MnO2 nanosheets for high-performance asymmetric supercapacitors

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Hierarchical nanocomposite composed of layered V2O5/PEDOT/MnO2 nanosheets for High-performance asymmetric supercapacitors Chun Xian Guo, Gamze Yilmaz, Shucheng Chen, Shaofeng Chen, Xianmao Lu

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S2211-2855(14)00287-0 http://dx.doi.org/10.1016/j.nanoen.2014.12.018 NANOEN636

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Received date: 22 September 2014 Revised date: 8 December 2014 Accepted date: 12 December 2014 Cite this article as: Chun Xian Guo, Gamze Yilmaz, Shucheng Chen, Shaofeng Chen, Xianmao Lu, Hierarchical nanocomposite composed of layered V2O5/ PEDOT/MnO2 nanosheets for High-performance asymmetric supercapacitors, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2014.12.018 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.

Hierarchical nanocomposite composed of layered V2O5/PEDOT/MnO2 nanosheets for high-performance asymmetric supercapacitors Chun Xian Guo, Gamze Yilmaz, Shucheng Chen, Shaofeng Chen, Xianmao Lu Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585

Keywords Redox reaction; layered V2O5; layered MnO2; conducting polymer; supercapacitor

Abstract We present here a tandem redox reaction strategy for building layered V2O5 (LVO), conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT), and layered MnO2 (LMO) into a sandwich structure LVO\PEDOT\LMO. The fabrication process consists of two redox reactions: i) the oxidizing polymerization of PEDOT on LVO nanosheets to form a conformal coating (LVO\PEDOT); and ii) the reduction of KMnO4 by PEDOT to generate LMO nanoplates that stacked onto the LVO\PEDOT (LVO\PEDOT\LMO). This approach to the fabrication of a complex structure eliminates the use of any extra toxic oxidizing/reducing agents. Using LVO aerogel as the starting material, the total reaction time can be as short as 10 min. Asymmetric supercapacitors built from LVO\PEDOT\LMO cathode and active carbon (AC) anode (LVO\PEDOT\LMO||AC) using Na2SO4 aqueous electrolyte showed an energy density of 39.2 Wh kg-1 (based on active materials), which is among the highest reported for supercapacitors with neutral aqueous electrolytes. The LVO\PEDOT\LMO||AC supercapacitors also offered high rate capability (21.7 Wh kg-1 at 2.2 kW kg-1) and good cycle stability (93.5 % capacitance retention after 3000 cycles). These results demonstrate that the green tandem redox

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reaction strategy is promising for the development of complex nanocomposite materials for advanced energy storage.

Introduction Layered materials, such as MoS2, graphite, Ni-Co double hydroxide, and clay minerals, can form strong chemical bonds in-plane but display weak out-of-plane bonding [16].They have found a wide range of applications in energy harvesting and storage, CO2 capture and conversion, chemical/biomedical sensing, and pollution control [7-11]. Layered vanadium pentoxide (LVO) and layered manganese oxide (LMO, birnessitetype MnO2), in particular, as two earth-abundant transition metal oxides have attracted increasing attention as electrode materials in electrochemical energy storage systems because of their high theoretical capacitances, wide potential window, and good compatibility with neutral aqueous electrolytes [12-20]. Compared with their highly crystalline forms such as single-crystal V2O5 and rutile MnO2 (β-MnO2), LVO and LMO offer relatively low material density and large electrochemical active surface area. More importantly, they allow facile intercalations of various molecules (e.g., H2O) and ions (e.g., Li+ and Na+) [21-24]. These intercalation reactions take place with a large free energy of formation, making LVO and LMO high-performance electrode materials for supercapacitors and lithium ion batteries [25-28]. In addition, during the intercalation process, vanadium and manganese can undergo a series of redox reactions due to the transitions of their different valence states [29-33]. Since these redox reactions occur at different potentials, combing LVO and LMO into a functional nanocomposite could provide synergistic effects for further enhanced capacitance. Nevertheless, such a nanocomposite has not been reported, possibly due to mismatched atomic/crystal structures and/or structural complexity of the two layered materials. Another challenge for LVO and LMO as electrode materials is their poor electrical conductivity, which results in high charge transfer resistance and limits their rate capability. Herein, we present a green and fast tandem redox reaction strategy for the fabrication of a nanocomposite material consisting of LVO and LMO, together with a conducing polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), in a sandwich structure LVO\PEDOT\LMO. Starting from porous LVO aerogel assembled from V2O5

