リチウムアミドの融解によるリチウムー室素系水素貯蔵材料の熱力学的特性変化

摘要

錯体系水素貯蔵材料の一つであるリチウムー窒素系水素貯蔵材料は，LiHとLiNH_2の混合物から構成されており，以下に示すように水素放出によってLi_2NHを生成することで可逆的に水素を吸蔵・放出する．%In this study, we used the Sieverts' method to measure the hydrogen pressure-composition isotherms of LiH and LiNH_2 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 -32 ±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 ± 5J · K~(-1)· (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.