Hydrogen Storage in Magnesium and Zinc Hydride: the Molecular Approach

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Efficient, safe and cheap storage of hydrogen is not only relevant for mobile applications but also for industry. Globally, research concentrates on several hydrogen storage materials among which MgH2 which reversibly can release and up-take H2. Although the H2 desorption temperature of 300 °C is high, small particles (< 1nm) have been calculated to release H2 at more practical temperatures (< 200 °C). We approach the problem at the molecular level and aim towards the synthesis of magnesium hydride clusters that are large from a molecular point of view but small for the materials chemist. These clusters function as model systems for bulk material and can be accurately studied at the atomic level by NMR or crystal structure determination. Our aim is to prepare a large variety of differently sized magnesium hydride clusters and confirm first indications for a relationship between cluster size and the H2-desorption temperature. At the same time we are interested in isolation of well-defined molecular decomposition products: these are likely multi-nuclear (> 2) low-valent Mg(I) complexes. In addition, independent synthetic approaches will be followed to synthesize larger Mg(I) clusters. Such molecular complexes allow for a detailed study of the hydrogen up-take process. Part of our investigations will be supported by our cooperation partners: Petra de Jongh (University of Utrecht) for Sievert and DSC measurements and Peter Sirsch (Universität Tübingen) for ab initio calculations. The close relationship between Mg and Zn chemistry, will steer part of the project towards syntheses of comparable zinc hydride clusters, their decomposition and the isolation of multi-nuclear (> 2) Zn(I) complexes. The Mg/Zn ratio can be an important factor for hydrogen storage properties and therefore also Zn-doped magnesium hydride clusters are our target. In addition, doping with first-row transition metals like Sc, Ti or Ni will be investigated. Although the larger part of our project concentrates on model systems and molecular understanding, part of our activities will deal with the bottom-up synthesis of more practical storage materials like MgH2 / ZnH2 mixtures, either in bulk or in the enclosed spaces of carbon nanofibers. It is our opinion that a combination of molecular research and solid state research can lead to detailed insights of the H2 elimination and up-take process and could be the key to development of new metal hydride materials for a convenient, safe and cheap storage of hydrogen.