Development of CeVO4/rGO Nanocomposite via Sonochemical Synthesis for High-Capacity Electrochemical Hydrogen Storage

INTRODUCTION
With the emergence and prominence of consequences resulting from excessive human use of fossil fuels, coupled with the knowledge that we will face a shortage of these fuels in the not-so-distant future, we are far from achieving global energy security [1-3]. Recently, the consequences of these predictions have prompted governments to dedicate a significant portion of their investment and research to the production and storage of safe and desirable clean energy. Consequently, proposed plans to provide energy through clean energy are frequently expressed and compared. As a result, these comparisons and experiments by researchers continuously refine methods, and modern approaches are also being developed [4-8]. Due to its unique features and potential to provide energy security, hydrogen is considered by scientists as the basis for the next generation of future energy systems and a promising alternative to today’s fossil fuels [9-13]. Currently, hydrogen production is possible and sustainable using three methods: fossil fuels (non-renewable sources), renewable sources, and electrochemical methods [14-19]. By purposefully choosing a method of storing and transporting hydrogen, based on the materials used, we can maximize its potential [8, 16, 20-22]. One promising method is the electrochemical storage of hydrogen, which has garnered significant attention due to its reversibility and the low operating temperature and pressure associated with the hydrogen absorption process in electrode materials [15, 16, 23]. In addition, the physical absorption of hydrogen is quickly reversible, and its enthalpy is lower than that of chemical absorption; this lower enthalpy helps reduce thermal control problems [23]. Because hydrogen storage is primarily based on chemical or physical surface absorption, the specific surface area and porosity of materials are the most critical parameters affecting absorption [15, 16]. Nanomaterials play a great role in hydrogen storage as the latest material tailoring technique due to their sole properties [12, 24]. Many types of research have been studied to identify suitable materials for hydrogen storage, including carbon nanotubes (CNTs), nanocomposites, metal-organic frameworks (MOFs), etc [25]. Hydrogen storage in solid materials and room conditions with complete safety has now been extended to hydrogen storage [16, 26 ,27]. The chronopotentiometry method is a three-electrode technique that is performed by applying current. Among the important groups of inorganic nanomaterials, CeVO4 from infrequent earth vanadate families has been extensively evaluated owing to its magnetic features, great electronic conductivity properties, proper specific surface area, and suitable electrochemical performance [12]. The suitable and porous composite structure of this material with substances like graphene oxide and graphite, it has significantly improved its performance. However, rGO with improved electrochemical properties as well as layered structure is suitable as an improved substrate for nanoparticles and increased efficiency and ease of storage [28]. The effect of rGO in increasing hydrogen storage can be seen in the works of researchers such as Valian et al. with the synthesis of new Li0.35La0.55TiO3/rGO nanocomposite and Guemou et al. who introduced a novel rGO-supported Ni-Nb nanocomposite observed [8, 29]. The sonochemical process that has benefited from green chemistry by displaying the attractive morphology of regular particle size and also creating more surface area is one of the simplest and most efficient methods that has been noticed in recent years for the synthesis of inorganic nanomaterials [30]. In our previous work [12], we synthesized CeVO4/rGO nanocomposite through hydrothermal method and investigated its performance in hydrogen storage, which was very favorable. Again, we believe that more work can be done about this composite. Therefore, here, in order to improve the storage efficiency, we synthesized CeVO4/rGO nanocomposite by a one-step sonochemical method as a simpler and faster method in the synthesis of high surface area nanomaterials. From hydrazine as a reduction factor GO to rGO, it enables rGO to perform one -step synthesis. Finally, the performance of the CeVO4/rGO nanocomposite was investigated by EDS, FTIR, XRD, BET, and FESEM analysis for hydrogen storing through an electrochemical process.

MATERIALS AND METHODS
Materials and instruments
Hydrochloric acid (HCl), ammonium metavanadate (NH4VO3), graphite, cerium (III) nitrate hexahydrate (Ce(NO3)3.6H2O), potassium hydroxide (KOH), KMnO4 (potassium permanganate), H2SO4 (sulfuric acid), H2O2 (hydrogen peroxide), and hydrazinium hydroxide (N2H2OH) without any further purification from Merck were procured and used. Fourier Transform Infrared Spectrometer analysis was done utilizing Magna-IR, spectrometer 550 550 Nicolet with a resolution of 0.125 cm in KBr pellets in the range of 400--4000 cm-1. FESEM images were recorded by MIRA3 FEGSEM. By Philips-X ’PertPro device using Ni-filtered Cu Kα radiation, X-ray diffraction patterns were prepared. A Philips XL30 microscope device was used to perform energy dispersion spectroscopy (EDS) analysis. BJH analysis was done to explore the specific surface area and pore size distribution. N2 adsorption/desorption (BET) analysis was obtained utilizing a Belsorp-mini device. Ultrasonic waves were produced using a 20 kHz ultrasonic device with a maximum output power of 400 watts and a probe (APU500) diameter of 12 mm.

