MnO2 nanocomposites for energy storage supercapacitor application

MnO2 nanocomposites for energy storage supercapacitor application

Journal Pre-proof Hydrothermal synthesis and characterization studies of α-Fe2O3/MnO2 nanocomposites for energy storage supercapacitor application Moh...

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Journal Pre-proof Hydrothermal synthesis and characterization studies of α-Fe2O3/MnO2 nanocomposites for energy storage supercapacitor application Mohamed Racik K, A. Manikandan, M. Mahendiran, J. Madhavan, M. Victor Antony Raj, M. Gulam Mohamed, T. Maiyalagan PII:

S0272-8842(19)33280-8

DOI:

https://doi.org/10.1016/j.ceramint.2019.11.091

Reference:

CERI 23460

To appear in:

Ceramics International

Received Date: 30 September 2019 Revised Date:

4 November 2019

Accepted Date: 11 November 2019

Please cite this article as: M. Racik K, A. Manikandan, M. Mahendiran, J. Madhavan, M. Victor Antony Raj, M.G. Mohamed, T. Maiyalagan, Hydrothermal synthesis and characterization studies of α-Fe2O3/ MnO2 nanocomposites for energy storage supercapacitor application, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.091. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

Hydrothermal synthesis and characterization studies of α-Fe2O3/MnO2 nanocomposites for energy storage supercapacitor application

Mohamed Racik. K1,2, A. Manikandan3, Mahendiran. M1,2, J. Madhavan1, M. Victor Antony Raj1, 2* M. Gulam Mohamed4, T. Maiyalagan5

1

2

3

Department of Physics, Loyola College (Autonomous), Chennai, India

Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai, India

Department of Chemistry, Bharath Institute of Higher Education and Research (BIHER), Bharath University, Chennai-600073, Tamil Nadu, India 4

5

Department of Physics, The New College, Chennai, India

Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, Chennai, India

*Corresponding author Email: [email protected] (M. Victor Antony Raj); *Co-authors

Email:

[email protected]

(Mohamed

[email protected];

[email protected]

[email protected]

M);

(Mahendiran.

[email protected]

(M.

(T. Maiyalagan)

1

(A.

[email protected]

Gulam

Mohamed);

Racik.

K);

Manikandan); (J.

Madhavan);

[email protected]

Abstract In this present study, semiconductor magnetic α-Fe2O3/MnO2 nanocomposites (NCs) were prepared by a facile hydrothermal (HT) method. The crystallographic structure, morphology, chemical configuration and magnetic features were analysed by X-ray powder diffraction (XRD), high resolution scanning electron microscope (HR-SEM), energy dispersive X-ray analysis (EDX), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) analyses. The as-prepared NCs were used as an electrode in energy storing supercapacitor was systematically examined. The electrochemical deeds of α-Fe2O3/MnO2 NCs was analysed by cyclic voltammetry (C-V) and galvanostatic charge–discharge (GCD) tests. The CV analysis of the NCs electrode showed a distinctive pseudocapacitive behaviour in 1M KOH solution. The NCs electrode reveals enhanced specific capacitance compared to plain α-Fe2O3 and MnO2 nanoparticles (NPs) and generates high specific capacitance of 216.35 Fg−1. Pseudocapacitor obtains of energy density 135.42 Wh kg-1 at power density of 6.399 kW kg-1, indicating the as-prepared α-Fe2O3/MnO2 NCs shows noteworthy high-energy, specific capacitance, power densities and long-standing cyclic stability with 89.2 % of preliminary capacitance reserved at 1A g-1 after 10000 cycles in judgement with the pure α-Fe2O3 and MnO2 NPs electrode. The α-Fe2O3/MnO2 NCs electrode having noteworthy electrochemical characteristics performance renders promising applications in energy storing systems.

Keywords: α-Fe2O3/MnO2 NCs; Hydrothermal; X-ray photoelectron spectroscopy; Magnetic Studies; Supercapacitor; Pseudocapacitance.

