Effect of synthesis procedures on hydrogen absorption-desorption kinetics of Mg2CoH5

I. GONZÁLEZ FERNÁNDEZ; F. C. GENNARI; G. O. MEYER

Lyon, Francia

Conferencia; WHEC 2006, 16th World Hydrogen Energy Conference; 2006

International Association for Hydrogen Energy

The study of the complex hydride Mg2CoH5 as a hydrogen storage material is justified by its high gravimetric (4.5 wt.%) and volumetric (about 100 kg H2. m-3) hydrogen densities, better absorption/desorption kinetics than pure magnesium, and low cost. However, the absence of the corresponding precursor (Mg2Co) as a stable phase limits the success of efficient production of the hydride and complicates the study of the resulting systems with hydrogen. In the present work, Mg2CoH5 was prepared using two different synthesis routes: conventional sintering (CS) [1] and reactive mechanical alloying (RMA) [2]. The evolution of the phases, their thermal stability and the microstructural modifications during the synthesis were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Hydrogen absorption and desorption kinetics as well as pressure-composition-temperature curves were measured using a Sievert-type volumetric equipment. The Mg2CoH5 was successfully prepared from magnesium and cobalt powders using both CS (450 ºC and 60 atm) [1] and RMA (room temperature and 5 atm) [2]. The yield of Mg2CoH5 was 65 wt.% and 50 wt.% for CS and RMA, respectively. During the formation of Mg2CoH5 we identified the presence of MgH2 and Mg6Co2H11 hydrides, depending on the synthesis procedure and the experimental conditions. In all cases, the decomposition of Mg2CoH5 leads to the formation of the MgCo intermetallic compound [2]. High hydrogen absorption rates were observed in both samples in the temperature range of 200 to 400ºC. On the other hand, hydrogen desorption rate at 250 ºC was slower for the sample produced by CS than that obtained from RMA. Correlation between hydrogen absorption/desorption kinetics and structural/microstructual characteristics of Mg2CoH5 with the synthesis procedure was analyzed. [1] P. Selvam, K. Yvon, Synthesis of Mg2FeH6, Mg2CoH5 y Mg2NiH4 by high-pressure sintering of the elements, Int. J. Hydrogen Energy 16 (1991) 615. [2] F. Gennari, F. Castro, Formation, composition and stability of Mg-Co compounds,  J. All. Comp. 396 (2005) 182-192.