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manganese dioxide nanoflakes
Author’s Accepted Manuscript Flexible and High Energy Density Asymmetrical Supercapacitors Based on Core/Shell Conducting Polymer Nanowires/ Manganese Dioxide Nanoflakes Weidong He, Chenggang Wang, Fuwei Zhuge, Xiaolong Deng, Xijin Xu, Tianyou Zhai www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30185-4 http://dx.doi.org/10.1016/j.nanoen.2017.03.045 NANOEN1873
To appear in: Nano Energy Received date: 21 November 2016 Revised date: 19 March 2017 Accepted date: 23 March 2017 Cite this article as: Weidong He, Chenggang Wang, Fuwei Zhuge, Xiaolong Deng, Xijin Xu and Tianyou Zhai, Flexible and High Energy Density Asymmetrical Supercapacitors Based on Core/Shell Conducting Polymer Nanowires/ Manganese Dioxide Nanoflakes, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.03.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Flexible and High Energy Density Asymmetrical Supercapacitors Based on Core/Shell Conducting Polymer Nanowires/ Manganese Dioxide Nanoflakes Weidong Hea, Chenggang Wanga, Fuwei Zhugeb, Xiaolong Denga, Xijin Xua,*, Tianyou Zhaib,*
a b
School of Physics and Technology, University of Jinan, Jinan 250022, P. R. China
State Key Laboratory of Material Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, P. R. China
[email protected], [email protected] *Corresponding address:
Abstract Hierarchically porous polypyrrole nanowires/manganese oxides nanoflakes (MnO2 NFs@PPy NWs) core/shell nanostructures were successfully constructed through a simple, convenient and environmentally friendly method by using PPy nanowires as the core buffer and K-Birnessite type MnO2 as the shell. The core/shell nanostructures effectively increase active surface areas and decrease the ion transmission distance, which is conducive to the efficient transfer of ions. The MnO2 NFs@PPy NWs core/shell nanostructures exhibited not only high specific capacitance (276 F g-1 at 2 A
g-1) but also excellent capacitance retained ratio of 72.5% under extreme charge/discharge conditions (200 F g-1 at 20 A g-1) due to the synergistic effect by combining the merits of MnO2 and PPy. Using such hierarchical nanostructure as the positive electrode, we further demonstrate that ultra-flexible asymmetrical supercapacitors (AFSCs) (MnO2@PPy//AC) possess excellent cycling stability (90.3% after 6000 cycles at 3 A g -1), mechanical flexibility, large voltage operation window (1.8~2.0 V vs. SCE) and high energy densities at all charge/discharge conditions (25.8 Wh kg-1 at the power density of 901.7 W kg-1, and 17.1 Wh kg-1 at the power density of 9000 W kg-1, respectively).
Keywords: ultra-flexible asymmetrical supercapacitors, MnO2@PPy core/shell nanostructures, synergistic effect, electrochemical performance
1. Introduction Flexible and lightweight supercapacitors (SCs), as a new class of energy storage devices, have received increasing attention due to their higher power density, moderate energy density and good operational safety [1-6]. For flexible SCs, electrode materials are the vital components that determining the overall performance of SCs, including the energy power density, lifetime, flexibility, and so on. Transitionmetal oxides (MnO2 [7], V2O5 [8], SnO2 [9], Co3O4 [10] etc.), polyoxometalates (NiCo2O4 [11], MnCo2O4 [12] etc.) and electronically conducting polymers
(polyaniline (PANI) [13], polypyrrole (PPy) [14], etc.) based on faradic redox charge storage have attracted significant attention because of their higher energy density [15]. As a competitive candidate, manganese oxides (MnO2) have been widely investigated due to their intriguing characteristics such as ultrahigh theoretical specific capacitance (1380 F g−1), low cost, environmental friendliness and abundance on earth [7, 16]. However, MnO2-based materials usually show poor electronic conductivity. They may also experience structural collapse or element dissolution during charge/discharge process, leading to the inadequate exploitation of active electrode materials [5, 17]. Considerable research efforts have been focused on fabricating new composite structures to overcome these disadvantages, for example, by combining MnO2 with highly conductive materials including metal nanostructures [18], carbon nanotubes [19], graphene [20] and conducting polymers [21]. Conducting polymers such as PPy, possess good electrical conductivity and are capable of improving the ion/electron exchange rate in SCs under high current charging/discharging operations [13, 22]. Designing an integrated synergic architecture that combines the advantages of MnO2 and polypyrrole is rationally considered here as very desirable strategy to achieve high quality electrode materials [23, 24]. Up to now, most reports can be summarized into one point, that is, the metal oxides were firstly synthesized and subsequently chemically coated by conductive polymers as shells [22, 25-27]. However, such structure presents several disadvantages: (a) the conductive polymer shells covered on surface prevent the full contact between the MnO2 core and electrolyte, resulting in a sacrifice of the energy
density and long-term cyclability of MnO2 [28]; (b) the morphologies of PPy-coated MnO2 are difficult to adjust, especially for the 3D nanostructures; (c) though conducting polymers shells greatly enhance specific capacitance, working ability at high current is urgent to be improved due to PPy structural instability [29, 30]. Besides, these nanostructures may become electrically isolated owing to the loose contacts between PPy shell and MnO2 core [31]. A new and facile method for design and fabrication of MnO2-based composites with highly-accessible surface areas, wide potential window, and high electronic conductivity for SCs still remain a challenge. In this work, we develop a green surfactant-free and scalable strategy to synthesize the hierarchically porous MnO2 NFs@PPy NWs core/shell nanostructures, which hold ultrahigh specific capacitance (276 F g-1 at 2 A g-1), good cycling performance (93.4% after 1500 cycles) and stable working performance even under the high current density (200 F g-1 at 20 A g-1). The nanoarchitecture also shows great promise as the electrode materials in asymmetrical flexible supercapacitors devices (AFSCs) that use activated carbon fiber cloth (ACFC) as the flexible structural supports. The assembled AFSCs possess an energy density of 17.1 W h kg-1 at an ultra-high power density of 9000 W kg-1 and the excellent mechanical flexibility that tolerates bending, folding and twisting within a wide operating voltage of 1.8 V. This present strategy, using a simple preparation method and low-cost raw materials will promote the fast development of flexible energy storage devices. 2. Experimental Section Synthesis of PPy NWs nanostructures: All reagents were of analytical grade and were
used without any further purification. In a typical procedure, 0.294 g methyl orange (MO) was dispersed in 180 ml FeCl36H2O (1.44 g) solution by ultrasonication. Then, 330 μL pyrrole monomers were added to the above suspension solution at room temperature with vigorous magnetic stirring for 24 h. The formed PPy black precipitate were washed with deionized water and absolute ethanol several times until the filtrate was colorless and neutral, and finally dried at 50 oC for 12 h in a vacuum oven. Synthesis of MnO2 NFs@PPy NWs core-shell structures and MnO2NFS on activated carbon fiber cloth (ACFC): In the typical process, 0.265 ml H2SO4 was added into 30 ml KMnO4 (0.79 g) solution by magnetic stirring. Then, the above mixture solution was added into 20 ml aqueous solution of 50 mg PPy NWs powders. Subsequently, the reaction system was placed in a water bath with a temperature of 85 o
C. The reaction time in this step was controlled to be 60 min to keep the nanowire
morphology of PPy after the reaction. The as-synthesized samples were then washed with deionized water and absolute ethanol, and the final products were dried at 60 oC for 12 h in vacuum. For comparison, ACFC (12 cm2) was washed by ultrasonically in deionized (DI) water, acetone, ethanol for 15 min, respectively, and dried in vacuum. Then, 0.265 ml H2SO4 was added into 50 ml KMnO4 (0.79 g) solution by magnetic stirring to form a homogeneous aqueous solution. Afterwards, a ACFC piece was dropped into the mixture, following the reaction system was placed in a water bath with a temperature of 85 oC for 60 min. The collected sample was rinsed several times, and dried at 60 oC for 12 h in vacuum.
Asymmetrical flexible supercapacitors (AFSCs): The working electrodes, negative and positive electrodes were prepared as follows: 75 wt.% sample was mixed with 15 wt.% acetylene black in an agate mortar until a homogeneous black powder was obtained. Then 10 wt.% polytetrafluoroethylene (PTFE) binder was added into the mixture to make a homogeneous slurry. The final slurry was pasted onto a piece of activated carbon fiber cloth (ACFC, 12 cm2), and the carbon cloth side (10.5 cm2) without coating with any materials, and dried for 12 hours at 60 oC in vacuum. Each electrode contained about 1~1.5 mg of electro-active material and had a geometric surface area of about 1×1.5 cm2. The AFSCs typically include a piece of activated carbon (TF-B520, ShangHai Carbosino Material Co., Ltd.) electrode (12 cm2) (AC@ACFC) as negative electrode, a piece of MnO2@PPy @ACFC (12 cm2) as positive electrode and separator typically made by the porous glassy fibrous paper. Meanwhile,
the electrodes and separator were soaked in 1M Na2SO4 for about 10 min.
