Rules and trends of metal cation driven hydride-transfer mechanisms in metal amidoboranes

Abstract

Group I and II metal amidoboranes have been identified as one of the promising families of materials for efficient H₂ storage. However, the underlying mechanism of the dehydrogenation of these materials is not well understood. Thus, the mechanisms and kinetics of H₂ release in metal amidoboranes are investigated using high level ab initio calculations and kinetic simulations. The metal plays the role of catalyst for the hydride transfer with formation of a metal hydride intermediate towards the dehydrogenation. In this process, with increasing ionic character of the metal hydride bond in the intermediate, the stability of the intermediate decreases, while the dehydrogenation process involving ionic recombination of the hydridic H with the protic H proceeds with a reduced barrier. Such correlations lead directly to a U-shaped relationship between the activation energy barrier for H₂ elimination and the ionicity of metal hydride bond. Oligomerized intermediates are formed by the chain reaction of the size-driven catalytic effects of metals, competing with the non-oligomerization pathway. The kinetic rates at low temperatures are determined by the maximum barrier height in the pathway (a Λ-shaped relation), while those at moderately high temperatures are determined by most of multiple-barriers. This requires kinetic simulations. At the operating temperatures of proton exchange membrane fuel cells, the metal amidoboranes with lithium and sodium release H₂ along both oligomerization and non-oligomerization paths. The sodium amidoboranes show the most accelerated rates, while others release H₂ at similar rates. In addition, we predict that the novel metal amidoborane-based adducts and mixtures would release H₂ with accelerated rates as well as with enhanced reversibility. This comprehensive study is useful for further developments of active metal-based better hydrogen storage materials.