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

Christopher Wolverton and Vidvuds Ozoliņš

Phys. Rev. B 75, 064101 – Published 1 February 2007

Abstract

Using first-principles density functional theory (DFT) calculations, we study the thermodynamics and crystal structure of calcium alanate, $Ca {(Al H_{4})}_{2}$ , and its decomposition products $Ca Al H_{5}$ , $Ca H_{2}$ , and $Ca {Al}_{2}$ . Using a large database of $A B_{2} C_{8}$ and $A B C_{5}$ structure types, we perform nearly 200 DFT calculations in an effort to predict the crystal structures of the $Ca {(Al H_{4})}_{2}$ and $Ca Al H_{5}$ phases. For the low-energy $T = 0 K$ 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 $Ca {(Al H_{4})}_{2}$ , we confirm the previously predicted $Ca B_{2} F_{8}$ -type structure as the stable phase. In addition, we uncover several phases (e.g., $β − Th {Mo}_{2} O_{8}$ -type, $Ag {Au}_{2} F_{8}$ -type, and $Pb {Re}_{2} O_{8}$ -type) very competitive in energy with the ground state structure. (ii) For $Ca Al H_{5}$ , we find the stable structure type to be the recently observed $α^{'} − Sr Al F_{5}$ -type, with $U Tl F_{5}$ -type, $Sr Fe F_{5}$ -type and $Ba Ga F_{5}$ -type structures being close in energy to the ground state. (iii) In agreement with recent experiments, our calculations show that the decomposition of $Ca {(Al H_{4})}_{2}$ is divided into a weakly exothermic step $[Ca {(Al H_{4})}_{2} \to Ca Al H_{5} + Al + 3 ∕ 2 H_{2}]$ , a weakly endothermic step $[Ca Al H_{5} \to Ca H_{2} + Al + 3 ∕ 2 H_{2}]$ , and a strong endothermic step $[Ca H_{2} + 2 Al \to Ca {Al}_{2} + H_{2}]$ . (iv) Including static $T = 0 K$ energies, zero-point energies, and the dynamic contributions of $H_{2}$ gas, the DFT-calculated $Δ H$ values for the first two decomposition steps ( $- 9$ and $+ 26 kJ ∕ mol$ $H_{2}$ at the observed decomposition temperatures $T \sim 127$ and $250 ° C$ , respectively) agree well with the experimental values recently reported ( $- 7$ and $+ 32 kJ ∕ mol$ $H_{2}$ ). Only the second step $[Ca Al H_{5} ∕ Ca H_{2}]$ 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 [ $Ca H_{2} + 2 Al \to Ca {Al}_{2} + H_{2}$ , $Δ H = 72 kJ ∕ mol$ $H_{2}$ ] with the pure decomposition of $Ca H_{2}$ [ $Ca H_{2} \to Ca + H_{2}$ , $Δ H = 171 kJ ∕ mol$ $H_{2}$ ] shows that the addition of Al provides a huge destabilizing effect on $Ca H_{2}$ , due to the formation of the strongly bound $Ca {Al}_{2}$ phase.