Effects of hydrogen blocking and trapping by point defects on hydrogen diffusivity in B2-type PdCu alloys: Modeling and experimental analysis

Thorough assessment of hydrogen and lattice defect interactions in hydrogen-permeable alloys is essential because of their significant influence on hydrogen diffusion, the dominant elementary process in hydrogen permeation. However, the underlying mechanisms are not adequately understood. In this study, theoretical and experimental analyses revealed the controlling factors leading to a significant reduction in the hydrogen diffusivity of B2-structured PdCu alloys, which are promising materials for low-temperature hydrogen purification in practical applications. Using a density functional theory (DFT)-informed kinetic Monte Carlo (KMC) approach, we quantitatively evaluated point defect (i.e., antisite Cu atoms and Cu vacancies) effects on the hydrogen diffusivity ( D ) of PdCu alloys. It was found that antisite Cu atoms decreased D in proportion to their number because of the blocking effect, whereas Cu vacancies strongly decreased D at temperatures below 500 K due to the trapping effect. Moreover, the D values predicted by KMC simulations considering Cu vacancy effects were in good agreement with their experimental counterparts. Thermal desorption spectroscopy measurements were performed to detect the presence of lattice defects that trapped hydrogen in the PdCu alloys. The results indicate that lattice defects with a hydrogen binding energy of 0.4 eV exist in the sample, and this value is comparable to the DFT result for the Cu vacancy. This suggests that point defects, analogous to Cu vacancies, were responsible for the significant reduction in D for PdCu alloys observed in experiments. This study contributes to a detailed understanding of the interplay between hydrogen and lattice defects in Pd-based alloys. • Point defects causing a severe reduction in H diffusivity in PdCu were identified. • The blocking effect of antisite Cu atoms moderately reduced the H diffusivity. • The trapping effect of Cu vacancies strongly reduced the H diffusivity below 500 K. • TDS measurements confirmed the presence of lattice defects trapping H in the sample. • Cu vacancies and double defects likely result in significantly reduced H diffusivity.