Activation methods for hydrogen storage alloys: A comprehensive review

Hydrogen storage alloys are promising materials for enabling the transition for the hydrogen economy. However, many alloys exhibit poor initial absorption kinetics or lose reactivity after air exposure, requiring activation procedures. This review first establishes a unified definition of the term “activation” as the process that enables a material to absorb hydrogen and also to achieve an optimal storage capacity, lower operating temperature or faster kinetics, depending on its target application. Subsequently, it provides a comprehensive overview of the main activation methods along with their proposed mechanisms. These activation methods include alloying and catalytic additions, mechanical processing, surface deposition, pulsed laser activation, and the most common activation method, thermal activation, along with its variations combining absorption-desorption cycles. They mainly act through surface modifications, since the surface plays a key role in several hydrogenation steps, such as H 2 dissociation and diffusion. These modifications include defect generation, dissociation of oxides or the prevention of their formation, and the formation of secondary phases that may enhance hydrogen diffusion and create catalytic sites for hydrogen dissociation. While activation effectively restores or enhances kinetics and capacity of hydrogen storage, its efficiency depends on many factors, such as alloy composition, type and oxide stability, and processing parameters. The best overall results regarding kinetics, gravimetric capacity, and oxidation resistance are achieved through integrated strategies, such as combining mechanical processing with metallic coating/addition of catalysts or, for example, combining alloying with thermal activating/cycling. Finally, future perspectives are highlighted, which include developing scalable, cost-effective, and faster activation strategies. • Unified definition of activation as a method to optimize hydrogen storage performance. • Overview of the main activation methods for hydrogen storage and their proposed working mechanisms. • Activation mechanisms of hydrogen absorption mainly act through surface modifications. • Surface plays a key role in H 2 dissociation and diffusion. • Combined activation approaches further improve hydrogen absorption kinetics and air stability.