Synthesis of CeVO4/rGO nanocomposite
GO was fabricated from graphite by the Hammers approach [31]. The first solution was prepared by dissolving 0.43 g of Ce(NO3)3.6H2O and 0.1 g of GO in 24 ml of distilled water. Then, 0.116 g of ammonium metavanadate (NH4VO3) has been dissolved in 24 ml of distilled water (secondary solution). The secondary solution was slowly added into the first solution while stirring. Then 8 ml of distilled water and 2 ml of hydrazine were added into the mentioned solution, and also stirred on a heater within 15 minutes. Finally, ultrasonic was performed with a power of 200 watts in one step and for 20 minutes. The synthesis steps are shown in Fig. 1. The storage capability of synthesized CeVO4/rGO nanocomposite was investigated by chrono-potentiometric approach in the presence of three electrodes, working electrode, reference electrode (Ag/AgCl), and counter electrode (Pt) in potassium hydroxide (KOH) solution. [30]. Fig. 2 shows the steps of electrochemical hydrogen storage.

RESULTS AND DISCUSSION
The XRD data of CeVO4/rGO nanocomposite is presented in Fig. 3. The absence of a signal at an angle of 2θ ≈ 10.6° reveals that GO is reduced [32]. The XRD pattern of Fig. 3 shows the tetragonal structure of CeVO4 (JCPDS = 120757) with lattice parameters a = 7.39 Å, b = 7.39 Å, and c = 6.48 Å. The EDS data of CeVO4/rGO nanocomposite is displayed in Fig. 4. Only V, Ce, O, and C signals can be apperceived in the EDS spectrum. Besides, there are no other impurities, and it is consistent with the XRD findings. To investigate the organic groups and chemical bonds of CeVO4/rGO nanocomposites, FT-IR spectroscopy is presented in Fig. 5. The signal at 794 cm-1 is assigned to asymmetric stretching vibrations of V-O [33]. The asymmetric stretching and bending vibration of the O-H bond in several signals near 3400 cm-1 and 1640 cm-1 is caused by H2O molecules adsorbed upon the external surface of CeVO4/rGO nanocomposites [34]. The conversion of graphene oxide to reduced graphene oxide can be confirmed by the absence of C=O as well as C-O-H bond peaks that emerge around 1715 cm-1 and also 1400 cm-1, correspondingly [35]. Fig. 6 is related to the FESEM data with different magnifications of the as-synthesized CeVO4/rGO nanocomposite. As can be seen, rod-like CeVO4 nanoparticles with an approximate diameter of 10 nm and a length of about 20 nm are spread on the graphene sheets. One of the characteristics of the formation of smaller particles size that leads to superior performance in hydrogen adsorption is the creation of a higher specific surface. One of the most key features of materials and nanocomposites to be suitable candidates for hydrogen storage is having a proper specific surface area and favorable porosity. In general, the amount of gas adsorbed on the adsorbent can be related to its surface area. The BET analysis in Fig. 7 displays the N2 adsorption/desorption isotherm as well as the pore size distribution of the synthesized nanocomposite using the Bart Joyner Holland (BJH) method. The isotherm observed in Fig. 7a corresponds to the type IV isotherm with H3 type residual loops, that represent mesoporous and solid particles with porous nature [36]. These residual loops are created in sheet-like structures and stacked compounds that do not show limiting absorption at high p/p0 [37-39]. Furthermore, the pore size distribution (BJH) in Fig. 7b confirms the meso-porosity of the sample. As can be seen in Fig. 7a, this isotherm has a hysteresis in the relative pressure between 0.7-1, which is a high relative pressure, the absorption increases and the holes are filled [40]. Also, the presence of hysteresis indicates the presence of hollow structures in the synthesized nanocomposite [41], which is consistent with SEM observations (Fig. 6). Specific area (as), total pore volume (Vt), and also mean pore diameter (Dm) for the synthesized nanocomposite are: 60.342 m2g-1, 0.11294cm3gˉ¹, and 8.5773 nm, respectively. In general, with the increase of the specific surface area of the nanocomposite, the amount of storage capacity increases because more surface area (as) can create more active storage sites.

