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⁺·2Mg²⁺·2H^-·[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 °C, hydrogen desorption commences with two major decomposition steps, with maximum H₂ desorption rates at 278 and 350 °C as measured by differential scanning calorimetry. The first step of decomposition results in the formation of Mg₂NiH_x (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 ΔH_des and ΔS_des of 83 kJ mol^-1 H₂ and 140 J K^-1 mol^-1 H₂, 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 °C. Despite the relatively high cost of Ni, the ability to effectively store hydrogen reversibly at operational temperatures of 318-568 °C should allow this material to be considered as a thermal energy storage material.