Hydrogen transport in thin films : Mg-MgH2 and Ti-TiH2 systems

Abstract

Hydrogen storage has become progressively important due to increasing energy demand. Magne-sium (Mg/MgH2) is one of the most promising elements of hydrogen uptake, however, the slow kinetics and need for high temperatures during dehydrogenation make this material challenging for mobile applications. Meanwhile, Titanium (Ti/TiH2/TiO2) draws attention due to its catalytic effect in hydrogenation of other metals with higher capacities. A comprehensive way to quantitatively char-acterize the kinetics of hydride formation in both systems (Mg and Ti) is shown here. A technique allowing a large range of pressures and temperatures (room temperature to 300 °C and from 0.05 bar up to 100 bar) is developed successfully. Thin films (50-1000 nm), deposited by ion beam sput-tering (PVD), are used because of their smooth surface and defined structure. In order to study hydrogen transport precisely, X-ray diffraction (XRD), electron microscopy (SEM/FIB/TEM) and electric resistance measurements are used. In the case of Mg, while a Pd coating is used as catalyst, the hydride is formed from the surface towards the substrate and transformation in the morpholo-gy is observed. Parabolic law is followed and the diffusion coefficient of hydrogen in MgH2 is ob-tained at room temperature (2.67 · 10-17 cm2/s). Additionally, a model is created to fit the experi-mental change in resistance during hydrogen loading and shows the changes in the behavior of thicker layers. The interface between Pd/Mg is discussed, since Mg5Pd2 and Mg6Pd are formed at high temperatures and are most dominant over dehydrogenation. However, at room temperature, this interface appears to be more stable. The activation energy of hydrogenation is calculated ex-perimentally from an Arrhenius plot to be equal to Ea = 22.6 ± 2.0 kJ/mol and the pre-factor D0 = 3904 cm2/s. Additional attention is given to magnesium hydride as an anode electrode in Li-ion bat-teries. TEM investigations of thin film electrodes demonstrate the complete lithiation of the mate-rial however, with drastic volume changes, leading to bad reversibility. In Ti the thin oxide layer naturally formed on the surface, appears to play a dominant role in the kinetics of hydrogen transport leading to a linear kinetics. A pressure dependency is observed, while an experimental evaluation of the permeation coefficient in the oxide is also discussed. Important information on the hydrogen transport is obtained in both systems, giving an input for further improvements of such hydrides.