C Nanoparticles on 3D ZnFe2O4

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Anchoring Ultrafine ZnFe2O4/C nanoparticles on 3D ZnFe2O4 nano-flakes for Boosting Cycle Stability and Energy Density of Flexible Asymmetric Supercapacitor Madagonda M Vadiyar, Sanjay S Kolekar, Jia-Yaw Chang, Zhibin Ye, and Anil V. Ghule ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06847 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017

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ACS Applied Materials & Interfaces

Anchoring Ultrafine ZnFe2O4/C nanoparticles on 3D ZnFe2O4 nano-flakes for Boosting Cycle Stability and Energy Density of Flexible Asymmetric Supercapacitor Madagonda M. Vadiyar,† Sanjay S. Kolekar,*† Jia-Yaw Chang,*‡ Zhibin Ye,£ and Anil V. Ghule*§ †

Analytical Chemistry and Material Science Research Laboratory, Department of Chemistry,

Shivaji University, Kolhapur 416004, Maharashtra, India Email: [email protected] ‡

Department of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei, 10607, Taiwan Email: [email protected] £

Bharti School of Engineering, Laurentian University, 935 Ramsey Lake Road, Sudbury,

Canada §

Green Nanotechnology Laboratory, Department of Chemistry, Shivaji University, Kolhapur

416004, Maharashtra, India Email: [email protected]

KEYWORDS:

Asymmetric

Supercapacitor,

Bio-Extract,

Nano-flakes@nanoparticle,

heterostructure, ZnFe2O4 thin films.

ACS Paragon Plus Environment

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ABSTRACT

The heterostructure-based metal oxide thin films are recognized as a leading material for a new generation, high performance and stable flexible supercapacitors. However, morphologies like nano-flakes, nanotubes, and nanorods etc. have been found to suffer from issues related to poor cycle stability and energy density. Thus, to circumvent these problems, herein, we have developed a low cost, high surface area and environmental benign self-assembled ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticles heterostructure electrode via anchoring ZnFe2O4 and carbon nanoparticles using in-situ bio-mediated green rotational chemical bath deposition approach for the first time. The synthesized ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure thin films demonstrate excellent specific capacitance of 1884 F g-1 at a current density of 5 mA cm-2. Additionally, all solid-state flexible asymmetric supercapacitor devices were designed based on ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure as a negative electrode and reduced graphene oxide (rGO) as a positive electrode. The device delivered maximum specific capacitance (347 F g-1) and energy density of 81 Wh kg-1 at a power density of 3.9 kW kg-1. Similarly, the asymmetric device exhibits ultralong cycle stability of 35000 cycles by losing only 2% capacitance. The excellent performance of the device is attributed to the self-assembled organization of the heterostructures. Moreover, the in-situ bio-mediated green strategy is also applicable for the synthesis of other metal oxide and carbon-based heterostructure electrodes.


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INTRODUCTION Development of low-cost and high-performance energy storage technologies have attracted great attention of the scientific communities in the recent years because of the tremendous increase in population, environmental pollution, global warming, and energy consumption in portable electronic devices.1-3 Lithium ion batteries were looked upon as promising alternative energy storage devices, however, these suffer from the limiting issues pertaining to low power density, low cycle stability, slow discharging rate, and safety. To circumvent these challenges, researchers drifted their focus towards the development of high-performance supercapacitors.4-6 Supercapacitors have great advantages such as high power density, quick charging-discharging ability, longer life cycle stability and better safety than batteries.7-9 Generally, supercapacitors store energy by two important charge storage mechanisms such as charge separation at electrode/electrolyte interfaces i.e. electric double layer capacitor (EDLCs) and charge storage by Faradic redox reactions i.e. pseudocapacitors.10-14 Among these, pseudocapacitors based on metal oxides, metal ferrites15,16 and metal sulfides17,18 have been explored as promising energy storage devices. These pseudocapacitors suffer from the limitations such as low energy density and smaller cycle stability.19 Thus, it is essential to improve the energy density and cycle stability of the pseudocapacitors by developing new systems of electrodes and electrolytes with optimized properties. Hence, most research efforts so far have been focused on exploring the synthesis of variety of electrode materials with unique structural morphologies such as nanoflakes, nano-rods, nano-wire, nanospheres and other unique structural morphologies such as onedimensional (1D) carbon hollow nanostructures.20-25 Nevertheless, one of the most important limitations of this type of unique morphology is the observed destruction of its original morphology after investigating its long-term cycle stability.21,26 This retards the effective