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1. Introduction The growth of universal economy in fast step demands a high energy to fulfill the need of developed world. The competent and non-polluting energy devices were desired to overcome the existing energy and environment based crisis [1-3]. Nowadays, tremendous efforts and researches were executed to devise energy stowing and conversion procedures from maintainable and renewable clean energy sources, due to the looming scarcity of fossil fuels and growing ecological concerns [4,5]. In recent years, supercapacitors or ultracapacitors is a noteworthy electrochemical energy storage system catching more attention, due to their elevated good stability, power density, low price, long cycling stability, non-hazardous to environment and other excellent properties [6]. The gaps among batteries, conservative solid state, electrolytic capacitors are appropriately filled by supercapacitors and they can play a vital role in supplementing or replacing batteries [7]. It is well known that, pseudocapacitors (PCs) and electric double-layer capacitors (EDLCs) were the categories of materials for electrode with respect to their charge storage mechanism [8, 9]. Mostly, EDLCs covers conductive porous carbon composite materials, such as graphene, super carbon, carbon spheres and activated carbon (AC) [10, 11]. Some of the prime pseudocapacitive materials namely, metal oxides, conducting polymers and hydroxide particles possess higher specific capacitances and energy densities, due to rapid, alterable, multi-electron, surface faradic reactions. [12-14]. In specific, there are certain metal oxides and low-price hydroxides, such as ZnO, α-Fe2O3, MnO2, Mn3O4, Co3O4, NiO, and Co(OH)2, were researched and reported as a potential materials for electrodes. Hematite (α-Fe2O3), is observed as a very stable oxide and a vital n-type semiconductor having hexagonal corundum structure, put forth promising in the field of photoelectronics, catalysis, gas detecting, field emission, energy storage, and waste water treatment, because of its excellent, abundant resource, environmental compatibility and

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advanced corrosion resistance [15]. Generally, α-Fe2O3 shows only weak or parasitic ferromagnetism between Morin and Neel temperature, which are fascinating magnetic features of the materials [16]. Moreover, transition metal oxides (TMOs) when engaged as energetic electrode resources for supercapacitor (SC) electrodes, display on PCs behaviors with long-standing cycling stability, due to poor electrical conductivity of α-Fe2O3. The experimentally observed specific capacitance of α-Fe2O3 is still very low compared to the theoretical value [17]. The previous literature study of supercapacitor shows that minimum amounts of additives can drastically impact the magnetic and opto-electronic properties of ferrimagnets by changing their environment and attention defects [18]. Therefore, the possibly for hybrid metal oxides, such as α-Fe2O3–MnO2, MnO2–NiO, Co3O4–MnO2 and V2O5–TiO2 to obtain enriched electrochemical routine by combining preferred purposes of discrete mechanisms and interfering increased combinational effects into the electrode scheme of PCs [19]. Manganese oxides exist in diverse arrangements and phases such as MnO2, MnO, Mn3O4 and Mn2O3. Electrochemical Energy Storage (EES) applications witness of MnO2 as a superior form of potential material, because of its high capacity/capacitance, low price and less pollution. By dissimilar solution-based production and fabricated for many MnO2-based nano and microstructures have been reported. Also, improved their electrochemical procedures, and are established in these arrangements with capable practical applications. Among them, solution-based route (chemical precipitation, electro-deposition, etc.), hydrothermal synthesis (HT) is the most widespread method for preparation of MnO2 and MnO2-based composites for EES application. The theoretical value of capacitance is exceptionally high (1100-1300 Fg-1) for MnO2 [20] and also it is observed that it reaches high experimental capacitance (600-800 Fg-1) with excellent cycling stability up to 1000s of cycles [21, 22]. Literature survey reveals that mixed oxide nanostructures were reported recently

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with high specific capacitance and stability. Supercapacitor electrodes with high performance, based on α-Fe2O3/MnO2 core-shell nanowire hetero structure (NHs) arrays were described by Sarkar et al. It was found that the current density of 1Ag-1 a specific capacitance of 801 Fg-1 observed for α-Fe2O3/MnO2 NHs electrode [23]. Nie et al. [24] reported core-shell typed heterostructures of α[email protected] NCs contains 60.1 wt% of MnO2, provides specific capacitance with higher value of 289 F g-1 at 1.0 Ag-1, good rate capability of 40% at 5Ag-1 and greater cycling stability of 85% up to 1200 cycles in comparison with pristine MnO2, emphasizing the benefits of such unique configuration combined by synergistic effect. Zhu [25] reported the preparation of core-shell [email protected] nanospindle by HT method, which showed the specific capacitance of 159 Fg-1 at 0.1 Ag-1 current density and specifically 97% capacitance retention after 5000 cycles in a 0.5 mol/L Potassium sulphate (K2SO4) electrolyte. In the view from the above considerations, herein α-Fe2O3/MnO2 NCs was synthesized by adapting facile HT route. The structural, morphology and magnetic belongings of bare α-Fe2O3, MnO2, and α-Fe2O3/MnO2 NCs were inspected by XRD, HRSEM, TEM, FT-IR and VSM performances. Electrochemical studies were inspected by CV, GCD and EIS techniques. The results emphasize that the α-Fe2O3/MnO2 NCs electrodes demonstrate good rate capability, power density, energy density and stability, which create this type of environment friendly, nanostructures a perfectly capable material for construction of supercapacitors electrode.