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nanosheets, the material fabrication includes two redox reactions in tandem: i) LVOcatalysed oxidizing polymerization of PEDOT to induce a conformal coating of PEDOT on LVO nanosheets; and ii) reduction of KMnO4 by PEDOT on LVO nanosheets to deposit LMO nanoplates. This tandem redox reaction strategy provides a time-saving characteristic with a total reaction time as short as 10 min (with LVO aerogel as the starting material), as well as a green approach without involving any toxic oxidizing/reducing agent. The resulting LVO\PEDOT\LMO has a hierarchical structure with PEDOT sandwiched between LMO nanoplates and LVO nanosheets, forming a porous aerogel composite. Such a unique structure would allow high electrochemical performance -- while LVO and LMO offer large specific capacitance, the incorporation of PEDOT reduces charge transfer resistance. To demonstrate the electrochemical activity of LVO\PEDOT\LMO, we fabricated asymmetric supercapacitors with activated carbon (AC) anode and Na2SO4 aqueous electrolyte. Asymmetric supercapacitors consisting of electrodes of different active materials can take the advantage of the different potential windows of the two electrodes and allow high operation voltage [34]. Relatively higher energy densities can thus be achieved for asymmetric supercapacitors than symmetric ones [34, 35]. In addition, asymmetric supercapacitors with neutral aqueous electrolytes are relatively environmentally friendly and can be assembled in ambient environment [17, 36]. Nevertheless, energy densities of current asymmetric supercapacitors with neutral aqueous electrolytes are typically below 30 Wh kg-1 [37-42]. Our LVO\PEDOT\LMO||AC supercapacitors provide a much higher energy density (39.2 Wh kg-1 based on active materials) than that of LVO\PEDOT||AC (28.9 Wh kg-1) and LVO||AC supercapacitors (10.8 Wh kg-1). This performance is among the highest reported for supercapacitors with neutral aqueous electrolytes. In addition, high rate capabilities (21.7 Wh kg-1 at 2.2 kW kg-1) as well as good cycle stability (93.5 % capacitance retention after 3000 cycles) were also attained for LVO\PEDOT\LMO||AC.

Materials and methods Material synthesis Commercial V2O5 powder (0.364 g, Sigma-Aldrich) and 30 mL water (18.2 MΩ·cm at 25 °C, Milli-Q) were mixed under vigorous magnetic stirring at room temperature. To the

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mixture, 5 mL hydrogen peroxide (30 wt% in water, Sigma-Aldrich) was quickly added with continuous stirring for 1h. The resultant solution was then transferred to a 40 mL autoclave and kept in an oven at 190 oC for 15 h. After cooling to room temperature, the highly viscous LVO solution was frozen by liquid nitrogen and dried under vacuum to obtain yellow-coloured LVO aerogels. LVO aerogels were put into 15 mL acetonitrile solution containing 0.25 mL 3,4-ethylenedioxythiophene (EDOT, 97%, Aldrich). The colour of the aerogels changed from yellow to green within 1 min. After 5 min, the colour became dark green, and no significant colour change was observed afterwards. The aerogels were then taken out of the acetonitrile solution, washed with ethanol and water for a couple of times, and dried at room temperature, resulting in LVO\PEDOT. Subsequently, LMO was deposited by soaking the as-prepared LVO\PEDOT in 25 mM KMnO4 (ACS reagent, ≥99.0%, Sigma-Aldrich) solution at room temperature for different times ranging from 5 to 60 mins, producing LVO\PEDOT\LMO. The materials were then taken out of the solution, washed with water for three times, followed by vacuum drying. Characterizations Field emission scanning electron microscopy (FESEM) was performed on a JEOL JSM6700F at an acceleration voltage of 10 kV. Transmission electron microscopic (TEM) and high-resolution transmission electron microscopic (HRTEM) images were recorded with a JEOL JEM-2010F microscope at an acceleration voltage of 200 kV. Elemental composition and distribution were investigated by energy dispersive X-ray spectroscopy (EDS) on JEOL JSM-6700F equipped with an Oxford/INCA EDS. Height profiles of the materials were examined using a Bruker Dimension ICON-PT atomic force microscope (AFM) with scanning mode at room temperature. X-ray diffraction (XRD) patterns were acquired using X-ray diffractometer (GADDS XRD system, Bruker AXS) with a CuKα source (λ=1.54 Å). X-ray photoelectron spectroscopy (XPS) characterizations were performed on a PHI Quantera x-ray photoelectron spectrometer with a chamber pressure of 5×10-9 torr, a spatial resolution of 30 µm and an Al cathode as the X-ray source to determine composition of the nanoparticles. Nitrogen adsorption/desorption experiments were carried out at 77.3 K by means of an Autosorb-1 analyzer.

