ELECTROCHEMICAL HYDROGENATION OF ALLOYS BASED ON Tb2Ni7 DOPED WITH LITHIUM AND MAGNESIUM ; ЕЛЕКТРОХІМІЧНЕ ГІДРУВАННЯ СПЛАВІВ НА ОСНОВІ Tb2Ni7, ЛЕГОВАНИХ ЛІТІЄМ ТА МАГНІЄМ

The α-Tb2Ni7-xMgx solid solution was investigated by X-ray powder diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy. Hydrogen sorption ability of composite alloys Tb22Ni78, Tb22Ni75.5Mg2.5 and Tb22Ni73Li2.5Mg2.5 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 Tb2Ni7 is characterized by a polymorphism. There are room-temperature phase α-Tb2Ni7 (structure type Ce2Ni7, space group P63/mmc) and high-temperature phase β-Tb2Ni7 (structure type Gd2Co7, space group R-3m). Phase analysis showed the formation of three electrochemical phases with Ce2Ni7-, Gd2Co7- and CaCu5-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 Tb22Ni78, Tb22Ni75.5Mg2.5 and Tb22Ni73Li2.5Mg2.5 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-Tb2Ni7 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) Å3; the change for the α-Tb2Ni7-xMgх (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) Å3. Four-component phase α-Tb2Ni7-2xLixMgx (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) Å3.Particle size of the electrode materials decreases during electrochemical processes. Appearance of amorphous halo on X-ray ...