2. Experimental 2.1 Preparation of α-Fe2O3 nanoparticles For the preparation of α-Fe2O3, 40 ml of DI water was taken to dissolve 1M of FeCl3.6H2O for the HT method. This solution was moved into a 100 ml Teflon coated

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autoclave filled with DI till 80% of the entire volume. This arrangement was subjected to constant temperature of 220°C for 5 h. Now the precipitates at the bottom are collected together and washed many times using DI water and methanol. Finally the obtained product was dried at 60˚C for 24h and calcinated at 600 ˚C for 2h.

2.2 Preparation of MnO2 nanoparticles Hydrothermal method is adapted to prepare MnO2. A homogeneous solution of MnO2 was obtained by dissolving stoichiometric ratio of MnCl2·4H2O and urea in 50 mL of DI water. Then this solution was loaded into Teflon coated autoclave and kept at 120°C for the duration of 24 hours. The powder obtained from this experiment was centrifuged and cleaned several times using DI water and methanol. At last the collected precipitate was obtained by heating at 80°C for 12 hour and calcined at 400°C for three hours.

2.3 Preparation of α-Fe2O3/MnO2 nanocomposite α-Fe2O3/MnO2 NCs was prepared by dissolving above prepared α-Fe2O3 in 20 mL of doubly DI water with continuous magnetic stirring and then MnCl2·4H2O and urea was mixed into the dispersion. This obtained mixed solution was stirred constantly for 1 hr and loaded into a 100 mL autoclave, which was treated at 120˚C for 24 hrs. The sediments of α-Fe2O3/MnO2 NCs settled at the bottom of the container are subjected to centrifuged using DI water and methanol. The final product is dehydrated for 12 hours at 80˚C and annealed at 400˚C for 4 hours.

2.4 Characterization techniques The crystal structure, phase formation and purity of the synthesized pristine powders and the α-Fe2O3/MnO2 NCs was confirmed by powder XRD analysis using Rigaku D/max-

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kA diffractometer (CuKα;

=1.54056 nm). FT-IR spectral study was performed in the

frequency range from 4000-400 cm-1 by Perkin-Elmer spectrometer to confirm the functional groups and co-ordination. Raman spectra for the prepared nanocomposites electrodes was taken out using the Horiba Jobin-Yvon spectrometer and the corresponding spectra was recorded from 100 to 1000 cm-1. The morphologies were observed by scanning electron microscopy (HRSEM, FEI Quanta FEG 200), and the TEM images were obtained on a Philips CM200FEG field emission microscope.

2.5 Electrochemical measurements Electrochemical properties of bare α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs were examined by Galvanostatic Charge and Discharge (GCD), cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). All electrochemical experimentations are executed at room temperature. Cyclic voltammograms of the electrodes were recorded on a potential range between -0.1 to 0.4 V (vs. Ag/AgCl) in different scan rates from 5 to 320 mV/s. The specific capacitance of the electrodes was expected over the charge/discharge curves as of the CP and CV measurements. The charge/discharge experiments were performed using chronopotentiometry by applying different constant current range between -0.1 and 0.4 V (vs. Ag/AgCl). The electrochemical impedance was also found in frequency region between 1Hz and 1MHz. The working electrode preparation consists of active material (80% by weight), polyvinylidene fluoride suspension (PVDF) (10%) and activated carbon (10%) ratio using the solvent of N-methyl-2-pyrrolidone to obtain in a gel form [26]. Then the gel from the mixture was coated on the surface of nickel foil sheet, dried at 80 ºC for 24h. All measurements were performed using KOH of 1M as an electrolyte under environmental conditions with electrode material prepared as a working electrode, Ag/AgCl as a reference electrode and platinum wire as a counter electrode.

