Solid-State Synthesis of Hydrogen Storage Mg₂Co Alloys via Bulk Mechanical Alloying

Abstract

Mg–Co system is employed to investigate the solid-state reactivity of Mg₂Co from the elemental powder mixture of magnesium and cobalt and to describe the hydrogenation and de-hydrogenation processes of Mg₂Co. Mg₂Co is successfully solid-state synthesized as a non-equilibrium phase compound by the bulk mechanical alloying. This solid-state reactivity is controlled by the repeated plastic flow of ductile elements in the powder mixture. This non-equilibrium phase, Mg₂Co is stable up to 833 K. Its hydrogenation and de-hydrogenation processes are described by DSC, PCT and XRD. These processes take place in multi-step with strong dependency on the holding temperature. Hydrogenation process from Mg₂Co is composed of two steps to Mg₂CoH₅ and dehydrogenation from Mg₂CoH₅, one step to Mg₂Co, at 633 K. Maximum hydrogen absorption capacity at 633 K is still limited to 2.03 mass% since hydrogenation and de-hydrogenation steps, working at higher temperature, are never activated without chemical modification to original Mg₂Co.