Cycling behavior of Mg-Fe-H system hydrides

A. BARUJ; J. PUZKIEL; F. C. GENNARI; P. ARNEODO LAROCHETTE

Reykjavik, Islandia

Conferencia; International Symposium on Metal-Hydrogen Systems (MH2008); 2008

MH2008 Organizing Commitee

Technological applications of hydride forming materials usually entail a large number of hydrogen absorption - desorption cycles. For this reason, a complete characterization of these materials should include a study of their properties under cycling conditions. Hydrides of the Mg-Fe-H system (MgH2 and Mg2FeH6) have been widely investigated due to their potential storage properties. However, there are few results in the literature about the cycling behavior of these materials when both hydride phases are present. Those studies were performed at considerably high temperatures (above 500 degC), and allowing for long (several hours) absorption and desorption times. In this work, samples of Mg-Fe elemental powders mixtures with three different compositions were studied, namely Mg2Fe, Mg3Fe and Mg15Fe. All of them were synthesized via reactive ball milling under hydrogen atmosphere. Additionally, Mg2Fe samples produced by means of ball milling in argon atmosphere were analyzed. The cycling process was carried out at constant temperatures of 375 degC and 400degC in a closed loop device. The time elapsed between two consecutive absorption stages was approximately 15 minutes. The evolution of both storage capacity and reaction kinetics was monitored during hydrogen absorption and desorption cycling. The samples prior and after cycling were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). During cycling, we have found that the materials display two different behaviors. Through the first cycles, which number depends on the sample characteristics and temperature, the storage capacity increases. After this initial stage, the materials slowly degrade with the increase in the number of pressure cycles. Typically, the storage capacity reduction after 1000 cycles at 400 degC is greater than that measured at 375 degC. We discuss the possible reasons for this behavior and the differences observed at these two temperatures based on the formation of the different hydride phases, their associated hydrogen sorption kinetics and their relative stability.