Studies on the hydrogen storage characteristic of La1−xCex(NiCoMnAlCuSiZr)5.7 with a B2 secondary phase

Abstract Six alloys with Ce partially substituting for La, having nominal alloy compositions of La 15− x Ce x Ni 68.7 Co 4.7 Mn 4.3 Al 5.6 Cu 1.2 Zr 0.2 Si 0.3 ( x  = 5, 6, 7, 8, 9, and 10), were prepared by induction melting. The structure, chemical composition, gaseous phase hydrogen storage, and electrochemical properties of these alloys in the as-cast and annealed (960 °C for 10 h in vacuum) states were investigated. It was found that both the hydrogen reactions in the gaseous phase and the electrochemistry of these Pr, Nd-free hyper-stoichiometric alloys were dominated by the “synergetic effect” between the AB 5 main phase and a B2 (close to AlMnNi 2 ) secondary phase (3–9 vol.%). As the abundance of the B2 secondary phase decreased with the annealing process, although the gaseous phase storage capacities improved, all the electrochemical properties (capacity, high-rate dischargeability, surface exchange current, and hydrogen diffusion in the bulk) deteriorated. Besides, with the increase in Ce-content (less La), the unit cell volume of the AB 5 main phase decreased, the c / a ratio in the same phase increased, the abundance of the B2 secondary phase increased, the hydrogen equilibrium pressure and the hydride entropy and enthalpy increased, and the maximum and reversible gaseous phase hydrogen storage capacities and the electrochemical discharge capacity increased first and then decreased. The surface exchange currents of the as-cast alloys were higher than those from the annealed alloys. Both the as-cast and annealed alloys showed higher hydrogen bulk diffusion coefficients (up to four times) than that from an AB 5 sample.