Effect of Particle Size on Hydrogen Release from Sodium Alanate Nanoparticles

Density functional theory and the cluster expansion method are used to model 2-10 nm sodium alanate (NaAlH4) nanoparticles and related decomposition products Na3AlH6, NaH, and Al. While bulk sodium alanate releases hydrogen in a two-step process, our calculations predict that below a certain size sodium alanate nanoparticles decompose in a single step directly to NaH, Al, and H-2 due to the effect of particle size on decomposition thermodynamics. This may explain why sodium alanate nanoparticles, unlike bulk sodium alanate, have been observed to release hydrogen in the operating temperature range of proton exchange membrane fuel cells. In addition, we identify low-energy surfaces that may be important for the dynamics of hydrogen storage and release from sodium alanate nanoparticles.