Kinetics restrictions on the formation of complex Mg-Co hydrides: a high-pressure differential scanning calorimetric study

M. G. VERÓN; G. O. MEYER; F. C. GENNARI

Reykjavik, Iceland

Simposio; International Symposium on Metal-Hydrogen Systems-Fundamentals and Applications; 2008

Metal-Hydrogen Systems Association

Ternary hydrides Mg2CoH5 and Mg6Co2H11 are attractive for hydrogen storage applications because of their high volumetric and gravimetric hydrogen densities (4.5 and 4.0 wt%, respectively). Besides, the Mg-Co-H system is attractive per se due to the formation of different hydrides phases including MgH2, the occurrence of a temperature induced tetragonal-cubic transformation of Mg2CoH5 and the absence of stable precursors in the rich-Mg composition zone of the Mg-Co phase diagram [1]. In the present work, a detailed analysis of the formation mechanism of Mg2CoH5 and Mg6Co2H11 at 2000 kPa of hydrogen pressure is presented based on high-pressure differential scanning calorimetry (HP-DSC) in combination with ex-situ X-ray diffraction (XRD) under controlled atmosphere. Ball milled Mg-Co mixtures with two different nominal compositions (1:1 and 2:1) were heated under non-isothermal conditions up to 300 and 400 °C. Based on the thermodynamic information available on the Mg-Co-H systems, the selected experimental conditions allow the formation of both complex Mg-Co hydrides [2,3]. However, depending on the temperature and the composition of the starting Mg-Co mixture, the selective formation of different hydrides is observed. Mg2CoH5 formation is favoured during heating up to 400 °C under hydrogen of the Mg-Co mixture after performing absorption/desorption cycles. Moreover, complex Mg6Co2H11 hydride formation is improved after absorption/desorption cycling at 300 °C. The role of the MgCo intermetallic in the formation mechanism of Mg2CoH5 is analysed in terms of the experimental constrains.