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 AB₂C₈ and ABC₅ 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=00.3em0exK 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., β-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 α^'-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∕2H₂], a weakly endothermic step [CaAlH₅→CaH₂+Al+3∕2H₂], and a strong endothermic step [CaH₂+2Al→CaAl₂+H₂]. (iv) Including static T=00.3em0exK energies, zero-point energies, and the dynamic contributions of H₂ gas, the DFT-calculated ΔH values for the first two decomposition steps (-9 and +260.3em0exkJ∕mol H₂ at the observed decomposition temperatures T~127 and 2500.2em0ex^∘C, respectively) agree well with the experimental values recently reported (-7 and +320.3em0exkJ∕mol H₂). 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+H₂, ΔH=720.3em0exkJ∕mol H₂] with the pure decomposition of CaH₂ [CaH₂→Ca+H₂, ΔH=1710.3em0exkJ∕mol H₂] shows that the addition of Al provides a huge destabilizing effect on CaH₂, due to the formation of the strongly bound CaAl₂ phase.