Project

Discipline

metal hydride; ionic conductor; Metal hydride; Borohydride; Hydrogen storage; Ionic conductivity; Powder diffraction; Crystal structure

Lead
L'hydrogène est reconnu comme un futur vecteur énergétique. Un défi majeur est le développement de moyens efficaces de stockage de l'hydrogène. Les borohydrures de métaux ont attiré beaucoup d'attention en raison de leur teneur en hydrogène extrêmement élevée et récemment aussi en raison de la conductivité ionique du lithium.
Lay summary
Contenu et objectifs du travail de recherche: L'utilisation des borohydrures métalliques est entravée par leur stabilité thermique soit trop élevée soit trop base. L'idée de ce projet est de mélanger des borohydrures des métaux alcalins (très stable) avec des borohydrures des métaux de transition (instable) et produire des borohydrures bimétalliques pour ajuster leur domaine de la stabilité. Nous cherchons activement des nouvelles composées et caractérisons leur structure cristalline, stabilité thermique et conductivité ionique.   Contexte scientifique et social du projet de recherche: Nous avons réussi de synthétisé plusieurs nouveaux borohydrures bi- et tri-métalliques avec la stabilité optimale et avec la conductivité ionique du lithium qui peut être optimisée par la suite en introduisant des lacunes sur les sites atomiques. Nous avons découvert des nouvelles familles des borohydrures tell que les pérovskites qui ouvrent la porte aux autres applications de ces composées tell que la luminescence et la photovoltaïque.

Direct link to Lay Summary

Last update: 27.09.2013

Active participation

Number	Title	Start	Funding scheme

The aim of the project is the development of new materials - bimetallic borohydrides - as hydrogen storage materials, and as superionic conductors. Hydrogen storage for mobile applications is still an open question. The cryogenic storage, compressed gas, chemical and physical storage are four directions intensively studied. The chemical storage, by chemical bonding in metal hydrides, seems to have the highest chance to follow the demanding application targets like for example those specified by US Dpt. of Energy: high volumetric and weight hydrogen capacities, hydrogen release temperatures, reversibility, low cost, safety. Among the materials used for hydrogen storage the compounds of light and/or transition metal M (1-4 period), boron and hydrogen, borohydrides, also called tetrahydridoborates, are very attractive due to their composition from light atoms, and high hydrogen content. However, the reversibility, and especially the hydrogen release temperatures Tdec between 60 and 120 deg C, are hard to achieve. Borohydrides are largely ionic compounds with a general formula M(BH4)n containing metallic cation and [BH4]- anion. Most of alkaline earth and alkaline metal borohydrides are too stable. On the other hand most borohydrides of 3d-metals are unstable and volatile. The tuning of the thermodynamic properties of borohydride-based hydrogen storage materials by the preparation of bimetallic (alkaline or alkaline earth and transition-metal) borohydrides has been suggested. We will follow this idea in our project, because we have already shown on the new compound LiSc(BH4)4 that it is a good direction, and we have good experience with borohydride preparation by mechanochemical synthesis - ball milling from alkaline metal borohydrides, and transition metal halides, both easy available. The method has the advantage of easy control of the synthesis product composition by milling of starting products in different stoichiometric ratios. In the project we will concentrate on a prospective work, investigation of different phase diagrams, and full crystal chemistry characterization by powder diffraction method, the field where we belong among the leaders in the methodology development (program Fox). We expect to find a material with suitable Tdec, which release only hydrogen without diborane (B2H6) evolution. To achieve that we have a possibility to tune the materials property not only by mixing the cations, but also the anions: substitution of [BH4]- by halide anions like Cl-. It has been shown that anion mixing modifies the thermal stability of borohydride phases.Besides he hydrogen storage properties we will search also for the superionic conductivity based on high mobility of Li+ cations as it was observed for HT hexagonal phase of LiBH4. It is possible to stabilize this superionic phase by anionic substitution using halides. The material with so high Li+ cations based electrical conductivity as of the order of 10-3 Scm-1 is of interest for applications in lithium batteries like solid electrolytes or anode/electrolyte interface. We expect to find the superionic properties also in other lithium containing borohydrides like LiSc(BH4)4 with the help of anionic substitution which modifies not only the stability of non-ambient phases, but also the anion/cation packing in the crystal.In this project we want to continue the research on borohydrides started in October 2009 within the second part of the SNSF project no. 200020-122123. We will also follow the methodology of ab-initio crystal structure determination from powder diffraction data as developed within our project no. 21-53847.98 (program Fox). We will also use, when needed, the expertise in the study of local order in disordered structures by the Pair Distribution Function (PDF) analysis, which we have developed within our projects no. 200021-107916 and 200020-122123.The synthesis work will be done in our laboratory by ball-milling at ambient and cryogenic temperatures. The structural analysis including temperature or pressure dependent studies will be done at powder diffraction installations at world leading synchrotron sources, such as Swiss-Norwegian Beamline (SNBL) at the ESRF in Grenoble or Swiss Light Source at the PSI Villigen. Since 2002 we have developed a very intense and successful collaboration between the University of Geneva (R. Cerný, H. Hagemann) and SNBL Grenoble (Y. Filinchuk) which is at the origin of 9 light metal borohydrides from 14 fully characterized up to now, including LiBH4 with highest hydrogen storage capacity (more than 18 wt.% of hydrogen), and Mg(BH4)2, the most complex one (55 independent atoms). We are convinced that such top rank research is important for a good image of Swiss science. The presented project is an active part of these studies, and follows the work that started eight years ago.