Effect of Li2CoMn3O8 Nanostructures Synthesized by Combustion

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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of Li2CoMn3O8 Nanostructures Synthesized by Combustion Method on Montmorillonite K10 as a Potential Hydrogen Storage Material Maryam Ghiyasiyan-Arani, and Masoud Salavati-Niasari J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02617 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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The Journal of Physical Chemistry

Effect of Li2CoMn3O8 Nanostructures Synthesized by Combustion Method on Montmorillonite K10 as a Potential Hydrogen Storage Material Maryam Ghiyasiyan-Arani, Masoud Salavati-Niasari* Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box.87317-51167, I. R. Iran * Corresponding author. Tel.: +98 315 5912383; Fax: +98 315 5913201; E-mail address: [email protected] ABSTRACT This paper outlines new design nanocomposites (Li2CoMn3O8/K10) for electrochemical hydrogen storage with emphasize on the optimize condition to achieve higher performance. Li2CoMn3O8/K10 nanocomposites were fabricated by loading different ratios of the Li2CoMn3O8 (5%, 10% and 20%) inside the montmorillonite K10. Electrochemical properties of the samples montmorillonite K10, Li2CoMn3O8 and the respective nanocomposites were studied by chronopotentiometry charge-discharge techniques in alkaline medium. The Li2CoMn3O8 nanostructures were synthesized by a facile combustion method in the presence of various carboxylic acids as fuel and capping agent. The influence of carboxylic acids on the size, morphology and homogeneity of the samples were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD), energy dispersive X-ray (EDX) and Fourier transform infrared (FT-IR) were applied to investigate purity and chemical compositions of samples. The electrochemical hydrogen storage performances of the samples were identified on the basis of discharge capacity at 15th cycle is 1041 mAhg-1 for pristine K10 and increases with a raise in ratio of Li2CoMn3O8. The discharge capacity for K10 by adding 20 wt. % nanoparticles rises to 1302 mAhg-1. Our results indicate that modifying by means of Li2CoMn3O8 nanoparticles can be a promising low-cost method to improve electrochemical performance of various hydrogen storage materials including montmorillonite K10. KEYWORDS: Electrochemical hydrogen storage; Montmorillonite K10; Li2CoMn3O8; Nanostructures. 1 ACS Paragon Plus Environment

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1.

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Introduction

The use of hydrogen as an energy carrier is beginning to accelerate in recent years. Hydrogen as a clean, safe and versatile source of energy plays an important role in decarbonizing transport. Developing renewable energy source affects on improving energy efficiency and decarbonizing the energy system. Among various fuels, hydrogen has many fascinating advantages including non-toxicity, its abundance and facile preparation 1-3

. Several techniques exist to store hydrogen such as high-pressure compression, chemisorption in metal or

complex hybrid4, 5, cryogenic liquid hydrogen, 6 and physisorptions7 in porous materials. Various compounds have been used as hosts for efficient hydrogen sorption. These include carbon based materials (CNT, graphene and etc.)8, metal organic frameworks (MOFs)9, Zeolites

10

, alloys,

11

and hydrides

12

. Outstanding

issue in hydrogen storage process is structure and morphology of the materials. Research has proven that compounds with high surface area, fine porosity, or layered structures have an excellent potential in storage functions

13

. Clay minerals with layered structure and high surface area can play an important role in

hydrogen storage mechanism. Although several studies have indicated that montmorillonite K10 clay may contribute to the store of energy, little attention has been paid to modify K10 and provide different types of K10 composites with the immense range of nanomaterials. Nanomaterials are an interdisciplinary field covering physics, chemistry, biology, materials science and engineering. Utilization of these nanomaterials are growing fast especially in various industrial areas with promising applications ranging from catalysis14 and sensing to solar cells15, supercapacitors16, antibacterial agents17and optics18. It is well known that size, structure and shape have significant influence on the final properties of the nanomaterials. The synthesis of nanoparticles with control over particle size, shape and crystalline nature has been one of the main challenges for several years. Various methods allow to achieve high purity products such as precipitation

19, 20

, sonochemical21, solid state22, sol-gel23, hydrothermal24, etc.