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Electrochemical measurements Cathodes were prepared by mixing active materials, carbon black and polytetrafluoroethylene (PTFE, 60 wt % dispersion in H2O) with a weight ratio of 7:2:1 via grinding and then pressed on nickel foam (0.08 mm), followed by drying at 80 oC under vacuum for 12 h. Materials loading of the cathodes are 1.2 mg cm-2. Anodes were prepared with the same procedures as that of cathodes except the use of activated carbon instead of LVO-based materials. Asymmetric supercapacitors were constructed using Swagelok-type cells with 1.0 M Na2SO4 aqueous solution as the electrolyte and Whatman filtration membrane as the separator. All electrochemical characterizations were measured using Autolab PGSTAT128N. Electrochemical behaviours of the cathodes were investigated in a three-electrode cell, in which platinum foil and SCE electrode were used as counter and reference electrodes, respectively. Supercapacitor performance was evaluated using two-electrode cell. Impedance analysis of supercapacitor was recorded under open circuit condition using a frequency range from 100 kHz to 100 mHz by applying a sine wave with an amplitude of 5.0 mV. Calculation Voltammetric specific capacitances (Cv, F g-1) were calculated from CV curves at different scan rates using equation C = (
 × )/(s×V), where I is the current density (A g-1), V is the potential (V), and s is the scan rate (V s-1). Charge-discharge specific capacitances (C, F g-1) were calculated from discharge profiles at different current ∆



densities using equation C = ∆ , where (A g-1) is the current density for chargedischarge measurements, ∆t is the discharge time, and ∆V is the potential change. Energy density E (Wh kg-1) of the supercapacitors was calculated using equation, E = (∆) 

, where C (F g-1) is specific capacitance, and ∆V (V) is the voltage window. Power 

density (P, W kg-1) of the supercapacitors is calculated using equation P =  , where E (Wh kg-1) is energy density of the supercapacitor, and ∆t (s) is the charge-discharge time. All calculations were based on the mass of active materials.

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Results and discussion Fabrication and characterizations of composite material of LVO\PEDOT\LMO The fabrication of LVO\PEDOT\LMO started from LVO aerogel prepared with hydrothermal method [43, 44]. Detailed procedures are provided in the Experimental Section. As shown in Figure 1a, the LVO aerogel has a cylinder shape with a diameter of 25 mm and a height of 20 mm. Scanning electron microscopy (SEM) images in Figure 1a1-a2 show that the LVO aerogel is mainly composed of interconnected LVO nanosheets with lateral sizes in micrometres. As revealed by atomic force microscopy (AFM) height profiles (Figure S1-2), these LVO nanosheets (average thickness of 10.5 nm) were assembled from ultrathin nanobelts with an average thickness of around 1.34 nm. PEDOT was polymerized onto LVO nanosheets by directly immersing the LVO aerogel in acetonitrile solution containing EDOT at room temperature. Within one minute, the colour of the aerogel changed from yellow to green (see the recorded video in Supporting Document). After 5 min, the colour became dark green; and no significant colour change was observed afterwards.

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Figure 1 Scheme and digital images showing the fabrication of (a) LVO aerogel, (b) LVO\PEDOT, and (c) LVO\PEDOT\LMO. The corresponding FESEM images with different magnifications are shown in (a1-a2) LVO aerogel, (b1-b2) LVO\PEDOT, and (c1c2) LVO\PEDOT\LMO. The resulting LVO\PEDOT retained the cylinder shape of the LVO aerogel (Figure 1b) and its porous structure (Figure 1b1-b2), while the nanosheet thickness increased to 24.2 nm due to the formation of PEDOT (Figure S3). Subsequently, LMO was deposited by immersing the as-prepared LVO\PEDOT in KMnO4 solution at room temperature to

