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(AlH4)(2), and its decomposition products CaAlH5, CaH2, and CaAl2. Using a large database of AB(2)C(8) and ABC(5) structure types, we perform nearly 200 DFT calculations in an effort to predict the crystal structures of the Ca(AlH4)(2) and CaAlH5 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(AlH4)(2), we confirm the previously predicted CaB2F8-type structure as the stable phase. In addition, we uncover several phases (e.g., beta-ThMo2O8-type, AgAu2F8-type, and PbRe2O8-type) very competitive in energy with the ground state structure. (ii) For CaAlH5, we find the stable structure type to be the recently observed alpha(')-SrAlF5-type, with UTlF5-type, SrFeF5-type and BaGaF5-type structures being close in energy to the ground state. (iii) In agreement with recent experiments, our calculations show that the decomposition of Ca(AlH4)(2) is divided into a weakly exothermic step [Ca(AlH4)(2)-> CaAlH5+Al+3/2H(2)], a weakly endothermic step [CaAlH5 -> CaH2+Al+3/2H(2)], and a strong endothermic step [CaH2+2Al -> CaAl2+H-2]. (iv) Including static T=0 K energies, zero-point energies, and the dynamic contributions of H-2 gas, the DFT-calculated Delta H values for the first two decomposition steps (-9 and +26 kJ/mol H-2 at the observed decomposition temperatures T similar to 127 and 250 degrees C, respectively) agree well with the experimental values recently reported (-7 and +32 kJ/mol H-2). Only the second step [CaAlH5/CaH2] 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 [CaH2+2Al -> CaAl2+H-2, Delta H=72 kJ/mol H-2] with the pure decomposition of CaH2 [CaH2 -> Ca+H-2, Delta H=171 kJ/mol H-2] shows that the addition of Al provides a huge destabilizing effect on CaH2, due to the formation of the strongly bound CaAl2 phase.