of Mn 3s is shown in Figure S6. The energy separation of Mn 3s remains almost the same at a value of 4.72 eV, confirming that the Mn4+ ions are dominant in all the products.27, 36 Figure 2d show the nitrogen adsorption/desorption isotherm of MnO2-NPs@Co/C sample. Surface area of the MnO2-


8

Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NPs@Co/C calculated from BET method is measured to be 223 m2g-1. Such large value is crucial for the enhancement of the power/energy density of supercapacitors.

37

From the high

resolution SEM and TEM images of MnO2-NPs@Co/C (Figure S7 and S8), it is clear that such hierarchical mesopores are mainly originating from the spaces of the adjacent nanoplatelets. The hybrids have a broad pore size distribution (inset of Figure 2d) of small mesopore (below 10 nm) from the spaces of the adjacent nanoplatelets. Such a hierarchical pore structure will be favorable to accelerate the ionic diffusion and charge transfer in redox process of MnO2. Electrical measurements of MnO2-NPs@Co/C films coated on silica substrate were performed in an Accent HL5500 Hall System. Electrical conductivity of MnO2-NPs@Co/C as a function of Co/C NBs mass loading is shown in Figure S9. Owing to the high conductivity of Co/C NBs (about 1720 Scm-1), the conductivity of MnO2-NPs@Co/C film rapidly increases when the percentage of Co/C NBs is increased. Obviously, the Co/C NBs have a significant effect on the conductivity of MnO2 spheres. With a Co/C loading of 19 wt%, the composites conductivity reaches 9.7×10-3 Scm-1, 122 folds larger than MnO2 spheres (7.98 ×10-5 Scm-1). Apparently, a highly conductive MnO2 network is formed across the embedded Co/C NBs, which allows efficient and easy access of charged ions into MnO2 spheres during electrochemical test.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

Figure 3. (a) CV curves of MnO2-NPs@Co/C in 1.0 M Na2SO4 electrolyte at different scan rates. (b) Galvanostatic charge/discharge curves of MnO2-NPs@Co/C, MnO2-NPs and bare Co/C NBs at a current density of 1 Ag-1. (c)

Galvanostatic charge/discharge curves of MnO2-

NPs@Co/C at different current densities. (d) Specific capacitance of MnO2-NPs@Co/C and MnO2-NPs at different current densities.

The electrochemical behavior of the MnO2-NPs@Co/C (19 wt% Co/C NBs) was measured in a two electrode configuration. The MnO2-NPs@Co/C based electrode was obtained by coating MnO2-NPs@Co/C particles on graphite paper. No other carbonaceous conductive agent was added. The mass loading of MnO2-NPs@Co/C on each electrode is 8.5 mg cm-2. Figure 3a shows cyclic voltammogram (CV) curves of MnO2-NPs@Co/C under scan rates from 0.03 Vs-1 to 0.1 Vs-1 with a potential window of 0-0.8 V. The CV curves of MnO2-NPs@Co/C are all nearly rectangular shaped when the scan rates increase up to 0.1 Vs-1. The absence of redox


10

Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peaks demonstrates the fast and reversible charge and discharge process at a constant rate. Simon et al have proposed that successive multiple redox reactions between Mn3+ and Mn4+ happen at a constant rate, and the rectangular shape of CV curves is the sum of various redox peaks.4 As a result, MnO2 exhibit ideal capacitive behavior, which is quite different with other pesudocapacitive materials such as nickel and cobalt oxides. Figure 3b shows galvanostatic charge/discharge curves of MnO2-NPs@Co/C, MnO2-NPs and bare Co/C NBs at 1 Ag-1. The discharging time for the MnO2-NPs@Co/C hybrid materials is much larger than that of pristine MnO2-NPs, demonstrating their higher charge capacity, which is consistent with their CV curves at 0.05 Vs-1 (Figure S10). The charge-discharge curves of MnO2-NPs@Co/C and pristine graphite paper at 1 M Na2SO4 aqueous solution are shown in Figure S11, showing that the capacitance contributed from the graphite paper is negligible. Considering the similar morphology of these two materials, such capacitive improvement is due to the decoration of Co/C NBs, which act as “hubs” among MnO2 spheres to promote electron transport between MnO2 and electrolyte. Pristine Co/C nanobeads exhibit very low charge capacity, which mainly originates from the surface electrosorption of the charged ions on the carbon layers. The charge/discharge curves of MnO2-NPs@Co/C with different current densities (Figure 3c) show perfect linear slopes, indicating excellent reversibility of the MnO2-NPs@Co/C spheres. The calculated specific capacitance of MnO2-NPs@Co/C and MnO2-NPs (for one electrode of symmetrical cells) were plotted in Figure 3d. Encouragingly, MnO2-NPs@Co/C electrodes deliver maximum capacitances of 1087, 960, 854, and 758 F g-1 at 0.5, 2, 6 and 20 A g-1, respectively, suggesting the excellent capacitive performance. In contrast, for the specific capacitance in MnO2-NPs electrode, 465 Fg-1 was achieved at a current density of 0.5 Ag-1, thus reaching only 43% of the value of MnO2-NPs@Co/C. When the current density further increased