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3. Results and discussion 3.1 Powder XRD analysis Fig. 1 depicts the XRD arrangements of α-Fe2O3, MnO2, and α-Fe2O3/MnO2 NCs. The noticeable peaks can be viewed at 2θ of 24.18˚, 33.20˚, 35.68˚, 40.92˚, 49.52˚, 54.13˚, 57.61˚, 62.50˚, 64.06˚, 72.04˚ and 75.58˚ and indexed (012), (104), (110), (113), (024), (116), (018), (214), (300), (119) and (220) planes, which can be perfectly matched with the diffraction peaks of hexagonal structured α-Fe2O3 (JCPDS card no. 33-0664) [27]. For MnO2, the peaks appear at 2θ of 28.76, 37.44, 42.78, 56.67, 59.33, and 72.24˚ is the planes of (110), (101), (111), (211), (220) and (301) crystal of tetragonal MnO2 (JCPDS card no. 81-2261) [28]. No peaks were further detected, specifying the greater degree of phase purity of α-Fe2O3 was prepared. XRD result assures the successful generation of α-Fe2O3/MnO2 NCs. The average particle size was calculated for α-Fe2O3 and MnO2 and obtained 33.89 nm and 35.89 nm, respectively. Entire diffraction peaks of α-Fe2O3/MnO2 NCs are recognized to the MnO2 having a cubic phase formation and other peaks representing to α-Fe2O3. The XRD results showed the coexistence of α-Fe2O3 and MnO2.

3.2 FT-IR analysis Vibrational atoms of α-Fe2O3, MnO2, and α-Fe2O3/MnO2 NCs were analyzed by FT-IR technique (Fig. 2) and the broad band at around 3432 cm-1 is attributed to O-H stretching vibrations, highlighting the nature of MnO2 hydrates. The absorption bands at around 1627 and 1036 cm-1 are fixed to Mn atoms along with O-H bending vibrations. The peaks placed around 538 cm-1 and 434 cm-1 are recognized to Mn-O vibrations. The peak at 3515 cm−1 is agreed as O–H stretching vibration. The vibrations bands at 540 and 466 cm−1 are attributed as Fe-O bond vibration of Fe2O3 [29]. The two solid bands at 538 and 470 cm-1 confirms the generation of α-Fe2O3/MnO2. The vibratory band related to the bonds Fe-O or

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O-Mn-O is indicated by the above mentioned bands [30]. Stretching vibrations of O-H adsorbed NCs surfaces by MnO2 and α-Fe2O3/MnO2 are present at the peak of 3446 cm-1. The peak around 1645 cm−1 is tentatively allocated to vibration of C–N bond and 1036 cm-1 is consigned to bending vibrations of O-H joined with Mn atoms. Thus, FT-IR spectrum of NCs confirms the incorporation of Fe2O3 NPs in MnO2. All the typical peaks perceived from the spectra also authorize the construction of α-Fe2O3/MnO2 NCs.

3.3 Raman Spectroscopy Analysis Raman spectra of pure MnO2, α-Fe2O3 and α-Fe2O3/MnO2 NCs (range 200–1400 cm-1) are shown in Fig. 3. Raman spectrum of MnO2 NPs has four major bands at 315, 368, 480 and 655 cm-1. The higher phonon mode frequency at 655 cm−1 (Ag modes), which is assigned to Mn-O stretching vibrations due to directly perpendicular to the double chains of MnO6 [edge-shared] octahedra, corresponding to tetragonal 2 × 2 tunnel structure of typical fingerprints [31-34]. α-Fe2O3 belongs to the D63d crystal space group and 7 phonon active lines are projected to be seen in the Raman spectrum, specifically two A1g and five Eg phonon modes. Bands at 224, 246, 294, 412, 497, and 614 cm-1, connected to α-Fe2O3 are observed at Raman Spectra. Bands at 224 and 497 cm-1 are allocated to A1g phonon modes. The bands at 294, 412 and 614 cm-1 are allocated to Eg phonon modes. The annealed product of α-Fe2O3 NPs was confirmed by this process. Since the product α-Fe2O3 phase like magnetite or maghemite, is high purity [35]. There are some reported results previously with α-Fe2O3 NPs [36]. Xu et al. [37] reported A1g (225, 498 cm-1), Eg (252, 293, 411, 612 cm-1) phonon modes in the Raman spectrum of α-Fe2O3 NPs. The prominent presence of pure MnO2 and α-Fe2O3 peaks in the analyzed Raman spectra of α-Fe2O3/MnO2 NCs reflects the generation of nanocomposites.

