Doping catalysts is a method for chemical reaction and material modification [
64,
65]. Generally, it can effectively improve reaction kinetics and the threshold value toward the target condition. Hanada et al. [
22] reported the catalytic effect of nanoparticle 3d-transition metals, including nanoparticles Fe, Co, Ni, and Cu, with mechanical milling. The desorption temperature of MgH
2 decreased to about 200°C successfully. The ball milling method proved to be an effective preparing technology which was regarded as a new technology called mechanochemical synthesis [
66]. All ingredients were prepared by ball-milling for different milling times, and a longer milling period led to finer particle sizes and a relatively low hydrogen capacity. It was proved that MgH
2 doped with
x(Ni) = 2% (
x is mole percent) could decompose and release at least
w(hydrogen) = 6.5% hydrogen from 150 to 250°C, and the reversibility attenuated at the lower operating temperature of 150°C with the Mg and Ni ensemble than that at 200°C with the growth of the MgNi
2 phase. Zhang et al. [
67] synthesized a novel complex transition metal oxide, TiVO
3.5, by using a solid-solution (Ti
0.5V
0.5)
3C
2 as a precursor, and studied the catalytic activity for the hydrogen storage reaction of MgH
2. The sample prepared by oxidation at 300°C exhibited the optimal catalytic performance. The addition of
w(catalysts) = 10% catalysts induced a 76°C reduction in the dehydrogenation temperature of MgH
2 from 267 to 197°C. Approximately
w(hydrogen) = 5% of the hydrogen was desorbed in 10 min at 250°C, the dehydrogenated sample could sorb approximately
w(hydrogen) = 3.9% hydrogen in 5 s at 100°C. The addition of TiVO
3.5 remarkably reduced the desorption activation energy from 153.5 to 62.4 kJ/mol, which was the most important reason for the reduced dehydrogenation temperature. They [
68] also studied the catalytic activity of carbon-supported nanocrystalline TiO
2 in the hydrogen storage reaction of MgH
2. A 95°C reduction in the dehydrogenation temperature was obtained. Zhou et al. [
69] also researched the desorption of MgH
2 doped with a series of Ti-based intermetallic alloys. Most catalysts could adjust the temperature of desorption and the kinetics of both desorption and sorption of hydrogen. Fig.5 and Fig.5 indicate that the desorption temperature after ultra-high-energy-high-pressure planetary milling decreases from 414 to 333°C compared with the as-milled undoped MgH
2. The Ti-based intermetallic dopants all show varying degrees of effectiveness in reducing the desorption temperature of MgH
2. Meanwhile, the equilibrium pressures and the van’t Hoff diagram revealed that the thermodynamics of original reaction remained the same after the additions of the Ti-based intermetallic alloys [
69]. The results shown in Fig.5 and Fig.5, clearly demonstrate that a relatively large hydrogen storage capacity could be sorbed by Mg at near-ambient-temperature and atmosphere hydrogen pressure, suggesting the strong effect of Ti-dopants that accelerate the sorption process. Among all systematic investigations, MgH
2 doped with TiMn
2 showed an extraordinary decomposition capacity of
w(hydrogen) = 5.1% at ambient temperature and 1 bar hydrogen pressure and did participate in both sorption and desorption reactions. Overall, doping metal catalysts by mechanical methods such as ball milling and cold rolling can provide a better kinetic parameter and meet the requirements of lower operation temperature of Mg-based materials. At present, transition metals are widely applied to hydrogen storage alloy. However, generated unexpected alloy lattice structures during the process may attenuate the hydrogen storage capacity and the reversibility due to the decline of available sites of hydrogen in the materials. Catalyst type, proportion, and process are three keys to the optimization of Mg-based hydrogen storage materials.