Hydrogen storage in calcium alanate: First-principles thermodynamics and crystal structures

Using first-principles density functional theory (DFT) calculations, we study the thermodynamics and crystal structure of calcium alanate, $\mathrm{Ca}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}$, and its decomposition products $\mathrm{Ca}\mathrm{Al}{\mathrm{H}}_{5}$, $\mathrm{Ca}{\mathrm{H}}_{2}$, and $\mathrm{Ca}{\mathrm{Al}}_{2}$. Using a large database of $A{B}_{2}{C}_{8}$ and $AB{C}_{5}$ structure types, we perform nearly 200 DFT calculations in an effort to predict the crystal structures of the $\mathrm{Ca}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}$ and $\mathrm{Ca}\mathrm{Al}{\mathrm{H}}_{5}$ phases. For the low-energy $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ phases, we perform DFT frozen-phonon calculations to ascertain the zero-point and vibrational entropy contributions to the thermodynamics of decomposition. We find the following: (i) For $\mathrm{Ca}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}$, we confirm the previously predicted $\mathrm{Ca}{\mathrm{B}}_{2}{\mathrm{F}}_{8}$-type structure as the stable phase. In addition, we uncover several phases (e.g., $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Th}{\mathrm{Mo}}_{2}{\mathrm{O}}_{8}$-type, $\mathrm{Ag}{\mathrm{Au}}_{2}{\mathrm{F}}_{8}$-type, and $\mathrm{Pb}{\mathrm{Re}}_{2}{\mathrm{O}}_{8}$-type) very competitive in energy with the ground state structure. (ii) For $\mathrm{Ca}\mathrm{Al}{\mathrm{H}}_{5}$, we find the stable structure type to be the recently observed ${\ensuremath{\alpha}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Sr}\mathrm{Al}{\mathrm{F}}_{5}$-type, with $\mathrm{U}\mathrm{Tl}{\mathrm{F}}_{5}$-type, $\mathrm{Sr}\mathrm{Fe}{\mathrm{F}}_{5}$-type and $\mathrm{Ba}\mathrm{Ga}{\mathrm{F}}_{5}$-type structures being close in energy to the ground state. (iii) In agreement with recent experiments, our calculations show that the decomposition of $\mathrm{Ca}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}$ is divided into a weakly exothermic step $[\mathrm{Ca}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}\ensuremath{\rightarrow}\mathrm{Ca}\mathrm{Al}{\mathrm{H}}_{5}+\mathrm{Al}+3∕2{\mathrm{H}}_{2}]$, a weakly endothermic step $[\mathrm{Ca}\mathrm{Al}{\mathrm{H}}_{5}\ensuremath{\rightarrow}\mathrm{Ca}{\mathrm{H}}_{2}+\mathrm{Al}+3∕2{\mathrm{H}}_{2}]$, and a strong endothermic step $[\mathrm{Ca}{\mathrm{H}}_{2}+2\mathrm{Al}\ensuremath{\rightarrow}\mathrm{Ca}{\mathrm{Al}}_{2}+{\mathrm{H}}_{2}]$. (iv) Including static $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ energies, zero-point energies, and the dynamic contributions of ${\mathrm{H}}_{2}$ gas, the DFT-calculated $\ensuremath{\Delta}H$ values for the first two decomposition steps ($\ensuremath{-}9$ and $+26\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ ${\mathrm{H}}_{2}$ at the observed decomposition temperatures $T\ensuremath{\sim}127$ and $250\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, respectively) agree well with the experimental values recently reported ($\ensuremath{-}7$ and $+32\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ ${\mathrm{H}}_{2}$). Only the second step $[\mathrm{Ca}\mathrm{Al}{\mathrm{H}}_{5}∕\mathrm{Ca}{\mathrm{H}}_{2}]$ has thermodynamics near the targeted range that might make a suitable on-board hydrogen storage reaction for hydrogen-fueled vehicles. (v) Comparing the enthalpies for final stage of decomposition [$\mathrm{Ca}{\mathrm{H}}_{2}+2\mathrm{Al}\ensuremath{\rightarrow}\mathrm{Ca}{\mathrm{Al}}_{2}+{\mathrm{H}}_{2}$, $\ensuremath{\Delta}H=72\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ ${\mathrm{H}}_{2}$] with the pure decomposition of $\mathrm{Ca}{\mathrm{H}}_{2}$ [$\mathrm{Ca}{\mathrm{H}}_{2}\ensuremath{\rightarrow}\mathrm{Ca}+{\mathrm{H}}_{2}$, $\ensuremath{\Delta}H=171\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ ${\mathrm{H}}_{2}$] shows that the addition of Al provides a huge destabilizing effect on $\mathrm{Ca}{\mathrm{H}}_{2}$, due to the formation of the strongly bound $\mathrm{Ca}{\mathrm{Al}}_{2}$ phase.