ZnO@MnO2 CoreâShell Nanofiber Cathodes for High Performance

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ZnO@MnO2 Core-Shell Nanofiber Cathodes for High Performance Asymmetric Supercapacitors A.V. Radhamani, K. M. Shareef, and M. S. Ramachandra Rao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08082 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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ACS Applied Materials & Interfaces

ZnO@MnO2 Core-Shell Nanofiber Cathodes for High Performance Asymmetric Supercapacitors A.V. Radhamani, 1 K. M. Shareef, 2 M. S. Ramachandra Rao1* 1

Nano Functional Materials Technology Centre, Material Science and Research Centre, Department of Physics, Indian Institute of Technology Madras, Chennai-600036, India

2

Conducting Polymer Lab, Department of Physics, Indian Institute of Technology Madras, Chennai-600 036, India.

* Corresponding author email ID: [email protected]

Keywords: Electrospinning; ZnO@MnO2 nanofibers; Asymmetric supercapacitor; aqueous electrolyte; Energy storage. ABSTRACT: Asymmetric supercapacitors (ASCs) with aqueous electrolyte medium have recently become the focus of increasing research. For high performance ASCs, selection of cathode materials play a crucial role and core-shell nanostructures are found to be of a good choice. We successfully synthesised, ZnO@MnO2 core-shell nanofibers (NFs) by modification of high-aspect-ratio-electrospun ZnO NFs hydrothermally with MnO2 nanoflakes. High conductivity of the ZnO NFs and the exceptionally high pseudocapacitive nature of MnO2 nanoflakes coating delivered a specific capacitance of 907 Fg-1 at 0.6 Ag-1 for the core-shell NFs. A simple and cost-effective ASC construction was demonstrated with ZnO@MnO2 NFs as a battery-type cathode material and a commercial-quality activated carbon as a capacitortype anode material. The fabricated device functioned very well in a voltage window of 0 – 2.0 V and a red-LED was illuminated using a single-celled fabricated ASC device. It was found to deliver a maximum energy density of 17 Whkg-1 and a power density of 6.5 kWkg-1 with capacitance retention of 94% and coulombic efficiency of 100%. The novel architecture of the ZnO@MnO2 core-shell nanofibrous material implies the importance of using simple design of fiber based electrode material by mere changes of core and shell counterparts.

INTRODUCTION Research related to energy production, storage and distribution is becoming increasingly essential due to the rapid consumption of non-renewable energy resources. 1 ACS Paragon Plus Environment


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Supercapacitors (SCs) are potential energy storage devices known for their high power density, long cycle life, excellent reversibility, safety, stability and environmental friendliness.1-3 However, in comparison to batteries (~100 Whkg-1), SCs have less energy density (~3-6 Whkg-1), thereby limiting their practical applications.4 This issue can be solved by extending cell voltage window and enhancing specific capacitance, thereby increasing the energy density (½CV2).5 Organic/ionic liquid electrolytes with high voltage windows of ~ 3-4 V could be used to extend cell voltage.6-8 Nevertheless, low ionic conductivity, toxicity, handling difficulty, and expense of the ionic liquid increases the need of an alternative method for voltage enhancement by preserving highly advantageous aqueous electrolyte medium.9,

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One of the best ways to tackle the above issue is by designing asymmetric

supercapacitors (ASCs), which are comprised of hybrid electrodes; battery type cathode which serves as an energy source and a capacitor type anode which serves as a power source; each electrode is characterised by different potential windows, thereby accounting for a widened cell voltage.11, 12 Based on their charge storage mechanism, SCs are generally categorized as electric double layer capacitors (EDLCs) and pseudocapacitors.13,

14

In EDLCs (e.g., carbonaceous

materials) charge storage occurs at the electrode-electrolyte interface via nanometer range charge separation; in pseudocapacitors (e.g., transition metal oxides and conducting polymers) reversible oxidation/reduction reactions are responsible for charge storage. Transition metal oxides, including MnO2, NiO, CuO, Co3O4, Fe3O4, and VOx have higher specific capacitance and energy density than carbonaceous and conducting polymer based materials.15 Among the various transition metal oxides, MnO2 is a very promising candidate due to its high theoretical capacitance (~1370 Fg-1), multiple oxidation states of Mn, ecofriendliness, natural abundance and low cost.16, 17 However, poor electronic conductivity (105

-10-6 Scm-1) and low rate performance of pristine MnO2 must be resolved before MnO2 can

be used in practical SC applications.18 The morphology, surface area and polymorphs also directly impact electrochemical characteristics.16, 19 Considerable research has been devoted for the design and development of various MnO2-conductive matrix/core-shell hybrid architectures in order to achieve the optimum performance.20 Hybrid core-shell configurations in which conductive matrix forms the core and MnO2 forms the shell are highly efficient due to synergistic contributions from MnO2 and conductive materials.21, 22 A range of morphologies that use hierarchical nanostructures with MnO2 as shell materials, such as, CuO@MnO223, Fe3O4@MnO224, CNT@MnO225, TiO2@MnO226, ZnCo2O4@MnO227 and Co3O4/SnO2@MnO228 have been reported and demonstrated improved electrochemical 2 ACS Paragon Plus Environment

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performance compared to their pristine counterpart. However, morphologies of core and shell could cause notable variation in SC performance.29 ZnO, a multifunctional material in electronic devices, sensors, and photocatalysis, has been well-studied due to its high electrical conductivity, special optoelectronic characteristics, and high chemical and thermal stability.30,

