Improved Dehydrogenation Properties of LiAlH4 by Addition of Nanosized CoTiO3

Despite the application of lithium aluminium hydride (LiAlH4) being hindered by its sluggish desorption kinetics and unfavourable reversibility, LiAlH4 has received special attention as a promising solid-state hydrogen storage material due to its hydrogen storage capacity (10.5 wt.%). In this work, investigated for the first time was the effect of the nanosized cobalt titanate (CoTiO3) which was synthesised via a solid-state method on the desorption behaviour of LiAlH4. Superior desorption behaviour of LiAlH4 was attained with the presence of a CoTiO3 additive. By means of the addition of 5, 10, 15 and 20 wt.% of CoTiO3, the initial desorption temperature of LiAlH4 for the first stage was reduced to around 115–120 °C and the second desorption stage was reduced to around 144–150 °C, much lower than for undoped LiAlH4. The LiAlH4-CoTiO3 sample also presents outstanding desorption kinetics behaviour, desorbing hydrogen 30–35 times faster than undoped LiAlH4. The LiAlH4-CoTiO3 sample could desorb 3.0–3.5 wt.% H2 in 30 min, while the commercial and milled LiAlH4 desorbs <0.1 wt.% H2. The apparent activation energy of the LiAlH4-CoTiO3 sample based on the Kissinger analysis was decreased to 75.2 and 91.8 kJ/mol for the first and second desorption stage, respectively, lower by 28.0 and 24.9 kJ/mol than undoped LiAlH4. The LiAlH4-CoTiO3 sample presents uniform and smaller particle size distribution compared to undoped LiAlH4, which is irregular in shape with some agglomerations. The experimental results suggest that the CoTiO3 additive promoted notable advancements in the desorption performance of LiAlH4 through the in situ-formed AlTi and amorphous Co or Co-containing active species that were generated during the desorption process.


Introduction
Hydrogen is a clean and renewable energy carrier and has become the most preferred alternative to fossil fuels for the future energy system. The advancement of secure and effective solid-state hydrogen storage that operates at low temperature was the focus in developing practical hydrogen as a potential energy carrier, offering a high capacity of hydrogen and excellent reversibility. Recently, alanates have been considered favourable materials for solid-state hydrogen storage with high hydrogen capacity and operating at low temperatures compared to other hydrides [1,2]. Among the alanates, lithium aluminium hydride (LiAlH 4 ) has attracted considerable research interest. LiAlH 4 offers 10.5 wt.% hydrogen content [3]. Below 250 • C, LiAlH 4 is able to liberate a theoretical capacity of 7.9 wt.% H 2 [4,5]. Basically, LiAlH 4 decomposes to LiH and Al in a three-step reaction as follows [6,7]: 3LiAlH 4 → Li 3 AlH 6 + 2Al + 3H 2 (1) (occurs at 150-175 • C, releasing 5.3 wt.% H 2 ) Li 3 AlH 6 → 3LiH + Al + 3/2H 2 (2) However, inadequate reversibility and unfavourable desorption kinetics of LiAlH 4 obstruct its progression to practical utilisation as a solid-state hydrogen storage medium [8]. Furthermore, the 2.6 wt.% H 2 in LiH as in Equation (3) is impracticable for applications owing to its high decomposition temperature (higher than 400 • C) [9]. To overcome these problems, several techniques have been introduced to accelerate the desorption behaviour of LiAlH 4for example, the use of the nanoconfinement method and high energy ball milling [10][11][12][13], the usage of additives [14][15][16][17] and destabilisation with other hydrides [18][19][20].
