Mg₂(Co_1/3Fe_1/3Ni_1/3) Processed by Ball-Milling/Annealing and High-Pressure Torsion for Hydrogen Storage, a Hydriding/Dehydriding Cycling Stability Testing

A mandatory prerequisite for a good hydrogen storage material is long-term stability in hydriding/dehydriding reactions, in a suitable temperature interval (250–350 °C for magnesium intermetallics). A 50-cycle hydriding/dehydriding stability test of two Mg₂(Co_1/3Fe_1/3Ni_1/3) materials is presented. Mg₂(Co_1/3Fe_1/3Ni_1/3) was processed progressively by ball milling and annealing, followed by high-pressure torsion. A comparison of the effects of the processing on the cycling test is presented. X-ray diffraction, scanning and transmission electron microscopy, and infrared characterization indicate the morphological and structural changes in the materials after production and cycling. The highest hydrogen storage was 3.55 wt.% and 3.25 wt.% for the ball-milled and annealed Mg₂(Co_1/3Fe_1/3Ni_1/3) and high-pressure torsion processed Mg₂(Co_1/3Fe_1/3Ni_1/3), respectively, at 15 bar and 300 °C. After 50 cycles of hydriding/dehydriding reactions, the hydriding onset temperature is 69 °C and 50 °C for the ball-milled and annealed Mg₂(Co_1/3Fe_1/3Ni_1/3) and high-pressure torsion processed Mg₂(Co_1/3Fe_1/3Ni_1/3), respectively. Meanwhile, the dehydriding onset temperatures are 257 °C and 223 °C, with hydrogen storage losses of 16% and 7.4% for the ball-milled and annealed Mg₂(Co_1/3Fe_1/3Ni_1/3) and the high-pressure torsion processed Mg₂(Co_1/3Fe_1/3Ni_1/3), respectively. Overall, the ball-milled and annealed Mg₂(Co_1/3Fe_1/3Ni_1/3) material presented better performance.