ChemMatCARS Nuggets - Chemistry

Hydrogenation of LaNiM₅H_x - A Model System for Safe Hydrogen Storage

If we are ever to shift over to a hydrogen-based fuel economy, we will need safe methods for storing hydrogen. Using solid powders with an enormous capacity for chemically trapping hydrogen and then releasing it later in a controlled manner is an attractive option.

The basic structure of (lanthanum nickel) LaNi₅ has been well characterized – especially as part of the broader study of nickel-metal hydride batteries, and its capacity to incorporate hydrogen into its structure has been known for a long time. While this material is not a serious contender for ‘real-life’ hydrogen storage systems there is a lot we can learn from it about the basic process of hydrogen uptake that could translate to economically viable materials.

Atomic structure of α (pure) LaNi₅ – when fully loaded with hydrogen the unit cell swells to form the β (hydrogenated LaNi₅H₆) phase with one hydrogen atom for every metal atom.

By using wide-angle x-ray scattering (WAXS) with synchrotron radiation, accurate maps of the alpha and beta phases and hence hydrogen uptake in small, well-characterized samples at accurately-known temperatures and pressures.

Without the small size and high-flux of the synchrotron beam it is almost impossible to resolve small sub-regions of a partially hydrogenated sample – this has in the past led to difficulties in resolving true intermediate phases from non-equilibrium mixtures of the un-hydrogenated (alpha) and hydrogentated (beta) phases. Thanks to this ability we have now found positive proof of an intermediate or ‘gamma’ phase – the existence of which has been the source of continuing debate.

Left: α- phase map (un-hydrogenated); Right: β- phase map (hydrogenated). Mapping the position of the (101) powder line against the position of the beam relative to the sample cell – we can see the distribution of partially hydrogenated material. Note that the H2 enters at bottom right.

« Back to top