Hydrogen absorption-desorption properties and hydrolysis performance of MgH2-Zr3V3O0.6Hx and MgH2-Zr3V3O0.6Hx-C composites

In this work, the catalytic effect of additions of η-Zr3V3O0.6 mixed suboxide and the title intermetallic composite with graphite on the properties of MgH2 in the processes of hydrogen storage and hydrogen generation has been studied. The hydride composites were obtained by high-energy reactive ball milling (RBM). The samples were characterized by X-ray diffraction and scanning electron microscopy. The cycling performance of hydrogen desorption and absorption as well as hydrogen generation in the hydrolysis reaction were studied. We found that during the reactive ball milling the studied composites are able in a few minutes to absorb ∼6.5 wt% of hydrogen. The addition of Zr3V3O0.6 and Zr3V3O0.6 + С catalysts not only increases the rates of hydrogen absorption and release, but also lowers the activation energy and the temperature of hydrogen desorption. For a composite containing 10 wt% of suboxide and 3 wt% of graphite, the activation energy of hydrogen desorption determined by the Kissinger method was very low, just 58 kJ/mol, and this value is among the lowest described in the reference publications. We also show that the intermetallic additive forms the Zr3V3O0.6H∼10 hydride, which results in a higher gravimetric capacity of the composite as H storage material. The improved kinetics of hydrogen exchange and increased hydrogenation capacity at modest operating temperatures of 150–200 °C appear to be characteristic for the composites containing suboxide and graphite additions. The reason for the advanced performance is in the different the morphology of the synthesized samples, particularly the graphite-containing composites showing a more developed dispergation of the material (the fraction with a particle size of 1–3 μm is >80 wt%). We furthermore show that the synthesized materials can be used in the hydrolysis process resulting in hydrogen generation. The amount of hydrogen released from the hydride Mg-Zr3V3O0.6-C composite in the hydrolysis reaction reaches 1364 ml/g (the conversion degree is ∼85 % for the process duration of 120 min) when using 0.04 M MgCl2 solutions.