Influence of doping elements on the electrochemical hydrogenation efficiency of Tb₂Ni₁₇-based phases

Chem. Met. Alloys 10 (2017) 61-68

https://doi.org/10.30970/cma10.0355

Vasyl KORDAN, Vitaliy NYTKA, Grzegorz KOVALCZYK, Agnieszka BALINSKA, Oksana ZELINSKA, Roman SERKIZ, Volodymyr PAVLYUK

Electrochemical hydrogenation of the phases Tb₂Ni_17-xM_x, x ≈ 1 (M = Li, Mg, Al, Ge, Sn, Sb, Bi, Co) that crystallize in the Th₂Ni₁₇-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 Tb₂Ni₁₇ 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 Tb₂Ni₁₆Li_0.4Mg_0.6 and Tb₂Ni_15.6Li_0.6Mg_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 [HTb₂(Ni,M)₄]. Electron microprobe analysis showed that the electrodes on the basis of Tb₂Ni_17‑xM_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).

The unit cell of Tb₂Ni_17-xM_xH_y hydrides.

Keywords

Intermetallic compound / Solid solution / Electrochemical hydrogenation / Electrode material /Ni-MH battery