Research Progress on Ti-Based Catalysts Modified Mg-Based Hydrogen Storage Materials

Facing severe issues such as shortage of fossil fuels and environmental pollution caused by greenhouse gas emissions，hydrogen energy is considered as the most promising alternative energy carrier. The storage and transportation of hydrogen are the most critical aspects，and Mg-based solid-state hydrogen storage materials have broad application prospects as high-energy-density hydrogen storage media，with the advantages of large hydrogen storage capacity，abundant resources，good reversibility and green environment protection. However，their stable thermodynamic properties and slow kinetic performance hinder the widespread commercial application. Catalysis is considered as one of the important means to improve the kinetics of Mg-based hydrogen storage systems. In general，a catalyst is considered as a reagent that can significantly reduce the activation energy of a reaction. Without changing the enthalpy and entropy changes，the addition of catalysts can significantly reduce the activation energy barrier of the reaction，which in turn improves the kinetic properties of the material and increases the hydrogen absorption/desorption rate. All the materials doped with 3d transition metals were found to possess better kinetic properties for hydrogen absorption/desorption than pure MgH2. One of them，Ti，as a member of the transition group metals，also exhibited good catalytic properties. This paper reviewed the modification effect of Ti-based catalysts on Mg-based hydrogen storage materials and its catalytic mechanism，including the rare metal Ti itself and its hydrides，halides，and oxides，all of which had positive effects on improving the hydrogen absorption/desorption kinetics. Ti could overcome the energy barrier of H2 molecular dissociation to a large extent，but the diffusion of H atoms was difficult due to the fact that too strong Ti-H bonds form. The hydrogen absorption reaction of Mg could be attributed to three steps：dissociation of hydrogen molecules；surface diffusion of hydrogen atoms；and internal diffusion of hydrogen atoms. It was found by calculation that Ti was not an excellent catalyst，but further experimental studies showed that Ti could easily form TiHx with H，i.e. elemental Ti could react with MgH2 to form TiH1.5，TiH1.971 and TiH2，and reduce the activation energy of hydrogen desorption of MgH2 significantly. Therefore，it was generally believed that TiH1.5，TiH1.971 and TiH2 were likely to play a catalytic role as active species throughout the hydrogen absorption/desorption process. Transition metal halides containing chloride and fluorine ions were considered to be effective catalysts，even better than titanium hydride. To investigate the catalytic effect of TiF3 on the hydrogen storage performance of MgH2，it was found that TiF3 also had excellent catalytic performance，especially in the kinetics of hydrogen absorption near room temperature. In addition，the catalytically enhanced kinetic properties persisted in the hydrogen absorption/desorption cycle，but the catalytic mechanisms had not been fully determined. In order to investigate the catalytic mechanisms，researchers prepared MgH2 samples containing various Ti-based catalysts and systematically investigated the catalytic effects of Ti-based catalysts（TiF3，TiCl3，TiO2，TiN and TiH2）on the hydrogen absorption/desorption performance of MgH2 and found that the functional anion F- was the reason for its remarkable catalytic effect. The further experimental results showed that among various TiO2 phases，the rutile phase showed the fastest hydrogen absorption kinetics and the highest capacity and had the best catalytic effect when the rutile-type TiO2 was added at 5%（mole fraction）. As for titanium oxide，it was currently believed that there was a relationship between the electronegativity of the oxide and the temperature of hydrogen release，but further experiments and calculations were still needed to solve the mystery of its catalysis. Each of the various hydrides，halides and oxides mentioned above had certain catalytic properties，and the effect of 1+1>2 could be achieved by making full use of various catalysts. In generally，multi-valence Ti-based composite catalysts（TiH2，TiO2 and TiCl3）were used as carriers for electron transfer between Mg2+ and H- as well as to provide catalytic active sites，thus improving the kinetic performance of MgH2 hydrogen absorption/ desorption. So far，the mechanisms of Ti-based catalysts were still far from being fully understood，and the physical factors governing the absorption/desorption rate were still elusive. However，various experiments and calculations show that Ti-based catalysts could indeed greatly improve the slow better kinetic performance of MgH2 and reduce the activation energy barrier of the reaction. In addition，it was likely that Ti-based catalysts could overcome the dilemma of strong Ti-H bond and replace precious metals to reduce the cost and achieve faster industrialization. In summary，Ti-based catalysts modified Mg-based hydrogen storage materials were efficient hydrogen storage materials with wide application prospects. They had the advantages of high hydrogen storage capacity，fast hydrogen absorption and desorption kinetics，and low reaction enthalpy. Although there were still some problems in the preparation and application of this material，such as the high equipment requirements for mechanical ball milling and the need for further improvement of the hydrogen absorption and desorption kinetics of MgH2，these problems were expected to be solved with further research. Ti-based catalysts modified Mg-based hydrogen storage materials would become an important component of future hydrogen storage technology. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.