An Overview of the Recent Advances of Additive-Improved Mg(BH 4 ) 2 for Solid-State Hydrogen Storage Material

: Recently, hydrogen (H 2 ) has emerged as a superior energy carrier that has the potential to replace fossil fuel. However, storing H 2 under safe and operable conditions is still a challenging process due to the current commercial method, i.e., H 2 storage in a pressurised and liquiﬁed state, which requires extremely high pressure and extremely low temperature. To solve this problem, research on solid-state H 2 storage materials is being actively conducted. Among the solid-state H 2 storage materials, borohydride is a potential candidate for H 2 storage owing to its high gravimetric capacity (majority borohydride materials release >10 wt% of H 2 ). Mg(BH 4 ) 2 , which is included in the borohydride family, shows promise as a good H 2 storage material owing to its high gravimetric capacity (14.9 wt%). However, its practical application is hindered by high thermal decomposition temperature (above 300 ◦ C), slow sorption kinetics and poor reversibility. Currently, the general research on the use of additives to enhance the H 2 storage performance of Mg(BH 4 ) 2 is still under investigation. This article reviews the latest research on additive-enhanced Mg(BH 4 ) 2 and its impact on the H 2 storage performance. The future prospect and challenges in the development of additive-enhanced Mg(BH 4 ) 2 are also discussed in this review paper. To the best of our knowledge, this is the ﬁrst systematic review paper that focuses on the additive-enhanced Mg(BH 4 ) 2 for solid-state H 2 storage.


Renewable and Sustainable Energy System
The rapid increase in energy consumption worldwide has already raised questions with regard to production issues, energy resource scarcity and serious environmental effects (ozone depletion, global warming, climate change, etc.) [1]. The earth's oil reserves are finite. Thus, they are not expected to last long. Moreover, it is expected that oil supply in the future will eventually peak and drop with the depletion of these reserves. On a global scale, the "greenhouse effect" is a key environmental issue due to the emission of carbon dioxide and other gases into the atmosphere. The major causes of such an impact are the burning of fossil fuels and deforestation [2].
Due to the productive relationship between clean energy and sustainable growth, renewable energy is often considered to be one of the most reliable solutions. The most urgent task is to take energy-saving measures in order to minimise energy consumption and change the demand situation that renewable energy must meet [3]. Many aspects can contribute to the realization of sustainable development. One of the most crucial aspects is the need to provide a completely sustainable energy [4][5][6] to reduce the world's dependence on non-renewable resources such as fossil fuel. For solid-state H 2 storage, chemisorption materials are preferable compared with physisorption materials owing to their high H 2 storage capacity and moderate temperature and pressure requirement. Chemisorption materials, especially metal hydrides and complex hydrides, have been thoroughly studied, owing to their significant H 2 density and dehydrogenation/rehydrogenation capabilities, while ignoring the requirement for high pressure or low temperature [35]. Previous studies have demonstrated that Mg-based hydrides are potential candidates for solid-state H 2 storage because of their excellent reversibility to H 2 absorption and desorption, as well as high H 2 storage capacity [36,37]. However, their high decomposition temperature and sluggish sorption kinetic properties hinder their commercial application. Meanwhile, researchers have revealed that complex hydrides have high gravimetric and volumetric capacities, enabling them to meet the capacity requirements of H 2 storage materials [38,39]. However, the disadvantage is that most complex hydride compounds do not have convenient H 2 desorption thermodynamics and/or kinetics and require high temperatures to desorb H 2 . Moreover, they usually contain impurities. The reversibility of the Ti-doped NaAlH 4 catalyst system was first studied by Bogdanovic and Schwickardi [40]. Since then, researchers have exerted significant effort to investigate the properties of complex hydrides as suitable candidates for solid-state H 2 storage [41][42][43][44][45].
Several theoretically predicted Mg(BH 4 ) 2 structures have been published, demonstrating an "outstanding divergence between experiment and theory" [60,77]. The high-pressure structures P-4, I4 1 /acd [78], I4 1 /amd [79] and Fddd [80] found by first-principle studies, for example, have been discovered to be more favourable than the experimentally determined P4 2 nm phase of the ultra-dense δ-Mg(BH 4 ) 2 [73]. These inconsistencies can lead to an erroneous assessment of thermodynamic properties, affecting the evaluation of Mg(BH 4 ) 2 s hydrogen storage characteristics. It was proposed that the small size of the Mg cation, together with the close proximity of BH 4 , leads to increased repulsive interactions and a condition in which the orientation of the anions plays a significant role [77,81]. Vibrational spectroscopy studies can be used to characterise the local structure of BH 4 , which can be linked to the stability of the compounds.
