Chemical hydrogen storage materials – boranes and silanes catalytic solvolysis and dehydrogenation: a mechanistic and regeneration perspective

Hydrogen (H₂) has gained a lot of interest as an alternative energy vector, to reduce greenhouse gas emission issues caused by the fossil fuel industry. However, to make hydrogen a real energy carrier in a decarbonated economy, a secure and sustainable supply chain is needed. This approach requires notably safe storage and efficient strategies for recycling of raw materials. We discuss in this survey the state-of-the-art in the field of chemical hydrogen storage (CHS) materials, considering two possible vectors: ammonia borane and hydrosilanes. Regardless of the vector, to achieve real use, it is necessary to understand both the performance of the system and its life cycle, which relates to catalysts structure, and the activation of chemical bonds with efficient and complete catalytic cycles. We give herein an overview of hydrolysis and/or alcoholysis from metals, using coordination complexes, molecular supported catalysts or other materials, including nanocatalysts, with a focus on mechanistic information and understanding. Notably, the studies related to these two vectors can be considered somewhat complementary. Thus, the set of bibliographic report on ammonia borane is very documented in efficient catalytic systems, while its recycling remains at a very early stage. In comparison, hydrosilanes have been much less addressed specifically as a vector for hydrogen, while their reactivity at the molecular scale benefits from a relevant understanding from coordination chemistry studies. In addition, both hydrosilane polymerization and solvolysis reaction enables the release of H₂, and produces by-products of which added value is already established. This opening the way to economical strategies where recycling can be optional. Nevertheless, the reversibility of hydrosilanes chemistry in H₂ uptake remains attractive and is another option to develop.