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

to 10 Ag-1, the corresponding value was dramatically reduced to 102 Fg-1, showing very poor rate capability. The mass loading of Co/C NBs is very important to determine the pore structure of MnO2-NPs (Table S1) and further impacts the specific capacitance of the composites (Figure S12, measured at 0.5 Ag-1). With a mass loading below 19 wt%, the introduction of Co/C NBs does not significantly impact the pore volume and surface area . All samples show similar porous structures. The 19 wt% mass amount of Co/C NBs appears to be optimal, exhibiting maximum specific capacitance (Figure S12). Further increase of the mass amount to 30 wt% results in a decrease of capacitance. Excess Co/C NBs can seal the nanopores of MnO2-NPs and greatly decrease the surface area and pore volume of the composites (Table S1), which prevents the effective contact between MnO2 and electrolyte. All these data clearly attest to the advantageous role of Co/C NBs in improving the pseudocapacitive activities of MnO2 spheres. The specific capacitance of MnO2-NPs@Co/C electrode described here is highly competitive with the values of MnO2 based composites in former works, which is shown in Table S2.18-20, 23, 38-41


12

Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Specific capacitance of the normalized specific capacitance of MnO2 at different current densities. (b) Nyquist plots of MnO2-NPs@Co/C and MnO2-NPs electrodes. (c) Cycling performance of MnO2-NPs@Co/C for 10000 cycles at 4 Ag-1. (d) Cycling stability of MnO2NPs@Co/C at progressively varied current densities.

To investigate the contribution of MnO2 to the total electrochemical performance of the MnO2-NPs@Co/C electrodes, the specific capacitance of MnO2 in the hybrids was obtained after subtracting the charge of the Co/C nanobeads according to their galvanostatic charge/discharge curves (see details in supporting information). The corresponding normalized capacitance of MnO2 as a function of current density is shown in Figure 4a. At 0.5 Ag-1, being optimal to achieve the best capacity performance, the value reaches up to 1240 Fg-1, which is 89% of the maximum theoretical value of 1380 Fg-1.42 When the current density is decreased to 0.2 Ag-1, current leakage becomes inevitable, which destroys the surface of electrode and leads to the detachment of MnO2. As a result, the measured capacitance is decreasing accordingly. At relatively high current density (e.g. 10 A g-1), only the surface of MnO2 nanoplatelets are involved into the redox process, resulting in the lower capacitance. Figure 4b presents the EIS curves of MnO2-NPs@Co/C and MnO2-NPs electrodes with a frequency range between 0.01 and 100 kHz. The corresponding equivalent circuit is also shown in Figure S13. As for the high frequency region, mainly two kinds of resistances have to be considered. The intercept Z’ axis represents the combined resistance (Rs) originating from the solution or electrolyte and the contact resistance between electrode material and current substrate.43-44 The semicircle stands for the transfer of charge at the interface between electrode and electrolyte. Through fitting, the obtained Rs and Rct values are 1.43 and 2.6 Ω, respectively, while the corresponding values of


