Hydrogen Storage Properties and Phase Variation Studies in the Destabilized CaH2+Si System

 

Hui Wu, Terrence J. Udovic, and John J. Rush

 

Many light-metal hydrides are known to have relatively high hydrogen-storage capacities (>5wt%), but most of them are too stable for practical applications and have sluggish sorption kinetics. Recently, more attention has been given to this class of hydrides in terms of (i) improving the kinetics through adding catalysts or preparing nanocrystalline hydrides using extended ball milling, and (ii) modifying the hydrogen-cycling thermodynamics using additives that could destabilize the metal hydrides by forming alloys or compounds in the hydrogenated and/or dehydrogenated states.  Here we present a detailed study of the hydrogen-storage properties of ball-milled mixtures of CaH2+Si powders using neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and neutron prompt-gamma activation analysis (PGAA). Alloying with Si is shown to destabilize the strongly bound CaH2. A single-phase CaSi alloy forms upon initial dehydrogenation at 630°C which was confirmed by Rietveld refinement and is in good agreement with the published structure. Subsequent hydrogenation of this alloy leads to various new CaSiHx ternary phases with different crystal structures and characteristic peaks in the vibrational spectra. The maximum hydrogen content was 1.9 wt% under a hydrogen pressure of 90 bar at 200°C with a stoichiometry of CaSiH1.2. The crystal structure of CaSiH1.2 was determined from Rietveld refinement of NPD data on the corresponding deuteride combined with vibrational spectra, showing a Pnma unit cell with two distinct H sites. A CaSiH0.14 composition was obtained after evacuating the hydrides at 200°C below 10-6 torr. Compared to pure CaSi (space group Cmcm), the lattice of CaSiH(D)0.15 was observed to undergo a small triclinic distortion. Combination of the NPD structure refinement and NVS data reveals a P1 structure with a single H(D) site in this ternary phase. In addition, we found that the CaSiH0.3 composition also shows a new orthorhombic crystal structure and a distinct vibrational spectrum which are different from CaSiH1.2 and CaSiH0.15. All these results show that CaH2 can be destabilized through alloy formation upon dehydration, and suggest the potential applications of CaSi for hydrogen storage. The experimental data including ternary phase structures, H sites, and phonon vibration spectra reveal the complexity of this metal hydride system and are essential to fully understand the hydrogen absorption properties of calcium silicide, and for better hydrogen storage applications in other analogue metal hydride systems.

 

 

Author Information

            Name:                           Hui Wu

            Mentor’s name:            Terrence J. Udovic

            Division:                       NIST Center for Neutron Research

            Laboratory:                   Materials Science and Engineering Laboratory

            Building:                       235

            Room:                           E118

            Mail Stop:                     8562

            Telephone #:                 301-975-2387

            Fax #:                            301-921-9847

            Email:                            huiwu@nist.gov

            Sigma Xi:                      not a member

            Category:                       Materials Science