Applicability and Limitations of Hydrogen Affinity As a Descriptor for Designing High-Entropy Alloys for Hydrogen Storage

Precise identification of high-entropy alloys (HEAs) from the vast chemical space remains a challenge across various application fields. Focusing on HEAs for hydrogen storage, this study investigates the applicability and limitations of the physically intuitive hydrogen affinity (ΔHaff) as an energetic descriptor for predicting the macroscopic performance, using comprehensive density functional theory simulations, statistical sampling, and experimental validation of V–Ti-based HEAs. Results reveal a near-equal thermodynamic preference for tetrahedral and octahedral hydrogen occupation sites in BCC HEAs and element-specific hydrogen selectivity at low concentrations. The developed ΔHaff descriptor exhibits strong linear correlations with hydrogen binding energy and hydride formation enthalpy, establishing a design principle targeting a medium ΔHaff value of approximately −25 kJ/mol H. Guided by this principle and Hume–Rothery rules, nine HEAs were designed and experimentally validated. Among them, V3Ti3Cr2Fe2 emerged as a promising candidate, demonstrating a reversible capacity of 2.1 wt % at room temperature. Together with literature data, ΔHaff shows a robust linear relationship with mean metallic radius as well as hydride desorption temperature for BCC alloys but fails to correlate with storage capacity. The limitation is mainly rooted in secondary phase formation, surface deactivation as well as significant selectivity of hydrogen binding at low H/M ratios. This work provides atomic insights into the mechanisms governing HEA performance and establishes directions for complex systems subject to multifaceted influences.