Chłopek et al. [55] and Matsunaga et al. [63,82] reported that the H 2 desorption of Mg(BH 4 ) 2 occurs in a two-step pathway as expressed in Equation (2): Several studies have shown that pure crystalline Mg(BH 4 ) 2 can release mostly pure H 2 at 300 • C [74,[83][84][85][86]. However, the preceding and subsequent experimental research has continuously demonstrated that decomposition occurs through a two-step pathway at a temperature beyond 200 • C [87][88][89][90]. Fichtner et al. [88] reported that the two-step decomposition of Mg(BH 4 ) 2 yielded activation energies of 311 ± 20 and 189 ± 15 kJ mol −1 , whereas Ibikunle et al. [91] only obtained a value of 155.2 kJ mol −1 [63]. The characteristic of Mg(BH 4 ) 2 is that it has a wide range of crystal structures, and the number of crystals is larger than that of any other known borohydrides. The experiment and theory are presented in Table 2.

Modification of the Mg(BH 4 ) 2 Properties
Although Mg(BH 4 ) 2 shows promise as a solid-state H 2 storage material, its high decomposition temperature, slow sorption kinetics and very stable thermodynamic properties hinder its commercial application. To modify the H 2 storage properties of Mg(BH 4 ) 2 , several methods have been employed, such as particle size reduction via ball milling [74,94], the use of catalysts or additives to improve sorption kinetics [74,[95][96][97] and alteration of the thermodynamic properties using the destabilised concept (mix of two or more complex metal hydrides) [98][99][100][101][102][103][104][105][106][107][108]. Among these methods, the addition of catalysts or additives to Mg(BH 4 ) 2 has been studied rigorously as it significantly improves the H 2 storage properties of Mg(BH 4 ) 2 . A catalyst or additive is generally used to increase the H 2 adsorption rate by reducing the activation energy of the dehydrogenation/rehydrogenation reaction. The obvious advantage of the addition of catalysts or additives is that it helps improve the kinetic performance, while the capacity loss is negligible [109]. The authors of this article reviewed the impact of the different groups of catalysts/additives on the H 2 storage properties of Mg(BH 4 ) 2 . In addition, the authors of this article also pointed out the role of catalysts/additives and their catalytic mechanism, and provided opinions on the future prospect and challenges with regard to Mg(BH 4 ) 2 as a suitable material for solid-state H 2 storage.

Metals
The addition of metal as a dopant to enhance the H 2 storage properties of complex metal hydrides has been widely studied [110,111]. The role of Al was investigated by Jiang et al. [112] on the dehydrogenation and rehydrogenation of Mg(BH 4 ) 2 . Table 3 presents the sorption properties of Mg(BH 4 ) 2 + Al, Mg(BH 4 ) 2 + LiH + Al, Mg(BH 4 ) 2 + 1/3(Li 3 AlH 6 ) + 2Al, Mg(BH 4 ) 2 + LiAlH 4 and pure Mg(BH 4 ) 2 . Figure 2a shows that the sample Mg(BH 4 ) 2 + Al and Mg(BH 4 ) 2 + LiH + Al desorption activity was comparable to Mg(BH 4 ) 2 , with improved the desorption kinetic and reduced the release temperature at the second step. In comparison to Mg(BH 4 ) 2 , sample Mg(BH 4 ) 2 + 1/3(Li 3 AlH 6 ) + 2Al had more desorption steps, which may be related to the decomposition of Li 3 AlH 6 . The process started at about 150 • C, which was 30 • C lower than that of Li 3 AlH 6 in pure LiAlH 4 . Two other desorption steps can be observed in sample Mg(BH 4 ) 2 + LiAlH 4 , which may cause the decomposition of LiAlH 4 to start at approximately 140 • C, which is 30 • C below the decomposition temperature of pure LiAlH 4 . At 350 • C, the time taken by pristine Mg(BH 4 ) 2 to release 90% of H 2 was 1911 s, whereas the other samples could complete 90% of desorption below 1700 s at the same temperature. As can be seen from Figure 2b, the rate of desorption of LiH-doped Mg(BH 4 ) 2 was significantly low compared with that of the sample containing Al and pure Mg(BH 4 ) 2 only. This indicates that Al plays a major role in the improvement of the release kinetics in the Al-doped Mg(BH 4 ) 2 samples. The Mg-B-Al-H system is considered to be somewhat reversible. Doped Mg(BH 4 ) 2 samples were able to absorb about 1.7, 5.1, 5.0 and 5.7 wt% of H 2 . The reversibility of the Mg-B-Al-H system is significantly associated with regenerated MgH 2 and LiBH 4 .  