Dynamically Staged Phase Transformation Mechanism of Co-Containing Rare Earth-Based Metal Hydrides with Unexpected Hysteresis Amelioration

Abstract

Solid hydrogen storage and supply systems with high density in a hydrogen refueling station are the critical factor to realize large-scale application of hydrogen energy, and one of the biggest challenges is how to ameliorate the extremely large pressure hysteresis of Ce-rich rare-earth-based metal hydrides. In this work, two series of La–Ce–Ca–Ni–Co alloys with single CaCu5-type structure and uniformly distributed elements were prepared via induction levitation melting. With increasing Co content in La0.3Ce0.5Ca0.2Ni5–xCox (x = 0, 0.5, 1.0, 1.5) alloys, a staged phase transformation occurs, contributing to a significant improvement in the pressure hysteresis without the monotonical sacrifice of dehydrogenation equilibrium pressure. An equilibrium-state thermal analysis method (ETA) is proposed and well verifies the thermodynamical phase transformation processes. Further first-principles calculations indicate that the enhanced charge synergy and increased charge transfer drive the conversion of staged phase transformation from dynamic to thermodynamically stable pathways. Thus, “dynamically staged phase transformation”, nominated as the dynamic realization of thermodynamic staged phase transformation, is more constructive for practical use. Consequently, the optimal composition of La0.25Ce0.55Ca0.2Ni4.5Co0.5 was developed with a saturated hydrogen storage capacity of 1.52 wt %, dehydrogenation equilibrium pressure of 10.68 MPa at 90 °C, and satisfactory cycling durability. The ETA method and composition design concept proposed in this work pave a convenient avenue for wider exploration of high-pressure hydrogen storage alloys.