The technique of high energy ball milling may exert a colossal effect on the behaviour of solid-state hydrogen storage materials by reducing the particle size that results in a larger surface area, hence shortening the distance of hydrogen diffusion [21,22]. The presence of additives, especially metal oxides, leads to a significant improvement, with a lower decomposition temperature and faster kinetic performance of LiAlH 4 due to enhanced catalytic support [23][24][25][26]. For instance, Li et al. [27] demonstrated satisfactory desorption behaviour of LiAlH 4 with the addition of NiCo 2 O 4 . The LiAlH 4 -NiCo 2 O 4 sample could release 4.95 wt.% H 2 at 130 • C in 150 min, while undoped LiAlH 4 only releases <1.0 wt.% H 2 within the same period. Then, Sulaiman et al. [28] showed that LiAlH 4 added with SrFe 12 O 19 began to decompose at temperatures of 80 • C (first stage) and 130 • C (second stage), which are lower than undoped LiAlH 4 . They also observed the formation of Fe, LiFeO 2 , Sr and Sr-containing species during the heating process, which are advantageous in boosting the desorption properties of LiAlH 4 . Furthermore, Li et al. [29] presented a study on the influence of CoFe 2 O 4 on the desorption of LiAlH 4 . Surprisingly, the performance of LiAlH 4 was significantly enhanced with rapid hydrogen desorption and low dehydrogenation activation energy. The LiAlH 4 -CoFe 2 O 4 sample could desorb 6.8 wt.% H 2 in 160 min, 6.1 wt.% higher than undoped LiAlH 4 . Moreover, the activation energy of the LiAlH 4 -CoFe 2 O 4 composites decreased by 42.4 kJ/mol (first stage) and 86.1 kJ/mol (second stage) compared to pure LiAlH 4 . The enhanced performance of the LiAlH 4 -CoFe 2 O 4 composite was attributed to the formation of Fe and Co species due to the reaction between LiAlH 4 and CoFe 2 O 4 . Then, another study revealed that the addition of TiO 2 could notably downshift the decomposition temperature of LiAlH 4 to 60 • C [30]. Desorption kinetics performance at 100 • C demonstrated that a TiO 2 -doped LiAlH 4 composite could desorb 5.2 wt.% H 2 in 30 min, whereas the commercial LiAlH 4 desorbed only <0.2 wt.% H 2 in the same time but at a higher temperature (120 • C). The addition of TiO 2 offers significant enhancement of the performance of LiAlH 4 that may be ascribed to the introduction of a high density of defects on the surfaces of the TiO 2 particles throughout the ball milling process.
Inspired by the superior performance of the metal oxide additives, it is motivating to discover the consequence of another metal oxide on the desorption behaviour of LiAlH 4 . The metal oxide additives may present outstanding performance compared to metal because of the bulk of oxides and the transition metal ions on the surface that may encounter different crystal fields due to the lack of ions at the surface of these oxides. This allows the electronic 3d state of the ions to be separated and is beneficial for the catalytic activity of transition metal oxides in the adsorption of gas molecules [31]. In this study, nanosized cobalt titanate (CoTiO 3 ) as an additive was added to LiAlH 4 by means of a ball milling process to ameliorate the desorption behaviour of LiAlH 4 . Nanosized approaches are favourable in decreasing the materials' particle size and reducing the hydrogen diffusion distance, accelerating the rate of hydrogen diffusion and nucleation sites for the desorption process [32]. Moreover, it is believed that nanosized CoTiO 3 could provide a superior performance of LiAlH 4 with low hydrogen release temperature and faster kinetics. Herein, different amounts of CoTiO 3 were milled together with LiAlH 4 to examine the desorption Nanomaterials 2022, 12, 3921 3 of 12 behaviour of LiAlH 4 as well as its kinetics behaviour. Moreover, the possible catalytic mechanism of the desorption process for this system has been discussed in depth.

Materials and Methods
Without any prior processing, LiAlH 4 (95% pure) from Sigma Aldrich was used. The nanosized CoTiO 3 was synthesised via the solid-state method, as explained in our previous work [33]. Then, various amounts of CoTiO 3 (5 wt.%, 10 wt.%, 15 wt.% and 20 wt.%) were doped with LiAlH 4 to study its influence on the desorption behaviour of LiAlH 4 . The composite was milled together at a speed of 400 rpm for one hour using a planetary ball mill (NQM-0.4).
A Sievert-type pressure-composition-temperature apparatus from the Advanced Materials Corporation was operated to carry out the hydrogen desorption kinetics experiment and the temperature-programmed desorption (TPD). The TPD test was carried out in a vacuum chamber and heated to 250 • C for the purpose of finding the initial desorption temperature. Experiments for the isothermal desorption tests were carried out at 90 • C and 1.0 atm H 2 pressure. Using a Mettler Toledo TGA/DSC 1, differential scanning calorimetry (DSC) measurements were carried out to analyse the thermal characteristics and determine the activation energy. For each characterisation, 6-8 mg of the sample was loaded in a crucible and heated to 300 • C with a flow of argon (50 mL/min) at several heating rates (15 • C/min, 20 • C/min, 25 • C/min and 30 • C/min).
The phase structure of the LiAlH 4 -CoTiO 3 sample after milling and after the desorption sample was examined using a Rigaku MiniFlex X-ray diffraction (XRD) system with Cu Kα radiation. The sample was tested at a rate of 2.00 • /min over diffraction angles of 20 • to 80 • . The sample's morphology and microstructure were then examined with a scanning electron microscope ((SEM; JEOL, Akishima, Tokyo, Japan) (JSM-6350LA)). The Fourier transform infrared (FTIR) spectrometer was tested in the range of 700 and 2000 cm −1 using IR Shimadzu Tracer-100.

