Improved hydrogen sorption kinetics in wet ball milled Mg hydrides

In this work, wet ball milling method is used in order to improve hydrogen sorption behavior due to its improved microstructure of solid hydrogen materials. Compared to traditional ball milling method, wet ball milling has benefits on improvement of MgH$_{2}$ microstructure and further influences on its hydrogen sorption behavior. With the help of solvent tetrahydrofuran (THF), wet ball milled MgH$_{2}$ powder has much smaller particle size and its specific surface area is 7 times as large as that of dry ball milled MgH2 powder. Although after ball milling the grain size is decreased a lot compared to as-received MgH$_{2}$ powder, the grain size of wet ball milled MgH$_{2}$ powder is larger than that of dry ball milled MgH$_{2}$ powder due to the lubricant effect of solvent THF during wet ball milling. The improved particle size and specific surface area of wet ball milled MgH$_{2}$ powder is found to be determining its hydrogen sorption kinetics especially at relatively low temperatures. And it also shows good cycling sorption behavior, which decides on its industrial applicability. With three different catalysts MgH$_{2}$ powder shows improved hydrogen sorption behavior as well as the cyclic sorption behavior. Among them, the Nb$_{2}$O$_{5}$ catalyst is found to be the most effective one in this work. Compared to the wet ball milled MgH$_{2}$ powder, the particle size and specific surface area of the MgH$_{2}$ powder with catalysts are similar to the previous ones, while the grain size of the MgH$_{2}$ with catalysts is much finer. In this case, two reasons for hydrogen sorption improvement are suggested: one is the reduction of the grain size. The other may be as pointed out in some literatures that formation of new oxidation could enhance the hydrogen sorption kinetics, which is also the reason why its hydrogen capacity is decreased compared to without catalysts. After further ball milling, the specific surface area of wet ball milled MgH$_{2}$ with Nb$_{2}$O$_{5}$ is much larger than the standard wet ball milled MgH$_{2}$ with Nb$_{2}$O$_{5}$ and the corresponding sorption behavior is also much improved. Furthermore, a simple model is built up in which the key parameter is main specific surface area and it follows the experimental desorption results quite well