Hydrogen Storage Behavior of Ti-Based High-Entropy Alloys: A Review

Titanium-based high-entropy alloys (HEAs) have emerged as promising solid-state hydrogen storage materials owing to their structural stability, tunable thermodynamics, and rapid kinetics. This review systematically examines the hydrogen storage behaviour of Ti-based HEAs across three structural categories: body-centered cubic (BCC) single-phase alloys, C14 Laves phase alloys, and multiphase systems containing CsCl-type structures. The high-entropy effect, severe lattice distortion, sluggish diffusion, and cocktail effect collectively enable these alloys to overcome key limitations of conventional materials, including difficult activation, poor reversibility, and insufficient cycling stability. Critical roles of constituent elements—Ti, V, Cr, Zr, Fe, and Mn—in modulating phase stability, hydrogen affinity, and kinetics are discussed. Empirical design parameters such as mixing entropy, valence electron concentration, atomic size mismatch, and electronegativity difference guide composition optimization. Advanced processing techniques, including mechanical alloying, high-pressure torsion, and melt spinning, enhance microstructural control and performance. Recent integration of CALPHAD, density functional theory, and machine learning has accelerated alloy discovery and design. Despite significant progress, challenges persist in balancing capacity with reversibility, mitigating surface oxidation, and ensuring processing consistency. This review provides a comprehensive framework for understanding structure–property relationships in Ti-based HEAs and outlines future directions toward practical hydrogen storage applications.