High-Performance Pseudocapacitor Electrodes Based on Î±-Fe2O3

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High Performance Pseudocapacitor Electrodes Based on #FeO / MnO Core – Shell Nanowire Heterostructure Arrays 2

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Debasish Sarkar, Gobinda Gopal Khan, Ashutosh Kumar Singh, and Kalyan Mandal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4039573 • Publication Date (Web): 05 Jul 2013 Downloaded from http://pubs.acs.org on July 17, 2013

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The Journal of Physical Chemistry

High Performance Pseudocapacitor Electrodes Based on α-Fe2O3 / MnO2 Core – Shell Nanowire Heterostructure Arrays Debasish Sarkar,†,‡ Gobinda Gopal Khan,*,#,‡ Ashutosh K. Singh, † and Kalyan Mandal † †

Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre

for Basic Sciences, Block JD, Sector III, Salt Lake City, Kolkata 700 098, India #

Center for Research in Nanoscience and Nanotechnology, University of Calcutta, Technology

Campus, Block JD2, Sector III, Salt Lake City, Kolkata 700 098, India KEYWORDS. α-Fe2O3 Nanowires, MnO2, Nano-heterostructures, Pseudocapacitor, Energy storage. ABSTRACT. A simple wet chemical technique has been employed to fabricate MnO2 nanolayer coated α-Fe2O3/MnO2 core–shell nanowire heterostructure arrays to prepare unique pseudocapacitor electrodes. The coating of MnO2 on α-Fe2O3 nanowires is triggered by the reduction of KMnO4 solutions by the metallic (Au) film on which the polycrystalline α-Fe2O3 nanowires have been grown electrochemically. This metallic film also acts as the current collector by making direct contact with the arrays of the 1D nano-heterostructures. The asprepared α-Fe2O3/MnO2 nano-heterostructures are found to exhibit excellent specific capacitance, high-energy density, high-power density, and long-term cyclic stability as compared

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with the bare α-Fe2O3 nanowires electrode. The unique geometry of the 1D nano-heterostructures with high effective surface area to allow faster redox reaction kinetics, the incorporation of two highly redox active material in the same structure and the porous surface structures of the heterostructure to allow facile electrolyte diffusion helps in the superior electrochemical performance of the α-Fe2O3/MnO2 nano-heterostructures. The maximum specific capacitance of 838 Fg-1 (based on pristine MnO2) has been achieved by cyclic voltammetry at a scan rate of 2 mVs-1 in 1 M KOH aqueous solution. The hybrid α-Fe2O3/MnO2 nanocomposite electrodes also exhibit good rate capability with excellent specific energy density of 17 Whkg-1 and specific power density of 30.6 kWkg-1 at a current density of 50 A g-1 and good long-term cycling stability (only 1.5% loss of its initial specific capacitance after 1000 cycles). These studies indicate that the α-Fe2O3/MnO2 nano-heterostructure architecture is very promising for next generation high-performance pseudocapacitors. INTRODUCTION Because of the growing concerns over the environmental pollution caused by the fossil fuels and in order to cope with the rapidly increasing global energy consumption, the development of alternative high-performance power sources, energy storage and delivery systems are of significant particular interest in various fields such as portable electronics and sensor systems .1, 2 In this context, the electrochemical capacitors, also known as supercapacitors or ultracapacitors, which can efficiently bridge the gap between high specific energy batteries and high specific power capacitors, have drawn intense research attention as the high specific energy storage systems for consumer electronics, hybrid vehicles and renewable backup power sources due to their superior power density, pulsed power supply characteristics, fast charge-discharge rates, simple principle and long cycle lifetime.3-7 However, based on the charge storage mechanism,


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supercapacitors (SCs) are classified into electrical double layer capacitors (EDLCs) and pseudocapacitors, where the charge is stored using the redox based Faradaic reactions.8, 9 Generally, the carbon based EDLCs have high power density but they suffer from low specific capacitance and rate capability.10 In this regard, the pseudocapacitors, based on the transition metal oxides or nitrides can provide higher specific capacitance with good charge/discharge rate capability and long-term cycling stability compared with the EDLCs.8, 11 However, still, the pseudocapacitors suffer from disadvantages like higher manufacturing cost and low electrical conductivity.12, 13 Therefore, active research works have been focused to design the high performance novel electrode material for the pseudocapacitors to achieve high specific surface area, good specific capacitance, high electronic conductivity, high power and energy densities and a fast cation intercalation/de-intercalation on the surface and into the electrode material. Specially, the ordered arrays of nanostructures are found to boost the electrochemical performance of the pseudocapacitor electrode through fast ion diffusion, stress relaxation, electron and mass transport .10, 14-16 The transition metal oxides (TMOs) like MnO2, nickel oxide, iron oxide, cobalt oxide, TiO2, MoO2 and VOx etc.15-20 are demonstrated as the ideal electrode materials for pseudocapacitors because of their variant oxidation states for efficient redox charge transfer.3, 14, 21 Both the MnO2 and hematite (α-Fe2O3) have already been studied extensively because of their superior performance as the active electrode materials for the pseudocapacitors compared with the conventional carbon-based materials. Moreover, the low cost, good stability, non toxic and environment friendly nature of the α-Fe2O3 and MnO2 coupled with their high redox activity makes them suitable for the extensive applications in pseudocapacitors.19, 22-31 However, the reported values of the specific capacitance for the nanostructured MnO2 and αFe2O3 electrodes are still far below their theoretical values (theoretical specific capacitance for MnO2 is ∼ 1,370 F g-1 32, 33 and the theoretical capacity of α-Fe2O3 is ∼ 1007 mAh g-1 34) as they


