Electrochemical hydrogen storage properties of Ce0.75Zr0.25O2 nanopowders synthesized by sol-gel method

Journal of Alloys and Compounds 790 (2019) 884e890

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Electrochemical hydrogen storage properties of Ce0.75Zr0.25O2 nanopowders synthesized by sol-gel method Ali Salehabadi a, *, Mardiana Idayu Ahmad a, Norhashimah Morad a, Masoud Salavati-Niasari b, Morteza Enhessari c a b c

Environmental Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800, Minden, Penang, Malaysia Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, P.O. Box 87317-51167, Iran Department of Chemistry, Naragh Branch, Islamic Azad University, Naragh, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 October 2018 Received in revised form 12 February 2019 Accepted 9 March 2019 Available online 11 March 2019

Solid-state hydrogen storage technology currently suffers from issues such as inadequate storage capacity, and instability, leading to low performances. Here we show a record of above 2000 mAh.g
1 (~7.8 wt% H) of discharge capacity of a binary metal oxides Ce0.75Zr0.25O2 nanopowders. The yellowish Ce0.75Zr0.25O2 nanopowders have been synthesized via a sol-gel method under thermal treatment of the molecular precursors of the, (NH4)2Ce(NO3)6 and C16H40O4Zr. The crystal structure and morphology evolution can be described by the cubic and highly pure structure and homogeneous nanoscales formation of Ce0.75Zr0.25O2, ranging from 35 to 60 nm. The formation of the Ce0.75Zr0.25O2 nanoparticles distinctly matches with the spectroscopy results. The activation energy (Ea) was calculated from the output of a reduction thermal programming profile at about 177 kJ mol
1 using Kissinger equation. Our work will promote the development of low-cost solid-state semiconductors based on transition metals, as a host for hydrogen sorption. © 2019 Elsevier B.V. All rights reserved.

Keywords: Nanomaterials Kinetic Kissinger Hydrogen storage Ce0.75Zr0.25O2

1. Introduction The shortage of fossil energy resources demands new energy carriers in the near future. One of the recent challenges in the automotive industry and academia is to discover and develop alternative fuel concepts for cars [1]. Cryo-compressed technology, which is a combination of compressed and physical hydrogen storage, is currently used in road vehicles; however, it has an insufficient cruising range [2]. Hydrogen is the most abundant and clean element of the universe. The gravimetric energy density of hydrogen is about 33.33 kWh/kg, which is much higher than gasoline (with approximately 12.7 kWh/kg), however, hydrogen has low volumetric energy density (~0.77 kWh/l). Therefore, a novel technique is urgently required in order to store huge volume of the hydrogen in a small area [3]. Solid-state hydrogen storage technologies are recently developed for storing hydrogen. Metal hydrides and complex hydrides

* Corresponding author. E-mail address: [email protected] (A. Salehabadi). https://doi.org/10.1016/j.jallcom.2019.03.160 0925-8388/© 2019 Elsevier B.V. All rights reserved.

are generally interstitial hydrides or hydrides of intermetallic compounds which are extremely suitable for hydrogen sorption. In alloys, the hydrogen atoms are located in interstitial sites among metal atoms and form metal hydrides. The electrochemical application of metal hydrides in hydrogen sorption can be considered as Eq. (1) [4]: MHx þ NiOOH 4 MHx-1 þ Ni(OH)2

(1)

The complex hydrides (known as borohydrides), well-known as potential solid-state hydrogen storage materials, have superior gravimetric hydrogen density as compared to metal hydrides [5]. Here, the hydrogen is covalently bound, therefore the hydrogen cannot be removed easily, unless due to the degradation of the whole compound. The mechanism of hydrogen sorption and desorption of the complex hydrides have not yet been determined, but it is theoretically known that the hydrogenation occurs by the formation of covalent bond, while dehydrogenation occurs via decomposition into several solid phase, as shown in Eq. (2) [6e8]: LiBH4 4 LiH þ B þ 3/2H2

(2)

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Materials with high surface area, such as carbon materials [9,10], zeolites [11,12], organic polymers [13,14], and metal organic frameworks [15,16], have also been discovered for effective storage of hydrogen. However, the increase in outer surface area is limited because the particles below a certain diameter are not stable. Mixed-metal oxides (MMOs) are novel materials recently discovered for solid-state hydrogen storage, owing to their physical properties and structural features [17,18]. The discharge capacity of the MMOs are reported to be higher than that of other hydrogen storage materials, due to availability of multi-level hydrogen storage sites [19]. The presence of the redox species in MMOs can enhance the hydrogen storage properties [20]. The reaction mechanism of typical MMOs is (Eq. (3)) [21]; AmBnOz þ xH2O þ ye
4 AmBnOz e Hx þ xOH


(3)