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utilization of the unique morphology based electrodes in the designing of the stable pseudocapacitors. Hence, the researchers have developed a variety of heterostructure based electrodes27 of one or two metal oxides in the form of hybrid core-shell structures such as metal oxide@metal oxide and carbon@metal oxide etc.28,29 For example, Co3O4@MnO2 nanosheets, Co3O4@SnO2 hollow structure, Fe2O3@SnO2 core-shell, NiCo2O4@MnMoO4 nanocolumns, Fe3O4@Fe2O3 core-shell nanorod, Fe2O3@NiO heterostructures, MnCo2O4@Ni(OH)2 core-shell flowers, ZnO/ZnS@Co3O4 heterostructures, CuCo2O4/CuO nanowires, Au@GeS nanosheetnanowires etc.30-34 Interestingly, it is noted that most of these core-shell heterostructures were fabricated using two or more different metal oxides or metal oxide/carbon hybrid materials. However, to the best of our knowledge, there are very few reports (nanorods-nanoflakes Li2Co2(MoO4)3)35 on the synthesis of same metal oxide with different morphological heterostructures.35 With this motivation, we have attempted the synthesis of a new system (ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructure) containing same metal oxide with two different morphologies using in-situ bio-mediated mechanochemical approach. Herein, we have successfully developed the self-assembled ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructure thin film arrays on a conducting flexible stainless steel mesh (FSSM) via anchoring of ZnFe2O4 and carbon nanoparticles by in-situ bio-mediated green rotational chemical bath deposition approach. Particularly, in this work, the ZnFe2O4 metal oxide is synthesized because of its low toxicity, excellent redox behavior, high surface area, large operational potential window, high specific capacitance, and energy density. This synthetic approach is a combination of our rotational chemical bath deposition method36 and Bengal gram bean (Cicer arietinum) extract (BGBE) mediated bio-green solution approach37 in one beaker system and is adapted to deposit the ZnFe2O4 nano-flake@ZnFe2O4/C nanoparticle


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heterostructure thin films. The advantages of this combination approach are good control over the desired heterostructured morphology within less time and spreading of uniform and small size ZnFe2O4/C nanoparticles over ZnFe2O4 nano-flakes. The synthesized ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructures thin film is directly used as binder-free negative electrode material in the fabrication of flexible solid-state asymmetric supercapacitor along with solid KOH-PVA gel electrolyte and rGO as the positive electrode. This device not only delivers maximum specific capacitance of 347 F g-1 but also exhibits high energy density (81 Wh kg-1) and ultra high cycle stability with 98% capacitance retention over 35000 cycles. EXPERIMENTAL SECTION Materials and Chemicals Bengal gram bean extract (containing biomolecules) and monoethanolamine (MEA) were used as complexing agents for synthesis of nanocrystalline ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure thin films. The AR grade zinc chloride (ZnCl2), ferrous chloride tetrahydrate (FeCl2.4H2O) and ammonia were used as received from Merck chemicals. Reduced graphene oxide (rGO) is purchased from Sigma-Aldrich. Preparation of Bengal Gram Bean Extract (BGBE) The Bengal gram bean extract (BGBE) was obtained by soaking 10 g of Bengal gram bean in 100 mL distilled water for 48 h at room temperature. Because of soaking and germination process of bean, the solution turns into faint yellow color.37 The solution was filtered using filter paper and the filtrate was used as an extract for the synthesis of ZnFe2O4 nano-


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flakes@ZnFe2O4/C nanoparticle heterostructure thin films using rotational chemical bath deposition approach. Synthesis of ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructures In a typical synthesis, an aqueous solution of ZnCl2 (0.1 M, 12.5 mL) and FeCl2.4H2O (0.2 M 12.5 mL) were prepared and mixed together under constant stirring rate in 50 mL beaker. After 15 min, 25 mL of Bengal gram bean extract (BGBE) was poured into the aqueous metal precursor solution. Furthermore, the whole solution was stirred for 30 min to complete dissolution of metals salts and put the beaker into the water bath. After completion of 30 min, 2.5 mL of monoethanolamine (MEA) followed by concentrated ammonia solution was added for adjustment of the pH 9.0 (± 0.5). On another hand, the flexible stainless steel mesh (300 mesh size) was cut into 1 cm × 3 cm and used as substrates. These substrates were initially soaked in detergent soap solution for 10 min. Subsequently, the substrates were also washed with double distilled water under ultrasonication followed by acetone cleaning. These ultrasonically cleaned substrates were immersed into the precursor solution containing a mixture of metal ion and BGBE extract. The temperature of bath solution was maintained to about 50 oC. During the deposition process, the substrates were rotated by applying the rotation speed of 55 rpm using gear motor. After 3 h, the completion of reaction took place and deposited substrates were removed from bath solution and washed with double distilled water. The as-prepared blue colored thin films were dried under hot air to obtain reddish colored films. After drying the ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles thin films were annealed at 550 oC for 5 h into closed muffle furnace under atmospheric air. For comparison, the similar depositions of pristine ZnFe2O4 nano-flakes and ZnFe2O4/C nanoparticles were also obtained by using MEA and BGBE solutions separately by keeping the identical conditions. The influence of BGBE on the


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formation of heterostructures is investigated by synthesizing the thin films by varying the quantity of extract viz, 0, 15, 20, 25, 30 and 35 mL, respectively. After annealing, the obtained ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles thin films were designated as ZF, ZF-15, ZF-20, ZF-25, ZF-30, and ZF-35, respectively. Materials characterization and electrochemical measurements The

morphologies

of

synthesized

ZnFe2O4

nano-flakes@ZnFe2O4/C

nanoparticle

heterostructures were characterized by field emission scanning electron microscopy (FE-SEM, HITACHI S4800, Japan) and transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). The pore size distributions and specific surface areas of the porous carbon nanofibers were evaluated by Brunauer Emmett Teller (BET) equation (NOVA1000e Quantachrome, USA). The chemical composition of heterostructures was determined by X-ray photoelectron spectroscopy (XPS, PHI-5300, USA). LabRAM HR with a 532 nm Raman spectrometer is used for point group analysis. X-ray Diffraction (XRD, D/Max 2500PC, Rigaku, Japan) is used for analysis of nanocrystalline structure and phase evaluation. TG Analyzer (SDT Q600 V20.9 Build 20) is used for thermal analysis of heterostructure. All the electrochemical measurements were carried out using electrochemical analyzer (CHI 608E, Shanghai Chen Hua Instrument Co., LTD, China) at room temperature. The detailed calculations and formula can be reviewed from the (Equations 1, 2, 3 and 4 Supporting Information). Fabrication of flexible solid state asymmetric supercapacitor Solid-state flexible asymmetric supercapacitor device was fabricated using the synthesized ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure electrode as negative electrode and rGO as a positive electrode (The positive electrode is also deposited on stainless steel mesh


