(509g) First-Principles Study of H2O Adsorption On the NaBH4(100) Surface

Sodium borohydride (NaBH₄) is a promising hydrogen storage material for niche applications due to its high gravimetric (10.7 wt%) and large volumetric (115 kg of H₂ m^-3) hydrogen content. The hydrolysis of NaBH₄ releases hydrogen with both fast kinetics and high extent of reaction under technical conditions by using steam deliquescence of NaBH₄. The NaBH₄ crystal is strongly hygroscopic, rapidly absorbing water vapor from the atmosphere and then dissolving into solution, even at fairly low relative humilities. The hydrolysis reaction takes place in solution at sufficiently high temperatures to produce H₂ and hydrated NaBO₂. To the best of our knowledge, there are no experimental data on the adsorption energy or structure of adsorbed water on NaBH₄ surfaces. It is difficult to characterize the initial adsorption processes on NaBH₄ surfaces from experiments because the adsorption and dissolution of the crystal occurs rapidly.  We have therefore used first-principles density functional theory (DFT) to probe the details of the initial adsorption process at the atomic level. Standard DFT functionals do not include an adequate description of van der Waals interactions, which are expected to be important for the energetics and structure of H₂O adsorption on NaBH₄. We have therefore used two different approaches for including van der Waals forces within DFT calculations and have compared our calculations with standard generalized gradient approximation DFT calculations. We have computed the adsorption energies and geometries for various coverages of H₂O on the NaBH₄(100) surface. We characterize the H₂O-H₂O and H₂O-surface interaction energies as the number of H₂O molecules on the surface increases. Diffusion of water on the surface is also an important process for cluster growth. We have used the Climbing Image-Nudged Elastic Band method to compute the diffusion pathways for an isolated water molecule on NaBH₄(100).