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both suffer from the poor electrical conductivity. Very recently, various MnO2 and α-Fe2O3 based nanohybrid structures like MnO2/NiO nanoflakes,19 MnO2/SnO2 NWs, 10 MnO2/CNT, 35, 36 MnO2/graphene, 8 reduced graphene oxide/MnO237, 38 and α-Fe2O3/graphene oxide,39 graphene/Fe2O3/polyaniline nanocomposites40 have been fabricated and demonstrated as the pseudocapacitors with enhanced electrochemical performance. In this back drop, here, we have fabricated a special kind of 1D core-shell type α-Fe2O3/MnO2 nano-heterostructures (NHs) architecture by coating the nanolayer of MnO2 on the surface of αFe2O3 nanowires (NWs) using a facile chemical route and have studied their electrochemical properties. Although, both the nanostructured MnO2 and α-Fe2O3 have been studied extensively as electrode material for pseudocapacitor, still there is no report on the electrochemical capacitive properties of the α-Fe2O3/MnO2 NHs, which might be superior material for electrode considering several basic requirements for the pseudocapacitors. The α-Fe2O3/MnO2 NHs have the following advantages as the electrode materials for pseudocapasitors: (1) both of the materials having high redox electroactivity are combined into one ordered nanostructure would potentially help to increase the value of the specific capacitance;19 (2) the effective surface area of the electrode has been enhanced by many orders of magnitude by using arrays of 1D NHs; (3) in this special architecture of 1D NHs, the thin layer of MnO2 coated on α-Fe2O3 NWs would provide a short ion diffusion path to enable the fast and reversible faradic reaction; (4) the electrical conductivity of α-Fe2O3 and MnO2 are in the range of 10-4 −10-5 and 10-5−10-6 Scm-1, respectively;16, 41-44 hence, α-Fe2O3 NWs situated at the core of the nanoheterostructures, being more electrically conducting compare to MnO2, would provide a path for the fast electron transport; and (5) α-Fe2O3 NWs would also create channels for the effective mass transport of electrolyte too. In this work, the α-Fe2O3/MnO2 NHs are found to exhibit superior capacitive


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performance with high specific capacitance of 838 Fg-1 and areal capacitance of 750 mFcm-2 at scan rate of 2 mVs-1. The 1D core-shell α-Fe2O3/MnO2 NHs electrodes exhibit high rate capability, high energy density, high power density and also long cycle stability, which make this type of environment friendly, nontoxic nano-heterostructures a perfectly promising material for fabrication of supercapacitors electrode. EXPERIMENTAL SECTION Synthesis of the α-Fe2O3 NWs The high-density array of α-Fe2O3 NWs were synthesized by the high-temperature oxidation of the metallic Fe NWs prepared by the template assisted electrochemical deposition. Highly ordered self-organized nanoporous anodic aluminium oxide (AAO) templates were fabricated by the controlled two-stage electrochemical anodization of high-purity aluminum foil. The 200 nm diameter porous AAO template with one side coated with a conductive gold (Au) metal layer grown by thermal evaporation technique was used as the working electrode to synthesize the highly ordered arrays of Fe NWs through electrodeposition. Software controlled three-electrode electrochemical cell and a power supply (potentiostat, AutoLab-30) was used for the electrochemical deposition of Fe NWs. A high-purity Pt wire and an Ag/AgCl calomel electrode were used as the counter and reference electrodes, respectively. The arrays of Fe NWs were grown in the pores of AAO using the aqueous solution of 120 g/L FeSO4.7H2O and 45 g/L H3BO3 as electrolyte at room temperature. Here, boric acid was used as buffer to control the electrodeposition process. The deposition of the Fe NWs was carried out by using a dc voltage of −1.03 V, following linear sweep voltammetry results. The pH of the electrolyte was maintained at 3.6. The electrodeposition was conducted for 10 min to prepare the Fe NWs. After the growth of the Fe NWs the template was removed by dissolving it in 2 M of NaOH solution. The open