Among all MMOs, transition metals such as Cerium (Ce) and Zirconium (Zr), form metal oxides (CeO2, Al2O3) and MMOs (Ce0.75Zr0.25O2), have been investigated as catalysts in the chemicals and petrochemicals reforming processes [22e24]. The MMOs of cerium and zirconium, applied as catalysts, have improved thermal profiles, large surface areas and high oxygen storage capacity [25]. However, the study of Ce0.75Zr0.25O2 from the standpoint of hydrogen energy has not been reported. The Ce0.75Zr0.25O2 is synthesized via impregnation [26], sorption-hydrolytic deposition [22], sol-gel [27], hydrothermal [23], and co-precipitation [24] methods. Among various standard techniques for the synthesis of the Ce0.75Zr0.25O2, the sol-gel process is one of the simplest technique which is successfully used in order to produce crystalline materials and high surface area compounds. Here we report a simple method for production of Ce0.75Zr0.25O2 nanoparticles via a continuous sol-gel technique at 600  C using (NH4)2Ce(NO3)6 (which then converts to Ce(C4H10O)4 after chemical treatments) and C16H40O4Zr. In addition, some fundamentally important criteria were chosen, covering structural, physical and kinetic properties of Ce0.75Zr0.25O2 nanoparticles. These criteria were further served in order to investigate the electrochemical hydrogen storage properties of Ce0.75Zr0.25O2 nanoparticles. 2. Experimental study 2.1. Materials Stearic acid (CH3(CH2)16COOH, bp ¼ 370  C, mp ¼ 68e70  C, Mw ¼ 284.48 g mol
1), ammonium cerium (IV) nitrate ((NH4)2Ce(NO3)6, assay ¼ 99.99%, Mw ¼ 548.22 g mol
1), and tetrabutyl zirconate (C16H40O4Zr, Mw ¼ 383.68 g mol
1) all purchased from Sigma-Aldrich and used without any further treatments. 2.2. Preparation of Ce0.75Zr0.25O2 nanoparticles Nanoscales binary metal oxides based cerium - zirconium were successfully synthesized via a sol-gel method. Primarily, stoichiometric amount of (NH4)2Ce(NO3)6 was dissolved in butanol to obtain an orangish solution. After complete dissolution, it was filtered to separate the solid residue (NH4NO3) from filtrate solution (Ce(OBu)4) according to Eq. (4); (NH4)2Ce(NO3)6 þ C4H10O / NH4NO3 þ Ce(C4H10O)4

(4)

The obtaining cerium (IV) butoxide and tetrabutyl zirconate (cationic sources) were dispersed in melted stearic acid and stirred for 1 h just above melting point of stearic acid (~75  C), and finally calcined at 600  C for 4 h. The fine-yellowish products was ground and packed for further analysis. Fig. 1 represents the process for

Fig. 1. Schematic representation of Ce0.75Zr0.25O2 nanoparticles preparation routs.

synthesis of Ce0.75Zr0.25O2 nanoparticles. 2.3. Characterization The structural analysis of the samples was characterized using an X-ray diffractometer (Rigaku D-max C III) in CuKa (k ¼ 1.5418 Å) radiation source in an incident radiation angle of 10e80 . The average crystallite size of the sample was calculated using Sherrer equation (Eq. (5));

L ¼ kl=bcosq:

(5)

where L is the average crystal size, k is the Scherrer constant (0.89), l is the X-ray wavelength (0.15418 nm), b is the full width at halfmaximum (FWHM), and q is the diffraction angle. The morphology of the Ce0.75Zr0.25O2 nanoparticles was determined using KYKY-EM3200 Scanning Electron Microscopy (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 Microscope (TEM) was performed - Philips CM30 TEM instrument. Spectroscopic analysis of the sample was carried out using a FT-IR JASCO-680 spectrophotometer in KBr pellets in the range of 4000e400 cm
1. To investigate the band gap energy, a diffuse reflectance UVevis spectroscopy (DRS-Shimadzu UV/ 3101 PC) was used in a range between 200 and 700 nm. The kinetic parameters were calculated by adsorption/desorption isotherms of N2 at
196  C using Chem-BET Pulsar TPR/TPD/BET (Toseye Hesgarsazan Asia Co., Iran). Typical degradation processes were investigated by temperature-programmed reduction (TPR) technique using a thermal conductivity detector (TCD) of a gas chromatograph (6890 plus, Toseye Hesgarsazan Asia Co., Iran). 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 Vs
1. Three-electrode setup was 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-

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electrode setup, the current flows between the counter electrode (CE) and the working electrode (WE), and the potential difference is measured between the reference electrode (RE) and the sample. All electrochemical tests were carried out using 1 mA at room temperature. The WE is a square copper sheet with an area of ~1 cm2 (1 cm  1 cm dimension). It is connected into the connection wire through a piece of copper tape. Prior to sample deposition, one side of the copper sheet is electrolyzed in the presence of copper sulfate. The empty side of the copper sheet is then covered by the sample, by dispersing the solution of nanoparticles in ethanol, using a dropper. Finally, physical pressure is applied to the electrode, to fix the materials onto the surface of the copper sheet. 3. Results and discussion 3.1. Structural analysis The crystalline structure of the sample was identified by XRD (Fig. 2a). An intense peak at 2q ¼ 28.88 corresponds to the plane of (1 1 1) of Ce0.75Zr0.25O2. The other peaks at 2q values of 33.38 , 48.05 , 57.04 , 59.78 , 70.34 and 77.83 correspond to (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), and (3 3 1) crystal planes of pure Ce0.75Zr0.25O2 with cubic phase (JCPDS 28-0271- space group Fm3m), respectively. The average crystallite size calculated by Scherrer equation was obtained to be around 7.45 nm. Further evidence of the composition and purity of Ce0.75Zr0.25O2 was obtained from EDX. The EDX spectrum of Ce0.75Zr0.25O2 nanostructures clearly shows that the sample is highly pure and contains only Ce, Zr and oxygen, as expected from the XRD pattern. The atomic percentage ratios of each compartment are Ce e 23.40, Zr e 8.39 and O e 64.54 in the sample (Fig. 2b). The surface morphology of the pure sample was investigated by microscopic analysis (Fig. 3a). In the respective micrographs,

