Effect of LaCoO3 Synthesized via Solid-State Method on the Hydrogen Storage Properties of MgH2

One of the ideal energy carriers for the future is hydrogen. It has a high energy density and is a source of clean energy. A crucial step in the development of the hydrogen economy is the safety and affordable storage of a large amount of hydrogen. Thus, owing to its large storage capacity, good reversibility, and low cost, Magnesium hydride (MgH2) was taken into consideration. Unfortunately, MgH2 has a high desorption temperature and slow ab/desorption kinetics. Using the ball milling technique, adding cobalt lanthanum oxide (LaCoO3) to MgH2 improves its hydrogen storage performance. The results show that adding 10 wt.% LaCoO3 relatively lowers the starting hydrogen release, compared with pure MgH2 and milled MgH2. On the other hand, faster ab/desorption after the introduction of 10 wt.% LaCoO3 could be observed when compared with milled MgH2 under the same circumstances. Besides this, the apparent activation energy for MgH2–10 wt.% LaCoO3 was greatly reduced when compared with that of milled MgH2. From the X-ray diffraction analysis, it could be shown that in-situ forms of MgO, CoO, and La2O3, produced from the reactions between MgH2 and LaCoO3, play a vital role in enhancing the properties of hydrogen storage of MgH2.


Introduction
Hydrogen is increasingly seen as an energy carrier owing to its non-toxicity, abundant resources, positive environmental impact, and high energy density [1][2][3]. Nowadays, searching for effective hydrogen storage technologies is commonly recognized as one of the major difficulties faced by the hydrogen economy [4,5]. Solid-state hydrogen storage materials have drawn a significant amount of interest because of their safety consideration, cheapness, and high gravimetric capacity [6]. Over the past decade, MgH 2 gained research interest due to its outstanding reversibility, low cost, non-toxicity, an abundance of resources, and high gravimetric hydrogen capacity (7.60 wt.%) [7][8][9]. Fortunately, practical applications of MgH 2 are severely restricted by the slow reaction kinetics and high dissociation temperature [10][11][12]. Recently, significant improvements have been made by producing nanocrystalline MgH 2 powders by the addition of metal oxide additives such as Nb 2 O 5 [13], TiO 2 [14], CoTiO 3 [15], CoMoO 4 [16], and MnMoO 4 [17], through ball milling method to enhance hydrogen storage performance of MgH 2 . Rare earth metals are considered as one of the most intriguing additives/catalysts used in solid-state materials. For example, Ismail [18] found that after the addition of 10 wt.% LaCl 3 into MgH 2 , hydrogen started to be released at 300 • C, 50 • C lower than with milled MgH 2 . It is revealed that the formation of MgCl 2 and Mg-La alloy during the heating process of the composites gives a vital role in enhancing the performance of hydrogen storage of MgH 2 . In our previous study [19], adding 10 wt.% LaFeO 3 to MgH 2 positively affected the hydrogen sorption properties of MgH 2 . Compared with pure MgH 2 , the introduction of 10 wt.% of LaFeO 3 reduced the desorption temperature by 120 • C. Further studies have exposed that ab/desorption kinetics of MgH 2 were improved by the formation of Fe, MgO, and La 2 O 3 phases during the heating process. For instance, Soni et al. [20] introduced LaF 3 into MgH 2 and proved that the samples started to release hydrogen at 320 • C, 40 • C lower than with milled MgH 2 . In addition, milled MgH 2 absorbed only 2.00 wt.% hydrogen in 2.5 min, while MgH 2 + LaF 3 could absorb 4.90 wt.% of H 2 under the same circumstances. Wu et al. [21] reported that adding LaNiO 3 significantly enhanced the desorption and absorption kinetics of MgH 2 . Further investigation revealed that in situ formations of Mg 2 NiH 4 and LaH 3 have a synergistic effect that can serve as a "hydrogen pump", hence enhancing the sorption kinetics of MgH 2 . The research led by Zhang and co-workers [22] discovered that after introducing LaNi 4.5 Mn 0.5 to MgH 2 , excellent catalytic activity was observed. Interestingly, at 300 • C, the composites could desorb 6.60 wt.% of H 2 in less than 360 s.