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(SSM) using SILAR method). The individual supercapacitive properties such as CV, GCD, and EIS of the positive electrode (rGO) are presented in detail in the supporting information Figure S1 (Supporting Information). The PVA-KOH gel electrolyte was synthesized by heating a mixture of PVA powder (5 g), KOH (5 g), and distilled water (50 mL) to 85 oC with stirring until the solution becomes clear. Initially, both the positive and negative electrodes and the separator (cellulose paper of thickness ~40 µm) were immersed in freshly prepared PVA-KOH gel electrolyte for 30 min. These two electrodes were put opposite and cellulose paper was sandwiched between them as shown in Figure S2 (Supporting Information). RESULTS AND DISCUSSION The details of the extraction process of BGBE extract and structure of enriching pectin biomolecules is schematically presented in Scheme S1 (Supporting Information). In addition, the formation mechanism of self-assembled ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures is systematically shown in Scheme 1.

Scheme 1 Formation mechanisms of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin films.


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In the formation of two different morphologies, both MEA and BGBE play an important role as a complexing agent. The higher bulk density (1.01 g/cm3) and higher pKa value (9.5) of MEA enables its fast complexation with metal ions followed by hydrolysis38,39 when compared to pectin-containing BGBE extract because of its slow complexation rate affected by low bulk density (0.89 g/cm3) and low pKa (3.4) values.40-42 Hence, the nanoflakes are deposited first on the surface of FSSM substrate followed by the deposition of nanoparticles which is taken to the advantage in this work. The van der Waals force of attraction holds both the nanoflakes@nanoparticle heterostructure compactly as shown in Scheme S2 (Supporting Information). At higher annealing (>550 oC) temperatures, the loosely bonded carbonaceous complex decompose/oxidize and oozes out creating porous structures, while the strongly bonded carbonaceous complex remains as amorphous carbon. The synthesized ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructure thin films were characterized using field emission scanning electron microscopy (FESEM). Figure 1a presents the top view FESEM images of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructure thin film which demonstrate the formation of the nano-flakes@nanoparticle morphology of 1 µm thickness, ~35 nm flakes diameter, and ~5 nm nanoparticles. Figure 1b represents the high-resolution SEM image, which reveals that the ZnFe2O4/C nanoparticles are uniformly spread and covering all over the nano-flakes. Furthermore, the formation of the ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film is confirmed using transmission electron microscopy (TEM). The TEM images provided in Figure 1c demonstrate the uniform spreading of nanoparticles over the nano-flakes. The particle size of the nanoparticles is about ~6 nm which is consistent with FESEM. The high-resolution transmission electron microscopy (HRTEM) image is represented in Figure 1d which reveals the lattice of


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(111) planes (d spacing 2.45 Å) and (220) planes (d spacing 1.48 Å) of the ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructure thin film.

Figure 1 (a) and (b) FESEM images at low and high magnifications (c) TEM images and (d) HRTEM of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle thin film heterostructure.

Moreover, the FESEM images for various ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure thin films obtained after varying mL (ZF-15, ZF-20, ZF-25, ZF-30, and ZF-35) of BGBE extract is presented in Figure S3 (Supporting Information). However, uniformly deposited ZF-30 is used for further study. The XRD pattern obtained from the ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructures thin film is presented in Figure 2a. The diffraction planes observed in XRD could be attributed to the cubic spinel structure of the ZnFe2O4. The planes corresponding to (220), (311), (511), (440) and (622) were observed at a 2θ


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angle of 30.0o, 35.4o, 56.8o, 63.2o and 74.0o, respectively15 and were in good agreement with the standard JCPDS card No-77-0011. In addition, the substrate diffraction peaks were also identified and designated as SSM peaks at a 2θ angle of 43.1o and 50.0o as shown in Figure 2a.15 There is no peak observed for carbon around 25.0o which confirms the amorphous nature of the carbon in the ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructure thin film.43 Similarly, XRD patterns obtained from individual ZnFe2O4 nano-flakes and ZnFe2O4/C nanoparticles showed characteristic peaks of ZnFe2O4 and were attributed to cubic spinel phase as presented in the Figure S4 (Supporting Information). Furthermore, in order to confirm the presence of carbon, Raman spectrum for the ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film is recorded and is presented in Figure 2b. The Raman spectrum reveals a broad single peak appearing at 1295 cm-1 along with a small peak at 1066 cm-1 which could be assigned to sp3 and sp2 hybridized amorphous graphitic carbon in the sample.44,45 In addition, the presence five peaks around a lower wave number of 221, 398, 465, 501 and 670 cm1

were assigned for five vibration modes of cubic spinel ZnFe2O4 nano-flakes@ZnFe2O4/C

nanoparticle thin film heterostructures.10 Among, the five vibration modes, the peaks above 600 cm-1 appeared due to AO4 vibration and is designated for A1g symmetry.36 However, the peak at 221 cm-1 is assigned to Eg symmetry, which is representative of M-O vibration in octahedral site. Rest of the peaks around 398, 465 and 501 cm-1 are assigned to the 3T2g symmetry of BO6 vibrations in the octahedral site.15 In addition, the Raman spectrum of individual ZnFe2O4 nanoflakes and ZnFe2O4/C nanoparticle supports the presence of carbon as a result of unreacted bimolecular carbon as shown in Figure S5 (Supporting Information). Furthermore, the presence of carbon is also supported by thermogravimetric analysis (TGA) of the ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructures thin film. Figure 2c presents the TGA profile


