Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9

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Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage Ali Salehabadi a, Masoud Salavati-Niasari a,*, Tahereh Gholami b, Asma Khoobi a a Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran b Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Islamic Republic of Iran

article info

abstract

Article history:

Although the technology of hydrogen energy heightened gradually, the application of bi-

Received 11 January 2018

nary metal oxides as a host for hydrogen sorption has not been widely established. Here we

Received in revised form

show, with a facial combustion method, the formation of Dy3Fe5O12 and DyFeO3 nano-

1 April 2018

structures with maximum average particle sizes ranging from 25 to 30 and 16e18 nm,

Accepted 3 April 2018

respectively. The physical properties of the samples were served which further reflect in

Available online xxx

hydrogen storage properties. The discharge capacities of Dy3Fe5O12 and DyFeO3 nanoparticles were obtained at 2000 and 2100 mA h/g, respectively. The hydrogen storage

Keywords:

properties

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Magnetism

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© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Hydrogen storage Dy3Fe5O12 DyFeO3 Nanostructures

Introduction The hydrogen storage technology is emerging from a variety of fuel cell applications such as stationary, mobile and portable power devices, but some deficiencies exist especially in the case of infrastructure deterioration of successful transition to the ‘Hydrogen Economy’ [1e4]. Hydrogen is

“secondary energy resources” which can be obtained from various sources such as biological, wind, water thermolysis, water electrolysis, water photosplitting, reforming of fossil fuels and methanol [5]. Hydrogen can be stored either by chemical storage [6,7] or by physisorption [8,9]. In chemical storage, the hydrogen generates through a chemical reaction. The most common examples include ammonia [10], metal hydrides [11,12],

* Corresponding author. E-mail address: [email protected] (M. Salavati-Niasari). https://doi.org/10.1016/j.ijhydene.2018.04.018 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Salehabadi A, et al., Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.018

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formic acid [13], carbohydrates [14], and liquid organic hydrogen carriers (LOHC) [15]. In hydrogen storage by physisorption, hydrogen molecules are adsorbed on the surface of the materials. The porous materials include carbon materials [16,17], zeolites [18], metal organic frameworks (MOFs) [19e21], covalent organic frameworks (COFs) [22], microporous metal coordination materials (MMOMs) [23], and organotransition metal complexes [2]. These are the most widely studied materials capable of surface sorption. Binary metal oxides (BMOs) with unique structures and properties have been reported for hydrogen energy storage [24e30]. BMOs are capable of adsorbing hydrogen due to the formation of stable chemical bonds between protons and oxygen as well as its structural defects. Perovskite and garnet-type materials are two sorts of BMOs which are adopted by materials of stoichiometry ABO3 and 0 0 M3M 2(XO4)3 (M and M are normally di- and tri-positive cations and X includes Si, Al, Ga, Ge, and Fe), respectively [31e35]. Dysprosium iron perovskite (DyFeO3) and garnet (Dy3Fe5O12) have been synthesized and reported for different electric and magnetic industrial applications. For example, DyFeO3 is known as multiferroics (MF) materials suggesting that the long-range magnetic and dipolar orders coexist [36]. Various synthesis methods are reported for preparation of DyFeO3 and Dy3Fe5O12 nanoparticles. Solid-state [37], sol-gel [38], and pulsed laser deposition [39] are the most renowned methods for synthesis of DyFeO3 nanoparticles, while Dy3Fe5O12 nanoparticles are synthesized through hydrothermal [40], ball-mill [41], and glycine assisted combustion [42] methods. Due to the structural features and physical properties of DyFeO3 and Dy3Fe5O12, these two materials can be used as hosts for hydrogen sorption, but the mentioned properties have not been directly highlighted. Although the technology of batteries have been widely adopted in vehicle, hand tools, and electric devices, but the reversible capacities of the materials used in these systems are low [43]. Therefore, it is critical to develop new type of hydrogen storage materials such as DyFeO3 and Dy3Fe5O12 which have improved electrochemical performance as well as lower cost. Here, a facial solution combustion method using a single fuel (maltose) was performed in order to attain rare-earth binary metal oxides such as Dy3Fe5O12 and DyFeO3. Primarily, the formation of nanoparticles and the respective magnetic properties were perused. Moreover, as the major objective of this work, the electrochemical hydrogen storage properties of as-synthesized nanoparticles were examined and compared.

