Hydrogen absorption and desorption properties of Mg/MgH2 with nanometric dispersion of small amounts of Nb(V) ethoxide

A study to determine the optimal content of Nb(V) ethoxide required to efficiently catalyze the H₂ sorption kinetics in the Mg/MgH₂ system is reported. The materials were synthesized by hand mixing different amounts of additive (from 0.10 to 1 mol%) to pre-milled MgH₂. Considering kinetics and capacity the best performance corresponds to a 0.25 mol% of Nb ethoxide concentration. With this material, a remarkable kinetic behavior with excellent reversibility is obtained: 5.3 wt% and 5.1 wt% of hydrogen are absorbed and desorbed respectively at 300 °C in 3 min. At 250 °C the material absorbs 5.2 wt% of hydrogen and releases 3.7 wt% in 10 min. Thermal desorption starts at 247 °C and peaks at 268 °C. The H₂ sorption properties of all the materials remain unchanged after 10 cycles of absorption and desorption at 300 °C, and the best material reversibly takes in and releases 5.3 wt% of H₂ during a 10 min combined cycle. The kinetic improvement of the hydrogen desorption and absorption properties is attributed to an enhancement of the kinetic processes that occur on the surface of the material, due to the excellent spreading of the liquid additive at nanometric level, as revealed by SEM/EDS and TEM/EELS.