Mechanisms for the enhanced hydrogen desorption performance of the TiF4-catalyzed Na2LiAlH6 used for hydrogenstorage

Abstract

A dramatic enhancement in the dehydrogenation/hydrogenation kinetics of Na₂LiAlH₆ was achieved by adding 5 wt% TiF₄. The starting temperature for dehydrogenation of the sample with TiF₄ added was lowered by 50 °C with respect to the pristine sample. In-depth investigations revealed that the addition of TiF₄ to Na₂LiAlH₆ not only enhanced the dehydrogenation kinetics, but also altered the dehydrogenation thermodynamics. The apparent activation energy and the desorption enthalpy change of the Na₂LiAlH₆ with 5 wt% TiF₄ added were calculated to be ca. 143.6 ± 2.7 kJ mol⁻¹ and 57.3 ± 2.3 kJ mol⁻¹ of H₂ by using Kissinger's approach and the van't Hoff equation, both lower than those of the pristine sample, respectively. We firmly believe that both Ti and F play critical roles in the dehydrogenation/hydrogenation process of the Na₂LiAlH₆ sample with added TiF₄ due to the formation of Ti–Al clusters, the substitution of F⁻ for H⁻ and the presence of LiF. All these are responsible for the significant improvement of hydrogen storage performances of the Na₂LiAlH₆ sample with the addition of TiF₄. This understanding provides us with insights into the dehydrogenation kinetic mechanisms of Na₂LiAlH₆ used for hydrogen storage and directs the optimization of high-performance catalysts for the hydrogen storage reaction of the alanates.