Then, the devices were assembled by sandwiching the separator between AC/ACFC and MnO2/PPy /ACFC electrodes. Finally, the assembled devices were carefully sealed using parafilm to ensure a stable performance of the device. The mass ratio of MnO2 electrode and AC electrode (around 1:1.4) was carefully chosen to balance the charge at each electrode. Characterizations: The compositions and morphologies of the as-prepared samples were characterized by a field emission scanning electron microscope (FESEM, FEI QUANTA FEG250) equipped with an energy dispersive x-ray spectroscopy (EDS, INCA MAX-50). The phases and structures were examined by X-ray diffraction
(XRD, D8-Advance, Bruker, using Cu Kα, λ=0.15418 nm). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed on a FEI Tecnai G2 F20 at an acceleration voltage of 200 kV. The average pore sizes, pore volumes, and specific surface areas of the samples were evaluated through measuring N2 adsorption-desorption isotherm with a NOVA touch LX4 apparatus. Electrochemical measurement: The electrochemical properties of the supercapacitor electrodes
were
evaluated
by
cyclic
voltammetry
(CV),
galvanostatic
charge/discharge (GCD) and electrochemical impedance spectrometry (EIS). The obtained GCD curves were used for the calculation of the specific capacitance (C), specific energy (E) and specific power (P) density according to the following equation:
C
It Vm
(1)
1 E CV 2 2 (2)
P
E t
(3) where I (mA) is the charge/discharge current, Δt (s) is the discharge time, m (mg) is the mass of active material for a single electrode or the total mass of active material of positive and negative electrodes, ΔV (V) is the voltage change excluding voltage drop (IR drop) at a constant discharge current. The electrochemical properties of the
electrodes were carried out using an electrochemical workstation (Metrohm Autolab PGSTAT302N) with a three-electrode configuration in a 1 M Na2SO4 aqueous solution. The prepared samples were used as the working electrodes, Pt foil as the counter electrode (CE) and a saturated calomel electrode (SCE) as the reference electrode (RE). The AFSCs were measured in a two-electrode cell configuration.
3. Results and Discussion Figure 1a shows the fabrication scheme for the MnO2@PPy hybrid nanostructure by using PPy nanowires as the starting template. Due to the reductive nature of PPy, MnO2 seeds are expected to form on the surface of PPy NWs in the initial stage by the following reaction pathway [32]: 4MnO4 3C H 2O 4MnO2 CO32 2HCO3-
(4) These seeds were found to grow into nano-flakes after keeping the reaction system at 85oC for a certain time. Low and high-magnification SEM images of the asprepared pure MnO2 NFs arrays (Figure 1b and c) reveal a high density and uniformity of NFs vertically achoring to the surface of carbon fiber. Figure 1d-e and f shows the SEM image of the PPy NW and the obtained hybrid nanostructures using a reaction time of 60min. In the enlarged Figure 1g, fine nanoflakes (NFs) were clearly seen to evolve on PPy NWs while keeping the overall NW like structure. Note that inappropriate selection of the reaction time or PPy NW concentration would lead to less NFs on NW or otherwise destroyed NW structures, as can be seen in Figure
S1. This is because PPy tend to be consumed during the reaction. Figure 1h and i shows the EDS spectra and elemental mapping taken from the enclosed area in Figure 1g. C, Mn, O, and N, K are found to the major elements in the final reaction product. The presence of N and C element signals from PPy again validates that PPy nanowires act as reactive templates. K was also detected because of its intercalation into -MnO2, which is consisted of 2D layers of edge-sharing MnO6 octahedra with K+ cations and water molecules in the interlayer space [17, 28]. The C: N: K: Mn ratio is estimated to be 15.66%: 7.03%: 3.07%: 25.6% from the composition analysis. Figure 1j shows the XRD patterns of the fabricated PPy NW and the final NFs@NW product to identify their crystalline phase. The broad peaks of PPy indicated their amorphous state after fabrication [33], while the peaks of final NFs@NWs structures were consistently indexed to the monoclinic K-Birnessite type δ-MnO2 (JCPDS no.801098) [34-37]. The 2D layer stacked structure of K-Birnessite MnO2 lead to the easy formation of thin MnO2 nanoflakes. With the typical 10 nm thickness of the nanoflakes and their large interstitial space, the prepared nanoflakes on PPy NW will not only provide abundant electrochemically active surface sites, but also facilitate the fast ion penetration in electrolyte for supercapacitor application. Figure 2a shows the close TEM examination of the MnO2 nanoflakes. The inset selected area electron diffraction (SAED) pattern contains multiple faint continuous diffraction rings, suggesting the polycrystalline nature of the as-prepared MnO2 NFs [38]. It is generally known that the BET surface areas and pore-size distributions directly correlate to the specific capacitance. Thus, we continued to examine the BET
surface area of prepared samples from the N2 adsorption and desorption isotherm (Figure 2b). The specific surface area of MnO2@PPy was found to be 166.65 m2 g−1. As a comparison, the specific surface area of the starting PPy NWs was only 17.81 m2 g−1 (Figure S2a). Loading MnO2 on the surface of PPy obviously increases the active surface area, which can potentially offer electrons and ions transport channels and avoid structural collapse during the discharge/charge process [39, 40]. Meanwhile, the MnO2 NFs@PPy NWs sample also shows wider pore size distributions (3~7 nm, Figure 2c) than that of pure PPy NWs (~1.97 nm, see Figure S2b). As the pore size significantly affects the rate performances of nanoporous materials [41] and the larger pores generally provide low-resistant pathways for the ions through the porous structure, as well as a shorter diffusion route, which can also effectively enhance the electronic conductivity and better rate performance [42]. Herein, the high surface area and large pore size achieved in the hierarchical MnO2 NFs@PPy NWs structure eventually endow it enhanced electrochemical performance in the following compared to other materials. CV measurements were firstly conducted on the ACFC, PPy NWs, pure MnO2 and the hybrid MnO2 NFs@PPy NWs structures to examine the synergetic integration of active surface area and conductive channels in the hierarchical nanostructure. Figure 3a shows the representative CV curves of ACFC, pure PPy, pure MnO2 and MnO2@PPy measured at 30 mV s-1. It is can be seen that the CV curve area of ACFC is negligible, indicating the minuscule specific capacitance. Moreover, the enlarged CV curves area of hybrid MnO2@PPy nanostructure suggests greater quantity of
charge storage than pure PPy and MnO2. For the electrode made by hybrid MnO2@PPy nanostructure, no obvious redox peaks are observed from 0 to 0.9 V, indicating that the electrochemical capacitive performances are mainly attributed to Faradaic redox processes [32, 43]. The charge storage mechanism of MnO2@PPy is thus proposed to involve surface adsorption/desorption and tunnel insertion/extraction
of electrolyte cations C H , Li , Na , K
in MnO2 and the reversible
doping/dedoping of anions D Cl , SO42 in PPy [32, 43]:
MnO 2 C e MnOOC (5)
PPy D
e PPy 0 D
(6) To examine the specific capacitance of the prepared electrodes, GCD curves were further measured at varied current densities. The results shown in Figure 3b were measured at a current density of 3.0 A g-1. The calculated specific capacitances for MnO2@PPy, pure PPy and MnO2 at 3.0 A g-1 using eq.1 are respectively 271, 23.3 and 147 F g-1. The highest specific capacitance was obtained in the hierarchical MnO2@PPy nanostructured electrode. In addition to this high specific capacitance, we also found that the prepared MnO2@PPy electrode exhibit excellent capacitance retained ratio (CRR) at high current density. Figure 3c summarizes the specific capacitance measurement results under varied current density from 1 to 30 A g-1. The prepared MnO2@PPy electrode exhibit the highest CRR of ~54.1% at 30 A g-1, which is much higher than the pure PPy (~21.2% at 10 A g-1) and pure MnO2 (~31.6% at 30
A g-1). The excellent capacitance performance of the MnO2@PPy electrode is thoroughly examined by changing the voltage scanning rate (5~150 mV s-1) and current density (1~30 A g-1) in CV and GCD measurements, as shown in Figure 3d and e. The highly linear and symmetric GCD curves in all current densities imply the excellent reversibility and good rate capability of the MnO2@PPy electrode. The corresponding CV and GCD curves of PPy NWs and MnO2 NFs were also revealed in Figure S3. To clearly visualize the capacitance performance of present MnO2@PPy electrode, we summarize in Table 1 the reported specific capacitance and capacitance retained ratio of previously reported MnO2 composite electrodes together with our measurement results. We found that our present MnO2@PPy electrode exhibit one of the best performances in the Table 1. To better understand the relationship between resistance behavior and electrochemical properties, EIS analysis of the electrodes were performed on the prepared samples. Apparently, the PPy electrode possesses the steepest slope of the curves and the smallest diameter of semicircle, meaning ideal capacitive performance and small the charge transfer resistance. We found that the present MnO2@PPy electrode allows not only faster ions diffusion of electrolyte within the pores of electrodes during redox reaction than pure MnO2, but also small internal resistances down to 1.3Ω, as evidenced by the Nyquist plot analysis in Figure S4a. This is ascribed to the larger surface area, pore volume and higher pore size distribution of the hierarchical nanostructure endowing more electron/ion paths for feasible ion transport and fast redox reaction. The above results all demonstrate the synergetic integration effect between the electrochemical active MnO2 nanoflake shell