Synthesis of dysprosium iron (III) oxides Both Dy3Fe5O12 and DyFeO3 are synthesized through a combustion method in the presence of a carbohydrate fuel (Maltose) in the stoichiometric amount of Dy(NO3)3.5H2O and Fe(NO3)3$9H2O. For the preparation of Dy3Fe5O12, appropriate amount of Dy(NO3)3.5H2O and Fe(NO3)3$9H2O were separately dissolved in 20 ml distilled water, followed by dissolution of the appropriate ratio of fuel (Dy3þ:fuel ¼ 1:2) in Dy(NO3)3.5H2O container. For preparation of DyFeO3, the stoichiometric ratio (1:1) of each cationic sources were primarily dissolved in distilled water and then maltose was added (Dy3þ:fuel ¼ 1:2). The above samples were both stirred at 70  C for 30 min. As dissolution completed and a clear brownish solution was obtained, the temperature of the heating system was increased in order to obtain a viscose solution. Concurrently, owing to the exothermic nature of a self-propagating combustion reaction, the temperature was suddenly exceeded to 200  C. As a result, NO2 gases evolved and fluffy materials formed. The obtaining materials were grinded and finally calcined at 750  C for 4 h. The schematic representation of preparation route is presented in Fig. 1. Similar trends were followed for the preparation of Dy3Fe5O12 and DyFeO3 nanoparticles except stoichiometric ratio of cationic sources.

Characterization The structural analysis of the samples was characterized using a X-ray diffractometer (Rigaku D-max C III) in CuKa (k ¼ 1.5418
A) radiation source. The XRD data for indexing and cell-parameter were collected in an incident radiation angle of 10e80 . The morphology of the nanopowders was determined using TESCAN (MIRA3) Field Emission Scanning Electron Microscopy (FE-SEM). Prior to morphological observations, the samples were coated with gold. Elemental analysis was carried out using an Energy Dispersive X-ray Spectroscope (EDX Hitachi S-4300). Moreover, a Transmission Electron

Experimental Materials Dysprosium Iron (III) Oxides were synthesized via a solutioncombustion method using maltose as a single fuel. The starting materials were all supplied in analytical pure grade. Dy(NO3)3.5H2O and Fe(NO3)3.9H2O were purchased from AlfaAesar (Mw ¼ 438.59 g mol1, mp ¼ 88.6  C) and SigmaAldrich (Mw ¼ 404 g mol1, mp ¼ 47  C), respectively.

Fig. 1 e Schematic representation of Dy3Fe5O12 and DyFeO3 nanostructures synthesis route.

Please cite this article in press as: Salehabadi A, et al., Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.018

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Microscope (TEM) was performed - Philips CM30 TEM instrument e which operated at an accelerating voltage of 120 kV. Spectroscopic analysis of the samples was carried out using a Fourier Transform Infrared (FTIR) Shimadzu Varian 4300 spectrophotometer in KBr pellets in the range of 4000e400 cm1. The magnetic properties of the samples were investigated using a Vibrating Sample Magnetometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran) at room temperature. The cyclic voltammograms (CV) of the samples were obtained from a probe solution of K3(Fe(CN)6)/K4(Fe(CN)6) in the 0.1 M phosphate buffer solution (pH 7.0) in a scan rate of 0.1 V1. Three electrodes setup were arranged in order to analyze the discharge capacities of the samples. The electrochemical cell was adjusted at room temperature in 6 M KOH electrolyte. In the three electrodes setup, the current flows between the counter electrode (CE) and the working electrode (WE), and the potential difference measures between the reference electrode (RE) and the sample. The electrochemical measurements were conducted using a porous copper sheet (surface area ~1  1 cm2). The porous copper sheet was coated without any glue or binder. The metal oxides were primarily dispersed in absolute alcohol and then coated on the surface of the copper sheet, were finally dried and pressed. All electrochemical tests were carried out in 1 mA at room temperature. The discharge capacities of the samples were calculated using Eq. (1); SC ¼ Itd =m