The electrochemical hydrogen storage ability of nanomaterials has been attracted a great attention due to their high surface and favorable absorption 25. The Li2CoMn3O8 is known as suitable cathode material for Li ion battery. In literatures the Li2CoMn3O8 nanostructures are synthesized via a ceramic method which is an

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extremely time-consuming process 26. Up to date, the morphological observations of nanoscaled Li2CoMn3O8 have not been widely investigated. Hydrogen storage mechanism in this type of oxides follows a redox process in conjunction with physisorption. In the Li2CoMn3O8, Mn4+ can be reduced to Mn3+, where charge balance is preserved by adsorption of H+ 27. On the other hand, owing to its unique structure and morphology of montmorillonite K10, it can be used as a host for hydrogen sorption via spillover mechanism28. Interaction of Montmorillonite K10 with Li2CoMn3O8 nanoparticles can further lead to the formation of new class of nanocomposites with superior hydrogen storage performances. Through this approach, the facile combustion method was selected in order to synthesize Li2CoMn3O8 nanostructures in the presence of various carboxylic acids (fuels and also capping agents). Furthermore, montmorillonite K10 were used as a host for Li2CoMn3O8 nanomaterials in order to enhance hydrogen storage performances. In the present context, an attempt has been developed to compare the discharge capacities of the modifier (Li2CoMn3O8 nanoparticles), host (montmorillonite K10) and their respective nanocomposites (Li2CoMn3O8/K10). In the K10 based nanocomposites, various amounts of nanoparticles were used as an effective variable. Within the framework of these criteria, chronopotentiometry and cyclic voltammetry measurements were used to declare the hydrogen storage properties of Li2CoMn3O8, montmorillonite K10 and Li2CoMn3O8/K10 nanocomposites. To the best of our knowledge, this is the first research to acquire proper hydrogen capacitor materials with low cost, simple and fast procedure. 2.

Experiments

2.1.

Materials and physical measurement

The starting chemicals used in preparing samples, including cobalt nitrate hexahydrate, manganese nitrate tetrahydrate, lithium nitrate, oxalic acid, malonic acid, succinic acid, citric acid, and trimesic acid were purchased from a Merck company and utilized as arrived without further purification. X-ray diffraction (XRD) patterns were recorded by a Philips-X’pertpro, X-ray diffractometer using Ni-filtered Cu Ka radiation. Fourier transform infrared (FT-IR) spectra were recorded on Nicolet Magna- 550 spectrometer in KBr pellets. Scanning electron microscopy (SEM) images were obtained on LEO-1455VP equipped with an energy



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dispersive X-ray spectroscopy. The EDX analysis with 20 kV accelerated voltage was administered. Transmission electron microscopy (TEM) image was obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. The N2 adsorption/desorption analysis (BET) was performed at -196°C using an automated gas adsorption analyzer (Tristar 3000, Micromeritics). Pore size distribution was calculated by using desorption branch of the isotherm by the Barrett, Joyner and Halenda (BJH) method. Magnetic properties were measured using a vibrating sample magnetometer 60 (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran). 2.2.

Synthesis of Li2CoMn3O8

Combustion method was used for synthesis of Li2CoMn3O8 nanostructures. For this purpose, different carboxylic acids were used as fuel and capping agent. At first, 1.23 g cobalt nitrate hexahydrate, 3.18 g manganese nitrate tetrahydrate and 0.58 g lithium nitrate in a stoichiometric ratio were dissolved in propylene glycol and well mixed up. Then succinic acid as fuel dissolved in propylene glycol and added to the above solution. The molar ratio of carboxylic acid to nitrate was 1:2. After heating the mixture, abrupt ignition occurred to produce a black powder. Finally, as-synthesized samples calcined at 700° C for 2 hours to remove organic compound. Table 1 listed the different carboxylic acids for the preparation of Li2CoMn3O8 to reach optimum condition. 2.3.

