Role of MgCo compound on the sorption properties of the Mg–Co milled mixtures

The influence of MgCo on the reaction paths during hydriding and dehydriding processes of Mg–Co mixtures was studied using a combined HP-DSC and XRD approach. Mg–Co mixtures with different compositions were mechanically milled under argon to prepare Mg–Co nanocomposites and then submitted to thermal treatment at 300 °C for 5 days to induce MgCo formation. The local Mg–Co composition in the milled and milled-heated samples determines the nature of the phases obtained after hydriding/dehydriding cycling. The formation of Mg6Co2H11, Mg2CoH5 and MgH2 hydrides occurs after the first hydriding stage of the 2Mg–Co and Mg–Co milled mixtures due to kinetic restrictions. On the contrary, Mg–Co milled-heated mixture exhibits the selective formation of Mg2CoH5 during first hydriding via two-step reaction. In the first one, MgCo disproportion to MgH2 and Co takes place simultaneously with Mg hydriding (<200 °C). The second step involves MgCo hydriding to Mg2CoH5 through MgH2 as intermediate phase (>200 °C). Dehydriding reaction is enhanced by dispersion of Co into Mg-matrix, which reduces more than 100 °C the hydrogen desorption temperature when compared with the Mg–Co milled sample without previous heating.