New materials for hydrogen storage - Multi-principal alloys forming hydrides

In today's environmental and energy context, hydrogen could become a clean energy carrier to achieve decarbonization of the economy and green growth in industry. Among the challenges to be met in deploying decarbonized hydrogen (production, distribution and transport, use), compact, safe and efficient storage remains a technique to be developed for practical applications. In order to simultaneously meet these key criteria, the method of storage in "solid" form in metal hydrides is very promising. Among the several types of hydridable materials currently under investigation, this article presents the results of a new class, multi-element alloys, also known as high-entropy alloys, which have recently shown very interesting performances. The study of reversible hydrogen absorption and desorption in these new alloys is fairly recent, some ten years old, with a turning point in 2016 marking the discovery of the TiVZrNbHf alloy capable of storing 2.5 H/M (hydrogen atom per metal atom). This value exceeds the 2.0 H/M in elemental metal hydrides or other conventional alloys. After a general introduction to the field, a description of the various methods used to synthesize and characterize these alloys is presented, followed by a review of the performance of the best compositions. Several aspects are addressed: maximum and reversible capacity, thermodynamic and kinetic properties, cycling stability. Composition possibilities in multidimensional phase diagrams are, however, extremely vast, and incremental experimental research limited. In the future, more research is needed to predict the best compositions and to rationalize experimentally observed trends using theoretical approaches.

A glossary of notations is provided at the end of the article.