Besides this, according to Juahir et al. [23], doping MgH 2 with Co 2 NiO lowered the starting hydrogen release and enhanced the ab/desorption kinetics of MgH 2 . According to their research, the formation of Co 1. 29 [24] came to the finding that CoFe 2 O 4 had the best catalytic performance in enhancing the hydrogen storage performance of MgH 2 . Furthermore, Cabo et al. [25] discovered that the addition of Co 3 O 4 and NiCo 2 O 4 additives decreased the starting desorption temperature of MgH 2 . In particular, Mandzhukova et al. [26] analyzed the effect of NiCo 2 O 4 on the kinetic performance of the Mg/MgH 2 system, and revealed that the kinetic properties of Mg were drastically enhanced. Liu et al. [27] used a reduction reaction method to synthesize Co@CNT and found that the doped samples began to release hydrogen at 324 • C, which was lower than that of the bulk samples (420 • C). Further research indicated that the energy barrier for hydrogen dissociation can be substantially reduced by Co and Co(II).
Motivated by previous research, two promising materials (La and Co) clearly demonstrate that LaCoO 3 improves hydrogen ab/desorption kinetics and lowers the MgH 2 desorption temperature. In this paper, different amounts of LaCoO 3 were milled together to make MgH 2 -x wt.% LaCoO 3 (where x is 5, 10, 15, and 20) composites. LaCoO 3 was used as an additive to see how this material affected the hydrogen sorption properties of MgH 2 . To date, this is the first research into the hydrogen storage performance of MgH 2 /LaCoO 3 composites for solid-state materials.

Materials and Methods
The LaCoO 3 material was synthesized by using citric acid (≥98% pure; Sigma Aldrich, St. Louis, MO, USA), lanthanum oxide (≥99.9% pure; Aldrich Chemical Compound, Milwaukee, WI, USA), and pure cobalt oxide (99.99% pure; Sigma Aldrich, St. Louis, MO, USA) as starting materials with 0.121 g, 0.081 g, and 0.040 g respectively. The powders were mixed and thoroughly ground in an agate mortar. The solid mixture was then placed into crucibles made of alumina and calcined at 950 • C in a furnace for 5 h. In a planetary ball mill (NQM-0.4), using stainless steel vials and 4 balls at 400 rpm with a 40:1 ball-to-powder ratio, the various weight percentage (5, 10, 15, and 20) of LaCoO 3 were milled together with MgH 2 (≥95% pure; Sigma Aldrich, St. Louis, MO, USA). This milling approach was carried out for 1 h in different directions (milling = 15 min, rest = 2 min, mill again = 15 min). In an argon atmosphere, the MBRAUN UNIlab glove box was used for all the preparations (including weighing).
Sievert-type pressure composition temperature (Advanced Materials Corporation, Pittsburgh, PA, USA) was used to investigate the temperature-programmed-desorption (TPD) and hydrogen ab/desorption kinetics for all of the samples. Approximately, Materials 2023, 16, 2449 3 of 14 400 mg of the samples were used in each test. All the samples were heated to 450 • C from ambient temperature for the TPD analyses at a rate of 5 • C/min. The absorption kinetics were carried out at 250 • C (33.0 atm), meanwhile, the desorption kinetics were evaluated at 300 • C (1.0 atm). To observe the hydrogen desorption behavior of MgH 2 , differential scanning calorimetry (DSC) was performed on a Mettler Toledo TGA/DSC 1. The samples were heated from 30 to 500 • C at rates of 15, 20, 25, and 30 • C/min under constant argon flow (50 mL). An alumina crucible in a glove box was filled with about 3-5 mg, and to prevent oxidation, the samples were then placed in a sealed glass bottle. To scrutinize the phase structure of the samples, X-ray diffraction (XRD) spectra were recorded in the range of 20 • -80 • using Cu-Kα radiation to analyze the phase structure of each sample. Scan speeds of 2.00 • /min were used for θ-2θ scans. Prior to this, a small portion of the samples was evenly distributed on a sample holder and sealed with scotch tape to prevent oxidation.
