Thermodynamic and electrochemical hydrogenation properties of LaNi_(5-x)In_x alloys

摘要

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 LaNi4_(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_5H_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 Hoffs (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 LaNi_(0.49)In_(0.1) alloy after 4-5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.