Electrochemical Hydrogen Storage Tests
Cyclic voltammograms of the synthesized nanomaterials, pure oxide nanostructure and the prepared nanocomposite were examined in the 2 molar potassium hydroxide electrolyte [42]. The cyclic voltammetry experiments were also performed with scan rate of 0.1 V/s [42]. The CV results of the two investigated nanostructure samples are depicted in Fig. 8. As can be seen, both samples prepared as electrode compounds for hydrogen storage exhibit the reversible curve in the potential range of -1 to -0.2 V. Table 1 gives the anodic and cathodic feedback information. It is clear that the anodic and cathodic currents of the nanocomposite sample are greater than the pure oxide nanostructure, which can demonstrate its better electrochemical features for energy storage [43]. It has been accepted that the presence of graphene-based compounds can have a positive effect on improving the electrochemical characteristics of nanomaterials owing to their favorable conductivity and proper structural properties [42]. In addition, the anodic and cathodic signals appearing in the CV curves denote the reversible adsorption/desorption process as well as adsorption of hydrogen upon the electrode surface, correspondingly [42, 43].
In this experimental work, the chrono-potentiometric approach was applied to measure and compare the hydrogen adsorption and desorption capacity of the synthesized nanomaterials, pure oxide nanostructure and the prepared nanocomposite (see Figs. 9 and 10). It has been reported that during the adsorption process, a number of water molecules in the electrolyte can be decomposed in the vicinity of the working electrode [43, 44]. H species in the electrolyte may be either adsorbed on the nanostructures coated on the working electrode or recombined on the electrode surface, leading to the formation of hydrogen molecules. These molecules will disperse on the electrode or create gas bubbles upon the electrode surface [43, 44]. It is worth noting that with the continuation of this process, the voltage can enhance and the adsorption in the nanostructures can continue. It has been reported that during the deposition step, the hydrogen species released from the nanostructures can combine with the hydroxyl species in the alkaline electrolyte and lead to the formation of water molecules. The reactions that can perform in these two stages are [16, 44, 45]:

ge- + Nanostructure + gH2O → gOH- + (gH + Nanostructure)

(gH + Nanostructure) + gOH- → ge- + Nanostructure + gH2O 

In order to check and measure the values of adsorption and desorption capacity, a constant current (one milliampere) is employed to start the process of hydrogen adsorption in nanostructures. As the process of hydrogen adsorption continues, the voltage can enhance. It has been observed that when the adsorption curve is flat, H species can be completely adsorbed upon the working electrode [16, 25, 44]. In this project, in order to explore the impact of rGO on the electrochemical storage capacity of the CeVO4 nanostructure that was studied in the previous research work [33], a nanocomposite sample was created and the performance of both samples (pure oxide and nanocomposite) was measured and compared at 15 cycles. Each cycle contains adsorption and desorption. In the case of both nanostructured samples, it is observed that the adsorption and desorption cycles can boost. In the case of nanocomposite sample, however, in all cycles, the measured adsorption capacity is greater than the adsorption capacity of the pure oxide sample (see Figs. 9 and 10). Pure oxide sample capacity can only be enhanced to about 4087 mAh/g after 15 adsorption-desorption cycles, while in the nanocomposite sample, capacity from 1051 mAh/g in the first cycle can reach about 4929 mAh/g in the 15th cycle. Adding reduced graphene oxide here improved the hydrogen storage capacity. As mentioned earlier, the BET outcomes showed that the specific surface area enhanced with the addition of reduced graphene oxide (see Fig. 5). In other words, the specific surface area of the pure oxide nanostructure is lower than that of the nanocomposite sample. Previous studies have demonstrated that having proper porosity and great specific area can play a very positive role in boosting the electrochemical storage capacity of hydrogen [16]. So, it seems that adding reduced graphene oxide, and as a result, the preparation of nanocomposite sample can be a beneficial approach to achieve better performance in electrochemical hydrogen storage. It is worth mentioning that the copper substrate without the presence of prepared nanostructures as electrode compounds exhibited a negligible capacity. This means that both synthesized nanostructures have a substantial effect on the obtained hydrogen storage capacities [46, 47].
In addition, the reason for the enhancement in discharge capacity with the increment in the number of adsorption-desorption cycles can be said to be the creation of more new sites for hydrogen adsorption and desorption in the working electrode [48]. Fig. 11 displays the last charge profiles of the both samples (pure oxide and nanocomposite) at room temperature. The appearance of different plateaus in the charge profile related to both nanostructured samples can demonstrate the existence of diverse sites for hydrogen absorption in the structure of these nanomaterials [49]. The above outcomes illustrate that the positive effect of adding reduced graphene oxide can provide alternative insights to produce nanocomposites with favorable structural and morphological characteristics for green energy storage uses very efficiently.

CONCLUSION
In this study, the CeVO4/rGO nanocomposite was fabricated by a one-pot sonochemical approach utilizing Ce(NO3)3.6H2O), NH4VO3, and GO as reactants and hydrazine as both reducing agent and OH- source. CeVO4/rGO nanocomposite was dried for 10 hours at 80 °C and then identified by EDS, FT-IR, XRD, BET, and FESEM analyses. The characterization results indicate that rod-like CeVO4 nanoparticles were formed on the reduced graphene oxide sheets. The results show the positive effect of adding reduced graphene oxide. This provides new insight into efficiently producing nanocomposites with favorable structural and morphological properties for green energy storage applications. Finally, the nanocomposite obtained by the chrono-potentiometric method was used for the electrochemical hydrogen storage. The hydrogen storage capacity was calculated to be 4929 mAh/g. The focus of current and future research is on the modification of existing materials and the development of new materials.

ACKNOWLEDGMENT
The authors would like to thank Semnan University Research Council and University of Bonab for the financial support of this work.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.