Ball milling is considered to be a useful technique to enhance the kinetics of H 2 reaction by altering the microstructures and surface properties [113]. Conversely, Li et al. [74] investigated the effect of ball milling and Ti-based additives on the dehydriding properties of Mg(BH 4 ) 2 . Compared with the initial dehydriding temperature of the as-synthesised Mg(BH 4 ) 2 , no significant difference was observed in the sample mixed with Ti, TiH 2 and TiB 2 , whereas the addition of TiO 2 reduced the initial dehydriding temperature by about 50 • C. However, the addition of TiCl 3 reduced the initial dehydriding temperature from 262 • C to 88 • C, which indicates a reduction of approximately 174 • C. The improvement may be due to the reaction between Mg(BH 4 ) 2 and TiCl 3 , forming unstable MgTi x (BH 4 )( 2+nx ), which is consistent with the study by Li et al. [114], in which ZrLi(BH 4 ) 5 was formed.
Recently, Wang et al. [115] investigated the influence of Ti nanoparticles on the dehydrogenation kinetic and reversibility of Mg(BH 4 ) 2 , and found that the Ti-doped Mg(BH 4 ) 2 sample showed better desorption kinetics than the undoped Mg(BH 4 ) 2 . The Ti-doped Mg(BH 4 ) 2 sample also showed a great improvement in terms of capacity released, in which at 270, 280, and 290 • C, as shown in Figure 3, the doped sample could desorb more H 2 within 35 h than Mg(BH 4 ) 2 does for each temperature. Moreover, the activation energy for hydrogen release from Mg(BH 4 ) 2 also decreased after the addition of Ti nanoparticles. Based on the Arrhenius plot, the activation for Ti-doped Mg(BH 4 ) 2 sample was 56.5 kJ/mol, which was lower than that of undoped Mg(BH 4 ) 2 (61.1 kJ/mol). From the experimental results, Wang et al. concluded that during the milling and heating process, Ti nanoparticles react with Mg(BH 4 ) 2 to generate in situ Ti-based species (TiH 1.924 and TiB 2 ). It is believed that TiB 2 could act as a heterogeneous nucleation agent, and TiH 1.924 could act as a hydrogen pump during the rehydrogenation process. Table 4 presents the effect of several metal catalysts on the desorption and absorption of Mg(BH 4 ) 2 .

Metal Oxides
Previous studies have demonstrated that among the catalysts/additives used to improve the H 2 storage properties of metal hydrides and complex hydrides, metal oxides are rigorously applied [94,117]. Zavorotynska et al. [118,119] focused on the study of cobalt-based additives used to enhance the sorption properties of Mg(BH 4 ) 2 in three cycles, as shown in Figure 4. The first cyclic absorption isotherm (Figure 4b) demonstrates that the grinding of undoped Mg(BH 4 ) 2 can increase the rehydrogenation kinetics by two to five times, whereas all additives, except Co 3 O 4 , decrease it.
In the second cycle, the desorption process of all samples ( Figure 4c) was even slower than that in the first cycle, as 90% of the desorption process was completed within 10 h. Although Co 2 B did not seem to have any major influence and CoF 3 slowed down the kinetic rate, Co 3 O 4 and CoCl 2 seemed to improve the absorption kinetics. In the third cycle, the desorption kinetics were quicker than those in the second cycle, with a slightly enhancing effect on CoF 3 and Co 2 B, whereas CoCl 2 and Co 3 O 4 were slightly negative. Overall, in the first cycle, the desorption kinetic rates were the highest, declining in the second cycle and increasing again in the third cycle. Most additives slightly improved the kinetics in the first cycle but had little effect in the second cycle. CoF 3 and Co 2 B slightly improved the reaction rate in the last cycle, whereas other additives declined the kinetics. Likewise, in the first cycle, the absorption kinetics were the fastest, and then they decreased significantly in the second cycle, whereas in the third cycle, the kinetic rate was the same as that in the first cycle. Additives mainly slowed down the absorption kinetics, and only Co 3 O 4 significantly enhanced the kinetics in all three cycles. It should be noted that the curve in Figure 4 behaved differently in all cycles, especially in desorption. This indicates that the reaction was controlled by a different mechanism, which explains the inconsistency of the reaction rate with the cycle.    Table 5 summarises the effect of several metal oxides on the desorption and absorption of the Mg(BH 4 ) 2 species.

