Synergistic and Competing Effects of Iron in TiVNbCr-Based High-Entropy Alloys for Reversible Hydrogen Storage.

Developing high-performance, cost-effective high-entropy alloys (HEAs) for reversible hydrogen storage is hindered by complex trade-offs between thermodynamic, kinetic, and stability properties. Herein, a CALPHAD-guided strategy is employed to systematically investigate the multifaceted role of Fe in TiVNbCr-based HEAs. Adding Fe effectively destabilizes the resulting hydride and accelerates desorption kinetics, culminating in a state-of-the-art reversible capacity of 2.31 wt% at 303 K for the Fe6 alloy. However, the influence of Fe is complex: while trace amounts catalytically enhance initial activation, higher concentrations promote a passivating surface oxide layer. Furthermore, long-term cycling demonstrates that Fe exacerbates lattice strain accumulation, leading to accelerated mechanical degradation. This work unravels the synergistic and competing effects of a single alloying element on surface chemistry, bulk thermodynamics, and structural evolution, establishing a holistic design paradigm that is essential for developing next-generation, practical hydrogen storage materials.