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in which the loss of carbon was observed above 600 oC and is attributed to the amorphous carbon content.46 Hence it was noted that the 550 oC annealed ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film contains amorphous graphitic carbon nanoparticles.46 In addition, the loss of water molecules is observed in TGA profile around the 50-150 oC, while the loss around 250-450 oC is due to decomposition of MEA and biomolecules of BGBE complexing agents. The weight loss around 500 oC is due to the conversion of impure Fe2O3 and ZnO phase into pure ZnFe2O4 cubic spinel phase.36 The same trends were observed in the TGA profile recorded for individual ZnFe2O4 nano-flakes and ZnFe2O4/C nanoparticles as shown in Figure S6 (Supporting Information).

Figure 2 (a) XRD pattern (b) Raman spectrum (c) TGA and DTA profiles and (d) N2 adsorptiondesorption isotherm (inset BJH pore size distribution) of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle thin film heterostructures.


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The porous nature, surface area and pore size distribution of the ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle thin film heterostructures are studied using Brunauer-EmmettTeller (BET) analysis. N2 adsorption-desorption isotherms were recorded after degassing the sample at 350 oC temperature and is presented in Figure 2d. From the figure, it is noted that the hysteresis loop appears from the relative pressure of 0.4-0.8. Hence, the ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructures thin film has mesoporous nature including small micropores. Therefore, the multipoint BET surface area for heterostructure is 235 m2 g-1, which is higher than the individual mesoporous ZnFe2O4 nano-flakes (90 m2 g-1) and ZnFe2O4/C nanoparticles (142 m2 g-1) as shown in Figure S7 (a) (Supporting Information). Average pore size distribution and pore volume is determined using BJH method and is noted to be ~3.5 nm (pore size) and 0.078 cc g-1, respectively as shown in the inset of Figure 2d. Moreover, the pore size (4.0 nm) of heterostructure is smaller than individual ZnFe2O4 nanoflakes (~5.7 nm) and ZnFe2O4/C nanoparticles (~7.8 nm) as shown in Figure S7 (b-c) (Supporting Information).


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Figure 3 (a-c) Low magnifications FESEM images (d-f) High-resolution FESEM images and (g-i) Lowresolution TEM images of bare ZnFe2O4 nano-flakes, ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure, and ZnFe2O4/C nanoparticles respectively.

To further confirm the heterostructure morphology, the FESEM images of individual ZnFe2O4 nano-flakes (MEA complexing agent) and ZnFe2O4/C nanoparticles (BGBE complexing agent) were taken and are presented in Figure 3 (a, d and c, f). The formation of pure nano-flakes in the presence of monoethanolamine (MEA) complexing agent confirms the reproducibility of rotational chemical bath deposition method which is reported in our previous work.36 On the other hand, small-sized nanoparticles of ZnFe2O4 and amorphous carbon are obtained when only BGBE extract is used as complexing agents. The advantage of using BGBE extract lies not only in reducing the size of the particles but also in forming the amorphous carbon on the decomposition of the carbonaceous organic matter when the substrate is annealed at 550 oC. The


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obtained ZnFe2O4/C nanoparticles were in good agreement with our previously reported work on TiO2/C nanoparticles.47 Similarly, the comparatively low-resolution TEM images obtained for individual ZnFe2O4 nano-flakes (MEA complexing agent) and ZnFe2O4/C nanoparticles (BGBE complexing agent) clearly indicate that the formation of nanoflakes is due to MEA and nanoparticles due to BGBE extract, respectively as shown in Figure 3 (g, h, and i). The BET surface area (235 m2 g-1) of the ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film is promising. Therefore, we have studied its chemical environment and oxidation states of the ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film using X-ray photoelectron spectroscopy (XPS) as shown in Figure 4 (a-d). The entire survey spectrum presented in the Figure S8 (Supporting Information) reveals the presence of Zn, Fe, O and C elements with appropriate binding energies.

Figure 4 (a) Core level spectrum of the Zn2p (b) Core level spectrum of the Fe2p (c) Core level spectrum of O1s and (d) Core level spectra of C1s of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle thin film heterostructures.