The morphology of the samples was examined using scanning electron microscopy (SEM; JEOL, Akishima, Tokyo, Japan) (JSM-6360LA). In a vacuum state, the gold spray was applied to the samples after being prepared on carbon tape. Moreover, to further examine the sample's chemical bond, a Shimadzu IRTracer-100, Kyoto, Japan Fourier Transform Infrared spectroscopy was used. Attenuated total reflectance (ATR) was used to measure the spectra at room temperature for 40 scans, between 2000 and 400 cm −1 , with a resolution of 4 cm −1 . At room temperature, Raman spectroscopy was performed using Renishaw Raman spectroscopy (532 nm radiation) extended with 0.1% power laser measurement.

Results and Discussion
Calcining the samples at 950 • C for 5 h yielded well-crystallized pure LaCoO 3 (JCPDF: , in the rhombohedral structure as shown in Figure 1a [28,29]. The crystallite sizes (L) were estimated at 20.85 nm, through the Scherrer formula as shown in Equation (1) below: where shape factor K = 0.94 constant, λ = X-ray used (0.154 nm), β (physical broadening) = full width at half the maximum, and θ = angle of Bragg's diffraction. The FTIR transmission of LaCoO 3 is indicated in Figure 1b. A peak at 508 cm −1 is ascribed to the Co-O bond as reported by Sarker and Razzaque [28], and Worayingyong et al. [30]. The La-O bond as confirmed by Radev et al. [31], corresponds to the peak at 410 cm −1 . The Raman spectra shown in Figure 1c illustrates typical characteristics of LaCoO 3 at 478 cm −1 , confirming the formation of La-O [32]. It can be evidenced that the pure LaCoO 3 was successfully synthesized by using the solid-state method based on the results of XRD, FTIR, and Raman spectroscopy. Meanwhile, Figure 1d shows the morphology of LaCoO 3 , where the agglomeration of particles of different sizes can be seen as indicated in a previous study [33]. Besides this, the particle sizes distribution (PSD) of the LaCoO 3 particle was calculated by using ImageJ (version 2022). Based on Figure 1e below, the PSD of LaCoO 3 was 82.71 µm.
The impact of LaCoO 3 on the desorption temperature of MgH 2 was measured using the TPD profile of gas desorption from the samples, as displayed in Figure 2a. Pure MgH 2 and milled MgH 2 both had onset desorption temperatures of 420 • C and 350 • C, respectively. It was discovered that the milling process had an impact on the decomposition of MgH 2 . According to Sokano et al. [34], milled MgH 2 has a lower onset desorption temperature, which is 328 • C, compared with that of pure MgH 2 (418 • C). However, the onset desorption temperature was shifted from 350 • C to a starting temperature below 325 • C when different weight percent of LaCoO 3 were added to MgH 2 . The onset desorption temperature of 5, 10, 15, and 20 wt.% of LaCoO 3 with MgH 2 was 316, 322, 310, and 323 • C, respectively. Meanwhile, the desorption capacity of 5, 10, 15, and 20 wt.% of LaCoO 3 with MgH 2 was 6.57, 6.06, 6.03, and 5.30 wt.%, respectively. A study led by Pandey et al. [35] proved that adding TiO 2 to MgH 2 lowered the onset desorption temperature to 335 • C, 55 • C lower than that of pure MgH 2 . Despite the fact that adding LaCoO 3 lowered the desorption temperature of MgH 2 , the hydrogen desorption capacity of xwt.% of LaCoO 3 (where x is 5, 10, 15, and 20 wt.%) with MgH 2 decreased due to the dead weight of LaCoO 3 . The impact of LaCoO3 on the desorption temperature of MgH2 was measured using the TPD profile of gas desorption from the samples, as displayed in Figure 2a. Pure MgH2 and milled MgH2 both had onset desorption temperatures of 420 °C and 350 °C, respectively. It was discovered that the milling process had an impact on the decomposition of MgH2. According to Sokano et al. [34], milled MgH2 has a lower onset desorption temperature, which is 328 °C, compared with that of pure MgH2 (418 °C). However, the onset desorption temperature was shifted from 350 °C to a starting temperature below 325 °C when different weight percent of LaCoO3 were added to MgH2. The onset desorption temperature of 5, 10, 15, and 20 wt.% of LaCoO3 with MgH2 was 316, 322, 310, and 323 °C, respectively. Meanwhile, the desorption capacity of 5, 10, 15, and 20 wt.% of LaCoO3 with MgH2 was 6.57, 6.06, 6.03, and 5.30 wt.%, respectively. A study led by Pandey et al. [35] proved that adding TiO2 to MgH2 lowered the onset desorption temperature to 335 °C, 55 °C lower than that of pure MgH2. Despite the fact that adding LaCoO3 lowered the desorption temperature of MgH2, the hydrogen desorption capacity of xwt.% of LaCoO3 (where x is 5, 10, 15, and 20 wt.%) with MgH2 decreased due to the dead weight of LaCoO3.
