Electrochemical hydrogenation of alloys based on Tb2Ni7 doped with lithium and magnesium

The α -Tb 2 Ni 7- x Mg x solid solution was investigated by X-ray powder diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy. Hydrogen sorption ability of composite alloys Tb 22 Ni 78 , Tb 22 Ni 75.5 Mg 2.5 and Tb 22 Ni 73 Li 2.5 Mg 2.5 was investigated using electrochemical hydrogenation. The alloys were synthesized by arc melting of pure components (5 wt. % excess of Li and Mg) under purified argon atmosphere. Morphology of the studied alloys surface was investigated using scanning electron microscope TESCAN Vega3 LMU. Quantitative composition of the samples was studied using Oxford Instruments energy-dispersive X-ray analyzer. The binary compound Tb 2 Ni 7 is characterized by a polymorphism. There are room-temperature phase α -Tb 2 Ni 7 (structure type Ce 2 Ni 7 , space group P 6 3 / mmc ) and high-temperature phase β -Tb 2 Ni 7 (structure type Gd 2 Co 7 , space group R -3 m ). Phase analysis showed the formation of three electrochemical phases with Ce 2 Ni 7 -, Gd 2 Co 7 - and CaCu 5 -type structure. Substitutions of Ni atoms by Mg or the statistical mixture of Li and Mg causes an increase of the unit cell parameters. The studied Tb 22 Ni 78 , Tb 22 Ni 75.5 Mg 2.5 and Tb 22 Ni 73 Li 2.5 Mg 2.5 alloys as electrode materials in nickel-metal hydride batteries are characterized by discharge capacities of 159, 171 and 180 mA·h/g, respectively. As a result of electrochemical hydrogenation, there is an isotropic increasing of the unit cell parameters. The unit cell parameters change for the α -Tb 2 Ni 7 compound after hydrogenation is: а = 4.9498(3)→4.955(1) Å, с = 24.126(3)→24.18(1) Å, V = 511.93(7)→ 514.2(3) Å 3 ; the change for the α-Tb 2 Ni 7- x Mg х ( x = 0,2) solid solution is: a = 4.9560(4)→ 4.9617(8) Å, с = 24.190(3)→24.221(6) Å, V = 514.56(9)→516.4(2) Å 3 . Four-component phase α -Tb 2 Ni 7-2 x Li x Mg x ( x = 0.2) demonstrates the biggest increase of the unit cell parameters: а = 4.9520(7)→4.9677(8) Å, с = 24.173(8)→24.275(6) Å, V = 513.3(2)→518.8(2) Å 3 . Particle size of the electrode materials decreases during electrochemical processes. Appearance of amorphous halo on X-ray powder patterns and a change of surface morphology confirmed the electrode material amorphization processes. Keywords : solid solution, X-ray diffraction, energy-dispersive X-ray spectroscopy, electrochemical hydrogenation.