(25b) First-Principles Study of Sodium Borohydride for Hydrogen Storage

Hydrogen storage continues to be a vexing problem for applications like hydrogen powered fuel cell vehicles. Complex hydrides of light elements are attractive as storage materials due to their high gravimetric and volumetric densities. The hydrolysis reaction of sodium borohydride with water releases 10.7 wt% hydrogen. The reaction mechanism and kinetic barriers associated with this reaction have not yet been conclusively elucidated, despite the number of studies on this material. We are developing a fundamental molecular level understanding of the hydrolysis reaction of NaBH4 using a combination of experiments and first-principles density functional theory (DFT) calculations. We first examined the ground state structure of the low pressure cubic α-NaBH4 crystal. Experimental x-ray diffraction data indicate partial occupancy of the hydrogen sites. We have therefore enumerated all possible structures under the constraint that the BH4 group is tetrahedral and from a series of calculations have found the lowest energy bulk structures. We computed the surface energies of the low Miller index surfaces and used the Wulff construction method to find the equilibrium crystal shape. The surfaces are important because the initial step in the hydrolysis reaction is the adsorption of H2O from the gas phase onto the surface. The (001) surface was found to have the lowest energy. We computed the adsorption energy and structure of H2O on the (001) surface with DFT. Possible reaction mechanisms on the surface were investigated with DFT molecular dynamics. We were unable to observe any reactions on the hydrated (001) surface of NaBH4 on picosecond time scales at high temperatures (~1000 K). We have investigated the role of defects on the reaction mechanism and have found that initial hydrolysis reactions can occur rapidly when Na vacancies are present on the NaBH4 surface. Vibrational modes of NaBH4 and related systems were calculated to provide data that directly complement experimental Raman measurements.