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form LVO\PEDOT\LMO (Figure 1c). The amount of LMO can be tailored by adjusting the LMO deposition time -- a short reaction time of 5 min is enough to form a layer of LMO nanoplates (evidences provided in the later part). With longer deposition times (>15 min), the LMO nanoplates grew into nanosheets with curling-up edges uniformly distributed on the LVO\PEDOT surface (Figure 1c2), while the porous structure was retained. The average thickness of the LMO nanoplates/nanosheets is around 2.0 nm (Figure S3). Energy dispersive X-ray spectroscopy (EDS) mappings of LVO\PEDOT\LMO revealed the homogeneous distribution of vanadium, sulphur, oxygen, and manganese elements (Figure S4), indicating the presence of LVO, LMO, and PEDOT. For deposition times of 5, 15, 30 and 60 min, the weight percentages of MnO2 in LVO\PEDOT\LMO increased from 10.4 to 15.0, 21.0 and 23.8%, respectively (Figure S5). For the sample with 15.0% of LMO, the weight percentages of LVO and PEDOT are 78.9% and 6.1%, respectively. The porous structure of the LVO\PEDOT\LMO composite was also investigated with N2 adsorption/desorption measurements (Figure S6). A specific surface area of 39.4 m2 g-1 was obtained based on the multipoint Brunauer-Emmett-Teller method, while the pore sizes are between 1.5 to 30 nm (inset of Figure S6). Structural and compositional analyses of the composite material were further performed with transmission electron microscopy (TEM, Figure 2). TEM images of LVO nanosheets confirm that they were assembled from nanobelts with a width less than 100 nm (Figure 2a1 and S7), consistent with the SEM and AFM characterizations. Highresolution TEM (HRTEM) image of LVO clearly shows the lattice fringes with a dspacing of 0.204 nm (Figure 2a2), corresponding to (220) planes of V2O5 [13]. LVO\PEDOT retained the nanosheet structure as that of LVO (Figure 2b1). While the (220) planes of V2O5 can still be observed for LVO\PEDOT (Figure 2b2), they are not as clear as those of LVO nanosheets without PEDOT. For LVO\PEDOT\LMO with 15.0 wt% LMO loading, the nanosheet structure was well retained (Figure 2c1). Highmagnification TEM image (Figure 2c2) shows that LMO nanoplates with an average size of around 5 nm are uniformly dispersed on the nanosheets. Lattices with interplanar distances of 0.253 and 0.212 nm (Figure 2c3) confirm the (200) and (112) planes of birnessite-type MnO2 [31, 45]. With the LMO mass loading increased to 23.8 wt%, 2.8-

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nm thick LMO nanoplates with curling-up edges were found for LVO\PEDOT\LMO (Figure 2d).

Figure 2 (a1-a2) TEM images of LVO nanosheets. (b1-b2) TEM images of LVO\PEDOT nanosheet. (c1-c2) TEM images of LVO\PEDOT\LMO nanosheet with an LMO loading of 15.0 wt%. (c3) A HRTEM image showing clear lattices of birnessite-type MnO2 of the sample. (d) TEM images of LVO\PEDOT\LMO with an LMO loading of 23.8 wt%, showing LMO nanoplates vertically grown on LVO\PEDOT. Inset of (d) shows an FESEM image of the same sample.

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X-ray diffraction pattern (XRD) of LVO aerogel in Figure 3a shows a series of diffraction peaks (00l) that are almost equidistant, suggesting a lamellar structure [4346]. The intense peak at 2θ of 6.299° corresponds to a d-spacing of 14.02 Å (d001), consistent with V2O5 layers intercalated with water molecules [12]. For LVO\PEDOT, the d001 spacing of LVO increased slightly to 14.30 Å (2θ = 6.176°), while the broad peak at 24.0° can be attributed to LVO. For LVO\PEDOT\LMO, the d001 spacing of LVO decreased to 10.72 Å (2θ = 8.235°). The interlayer spacing of LVO with intercalated water (V2O5·nH2O) decreases with reduced n value [15]. Therefore, it is likely that some intercalated water molecules between LVO layers were lost during the LMO deposition, causing reduced d001 spacing of LVO in LVO\PEDOT\LMO. The diffraction peaks of LMO in LVO\PEDOT\LMO were barely noticeable, possibly due to the poor crystal structure of LMO in comparison with LVO, which shows strong and sharp peaks. However, after the etching of LVO from the LVO\PEDOT\LMO by H2O2, two peaks were revealed at 2θ of 12.2° and 24.5° (Figure 3b), corresponding to (001) and (002) of LMO (birnessite-type MnO2, JCPDS 80-1098), respectively [47, 48]. The layered structure of LVO and LMO was also examined through HRTEM by imaging the curling-up edges of the nanosheets. Before the coating of PEDOT, LVO nanosheet edges show well-defined layers with an inter-layer distance of around 1.15 nm (Figure 3c); while for LVO\PEDOT and LVO\PEDOT\LMO, the inter-layer distances of LVO became 1.22 nm (Figure 3d) and 0.96 nm (Figure 3e), respectively. The change in inter-layer distance of LVO in the three materials is consistent with the XRD results. For LMO (23.8 wt% loading) in LVO\PEDOT\LMO, the HRTEM of the nanosheet edges also reveal a layered structure with an inter-layer spacing of around 0.72 nm (Figure 3f), matching well with LMO d001 planes.