inhomogeneous distributions of the particles can be observed, where the particles are agglomerated in some areas. The particle size distribution probabilities of the Ce0.75Zr0.25O2 are presented in Fig. 3b. The graph reveals that the longitudinal lengths of the particles are in the range of 40e45 nm, with around 70% of distribution probability in this range. The transmission electron micrograph of the Ce0.75Zr0.25O2 nanoparticles shows spherical particles (Fig. 3c). Strong contrast and lattice fringes in the samples demonstrate that the particles are highly dense and crystalline. Here, the size of the particles is in the range of 11e16 nm, which clearly confirms the presence of agglomerated particles in the SEM micrograph. The FTIR spectrum of the pure Ce0.75Zr0.25O2 nanoparticles is shown in Fig. 4a. For hydrogen storage, the hydrogen may be located near to metal ions, and also at oxygen vacancies [28,29]. In the FTIR spectrum of the Ce0.75Zr0.25O2 nanoparticles, the CeeO and ZreO bonds are located below 1000 cm
1, and eOH bonds are also invariably found (at 3434 and 1640 cm
1), suggesting an additional site for hydrogen sorption at oxygen site. On this spectrum, an intense band at around 541 cm
1 is assigned to the vibration mode of the face-centered cubic Ce e Oe Ce stretching due to the symmetrical breathing mode of the O atoms around Ce ions [30]. In addition, two absorptions at 637 and 427 cm
1 are assigned to the stretching vibration frequency of ZreO [31]. Sing and coworkers [32] reported the hydrogenation mechanism of MgxZn1-xO and expressed that a favourable site for hydrogen sorption is near to oxygen vacancies. They also expressed that the hydrogen uptake in ZnO and Zn (Mg) O takes place due to association of hydrogen molecules to hydrogen atoms, and concluded that a higher storage capacity is due to higher unit cell volume as a result of enlargement of hydrogen containing bonds. Single peaks in bending and symmetric stretching exhibit pure crystallinity. A series of peaks at around 1580, 1427, and 839 cm
1 are associated to cerium e oxygen (CeeO) stretching vibration frequencies [33]. As mentioned before, a broad absorption at 3434 cm
1 and weak band at 1640 cm
1 are assigned to the stretching vibration mode of hydroxyl group (n OeH) and the bending vibration mode of adsorbed water on the surface of the particles, respectively [34]. The band at 1640 cm
1 is complex and contributes from both multilayer physisorbed and lone-pair Lewis-coordinated H2O [35]. Energy storage devices can affect based on a wide variety of physical properties such as electric fields in capacitors, and chemical reactions in batteries [36]. The vacancies for hydrogen sorption exist throughout the materials, and are determined by their physical properties such as band-gap, conductivity, magnetism, etc [37]. The band gap energy (Eg) of the Ce0.75Zr0.25O2 nanoparticles is calculated from its respective DRS (Fig. 4b). The plot of (ahn)2 or (ahn)1/2 versus hn shows a straight line, indicating a direct or indirect transition, respectively. By extrapolating the straight portion of the curve obtained from the above plot, the direct bandgap of Ce0.75Zr0.25O2 nanoparticles was obtained to be around 3.25 eV (Fig. 4b: inset). The band gap energy can also be determined from the wavenumber at the straight portion of the UV-DRS intense peak using (l ¼ 1240/Eg) equation, where, l and Eg are wavenumber (nm) and band gap energy (eV), respectively. The band gap energy calculated from above equation is obtained to be 3.20 eV (l ¼ 372 nm). The results are clearly complementing each other. 3.2. Thermal reduction programming (TPR) and kinetic study

Fig. 2. (a) XRD pattern and (b) EDX spectrum of Ce0.75Zr0.25O2 nanoparticles.

Certain aspects of the kinetics and mechanism of the thermal dissociation of solid metal oxides, either radical or molecular, were first considered by Malinin et al. [38]. The reduction behavior of the Ce0.75Zr0.25O2 nanoparticles was identified using H2 TPR in three scan rates (5, 10 and 20  C/min) (Fig. 5a). Three reduction areas are

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Fig. 3. (a) SEM micrograph, (b) distribution probability of the particle diameters, and (c) TEM images of Ce0.75Zr0.25O2 nanoparticles synthesized at 600  C.

