Effective synthesis of Mg2CoH5 by reactive mechanical milling and its hydrogen sorption behavior after cycling

Mg2CoH5 was synthesized by reactive mechanical milling (RMM) under hydrogen atmosphere (0.5 MPa) from 2MgH2–Co and 3MgH2–Co mixtures, with a yield >80%. The microstructure, structure and thermal behavior of the phases formed during the processing were investigated by transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Kinetic properties of the reaction with hydrogen of the 2MgH2–Co and 3MgH2–Co mixtures after RMM were evaluated using modified Sieverts-type equipment. The 3MgH2–Co mixture showed better properties for storage applications, with its highest rate of hydrogen absorption and desorption at 300 °C, its storage capacity of about 3.7 wt% in less than 100 s, and good stability after cycling. Although the starting material presents Mg2CoH5 as majority phase, the cycling leads to disproportion between Mg and Co. We obtained a mixture of Mg2CoH5, Mg6Co2H11 and MgH2 hydrides, as well as other phases such as Co and/or Mg, depending on experimental conditions.