Thermodynamic Property Change in Li-N-H Hydrogen Storage System by Melting Lithium Amide

In this study, we used the Sieverts' method to measure the hydrogen pressure-composition isotherms of LiH and LiNH2 mixed in a molar ratio of 2 : 1 in order to obtain the standard reaction enthalpy and entropy of the hydrogenation reaction. The amounts of hydrogen absorbed and desorbed by the mixture were equal to the theoretical value at temperatures ranging from 653 K to 873 K (which were higher than the melting temperature of the mixture), where the standard reaction enthalpy and entropy were -3 +/- 3 kJ . (mol H-2)(-1) and -68 +/- 4 J . K-1.(mol H-2)(-1), respectively. On the other hand, the amounts were less than the theoretical value at 553 K and 623 K (which were lower than the melting temperature) due to the incomplete reaction, indicating that these isotherms would not afford reliable estimates of the standard reaction enthalpy and entropy. Hence, we instead calculated the standard enthalpy and entropy below the melting temperature using the values above the melting temperature and the standard enthalpy and entropy of fusion of the mixture. The values thus obtained, -49 +/- 3 kJ (mol H-2)(-1) and -95 +/- 5 J K (mol H-2)(-1), respectively, are significantly different from those previously reported. Here, the differences are discussed from the standpoint of the difficulty in measuring the hydrogen pressure-composition isotherms at lower temperatures as well as the validity of the method.