Fig. 4. (a) FTIR spectrum, and (b) DRS spectrum (inset: Tauc plot) of Ce0.75Zr0.25O2 nanoparticles synthesized at 600  C.

Fig. 5. (a) TPR thermograms, and (b) Kissinger plot of Ce0.75Zr0.25O2 nanoparticles.

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formed from the H2 consumption peaks. The first consumption appears at temperatures below 450  C, the second reduction (maximum reduction) processes occur in the range of 450e650  C and the last reduction peak emerge at above 650  C. In the 10 and 20  C/min scan rates, similar H2 reduction profiles can be observed. The first and second reductions are attributed to the reduction of absorbed surface oxygen species, and reduction of oxygen on the ceria surface (CeO2 / Ce3O4), respectively, while the reduction of the bulk ceria occurs at higher temperature [23]. Upon increasing the scan rates, the Tmax shifts to higher temperatures. This is attributed to the heat transfer lag or slow heat diffusion at higher heating rates, which causes a slow equilibrium between the sample and its environment [39]. Chem-BET Pulsar TPR/TPD/BET was used to study the specific surface area (SSA) of the Ce0.75Zr0.25O2 nanoparticles. It was observed that at ambient temperature and pressure, the specific surface area is around 68.39 m2/g. Kissinger plot (Fig. 5b) was performed in order to measure the kinetic parameters of thermal profile using Eq. (6);

. In b T 2max ¼
Ea =RTmax þ In AR=Ea

(6)

where, b, Tmax, Ea,R, and A are the heating rate (K/min), rate temperature (K), activation energy, gas constant and pre-exponential factor, respectively. The Ea and A could be obtained from the slope and the intersection of the Kissinger plot, respectively, via the plot of {ln b/T2max} versus {1/Tmax}. The activation energy (Ea) of Ce0.75Zr0.25O2 nanoparticles was calculated to be around 177.13 kJ mol
1. 3.3. Hydrogen storage properties Chronopotentiometry (CP) and cyclic voltammetry (CV) are two important key enablers in electrochemical studies. In the CV curve (Fig. 6a), a reduced peak is due to the water decomposition reaction on the basis of the Faradic hydrogen formation as Eq. (7); H3Oþ þ e
/ H þ H2O

(7)

The anodic (Ipa) of Ce0.75Zr0.25O2 nanoparticles grows at 41.18 mA, while the cathodic peak currents (Ipc) appears at
37.14 mA. In addition the cathodic (Epc) and anodic (Epa) peak potentials of the sample, are located at 0.32 V and
0.03 V. The DECe0.75Zr0.25O2 is higher than DEGCE. The glassy carbon electrode (GCE) itself has Epc and Epa of 0.087 V and 0.29 V, respectively. Evaluation of the hydrogen storage capacity of the Ce0.75Zr0.25O2 nanoparticles was estimated using chronopotentiometry measurement via a series of charge-discharge sequences. The reaction profile based hydrogen sorption can be described as (Eq. (8)); Ce0.75Zr0.25O2 þ xH2O þ xe
4 Ce0.75Zr0.25O2 - Hx þ xOH


(8)

In this reaction, the hydrogen migrates from the dissociated electrolyte to the working electrode where it is absorbed; as a result, an electron is generated in the reverse direction. The storage capacity (SC) is calculated using Eq. (9);

SC ¼ Itd =m

(9)

where, I, td and m are current (mA), discharge time (hours) and mass of the coating materials (g), respectively. Fig. 6b shows 15th cycles of discharging process of the Ce0.75Zr0.25O2 nanoparticles. The discharge capacity is enhanced from 550 (1st cycle) to 2200 mAh/g (15th cycle). It must be mentioned that the discharge capacity can be affected by the specific surface area (porosity),

Fig. 6. (a) Cyclic voltamograms, and (b) fifteen discharge capacity sequences of Ce0.75Zr0.25O2 nanoparticles synthesized at 600  C.

structural functionalities, and the presence of redox species. A high value of discharge capacity of Ce0.75Zr0.25O2 nanoparticles is due to the presence of redox species [21], therefore (Eq. (10)); Ce0.75ZrIV0.25O2 þ xH2O þ xe
4 Ce0.75 ZrIV0.25-yZrIIIyO2 Hx þ xOH


(10)

Moreover, the structural vacancies in mixed metal oxides like Ce0.75Zr0.25O2 could be replaced by hydrogen atoms. This replacement could be resided on oxygen ions to form substitutional OH
ion defects [21]. As mentioned before, electrochemical hydrogen adsorption (Had.) occurs as Eq. (8). According to this equation, one electron is transferred for each hydrogen atom, therefore coulomb counting is used to determine the hydrogen content [40]. The hydrogen content (wt % H) of the sample is calculated to be around 7.8%. Mixed metal oxides (MMOs) have been proven to be potentially viable for use as electrochemical hydrogen storage. In many cases, the MMOs show a very high discharge capacities, as compared to other reported materials. For example, the discharge capacities for Ba2Co9O14 is 850 mAh/g [41], ZnOeCeO2 is 2400 mAh/g [42], Fe2O3eCeO2 is > 5000 mAh/g [43], Li2CoMn3O8 is 2000 mAh/g [44], and Sr3Al2O6 is 2500 mAh/g [45]. In MMOs based materials, in addition to their structural and morphological properties, the activities of the ions play an important role in the electrochemical properties. Therefore, these multidisciplinary properties of the MMOs make them ideal for hydrogen storage. Previous investigations on Ce and/or Zr have implied the potential of these MMOs/MOs oxides for energy storage. Sangsefidi