Preparation of Li2CoMn3O8/K10 nanocomposites

Li2CoMn3O8/K10 nanocomposites were prepared as follows; 1g montmorillonite K10 were dispersed in isopropyl alcohol by using ultrasonic irradiation for 10 min and then stirred for more time. Then, Li2CoMn3O8 (5 wt. %, 10 wt. % and 20 wt. %) ultrasonically dispersed in isopropyl alcohol and was added to the mixture. This mixture sonicated for 10 min and stirred more time. Eventually, as-prepared samples filtered and separated from solution. The obtained powder dried at 60° C in an oven. 2.4.

Electrochemical measurement


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Hydrogen storage capacity and electrochemical properties of each sample were tested by chronopotentiometry technique in three electrodes electrochemical cell consisting of Li2CoMn3O8 (working electrode), Pt electrode (counter electrode), and Ag/AgCl electrode (reference electrode) with 6 M KOH solution as electrolyte. Working electrode was prepared by coating thin layer of as-made powder samples on the pure copper plate substrate. The coating solution was prepared by dispersing as-synthesized nanostructures in ethanol, which was sonicated 15 min in an ultrasonic bath to obtain a uniform dispersion. This solution pipetted on the surface of copper substrate and dried at 80° C without any binder to achieve a thin layer. The electrochemical cell was set up at room temperature. The potential of working electrode was recorded versus reference electrode by applying the constant current between the working and counter electrodes. 3.

Results and discussion

3.1.

X-Ray diffraction patterns

Crystallinity and phase purity of as-prepared samples were confirmed by XRD pattern. Figure 1 points out XRD pattern of pristine Li2CoMn3O8 nanostructures synthesized in presence of succinic acid (Sample No. 3). The crystal system of Li2CoMn3O8 is cubic (JCPDS 48-0261) with space group of Fd3m and cell constants a=b=c= 8.1429. The crystallite diameter (Dc) of Li2CoMn3O8 obtained using the Scherrer equation

29

; D=

Kλ/βcosθ; where β is the breadth of the observed diffraction line at its half intensity maximum, K is the socalled shape factor, which usually takes a value of about 0.9, and λ is the wave-length of X-ray source used in XRD. Assessed crystalline domain size has been acquired to be 25.34 nm for Li2CoMn3O8. Figure 2a shows the XRD pattern of K10 in the 2θ range of 5-80 degree. This diffractogram can be illustrated by the crystal structure of montmorillonite clay (JCPDS 03-0015), and a sharp peak at 2θ=26.70° is observed in peak list which is originated from quartz (JCPDS 74-1811) as impurity phase. The K10 comprises peaks at 6.17, 19.91, 27.87, 34.96, 45.65, 50.17, 55 and 61.88 degree corresponding to montmorillonite with chemical formula of (Na, Ca)0.3 (Al, Mg)2 Si4O10 (OH)2 ·xH2O and crystalline form of bentonite that confirms the composition of K10 30. To probe the interaction between Li2CoMn3O8 as modifier in K10 structure, XRD technique has been also carried out. Diffraction peaks of Li2CoMn3O8 /K10, shown in Figure 2b, indicating that crystal structure 5 ACS Paragon Plus Environment


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of montmorillonite existed and presence of Li2CoMn3O8 could be identified by the appearance of sharp peaks at 2θ=19.16, 36.79, 44.69, 59.11, 64.99, and 68.31 degree. According to Figure 2b, structure of K10 layers has been destroyed due to disappearing some reflection peaks. By comparing d-spacing for K10 sheets in pure K10 and Li2CoMn3O8 /K10 nanocomposites, it is found out that d-spacing is almost constant. 3.2.

