Improving dehydrogenation properties of MgH2 by addition of dihydrogen complexes
Résumé
Although hydrogen is widely recognized as a promising energy carrier for the transportation sector, widespread adoption of hydrogen as alternative to fossil fuels depends critically on the ability to store hydrogen at adequate densities, as well as release hydrogen at sufficient rates [1]. Applicable devices are far from a valuable technology, and advanced research is still required. In this context, the presented study focused on the hydrogen storage in solid form. The ideal material for such application should reversibly store a significant amount of hydrogen under moderate conditions of pressures and temperatures. However, the most promising systems such as metal hydrides still bind hydrogen too strongly and often suffer from poor hydrogen kinetics and/or lack of reversibility.
Firstly, in this work, the knowledge gained with high-energy ball-milling [2-4] was exploited using magnesium as a hydrogen storage material. For this, systematic ball-milling operations were done using a planetary miller with different balls to MgH2 volume ratio. Microstructure, phase composition and morphology of fresh and processed MgH2 powders were fully characterized by XRD, XPS and SEM. Temperature programmed desorption (TPD) technique was used to investigate the hydrogen desorption kinetics and a PCT device allowed to build the absorption/desorption isotherms.
The results obtained show that nanocrystalline MgH2 formed by a ball-milling process leads to improved properties comparing with conventional polycrystalline MgH2. Surprisingly, the single feature of hydrogen desorption obtained immediately after ball milling, split with time into two and three desorption peaks, respectively, with the onset temperature at lower values. The characteristics conducting to such behavior will be discussed in direct correlation with the stability of the small size particles reached through nanostructuring against agglomeration, sintering and oxidation.
Secondly, as the temperature of desorption was not decreased enough by ball-milling for PEMFC applications, dihydrogen complexes of ruthenium were added to MgH2. Dihydrogen complexes in which dihydrogen is coordinated to a transition metal centre without H-H bond breaking represent an ideal class in terms of binding strength midway between metal hydrides and physisorption [5,6]. The concept that increasing the number of dihydrogen ligands for a better hydrogen release will be demonstrated in this work. The efficiency of using dihydrogen complexes of ruthenium as doping agent to MgH2 will be discussed.