and the conductive PPy NW core in supercapacitors. Next, the long-term cycling performances of composite MnO2@PPy electrodes and pure PPy were examined by GCD measurements at a current density of 3 A g -1 for 1500 cycles, as shown in Figure 3f. For pure PPy NWs, the specific capacitance decreased to 65.4% of initial capacitance after 1500 cycles (inset in Figure 3f.). For the MnO2@PPy electrode, the specific capacitance increased continuously from its initial value of 271 to 277 F g-1 in the first 150 cycles and then decreased gradually, and maintaining 93.4% of the initial value after a continuous cycling of 1500 times. The increase of specific capacitance in the beginning may be related to the activation effect of electrochemical cycling. Similar phenomena has been observed by Jiang et al. [52], Jin et al. [44] and An et al. [53]. The good cycle stability of MnO2 NFs@PPy NWs could be ascribed to the unique nanoflakes heterostructure that can accommodate structure collapse of the electrode materials during long-term cycles. Besides, the structural robustness of the hierarchical structure can also effectively prevent the aggregation and restacking of PPy NWs and MnO2 NFs during the cycling process. The inset of Figure 3f shows the digital picture of the electrolyte after the measurement. After 1500 cycles, the color of the electrolyte with pure PPy electrode (inset of Figure 3f-1) turns into yellow due to the dissolving of PPy in electrolyte. In contrast, no obvious color change can be observed for the electrolyte with composite electrode, which suggests the good cycle stability of the MnO2@PPy electrode. Figure S4b shows the first and last three cycles of MnO2@PPy electrode material. The good cycling performance, together with the high specific capacitance and
capacitance retained ratio discussed in the above, all imply a good potential of the present
MnO2@PPy
composite
electrode
in
high
energy/power
density
supercapacitors. By using the above MnO2@PPy composite electrode, we demonstrate in the following the assembly of highly flexible AFSC with wide operation voltage window, energy/power density and stability. Figure 4a shows the schematic illustration of the structure of the AFSC, by using MnO2@PPy as the positive electrode, AC the negative electrode, porous glassy fibrous paper as the separator. The electrode material is attached on carbon fiber cloth. Parafilm is used to seal the assembled electrodes to ensure stable operation of the AFSC devices. All the above components exhibit excellent the mechanical stability under different bending conditions, no matter being stretched, kinked or bent (Figure S5). The commercial AC is chosen as negative, material, which has specific capacitances of AC were calculated to be 217, 202, 198 192, 187, and 184 F g−1 at current densities of 1, 2, 3, 5, 8 and 10 A g−1, respectively. The corresponding CV and GCD curves of AC electrode at different scan rates and current densities between -0.9 and 0 V (vs. SCE) are shown in Figure S6a and b. To balance the stored charge at the anode and cathode, mass loadings of AC and MnO2@PPy on the fiber cloth are determined to be 1.4 and 1 mg cm −2, respectively, based on their measured specific capacitance values at 2 A g−1. Figure 4c shows CV curves of MnO2 NFs@PPy NWs and AC electrodes at a scan rate of 30 mV s−1. To estimate the best operating potential of the as-prepared AFSCs, a series of CV curves with different potential windows are collected at 30 mV s-1 as shown in
Figure 4d. The potential window of the device is possible to be extended to as high as 2.0 V without obvious polarization curves. The wide and stable operation voltage of present assembled ASFC make it very suitable for portable electronic devices. Figure 5a and b shows the CV and GCD curves of the assembled AFSCs measured at varied voltage scanning rate and current densities. The as-prepared AFSCs exhibit an ideal capacitive behavior with quasi-rectangular CV curves and feeble redox peaks at all scan rates from 10 to 200 mV s -1 within a potential window of 1.8 V, indicating the presence of both pseudocapacitive and electric double layer capacitor (EDLC) type properties. The GCD curves of the supercapacitor device are seen to maintain a good symmetry even at a high current density of 10 A g-1, demonstrating its rapid charge-discharge properties and good rate capability. The specific capacitance calculated based on the total mass of both positive and negative electrodes at different current densities, reaches a specific capacitance of around 57 F g-1 at a current density of 1 A g-1 (Fig. S7a). The result of EIS measurement on AFSCs is shown in Figure S7b. The straight line in the low-frequency region reveals the ideal capacitive behavior of the device. The arc in the high-frequency area has a small radius, indicating low charge transport resistance (Rct). The bulk solution resistance (Rs) was determined by x intercept on real axis (Z,). The value of Rs and Rct is 1.2 and 2.2, respectively, which prove the high electrical conductivity of both electrodes and the high ionic conductivity of Na2SO4 electrolyte. In consideration of practical use, the long-term cycling stability of MnO2@PPy AFSCs was further measured at current density of 3 A g-1 up to 6000 cycles, and is shown in Figure 5c. The specific
capacitance of the device retains 47.2 F g-1 after 6000 cycles, corresponding to 90.3% of its initial specific capacitance (42.6 F g-1). From the discharge curves shown in Figure 5d, the energy densities and power densities of MnO2 NFs@PPy NWs AFSCs are calculated based on eq.2, and 3, respectively. Figure 5d shows the corresponding calculated Ragone plots, the AFSCs demonstrated in this work delivered an energy density of as high as 25.8 W h kg-1 at the power density of 901.7 W kg-1, which maintained at 17.1 W h kg-1 at an extremely high power density of 9000 W kg-1. Compared to the other electrode materials shown together in the figure, this performance is found superior to that of Co3O4/PPy/MnO2//AC
[18],
MnO2//Fe2O3/Ppy
[23],
MnO2/TiO2//AC
[46],
NiCo2O4/MnO2 NWs//AG (activated graphenes) [47], MnO2/PPy//V2O5/PPy [54], V2O5/GO//rGO [55], CuO/MnO2//MEGO (activated microwave exfoliated graphite oxide) [56], MnO2/C//V2O5-CNT [57], MnO2//Bi2O3 [58], GH(graphene hydrogel)PANI/GP(graphene paper)//GH−PANI/GP [59]. This results demonstrate the bright potential of present MnO2@PPy composite electrode material in high power supercapacitors. Finally, the excellent flexibility of AFSCS is revealed by deforming the test device under various bending states. CV analysis was performed at the bending angles of 0°, 90°, 180°, as shown in Figure 5e. There is a slight increase in capacitance retention and almost completely overlapped CV curves (Fig. 5e). In this case, folding and bending may may reduce the distance between two electrodes, which will help to improve the conductivity and ion transport. The anode and cathode electrodes will
neither detach nor contact with each other in experiment, indicating remarkable mechanical flexibility and stability of our device. The concise structure of present AFSC would greatly simplify the future design of portable/flexible electronic devices. The feasible integration ability of present AFSC for higher operation voltages is further demonstrated by a serial connection of two devices. Figure 5f shows the GCD curves of a single AFSCs and a pair of AFSCs connected in series at current density of 3 A g-1. The connected pair can be steadily operated between 0 and 3.6 V. The inset figure shows a digital image of powered light-emitting-diode with a working voltage of 3.5 V. Figure S7c shows a LED was successfully lighten by two serially connected AFSC cells. The merits of present hierarchical nanostructure can be summarized briefly as follows: (a) MnO2 nanoflakes (MnO2 NFs) are well-grown and well-dispersed on novel polypyrrole nanowires, which build a reliable conductive network for fast electron transport throughout the electrode; (b) Porous MnO2 NFs@PPy NWs core/shell structures significantly enhance the effective utilization of active materials leading to more contact area with electrolyte; (c) PPy NWs serve as both the backbone and conductive pathway for MnO2 NFs, the structure create short distances and additional channels for ion diffusion and charge transfer; (d) PPy NWs and MnO2 NFs@PPy NWs nanostructures are formed from low-cost materials without involving organic solvent and additives. Furthermore, the preparation procedures are straightforward and convenient via stirring process at room temperature and water bath synthesis, which provides a simple, economic and green approach to fabricate an
important kind of metal oxide/conducting polymer nanocomposites. These features highlight the capability of 3D hybrid electrode to meet the requirements of long-cycle lifetime, good rate capability and low internal resistance, which are important merits for practical energy storage devices. 4. Conclusion In summary, we take PPy NWs as a buffer and K-Birnessite type MnO2 NFs as a shell to construct a novel hierarchically porous MnO2 NFs@PPy NWs core/shell heterostructures through a new preparation method. MnO2 NFs@PPy NWs core/shell nanostructures exhibit not only high specific capacitance and good cycle stability, but also low and stable internal resistance. Moreover, the excellent capacitance retained ratios are72.5% (276~200 F g-1), when the current densities range from 2 A g-1 to 20 A g-1, indicating the material being suitable for working under high current. The assembled asymmetrical flexible supercapacitors (AFSCs) (MnO2/PPy//AC) give rise to a maximum energy density of 25.8 W h kg-1 at power density of around 901.7W kg1
and maintains 17.1 W h kg-1 at an extremely high power density of 9000 W kg-1 with
an excellent cycling performance. Furthermore, the as-assembled device shows negligible capacitance degradation at different bending state, indicating excellent mechanical flexibility and stability. These results imply that PPy can be regarded as an ideal conductive substrate, and can greatly enhance the electrochemical performance of MnO2. More importantly, the method may be suitable for the largescale and environment-friendly production of MnO2/PPy hybrid composites and be easily generalized to grow other mesoporous metal oxides nanostructure on
conducting polymers for the fabrication of high-performance energy-storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11304120, 51672109, 21505050), National Basic Research Foundation of China (2015CB932600),
Program
for
HUST
Interdisciplinary
Innovation
Team
(2015ZDTD038) and the Fundamental Research Funds for the Central University. Here authors also want to thank Analytical and Testing Center in Huazhong University of Science and Technology for the technical support.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at Nano Energy
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Figure 1. Synthesis and characterization of MnO2 NFs@PPy NWs core/shell nanostructures. a) Schematic diagrams for the synthesis of MnO2 NFs@PPy NWs core/shell
nanostructures.