Results and Discussions
The influence of CoTiO 3 on the initial desorption temperature of LiAlH 4 was characterised by the TPD experiment, as presented in Figure 1. By referring to the figure, the addition of CoTiO 3 results in superior desorption performance of LiAlH 4 . Commercial LiAlH 4 begins to release hydrogen at 145 • C (first stage) and 175 • C (second stage) releasing around 7.4 wt.% H 2 . After 1 h of milling, the temperature to release hydrogen was similar to the commercial LiAlH 4 , demonstrating that the milling process had a negligible influence on the desorption temperature of LiAlH 4 . Contrarily, by adding different amounts of CoTiO 3 (5, 10, 15 and 20 wt.%), the hydrogen release initiates at a much lower temperature than commercial and milled LiAlH 4 . The LiAlH 4 + 5 wt.% CoTiO 3 sample began to release hydrogen at 120 • C (first stage) and 150 • C (second stage), releasing 6.9 wt.% H 2 . Furthermore, increasing the amount of CoTiO 3 to 10 wt.% resulted in a decrease in hydrogen release temperature to 115 • C (first stage) and 145 • C (second stage), releasing 6.2 wt.% H 2 . Adding 15 wt.% of CoTiO 3 resulted in a similar temperature for hydrogen release as the LiAlH 4 + 10 wt.% CoTiO 3 sample. For the LiAlH 4 + 20 wt.% CoTiO 3 sample, hydrogen release occurs at 118 • C (first stage) and 144 • C (second stage), releasing 6.0 wt.% H 2 . The hydrogen release temperature for all of the CoTiO 3 -doped LiAlH 4 samples was much lower for than the undoped LiAlH 4 . Notably, with the amount of hydrogen released being slightly reducing compared to the undoped LiAlH 4, the presence of CoTiO 3 provides positive contributions in lowering the temperature for the LiAlH 4 to release hydrogen. The amount of hydrogen released for all of the CoTiO 3 -doped LiAlH 4 samples was slightly reduced compared to undoped LiAlH 4 due to the deadweight of CoTiO 3 that does not hold any hydrogen [10,34]. provides positive contributions in lowering the temperature for the LiAlH4 to release hydrogen. The amount of hydrogen released for all of the CoTiO3-doped LiAlH4 samples was slightly reduced compared to undoped LiAlH4 due to the deadweight of CoTiO3 that does not hold any hydrogen [10,34]. Additionally, Figure 2 demonstrates the desorption kinetic performance of LiAlH4 with and without CoTiO3 at 90 °C. As shown in Figure 2, the LiAlH4 + xCoTiO3 (x = 5, 10, 15 and 20 wt.%) releases a large amount of hydrogen in a short period of time. Within 30 min, the LiAlH4-CoTiO3 sample releases 3.0-3.5 wt.% H2. However, the undoped LiAlH4 shows sluggish kinetics , with the ability to release only <0.1 wt.% H2 within the same period. The addition of CoTiO3 results in accelerated desorption kinetics which are 30-35 times faster than undoped LiAlH4. The rapid hydrogen release from the LiAlH4-CoTiO3 sample could be related to the creation of surface defects and active materials as a result of a reaction between LiAlH4 and CoTiO3. Therefore, it can be concluded that the kinetic behaviour of LiAlH4 can be enhanced by introducing CoTiO3. Considering the initial desorption temperature and the desorption kinetics performance, the LiAlH4 + 10 wt.% CoTiO3 sample is selected to further examine the effect of CoTiO3 on the desorption behaviour of LiAlH4.