(1)

where, I, td and m are charge/discharge current (mA), discharge time (hours) and active mass (g), respectively. All obtaining results were normalized, accordingly.

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Results and discussion Structural analysis and magnetic properties A wealth of useful and accurate information about the bulk structure of the samples is obtained by X-ray diffraction (Fig. 2). Phase identification and determination of lattice parameters show that the major phases of the samples are wellmatched to JCPDS 23-0237 (Dy3Fe5O12) and 89-6645 (DyFeO3); however, a small quantity of impurities are assigned to each composition. A cubic crystal system with Ia3d space group is defined for garnet-type oxides, while an orthorhombic Pnma space group is reported for perovskite-type oxides. The major phases in the annealed Dy3Fe5O12 consist of diffraction peaks at 2q equal to 27.09 (4 0 0), 32.48 (4 2 0), 35.66 (4 2 2), 53.42 (6 4 0), 55.59 (6 4 2) and 75.19 (8 4 2). Alongside, a minor phase of DyFeO3 can be observed at 26.24 , 34.10 , 48.95 , 47.90 . The XRD pattern of the annealed DyFeO3 reveals that the DyFeO3 holds a dual-phase structure, composed of a main phase of DyFeO3 and a minor phase of Fe2O3. The diffraction peaks of DyFeO3 are well-fitted to 2q equal to 32.44 (2 0 0), 33.37 (1 2 1), 35.78 (0 0 2). The average crystallite sizes in all the compositions were determined using Scherrer equation (Eq. (2)); L ¼ 0:9l=bcos q

(2)

where l, b, and q are the wavelength of radiation, the width of the XRD peak at its half maximum intensity (FWHM), and the Bragg diffraction angle, respectively. From this equation, the average crystallite sizes of Dy3Fe5O12 and DyFeO3 nanoparticles calculate to be around 25.3 and 17.5 nm, respectively.

Fig. 2 e XRD patterns of Dy3Fe5O12 and DyFeO3 nanostructures calcined at 750  C.

Please cite this article in press as: Salehabadi A, et al., Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.018

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Fig. 3 e FTIR spectra of (a) Dy3Fe5O12 and (b) DyFeO3 nanoparticles.

FT-IR spectra of the samples are shown in Fig. 3. A series of strong bands of metal-oxygen (M-O) stretching vibrations and oxygen-metal-oxygen (O-M-O) bending between 400 and 800 cm1 can be observed in Dy3Fe5O12 nanostructures (Fig. 3a). Two bands at 558 and 442 cm1 are assigned to Fe-O

stretching vibration. It is important to mention that the symmetrical stretching vibration of Dy-O is also formed at around 558 cm1. A broad absorption at around 3432 cm1 attributes to stretching vibration of the hydroxyl group. In the FTIR spectrum of DyFeO3 (Fig. 3b), a band at about 3440 cm1 ascribes to the stretching modes and H-O-H bending vibration of the free or absorbed water moisture from the atmosphere. The absorption bands in the frequency range of 400e800 cm1 correspond to metal-oxygen (Dy-O, Fe-O) and metal-oxygen-metal (M-O-M) stretching vibrations. Moreover, a vibration frequency at 607 cm1 corresponds to Fe3þd O2 in tetrahedral site [40]. Moreover, the presence of small double shoulder peaks at 540 (overlapped) and 433 cm1 are known as a fingerprint of Fe2O3 [44]. The morphologies and the particle sizes of the materials are shown in Fig. 4. The FESEM micrographs clearly indicate that the Dy3Fe5O12 nanoparticles are formed inhomogeneously with irregular shapes (Fig. 4a). However, the particles are agglomerated in some areas. The chemical composition of the Dy3Fe5O12 nanoparticles is determined by the energy dispersive X-ray (EDX) spectroscopy (Fig. 4a: inset). The elemental analysis confirms the presence of Dy, Fe, and O in this composition. The surface morphology of

