Complex hydrides as thermal energy storage materials: characterisation and thermal decomposition of Na2Mg2NiH6

Complex transition metal hydrides have been identified as being materials for multi-functional applications holding potential as thermal energy storage materials, hydrogen storage materials and optical sensors. Na2Mg2NiH6 (2Na(+) center dot 2Mg(2+) center dot 2H- center dot [NiH4](4-)) is one such material. In this study, the decomposition pathway and thermodynamics have been explored for the first time, revealing that at 225 degrees C, hydrogen desorption commences with two major decomposition steps, with maximum H-2 desorption rates at 278 and 350 degrees C as measured by differential scanning calorimetry. The first step of decomposition results in the formation of Mg2NiHx (x < 0.3) and NaH, before these compounds decompose into Mg2Ni and Na, respectively. PCI analysis of Na2Mg2NiH6 has determined the thermodynamics of decomposition for the first step to have a Delta H-des and Delta S-des of 83 kJ mol(-1) H-2 and 140 J K-1 mol(-1) H-2, respectively. Hydrogen cycling of the first step has been achieved for 10 cycles without any significant reduction in hydrogen capacity, with complete hydrogen desorption within 20 min at 395 degrees C. Despite the relatively high cost of Ni, the ability to effectively store hydrogen reversibly at operational temperatures of 318-568 degrees C should allow this material to be considered as a thermal energy storage material.