Kinetic improvement of H2 absorption and desorption properties in Mg/MgH2 by using niobium ethoxide as additive

Abstract

The hydrogen absorption and desorption properties of a MgH2 – 1 mol.% Nb(V)ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH2. Nb ethoxide reacts with MgH2 during heating, releasing C2H6 and H2, and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 °C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH2 without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H2 to 94 kJ/mol H2, and from 176 kJ/mol H2 to 75 kJ/mol H2, respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 °C. The kinetic improvement is attributed to the fine distribution of amorphous/nanometric NbHx achieved by the dispersion of the liquid additive.