Stability analysis of doped materials for reversible hydrogen storage in destabilized metal hydrides

Abstract

Thermodynamic calculations with first principles density functional theory are carried out to estimate the free energies of various doped metal hydride systems. We consider doped destabilized hydride reactions of the form ${X}_{1--x}{Y}_{x}{\mathrm{H}}_{2}+2\mathrm{Li}\mathrm{B}{\mathrm{H}}_{4}\ensuremath{\rightarrow}{X}_{1--x}{Y}_{x}{\mathrm{B}}_{2}+2\mathrm{Li}\mathrm{H}+4{\mathrm{H}}_{2}$, where $X$,$Y=\mathrm{Mg}$, Sc, or Ti. We have evaluated the zero temperature enthalpies, without inclusion of zero point energies, for 18 different doped systems. Most systems are found to be unstable with respect to phase separation at $0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We have included configurational entropy to estimate the temperature at which the doped systems become stable. Most doped compounds are estimated to remain unstable with respect to phase segregation up to temperatures that are too high to be of practical interest. We have computed the phonon density of states for the $X=\mathrm{Sc}$, $Y=\mathrm{Ti}$ system and find that this system is stable with respect to phase segregation at $T>435\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We have computed the van't Hoff plot for ${\mathrm{Sc}}_{7}{\mathrm{H}}_{16}\mathrm{Ti}+16\mathrm{Li}\mathrm{B}{\mathrm{H}}_{4}\ensuremath{\rightarrow}{\mathrm{Sc}}_{7}{\mathrm{B}}_{16}\mathrm{Ti}+16\mathrm{Li}\mathrm{H}+32{\mathrm{H}}_{2}$ and compared this to the undoped reaction. Doping increases the vapor pressure at a given temperature, but only by a factor of 2--4.