The effect of CoTiO3 on the thermal behaviour of LiAlH4 was characterised by DSC, as indicated in Figure 3. By referring to Figure 3, the thermal properties of as-milled LiAlH4 consist of four peaks attributed to the two endothermic and two exothermic peaks. The first exothermic peak (140 °C) was due to the interaction of hydroxyl impurities at the surface of LiAlH4, while the first endothermic peak (163 °C) is attributed to the melting of LiAlH4. The second exothermic peak (175 °C) is ascribed to the process of LiAlH4 decomposing to Li3AlH6 and Al, while the second endothermic peak (230 °C) relates to the process of Li3AlH6 decomposing to LiH and Al. Upon the addition of CoTiO3, the number of peaks was decreased to two, namely one endothermic peak and one exothermic peak, respectively, that occur at a lower temperature than for milled LiAlH4. The exothermic peak (LiAlH4 decomposition) occurs at 105 °C and the endothermic peak (Li3AlH6 decomposition) occurs at 196 °C. Implying that the melt of LiAlH4 is inhibited after the addition of the nanosized CoTiO3, the LiAlH4 + 10 wt.% CoTiO3 sample begins to decompose prior to melting. These observations are consistent with the previous reports [35,36]. Additionally, Figure 2 demonstrates the desorption kinetic performance of LiAlH 4 with and without CoTiO 3 at 90 • C. As shown in Figure 2, the LiAlH 4 + xCoTiO 3 (x = 5, 10, 15 and 20 wt.%) releases a large amount of hydrogen in a short period of time. Within 30 min, the LiAlH 4 -CoTiO 3 sample releases 3.0-3.5 wt.% H 2 . However, the undoped LiAlH 4 shows sluggish kinetics, with the ability to release only <0.1 wt.% H 2 within the same period. The addition of CoTiO 3 results in accelerated desorption kinetics which are 30-35 times faster than undoped LiAlH 4 . The rapid hydrogen release from the LiAlH 4 -CoTiO 3 sample could be related to the creation of surface defects and active materials as a result of a reaction between LiAlH 4 and CoTiO 3 . Therefore, it can be concluded that the kinetic behaviour of LiAlH 4 can be enhanced by introducing CoTiO 3 . Considering the initial desorption temperature and the desorption kinetics performance, the LiAlH 4 + 10 wt.% CoTiO 3 sample is selected to further examine the effect of CoTiO 3 on the desorption behaviour of LiAlH 4 .  The effect of CoTiO 3 on the thermal behaviour of LiAlH 4 was characterised by DSC, as indicated in Figure 3. By referring to Figure 3, the thermal properties of as-milled LiAlH 4 consist of four peaks attributed to the two endothermic and two exothermic peaks. The first exothermic peak (140 • C) was due to the interaction of hydroxyl impurities at the surface of LiAlH 4 , while the first endothermic peak (163 • C) is attributed to the melting of LiAlH 4 . The second exothermic peak (175 • C) is ascribed to the process of LiAlH 4 decomposing to Li 3 AlH 6 and Al, while the second endothermic peak (230 • C) relates to the process of Li 3 AlH 6 decomposing to LiH and Al. Upon the addition of CoTiO 3 , the number of peaks was decreased to two, namely one endothermic peak and one exothermic peak, respectively, that occur at a lower temperature than for milled LiAlH 4 . The exothermic peak (LiAlH 4 decomposition) occurs at 105 • C and the endothermic peak (Li 3 AlH 6 decomposition) occurs at 196 • C. Implying that the melt of LiAlH 4 is inhibited after the addition of the nanosized CoTiO 3 , the LiAlH 4 + 10 wt.% CoTiO 3 sample begins to decompose prior to melting. These observations are consistent with the previous reports [35,36].  To scrutinise the impact of nanosized CoTiO3 addition on the desorption activation energy of LiAlH4, the DSC measurement was performed at several heating rates. The DSC profiles of as-milled LiAlH4 and LiAlH4 + 10 wt.% CoTiO3 at several heating rates are shown in Figure 4. As depicted in the figure, the decomposition peak of LiAlH4 + 10 wt.% CoTiO3 occurs at a lower temperature than that of undoped LiAlH4. The apparent activation energy (EA) could be fitted from the DSC curve and calculated by Kissinger analysis as follows: where β is the heating rate, Tp is the endothermic peak related to the decomposition temperature, EA is the activation energy, R is the gas constant and A is a linear constant. To scrutinise the impact of nanosized CoTiO 3 addition on the desorption activation energy of LiAlH 4 , the DSC measurement was performed at several heating rates. The DSC profiles of as-milled LiAlH 4 and LiAlH 4 + 10 wt.% CoTiO 3 at several heating rates are shown in Figure 4. As depicted in the figure, the decomposition peak of LiAlH 4 + 10 wt.