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However, the core level spectrum of Zn2p demonstrates two binding peaks around 1025.1 and 1045.3 eV and these are differentiated with the spin-orbit splitting energy of 23.0 eV as shown in Figure 4a. The core level spectrum of Fe2p is shown in Figure 4b and from the figure, it is noted that there are three peaks at 711.8, 725.5, and 720.5 eV, respectively in accordance with an oxidation state of Fe3+ of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film. The core level spectra of O1s (Figure 3c) shows binding energy peaks at 530.4 eV which could be attributed to M-O bonding and lastly the carbon core level spectrum shows peak at 286.5 eV, which on deconvolution showed five sub-peak at different B.E. of 290.2 eV (-COOgroup), 287.7 eV (-C=O group), 286.9 eV (-C-O), 286.0 eV (sp3 hybrid carbon) and 285.6 eV (sp2 carbon) revealing the amorphous sp3 hybridized graphitic carbon as shown in Figure 3d. The comparative EDAX elemental analysis of ZnFe2O4 nano-flakes, ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructures, and ZnFe2O4/C nanoparticles confirms the presence of Zn, Fe, O, and C respectively as shown in Figure S9 (Supporting Information). Due

to

interesting

structural

and

morphological

properties

of

ZnFe2O4

nano-

flakes@ZnFe2O4/C nanoparticle heterostructures thin films we used it as an electrode material for supercapacitor application. Initially, individual electrochemical properties were studied in a conventional three-electrode system with ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film as a working electrode, against SCE reference and Pt wire as counter electrodes in 6 M KOH electrolyte.20 The cyclic voltammetry (CV) measurements at various scan rates (Figure 5 (a-c)) of individual electrodes such as ZnFe2O4 nano-flakes, ZnFe2O4/C nanoparticles, and ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle heterostructures thin film shows

well-defined

oxidation

and

reduction

peaks

indicating

the

pseudocapacitive

characteristics. This redox behavior is attributed to the Feo/Fe2+/Fe3+ or Fe3+/Fe2+ electrochemical


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reactions at the surface of the electrode.48 Interestingly, the heterostructure (nanoflakes@nanoparticle) provides better current density and area under the CV curve, which indicates that the specific capacitance of the heterostructure is higher than individual ZnFe2O4 nano-flakes and ZnFe2O4/C nanoparticles.

Figure 5 (a-c) Cyclic voltammetry (CV) curves (d-f) Galvanostatic charge-discharge (GCD) curves (g) plot -1

-2

of specific capacitance (F g ) vs. current density (mA cm ) (h) Cycle stability over 18000 cycles (i) Nyquist plots of individual ZnFe2O4 nano-flakes (black line in g-i), ZnFe2O4/C nanoparticles (red line g-i) and ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure (blue line in g-i).


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To evaluate further surface electrochemistry, galvanostatic charge-discharge measurements (GCD) at various current densities (5-25 mA cm-2) were recorded individually of ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticles heterostructure, ZnFe2O4 nano-flakes, and ZnFe2O4/C nanoparticles as shown in Figure 5 (d-f). Interestingly all the GCD profiles demonstrate ideal pseudocapacitive properties. Moreover, from the figure (Figure 5 (d-f)) the discharging time of individual ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticles heterostructure electrode is higher which gives high specific capacitance of 1884 F g-1 at 5 mA cm-2 current density, which is higher than ZnFe2O4 nano-flakes (718 F g-1), ZnFe2O4/C nanoparticles (1084 F g-1) and that of earlier reports. Figure 5 (d) and (e) are the GCD curves for individual ZnFe2O4 nano-flakes and ZnFe2O4/C nanoparticles electrodes which are able to achieve potential window of -1.2 to 0.0 V only and current densities up to 5-15 mA cm-2. It is important to note that the GCD curves of the nano-flakes and nanoparticles are not able to achieve the potential range of -1.2 to 0.0 V above 15 mA cm-2 current density and thus the measurement was restricted to 15 mA cm-2. On another hand, ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure electrode achieves the potential window range (-1.2 to 0.0 V) even after higher current density of 25 mA cm-2 indicating the excellent electrochemical performance of the heterostructures electrode materials. This enhancement in the current density may be due to the increased surface area and ion diffusion in the porous heterostructures electrode. This also supports the improved performance of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure electrode. The specific capacitances of the electrodes decrease when the current densities were increased (Figure 5 (g)). The observed higher specific capacitance at low current density can be attributed to the efficient rate of electrolytic ions diffusion and migration into the pores of the active materials. On the other hand, at higher current densities, the rate of diffusion and migration of electrolytic ions is


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faster which limits their access into the pores of the material thereby reducing the specific capacitance. This is a common phenomenon and is in agreement with the earlier literature report. The cycle stability plots of individual electrodes are presented in Figure 5 (h). From the figure, it is noted that the ZnFe2O4nanoflakes@ZnFe2O4/C nanoparticles electrode provides long cycle stability (18000 cycle with 99 % retention), which is marginally higher than the individual ZnFe2O4 nanoflakes (12000 with 83% retention) and ZnFe2O4/C nanoparticles (14000 with 92% retention) based electrodes, respectively. The frequency response of the individual electrodes was studied using the electrochemical impedance spectroscopy (EIS) measurement as shown in Figure 5 (i). The high-resolution Nyquist plots in the inset show minimum equivalent series resistance (ESR) and charge transfer resistance (Rct) for ZnFe2O4nanoflakes@ZnFe2O4/C nanoparticles heterostructure, which confirms the pseudocapacitive nature of the electrodes. To further evaluate the supercapacitive performance, a solid-state flexible asymmetric supercapacitor device is fabricated using synthesized ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticles heterostructure thin film as negative electrode (high voltage window -1.15 to 0.0 V) and rGO as positive electrode (high specific capacitance of 583 F g-1 at 5 mA cm-2 current density Figure S1) using PVA-KOH solid gel electrolyte. The schematic and actual photo of the asymmetric device is as shown in Figure S2 (Supporting Information). The performance of the device is evaluated through CV and GCD measurements, as shown in Figure 6. To optimize the performance of the asymmetric device, the charge between positive and negative electrodes should be balanced and the ratio of these two electrodes is calculated to be 4.6 (details in equations 4-6 Supporting Information). Figure 6a presents the CV curves of designed asymmetric device collected at 10 mV s-1 scan rates with operational cell voltage ranging from 1.15 V to 0.5 V; indicate that the fabricated device is stable up to an operational cell voltage of