Metal Halides
Metal halides are a common type of catalyst/additive used to improve the H 2 storage properties of metal hydrides and complex hydrides [122,123]. Al-Kukhun et al. [124] investigated the influence of VCl 3 , NbF 5  Wang et al. [125] also studied the influence of NbF 5 on amorphous Mg(BH 4 ) 2 that formed in situ to enhance H 2 storage performance. They found that the initial decomposition temperature of pure Mg(BH 4 ) 2 was 283 • C, whereas amorphous Mg(BH 4 ) 2 and NbF 5 -doped amorphous Mg(BH 4 ) 2 began to release H 2 at 127 • C and 120 • C, respectively. However, as a result, the amount of H 2 released was slightly decreased to 10.04 wt% for the NbF 5 -doped amorphous Mg(BH 4 ) 2 sample as compared with 10.28 and 10.80 wt% for pure Mg(BH 4 ) 2 and amorphous Mg(BH 4 ) 2 , respectively. From the FTIR result, it was proven that the intermediate MgB 12 H 12 is regenerated during the hydrogenation process. The formation of intermediate MgB 12 H 12 is believed to exert negative effects on the reversibility process in the Mg(BH 4 ) 2 . However, Yan et al. [126] proved that the intermediate MgB 12 H 12 does not form as a dehydrogenation product of Mg(BH 4 ) 2 in their study. The results also indicated that the addition of NbF 5 changed the dehydrogenation pathway of Mg(BH 4 ) 2 and two new species (MgF 2 and NbB 2 ) that formed in situ during the dehydrogenation process. It is believed that the MgF 2 and NbB 2 species play a major catalytic role in improving the H 2 storage properties of NbF 5 -doped Mg(BH 4 ) 2 composite.
Another study by Newhouse et al. [127] focused on the H 2 storage properties of Mg(BH 4 ) 2 with ScCl 3 and TiF 3 as catalysts. The addition of 5 mol% TiF 3 and ScCl 3 can significantly increase both the amount of H 2 releases and the rate of H 2 desorption of Mg(BH 4 ) 2 . Mg(BH 4 ) 2 added with the catalyst released 9.7 wt% of H 2 after being placed at 300 • C for 17 h, whereas pristine Mg(BH 4 ) 2 desorbed 7.7 wt% of H 2 . Compared with the undoped sample that required 10 h, the sample with additives had 95% of the H 2 completely desorbed in the first 2 h. The dehydrogenated species, MgB 2 , fully absorbed the H 2 for the sample with and without additives. The formation of Mg(BH 4 ) 2 after the hydrogenation process was confirmed via NMR spectroscopy, Raman spectroscopy and power X-ray diffraction. However, the use of additives was beneficial to the formation of stable B-H 2 intermediates during the absorption process.
