Hydrogenography of Mg yNi 1− yH x gradient thin films: Interplay between the thermodynamics and kinetics of hydrogenation

Abstract

We monitor the hydrogenation of PTFE/Pd-capped Mg y Ni 1− y H x gradient thin films using the change in optical transmission as a function of time, temperature and hydrogen pressure, to study the relation between kinetics and thermodynamics of hydrogenation of this multiphase hydride system. The interplay between kinetics and thermodynamics is used to extrapolate the hydrogenation equilibrium pressures via the H-absorption rate. Pressure–optical-transmission–isotherms determined independently at different temperatures provide a cross-check for the equilibrium pressure and the enthalpy of H-absorption. We find that the hydrogenation reaction is destabilized with respect to the Mg 2Ni → Mg 2NiH 4 reaction for Mg fractions close to the Mg 2Ni–Mg eutectic point. From a comparison with calculated enthalpies obtained from density functional theory, we conclude that the experimentally observed destabilization originates from the well-mixed microstructure around the eutectic point.