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arrays of Fe NWs grown on the Au layer were finally oxidized to form α-Fe2O3 NWs by heating them at 400°C for 2 h in oxygen atmosphere. Coating of MnO2 on α-Fe2O3 NWs The nanolayer of MnO2 was grown on the surface of α-Fe2O3 NWs through the reduction of Mn (VII) of the KMnO4 solution according to the following chemical reaction where the electrons for reduction come from the metallic Au substrate underneath the NWs arrays.10 MnO 4− + 4H + + 3e- → MnO 2 + 2H 2 O It was observed that a thin black layer of was formed on the surface of Au film after the reaction was over.10 This layer was characterized as Mn-Au oxide layer by energy dispersive xray spectroscopy analysis. Hence, the partial oxidation of the polycrystalline metallic Au film was confirmed, which indicates that the electrons for reduction came from the Au film. For coating process, 5 mmol of KMnO4 and 5 mmol of H2SO4 were mixed with 50 mL of deionized water to prepare the precursor solution. To fabricate MnO2 coated α-Fe2O3 NWs, the α-Fe2O3 NWs grown on the metallic Au substrate was immersed into the precursor solution maintained at 85°C for 10 min. The thickness of MnO2 nanolayer on the α-Fe2O3 NWs can be easily controlled by adjusting the immersion time. Then the MnO2 coated α-Fe2O3 NWs were rinsed with deionized water and subsequently annealed at 400°C for 30 min in oxygen atmosphere.

Characterization The crystallographic nature of the as-prepared α-Fe2O3 NWs as well as the αFe2O3/MnO2 core–shell NW heterostructure arrays were analysed by x-ray diffraction (XRD, Panalytical X’Pert Pro diffractometer). The account of the chemical composition and elemental composition of the NW heterostructure was investigated by energy dispersive x-ray (EDAX) and x-ray photoelectron spectroscopy (XPS). The morphology and the structure of the arrays of α-


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Fe2O3 NWs and α-Fe2O3/MnO2 core–shell NW heterostructure were studied by scanning electron microscope (SEM, FEIQuanta-200 Mark-2), transmission electron microscope (TEM, FEI TECNAI G2 TF20ST) and scanning transmission electron microscopy (STEM). The crystalline structure of the NW heterostructure was further investigated by the high-resolution TEM (HRTEM).

Electrochemical Measurements The electrochemical properties of the samples were investigated with cyclic voltammetry (CV) and galvanostatic (GV) charge/discharge tests by using a software controlled conventional three-electrode electrochemical cell (potentiostat, AutoLab-30) consisted of the as-prepared samples as the working electrode, Ag/AgCl as the reference electrode and the Pt wire as the counter electrode. The area of the working electrode was 0.5 cm2. All the electrochemical measurements were performed in a 1M KOH solution at room temperature. The CV measurements were performed at different scan rates of 2, 5, 10, 20, 50, and 100 mVs-1 at room temperature. GV charge/discharge measurements were conducted at various current densities of 1, 2, 3, 5, 10, 25 and 50 Ag-1 to evaluate the specific capacitance, power density and energy density. A potential window in the range from -0.1 to 0.6 V was used in all the measurements. The constant current charge/discharge method was also employed to test the long-rate capability of the electrode materials. The long-cycle stability of the electrodes was tested by CV measurement during 1500 cycles at a scan rate of 100 mVs-1.

RESULTS AND DISCUSSION. The photograph of the as-synthesized α-Fe2O3/MnO2 core– shell NHs is shown in Figure 1a. Figure 1b shows the scanning electron microscopy (SEM) image of the well aligned arrays of as-prepared 1D α-Fe2O3/MnO2 core–shell NHs grown perpendicular to the supporting Au substrate. It is evident from Figure 1b that the length and the diameter of the 1D NHs are uniform in nature. The transmission electron microscope (TEM) and


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the scanning transmission electron microscope (STEM) micrographs of the α-Fe2O3/MnO2 core– shell NHs shown in Figure 1c and 1d, respectively, clearly shows the formation of core-shell type NW heterostructure with a nearly uniform coating of the MnO2 nanolayer on the surface of the 200 nm diameter α-Fe2O3 NWs. It is also evident from figure that the thickness of the MnO2 nanolayer grown on α-Fe2O3 NWs varies in between 40-60 nm. The STEM image (Figure 1d) clearly indicates the formation of good quality α-Fe2O3/MnO2 core–shell NHs. The crystallographic nature of the arrays of α-Fe2O3/MnO2 core–shell NHs investigated by XRD (Figure S1, Supporting Information) indicates that the as grown MnO2 nanolayer on α-Fe2O3 NWs is poorly crystallized, 10, 45 as there is no peak corresponding to MnO2 in the XRD pattern. The diffraction peaks in the XRD pattern appear from the pure rhombohedral phase (space group

