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.