Frequency-domain hydriding and dehydriding kinetics of LaNi 5H x

Abstract

The kinetics of hydrogen absorption and desorption by LaNi 5 have been studied at several values of the hydrogen-to-metal ratio near the absorption and desorption plateaux. The dependence of the hydrogen-to-metal ratio on frequency rather than time was measured, using small sinusoidal perturbations of the hydrogen gas pressure. In contrast to the usual complex behaviour in the time domain (mixed order kinetics, rate dependent on pressure) the hydride responded linearly and the results are well described by the frequency dependences of two concurrent and one consecutive first-order reaction steps. Relaxation times (proportional to the reciprocal of reaction rate constant) ranging from a few seconds to nearly 300 s were determined for the three reaction steps, depending on the hydrogen content. In the linear regime of these experiments, the kinetics of absorption and desorption are generally considerably slower near the desorption plateau than near the absorption plateau. Some candidate physical reaction steps are discussed and it is conjectured that the three relaxation times are due to chemi(de)sorption (consecutive) and diffusion parallel and perpendicular to the c axis (concurrent) in a surface phase, probably α-LaNi 5H x, through which all the atomic flux of H enters and leaves the metal hydride.