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Among the various reported

core-shell structures, ZnO is an ideal material for forming ‘core’ as it offers effective pathways for electron transport and strong mechanical support (chemical-thermal stability).32 Only a few studies have reported use of ZnO@MnO2 nanostructures as SC electrodes. Yang et al.31 synthesized ZnO@MnO2 nanocables via high temperature annealing and the hydrogenation method, achieving an improved capacitance of 138.7 mFcm-2 at a current density of 1 mAcm-2. Sun et al.33 reported ZnO@MnO2 nanostructures with an areal capacitance of 31.3 mFcm-2. Li et al.34 developed a ZnO nanorod/MnO2 nanowire hybrid assembly with a specific capacitance of 747 Fg-1 at a scan rate of 2 mVs-1. A wide range of ZnO, MnO2 and ZnO@MnO2 nanostructures can be prepared by various methods, including hydrothermal, sol-gel, co-precipitation, electrodeposition and solvo-thermal methods.33, 35, 36 Electrospinning has garnered specific attention due to its simplicity, versatility and cost effectiveness.37,

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Electrospun nanofibers (NFs) are uniquely characterised by their high

aspect ratio, large surface-to-volume ratio, granular and fibrous morphology, high mechanical strength and good crystallinity, which makes them ideal candidates for energy storage devices such as batteries, fuel cells, supercapacitors and solar cells. Nanofibrous morphology generates free pathways for electron movement and hence ideal morphology for core materials in core-shell configuration.39, 40 To the best of our knowledge, this is the first report on the synthesis of NF-based ZnO@MnO2 core-shell nanostructures and the fabrication of a lab-scale prototype ASC to demonstrate it as a potential cathode material for energy applications. Zilong et al. 41 reported ASC assembly comprised of a ZnO/MnO2 core–shell nanorod array prepared by colloid pretreatment method followed by electrodeposition, resulting in a volumetric capacitance of 0.52 Fcm-3 at a scan rate of 10 mVs-1 in a 0-1.8 V potential window. This paper describes preparation of high conductive and high aspect ratio electrospun ZnO NFs by annealing zinc acetate-polyvinyl alcohol fibrous mat at a suitable temperature and synthesis of ZnO@MnO2 core-shell NFs. A simple, inexpensive ASC assembly was made in 1 M Na2SO4 aqueous electrolyte solution, by suitably loading ZnO@ MnO2 NF and activated carbon (AC) composite as a battery-type cathode material and a high surface area AC as a capacitive-type anode material. An extended voltage window of 2 V was achieved, and a red LED was 3 ACS Paragon Plus Environment


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illuminated for approximately one minute. Results obtained from various analytical and electrochemical methods are presented in the following sections. RESULTS AND DISCUSSIONS As-spun ZnAc-PVA NFs were analysed using thermogravimetric Analysis (TGA) and differential scanning calorimetry (DSC) (Figure 1a) to determine the suitable calcination temperature to obtain ZnO pure phase. Thermal decomposition of ZnAc-PVA was observed to proceed through three steps. An initial weight loss of ~ 8% below 100 °C corresponded to the evaporation of water. In the temperature range of 100-250 °C, slow intramolecular decomposition (weight loss ~ 44%) of PVA, dehydration of crystal water, and partial decomposition of ZnAc occurred while rapid weight loss (~26%) began at 385 °C, indicating two step intermolecular decomposition of ZnAc and PVA.42 Complete decomposition of PVA took place at 500 °C, resulting in pure ZnO phase. Further increase in temperature caused negligible weight of the samples. The above observation was verified by an endothermic peak corresponding to solvent evaporation at ~ 110 °C, and three exothermic peaks at temperatures, 247 °C, 343 °C, and 439 °C corresponded to ZnAc and PVA decompositions. Thermal analysis showed that a calcination temperature of 500 °C is required for phase pure ZnO formation. TGA and DSC data of pure PVA is shown in Figure S1, leading to the inference that the relative content of ZnO retained in addition to carbon was ~ 24 %. It is also reasonable to conclude from Figure 1a that ZnO NFs show a very good thermal stability up to 800 °C. Thermal stability of ZnO@MnO2 core-shell NFs is shown in Figure 1b, in which a weight loss of ~ 25 % corresponds to evaporation of absorbed water molecules and dehydration of trapped water molecules occurring in the temperature range of 30 - 140 °C. A weight loss of only ~ 5 % was observed with further increase in temperature to 400 °C. A dip in TGA and DSC at ~ 500 oC indicates a MnO2 to Mn2O3 transformation hence the synthesised core-shell NFs shows very good thermal stability up to 400 oC. Phase purity of ZnO and ZnO@MnO2 NFs were analysed by X-ray diffraction studies (Figure 1c). All X-ray reflections related to ZnO NFs were assigned to a single face hexagonal phase of ZnO (ICDD No. 98-001-1316) with space group P63mc and lattice parameters 3.243 Å (=a=b) and 5.195 Å (=c). Diffraction peaks of ZnO@MnO2 NFs were assigned to the majority phases of ZnO, birnessite δ-MnO2 and minority phases of spinel ZnMn2O4. The outer surface of ZnO NFs typically contains MnO2, whereas spinel phase may grow at the interface of ZnO and MnO2. Formation of birnessite δ-MnO2 nanoflakes on ZnO outer surfaces was further confirmed from FESEM images. Birnessite δ-MnO2 (ICDD No. 4 ACS Paragon Plus Environment

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98-010-4520) exhibits hexagonal crystal phase with lattice parameters; 2.851 Å (a), 21.493 Å (c) and space group of R-3m with two-dimensional (2D) interlayer structure comprised of edge shared MnO6 which forms a tunnel/interlayer gap of size ~7.2 Å. Raman spectroscopy was used to analyse vibrational modes of ZnO and ZnO@MnO2 core-shell NFs, as shown in Figure 1d, in order to understand more about their crystalline quality. Prominent Raman vibrational modes at 329, 381, 435, 581 cm-1 were assigned to E2(high)-E2(Low), A1(TO), E2(High), A1(LO) modes of hexagonal ZnO with wurtzite crystal structure.43,