R3c) of the α-Fe2O3 NWs (JCPDS file no. 89-0597, a = 5.039 Å, c = 13.77 Å) and the metallic Au layer underneath the NHs.41 More information about the crystallographic nature of the MnO2 nanolayer can be revealed from the HRTEM micrograph (Figure 1e), which clearly indicates the amorphous/poor crystalline nature of the as-prepared MnO2 nanolayer. The energy dispersive xray (EDAX) spectrum of the α-Fe2O3/MnO2 core–shell NHs, shown in Figure 1f, clearly indicates the presence of Fe, Mn and O in the NHs, where the Fe:Mn ratio was found to be 2:1. The chemical composition and the valence state of the elements in the as-prepared αFe2O3/MnO2 core–shell NHs have been further investigated by the x-ray photoelectron spectroscopy (XPS) (Figure S2, Supporting Information). The peaks of Mn 2p3/2 and Mn 2p1/2 located at 642 and 653.8 eV, respectively, with an energy separation of 11.8 eV, are in good agreement with reported data of Mn 2p3/2 and Mn 2p1/2 in MnO2. 46 The Fe 2p3/2 and Fe 2p1/2 peaks centered at 711.4 and 725 eV, respectively, with another tiny peak at 719.1 eV correspond to the Fe3+ oxidation state of Fe in α-Fe2O3 NWs. 41


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Figure 1. The photograph of the as-prepared α-Fe2O3/MnO2 core–shell NHs. The (b) SEM, (c) TEM and (c) STEM micrographs of the as-prepared α-Fe2O3/MnO2 core–shell NHs. (e) The HRTEM image of the MnO2 nanolayer grown on the α-Fe2O3 NWs. (f) The EDAX spectrum of the α-Fe2O3/MnO2 core–shell NHs. The electrochemical properties of the as-prepared α-Fe2O3 NWs and the α-Fe2O3/MnO2 NHs were investigated by a three electrode system in 1 M KOH solution. The Faradic redox reactions, as represented by the cyclic voltammetry (CV) curves of the as-prepared α-Fe2O3 NWs at different scan rates from 2 to 100 mVs-1 are shown in the Figure 2a. All the cyclic voltammograms show distinct pair of redox peaks during the anodic and cathodic sweeps within a voltage window between -0.1 to 0.6 V. These redox peaks arise due to conversion between different oxidation states of iron [Fe (II) ↔ Fe (III)]. The possible anodic and cathodic reactions can be written as the following reversible reaction: Fe2 O3 + M + + e − ↔ Fe 2 O3 M , M + = K + orH 3O +

(1)

The shape of the CV curves represents the pseudocapacitive behavior of the hematite NWs, which is significantly different from the typical rectangular CV curves characteristics of the


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EDLCs.3, 18 Pseudocapacitance of α-Fe2O3 NWs arises due to the interaction of the Fe3+ ions with the K+ ions of electrolyte which intercalates/deintercalates at the surface of the α-Fe2O3 NWs and also into it through the channels and the pores in the crystal structure of the NWs.3, 18 The intercalation or adsorption of the ions mainly happens at the surface of the electrodes. The surface serves as the sites for the intercalations/deintercalations of the alkali ions. Intercalation is an adsorption process and the electrodes having higher surface area and pores would facilitate the adsorption of ions in every possible pore which leads to higher capacitive performance of the electrode. However, the CV curve changes much with increasing scan rate characterized by increasing area under the curve representing the higher capacitive performance and continuous shifting of anodic and cathodic peaks towards higher and lower potentials, respectively leading to a larger potential separation between oxidation and reduction peaks. It is evident from Figure 2a that all the CV curves exhibit a current leap especially at the upper potential limit which indicates an obvious electrochemical oxidation process. Figure 2b shows the CV curves of the α-Fe2O3/MnO2 NHs measured at different scan rates and within the same voltage window as that of the hematite NWs. Here, distinct pair of redox peaks can also be observed even at scan rate of 100 mV s-1, indicating that the α-Fe2O3/MnO2 NHs are beneficial for fast redox reactions similar to the α-Fe2O3 NW electrodes. The area under the CV curves increases with the increase in scan rate voltage, which indicates the higher capacitive charge storage capability of this nanocomposite. Comparison between the CV curves of α-Fe2O3/MnO2 NHs and α-Fe2O3 NWs at scan rate of 100 mV s-1 is shown in Figure 2c. The larger area of the CV curves in α-Fe2O3/MnO2 NHs as compared to α-Fe2O3 NWs at a particular scan rate indicates higher capacitive capability of the former electrode. Nearly thirteen times enhancement in current density has been observed in case of αFe2O3/MnO2 NHs over the as prepared α-Fe2O3 NWs. This indicates significant enhancement in


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capacitive performance of α-Fe2O3 NWs after MnO2 coating. However, in this case no obvious current leap at the upper potential limit of the CV curves has been observed, which indicates that the electrochemical oxidation process is completed. Furthermore, it can be seen that the current of the α-Fe2O3 electrode and α-Fe2O3/MnO2 nanocomposite electrode responds rapidly to the switching potential, particularly at the potential switching point of 0.6 V, representing smaller equivalent series resistance (ESR) of both the electrodes which is very important to achieve long rate capability and high power density.6 To have a quantitative insight of the capacitive performance of the electrode materials, the areal capacitance (Ca, mF cm-2) and specific capacitance (Csp, F g-1) of all the nanostructure electrodes were calculated according to the following equations: Ca =

I I ….. (2) and Csp = ….. (3); fA fm

where, ‘I(A)’ is the average cathodic current of the CV loop, ‘f (V s-1)’ is the scan rate, ‘A(cm2)’ is the area of the working electrode and ‘m (g)’ is the mass of the redox active material of the electrode.


