Hydrogen storage properties of the Li-Mg-B-N-H system, effects of starting materials stoichiometry

FAGIANO F. ; AMICA, G.; ARNEODO LAROCHETTE, P.

Buenos Aires

Congreso; WCCE11 - CIBIQ2023 - GS10-Global Symposium on Green hydrogen; 2023

Solid-state hydrogen storage is a safe and efficient method which provides improved volumetric energy densities at moderate pressures and temperatures. Complex hydrides have high gravimetric and volumetric storage capacities, but they are thermodynamically very stable. The formation of composite materials from hydrides and complex hydrides allows the release of hydrogen at a lower temperature [1]. The Mg(NH 2 ) 2 -2LiH composite possesses high reversible hydrogen storage capacity and suitable thermodynamic properties. However, for technological applications, an important barrier is the poor kinetic rate during the dehydrogenation process. This can be improved with different approaches including the addition of catalysts and the reduction of the particle size. The 2LiNH 2 :MgH 2 :LiBH 4 composite has shown an autocatalyzed reaction pathway, responsible for fast kinetics and low desorption temperatures [2]. The aim of this work is to synthesize the Mg(NH 2 ) 2 -2LiH-Li 4 (NH 2 ) 3 BH 4 composite (Li-Mg-B-N-H system) and evaluate the effects of the stoichiometry of the starting materials on the hydrogen storage properties of the system. The starting materials were LiNH 2 :MgH 2 :LiBH 4 and the molar ratios studied were 2:1:0.2, 2.6:1:0.2, 2:1.5:0.2 and 2:1:0.1. The synthesis was carried out by mechanical milling of the LiNH 2 -MgH 2 -LiBH 4 mixture for 20 h, followed by thermal treatment for 0.5 h at 200 °C under 6000 kPa of hydrogen. To evaluate the interaction of the materials with hydrogen, a Sieverts-type volumetric equipment was used, where dehydrogenation and hydrogenation cycles were performed. Dehydrogenation curves were obtained at a constant temperature of 200 °C. The rehydrogenation was performed at 200 °C with a constant hydrogen pressure of 6000 kPa. Structural properties of the as-milled and as-cycled samples were studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR). After mechanical milling and subsequent thermal treatment, all the desired products were identified in the 2:1:0.2 and 2.6:1:0.2 systems, while in the 2:1.5:0.2 system the Li 4 (NH 2 ) 3 BH 4 was not identified. In the 2.6:1:0.2 system a LiNH 2 excess was detected. Sample composition was studied by the Rietveld method. The 2.6:1:0.2, 2:1:0.2 and 2:1:0.1 systems show similar stability under cycling. The capacity of the 2:1.5:0.2 system drops sharply after the second cycle. The 2:1:0.2 and 2:1:0.1 systems have the highest hydrogen storage capacity (4.2% w/w H 2 ) and the 2:1:0.2 system presents the highest desorption rate. At 160 ºC the dehydrogenation time is 3 h. These results are interesting from a technological point of view and for potential mobile applications.