Energy Barriers for H2 Uptake and Release by Defects in Boron Nitride

Density functional theory simulations (DFT) have been used in combination with the climbing image nudged elastic band method (CI-NEB) to determine the energy barriers required to dissociate H2 at h-BN defects and edges and evaluate the potential for reacted H species to hop to adjacent available sites. Four types of defects were selected based on previous work in which ab initio thermodynamics was used to identify specific defects with optimal thermodynamics for reversible H2 storage, that is, the defects would dissociate molecular H2 while not binding H species too strongly so that hydrogen release is possible. The four defects investigated are the N and B monovacancies, which only offer either B or N defective sites, respectively, as well as larger cluster vacancy defects named 3V(1B2N) and hexagonal 6V(3B3N), which present a mixture of B and N defect sites available for H binding. Three edges, such as the pristine armchair BN edge, zigzag B, and zigzag N edges, were also investigated. While homogeneous H2 dissociation by B and N monovacancies has energy barriers larger than 0.4 eV, we found that mixed BN-terminated defects have energy barriers lower than 0.4 eV for H2 dissociation. In particular, H2 dissociation by the hexagonal 6V(3B3N) defect, exclusively made of Frustrated Lewis Pairs (FLPs), is almost barrierless. The energy barrier for subsequent H hopping to available adjacent B or N sites of the defect was also calculated. It was found that the hopping of H species generally involves high energy barriers ranging from 0.26 to 2.57 eV. In the case of edges, all dissociation reactions were found to be almost barrierless, suggesting that h-BN edges efficiently split H2. This work completes previous thermodynamic theoretical investigations and provides a more comprehensive picture of the H2 reaction with defective h-BN for potential H2 storage and recovery.