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3.4 Morphological Analysis Fig. 4 displays the HRSEM images of α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs confirming that the synthesized samples having non-uniform size. The shape of the NCs varied from spherical to rod like morphology, post reaction was employed for 24 h [38]. The HRSEM images reveal that the presence of spherical nanoparticles likes structure, as a result of the aggregation of NPs and their agglomeration. The magnetic interactions between the particles collect them together [39, 40]. The elemental or chemical compositions of α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs are shown in Fig. 5 (a,b), which reveal the existence of stoichiometric quantity of O, Mn and O, Fe in MnO2 and α-Fe2O3 respectively and Fig. 5(c) is an evidence for the presence of Mn, Fe, and O in α-Fe2O3 /MnO2 NCs. The obtained molar ratio is confirmed the formation of MnO2, α-Fe2O3 and α-Fe2O3 /MnO2 NCs (Fig. 5 (a-c) inset). The structure and morphology features of α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs are defined by HR-TEM analysis and the images were shown in Fig. 6(a-c). From the highly magnified TEM image, it also vivid that the formation of spherical shaped particles with diameter in the ranges of 25-50 nm (Fig. 6). The SAED pattern of pristine α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs were exposed in Fig. 7(a-c) and confirmed the powders were well crystallized in nature.

3.5 X-ray photoelectron spectra (XPS) analysis The configuration of elements, electronic and chemical states of α-Fe2O3/MnO2 NCs was decided by XPS analysis. Fig. 8(a) depicts the comprehensive survey spectrum of αFe2O3/MnO2 NCs and Fig. 8(b-d) displays the individual scanned spectra of C 1s, Mn 2p, Fe 2p and O 1s elements. Fig. 8(a) depicts the wide-scan XPS survey spectrum of NCs makes evident the existence of Mn, Fe, and O elements in the NCs. In Fig. 8(c), two prominent

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peaks at 708.95 and 722.56 eV were detected consistent to Fe 2p3/2 and Fe 2p1/2 respectively. A satellite peak was spotted in gap between two prominent peaks, which is approved that the prepared α-Fe2O3 NPs belonged to α-Fe2O3 as opposed to Fe3O4 [41]. The survey spectra from Fig. 8(d) shows that the obtainable O and Mn, with C from the reference. Moreover, demonstrating the formation of MnO2 and the molar rate of Mn and O is 1:2 in the survey spectra. There are two XPS binding vitalities of Mn 2p1/2 and Mn 2p3/2 at 652.72 eV and 641.20 eV were witnessed for MnOX. Realising, the separation of binding energy is 11.5 eV among the Mn 2p1/2 and Mn 2p3/2 states. Moreover, Mn 2p1/2 and Mn 2p3/2 can be separated into two characteristic peaks such as Mn4+ and Mn3+ ions. The higher oxidation state Mn type is more active for the electrochemical performance over Mn-based electrodes. The O 1s spectra of the α-Fe2O3/MnO2 NCs in Fig. 8(e) has two kinds of notable surface oxygen type. The minor binding energy peak at 528.0–530.0 eV strength be entrusted to the superficial lattice oxygen (Oβ), such as imperfect oxides or shallow oxygen ions attached to Mn in a coordinately soaked atmosphere, and the advanced binding energy peak at 530.0– 532.0 eV could be its place to the superficial chemisorbed oxygen (Oα) [42]. Generally, Oα is more reactive than the Oβ, due to the higher mobility of Oα. In Fig. 8(e), the peak at 528.52 eV could be allotted to O2− and the peak at a advanced binding energy (530.73 eV) might be endorsed to superficial hydroxyl (OH) groups [43].

3.6 Magnetic Studies Using vibrating sample magnetometer, the magnetic characteristic measurements of magnetization (M) versus applied field (H) of as-synthesized α-Fe2O3, MnO2, and αFe2O3/MnO2 NCs were recorded at room temperature (RT) and the hysteresis (M-H) loops are presented in Fig. 9(a-c). From the VSM hysteresis loop graphs, the magnetic characteristic parameter includes coercivity (Hc), remanent (Mr) and saturation

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magnetization (Ms) was calculated. It was found that 0.446 emu/g, 0.1452 emu/g and 3777.9 Oe for Ms, Mr and Hc, respectively for α-Fe2O3 NPs from Fig. 9(a). This reveals the existence of weak ferromagnetism in α-Fe2O3 NPs. This can also be accredited to the surface effects includes spin canting, decrease in Fe content, partial oxidation and deviations from stoichiometry [44, 45]. Similar behaviour of weak ferromagnetic portent was also detected for BiFeO3 [35]. The M-H loop of MnO2 is presented in Fig. 9(b), Ms and Hc of MnO2 NPs was found to be 0.830 emu/g and 62.73 Oe respectively. At low peripheral field, the MnO2 NPs reveals high Hc (62.73Oe), characteristic performance of ferromagnetism. From the viewpoint of magnetic connections, only Mn4+–O2−–Mn4+ is in ferromagnetic coupling, decides magnetic response of MnO2, which depends on the valence ion distribution of Mn in the [Mn2]O4 framework [46]. Fig. 9(c) portrays the M-H loop of α-Fe2O3/MnO2 NCs and the Ms and Hc of NCs was found to be 0.896 emu/g and 1255.36 Oe respectively.

