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

## TL;DR

This paper explores how hydrogen is stored in specific metal compounds, identifying which structures can hold more hydrogen and offering guidelines for improving storage capacity.

## Contribution

The study introduces a high-throughput search method to identify binary C15 Laves phases with enhanced hydrogen storage capacity.

## Key findings

- ZrMo2 accommodates hydrogen only in A2B2 tetrahedral sites, while ZrV2 accommodates hydrogen in both A2B2 and AB3 tetrahedral sites.
- Hydrogen atoms in ZrV2 are separated by distances close to the Switendick criterion, allowing for higher storage capacity.
- A high-throughput search identifies several binary C15 Laves phases with potential for multi-site hydrogen accommodation.

## 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.