Microstructure characteristics and hydrolysis mechanism of Mg–Ca alloy hydrides for hydrogen generation

• After hydrogenation, Mg–Ca alloys change into mixtures of MgH2 and Ca4Mg3H14.
• The hydrolysis of Ca4Mg3H14 and solubility of Ca(OH)2 improve the hydrogen release.
• Ca content is responsible for the specific surface areas of Mg–Ca alloy hydrides.
• The increment of Ca content lowers the activation energies of Mg–Ca alloy hydrides.

The microstructure characteristics and hydrolysis mechanism of Mg–Ca alloy hydrides were investigated in this paper. It was found that x wt.% Ca–Mg alloys (x = 10, 20 and 30) hydrogenated after ball-milling are mainly composed of MgH2 and Ca4Mg3H14 phases, and they show much better hydrolysis properties than pure MgH2 at the temperature range of 25–70 °C. The superior performances of Mg–Ca alloy hydrides may be attributed to an easy hydrolysis nature of Ca4Mg3H14 and a high solubility of the by-product Ca(OH)2, which provides gates to make water penetrate deeply inside the particles. Moreover, increasing the content of Ca enhances the hydrolysis performances of Mg–Ca alloy hydrides further due to the formation of more Ca4Mg3H14 and larger specific surface areas as well. The best comprehensive hydrolysis performances are obtained by 30 wt.% Ca–Mg alloy hydride, which releases 1419.8 mL g−1 hydrogen within 1 h at 70 °C and its conversion yield is about 95%. The hydrogen generation kinetics of Mg–Ca alloy hydrides was also investigated and it showed that the activation energies decrease with the increase of the Ca content, which is in accordance with their hydrolysis properties.