Tailoring hydrogen diffusion pathways in Mg-Ni alloys through Gd addition: A combined experimental and computational study

Magnesium-based alloys represent a promising avenue for solid-state hydrogen storage, yet their practical implementation is impeded by sluggish kinetics and thermodynamic stability. This study presents a systematic investigation into the effects of gadolinium (Gd) and nickel (Ni) content on the microstructure and hydrogen storage properties of Mg-Ni-Gd alloys. Through advanced characterization techniques and density functional theory calculations, we elucidate that Gd preferentially occupies sites in the Mg2Ni phase rather than the Mg matrix, facilitating hydrogen diffusion. The Mg-7.8Ni-2.2Gd alloy demonstrates superior performance, absorbing 5.8 wt% hydrogen at 300 degrees C, compared to 5.3 wt% for Mg-10Ni under identical conditions. Remarkably, this composition exhibits excellent low-temperature kinetics, absorbing 4.5 wt% hydrogen within 30 min at 120 degrees C and maintaining 99 % capacity retention after 128 cycles. Post-initial hydrogenation, Gd accumulates at Mg2Ni surfaces, amplifying its role as a "hydrogen pump". Kinetic analysis using the Chou model for absorption and the Johnson-Mehl-Avrami-Kolmogorov model for desorption provides quantitative insights into the improved hydrogen storage mechanisms. Our computational studies corroborate these experimental findings, revealing that Gd doping in Mg2Ni substantially improves the hydrogen absorption ability, and surface penetration barrier of H atom deeply decreased. This work not only advances our understanding of the complex interplay between composition, microstructure, and hydrogen storage properties in Mg-based alloys but also establishes design principles for high-performance hydrogen storage materials.