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chemical reactions ((NH4)2Ce(NO3)6 / Ce(OBt)4 þ Zr(OBt)4 / Ce0.75Zr0.25O2). The structural analysis of the sample demonstrated a pure and nanoscales Ce0.75Zr0.25O2. TPR analysis was used to probe kinetic activities of the sample. The activation energy of Ce0.75Zr0.25O2 nanoparticles, obtained using Kissinger plot, was found to be approximately 177 kJ/mol. The hydrogen storage performance of the sample was also recorded in a series of discharge sequences. The maximum discharge capacity was measured at around 2200 mAh/g. Owing to the physico-chemical and electrochemical performances of the Ce0.75Zr0.25O2 nanoparticles, this MMO can potentially use as a host for hydrogen sorption. The requirements for on-board hydrogen storage are highly important, in order to guarantee the cruising range for road vehicles. In addition, further computational investigations are required to propose the exact electrochemical hydrogen storage mechanisms of the binary metal oxides with structural oxygen defects. Fig. 7. Maximum charge and discharge curves of Ce0.75Zr0.25O2 nanoparticles synthesized at 600  C.

et al. [46] reported an assembly of CuO and CeO2. They found a high discharge capacity of around 2450 mAh.g
1 for CuO/CeO2 nanocomposites, higher than CeO2 nanoparticles itself with 2150 mAh.g
1, due to the larger surface area of the nanocomposites. The hydrogen storage performance of the composites containing MMOs is higher than their pristine substrates. For example, in a study of hydrogen storage Co0.9Cu0.1Si alloy system, the discharge capacities for the Co0.9Cu0.1Si/a-Fe2O3 composites is observed to be higher than that of the Co0.9Cu0.1Si alloy and reached a maximum discharge at 605.9 mA h/g for 5 wt% content of a-Fe2O3 [47]. The electrochemical responses of various Mg80X20 (X ¼ Sc, Ti, V, Cr) are compared and reported by Niessen et al. [40]. during initial hydrogen insertion (charging) and hydrogen extraction (discharging) process. They expressed a high discharge performance and efficiency of this system, much higher than commercially used metal hydride (MH) electrodes developed with hydrogen storage capacities of about 300 mAh/g, corresponding to 1.1 wt % hydrogen content. The gravimetrical storage capacities of Mg80Sc20, Mg80Ti20, Mg80V20, and Mg80Cr20 are determined to be 1790 mAh/g, 1750 mAh/g, 1700 mAh/g, and 1325 mAh/g, corresponding to 6.7, 6.5, 6.4, and 4.9 wt % H, respectively. The charge-discharge efficiency (%Eff.) of the last sequences shows a competitive efficiency of around 62% (Fig. 7) with 3550 mAh/g and 2200 mAh/g of charging and discharging capacities. Multiple plateaus of potential in the charge sequences indicate multiple electrochemical charging processes i.e. different hydrogen adsorption sites in this structure [48,49]. It must be mentioned that the oxidation of copper sheet (working electrode) cannot affect the final discharge properties. The initial discharge capacity of the copper electrode is at 0.2 mAh/ g, while after 15 cycles this profile is just enhanced to 0.75 mAh/g [19]. In the discharging curve of Ce0.75Zr0.25O2 nanoparticles (Fig. 6b), the plateau of potential indicates the presence of various hydrogen adsorption sites. These potential plateaus occur when protonoxygen in the metal oxides forms chemical bonds [50]. Recent trends of hydrogen storage materials clearly indicated that the mixed metal oxides can be potentially used to provide efficient hydrogen storage systems.

4. Conclusions Ce0.75Zr0.25O2 nanoparticles were successfully synthesized via a sol-gel method using stearic acid with a series of proposed