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

MnO2-NPs are 5.2 and 7.6 Ω. The uniformly distributed Co/C nanobeads, which act as a highly conductive hub, not only facilitate the charge transportation on the surface of MnO2, but also significantly improve the ion mobility from MnO2 to graphite paper attributing to the high electrical conductivity of Co/C nanobeads (1720 Scm-1). Therefore, compared with pristine MnO2-NPs, both Rs and Rct are obviously decreased, which is favorable for fast chargedischarge and high power performance. Furthermore, from the corresponding equivalent circuit as shown in Figure S13, the calculated Warburg (W) values for MnO2-NPs@Co/C and MnO2NPs are 12.68 and 13.75 Ω, respectively. Because W is determined from the structure of the active materials, the similar values of W indicates that both samples show similar porous structure.45 The variation of measured capacitance as a function of cycling number under 4 Ag-1 current density was shown in Figure 4c. Even after 10000 cycles, the specific capacitance is still retained at 89.1%. In comparison, pristine MnO2-NPs only exhibit 57.2% retention after 10000 cycles. These results demonstrate the superior cycling stability of MnO2-NPs@Co/C over MnO2-NPs. Compared with 2D structured nanocomposites, the 3D structures of MnO2-NPs@Co/C ensures strong binding between MnO2 and Co/C components. As a result, during charge-discharge, Co/C nanobeads will not easily detach from the MnO2 matrix and therefore show impressive stability. The MnO2 nanoplatelets grown on the carbon layers of Co/C nanobeads provide faster channel for the ion transportation in electrolyte, while the volume change can be significantly restricted.46 In addition, the advantages of MnO2-NPs@Co/C as electrodes for supercapacitors is further evidenced by the cycling stability of the electrode at progressively increased and decreased current density (Figure 4d). Suffering from a sudden current change, the fabricated MnO2NPs@Co/C still maintain high cycle stability. Therefore, the obtained MnO2-NPs@Co/C feature


14

Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


not only superior rate capability but also good long-term cycle life, which plays an important role in industrial applications.

Figure 5. (a) Galvanostatic charge/discharge curves of MnO2-NPs@Co/C at different current densities in the electrolyte of 1M LiPF6/EC-DEC. (b-c) Specific capacitance as a function of current density and cycling performance of MnO2-NPs@Co/C in 1M LiPF6/EC-DEC electrolyte. (d) Ragone plots of MnO2-NPs@Co/C in both aqueous and non-aqueous electrolytes compared with the values of similar MnO2 based supercapacitors.

It is well known that MnO2 usually suffers from poor specific capacitance in organic electrolytes owing to its low ionic conductivities.47 In this study, MnO2-NPs@Co/C based supercapacitor with organic electrolyte (1M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC), volume

ratio of 1:1) was also fabricated, using the same symmetrical

configuration as aqueous electrolyte. The corresponding charge-discharge curves at different


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

current densities are shown in Figure 5a. Owing to the non-aqueous nature of the electrolyte, the operation voltages can reach up to 1.6 V. As a function of current density, the calculated specific capacitance is plotted in Figure 5b. At 2 Ag-1, the specific capacitance of the MnO2-NPs@Co/C supercapacitor reaches 379 Fg-1. The specific capacitance still retains 122 Fg-1 at 20 Ag-1, comparable to the values of similar MnO2-based supercapacitors in aqueous electrolytes.48 The charge-discharge curves of MnO2-NPs@Co/C and pristine graphite paper at 1 M LiPF6 in ECDEC solution are also shown in Figure S14. Similar as for aqueous electrolyte, the capacitance contribution from graphite paper is negligible in organic electrolyte. Figure 5c shows the cycling stability of the MnO2-NPs@Co/C supercapacitor with LiPF6/EC-DEC electrolyte. A gradual decrease to 82% of the initial value after 1000 cycles can be seen in this plot, thus showing acceptable cycling life in an organic electrolyte. The energy and power density are generally employed to investigate the performance of supercapacitors. The specific Ragone plot of MnO2NPs@Co/C symmetrical supercapacitor either in Na2SO4 aqueous electrolyte or non-aqueous LiPF6/EC-DEC electrolytes is shown in Figure 5d. To make a fair comparison, all the data in this table are normalized with the mass of MnO2 in the whole cell. The maximum energy density in these two electrolytes is 24 Wh kg-1 and 33 Wh kg-1, respectively. The higher energy density in organic electrolyte is attributed to its wider voltage window (0-1.6 V) than aqueous electrolyte (0-0.8 V). From Figure 5d, the calculated values of energy and power density in both electrolytes are not only superior than other MnO2 based symmetrical supercapacitors, such as Au-MnO2CNT,18 MnO2 nanowire-graphene49 and graphene-MnO2-CNT,50 but also highly competitive to asymmetrical supercapacitors, such as activate carbon//MnO2,51 PEDOT//MnO252 and graphene hydrogel//MnO2.53


16

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a,c) Specific capacitance as a function of scan rate (inverse square root) for MnO2NPs@Co/C and MnO2-NPs electrodes in (a) aqueous and (c) organic solvent electrolytes. (b,d) specific (inverse) capacitance as a function of scan rate (square root) for MnO2-NPs@Co/C and MnO2-NPs electrodes in (b) aqueous and (d) organic electrolytes. Inset: the linear part at low scan rates.