The effects of metal fluorides (CaF 2 , ZnF 2 and TiF 3 ) as additives on the H 2 release process of Mg(BH 4 ) 2 and its corresponding microstructure evolution were explored by Zhang et al. [128]. It can be seen from Figures 7a and 8a that the first peak that appeared near 150 • C was the polymorphic transition to ε-Mg(BH 4 ) 2 from γ-Mg(BH 4 ) 2 , with 0.65 wt% of H 2 released. The second transformation occurred at 200 • C, which was the polymorphic transition to β-Mg(BH 4 ) 2 , releasing 0.35 wt% of H 2 . When the sample was kept at 295 • C for 5 h, a large quantity of H 2 (8.8 wt%) was liberated, as shown in Figure 8a. In comparison with pure Mg(BH 4 ) 2 , the addition of fluoride triggered the reduction of the decomposition temperature. The addition of CaF 2 demonstrated an identical pattern to that of pure Mg(BH 4 ) 2 , whereas TiF 3 and ZnF 2 reduced the onset temperature to 50 • C. However, under the same conditions, 4.5 wt% of H 2 was released during the transformation from γ-Mg(BH 4 ) 2 to the amorphous Mg(BH 4 ) 2 , as compared with 9.80 wt% for pristine Mg(BH 4 ) 2 . From the TEM results, all the additives reacted with amorphous Mg(BH 4 ) 2 during the dehydrogenation process, confirming that CaF 2 , ZnF 2 and TiF 3 acted as additives rather than as catalysts in this study.  Another study by Kumar et al. [129] focused on ZrCl 4 -catalysed Mg(BH 4 ) 2 and its thermal dehydrogenation properties. As can be seen from Figure 9, the dehydrogenation process of pure Mg(BH 4 ) 2 was performed in three steps between 230 • C and 450 • C, releasing approximately 11.0 wt% of H 2 . The first dehydrogenation reaction started at 230 • C and peaked at 302 • C, releasing 4.90 wt% of H 2 . Mass spectroscopy (MS) analysis revealed that the second dehydrogenation process occurred at 330 • C, releasing 3.4 wt% of H 2 . The third endothermic events were associated with the dehydrogenation of MgH 2 generated in situ, which occurred between 365 • C and 450 • C, with 2.6 wt% of H 2 released. Compared with pure Mg(BH 4 ) 2 , the onset temperature of catalysed Mg(BH 4 ) 2 was lower. The dehydrogenation of Mg(BH 4 ) 2 with a catalyst began at 197 • C and peaked at 289 • C, releasing 5.6 wt% of H 2 . Whereas ZrCl 4 exerted mild catalytic effects at 289 • C and 337 • C in the first and second dehydrogenation phases, a solder peak observed at 360 • C indicated an impressive outcome of catalyst in the dehydrogenation of MgH 2 formed in situ. According to the Kissinger analysis, the addition of ZrCl 4 reduced the activation energy of H 2 released from Mg(BH 4 ) 2 for the first and second steps. The XPS result indicated that ZrCl 4 reduced to ZrCl 3 and metallic Zr during the milling process. It is believed that ZrCl 3 and metallic Zr formed in situ act as real catalysts in enhancing the dehydrogenation properties of the ZrCl 4 -doped Mg(BH 4 ) 2 composite system. Bardaji et al. [130] studied the effect of different metal chlorides (PdCl 2 , TiCl 3 , VCl 3 , MoCl 3 , RuCl 3 , CeCl 3 and NbCl 5 ) on the dehydrogenation properties of Mg(BH 4 ) 2 . They found that the H 2 desorption of Mg(BH 4 ) 2 was slightly improved by the addition of PdCl 2 , CeCl 3 , VCl 3 , MoCl 3 and RuCl 3 . The initial decomposition temperature was reduced even further by adding NbCl 5 and TiCl 3 . TiCl 3 -and NbCl 5 -doped samples were able to desorb 6.3 and 5.5 wt% of H 2 at 300 • C, respectively, which indicates more than 50% of Mg(BH 4 ) 2 H 2 storage capacity. The co-catalyst Ti-Nb nanocomposite (mix of TiCl 3 and NbCl 5 ) had a positive influence on the dehydrogenation properties of Mg(BH 4 ) 2 (decreased up to 125 • C). However, no improvement was observed on the reversibility of Mg(BH 4 ) 2 after the addition of catalysts.
Recently, Zheng et al. [131] investigated the effect of dual-cation transition metal fluorides K 2 NbF 7 and K 2 TiF 6 on the reversible hydrogen absorption/desorption properties of Mg(BH 4 ) 2 . They discovered that 3% K 2 TiF 6 and 3% K 2 NbF 7 -doped Mg(BH 4 ) 2 had an initial desorption temperature of 105 • C and 118 • C, respectively, which is approximately 200 • C lower than undoped Mg(BH 4 ) 2 . Meanwhile, the partial reversibility of 3% K 2 TiF 6doped Mg(BH 4 ) 2 was enhanced to 2.7 wt% at 280 • C in 250 min, which is higher comparing to that of undoped Mg(BH 4 ) 2 . From the XRD, FTIR and 11 B nuclear magnetic resonance results, Zheng et al. concluded that the improvement of reversibility performance of Mg(BH 4 ) 2 with the presence of K 2 TiF 6 is due to the active hydrides species, KBH 4 and TiH 2 . These active hydride species are formed during the dehydrogenation process by the reaction of Mg(BH 4 ) 2 and K 2 TiF 6 and furthermore act as a real catalysing agents to accelerate the re-generation of Mg(BH 4 ) 2 . Table 6 presents the influence of several metal halide additives on the H 2 storage properties of Mg(BH 4 ) 2 .