b)
and
c)
SEM
images
of
pure
MnO2
nan
oflakes on ACFC. d) and e) SEM images of PPy nanowires. f) and g) SEM images of
MnO2@PPy core/shell nanostructures. h) EDS spectra of MnO2@PPy core/shell nanostructures. i) EDS elemental mapping of MnO2@PPy core/shell nanostructures. j) XRD patterns of the as-synthesized PPy and MnO2@PPy
Figure 2. a) High-magnification TEM images of MnO2@PPy core/shell
nanostructures (inset showing the SAED pattern). b) Nitrogen adsorption and desorption isotherms for MnO2@PPy core/shell nanostructures; c) the corresponding BJH pore size distributions.
Figure 3. a) CV curves of pure MnO2, pure PPy ACFC and MnO2@PPy measured at 30 mV s-1. b) GCD curves of pure MnO2, pure PPy and MnO2@PPy measured at 3A g-1. c) Discharge current density dependence of the specific capacitance. d) CV curves of the MnO2@PPy electrode material at various scan rates between 5.0 and 150 mV s 1
. e) Charge/discharge curves of the MnO2@PPy electrode material at various current
densities ranging from 1 to 30 A g-1. f) Cycle life of pure PPy, MnO2@PPy electrodes at a current density of 3 A g -1; 1) and 2) the color change of electrolyte (pure PPy, MnO2@PPy) after 1500 cycles).
Figure 4. a) Schematic diagrams for designed flexible hybrid devices. b) Digital images for asymmetrical flexible supercapacitors device. c) CV curves collected for MnO2@PPy min and AC electrodes at a scan rate of 30 mV s−1. d) CV curves collected in various potential windows at 30 mV s-1.
Figure 5. Electrochemical characterizations of MnO2@PPy asymmetrical flexible supercapacitors (AFSCs): a) The CV curves of AFSCs at various scan rates ranging from 10 to 200 mV s-1. b) Galvanostatic charge/discharge profiles at various current densities in the voltage range of 0 to 1.8 V. c) cycling stability test of the AFSCs measured at a current density of 3 A / g. d) Ragone plot of MnO2@PPy AFSCs. e) cycling performance of MnO2@PPy AFSCs at different bending states (the inset shows the CV curves at different bending states). f) Alvanostatic charge/discharge curves of a single SCs and two SCs connected in series (the inset shows the magnification of light bulbs).
Weidong He received his B. S. degree in college of physics and Engineering Qufu Normal universiy in 2015. He is now pursing his M.S. degree under the supervision of Prof. Xijin Xu in School of physics and technology, University of Jinan, P.R. China. His main interest is the design and fabrication three-dimensional nanomaterials for energy storage and electrochemistry.
Chenggang Wang received his B. S. degree in School of Physics and Technology from University of Jinan in 2014. Currently, he is a MSc student majoring in Functional Micro/nano Materials and Devices at Jinan University, China. His current research is focused on improving the performance of supercapacitors’ device.
Fuwei Zhuge received his PhD degree from Shanghai Institute of Ceramics, Chinese Academic of Sciences in 2011. Then he joined Osaka university and Kyushu university as postdoctoral researcher. He is now an associated professor in the school of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include controllable fabrication and novel
application of nanomaterials in energy storage and conversion devices.
Dr Xiaolong Deng is an Assistant Professor of the School of Physics and Technology at University of Jinan. He graduated from Northwestern Polytechnical University with a BSc in 2006, and received MEng from Shanghai University in 2009. He received PhD from Konkuk University in 2013. Dr Deng’s research interests focus on synthesis and characterization of advanced sub-micro/nanomaterials for photocatalysis, conversion and energy storage applications.
Xijin Xu received his Ph.D. degree in Institute of Solid State Physics, Chinese Academy of Sciences in 2007. He conducted his postdoctoral research at Nanyang Technological University, Singapore in 2007 and then joined National Institute for Materials Science, Japan (2008-2010) and Griffith University in Australia. Currently, he is a full professor in University of Jinan. He is the author or co-author of more than 90 research articles in peer reviewed journals and 2 book chapters. His most recent research interests include the synthesis and characterization of functional micro/nanostructures and their applications in environmental remediation and energy storages.
Tianyou Zhai received his B. S. degree in chemistry from Zhengzhou University in 2003, and then received his Ph.D. degree in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the supervision of Prof. Jiannian Yao in 2008. Afterwards he joined in National Institute for Materials Science (NIMS) as a JSPS postdoctoral fellow of Prof. Yoshio Bando’s group and then as an ICYS-MANA researcher within NIMS. Currently, he is a Chief Professor of School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include the controlled synthesis and exploration of fundamental physical properties of inorganic functional nanomaterials, as well as their promising applications in energy science, electronics and optoelectronics. He has authored and co-authored about 140 peer-reviewed journal articles, 5 book chapters, co-edited 1 book on nanotechnology, and held 7 patents. His publications have been cited more than 6700 times (H-index is 45).
Table 1 Summary of electrochemical properties with MnO2 composite electrode materials for supercapacitor reported in recent papers.
materials MnO2@NG Mn3O4@G MnO2@PANI MnCo2O4.5@MnO2 MnO2@H2Ti3O7 MnO2@NiCo2O4 MnO2@CNT@G Ni(OH)2@MnO2 PANI@MnO2 PPy covered MnO2NT PPy covered MnO2 PPy covered MnO2NW NG@MnO2 G@MnO2
current density 2 A g-1 2 / 10 A g-1 5 A g-1 10 A g-1 2 A g-1 8 A g-1 1 / 3 A g-1 20 A g-1 20 A g-1
Sc / F g-1
Ref.
5M LiCl 1M Na2SO4 0.5M Na2SO4 1M Na2SO4 1M Na2SO4 2M KOH 1M Li2SO4 1M Na2SO4 1M Na2SO4
current collector Graphite paper Stainless stell Ni foam Platinum foil Platinum foil Ni foam Ni foam Graphite paper Graphite paper
150.8 210 / 141 186 211.2 90 258 251 / 150 163.3 134
[33] [44] [45] [31] [46] [47] [48] [35] [28]
2M KCl
Ni foam
15 A g-1
215
[32]
electrolyte
-2
1M Na2SO4
Graphite paper
2 mA.cm
141
[27]
1M Na2SO4
Ni foam
10 A g-1
150
[25]
192.5 194
[49] [50]
1M Na2SO4 1M Na2SO4
Ni foam Platinum
-1
2Ag 1A g-1
α-MnO2 @ δ-MnO2 MnO2 cover PPy
substrate Ni foam 20 A g-1
6 M KOH 1M Na2SO4
ACFC
153.8
1 / 2 / 10 / 20 / 30 A g-1
[51] 325 / 276 244 / 200 / 170
This work
(Note: Sc= Specific capacitance NG = nitrogen-doped graphite G = graphite PANI = polyaniline CNT = carbon nanotubes NT = nanotubes NW = nanowires ACFC=
activated carbon fiber cloth)
Highlight
A simple, economic and green approach was developed for the fabrication of hierarchical
MnO2
nanoflakes@PPy
nanowire
core/shell
structured
nanocomposites.
Synergetic combination of the chemical stable MnO2 shell and the conductive PPy core is successfully demonstrated for high power density and persistent electrochemical operation.