3.7 Electrochemical Studies Electrochemical properties of α-Fe2O3/MnO2 NCs were evaluated using 3-electrode system in 1M aqueous KOH electrolyte with counter electrode (platinum foil) and the reference electrode (Ag/AgCl). For assessment, electrochemical behavior of pristine α-Fe2O3 and MnO2 were also investigated. Fig. 10 shows the CV curves of α-Fe2O3, MnO2 and αFe2O3/MnO2 NCs. The CV electrode measurement was carried out in a range -0.1 V to 0.4 V. Fig. 10(a) illustrates the CV diagrams of pristine α-Fe2O3 electrode. CV curves showed at several scan rates from 5–320 mV-1 over the voltage range of -0.1–0.4 V, representing parallel PCs response and significant rate capability. Additionally, when increasing of scan rates, the redox peaks scan sweep and increased when current intensity areas turn larger [47-49]. Moreover, enormously shifting in anodic peaks towards a positive (+ve) potential owing to electroactive sample polarization, while the cathodic peaks relocated

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on the way to a negative (–ve) potential. Fig. 10(b) depicts the pristine MnO2 CV curve, which appears as a rectangular shape with a pseudocapacitance, accredited to the reversible redox reaction between Mn3+ and Mn+4. The plot shape is not effortlessly rectangular, pointing out unconventionality from perfect behavior of the supercapacitor [50]. The electrode of good charging rate capability is specified by the slight shape change in CV curves. Fig. 10(c) appeared CV curves of pristine MnO2 electrode materials with several mass loading of MnO2 at 5–320 mV-1 scan rates. It could be comprehended that the CV curves of pristine MnO2 electrode exhibits relatively outward redox peaks, which is the suggestion of characteristic PSc performance of MnO2 electrode [51]. The charge-discharge (GCD) method is adapted to examine the capacitive behaviors of as-synthesized samples and Fig. 11 shows the results of GCD behavior of pristine α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs performed the density of 1 Ag-1 and the voltage maintained from -0.1–0.4 V and the specific capacitance can be obtained by the equation: Specific capacitance= CGCD = Where i/m is the current density (Ag-1),

(1)

stand for potential range and

corresponds to

discharge time The charge/discharge process of α-Fe2O3/MnO2 NCs electrode is understood by charge /discharge curves and it exhibits equilateral triangle shapes indicates the reversibility during the process [52]. The calculated specific capacitances of α-Fe2O3/MnO2 are 216.35, 128.47, 77.80 and 60.93 F g−1 at current densities of 4, 2, 1 and 0.5 A/g, respectively (Fig. 11c). And those of α-Fe2O3 are 116.99, 58.61, 38.30 and 29.72 F g−1 at current densities of 4, 2, 1 and 0.5 A/g respectively (Fig. 11a). The specific capacitances of MnO2 at 4, 2, 1 and 0.5 A/g current densities were calculated as 37.47, 45.15, 66.78, and 175.51 F/g respectively (Fig. 11b) which are higher than the values reported in literatures.

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This result shows the specific capacitance of α-Fe2O3/MnO2 NCs electrode is larger than pristine α-Fe2O3 and MnO2 electrode, because of MnO2 spherical NPs were well covered on the surface of α-Fe2O3 NPs to give composites with larger surface area [53]. A better specific capacitance is obtained for the produced material than that of MnO2 electrode based composites. Equations 2 and 3 were used to analyse the power density and energy density of the NCs and the results are tabulated in Table 1. (2)

(3)

