Catalytic Modification Of MgH₂ Hydrogen Storage Material By Transition Metal Oxides And Its Mechanism

Magnesium hydride has the advantages of high hydrogen capacity,good reversibility,and low cost,and MgH₂ is considered to be one of the most promising solid-state hydrogen storage materials.However,its high working temperature and slow de/absorption kinetics seriously hinder the practical application of MgH₂ in the field of hydrogen energy.In order to reduce the temperature of MgH₂ hydrogen absorption and desorption,increase its hydrogen absorption and desorption rate,and improve its reversible hydrogen absorption and desorption performance,introducing a catalyst into MgH₂ is an effective means to improve its hydrogen storage performance.Among all the currently available catalysts,transition metal-based additives are typical catalysts with high electronegativity and multivalence,which have been shown to significantly improve the de/absorption kinetics of MgH₂.Among them,transition metal oxides are the most common and easiest to use catalysts.In view of the above problems and research status,in this paper,the transition metal oxide V₂O₅catalyst and KH-modified TiO₂ catalyst were added to MgH₂,and the effects of the two catalysts on the hydrogen storage performance of MgH₂ and their mechanism were systematically studied.The transition metal oxide V₂O₅ was introduced into MgH₂ by ball milling.The experimental results show that the hydrogen storage performance of MgH₂ system is closely related to the amount of V₂O₅ added.V₂O₅ in different mass ratios（2.5 wt%,5wt%,10 wt%）added to MgH₂.The MgH₂ sample had the best comprehensive performance by TPD-MS and volumetric dehydrogenation tests when the added amount of V₂O₅ was 5 wt%.The dehydrogenation of MgH₂ with 5 wt%V₂O₅ starts at175°C,which is 89°C lower than that of pure MgH₂.When the temperature increased to 300℃,the system could release 6.8 wt%H₂.At the same time,the sample can release 6.4 wt%H₂ at 300°C for 10 min isothermally,while pure MgH₂ can hardly release a large amount of H₂ under the same conditions.After dehydrogenation of the MgH₂+5 wt%V₂O₅ system,at room temperature and a hydrogen pressure of 30 bar,2.1 and 3.8 wt%H₂ were absorbed within 30 and 180 min,respectively.It can be seen that the addition of V₂O₅ significantly improved the de/absorption kinetics of MgH₂.The dehydrogenation kinetics test results show that the addition of 5 wt%V₂O₅ can significantly reduce the dehydrogenation activation energy of MgH₂,but has little effect on the thermodynamic properties of MgH₂.Meanwhile,by studying the mechanism of V₂O₅ promoting the hydrogen storage performance of MgH₂,it was found that V₂O₅ could be reduced by MgH₂ to metal oxides and metal vanadium nanoparticles with lower oxidation states than their starting materials during the reaction.The disordered structure and interfacial effect of this low oxidation state oxide and metallic vanadium nanoparticles facilitate hydrogen diffusion and hydride nucleation,thereby enhancing the hydrogen storage performance of MgH₂.The black K-TiO_2-x catalyst with surface defects was prepared by modifying TiO₂ with KH.The effects of different molar ratios of KH and TiO₂ on the hydrogen storage properties of MgH₂ were discussed,and the size,structure and morphology of K-TiO_2-x and its catalytic mechanism on MgH₂ were studied in detail.Compared with pure MgH₂ under the same conditions,the addition of 5 wt%K-TiO_2-x to MgH₂ can significantly improve the de/absorption behavior of MgH₂.The composite system absorbed 5.3 wt%and 6.0 wt%of hydrogen in the first 30 and 180 min at room temperature and hydrogen pressure of 80 bar,respectively.The room temperature hydrogen absorption performance of this sample was at a high level in the literature.After increasing the absorption temperature to 230°C,7.1 wt%H₂ can be absorbed within the first 60 s at hydrogen pressure of 30 bar.Meanwhile,hydrogen can be released reversibly starting from 194°C,which is～96°C lower than that of pure MgH₂without catalyst addition,and the system can also release～6.5 wt%H₂ after 30 cycles.In the study of each reaction process of MgH₂-[K-TiO_2-x]system,it was found that"K₂Ti₂O₃"pseudo-Ti²⁺complex existed in MgH₂-[K-TiO_2-x]in the process of hydrogen absorption.In situ release during hydrogen uptake;this pseudo-complex is considered as a catalyst that enhances the hydrogen uptake performance of MgH₂ by providing nucleation sites,oxygen vacancies,and hydrogen diffusion channels at its boundary with Mg/MgH₂.