EDX and FT-IR

In order to further understand chemical purity and elemental composition, Figure 3(a, b) indicates EDX analysis of Li2CoMn3O8 and Li2CoMn3O8/K10. The lines of Co, Mn and O are obviously observed in Li2CoMn3O8 nanoparticles. According to Figure 3b, it is known that the presence of Ca, Na, Al, Mg and Si is related to K10 chemical composition. Figure 3b shows the elemental analysis of Li2CoMn3O8/K10 nanocomposites which clearly confirms the presence of Li2CoMn3O8 over the K10 layers. FT-IR spectra of Li2CoMn3O8, K10, and Li2CoMn3O8/K10 nanocomposites have been demonstrated in Figure 4 (a-c). The absorption around 3441 cm-1 and 1631 cm-1 in all samples can be assigned to the stretching and bending vibration of the hydrogen bonded OH groups of the adsorbed water. In the spectrum of pristine Li2CoMn3O8, the spectral reflection around 621 cm-1 and 517 cm-1 attributed to a metal-oxygen band of Li-O bending vibration mode and Li-Mn-O stretching vibration mode respectively31(Figure4a). As it is seen in the IR spectrum of K10 (Figure 4b), Si-O-Si stretching vibration strong band has been appeared at 1040 cm-1. The band situated at 799 cm-1 corresponds to skeletal vibrations of quartz. The band near 3625 cm-1 assigned to Al2OH group of octahedral layer. The observed peaks prior to 1000 cm-1 are related to Al-ІV tetrahedral32, 33. Li2CoMn3O8/K10 nanocomposites showed the bands of both Li2CoMn3O8 and K10, which are consistent with XRD and EDX results (Figure 4c). 3.3.

Study of morphology: SEM and TEM

SEM images of Li2CoMn3O8 samples, which have been provided in the presence of oxalic acid, Malonic acid, succinic acid (as dicarboxylic acid), citric acid and trimesic acid (as tricarboxylic acid) are shown in Figure 5 (a-e) respectively. The effect of various carboxylic acids, as both fuel and capping agent, was examined on


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morphology, size, and uniformity of as-prepared samples. By comparing SEM images, the effect of capping agents in the formation of small particles is known, but exposed with the problem of particle agglomeration. It can be seen in Figure 5(a, e) that the product included agglomerated small particles in the presence of oxalic acid and trimessic acid. However, using citric acid to synthesis of Li2CoMn3O8 causes large structure (Figure 5d) with several particles on its surface. In particular, we find that utilizing succinic acid causes an optimal condition and favorable particle size of Li2CoMn3O8 nanostructures. The probable mechanism for Li2CoMn3O8 formation is as follows; carboxylic acid as fuel and reductant and nitrates as oxidant will generate oxidation-reduction reaction into combustion. Immense heat, CO2 and N2 gas are released during this process34. Carboxylic acid can form chelating complex with metal ions and these complexes can be stabilized in propylene glycol. Propylene glycol as a cross-linking agent with two alcoholic functional groups can connect to carboxylic acid functional groups (-COOH) and generate a polyester resin. Propylene glycol is known as multifunctional materials with polar characteristics which can affect crystal facets of the Li2CoMn3O8 by limiting the unwanted crystal growth, leading to the formation of uniform and small particles. The open structure of diverse carboxylic acids used in this work are shown in Table1. The structure of metal carboxylate formed by bidentate or tridentate acid impacts the particle size. Oxalic acid which is too short may form a linear complex while malonic acid or succinic acid may coordinate due to bending configuration to form a suitable ring structure. These rings have surrounded particles to prevent agglomeration and particle growth35. Citric acid and trimesic acid as tridentate acid able to react with two hydroxyl groups of propylene glycol and stabilized by chelating and polymerizing process, but formation of large polymeric structure in reaction environment, due to massive hydrocarbon ring or chain, led to nonuniform and large construction. Succinic acid with moderate steric hindrance effect can generate optimum conditions to achieve the desired structure. Particle size distribution histograms for selected product have been plotted and compared with each other as well. Figure 6 (a-d) asserts a clear trend in mean diameter size of samples made by several complexing agents such as oxalic acid, malonic acid, succinic acid, and trimessic acid correspondingly. These results confirm that the nanostructures prepared in the presence of succinic acid 7 ACS Paragon Plus Environment