Fig. 4 e FESEM micrographs and EDX spectra of (a) Dy3Fe5O12 and (b) DyFeO3 nanoparticles. Please cite this article in press as: Salehabadi A, et al., Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.018

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the DyFeO3 (Fig. 4b) is composed of the particles with a narrow distribution and approximately congruent shape and size. The respective EDX spectrum affirms the presence of Dy, Fe, and O in its appropriate energy levels (Fig. 4b: inset). TEM investigations have also provided insight into the morphology of the samples (Fig. 5). TEM images clearly reveal that both Dy3Fe5O12 and DyFeO3 particles are spherical, with a homogeneous texture. The particle sizes of Dy3Fe5O12 nanoparticles are slightly larger than DyFeO3. The former particle diameters are in the range of 10e35 nm (Fig. 5a) and the later are in the range of 10e26 nm (Fig. 5b). The most of the particles of Dy3Fe5O12 and DyFeO3 are in the range of 25e30 nm and 16e18 nm, respectively. In order to understand the magnetic properties of the samples, the samples were placed in a uniform magnetic field and their electrical signals collected using a vibrating sample magnetometer (VSM). Fig. 6 shows typical magnetization curve (M-H curve) of Dy3Fe5O12 and DyFeO3 nanoparticles at room temperature. The results indicate that the samples are ferromagnetic at room temperature. The hysteresis loop of Dy3Fe5O12 nanoparticles (320 to þ382 Oe) is much larger than DyFeO3 nanoparticles (63 to þ5.4 Oe). Larger coercive force can retain the large fraction of the saturation field as driving force removed. On the other hand, narrow hysteresis loop implies a small dissipated energy [45]. It is reported that the

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DyFeO3 is a multiferroic material (coupling of magnetic and electric order parameters) with strong magnetoelectric effect [38,46].

Electrochemical hydrogen storage performances Primarily, using a Cyclic Voltammetry (CV), the electrochemical performances of the samples were analyzed in order to investigate the electrochemical characteristics of the electrodes. Furthermore, the galvanostatic charge-discharge curves of the samples were recorded to calculate the hydrogen storage capacities in a three-electrode setup, the working electrode, the reference electrode (Ag/AgCl), and the counter electrode (Pt). Fig. 7 shows the CVs of GCE, Dy3Fe5O12, and DyFeO3 nanoparticles at operating potential of 0.3e0.8 V. The voltammograms were obtained in a probe solution of K3(Fe(CN)6)/ K4(Fe(CN)6) in the 0.1 M phosphate buffer solution in a constant scan rate of 0.1 V1. It is obvious that the electrochemical performance of GCE is superior as compared to Dy3Fe5O12, and DyFeO3. The cyclic voltammogram of GCE (blank) represents a couple of well-defined redox peaks. It is obvious that after coating the surface of the GCE with dysprosium iron (III) oxide nanoparticles, the peak currents of the ferri/ferrocyanide redox couple are decreased. The anodic peak currents (Ipa) of the glassy carbon electrode (GCE) is around 49.12 mA, higher

Fig. 5 e TEM images and particles size histograms of (a) Dy3Fe5O12 and (b) DyFeO3 nanoparticles. Please cite this article in press as: Salehabadi A, et al., Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.018