Trasatti and Lee have demonstrated an elegant method54-55 to quantitatively isolate the the capacitive elements from the diffusion controlled insertion processes.56 By plotting capacity as a function of scan rate, the capacitive contributions can be easily segregated. It is well known that when scan rate increases, specific capacitance decreases correspondingly. After plotting the data as a function of v-1/2 or v1/2 (Figure 6), the values of Csp can be extrapolated to v = 0 and v = ∞. If every reaction is prolonged sufficiently, Csp at 0 mV s-1 represents the total capacitance (CT), while the Csp at infinity is the charge stored at the surface (double-layer and Faradaic


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

pseudocapacitive charging, Csur). The capacitance derived from the insertion process (Cins) can be calculated after subtracting the surface capacitance from the total value (CT-Csur). Initially, the maximum capacitance associated with the surface charges (Faradaic pseudocapacitive charging in this case) can be obtained by plotting Csp, as a function of the scan rate (the inverse square root), υ-1/2, for MnO2-NPs@Co/C and MnO2-NPs (Figure 6a,c). At low scan rates, a linear fit of the data can be appraised, while at high scan rates linearity deviates considerably, being due to the resistance drops and irreversible redox transitions originated from MnO2. Applying a linear plot, the intercept (υ-1/2 = 0) demonstrates the maximum capacitance (blue area of bar graph in Figure 7). Moreover, the capacitance (inverse) as a function of the scan rate (the square root of ) for both samples are shown in Figure 6b,d. After extrapolating the curve at υ1/2 = 0, the total maximum capacitance can be obtained. As a result, the insertion process capacitance (Cins) is calculated (CT-Csur), as shown in red area of bar graph in Figure 7.

Figure 7. Bar graph of specific capacitance (Csp) of MnO2-NPs@Co/C and MnO2-NPs electrodes with Faradaic insertion capacity (red) and Faradaic pseudocapacitive charging (blue) derived from Trasatti's method.


18

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


These results demonstrate that the higher capacitance of MnO2-NPs@Co/C over MnO2-NPs in aqueous solvent originate from an increase in surface pseudocapacitive charge storage (455 to 935 Fg-1). Such improved capacitive charge storage is mainly due to the existence of Co/C nanobeads, which act as a hub to significantly facilitate the surface charge transfer. Owing to the low ion transfer rate in organic electrolyte, the impact of Co/C in charge transportation is more obvious than that in aqueous solvent. Surface capacitive charge storage in MnO2-NPs@Co/C is 175 Fg-1, 1.7 times higher than the value of MnO2-NPs (63 Fg-1). Such a higher surface charge value largely increase the capacitance of MnO2-NPs@Co/C in organic electrolyte. The mechanism analysis put forward quantitatively confirms the fully utilization of pseudocapacity of MnO2 in MnO2-NPs@Co/C sample. 4. CONCLUSION The incorporation of Co/C NBs into MnO2 nanoplatelets greatly accelerates the ionic diffusion and charge transfer in its redox processes, allowing for a high utilization efficiency of MnO2. The specific capacitance of the composites based on MnO2 constituent (81 wt% with a commercial mass loading of 8.5 mg cm-2) reaches a maximum value of 1240 Fg-1, i.e. 89% of the maximum theoretical value of MnO2. Moreover, such symmetrical device achieves high energy density in both aqueous Na2SO4 (24 Wh kg-1) and organic LiPF6/EC-DEC (33 Wh kg-1) electrolytes, and no significant capacitance decay occurred after even 10000 charge/discharge cycles. The much lower price of Co/C NBs compared to noble metals combined with the ultrahigh specific capacitances and excellent cycling stability makes MnO2-NPs@Co/C hybrid structures promising candidates as electrodes in supercapacitors with high electrical activity.