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have best uniformity and lowest average particle size among all the Li2CoMn3O8 structures. The mean size of particles prepared by succinic acid is 39.73 nm even less than that of the other samples. In order to further verification for morphology and structure of selected nanoparticles, imaging transmission electron microscopy was carried out on succinic acid sample. Figure 7 (a-d) presents different scales of nanoparticles with the size range of 30-80 nm. TEM images clarified spherical nanoparticles, which perfectly match the SEM images of Li2CoMn3O8 nanostructures. The morphology of the montmorillonite K10 modified by Li2CoMn3O8 nanoparticles shows in Figure 8. The micrographs clearly indicate that the Li2CoMn3O8 nanoparticles are well dispersed on K10 plates. It is clear that the layers are expanded and the Li2CoMn3O8 nanoparticles are dispersed on the surface of K10 layers. High resolution transmission electron microscopy (HRTEM) of Li2CoMn3O8/K10 nanocomposites for further investigation is demonstrated in Figure 9. Figure 9 (a-c) revealed Li2CoMn3O8 nanoparticles distribution over K10 layers culminated in the confirming efficient inclusion process. Interaction between hydroxyl groups on the surface of K10 and nanoparticles through hydrogen bonding causes to link particle to K10 layers. Lattice fringe spacing of Li2CoMn3O8 nanoparticles inside K10 is measured about 0.316 nm and can be observed in Figure 9d. Hence, the outcome of SEM and TEM images for Li2CoMn3O8/K10 nanocomposites confirmed impressive inclusion Li2CoMn3O8 nanoparticles over K10 layers. 3.4.

BET analysis

BET method is standard instrumentation to evaluate the surface area and pore volume of as-made samples from nitrogen adsorption isotherms measured at 77 K. Figure 10 pinpoints adsorption/desorption isotherms and BJH plots of Li2CoMn3O8 and Li2CoMn3O8 /K10 nanocomposites. From the isotherm of both Li2CoMn3O8 and Li2CoMn3O8 /K10 (Figure 10a, c), the researchers categorized as a type ІV isotherm, which indicates solids consisting of particles with slit shape pore in conjunction with non-uniform shape or size (type H3)36. The adsorption on the mesoporous solids powders most frequently gives the type ІV isotherm. Also, the H3 type hysteresis is usually observed in plate like particles or aggregated or agglomerated particles


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giving rise to slit shaped pores with non-uniform size

37, 38

. The pore size distribution for Li2CoMn3O8 and

Li2CoMn3O8 /K10 are presented in Figure 10b, d showing broad pore size distribution with maximum around pores of 2 nm diameter. Table 2 summarizes the specific surface areas, total pore volumes and average pore diameters of Li2CoMn3O8 nanoparticles and Li2CoMn3O8 /K10 nanocomposites obtained from BET results. 3.5. Magnetic properties The magnetic properties of Li2CoMn3O8 and Li2CoMn3O8 /K10 were compared by means of using vibrating sample magnetometer (VSM) at room temperature. In this work, investigation of the magnetic properties of Li2CoMn3O8 was conducted for the first time in order to correlate between magnetic and electrical parameters. Figure 11a exhibits the magnetic behavior of the Li2CoMn3O8 nanoparticles prepared with succinic acid. The hysteresis loop shows dual behavior, ferromagnetic in low fields and paramagnetic behavior in up fields. Some previous reports confirmed twofold behaviors in magnetic properties39.The maximum saturation magnetization (Ms), remanent magnetization (Mr), and coactivity (Hc) for sample No. 3 are 4.53 (emu/g), -0.0763(emu/g) and 131.26(Oe) respectively. Figure 11b shows the M-H curve of Li2CoMn3O8 /K10 nanocomposites at room temperature. The hysteresis loop of nanocomposites is smaller than that in pristine Li2CoMn3O8. Narrow hysteresis loop implies a small dissipated energy. It is renowned the large fraction of saturation field can be sustained in the presence of large coercive force as driving force removed40. 3.6. Electrochemical measurement 3.6.1 Cyclic Voltammetry behaviors In order to comprehend the electrochemical performance of the as-prepared samples, a set of CVs were recorded (Figure 12). This test was evaluated by copper substrate coated by the as-synthesized samples in 1.5M KOH solution. The current measured between the working and counter (Pt) electrodes by applying a potential between the working and the reference (Ag/AgCl) electrodes. Figure 12 indicates the cyclic voltammograms of K10, Li2CoMn3O8 and Li2CoMn3O8/K10 hydrogen capacitor cell at a scan rate of 0.1 Vs-1.