% CoTiO 3 occurs at a lower temperature than that of undoped LiAlH 4 . The apparent activation energy (E A ) could be fitted from the DSC curve and calculated by Kissinger analysis as follows: where β is the heating rate, T p is the endothermic peak related to the decomposition temperature, E A is the activation energy, R is the gas constant and A is a linear constant. Then, the activation energy is determined based on the Kissinger plot of ln [β/T p 2 ] versus 1000/T p , as in Figure 5. The activation energy of the undoped LiAlH 4 for the first and second desorption stage are 103.2 and 116.7 kJ/mol, respectively, based on the Kissinger plot. By adding nanosized CoTiO 3 , the activation energy of LiAlH 4 was notably decreased to 75.2 and 91.8 kJ/mol for the first and second desorption stage, respectively, being lowered by 27% for the first stage and 21% for the second stage compared to undoped LiAlH 4 . The reduced activation energy was in good agreement with the reduced decomposition temperature. From these results, it can be clearly seen that the introduction of CoTiO 3 as an additive remarkably reduced the activation energy of LiAlH 4 , demonstrating that CoTiO 3 has a favourable impact on the desorption behaviours of LiAlH 4 .  Then, the activation energy is determined based on the Kissinger plot of ln [β/Tp 2 ] versus 1000/Tp, as in Figure 5. The activation energy of the undoped LiAlH4 for the first and second desorption stage are 103.2 and 116.7 kJ/mol, respectively, based on the Kissinger plot. By adding nanosized CoTiO3, the activation energy of LiAlH4 was notably decreased to 75.2 and 91.8 kJ/mol for the first and second desorption stage, respectively, being lowered by 27% for the first stage and 21% for the second stage compared to undoped LiAlH4. The reduced activation energy was in good agreement with the reduced decomposition temperature. From these results, it can be clearly seen that the introduction of CoTiO3 as an additive remarkably reduced the activation energy of LiAlH4, demonstrating that CoTiO3 has a favourable impact on the desorption behaviours of LiAlH4.  Figure 6 displays the SEM micrograph of commercial LiAlH4, as-milled LiAlH4 and LiAlH4 + 10 wt.% CoTiO3 sample. By referring to Figure 6a, commercial LiAlH4 consists of large uniform and blocky-shaped particles. After undergoing the milling process of one hour (Figure 6b), the blocky shape was broken into non-uniform smaller particles, but with some agglomerations. Meanwhile, after the addition of CoTiO3, as shown in Figure  6c, the particle was transformed to a finer shape and size. The morphology of the composite sample (CoTiO3-doped LiAlH4) presents more uniform distributions with fewer agglomerations. The difference in morphology between the milled LiAlH4 and LiAlH4 + 10 wt.% CoTiO3 sample may be due to the presence of an additive that acts as a lubricant and prevents the agglomerations of the sample. These superfine particles could facilitate the rapid hydrogen desorption of LiAlH4 [32]. As reported previously, metal oxide-based  Then, the activation energy is determined based on the Kissinger plot of ln [β versus 1000/Tp, as in Figure 5. The activation energy of the undoped LiAlH4 for the and second desorption stage are 103.2 and 116.7 kJ/mol, respectively, based on the Ki ger plot. By adding nanosized CoTiO3, the activation energy of LiAlH4 was notably creased to 75.2 and 91.8 kJ/mol for the first and second desorption stage, respectively ing lowered by 27% for the first stage and 21% for the second stage compared to undo LiAlH4. The reduced activation energy was in good agreement with the reduced dec position temperature. From these results, it can be clearly seen that the introductio CoTiO3 as an additive remarkably reduced the activation energy of LiAlH4, demonstra that CoTiO3 has a favourable impact on the desorption behaviours of LiAlH4.  Figure 6 displays the SEM micrograph of commercial LiAlH4, as-milled LiAlH4 LiAlH4 + 10 wt.% CoTiO3 sample. By referring to Figure 6a, commercial LiAlH4 con of large uniform and blocky-shaped particles. After undergoing the milling process of hour (Figure 6b), the blocky shape was broken into non-uniform smaller particles with some agglomerations. Meanwhile, after the addition of CoTiO3, as shown in Fi 6c, the particle was transformed to a finer shape and size. The morphology of the com site sample (CoTiO3-doped LiAlH4) presents more uniform distributions with fewe glomerations. The difference in morphology between the milled LiAlH4 and LiAlH4 wt.% CoTiO3 sample may be due to the presence of an additive that acts as a lubricant prevents the agglomerations of the sample. These superfine particles could facilitate rapid hydrogen desorption of LiAlH4 [32]. As reported previously, metal oxide-b   (Figure 6b), the blocky shape was broken into non-uniform smaller particles, but with some agglomerations. Meanwhile, after the addition of CoTiO 3 , as shown in Figure 6c, the particle was transformed to a finer shape and size. The morphology of the composite sample (CoTiO 3 -doped LiAlH 4 ) presents more uniform distributions with fewer agglomerations. The difference in morphology between the milled LiAlH 4 and LiAlH 4 + 10 wt.% CoTiO 3 sample may be due to the presence of an additive that acts as a lubricant and prevents the agglomerations of the sample. These superfine particles could facilitate the rapid hydrogen desorption of LiAlH 4 [32]. As reported previously, metal oxide-based additives have shown good lubricating performance [37,38]. This indicates that the addition of CoTiO 3 is favourable in constraining the agglomeration of the LiAlH 4 and results in a smaller and finer shape of the particle. Figure 7 presents the particle distribution size of commercial LiAlH 4 , as-milled LiAlH 4 and LiAlH 4 + 10 wt.