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, Ca (Al H4) 2, and its decomposition products CaAl H5, Ca H2, and Ca Al2. Using a large database of A B2 C8 and AB C5 structure types, we perform nearly 200 DFT calculations in an effort to predict the crystal structures of the Ca (Al H4) 2 and CaAl H5 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 H4) 2, we confirm the previously predicted Ca B2 F8 -type structure as the stable phase. In addition, we uncover several phases (e.g., β-Th Mo2 O8 -type, Ag Au2 F8 -type, and Pb Re2 O8 -type) very competitive in energy with the ground state structure. (ii) For CaAl H5, we find the stable structure type to be the recently observed α′ -SrAl F5 -type, with UTl F5 -type, SrFe F5 -type and BaGa F5 -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 H4) 2 is divided into a weakly exothermic step [Ca (Al H4) 2 →CaAl H5 +Al+3/2 H2], a weakly endothermic step [CaAl H5 →Ca H2 +Al+3/2 H2], and a strong endothermic step [Ca H2 +2Al→Ca Al2 + H2]. (iv) Including static T=0 K energies, zero-point energies, and the dynamic contributions of H2 gas, the DFT-calculated ΔH values for the first two decomposition steps (-9 and +26 kJ/mol H2 at the observed decomposition temperatures T∼127 and 250°C, respectively) agree well with the experimental values recently reported (-7 and +32 kJ/mol H2). Only the second step [CaAl H5 /Ca H2] 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 H2 +2Al→Ca Al2 + H2, ΔH=72 kJ/mol H2] with the pure decomposition of Ca H2 [Ca H2 →Ca+ H2, ΔH=171 kJ/mol H2] shows that the addition of Al provides a huge destabilizing effect on Ca H2, due to the formation of the strongly bound Ca Al2 phase.