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Figure 3 (a) XRD patterns of LVO aerogel, LVO\PEDOT, and LVO\PEDOT\LMO. (b) XRD pattern of LVO\PEDOT\LMO sample after etching V2O5. HRTEM images showing layered structure of V2O5 in (c) LVO aerogel, (d) LVO\PEDOT, and (e) LVO\PEDOT\LMO. (f) HRTEM image exhibiting the layered structure of birnessite-type MnO2 nanosheets in LVO\PEDOT\LMO with an LMO loading of 23.8 wt%.

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Tandem redox reaction mechanisms It is important to uncover the reaction process of the tandem redox reactions involved in the fabrication of LVO\PEDOT\LMO. Such understanding may facilitate the extension of this strategy to the preparation of other functional composite materials. Of the two redox reactions in tandem, the former is the polymerization of PEDOT on LVO aerogel by oxidizing EDOT monomers. Since no additional oxidizing agent was added during the polymerization, one possibility is that LVO aerogel itself acts the oxidizing agent. However, LVO well retained its element composition and valance after PEDOT deposition, as observed from XPS V2p spectra shown in Figure 4a. It has been reported that the vanadium centers in V2O5 can activate oxygen molecules for the oxidization of organic molecules [49-51]. Therefore, in the present work, the LVO should functionalize as catalyst for oxygen to oxidize EDOT, forming PEDOT (Figure 4d1). It is worth noting that typically it takes several hours or even longer for the oxidation of organic molecules with conventional V2O5 materials [49]. The fast polymerization of PEDOT on LVO nanosheets in our work (5 min) may be attributed to the porous structure and large surface area of LVO aerogel, allowing efficient contact with EDOT and oxygen molecules. The second redox reaction is the reduction of KMnO4 by PEDOT coated on LVO (Figure 4d2). The distance of the two oxygen atoms in the 6-atom ring of PEDOT is ~2.94 Å, equal to the distance between two Mn atoms of the MnO6 octahedra in birnessite-type MnO2 [48]. In addition, the affinity of Mn atoms of LMO to oxygen atoms of PEDOT molecules is favorable due to their native electron characteristics. The matching atomic and electronic structures make it favorable for PEDOT serving as nucleation sites to guild growth of LMO. During redox exchange reaction, thiophene sulphur of PEDOT can reduce MnO4- to form MnO2 and sulfoxide/sulfone, as confirmed from XPS S2p peaks of LVO\PEDOT\LMO (Figure 4b). Based on the XPS Mn3s (Figure S8), the oxidation state of Mn in the LMO was determined to be 3.64 by employing the method reported by Toupin and coworkers [52]. This oxidation number suggests the coexistence of both Mn4+ and Mn3+ for LMO in the LVO/PEDOT/LMO. Similar redox reactions used to deposit manganese oxide have been recently reported [22, 53].

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Figure 4 (a) XPS V2p spectra of LVO and LVO\PEDOT. (b) XPS S2p spectra of LVO\PEDOT and LVO\PEDOT\LMO. LVO\PEDOT\LMO displays a peak centred at 168.0 eV, attributing to the sulfoxide/sulfone groups. (c) Tandem redox reactions for the fabrication of LVO\PEDOT\LMO: (c1) reaction 1: LVO-activated oxygen molecules to oxidize EDOT for PEDOT polymerization; (c2) reaction 2: PEDOT-reduced MnO4- for LMO deposition. Three-electrode characterizations The electrochemical behaviours of the materials were investigated in a three-electrode cell. Working electrodes were prepared by pressing a ternary mixture of the as-prepared