Where, Ed is energy density (Wh kg−1), ∆V is potential window of discharge (V), Cs is specific capacitance (F g−1), ∆t is discharge time (s) and Pd is power density (W kg−1). With the increase of power density the energy density is also on the increase. This indicates that the discharging capacity of the pseudo-capacitor is directly proportional to discharge current. For pure α-Fe2O3 at a current density of 4 Ag−1 is of energy density 66.0506 Wh kg−1 and power density of 6.397 kW kg−1. MnO2 exhibits the energy density and power density of 83.2695 Wh kg−1 and 7.989 kW kg−1 for the same current density of 4 Ag−1. Then αFe2O3/MnO2 NCs given a superior energy density of 135.4204 Wh kg−1 and power density of 6.399 kW kg−1 in the same current density of 4 Ag−1 (Table 1). Table 2 summaries the comparisons of specific capacitance performance of α-Fe2O3/MnO2 NCs prepared by this present work exposed good cycling performance than some of the previously reported papers [54]. From GCD plot, Coulombic efficiency was calculated using the following equation: Coulombic efficiency = [discharging time (tD)/charging time (tC)]

100%.

(4)

Coulombic efficiency of α-Fe2O3/MnO2 NCs is 102%. As exposed in Fig. 13, the αFe2O3/MnO2 nanosphere-like structure has admirable long-standing electrochemical and higher stability with a very small decrease to 85.1%, even after 10000 cycles. 14

Specific capacitance vs increasing current density was conspired in Fig. 12. Since the higher current density obstructs the availability of ions from towards the inside into the inner core of the electrode NCs material the specific capacitance is inversely proportionate to the current density. Slow diffusion restricts the transport of ions and the ions present at the outer surfaces are used as charge storages. The obtained specific capacitance values for the α[email protected] NCs electrode are 216.35, 128.47, 77.80, and 60.93 F/g for various current densities like 0.5, 1, 2 and 4 A/g respectively. The obtained results instruct us that α[email protected] NCs electrode has remarkable specific capacitance at lesser current densities and advanced energy and power densities at higher current density of 4 Ag-1. EIS analysis helped us to understand the electrochemical actions of the α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs electrodes. Fig. 14 shows The Nyquist impedance plot for the NCs electrode. This plot contains a linear line (lower frequency region) and semicircle (higher frequency region), shows the characteristics of the resistance of oxide-electrolyte interface. The lower frequency region of Warburg impedance designates the diffusive resistance of the OH- ions inside the electrode material. The plot displays the linear line denotes the reversible Faradaic redox reaction. The results point out that the α-Fe2O3 /MnO2 NCs is a proper electrode for supercapacitors.

4. Conclusion In summary, a hydrothermal route was implemented to construct α-Fe2O3/MnO2 NCs. The positive synergism between α-Fe2O3 and MnO2 NPs brings the formation of electrically conductive, thermally stable and mechanically robust composite. Additionally, the composite was employed as a working electrode material to establish its probable application as a favorable electrode substance for high efficient hybrid supercapacitors. The XRD and FT-IR results of α-Fe2O3, MnO2 and the α-Fe2O3/MnO2 NCs supports the presence of α-Fe2O3

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/MnO2 and the relation exists between α-Fe2O3 - MnO2 nanocrystalline. The electrochemical characteristic achievements of organized electrodes were studied through CV and galvanostatic charge/discharge (GCD) test. The discharge curve of the as-synthesized NCs is linear in nature and significant IR drop very clearly shows a good capacitive performance of of α-Fe2O3 /MnO2 which is inferred from the CP curves of the materials. The α-Fe2O3/MnO2 NCs exhibits a high specific capacity of 216 Fg-1 at a current density of 0.5 Ag-1, energy density of 7.5121Wh⋅kg−1and power density value measured of 0.118 kW kg-1. The presence of MnO2 with high surface area in the α-Fe2O3/MnO2 NCs, good interaction between α-Fe2O3 and MnO2 give a superior structure easy electrolyte accessibility results in the better capacitance of α-Fe2O3/MnO2 NCs in comparison with pure α-Fe2O3 and MnO2. EIS analysis proposes high capacitance, high phase homogeneity and improved chargetransfer characteristics. We find all the above mentioned results are in line with this EIS analysis. It proves that α-Fe2O3/MnO2 NCs material can be an economical anode material for next generation super-capacitor applications.

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Figures

Fig. 1. XRD patterns of pure α-Fe2O3, MnO2, and α-Fe2O3/MnO2 NCs.

24

Fig. 2: FTIR spectrum of α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs.

25

Fig. 3: Raman Spectrum of α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs.

26

Fig. 4: HRSEM images for (a) α-Fe2O3 (b) MnO2 and (c) α-Fe2O3/MnO2 NCs.