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1.6 V. Figure 6b shows CV curves at different potential windows (0.0 to 1.6V) which indicating high voltage stability of the asymmetric supercapacitor. Furthermore, Figure 6c displays CV curves of the device recorded at different scan rates (5-100 mV s-1). All of the CV profiles show identical shapes that are indicative of good rate capability and typical supercapacitive properties. Additionally, the galvanostatic charge-discharge (GCD) profiles at various current densities (310 mA cm-2) show identical shape, further confirming the better performance of asymmetric supercapacitor device as shown in Figure 6d. Moreover, the device shows maximum specific capacitance of 347 F g-1 at 3 mA cm-2 current density and the value is substantially larger than that of the previously reported asymmetric devices as shown in Table 1. Figure 6e presents specific capacitance (F g-1) vs. current densities (mA cm-2) plot of asymmetric device which reveals decrease in specific capacitance (347, 301, 281, 267, 250, 212 and 187 F g-1 at current densities 3, 3.5, 4, 4.5, 5, 8 and 10 mA cm-2 ) as current densities increases. The observed higher specific capacitance at low current density can be attributed to the efficient rate of electrolytic ions diffusion and migration into the pores of the active materials. On the other hand, at higher current densities, the rate of diffusion and migration of electrolytic ions is faster which limits their access into the pores of the material thereby reducing the specific capacitance. It is important to note that the coulombic efficiency is influenced by the employed electrolyte. In the case of individual electrode, highly concentrated 6 M KOH electrolyte was used which shows low coulombic efficiency. On the other hand, in the asymmetric supercapacitor device 94% coulombic efficiency is noted wherein two electrode system and solid PVA-KOH gel electrolyte was used which reduces the direct contact with current collectors thereby improving the coulombic efficiency. Furthermore, it is important to note that the main cause of the decrease in coulombic efficiency of the electrode in standard three electrode configuration might be due to


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the high internal resistance of the stainless steel mesh substrate, fast irreversible redox reaction Fe3+/Fe2+ or shuttle effect of ions may be responsible for lowering the coulombic efficiency. In addition, the negative electrode (ZnFe2O4 Nano-flakes@ZnFe2O4/C nanoparticles electrode) operates at the high potential window (>1 V). However, beyond 1 V, unexpected side reactions and mechanical degradation of nanoparticles from the surface of nanoflakes of the electrodes might possibly contribute to minimizing the coulombic efficiency. Table 1 Comparative study of our asymmetric supercapacitor device performance with various asymmetric supercapacitor device performances made by metal oxides and carbon based electrodes. Device Materials

Nanostructure of electrodes

Electrolyte

Polyhedral

6M KOH

Cell voltage (V) 1.6 V

Nanowires

3M NaOH

1.8 V

Nanosheets@N anoparticle Nanorods

PVA-KOH

1.7 V

1 M Na2SO4

2.0 V

Combustion method Hydrothermal

Nanoparticles

1M KOH

1.5 V

Nanorods

PVA-KOH

1.25 V

Fe2O3/rGO//Mn3O4//r GO ZnFe2O4//ZnFe2O4

Hydrothermal

Nanoparticles

3M KOH

1.8 V

SILAR

Nanoplate

PVA-LiClO4

1.0 V

Fe2O3PEDOT//Fe2O3PE DOT

Sonochemical method

Nanoparticles

PVA-H2SO4

1.0 V

ZnFe2O4//Ni(OH)2

R-CBD

Nano-flakes

6M KOH

1.6 V

ZnFe2O4//Mn3O4

SILAR

Nanospheres

6M KOH

1.6 V

ZnFe2O4@ZnFe2O4/C// rGO

Bio mediated R-CBD

Nanoflakes@N anoparticles

PVA-KOH

1.6 V

Co3O4//Carbon CoO@Ppy//AC MnO2/FeCo2O4//MnO2 /FeCo2O4 Fe3O4@Fe2O3//Fe3O4@ MnO2 CoFe2O4//rGO NiO//Fe2O3

Synthesis Methods Solution approach Solution approach Hydrothermal Hydrothermal

Specific capacitance

Cycle stability with retention

101 F g-1 at 2 A g-1 4.43 mF cm-2 at 1 mA cm-2 2.56 mF cm-2 at 2 mA cm-2 1.49 mF cm-2 at 1.25 mA cm-2 38 F g-1 at 3 mA cm-2 57 F g-1 at 4 mA cm-2 178 F g-1 at 1 mV s-1 32 F g-1 at 1 A g-1 167 F g-1 at 10 A g-1

2000 cycle, retention 89% 20000 cycles retention 91% 1500 cycles, retention 94% 5000 cycles, retention 92 % 3000 cycles, retention 67% 10000 cycles, retention 85% 10000 cycle, retention 83% 1000 cycles, retention 66 % 1000 cycles, retention 98.5% 8000 cycles, retention 88% 3000 cycles, retention 74% 35000 cycles, retention 98%

118 F g-1 at 5 mA cm-2 81 F g-1 at 2 mA cm-2 347 F g-1 at 1 mA cm-2

Energy density (Wh kg-1) 36 Wh kg-1

Ref. No.

