Nanocomposites and nanomaterials

Hydrogen sorption/desorption kinetics in the Mg-C system
Lakhnik A.M., Kirian I.M., Rud A.D. 
G.V. Kurdyumov Institute for Metal Physics of N.A.S. of Ukraine
36 Academician Vernadsky Boulevard Kyiv 03142, Ukraine
E-mail: kiryan.inna@gmail.com
Magnesium occupies a special place among metals due to its high hydrogen capacity of 7.67%. But its use as a hydrogen storage medium is limited by several factors: high operating temperatures, slow kinetics of interaction with hydrogen and low cyclic stability. A wide range of methods has been proposed to overcome these shortcomings. Mechanical activation of magnesium with various additives occupies a special place as it allows controlling the synthesis of nanoscale systems, which are characterized by more "soft" modes of sorption/desorption of hydrogen.
Samples Mg-10 wt% C (C – graphite with different specific surface areas) were prepared by ball milling under a hydrogen atmosphere at 400 rpm for 30 h. Initial hydrogen pressure in the milling vial was ~1 MPa. Every one hour the grinding process was stopped and a new portion of hydrogen gas was added. This technique allowed the control of the hydrogen absorption process. During grinding, the mixture exhibited high activity of interaction with hydrogen. This approach allowed obtaining highly active powder mixtures, which are characterized by fast kinetics of interaction with hydrogen and high capacity.
The sorption properties of the composite Mg-10 wt% C after reactive ball milling have been tested by Sieverts technique. The sorption/desorption kinetics of hydrogen was studied both under conditions of linear heating at a constant rate and under isothermal conditions. A systematic study of the composite Mg-10 wt. % C showed that its ability to sorption/desorption hydrogen and kinetics depend on the BET surface area of graphite. The addition of graphite with a BET surface area of 8.135 m2g-1 showed the best catalytic effect on the kinetics of the interaction of H2 with Mg. After activation sample can relies 4.24 wt.% H2 within 18 min at 270 °C, 4.48 wt.% for 14 min at 280 °C, 4.66 wt.% for 9 min at 300 °C, and 5.19 wt.% for 8 min at 320 °C. The activation energy of the hydrogen desorption/sorption process was determined by the Kissinger method. Desorption activation energies Ea determined for the sample before and after activation are 165 and 78 kJ/mol, respectively. The analysis of hydrogen sorption curves obtained under isothermal conditions at a temperature of 175, 200, 225, 250, and 300 °C was performed. It was found that at a minimum temperature of 175 °C the sample absorbed 2.8 wt.% H2 for 34 min, and at a temperature of 300 °C the composite absorbed 4.56 wt.% H2 for 14 min. The sorption activation energy Ea for composite is 111 kJ/mol.
The work was carried out within the framework of the youth project of the National Academy of Sciences of Ukraine № 06/01-2022(03).