First-Principles Calculations of van't Hoff Plots for Novel Hydrogen-Storage Materials

A van't Hoff plot, log(P) vs. 1/T, provides information on the free-energy change in a reaction and is widely used to characterize hydrogen-storage materials. Recently, a new reaction of LiBH$_4$ destabilized by MgH$_2$ (yielding over 11 wt.\% of H$_2$) was proposed.\footnote{J.J. Vajo et al., J. of Phys. Chem. B 109, p.3719 (2005)} Here we investigate this reaction and its products by first-principles calculations and construct the van't Hoff plot for a direct comparison to experiment. Although it is often assumed that there is a constant slope in the van't Hoff plot for ease of interpretation, we find an important non-linearity arising from temperature-dependent vibrational entropy difference, etc. This non-linearity can be critical for an accurate comparison to experimental data, and between various reactions to determine optimal hydrogen-storage systems. Including these effects, we find agreement with recent measurements.