By using the hierarchical MnO2 nanoflakes @PPy Nanowires core/shell nanostructures as positive electrode, ultra-flexible and high energy density asymmetrical supercapacitors were successfully achieved
PII: DOI: Reference:
S2211-2855(17)30185-4 http://dx.doi.org/10.1016/j.nanoen.2017.03.045 NANOEN1873
To appear in: Nano Energy Received date: 21 November 2016 Revised date: 19 March 2017 Accepted date: 23 March 2017 Cite this article as: Weidong He, Chenggang Wang, Fuwei Zhuge, Xiaolong Deng, Xijin Xu and Tianyou Zhai, Flexible and High Energy Density Asymmetrical Supercapacitors Based on Core/Shell Conducting Polymer Nanowires/ Manganese Dioxide Nanoflakes, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.03.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Flexible and High Energy Density Asymmetrical Supercapacitors Based on Core/Shell Conducting Polymer Nanowires/ Manganese Dioxide Nanoflakes Weidong Hea, Chenggang Wanga, Fuwei Zhugeb, Xiaolong Denga, Xijin Xua,*, Tianyou Zhaib,*
a b
School of Physics and Technology, University of Jinan, Jinan 250022, P. R. China
State Key Laboratory of Material Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, P. R. China
[email protected], [email protected] *Corresponding address:
Abstract Hierarchically porous polypyrrole nanowires/manganese oxides nanoflakes (MnO2 NFs@PPy NWs) core/shell nanostructures were successfully constructed through a simple, convenient and environmentally friendly method by using PPy nanowires as the core buffer and K-Birnessite type MnO2 as the shell. The core/shell nanostructures effectively increase active surface areas and decrease the ion transmission distance, which is conducive to the efficient transfer of ions. The MnO2 NFs@PPy NWs core/shell nanostructures exhibited not only high specific capacitance (276 F g-1 at 2 A
g-1) but also excellent capacitance retained ratio of 72.5% under extreme charge/discharge conditions (200 F g-1 at 20 A g-1) due to the synergistic effect by combining the merits of MnO2 and PPy. Using such hierarchical nanostructure as the positive electrode, we further demonstrate that ultra-flexible asymmetrical supercapacitors (AFSCs) (MnO2@PPy//AC) possess excellent cycling stability (90.3% after 6000 cycles at 3 A g -1), mechanical flexibility, large voltage operation window (1.8~2.0 V vs. SCE) and high energy densities at all charge/discharge conditions (25.8 Wh kg-1 at the power density of 901.7 W kg-1, and 17.1 Wh kg-1 at the power density of 9000 W kg-1, respectively).
Keywords: ultra-flexible asymmetrical supercapacitors, MnO2@PPy core/shell nanostructures, synergistic effect, electrochemical performance
1. Introduction Flexible and lightweight supercapacitors (SCs), as a new class of energy storage devices, have received increasing attention due to their higher power density, moderate energy density and good operational safety [1-6]. For flexible SCs, electrode materials are the vital components that determining the overall performance of SCs, including the energy power density, lifetime, flexibility, and so on. Transitionmetal oxides (MnO2 [7], V2O5 [8], SnO2 [9], Co3O4 [10] etc.), polyoxometalates (NiCo2O4 [11], MnCo2O4 [12] etc.) and electronically conducting polymers
(polyaniline (PANI) [13], polypyrrole (PPy) [14], etc.) based on faradic redox charge storage have attracted significant attention because of their higher energy density [15]. As a competitive candidate, manganese oxides (MnO2) have been widely investigated due to their intriguing characteristics such as ultrahigh theoretical specific capacitance (1380 F g−1), low cost, environmental friendliness and abundance on earth [7, 16]. However, MnO2-based materials usually show poor electronic conductivity. They may also experience structural collapse or element dissolution during charge/discharge process, leading to the inadequate exploitation of active electrode materials [5, 17]. Considerable research efforts have been focused on fabricating new composite structures to overcome these disadvantages, for example, by combining MnO2 with highly conductive materials including metal nanostructures [18], carbon nanotubes [19], graphene [20] and conducting polymers [21]. Conducting polymers such as PPy, possess good electrical conductivity and are capable of improving the ion/electron exchange rate in SCs under high current charging/discharging operations [13, 22]. Designing an integrated synergic architecture that combines the advantages of MnO2 and polypyrrole is rationally considered here as very desirable strategy to achieve high quality electrode materials [23, 24]. Up to now, most reports can be summarized into one point, that is, the metal oxides were firstly synthesized and subsequently chemically coated by conductive polymers as shells [22, 25-27]. However, such structure presents several disadvantages: (a) the conductive polymer shells covered on surface prevent the full contact between the MnO2 core and electrolyte, resulting in a sacrifice of the energy
density and long-term cyclability of MnO2 [28]; (b) the morphologies of PPy-coated MnO2 are difficult to adjust, especially for the 3D nanostructures; (c) though conducting polymers shells greatly enhance specific capacitance, working ability at high current is urgent to be improved due to PPy structural instability [29, 30]. Besides, these nanostructures may become electrically isolated owing to the loose contacts between PPy shell and MnO2 core [31]. A new and facile method for design and fabrication of MnO2-based composites with highly-accessible surface areas, wide potential window, and high electronic conductivity for SCs still remain a challenge. In this work, we develop a green surfactant-free and scalable strategy to synthesize the hierarchically porous MnO2 NFs@PPy NWs core/shell nanostructures, which hold ultrahigh specific capacitance (276 F g-1 at 2 A g-1), good cycling performance (93.4% after 1500 cycles) and stable working performance even under the high current density (200 F g-1 at 20 A g-1). The nanoarchitecture also shows great promise as the electrode materials in asymmetrical flexible supercapacitors devices (AFSCs) that use activated carbon fiber cloth (ACFC) as the flexible structural supports. The assembled AFSCs possess an energy density of 17.1 W h kg-1 at an ultra-high power density of 9000 W kg-1 and the excellent mechanical flexibility that tolerates bending, folding and twisting within a wide operating voltage of 1.8 V. This present strategy, using a simple preparation method and low-cost raw materials will promote the fast development of flexible energy storage devices. 2. Experimental Section Synthesis of PPy NWs nanostructures: All reagents were of analytical grade and were
used without any further purification. In a typical procedure, 0.294 g methyl orange (MO) was dispersed in 180 ml FeCl36H2O (1.44 g) solution by ultrasonication. Then, 330 μL pyrrole monomers were added to the above suspension solution at room temperature with vigorous magnetic stirring for 24 h. The formed PPy black precipitate were washed with deionized water and absolute ethanol several times until the filtrate was colorless and neutral, and finally dried at 50 oC for 12 h in a vacuum oven. Synthesis of MnO2 NFs@PPy NWs core-shell structures and MnO2NFS on activated carbon fiber cloth (ACFC): In the typical process, 0.265 ml H2SO4 was added into 30 ml KMnO4 (0.79 g) solution by magnetic stirring. Then, the above mixture solution was added into 20 ml aqueous solution of 50 mg PPy NWs powders. Subsequently, the reaction system was placed in a water bath with a temperature of 85 o
C. The reaction time in this step was controlled to be 60 min to keep the nanowire
morphology of PPy after the reaction. The as-synthesized samples were then washed with deionized water and absolute ethanol, and the final products were dried at 60 oC for 12 h in vacuum. For comparison, ACFC (12 cm2) was washed by ultrasonically in deionized (DI) water, acetone, ethanol for 15 min, respectively, and dried in vacuum. Then, 0.265 ml H2SO4 was added into 50 ml KMnO4 (0.79 g) solution by magnetic stirring to form a homogeneous aqueous solution. Afterwards, a ACFC piece was dropped into the mixture, following the reaction system was placed in a water bath with a temperature of 85 oC for 60 min. The collected sample was rinsed several times, and dried at 60 oC for 12 h in vacuum.
Asymmetrical flexible supercapacitors (AFSCs): The working electrodes, negative and positive electrodes were prepared as follows: 75 wt.% sample was mixed with 15 wt.% acetylene black in an agate mortar until a homogeneous black powder was obtained. Then 10 wt.% polytetrafluoroethylene (PTFE) binder was added into the mixture to make a homogeneous slurry. The final slurry was pasted onto a piece of activated carbon fiber cloth (ACFC, 12 cm2), and the carbon cloth side (10.5 cm2) without coating with any materials, and dried for 12 hours at 60 oC in vacuum. Each electrode contained about 1~1.5 mg of electro-active material and had a geometric surface area of about 1×1.5 cm2. The AFSCs typically include a piece of activated carbon (TF-B520, ShangHai Carbosino Material Co., Ltd.) electrode (12 cm2) (AC@ACFC) as negative electrode, a piece of MnO2@PPy @ACFC (12 cm2) as positive electrode and separator typically made by the porous glassy fibrous paper. Meanwhile,
the electrodes and separator were soaked in 1M Na2SO4 for about 10 min.