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A high

intense peak with small full width at half maximum (F.W.H.M) showed good crystallinity of electrospun ZnO NFs. For the core shells , vibrational modes at 506, 560 cm-1 were in good agreement with the major intrinsic vibrational modes of birnessite δ-MnO245, while 660, 670 cm-1 were related to ZnMn2O4 vibrational modes . Therefore, the Raman results agreed with XRD observations. Figures 2a-2d show field emission scanning electron micrographs of the as-spun ZnAc-PVA nanofibrous mat at various PVA-to-ZnAc loading ratios (x) (i.e., 5, 2, 1, and 0. 5). The diameter, interconnectivity, branching and surface defects associated with the PVAZnAc NF varied in each case, which in turn reflected in the final morphology of the annealed product. Factors such as surface tension of solvent mixtures, polymer structure, viscosity of the solvent, solution volatility, solution conductivity, and electric potential affect as-spun fiber morphology.46 In the present study ZnAc loading was varied by keeping rest of the parameters constant. A slight change in the ZnAc concentration caused a change in molecular weight and viscosity, surface tension, solution conductivity, and solvent dielectric effect. At high PVA- ZnAc loading conditions (x = 5), NFs with diameters of ~ 250 nm were formed with a high degree of surface roughness and stuck together, forming large-sized junctions. At x = 2 ratio, clear surface defects were observed in addition to non-uniform diameter distribution. High loading ratio values of 1 and 0.5 resulted in highly surface defected NFs with almost uniform diameters of ~ 400 nm. In general, random distribution of web-like morphologies with high aspect ratio was due to bending instability and partial evaporation of water solvent37, 38 FESEM images of NFs calcined at 500 °C at a slow heating rate of 2 °Cmin-1 are shown in Figures 3a-3d, which depicts morphological variation in ZnO formation with respect to PVA-ZnAc concentrations. A close investigation of the annealed fibers revealed that they were all comprised of a large number of interconnected nano-grains as a result of the slow decomposition of polymer template and other organic materials during annealing. A rapid heating rate and high temperature can theoretically spoil all nanofibric morphological 5 ACS Paragon Plus Environment


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properties of the material.47 Control experiments have been conducted to determine the optimum temperature and heating rate for the high aspect ratio NFs. At a PVA-ZnAc loading ratio of x = 5, thick clusters of ZnO nanoparticles were observed with less aspect ratio for the resulting fibers. A mixed morphology comprised of nanorods and granular NFs was observed at a ratio of x = 2, whereas an interconnected network of nanoparticles, which occasionally formed NFs with high aspect ratios, were observed at x = 1. At a comparatively lower PVAZnAc loading ratio (i.e., at x = 0.5), granular NFs with a high aspect ratio of ~ 1000 (~ 300 µm / 300 nm) were observed and selected to make the core-material in the core-shell hybrid configuration since they provide a large number of surface sites for MnO2 nucleation. TEM analysis on the x = 0.5 case is shown in Figures 3e and 3f. Granular morphology of the ZnO NFs (Figure 3e) with perfect hexagonal granular particles was evident from the SAED pattern, as shown in the inset of Figure 3e. The SAED pattern in Figure 3f demonstrates the polycrystalline nature of NFs containing many single crystals of ZnO granular particles. The lattice- resolved fringes from a single crystalline ZnO showed a‘d’ spacing of 2.8 Å corresponding to (010) plane (inset Fig.3f). One ZnO NFs, that retained a high fibrous nature and granularity, was selected from the various morphologies of synthesised ZnO NFs for MnO2 coating via the hydrothermal method (Figure 4) because it provides large surface sites for nucleation. As KMnO4 is a strong oxidizing agent, it easily reduces Mn oxidation state from +7 to +4 in an aqueous medium. The reaction involved in the formation of MnO2 nanoflakes is represented as 2KMnO4 + 2H2O 2MnO2 + 2K+ + 4OH- + O2

(1)

The hydrothermal temperature and time duration for the shell coating without decomposition of the core-material were optimised as 150 °C and 5 h respectively. Under hydrothermal conditions, the Ostwald ripening process48 occurred, causing the growth of MnO2 nanoflakes with lamellar structure. Typical scanning electron micrographs of core-shell NFs at various magnifications are shown in Figures 5a and 5b. As shown in the figures, NF morphology was retained with a few nanometer thick coating of MnO2 nanoflakes on the ZnO nanofibrous surface. The MnO2 nanoflakes with high surface area is highly beneficial for rapid and efficient electronic transportation between electrolyte and ZnO NFs. EDS analysis of coreshell NFs is shown in Figure S2, highlighting the presence of Zn, O and Mn. The crystal structure and textural characteristics of core-shell NFs were further analysed by TEM images and selected area electron diffraction patterns (Figures 5c-d). Investigation of an isolated cross-sectional NF in Figure 5c shows granular particles of ZnO with diameters of ~ 150 nm with uniformly and strongly bound MnO2 nanoflakes on the 6 ACS Paragon Plus Environment

