Observation of hydrogen absorption/desorption reaction processes in Li–Mg–N–H system by in-situ X-ray diffractmetry

Abstract

The in-situ XRD measurements on dehydrogenation/rehydrogenation of the Li-Mg-N-H system were performed in this work. The ballmilled mixture of 8LiH and 3Mg(NH 2 ) 2 as a hydrogenated phase gradually changed into Li 2 NH as a dehydrogenated phase during heat-treatment at 200 • C in vacuum for 50 h. Neither Mg-related phases nor other intermediate phases were recognized in the dehydrogenated phase. With respect to the hydrogenation process, the dehydrogenated state gradually returned to the mixed phase of the LiH and Mg(NH 2 ) 2 without appearance of any intermediate phases during heat treatment at 200 • C under 5 MPa H 2 for 37 h and during slow cooling down to room temperature through 24 h. In the hydrogenation process at 200 • C under 1 MPa H 2 , however, the growing up of the LiNH 2 and LiH phase was observed in the XRD profiles before the 3Mg(NH 2 ) 2 and 8LiH phases were formed as the final hydrogenated state. This indicates that the LiNH 2 and LiH phase essentially appears as an intermediate state in the Li-Mg-N-H system composed of 3Mg(NH 2 ) 2 and 8LiH.

Electronic structure, chemical bond and thermal stability of hydrogen absorber Li2MgN2H2

“…Therefore, the reaction enthalpies for eq. (1) are 60.3 kJ/mol, and this agrees well with the experimental result of 44.1 kJ/mol [20] . All the results were calculated at zero absolute temperature and the Zero-point energy corrections were not taken into account any of the formation enthalpies in this study.…”

“…that the temperature to desorb hydrogen at 1.0 bar equilibrium pressure is 90℃ [20] , which is close to the practical usage temperature of the proton exchange membrane fuel cell. Another 3Mg(NH 2 ) 2 +8LiH system has higher temperature for hydrogen desorption in order to obtain the products of magnesium nitride [14] , but this system still focuses on its higher H content and could be applied under the condition of higher temperature.…”

“…Among the modified hydrogen systems, Li-Mg-N-H system shows great properties: the temperature of hydrogen reaction decreases from 300℃ to lower than 200℃ [11][12][13][14] , and the kinetics of hydrogen reaction is also improved [15,16] . Moreover, the result of van't Hoff plot of Mg(NH 2 ) 2 +2LiH [17] or 2LiNH 2 +MgH 2 [18,19] indicates that the temperature to desorb hydrogen at 1.0 bar equilibrium pressure is 90℃ [20] , which is close to the practical usage temperature of the proton exchange membrane fuel cell. Another 3Mg(NH 2 ) 2 +8LiH system has higher temperature for hydrogen desorption in order to obtain the products of magnesium nitride [14] , but this system still focuses on its higher H content and could be applied under the condition of higher temperature.…”

Dehydrogenation reaction of Li–Mg–N–H systems studied by in situ synchrotron powder X-ray diffraction and powder neutron diffraction

“…Both reactions probably contain more than two steps with formation of intermediate phases. From in situ PXD data Nakagawa et al reported that dehydrogenation reaction of 3Mg(NH 2 ) 2 with 8LiH under vacuum formed Li 2 NH around 200 • C, but no phase containing Mg was observed in the diffraction pattern [16]. Leng et al reported that the same reaction had 4Li 2 NH + Mg 3 N 2 as the end product above 400 • C [10,14].…”

Crystal structure change in the dehydrogenation process of the Li–Mg–N–H system

“…The numerous researches have been reported as to the reversible reaction mechanism of the Li-Mg-N-H system [12][13][14][15][16]. Especially, as for the intermediate phases during the reaction process, in situ measurement of structural change [17][18][19][20][21], theoretical calculation [22,23] and thermal stability [24] have been advanced. However, the reversible reaction mechanism under hydrogen pressure has not been sufficiently clarified, yet.…”