Reactive Ball Milling-Induced Improvement in Hydrogen Storage Performance of Mg-Co Alloys

Abstract

The ability of magnesium-based hydrogen storage materials to store and release hydrogen is hampered by their relatively slow rates. To address this, Mg-x% Co (x = 5, 10, 15) magnesium-rich hydrogen storage alloys were fabricated using reactive ball milling. The resulting alloys were multiphase, composed of Mg, Co, MgCo₂, and Mg₂CoH₅, which provided nucleation sites and improved hydrogen uptake. The reduced particle size and surface microcracks and pores enhanced hydrogen diffusion. The Mg-15%Co alloy had the ability to take in 4.72 wt.% hydrogen in 1 minute when the temperature was set at 350°C. Hydrogenation resulted in the formation of a biphasic structure composed of Mg₂CoH₅ and Mg₆Co₂H₁₁. This structure destabilized the Mg-H bond and improved the kinetics of hydrogen release. The hydrogen release capacity first increased and then decreased with Co content, with Mg-10%Co showing the best performance. Due to the combined action of ball milling and Co catalysis, the Mg-10%Co alloy had an activation energy for hydrogen release of 113.65 kJ/mol, making it the alloy with the lowest such activation energy in the set. Differential scanning calorimetry (DSC) analysis confirms that cobalt addition reduces the hydrogen release peak temperature of Mg by 16.58–29.67 °C, with the Mg-10% Co alloy exhibiting peak temperatures of 410.72–425.53 °C, attributed to synergistic decomposition of MgH₂ with intermediate hydrides like Mg₂CoH₅ and Mg₆Co₂H₁₁. The findings of this study indicate that alloying Mg-based alloys with transition elements while performing ball milling is a practical method for enhancing their hydrogen storage and release kinetics.