% CoTiO 3 sample evaluated by the Image J software. The average particle size of the commercial LiAlH 4 , as-milled LiAlH 4 and LiAlH 4 + 10 wt.% CoTiO 3 sample was calculated to be~44, 0.5 and 0.3 µm, respectively. The morphological alteration and significant size reduction are responsible for the large surface defects and the expansion of grain boundaries across the composite's surface [39]. Resulting in more reactions for the nucleation sites and better diffusion channels of hydrogen being achieved with amplified grain boundaries that enhance the desorption behaviour of the LiAlH 4 -CoTiO 3 sample with rapid desorption kinetics and lower activation energy, accelerating the rate of hydrogen diffusion [40], the reduction in particle size also results in a shorter hydrogen diffusion path. Similarly, other findings also found that smaller and finer particle distributions demonstrated superior dehydriding performance of LiAlH 4 [36,41]. additives have shown good lubricating performance [37,38]. This indicates that the addition of CoTiO3 is favourable in constraining the agglomeration of the LiAlH4 and results in a smaller and finer shape of the particle.  Figure 7 presents the particle distribution size of commercial LiAlH4, as-milled LiAlH4 and LiAlH4 + 10 wt.% CoTiO3 sample evaluated by the Image J software. The average particle size of the commercial LiAlH4, as-milled LiAlH4 and LiAlH4 + 10 wt.% CoTiO3 sample was calculated to be ~44, 0.5 and 0.3 µm, respectively. The morphological alteration and significant size reduction are responsible for the large surface defects and the expansion of grain boundaries across the composite's surface [39]. Resulting in more reactions for the nucleation sites and better diffusion channels of hydrogen being achieved with amplified grain boundaries that enhance the desorption behaviour of the LiAlH4-CoTiO3 sample with rapid desorption kinetics and lower activation energy, accelerating the rate of hydrogen diffusion [40], the reduction in particle size also results in a shorter hydrogen diffusion path. Similarly, other findings also found that smaller and finer particle distributions demonstrated superior dehydriding performance of LiAlH4 [36,41]. To discover the reaction mechanism during the milling process, XRD analysis was carried out as presented in Figure 8. Figure 8a shows the XRD peaks of commercial LiAlH4, and it appears that only peaks of LiAlH4 were discovered, indicating the high purity of LiAlH4. For the milled sample (Figure 8b), similar to commercial LiAlH4, only To discover the reaction mechanism during the milling process, XRD analysis was carried out as presented in Figure 8. Figure 8a shows the XRD peaks of commercial LiAlH 4 , and it appears that only peaks of LiAlH 4 were discovered, indicating the high purity of LiAlH 4 . For the milled sample (Figure 8b), similar to commercial LiAlH 4 , only peaks of LiAlH 4 were detected, suggesting that, similar to a previous report [42], LiAlH 4 maintains high stability throughout the milling process. Meanwhile, for the LiAlH 4 + 10 wt.% CoTiO 3 , as shown in Figure 8c, only LiAlH 4 peaks were visible, while the peaks of CoTiO 3 were not identified by the XRD, indicating that the amount of CoTiO 3 was too small to be detected by the XRD. This phenomenon was comparable to a prior work where several additives such as Ti 3 C 2 and FeCl 2 were not detected by XRD [2,43]. To discover the reaction mechanism during the milling process, XRD analysis was carried out as presented in Figure 8. Figure 8a shows the XRD peaks of commercial LiAlH4, and it appears that only peaks of LiAlH4 were discovered, indicating the high purity of LiAlH4. For the milled sample (Figure 8b), similar to commercial LiAlH4, only peaks of LiAlH4 were detected, suggesting that, similar to a previous report [42], LiAlH4 maintains high stability throughout the milling process. Meanwhile, for the LiAlH4 + 10 wt.% CoTiO3, as shown in Figure 8c, only LiAlH4 peaks were visible, while the peaks of CoTiO3 were not identified by the XRD, indicating that the amount of CoTiO3 was too small to be detected by the XRD. This phenomenon was comparable to a prior work where several additives such as Ti3C2 and FeCl2 were not detected by XRD [2,43].   Figure 9 depicts the FTIR spectra of commercial LiAlH 4 , as-milled LiAlH 4 and LiAlH 4 + 10 wt.% CoTiO 3 sample. FTIR analysis was performed to determine the existence of Li 3 AlH 6 after the milling process. All three samples exhibit two distinct region modes around 800-900 cm −1 , corresponding to the Li-Al-H bending mode, and 1600-1800 cm −1 , attributed to the Al-H stretching mode. However, for the LiAlH 4 + 10 wt.% CoTiO 3 sample (Figure 9c), an extra absorbance peak around 1400 cm −1 was identified, indicating the presence of the Al-H stretching mode corresponding to the Li 3 AlH 6 . This result proves that LiAlH 4 was slightly decomposed to Li 3 AlH 6 during the milling process of LiAlH 4 and CoTiO 3 .
