Preparation and study of characteristics of LiCoO2/Fe3O4/Li2B2O4 nanocomposites as ideal active materials for electrochemical hydrogen storage

The nanocomposites of LiCoO2/Fe3O4/Li2B2O4 were designed by the Pechini route using different fuels for the optimization of their morphology and structure. Compared to other fuels, citric acid can act as both an ideal fuel and a capping agent. The ratio of the EG : H2O mixture is another parameter, which was studied in terms of its effects on the structural characterization. The optimized sample with a rod shape was selected to compare with the bulk sample through electrochemical hydrogen storage capacity. The discharge capacity for rod-shaped nanocomposites measured was 1284 mA h g−1. However, the discharge capacity for the bulk morphology was calculated to be about 694 mA h g−1. The magnetic, electrochemical and structural analyses were performed to investigate the properties of LiCoO2/Fe3O4/Li2B2O4 nanocomposites.


Introduction
At present, the usage of fossil fuels has been restricted due to the release of pollutants from their combustion and the defective origins. In order to meet the energy demands, researchers are designing prevalent and up-to-date replacement sources for energy storage. Electrochemical hydrogen sorption is a potential technique for energy storage in the nano-scale compounds in terms of reproducibility, safety and clean sources because of their availability, supreme prociency, simple technique and low pollution. Designing novel active materials with improved efficiency, amplied structure or compound, and sustainable consumption is a crucial target in energy storage elds. [1][2][3] The structural stability, structural chemistry, mechanical attributes, and physical properties are remarkable factors for the selection of energy storage materials. The selection of suitable active materials and the design of an improved structure are important tasks for achieving high performance in electrochemical energy storage. Therefore, metal oxide-supported materials are key options to design potential active materials for electrodes utilized in energy storage. [4][5][6] Nano-scale structures with different compositions are synthesized for the energy storage application via chemical procedures, which are simple and afford high yield products.
These chemical methods include ultrasonic-based precipitation, microwave-based precipitation, fuel-assisted sol-gel method, solvothermal and thermal treatment routes. 7-10 The metal oxide materials applied in the energy storage elds are comprehensive such as borate, vanadate, spinels, and perovskites. The central metals in these structures usually are cobalt, iron, nickel, magnesium, zinc, etc. The important factor for the selection of the central metal is the ability to change the oxidation number in the redox process during electrochemical charge and discharge. [11][12][13][14][15] In the recent study, the ternary nanocomposites of LiCoO 2 / Fe 3 O 4 /Li 2 B 2 O 4 were designed by the one-step Pechini procedure by changing the operational synthesis parameters to achieve an ideal morphology. The ideal morphology of rod-shape in the nano-scale was achieved using citric acid and compared with the bulk structures in terms of electrochemical hydrogen sorption properties.  Table 1. Then, the chosen fuel materials were added to the above-mentioned mixture. Four types of structures such as EDTA, phthalic acid, citric acid and tartaric acid could play the dual role in the Pechini reaction as a chelating agent and as a fuel to control the morphology and perform the synthesis reaction. Ethylene diamine was applied in order to control the pH of the reaction at 10. The mixture was heated at 80 C to form a gel. The obtained gel was dried for 4 h in an oven at 100 C. Eventually, the dried gel was heat-treated for 4 h at 700 C to obtain the products. In order to investigate the hydrogen sorption capacity of the samples, the electrochemical chronopotentiometry technique was utilized in the assembled cell using Pt (counter electrode), Ag/AgCl (reference electrode) and working electrodes in the electrolyte medium of KOH (2 M). The applied current density is constant 1 mA. Therefore, the potentials of the working electrode versus reference electrode were recorded by applying constant current between the working and counter electrodes. The working electrode was prepared by coating a thin layer of fabricated nanocomposites on the copper substrate, which dried at 60 C to form a uniform layer on the copper foam.

