By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter
Research into hydride materials for energy applications typically focuses on maximizing gravimetric storage density and ion transport. However, the requirements for stationary applications such as fuel cells can be significantly different and amenable to a broader class of potential materials. In work recently published online in Nature Materials, Maarten Verbraeken of John Irvine’s research group at the University of Saint Andrews, and coworkers, characterize the transport properties and phase behavior of barium hydride at high temperature, and report very high hydride conductivity.
The authors look at the sequence of alkaline earth hydrides and report a first-order, reversible phase transition in BaH2 above 500 oC. Neutron diffraction studies reveal that hydrogen occupancy splits from a higher-symmetry 2d site to a lower-symmetry 4f site upon going from the low-temperature to high-temperature phase. This results in a doubling of charge carriers in the (002) plane (though not an anticipated decrease in the activation energy for hydride transport due to not-positively-identified competing effects). The net result is a material with charge-carrying properties defined purely by ionic conduction and at a level an order of magnitude (0.2 S cm-1) better than competing oxide and perovskite structures around 600 oC. These materials could thus have promising applications as separator membranes, hydrogen fuel cell components, and in other electrochemical uses.
[The figure shows the splitting of one of the hydrogen (deuterium) sites (identified in green as D1) (d) into two sites (f). Reprinted by permission from Macmillan Publishers Ltd: [Nature Materials] M. C. Verbraeken, et al., Nat. Mater., advance online publication, 8 December 2014, DOI: 10.1038/NMAT4136 copyright 2014]