Exploring Detailed Reaction Pathways for Hydrogen Storage with Borohydrides Using DFT Calculations

Borohydrides have seen renewed interest due to their ability to store and carry a high weight percentage of hydrogen, making them desirable materials for large-scale and long-duration energy storage applications. In this work, we present a systematic computational study of the thermochemical feasibility of a series of mechanistic pathways that can lead to the formation of the observed boron clusters. This allowed us to identify predominant pathways for buildup, following a complex network of pathways. We find that the preferred mechanisms are different for smaller and larger clusters. The primary mechanism for buildup in clusters up to ∼5 boron atoms is by addition of BH4–/BH3 species, followed by loss of H2 or H–. For larger clusters, fusion reactions dominate, again followed by the loss of H2 or H–. A few species were identified as possible branching points, where the trends for favorability change; B2H7– as an initiation step and B4H7–, B3H8–, and B9H132– as minima on the potential energy surface are most notable. This work demonstrates that borohydride clusters feature a wealth of pathways for realizing the desired products (B10H102– or B12H122–), the redundancy of which may help avoid energetic sinks or kinetic bottlenecks. This robustness in reactivity has the potential to make them a versatile H2 storage material.