Dynamics of hydrogen in intermetallic hydrides

Hydrogen diffusion in metal-hydrogen systems via NMR and DFT

“…The relaxation measurements result in a lower activation energy value (0.11 eV), as compared to the diffusion measurements (0.20 eV). Such a discrepancy is not exceptional [61] as the two methods probe effectively different timescales. Moreover, the E a value determined from T 1 (1/T) is affected by the use of the most appropriate model, whereas for diffusion measurements the E a value is determined directly from the D(T) slope.…”

Deuterium exchange dynamics in Zr2NiD4.8 studied by 2H MAS NMR spectroscopy

“…The kinetics of the hydrogen absorption and desorption processes will often determine the conditions for successful and efficient operation of proposed devices. Knowledge of the hydrogen diffusion parameters can provide valuable insights during selection and development of appropriate candidates in addition to gaining better understanding of the role of crystal structures and compositions on the fundamental transport mechanisms in the hydride phases [4,5] Nuclear magnetic resonance (NMR) provides information on both local (e.g., hops among a few closely separated sites or rotations of covalently bonded ions) and long-range (e.g., translations) motions from behavior of spectra and several kinds of relaxation times as described in various reviews [6,7]. The most common approach to obtain diffusion parameters using NMR is to measure the spectra or relaxation times over a temperature range appropriate for a given hydride, which are generally analyzed by well-developed microscopic models based upon an Arrhenius relations [6] with an attempt frequency and activation energy (E a ).…”

“…The most common approach to obtain diffusion parameters using NMR is to measure the spectra or relaxation times over a temperature range appropriate for a given hydride, which are generally analyzed by well-developed microscopic models based upon an Arrhenius relations [6] with an attempt frequency and activation energy (E a ). Over the years, NMR studies have been performed on nearly all binary metal hydrides and many intermetallic and alloy hydrides [4,5,6,7]. Unfortunately, these measurements are often insufficient to given more detailed assessments of the specific roles from multiple H-site occupancies on the diffusion processes in the more complicated host structures.…”

Metallic Hydrides I: Hydrogen Storage and Other Gas-Phase Applications

“…8 The D value for MgH 2 was from an x-ray photoelectron spectroscopy study by Spatz et al 9 When D 1.7 ϫ 10 Ϫ8 cm 2 /s, the mean time (d 2 /6D) for a hydrogen atom to diffuse d 10 m is 10 s. Since the particle dimensions of most activated metal hydrides are in the range of 1-100 m, complete transport of hydrogen through these particles would normally occur within a minute or so. Bulk diffusion should be adequate for PdH 0.7 , LaH 2.92 , ␤-TiH 0.7 , LaNi 5 H 6 , and ZrV 2 H 5 at temperatures above 250 K, or above ϳ600 K for ZrH 1.58 or LaH 2.0 .…”

“…7 The final contributor to kinetics is the hydrogen-diffusion rate within the alloy and hydride phases. Figure 1 shows the intrinsic diffusion coefficients (D) for representative hydrides, obtained mostly by nuclear magnetic resonance 8 or quasi- 8 or quasi-elastic neutron scattering. 8 The D value for MgH 2 was from an x-ray photoelectron spectroscopy study by Spatz et al 9 The horizontal dotted line at D 1 ϫ 10 Ϫ8 represents the approximate conditions of a hydrogen atom diffusing 10 m in 10 s.…”