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surface with a thicknesses of ~ 9 nm. The SAED pattern shown in the Figure 5d indicates the crystalline nature of ZnO nanoparticles, which are part of the granular fibers. The sharp diffraction spots could be indexed to the (002) and (110) atomic planes of the hexagonal wurtzite phase of ZnO. ZnO@MnO2 NFs are highly polycrystalline in nature, as revealed by ring pattern in the left inset of Figure 5d. Lattice resolved images in which single-grain crystal growth occurred along the (001) plane with an interplanar spacing of 5.1 Å is shown in the bottom right inset of Figure 5d. The elemental oxidation state and phase purity of the ZnO@MnO2 NFs were analysed locally using XPS (Figure S3). Survey scan spectrum in Figure S3 (a) shows the presence of manganese (Mn), oxygen (O) and zinc (Zn) elements. Core-level of Mn 2p (Figure S3 (b)) corresponds to two peaks, one at 642.7 eV (Mn2p3/2) and other at 654.2 eV (Mn2p1/2). The observed energy separation of 11.5 eV between Mn 2p peaks is equivalent to Mn4+ ions and hence the Mn-phase could be confirmed as MnO2.49 Binding energies of oxygen for different bonding states such as Mn-O-Mn, Mn-OH, H-O-H can be evident by deconvoluting the O1s peak (Figure S3 (b)). Zn 2p also well resolved in to two strong peaks at 1021.0 and 1044.0 eV which corresponds to Zn 2p3/2 and Zn 2p1/2 levels respectively. Binding energy peaks of Zn 2p confirms the Zn2+ nature in the phases of pristine ZnO or/and interfacial ZnMn2O4. Electrochemical performance and impedance analysis of ZnO NFs (working electrode composed of 80% ZnO+15% AC+5% PVDF) in three electrode configurations were evaluated to understand the capacitance and conductivity contribution to core-shell performance. CV plots (Figure S4 (a)) at various scan rates showed a negligible charge storage capacity in which the maximum specific current obtained was in the range of few tens of µA. Specific capacitance estimated from GCD (inset of Figure S4 (a)) profiles was 0.32 Fg-1 at a lower current density of 0.1 Ag-1, confirming the inactiveness of ZnO towards redox/surface adsorption of ions. The high conductivity of ZnO fibers revealed by the lower dc resistance value ~8.6 Ω (Figure S4 (b)). Typical CV plots (Figure 6a) of the ZnO@MnO2AC composite electrode were recorded in the potential window of 0 – 1 V at various scan rates. CV curves displayed predominant pseudocapacitive behaviour over the double layer contribution. MnO2 nanoflakes can only contribute significantly to pseudocapacitnace whereas contribution from ZnMn2O4 phase at the interface is very small50 compared to coreshell NFs. These distorted CV curves were due to the electric double layer effect of AC and Faradaic oxidation/reduction reactions of ZnO@MnO2 NFs. Oxidation and reduction peaks were vaguely seen and exact location was difficult to determine from the curves. However,

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the slope of the curves indicated that reduction peaks widened with respect to scan rates. Typical pseudocapacitive redox reaction in the birnessite δ-MnO251 was represented as +

-

MnO2 + H + e +

MnOOH

-

MnO2 + Na + e

(2)

MnOONa

(3)

GCD profiles of ZnO@MnO2 NFs were recorded and plotted for current densities of 6, 12, 23, and 35 Ag-1, as shown in Figure 6b. Performances for slightly lower current densities (0.6 and 1 Ag-1) are shown in the inset of Figure 6b (top right). The nonlinear nature of GCD curves further demonstrated the pseudocapacitive behavior of birnessite δ-MnO2. As indicated by the inset of Figure. 6b (bottom right), IR drop contribution which comes from the equivalent series resistance of the cell increased slightly with current density. The value of IR drop reached to ~ 0.1 V for an applied current density up to 12 Ag-1 and reached to ~ 0.2 V when the current density was 35 Ag-1. Potential drop at a higher current rate could be attributed to blockage at electrode pores during gas evolution, electric breakdown of electrode-current collector interface possibly due to over oxidation, or

dissolution of

electrode in electrolyte solutions.52 Specific capacitance (Cs) was calculated from GCD curves without including any ‘IR’ voltage drop contribution as what impact for a device applications is the increase or decrease in specific values, Cs =

I ∆t m∆ V

(4)

where I is the current density (Ag-1), ∆t is the time of discharge (s), and ∆V is the potential window (V). A specific capacitance of 907 Fg-1 was observed at a low current density of 0.6 Ag-1. A remarkable specific capacitance of 210 Fg-1 at a high current density of 35 Ag-1 showed excellent high rate capability of the synthesised core-shell material. Specific capacitance variation as a function of current density was calculated from GCD curves, as shown in Figure 6c. In spite of the slight voltage drop, the material showed an efficient performance at high current density. Effective energy density (Es) in Whkg-1 and power density (Ps) in kWkg-1 were estimated by using the equations:

Cs ∆V 2 Es = 7.2 Ps =

3.6 Es V 2 = R ∆t 8

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(5) (6)

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A maximum energy density of 126 Whkg-1 and a power density of 10.8 kWkg-1 were observed at current densities of 0.5 Ag-1 and 35 Ag-1, respectively. Es and Ps variation with respect to applied current density are shown in Figure S5(a). Nyquist plot analysis (Figure S5 (b)) on the working electrode material shows a dc resistance of 5.6 Ω and a charge-transfer resistance of 22.8 Ω. Charge-discharge cycling was conducted for approximately 5000 cycles at a high current density of 30 Ag-1. Material performance in terms of capacity retention (%) and coulombic efficiency (%) were calculated (Figure 6d). Columbic efficiency of a system with charging time tc and discharging time td can be evaluated as ɳ = (tc/td) x 100. The symmetric nature of GCD cycles shows excellent coulombic efficiency. A slight fluctuation (< 5%) was observed in the capacitance for the first 2500 cycles, and 97% capacity retention was observed for the next 2500 cycles. Morphology of cycled samples retains the core-shell structure (inset of Figure 6d), which further confirms high cyclic stability of the material. Overall capacitance retention was very promising for such high current-rated chargedischarge cycling. Performance of ZnO@MnO2 NFs in the 3 electrode configurations was compared to similar core-shell nanostructures22,

31-34, 53-56

and tabulated in Table S1.

Performance of the NF based core-shell nanostructures showed superior performance compared to other geometries. A prototype ASC device was assembled using commercially available AC as negative electrode material, to demonstrate the capability of the synthesised ZnO@MnO2 core-shell NFs as cathode material. In order to assemble an ASC with high performance parameters, + − coulombic charge balance i.e., Qcathode = QAnode had to be maintained in both ASC electrodes.