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In all cases, the CV curves display good reversible shape within the potential region -0.9 – -0.3 V. The anodic and cathodic peak current for Li2CoMn3O8 is 628.28µA and -1179.4 µA respectively. Anodic and cathodic peak potential of this sample was gauged -0.467V and -.693V. From the cyclic voltammograms of Li2CoMn3O8/K10 note that peak currents were measured 549.47 µA and -759.68 µA for anodic and cathodic potential -0.451 V and -0.708 V respectively. These parameters for Pristine K10 are less than Li2CoMn3O8 and Li2CoMn3O8/K10. It is fascinating to note that adding Li2CoMn3O8 nanoparticles in the K10 structures, improve the electrochemical properties of nanocomposites. Because of low loading Li2CoMn3O8in K10, negligible changes in cyclic voltammogrames can be seen. Table 4 declares data on the electrode response of samples from CV observation. Consequently, the two potential peaks in both anodic and cathodic processed for Li2CoMn3O8/ K10 significantly reflected favorable electrochemical behavior for hydrogen storage. Moreover, cathodic peak is related to adsorption of hydrogen on Li2CoMn3O8/ K10 electrodes and anodic peak can be attributed to reversible electrochemical hydrogen adsorption/desorption reaction occurring on the as-made electrodes 41. 3.6.2 Hydrogen Storage performance The electrochemical attributes of products as hydrogen capacitor electrodes were inquired by chargedischarge chronopotentiometry in three-electrode cell. In this cell, the constant current was applied between the working and counter electrodes. Afterwards, the potential difference between the working and the reference electrodes was modulated. The storage capacity (SC) of samples is computed from the following formula (Eq. 1); =

(1)

Where I is the charge/discharge current (mA), td is discharge time (h) and m is the active mass (g)42. According to charge-discharge consequence, capacity of Li2CoMn3O8 electrode can be dependent on regulated current's value. Effect of current alteration on discharge capacity of Li2CoMn3O8 is presented in Figure 13a, b. By increasing applied current, discharge capacity of Li2CoMn3O8 nanostructures increased from 80 to 2248 mAhg-1 after 15 cycles for I=0.5 mA and I=1mA respectively. Influence of Li2CoMn3O8 as 10 ACS Paragon Plus Environment

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modifier in K10 structure were investigated in presence of various amount of Li2CoMn3O8 nanostructures. Figure 14a-d shows a clear trend in discharge capacity of samples which has 0 wt. %, 5 wt. %, 10 wt. % and 20 wt. % of Li2CoMn3O8 correspondingly. It is apparent from Figure 14a that discharge capacity of K10 without any additive is 1041 mAhg-1 after 15 cycles. By adding Li2CoMn3O8 to K10, discharge capacity increased from 1078 to 1302 mAhg-1 with increased dosage of additive from 5% to 20%. Table 3 summarized the data on capacity of K10 in absence and presence of various amount of Li2CoMn3O8 additive. As a result, discharge capacity of clay can be modified by adding Li2CoMn3O8 and the best performance was achieved in presence of 20 wt. % of nanostructures. Curves of discharge capacity of blank copper substrate before coating indicated in Figure 15. From the graph, it can be noted that favorable capacity of copper substrate in presence of coated nanomaterial layer is due to interaction of as-made product and their electrochemical properties. Consequently, the mechanism of hydrogen capacitor function is as follows; in typical mixed metal oxides (Ψ), the generated H+ from aqueous solution adsorbs on the surface of the sample. The overall reaction mechanism for a mixed metal oxide can be proposed on the basis of the following reaction (Eq. 2). In the charging process, electrolyte containing 6M alkali KOH dissociated to OH- and H+. Generated H+ migrate to working electrode and adsorb on surface of sample (cathode reaction).