Acknowledgments The authors are grateful to the Universiti Sains Malaysia, Malaysia for supporting this work in the form of Postdoctoral fellow. References [1] H. Fayaz, R. Saidur, N. Razali, F.S. Anuar, A.R. Saleman, M.R. Islam, An overview of hydrogen as a vehicle fuel, Renew. Sustain. Energy Rev. 16 (2012) 5511e5528, https://doi.org/10.1016/j.rser.2012.06.012. [2] M. Caponigro, Handbook on Renewable Energy Sources, 2011. [3] B. Hobein, R. Kruger, Physical hydrogen storage technologies - a current overview, in: D. Stolten (Ed.), Hydrog. Fuel Cells; Fundam. Technol. Apllications, Wiley-VCH, Germany, 2010, pp. 377e393. [4] E. Akiba, Metal hydrides, in: D. Stolten (Ed.), Hydrog. Fuel Cells; Fundam. Technol. Apllications, Wiley-VCH, Germany, 2010, pp. 395e413. [5] R. Amirante, E. Cassone, E. Distaso, P. Tamburrano, Overview on recent developments in energy storage: mechanical , electrochemical and hydrogen technologies, Energy Convers. Manag. 132 (2016) 372e387, https://doi.org/ 10.1016/j.enconman.2016.11.046. [6] A. Borgschulte, R. Gremaud, O. Friedrichs, P. Mauron, A. Remhof, A. Zuttel, Complex hydrides, in: Hydrog. Fuel Cells; Fundam. Technol. Apllications, 2010, pp. 415e427. [7] W. Cai, J. Hou, S. Huang, J. Chen, Y. Yang, P. Tao, L. Ouyang, H. Wang, X. Yang, Altering the chemical state of boron towards the facile synthesis of LiBH4 via hydrogenating lithium compound-metal boride mixture, Renew. Energy 134 (2019) 235e240, https://doi.org/10.1016/J.RENENE.2018.11.042. [8] M. Bilen, O. Yılmaz, M. Gürü, Synthesis of LiBH4 from LiBO2 as hydrogen carrier and its catalytic dehydrogenation, Int. J. Hydrogen Energy. 40 (2015) 15213e15217, https://doi.org/10.1016/J.IJHYDENE.2015.02.085.
guin, Carbon materials for the electrochemical storage of [9] E. Frackowiak, F. Be energy in capacitors, Carbon N. Y. 39 (2001) 937e950, https://doi.org/ 10.1016/S0008-6223(00)00183-4. [10] R. Jin, G. Li, Z. Zhang, L.-X. Yang, G. Chen, Carbon coated flower like Bi2S3 grown on nickel foam as binder-free electrodes for electrochemical hydrogen and Li-ion storage capacities, Electrochim. Acta 173 (2015) 458e464, https:// doi.org/10.1016/j.electacta.2015.05.021. [11] J. Weitkamp, Fuels - hydrogen storage - zeolites, in: J. Garche (Ed.), Encycl. Electrochem. Power Sources, first ed., Elsevier, Netherland, 2009, pp. 497e503, https://doi.org/10.1016/B978-044452745-5.00330-0. [12] C.L. Garcia, J.A. Lercher, Adsorption of hydrogen sulfide on ZSM 5 zeolites, J. Phys. Chem. 96 (1992) 2230e2235, https://doi.org/10.1021/j100184a038.
chet, F. Svec, Nanoporous polymers for hydrogen storage, [13] J. Germain, J.M.J. Fre Small 5 (2009) 1098e1111, https://doi.org/10.1002/smll.200801762. [14] M.Y. Song, E. Choi, Y.J. Kwak, Preparation of a Mg-Based alloy with a high hydrogen-storage capacity by adding a polymer CMC via milling in a hydrogen atmosphere, Int. J. Hydrogen Energy. 44 (2019) 3779e3789, https:// doi.org/10.1016/J.IJHYDENE.2018.12.081. [15] H.W. Langmi, J. Ren, N.M. Musyoka, Metal-organic frameworks for hydrogen storage, in: Compend. Hydrog. Energy, 2016, pp. 163e188, https://doi.org/ 10.1016/B978-1-78242-362-1.00007-9. [16] Y. Zhao, F. Liu, J. Tan, P. Li, Z. Wang, K. Zhu, X. Mai, H. Liu, X. Wang, Y. Ma, Z. Guo, Preparation and hydrogen storage of Pd/MIL-101 nanocomposites, J. Alloys Compd. 772 (2019) 186e192, https://doi.org/10.1016/ J.JALLCOM.2018.09.045. [17] A. Salehabadi, M. Salavati-Niasari, T. Gholami, Green and facial combustion synthesis of Sr3Al2O6 nanostructures; a potential electrochemical hydrogen storage material, J. Clean. Prod. 171 (2018) 1e9. [18] A. Salehabadi, F. Sarrami, M. Salavati-Niasari, T. Gholami, D. Spagnoli,