Then, the devices were assembled by sandwiching the separator between AC/ACFC and MnO2/PPy /ACFC electrodes. Finally, the assembled devices were carefully sealed using parafilm to ensure a stable performance of the device. The mass ratio of MnO2 electrode and AC electrode (around 1:1.4) was carefully chosen to balance the charge at each electrode. Characterizations: The compositions and morphologies of the as-prepared samples were characterized by a field emission scanning electron microscope (FESEM, FEI QUANTA FEG250) equipped with an energy dispersive x-ray spectroscopy (EDS, INCA MAX-50). The phases and structures were examined by X-ray diffraction
(XRD, D8-Advance, Bruker, using Cu Kα, λ=0.15418 nm). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed on a FEI Tecnai G2 F20 at an acceleration voltage of 200 kV. The average pore sizes, pore volumes, and specific surface areas of the samples were evaluated through measuring N2 adsorption-desorption isotherm with a NOVA touch LX4 apparatus. Electrochemical measurement: The electrochemical properties of the supercapacitor electrodes
were
evaluated
by
cyclic
voltammetry
(CV),
galvanostatic
charge/discharge (GCD) and electrochemical impedance spectrometry (EIS). The obtained GCD curves were used for the calculation of the specific capacitance (C), specific energy (E) and specific power (P) density according to the following equation:
C
It Vm
(1)
1 E CV 2 2 (2)
P
E t
(3) where I (mA) is the charge/discharge current, Δt (s) is the discharge time, m (mg) is the mass of active material for a single electrode or the total mass of active material of positive and negative electrodes, ΔV (V) is the voltage change excluding voltage drop (IR drop) at a constant discharge current. The electrochemical properties of the
electrodes were carried out using an electrochemical workstation (Metrohm Autolab PGSTAT302N) with a three-electrode configuration in a 1 M Na2SO4 aqueous solution. The prepared samples were used as the working electrodes, Pt foil as the counter electrode (CE) and a saturated calomel electrode (SCE) as the reference electrode (RE). The AFSCs were measured in a two-electrode cell configuration.
3. Results and Discussion Figure 1a shows the fabrication scheme for the MnO2@PPy hybrid nanostructure by using PPy nanowires as the starting template. Due to the reductive nature of PPy, MnO2 seeds are expected to form on the surface of PPy NWs in the initial stage by the following reaction pathway [32]: 4MnO4 3C H 2O 4MnO2 CO32 2HCO3-
(4) These seeds were found to grow into nano-flakes after keeping the reaction system at 85oC for a certain time. Low and high-magnification SEM images of the asprepared pure MnO2 NFs arrays (Figure 1b and c) reveal a high density and uniformity of NFs vertically achoring to the surface of carbon fiber. Figure 1d-e and f shows the SEM image of the PPy NW and the obtained hybrid nanostructures using a reaction time of 60min. In the enlarged Figure 1g, fine nanoflakes (NFs) were clearly seen to evolve on PPy NWs while keeping the overall NW like structure. Note that inappropriate selection of the reaction time or PPy NW concentration would lead to less NFs on NW or otherwise destroyed NW structures, as can be seen in Figure
S1. This is because PPy tend to be consumed during the reaction. Figure 1h and i shows the EDS spectra and elemental mapping taken from the enclosed area in Figure 1g. C, Mn, O, and N, K are found to the major elements in the final reaction product. The presence of N and C element signals from PPy again validates that PPy nanowires act as reactive templates. K was also detected because of its intercalation into -MnO2, which is consisted of 2D layers of edge-sharing MnO6 octahedra with K+ cations and water molecules in the interlayer space [17, 28]. The C: N: K: Mn ratio is estimated to be 15.66%: 7.03%: 3.07%: 25.6% from the composition analysis. Figure 1j shows the XRD patterns of the fabricated PPy NW and the final NFs@NW product to identify their crystalline phase. The broad peaks of PPy indicated their amorphous state after fabrication [33], while the peaks of final NFs@NWs structures were consistently indexed to the monoclinic K-Birnessite type δ-MnO2 (JCPDS no.801098) [34-37]. The 2D layer stacked structure of K-Birnessite MnO2 lead to the easy formation of thin MnO2 nanoflakes. With the typical 10 nm thickness of the nanoflakes and their large interstitial space, the prepared nanoflakes on PPy NW will not only provide abundant electrochemically active surface sites, but also facilitate the fast ion penetration in electrolyte for supercapacitor application. Figure 2a shows the close TEM examination of the MnO2 nanoflakes. The inset selected area electron diffraction (SAED) pattern contains multiple faint continuous diffraction rings, suggesting the polycrystalline nature of the as-prepared MnO2 NFs [38]. It is generally known that the BET surface areas and pore-size distributions directly correlate to the specific capacitance. Thus, we continued to examine the BET
surface area of prepared samples from the N2 adsorption and desorption isotherm (Figure 2b). The specific surface area of MnO2@PPy was found to be 166.65 m2 g−1. As a comparison, the specific surface area of the starting PPy NWs was only 17.81 m2 g−1 (Figure S2a). Loading MnO2 on the surface of PPy obviously increases the active surface area, which can potentially offer electrons and ions transport channels and avoid structural collapse during the discharge/charge process [39, 40]. Meanwhile, the MnO2 NFs@PPy NWs sample also shows wider pore size distributions (3~7 nm, Figure 2c) than that of pure PPy NWs (~1.97 nm, see Figure S2b). As the pore size significantly affects the rate performances of nanoporous materials [41] and the larger pores generally provide low-resistant pathways for the ions through the porous structure, as well as a shorter diffusion route, which can also effectively enhance the electronic conductivity and better rate performance [42]. Herein, the high surface area and large pore size achieved in the hierarchical MnO2 NFs@PPy NWs structure eventually endow it enhanced electrochemical performance in the following compared to other materials. CV measurements were firstly conducted on the ACFC, PPy NWs, pure MnO2 and the hybrid MnO2 NFs@PPy NWs structures to examine the synergetic integration of active surface area and conductive channels in the hierarchical nanostructure. Figure 3a shows the representative CV curves of ACFC, pure PPy, pure MnO2 and MnO2@PPy measured at 30 mV s-1. It is can be seen that the CV curve area of ACFC is negligible, indicating the minuscule specific capacitance. Moreover, the enlarged CV curves area of hybrid MnO2@PPy nanostructure suggests greater quantity of
charge storage than pure PPy and MnO2. For the electrode made by hybrid MnO2@PPy nanostructure, no obvious redox peaks are observed from 0 to 0.9 V, indicating that the electrochemical capacitive performances are mainly attributed to Faradaic redox processes [32, 43]. The charge storage mechanism of MnO2@PPy is thus proposed to involve surface adsorption/desorption and tunnel insertion/extraction
of electrolyte cations C H , Li , Na , K
in MnO2 and the reversible
doping/dedoping of anions D Cl , SO42 in PPy [32, 43]:
MnO 2 C e MnOOC (5)
PPy D
e PPy 0 D
(6) To examine the specific capacitance of the prepared electrodes, GCD curves were further measured at varied current densities. The results shown in Figure 3b were measured at a current density of 3.0 A g-1. The calculated specific capacitances for MnO2@PPy, pure PPy and MnO2 at 3.0 A g-1 using eq.1 are respectively 271, 23.3 and 147 F g-1. The highest specific capacitance was obtained in the hierarchical MnO2@PPy nanostructured electrode. In addition to this high specific capacitance, we also found that the prepared MnO2@PPy electrode exhibit excellent capacitance retained ratio (CRR) at high current density. Figure 3c summarizes the specific capacitance measurement results under varied current density from 1 to 30 A g-1. The prepared MnO2@PPy electrode exhibit the highest CRR of ~54.1% at 30 A g-1, which is much higher than the pure PPy (~21.2% at 10 A g-1) and pure MnO2 (~31.6% at 30
A g-1). The excellent capacitance performance of the MnO2@PPy electrode is thoroughly examined by changing the voltage scanning rate (5~150 mV s-1) and current density (1~30 A g-1) in CV and GCD measurements, as shown in Figure 3d and e. The highly linear and symmetric GCD curves in all current densities imply the excellent reversibility and good rate capability of the MnO2@PPy electrode. The corresponding CV and GCD curves of PPy NWs and MnO2 NFs were also revealed in Figure S3. To clearly visualize the capacitance performance of present MnO2@PPy electrode, we summarize in Table 1 the reported specific capacitance and capacitance retained ratio of previously reported MnO2 composite electrodes together with our measurement results. We found that our present MnO2@PPy electrode exhibit one of the best performances in the Table 1. To better understand the relationship between resistance behavior and electrochemical properties, EIS analysis of the electrodes were performed on the prepared samples. Apparently, the PPy electrode possesses the steepest slope of the curves and the smallest diameter of semicircle, meaning ideal capacitive performance and small the charge transfer resistance. We found that the present MnO2@PPy electrode allows not only faster ions diffusion of electrolyte within the pores of electrodes during redox reaction than pure MnO2, but also small internal resistances down to 1.3Ω, as evidenced by the Nyquist plot analysis in Figure S4a. This is ascribed to the larger surface area, pore volume and higher pore size distribution of the hierarchical nanostructure endowing more electron/ion paths for feasible ion transport and fast redox reaction. The above results all demonstrate the synergetic integration effect between the electrochemical active MnO2 nanoflake shell