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Figure 2. Cyclic voltammetry curves of the as prepared (a) α-Fe2O3 NWs and (b) α-Fe2O3/MnO2 NHs at different scan rates in a 1M KOH solution. (c) Comparison between the CV curves of αFe2O3/MnO2 NHs and α-Fe2O3 NWs at scan rate of 100 mVs-1. (d) Variation of specific capacitance (Cs) and areal capacitance (Ca) as a function of scan rate of α-Fe2O3/MnO2 NHs and

α-Fe2O3 NWs (Inset). Figure 2d shows the plot of the calculated values of Csp and Ca of the α-Fe2O3/MnO2 NHs electrode as a function of scan rate whereas the corresponding variation for α-Fe2O3 NWs electrode is shown in the inset of Figure 2d. Maximum values of the Csp and Ca of the asprepared α-Fe2O3 NWs electrode were found to be nearly 8.3 Fg-1 and 6.4 mFcm-2, respectively at scan rate of 2 mVs-1. However, after MnO2 coating over the α-Fe2O3 NWs significant enhancement in the values of Csp and Ca has been observed. For α-Fe2O3/MnO2 nanocomposite


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electrode, the values of Csp and Ca are found to be nearly 838 Fg-1 and 750 mFcm-2, respectively at scan rate of 2 mVs-1. Though in case of α-Fe2O3/MnO2 NHs electrode MnO2 resides on the surface in contact with the electrolyte for charge storage process, effect of α-Fe2O3 cannot be neglected because of its finite/considerable capacitance values which will be clarified further when we discuss the galvanostatic charging/discharging process of these electrodes. However, the significant enhancement of the capacitance of the α-Fe2O3/MnO2 NHs electrode is believed to be because of the MnO2 nanolayer present on the surface of α-Fe2O3 NWs. The value of Csp for the α-Fe2O3/MnO2 NHs electrode is found to be much higher than that of the other reported MnO2 and MnO2 based nanocomposite electrode such as, thin films of MnO2 (~ 698 Fg-1), SnO2@MnO2 nanowire (~ 637 Fg-1, calculated from CV loops), 10 MnO2/ZTO/CMF hybrid composite (~ 621.6 Fg-1) 47 and TiN/MnO2 nanotubes (~ 480 Fg-1).48 Furthermore, we also have grown the MnO2 thin films on Cu substrate and for such bulk film the specific capacitance was found to be ~ 468 Fg-1. The details of the electrochemical measurements for the MnO2 thin film is not provided here because of the space limitation. Therefore, the Csp of the α-Fe2O3/MnO2 NHs electrode is found to be much higher than the pure MnO2 film prepared in our experiment also. The specific and areal capacitance of both the nanostructure electrodes decreases with the increase of scan rates. This is because of the fact that higher scan rate limits the accessibility of ions inside every pores of the electrode and only the outermost portion of electrode is utilized for the ion diffusion. However, at higher scan rate of 100 mVs-1 the specific capacitance remains almost 95 Fg-1 for α-Fe2O3/MnO2 nanocomposite electrode whereas the pristine α-Fe2O3 NWs retain nearly 60% of its initial charge, as evident from the inset of Figure 2d. This high rate capability of the pristine α-Fe2O3 NWs electrode is mainly due to its higher conductivity and porous structure evidenced in our earlier studies (Figure S3, Supporting information). 50 These pores present on the surface of the α-Fe2O3 NWs enhance the accessibility of ions inside the


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NWs which can even possible at higher scan rates rendering high rate capability of specific capacitance. In case of the α-Fe2O3/MnO2 NHs electrode the charge storage mechanism in MnO2 is conducted through surface adsorption of electrolyte cations as well as proton incorporation and can be described by following equation:

MnO 2 + xK + + yH + + ( x + y )e− ↔ MnOOK x H y

(4)

