Electrochemical hydrogenation of the R2Zn17-xMnx phases (R = Y, La, Gd, Tb)

Abstract

The electrochemical hydrogenation of the solid solutions R 2 Zn 17-x Mn x , x = 0.5-0.9 (R = Y, La, Gd, Tb) was studied in Ni-MH battery prototypes for the first time. Phase analysis of the electrode materials on the basis of these compounds was carried out before and after hydrogenation by X-ray powder diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray fluorescence spectroscopy. All the majority phases crystallize in the hexagonal Th 2 Ni 17 -type structure. As electrodes the alloys showed similar electrochemical behavior and the amount of deintercalated hydrogen increased in the order La < Y < Gd < Tb. The largest degree of amorphization and formation of oxides, such as ZnO and R 2 O 3 , were observed for the Y-and La-containing alloys after 50 cycles of hydrogenation/dehydrogenation. The geometrically most advantageous sites are octahedral voids in Wyckoff position 6h of the initial structures with a coordination polyhedron [HR 2 (Zn,Mn) 4 ] for the H-atom. Solid solution / Th 2 Ni 17 -type structure / Electrochemical hydrogenation / Ni-MH battery