The isothermal absorption measurement of the milled MgH2 and xwt.% of LaCoO3 (where x is 5, 10, 15, and 20 wt.%) with MgH2 was further conducted under 33.0 atm at 250 °C, as depicted in Figure 2b. The result proved that adding 5, 10, 15, and 20 wt.% of LaCoO3 with MgH2 could absorb 7.30, 7.30, 6.99, and 5.49 wt.%, respectively, within 20 min. Meanwhile, milled MgH2 could only absorb 6.68 wt.% under the same circumstances. The amount of hydrogen absorption for 20 wt.% of LaCoO3 with MgH2 was lower by 1.19 wt.%, compared with that of milled MgH2. This was due to the possibility that too much additive in the composite may block the diffusion path of hydrogen [36]. A previous study reported by Sulaiman et al. [37] indicated that the amount of Na3FeF6 affects the hydrogen absorption behavior of MgH2. The addition of excess Na3FeF6 catalyst into MgH2 obstructs the hydrogen diffusion by blocking the diffusion path, which limits the Mg-H reaction. However, faster absorption kinetics of MgH2 could be seen within 4 min after the addition of 20 wt.% LaCoO3. As evidenced by the above experimental results, the hydrogen ab- The isothermal absorption measurement of the milled MgH 2 and xwt.% of LaCoO 3 (where x is 5, 10, 15, and 20 wt.%) with MgH 2 was further conducted under 33.0 atm at 250 • C, as depicted in Figure 2b. The result proved that adding 5, 10, 15, and 20 wt.% of LaCoO 3 with MgH 2 could absorb 7.30, 7.30, 6.99, and 5.49 wt.%, respectively, within 20 min. Meanwhile, milled MgH 2 could only absorb 6.68 wt.% under the same circumstances. The amount of hydrogen absorption for 20 wt.% of LaCoO 3 with MgH 2 was lower by 1.19 wt.%, compared with that of milled MgH 2 . This was due to the possibility that too much additive in the composite may block the diffusion path of hydrogen [36]. A previous study reported by Sulaiman et al. [37] indicated that the amount of Na 3 FeF 6 affects the hydrogen absorption behavior of MgH 2 . The addition of excess Na 3 FeF 6 catalyst into MgH 2 obstructs the hydrogen diffusion by blocking the diffusion path, which limits the Mg-H reaction. However, faster absorption kinetics of MgH 2 could be seen within 4 min after the addition of 20 wt.% LaCoO 3 . As evidenced by the above experimental results, the hydrogen absorption kinetics of MgH 2 can be improved by the presence of LaCoO 3 .
To compare hydrogen desorption properties of different weight percentages of LaCoO 3 with MgH 2 and milled MgH 2 , an isothermal desorption test was conducted at 300 • C for 1 h, as presented in Figure 2c. strated faster desorption kinetics than milled MgH2. An amount of 5 wt.% of LaCoO3 with MgH2, and 10 wt.% of LaCoO3 with MgH2 released H2 at approximately 2.46 and 3.24 wt.%, respectively. In addition, 15 wt.% of LaCoO3 with MgH2, and 20 wt.% of LaCoO3 with MgH2 desorbed 2.01 wt.% and 4.53 wt.% of H2, respectively. However, milled MgH2 only released 0.34 wt.% of H2 under the same circumstances. Table 1 summarizes the onset desorption temperature, the capacity of absorption kinetics at 250 °C, and desorption kinetics at 300 °C for pure MgH2, milled MgH2, and composites of different LaCoO3 weight percentages with MgH2. Considering the onset desorption temperature, absorption and desorption kinetics of each sample, 10 wt.% of LaCoO3 with MgH2 composites were chosen for further investigation.    To obtain a greater understanding of the kinetic mechanism in hydrogen storage materials, kinetic models were used to describe absorption and desorption behaviors. In this study, the kinetic mechanism was investigated by using the Johnson-Mehl-Avrami (JMA) and Contracting Volume (CV) equations as can be seen in Table 2 [38]. The absorption and desorption kinetic curves for 10 wt.% LaCoO 3 with MgH 2 composites are illustrated in Figure 3a,b below. The kinetic curves for the composites were calculated for the reacted fraction in the range from 0 to 80%. Based on the figure below, the absorption process at 250 • C can be best described by the CV 3D decrease surface while the desorption process at 300 • C can be best described by the JMA 2D.
