Crystal structure and hydrogen storage properties of ZrNbFeCo medium-entropy alloy

To make hydrogen a more viable energy carrier, various solutions for hydrogen storage have been developed, with significant recent progress in developing new high-entropy alloys (HEAs) that exhibit attractive hydrogen storage properties. In this paper, we investigated the crystal structure and hydrogen storage properties of a new medium-entropy alloy (MEA) ZrNbFeCo, designed using a combination of semi-empirical parameters and thermodynamic calculations via the CALPHAD method. The alloy was synthesized by arc melting under an argon atmosphere and subsequently characterized using comprehensive techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These analyses revealed the presence of a major C14 Laves phase, with a compositional gradient and grain sizes ranging from microscale to nanoscale. The hydrogen storage properties were evaluated using pressure-composition isotherms (PCI) and kinetics curves. After a simple activation procedure, the alloy formed a C14 hydride and exhibited excellent properties to act as a vessel for hydrogen storage at room temperature. Under these conditions, the alloy was able to absorb up to 1.2 wt% of hydrogen (hydrogen-to-metal ratio of H/M ∼ 0.9), with fast absorption kinetics, reaching around 87 % of its maximum capacity after just 60s. The alloy also exhibited full reversibility and great stability through multiple absorption-desorption cycles, absorbing an average content of 1.1 wt% of hydrogen (H/M ∼ 0.82) after 8 cycles. The present results demonstrate that it is possible to practically employ semi-empirical and thermodynamics calculations, originally developed for HEAs, to develop new MEAs that exhibit appropriate microstructure and excellent hydrogen storage properties at room temperature.