To explore the specific mechanism and catalytic activity of CoTiO 3 that contributed to the superior desorption behaviours of LiAlH 4 , the 10 wt.% and 20 wt.% CoTiO 3 -doped LiAlH 4 samples after desorption at 250 • C were analysed using XRD, as depicted in Figure 10. After the desorption process of the 10 wt.% CoTiO 3 -doped LiAlH 4 sample (Figure 10a), the main peaks detected are the dehydrogenated products of LiAlH 4 , which are LiH and Al, denoting that the complete desorption process of LiAlH 4 occurs. Moreover, the peaks of AlTi were also detected, indicating that there is a reaction between LiAlH 4 and CoTiO 3 . However, the peaks of Co or Co-containing species were not identified. The additional characterisation was carried out with 20 wt.% of CoTiO 3 (Figure 10b). Similarly, the main peaks of LiH and Al were observed with the additional peak of AlTi, and the fact that no peaks of Co or Co-containing species were identified after the desorption process may be due to the amorphous form of Co or Co-containing species.
Li3AlH6 after the milling process. All three samples exhibit two distinct region modes around 800-900 cm −1 , corresponding to the Li-Al-H bending mode, and 1600-1800 cm −1 , attributed to the Al-H stretching mode. However, for the LiAlH4 + 10 wt.% CoTiO3 sample (Figure 9c), an extra absorbance peak around 1400 cm −1 was identified, indicating the presence of the Al-H stretching mode corresponding to the Li3AlH6. This result proves that LiAlH4 was slightly decomposed to Li3AlH6 during the milling process of LiAlH4 and CoTiO3. To explore the specific mechanism and catalytic activity of CoTiO3 that contributed to the superior desorption behaviours of LiAlH4, the 10 wt.% and 20 wt.% CoTiO3-doped LiAlH4 samples after desorption at 250 °C were analysed using XRD, as depicted in Figure  10. After the desorption process of the 10 wt.% CoTiO3-doped LiAlH4 sample (Figure 10a), the main peaks detected are the dehydrogenated products of LiAlH4, which are LiH and Al, denoting that the complete desorption process of LiAlH4 occurs. Moreover, the peaks of AlTi were also detected, indicating that there is a reaction between LiAlH4 and CoTiO3. However, the peaks of Co or Co-containing species were not identified. The additional characterisation was carried out with 20 wt.% of CoTiO3 (Figure 10b). Similarly, the main peaks of LiH and Al were observed with the additional peak of AlTi, and the fact that no peaks of Co or Co-containing species were identified after the desorption process may be due to the amorphous form of Co or Co-containing species.
Referring to the results, it is noteworthy to state that the desorption behaviours of LiAlH4 were improved due to the in situ formation of active species during the heating process. In this study, the formation of AlTi and Co or Co-containing species was believed to contribute to the enhanced desorption behaviours of LiAlH4. This phenomenon agrees with a previous study that also reported the formation of AlTi after the dehydrogenation process of LiAlH4 [3,44]. The in situ formation of AlTi is beneficial in ameliorating the desorption behaviour of LiAlH4 by catalysing the dehydrogenation through reaction in Equations (1) and (2). In addition, the study performed by Wohlwend et al. [45] revealed the superior performance of LiAlH4 with the addition of Ti-based additives. The Ti atom reacts strongly with the LiAlH4 surface and reduces the H binding energy. The reduction in binding energy may be a result of the correlation of the charge transfer change between Al and H that results in the accelerated kinetic performance of LiAlH4. It denotes that Co or Co-containing species also occupy a dominant role in improving the desorption behaviour of LiAlH4, even though the Co or Co-containing phase was not detected. For instance, a previous study discovered that the presence of Co2O3 exhibited a significant enhancement in the desorption of LiAlH4 compared to undoped LiAlH4 [46]. Li et al. [29] also indicated that the formation of Co-containing species after the desorption process of LiAlH4 provides a positive impact on the desorption behaviour of LiAlH4. Therefore, it is reasonable to infer that the formation of these active species contributes to the notable advancement of the desorption behaviour of LiAlH4. By shortening the length of diffusion of the reaction ions [29,41], these in situ-formed AlTi and Co or Co-containing species are responsible for boosting the desorption behaviour of LiAlH4 by taking part as the active sites for the nucleation and creation of the dehydrogenation yield. However, further characterisation using X-ray photoelectron spectroscopy and transmission electron microscopy is needed to evaluate the actual impact and mechanism of the LiAlH4-CoTiO3 system.