To obtain a greater understanding of the kinetic mechanism in hydrogen storage materials, kinetic models were used to describe absorption and desorption behaviors. In this study, the kinetic mechanism was investigated by using the Johnson-Mehl-Avrami (JMA) and Contracting Volume (CV) equations as can be seen in Table 2 [38].  The absorption and desorption kinetic curves for 10 wt.% LaCoO3 with MgH2 composites are illustrated in Figure 3a,b below. The kinetic curves for the composites were calculated for the reacted fraction in the range from 0 to 80%. Based on the figure below, the absorption process at 250 °C can be best described by the CV 3D decrease surface while the desorption process at 300 °C can be best described by the JMA 2D.  Table 2 for (a) absorption kinetics at 250 °C and (b) desorption kinetics at 300 °C of 10 wt.% of LaCoO3 doped MgH2. Figure 3. The resulting calculation of different kinetic equations is described in Table 2 for (a) absorption kinetics at 250 • C and (b) desorption kinetics at 300 • C of 10 wt.% of LaCoO 3 doped MgH 2 .

Integrated Equation
DSC analyses were used to look into the effect of LaCoO 3 on the desorption kinetics of MgH 2 . Figure 4a exhibits the DSC curves of milled MgH 2 , while Figure 4b indicates the 10 wt.% of LaCoO 3 with MgH 2 composites heated at various heating rates. As the heating rises, the hydrogen desorption peaks move to a higher temperature. An endothermic peak for both samples was detected in Figure 4c for a heating rate of 25 • C/min, revealing that the decomposition from MgH 2 to Mg had occurred. As indicated in Figure 4c, the endothermic peak for milled MgH 2 was 433 • C, while the temperature was shifted to a lower temperature after 10 wt.% of LaCoO 3 was added (415 • C). From the results obtained for 10 wt.% of LaCoO 3 with MgH 2 , the desorption peak temperature by DSC and TPD were 415 • C and 322 • C, respectively. This was due to the different atmospheres and heating rates as explained in our previous research [39,40]. A similar outcome had been observed by Verma et al. [41], which revealed that different desorption temperatures for DSC and TPD could be detected when the experiment was conducted under different circumstances. temperature after 10 wt.% of LaCoO3 was added (415 °C). From the results obtained for 10 wt.% of LaCoO3 with MgH2, the desorption peak temperature by DSC and TPD were 415 °C and 322 °C, respectively. This was due to the different atmospheres and heating rates as explained in our previous research [39,40]. A similar outcome had been observed by Verma et al. [41], which revealed that different desorption temperatures for DSC and TPD could be detected when the experiment was conducted under different circumstances.