Figure 7a shows typical CV plots of AC and ZnO@MnO2-AC electrodes, recorded at a scan rate of 10 mVs-1 in a potential window of 0 – -1 V and 0 – +1 V, respectively, against the Ag/AgCl reference electrode. The rectangular-shaped CV curve of the AC showed an electric double layer capacitance, while the distorted CV curve of ZnO@MnO2-AC displayed the pseudocapacitance behaviour in addition to the double layer effect. Because the AC and ZnO@MnO2-AC can store electric charges in the potential windows of 0 – -1 V and 0 – +1 V respectively, with respect to the Ag/AgCl electrode, the ASC potential window designed from these electrodes was the sum of potential across capacitor-type and potential across hybrid battery-type electrode, potentially able to operate from 0 – +2 V.57 Maximum achievable potential windows of electrodes were drawn from the corresponding CV plots, and the appropriate mass loading ratio of the electrode materials for designing ASC with highest performance was evaluated from the GCD curve (Figure S6). Mass loading ratio must



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be determined for the two electrodes that differ in specific capacitance and operating potential window in order to balance their charge storage capacities. The mass ratio was calculated from the following equations to obtain charge balance58:

m+ Cs − ∆V− = m− Cs + ∆V+

(7)

where m+ and m- are masses on positive (cathode) and negative (anode) electrodes of the ASC, Cs+ and Cs- are specific capacitances of the electrode materials in the potential windows of ∆V+ and ∆V- respectively, measured against a reference electrode in the three electrode configurations. The mass loading ratio of the positive and negative electrodes was estimated as ~5.7 according to equation (7). Approximately 1.7 mg of negative electrode (AC) and 0.3 mg of positive electrode (ZnO@MnO2-AC) were used to maintain the charge neutrality of ASC. Electrochemical assessment of ASC was carried out on the basis of total mass loading on both electrodes. Figure 7b shows cyclic voltammograms of the fabricated ASC device recorded at a scan rate of 50 mVs-1 for various voltage windows. All CV and GCD characteristics were studied in the voltage window of 0-2 V. Figure 7c shows CV profiles for the ASC in the voltage window of 0-2 V at various scan rates of 5, 10, 20, 50, and 100 mVs-1. All CV plots had an almost identical shape and retained a pair of oxidation and reduction peaks. Redox peak separations in the CV plots increased with increasing scan rates. Oxidation and reduction peaks were located at 0.98 V and 0.78 V, respectively, for a low scan rate of 5 mVs-1. The minor differences in peak location at lower scan rates implied good reversibility of the involved chemical reactions. Redox peaks which are characteristic of the pseudocapacitive behaviour of an ASC, dominate the double layer contribution from the AC, as evidenced by the CV characteristics of the individual electrodes (Figure 7a). GCD curves of the ASC are plotted in Figure 7d at different current densities in the voltage window of 0-2 V. The nonlinear nature of the charge-discharge curves supported the rapid and reversible oxidation/reduction peaks demonstrated in the CV plots. The specific capacitance (Cs), energy density (Es), and power density (Ps) of the ASC (expressed in Fg-1, Whkg-1 and kWkg1

, respectively) were estimated from the GCD curves using equations (4), (5), and (6)

respectively, where the symbols have their usual meanings except ‘m’, which denotes the total mass loading on both electrodes (expressed in grams). Specific capacitances at various current densities were calculated and plotted in Figure 7e. A specific capacitance of ~ 31 Fg-1 (31 mFcm-2) was attained at a current density of ~ 0.4 Ag-1 (0.8 mAcm-2), gradually reducing to ~ 7.5 Fg-1 at a high current density of ~ 6 Ag-1. The observed specific capacitance was 10 ACS Paragon Plus Environment

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limited by the selection of AC and could be increased further by proper selection of anode materials. Theoretical specific capacitance for ASC was calculated as, 1 1 1 = + Cs C+ C− C ASC =

Cs ( m+ + m− )

(8)

(9)

where C+ and C- are the specific capacitances of positive and negative electrodes, respectively, at identical current rates in three electrode configurations, and CASC is the resultant specific capacitance expected on two electrodes, and (m+ + m-) is the total mass loading on both electrodes. Therefore, the selection of negative electrode has a significant role in deciding the overall capacitance since Cs and CASC will always be lesser than C-. Approximately 50 % of the theoretical capacitance (~ 64 Fg-1 at 0.4 Ag-1) was retained in the fabricated device. The specific capacitance, energy density, and power density of the fabricated device was compared to the previously reported ASC or symmetric SCs 22, 32 based on ZnO@MnO2 core-shell nanostructures as cathode material, as shown in Table S2. The energy density and power density of the ASC was plotted against the applied current densities, as shown in Figure 7f. A maximum energy density of ~ 17 Whkg-1 and a power density of 6.5 kWkg-1 were observed for the device. Cyclic stability is an important factor that determines its industrial applications. The fabricated ASC was subjected to chargedischarge cycling of ~ 3000, at a current density of 1 Ag-1. Figure 8a shows the capacity retention and columbic efficiency of the ASC. The last five cycles of the GCD are plotted in Figure 8b. Results showed that 94% of the specific capacitance was retained, even after 3000 cycles, with a cent % columbic efficiency (η = td tc X 100 ). Excellent energy density, cyclic stability, and columbic efficiency of the ASC was attributed to the ZnO@MnO2 core-shell nanofibrous configuration and the high surface area of AC. The dominance of pseudocapacitive behaviour of core-shell NFs over the double layer nature of AC was justified by its unique structural properties. The surface-coated MnO2 nanoflakes provide a large effective area for ionic surface adsorption and intercalation/de-intercalation, while ZnO and the AC increased the speed of electronic transport from poorly conducting MnO2 to the current collector. Electrochemical impedance spectroscopy further revealed the internal resistance and the charge transport process of the ASC. Figure 8c shows a Nyquist plot of the ASC under open circuit potential (OCP). Two distinct regions in the EIS plot i.e., a distorted semicircle 11 ACS Paragon Plus Environment