From equations (1) and (4) it can be seen that both the cations and electrons take part in redox reactions of α-Fe2O3 and α-Fe2O3/MnO2 nanocomposites. Therefore, it can be concluded that the extent of diffusion of cations on the surface and into the electrode materials followed by the transport of electrons to the current collectors actually decides the pseudocapacitive performance of the electrodes. Increase in specific capacitance after MnO2 modification of the hematite NWs may be attributed to the fast reversible redox activity of MnO2 and unique 1D core/shell structure of α-Fe2O3/MnO2 NHs. In this core/shell structure, the high surface area of the thin MnO2 nanolayer serves as the huge platform for intercalation/deintercalation of the K+ ions of electrolyte whereas the core consist of α-Fe2O3 NWs having higher conductivity than MnO2 41-45 serves as a fast path for electron transport to the current collector. The thin nanolayer of MnO2 also serves as the shorter diffusion path for the ions, which can easily diffuse through MnO2 to intercalate into highly porous and highly redox active α-Fe2O3 core providing the high specific capacitance of the NHs at lower scan rates. However, the only 13% capacitance retention of the

α-Fe2O3/MnO2 NHs electrode at higher scan rates can be ascribed to the non-uniformity of MnO2 nanolayer over the α-Fe2O3 NWs as evidenced from the TEM image. Relatively thick MnO2 layer on some places over α-Fe2O3 NWs limits the movement of electrons through it to the core due to its lower conductivity (10-5 – 10-6 Scm-1) together with the lesser ion diffusion into the electrode material at higher scan rates lead to substantial decrease of capacitance at higher scan rates.


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Furthermore, in the thicker portion, specifically the MnO2 layer far from the surface layer cannot take part in the ion diffusion effectively because of its low proton diffusion constant (~10-13 cm2/ (Vs)).50 However, this type of unique 1D core/shell structure of the α-Fe2O3/MnO2 NHs, where both of the materials are redox active and porous in nature has its extra advantage for the fabrication of NWs based supercapacitors. It is interesting to observe that the shape of the CV curves for both of the α-Fe2O3 NWs and αFe2O3/MnO2 NHs electrodes change slightly when the scan rate changes from 2 to 100 mVs-1 indicating that the electrodes possess low polarization because of their higher conductivity. Furthermore, a linear relation between the peak current (I ) of CV loops at different scan rates with the square root of scan rate voltage (f 1/2) has been observed for both the electrodes (Figure S4, Supporting Information). This type of linear relation indicates a fast electron transfer rate during the redox reactions and thus these redox reactions are diffusion-controlled process rather than a kinetic one. 51, 52


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Figure 3. Constant current charge/discharge curves of the as-prepared (a) α-Fe2O3 NWs and (b)

α-Fe2O3/MnO2 NHs at different current density. (c) Comparison between the charge/discharge curves of the α-Fe2O3 NWs and α-Fe2O3/MnO2 NHs at the current density of 1 Ag-1. (d) Plotted curve of specific capacitance and energy density of both type of capacitors as a function of current density. The electrochemical performance of these 1D asymmetric supercapacitors has been investigated further by galvanostatic (GV) charge/discharge method which is very important for this type of supercapacitor electrodes. As the current density is one of the important parameter that controls the capacitive performance of the electrode, the GV charge/discharge curves of the

α-Fe2O3 NWs and the α-Fe2O3/MnO2 NHs in 1M KOH solution have been recorded at different current densities and are shown in Figure 3a and 3b, respectively. It is found that the charge/discharge curves of the α-Fe2O3 NWs are not ideally linear; the discharging curves are


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very much symmetric as compared with the charging one with a very low voltage drop even at high current density of 2 Ag-1 indicating low internal resistance of the α-Fe2O3 NWs electrode. The charging/discharging curves of the α-Fe2O3/MnO2 NHs electrode are not symmetric too but substantially prolonged over the α-Fe2O3 NWs electrode indicating the enhanced capacitive behavior of the α-Fe2O3 NWs by MnO2 coating. The discharging process of the α-Fe2O3/MnO2 NHs electrode at different current densities shows two stages; a fast voltage drop which increases with increasing current density followed by a slow discharge. The fast voltage drop is related to the internal resistance of the electrodes however the slow discharge with nonlinear slope represents that the faradic reactions occur on the surface of the α-Fe2O3/MnO2 NHs electrode. Figure 3c shows the comparison of charging/discharging behavior of both types of electrodes at a constant current density of 1Ag-1. Higher charging/discharging time for the same voltage window (here 0.7 V) of the α-Fe2O3/MnO2 NHs electrode as compared to α-Fe2O3 NWs represents the superior capacitive performance of the former electrode. The discharge specific capacitance of the asymmetric supercapacitors has been calculated from the discharge curves using following equation:

Csp =

I ∆t ……. (5); m∆V

where ‘I(A)’ is the discharge current, ‘∆t (s)’ is the discharge time consumed in the potential range of ‘∆V(V)’, ‘m(g)’ is the mass of the active material (or mass of the electrode materials) and ∆V is the potential window. Variation of specific capacitance calculated from the discharge curves as a function of current density for the α-Fe2O3 NWs and α-Fe2O3/MnO2 NHs electrodes are shown in Figure 3d. Maximum values of specific capacitance for the α-Fe2O3 NWs and α-Fe2O3/MnO2 NHs electrodes have been found to be nearly 42.7 and 801 Fg-1, respectively, at a current density of 1