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With constant applied scan rate, the separation between cathodic (Epc) and anodic (Epa) peak potentials of DyFeO3 are larger than Dy3Fe5O12 (DE DyFeO3>DE Dy3Fe5O12). The glassy carbon electrode (GCE) itself has Epc and Epa of 0.11 V and 0.28 V, respectively. The responses of the electrodes are summarized in Table 1. Fifteen cycles discharge curves of the samples are shown in Fig. 8. Discharge performance of the materials plays an important role in batteries technology. In metal oxides, generally a two-step reaction can be proposed during the electrochemical charging/discharging processes. The first step arises on the surface of the particles over a few atomic layers of the materials, while the second step betides inside the particles i.e. the diffusion of H-atom. In a typical mixed metal oxide, the generated hydrogen from aqueous solution adsorbs on the surface of the sample. The overall reaction mechanism for a mixed metal oxides (▪) can be proposed on the basis of the following reaction Eq. (3);

Fig. 6 e M-H curves of (a) Dy3Fe5O12 and (b) DyFeO3 nanoparticles (inset: localized M-H curve of (b)).

▪þxH2 O þ xe 4▪  Hx þxOH

(3)

During the charging process, the electrolyte dissociates (Eq. (4)) and the sample adsorbs hydrogen. The same mechanism has been reported for various structures of nanoscale binary metal oxides [29,49,50]. H2 O þ e /H þ OH

Fig. 7 e Cyclic voltammograms of GCE, Dy3Fe5O12, and DyFeO3 nanoparticles.

than Dy3Fe5O12 (35.48 mA) and DyFeO3 (41.64 mA) nanoparticles, while the cathodic peak currents (Ipc) of GCE, Dy3Fe5O12, and DyFeO3 are located at 48.92, 35.52 and 41.78 mA, respectively. Any change in the electrode behavior can be due to the presence of nanoparticles at the surface of the GCE. Depression in the peak currents suggests that the surface of the GCE is blocked by dysprosium iron (III) oxides, which caused by the diffusion limitation of the redox couples. A comparison between the CVs of the samples (Dy3Fe5O12, and DyFeO3) indicates inherent features of the ferri/ferrocyanide redox, such as enhanced porosity of the DyFeO3 nanoparticles [47,48].

Triple flat potential plateaus in the discharge pattern of the Dy3Fe5O12 nanoparticles (Fig. 8a) can be due to the multiple hydrogen adsorption sites [2], which generated owing to the special structural defects, roughness, bonding, etc [51]. In the discharge curves of DyFeO3 nanoparticles, a flat and long potential plateau can be observed due to the formation of stable chemical bonds between protons and oxygen in the oxide (Fig. 8b). It is reported that the vacancies in protonconductive perovskite-type oxides could be replaced by protons, which resided on oxygen ions to form substitution OH ion defects [43]. Fig. 8c shows the discharge capacity of the uncoated copper sheet. The initial discharge capacity of the copper electrode is at 0.2 mA h/g, while after 15 cycles this profile enhanced to 0.75 mA h/g. It can be concluded that the oxidation of copper sheet (substrate) does not affect the overall discharge capacities of the coated copper sheets. The maximum discharge capacity of the Dy3Fe5O12 and DyFeO3 nanoparticles can be observed at 2000 and 2100 mA h/g after 15 cycles, respectively. As previously mentioned, the mechanism of hydrogen adsorption is mostly physisorption; however, due to the presence of redox species, additional reaction mechanism can be also proposed [28,43,52] as Eq. (5); ðIIIÞ FenðIIÞ Oz  Hx þxOH Dyx FeyðIIIÞ Oz þxH2 O þ xe 4 Dyx Feyn

Table 1 e Electrode responses of the Dy3Fe5O12, DyFeO3 and GCE from CV observations. Response

Ipa (mA)

Ipc (mA)

Epa (V)

Epc (V)

35.48 41.64 49.12

35.52 41.78 48.92

0.31 0.35 0.28

0.08 0.06 0.11

Electrode Dy3Fe5O12 DyFeO3 Glassy carbon

(4)

