The catalytic reactions in the Cu–Li–Mg–H high capacity hydrogen storage system

Abstract

A family of hydrides, including the high capacity MgH₂ and LiH, is reported. The disadvantages these hydrides normally display (high absorption/desorption temperatures and poor kinetics) are mitigated by Cu-hydride catalysis. This paper reports on the synthesis of novel CuLi_0.08Mg_1.42H₄ and CuLi_0.08Mg_1.92H₅ hydrides, which are structurally and thermodynamically characterized for the first time. The CuLi_0.08Mg_1.42H₄ hydride structure in nanotubes is able to hold molecular H₂, increasing the gravimetric and volumetric capacity of this compound. The catalytic effect these compounds show on hydride formation and decomposition of CuMg₂ and Cu₂Mg/MgH₂, Li and LiH, Mg and MgH₂ is analyzed. The Gibbs energy, decomposition temperature, and gravimetric capacity of the reactions occurring within the Cu–Li–Mg–H system are presented for the first time. First principles and phonon calculations are compared with experiments, including neutron spectroscopy. It is demonstrated that the most advantageous sample contains CuLi_0.08Mg_1.92 and (Li) ∼ Li₂Mg₃; it desorbs/absorbs hydrogen according to the reaction, 2CuLi_0.08Mg_1.42H₄ + 2Li + 4MgH₂ ↔ 2CuLi_0.08Mg_1.92 + Li₂Mg₃ + 8H₂ at 114 °C (5.0 wt%) – 1 atm, falling within the proton exchange membrane fuel cell applications window. Finally the reaction 2CuLi_0.08Mg_1.42H₄ + MgH₂ ↔ 2CuLi_0.08Mg_1.92 + 5H₂ at 15 °C (4.4 wt%) – 1 atm is found to be the main reaction of the samples containing CuLi_0.08Mg_1.92 that were analyzed in this study.