First-Principles Thermodynamics of Hydrogen Absorption in Binary C15 Laves Phases.

Abstract

Intermetallic compounds are attractive candidates for hydrogen storage applications. This study investigates the thermodynamics of hydrogen absorption in binary AB₂ Laves phases with the C15 crystal structure. First-principles calculations, cluster expansion models, and statistical mechanics simulations are employed to determine the pressure-composition isotherms for two prototypical Laves phases: ZrMo₂ and ZrV₂. Our calculations show that ZrMo₂ accommodates hydrogen exclusively within A₂B₂ coordinated tetrahedral sites. In contrast, ZrV₂ accommodates hydrogen over both A₂B₂ and AB₃ coordinated tetrahedral sites. Finite-temperature simulations reveal that hydrogen atoms can occupy neighboring edge-sharing tetrahedra and are separated by a distance close to the Switendick criterion in ZrV₂. The occupation of both interstitial site types increases the hydrogen storage capacity of ZrV₂ as compared to ZrMo₂. Building on this insight, we perform a high-throughput search of binary C15 Laves phases and identify several promising candidates that can accommodate hydrogen across multiple interstitial sites. The results of this study provide chemical guidelines for tuning the hydrogen storage capacity of intermetallic compounds.