(5)

In the discharge profiles, the discharge capacities enhanced gradually with increase in cycle numbers. As the reaction proceeds, more hydrogen atoms incorporated into suitable interstices, therefore, various hydrogen adsorption sites generates in the working electrode [53]. The hydrogen storage in mixed metal oxides is a step-wise electrochemical process, manifesting different potential plateaus in the charge

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Fig. 8 e Fifteen cycles discharge sequences of (a) Dy3Fe5O12, (b) DyFeO3 nanoparticles and (c) copper sheet (blank).

and discharge profiles. A step-wise electrochemical process produces various distinct sites for hydrogen sorption; on the surface, into the nanopores of bulk and/or into the interlayer [54]. Comparing electrochemical discharge capacities of the asreported mixed metal oxides have revealed enhanced discharge capacities of dysprosium iron (III) oxides. For instance, in our previous reports, the hydrogen storage performances of spinel BaAl2O4 [49], polycrystalline Sr3Al2O6 [29], spinel NiAl2O4/NiO/TiO2 nanocomposites [30], and Dy3Al5O12 nanogarnets [55] were obtained at around 1000 mA h/g, 2500 mA h/g, 3000 mA h/g, and 3137 mA h/g, respectively. It is renowned that the structure and the morphology play crucial roles in hydrogen storage performance of the host materials. In a study on hydrogen storage proton-conductive perovskites (ACe1-xMxO3-d; A ¼ Sr or Ba, M ¼ rare earth element), Esaka and his coworkers [56] reported that the cerium itself can charge/discharge some hydrogen, due to the reducible characteristic of cerium (Ce4þ / Ce3þ). They proposed a reaction mechanism as Eq. (6); ACe4þ 0:95 M0:05 O3d þxH2 O þ xe 3þ  1þ 4ACe4þ 0:95x Cex M0:05 O3d Hx þxOH

(6)

According to another conceptual framework, in the presence of Fe2O3, subsidiary hydrogen sorption reaction mechanisms can be proposed [52]. A charging process via Eqs. (7) and (8);

Fe2 O3 þ2Hþ þ2e / FeðOHÞ2 þ FeO

(7)

FeO þ 2Hþ þ2e /Fe þ H2 O

(8)

and discharging mechanism through Eqs. (9)e(11); Fe þ 2H2 O  2e /FeðOHÞ2 þ2Hþ 3FeðOHÞ2  2e / Fe3 O4 þ2H2 O þ 2Hþ

(9) (10)

or FeðOHÞ2  e /FeOOH þ Hþ

(11)

Conclusion In the current study, nanoscale dysprosium iron (III) oxides were successfully synthesized via a solution combustion method using maltose at 750  C. The structural and morphological observations were confirmed the formation of Dy3Fe5O12 and DyFeO3 nanoparticles alongside with small quantities of DyFeO3 and Fe2O3, respectively. Slightly higher electrochemical performances of DyFeO3 nanoparticles (as observed in its CV results) were observed compared to Dy3Fe5O12 nanoparticles. The CV results were further reflected in the hydrogen storage performance of the samples, i.e. higher discharge capacity of DyFeO3 nanoparticles.

Please cite this article in press as: Salehabadi A, et al., Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.018

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Impressive discharge capacity profiles were indicated that the Dy3Fe5O12 and DyFeO3 nanoparticles are both suitable for Hsorption. It must be mentioned that further experimental and computational investigations are required in order to account for electrochemical hydrogen storage mechanisms especially for binary metal oxides.

Acknowledgement Authors are grateful to the council of Iran nanotechnology initiative council, Iran National Elites Foundation, Iran National Science Foundation (INSF) and University of Kashan for supporting this work by Grant No (159271/8990).

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Please cite this article in press as: Salehabadi A, et al., Dy3Fe5O12 and DyFeO3 nanostructures: Green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.018