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at high frequency region and a straight line with a positive slope towards low frequency region represent the charge transfer process and the capacitive nature of the device respectively. The impedance plot was resolved using Randles circuit to obtain the equivalent components. The equivalent series resistance was found to be 3.2 Ω and the contribution can stem from electrolyte resistance, electrode-electrolyte interfacial resistance, electrode’s internal resistance, and contact resistance between the electrode and current collector. Inset in Figure 8c shows the obtained equivalent circuit, consisting of bulk solution resistance (Rs), double layer capacitance (Cdl), Warburg impedance of electrolyte ion diffusion, charge transfer resistance (Rct), and psedocapacitance element (CPE1). Experimentally observed data with equivalent circuit are shown in the insets of Figure 8c and the values of Rs and Rct estimated from the equivalent circuit are 3.2 Ω and 1.7 Ω respectively. The low value of Rct was attributed to the significant interfacial conductivity of ZnO@MnO2 core-shell NFs due to high conductivity and surface area of ZnO NFs and AC. The large slope of the straight line in the low frequency region of the Nyquist plot was attributed to Warburg behaviour, which arose from the rapid ionic diffusion across the electrode-electrolyte interface. Figure 8d represents the Bode plot of the ASC device. The measured phase angle of 76o at the low frequency region was correlated to the contribution from the pseudocapacitance and double layer capacitance. Imaginary part of the capacitance is an important parameter that offers information about energy loss in the ASC during any irreversible process. The imaginary part of capacitance was estimated from the following equation59,

C '' (ω ) =

Z '(ω )

ω Z (ω )

2

(10)

where Z '(ω ) is the real part of the complex impedance, ω is the perturbing ac frequency, and Z (ω) is the modulus of the impedance. The estimated energy loss was plotted against the frequency, as shown in the inset of Figure 8d. The response or knee frequency (f0) of the SC was ~ 25 Hz at which maximum capacitance was delivered. Response frequency divided the energy loss curve in to two regions: capacitive where f < fo and resistive where f > fo. The corresponding relaxation time constant, or SC figure of merit (Ʈo = 1/fo) was 40 ms. The small value of time constant implied a fast discharge rate and supported the higher power density of ASC59. The fabricated prototype ASC was connected to a red LED (GaAsP: red LED with VF = 1.8 V at 20 mA) and was illuminated for ~1 min. A schematic of the entire process involved is depicted in Figure 9a and the glowing LED at various time spans is shown in 12 ACS Paragon Plus Environment

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Supplementary video V1. The power density and energy density of the ASC device, as shown in a Ragone plot (Figure 9b), was compared to other potential energy storage devices. The packaged device showed a maximum energy density of 17 Whkg-1 at a power density of 393 Wkg-1 at a current density of ~ 0.4 Ag-1. These ASC device values were superior to many symmetric SCs and ASCs constructed from MnO2. Furthermore, cyclic stability of the material ensured the stability and minimal damage of the electrode material for the redox reactions. Therefore, the core-shell fibers were of very good choice with substantial hope for future supercapacitive energy storage devices. Voltage window extensions of ASC in aqueous electrolytes can be explained as follows. The stable voltage window of aqueous electrolytes were given by V=∆G/nF=1.2 V, where ∆G is Gibb’s free energy for the water splitting reaction (~237 kJmol-1), n is the number of electrons involved in the reaction, and F is Faraday’s constant. Hence, the operating voltage window in an aqueous electrolyte and symmetric configuration of MnOxǁMnOx is usually limited by the oxygen evolution reaction at the positive electrodes and the irreversible reduction reaction of Mn4+ to Mn3+ at the negative electrodes.60 However, the high over-potentials for the H2 and O2 evolution at the positive and negative electrodes, respectively, extended the thermodynamically stable potential window limit (~1.2 V) in an asymmetric configuration, thereby causing the high energy density compared to symmetric capacitors. Water splitting is an important issue at high operating voltage windows (>2 V)61, i.e., at highly reducing conditions (low applied potential) H2O is reduced to H2 2H2O+2e- H2 (g) + 2OH

(11)

and at highly oxidizing conditions (high applied potential) H2O is oxidized to O2 6H2O 2O2 (g) + 4H3O+ + e-

(12)

The above process continues as long as water is available to be reduced/oxidized. This study showed that the voltage window could be extended up to 2.6 V with a very less amount of O2 evolution. The corresponding cyclic voltammetry profile is shown in Figure S7. A high voltage window (~2.6 V) was observed since both electrode materials consisted of AC, which acted as a catalyst in the recombination of H2 and O2 as C + H CHads

(13)

where CHads is the adsorbed hydrogen complex formed with C. During the discharge process hydrogen is released from the AC surface and combines with OH- ions to form water. The production of unwanted gases can also be handled by the addition of other catalysts in nonAC systems.62



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Core-shell NFs that form the cathode material can also be coupled with a variety of alternative pseudocapacitive materials, such as SnO2 and TiO2 (other than AC/graphene), which possess complementary potential windows. These kind of coupling in which both electrodes are based on NFs open up a new avenue for further development of high performance SCs. Core-shell architectures are highly favourable for meeting the desired ASC performance which would have otherwise been limited by poor electron transfer kinetics of the pseudocapacitive material. Superior material performance of ZnO@MnO2 NFs can be attributed to the following factors: (i) ZnO NFs offers high material stability, easier path for electronic transport, and a large number of surface sites for MnO2 nucleation; (ii) MnO2 nanoflakes can shorten the electrolyte ion diffusion length and guarantee maximum utility of the MnO2 active material; (iii) Redox reaction kinetics is further enhanced by the porous nature of the core-shell heterostructures, which aids proper wettability of the electrode in an electrolyte solution, ensuring maximum electrode-electrolyte contact. Lab-scale prototype ASC device using AC demonstrated infinite possibilities for the exploitation of core-shell nanostructures by efficiently selecting potential anode materials. The LED glowing time could be enhanced by connecting two such cells in series to obtain a higher voltage range. Synthesised ZnO NFs and core-shell NFs could find potential applications in gas sensors, drug delivery, water purification and many novel areas requiring further investigation.