where A = linear constant, Tp = peak temperature in the DSC curve, R = gas constant, and β = heating rate of the samples. Hence, the EA of the thermal decomposition for 10 wt.% of LaCoO3 with MgH2 based on Equation (2) was approximately 90 kJ/mol, as demonstrated in Figure 5. Conversely, the EA for milled MgH2 was only 133 kJ/mol. The EA of 10 wt.% of LaCoO3 with MgH2 was lower than those of the other additives from previous studies, such as MgH2-KNbO3 [42], MgH2-Co@C [43], and MgH2-SrFe12O19 [6]. According to the findings, overcoming the barrier for converting MgH2 into Mg requires an EA of 90 kJ/mol, The Kissinger method was used for both samples to evaluate the effect of LaCoO 3 additives on desorption apparent activation energy (E A ), as shown in Equation (2) below where A = linear constant, T p = peak temperature in the DSC curve, R = gas constant, and β = heating rate of the samples. Hence, the E A of the thermal decomposition for 10 wt.% of LaCoO 3 with MgH 2 based on Equation (2) was approximately 90 kJ/mol, as demonstrated in Figure 5. Conversely, the E A for milled MgH 2 was only 133 kJ/mol. The E A of 10 wt.% of LaCoO 3 with MgH 2 was lower than those of the other additives from previous studies, such as MgH 2 -KNbO 3 [42], MgH 2 -Co@C [43], and MgH 2 -SrFe 12 O 19 [6]. According to the findings, overcoming the barrier for converting MgH 2 into Mg requires an E A of 90 kJ/mol, in the presence of 10 wt.% LaCoO 3 . It is also worth noting that LaCoO 3 additives lower the desorption peak temperature during the desorption processes of MgH 2 . The morphologies of pure MgH 2 , milled MgH 2 , and 10 wt.% of LaCoO 3 with MgH 2 were investigated by SEM as shown in Figure 6. The SEM of pure MgH 2 shown in Figure 6a revealed that the particles have a larger size and flakelike shapes, as reported by Czujko et al. [44]. Even though the samples were analyzed at different magnifications, as displayed in Figure 6b,d, both samples suggest that ball milling produces inhomogeneity, some agglomeration, and reduction in the MgH 2 samples caused by the ball collision. Smaller particle sizes for the milled MgH 2 suggests an enhancement in the desorption temperature of MgH 2 . Besides, Shang and colleagues [45] indicated that the particle sizes in the submicron range can be achieved by milling Mg with or without the presence of additives. Compared with milled MgH 2 samples, it was obvious that particles the size of 10 wt.% of LaCoO 3 with MgH 2 , as illustrated in Figure 6c,e, became smaller and less agglomerated, which may accelerate the desorption and absorption kinetics of MgH 2 due to the increase of specific surface area, even when the samples were investigated at different magnifications. Chawla et al. [46] exposed that mechanical milling of Mg with PdCl 2 increases the surface area, resulting in a reduction in hydrogen atom diffusion length and an improvement in hydrogen ab/desorption kinetics. A similar outcome was also reported by Zinsou and co-workers [47] which also revealed that smaller particles are expected to release hydrogen at a lower temperature than samples that have larger particle sizes.
Materials 2023, 16, 2449 8 of 14 in the presence of 10 wt.% LaCoO3. It is also worth noting that LaCoO3 additives lower the desorption peak temperature during the desorption processes of MgH2. The morphologies of pure MgH2, milled MgH2, and 10 wt.% of LaCoO3 with MgH2 were investigated by SEM as shown in Figure 6. The SEM of pure MgH2 shown in Figure  6a revealed that the particles have a larger size and flakelike shapes, as reported by Czujko et al. [44]. Even though the samples were analyzed at different magnifications, as displayed in Figure 6b,d, both samples suggest that ball milling produces inhomogeneity, some agglomeration, and reduction in the MgH2 samples caused by the ball collision. Smaller particle sizes for the milled MgH2 suggests an enhancement in the desorption temperature of MgH2. Besides, Shang and colleagues [45] indicated that the particle sizes in the submicron range can be achieved by milling Mg with or without the presence of additives. Compared with milled MgH2 samples, it was obvious that particles the size of 10 wt.