The Mg(NH2)2-2LiH composite catalyzed by lithium fast-ion conductor: enhanced hydrogen sorption properties and dehydrogenation pathway via two-controlling mechanisms

G. AMICA; F. COVA; LAROCHETTE, P. ARNEODO; F. C. GENNARI

Río de Janeiro

Conferencia; WHEC 2018 (World Hydrogen Energy Conference); 2018

International Association for Hydrogen Energy (IAHE)

Hydrogen storage remains one of the most challenging technological barriers to the advancement of hydrogen fuel cell technologies for mobile applications. Recently, among the hydrogen storage systems based on complex hydrides, the Mg(NH2)2-LiH composite has proved to be a propitious candidate due to its good reversibility, moderate operating temperatures, relatively high hydrogen storage capacity (5.5 wt% H) and suitable ∆H (~44.1 kJ mol-1 H2), determining a desorption temperature lower than 100 °C at atmospheric pressure. As Li4(NH2)3BH4 is well known as a lithium fast-ion conductor, the hydrogenation and dehydrogenation kinetics and thermodynamic properties of the Mg(NH2)2-LiH system modified by addition of Li4(NH2)3BH4 were studied(1). The effect of repetitive hydrogen sorption cycling on the kinetic and thermodynamic performance was evaluated. The dehydrogenation rate in the doped composite was twice as in the un-doped one at 200 °C, while hydrogenation was 20 times faster. A reduction in about 9% of the activation energy was attributed to the catalytic effect of Li4(NH2)3BH4. Thermodynamic studies revealed a variation in the pressure composition isotherm curves between the first dehydrogenation cycle and the subsequent. The doped composite showed a sloped plateau region at higher equilibrium pressure in regard to the flat plateau of the un-doped composite. Detailed structural investigations revealed the effective influence of Li4(NH2)3BH4 in different reactions: the irreversible dehydrogenation in the presence of MgH2 and the reversible hydrogen release when reacts with Li2Mg2(NH)3. Its beneficial role was associated with the weakening of the N-H bond and the mobile small ions mass transfer enhancing. Moreover, it was determined that a single mechanism model was not able to explain the behavior of the reaction at all reacted fractions simultaneously. Instead, the combination of two controlling mechanisms: nucleation and grow (at low conversions) and a 3D diffusion model (at high conversions) showed good accuracy(2). With pressure increase, the mechanism change occurs later during desorption due to the higher difficulty in creating nucleation points on a surface exposed to a higher concentration of hydrogen. The equations deduced allowed the prediction of hydrogen desorption rate as a function of pressure and the reacted fraction in the Li4(NH2)3BH4 doped Mg(NH2)2-LiH composite.(