Low-Temperature Activation of Air-Exposed TiFe0.9Mn0.1Ce0.04Zr0.02 Alloy: Possible Role of a Mn/Zr-Enriched Secondary Region

In this work, a TiFe0.9Mn0.1Ce0.04Zr0.02 hydrogen storage alloy was prepared by vacuum arc melting. The as-cast alloy was crushed into powder in air and then stored in air for 2 days before hydrogenation measurements, which simulated realistic handling conditions. Microstructural characterization revealed that the alloy consisted of a TiFe-based matrix, a Ce-rich phase, and an additional Mn/Zr-enriched secondary region induced by minor Zr addition. X-ray Photoelectron Spectroscopy (XPS) confirmed the formation of surface oxides after air exposure, indicating that the subsequent activation proceeded from an oxidized surface. The alloy exhibited remarkably improved activation behavior under these realistic conditions. Without any vacuum heat pre-treatment, direct activation was achieved at 60 °C within 1 h under 4 MPa hydrogen. This Mn/Zr-enriched secondary region is proposed to enable the low-temperature activation by preferentially absorbing hydrogen, which may crack the surface oxide layer and exposed fresh catalytic surfaces. The maximum hydrogen storage capacity reached 1.71 wt.% at room temperature. Hydrogen absorption and desorption kinetics and thermodynamics were systematically investigated at different temperatures. The fitted negative activation energy for absorption is apparent and protocol-dependent, reflecting the temperature-dependent driving force at fixed pressure. Alloy tablets with the same nominal composition were also prepared and tested, as shaping into tablets is a necessary step for pilot-scale hydrogen storage applications. The results demonstrate that the introduced Mn/Zr-enriched secondary region facilitates initial hydrogenation without significantly compromising the alloy’s thermodynamic properties. These findings indicate that inducing a Mn/Zr-enriched secondary region by minor Zr addition is a viable strategy for enhancing the low-temperature activation performance of TiFe-based hydrogen storage alloys under realistic air-handling conditions.