890

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

A. Salehabadi et al. / Journal of Alloys and Compounds 790 (2019) 884e890 A. Karton, Dy3Al2(AlO4)3 ceramic nanogarnets: sol-gel auto-combustion synthesis, characterization and joint experimental and computational structural analysis for electrochemical hydrogen storage performances, J. Alloys Compd. 744 (2018) 574e582, https://doi.org/10.1016/j.jallcom.2018.02.117. A. Salehabadi, M. Salavati-Niasari, T. Gholami, A. Khoobi, Dy3Fe5O12 and DyFeO3 nanostructures: green and facial auto-combustion synthesis, characterization and comparative study on electrochemical hydrogen storage, Int. J. Hydrogen Energy. 43 (2018) 9713e9721, https://doi.org/10.1016/ J.IJHYDENE.2018.04.018. A. Salehabadi, M. Salavati-Niasari, M. Ghiyasiyan-Arani, Self-assembly of hydrogen storage materials based multi-walled carbon nanotubes (MWCNTs) and Dy3Fe5O12 (DFO) nanoparticles, J. Alloys Compd. 745 (2018) 789e797, https://doi.org/10.1016/J.JALLCOM.2018.02.242. G. Deng, Y. Chen, M. Tao, C. Wu, X. Shen, H. Yang, M. Liu, Study of the electrochemical hydrogen storage properties of the proton-conductive perovskite-type oxide LaCrO3 as negative electrode for Ni/MH batteries, Electrochim. Acta 55 (2010) 884e886, https://doi.org/10.1016/ j.electacta.2009.06.071. T.B. Shoynkhorova, V.N. Rogozhnikov, P.A. Simonov, P.V. Snytnikov, A.N. Salanov, A.V. Kulikov, E.Y. Gerasimov, V.D. Belyaev, D.I. Potemkin, V.A. Sobyanin, Highly dispersed Rh/Ce0.75Zr0.25O2-d-ƞ-Al2O3/FeCrAl wire mesh catalyst for autothermal n-hexadecane reforming, Mater. Lett. 214 (2018) 290e292, https://doi.org/10.1016/J.MATLET.2017.12.017. S. Ji, Z. Xiao, H. Zhang, L. Li, G. Li, L. Wang, G. Liu, Catalytic steam reforming of n-dodecane over high surface area Ce0.75Zr0.25O2 supported Ru catalysts, Int. J. Hydrogen Energy. 42 (2017) 29484e29497, https://doi.org/10.1016/ J.IJHYDENE.2017.10.085. J. Chen, Q. Wu, J. Zhang, J. Zhang, Effect of preparation methods on structure and performance of Ni/Ce0.75Zr0.25O2 catalysts for CH4eCO2 reforming, Fuel 87 (2008) 2901e2907, https://doi.org/10.1016/J.FUEL.2008.04.015. S.R. Dhage, S.P. Gaikwad, P. Muthukumar, V. Ravi, Synthesis of Ce0.75Zr0.25O2 at 100 C, 2005, https://doi.org/10.1016/ j.ceramint.2004.04.009. H. Hu, K. Zha, H. Li, L. Shi, D. Zhang, In situ DRIFTs investigation of the reaction mechanism over MnOx-MOy/Ce0.75Zr0.25O2 (M ¼ Fe, Co, Ni, Cu) for the selective catalytic reduction of NOx with NH3, Appl. Surf. Sci. 387 (2016) 921e928, https://doi.org/10.1016/J.APSUSC.2016.07.022. Z. Ma, X. Wu, H. H€ arelind, D. Weng, B. Wang, Z. Si, NH3-SCR reaction mechanisms of NbOx/Ce0.75Zr0.25O2 catalyst: DRIFTS and kinetics studies, J. Mol. Catal. A Chem. 423 (2016) 172e180, https://doi.org/10.1016/ J.MOLCATA.2016.06.023. € pe, Y. Nishi, B. Govoreanu, Hydrogen-induced oxygen K. Jung, B. Magyari-Ko vacancy bistability and its impact on RRAM device operation, IEEE Electron. Device Lett. 38 (2017) 728e731, https://doi.org/10.1109/LED.2017.2693368. M. Lindgren, I. Panas, Oxygen vacancy formation, mobility, and hydrogen pick-up during oxidation of zirconium by water, Oxid. Metals 87 (2017) 355e365, https://doi.org/10.1007/s11085-016-9695-z. G. Li, R.L. Smith, H. Inomata, Synthesis of nanoscale Ce 1-x Fe x O 2 solid solutions via a low-temperature approach, J. Am. Chem. Soc. 123 (2001) 11091e11092, https://doi.org/10.1021/ja016502. J. Tan, Y. Su, T. Hu, Q. Yu, R. Tursun, Q. Li, Y. Xi, Preparation and conductivity of Sc2O3eCeO2eZrO2, Solid State Ionics 292 (2016) 22e26, https://doi.org/ 10.1016/J.SSI.2016.03.018. J. Singh, M.S.L. Hudson, S.K. Pandey, R.S. Tiwari, O.N. Srivastava, Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires, Int. J. Hydrogen Energy. 37 (2012) 3748e3754, https://doi.org/10.1016/ j.ijhydene.2011.04.010. G. Wang, Q. Mu, T. Chen, Y. Wang, Synthesis, characterization and photoluminescence of CeO2 nanoparticles by a facile method at room temperature, J. Alloys Compd. 493 (2010) 202e207, https://doi.org/10.1016/ j.jallcom.2009.12.053. T. Gholami, M. Salavati-Niasari, A. Salehabadi, M. Amiri, M. Sabani, M. Rezaei, Electrochemical hydrogen storage properties of NiAl2O4/NiO nanostructures using TiO2, SiO2 and graphene by auto-combustion method using green tea extract, Renew. Energy 115 (2018) 199e207, https://doi.org/10.1016/

