SYNTHESIS AND ELECTROCHEMICAL HYDROGENATION OF Tb2Co17-х-ySbxLiy AND Tb2Co17-х-yAlxMgy PHASES

Abstract

Metal hydride batteries have a high sorption ability to the reverse hydrogen absorption, moderate pressure and temperature of the processes of hydrogen sorption-desorption and stability to cycling. We synthesized doped derivatives of binary intermetallic compound consisting terbium with cobalt to establish electrochemical properties when using them as anode materials in prototypes of nickel metal hydride batteries. The samples were made of high purity metals in an electric arc furnace in an argon atmosphere. Phase analysis was performed using X-ray diffraction data obtained on the DRON-2.0M powder diffractometer (Fe Kα-radiation). Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray fluorescence spectroscopy (XRF) were used to determine the quantitative and qualitative composition of the alloys. Electrochemical hydrogenation was performed in Swagelok cell two-electrode prototypes of chemical sources of electric current on a two-channel MTech G410-2 galvanostat in galvanostatic mode at a current density of 1.0 mA/cm2. The elemental composition of the Tb2Co16.5Sb0.2Li0.3 and Tb2Co16Al0.5Mg0.5 samples before and after hydrogenation was determined by the XRF method, a slight decrease in the content of terbium on the surface was found. Phase analysis of diffractograms of both samples before and after electrochemical hydrogenation showed that they contain hexagonal phases with a stoichiometry of 2:17. Only a phase with a structure of the Th2Ni17 type was found in the Tb2Co16.5Sb0.2Li0.3 alloy. In the Tb2Co16Al0.5Mg0.5 sample, in addition to the main phase with Th2Ni17 structure, the existence of another hexagonal phase with a similar crystal structure (TbCo5-x-yMgxAly) was detected. The results of X-ray diffraction of the Tb2Co16Al0.5Mg0.5 sample are fully consistent with the results of EDX analysis. The Tb2Co16.5Sb0.2Li0.3-based electrode undergoes significant amorphization due to the active hydrogenation/dehydrogenation process, but the hydrogen absorption properties remain acceptable. The cell parameters after hydrogenation increase, which indicates the inclusion of hydrogen atoms into the structure. The amount of deintercalated Hydrogen under experimental conditions is approximately 0.40 H/f.u. The Coulomb efficiency on the 10th cycle is equal to 81.3%, and on the 20th cycle is 90.2%. The Ni-MH battery prototype with the Tb2Co16Al0.5Mg0.5-based electrode demonstrates good reproducibility and Coulomb efficiency. The amount of deintercalated Hydrogen under experimental conditions is approximately 0.44 H/f.u. The Coulomb efficiency on the 10th cycle is 95.5%, and on the 20th is equal to 93.4%. Due to the amorphization processes as the number of cycles increases, the charging plateau voltage increases and a small decrease in the capacitive parameter τ is observed similarly to the Li,Sb-containing electrode. As electrode materials both alloys showed corrosion resistance in the electrolyte solution. Keywords: terbium; cobalt; solid solutions; crystal structure; electrochemical hydrogenation; anode materials.