NMR Hydrogen Storage Systems: Ionic Hydrides and Mobile Species

In the past, hydrogen storage solids were almost exclusively interstitial metallic hydrides. These are noted for their generally good hydrogen diffusion kinetics; here we report ωH, the rate of atomic-level hydrogen hopping events. This is nicely demonstrated in Figure 1, where ωH appears on a logarithmic scale for the prototypical ionic system MgH2 and in the metallic systems ScH2, Mg-ScHx, and LaNi5H6.8. Clearly, MgH2 has much slower dynamics than the metals, and a much higher activation energy. Remarkably, MgH2 can be converted from the rutile (ionic) structure to the fluorite (metallic) structure with as little as 20% Sc, though Figure 1 is for 35% Sc; the H hopping in the metallic phase is much faster than in MgH2 and is even a bit faster than in ScH2. While the metallic hydrides show good H kinetics, an essential feature of any hydrogen storage system, the mass-fraction of H, is too small. Thus, interest has turned to the lightweight hydrides, such as LiH, MgH2 (7.6 w/w%), NaMgH3, and LiBH4 (18 w/w%). These are all ionic or complex hydrides. We examined coarsegrained MgH2 as the prototypical ionic hydride. The rate ωH of H hopping remains too slow to narrow the hydrogen NMR line up to 400°C, so ωH <10 5 s. This confirms the reputation of MgH2 for slow kinetics – rehydriding Mg metal often is halted once a thin skin of MgH2 forms and blocks further reaction progress. To detect and measure such slow motions, we turned to the ultraslow motion experiment of Ailion and Slichter. Standard spin Zeeman-order is converted to spin dipolar-order at the start of the experiment. This order is found to decay with time constant T1D. Because dipolar order is a correlation between a spin’s orientation and the local dipolar field from its neighbors, and because the local field varies from site-to-site with little correlation, a single atomic jump destroys a given spin’s contribution to the dipolar order. Thus, the measured relaxation rate 1/T1D is essentially equal to the rate of atomic jumps for a typical H atom.