and the conductive PPy NW core in supercapacitors. Next, the long-term cycling performances of composite MnO2@PPy electrodes and pure PPy were examined by GCD measurements at a current density of 3 A g -1 for 1500 cycles, as shown in Figure 3f. For pure PPy NWs, the specific capacitance decreased to 65.4% of initial capacitance after 1500 cycles (inset in Figure 3f.). For the MnO2@PPy electrode, the specific capacitance increased continuously from its initial value of 271 to 277 F g-1 in the first 150 cycles and then decreased gradually, and maintaining 93.4% of the initial value after a continuous cycling of 1500 times. The increase of specific capacitance in the beginning may be related to the activation effect of electrochemical cycling. Similar phenomena has been observed by Jiang et al. [52], Jin et al. [44] and An et al. [53]. The good cycle stability of MnO2 NFs@PPy NWs could be ascribed to the unique nanoflakes heterostructure that can accommodate structure collapse of the electrode materials during long-term cycles. Besides, the structural robustness of the hierarchical structure can also effectively prevent the aggregation and restacking of PPy NWs and MnO2 NFs during the cycling process. The inset of Figure 3f shows the digital picture of the electrolyte after the measurement. After 1500 cycles, the color of the electrolyte with pure PPy electrode (inset of Figure 3f-1) turns into yellow due to the dissolving of PPy in electrolyte. In contrast, no obvious color change can be observed for the electrolyte with composite electrode, which suggests the good cycle stability of the MnO2@PPy electrode. Figure S4b shows the first and last three cycles of MnO2@PPy electrode material. The good cycling performance, together with the high specific capacitance and
capacitance retained ratio discussed in the above, all imply a good potential of the present
MnO2@PPy
composite
electrode
in
high
energy/power
density
supercapacitors. By using the above MnO2@PPy composite electrode, we demonstrate in the following the assembly of highly flexible AFSC with wide operation voltage window, energy/power density and stability. Figure 4a shows the schematic illustration of the structure of the AFSC, by using MnO2@PPy as the positive electrode, AC the negative electrode, porous glassy fibrous paper as the separator. The electrode material is attached on carbon fiber cloth. Parafilm is used to seal the assembled electrodes to ensure stable operation of the AFSC devices. All the above components exhibit excellent the mechanical stability under different bending conditions, no matter being stretched, kinked or bent (Figure S5). The commercial AC is chosen as negative, material, which has specific capacitances of AC were calculated to be 217, 202, 198 192, 187, and 184 F g−1 at current densities of 1, 2, 3, 5, 8 and 10 A g−1, respectively. The corresponding CV and GCD curves of AC electrode at different scan rates and current densities between -0.9 and 0 V (vs. SCE) are shown in Figure S6a and b. To balance the stored charge at the anode and cathode, mass loadings of AC and MnO2@PPy on the fiber cloth are determined to be 1.4 and 1 mg cm −2, respectively, based on their measured specific capacitance values at 2 A g−1. Figure 4c shows CV curves of MnO2 NFs@PPy NWs and AC electrodes at a scan rate of 30 mV s−1. To estimate the best operating potential of the as-prepared AFSCs, a series of CV curves with different potential windows are collected at 30 mV s-1 as shown in
Figure 4d. The potential window of the device is possible to be extended to as high as 2.0 V without obvious polarization curves. The wide and stable operation voltage of present assembled ASFC make it very suitable for portable electronic devices. Figure 5a and b shows the CV and GCD curves of the assembled AFSCs measured at varied voltage scanning rate and current densities. The as-prepared AFSCs exhibit an ideal capacitive behavior with quasi-rectangular CV curves and feeble redox peaks at all scan rates from 10 to 200 mV s -1 within a potential window of 1.8 V, indicating the presence of both pseudocapacitive and electric double layer capacitor (EDLC) type properties. The GCD curves of the supercapacitor device are seen to maintain a good symmetry even at a high current density of 10 A g-1, demonstrating its rapid charge-discharge properties and good rate capability. The specific capacitance calculated based on the total mass of both positive and negative electrodes at different current densities, reaches a specific capacitance of around 57 F g-1 at a current density of 1 A g-1 (Fig. S7a). The result of EIS measurement on AFSCs is shown in Figure S7b. The straight line in the low-frequency region reveals the ideal capacitive behavior of the device. The arc in the high-frequency area has a small radius, indicating low charge transport resistance (Rct). The bulk solution resistance (Rs) was determined by x intercept on real axis (Z,). The value of Rs and Rct is 1.2 and 2.2, respectively, which prove the high electrical conductivity of both electrodes and the high ionic conductivity of Na2SO4 electrolyte. In consideration of practical use, the long-term cycling stability of MnO2@PPy AFSCs was further measured at current density of 3 A g-1 up to 6000 cycles, and is shown in Figure 5c. The specific
capacitance of the device retains 47.2 F g-1 after 6000 cycles, corresponding to 90.3% of its initial specific capacitance (42.6 F g-1). From the discharge curves shown in Figure 5d, the energy densities and power densities of MnO2 NFs@PPy NWs AFSCs are calculated based on eq.2, and 3, respectively. Figure 5d shows the corresponding calculated Ragone plots, the AFSCs demonstrated in this work delivered an energy density of as high as 25.8 W h kg-1 at the power density of 901.7 W kg-1, which maintained at 17.1 W h kg-1 at an extremely high power density of 9000 W kg-1. Compared to the other electrode materials shown together in the figure, this performance is found superior to that of Co3O4/PPy/MnO2//AC
[18],
MnO2//Fe2O3/Ppy
[23],
MnO2/TiO2//AC
[46],
NiCo2O4/MnO2 NWs//AG (activated graphenes) [47], MnO2/PPy//V2O5/PPy [54], V2O5/GO//rGO [55], CuO/MnO2//MEGO (activated microwave exfoliated graphite oxide) [56], MnO2/C//V2O5-CNT [57], MnO2//Bi2O3 [58], GH(graphene hydrogel)PANI/GP(graphene paper)//GH−PANI/GP [59]. This results demonstrate the bright potential of present MnO2@PPy composite electrode material in high power supercapacitors. Finally, the excellent flexibility of AFSCS is revealed by deforming the test device under various bending states. CV analysis was performed at the bending angles of 0°, 90°, 180°, as shown in Figure 5e. There is a slight increase in capacitance retention and almost completely overlapped CV curves (Fig. 5e). In this case, folding and bending may may reduce the distance between two electrodes, which will help to improve the conductivity and ion transport. The anode and cathode electrodes will
neither detach nor contact with each other in experiment, indicating remarkable mechanical flexibility and stability of our device. The concise structure of present AFSC would greatly simplify the future design of portable/flexible electronic devices. The feasible integration ability of present AFSC for higher operation voltages is further demonstrated by a serial connection of two devices. Figure 5f shows the GCD curves of a single AFSCs and a pair of AFSCs connected in series at current density of 3 A g-1. The connected pair can be steadily operated between 0 and 3.6 V. The inset figure shows a digital image of powered light-emitting-diode with a working voltage of 3.5 V. Figure S7c shows a LED was successfully lighten by two serially connected AFSC cells. The merits of present hierarchical nanostructure can be summarized briefly as follows: (a) MnO2 nanoflakes (MnO2 NFs) are well-grown and well-dispersed on novel polypyrrole nanowires, which build a reliable conductive network for fast electron transport throughout the electrode; (b) Porous MnO2 NFs@PPy NWs core/shell structures significantly enhance the effective utilization of active materials leading to more contact area with electrolyte; (c) PPy NWs serve as both the backbone and conductive pathway for MnO2 NFs, the structure create short distances and additional channels for ion diffusion and charge transfer; (d) PPy NWs and MnO2 NFs@PPy NWs nanostructures are formed from low-cost materials without involving organic solvent and additives. Furthermore, the preparation procedures are straightforward and convenient via stirring process at room temperature and water bath synthesis, which provides a simple, economic and green approach to fabricate an
important kind of metal oxide/conducting polymer nanocomposites. These features highlight the capability of 3D hybrid electrode to meet the requirements of long-cycle lifetime, good rate capability and low internal resistance, which are important merits for practical energy storage devices. 4. Conclusion In summary, we take PPy NWs as a buffer and K-Birnessite type MnO2 NFs as a shell to construct a novel hierarchically porous MnO2 NFs@PPy NWs core/shell heterostructures through a new preparation method. MnO2 NFs@PPy NWs core/shell nanostructures exhibit not only high specific capacitance and good cycle stability, but also low and stable internal resistance. Moreover, the excellent capacitance retained ratios are72.5% (276~200 F g-1), when the current densities range from 2 A g-1 to 20 A g-1, indicating the material being suitable for working under high current. The assembled asymmetrical flexible supercapacitors (AFSCs) (MnO2/PPy//AC) give rise to a maximum energy density of 25.8 W h kg-1 at power density of around 901.7W kg1
and maintains 17.1 W h kg-1 at an extremely high power density of 9000 W kg-1 with
an excellent cycling performance. Furthermore, the as-assembled device shows negligible capacitance degradation at different bending state, indicating excellent mechanical flexibility and stability. These results imply that PPy can be regarded as an ideal conductive substrate, and can greatly enhance the electrochemical performance of MnO2. More importantly, the method may be suitable for the largescale and environment-friendly production of MnO2/PPy hybrid composites and be easily generalized to grow other mesoporous metal oxides nanostructure on
conducting polymers for the fabrication of high-performance energy-storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11304120, 51672109, 21505050), National Basic Research Foundation of China (2015CB932600),
Program
for
HUST
Interdisciplinary
Innovation
Team
(2015ZDTD038) and the Fundamental Research Funds for the Central University. Here authors also want to thank Analytical and Testing Center in Huazhong University of Science and Technology for the technical support.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at Nano Energy
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Figure 1. Synthesis and characterization of MnO2 NFs@PPy NWs core/shell nanostructures. a) Schematic diagrams for the synthesis of MnO2 NFs@PPy NWs core/shell
nanostructures.
