Thermodynamic and electrochemical hydrogenation properties of LaNi5 − xInx alloys

Abstract

Abstract Hydrogenation properties of LaNi 5 − x In x 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 Sievert's type apparatus in 300–400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi 5 significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi 5 . The LaNi 4.9 In 0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the α -phase (hydrogen solid solution) and β -phase (LaNi 5 H 6 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 H 2 -pressure/composition ( p–c ) isotherms at several temperatures, applying the Van't Hoff's (ln p − 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 LaNi 4.9 In 0.1 alloy after 4–5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.