ASSOCIATED CONTENT


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

Supporting Information. Detailed calculations of specific capacitance, supplementary characterization figures and additional electrochemical plots (Figure S1-S14), tables for pore characteristics and comparison of the electrochemical performance (Table S1-S2),

these

materials are available free of charge in supporting information via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENTS This work was supported by the EU-ITN network Mag(net)icFun (PITN-GA-2012–290248), and NSF of China (Grants 91122034, 51125006, and 61376056). We are grateful to Ms. C. Meese and Prof. R. Witzgall in Department of Biology for SEM, Prof. Zweck in Department of Physics for TEM, Prof. Javier (ETH Zurich) for HAADF-STEM measurement, respectively.

REFERENCES 1. 2. 3.

4. 5.

Winter, M.; Brodd, R. J., What are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104 (10), 4245-4270. Burke, A., Ultracapacitors: Why, How, and Where is the Technology. J. Power Sources 2000, 91 (1), 37-50. Hu, C.-C.; Chang, K.-H.; Lin, M.-C.; Wu, Y.-T., Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors. Nano Lett. 2006, 6 (12), 2690-2695. Simon, P.; Gogotsi, Y., Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7 (11), 845854. Wang, G.; Zhang, L.; Zhang, J., A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41 (2), 797.


20

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6.

7.

8. 9.

10.

11. 12.

13.

14.

15.

16.

17. 18.

19.

20.

21.


Wang, D.-W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H.-M., 3D Aperiodic Hierarchical Porous Graphitic Carbon Material for High-Rate Electrochemical Capacitive Energy Storage. Angew. Chem. Int. Ed. 2008, 47(2): 373-376. Yuan, C.; Zhang, X.; Su, L.; Gao, B.; Shen, L., Facile Synthesis and Self-assembly of Hierarchical Porous NiO Nano/Micro Spherical Superstructures for High Performance Supercapacitors. J. Mater. Chem. 2009, 19 (32), 5772. Wei, W.; Cui, X.; Chen, W.; Ivey, D. G., Manganese Oxide-Based Materials as Electrochemical Supercapacitor Electrodes. Chem. Soc. Rev. 2011, 40 (3), 1697-1721. Yu, P.; Zhang, X.; Chen, Y.; Ma, Y.; Qi, Z., Preparation and Pseudo-capacitance of BirnessiteType MnO2 Nanostructures via Microwave-Assisted Emulsion Method. Mater. Chem. Phys. 2009, 118 (2), 303-307. Wang, X.; Yuan, A.; Wang, Y., Supercapacitive Behaviors and Their Temperature Dependence of Sol–Gel Synthesized Nanostructured Manganese Dioxide in Lithium Hydroxide Electrolyte. J. Power Sources 2007, 172 (2), 1007-1011. Zolfaghari, A.; Ataherian, F.; Ghaemi, M.; Gholami, A., Capacitive Behavior of Nanostructured MnO2 Prepared by Sonochemistry Method. Electrochim. Acta 2007, 52 (8), 2806-2814. Ghaemi, M.; Ataherian, F.; Zolfaghari, A.; Jafari, S., Charge Storage Mechanism of Sonochemically Prepared MnO2 as Supercapacitor Electrode: Effects of Physisorbed Water and Proton Conduction. Electrochim. Acta 2008, 53 (14), 4607-4614. Bao, L.; Zang, J.; Li, X., Flexible Zn2SnO4/MnO2 Core/shell Nanocable-Carbon Microfiber Hybrid Composites for High-Performance Supercapacitor Electrodes. Nano Lett. 2011, 11 (3), 1215-1220. Yu, G.; Hu, L.; Vosgueritchian, M.; Wang, H.; Xie, X.; McDonough, J. R.; Cui, X.; Cui, Y.; Bao, Z., Solution-Processed Graphene/MnO2 Nanostructured Textiles for High-Performance Electrochemical Capacitors. Nano Lett. 2011, 11 (7), 2905-2911. Pang, H.; Wang, S.; Li, G.; Ma, Y.; Li, J.; Li, X.; Zhang, L.; Zhang, J.; Zheng, H., Cu Superstructures Fabricated Using Tree Leaves and Cu–MnO2 Superstructures for High Performance Supercapacitors. J. Mater. Chem.A 2013, 1 (16), 5053-5060. Liu, R.; Duay, J.; Lee, S. B., Electrochemical Formation Mechanism for the Controlled Synthesis of Heterogeneous MnO2/Poly (3, 4-ethylenedioxythiophene) Nanowires. ACS nano 2011, 5 (7), 5608-5619. Liu, J.-S.; Hu, Y., Characterization of Manganese Oxides/Au Nanocompsite Thin Films for Supercapacitor. Int. J. Electrochem. Sci. 2013, 8, 9231-9241. Reddy, A. L. M.; Shaijumon, M. M.; Gowda, S. R.; Ajayan, P. M., Multisegmented AuMnO2/Carbon Nanotube Hybrid Coaxial Arrays for High-power Supercapacitor Applications. J. Phys. Chem. C 2009, 114 (1), 658-663. Wen, L.; Mi, Y.; Wang, C.; Fang, Y.; Grote, F.; Zhao, H.; Zhou, M.; Lei, Y., Cost‐effective Atomic Layer Deposition Synthesis of Pt Nanotube Arrays: Application for High Performance Supercapacitor. Small 2014, 10 (15), 3162-3168. Zhang, G.; Zheng, L.; Zhang, M.; Guo, S.; Liu, Z.-H.; Yang, Z.; Wang, Z., Preparation of Agnanoparticle-loaded MnO2 Nanosheets and Their Capacitance Behavior. Energy Fuels 2011, 26 (1), 618-623. Yan, W.; Kim, J. Y.; Xing, W.; Donavan, K. C.; Ayvazian, T.; Penner, R. M., Lithographically Patterned Gold/manganese Dioxide Core/shell Nanowires for High Capacity, High rate, and High Cyclability Hybrid Electrical Energy Storage. Chem. Mater. 2012, 24 (12), 2382-2390.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