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materials (LVO aerogel, LVO\PEDOT, or LVO\PEDOT\LMO), acetylene black, and PTFE with a weight ratio of 7:2:1 onto nickel mesh. Figure 5a presents the CV curves with a potential range of 0-0.8 V (vs. SCE) at 25 mV s-1 in 1 M Na2SO4 aqueous solution. LVO electrode exhibits quasi-rectangular shape with several pairs of redox peaks, which should be from the redox reactions of vanadium of different valence states. Compared with LVO aerogel, LVO\PEDOT displays higher current density, which should be attributed to capacitance enhancement from the electrochemical doping/dedoping process of SO42− ions of PEDOT [54, 55] and better LVO utilization via PEDOT-improved charge transfer. In the case of LVO\PEDOT\LMO, it shows further increased current density compared with both LVO and LVO\PEDOT, indicating an enhanced energy storage capability. The specific capacitances of the electrodes were calculated from CV curves according to C = ( ∫ I × dV ) / (s ×V ) , where I is the current density (A g-1), V is the potential (V), and s is the scan rate (V s-1). The LMO loading considerably affects the specific capacitance of LVO\PEDOT\LMO. While the specific capacitance initially increased with LMO loading and reached the maximum value of 215.6 F g-1 at a loading of 15.0 wt.%, it decreased with further increased loading of LMO (Figure 5b). Considering the weight ratio of each component in LVO\PEDOT\LMO with LMO loading of 15.0 wt.%, the capacitance contribution from LVO, PEDOT and LMO is estimated to be 43.2%, 20.2% and 36.6%, of which detailed calculation is provided in the supporting information.

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Figure 5 (a) CV curves of LVO, LVO\PEDOT, and LVO\PEDOT\LMO at 25 mV s-1 in 1 M Na2SO4 aqueous solution. (b) Effect of LMO weight percentage on the specific capacitance of LVO\PEDOT\LMO. (c) Specific capacitances at different scan rates

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ranging from 2 to 250 mV s-1. The inset of (c) shows the specific capacitances at low scan rates from 0.5 to 5 mV s-1. (d) Galvanostatic charge–discharge curves of LVO, LVO\PEDOT, and LVO\PEDOT\LMO at 1 A g-1. (e) Galvanostatic charge–discharge curves of LVO\PEDOT\LMO at different current densities. (f) Electrochemical impedance spectra of the three electrodes. Figure 5c shows the specific capacitances of the three electrode materials at different scan rates ranging from 0.5 to 250 mV s-1. At all scan rates, LVO\PEDOT\LMO exhibits the highest specific capacitance. Specifically, at 2 mV s-1, LVO\PEDOT\LMO has a specific capacitance of 266.4 F g-1, higher that of LVO\PEDOT (228.0 F g-1) and LVO aerogel (161.6 F g-1). At a higher scan rate of 100 mV s-1, LVO\PEDOT\LMO still retains a specific capacitance of 182.6 F g-1, superior to that of LVO\PEDOT (146.8 F g1

) and LVO aerogel (85.2 F g-1). LVO\PEDOT\LMO also shows good rate capability.

With the scan rate increased from 2 to 250 mV s-1, a capacitance rention of 58.6% was achieved for LVO\PEDOT\LMO; while only 29.8% and 53.2% of the capacitances were retained for LVO and LVO\PEDOT, respectively (Figure S9). The galvanostatic chargedischarge curves of LVO\PEDOT\LMO at current densities up to 10 A g-1 are nearly symmetrical (Figure 5d and 5e), further confirming the good electrochemical reversibility and rate capability. Impedance analysis indicates that the charge transfer resistance of LVO\PEDOT is less than half of that of LVO (Figure 5f). After the deposition of LMO, the charge transfer resistance slightly increased, but still much smaller than that of LVO. The good electric conductivity of PEDOT and the mesoporous structure of the composite should promote the enhanced charge transfer and ion transport, leading to the improved electrochemical performance. It is worth noting that PEDOT was partially oxidized during the deposition of LMO. As calculated from XPS S2p curve in Figure 4b, the ratio of oxidized PEDOT over PEDOT is around 1:4, indicating only a small part (20%) of the PEDOT was oxidized. Therefore, the partially oxidized PEDOT may not affect the electrochemical performance of LVO\PEDOT\LMO significantly. In addition, a previous work using redox exchange reaction to deposit MnO2 on PEDOT reported that the redox reaction and charge-discharge processes of the resulting material might be improved compared to that of plain PEDOT [22]. Nevertheless, further study may be

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necessary to unveil the effect of oxidized PEDOT on the electrochemical behaviours of the materials synthesized using the redox exchange method. Performance evolution of asymmetric supercapacitors We further fabricated asymmetric supercapacitors using the layered materials as the cathode and activated carbon (AC) as the anode with 1 M Na2SO4 aqueous electrolyte (see detailed device fabrication process in the Experimental Section). The configuration of the asymmetric supercapacitors is shown in Figure 6a. To maximize device performance, the mass loadings of cathode and anode were optimized based on charge balance Q+ = Q- [56, 57], where the charges stored at the cathode (Q+) and anode (Q-) were determined by galvanostatic charge-discharge curves of the cathode (0-0.8V) and anode (-1.0 to 0 V) as shown in Figure S10. Based on charge balance, the anode/cathode mass ratios for asymmetric supercapacitors LVO||AC, LVO\PEDOT||AC, and LVO\PEDOT\LMO||AC were determined to be 0.92, 1.32 and 1.50, respectively.

