New insights into the mechanism of activation and hydrogen absorption of (2LiNH2-MgH2)
- 作者：Weifang Luo ; wluo@sandia.gov ; Vitalie Stavila ; Leonard E. Klebanoff
- 关键词：Hydrogen storage material ; Li&ndash ; Mg&ndash ; NH2 storage system ; Reaction mechanism ; absorption/desorption rates ; Catalysis
- 刊名：International Journal of Hydrogen Energy
- 出版年：2012
- 期刊代码：42_03603199
- 类别：ch
- 出版时间：April, 2012
- 卷：37
- 期：8
- 页码：6646-6652
- 文件大小：739 K
摘要
2LiNH2-MgH2 is considered an attractive material for reversible hydrogen storage. In an attempt to improve the hydrogen storage characteristics of the 2LiNH2-MgH2 system, the activation mechanism of the material, as well as the improvement in hydrogen absorption rates through doping were explored. Differential Scanning Calorimetry (DSC) investigations reveal that the initial and irreversible conversion process is exothermic, indicating it is energetically favorable for this initial conversion of the starting material. The exothermicity of this first step explains why the original starting material (2LiNH2聽+聽MgH2) is never regenerated during re-hydrogenation of the desorbed product. Adding catalytic amounts (<4聽mol%) of potassium hydride (KH) significantly increases the hydrogen absorption rate of the desorbed material, and has a less dramatic effect on the聽kinetics of hydrogen desorption. Pressure-Composition-Temperature (PCT) studies for the KH-catalyzed material indicate a substantial hydrogen equilibrium pressure of 20聽atm at 180聽掳C. The fast absorption rate obtained via using KH catalysis allows a more accurate equilibrium measurement. The changes of enthalpy and entropy for the conversion of catalyzed (2LiH聽+聽Mg(NH2)2) to Li2Mg(NH)2 with hydrogen release were determined from the van鈥檛 Hoff plot. These values for the enthalpy and entropy of hydrogen desorption are 鈭咹聽=聽40聽kJ/mol H2, and 鈭哠聽=聽99聽J/K-mole H2, respectively. This 鈭咹 value is similar to that reported previously by Wang et聽al. for the K-catalyzed material and previous measurements for the un-catalyzed material. The similarity of the 鈭咹 values for both K-doped and un-doped material confirms KH is acting catalytically and not thermodynamically.