22.

23. 24.

25. 26.

27.

28.

29. 30.

31. 32.

33.

34.

35.

36.

Page 22 of 24

Wu, X. M.; He, Z. Q.; Chen, S.; Ma, M. Y.; Xiao, Z. B.; Liu, J. B., Silver-doped Lithium Manganese Oxide Thin Films Prepared by Solution Deposition. Mater. Lett. 2006, 60 (20), 24972500. Lang, X.; Hirata, A.; Fujita, T.; Chen, M., Nanoporous Metal/oxide Hybrid Electrodes for Electrochemical Supercapacitors. Nat. Nanotechnol. 2011, 6 (4), 232-236. Grass, R. N.; Athanassiou, E. K.; Stark, W. J., Covalently Functionalized Cobalt Nanoparticles as a Platform for Magnetic Separations in Organic Synthesis. Angew. Chem. Int. Ed. 2007, 46 (26), 4909-4912. Stark, W. J.; Mädler, L.; Maciejewski, M.; Pratsinis, S. E.; Baiker, A., Flame Synthesis of Nanocrystalline Ceria–Zirconia: Effect of Carrier Liquid. Chem. Commun. 2003, (5), 588-589. Rossier, M.; Koehler, F. M.; Athanassiou, E. K.; Grass, R. N.; Aeschlimann, B.; Günther, D.; Stark, W. J., Gold Adsorption on the Carbon Surface of C/Co Nanoparticles allows Magnetic Extraction from Extremely Diluted Aqueous Solutions. J. Mater. Chem. 2009, 19 (43), 82398243. Zhi, J.; Mitchell, S.; Pérez‐Ramírez, J.; Reiser, O., Hierarchically Structured MnO2‐Co/C Nanocomposites: Highly Efficient and Magnetically Recyclable Catalysts for the Aerobic Oxidation of Alcohols. ChemCatChem 2015, 7 (17), 2585-2589. Lutsev, L. V.; Yakovlev, S. V.; Zvonareva, T. K.; Alexeyev, A. G.; Starostin, A. P.; Kozyrev, S. V., Microwave Properties of Granular Amorphous Carbon Films with Cobalt Nanoparticles. J. Appl. Phys. 2005, 97 (10), 104327. Wang, L.; Yu, Y.; Chen, P.-C.; Chen, C.-H., Electrospun Carbon–cobalt Composite Nanofiber as an Anode Material for Lithium Ion Batteries. Scr. Mater. 2008, 58 (5), 405-408. Barakat, N. A. M.; Abdelkareem, M. A.; Kim, H. Y., Ethanol Electro-oxidation Using CadmiumDoped Cobalt/carbon Nanoparticles as Novel Non Precious Electrocatalyst. Appl. Catal., A 2013, 455, 193-198. Fu, X.; Feng, J.; Wang, H.; Ng, K. M., Room Temperature Synthesis of a Novel γ-MnO2 Hollow Structure for Aerobic Oxidation of Benzyl Alcohol. Nanotechnol 2009, 20 (37), 375601. Zhai, T.; Xie, S.; Yu, M.; Fang, P.; Liang, C.; Lu, X.; Tong, Y., Oxygen Vacancies Enhancing Capacitive Properties of MnO2 Nanorods for Wearable Asymmetric Supercapacitors. Nano Energy 2014, 8, 255-263. Kainz, Q. M.; Linhardt, R.; Grass, R. N.; Vilé, G.; Pérez-Ramírez, J.; Stark, W. J.; Reiser, O., Palladium Nanoparticles Supported on Magnetic Carbon-Coated Cobalt Nanobeads: Highly Active and Recyclable Catalysts for Alkene Hydrogenation. Adv. Funct. Mater. 2014, 24 (14), 2020-2027. Kumar, R.; Singh, R. K.; Dubey, P. K.; Singh, D. P.; Yadav, R. M., Self-Assembled Hierarchical Formation of Conjugated 3D cobalt Oxide Nanobead–CNT–graphene Nanostructure Using Microwaves for High-performance Supercapacitor Electrode. ACS Appl. Mater. Interfaces 2015, 7 (27), 15042-15051. Kumar, R.; Kim, H.-J.; Park, S.; Srivastava, A.; Oh, I.-K., Graphene-wrapped and Cobalt OxideIntercalated Hybrid for Extremely Durable Super-capacitor with Ultrahigh Energy and Power Densities. Carbon 2014, 79, 192-202. Kong, D.; Luo, J.; Wang, Y.; Ren, W.; Yu, T.; Luo, Y.; Yang, Y.; Cheng, C., Three‐Dimensional Co3O4@ MnO2 Hierarchical Nanoneedle Arrays: Morphology Control and Electrochemical Energy Storage. Adv. Funct. Mater. 2014, 24 (24), 3815-3826.