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Figure 6 (a) Configuration of the asymmetric supercapacitors. (b) CV curves of asymmetric supercapacitors with a voltage window of 1.8 V at a scan rate of 25 mV s-1. (c) Charge-discharge measurements of asymmetric supercapacitors at a current density of 1.2 A g-1. (d) Ragone plots of the asymmetric supercapacitors calculated from charge-discharge curves based on the total mass of active materials. (e) Cycle performance of the asymmetric supercapacitors at 6 A g-1. (f) Digital images of a green LED powered by two LVO\PEDOT\LMO||AC supercapacitors connected in series.

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The asymmetric supercapacitors were tested by CV with a voltage window of 1.8 V. At different scan rates, LVO\PEDOT\LMO||AC shows larger specific currents than that of both LVO||AC and LVO\PEDOT||AC. Specifically, at a low scan rate of 0.5 mV s-1, LVO\PEDOT\LMO||AC provides a specific capacitance of 85.7 F g-1, much higher than that of LVO\PEDOT||AC (63.5 F g-1) and LVO||AC (36.3 F g-1). For LVO||AC, LVO\PEDOT||AC, and LVO\PEDOT\LMO||AC at a scan rate of 25 mV s-1, the specific capacitances are still 17.5, 34.2 and 41.8 F g-1, respectively. The electrochemical performance of the devices was also compared based on charge-discharge measurements (Figure 6c). The specific capacitances at 1.2 A g-1 are 15.6, 39.2 and 59.2 F g-1 for LVO||AC, LVO\PEDOT||AC and LVO\PEDOT\LMO||AC supercapacitors, respectively, further proving the enhanced energy storage capability for the materials by combing LMO, LVO, and PEDOT. Ragone plots from the charge-discharge measurements of LVO||AC, LVO\PEDOT||AC and LVO\PEDOT\LMO||AC supercapacitors at various current densities from 0.3 to 9.0 A g-1 are summarized in Figure 6d. Based on the total mass of electrode active materials including both cathode and anode, a high energy density of 39.2 Wh kg-1 was achieved for the LVO\PEDOT\LMO||AC supercapacitor. This energy density is higher than of LVO\PEDOT||AC (28.9 Wh kg-1) and more than three times as high as that of the LVO||AC (10.8 Wh kg-1). LVO\PEDOT\LMO||AC also shows better rate capability (21.7 Wh kg-1 at a power density of 2.2 kW kg-1) than that of LVO\PEDOT||AC (15.8 Wh kg-1 at 2.2 kW kg-1) and LVO||AC (4.9 Wh kg-1 at 2.2 kW kg-1). It is worth noting that the energy density of LVO\PEDOT\LMO||AC supercapacitor (39.2 Wh kg-1) is more than three times higher than that (11.6 Wh kg-1) of previously reported asymmetric supercapacitors of CNTs\DNA\MnO2||CNTs/DNA with the Na2SO4 aqueous electrolyte [36]. It is also higher than reported supercapacitors based on neutral aqueous electrolyte including graphite foam/Co3O4/PEDOT/MnO2||graphite foam/activated graphene (9.8 Wh kg-1) [37], coaxial nanowires of PEDOT/MnO2||PEDOT nanowires (~9.8 Wh kg-1) [38], MnO2 nanotubes||activated graphene (20.0 Wh kg-1) [17], graphene/MnO2/carbon nanotubes||graphene/AC supercapacitor (24.0 Wh kg-1) [42], amorphous MnO2 nanoparticles||graphene (25.2 Wh kg-1) [39], V2O5 nanofibers||polyaniline nanofibers