27

Fig. 5: EDX spectrum of (a) α-Fe2O3, (b) MnO2 and (c) α α-Fe2O3/MnO2 NCs.

28

Fig. 6: TEM images of (a) α-Fe2O3 (b) MnO2, and (c) α-Fe2O3/MnO2 NCs.

29

Fig. 7: SAED Pattern of (a) α-Fe2O3, (b) MnO2 and (c) α-Fe2O3/MnO2 NCs.

30

Fig. 8: XPS spectra of α-Fe2O3/MnO2 NCs (a) total XPS survey scan spectrum and (b–e) deconvoluted XPS spectra of C1s, Fe 2p, Mn 2p and O 1s respectively.

31

Fig. 9: Magnetic hysteresis curves of (a) α-Fe2O3 (b) MnO2, and (c) α-Fe2O3/MnO2 NCs.

32

Fig. 10: Cyclic voltammograms of (a) α-Fe2O3 (b) MnO2, and (c) α-Fe2O3/MnO2 NCs at different scan rates.

33

Fig. 11: Charge–discharge curves at different current densities of (a) α-Fe2O3 (b) MnO2, and (c) α-Fe2O3/MnO2 NCs.

34

Fig. 12: Variation of specific capacitance with current density of pure of α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs electrodes using GCD technique.

35

Fig. 13: Cycling performance of the α-Fe2O3, MnO2 and α-Fe2O3/MnO2 NCs electrodes

36

Fig. 14: Nyquist plots of c.electrodes.

37

Table 1. Specific capacitance, energy and power density values of α-Fe2O3, MnO2, and α-Fe2O3/MnO2 NCs.

Sample

Specific

Energy density

Power density

Capacitance

(Wh⋅kg−1)

(kW⋅kg−1)

(Fg-1)

α-Fe2O3

MnO2

0.5 Ag-1

116.99

4.0624

0.109

1 Ag-1

58.61

8.1405

0.398

2 Ag-1

38.30

21.2782

1.599

4 Ag-1

29.72

66.0506

6.397

0.5 Ag-1

175.51

6.0904

0.124

1 Ag-1

66.77

9.275

0.589

2 Ag-1

45.16

25.032

1.998

4 Ag-1

37.48

83.269

7.987

7.5121

0.118

0.5 Ag-1

α[email protected]

216.35

1 Ag-1

128.47

17.8439

0.409

2 Ag-1

77.80

43.2228

1.799

4 Ag-1

60.93

135.4204

6.399

38

Table 2: Maximum capacitance value of α-Fe2O3/MnO2 NCs in comparison with the literature. Electrode

Method

Morphology

Electrolyte

material

α –Fe2O3/MnO2

Hydrothermal

Nanorods

Na2SO4 (1M)

Capacitance

Ref.

(F g-1)

No

227mFg-1

44 2

core shell

(0.5mA/cm ) Nanotubes

KOH (3M)

289.9 Fg-1 (1Ag-1)

21

Hydrothermal

Nanospindle

K2SO4 (0.5M)

159 Fg-1 (0.1Ag-1)

22

Hydrothermal

Nanospheres

Li2SO4 (1 M)

255Fg-1 (1 Ag-1)

45

MnO2/graphene

Hydrothermal

Honeycomb

KOH (1M)

278.5 Fg-1(1Ag-1)

46

[email protected]

Solvothermal

Nano particle

KOH (2M)

274 Fg-1 (1 Ag-1)

47

graphene

method

[email protected]

Hydrothermal

Nanorod

Na2SO4 (1M)

285 Fg-1(2.5Ag-1)

48

α –Fe2O3 /graphene

Hydrothermal

Spherical

Na2SO4 (1M)

306.9 Fg-1 (3Ag-1)

49

α –Fe2O

Co-precipitate

Nanosheets

Li2SO4(1 M)

116 Fg-1 (0.75 Ag-1)

50

electrochemical

method

Nanoflakes

LiClO4 (0.5M)

261 Fg-1(1Ag-1)

51

Spherical

KOH (1M)

216.35(0.5 Ag-1)

This

Hierarchical α –

Ultrasonic

[email protected]

Treatment

Core shell MnO2 @Fe2O3 nanospindles [email protected] hybrids

capacitor electrodes MnO2 and Fe2O3

Chemical bath

on carbon fibers

deposition

α –Fe2O3/MnO2

Hydrothermal

nanocomposites

Work

39

Conflict of Interest Form

o

All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

o

This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

o

The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript