Crystallographic and thermodynamic study of La0.55Y0.45Ni5-H2, a candidate system for hydrogen buffer tanks

Electrical vehicles (EV) powered by a fuel cell will need a buffer tank to supply hydrogen especially for cold start and/or to overcome the delayed response time in the hydrogen supply system (reformer, dehydrogenation, etc.). To allow hydrogen supply at low temperature, the crystallographic and thermodynamic properties of the system La0.55Y0.45Ni5-H2 have been investigated. The pressure-composition-isotherm (PCI) curves have been carried out between 253 and 318 K. The compound La0.55Y0.45Ni5 can deliver 5.4 H/f.u. (1.3 wt.%) under 0.35 MPa at 253 K. At room temperature, between 0.6 and 3.3 H/f.u., a well-defined plateau is observed in the PCI curve. However, for capacity values larger than 3.3 H/f.u., a step is observed followed by a sloppy domain. Investigation by neutron diffraction at various states of charge has allowed determination of the La0.55Y0.45Ni5-D2 phase diagram. The existence of an intermediate γ phase has been established. The crystal structures of the α,γ and β phases have been determined. The occurrence of the γ phase could significantly increase the cycle life of the hydride by decreasing the constraints which are generated during absorption/desorption cycling as previously observed in other LaNi5-type systems.