% of LaCoO3 with MgH2, as illustrated in Figure 6c,e, became smaller and less agglomerated, which may accelerate the desorption and absorption kinetics of MgH2 due to the increase of specific surface area, even when the samples were investigated at different magnifications. Chawla et al. [46] exposed that mechanical milling of Mg with PdCl2 increases the surface area, resulting in a reduction in hydrogen atom diffusion length and an improvement in hydrogen ab/desorption kinetics. A similar outcome was also reported by Zinsou and co-workers [47] which also revealed that smaller particles are expected to release hydrogen at a lower temperature than samples that have larger particle sizes. Particle size distribution was calculated by using Image J. The particle size distribution for pure MgH2 is generally known to be 70 µm, as displayed in Figure 7a. Meanwhile, based on Figure 7b, the particle size distribution of MgH2 started to decrease after MgH2 was milled for 1 h (approximately 0.34 µm). Xiao and colleagues [48] have described that the particle size of commercial MgH2 significantly reduced to ~300 nm after MgH2 was Particle size distribution was calculated by using Image J. The particle size distribution for pure MgH 2 is generally known to be 70 µm, as displayed in Figure 7a. Meanwhile, based on Figure 7b, the particle size distribution of MgH 2 started to decrease after MgH 2 was milled for 1 h (approximately 0.34 µm). Xiao and colleagues [48] have described that the particle size of commercial MgH 2 significantly reduced to~300 nm after MgH 2 was milled. However, it is safer to assume that longer milling times would not result in a reduction in particle size as revealed by Rahmaninasab et al. [49]. Rahmaninasab et al. [49] also reported that the particle size of MgH 2 increased to 372 nm after the samples were milled for 40 h. Moreover, the particles of 10 wt.% of LaCoO 3 with MgH 2 were similar in size and less agglomerated, compared with those of milled MgH 2 , and most of the particles were single particles with 0.13 µm for particles size distribution, as indicated in Figure 7c. The addition of additive and milling methods effectively alters the distribution of MgH 2 . According to Si et al. [50], the increase in the small particles size was caused by the addition of Ni particles, which potentially improved the hydrogen storage performance of MgH 2 . Therefore, adding 10 wt.% of LaCoO 3 reduces the particles size and shortens the diffusion length of MgH 2 , and contributes in improving MgH 2 performance as observed.  Figure 8 presents the FTIR spectra of the samples before and after being doped with 10 wt.% LaCoO3. As seen from the figure below, the obvious signature bands for the Mg-H bending and Mg-H stretching are located between 400-800 cm −1 and 900-1200 cm −1 , respectively, for pure MgH2, milled MgH2, and 10 wt.% of LaCoO3 with MgH2. The results suggest that no obvious reactions occurred due to its relatively low content of LaCoO3. Moreover, after adding 10 wt.% of LaCoO3 to MgH2, the bending bands of the samples tend to shift to lower wavenumbers, implying the weakness of the Mg-H bonds as proposed in a previous study [51].  Figure 8 presents the FTIR spectra of the samples before and after being doped with 10 wt.% LaCoO 3 . As seen from the figure below, the obvious signature bands for the Mg-H bending and Mg-H stretching are located between 400-800 cm −1 and 900-1200 cm −1 , respectively, for pure MgH 2 , milled MgH 2 , and 10 wt.% of LaCoO 3 with MgH 2 . The results suggest that no obvious reactions occurred due to its relatively low content of LaCoO 3 . Moreover, after adding 10 wt.% of LaCoO 3 to MgH 2 , the bending bands of the samples tend to shift to lower wavenumbers, implying the weakness of the Mg-H bonds as proposed in a previous study [51]. Figure 8 presents the FTIR spectra of the samples before and after being doped with 10 wt.% LaCoO3. As seen from the figure below, the obvious signature bands for the Mg-H bending and Mg-H stretching are located between 400-800 cm −1 and 900-1200 cm −1 , respectively, for pure MgH2, milled MgH2, and 10 wt.% of LaCoO3 with MgH2. The results suggest that no obvious reactions occurred due to its relatively low content of LaCoO3. Moreover, after adding 10 wt.% of LaCoO3 to MgH2, the bending bands of the samples tend to shift to lower wavenumbers, implying the weakness of the Mg-H bonds as proposed in a previous study [51].  The XRD pattern shown here in Figure 9 was used to clarify the mechanism of 10 wt.% of LaCoO 3 on MgH 2 hydrogen storage performance. After 10 wt.% of LaCoO 3 with MgH 2 was milled for 1 h, as shown in Figure 9a, the peaks of the composites mainly corresponded to parent materials which are MgH 2 and LaCoO 3 , indicating that LaCoO 3 had not reacted with MgH 2 during the milling process. Based on Figure 9b, after the composites were heated up to 450 • C, MgH 2 peaks completely disappeared and transformed to Mg, implying that the decomposition process had occurred completely. A new peak of CoO, La 2 O 3 , and MgO was detected. The 10 wt.