Ψ+ x H2O+ x e- ↔ Ψ-Hx + x OH-

(2)

During the charging process, the electrolyte dissociates (Eq. 3) and the sample adsorbs hydrogen. The same mechanism has been reported for various structures of nanoscale binary metal oxides. H2O + e- ↔ H + OH-

(3)

Anodic reaction of charging process occurs in the counter electrodes via an oxidization process. In discharge direction, H2 migrate from working electrode under alkaline circumstance and becomes water again while freeing an electron43. In addition to physisorption mechanism, spillover effect is also involved in the storage process. In this mechanism, adsorbed hydrogen on Li2CoMn3O8 migrates from nanoparticles to surface of K10 support and hydrogen can transport on a surface to another surface44. Schematic diagram of spillover



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mechanism of Li2CoMn3O8/K10 has been depicted in schematic1. The charge-discharge mechanism in Li2CoMn3O8 structures involves reduction of Mn ions through the following reaction (Eq. 4); ()

()

()

+ + ↔

()

+

(4)

It seems in this equation that some of Mn4+ as central metal is reduced to Mn3+and then, adsorbing hydrogen can balance the total charge of Li2CoMn3O8. The structure favored formation of OH- ions with a reduction of Mn4+ to Mn3+ over the formation of Mn-H bonds. In a study on hydrogen storage mixed metal oxide (DyxFeyOz), Salehabadi et.al.

40

reported that the iron itself can charge/discharge some hydrogen, due to the

reducible characteristic of iron (Fe3+ / Fe2+). They proposed a reaction mechanism as Eq. 5. ()

()

()

+ + ↔ ! ! − +

(5)

Also, in other case, hydrogen storage mechanism has been proposed for perovskite type oxide BaCe1-xNdxO3z,

as a result of that, the Ce4+ reduced to Ce3+ and Ce-H bonds formed

45

. Several materials follow redox

process to store energy devoted to the interdisciplinary applications such as Li-Ion Battery, Ni-MH battery and electrochemical hydrogen storage46-48. Cooperating among these three mechanism consisting redox process, physisorption, and spillover in designed Li2CoMn3O8 /K10 causes the hydrogen storage function. 4.

Conclusion

The aim of this project was to assess the electrochemical hydrogen storage of Li2CoMn3O8 /K10 nanocomposites. The uniform Li2CoMn3O8 nanostructures were synthesized via a low-cost, facile and rapid carboxylic acid assisted-combustion method. The Li2CoMn3O8 nanostructures were further used as montmorillonite modifier in order to prepare Li2CoMn3O8 /K10 nanocomposites. Further investigations and experimental observations into the hydrogen storage activities of nanoclay based nanocomposites is strongly recommended. The results indicate that upon increasing 5, 10 and 20 wt. % Li2CoMn3O8 nanostructures into the silicate layers, the discharge capacity enhance to 1302 mAhg-1 as compared to pristine K10 (1041 mAhg1

).

Acknowledgements


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Financial support from the Iran National Science Foundation (INSF) and University of Kashan, Grant No (159271/8990) is gradually acknowledged.



