Hydrogen Motion in Magnesium Hydride by NMR

Abstract

In coarse-grained MgH 2 , the diffusive motion of hydrogen remains too slow (<10 5 hops s -1 ) to narrow the H NMR line up to 400 °C. Slow-motion dipolar relaxation time T 1D measurements reveal the motion, with hopping rate ω H from 0.1 to 430 s -1 over the range of 260 to 400 °C, the first direct measurement of H hopping in MgH 2 . The ω H data are described by an activation energy of 1.72 eV (166 kJ/mol) and attempt frequency of 2.5 × 10 15 s -1 . In ball-milled MgH 2 with 0.5 mol % added Nb 2 O 5 catalyst, line-narrowing is evident already at 50 °C. The line shape shows distinct broad and narrow components corresponding to immobile and mobile H, respectively. The fraction of mobile H grows continuously with temperature, reaching ∼30% at 400 °C. This demonstrates that this material's superior reaction kinetics are due to an increased rate of H motion, in addition to the shorter diffusion paths from ball-milling. In ball-milled MgH 2 without additives, the line-narrowed component is weaker and is due, at least in part, to trapped H 2 gas. The spin-lattice relaxation rates T 1 -1 of all materials are compared, with ball-milling markedly increasing T 1 -1 . The weak temperature dependence of T 1 -1 suggests a mechanism with paramagnetic relaxation centers arising from the mechanical milling.

Ion dynamics in solid electrolytes for lithium batteries

“…At room temperature the orthorhombic modification is stable that exhibits only a poor ion conductivity. While the Li ions are rather immobile on the MHz time scale, fast reorientations of the BH 4 units are seen through 11 B, 1 H NMR, and indirectly through the 7 Li spins [108,109]. At T hex./ortho ≈ 381 K ortho-LiBH 4 reversibly transforms into its hexagonal modification, see Fig.…”

1H NMR Studies of MgH2

“…The spin-echo signals were accumulated by using the π/2 -τ -π pulse sequence with cycling phase so as to cancel FID signals. Figure 1 shows FT-NMR spectra of 1 H FID signals of MgH 2 with cooling from 289 K to 77.3 K. The spectra of the present sample are superpositions of narrow and broad components, which resemble the spectra for coarse-grained and ball-milled (bm) MgH 2 with Nb 2 O 5 additive [2]. The temperature dependence of the two components of the present sample below room temperature resembles that of the coarse-bm MgH 2 above room temperature.…”

“…The reactions need heat treatment above about 280 • C, which is too high for practical uses. The reduction of grain particle sizes from ball-milling and catalytic additives such as Nb 2 O 5 have been reported to achieve great improvements to the reaction kinetics of MgH 2 [2]. The hopping of proton ions through the material has been believed to be crucial to the dehydriding and rehydriding reactions.…”

Reactivity enhancement of oxide skins in reversible Ti-doped NaAlH4

“…[58][59][60] Following our line of argumentation, breaking the surface oxide skin should have a similar effect on hydrogen sorption. Indeed, Na 3 AlH 6 and NaAlH 4 can both be formed by ball milling of NaH and Al at moderate hydrogen pressures and room temperature.…”