% of LaCoO 3 with MgH 2 during the absorption process at 250 • C had also been conducted, as shown in Figure 9c. The Mg peaks were converted to MgH 2 , revealing that the hydrogen absorption process had completely occurred. However, the peak of CoO, La 2 O 3 , and MgO were still detected. The following equation (Equation (3)) can be used to predict the reaction between 10 wt.% of LaCoO 3 with MgH 2 : The XRD pattern shown here in Figure 9 was used to clarify the mechanism of 10 wt.% of LaCoO3 on MgH2 hydrogen storage performance. After 10 wt.% of LaCoO3 with MgH2 was milled for 1 h, as shown in Figure 9a, the peaks of the composites mainly corresponded to parent materials which are MgH2 and LaCoO3, indicating that LaCoO3 had not reacted with MgH2 during the milling process. Based on Figure 9b, after the composites were heated up to 450 °C, MgH2 peaks completely disappeared and transformed to Mg, implying that the decomposition process had occurred completely. A new peak of CoO, La2O3, and MgO was detected. The 10 wt.% of LaCoO3 with MgH2 during the absorption process at 250 °C had also been conducted, as shown in Figure 9c. The Mg peaks were converted to MgH2, revealing that the hydrogen absorption process had completely occurred. However, the peak of CoO, La2O3, and MgO were still detected. The following equation (Equation (3)) can be used to predict the reaction between 10 wt.% of LaCoO3 with MgH2: MgH2 + 2LaCoO3 → 2CoO + La2O3 + MgO + H2 (3) A prior study conducted by Rahman et al. [42] discovered that adding KNbO3 greatly reduced the onset desorption temperature from 370 °C to 327 °C, and lowered the EA by 61 kJ/mol, when compared with milled MgH2. Additionally, when as-synthesized Co@C was added to MgH2, the desorption temperature was reduced by 99 °C compared with A prior study conducted by Rahman et al. [42] discovered that adding KNbO 3 greatly reduced the onset desorption temperature from 370 • C to 327 • C, and lowered the E A by 61 kJ/mol, when compared with milled MgH 2 . Additionally, when as-synthesized Co@C was added to MgH 2 , the desorption temperature was reduced by 99 • C compared with milled MgH 2 [43]. Furthermore, at 300 • C, MgH 2 -Co@C composites absorbed 5.96 wt.% of H 2 in 10 min, and desorbed 5.74 wt.% of H 2 in 1 h. Another metal oxide catalyst, SrFe 12 O 19 , also exhibited superior performance for MgH 2 [6]. In comparison to milled MgH 2 , the introduction of SrFe 12 O 19 decreased the E A and onset desorption temperature from 350 • C to 270 • C, and 133.31 kJ/mol to 114.22 kJ/mol, respectively. According to the findings, the in-situ formation of SrO, MgFe 2 O 4 , and Fe plays a crucial role in enhancing the hydrogen storage properties of MgH 2 . In this study, according to the results of the onset desorption temperature, ab/desorption kinetics, and the activation energy, adding the LaCoO 3 (metal oxides) additive greatly enhances the hydrogen storage performance of MgH 2 . The onset desorption temperature was reduced by 28 • C and the E A was lowered by 43 kJ/mol. In addition, this composite could absorb 7.30 wt.% of H 2 at 250 • C and desorb 3.24 wt.% of H 2 at 300 • C, which is faster in kinetics compared with milled MgH 2 . The further study exposed that in situ-generated CoO, La 2 O 3 , and MgO could play a significant role in enhancing the dehydrogenation performance of MgH 2 .
According to Lee et al. [52], adding CoO has the best impact on the hydrogen sorption properties of Mg. Reactive grinding of Mg with Co/CoO causes defects and cracks in the surface of Mg particles, thus shortening diffusion distances between the samples. In addition, adding a minimal amount of CoO diminishes the dehydrogenation temperature and speeds up the dehydrogenation rate of LiBH 4 .NH 3 -3LiH system [53]. Furthermore, adding La 2 O 3 to MgH 2 improves the sorption properties [54]. Our previous study [19] also found that the introduction of 10 wt.% LaFeO 3 positively affected the sorption kinetics of MgH 2 . After the addition of LaFeO 3 , the E A decreased by 32 kJ/mol. Besides this, when MgH 2 was milled with MgO, the hydrogen kinetics were dramatically improved compared with MgH 2 [55]. At 300 • C, faster hydrogen absorption and desorption kinetics can be accomplished in <100 s. Based on the discussions above, the introduction of LaCoO 3 lowered the onset desorption temperature and enhanced the kinetic properties of MgH 2 via the formation of in situ-generated CoO, La 2 O 3 , and MgO.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.