b)
and
c)
SEM
images
of
pure
MnO2
nan
oflakes on ACFC. d) and e) SEM images of PPy nanowires. f) and g) SEM images of
MnO2@PPy core/shell nanostructures. h) EDS spectra of MnO2@PPy core/shell nanostructures. i) EDS elemental mapping of MnO2@PPy core/shell nanostructures. j) XRD patterns of the as-synthesized PPy and MnO2@PPy
Figure 2. a) High-magnification TEM images of MnO2@PPy core/shell
nanostructures (inset showing the SAED pattern). b) Nitrogen adsorption and desorption isotherms for MnO2@PPy core/shell nanostructures; c) the corresponding BJH pore size distributions.
Figure 3. a) CV curves of pure MnO2, pure PPy ACFC and MnO2@PPy measured at 30 mV s-1. b) GCD curves of pure MnO2, pure PPy and MnO2@PPy measured at 3A g-1. c) Discharge current density dependence of the specific capacitance. d) CV curves of the MnO2@PPy electrode material at various scan rates between 5.0 and 150 mV s 1
. e) Charge/discharge curves of the MnO2@PPy electrode material at various current
densities ranging from 1 to 30 A g-1. f) Cycle life of pure PPy, MnO2@PPy electrodes at a current density of 3 A g -1; 1) and 2) the color change of electrolyte (pure PPy, MnO2@PPy) after 1500 cycles).
Figure 4. a) Schematic diagrams for designed flexible hybrid devices. b) Digital images for asymmetrical flexible supercapacitors device. c) CV curves collected for MnO2@PPy min and AC electrodes at a scan rate of 30 mV s−1. d) CV curves collected in various potential windows at 30 mV s-1.
Figure 5. Electrochemical characterizations of MnO2@PPy asymmetrical flexible supercapacitors (AFSCs): a) The CV curves of AFSCs at various scan rates ranging from 10 to 200 mV s-1. b) Galvanostatic charge/discharge profiles at various current densities in the voltage range of 0 to 1.8 V. c) cycling stability test of the AFSCs measured at a current density of 3 A / g. d) Ragone plot of MnO2@PPy AFSCs. e) cycling performance of MnO2@PPy AFSCs at different bending states (the inset shows the CV curves at different bending states). f) Alvanostatic charge/discharge curves of a single SCs and two SCs connected in series (the inset shows the magnification of light bulbs).
Weidong He received his B. S. degree in college of physics and Engineering Qufu Normal universiy in 2015. He is now pursing his M.S. degree under the supervision of Prof. Xijin Xu in School of physics and technology, University of Jinan, P.R. China. His main interest is the design and fabrication three-dimensional nanomaterials for energy storage and electrochemistry.
Chenggang Wang received his B. S. degree in School of Physics and Technology from University of Jinan in 2014. Currently, he is a MSc student majoring in Functional Micro/nano Materials and Devices at Jinan University, China. His current research is focused on improving the performance of supercapacitors’ device.
Fuwei Zhuge received his PhD degree from Shanghai Institute of Ceramics, Chinese Academic of Sciences in 2011. Then he joined Osaka university and Kyushu university as postdoctoral researcher. He is now an associated professor in the school of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include controllable fabrication and novel
application of nanomaterials in energy storage and conversion devices.
Dr Xiaolong Deng is an Assistant Professor of the School of Physics and Technology at University of Jinan. He graduated from Northwestern Polytechnical University with a BSc in 2006, and received MEng from Shanghai University in 2009. He received PhD from Konkuk University in 2013. Dr Deng’s research interests focus on synthesis and characterization of advanced sub-micro/nanomaterials for photocatalysis, conversion and energy storage applications.
Xijin Xu received his Ph.D. degree in Institute of Solid State Physics, Chinese Academy of Sciences in 2007. He conducted his postdoctoral research at Nanyang Technological University, Singapore in 2007 and then joined National Institute for Materials Science, Japan (2008-2010) and Griffith University in Australia. Currently, he is a full professor in University of Jinan. He is the author or co-author of more than 90 research articles in peer reviewed journals and 2 book chapters. His most recent research interests include the synthesis and characterization of functional micro/nanostructures and their applications in environmental remediation and energy storages.
Tianyou Zhai received his B. S. degree in chemistry from Zhengzhou University in 2003, and then received his Ph.D. degree in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the supervision of Prof. Jiannian Yao in 2008. Afterwards he joined in National Institute for Materials Science (NIMS) as a JSPS postdoctoral fellow of Prof. Yoshio Bando’s group and then as an ICYS-MANA researcher within NIMS. Currently, he is a Chief Professor of School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include the controlled synthesis and exploration of fundamental physical properties of inorganic functional nanomaterials, as well as their promising applications in energy science, electronics and optoelectronics. He has authored and co-authored about 140 peer-reviewed journal articles, 5 book chapters, co-edited 1 book on nanotechnology, and held 7 patents. His publications have been cited more than 6700 times (H-index is 45).
Table 1 Summary of electrochemical properties with MnO2 composite electrode materials for supercapacitor reported in recent papers.
materials MnO2@NG Mn3O4@G MnO2@PANI MnCo2O4.5@MnO2 MnO2@H2Ti3O7 MnO2@NiCo2O4 MnO2@CNT@G Ni(OH)2@MnO2 PANI@MnO2 PPy covered MnO2NT PPy covered MnO2 PPy covered MnO2NW NG@MnO2 G@MnO2
current density 2 A g-1 2 / 10 A g-1 5 A g-1 10 A g-1 2 A g-1 8 A g-1 1 / 3 A g-1 20 A g-1 20 A g-1
Sc / F g-1
Ref.
5M LiCl 1M Na2SO4 0.5M Na2SO4 1M Na2SO4 1M Na2SO4 2M KOH 1M Li2SO4 1M Na2SO4 1M Na2SO4
current collector Graphite paper Stainless stell Ni foam Platinum foil Platinum foil Ni foam Ni foam Graphite paper Graphite paper
150.8 210 / 141 186 211.2 90 258 251 / 150 163.3 134
[33] [44] [45] [31] [46] [47] [48] [35] [28]
2M KCl
Ni foam
15 A g-1
215
[32]
electrolyte
-2
1M Na2SO4
Graphite paper
2 mA.cm
141
[27]
1M Na2SO4
Ni foam
10 A g-1
150
[25]
192.5 194
[49] [50]
1M Na2SO4 1M Na2SO4
Ni foam Platinum
-1
2Ag 1A g-1
α-MnO2 @ δ-MnO2 MnO2 cover PPy
substrate Ni foam 20 A g-1
6 M KOH 1M Na2SO4
ACFC
153.8
1 / 2 / 10 / 20 / 30 A g-1
[51] 325 / 276 244 / 200 / 170
This work
(Note: Sc= Specific capacitance NG = nitrogen-doped graphite G = graphite PANI = polyaniline CNT = carbon nanotubes NT = nanotubes NW = nanowires ACFC=
activated carbon fiber cloth)
Highlight
A simple, economic and green approach was developed for the fabrication of hierarchical
MnO2
nanoflakes@PPy
nanowire
core/shell
structured
nanocomposites.
Synergetic combination of the chemical stable MnO2 shell and the conductive PPy core is successfully demonstrated for high power density and persistent electrochemical operation.
By using the hierarchical MnO2 nanoflakes @PPy Nanowires core/shell nanostructures as positive electrode, ultra-flexible and high energy density asymmetrical supercapacitors were successfully achieved