43.5 Wh kg-1

50

93 Wh kg -1

51

26.6 Wh kg-1

52

12 Wh kg-1

53

12.4 Wh kg-1

54

0.056 Wh cm-2 4.5 Wh kg-1

55

136 Wh kg-1

57

42 Wh kg-1

15

28 Wh kg-1

16

81 Wh kg-1

Prese nt Work

49

56

In addition, the Figure 6f presents CV profiles (100 mV s-1 scan rate) of asymmetric supercapacitor device at various bending angles of 30o, 60o, 120o, 150o, and 180o, respectively. All the CV curves were consistent and did not show any significant change with similar specific


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capacitances and negligible variation under the different bending state as shown in Figure S10 (Supporting Information). Furthermore, the asymmetric device exhibits extraordinary and ultra long cycle stability by retaining 98% of its original capacitance over 35000 cycles as shown in Figure 6g. This long-term cycle stability is attributed to the heterostructures of ZnFe2O4 nanoflakes@ZnFe2O4 nanoparticles and rGO thin films, which provides large surface area and a maximum number of electrochemical reaction sites. We explored to improve the surface area and electrical conductivity of ZnFe2O4 electrode by anchoring the ultrafine carbon nanoparticles over 3D nanoflakes. These smaller carbon nanoparticles not only enhance the surface area but also the electrical conductivity. The ultrafine ZnFe2O4 and carbon particles get uniformly anchored onto the 3D ZnFe2O4 nanoflakes. The ultrafine nanoparticles increase the surface area and also shorten the ion diffusion length of the electrode materials, which enhances the energy storage efficiency similar to that in the case of core-shell materials. Due to this type of ultrafine nanoparticles, the decomposition of nanoflakes after long-term cycle stability is almost reduced or it is negligible and hence ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles electrode improves the cycle stability of the electrode materials. In addition, the electrical conductivity (Fig. S14, supporting information) of heterostructure electrode also improves as compared to individual nanoflakes and nanoparticle based electrode. These ultrafine nanoparticles increase the surface area and also shorten the ion diffusion length of the electrode materials which enhance the energy storage efficiency. One more advantage of this type of anchoring of ultrafine nanoparticles over nanoflakes is it strongly reduces the decomposition of 3D nanoflakes after long-term cycle stability and hence ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles electrode improving the cycle stability of electrode materials. The inset of Figure 6g shows the last 15 cycles which support the long-term cycle stability. This indicates that the device possesses


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excellent structural integrity even on bending. These exciting results show its fascinating mechanical flexibilities and potential for flexible energy storage devices. In addition, the FESEM images of ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticles heterostructure thin film were recorded after long term 18000 cycles to investigate stability and are presented in Figure 6h and Figure S11 (a-b), respectively. The images at low and high magnifications revealed intact heterostructures being maintained with a small detachment of nanoparticles on the surface. There is the possibility of polarization and distortion in the ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles electrode which might be attributed to the impurity induced redox reactions. i. e. impurities like unreacted functional groups of biomolecules in the carbon nanoparticles and shuttle effect of Fe2+ or Fe3+ ion contributed from the highly conducting stainless steel substrate which is annealed at 550 oC. Interestingly, no destruction of nanoflakes is noted which formed the base of the heterostructure, thus supporting the fact that the heterostructure electrodes demonstrate long-term cycle stability. To further understand the supercapacitive properties of the device, electrochemical impedance spectroscopy (EIS) was studied. The Nyquist plot and Bode plots for the two electrode system are presented in Figure S12 (Supporting Information). The Nyquist plot was made up of two parts, one is quasi-semicircle and another one is a straight line in the high and low-frequency regions, respectively. The quasi-semicircle represents an electron-transfer-limited process, while the straight slope could be attributed to the diffusion limited electron-transfer process. Additionally, the fitted equivalent circuit shows smaller solution resistance (Rs), charge transfer resistance (Rct) and Warburg resistance (Rw), respectively. This clearly reveals that the conductivity of ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles can be greatly improved by carbon nanoparticles in the heterostructures. The analysis of Bode plot demonstrates smaller


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phase angle (-35o) and relaxation time (τ) 0.1 s confirming excellent energy storage properties of asymmetric supercapacitor device as shown in Figure S12 (b). Self-discharge measurement of our asymmetric device is also studied and the device shows small voltage decay due to leakage current. In self-discharge process, the device is fully charged to Vmax (1.6 V) as shown in Figure S13 (Supporting Information). The device took more than 15 h to drop to 1.2 V, which is better than that reported earlier for carbon-based ASC devices.

Figure 6 (a) CV profiles of ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle and rGO at scan rates of 10 -1

-1

mV s (b) CV profiles at various potential voltage (V) (c) CV profiles at various scan rates (5-100 mV s ) -2

within 0.0 V to 1.6 V cell voltage (d) Charge-discharge profiles at various current densities (3-10 mA cm ) -1

-2

within 0.0 V to 1.6 V cell voltage (e) Plot of specific capacitances (F g ) vs. current density (mA cm ) (f)


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o

o

o

o

o

CV profiles at various bending angles (30 , 60 , 90 , 120 and 180 ) (g) Cycle stability over 35000 cycles with 98% capacitance retention (Inset shows last 15 cycles) (h) High magnification FESEM image after 18000 cycle stability of SSM/ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle//rGO nanoparticles/SSM).