22

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

37.

38.

39.

40.

41.

42.

43.

44.

45.

46.

47.

48. 49.

50.

51.


Hou, J.; Li, Y.; Mao, M.; Ren, L.; Zhao, X., Tremendous Effect of the Morphology of BirnessiteType Manganese Oxide Nanostructures on Catalytic Activity. ACS Appl. Mat. Interfaces 2014, 6 (17), 14981-14987. Li, Q.; Lu, X.-F.; Xu, H.; Tong, Y.-X.; Li, G.-R., Carbon/MnO2 double-walled nanotube arrays with fast ion and electron transmission for high-performance supercapacitors. ACS Appl. Mat. Interfaces 2014, 6 (4), 2726-2733. Li, H.; Jiang, L.; Cheng, Q.; He, Y.; Pavlinek, V.; Saha, P.; Li, C., MnO2 Nanoflakes/hierarchical Porous Carbon Nanocomposites for High-performance Supercapacitor Electrodes. Electrochim. Acta 2015, 164, 252-259. Wang, H. Y.; Xiao, F. X.; Yu, L.; Liu, B.; Lou, X. W. D., Hierarchical α‐MnO2 Nanowires@ Ni1‐xMnxOy Nanoflakes Core–Shell Nanostructures for Supercapacitors. small 2014, 10 (15), 3181-3186. Wang, J.-G.; Yang, Y.; Huang, Z.-H.; Kang, F., Rational synthesis of MnO2/conducting Polypyrrole@carbon Nanofiber Triaxial Nano-cables for High-performance Supercapacitors. J. Mater. Chem. 2012, 22 (33), 16943-16949. Brousse, T.; Toupin, M.; Dugas, R.; Athouël, L.; Crosnier, O.; Bélanger, D., Crystalline MnO2 as Possible Alternatives to Amorphous Compounds in Electrochemical Supercapacitors. J. Electrochem. Soc. 2006, 153 (12), A2171-A2180. Zhi, J.; Zhao, W.; Liu, X.; Chen, A.; Liu, Z.; Huang, F., Highly Conductive Ordered Mesoporous Carbon Based Electrodes Decorated by 3D Graphene and 1D Silver Nanowire for Flexible Supercapacitor. Adv. Funct. Mater. 2014, 24 (14), 2013-2019. Zhi, J.; Deng, S.; Wang, Y.; Hu, A., Highly Ordered Metal Oxide Nanorods inside Mesoporous Silica Supported Carbon Nanomembranes: High Performance Electrode Materials for Symmetrical Supercapacitor Devices. J. Phys. Chem. C 2015, 119 (16), 8530-8536. Shang, Y.; Yu, Z.; Xie, C.; Xie, Q.; Wu, S.; Zhang, Y.; Guan, Y., A Facile Fabrication of MnO2/graphene Hybrid Microspheres with a Porous Secondary Structure for High Performance Supercapacitors. J. Solid State Electrochem. 