The onset of dehydrogenation in solid Ammonia Borane, an ab-initio metadynamics study

TL;DR

This study uses ab-initio molecular dynamics and metadynamics to elucidate the atomistic mechanisms of hydrogen release in solid ammonia borane, revealing the formation of ammonia diborane and an autocatalytic hydrogen production cycle.

Contribution

It introduces a novel computational approach combining ab-initio MD and metadynamics to model complex solid-state dehydrogenation processes.

Findings

Ammonia diborane formation triggers NH4+ release.

NH4+ initiates an autocatalytic H2 production cycle.

Multiple molecules are essential for the reaction pathway.

Abstract

The discovery of effective hydrogen storage materials is fundamental for the progress of a clean energy economy. Ammonia borane ($\mathrm{H_3BNH_3}$) has attracted great interest as a promising candidate but the reaction path that leads from its solid phase to hydrogen release is not yet fully understood. To address the need for insights in the atomistic details of such a complex solid state process, in this work we use \textit{ab-initio} molecular dynamics and metadynamics to study the early stages of AB dehydrogenation. We show that the formation of ammonia diborane ($\mathrm{H_3NBH_2(}$$\mu$$\mathrm{-H)BH_3}$) leads to the release of $\mathrm{NH_4^+}$, which in turn triggers an autocatalytic $\mathrm{H_2}$ production cycle. Our calculations provide a model for how complex solid state reactions can be theoretically investigated and rely upon the presence of multiple ammonia borane molecules, as substantiated by standard quantum-mechanical simulations on a cluster.