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Hydrogen dynamics in complex hydrides

Applicant Remhof Arndt
Number 119972
Funding scheme Project funding (Div. I-III)
Research institution Mobilität, Energie und Umwelt Empa
Institution of higher education Swiss Federal Laboratories for Materials Science and Technology - EMPA
Main discipline Condensed Matter Physics
Start/End 01.04.2008 - 31.03.2011
Approved amount 194'102.00
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Keywords (10)

complex hydrides; structure; dynamics; diffusion; Hydrogen Storage; Solid State Diffusion; Hydrogen Mobility; Neutron Spectroscopy; Neutron Diffraction; Nuclear Magnetic Resonance

Lay Summary (English)

Lead
Lay summary
A future hydrogen economy requires dense, safe, efficient and reversible hydrogen storage materials. Hydrogen as an energy carrier is difficult to store because hydrogen is a gas at ambient conditions. Hydrogen can be stored by different methods and phenomena: high pressure gas cylinders (up to 800 bar), liquid hydrogen in cryogenic tanks (at 21 K), ad-sorbed hydrogen on materials with a large specific surface area (at T< 100 K), absorbed on interstitial sites in a host metal (at ambient pressure and temperature), chemically bond in covalent and ionic compounds (at ambient pressure), or through oxidation of reactive metals e.g. Li, Na, Mg, Al, Zn with water.
Of the different storage methods, metal hydrides and complex hydrides exhibit the largest volumetric hydrogen density up to 150 kg m-3, which corresponds to more than twice the hydrogen density of liquid hydrogen. The gravimetric hy-drogen density in transition metal hydrides is limited by the mass of the respective metals. Light-weight p-element complex hydrides such as. M[AlH4]x and M[BH4]x exhibit a gravimetric hydrogen density of up to 20 mass%. Although many of these compounds are well known for their chemical behaviour, very little is known about their physical proper-ties, e.g. structure, thermodynamic stability, electronic structure and hydrogen diffusion.
In this project we propose to investigate the hydrogen mobility and the hydrogen dynamics of a series of p-element complex hydrides and their influence on the stability and thermodynamic properties of the respective hydrides. In order to achieve the results we will combine neutron diffraction as well as inelastic and quasielastic neutron spectroscopy measurements at SINQ (Paul Scherrer Institut, Villigen, AG), at BENSC (Berlin, Germany) and at ISIS (Didcot, United Kingdom) with nuclear magnetic resonance measurements. The element specific dynamics of the constituent elements will be investigated in collaboration with the ETH Zürich. Complementary X-ray diffraction measurements will be car-ried out at our own laboratory source as well as at SLS (Paul Scherrer Institut, Villigen, AG) and ESRF (Grenoble, France). We will combine experimental investigations with theoretical modelling in order to understand the local envi-ronment of the hydrogen in the lattice (type of bonding of hydrogen) and to develop a model to describe the stability of the hydrides. The main results will be: 1) The structure and the electronic charge density distribution of the p-element complex hydrides, 2) the stability (Van’t Hoff plot) of the complexes, 3) the hydrogen mobility (self-diffusion) and permeation 4) the hydrogen sorption mechanisms in complex hydrides.
Direct link to Lay Summary Last update: 21.02.2013

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Number Title Start Funding scheme
134442 Hydrogen dynamics in complex hydrides 01.04.2011 Project funding (Div. I-III)

Abstract

A future hydrogen economy requires dense, safe, efficient and reversible hydrogen storage materials. Hydrogen as an energy carrier is difficult to store because of the low critical temperature of 33K, i.e. hydrogen is a gas at room temperature. For mobile and in many cases also for stationary applications the volumetric and gravimetric density of hydrogen in a storage material is crucial. Hydrogen can be stored by six different methods and phenomena: high pressure gas cylinders (up to 800 bar), liquid hydrogen in cryogenic tanks (at 21 K), adsorbed hydrogen on materials with a large specific surface area (at T< 100 K), absorbed on interstitial sites in a host metal (at ambient pressure and temperature), chemically bond in covalent and ionic compounds (at ambient pressure), or through oxidation of reactive metals e.g. Li, Na, Mg, Al, Zn with water. Metal hydrides and complex hydrides exhibit a large volumetric hydrogen density up to 150 kg m-3, which corresponds to more than twice the hydrogen density of liquid hydrogen. The gravimetric hydrogen density in transition metal hydrides is limited by the mass of the respective metals. Light-weight p-element complex hydrides such as. M[AlH4]x and M [BH4]x exhibit a gravimetric hydrogen density of up to 20 mass%. Although many of these compounds are well known for their chemical behaviour, very little is known about their physical properties, e.g. structure, thermodynamic stability, electronic structure and hydrogen diffusion.In this project we propose to investigate the structure and the hydrogen mobility of a series of p-element complex hydrides (e.g. LiBH4, NaBH4, KBH4, Mg(BH4)2, Ca(BH4)2 and Ti(BH4)3 and their influence on the stability and thermodynamic properties of the respective hydrides. In order to achieve the results we will perform neutron diffraction as well as inelastic and quasielastic neutron measurements at SINQ (accepted proposal) and at BENSC (beamtime for test measurements allocated). Complementary X-ray diffraction measurements will be carried out at our own laboratory source as well as at SLS and ESRF (submitted proposals). The thermodynamical properties as well as the hydrogen adsorption/desorption will be investigated by differential-scanning calorimetry and thermal desorption spectroscopy, both under applied hydrogen atmosphere. We will combine experimental investigations with theoretical modelling in order to understand the local environment of the hydrogen in the lattice (type of bonding of hydrogen) and to develop a model to describe the stability of the hydrides. The main results will be: 1) The structure and the electronic charge density distribution of the p-element complex hydrides, 2) the stability (Van’t Hoff plot) of the complexes, 3) the hydrogen mobility (self-diffusion) and permeation 4) the hydrogen sorption mechanisms in complex hydrides. The p-element complex hydrides represent a new and very promising class of hydrides with a gravimetric hydrogen density one order of magnitude larger than what we know from metallic hydrides. The knowledge of the properties of the complex hydrides and the understanding of the hydrogen sorption mechanisms will greatly improve the scientific basis of hydrogen storage in solids.
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