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Ag-1. The value of Csp for α-Fe2O3/MnO2 NHs electrode is consistent with the value as calculated from the CV curves. However, in case of α-Fe2O3 NWs electrode it is much higher than that obtained from CV loops. This might be because at low current density the cations can intercalate more effectively into the pores of the α-Fe2O3 NWs. Particularly these pores of the NWs based electrodes can serve as “ion-buffering reservoirs” which can supply the OH- ions to sustain sufficient redox reactions at higher current densities for energy storage.53 From the Figure 3c, it is evident that the Coulombic efficiency (η = discharging time/charging time), which is the measure of competence of ion/charge transfer during an electrochemical reaction is much higher in case of

α-Fe2O3/MnO2 NHs electrode as compared with the α-Fe2O3 NWs electrode. The specific capacitance decreases with the increase in current density for both the electrodes and tends to stabilize after current density of 3 Ag-1. This behavior is similar to that one as observed in the case of increasing scan rate voltage. This decrease in specific capacitance is mainly due to the limited accessible areas for ions for diffusion with increasing current density. At lower current density, ions can easily penetrate into the innermost portion of the electrode material through almost every available pores and channels resulting higher capacitive performance. However, only the outer surface of the electrode material can be utilized by ions at higher scan rates. Interestingly, in this charging/discharging process the capacitance retention of the electrodes is much higher than that observed from the CV analysis. Nearly 68% and 52% capacity retention have been observed for α-Fe2O3/MnO2 NHs and α-Fe2O3 NWs electrode, respectively at a current density of 50 Ag-1 demonstrating the relatively good high-rate capability of these electrodes. Energy density and power density are another two important parameters that defines the performance of an electrochemical capacitor. A good electrochemical supercapacitor should possess a high energy density or high specific capacitance at higher current densities. Here, in our studies we also have investigated the performance of the prepared asymmetric supercapacitor


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electrode materials in terms of these above parameters. The energy density and the power density of both types of electrodes have been calculated according to the following equations:

1 E E = Csp (∆V ) 2 …… (6) and P = …… (7); 2 t where, ‘E (WhKg-1)’, ‘Csp (F g-1)’, ‘∆V (V)’, ‘t (s)’ and ‘P (kWkg-1)’ are the energy density, specific capacitance, potential window of discharge, time of discharge and power density, respectively. The variation of ‘E’ with current density for both of the electrode materials is shown in Figure 3d. Nearly 18 times enhancement of energy density has been observed for the αFe2O3/MnO2 NHs electrode (54 Whkg-1) compare to the α-Fe2O3 NWs electrode (2.9 Whkg-1) at a current density of 1 Ag-1. The energy density of α-Fe2O3/MnO2 NHs electrode remains 17 Whkg-1 at the current density as high as 50 Ag-1, indicating good energy storage capability once again.


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Figure 4. Ragone plot of the asymmetric supercapacitors consisting of α-Fe2O3 NWs (■ black one) and α-Fe2O3/MnO2 NHs (■ red one) as cathode and Pt as anode in comparison with various electrical energy storage devices. Times shown are the time constants of the devices as obtained by dividing energy density by power. Figure 4 shows the Ragone plot (power density vs. energy density) of the supercapacitors based on α-Fe2O3 NWs and α-Fe2O3/MnO2 NHs electrodes derived from the constant current charging/discharging curves in comparison with some advanced aqueous-based supercapacitors. The power density of α-Fe2O3 NWs electrode is found to be almost 17.4 kWkg-1 at an energy density of 1.5 Whkg-1, which is competitive with the Ni-MH batteries and significantly improved as compared with the other currently available electrochemical capacitors. Interestingly, this amount of power density of pristine α-Fe2O3 NWs electrode can solely fulfill the power target (15 kWkg-1) of the Partnership for a New Generation of Vehicles (PNGV).10, 29 Moreover, the energy density reaches as high as 54.4 Whkg-1 at a power density of 700 Wkg-1 for the αFe2O3/MnO2 NHs supercapacitor. However, the α-Fe2O3/MnO2 nanocomposite electrode can still


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possess a specific energy of 17 Whkg-1 as the power density increases even as high as 30.6 kWkg-1. This type of high power and energy performance of the α-Fe2O3/MnO2 NHs electrode is much more superior to the Ni-MH batteries and other conventional electrochemical supercapacitors and even highly competitive with the Li-ion batteries as is also evident from the Ragone plot.