Characterization details
In order to certify the phase purity of the designed nanocomposites, the XRD diffractograms were recorded via the X-ray diffraction technique. As shown in Fig. 1, the three components existed in the texture of composites. LiCoO 2 (JCPDS 16-0427), Fe 3 O 4 (JCPDS 89-0951) and Li 2 B 2 O 4 (JCPDS 11-0407) items can match with the displayed diffracted peaks. The sharp diffracted peaks present a highly crystallized structure for designed nanocomposites. The EDS result for sample 3 is presented in Fig. 2. The result of the energy dispersive spectroscopy conrms that the nanocomposites are composed of B, Co, Fe and O elements (Fig. 2). It is well-known that the EDS pattern reveals the high purity of the product.     The achieved prole for sample 3 is presented in Fig. 3. The situated peaks at 405, 684 and 1132 cm À1 related to Li 2 B 2 O 4 structures in terms of vibrating Li-O, bending B-O-B and stretching B-O bonds, respectively. The stretching mode of the [CoO 6 ] octahedron is supported by the peak located at 622 cm À1 , which corresponds to the LiCoO 2 structure. The observed weak peak at 580 cm À1 can be correlated to the Fe 3 O 4 structure. Also, the remaining molecules of water and carboxylic acid create the bonds of O-H and C-O at 3425, 1689 and 1454 cm À1 . 11,16,17 The control of the morphology is a signicant issue in the design of nanomaterials, which are applied for energy storage applications. In this study, the effects of diverse fuels on the morphology of the as-synthesized nanocomposites were compared as the rst parameter due to the chelating agent role of chosen fuel structures. Fig. 4a-d display the FE-SEM images of the designed nanocomposites in the presence of EDTA, phthalic acid, citric acid, and tartaric acid, respectively. The EDTA-assisted Pechini-synthesized nanocomposites having agglomerated structures are shown in Fig. 4a. The FE-SEM images of the as-fabricated nanocomposites using phthalic acid and tartaric acid ( Fig. 4b and d) show the formation of large structures in microscale. As shown in Fig. 4c, the nanocomposites fabricated using citric acid possessed rod-shaped structures in nano-scale. The second parameter for controlling the morphology in this study is the ratio of EG : H 2 O in the reaction medium owing to the role of EG as a gelling agent.   exhibit large and agglomerated structures ( Fig. 5a and b). However, sample 7 (Fig. 5c) prepared in the mixture of EG : H 2 O (4 : 1) shows porous structures. Comparing the FE-SEM results, it is found that the designed nanocomposites in the presence of citric acid and the ratio of 1 : 1 for EG : H 2 O present an ideal morphology and size, which was chosen as the optimum sample (sample 3). To further analyze the morphology and shape of the optimized sample, the TEM images were obtained for sample 3, as shown in Fig. 6. The results of the obtained TEM images conrm the morphology of sample 3.
The porosity and surface characteristics were determined using the BET method for sample 3, which is presented in Fig. 7. The BET method was also applied to assess the surface area and pore volume of the designed nanocomposites with rod-shaped structures, as shown in the nitrogen adsorption isotherms measured at 77 K. The specic surface area, pore volume, and average pore size for sample 3 is 1.1896 m 2 g À1 , 0.010677 cm 3 g À1 , and 35.902 nm, respectively. The pore size distribution of the nanocomposites is displayed in Fig. 7b. Finally, BET results affirm that the surface area and porosity are important issues for the facile ion transportation to increase the electrochemical performance of the active materials in the electrode matrix. 18 The properties of LiCoO 2 /Fe 3 O 4 /Li 2 B 2 O 4 nanocomposites in terms of magnetic ability were studied by the VSM technique. The hysteresis loop for sample 3 is displayed in Fig. 8. It can be seen that the plots illustrate a ferromagnetic behavior. Also, the maximum saturation magnetization (Ms) obtained for LiCoO 2 / Fe 3 O 4 /Li 2 B 2 O 4 nanocomposites with the rod-shaped structure is 0.388 emu.