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Figure captions: Figure 1. XRD pattern of pristine Li2CoMn3O8 nanostructures Figure 2. XRD patterns of (a) pristine K10 and (b) Li2CoMn3O8/K10 nanocomposites Figure 3.EDX patterns of (a) pristine K10 and (b) Li2CoMn3O8/K10 nanocomposites Figure 4. FT-IR spectra of (a) pure Li2CoMn3O8 nanostructures, (b) pristine K10 and (c) Li2CoMn3O8/K10 nanocomposites Figure 5. SEM images of pristine Li2CoMn3O8 nanostructures prepared by various carboxylic acids (a) oxalic acid, (b) malonic acid, (c) succinic acid, (d) citric acid and, (e) trimesic acid Figure 6. Particle size distribution histograms of Li2CoMn3O8 nanostructures prepared in (a) oxalic acid, (b) malonic acid, (c) succinic acid, (d) trimesic acid Figure 7. TEM images of pristine Li2CoMn3O8 nanostructures in different magnifications Figure 8. SEM images of Li2CoMn3O8/K10 nanocomposites in different magnifications Figure 9.TEM and HRTEM images of Li2CoMn3O8/K10 nanocomposites in different magnifications Figure 10. N2 adsorption/desorption isotherms and BJH pore size distributions of (a, b) Li2CoMn3O8 nanostructure and (c, d) Li2CoMn3O8/K10 nanocomposites Figure 11. Magnetization versus utilized magnetic field at room temperature for (a) pristine Li2CoMn3O8 nanostructures and (b) Li2CoMn3O8/K10 nanocomposites Figure 12. CV curves of blank electrode (copper), Li2CoMn3O8 nanostructures, K10 and Li2CoMn3O8/K10 nanocomposites



Page 20 of 42

Figure 13. Fifteen cycles discharge capacity profiles of Li2CoMn3O8 nanostructures in two currents (a) 0.5 mA and (b) 1mA Figure 14. Fifteen cycles discharge capacity profiles of (a) pristine K10, (b) 5 wt. % (c) 10wt. % and (d) 20 wt.% of Li2CoMn3O8/K10 Figure 15. Fifteen cycles discharge capacity profiles of blank copper substrate Schematic 1. Schematic representation of proposed spillover mechanism


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Table 1. Preparation condition for Li2CoMn3O8 nanostructures

Sample No.

Type of Fuel

Carboxylic acid Structure

SEM Figure

O

1

OH

Oxalic acid

5a

HO

O

O

2

O

Malonic acid

5b HO

OH O

3

OH

Succinic acid

HO

5c

O O

OH

O

4

O

Citric acid

5d HO

OH OH

O

5

OH

Trimesic acid

5e HO

O

O


OH


Page 22 of 42

Table 2. Summary of surface characteristic for Li2CoMn3O8 nanoparticles and Li2CoMn3O8/K10 nanocomposite

Sample

SBET (m2/g) Vtotal (cm3/g) SBJH (m2/g) Vmeso(cm3/g)

Davg (nm)

Li2CoMn3O8

7.7365

0.030402

8.2098

0.030175

15.719

Li2CoMn3O8/K10

195.06

0.307

189.19

0.2946

6.2952


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Table 3. Summary of discharge capacity results for hydrogen storage measurements No.

Sample

Current (mA)

Discharge capacity (mAhg-1)

1

Li2CoMn3O8

0.5

80

2

Li2CoMn3O8

1

2248

3

K10

1

1041

4

5 wt. % Li2CoMn3O8/K10

1

1078

5


1

1235

6


1

1302

7

Copper substrate

1

12



Page 24 of 42

Table 4. summary of electrode response of as-provided samples from CV observation Electrode

IPa

IPc

EPa

EPc

K10

467.22

-786.67

-0.457

-0.699

Li2CoMn3O8/K10

549.47

-759.68

0.451

-0.708

Li2CoMn3O8

628.28

-1179.4

-0.467

-0.693


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure. 5



Figure 6


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Figure 7



Figure 8


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Figure 9



Figure 10


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Figure 11



Figure 12


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Figure 13



Figure 14


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Figure 15



Schematic 1


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TOC Graphic



159x230mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Effect of Li2CoMn3O8 Nanostructures Synthesized by Combustion

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