Mathematical model for the plateau region of P–C-isotherms of hydrogen-absorbing alloys using hydrogen reaction kinetics

A theoretical treatment is presented to account for the two-phase (α–β) region of pressure-composition (P–C) isotherms of hydrogen-absorbing alloys, i.e., the dependence of equilibrium pressure on the hydrogen concentration in hydrogen-absorbing alloys at different temperatures. The model is based upon a consideration of the hydrogen–metal reaction kinetics (i.e., static equilibrium condition between adsorbed and absorbed hydrogen at the particle interface) and the hydrogen–hydrogen (H–H) interaction kinetics. The relationship between the slope, the pressure level and the length of plateau region of the P–C isotherms can be derived from this expression and this then enables us to understand the hydrogen reaction mechanism in the alloy powders. The model is found to fit very well with experimental data obtained on LaNi4.7Al0.3. It can be used to analyse the enthalpies for the hydride decomposition and hydride formation processes, and the slope changes for the two-phase (α and β) coexistence region. The average enthalpies (H/M=0.02 to 0.9) for hydride decomposition and hydride formation of the LaNi4.7Al0.3 alloy are ΔHd=34.5 kJ mol H2−1 and ΔHa=−31.8 kJ mol H2−1. The effect of lateral H–H interactions on the values of equilibrium pressure and variation of slope with temperature are discussed. H–H interactions contribute to the slope, and the slope of P–C isotherms will become lower when the degree of H–H interaction is high in the LaNi4.7Al0.3 alloy. The H–H interactions do not contribute to the hysteresis of P–C isotherms for hydrogen-absorbing alloys. It is proposed that hysteresis originates from energy differences rather than from entropy differences.