Electrochemical hydrogen storage
Size and morphology are signicant factors in energy storage materials. The active materials in the nanoscale can show higher performance than bulk materials due to the surface effects between the electrolyte and active sites in the nanomaterials. The hydrogen sorption capacity for the obtained nanocomposites of LiCoO 2 /Fe 3 O 4 /Li 2 B 2 O 4 with rod-shaped structures was compared with that of bulk structures. For this purpose, chronopotentiometry tests were conducted in the three-electrode cell (including Pt, Ag/AgCl and working electrodes) in the electrolyte medium of KOH (2 M). The applied current density is constant 1 mA. The discharge capacity for the chosen samples was computed using the C ¼ IDt/mDV formula. 19 The discharge capacity for rod-shaped nanocomposites (sample 3) was 1284 mA h g À1 aer 15 cycles. However, the bulk sample 2 had a discharge capacity of 694 mA h g À1 aer 15 cycles ( Fig. 9a and b).
Diverse mechanisms for the hydrogen storage of metal oxide materials have been proposed, which comprise the existence of layered structures, morphology and physisorption process. [20][21][22] The morphology and the shape of electrode materials have an incredible impact on the energy storage prociency. The borate compounds have open channels and layered structures, which are appropriate for storing the ions and transfer them. 23 The structure of LiCoO 2 /Fe 3 O 4 /Li 2 B 2 O 4 nanocomposites assigns facile, rapid and efficacious electron transportation through the electrolyte due to easy access of the electrolyte with the structure of metal oxides. The one-dimensional structures such as wires, rods, or tubes are the most efficient shape for improving the  electrochemical prociency due to the increased surface areas and the shortened ion diffusion paths. The hydrogen storage outcomes gure out a fast surface charge delivery mechanism and higher ion diffusion performance in the electrode materials with the rod-shaped morphology (sample 3), which has a small diameter. 24,25 The comparative hydrogen sorption study between nano rod-shape and bulk structures of LiCoO 2 /Fe 3 O 4 / Li 2 B 2 O 4 nanocomposites are shown in Fig. 9. Also, Fig. 10 shows the cycling performance for samples 2 and 3. On the other hand, physical adsorption occurs in the metal oxides during the charge and discharge processes. The electrolyte of 2 M KOH separates into OH À and H + (eqn (1)). Created H + migrates to the working electrode and adsorbs on the surface of the active materials (cathode reaction). The anodic reaction of the charging process occurs in the Pt electrode through an oxidization mechanism. In discharge direction, H 2 transports from the working electrode under the alkaline circumstance and becomes water again while freeing an electron. The generated H + migrates to the surface of active materials in the fabricated working electrode, which can be adsorbed on the surface of metal oxides. The stored H 2 in the working electrode should be migrating in the opposite direction when the cell is discharged and converted into water. 26 The Heyrovsky process occurred due to the suitable electrocatalyst activity of the as-synthesized electrode materials. The synergic effects of Tafel (eqn (3)) and Heyrovsky (eqn (2)) processes led to hydrogen storage in the active materials of LiCoO 2 /Fe 3 O 4 /Li 2 B 2 O 4 nanocomposites. According to the electrochemical outcomes, LiCoO 2 /Fe 3 O 4 /Li 2 B 2 O 4 nanocomposites have a potential for hydrogen sorption as novel active materials in energy storage elds due to physisorption and rod-shaped morphology.

Conclusion
In summary, the Pechini-synthesized nanocomposites of LiCoO 2 /Fe 3 O 4 /Li 2 B 2 O 4 were successfully obtained in the ideal condition using citric acid as a fuel. The hydrogen sorption efficiency is compared with that of the bulk structure and the results show an increase in the discharge capacity from 694 to 1284 from bulk to the nano-sized rod-shaped morphology. Therefore, the magnetic nanocomposites of LiCoO 2 /Fe 3 O 4 / Li 2 B 2 O 4 as a novel candidate can play a suitable role in the energy storage elds.

Conflicts of interest
The authors declare that there are no conicts of interest regarding the publication of this manuscript.