Figure 7a shows the GCD profiles of single and two asymmetric supercapacitors connected in a series. The two cells in series show a maximum of 3.2 V potential at a same current density of 4.5 mA cm-2 indicating that the capacitive performance of each device is well maintained. The overall performance of the asymmetric supercapacitor device is investigated by calculating the energy density (Wh kg-1) and power density (kW kg-1) and is presented in Ragone plot (Figure 7b) (details of equations 3, 4 Supporting Information). Interestingly, the asymmetric device delivered a maximum energy density of 81 Wh kg-1 at a power density of 3.9 kW kg-1, indicating excellent energy storage ability of the device. Even at a very high power density of 13.0 kW kg-1, the device delivered the energy density of 44 Wh kg-1.

Figure 7 (a) GCD profiles of single and two supercapacitor devices connected in series (b) Ragone plot -1

-1

of Energy density (Wh kg ) vs. Power density (kW kg ) of fabricated flexible (SSM/ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle//rGO/SSM) asymmetric supercapacitor device (c) Actual photos of


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ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticle electrode at various flexibility states (d) Actual photos of 2 cm x 1 cm electrodes and two devices powering the white LED.

These obtained values are significantly higher than the earlier reports in the literature such as, FeCo2O4//FeCo2O4 (30.9 Wh kg-1),58 Fe2O3/ GH//GH-III (25.6 Wh kg-1),59 MnO2@Fe2O3//AC (43 Wh kg-1),60 ZnFe2O4//ZnFe2O4 (4.5 Wh kg-1),56 ZnFe2O4//Ni(OH)2 (33 Wh kg-1),61 ZnFe2O4//Mn3O4 (28 Wh kg-1),16 ZnFe2O4//Ni(OH)2 (42 Wh kg-1),15 CoFe2O4//rGO (12.5 Wh kg1 53

),

and PEDOT@MnO2//C@Fe3O4 (43 Wh kg-1).62 The mechanical flexibility of both

individual ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticles heterostructure and rGO electrodes (positive electrode) is presented using actual photos (Figure 7c) in different states (straight, bent and twist) indicating the good flexibility of the electrodes with high surface area. Furthermore, two asymmetric supercapacitor cells in a series are able to glow white LED (3.0 V) (Figure 7 (d)) after charging only for 30 s. In this way, the solid-state flexible asymmetric supercapacitor device is made by in-situ derived and self-assembled ZnFe2O4 nano-flakes@ZnFe2O4/C nanoparticles heterostructure and rGO nanoparticles which show excellent performance for both single electrode and the device. CONCLUSIONS In summary, an alternative and attractive in-situ green strategy is reported for the first time, which remarkably boosts the energy storage properties and ultralong-term cycle stability of selfassembled ZnFe2O4 nano-flakes@ZnFe2O4/C heterostructures thin films. Compared to a pristine ZnFe2O4 nano-flakes electrode and a ZnFe2O4/C nanoparticles electrode, the ZnFe2O4 nanoflakes@ZnFe2O4/C heterostructures thin films based electrode exhibit an enhanced specific capacitance of 1884 F g-1 at a high current density of 5 mA cm-2. Moreover, the ZnFe2O4 nanoflakes@ZnFe2O4/C nanoparticle heterostructures thin films electrode shows exceptionally long-


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term cycle stability with retention of more than 99% even after 18000 cycles, is the best cycling stability achieved so far by iron oxide (Fe2O3, Fe3O4, and MFe2O4) based electrodes. Additionally, flexible high-performance solid-state asymmetric supercapacitor device based on the ZnFe2O4 nano-flakes@ZnFe2O4/C heterostructures thin films electrode as the negative and rGO nanoparticles electrode as the positive is fabricated and the device performance demonstrated a high specific capacitance (347 F g-1) and a high energy density of 81 Wh kg-1 with an impressive rate capability. Furthermore, an asymmetric device exhibited extraordinary long life cycle stability by showing only 2% loss of its initial capacitance (98% retention over 35000 cycles). This demonstration of the development of stable, high capacitive and high energy ZnFe2O4 nano-flakes@ZnFe2O4/C heterostructures based electrodes offers new opportunities for iron oxide materials in constructing the high-performance asymmetric device. Interestingly, the reported in-situ bio-mediated green approach is not only applicable for the preparation of ZnFe2O4 heterostructures but would also be applicable to prepare other metal oxides heterostructures such NiCo2O4, ZnCo2O4, CoFe2O4, NiFe2O4, NiO, Co3O4, MnFe2O4, and MnO2 etc. and the work in that direction is underway. ASSOCIATED CONTENT Supporting Information: Equations related to specific capacitance, energy density and power density, schematic of Bengal gram bean extract, pectin composition, and metal-pectin complex. Electrochemical properties of rGO are also included and FESEM images at various concentration of BGBE extracts, XRD, Raman, TGA/DTA, XPS, EDS pattern and BET analysis of individual electrodes. Nyquists and Bodes plot along with self-discharging plots of asymmetric devices is included.


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AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected] * Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Authors are thankful to UGC, New Delhi (F. No. 41-249/2012 (SR), UGC-SAP and DSTFIST, PURSE for financial support and instrument facilities at the Department of Chemistry, Shivaji University, Kolhapur. Author MMV is thankful for UGC-BSR Meritorious Students Fellowship. JYC is thankful to Ministry of Science and Technology, R.O.C. Contract No. 1022628-M-011-001-MY3. REFERENCES (1)

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TOC


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C Nanoparticles on 3D ZnFe2O4

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