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(26.7 Wh kg-1) [40], and carbon nanofibers/graphene||carbon nanofibers/graphene (29.1 Wh kg-1) [41], as well as higher than or close to some recently reported aqueous base/acid or organic electrolyte-based supercapacitors such as MnO2/porous activated graphene||porous activated graphene (24.3 Wh kg-1) [58] and Co3O4/PPy/MnO2 composites [8, 59]. Figure 6e shows the cycling stability of the supercapacitors measured by chargedischarge cycles at 6 A g-1. After 3000 cycles, a capacitance retention of 93.5 % was observed for LVO\PEDOT\LMO||AC. The retention is higher than that of LVO\PEDOT||AC (87.6%) and LVO||AC (57.2%). Cycling stability is related to the chemical/physical stability of the electrode materials. This result indicates that the combination of LVO, LMO and PEDOT can promote the chemical/physical stability of the composite for the good cycling performance. To further demonstrate the potential application, we used two LVO\PEDOT\LMO||AC supercapacitors connected in series with a working voltage larger than 3 V to drive a green light emitting diode (LED) light. The LED can be powered up for more than 10 mins (Figure 6f), showing the potential of the layered nanocomposite LVO\PEDOT\LMO as a high-performance energy storage material.

Conclusion In summary, we report the fabrication of a functional nanocomposite LVO\PEDOT\LMO using a tandem redox reaction strategy. The LVO\PEDOT\LMO has a sandwich structure with PEDOT filled between LVO nanosheets and LMO nanoplates. The material fabrication mainly consists of two redox reactions in tandem: LVO nanosheetcatalyzed oxidizing polymerization of PEDOT which coated conformally on the LVO nanosheets; and reduction of MnO4- by PEDOT to generate LMO nanoplates on LVO\PEDOT, leading to the formation of a sandwich composite LVO\PEDOT\LMO. This strategy offers a green approach to the fabrication of a complex structure without involving any toxic oxidizing/reducing agent. The reaction time is as short as 10 min using LVO aerogel as the starting material. Asymmetric supercapacitors LVO\PEDOT\LMO||AC using Na2SO4 aqueous electrolyte show an energy density of 39.2 Wh kg-1 (based on the mass of active materials), much higher than of that of

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LVO\PEDOT||AC (28.9 Wh kg-1) and LVO||AC (10.8 Wh kg-1). This energy density is among the highest reported for supercapacitors with neutral aqueous electrolytes. In addition, LVO\PEDOT\LMO||AC offer high rate capabilities (21.7 Wh kg-1 at 2.2 kW kg-1) as well as good cycle stability (a capacitance retention of 93.5 % after 3000 cycles). This study demonstrates the effectiveness of this tandem redox reaction strategy for the fabrication of layered nanocomposite materials for high-performance supercapacitors.

Acknowledgements We thank the financial support by Ministry of Education, Singapore (Grant# R279-000391-112) and National Research Foundation CRP program (Grant# R279-000-337281).

Appendix A. Supporting information Supplementary data associated with this article can be found in the on line version at http://dx.doi.org/10.1016/.

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Graphical Abstract

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Author Biography

Dr. Chunxian Guo is currently a postdoctoral fellow in the Department of Chemical & Biomolecular Engineering, National University of Singapore. He received his Bachelor and PhD degrees from Nanyang Technological University, Singapore in 2007 and 2011, respectively. He works on functional composite materials composed of metal oxides, conducting polymers and nanostructured carbons, as well as their applications for supercapacitors and electrocatalysts.

Gamze Yilmaz is a PhD candidate in Department of Chemical & Biomolecular Engineering, National University of Singapore. She works on nanoporous metal oxides for high-performance supercapacitors.

Shucheng Chen was a research engineer in the Department of Chemical & Biomolecular Engineering, National University of Singapore. He is currently a PhD

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candidate at Stanford University. He worked on polymer-functionalized metal oxide nanoparticles.

Dr. Shaofeng Chen worked as a postdoctoral fellow in the Department of Chemical & Biomolecular Engineering, National University of Singapore. He received his PhD degree from Department of Chemistry, University of Science and Technology of China in 2009. He works on layered materials for supercapacitors and biomineralizations.

Dr. Xianmao Lu is an assistant professor in the Department of Chemical & Biomolecular Engineering at National University of Singapore (NUS). Before he joined NUS, he was a postdoctoral research fellow at University of Washington and Washington University. He received his PhD in Chemical Engineering from the University of Texas at Austin, where he started his research in nanomaterials. His current research interest is mainly on controlled growth of metal and metal oxide nanomaterials for applications in energy and environmental technologies. Highlights Layered V2O5/PEDOT/MnO2 nanosheets were sandwiched into a hierarchical structure. The fabrication was based on a green and fast tandem redox reaction strategy. Supercapacitors using neutral aqueous electrolyte showed a high energy density of 39.2 Wh kg-1. High rate capability and good cycle stability were demonstrated.

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