Continious series of solid solutions Tb2Ni17-xCox, 0 ≤ x ≤ 17. Structural and electrochemical characteristics of alloys

Synthesis, study of the phase, structural and electrochemical characteristics of the alloys from homogeneity range of the continious series of solid solutions Tb 2 Ni 17- x Co x (0 ≤  x  ≤ 17) was carried out in the prototype of NiMH battery for the purpose of search of new materials for modern electrical energy sources. The samples for investigation were synthesized by arc melting of pure components under an argon atmosphere with further annealing at 600 oC for one month in evacuated silica ampules. X-ray powder diffraction, scanning electron microscopy, X-ray fluorescent and energy dispersive X-ray spectroscopy were used to examine the alloys before and after electrochemical studies. The process of electrochemical hydrogenation of the samples was carried out in two-electrode “Swagelok-cell” prototype of NiMH battery. Studied alloys were used as anode materials (during discharging of the batteries). A mixture of Ni(OH) 2 and graphite served as cathode material (during discharging of the batteries). The anode and cathode materials were soaked by 6 M solution of KOH as electrolite and separated by pressed cellulose to avoid contacts between them. During charging we obtained the hydride of inclusion on the basis of Tb 2 Ni 17- x Co x phases. The process of hydrogenation is characterized by one stage, namely the inclusion of H-atoms into vacant tetrahedral and octahedral voids. At the small hydrogen concentration its atoms occupy the octahedral voids (6 h ) formed by two Tb atoms and four (Ni, Co) atoms. As electrode materials, the studied phases are relatively stable in the electrolyte solution. Increasing of the cobalt content improves the electrode capacity, but decreases the corrosion stability of the electrodes to the action of the electrolyte that is confirmed by the decreasing of crystallinity, passivation of the surfaces and changing of quantitative compositions of the samples. The amount of mobile hydrogen under the same conditions (during discharge) is the following: 1.53 H/f.u. for Tb 2 Ni 17 , 1.83 H/f.u. for Tb 2 Ni 12 . 2 Co 4 . 8 and 2.08 H/f.u. for Tb 2 Ni 6 . 5 Co 10 . 5 . The specific capacity does not exceed 60 mA∙h/g. The electrochemical processes in all cases led to changing of the surface morphology, reducing of the grains size. Significant amorphization was observed for electrodes with higher content of Co. Keywords : intermetallic compound, continious series of solid solutions, electrochemical hydrogenation, nickel metal hydride batteries.