Thermodynamic and electrochemical hydrogenation properties of LaNi5 − xInx alloys

Hydrogenation properties of LaNi5 − xInx alloys (x = 0.1, 0.2 and 0.5) were examined by their direct reaction with gaseous hydrogen and by cathodic charging in 6 M KOH solution. The gas phase measurements were carried out using Sievertʹs type apparatus in 300–400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi5 significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi5. The LaNi4.9In0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the α-phase (hydrogen solid solution) and β-phase (LaNi5H6 hydride), formation of a new σ*-hydride phase was postulated at the hydrogen content extended over the region of H/f.u. = 1.3–1.8. Thermodynamic functions: enthalpy and entropy of the hydrogen absorption process were calculated from the H2-pressure/composition (p–c) isotherms at several temperatures, applying the Vanʹt Hoffʹs (lnp − 1/T) dependence. Electrochemical galvanostatic hydrogenation experiments at 185 mA/g charge/discharge rate revealed the greatest discharge current capacity of 319 mAh/g for LaNi4.9In0.1 alloy after 4–5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.