CONCLUSIONS ZnO@MnO2 core-shell NFs were prepared via electrospinning and the hydrothermal method for use as a potential cathode material for ASCs. Core-shell NFs in three electrode configuration showed a specific capacitance of 907 Fg-1 at a current density of 0.6 Ag-1. High capacity retention of 97% with excellent coulombic efficiency was observed after 5000 cycles even at a higher current density of 30 Ag-1. A fully packaged cell of ASC was fabricated, and the resultant device was charged and discharged safely in a potential window of 0-2 V. ASC device delivered a maximum energy density of 17 Whkg-1 and a power density of 6.5 kWkg-1 with capacitance retention of 94% and coulombic efficiency of 100%. The small relaxation time constant of ~ 40 ms revealed fast discharge characteristics of the cell. A red LED was successfully illuminated after charging the cell to a potential of 2 V. Hierarchical structures comprised of electrospun NFs as electrode material open up new avenues for the synthesis of a variety of nanostructures for high performance supercapacitor applications.

METHOD:


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All chemicals used in this study were of analytical grade and utilised without further treatments. Zinc (II) acetate Zn (CH3COO) 2.4H2O (referred to as ZnAc), polyvinyl alcohol (PVA) [CH2CH (OH)]

n

(molecular weight ~1, 25,000) bought from Alfa Aesar, and de-

ionised (DI) water were used to prepare precursor spinning solution. A high surface area AC (surface area ~ 600 m2g-1) purchased from Kuraray chemicals was used as the carbon source for mixing with the working electrode material and fabricating anode. Poly vinylidene fluoride (PVDF) and N-methylpyrrolidinone (NMP) were bought from Sigma-Aldrich. In a typical synthesis method, 0.48 g of PVA was allowed to completely dissolve in 3 ml of DI water by stirring the solution for approximately 5 h at 60 °C on a hot plate. A pre-synthesized ZnAc solution (prepared in DI water) was added to the resulting PVA dispersion and stirred for another 5 h for uniform mixing (Complete experimental details are given in the ESI). Three other spinning solutions were made by varying the ZnAc loading with identical parameters in order to understand morphological variation and select a favourable one. The as-prepared ZnAc-PVA sol-gel was used in an electrospinning apparatus (PICO India) at an electric potential of 30 kV with a flow rate of 0.2 mLh-1. As-spun fibers were collected on a stainless steel foil wounded on a drum collector at a distance of 17 cm away from the syringe needle tip. Then, as-spun fibers were calcined at 500 °C for 2 h at a heating rate of 2 °Cmin-1, resulting in the formation of ZnO NFs. In a typical procedure, a known amount of ZnO NFs was dissolved in 6 mM KMnO4 solution using DI water as solvent; the resulting mixture was stirred uniformly for 0.5 h before being transferred into a 100 ml autoclave. The hydrothermal temperature and time duration to form ZnO@MnO2 NFs were optimized as 150 °C for 5 h (A detailed description of spinning condition and hydrothermal experiment are given in the ESI.) As-spun ZnAc-PVA NFs, annealed ZnO NFs, and ZnO@MnO2 NFs were characterised by microscopic and spectroscopic tools in addition to thermogravimetric and X-



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ray diffraction studies. Powder X-ray diffraction pattern was recorded at room temperature over a range of 10°-70° using a PANalytical X'pert Pro diffractometer with a nickel- filtered Cu Kα radiation with wavelength of 1.54 Å. A Horiba Jobin Yvon HR 800 instrument equipped with an optical microscope and 100X objective lens was used for Raman spectroscopic analysis. Resolution of the instrument was 2 cm-1, and the laser wavelength was 632.8 nm (He-Ne laser). A minimum laser power of ~ 1 mW was used to avoid thermally induced phase transitions and the subsequent conversion of MnO2 shell material into Mn2O3/Mn3O4. Thermogravimetric and differential scanning calorimetric analysis was carried out simultaneously using TGA, SDT Q600 TA instrument under air atmosphere in the temperature range of 30 - 800 °C at a heating rate of 20 °Cmin-1. Morphologies of NFs were studied using a field emission scanning electron microscope (FEI Quanta FEG 300) equipped with an energy dispersive X-ray spectrometer (EDS) at an accelerating voltage of 20 kV and a transmission electron microscope (Philips CM20 TEM at 200 kV) for high resolution images and lattice spacing calculations. Elemental composition and chemical state of the core-shell NFs were analysed using X-ray photoelectron spectroscopy (SPECS GmBH) with Al Kα X-rays. Exact mass loading on the working electrode materials were obtained with a METTLER TOLEDO XS105 analytical balance. Electrochemical performance of the ZnO NF-AC composite was studied to understand the conductivity response and capacitance contribution to the hybrid core-shell structure.ZnO@MnO2-AC, which is a cathode material for ASCs, was studied at room temperature in three electrode configurations using Ag/AgCl as the reference electrode and platinum-rod as the counter electrode in a 1 M Na2SO4 aqueous electrolyte over a potential window of 0-1 V. ASC was then designed from capacitive-type negative electrode prepared from commercially purchased AC and a Faradaic battery-type positive electrode prepared from a composite of AC and ZnO@MnO2 NFs (ZnO@MnO2-AC) on a Toray carbon paper.


