Achieving both high hydrogen capacity and low decomposition temperature of the metastable AlH3 by proper ball milling with TiB2

Abstract

AlH3 is a metastable hydride with a high hydrogen density of 10.1 wt% and it can release hydrogen at a low temperature of 150–200 °C. Many additives (e.g., NbF5, TiF3, etc.) introduced by ball milling can significantly reduce the decomposition temperature of AlH3, but often simultaneously decrease the available hydrogen capacity. In this work, TiB2 was introduced by ball milling to improve the decomposition performance of AlH3. AlH3 + x wt% TiB2 (x = 2.5, 5, 7.5, 10) composites were prepared by ball milling, and the milling conditions were optimized. It was shown that the decomposition performance of the AlH3 + 2.5 wt% TiB2 ball milled at 225 rpm for 108 min is the best. The onset decomposition temperature is 78 °C, which is 60 °C lower than that of pure AlH3. The decomposition is terminated at 130 °C with 8.5 wt% of hydrogen is obtained. In addition, 5.3 wt% of hydrogen can be released within 200 min at constantly 80 °C. Under the same conditions, ball-milled AlH3 can hardly release any hydrogen. The activation energy calculated by the Kissinger's method is 86 kJ mol−1, which was 28 kJ mol−1 lower than that of ball-milled AlH3. Catalytic mechanism study reveals that the Al2O3 layers on the surface of AlH3 will interact with TiB2 to form Al–Ti–B solid solution, resulting in lattice distortion. Through lattice activation, the decomposition kinetics of AlH3 is improved. This work provides an efficient strategy to achieve both high hydrogen capacity and low decomposition temperature of metastable AlH3 by proper ball milling with metal borides.