2015, 19 (4), 949-956. Kumar, R.; Singh, R. K.; Dubey, P. K.; Singh, D. P.; Yadav, R. M.; Tiwari, R. S., Freestanding 3D Graphene–Nickel Encapsulated Nitrogen‐Rich Aligned Bamboo Like Carbon Nanotubes for High‐Performance Supercapacitors with Robust Cycle Stability. Adv. Mater. Interfaces 2015, 2 (15). Chen, L.; Kang, J.; Hou, Y.; Liu, P.; Fujita, T.; Hirata, A.; Chen, M., High-energy-density Nonaqueous MnO2@ nanoporous Gold Based Supercapacitors. J. Mater. Chem. A 2013, 1 (32), 9202-9207. Xu, C.; Kang, F.; Li, B.; Du, H., Recent Progress on Manganese Dioxide based Supercapacitors. J. Mater. Res. 2011, 25 (08), 1421-1432. Wu, Z.-S.; Ren, W.; Wang, D.-W.; Li, F.; Liu, B.; Cheng, H.-M., High-energy MnO2 Nanowire/graphene and Graphene Asymmetric Electrochemical Capacitors. ACS nano 2010, 4 (10), 5835-5842. Cheng, Y.; Lu, S.; Zhang, H.; Varanasi, C. V.; Liu, J., Synergistic Effects from Graphene and Carbon Nanotubes Enable Flexible and Robust Electrodes for High-performance Supercapacitors. Nano Lett. 2012, 12 (8), 4206-4211. Cottineau, T.; Toupin, M.; Delahaye, T.; Brousse, T.; Belanger, D., Nanostructured Transition Metal Oxides for Aqueous Hybrid Electrochemical Supercapacitors. Appl. Phys. A 2006, 82 (4), 599-606.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

52. 53.

54. 55.

56.

Page 24 of 24

Khomenko, V.; Raymundo-Pinero, E.; Frackowiak, E.; Beguin, F., High-voltage Asymmetric Supercapacitors Operating in Aqueous Electrolyte. Appl. Phys. A 2006, 82 (4), 567-573. Gao, H.; Xiao, F.; Ching, C. B.; Duan, H., High-performance Asymmetric Supercapacitor based on Graphene Hydrogel and Nanostructured MnO2. ACS Appl. Mat. Interfaces 2012, 4 (5), 28012810. Ardizzone, S.; Fregonara, G.; Trasatti, S., “Inner” and “Outer” Active Surface of RuO2 Electrodes. Electrochim. Acta 1990, 35 (1), 263-267. Duay, J.; Sherrill, S. A.; Gui, Z.; Gillette, E.; Lee, S. B., Self-limiting Electrodeposition of Hierarchical MnO2 and M (OH) 2/MnO2 Nanofibril/Nanowires: Mechanism and Supercapacitor Properties. Acs Nano 2013, 7 (2), 1200-1214. Shao, J.; Zhou, X.; Liu, Q.; Zou, R.; Li, W.; Yang, J.; Hu, J., Mechanism Analysis of the Capacitance Contributions and Ultralong Cycling-stability of the Isomorphous MnO2@ MnO2 Core/shell Nanostructures for Supercapacitors. J. Mater. Chem.A 2015, 3 (11), 6168-6176.

TABLE OF CONTENTS


24

Hierarchical MnO2 Spheres Decorated by Carbon-Coated Cobalt

Recommend Documents