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The positive electrode of ASC was designed by dispersing ZnO@MnO2, AC, and PVDF in NMP in the weight ratio of 70: 25: 5; the negative electrode was prepared from AC and PVDF in the weight ratio of 95: 5. Uniformly mixed slurry of the electrode materials was then brush-coated on Toray carbon papers and allowed to dry overnight in an oven at 60 °C. A Whatman filter paper soaked in 1M Na2SO4 solution was used as a separator with stainless steel foils as current collectors. Electrochemical performance of the ASC was evaluated at room temperature in a two-electrode set up using an Ivium Compact Stat. electrochemical workstation. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) cycles were performed in a potential window of 0–2.0 V. Electrochemical impedance of the device was analysed in the frequency range of 0.1–100 kHz with 5 mV.

ASSOCIATED CONTENT Detailed description of making spinning solution and synthesis of core-shell NFs; TGA-DSC analysis of as-spun PVA NFs; EDS analysis on ZnO@MnO2 NFs; XPS studies on ZnO@MnO2 NFs; CV and GCD profiles of pure ZnO NFs and Nyquist plot; Ragone and Nyquist plot for core-shell NFs; Performance comparison of core-shell NFs in three electrode and two electrode (ASC) configurations with recent literature; charge-discharge curves of individual electrodes (AC and ZnO@MnO2-AC composite) against Ag/AgCl electrode at 0.6 Ag-1; CV profiles of ASC in a voltage window of 0-2.6 V; and Supplementary video showing the illumination of an LED.

ACKNOWLEDGEMENT The authors acknowledge the support from DST (Grant No: SR/NM/NAT/02-2005)

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Figure captions


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Figure 1 TGA-DSC analysis for the (a) as-spun ZnAc-PVA NFs, (b) ZnO@MnO2 NFs, (c) X-ray diffraction patterns of ZnO NFs and ZnO@MnO2 core-shell NFs, compared to standard ZnO reference patterns, and (d) Raman spectrum of ZnO and ZnO@MnO2 NFs. Figure 2 Field emission scanning electron micrographs of as-spun NFs with various PVA-to-ZnAc ratios (x): (a) 5, (b) 2, (c) 1, and (d) 0.5. Figure 3 Field emission scanning electron micrographs of annealed NFs depicting morphological variation with various PVA-to-ZnAc ratios: (a) 5, (b) 2, (c) 1, and (d) 0.5., HRTEM images of NFs for the ratio x = 0.5 shows (e) granular morphology; inset shows the SAED pattern from isolated grains, and (f) SAED from several grains of a single NF showing polycrystalline nature with lattice-resolved fringes for the single grain in the inset. Figure 4 Stepwise schematic for the preparation of ZnO-MnO2 core-shell NFs. Figure 5 High resolution scanning electron micrographs of ZnO@MnO2 NFs at three magnifications are shown in (a), (b), and the inset of (b). MnO2 nano-flakes were almost uniformly grown on ZnO NFs. Transmission electron microscopy images: (c) cross-sectional images of ZnO/MnO2 core-shell with few nanometer thick MnO2 nanostructures, and (d) SAED pattern of a single ZnO spherical nanoparticle belonging to cross-sectional case with corresponding high resolution lattice fringe images on the bottom right inset and polycrystalline SAED pattern from the core-shell structures on the left inset. Figure 6 Electrochemical performance of ZnO@MnO2 core-shell using three electrode configurations against Ag/AgCl electrode in 1M Na2SO4 electrolyte: (a) cyclic voltammograms at various scan rates, (b) GCD profiles at various current rates, (c) rate performance at various current densities, and (d) cyclic stability tests for 5000 charge-discharge cycles in which capacity retention and coulombic efficiency with cycling was shown with charge-discharge profiles for the last five cycles in the inset. Figure 7 (a) Cyclic voltammograms of AC and ZnO@MnO2 core-shell NFs in three electrode configurations against Ag/AgCl electrode showing corresponding operational voltage window., Electrochemical performance of ASC: (b) cyclic voltammograms at various voltage windows at a scan rate of 50 mVs-1, (c) cyclic voltammetry at various scan rates but the voltage window of 2 V, (d) GCD profiles at various current densities in a voltage window of 2 V, (e) supercapacitance variation at various current densities, and (f) energy density and power density variation at various current densities. Figure 8 (a) Capacity retention and coulombic efficiency of the ASC at a current density of 1 Ag-1, (b) last five cycles of 3000 charge-discharge profiles, (c) experimental and simulated Nyquist plot measured with the expanded view of high frequency region and the equivalent circuit in the insets and



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(d) Bode plot of phase angle versus frequency and Bode plot of impedance versus frequency with the energy loss plot against frequency in the inset.

Figure 9 (a) Illumination of a red LED by a lab-scale prototype ASC device and (b) power density versus energy density for ACǁZnO@MnO2-AC asymmetric SC in a Ragone plot with conventional capacitors, SCs, conventional batteries and fuel cells.

Graphical abstract

Figure 1


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Figure 2



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Figure 3

Figure 4

Figure 5


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Figure 7


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Figure 8

Figure 9



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Figure 1 TGA-DSC analysis for the (a) as-spun ZnAc-PVA NFs, (b) ZnO@MnO2 NFs, (c) X-ray diffraction patterns of ZnO NFs and ZnO@MnO2 core-shell NFs, compared to standard ZnO reference patterns, and (d) Raman spectrum of ZnO and ZnO@MnO2 NFs. 112x81mm (300 x 300 DPI)


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Fig.2 Field emission scanning electron micrographs of as-spun NFs with various PVA-to-ZnAc ratios (x): (a) 5, (b) 2, (c) 1, and (d) 0.5. 83x71mm (150 x 150 DPI)



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ZnO@MnO2 CoreâShell Nanofiber Cathodes for High Performance

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