Carbon-Based Materials
There are several carbon-based materials that have been used as catalysts/additives for the Mg(BH 4 ) 2 , such as CNTs [132][133][134], carbon nanocomposites [135], two-dimensional MXene [136,137] and graphene [97,138]. The effects of CNTs on the microstructure development and H 2 storage performance of Mg(BH 4 ) 2 were investigated by Jiang et al. [133]. The addition of CNTs reduced the decomposition temperature but suffered a decrease in H 2 desorption capacity from 9.82 to 5.98 wt%. The quickest and most effective H 2 desorption process was shown by Mg(BH 4 ) 2 doped with 5 wt% CNTs, where the initial temperature decreased to a lower temperature of about 120 • C compared with 155 • C for pristine Mg(BH 4 ) 2 . The dehydriding kinetics for the sample were recorded at varying temperatures ranging from 200 • C to 300 • C. As the temperature increased, the H 2 storage capability increased, whereas the dehydrogenation kinetics accelerated. Mg(BH 4 ) 2 doped with 5 wt% CNTs released 1.32 wt% of H 2 in 30 min at a temperature of 200 • C and increased to as high as 6.04 wt% of H 2 at 300 • C for the same period. After the addition of CNTs, the activation energy, E a , for H 2 desorption was found to be only 130.2 kJ mol −1 , which was significantly smaller than that of H 2 desorption for pristine Mg(BH 4 ) 2 , which was 451.6 kJ mol −1 . From the results obtained, Jiang et al. [133] concluded that the improvement of the kinetic properties of CNT-doped Mg(BH 4 ) 2 was due to the change in the reaction pathway.
Yan et al. [135] studied the effect of carbon nanocomposites on the reversibility of Mg(BH 4 ) 2 . The Mg(BH 4 ) 2 -carbon nanocomposites were synthesised by the milling of MgH 2 nanoparticles supported on carbon aerogel under a B 2 H 6 -H 2 atmosphere. The result indicated that the decomposition temperature of as-synthesised Mg(BH 4 ) 2 -carbon nanocomposites decreased to 160 • C compared with the undoped sample. For the rehydrogenation process, the formation of Mg(BH 4 ) 2 occurred under moderate pressure and temperature (80 to 150 bar H 2 and 200 • C). From the Kissinger analysis, the apparent activation energy for the dehydrogenation process in the Mg(BH 4 ) 2 -carbon aerogel sample was reduced by 238 kJ mol −1 compared with undoped Mg(BH 4 ) 2 , as presented in Figure 10. According to Yan et al., the significantly improved H 2 storage properties of Mg(BH 4 ) 2 catalysed with carbon nanocomposites may be due to the effect of nanoengineering modification. It is believed that the smaller the particle size, the more the H 2 storage properties of complex metal hydrides are improved, as proven in the previous studies on LiBH 4 and MgH 2 nanocomposite systems [139][140][141]. Zheng et al. [136] introduced two-dimensional MXene Ti 3 C 2 to Mg(BH 4 ) 2 using a simple ball-milling technique to enhance Mg(BH 4 ) 2 dehydrogenation performance. The TPD test revealed that the initial H 2 desorption temperature of undoped Mg(BH 4 ) 2 was 286.7 • C. A significant decrease in the desorption temperature can be observed in Mg(BH 4 ) 2 -xTi 3 C 2 hybrids (x = 30, 40 and 50 wt%), where H 2 starts to release at much lower temperatures of 133 • C, 125 • C and 116 • C, respectively. Isothermal dehydrogenation tests were conducted, in which, within 10 min after the temperature reached 330 • C, Mg(BH 4 ) 2 -40Ti 3 C 2 released 10.71 wt% of H 2 . In the same process, only 5.28 wt% of H 2 was detected in the pristine Mg(BH 4 ) 2 . The entire dehydrogenation process of Mg(BH 4 ) 2 -40Ti 3 C 2 can be completed within 1 h, releasing 11 wt% of H 2 . Contrarily, the pristine Mg(BH 4 ) 2 released only 9.10 wt% of H 2 in 2 h, as presented in Figure 11. The improvement of the dehydrogenation properties of 40Ti 3 C 2 -doped Mg(BH 4 ) 2 composite may be due to the synergetic catalytic effect of Ti 3 C 2 . According to recent reports [142,143], the metal titanium formed in situ can serve as a catalytic site, lowering the energy barrier for H-atom diffusion and weakening the stable ionic bond between Mg 2+ and [BH 4 ] − . Table 7 presents the effect of carbon-based metal additives on the desorption and absorption kinetic of the Mg(BH 4 ) 2 .

