Nuclear magnetic resonance study of hydrogen diffusion in palladium-silver alloys

doi:10.1016/0022-5088(76)90033-3

Journal of the Less Common Metals

Volume 49, September–October 1976, Pages 159-168

https://doi.org/10.1016/0022-5088(76)90033-3 Get rights and content

Abstract

The diffusion of hydrogen in the α′ phase of Pd and PdAg alloys with up to 17% Ag has been studied by proton magnetic resonance. Proton spin-lattice relaxation times, T₁, have been measured over the range ~200 K to ~400 K and the diffusion coefficient, D, was also measured directly by the pulsed-field-gradient method in the range ~300 K to ~400 K. The two sets of measurements taken together are consistent with random jumps between nearest neighbour octahedral sites, and are inconsistent with jumps between tetrahedral sites. For a fixed hydrogen concentration, D decreases, d₀ increases by a factor of about 2, and the activation energy increases from 0.23 ± 0.01 eVto 0.28 ± 0.02 eV on addition of 17 at.% Ag.

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Solid-state hydrogen storage materials undergo complex phase transformations in which kinetics are often limited by hydrogen diffusion that significantly changes during hydrogen uptake and release. Here we perform robust statistically-averaged molecular dynamics simulations to obtain a well-converged analytical expression for hydrogen diffusivity in bulk palladium that is valid throughout all stages of the reaction. Our studies confirm the experimentally observed dependence of the diffusivity on concentration and temperature and elucidate the underlying physics. Whereas at low hydrogen concentrations, a single dilute hopping barrier dominates, at high hydrogen concentrations, diffusion exhibits multiple hopping barriers corresponding to hydrogen-rich and hydrogen-poor local environments.
<sup>1</sup>H NMR spectra and echoes in Pd-H and Pd-Ag-H alloys
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The paper presents the experimental results of rigid-lattice proton magnetic resonance spectra and second moments in Pd–H, Pd_0.90Ag_0.10–H, Pd_0.80Ag_0.20–H, Pd_0.75Ag_0.25–H, and Pd_0.65Ag_0.35–H alloys at T = 2.4 K in a wide hydrogen concentration range. Free induction decay (FID), solid and inhomogeneous echoes were detected and interpreted. The interpretation based on dipole–dipole interaction of homogeneously distributed proton spin system is satisfactory at high hydrogen concentration, but an inhomogeneous field of paramagnetic origin (most probably coming from Fe impurities) plays the dominant role at small hydrogen content. An unexpected correlation was found between the macroscopic magnetic susceptibility and the second moment coming from the inhomogeneous internal field.
<sup>1</sup>H NMR analysis of nuclear relaxation mechanisms in Pd-H and Pd-Ag-H alloys
2005, Journal of Alloys and Compounds
Crystalline Pd_1−xAg_x–H_y ( $x = 0 - 0.35$ ) alloys were studied as model systems representing a chemically disordered system for hydrogen-storage materials. Extensive ${}^{1}H$ relaxation rate ( $R_{1}$ , $R_{1 ρ}$ and $R_{2}$ ) measurements were carried out in a wide temperature range of 2.4–400 K. Palladium–silver alloys of silver concentration as high as 35 at.% were studied by NMR spectroscopy for the first time. The relaxation mechanisms magnetic dipolar relaxation (hydrogen diffusion), Korringa relaxation, relaxation by paramagnetic impurity ions and cross-relaxation to quadrupolar metal nuclei were identified and characterized. The presence of cross-relaxation between ${}^{1}H$ and quadrupolar ${}^{105}{Pd}$ nuclear spins was assumed and demonstrated for the first time in Pd–H and Pd_1−xAg_x–H systems. More than one jump processes of diffusing hydrogen were found to be present in the studied palladium–silver alloys. Only lower estimated limit values of Korringa constants could be obtained since no frequency independent $R_{1}$ versus T region was found. These values are significantly higher than the ones reported by others.
The existence of more than one jump process of hydrogen in palladium-silver alloys - An NMR study
2002, Journal of Alloys and Compounds
The hydrogen dynamics in the α-phase of Pd_1−xAg_xH_y has been studied by proton nuclear magnetic resonance (NMR) measurements. Jump frequencies of the hydrogen atoms in the metal lattice have been deduced from the dipolar spin-lattice relaxation rates Γ_1d by applying the BPP model. In PdH_y, the temperature dependence of Γ_1d is well described by a single jump process for all hydrogen atoms. In contrast, in the palladium–silver alloys Pd_1−xAg_xH_y, the Γ_1d data indicate more than one jump process with different activation energies. Two and three processes could be separated for x=0.1 and x=0.3, respectively. In both alloys, the diffusion process with the lowest activation energy (E_a=226 meV) is comparable to that in pure palladium. The jump processes with higher activation energies occur more frequently in Pd_0.7Ag_0.3 than in Pd_0.9Ag_0.1. This indicates higher energy-barriers for jumps between O-sites in silver-rich environments. Direct measurements of the long-range diffusion coefficients D were performed by pulsed-field-gradient (PFG)-NMR at temperatures up to 450 K. For both Pd_1−xAg_xH_ysamples, D(T) is well represented by a single Arrhenius law with the diffusion parameters D₀=3.0×10⁻⁷ m² s⁻¹ and E_a=230 meV for x=0.1 and D₀=2.1×10⁻⁷ m² s⁻¹ and E_a=260 meV for x=0.3. The diffusivities calculated from the jump frequencies of the hydrogen atoms are in good agreement with these PFG results and also with measurements using the time-lag technique.
Concentration-distance profiles resulting from the hydrogen-charging of metal lattices under high fugacity conditions
1999, Acta Materialia
In the hydrogenation of metals under high fugacity conditions (such as electrolytic charging), non-steady state diffusion produces concentration–distance profiles which may not be calculated by assuming simple diffusion behavior. Such profiles have been calculated using finite difference methods by considering specific models for the concentration-perturbation of the appropriate jump activation barriers. The results show that much greater H concentrations are produced for a given penetration distance than would be expected by assuming fickian diffusion.
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^☆: Presented at the Meeting on “Hydrogen in Metals”, held at the University of Birmingham, United Kingdom, on January 5–6, 1976, under the auspices of the Chemical Society (Faraday Division).

^∗∗: Permanent address: Israel Atomic Energy Commission, Soreq Nuclear Research Centre, Yavne, Israel.

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Nuclear magnetic resonance study of hydrogen diffusion in palladium-silver alloys☆

Abstract

Phys. Rev. Lett.

Z. Phys. Chem., (N.F.)

Ber. Bunsenges. Phys. Chem.

Ber. Bunsenges. Phys. Chem.

Phys. Rev. B

J. Phys. C: Solid State Phys.

J. Phys. C: Solid State Phys.

J. Magn. Reson.

J. Phys. C: Solid State Phys.