The electrochemical hydrogenation of Tb 2 Ni 17-x M x phases (M = Mg, Sn) was studied for the first time. Both compounds have the hexagonal Th 2 Ni 17-type structure and small homogeneity ranges, x = 0-1 for TbNi 17-x Sn x and x = 0-1.5 for TbNi 17-x Mg x. Under the conditions of the experiment pure Tb 2 Ni 17 absorbed approximately 0.55 H/f.u. The Mg-containing phase absorbed approximately 1.14 H/f.u. and the Sn-containing phase ~0.63 H/f.u. In all cases intercalation of hydrogen occurred in octahedral voids 6h of the initial structures, so the coordination polyhedron of the H-atom is an octahedron [HTb 2 M 4 ]. The Tb 2 Ni 17-x Mg x phase absorbed the largest amount of hydrogen because magnesium, like rare-earth and transition metals, is able to absorb hydrogen and a combination of these elements leads to better hydrogen absorption. Electron microprobe analysis showed that the electrodes on the basis of Tb 2 Ni 17-x M x were stable in the electrolyte during the electrochemical processes, i.e. the qualitative and quantitative composition of the observed phases remained unchanged. Intermetallic compound / Ni-MH battery / Electrochemical hydrogenation

Electrochemical hydrogenation of the phases Tb 2 Ni 17-x M x , x ≈ 1 (M = Li, Mg, Al, Ge, Sn, Sb, Bi, Co) that crystallize in the Th 2 Ni 17-type structure was investigated for the first time. The phases containing the s-element Li or Mg, or a mixture of these (Li, Mg) as doping element showed the best Coulomb efficiency. Under the conditions of the experiment (10 mA•h charge) pure Tb 2 Ni 17 absorbed approximately 0.67 H/f.u. (50.0 % efficiency), the Li-containing phase approximately 1.43 H/f.u. (86.0 %), the Mg-containing phase 1.37 H/f.u. (76.3 %), and the phases with a mixture of Li and Mg, namely Tb 2 Ni 16 Li 0.4 Mg 0.6 and Tb 2 Ni 15.6 Li 0.6 Mg 0.8 , 1.46 H/f.u. (91.5 %) and 1.50 H/f.u. (95.0 %), respectively. The phases with p-elements such as Al, Ge, Sn revealed interaction of the surface with the electrolyte, but showed structural and corrosion stability over 30 charge-discharge cycles. The Al-containing phase absorbed 1.09 H/f.u. (67.8 % efficiency), the Ge-containing phase 1.05 H/f.u. (63.1 %), the Sn-containing phase 0.76 H/f.u. (52.5 %). the Sb-containing phase 1.24 H/f.u. (75.5 %), and the Bi-containing phase 1.46 H/f.u. (79.8 %). Cobalt was added to the initial binary compound in larger amounts (up to 26.3 at.%) because smaller quantities did not increase the amount of absorbed hydrogen, but even high Co contents did not affect the results significantly (0.98 H/f.u., 59.5 % efficiency). In all cases intercalation of hydrogen occurred in octahedral voids (Wyckoff position 6h) of the initial structures, i.e. the coordination polyhedron of the H-atoms was an octahedron [HTb 2 (Ni,M) 4 ]. Electron microprobe analysis showed that the electrodes on the basis of Tb 2 Ni 17-x M x were stable in the electrolyte over 30-50 cycles of electrochemical processes. Cyclic voltamperometry, impedance measurements and corrosion studies of the electrode materials also confirmed their stability in alkaline solutions of electrolyte (6 M KOH).