Synthesis, phase analysis and electrochemical hydrogenation of an alloy based on LaNi5 doped with Zr, Mg and Sn

Phase composition and structural characteristics of a high-entropy alloy La 11.7 Zr 5.0 Ni 73.3 Mg 5.0 Sn 5.0 with the main phase La 0.92 Zr 0.08 Ni 4.40 Mg 0.15 Sn 0.45 , a doped derivative of the binary intermetallic compound LaNi 5 (CaCu 5 -type structure, space group P 6/ mmm ), were investigated using powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The alloy was synthesised by arc melting of pure components in stoichiometric composition, with the exception of Mg, which was taken in excess of 5 wt.% under a purified argon atmosphere. After that, the alloy was further annealed at 400 °C for two weeks in sealed evacuated silica tubes and later was quenched in cold water without breaking the ampoules. Phase analysis of the sample was carried out based on powder data obtained on a diffractometer DRON-2.0M (Fe K α-radiation) using the PowderCell program. Full-profile refinement of the powder patterns was carried out using the Rietveld method with the FullProf software. The quantitative and qualitative composition of the phases and the morphology of the sample surface before and after electrochemical hydrogenation were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy on the TescanVega3 LMU electron microscope with the Oxford Instruments Aztec ONE EDX system. Electrochemical hydrogenation of the alloy serving as a negative electrode was conducted in a two-electrode prototype of the chemical power source “Swagelok-cell”. Freshly synthesised nickel(II) hydroxide mixed with graphite powder (9:1 by weight) was used as a cathode material. A 6 M potassium hydroxide solution was used as the electrolyte. The cathode and anode spaces of the battery were separated by a separator made of pressed cellulose. The charging and discharging processes of the assembled battery prototype were carried out at a current density of 1.0 mA/cm 2 over 30 charge-discharge cycles in a two-channel galvanostat MTech G410-2. Electrochemical hydrogenation of the investigated alloy confirmed its ability to intercalate hydrogen (up to 0.213 H/f.u.) demonstrating high efficiency ( η = 96 %) and discharging time ( t disch ≈ 3.8 h), which was also proved by the increase in the volume of the unit cell of the main phase due to hydride formation (Δ V / V = 0.057), obtained from a full-profile refinement of X-ray data. After the electrochemical processes, an increase in the porosity of the grain surface and partial amorphisation of the anode material were observed while its crystal structure and composition remained stable throughout the entire period of electrochemical hydrogenation. The nominal voltage of the studied battery was in the range of 1.20–1.25 V, which is a typical value for nickel-metal hydride batteries. Keywords: intermetallic compound, electrochemical hydrogenation, electrode material, nickel-metal hydride battery.