Future Prospects and Challenges
Mg(BH 4 ) 2 is a promising material for H 2 storage in a solid-state form owing to its high gravimetric capacity (14.9 wt%). However, its drawbacks, such as high decomposition temperature, sluggish sorption kinetics and stable thermodynamic properties, need to be solved prior to its commercial application as a medium for solid-state H 2 storage materials. Moreover, the release of other gases, such as diborane, during the dehydrogenation process is also unpleasant. Previous studies indicate that the H 2 storage properties of Mg(BH 4 ) 2 improves when doped with catalysts/additives. Different types of materials have been utilised as catalysts/additives to reduce the decomposition temperature, improve the sorption kinetics and alter the thermodynamic properties of Mg(BH 4 ) 2 . The critical aspect is to adjust the thermodynamic stability to ensure the reversibility of Mg(BH 4 ) 2 and improve the kinetics at temperatures close to the acceptable and operable range. These problems need to be resolved in future research so that a favourable and convenient H 2 storage system can be realised. Some standards that should be focused on are as follows: (a) Reduce the size of the particle of Mg(BH 4 ) 2 . Nanostructured materials contain a large surface area, which is conducive to the uniform distribution of the catalyst, inducing more defects on the surface of the particles, uniformly mixing different composite hydrides, making the product heterogeneous nucleation and perhaps reducing the enthalpy of the reaction. (b) Lower the activation barrier, E a , for the H 2 released from Mg(BH 4 ) 2 , to understand the role of catalyst/additive and the mechanism of destabilization reactions in changing the properties of H 2 bonds to achieve a low temperature. (e) The degradation of H 2 storage capacity during the cycling process (rehydrogenation/dehydrogenation process) for the second stage of Mg(BH 4 ) 2 is also a crucial problem. It is important to find a suitable catalyst/additive that can prevent the decline in the H 2 storage capacity during the cycling process. (f) The release of diborane gas, which is toxic, might also have a negative effect. To prevent the release of diborane gas, the reactive hydride composite approach (mix of two or three hydride materials) might offer a good solution through the reaction between hydrides during the milling process or dehydrogenation process.

Conclusions
This review provides the latest developments in additive-enhanced Mg(BH 4 ) 2 for solidstate H 2 storage material. Mg(BH 4 ) 2 has shown to be promising as a potential candidate for H 2 storage owing to its high gravimetric capacity, which is 14.9 wt%. However, the application of Mg(BH 4 ) 2 is hindered by its high decomposition temperature, sluggish sorption kinetic and reversibility at a moderate temperature. To overcome these drawbacks, numerous studies have been conducted, proposing few alternatives. Whether using the ball-milling technique to reduce the particle size or by doping with catalyst/additive to reduce the activation energy of H 2 release from Mg(BH 4 ) 2 , both approaches have been proven to enhance the H 2 storage capacity of Mg(BH 4 ) 2 . Many studies on the ball-milling technique have demonstrated that a smaller particle size can provide a wider surface area for H 2 desorption. Other research has focused on the catalyst's/additive's efficiency in improving the H 2 storage performance of Mg(BH 4 ) 2 . This article has discussed the advantages of various types of additives and their effects on the storage performance of Mg(BH 4 ) 2 . The addition of the additive to Mg(BH 4 ) 2 provides a lower activation energy for H 2 release, therefore decreasing the decomposition temperature and improving the desorption properties of Mg(BH 4 ) 2 . To enhance the potential of Mg(BH 4 ) 2 in advanced H 2 storage systems, further research and exploration are needed, especially in the use of proper catalysts/additives or engineering modification to ensure the whole Mg(BH 4 ) 2 (first and second stages) is reversible to H 2 absorption and desorption under moderate conditions. It is also suggested to explore a new catalyst/additive for Mg(BH 4 ) 2 to prevent the decline in H 2 storage capacity during the cycling process and to employ a reactive hydride composite approach to prevent the release of diborane gas during the dehydrogenation process of Mg(BH 4 ) 2 .

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