Synthesis of CsH and its effect on the hydrogen storage properties of the Mg(NH2)2-2LiH system

• CsH is synthesized by a molten ball milling technique within H2 atmosphere.
• Adding CsH significantly improves hydrogen storage properties of Mg(NH2)2-2LiH.
• The 0.08CsH-added sample reversibly stores 4.62 wt% hydrogen starting at 70 °C.
• A good cycling stability is achieved for the 0.08CsH-added sample.
• CsH first remains unchanged and then participates in the reaction upon heating.

Cesium hydride (CsH) was successfully synthesized by ball milling Cs under a hydrogen atmosphere of 50 bar at 50 °C. The effect of the prepared CsH on the hydrogen storage properties of the Mg(NH2)2-2LiH system was systematically investigated. The Mg(NH2)2-2LiH-0.08CsH composite exhibited optimal hydrogen storage properties because it reversibly stores approximately 4.62 wt% hydrogen with a dehydrogenation onset temperature of 70 °C via a two-stage reaction. At 150 °C, approximately 80% of the reversible capacity was quickly released from the Mg(NH2)2-2LiH-0.08CsH composite within 100 min. The fully dehydrogenated CsH-containing sample began to absorb hydrogen at 55 °C and took up approximately 4.58 wt% hydrogen at 130 °C. A cycling analysis indicated that the CsH-containing Mg(NH2)2-2LiH system exhibited good reversible hydrogen storage abilities. Detailed mechanistic studies revealed that during the initial heating process, CsH gradually reacted with Mg(NH2)2 to afford CsMg(NH)(NH2), and CsH acted as a catalyst to reduce the activation energy barrier of the first dehydrogenation step. As the operating temperature increased, CsMg(NH)(NH2) as a reactant participated in a second dehydrogenation step to decrease the desorption enthalpy change. This behavior reasonably explains the significantly improved hydrogen storage properties of the CsH-containing Mg(NH2)2-2LiH system.