J.RENENE.2017.08.037. [35] G. Cerrato, S. Bordiga, S. Barbera, C. Morterra, Surface characterization of monoclinic ZrO2: I. Morphology, FTIR spectral features, and computer modelling, Appl. Surf. Sci. 115 (1997) 53e65, https://doi.org/10.1016/S01694332(96)00586-7. [36] T.P. Holme, F.B. Prinz, A. Iancu, Energy Storage Device with Large Charge Separation - Google Patents, US20120313589A1, 2016. https://patents.google. com/patent/US20120313589A1/en? oq¼Energyþstorageþdeviceþwithþlargeþchargeþseparation. (Accessed 25 January 2019). [37] G. Li, G.R. Blake, T.T.M. Palstra, Vacancies in functional materials for clean energy storage and harvesting: the perfect imperfection, Chem. Soc. Rev. 46 (2017) 1693e1706, https://doi.org/10.1039/C6CS00571C. [38] G.V. Malinin, Y.M. Tolmachev, Thermal decomposition of solid metal oxides, Russ. Chem. Rev. 44 (1975) 392e397, https://doi.org/10.1070/ RC1975v044n05ABEH002349. [39] A. Salehabadi, M. Abu Bakar, Poly(3-hydroxybutyrate )organo modified montmorillonite nanohybrids; preparation and characterization, Adv. Mater. Res. 623 (2013) 263e270, https://doi.org/10.4028/www.scientific.net/AMR. .622-623.263. [40] R.A.H. Niessen, P.H.L. Notten, Electrochemical hydrogen storage characteristics of thin film MgX (X¼Sc, Ti, V, Cr) compounds, Electrochem. Solid State Lett. 8 (2005) A534, https://doi.org/10.1149/1.2012238. [41] F.S. Razavi, M.S. Morassaei, A. Salehabadi, M. Ghiyasiyan-Arani, M. SalavatiNiasari, Structural characterization and electrochemical hydrogen sorption performances of the polycrystalline Ba2Co9O14 nanostructures, J. Alloys Compd. 777 (2019) 252e258, https://doi.org/10.1016/J.JALLCOM.2018.11.019. [42] F.S. Sangsefidi, M. Salavati-Niasari, M. Ghasemifard, M. Shabani-Nooshabadi, Study of hydrogen storage performance of ZnOeCeO2 ceramic nanocomposite and the effect of various parameters to reach the optimum product, Int. J. Hydrogen Energy. 43 (2018) 22955e22965, https://doi.org/10.1016/ J.IJHYDENE.2018.10.082. [43] F.S. Sangsefidi, M. Salavati-Niasari, Fe 2 O 3 eCeO 2 ceramic nanocomposite oxide: characterization and investigation of the effect of morphology on its electrochemical hydrogen storage capacity, ACS Appl. Energy Mater. 1 (2018) 4840e4848, https://doi.org/10.1021/acsaem.8b00907. [44] M. Ghiyasiyan-Arani, M. Salavati-Niasari, Effect of Li 2 CoMn 3 O 8 nanostructures synthesized by a combustion method on montmorillonite K10 as a potential hydrogen storage material, J. Phys. Chem. C 122 (2018) 16498e16509, https://doi.org/10.1021/acs.jpcc.8b02617. [45] A. Salehabadi, M. Salavati-Niasari, T. Gholami, Green and facial combustion synthesis of Sr3Al2O6 nanostructures; a potential electrochemical hydrogen storage material, J. Clean. Prod. 171 (2018) 1e9, https://doi.org/10.1016/ J.JCLEPRO.2017.09.250. [46] F.S. Sangsefidi, M. Salavati-Niasari, H. Khojasteh, M. Shabani-Nooshabadi, Synthesis, characterization and investigation of the electrochemical hydrogen storage properties of CuOeCeO2 nanocomposites synthesized by green method, Int. J. Hydrogen Energy. 42 (2017) 14608e14620, https://doi.org/ 10.1016/j.ijhydene.2017.04.103. [47] S. Pengpanich, V. Meeyoo, T. Rirksomboon, J. Schwank, iso-Octane partial oxidation over Ni-Sn/Ce0.75Zr0.25O2 catalysts, Catal. Today 136 (2008) 214e221, https://doi.org/10.1016/J.CATTOD.2008.01.018. [48] F.M. Mulder, T.J. Dingemans, H.G. Schimmel, A.J. Ramirez-Cuesta, G.J. Kearley, Hydrogen adsorption strength and sites in the metal organic framework MOF5: comparing experiment and model calculations, Chem. Phys. 351 (2008) 72e76, https://doi.org/10.1016/j.chemphys.2008.03.034. [49] B. Zhang, X. Ye, W. Hou, Y. Zhao, Y. Xie, Biomolecule-assisted synthesis and electrochemical hydrogen storage of Bi2S3 flowerlike patterns with wellaligned nanorods, J. Phys. Chem. B 110 (2006) 8978e8985, https://doi.org/ 10.1021/jp060769j. [50] G. Deng, Y. Chen, M. Tao, C. Wu, X. Shen, H. Yang, M. Liu, Electrochemical properties and hydrogen storage mechanism of perovskite-type oxide LaFeO3 as a negative electrode for Ni/MH batteries, Electrochim. Acta 55 (2010) 1120e1124, https://doi.org/10.1016/j.electacta.2009.09.078.