Figure 5. (a) Cyclic voltammetry curves of different cycles of α-Fe2O3/MnO2 composite capacitor measured in a 1M KOH solution at a scan rate of 100 mVs-1. (b) Variation of specific capacitance of α-Fe2O3/MnO2 composite with cycle number for their long cycle stability test. The long cycle life is another important parameter for investigating the performance of a supercapacitor. The long term cycle ability of the α-Fe2O3/MnO2 NHs electrode was evaluated by repeating the CV test at a scan rate of 100 mVs-1 for 1500 cycles, as shown in Figure 5a. All the CV curves are nearly symmetric in nature and the curves after 50 cycles are almost overlapping indicating good cyclic capability of the 1D nanocomposite pseudocapacitor (the corresponding charging/discharging curves for first 10 cycles of α-Fe2O3/MnO2 core–shell NHs at a current density of 1 Ag-1is provided in Figure S5, Supporting Information). The specific capacitance, calculated using Eq. (3), as a function of cycle number is presented in Figure 5b. It is evident from the figure that the specific capacitance remains almost constant at the beginning (~ 93 Fg-1


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up to 25 cycles), then suddenly jumps to a value of ~ 249 Fg-1 after 50 cycles. Specific capacitance increases up to a maximum value of 256.1 Fg-1 after 800 cycles, then subsequently decreases slightly and becomes almost constant at a value of 253 Fg-1 at the end of 1500 cycles. It is interesting that the specific capacitance increases almost 2.6 times after 50 cycles. This phenomenon is indicative of the fact that there is an initial activation process for the faradic pseudocapacitance of the α-Fe2O3/MnO2 NHs electrodes.15, 51 After 800 cycles specific capacitance decreases only 1.5% of the maximum value of capacitance. This decrease of capacitance can be ascribed to the mechanical expansion of MnO2 nanolayer due to continuous ion insertion/deinsertion process or dissolution of some amount of MnO2 into electrolyte. However, the electrolyte remains transparent after 1500 cycles, indicates minimal dissolution of MnO2 into the electrolyte solution after the long-term cycling test which has been considered as the main reason behind the capacitance loss of MnO2 based supercapacitors. Therefore, this study suggests that the α-Fe2O3/MnO2 NHs electrode is very stable during the long term cycling test.

CONCLUSIONS. In this report, arrays of 1D core-shell α-Fe2O3/MnO2 nanoheterostructures have been fabricated by a facile wet chemical technique by coating MnO2 nanolayer on α-Fe2O3 NWs synthesized by template assisted electrochemical route. The coated MnO2 nanolayer is amorphous/poorly crystalline in nature. Both the α-Fe2O3 NWs and MnO2/α-Fe2O3 NHs have been demonstrated as the electrode material to use them in pseudocapacitors. The α-Fe2O3 NWs electrode exhibits specific capacity of nearly 42 Fg-1 at a current density of 1 Ag-1 due to its highly porous structure which allows the cations to diffuse more into the electrode material. The specific capacitance is found to increase remarkably when the surface of α-Fe2O3 NWs is modified with MnO2 nanolayer. Nearly 801 Fg-1 of specific capacitance have been observed for the α-Fe2O3/MnO2 NHs electrode at a current density of 1 Ag-1. The superior electrochemical


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performance of the MnO2/α-Fe2O3 NHs can be ascribed to the unique 1D core/shell structure of the nanocomposite, where the shell consists of highly redox active MnO2 and core consists of porous, conductive α-Fe2O3. Moreover, the thin layer of MnO2 serves as the large platform for ion diffusion whereas the conductive core provides the highway for fast transport of electrons. Transparent electrolyte solution after 1500 electrochemical cycles indicates minimal dissolution of MnO2 i.e. long cycle stability of the α-Fe2O3/MnO2 NHs. The energy density and power density of the α-Fe2O3/MnO2 NHs electrode have been found to be 17 Whkg-1 and 30.6 kWkg-1, respectively at a current density of 50 Ag-1. Nearly 68% capacitive retention at a current density of 50 Ag-1 indicates the good rate capability of the nanocomposite electrode. Here, it is found that the unique 1D core/shell α-Fe2O3/MnO2 nanoheterostructure made of environment friendly, lightweight, extremely low cost, highly redox active and nontoxic materials is highly efficient in energy conversion and storage systems for the effective use of renewable energy.

AUTHOR INFORMATION

Corresponding Author * [email protected]

Author Contributions ‡These authors contributed equally to this work.

ACKNOWLEDGMENT One of the authors, Debasish Sarkar thanks Council of Scientific and Industrial Research (CSIR), Government of India, for providing financial support through senior research fellowship. Author


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Gobinda Gopal Khan is thankful to Department of Science and Technology (DST), Government of India, for providing research support through ‘INSPIRE Faculty Award’ (IFA12-ENG-09).

Supporting Information. The XRD and XPS characterization of the α-Fe2O3/MnO2 NHs; the SEM micrograph of the as-prepared α-Fe2O3 NWs; The peak current (I ) vs. square root of scan rate (f) plot for both types of capacitors measured using CV analysis and the Charging/discharging curves for first 10 cycles of α-Fe2O3/MnO2 core–shell NHs at a current density of 1 Ag-1 are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.”

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High-Performance Pseudocapacitor Electrodes Based on Î±-Fe2O3

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