Conclusions
The introduction of nanosized CoTiO3 that was synthesised via a solid-state method significantly ameliorates the desorption behaviour of LiAlH4. The LiAlH4-CoTiO3 sample began to release hydrogen at around 115-120 °C and 144-150 °C for the first and second desorption stages, respectively, lower than for undoped LiAlH4. The addition of nanosized CoTiO3 also results in accelerated desorption kinetics with the ability to desorb hydrogen 30-35 times faster than undoped LiAlH4. The rapid hydrogen desorption performance of LiAlH4 when added with CoTiO3 may be due to the lower activation energy of the LiAlH4-CoTiO3 sample, which was calculated to be 75.2 kJ/mol (first stage) and 91.8 kJ/mol (second stage), reduced by 28.0 and 24.9 kJ/mol compared to undoped LiAlH4. The nanosized CoTiO3 was also beneficial in the reduction of LiAlH4 particle size during the milling process. Smaller particle size is favourable in reducing the hydrogen diffusion length, which then results in superior desorption performance. The enhanced desorption behaviour of LiAlH4 was also due to the synergetic effect of the in situ formation of AlTi and Co or Co-containing species during the heating process of LiAlH4 and CoTiO3. By reducing the initial desorption temperature and activation energy, the addition of nanosized CoTiO3 remarkably ameliorates the desorption behaviour of LiAlH4, resulting in finer and smaller particles and accelerating the desorption kinetic behaviour. These findings shed light on the preparation of LiAlH4 hydrogen storage systems for mobile appli- Referring to the results, it is noteworthy to state that the desorption behaviours of LiAlH 4 were improved due to the in situ formation of active species during the heating process. In this study, the formation of AlTi and Co or Co-containing species was believed to contribute to the enhanced desorption behaviours of LiAlH 4 . This phenomenon agrees with a previous study that also reported the formation of AlTi after the dehydrogenation process of LiAlH 4 [3,44]. The in situ formation of AlTi is beneficial in ameliorating the desorption behaviour of LiAlH 4 by catalysing the dehydrogenation through reaction in Equations (1) and (2). In addition, the study performed by Wohlwend et al. [45] revealed the superior performance of LiAlH 4 with the addition of Ti-based additives. The Ti atom reacts strongly with the LiAlH 4 surface and reduces the H binding energy. The reduction in binding energy may be a result of the correlation of the charge transfer change between Al and H that results in the accelerated kinetic performance of LiAlH 4 . It denotes that Co or Co-containing species also occupy a dominant role in improving the desorption behaviour of LiAlH 4 , even though the Co or Co-containing phase was not detected. For instance, a previous study discovered that the presence of Co 2 O 3 exhibited a significant enhancement in the desorption of LiAlH 4 compared to undoped LiAlH 4 [46]. Li et al. [29] also indicated that the formation of Co-containing species after the desorption process of LiAlH 4 provides a positive impact on the desorption behaviour of LiAlH 4 . Therefore, it is reasonable to infer that the formation of these active species contributes to the notable advancement of the desorption behaviour of LiAlH 4 . By shortening the length of diffusion of the reaction ions [29,41], these in situ-formed AlTi and Co or Co-containing species are responsible for boosting the desorption behaviour of LiAlH 4 by taking part as the active sites for the nucleation and creation of the dehydrogenation yield. However, further characterisation using X-ray photoelectron spectroscopy and transmission electron microscopy is needed to evaluate the actual impact and mechanism of the LiAlH 4 -CoTiO 3 system.

Conclusions
The introduction of nanosized CoTiO 3 that was synthesised via a solid-state method significantly ameliorates the desorption behaviour of LiAlH 4 . The LiAlH 4 -CoTiO 3 sample began to release hydrogen at around 115-120 • C and 144-150 • C for the first and second desorption stages, respectively, lower than for undoped LiAlH 4 . The addition of nanosized CoTiO 3 also results in accelerated desorption kinetics with the ability to desorb hydrogen 30-35 times faster than undoped LiAlH 4 . The rapid hydrogen desorption performance of LiAlH 4 when added with CoTiO 3 may be due to the lower activation energy of the LiAlH 4 -CoTiO 3 sample, which was calculated to be 75.2 kJ/mol (first stage) and 91.8 kJ/mol (second stage), reduced by 28.0 and 24.9 kJ/mol compared to undoped LiAlH 4 . The nanosized CoTiO 3 was also beneficial in the reduction of LiAlH 4 particle size during the milling process. Smaller particle size is favourable in reducing the hydrogen diffusion length, which then results in superior desorption performance. The enhanced desorption behaviour of LiAlH 4 was also due to the synergetic effect of the in situ formation of AlTi and Co or Co-containing species during the heating process of LiAlH 4 and CoTiO 3 . By reducing the initial desorption temperature and activation energy, the addition of nanosized CoTiO 3 remarkably ameliorates the desorption behaviour of LiAlH 4 , resulting in finer and smaller particles and accelerating the desorption kinetic behaviour. These findings shed light on the preparation of LiAlH 4 hydrogen storage systems for mobile applications.

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