Influence of nickel and silicon addition on the deuterium siting and mobility in fcc Mg-Ti hydride studied with 2H MAS NMR.

Fluorite-structured Mg–Ti hydrides are interesting for hydrogen storage applications because of their high gravimetric hydrogen storage capacity, and improved (de)hydrogenation kinetics compared to MgH2. In the present study we have investigated the potential catalytic effect of Ni and Si as third element on the siting and mobility of electrochemically loaded deuterium in ball-milled Mg0.63Ti0.27Ni0.10 and Mg0.63Ti0.27Si0.10 alloys. Magic angle spinning (MAS) 2H NMR reveals that Ni and Si induce new types of deuterium sites in addition to the Mg-rich and Ti-rich sites already present in Mg0.65Ti0.35D1.2. 2D exchange NMR spectroscopy shows a substantial deuterium exchange between the various types of sites, which reflects their close interconnectivity in the crystal structure. Furthermore, the time scale and temperature dependence of the deuterium mobility have been quantified by 1D exchange NMR. The obtained effective residence times for deuterium atoms in the Mg-rich and Ti-rich nanodomains in Mg0.65Ti0.35D1.2, Mg0.63Ti0.27Ni0.10D1.3, and Mg0.63Ti0.27Si0.10D1.1 at 300 K are 0.4, 0.3, and 0.8 s, respectively, and the respective apparent activation energies 17, 21, and 27 kJ mol–1. The addition of Ni promotes deuterium mobility inside Mg–Ti hydrides, which is in agreement with the observed catalytic effect of Ni on the electrochemical (de)hydrogenation of these materials.